Interactions of primary fibroblasts and keratinocytes with extracellular matrix proteins: contribution of alpha2beta1 integrin.

The alpha2beta1 integrin is a collagen-binding protein with very high affinity for collagen I. It also binds several other collagens and laminins and it is expressed by many cells, including keratinocytes and fibroblasts in the skin. In the past, alpha2beta1 integrin was suggested to be responsible for cell attachment, spreading and migration on monomeric collagen I and contraction of three-dimensional collagen lattices. In view of these functions, normal development and fertility in integrin alpha2-deficient mice, which we generated by targeting the integrin alpha2 gene, came as a surprise. This suggested the existence of compensatory mechanisms that we investigate here using primary fibroblasts and keratinocytes isolated from wild-type and alpha2-deficient mice, antibodies blocking integrin function and downregulation of integrin alpha2 expression. The results show that the alpha2beta1 integrin is absolutely required for keratinocyte adhesion to collagens whereas for fibroblasts other collagen-binding integrins partially back-up the lack of alpha2beta1 in simple adhesion to collagen monomers. A prominent requirement for alpha2beta1 integrins became apparent when fibroblasts executed mechanical tasks of high complexity in three-dimensional surroundings, such as contracting free-floating collagen gels and developing isometric forces in tethered lattices. The deficits observed for alpha2-deficient fibroblasts appeared to be linked to alterations in the distribution of force-bearing focal adhesions and deregulation of Rho-GTPase activation.