Abstract
During development, neurons form extensive synaptic connectivity with their fate-determined targets. Ectopic synapse formation with aberrant targets underlie various neurological disorders including autism. While we are beginning to elucidate the underlying mechanisms by which extracellular ligand-receptor interactions enhance synapse specificity by inhibiting synaptogenesis, our knowledge about their intracellular mechanisms remains limited. Here we show that Rap2 GTPase (rap-2) and its effector, TNIK (mig-15), act downstream of Plexin (plx-1) signaling to restrict presynaptic assembly and to form tiled synaptic innervation of two cholinergic motor neurons (DA8 and DA9) in C. elegans. Both constitutively GTP- and GDP-forms of rap-2 mutants exhibit similar synaptic tiling defects as plx-1 mutants, suggesting that cycling of the RAP-2 nucleotide state is critical for synapse inhibition. Consistently, RAP-2 activity is suppressed in a short segment of axon lacking synapses where PLX-1 is enriched. Excessive ectopic synapse formation in mig-15 mutants causes expansion of the synaptic domains in DA8 and DA9, which induces a severe synaptic tiling defect. Conversely, overexpression of mig-15 strongly inhibited synapse formation, which suggests that mig-15 is a negative regulator of synapse formation. These results reveal that subcellular regulation of small GTPase activity by Plexin shapes proper synapse patterning in vivo.
