Two novel alleles of tottering with distinct Ca(v)2.1 calcium channel neuropathologies

The calcium channel CACNA1A gene encodes the pore-forming, voltage-sensitive subunit of the voltage-dependent calcium Ca(v)2.1 type channel. Mutations in this gene have been linked to several human disorders, including familial hemiplegic migraine, episodic ataxia 2 and spinocerebellar ataxia type 6. The mouse homologue, Cacna1a, is associated with the tottering, Cacna1a^tg, mutant series. Here we describe two new missense mutant alleles, Cacna1a^tg-4J and Cacna1a^Tg-5J. The Cacna1a^tg-4J mutation is a valine to alanine mutation at amino acid 581, in segment S5 of domain II. The recessive Cacna1a^tg-4J mutant exhibited the ataxia, paroxysmal dyskinesia and absence seizures reminiscent of the original tottering mouse. The Cacna1a^tg-4J mutant also showed altered activation and inactivation kinetics of the Ca(v)2.1 channel, not previously reported for other tottering alleles. The semi-dominant Cacna1a^Tg-5J mutation changed a conserved arginine residue to glutamine at amino acid 1252 within segment S4 of domain III. The heterozygous mouse was ataxic and homozygotes rarely survived. The Cacna1a^Tg-5J mutation caused a shift in both voltage activation and inactivation to lower voltages, showing that this arginine residue is critical for sensing Ca(v)2.1 voltage changes. These two tottering mouse models illustrate how novel allelic variants can contribute to functional studies of the Ca(v)2.1 calcium channel.