The synaptophysin-synaptobrevin complex is developmentally upregulated in cultivated neurons but is absent in neuroendocrine cells

GFP nanobodies reveal recently-exocytosed pHluorin molecules

Neurotransmitter release requires vesicle recycling, which consists of exocytosis, endocytosis and the reformation of new fusion-competent vesicles. One poorly understood aspect in this cycle is the fate of the vesicle proteins after exocytosis, when they are left on the plasma membrane. Such proteins are often visualized by coupling to pH-sensitive GFP moieties (pHluorins). However, pHluorin imaging is typically limited by diffraction to spots several-fold larger than the vesicles. Here we show that pHuorin-tagged vesicle proteins can be easily detected using single-domain antibodies (nanobodies) raised against GFP. By coupling the nanobodies to chemical fluorophores that were optimal for super-resolution imaging, we could analyze the size and intensity of the groups of pHluorin-tagged proteins under a variety of conditions, in a fashion that would have been impossible based solely on the pHluorin fluorescence. We conclude that nanobody-based pHluorin detection is a promising tool for investigating post-exocytosis events in neurons.

Caffeic acid protects mice from memory deficits induced by focal cerebral ischemia

Brain ischemia pathophysiology involves a complex cascade of events such as inflammation and oxidative stress that lead to neuronal loss and cognitive deficits. Caffeic acid (CA) is a natural phenolic compound with antioxidant and anti-inflammatory properties. To evaluate the neuroprotective efficacy of this compound in mice subjected to a permanent middle cerebral artery occlusion, animals were pretreated and post-treated with CA, 2, 20, and 60 mg/kg/day, intraperitoneally, at 24, 48, 72, 96, or 120 h after ischemia. Animals were evaluated at 24 h after the permanent middle cerebral artery occlusion for brain infarction and neurological deficit score. At 72 h after the occlusion, animals were evaluated for locomotor activity, working memory, and short-term aversive memory; long-term aversive memory was evaluated 24 h after the evaluation of short-term aversive memory. Finally, at 120 h after the event, spatial memory and the expression levels of synaptophysin (SYP), SNAP-25, and caspase 3 were evaluated. The treatment with CA reduced the infarcted area and improved neurological deficit scores. There was no difference in locomotor activity between groups. The working, spatial, and long-term aversive memory deficits improved with CA. Furthermore, western blotting data showed that the expression of SYP, which correlates with synaptic formation and function, decreased after ischemic insult, and CA inhibited the reduction of SYP expression. Ischemia also increased, and CA treatment decreased, caspase 3 expression. These results suggest that CA exerts neuroprotective and antidementia effects, at least in part, by preventing the loss of neural cells and synapses in ischemic brain injury.

The SNARE Sec22b has a non-fusogenic function in plasma membrane expansion

Development of the nervous system requires extensive axonal and dendritic growth during which neurons massively increase their surface area. Here we report that the endoplasmic reticulum (ER)-resident SNARE Sec22b has a conserved non-fusogenic function in plasma membrane expansion. Sec22b is closely apposed to the plasma membrane SNARE syntaxin1. Sec22b forms a trans-SNARE complex with syntaxin1 that does not include SNAP23/25/29, and does not mediate fusion. Insertion of a long rigid linker between the SNARE and transmembrane domains of Sec22b extends the distance between the ER and plasma membrane, and impairs neurite growth but not the secretion of VSV-G. In yeast, Sec22 interacts with lipid transfer proteins, and inhibition of Sec22 leads to defects in lipid metabolism at contact sites between the ER and plasma membrane. These results suggest that close apposition of the ER and plasma membrane mediated by Sec22 and plasma membrane syntaxins generates a non-fusogenic SNARE bridge contributing to plasma membrane expansion, probably through non-vesicular lipid transfer.

