Native Functions of the Androgen Receptor are Essential to Pathogenesis in a Drosophila Model of Spinobulbar Muscular Atrophy

Spinobulbar muscular atrophy (SBMA) is a progressive, late-onset disease characterized by degeneration of motor neurons in the brainstem and spinal cord. The disease is caused by expansion of a polyglutamine tract in the androgen receptor (AR) and is dependent on exposure to AR ligand. The expanded polyglutamine tract confers toxic function to the protein through unknown mechanisms, although the ligand-dependent nature of SBMA suggests that the mechanism of pathogenesis may be tied to ligand-dependent alterations in AR function. However, whether toxicity is mediated by native AR function or a novel AR function is unknown. We systematically investigated ligand-dependent modifications of AR in a Drosophila model of SBMA. We demonstrate in vivo that nuclear translocation of mutant AR is necessary but not sufficient for toxicity and that DNA binding by AR is necessary for toxicity. Mutagenesis studies demonstrated that a functional AF-2 domain is essential for toxicity, a finding corroborated by a genetic screen that identified AF-2 interactors as dominant modifiers of degeneration. As proof of this principle, we perform epistasis experiments using the AR coregulator limpet, which we find modifies polyglutamine-expanded AR toxicity in an AF-2-dependent manner. In addition, we use expression profiling to examine the molecular phenotype of polyglutamine-expanded AR degeneration, revealing that expression of wild-type AR results in a molecular phenotype that is very similar to that caused by polyglutamine-expanded AR. These findings suggest that expanded-polyglutamine AR toxicity may be mediated by amplification of normal function, a mechanism that may be broadly applicable to other polyglutamine diseases

FOXO3a is broadly neuroprotective in vitro and in vivo against insults implicated in motor neuron diseases

Aging is a risk factor for the development of adult-onset neuro-degenerative diseases. While some of the molecular pathways regulating longevity and stress resistance in lower organisms are defined (i.e., those activating the transcriptional regulators DAF-16 and HSF-1 in C. elegans), their relevance to mammals and disease susceptibility are unknown. We studied the signaling controlled by the mammalian homolog of DAF-16, FOXO3a, in model systems of motor neuron disease. Neuron death elicited in vitro by excitotoxic insult or the expression of mutant SOD1, mutant p150(glued) or polyQ expanded androgen receptor was abrogated by expression of nuclear-targeted FOXO3a. We identify a compound (Psammaplysene A, PA) that increases nuclear localization of FOXO3a in vitro and in vivo and show that PA also protects against these insults in vitro. Administration of PA to invertebrate model systems of neurodegeneration similarly blocked neuron death in a DAF-16/FOXO3a-dependent manner. These results indicate that activation of the DAF-16/FOXO3a pathway, genetically or pharmacologically, confers protection against the known causes of motor neuron diseases

Native Functions of the Androgen Receptor are Essential to Pathogenesis in a Drosophila Model of Spinobulbar Muscular Atrophy

Spinobulbar muscular atrophy (SBMA) is a progressive, late-onset disease characterized by degeneration of motor neurons in the brainstem and spinal cord. The disease is caused by expansion of a polyglutamine tract in the androgen receptor (AR) and is dependent on exposure to AR ligand. The expanded polyglutamine tract confers toxic function to the protein through unknown mechanisms, although the ligand-dependent nature of SBMA suggests that the mechanism of pathogenesis may be tied to ligand-dependent alterations in AR function. However, whether toxicity is mediated by native AR function or a novel AR function is unknown. We systematically investigated ligand-dependent modifications of AR in a Drosophila model of SBMA. We demonstrate in vivo that nuclear translocation of mutant AR is necessary but not sufficient for toxicity and that DNA binding by AR is necessary for toxicity. Mutagenesis studies demonstrated that a functional AF-2 domain is essential for toxicity, a finding corroborated by a genetic screen that identified AF-2 interactors as dominant modifiers of degeneration. As proof of this principle, we perform epistasis experiments using the AR coregulator limpet, which we find modifies polyglutamine-expanded AR toxicity in an AF-2-dependent manner. In addition, we use expression profiling to examine the molecular phenotype of polyglutamine-expanded AR degeneration, revealing that expression of wild-type AR results in a molecular phenotype that is very similar to that caused by polyglutamine-expanded AR. These findings suggest that expanded-polyglutamine AR toxicity may be mediated by amplification of normal function, a mechanism that may be broadly applicable to other polyglutamine diseases

Autophagy and the ubiquitin-proteasome system: Collaborators in neuroprotection

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Native functions of the androgen receptor are essential to pathogenesis in a drosophila model of spinobulbar muscular atrophy

Spinobulbar muscular atrophy (SBMA) is a neurodegenerative disease caused by expansion of a polyglutamine tract in the androgen receptor (AR). This mutation confers toxic function to AR through unknown mechanisms. Mutant AR toxicity requires binding of its hormone ligand, suggesting that pathogenesis involves ligand-induced changes in AR. However, whether toxicity is mediated by native AR function or a novel AR function is unknown. We systematically investigated events downstream of ligand-dependent AR activation in a Drosophila model of SBMA. We show that nuclear translocation of AR is necessary but not sufficient for toxicity and that DNA binding by AR is necessary for toxicity. Mutagenesis studies demonstrated that a functional AF-2 domain is essential for toxicity, a finding corroborated by a genetic screen that identified AF-2 interactors as dominant modifiers of degeneration. These findings indicate that SBMA pathogenesis is mediated by misappropriation of native protein function, a mechanism that may apply broadly to polyglutamine diseases

B2 attenuates polyglutamine-expanded androgen receptor toxicity in cell and fly models of spinal and bulbar muscular atrophy

Expanded polyglutamine tracts cause neurodegeneration through a toxic gain of function mechanism. Generation of inclusions is a common feature of polyglutamine diseases and other protein misfolding disorders. Inclusion formation is likely to be a defensive response of the cell to the presence of unfolded protein. Recently, the compound B2 has been shown to increase inclusion formation and decrease toxicity of polyglutamine-expanded huntingtin in cultured cells. We explored the effect of B2 on spinal and bulbar muscular atrophy (SBMA). SBMA is caused by expansion of polyglutamine in the androgen receptor (AR) and is characterized by the loss of motor neurons in the brainstem and spinal cord. We found that B2 increases the deposition of mutant AR into nuclear inclusions without altering the ligand-induced aggregation, expression, or subcellular distribution of the mutant protein. The effect of B2 on inclusions was associated with a decrease in AR transactivation function. Importantly, we show that B2 reduces mutant AR toxicity in cell and fly models of SBMA, further supporting the idea that accumulation of polyglutamine-expanded protein into inclusions is protective. Our findings suggest B2 as a novel approach to therapy for SBMA