Heterogeneity of glutamatergic and GABAergic release machinery in cerebral cortex: analysis of synaptogyrin, vesicle-associated membrane protein, and syntaxin

Time to Wake Up! The Ongoing Search for General Anesthetic Reversal Agents

How general anesthetics work remains a topic of ongoing study. A parallel field of research has sought to identify methods to reverse general anesthesia. Reversal agents could shorten patients' recovery time and potentially reduce the risk of postoperative complications. An incomplete understanding of the mechanisms of general anesthesia has hampered the pursuit for reversal agents. Nevertheless, the search for reversal agents has furthered understanding of the mechanisms underlying general anesthesia. The study of potential reversal agents has highlighted the importance of rigorous criteria to assess recovery from general anesthesia in animal models, and has helped identify key arousal systems (e.g., cholinergic, dopaminergic, and orexinergic systems) relevant to emergence from general anesthesia. Furthermore, the effects of reversal agents have been found to be inconsistent across different general anesthetics, revealing differences in mechanisms among these drugs. The presynapse and glia probably also contribute to general anesthesia recovery alongside postsynaptic receptors. The next stage in the search for reversal agents will have to consider alternate mechanisms encompassing the tripartite synapse.

Syntabulin regulates neuronal excitation/inhibition balance and epileptic seizures by transporting syntaxin 1B

Epilepsy is a widespread neurological disorder affecting more than 65 million people, but the mechanisms of epilepsy remains unknown. Abnormal synaptic transmission has a crucial role in the occurrence and development of epilepsy. Here, we found that syntabulin, a neuronal transporter, was mainly localized in neurons, and its expression was increased in epileptic tissues. Knockdown of syntabulin increased susceptibility and severity of epilepsy, whereas overexpression of syntabulin had the opposite effect. Mechanistically, in the epileptic brain tissue, syntabulin mainly translocated syntaxin 1B (STX1B) rather than syntaxin 1A (STX1A) to the presynaptic membrane, which resulted in increased presynaptic transmitter release. Further studies showed that syntabulin had a more significant effect on presynaptic functionality of GABAergic activity over that of excitatory synapses and resulted in an excitation/inhibition (E/I) imbalance, thereby regulating the epileptic phenotype. In addition, we found that the increased expression of syntabulin in epileptic brain tissue was mainly regulated by transcription factor TFAP2A. In summary, syntabulin plays a protective role in epilepsy by maintaining a proper E/I balance in the hippocampus.

Synaptophysin-dependent synaptobrevin-2 trafficking at the presynapse-Mechanism and function

Synaptobrevin-2 (Syb2) is a soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) that is essential for neurotransmitter release. It is the most numerous protein on a synaptic vesicle (SV) and drives SV fusion via interactions with its cognate SNARE partners on the presynaptic plasma membrane. Synaptophysin (Syp) is the second most abundant protein on SVs; however, in contrast to Syb2, it has no obligatory role in neurotransmission. Syp interacts with Syb2 on SVs, and the molecular nature of its interaction with Syb2 and its physiological role has been debated for decades. However, recent studies have revealed that the sole physiological role of Syp at the presynapse is to ensure the efficient retrieval of Syb2 during SV endocytosis. In this review, current theories surrounding the role of Syp in Syb2 trafficking will be discussed, in addition to the debate regarding the molecular nature of their interaction. A unifying model is presented that describes how Syp controls Syb2 function as part of an integrated mechanism involving key molecular players such as intersectin-1 and AP180/CALM. Finally, key future questions surrounding the role of Syp-dependent Syb2 trafficking will be posed, with respect to brain function in health and disease.

Interneuronal exchange and functional integration of synaptobrevin via extracellular vesicles

Recent studies have investigated the composition and functional effects of extracellular vesicles (EVs) secreted by a variety of cell types. However, the mechanisms underlying the impact of these vesicles on neurotransmission remain unclear. Here, we isolated EVs secreted by rat and mouse hippocampal neurons and found that they contain synaptic-vesicle-associated proteins, in particular the vesicular SNARE (soluble N-ethylmaleimide-sensitive factor [NSF]-attachment protein receptor) synaptobrevin (also called VAMP). Using a combination of electrophysiology and live-fluorescence imaging, we demonstrate that this extracellular pool of synaptobrevins can rapidly integrate into the synaptic vesicle cycle of host neurons via a CD81-dependent process and selectively augment inhibitory neurotransmission as well as specifically rescue neurotransmission in synapses deficient in synaptobrevin. These findings uncover a novel means of interneuronal communication and functional coupling via exchange of vesicular SNAREs.

hiPSC-Derived Neurons Provide a Robust and Physiologically Relevant In Vitro Platform to Test Botulinum Neurotoxins

Botulinum neurotoxins (BoNTs) are zinc metalloproteases that block neurotransmitter release at the neuromuscular junction (NMJ). Their high affinity for motor neurons combined with a high potency have made them extremely effective drugs for the treatment of a variety of neurological diseases as well as for aesthetic applications. Current in vitro assays used for testing and developing BoNT therapeutics include primary rodent cells and immortalized cell lines. Both models have limitations concerning accuracy and physiological relevance. In order to improve the translational value of preclinical data there is a clear need to use more accurate models such as human induced Pluripotent Stem Cells (hiPSC)-derived neuronal models. In this study we have assessed the potential of four different human iPSC-derived neuronal models including Motor Neurons for BoNT testing. We have characterized these models in detail and found that all models express all proteins needed for BoNT intoxication and showed that all four hiPSC-derived neuronal models are sensitive to both serotype A and E BoNT with Motor Neurons being the most sensitive. We showed that hiPSC-derived Motor Neurons expressed authentic markers after only 7 days of culture, are functional and able to form active synapses. When cultivated with myotubes, we demonstrated that they can innervate myotubes and induce contraction, generating an in vitro model of NMJ showing dose-responsive sensitivity BoNT intoxication. Together, these data demonstrate the promise of hiPSC-derived neurons, especially Motor Neurons, for pharmaceutical BoNT testing and development.