The α-subunit of the trimeric GTPase Go2 regulates axonal growth

The Goα splice variants Go1α and Go2α are subunits of the most abundant G-proteins in brain, Go1 and Go2. Only a few interacting partners binding to Go1α have been described so far and splice variant-specific differences are not known. Using a yeast two-hybrid screen with constitutively active Go2α as bait, we identified Rap1GTPase activating protein (Rap1GAP) and Girdin as interacting partners of Go2α, which was confirmed by co-immunoprecipitation. Comparison of subcellular fractions from brains of wild type and Go2α-/- mice revealed no differences in the overall expression level of Girdin or Rap1GAP. However, we found higher amounts of active Rap1-GTP in brains of Go2α deficient mutants, indicating that Go2α may increase Rap1GAP activity, thereby effecting the Rap1 activation/deactivation cycle. Rap1 has been shown to be involved in neurite outgrowth and given a Rap1GAP-Go2α interaction, we found that the loss of Go2α affected axonal outgrowth. Axons of cultured cortical and hippocampal neurons prepared from embryonic Go2α-/- mice grew longer and developed more branches than those from wild-type mice. Taken together, we provide evidence that Go2α regulates axonal outgrowth and branching.

The Onecut transcription factor HNF-6 regulates in motor neurons the formation of the neuromuscular junctions

The neuromuscular junctions are the specialized synapses whereby spinal motor neurons control the contraction of skeletal muscles. The formation of the neuromuscular junctions is controlled by a complex interplay of multiple mechanisms coordinately activated in motor nerve terminals and in their target myotubes. However, the transcriptional regulators that control in motor neurons the genetic programs involved in neuromuscular junction development remain unknown. Here, we provide evidence that the Onecut transcription factor HNF-6 regulates in motor neurons the formation of the neuromuscular junctions. Indeed, adult Hnf6 mutant mice exhibit hindlimb muscle weakness and abnormal locomotion. This results from defects of hindlimb neuromuscular junctions characterized by an abnormal morphology and defective localization of the synaptic vesicle protein synaptophysin at the motor nerve terminals. These defects are consequences of altered and delayed formation of the neuromuscular junctions in newborn mutant animals. Furthermore, we show that the expression level of numerous regulators of neuromuscular junction formation, namely agrin, neuregulin-2 and TGF-ß receptor II, is downregulated in the spinal motor neurons of Hnf6 mutant newborn animals. Finally, altered formation of neuromuscular junction-like structures in a co-culture model of wildtype myotubes with mutant embryonic spinal cord slices is rescued by recombinant agrin and neuregulin, indicating that depletion in these factors contributes to defective neuromuscular junction development in the absence of HNF-6. Thus, HNF-6 controls in spinal motor neurons a genetic program that coordinates the formation of hindlimb neuromuscular junctions.

Progesterone influences postischemic synaptogenesis in the CA1 region of the hippocampus in rats

Synaptogenesis is considered necessary for learning and memory. Recently, it has been suggested that progesterone (PROG) effects synaptogenesis of the cerebellar Purkinje cell, helps alleviate symptoms of multiple sclerosis and helps arrest spinal cord neurodegeneration. However, it is unclear whether PROG influences synaptic plasticity in central nervous system neurons after global cerebral ischemia. The purpose of the present study was to reveal PROG's influence on postischemic synaptogenesis in the CA1 region of the hippocampus in rats. Global cerebral ischemia was induced in male Sprague-Dawley rats by the 4-vessel occlusion (4-VO) method. To determine the amount of synaptogenesis, growth-associated protein 43 (GAP-43) and synaptophysin (SYP) expression were examined by immunohistochemical, reverse transcription-polymerase chain reaction (RT-PCR) and western blot techniques. Histological and behavioral tests were used to indicate the effect of PROG on global cerebral ischemia. Recovery times were 3, 7, 14, 21, and 35 days after surgery. We found that PROG increased the expression of GAP-43 and SYP. In addition, there was a significant increase in neuronal cell density and improvement ability to remain on an accelerating rotarod observed in the 4-VO rats treated with PROG compared to vehicle. We propose that PROG helped enable synaptogenesis in the CA1 region of therat hippocampus after global cerebral ischemia in rats.

