Actin-Based Adhesion Modules Mediate Cell Interactions with the Extracellular Matrix and Neighboring Cells

Abstract

Cell adhesions link cells to the extracellular matrix (ECM) and to each other and depend on interactions with the actin cytoskeleton. Both cell-ECM and cell-cell adhesion sites contain discrete, yet overlapping, functional modules. These modules establish physical associations with the actin cytoskeleton, locally modulate actin organization and dynamics, and trigger intracellular signaling pathways. Interplay between these modules generates distinct actin architectures that underlie different stages, types, and functions of cell-ECM and cell-cell adhesions. Actomyosin contractility is required to generate mature, stable adhesions, as well as to sense and translate the mechanical properties of the cellular environment into changes in cell organization and behavior. Here, we review the organization and function of different adhesion modules and how they interact with the actin cytoskeleton. We highlight the molecular mechanisms of mechanotransduction in adhesions and how adhesion molecules mediate cross talk between cell-ECM and cell-cell adhesion sites.

Figure 1.

Organization of functional modules at cell adhesions. (A) Cell–ECM and cell–cell adhesions are compartmentalized in modules with distinct functional properties. Physical linkage between the actin cytoskeleton (gray) and transmembrane adhesion receptors (orange) is mediated by the actin linkage module (cyan), which contains actin-binding molecules. Regulation of this linkage and local actin organization is mediated by components of the actin regulatory (blue) and signaling (yellow) modules. (B) Molecular interactions of proteins in the different adhesion modules between the actin cytoskeleton (gray) and adhesion receptors (orange). Functional modules at both adhesion sites are spatially segregated (with partial overlap) and, in cell–ECM adhesions (left), are organized within vertical organizational strata between the plasma membrane and the actin cytoskeleton. Arp, Actin-related protein; ECM, extracellular matrix; EPLIN, “epithelial protein lost in neoplasm” (LIMA1); FAK, focal adhesion kinase; ILK, integrin-linked kinase; SFK, Src-family kinases.

Figure 2.

Actin cytoskeleton architecture dictates the organization of cell–ECM adhesion. Diffraction-limited nascent adhesions emerge within the dendritic actin network, which is promoted by the action of the Arp2/3 complex in the lamellipodium of motile cells. Nascent adhesions are precursors for larger focal complexes (∼1 µm) located along the lamellipodium–lamellum interface and elongated focal adhesions (FAs) (1–5 µm) that are associated with actin filament bundles in the lamellum. The size and stability of actin filament bundles correlates with the size of FAs—small actin filament bundles located at the leading edge of the lamellum (associated with α-actinin and myosin IIA) correlate with small FAs, whereas large, stable (myosin IIB decorated) actin fibers along the center and rear of the cell promote large FAs. Transverse actin arcs that connect actin filaments and fibers presumably form in the front of the lamellipodium when myosin IIA emerges and bundles actin fibers in an arc that then recedes backward and couples with nascent adhesions as the edge protrudes. This arc eventually joins other transverse arcs in the lamellum.

Figure 3.

Stages of cell–cell contact formation coincide with distinct organizations of the actin cytoskeleton, molecular compositions, and directions of forces. (A) Nascent cell–cell adhesions are connected to radial actin bundles lying perpendicular to sites of contact. These sites contain high levels of active Rac1 and Src, and actin-regulatory proteins (Arp2/3, cortactin, VASP, formin-1) that cooperate to generate a branched actin network required for contact expansion. α-catenin is under tension, which results in association with actin, directly through a catch–bond interaction, and vinculin. (B) At mature adhesions, the peri-junctional actin cytoskeleton is rearranged into bundles parallel to the plasma membrane. RhoA activity increases, which activates myosin II, thereby generating tensile forces required for adhesion maturation. Locally high concentrations of α-catenin presumably recruit actin binding and bundling proteins (EPLIN and α-actinin), and α-catenin itself forms homodimers that bind and bundle actin. EPLIN, epithelial protein lost in neoplasm (LIMA1); MLCK, myosin light-chain kinase; VASP, vasodilator-stimulated phosphoprotein.

Figure 4.

Mechanisms of mechanotransduction in adhesions. (A) Multiple key components within adhesions mediate force sensing and transmission. This is through force-induced conformational changes that can expose cryptic sites containing residues that can become phosphorylated or provide binding sites for other proteins. In addition, a number of proteins in adhesion complexes interact through a catch bond–binding mechanism in which force increases the bond lifetime, as reported for homotypic trans binding between cadherin extracellular domains and α-catenin–actin interactions. (B) Force sensing and transmission at cell–ECM and cell–cell adhesions converges on myosin II activity. Cell adhesions initiate signaling cascades that activate members of the Rho family of GTPases (RhoA, Cdc42), among other proteins, which activate myosin II. Actomyosin contractility results in traction forces that are transmitted through adhesion complexes to the ECM or neighboring cells—this is commonly referred to as “mechanotransduction.” Furthermore, adhesions sense mechanical forces in their microenvironment (“mechanosensing”), such as substrate stiffness and pulling forces from neighboring cells, and translate those forces into biochemical signals that, in turn, activate myosin II to regulate the forces transmitted through adhesions. Both mechanotransduction and mechanosensing occur in a dynamic, tightly regulated feedback loop mediated by myosin II activity and actin organization. ECM, extracellular matrix.

Figure 5.

Schematic diagram of the states that comprise the “molecular clutch” model at cell–ECM adhesions. (A) In an unengaged clutch, the actin cytoskeleton is uncoupled from the cell membrane and integrin adhesion receptors and manifests as fast actin retrograde flux and low traction transmitted to the ECM. The dissociation between the actin cytoskeleton and the membrane is mediated by adhesion molecules within the actin linkage domain (cyan) that could either be associated with actin and the actin-regulatory module (blue) (1), integrin and the signaling module (yellow) (2), or integrin and the ECM (3). (B) In a fully engaged clutch, strong coupling occurs between the actin cytoskeleton, integrin, and various intracellular adhesion components. This presents as fast actin retrograde flows and high forces transmitted to the ECM. However, the differential coupling of the retrograde speeds of adhesion components with actin or integrin indicates that an intermediate and partial clutch engagement exists (C). This arises from molecular slippage that occurs at the actin linkage module level between the actin regulatory domain (1), the signaling domain (2), or integrin–ECM binding (3), and it presents as intermediate actin retrograde speeds and traction forces. The level of clutch engagement correlates with protrusion, as indicated in the diagram.

Figure 6.

Fluorescent images of an osteosarcoma U2OS cell expressing GFP–vinculin and mCh–talin showing colocalization at cell–ECM adhesions, and Madin–Darby canine kidney epithelial (MDCK) cells immunostained for E-cadherin and β-catenin showing their colocalization at cell–cell adhesions.