Endocytosis of β1 integrins is an early event in migration promoted by the cell adhesion molecule L1

Directional cell motility is a complex process requiring orchestration of signals from diverse cell adhesion receptors for proper organization of neuronal groups in the brain. The L1 cell adhesion molecule potentiates integrin-dependent migration of neuronal cells and stimulates integrin endocytosis but its mechanism of action is unclear. The hypothesis was investigated that L1 stimulates cell motility by modulating surface levels of integrins through intracellular trafficking using a model cell system. Antibody-induced clustering of L1, which mimics ligand binding, induced formation of cell surface complexes of L1 and β1 integrins in L1-expressing HEK293 cells. L1 formed cell surface complexes with integrin β1 and α3 subunits but not with integrin α1. Following cell surface clustering, β1 integrins and L1 became rapidly internalized into Rab5+ early endosomes. Internalization of L1 and β1 integrins was prevented by treatment with monodansyl cadaverine (MDC), an inhibitor of clathrin-dependent endocytosis, and by deletion of the AP2/clathrin binding motif (RSLE) from the L1 cytoplasmic domain. MDC treatment coordinately inhibited L1-potentiated haptotactic migration of HEK293 cells to fibronectin in Transwell assays. These results suggested that downregulation of adhesive complexes of L1 and β1 integrin at the plasma membrane by clathrin-mediated endocytosis is a potential mechanism for enhancing cell motility.