Synaptic Network Activity Induces Neuronal Differentiation of Adult Hippocampal Precursor Cells through BDNF Signaling

Adult hippocampal neurogenesis is regulated by activity. But how do neural precursor cells in the hippocampus respond to surrounding network activity and translate increased neural activity into a developmental program? Here we show that long-term potentiation (LTP)-like synaptic activity within a cellular network of mature hippocampal neurons promotes neuronal differentiation of newly generated cells. In co-cultures of precursor cells with primary hippocampal neurons, LTP-like synaptic plasticity induced by addition of glycine in Mg(2+)-free media for 5 min, produced synchronous network activity and subsequently increased synaptic strength between neurons. Furthermore, this synchronous network activity led to a significant increase in neuronal differentiation from the co-cultured neural precursor cells. When applied directly to precursor cells, glycine- and Mg(2+)-free solution did not induce neuronal differentiation. Synaptic plasticity-induced neuronal differentiation of precursor cells was observed in the presence of GABAergic neurotransmission blockers but was dependent on NMDA-mediated Ca(2+) influx. Most importantly, neuronal differentiation required the release of brain-derived neurotrophic factor (BDNF) from the underlying substrate hippocampal neurons as well as TrkB receptor phosphorylation in precursor cells. This suggests that activity-dependent stem cell differentiation within the hippocampal network is mediated via synaptically evoked BDNF signaling.

Protein tyrosine phosphatases expression during development of mouse superior colliculus

Protein tyrosine phosphatases (PTPs) are key regulators of different processes during development of the central nervous system. However, expression patterns and potential roles of PTPs in the developing superior colliculus remain poorly investigated. In this study, a degenerate primer-based reverse transcription-polymerase chain reaction (RT-PCR) approach was used to isolate seven different intracellular PTPs and nine different receptor-type PTPs (RPTPs) from embryonic E15 mouse superior colliculus. Subsequently, the expression patterns of 11 PTPs (TC-PTP, PTP1C, PTP1D, PTP-MEG2, PTP-PEST, RPTPJ, RPTPε, RPTPRR, RPTPσ, RPTPκ and RPTPγ) were further analyzed in detail in superior colliculus from embryonic E13 to postnatal P20 stages by quantitative real-time RT-PCR, Western blotting and immunohistochemistry. Each of the 11 PTPs exhibits distinct spatiotemporal regulation of mRNAs and proteins in the developing superior colliculus suggesting their versatile roles in genesis of neuronal and glial cells and retinocollicular topographic mapping. At E13, additional double-immunohistochemical analysis revealed the expression of PTPs in collicular nestin-positive neural progenitor cells and RC-2-immunoreactive radial glia cells, indicating the potential functional importance of PTPs in neurogenesis and gliogenesis.

Sept8 controls the binding of vesicle-associated membrane protein 2 to synaptophysin

Septins, a conserved family of GTP/GDP-binding proteins, are present in organisms as diverse as yeast and mammals. We analyzed the distribution of five septins, Sept6, Sept7, Sept8, Sept9 and Sept11, in various rat tissues by western blot analyses and found all septins to be expressed in brain. We also examined the developmental changes of expression of these septins in the rat brain and found that the level of Sept8 increased during post-natal development. Morphological analyses revealed that Sept8 is enriched at pre-synapses. Using yeast two-hybrid screening, we identified vesicle-associated membrane protein 2 (VAMP2), a soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE), as an interacting protein for Sept8. Synaptophysin is reported to associate with and recruit VAMP2 to synaptic vesicles and dissociate prior to forming the SNARE complex consisting of VAMP2, syntaxin and synaptosome-associated protein of 25 kDa. We showed that Sept8 suppresses the interaction between VAMP2 and synaptophysin through binding to VAMP2. In addition, we found that Sept8 forms a complex with syntaxin1A, and the Sept8-VAMP2 interaction is disrupted by synaptosome-associated protein of 25 kDa. These results suggest that Sept8 may participate in the process of the SNARE complex formation and subsequent neurotransmitter release.

