Characterization of cellular and transgenic models of spinal and bulbar muscular atrophy

X-linked Spinal and Bulbar Muscular Atrophy (SBMA, Kennedy\u27s disease) is a slowly progressive neuromuscular disorder caused by an expanded polyglutamine repeat in the androgen receptor (AR) and characterized by a loss of motor neurons in the spinal cord and brainstem. SBMA is one of a group of progressive neurodegenerative diseases resulting from a polyglutamine repeat expansion. The disease is marked by the presence of AR-containing ubiquitinated neuronal intranuclear inclusions (NII) in motor neurons of the anterior horn of the spinal cord. We have developed cells and transgenic mice modeling aspects of SBMA to study the pathogenesis of the disease. Transgenic mice expressing a truncated form of the androgen receptor containing a highly expanded polyglutamine repeat developed gait abnormalities accompanied by the formation of ubiquitinated NII. NII sequestered molecular chaperones and CREB-binding protein. The lack of muscle and neuronal degeneration indicated that symptoms resulted from neuronal dysfunction rather than loss. This was the first mouse model expressing mutant AR to develop signs of neurological disease. We also created cellular models of SBMA using the entire AR protein. Transient expression of full-length AR was accompanied by the formation of an aberrant protein species containing portions of the amino- and carboxyl-termini of the androgen receptor. Further characterization of this AR species indicated that it forms independently of the full-length protein and is not likely a proteolytic fragment associated with SBMA pathogenesis. We went on to develop an inducible cell model of SBMA that reproduced the formation of intranuclear inclusions in the absence of the fragment species detected in transient transfection studies. PC12 cells expressing highly expanded AR formed ubiquitinated granular intranuclear inclusions containing only amino-terminal epitopes of the AR. Dihydrotestosterone treatment of mutant AR-expressing cells resulted in increased inclusion load. This cell model is the first to mimic the formation of ubiquitinated intranuclear inclusions containing the amino-terminal portion of AR observed in patient tissue and reveals a role for ligand in the pathogenesis of SBMA. This cell model system will be analyzed further to determine pathways of cellular dysfunction and to test potential therapeutics for the treatment of SBMA and other polyglutamine diseases