Tetanus toxin blocks the exocytosis of synaptic vesicles clustered at synapses but not of synaptic vesicles in isolated axons

AMPA and GABAA receptor nanodomains assemble in the absence of synaptic neurotransmitter release

Postsynaptic neurotransmitter receptors and their associated scaffolding proteins assemble into discrete, nanometer-scale subsynaptic domains (SSDs) within the postsynaptic membrane at both excitatory and inhibitory synapses. Intriguingly, postsynaptic receptor SSDs are mirrored by closely apposed presynaptic active zones. These trans-synaptic molecular assemblies are thought to be important for efficient neurotransmission because they concentrate postsynaptic receptors near sites of presynaptic neurotransmitter release. While previous studies have characterized the role of synaptic activity in sculpting the number, size, and distribution of postsynaptic SSDs at established synapses, it remains unknown whether neurotransmitter signaling is required for their initial assembly during synapse development. Here, we evaluated synaptic nano-architecture under conditions where presynaptic neurotransmitter release was blocked prior to, and throughout synaptogenesis with tetanus neurotoxin (TeNT). In agreement with previous work, neurotransmitter release was not required for the formation of excitatory or inhibitory synapses. The overall size of the postsynaptic specialization at both excitatory and inhibitory synapses was reduced at chronically silenced synapses. However, both AMPARs and GABAARs still coalesced into SSDs, along with their respective scaffold proteins. Presynaptic active zone assemblies, defined by RIM1, were smaller and more numerous at silenced synapses, but maintained alignment with postsynaptic AMPAR SSDs. Thus, basic features of synaptic nano-architecture, including assembly of receptors and scaffolds into trans-synaptically aligned structures, are intrinsic properties that can be further regulated by subsequent activity-dependent mechanisms.

Advancements in the Quest to Map, Monitor, and Manipulate Neural Circuitry

Neural circuits and the cells that comprise them represent the functional units of the brain. Circuits relay and process sensory information, maintain homeostasis, drive behaviors, and facilitate cognitive functions such as learning and memory. Creating a functionally-precise map of the mammalian brain requires anatomically tracing neural circuits, monitoring their activity patterns, and manipulating their activity to infer function. Advancements in cell-type-specific genetic tools allow interrogation of neural circuits with increased precision. This review provides a broad overview of recombination-based and activity-driven genetic targeting approaches, contemporary viral tracing strategies, electrophysiological recording methods, newly developed calcium, and voltage indicators, and neurotransmitter/neuropeptide biosensors currently being used to investigate circuit architecture and function. Finally, it discusses methods for acute or chronic manipulation of neural activity, including genetically-targeted cellular ablation, optogenetics, chemogenetics, and over-expression of ion channels. With this ever-evolving genetic toolbox, scientists are continuing to probe neural circuits with increasing resolution, elucidating the structure and function of the incredibly complex mammalian brain.

Transsynaptic N-Cadherin Adhesion Complexes Control Presynaptic Vesicle and Bulk Endocytosis at Physiological Temperature

At mammalian glutamatergic synapses, most basic elements of synaptic transmission have been shown to be modulated by specific transsynaptic adhesion complexes. However, although crucial for synapse homeostasis, a physiological regulation of synaptic vesicle endocytosis by adhesion molecules has not been firmly established. The homophilic adhesion protein N-cadherin is localized at the peri-active zone, where the highly temperature-dependent endocytosis of vesicles occurs. Here, we demonstrate an important modulatory role of N-cadherin in endocytosis at near physiological temperature by synaptophysin-pHluorin imaging. Different modes of endocytosis including bulk endocytosis were dependent on N-cadherin expression and function. N-cadherin modulation might be mediated by actin filaments because actin polymerization ameliorated the knockout-induced endocytosis defect. Using super-resolution imaging, we found strong recruitment of N-cadherin to glutamatergic synapses upon massive vesicle release, which might in turn enhance vesicle endocytosis. This provides a novel, adhesion protein-mediated mechanism for efficient coupling of exo- and endocytosis.

Semimechanistic Population Pharmacokinetic Modeling to Investigate Amyloid Beta Trafficking and Accumulation at the BBB Endothelium

Elevated exposure to toxic amyloid beta (Aβ) peptides and consequent blood-brain barrier (BBB) dysfunction are believed to promote vasculopathy in Alzheimer's disease (AD). However, the accumulation kinetics of different Aβ isoforms within the BBB endothelium and how it drives BBB dysfunction are not clearly characterized. Using single positron emission computed tomography (SPECT)-computed tomography (CT) dynamic imaging coupled with population pharmacokinetic modeling, we investigated the accumulation kinetics of Aβ40 and Aβ42 in the BBB endothelium. Brain clearance was quantified after intracerebral administration of ¹²⁵I-Aβ, and BBB-mediated transport was shown to account for 54% of ¹²⁵I-Aβ40 total clearance. A brain influx study demonstrated lower values of both maximal rate (V_max) and Michaelis constant (K_m) for ¹²⁵I-Aβ42 compared to ¹²⁵I-Aβ40. Validated by a transcytosis study in polarized human BBB endothelial cell (hCMEC/D3) monolayers, model simulations demonstrated impaired exocytosis was responsible for inefficient permeability and enhanced accumulation of Aβ42 in the BBB endothelium. Further, both isoforms were shown to disrupt the exocytosis machinery of BBB endothelial cells so that a vicious cycle could be generated. The validated model was able to capture changes in Aβ steady-state levels in plasma as well as the brain during AD progression and allowed us to predict the kinetics of Aβ accumulation in the BBB endothelium.

Cytoskeleton and Membrane Organization at Axon Branches

Axon branching is a critical process ensuring a high degree of interconnectivity for neural network formation. As branching occurs at sites distant from the soma, it is necessary that axons have a local system to dynamically control and regulate axonal growth. This machinery depends on the orchestration of cellular functions such as cytoskeleton, subcellular transport, energy production, protein- and membrane synthesis that are adapted for branch formation. Compared to the axon shaft, branching sites show a distinct and dynamic arrangement of cytoskeleton components, endoplasmic reticulum and mitochondria. This review discusses the regulation of axon branching in the context of cytoskeleton and membrane remodeling.

Tetanus and tetanus neurotoxin: From peripheral uptake to central nervous tissue targets

Tetanus is a deadly but preventable disease caused by a protein neurotoxin produced by Clostridium tetani. Spores of C. tetani may contaminate a necrotic wound and germinate into a vegetative bacterium that releases a toxin, termed tetanus neurotoxin (TeNT). TeNT enters the general circulation, binds to peripheral motor neurons and sensory neurons, and is transported retroaxonally to the spinal cord. It then enters inhibitory interneurons and blocks the release of glycine or GABA causing a spastic paralysis. This review attempts to correlate the metalloprotease activity of TeNT and its trafficking and localization into the vertebrate body to the nature and sequence of appearance of the symptoms of tetanus.

