The altered huntingtin protein responsible for HD has been shown to contain many more molecules of the amino acid glutamine than regular huntingtin. This abundance of glutamine is due to repetitive copies of the CAG codon in the Huntington gene. The extended glutamine tracts of these proteins have affinity for one another, and tend to “stick together,” leading to the formation of “clumps” or aggregations of the protein in the cell’s nucleus. These protein aggregations are often referred to as neuronal inclusions (NIs).
It is not absolutely clear whether NIs are the cause or the result of HD, or whether they might even be a defense mechanism against it. Scientists are not certain whether the NIs themselves are toxic, or whether the intermediates or building blocks in the aggregation process are the toxic agents. These questions aside, there is mounting evidence supporting NIs as a primary mediator of cellular toxicity in Huntington’s disease.
There are two major lines of thought regarding the toxic mechanisms of polyglutamine protein aggregation. One is that other normal molecules with glutamine tracts get “trapped” within the NIs, and are therefore prevented from performing their normal functions. Another is that the NIs “clog” the Ubiquitin-Proteasome System (UPS), a kind of disposal system that is essential for normal cell function. For more information on the role of protein aggregation in the progression of HD, click here.
Scientists have shown that reducing the amount of protein aggregation in the cell may be beneficial for patients with HD. The drugs listed under the “Protein aggregation” navigation menu potentially reduce the amount of NIs in the cell, and are therefore being researched as possible treatments for HD.
-E. Tan, 9-21-01
Compounds that have the ability to regulate the transcription of specific sets of genes are being proposed as possible candidates for treating HD. Some of these compounds may be able to regulate genes to produce proteins that could delay various HD symptoms. For example, some of the drugs could induce the transcription of genes responsible for the production of proteins with anti-inflammatory effects. Because inflammation has been found to play a role in the destructive effects associated with HD, the production of proteins with anti-inflammatory effects could be beneficial for people with HD. The drugs listed under the “changes in gene transcription” subcategory act to induce gene transcription and increase production of proteins beneficial for people with HD, and are therefore being looked into as possible HD treatments.
-E. Tan, 11-21-01
The mutant huntingtin protein has been found to disrupt cellular metabolism, the process by which cells make energy. It interacts with key proteins needed to produce energy and causes damage to mitochondria, the ‘energy factory’ of the cell. Mitochondria produce energy in the form of molecules known as ATP (for “adenosine triphosphate”). The amount of ATP available to cells is lower in Huntington’s Disease (HD), which makes cells more susceptible to damage by toxic compounds. Scientists are looking into drugs and supplements that increase the amount of energy available in cells, as they might be possible candidates for treating HD. This article explains how huntingtin affects cellular metabolism, which is important for understanding how these drugs may improve energy production in the cell.
The Basics of Energy Metabolism
Energy metabolism is a process by which the food we eat is broken down by various enzymes in order to produce a molecule called ATP, the energy source of the cell. The pathway by which ATP is produced depends on the availability of oxygen in cells. If there is a sufficient amount of oxygen, aerobic respiration takes place in the mitochondria and large amounts of ATP are produced. If there is not enough oxygen in cells, anaerobic respiration is instead performed, which produces a smaller amount of ATP. Thus, aerobic respiration is a more efficient process because it produces more energy from the food we eat.
Glycolysis is a series of reactions that begins the process of metabolism in all cells. It takes place in the cytosol (sometimes also called “cytoplasm”), which is the fluid portion of the cell.
The important molecular product of glycolysis is called pyruvate, which can undergo either aerobic or anaerobic respiration. If sufficient oxygen is present, pyruvate gets transported to the mitochondria where it undergoes aerobic respiration. Each step of this process helps convert the food we eat from one molecule to another until ATP is produced as the end product.
HD and Cellular Metabolism
Exactly how mutant huntingtin interferes with energy production is unknown, but studies have revealed that it interacts with a variety of key proteins involved in energy metabolism. For example, the altered huntingtin protein interacts with a molecule known as GAPDH (which stands for glyceraldehyde-3-phosphate dehydrogenase), a key enzyme in glycolysis, the early part of metabolism described above. Huntingtin’s interaction with GAPDH partially prevents it from working properly. Research suggests that GAPDH interacts preferentially with small subunits of huntingtin protein rather than the full length protein. But this is precisely what the altered huntingtin becomes in people with HD: the altered huntingtin protein is readily cleaved into small pieces by proteins called caspases. (Click here to read more about caspases, or here for a figure depicting the effects of caspases in a nerve cell.) As HD progresses, cleavage by caspases is enhanced, generating more protein fragments. These fragments then interact with GAPDH and inhibit its activity, which leads to lower amounts of ATP available in cells and eventually causes cell death.
The mutant huntingtin protein is believed to have a greater impact on cellular metabolism when it has a longer glutamine tail, which happens when an individual’s copy of the HD allele has a longer segment of CAG repeats. Cells engineered to express huntingtin with particularly long polyglutamine tails were significantly worse at making ATP than cells expressing huntingtin with medium-length polyglutamine tails.
Damage to Mitochondria
Aside from interfering with one of the enzymes involved in glycolysis, mutant huntingtin also interferes with oxidative phosphorylation, the final step in aerobic respiration. Specifically, mutant huntingtin makes the electron transport chain less efficient. The electron transport chain is a series of protein complexes that are found in the membrane of mitochondria, and is a vital component of oxidative phosphorylation. The protein complexes are named Complex I, II, III, and IV. As electrons are transported from one complex to another, protons (H+) are pumped out into the space between the inner and outer membrane of the mitochondria. As protons are pumped into the space between the two membranes, a proton gradient forms – more protons are present in the space between the two membranes. The proton gradient is essential in ATP production. The protons that accumulate between the two membranes are then transported through a molecule called ATP synthase. ATP synthase then produces ATP molecules that the cell uses as its source of energy.
Most studies report that HD cells exhibit reduced activity in complex II and III. A few studies have also reported decreased activity in complex I as well. Scientists are still not certain how the huntingtin protein interacts with these protein complexes. They currently speculate that that the altered huntingtin protein may indirectly interfere with these complexes by interacting with other molecules involved in the electron transport chain. As the altered huntingtin protein disrupts this step of metabolism, the cell experiences more energy deficits, with some experiments suggesting that neurons in the striatum, a region of the brain heavily affected in HD, make 30% less ATP than non-HD neurons. This makes those brain cells more susceptible to damage by toxic substances such as glutamate.
In summary, because of damage to mitochondria in neurons of people with HD, aerobic respiration is less efficient and therefore produces less energy. Compounds that target different parts of the pathways of aerobic respiration are currently being studied to determine if they increase the energy supply available to cells and may therefore be potential drugs for HD.
As mentioned earlier, anaerobic respiration occurs when there is not enough oxygen available to cells. Anaerobic energy producing pathways are called fermentation. Organisms that do not need oxygen in order to grow and survive rely on fermentation as their main source of energy. Examples of such organisms include bacteria. During exercise, our skeletal muscles also rely on fermentation for energy during the few moments when insufficient amounts of oxygen are available. Fermentation produces lower amounts of energy and releases various by-products. In the muscle, the by- products of fermentation include molecules called lactate (also known as lactic acid). The accumulation of lactic acid is what makes our muscles hurt when we exercise. A summary of the steps involved in anaerobic respiration is shown below.
If you remember, the altered huntingtin protein has been found to partially inhibit the activity of the GAPDH enzyme, resulting in impairments in glycolysis. Given that fermentation requires the products of glycolysis in order to occur, how then can fermentation still occur in HD cells? It turns out that partial inhibition of GAPDH still allows some fermentation to occur, although complete inhibition would block glycolysis, and consequently, fermentation.
The altered huntingtin protein has been found to interfere with an enzyme involved in glycolysis and the electron transport chain. As a consequence, more fermentation occurs relative to aerobic respiration. Studies have reported that people with HD have increased brain lactate levels, indicating damage to mitochondria and impaired energy metabolism. Lactate levels are often used in studies to measure the efficiency of a drug or supplement. Lower lactate levels after treatment is seen as an indication of improved metabolism in cells.
The Big Picture
So what do defects in energy metabolism mean for people with HD? Brain scans reveal that people with HD metabolize glucose more slowly in certain parts of the brain. One of those regions, the basal ganglia, is responsible for controlling movements. Patients with particularly impaired metabolism in the basal ganglia have worse motor symptoms and lower functional capacity. Moreover, some scientists think that defective energy metabolism is partly responsible for the weight loss that many people with HD experience, as described in more detail here.
The drugs outlined in this “Abnormalities in Energy Metabolism” section are meant to boost energy, and hopefully reverse some of the effects described in this article.
-E. Tan, 9-21-01, updated M. Hedlin 12.22.11
Autophagy is a process by which a cell breaks down and recycles its own components. In normally functioning animal cells, autophagy occurs at a very low level. Autophagy pathways are activated when a cell is running low on nutrients. The cell breaks down already existing proteins and other cell components into their basic building block components so that they can be reused to maintain essential cellular functions. There is also evidence to suggest that autophagy can be used by the cell to break down misfolded proteins.
The induction of autophagy in Huntington disease (HD) cells results in the accelerated breakdown of huntingtin aggregates and has been shown to have neuroprotective effects. It is currently unknown whether huntingtin aggregates are the cause or result of HD, but nerve cells that build up huntingtin aggregates often die. To read more about huntingtin protein aggregation and its role in HD, click here.
The Process of Autophagy ^
The part (or parts) of the cell that is to be degraded is first engulfed by a double membrane to separate it from the rest of the cell; the resulting membrane-enclosed bubble of cytosol (along with all the proteins the bubble contains) becomes what is called the autophagosome. The autophagosome eventually fuses with a cellular organelle called a lysosome, a much larger membrane-enclosed bubble that contains a variety of enzymes that can break down many types of cellular components (which is why lysosomes are sometimes referred to as the “garbage disposals” of the cell). These enzymes only work in a very acidic environment, so the pH inside lysosomes is much lower than the neutral pH in the rest of the cell. This pH barrier, as well as the physical barrier of the organelle membrane, protects the rest of the cell from being degraded should the enzymes somehow leak out. Once the contents of the autophagosome are delivered to the lysosome, the lysosomal enzymes break down the new contents, which can then be recycled and reused within the cell.
Until a couple of years ago, it was believed that the main mechanism by which the nerve cell got rid of huntingtin aggregates involved what is called the ubiquitin-proteasome system, which is responsible for tagging and degrading improperly formed proteins. However, recent research shows that proteins with abnormally expanded stretches of the amino acid glutamine, like the altered huntingtin protein (which is associated with HD), are also disposed of by the process of autophagy. In this process, the aggregated proteins are gathered up and transported to the lysosome, where they are broken down and their component amino acids are recycled. Studies of nerve cells have shown that the mutant huntingtin protein can often be found in autophagosomes, the membrane-bound sacs that carry cell parts to the lysosome for degradation.
Researchers have investigated whether proteins with expanded sections of the amino acids glutamine and alanine could be degraded by cells using the process of autophagy. They compared autophagy with the ubiquitin-proteasome process, which was originally thought to be the only process by which these harmful proteins are degraded. The researchers used cells that expressed these proteins and tagged them with green fluorescent protein (GFP) in order to visualize their fate within the cells. GFP allows researchers to see the amount and the location of a specific protein present in the cell because it fluoresces, or glows, when viewed under a special microscope. To study how huntingtin aggregates are broken down by the cell, they used cells that produced, or expressed, part of the HD allele that contained either 55 or 74 CAG repeats. (To read more about the huntingtin protein, click here.)
To determine whether autophagy is indeed a key process in the clearance of huntingtin aggregates, the researchers first used two different compounds to inhibit autophagy at different points of the process and observed the effect on aggregate formation. The first compound they used inhibits autophagy by preventing a membrane from surrounding the cell contents that are about to be degraded; if the autophagosome cannot form, the contents cannot be delivered to the lysosome to be broken down. The second compound they used prevents the autophagosome from fusing with the lysosome and releasing its contents, which also prevents autophagy from occurring. Treatment with these compounds resulted in visibly higher levels of huntingtin aggregates in cell cultures, which showed that autophagy does play a role in the breakdown of aggregates. Along with the increase in aggregates, the researchers also saw increased cell death when the cells were treated with autophagy-inhibiting compounds.
The researchers also tested the role of the ubiquitin-proteasome system in reducing protein aggregation in the same cell cultures. Most previous experiments have used a certain compound to inhibit the proteasome that is thought to inhibit the function of the lysosome as well. Because they wanted to test the role of the proteasome only, the researchers used a different compound that inhibits the proteasome and has no effect on lysosomes. They found that inhibiting the proteasome increased aggregate formation in one cell line but not in another. While these results are somewhat inconclusive, they may suggest that the ubiquitin-proteasome process is not the main mechanism by which cells get rid of the disease-state huntingtin protein. More research about the role of autophagy in degrading mutant huntingtin needs to be done.
Several drugs are known to modulate the process of autophagy in different ways. The hope is that drugs which promote autophagy will aid nerve cells in breaking down huntingtin aggregates and help to protect the cells. Research is being done to identify the effectiveness of different types of drugs.
For further reading^
- Raught, et al. “The target of rapamycin (TOR) proteins.” Proceedings of the National Academy of Sciences of the United States of America. 2001 Jun 19;98(13):7037-44.
Short paper which describes various functions of target of rapamycin (TOR) proteins in fairly technical writing.
- Ravikumar, et al. “Aggregate-prone proteins with polyglutamine and polyalanine expansions are degraded by autophagy.” Human Molecular Genetics. 2002 May 1;11(9):1107-17.
2001 Jun 19;98(13):7037-44.
Fairly technical article which describes experiments aimed to discover whether or not proteins with multiple amino acid repeats could be controlled through the process of autophagy.
- Ravikumar, et al. “Inhibition of mTOR induces autophagy and reduces toxicity of polyglutamine expansions in fly and mouse models of Huntington disease.” Nature Genetics 2004 Jun 36(6):585-95.
