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TRiC and Huntingtin Protein Aggregation

In Huntington’s disease (HD), an abnormal increase in the number of CAG repeats in the mutant Huntington gene corresponds to a long tract of glutamine amino acids in the huntingtin protein (Htt). This excessively long glutamine tract is sticky and leads to the formation of protein aggregates in brain cells. Whether these aggregates are toxic only upon formation or are formed as a cellular defense mechanism against free toxic mutant huntingtin protein (mHtt) is still hotly debated (more information about huntingtin protein aggregation can be found here); however, what is certain is that these protein aggregates are directly linked to neuron dysfunction and death. Moreover, a larger number of CAG repeats is clearly correlated with earlier disease onset and increased severity.

Although these findings suggest that HD is solely a result of the CAG expansion and subsequent protein aggregation, several observations indicate that other factors are also at play. For example, patients with the same number of CAG repeats may exhibit different disease patterns. In addition, there are genes other than the Huntington gene that also contain a CAG repeat region. Some of these genes may have a long region of upwards of 50 CAG repeats, but still produce functional non-harmful proteins (recall that HD can occur in patients with as few as 36 CAG repeats in the Huntington gene).  Evidently, there are other cellular mechanisms, though not well understood, that govern how the expanded number of CAG repeats translates into the toxic behavior of mHtt and aggregates seen in the specific case of HD.

Recently, researchers have found increasing evidence that the cellular mechanisms contributing to neurodegenerative diseases resulting from protein aggregates involve disruptions to the quality control machinery in the cell responsible for identifying and eliminating these toxic proteins. In particular, a protein known as TRiC (TCP-1 ring complex) has been found to be involved in controlling whether proteins with a long glutamine tract like Htt will fold and aggregate.  This article will first review how certain classes of proteins can control protein folding, then go over the case of TRiC and its role in Htt folding, and finally discuss how this knowledge might be helpful for researching HD therapeutics.

Protein Folding and Molecular Chaperones^

To understand how TRiC is involved in huntingtin (Htt) protein aggregation, it is useful to first review the players that make proteins in the cell. First, mRNA instructions are made based on the DNA blueprint in the nucleus, or the control center, of the cell. These instructions are then sent out into the cytoplasm of the cell and are read by structures known as ribosomes, the actual molecular machines that make proteins. Proteins are synthesized as long chains of amino acid building blocks, which are assembled one by one as the ribosome reads the mRNA instructions. For a more detailed breakdown of the protein synthesis process, go here.

However, proteins are not functional as a linear chain; they need to take on a certain shape or conformation in order to exert their proper functions. To assist in this process known as protein folding, proteins known as molecular chaperones bind to chains of amino acids that are newly made by ribosomes. Molecular chaperones, as their name suggests, are present during the protein-making process to ensure that newly made proteins behave normally and appropriately for their intended function. They do this by binding to the amino acid chain, thereby stabilizing it and facilitating proper folding. Another function that chaperones perform is to prevent folding errors and inappropriate aggregation, by recognizing incorrectly folded proteins and targeting them for repair or elimination. More information about molecular chaperones and proper protein folding can be found here.

Proteins folding incorrectly can lead to protein aggregation and disease. Moreover, there is a natural decline in a cell’s ability to continually make and fold protein correctly as one ages. As a result, in the study of HD, many scientists have focused their attention to the study of molecular chaperones and their role in responding to protein aggregation. TRiC is a molecular chaperone that has recently been found to be directly involved in regulating aggregation seen in proteins containing a long tract of glutamine amino acids, such as Htt.

What is TRiC and how is it involved in protein folding?^

TCP-1 ring complex, or TRiC, is a type of molecular chaperone that is composed of 8 different protein subunits, called CCT1, CCT2, all the way to CCT8 (CCT = chaperonin containing TCP-1). Together, these CCT subunits form a structure resembling two donuts stacked on top of each other.  Like other molecular chaperones, TRiC’s primary role in the cell is to act as a protein-folding machine: they bind to a specific set of newly made proteins and fold them into shapes that allow them to function correctly. This process occurs first with the unfolded protein entering and binding into the center of one of the rings in TRiC. TRiC then closes up the ring with the protein inside. During this time when the protein is enveloped in TRiC, protein folding occurs. After the protein is folded, TRiC can open up its ring again and eject the folded protein. In addition to huntingtin (Htt), some other proteins that are known to be regulated by TRiC include actin and tubulin, which are very important proteins that maintain the structural framework of the cell.

In order to identify molecular chaperones that are involved in proper folding of proteins with glutamine repeats, researchers looked at a roundworm often used in lab research. This 2006 study examined which proteins are important for minimizing aggregation of proteins with a polyglutamine expansion (“poly” means “many”, so “polyglutamine” means many glutamines). The researchers achieved this by using a roundworm that was genetically engineered to produce yellow fluorescent protein containing a polyglutamine expansion of 40 glutamine repeats. Then, they suppressed each gene in the roundworm one by one and looked for changes in the polyglutamine expanded fluorescent protein. The logic behind this experiment is that if a gene is important for preventing proteins with polyglutamine expansions, like Htt, from aggregating, then blocking that gene would increase the amount of protein aggregation. As you might have expected, one of the genes that the scientists identified through this experiment is the one that encodes the molecular chaperone TRiC.

Although roundworm models are easy to manipulate in the lab, it is much more relevant to study Htt aggregation in human cells. Consistent with the previous result, it has also been found that blocking TRiC in human cells will result in increased aggregation of proteins with an expanded glutamine repeat region.

How does TRiC actually suppress huntingtin aggregation?^

So far, we’ve discussed how TRiC is involved in folding newly formed proteins, but how does TRiC prevent already formed mutant huntingtin (mHtt) from aggregating?  Studies in this area of research first found that TRiC is able to directly bind to the Htt protein. This result aligns with what we previously knew about TRiC – that it acts as a molecular chaperone by assisting in protein folding. In particular, scientists have found, through several different experiments, that CCT1, one of the eight subunits forming TRiC, is especially important for binding to Htt. First, an in vivo study of the brain tissue of a transgenic mouse expressing mHtt revealed that TRiC and, specifically, CCT1 directly interacts with the Htt protein. This is very strong evidence indicating that TRiC regulates Htt activity in the brain in some way. Furthermore, by suppressing the CCT1 subunit alone, researchers still observe the same increase in Htt aggregation as when TRiC is suppressed. Even more significantly, expressing more CCT1 in cells can actually decrease the amount of Htt aggregation and toxicity in human cell culture models. All of these experiments suggest that TRiC can somehow control Htt aggregation through the CCT1 subunit.

Recent studies conducted between 2009 and 2013 from Judith Frydman’s lab at Stanford have shed light on how TRiC might control Htt aggregation through the CCT1 subunit.  First, researchers from the lab found that CCT1 binds to the Htt protein on a region of 17 amino acids, called N17, right next to the tract of glutamine repeats. This N17 region is significant because deleting this region in an Htt protein with 51 glutamine repeats appears to slow down the toxic protein aggregation process normally observed. In other words, the N17 region right next to the glutamine tract plays a role in speeding up the process of Htt protein aggregation.  Scientists also found that the N17 region of one Htt protein can bind to the N17 region and the glutamine tract of other Htt proteins.  Based on this evidence, they hypothesize that perhaps the N17 region of the Htt protein acts as a sticky end that other Htt proteins can attach onto and gradually form aggregates.

TRiC is another protein that can bind to the N17 region of Htt through its CCT1 subunit. Using advanced imaging techniques, the Stanford researchers found that when Htt and TRiC were put together, TRiC can either encapsulate single Htt proteins or small Htt aggregates in its ring chamber. It can also attach onto the tips of larger Htt aggregate fibers. From this experiment, the researchers concluded that TRiC can inhibit Htt aggregation perhaps by blocking the N17 region of Htt protein from endlessly sticking to other Htt proteins to form large, fibrous protein aggregates associated with neuron death. In the presence of TRiC, Htt will be maintained as a small single protein or small protein aggregates that are not toxic to the cell.

How can these findings about TRiC translate to an HD therapeutic?^

Research about TRiC is still in its preliminary stages, since much of the mechanisms behind TRiC’s ability to suppress huntingtin (Htt) aggregation remains to be understood. However, the fact that we do know TRiC and specifically its CCT1 subunit are potent inhibitors of aggregation is useful for researchers who are thinking of ways to stop the formation of dangerous aggregates that are toxic to brain cells.

A study published in 2012 by Sontag et al. took this knowledge and asked the following question: because cells that express more of the CCT1 subunit had less Htt protein aggregation, can this CCT1 subunit be applied as a drug and be directly delivered to brain cells? In order to answer this question, the researchers used cell models of HD and first showed that a modified fragment of CCT1 can penetrate the cell membrane and enter the cell to exert its effects. With regards to therapeutic benefits, the researchers observed that when higher doses of CCT1 were applied to rat cells engineered to express mHtt, there was a corresponding decrease in the amount of visible Htt aggregates formed. The scientists followed up with another experiment where they applied CCT1 to cells of HD mice that were removed from the striatum, the brain area most affected in HD.  They obtained perhaps an even more encouraging result: the cells receiving CCT1 treatment exhibited a higher survival rate compared to cells without treatment.  This suggests that CCT1 may exert therapeutic benefits for cells expressing mHtt beyond just suppressing protein aggregation.

Conclusion^

The above research is simply a small-scale experiment that demonstrates the potential that this line of research can have for HD therapeutics.  Evidently, there is a still a lot that we don’t know about how molecular chaperones like TRiC can suppress huntingtin (Htt) aggregation.  The experiments up to this point have all been performed with cells in laboratory settings, and these results will have to be verified in subsequent animal studies before any conclusions can be made about their actual therapeutic benefit.  However, studying TRiC has emphasized the importance of looking beyond the infamous glutamine repeats in Htt and looking at regions such as N17 to understand the mechanism of Htt aggregation and toxicity. Furthermore, the fact that direct protein delivery to the brain is already being done in mice for Parkinson’s disease research could lead the way for testing CCT1 in HD mouse models as well.

Further Reading^

Meyer AS, Gillespie JR, Walther D, Millet IS, Doniach S, Frydman J. (2003) Closing the Folding Chamber of the Eukaryotic Chaperonin Requires the Transition State of ATP Hydrolysis. Cell. 113(3): 369-381.

Behrands C, Langer CA, Boteva R, Böttcher UM, Stemp MJ, Schaffer G, Rao BR, Giese A, Kretzschmar H, Siegers K, Hartl FU. (2006). Chaperonin TRiC Promotes the Assembly of polyQ Expansion Proteins into Nontoxic Oligomers. Mol Cell. 23(6): 887-897.

Tam S, Geller R, Spiess C, Frydman J. (2006). The chaperonin TRiC controls polyglutamine aggregation and toxicity through subunit-specific interactions. Nat. Cell Biol. 8(10): 1155-1162.

Tam S, Spiess C, Auyeung W, Joachimiak L, Chen B, Poirier MA, Frydman J. (2009) The Chaperonin TRiC Blocks a Huntingtin Sequence Element that promotes the Conformational Switch to Aggregation. Nat. Struct. Mol. Biol. 16(12): 1279-1285.

Shen K, Frydman J. (2013). The interplay between the chaperonin TRiC and N-terminal region of Huntingtin mediates Huntington’s Disease aggregation and pathogenesis. Protein Quality Control in Neurodegenerative Diseases. Eds. Morimoto  RI, Christen Y. 121-132.

Shahmoradian SH, Galaz-Montoya JG, Schmid MF, Cong Y, Ma B, Spiess C, Frydman J, Ludtke SJ, Chiu W. (2013). TRiC’s tricks inhibit huntingtin aggregation. eLife. 2:e00710.

Sontag EM, Joachimiak LA, Tan Z, Tomlinson A, Housman DE, Glabes CG, Potkin SG, Frydman J, Thompson LM. (2012). Exogenous delivery of chaperonin subunit fragment ApiCCT1 modulates mutant Huntingtin cellular phenotypes. Proc. Natl. Acad. Sci. USA. 110(8): 3077-3082.

