Huntington's Disease Society of America

Research

Form of the HD Protein Associated with Neurodegeneration is Identified

Dr. Steven Finkbeiner, Professor of Neurology

and Physiology at the University of California,

San Francisco, and Gladstone Institute

Dr. Steven Finkbeiner, and colleagues have identified the form of the HD protein associated with neurodegeneration. This is the result of research that began in 2003 with funding from HDSA, that continued over the years with funding from additional sources, and that could only have been completed with the development of a variety of new technologies.

In 1997, large aggregates of the HD protein were found first in the brains of HD mice and then in brain tissue samples from deceased HD patients. At first it was hypothesized that these aggregates, also called inclusion bodies, were the cause of the disease but cumulative research has suggested that they are a coping mechanism. More recent research has pointed to tiny, hard to detect aggregates called oligomers as problematic and suggested that the large inclusion bodies may be protecting the cell by sequestering them.

Dr. Finkbeiner and his colleagues have shown that the toxic species of the protein can be identified through antibodies that are specific to the different conformations (shapes) of the various species of the HD protein. First, they found antibodies for each conformation. Then they determined the survival times associated with each form by tracking thousands of neurons over long periods of time using an automated microscope. This generated an enormous amount of data and a modified form of a statistical technique called Cox analysis was used to determine which species best predicted cell death or survival. Finally, they determined the biochemical and structural characteristics of the toxic species.

One antibody, 3B5H10, was found to recognize a species of the HD protein that best predicts neuronal death. This species is found in monomers (single polypeptides) and possibly oligomeric forms of the protein but not in the larger aggregated forms, such as the inclusion bodies. The antibody recognizes a compact, structured portion of the polyglutamine that is only minimally present in the normal protein and is either exposed or created in the HD protein.

Interestingly and surprisingly, when pre-formed oligomers were exposed to 3B5H10, they dissolved. They also discovered as those large inclusion bodies form there is a corresponding loss of intraneuronal 3B5H10 binding, suggesting that inclusion body formation is somehow protecting the neurons either by reducing, masking or refolding the toxic protein part.

There are many pathogenic processes in Huntington’s disease, a number of therapeutic targets which have been proposed and a number of drugs discovered or developed to address them, leading to a growing pipeline of potential treatments. Prioritizing for clinical trial those drugs which address the most significant, upstream pathogenic processes is critically important in developing effective treatments. The value of a study such as this one is that researchers were able to follow the development of the disease process in numerous cells over time, distinguish between the toxic and nontoxic forms of the protein and identify an important target for drug development.

“Effective treatments for diseases such as Huntington's and Alzheimer's have been slow to develop,” said Dr. Finkbeiner, whose research at Gladstone investigates the interactions between genes, neurons and memory. “We hope that our newfound understanding of precisely which misfolded proteins contribute to disease symptoms will speed up drug development for sufferers. Now that our experiments have identified—at a cellular level—when neurons will die, it will be easier to develop drugs that target the toxic form of htt [the huntingtin protein] that causes Huntington's symptoms.”

“Dr. Finkbeiner's powerful and precise approach could enable the development of pharmaceutical treatments for other diseases as well,” added Lennart Mucke, MD, who directs neurological research at Gladstone. “These methods are applicable to many devastating neurodegenerative conditions, especially Parkinson's disease and Alzheimer's.”

Reference:

Jason Miller, Montserrat Arrasate, Elizabeth Brooks, Clare Peters Libeu, Justin Legleiter, Danny Hatters, Jessica Curtis, Kenneth Cheung, Preethi Krishnan, Siddhartha Mitra, Kartika Widjaja, Benjamin A Shaby, Gregor P Lotz, Yvonne Newhouse, Emily J Mitchell, Alex Osmand, Michelle Gray, Vanitha Thulasiramin, Frédéric Saudou, Mark Segal, X William Yang, Eliezer Masliah, Leslie M Thompson, Paul J Muchowski, Karl H Weisgraber & Steven Finkbeiner. “Identifying polyglutamine protein species in situ that best predict neurodegeneration.” Nature Chemical Biology 2011 Oct 30. [Epub ahead of print].

- Marsha L. Miller, Ph.D., November 11, 2011