CELL-ADHESION TO LAMININ-1 OR LAMININ-5 INDUCES ISOFORM-SPECIFIC CLUSTERING OF INTEGRINS AND OTHER FOCAL ADHESION COMPONENTS

Laminin 1 (alpha 1 beta 1 gamma 1) and laminin 5 (alpha 3 beta 3 gamma 2) induce cell adhesion with different involvement of integrins: both are ligands for the alpha 6 beta 1 integrin, while alpha 3 beta 1 int egrin has affinity for laminin 5 only. These two laminin isoforms ther efore provide good models to investigate whether alpha 3 beta 1 and al pha 6 beta 1 integrins play different roles in signal transduction and in focal adhesion formation. Laminin 1 or 5 induced adhesion of norma l human skin fibroblasts to a similar extent but promoted different ov erall cell shapes. On laminin 1 the fibroblasts formed mainly filopodi a-like structures, while on laminin 5 they developed lamellipodias. St aining of fibrillar actin with fluorescein-phalloidin revealed a simil ar 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, whi le on laminin 5 they appeared as dots or streaks clustered on a long p ortion of actin microfilaments. To test whether the differing affinity of laminin 1 or 5 for alpha 3 beta 1 integrin would explain the forma tion of morphologically different focal adhesions, cells were seeded o n laminin 1 under conditions in which alpha 3 beta 1 integrins were oc cupied by a function-blocking antibody, This resulted in the formation of focal adhesions similar to that observed on laminin 5, where the i ntegrin is occupied by its natural ligand. These results provide the f irst evidence for a crosstalk between alpha 3 beta 1 and alpha 6 beta 1 integrins and indicate that occupancy of alpha 3 beta 1 integrins re sults in a trans-dominant regulation of alpha 6 beta 1 integrin cluste ring and of focal adhesions. It suggests that recruitment of integrins and cytoskeletal linker proteins are laminin isoform-specific and tha t tissue specific expression of laminin isoforms might modulate cell b ehavior by the activation of distinct sets of integrins and by the ind uction of distinct molecular assemblies within the cell adhesion signa ling complexes.