– J. Choi, 02-03-14

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Genome Editing

Although the pathology of Huntington’s disease (HD) is still not completely understood, we know that HD is a genetic disorder where the root cause of every HD case is a longer-than-normal series of three repeated DNA base pairs, CAG, in the HD gene. A DNA sequence provides the instructions for the cell to make mRNA (messenger RNA), which in turn contains the instructions for making a protein – the building blocks and machines of cells. The process of using the information contained in genetic material (DNA and RNA) to form protein is called gene expression. Genetic changes in the HD gene sequence are thus propagated into the mRNA sequence and result in production of a mutated version of the huntingtin protein that ultimately results in degeneration of brain cells.

In order to develop a therapy that prevents the production of the mutant huntingtin protein, many scientists are currently using a technology known as gene silencing. This approach creates molecules that directly target and bind to the mRNA copies that contain the instructions for producing huntingtin protein. These molecules then use the cell’s own molecular machinery to destroy the problematic mRNA instructions (for more information about gene silencing, click here).

Gene silencing is a very promising approach, and many scientists are focusing their efforts on conducting studies and clinical trials to assess the feasibility and efficacy of gene silencing treatments for HD and other diseases. However, it has recently become possible to go one step further in manipulating gene expression. Instead of targeting the intermediate mRNA copies as in gene silencing, some scientists are pursuing strategies that will directly modify the DNA blueprint in cells and ultimately living organisms. This article will discuss this nascent technology known as genome editing and its potential as an HD therapeutic.

What is Genome Editing?^

Just as editing text involves adding, removing, or replacing words, genome editing is an approach in which the genome sequence is directly changed by adding, replacing, or removing DNA bases. However, the genome is relatively resistant to change. DNA in the body is not only responsible for encoding all of the necessary functions within a cell, but it is also crucial to determining differences between individuals. If DNA could be easily altered, many essential cell functions would be disrupted in undesirable ways. To deter any changes from being inadvertently made to DNA, cells have inherent mechanisms to proofread and repair their genetic code. These repair mechanisms and the overall stability of DNA is what makes genome editing such a novel approach and so difficult to achieve.

Remarkably, researchers have been able to take advantage of the cell’s DNA repair mechanisms to achieve genome editing. To accomplish this, scientists can use artificially engineered enzymes called nucleases to cleave DNA strands. In effect, these nucleases act as molecular scissors that form a break in the DNA double-stranded helix. Once a break is introduced in the DNA, the cell will detect a problem in its genetic code and quickly activate its repair machinery.

There are two major methods by which a cell can repair a break in its DNA. First, the cell can employ various enzymes to directly join the two ends of the DNA break back together. This process, known as nonhomologous end-joining, is very error-prone and often results in mutations – such as small insertions or deletions of nucleotides – in the resulting DNA strand. These small mutations can be neutral, but they can also render the entire gene in that location nonfunctional, achieving the disruption or knockout of the gene.

Second, the cell can also repair a DNA break by using another DNA sequence as a template. In genome editing applications, a DNA sequence can be designed to be inserted along with a nuclease, such that when a cut is made in the DNA, the cell’s own repair mechanisms can use the DNA sequence supplied to replace an existing DNA sequence as it repairs the break. This method allows scientists to directly change genetic information in cells by introducing a correct version of a DNA sequence to replace an unwanted mutation.

How can a specific gene be edited?^

By using a cell’s own repair mechanisms, scientists can disrupt or correct a mutation by genome editing, and both approaches could prove useful in the context of HD treatment. However, there must be a way to direct a nuclease to the desired location where a DNA break is to be introduced. To address this issue, many different types of nucleases have been developed. All nucleases consist of 2 components – the nuclease itself that is responsible for DNA cleavage and a secondary component responsible for recognizing a specific DNA sequence. There are three main classes of nucleases engineered for genomic editing purposes:

Zinc finger nucleases (ZFNs)^

ZFNs consist of a nuclease component linked to a DNA-binding component derived from an array of zinc finger proteins. Each zinc finger protein can bind three nucleotides, so combinations of zinc fingers linked together can be designed to recognize specific genomic sequences.

Transcription activator-like effector nucleases (TALENs)^

TALENs are very similar to ZFNs in that they also have a nuclease domain linked to a DNA recognition domain. The difference lies in the fact that in TALENs, the DNA recognition domain is a series of amino acid repeats. Each repeat corresponds to a single nucleotide base (A, G, C, or T), and TALENs can be designed to have different combinations of repeats to recognize specific genomic sequences.

CRISPR/Cas System^

The CRISPR/Cas system employs a nuclease called Cas9 to introduce a DNA double strand break. Unlike ZFNs or TALENs, this approach does not use a protein-based DNA recognition domain. In order to guide the Cas9 nuclease to a specific DNA binding site, an RNA sequence is designed to precisely bind to a complementary DNA sequence, allowing for the Cas9 nuclease to make a cut.

Meganucleases^

Unlike ZFNs or TALENs, each meganuclease has a long recognition sequence that allows them to make DNA breaks at specific sites. However, these long recognition sequences are naturally defined and cannot be engineered, thus meganucleases can only be used for some target genetic sequences.

Can genome editing be used therapeutically?^

Just like gene silencing, genome editing is already being used by scientists as one of their many tools to develop cell and animal models for studying different diseases. Is there a possibility that genome editing can be used in humans to cure genetic diseases like HD? Preliminary research suggests that genome editing may be a promising therapeutic approach, but more work is needed prior to clinical testing in humans.

Research has shown that simply delivering engineered zinc finger proteins (ZFPs) that do not have any nuclease activity is able to reduce the levels of mutant huntingtin. A study published in 2012 by a research group in Spain found that ZFPs can be designed to bind longer CAG repeats more strongly than shorter repeats, which means that these ZFPs could specifically recognize the mutant huntingtin gene with the CAG expansion. They further demonstrated that ZFPs reduced the levels of mutant huntingtin by 95% without affecting the levels of the wild-type huntingtin protein in an in vitro model of HD using mouse cells expressing a human version of the mutant HD gene. Moreover, they were able to demonstrate similar results in an HD mouse model, where ZFP treatment reduced the level of mutant huntingtin up to 60% and motor performance as measured on a rotarod was significantly improved. This study not only demonstrated that ZFPs can specifically bind to the mutant huntingtin gene, but also suggested that ZFPs can accomplish gene silencing by simply binding to the DNA and preventing the gene from being transcribed. These findings support the use of zinc finger nucleases (ZFNs), which could add to the repressive effect of ZFPs by actually disrupting or correcting the mutant gene.

ZFNs have been tested as a therapeutic approach in other diseases. For example, Sangamo Biosciences, a biopharmaceutical company, has explored the potential of ZFNs in treating hemophilia, a genetic disorder in which the ability of the blood to clot is impaired. The researchers used ZFNs to replace the mutated gene responsible for causing hemophilia with a correct gene that allows for normal function in a mouse model of hemophilia, and found that clotting times of the mice returned to normal after treatment. This study suggests that ZFNs are a viable strategy for correcting the genome in genetic diseases such as HD.

Conclusion^

Many factors will still need to be considered before genome editing can be used as a viable therapeutic option for HD. Designing nucleases to be specific for one genetic sequence is a difficult and often expensive process, as it requires linking together the right combination of zinc fingers in ZFNs or the right combination of amino acid repeats in TALENs. Moreover, just like in gene silencing, DNA recognition by ZFNs or TALENs is not perfect and can result in off-target effects (binding to the wrong genetic sequence). Genome editing is still in its early stages, and it will be awhile before we will know if it can be used as a gene therapy for HD patients. But, many scientists are pursuing this new avenue of research with promising results.

Further Reading^

Jasin, Maria. (1996) Genetic manipulation of genomes with rare-cutting endonucleases. Trends in Genetics. 12(6): 224-228.

de Souza, Natalie. (2012) Primer: genome editing with engineered nucleases. Nature Methods. 9: 27.

Hsu PD, Scott DA, Weinstein JA, Ran FA, Konermann S, Agarwala V, Li Y, Fine EJ, Wu X, Shalem O, Cradick TJ, Marraffini LA, Bao G, Zhang F. (2013) DNA targeting specificity of RNA-guided Cas9 nucleases. Nature Biotech. 31: 827-832.

Carroll, Jeff. (2011) Cut-and-paste DNA: fixing mutations with ‘genome editing’. HDBuzz. http://en.hdbuzz.net/038.

Sangamo Biosciences. ZFP nucleases: http://www.sangamo.com/technology/zf-nucleases. Huntington’s disease: http://www.sangamo.com/pipeline/huntingtons-disease.html.

Garriga-Canut¬¬¬ M, Agustin-Pavón C, Hermann F, Sánchez A, Dierssen M, Fillat C, Isalan M. (2012) Synthetic zinc finger repressors reduce mutant huntingtin expression in the brain of R6/2 mice. Proc Natl Acad Sci USA. 109(45): E3136-E3145.

-J. Choi, 10-24-13

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Gangliosides

Contrary to what one may think, the brain is the most fat-rich organ in the body.  Aside from being an efficient way to store energy from the food we eat, fat molecules, known as lipids, have many variations in their structure, allowing for a correspondingly large number of additional functions.  For example, some lipids are integral structural components of the membrane that encloses all cells, while others act as hormones that serve as important chemical messengers between different parts of the body.  In the brain and the rest of the central nervous system, lipids play critical roles as signaling molecules that trigger processes such as forming new neuronal connections and promoting brain repair.  Moreover, lipids are a significant component of myelin, the coating around axons that insulates the transmission of electric signals between brain cells just like insulation on electrical wires.  For more information about the role of fats in the nervous system and in HD, click here.

Recently, one family of lipids known as gangliosides has emerged as a potentially important player in HD progression.  Specifically, researchers found lower levels of one type of ganglioside in not only different types of HD mouse models, but also cells isolated from HD patients and postmortem human HD brain samples.  This article describes how gangliosides normally function in the nervous system, how ganglioside function may be disrupted by HD, and how these findings might be useful in the development of a viable HD therapeutic.

Gangliosides and Diseases of the Nervous System^

Gangliosides are a family of lipids first identified in 1942 and so named because they were isolated from ganglion cells of the brain.  Although gangliosides have since been found in cells throughout the body, they are most concentrated in the nervous system, where they seem to exhibit important effects in cell signaling and neuroprotection.  However, their functions are not well understood.

One way of assessing the importance of a molecule in any biological system is by looking at the consequences if the molecule is no longer present.  In the case of gangliosides, many characterized neurological disorders have been linked to defects in ganglioside production.  For example, Guillain-Barré syndrome, an acute inflammatory disease that affects the peripheral nervous system, is caused by the production of antibodies by the immune system that specifically target gangliosides in the body. This type of autoimmune disease, wherein one’s own immune system inadvertently destroys a component of the body, damages the nerve axons that are responsible for nerve signaling and can result in patient paralysis.   Moreover, a genetic mutation that impairs an enzyme important to the production of one type of ganglioside, resulting in complete loss of that ganglioside, leads to a severe infantile-onset epilepsy syndrome characterized by symptoms such as brain atrophy, seizures, and chorea, all of which are symptoms associated with juvenile HD (for more information about juvenile HD, click here).  Since a consequence of the loss of gangliosides is neurodegeneration, gangliosides are thought to play important neuroprotective roles.  In support of this theory, scientists who have genetically engineered mice lacking certain types of gangliosides have found that these mice exhibit severe neurodegeneration and accompanying motor defects that resemble those of HD mouse models.

On the other end of the spectrum, increased levels of gangliosides in nerve cells can result from overproduction of the lipid or problems in its degradation, leading to abnormal buildup.  The genetic disorder Tay-Sachs disease, found mainly in Jewish populations, results from the harmful accumulation of gangliosides in the nerve cells of the brain and other tissues.  Tay-Sachs disease is caused by a genetic mutation that impairs proper degradation of gangliosides. Buildup of the lipid causes nerve cells to become swollen, leading to deterioration of cognitive and motor skills.  Improper control of ganglioside levels has also been observed in cases of Alzheimer’s disease, a neurodegenerative disease characterized by protein aggregates (for more information about Alzheimer’s disease, click here).  Researchers have found that gangliosides bind with amyloid β-protein and facilitate the production of amyloid β-protein aggregates that accumulate in the brains of some Alzheimer’s disease patients.  What is clear is that both the deficiency and excess of gangliosides in the nervous system result in neurodegenerative defects, suggesting that the careful maintenance of ganglioside levels is important for neuronal function.

