Vesicles derived via AP-3-dependent recycling contribute to asynchronous release and influence information transfer

Role of adaptor protein complexes in generating functionally distinct synaptic vesicle pools

The synaptic vesicle (SV) cycle ensures the release of neurotransmitters and the replenishment of SVs to sustain neuronal activity. Multiple endocytosis and sorting pathways contribute to the recapture of the SV membrane and proteins after fusion. Adaptor protein (AP) complexes are among the critical components of the SV retrieval machinery. The canonical clathrin adaptor AP2 ensures the replenishment of most SVs across many neuronal populations. An alternative AP1/AP3-dependent process mediates the formation of a subset of SVs that differ from AP2 vesicles in molecular composition and respond preferentially during higher frequency firing. Furthermore, recent studies show that vesicular transporters for different neurotransmitters depend to a different extent on the AP3 pathway and this affects the release properties of the respective neurotransmitters. This review focuses on the current understanding of the AP-dependent molecular and functional diversity among SVs. We also discuss the contribution of these pathways to the regulation of neurotransmitter release across neuronal populations.

Functional subtypes of synaptic dynamics in mouse and human

Synapses preferentially respond to particular temporal patterns of activity with a large degree of heterogeneity that is informally or tacitly separated into classes. Yet, the precise number and properties of such classes are unclear. Do they exist on a continuum and, if so, when is it appropriate to divide that continuum into functional regions? In a large dataset of glutamatergic cortical connections, we perform model-based characterization to infer the number and characteristics of functionally distinct subtypes of synaptic dynamics. In rodent data, we find five clusters that partially converge with transgenic-associated subtypes. Strikingly, the application of the same clustering method in human data infers a highly similar number of clusters, supportive of stable clustering. This nuanced dictionary of functional subtypes shapes the heterogeneity of cortical synaptic dynamics and provides a lens into the basic motifs of information transmission in the brain.

Interrogation and validation of the interactome of neuronal Munc18-interacting Mint proteins with AlphaFold2

Munc18-interacting proteins (Mints) are multidomain adaptors that regulate neuronal membrane trafficking, signaling, and neurotransmission. Mint1 and Mint2 are highly expressed in the brain with overlapping roles in the regulation of synaptic vesicle fusion required for neurotransmitter release by interacting with the essential synaptic protein Munc18-1. Here, we have used AlphaFold2 to identify and then validate the mechanisms that underpin both the specific interactions of neuronal Mint proteins with Munc18-1 as well as their wider interactome. We found that a short acidic α-helical motif within Mint1 and Mint2 is necessary and sufficient for specific binding to Munc18-1 and binds a conserved surface on Munc18-1 domain3b. In Munc18-1/2 double knockout neurosecretory cells, mutation of the Mint-binding site reduces the ability of Munc18-1 to rescue exocytosis, and although Munc18-1 can interact with Mint and Sx1a (Syntaxin1a) proteins simultaneously in vitro, we find that they have mutually reduced affinities, suggesting an allosteric coupling between the proteins. Using AlphaFold2 to then examine the entire cellular network of putative Mint interactors provides a structural model for their assembly with a variety of known and novel regulatory and cargo proteins including ADP-ribosylation factor (ARF3/ARF4) small GTPases and the AP3 clathrin adaptor complex. Validation of Mint1 interaction with a new predicted binder TJAP1 (tight junction-associated protein 1) provides experimental support that AlphaFold2 can correctly predict interactions across such large-scale datasets. Overall, our data provide insights into the diversity of interactions mediated by the Mint family and show that Mints may help facilitate a key trigger point in SNARE (soluble N-ethylmaleimide-sensitive factor attachment receptor) complex assembly and vesicle fusion.

Adaptor protein-3 produces synaptic vesicles that release phasic dopamine

The burst firing of midbrain dopamine neurons releases a phasic dopamine signal that mediates reinforcement learning. At many synapses, however, high firing rates deplete synaptic vesicles (SVs), resulting in synaptic depression that limits release. What accounts for the increased release of dopamine by stimulation at high frequency? We find that adaptor protein-3 (AP-3) and its coat protein VPS41 promote axonal dopamine release by targeting vesicular monoamine transporter VMAT2 to the axon rather than dendrites. AP-3 and VPS41 also produce SVs that respond preferentially to high-frequency stimulation, independent of their role in axonal polarity. In addition, conditional inactivation of VPS41 in dopamine neurons impairs reinforcement learning, and this involves a defect in the frequency dependence of release rather than the amount of dopamine released. Thus, AP-3 and VPS41 promote the axonal polarity of dopamine release but enable learning by producing a distinct population of SVs tuned specifically to high firing frequency that confers the phasic release of dopamine.

