An integrin and Rho GTPase-dependent pinocytic vacuole mechanism controls capillary lumen formation in collagen and fibrin matrices.

A major question that remains unanswered concerning endothelial cell (EC) morphogenesis is how lumens are formed in three-dimensional extracellular matrices (ECMs). Studies from many laboratories have revealed a critical role for an ECM-integrin-cytoskeletal signaling axis during EC morphogenesis. We have discovered a mechanism involving intracellular vacuole formation and coalescence that is required for lumen formation in several in vitro models of morphogenesis. In addition, a series of studies have observed vacuoles in vivo during angiogenic events. These vacuoles form through an integrin-dependent pinocytic mechanism in either collagen or fibrin matrices. In addition, we have shown that the Cdc42 and Rac1 guanosine triphosphatases (GTPases), which control actin and microtubule cytoskeletal networks, are required for vacuole and lumen formation. These GTPases are also known to regulate integrin signaling and are activated after integrin-matrix interactions. Furthermore, the expression of green fluorescent protein-Rac1 or -Cdc42 chimeric proteins in ECs results in the targeting of these fusion proteins to intracellular vacuole membranes during lumen formation. Thus, a matrix-integrin-cytoskeletal signaling axis involving both the Cdc42 and Rac1 GTPases regulates the process of EC lumen formation in three-dimensional collagen or fibrin matrices.