A Novel Form of CaV1.4 Ca2+ Channel Regulation Revealed by Alternative Splicing and a Mutation Causing Congenital Stationary Night Blindness

Abstract

To support its role in mediating tonic exocytosis at the photoreceptor synapse, Cav1.4 L- type Ca2+ channels undergo weak Ca2+-dependent inactivation (CDI). The mechanism involves a C-terminal automodulatory domain (CTM) that competes with calmodulin (CaM) binding to the channel. A mutation that causes congenital stationary night blindness (CSNB2), K1591X, causes premature truncation of the channel just downstream of the CaM binding region and removal of the entire CTM. The mutant channels (Cav1.4K1591X) exhibit strong CDI and activation at more negative voltages than full-length channels similar to a splice variant (Cav1.4Δex47) that lacks just a portion of the CTM. To determine if selective deletion of exon 47 causes distinct regulation of CDI and activation as compared to the K1591X mutation, we compared the properties of Cav1.4K1591X and Cav1.4Δex47 in electrophysiological recordings of transfected HEK293T cells. We found key differences in how CaM regulates CDI of these channels. First, Cav1.4K1591X undergoes significantly faster CDI than Cav1.4Δex47. Second, CDI of Cav1.4Δex47 can be suppressed by dominant negative expression of CaM mutants that cannot bind Ca2+ in either the C-terminal (CaM34) or N-terminal (CaM12) lobe, whereas only CaM34 blunts CDI of Cav1.4K1591X. Third, CaM12 reverses the negative shift in activation of Cav1.4Δex47 but not of Cav1.4K1591X. We conclude that the deletion of exon 47 alters how CaM functionally interacts with the channel in a way that is not reproduced by the K1591X mutation, which may contribute to the pathological effects of K1591X for Ca2+ signaling at the photoreceptor synapse.