Adaptor protein AP-3 produces synaptic vesicles that release at high frequency by recruiting phospholipid flippase ATP8A1

Neural systems encode information in the frequency of action potentials, which is then decoded by synaptic transmission. However, the rapid, synchronous release of neurotransmitters depletes synaptic vesicles (SVs), limiting release at high firing rates. How then do synapses convey information about frequency? Here, we show in mouse hippocampal neurons and slices that the adaptor protein AP-3 makes a subset of SVs that respond specifically to high-frequency stimulation. Neurotransmitter transporters slot onto these SVs in different proportions, contributing to the distinct properties of release observed at different excitatory synapses. Proteomics reveals that AP-3 targets the phospholipid flippase ATP8A1 to SVs; loss of ATP8A1 recapitulates the defect in SV mobilization at high frequency observed with loss of AP-3. The mechanism involves recruitment of synapsin by the cytoplasmically oriented phosphatidylserine translocated by ATP8A1. Thus, ATP8A1 enables the subset of SVs made by AP-3 to release at high frequency.

Adaptor Protein-3 Produces Synaptic Vesicles that Release Phasic Dopamine

The burst firing of midbrain dopamine neurons releases a phasic dopamine signal that mediates reinforcement learning. At many synapses, however, high firing rates deplete synaptic vesicles (SVs), resulting in synaptic depression that limits release. What accounts for the increased release of dopamine by stimulation at high frequency? We find that adaptor protein-3 (AP-3) and its coat protein VPS41 promote axonal dopamine release by targeting vesicular monoamine transporter VMAT2 to the axon rather than dendrites. AP-3 and VPS41 also produce SVs that respond preferentially to high frequency stimulation, independent of their role in axonal polarity. In addition, conditional inactivation of VPS41 in dopamine neurons impairs reinforcement learning, and this involves a defect in the frequency dependence of release rather than the amount of dopamine released. Thus, AP-3 and VPS41 promote the axonal polarity of dopamine release but enable learning by producing a novel population of SVs tuned specifically to high firing frequency that confers the phasic release of dopamine.

Two forms of asynchronous release with distinctive spatiotemporal dynamics in central synapses

Asynchronous release is a ubiquitous form of neurotransmitter release that persists for tens to hundreds of milliseconds after an action potential. How asynchronous release is organized and regulated at the synaptic active zone (AZ) remains debatable. Using nanoscale-precision imaging of individual release events in rat hippocampal synapses, we observed two spatially distinct subpopulations of asynchronous events, ~75% of which occurred inside the AZ and with a bias towards the AZ center, while ~25% occurred outside of the functionally defined AZ, that is, ectopically. The two asynchronous event subpopulations also differed from each other in temporal properties, with ectopic events occurring at significantly longer time intervals from synchronous events than the asynchronous events inside the AZ. Both forms of asynchronous release did not, to a large extent, utilize the same release sites as synchronous events. The two asynchronous event subpopulations also differ from synchronous events in some aspects of exo-endocytosis coupling, particularly in the contribution from the fast calcium-dependent endocytosis. These results identify two subpopulations of asynchronous release events with distinctive organization and spatiotemporal dynamics.

Inositol hexakisphosphate kinases differentially regulate trafficking of vesicular glutamate transporters 1 and 2

Inositol pyrophosphates have been implicated in cellular signaling and membrane trafficking, including synaptic vesicle (SV) recycling. Inositol hexakisphosphate kinases (IP6Ks) and their product, diphosphoinositol pentakisphosphate (PP-IP₅ or IP7), directly and indirectly regulate proteins important in vesicle recycling by the activity-dependent bulk endocytosis pathway (ADBE). In the present study, we show that two isoforms, IP6K1 and IP6K3, are expressed in axons. The role of the kinases in SV recycling are investigated using pharmacologic inhibition, shRNA knockdown, and IP6K1 and IP6K3 knockout mice. Live-cell imaging experiments use optical reporters of SV recycling based on vesicular glutamate transporter isoforms, VGLUT1- and VGLUT2-pHluorins (pH), which recycle differently. VGLUT1-pH recycles by classical AP-2 dependent endocytosis under moderate stimulation conditions, while VGLUT2-pH recycles using AP-1 and AP-3 adaptor proteins as well. Using a short stimulus to release the readily releasable pool (RRP), we show that IP6K1 KO increases exocytosis of both VGLUT1-and VGLUT2-pH, while IP6K3 KO decreases the amount of both transporters in the RRP. In electrophysiological experiments we measure glutamate signaling with short stimuli and under the intense stimulation conditions that trigger bulk endocytosis. IP6K1 KO increases synaptic facilitation and IP6K3 KO decreases facilitation compared to wild type in CA1 hippocampal Schaffer collateral synapses. After intense stimulation, the rate of endocytosis of VGLUT2-pH, but not VGLUT1-pH, is increased by knockout, knockdown, and pharmacologic inhibition of IP6Ks. Thus IP6Ks differentially affect the endocytosis of two SV protein cargos that use different endocytic pathways. However, while IP6K1 KO and IP6K3 KO exert similar effects on endocytosis after stimulation, the isoforms exert different effects on exocytosis earlier in the stimulus and on the early phase of glutamate release. Taken together, the data indicate a role for IP6Ks both in exocytosis early in the stimulation period and in endocytosis, particularly under conditions that may utilize AP-1/3 adaptors.