Gangliosides and HD^

Taking into account the above observations that abnormal ganglioside levels are often implicated in diseases of the nervous system, scientists began to question whether HD may also involve ganglioside irregularities.  It was found in 2010 that the production of GM1, a specific type of ganglioside, appears to be impaired not only in different cell models of HD, but also in cells directly derived from human HD patients.  In this study, Simonetta Sipione and her research group at the University of Alberta first used an in vitro model of HD by growing rat striatal cells that have been engineered to express mutant huntingtin.  By using a protein marker that specifically identifies the GM1 ganglioside, they found that the levels of GM1 in the cells expressing mutant huntingtin were significantly lower compared to normal cells.  More importantly, they observed the same results when they performed the experiment on skin cells isolated directly from human HD patients.  The researchers also found that the levels of an enzyme involved in the production of GM1 were decreased, suggesting that the reason for the lowered levels of GM1 in HD is because of defects in GM1 production.

To determine whether the decreased levels of GM1 have any effects on striatal neuron survival, Sipione’s group of researchers administered GM1 to the cells that express mutant huntingtin.  They found that the recovery of GM1 levels in the cells was accompanied by a drop in the number of cells undergoing apoptosis, or programmed cell death.  On the other hand, when they added a molecule that lowers the amount of GM1 in normal cells, they observed an increase in the number of cells that underwent apoptosis.  These two experiments suggest that GM1 may be an important protective factor in HD – the presence of GM1 may protect cells in the face of stress, but mutant huntingtin leads to decreased levels of GM1.  GM1 deficiency in turn contributes to increased cellular apoptosis that corresponds to neuronal loss in HD.

In a second follow-up study, Sipione’s research group asked whether GM1 could be used as a potential therapeutic in HD mouse models, given the above observations that suggest a potential neuroprotective role.  To answer this question, Sipione used a well-characterized type of HD mouse model that contains a copy of the human mutant huntingtin gene in its genome.  This HD mouse mirrors many of the motor and cognitive symptoms of HD seen in human patients and provides a model of the disease that is useful for testing potential therapeutics (for more information about animal models of HD, click here).  Importantly, these HD mice also exhibited low levels of GM1 compared to wild-type control mice.  The researchers applied therapeutic GM1 by infusing the lipid into the mouse brains. They report that mice already beginning to exhibit HD motor symptoms before the treatment had restored normal motor control in four different test of motor function.  This result is particularly remarkable because the GM1 treatment began following the appearance of symptoms in mice, yet still resulted in complete motor recovery.  Post-symptomatic treatments are important when developing a human therapy because of the difficulties involved in treating an individual carrying the HD mutation throughout their lifetime.

Finally, in light of these encouraging in vitro and in vivo results, the researchers were interested in how GM1 might be causing such a drastic improvement in motor control in HD mice.  The researchers found that GM1 treatment caused a change in the huntingtin protein.  Specifically, they found that GM1-treated brain cells express higher levels of huntingtin protein that has been modified with phosphate tags at two specific amino acid sites within the protein.  Proteins that have been tagged with phosphates, through a process known as phosphorylation, often demonstrate altered activity depending on the specific location of the tag. In the case of the mutant huntingtin protein, studies in mouse models have found that if phosphates are tagged on two specific locations on the huntingtin protein – serine 13 and serine 16 – the toxicity of the mutant huntingtin protein is significantly decreased.  Therefore, the observation that GM1 causes the addition of these phosphate tags at these particular sites of mutant huntingtin raises the possibility that GM1 improves motor control in mice by making the mutant huntingtin protein less harmful to neuronal cells.

Conclusion^

The current research on gangliosides, especially GM1, is very promising for the development of a potential therapeutic for HD, but there remain some challenges.  The current experiments have only tested GM1 treatment in cell and mouse models of HD and the transition into a human study will not necessarily yield results that are as encouraging.  However, ganglioside treatments for other neurological disorders, such as Parkinson’s disease, Alzheimer’s disease, and stroke, have already reached clinical trials.  These studies have met the first criteria of clinical trials by demonstrating that long-term ganglioside treatment does not pose any safety concerns (for more information about clinical trials, click here). While it has yet to be shown if the ganglioside treatments are effective in any of the neurological disorders, these studies, if successful, could pave the way for beginning clinical trials to test the therapeutic value of ganglioside treatment for HD.

Further Reading^

Posse de Chaves E, Sipione S. (2010).  Sphingolipids and gangliosides of the nervous system in membrane function and dysfunction.  FEBS Lett.  584:  1748-1759.

Christie W.  (2013).  Gangliosides:  Structure, Occurence, Biology, and Analysis. AOCS Lipid Library.  http://lipidlibrary.aocs.org/lipids/gang/index.htm.

Yu RK, Ariga T, Yanagisawa M, Zeng G. (2008).  Gangliosides in the nervous system: Biosynthesis and degradation. in Glycoscience (Fraser-Reid, B.; Tatsuka, K.; Thiem, J. ed.) Springer-Verlag. Berlin-Heiderberg, Germany. pp.1671-1695.

Yamashita T, Wu Y-P, Sandhoff R, Werth N, Mizukami H, Ellis JM, Dupree JL, Geyer R, Sandhoff K, Proia RL.  (2005).  Interruption of ganglioside synthesis produces central nervous system degeneration and altered axon-glial interactions.  Proc Natl Acad Sci USA.  102:  2725-2730.

Maglione V, Marchi P, Di Pardo A, Lingrell S, Horkey M, Tidmarsh E, Sipione S. (2010).  Impaired ganglioside metabolism in Huntington’s disease and  neuroprotective role of GM1. J Neurosci.  30: 4072-4080.

Di Pardo A, MaglioneV, Alpaugh M, HorkeyM, Atwal RS, Sassone J, Ciammola A, Steffan JS, Fouad K, Truant R, Sipione S.  (2012). Ganglioside GM1 induces phosphorylation of mutant huntingtin and restores normal motor behavior in Huntington disease mice.  Proc Natl Acad Sci USA.  109: 3528-3533.

Carroll J.  (2012)  Special ‘brain fat’ injection helps HD mice. HDBuzz.  Web:  http://en.hdbuzz.net/072.

-J. Choi, 7-31-13

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Vitamin D3 (cholecalciferol)

Vitamin D has been called the “miracle vitamin” by many health experts due to mounting discoveries of its significance in promoting health and fighting numerous diseases, including cancer, heart disease, and diabetes. It may also be therapeutic for neurodegenerative diseases, which may be relevant to Huntington’s disease (HD). This particular vitamin is found in many food sources, including milk, eggs, and fish, and it can also be produced by the skin through sunlight exposure. While vitamin D is widely known for its role in maintaining strong and healthy bones by helping the body absorb calcium, it is much more than a bone-protecting vitamin. Research for the past few decades has shed light on the protective effects of vitamin D on immune and neural cells and has implicated a deficiency of vitamin D as a risk factor for various brain diseases. This article will focus primarily on a form of vitamin D called vitamin D3, also known as cholecalciferol, and how the vitamin may be protective against neurodegenerative diseases such as HD.

What is vitamin D?^

The term “vitamin D” actually refers to a group of fat-soluble vitamins. There are five different forms of vitamin D, but the two major forms are vitamin D2 (ergocalciferol) and vitamin D3 (cholecalciferol). Vitamin D2 is produced by plants, while vitamin D3 is produced by the skin of animals in response to sunlight (UV light) exposure. UV light reacts with an enzyme called 7-dehydrocholesterol to create pre-vitamin D, which rearranges its structure to form vitamin D3. An enzyme then converts vitamin D3 into a compound called calcitriol, which is the active form of vitamin D that is responsible for the numerous health benefits.

What is its mechanism of action?^

After its conversion from Vitamin D3, calcitriol exerts its effects on the body by binding to and activating vitamin D receptors (VDRs), which are located in the nuclei of target cells. Once activated, VDRs can function as transcription factors that bind to cellular DNA and control gene expression, ultimately triggering a biological response.

The major physiological role of vitamin D is to facilitate the intestinal absorption of calcium, by stimulating the expression of proteins involved in calcium transport. Vitamin D also plays a crucial role in providing the proper balance of minerals necessary for bone growth and function. However, it turns out that VDRs are present in the cells of most organs in the body, suggesting that there is wide diversity in the types of responses that vitamin D3 can promote.

Vitamin D3 and the brain^

Initially it was believed that only the liver and kidneys contained the enzyme responsible for producing calcitriol from vitamin D3. It is now known that many tissues, including the brain, contain this enzyme. In addition, VDRs are widely present throughout the brain, implicating vitamin D3 as a contributor to a variety of neural processes. Several of these processes are thought to be neuroprotective.

Studies have indicated that calcitriol may possess antioxidant properties and also strengthens the role of existing antioxidants in the body. For instance, Garcion et al. (1997) demonstrated that calcitriol acts similarly to traditional antioxidant nutrients by inhibiting an enzyme called inducible nitric oxide synthase (iNOS), which is overactive in patients with Alzheimer’s and Parkinson’s disease. Baas et al. (2000) showed that calcitriol increases levels of glutathione, a natural antioxidant which protects oligodendrocytes, which are brain cells that provide support and insulation for neurons.

Calcitriol can also protect neurons by producing neurotrophins, including neurotrophin-3 (NT-3), glial cell derived neurotrophic factor (GDNF), and nerve growth factor (NGF) that promote the survival of neurons in aging and with neurological injury. As shown in studies by Naveilhan et al. (1993) and Neveu et al. (1994), calcitriol increases levels of GDNF and NT-3. NT-3 protects nerve transmission and synaptic plasticity, and GDNF influences the survival and differentiation of dopamine-producing cells. In animal models of Parkinson’s disease, treatment with calcitriol increased GDNF levels and reduced oxidative stress (Wang et al., 2001). On the other hand, in newborn rodents, depleting vitamin D3 while they were in their mother’s uterus reduced levels of GDNF and NGF and caused damaging structural brain changes (Becker et al., 2005). (To read more about neurotrophins, click here.)

Vitamin D3 and neurodegenerative disorders^

While there has not been much research focused on its potential role in HD, vitamin D3 deficiency has been implicated as serving a role in a number of neurodegenerative disorders.
For instance, there is compelling evidence that low levels of vitamin D3 are a risk factor for multiple sclerosis (MS), a disease in which the immune system attacks the central nervous system and causes demyelination and axon degeneration. The prevalence of MS is linked with decreasing exposure to solar UV radiation, and a study by Munger et al. suggests that high circulating levels of vitamin D3 correspond to a lower risk of MS.

There is also evidence that vitamin D3 deficiency is relevant for Parkinson’s disease and Alzheimer’s disease. The greatest number of VDRs are found in the substantia nigra, the portion of the brain that primarily degenerates in Parkinson’s disease and can also be affected in HD. Treating substantia nigra neurons with vitamin D3 protects them from Parkinson-like insults (Shinpo et al., 2000). In Alzheimer’s disease, a condition characterized by dementia and neuron loss in the hippocampus, some evidence suggests that there may be a vitamin D3 deficiency early in the disease (Landfield et al., 1991), and, in aging rats, treating with calcitriol reduced hippocampus shrinkage and prevented decreases in neuron density (Landfield and Cadwallader-Neal, 1998). Although more extensive research into this area is needed, these results suggest that vitamin D3 could have a potential role in the prevention of neurodegenerative disorders.

Conclusion^

Despite the many exciting findings about this “miracle vitamin” over the years, determining the many health benefits of vitamin D3 is still an active area of research. While the extent to which vitamin D3 contributes to neural processes is not clearly understood, there is currently much evidence to support a neuroprotective role for vitamin D3 in the brain, as well as promising evidence that it may have preventative effects against neurodegenerative disorders.

Works Cited^

Baas, D et al. “Rat oligodendrocytes express the vitamin D(3) receptor and respond to 1,25-dihydroxyvitamin D(3).” Glia 31.1 (2000): 59–68. Print.

Becker, Axel et al. “Transient prenatal vitamin D deficiency is associated with subtle alterations in learning and memory functions in adult rats.” Behavioural brain research 161.2 (2005): 306–312.

