Human Molecular Genetics Advance Access published online on March 11, 2009
Human Molecular Genetics, doi:10.1093/hmg/ddp115
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Cytoplasmic Retention of Polyglutamine-Expanded Androgen Receptor Ameliorates Disease via Autophagy in a Mouse Model of Spinal and Bulbar Muscular Atrophy
1 Department of Biochemistry and Molecular Biology, Thomas Jefferson University, Philadelphia, PA 2 Gladstone Institute of Neurological Disease, San Francisco, CA 3 Departments of Neurology and Physiology, University of California, San Francisco, CA
* Address Correspondence to: Diane E. Merry, Ph.D. Thomas Jefferson University, Department of Biochemistry and Molecular Biology, 228 Bluemle Life Sciences Building, 223 S. 10th Street, Philadelphia, PA 19107, Tel: 215-503-4907, Fax: 215-923-9162, Email: diane.merry{at}jefferson.edu
Received January 13, 2009; Revised February 19, 2009; Accepted March 9, 2009
The nucleus is the primary site of protein aggregation in many polyglutamine diseases, suggesting a central role in pathogenesis. In SBMA, the nucleus is further implicated by the critical role for disease of androgens, which promote the nuclear translocation of the mutant androgen receptor (AR). To clarify the importance of the nucleus in SBMA, we genetically manipulated the nuclear localization signal of the polyglutamine-expanded AR. Transgenic mice expressing this mutant AR displayed inefficient nuclear translocation and substantially improved motor function compared with SBMA mice. While we found that nuclear localization of polyglutamine-expanded AR is required for SBMA, we also discovered, using cell models of SBMA, that it is insufficient for both aggregation and toxicity and requires androgens for these disease features. Through our studies of cultured motor neurons, we further found that the autophagic pathway was able to degrade cytoplasmically retained expanded AR and represents an endogenous neuroprotective mechanism. Moreover, pharmacologic induction of autophagy rescued motor neurons from the toxic effects of even nuclear-residing mutant AR, suggesting a therapeutic role for autophagy in this nucleus-centric disease. Thus, our studies firmly establish that polyglutamine-expanded AR must reside within nuclei in the presence of its ligand to cause SBMA. They also highlight a mechanistic basis for the requirement for nuclear localization in SBMA neurotoxicity, namely the lack of mutant AR removal by the autophagic protein degradation pathway.