The L1 adhesion molecule plays an important role in axon guidance and cell migration in the nervous system. L1 is also expressed by many human carcinomas. In addition to cell surface expression, the L1 ectodomain can be released by a metalloproteinase, but the biological function of this process is unknown. Here we demonstrate that membrane-proximal cleavage of L1 can be detected in tumors and in the developing mouse brain. The shedding of L1 involved a disintegrin and metalloproteinase (ADAM)10, as transfection with dominant-negative ADAM10 completely abolishes L1 release. L1-transfected CHO cells (L1-CHO) showed enhanced haptotactic migration on fibronectin and laminin, which was blocked by antibodies to αvβ5 and L1. Migration of L1-CHO cells, but not the basal migration of CHO cells, was blocked by a metalloproteinase inhibitor, indicating a role for L1 shedding in the migration process. CHO and metalloproteinase-inhibited L1-CHO cells were stimulated to migrate by soluble L1-Fc protein. The induction of migration was blocked by αvβ5-specific antibodies and required Arg-Gly-Asp sites in L1. A 150-kD L1 fragment released by plasmin could also stimulate CHO cell migration. We propose that ectodomain-released L1 promotes migration by autocrine/paracrine stimulation via αvβ5. This regulatory loop could be relevant for migratory processes under physiological and pathophysiological conditions.

The ectodomain of certain transmembrane molecules can be released by proteolysis, and the solubilized antigens often exert important biological functions. We demonstrated before that the L1 adhesion molecule is shed from the cell surface. Here we show that L1 release in AR breast carcinoma cells is mediated by a member of the disintegrin metalloproteinase (ADAM) family of proteinases. Up-regulation of L1 shedding by phorbol ester or pervanadate involved distinct mechanisms. Pervanadate induced shedding and rounding-up of cells from the substrate, which was blocked by the Src kinase inhibitor PP2. Tyr phosphorylation of the L1 cytoplasmic tail and the Src kinase Fyn was observed following pervanadate treatment. Up-regulation of L1 release and activation of Fyn occurred also when cells were detached by EDTA suggesting that the regulation of L1 shedding by this pathway was linked to cell morphology and adhesion. The phorbol 12-myristate 13-acetate-induced shedding was inhibited by the protein kinase C inhibitor bisindolylmaleimide I and by PD98059, a specific inhibitor of the mitogen-activated protein kinase pathway. Soluble L1 binds to the proteoglycan neurocan and in bound form could support integrin-mediated cell adhesion and migration. We propose that the release of cell-associated adhesion molecules such as L1 may be relevant to promote cell migration.Many transmembrane proteins can undergo cleavage and release of their ectodomain into the medium (for review see Refs. 1-3). These proteins are diverse in structure and function and comprise molecules such as TNF-␣ 1 (4 -8), FasL (9), interleukin-6 receptor (10, 11), L-selectin (12-14), pro-TGF-␣, and the ␤-amyloid precursor protein (15-17). Although there is evidence for a physiological function of many released molecules, a general significance for ectodomain shedding is still disputed (2). Recently, members of the ADAM metalloproteinase family, which are membrane proteins composed of a disintegrin and metalloproteinase domain, were shown to be important in ectodomain release (for review see Refs. 3 and 18). TACE/ADAM17 mediates the membrane release of TNF-␣, Lselectin, TGF-␣ (8), and TRANCE (tumor necrosis factor-related activation-induced cytokine), a TNF family member involved in osteoclastogenesis and dendritic cell survival (19). ADAM10/Kuz has been described as an ␣-secretase for the cleavage of ␤-amyloid precursor protein (20), and ADAM9 is involved in the phorbol ester-induced ectodomain shedding of membrane-anchored heparin-binding EGF-like growth factor (HB-EGF) (21, 22). Certain released molecules can be cleaved by more than one enzyme, and some enzymes can cleave more than one substrate. For example, TNF-␣ cleavage can also be mediated by ADAM10 (23), and ␣-secretase activity for ␤-amyloid precursor protein has been attributed to TACE/ADAM17 (24) and ADAM9 (25). It is not known how the protease(s) select their substrate, because consensus cleavage sites have not been identified. In addition to the proteolytic function, some members of the ADAM famil...

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 ␣5␤1-or ␣v␤3-mediated integrin binding and that both RGD sites are essential. Cooperation of RGD sites with neighboring domains are necessary for ␣ 5 ␤ 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.