Cooperation of Syd-1 with Neurexin synchronizes pre- with postsynaptic assembly

Presynaptic regulators in memory formation

The intricate molecular and structural sequences guiding the formation and consolidation of memories within neuronal circuits remain largely elusive. In this study, we investigate the roles of two pivotal presynaptic regulators, the small GTPase Rab3, enriched at synaptic vesicles, and the cell adhesion protein Neurexin-1, in the formation of distinct memory phases within the Drosophila mushroom body Kenyon cells. Our findings suggest that both proteins play crucial roles in memory-supporting processes within the presynaptic terminal, operating within distinct plasticity modules. These modules likely encompass remodeling and maturation of existing active zones (AZs), as well as the formation of new AZs.

Skewing information flow through pre- and postsynaptic plasticity in the mushroom bodies of Drosophila

Animal brains need to store information to construct a representation of their environment. Knowledge of what happened in the past allows both vertebrates and invertebrates to predict future outcomes by recalling previous experience. Although invertebrate and vertebrate brains share common principles at the molecular, cellular, and circuit-architectural levels, there are also obvious differences as exemplified by the use of acetylcholine versus glutamate as the considered main excitatory neurotransmitters in the respective central nervous systems. Nonetheless, across central nervous systems, synaptic plasticity is thought to be a main substrate for memory storage. Therefore, how brain circuits and synaptic contacts change following learning is of fundamental interest for understanding brain computations tied to behavior in any animal. Recent progress has been made in understanding such plastic changes following olfactory associative learning in the mushroom bodies (MBs) of Drosophila A current framework of memory-guided behavioral selection is based on the MB skew model, in which antagonistic synaptic pathways are selectively changed in strength. Here, we review insights into plasticity at dedicated Drosophila MB output pathways and update what is known about the plasticity of both pre- and postsynaptic compartments of Drosophila MB neurons.

Targeting the intracellular neurexin interactome by in vivo proximity ligation

In a recent study, Profes, Tiroumalechetty, and colleagues used the in vivo proximity ligation technique TurboID to scrupulously characterize the interactome of the intracellular domain (ICD) of neurexin, revealing that this domain may be involved in presynaptic actin assembly by interacting with actin-associated proteins.

Complex formation of immunoglobulin superfamily molecules Side-IV and Beat-IIb regulates synaptic specificity

Neurons establish specific synapses based on the adhesive properties of cell-surface proteins while also retaining the ability to form synapses in a relatively non-selective manner. However, comprehensive understanding of the underlying mechanism reconciling these opposing characteristics remains incomplete. Here, we have identified Side-IV/Beat-IIb, members of the Drosophila immunoglobulin superfamily, as a combination of cell-surface recognition molecules inducing synapse formation. The Side-IV/Beat-IIb combination transduces bifurcated signaling with Side-IV's co-receptor, Kirre, and a synaptic scaffold protein, Dsyd-1. Genetic experiments and subcellular protein localization analyses showed the Side-IV/Beat-IIb/Kirre/Dsyd-1 complex to have two essential functions. First, it narrows neuronal binding specificity through Side-IV/Beat-IIb extracellular interactions. Second, it recruits synapse formation factors, Kirre and Dsyd-1, to restrict synaptic loci and inhibit miswiring. This dual function explains how the combinations of cell-surface molecules enable the ranking of preferred interactions among neuronal pairs to achieve synaptic specificity in complex circuits in vivo.

Characterization of the intracellular neurexin interactome by in vivo proximity ligation suggests its involvement in presynaptic actin assembly

Neurexins are highly spliced transmembrane cell adhesion molecules that bind an array of partners via their extracellular domains. However, much less is known about the signaling pathways downstream of neurexin's largely invariant intracellular domain (ICD). Caenorhabditis elegans contains a single neurexin gene that we have previously shown is required for presynaptic assembly and stabilization. To gain insight into the signaling pathways mediating neurexin's presynaptic functions, we employed a proximity ligation method, endogenously tagging neurexin's intracellular domain with the promiscuous biotin ligase TurboID, allowing us to isolate adjacent biotinylated proteins by streptavidin pull-down and mass spectrometry. We compared our experimental strain to a control strain in which neurexin, endogenously tagged with TurboID, was dispersed from presynaptic active zones by the deletion of its C-terminal PDZ-binding motif. Selection of this control strain, which differs from the experimental strain only in its synaptic localization, was critical to identifying interactions specifically occurring at synapses. Using this approach, we identified both known and novel intracellular interactors of neurexin, including active zone scaffolds, actin-binding proteins (including almost every member of the Arp2/3 complex), signaling molecules, and mediators of RNA trafficking, protein synthesis and degradation, among others. Characterization of mutants for candidate neurexin interactors revealed that they recapitulate aspects of the nrx-1(-) mutant phenotype, suggesting they may be involved in neurexin signaling. Finally, to investigate a possible role for neurexin in local actin assembly, we endogenously tagged its intracellular domain with actin depolymerizing and sequestering peptides (DeActs) and found that this led to defects in active zone assembly. Together, these results suggest neurexin's intracellular domain may be involved in presynaptic actin-assembly, and furthermore highlight a novel approach to achieving high specificity for in vivo proteomics experiments.

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.

An antagonism between Spinophilin and Syd-1 operates upstream of memory-promoting presynaptic long-term plasticity

We still face fundamental gaps in understanding how molecular plastic changes of synapses intersect with circuit operation to define behavioral states. Here, we show that an antagonism between two conserved regulatory proteins, Spinophilin (Spn) and Syd-1, controls presynaptic long-term plasticity and the maintenance of olfactory memories in Drosophila. While Spn mutants could not trigger nanoscopic active zone remodeling under homeostatic challenge and failed to stably potentiate neurotransmitter release, concomitant reduction of Syd-1 rescued all these deficits. The Spn/Syd-1 antagonism converged on active zone close F-actin, and genetic or acute pharmacological depolymerization of F-actin rescued the Spn deficits by allowing access to synaptic vesicle release sites. Within the intrinsic mushroom body neurons, the Spn/Syd-1 antagonism specifically controlled olfactory memory stabilization but not initial learning. Thus, this evolutionarily conserved protein complex controls behaviorally relevant presynaptic long-term plasticity, also observed in the mammalian brain but still enigmatic concerning its molecular mechanisms and behavioral relevance.

