The presynaptic dense projection of the Caenorhabditis elegans cholinergic neuromuscular junction localizes synaptic vesicles at the active zone through SYD-2/liprin and UNC-10/RIM-dependent interactions

Neuromuscular Transmission in a Biological Context

Neuromuscular transmission is the process by which motor neurons activate muscle contraction and thus plays an essential role in generating the purposeful body movements that aid survival. While many features of this process are common throughout the Animal Kingdom, such as the release of transmitter in multimolecular "quanta," and the response to it by opening ligand-gated postsynaptic ion channels, there is also much diversity between and within species. Much of this diversity is associated with specialization for either slow, sustained movements such as maintain posture or fast but brief movements used during escape or prey capture. In invertebrates, with hydrostatic and exoskeletons, most motor neurons evoke graded depolarizations of the muscle which cause graded muscle contractions. By contrast, vertebrate motor neurons trigger action potentials in the muscle fibers which give rise to all-or-none contractions. The properties of neuromuscular transmission, in particular the intensity and persistence of transmitter release, reflect these differences. Neuromuscular transmission varies both between and within individual animals, which often have distinct tonic and phasic subsystems. Adaptive plasticity of neuromuscular transmission, on a range of time scales, occurs in many species. This article describes the main steps in neuromuscular transmission and how they vary in a number of "model" species, including C. elegans , Drosophila , zebrafish, mice, and humans. © 2024 American Physiological Society. Compr Physiol 14:5641-5702, 2024.

Liprin-α proteins are master regulators of human presynapse assembly

The formation of mammalian synapses entails the precise alignment of presynaptic release sites with postsynaptic receptors but how nascent cell-cell contacts translate into assembly of presynaptic specializations remains unclear. Guided by pioneering work in invertebrates, we hypothesized that in mammalian synapses, liprin-α proteins directly link trans-synaptic initial contacts to downstream steps. Here we show that, in human neurons lacking all four liprin-α isoforms, nascent synaptic contacts are formed but recruitment of active zone components and accumulation of synaptic vesicles is blocked, resulting in 'empty' boutons and loss of synaptic transmission. Interactions with presynaptic cell adhesion molecules of either the LAR-RPTP family or neurexins via CASK are required to localize liprin-α to nascent synaptic sites. Liprin-α subsequently recruits presynaptic components via a direct interaction with ELKS proteins. Thus, assembly of human presynaptic terminals is governed by a hierarchical sequence of events in which the recruitment of liprin-α proteins by presynaptic cell adhesion molecules is a critical initial step.

Cell non-autonomous signaling through the conserved C. elegans glycopeptide hormone receptor FSHR-1 regulates cholinergic neurotransmission

Modulation of neurotransmission is key for organismal responses to varying physiological contexts such as during infection, injury, or other stresses, as well as in learning and memory and for sensory adaptation. Roles for cell autonomous neuromodulatory mechanisms in these processes have been well described. The importance of cell non-autonomous pathways for inter-tissue signaling, such as gut-to-brain or glia-to-neuron, has emerged more recently, but the cellular mechanisms mediating such regulation remain comparatively unexplored. Glycoproteins and their G protein-coupled receptors (GPCRs) are well-established orchestrators of multi-tissue signaling events that govern diverse physiological processes through both cell-autonomous and cell non-autonomous regulation. Here, we show that follicle stimulating hormone receptor, FSHR-1, the sole Caenorhabditis elegans ortholog of mammalian glycoprotein hormone GPCRs, is important for cell non-autonomous modulation of synaptic transmission. Inhibition of fshr-1 expression reduces muscle contraction and leads to synaptic vesicle accumulation in cholinergic motor neurons. The neuromuscular and locomotor defects in fshr-1 loss-of-function mutants are associated with an underlying accumulation of synaptic vesicles, build-up of the synaptic vesicle priming factor UNC-10/RIM, and decreased synaptic vesicle release from cholinergic motor neurons. Restoration of FSHR-1 to the intestine is sufficient to restore neuromuscular activity and synaptic vesicle localization to fshr-1- deficient animals. Intestine-specific knockdown of FSHR-1 reduces neuromuscular function, indicating FSHR-1 is both necessary and sufficient in the intestine for its neuromuscular effects. Re-expression of FSHR-1 in other sites of endogenous expression, including glial cells and neurons, also restored some neuromuscular deficits, indicating potential cross-tissue regulation from these tissues as well. Genetic interaction studies provide evidence that downstream effectors gsa-1 / Gα S , acy-1 /adenylyl cyclase and sphk-1/ sphingosine kinase and glycoprotein hormone subunit orthologs, GPLA-1/GPA2 and GPLB-1/GPB5, are important for FSHR-1 modulation of the NMJ. Together, our results demonstrate that FSHR-1 modulation directs inter-tissue signaling systems, which promote synaptic vesicle release at neuromuscular synapses.

Protein-lipid interactions drive presynaptic assembly upstream of cell adhesion molecules

Textbook models of synaptogenesis position cell adhesion molecules such as neurexin as initiators of synapse assembly. Here we discover a mechanism for presynaptic assembly that occurs prior to neurexin recruitment, while supporting a role for neurexin in synapse maintenance. We find that the cytosolic active zone scaffold SYD-1 interacts with membrane phospholipids to promote active zone protein clustering at the plasma membrane, and subsequently recruits neurexin to stabilize those clusters. Employing molecular dynamics simulations to model intrinsic interactions between SYD-1 and lipid bilayers followed by in vivo tests of these predictions, we find that PIP2-interacting residues in SYD-1's C2 and PDZ domains are redundantly necessary for proper active zone assembly. Finally, we propose that the uncharacterized yet evolutionarily conserved short γ isoform of neurexin represents a minimal neurexin sequence that can stabilize previously assembled presynaptic clusters, potentially a core function of this critical protein.

CaMKII mediates sexually dimorphic synaptic transmission at neuromuscular junctions in C. elegans

Sexually dimorphic behaviors are ubiquitous throughout the animal kingdom. Although both sex-specific and sex-shared neurons have been functionally implicated in these diverse behaviors, less is known about the roles of sex-shared neurons. Here, we discovered sexually dimorphic cholinergic synaptic transmission in C. elegans occurring at neuromuscular junctions (NMJs), with males exhibiting increased release frequencies, which result in sexually dimorphic locomotion behaviors. Scanning electron microscopy revealed that males have significantly more synaptic vesicles (SVs) at their cholinergic synapses than hermaphrodites. Analysis of previously published transcriptome identified the male-enriched transcripts and focused our attention on UNC-43/CaMKII. We ultimately show that differential accumulation of UNC-43 at cholinergic neurons controls axonal SV abundance and synaptic transmission. Finally, we demonstrate that sex reversal of all neurons in hermaphrodites generates male-like cholinergic transmission and locomotion behaviors. Thus, beyond demonstrating UNC-43/CaMKII as an essential mediator of sex-specific synaptic transmission, our study provides molecular and cellular insights into how sex-shared neurons can generate sexually dimorphic locomotion behaviors.

