Focal adhesion-mediated cell anchoring and migration: from in vitro to in vivo

Abstract

Cell-extracellular matrix interactions have been studied extensively using cells cultured in vitro. These studies indicate that focal adhesion (FA)-based cell-extracellular matrix interactions are essential for cell anchoring and cell migration. Whether FAs play a similarly important role in vivo is less clear. Here, we summarize the formation and function of FAs in cultured cells and review how FAs transmit and sense force in vitro. Using examples from animal studies, we also describe the role of FAs in cell anchoring during morphogenetic movements and cell migration in vivo. Finally, we conclude by discussing similarities and differences in how FAs function in vitro and in vivo.

Keywords: Cell adhesion; Cell migration; Focal adhesion; Integrin.

Fig. 1.

Overview of focal adhesions. In culture, cells form focal adhesions in order to adhere to their substrate. This adhesion is mediated by integrins, which form a bridge between the extracellular matrix (ECM) outside the cell and the actomyosin network inside the cell, acting via various adaptor proteins.

Fig. 2.

Properties of nascent adhesions and focal adhesions. In a migrating cell in culture, nascent adhesions form at the leading edge of the cell. Nascent adhesions are small, short-lived adhesion complexes that do not require myosin II activity. Some nascent adhesions grow in size, recruit additional proteins and mature into focal adhesions in a myosin II-dependent manner.

Fig. 3.

Molecular clutch model. The binding of integrin to the ECM recruits talin and couples it to the retrograde flowing actin network. An increase in tension across talin exposes vinculin-binding sites in talin and recruits vinculin. Vinculin couples talin to the actin network, strengthens the bridge between the ECM and the actin network, and slows retrograde actin flow.

Fig. 4.

Focal adhesion-mediated cell anchorage in vivo. (A) Optic cup morphogenesis in fish (zebrafish and medaka). The basal sides of retinal neuroepithelial cells in the optic vesicle form focal adhesion-like clusters and attach to the ECM. Myosin II-mediated constriction of the basal cell sides indents the epithelium and induces its cup shape-like form (bottom left). Defects in cell-ECM interactions block this process (bottom right). (B) Drosophila dorsal closure. Focal adhesion proteins localize to the basal sides of amnioserosa cells and epidermal cells. To close the gap in the epidermis, amnioserosa cells use myosin II to constrict their apical sides. This pulls the two sides of the epidermis towards each other. Focal adhesion-like structures on the basal cell sides tether the tissue to the yolk cell (bottom left). Without these tethers, the amnioserosa separates from the yolk cell and the epidermis (bottom right). Dashed line indicates the plane shown in the cross-sections below. (C) Myotendinous junction formation in Drosophila flight muscles. Muscle cells and tendon cells adhere to each other by binding to the ECM via integrins. Defective cell-ECM adhesion results in rupture of the muscle-tendon connection. (D) Wing formation in Drosophila. The fly wing forms via outgrowth and apposition of two epithelia from the initially planar epithelium of the wing disc (left). The apposed epithelia adhere initially and then separate again and only stay connected to each other via long protrusions. These long protrusions then pull the two epithelia together for final adhesion and re-apposition of the wing. Failure of the epithelia to adhere for initial or final apposition leads to wing blisters. BM, basement membrane.

Fig. 5.

The role of focal adhesions in cell migration in vivo. (A) Distal tip cell migration in C. elegans. The two distal tip cells are located at each end of the gonad and migrate along a U-shaped route through the embryo (top). During their migration, distal tip cells express ina-1 and pat-2, the sole two α integrin subunits found in worms. These two α integrin subunits serve different functions: ina-1 is required for the generation of traction and pat-2 for pathfinding. Therefore, distal tip cells lacking ina-1 are slowed down and do not reach their target (middle) whereas distal tip cells lacking pat-2 are less directional and veer off their migratory course (bottom). BM, basement membrane. (B) Enteric neural crest cell (ENCC) migration in mice. A large population of ENCCs (top, yellow cells indicated by thick red arrows; the location of the smaller population is indicated by the thin red arrow) migrates from the dorsal side of the neural tube to the foregut, then to the hindgut. By embryonic day (E) 12.5 (middle) this population colonizes the entire length of the gut. Depletion of integrin β1 from ENCCs (e.g. in Itgb1^−/− embryos; bottom) results in abnormal migration and a premature stop around the cecum, the junction between the fore- and hindgut. Black arrows indicate direction of migration. (C) Hemocyte migration in fly embryos. Hemocytes (yellow) rely on integrin-mediated adhesion for efficient migration but not for directionality. Because they are slowed in the absence of integrin activity, hemocytes fail to spread through the embryo in myospheroid (mys/integrin beta) mutants and extend more filopodia than in wild-type embryos. (D) Dendritic cell migration in mice. Focal adhesion-mediated cell-ECM adhesion is required for dendritic cells to extravasate from blood vessels (BV; top). By contrast, interstitial migration in the dermis, entry into lymphatic vessels (LV) and migration in the lymph node (LN) do not require integrins; as such, these processes are unaffected in integrin a_v^flox/flox, integrin b₁^flox/flox, integrin b₂^−/−, integrin b₇^−/−, Mx1:Cre^+/− or Tln1^−/− mutants whereas extravasion is blocked (bottom).