Abstract
Platelet endothelial cell adhesion molecule (PECAM-1) has been implicated in angiogenesis through processes that involve stimulation of endothelial cell motility. Previous studies suggest that PECAM-1 tyrosine phosphorylation mediates the recruitment and then activation of the tyrosine phosphatase SHP-2, which in turn promotes the turnover of focal adhesions and the extension of filopodia, processes critical to cell motility. While these studies have implicated PECAM-1-dependent signaling in PECAM-1-mediated cell motility, the involvement of PECAM-1 ligand binding in cell migration is undefined. Therefore to investigate the role of PECAM-1 binding interactions in cell motility, mutants of PECAM-1 were generated in which either homophilic or heparin/glycosaminoglycan (GAG)-mediated heterophilic binding had been disabled and then expressed in an endothelial cell surrogate. We found that the ability of PECAM-1 to stimulate cell migration, promote filopodia formation and trigger Cdc42 activation were lost if PECAM-1-dependent homophilic or heparin/GAG-dependent heterophilic ligand binding was disabled. We further observed that PECAM-1 concentrated at the tips of extended filopodia, an activity that was diminished if homophilic, but not heparin/GAG-mediated heterophilic binding had been disrupted. Similar patterns of activities were seen in mouse endothelial cells treated with antibodies that specifically block PECAM-1-dependent homophilic or heterophilic adhesion. Together these data provide evidence for the differential involvement of PECAM-1-ligand interactions in PECAM-1-dependent motility and the extension of filopodia.