Presynaptic Cytomatrix Proteins

The Cytomatrix Assembled at the active Zone (CAZ) of a presynaptic terminal displays electron-dense appearance and defines the center of the synaptic vesicle release. The protein constituents of CAZ are multiple-domain scaffolds that interact extensively with each other and also with an ensemble of synaptic vesicle proteins to ensure docking, fusion, and recycling. Reflecting the central roles of the active zone in synaptic transmission, CAZ proteins are highly conserved throughout evolution. As the nervous system increases complexity and diversity in types of neurons and synapses, CAZ proteins expand in the number of gene and protein isoforms and interacting partners. This chapter summarizes the discovery of the core CAZ proteins and current knowledge of their functions.

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

Synapse formation is locally determined by transmembrane proteins, yet synaptic material is synthesized remotely and undergoes processive transport in axons. How local synaptogenic signals intercept synaptic cargo in transport to promote its delivery and synapse formation is unknown. We found that the control of synaptic cargo delivery at microtubule (MT) minus ends mediates pro- and anti-synaptogenic activities of presynaptic neurexin and frizzled in C. elegans and identified the atypical kinesin VAB-8/KIF26 as a key molecule in this process. VAB-8/KIF26 levels at synaptic MT minus ends are controlled by frizzled and neurexin; loss of VAB-8 mimics neurexin mutants or frizzled hyperactivation, and its overexpression can rescue synapse loss in these backgrounds. VAB-8/KIF26 is required for the synaptic localization of other minus-end proteins and promotes the pausing of retrograde transport to allow delivery to synapses. Consistently, reducing retrograde transport rescues synapse loss in vab-8 and neurexin mutants. These results uncover a mechanistic link between synaptogenic signaling and axonal transport.

Local BMP signaling: A sensor for synaptic activity that balances synapse growth and function

Synapse development is coordinated by intercellular communication between the pre- and postsynaptic compartments, and by neuronal activity itself. In flies as in vertebrates, neuronal activity induces input-specific changes in the synaptic strength so that the entire circuit maintains stable function in the face of many challenges, including changes in synapse number and strength. But how do neurons sense synapse activity? In several studies carried out using the Drosophila neuromuscular junction (NMJ), we demonstrated that local BMP signaling provides an exquisite sensor for synapse activity. Here we review the main features of this exquisite sensor and discuss its functioning beyond monitoring the synapse activity but rather as a key controller that operates in coordination with other BMP signaling pathways to balance synapse growth, maturation and function.

Influence of T-Bar on Calcium Concentration Impacting Release Probability

The relation of form and function, namely the impact of the synaptic anatomy on calcium dynamics in the presynaptic bouton, is a major challenge of present (computational) neuroscience at a cellular level. The Drosophila larval neuromuscular junction (NMJ) is a simple model system, which allows studying basic effects in a rather simple way. This synapse harbors several special structures. In particular, in opposite to standard vertebrate synapses, the presynaptic boutons are rather large, and they have several presynaptic zones. In these zones, different types of anatomical structures are present. Some of the zones bear a so-called T-bar, a particular anatomical structure. The geometric form of the T-bar resembles the shape of the letter “T” or a table with one leg. When an action potential arises, calcium influx is triggered. The probability of vesicle docking and neurotransmitter release is superlinearly proportional to the concentration of calcium close to the vesicular release site. It is tempting to assume that the T-bar causes some sort of calcium accumulation and hence triggers a higher release probability and thus enhances neurotransmitter exocytosis. In order to study this influence in a quantitative manner, we constructed a typical T-bar geometry and compared the calcium concentration close to the active zones (AZs). We compared the case of synapses with and without T-bars. Indeed, we found a substantial influence of the T-bar structure on the presynaptic calcium concentrations close to the AZs, indicating that this anatomical structure increases vesicle release probability. Therefore, our study reveals how the T-bar zone implies a strong relation between form and function. Our study answers the question of experimental studies (namely “Wichmann and Sigrist, Journal of neurogenetics 2010”) concerning the sense of the anatomical structure of the T-bar.

Function of SYDE C2-RhoGAP family as signaling hubs for neuronal development deduced by computational analysis

Recent investigations of neurological developmental disorders have revealed the Rho-family modulators such as Syde and its interactors as the candidate genes. Although the mammalian Syde proteins are reported to possess GTPase-accelerating activity for RhoA-family proteins, diverse species-specific substrate selectivities and binding partners have been described, presumably based on their evolutionary variance in the molecular organization. A comprehensive in silico analysis of Syde family proteins was performed to elucidate their molecular functions and neurodevelopmental networks. Predicted structural modeling of the RhoGAP domain may account for the molecular constraints to substrate specificity among Rho-family proteins. Deducing conserved binding motifs can extend the Syde interaction network and highlight diverse but Syde isoform-specific signaling pathways in neuronal homeostasis, differentiation, and synaptic plasticity from novel aspects of post-translational modification and proteolysis.

Kinesin-3 mediated axonal delivery of presynaptic neurexin stabilizes dendritic spines and postsynaptic components

The functional properties of neural circuits are defined by the patterns of synaptic connections between their partnering neurons, but the mechanisms that stabilize circuit connectivity are poorly understood. We systemically examined this question at synapses onto newly characterized dendritic spines of C. elegans GABAergic motor neurons. We show that the presynaptic adhesion protein neurexin/NRX-1 is required for stabilization of postsynaptic structure. We find that early postsynaptic developmental events proceed without a strict requirement for synaptic activity and are not disrupted by deletion of neurexin/nrx-1. However, in the absence of presynaptic NRX-1, dendritic spines and receptor clusters become destabilized and collapse prior to adulthood. We demonstrate that NRX-1 delivery to presynaptic terminals is dependent on kinesin-3/UNC-104 and show that ongoing UNC-104 function is required for postsynaptic maintenance in mature animals. By defining the dynamics and temporal order of synapse formation and maintenance events in vivo, we describe a mechanism for stabilizing mature circuit connectivity through neurexin-based adhesion.

Spatial and Temporal Gene Function Studies in Rodents: Towards Gene-Based Therapies for Autism Spectrum Disorder

Autism spectrum disorder (ASD) is a complex neurodevelopmental condition that is characterized by differences in social interaction, repetitive behaviors, restricted interests, and sensory differences beginning early in life. Especially sensory symptoms are highly correlated with the severity of other behavioral differences. ASD is a highly heterogeneous condition on multiple levels, including clinical presentation, genetics, and developmental trajectories. Over a thousand genes have been implicated in ASD. This has facilitated the generation of more than two hundred genetic mouse models that are contributing to understanding the biological underpinnings of ASD. Since the first symptoms already arise during early life, it is especially important to identify both spatial and temporal gene functions in relation to the ASD phenotype. To further decompose the heterogeneity, ASD-related genes can be divided into different subgroups based on common functions, such as genes involved in synaptic function. Furthermore, finding common biological processes that are modulated by this subgroup of genes is essential for possible patient stratification and the development of personalized early treatments. Here, we review the current knowledge on behavioral rodent models of synaptic dysfunction by focusing on behavioral phenotypes, spatial and temporal gene function, and molecular targets that could lead to new targeted gene-based therapy.

