Huntington's disease (HD) is an untreatable inherited adult onset neurodegenerative disease caused by an abnormal expansion in the polyglutamine (polyQ) tract in the Huntington (Htt) protein. The polyQ expansion makes mutant Htt (mHtt) prone to aggregate, but the relationship between aggregation and neurodegeneration has been difficult to understand. In the previous funding period, we invented a new imaging platform to study a neuron model of HD and discovered that the aggregation of mHtt into visible deposits called inclusion bodies (IBs) appears to be a coping response. Recently, we developed new methods to study neuronal protein homeostasis in the context of mHtt in live neurons. Surprisingly, we showed that neurons recognize and target mHtt for accelerated degradation in vitro and in vivo. Remarkably, we found that the protein homeostasis and clearance capacity of different types of neurons varies and significantly predicts their susceptibility to mHtt. We discovered that a major protein clearance pathway called autophagy is differentially regulated in neurons. We found small molecules that stimulate neuronal autophagy and showed that they lower mHtt levels and protect them from neurodegeneration. These findings have led to the overarching hypothesis that motivates this proposal: the demand made by aggregation-prone mHtt on the protein homeostasis network of susceptible neurons exceeds their capacity leading to neurodegeneration. In the first Aim, we will investigate how neurons recognize and respond to misfolded protein. The highly conserved heat shock response mediates responses to misfolded proteins in most cells, but it seems different and ineffective in neurons.
In Aim 2, we will test two major hypotheses about how protein dyshomeostasis leads to neurodegeneration-that inadequate capacity created by the demands of mHtt (1) leads to cell-wide protein misfolding and deleterious loss-of-function of metastable proteins or (2) a competition for flux through critical clearance pathways such as mitochondria and accumulation of substrates, such as dysfunctional mitochondria.
In Aim 3, we propose to adapt our powerful in vitro single cell longitudinal methods to study neuronal protein homeostasis and Htt metabolism in vivo. The results could help validate the protein dyshomeostasis model of HD and lead to new therapies.

Public Health Relevance

This is a competing renewal of a highly productive research program on Huntington's disease, which led to the development of a novel imaging platform for cell biology. With it, we made the surprising discovery that neurons with mutant huntingtin (mHtt) that form inclusion bodies (IB) live longer than those that don't form an IB. We also discovered that neurons regulate protein homeostasis differently than non-neuronal cells in key ways that could be relevant to neurodegenerative disease. In this proposal, we will dig deeper to understand the homeostatic mechanisms neurons have for responding to protein misfolding and test two key ideas for how protein dyshomeostasis causes neurodegeneration. During the previous funding period, we discovered compounds that stimulate an important component of the protein homeostasis system, a clearance pathway called autophagy. We showed that they protect neurons from mHtt-induced degeneration in vitro and in this proposal we will develop ways to adapt our powerful in vitro single cell methods for studying protein homeostasis in vivo.

Agency
National Institute of Health (NIH)
Institute
National Institute of Neurological Disorders and Stroke (NINDS)
Type
Research Project (R01)
Project #
2R01NS039074-10A1
Application #
8507056
Study Section
Cell Death in Neurodegeneration Study Section (CDIN)
Program Officer
Sutherland, Margaret L
Project Start
2000-07-01
Project End
2017-12-31
Budget Start
2013-02-15
Budget End
2013-12-31
Support Year
10
Fiscal Year
2013
Total Cost
$417,813
Indirect Cost
$199,063
Name
J. David Gladstone Institutes
Department
Type
DUNS #
099992430
City
San Francisco
State
CA
Country
United States
Zip Code
94158
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Sathasivam, Kirupa; Lane, Amin; Legleiter, Justin et al. (2010) Identical oligomeric and fibrillar structures captured from the brains of R6/2 and knock-in mouse models of Huntington's disease. Hum Mol Genet 19:65-78

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