C. elegans Clarinet/CLA-1 recruits RIMB-1/RIM-binding protein and UNC-13 to orchestrate presynaptic neurotransmitter release

Synaptic transmission requires the coordinated activity of multiple synaptic proteins that are localized at the active zone (AZ). We previously identified a Caenorhabditis elegans protein named Clarinet (CLA-1) based on homology to the AZ proteins Piccolo, Rab3-interactingmolecule (RIM)/UNC-10 and Fife. At the neuromuscular junction (NMJ), cla-1 null mutants exhibit release defects that are greatly exacerbated in cla-1;unc-10 double mutants. To gain insights into the coordinated roles of CLA-1 and UNC-10, we examined the relative contributions of each to the function and organization of the AZ. Using a combination of electrophysiology, electron microscopy, and quantitative fluorescence imaging we explored the functional relationship of CLA-1 to other key AZ proteins including: RIM1, Cav2.1 channels, RIM1-binding protein, and Munc13 (C. elegans UNC-10, UNC-2, RIMB-1 and UNC-13, respectively). Our analyses show that CLA-1 acts in concert with UNC-10 to regulate UNC-2 calcium channel levels at the synapse via recruitment of RIMB-1. In addition, CLA-1 exerts a RIMB-1-independent role in the localization of the priming factor UNC-13. Thus C. elegans CLA-1/UNC-10 exhibit combinatorial effects that have overlapping design principles with other model organisms: RIM/RBP and RIM/ELKS in mouse and Fife/RIM and BRP/RBP in Drosophila. These data support a semiconserved arrangement of AZ scaffolding proteins that are necessary for the localization and activation of the fusion machinery within nanodomains for precise coupling to Ca2+ channels.

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.

Single-Molecule Localization Microscopy of Presynaptic Active Zones in Drosophila melanogaster after Rapid Cryofixation

Single-molecule localization microscopy (SMLM) greatly advances structural studies of diverse biological tissues. For example, presynaptic active zone (AZ) nanotopology is resolved in increasing detail. Immunofluorescence imaging of AZ proteins usually relies on epitope preservation using aldehyde-based immunocompetent fixation. Cryofixation techniques, such as high-pressure freezing (HPF) and freeze substitution (FS), are widely used for ultrastructural studies of presynaptic architecture in electron microscopy (EM). HPF/FS demonstrated nearer-to-native preservation of AZ ultrastructure, e.g., by facilitating single filamentous structures. Here, we present a protocol combining the advantages of HPF/FS and direct stochastic optical reconstruction microscopy (dSTORM) to quantify nanotopology of the AZ scaffold protein Bruchpilot (Brp) at neuromuscular junctions (NMJs) of Drosophila melanogaster. Using this standardized model, we tested for preservation of Brp clusters in different FS protocols compared to classical aldehyde fixation. In HPF/FS samples, presynaptic boutons were structurally well preserved with ~22% smaller Brp clusters that allowed quantification of subcluster topology. In summary, we established a standardized near-to-native preparation and immunohistochemistry protocol for SMLM analyses of AZ protein clusters in a defined model synapse. Our protocol could be adapted to study protein arrangements at single-molecule resolution in other intact tissue preparations.

A lipid transfer protein ensures nematode cuticular impermeability

The cuticle of C. elegans is impermeable to chemicals, toxins, and pathogens. However, increased permeability is a desirable phenotype because it facilitates chemical uptake. Surface lipids contribute to the permeability barrier. Here, we identify the lipid transfer protein GMAP-1 as a critical element setting the permeability of the C. elegans cuticle. A gmap-1 deletion mutant increases cuticular permeability to sodium azide, levamisole, Hoechst, and DiI. Expressing GMAP-1 in the hypodermis or transiently in the adults is sufficient to rescue this gmap-1 permeability phenotype. GMAP-1 protein is secreted from the hypodermis to the aqueous fluid filling the space between collagen fibers of the cuticle. In vitro, GMAP-1 protein binds phosphatidylserine and phosphatidylcholine while in vivo, GMAP-1 sets the surface lipid composition and organization. Altogether, our results suggest GMAP-1 secreted by hypodermis shuttles lipids to the surface to form the permeability barrier of C. elegans.

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GMAP-1 is secreted by the hypodermis toward the cuticle of Caenorhabditis elegans

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GMAP-1 binds and shuttle phosphoglycerides

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GMAP-1 sets the lipid composition of the cuticle

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While healthy, gmap-1 mutant displays high cuticular permeability

Ultrastructural analysis of wild-type and RIM1α knockout active zones in a large cortical synapse

Rab3A-interacting molecule (RIM) is crucial for fast Ca²⁺-triggered synaptic vesicle (SV) release in presynaptic active zones (AZs). We investigated hippocampal giant mossy fiber bouton (MFB) AZ architecture in 3D using electron tomography of rapid cryo-immobilized acute brain slices in RIM1α^-/- and wild-type mice. In RIM1α^-/-, AZs are larger with increased synaptic cleft widths and a 3-fold reduced number of tightly docked SVs (0-2 nm). The distance of tightly docked SVs to the AZ center is increased from 110 to 195 nm, and the width of their electron-dense material between outer SV membrane and AZ membrane is reduced. Furthermore, the SV pool in RIM1α^-/- is more heterogeneous. Thus, RIM1α, besides its role in tight SV docking, is crucial for synaptic architecture and vesicle pool organization in MFBs.

Hypothesis Relating the Structure, Biochemistry and Function of Active Zone Material Macromolecules at a Neuromuscular Junction

This report integrates knowledge of in situ macromolecular structures and synaptic protein biochemistry to propose a unified hypothesis for the regulation of certain vesicle trafficking events (i.e., docking, priming, Ca²⁺-triggering, and membrane fusion) that lead to neurotransmitter secretion from specialized “active zones” of presynaptic axon terminals. Advancements in electron tomography, to image tissue sections in 3D at nanometer scale resolution, have led to structural characterizations of a network of different classes of macromolecules at the active zone, called “Active Zone Material’. At frog neuromuscular junctions, the classes of Active Zone Material macromolecules “top-masts”, “booms”, “spars”, “ribs” and “pins” direct synaptic vesicle docking while “pins”, “ribs” and “pegs” regulate priming to influence Ca²⁺-triggering and membrane fusion. Other classes, “beams”, “steps”, “masts”, and “synaptic vesicle luminal filaments’ likely help organize and maintain the structural integrity of active zones. Extensive studies on the biochemistry that regulates secretion have led to comprehensive characterizations of the many conserved proteins universally involved in these trafficking events. Here, a hypothesis including a partial proteomic atlas of Active Zone Material is presented which considers the common roles, binding partners, physical features/structure, and relative positioning in the axon terminal of both the proteins and classes of macromolecules involved in the vesicle trafficking events. The hypothesis designates voltage-gated Ca²⁺ channels and Ca²⁺-gated K⁺ channels to ribs and pegs that are connected to macromolecules that span the presynaptic membrane at the active zone. SNARE proteins (Syntaxin, SNAP25, and Synaptobrevin), SNARE-interacting proteins Synaptotagmin, Munc13, Munc18, Complexin, and NSF are designated to ribs and/or pins. Rab3A and Rabphillin-3A are designated to top-masts and/or booms and/or spars. RIM, Bassoon, and Piccolo are designated to beams, steps, masts, ribs, spars, booms, and top-masts. Spectrin is designated to beams. Lastly, the luminal portions of SV2 are thought to form the bulk of the observed synaptic vesicle luminal filaments. The goal here is to help direct future studies that aim to bridge Active Zone Material structure, biochemistry, and function to ultimately determine how it regulates the trafficking events in vivo that lead to neurotransmitter secretion.

Neuronal Menin Overexpression Rescues Learning and Memory Phenotype in CA1-Specific α7 nAChRs KD Mice

The perturbation of nicotinic cholinergic receptors is thought to underlie many neurodegenerative and neuropsychiatric disorders, such as Alzheimer's and schizophrenia. We previously identified that the tumor suppressor gene, MEN1, regulates both the expression and synaptic targeting of α7 nAChRs in the mouse hippocampal neurons in vitro. Here we sought to determine whether the α7 nAChRs gene expression reciprocally regulates the expression of menin, the protein encoded by the MEN1 gene, and if this interplay impacts learning and memory. We demonstrate here that α7 nAChRs knockdown (KD) both in in vitro and in vivo, initially upregulated and then subsequently downregulated menin expression. Exogenous expression of menin using an AAV transduction approach rescued α7 nAChRs KD mediated functional and behavioral deficits specifically in hippocampal (CA1) neurons. These effects involved the modulation of the α7 nAChR subunit expression and functional clustering at the synaptic sites. Our data thus demonstrates a novel and important interplay between the MEN1 gene and the α7 nAChRs in regulating hippocampal-dependent learning and memory.

