Biochemical Signaling Mechanisms

Over the past decade, great advances have been made in our understanding of the chemical factors that drive cell motility, as well as the biochemical signaling cascades that mediate cellular responses, as described in several excellent recent reviews (see Bibliography). In particular, the Rho family of small GTPases, consisting of Rho, Rac, and Cdc42, have been shown to regulate signal transduction pathways that link extracellular signals through membrane receptors to changes in cytoskeletal structure and organization that drive cell locomotion. For instance, in 1992, Hall and coworkers demonstrated that constitutively active (GTpase-deficient) mutants of Rac induce lamellipodia assembly in fibroblasts, whereas active Rho and Cdc42 mutants promote formation of stress fibers and filopodia, respectively. Similar effects have been observed in various endothelial cells as well.

In fact, Rac has been found to be essential for the migration of all cells examined thus far. Studies in macrophage, for example, have demonstrated that downregulation of Rac activity stalls cell migration while downregulation of Cdc42 renders cells unresponsive to the direction of a chemotactic gradient, causing them instead to move via a random walk. Studies utilizing fluorescence resonance energy transfer (FRET) microscopy have revealed that while Rac protein is distributed homogeneously throughout migratory fibroblasts, the active GTP-bound form is localized to the leading edge of the cell. Rac and Cdc42 may therefore act locally to govern where actin polymerization is activated at the periphery of the cell.

Analysis of Rho GTPase signaling has revealed a vast array of downstream effector proteins (at least 60 at present) that regulate many different cell processes. Rac is activated by guanine nucleotide exchange factors (GEFs) such as Trio, Vav, and Sos-1 and repressed by GTPase-activating proteins (GAPs) such as chimerin. Several factors downstream of Rac act on cytoskeletal structure and other aspects of cell migration. Perhaps the best characterized of these effectors is p21-activated kinase (Pak), a serine/threonine kinase that is activated upon binding to the GTP-bound form of Rac or Cdc42. Pak provides a direct link from Rac to cell motility through phosphorylation and activation of LIM kinase, which phosphorylates and represses the actin depolymerizing factor, cofilin. Pak may also regulate cell traction forces by promoting myosin light-chain phosphory-lation and increasing cytoskeletal contractility, although these results have been controversial and may depend on the cell type examined.

The Rho GTPases also impact directional migration through the regulation of focal adhesion dynamics. In migrating cells, Rac is required for the formation of new focal adhesion complexes at the leading edge of the cell, whereas Rho is essential for the maturation of these nascent adhesive contacts into fully developed focal adhesions. Rac may regulate adhesion turnover indirectly by antagonizing Rho through its downstream effector Pakl, which localizes to focal adhesions upon activation. The mechanism by which Pakl regulates adhesion dynamics is not yet clear but may involve the action of LIM kinase or the phosphoryla-tion of myosin light chain.

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Essentials of Human Physiology

Essentials of Human Physiology

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