All posts in Autophagy

About Autophagy

Autophagy is a process by which a cell breaks down and recycles its own components. In normally functioning animal cells, autophagy occurs at a very low level. Autophagy pathways are activated when a cell is running low on nutrients. The cell breaks down already existing proteins and other cell components into their basic building block components so that they can be reused to maintain essential cellular functions. There is also evidence to suggest that autophagy can be used by the cell to break down misfolded proteins.

The induction of autophagy in Huntington disease (HD) cells results in the accelerated breakdown of huntingtin aggregates and has been shown to have neuroprotective effects. It is currently unknown whether huntingtin aggregates are the cause or result of HD, but nerve cells that build up huntingtin aggregates often die. To read more about huntingtin protein aggregation and its role in HD, click here.

The Process of Autophagy ^

The part (or parts) of the cell that is to be degraded is first engulfed by a double membrane to separate it from the rest of the cell; the resulting membrane-enclosed bubble of cytosol (along with all the proteins the bubble contains) becomes what is called the autophagosome. The autophagosome eventually fuses with a cellular organelle called a lysosome, a much larger membrane-enclosed bubble that contains a variety of enzymes that can break down many types of cellular components (which is why lysosomes are sometimes referred to as the “garbage disposals” of the cell). These enzymes only work in a very acidic environment, so the pH inside lysosomes is much lower than the neutral pH in the rest of the cell. This pH barrier, as well as the physical barrier of the organelle membrane, protects the rest of the cell from being degraded should the enzymes somehow leak out. Once the contents of the autophagosome are delivered to the lysosome, the lysosomal enzymes break down the new contents, which can then be recycled and reused within the cell.

Until a couple of years ago, it was believed that the main mechanism by which the nerve cell got rid of huntingtin aggregates involved what is called the ubiquitin-proteasome system, which is responsible for tagging and degrading improperly formed proteins. However, recent research shows that proteins with abnormally expanded stretches of the amino acid glutamine, like the altered huntingtin protein (which is associated with HD), are also disposed of by the process of autophagy. In this process, the aggregated proteins are gathered up and transported to the lysosome, where they are broken down and their component amino acids are recycled. Studies of nerve cells have shown that the mutant huntingtin protein can often be found in autophagosomes, the membrane-bound sacs that carry cell parts to the lysosome for degradation.

Researchers have investigated whether proteins with expanded sections of the amino acids glutamine and alanine could be degraded by cells using the process of autophagy. They compared autophagy with the ubiquitin-proteasome process, which was originally thought to be the only process by which these harmful proteins are degraded. The researchers used cells that expressed these proteins and tagged them with green fluorescent protein (GFP) in order to visualize their fate within the cells. GFP allows researchers to see the amount and the location of a specific protein present in the cell because it fluoresces, or glows, when viewed under a special microscope. To study how huntingtin aggregates are broken down by the cell, they used cells that produced, or expressed, part of the HD allele that contained either 55 or 74 CAG repeats. (To read more about the huntingtin protein, click here.)

To determine whether autophagy is indeed a key process in the clearance of huntingtin aggregates, the researchers first used two different compounds to inhibit autophagy at different points of the process and observed the effect on aggregate formation. The first compound they used inhibits autophagy by preventing a membrane from surrounding the cell contents that are about to be degraded; if the autophagosome cannot form, the contents cannot be delivered to the lysosome to be broken down. The second compound they used prevents the autophagosome from fusing with the lysosome and releasing its contents, which also prevents autophagy from occurring. Treatment with these compounds resulted in visibly higher levels of huntingtin aggregates in cell cultures, which showed that autophagy does play a role in the breakdown of aggregates. Along with the increase in aggregates, the researchers also saw increased cell death when the cells were treated with autophagy-inhibiting compounds.

The researchers also tested the role of the ubiquitin-proteasome system in reducing protein aggregation in the same cell cultures. Most previous experiments have used a certain compound to inhibit the proteasome that is thought to inhibit the function of the lysosome as well. Because they wanted to test the role of the proteasome only, the researchers used a different compound that inhibits the proteasome and has no effect on lysosomes. They found that inhibiting the proteasome increased aggregate formation in one cell line but not in another. While these results are somewhat inconclusive, they may suggest that the ubiquitin-proteasome process is not the main mechanism by which cells get rid of the disease-state huntingtin protein. More research about the role of autophagy in degrading mutant huntingtin needs to be done.

Several drugs are known to modulate the process of autophagy in different ways. The hope is that drugs which promote autophagy will aid nerve cells in breaking down huntingtin aggregates and help to protect the cells. Research is being done to identify the effectiveness of different types of drugs.