Structural Analysis of the Caenorhabditis elegans Dauer Larval Anterior Sensilla by Focused Ion Beam-Scanning Electron Microscopy

At the end of the first larval stage, the nematode Caenorhabditis elegans developing in harsh environmental conditions is able to choose an alternative developmental path called the dauer diapause. Dauer larvae exhibit different physiology and behaviors from non-dauer larvae. Using focused ion beam-scanning electron microscopy (FIB-SEM), we volumetrically reconstructed the anterior sensory apparatus of C. elegans dauer larvae with unprecedented precision. We provide a detailed description of some neurons, focusing on structural details that were unknown or unresolved by previously published studies. They include the following: (1) dauer-specific branches of the IL2 sensory neurons project into the periphery of anterior sensilla and motor or putative sensory neurons at the sub-lateral cords; (2) ciliated endings of URX sensory neurons are supported by both ILso and AMso socket cells near the amphid openings; (3) variability in amphid sensory dendrites among dauers; and (4) somatic RIP interneurons maintain their projection into the pharyngeal nervous system. Our results support the notion that dauer larvae structurally expand their sensory system to facilitate searching for more favorable environments.

SNARE Regulatory Proteins in Synaptic Vesicle Fusion and Recycling

Membrane fusion is a universal feature of eukaryotic protein trafficking and is mediated by the soluble N-ethylmaleimide sensitive factor attachment protein receptor (SNARE) family. SNARE proteins embedded in opposing membranes spontaneously assemble to drive membrane fusion and cargo exchange in vitro. Evolution has generated a diverse complement of SNARE regulatory proteins (SRPs) that ensure membrane fusion occurs at the right time and place in vivo. While a core set of SNAREs and SRPs are common to all eukaryotic cells, a specialized set of SRPs within neurons confer additional regulation to synaptic vesicle (SV) fusion. Neuronal communication is characterized by precise spatial and temporal control of SNARE dynamics within presynaptic subdomains specialized for neurotransmitter release. Action potential-elicited Ca²⁺ influx at these release sites triggers zippering of SNAREs embedded in the SV and plasma membrane to drive bilayer fusion and release of neurotransmitters that activate downstream targets. Here we discuss current models for how SRPs regulate SNARE dynamics and presynaptic output, emphasizing invertebrate genetic findings that advanced our understanding of SRP regulation of SV cycling.

Transcriptional Control of Parallel-Acting Pathways That Remove Specific Presynaptic Proteins in Remodeling Neurons

Synapses are actively dismantled to mediate circuit refinement, but the developmental pathways that regulate synaptic disassembly are largely unknown. We have previously shown that the epithelial sodium channel ENaC/UNC-8 triggers an activity-dependent mechanism that drives the removal of presynaptic proteins liprin-α/SYD-2, Synaptobrevin/SNB-1, RAB-3, and Endophilin/UNC-57 in remodeling GABAergic neurons in Caenorhabditis elegans (Miller-Fleming et al., 2016). Here, we report that the conserved transcription factor Iroquois/IRX-1 regulates UNC-8 expression as well as an additional pathway, independent of UNC-8, that functions in parallel to dismantle functional presynaptic terminals. We show that the additional IRX-1-regulated pathway is selectively required for the removal of the presynaptic proteins, Munc13/UNC-13 and ELKS, which normally mediate synaptic vesicle (SV) fusion and neurotransmitter release. Our findings are notable because they highlight the key role of transcriptional regulation in synapse elimination during development and reveal parallel-acting pathways that coordinate synaptic disassembly by removing specific active zone proteins.SIGNIFICANCE STATEMENT Synaptic pruning is a conserved feature of developing neural circuits but the mechanisms that dismantle the presynaptic apparatus are largely unknown. We have determined that synaptic disassembly is orchestrated by parallel-acting mechanisms that target distinct components of the active zone. Thus, our finding suggests that synaptic disassembly is not accomplished by en masse destruction but depends on mechanisms that dismantle the structure in an organized process.

PTP-3 phosphatase promotes intramolecular folding of SYD-2 to inactivate kinesin-3 UNC-104 in neurons

UNC-104 is the Caenorhabditis elegans homolog of kinesin-3 KIF1A known for its fast shuffling of synaptic vesicle protein transport vesicles in axons. SYD-2 is the homolog of liprin-α in C. elegans known to activate UNC-104; however, signals that trigger SYD-2 binding to the motor remain unknown. Because SYD-2 is a substrate of PTP-3/LAR PTPR, we speculate a role of this phosphatase in SYD–2-mediated motor activation. Indeed, coimmunoprecipitation assays revealed increased interaction between UNC-104 and SYD-2 in ptp-3 knockout worms. Intramolecular FRET analysis in living nematodes demonstrates that SYD-2 largely exists in an open conformation state in ptp-3 mutants. These assays also revealed that nonphosphorylatable SYD-2 (Y741F) exists predominately in folded conformations, while phosphomimicking SYD-2 (Y741E) primarily exists in open conformations. Increased UNC-104 motor clustering was observed along axons likely as a result of elevated SYD-2 scaffolding function in ptp-3 mutants. Also, both motor velocities as well as cargo transport speeds were visibly increased in neurons of ptp-3 mutants. Lastly, epistatic analysis revealed that PTP-3 is upstream of SYD-2 to regulate its intramolecular folding.

Overexpression of an ALS-associated FUS mutation in C. elegans disrupts NMJ morphology and leads to defective neuromuscular transmission

The amyotrophic lateral sclerosis (ALS) neurodegenerative disorder has been associated with multiple genetic lesions, including mutations in the gene for fused in sarcoma (FUS), a nuclear-localized RNA/DNA-binding protein. Neuronal expression of the pathological form of FUS proteins in Caenorhabditis elegans results in mislocalization and aggregation of FUS in the cytoplasm, and leads to impairment of motility. However, the mechanisms by which the mutant FUS disrupts neuronal health and function remain unclear. Here we investigated the impact of ALS-associated FUS on motor neuron health using correlative light and electron microscopy, electron tomography, and electrophysiology. We show that ectopic expression of wild-type or ALS-associated human FUS impairs synaptic vesicle docking at neuromuscular junctions. ALS-associated FUS led to the emergence of a population of large, electron-dense, and filament-filled endosomes. Electrophysiological recording revealed reduced transmission from motor neurons to muscles. Together, these results suggest a pathological effect of ALS-causing FUS at synaptic structure and function organization.This article has an associated First Person interview with the first author of the paper.

