Asymmetric PI(3,4,5)P3and Akt Signaling Mediates Chemotaxis of Axonal Growth Cones

Abstract

The action of many extracellular guidance cues on axon pathfinding requires Ca²⁺influx at the growth cone (Hong et al., 2000; Nishiyama et al., 2003; Henley and Poo, 2004), but how activation of guidance cue receptors leads to opening of plasmalemmal ion channels remains largely unknown. Analogous to the chemotaxis of amoeboid cells (Parent et al., 1998; Servant et al., 2000), we found that a gradient of chemoattractant triggered rapid asymmetric PI(3,4,5)P₃accumulation at the growth cone's leading edge, as detected by the translocation of a GFP-tagged binding domain of Akt inXenopus laevisspinal neurons. Growth cone chemoattraction required PI(3,4,5)P₃production and Akt activation, and genetic perturbation of polarized Akt activity disrupted axon pathfindingin vitroandin vivo. Furthermore, patch-clamp recording from growth cones revealed that exogenous PI(3,4,5)P₃rapidly activated TRP (transient receptor potential) channels, and asymmetrically applied PI(3,4,5)P₃was sufficient to induce chemoattractive growth cone turning in a manner that required downstream Ca²⁺signaling. Thus, asymmetric PI(3,4,5)P₃elevation and Akt activation are early events in growth cone chemotaxis that link receptor activation to TRP channel opening and Ca²⁺signaling. Altogether, our findings reveal that PI(3,4,5)P₃elevation polarizes to the growth cone's leading edge and can serve as an early regulator during chemotactic guidance.