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Increasing evidence indicates that guidance molecules used during development for cellular and axonal navigation also play roles in synapse maturation and homeostasis. In C. elegans the netrin receptor UNC-40/DCC controls the growth of dendritic-like muscle cell extensions towards motoneurons and is required to recruit type A GABA receptors (GABA A Rs) at inhibitory neuromuscular junctions. Here we show that activation of UNC-40 assembles an intracellular synaptic scaffold by physically interacting with FRM-3, a FERM protein orthologous to FARP1/2. FRM-3 then recruits LIN-2, the ortholog of CASK, that binds the synaptic adhesion molecule NLG-1/Neuroligin and physically connects GABA A Rs to prepositioned NLG-1 clusters. These processes are orchestrated by the synaptic organizer CePunctin/MADD-4, which controls the localization of GABA A Rs by positioning NLG-1/neuroligin at synapses and regulates the synaptic content of GABA A Rs through the UNC-40-dependent intracellular scaffold. Since DCC is detected at GABA synapses in mammals, DCC might also tune inhibitory neurotransmission in the mammalian brain. The netrin receptor UNC-40/DCC is required to recruit GABA A R at neuromuscular junctions in C. elegans . Here, the authors show that UNC-40/DCC assembles an intracellular synaptic scaffold, regulating the content of GABA A R and inhibitory neurotransmission.

Two presynaptically secreted isoforms of the protein Punctin in the nematode Caenorhabditis elegans determine the postsynaptic accumulation of acetylcholine versus GABA receptors, raising the question of whether the related human punctin-2 gene, which has been associated with schizophrenia, may also control synaptic organization. Punctin is a synaptic organizer in C. elegans Most neurons receive myriads of inputs from excitatory and inhibitory neurons, and each presynaptic neurotransmitter must find the appropriate receptor on the facing, postsynaptic surface. Jean-Louis Bessereau and colleagues show that two presynaptically secreted isoforms of the protein Punctin determine the postsynaptic accumulation of acetylcholine versus GABA receptors in the nematode Caenorhabditis elegans . Whether the related human punctin-2 gene, which has been associated with schizophrenia, is also involved in the control of synaptic organization remains to be investigated. Because most neurons receive thousands of synaptic inputs, the neuronal membrane is a mosaic of specialized microdomains where neurotransmitter receptors cluster in register with the corresponding presynaptic neurotransmitter release sites. In many cases the coordinated differentiation of presynaptic and postsynaptic domains implicates trans-synaptic interactions between membrane-associated proteins such as neurexins and neuroligins 1 , 2 , 3 . The Caenorhabditis elegans neuromuscular junction (NMJ) provides a genetically tractable system in which to analyse the segregation of neurotransmitter receptors, because muscle cells receive excitatory innervation from cholinergic neurons and inhibitory innervation from GABAergic neurons 4 . Here we show that Ce-Punctin / madd-4 (ref. 5 ), the C. elegans orthologue of mammalian punctin-1 and punctin-2 , encodes neurally secreted isoforms that specify the excitatory or inhibitory identity of postsynaptic NMJ domains. These proteins belong to the ADAMTS (a disintegrin and metalloprotease with thrombospondin repeats)-like family, a class of extracellular matrix proteins related to the ADAM proteases but devoid of proteolytic activity 6 . Ce-Punctin deletion causes the redistribution of synaptic acetylcholine and GABA A (γ-aminobutyric acid type A) receptors into extrasynaptic clusters, whereas neuronal presynaptic boutons remain unaltered. Alternative promoters generate different Ce-Punctin isoforms with distinct functions. A short isoform is expressed by cholinergic and GABAergic motoneurons and localizes to excitatory and inhibitory NMJs, whereas long isoforms are expressed exclusively by cholinergic motoneurons and are confined to cholinergic NMJs. The differential expression of these isoforms controls the congruence between presynaptic and postsynaptic domains: specific disruption of the short isoform relocalizes GABA A receptors from GABAergic to cholinergic synapses, whereas expression of a long isoform in GABAergic neurons recruits acetylcholine receptors to GABAergic NMJs. These results identify Ce-Punctin as a previously unknown synaptic organizer and show that presynaptic and postsynaptic domain identities can be genetically uncoupled in vivo . Because human punctin-2 was identified as a candidate gene for schizophrenia 7 , ADAMTS-like proteins may also control synapse organization in the mammalian central nervous system.

Nicotine exposure elicits diverse behavioral changes, yet the underlying neural pathways and molecular mechanisms remain incompletely understood. Here, we demonstrate that chronic nicotine exposure markedly increases both the initiation and duration of reversals in Caenorhabditis elegans . Strikingly, these reversals were tightly coupled with the rhythmic body contractions of the defecation motor program (DMP). Through pharmacological, genetic, in situ electrophysiological, and calcium imaging analyses, we show that nicotine enhances the activity of the AVA interneuron via selective upregulation of ACR-16, a nicotinic ACh receptor critical for nicotine-induced motor coupling. Ablation of touch receptor neurons (TRNs) or inhibition of TRNs-mediated mechanosensation completely abolished this coupling. Furthermore, optogenetic activation of TRNs in nicotine-treated animals evoked stronger AVA depolarization, and nicotine amplified gentle touch-evoked reversals. Together, these findings reveal a potential interoceptive effect of nicotine mediated by sensitization of the TRNs-AVA mechanosensory pathway, providing new insight into the neural and molecular basis of nicotine’s modulation of sensory-motor coupling.

