David R. Critchley

Rapid leukocyte migration by integrin-independent flowing and squeezing

All metazoan cells carry transmembrane receptors of the integrin family, which couple the contractile force of the actomyosin cytoskeleton to the extracellular environment. In agreement with this principle, rapidly migrating leukocytes use integrin-mediated adhesion when moving over two-dimensional surfaces. As migration on two-dimensional substrates naturally overemphasizes the role of adhesion, the contribution of integrins during three-dimensional movement of leukocytes within tissues has remained controversial. We studied the interplay between adhesive, contractile and protrusive forces during interstitial leukocyte chemotaxis in vivo and in vitro. We ablated all integrin heterodimers from murine leukocytes, and show here that functional integrins do not contribute to migration in three-dimensional environments. Instead, these cells migrate by the sole force of actin-network expansion, which promotes protrusive flowing of the leading edge. Myosin II-dependent contraction is only required on passage through narrow gaps, where a squeezing contraction of the trailing edge propels the rigid nucleus.

Focal adhesions - the cytoskeletal connection.

Cellular contacts with the extracellular matrix are regulated by the Rho family of GTPases through their effects on both the actin and the microtubule cytoarchitecture. Recent genetic, biochemical and structural data have highlighted the role played by a subset of actin-binding proteins in coupling integrins to cytoskeletal actin and in assembling signalling complexes that are important for cell motility and cell proliferation.

Structural determinants of integrin recognition by talin.

The binding of cytoplasmic proteins, such as talin, to the cytoplasmic domains of integrin adhesion receptors mediates bidirectional signal transduction. Here we report the crystal structure of the principal integrin binding and activating fragment of talin, alone and in complex with fragments of the beta 3 integrin tail. The FERM (four point one, ezrin, radixin, and moesin) domain of talin engages integrins via a novel variant of the canonical phosphotyrosine binding (PTB) domain-NPxY ligand interaction that may be a prototype for FERM domain recognition of transmembrane receptors. In combination with NMR and mutational analysis, our studies reveal the critical interacting elements of both talin and the integrin beta 3 tail, providing structural paradigms for integrin linkage to the cell interior.

Calpain-mediated proteolysis of talin regulates adhesion dynamics

Dynamic regulation of adhesion complexes is required for cell migration and has therefore emerged as a key issue in the study of cell motility. Recent progress has been made in defining some of the molecular mechanisms by which adhesion disassembly is regulated, including the contributions of adhesion adaptor proteins and tyrosine kinases. However, little is known about the potential contribution of proteolytic mechanisms to the regulation of adhesion complex dynamics. Here, we show that proteolysis of talin by the intracellular calcium-dependent protease calpain is critical for focal adhesion disassembly. We have generated a single point mutation in talin that renders it resistant to proteolysis by calpain. Quantification of adhesion assembly and disassembly rates demonstrates that calpain-mediated talin proteolysis is a rate-limiting step during adhesion turnover. Furthermore, we demonstrate that disassembly of other adhesion components, including paxillin, vinculin and zyxin, is also dependent on the ability of calpain to cleave talin, suggesting a general role for talin proteolysis in regulating adhesion turnover. Together, these findings identify calpain-mediated proteolysis of talin as a mechanism by which adhesion dynamics are regulated.

Two-piconewton slip bond between fibronectin and the cytoskeleton depends on talin

Mechanical forces on matrix–integrin–cytoskeleton linkages are crucial for cell viability, morphology and organ function. The production of force depends on the molecular connections from extracellular-matrix–integrin complexes to the cytoskeleton. The minimal matrix complex causing integrin–cytoskeleton connections is a trimer of fibronectins integrin-binding domain FNIII7-10 (ref. 4). Here we report a specific, molecular slip bond that was broken repeatedly by a force of 2 pN at the cellular loading rate of 60 nm s-1; this occurred with single trimer beads but not with monomer. Talin1, which binds to both integrins and actin filaments in vitro, is required for the 2-pN slip bond and rapid cytoskeleton binding. Further, inhibition of fibronectin binding to αvβ3 and deletion of β3 markedly decreases the 2-pN force peak. We suggest that talin1 initially forms a molecular slip bond between closely packed fibronectin–integrin complexes and the actin cytoskeleton, which can apply a low level of force to fibronectin until many bonds form or a signal is received to activate a force response.