Zebrafish Syndromic Albinism Models as Tools for Understanding and Treating Pigment Cell Disease in Humans

Simple Summary

Zebrafish (Danio rerio) is an emerging model for studying many diseases, including disorders originating in black pigment cells, melanocytes. In this review of the melanocyte literature, we discuss the current knowledge of melanocyte biology relevant to understanding different forms of albinism and the potential of the zebrafish model system for finding novel mechanisms and treatments.

Abstract

Melanin is the pigment that protects DNA from ultraviolet (UV) damage by absorbing excess energy. Melanin is produced in a process called melanogenesis. When melanogenesis is altered, diseases such as albinism result. Albinism can result in an increased skin cancer risk. Conversely, black pigment cell (melanocyte) development pathways can be misregulated, causing excessive melanocyte growth that leads to melanoma (cancer of melanocytes). Zebrafish is an emerging model organism used to study pigment disorders due to their high fecundity, visible melanin development in melanophores (melanocytes in mammals) from 24 h post-fertilization, and conserved melanogenesis pathways. Here, we reviewed the conserved developmental pathways in zebrafish melanophores and mammalian melanocytes. Additionally, we summarized the progress made in understanding pigment cell disease and evidence supporting the strong potential for using zebrafish to find novel treatment options for albinism.

Cerebellar ataxia and myeloradiculopathy associated with AP3B2 antibody: a case report and literature review

BACKGROUND

AP3B2 is one of the subunits of vesicle coat protein AP3 and is specifically expressed in central nervous system neurons. AP3B2 antibody has been reported in patients with autoimmune cerebellar ataxia and various extracerebellar symptoms. However, there have been few reports on its clinical features and treatment response.

METHODS

We report a 47-year-old man with AP3B2 antibody who presented with insidious-onset paresthesia and gait disturbance. His serum and cerebrospinal fluid (CSF) showed reactivity with the cytoplasm of Purkinje cells and granular layer synapses comparable to the reported specific pattern of anti-AP3B2 IgG, and this was confirmed by a cell-based assay. His symptoms improved after the administration of intravenous immunoglobulin, and oral prednisone and mycophenolate mofetil. Extensive examination and long-term follow-up showed no evidence of malignancy. A literature review was included to emphasize the neurological syndrome associated with this rare autoantibody.

RESULTS

Eleven cases with AP3B2 antibody, including our patient, were identified. The diversity of symptoms, including cerebellar and sensory ataxia, paresthesia, and weakness, was in line with the extensive binding of AP3B2 antibody to the spinal cord gray matter, dorsal root ganglia, cerebellar cortex, and nucleus. In the CSF, half of patients had elevated white blood cell counts, increased protein concentrations, or CSF-specific oligoclonal bands. All previous cases had subacute onsets and no improvement was noted after immunotherapy.

CONCLUSION

Our case indicated that disorders associated with AP3B2 antibody can also start insidiously. Immunotherapy is warranted given the possibility of clinical improvement.

Ultrastructural Imaging of Activity-Dependent Synaptic Membrane-Trafficking Events in Cultured Brain Slices

Electron microscopy can resolve synapse ultrastructure with nanometer precision, but the capture of time-resolved, activity-dependent synaptic membrane-trafficking events has remained challenging, particularly in functionally distinct synapses in a tissue context. We present a method that combines optogenetic stimulation-coupled cryofixation ("flash-and-freeze") and electron microscopy to visualize membrane trafficking events and synapse-state-specific changes in presynaptic vesicle organization with high spatiotemporal resolution in synapses of cultured mouse brain tissue. With our experimental workflow, electrophysiological and "flash-and-freeze" electron microscopy experiments can be performed under identical conditions in artificial cerebrospinal fluid alone, without the addition of external cryoprotectants, which are otherwise needed to allow adequate tissue preservation upon freezing. Using this approach, we reveal depletion of docked vesicles and resolve compensatory membrane recycling events at individual presynaptic active zones at hippocampal mossy fiber synapses upon sustained stimulation.

