Differentiation-specific isoform mRNA expression of the calmodulin-dependent plasma membrane Ca²⁺-ATPase

The functional significance of the isoform diversity of the calmodulin-dependent plasma membrane Ca²⁺-ATPase (PMCA) is largely unknown. To determine whether the mRNA synthesis of different isoforms of the enzyme is regulated in a differentiation-specific manner, we investigated the expression of isoform-specific mRNAs in muscle and neuronal cells during differentiation by reverse transcription PCR. In the rat, the ubiquitous PMCA splicing variants 1b and 4b formed the typical PMCA isoform pattern of L6 myoblasts, the heart-derived cell line H9c2(2-1), two different fibroblast cell lines (FR and NRK-49F), smooth muscle, and endothelial cells. In addition to these two enzymes, novel expression of the splicing variants 1e, 1d, and 4a was induced during myogenic differentiation of L6 and H9c2(2-1) cells. A similar isoform subtype switch could be detected during differentiation of the neuronal PC-12 cells induced by nerve growth factor (NGF). The isoform-specific mRNAs 1c, 1d, and 4a were not expressed in cells other than myocytes and neurons, and therefore may be specific for excitable cells. The mRNA for isoform 1d was heart- and skeletal muscle-specific. To determine whether expression of a differentiation-specific PMCA mRNA pattern is under control of a myogenic determination factor, myogenin was constitutively expressed in rat fibroblasts. These cells converted to multinucleated myotubes, which displayed the PMCA isoform-specific mRNAs 1e, 1d, and 4a, typical of differentiated muscle cells. We conclude that: 1) the distribution of the various PMCA isoform-specific mRNAs and their splicing variants is cell type- and development-specific; 2) expression of the myogenic determination factor myogenin is sufficient to direct alternative splicing generating muscle-specific PMCA mRNA species; and 3) PMCA isoforms and/or splicing variants may play a role in determining functions of terminally differentiated muscle and neuronal cells and possibly during the differentiation process itself.