Actin network

Proteins that cross-link actin filaments bind to their sides to produce bundles or three-dimensional networks (Otto, 1994). In microvilli, approximately 20 actin filaments of the core are cross-linked by villin (95 kD) and fimbrin (68 kD) in helical array to form a compact bundle (Figure 5). Filamin (2 x 250 kD) induces the formation of an actin network with gel formation. By immunofluorescence microscopy, this ABP is found in the ruffled, motile edge of cultured cells, where only actin filaments are abundant. 

Stress fibers are parallel bundles of actin filaments that develop in the cytoplasm of fibroblasts from the cortical actin network in response to mechanical tension. These often bind to the plasma membrane at focal contacts and, through transmembrane linker glycoproteins, to the extracellular matrix. Thus, actin filaments of stress fibers indirectly Join to the inner face of the plasma membrane through molecular assemblies of attachment proteins, which include an actin-capping protein, a-actinin, vinculin, and talin (Small, 1988). 

Assembly and disassembly of the actin network is controlled at two major steps (i) the length of individual actin filaments and (ii) the extent to which these filaments are cross-linked to each other (Fig. 4.2). Filament length itself is controlled by the rates of ... 

Figure 4.2. Control of the assembly of the actin network. The actin network may be regulated by (1) the rate of formation of new nucleation centres (composed of three actin monomers) (2) filament formation from these nucleation centres (3) extension of filaments (4) joining of filaments end to end (5) cross-linking of filaments. All of these steps are reversible.

Vinculin is one of a number of proteins that bind the actin network to the plasma membrane. It is a 130-kDa protein that is phosphorylated at a tyrosine residue by a kinase that is controlled by the src gene. Vinculin function is Ca2+-dependent, and it is found in close association with a-actinin. 

Assembly of the actin network merely by interaction with these binding proteins can itself account for pseudopodia formation and propulsive movement. However, there is some evidence to suggest that F-actin-myosin interactions are required for vectorial movement hence it has been demonstrated that pseudopodia contain filament networks comprising actin and myosin. Myosin plays a role in the contractile movement of neutrophils in a... 

Yuruker, B., Niggli, V. (1992). a- Actinin and vinculin in human neutrophils Reorganization during adhesion and relation to the actin network. /. Cell Sci. 101,403-14. 

The work that follows pertains primarily to actin networks. Many proteins within a cell are known to associate with actin. Among these are molecules which can initiate or terminate polymerization, intercalate with and cut chains, crosslink or bundle filaments, or induce network contraction (i.e., myosin) (A,11,12). The central concern of this paper is an exploration of the way that such molecular species interact to form complex networks. Ultimately we wish to elucidate the biophysical linkages between molecular properties and cellular function (like locomotion and shape differentiation) in which cytoskeletal structures are essential attributes. Here, however, we examine the iri vitro formation of cytoplasmic gels, with an emphasis on delineating quantitative assays for network constituents. Specific attention is given to gel volume assays, determinations of gelation times, and elasticity measurements. 

The function of spectrin superfamily proteins is particularly evident when taken in context of their cellular localization. They often form flexible links or structures that allow interactions with the cellular cyto-skeletal architecture and the membrane. In both spectrin and dystrophin, such a function is performed, but the spectrin repeats of these molecules are also able to interact with actin and contribute to binding. A portion of the dystrophin rod domain that spans residues 11-17 contains a number of basic repeats that allow a lateral interaction with filamentous actin (Rybakova et al., 2002). The homologous utrophin can also interact laterally with actin. This interaction is distinct from that of dystrophin, as the utrophin rod domain lacks the basic repeat cluster and associates with actin via the first ten spectrin repeats (Rybakova et al., 2002). /3-Spectrin also exhibits an extended contact with actin via the first spectrin repeat. In this situation, it was found that the extended contact increased the association of the adjacent ABD with actin (Li and Bennett, 1996). In conjunction with this interaction, it has been found that the second repeat is also required for maximal interaction with adducin (Li and Bennett, 1996), a protein localized at the spectrin-actin junction that is believed to contribute to the assembly of this structure in the membrane skeletal network (Gardner and Bennett, 1987). In the erythrocyte cytoskeletal lattice, /3-spectrin interacts with ankyrin, which in turn binds to the cytoplasmic domain of the membrane-associated anion exchanger. This indirect link to the cellular membrane occurs via repeat 15 of /3-spectrin (Kennedy et al., 1991) and is largely responsible for the attachment of the spectrin-actin network to the erythrocyte membrane (reviewed in Bennett and Baines, 2001). A much larger number of direct links to transmembrane proteins have been determined for the spectrin repeats of o-actinin (reviewed in Djinovic-Carugo et al, 2002). 