This technical paper describing the effects of mTOR inhibition was cited in the “mTOR and HD” section.
- Sarkar S., et al. “Rapamycin and mTOR-independent autophagy inducers ameliorate toxicity of polyglutamine-expanded huntingtin and related proteinopathies.” Cell Death and Differentiation advance online publication, 18 July 2008; doi:10.1038/cdd.2008.110.
Very technical paper which describes the effects of autophagy inducers in controlling HD and other diseases caused by malformed proteins.
- Thoreen, et al. “Huntingtin aggregates ask to be eaten.” Nature Genetics. 2002 Jun;36(6):553-4.
Less technical article that describes the role of autophagy in controlling mutant huntingtin aggregates.
- Williams et al. “Novel targets for Huntington’s Disease in an mTOR-independent autophagy pathway.” 2008 May;5(4):295-305
Less technical article which reviewed the role of calpains in HD and different autophagy-inducing therapies was cited in the “Calpains and HD” and the “Combination Therapies” sections.
A. Pipathsouk, 4/24/09
Inflammation, what we commonly know as the swelling, redness, heat, and pain that often accompany injuries, is one of our body’s most important natural defense mechanisms against internal and external threats. The inflammatory process protects our body from damage and disease by releasing cells and mediators that combat foreign substances and help prevent infection. However, these same inflammatory elements can also be deadly to the body when “switched on” too long, a condition known as chronic inflammation. Research has indicated that chronic inflammation is common in the nerve cells of patients with HD, and that it may be a powerful mediator of HD’s neurodegenerative damage.
Because the constant pain that often accompanies chronic inflammation is often a source of complaint in many diseases, there are a relatively large number of anti-inflammatory drugs and therapies available. However, most of these treatments target inflammation at a general level, and were not designed to block the inflammatory response specifically in the nerve cells affected by HD. As such, they can lead to unwanted side effects. The following anti-inflammatory drugs and supplements presented below have either the theoretical potential to alleviate inflammatory damage in the brains of patients with HD, or have been tested on other neurological diseases in which inflammation is a disease mechanism, such as Alzheimer’s disease. Some experiments and/or clinical trials of these treatments have been done on either animals or human patients with HD.
Important information about Huntington’s disease and inflammation^
Studies of the HD brain indicate that long-term inflammation plays a significant role in the progression of HD. Given this finding, scientists are trying to understand the specific role of inflammation and are investigating the possibility of anti-inflammatory drugs as HD therapies.
The process of inflammation can be thought of as our body going into battle. Both inflammation and wars are responses to outside threats. Inflammation is a complex process that causes swelling, redness, warmth, and pain. It’s our body’s natural response to injury and plays an important role in healing and fighting infection. Similar to war, inflammation has its own troops: immune cells that secrete various molecules and enzymes that kill foreign invaders. Inflammation destroys and kills the injury-causing agent through a variety of mechanisms. Short-term inflammation protects the body from damage and disease. However, long-term or chronic inflammation, much like a drawn-out war, can lead to damage, not only to the foreign substances, but to the body itself as well.
Studies of the HD brain indicate that chronic inflammation plays a significant role in the progression of HD. Our body’s immune system has the ability to recognize foreign substances and launch various defense mechanisms to get rid of these potentially harmful substances. Scientists believe that the immune system recognizes the expanded glutamine tract in the altered huntingtin protein as “foreign” and tries to get rid of it, resulting in chronic inflammation and damage.
Studies have also shown that excitotoxic amino acids such as glutamate induce a direct activation and proliferation of cells involved in inflammation. Since glutamate activity is also implicated in the progression of HD, it is possible that the glutamate molecules in the HD brain induce an inflammatory response.
The inflammatory response results in the activation of various types of cells and the production of different molecules that can lead to cell death. An example of cells activated by the inflammatory response are the microglia (a type of immune cell) which have been found to be highly activated in the HD brain.
What are glial cells?^
Nerve cell bodies and axons are surrounded by glial cells. Glial cells outnumber nerve cells by about five to one in the nervous system. Although their names come from the Greek word for glue, glial cells do not actually hold other cells together. Furthermore, glial cells do not conduct nerve impulses, and are thus not essential for processing information. Rather, they serve as supporting elements to the brain and act as scavengers, removing debris after injury or neuronal death. Two types of glial cells produce the fatty coating that covers large axons of the nerve cells.
There are many different types of glial cells in the nervous system. Glial cells such as the oligodendrocytes produce the fatty coating in nerve cells, the astrocytes maintain ionic balance, while the microglia get rid of unwanted substances.
Glial cells and HD^
Research has shown that there is a marked increase in microglia in the HD brain. Microglia play the role of immune cells in the brain. They are sometimes called “brain macrophages” because they perform many of the same functions that macrophages in our body do. Macrophages are immune cells found all over our body that act as scavengers, engulfing dead cells, foreign substances, and other debris.
In the brain, the microglia act as macrophages, getting rid of unwanted substances by engulfing them and “eating” them.
Microglia are normally inactive. They become active in the brain following a variety of debilitating events such as infection, trauma, and decreased blood and oxygen flow. Once activated, the microglia are then able to remove dying neurons and other cells.
In the HD brain, an increase in activated microglia is found along the vicinity of nerve cells that contain neuronal inclusions (NIs) – accumulation of the huntingtin protein. This finding suggests that the huntingtin protein accumulation influences the activation of reactive microglia. Nerve cell injury due to excitotoxins such as glutamate also induces long-term microglial activation in the brain. Excitotoxins are excitatory amino acids found in increased concentrations in the nervous system and cause damage and cell death. (For more on excitotoxins, click here.)
Microglia and other inflammatory mediators^
Aside from engulfing foreign substances, activated microglia are also capable of producing various substances that act as mediators of the inflammatory response. Although these mediators play an important role in inflammation, they are also potentially neurotoxic substances that can contribute to widespread central nervous system injury. Examples of inflammatory mediators include free radicals, proteases, excitatory amino acids, complement proteins, cytokines, and certain prostaglandins. These substances are the microglia’s “weapons”: they act to kill the foreign substance that invade our body. However, as stated before, chronic inflammation results in chronic release of these substances, which can eventually lead to considerable damage and cell death.
The exact mechanisms of these substances are not covered in this section. More information on each of the various inflammatory mediators can be found in various sources listed in the references section.
These mediators each have different roles in the inflammatory response, but for our purposes, it is sufficient to know that all of them contribute to inflammation and are found in increased concentrations in the brains of people with neurological diseases such as HD and AD. Drugs that could lower the concentrations of these molecules are therefore attractive treatment agents for people with diseases where inflammation plays a prominent role.
Inflammation and disease^
Inflammation, whether in the brain or in other parts of the body, is almost always a secondary response to some primary disease-causing substance or event. Despite the fact that inflammation is a secondary response, it is still an important mechanism that can protect or damage the cell, depending on its severity and length of occurrence. In head trauma, for example, the blow to the head is the primary event. However, what may be of greater concern is the secondary inflammatory response that will result from the primary event. When it continues for a long period of time, inflammation is likely to cause more neuron loss than the initial injury. Given that chronic inflammation has been reported in the brains of people with HD, anti-inflammatory compounds that will delay the inflammatory response or eradicate it altogether may be potential HD treatments to consider.
Various inflammatory mediators are released by our immune cells during times when harmful agents invade our body. Long-term release of some of these inflammatory mediators has been observed in the cells of people with HD. An understanding of how these mediators work and how to block their release could be helpful in looking for ways to delay the progression of HD.
Our body must defend itself against many different disease-causing substances such as viruses, bacteria, and parasites, as well as tumors and a number of various harmful agents. To combat these disease-causing substances or events, our body has developed many mechanisms to defend itself against such an “attack.” One of the ways by which our body protects itself is by triggering an inflammatory response. Early scientists considered inflammation as our body’s primary defense system. However, inflammation is more than just a simple defense system, because when left unchecked, it could lead to debilitating diseases such as arthritis or even death. Long-term inflammation is also linked to the progression of neurological diseases such as Alzheimer’s Disease and Huntington’s Disease.
The development of inflammatory reactions is controlled by various molecules released by our body’s immune cells. Our immune cells act as the body’s “soldiers”, and they guard the body against attack by releasing “weapons” in the form of inflammatory mediators. One type of immune cell found to be present in extraordinarily high concentrations in the HD brain is the microglia. The microglia have been observed to release various inflammatory mediators that contribute to the long-term occurrence of inflammation in the HD brain, resulting in damage and cell death.
We will go over some of the most common inflammatory mediators released by the body’s immune cells in order to understand how the inflammatory response works. In general, most of the mediators that we will talk about in this section have one of two roles: amplification of the immune response, or destruction of the foreign substance.
This section will discuss the following inflammatory mediators: Free Radicals, Excitotoxins, Complement, Cytokines, Prostaglandins.
Free radicals are atoms or molecules that are highly reactive with other cellular structures because they contain unpaired electrons. As free radicals react with cellular structures, they lead to cellular injury and eventually, cell death. Free radicals may also trigger activation of various proteins that in turn activate the inflammatory response.
Although the majority of the research on HD focuses on free radical generation due to impaired electron transport chain functioning, the concept of free radical toxicity actually has its roots in inflammation biology. (Click here for more information on free radicals and antioxidants.) The secretion of reactive oxygen and nitrogen free radical species by inflammatory cells is a major mechanism for attacking foreign substances. Large amounts of free radicals are produced by activated microglia, and chronic release of free radicals result in neuronal injury and cell death.
Excitotoxins such as glutamate and quilonic acid are excitatory molecules that are released by immune cells and are known to cause damage to the body. They can also result in cognitive impairment when found in increased concentrations in the brain. Glutamate has specifically been found to initiate various mechanisms that ultimately lead to cell death. (For more information on glutamate, click here.)
Complement is a set of many proteins activated in sequence when cells are exposed to a foreign substance. Once the proteins are activated, nine of them come together to form the membrane attack complex (MAC). When assembled on a cell membrane, MAC forms a ring-like structure that allows the movement of ions and small molecules into and out of the cell, disrupting the normal state of the cell.
The MAC creates a pore that allows the movement of various ions and substances into and out of the cell, resulting in cell damage.
The complement system is a potent mechanism for initiating and amplifying inflammation. One of the most damaging effects induced by the formation of MAC is the entry of calcium ions (Ca2+) into the cell. The Ca2+ ions are capable of activating various Ca2+-dependent proteins that contribute to cell death. If a sufficient number of MACs have assembled on the cell, cell death eventually occurs.
Studies have reported that a number of complement proteins are expressed at a higher level in HD brains compared to non-HD brains. The increased number of activated microglia induced by the altered huntingtin protein most likely causes the higher levels of complement proteins in HD brains.
Cytokines are proteins that are secreted by various types of immune cells and serve as signaling chemicals. The central role of cytokines is to control the direction, amplitude, and duration of the inflammatory response.
There are two main groups of cytokines: pro-inflammatory and anti-inflammatory. Pro-inflammatory cytokines are produced predominantly by activated immune cells such as microglia and are involved in the amplification of inflammatory reactions. Anti-inflammatory cytokines are involved in the reduction of inflammatory reactions. Table 1 lists some of the most common proinflammatory and inflammatory cytokines.
Table 1: List of common pro-inflammatory and anti-inflammatory cytokines
Prostaglandins are produced in most tissues of the body and have varying actions. They are short-lived, hormone-like chemicals that regulate cellular activities on a moment-to-moment basis. Prostaglandins fall into 3 series – PG1, PG2, and PG3. PG1 and PG3 are known to have anti-inflammatory effects as they decrease inflammation, increase oxygen flow, prevent cell aggregation, and decrease pain. PG2 are known to have pro-inflammatory effects, since their effects are opposite to those of PG1 and PG3. Table 2 shows a comparison of the effects of the different prostaglandins.
Table 2: Prostaglandins
Because of the negative effects of chronic inflammation, it is speculated that people with HD would most likely benefit from an increase in Series 1 and 3 prostaglandins and a decrease in Series 2 prostaglandins.
For further reading^
- Inflammation: http://nic.savba.sk/logos/books/scientific/node4.html
This page contains detailed information about inflammation. It goes over all phases of the inflammatory response in a technical manner. Although not as easy to understand as other sites, this page has vast amounts of information for the person seeking to understand the many aspects of inflammation.
- Mulitple Sclerosis Glossary: http://www.albany.net/~tjc/gloss.html
This page contains a glossary of words relevant to multiple sclerosis (MS) – an auto-immune disease that affects the nervous system. It is useful for looking up terms and concepts about the nervous system and immune system.
- Neuroinflammation Working Group. “Inflammation and Alzheimer’s Disease.” Neurobiology of Aging. 2000; 21: 383-421.
This article contains detailed, comprehensive information on the inflammatory response and Alzheimer’s Disease (AD). It has information on the many studies done on the role of inflammation on the pathology of AD as well as the trials conducted on various anti-inflammatory compounds.
- Sapp, et al. “Early and Progressive Accumulation of Reactive Microglia in the Huntington Disease Brain”.Journal of Neuropathology and Experimental Neurology. 2001; 60(2): 161-172.
This article contains information on a study done that investigated the presence of reactive microglia in postmortem brains of people with HD. The study reported that increased levels of reactive microglia are present in HD brains.
- Singhrao, et al. “Increased Complement Biosynthesis By Microglia and Complement Activation on Neurons in Huntington’s Disease.” Experimental Neurology. 1999 Oct; 159(2):362-376.
This article contains the full details on a study done to investigate the levels of inflammatory response proteins in HD nerve cells. The study reported that increased levels of those proteins are found in HD nerve cells.
