Liprin-α3 controls vesicle docking and exocytosis at the active zone of hippocampal synapses

Intracellular protein-lipid interactions drive presynaptic assembly prior to neurexin recruitment

Neurexin cell-adhesion molecules regulate synapse development and function by recruiting synaptic components. Here, we uncover a mechanism for presynaptic assembly that precedes neurexin recruitment, mediated by interactions between cytosolic proteins and membrane phospholipids. Developmental imaging in C. elegans reveals that the intracellular active zone protein SYD-1 accumulates at nascent presynapses prior to its binding partner neurexin. Combining molecular dynamics simulations to model intrinsic interactions between SYD-1 and lipid bilayers with biochemical and in vivo validation of these predictions, we find that PIP2-interacting residues in the SYD-1 C2 domain are required for active zone assembly. Genetic perturbation of a PIP2-generating enzyme disrupts synaptic SYD-1 accumulation, while the PIP2-interacting domain of mammalian RIM1 can compensate for the SYD-1 C2 domain, suggesting functional homology between these proteins. Finally, we propose that the evolutionarily conserved γ-neurexin isoform represents a minimal neurexin sequence that stabilizes nascent presynaptic assemblies, potentially a core function of this isoform.

Interplay between membranes and biomolecular condensates in the regulation of membrane-associated cellular processes

Liquid‒liquid phase separation (LLPS) has emerged as a key mechanism for organizing cellular spaces independent of membranes. Biomolecular condensates, which assemble through LLPS, exhibit distinctive liquid droplet-like behavior and can exchange constituents with their surroundings. The regulation of condensate phases, including transitions from a liquid state to gel or irreversible aggregates, is important for their physiological functions and for controlling pathological progression, as observed in neurodegenerative diseases and cancer. While early studies on biomolecular condensates focused primarily on those in fluidic environments such as the cytosol, recent discoveries have revealed their existence in close proximity to, on, or even comprising membranes. The aim of this review is to provide an overview of the properties of membrane-associated condensates in a cellular context and their biological functions in relation to membranes.

Combined expansion and STED microscopy reveals altered fingerprints of postsynaptic nanostructure across brain regions in ASD-related SHANK3-deficiency

Synaptic dysfunction is a key feature of SHANK-associated disorders such as autism spectrum disorder, schizophrenia, and Phelan-McDermid syndrome. Since detailed knowledge of their effect on synaptic nanostructure remains limited, we aimed to investigate such alterations in ex11|SH3 SHANK3-KO mice combining expansion and STED microscopy. This enabled high-resolution imaging of mosaic-like arrangements formed by synaptic proteins in both human and murine brain tissue. We found distinct shape-profiles as fingerprints of the murine postsynaptic scaffold across brain regions and genotypes, as well as alterations in the spatial and molecular organization of subsynaptic domains under SHANK3-deficient conditions. These results provide insights into synaptic nanostructure in situ and advance our understanding of molecular mechanisms underlying synaptic dysfunction in neuropsychiatric disorders.

Distinct active zone protein machineries mediate Ca2+ channel clustering and vesicle priming at hippocampal synapses

Action potentials trigger neurotransmitter release at the presynaptic active zone with spatiotemporal precision. This is supported by protein machinery that mediates synaptic vesicle priming and clustering of CaV2 Ca2+ channels nearby. One model posits that scaffolding proteins directly tether vesicles to CaV2s; however, here we find that at mouse hippocampal synapses, CaV2 clustering and vesicle priming are executed by separate machineries. CaV2 nanoclusters are positioned at variable distances from those of the priming protein Munc13. The active zone organizer RIM anchors both proteins but distinct interaction motifs independently execute these functions. In transfected cells, Liprin-α and RIM form co-assemblies that are separate from CaV2-organizing complexes. At synapses, Liprin-α1-Liprin-α4 knockout impairs vesicle priming but not CaV2 clustering. The cell adhesion protein PTPσ recruits Liprin-α, RIM and Munc13 into priming complexes without co-clustering CaV2s. We conclude that active zones consist of distinct machineries to organize CaV2s and prime vesicles, and Liprin-α and PTPσ specifically support priming site assembly.

The intracellular C-terminus confers compartment-specific targeting of voltage-gated calcium channels

To achieve the functional polarization that underlies brain computation, neurons sort protein material into distinct compartments. Ion channel composition, for example, differs between axons and dendrites, but the molecular determinants for their polarized trafficking remain obscure. Here, we identify mechanisms that target voltage-gated Ca2+ channels (CaVs) to distinct subcellular compartments. In hippocampal neurons, CaV2s trigger neurotransmitter release at the presynaptic active zone, and CaV1s localize somatodendritically. After knockout of all three CaV2s, expression of CaV2.1, but not CaV1.3, restores neurotransmitter release. We find that chimeric CaV1.3s with CaV2.1 intracellular C-termini localize to the active zone, mediate synaptic vesicle exocytosis, and render release sensitive to CaV1 blockers. This dominant targeting function of the CaV2.1 C-terminus requires the first EF hand in its proximal segment, and replacement of the CaV2.1 C-terminus with that of CaV1.3 abolishes CaV2.1 active zone localization and function. We conclude that CaV intracellular C-termini mediate compartment-specific targeting.

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.

Exploring the novel duo of Reticulocalbin, and Sideroflexin as future biomarker candidates for Exacerbated Chronic Obstructive Pulmonary Disease

BACKGROUND

COPD is a complex respiratory disorder with high morbidity and mortality rates. Even with the current conventional diagnostic methods, including circulating inflammatory biomarkers, underdiagnosis rates in COPD remain as high as 70%. Our study was a comparative cross-sectional study that aimed to address the diagnostic challenges by identifying future biomarker candidates in COPD variants.

METHODS

This study used a label-free plasma proteomics approach that combined mass spectrometric data with bioinformatics to shed light on the functional roles of differentially expressed proteins in the COPD lung microenvironment. The predictive capacity of the screened proteins was assessed using Receiver Operating Characteristic (ROC) curves, with Western blot analysis validating protein expression patterns in an independent cohort.

RESULTS

Our study identified three DEPs-reticulocalbin-1, sideroflexin-4, and liprinα-3 that consistently exhibited altered expression in COPD exacerbation. ROC analysis indicated strong predictive potential, with AUC values of 0.908, 0.715, and 0.856 for RCN1, SFXN4, and LIPα-3, respectively. Validation through Western blot analysis confirmed their expression patterns in an independent validation cohort.

CONCLUSIONS

Our study discovered a novel duo of proteins reticulocalbin-1, and sideroflexin-4 that showed potential as valuable future biomarkers for the diagnosis and clinical management of COPD exacerbations.

