Integrins in point contacts mediate cell spreading: Factors that regulate integrin accumulation in point contacts vs. focal contacts

Abstract

We have studied the function and distribution of the α1β1,, α5β1 and α6β1 heterodimers on type-1 astrocytes with antibodies specific for integrin subunits (α1, α5, α6, and β1). The α1β1 heterodimer mediates adhesion to laminin and collagen, the α5β1 to fibronectin in an RGD-dependent manner. The α5β1 integrin is found in focal contacts in long-term cultures of well-spread astrocytes colocalizing with vinculin and the termini of actin stress fibers. α1β1 heterodimers can occasionally be found as small aggregates within focal contacts but they do not accumulate there. Instead, α1β1 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 α1-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 α1β1 integrins are found only in point contacts in astrocytes, the α6β1 integrin, another laminin receptor, is localized within focal contacts. Moreover, α1β1 heterodimers accumulate in focal contacts in fibroblasts. Thus, the α 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.