Regulation of growth cone actin dynamics by ADF/cofilin.

Nervous system development is reliant on neuronal pathfinding, the process in which axons are guided to their target cells by specific extracellular cues. The ability of neurons to extend over long distances in response to environmental guidance signals is made possible by the growth cone, a highly motile structure found at the end of neuronal processes. Growth cones detect directional cues and respond with either attractive or repulsive movements. The motility of growth cones is dependent on rapid reorganization of the actin cytoskeleton, presumably mediated by actin-associated proteins under the control of incoming guidance signals. This article reviews how one such family of proteins, the ADF/cofilins, are emerging as key regulators of growth cone actin dynamics. These proteins are essential for rapid actin turnover in a variety of different cell types. ADF/cofilins are heavily co-localized with actin in growth cones and are necessary for neurite outgrowth. ADF/cofilin activities are regulated through reversible phosphorylation by LIM kinases and slingshot phosphatases. LIM kinases are downstream effectors of the Rho GTPases Rho, Rac, and Cdc42. Growing evidence suggests that extracellular guidance cues may locally alter actin dynamics by regulating the activity of LIM kinase and ADF/cofilin phosphatases via the Rho GTPases. In this way, ADF/cofilins and their upstream effectors may be pivotal to our understanding of how guidance information is translated into physical alterations of the growth cone actin cytoskeleton.