Synapse development and maturation at the drosophila neuromuscular junction

Synapses are the sites of neuron-to-neuron communication and form the basis of the neural circuits that underlie all animal cognition and behavior. Chemical synapses are specialized asymmetric junctions between a presynaptic neuron and a postsynaptic target that form through a series of diverse cellular and subcellular events under the control of complex signaling networks. Once established, the synapse facilitates neurotransmission by mediating the organization and fusion of synaptic vesicles and must also retain the ability to undergo plastic changes. In recent years, synaptic genes have been implicated in a wide array of neurodevelopmental disorders; the individual and societal burdens imposed by these disorders, as well as the lack of effective therapies, motivates continued work on fundamental synapse biology. The properties and functions of the nervous system are remarkably conserved across animal phyla, and many insights into the synapses of the vertebrate central nervous system have been derived from studies of invertebrate models. A prominent model synapse is the Drosophila melanogaster larval neuromuscular junction, which bears striking similarities to the glutamatergic synapses of the vertebrate brain and spine; further advantages include the simplicity and experimental versatility of the fly, as well as its century-long history as a model organism. Here, we survey findings on the major events in synaptogenesis, including target specification, morphogenesis, and the assembly and maturation of synaptic specializations, with a emphasis on work conducted at the Drosophila neuromuscular junction.

Expansion microscopy of C. elegans

We recently developed expansion microscopy (ExM), which achieves nanoscale-precise imaging of specimens at ~70 nm resolution (with ~4.5x linear expansion) by isotropic swelling of chemically processed, hydrogel-embedded tissue. ExM of C. elegans is challenged by its cuticle, which is stiff and impermeable to antibodies. Here we present a strategy, expansion of C. elegans (ExCel), to expand fixed, intact C. elegans. ExCel enables simultaneous readout of fluorescent proteins, RNA, DNA location, and anatomical structures at resolutions of ~65-75 nm (3.3-3.8x linear expansion). We also developed epitope-preserving ExCel, which enables imaging of endogenous proteins stained by antibodies, and iterative ExCel, which enables imaging of fluorescent proteins after 20x linear expansion. We demonstrate the utility of the ExCel toolbox for mapping synaptic proteins, for identifying previously unreported proteins at cell junctions, and for gene expression analysis in multiple individual neurons of the same animal.

Quantitative basis of meiotic chromosome synapsis analyzed by electron tomography

The synaptonemal complex is a multiprotein complex, which mediates the synapsis and recombination between homologous chromosomes during meiosis. The complex is comprised of two lateral elements and a central element connected by perpendicular transverse filaments (TFs). A 3D model based on actual morphological data of the SC is missing. Here, we applied electron tomography (ET) and manual feature extraction to generate a quantitative 3D model of the murine SC. We quantified the length (90 nm) and width (2 nm) of the TFs. Interestingly, the 80 TFs/µm are distributed asymmetrically in the central region of the SC challenging available models of SC organization. Furthermore, our detailed 3D topological analysis does not support a bilayered organization of the central region as proposed earlier. Overall, our quantitative analysis is relevant to understand the functions and dynamics of the SC and provides the basis for analyzing multiprotein complexes in their morphological context using ET.

Electron tomography of mouse LINC complexes at meiotic telomere attachment sites with and without microtubules

Telomere movements during meiotic prophase I facilitate synapsis and recombination of homologous chromosomes. Hereby, chromosome movements depend on the dynamic attachment of meiotic telomeres to the nuclear envelope and generation of forces that actively move the telomeres. In most eukaryotes, forces that move telomeres are generated in the cytoplasm by microtubule-associated motor proteins and transduced into the nucleus through the LINC complexes of the nuclear envelope. Meiotic LINC complexes, in mouse comprised of SUN1/2 and KASH5, selectively localize to the attachment sites of meiotic telomeres. For a better understanding of meiotic telomere dynamics, here we provide quantitative information of telomere attachment sites that we have generated with the aid of electron microscope tomography (EM tomography). Our data on the number, length, width, distribution and relation with microtubules of the reconstructed structures indicate that an average number of 76 LINC complexes would be required to move a telomere attachment site.

RIMB-1/RIM-Binding Protein and UNC-10/RIM Redundantly Regulate Presynaptic Localization of the Voltage-Gated Calcium Channel in Caenorhabditis elegans

Presynaptic active zones (AZs) contain many molecules essential for neurotransmitter release and are assembled in a highly organized manner. A network of adaptor proteins known as cytomatrix at the AZ (CAZ) is important for shaping the structural characteristics of AZ. Rab3-interacting molecule (RIM)-binding protein (RBP) family are binding partners of the CAZ protein RIM and also bind the voltage-gated calcium channels (VGCCs) in mice and flies. Here, we investigated the physiological roles of RIMB-1, the homolog of RBPs in the nematode Caenorhabditis elegans RIMB-1 is expressed broadly in neurons and predominantly localized at presynaptic sites. Loss-of-function animals of rimb-1 displayed slight defects in motility and response to pharmacological inhibition of synaptic transmission, suggesting a modest involvement of rimb-1 in synapse function. We analyzed genetic interactions of rimb-1 by testing candidate genes and by an unbiased forward genetic screen for rimb-1 enhancer. Both analyses identified the RIM homolog UNC-10 that acts together with RIMB-1 to regulate presynaptic localization of the P/Q-type VGCC UNC-2/Ca_v2. We also find that the precise localization of RIMB-1 to presynaptic sites requires presynaptic UNC-2/Ca_v2. RIMB-1 has multiple FN3 and SH3 domains. Our transgenic rescue analysis with RIMB-1 deletion constructs revealed a functional requirement of a C-terminal SH3 in regulating UNC-2/Ca_v2 localization. Together, these findings suggest a redundant role of RIMB-1/RBP and UNC-10/RIM to regulate the abundance of UNC-2/Ca_v2 at the presynaptic AZ in C. elegans, depending on the bidirectional interplay between CAZ adaptor and channel proteins.SIGNIFICANCE STATEMENT Presynaptic active zones (AZs) are highly organized structures for synaptic transmission with characteristic networks of adaptor proteins called cytomatrix at the AZ (CAZ). In this study, we characterized a CAZ protein RIMB-1, named for RIM-binding protein (RBP), in the nematode Caenorhabditis elegans Through systematic analyses of genetic interactions and an unbiased genetic enhancer screen of rimb-1, we revealed a redundant role of two CAZ proteins RIMB-1/RBP and UNC-10/RIM in regulating presynaptic localization of UNC-2/Ca_v2, a voltage-gated calcium channel (VGCC) critical for proper neurotransmitter release. Additionally, the precise localization of RIMB-1/RBP requires presynaptic UNC-2/Ca_v2. These findings provide new mechanistic insight about how the interplay among multiple CAZ adaptor proteins and VGCC contributes to the organization of presynaptic AZ.

Nanomachinery Organizing Release at Neuronal and Ribbon Synapses

A critical aim in neuroscience is to obtain a comprehensive view of how regulated neurotransmission is achieved. Our current understanding of synapses relies mainly on data from electrophysiological recordings, imaging, and molecular biology. Based on these methodologies, proteins involved in a synaptic vesicle (SV) formation, mobility, and fusion at the active zone (AZ) membrane have been identified. In the last decade, electron tomography (ET) combined with a rapid freezing immobilization of neuronal samples opened a window for understanding the structural machinery with the highest spatial resolution in situ. ET provides significant insights into the molecular architecture of the AZ and the organelles within the presynaptic nerve terminal. The specialized sensory ribbon synapses exhibit a distinct architecture from neuronal synapses due to the presence of the electron-dense synaptic ribbon. However, both synapse types share the filamentous structures, also commonly termed as tethers that are proposed to contribute to different steps of SV recruitment and exocytosis. In this review, we discuss the emerging views on the role of filamentous structures in SV exocytosis gained from ultrastructural studies of excitatory, mainly central neuronal compared to ribbon-type synapses with a focus on inner hair cell (IHC) ribbon synapses. Moreover, we will speculate on the molecular entities that may be involved in filament formation and hence play a crucial role in the SV cycle.

