Role of Alternative Splicing in Generating Isoform Diversity Among Plasma Membrane Calcium Pumps

Abstract

Calcium pumps of the plasma membrane (also known as plasma membrane Ca²⁺-ATPases or PMCAs) are responsible for the expulsion of Ca²⁺ from the cytosol of all eukaryotic cells. Together with Na⁺/Ca²⁺ exchangers, they are the major plasma membrane transport system responsible for the long-term regulation of the resting intracellular Ca²⁺concentration. Like the Ca²⁺ pumps of the sarco/endoplasmic reticulum (SERCAs), which pump Ca²⁺ from the cytosol into the endoplasmic reticulum, the PMCAs belong to the family of P-type primary ion transport ATPases characterized by the formation of an aspartyl phosphate intermediate during the reaction cycle. Mammalian PMCAs are encoded by four separate genes, and additional isoform variants are generated via alternative RNA splicing of the primary gene transcripts. The expression of different PMCA isoforms and splice variants is regulated in a developmental, tissue- and cell type-specific manner, suggesting that these pumps are functionally adapted to the physiological needs of particular cells and tissues. PMCAs 1 and 4 are found in virtually all tissues in the adult, whereas PMCAs 2 and 3 are primarily expressed in excitable cells of the nervous system and muscles. During mouse embryonic development, PMCA1 is ubiquitously detected from the earliest time points, and all isoforms show spatially overlapping but distinct expression patterns with dynamic temporal changes occurring during late fetal development. Alternative splicing affects two major locations in the plasma membrane Ca²⁺ pump protein: the first intracellular loop and the COOH-terminal tail. These two regions correspond to major regulatory domains of the pumps. In the first cytosolic loop, the affected region is embedded between a putative G protein binding sequence and the site of phospholipid sensitivity, and in the COOH-terminal tail, splicing affects pump regulation by calmodulin, phosphorylation, and differential interaction with PDZ domain-containing anchoring and signaling proteins. Recent evidence demonstrating differential distribution, dynamic regulation of expression, and major functional differences between alternative splice variants suggests that these transporters play a more dynamic role than hitherto assumed in the spatial and temporal control of Ca²⁺ signaling. The identification of mice carrying PMCA mutations that lead to diseases such as hearing loss and ataxia, as well as the corresponding phenotypes of genetically engineered PMCA “knockout” mice further support the concept of specific, nonredundant roles for each Ca²⁺ pump isoform in cellular Ca²⁺ regulation.