Spinal and bulbar muscular atrophy (SBMA) is a degenerative disorder of lower motor neurons and skeletal muscle caused by a CAG/glutamine tract expansion in the androgen receptor (AR) gene. The polyglutamine AR (polyQ AR) undergoes hormone-dependent nuclear translocation and unfolding, steps that are essential to toxicity and to the development of progressive muscle weakness in men. Although the disease causing mutation was identified over two and a half decades ago, only supportive therapies are currently available to SBMA patients. Model systems that have been used to study disease pathogenesis show hormone and glutamine length-dependent changes in an array of downstream pathways, supporting a role for divergent mechanisms in toxicity. These observations prompted us to focus instead on understanding the proximal mechanisms that regulate degradation of the mutant androgen receptor protein in hopes of harnessing these for the discovery of effective treatments. However, these clearance pathways are incompletely defined, and this lack of knowledge hinders the development of disease-modifying therapies. The objective of this application is to define the role of Hsp70 in the protein quality control decisions that govern degradation of the full-length polyQ AR. The scientific premise of this application is that the Hsp70, acting through the Hsp90/Hsp70-based machinery and the Hsp70/Hsp110 disaggregase machinery, plays a critical role in controlling polyQ AR degradation through the proteasome. This premise provides the foundation for our central hypothesis that Hsp70 targeted strategies will promote ubiquitination and clearance of the mutant protein. This hypothesis springs from our preliminary data showing that association with Hsp90 stabilizes the polyQ AR, while unfolding of the mutant protein leads to ubiquitination by Hsp70-dependent E3 ligases. Moreover, we will build upon our published studies demonstrating that allosteric regulation of Hsp70 to increase binding to misfolded proteins enhances clearance of the polyQ AR in cells and alleviates toxicity in a Drosophila model. The rationale of the proposed work is that establishing the mechanisms that regulate polyQ AR degradation will identify targets that can be exploited for the development of new therapies. Structural, biochemical, genetic and pharmacological approaches will be used to characterize the Hsp70-CHIP complex with the polyQ AR that regulates protein triage (Aim 1), establish the effects of genetic and small molecule allosteric regulators of Hsp70 in SBMA models (Aim 2), and determine effects of polyQ AR expression on ubiquitin-proteasome pathway function (Aim 3). These studies are expected to characterize the structure and function of the cellular machinery that regulates polyQ AR degradation and provide proof-of-concept data supporting a therapeutic approach centered on targeting Hsp70. As this chaperone also regulates quality control decisions governing the turnover of other mutant proteins that cause neurodegeneration, we expect that the approaches defined here will inform therapeutic strategies that will be applicable to other age-dependent neurological diseases.
The relevance of the proposed studies to public health is that they will define mechanisms by which Hsp70 targeted allosteric regulators promote degradation of the polyQ AR, the mutant protein that causes SBMA. We will test approaches to promote polyQ AR clearance, with the expectation that they will ameliorate the SBMA phenotype in model systems and speed the advance toward disease-modifying therapies.
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