MADD-4/Punctin and Neurexin Organize C. elegans GABAergic Postsynapses through Neuroligin

UNC-30/PITX coordinates neurotransmitter identity with postsynaptic GABA receptor clustering

Terminal selectors are transcription factors that control neuronal identity by regulating expression of key effector molecules, such as neurotransmitter biosynthesis proteins and ion channels. Whether and how terminal selectors control neuronal connectivity is poorly understood. Here, we report that UNC-30 (PITX2/3), the terminal selector of GABA nerve cord motor neurons in Caenorhabditis elegans, is required for neurotransmitter receptor clustering, a hallmark of postsynaptic differentiation. Animals lacking unc-30 or madd-4B, the short isoform of the motor neuron-secreted synapse organizer madd-4 (punctin/ADAMTSL), display severe GABA receptor type A (GABAAR) clustering defects in postsynaptic muscle cells. Mechanistically, UNC-30 acts directly to induce and maintain transcription of madd-4B and GABA biosynthesis genes (e.g. unc-25/GAD, unc-47/VGAT). Hence, UNC-30 controls GABAA receptor clustering in postsynaptic muscle cells and GABA biosynthesis in presynaptic cells, transcriptionally coordinating two crucial processes for GABA neurotransmission. Further, we uncover multiple target genes and a dual role for UNC-30 as both an activator and a 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 variants.

Neurexin drives Caenorhabditis elegans avoidance behavior independently of its post-synaptic binding partner neuroligin

Neurexins and their canonical binding partners, neuroligins, are localized to neuronal pre-, and post-synapses, respectively, but less is known about their role in driving behaviors. Here, we use the nematode C. elegans to show that neurexin, but not neuroligin, is required for avoiding specific chemorepellents. We find that adults with knockouts of the entire neurexin locus exhibit a strong avoidance deficit in response to glycerol and a weaker defect in response to copper. Notably, the C. elegans neurexin (nrx-1) locus, like its mammalian homologs, encodes multiple isoforms, α and γ. Using isoform-specific mutations, we find that the γ isoform is selectively required for glycerol avoidance. Next, we used transgenic rescue experiments to show that this isoform functions at least partially in the nervous system. We also confirm that the transgenes are expressed in the neurons and observe protein accumulation in neurites. Furthermore, we tested whether these mutants affect the behavioral responses of juveniles. We find that juveniles (4th larval stages) of mutants knocking out the entire locus or the α-isoforms, but not γ-isoform, are defective in avoiding glycerol. These results suggest that the different neurexin isoforms affect chemosensory avoidance behavior in juveniles and adults, providing a general principle of how isoforms of this conserved gene affect behavior across species.

RIM and RIM-Binding Protein Localize Synaptic CaV2 Channels to Differentially Regulate Transmission in Neuronal Circuits

At chemical synapses, voltage-gated Ca2+ channels (VGCCs) translate electrical signals into a trigger for synaptic vesicle (SV) fusion. VGCCs and the Ca2+ microdomains they elicit must be located precisely to primed SVs to evoke rapid transmitter release. Localization is mediated by Rab3-interacting molecule (RIM) and RIM-binding proteins, which interact and bind to the C terminus of the CaV2 VGCC α-subunit. We studied this machinery at the mixed cholinergic/GABAergic neuromuscular junction of Caenorhabditis elegans hermaphrodites. rimb-1 mutants had mild synaptic defects, through loosening the anchoring of UNC-2/CaV2 and delaying the onset of SV fusion. UNC-10/RIM deletion much more severely affected transmission. Although postsynaptic depolarization was reduced, rimb-1 mutants had increased cholinergic (but reduced GABAergic) transmission, to compensate for the delayed release. This did not occur when the excitation-inhibition (E-I) balance was altered by removing GABA transmission. Further analyses of GABA defective mutants and GABAA or GABAB receptor deletions, as well as cholinergic rescue of RIMB-1, emphasized that GABA neurons may be more affected than cholinergic neurons. Thus, RIMB-1 function differentially affects excitation-inhibition balance in the different motor neurons, and RIMB-1 thus may differentially regulate transmission within circuits. Untethering the UNC-2/CaV2 channel by removing its C-terminal PDZ ligand exacerbated the rimb-1 defects, and similar phenotypes resulted from acute degradation of the CaV2 β-subunit CCB-1. Therefore, untethering of the CaV2 complex is as severe as its elimination, yet it does not abolish transmission, likely due to compensation by CaV1. Thus, robustness and flexibility of synaptic transmission emerge from VGCC regulation.

Nematode Extracellular Protein Interactome Expands Connections between Signaling Pathways

Multicellularity was accompanied by the emergence of new classes of cell surface and secreted proteins. The nematode C. elegans is a favorable model to study cell surface interactomes, given its well-defined and stereotyped cell types and intercellular contacts. Here we report our C. elegans extracellular interactome dataset, the largest yet for an invertebrate. Most of these interactions were unknown, despite recent datasets for flies and humans, as our collection contains a larger selection of protein families. We uncover new interactions for all four major axon guidance pathways, including ectodomain interactions between three of the pathways. We demonstrate that a protein family known to maintain axon locations are secreted receptors for insulins. We reveal novel interactions of cystine-knot proteins with putative signaling receptors, which may extend the study of neurotrophins and growth-factor-mediated functions to nematodes. Finally, our dataset provides insights into human disease mechanisms and how extracellular interactions may help establish connectomes.

The neuron-specific IIS/FOXO transcriptome in aged animals reveals regulatory mechanisms of cognitive aging

Cognitive decline is a significant health concern in our aging society. Here, we used the model organism C. elegans to investigate the impact of the IIS/FOXO pathway on age-related cognitive decline. The daf-2 Insulin/IGF-1 receptor mutant exhibits a significant extension of learning and memory span with age compared to wild-type worms, an effect that is dependent on the DAF-16 transcription factor. To identify possible mechanisms by which aging daf-2 mutants maintain learning and memory with age while wild-type worms lose neuronal function, we carried out neuron-specific transcriptomic analysis in aged animals. We observed downregulation of neuronal genes and upregulation of transcriptional regulation genes in aging wild-type neurons. By contrast, IIS/FOXO pathway mutants exhibit distinct neuronal transcriptomic alterations in response to cognitive aging, including upregulation of stress response genes and downregulation of specific insulin signaling genes. We tested the roles of significantly transcriptionally-changed genes in regulating cognitive functions, identifying novel regulators of learning and memory. In addition to other mechanistic insights, a comparison of the aged vs young daf-2 neuronal transcriptome revealed that a new set of potentially neuroprotective genes is upregulated; instead of simply mimicking a young state, daf-2 may enhance neuronal resilience to accumulation of harm and take a more active approach to combat aging. These findings suggest a potential mechanism for regulating cognitive function with age and offer insights into novel therapeutic targets for age-related cognitive decline.

Structural and functional characterization of the IgSF21-neurexin2α complex and its related signaling pathways in the regulation of inhibitory synapse organization

The prevailing model behind synapse development and specificity is that a multitude of adhesion molecules engage in transsynaptic interactions to induce pre- and postsynaptic assembly. How these extracellular interactions translate into intracellular signal transduction for synaptic assembly remains unclear. Here, we focus on a synapse organizing complex formed by immunoglobulin superfamily member 21 (IgSF21) and neurexin2α (Nrxn2α) that regulates GABAergic synapse development in the mouse brain. We reveal that the interaction between presynaptic Nrxn2α and postsynaptic IgSF21 is a high-affinity receptor-ligand interaction and identify a binding interface in the IgSF21-Nrxn2α complex. Despite being expressed in both dendritic and somatic regions, IgSF21 preferentially regulates dendritic GABAergic presynaptic differentiation whereas another canonical Nrxn ligand, neuroligin2 (Nlgn2), primarily regulates perisomatic presynaptic differentiation. To explore mechanisms that could underlie this compartment specificity, we targeted multiple signaling pathways pharmacologically while monitoring the synaptogenic activity of IgSF21 and Nlgn2. Interestingly, both IgSF21 and Nlgn2 require c-jun N-terminal kinase (JNK)-mediated signaling, whereas Nlgn2, but not IgSF21, additionally requires CaMKII and Src kinase activity. JNK inhibition diminished de novo presynaptic differentiation without affecting the maintenance of formed synapses. We further found that Nrxn2α knockout brains exhibit altered synaptic JNK activity in a sex-specific fashion, suggesting functional linkage between Nrxns and JNK. Thus, our study elucidates the structural and functional relationship of IgSF21 with Nrxn2α and distinct signaling pathways for IgSF21-Nrxn2α and Nlgn2-Nrxn synaptic organizing complexes in vitro. We therefore propose a revised hypothesis that Nrxns act as molecular hubs to specify synaptic properties not only through their multiple extracellular ligands but also through distinct intracellular signaling pathways of these ligands.