Paraventricular, subparaventricular and periventricular hypothalamic IRS4-expressing neurons are required for normal energy balance

Understanding the neural components modulating feeding-related behavior and energy expenditure is crucial to combating obesity and its comorbidities. Neurons within the paraventricular nucleus of the hypothalamus (PVH) are a key component of the satiety response; activation of the PVH decreases feeding and increases energy expenditure, thereby promoting negative energy balance. In contrast, PVH ablation or silencing in both rodents and humans leads to substantial obesity. Recent studies have identified genetically-defined PVH subpopulations that control discrete aspects of energy balance (e.g. oxytocin (OXT), neuronal nitric oxide synthase 1 (NOS1), melanocortin 4-receptor (MC4R), prodynorphin (PDYN)). We previously demonstrated that non-OXT NOS1^PVH neurons contribute to PVH-mediated feeding suppression. Here, we identify and characterize a non-OXT, non-NOS1 subpopulation of PVH and peri-PVH neurons expressing insulin-receptor substrate 4 (IRS4^PVH) involved in energy balance control. Using Cre-dependent viral tools to activate, trace and silence these neurons, we highlight the sufficiency and necessity of IRS4^PVH neurons in normal feeding and energy expenditure regulation. Furthermore, we demonstrate that IRS4^PVH neurons lie within a complex hypothalamic circuitry that engages distinct hindbrain regions and is innervated by discrete upstream hypothalamic sites. Overall, we reveal a requisite role for IRS4^PVH neurons in PVH-mediated energy balance which raises the possibility of developing novel approaches targeting IRS4^PVH neurons for anti-obesity therapies.

A Targeted Mutation Disrupting Mitochondrial Complex IV Function in Primary Afferent Neurons Leads to Pain Hypersensitivity Through P2Y1 Receptor Activation

As mitochondrial dysfunction is evident in neurodegenerative disorders that are accompanied by pain, we generated inducible mutant mice with disruption of mitochondrial respiratory chain complex IV, by COX10 deletion limited to sensory afferent neurons through the use of an Advillin Cre-reporter. COX10 deletion results in a selective energy-deficiency phenotype with minimal production of reactive oxygen species. Mutant mice showed reduced activity of mitochondrial respiratory chain complex IV in many sensory neurons, increased ADP/ATP ratios in dorsal root ganglia and dorsal spinal cord synaptoneurosomes, as well as impaired mitochondrial membrane potential, in these synaptoneurosome preparations. These changes were accompanied by marked pain hypersensitivity in mechanical and thermal (hot and cold) tests without altered motor function. To address the underlying basis, we measured Ca²⁺ fluorescence responses of dorsal spinal cord synaptoneurosomes to activation of the GluK1 (kainate) receptor, which we showed to be widely expressed in small but not large nociceptive afferents, and is minimally expressed elsewhere in the spinal cord. Synaptoneurosomes from mutant mice showed greatly increased responses to GluK1 agonist. To explore whether altered nucleotide levels may play a part in this hypersensitivity, we pharmacologically interrogated potential roles of AMP-kinase and ADP-sensitive purinergic receptors. The ADP-sensitive P2Y₁ receptor was clearly implicated. Its expression in small nociceptive afferents was increased in mutants, whose in vivo pain hypersensitivity, in mechanical, thermal and cold tests, was reversed by a selective P2Y₁ antagonist. Energy depletion and ADP elevation in sensory afferents, due to mitochondrial respiratory chain complex IV deficiency, appear sufficient to induce pain hypersensitivity, by ADP activation of P2Y₁ receptors.

Pleomorphic linkers as ubiquitous structural organizers of vesicles in axons

Many cellular processes depend on a precise structural organization of molecular components. Here, we established that neurons grown in culture provide a suitable system for in situ structural investigations of cellular structures by cryo-electron tomography, a method that allows high resolution, three-dimensional imaging of fully hydrated, vitrified cellular samples. A higher level of detail of cellular components present in our images allowed us to quantitatively characterize presynaptic and cytoskeletal organization, as well as structures involved in axonal transport and endocytosis. In this way we provide a structural framework into which information from other methods need to fit. Importantly, we show that short pleomorphic linkers (tethers and connectors) extensively interconnect different types of spherical vesicles and other lipid membranes in neurons imaged in a close-to-native state. These linkers likely serve to organize and precisely position vesicles involved in endocytosis, axonal transport and synaptic release. Hence, structural interactions via short linkers may serve as ubiquitous vesicle organizers in neuronal cells.

A New Behavioral Test and Associated Genetic Tools Highlight the Function of Ventral Abdominal Muscles in Adult Drosophila

The function of the nervous system in complex animals is reflected by the achievement of specific behaviors. For years in Drosophila, both simple and complex behaviors have been studied and their genetic bases have emerged. The neuromuscular junction is maybe one of the prototypal simplest examples. A motor neuron establishes synaptic connections on its muscle cell target and elicits behavior: the muscle contraction. Different muscles in adult fly are related to specific behaviors. For example, the thoracic muscles are associated with flight and the leg muscles are associated with locomotion. However, specific tools are still lacking for the study of cellular physiology in distinct motor neuron subpopulations. Here we decided to use the abdominal muscles and in particular the ventral abdominal muscles (VAMs) in adult Drosophila as new model to link a precise behavior to specific motor neurons. Hence, we developed a new behavioral test based on the folding movement of the adult abdomen. Further, we performed a genetic screen and identify two specific Gal4 lines with restricted expression patterns to the adult motor neurons innervating the VAMs or their precursor cells. Using these genetic tools, we showed that the lack of the VAMs or the loss of the synaptic transmission in their innervating motor neurons lead to a significant impairment of the abdomen folding behavior. Altogether, our results allow establishing a direct link between specific motor neurons and muscles for the realization of particular behavior: the folding behavior of the abdomen in Drosophila.

Synaptic input as a directional cue for migrating interneuron precursors

During CNS development, interneuron precursors have to migrate extensively before they integrate in specific microcircuits. Known regulators of neuronal motility include classical neurotransmitters, yet the mechanisms that assure interneuron dispersal and interneuron/projection neuron matching during histogenesis remain largely elusive. We combined time-lapse video microscopy and electrophysiological analysis of the nascent cerebellum of transgenic Pax2-EGFP mice to address this issue. We found that cerebellar interneuronal precursors regularly show spontaneous postsynaptic currents, indicative of synaptic innervation, well before settling in the molecular layer. In keeping with the sensitivity of these cells to neurotransmitters, ablation of synaptic communication by blocking vesicular release in acute slices of developing cerebella slows migration. Significantly, abrogation of exocytosis primarily impedes the directional persistence of migratory interneuronal precursors. These results establish an unprecedented function of the early synaptic innervation of migrating neuronal precursors and demonstrate a role for synapses in the regulation of migration and pathfinding.

The role of spontaneous neurotransmission in synapse and circuit development

In the past, the spontaneous release of neurotransmitter from presynaptic terminals has been thought of as a side effect of evoked release, with little functional significance. As our understanding of the process of spontaneous release has increased over time, this notion has gradually changed. In this review, we focus on the importance of this form of release during neuronal development, a time of extreme levels of plasticity that includes the growth of dendrites and axons as well as the formation of new synaptic contacts. This period also encompasses high levels of neurotransmitter release from growing axons, and recent studies have found that spontaneous transmitter release plays an important role in shaping neuronal morphology as well as modulating the properties of newly forming synaptic contacts in the brain. Here, we bring together the latest findings across different species to argue that the spontaneous release of neurotransmitter is an important player in the wiring of the brain during development.