-E. Tan, 9/21/01 and P. Chang, 5/6/03
Glutamate is a powerful excitatory neurotransmitter that is released by nerve cells in the brain. It is responsible for sending signals between nerve cells, and under normal conditions it plays an important role in learning and memory
The X-Linked Inhibitor of Apoptosis Protein (XIAP) gene is a gene present in normal body cells that inhibits the activity of caspases 9, 3, and 7. A caspase is an enzyme that degrades proteins and is involved in certain types of cell death, also known as apoptosis. In Huntington’s disease (HD), the presence of mutant huntingtin clumps, or aggregates, activates caspases. Once a caspase is activated, it can cut mutant huntingtin protein into smaller pieces, making the mutant huntingtin protein more toxic and causing brain cells to die. Remember that some evidence exists that apoptosis of brain cells is the root of the neurodegenerative problems in HD. To learn more about cell death in HD, click here.
Because the XIAP gene inhibits caspase activity, it can prevent cell death, which means it has great potential in the treatment of neurodegenerative diseases such as HD. The goal of XIAP gene therapy is to inject this gene into cells that are affected by HD so that apoptosis does not occur.
How has XIAP gene therapy been shown to prevent apoptosis?^
In 2005, researchers working for the pharmaceutical company Neurologix Incorporated tested the potential of the XIAP gene both in vitro (outside of a living organism) and in vivo (inside of a living organism). In vitro, the scientists added the XIAP gene (also known as dXIAP) to brain cells that were designed to have the HD mutation. The study found that the addition of dXIAP significantly decreased the number of cells that died due to apoptosis. The scientists also confirmed the potential of XIAP gene therapy in rat models. The rats were engineered to have symptoms of Parkinson’s disease, a disease that, like HD, results in brain cell death. Researchers found that the neurons of rats that were injected with dXIAP were protected against apoptosis.
In the same study, the scientists also aimed to determine more specifically how XIAP gene therapy works. To do this, they engineered four mutant versions of the dXIAP gene. In each mutant version, a different small section of the gene was mutated so that its function was disrupted. Each of these mutant versions was injected into HD-mutant cells to test their effectiveness. The scientists found that only a mutation in a section called BIR3 prevented dXIAP from effectively stopping cell death. This means that the BIR3 section, specifically, is most crucial to the success of XIAP gene therapy. This was useful because the scientists already knew that the BIR3 domain commonly interacts with caspase 9. Thus they concluded that the neuroprotective effect of the XIAP gene may primarily work by stopping the activity of caspase 9 rather than caspase 3 or 7.
What’s the potential of XIAP gene therapy in HD?^
Neurologix Incorporated has taken the initiative to determine how the XIAP protein could be used to treat neurodegenerative symptoms in HD patients. In 2008, the company was given exclusive rights to use XIAP to develop a treatment for HD. As mentioned above, Neurologix has already shown that XIAP gene therapy has some effect in preventing cell death in rodents, but it is not yet ready to be tested in humans. Many more experiments with rodent models of HD have to be done before XIAP gene therapy can even be considered a possible treatment for HD in humans. Neurologix does plan to conduct clinical trials in humans, but it is unknown when they will be ready and able to begin these trials.
For Further Reading^
-C. Garnett, 2-28-10
Because of the ability of neurotrophic factors (NTFs) to protect dying neurons, scientists believe that these proteins could one day be used to treat neurodegenerative disorders such as Huntington’s disease (HD) and Parkinson’s disease. One NTF currently being examined is neurturin, a member of the Glial cell line-derived neurotrophic factor (GDNF) family. Studies performed in living organisms ( in vivo) suggest that neurturin is not essential for survival. Mice born without neurturin are able to grow, reproduce and survive similar to mice born with neurturin. However, these in vivo studies of mice provide evidence that neurturin is essential for certain neural functions, such as controlling the sensory nerves. Additionally, neurturin has been shown to promote the survival of certain neurons in vitro, including those found in the sympathetic nervous system, the dorsal root ganglion, and the midbrain.
Does neurturin affect the progression of HD?^
Neurturin and its receptor can be found in the striatum, the region of the brain that is greatly affected by HD (Click here to see our section on the effects of HD on striatal neurons). One in vivo study compared the protective effects of neurturin and GDNF (the namesake NTF of the GDNF family) by engineering cells to serve as NTF production factories and grafting, or transplanting, these cells into mice. These mice were then given injections of chemicals intended to mimic the excitotoxic model of HD (Click here to see our section on the excitotoxic model). Neurturin was not only more effective than GDNF at rescuing a specific type of striatal neurons, but the former NTF also reduced the extent of neuronal damage caused by excitotoxic damage. Interestingly, the study found that neurturin and GDNF interacted with striatal neurons in different ways, suggesting that these factors may work together to protect these neurons. Indeed, GDNF has been found to be more effective than neurturin at protecting certain populations of striatal interneurons, nerve cells that connect afferent neurons (those that carry sensory information to the brain) and efferent neurons (those that carry nerve impulses away from the brain). Future research may look at ways of combining different NTFs to more effectively preserve damaged neurons.
Can neurturin one day be used to treat human patients with neurodegenerative diseases?^
The therapeutic application of neurturin is currently being investigated in a series of clinical trials run by the drug company Ceregene. A major challenge to the therapeutic use of neurturin and other NTFs is figuring out how to sustainably deliver these compounds into the brain. Because NTFs do not cross the blood-brain barrier, they cannot be administered orally. One proposed method has been the use of viral vectors to deliver a gene engineered to over-express neurturin into the striatum (For more information on viral vectors, click here). These genes can be thought of as neuturin factories, designed to increase the levels of neuturin produced by these cells. Once introduced, viral vectors with these genes have been shown to consistently and selectively deliver neurturin to dying neurons in cultures. Scientists at Ceregene have demonstrated that the viral vector delivery of neuturin (trade name: CERE-120) protected damaged neurons in mice and monkey models of Parkinson’s disease. Based on these results, CERE-120 for Parkinson’s disease is currently being evaluated in Phase II clinical trials. However, recent results have not been encouraging—patients treated with CERE-120 failed to show significant improvements over those who did not receive treatment. As a result, Ceregene is currently evaluating their future plans for CERE-120.
CERE-120 has also been proposed as a potential treatment for HD. The administration of CERE-120 to mouse models of HD showed evidence of both structural and functional protection of nerve cells—the mice not only showed decreased rates of neuron death, but also exhibited improved motor control. Positive results have been observed both in transgenic HD rodents, as well as rodents chemically induced to show symptoms of HD. The use of CERE-120 in humans to treat HD is currently being evaluated in pre-clinical development. Updates on the progress of CERE-120 will be added to this page as necessary.
For Further Reading^
- Alberch, J., Pérez-Navarro, E., & Canals, J.M. (2002) Neuroprotection by neurotrophins and GDNF family members in the excitotoxic model of Huntington’s Disease. Brain Research Bulletin 57(6): 817-822.
- This paper reviews the research on the potential of NTFs in the GDNF family to protect neurons in animal models of Huntington’s disease. Primarily written for scientists.
- Ceregene. Pipeline. http://www.ceregene.com/pipeline.asp. Accessed October 7, 2009.
- Gasmi, M., Brandon, E.P., Hergoz, C.D. et al. (2007) AAV2-mediated delivery of human neurturin to the rat nigrostriatal system: Long-term efficacy and tolerability of CERE-120 for Parkinson’s disease. Neurobiology of Disease 27: 67-70.
- Heuckeroth, R., Enomoto, H., Grider, J., et al. (1999) Gene targeting reveals a critical role of neurturin in the development and maintenance of enteric, sensory, and parasympathetic neurons. Neuron 22(2): 253-263.
- This article seeks to characterize the normal functions of neurturin by examining mice incapable of producing this NTF. Although the language is technical at times, the article is pretty easy to understand.
- Kordower, J.H., Hergoz, C.D., Dass, Biplob, et al. (2006) Delivery of neurturin by AAV2 (CERE-120)-mediated gene transfer provides structural and functional neuroprotection and neurorestoration in MPTP-treated monkeys. Annals of neurology 60: 706-715.
- This technical article examines the effectiveness of CERE-120 in monkey models of PD. New treatments are usually tested on monkeys before they go into clinical trials.
- Pérez-Navarro, E., Akerud, P., Marco, S., et al. (2000) Neurturin protects striatal projection neurons but not interneurons in a rat model of Huntington’s Disease. Neuroscience 98(1): 89-96.
- This article investigates the ability of neurturin to protect striatal neurons in rodent models of HD. The language can get very technical, but its conclusions are very clear and easy to understand.
- Ramaswamy, S., McBride, J.L., Han, I., et al. (2008) Intrastriatal CERE-120 (AAV-Neurturin) protects striatal and cortical neurons and delays motor deficits in a transgenic mouse model of Huntington’s disease. Neurobiology of Disease 34: 40-50.
- This article examines the effectiveness of CERE-120 in the treatment of transgenic mice with the mutated Huntington gene. The introduction is pretty accessible to all readers.
- Ramaswamy, S., McBride, J.L., Hergoz, C.D. (2007) Neurturin gene therapy improves motor function and prevents death of striatal neurons in a 3-nitropropionic acid rat model of Huntington’s disease. Neurobiology of Disease 26: 375-384.
- This article uses CERE-120 to treat rats chemically induced to exhibit HD-like symptoms. The writing is quite technical throughout.
-Y. Lu, 1-17-10
There is ample evidence that Huntington’s disease is associated with a specific genetic mutation that produces an expanded polyglutamine chain in the huntingtin protein. This mutation causes huntingtin to become a misfolded protein with an altered shape. One of the hallmarks of HD is the build-up of short, broken fragments of the altered huntingtin protein in the nucleus of the nerve cell. There are many theories regarding the actual role of these fragments of altered huntingtin protein in the nerve cell’s nucleus. However, many scientists believe that the accumulation of these fragments in the nucleus directly underlies the death of nerve cells in HD. Nerve cell death is responsible for the many cognitive, behavioral, and motor symptoms of HD (for more information about HD symptoms, click here.
The nucleus of a mammalian cell is enclosed by a nuclear envelope, a membrane that features many small openings or “pores.” (The nuclear membrane and its pores can be seen in Segment 4 of the “Basics of HD” video: click here to view that segment.) These pores allow different molecules to move back and forth between the nucleus and the cytoplasm. But these pores are very small and allow only smaller molecules to cross the nuclear envelope. Larger molecules require other, more complex mechanisms to be transported into the nucleus, and these mechanisms often take longer as well.
The altered huntingtin protein associated with HD normally resides in the cytoplasm of a nerve cell because it is too big to be able to easily cross the envelope into the nucleus. But when that intact protein is cut up into small fragments, those fragments can easily move into the nucleus and cause dangerous problems for the cell. Proteases are a family of proteins that break up other proteins into smaller pieces. Studies have shown that a specific group of proteases called caspases play a big role in cutting up altered huntingtin into small fragments that can move from the cytoplasm into the nucleus.
A recent study from the lab of Michael Hayden at the University of British Columbia has shown that a particular caspase protein, named caspase-6, may be responsible for the type of huntingtin fragments that lead to nerve cell death and symptoms in HD. In the study, scientists used a mouse model of HD and changed the altered huntingtin protein so that caspase-6 could no longer cut it into fragments. They found that these mice showed no evidence of nerve cell death and they never developed any symptoms of HD. This finding suggests that a drug that inhibits the activity of caspase-6 may be a treatment for HD.
Background on caspases and HD^
Caspase-6 is not the only protein to cut up altered huntingtin into fragments. Previous studies have shown that caspases can be divided into three rough categories (for more on caspases, click here). There are “ICE-like” caspases (named for their similarity to another kind of protease called the interleukin-1b converting enzyme), “initiator” caspases, and “effector” caspases. ICE-like caspases include caspase-1, 4, and 5. These three seem to play a role in fragmenting proteins involved in processes like inflammation, rather than fragmenting the huntingtin protein. Initiator caspases include caspase-3, 7, and 2, and convert the inactive form of an effector caspase to an active form by cutting off one or two small fragments from the inactive effector caspase. The three effector caspases include caspase-6, 8, and 9, which are the caspases that (when activated by the initiator caspases) actually break down most other proteins.
But there is much overlap between all the caspases, and some fit in more than one category. Furthermore, each of these nine caspases have different target sites where they interact with other proteins. Target sites are specific short sequences of amino acids within a protein where the caspase cuts the protein. Studies have shown that caspase-1, 3, and 6 all target altered huntingtin protein, but they do so at different target sites. The altered huntingtin protein has three locations that have the right target sequences for cleavage by caspase-1. However, for unknown reasons, caspase-1 does not fragment the altered huntingtin protein very much. There are four sites in the altered huntingtin protein that serve as targets for caspase-3. Two of them are active, and caspase-3 does indeed fragment the huntingtin protein at these points. The other two sites are considered “silent” because caspase-3 does not use those targets to fragment huntingtin. Finally, there is only one site that caspase-6 can target, and it’s an active site, so caspase-6 does fragment the huntingtin protein.
The Hayden lab study^
While it was known for some time that both caspase-3 and caspase-6 break down huntingtin protein into fragments, it was not known if all of the resulting fragments enter the nucleus and cause nerve cell death. It seemed possible that the fragments that were particularly toxic to the nerve cell were specifically generated by one of the two caspases. So in their study, Hayden and co-workers used a mouse model of HD, and mutated the altered huntingtin protein so that either the caspase-3 or caspase-6 protein would not find its usual target. This involved changing the specific amino acid sequence that caspase-3 (or -6) usually targets, and only changing that part so that the rest of the huntingtin protein acts the same. Most of the target sites for caspases are only 4 amino acids long, so it is not difficult to selectively change that part.
Hayden’s group generated one mouse that had all four of the caspase-3 target sites changed and inactivated, one mouse that had the single caspase-6 target sites changed and inactivated, and one mouse that had all of the caspase-3 sites and the caspase-6 site changed and inactivated. They tested all of the types of mice to make sure that they were expressing similar amounts of the huntingtin protein, and that the expanded polyglutamine chains were roughly the same length. In so doing, the researchers ensured that the main difference between these mice was the ability for caspases-3 and 6 to fragment the huntingtin protein.
