Contractile proteins in endothelial cells.

TLDR: An inverse correlation between stress-fiber density and cell motility is demonstrated and is conclusively demonstrated.

Abstract: Although it is generally accepted that cytoplasmic actomyosin interactions generate force for cellular and subcellular movements, the nature of the tensionproducing mechanism has not been revealed.'-3 To reach this goal, biochemists must learn what milieu regulates the assembly and dismantling of the motor,4 while molecular anatomists must reveal the precise form and arrangement of the component parts and the manner in which they are anchored within cells and tissues to produce functional motility in vivo. In addition to their presumed role in cell motility, the biophysical properties of cytoplasmic actomyosin and the associated proteins,1,2,4,5 contribute to the maintenance of cytoplasmic structure during motility and quiescence. Light microscopic studies using time-lapse videotapes to document cell movements prior to the localization of the contractile proteins with specific markers indicate that when actomyosin is involved in structural support of the cytoplasm its form and distribution differ from those seen during cell movements.6 Fast-moving cells display a diffuse or randomized distribution of contractile proteins (FIGURE 1)and slow-moving cells possess combinations of diffuse and fibrous straining patterns (FIGURE 2). By contrast, immobilized cells possess mostly fibrous staining for actin and myosin (FIGURE 3). These results conclusively demonstrate an inverse correlation between stress-fiber density and cell motility.6 Thus, although it has been demonstrated that stress fibers possess an assortment of contractile proteins as well as contractile potential in model systems in uitro,7.8 it is unlikely they are important for moving cells in vivo.6 What role do stress fibers play in uivo? Until this time, little was known regarding the existence and function of these bundles of contractile proteins. Only in the vascular endothelium had bundles of 6-7 nm (actin) filaments been ob~erved,~ but it was not known whether they also contained the other contractile proteins localized to stress fibers of cultured cells (e.g. myosin and cr-actinin).lo We searched in tissues for cells with a microfilamentous bundle system comparable to the stress fibers of immobilized cells grown in tissue culture,