Garcion, E et al. “1,25-Dihydroxyvitamin D3 inhibits the expression of inducible nitric oxide synthase in rat central nervous system during experimental allergic encephalomyelitis.” Brain research. Molecular brain research 45.2 (1997): 255–267. Print.

Landfield, P W et al. “Phosphate/calcium alterations in the first stages of Alzheimer’s disease: implications for etiology and pathogenesis.” Journal of the neurological sciences 106.2 (1991): 221–229. Print.

Landfield, P W, and L Cadwallader-Neal. “Long-term treatment with calcitriol (1,25(OH)2 vit D3) retards a biomarker of hippocampal aging in rats.” Neurobiology of aging 19.5 (1998): 469–477. Print.

Naveilhan, P et al. “Expression of 25(OH) vitamin D3 24-hydroxylase gene in glial cells.” Neuroreport 5.3 (1993): 255–257. Print.

Neveu, I et al. “1,25-dihydroxyvitamin D3 regulates NT-3, NT-4 but not BDNF mRNA in astrocytes.” Neuroreport 6.1 (1994): 124–126. Print.

Shinpo, Kazuyoshi et al. “Effect of 1,25-dihydroxyvitamin D3 on Cultured Mesencephalic Dopaminergic Neurons to the Combined Toxicity Caused by L-buthionine Sulfoximine and 1-methyl-4-phenylpyridine.” Journal of Neuroscience Research 62.3 (2000): 374–382. Web. 7 Apr. 2013.

Wang, J Y et al. “Vitamin D(3) attenuates 6-hydroxydopamine-induced neurotoxicity in rats.” Brain research 904.1 (2001): 67–75. Print.

J. Nguyen 2013

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Melatonin

It has long been known that melatonin, a hormone produced in the brain, plays an important role in regulating the body’s natural sleep-wake cycle by causing drowsiness and inducing sleep. The pineal gland, a small structure located beneath the center of the brain produces and releases melatonin in response to the intensity and type of light detected by the eyes. Darkness causes increased melatonin release, while light inhibits melatonin release. What results is a daytime decline and nighttime rise in melatonin levels that mirrors waking and sleeping. Thus, melatonin plays an important role as a regulator of the biological clock. Because of this, melatonin is sometimes prescribed as a treatment for sleep disorders. Disruption of normal sleep is a common symptom in HD, and more information about sleep and HD can be found here.

Outside of sleep regulation, melatonin is also involved in many human bodily processes including learning, memory, and aging. Some of these functions are brought about by the antioxidant properties of the hormone itself, while others are attributed to melatonin binding with its receptor proteins, MT1 and MT2. There has been a great deal of interest in studying these additional benefits of melatonin, since it is already an FDA-approved drug.  In terms of HD specifically, researchers have recently identified melatonin as a neuroprotective agent due to its role in inhibiting the neuronal death characteristic of HD.  Since melatonin acts through so many different possible mechanisms, how exactly can melatonin produce its therapeutic effects against HD and affect disease progression?

Melatonin as an Antioxidant^

Free Radical Damage in HD^

In order to understand the role of antioxidants like melatonin in HD, we must first briefly review free radical damage, a phenomenon implicated in HD-associated neuron death.  Free radicals are highly reactive molecules that are natural byproducts of biochemical processes in the body, but high levels of free radicals can be toxic to cells in the body because they cause oxidative damage. Neuron cells in the brain, seem to be particularly susceptible to oxidative damage. For more information about free radicals and how they damage cells, click here.

One cause of free radical excess is glutamate excitotoxicity. Glutamate is an important neurotransmitter, a chemical signal used by neurons to communicate with each other.  Normally, binding of glutamate to NMDA receptors on neurons is responsible for processes such as learning and memory in the brain. Scientists believe that in HD, the mutant huntingtin protein causes problems that lead to excess binding of glutamate (for more information about NMDA receptors and glutamate toxicity, click here).  One of the eventual results of this defect is the overproduction of free radicals.

Another mechanism that can lead to free radical damage in HD is dysfunction of the mitochondria, the parts of the cell that act as energy power plants. A normal byproduct of the mitochondria producing energy for the cell is the generation of free radicals. However, if mitochondria are defective they may overproduce free radicals, leading to increased damage in the cell.  This is one possible explanation for the mitochondrial defects observed in some patients with HD.

Melatonin’s Antioxidative Properties^

As a defense mechanism against harmful free radicals that are normally produced, the body employs molecules known as free radical scavengers, which we more commonly refer to as antioxidants.  As their name implies, free radical scavengers encounter free radicals and detoxify them before they can damage cells or tissues.  Melatonin, in addition to being a hormone that regulates sleep-wake cycles, is also a potent antioxidant.  Its antioxidant properties go beyond free radical scavenging.  There is evidence suggesting that melatonin can stabilize cell membranes and thereby increase the cell’s resistance against free radicals, that it can stimulate cellular production of other antioxidants, and that it may even play a role in directly inhibiting production of certain types of free radicalsMelatonin is also among the few antioxidants that can cross the blood-brain barrier, thus extending its protective properties to neurons in the brain.

Much research has been done to determine whether melatonin is truly effective for reducing oxidative damage.  In order to mimic neuronal damage as a result of uncontrolled free radical levels in HD, scientists use chemicals such as quinolinic acid and 3-nitropropionic acid.  The former is a molecule that binds to NMDA receptors which mimics glutamate, while the latter interrupts mitochondrial activity, and injection of either chemical into animal models result in oxidative damage similar to the neuropathology seen in brains of HD patients.  In a 1999 study performed by Southgate et al., rat brains were treated with quinolinic acid, which caused oxidative damage to cell membranes.  However, after the addition of melatonin, the amount of damage decreased significantly, suggesting that melatonin has protective antioxidative effects.  In a more recent 2005 study by Nam et al., 3-nitropropionic acid was injected directly into the striatum of rats, creating neuronal lesions similar to those seen in HD.  Treatment with melatonin reduced the amount of free radical damage to levels comparable to control mice that did not have a 3-nitropropionic acid injection, and significantly decreased the size of neuronal lesions in 3-nitropropionic acid injected mice.  All of these studies provide evidence that melatonin has protective effects against these chemically-induced HD models through antioxidant action. However, it is important to remember that these chemical treatments do not precisely replicate HD and further research must be done to demonstrate the effects of melatonin on oxidative damage in HD.

Melatonin and Melatonin Receptor Interactions^

The reasons why melatonin demonstrates neuroprotective effects are still unknown, but aside from its antioxidant properties, researchers have also shown that the interaction of the hormone with its receptors could be another way it protects the brain from damage by HD.  Like all hormones, melatonin is a chemical signal, and in order to bring about responses, melatonin must bind to one of its two receptors in the body – MT1 and MT2. Since altered levels of MT1 and MT2 receptors have been observed in other neurodegenerative disorders such as Alzheimer’s and Parkinson’s disease, it is possible that the melatonin receptors can play a role in HD as well.

In 2011, Friedlander’s research group conducted a study on the interaction between melatonin and its receptors in HD models and the therapeutic potential of melatonin treatment.  Their results shed light on the means by which melatonin might protect neurons.  Firstly, they found that in addition to its free radical scavenging abilities, melatonin treatment in vitro also blocked cellular pathways that lead to neuron death, or apoptosis, which some research suggests is increased by the mutant huntingtin protein in HD.  More importantly, they found that this effect is achieved by melatonin binding specifically to MT1 receptor.  However, in brain tissue samples of both mice carrying the mutant huntingtin gene and patients with HD, levels of MT1 receptor seemed to decline along with HD progression – samples that had more severe, late-stage HD pathology also had significantly lower levels of MT1.  This fact, coupled with the earlier findings that melatonin provides cell survival signals via the MT1 receptor, suggests that this interaction is disrupted in HD and is a potential target for HD therapeutics.

The Friedlander research group subsequently tested the effect of melatonin treatment in HD mouse models.  In contrast to the earlier animal models mentioned which were induced by chemicals to exhibit HD-like symptoms, they chose to employ transgenic mice that carry a version of the mutant huntingtin gene.  Daily melatonin treatment showed a beneficial effect on HD mice.  Not only did melatonin-treated mice retain normal movement control longer before onset of disease, but they also survived 21% longer than HD mice with no treatment.  Brain tissues were also less damaged in melatonin-treated mice than in untreated mice.  However, some key characteristics of HD – such as weight loss and levels of mutant huntingtin protein aggregates – remained unaffected by melatonin.  Nevertheless, the results of this in vivo study was a physiological reflection of the in vitro findings that melatonin binding with MT1 may help protect neurons by inhibiting certain pathways that cause cell death.

Conclusion^

Melatonin’s antioxidative properties and its interaction with MT1 receptors both seem to contribute to its protection of neurons in HD and make it a promising therapeutic candidate for the disease.  However, research on melatonin’s effect on HD progression is still in its early stages and further work must be done to validate these results before melatonin can potentially be studied in clinical trials.  On the bright side, the fact that melatonin is already an approved drug for treating certain sleep-related conditions could expedite the approval process required for future clinical studies in human patients.

Further Reading^

  1. Wang X, Zhu S, Pei Z, Drozda M, Stavrovskaya IG, Del Signore SJ, Cormier K, Shimony EM, Wang H, Ferrante RJ, Kristal BS, Friedlander RM (2008)  Inhibitors of cytochrome c release with therapeutic potential for Huntington’s Disease. J Neuro 28: 9473-9485.
  2. Reiter RJ, Manchester LC, Tan D-X (2010)  Neurotoxins:  Free radical mechanisms and melatonin protection. Curr Neuropharmacol 8: 194-210.
  3. Reiter RJ, Cabrera J, Sainz RM, Mayo JC, Manchester LC, Tan D-X (1999) Melatonin as a pharmacological agent against neuronal loss in experimental models of Huntington’s disease, Alzheimer’s disease, and Parkinsonism. Annals of New York Academy of Sciences 890: 471-485.
  4. Southgate G, Daya S (1999) Melatonin reduces quniolinic acid-induced lipid peroxidation in rat brain homogenate. Metabolic Brain Disease 14: 165-171.
  5. Nam E, Lee SM, Koh SE, Joo WS, Maeng S, Im HI, Kim YS (2005) Melatonin protects against neuronal damage induced by 3-nitropropionic acid in rat striatum. Brain Research 1046: 90-96.
  6. Wang X, Siranni A, Pei Z, Cormier K, Smith K, Jiang J, Zhou S, Wang H, Zhao R, Yano H, Kim JE, Li W, Kristal BS, Ferrante RJ, Friedlander RM (2011) The melatonin MT1 receptor axis modulates mutant huntingtin-mediated toxicity. J Neuro 31: 14496-14507.

-J. Choi, 1-27-13


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    Lithium

    Background^

    Lithium is a soft, light metal that is used in various industries, including in the production of ceramics, glass, and batteries. It is found in trace amounts in all living organisms. While it is not necessary for survival, lithium does play some role in the human body since the lithium ion (Li+ ) has neurological effects. In medicine, Li+ is used to treat psychiatric disorders, specifically to stabilize mood and treat mania symptoms of bipolar disorder, a mood disorder characterized by alternating episodes of depression and mania.

    The method by which lithium affects the brain to influence mood remains unclear but several mechanisms have been suggested. Scientists believe that lithium could stabilize mood by regulating levels of glutamate, the main excitatory neurotransmitter in the brain (For more on glutamate, click here.), or by interacting with nitric oxide, a gaseous signaling molecule. It could also work by altering the body’s circadian rhythm (biological clock).

    HD and Lithium^

    In recent years, researchers have investigated lithium as a potential treatment for HD because of its ability to regulate glutamate levels. Several studies have evaluated the effects of lithium on rat models of HD.

    In the Wei et al. (2001) study, rats were injected with a lithium solution or with a control saline solution daily. After 16 days, the researchers infused the rats’ brains with quinolinic acid (QA), a chemical that has neurotoxic effects and is an agonist that activates the glutamate NMDA receptors. QA injections produce rats with lesions that lead to HD-like symptoms because one potential cause of HD pathology is over-activation of NMDA receptors due to high concentrations of glutamate. This over-activation can cause neuron death. Results showed that the brains of rats that received pre-treatment with lithium contained significantly smaller lesions (40-50%) than those treated with the control solution. Since lithium inhibits excessive NMDA receptor function, it could potentially counteract over-activation of NMDA receptors that occurs in the HD brain (For more on NMDA receptors and its role in HD, click here.). Nevertheless, it remains unclear how long the rats must be treated with lithium in order to sustain these positive effects. Future studies need to be conducted to answer this question.