Neuroligin-1 Is a Mediator of Methylmercury Neuromuscular Toxicity

Methylmercury (MeHg) is a developmental toxicant capable of eliciting neurocognitive and neuromuscular deficits in children with in utero exposure. Previous research in Drosophila melanogaster uncovered that developmental MeHg exposure simultaneously targets the developing musculature and innervating motor neuron in the embryo, along with identifying Drosophila neuroligin 1 (nlg1) as a gene associated with developmental MeHg sensitivity. Nlg1 and its transsynaptic partner neurexin 1 (Nrx1) are critical for axonal arborization and NMJ maturation. We investigated the effects of MeHg exposure on indirect flight muscle (IFM) morphogenesis, innervation, and function via flight assays and monitored the expression of NMJ-associated genes to characterize the role of Nlg1 mediating the neuromuscular toxicity of MeHg. Developmental MeHg exposure reduced the innervation of the IFMs, which corresponded with reduced flight ability. In addition, nlg1 expression was selectively reduced during early metamorphosis, whereas a subsequent increase was observed in other NMJ-associated genes, including nrx1, in late metamorphosis. Developmental MeHg exposure also resulted in persistent reduced expression of most nlg and nrx genes during the first 11 days of adulthood. Transgenic modulation of nlg1 and nrx1 revealed that developing muscle is particularly sensitive to nlg1 levels, especially during the 20-36-h window of metamorphosis with reduced nlg1 expression resulting in adult flight deficits. Muscle-specific overexpression of nlg1 partially rescued MeHg-induced deficits in eclosion and flight. We identified Nlg1 as a muscle-specific, NMJ structural component that can mediate MeHg neuromuscular toxicity resulting from early life exposure.

Drosophila Models Rediscovered with Super-Resolution Microscopy

With the advent of super-resolution microscopy, we gained a powerful toolbox to bridge the gap between the cellular- and molecular-level analysis of living organisms. Although nanoscopy is broadly applicable, classical model organisms, such as fruit flies, worms and mice, remained the leading subjects because combining the strength of sophisticated genetics, biochemistry and electrophysiology with the unparalleled resolution provided by super-resolution imaging appears as one of the most efficient approaches to understanding the basic cell biological questions and the molecular complexity of life. Here, we summarize the major nanoscopic techniques and illustrate how these approaches were used in Drosophila model systems to revisit a series of well-known cell biological phenomena. These investigations clearly demonstrate that instead of simply achieving an improvement in image quality, nanoscopy goes far beyond with its immense potential to discover novel structural and mechanistic aspects. With the examples of synaptic active zones, centrosomes and sarcomeres, we will explain the instrumental role of super-resolution imaging pioneered in Drosophila in understanding fundamental subcellular constituents.

Liprin-α-Mediated Assemblies and Their Roles in Synapse Formation

Brain's functions, such as memory and learning, rely on synapses that are highly specialized cellular junctions connecting neurons. Functional synapses orchestrate the assembly of ion channels, receptors, enzymes, and scaffold proteins in both pre- and post-synapse. Liprin-α proteins are master scaffolds in synapses and coordinate various synaptic proteins to assemble large protein complexes. The functions of liprin-αs in synapse formation have been largely uncovered by genetic studies in diverse model systems. Recently, emerging structural and biochemical studies on liprin-α proteins and their binding partners begin to unveil the molecular basis of the synaptic assembly. This review summarizes the recent structural findings on liprin-αs, proposes the assembly mechanism of liprin-α-mediated complexes, and discusses the liprin-α-organized assemblies in the regulation of synapse formation and function.

Antagonistic interactions between two Neuroligins coordinate pre- and postsynaptic assembly

As a result of developmental synapse formation, the presynaptic neurotransmitter release machinery becomes accurately matched with postsynaptic neurotransmitter receptors. Trans-synaptic signaling is executed through cell adhesion proteins such as Neurexin::Neuroligin pairs but also through diffusible and cytoplasmic signals. How exactly pre-post coordination is ensured in vivo remains largely enigmatic. Here, we identified a "molecular choreography" coordinating pre- with postsynaptic assembly during the developmental formation of Drosophila neuromuscular synapses. Two presynaptic Neurexin-binding scaffold proteins, Syd-1 and Spinophilin (Spn), spatio-temporally coordinated pre-post assembly in conjunction with two postsynaptically operating, antagonistic Neuroligin species: Nlg1 and Nlg2. The Spn/Nlg2 module promoted active zone (AZ) maturation by driving the accumulation of AZ scaffold proteins critical for synaptic vesicle release. Simultaneously, these regulators restricted postsynaptic glutamate receptor incorporation. Both functions of the Spn/Nlg2 module were directly antagonized by Syd-1/Nlg1. Nlg1 and Nlg2 also had divergent effects on Nrx-1 in vivo motility. Concerning diffusible signals, Spn and Syd-1 antagonistically controlled the levels of Munc13-family protein Unc13B at nascent AZs, whose release function facilitated glutamate receptor incorporation at assembling postsynaptic specializations. As a result, we here provide direct in vivo evidence illustrating how a highly regulative and interleaved communication between cell adhesion protein signaling complexes and diffusible signals allows for a precise coordination of pre- with postsynaptic assembly. It will be interesting to analyze whether this logic also transfers to plasticity processes.

A Picture Worth a Thousand Molecules-Integrative Technologies for Mapping Subcellular Molecular Organization and Plasticity in Developing Circuits

A key challenge in developmental neuroscience is identifying the local regulatory mechanisms that control neurite and synaptic refinement over large brain volumes. Innovative molecular techniques and high-resolution imaging tools are beginning to reshape our view of how local protein translation in subcellular compartments drives axonal, dendritic, and synaptic development and plasticity. Here we review recent progress in three areas of neurite and synaptic study in situ-compartment-specific transcriptomics/translatomics, targeted proteomics, and super-resolution imaging analysis of synaptic organization and development. We discuss synergies between sequencing and imaging techniques for the discovery and validation of local molecular signaling mechanisms regulating synaptic development, plasticity, and maintenance in circuits.