Calcium-sensing-receptor (CaSR) controls IL-6 secretion in metastatic breast cancer MDA-MB-231 cells by a dual mechanism revealed by agonist and inverse-agonist modulators

IL-6 is a tightly controlled pleiotropic cytokine with hormone-like properties whose levels are frequently altered in cancer and inflammatory diseases. In highly invasive MDA-MB-231 breast cancer cells, basal activity of endogenously expressed calcium sensing receptor (CaSR) promotes IL-6 secretion. Interestingly, upon agonist stimulation, CaSR reduces IL-6 levels whereas it promotes secretion of various other cytokines and growth factors, raising intriguing questions about how CaSR signaling modulates IL-6 secretion. Here, using NPS-2143, which acted as an inverse agonist, we show that IL-6 secretion promoted by constitutive activity of CaSR is mechanistically linked to Gαs/PKC, MEK1/2 and mTORC1 signaling pathways, integrated by transactivated EGFR. On the other hand, agonist-stimulated CaSR engages in a Rab11a-dependent trafficking pathway critical to inhibit constitutive IL-6 secretion via the PI3K/AKT and PKC signaling pathways. These results support the emerging potential of CaSR as a therapeutic target in metastatic breast cancer whose pharmacological modulation would reduce IL-6.

Beyond the cytoskeleton: The emerging role of organelles and membrane remodeling in the regulation of axon collateral branches

The generation of axon collateral branches is a fundamental aspect of the development of the nervous system and the response of axons to injury. Although much has been discovered about the signaling pathways and cytoskeletal dynamics underlying branching, additional aspects of the cell biology of axon branching have received less attention. This review summarizes recent advances in our understanding of key factors involved in axon branching. This article focuses on how cytoskeletal mechanisms, intracellular organelles, such as mitochondria and the endoplasmic reticulum, and membrane remodeling (exocytosis and endocytosis) contribute to branch initiation and formation. Together this growing literature provides valuable insight as well as a platform for continued investigation into how multiple aspects of axonal cell biology are spatially and temporally orchestrated to give rise to axon branches. © 2016 Wiley Periodicals, Inc. Develop Neurobiol 76: 1293-1307, 2016.

LRRK2 phosphorylates pre-synaptic N-ethylmaleimide sensitive fusion (NSF) protein enhancing its ATPase activity and SNARE complex disassembling rate

Background

Lrrk2, a gene linked to Parkinson’s disease, encodes a large scaffolding protein with kinase and GTPase activities implicated in vesicle and cytoskeletal-related processes. At the presynaptic site, LRRK2 associates with synaptic vesicles through interaction with a panel of presynaptic proteins.

Results

Here, we show that LRRK2 kinase activity influences the dynamics of synaptic vesicle fusion. We therefore investigated whether LRRK2 phosphorylates component(s) of the exo/endocytosis machinery. We have previously observed that LRRK2 interacts with NSF, a hexameric AAA+ ATPase that couples ATP hydrolysis to the disassembling of SNARE proteins allowing them to enter another fusion cycle during synaptic exocytosis. Here, we demonstrate that NSF is a substrate of LRRK2 kinase activity. LRRK2 phosphorylates full-length NSF at threonine 645 in the ATP binding pocket of D2 domain. Functionally, NSF phosphorylated by LRRK2 displays enhanced ATPase activity and increased rate of SNARE complex disassembling. Substitution of threonine 645 with alanine abrogates LRRK2-mediated increased ATPase activity.

Conclusions

Given that the most common Parkinson’s disease LRRK2 G2019S mutation displays increased kinase activity, our results suggest that mutant LRRK2 may impair synaptic vesicle dynamics via aberrant phosphorylation of NSF.

Electronic supplementary material

The online version of this article (doi:10.1186/s13024-015-0066-z) contains supplementary material, which is available to authorized users.

Selected SNARE proteins are essential for the polarized membrane insertion of igf-1 receptor and the regulation of initial axonal outgrowth in neurons

The establishment of polarity necessitates initial axonal outgrowth and, therefore, the addition of new membrane to the axon’s plasmalemma. Axolemmal expansion occurs by exocytosis of plasmalemmal precursor vesicles (PPVs) primarily at the neuronal growth cone. Little is known about the SNAREs family proteins involved in the regulation of PPV fusion with the neuronal plasmalemma at early stages of differentiation. We show here that five SNARE proteins (VAMP2, VAMP4, VAMP7, Syntaxin6 and SNAP23) were expressed by hippocampal pyramidal neurons before polarization. Expression silencing of three of these proteins (VAMP4, Syntaxin6 and SNAP23) repressed axonal outgrowth and the establishment of neuronal polarity, by inhibiting IGF-1 receptor exocytotic polarized insertion, necessary for neuronal polarization. In addition, stimulation with IGF-1 triggered the association of VAMP4, Syntaxin6 and SNAP23 to vesicular structures carrying the IGF-1 receptor and overexpression of a negative dominant form of Syntaxin6 significantly inhibited exocytosis of IGF-1 receptor containing vesicles at the neuronal growth cone. Taken together, our results indicated that VAMP4, Syntaxin6 and SNAP23 functions are essential for regulation of PPV exocytosis and the polarized insertion of IGF-1 receptor and, therefore, required for initial axonal elongation and the establishment of neuronal polarity.

Building a Terminal: Mechanisms of Presynaptic Development in the CNS

To create a presynaptic terminal, molecular signaling events must be orchestrated across a number of subcellular compartments. In the soma, presynaptic proteins need to be synthesized, packaged together, and attached to microtubule motors for shipment through the axon. Within the axon, transport of presynaptic packages is regulated to ensure that developing synapses receive an adequate supply of components. At individual axonal sites, extracellular interactions must be translated into intracellular signals that can incorporate mobile transport vesicles into the nascent presynaptic terminal. Even once the initial recruitment process is complete, the components and subsequent functionality of presynaptic terminals need to constantly be remodeled. Perhaps most remarkably, all of these processes need to be coordinated in space and time. In this review, we discuss how these dynamic cellular processes occur in neurons of the central nervous system in order to generate presynaptic terminals in the brain.

Spontaneous Neurotransmitter Release Shapes Dendritic Arbors via Long-Range Activation of NMDA Receptors

Spontaneous neurotransmitter release is a core element of synaptic communication in mature neurons, but despite exceptionally high levels of spontaneous vesicle cycling occurring in developing axons, little is known of its function during this period. We now show that high-level, spontaneous axonal release of the neurotransmitter glutamate can signal at long range to NMDA receptors on developing dendrites, prior to synapse formation and, indeed, axodendritic contact. Blockade of NMDA signaling during this early period of spontaneous vesicle cycling leads to a reduction in dendritic arbor complexity, indicating an important role for early spontaneous release in dendritic arbor growth.

