Attractive axon guidance involves asymmetric membrane transport and exocytosis in the growth cone

Asymmetric elevation of the Ca²⁺ concentration in the growth cone can mediate both attractive and repulsive axon guidance. Ca²⁺ signals that are accompanied by Ca²⁺-induced Ca²⁺ release (CICR) trigger attraction, whereas Ca²⁺ signals that are not accompanied by CICR trigger repulsion. The molecular machinery downstream of Ca²⁺ signals, however, remains largely unknown. Here we report that asymmetric membrane trafficking mediates growth cone attraction. Local photolysis of caged Ca²⁺, together with CICR, on one side of the growth cone of a chick dorsal root ganglion neuron facilitated the microtubule-dependent centrifugal transport of vesicles towards the leading edge and their subsequent vesicle-associated membrane-protein 2 (VAMP2)-mediated exocytosis on the side with an elevated Ca²⁺ concentration. In contrast, Ca²⁺ signals without CICR had no effect on the vesicle transport. Furthermore, pharmacological inhibition of VAMP2-mediated exocytosis prevented growth cone attraction, but not repulsion. These results strongly suggest that growth cone attraction and repulsion are driven by distinct mechanisms, rather than using the same molecular machinery with opposing polarities.