    Another study by Senatorov et al. (2004) used a similar QA-infused rat model of HD but instead injected rats with either lithium or saline control twice, once 24 hours prior to, and 1 hour after, QA infusion. Seven days later, lithium treatment again decreased lesions by 40% as compared to the control. In addition to its role in preventing neuronal death, the researchers believe lithium also has ability to produce new neurons in the hippocampus, a brain area involved in learning and memory.

    Side Effects^

    Lithium has numerous side effects and can be toxic at high doses. The most common side effects are nausea, headaches, and hand tremor. Because lithium is a salt, it can also cause electrolyte imbalance and dehydration.

    Conclusion^

    Research on lithium and HD is still in its early stages, as studies with HD patients have yet to be conducted. However, research on lithium in rat models of HD has yielded promising results so far.

    For further reading:^

    1. Wei et al. “Lithium suppresses excitotoxicity-induced striatal lesions in a rat model of Huntington’s disease.” Neuroscience, Volume 106, Issue 3, 27 September 2001, Pages 603-612.

    2. Senatorov et al. “Short-term lithium treatment promotes neuronal survival and proliferation in rat striatum infused with quinolinic acid, an excitotoxic model of Huntington’s disease.” Molecular Psychiatry (2004) 9, 371–385.

    – A. Zhang, 08-21-12

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    Gene Silencing

    Huntington’s disease (HD) is a genetic disease due to the abnormal CAG expansion of the mutated huntingtin gene.  The mutated gene instructs cells in the body to produce a version of the huntingtin protein that ultimately leads to neuronal damage in the brain and creates the symptoms of HD.  Currently, available HD therapies can only provide relief of symptoms, but what if there were a way to eliminate the root of the problem by correcting the mutated gene or preventing the production of the “bad” huntingtin protein?  We are still years away from being able to directly edit the genome in human patients, but much progress has been made in the field of gene silencing, a scientific technique which can hinder or stop the production of a protein.  This article discusses the mechanism of gene silencing, its potential as an HD therapeutic, and challenges this possible therapy faces on the path to clinical implementation.

    What is Gene Silencing?^

    As the name implies, gene silencing is a technique that aims to reduce or eliminate the production of a protein from its corresponding gene.  Genes are sections of DNA that contain the instructions for making proteins.  Proteins are essential molecules that perform an array of functions including signaling between cells, speeding up biochemical reactions, and providing structural support for the cell.  Each gene is responsible for producing a corresponding protein in a two-step process.  First, a copy of the information encoded in a gene is made in the form of messenger RNA (mRNA), a process known as transcription.  This occurs in the nucleus of the cell, the cellular structure where all of the cell’s genetic material is contained.  The mRNA subsequently travels out of the nucleus, and the genetic information it carries is used to produce a specific protein, a process known as translation.  (For more information about proteins and how they are made, click here.)

    Instead of directly editing DNA or inhibiting the transcription process, the key idea behind gene silencing is intervening in gene expression prior to translation.  By designing a molecule that can specifically identify and breakdown the mRNA carrying instructions for making a certain protein, scientists have been able to effectively decrease levels of that protein.  Imagine the gene silencing molecule as a censor and mRNA as messages from genes that are broadcast into proteins:  the molecule will censor out a specified mRNA message, preventing the corresponding protein from being broadcast into the cell, and thus silencing the gene that is providing these instructions.  The ability to significantly lower the levels of a specific protein opens up many possibilities in scientific research and drug development, since proteins are critically involved in the proper function and structure of cells.

    Types of Gene Silencing Techniques^

    There are various gene silencing methods currently employed in research and being developed as potential disease therapeutics.  Nearly all of them involve disabling the function of mRNA by preventing it from being translated into a protein.  However, they differ in the design of the molecule used to disrupt mRNA and the manner of mRNA breakdown.  As a result, different silencing methods have specific advantages and drawbacks.  Two of the leading and most understood methods of gene silencing are RNA interference (RNAi) and antisense oligonucleotides (ASOs).

    RNA Interference

    In RNAi, the molecules that identify the target mRNA are called small-interfering RNAs (siRNAs). Unlike normal single-stranded RNA found in cells – such as mRNA – siRNAs are short, synthetically made double-stranded RNA molecules designed to pair with a specific mRNA strand.  This association of the siRNAs with a particular target mRNA causes the breakdown of the target mRNA by recruiting other proteins that degrade the mRNA target.  Because siRNAs are double-stranded, they are more stable and less susceptible to degradation than ASOs, allowing them to continue to perform their silencing function for a longer period of time in the cell.  For a more detailed description of how RNAi works, click here.

    Antisense Oligonucleotides

    Similar to siRNAs, ASOs are engineered by scientists to associate with a target mRNA strand.  The binding of the ASO to mRNA directs a protein to breakdown the mRNA.  However, unlike siRNAs, ASOs are smaller, single-stranded RNA molecules.  As mentioned above, single-stranded RNAs are not as stable as double-stranded ones; thus, ASOs are often chemically modified to increase their durability in a biological environment.  However, their smaller size and chemical structure allow ASOs to be transported in cells and living tissues much more effectively than siRNAs.  For a more detailed description of how ASOs work, click here.

    Is one gene silencing method better than the other?

    In terms of developing a drug therapy based on gene silencing, how do RNAi and ASOs compare to each other in effectiveness?  In cell culture experiments, gene silencing is often used to intentionally decrease levels of a certain protein for research purposes.  In such applications, siRNAs have sometimes been shown to produce stronger and longer lasting gene silencing than ASOs.  However, when developing silencing therapeutics, the strength and duration of gene silencing needed for treatment may vary; sometimes a shorter-acting or less complete gene silencing may be required.  Furthermore, when considering the efficacy of each method in live animal models, the results are not as clear-cut.  For example, as mentioned earlier, ASOs can often be distributed more easily than siRNAs throughout the target tissue because of their size and structure.  This observation would be expected to simplify delivery and lower costs of a therapeutic application.  The fact that there is no definitive answer to which gene silencing method is more effective has resulted in continued active research and development of both areas.

    Gene Silencing and HD^

    HD is characterized by a mutation causing excess CAG repeats in the Huntington gene and the consequent production of the mutant huntingtin protein results in disease. As such, silencing of the mutant version of the huntingtin gene is a potential therapeutic strategy for HD treatment.  Indeed, HD and other related neurodegenerative diseases involving mutant CAG repeats, such as spinobulbar muscular atrophy and some types of spinocerebellar ataxias, have been at the frontier of the therapeutic development of gene silencing (for more information about trinucleotide repeat disorders, click here).

    Approaches to Huntingtin Gene Silencing

    Recall that everyone’s DNA is composed of two copies of a gene, called alleles, one from each parent.  In the majority of individuals with HD, one copy of the gene is mutated with excess CAG repeats, while the other copy is an allele with a number of CAG repeats within the normal range.  As a result, the body not only produces the mutant version of the huntingtin protein, but also makes the normal protein.  When considering gene silencing as a therapeutic approach to HD, it is crucial to think about the difference between silencing huntingtin mRNA in general and selectively disrupting mRNA that encodes for the mutant, and not the normal, huntingtin protein.

    The huntingtin protein has many roles in proper development. Studies in mouse models have shown that completely eliminating huntingtin protein results in mice that do not survive past the embryonic stages of development, while mice that were induced to lose huntingtin after birth experienced severe neuronal degeneration (for more information about the function of wild-type huntingtin protein, click here). Thus, it is important for scientists to develop gene silencing drugs that specifically target mutant huntingtin mRNA.  This type of approach is known as allele-specific gene silencing.

    While it may seem straightforward to target the excess CAG repeats to specifically decrease the levels of mutant huntingtin, it is important to remember that the molecules used for silencing are short RNA sequences, about 25 nucleotides in length.  Hence, they cannot effectively distinguish between the size of the normal and the expanded CAG repeats of the huntingtin gene. This is particularly true when trying to differentiate between 30 CAGs and 40 CAGs, CAG repeat ranges that are near normal.  To get around this obstacle, scientists are developing an approach that identifies single nucleotide polymorphisms (SNPs) – changes in a single nucleotide in the DNA sequence –closely linked with the mutant gene and not the normal allele.  SNPs are mutations that differ by a single nucleotide (e.g. ‘A’ à ‘C’) and result in the genetic variation between individuals.  What researchers have found is that many HD patients have common SNPs that are associated with the mutated huntingtin allele. Using silencing molecules to identify these SNPs provides a potential approach to allele-specific silencing of the mutant gene.

    A recent study explored allele-specific silencing of the mutant huntingtin protein by targeting associated SNPs.  Since not all HD patients have the same SNPs, they sought to develop a panel of ASOs to maximize the coverage of the HD population.  They found that 85% of HD patients can be covered by targeting as few as three SNPs.  Moreover, injecting ASOs targeting an HD-associated SNP into HD mice showed a greater than 50% decrease in mutant huntingtin protein, while normal mice receiving the same treatment showed only a 3% drop in huntingtin levels.  These results indicate a relatively strong and selective silencing effect on mutant huntingtin.  Although further work must be done to expand coverage of the HD population by this approach and to assess its therapeutic efficacy outside of animal models, this study represents an initial step forward toward using allele-specific gene silencing as an HD therapeutic.

    Although the above results are encouraging, some researchers have suggested using nonallele-specific silencing of huntingtin protein because  of the lack of a single SNP that will specifically target mutant huntingtin in all HD patients.  In this approach, instead of trying to decrease levels of the mutant huntingtin only, both the normal and mutated versions of the huntingtin protein are targeted and decreased.  This method is also advantageous because instead of employing different silencing molecules for different SNPs in different individuals, a single therapy can be developed for all HD patients, thus minimizing costs.  Although huntingtin has important functions in the body, interestingly, a study of nonallele-specific silencing using siRNA in HD mouse models found that when both mutant huntingtin and normal huntingtin levels were decreased by 75%, the treated mice demonstrated improved motor control and increased survival compared to controls.  This result suggests that nonallele-specific silencing may be a beneficial therapeutic for HD.  An important caveat to consider is that many therapies that show an effect in HD mouse models may not directly translate to humans.  For example, the mouse brain may be better able to tolerate a decrease in normal huntingtin than the human brain.  In any case, since both allele-specific and nonallele-specific silencing methods have their pros and cons, both continue to be under investigation as potential therapeutic options.

    Challenges to Gene Silencing Therapeutics^

    Even though gene silencing is a promising strategy for treating HD, there are still many hurdles to overcome before it can be applied in the clinic.  First and foremost, gene-silencing molecules have to be effectively delivered to the relevant parts of the body, which, in the case of HD, are the afflicted areas of the brain.  The blood-brain barrier prevents passage into the brain of most molecules that are injected or absorbed into the blood, making drug delivery difficult.  Some methods that scientists have used in animal models include direct injection of the silencing drug or implanting pumps that infuse the molecules into the brain.

    Once past the blood-brain barrier, silencing molecules have to locate neurons and other affected cells and enter these cells to silence huntingtin expression.  As mentioned earlier, due to their structure, ASOs distribute and enter cells more effectively than siRNAs.  To effectively deliver siRNAs into cells, scientists currently use viral-based delivery systems, which essentially take advantage of the machinery viruses use to infect our cells.  One of the drawbacks of using a viral delivery mechanism is the potential for an immune response against the molecules.  As an alternative to this method, Dr. Jan Nolta’s group at the University of California Davis has begun studying the possibility of using mesenchymal stem cells (MSCs) as a delivery system for siRNA (for more information about MSCs, click here).  A possible advantage of using a viral or stem cell delivery system is that they might be able to become a production facility for siRNA molecules, allowing long-term therapeutic treatment of HD, a great benefit for a chronic illness. Ongoing research is currently investigating whether this theoretical possibility could become a reality.