UNC-30/PITX coordinates neurotransmitter identity with postsynaptic GABA receptor clustering

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 C. elegans , is required for NT receptor clustering, a hallmark of postsynaptic differentiation. Animals lacking unc-30 or madd-4B, the short isoform of the MN-secreted synapse organizer madd-4 ( Punctin/ADAMTSL ), display severe GABA receptor type A (GABA A R) clustering defects in postsynaptic muscle cells. Mechanistically, UNC-30 acts directly to induce and maintain transcription of madd-4B and GABA biosynthesis genes (e.g., unc-25/GAD , unc-47/VGAT ). Hence, UNC-30 controls GABA A 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.

Conserved autism-associated genes tune social feeding behavior in C. elegans

Animal foraging is an essential and evolutionarily conserved behavior that occurs in social and solitary contexts, but the underlying molecular pathways are not well defined. We discover that conserved autism-associated genes (NRXN1(nrx-1), NLGN3(nlg-1), GRIA1,2,3(glr-1), GRIA2(glr-2), and GLRA2,GABRA3(avr-15)) regulate aggregate feeding in C. elegans, a simple social behavior. NRX-1 functions in chemosensory neurons (ADL and ASH) independently of its postsynaptic partner NLG-1 to regulate social feeding. Glutamate from these neurons is also crucial for aggregate feeding, acting independently of NRX-1 and NLG-1. Compared to solitary counterparts, social animals show faster presynaptic release and more presynaptic release sites in ASH neurons, with only the latter requiring nrx-1. Disruption of these distinct signaling components additively converts behavior from social to solitary. Aggregation induced by circuit activation is also dependent on nrx-1. Collectively, we find that aggregate feeding is tuned by conserved autism-associated genes through complementary synaptic mechanisms, revealing molecular principles driving social feeding.

Distinct neurexin isoforms cooperate to initiate and maintain foraging activity

Neurexins are synaptic adhesion molecules that play diverse roles in synaptic development, function, maintenance, and plasticity. Neurexin genes have been associated with changes in human behavior, where variants in NRXN1 are associated with autism, schizophrenia, and Tourette syndrome. While NRXN1, NRXN2, and NRXN3 all encode major α and β isoforms, NRXN1 uniquely encodes a γ isoform, for which mechanistic roles in behavior have yet to be defined. Here, we show that both α and γ isoforms of neurexin/nrx-1 are required for the C. elegans behavioral response to food deprivation, a sustained period of hyperactivity upon food loss. We find that the γ isoform regulates initiation and the α isoform regulates maintenance of the behavioral response to food deprivation, demonstrating cooperative function of multiple nrx-1 isoforms in regulating a sustained behavior. The γ isoform alters monoamine signaling via octopamine, relies on specific expression of NRX-1 isoforms throughout the relevant circuit, and is independent of neuroligin/nlg-1, the canonical trans-synaptic partner of nrx-1. The α isoform regulates the pre-synaptic structure of the octopamine producing RIC neuron and its maintenance role is conditional on neuroligin/nlg-1. Collectively, these results demonstrate that neurexin isoforms can have separate behavioral roles and act cooperatively across neuronal circuits to modify behavior, highlighting the need to directly analyze and consider all isoforms when defining the contribution of neurexins to behavior.

Post-synaptic GABAA receptors potentiate transmission by recruiting CaV2 channels to their inputs

We describe a retrograde synaptic signal at the C. elegans GABAergic neuromuscular junction. At this synapse, GABA release is controlled by two voltage-activated calcium channels (UNC-2/CaV2 and EGL-19/CaV1), and muscle responses are mediated by a single GABA receptor (UNC-49/GABAA). Mutations inactivating UNC-49 or those preventing UNC-49 synaptic clustering cause retrograde defects in GABAergic motor neurons, whereby UNC-2/CaV2 levels at active zones, UNC-2 current, and pre-synaptic GABA release are decreased. Inactivating post-synaptic GABAA receptors has no effect on GABA neuron EGL-19/CaV1 levels nor on several other pre-synaptic markers. The effect of GABAA receptors on pre-synaptic strength is not a consequence of decreased GABA transmission and is input selective. Finally, pre-synaptic UNC-2/CaV2 levels are increased when post-synaptic GABAA receptors are increased but are unaffected by increased extra-synaptic receptors. Collectively, these results suggest that clustered post-synaptic GABAA receptors adjust the strength of their inputs by recruiting CaV2 to contacting active zones.

UNC-43/CaMKII-triggered anterograde signals recruit GABAARs to mediate inhibitory synaptic transmission and plasticity at C. elegans NMJs

Disturbed inhibitory synaptic transmission has functional impacts on neurodevelopmental and psychiatric disorders. An essential mechanism for modulating inhibitory synaptic transmission is alteration of the postsynaptic abundance of GABA_ARs, which are stabilized by postsynaptic scaffold proteins and recruited by presynaptic signals. However, how GABAergic neurons trigger signals to transsynaptically recruit GABA_ARs remains elusive. Here, we show that UNC-43/CaMKII functions at GABAergic neurons to recruit GABA_ARs and modulate inhibitory synaptic transmission at C. elegans neuromuscular junctions. We demonstrate that UNC-43 promotes presynaptic MADD-4B/Punctin secretion and NRX-1α/Neurexin surface delivery. Together, MADD-4B and NRX-1α recruit postsynaptic NLG-1/Neuroligin and stabilize GABA_ARs. Further, the excitation of GABAergic neurons potentiates the recruitment of NLG-1-stabilized-GABA_ARs, which depends on UNC-43, MADD-4B, and NRX-1. These data all support that UNC-43 triggers MADD-4B and NRX-1α, which act as anterograde signals to recruit postsynaptic GABA_ARs. Thus, our findings elucidate a mechanism for pre- and postsynaptic communication and inhibitory synaptic transmission and plasticity.

Function of cell adhesion molecules in differentiation of ray sensory neurons in C. elegans

For proper functioning of the nervous system, it is crucial that neurons find their appropriate partners and build the correct neural connection patterns. Although cell adhesion molecules (CAMs) have been studied for many years as essential players in neural connections, we have yet to unravel the code by which CAMs encode synaptic specificity. We analyzed the effects of mutations in CAM genes on the morphology and synapses of a set of sensory neurons in the Caenorhabditis elegans male tail. B-type ray sensory neurons express 10 genes encoding CAMs. We examined the effect on axon trajectory and localization of pre-synaptic components in viable mutants of nine of these. We found axon trajectory defects in mutants of UNC-40/DCC, SAX-3/ROBO, and FMI-1/Flamingo/Celsr1. None of the mutations caused loss of pre-synaptic components in axons, and in several the level even appeared to increase, suggesting possible accumulation of pre-synapses. B-type sensory neurons fasciculate with a second type of ray sensory neuron, the A-type, in axon commissures. We found a CAM expressed in A-type functions additively with a CAM expressed in B-type in axon guidance, and lack of a CAM expressed in B-type affected A-type axon guidance. Overall, single and multiple mutants of CAM genes had limited effects on ray neuron trajectories and accumulation of synaptic components.

