Presynaptic Cdc42 Signaling-mediated Rapid Maturation Of Silent Synapses

Maturation of presynaptic transmitter secretion machinery and trafficking ofpostsynaptic receptors are two critical steps in synaptogenesis and synapticdevelopment of central nervous system (CNS). Silent synapses are frequentlyfound in developmental brain and are generally refered to synapses containingonly postsynaptic NMDA receptors but no AMPA receptors and thus are notfunctional under physiological condition due to the voltage-dependent Mg2+blockage of NMDA receptors. Unsilent such non-functional synapses aregenerally believed caused by trafficking AMPA receptors to postsynapticmembrane. Although the contribution of presynaptic cell in unsilencing hasbeen addressed, the mechanism of activation and related molecular signaling isunclear.Using double patch recording from cultured hippocampal neurons, we foundthat during early development stage many pairs lack both NMDA and AMPAreceptors mediated response. A brief train of presynaptic action potentialsrapidly converts these non-functional contacts into functional synapses by turningon presynaptic glutamate release. The enhanced release was confirmed by amarked increase in the number of depolarization-induced FM4-64 puncta in thepresynaptic axon. This rapid presynaptic maturation can be abolished bytreatments that interfered with presynaptic BDNF/Cdc42 signaling or actinpolymerization. Activation of Cdc42 by applying bradykinin mimicked theeffect of electrical activity in promoting synaptic maturation. Furthermore,activity-induced increase in presynaptic actin polymerization, as revealed byincreased concentration of actin-YFP at axon boutons, was abolished by inhibitingBDNF and Cdc42 signaling. The activity-induced increase in readily releasablevesicles in active zone was also observed by electron microscopy.Taken together, our study reveals a novel mechanism of unsilencing that mayplay important roles in synaptogenesis and neural plasticity.