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New role for spontaneous neurotransmitter release in dendritic arbor formation

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Vesicular glutamate can activate distant “long-range” dendritic NMDA receptors

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Presynaptic and postsynaptic machinery operate before synaptic contact

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Spontaneous glutamate release may provide local dendritic guidance or branching cue

Structural and functional substrates of tetanus toxin in an animal model of temporal lobe epilepsy

The effects of tetanus toxin (TeNT) both in the spinal cord, in clinical tetanus, and in the brain, in experimental focal epilepsy, suggest disruption of inhibitory synapses. TeNT is a zinc protease with selectivity for Vesicle Associated Membrane Protein (VAMP; previously synaptobrevin), with a reported selectivity for VAMP2 in rats. We found spatially heterogeneous expression of VAMP1 and VAMP2 in the hippocampus. Inhibitory terminals in stratum pyramidale expressed significantly more VAMP1 than VAMP2, while glutamatergic terminals in stratum radiatum expressed significantly more VAMP2 than VAMP1. Intrahippocampal injection of TeNT at doses that induce epileptic foci cleaved both isoforms in tissue around the injection site. The cleavage was modest at 2 days after injection and more substantial and extensive at 8 and 16 days. Whole-cell recordings from CA1 pyramidal cells close to the injection site, made 8-16 days after injection, showed that TeNT decreases spontaneous EPSC frequency to 38 % of control and VAMP2 immunoreactive axon terminals to 37 %. In contrast, TeNT almost completely abolished both spontaneous and evoked IPSCs while decreasing VAMP1 axon terminals to 45 %. We conclude that due to the functional selectivity of the toxin to the relative sparing of excitatory synaptic transmission shifts the network to pathogenically excitable state causing epilepsy.

Intracellular distribution of amyloid beta peptide and its relationship to the lysosomal system

Background

Amyloid beta peptide (Aβ) is the main component of extraneuronal senile plaques typical of Alzheimer’s disease (AD) brains. Although Aβ is produced by normal neurons, it is shown to accumulate in large amounts within neuronal lysosomes in AD. We have recently shown that under normal conditions the majority of Aβ is localized extralysosomally, while oxidative stress significantly increases intralysosomal Aβ content through activation of macroautophagy. It is also suggested that impaired Aβ secretion and resulting intraneuronal increase of Aβ can contribute to AD pathology. However, it is not clear how Aβ is distributed inside normal neurons, and how this distribution is effected when Aβ secretion is inhibited.

Methods

Using retinoic acid differentiated neuroblastoma cells and neonatal rat cortical neurons, we studied intracellular distribution of Aβ by double immunofluorescence microscopy for Aβ₄₀ or Aβ₄₂ and different organelle markers. In addition, we analysed the effect of tetanus toxin-induced exocytosis inhibition on the intracellular distribution of Aβ.

Results

Under normal conditions, Aβ was found in the small cytoplasmic granules in both neurites and perikarya. Only minor portion of Aβ was colocalized with trans-Golgi network, Golgi-derived vesicles, early and late endosomes, lysosomes, and synaptic vesicles, while the majority of Aβ granules were not colocalized with any of these structures. Furthermore, treatment of cells with tetanus toxin significantly increased the amount of intracellular Aβ in both perikarya and neurites. Finally, we found that tetanus toxin increased the levels of intralysosomal Aβ although the majority of Aβ still remained extralysosomally.

Conclusion

Our results indicate that most Aβ is not localized to Golgi-related structures, endosomes, lysosomes secretory vesicles or other organelles, while the suppression of Aβ secretion increases intracellular intra- and extralysosomal Aβ.

Acute knockdown of AMPA receptors reveals a trans-synaptic signal for presynaptic maturation

Newly formed glutamatergic synapses often lack postsynaptic AMPA-type glutamate receptors (AMPARs). Aside from 'unsilencing' the postsynaptic site, however, the significance of postsynaptic AMPAR insertion during synapse maturation remains unclear. To investigate the role of AMPAR in synapse maturation, we used RNA interference (RNAi) to knockdown AMPARs in cultured hippocampal neurons. Surprisingly, loss of postsynaptic AMPARs increased the occurrence of presynaptically inactive synapses without changing the release probability of the remaining active synapses. Additionally, heterologous synapses formed between axons and AMPAR-expressing HEK cells develop significantly fewer inactive presynaptic terminals. The extracellular domain of the AMPAR subunit GluA2 was sufficient to reproduce this effect at heterologous synapses. Indeed, the retrograde signalling by AMPARs is independent of their channel function as RNAi-resistant AMPARs restore synaptic transmission in neurons lacking AMPARs despite chronic receptor antagonist treatment. Our findings suggest that postsynaptic AMPARs perform an organizational function at synapses that exceeds their standard role as ionotropic receptors by conveying a retrograde trans-synaptic signal that increases the transmission efficacy at a synapse.

Integrin signaling switches the cytoskeletal and exocytic machinery that drives neuritogenesis

Neurons establish their unique morphology by elaborating multiple neurites that subsequently form axons and dendrites. Neurite initiation entails significant surface area expansion, necessitating addition to the plasma membrane. We report that regulated membrane delivery coordinated with the actin cytoskeleton is crucial for neuritogenesis and identify two independent pathways that use distinct exocytic and cytoskeletal machinery to drive neuritogenesis. One pathway uses Ena/VASP-regulated actin dynamics coordinated with VAMP2-mediated exocytosis and involves a novel role for Ena/VASP in exocytosis. A second mechanism occurs in the presence of laminin through integrin-dependent activation of FAK and src and uses coordinated activity of the Arp2/3 complex and VAMP7-mediated exocytosis. We conclude that neuritogenesis can be driven by two distinct pathways that differentially coordinate cytoskeletal dynamics and exocytosis. These regulated changes and coordination of cytoskeletal and exocytic machinery may be used in other physiological contexts involving cell motility and morphogenesis.

Molecular mechanisms of presynaptic differentiation

Information processing in the nervous system relies on properly localized and organized synaptic structures at the correct locations. The formation of synapses is a long and intricate process involving multiple interrelated steps. Decades of research have identified a large number of molecular components of the presynaptic compartment. In addition to neurotransmitter-containing synaptic vesicles, presynaptic terminals are defined by cytoskeletal and membrane specializations that allow highly regulated exo- and endocytosis of synaptic vesicles and that maintain precise registration with postsynaptic targets. Functional studies at multiple levels have revealed complex interactions between the transport of vesicular intermediates, the presynaptic cytoskeleton, growth cone navigation, and synaptic targets. With the advent of finer anatomical, physiological, and molecular tools, great insights have been gained toward the mechanistic dissection of functionally redundant processes controlling the specificity and dynamics of synapses. This review highlights the recent findings pertaining to the cellular and molecular regulation of presynaptic differentiation.

Calpain activity contributes to the control of SNAP-25 levels in neurons

Calpains are a family of calcium-dependent proteases with abundant expression in the CNS, and potent in cleaving some synaptic components. Assessment of calpain activity by its fluorescent substrate, Boc-Leu-Met-CMAC, revealed that cultured neurons display a significant level of constitutive enzyme activity. Notably, calpain activity differs in distinct neuronal populations, with a significantly higher level of activity in GABAergic cells. Using selectively-enriched cultures of fast-spiking GABAergic interneurons, we show that calpain activity partially contributes to the post-translational down regulation of SNAP-25, a calpain substrate, in differentiated GABA cells. In addition, we demonstrate that SNAP-25 is cleaved by calpain in response to acute seizures induced by intraperitoneal kainate injection in vivo. These data indicate that calpains in neurons are active even at physiological calcium concentrations and that different levels of calpain activation in selected neuron subtypes may contribute to the pattern of synaptic protein expression.