Synaptogenesis: unmasking molecular mechanisms using Caenorhabditis elegans

The nematode Caenorhabditis elegans is a research model organism particularly suited to the mechanistic understanding of synapse genesis in the nervous system. Armed with powerful genetics, knowledge of complete connectomics, and modern genomics, studies using C. elegans have unveiled multiple key regulators in the formation of a functional synapse. Importantly, many signaling networks display remarkable conservation throughout animals, underscoring the contributions of C. elegans research to advance the understanding of our brain. In this chapter, we will review up-to-date information of the contribution of C. elegans to the understanding of chemical synapses, from structure to molecules and to synaptic remodeling.

Structure and evolution of neuronal wiring receptors and ligands

One of the fundamental properties of a neuronal circuit is the map of its connections. The cellular and developmental processes that allow for the growth of axons and dendrites, selection of synaptic targets, and formation of functional synapses use neuronal surface receptors and their interactions with other surface receptors, secreted ligands, and matrix molecules. Spatiotemporal regulation of the expression of these receptors and cues allows for specificity in the developmental pathways that wire stereotyped circuits. The families of molecules controlling axon guidance and synapse formation are generally conserved across animals, with some important exceptions, which have consequences for neuronal connectivity. Here, we summarize the distribution of such molecules across multiple taxa, with a focus on model organisms, evolutionary processes that led to the multitude of such molecules, and functional consequences for the diversification or loss of these receptors.

Channel-independent function of UNC-9/Innexin in spatial arrangement of GABAergic synapses in C. elegans

Precise synaptic connection of neurons with their targets is essential for the proper functioning of the nervous system. A plethora of signaling pathways act in concert to mediate the precise spatial arrangement of synaptic connections. Here we show a novel role for a gap junction protein in controlling tiled synaptic arrangement in the GABAergic motor neurons in Caenorhabditis elegans, in which their axons and synapses overlap minimally with their neighboring neurons within the same class. We found that while EGL-20/Wnt controls axonal tiling, their presynaptic tiling is mediated by a gap junction protein UNC-9/Innexin, that is localized at the presynaptic tiling border between neighboring dorsal D-type GABAergic motor neurons. Strikingly, the gap junction channel activity of UNC-9 is dispensable for its function in controlling tiled presynaptic patterning. While gap junctions are crucial for the proper functioning of the nervous system as channels, our finding uncovered the novel channel-independent role of UNC-9 in synapse patterning.

CASK and FARP localize two classes of post-synaptic ACh receptors thereby promoting cholinergic transmission

Changes in neurotransmitter receptor abundance at post-synaptic elements play a pivotal role in regulating synaptic strength. For this reason, there is significant interest in identifying and characterizing the scaffolds required for receptor localization at different synapses. Here we analyze the role of two C. elegans post-synaptic scaffolding proteins (LIN-2/CASK and FRM-3/FARP) at cholinergic neuromuscular junctions. Constitutive knockouts or muscle specific inactivation of lin-2 and frm-3 dramatically reduced spontaneous and evoked post-synaptic currents. These synaptic defects resulted from the decreased abundance of two classes of post-synaptic ionotropic acetylcholine receptors (ACR-16/CHRNA7 and levamisole-activated AChRs). LIN-2's AChR scaffolding function is mediated by its SH3 and PDZ domains, which interact with AChRs and FRM-3/FARP, respectively. Thus, our findings show that post-synaptic LIN-2/FRM-3 complexes promote cholinergic synaptic transmission by recruiting AChRs to post-synaptic elements.

madd-4 plays a critical role in light against Bursaphelenchus xylophilus

Bursaphelenchus xylophilus is a notorious invasive species, causing extensive losses to pine ecosystems globally. Previous studies had shown that the development of B. xylophilus was seriously suppressed by light. However, the mechanism involved in the inhibition is unknown. Here, it is the first report that Bxy-madd-4 is a light-regulated gene, plays a potential role in B. xylophilus in responding to the blue light. Transcriptome sequencing revealed that the expression level of Bxy-madd-4 declined by 86.39% under blue light. The reverse transcription quantitative real-time PCR results were in accord with the transcriptome sequencing, confirming the expression level of Bxy-madd-4 was suppressed by blue light. Bxy-madd-4 promoter::mCherry reporter constructed in Caenorhabditis elegans were utilized to mimic the spatiotemporal expression patterns of Bxy-madd-4. Bxy-madd-4A promoter activity had a strong continuity throughout all development stages in C. elegans. Further RNA interference indicated that only 36.8% of the Bxy-madd-4 dsRNA treated embryos were hatched. Moreover, 71.6% of the hatched nematodes were abnormal, such as particles on the body surface and concave tissues. Our findings contribute towards a better understanding of the mechanism of light against the destructive invasive nematode, providing a promising hint for control of the destructive invasive nematode.

Synapse Formation and Function Across Species: Ancient Roles for CCP, CUB, and TSP-1 Structural Domains

The appearance of synapses was a crucial step in the creation of the variety of nervous systems that are found in the animal kingdom. With increased complexity of the organisms came a greater number of synaptic proteins. In this review we describe synaptic proteins that contain the structural domains CUB, CCP, or TSP-1. These domains are found in invertebrates and vertebrates, and CUB and CCP domains were initially described in proteins belonging to the complement system of innate immunity. Interestingly, they are found in synapses of the nematode C. elegans, which does not have a complement system, suggesting an ancient function. Comparison of the roles of CUB-, CCP-, and TSP-1 containing synaptic proteins in various species shows that in more complex nervous systems, these structural domains are combined with other domains and that there is partial conservation of their function. These three domains are thus basic building blocks of the synaptic architecture. Further studies of structural domains characteristic of synaptic proteins in invertebrates such as C. elegans and comparison of their role in mammals will help identify other conserved synaptic molecular building blocks. Furthermore, this type of functional comparison across species will also identify structural domains added during evolution in correlation with increased complexity, shedding light on mechanisms underlying cognition and brain diseases.

Excitatory and inhibitory receptors utilize distinct post- and trans-synaptic mechanisms in vivo

Ionotropic neurotransmitter receptors at postsynapses mediate fast synaptic transmission upon binding of the neurotransmitter. Post- and trans-synaptic mechanisms through cytosolic, membrane, and secreted proteins have been proposed to localize neurotransmitter receptors at postsynapses. However, it remains unknown which mechanism is crucial to maintain neurotransmitter receptors at postsynapses. In this study, we ablated excitatory or inhibitory neurons in adult mouse brains in a cell-autonomous manner. Unexpectedly, we found that excitatory AMPA receptors remain at the postsynaptic density upon ablation of excitatory presynaptic terminals. In contrast, inhibitory GABA_A receptors required inhibitory presynaptic terminals for their postsynaptic localization. Consistent with this finding, ectopic expression at excitatory presynapses of neurexin-3 alpha, a putative trans-synaptic interactor with the native GABA_A receptor complex, could recruit GABA_A receptors to contacted postsynaptic sites. These results establish distinct mechanisms for the maintenance of excitatory and inhibitory postsynaptic receptors in the mature mammalian brain.