One way to test for nerve cell death is simply to measure the weight of the brain at a certain age in HD mice and compare it with the weight of the brain in other strains of mice. The less the brain weighs, the more you can assume there is nerve cell death. Previous studies have shown that mice with altered huntingtin protein (that can be targeted by both caspase-3 and 6) lose about 10% of their brain mass as compared to healthy, wild-type mice without the altered huntingtin protein. This loss of brain mass can be attributed to nerve cell death due to the HD associated protein. The first thing that the Hayden group observed was that the mice with altered huntingtin protein resistant to both caspase-3 and caspase-6 did not have that 10% loss of brain mass. Instead, the mice were much more similar to the healthy, wild-type mice.
Then, to determine which of the caspases—3, 6, or both—were necessary for brain mass loss, Hayden and coworkers tested each of the other two mouse lines they had generated. They found that the mice with huntingtin protein resistant to caspase-3 cleavage had similar brain mass loss as mice with the HD associated huntingtin protein. In other words, fragments generated by caspase-3 are not the fragments that cause nerve cell death. But these mice still generated fragments due to caspase-6.
Next, they tested the mice with huntingtin protein resistant to caspase-6 cleavage, and they found that these mice had no significant brain mass loss. They were similar to healthy, wild-type mice and to the mice that had huntingtin protein resistant to both caspase-3 and 6 cleavage. Notably, these mice were still generating fragments due to caspase-3. But since these mice had no evidence of brain mass loss, it is evident that fragments selectively generated by the action of caspase-6, but not caspase-3, are toxic and cause nerve cell death.
Additionally, Hayden and his group tested the motor coordination of each type of mouse. What they found was that both the mice resistant to caspase-6 action and the mice resistant to caspase-3 and -6 action, were able to perform normally, just like healthy, wild-type mice. The mice resistant to only caspase-3 action performed poorly, just like mice with the regular HD-associated huntingtin protein. This result shows that selective inhibition of caspase-6 not only prevents brain mass loss, it also prevents motor symptoms of HD.
Finally, they looked specifically at the location of fragments of huntingtin protein within the nerve cell. Mice with the HD-associated huntingtin protein and mice that have caspase-3 resistance (but generate fragments cut by caspase-6) both have fragments that enter the nucleus early in the mouse’s lifetime. Both healthy, wild-type mice and mice that are resistant to caspace-6 (but generate caspase-3 fragments), show little to no signs of huntingtin fragments entering the nucleus. In the caspase-6 resistant mice, researchers saw some fragments enter the cell very late in life, but they still did not cause nerve cell death or symptoms. This points to the idea that it is the action of fragments (created selectively by caspase-6) inside the nucleus that causes toxicity and nerve cell death. If fragments created selectively by caspase-6 are the ones to enter the nucleus, then caspase-6 inhibition might prevent that toxicity and might prevent HD symptoms.
Directions for the future^
A few uncertainties remain to be considered in the Hayden Lab study. Most significantly, it is unknown whether altering the specific amino acid sites that caspase-3 and -6 target has any effect on the rest of the huntingtin protein itself. Perhaps in addition to being targets for caspases, those sites determine huntingtin structure, stability, or clearance. If so, we cannot know whether the lab’s findings on the role of caspase-6 would hold true in human patients. Furthermore, caspase-6 might have other important functions in the cell that an inhibitory drug would impede. Finally, we do not know if there are other caspases or caspase sites that play a significant role in creating the specific huntingtin fragments that lead to nerve cell death in humans. More work will have to be done to answer all of these questions and ensure that any caspase-6 inhibitors developed as drugs are safe and effective.
Nevertheless, this research has excited the HD community. Finding a drug to inhibit the action of caspase-6 seems a promising direction for treatments, and the work that must be done is very practical and logical. Researchers will look for chemical compounds that might inhibit caspase-6, test them in mice with the human version of HD, and if successful, then move on to clinical trials. Hayden says he hopes to start human clinical trials within five years. In fact, at the Cure HD Initiative (CHDI), a nonprofit drug development research organization for Huntington’s disease, efforts are already underway to develop a safe, effective caspase-6 inhibitor. This area of research will be important to watch for the next few years.
For further reading^
- Graham RK, et al. (2006). Cleavage at the caspase-6 site is required for neuronal dysfunction and degeneration due to mutant huntingtin. Cell. Jun 16;125(6):1179-91
This is the main paper discussed in this article: a fairly technical research paper.
- Slow EJ, et al. (2003). Selective striatal neuronal loss in a YAC128 mouse model of Huntington disease. Hum Mol Genet. Jul 1;12(13):1555-67.
This paper discusses the creation of the mouse model used in the study described by Graham et al (2006).
- Thornberry NA, et al. (1997). A combinatorial approach defines specificities of members of the caspase family and granzyme: Functional relationships established for key mediators of apoptosis. J Biol Chem. Jul 18;272(29):17907-11.
A more general review of caspases and their three different functions. Still technical, but more comprehensible.
- Wellington CL, et al. (2002). Caspase cleavage of mutant huntingtin precedes neurodegeneration in Huntington’s disease. J Neurosci. Sep 15;22(18):7862-72.
A preliminary study of the role of caspases in a different model system of HD
- Gutekunst CA, et al. (1999). Nuclear and neuropil aggregates in Huntington’s disease: relationship to neuropathology. J Neurosci. Apr 1;19(7):2522-34.
This paper is a technical but readable research article about where it is in the nerve cell that huntingtin protein and huntingtin fragments tend to localize.
Drug Summary: Fluoxetine (also known as Prozac) is part of the class of drugs known as selective serotonin reuptake inhibitors (SSRIs) . It is usually prescribed to treat depression and obsessive-compulsive disorder (OCD) in people with and without HD. While fluoxetine has traditionally been used to treat behavioral symptoms, recent observations indicate that it may also be helpful in treating other aspects of HD.
Research on Fluoxetine
Como et al. (1997) performed a randomized, double-blind study of 30 nondepressed patients with HD. 17 subjects received fluoxetine for 4 months, while 13 received placebo. The study did not find that fluoxetine was helpful; patients receiving fluoxetine did not demonstrate improvements in motor or cognitive symptoms, and did not improve in measures of functional capacity (the ability to perform day-to-day tasks. While this study suggests that fluoxetine is not helpful for nondepressed people with HD, the researchers had previously reported that treating 8 depressed HD patients with fluoxetine helped those patients deal with symptoms much better – so fluoxetine may be useful to treat depression in HD.
DeMarchi, et al. (2001) observed two people with HD who were given fluoxetine for psychiatric issues. They tested the two patients each month using the HD motor rating scale (HDMRS) to measure movement abilities, and the mini mental state examination (MMSE) for cognitive (or thinking) abilities. These tests were also accompanied by psychiatric and neurological examinations.
The first case study was on a 60-year-old woman who had symptoms of HD beginning in her mid-forties and symptoms of OCD beginning at age 25. She had not been successfully treated for her chorea, declining cognitive functioning, or aggressive behavior. Before treatment, her symptoms were so bad that her speech could not be understood and her cognitive functioning so impaired that she could not even take the MMSE. She was given the HDMRS, and her motor functioning scored 20 on a scale of 25 (with 0 as the least impaired, and 25 as the most impaired). She began treatment with fluoxetine, and after a month she was clearly less agitated and had a better mood. Her motor performance progressively improved during treatment, scoring 12 on the HDMRS after 4-6 months. The improvement in her motor functioning allowed her to begin walking again and speak coherently. Perhaps most surprising was her improvement in cognitive functioning. Cognitive improvement began after about 4-6 months of treatment, and after about a year she could take the MMSE and scored 12 out of 25. She continues to improve 6 years after beginning treatment with fluoxetine. Additionally, her movement became worse during the two periods in which she stopped taking the medication.
The second case study was on a 55-year-old woman who had symptoms of HD for the past 8 years. Her main symptom was the involuntary movements characteristic of HD, and she mostly retained her cognitive functioning. When tested before treatment began she received a score of 13 on the HDMRS and 19 on MMSE. She began treatment with fluoxetine and another drug to treat her insomnia (since fluoxetine was making the insomnia worse). She began to improve in her motor performance after about 2 months of treatment and reached the height of her improvement after 6 months, with a score of 8 on the HDMRS. She maintained this level of motor functioning for the next year and was able to return to her job. The patient did not change significantly in cognitive functioning, maintaining a score of 20 on the MMSE for as long as she was observed. She went off of the medication for a period of 3 months after a year of treatment and her motor performance deteriorated during this time. When she started taking fluoxetine again, she regained her previous level of motor functioning.
These two case studies show that fluoxetine may be beneficial to people with HD who have not responded well to other treatments for both behavioral and movement symptoms. The motor functioning probably improved as a result of increased serotonin signaling in the brain. It is unclear how the patient in Case One had such impressive cognitive improvement; this has never been seen before and may only be partially due to the beneficial effects of serotonin. It is possible that the reason why fluoxetine was so helpful in these two cases has to do with them both having a history of OCD in their families. In other words, a possible reason for success in these cases had to do with improvements in their OCD rather than, or in addition to, HD. It is important to note that this study only represents two cases of people with HD. More research needs to be done before making assessments about fluoxetine’s effect on people with HD. However, judging by these cases, fluoxetine may at least be helpful to people with HD who also have a family history of OCD.
Grote et al. (2005) studied fluoxetine in a mouse model of HD, and found that fluoxetine might help fight some of the effects of the disease. R6/1 mice were either treated with fluoxetine or a placebo. Scientists found that there was no improvement in motor symptoms, but HD mice treated with fluoxetine had improvements in cognitive symptoms; untreated HD mice tend to repeatedly explore the same paths in a maze, and the treated HD mice behaved more like normal mice by exploring the maze more thoroughly. Treated HD mice also showed fewer symptoms of depression than untreated HD mice.
The results were more than just behavioral; when the researchers looked at the brains of these mice, they found that fluoxetine reversed many of the problems HD causes in the brain. Treated HD mice had much larger dentate gyruses than untreated HD mice, and had an increase in neurogenesis.
Altogether, research results on fluoxetine are mixed; an animal study reports some improvement, while a small clinical trial does not.
For further reading
- Como PG, Rubin AJ, O’Brien CF, Lawler K, Hickey C, Rubin AE, Henderson R, McDermott MP, McDermott M, Steinberg K, Shoulson I. A controlled trial of fluoxetine in nondepressed patients with Huntington’s disease. Mov Disord. 1997 May;12(3):397-401. This medium-difficulty study describes the clinical trial of fluoxetine
- DeMarchi, et al. Fluoxetine in the treatment of Huntington’s disease. 2001. Psychopharmacology 153: 264-266. This is a scientific article of medium difficulty that describes two case studies of people with HD that were treated successfully with fluoxetine.
- Grote HE, Bull ND, Howard ML, van Dellen A, Blakemore C, Bartlett PF, Hannan AJ. Cognitive disorders and neurogenesis deficits in Huntington’s disease mice are rescued by fluoxetine. Eur J Neurosci. 2005 Oct;22(8):2081-8. This technical article discusses how fluoxetine improved some symptoms in a mouse model of HD
-K. Taub, 1-29-06, updated by M. Hedlin 8.9.11
Drug Summary: Paroxetine (also known as Paxil) is part of the class of drugs known as selective serotonin reuptake inhibitors (SSRIs). Paroxetine is a commonly prescribed drug for depression and severe anxiety in people with and without HD. While it has traditionally been used to treat psychiatric disorders, new research suggests that it may also be helpful in treating the symptoms and slowing the progression of HD.
Research on Paroxetine
Duan, et al. (2004) noted that SSRIs are very helpful in treating psychiatric symptoms in people with HD, yet no one had tested how they might affect neurodegeneration and the progression of the disease. These researchers created four different experimental groups of mice: transgenic (mouse models of HD) and nontransgenic mice that were injected with paroxetine, and transgenic and nontransgenic mice that were injected with a placebo. The progression of the disease was observed in the mice each day, and they were weighed each week. Their motor performance was tested by placing them on a rotating rod and recording the amount of time that they were able to stay on. Levels of different brain chemicals were measured, including serotonin and the related molecule 5-HIAA.
The mice received treatment (or placebo) starting at eight weeks of age. At 14 weeks it was found that HD mice had lower levels of serotonin and 5-HIAA in the striatum compared to the nontransgenic mice. Serotonin was increased in both transgenic and nontransgenic mice that received paroxetine. Furthermore, administration of serotonin did not affect the levels of any other important brain chemicals tested, such as dopamine.
Paroxetine also delayed the beginning of behavioral symptoms by an average of 2 weeks in the HD mice, and even helped them survive for an average of 15 days longer than the previous maximum life span (this is a significant amount of time in the life of a mouse). While weight loss is a problem both for HD mice and people with HD, paroxetine slowed the loss of weight in HD mice compared to untreated HD mice. At 16 weeks the HD mice treated with paroxetine performed significantly better than the untreated HD mice on the motor tests.
In order to assess the drug’s effect on the neurodegenerative process, the researchers examined the brains of HD mice and compared them to nontransgenic mice. While untreated HD mice showed deterioration of the brain with larger lateral ventricles and a thinner cerebral cortex, HD mice treated with paroxetine had less enlarged ventricles. Having less enlarged ventricles means that the HD mice treated with paroxetine lost fewer nerve cells in their brains. (For more information on HD and the brain, click here.)
This study suggests that paroxetine not only improves serotonin signaling in HD mice, but it also slows neurodegeneration and improves overall survival. This slowing of the neurodegenerative process may also help to slow the typical weight loss caused by HD. By using an SSRI such as paroxetine to increase serotonin in the brain, serotonin-induced signaling is increased, as is the expression of the important brain-derived neurotrophic factor (BDNF) . (For more information on BDNF, click here.) Additionally, this research found that paroxetine was helpful when given both before and after the onset of motor symptoms. This finding is important because it may indicate that paroxetine might still have beneficial effects even if it is given later in the course of the disease. SSRIs appear to be safe for long-term use in humans, so starting paroxetine early should not rule out its later use as well.