Selective regulation of the perinuclear distribution of glucose transporter 4 (GLUT4) by insulin signals in muscle cells

Insulin regulates glucose transporter 4 (GLUT4) availability at the surface of muscle and adipose cells. In L6 myoblasts, stably expressed GLUT4myc is detected mostly in a perinuclear region. In unstimulated cells, about half of perinuclear GLUT4myc colocalizes with the transferrin receptor (TfR). Insulin stimulation selectively decreased the perinuclear colocalization of GLUT4myc with TfR determined by 3D-reconstruction of fluorescence images. Perinuclear GLUT4myc adopted two main distributions defined morphometrically as 'conical' and 'concentric'. Insulin rapidly reduced the proportion of cells with conical in favor of concentric perinuclear GLUT4myc distributions in association with the gain in surface GLUT4myc. Upon removal of insulin, the GLUT4myc perinuclear distribution and surface levels reversed in parallel. In contrast, hypertonicity (which like insulin elevates surface GLUT4myc) did not elicit perinuclear GLUT4myc redistribution. Insulin also caused redistribution of perinuclear vesicle-associated membrane protein-2 (VAMP2), without alteration of perinuclear TfR and VAMP3. Inhibitory mutants of phosphatidylinositol-3 kinase (Deltap85) or Akt substrate AS160 (AS160-4P) prevented insulin-mediated perinuclear GLUT4myc redistribution. Tetanus toxin expression did not prevent the perinuclear GLUT4myc redistribution, suggesting that redistribution is independent of GLUT4myc fusion with the plasma membrane. We propose that insulin causes selective, dynamic relocalization of perinuclear GLUT4myc and VAMP2 and perinuclear GLUT4myc redistribution is a direct target of insulin-derived signals.

Synaptic vesicle protein trafficking at the glutamate synapse

Expression of the integral and associated proteins of synaptic vesicles is subject to regulation over time, by region, and in response to activity. The process by which changes in protein levels and isoforms result in different properties of neurotransmitter release involves protein trafficking to the synaptic vesicle. How newly synthesized proteins are incorporated into synaptic vesicles at the presynaptic bouton is poorly understood. During synaptogenesis, synaptic vesicle proteins sort through the secretory pathway and are transported down the axon in precursor vesicles that undergo maturation to form synaptic vesicles. Changes in protein content of synaptic vesicles could involve the formation of new vesicles that either mix with the previous complement of vesicles or replace them, presumably by their degradation or inactivation. Alternatively, new proteins could individually incorporate into existing synaptic vesicles, changing their functional properties. Glutamatergic vesicles likely express many of the same integral membrane proteins and share certain common mechanisms of biogenesis, recycling, and degradation with other synaptic vesicles. However, glutamatergic vesicles are defined by their ability to package glutamate for release, a property conferred by the expression of a vesicular glutamate transporter (VGLUT). VGLUTs are subject to regional, developmental, and activity-dependent changes in expression. In addition, VGLUT isoforms differ in their trafficking, which may target them to different pathways during biogenesis or after recycling, which may in turn sort them to different vesicle pools. Emerging data indicate that differences in the association of VGLUTs and other synaptic vesicle proteins with endocytic adaptors may influence their trafficking. These observations indicate that independent regulation of synaptic vesicle protein trafficking has the potential to influence synaptic vesicle protein composition, the maintenance of synaptic vesicle pools, and the release of glutamate in response to changing physiological requirements.

Spontaneous glutamate release controls NT-3-dependent development of hippocampal calbindin-D28k phenotype through activation of sodium channels ex vivo

Functional NMDA and AMPA ionotropic glutamate receptors are expressed in embryonic hippocampal glutamatergic pyramidal neurons prior to synapse formation but their function and mechanisms of action are still unclear. At the same time, these neurons develop their calbindin-D(28k) phenotype through an activity-dependent NT-3 autocrine loop. Using single-neuron microcultures, we show here that immature pyramidal neurons spontaneously secreted glutamate and that chronic blockade of either NMDA or AMPA receptors down-regulated the number of calbindin-D(28k)-positive pyramidal neurons without affecting neuronal survival. This antagonistic effect of glutamate ionotropic receptors was mimicked by anti-TrkC antibodies and reversed by the application of NT-3. Similar results were obtained in ex vivo embryonic hippocampal slice cultures. Moreover, glutamate receptor blockade inhibited the generation of spontaneous sodium-driven action potentials which, in turn, regulate both the endogenous secretion of NT-3 and the calbindin-D(28k) phenotype acquisition. Altogether, these results suggest an unexpected role for glutamate in the development of the physiological and biochemical properties of hippocampal pyramidal neurons and support the idea that glutamate may underlie an activity-dependent mode of differentiation prior to synapse formation.

Share and share alike: trading of presynaptic elements between central synapses

Central presynaptic terminals harbour synaptic vesicles (SVs) and synapse-specific proteins necessary for neurotransmission. Classically, these elements were thought to reside more or less stably at individual mature synapses, giving rise to the idea that each terminal was essentially an independent functional unit. However, emerging evidence from fluorescence imaging studies in hippocampal cultured neurons is now challenging this view, suggesting that neighbouring synapses along axons share vesicles, and also other synaptic elements, at high levels. This raises the possibility that control of import and export might be an important regulatory target for the maintenance of release sites, modulation of synaptic efficacy and formation of new synaptic contacts. Here, temporal synaptic stability and the functional consequences for presynaptic operation will be considered.

Dynamic aspects of CNS synapse formation

The mammalian central nervous system (CNS) requires the proper formation of exquisitely precise circuits to function correctly. These neuronal circuits are assembled during development by the formation of synaptic connections between thousands of differentiating neurons. Proper synapse formation during childhood provides the substrate for cognition, whereas improper formation or function of these synapses leads to neurodevelopmental disorders, including mental retardation and autism. Recent work has begun to identify some of the early cellular events in synapse formation as well as the molecular signals that initiate this process. However, despite the wealth of information published on this topic in the past few years, some of the most fundamental questions about how, whether, and where glutamatergic synapses form in the mammalian CNS remain unanswered. This review focuses on the dynamic aspects of the early cellular and molecular events in the initial assembly of glutamatergic synapses in the mammalian CNS.

A null mutation in VAMP1/synaptobrevin is associated with neurological defects and prewean mortality in the lethal-wasting mouse mutant

The soluble N-ethylmaleimide sensitive factor attachment receptors are a large family of membrane-associated proteins that are critical for Ca(2+)-mediated synaptic vesicle release. This family includes the VAMP, synaptosomal-associated protein, and syntaxin proteins. In this report, we describe a mutation in vesicle-associated membrane protein 1(VAMP1)/synaptobrevin in the mouse neurological mutant lethal-wasting (lew). The lethal-wasting mutant phenotype is characterized by a general lack of movement and wasting, eventually leading to death before weaning. Mutants are visibly immobile and lay on their side by postnatal day 10 (P10). Before this stage, mutants can be identified by a failure to attempt to right themselves. Affected mice die on average at P15. We used a positional cloning strategy to identify the mutation associated with this neurological phenotype. Lethal wasting had previously been linked to chromosome 6. We further narrowed the genetic disease interval and selected a small number of candidate genes for mutation screening. Genes were evaluated by quantitative reverse transcription-polymerase chain reaction (RT-PCR) to detect differences in their expression levels between control and mutant brain ribonucleic acid (RNA) samples. VAMP1 mRNA was found to be significantly downregulated in the lethal-wasting brain compared to wild-type littermates. Subsequently, a nonsense mutation was identified in the coding region of the gene. This mutation is predicted to truncate approximately half of the protein; however, Western blot analysis showed that no protein is detectable in the mutant. VAMP1 is selectively expressed in the retina and in discrete areas of the brain including the zona incerta and rostral periolivary region, although no gross histological abnormalities were observed in these tissues. Taken together, these data indicate that VAMP1 has a vital role in a subset of central nervous system tissues.

