The Arp2/3 Activators WAVE and WASP Have Distinct Genetic Interactions With Rac GTPases inCaenorhabditis elegansAxon Guidance

Abstract

In the developing nervous system, axons are guided to their targets by the growth cone. Lamellipodial and filopodial protrusions from the growth cone underlie motility and guidance. Many molecules that control lamellipodia and filopodia formation, actin organization, and axon guidance have been identified, but it remains unclear how these molecules act together to control these events. Experiments are described here that indicate that, in Caenorhabditis elegans, two WH2-domain-containing activators of the Arp2/3 complex, WVE-1/WAVE and WSP-1/WASP, act redundantly in axon guidance and that GEX-2/Sra-1 and GEX-3/ Kette, molecules that control WAVE activity, might act in both pathways. WAVE activity is controlled by Rac GTPases, and data are presented here that suggest WVE-1/WAVE and CED-10/Rac act in parallel to a pathway containing WSP-1/WASP and MIG-2/RhoG. Furthermore, results here show that the CED-10/ WVE-1 and MIG-2/WSP-1 pathways act in parallel to two other molecules known to control lamellipodia and filopodia and actin organization, UNC-115/abLIM and UNC-34/Enabled. These results indicate that at least three actin-modulating pathways act in parallel to control actin dynamics and lamellipodia and filopodia formation during axon guidance (WASP-WAVE, UNC-115/abLIM, and UNC-34/Enabled). T HE growth cone of an extending axon senses and responds to extracellular guidance cues that direct axon pathfinding in the developing nervous system (Mortimer et al. 2008). Growth cone motility and guidance are mediated by the dynamic extension and retraction of lamellipodia and filopodia that are themselves the result of actin cytoskeleton-plasma membrane dynamics and interactions (Gallo and Letourneau 2004;Zhou and Cohan 2004;Pak et al. 2008). The peripheral region of the growth cone is rich in dynamic, actin-based structures including bundled microfilaments in filopodia and a lamellipodial-like meshwork of actin filaments . Multiple actin regulatory molecules are required to drive the formation of these distinct domains of actin cytoskeletal architecture in the growth cone, and the control of actin dynamics in the growth cone in response to guidance signals is an area of much interest and active study.Rac GTPases are key regulators of cytoskeletal dynamics and cellular protrusion and have been shown to control axon guidance as well as other morphogenetic events (Lundquist 2003;Watabe-Uchida et al. 2006). In Caenorhabditis elegans, the Rac GTPase CED-10 has overlapping function with the Rac-like GTPase MIG-2 in axon guidance (Lundquist et al. 2001). While MIG-2-like molecules (Mtl in Drosophila) (Hakeda-Suzuki et al. 2002) are not found in vertebrates, MIG-2 might be the C. elegans functional equivalent of vertebrate RhoG (Debakker et al. 2004). Previous work also showed that the UNC-115/abLIM actin binding protein might act downstream of Rac signaling to control lamellipodia and filopodia formation Struckhoff and Lundquist 2003;Yang and Lundquist 2005). The Enabled actin-regulatory molecule is involved in filopodi...

An UNC-40 pathway directs postsynaptic membrane extension inCaenorhabditis elegans

“…By contrast, WASP (Wasl) is auto-inhibited and only able to stimulate the Arp2/3 complex upon binding activated Rho, Cdc42, or other positive regulators (Abe et al, 2003;Symons et al, 1996;Weaver et al, 2003). The C. elegans orthologs of Nap1, WAVE and WASP are encoded by gex-3, wve-1 and wsp-1, respectively (Sawa et al, 2003;Soto et al, 2002;Withee et al, 2004), which have recently been shown to play a role in axon guidance (Shakir et al, 2008). Consistent with our finding that gex-2 is required for muscle arm extension, we found that animals compromised for gex-3 and wve-1 function have fewer muscle arms than do controls (Fig.…”

Control of developmental networks by Rac/Rho small GTPases: How cytoskeletal changes during embryogenesis are orchestrated

“…CED‐10/Rac activates the WAVE/SCAR complex, which serves as an actin‐nucleation promoting factor to activate the Arp2/3 complex for the polymerization of short, branched F‐actin (Fig. 2) 33, 40, 41. Loss of CED‐10/Rac, any WAVE/SCAR component, or the Arp2/3 complex, blocks cell migrations of the embryonic epidermis, whereas cell differentiation remains unaffected, in all these cases the ventral epidermal cells remain unattached on both sides of the embryo.…”

“…

WASP‐WAVE: Genetic studies revealed overlapping roles of Rac‐like downstream effectors wve‐1/WAVE (a member of the WRC) and wsp‐1/WASP (Wiskott Aldrich syndrome protein) during axon guidance. wve‐1 /WAVE and wsp‐1 /WASP single mutants had mild effects on axon guidance, whereas wve‐1; wsp‐1 double mutants displayed severe defects 41. Both proteins have C‐terminal WH2 domains known to function as Arp2/3 activators in other systems.

…”

“…wve‐1 /WAVE and wsp‐1 /WASP single mutants had mild effects on axon guidance, whereas wve‐1; wsp‐1 double mutants displayed severe defects 41. Both proteins have C‐terminal WH2 domains known to function as Arp2/3 activators in other systems.…”

“…Both proteins have C‐terminal WH2 domains known to function as Arp2/3 activators in other systems. Genetic experiments suggest that WVE‐1/WAVE acts in the CED‐10/Rac pathway and WSP‐1/WASP acts downstream MIG‐2/RhoG in two parallel pathways, since double mutants ced‐10; wve‐1 or mig‐2; wsp‐1 showed few defects, whereas mig‐2; wve‐1 or ced‐10; wsp‐1 mutants had much more severe axon guidance defects 41.…”