Talin depletion reveals independence of initial cell spreading from integrin activation and traction

Cell spreading, adhesion and remodelling of the extracellular matrix (ECM) involve bi-directional signalling and physical linkages between the ECM, integrins and the cell cytoskeleton. The actin-binding proteins talin1 and 2 link ligand-bound integrins to the actin cytoskeleton and increase the affinity of integrin for the ECM. Here we report that depletion of talin2 in talin1-null (talin1−/−) cells did not affect the initiation of matrix-activated spreading or Src family kinase (SFK) activation, but abolished the ECM–integrin–cytoskeleton linkage and sustained cell spreading and adhesion. Specifically, focal adhesion assembly, focal adhesion kinase (FAK) signalling and traction force generation on substrates were severely affected. The talin1 head domain restored β1 integrin activation but only full-length talin1 restored the ECM–cytoskeleton linkage and normal cytoskeleton organization. Our results demonstrate three biochemically distinct steps in fibronectin-activated cell spreading and adhesion: 1) fibronectin–integrin binding and initiation of spreading, 2) fast cell spreading and 3) focal adhesion formation and substrate traction. We suggest that talin is not required for initial cell spreading. However, talin provides the important mechanical linkage between ligand-bound integrins and the actin cytoskeleton required to catalyse focal adhesion-dependent pathways.

The structure and function of α-actinin

OE-Actinin is an actin binding and crosslinking protein found in both muscle and non-muscle cells at points where actin is anchored to a variety of intracellular structures. It is a dimer, probably a homodimer, with a subunit molecular weight of 94-103 kDa (Endo & Masaki, 1982; Landon et al., 1985; Baron et al., 1987a), the subunits being anti-parallel in orientation (Wallraff et al., 1986; Imamura et al., 1988). It is visualized as a long rod-shaped molecule in the electron microscope, 3-4nm wide by 30-40nm in length (Podlubnaya et al., 1975; Bretscher et al., 1979; Imamura et al., 1988). A number of distinct isoforms of 0c-actinin have been characterized including the skeletal and smooth muscle isoforms (Endo & Masaki, 1982), and the non-muscle isoforms isolated from brain (D~ihaiman & Bamburg, 1984), macrophages (Bennett et al., 1984), platelets (Landon et al., 1985) and cultured fibroblasts (Burridge & Feramisco, 1981). The only clear functional difference between these isoforms is that binding of the various nonmuscle ~-actinins to actin is calcium sensitive, whereas binding of the muscle isoforms is calcium insensitive (Burridge & Feramisco, 1981).

Structural basis for vinculin activation at sites of cell adhesion

Vinculin is a highly conserved intracellular protein with a crucial role in the maintenance and regulation of cell adhesion and migration. In the cytosol, vinculin adopts a default autoinhibited conformation. On recruitment to cell–cell and cell–matrix adherens-type junctions, vinculin becomes activated and mediates various protein–protein interactions that regulate the links between F-actin and the cadherin and integrin families of cell-adhesion molecules. Here we describe the crystal structure of the full-length vinculin molecule (1,066 amino acids), which shows a five-domain autoinhibited conformation in which the carboxy-terminal tail domain is held pincer-like by the vinculin head, and ligand binding is regulated both sterically and allosterically. We show that conformational changes in the head, tail and proline-rich domains are linked structurally and thermodynamically, and propose a combinatorial pathway to activation that ensures that vinculin is activated only at sites of cell adhesion when two or more of its binding partners are brought into apposition.

The structure of an integrin/talin complex reveals the basis of inside‐out signal transduction

Fundamental to cell adhesion and migration, integrins are large heterodimeric membrane proteins that uniquely mediate inside‐out signal transduction, whereby adhesion to the extracellular matrix is activated from within the cell by direct binding of talin to the cytoplasmic tail of the β integrin subunit. Here, we report the first structure of talin bound to an authentic full‐length β integrin tail. Using biophysical and whole cell measurements, we show that a specific ionic interaction between the talin F3 domain and the membrane–proximal helix of the β tail disrupts an integrin α/β salt bridge that helps maintain the integrin inactive state. Second, we identify a positively charged surface on the talin F2 domain that precisely orients talin to disrupt the heterodimeric integrin transmembrane (TM) complex. These results show key structural features that explain the ability of talin to mediate inside‐out TM signalling.

Talins and kindlins: partners in integrin-mediated adhesion

Integrin receptors provide a dynamic, tightly-regulated link between the extracellular matrix (or cellular counter-receptors) and intracellular cytoskeletal and signalling networks, enabling cells to sense and respond to their chemical and physical environment. Talins and kindlins, two families of FERM–domain proteins, bind the cytoplasmic tail of integrins, recruit cytoskeletal and signalling proteins involved in mechanotransduction and synergize to activate integrin binding to extracellular ligands. New data reveal the domain structure of full-length talin, provide insights into talin-mediated integrin activation and show that RIAM recruits talin to the plasma membrane, whereas vinculin stabilizes talin in cell–matrix junctions. How kindlins act is less well-defined, but disease-causing mutations show that kindlins are also essential for integrin activation, adhesion, cell spreading and signalling.