Neural Tissue Homeostasis and Repair Is Regulated via CS and DS Proteoglycan Motifs

Chondroitin sulfate (CS) is the most abundant and widely distributed glycosaminoglycan (GAG) in the human body. As a component of proteoglycans (PGs) it has numerous roles in matrix stabilization and cellular regulation. This chapter highlights the roles of CS and CS-PGs in the central and peripheral nervous systems (CNS/PNS). CS has specific cell regulatory roles that control tissue function and homeostasis. The CNS/PNS contains a diverse range of CS-PGs which direct the development of embryonic neural axonal networks, and the responses of neural cell populations in mature tissues to traumatic injury. Following brain trauma and spinal cord injury, a stabilizing CS-PG-rich scar tissue is laid down at the defect site to protect neural tissues, which are amongst the softest tissues of the human body. Unfortunately, the CS concentrated in gliotic scars also inhibits neural outgrowth and functional recovery. CS has well known inhibitory properties over neural behavior, and animal models of CNS/PNS injury have demonstrated that selective degradation of CS using chondroitinase improves neuronal functional recovery. CS-PGs are present diffusely in the CNS but also form denser regions of extracellular matrix termed perineuronal nets which surround neurons. Hyaluronan is immobilized in hyalectan CS-PG aggregates in these perineural structures, which provide neural protection, synapse, and neural plasticity, and have roles in memory and cognitive learning. Despite the generally inhibitory cues delivered by CS-A and CS-C, some CS-PGs containing highly charged CS disaccharides (CS-D, CS-E) or dermatan sulfate (DS) disaccharides that promote neural outgrowth and functional recovery. CS/DS thus has varied cell regulatory properties and structural ECM supportive roles in the CNS/PNS depending on the glycoform present and its location in tissue niches and specific cellular contexts. Studies on the fruit fly, Drosophila melanogaster and the nematode Caenorhabditis elegans have provided insightful information on neural interconnectivity and the role of the ECM and its PGs in neural development and in tissue morphogenesis in a whole organism environment.

The HSPG syndecan is a core organizer of cholinergic synapses

The extracellular matrix has emerged as an active component of chemical synapses regulating synaptic formation, maintenance, and homeostasis. The heparan sulfate proteoglycan (HSPG) syndecans are known to regulate cellular and axonal migration in the brain. They are also enriched at synapses, but their synaptic functions remain more elusive. Here, we show that SDN-1, the sole orthologue of syndecan in C. elegans, is absolutely required for the synaptic clustering of homomeric α7-like acetylcholine receptors (AChRs) and regulates the synaptic content of heteromeric AChRs. SDN-1 is concentrated at neuromuscular junctions (NMJs) by the neurally secreted synaptic organizer Ce-Punctin/MADD-4, which also activates the transmembrane netrin receptor DCC. Those cooperatively recruit the FARP and CASK orthologues that localize α7-like-AChRs at cholinergic NMJs through physical interactions. Therefore, SDN-1 stands at the core of the cholinergic synapse organization by bridging the extracellular synaptic determinants to the intracellular synaptic scaffold that controls the postsynaptic receptor content.

The Prop1-like homeobox gene unc-42 specifies the identity of synaptically connected neurons

Many neuronal identity regulators are expressed in distinct populations of cells in the nervous system, but their function is often analyzed only in specific isolated cellular contexts, thereby potentially leaving overarching themes in gene function undiscovered. We show here that the Caenorhabditis elegans Prop1-like homeobox gene unc-42 is expressed in 15 distinct sensory, inter- and motor neuron classes throughout the entire C. elegans nervous system. Strikingly, all 15 neuron classes expressing unc-42 are synaptically interconnected, prompting us to investigate whether unc-42 controls the functional properties of this circuit and perhaps also the assembly of these neurons into functional circuitry. We found that unc-42 defines the routes of communication between these interconnected neurons by controlling the expression of neurotransmitter pathway genes, neurotransmitter receptors, neuropeptides, and neuropeptide receptors. Anatomical analysis of unc-42 mutant animals reveals defects in axon pathfinding and synaptic connectivity, paralleled by expression defects of molecules involved in axon pathfinding, cell-cell recognition, and synaptic connectivity. We conclude that unc-42 establishes functional circuitry by acting as a terminal selector of functionally connected neuron types. We identify a number of additional transcription factors that are also expressed in synaptically connected neurons and propose that terminal selectors may also function as 'circuit organizer transcription factors' to control the assembly of functional circuitry throughout the nervous system. We hypothesize that such organizational properties of transcription factors may be reflective of not only ontogenetic, but perhaps also phylogenetic trajectories of neuronal circuit establishment.

Extracellular Matrix Muscle Arm Development Defective Protein Cooperates with the One Immunoglobulin Domain Protein To Suppress Precocious Synaptic Remodeling

Synaptic remodeling plays important roles in health and neural disorders. Although previous studies revealed that several transcriptional programs control synaptic remodeling in the nematode Caenorhabditis elegans, the molecular mechanisms of the dorsal D-type (DD) synaptic remodeling are poorly understood. Here we show that extracellular matrix molecule muscle arm development defective protein-4 (MADD-4) cooperates with the one immunoglobulin domain protein-1 (OIG-1) to defer precocious DD synaptic remodeling. Specifically, loss of MADD-4 exhibited the precocious DD synaptic remodeling. The long isoform MADD-4L is dynamically expressed while the short isoform MADD-4B is persistently expressed in DD neurons of L1 stage. In the unc-30 mutant lacking the Pitx-type homeodomain transcription factor UNC-30, the expression levels of both MADD-4B and -L isoforms were dramatically downregulated in DD neurons of the L1 stage. Our further data showed that MADD-4B and -L isoforms physically interact with OIG-1 and madd-4 acts in the oig-1 genetic pathway to modulate the DD synaptic remodeling. Our findings demonstrated that the extracellular matrix plays a novel role in synaptic plasticity.

Investigating autism associated genes in C. elegans reveals candidates with a role in social behaviour

Autism spectrum disorder (ASD) is a neurodevelopmental disorder characterised by a triad of behavioural impairments and includes disruption in social behaviour. ASD has a clear genetic underpinning and hundreds of genes are implicated in its aetiology. However, how single penetrant genes disrupt activity of neural circuits which lead to affected behaviours is only beginning to be understood and less is known about how low penetrant genes interact to disrupt emergent behaviours. Investigations are well served by experimental approaches that allow tractable investigation of the underpinning genetic basis of circuits that control behaviours that operate in the biological domains that are neuro-atypical in autism. The model organism C. elegans provides an experimental platform to investigate the effect of genetic mutations on behavioural outputs including those that impact social biology. Here we use progeny-derived social cues that modulate C. elegans food leaving to assay genetic determinants of social behaviour. We used the SAFRI Gene database to identify C. elegans orthologues of human ASD associated genes. We identified a number of mutants that displayed selective deficits in response to progeny. The genetic determinants of this complex social behaviour highlight the important contribution of synaptopathy and implicates genes within cell signalling, epigenetics and phospholipid metabolism functional domains. The approach overlaps with a growing number of studies that investigate potential molecular determinants of autism in C. elegans. However, our use of a complex, sensory integrative, emergent behaviour provides routes to enrich new or underexplored biology with the identification of novel candidate genes with a definable role in social behaviour.

Specific heparan sulfate modifications stabilize the synaptic organizer MADD-4/Punctin at C. elegans neuromuscular junctions

Heparan sulfate proteoglycans contribute to the structural organization of various neurochemical synapses. Depending on the system, their role involves either the core protein or the glycosaminoglycan chains. These linear sugar chains are extensively modified by heparan sulfate modification enzymes, resulting in highly diverse molecules. Specific modifications of glycosaminoglycan chains may thus contribute to a sugar code involved in synapse specificity. Caenorhabditis elegans is particularly useful to address this question because of the low level of genomic redundancy of these enzymes, as opposed to mammals. Here, we systematically mutated the genes encoding heparan sulfate modification enzymes in C. elegans and analyzed their impact on excitatory and inhibitory neuromuscular junctions. Using single chain antibodies that recognize different heparan sulfate modification patterns, we show in vivo that these two heparan sulfate epitopes are carried by the SDN-1 core protein, the unique C. elegans syndecan orthologue, at neuromuscular junctions. Intriguingly, these antibodies differentially bind to excitatory and inhibitory synapses, implying unique heparan sulfate modification patterns at different neuromuscular junctions. Moreover, while most enzymes are individually dispensable for proper organization of neuromuscular junctions, we show that 3-O-sulfation of SDN-1 is required to maintain wild-type levels of the extracellular matrix protein MADD-4/Punctin, a central synaptic organizer that defines the identity of excitatory and inhibitory synaptic domains at the plasma membrane of muscle cells.