While these results are hopeful, it is important to remember that the study was conducted on mice made to look like they have HD, not on humans or people with HD themselves. Even if paroxetine is relatively safe for use in human use, it nevertheless may not help neurodegeneration in humans the same way that it is indicated in the mouse study. Overall, the use of paroxetine to treat both the symptoms and progression of HD is a promising idea that needs more investigation.
For further reading
- Duan, et al. Paroxetine retards disease onset and progression in Huntington mutant mice. 2004. Annals of Neurology 55(4): 590-594.
This is a scientific article that describes a study done on mice treated with paroxetine.
Recent research has shown that levels of serotonin in the brains of HD mice are lower than normal. A common and effective way to increase the amount of serotonin in the brain is by prescribing a class of drugs known as selective serotonin reuptake inhibitors (SSRIs).
How do SSRIs work?
In order to understand how SSRIs increase the amount of serotonin signaling in the brain, we must first understand how neurotransmitters like serotonin work. Neurotransmitters are important molecules in the brain that help nerve cells communicate with each other. A message is passed within a nerve cell electrically, but when it comes to the end of the nerve cell and the message must be passed to another nerve cell, the message must be converted to a chemical signal. This is where neurotransmitters come in: they are the chemicals that carry the message between nerve cells. The space between two nerve cells is called the synapse. The nerve cell that is sending the message is called the presynaptic cell and the nerve cell that is receiving the message is called the postsynaptic cell. When the presynaptic cell gets the signal to pass on the message, it releases the stored neurotransmitters into the synapse. Once in the synapse, the neurotransmitters can be taken up by receptors on the postsynaptic cell, and the message begins to be passed through the new cell. In order to prevent too much signaling, the neurotransmitter cannot stay in the synapse for too long. The presynaptic cell begins to take back the neurotransmitter, storing it for the next time that a message needs to be passed across the synapse. This recycling of neurotransmitter is called “reuptake.” (For more information on the neurobiology of HD, click here.)
When the nerve cells of the brain produce less serotonin, there is decreased serotonin signaling. Serotonin signaling is decreased simply because there are not enough serotonin molecules to interact with the receptors on the cells. Instead of figuring out how to make more serotonin, the amount of serotonin signaling can be increased by preventing the reuptake of the neurotransmitter back into the presynaptic cell. By allowing the serotonin more time in the synapse, there is a better chance that the proper amount of interactions will occur with the postsynaptic cell to pass on the message. This mechanism is where SSRIs come in: they block parts of the presynaptic cell so that less serotonin can be recycled, allowing it to spend more time in the synapse to pass on the signal.
Histone deacetylases (HDACs) are enzymes involved in expression of DNA. Blocking these enzymes with HDAC inhibitors such as sodium butyrate and suberoylanilide hydroxamic acid (SAHA) has beneficial effects on fruit-fly and mouse models of HD. Why are HDACs significant in the study of HD and how could they lead to HD therapies? To answer these questions, we will explore histones and their role in gene expression, learn about certain histone-modifying enzymes, and then address how regulation of these enzymes can have potentially beneficial effects for an individual with HD.
Histones and DNA^
If you were to stretch out all of the DNA in the human genome, it would span approximately 102 centimeters. This length is tremendous given that the average diameter of a mammalian cell’s nucleus, which is where DNA is stored, is only 5 micrometers. Given that the cell’s total length of DNA is over 20,000 times longer its storage space, fitting DNA into the nucleus requires packing it within a tightly-condensed state known as chromatin. In its chromatin form, DNA is organized in structures called nucleosomes, which consist of both DNA and proteins known as histones. Each nucleosome has 147 base pairs of DNA wrapped around a scaffolding of eight histones like twine around a spool. Thus,
histones provide the structural support for packaging DNA.
Not only do histones provide a backbone for packaging DNA, they also regulate the function of DNA, which is to replicate the body’s cells and to transcribe all of the body’s protein building blocks. When the cell needs to divide or transcribe its genes, signaling proteins are sent to interact with the “tail” of amino acids on the histones, causing them to unwind their DNA. When DNA is unwound, its distinct strands can be accessed by the cell’s replication or transcription machinery.
DNA accessibility can be regulated by histones by adding and then subsequently removing molecules such as acetyl groups, methyl groups, and phosphate groups to or from specific sites on the histone tail. These histone modifications are performed by enzymes that are specialized to attach or remove a specific group. For example, enzymes known as histone methyltransferases are responsible for attaching methyl groups to histones. These attached groups are small compared to the size of the structure they are attached to, but their presence or absence on the histone have a big effect on the accessibility of the DNA in its tightly-condensed chromatin form. For instance, adding a methyl group to the ninth lysine on histone 3 represses transcription, while adding it to the seventeenth arginine activates transcription. The significant effect of this methyl group and other histone modifiers has been attributed to two different mechanisms:
- The binding of modifiers disrupts contacts between nucleosomes, resulting in the unraveling of the DNA.
- The bound modifiers attract or repel other proteins that initiate a variety of enzymatic activities (e.g. prepare the DNA for transcription).
Clearly, it is very important for cells to precisely control which modifications go on which part of the histone. The huge variety of different combinations of attached groups has led scientists to suggest that these modifications serve as a “code” that tells the cell what to do with the DNA wound around the histone.
Histone acetylation and HD^
Because the chromatin structure is so important in regulating in the accessibility of DNA, histones and the enzymes that modify them play significant roles in controlling which genes are expressed and which are turned off. This method of regulation is very relevant in the nervous system, which depends on the coordinated expression of many different genes in order to develop and maintain itself. For example, recall that all neurons are descendants of embryonic stem cells, cellular progenitors that have the potential to turn into many other types of cells. What these stem cells ultimately become depends on the signals that they receive from their environment. (For more information on stem cells, click here.) One way that these signals may “tell” stem cells what to become is through histone modifications. This makes sense when you consider the importance of chromatin structure in determining the access of genes—turning different genes “on” and “off” results in stem cells taking on different fates. As we will soon see, in an organ as critical and complex as the brain, even small changes in gene expression can have big effects on function.
The addition of a chemical structure known as an acetyl group to histones is associated with activating transcription. This addition, which is known as acetylation, is performed by enzymes known as histone acetyltransferases (HATs) and produces a more transcriptionally-active form of chromatin. Conversely, the removal of attached acetyl groups by enzymes called histone deacetylases (HDACS) represses transcription. Varying the amounts of active HATs and HDACs allows the cell to control the accessibility of its chromatin, and thus which genes are turned on and when. One simple way to think about this relationship is a seesaw, with the left side signifying a certain gene is “on” and the right side signifying that a gene is “off.” If there are more HATs on the left side than there are HDACs on the other side, more histones will be acetylated than not and the seesaw will tilt towards the gene being transcribed. On the other hand, if HDACs outnumber HATs, acetyl groups will be removed faster than they are added and the transcription of the gene is repressed. Of course, the control of gene transcription is more complex than a seesaw of HDACs and HATs. However, this model provides a simple way of understanding what can go wrong if the balance between HDACs and HATs goes changes, as what appears to happen in HD.
Mutant huntingtin and acetylation: ^
An imbalance in histone modifications occurs in HD and researchers are trying to correct this problem as a therapeutic strategy. The mutated version of the huntingtin protein has been shown to disrupt transcription. (For more information about mutant huntingtin, click here.) Many genes important for the proper functioning of neurons, such as those responsible for neurotransmitter signaling, are down-regulated, or show reduced expression, in the brains of HD patients and animal models. The reduced output of these genes plays a major role in the massive neurodegeneration seen in HD and is likely at least partially due to mutant huntingtin’s effect on histones. Experiments examining the chromatin of genes known to be downregulated in both HD cell lines and HD mice and found that their histones contained fewer acetyl groups than usual. Recalling the well-established role that acetylation plays in transcriptional activation, these results provide a way of understanding how transcription is affected in HD.
Further investigations have provided more insight into what actually causes the reduced acetylation seen in HD. Mutant huntingtin has been observed to interact with both HATs, the enzymes that add acetyl groups, and HDACs, the enzymes that remove them. While there are some indications that mutant huntingtin may directly act on HATs to de-active them, there has been more evidence suggesting that the mutated protein affects the histone-modifying enyzmes by making them unavailable to do their jobs. This latter mechanism is a recurring one in HD—mutant huntingtin sequesters transcription factors and co-activators (molecules that help transcription factors bind to DNA), resulting in abnormal gene expression. But what happened to these coactivators and transcription factors? In order to answer this question, we need to review some of the disease mechanisms that take place in the cells with mutant huntingtin.
A Brief Review of HD Mechanisms
The expanded CAG repeat found in the HD gene produces a mutant form of the huntingtin protein. At some point in the life of mutant huntingtin, enzymes known as caspases cleave the protein, producing huntingtin fragments. These fragments are transported into the nucleus of the HD brain cell, or neuron, where they aggregate to form neuronal inclusions (NIs) in the nucleus. Figure 3 shows a diagram depicting the formation of NIs from the altered huntingtin protein. (For more information about the mutant huntingtin protein and protein aggregation, click here.)
Recent studies showed that the NIs “trap” various co-activators and transcription factors and prevent them from doing their jobs. This discovery led scientists to speculate that one of the ways by which HD progresses is through the loss of transcription of several key genes essential for cell survival. It was then important for scientists to try to identify which molecules are being trapped by the NIs so that this molecular “trapping” can be counteracted.
One such molecule called CREB-binding protein (CBP), which is an HDAC, appears to associate with NIs. CBP is a co-activator because it induces histones to adopt a more open chromatin configuration, allowing transcription factors to bind. If CBPs are trapped by the NIs, transcription factors can’t access DNA and certain genes are not transcribed. Sequestering CBP in HD especially affects p53, a transcription factor that regulates neuronal death, but is better known for its role as a tumor-suppressor protein. When few CBP molecules are available to co-activate p53, it is unable to access and bind to DNA. Without p53, abnormal gene transcription and expression occurs, and scientists speculate that this error in transcription may lead to cell death.
In summary, the altered huntingtin protein forms NIs that trap coactivators such as CBP. Loss of CBP results in the loss of histone acetylation, which in turn results in the inability of the transcription factor p53 to bind to DNA. Drugs such as HDAC inhibitors that could compensate for the loss of the coactivator CBP could, in theory, be possible treatments for HD. The following section summarizes some of the recent studies that test this theory using animal models.
The disruption of localized processes, such as p53 binding, by mutant huntingtin results in global changes to the cell. Gene profiling studies, which examine the expression of a tremendous number of genes, have found significant differences in the messenger RNAs (mRNAs) that are produced in individuals with HD compared to those unaffected by the disease. Recall that mRNAs are transcribed from DNA and form the genetic code that is “read” to produce proteins. The changes seen in the expression profiles of individuals affected by HD suggest that the disease impacts transcription at a global level.
The disruption of localized processes, such as p53 binding, by mutant huntingtin results in global changes to the cell. Gene profiling studies, which examine the expression of a tremendous number of genes, have found significant differences in the messenger RNAs (mRNAs) that are produced in individuals with HD compared to those unaffected by the disease. Recall that mRNAs are transcribed from DNA and form the genetic code that is “read” to produce proteins. The changes seen in the expression profiles of individuals affected by HD suggest that the disease impacts transcription at a global level.
From bench to bedside: ^
Because of the strong evidence linking histone dysregulation to the disrupted transcription seen in HD, scientists have hypothesized that addressing the imbalances in histone modifications may improve the symptoms of HD. Research been particularly focused on a diverse group of molecules known as HDAC inhibitors, which prevent the deacetylases from removing acetyl groups.
As mentioned above, CBP gets “trapped” in NIs which disrupts DNA transcription. Steffan, et al. (2001) looked into the possibility of reversing or preventing CBP aggregation in NIs. CBP functions as a HAT but this function is lost when it is “trapped” in the NIs. This loss of HAT function (or acetyltransferase activity) may be counteracted by inhibiting HDACs (or reducing deacetylase activity). In other words, blocking the inhibitor of transcription would make up for the low levels of transcription activators. To test this idea, Steffan, et al. used a Drosophila (fruit fly) model of HD, which was genetically modified to express mutant huntingtin. These HD mutant flies show degeneration of nerve cells and decreased survival rates similar to what is observed in people with HD.
To assess the efficacy of reducing deacetylase activity in treating HD, the flies received food containing various HDAC inhibitors, including sodium butyrate and suberoylanilide hydroxamic acid (SAHA). After treatment, the researchers discovered that the flies fed SAHA showed increased histone acetylation compared to untreated flies as well as a slowing in the progression of neurodegeneration. SAHA also increased survival rates: 70% of untreated flies showed early death compared to 45% of SAHA-treated flies. The results of this study suggest that mutant huntingtin reduces levels of acetylation and transcription by sequestering co-activators (such as CBP and others) and trapping them into aggregates.
Similar studies have been carried out in mouse models of HD, which are better than invertebrate models in assessing how the disease progresses in humans. In 2003, Hockly et al. found that HD mice that were administered SAHA through their drinking water performed better on the Rotarod, a revolving rod that is used to assess motor coordination, than HD mice that were given a placebo.
These effects of SAHA have also been investigated at the molecular level, an important validation step that confirms that the drug is doing what it is supposed to do in the cells of HD mice. Experiments performed by Mielcarek et al. in 2011 showed that SAHA reduced the levels of HDAC4, a class of histone deacetylases, in the cortex and brain stem of HD mice. Recalling the see-saw model of histone acetylation (see above), lower concentrations of HDAC would be expected to result in higher levels of histone acetlyation and thus transcriptional activation—genes being turned “on.” The potentially positive effects SAHA could be seen in the significant reduction of mutant huntingtin aggregation and the partial restoration of the HD mice’s levels of brain-derived neutrophic factor (BDNF), a protein that is important for the survival and growth of certain neurons and is inhibited in HD. (For more information on BDNF, click here.) Despite these promising results, the investigators also found that both wild-type and HD mice treated with SAHA showed significant weight loss, a side-effect that needs to be taken into account if the chemical enters clinical trials.