Transcriptomic and epigenomic dynamics associated with development of human iPSC-derived GABAergic interneurons

GABAergic interneurons (GINs) are a heterogeneous population of inhibitory neurons that collectively contribute to the maintenance of normal neuronal excitability and network activity. Identification of the genetic regulatory elements and transcription factors that contribute toward GIN function may provide new insight into the pathways underlying proper GIN activity while also indicating potential therapeutic targets for GIN-associated disorders, such as schizophrenia and epilepsy. In this study, we examined the temporal changes in gene expression and chromatin accessibility during GIN development by performing transcriptomic and epigenomic analyses on human induced pluripotent stem cell-derived neurons at 22, 50 and 78 days (D) post-differentiation. We observed 13 221 differentially accessible regions (DARs) of chromatin that associate with temporal changes in gene expression at D78 and D50, relative to D22. We also classified families of transcription factors that are increasingly enriched at DARs during differentiation, indicating regulatory networks that likely drive GIN development. Collectively, these data provide a resource for examining the molecular networks regulating GIN functionality.

Seizing the moment: Zebrafish epilepsy models

Zebrafish are now widely accepted as a valuable animal model for a number of different central nervous system (CNS) diseases. They are suitable both for elucidating the origin of these disorders and the sequence of events culminating in their onset, and for use as a high-throughput in vivo drug screening platform. The availability of powerful and effective techniques for genome manipulation allows the rapid modelling of different genetic epilepsies and of conditions with seizures as a core symptom. With this review, we seek to summarize the current knowledge about existing epilepsy/seizures models in zebrafish (both pharmacological and genetic) and compare them with equivalent rodent and human studies. New findings obtained from the zebrafish models are highlighted. We believe that this comprehensive review will highlight the value of zebrafish as a model for investigating different aspects of epilepsy and will help researchers to use these models to their full extent.

Enhanced functional recovery by levodopa is associated with decreased levels of synaptogyrin following stroke in aged mice

Levodopa is a precursor to dopamine that has been shown to improve functional recovery following stroke partly achieved through mechanisms of brain plasticity. This study investigates if dopamine might affect plasticity by having a direct effect on synaptic plasticity through alterations in neurotransmitter release and re-uptake. Synaptogyrin is a synaptic vesicle protein that has been suggested to be involved in dopamine re-uptake in the synaptic terminal. Therefore, we investigated if levodopa has an effect on the expression of synaptogyrin 1. Thy1-YFP mice were subjected to photothrombosis as an experimental model of stroke. Starting two days after surgery they were treated with either levodopa or a vehicle solution (saline) on a daily basis until day seven following surgery. On day seven they were sacrificed and their brains stained for Dopamine 1 receptor (D1R), Dopamine 2 receptor (D2R) and Parvalbumin (PV). Neu-N stainings were used to estimate infarct size. A second group of mice were subjected to photothrombosis and also treated with either levodopa or a vehicle solution in the same manner as previously mentioned. On day seven they were then sacrificed, and samples of brain tissue were taken for protein determination. Western blots were carried out to investigate possible differences in synaptogyrin expression between the two groups. Immunofluorescent stains showed the presence of dopamine receptors on the YFP-positive neurons and on PV-expressing neurones. Our Western Blot analysis showed a significant decrease in the expression of synaptogyrin in levodopa-treated mice. Our stains showed co-localisation with Thy-1 neurones and PV-expressing neurones for both D1 and D2 receptors. This indicates that dopamine has the ability to bind to, and directly influence cortical neurons, as well as inhibitory interneurons. We discovered a considerable decrease in synaptogyrin expression through levodopa treatment, suggesting that this might be a mechanism for regulating brain plasticity.

Short-term plasticity at cerebellar granule cell to molecular layer interneuron synapses expands information processing

Information processing by cerebellar molecular layer interneurons (MLIs) plays a crucial role in motor behavior. MLI recruitment is tightly controlled by the profile of short-term plasticity (STP) at granule cell (GC)-MLI synapses. While GCs are the most numerous neurons in the brain, STP diversity at GC-MLI synapses is poorly documented. Here, we studied how single MLIs are recruited by their distinct GC inputs during burst firing. Using slice recordings at individual GC-MLI synapses of mice, we revealed four classes of connections segregated by their STP profile. Each class differentially drives MLI recruitment. We show that GC synaptic diversity is underlain by heterogeneous expression of synapsin II, a key actor of STP and that GC terminals devoid of synapsin II are associated with slow MLI recruitment. Our study reveals that molecular, structural and functional diversity across GC terminals provides a mechanism to expand the coding range of MLIs.

Synaptic vesicle protein 2: A multi-faceted regulator of secretion

Synaptic Vesicle Protein 2 (SV2) comprises a recently evolved family of proteins unique to secretory vesicles that undergo calcium-regulated exocytosis. In this review we consider SV2s' structural features, evolution, and function and discuss its therapeutic potential as the receptors for an expanding class of drugs used to treat epilepsy and cognitive decline.