Synaptic Properties and Plasticity Mechanisms of Invertebrate Tonic and Phasic Neurons

Defining neuronal cell types and their associated biophysical and synaptic diversity has become an important goal in neuroscience as a mechanism to create comprehensive brain cell atlases in the post-genomic age. Beyond broad classification such as neurotransmitter expression, interneuron vs. pyramidal, sensory or motor, the field is still in the early stages of understanding closely related cell types. In both vertebrate and invertebrate nervous systems, one well-described distinction related to firing characteristics and synaptic release properties are tonic and phasic neuronal subtypes. In vertebrates, these classes were defined based on sustained firing responses during stimulation (tonic) vs. transient responses that rapidly adapt (phasic). In crustaceans, the distinction expanded to include synaptic release properties, with tonic motoneurons displaying sustained firing and weaker synapses that undergo short-term facilitation to maintain muscle contraction and posture. In contrast, phasic motoneurons with stronger synapses showed rapid depression and were recruited for short bursts during fast locomotion. Tonic and phasic motoneurons with similarities to those in crustaceans have been characterized in Drosophila, allowing the genetic toolkit associated with this model to be used for dissecting the unique properties and plasticity mechanisms for these neuronal subtypes. This review outlines general properties of invertebrate tonic and phasic motoneurons and highlights recent advances that characterize distinct synaptic and plasticity pathways associated with two closely related glutamatergic neuronal cell types that drive invertebrate locomotion.

Postsynaptic cAMP signalling regulates the antagonistic balance of Drosophila glutamate receptor subtypes

The balance among different subtypes of glutamate receptors (GluRs) is crucial for synaptic function and plasticity at excitatory synapses. However, the mechanisms balancing synaptic GluR subtypes remain unclear. Herein, we show that the two subtypes of GluRs (A and B) expressed at Drosophila neuromuscular junction synapses mutually antagonize each other in terms of their relative synaptic levels and affect subsynaptic localization of each other, as shown by super-resolution microscopy. Upon temperature shift-induced neuromuscular junction plasticity, GluR subtype A increased but subtype B decreased with a timecourse of hours. Inhibition of the activity of GluR subtype A led to imbalance of GluR subtypes towards more GluRIIA. To gain a better understanding of the signalling pathways underlying the balance of GluR subtypes, we performed an RNA interference screen of candidate genes and found that postsynaptic-specific knockdown of dunce, which encodes cAMP phosphodiesterase, increased levels of GluR subtype A but decreased subtype B. Furthermore, bidirectional alterations of postsynaptic cAMP signalling resulted in the same antagonistic regulation of the two GluR subtypes. Our findings thus identify a direct role of postsynaptic cAMP signalling in control of the plasticity-related balance of GluRs.

Summary: The antagonistic balance of GluR subtypes, which is associated with synaptic plasticity, is regulated by cAMP signalling in postsynaptic muscles of Drosophila NMJ synapses.

A Presynaptic Perspective on Transport and Assembly Mechanisms for Synapse Formation

Neurons are highly polarized cells with a single axon and multiple dendrites derived from the cell body to form tightly associated pre- and postsynaptic compartments. As the biosynthetic machinery is largely restricted to the somatodendritic domain, the vast majority of presynaptic components are synthesized in the neuronal soma, packaged into synaptic precursor vesicles, and actively transported along the axon to sites of presynaptic biogenesis. In contrast with the significant progress that has been made in understanding synaptic transmission and processing of information at the post-synapse, comparably little is known about the formation and dynamic remodeling of the presynaptic compartment. We review here our current understanding of the mechanisms that govern the biogenesis, transport, and assembly of the key components for presynaptic neurotransmission, discuss how alterations in presynaptic assembly may impact nervous system function or lead to disease, and outline key open questions for future research.

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.

Structural Remodeling of Active Zones Is Associated with Synaptic Homeostasis

Perturbations to postsynaptic glutamate receptors (GluRs) trigger retrograde signaling to precisely increase presynaptic neurotransmitter release, maintaining stable levels of synaptic strength, a process referred to as homeostatic regulation. However, the structural change of homeostatic regulation remains poorly defined. At wild-type Drosophila neuromuscular junction synapse, there is one Bruchpilot (Brp) ring detected by superresolution microscopy at active zones (AZs). In the present study, we report multiple Brp rings (i.e., multiple T-bars seen by electron microscopy) at AZs of both male and female larvae when GluRs are reduced. At GluRIIC-deficient neuromuscular junctions, quantal size was reduced but quantal content was increased, indicative of homeostatic presynaptic potentiation. Consistently, multiple Brp rings at AZs were observed in the two classic synaptic homeostasis models (i.e., GluRIIA mutant and pharmacological blockade of GluRIIA activity). Furthermore, postsynaptic overexpression of the cell adhesion protein Neuroligin 1 partially rescued multiple Brp rings phenotype. Our study thus supports that the formation of multiple Brp rings at AZs might be a structural basis for synaptic homeostasis.SIGNIFICANCE STATEMENT Synaptic homeostasis is a conserved fundamental mechanism to maintain efficient neurotransmission of neural networks. Active zones (AZs) are characterized by an electron-dense cytomatrix, which is largely composed of Bruchpilot (Brp) at the Drosophila neuromuscular junction synapses. It is not clear how the structure of AZs changes during homeostatic regulation. To address this question, we examined the structure of AZs by superresolution microscopy and electron microscopy during homeostatic regulation. Our results reveal multiple Brp rings at AZs of glutamate receptor-deficient neuromuscular junction synapses compared with single Brp ring at AZs in wild type (WT). We further show that Neuroligin 1-mediated retrograde signaling regulates multiple Brp ring formation at glutamate receptor-deficient synapses. This study thus reveals a regulatory mechanism for synaptic homeostasis.

The conserved microRNA miR-34 regulates synaptogenesis via coordination of distinct mechanisms in presynaptic and postsynaptic cells

Micro(mi)RNA-based post-transcriptional regulatory mechanisms have been broadly implicated in the assembly and modulation of synaptic connections required to shape neural circuits, however, relatively few specific miRNAs have been identified that control synapse formation. Using a conditional transgenic toolkit for competitive inhibition of miRNA function in Drosophila, we performed an unbiased screen for novel regulators of synapse morphogenesis at the larval neuromuscular junction (NMJ). From a set of ten new validated regulators of NMJ growth, we discovered that miR-34 mutants display synaptic phenotypes and cell type-specific functions suggesting distinct downstream mechanisms in the presynaptic and postsynaptic compartments. A search for conserved downstream targets for miR-34 identified the junctional receptor CNTNAP4/Neurexin-IV (Nrx-IV) and the membrane cytoskeletal effector Adducin/Hu-li tai shao (Hts) as proteins whose synaptic expression is restricted by miR-34. Manipulation of miR-34, Nrx-IV or Hts-M function in motor neurons or muscle supports a model where presynaptic miR-34 inhibits Nrx-IV to influence active zone formation, whereas, postsynaptic miR-34 inhibits Hts to regulate the initiation of bouton formation from presynaptic terminals.