Thomas et al. (2008): Researchers at The Scripps Research Institute in California have developed an HDAC inhibitor that staves off disease progression in a mouse model of HD. HD mice treated with the drug, called HDACi (HDAC inhibitor) 4b, had significant improvements in movement and coordination, and lost less weight. When scientists looked at the brains of the HD mice, they found that HDACi 4b countered some of the negative effects that HD causes. HD mice usually have a smaller striatum and larger ventricles, and have smaller brains overall. (For more information on HD’s effect on the brain, click here.) However, HD mice treated with HDACi 4b had brains that were just as large as the brains of normal mice, and had a normal-sized striatum and ventricles.
Notably, HDACi 4b has very low toxicity – which is important because many HDAC inhibitors had toxic side effects in mice, and therefore can’t be used in humans. The researchers suggest that HDACi 4b should be studied in humans, as it shows potential to help people with Huntington’s disease.
The results of these studies have been replicated and adapted in subsequent studies looking at other HDAC inhibitors. Taken together, the body of evidence presents a compelling case that this class of molecules could be effective in treating HD. However, there are significant challenges in translating basic science research into clinical therapies. (For more information on clinical trials, click here.) In 2006, researchers concluded Phase II of a clinical trial examining the safety and tolerability of phenylbutyrate, a HDAC inhibitor that has been found to increase acetylation and improve motor function in mice models of HD. As of the end of 2011, it is unclear whether phenylbutyrate is still actively being considered as a treatment for HD, although scientists have been publishing based on the results of the Phase II study as recently as 2010.
The development of HDAC inhibitors into viable drugs to treat HD can draw from ongoing research into their use as therapies for cancer. Two HDAC inhibitors (including SAHA, the molecule tested in fly and mouse models of HD) are FDA-approved for the treatment of cutaneous T-cell lymphoma, a cancer of a class of immune cells. Many more are being investigated in clinical trials for a range of cancers, an encouraging indication that these drugs are regarded as generally safe for use. But despite their use in current treatments, scientists are still unclear about how increasing histone acetylation improves clinical outcomes. For example, there is now evidence that the majority of molecules targeted by HDACs for acetyl group removal are not histones, but other protein complexes important for cellular function. Clearly, scientists still have an incomplete understanding of how HDAC inhibitors work at the molecular level.
While further research is needed to determine whether HDAC inhibitors can be developed into a safe and effective drug for HD, early results suggest that these molecules hold promise in improving the lives of individuals affected by this terrible disease.
For further reading^
- Steffan, et al. “Histone deacetylase inhibitors arrest polyglutamine-dependent neurodegeneration in Drosophila”. 2001. Nature 413: 739-743.
Treatment with HDAC inhibitors increased survival rates in a Drosophila model of HD.
- Thomas EA, Coppola G, Desplats PA, Tang B, Soragni E, Burnett R, Gao F, Fitzgerald KM, Borok JF, Herman D, Geschwind DH, Gottesfeld JM. The HDAC inhibitor 4b ameliorates the disease phenotype and transcriptional abnormalities in Huntington’s disease transgenic mice. Proc Natl Acad Sci U S A. 2008 Oct 7;105(40):15564-9. Epub 2008 Sep 30. A technical article describing the effect of HDACi 4b on HD mice
- Gray SG. Targeting Huntington’s disease through histone deacetylases. 2011. Clinical Epigenetics 2: 257-277.
This technical article provides a broad review of histone deacetylases and their role in Huntington’s disease.
- Butler R, Bates G. Histone deacetylase inhibitors as therapeutics for polyglutamine disorders. 2006. Nature Reviews Neuroscience 7:785-796.
This is another technical review of how histone deacetylase inhibitors may prove useful in treating diseases such as HD.
- Kouzarides T. Chromatin Modifications and Their Function. 2007. Cell 128:693-705
This review article provides a good overview of how the addition and removal of different groups affect chromatin structure and function.
- Hockly E, Richon VM, Woodman B et al. 2003. Proceedings of the National Academy of Sciences. 100(4): 2041-2046.
This is an article from the primary literature about testing SAHA on mice models of HD. that is targeted towards individuals with a science background. Although it may be difficult to understand, the introduction and conclusion are readable.
- Mielcarek M, Benn CL, Franklin SA et al. 2011. PLoS ONE 6(11): 1-10.
This is another article from the primary literature about SAHA in mice models of HD.
-E. Tan, 11-21-01, updated by M. Hedlin, 10-6-11, Y. Lu, 2-27-12
-Histone update by Y. Lu, 2-27-12
Drug Summary: Memantine is an anti-glutamate and energy-buffering drug. As an NMDA antagonist, memantine prevents the neurotransmitter glutamate from leading to nerve cell degeneration by inhibiting glutamate´s binding to the receptor. Memantine has been clinically used to treat dementia and Alzheimer´s disease. Current research on its effects in other diseases of the central nervous system (CNS), including HD, looks promising because memantine appears to be well-tolerated, and may help learning. It is possible that memantine may even be able to disrupt the progression of HD.
Mechanism of Action^
According to a theory known as the excitotoxicity theory, lower energy levels in the nerve cells of people with HD cause them to be overly sensitive to glutamate. As a result, even normal levels of glutamate can overactivate the glutamate receptors on the nerve cells. When these receptors (also known as NMDA receptors) are activated, calcium ions enter the nerve cells. Excessive activation causes a buildup of these calcium ions, which then leads to the death of the nerve cell. (For more on the excitotoxicity theory, click here.)
HD researchers believe that memantine may have strong potential to slow the progression of HD by decreasing the NMDA receptor´s sensitivity to glutamate. Memantine is an NMDA antagonist. As an antagonist, memantine prevents the excessive binding of glutamate to NMDA receptors, inhibiting the pathway to excessive NMDA activation and nerve cell death. Memantine is also a non-competitive antagonist. “Non-competitive” means that memantine binds to a site on the NMDA receptor that is different from glutamate´s binding site. By binding to one portion of the NMDA receptor, memantine changes the overall shape of the receptor, making it more difficult for glutamate to bind to the other portion of the receptor.
Memantine differs from other NMDA non-competitive antagonists in that it allows the NMDA receptor to undergo physiological activity required for normal nerve cell functioning, while at the same time preventing the receptor from the over-activation that leads to nerve cell death. This is important because NMDA receptors still need to be activated to allow the entry of calcium ions, which facilitate learning and memory. But once again, too much activation of the receptor can lead to nerve cell death. Two properties of memantine allow the NMDA receptors to be activated to the optimal level, which allows learning but prevents nerve cell death.
The first property of memantine prevents nerve cell death by decreasing the NMDA receptor´s sensitivity to glutamate. When glutamate binds to the receptor, it increases the cell´s electrical charge. The electrical charge inside the cell first needs to rise to a specific value before the magnesium ion leaves the receptor so that calcium can now enter. In people with HD, the over-excitation by glutamate causes the magnesium ion to leave too easily, allowing the influx of calcium ions responsible for nerve cell death. On the other hand, memantine is not as sensitive as the magnesium ion towards an electrical charge. That is, more glutamate needs to bind to the receptor before memantine will leave the receptor, thereby allowing calcium ions to enter. This is an advantage for those with HD, because memantine can block the pathological pathway by not responding as easily to an excessive amount of glutamate.
Besides inhibiting over-activation by glutamate, the second property of memantine still enables the physiological pathway to learning and memory. Memantine has “fast blocking/unblocking kinetics.” This means that, after glutamate strongly activates the receptor, memantine is still capable of quickly unbinding the receptor, thereby allowing calcium to enter the nerve cell. The fast kinetics of memantine is what allows an appropriate amount of calcium to enter the nerve cell, a process necessary for learning and memory.
Memantine has been clinically used in the treatment of dementia and Alzheimer´s disease. In studies general to all chronic neurodegenerative diseases, therapeutic doses of memantine inhibit disruption of spatial learning and aid learning in general through prevention of the pathological pathway discussed earlier. Researchers are currently testing its efficacy in treating other CNS disorders, including HD. Discussed later in this article, a clinical study on treatment of HD with memantine has also discovered benefits in its ability to slow the progression of HD.
Clinicians have used memantine to treat over 200,000 patients for mostly dementia over the last fifteen years. Although memantine has been well-tolerated in humans, in animals it has produced side effects characteristic of other NMDA receptor antagonists. For instance, memantine can impair the ability to control muscular movements (ataxia), muscle relaxation (myorelaxation), and is sometimes known to cause amnesia. However, these side effects were only seen at high dosages (greater than or equal to 20mg/kg per day)-dosages far higher than the usual 5mg/kg per day used in humans for therapy. In humans, high doses of memantine have been known to result in psychosis in some rare instances. At therapeutic (low-level) doses, memantine does not display the negative side effects found in other NMDA receptor antagonists.
Other tested side effects are drug dependency and abuse. There is some evidence to show that memantine can lead to dependence in animals. A dependency on memantine appeared in rats and monkeys but only at high doses. However, researchers have widely agreed that memantine has little abuse potential based on the many years it has been clinically used, recent clinical studies, and zero reports of abuse in humans.
Research on Memantine^
Beister, et al. (2004) conducted a two-year-long study with twenty-seven HD patients recruited from two different clinics. Each patient took up to a maximum of 30mg of memantine per day, depending on his/her individual tolerance for the drug. (Note: mg/day should not be confused with the units mg/kg per day that was used to specify therapeutic-level doses.)
Rating scales established in the HD medical literature measured the progression of HD. The Scale of Abnormal Involuntary Movements, the HD Rating Scale, and standardized video recordings evaluated chorea. For instance, in the videotapes, chorea was measured for the arms, legs, head, and trunk each on a three point scale, with 1 = slight, 2 = moderate, 3 = severe, and half-points possibly assigned. The scores for the different body parts were then averaged together.
The Clinical Global Impression (CGI) scale, the HD Activities of Daily Living (HD-ADL) scale, and the Total Functioning Capacity (TFC) of the HD Functional Capacity Scale were used to measure deteriorations. For example, HD-ADL consists of seventeen items that track the progress of HD through assessing a person with HD´s capabilities in taking care of him/herself in various areas, such as eating, dressing, taking medicine, and maintaining relationships. The person´s capability in each area is evaluated on a 3 point scale, with 0 indicating normal ability and 3 indicating necessary help from others required. The points for each area are summed to get the total HD-ADL score.
The Total Motor Score of the Unified HD Rating Scale (UHDRS) measured motor functioning. The Total Motor Score is reached by summing up points for certain movements, such as being able to carry out a sequence of hand movements or the velocity in moving a certain way. Scores for each movement are graded on a 4 point scale, with 0 being normal and 4 being unable to execute.
Psychometric tests, such as the Short Syndrome Test (SKT), the Brief Test of General Intelligence (KAI), and the Trail-making test were used to measure cognitive abilities.
The results following a two year treatment with memantine suggest that memantine has the ability to slow the progression of HD. Untreated people with HD in the Huntington Study Group (1996) experienced a 21.2% decrease in motor function over two years according to the Total Motor Score of the UHDRS. In comparison, treated patients experienced a decline of only 4.3%.
The scores on competence in daily living tasks also show memantine´s benefits. Researchers compared their results measured by HD-ADL with results measured by TFC of the HD Functional Capacity Scale because the two are similar enough to produce comparable outcomes. Untreated people with HD had a decrease in ability of daily living tasks, demonstrated by their average decline of 0.5 points over six months on the TFC scale. On the other hand, people with HD who received memantine treatment actually gained ability in daily living tasks, with an average increase of 0.28 points. These results translate to a 15.4% decrease in competency of daily living tasks over two years in untreated people with HD but a 9.3% reduction in progression of incompetence in daily living tasks in treated people with HD.
With no statistically significant changes in SKT and KAI, psychometric testing showed no deterioration of cognition in the treated participants.
Furthermore, in the second year of treatment (between 12 and 24 months), there were no significant changes in CGI and HD-ADL scores. This score stability indicates a reduction in progression of deterioration. It also interestingly suggests that memantine´s ability to prevent HD progression is expressed only after long treatment with memantine.
Overall, the researchers concluded that memantine has good potential to slow the progression of HD. However, more studies need to be conducted with control groups to serve as a comparison (control groups do not get treatment, they take a placebo) in order to verify the study´s findings.
Forest Pharmaceuticals, Inc. (2010) ran a phase II clinical trial in which 50 people with mild to moderate Huntington’s disease received either 10 mg of memantine or a placebo twice daily for 12 weeks. Then, for the next 12 weeks, all participants took memantine. When compared to patients taking placebo, patients taking memantine showed improvements on tests of memory and attention, but performed worse on tests measuring motor symptoms. Larger studies will be necessary to confirm these findings.
For further reading^
- Beister, et al. “The N-methyl-D-asparate antagonist memantine retards progression of Huntington´s disease.” Journal of Neural Transmission Supplement. 2004 Supplement; (68): 117-22.
This fairly technical article presents the complete details of the study conducted by Beister, et al. The article concludes that memantine has good potential to slow the progression of HD, but more studies still need to be conducted to confirm results.
- Parsons, et al. “Memantine is a clinically well tolerated N-methyl-D-asparate (NMDA) receptor antagonist-a review of preclinical data.” Neuropharmacology. 1999, Jun; 38(6): 735-67. Review.
This is a highly technical article that summarizes the findings on memantine in its usage for a variety of diseases, including HD. The article also explains in detail memantine´s mechanism and tolerability.
This website is easy to understand but centers on memantine´s use for Alzheimer´s disease. However, the website clearly explains memantine´s mechanism as well as provides many research studies. The research posted under “Studies & Literature” is helpful in understanding more about memantine´s effects and good tolerability.
- Palmer GC. “Neuroprotection by NMDA receptor antagonists in a variety of neuropathologies.” Current Drug Targets. 2001 Sep; 2(3): 241-71. Review.
