Synapse Formation in Monosynaptic Sensory–Motor Connections Is Regulated by Presynaptic Rho GTPase Cdc42

Abstract

Spinal reflex circuit development requires the precise regulation of axon trajectories, synaptic specificity, and synapse formation. Of these three crucial steps, the molecular mechanisms underlying synapse formation between group Ia proprioceptive sensory neurons and motor neurons is the least understood. Here, we show that the Rho GTPase Cdc42 controls synapse formation in monosynaptic sensorymotor connections in presynaptic, but not postsynaptic, neurons. In mice lacking Cdc42 in presynaptic sensory neurons, proprioceptive sensory axons appropriately reach the ventral spinal cord, but significantly fewer synapses are formed with motor neurons compared with wild-type mice. Concordantly, electrophysiological analyses show diminished EPSP amplitudes in monosynaptic sensory-motor circuits in these mutants. Temporally targeted deletion of Cdc42 in sensory neurons after sensory-motor circuit establishment reveals that Cdc42 does not affect synaptic transmission. Furthermore, addition of the synaptic organizers, neuroligins, induces presynaptic differentiation of wild-type, but not Cdc42-deficient, proprioceptive sensory neurons in vitro. Together, our findings demonstrate that Cdc42 in presynaptic neurons is required for synapse formation in monosynaptic sensory-motor circuits.

Requirement for Dicer in Maintenance of Monosynaptic Sensory-Motor Circuits in the Spinal Cord

“…In situ hybridizations were performed according to standard protocols (Imai et al, 2016). Template DNAs were cloned into the pCRII vector (Life Technologies) by PCR using following primers.…”

“…Dissections of spinal cords and electrophysiological recordings have been previously described in detail (Imai et al, 2016, Fukuhara et al, 2013). Briefly, spinal cords from P5–P7 newborn pups were hemisectioned in oxygenated aCSF bath.…”

Molecular mechanisms underlying monosynaptic sensory‐motor circuit development in the spinal cord

“…After Ia proprioceptive sensory afferents find their appropriate target motor neurons, the synapses mature. A recent study in mice showed that loss of Cdc42, a small GTPase, caused defects in synapse formation in monosynaptic sensory-motor connections (Imai et al, 2016b). Although Cdc42 in post-synaptic neurons has been shown to control synapse formation in other regions of the CNS, knocking out Cdc42 in motor neurons (the post-synaptic neurons in monosynaptic sensory-motor connections) did not affect sensory-motor synaptogenesis (Irie and Yamaguchi, 2002; Murakoshi et al, 2011; Kim et al, 2014; Hedrick et al, 2016).…”

Glia Promote Synaptogenesis through an IQGAP PES-7 in C. elegans

“…In mammals, the homolog CDC42 also regulates dendritic spine formation and plasticity (Tashiro et al, 2000;Tolias et al, 2011;Hedrick et al, 2016;Martin-Vilchez et al, 2017;Murakoshi et al, 2011;Bijata et al, 2017). Recently, CDC42 was also found to be required for presynaptic development in Drosophila and mice (Imai et al, 2016;Rodal et al, 2008), indicating that its roles in presynaptic assembly are evolutionarily conserved.…”