Although micro(mi)RNA-based post-transcriptional regulatory mechanisms have been implicated in the assembly and modulation of synaptic connections, few miRNAs have been identified that control synapse formation. Here, authors performed an unbiased screen for novel regulators of synapse morphogenesis at the Drosophila larval neuromuscular junction and discovered that miR-34 inhibits Nrx-IV to influence active zone formation, whereas, postsynaptic miR-34 inhibits Hts to regulate the initiation of bouton formation from presynaptic terminals.

The calcium channel subunit α2δ-3 organizes synapses via an activity-dependent and autocrine BMP signaling pathway

Synapses are highly specialized for neurotransmitter signaling, yet activity-dependent growth factor release also plays critical roles at synapses. While efficient neurotransmitter signaling relies on precise apposition of release sites and neurotransmitter receptors, molecular mechanisms enabling high-fidelity growth factor signaling within the synaptic microenvironment remain obscure. Here we show that the auxiliary calcium channel subunit α2δ-3 promotes the function of an activity-dependent autocrine Bone Morphogenetic Protein (BMP) signaling pathway at the Drosophila neuromuscular junction (NMJ). α2δ proteins have conserved synaptogenic activity, although how they execute this function has remained elusive. We find that α2δ-3 provides an extracellular scaffold for an autocrine BMP signal, suggesting a mechanistic framework for understanding α2δ's conserved role in synapse organization. We further establish a transcriptional requirement for activity-dependent, autocrine BMP signaling in determining synapse density, structure, and function. We propose that activity-dependent, autocrine signals provide neurons with continuous feedback on their activity state for modulating both synapse structure and function.

The Long and Short of It: A Dwarf Neurexin Suffices for Synapse Assembly

Neurexins have been established as a major coordinator of synapse assembly, functioning through interactions with postsynaptic cell adhesion molecules. Kurshan et al. (2018) now show that a C. elegans "dwarf neurexin" lacking its extracellular interaction domains can conduct synapse formation independent of postsynaptic partners.

Serial Synapse Formation through Filopodial Competition for Synaptic Seeding Factors

Following axon pathfinding, growth cones transition from stochastic filopodial exploration to the formation of a limited number of synapses. How the interplay of filopodia and synapse assembly ensures robust connectivity in the brain has remained a challenging problem. Here, we developed a new 4D analysis method for filopodial dynamics and a data-driven computational model of synapse formation for R7 photoreceptor axons in developing Drosophila brains. Our live data support a "serial synapse formation" model, where at any time point only 1-2 "synaptogenic" filopodia suppress the synaptic competence of other filopodia through competition for synaptic seeding factors. Loss of the synaptic seeding factors Syd-1 and Liprin-α leads to a loss of this suppression, filopodial destabilization, and reduced synapse formation. The failure to form synapses can cause the destabilization and secondary retraction of axon terminals. Our model provides a filopodial "winner-takes-all" mechanism that ensures the formation of an appropriate number of synapses.

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.

Liprin alfa 2 gene expression is increased by cannabis use and associated with neuropsychological function

The relation of heavy cannabis use with decreased neuropsychological function has frequently been described but the underlying biological mechanisms are still largely unknown. This study investigates the relation of cannabis use with genome wide gene expression and subsequently examines the relations with neuropsychological function. Genome-wide gene expression in whole blood was compared between heavy cannabis users (N = 90) and cannabis naïve participants (N = 100) that were matched for psychotic like experiences. The results were validated using quantitative real-time PCR. Psychotic like experiences were assessed using the Comprehensive Assessment of Psychotic Experiences (CAPE). Neuropsychological function was estimated using four subtasks of the Wechsler Adult Intelligence Scale (WAIS). Subsequent in vitro studies in monocytes and a neuroblastoma cell line investigated expression changes in response to two major psychotropic components of cannabis; tetrahydrocannabinol (THC) and cannabidiol (CBD). mRNA expression of Protein Tyrosine Phosphatase Receptor Type F Polypeptide-Interacting-Protein Alpha-2 (PPFIA2) was significantly higher in cannabis users (LogFold Change 0.17) and confirmed by qPCR analysis. PPFIA2 expression level was negatively correlated with estimated intelligence (B=-22.9, p = 0.002) also in the 100 non-users (B=-28.5, p = 0.037). In vitro exposure of monocytes to CBD led to significant increase in PPFIA2 expression. However, exposure of monocytes to THC and neuroblastoma cells to THC or CBD did not change PPFIA2 expression. Change in PPFIA2 gene expression in response to cannabinoids is a putative mechanism by which cannabis could influence neuropsychological functions. The findings warrant further exploration of the role of PPFIA2 in cannabis induced changes of neuropsychological function, particularly in relation to CBD.

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.

The Tenets of Teneurin: Conserved Mechanisms Regulate Diverse Developmental Processes in the Drosophila Nervous System

To successfully integrate a neuron into a circuit, a myriad of developmental events must occur correctly and in the correct order. Neurons must be born and grow out toward a destination, responding to guidance cues to direct their path. Once arrived, each neuron must segregate to the correct sub-region before sorting through a milieu of incorrect partners to identify the correct partner with which they can connect. Finally, the neuron must make a synaptic connection with their correct partner; a connection that needs to be broadly maintained throughout the life of the animal while remaining responsive to modes of plasticity and pruning. Though many intricate molecular mechanisms have been discovered to regulate each step, recent work showed that a single family of proteins, the Teneurins, regulates a host of these developmental steps in Drosophila – an example of biological adaptive reuse. Teneurins first influence axon guidance during early development. Once neurons arrive in their target regions, Teneurins enable partner matching and synapse formation in both the central and peripheral nervous systems. Despite these diverse processes and systems, the Teneurins use conserved mechanisms to achieve these goals, as defined by three tenets: (1) transsynaptic interactions with each other, (2) membrane stabilization via an interaction with and regulation of the cytoskeleton, and (3) a role for presynaptic Ten-a in regulating synaptic function. These processes are further distinguished by (1) the nature of the transsynaptic interaction – homophilic interactions (between the same Teneurins) to engage partner matching and heterophilic interactions (between different Teneurins) to enable synaptic connectivity and the proper apposition of pre- and postsynaptic sites and (2) the location of cytoskeletal regulation (presynaptic cytoskeletal regulation in the CNS and postsynaptic regulation of the cytoskeleton at the NMJ). Thus, both the roles and the mechanisms governing them are conserved across processes and synapses. Here, we will highlight the contributions of Drosophila synaptic biology to our understanding of the Teneurins, discuss the mechanistic conservation that allows the Teneurins to achieve common neurodevelopmental goals, and present new data in support of these points. Finally, we will posit the next steps for understanding how this remarkably versatile family of proteins functions to control multiple distinct events in the creation of a nervous system.

