Expression and functional activity of four myocardin isoforms

Myocardin (MYOCD) is an essential component of a molecular switch for the expression of contractile genes in smooth muscle and cardiac muscle cells. The Myocd gene comprises at least fifteen exons, including two alternately spliced exons designated 2a and 10a. We investigated tissue-specific Myocd expression in mouse, rat and human tissues to determine the conservation in expression of each Myocd splice variant and to ascertain whether any functional differences exist among MYOCD isoforms. Conventional and quantitative RT-PCR revealed the dominant expression of Myocd exon 2a (Myocd_v3) in smooth muscle cell (SMC)-rich tissues (aorta and bladder) with little expression in heart across all species studied. Each species of heart showed primarily a longer version of Myocd (Myocd_v1) without exon 2a. While exclusion of exon 2a was common in all cardiac muscle samples, exon skipping of Myocd exon 10a was a rare event in both cardiac muscle and SMC tissues. In general, all four MYOCD isoforms showed comparable stimulation of SMC promoters. On the other hand, Myocd_v1 and Myocd_v2 were more active than Myocd_v3 and Myocd_v4 in stimulating cardiac muscle promoters and Myocd_v1's activity was augmented in the presence of the cardiac transcription factor, MEF2C. Importantly, whereas all four MYOCD isoforms similarly induced expression of endogenous SMC genes in a prostate tumor cell line (LNCaP), none could induce the endogenous expression of specific cardiac markers. These results are the first to show relative expression and activities of the major myocardin isoforms across disparate species. We propose a new myocardin nomenclature reflecting the dominant splice variants expressed in cardiac muscle (Myocd_v1 and v2) versus SMC-rich tissues (Myocd_v3 and v4).