A syndromic neurodevelopmental disorder caused by rare variants in PPFIA3

PPFIA3 encodes the protein-tyrosine phosphatase, receptor-type, F-polypeptide-interacting-protein-alpha-3 (PPFIA3), which is a member of the LAR-protein-tyrosine phosphatase-interacting-protein (liprin) family involved in synapse formation and function, synaptic vesicle transport, and presynaptic active zone assembly. The protein structure and function are evolutionarily well conserved, but human diseases related to PPFIA3 dysfunction are not yet reported in OMIM. Here, we report 20 individuals with rare PPFIA3 variants (19 heterozygous and 1 compound heterozygous) presenting with developmental delay, intellectual disability, hypotonia, dysmorphisms, microcephaly or macrocephaly, autistic features, and epilepsy with reduced penetrance. Seventeen unique PPFIA3 variants were detected in 18 families. To determine the pathogenicity of PPFIA3 variants in vivo, we generated transgenic fruit flies producing either human wild-type (WT) PPFIA3 or five missense variants using GAL4-UAS targeted gene expression systems. In the fly overexpression assays, we found that the PPFIA3 variants in the region encoding the N-terminal coiled-coil domain exhibited stronger phenotypes compared to those affecting the C-terminal region. In the loss-of-function fly assay, we show that the homozygous loss of fly Liprin-α leads to embryonic lethality. This lethality is partially rescued by the expression of human PPFIA3 WT, suggesting human PPFIA3 function is partially conserved in the fly. However, two of the tested variants failed to rescue the lethality at the larval stage and one variant failed to rescue lethality at the adult stage. Altogether, the human and fruit fly data reveal that the rare PPFIA3 variants are dominant-negative loss-of-function alleles that perturb multiple developmental processes and synapse formation.

The intracellular C-terminus confers compartment-specific targeting of voltage-gated Ca2+ channels

To achieve the functional polarization that underlies brain computation, neurons sort protein material into distinct compartments. Ion channel composition, for example, differs between axons and dendrites, but the molecular determinants for their polarized trafficking remain obscure. Here, we identify the mechanisms that target voltage-gated Ca2+ channels (CaVs) to distinct subcellular compartments. In hippocampal neurons, CaV2s trigger neurotransmitter release at the presynaptic active zone, and CaV1s localize somatodendritically. After knockout of all three CaV2s, expression of CaV2.1, but not of CaV1.3, restores neurotransmitter release. Chimeric CaV1.3 channels with CaV2.1 intracellular C-termini localize to the active zone, mediate synaptic vesicle exocytosis, and render release fully sensitive to blockade of CaV1 channels. This dominant targeting function of the CaV2.1 C-terminus requires an EF hand in its proximal segment, and replacement of the CaV2.1 C-terminus with that of CaV1.3 abolishes CaV2.1 active zone localization. We conclude that the intracellular C-termini mediate compartment-specific CaV targeting.

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.

Molecular definition of distinct active zone protein machineries for Ca2+ channel clustering and synaptic vesicle priming

Action potentials trigger neurotransmitter release with minimal delay. Active zones mediate this temporal precision by co-organizing primed vesicles with CaV2 Ca2+ channels. The presumed model is that scaffolding proteins directly tether primed vesicles to CaV2s. We find that CaV2 clustering and vesicle priming are executed by separate machineries. At hippocampal synapses, CaV2 nanoclusters are positioned at variable distances from those of the priming protein Munc13. The active zone organizer RIM anchors both proteins, but distinct interaction motifs independently execute these functions. In heterologous cells, Liprin-α and RIM from co-assemblies that are separate from CaV2-organizing complexes upon co-transfection. At synapses, Liprin-α1-4 knockout impairs vesicle priming, but not CaV2 clustering. The cell adhesion protein PTPσ recruits Liprin-α, RIM and Munc13 into priming complexes without co-clustering of CaV2s. We conclude that active zones consist of distinct complexes to organize CaV2s and vesicle priming, and Liprin-α and PTPσ specifically support priming site assembly.

The CEP170B-KIF2A complex destabilizes microtubule minus ends to generate polarized microtubule network

Microtubule (MT) minus ends are stabilized by CAMSAP family proteins at noncentrosomal MT-organizing centers. Despite progress in identifying diverse positive regulators, knowledge on the negative regulation of the MT minus-end distribution is lacking. Here, we identify CEP170B as a MT minus-end-binding protein that colocalizes with the microtubule-stabilizing complex at the cortical patches. CEP170B depends on the scaffold protein liprin-α1 for its cortical targeting and requires liprin-α1-bound PP2A phosphatase for its MT localization. CEP170B excludes CAMSAPs-stabilized MT minus ends from the cell periphery in HeLa cells and the basal cortex in human epithelial cells and is required for directional vesicle trafficking and cyst formation in 3D culture. Reconstitution experiments demonstrate that CEP170B autonomously tracks growing MT minus ends and blocks minus-end growth. Furthermore, CEP170B in a complex with the kinesin KIF2A acts as a potent MT minus-end depolymerase capable of antagonizing the stabilizing effect of CAMSAPs. Our study uncovers an antagonistic mechanism for controlling the spatial distribution of MT minus ends, which contributes to the establishment of polarized MT network and cell polarity.

The role of RIM in neurotransmitter release: promotion of synaptic vesicle docking, priming, and fusion

There are many special sites at the end of a synapse called active zones (AZs). Synaptic vesicles (SVs) fuse with presynaptic membranes at these sites, and this fusion is an important step in neurotransmitter release. The cytomatrix in the active zone (CAZ) is made up of proteins such as the regulating synaptic membrane exocytosis protein (RIM), RIM-binding proteins (RIM-BPs), ELKS/CAST, Bassoon/Piccolo, Liprin-α, and Munc13-1. RIM is a scaffold protein that interacts with CAZ proteins and presynaptic functional components to affect the docking, priming, and fusion of SVs. RIM is believed to play an important role in regulating the release of neurotransmitters (NTs). In addition, abnormal expression of RIM has been detected in many diseases, such as retinal diseases, Asperger's syndrome (AS), and degenerative scoliosis. Therefore, we believe that studying the molecular structure of RIM and its role in neurotransmitter release will help to clarify the molecular mechanism of neurotransmitter release and identify targets for the diagnosis and treatment of the aforementioned diseases.