The Control of Neuronal Calcium Homeostasis by SNAP-25 and its Impact on Neurotransmitter Release

The process of neurotransmitter release is central to the control of cell-to-cell communication in brain. SNAP-25 is a component of the SNARE complex, which, together with syntaxin-1 and synaptobrevin, mediates synaptic vesicle fusion with the plasma membrane. The genetic ablation of the protein or its proteolytic cleavage by botulinum neurotoxins results in a complete block of synaptic transmission. In the last years, several evidences have indicated that SNAP-25 also plays additional modulatory roles in neurotransmission through the control of voltage-gated calcium channels and presynaptic calcium ion concentration. Consistently, reduced levels of the protein affect presynaptic calcium homeostasis and result in pathologically enhanced glutamate exocytosis. The SNAP-25-dependent alterations of synaptic calcium dynamics may have direct impact on the development of neuropsychiatric disorders where the Snap-25 gene has been found to be involved.

Expression of Neurofilament Subunits at Neocortical Glutamatergic and GABAergic Synapses

Neurofilaments (NFs) are neuron-specific heteropolymers that have long been considered as structural proteins. However, it has recently been documented that they may play a functional role at synapses. Indeed, the four NF subunits—NFL, NFM, NFH and α-internexin—are integral components of synapses in the striatum and hippocampus, since their elimination disrupts synaptic plasticity and impairs social memory, an observation that might have important implications for some neuropsychiatric diseases. Here, we studied NFs localization in VGLUT1-, VGLUT2-, VGAT-, PSD-95- and gephyrin-positive (+) puncta, and in glutamatergic and GABAergic synapses in the cerebral cortex of adult rats. Synapses were identified by pre- and postsynaptic markers: glutamatergic synapses by VGLUT1+ or VGLUT2+ puncta contacting PSD-95+ puncta; and GABAergic synapses by VGAT+ puncta contacting gephyrin+ puncta. In VGLUT1 glutamatergic synapses NF showed a greater expression in the compartment labeled by postsynaptic markers (20%–30%) than in those labeled by presynaptic markers (10%–20%), whereas in GABAergic synapses a similar expression was detected in both compartments (20%–30%). Moreover, NF expression was higher in the GABAergic (20%–30%) than in the glutamatergic (10%–15%) compartments labeled by presynaptic markers. Finally, a higher colocalization of VGLUT1+, VGLUT2+ and VGAT+ puncta with NFs was seen when presynaptic puncta contacted elements labeled by postsynaptic markers. These findings show that the four NF subunits are expressed at some neocortical synapses, and contribute to glutamatergic and GABAergic synapse heterogeneity.

Dissecting Brain Networks Underlying Alcohol Binge Drinking Using a Systems Genomics Approach

Alcohol use disorder (AUD) is a complex psychiatric disorder with strong genetic and environmental risk factors. We studied the molecular perturbations underlying risky drinking behavior by measuring transcriptome changes across the neurocircuitry of addiction in a genetic mouse model of binge drinking. Sixteen generations of selective breeding for high blood alcohol levels after a binge drinking session produced global changes in brain gene expression in alcohol-naïve High Drinking in the Dark (HDID-1) mice. Using gene expression profiles to generate circuit-level hypotheses, we developed a systems approach that integrated regulation of gene coexpression networks across multiple brain regions, neuron-specific transcriptional signatures, and knowledgebase analytics. Whole-cell, voltage-clamp recordings from nucleus accumbens shell neurons projecting to the ventral tegmental area showed differential ethanol-induced plasticity in HDID-1 and control mice and provided support for one of the hypotheses. There were similarities in gene networks between HDID-1 mouse brains and postmortem brains of human alcoholics, suggesting that some gene expression patterns associated with high alcohol consumption are conserved across species. This study demonstrated the value of gene networks for data integration across biological modalities and species to study mechanisms of disease.

The genetics and molecular biology of fever-associated seizures or epilepsy

Fever-associated seizures or epilepsy (FASE) is primarily characterised by the occurrence of a seizure or epilepsy usually accompanied by a fever. It is common in infants and children, and generally includes febrile seizures (FS), febrile seizures plus (FS+), Dravet syndrome (DS) and genetic epilepsy with febrile seizures plus (GEFSP). The aetiology of FASE is unclear. Genetic factors may play crucial roles in FASE. Mutations in certain genes may cause a wide spectrum of phenotypical overlap ranging from isolated FS, FS+ and GEFSP to DS. Synapse-associated proteins, postsynaptic GABAA receptor, and sodium channels play important roles in synaptic transmission. Mutations in these genes may involve in the pathogenesis of FASE. Elevated temperature promotes synaptic vesicle (SV) recycling and enlarges SV size, which may enhance synaptic transmission and contribute to FASE occurring. This review provides an overview of the loci, genes, underlying pathogenesis and the fever-inducing effect of FASE. It may provide a more comprehensive understanding of pathogenesis and contribute to the clinical diagnosis of FASE.

