Alternative splicing generates a novel truncated Cav1.2 channel in neonatal rat heart

L-type Cav1.2 Ca(2+) channel undergoes extensive alternative splicing, generating functionally different channels. Alternatively spliced Cav1.2 Ca(2+) channels have been found to be expressed in a tissue-specific manner or under pathological conditions. To provide a more comprehensive understanding of alternative splicing in Cav1.2 channel, we systematically investigated the splicing patterns in the neonatal and adult rat hearts. The neonatal heart expresses a novel 104-bp exon 33L at the IVS3-4 linker that is generated by the use of an alternative acceptor site. Inclusion of exon 33L causes frameshift and C-terminal truncation. Whole-cell electrophysiological recordings of Cav1.233L channels expressed in HEK 293 cells did not detect any current. However, when co-expressed with wild type Cav1.2 channels, Cav1.233L channels reduced the current density and altered the electrophysiological properties of the wild type Cav1.2 channels. Interestingly, the truncated 3.5-domain Cav1.233L channels also yielded a dominant negative effect on Cav1.3 channels, but not on Cav3.2 channels, suggesting that Cavβ subunits is required for Cav1.233L regulation. A biochemical study provided evidence that Cav1.233L channels enhanced protein degradation of wild type channels via the ubiquitin-proteasome system. Although the physiological significance of the Cav1.233L channels in neonatal heart is still unknown, our report demonstrates the ability of this novel truncated channel to modulate the activity of the functional Cav1.2 channels. Moreover, the human Cav1.2 channel also contains exon 33L that is developmentally regulated in heart. Unexpectedly, human exon 33L has a one-nucleotide insertion that allowed in-frame translation of a full Cav1.2 channel. An electrophysiological study showed that human Cav1.233L channel is a functional channel but conducts Ca(2+) ions at a much lower level.