Focal adhesion kinase regulates smooth muscle cell recruitment to the developing vasculature.

OBJECTIVE: The investment of newly formed endothelial cell tubes with differentiated smooth muscle cells (SMC) is critical for appropriate vessel formation, but the underlying mechanisms remain unknown. We previously showed that depletion of focal adhesion kinase (FAK) in the nkx2.5 expression domain led to aberrant outflow tract (OFT) morphogenesis and strove herein to determine the cell types and mechanisms involved. METHODS AND
RESULTS: We crossed fak(loxp) targeted mice with available Cre drivers to deplete FAK in OFT SMC (FAK(wnt) and FAK(nk)) or coronary SMC (FAK(cSMC)). In each case, depletion of FAK led to defective vasculogenesis that was incompatible with postnatal life. Immunohistochemical analysis of the mutant vascular structures revealed that FAK was not required for progenitor cell proliferation, survival, or differentiation into SMC but was necessary for subsequent SMC recruitment to developing vasculature. Using a novel FAK-null SMC culture model, we found that depletion of FAK did not influence SMC growth or survival, but blocked directional SMC motility and invasion toward the potent endothelial-derived chemokine, platelet-derived growth factor PDGFBB. FAK depletion resulted in unstable lamellipodial protrusions due to defective spatial-temporal activation of the small GTPase, Rac-1, and lack of Rac1-dependent recruitment of cortactin (an actin stabilizing protein) to the leading edge. Moreover, FAK null SMC exhibited a significant reduction in stimulated extracellular matrix degradation.
CONCLUSIONS: FAK drives PDGFBB-stimulated SMC chemotaxis/invasion and is essential for SMC to appropriately populate the aorticopulmonary septum and the coronary vascular plexus.