The migration of ECs requires the cells to "crawl" and "burrow" through the surrounding tissue matrix. Motility is dependent on the intracellular cytoskeleton, which is composed principally of actin and myosin. As in VSMCs, contraction/relaxation is mediated principally by Ca2+ mobilization, phosphorylation of myosin light-chain kinase (MLCK) and the rho kinases. Motility and migration are orchestrated by the focal adhesion complexes that link sites of adhesion with intracellular activity of actin. Components of a focal adhesion, which are complex dynamic structures, include the pi and b3 integrins, vinculin, tensin, talin, zyxin, a-actinin, and focal adhesion kinase (FAK).
In order for motility to take place, migrating ECs need a fulcrum of attachment, which is achieved through adhesion of ECs to the matrix proteins via specialized adhesion molecules: (1) the integrins, including the P1 series and the vitronectin receptor; the integrins consist of heterodimeric transmembrane proteins with an extracellular domain and an intracellular domain that is linked to the cytoskeleton; (2) the immunoglobulin superfamily, including intracellular adhesion molecule (ICAM), vascular cell adhesion molecule (VCAM), and platelet endothelial cell adhesion molecule-1 (PECAM-1); (3) the selectins (e.g., E- and P-selectin); and (4) the cadherins that form complexes with catenins, which are components of focal adhesions. The matrix proteins bound by EC integrins are principally vitronectin, laminin, fibrinogen, collagen, thrombospondin, von Willebrand factor, and fibronectin.
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