The orphan receptor GPR17 identified as a new dual uracil nucleotides/cysteinyl-leukotrienes receptor

Nucleotides and cysteinyl-leukotrienes (CysLTs) are unrelated signaling molecules inducing multiple effects through separate G-protein-coupled receptors: the P2Y and the CysLT receptors. Here we show that GPR17, a Gi-coupled orphan receptor at intermediate phylogenetic position between P2Y and CysLT receptors, is specifically activated by both families of endogenous ligands, leading to both adenylyl cyclase inhibition and intracellular calcium increases. Agonist-response profile, as determined by [(35)S]GTPgammaS binding, was different from that of already known CysLT and P2Y receptors, with EC(50) values in the nanomolar and micromolar range, for CysLTs and uracil nucleotides, respectively. Both rat and human receptors are highly expressed in the organs typically undergoing ischemic damage, that is, brain, heart and kidney. In vivo inhibition of GPR17 by either CysLT/P2Y receptor antagonists or antisense technology dramatically reduced ischemic damage in a rat focal ischemia model, suggesting GPR17 as the common molecular target mediating brain damage by nucleotides and CysLTs. In conclusion, the deorphanization of GPR17 revealed a dualistic receptor for two endogenous unrelated ligand families. These findings may lead to dualistic drugs of previously unexplored therapeutic potential.

GABAergic miniature postsynaptic currents in septal neurons show differential allosteric sensitivity after binge-like ethanol exposure

Binge-like ethanol treatment of septal neurons blunts GABAAR-mediated miniature postsynaptic currents (mPSCs), suggesting it arrests synaptic development. Ethanol may disrupt postsynaptic maturation by blunting feedback signaling through immature GABAARs. Here, the impact of ethanol on the sensitivity of mPSCs to zolpidem, zinc and 3alpha-hydroxy-5alpha-pregnan-20-one (3alpha-OH-DHP) was tested. The decay phase of mPSCs showed concentration-dependent potentiation by zolpidem (0.03-100 microM), which was substantially blunted after ethanol exposure. Since zolpidem potentiation exhibited a substantial age-dependent increase in untreated neurons, this finding supported the idea that ethanol arrests synaptic development. GABAAR alpha1 subunit protein also increased with age in untreated neurons, paralleling enhanced sensitivity to zolpidem. Surprisingly, alpha1 levels were not reduced by binge ethanol even though mPSCs were relatively zolpidem-insensitive. Zinc (3-30 microM) decreased mPSC parameters in a concentration- and age-related manner with older untreated cells showing less inhibition. However, there was no increase in mPSC zinc sensitivity after binge ethanol as would be expected if a general arrest of synaptic maturation had occurred. 3alpha-OH-DHP (3-1000 nM) induced concentration-dependent potentiation of mPSC decay. Although potentiation was age-independent, binge ethanol treatment exaggerated sensitivity to this neurosteroid. Finally, chronic picrotoxin pretreatment (100 microM) intended to mimic GABAAR inhibition from ethanol pretreatment did not significantly change mPSC modulation by zolpidem, zinc or 3alpha-OH-DHP. These results suggest that binge ethanol treatment selectively arrests a subset of processes important for maturation of postsynaptic GABAA Rs. However, it is unlikely that ethanol causes a broad arrest of postsynaptic development through a direct inhibition of GABAAR signaling.

Synaptic Activity Regulates Interstitial Fluid Amyloid-β Levels In Vivo

Aggregation of the amyloid-beta (Abeta) peptide in the extracellular space of the brain is central to Alzheimer's disease pathogenesis. Abeta aggregation is concentration dependent and brain region specific. Utilizing in vivo microdialysis concurrently with field potential recordings, we demonstrate that Abeta levels in the brain interstitial fluid are dynamically and directly influenced by synaptic activity on a timescale of minutes to hours. Using an acute brain slice model, we show that the rapid effects of synaptic activity on Abeta levels are primarily related to synaptic vesicle exocytosis. These results suggest that synaptic activity may modulate a neurodegenerative disease process, in this case by influencing Abeta metabolism and ultimately region-specific Abeta deposition. The findings also have important implications for treatment development.

Analysis of SNAP-25 immunoreactivity in hippocampal inhibitory neurons during development in culture and in situ

Synaptosomal associated protein of 25 kDa (SNAP-25) is a component of the soluble N-ethylmaleimide-sensitive fusion protein (NSF) attachment protein receptor (SNARE) complex which plays a central role in synaptic vesicle exocytosis. We have previously demonstrated that adult rat hippocampal GABAergic synapses, both in culture and in brain, are virtually devoid of SNAP-25 immunoreactivity and are less sensitive to the action of botulinum toxin type A, which cleaves this SNARE protein [Neuron 41 (2004) 599]. In the present study, we extend our findings to the adult mouse hippocampus and we also provide demonstration that hippocampal inhibitory synapses lacking SNAP-25 labeling belong to parvalbumin-, calretinin- and cholecystokinin-positive interneurons. A partial colocalization between SNAP-25 and glutamic acid decarboxylase is instead detectable in developing mouse hippocampus at P0 and, at a lesser extent, at P5. In rat embryonic hippocampal cultures at early developmental stages, SNAP-25 immunoreactivity is detectable in a percentage of GABAergic neurons, which progressively reduces with time in culture. Consistent with the presence of the substrate, botulinum toxin type A is partially effective in inhibiting synaptic vesicle recycling in immature GABAergic neurons. Since SNAP-25, beside its role as a SNARE protein, is involved in additional processes, such as neurite outgrowth and regulation of calcium dynamics, the presence of higher levels of the protein at specific stages of neuronal differentiation may have implications for the construction and for the functional properties of brain circuits.

