INTEGRINS IN POINT CONTACTS MEDIATE CELL SPREADING - FACTORS THAT REGULATE INTEGRIN ACCUMULATION IN POINT CONTACTS VS FOCAL CONTACTS

We have studied the function and distribution of the alpha1beta1, alpha5beta1 and alpha6beta1 heterodimers on type-1 astrocytes with antibodies specific for integrin subunits alpha1, alpha5, alpha6, and beta1). The alpha1beta1 heterodimer mediates adhesion to laminin and collagen, the alpha5beta1 to fibronectin in an RGD-dependent manner. The alpha5beta1 integrin is found in focal contacts in long-term cultures of well-spread astrocytes colocalizing with vinculin and the termini of actin stress fibers. Alpha1beta1 heterodimers can occasionally be found as small aggregates within focal contacts but they do not accumulate there. Instead, alpha1beta1 integrins are found in punctate deposits called point contacts which are distributed over the upper and the lower cell surfaces whether laminin, collagen, fibronectin or polylysine is used as a substratum. Unlike focal contacts, point contacts contain clathrin but rarely codistribute with actin or vinculin. Two observations indicate that these point contacts are functional. First, mAb 3A3, directed against the rat alpha1 subunit, inhibits the attachment of astrocytes to laminin and collagen. Second, during the spreading of astrocytes, a band of point contacts forms around the cell perimeter at a time when no focal contacts are visible. While alpha1beta1 integrins are found only in point contacts in astrocytes, the alpha6beta1 integrin, another laminin receptor, is localized within focal contacts. Moreover, alpha1beta1 heterodimers accumulate in focal contacts in fibroblasts. Thus, the alpha subunit contributes, independent of its ligand, to functional integrin heterodimer accumulation in focal contacts or in point contacts. This accumulation varies among different cell types with apparently identical heterodimers as well as with the motile state (spreading vs. flattened) of the same cells.