This is a highly technical article that reviews memantine´s mechanism against glutamate toxicity. It is not very useful in understanding memantine´s effects on HD in particular.
- Proc. of Fourth Annual Huntington Disease Clinical Research Symposium, San Pavilion Ballroom at the Hyatt Regency La Jolla at Aventine, San Diego. This technical report describes the results of the phase II clinical trial on memantine
– C. A. Chen, 05.02.05, Updated by M. Hedlin on 9.13.11
Drug Summary: Lamotrigine belongs to a group of medications called anticonvulsants, which are used to control seizure disorders. Lamotrigine acts on the central nervous system to control the number and severity of seizures. It is thought to suppress the activity of certain parts of the brain and the abnormal firing of nerve cells that cause seizures. In psychiatry, lamotrigine may be used as a mood stabilizer. In the laboratory, researchers have found that lamotrigine also inhibits release of the neurotransmitter glutamate. This is important because glutamate may play a role in nerve cell degeneration in the brains of people with HD, so reducing the amount of glutamate released makes lamotrigine a potential treatment for HD.
Problem: Glutamate sensitivity^
Many factors contribute to the degeneration and death of nerve cells in people with HD. One aspect of HD is that nerve cells are particularly sensitive to glutamate. Glutamate is a neurotransmitter that is used to pass messages along from one nerve cell to another. (For more information on glutamate and HD click here.) Researchers have observed that because glutamate receptors in some nerve cells of people with HD are more sensitive than in people without HD, they are activated more frequently than normal receptors. This increased activity and sensitivity to glutamate has been associated with nerve cell death.
One way to prevent the overstimulation of a nerve cell by glutamate is to inhibit glutamate release from the nerve cells that communicate with it. In order to understand this kind of treatment, we must first understand the steps involved in the nerve impulse. (For more information on how nerve impulses work, click here.) It is important that we understand the steps of the nerve impulse because different treatments can be used to inhibit glutamate release by interfering at different steps. A nerve impulse involves receiving a message at one end of a cell and transmitting it via an electric signal to the other end of the cell. Neurotransmitters such as glutamate are stored at the end of the cell and are released in the last step. They act as a chemical signal, transmitting the message to a neighboring cell.
An important step in the electrical transmission of the nerve impulse involves sodium (Na+) channels. Most of the time, charged particles called ions line up along the inside and outside of the nerve cell membrane, giving the membrane a small electric voltage. Many different types of channels are located in the membrane, acting like guards at an exclusive community, only letting certain molecules in and out. Some of these channels open or close depending on what the membrane voltage is. One of these voltage-gated channels is the sodium channel, and it opens when the inside of the membrane becomes more electrically positive than usual. When the channel opens, sodium ions are free to enter the cell and continue the messaging cascade that ultimately leads to the release of neurotransmitters such as glutamate.
After the sodium channel lets enough sodium into the cell so that it reaches a maximum voltage, the channel temporarily becomes inactivated. An inactivated channel means that not only can no more sodium get through to relay the current message, but also the channel cannot be immediately reset, and thus will let no new messages be relayed. This intermediate stage between open and closed is called the refractory period. The sodium channel returns to the closed position only after the membrane voltage returns to a normal level (restoring the normal voltage involves the exit and entry of different ions). Once the channel is back in the closed position it can be opened again when the voltage rises enough. (See figure L-5 for a representation of the different sodium channel positions.)
How can lamotrigine reduce glutamate release?^
Studies have shown that lamotrigine may inhibit the release of glutamate. While lamotrigine may act in several different ways, it is primarily thought to act as an anti-glutamate drug by interfering with sodium channels. These channels are a necessary step in the nerve impulse and for normal release of glutamate by a nerve cell. In this way, lamotrigine’s inhibition of glutamate release is similar to that of the drug riluzole. (For more information on riluzole click here.)
Lamotrigine exerts its effects during the refractory period by binding to sodium channels. In overactive nerve cells such as in people with seizure disorders or HD, it takes longer for sodium channels to transition from the open period to the inactivated refractory period. An extended open period is what allows so much glutamate to be released in overactive cells. Lamotrigine targets these overactive cells that are slow to inactivate, leaving normal areas of the brain unaffected. Lamotrigine acts by prolonging the inactive refractory period so that sodium channels cannot return to the closed position. Since the channel must first be closed before it can be re-opened, prolonging the inactive period decreases the time of the open period, thus decreasing glutamate release. To put it another way, during the inactive refractory period, no more sodium can get in, so the membrane’s voltage is stabilized. When sodium is kept out, no more messages can be relayed, and thus no more glutamate is released. Therefore, lamotrigine inhibits glutamate release by interfering with sodium channels.
Research on lamotrigine^
Kremer, et al. (1999) recognized that prolonged exposure to glutamate leads to the gradual decline and death of nerve cells in diseases such as HD. They therefore hypothesized that inhibiting the release of glutamate would prevent or at least slow the progression of HD. Lamotrigine is known to inhibit glutamine release in vitro, and has been successfully applied to protect nerve cells in other experiments using animal models. Building on these results, the researchers ran a clinical trial on humans lasting 30 months to see if lamotrigine would slow the progression of HD in people who had experienced physical symptoms for less than five years.
The researchers studied the effects of lamotrigine on 28 people with HD; they also gave a placebo to 27 people with HD to control for psychological effects of treatment as well as to have a comparison group. This was a double-blind study, meaning neither the researchers nor the patients knew which group received the lamotrigine and which received the placebo. (The purpose of a double-blind study is to remove any experimenter or patient bias in evaluating the treatment.) The efficacy of the drug was primarily measured using the total functional capacity (TFC) scale. Patients were also assessed using a variety of cognitive and physical tests.
Over the course of the 30 months of the study, both groups significantly declined in their TFC scores, without any significant difference between the group receiving lamotrigine treatment and the group receiving a placebo pill. This led the researchers to conclude that lamotrigine is not effective in slowing the progression of HD. However, there was slightly less deterioration in terms of the physical symptoms known as chorea in the group receiving lamotrigine. Also, when asked about their various symptoms (mood, physical, etc.), a larger percentage of patients in the group receiving lamotrigine reported an improvement. Despite this perception, both groups declined in their performance on physical tasks. In addition, not much change was observed in the cognitive tests, although the placebo group performed better than the lamotrigine group on one test due to better learning.
Sixteen (of 28) people receiving lamotrigine treatment reported several side effects, including nausea, skin rash, insomnia, and severe depression. Eight (of 27) people receiving a placebo reported mild side effects.
While the study reported the overall inefficacy of lamotrigine, it is important to consider the relatively small sample size and the fact that deterioration varied widely among participants. This is why the researchers have not fully ruled out lamotrigine’s ability to treat early HD. The positive results of the study (decreased chorea and improved symptoms such as mood) may be a result of what lamotrigine is already used for – as an anticonvulsant and mood elevator. A possible reason why the clinical results on humans were not as favorable as those on animals is because the effective dose in animals is much too high for humans to tolerate. Increasing the dose in people is not an option because of the harmful side effects associated with the drug.
Higgins, et al. (2002) also focused on decreasing the amount of glutamate released in nerve cells. Since lamotrigine is known to inhibit the release of glutamate, this group tested the safety of various doses of the drug and how well it was tolerated in HD patients. They conducted an open-label study, meaning that the patients knew they were receiving an actual drug and not a placebo. Over the course of seven weeks the researchers increased the amount of lamotrigine given and then continued giving the maximum dose up to six months. The effects of the drug were tested using the Unified Huntington’s disease Rating Scale (UHDRS) and cognitive tests.
The researchers studied only twenty people with HD and ended up collecting data from fifteen (two people’s symptoms got worse while three people did not report back). The researchers did not find any changes in the UHDRS (this includes motor, functional, and behavioral aspects of HD). However, significant improvements were seen in two parts of the cognitive tests, Verbal Fluency and Symbol Digit Modalities.
Overall, the researchers found that the patients were able to tolerate the drug well and that it was safe to use. They were not able to reproduce the results seen in a previous study that found lamotrigine could reduce chorea. Researchers will need to follow up on this study with a longer lasting investigation that is not open-label and includes more patients.
For further reading^
- Kremer, et al. Influence of lamotrigine on progression of early Huntington’s disease. 1999. Neurology 53(5): 1000. Online.
This is a research article about a clinical trial of lamotrigine and HD. It describes the study’s methods and results in great detail and is directed toward a scientific audience.
- Higgins, et al. Safety and tolerability of lamotrigine in Huntington’s disease. 2002. Movement Disorders 17(S5): S324.
This is a short description of medium difficulty of a clinical trial using lamotrigine as presented at the 7th international congress of Parkinson’s disease and movement disorders.
- Hurley, Stephen C. Lamotrigine update and its use in mood disorders. 2002. The Annals of Pharmacotherapy 36(5): 860-873. Online.
This article reviews known information about lamotrigine and evaluates its use in treating mood disorders. It is not directly related to HD, but the section on pharmacology on page 861 is helpful in understanding how lamotrigine works on nerve cells.
-K. Taub, 11/21/04
Update: Riluzole is no longer considered to be a promising avenue of research; it failed a phase III clinical trial in 2007. The trial ran for 3 years and included 537 adult HD patients, who were randomly assigned to either the treatment group (receiving 50 mg of riluzole twice a day) or the control group (which received a placebo instead). The 379 patients who completed the study were measured with the Unified Huntington’s Disease Rating Scale (UHDRS), a test commonly used in clinical trials to measures factors such as motor control, independence, and mental function. The scientists performing the study concluded that riluzole has no benefit for the treatment of HD, as it was not significantly better than the placebo; it does not slow the progression of HD, nor does it improve symptoms.
Previous studies found some improvement in motor control for patients who took riluzole. However, these studies were complicated by the fact that other drugs, such as antipsychotics, were taken at the same time to control chorea. Therefore, this study was careful to look at the effects of riluzole separate from all other treatments; patients who participated in the study exclusively used drugs prescribed for the study.
For more information, click here.
Drug Summary: Riluzole has been shown to have energy-buffering and anti-glutamate properties. It has been associated with increased energy metabolism efficiency and inhibition of glutamate activity, and is currently used as a treatment for Amyotrophic Lateral Sclerosis (ALS), a disease that is also hypothesized to involve glutamate toxicity. Huntington’s disease is associated with these both problems in energy metabolism and glutamate toxicity; let us discuss some of these problems and the ways in which riluzole might alleviate them.
Problem: Aerobic inefficiency^
Energy metabolism is the process by which cells produce energy. Normally, cells prefer a form of energy metabolism called aerobic respiration due to its efficiency and high-energy yield. The altered huntingtin protein in people with HD is believed to interfere with aerobic respiration, resulting in the inability of HD cells to perform aerobic respiration efficiently. Instead, HD cells must resort to anaerobic respiration, another form of energy metabolism that is less efficient. This impairment in energy metabolism results in various negative effects that eventually lead to cell death.
Studies have reported that riluzole treatment improves motor abnormalities associated with administration of a toxin that blocks energy metabolism. The improvements indicate that riluzole may have positive effects on cells with defective metabolism. However, the mechanism by which riluzole improves energy metabolism is still unknown.
Problem: Glutamate Sensitivity^
One of the effects of the impairment in energy metabolism in HD cells is an increased sensitivity to glutamate. Glutamate is one of the major neurotransmitters in the nervous system, used to transmit messages from nerve cell to another. (For more on glutamate, click here.) Increased activation of receptors that receive glutamate has been observed in people with HD. Increased glutamate activity, in turn, has been associated with nerve cell death.
Studies have demonstrated that riluzole may act as an anti-glutamate drug in two ways: 1) by inhibiting the release of glutamate and 2) by interfering with the effects of glutamate on nerve cells.
It is thought that riluzole inhibits the release of glutamate by interfering with sodium (Na+) channels that are required for normal glutamate release. Figure L-3 shows how riluzole inhibits glutamate release.
The mechanism by which riluzole disrupts the effects of glutamate on target cells is slightly more complicated. Let us first go over what happens in a normal glutamate-receiving cell in order to understand the effects of riluzole on these cells in a patient with HD.
Various types of glutamate receptors are found in nerve cells. One type of glutamate receptor allows the entry of ions into the cell upon glutamate binding, resulting in various changes inside the cell. Among these receptors are NMDA receptors, discussed in the section HD and Glutamate. A second type of glutamate receptors causes cellular changes by initiating a messenger cascade, which involves the activation and deactivation of various molecules and pathways that can cause changes inside the nerve cell.
In a messenger cascade, the binding of glutamate is a “message” that is being sent to the nerve cell. This message is passed on from one molecule to another, until it reaches its final destination. Scientists have discovered that glutamate binding “tells” the cell to release calcium from its stores.
In HD cells, the overactivation of the glutamate receptors results in overactivation of the messenger cascades and consequently, increased calcium release. High amounts of calcium in the nerve cells are known to cause cell death, which is one possible explanation of how HD nerve cells die. Figure L-4 shows a diagram depicting the molecules involved in the messenger cascade as well as the final effects of the cascade.
Riluzole may disrupt glutamate activity by interfering with the activity of certain proteins involved in the messenger cascade. Once the cascade is inhibited, changes induced by glutamate such as calcium release and the associated cell death might eventually be delayed.
Research on Riluzole^
Bensimon, et al. (1994) hypothesized that riluzole may have beneficial effects on people with diseases such as amyotrophic lateral sclerosis (ALS) which involve overactivation of glutamate receptors. ALS is a progressive and fatal disorder affecting nerve cells. The cause of the disease is unknown, and no treatment is available that influences survival.
Many hypotheses about the cause of the disease are currently being studied. One of these hypotheses involves glutamate. Studies have reported that increased glutamate concentrations in the brain result in nerve cell death. Given this possible role of glutamate in ALS progression, the researchers sought to assess the effects of riluzole in people with ALS.
The researchers conducted a trial in 155 participants with ALS in France for one year. The participants were given either 50-mg of riluzole twice a day or a placebo. Survival and changes in ability to function were used as tests for the drug’s effectiveness. A secondary test used to examine the drug’s effectiveness was change in muscle strength.