Binge ethanol exposure delays development of GABAergic miniature postsynaptic currents in septal neurons

Whole cell GABA(A)R currents of septal neurons isolated from rat pups increase rapidly during the first weeks of life when inhibitory synapses are forming. Early postnatal binge ethanol intubation on days 4-9 delays this maturational up-regulation in septal neurons isolated several days later suggesting inhibitory synapse formation could be disrupted [S.-H. Hsiao, J.L. Acevedo, D.W. DuBois, K.R. Smith, J.R. West, G.D. Frye, Early postnatal ethanol intubation blunts GABA(A) receptor upregulation and modifies 3alpha-hydroxy-5alpha-pregnan-20-one sensitivity in rat MS/DB neurons, Brain Res. Dev. Brain Res. 130 (2001) 25-40]. Surprisingly, whole cell GABA(A)R function does not increase rapidly when septal neurons are grown for the same period in vitro and is not blunted by comparable ethanol exposure of the cultures [S.-H. Hsiao, D.W. DuBois, R.C. Miranda, G.D. Frye, Critically timed ethanol exposure reduces GABA(A)R function on septal neurons developing in vivo but not in vitro, Brain Res Dev. Brain Res. 1008 (2004) 69-80]. Because GABAergic miniature postsynaptic currents (mPSCs) show parallel patterns of maturation whether cortical neurons are growing in vivo or in vitro [D.D. Dunning, C.L. Hoover, I. Soltesz, M.A. Smith, D.K. ODowd, GABA(A) receptor-mediated miniature postsynaptic currents and alpha-subunit expression in developing cortical neurons, J. Neurophysiol. 82 (1999) 3286-3297], we examined the impact of binge ethanol exposure on synaptic receptors activated by these currents in septal cultures. Binge ethanol treatment of embryonic septal neurons over 6-11 days in vitro (DIV) slightly reduced GABA(A)R-mediated mPSC amplitude and frequency, but also substantially slowed decay kinetics when mPSCs were recorded later on DIV 13-18. Decreased frequency and slowed mPSC decay kinetics after ethanol were consistent with parameters measured in immature neurons. Untreated septal neurons exhibited decreased mPSC amplitude and frequency with acute 30-100 mM ethanol, without changing decay kinetics suggesting a direct inhibition of postsynaptic receptors. Sustained inhibition of GABA(A)Rs with 100 microM picrotoxin on DIV 6-11 decreased mPSC amplitude and frequency and slowed decay kinetics similar to binge ethanol exposure. These results suggest that binge ethanol exposure delays mPSC maturation by interfering with trophic postsynaptic GABA(A)R signaling during the early development of septal neurons.

Snake presynaptic neurotoxins with phospholipase A2 activity induce punctate swellings of neurites and exocytosis of synaptic vesicles

The mechanisms of action of four snake presynaptic phospholipase A2 neurotoxins were investigated in cultured neurons isolated from various parts of the rat brain. Strikingly, physiological concentrations of notexin, beta-bungarotoxin, taipoxin or textilotoxin induced a dose-dependent formation of discrete bulges at various sites of neuronal projections. Neuronal bulging was paralleled by the redistribution of the two synaptic vesicle markers synaptophysin I (SypI) and vesicle-attached membrane protein 2 (VAMP2) to the bulges, and by the exposure of the luminal domain of synaptotagmin on the cell surface. These neurotoxins induced glutamate release from cultured neurons similarly to the known evoked release of acetylcholine from neuromuscular junctions. In addition, partial fragmentation of F-actin and neurofilaments was observed in neurons, but not in astrocytes. These findings indicate that these snake presynaptic neurotoxins act with by same mechanism and that the observed phenotype results from the fusion of synaptic vesicles with the plasma membrane not balanced by an adequate membrane retrieval. These changes closely resemble those occurring at neuromuscular junctions of intoxicated animals and fully qualify these primary neuronal cultures as pertinent models for studying the molecular mode of action of these neurotoxins.

Synaptic structural modification following changes in activity induced by tetanus neurotoxin in cat abducens neurons

A low or a high dose of tetanus neurotoxin (TeNT) injected in the lateral rectus muscle of the cat causes respectively, functional block of inhibitory synapses only or of both inhibitory and excitatory synapses simultaneously in abducens neurons (González-Forero et al. [2003] J. Neurophysiol. 89:1878-1890). As a consequence, neuronal firing activity increases (at low dose) or decreases (at high dose). We investigated possible structural modifications of inhibitory synapses in response to these activity alterations induced by TeNT. We used immunofluorescence against postsynaptic (gephyrin) and presynaptic (vesicular gamma-aminobutyric acid [GABA] transporter [VGAT]) markers of inhibitory synapses in combination with cell type markers for abducens motoneurons (calcitonin gene-related peptide or choline acetyltransferase) or internuclear neurons (calretinin). Seven days after high-dose treatment, the number of gephyrin-immunoreactive (IR) clusters per 100 microm of membrane perimeter was reduced on the soma of abducens motoneurons by 55.3% and by 60.1% on internuclear neurons. Proximal dendritic gephyrin-IR clusters were also significantly altered but to a lesser degree. Partial synaptic re-establishment was observed 15 days post injection, and complete recovery occurred after 42 days. Coverage by VGAT-IR terminals was reduced in parallel. In contrast, a low dose of TeNT caused no structural alterations. With electron microscopy we estimated that overall synaptic coverage was reduced by 40% in both types of neurons after a high dose of TeNT. However, F-type terminals with postsynaptic gephyrin were preferentially lost. Thus, the ratio between F and S terminals diminished from 1.28 to 0.39 on motoneurons and from 1.26 to 0.47 on internuclear neurons. These results suggest that the maintenance of proximal inhibitory synaptic organization on central neurons is best related to neuronal activity and not to the level of inhibitory synaptic function, which was equally blocked by the high or low dose of TeNT.

SNAP-25 modulation of calcium dynamics underlies differences in GABAergic and glutamatergic responsiveness to depolarization

SNAP-25 is a component of the SNARE complex implicated in synaptic vesicle exocytosis. In this study, we demonstrate that hippocampal GABAergic synapses, both in culture and in brain, lack SNAP-25 and are resistant to the action of botulinum toxins type A and E, which cleave this SNARE protein. Relative to glutamatergic neurons, which express SNAP-25, GABAergic cells were characterized by a higher calcium responsiveness to depolarization. Exogenous expression of SNAP-25 in GABAergic interneurons lowered calcium responsiveness, and SNAP-25 silencing in glutamatergic neurons increased calcium elevations evoked by depolarization. Expression of SNAP-25(1-197) but not of SNAP-25(1-180) inhibited calcium responsiveness, pointing to the involvement of the 180-197 residues in the observed function. These data indicate that SNAP-25 is crucial for the regulation of intracellular calcium dynamics and, possibly, of network excitability. SNAP-25 is therefore a multifunctional protein that participates in exocytotic function both at the mechanistic and at the regulatory level.

The presynaptic release apparatus is functional in the absence of dendritic contact and highly mobile within isolated axons

Whether contact of an axon with a dendrite is a necessary inductive signal for the assembly of functional presynaptic machinery is controversial. Combining FM1-43 imaging with retrospective immunocytochemistry, we observe many functional synaptic vesicle (SV) release sites lacking postsynaptic specializations in cultured hippocampal neurons. These "orphan" release sites share the same exocytic machinery and mechanisms of endocytic recycling as mature synaptic sites. Moreover, quantitative analysis of FM1-43 destaining at these orphan release sites reveals similar kinetics with slightly lower release probabilities. Time-lapse imaging of FM1-43 reveals that orphans are generated by complete or partial mobilization of synaptic release sites that retain their functionality in transit. Orphan clusters fuse with existing synaptic release sites or form novel release sites onto dendrites. Mobilization and stabilization of orphan boutons to new sites of dendritic contact may represent a necessary presynaptic counterpart to postsynaptic changes observed during development and plasticity in the CNS.