Synaptogenic pathways

During synaptogenesis, presynaptic and postsynaptic assembly are driven by diverse molecular mechanisms, mediated by intrinsic as well as extrinsic factors. How these processes are initiated and coordinated are open questions. Synapse specificity, or synaptic partner selection, is widely understood to be determined by the trans-synaptic binding of cell adhesion molecules. However, in vivo evidence that cell adhesion molecules subsequently function to initiate synapse assembly, as initially proposed, is lacking. Here, we present a summary of our current understanding of synaptogenic pathways that mediate presynaptic and postsynaptic assembly and the coordination of these processes.

Molecular architecture of the presynaptic terminal

Neurotransmitter release at the presynaptic terminal is one of the fundamental processes in neuronal communication. It is a complex process comprising signaling pathways that exert a precise spatio-temporal coordination to prepare and bring synaptic vesicles to exocytosis. While many molecular components involved have been identified, their direct observation at different stages of the neurotransmitter release is lacking. Three-dimensional imaging by electron tomography provided remarkable views of the synaptic vesicles and the cytomatrix. Imaging fully hydrated, vitrified samples allowed a direct visualization, precise localization and a quantitative characterization of pleomorphic synaptic vesicle-bound complexes in situ, as well as the elucidation of their function in the neurotransmitter release.

Sentryn Acts with a Subset of Active Zone Proteins To Optimize the Localization of Synaptic Vesicles in Caenorhabditis elegans

Synaptic vesicles (SVs) transmit signals by releasing neurotransmitters from specialized synaptic regions of neurons. In the synaptic region, SVs are tightly clustered around small structures called active zones. The motor KIF1A transports SVs outward through axons until they are captured in the synaptic region. This transport must be guided in the forward direction because it is opposed by the dynein motor, which causes SVs to reverse direction multiple times en route. The core synapse stability (CSS) system contributes to both guided transport and capture of SVs. We identified Sentryn as a CSS protein that contributes to the synaptic localization of SVs in Caenorhabditis elegans Like the CSS proteins SAD Kinase and SYD-2 (Liprin-α), Sentryn also prevents dynein-dependent accumulation of lysosomes in dendrites in strains lacking JIP3. Genetic analysis showed that Sentryn and SAD Kinase each have at least one nonoverlapping function for the stable accumulation of SVs at synapses that, when combined with their shared functions, enables most of the functions of SYD-2 (Liprin-α) for capturing SVs. Also like other CSS proteins, Sentryn appears enriched at active zones and contributes to active zone structure, suggesting that it is a novel, conserved active zone protein. Sentryn is recruited to active zones by a process dependent on the active zone-enriched CSS protein SYD-2 (Liprin-α). Our results define a specialized group of active zone enriched proteins that can affect motorized transport throughout the neuron and that have roles in both guided transport and capture of SVs.

Sentryn and SAD Kinase Link the Guided Transport and Capture of Dense Core Vesicles in Caenorhabditis elegans

Dense core vesicles (DCVs) can transmit signals by releasing neuropeptides from specialized synaptic regions called active zones. DCVs reach the active zone by motorized transport through a long axon. A reverse motor frequently interrupts progress by taking DCVs in the opposite direction. "Guided transport" refers to the mechanism by which outward movements ultimately dominate to bring DCVs to the synaptic region. After guided transport, DCVs alter their interactions with motors and enter a "captured" state. The mechanisms of guided transport and capture of DCVs are unknown. Here, we discovered two proteins that contribute to both processes in Caenorhabditis elegans SAD kinase and a novel conserved protein we named Sentryn are the first proteins found to promote DCV capture. By imaging DCVs moving in various regions of single identified neurons in living animals, we found that DCV guided transport and capture are linked through SAD kinase, Sentryn, and Liprin-α. These proteins act together to regulate DCV motorized transport in a region-specific manner. Between the cell body and the synaptic region, they promote forward transport. In the synaptic region, where all three proteins are highly enriched at active zones, they promote DCV pausing by inhibiting transport in both directions. These three proteins appear to be part of a special subset of active zone-enriched proteins because other active zone proteins do not share their unique functions.

Decoding the intensity of sensory input by two glutamate receptors in one C. elegans interneuron

How neurons are capable of decoding stimulus intensity and translate this information into complex behavioral outputs is poorly defined. Here, we demonstrate that the C. elegans interneuron AIB regulates two types of behaviors: reversal initiation and feeding suppression in response to different concentrations of quinine. Low concentrations of quinine are decoded in AIB by a low-threshold, fast-inactivation glutamate receptor GLR-1 and translated into reversal initiation. In contrast, high concentrations of quinine are decoded by a high-threshold, slow-inactivation glutamate receptor GLR-5 in AIB. After activation, GLR-5 evokes sustained Ca²⁺ release from the inositol 1,4,5-trisphosphate (IP₃)-sensitive Ca²⁺ stores and triggers neuropeptide secretion, which in turn activates the downstream neuron RIM and inhibits feeding. Our results reveal that distinct signal patterns in a single interneuron AIB can encode differential behavioral outputs depending on the stimulus intensity, thus highlighting the importance of functional mapping of information propagation at the single-neuron level during connectome construction.

Coupling the Structural and Functional Assembly of Synaptic Release Sites

Information processing in our brains depends on the exact timing of calcium (Ca²⁺)-activated exocytosis of synaptic vesicles (SVs) from unique release sites embedded within the presynaptic active zones (AZs). While AZ scaffolding proteins obviously provide an efficient environment for release site function, the molecular design creating such release sites had remained unknown for a long time. Recent advances in visualizing the ultrastructure and topology of presynaptic protein architectures have started to elucidate how scaffold proteins establish “nanodomains” that connect voltage-gated Ca²⁺ channels (VGCCs) physically and functionally with release-ready SVs. Scaffold proteins here seem to operate as “molecular rulers or spacers,” regulating SV-VGCC physical distances within tens of nanometers and, thus, influence the probability and plasticity of SV release. A number of recent studies at Drosophila and mammalian synapses show that the stable positioning of discrete clusters of obligate release factor (M)Unc13 defines the position of SV release sites, and the differential expression of (M)Unc13 isoforms at synapses can regulate SV-VGCC coupling. We here review the organization of matured AZ scaffolds concerning their intrinsic organization and role for release site formation. Moreover, we also discuss insights into the developmental sequence of AZ assembly, which often entails a tightening between VGCCs and SV release sites. The findings discussed here are retrieved from vertebrate and invertebrate preparations and include a spectrum of methods ranging from cell biology, super-resolution light and electron microscopy to biophysical and electrophysiological analysis. Our understanding of how the structural and functional organization of presynaptic AZs are coupled has matured, as these processes are crucial for the understanding of synapse maturation and plasticity, and, thus, accurate information transfer and storage at chemical synapses.

Automated classification of synaptic vesicles in electron tomograms of C. elegans using machine learning

Synaptic vesicles (SVs) are a key component of neuronal signaling and fulfil different roles depending on their composition. In electron micrograms of neurites, two types of vesicles can be distinguished by morphological criteria, the classical “clear core” vesicles (CCV) and the typically larger “dense core” vesicles (DCV), with differences in electron density due to their diverse cargos. Compared to CCVs, the precise function of DCVs is less defined. DCVs are known to store neuropeptides, which function as neuronal messengers and modulators [1]. In C. elegans, they play a role in locomotion, dauer formation, egg-laying, and mechano- and chemosensation [2]. Another type of DCVs, also referred to as granulated vesicles, are known to transport Bassoon, Piccolo and further constituents of the presynaptic density in the center of the active zone (AZ), and therefore are important for synaptogenesis [3].