Cadmium desynchronizes neurotransmitter release in the neuromuscular junction: Key role of ROS

Cd²⁺ is one of the most widespread environmental pollutants and its accumulation in central and peripheral nervous systems leads to neurotoxicity as well as aggravation of common neurodegenerative diseases. Mechanism of the Cd²⁺ toxicity is far from being resolved. Here, using microelectrode recordings of postsynaptic responses and fluorescent redox indicators we studied the effect of Cd²⁺ in the submicromolar range on timing of neurotransmitter release and oxidative status in two functionally different compartments of the same frog motor nerve terminal. Cd²⁺ (0.1-1 μM) acting as typical voltage-gated Ca²⁺channel (VGCC) antagonist decreased neurotransmitter release in both distal and proximal parts of the nerve terminal, but in contrast to the VGCC blockers Cd²⁺(0.1-0.5 μM) desynchronized the release selectively in the distal region. The latter action of Cd²⁺ was completely prevented by inhibitor of NADPH-oxidase and antioxidants, including mitochondrial specific, as well as redox-sensitive TRPV1 channel blocker. Cd²⁺ markedly increased levels of mitochondrial reactive oxygen species (ROS) in both the distal and proximal compartments of the nerve terminal, which was associated with lipid peroxidation mainly in the distal region. Zn²⁺, whose transport systems translocate Cd²⁺, markedly enhanced the effects of Cd²⁺ on both the mitochondrial ROS levels and timing of neurotransmitter release. Furthermore, in the presence of Zn²⁺ ions, Cd²⁺ also desynchronized the neurotransmitter release in the proximal region. Thus, in synapses Cd²⁺ at very low concentrations can increase mitochondrial ROS, lipid peroxidation and disturb the timing of neurotransmitter release via a ROS/TRPV-dependent mechanism. Desynchronization of neurotransmitter release and synaptic oxidative stress could be early events in Cd²⁺ neurotoxicity.

Presynaptic dysfunction in neurodevelopmental disorders: Insights from the synaptic vesicle life cycle

The activity-dependent fusion, retrieval and recycling of synaptic vesicles is essential for the maintenance of neurotransmission. Until relatively recently it was believed that most mutations in genes that were essential for this process would be incompatible with life, because of this fundamental role. However, an ever-expanding number of mutations in this very cohort of genes are being identified in individuals with neurodevelopmental disorders, including autism, intellectual disability and epilepsy. This article will summarize the current state of knowledge linking mutations in presynaptic genes to neurodevelopmental disorders by sequentially covering the various stages of the synaptic vesicle life cycle. It will also discuss how perturbations of specific stages within this recycling process could translate into human disease. Finally, it will also provide perspectives on the potential for future therapy that are targeted to presynaptic function.

Slow-decaying presynaptic calcium dynamics gate long-lasting asynchronous release at the hippocampal mossy fiber to CA3 pyramidal cell synapse

Action potentials trigger two modes of neurotransmitter release, with a fast synchronous component and a temporally delayed asynchronous release. Asynchronous release contributes to information transfer at synapses, including at the hippocampal mossy fiber (MF) to CA3 pyramidal cell synapse where it controls the timing of postsynaptic CA3 pyramidal neuron firing. Here, we identified and characterized the main determinants of asynchronous release at the MF–CA3 synapse. We found that asynchronous release at MF–CA3 synapses can last on the order of seconds following repetitive MF stimulation. Elevating the stimulation frequency or the external Ca²⁺ concentration increased the rate of asynchronous release, thus, arguing that presynaptic Ca²⁺ dynamics is the major determinant of asynchronous release rate. Direct MF bouton Ca²⁺ imaging revealed slow Ca²⁺ decay kinetics of action potential (AP) burst‐evoked Ca²⁺ transients. Finally, we observed that asynchronous release was preferentially mediated by Ca²⁺ influx through P/Q‐type voltage‐gated Ca²⁺ channels, while the contribution of N‐type VGCCs was limited. Overall, our results uncover the determinants of long‐lasting asynchronous release from MF terminals and suggest that asynchronous release could influence CA3 pyramidal cell firing up to seconds following termination of granule cell bursting.