    There are other concerns associated with gene silencing therapeutics.  For example, researchers have observed that high dosages of silencing molecules could have a toxic effect, highlighting the importance of finding an optimal dosage that is safe and effective.  In addition, there is the possibility for ASOs and siRNAs to accidentally bind to an undesired mRNA (mRNA coding for a protein that is not huntingtin).  To deal with this so-called off-target gene silencing phenomenon, researchers are studying the selectivity of how siRNAs and ASOs bind to the huntingtin mRNA, in order to better develop specific, effective, and safe HD gene silencing therapeutics.

    Conclusion^

    Gene silencing as a therapeutic strategy is a highly active area of research and may one day yield an effective treatment for HD, since it acts by directly reducing the production of the mutant huntingtin protein.  However, the technology is still in preclinical stages, and there remain many issues to address and resolve before it can be approved for clinical trials.  Delivery methods, dosages, and selectivity of gene silencing drugs must be optimized to ensure safety and efficacy of treatment.  Clinical trials have begun using gene silencing for therapeutic applications in other diseases. These studies will help to inform current efforts to develop gene silencing for HD treatment.

    Further Reading^

    1.  Bennett CF, Swayze EE.  RNA Targeting Therapeutics: Molecular Mechanisms of Antisense Oligonucleotides as a Therapeutic Platform.  Annu. Rev. Pharmacol. Toxicol. 2010. 50:259–93.

    This is a technical article published by Isis Pharmaceuticals that gives an in-depth review of the mechanisms and pharmacology involved in developing RNA-targeting gene silencing therapeutics.

    2.  Boudreau RL, McBride JL, Martins I, Shen S, Xing Y, Carter BJ, Davidson BL.  Nonallele-specific silencing of mutant and wild-type Huntingtin demonstrates therapeutic efficacy in Huntington’s disease mice. Molecular Therapy (2009) 17 6, 1053–1063.

    A technical article that explains the potential benefit of nonallele-specific silencing in HD mouse models.

    3.  Boudreau RL, Rodriguez-Lebrón E, Davidson, BL.  RNAi medicine for the brain: progresses and challenges.  Hum Mol Genet. 2011 Apr 15;20(R1):R21-7.

    A medium-difficulty article that discusses the development of RNAi as a therapeutic, current preclinical data, and the key challenges that remain for its clinical implementation.

    4.  Carroll JB, Warby SC, Southwell AL, Doty CN, Greenlee S, Skotte N, Hung G, Bennett CF, Freier SM, Hayden MR.  Potent and selective antisense oligonucleotides targeting single-nucleotide polymorphisms in the Huntington disease gene/ allele-specific silencing of mutant huntingtin.  Mol Ther. 2011 Dec;19(12):2178-85.

    A technical article that explains the results of a study to develop a panel of ASOs for allele-specific gene silencing of mutant huntingtin in mouse models.

    5.  Dessy A, Gorman JM.  The emerging therapeutic role of RNA interference in disorders of the central nervous system.  Clinical Pharmacology & Therapeutics (2011) 89 3, 450–454.

    A medium-difficulty article that gives a broad overview of the current status of RNAi as a developing therapy for neurodegenerative diseases.

    6.  Scholefield J, Wood MJ.  Therapeutic gene silencing strategies for polyglutamine disorders.  Trends Genet. 2010 Jan;26(1):29-38.

    A technical article that reviews the mechanism of gene silencing and discusses therapeutic studies that have been done and challenges that remain to be addressed for allele-specific silencing of polyglutamine disorders.

    7.  Sah DWY, Aronin N.  Oligonucleotide therapeutic approaches for Huntington disease.  J Clin Invest. 2011;121(2):500–507.

    Another technical article that explains and compares the various approaches to gene silencing therapeutics for HD.

    8.  Sass, Meghan; Aronin, Neil. “RNA- and DNA- Based Therapies for Huntington’s Disease.” Neurobiology of Huntington’s Disease: Applications to Drug Discovery. Ed. Donald C. Lo and Robert E. Hughes. Boca Raton: CRC Press, 2010.

    A technical article that broadly covers the mechanisms, current studies, and challenges of both RNAi and ASO therapeutics for HD.

      J. Choi 04.04.12


      More

      Antidepressants and HD

      Introduction to Antidepressants^

      Antidepressants are medications that are used to treat depression by improving symptoms such as mood, sleep, appetite and concentration. There are many different types of antidepressants, and they are classified based on how they affect the brain. Broadly, antidepressants work by increasing the amount of a certain neurotransmitter (a chemical messenger) in the brain. Most relevant to Huntington’s Disease (HD) is a class of antidepressant called SSRIs, or selective serotonin reuptake inhibitors. SSRIs increase the effect of the neurotransmitter called serotonin. Normally, serotonin transmits chemical messages to a postsynaptic cell when it is released from a presynaptic neuron into a synapse, which is the space between two nerve cells. To stop serotonin’s action, the presynaptic neuron re-absorbs the serotonin it just released. SSRIs block this re-uptake which increases the amount of serotonin present in the synapse and magnifies its effects. For more information about SSRIs, click here.

      Serotonin is mostly present in intestinal membranes and the central nervous system (CNS). For more information on serotonin, click here. It has a wide range of functions in the CNS including the regulation of mood, appetite, sleep, behavior, learning, memory and muscle contraction. In the brain, there are more receptors for serotonin than any other neurotransmitter, which emphasizes the widespread effects of serotonin. Recently, researchers have been attracted to the idea of using SSRIs as a potential treatment for HD. The mutant HD gene has been found to reduce the number and activity of serotonin receptors, and SSRIs may be a way to overcome the reduction in serotonin signaling. SSRIs are also an attractive drug because they are known to have fewer side effects than other classes of antidepressants. The most common side effects of SSRIs include upset GI tract, diarrhea, restlessness, weight loss or insomnia.

      Using SSRIs to treat HD may address both the psychiatric and neurological abnormalities in HD patients. HD patients have commonly exhibited psychiatric symptoms both before and after diagnosis, such as depression, hostility, obsessive–compulsiveness, anxiety, interpersonal sensitivity, phobic anxiety, and psychoticism. As stated above, SSRIs are normally used to treat depression and severe anxiety disorders. However, current research suggests that SSRIs not only can help treat depression, but also may have therapeutic potential as neuroprotective agents.

      SSRIs and HD: From Animals Models to Clinical Trials^

      In 2005, researchers studied the effects of a SSRI for the first time in huntingtin mutant mice. These scientists found that the administration of paroxetine, a widely prescribed antidepressant drug (and SSRI) increased serotonin levels, delayed onset of neuronal degeneration and motor dysfunction, improved energy metabolism, and increased mouse lifetime. The researchers did not investigate how paroxetine could have exerted these effects on the mice. For more information on paroxetine, click here. The researchers also observed that there were few or no side effects of the drug in mice. Most importantly, this study was the first to demonstrate the positive effects of SSRIs on neurological aspects of HD, calling for further investigation of these effects in both mice models and HD patients.

      Another study conducted in 2005 investigated the relationship between SSRIs and neurogenesis, the birth of new neurons, in HD mice. In this study, researchers used another SSRI, fluoxetine (also known by the tradename Prozac), to determine whether it would promote neurogenesis and mitigate HD symptoms in a mouse model of the disease. Through extensive behavioral testing of the mice, the researchers demonstrated that flouxetine did not affect motor activity or body weight, but did improve cognitive function and “reversed” a depressive phenotype of HD mice. Furthermore, the flouxetine-treated mice displayed a considerable increase in neurogenesis and volume of the dentate gyrus. The dentate gyrus is a part of the hippocampus, which is a region of the brain thought to contribute to memory formation. The growth in the volume of the denate gyrus of the hippocampus in mice treated with flouxetine was so significant that it was comparable to the size of the denate gyrus in mice without HD.

      Many recent studies have emphasized the role of the hippocampus in depression. The finding that SSRIs are able to target both neurological symptoms and those of depression implies a link between the two. In the fluoxetine study, the researchers propose a possible mechanism relating neurogenesis with the action of antidepressants in the body. They suggest that antidepressant stimulation of neurogenesis may act through the increased expression of neurotrophic factors such as BDNF, or brain-derived neurotrophic factor. BDNF is required for neurons to survive and regenerate. The loss of BDNF in the brains of HD patients and mouse models has been shown to play a crucial role in the development of the disease. For more information about BDNF click here.

      Interestingly, serotonin stimulates the expression of BDNF, and BDNF enhances the growth and survival of neurons that release serotonin. Because Huntington’s patients have decreased levels of both BDNF and serotonin, this interaction could play an important role in the pathogenesis of HD. Peng and Masuda, researchers at Johns Hopkins, decided to further investigate the impact of SSRIs on BDNF levels and neurogenesis in mice. These scientists used yet another SSRI, sertraline, which has also been widely used in the treatment of depression. Their study concluded that sertraline prolongs survival, improves motor performance, and decreases brain atrophy in HD mice. Furthermore, it showed that sertraline significantly increased BDNF protein levels in HD mice, and that the effective levels of sertraline in mice are comparable in humans—providing a case for the testing of sertraline in HD patients.

      Due to the evidence in HD mouse models supporting the use of SSRIs to treat HD, the University of Iowa facilitated a randomized, double-blind placebo controlled clinical trial to test the efficacy of citalopram, also an SSRI, in HD patients. This clinical trial is currently in phase II, and researchers are now recruiting more participants. For more information about clinical trials, click here.

      Conclusion^

      In conclusion, many different SSRIs have consistently been shown to increase neurogenesis, motor control, cognitive ability, and brain metabolism in mouse models of HD. It is likely that SSRIs such as sertraline influence neurogenesis via increasing BDNF, a neurotrophic factor essential to neuron growth and survival that has been found in abnormally low levels in HD patients. Thus far, the data investigating the relationship between SSRIs and HD are very promising. It is also encouraging that SSRIs have been tolerated for long-term treatment in humans without significant side effects for depression—suggesting (but not proving) the safety of SSRI treatment for HD patients. Clinical trials such as the one being conducted on citalopram are necessary in order to confirm the safety and efficacy of SSRIs for HD patients. If the findings in mouse models translate to human medicine, this promising avenue of research may allow for SSRIs to be co-opted for Huntington’s Disease.

      Further Reading^

      1) Visit HD drug works for specific information about different categories of antidepressants

      2) This article gives an in depth discussion (not too complex) about various psychiatric symptoms of HD and drugs commonly used to treat them.

      Works Cited^

      Duan W, Peng Q, Zhao M, Ladenheim B, Masuda N, Cadet JL, Ross CA. Sertraline Retards Progression and Improves Survival in a Mouse Model of Huntington’s Disease. Society for Neuroscience.s 2005.

      Duan W, Guo Z, Jiang H, Ladenheim B, Xu X, Cadet JL, Mattson MP. Paroxetine retards disease onset and progression in Huntingtin mutant mice. Ann Neurol 2004 Apr;55(4):590-4.

      Duff K., Paulsen J.S., Beglinger L.J., et al. Psychiatric Symptoms in Huntington’s Disease before Diagnosis: The Predict-HD Study. Biological Psychiatry, 62(12): 1341-1346

      Lazic SE, Grote HE, Blakemore C, Hannan AJ, van Dellen A, Phillips W, Barker RA. Neurogenesis in the R6/1 transgenic mouse model of Huntington’s disease: effects of environmental enrichment. Eur J Neurosci 2006 Apr;23(7):1829-38. PubMed abstract

      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.


      P. Bakhai 9.13.11 More

      Retinoic Acid (RA)

      Recent research has spotlighted retinoic acid (RA) as an intriguing possibility for further exploration as a treatment for Huntington’s disease.  Retinoic acid is derived from a compound that we know as Vitamin A, which is fat-soluble and primarily found in two forms: retinol and carotenoids.  Retinoic acid is synthesized in the body from retinol, which is derived from a precursor found in animal foods such as milk and eggs.  Once in the body, the precursor is converted to retinol, which then undergoes a series of reactions to form RA.