Synaptogyrin-2 Promotes Replication of a Novel Tick-borne Bunyavirus through Interacting with Viral Nonstructural Protein NSs

Synaptogyrin-2 is a non-neuronal member of the synaptogyrin family involved in synaptic vesicle biogenesis and trafficking. Little is known about the function of synaptogyrin-2. Severe fever with thrombocytopenia syndrome (SFTS) is an emerging infectious disease characterized by high fever, thrombocytopenia, and leukocytopenia with high mortality, caused by a novel tick-borne phlebovirus in the family Bunyaviridae. Our previous studies have shown that the viral nonstructural protein NSs forms inclusion bodies (IBs) that are involved in viral immune evasion, as well as viral RNA replication. In this study, we sought to elucidate the mechanism by which NSs formed the IBs, a lipid droplet-based structure confirmed by NSs co-localization with perilipin A and adipose differentiation-related protein (ADRP). Through a high throughput screening, we identified synaptogyrin-2 to be highly up-regulated in response to SFTS bunyavirus (SFTSV) infection and to be a promoter of viral replication. We demonstrated that synaptogyrin-2 interacted with NSs and was translocated into the IBs, which were reconstructed from lipid droplets into large structures in infection. Viral RNA replication decreased, and infectious virus titers were lowered significantly when synaptogyrin-2 was silenced in specific shRNA-expressing cells, which correlated with the reduced number of the large IBs restructured from regular lipid droplets. We hypothesize that synaptogyrin-2 is essential to promoting the formation of the IBs to become virus factories for viral RNA replication through its interaction with NSs. These findings unveil the function of synaptogyrin-2 as an enhancer in viral infection.

Differential expression of metabotropic glutamate and GABA receptors at neocortical glutamatergic and GABAergic axon terminals

Metabotropic glutamate (Glu) receptors (mGluRs) and GABAB receptors are highly expressed at presynaptic sites. To verify the possibility that the two classes of metabotropic receptors contribute to axon terminals heterogeneity, we studied the localization of mGluR1α, mGluR5, mGluR2/3, mGluR7, and GABAB1 in VGLUT1-, VGLUT2-, and VGAT- positive terminals in the cerebral cortex of adult rats. VGLUT1-positive puncta expressed mGluR1α (∼5%), mGluR5 (∼6%), mGluR2/3 (∼22%), mGluR7 (∼17%), and GABAB1 (∼40%); VGLUT2-positive terminals expressed mGluR1α (∼10%), mGluR5 (∼11%), mGluR2/3 (∼20%), mGluR7 (∼28%), and GABAB1 (∼25%); whereas VGAT-positive puncta expressed mGluR1α (∼27%), mGluR5 (∼24%), mGluR2/3 (∼38%), mGluR7 (∼31%), and GABAB1 (∼19%). Control experiments ruled out the possibility that postsynaptic mGluRs and GABAB1 might have significantly biased our results. We also performed functional assays in synaptosomal preparations, and showed that all agonists modify Glu and GABA levels, which return to baseline upon exposure to antagonists. Overall, these findings indicate that mGluR1α, mGluR5, mGluR2/3, mGluR7, and GABAB1 expression differ significantly between glutamatergic and GABAergic axon terminals, and that the robust expression of heteroreceptors may contribute to the homeostatic regulation of the balance between excitation and inhibition.

Botulinum toxin B increases intrinsic muscle activity in organotypic spinal cord-skeletal muscle co-cultures

In organotypic spinal cord-skeletal muscle co-cultures, motoneurons are driven by locomotor commands and induce contractions in surrounding muscle fibres. Using these co-cultures, it has been shown that effects of organophosphorus compounds on neuromuscular synapses can be determined in vitro. In the present study we aimed to extend this in vitro tool for pharmacologic testing of botulinum toxin B. This neurotoxin is widely used for the treatment of dystonia. Besides its effects on the neuromuscular junction, botulinum toxins may also act at centrally located synapses. Incubation with botulinum toxin B (Neurobloc(®)) induced a significant increase in muscular activity after 24, 48 and 72h. Application of the NMDA- and AMPA-receptor antagonists AP5 (20μM) and CNQX (15μM) induced a similar augmentation of muscle activity after 48 and 72h, respectively. Administration of the glycine- and GABA(A)-receptor antagonists strychnine (1μM) and bicuculline (100μM) did not alter intrinsic muscle activity. In contrast, application of a non-depolarizing muscle relaxant rocuronium bromide reduced the muscle activity in a dose-dependent manner. Our findings suggest that glutamatergic synapses in the spinal cord are more sensitive to botulinum toxin B than synaptic contacts between spinal motoneurons and muscle fibres.

Differential maturation of vesicular glutamate and GABA transporter expression in the mouse auditory forebrain during the first weeks of hearing

Vesicular transporter proteins are an essential component of the presynaptic machinery that regulates neurotransmitter storage and release. They also provide a key point of control for homeostatic signaling pathways that maintain balanced excitation and inhibition following changes in activity levels, including the onset of sensory experience. To advance understanding of their roles in the developing auditory forebrain, we tracked the expression of the vesicular transporters of glutamate (VGluT1, VGluT2) and GABA (VGAT) in primary auditory cortex (A1) and medial geniculate body (MGB) of developing mice (P7, P11, P14, P21, adult) before and after ear canal opening (~P11-P13). RNA sequencing, in situ hybridization, and immunohistochemistry were combined to track changes in transporter expression and document regional patterns of transcript and protein localization. Overall, vesicular transporter expression changed the most between P7 and P21. The expression patterns and maturational trajectories of each marker varied by brain region, cortical layer, and MGB subdivision. VGluT1 expression was highest in A1, moderate in MGB, and increased with age in both regions. VGluT2 mRNA levels were low in A1 at all ages, but high in MGB, where adult levels were reached by P14. VGluT2 immunoreactivity was prominent in both regions. VGluT1 (+) and VGluT2 (+) transcripts were co-expressed in MGB and A1 somata, but co-localization of immunoreactive puncta was not detected. In A1, VGAT mRNA levels were relatively stable from P7 to adult, while immunoreactivity increased steadily. VGAT (+) transcripts were rare in MGB neurons, whereas VGAT immunoreactivity was robust at all ages. Morphological changes in immunoreactive puncta were found in two regions after ear canal opening. In the ventral MGB, a decrease in VGluT2 puncta density was accompanied by an increase in puncta size. In A1, perisomatic VGAT and VGluT1 terminals became prominent around the neuronal somata. Overall, the observed changes in gene and protein expression, regional architecture, and morphology relate to-and to some extent may enable-the emergence of mature sound-evoked activity patterns. In that regard, the findings of this study expand our understanding of the presynaptic mechanisms that regulate critical period formation associated with experience-dependent refinement of sound processing in auditory forebrain circuits.

