The distribution of actin microfilaments in endothelial cells in normal confluent monolayers is characterized by a dense peripheral band (DPB) of actin microfilament bundles with some central microfilament bundles or ‘stress fibers’. In both the in vitro and in vivo low flow conditions, there is an increase in dense peripheral bands with fewer stress fibers while cells under normal physiologic flow form less dense peripheral bands with more stress fibers. Elevated shear stress results in prominent stress fibers which are gradually lost when shear stress is returned to normal. Stress fibers are contractile and are associated with sites of strong substrate anchorage. Actin microfilaments are also present in the ‘lamellipodia’ and ‘filopodia’ that form at the cell periphery and are required for spreading and motility. These microfilaments play a role in force-generation “machinery” of cytoskeleton and cell-substratum and cell-cell adhesion. They are also essential in maintaining the structural integrity of the endothelium. Formations of these actin filament arrays are regulated by members of the Rho family of small GTPases: Rho (stress fiber formation), Rac (lamellipodia formation) and Cdc42 (filopodia formation).
Fig. 1 Structure of microfilaments.
The structure of actin microfilaments is dynamic in nature and thus is well suited to regulate cell shape and cell motility. This is achieved because actin microfilaments are composed of two filaments of spherical monomers which wind around each other to form a 7 nm helix of filamentous actin or F-actin. Each actin subunit has defined polarity and polymerizes head to tail. Polymerization occurs by addition of actin monomers with bound ATP whereas hydrolysis of bound ATP promotes actin depolymerization. Due to the polarity of the subunits, one end (plus end) assembles faster than the other (minus end) (Pollard, 1984). When the rate of polymerization at one end is equal to the rate of depolymerization at the other end, then the actin is at a steady state and ‘treadmills’.
It is important to note that the properties and functions of actin microfilaments have already been shown to be influenced by a variety of proteins that bind to the actin. Some of the identified actin binding proteins include Arp2/3, a stable complex of seven subunits - two actin-related proteins (Arp2 and Arp3) and five novel proteins (p40, p35, p19, p18, and p14). It is an actin nucleator complex which drives the leading edge extension of actin microfilaments by concentrating at the leading edges catalyzing the growth of branched actin networks. The Arp2/3 complex functions together with a severing protein called Cofilin which produces numerous short actin filaments. Arp2/3 then caps the barbed ends for actin nucleation and polymerization. Several factors enhance the nucleation of three actin monomers which is the rate-limiting step of actin polymerization. Among these factors is a family of regulatory proteins, including Wiskott-Aldrich syndrome protein (WASP), neural (N)-WASP, and WASP family verprolin-homologous (WAVE) proteins, which relay signals from the small GTPases, Cdc42 and Rac, to the Arp2/3 complex, thus regulating actin nucleation.
Profilin is another ubiquitous cytoplasmic protein which regulates actin polymerization. Profilin binds to phosphatidylinositol (4, 5)-bis-phosphate (PIP2), N-WASP, and Arp2/3 complex and associates with actin monomers in a 1:1 stoichiometry. Profilin also functions together with formin, an actin-barbed-end-associated protein, to regulate actin nucleation and elongation without branching or contribution from the Arp2/3 complex.