Early exposure to general anesthesia causes significant neuronal deletion in the developing rat brain

Frequent exposure of children to general anesthesia is common practice in modern medicine. Although previously unrecognized, recent in vitro and in vivo animal studies suggest that exposure to clinically relevant general anesthetics at the peak of brain development could be detrimental to immature mammalian neurons, as demonstrated by massive and widespread apoptotic neurodegeneration. The survival of the developing neurons presumably depends on proper and timely formation of synapses, for which synaptic proteins (e.g., synaptophysin, synaptobrevin, amphiphysin, synaptosomal-associated protein 25 [SNAP-25], and Ca(2+)/calmodulin-dependent protein kinase II [CaM kinase II]) are crucially important. Overinhibition of developing neurons impairs synaptic protein function and activity-induced synaptic plasticity, which could in turn result in permanent neuronal loss. To examine the effects of general anesthesia, the pharmacological agents known to cause extensive neuronal inhibition, on synaptic proteins, and neuronal survival at the peak of synaptogenesis, we exposed 7-day-old rat pups to general anesthesia (midazolam, 9 mg/kg of body weight, subcutaneously, followed by 6 h of nitrous oxide 75 vol% and isoflurane 0.75 vol%). We found that this general anesthesia causes permanent neuronal deletion in the most vulnerable brain regions-the cerebral cortex and the thalamus-while transiently modulating protein levels of synaptophysin, synaptobrevin, amphiphysin, SNAP-25, and CaM kinase II.

Statins increase neurogenesis in the dentate gyrus, reduce delayed neuronal death in the hippocampal CA3 region, and improve spatial learning in rat after traumatic brain injury

Traumatic brain injury (TBI) remains a major public health problem globally. Presently, there is no way to restore cognitive deficits caused by TBI. In this study, we seek to evaluate the effect of statins (simvastatin and atorvastatin) on the spatial learning and neurogenesis in rats subjected to controlled cortical impact. Rats were treated with atorvastatin and simvastatin 1 day after TBI and daily for 14 days. Morris water maze tests were performed during weeks 2 and 5 after TBI. Bromodeoxyuridine (BrdU; 50 mg/kg) was intraperitoneally injected 1 day after TBI and daily for 14 days. Brain tissue was processed for immunohistochemical staining to identify newly generated cells and vessels. Our data show that (1) treatment of TBI with statins improves spatial learning on days 31-35 after onset of TBI; (2) in the non-neurogenic region of the hippocampal CA3 region, statin treatment reduces the neuronal loss after TBI, demonstrating the neuroprotective effect of statins; (3) in the neurogenic region of the dentate gyrus, treatment of TBI with statins enhances neurogenesis; (4) statin treatment augments TBI-induced angiogenesis; and (5) treatment with simvastatin at the same dose provides a therapeutic effect superior to treatment with atorvastatin. These results suggest that statins may be candidates for treatment of TBI.

Synaptic tetraspan vesicle membrane proteins are conserved but not needed for synaptogenesis and neuronal function in Caenorhabditis elegans

Tetraspan vesicle membrane proteins (TVPs) comprise a major portion of synaptic vesicle proteins, yet their contribution to the synaptic vesicle cycle is poorly understood. TVPs are grouped in three mammalian gene families: physins, gyrins, and secretory carrier-associated membrane proteins (SCAMPs). In Caenorhabditis elegans, only a single member of each of these families exists. These three nematode TVPs colocalize to the same vesicular compartment when expressed in mammalian cells, suggesting that they could serve overlapping functions. To examine their function, C. elegans null mutants were isolated for each gene, and a triple mutant was generated. Surprisingly, these animals develop normally and exhibit normal neuronal architecture and synaptic contacts. In addition, functions of the motor and sensory systems are normal as determined by pharmacological, chemotaxis, and thermotaxis assays. Finally, direct electrophysiological analysis of the neuromuscular junction revealed no phenotype in the TVP mutants. We therefore conclude that TVPs are not needed for the basic neuronal machinery and instead may contribute to subtle higher order functions.