      Retinoic acid is a biological molecule that regulates gene expression throughout the body and is crucial for cell differentiation and proliferation.  These functions of RA have made it a useful treatment for skin diseases and cancer.  In addition to the body, RA may regulate the expression of many proteins in the brain.  Most proteins in the RA signaling pathway have been identified in the brain, including many that are adversely affected in HD.  Nevertheless, little is currently known about where RA acts in the brain, what specific proteins it affects, or how its signaling relates to brain function, so much work still needs to be done to identify RA’s functions.

      medium

      RA Signaling and Function^

      Some evidence suggests that RA may play a role in gene transcription and cell differentiation. RA binds to specialized receptors in the cell nucleus, where transcription takes place, and can activate a large number of molecules within the cell.  These molecules include enzymes, transcription factors, and inflammatory agents (i.e. cytokines and cytokine receptors), all of which are important in regulating crucial biological functions.  RA may, for instance, send a signal that causes stem cells to become neurons.  In fact the compound has been used in research for years, as a differentiation agent that can produce many types of early neural cells.  When RA is absent, its receptors can also act as gene repressors, turning off gene transcription and thus inhibiting certain functions.

      RA was first recognized as a key mediator of early development.  It forms a concentration gradient across developing embryonic tissues and thereby governs the pattern of gene expression. This pattern regulation promotes development of different parts of the body depending on what genes are switched on. This property of RA was demonstrated by an experiment conducted on tadpole embryos.  In this experiment, researchers noticed that concentrations of RA were much higher in the posterior (back or tail) end of the tadpole embryo than in the anterior (front or head) end.  They exposed the embryo to elevated RA, and as a result, certain structures in the anterior end (such as the brain) failed to develop properly.  This finding implied that different parts of the embryo required different amounts of RA, and receiving more or less than the required amount interfered with vital developmental processes (Altaba & Jessell, 1991).

      It was later discovered that RA acts in a similar fashion in the adult brain, in which gene transcription is also controlled through varying RA concentrations.  Several important functions found to be regulated by the RA pathway include spatial learning, long-term potentiation (LTP), synaptic plasticity, and nerve regeneration.

      Implications for HD^

      RA may play a role in motor disorders such as Parkinson’s disease (PD) and HD since it is involved in the function of the striatum, a brain structure involved in planning and execution of movement and that undergoes cell death in HD.  RA promotes neuron formation and differentiation in the striatum, which receives RA from the substantia nigra, a midbrain structure that is also important for motor planning.  Neurons in the substantia nigra express high levels of an enzyme that catalyzes RA synthesis.  This enzyme travels along the neurons extending from the substantia nigra to the striatum, where it can help generate RA.  In studies of HD transgenic mice, it was found that more than 20% of the genes in the striatum that showed diminished expression contained elements of the RA signaling pathway.  This result could suggest that some type of defect in the RA pathway contributes to HD.  Further research is needed to determine whether a defective RA pathway could relate to HD motor symptoms.

      Several discoveries suggest that RA may be able to treat key pathological symptoms of HD, making it a candidate therapeutic for the disease.  For instance, huntingtin aggregates, which form in the brains of HD patients, interfere with RA signaling.  These aggregates disrupt the activity of a protein called PGC1-alpha, which in turn disturbs the protein PPAR-delta, which mediates cellular responses to RA.  PPAR-delta function is significantly diminished in HD mouse models, implicating PPAR-delta in the pathology of the disease.  (You can read more about this study here). Drugs that target the RA signaling pathway are currently on the market to treat tumors and cancers such as leukemia.  Further investigation of RA and the RA signaling pathway could open the door to a potential treatment for HD, maybe by enhancing the retinoic acid pathway through PPAR-delta.  Proof of such a hypothesis remains a topic of developing preliminary research.

      For Further Reading^

      Altaba, A. Ruiz i, and T. Jessell. “Retinoic acid modifies mesodermal patterning in early Xenopus embryos.” Genes & Development 5 (1991): 175-187.  This study has a lot of technical language, but most of the key points have been summarized above in the RA Signaling and Function section.

      Duester, Gregg. “Retinoic Acid Synthesis and Signaling during Early Organogenesis.” Cell 134.6 (2008): 921-931. Web. 13 Apr 2011. <http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2632951/pdf/nihms76219.pdf>.  This is a very in-depth article about the synthesis and function of RA.  Some technical language.

      Luthi-Carter, R, A Strand, NL Peters, SM Solano, and others. “Decreased expression of striatal signaling genes in a mouse model of Huntington.” Hum Mol Genet 9.9 (2000): 1259-71. Web. 13 Apr 2011. <http://hmg.oxfordjournals.org/content/9/9/1259.full>.  The main point to take away from this study is that “mutant huntingtin directly or indirectly reduces the expression of a distinct set of genes involved in signaling pathways known to be critical to striatal neuron function.”

      Mey, Jörg, & McCaffery, Peter. Retinoic acid signaling in the nervous system of adult vertebrates. The Neuroscientist 10.5 (2004). Web. 13 Apr 2011. <http://www.accutaneaction.com/Studies/2004_Mey.pdf>.  This contains some technical language, but it has a lot of interesting information about RA research and treatment possibilities.

      J. Nguyen 2011

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      Pridopidine (Huntexil, ACR-16)

      Drug Summary: Pridopidine, also known as Huntexil or ACR-16, is a dopamine stabilizer intended to improve voluntary movements and reduce chorea. Initial clinical trials – the MermaiHD and HART studies – show promising results, but drug regulation agencies have requested another trial before pridopidine can be sold to the general public.

      control relevant

      Dopamine in the HD Brain^

      The brain plays a delicate balancing act: it needs to maintain the right levels of many brain chemicals in order to orchestrate movements and execute thoughts. In people with Huntington’s disease (HD), that balance is threatened; the brain has trouble regulating neurotransmitters, chemicals in the brain that transfer messages between neurons. This causes miscommunication between different parts of the brain. As a result, people with HD have less control over behaviors and movements that are usually directed by the affected neurotransmitters, as described in greater detail here. Pridopidine, which is being investigated as a treatment for the motor symptoms of HD, is thought to restore the balance of neurotransmitters that the brain needs to function.

      Specifically, pridopidine is believed to work by stabilizing levels of the neurotransmitter dopamine in the brain. Dopamine has a number of different roles depending on what part of the brain it acts on, but in the HD brain, the most relevant function is its effects on motion. Dopamine in the striatum, a part of the brain responsible for planning and controlling movements, helps coordinate voluntary motions (like walking or waving) and prevent involuntary motions (like the unwanted dance-like movements of chorea), as discussed in more detail here.

      However, sometimes there’s too much of a good thing. When there’s too much dopamine in certain parts of the striatum, the brain has trouble stopping involuntary movements, which causes chorea. On the other hand, when there’s too little, the brain can’t start voluntary movements, and the symptoms – such as stiffness, staggering, and difficulties speaking – get in the way of everyday life. The brain walks a tightrope as it tries to maintain the right balance of neurotransmitters, and the slightest disturbance can cause movements to falter (Andre et al., 2011).

      How Pridopidine Works^

      As a dopamine stabilizer, pridopidine is thought to reduce the effects of dopamine when there’s too much, and increase its effects when there’s too little. When dopamine levels are too high, pridopidine interacts with dopamine receptors, which act as the “ears” the neuron uses to “hear” dopamine. These receptors have a very specific shape that allows them to bind and recognize dopamine, and when levels of dopamine are high, the receptors change shape as they become more active. Pridopidine is particularly attracted to the “active” form of the receptor and lodges itself in the spot where dopamine usually binds, preventing dopamine from interacting with the receptor. In this way, pridopidine blocks the dopamine receptor from sensing and responding to dopamine when dopamine levels are too high (Pontel et al., 2010).

      Conversely, when levels of dopamine are low, pridopidine has a round-about way of increasing dopamine production. HD affects more than just dopamine: low levels of the neurotransmitter glutamate in a region of the brain called the cortex  are also associated with the disease (Pontel et al., 2010). The cortex is the part of our brain that helps us think and plan, and tells the striatum what voluntary movements to perform. Pridopidine raises glutamate levels in the cortex, allowing it to communicate better with the striatum. This increases dopamine levels in the parts of the striatum that had too little. By increasing glutamate signaling in the cortex, pridopidine increases dopamine levels in certain parts of the striatum, allowing voluntary movements to occur (Andre et al., 2011).

      Pridopidine therefore plays two opposing roles in the brain, which stabilize dopamine levels. In this way, pridopidine is thought to help the brain reestablish a normal balance of neurotransmitters, and thus regain control over motion.

      Research on HD^

      Neurosearch, a pharmaceutical company based in Sweden, has conducted two different clinical trials on pridopidine.

      MermaiHD (2009)^

      The MermaiHD study was a phase III clinical trial, conducted in 32 centers spread across eight countries in Europe. 437 HD patients were randomly assigned to one of three groups: one treatment group received 45 mg of pridopidine once per day; the second treatment group received 45 mg of pridopidine twice per day; the control group received a placebo. To prevent potential bias, MermaiHD was a double-blind study; neither doctors nor patients knew whether the patient was receiving pridopidine.

      After 6 months, patients were given the opportunity to continue participating in the study for another 6 months. In this “open-label” phase, the 357 patients who opted to proceed took 45 mg of pridopidine twice daily – no patients were given placebo. The purpose of the open-label segment of the study was to test whether pridopidine is safe and effective for longer periods of time.

      Preliminary results suggest that pridopidine might help HD patients control motor symptoms. Doctors measured patients’ progress using the modified Motor Score (mMS), which tests a patient’s ability to perform voluntary movements. Results suggest that patients taking pridopidine performed better on the mMS; patients taking 45 mg of pridopidine twice daily averaged a 1.0 point improvement on the test. However, the results of the mMS did not reach the goals that the scientists had set out to prove: these results reached a statistical significance level of p=0.042. This means that there is a 4.2% probability that pridopidine is no better than a placebo, and that these results occurred by chance; they had originally aimed for a p=0.025.

      However, further data analysis indicates that pridopidine may still hold promise. The mMS is just a subsection of a more widely-used test called the Unified Huntington’s Disease Rating Scale (UHDRS), which is described in more detail here. When measured on the motor category of the UHDRS, a test called the UHDRS-TMS, the results were very significant: Patients taking 45 mg of pridopidine twice per day had a 3.0 point improvement, at a statistical significance level of p=0.004. To put that in perspective, HD patients generally experience a 3-point annual decline in their UHDRS-TMS score. This strongly indicates that pridopidine improves motor symptoms of HD.

      Furthermore, pridopidine did not appear to have notable side effects, and didn’t make other symptoms of the disease worse. This was a concern because other treatments, such as tetrabenazine, sometimes cause depression and other side effects if they change neurotransmitters too much in the wrong parts of the brain, as described here.

      HART (2010)^

      In the HART study, Neurosearch and the Huntington Study Group teamed up to study pridopidine further. The HART study was a phase IIb clinical trial, which measures how well a drug works at the prescribed dose. The study was also conducted to see whether pridopidine is effective and safe, and to establish an optimal dose. HART enrolled 227 patients, and was run in 28 centers across America and Canada. Like the MermaiHD study, the HART study was randomized, double-blind, and placebo-controlled.

      To determine the dose, there was one placebo group and three treatment groups; patients received 10 mg, 22.5 mg, or 45 mg of pridopidine twice per day.

      After just 12 weeks, a significant effect was seen in the group taking the largest dose, 45 mg. Total motor function, as measured by the UHDRS-TMS, improved by 2.8 points, which was statistically significant with a p=0.039. Again, the original test – the mMS – did not show statistical significance, though it did show a strong trend with p=0.078.

      The HART study backed up the findings of the MermaiHD study and also helped scientists determine which dose of pridopidine is most effective. This study will continue in an open-label phase, where patients who participated in HART are given the opportunity to continue taking pridopidine until the U.S. Food and Drug Administration (FDA) decides whether or not to approve pridopidine.

      Conclusions^

      Pridopidine significantly improves motor function, and has a positive effect on both voluntary and involuntary motor actions. Furthermore, it is very well tolerated, even when patients are taking other drugs, such as antipsychotics. However, pridopidine isn’t a “miracle drug” – while the findings are very hopeful, the drug has only been shown to improve motor symptoms; there is no evidence that it can “cure” the disease. Also, pridopidine’s effects seem to be limited to motor symptoms; patients experienced no significant changes in cognition, mood, or general ability to function in day-to-day life.