Increasing evidence indicates that guidance molecules used during development for cellular and axonal navigation also play roles in synapse maturation and homeostasis. In C. elegans the netrin receptor UNC-40/DCC controls the growth of dendritic-like muscle cell extensions towards motoneurons and is required to recruit type A GABA receptors (GABA Rs) at inhibitory neuromuscular junctions. Here we show that activation of UNC-40 assembles an intracellular synaptic scaffold by physically interacting with FRM-3, a FERM protein orthologous to FARP1/2. FRM-3 then recruits LIN-2, the ortholog of CASK, that binds the synaptic adhesion molecule NLG-1/Neuroligin and physically connects GABA Rs to prepositioned NLG-1 clusters. These processes are orchestrated by the synaptic organizer CePunctin/MADD-4, which controls the localization of GABA Rs by positioning NLG-1/neuroligin at synapses and regulates the synaptic content of GABA Rs through the UNC-40-dependent intracellular scaffold. Since DCC is detected at GABA synapses in mammals, DCC might also tune inhibitory neurotransmission in the mammalian brain.

A critical accomplishment in the rapidly developing field of regenerative medicine will be the ability to foster repair of neurons severed by injury, disease, or microsurgery. In C. elegans, individual visualized axons can be laser-cut in vivo and neuronal responses to damage can be monitored to decipher genetic requirements for regeneration. With an initial interest in how local environments manage cellular debris, we performed femtosecond laser axotomies in genetic backgrounds lacking cell death gene activities. Unexpectedly, we found that the CED-3 caspase, well known as the core apoptotic cell death executioner, acts in early responses to neuronal injury to promote rapid regeneration of dissociated axons. In ced-3 mutants, initial regenerative outgrowth dynamics are impaired and axon repair through reconnection of the two dissociated ends is delayed. The CED-3 activator, CED-4/Apaf-1, similarly promotes regeneration, but the upstream regulators of apoptosis CED-9/Bcl2 and BH3-domain proteins EGL-1 and CED-13 are not essential. Thus, a novel regulatory mechanism must be utilized to activate core apoptotic proteins for neuronal repair. Since calcium plays a conserved modulatory role in regeneration, we hypothesized calcium might play a critical regulatory role in the CED-3/CED-4 repair pathway. We used the calcium reporter cameleon to track in vivo calcium fluxes in the axotomized neuron. We show that when the endoplasmic reticulum calcium-storing chaperone calreticulin, CRT-1, is deleted, both calcium dynamics and initial regenerative outgrowth are impaired. Genetic data suggest that CED-3, CED-4, and CRT-1 act in the same pathway to promote early events in regeneration and that CED-3 might act downstream of CRT-1, but upstream of the conserved DLK-1 kinase implicated in regeneration across species. This study documents reconstructive roles for proteins known to orchestrate apoptotic death and links previously unconnected observations in the vertebrate literature to suggest a similar pathway may be conserved in higher organisms.

We have isolated the Podospora anserina TOR gene. The PaTOR protein displayed strong identities with TOR proteins from other eukaryotes especially in the FRB domain and the kinase domain. Genome analysis suggests that a single TOR gene exists in Podospora. The serine residue known to be one site of missense mutations conferring rapamycin resistance in other organisms is conserved in the PaTOR protein (S1895). A PaTOR-S1895R mutated allele has been constructed and introduced in the wild-type strain, as expected strains expressing the PaTOR-S1895R gene become resistant to rapamycin. The dominance of the PaTOR-S1895R allele indicates that apparently the mutation does not impair the kinase activity. We confirm that all cytological modifications associated with rapamycin treatment in Podospora are indeed mediated by PaTOR. We conclude that the PaTOR gene is likely to be essential and that rapamycin treatment might be useful to further investigate rapamycin-sensitive TOR functions in Podospora and especially newly identified rapamycin-sensitive functions such as the autophagy-independent control of vacuole remodeling and septation.

The localization and clustering of neurotransmitter receptors at appropriate postsynaptic sites is a key step in the control of synaptic transmission. Here, we identify a novel paradigm for the synaptic localization of an ionotropic acetylcholine receptor (AChR) based on the direct interaction of its extracellular domain with a cell adhesion molecule of the IgLON family. Our results show that RIG-5 and ZIG-8, which encode the sole IgLONs in are tethered in the pre- and postsynaptic membranes, respectively, and interact through their first immunoglobulin-like (Ig) domains. In addition, ZIG-8 traps ACR-16 via a direct interaction between the ZIG-8 Ig2 domain and the base of the large extracellular AChR domain. Such mechanism has never been reported, but all these molecules are conserved during evolution. Similar interactions may directly couple Ig superfamily adhesion molecules and members of the large family of Cys-loop ionotropic receptors, including AChRs, in the mammalian nervous system, and may be relevant in the context of IgLON-associated brain diseases.

Terminal selectors are transcription factors that control neuronal identity by regulating the expression of key effector molecules, such as neurotransmitter (NT) biosynthesis proteins, ion channels and neuropeptides. Whether and how terminal selectors control neuronal connectivity is poorly understood. Here, we report that UNC-30 (PITX2/3), the terminal selector of GABA motor neuron identity in , is required for NT receptor clustering, a hallmark of postsynaptic differentiation. Animals lacking or the short isoform of the MN-secreted synapse organizer ( ), display severe GABA receptor type A (GABA R) clustering defects in postsynaptic muscle cells. Mechanistically, UNC-30 acts directly to induce and maintain transcription of and GABA biosynthesis genes (e.g., , ). Hence, UNC-30 controls GABA R clustering on postsynaptic muscle cells and GABA biosynthesis in presynaptic cells, transcriptionally coordinating two critical processes for GABA neurotransmission. Further, we uncover multiple target genes and a dual role for UNC-30 both as an activator and repressor of gene transcription. Our findings on UNC-30 function may contribute to our molecular understanding of human conditions, such as Axenfeld-Rieger syndrome, caused by PITX2 and PITX3 gene mutations.