Transcriptome comparison of murine wild-type and synaptophysin-deficient retina reveals complete identity

Loss of synaptophysin, one of the major synaptic vesicle membrane proteins, is surprisingly well tolerated in knockout mice. To test whether compensatory gene transcription accounts for the apparent lack of functional deficiencies, comparative transcriptome analyses were carried out. The retina was selected as the most suitable tissue since morphological alterations were observed in mutant photoreceptors, most notably a reduction of synaptic vesicles and concomitant increase in clathrin-coated vesicles. Labeled cRNA was prepared in triplicate from retinae of age- and sex-matched wild-type and mutant litter mates and hybridized to high-density microarray chips. Only three differentially expressed RNAs were identified in this way, one of which was synaptophysin. Further validation by quantitative RT-PCR could only corroborate the results for the latter. We therefore conclude that, despite the distinct morphological phenotype, no significant changes in gene expression are detectable in synaptophysin-deficient animals and that therefore compensatory mechanisms are either pre-existent and/or act at the posttranscriptional level.

Immunohistochemical and microarray analyses of a mouse model for the smith-lemli-opitz syndrome

The Smith-Lemli-Opitz syndrome is a mental retardation/malformation syndrome with behavioral components of autism. It is caused by a deficiency in 3beta-hydroxysteroid-Delta7-reductase (DHCR7), the enzyme required for the terminal enzymatic step of cholesterol biosynthesis. The availability of Smith-Lemli-Opitz syndrome mouse models has made it possible to investigate the genesis of the malformations associated with this syndrome. Dhcr7 gene modification (Dhcr7-/-) results in neonatal lethality and multiple organ system malformations. Pathology includes cleft palate, pulmonary hypoplasia, cyanosis, impaired cortical response to glutamate, and hypermorphic development of hindbrain serotonergic neurons. For the current study, hindbrain regions microdissected from gestational day 14 Dhcr7-/-, Dhcr7+/- and Dhcr7+/+ fetuses were processed for expression profiling analyses using Affymetrix oligonucleotide arrays and filtered using statistical significance (S-score) of change in gene expression. Of the 12,000 genes analyzed, 91 were upregulated and 98 were downregulated in the Dhcr7-/- hindbrains when compared to wild-type animals. Fewer affected genes, representing a reduced affect on these pathways, were identified in heterozygous animals. Hierarchical clustering identified altered expression of genes associated with cholesterol homeostasis, cell cycle control and apoptosis, neurodifferentiation and embryogenesis, transcription and translation, cellular transport, neurodegeneration, and neuronal cytoskeleton. Of particular interest, Dhcr7 gene modification elicited dynamic changes in genes involved in axonal guidance. In support of the microarray findings, immunohistochemical analyses of the netrin/deleted in colorectal cancer axon guidance pathway illustrated midline commissural deficiencies and hippocampal pathfinding errors in Dhcr7-/- mice. The results of these studies aid in providing insight into the genesis of human cholesterol-related birth defects and neurodevelopmental disorders and highlight specific areas for future investigation.

The C-terminal transmembrane region of synaptobrevin binds synaptophysin from adult synaptic vesicles

Synaptophysin and synaptobrevin are abundant membrane proteins of neuronal small synaptic vesicles. In mature, differentiated neurons they form the synaptophysin/synaptobrevin (Syp/Syb) complex. Synaptobrevin also interacts with the plasma membrane-associated proteins syntaxin and SNAP25, thereby forming the SNARE complex necessary for exocytotic membrane fusion. The two complexes are mutually exclusive. Synaptobrevin is a C-terminally membrane-anchored protein with one transmembrane domain. While its interaction with its SNARE partners is mediated exclusively by its N-terminal cytosolic region it has been unclear so far how binding to synaptophysin is accomplished. Here, we show that synaptobrevin can be cleaved in its synaptophysin-bound form by tetanus toxin and botulinum neurotoxin B, or by botulinum neurotoxin D, leaving shorter or longer C-terminal peptide chains bound to synaptophysin, respectively. A recombinant, C-terminally His-tagged synaptobrevin fragment bound to nickel beads specifically bound synaptophysin, syntaxin and SNAP25 from vesicular detergent extracts. After cleavage by tetanus toxin or botulinum toxin D light chain, the remaining C-terminal fragment no longer interacted with syntaxin or SNAP 25. In contrast, synaptophysin was still able to bind to the residual C-terminal synaptobrevin cleavage product. In addition, the His-tagged C-terminal synaptobrevin peptide 68-116 was also able to bind synaptophysin in detergent extracts from adult brain membranes. These data suggest that synaptophysin interacts with the C-terminal transmembrane part of synaptobrevin, thereby allowing the N-terminal cytosolic chain to interact freely with the plasma membrane-associated SNARE proteins. Thus, by binding synaptobrevin, synaptophysin may positively modulate neurotransmission.

