Muscle β1D Integrin Reinforces the Cytoskeleton–Matrix Link: Modulation of Integrin Adhesive Function by Alternative Splicing (Fragment)

Alexey M. Belkin, S. Francesco Retta. Department of Cell Biology and Anatomy, University of North Carolina at Chapel Hill, North Carolina 27599.


Expression of muscle-specific b 1D integrin with an alternatively spliced cytoplasmic domain in CHO and GD25, b 1 integrin-minus cells leads to their phenotypic conversion.B 1D-transfected nonmuscle cells display rounded morphology, lack of pseudopodial activity, retarded spreading, reduced migration, and significantly enhanced contractility compared with their b 1A-expressing counterparts. The transfected b 1D is targeted to focal adhesions and efficiently displaces the endogenous b 1A and a v b 3 integrins from the sites of cell–matrix contact. This displacement is observed on several types of extracellular matrix substrata and leads to elevated stability of focal adhesions in b 1D transfectants. Whereas a significant part of cellular b 1A integrin is extractable in digitonin, the majority of the transfected b 1D is digitonin-insoluble and is strongly associated with the detergent-insoluble cytoskeleton.

Increased interaction of b 1D integrin with the actin cytoskeleton is consistent with and might be mediated by its enhanced binding to talin. In contrast, b 1A interacts more strongly with a-actinin, than b 1D. Inside-out driven activation of the b 1D ectodomain increases ligand binding and fibronectin matrix assembly by b 1D transfectants. Phenotypic effects of b 1D integrin expression in nonmuscle cells are due to its enhanced interactions with both cytoskeletal and extracellular ligands. They parallel the transitions that muscle cells undergo during differentiation. Modulation of b 1 integrin adhesive function by alternative splicing serves as a physiological mechanism reinforcing the cytoskeleton–matrix link in muscle cells. This reflects the major role for b 1D integrin in muscle, where extremely stable association is required for contraction.

Integrins are a large family of transmembrane heterodimeric receptors that play a key role in cell adhesion to extracellular matrix (Hynes, 1992). Integrin receptors serve a dual purpose, linking extracellular matrix to the actin cytoskeleton and providing bidirectional transmission of signals between the extracellular matrix and the cytoplasm (Schwartz et al., 1995; Yamada and Miyamoto, 1995; Burridge and Chrzanowska-Wodnicka, 1996). At least two major actin-binding proteins, talin and a -actinin, are thought to interact directly with the cytoplasmic domain of several b subunits, providing a link to the actin cytoskeleton (Horwitz et al., 1986; Otey et al., 1990; Hemler et al., 1994). Among integrins, b 1 is typically the most abundant and ubiquitously expressed subunit associated with a number of subunits to form distinct heterodimers. These interact with a variety of extracellular matrix and cell adhesion molecules (Hynes, 1992). The entire structure of the b 1 integrin cytoplasmic domain is critical for integrin– cytoskeleton interaction (Hayashi et al., 1990; LaFlamme et al., 1992; Reszka et al., 1992; Ylanne et al., 1993; Lewis and Schwartz, 1995).