Integrin and neurocan binding to L1 involves distinct Ig domains

The cell adhesion molecule L1, a 200-220-kDa type I membrane glycoprotein of the Ig superfamily, mediates many neuronal processes. Originally studied in the nervous system, L1 is expressed by hematopoietic and many epithelial cells, suggesting a more expanded role. L1 supports homophilic L1-L1 and integrin-mediated cell binding and can also bind with high affinity to the neural proteoglycan neurocan; however, the binding site is unknown. We have dissected the L1 molecule and investigated the cell binding ability of Ig domains 1 and 6. We report that RGD sites in domain 6 support alpha5beta1- or alphavbeta3-mediated integrin binding and that both RGD sites are essential. Cooperation of RGD sites with neighboring domains are necessary for alpha(5)beta(1). A T cell hybridoma and activated T cells could bind to L1 in the absence of RGDs. This binding was supported by Ig domain 1 and mediated by cell surface-exposed neurocan. Lymphoid and brain-derived neurocan were structurally similar. We also present evidence that a fusion protein of the Ig 1-like domain of L1 can bind to recombinant neurocan. Our results support the notion that L1 provides distinct cell binding sites that may serve in cell-cell or cell-matrix interactions.

The L1 adhesion molecule supports alpha v beta 3-mediated migration of human tumor cells and activated T lymphocytes

The L1 adhesion molecule is a member of the immunoglobulin superfamily which is expressed by neural and hematopoietic cells. L1 is primarily a cell surface molecule but in its released form it becomes embedded in the extracellular matrix. In addition to the established L1-L1 homotypic interaction, L1 can bind to alpha v beta 3 in the human. The 6th Ig-like domain is critical for this function. We now demonstrate that a fusion protein containing the 6th Ig-like domain of L1 (6.L1-Fc) can support the migration of human MED-B1 (alpha v beta 3+) but not of Nalm-6 cells (alpha 5 beta 1+). The migration was blocked in the presence of a mab to alpha v beta 3 and was not seen on a 6.L1-Fc in which the RGD site was mutated. Activation of human T lymphocytes in the presence of PHA and PMA led to the induction of alpha v beta 3 and alpha v beta 5 expression and concomitantly induced migration of the cells on 6.L1-Fc. The migration was blocked by mabs to alpha v beta 3 but not to alpha v beta 5. Our results suggest that L1 exposed at the cell surface or as a matrix constituent can serve as a potent substrate for alpha v beta 3 mediated cell migration.