Altered frequency-dependent inactivation and steady-state inactivation of polyglutamine-expanded alpha1A in SCA6

Spinocerebellar ataxia type 6 (SCA6) is a neurodegenerative disease of the cerebellum and inferior olives characterized by a late-onset cerebellar ataxia and selective loss of Purkinje neurons. SCA6 arises from an expansion of the polyglutamine tract located in exon 47 of the alpha(1A) (P/Q-type calcium channel) gene from a nonpathogenic size of 4 to 18 glutamines (CAG(4-18)) to CAG(19-33) in SCA6. The molecular basis of SCA6 is poorly understood. To date, the biophysical properties studied in heterologous systems support both a gain and a loss of channel function in SCA6. We studied the behavior of the human alpha(1A) isoform, previously found to elicit a gain of function in disease, focusing on properties in which the COOH terminus of the channel is critical for function: we analyzed the current properties in the presence of beta(4)- and beta(2a)-subunits (both known to interact with the alpha(1A) COOH terminus), current kinetics of activation and inactivation, calcium-dependent inactivation and facilitation, voltage-dependent inactivation, frequency dependence, and steady-state activation and inactivation properties. We found that SCA6 channels have decreased activity-dependent inactivation and a depolarizing shift (+6 mV) in steady-state inactivation properties consistent with a gain of function.