Coordinate expression of α-tropomyosin and caldesmon isoforms in association with phenotypic modulation of smooth muscle cells

Abstract

Isoform diversity of tropomyosin is generated from the limited genes by a combination of differential transcription and alternative splicing. In the case of the α-tropomyosin (α-TM) gene, exert 2a rather than exon 2b is specifically spliced in α-TM-SM mRNA, which is one of the major tropomyosin isoforms in smooth muscle cells. Here we demonstrate that expressions of α- tropomyosin and caldesmon isoforms are coordinately regulated in association with phenotypic modulation of smooth muscle cells. Molecular cloning and Western and Northern blottings have revealed that in addition to the down- regulation of β-TM-SM, α-TM-SM converted to α-TM-F1 and α-TM-F2 by a selectional change from exon 2a to exon 2b during dedifferentiation of smooth muscle cells in culture. Simultaneously, a change of caldesmon isoforms from high M(r) type to low M(r) type was also observed by alternative selection between exons 3b and 4 in the caldesmon gene during this process. In contrast, cultured smooth muscle cells maintaining a differentiated phenotype continued to express α-TM-SM, β-TM-SM, and high M(r) caldesmon. In situ hybridization revealed specific coexpression of α-TM-SM and high M(r) caldesmon in smooth muscle in developing embryos. These results suggest a common splicing mechanism for phenotype-dependent expression of tropomyosin and caldesmon isoforms in both visceral and vascular smooth muscle cells.