GABAergic synapses suppress intestinal innate immunity via insulin signaling in Caenorhabditis elegans

GABAergic neurotransmission constitutes a major inhibitory signaling mechanism that plays crucial roles in central nervous system physiology and immune cell immunomodulation. However, its roles in innate immunity remain unclear. Here, we report that deficiency in the GABAergic neuromuscular junctions (NMJs) of Caenorhabditis elegans results in enhanced resistance to pathogens, whereas pathogen infection enhances the strength of GABAergic transmission. GABAergic synapses control innate immunity in a manner dependent on the FOXO/DAF-16 but not the p38/PMK-1 pathway. Our data reveal that the insulin-like peptide INS-31 level was dramatically decreased in the GABAergic NMJ GABA_AR-deficient unc-49 mutant compared with wild-type animals. C. elegans with ins-31 knockdown or loss of function exhibited enhanced resistance to Pseudomonas aeruginosa PA14 exposure. INS-31 may act downstream of GABAergic NMJs and in body wall muscle to control intestinal innate immunity in a cell-nonautonomous manner. Our results reveal a signaling axis of synapse-muscular insulin-intestinal innate immunity in vivo.

Male pheromones modulate synaptic transmission at the C. elegans neuromuscular junction in a sexually dimorphic manner

The development of functional synapses in the nervous system is important for animal physiology and behaviors, and its disturbance has been linked with many neurodevelopmental disorders. The synaptic transmission efficacy can be modulated by the environment to accommodate external changes, which is crucial for animal reproduction and survival. However, the underlying plasticity of synaptic transmission remains poorly understood. Here we show that in Caenorhabditis elegans, the male environment increases the hermaphrodite cholinergic transmission at the neuromuscular junction (NMJ), which alters hermaphrodites' locomotion velocity and mating efficiency. We identify that the male-specific pheromones mediate this synaptic transmission modulation effect in a developmental stage-dependent manner. Dissection of the sensory circuits reveals that the AWB chemosensory neurons sense those male pheromones and further transduce the information to NMJ using cGMP signaling. Exposure of hermaphrodites to the male pheromones specifically increases the accumulation of presynaptic CaV2 calcium channels and clustering of postsynaptic acetylcholine receptors at cholinergic synapses of NMJ, which potentiates cholinergic synaptic transmission. Thus, our study demonstrates a circuit mechanism for synaptic modulation and behavioral flexibility by sexual dimorphic pheromones.

Neurexins: molecular codes for shaping neuronal synapses

The function of neuronal circuits relies on the properties of individual neuronal cells and their synapses. We propose that a substantial degree of synapse formation and function is instructed by molecular codes resulting from transcriptional programmes. Recent studies on the Neurexin protein family and its ligands provide fundamental insight into how synapses are assembled and remodelled, how synaptic properties are specified and how single gene mutations associated with neurodevelopmental and psychiatric disorders might modify the operation of neuronal circuits and behaviour. In this Review, we first summarize insights into Neurexin function obtained from various model organisms. We then discuss the mechanisms and logic of the cell type-specific regulation of Neurexin isoforms, in particular at the level of alternative mRNA splicing. Finally, we propose a conceptual framework for how combinations of synaptic protein isoforms act as 'senders' and 'readers' to instruct synapse formation and the acquisition of cell type-specific and synapse-specific functional properties.

Neuroligin dependence of social behaviour in Caenorhabditis elegans provides a model to investigate an autism-associated gene

Autism spectrum disorder (ASD) is characterized by a triad of behavioural impairments including social behaviour. Neuroligin, a trans-synaptic adhesion molecule, has emerged as a penetrant genetic determinant of behavioural traits that signature the neuroatypical behaviours of autism. However, the function of neuroligin in social circuitry and the impact of genetic variation to this gene is not fully understood. Indeed, in animal studies designed to model autism, there remains controversy regarding the role of neuroligin dysfunction in the expression of disrupted social behaviours. The model organism, Caenorhabditis elegans, offers an informative experimental platform to investigate the impact of genetic variants on social behaviour. In a number of paradigms, it has been shown that inter-organismal communication by chemical cues regulates C. elegans social behaviour. We utilize this social behaviour to investigate the effect of autism-associated genetic variants within the social domain of the research domain criteria. We have identified neuroligin as an important regulator of social behaviour and segregate the importance of this gene to the recognition and/or processing of social cues. We also use CRISPR/Cas9 to edit an R-C mutation that mimics a highly penetrant human mutation associated with autism. C. elegans carrying this mutation phenocopy the behavioural dysfunction of a C. elegans neuroligin null mutant, thus confirming its significance in the regulation of animal social biology. This highlights that quantitative behaviour and precision genetic intervention can be used to manipulate discrete social circuits of the worm to provide further insight into complex social behaviour.

The Ig-like domain of Punctin/MADD-4 is the primary determinant for interaction with the ectodomain of neuroligin NLG-1

Punctin/MADD-4, a member of the ADAMTSL extracellular matrix protein family, was identified as an anterograde synaptic organizer in the nematode Caenorhabditis elegans. At GABAergic neuromuscular junctions, the short isoform MADD-4B binds the ectodomain of neuroligin NLG-1, itself a postsynaptic organizer of inhibitory synapses. To identify the molecular bases of their partnership, we generated recombinant forms of the two proteins and carried out a comprehensive biochemical and biophysical study of their interaction, complemented by an in vivo localization study. We show that spontaneous proteolysis of MADD-4B first generates a shorter N-MADD-4B form, which comprises four thrombospondin (TSP) domains and one Ig-like domain and binds NLG-1. A second processing event eliminates the C-terminal Ig-like domain along with the ability of N-MADD-4B to bind NLG-1. These data identify the Ig-like domain as the primary determinant for N-MADD-4B interaction with NLG-1 in vitro We further demonstrate in vivo that this Ig-like domain is essential, albeit not sufficient per se, for efficient recruitment of GABA_A receptors at GABAergic synapses in C. elegans The interaction of N-MADD-4B with NLG-1 is also disrupted by heparin, used as a surrogate for the extracellular matrix component, heparan sulfate. High-affinity binding of heparin/heparan sulfate to the Ig-like domain may proceed from surface charge complementarity, as suggested by homology three-dimensional modeling. These data point to N-MADD-4B processing and cell-surface proteoglycan binding as two possible mechanisms to regulate the interaction between MADD-4B and NLG-1 at GABAergic synapses.

Paracrine Role for Somatostatin Interneurons in the Assembly of Perisomatic Inhibitory Synapses

GABAergic interneurons represent a heterogenous group of cell types in neocortex that can be clustered based on developmental origin, morphology, physiology, and connectivity. Two abundant populations of cortical GABAergic interneurons include the low-threshold, somatostatin (SST)-expressing cells and the fast-spiking, parvalbumin (PV)-expressing cells. While SST⁺ and PV⁺ interneurons are both early born and migrate into the developing neocortex at similar times, SST⁺ cells are incorporated into functional circuits prior to PV⁺ cells. During this early period of neural development, SST⁺ cells play critical roles in the assembly and maturation of other cortical circuits; however, the mechanisms underlying this process remain poorly understood. Here, using both sexes of conditional mutant mice, we discovered that SST⁺ interneuron-derived Collagen XIX, a synaptogenic extracellular matrix protein, is required for the formation of GABAergic, perisomatic synapses by PV⁺ cells. These results, therefore, identify a paracrine mechanism by which early-born SST⁺ cells orchestrate inhibitory circuit formation in the developing neocortex.SIGNIFICANCE STATEMENT Inhibitory interneurons in the cerebral cortex represent a heterogenous group of cells that generate the inhibitory neurotransmitter GABA. One such interneuron type is the low-threshold, somatostatin (SST)-expressing cell, which is one of the first types of interneurons to migrate into the cerebral cortex and become incorporated into functional circuits. In addition, to contributing important roles in controlling the flow of information in the adult cerebral cortex, SST⁺ cells play important roles in the development of other neural circuits in the developing brain. Here, we identified an extracellular matrix protein that is released by these early-born SST⁺ neurons to orchestrate inhibitory circuit formation in the developing cerebral cortex.