To better understand the role of different types of SVs, we present here a new automated approach to classify vesicles. We combine machine learning with an extension of our previously developed vesicle segmentation workflow, the ImageJ macro 3D ART VeSElecT. With that we reliably distinguish CCVs and DCVs in electron tomograms of C. elegans NMJs using image-based features. Analysis of the underlying ground truth data shows an increased fraction of DCVs as well as a higher mean distance between DCVs and AZs in dauer larvae compared to young adult hermaphrodites. Our machine learning based tools are adaptable and can be applied to study properties of different synaptic vesicle pools in electron tomograms of diverse model organisms.

An Input-Specific Orphan Receptor GPR158-HSPG Interaction Organizes Hippocampal Mossy Fiber-CA3 Synapses

Pyramidal neuron dendrites integrate synaptic input from multiple partners. Different inputs converging on the same dendrite have distinct structural and functional features, but the molecular mechanisms organizing input-specific properties are poorly understood. We identify the orphan receptor GPR158 as a binding partner for the heparan sulfate proteoglycan (HSPG) glypican 4 (GPC4). GPC4 is enriched on hippocampal granule cell axons (mossy fibers), whereas postsynaptic GPR158 is restricted to the proximal segment of CA3 apical dendrites receiving mossy fiber input. GPR158-induced presynaptic differentiation in contacting axons requires cell-surface GPC4 and the co-receptor LAR. Loss of GPR158 increases mossy fiber synapse density but disrupts bouton morphology, impairs ultrastructural organization of active zone and postsynaptic density, and reduces synaptic strength of this connection, while adjacent inputs on the same dendrite are unaffected. Our work identifies an input-specific HSPG-GPR158 interaction that selectively organizes synaptic architecture and function of developing mossy fiber-CA3 synapses in the hippocampus.

Understanding the Molecular Basis of Salt Sequestration in Epidermal Bladder Cells of Chenopodium quinoa

Soil salinity is destroying arable land and is considered to be one of the major threats to global food security in the 21st century. Therefore, the ability of naturally salt-tolerant halophyte plants to sequester large quantities of salt in external structures, such as epidermal bladder cells (EBCs), is of great interest. Using Chenopodium quinoa, a pseudo-cereal halophyte of great economic potential, we have shown previously that, upon removal of salt bladders, quinoa becomes salt sensitive. In this work, we analyzed the molecular mechanism underlying the unique salt dumping capabilities of bladder cells in quinoa. The transporters differentially expressed in the EBC transcriptome and functional electrophysiological testing of key EBC transporters in Xenopus oocytes revealed that loading of Na⁺ and Cl^- into EBCs is mediated by a set of tailored plasma and vacuole membrane-based sodium-selective channel and chloride-permeable transporter.

Active Zone Material-Directed Orientation, Docking, and Fusion of Dense Core Vesicles Alongside Synaptic Vesicles at Neuromuscular Junctions

Active zone material is an organelle that is common to active zones along the presynaptic membrane of chemical synapses. Electron tomography on active zones at frog neuromuscular junctions has provided evidence that active zone material directs the docking of synaptic vesicles (SVs) on the presynaptic membrane at this synapse. Certain active zone material macromolecules connect to stereotypically arranged macromolecules in the membrane of undocked SVs, stably orienting a predetermined fusion domain of the vesicle membrane toward the presynaptic membrane while bringing and holding the two membranes together. Docking of the vesicles is required for the impulse-triggered vesicle membrane-presynaptic membrane fusion that releases the vesicles’ neurotransmitter into the synaptic cleft. As at other synapses, axon terminals at frog neuromuscular junctions contain, in addition to SVs, vesicles that are larger, are much less frequent and, when viewed by electron microscopy, have a distinctive electron dense core. Dense core vesicles at neuromuscular junctions are likely to contain peptides that are released into the synaptic cleft to regulate formation, maintenance and behavior of cellular apparatus essential for synaptic impulse transmission. We show by electron tomography on axon terminals of frog neuromuscular junctions fixed at rest and during repetitive impulse transmission that dense core vesicles selectively dock on and fuse with the presynaptic membrane alongside SVs at active zones, and that active zone material connects to the dense core vesicles undergoing these processes in the same way it connects to SVs. We conclude that undocked dense core vesicles have a predetermined fusion domain, as do undocked SVs, and that active zone material directs oriented docking and fusion of these different vesicle types at active zones of the presynaptic membrane by similar macromolecular interactions.

Vesicle sub-pool organization at inner hair cell ribbon synapses

The afferent inner hair cell synapse harbors the synaptic ribbon, which ensures a constant vesicle supply. Synaptic vesicles (SVs) are arranged in morphologically discernable pools, linked via filaments to the ribbon or the presynaptic membrane. We propose that filaments play a major role in SV resupply and exocytosis at the ribbon. Using advanced electron microscopy, we demonstrate that SVs are organized in sub-pools defined by the filament number per vesicle and its connections. Upon stimulation, SVs increasingly linked to other vesicles and to the ribbon, whereas single-tethered SVs dominated at the membrane. Mutant mice for the hair cell protein otoferlin (pachanga, Otof ^Pga/Pga ) are profoundly deaf with reduced sustained release, serving as a model to investigate the SV replenishment at IHCs. Upon stimulation, multiple-tethered and docked vesicles (rarely observed in wild-type) accumulated at Otof ^Pga/Pga active zones due to an impairment downstream of docking. Conclusively, vesicles are organized in sub-pools at ribbon-type active zones by filaments to support vesicle supply, transport, and finally release.

A Modular Organization of LRR Protein-Mediated Synaptic Adhesion Defines Synapse Identity

Pyramidal neurons express rich repertoires of leucine-rich repeat (LRR)-containing adhesion molecules with similar synaptogenic activity in culture. The in vivo relevance of this molecular diversity is unclear. We show that hippocampal CA1 pyramidal neurons express multiple synaptogenic LRR proteins that differentially distribute to the major excitatory inputs on their apical dendrites. At Schaffer collateral (SC) inputs, FLRT2, LRRTM1, and Slitrk1 are postsynaptically localized and differentially regulate synaptic structure and function. FLRT2 controls spine density, whereas LRRTM1 and Slitrk1 exert opposing effects on synaptic vesicle distribution at the active zone. All LRR proteins differentially affect synaptic transmission, and their combinatorial loss results in a cumulative phenotype. At temporoammonic (TA) inputs, LRRTM1 is absent; FLRT2 similarly controls functional synapse number, whereas Slitrk1 function diverges to regulate postsynaptic AMPA receptor density. Thus, LRR proteins differentially control synaptic architecture and function and act in input-specific combinations and a context-dependent manner to specify synaptic properties.

Transient and Partial Nuclear Lamina Disruption Promotes Chromosome Movement in Early Meiotic Prophase

Meiotic chromosome movement is important for the pairwise alignment of homologous chromosomes, which is required for correct chromosome segregation. Movement is driven by cytoplasmic forces, transmitted to chromosome ends by nuclear membrane-spanning proteins. In animal cells, lamins form a prominent scaffold at the nuclear periphery, yet the role lamins play in meiotic chromosome movement is unclear. We show that chromosome movement correlates with reduced lamin association with the nuclear rim, which requires lamin phosphorylation at sites analogous to those that open lamina network crosslinks in mitosis. Failure to remodel the lamina results in delayed meiotic entry, altered chromatin organization, unpaired or interlocked chromosomes, and slowed chromosome movement. The remodeling kinases are delivered to lamins via chromosome ends coupled to the nuclear envelope, potentially enabling crosstalk between the lamina and chromosomal events. Thus, opening the lamina network plays a role in modulating contacts between chromosomes and the nuclear periphery during meiosis.

