Contractile Proteins in I. Immunoperoxidase Pericytes. Localization of Tropomyosin

In these studies we have compared the relative amounts and isoforms of tropomyosin in capillary and postcapillary venule pericytes, endothelial cells, and vascular smooth muscle cells in four rat microvascular beds: heart, diaphragm, pancreas, and the intestinal mucosa. The results, obtained by in situ immunoperoxidase localization, indicate that (a) tropomyosin is present in capillary and postcapillary venule pericytes in relatively high concentration; (b) the tropomyosin content of pericytes appears to be somewhat lower than in vascular smooth muscle cells but higher than in endothelia and other vessel-associated cells; and (c) pericytes, unlike endothelia and other nonmuscle cells, contain detectable levels of tropomyosin immunologically related to the smooth muscle isoform. These results and our previous findings concerning the presence of a cyclic GMP-dependent protein kinase (Joyce, N., P. DeCamilli, and J. Boyles, 1984, Microvasc. Res. 28:206-219) in pericytes demonstrate that these cells contain significant amounts of at least two proteins important for contraction regulation. Taken together, the evidence suggests that pericytes are contractile elements related to vascular smooth muscle cells, possibly involved, as are the latter, in the regulation of blood flow through the microvasculature. Pericytes are polymorphic cells closely associated with the walls of capillaries and postcapillary venules. They have a cell body which is often highly elongated, and multiple branching foot processes which partially encircle the vessel wall. Pericytes can be distinguished from other perivascular cells, such as adventitial fibroblasts, by their location within the basement membrane of the vessels, and by the close apposition of the tips of their processes to the underlying endothelium. Electron microscopic studies have identified pericytes in a number of tissues and organs (I0, 12, 20, 36). They may, in fact, be common to all microvascular beds, although their relative frequency and distribution appear to vary from one microvascular bed to another (32, 33). The function of pericytes is still unclear; however, morphologic evidence suggests that they may be contractile cells related to vascular smooth muscle. Numerous micro filaments form a continuous plate in the adluminal cytoplasm of their cell body and extend from it into the foot processes, where, in the more distal segments, they fill the cytoplasm to the relative exclusion of other subeellular components. In addition, the foot processes have densities that appear similar to the attachment plaques and dense bodies of smooth muscle cells (29, 32, 34). The presence of such structures within circumferentially oriented pericyte processes suggests a potenTHE JOURNAL OF CELt BIOLOGY • VOLUME 100 MAY 1985 1379-1386 © The Rockefeller University Press . 0021-9525185/05/1379/08 $1.00 tial contractile function that may contribute to the regulation of blood flow in the microvasculature (12, 19, 24, 37). Since the morphologic findings mentioned above are simply suggestive, biochemical evidence is needed to establish the presence of significant amounts of essential contractile and regulatory proteins in these cells as a necessary basis for a contractile function. Actin has already been identified in pericytes (12, 14, 21, 35), and, in the companion paper, we present evidence for the presence in these cells of myosin in relatively high concentration (18). Pericytes and vascular smooth muscle cells are related by a similar topography in the wall of the vessel (1), and a biochemical relationship between the two cell types is suggested by the immunocytochemical localization in both cells of cyclic GMP-dependent protein kinase (17), an enzyme believed to function in the regulation of smooth muscle cell contractility (13, 28, 30), and of the intermediate filament proteins desmin and vimentin (9). To explore this relationship further and to provide evidence for the presence in pericytes of additional contraction-associated proteins, we extended the inquiry to tropomyosin, a protein considered essential for the regulation of cellular contractility. We took advantage of the fact that this highly conserved molecule exists in several isoforms, apparently distributed preferentially to specific cell types (7, 1379 on Jne 9, 2017 D ow nladed fom Published May 1, 1985