Rare variants in PPFIA3 cause delayed development, intellectual disability, autism, and epilepsy

PPFIA3 encodes the Protein-Tyrosine Phosphatase, Receptor-Type, F Polypeptide-Interacting Protein Alpha-3 (PPFIA3), which is a member of the LAR protein-tyrosine phosphatase-interacting protein (liprin) family involved in synaptic vesicle transport and presynaptic active zone assembly. The protein structure and function are well conserved in both invertebrates and vertebrates, but human diseases related to PPFIA3 dysfunction are not yet known. Here, we report 14 individuals with rare mono-allelic PPFIA3 variants presenting with features including developmental delay, intellectual disability, hypotonia, autism, and epilepsy. To determine the pathogenicity of PPFIA3 variants in vivo , we generated transgenic fruit flies expressing either human PPFIA3 wildtype (WT) or variant protein using GAL4-UAS targeted gene expression systems. Ubiquitous expression with Actin-GAL4 showed that the PPFIA3 variants had variable penetrance of pupal lethality, eclosion defects, and anatomical leg defects. Neuronal expression with elav-GAL4 showed that the PPFIA3 variants had seizure-like behaviors, motor defects, and bouton loss at the 3 ^rd instar larval neuromuscular junction (NMJ). Altogether, in the fly overexpression assays, we found that the PPFIA3 variants in the N-terminal coiled coil domain exhibited stronger phenotypes compared to those in the C-terminal region. In the loss-of-function fly assay, we show that the homozygous loss of fly Liprin- α leads to embryonic lethality. This lethality is partially rescued by the expression of human PPFIA3 WT, suggesting human PPFIA3 protein function is partially conserved in the fly. However, the PPFIA3 variants failed to rescue lethality. Altogether, the human and fruit fly data reveal that the rare PPFIA3 variants are dominant negative loss-of-function alleles that perturb multiple developmental processes and synapse formation.

Antioxidant Activity of Crocodile Oil (Crocodylus siamensis) on Cognitive Function in Rats

Crocodile oil (CO) is rich in monounsaturated fatty acids and polyunsaturated fatty acids. The antioxidant activity and cognitive effect of monounsaturated fatty acids and polyunsaturated fatty acids have been largely reported. This work aimed to investigate the effect of CO on antioxidant activity and cognitive function in rats. Twenty-one rats were divided into three treatment groups: (1) sterile water (NS), (2) 1 mL/kg of CO (NC1), and (3) 3 mL/kg of CO (NC3). Rats underwent oral gavage once daily for 8 weeks. CO treatment decreased the triglycerides level significantly compared with that in the NS group. CO had a free radical scavenging ability greater than that of olive oil but had no effect on levels of antioxidant markers in the brain. Expression of unique proteins in the CO-treatment group were correlated with the detoxification of hydrogen peroxide. Rats in the NC1 group had better memory function than rats in the NC3 group. Expression of unique proteins in the NC1 group was correlated with memory function. However, CO did not cause a decline in cognitive function in rats. CO can be an alternative dietary oil because it has a hypolipidemia effect and antioxidant activity. In addition, CO did not cause a negative effect on cognitive function.

Interactions between Membraneless Condensates and Membranous Organelles at the Presynapse: A Phase Separation View of Synaptic Vesicle Cycle

Action potential-induced neurotransmitter release in presynaptic boutons involves coordinated actions of a large list of proteins that are associated directly or indirectly with membrane structures including synaptic vesicles and plasma membranes. These proteins are often highly abundant in different synaptic bouton sub-compartments, and they rarely act alone. Instead, these proteins interact with each other forming intricate and distinct molecular complexes. Many of these complexes form condensed clusters on membrane surfaces. This review summarizes findings in recent years showing that many of presynaptic protein complex assemblies are formed via phase separation. These protein condensates extensively interact with lipid membranes via distinct modes, forming various mesoscale structures by different mode of organizations between membraneless condensates and membranous organelles. We discuss that such mesoscale interactions could have deep implications on mobilization, exocytosis, and retrieval of synaptic vesicles.

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.

A novel homozygous truncating variant in PPFIBP1 further delineates PPFIBP1-associated neurodevelopmental disorder

Neurodevelopmental disorders (NDDs) are classified as a group of disorders affecting function and development of the brain and having wide clinical variability. Herein, we describe two affected individuals segregating a recessive NDD. The affected individuals exhibited phenotypes such as global developmental delay (GDD), intellectual disability (ID), microcephaly and speech delay. Whole-exome sequencing (WES) followed by bidirectional Sanger sequencing techniques identified a homozygous nonsense variant (c.466C > T; p.Gln156*) in the PPFIBP1 gene (NM_003622.4) that segregated with the disease phenotype. Further, to elucidate the effect of the variant on protein structure, 3D protein modelling was performed for the mutant and normal protein that suggested substantial reduction of the mutant protein. Our data support the evidence that PPFIBP1 has a pivotal role in neurodevelopment in humans, and loss-of-function variants cause clinically variable neurodevelopmental phenotypes.

Munc13 supports fusogenicity of non-docked vesicles at synapses with disrupted active zones

Active zones consist of protein scaffolds that are tightly attached to the presynaptic plasma membrane. They dock and prime synaptic vesicles, couple them to voltage-gated Ca²⁺ channels, and direct neurotransmitter release toward postsynaptic receptor domains. Simultaneous RIM + ELKS ablation disrupts these scaffolds, abolishes vesicle docking, and removes active zone-targeted Munc13, but some vesicles remain releasable. To assess whether this enduring vesicular fusogenicity is mediated by non-active zone-anchored Munc13 or is Munc13-independent, we ablated Munc13-1 and Munc13-2 in addition to RIM + ELKS in mouse hippocampal neurons. The hextuple knockout synapses lacked docked vesicles, but other ultrastructural features were near-normal despite the strong genetic manipulation. Removing Munc13 in addition to RIM + ELKS impaired action potential-evoked vesicle fusion more strongly than RIM + ELKS knockout by further decreasing the releasable vesicle pool. Hence, Munc13 can support some fusogenicity without RIM and ELKS, and presynaptic recruitment of Munc13, even without active zone anchoring, suffices to generate some fusion-competent vesicles.

Alterations of presynaptic proteins in autism spectrum disorder

The expanded use of hypothesis-free gene analysis methods in autism research has significantly increased the number of genetic risk factors associated with the pathogenesis of autism. A further examination of the implicated genes directly revealed the involvement in processes pertinent to neuronal differentiation, development, and function, with a predominant contribution from the regulators of synaptic function. Despite the importance of presynaptic function in synaptic transmission, the regulation of neuronal network activity, and the final behavioral output, there is a relative lack of understanding of the presynaptic contribution to the pathology of autism. Here, we will review the close association among autism-related mutations, autism spectrum disorders (ASD) phenotypes, and the altered presynaptic protein functions through a systematic examination of the presynaptic risk genes relating to the critical stages of synaptogenesis and neurotransmission.

