Calcium Channels and Hereditary Ataxia

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The longterm objective of this study is to define the role of the P/Q-type voltage-gated channel in Purkinje-cell degeneration and hereditary ataxia. P/Q channels are membrane proteins that play a crucial role in membrane excitability and triggering synaptic transmission. The P/Q a subunit (a1A) encoded by the gene, CACNA1A, has been implicated in familial migraine, hereditary episodic ataxia and spinocerebellar ataxia type 6 (SCA6). SCA6, a form of autosomal dominant progressive ataxia associated with nearly selective Purkinje cell degeneration, bears the putative pathogenic mutation in the long C terminus of a novel spliceform of the a1A mRNA. This study proposes to investigate the distribution of this a1A subunit spliceform and the pathogenicity of the CAG repeat expansion to which the disease is attributed. Using antisera specific for the long splice form, fusion proteins composed of the long C terminus and chimeric full length a1A subunits bearing the long tail with and without expanded CAG repeats we propose to: 1) Characterize the distribution and localization of the wild type and SCA6 mutant long splice form of the a1A subunit of the P/Q-type voltage-gated Ca2+ channel in brain and transfected cells and in relation to b subunit subtypes. 2) Characterize the effect of the SCA6 P/Q channel mutation and b subunit subtype on P/Q channel kinetics, and on Ca2+ overload and calcium-coupled pathways. 3) Develop mice that express the a1A subunit with SCA6-associated polyglutamine expansions to investigate the role of these mutations in the development of ataxia and Purkinje cell dysfunction in transgenic mice. These studies will provide new information about the biology of Ca+2 channels and may aid in understanding the basis for selective vulnerability of Purkinje cells in SCA6. Identification of an effect of an elongated polyglutamine tract in the a1A C terminus on specific Ca+2 channel properties or on Purkinje cell viability would help elucidate the pathogenesis of SCA6 and increase our understanding of P/Q-type Ca+2 channel function.