Synaptobrevin-2-like immunoreactivity is associated with vesicles at synapses in rat circumvallate taste buds

Synaptobrevin is a vesicle-associated membrane protein (VAMP) that is believed to play a critical role with presynaptic membrane proteins (SNAP-25 and syntaxin) during regulated synaptic vesicle docking and exocytosis of neurotransmitter at the central nervous system. Synaptic contacts between taste cells and nerve processes have been found to exist, but little is known about synaptic vesicle docking and neurotransmitter release at taste cell synapses. Previously we demonstrated that immunoreactivity to SNAP-25 is present in taste cells with synapses. Our present results show that synaptobrevin-2-like immunoreactivity (-LIR) is present in approximately 35% of the taste cells in rat circumvallate taste buds. Synaptobrevin-2-LIR colocalizes with SNAP-25-, serotonin-, and protein gene product 9.5-LIR. Synaptobrevin-2-LIR also colocalizes with immunoreactivity for type III inositol 1,4,5-triphosphate receptor (IP3R3), a taste-signaling molecule in taste cells. All IP3R3-LIR taste cells express synaptobrevin-2-LIR. However, approximately 27% of the synaptobrevin-2-LIR taste cells do not display IP3R3-LIR. We believe, based on ultrastructural and biochemical features, that both type II and type III taste cells display synaptobrevin-2-LIR. All of the synapses that we observed from taste cells onto nerve processes express synaptobrevin-2-LIR, as well as some taste cells without synapses. By using colloidal gold immunoelectron microscopy, we found that synaptobrevin-2-LIR is associated with synaptic vesicles at rat taste cell synapses. The results of this study suggest that soluble NSF attachment receptor (SNARE) machinery may control synaptic vesicle fusion and exocytosis at taste cell synapses.

Expression of the vesicular glutamate transporters during development indicates the widespread corelease of multiple neurotransmitters

Three closely related proteins transport glutamate into synaptic vesicles for release by exocytosis. Complementary patterns of expression in glutamatergic terminals have been reported for VGLUT1 and VGLUT2. VGLUT3 shows expression by many cells not considered to be glutamatergic. Here we describe the changes in VGLUT expression that occur during development. VGLUT1 expression increases gradually after birth and eventually predominates over the other isoforms in telencephalic regions. Expressed at high levels shortly after birth, VGLUT2 declines with age in multiple regions, in the cerebellum by 14-fold. In contrast, Coexpression of the two isoforms occurs transiently during development as well as permanently in a restricted subset of glutamatergic terminals in the adult. VGLUT3 is transiently expressed at high levels by select neuronal populations, including terminals in the cerebellar nuclei, scattered neurons in the cortex, and progenitor-like cells, implicating exocytotic glutamate release in morphogenesis and development. VGLUT3 also colocalizes extensively during development with the neuronal vesicular monoamine transporter VMAT2, with the vesicular acetylcholine transporter VAChT, and with the vesicular gamma-aminobutyric acid transporter VGAT. Such coexpression occurs particularly at some specific developmental stages and is restricted to certain sets of cells. In skeletal muscle, VGLUT3 localizes to granular organelles in the axon terminal as well as in the muscle sarcoplasm. The results suggest novel mechanisms and roles for regulated transmitter release.

In vivo evidence for an activity-independent maturation of AMPA/NMDA signaling in the developing hippocampus

Correlated pre- and postsynaptic activity is thought to promote maturation of excitatory synapses in the developing brain by directing AMPA receptors to pure NMDA synapses. However, this hypothesis has not been tested in vivo. Here, we have performed such test by inhibiting correlated neural activity in vivo using a single injection of tetanus toxin into the rat hippocampal CA1 area at postnatal day 1. When examined in the acute slice preparation (1-7 days post-injection), there was a strong reduction, down to 20% of control level, in the frequency of glutamatergic and GABAergic spontaneous postsynaptic currents (sPSCs). This activity deprivation led to a growth retardation of CA1 pyramidal neurons and to markedly faster decay kinetics of NMDA sPCSs. However, it did not alter the relationship between AMPA and NMDA sPSCs with respect to either their frequency or amplitude. Thus, although critical for certain aspects of neuronal development, correlated neural activity in the neonatal hippocampus does not seem to promote incorporation of AMPA receptors at pure NMDA synapses.

Mechanisms of Synapse Assembly and Disassembly

The mechanisms that govern synapse formation and elimination are fundamental to our understanding of neural development and plasticity. The wiring of neural circuitry requires that vast numbers of synapses be formed in a relatively short time. The subsequent refinement of neural circuitry involves the formation of additional synapses coincident with the disassembly of previously functional synapses. There is increasing evidence that activity-dependent plasticity also involves the formation and disassembly of synapses. While we are gaining insight into the mechanisms of both synapse assembly and disassembly, we understand very little about how these phenomena are related to each other and how they might be coordinately controlled to achieve the precise patterns of synaptic connectivity in the nervous system. Here, we review our current understanding of both synapse assembly and disassembly in an effort to unravel the relationship between these fundamental developmental processes.

Targeted expression of tetanus toxin reveals sets of neurons involved in larval locomotion in Drosophila

The Drosophila larva is widely used for studies of neuronal development and function, yet little is known about the neuronal basis of locomotion in this model organism. Drosophila larvae crawl over a plain substrate by performing repetitive waves of forward peristalsis alternated by brief episodes of head swinging and turning. To identify sets of central and peripheral neurons required for the spatial or temporal pattern of larval locomotion, we blocked neurotransmitter release from defined populations of neurons by targeted expression of tetanus toxin light chain (TeTxLC) with the GAL4/UAS system. One hundred fifty GAL4 lines were crossed to a UAS-TeTxLC strain and a motion-analysis system was used to identify larvae with abnormal movement patterns. Five lines were selected that show discrete locomotor defects (i.e., increased turning and pausing) and these defects are correlated with diverse sets of central neurons. One line, 4C-GAL4, caused an unusual circling behavior that is correlated with approximately 200 neurons, including dopaminergic and peptidergic interneurons. Expression of TeTxLC in all dopaminergic and serotonergic but not in peptidergic neurons, caused turning deficits that are similar to those of 4C-GAL4/TeTxLC larvae. The results presented here provide a basis for future genetic studies of motor control in the Drosophila larva.

Morphological and molecular heterogeneity in release sites of single neurons

We have previously shown that labelling intensities for synaptic proteins vary strongly among synaptic boutons. Here we addressed the questions as to whether there are heterogeneous levels of integral membrane synaptic vesicle proteins at distinct active release sites of single neurons and if these sites possess the ultrastructural features of synapses. By double-immunostaining with specific antibodies against synaptophysin, synaptotagmin I, VAMP1 and VAMP2, we identified different relative levels of these integral membrane proteins of synaptic vesicles in comparison to boutons of the same rat cortical neuron. This heterogeneity could also be observed between the two isoforms VAMP1 and VAMP2. By studying pairs of these proteins implicated in neurotransmitter release, including both VAMP isoforms, we also show that the sites that contained predominantly one protein were nevertheless functional, as they internalized and released FM1-43 upon potassium stimulation. Using electron microscopy, we show that these active sites could have either synaptic specializations, or the features of vesicle-containing varicosities without a postsynaptic target. Different varicosities of the same neuron showed different intensities for synaptic vesicle proteins; some varicosities were capable of internalizing and releasing FM1-43, while others were silent. These results show that integral membrane synaptic vesicle proteins are differentially distributed among functional release sites of the same neuron.