Functional Characterization of Alternative Splicing in the C Terminus of L-type CaV1.3 Channels*

Background: Alternative splicing generates calcium channel splice variants with altered electrophysiological properties. Results: Exclusion of exons encoding the IQb domain or proximal/distal domains attenuates Ca2+-dependent inactivation of the CaV1.3 channels. Conclusion: Alternative splicing at the C terminus alters the critical Ca2+ inhibitory feedback property of CaV1.3 channels. Significance: Alternative splicing is an exquisite mechanism for customizing channel function within diverse biological niches. CaV1.3 channels are unique among the high voltage-activated Ca2+ channel family because they activate at the most negative potentials and display very rapid calcium-dependent inactivation. Both properties are of crucial importance in neurons of the suprachiasmatic nucleus and substantia nigra, where the influx of Ca2+ ions at subthreshold membrane voltages supports pacemaking function. Previously, alternative splicing in the CaV1.3 C terminus gives rise to a long (CaV1.342) and a short form (CaV1.342A), resulting in a pronounced activation at more negative voltages and faster inactivation in the latter. It was further shown that the C-terminal modulator in the CaV1.342 isoforms modulates calmodulin binding to the IQ domain. Using splice variant-specific antibodies, we determined that protein localization of both splice variants in different brain regions were similar. Using the transcript-scanning method, we further identified alternative splicing at four loci in the C terminus of CaV1.3 channels. Alternative splicing of exon 41 removes the IQ motif, resulting in a truncated CaV1.3 protein with diminished inactivation. Splicing of exon 43 causes a frameshift and exhibits a robust inactivation of similar intensity to CaV1.342A. Alternative splicing of exons 44 and 48 are in-frame, altering interaction of the distal modulator with the IQ domain and tapering inactivation slightly. Thus, alternative splicing in the C terminus of CaV1.3 channels modulates its electrophysiological properties, which could in turn alter neuronal firing properties and functions.