Cell adhesion to laminin 1 or 5 induces isoform-specific clustering of integrins and other focal adhesion components.

Laminin 1 (α1β1γ1) and laminin 5 (α3β3γ2) induce cell adhesion with different involvement of integrins: both are ligands for the α6β1 integrin, while α3β1 integrin has affinity for laminin 5 only. These two laminin isoforms therefore provide good models to investigate whether α3β1 and α6β1 integrins play different roles in signal transduction and in focal adhesion formation. Laminin 1 or 5 induced adhesion of normal human skin fibroblasts to a similar extent but promoted different overall cell shapes. On laminin 1 the fibroblasts formed mainly filopodia-like structures, while on laminin 5 they developed lamellipodias. Staining of fibrillar actin with fluoresceinphalloidin revealed a similar organisation of the actin cytoskeleton on both substrates. However, integrin subunits and several cytoskeletal linker proteins, including vinculin, talin, and paxillin, showed an isoform-specific arrangement into focal adhesions. On laminin 1 they were recruited into thick and short aggregates localized at the termini of actin stress fibers, while on laminin 5 they appeared as dots or streaks clustered on a long portion of actin microfilaments. To test whether the differing affinity of laminin 1 or 5 for α3β1 integrin would explain the formation of morphologically different focal adhesions, cells were seeded on laminin 1 under conditions in which α3β1 integrins were occupied by a function-blocking antibody. This resulted in the formation of focal adhesions similar to that observed on laminin 5, where the integrin is occupied by its natural ligand. These results provide the first evidence for a crosstalk between α3β1 and α6β1 integrins and indicate that occupancy of α3β1 integrins results in a trans-dominant regulation of α6β1 integrin clustering and of focal adhesions. It suggests that recruitment of integrins and cytoskeletal linker proteins are laminin isoform-specific and that tissue specific expression of laminin isoforms might modulate cell behavior by the activation of distinct sets of integrins and by the induction of distinct molecular assemblies within the cell adhesion signaling complexes.