The road to lysosome-related organelles: Insights from Hermansky-Pudlak syndrome and other rare diseases

Lysosome-related organelles (LROs) comprise a diverse group of cell type-specific, membrane-bound subcellular organelles that derive at least in part from the endolysosomal system but that have unique contents, morphologies and functions to support specific physiological roles. They include: melanosomes that provide pigment to our eyes and skin; alpha and dense granules in platelets, and lytic granules in cytotoxic T cells and natural killer cells, which release effectors to regulate hemostasis and immunity; and distinct classes of lamellar bodies in lung epithelial cells and keratinocytes that support lung plasticity and skin lubrication. The formation, maturation and/or secretion of subsets of LROs are dysfunctional or entirely absent in a number of hereditary syndromic disorders, including in particular the Hermansky-Pudlak syndromes. This review provides a comprehensive overview of LROs in humans and model organisms and presents our current understanding of how the products of genes that are defective in heritable diseases impact their formation, motility and ultimate secretion.

Distinct Calcium Sources Define Compartmentalized Synaptic Signaling Domains

Nervous system communication relies on neurotransmitter release for synaptic transmission between neurons. Neurotransmitter is contained within vesicles in presynaptic terminals and intraterminal calcium governs the fundamental step of their release into the synaptic cleft. Despite a common dependence on calcium, synaptic transmission and its modulation varies highly across the nervous system. The precise mechanisms that underlie this heterogeneity, however, remain unclear. The present review highlights recent data that reveal vesicles sourced from separate pools define discrete modes of release. A rich diversity of regulatory machinery may further distinguish the different forms of vesicle release, including presynaptic proteins involved in trafficking, alignment, and exocytosis. These multiple vesicle release mechanisms and vesicle pools likely depend on the arrangement of vesicles in relation to specific calcium entry pathways that create compartmentalized spheres of calcium influence (i.e., domains). This diversity permits release specialization. This review details examples of how individual neurons rely on multiple calcium sources and unique regulatory schemes to provide differential release and discrete modulation of neurotransmitter release from specific vesicle pools-as part of network signal integration.

Autoimmune gait disturbance accompanying adaptor protein-3B2-IgG

Objective

To describe phenotypes, treatment response, and outcomes of autoimmunity targeting a synaptic vesicle coat protein, the neuronal (B2) form of adaptor protein–3 (AP3).

Methods

Archived serum and CSF specimens (from 616,025 screened) harboring unclassified synaptic antibodies mimicking amphiphysin–immunoglobulin G (IgG) on tissue-based indirect immunofluorescence assay (IFA) were re-evaluated for novel IgG staining patterns. Autoantigens were identified by western blot and mass spectrometry. Recombinant western blot and cell-binding assay (CBA) were used to confirm antigen specificity. Clinical data were obtained retrospectively.

Results

Serum (10) and CSF (6) specimens of 10 patients produced identical IFA staining patterns throughout mouse nervous system tissues, most prominently in cerebellum (Purkinje neuronal perikarya, granular layer synapses, and dentate regions), spinal cord gray matter, dorsal root ganglia, and sympathetic ganglia. The antigen revealed by mass spectrometry analysis and confirmed by recombinant assays (western blot and CBA) was AP3B2 in all. Of 10 seropositive patients, 6 were women; median symptom onset age was 42 years (range 24–58). Clinical information was available for 9 patients, all with subacute onset and rapidly progressive gait ataxia. Neurologic manifestations were myeloneuropathy (3), peripheral sensory neuropathy (2), cerebellar ataxia (2), and spinocerebellar ataxia (2). Five patients received immunotherapy; none improved, but they did not worsen over the follow-up period (median 36 months; range 3–94). Two patients (both with cancer) died. One of 50 control sera was positive by western blot only (but not by IFA or CBA).

Conclusion

AP3B2 (previously named β-neuronal adaptin-like protein) autoimmunity appears rare, is accompanied by ataxia (sensory or cerebellar), and is potentially treatable.

Synaptic Vesicle Recycling Pathway Determines Neurotransmitter Content and Release Properties

In contrast to temporal coding by synaptically acting neurotransmitters such as glutamate, neuromodulators such as monoamines signal changes in firing rate. The two modes of signaling have been thought to reflect differences in release by different cells. We now find that midbrain dopamine neurons release glutamate and dopamine with different properties that reflect storage in different synaptic vesicles. The vesicles differ in release probability, coupling to presynaptic Ca2+ channels and frequency dependence. Although previous work has attributed variation in these properties to differences in location or cytoskeletal association of synaptic vesicles, the release of different transmitters shows that intrinsic differences in vesicle identity drive different modes of release. Indeed, dopamine but not glutamate vesicles depend on the adaptor protein AP-3, revealing an unrecognized linkage between the pathway of synaptic vesicle recycling and the properties of exocytosis. Storage of the two transmitters in different vesicles enables the transmission of distinct signals.

