Pathways of neurodegeneration in SBMA

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The long-term goal is to understand the molecular basis of spinobulbar muscular atrophy (SBMA) so that treatment may be developed. SBMA is a hereditary disease characterized by progressive loss of motor neurons in the brainstem and spinal cord. SBMA is caused by trinucleotide (CAG) repeat expansion in the androgen receptor (AR) gene leading to polyglutamine expansion in AR protein. Eight other diseases have the same kind of mutation: Huntington's disease, DRPLA, and six forms of spinocerebellar ataxia. Expanded polyglutamine itself is toxic. However, despite widespread and overlapping expression of the disease proteins, these disorders are distinguished by selective vulnerability of different populations of neurons. This observation indicates that expanded polyglutamine cannot be the sole determinant of neurodegeneration. The first aim of this proposal will address the role of discrete AR functional domains in the initiation and pathogenesis of SBMA. Another important determinant in the initiation and progression of polyglutamine disease is that status of the cellular mechanisms responsible for protecting cells from toxic proteins. Two pathways of protein degradation, the ubiquitin-proteasome system (UPS) and autophagy, have been implicated as playing important roles in polyglutamine diseases. The UPS is a multi-component system that coordinates timely and specific degradation of intracellular protein. Autophagy is a lysosome-mediated catabolic pathway and is the primary means of bulk degradation of cytoplasmic components. In the second two aims of this proposal, we will test specific hypotheses relating to the role of the UPS and autophagy in the pathogenesis of SBMA. Aim 1: Defining the contributions of specific AR domains to polyglutamine-expanded AR toxicity. Aim 2: To test the hypothesis that SBMA pathogenesis is associated with impaired UPS function in vivo. Aim 3: To determine the role of autophagy in SBMA pathogenesis using histological and genetic approaches