      Individually, neither MermaiHD nor HART lived up to the original standards the researchers had set out to meet. However, statistical significance was reached when the results of the two studies were combined, and when the UHDRS-TMS was used to evaluate patients. Based on these results, Neurosearch lobbied the FDA, which regulates American drugs, and the European Medicines Agency (EMA), which regulates European drugs, to accept pridopidine as a treatment for HD. However, both organizations have asked for another phase III clinical trial to validate that pridopidine lives up to these promises. Neurosearch has declared that it will carry out a further trial, but has not yet announced further details. If it successfully passes this trial, the FDA and EMA would be likely to allow pridopidine to start being marketed as a treatment for HD.

      Bibliography^

      1. André VM, Cepeda C, Levine MS. Dopamine and glutamate in Huntington’s disease: A balancing act. CNS Neurosci Ther. 2010 Jun;16(3):163-78. Epub 2010 Apr 8. Review. This article discusses dopamine and glutamate signaling in the brain, and is very technical.
      2. Miller, Marsha L. “The American ACR16 Trial Results.” HDAC.org. Huntington’s Disease Advocacy Center, 14 Oct. 2010. Web. 5 July 2011. This article discusses the MermaiHD and HART studies, and is moderately difficult.
      3. Ponten H, Kullingsjö J, Lagerkvist S, Martin P, Pettersson F, Sonesson C, Waters S, Waters N. In vivo pharmacology of the dopaminergic stabilizer pridopidine. Eur J Pharmacol. 2010 Oct 10;644(1-3):88-95. Epub 2010 Jul 24. This highly technical article discusses how Pridopidine is believed to work in the brain.

      M. Hedlin, 7.16.11

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      About Disease Mechanisms

      Welcome to the “Drugs and Supplements” section of the HOPES website! Articles within this section will frequently use the terms “treatment” and “cure.” Please note, though, that the word “treatment” must not be confused with the word “cure,” for there is currently no medical cure for Huntington’s Disease (HD). However, while no existing drugs can actually stop or reverse the neurological degeneration that HD causes, most if not all HD patients can benefit from the management of certain symptoms associated with HD, such as chorea, psychosis, and depression. There are well-tested drugs that have been developed to treat these symptoms, and such therapies may help ease the frustration, embarrassment, and emotional pain that often accompanies HD. In general, these drugs can greatly enhance the quality of life for HD patients and their families. Information on such symptoms and their treatments can be found by clicking here.

      The “Drugs and Supplements” section of the website focuses on identifying and explaining the current research on various potential treatments aimed at stopping the progression of HD. We must make it clear that HOPES does not advocate the use of any of the therapies described without the consent or approval of a doctor, and cannot provide medical advice of any kind. However, we do hope that patients with HD, their friends, and their families will ask doctors and HD specialists about both the possibilities presented here and any new developments in this active area of investigation. In the past decade, intensive research efforts have given scientists a much better understanding of how HD damages the brain. With this improved insight, researchers throughout the world are actively investigating and testing the efficacy of drugs that target one or more of these mechanisms of pathology. Their hope is that interfering with HD’s damaging pathways can help delay, stop, or reverse the course of the disease.

      This section is organized by disease mechanism. A potential contributor to HD’s damaging effects is introduced, followed by the drugs and supplements currently being studied to counter a given mechanism. For your convenience, a simple alphabetical list of these drugs can also be found by clicking here. Each drug profile also has a short drug summary that avoids potentially difficult and confusing details.

      The therapies introduced in this section are at various stages of investigation. Some have been at the heart of well-controlled experiments on mouse or fruit fly models of Huntington’s disease, and others have shown some efficacy in treating disorders with disease mechanisms similar to HD’s such as Alzheimer’s Disease or Spinobulbar Muscular Atrophy (SBMA). There are also many that are being studied simply for their theoretical potential in alleviating HD’s degenerative damage. Because many of the treatments profiled here have not yet passed or even begun clinical trials, and many affect broad biological systems, HOPES must again stress that unsupervised testing could be potentially hazardous. None of these therapies should be tried without exclusive consultation with an HD specialist.

      Most scientists believe that a combination of treatments, rather than a single treatment, will be needed in order to cure HD. The different therapies could work together to combat different aspects of HD’s damage mechanisms, and hopefully halt or reverse its progression. Similar therapeutic approaches are currently being used with varying levels of success to control cancer and AIDS.

      While this section focuses on the potential drug and supplement candidates for treating HD, various potential treatment options beyond drugs also exist. Recent advances in gene therapy, stem cell research, and neurosurgery are some of the potential long-term solutions being studied. Some of these therapies are becoming available for clinical trials, while others still require further development. (For more information on Huntington’s Disease research in America and in your region, click here.)

      More

      About Huntington's Disease and Serotonin

      Serotonin (also known as 5-HT) is a neurotransmitter used to communicate important information between nerve cells. Serotonin is sometimes referred to as the “feel good” neurotransmitter owing to its association with elevated mood levels. It also has many other functions in the central nervous system including roles in sleep, depression, memory, pain, and aggression. Recent studies on mice indicate that serotonin signaling is significantly reduced in mice models of HD compared to mice without HD. Having less serotonin and the products made from serotonin may greatly impact on the progression of HD. Because the connection between HD and serotonin signaling is a fairly new development, much more investigation needs to be done before there are clear answers. Decreased serotonin may be a contributor to the development of HD or it may simply be a result of another disease mechanism. Regardless of the cause for reduced serotonin, diminished signaling in the mouse model of HD may explain some of the common behavioral symptoms associated with HD in people. (For more information about the behavioral symptoms associated with HD, click here.)

      Decreased serotonin is associated with several diseases, most notably depression. Consequently, a number of drugs are already available to help bring serotonin back to normal levels in the body. The main class of drugs for this purpose is called selective serotonin reuptake inhibitors (SSRIs). Recent research has shown that, in addition to alleviating symptoms associated with HD such as depression, SSRIs may also help delay the onset of HD and prevent the degeneration of nerve cells.

      Researchers have found elevated serotonin in the brains of people with HD after death, but serotonin levels in a mouse model of HD actually have decreased levels in all different age groups. However, this discrepancy does not necessarily mean that the mouse data is wrong. First of all, it is difficult to interpret human HD samples taken after death due to the large amount of nerve cell loss that occurred in life. Most of what is known about serotonin and HD in living brains comes from mouse studies because it is easier and more ethical to experiment on mice that are made to look like they have HD than it is to study humans with HD. It is important to keep this fact in mind when discussing the findings from these studies because the results from experiments on mouse models do not always translate perfectly to people with HD. Mouse studies are an imperfect but important tool in learning about HD.

      One group of researchers found that, compared to non-HD mice, HD mice had only 50% of the amount of serotonin by age 12 weeks in the striatum, hippocampus, and brainstem. They also found that a related molecule derived from serotonin called 5-hydroxyindoleacetic acid (5-HIAA) is also decreased in the brain. Decreased serotonin and 5-HIAA both before and after symptoms began to appear probably indicates that the serotonin system starts malfunctioning way before the serotonin levels are decreased.

      Another group of researchers tested the hypothesis that the serotonin system starts malfunctioning before it is observable with decreased serotonin. Since serotonin levels could not be used as a marker in this experiment, they tested the rate-limiting enzyme in the synthesis of serotonin, tryptophan hydroxylase (TPH). A rate-limiting enzyme is the slowest step in the creation of a molecule, and often the most important, because it requires additional energy and is highly regulated. The rate-limiting enzyme can have the biggest effect on the final product, so if something is wrong with it, the effect of this malfunction will translate down the chain to the end product. You can think of synthesis as a row of dominoes, with the goal to knock down the final domino. If one of the dominoes is missing or too small to reach the next one, the rest of the dominoes in the chain will not fall down and you will not achieve your goal. As a rate-limiting enzyme, TPH is essential to the overall production of serotonin. An alteration of TPH can therefore lead to decreased levels of serotonin in the brain overall.

      Researchers have tested both the levels of TPH and its enzymatic activity. Despite normal levels of TPH, they found the activity of this enzyme was significantly diminished. This finding means that while the enzyme was present, it was not functioning properly. TPH activity was 62% less than normal before symptoms were present at 4 weeks and 86% less than normal in symptomatic 12 week old mice. These results indicate that TPH is severely damaged and account for the decreased levels of its product, serotonin.

      Often, in order to compensate for decreased levels of a neurotransmitter like serotonin, the brain increases the number of receptors for that specific neurotransmitter. While this sort of “upregulation” of serotonin receptors occurs in Parkinson’s disease, it was not found to occur in the brains of HD mice. In fact, receptor binding was significantly decreased in several important areas of the brain.

      We must now ask, “Why is TPH activity decreased?” The obvious answer might be that mutant huntingtin protein prevents TPH from doing its job; however, this appears unlikely. The researchers tested this hypothesis and found that the expanded polyglutamine section of the mutant huntingtin protein does not interact with TPH. Another possibility comes from the fact that TPH is very sensitive to free radical damage by reactive oxygen species. (For more information on free radical damage, click here.) It is already known that free radical damage plays a role in the progression of HD, so it is very possible that it contributes to decreased serotonin by interfering with TPH. More evidence about the role of free radicals comes from the decreased activity of TPH. Since TPH uses tryptophan to create certain products, when TPH doesn’t work, this pathway is disrupted. This disruption results in increased levels of 3-hydroxykynurenin (3HK), which makes free radicals. These free radicals can then go on to further damage TPH and many other molecules in the brain.

      Now that researchers have an idea of the problem, they can begin to investigate ways to fix it. First, it must be determined whether TPH activity is also decreased in human brains, since so far it has only been tested in mice. If it is also decreased in humans, that could explain why depression is apparent before any of the motor symptoms of HD. If the current hypotheses from the mouse studies are correct, symptoms may be prevented or at least delayed by treating people at risk for HD with early antioxidants and SSRIs to keep TPH activity and serotonin levels normal. It has already been shown that one type of SSRI helped to increase TPH activity in rats.

      For further reading

      1. Reynolds, et al. Brain neurotransmitter deficits in mice transgenic for the Huntington’s disease mutation. 1999. Journal of Neurochemistry 72: 1773-1776. This is a technical scientific article that discusses changes in several different neurotransmitters in the HD mouse model, including serotonin.
      2. Yohrling, et al. Inhibition of tryptophan hydroxylase activity and decreased 5-HT1Areceptor binding in a mouse model of Huntington’s disease. 2002. Journal of Neurochemistry 82: 1416-1423. This is a very technical scientific article that tested the activity of TPH, an important enzyme in the synthesis of serotonin, in a mouse model of HD.
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      About Free Radical Damage

      Aside from impaired energy production, damage to the mitochondria leads also to increased production of toxic molecules called free radicals. Compounds called antioxidants act as free radical scavengers by initiating reactions that make free radicals non-toxic to cells. Evidence indicates that damage by free radicals is a contributing factor to the pathology of HD. Consequently, compounds with antioxidant properties are being studied to see if they can serve as possible treatments for HD.

      Free Radicals and Antioxidants

      Free radicals are atoms or molecules that are highly reactive with other cellular structures because they contain unpaired electrons. Free radicals are natural by-products of ongoing biochemical reactions in the body, including ordinary metabolic processes and immune system responses. Free radical-generating substances can be found in the food we eat, the drugs and medicines we take, the air we breathe, and the water we drink. These substances include fried foods, alcohol, tobacco smoke, pesticides, air pollutants, and many more. Free radicals can cause damage to parts of cells such as proteins, DNA, and cell membranes by stealing their electrons through a process called oxidation. (This is why free radical damage is also called “oxidative damage.”) When free radicals oxidize important components of the cell, those components lose their ability to function normally, and the accumulation of such damage may cause the cell to die. Numerous studies indicate that increased production of free radicals causes or accelerates nerve cell injury and leads to disease.

      Antioxidants , also known as “free radical scavengers,” are compounds that either reduce the formation of free radicals or react with and neutralize them. Antioxidants often work by donating an electron to the free radical before it can oxidize other cell components. Once the electrons of the free radical are paired, the free radical is stabilized and becomes non-toxic to cells. Therapy aimed at increasing the availability of antioxidants in cells may be effective in preventing or slowing the course of neurological diseases like HD.

      -E. Tan, 9-21-01

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