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Upon meiotic entry, the protein network of the nuclear lamina is opened up/loosened

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Weakening of lamin crosslinks triggers dramatic chromatin reorganization

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Failure to remodel lamina crosslinking can lead to aberrant chromosome forms

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Meiotic phosphorylation of lamin is essential for maintaining chromosome integrity

Monoamines differentially modulate neuropeptide release from distinct sites within a single neuron pair

Monoamines and neuropeptides often modulate the same behavior, but monoaminergic-peptidergic crosstalk remains poorly understood. In Caenorhabditis elegans, the adrenergic-like ligands, tyramine (TA) and octopamine (OA) require distinct subsets of neuropeptides in the two ASI sensory neurons to inhibit nociception. TA selectively increases the release of ASI neuropeptides encoded by nlp-14 or nlp-18 from either synaptic/perisynaptic regions of ASI axons or the ASI soma, respectively, and OA selectively increases the release of ASI neuropeptides encoded by nlp-9 asymmetrically, from only the synaptic/perisynaptic region of the right ASI axon. The predicted amino acid preprosequences of genes encoding either TA- or OA-dependent neuropeptides differed markedly. However, these distinct preprosequences were not sufficient to confer monoamine-specificity and additional N-terminal peptide-encoding sequence was required. Collectively, our results demonstrate that TA and OA specifically and differentially modulate the release of distinct subsets of neuropeptides from different subcellular sites within the ASIs, highlighting the complexity of monoaminergic/peptidergic modulation, even in animals with a relatively simple nervous system.

EM Tomography of Meiotic LINC Complexes

Electron microscope (EM) tomography is a powerful technique that enables the three-dimensional analysis of subcellular structures at high resolution. We have applied this method to the quantitative analysis of LINC complex distribution and interaction with the cytoskeleton in meiotic cells from male mice. In this chapter, we describe methods to generate and analyze the tomograms.

Clarinet (CLA-1), a novel active zone protein required for synaptic vesicle clustering and release

Active zone proteins cluster synaptic vesicles at presynaptic terminals and coordinate their release. In forward genetic screens, we isolated a novel Caenorhabditis elegans active zone gene, clarinet (cla-1). cla-1 mutants exhibit defects in synaptic vesicle clustering, active zone structure and synapse number. As a result, they have reduced spontaneous vesicle release and increased synaptic depression. cla-1 mutants show defects in vesicle distribution near the presynaptic dense projection, with fewer undocked vesicles contacting the dense projection and more docked vesicles at the plasma membrane. cla-1 encodes three isoforms containing common C-terminal PDZ and C2 domains with homology to vertebrate active zone proteins Piccolo and RIM. The C-termini of all isoforms localize to the active zone. Specific loss of the ~9000 amino acid long isoform results in vesicle clustering defects and increased synaptic depression. Our data indicate that specific isoforms of clarinet serve distinct functions, regulating synapse development, vesicle clustering and release.

The Structure of the Synaptic Vesicle-Plasma Membrane Interface Constrains SNARE Models of Rapid, Synchronous Exocytosis at Nerve Terminals

Contemporary models of neurotransmitter release invoke direct or indirect interactions between the Ca²⁺ sensor, synaptotagmin and the incompletely zippered soluble, N-ethyl-maleimide-sensitive factor attachment protein receptor (SNARE) complex. However, recent electron microscopic (EM) investigations have raised pragmatic issues concerning the mechanism by which SNAREs trigger membrane fusion at nerve terminals. The first issue is related to the finding that the area of contact between a “fully primed” synaptic vesicle and the plasma membrane can exceed 600 nm². Approximately four-thousands lipid molecules can inhabit this contact zone. Thus, renewed efforts will be needed to explain how the zippering of as few as two SNARE complexes mobilizes these lipids to achieve membrane fusion. The second issue emerges from the finding that “docking filaments” are sandwiched within the area of vesicle-plasma membrane contact. It is challenging to reconcile the location of these filaments with SNARE models of exocytosis. Instead, this commentary outlines how these data are more compatible with a model in which a cluster of synaptotagmins catalyzes exocytotic membrane fusion.

Active zone proteins are transported via distinct mechanisms regulated by Par-1 kinase

Disruption of synapses underlies a plethora of neurodevelopmental and neurodegenerative disease. Presynaptic specialization called the active zone plays a critical role in the communication with postsynaptic neuron. While the role of many proteins at the active zones in synaptic communication is relatively well studied, very little is known about how these proteins are transported to the synapses. For example, are there distinct mechanisms for the transport of active zone components or are they all transported in the same transport vesicle? Is active zone protein transport regulated? In this report we show that overexpression of Par-1/MARK kinase, a protein whose misregulation has been implicated in Autism spectrum disorders (ASDs) and neurodegenerative disorders, lead to a specific block in the transport of an active zone protein component- Bruchpilot at Drosophila neuromuscular junctions. Consistent with a block in axonal transport, we find a decrease in number of active zones and reduced neurotransmission in flies overexpressing Par-1 kinase. Interestingly, we find that Par-1 acts independently of Tau-one of the most well studied substrates of Par-1, revealing a presynaptic function for Par-1 that is independent of Tau. Thus, our study strongly suggests that there are distinct mechanisms that transport components of active zones and that they are tightly regulated.

Synapses consist of pre- and postsynaptic partners. Proper function of active zones, a presynaptic component of synapse, is essential for efficacious neuronal communication. Disruption of neuronal communication is an early sign of both neurodevelopmental as well as neurodegenerative diseases. Since proteins that reside in active zones are used so frequently during the neuronal communication, they must be constantly replenished to maintain active zones. Axonal transport of these proteins plays an important role in replenishing these vital components necessary for the health of active zones. However, the mechanisms that transport components of active zones are not well understood. Our data suggest that there are distinct mechanisms that transport various active zone cargoes and this process is likely regulated by kinases. Further, our data show that disruption in the transport of one such active zone components causes reduced neuronal communication emphasizing the importance of the process of axonal transport of active zone protein(s) for neuronal communication. Understanding the processes that govern the axonal transport of active zone components will help dissect the initial stages of pathogenesis in both neurodevelopmental and neurodegenerative diseases.

Fast cAMP Modulation of Neurotransmission via Neuropeptide Signals and Vesicle Loading

Cyclic AMP (cAMP) signaling augments synaptic transmission, but because many targets of cAMP and protein kinase A (PKA) may be involved, mechanisms underlying this pathway remain unclear. To probe this mechanism, we used optogenetic stimulation of cAMP signaling by Beggiatoa-photoactivated adenylyl cyclase (bPAC) in Caenorhabditis elegans motor neurons. Behavioral, electron microscopy (EM), and electrophysiology analyses revealed cAMP effects on both the rate and on quantal size of transmitter release and led to the identification of a neuropeptidergic pathway affecting quantal size. cAMP enhanced synaptic vesicle (SV) fusion by increasing mobilization and docking/priming. cAMP further evoked dense core vesicle (DCV) release of neuropeptides, in contrast to channelrhodopsin (ChR2) stimulation. cAMP-evoked DCV release required UNC-31/Ca²⁺-dependent activator protein for secretion (CAPS). Thus, DCVs accumulated in unc-31 mutant synapses. bPAC-induced neuropeptide signaling acts presynaptically to enhance vAChT-dependent SV loading with acetylcholine, thus causing increased miniature postsynaptic current amplitudes (mPSCs) and significantly enlarged SVs.