Bi-allelic loss-of-function variants in PPFIBP1 cause a neurodevelopmental disorder with microcephaly, epilepsy, and periventricular calcifications

PPFIBP1 encodes for the liprin-β1 protein, which has been shown to play a role in neuronal outgrowth and synapse formation in Drosophila melanogaster. By exome and genome sequencing, we detected nine ultra-rare homozygous loss-of-function variants in 16 individuals from 12 unrelated families. The individuals presented with moderate to profound developmental delay, often refractory early-onset epilepsy, and progressive microcephaly. Further common clinical findings included muscular hyper- and hypotonia, spasticity, failure to thrive and short stature, feeding difficulties, impaired vision, and congenital heart defects. Neuroimaging revealed abnormalities of brain morphology with leukoencephalopathy, ventriculomegaly, cortical abnormalities, and intracranial periventricular calcifications as major features. In a fetus with intracranial calcifications, we identified a rare homozygous missense variant that by structural analysis was predicted to disturb the topology of the SAM domain region that is essential for protein-protein interaction. For further insight into the effects of PPFIBP1 loss of function, we performed automated behavioral phenotyping of a Caenorhabditis elegans PPFIBP1/hlb-1 knockout model, which revealed defects in spontaneous and light-induced behavior and confirmed resistance to the acetylcholinesterase inhibitor aldicarb, suggesting a defect in the neuronal presynaptic zone. In conclusion, we establish bi-allelic loss-of-function variants in PPFIBP1 as a cause of an autosomal recessive severe neurodevelopmental disorder with early-onset epilepsy, microcephaly, and periventricular calcifications.

Rebuilding essential active zone functions within a synapse

Presynaptic active zones are molecular machines that control neurotransmitter secretion. They form sites for vesicle docking and priming and couple vesicles to Ca²⁺ entry for release triggering. The complexity of active zone machinery has made it challenging to determine its mechanisms in release. Simultaneous knockout of the active zone proteins RIM and ELKS disrupts active zone assembly, abolishes vesicle docking, and impairs release. We here rebuild docking, priming, and Ca²⁺ secretion coupling in these mutants without reinstating active zone networks. Re-expression of RIM zinc fingers recruited Munc13 to undocked vesicles and rendered the vesicles release competent. Action potential triggering of release was reconstituted by docking these primed vesicles to Ca²⁺ channels through attaching RIM zinc fingers to Ca_Vβ4-subunits. Our work identifies an 80-kDa β4-Zn protein that bypasses the need for megadalton-sized secretory machines, establishes that fusion competence and docking are mechanistically separable, and defines RIM zinc finger-Munc13 complexes as hubs for active zone function.

Molecular and functional architecture of striatal dopamine release sites

Despite the importance of dopamine for striatal circuit function, mechanistic understanding of dopamine transmission remains incomplete. We recently showed that dopamine secretion relies on the presynaptic scaffolding protein RIM, indicating that it occurs at active zone-like sites similar to classical synaptic vesicle exocytosis. Here, we establish using a systematic gene knockout approach that Munc13 and Liprin-α, active zone proteins for vesicle priming and release site organization, are important for dopamine secretion. Furthermore, RIM zinc finger and C₂B domains, which bind to Munc13 and Liprin-α, respectively, are needed to restore dopamine release after RIM ablation. In contrast, and different from typical synapses, the active zone scaffolds RIM-BP and ELKS, and RIM domains that bind to them, are expendable. Hence, dopamine release necessitates priming and release site scaffolding by RIM, Munc13, and Liprin-α, but other active zone proteins are dispensable. Our work establishes that efficient release site architecture mediates fast dopamine exocytosis.

Liprins in oncogenic signaling and cancer cell adhesion

Liprins are a multifunctional family of scaffold proteins, identified by their involvement in several important neuronal functions related to signaling and organization of synaptic structures. More recently, the knowledge on the liprin family has expanded from neuronal functions to processes relevant to cancer progression, including cell adhesion, cell motility, cancer cell invasion, and signaling. These proteins consist of regions, which by prediction are intrinsically disordered, and may be involved in the assembly of supramolecular structures relevant for their functions. This review summarizes the current understanding of the functions of liprins in different cellular processes, with special emphasis on liprins in tumor progression. The available data indicate that liprins may be potential biomarkers for cancer progression and may have therapeutic importance.

Liquid-Liquid Phase Separation at the Plasma Membrane-Cytosol Interface: Common Players in Adhesion, Motility, and Synaptic Function

Networks of scaffold proteins and enzymes assemble at the interface between the cytosol and specific sites of the plasma membrane, where these networks guide distinct cellular functions. Some of these plasma membrane-associated platforms (PMAPs) include shared core components that are able to establish specific protein-protein interactions, to produce distinct supramolecular assemblies regulating dynamic processes as diverse as cell adhesion and motility, or the formation and function of neuronal synapses. How cells organize such dynamic networks is still an open question. In this review we introduce molecular networks assembling at the edge of migrating cells, and at pre- and postsynaptic sites, which share molecular players that can drive the assembly of biomolecular condensates. Very recent experimental evidence has highlighted the emerging role of some of these multidomain/scaffold proteins belonging to the GIT, liprin-α and ELKS/ERC families as drivers of liquid-liquid phase separation (LLPS). The data point to an important role of LLPS: (i) in the formation of PMAPs at the edge of migrating cells, where LLPS appears to be involved in promoting protrusion and the turnover of integrin-mediated adhesions, to allow forward cell translocation; (ii) in the assembly of the presynaptic active zone and of the postsynaptic density deputed to the release and reception of neurotransmitter signals, respectively. The recent results indicate that LLPS at cytosol-membrane interfaces is suitable not only for the regulation of active cellular processes, but also for the continuous spatial rearrangements of the molecular interactions involved in these dynamic processes.

Proteomic Analysis Unveils Expressional Changes in Cytoskeleton- and Synaptic Plasticity-Associated Proteins in Rat Brain Six Months after Withdrawal from Morphine

Drug withdrawal is associated with abstinence symptoms including deficits in cognitive functions that may persist even after prolonged discontinuation of drug intake. Cognitive deficits are, at least partially, caused by alterations in synaptic plasticity but the precise molecular mechanisms have not yet been fully identified. In the present study, changes in proteomic and phosphoproteomic profiles of selected brain regions (cortex, hippocampus, striatum, and cerebellum) from rats abstaining for six months after cessation of chronic treatment with morphine were determined by label-free quantitative (LFQ) proteomic analysis. Interestingly, prolonged morphine withdrawal was found to be associated especially with alterations in protein phosphorylation and to a lesser extent in protein expression. Gene ontology (GO) term analysis revealed enrichment in biological processes related to synaptic plasticity, cytoskeleton organization, and GTPase activity. More specifically, significant changes were observed in proteins localized in synaptic vesicles (e.g., synapsin-1, SV2a, Rab3a), in the active zone of the presynaptic nerve terminal (e.g., Bassoon, Piccolo, Rims1), and in the postsynaptic density (e.g., cadherin 13, catenins, Arhgap35, Shank3, Arhgef7). Other differentially phosphorylated proteins were associated with microtubule dynamics (microtubule-associated proteins, Tppp, collapsin response mediator proteins) and the actin–spectrin network (e.g., spectrins, adducins, band 4.1-like protein 1). Taken together, a six-month morphine withdrawal was manifested by significant alterations in the phosphorylation of synaptic proteins. The altered phosphorylation patterns modulating the function of synaptic proteins may contribute to long-term neuroadaptations induced by drug use and withdrawal.

