Pip5k1γ regulates axon formation by limiting Rap1 activity

During their differentiation, neurons establish a highly polarized morphology by forming axons and dendrites. Cortical and hippocampal neurons initially extend several short neurites that all have the potential to become an axon. One of these neurites is then selected as the axon by a combination of positive and negative feedback signals that promote axon formation and prevent the remaining neurites from developing into axons. Here, we show that Pip5k1γ is required for the formation of a single axon as a negative feedback signal that regulates C3G and Rap1 through the generation of phosphatidylinositol-4,5-bisphosphate (PI(4,5)P₂). Impairing the function of Pip5k1γ results in a hyper-activation of the Fyn/C3G/Rap1 pathway, which induces the formation of supernumerary axons. Application of a hyper-osmotic shock to modulate membrane tension has a similar effect, increasing Rap1 activity and inducing the formation of supernumerary axons. In both cases, the induction of supernumerary axons can be reverted by expressing constitutively active Pip5k. Our results show that PI(4,5)P₂-dependent membrane properties limit the activity of C3G and Rap1 to ensure the extension of a single axon.