Change in plasma membrane Ca2(+)-ATPase splice-variant expression in response to a rise in intracellular Ca2+

Most eukaryotic genes are divided into introns and exons. Upon transcription, the intronic segments are eliminated and the exonic sequences spliced together through a series of complex processing events. Alternative splicing refers to the optional inclusion or exclusion of specific exons in transcripts derived from a single gene, which leads to structural and functional changes in the encoded proteins. Although many components of the machinery directing the physical excision of introns and joining of exons have been elucidated in recent years, the signaling pathways regulating the activity of the machinery remain largely unexplored.

A calcium-mediated signaling pathway regulates alternative splicing at a specific site of human plasma membrane calcium pump-2 transcripts. This site consists of three exons, which are differentially used in a tissue-specific manner. In IMR32 neuroblastoma cells, a transient elevation of intracellular calcium changed the predominant pattern from one in which all three exons are included to the coexpression of a variant including only the third exon. Western-blot analysis demonstrated that the newly expressed mRNAs are faithfully translated. Once induced, the new splicing pattern was maintained over multiple cell divisions. Protein synthesis was not required to induce the alternative splice change, indicating that all components necessary for a rapid cellular response are present in the cells.

Calcium signaling exerts a direct influence on the regulation of alternative splicing. Notably, a calcium-mediated change in the expression of alternatively spliced variants of a calcium regulatory protein was discovered. The change in splicing occurs quickly, is persistent but reversible and leads to a corresponding change in protein expression. The specific nature in which differently spliced protein variants are expressed, and now the fact that their expression can be regulated by distinct intracellular signaling pathways, suggests that the regulation of alternative splicing by physiological stimuli is a widespread regulatory mechanism by which a cell may coordinate its responses to environmental cues.