For a cell to move, it must adhere to a substrate and exert traction. Adhesion occurs at specific foci at which the actin cytoskeleton on the inside of the cell is linked via transmembrane receptors (integrins) to the extracellular matrix on the outside. These adhesion sites are composed of complexes of more than 50 different proteins (Geiger et al., 2009), including structural, signaling and adaptor molecules:
Highly simplified schematic illustration of the organisation of a focal adhesion. Transmembrane integrins (alpha/beta) bind to matrix ligands on the outside of the cell, and to a complex of molecules inside the cell that link to actin filaments. At focal adhesions, the actin filaments are bundled by actin filament cross-linkers, including the contractile protein myosin. Tension in the bundle, generated by myosin, is required to maintain the clustering of integrins and the integrity of focal adhesions (Burridge and Chrzanowska-Wodnicka, 1996).
Adhesion foci can be visualised in living cells by tagging single proteins belonging to the adhesion complex with a fluorescent probe. The adhesion sites are initiated under lamellipodia and filopodia as focal complexes (see figure in Rho-GTPases). Focal complexes can either form and dissolve, with a lifetime of around 1 — 2mins, or can persist and differentiate into larger focal adhesions (see Rho-GTPases). Examples of this differentiation from focal complexes to focal adhesions is shown in the videos of the next two figures. Focal complexes and focal adhesions formed at the advancing cell front remain stationary, relative to the substrate.
The formation of substrate adhesion sites in a migrating goldfish fibroblast. The cell was transfected with GFP-actin (green) and microinjected with rhodamine-tagged vinculin (an adhesion component; red). The protruding cell front is marked by a diffuse band of actin filaments (the lamellipodium), which contains radial filament bundles (filopodia) that project beyond the cell edge. Different types of adhesion foci (red) can be distiguished: small foci in association with lamellipodia and filopodia (focal complexes) and, behind the lamellipodium, larger foci associated with actin filament bundles (focal adhesions). Focal adhesions are also observed at the periphery of retracting cell edges (bottom region of figure). Focal complexes and focal adhesions in the advancing front remain stationary, relative to the substrate, whereas, focal adhesions at the retracting edges can slide. (Video was produced by Olga Krylyshkina: from Small et al., 2002).
A highly enlarged detail of the video in the figure above, showing the origination of adhesion foci (focal complexes) in association with lamellipodia and filopodia.
Development of early adhesion sites underneath filopodia in a fish fibroblast transfected with mCherry-actin (red) and GFP-paxillin (another adhesion protein; green). (Nemethova et al, JCB 2008)
If protrusion of a front ceases, the cell edge retracts to the level of the outermost focal adhesions. Further retraction results in the sliding and eventual detachment of these adhesion sites. This is the scenario at the rear and flanks of a migrating cell (Rid et al., 2005):
Fish fibroblast transfected with GFP-zyxin, showing sliding focal adhesions
Different cell types use different adhesion strategies to move. The faster moving cell types show a higher proportion of focal complexes as compared to focal adhesions. Examples of alternative adhesion strategies are shown for a mouse melanoma cell and for a fish keratocyte in the following figures:
Adhesion dynamics at the rapidly migrating front of a B16 mouse melanoma cell moving on laminin. The cell was transfected with GFP-VASP, which is recruited to adhesion foci, as well as to the very tip of advancing lamellipodia. Focal complexes are formed behind the cell front that turnover within 1-2mins. Very few develop into focal adhesions (see top right at stationary cell edge). (Rottner et al., 1999)
Adhesion dynamics in a migrating fish keratocyte. Short-lived focal complexes form under the advancing lamellipodium. Larger, sliding adhesions, similar to focal adhesions, are formed at retracting edges. These latter adhesions are however short-lived, since the cell traverses its own length within 2–3 mins. Video courtesy of Kurt Anderson (Anderson and Cross, 2000).
- Anderson, K. I., Cross, R. (2000). Contact dynamics during keratocyte motility. Curr. Biol. 10, 253–260.
- Burridge, K., Chrzanowska-Wodnicka, M. (1996). Focal adhesions, contractility, and signaling. Annu Rev Cell Dev Biol. 12, 463–518.
- Geiger, B., Spatz, J. P., Bershadsky A. D. (2009). Environmental sensing through focal adhesions. Nat Rev Mol Cell Biol. 10, 121–133.
- Nemethova, M., Auinger, S., Small, J. V. (2008). Building the actin cytoskeleton: filopodia contribute to the construction of contractile bundles in the lamella. J Cell Biol. 180, 1233–1244.
- Rid R., Schiefermeier N., Grigoriev I., Small J. V., Kaverina I. (2005). The last but not the least: the origin and significance of trailing adhesions in fibroblastic cells. Cell Motil Cytoskeleton. 61, 161–71.
- Rottner, K., Behrendt, B., Small, J. V., Wehland, J. (1999). VASP dynamics during lamellipodia protrusion. Nat. Cell. Biol. 1, 321–322.
- Small, J.V., Geiger, B., Kaverina, I., Bershadsky, A. (2002). How do microtubules guide migrating cells? Nat. Rev. Mol. Cell Biol. 3, 957–64.