Functional mosaic organization of neuroligins in neuronal circuits

Complex brain circuitry with feedforward and feedback systems regulates neuronal activity, enabling neural networks to process and drive the entire spectrum of cognitive, behavioral, sensory, and motor functions. Simultaneous orchestration of distinct cells and interconnected neural circuits is underpinned by hundreds of synaptic adhesion molecules that span synaptic junctions. Dysfunction of a single molecule or molecular interaction at synapses can lead to disrupted circuit activity and brain disorders. Neuroligins, a family of cell adhesion molecules, were first identified as postsynaptic-binding partners of presynaptic neurexins and are essential for synapse specification and maturation. Here, we review recent advances in our understanding of how this family of adhesion molecules controls neuronal circuit assembly by acting in a synapse-specific manner.

The netrin receptor UNC-40/DCC assembles a postsynaptic scaffold and sets the synaptic content of GABAA receptors

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_ARs) 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_ARs to prepositioned NLG-1 clusters. These processes are orchestrated by the synaptic organizer CePunctin/MADD-4, which controls the localization of GABA_ARs by positioning NLG-1/neuroligin at synapses and regulates the synaptic content of GABA_ARs 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_AR at neuromuscular junctions in C. elegans. Here, the authors show that UNC-40/DCC assembles an intracellular synaptic scaffold, regulating the content of GABA_AR and inhibitory neurotransmission.

Trafficking and Activity of Glutamate and GABA Receptors: Regulation by Cell Adhesion Molecules

The efficient targeting of ionotropic receptors to postsynaptic sites is essential for the function of chemical excitatory and inhibitory synapses, constituting the majority of synapses in the brain. A growing body of evidence indicates that cell adhesion molecules (CAMs), which accumulate at synapses at the earliest stages of synaptogenesis, are critical for this process. A diverse variety of CAMs assemble into complexes with glutamate and GABA receptors and regulate the targeting of these receptors to the cell surface and synapses. Presynaptically localized CAMs provide an additional level of regulation, sending a trans-synaptic signal that can regulate synaptic strength at the level of receptor trafficking. Apart from controlling the numbers of receptors present at postsynaptic sites, CAMs can also influence synaptic strength by modulating the conductivity of single receptor channels. CAMs thus act to maintain basal synaptic transmission and are essential for many forms of activity dependent synaptic plasticity. These activities of CAMs may underlie the association between CAM gene mutations and synaptic pathology and represent fundamental mechanisms by which synaptic strength is dynamically tuned at both excitatory and inhibitory synapses.

Neurexins cluster Ca2+ channels within the presynaptic active zone

To achieve ultrafast neurotransmission, neurons assemble synapses with highly organized presynaptic and postsynaptic nanomachines that are aligned by synaptic adhesion molecules. How functional assembly of presynaptic active zones is controlled via trans-synaptic interactions remains unknown. Here, we conditionally deleted all three neurexin adhesion molecules from presynaptic neurons of the calyx of Held in the mouse auditory system, a model synapse that allows precise biophysical analyses of synaptic properties. The pan-neurexin deletion had no effect on synapse development or the basic release machinery, but dramatically impaired fast neurotransmitter release. The overall properties of presynaptic calcium ion channels appeared normal, as reflected by the similar characteristics of calcium currents recorded at the nerve terminals. However, the pan-neurexin deletion significantly impaired the tight coupling of calcium influx to exocytosis, thereby suppressing neurotransmitter release. Furthermore, the pan-neurexin deletion reduced the function of calcium-activated BK potassium channels, whose activation depends on their tight association with presynaptic calcium channels. Together, these results suggest that neurexins perform a major function at the calyx synapse in coupling presynaptic calcium channels to release sites.

Graphene oxide disrupts the protein-protein interaction between Neuroligin/NLG-1 and DLG-1 or MAGI-1 in nematode Caenorhabditis elegans

Graphene oxide (GO) is a carbon-based engineered nanomaterial (ENM). Using Caenorhabditis elegans as an animal model, we investigated the effect of GO exposure on protein-protein interactions. In nematodes, NLG-1/Neuroligin, a postsynaptic protein, acted only in the neurons to regulate the GO toxicity. In the neurons, DLG-1, a PSD-95 protein, and MAGI-1, a S-SCAM protein, were identified as the downstream targets of NLG-1 in the regulation of GO toxicity. PKC-1, a serine/threonine protein kinase C, further acted downstream of neuronal DLG-1 and MAGI-1 to regulate the GO toxicity. Co-immunoprecipitation analysis demonstrated the protein-protein interaction between NLG-1 and DLG-1 or MAGI-1. After GO expression, this protein-protein interaction between NLG-1 and DLG-1 or MAGI-1 was significantly inhibited. Therefore, our data raised the evidence to suggest the potential of GO exposure in disrupting protein-protein interactions in organisms.

Molecular Architecture of Genetically-Tractable GABA Synapses in C. elegans

Inhibitory synapses represent a minority of the total chemical synapses in the mammalian brain, yet proper tuning of inhibition is fundamental to shape neuronal network properties. The neurotransmitter γ-aminobutyric acid (GABA) mediates rapid synaptic inhibition by the activation of the type A GABA receptor (GABA_AR), a pentameric chloride channel that governs major inhibitory neuronal transduction in the nervous system. Impaired GABA transmission leads to a variety of neuropsychiatric diseases, including schizophrenia, autism, epilepsy or anxiety. From an evolutionary perspective, GABA_AR shows remarkable conservations, and are found in all eukaryotic clades and even in bacteria and archaea. Specifically, bona fide GABA_ARs are found in the nematode Caenorhabditis elegans. Because of the anatomical simplicity of the nervous system and its amenability to genetic manipulations, C. elegans provide a powerful system to investigate the molecular and cellular biology of GABA synapses. In this mini review article, we will introduce the structure of the C. elegans GABAergic system and describe recent advances that have identified novel proteins controlling the localization of GABA_ARs at synapses. In particular, Ce-Punctin/MADD-4 is an evolutionarily-conserved extracellular matrix protein that behaves as an anterograde synaptic organizer to instruct the excitatory or inhibitory identity of postsynaptic domains.

Stress-Induced Neuron Remodeling Reveals Differential Interplay Between Neurexin and Environmental Factors in Caenorhabditis elegans

Neurexins are neuronal adhesion molecules important for synapse maturation, function, and plasticity. Neurexins have been genetically associated with neurodevelopmental disorders, including autism spectrum disorders (ASDs) and schizophrenia, but can have variable penetrance and phenotypic severity. Heritability studies indicate that a significant percentage of risk for ASD and schizophrenia includes environmental factors, highlighting a poorly understood interplay between genetic and environmental factors. The singular Caenorhabditis elegans ortholog of human neurexins, nrx-1, controls experience-dependent morphologic remodeling of a GABAergic neuron in adult males. Here, I show remodeling of this neuron's morphology in response to each of three environmental stressors (nutritional, heat, or genotoxic stress) when applied specifically during sexual maturation. Increased outgrowth of axon-like neurites following adolescent stress is the result of an altered morphologic plasticity in adulthood. Despite remodeling being induced by each of the three stressors, only nutritional stress affects downstream behavior and is dependent on neurexin/nrx-1 Heat or genotoxic stress in adolescence does not alter behavior despite inducing GABAergic neuron remodeling, in a neurexin/nrx-1 independent fashion. Starvation-induced remodeling is also dependent on neuroligin/nlg-1, the canonical binding partner for neurexin/nrx-1, and the transcription factors FOXO/daf-16 and HSF1/hsf-1hsf-1 and daf-16, in addition, each have unique roles in remodeling induced by heat and UV stress. The differential molecular mechanisms underlying GABAergic neuron remodeling in response to different stressors, and the disparate effects of stressors on downstream behavior, are a paradigm for understanding how genetics, environmental exposures, and plasticity may contribute to brain dysfunction in ASDs and schizophrenia.

