Ca2+ induces macropinocytosis via F-actin depolymerization during growth cone collapse

Growth cone collapse occurs in repulsive axon guidance and is accompanied by a reduction in the surface area of the plasma membrane of growth cones. However, the mechanism of this reduction is unclear. Here, we show that during growth cone collapse, caffeine-induced Ca2+ release from ryanodine-sensitive Ca2+ stores triggers the formation of large vacuoles in growth cones by macropinocytosis, a clathrin-independent endocytosis for the massive retrieval of the cellular plasma membrane, and subsequent retrograde membrane transport. We observed a significant correlation of the area of caffeine-induced macropinosomes with growth cone collapse. We also detected macropinocytosis induced by semaphorin 3A, a typical repulsive cue, and correlation between the area of semaphorin 3A-induced macropinocytic vacuoles and growth cone collapse. Moreover, jasplakinolide, an inhibitor of F-actin depolymerization, blocked caffeine-induced macropinocytosis. We propose that the coordinated regulation of actin cytoskeletal reorganization and macropinocytosis-mediated retrograde membrane trafficking may contribute to Ca2+-induced axon growth inhibition.