Coupling of microtubule motors with AP-3 generated organelles in axons by NEEP21 family member calcyon

Transport of late endosomes and lysosome-related organelles (LE/LROs) in axons is essential for supplying synaptic cargoes and for removing damaged macromolecules. Defects in this system are implicated in a range of human neurodegenerative and neurodevelopmental disorders. The findings reported here identify a novel mechanism regulating LE/LRO transport based on the coordinated coupling of microtubule motors and vesicle coat proteins to the neuron-enriched, transmembrane protein calcyon (Caly). We found that the cytoplasmic C-terminus of Caly pulled down proteins involved in microtubule-dependent transport (DIC, KIF5A, p150Glued, Lis1) and organelle biogenesis (AP-1 and AP-3) from the brain. In addition, RNA interference-mediated knockdown of Caly increased the percentage of static LE/LROs labeled by LysoTracker in cultured dorsal root ganglion axons. In contrast, overexpression of Caly stimulated movement of organelles positive for LysoTracker or the AP-3 cargo GFP-PI4KIIα. However, a Caly mutant (ATEA) that does not bind AP-3 was unable to pull down motor proteins from brain, and expression of the ATEA mutant failed to increase either LE/LRO flux or levels of associated dynein. Taken together, these data support the hypothesis that Caly is a multifunctional scaffolding protein that regulates axonal transport of LE/LROs by coordinately interacting with motor and vesicle coat proteins.

Sex-Specific Consequences of Neonatal Stress on Cardio-Respiratory Inhibition Following Laryngeal Stimulation in Rat Pups

The presence of liquid near the larynx of immature mammals triggers prolonged apneas with significant O2 desaturations and bradycardias. When excessive, this reflex (the laryngeal chemoreflex; LCR) can be fatal. Our understanding of the origins of abnormal LCR are limited; however, perinatal stress and male sex are risk factors for cardio-respiratory failure in infants. Because exposure to stress during early life has deleterious and sex-specific consequences on brain development it is plausible that respiratory reflexes are vulnerable to neuroendocrine dysfunction. To address this issue, we tested the hypothesis that neonatal maternal separation (NMS) is sufficient to exacerbate LCR-induced cardio-respiratory inhibition in anesthetized rat pups. Stressed pups were separated from their mother 3 h/d from postnatal days 3 to 12. At P14-P15, pups were instrumented to monitor breathing, O2 saturation (Spo2), and heart rate. The LCR was activated by water injections near the larynx (10 µl). LCR-induced apneas were longer in stressed pups than controls; O2 desaturations and bradycardias were more profound, especially in males. NMS increased the frequency and amplitude of spontaneous EPSCs (sEPSCs) in the dorsal motor nucleus of the vagus (DMNV) of males but not females. The positive relationship between corticosterone and testosterone observed in stressed pups (males only) suggests that disruption of neuroendocrine function by stress is key to sex-based differences in abnormal LCR. Because testosterone application onto medullary slices augments EPSC amplitude only in males, we propose that testosterone-mediated enhancement of synaptic connectivity within the DMNV contributes to the male bias in cardio-respiratory inhibition following LCR activation in stressed pups.

Integration of Synaptic Vesicle Cargo Retrieval with Endocytosis at Central Nerve Terminals

Central nerve terminals contain a limited number of synaptic vesicles (SVs) which mediate the essential process of neurotransmitter release during their activity-dependent fusion. The rapid and accurate formation of new SVs with the appropriate cargo is essential to maintain neurotransmission in mammalian brain. Generating SVs containing the correct SV cargo with the appropriate stoichiometry is a significant challenge, especially when multiple modes of endocytosis exist in central nerve terminals, which occur at different locations within the nerve terminals. These endocytosis modes include ultrafast endocytosis, clathrin-mediated endocytosis (CME) and activity-dependent bulk endocytosis (ADBE) which are triggered by specific patterns of neuronal activity. This review article will assess the evidence for the role of classical adaptor protein complexes in SV retrieval, discuss the role of monomeric adaptors and how interactions between specific SV cargoes can facilitate retrieval. In addition it will consider the evidence for preassembled plasma membrane cargo complexes and their role in facilitating these endocytosis modes. Finally it will present a unifying model for cargo retrieval at the presynapse, which integrates endocytosis modes in time and space.

