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

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 unknown(3). Previous studies ha ve shown that the level and temporal patterns of cytoplasmic Ca2+ can regul ate the rate of growth-cone extension in vitro(4-8) and in vivo(9). Here we report that Ca2+ also mediates the turning behaviour of the growth cones o f cultured Xenopus neurons that are induced by an extracellular gradient of netrin-1, an established diffusible guidance factor in vivo(1,10). The net rin-1-induced turning response depends on Ca2+ influx through plasma membra ne Ca2+ channels, as well as Ca2+-induced Ca2+ release from cytoplasmic sto res(11). Reduction of Ca2+ signals by blocking either of these two Ca2+ sou rces converted the netrin-1-induced response from attraction to repulsion. Activation of Ca2+-induced Ca2+ release from internal stores with a gradien t 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 mediat e growth-cone guidance by netrin-1, and different patterns of Ca2+ elevatio n trigger attractive and repulsive turning responses.