Junctional adhesion molecule-A (JAM-A) is a member of the immunoglobulin family with diverse functions in epithelial cells, including cell migration, cell contact maturation, and tight junction formation. In endothelial cells, JAM-A has been implicated in basic fibroblast growth factor (bFGF)-regulated angiogenesis through incompletely understood mechanisms. In this paper, we identify tetraspanin CD9 as novel binding partner for JAM-A in endothelial cells. CD9 acts as scaffold and assembles a ternary JAM-A-CD9-αvβ3 integrin complex from which JAM-A is released upon bFGF stimulation. CD9 interacts predominantly with monomeric JAM-A, which suggests that bFGF induces signaling by triggering JAM-A dimerization. Among the two vitronectin receptors, αvβ3 and αvβ5 integrin, which have been shown to cooperate during angiogenic signaling with bFGF and vascular endothelial growth factor (VEGF), respectively, CD9 links JAM-A specifically to αvβ3 integrin. In line with this, knockdown of CD9 blocks bFGF- but not VEGF-induced ERK1/2 activation. JAM-A or CD9 knockdown impairs endothelial cell migration and tube formation. Our findings indicate that CD9 incorporates monomeric JAM-A into a complex with αvβ3 integrin, which responds to bFGF stimulation by JAM-A release to regulate mitogen-activated protein kinase (MAPK) activation, endothelial cell migration, and angiogenesis. The data also provide new mechanistic insights into the cooperativity between bFGF and αvβ3 integrin during angiogenic signaling.
Essential Role of Junctional Adhesion Molecule-1 in Basic Fibroblast Growth Factor–Induced Endothelial Cell Migration