Coordinated bi-directional trafficking of synaptic vesicle and active zone proteins in peripheral nerves

Synaptic transmission is mediated by neurotransmitters that are stored in synaptic vesicles (SV) and released at the synaptic active zone (AZ). While in recent years major progress has been made in unraveling the molecular machinery responsible for SV docking, fusion and exocytosis, the mechanisms governing AZ protein and SV trafficking through axons still remain unclear. Here, we performed stop-flow nerve ligation to examine axonal trafficking of endogenous AZ and SV proteins. Rat sciatic nerves were collected 1 h, 3 h and 8 h post ligation and processed for immunohistochemistry and electron microscopy. First, we followed the transport of an integral synaptic vesicle protein, SV2A and a SV-associated protein involved in SV trafficking, Rab3a, and observed that while SV2A accumulated on both sides of ligation, Rab3a was only noticeable in the proximal segment of the ligated nerve indicating that only SV trans-membrane protein SV2A displayed a bi-directional axonal transport. We then demonstrate that multiple AZ proteins accumulate rapidly on either side of the ligation with a timescale similar to that of SV2A. Overall, our data uncovers an unexpected robust bi-directional, coordinated -trafficking of SV and AZ proteins in peripheral nerves. This implies that pathological disruption of axonal trafficking will not only impair trafficking of newly synthesized proteins to the synapse but will also affect retrograde transport, leading to neuronal dysfunction and likely neurodegeneration.

UNC-2 CaV2 Channel Localization at Presynaptic Active Zones Depends on UNC-10/RIM and SYD-2/Liprin-α in Caenorhabditis elegans

Presynaptic active zone proteins couple calcium influx with synaptic vesicle exocytosis. However, the control of presynaptic calcium channel localization by active zone proteins is not completely understood. In a Caenorhabditis elegans (C. elegans) forward genetic screen, we find that UNC-10/RIM (Rab3-interacting molecule) and SYD-2/Liprin-α regulate presynaptic localization of UNC-2, the CaV2 channel ortholog. We further quantitatively analyzed live animals using endogenously GFP-tagged UNC-2 and active zone components. Consistent with the interaction between RIM and CaV2 in mammals, the intensity and number of UNC-2 channel puncta at presynaptic terminals were greatly reduced in unc-10 mutant animals. To understand how SYD-2 regulates presynaptic UNC-2 channel localization, we analyzed presynaptic localization of endogenous SYD-2, UNC-10, RIMB-1/RIM-BP (RIM binding protein), and ELKS-1. Our analysis revealed that although SYD-2 is the most critical for active zone assembly, loss of SYD-2 function does not completely abolish presynaptic localization of UNC-10, RIMB-1, and ELKS-1, suggesting an existence of SYD-2-independent active zone assembly. UNC-2 localization analysis in double and triple mutants of active zone components show that SYD-2 promotes UNC-2 localization by partially controlling UNC-10 localization, and ELKS-1 and RIMB-1 also contribute to UNC-2 channel localization. In addition, we find that core active zone proteins are unequal in their abundance. Although the abundance of UNC-10 at the active zone is comparable to UNC-2, SYD-2 and ELKS-1 are twice more and RIMB-1 four times more abundant than UNC-2. Together our data show that UNC-10, SYD-2, RIMB-1, and ELKS-1 control presynaptic UNC-2 channel localization in redundant yet distinct manners.SIGNIFICANCE STATEMENT Precise control of neurotransmission is dependent on the tight coupling of the calcium influx through voltage-gated calcium channels (VGCCs) to the exocytosis machinery at the presynaptic active zones. However, how these VGCCs are tethered to the active zone is incompletely understood. To understand the mechanism of presynaptic VGCC localization, we performed a C. elegans forward genetic screen and quantitatively analyzed endogenous active zones and presynaptic VGCCs. In addition to RIM, our study finds that SYD-2/Liprin-α is critical for presynaptic localization of VGCCs. Yet, the loss of SYD-2, a core active zone scaffolding protein, does not completely abolish the presynaptic localization of the VGCC, showing that the active zone is a resilient structure assembled by redundant mechanisms.

Presynaptic bouton compartmentalization and postsynaptic density-mediated glutamate receptor clustering via phase separation

Neuronal synapses encompass three compartments: presynaptic axon terminal, synaptic cleft, and postsynaptic dendrite. Each compartment contains densely packed molecular machineries that are involved in synaptic transmission. In recent years, emerging evidence indicates that the assembly of these membraneless substructures or assemblies that are not enclosed by membranes are driven by liquid-liquid phase separation. We review here recent studies that suggest the phase separation-mediated organization of these synaptic compartments. We discuss how synaptic function may be linked to its organization as biomolecular condensates. We conclude with a discussion of areas of future interest in the field for better understanding of the structural architecture of neuronal synapses and its contribution to synaptic functions.

PKC-phosphorylation of Liprin-α3 triggers phase separation and controls presynaptic active zone structure

The active zone of a presynaptic nerve terminal defines sites for neurotransmitter release. Its protein machinery may be organized through liquid-liquid phase separation, a mechanism for the formation of membrane-less subcellular compartments. Here, we show that the active zone protein Liprin-α3 rapidly and reversibly undergoes phase separation in transfected HEK293T cells. Condensate formation is triggered by Liprin-α3 PKC-phosphorylation at serine-760, and RIM and Munc13 are co-recruited into membrane-attached condensates. Phospho-specific antibodies establish phosphorylation of Liprin-α3 serine-760 in transfected cells and mouse brain tissue. In primary hippocampal neurons of newly generated Liprin-α2/α3 double knockout mice, synaptic levels of RIM and Munc13 are reduced and the pool of releasable vesicles is decreased. Re-expression of Liprin-α3 restored these presynaptic defects, while mutating the Liprin-α3 phosphorylation site to abolish phase condensation prevented this rescue. Finally, PKC activation in these neurons acutely increased RIM, Munc13 and neurotransmitter release, which depended on the presence of phosphorylatable Liprin-α3. Our findings indicate that PKC-mediated phosphorylation of Liprin-α3 triggers its phase separation and modulates active zone structure and function.

ELKS1 Captures Rab6-Marked Vesicular Cargo in Presynaptic Nerve Terminals

Neurons face unique transport challenges. They need to deliver cargo over long axonal distances and to many presynaptic nerve terminals. Rab GTPases are master regulators of vesicular traffic, but essential presynaptic Rabs have not been identified. Here, we find that Rab6, a Golgi-derived GTPase for constitutive secretion, associates with mobile axonal cargo and localizes to nerve terminals. ELKS1 is a stationary presynaptic protein with Golgin homology that binds to Rab6. Knockout and rescue experiments for ELKS1 and Rab6 establish that ELKS1 captures Rab6 cargo. The ELKS1-Rab6-capturing mechanism can be transferred to mitochondria by mistargeting ELKS1 or Rab6 to them. We conclude that nerve terminals have repurposed mechanisms from constitutive exocytosis for their highly regulated secretion. By employing Golgin-like mechanisms with anchored ELKS extending its coiled-coils to capture Rab6 cargo, they have spatially separated cargo capture from fusion. ELKS complexes connect to active zones and may mediate vesicle progression toward release sites.