The role of non-canonical SNAREs in synaptic vesicle recycling

An increasing number of studies suggest that distinct pools of synaptic vesicles drive specific forms of neurotransmission. Interspersed with these functional studies are analyses of the synaptic vesicle proteome which have consistently detected the presence of so-called “non-canonical” SNAREs that typically function in fusion and trafficking of other subcellular structures within the neuron. The recent identification of certain non-canonical vesicular SNAREs driving spontaneous (e.g., VAMP7 and vti1a) or evoked asynchronous (e.g., VAMP4) release integrates and corroborates existing data from functional and proteomic studies and implies that at least some complement of non-canonical SNAREs resident on synaptic vesicles function in neurotransmission. Here, we discuss the specific roles in neurotransmission of proteins homologous to each member of the classical neuronal SNARE complex consisting of synaptobrevin2, syntaxin-1 and SNAP-25.

Construction and analysis of the protein-protein interaction networks based on gene expression profiles of Parkinson's disease

Background

Parkinson's Disease (PD) is one of the most prevailing neurodegenerative diseases. Improving diagnoses and treatments of this disease is essential, as currently there exists no cure for this disease. Microarray and proteomics data have revealed abnormal expression of several genes and proteins responsible for PD. Nevertheless, few studies have been reported involving PD-specific protein-protein interactions.

Results

Microarray based gene expression data and protein-protein interaction (PPI) databases were combined to construct the PPI networks of differentially expressed (DE) genes in post mortem brain tissue samples of patients with Parkinson's disease. Samples were collected from the substantia nigra and the frontal cerebral cortex. From the microarray data, two sets of DE genes were selected by 2-tailed t-tests and Significance Analysis of Microarrays (SAM), run separately to construct two Query-Query PPI (QQPPI) networks. Several topological properties of these networks were studied. Nodes with High Connectivity (hubs) and High Betweenness Low Connectivity (bottlenecks) were identified to be the most significant nodes of the networks. Three and four-cliques were identified in the QQPPI networks. These cliques contain most of the topologically significant nodes of the networks which form core functional modules consisting of tightly knitted sub-networks. Hitherto unreported 37 PD disease markers were identified based on their topological significance in the networks. Of these 37 markers, eight were significantly involved in the core functional modules and showed significant change in co-expression levels. Four (ARRB2, STX1A, TFRC and MARCKS) out of the 37 markers were found to be associated with several neurotransmitters including dopamine.

Conclusion

This study represents a novel investigation of the PPI networks for PD, a complex disease. 37 proteins identified in our study can be considered as PD network biomarkers. These network biomarkers may provide as potential therapeutic targets for PD applications development.

Targeting synaptic pathology with a novel affinity mass spectrometry approach

We report a novel strategy for studying synaptic pathology by concurrently measuring levels of four SNARE complex proteins from individual brain tissue samples. This method combines affinity purification and mass spectrometry and can be applied directly for studies of SNARE complex proteins in multiple species or modified to target other key elements in neuronal function. We use the technique to demonstrate altered levels of presynaptic proteins in Alzheimer disease patients and prion-infected mice.

Prenatal inhibition of the kynurenine pathway leads to structural changes in the hippocampus of adult rat offspring

Glutamate receptors for N-methyl-d-aspartate (NMDA) are involved in early brain development. The kynurenine pathway of tryptophan metabolism includes the NMDA receptor agonist quinolinic acid and the antagonist kynurenic acid. We now report that prenatal inhibition of the pathway in rats with 3,4-dimethoxy-N-[4-(3-nitrophenyl)thiazol-2-yl]benzenesulphonamide (Ro61-8048) produces marked changes in hippocampal neuron morphology, spine density and the immunocytochemical localisation of developmental proteins in the offspring at postnatal day 60. Golgi–Cox silver staining revealed decreased overall numbers and lengths of CA1 basal dendrites and secondary basal dendrites, together with fewer basal dendritic spines and less overall dendritic complexity in the basal arbour. Fewer dendrites and less complexity were also noted in the dentate gyrus granule cells. More neurons containing the nuclear marker NeuN and the developmental protein sonic hedgehog were detected in the CA1 region and dentate gyrus. Staining for doublecortin revealed fewer newly generated granule cells bearing extended dendritic processes. The number of neuron terminals staining for vesicular glutamate transporter (VGLUT)-1 and VGLUT-2 was increased by Ro61-8048, with no change in expression of vesicular GABA transporter or its co-localisation with vesicle-associated membrane protein-1. These data support the view that constitutive kynurenine metabolism normally plays a role in early embryonic brain development, and that interfering with it has profound consequences for neuronal structure and morphology, lasting into adulthood.

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.