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.

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.

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.

Postsynaptic Syntaxin 4 negatively regulates the efficiency of neurotransmitter release

Signaling from the postsynaptic compartment regulates multiple aspects of synaptic development and function. Syntaxin 4 (Syx4) is a plasma membrane t-SNARE that promotes the growth and plasticity of Drosophila neuromuscular junctions (NMJs) by regulating the localization of key synaptic proteins in the postsynaptic compartment. Here, we describe electrophysiological analyses and report that loss of Syx4 leads to enhanced neurotransmitter release, despite a decrease in the number of active zones. We describe a requirement for postsynaptic Syx4 in regulating several presynaptic parameters, including Ca2+ cooperativity and the abundance of the presynaptic calcium channel Cacophony (Cac) at active zones. These findings indicate Syx4 negatively regulates presynaptic neurotransmitter release through a retrograde signaling mechanism from the postsynaptic compartment.

Characterization of developmental and molecular factors underlying release heterogeneity at Drosophila synapses

Neurons communicate through neurotransmitter release at specialized synaptic regions known as active zones (AZs). Using biosensors to visualize single synaptic vesicle fusion events at Drosophila neuromuscular junctions, we analyzed the developmental and molecular determinants of release probability (P_r) for a defined connection with ~300 AZs. P_r was heterogeneous but represented a stable feature of each AZ. P_r remained stable during high frequency stimulation and retained heterogeneity in mutants lacking the Ca²⁺ sensor Synaptotagmin 1. P_r correlated with both presynaptic Ca²⁺ channel abundance and Ca²⁺ influx at individual release sites. P_r heterogeneity also correlated with glutamate receptor abundance, with high P_r connections developing receptor subtype segregation. Intravital imaging throughout development revealed that AZs acquire high P_r during a multi-day maturation period, with P_r heterogeneity largely reflecting AZ age. The rate of synapse maturation was activity-dependent, as both increases and decreases in neuronal activity modulated glutamate receptor field size and segregation.

To send a message to its neighbor, a neuron releases chemicals called neurotransmitters into the gap – or synapse – between them. The neurotransmitter molecules bind to proteins on the receiver neuron called receptors. But what causes the sender neuron to release neurotransmitter in the first place? The process starts when an electrical impulse called an action potential arrives at the sender cell. Its arrival causes channels in the membrane of the sender neuron to open, so that calcium ions flood into the cell. The calcium ions interact with packages of neurotransmitter molecules, known as synaptic vesicles. This causes some of the vesicles to empty their contents into the synapse.

But this process is not particularly reliable. Only a small fraction of action potentials cause vesicles to fuse with the synaptic membrane. How likely this is to occur varies greatly between neurons, and even between synapses formed by the same neuron. Synapses that are likely to release neurotransmitter are said to be strong. They are good at passing messages from the sender neuron to the receiver. Synapses with a low probability of release are said to be weak. But what exactly differs between strong and weak synapses?

Akbergenova et al. studied synapses between motor neurons and muscle cells in the fruit fly Drosophila. Each motor neuron forms several hundred synapses. Some of these synapses are 50 times more likely to release neurotransmitter than others. Using calcium imaging and genetics, Akbergenova et al. showed that sender cells at strong synapses have more calcium channels than sender cells at weak synapses. The subtypes and arrangement of receptor proteins also differ between the receiver neurons of strong versus weak synapses. Finally, studies in larvae revealed that newly formed synapses all start out weak and then gradually become stronger. How fast this strengthening occurs depends on how active the neuron at the synapse is.

This study has shown, in unprecedented detail, key molecular factors that make some fruit fly synapses more likely to release neurotransmitter than others. Many proteins at synapses of mammals resemble those at fruit fly synapses. This means that similar factors may also explain differences in synaptic strength in the mammalian brain. Changes in the strength of synapses underlie the ability to learn. Furthermore, many neurological and psychiatric disorders result from disruption of synapses. Understanding the molecular basis of synapses will thus provide clues to the origins of certain brain diseases.

Clarin-1 gene transfer rescues auditory synaptopathy in model of Usher syndrome

Clarin-1, a tetraspan-like membrane protein defective in Usher syndrome type IIIA (USH3A), is essential for hair bundle morphogenesis in auditory hair cells. We report a new synaptic role for clarin-1 in mouse auditory hair cells elucidated by characterization of Clrn1 total (Clrn1ex4-/-) and postnatal hair cell-specific conditional (Clrn1ex4fl/fl Myo15-Cre+/-) knockout mice. Clrn1ex4-/- mice were profoundly deaf, whereas Clrn1ex4fl/fl Myo15-Cre+/- mice displayed progressive increases in hearing thresholds, with, initially, normal otoacoustic emissions and hair bundle morphology. Inner hair cell (IHC) patch-clamp recordings for the 2 mutant mice revealed defective exocytosis and a disorganization of synaptic F-actin and CaV1.3 Ca2+ channels, indicative of a synaptopathy. Postsynaptic defects were also observed, with an abnormally broad distribution of AMPA receptors associated with a loss of afferent dendrites and defective electrically evoked auditory brainstem responses. Protein-protein interaction assays revealed interactions between clarin-1 and the synaptic CaV1.3 Ca2+ channel complex via the Cavβ2 auxiliary subunit and the PDZ domain-containing protein harmonin (defective in Usher syndrome type IC). Cochlear gene therapy in vivo, through adeno-associated virus-mediated Clrn1 transfer into hair cells, prevented the synaptic defects and durably improved hearing in Clrn1ex4fl/fl Myo15-Cre+/- mice. Our results identify clarin-1 as a key organizer of IHC ribbon synapses, and suggest new treatment possibilities for USH3A patients.

Neurexin-Neuroligin 1 regulates synaptic morphology and functions via the WAVE regulatory complex in Drosophila neuromuscular junction

Neuroligins are postsynaptic adhesion molecules that are essential for postsynaptic specialization and synaptic function. But the underlying molecular mechanisms of neuroligin functions remain unclear. We found that Drosophila Neuroligin 1 (DNlg1) regulates synaptic structure and function through WAVE regulatory complex (WRC)-mediated postsynaptic actin reorganization. The disruption of DNlg1, DNlg2, or their presynaptic partner neurexin (DNrx) led to a dramatic decrease in the amount of F-actin. Further study showed that DNlg1, but not DNlg2 or DNlg3, directly interacts with the WRC via its C-terminal interacting receptor sequence. That interaction is required to recruit WRC to the postsynaptic membrane to promote F-actin assembly. Furthermore, the interaction between DNlg1 and the WRC is essential for DNlg1 to rescue the morphological and electrophysiological defects in dnlg1 mutants. Our results reveal a novel mechanism by which the DNrx-DNlg1 trans-synaptic interaction coordinates structural and functional properties at the neuromuscular junction.

