Rap2 and TNIK control Plexin-dependent tiled synaptic innervation in C. elegans

Abstract

During development, neurons form synapses with their fate-determined targets. While we begin to elucidate the 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 genetically downstream of Plexin (plx-1) to restrict presynaptic assembly and to form tiled synaptic innervation in C. elegans. Both constitutively GTP- and GDP-forms of rap-2 mutants exhibit synaptic tiling defects as plx-1 mutants, suggesting that cycling of the RAP-2 nucleotide state is critical for synapse inhibition. Consistently, PLX-1 suppresses local RAP-2 activity. Excessive ectopic synapse formation in mig-15 mutants causes a severe synaptic tiling defect. Conversely, overexpression of mig-15 strongly inhibited synapse formation, suggesting 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.

A homozygous missense variant in CACNB4 encoding the auxiliary calcium channel beta4 subunit causes a severe neurodevelopmental disorder and impairs channel and non-channel functions

“…In CNS synapses Tnik is concentrated in the postsynaptic density [81]. However, decreases in the frequency of miniature excitatory postsynaptic currents in Tnik -/mice and Tnik's ability to regulate the number of synapses in Caenorhabditis elegans suggest a role of Tnik in presynaptic processes [81,94]. Therefore, a possible functional link between TNIK and presynaptic calcium channels commonly regulating neurotransmitter release may exist.…”

Sustained expression of unc-4 homeobox gene and unc-37/Groucho in postmitotic neurons specifies the spatial organization of the cholinergic synapses in C. elegans

“…The DA neurons extend axons within the dorsal nerve cord, where they form en passant synapses at specific subaxonal regions onto the dorsal body wall muscles ( White et al, 1976 ). Previous electron microscopy reconstruction and fluorescent microscopy has revealed the tiled synaptic innervation pattern between neighboring DA neurons: the axonal region with synapses (synaptic domain) of a DA neuron does not overlap with those of its neighboring DA neurons ( Chen et al, 2018 ; Mizumoto and Shen, 2013a ; White et al, 1976 ). In this study we generated a new transgene ( mizIs3 ) with improved color contrast between the two most posterior DA neurons, DA8 and DA9, under the fluorescent microscope.…”

“…DA neurons exhibit a unique ‘tiled’ synaptic innervation pattern, in which the synaptic domain of a DA neuron does not overlap with those from the neighboring DA neurons ( White et al, 1976 ; Figure 1A ). Previously, we have shown that the interaxonal interaction between two DA neurons (DA8 and DA9) mediated by Semaphorin–Plexin signaling establishes the synaptic tiling of DA8 and DA9 ( Chen et al, 2018 ; Mizumoto and Shen, 2013a ). In the loss of function mutants of unc-4 and unc-37, we show that the tiled synaptic innervation between DA8 and DA9 is severely disrupted, similar to the mutants of Semaphorin-Plexin signaling components.…”

Neurexin and Frizzled intercept axonal-transport at microtubule minus-ends to control synapse formation

“…The C. elegans cholinergic motor neuron DA9 resides in the preanal ganglion, from where it extends a dendrite in the ventral cord and an axon that traverses across to the dorsal cord and grows anteriorly, establishing ∼25 en passant presynaptic sites onto dorsal muscles at stereotypic positions (Figure 1A) (White et al, 1986). Expression of the active zone protein Clarinet-1 (CLA-1), homolog of mammalian Piccolo and Bassoon, and the SVP marker RAB-3 allow to visualize DA9 synapses at a single-cell resolution (Chen et al, 2018; Kurshan et al, 2018; Xuan et al, 2017) (Figure 1B). In DA9, the sole C. elegans Neurexin, nrx-1 , is required presynaptically for at least two known functions: clustering of RAB-3 vesicles and preventing loss of proximal synaptic boutons (Kurshan et al, 2018) (Figure 1E).…”