Distinct Calcium Sources Support Multiple Modes of Synaptic Release from Cranial Sensory Afferents

Most craniosensory afferents have unmyelinated axons expressing TRP Vanilloid 1 (TRPV1) receptors in synaptic terminals at the solitary tract nucleus (NTS). Neurotransmission from these synapses is characterized by substantial asynchronous EPSCs following action potential-synched EPSCs and high spontaneous rates that are thermally sensitive. The present studies blocked voltage-activated calcium channels (CaV) using the nonselective CaV blocker Cd(2+) or the specific N-type blocker ω-conotoxin GVIA to examine the calcium dependence of the synchronous, asynchronous, spontaneous, and thermally gated modes of release. In rat brainstem slices containing caudal NTS, shocks to the solitary tract (ST) triggered synchronous ST-EPSCs and trailing asynchronous EPSCs. Cd(2+) or GVIA efficiently reduced both synchronous and asynchronous EPSCs without altering spontaneous or thermal-evoked transmission. Activation of TRPV1 with either the selective agonist resiniferatoxin (150 pm) or temperature augmented basal sEPSC rates but failed to alter the synchronous or asynchronous modes of release. These data indicate that calcium sourced through TRPV1 has no access to the synchronous or asynchronous release mechanism(s) and conversely that CaV-sourced calcium does not interact with the thermally evoked mode of release. Buffering intracellular calcium with EGTA-AM or BAPTA-AM reduced asynchronous EPSC rates earlier and to a greater extent than synchronous ST-EPSC amplitudes without altering sEPSCs or thermal sensitivity. Buffering therefore distinguishes asynchronous vesicles as possessing a highly sensitive calcium sensor located perhaps more distant from CaV than synchronous vesicles or thermally evoked vesicles from TRPV1. Together, our findings suggest separate mechanisms of release for spontaneous, asynchronous and synchronous vesicles that likely reside in unique, spatially separated vesicle domains.

SIGNIFICANCE STATEMENT

Most craniosensory fibers release glutamate using calcium entry from two sources: CaVs and TRPV1. We demonstrate that calcium segregation distinguishes three vesicle release mechanisms. Most surprisingly, asynchronous release is associated with CaV and not TRPV1 calcium entry. This reveals that asynchronous release is an additional and separate phenotypic marker of unmyelinated afferents rather than operated by TRPV1. The functional independence of the two calcium sources expands the regulatory repertoire of transmission and imbues these inputs with additional modulation targets for synaptic release not present at conventional CaV synapses. Peptides and lipid mediators may target one or both of these calcium sources at afferent terminals within the solitary tract nucleus to independently modify release from distinct, functionally segregated vesicle pools.

Molecular Machines Regulating the Release Probability of Synaptic Vesicles at the Active Zone

The fusion of synaptic vesicles (SVs) with the plasma membrane of the active zone (AZ) upon arrival of an action potential (AP) at the presynaptic compartment is a tightly regulated probabilistic process crucial for information transfer. The probability of a SV to release its transmitter content in response to an AP, termed release probability (Pr), is highly diverse both at the level of entire synapses and individual SVs at a given synapse. Differences in Pr exist between different types of synapses, between synapses of the same type, synapses originating from the same axon and even between different SV subpopulations within the same presynaptic terminal. The Pr of SVs at the AZ is set by a complex interplay of different presynaptic properties including the availability of release-ready SVs, the location of the SVs relative to the voltage-gated calcium channels (VGCCs) at the AZ, the magnitude of calcium influx upon arrival of the AP, the buffering of calcium ions as well as the identity and sensitivity of the calcium sensor. These properties are not only interconnected, but can also be regulated dynamically to match the requirements of activity patterns mediated by the synapse. Here, we review recent advances in identifying molecules and molecular machines taking part in the determination of vesicular Pr at the AZ.

Asynchronous presynaptic glutamate release enhances neuronal excitability during the post-spike refractory period

KEY POINTS

Many excitatory synapses in the brain release glutamate with both synchronous and asynchronous components. Immediately following an action potential, neurons display a reduced excitability due to the post-spike afterhyperpolarization (AHP). This gives rise to a relative refractory period. When an action potential is evoked by glutamate synaptic input possessing asynchronous release, the delayed glutamate release events act to depolarize the neuron during the AHP and overcome the relative refractory period. These results demonstrate a new role for asynchronous release in regulating post-spike excitability and the relative refractory period in central neurons.