Simulation of glucose transport and glucose transporter translocation from intracellular stores to the plasma membrane in muscle cells by vanadate and peroxovan-adate involve a mechanism independent of PI-3K and protein kinase C systems utilized for stimulation of these processes by insulin. The transport of GLUT4 to the plasma membrane in muscle cells growing in culture after stimulation by vanadate, peroxovanadate, or insulin all require an intact actin network [138], Sometimes, the insulin-like action of vanadium is accompanied by overall stimulation of actual metabolic pathways. One example of this is the stimulation of the pentose phosphate pathway observed when vanadate promotes the incorporation of glucose into lipids, an antilipogenic effect [139],... 

A growing number of proteins have been identified to be involved in the formation of TJ (Fig. 3.5). These proteins may be divided into three groups, namely the integral TJ proteins involved in the cell-cell contacts, the TJ plaque proteins, which connect integral TJ proteins to the actin network of the cytos-keleton, and various cytosolic and nuclear proteins interacting with TJ plaque... 

Fluoroaluminate complexes can mimic the action of many hormones, neurotransmitters, and growth factors. G-protein-mediated cell responses are key steps in neurotransmission and intercellular signaling in the brain [20], and TFA acts as an active stimulatory species [21]. Exposure of osteoblasts to TFA results in a marked potentiation of intracellular orthophosphate transport, alluding to the anion s ability to increase bone mineralization [22]. Brief exposure to aluminum fluoride complexes induces prolonged enhancement of synaptic transmission [23] and can potentially affect the activity of many other ion channels and enzymes in the kidney [24]. Rapid and dynamic changes of the cytoskeletal actin network are of vital importance to the motility of many cells, and TFA induction effects a pronounced and sustained... 

Suetsugu, S. and Takenawa, T., 2003, Regulation of cortical actin networks in cell migration. Inti. Rev. Cytol. 229 245-286. 

Lipid raft domains of plasma membranes are enriched in cholesterol and sphingolipids. As a consequence, compounds that extract or sequester cholesterol, such as fS-cyclodextrins, nystatin, and filipin, can block selectively endocytosis of cholera toxin, GPI-linked proteins, and other receptors that associate with lipid rafts and caveolae. However, cholesterol is also critical for CME, secretion of proteins, and the actin network. Therefore, conditions designed to affect selectively raft-mediated endocytosis by perturbing cholesterol levels must be carefully controlled to avoid disrupting other mechanisms of endocytosis (40). 

While actin nucleation represents one key control point in determining the dynamic behavior of cellular actin networks, an equally important point of control is filament disassembly. Only by maintaining actin polymers in a state of rapid turnover can cells maintain a pool of assembly-competent actin subunits for new growth and reorganize their networks rapidly in response to signals. Replenishment of subunits is accelerated by cellular factors that selectively destabilize and depolymerize the older (ADP-bound) filaments in networks. Cofilin (also called ADF) plays a central role in this process and recendy it has emerged that coronin assists cofilin in driving these events. 

Cofilins are a widely conserved family of proteins that accelerate actin network disassembly and arc required for dynamic actin-based processes, including cell motility, endocytosis and cytokinesis. Cofilin binds to the sides of actin filaments in a cooperative manner and increases the twist of filaments, leading to filament severing and disassembly. Cofilin promotes filament disassembly in concert with several other conserved actin-binding proteins, each of which makes... 

Gandhi M, Goode BL. Coronin promotes in vivo turnover of actin networks by nucleotide-sensitive regulation of cofilin effects on F-actin. submitted. 

Udo H, Jin I, Kim JH, Li HL, Youn T, Hawkins RD, Kandel ER, Bailey CH (2005) Serotonin-induced regulation of the actin network for learning-related synaptic growth requires Cdc42, N-WASP, and PAK in Aplysia sensory neurons. Neuron 45 887-901. 

DAPI (4, 6-Diamidine-2-phenylindole dihydrochloride), a fluorescent stain that specifically binds double stranded DNA, is widely used to specifically localize nuclei in developing embryos and as a counterstain. Phalloidin is a plant toxin that binds filamentous actin, thus allowing the visualization of cell borders by highlighting the F-actin network along cell membranes. Both are toxic and should be handled with care. 

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