Ca²⁺-dependent regulation of Rho GTPases triggers turning of nerve growth cones

Cytoplasmic Ca²⁺ elevation and changes in Rho GTPase activity are both known to mediate axon guidance by extracellular factors, but the causal relationship between these two events has been unclear. Here we show that direct elevation of cytoplasmic Ca²⁺ by extracellular application of a low concentration of ryanodine, which activated Ca²⁺ release from intracellular stores, upregulated Cdc42/Rac, but downregulated RhoA, in cultured cerebellar granule cells and human embryonic kidney 293T cells. Chemoattractive turning of the growth cone triggered by a gradient of ryanodine was blocked by overexpression of mutant forms of Cdc42 but not of RhoA in Xenopus spinal cord neurons. Furthermore, Ca²⁺-induced GTPase activity correlated with activation of protein kinase C and required a basal activity of Ca ²⁺/calmodulin-dependent protein kinase II. Thus, Rho GTPases may mediate axon guidance by linking upstream Ca²⁺ signals triggered by guidance factors to downstream cytoskeletal rearrangements.