ABSTRACT

Post-spike afterhyperpolarizations (AHPs) functionally inhibit neuronal excitability for tens to hundreds of milliseconds following each action potential. This imposes a relative refractory period during which synaptic excitation is less effective at evoking spikes. Here we asked whether some synapses have mechanisms in place that allow them to overcome the AHP and drive spiking in target cells during this period of reduced excitability. We examined glutamate synapses onto oxytocin and vasopressin neurons in the paraventricular nucleus of the hypothalamus. These synapses can display pronounced asynchronous glutamate release following a single presynaptic spike, with the time course of release being similar to that of the post-spike AHP. To test whether asynchronous release is more effective at overcoming the relative refractory period, we evoked a single action potential with either a brief synchronous depolarization or an asynchronous potential and then assessed excitability at multiple time points following the spike. Neurons receiving asynchronous depolarizing synaptic inputs had a shorter relative refractory period than those receiving synchronous depolarizations. Our data demonstrate that synapses releasing glutamate in an asynchronous and delayed manner are ideally adapted to counter the AHP. By effectively overcoming the relative refractory period, the kinetics of excitatory synaptic input can play an important role in controlling post-spike excitability.

Functionally heterogeneous synaptic vesicle pools support diverse synaptic signalling

Synaptic communication between neurons is a highly dynamic process involving specialized structures. At the level of the presynaptic terminal, neurotransmission is ensured by fusion of vesicles to the membrane, which releases neurotransmitter in the synaptic cleft. Depending on the level of activity experienced by the terminal, the spatiotemporal properties of calcium invasion will dictate the timing and the number of vesicles that need to be released. Diverse presynaptic firing patterns are translated to neurotransmitter release with a distinct temporal feature. Complex patterns of neurotransmitter release can be achieved when different vesicles respond to distinct calcium dynamics in the presynaptic terminal. Specific vesicles from different pools are recruited during various modes of release as the particular molecular composition of their membrane proteins define their functional properties. Such diversity endows the presynaptic terminal with the ability to respond to distinct physiological signals via the mobilization of specific subpopulation of vesicles. There are several mechanisms by which a diverse vesicle population could be generated in single presynaptic terminals, including distinct recycling pathways that utilize various adaptor proteins. Several additional factors could potentially contribute to the development of a heterogeneous vesicle pool such as specialized release sites, spatial segregation within the terminal and specialized delivery pathways. Among these factors molecular heterogeneity plays a central role in defining the functional properties of different subpopulations of vesicles.

Molecular underpinnings of synaptic vesicle pool heterogeneity

Neuronal communication relies on chemical synaptic transmission for information transfer and processing. Chemical neurotransmission is initiated by synaptic vesicle fusion with the presynaptic active zone resulting in release of neurotransmitters. Classical models have assumed that all synaptic vesicles within a synapse have the same potential to fuse under different functional contexts. In this model, functional differences among synaptic vesicle populations are ascribed to their spatial distribution in the synapse with respect to the active zone. Emerging evidence suggests, however, that synaptic vesicles are not a homogenous population of organelles, and they possess intrinsic molecular differences and differential interaction partners. Recent studies have reported a diverse array of synaptic molecules that selectively regulate synaptic vesicles' ability to fuse synchronously and asynchronously in response to action potentials or spontaneously irrespective of action potentials. Here we discuss these molecular mediators of vesicle pool heterogeneity that are found on the synaptic vesicle membrane, on the presynaptic plasma membrane, or within the cytosol and consider some of the functional consequences of this diversity. This emerging molecular framework presents novel avenues to probe synaptic function and uncover how synaptic vesicle pools impact neuronal signaling.

VAMP4 Is an Essential Cargo Molecule for Activity-Dependent Bulk Endocytosis

The accurate formation of synaptic vesicles (SVs) and incorporation of their protein cargo during endocytosis is critical for the maintenance of neurotransmission. During intense neuronal activity, a transient and acute accumulation of SV cargo occurs at the plasma membrane. Activity-dependent bulk endocytosis (ADBE) is the dominant SV endocytosis mode under these conditions; however, it is currently unknown how ADBE mediates cargo retrieval. We examined the retrieval of different SV cargo molecules during intense stimulation using a series of genetically encoded pH-sensitive reporters in neuronal cultures. The retrieval of only one reporter, VAMP4-pHluorin, was perturbed by inhibiting ADBE. This selective recovery was confirmed by the enrichment of endogenous VAMP4 in purified bulk endosomes formed by ADBE. VAMP4 was also essential for ADBE, with a cytoplasmic di-leucine motif being critical for this role. Therefore, VAMP4 is the first identified ADBE cargo and is essential for this endocytosis mode to proceed.

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VAMP4 is the first identified ADBE cargo

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VAMP4 is essential for ADBE

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Most synaptic vesicle cargoes are not selectively recovered by ADBE