Laminin G Domains, their Receptors, and Activation of Intracellular Pathways

Sammanfattning: Basement membranes are sheet like structures underlying epithelial, endothelial, adipocytes, muscle and peripheral nerve cells. Laminins are an important family of basement membrane proteins implicated in various biological functions through their interactions with cell surface receptors. Laminins are trimers composed of a, b and g chains. Integrins, dystroglycan and other transmembrane glycoproteins such as syndecans have been identified as cell surface receptors for laminins. The regulation of these interactions is important for many biological processes including cell adhesion, proliferation, differentiation, migration, cell survival and angiogenesis. These diverse activities are likely to be mediated by receptors inducing intracellular signaling pathways. Many receptor-binding sites have been localized to the LG domains of the laminin a chains. Using recombinant proteins of laminin a5 chain, we localized integrin a3?1 and a6?1 binding sites to laminin a5LG1-3, and a-dystroglycan binding to laminin a5LG4-5. We found that recombinant laminin a4LG4-5 promoted endothelial cell adhesion through the interaction with integrin a3?1 and a6?1 but not integrin av?3. Laminin-10/11 was shown to promote epithelial cell adhesion through the interaction with both integrin a3?1 and a6?1. Integrin a6A?1 could induce laminin-1 and laminin-10/11 mediated ERK activation. The dystroglycan-binding domains of both laminin-1 and laminin-10/11 suppressed integrin a6A?1 mediated ERK activation. These results indicate opposing roles of integrin a6A?1 and a-dystroglycan in laminin mediated the ERK activation. Laminin isoforms were also found to play different roles in regulating cell adhesion, spreading, proliferation and ERK activation through the interaction with integrin a6A?4. The importance of laminin a4 chain for angiongensis was studied using both in vitro and in vivo angiogenesis model systems. Laminin a4LG4-5 inhibited endothelial cell tube formation and wound closure. It also showed an inhibitory effect on angiogenesis. Using monoclonal antibodies produced by us, we found that both intact and proteolytically processed forms of the a4 chain exist in vivo.