Finding functions of phase separation in the presynapse

Synapses are the basic units of neuronal communication. Understanding how synapses assemble and function is therefore essential to understanding nervous systems. Decades of study have identified many molecular components and functional mechanisms of synapses. Recently, an additional level of synaptic protein organization has been identified: phase separation. In the presynapse, components of the central active zone and a synaptic vesicle-clustering factor have been shown to form liquid-liquid phase-separated condensates or hydrogels. New in vivo functional studies have directly tested how phase separation impacts both synapse formation and function. Here, we review this emerging evidence for in vivo functional roles of phase separation at the presynapse and discuss future functional studies necessary to understand its complexity.

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.

Intact synapse structure and function after combined knockout of PTPδ, PTPσ, and LAR

It has long been proposed that leukocyte common antigen-related receptor protein tyrosine phosphatases (LAR-RPTPs) are cell-adhesion proteins that control synapse assembly. Their synaptic nanoscale localization, however, is not established, and synapse fine structure after knockout of the three vertebrate LAR-RPTPs (PTPδ, PTPσ, and LAR) has not been tested. Here, superresolution microscopy reveals that PTPδ localizes to the synaptic cleft precisely apposed to postsynaptic scaffolds of excitatory and inhibitory synapses. We next assessed synapse structure in newly generated triple-conditional-knockout mice for PTPδ, PTPσ, and LAR, complementing a recent independent study of synapse function after LAR-RPTP ablation (Sclip and Südhof, 2020). While mild effects on synaptic vesicle clustering and active zone architecture were detected, synapse numbers and their overall structure were unaffected, membrane anchoring of the active zone persisted, and vesicle docking and release were normal. Hence, despite their localization at synaptic appositions, LAR-RPTPs are dispensable for presynapse structure and function.

Oligomerized liprin-α promotes phase separation of ELKS for compartmentalization of presynaptic active zone proteins

Synaptic scaffold proteins (e.g., liprin-α, ELKS, RIM, and RIM-BP) orchestrate ion channels, receptors, and enzymes at presynaptic terminals to form active zones for neurotransmitter release. The underlying mechanism of the active zone assembly remains elusive. Here, we report that liprin-α proteins have the potential to oligomerize through the N-terminal coiled-coil region. Our structural and biochemical characterizations reveal that a gain-of-function mutation promotes the self-assembly of the coiled coils in liprin-α2 by disrupting intramolecular interactions and promoting intermolecular interactions. By enabling multivalent interactions with ELKS proteins, the oligomerized coiled-coil region of liprin-α2 enhances the phase separation of the ELKS N-terminal segment. We further show that liprin-α2, by regulating the interplay between two phase separations of ELKS and RIM/RIM-BP, controls the protein distributions. These results imply that the complicated protein-protein interactions allow liprin-α to function with the active zone scaffolds and compartmentalize protein assemblies to achieve comprehensive functions in the active zone.

Impact of RIM-BPs in neuronal vesicles release

Accurate signal transmission between neurons is accomplished by vesicle release with high spatiotemporal resolution in the central nervous system. The vesicle release occurs mainly in the active zone (AZ), a unique area on the presynaptic membrane. Many structural proteins expressed in the AZ connect with other proteins nearby. They can also regulate the precise release of vesicles through protein-protein interactions. RIM-binding proteins (RIM-BPs) are one of the essential proteins in the AZ. This review summarizes the structures and functions of three subtypes of RIM-BPs, including the interaction between RIM-BPs and other proteins such as Bassoon and voltage-gated calcium channel, their significance in stabilizing the AZ structure in the presynaptic region and collecting ion channels, and ultimately regulating the fusion and release of neuronal vesicles.

Proper synaptic adhesion signaling in the control of neural circuit architecture and brain function

Trans-synaptic cell-adhesion molecules are critical for governing various stages of synapse development and specifying neural circuit properties via the formation of multifarious signaling pathways. Recent studies have pinpointed the putative roles of trans-synaptic cell-adhesion molecules in mediating various cognitive functions. Here, we review the literature on the roles of a diverse group of central synaptic organizers, including neurexins (Nrxns), leukocyte common antigen-related receptor protein tyrosine phosphatases (LAR-RPTPs), and their associated binding proteins, in regulating properties of specific type of synapses and neural circuits. In addition, we highlight the findings that aberrant synaptic adhesion signaling leads to alterations in the structures, transmission, and plasticity of specific synapses across diverse brain areas. These results seem to suggest that proper trans-synaptic signaling pathways by Nrxns, LAR-RPTPs, and their interacting network is likely to constitute central molecular complexes that form the basis for cognitive functions, and that these complexes are heterogeneously and complexly disrupted in many neuropsychiatric and neurodevelopmental disorders.

Unraveling the Nanoscopic Organization and Function of Central Mammalian Presynapses With Super-Resolution Microscopy

The complex, nanoscopic scale of neuronal function, taking place at dendritic spines, axon terminals, and other minuscule structures, cannot be adequately resolved using standard, diffraction-limited imaging techniques. The last couple of decades saw a rapid evolution of imaging methods that overcome the diffraction limit imposed by Abbe's principle. These techniques, including structured illumination microscopy (SIM), stimulated emission depletion (STED), photo-activated localization microscopy (PALM), and stochastic optical reconstruction microscopy (STORM), among others, have revolutionized our understanding of synapse biology. By exploiting the stochastic nature of fluorophore light/dark states or non-linearities in the interaction of fluorophores with light, by using modified illumination strategies that limit the excitation area, these methods can achieve spatial resolutions down to just a few tens of nm or less. Here, we review how these advanced imaging techniques have contributed to unprecedented insight into the nanoscopic organization and function of mammalian neuronal presynapses, revealing new organizational principles or lending support to existing views, while raising many important new questions. We further discuss recent technical refinements and newly developed tools that will continue to expand our ability to delve deeper into how synaptic function is orchestrated at the nanoscopic level.

Roles of type IIa receptor protein tyrosine phosphatases as synaptic organizers

Neurons establish circuits for brain functions such as cognition, emotion, learning, and memory. Their connections are mediated by synapses, which are specialized cell-cell adhesions responsible for neuronal signal transmission. During neurodevelopment, synapse formation is triggered by interactions of cell adhesion molecules termed synaptic organizers or synapse organizers. Type IIa receptor protein tyrosine phosphatases (IIa RPTPs; also known as leukocyte common antigen-related receptor tyrosine phosphatases or LAR-RPTPs) play important roles in axon guidance and neurite extension, and also serve as presynaptic organizers. IIa RPTPs transsynaptically interact with multiple sets of postsynaptic organizers, mostly in a splicing-dependent fashion. Here, we review and update research progress on IIa RPTPs, particularly regarding their functional roles in vivo demonstrated using conditional knockout approach and structural insights into their extracellular and intracellular molecular interactions revealed by crystallography and other biophysical techniques. Future directions in the research field of IIa RPTPs are also discussed, including recent findings of the molecular assembly mechanism underlying the formation of synapse-specific nanostructures essential for synaptic functions.

