Regulation of fibronectin matrix assembly and capillary morphogenesis in endothelial cells by Rho family GTPases

Fibronectin (FN) fibrillogenesis is an essential biological process mediated by α5β1 integrin and cellular contractile forces. Assembly of a FN matrix by activated endothelial cells occurs during angiogenic blood vessel remodeling and signaling components that control this event represent attractive therapeutic targets. Here we examined the role of individual Rho GTPases in FN matrix remodeling by selectively attenuating their expression in cultured endothelial cells. Whereas pharmacological ablation of myosin-regulated contractility abrogated matrix assembly, no significant decrease was detected in the amount of FN deposited by RhoA, RhoB-, RhoC-, Rac1-, or Cdc42-depleted cells. Rather, distinct differences in fiber arrangement were observed. Most strikingly, RhoA silenced cells assembled a fine FN meshwork beneath α5β1 integrin-based fibrillar adhesions, in the absence of classical focal adhesions and actin stress fibers, indicating that α5β1 integrin translocation and FN fibril elongation can occur in low tension states such as those encountered by newly-forming vessels in tissue. In contrast, highly contractile Cdc42-deficient cells deposited FN globules and Rac-deficient cells assembled long arrays, reflecting their increased motility. We propose that regulation of FN scaffolds by Rho GTPase signaling impacts bidirectional communications and mechanical interactions between endothelial cells and their extracellular matrix during vascular morphogenesis.