For further reading^

  • Raught, et al. “The target of rapamycin (TOR) proteins.” Proceedings of the National Academy of Sciences of the United States of America. 2001 Jun 19;98(13):7037-44.
    Short paper which describes various functions of target of rapamycin (TOR) proteins in fairly technical writing.
  • Ravikumar, et al. “Aggregate-prone proteins with polyglutamine and polyalanine expansions are degraded by autophagy.” Human Molecular Genetics. 2002 May 1;11(9):1107-17.
    2001 Jun 19;98(13):7037-44.
    Fairly technical article which describes experiments aimed to discover whether or not proteins with multiple amino acid repeats could be controlled through the process of autophagy.
  • Ravikumar, et al. “Inhibition of mTOR induces autophagy and reduces toxicity of polyglutamine expansions in fly and mouse models of Huntington disease.” Nature Genetics 2004 Jun 36(6):585-95.
    This technical paper describing the effects of mTOR inhibition was cited in the “mTOR and HD” section.
  • Sarkar S., et al. “Rapamycin and mTOR-independent autophagy inducers ameliorate toxicity of polyglutamine-expanded huntingtin and related proteinopathies.” Cell Death and Differentiation advance online publication, 18 July 2008; doi:10.1038/cdd.2008.110.
    Very technical paper which describes the effects of autophagy inducers in controlling HD and other diseases caused by malformed proteins.
  • Thoreen, et al. “Huntingtin aggregates ask to be eaten.” Nature Genetics. 2002 Jun;36(6):553-4.
    Less technical article that describes the role of autophagy in controlling mutant huntingtin aggregates.
  • Williams et al. “Novel targets for Huntington’s Disease in an mTOR-independent autophagy pathway.” 2008 May;5(4):295-305
    Less technical article which reviewed the role of calpains in HD and different autophagy-inducing therapies was cited in the “Calpains and HD” and the “Combination Therapies” sections.

A. Pipathsouk, 4/24/09

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Verapamil

Verapamil is a currently available FDA-approved drug traditionally used to treat irregular heartbeats (arrhythmias) and high blood pressure by relaxing blood vessels. It has been discovered that verapamil can also modulate autophagy, a process by which cells gets rid of unwanted proteins and damaged cellular components. If this process is disrupted as it is in Huntington’s Disease (HD) and other neurodegenerative disorders then cellular “trash” can accumulate and harm brain cells. Thus, Verapamil’s effects on autophagy opens the door for its use in treating HD where the formation of protein aggregates is characteristic. To learn more about autophagy, click here. To learn more about the role of protein aggregates in HD, click here.

Verapamil was one of five L-type Ca+2 channel antagonists initially screened to test for its efficacy in modulating autophagy. L-type Ca+2 channels are specialized high-voltage ion channels found on the dendritic spines of cortical neurons. For more information about the different parts of the brain, see the brain tutorial here. Verapamil and other calcium channel inhibitors may regulate autophagy by limiting the amount of calcium that can enter the cell. High levels of intracellular Ca+2 can up-regulate autophagy by activating calpains, which are enzymes that aid in protein breakdown. Interestingly, some studies have found that calpain activity is increased in HD cells and can chop the mutant huntingtin protein into smaller fragments which allows it to enter the nucleus of neurons leading to toxicity.

Verapamil may block this toxicity by preventing calcium from entering the neuron. This lower concentration of calcium can reduce calpain activity, which can in turn increase autophagy. In HD, more autophagy means that more of the mutant huntingtin protein is cleared and fewer aggregates formed. To learn more about aggregate formation and its role in HD, click here.

The Promise of Verapamil in treating HD

Verapamil was tested for its ability to induce autophagy in several HD cellular and mouse models. To learn more about mouse models, click here. The first, and simplest model used was the cell model. Rat-derived neuronal cells were engineered to express huntingtin aggregates and some were treated with verapamil. Cells exposed to the drug showed a greater degree of aggregate clearance than cells that were not.

The next model used to explore the effects of verapamil on HD was the fruit fly, a commonly used model for many experiments. The development of the eyes in flies expressing the mutant huntingtin protein is altered, which causes the photorecetors to become disorganized and to degenerate. Flies given verapamil had less severe degeneration, than control flies did.

The next animal model of HD tested was the zebrafish. Zebrafish expressing mutant huntingtin form aggregates in their eyes and optic nerve. As in human HD, cells that form aggregates are more likely to die. Zebrafish administered verapamil had fewer aggregates.

Despite all of these experiments indicating a neuroprotective role for verapamil, the process for approving the use of verapamil in treating HD is still in very early stages. Although the results of preliminary studies are very promising, many more trials and more research needs to be done before using verapamil in HD treatments.

Further reading

  • http://www.nlm.nih.gov/medlineplus/druginfo/medmaster/a684030.html
    Gafni J., and L. Ellerby. “Calpain Activation in Huntington’s Disease.” Journal of Neuroscience. 2002 June; 22(12):4842-4849.
    This technical paper explained how calpain activation breaks huntingtin protein into pieces small enough to enter the nucleus and lead to toxicity in HD cells.
  • Sarkar S., et al. “Rapamycin and mTOR-independent autophagy inducers ameliorate toxicity of polyglutamine-expanded huntingtin and related proteinopathies.” Cell Death and Differentiation advance online publication, 18 July 2008; doi:10.1038/cdd.2008.110.
    This review paper explained the relationship between aggregate formation in several neurological diseases and the role in autophagy in protecting against these diseases. It also explained several animal models of HD.
  • Williams et al. “Novel targets for Huntington’s Disease in an mTOR-independent autophagy pathway.” Nature Chemical Biology. 2008 May;5(4):295-305
    This paper explained the testing of a number of potential HD drugs though targeting the autophagy mechanisms within cells.

A. Pipathsouk, 5/21/2009

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