A key function for alphav containing integrins in mesodermal cell migration during Pleurodeles waltl gastrulation

During cleavage of Pleurodeles waltl amphibian embryos, inner cells of the blastocoel roof (presumptive ectodermal and mesodermal cells) organize a fibrillar extracellular matrix (ECM) containing fibronectin on their basal surface by a beta1-integrin-dependent process. This matrix is used as a migratory substrate by mesodermal cells during gastrulation. While alpha5beta1 integrin is expressed on both ectodermal and mesodermal cell surface, we have shown previously that alphav containing integrins are essentially restricted to the surface of mesodermal cells (Alfandari, D., Whittaker, C. A., DeSimone, D. W., and Darribère, T., Dev. Biol. 170, 249-261, 1995). To investigate the function of alphav integrins during gastrulation, we have generated a function blocking antibody directed against the extracellular domain of the Pleurodeles integrin alphav subunit. The antibody did not prevent fibronectin fibril formation, whereas an antibody against the alpha5beta1 integrin did. When injected into the blastocoel, the antibody against integrin alphav subunit perturbed gastrulation and further development in a stage-dependent manner. Developmental defects were correlated to an abnormal positioning of the mesoderm layer. In vitro, the antibody blocked spreading of mesodermal cell to fibronectin or blastocoel roof ECM but not their attachment. In contrast, the antibody directed against the alpha5beta1 integrin inhibited both cell attachment and spreading to the same substrates. We propose that the alpha5beta1 integrin is required for fibronectin assembly into fibrils and mesodermal cell attachment to the blastocoel roof ECM, while the alphav containing integrins are necessary for cell spreading, and possibly migration, on this complex network.