Emerging Roles of Synapse Organizers in the Regulation of Critical Periods

Experience remodels cortical connectivity during developmental windows called critical periods. Experience-dependent regulation of synaptic strength during these periods establishes circuit functions that are stabilized as critical period plasticity wanes. These processes have been extensively studied in the developing visual cortex, where critical period opening and closure are orchestrated by the assembly, maturation, and strengthening of distinct synapse types. The synaptic specificity of these processes points towards the involvement of distinct molecular pathways. Attractive candidates are pre- and postsynaptic transmembrane proteins that form adhesive complexes across the synaptic cleft. These synapse-organizing proteins control synapse development and maintenance and modulate structural and functional properties of synapses. Recent evidence suggests that they have pivotal roles in the onset and closure of the critical period for vision. In this review, we describe roles of synapse-organizing adhesion molecules in the regulation of visual critical period plasticity and we discuss the potential they offer to restore circuit functions in amblyopia and other neurodevelopmental disorders.

Gain-of-function mutations in the UNC-2/CaV2α channel lead to excitation-dominant synaptic transmission in Caenorhabditis elegans

Mutations in pre-synaptic voltage-gated calcium channels can lead to familial hemiplegic migraine type 1 (FHM1). While mammalian studies indicate that the migraine brain is hyperexcitable due to enhanced excitation or reduced inhibition, the molecular and cellular mechanisms underlying this excitatory/inhibitory (E/I) imbalance are poorly understood. We identified a gain-of-function (gf) mutation in the Caenorhabditis elegans CaV2 channel α1 subunit, UNC-2, which leads to increased calcium currents. unc-2(zf35gf) mutants exhibit hyperactivity and seizure-like motor behaviors. Expression of the unc-2 gene with FHM1 substitutions R192Q and S218L leads to hyperactivity similar to that of unc-2(zf35gf) mutants. unc-2(zf35gf) mutants display increased cholinergic and decreased GABAergic transmission. Moreover, increased cholinergic transmission in unc-2(zf35gf) mutants leads to an increase of cholinergic synapses and a TAX-6/calcineurin-dependent reduction of GABA synapses. Our studies reveal mechanisms through which CaV2 gain-of-function mutations disrupt excitation-inhibition balance in the nervous system.

Intrinsic and extrinsic mechanisms of synapse formation and specificity in C. elegans

Precise neuronal wiring is critical for the function of the nervous system and is ultimately determined at the level of individual synapses. Neurons integrate various intrinsic and extrinsic cues to form synapses onto their correct targets in a stereotyped manner. In the past decades, the nervous system of nematode (Caenorhabditis elegans) has provided the genetic platform to reveal the genetic and molecular mechanisms of synapse formation and specificity. In this review, we will summarize the recent discoveries in synapse formation and specificity in C. elegans.

An Upconversion Nanoparticle Enables Near Infrared-Optogenetic Manipulation of the Caenorhabditis elegans Motor Circuit

Near-infrared (NIR) light penetrates tissue deeply, but its application to motor behavior stimulation has been limited by the lack of known genetic NIR light-responsive sensors. We designed and synthesized a Yb³⁺/Er³⁺/Ca²⁺-based lanthanide-doped upconversion nanoparticle (UCNP) that effectively converts 808 nm NIR light to green light emission. This UCNP is compatible with Chrimson, a cation channel activated by green light; as such, it can be used in the optogenetic manipulation of the motor behaviors of Caenorhabditis elegans. We show that this UCNP effectively activates Chrimson-expressing, inhibitory GABAergic motor neurons, leading to reduced action potential firing in the body wall muscle and resulting in locomotion inhibition. The UCNP also activates the excitatory glutamatergic DVC interneuron, leading to potentiated muscle action potential bursts and active reversal locomotion. Moreover, this UCNP exhibits negligible toxicity in neural development, growth, and reproduction, and the NIR energy required to elicit these behavioral and physiological responses does not activate the animal's temperature response. This study shows that UCNP provides a useful integrated optogenetic toolset, which may have wide applications in other experimental systems.

Neuroligin tuning of pharyngeal pumping reveals extrapharyngeal modulation of feeding in Caenorhabditis elegans

The integration of distinct sensory modalities is essential for behavioural decision making. In C aenorhabditis elegans, this process is coordinated by neural circuits that integrate sensory cues from the environment to generate an appropriate behaviour at the appropriate output muscles. Food is a multimodal cue that impacts the microcircuits to modulate feeding and foraging drivers at the level of the pharyngeal and body wall muscle, respectively. When food triggers an upregulation in pharyngeal pumping, it allows the effective ingestion of food. Here, we show that a C elegans mutant in the single gene orthologous to human neuroligins, nlg-1, is defective in food-induced pumping. This was not due to an inability to sense food, as nlg-1 mutants were not defective in chemotaxis towards bacteria. In addition, we found that neuroligin is widely expressed in the nervous system, including AIY, ADE, ALA, URX and HSN neurons. Interestingly, despite the deficit in pharyngeal pumping, neuroligin was not expressed within the pharyngeal neuromuscular network, which suggests an extrapharyngeal regulation of this circuit. We resolved electrophysiologically the neuroligin contribution to the pharyngeal circuit by mimicking food-dependent pumping and found that the nlg-1 phenotype is similar to mutants impaired in GABAergic and/or glutamatergic signalling. We suggest that neuroligin organizes extrapharyngeal circuits that regulate the pharynx. These observations based on the molecular and cellular determinants of feeding are consistent with the emerging role of neuroligin in discretely impacting functional circuits underpinning complex behaviours.

Intracellular protein complexes involved in synapse assembly in presynaptic neurons

The presynaptic active zone, composed of evolutionarily conserved protein complexes, is a specialized area that serves to orchestrate precise and efficient neurotransmitter release by organizing various presynaptic proteins involved in mediating docking and priming of synaptic vesicles, recruiting voltage-gated calcium channels, and modulating presynaptic nerve terminals with aligned postsynaptic structures. Among membrane proteins localized to active zone, presynaptic neurexins and LAR-RPTPs (leukocyte common antigen-related receptor tyrosine phosphatase) have emerged as hubs that orchestrate both shared and distinct extracellular synaptic adhesion pathways. In this chapter, we discuss intracellular signaling cascades involved in recruiting various intracellular proteins at both excitatory and inhibitory synaptic sites. In particular, we highlight recent studies on key active zone proteins that physically and functionally link these cascades with neurexins and LAR-RPTPs in both vertebrate and invertebrate model systems. These studies allow us to build a general, universal view of how presynaptic active zones operate together with postsynaptic structures in neural circuits.

γ-Neurexin and Frizzled Mediate Parallel Synapse Assembly Pathways Antagonized by Receptor Endocytosis

Synapse formation defines neuronal connectivity and is thus essential for neuronal circuit assembly. Trans-synaptic interactions of cell adhesion molecules are thought to induce synapse assembly. Here we demonstrate that a recently discovered and conserved short form of neurexin, γ-neurexin, which lacks canonical extracellular domains, is nonetheless sufficient to promote presynaptic assembly in the nematode C. elegans. γ- but not α-neurexin is required for assembling active zone components, recruiting synaptic vesicles, and clustering calcium channels at release sites to promote evoked synaptic transmission. Furthermore, we find that neurexin functions in parallel with the transmembrane receptor Frizzled, as the absence of both proteins leads to an enhanced phenotype-the loss of most synapses. Frizzled's pro-synaptogenic function is independent of its ligand, Wnt. Wnt binding instead eliminates synapses by inducing Frizzled's endocytosis and the downregulation of neurexin. These results reveal how pro- and anti-synaptogenic factors converge to precisely sculpt circuit formation in vivo.

