Transcriptional Coordination of Synaptogenesis and Neurotransmitter Signaling

Abstract

SUMMARY During nervous system development, postmitotic neurons face the challenge of generating and structurally organizing specific synapses with appropriate synaptic partners. An important unexplored question is whether the process of synaptogenesis is coordinated with the adoption of specific signaling properties of a neuron. Such signaling properties are defined by the neurotransmitter system that a neuron uses to communicate with postsynaptic partners, by the neurotransmitter receptor type used to receive input from presynaptic neurons and, potentially, by other sensory receptors that activate a neuron. Elucidating the mechanisms that coordinate synaptogenesis, neuronal activation and neurotransmitter signaling in a postmitotic neuron represents one key approach to understand how neurons develop as functional units. Using the SAB class of Caenorhabditis elegans motor neurons as a model system, we show here that the phylogenetically conserved COE-type transcription factor UNC-3 is required for synaptogenesis. UNC-3 directly controls the expression of the ADAMTS-like protein MADD-4/Punctin, a presynaptically secreted synapse-organizing molecule that clusters postsynaptic receptors. UNC-3 also controls the assembly of presynaptic specializations and ensures the coordinated expression of enzymes and transporters that define the cholinergic neurotransmitter identity of the SAB neurons. Furthermore, synaptic output properties of the SAB neurons are coordinated with neuronal activation and synaptic input, as evidenced by UNC-3 also regulating the expression of ionotropic neurotransmitter receptors and putative stretch receptors. Our study shows how synaptogenesis and distinct, function-defining signaling features of a postmitotic neuron are hardwired together through coordinated transcriptional control.

Excitatory neurons sculpt GABAergic neuronal connectivity in the C. elegans motor circuit

“…3B). However, consistent with a prior study (Kratsios et al, 2015), we found that overall muscle morphology is preserved -muscle arms extend normally to the nerve cord and the extent of muscle membrane contact with the nerve cord region is not appreciably altered (Fig. 3D).…”

“…The remaining cholinergic transmission might reflect activity of cholinergic VC motor neurons, which do not require unc-3 for expression of cholinergic markers (Kratsios et al, 2012). Prior work identified a requirement for unc-3 in the clustering of postsynaptic ionotropic (nicotinic) acetylcholine receptors (iAChRs) in muscles (Kratsios et al, 2015). Confirming these observations, we find that the distribution of muscle iAChRs (visualized with ACR-16::GFP) is disrupted by mutation of unc-3 (Fig.…”

“…Secretion of MADD-4 from cholinergic motor neurons is required for proper iAChR localization at neuromuscular synapses (Pinan-Lucarre et al, 2014). Prior work showed that UNC-3 transcriptional regulation of madd-4 is essential for this process (Kratsios et al, 2015). To test whether MADD-4 is similarly required at synaptic connections between cholinergic and GABAergic motor neurons, we examined ACR-12 clustering in GABAergic neurons of madd-4 mutants.…”

Diversification of C. elegans Motor Neuron Identity via Selective Effector Gene Repression

“…We initially chose two different reporter transgenes for our genetic screens: ( a ) a reporter for unc-129 , a TGFβ-family member gene which is a direct target of UNC-3 (Kratsios et al, 2015) expressed in DA and DB classes of cholinergic VNC MNs (but not VA, VB and AS classes, where unc-3 actively promotes cholinergic identity yet is apparently not sufficient to induce unc-129 expression); ( b ) a reporter for unc-53 , a novel unc-3 -dependent gene encoding a cytoskeletal protein expressed in DA and AS, but not VA, DB or VB MN classes (Fig.4E,F). We mutagenized these reporter strains with ethyl methanesulfonate (EMS) and screened ~70,760 haploid genomes for expression defects of the unc-129 reporter and ~4500 haploid genomes for expression defects of the unc-53 reporter.…”

Preventing Illegitimate Extrasynaptic Acetylcholine Receptor Clustering Requires the RSU-1 Protein

“…Each process innervates four cells in each quadrant and makes ϳ15-20 NMJs ( Fig. 4A; Zhao and Nonet, 2000; Kratsios et al, 2015). To determine whether rsu-1 ectopic clusters might be at ectopic synapses, we visualized SAB presynaptic boutons using the vesicle-associated membrane protein (VAMP) fused to GFP (Nonet, 1999).…”