Biochemical characterization of contractile proteins of rat cultured mesangial cells

We examined characteristics of contractile proteins of rat cultured mesangial cells. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis revealed that actomyosin was a major protein component in the extract of mesangial cells. By Western blot, tropomyosin, caldesmon, alpha-actinin, and vinculin were recognized in mesangial cells. The molecular masses of alpha-actinin and vinculin were the same as aortic smooth muscle. Mesangial cells contained five tropomyosin isoforms: TM-1, -2, -3, -4, and -5. TM-1 was present as a major isoform in mesangial cells, and it had an immunological cross reactivity with beta-tropomyosin of aortic smooth muscle. On the other hand, the molecular mass of caldesmon was similar to dermal fibroblasts, and differed from aortic smooth muscle. Immunofluorescent studies showed that the staining patterns of tropomyosin and caldesmon between cultured mesangial cells and cultured vascular smooth muscle cells were somewhat different. From these results, we conclude that mesangial cells abound in contractile proteins, and that the compositions of these proteins are similar to those of aortic smooth muscle with minor differences. Thus, this study appears to biochemically support a hypothesis that mesangial cells are derived from vascular smooth muscle cells, but with a minor modification in their evolution.