Ordered arrangement of dendrites within a C. elegans sensory nerve bundle

Biological systems are organized into well-ordered structures and can evolve new patterns when perturbed. To identify principles underlying biological order, we turned to C. elegans for its simple anatomy and powerful genetics. We developed a method to quantify the arrangement of three dendrites in the main sensory nerve bundle, and found that they exhibit a stereotyped arrangement throughout larval growth. Dendrite order does not require prominent features including sensory cilia and glial junctions. In contrast, loss of the cell adhesion molecule (CAM) CDH-4/Fat-like cadherin causes dendrites to be ordered randomly, despite remaining bundled. Loss of the CAMs PTP-3/LAR or SAX-7/L1CAM causes dendrites to adopt an altered order, which becomes increasingly random as animals grow. Misexpression of SAX-7 leads to subtle but reproducible changes in dendrite order. Our results suggest that combinations of CAMs allow dendrites to self-organize into a stereotyped arrangement and can produce altered patterns when perturbed.

The Adhesion-GPCR BAI1 Promotes Excitatory Synaptogenesis by Coordinating Bidirectional Trans-synaptic Signaling

Excitatory synapses are specialized cell-cell contacts located on actin-rich dendritic spines that mediate information flow and storage in the brain. The postsynaptic adhesion-G protein-coupled receptor (A-GPCR) BAI1 is a critical regulator of excitatory synaptogenesis, which functions in part by recruiting the Par3-Tiam1 polarity complex to spines, inducing local Rac1 GTPase activation and actin cytoskeletal remodeling. However, a detailed mechanistic understanding of how BAI1 controls synapse and spine development remains elusive. Here, we confirm that BAI1 is required in vivo for hippocampal spine development, and we identify three distinct signaling mechanisms mediating BAI1's prosynaptogenic functions. Using in utero electroporation to sparsely knock down BAI1 expression in hippocampal pyramidal neurons, we show that BAI1 cell-autonomously promotes spinogenesis in the developing mouse brain. BAI1 appears to function as a receptor at synapses, as its extracellular N-terminal segment is required for both its prospinogenic and prosynaptogenic functions. Moreover, BAI1 activation with a Stachel-derived peptide, which mimics a tethered agonist motif found in A-GPCRs, drives synaptic Rac1 activation and subsequent spine and synapse development. We also reveal, for the first time, a trans-synaptic function for BAI1, demonstrating in a mixed-culture assay that BAI1 induces the clustering of presynaptic vesicular glutamate transporter 1 (vGluT1) in contacting axons, indicative of presynaptic differentiation. Finally, we show that BAI1 forms a receptor complex with the synaptogenic cell-adhesion molecule Neuroligin-1 (NRLN1) and mediates NRLN1-dependent spine growth and synapse development. Together, these findings establish BAI1 as an essential postsynaptic A-GPCR that regulates excitatory synaptogenesis by coordinating bidirectional trans-synaptic signaling in cooperation with NRLN1.SIGNIFICANCE STATEMENT Adhesion-G protein-coupled receptors are cell-adhesion receptors with important roles in nervous system development, function, and neuropsychiatric disorders. The postsynaptic adhesion-G protein-coupled receptor BAI1 is a critical regulator of dendritic spine and excitatory synapse development. However, the mechanism by which BAI1 controls these functions remains unclear. Our study identifies three distinct signaling paradigms for BAI1, demonstrating that it mediates forward, reverse, and lateral signaling in spines. Activation of BAI1 by a Stachel-dependent mechanism induces local Rac1 activation and subsequent spinogenesis/synaptogenesis. BAI1 also signals trans-synaptically to promote presynaptic differentiation. Furthermore, BAI1 interacts with the postsynaptic cell-adhesion molecule Neuroligin-1 (NRLN1) and facilitates NRLN1-dependent spine growth and excitatory synaptogenesis. Thus, our findings establish BAI1 as a functional synaptogenic receptor that promotes presynaptic and postsynaptic development in cooperation with synaptic organizer NRLN1.

Neurexin directs partner-specific synaptic connectivity in C. elegans

In neural circuits, individual neurons often make projections onto multiple postsynaptic partners. Here, we investigate molecular mechanisms by which these divergent connections are generated, using dyadic synapses in C. elegans as a model. We report that C. elegans nrx-1/neurexin directs divergent connectivity through differential actions at synapses with partnering neurons and muscles. We show that cholinergic outputs onto neurons are, unexpectedly, located at previously undefined spine-like protrusions from GABAergic dendrites. Both these spine-like features and cholinergic receptor clustering are strikingly disrupted in the absence of nrx-1. Excitatory transmission onto GABAergic neurons, but not neuromuscular transmission, is also disrupted. Our data indicate that NRX-1 located at presynaptic sites specifically directs postsynaptic development in GABAergic neurons. Our findings provide evidence that individual neurons can direct differential patterns of connectivity with their post-synaptic partners through partner-specific utilization of synaptic organizers, offering a novel view into molecular control of divergent connectivity.

Nervous systems are complex networks of interconnected cells called neurons. These networks vary in size from a few hundred cells in worms, to tens of billions in the human brain. Within these networks, each individual neuron forms connections – called synapses – with many others. But these partner neurons are not necessarily alike. In fact, they may be different cell types. How neurons form distinct connections with different partner cells remains unclear.

Part of the answer may lie in specialized proteins called cell adhesion molecules. These proteins occur on the cell surface and enable neurons to recognize one another. This helps ensure that the cells form appropriate connections via synapses. Cell adhesion molecules are therefore also known as synaptic organizers. Philbrook et al. have now examined the role of synaptic organizers in wiring up the nervous system of the nematode worm and model organism Caenorhabditis elegans.

Motor neurons form connections with two types of partner cell: muscle cells and neurons. Philbrook et al. screened C. elegans that have mutations in genes encoding various synaptic organizers. This revealed that a protein called neurexin must be present for motor neurons to form synapses with other neurons. By contrast, neurexin is not required for the same neurons to establish synapses with muscles. Philbrook et al. found that neuron-to-neuron synapses arise at specialized finger-like projections. These resemble the dendritic spines at which synapses form in the brains of mammals, and had not been previously identified in C. elegans. In worms that lack neurexin, these spine-like structures do not form correctly, disrupting the formation of neuron-to-neuron connections.

Previous work has implicated neurexin in synapse formation in the mammalian brain. But this is the first study to reveal a role for neurexin in establishing partner-specific synaptic connections. Mutations in synaptic organizers, including neurexin, contribute to disorders of brain development. These include schizophrenia and autism spectrum disorders. Learning more about how neurexin helps establish specific synaptic connections may help us understand how these disorders arise.

The Claudin-like Protein HPO-30 Is Required to Maintain LAChRs at the C. elegans Neuromuscular Junction

Communications across chemical synapses are primarily mediated by neurotransmitters and their postsynaptic receptors. There are diverse molecular systems to localize and regulate the receptors at the synapse. Here, we identify HPO-30, a member of the claudin superfamily of membrane proteins, as a positive regulator for synaptic localization of levamisole-dependent AChRs (LAChRs) at the Caenorhabditis elegans neuromuscular junction (NMJ). The HPO-30 protein localizes at the NMJ and shows genetic and physical association with the LAChR subunits LEV-8, UNC-29, and UNC-38. Using genetic and electrophysiological assays in the hermaphrodite C. elegans, we demonstrate that HPO-30 functions through Neuroligin at the NMJ to maintain postsynaptic LAChR levels at the synapse. Together, this work suggests a novel function for a tight junction protein in maintaining normal receptor levels at the NMJ.

SIGNIFICANCE STATEMENT Claudins are a large superfamily of membrane proteins. Their role in maintaining the functional integrity of tight junctions has been widely explored. Our experiments suggest a critical role for the claudin-like protein, HPO-30, in maintaining synaptic levamisole-dependent AChR (LAChR) levels. LAChRs contribute to <20% of the acetylcholine-mediated conductance in adult Caenorhabditis elegans; however, they play a significant functional role in worm locomotion. This study provides a new perspective in the study of LAChR physiology.