Neurexin and Neuroligin-based adhesion complexes drive axonal arborisation growth independent of synaptic activity

Building arborisations of the right size and shape is fundamental for neural network function. Live imaging in vertebrate brains strongly suggests that nascent synapses are critical for branch growth during development. The molecular mechanisms underlying this are largely unknown. Here we present a novel system in Drosophila for studying the development of complex arborisations live, in vivo during metamorphosis. In growing arborisations we see branch dynamics and localisations of presynaptic proteins very similar to the ‘synaptotropic growth’ described in fish/frogs. These accumulations of presynaptic proteins do not appear to be presynaptic release sites and are not paired with neurotransmitter receptors. Knockdowns of either evoked or spontaneous neurotransmission do not impact arbor growth. Instead, we find that axonal branch growth is regulated by dynamic, focal localisations of Neurexin and Neuroligin. These adhesion complexes provide stability for filopodia by a ‘stick-and-grow’ based mechanism wholly independent of synaptic activity.

Drosophila Syd-1 Has RhoGAP Activity That Is Required for Presynaptic Clustering of Bruchpilot/ELKS but Not Neurexin-1

Syd-1 proteins are required for presynaptic development in worm, fly, and mouse. Syd-1 proteins in all three species contain a Rho GTPase activating protein (GAP)-like domain of unclear significance: invertebrate Syd-1s are thought to lack GAP activity, and mouse mSYD1A has GAP activity that is thought to be dispensable for its function. Here, we show that Drosophila melanogaster Syd-1 can interact with all six fly Rhos and has GAP activity toward Rac1 and Cdc42. During development, fly Syd-1 clusters multiple presynaptic proteins at the neuromuscular junction (NMJ), including the cell adhesion molecule Neurexin (Nrx-1) and the active zone (AZ) component Bruchpilot (Brp), both of which Syd-1 binds directly. We show that a mutant form of Syd-1 that specifically lacks GAP activity localizes normally to presynaptic sites and is sufficient to recruit Nrx-1 but fails to cluster Brp normally. We provide evidence that Syd-1 participates with Rac1 in two separate functions: (1) together with the Rac guanine exchange factor (RacGEF) Trio, GAP-active Syd-1 is required to regulate the nucleotide-bound state of Rac1, thereby promoting Brp clustering; and (2) Syd-1, independent of its GAP activity, is required for the recruitment of Nrx-1 to boutons, including the recruitment of Nrx-1 that is promoted by GTP-bound Rac1. We conclude that, contrary to current models, the GAP domain of fly Syd-1 is active and required for presynaptic development; we suggest that the same may be true of vertebrate Syd-1 proteins. In addition, our data provide new molecular insight into the ability of Rac1 to promote presynaptic development.

Genetic interaction of DISC1 and Neurexin in the development of fruit fly glutamatergic synapses

Originally identified at the breakpoint of a (1;11)(q42.1; q14.3) chromosomal translocation in a Scottish family with a wide range of mental disorders, the DISC1 gene has been a focus of intensive investigations as an entry point to study the molecular mechanisms of diverse mental dysfunctions. Perturbations of the DISC1 functions lead to behavioral changes in animal models, which are relevant to psychiatric conditions in patients. In this work, we have expressed the human DISC1 gene in the fruit fly (Drosophila melanogaster) and performed a genetic screening for the mutations of psychiatric risk genes that cause modifications of DISC1 synaptic phenotypes at the neuromuscular junction. We found that DISC1 interacts with dnrx1, the Drosophila homolog of the human Neurexin (NRXN1) gene, in the development of glutamatergic synapses. While overexpression of DISC1 suppressed the total bouton area on the target muscles and stimulated active zone density in wild-type background, a partial reduction of the dnrx1 activity negated the DISC1–mediated synaptic alterations. Likewise, overexpression of DISC1 stimulated the expression of a glutamate receptor component, DGLURIIA, in wild-type background but not in the dnrx1 heterozygous background. In addition, DISC1 caused mislocalization of Discs large, the Drosophila PSD-95 homolog, in the dnrx1 heterozygous background. Analyses with a series of domain deletions have revealed the importance of axonal localization of the DISC1 protein for efficient suppression of DNRX1 in synaptic boutons. These results thus suggest an intriguing converging mechanism controlled by the interaction of DISC1 and Neurexin in the developing glutamatergic synapses.

Fruit fly models uncover a potential new mechanism by which two schizophrenia risk factor genes interact to alter synaptic junctions. DISC1 gene alterations have previously been linked to psychiatric anomalies, although the gene has not been formally recognized as a schizophrenia risk factor. A US-Japan research collaboration led by the University of Tsukuba’s Katsuo Furukubo-Tokunaga expressed human DISC1 in fruit fly synapses to better understand the changes that take place when gene disruption leads to overexpression. The team found that overexpression of DISC1 affected the expression of the fruit fly counterpart to human ‘neurexin,’ a known risk factor for conditions including schizophrenia and autism spectrum disorders. The interaction between neurexin and DISC1 also influenced other synapse-altering genes. Further research is warranted to explore the roles of DISC1 and neurexin in psychiatric disease.

Presynaptic LRP4 promotes synapse number and function of excitatory CNS neurons

Precise coordination of synaptic connections ensures proper information flow within circuits. The activity of presynaptic organizing molecules signaling to downstream pathways is essential for such coordination, though such entities remain incompletely known. We show that LRP4, a conserved transmembrane protein known for its postsynaptic roles, functions presynaptically as an organizing molecule. In the Drosophila brain, LRP4 localizes to the nerve terminals at or near active zones. Loss of presynaptic LRP4 reduces excitatory (not inhibitory) synapse number, impairs active zone architecture, and abolishes olfactory attraction - the latter of which can be suppressed by reducing presynaptic GABA_B receptors. LRP4 overexpression increases synapse number in excitatory and inhibitory neurons, suggesting an instructive role and a common downstream synapse addition pathway. Mechanistically, LRP4 functions via the conserved kinase SRPK79D to ensure normal synapse number and behavior. This highlights a presynaptic function for LRP4, enabling deeper understanding of how synapse organization is coordinated.

