Calcium signalling in the guidance of nerve growth by netrin-1

Abstract

Pathfinding by growing axons in the developing nervous system is guided by diffusible or bound factors that attract or repel the axonal growth cone^1,2. The cytoplasmic signalling mechanisms that trigger the responses of the growth cone to guidance factors are mostly unknown³. Previous studies have shown that the level and temporal patterns of cytoplasmic Ca²⁺ can regulate the rate of growth-cone extension in vitro^4,5,6,7,8 and in vivo⁹. Here we report that Ca²⁺ also mediates the turning behaviour of the growth cones of cultured Xenopus neurons that are induced by an extracellular gradient of netrin-1, an established diffusible guidance factor in vivo^1,10. The netrin-1-induced turning response depends on Ca²⁺ influx through plasma membrane Ca²⁺ channels, as well as Ca²⁺-induced Ca²⁺ release from cytoplasmic stores¹¹. Reduction of Ca²⁺ signals by blocking either of these two Ca²⁺ sources converted the netrin-1-induced response from attraction to repulsion. Activation of Ca²⁺-induced Ca²⁺ release from internal stores with a gradient of ryanodine in the absence of netrin-1 was sufficient to trigger either attractive or repulsive responses, depending on the ryanodine concentration used. These results support the model that cytoplasmic Ca²⁺ signals mediate growth-cone guidance by netrin-1, and different patterns of Ca²⁺ elevation trigger attractive and repulsive turning responses.