Cofilin is a pH sensor for actin free barbed end formation: role of phosphoinositide binding

Abstract

Newly generated actin free barbed ends at the front of motile cells provide sites for actin filament assembly driving membrane protrusion. Growth factors induce a rapid biphasic increase in actin free barbed ends, and we found both phases absent in fibroblasts lacking H+ efflux by the Na-H exchanger NHE1. The first phase is restored by expression of mutant cofilin-H133A but not unphosphorylated cofilin-S3A. Constant pH molecular dynamics simulations and nuclear magnetic resonance (NMR) reveal pH-sensitive structural changes in the cofilin C-terminal filamentous actin binding site dependent on His133. However, cofilin-H133A retains pH-sensitive changes in NMR spectra and severing activity in vitro, which suggests that it has a more complex behavior in cells. Cofilin activity is inhibited by phosphoinositide binding, and we found that phosphoinositide binding is pH-dependent for wild-type cofilin, with decreased binding at a higher pH. In contrast, phosphoinositide binding by cofilin-H133A is attenuated and pH insensitive. These data suggest a molecular mechanism whereby cofilin acts as a pH sensor to mediate a pH-dependent actin filament dynamics.

Regulation of Early Neurite Morphogenesis by the Na+/H+ Exchanger NHE1

“…NHE1-dependent elevations in growth cone pH i could affect actin cytoskeletal remodeling by altering the state of actin polymerization directly and/or by modulating the activities of actin-binding and other proteins that together regulate the dynamic reorganization of the actin cytoskeleton (Srivastava et al, 2007(Srivastava et al, , 2008. For example, increases in pH i promote not only the activation of actin depolymerizing factor and cofilin, which enhance filopodial dynamics and neurite outgrowth, but also their translocation to the leading edge of migrating cells (Bernstein et al, 2000;Frantz et al, 2008). In addition, H ϩ efflux by NHE1 promotes the activation of Cdc42 (Frantz et al, 2007), a finding of particular interest given the importance of this Rho-family GTPase for promoting filopodia formation and neurite outgrowth (da Silva and Dotti, 2002;Govek et al, 2005;Heasman and Ridley, 2008) (see below).…”

Cellular functions of the ADF/cofilin family at a glance

“…Firstly, the cofilin-induced clustering of PIP 2 (see above) is sensitive to pH, with higher pH inhibiting clustering; this can decrease PIP 2 density at the membrane, leading, in turn, to increased local cofilin release and activation, and pH-dependent membrane protrusions and motility in response to growth factor stimulation (Frantz et al, 2008;Zhao et al, 2010). Secondly, cofilin activities, with respect to actin severing and filament depolymerization, are regulated by pH, with cofilin being more potent at increased pH ( pH 8) (Yeoh et al, 2002).…”

HIV-1 Tat inhibits phagocytosis by preventing the recruitment of Cdc42 to the phagocytic cup

“…This actin polymerization sustains the formation of membrane extensions called pseudopods that will engulf the target particle to form the phagosome. The phosphoinositide PI(4,5)P 2 seems to play multiple roles at the phagocytic cup, modulating actin polymerization by affecting actin-capping proteins 4 , regulating cytoskeleton plasma membrane adhesion 5 and enabling the recruitment of Rac1 and Cdc42 during FcgR receptor-mediated phagocytosis 6 .…”