Vesicle Tethering on the Surface of Phase-Separated Active Zone Condensates

Tethering of synaptic vesicles (SVs) to the active zone determines synaptic strength, although the molecular basis governing SV tethering is elusive. Here, we discover that small unilamellar vesicles (SUVs) and SVs from rat brains coat on the surface of condensed liquid droplets formed by active zone proteins RIM, RIM-BP, and ELKS via phase separation. Remarkably, SUV-coated RIM/RIM-BP condensates are encapsulated by synapsin/SUV condensates, forming two distinct SUV pools reminiscent of the reserve and tethered SV pools that exist in presynaptic boutons. The SUV-coated RIM/RIM-BP condensates can further cluster Ca²⁺ channels anchored on membranes. Thus, we reconstitute a presynaptic bouton-like structure mimicking the SV-tethered active zone with its one side attached to the presynaptic membrane and the other side connected to the synapsin-clustered SV condensates. The distinct interaction modes between membraneless protein condensates and membrane-based organelles revealed here have general implications in cellular processes, including vesicular formation and trafficking, organelle biogenesis, and autophagy.

Disentangling the Roles of RIM and Munc13 in Synaptic Vesicle Localization and Neurotransmission

Efficient neurotransmitter release at the presynaptic terminal requires docking of synaptic vesicles to the active zone membrane and formation of fusion-competent synaptic vesicles near voltage-gated Ca²⁺ channels. Rab3-interacting molecule (RIM) is a critical active zone organizer, as it recruits Ca²⁺ channels and activates synaptic vesicle docking and priming via Munc13-1. However, our knowledge about Munc13-independent contributions of RIM to active zone functions is limited. To identify the functions that are solely mediated by RIM, we used genetic manipulations to control RIM and Munc13-1 activity in cultured hippocampal neurons from mice of either sex and compared synaptic ultrastructure and neurotransmission. We found that RIM modulates synaptic vesicle localization in the proximity of the active zone membrane independent of Munc13-1. In another step, both RIM and Munc13 mediate synaptic vesicle docking and priming. In addition, while the activity of both RIM and Munc13-1 is required for Ca²⁺-evoked release, RIM uniquely controls neurotransmitter release efficiency. However, activity-dependent augmentation of synaptic vesicle pool size relies exclusively on the action of Munc13s. Based on our results, we extend previous findings and propose a refined model in which RIM and Munc13-1 act in overlapping and independent stages of synaptic vesicle localization and release.

SIGNIFICANCE STATEMENT The presynaptic active zone is composed of scaffolding proteins that functionally interact to localize synaptic vesicles to release sites, ensuring neurotransmission. Our current knowledge of the presynaptic active zone function relies on structure-function analysis, which has provided detailed information on the network of interactions and the impact of active zone proteins. Yet, the hierarchical, redundant, or independent cooperation of each active zone protein to synapse functions is not fully understood. Rab3-interacting molecule and Munc13 are the two key functionally interacting active zone proteins. Here, we dissected the distinct actions of Rab3-interacting molecule and Munc13-1 from both ultrastructural and physiological aspects. Our findings provide a more detailed view of how these two presynaptic proteins orchestrate their functions to achieve synaptic transmission.

Nanoscale Organization of Vesicle Release at Central Synapses

Presynaptic boutons support neurotransmitter release with nanoscale precision at sub-millisecond timescales. Studies over the past two decades have revealed a rich tapestry of molecular players governing synaptic vesicle fusion at highly specialized release sites in the active zone (AZ). However, the spatiotemporal organization of release at active synapses remains elusive, in part owing to the extremely small size of the AZ and the limited resolution of conventional approaches. Recent advances in fluorescence nanoscopy have revolutionized direct investigation of presynaptic release organization and dynamics. We discuss here recent nanoscopy-based studies of the molecular architecture, the spatial organization and dynamic regulation of release sites, and the mechanisms of release site replenishment. These findings have uncovered previously unknown levels of structural and functional organization at central synapses, with important implications for synaptic transmission and plasticity.

Role of the active zone protein, ELKS, in insulin secretion from pancreatic β-cells

Background

Insulin is stored within large dense-core granules in pancreatic beta (β)-cells and is released by Ca²⁺-triggered exocytosis with increasing blood glucose levels. Polarized and targeted secretion of insulin from β-cells in pancreatic islets into the vasculature has been proposed; however, the mechanisms related to cellular and molecular localization remain largely unknown. Within nerve terminals, the Ca²⁺-dependent release of a polarized transmitter is limited to the active zone, a highly specialized area of the presynaptic membrane. Several active zone-specific proteins have been characterized; among them, the CAST/ELKS protein family members have the ability to form large protein complexes with other active zone proteins to control the structure and function of the active zone for tight regulation of neurotransmitter release. Notably, ELKS but not CAST is also expressed in β-cells, implying that ELKS may be involved in polarized insulin secretion from β-cells.

Scope of review

This review provides an overview of the current findings regarding the role(s) of ELKS and other active zone proteins in β-cells and focuses on the molecular mechanism underlying ELKS regulation within polarized insulin secretion from islets.

Major conclusions

ELKS localizes at the vascular-facing plasma membrane of β-cells in mouse pancreatic islets. ELKS forms a potent insulin secretion complex with L-type voltage-dependent Ca²⁺ channels on the vascular-facing plasma membrane of β-cells, enabling polarized Ca²⁺ influx and first-phase insulin secretion from islets. This model provides novel insights into the functional polarity observed during insulin secretion from β-cells within islets at the molecular level. This active zone-like region formed by ELKS at the vascular side of the plasma membrane is essential for coordinating physiological insulin secretion and may be disrupted in diabetes.

Synapse and Active Zone Assembly in the Absence of Presynaptic Ca2+ Channels and Ca2+ Entry

Presynaptic Ca_V2 channels are essential for Ca²⁺-triggered exocytosis. In addition, there are two competing models for their roles in synapse structure. First, Ca²⁺ channels or Ca²⁺ entry may control synapse assembly. Second, active zone proteins may scaffold Ca_V2s to presynaptic release sites, and synapse structure is Ca_V2 independent. Here, we ablated all three Ca_V2s using conditional knockout in cultured hippocampal neurons or at the calyx of Held, which abolished evoked exocytosis. Compellingly, synapse and active zone structure, vesicle docking, and transsynaptic nano-organization were unimpaired. Similarly, long-term blockade of action potentials and Ca²⁺ entry did not disrupt active zone assembly. Although Ca_V2 knockout impaired the localization of β subunits, α2δ-1 localized normally. Rescue with Ca_V2 restored exocytosis, and Ca_V2 active zone targeting depended on the intracellular C-terminus. We conclude that synapse assembly is independent of Ca_V2s or Ca²⁺ entry through them. Instead, active zone proteins recruit and anchor Ca_V2s via Ca_V2 C-termini.