HSF1 transcriptional activity mediates alcohol induction of Vamp2 expression and GABA release

Many central synapses are highly sensitive to alcohol, and it is now accepted that short-term alterations in synaptic function may lead to longer-term changes in circuit function. The regulation of postsynaptic receptors by alcohol has been well studied, but the mechanisms underlying the effects of alcohol on the presynaptic terminal are relatively unexplored. To identify a pathway by which alcohol regulates neurotransmitter release, we recently investigated the mechanism by which ethanol induces Vamp2, but not Vamp1, in mouse primary cortical cultures. These two genes encode isoforms of synaptobrevin, a vesicular soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) protein required for synaptic vesicle fusion. We found that alcohol activates the transcription factor heat shock factor 1 (HSF1) to induce Vamp2 expression, while Vamp1 mRNA levels remain unaffected. As the Vamp2 gene encodes a SNARE protein, we then investigated whether ethanol exposure and HSF1 transcriptional activity alter neurotransmitter release using electrophysiology. We found that alcohol increased the frequency of γ-aminobutyric acid (GABA)-mediated miniature IPSCs via HSF1, but had no effect on mEPSCs. Overall, these data indicate that alcohol induces HSF1 transcriptional activity to trigger a specific coordinated adaptation in GABAergic presynaptic terminals. This mechanism could explain some of the changes in synaptic function that occur soon after alcohol exposure, and may underlie some of the more enduring effects of chronic alcohol intake on local circuit function.

Proconvulsant actions of intrahippocampal botulinum neurotoxin B in the rat

Botulinum neurotoxins (BoNTs) may affect the excitability of brain circuits by inhibiting neurotransmitter release at central synapses. There is evidence that local delivery of BoNT serotypes A and E, which target SNAP-25, a component of the release machinery specific to excitatory synapses, can inhibit seizure generation. BoNT serotype B (BoNT/B) targets VAMP2, which is expressed in both excitatory and inhibitory terminals. Here we assessed the effects of unilateral intrahippocampal infusion of BoNT/B in the rat on intravenous pentylenetetrazol (PTZ) seizure thresholds, and on the expression of spontaneous behavioral and electrographic seizures. Infusion of BoNT/B (500 and 1,000 unit) by convection-enhanced delivery caused a reduction in myoclonic twitch and clonic seizure thresholds in response to intravenous PTZ beginning about 6 days after the infusion. Handling-evoked and spontaneous convulsive seizures were observed in many BoNT/B-treated animals but not in vehicle-treated controls. Spontaneous electrographic seizure discharges were recorded in the dentate gyrus of animals that received local BoNT/B infusion. In addition, there was an increased frequency of interictal epileptiform spikes and sharp waves at the same recording site. BoNT/B-treated animals also exhibited tactile hyperresponsivity in comparison with vehicle-treated controls. This is the first demonstration that BoNT/B causes a delayed proconvulsant action when infused into the hippocampus. Local infusion of BoNT/B could be useful as a focal epilepsy model.

Heterogeneity of presynaptic proteins: do not forget isoforms

Analysis of presynaptic protein expression in glutamatergic and GABAergic central synapses performed in several laboratories and with different techniques is unveiling a complex scenario, largely because each presynaptic protein exists in several isoforms. The interpretation of these findings is generally based on the notion that each synapse and each synaptic vesicle contains one of the isoforms of each family of presynaptic proteins. We verified whether this interpretation is tenable by performing triple labeling and immunoisolation studies with the aim of detecting two isoforms of a given presynaptic protein in glutamatergic or GABAergic axon terminals and/or synaptic vesicles (SVs). Here, we show that: (1) the possibility that not all families of presynaptic proteins are expressed in all terminals must be taken into serious account; (2) the expression of a given protein isoform in a terminal does not exclude the expression of other isoforms of the same protein in the same terminal and in the same vesicle. These conclusions open new and interesting problems; their experimental analysis might improve our understanding of the physiology and pathophysiology of central synapses.

Presynaptic inhibition of the release of multiple major central nervous system neurotransmitter types by the inhaled anaesthetic isoflurane

BACKGROUND

Presynaptic effects of general anaesthetics are not well characterized. We tested the hypothesis that isoflurane exhibits transmitter-specific effects on neurotransmitter release from neurochemically and functionally distinct isolated mammalian nerve terminals.

METHODS

Nerve terminals from adult male rat brain were prelabelled with [(3)H]glutamate and [(14)C]GABA (cerebral cortex), [(3)H]norepinephrine (hippocampus), [(14)C]dopamine (striatum), or [(3)H]choline (precursor of [(3)H]acetylcholine; striatum). Release evoked by depolarizing pulses of 4-aminopyridine (4AP) or elevated KCl was quantified using a closed superfusion system.

RESULTS

Isoflurane at clinical concentrations (<0.7 mM; ~2 times median anaesthetic concentration) inhibited Na(+) channel-dependent 4AP-evoked release of the five neurotransmitters tested in a concentration-dependent manner. Isoflurane was a more potent inhibitor [expressed as IC(50) (SEM)] of glutamate release [0.37 (0.03) mM; P<0.05] compared with the release of GABA [0.52 (0.03) mM], norepinephrine [0.48 (0.03) mM], dopamine [0.48 (0.03) mM], or acetylcholine [0.49 (0.02) mM]. Inhibition of Na(+) channel-independent release evoked by elevated K(+) was not significant at clinical concentrations of isoflurane, with the exception of dopamine release [IC(50)=0.59 (0.03) mM].