Unique versus Redundant Functions of Neuroligin Genes in Shaping Excitatory and Inhibitory Synapse Properties

Neuroligins are evolutionarily conserved postsynaptic cell adhesion molecules that interact with presynaptic neurexins. Neurons express multiple neuroligin isoforms that are targeted to specific synapses, but their synaptic functions and mechanistic redundancy are not completely understood. Overexpression or RNAi-mediated knockdown of neuroligins, respectively, causes a dramatic increase or decrease in synapse density, whereas genetic deletions of neuroligins impair synapse function with only minor effects on synapse numbers, raising fundamental questions about the overall physiological role of neuroligins. Here, we have systematically analyzed the effects of conditional genetic deletions of all major neuroligin isoforms (i.e., NL1, NL2, and NL3), either individually or in combinations, in cultured mouse hippocampal and cortical neurons. We found that conditional genetic deletions of neuroligins caused no change or only a small change in synapses numbers, but strongly impaired synapse function. This impairment was isoform specific, suggesting that neuroligins are not functionally redundant. Sparse neuroligin deletions produced phenotypes comparable to those of global deletions, indicating that neuroligins function in a cell-autonomous manner. Mechanistically, neuroligin deletions decreased the synaptic levels of neurotransmitter receptors and had no effect on presynaptic release probabilities. Overexpression of neuroligin-1 in control or neuroligin-deficient neurons increased synaptic transmission and synapse density but not spine numbers, suggesting that these effects reflect a gain-of-function mechanism; whereas overexpression of neuroligin-3, which, like neuroligin-1 is also targeted to excitatory synapses, had no comparable effect. Our data demonstrate that neuroligins are required for the physiological organization of neurotransmitter receptors in postsynaptic specializations and suggest that they do not play a major role in synapse formation.SIGNIFICANCE STATEMENT Human neuroligin genes have been associated with autism, but the cellular functions of different neuroligins and their molecular mechanisms remain incompletely understood. Here, we performed comparative analyses in cultured mouse neurons of all major neuroligin isoforms, either individually or in combinations, using conditional knockouts. We found that neuroligin deletions did not affect synapse numbers but differentially impaired excitatory or inhibitory synaptic functions in an isoform-specific manner. These impairments were due, at least in part, to a decrease in synaptic distribution of neurotransmitter receptors upon deletion of neuroligins. Conversely, the overexpression of neuroligin-1 increased synapse numbers but not spine numbers. Our results suggest that various neuroligin isoforms perform unique postsynaptic functions in organizing synapses but are not essential for synapse formation or maintenance.

Conditional Deletion of All Neurexins Defines Diversity of Essential Synaptic Organizer Functions for Neurexins

Neurexins are recognized as key organizers of synapses that are essential for normal brain function. However, it is unclear whether neurexins are fundamental building blocks of all synapses with similar overall functions or context-dependent specifiers of synapse properties. To address this question, we produced triple cKO (conditional knockout) mice that allow ablating all neurexin expression in mice. Using neuron-specific manipulations combined with immunocytochemistry, paired recordings, and two-photon Ca2+ imaging, we analyzed excitatory synapses formed by climbing fibers on Purkinje cells in cerebellum and inhibitory synapses formed by parvalbumin- or somatostatin-positive interneurons on pyramidal layer 5 neurons in the medial prefrontal cortex. After pan-neurexin deletions, we observed in these synapses severe but dramatically different synaptic phenotypes that ranged from major impairments in their distribution and function (climbing-fiber synapses) to large decreases in synapse numbers (parvalbumin-positive synapses) and severe alterations in action potential-induced presynaptic Ca2+ transients (somatostatin-positive synapses). Thus, neurexins function primarily as context-dependent specifiers of synapses.

Presynaptic morphogenesis, active zone organization and structural plasticity in Drosophila

Effective adaptation of neural circuit function to a changing environment requires many forms of plasticity. Among these, structural plasticity is one of the most durable, and is also an intrinsic part of the developmental logic for the formation and refinement of synaptic connectivity. Structural plasticity of presynaptic sites can involve the addition, remodeling, or removal of pre- and post-synaptic elements. However, this requires coordination of morphogenesis and assembly of the subcellular machinery for neurotransmitter release within the presynaptic neuron, as well as coordination of these events with the postsynaptic cell. While much progress has been made in revealing the cell biological mechanisms of postsynaptic structural plasticity, our understanding of presynaptic mechanisms is less complete.

Clinical genomics expands the morbid genome of intellectual disability and offers a high diagnostic yield

Intellectual disability (ID) is a measurable phenotypic consequence of genetic and environmental factors. In this study, we prospectively assessed the diagnostic yield of genomic tools (molecular karyotyping, multi-gene panel and exome sequencing) in a cohort of 337 ID subjects as a first-tier test and compared it with a standard clinical evaluation performed in parallel. Standard clinical evaluation suggested a diagnosis in 16% of cases (54/337) but only 70% of these (38/54) were subsequently confirmed. On the other hand, the genomic approach revealed a likely diagnosis in 58% (n=196). These included copy number variants in 14% (n=54, 15% are novel), and point mutations revealed by multi-gene panel and exome sequencing in the remaining 43% (1% were found to have Fragile-X). The identified point mutations were mostly recessive (n=117, 81%), consistent with the high consanguinity of the study cohort, but also X-linked (n=8, 6%) and de novo dominant (n=19, 13%). When applied directly on all cases with negative molecular karyotyping, the diagnostic yield of exome sequencing was 60% (77/129). Exome sequencing also identified likely pathogenic variants in three novel candidate genes (DENND5A, NEMF and DNHD1) each of which harbored independent homozygous mutations in patients with overlapping phenotypes. In addition, exome sequencing revealed de novo and recessive variants in 32 genes (MAMDC2, TUBAL3, CPNE6, KLHL24, USP2, PIP5K1A, UBE4A, TP53TG5, ATOH1, C16ORF90, SLC39A14, TRERF1, RGL1, CDH11, SYDE2, HIRA, FEZF2, PROCA1, PIANP, PLK2, QRFPR, AP3B2, NUDT2, UFC1, BTN3A2, TADA1, ARFGEF3, FAM160B1, ZMYM5, SLC45A1, ARHGAP33 and CAPS2), which we highlight as potential candidates on the basis of several lines of evidence, and one of these genes (SLC39A14) was biallelically inactivated in a potentially treatable form of hypermanganesemia and neurodegeneration. Finally, likely causal variants in previously published candidate genes were identified (ASTN1, HELZ, THOC6, WDR45B, ADRA2B and CLIP1), thus supporting their involvement in ID pathogenesis. Our results expand the morbid genome of ID and support the adoption of genomics as a first-tier test for individuals with ID.