3. Actin Cytoskeleton
The crawling movement of cells is driven by the continuous reorganization and turnover of the actin cytoskeleton. Two abilities of actin filaments are exploited by the cell in order to move: the ability to push by polymerization and the ability to contract by interacting with myosin. Actin polymerization drives the extension of sheet-like and rod-like protrusions at the cell front, termed respectively lamellipodia and filopodia. Behind the protruding front actin interacts with myosin to form contractile arrays that drive the translocation of the trailing cell body.
Figure 3-1: Schematic representation of the actin cytoskeleton in a polarised fibroblast. The different organisational forms of actin filaments are depicted: diagonal actin filament meshwork in the lamellipodium, with associated radial bundles (projecting filopodium and non-projecting microspike); contractile bundles of actin (stress fiber) in the cell body and at the cell edge; and a loose actin network throughout the cell. Arc-shaped bundles are sometimes observed that move inwards under the dorsal cell surface (arc). The diagram shows an idealized cell; in reality the actin arrays are interconnected in various combinations and geometries. Sites of adhesion of the cell with the substrate are also indicated, in red. The flat region behind the lamellipodium and in front of the nucleus (N) has been termed the lamella. At the cell front, in lamellipodia and filopodia, actin filaments are all polarized in one direction, with their fast polymerizing ends forwards (for pushing); in the body of the cytoskeleton, actin filaments form bipolar assemblies with myosin to form contractile arrays (for retracting).Click on picture to enlarge
Figure 3-2. Actin cytoskeleton of different cell types. A, Fish keratocyte visualized by labelling actin with fluorescent phalloidin. B, Video sequence of a fish fibroblast expressing mCherry-actin (red) and myosin light chain (green). Myosin serves with actin in retraction of the cell body and is mainly excluded from the protruding lamellipodia and filopodia. Bar, 10µm. C, Video sequence of a B16 melanoma cell expressing GFP-actin. C, courtesy of Klemens Rottner
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The different parts of the actin cytoskeleton can be visualised in the electron microscope after extracting cells with detergents to remove membranes, organelles and soluble components of the cytoplasm (Figures 3-3A-D). In the examples shown the cytoskeleton filaments have been contrasted by the negative staining method (Small and Celis, 1978a; Small, 1981; Small et al., 1982).
Figure 3-3A: Electron micrograph of a fibroblast that was grown on a thin, plastic support film and extracted with detergent to reveal the cytoskeleton. Parts of the actin cytoskeleton corresponding to Fig. 3-1 are marked: Lp, lamellipodium; Fp, filopodium; SF, stress fibre.
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Figures 3-3B-D: Details of different parts of the actin cytoskeleton in a fibroblast prepared as in Fig. 3-3A. The cytoskeleton was dried in a heavy metal stain to reveal filaments in negative contrast. B and C show examples of the actin filament network in the lamellipodium, with an included filopodium in C. D shows a region deeper in the cytoplasm that includes stress fibre bundles of actin (a), a background network of actin filaments, microtubules (mt) and intermediate filaments (if).
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Many different proteins associate with actin and are generally classified as actin binding-, actin associated- or actin modulating-proteins. They play key roles in determining the organisation of actin filaments into the different organisational forms found in the actin cytoskeleton.