After 12 months, 58 percent in the placebo group were still alive, compared with 74 percent in the riluzole group. The deterioration of muscle strength and functional ability was significantly slower in the riluzole group than in the placebo group.
Side effects of riluzole included stiffness, mild increase in blood pressure, and increase in the levels of the enzyme aminotransferase, which sometimes result in elevations of toxic ammonia. High levels of ammonia have been associated with brain damage, although the reason for ammonia toxicity is still unknown. While aminotransferase elevations were more frequent with riluzole treatment, the elevations were well tolerated and did not cause severe adverse effects in most of the participants in this study. More studies need to be conducted to understand this side effect of riluzole.
On the whole, it appears that these reported side effects may worsen the quality of life, but such consequences may be outweighed by the effect of the drug in improving muscle function and survival rates. The mechanism by which riluzole improves muscle function and survival rates is still unknown. However, the results of this study indicate that riluzole may have a beneficial effect in people with diseases that involve glutamate toxicity such as ALS and HD.
Rosas, et al. (1999) hypothesized that riluzole treatment may have beneficial effects in people with HD. The researchers conducted a 6-week trial of riluzole in eight participants with HD. The participants were treated with 50 mg of riluzole twice a day and were observed for changes in chorea (involuntary dance-like movements), dystonia (prolonged muscle contractions), and total functional capacity (TFC) scores. TFC is a standardized scale used to assess the capacity to work, handle finances, perform domestic chores and self-care tasks, and live independently. The brain lactate evels of the participants were also studied. Lactate is a by-product of anaerobic metabolism that is often used as a measure of energy metabolism efficiency in cells. Low lactate levels would indicated high aerobic respiration and high energy yields. High lactate levels on the other hand, would indicate that cells are unable to perform aerobic respiration and had to resort to the less-efficient anaerobic respiration instead. Changes in lactate levels were then used by the researchers to test the effects of riluzole on energy metabolism.
The researchers found that the chorea rating score of the participants who took riluzole improved by 35% compared to their scores before treatment. Discontinuation of treatment resulted in worsened chorea, indicating that riluzole was indeed associated with the improved chorea. No significant changes were seen on the dystonia or TFC scores.
Lactate levels were lower in the riluzole-treated participants compared to their levels before treatment. However, the researchers reported concerns about inaccuracies in lactate measurements due to limitations in their instruments and measuring methods. Whether or not the decreased lactate levels associated with riluzole indicate improved energy metabolism remains to be determined.
In this study, no significant adverse effects were observed after 6 weeks of treatment. The most frequent side effect was diarrhea; other symptoms quickly resolved without the need for medical intervention.
The results of this study also suggest a possible role for riluzole in the treatment of chorea in people with HD. However, the mechanism by which riluzole might alter or prevent disease progression is still ambiguous. More studies need to be conducted to determine whether and how riluzole can slow the progression of HD and protect nerve cells.
For further reading^
- Bensimon, et al. “A Controlled Trial of Riluzole in Amyotrophic Lateral Sclerosis (ALS).” The New England Journal of Medicine. 1994; 330(9): 585-591. Online.
This study reported that riluzole treatment resulted in increased survival rates and improved muscle function in people with ALS.
- Rosas, et al. “Riluzole Therapy in Huntington’s Disease (HD).” Movement Disorders. 1999; 14(2): 326-330.
This study reproted that riluzole treatment resulted in decreased chorea and lactate levels in people with HD.
- Landwehrmeyer GB, Dubois B, de Yébenes JG, Kremer B, Gaus W, Kraus PH, Przuntek H, Dib M, Doble A, Fischer W, Ludolph AC; European Huntington’s Disease Initiative Study Group. Riluzole in Huntington’s disease: a 3-year, randomized controlled study. Ann Neurol. 2007 Sep;62(3):262-72.
This study concluded that Riluzole has no benefit for HD.
-E. Tan, 1-15-02, updated by M. Hedlin 7-1-11
Drug Summary: Remacemide (RMC) is a drug that HD researchers hope can alleviate glutamate toxicity in the brains of HD patients. Remacemide is an NMDA antagonist – it inhibits the binding of glutamate to NMDA receptors, preventing glutamate from exerting its toxic effects on the nerve cell. Although, it has been shown to transiently improve motor performance in mouse models of HD, the few human clinical trials that have been performed have not produced statistically significant improvements in brain or motor function. Patients have also experienced side effects such as lightheadedness, dizziness, vomiting, nausea, and gastrointestinal disturbance.
The lowered amount of energy available in the nerve cells of patients with HD is thought to cause NMDA receptors to be oversensitive to glutamate. Therefore, normal physiological levels of glutamate can cause overexcitation of the NMDA receptor, leading to the influx of calcium ions into the cell. Excess calcium ion entry can lead to cell death through a combination of events. (For more information, click here.)
Remacemide, sometimes referred to as Remacemide Hydrochloride, is under investigation as a treatment for HD because it acts as a non-competitive inhibitor of the NMDA receptor. This means that remacemide decreases the receptor’s ability to bind glutamate by docking to a site on the receptor other than the glutamate binding site, and changing the shape of the receptor such that glutamate has a difficult time binding. Researchers hope that by inhibiting the NMDA receptor, the toxic effects of glutamate in the neurons of patients with HD can be lessened.
Clinical trials have examined the effectiveness of remacemide in curbing or stopping the neurodegenerative effects of HD in humans. Although remacemide treatment has not produced statistically significant improvement in these trials, in some patients it seems to transiently improve certain motor symptoms caused by HD such as chorea. Side effects such as dizziness, nausea, vomiting, lightheadedness, and gastrointestinal disturbances tended to accompany treatment.
Experiments done on mouse models of HD have been more positive.
Research on Remacemide^
Kieburtz, et al. (1996) conducted a study on the effects of remacemide in 31 participants in the early-stages of HD. The study was conducted over a 5-week period and the participants were divided into three treatment groups:
• 10 received 200 mg of remacemide per day
• 10 received 600 mg of remacemide per day
• 11 received a placebo (no medication at all)
The total functional capacity (TFC) of the participants was used as the criteria of the drug’s effectiveness. TFC is a standardized scale used to assess capacity to work, handle finances, perform domestic chores and self-care tasks, and live independently. The TFC scale ranges from 13 (normal) to 0 (severe disability). The HD Motor Rating Scale (HDMRS) was also used to assess the motor capabilities of the participants. The HDMRS consists of 14 items that assess the relevant motor features of HD including chorea and other motor functions. Other psychological tests were also conducted to measure the effectiveness of the drug in improving cognitive function.
Following treatment, the researchers concluded that there was no statistically significant difference between the three treatment groups. However, a trend towards improvement in chorea was observed among the participants who received 200 mg of remacemide per day. No major side effects were observed in most of the participants. However, one of the participants who received 600 mg/day did not complete the study due to persistent nausea and vomiting, which was believed to be a result of the medication.
The researchers concluded that remacemide could have short-term effects in improving chorea experienced by people in the early stages of HD. No statistically significant changes in cognitive performances were seen in the treatment groups. Larger, long-term controlled studies of remacemide are needed to determine the duration of tolerability and potential benefits of remacemide and other NMDA blockers.
The Huntington Study Group (2001) conducted a clinical trial involving 347 early-stage HD patients at 23 sites in the United States and Canada, monitored between July 1997 and June 1998. Participants in the study were assigned to four different treatments:
• 25% received remacemide (200 mg thrice a day)
• 25% received CoQ10 (300 mg twice a day)
• 25% received a combination of remacemide and CoQ10
• 25% received a placebo (no medication at all)
The primary measure of the drug’s effectiveness was change in total functional capacity (TFC) of the people with HD. A score of 13 represents a normal degree of function and a score of 0 represents a severely disabled state. The average TFC score of the participants before the study was 10.2. None of the treatments significantly altered the decline in TFC.
The condition of the participants who were treated with remacemide worsened by 2.3 points on the TFC scale, showing that the drug had no beneficial effect on slowing the functional decline experienced by people with HD. However, there was a trend toward an improvement in the degree of chorea in the participants treated with remacemide. Although this effect was not statistically significant, the effect was seen during the patient’s first visit after treatment began, suggesting that remacemide may decrease chorea. These findings suggest that antiglutamate therapies could be useful in controlling chorea even if they have no impact on slowing functional decline. However, remacemide was associated with side effects that included dizziness, lightheadedness and nausea. A trend towards a decrease in TFC decline was seen in the participants treated with CoQ10. (For information on CoQ10, click here.)
Ferrante et al. (2002) studied the potential therapeutic effects of remacemide, coenzyme Q10, and the combination of the two drugs on transgenic mouse models of Huntington’s Disease. They found that oral administration of either coenzyme Q10 or remacemide significantly extended survival and delayed the development of motor deficits, weight loss, cerebral atrophy, and neuronal intranuclear inclusions in the R6/2 transgenic mouse model of HD. The combined treatment, using CoQ10 and remacemide together, was even more effective than either compound alone.
For further reading^
- Kieburtz, et al. “A controlled trial of remacemide hydrochloride in Huntington’s disease.” Movement Disorders. 1996, May; 11(3): 273-7.
This article contains the full details on the study by Kieburtz, et al.
- The Huntington Study Group. “A randomized, placebo-controlled trial of coenzyme Q10 and remacemide in Huntington’s disease.” Neurology. 2001, Aug 14; 57(3): 397-404.
This article contains details on the study done by The Huntington Study Group.
- Schilling, et al. “Coenzyme Q10 and remacemide hydrochloride ameliorate motor deficits in a Huntington’s disease transgenic mouse model.” Neuroscience Letters. 2001, Nov 27; 315(3): 149-153.
- Ferrante, et al. “Therapeutic Effects of Coenzyme Q10 and Remacemide in Transgenic Mouse Models of Huntington’s Disease.” Journal of Neurosience. 2002, Mar 1; 22(5): 1592-1598.
-P. Chang, 7/5/04
Drug Summary: Mithramycin (also known as MIT and plicamycin) is an antibiotic that binds to DNA to regulate transcription. It attaches to specific regions of DNA that are rich in guanine and cytosine. While it is currently prescribed for the treatment of certain types of cancer and a few other conditions, recent research shows that it is helpful in treating motor symptoms and prolonging life in a mouse model of HD.
How could Mithramycin treat HD?^
Normally in the course of Huntington’s disease certain genes are prevented from being expressed in their normal protein products. This abnormal repression of genes is referred to as transcriptional dysregulation. Remember that “transcription” is the process by which the information of DNA is copied into messengers that are then used as templates for protein synthesis. Many of the genes that are prevented from being expressed are important for nerve cell health and survival. So, when these genes are blocked from producing their respective proteins, they cannot help to prevent the neurodegeneration that is typical of HD. This repression is caused by the mutant huntingtin protein, which interacts with molecules that would normally aid in the transcription of these helpful genes. The proteins that are encoded by the repressed genes have a wide variety of functions, which may explain why there are so many different symptoms associated with HD. One possible way to prevent many of these symptoms is to restore normal transcription, which is the proposed function of the drug mithramycin.
How does Mithramycin work?^
A number of hypotheses exist for how mithramycin acts in the body, but a group of researchers recently discovered what may be the key mechanism by which it prevents gene repression. One way that genes are repressed, or “silenced,” is through a process called methylation. Before explaining exactly what methylation is, we will first review how DNA is organized.
The entire DNA code is extremely long, and in order to be able to fit it into each cell, it needs to be very tightly compacted into chromosomes. (For more information on chromosomes, click here.) To accomplish this compression, the DNA is wound around structures called nucleosomes. You can think of a nucleosome as a spool and the DNA as the thread. Nucleosomes are made up of smaller proteins called histones. There are four different core histones (H2A, H2B, H3, and H4), and two of each are present in every nucleosome, along with one helper histone (H1). Histones play a very important role in the regulation of transcription.
By controlling access to DNA, histones determine if and when transcription occurs. When the histones keep the DNA tightly wound up, transcription factors cannot access the DNA, and it therefore cannot be transcribed. Different chemical groups (a chemical group can be a single atom or a small molecule) can be added to the histones in specific spots, causing the DNA to coil up to prevent transcription or causing the DNA to uncoil to allow transcription.
One way to control transcription is by methylation. In methylation, a chemical group called a methyl group is added to histone H3 or H4 at specific spots. When a histone is methylated at one of these spots, a protein called HP1 recognizes this signal and binds to the methyl group. These HP1 proteins also recognize each other and bind together, winding the DNA up into the coiled form of chromosomes in the process. Remember that in this coiled form, transcription factors cannot bind to the DNA so transcription cannot occur: the genes have been silenced. Methylation is a fairly permanent way of controlling transcription, so it can be devastating for an important section of the DNA to be wrongly methylated.
According to an important hypothesis, mithramycin helps restore normal transcription by regulating methylation. Researchers have discovered in a mouse model of HD that histone H3 was hypermethylated (methylated more that usual) at its ninth amino acid (recall that histones are proteins made up of amino acids in a long string or “chain”). It is already well known that increased methylation at this particular spot affects transcription. After conducting several experiments, the researchers concluded that mithramycin exerts its neuroprotective effects by preventing this hypermethylation and restoring normal transcription. Exactly how mithramycin prevents the hypermethylation has yet to be decisively determined, but the researchers have proposed a likely explanation.
Mithramycin is known to bind to areas of DNA that are rich in guanine (G) and cytosine (C), two DNA bases. (For more information on DNA bases, click here.) By binding to a GC-rich section of DNA, mithramycin likely prevents another molecule, which plays a role in methylation, from binding. Such molecules that aid in methylation could be either transcription factors or a type of enzyme called histone methyltransferase (HMT). HMT adds methyl groups to histones, causing the DNA to coil up as noted earlier and make it inaccessible for transcription. Transcription factors can also play a role in gene silencing by recruiting HMT. One type of transcription factor (TF) is already known to recruit a