Effects of brain-derived neurotrophic factor (BDNF) on glial cells and serotonergic neurones during development

Serotonergic neurones are among the first to develop in the central nervous system. Their survival and maturation is promoted by a variety of factors, including serotonin itself, brain-derived neurotrophic factor (BDNF) and S100beta, an astrocyte-specific Ca(2+) binding protein. Here, we used BDNF-deficient mice and cell cultures of embryonic raphe neurones to determine whether or not BDNF effects on developing serotonergic raphe neurones are influenced by its action on glial cells. In BDNF-/- mice, the number of serotonin-immunoreactive neuronal somata, the amount of the serotonin transporter, the serotonin content in the striatum and the hippocampus, and the content of 5-hydroxyindoleacetic acid in all brain regions analysed were increased. By contrast, reduced immunoreactivity was found for myelin basic protein (MBP) in all brain areas including the raphe and its target region, the hippocampus. Exogenously applied BDNF increased the number of MBP-immunopositive cells in the respective culture systems. The raphe area displayed selectively reduced immunoreactivity for S100beta. Accordingly, S100beta was increased in primary cultures of pure astrocytes by exogenous BDNF. In glia-free neuronal cultures prepared from the embryonic mouse raphe, addition of BDNF supported the survival of serotonergic neurones and increased the number of axon collaterals and primary dendrites. The latter effect was inhibited by the simultaneous addition of S100beta. These results suggest that the presence of BDNF is not a requirement for the survival and maturation of serotonergic neurones in vivo. BDNF is, however, required for the local expression of S100beta and production of MBP. Therefore BDNF might indirectly influence the development of the serotonergic system by stimulating the expression of S100beta in astrocytes and the production MBP in oligodendrocytes.

Stimulus-dependent dynamic homo- and heteromultimerization of synaptobrevin/VAMP and synaptophysin

Synaptophysin and synaptobrevin/VAMP are abundant synaptic vesicle proteins that form homo- and heterooligomers. We now use chemical cross-linking in synaptosomes, pinched-off nerve terminals that are capable of stimulus-dependent neurotransmitter release, to investigate whether these complexes are regulated. We show that in synaptosomes treated with three stimuli that induce exocytosis (a depolarizing K(+) solution, the excitatory neurotoxin alpha-latrotoxin, or the Ca(2+)-ionophore ionomycin), the homo- and heteromultimerization of synaptophysin and synaptobrevin is increased up to 6-fold. Whereas at rest less than 10% of the total synaptobrevin and synaptophysin could be chemically cross-linked into homo- and heteromeric complexes, after stimulation up to 25% of synaptobrevin and synaptophysin are present in homo- and heteromultimers, suggesting that a large fraction of these synaptic vesicle proteins physiologically participate in such complexes. The increase in multimerization of synaptophysin and synaptobrevin was only observed in intact but not in lysed synaptosomes and could not be inhibited by general kinase or phosphatase inhibitors. The stimulus dependence of synaptophysin and synaptobrevin multimers indicates that the complexes are not composed of a fixed multisubunit structure, for example, as an ion channel, but represent distinct functional states of synaptobrevin and synaptophysin that are modulated in parallel with synaptic vesicle exo- and endocytosis.