Assembly of the presynaptic active zone

In a presynaptic nerve terminal, the active zone is composed of sophisticated protein machinery that enables secretion on a submillisecond time scale and precisely targets it toward postsynaptic receptors. The past two decades have provided deep insight into the roles of active zone proteins in exocytosis, but we are only beginning to understand how a neuron assembles active zone protein complexes into effective molecular machines. In this review, we outline the fundamental processes that are necessary for active zone assembly and discuss recent advances in understanding assembly mechanisms that arise from genetic, morphological and biochemical studies. We further outline the challenges ahead for understanding this important problem.

Structural insights into selective interaction between type IIa receptor protein tyrosine phosphatases and Liprin-α

Synapse formation is induced by transsynaptic interaction of neuronal cell-adhesion molecules termed synaptic organizers. Type IIa receptor protein tyrosine phosphatases (IIa RPTPs) function as presynaptic organizers. The cytoplasmic domain of IIa RPTPs consists of two phosphatase domains, and the membrane-distal one (D2) is essential for synapse formation. Liprin-α, which is an active zone protein critical for synapse formation, interacts with D2 via its C-terminal domain composed of three tandem sterile alpha motifs (tSAM). Structural mechanisms of this critical interaction for synapse formation remain elusive. Here, we report the crystal structure of the complex between mouse PTPδ D2 and Liprin-α3 tSAM at 1.91 Å resolution. PTPδ D2 interacts with the N-terminal helix and the first and second SAMs (SAM1 and SAM2, respectively) of Liprin-α3. Structure-based mutational analyses in vitro and in cellulo demonstrate that the interactions with Liprin-α SAM1 and SAM2 are essential for the binding and synaptogenic activity.

Cataloguing and Selection of mRNAs Localized to Dendrites in Neurons and Regulated by RNA-Binding Proteins in RNA Granules

Spatiotemporal translational regulation plays a key role in determining cell fate and function. Specifically, in neurons, local translation in dendrites is essential for synaptic plasticity and long-term memory formation. To achieve local translation, RNA-binding proteins in RNA granules regulate target mRNA stability, localization, and translation. To date, mRNAs localized to dendrites have been identified by comprehensive analyses. In addition, mRNAs associated with and regulated by RNA-binding proteins have been identified using various methods in many studies. However, the results obtained from these numerous studies have not been compiled together. In this review, we have catalogued mRNAs that are localized to dendrites and are associated with and regulated by the RNA-binding proteins fragile X mental retardation protein (FMRP), RNA granule protein 105 (RNG105, also known as Caprin1), Ras-GAP SH3 domain binding protein (G3BP), cytoplasmic polyadenylation element binding protein 1 (CPEB1), and staufen double-stranded RNA binding proteins 1 and 2 (Stau1 and Stau2) in RNA granules. This review provides comprehensive information on dendritic mRNAs, the neuronal functions of mRNA-encoded proteins, the association of dendritic mRNAs with RNA-binding proteins in RNA granules, and the effects of RNA-binding proteins on mRNA regulation. These findings provide insights into the mechanistic basis of protein-synthesis-dependent synaptic plasticity and memory formation and contribute to future efforts to understand the physiological implications of local regulation of dendritic mRNAs in neurons.

PSA-NCAM Colocalized with Cholecystokinin-Expressing Cells in the Hippocampus Is Involved in Mediating Antidepressant Efficacy

The extracellular glycan polysialic acid linked to neural cell adhesion molecule (PSA-NCAM) is principally expressed in the developing brain and the adult neurogenic regions. Although colocalization of PSA-NCAM with cholecystokinin (CCK) was found in the adult brain, the role of PSA-NCAM remains unclear. In this study, we aimed to elucidate the functional significance of PSA-NCAM in the CA1 region of the male mouse hippocampus. Combined fluorescence in situ hybridization and immunohistochemistry showed that few vesicular glutamate transporter 3-negative/CCK-positive (VGluT3^-/CCK⁺) cells were colocalized with PSA-NCAM, but most of the VGluT3⁺/CCK⁺ cells were colocalized with PSA-NCAM. The somata of PSA-NCAM⁺/CCK⁺ cells were highly innervated by serotonergic boutons than those of PSA-NCAM^-/CCK⁺ cells. The expression ratios of 5-HT_3A receptors and p11, a serotonin receptor-interacting protein, were higher in PSA-NCAM⁺/CCK⁺ cells than in PSA-NCAM^-/CCK⁺ cells. Pharmacological digestion of PSA-NCAM impaired the efficacy of antidepressant fluoxetine (FLX), a selective serotonin reuptake inhibitor, but not the efficacy of benzodiazepine anxiolytic diazepam. A Western blot showed that restraint stress decreased the expressions of p11 and mature brain-derived neurotrophic factor (BDNF), and FLX increased them. Interestingly, the FLX-induced elevation of expression of p11, but not mature BDNF, was impaired by the digestion of PSA-NCAM. Quantitative reverse transcription-polymerase chain reaction showed that restraint stress reduced the expression of polysialyltransferase ST8Sia IV and FLX elevated it. Collectively, PSA-NCAM colocalized with VGluT3⁺/CCK⁺ cells in the CA1 region of the hippocampus may play a unique role in the regulation of antidepressant efficacy via the serotonergic pathway.SIGNIFICANCE STATEMENT Polysialic acid (PSA) is composed of eight or more α2,8-linked sialic acids. Here, we examined the functional significance of polysialic acid linked to the neural cell adhesion molecule (PSA-NCAM) in the adult mouse hippocampus. Few vesicular glutamate transporter 3-negative/cholecystokinin-positive (VGluT3^-/CCK⁺) cells were colocalized with PSA-NCAM, but most of the VGluT3⁺/CCK⁺ cells were colocalized with PSA-NCAM. The expression ratios of 5-HT_3A receptors and p11, a serotonin receptor-interacting protein, were higher in PSA-NCAM⁺/CCK⁺ cells than in PSA-NCAM^-/CCK⁺ cells. The efficacy of antidepressants, but not anxiolytics, was impaired by the digestion of PSA-NCAM. The antidepressant-induced increase in p11 expression was inhibited following PSA-NCAM digestion. We hence hypothesize that PSA-NCAM colocalized with VGluT3⁺/CCK⁺ cells may play a unique role in regulating antidepressant efficacy.