FIJI Macro 3D ART VeSElecT: 3D Automated Reconstruction Tool for Vesicle Structures of Electron Tomograms

Automatic image reconstruction is critical to cope with steadily increasing data from advanced microscopy. We describe here the Fiji macro 3D ART VeSElecT which we developed to study synaptic vesicles in electron tomograms. We apply this tool to quantify vesicle properties (i) in embryonic Danio rerio 4 and 8 days past fertilization (dpf) and (ii) to compare Caenorhabditis elegans N2 neuromuscular junctions (NMJ) wild-type and its septin mutant (unc-59(e261)). We demonstrate development-specific and mutant-specific changes in synaptic vesicle pools in both models. We confirm the functionality of our macro by applying our 3D ART VeSElecT on zebrafish NMJ showing smaller vesicles in 8 dpf embryos then 4 dpf, which was validated by manual reconstruction of the vesicle pool. Furthermore, we analyze the impact of C. elegans septin mutant unc-59(e261) on vesicle pool formation and vesicle size. Automated vesicle registration and characterization was implemented in Fiji as two macros (registration and measurement). This flexible arrangement allows in particular reducing false positives by an optional manual revision step. Preprocessing and contrast enhancement work on image-stacks of 1nm/pixel in x and y direction. Semi-automated cell selection was integrated. 3D ART VeSElecT removes interfering components, detects vesicles by 3D segmentation and calculates vesicle volume and diameter (spherical approximation, inner/outer diameter). Results are collected in color using the RoiManager plugin including the possibility of manual removal of non-matching confounder vesicles. Detailed evaluation considered performance (detected vesicles) and specificity (true vesicles) as well as precision and recall. We furthermore show gain in segmentation and morphological filtering compared to learning based methods and a large time gain compared to manual segmentation. 3D ART VeSElecT shows small error rates and its speed gain can be up to 68 times faster in comparison to manual annotation. Both automatic and semi-automatic modes are explained including a tutorial.

3D Analysis of Synaptic Ultrastructure in Organotypic Hippocampal Slice Culture by High-Pressure Freezing and Electron Tomography

Transmission electron microscopy serves as a valuable tool for synaptic structure-function analyses aimed at identifying morphological features or modifications associated with specific developmental stages or dysfunctional synaptic states. By utilizing cryo-preparation techniques to minimize the introduction of structural artifacts during sample preparation, and electron tomography to reconstruct the 3D ultrastructural architecture of a synapse, the spatial organization and morphological properties of synaptic organelles and subcompartments can be quantified with unparalleled precision. In this chapter, we present an experimental approach combining organotypic slice culture, high-pressure freezing, automated freeze-substitution, and electron tomography to investigate spatial relationships between synaptic vesicles and active zone release sites in synapses from lethal mouse mutants.

Fife organizes synaptic vesicles and calcium channels for high-probability neurotransmitter release

Fife is a Piccolo-RIM–related protein that regulates neurotransmission and motor behavior through an unknown mechanism. Here, Bruckner et al. show that Fife organizes synaptic vesicle docking and coupling to calcium channels to establish and modulate synaptic strength.

The strength of synaptic connections varies significantly and is a key determinant of communication within neural circuits. Mechanistic insight into presynaptic factors that establish and modulate neurotransmitter release properties is crucial to understanding synapse strength, circuit function, and neural plasticity. We previously identified Drosophila Piccolo-RIM-related Fife, which regulates neurotransmission and motor behavior through an unknown mechanism. Here, we demonstrate that Fife localizes and interacts with RIM at the active zone cytomatrix to promote neurotransmitter release. Loss of Fife results in the severe disruption of active zone cytomatrix architecture and molecular organization. Through electron tomographic and electrophysiological studies, we find a decrease in the accumulation of release-ready synaptic vesicles and their release probability caused by impaired coupling to Ca²⁺ channels. Finally, we find that Fife is essential for the homeostatic modulation of neurotransmission. We propose that Fife organizes active zones to create synaptic vesicle release sites within nanometer distance of Ca²⁺ channel clusters for reliable and modifiable neurotransmitter release.

Three-dimensional imaging of Drosophila motor synapses reveals ultrastructural organizational patterns

We combined cryo-preservation of intact Drosophila larvae and electron tomography with comprehensive segmentation of key features to reconstruct the complete ultrastructure of a model glutamatergic synapse in a near-to-native state. Presynaptically, we detail a complex network of filaments that connects and organizes synaptic vesicles. We link the complexity of this synaptic vesicle network to proximity to the active zone cytomatrix, consistent with the model that these protein structures function together to regulate synaptic vesicle pools. We identify a net-shaped network of electron-dense filaments spanning the synaptic cleft that suggests conserved organization of trans-synaptic adhesion complexes at excitatory synapses. Postsynaptically, we characterize a regular pattern of macromolecules that yields structural insights into the scaffolding of neurotransmitter receptors. Together, these analyses reveal an unexpected level of conservation in the nanoscale organization of diverse glutamatergic synapses and provide a structural foundation for understanding the molecular machines that regulate synaptic communication at a powerful model synapse.

Puzzling out presynaptic differentiation

Proper brain function in the nervous system relies on the accurate establishment of synaptic contacts during development. Countless synapses populate the adult brain in an orderly fashion. In each synapse, a presynaptic terminal loaded with neurotransmitters-containing synaptic vesicles is perfectly aligned to an array of receptors in the postsynaptic membrane. Presynaptic differentiation, which encompasses the events underlying assembly of new presynaptic units, has seen notable advances in recent years. It is now consensual that as a growing axon encounters the receptive dendrites of its partner, presynaptic assembly will be triggered and specified by multiple postsynaptically-derived factors including soluble molecules and cell adhesion complexes. Presynaptic material that reaches these distant sites by axonal transport in the form of pre-assembled packets will be retained and clustered, ultimately giving rise to a presynaptic bouton. This review focuses on the cellular and molecular aspects of presynaptic differentiation in the central nervous system, with a particular emphasis on the identity of the instructive factors and the intracellular processes used by neuronal cells to assemble functional presynaptic terminals. We provide a detailed description of the mechanisms leading to the formation of new presynaptic terminals. In brief, soma-derived packets of pre-assembled material are trafficked to distant axonal sites. Synaptogenic factors from dendritic or glial provenance activate downstream intra-axonal mediators to trigger clustering of passing material and their correct organization into a new presynaptic bouton. This article is part of a mini review series: "Synaptic Function and Dysfunction in Brain Diseases".

A Network of Three Types of Filaments Organizes Synaptic Vesicles for Storage, Mobilization, and Docking

Synaptic transmission between neurons requires precise management of synaptic vesicles. While individual molecular components of the presynaptic terminal are well known, exactly how the molecules are organized into a molecular machine serving the storage and mobilization of synaptic vesicles to the active zone remains unclear. Here we report three filament types associated with synaptic vesicles in glutamatergic synapses revealed by electron microscope tomography in unstimulated, dissociated rat hippocampal neurons. One filament type, likely corresponding to the SNAREpin complex, extends from the active zone membrane and surrounds docked vesicles. A second filament type contacts all vesicles throughout the active zone and pairs vesicles together. On the third filament type, vesicles attach to side branches extending from the long filament core and form vesicle clusters that are distributed throughout the vesicle cloud and along the active zone membrane. Detailed analysis of presynaptic structure reveals how each of the three filament types interacts with synaptic vesicles, providing a means to traffic reserved and recycled vesicles from the cloud of vesicles into the docking position at the active zone.

SIGNIFICANCE STATEMENT

The formation and release of synaptic vesicles has been extensively investigated. Explanations of the release of synaptic vesicles generally begin with the movement of vesicles from the cloud into the synaptic active zone. However, the presynaptic terminal is filled with filamentous material that would appear to limit vesicular diffusion. Here, we provide a systematic description of three filament types connecting synaptic vesicles. A picture emerges illustrating how the cooperative attachment and release of these three filament types facilitate the movement of vesicles to the active zone to become docked in preparation for release.