CONCLUSIONS

Isoflurane inhibited the release of the major central nervous system neurotransmitters with selectivity for glutamate release, consistent with both widespread inhibition and nerve terminal-specific presynaptic effects. Glutamate release was most sensitive to inhibition compared with GABA, acetylcholine, dopamine, and norepinephrine release due to presynaptic specializations in ion channel expression, regulation, and/or coupling to exocytosis. Reductions in neurotransmitter release by volatile anaesthetics could contribute to altered synaptic transmission, leading to therapeutic and toxic effects involving all major neurotransmitter systems.

The sodium-driven chloride/bicarbonate exchanger in presynaptic terminals

The sodium-driven chloride/bicarbonate exchanger (NDCBE), a member of the SLC4 family of bicarbonate transporters, was recently found to modulate excitatory neurotransmission in hippocampus. By using light and electron microscopic immunohistochemistry, we demonstrate here that NDCBE is expressed throughout the adult rat brain, and selectively concentrates in presynaptic terminals, where it is closely associated with synaptic vesicles. NDCBE is in most glutamatergic axon terminals, and is also present in the terminals of parvalbumin-positive γ-aminobutyric acid (GABA)ergic cells. These findings suggest that NDCBE can regulate glutamatergic transmission throughout the brain, and point to a role for NDCBE as a possible regulator of GABAergic neurotransmission.

Deep molecular diversity of mammalian synapses: why it matters and how to measure it

Pioneering studies in the middle of the twentieth century revealed substantial diversity among mammalian chemical synapses and led to a widely accepted classification of synapse type on the basis of neurotransmitter molecule identity. Subsequently, powerful new physiological, genetic and structural methods have enabled the discovery of much deeper functional and molecular diversity within each traditional neurotransmitter type. Today, this deep diversity continues to pose both daunting challenges and exciting new opportunities for neuroscience. Our growing understanding of deep synapse diversity may transform how we think about and study neural circuit development, structure and function.

Role of GABA release from leptin receptor-expressing neurons in body weight regulation

It is well established that leptin regulates energy balance largely through isoform B leptin receptor-expressing neurons (LepR neurons) in the brain and that leptin activates one subset of LepR neurons (leptin-excited neurons) while inhibiting the other (leptin-inhibited neurons). However, the neurotransmitters released from LepR neurons that mediate leptin action in the brain are not well understood. Previous results demonstrate that leptin mainly acts on γ-aminobutyric acid (GABA)ergic neurons to reduce body weight, and that leptin activates proopiomelanocortin neuron activity by reducing GABA release onto these neurons, suggesting a body weight-promoting role for GABA released from leptin-inhibited neurons. To directly examine the role of GABA release from LepR neurons in body weight regulation, mice with disruption of GABA release specifically from LepR neurons were generated by deletion of vesicular GABA transporter in LepR neurons. Interestingly, these mice developed mild obesity on chow diet and were sensitive to diet-induced obesity, which were associated with higher food intake and lower energy expenditure. Moreover, these mice showed blunted responses in both food intake and body weight to acute leptin administration. These results demonstrate that GABA plays an important role in mediating leptin action. In combination with the previous studies that leptin reduces GABA release onto proopiomelanocortin neurons through leptin-inhibited neurons and that disruption of GABA release from agouti gene-related protein neurons, one subset of LepR-inhibited neurons, leads to a lean phenotype, our results suggest that, under our experimental conditions, GABA release from leptin-excited neuron dominates over leptin-inhibited ones.

Dynamics of subcellular proteomes during brain development

Many neurological disorders are caused by perturbations during brain development, but these perturbations cannot be readily identified until there is comprehensive description of the development process. In this study, we performed mass spectrometry analysis of the synaptosomal and mitochondrial fractions from three rat brain regions at four postnatal time points. To quantitate our analysis, we employed (15)N labeled rat brains using a technique called SILAM (stable isotope labeling in mammals). We quantified 167429 peptides and identified over 5000 statistically significant changes during development including known disease-associated proteins. Global analysis revealed distinct trends between the synaptic and nonsynaptic mitochondrial proteomes and common protein networks between regions each consisting of a unique array of expression patterns. Finally, we identified novel regulators of neurodevelopment that possess the identical temporal pattern of known regulators of neurodevelopment. Overall, this study is the most comprehensive quantitative analysis of the developing brain proteome to date, providing an important resource for neurobiologists.

Analysis of Synaptotagmin, SV2, and Rab3 Expression in Cortical Glutamatergic and GABAergic Axon Terminals

We investigated whether cortical glutamatergic and GABAergic release machineries can be differentiated on the basis of the nature and amount of proteins they express, by performing a quantitative analysis of the degree of co-localization of synaptotagmin (SYT) 1 and 2, synaptic vesicle protein 2 (SV2) A and B, and Rab3a and c in VGLUT1+, VGLUT2+, and VGAT+ terminals and synaptic vesicles (SVs) in rat cerebral cortex. Co-localization studies showed that VGLUT1 puncta had high levels of SV2A and B and of Rab3c, intermediate levels of SYT1, and low levels of SYT2 and Rab3c; VGLUT2 puncta exhibited intermediate levels of all presynaptic proteins studied; whereas vesicular GABA transporter (VGAT) puncta had high levels of SV2A and SYT2, intermediate levels of SYT1, Rab3a, and Rab3c, and low levels of SV2B. Since SV2B is reportedly expressed by glutamatergic neurons and we observed SV2B expression in VGAT puncta, we performed electron microscopic studies and found SV2B positive axon terminals forming symmetric synapses. Immunoisolation studies showed that the expression levels of the protein isoforms varied in the three populations of SVs. Expression of SYT1 was highest in VGLUT1–SVs, while SYT2 expression was similar in the three SV groups. Expression of SV2A was similarly high in all three SV populations, except for SV2B levels that were very low in VGAT SVs. Finally, Rab3a levels were similar in the three SV groups, while Rab3c levels were highest in VGLUT1–SVs. These quantitative results extend our previous studies on the differential expression of presynaptic proteins involved in neurotransmitter release in GABAergic and glutamatergic terminals and indicate that heterogeneity of the respective release machineries can be generated by the differential complement of SV proteins involved in distinct stages of the release process.

