Activity-dependent synaptogenesis:: Regulation by a CaM-kinase kinase/CaM-kinase I/βPIX signaling complex

Neuronal activity augments maturation of mushroom-shaped spines to form excitatory synapses, thereby strengthening synaptic transmission. We have delineated a Ca2+-signaling pathway downstream of the NMDA receptor that stimulates calmodulin-dependent kinase kinase (CaMKK) and CaMKI to promote formation of spines and synapses in hippocampal neurons. CaMKK and CaMKI form a multiprotein signaling complex with the guanine nucleotide exchange factor (GEF) beta PIX and GIT1 that is localized in spines. CaMKI-mediated phosphorylation of Ser516 in beta PIX enhances its GEF activity, resulting in activation of Rac1, an established enhancer of spinogenesis. Suppression of CaMKK or CaMKI by pharmacological inhibitors, dominant-negative (dn) constructs and siRNAs, as well as expression of the beta PIX Ser516Ala mutant, decreases spine formation and mEPSC frequency. Constitutively-active Pak1, a downstream effector of Rac1, rescues spine inhibition by dnCaMKI or beta PIX S516A. This activity-dependent signaling pathway can promote synapse formation during neuronal development and in structural plasticity.