VAMP-2, SNAP-25A/B and syntaxin-1 in glutamatergic and GABAergic synapses of the rat cerebellar cortex

BACKGROUND

The aim of this study was to assess the distribution of key SNARE proteins in glutamatergic and GABAergic synapses of the adult rat cerebellar cortex using light microscopy immunohistochemical techniques. Analysis was made of co-localizations of vGluT-1 and vGluT-2, vesicular transporters of glutamate and markers of glutamatergic synapses, or GAD, the GABA synthetic enzyme and marker of GABAergic synapses, with VAMP-2, SNAP-25A/B and syntaxin-1.

RESULTS

The examined SNARE proteins were found to be diffusely expressed in glutamatergic synapses, whereas they were rarely observed in GABAergic synapses. However, among glutamatergic synapses, subpopulations which did not contain VAMP-2, SNAP-25A/B and syntaxin-1 were detected. They included virtually all the synapses established by terminals of climbing fibres (immunoreactive for vGluT-2) and some synapses established by terminals of parallel and mossy fibres (immunoreactive for vGluT-1, and for vGluT-1 and 2, respectively). The only GABA synapses expressing the SNARE proteins studied were the synapses established by axon terminals of basket neurons.

CONCLUSION

The present study supplies a detailed morphological description of VAMP-2, SNAP-25A/B and syntaxin-1 in the different types of glutamatergic and GABAergic synapses of the rat cerebellar cortex. The examined SNARE proteins characterize most of glutamatergic synapses and only one type of GABAergic synapses. In the subpopulations of glutamatergic and GABAergic synapses lacking the SNARE protein isoforms examined, alternative mechanisms for regulating trafficking of synaptic vesicles may be hypothesized, possibly mediated by different isoforms or homologous proteins.

Synapsins: from synapse to network hyperexcitability and epilepsy

The synapsin family in mammals consists of at least 10 isoforms encoded by three distinct genes and composed by a mosaic of conserved and variable domains. Synapsins, although not essential for the basic development and functioning of neuronal networks, are extremely important for the fine-tuning of SV cycling and neuronal plasticity. Single, double and triple synapsin knockout mice, with the notable exception of the synapsin III knockout mice, show a severe epileptic phenotype without gross alterations in brain morphology and connectivity. However, the molecular and physiological mechanisms underlying the pathogenesis of the epileptic phenotype observed in synapsin deficient mice are still far from being elucidated. In this review, we summarize the current knowledge about the role of synapsins in the regulation of network excitability and about the molecular mechanism leading to epileptic phenotype in mouse lines lacking one or more synapsin isoforms. The current evidences indicate that synapsins exert distinct roles in excitatory versus inhibitory synapses by differentially affecting crucial steps of presynaptic physiology and by this mean participate in the determination of network hyperexcitability.

Lack of change in markers of presynaptic terminal abundance alongside subtle reductions in markers of presynaptic terminal plasticity in prefrontal cortex of schizophrenia patients

BACKGROUND

Reduced synaptic connectivity in frontal cortex may contribute to schizophrenia symptoms. While altered messenger RNA (mRNA) and protein expression of various synaptic genes have been found, discrepancies between studies mean a generalizable synaptic pathology has not been identified.

METHODS

We determined if mRNAs encoding presynaptic proteins enriched in inhibitory (vesicular gamma-aminobutyric acid transporter [VGAT] and complexin 1) and/or excitatory (vesicular glutamate transporter 1 [VGluT1] and complexin 2) terminals are altered in the dorsolateral prefrontal cortex of subjects with schizophrenia (n = 37 patients, n = 37 control subjects). We also measured mRNA expression of markers associated with synaptic plasticity/neurite outgrowth (growth associated protein 43 [GAP43] and neuronal navigators [NAVs] 1 and 2) and mRNAs of other synaptic-associated proteins previously implicated in schizophrenia: dysbindin and vesicle-associated membrane protein 1 (VAMP1) mRNAs using quantitative polymerase chain reaction.

RESULTS

No significant changes in complexin 1, VGAT, complexin 2, VGluT1, dysbindin, NAV2, or VAMP1 mRNA expression were found; however, expression of mRNAs associated with plasticity/cytoskeletal modification (GAP43 and NAV1) was reduced in schizophrenia. Although dysbindin mRNA did not differ in schizophrenia compared with control subjects, dysbindin mRNA positively correlated with GAP43 and NAV1 in schizophrenia but not in control subjects, suggesting low levels of dysbindin may be linked to reduced plasticity in the disease state. No relationships between three dysbindin genetic polymorphisms previously associated with dysbindin mRNA levels were found.

CONCLUSIONS

A reduction in the plasticity of synaptic terminals supports the hypothesis that their reduced modifiability may contribute to neuropathology and working memory deficits in schizophrenia.