The synaptophysin/synaptobrevin complex dissociates independently of neuroexocytosis

Synaptophysin is one of the most abundant membrane proteins of small synaptic vesicles. In mature nerve terminals it forms a complex with the vesicular membrane protein synaptobrevin, which appears to modulate synaptobrevin's interaction with the plasma membrane-associated proteins syntaxin and SNAP25 to form the SNARE complex as a prerequisite for membrane fusion. Here we show that synaptobrevin is preferentially cleaved by tetanus toxin while bound to synaptophysin or when existing as a homodimer. The synaptophysin/synaptobrevin complex is, however, not affected when neuronal secretion is blocked by botulinum A toxin which cleaves SNAP25. Excessive stimulation with alpha-latrotoxin or Ca(2+)-ionophores dissociates the synaptophysin/synaptobrevin complex and increases the interaction of the other SNARE proteins. The stimulation-induced dissociation of the synaptophysin/synaptobrevin complex is not inhibited by pre-incubating neurones with botulinum A toxin, but depends on extracellular calcium. However, the synaptophysin/synaptobrevin complex cannot be directly dissociated by calcium alone or in combination with magnesium. The dissociation of synaptobrevin from synaptophysin appears to precede its interaction with the other SNARE proteins and does not depend on the final fusion event. This finding further supports the modulatory role the synaptophysin/synaptobrevin complex may play in mature neurones.

Synaptophysin I controls the targeting of VAMP2/synaptobrevin II to synaptic vesicles

Synaptic vesicle (SV) proteins are synthesized at the level of the cell body and transported down the axon in membrane precursors of SVs. To investigate the mechanisms underlying sorting of proteins to SVs, fluorescent chimeras of vesicle-associated membrane protein (VAMP) 2, its highly homologous isoform VAMP1 and synaptotagmin I (SytI) were expressed in hippocampal neurons in culture. Interestingly, the proteins displayed a diffuse component of distribution along the axon. In addition, VAMP2 was found to travel in vesicles that constitutively fuse with the plasma membrane. Coexpression of VAMP2 with synaptophysin I (SypI), a major resident of SVs, restored the correct sorting of VAMP2 to SVs. The effect of SypI on VAMP2 sorting was dose dependent, being reversed by increasing VAMP2 expression levels, and highly specific, because the sorting of the SV proteins VAMP1 and SytI was not affected by SypI. The cytoplasmic domain of VAMP2 was found to be necessary for both the formation of VAMP2-SypI hetero-dimers and for VAMP2 sorting to SVs. These data support a role for SypI in directing the correct sorting of VAMP2 in neurons and demonstrate that a direct interaction between the two proteins is required for SypI in order to exert its effect.

The expression pattern and assembly profile of synaptic membrane proteins in ribbon synapses of the developing mouse retina

In the present study, we generated a systematic overview of the expression pattern and assembly profile of synaptic membrane proteins in ribbon synapses of the developing mouse retina. Using indirect immunofluorescence microscopy, we analyzed the spatial and temporal distribution of 11 important membrane and membrane-associated synaptic proteins (syntaxin 1/3, SNAP-25, synaptobrevin 2, synaptogyrin, synaptotagmin I, SV2A, SV2B, Rab3A, clathrin light chains, CSP and neuroligin I) during synaptogenesis. The temporospatial distribution of these synaptic proteins was "normalized" by the simultaneous visualization of the synaptic vesicle protein synaptophysin, which served as an internal reference protein. We found that expression of various synaptic membrane proteins started at different time points and changed progressively during development. At early stages of development synaptic vesicle membrane proteins at extrasynaptic locations did not always colocalize with synaptophysin, indicating that these proteins probably do not reside in the same transport vesicles. Despite a non-synchronized onset of protein expression, clustering and colocalization of all synaptic membrane proteins at ribbon synapses roughly occurred in the same time window (between day 4 after birth, P4, and P5). Thus, the basic synaptic membrane machinery is already present in ribbon synapses before the well-known complete morphological maturation of ribbon synapses between P7 and P12. We conclude that ribbon synapse formation is a multistep process in which the concerted recruitment of synaptic membrane proteins is a relatively early event and clearly not the final step.

The synaptophysin/synaptobrevin interaction critically depends on the cholesterol content

Synaptophysin interacts with synaptobrevin in membranes of adult small synaptic vesicles. The synaptophysin/synaptobrevin complex promotes synaptobrevin to built up functional SNARE complexes thereby modulating synaptic efficiency. Synaptophysin in addition is a cholesterol-binding protein. Depleting the membranous cholesterol content by filipin or beta-methylcyclodextrin (beta-MCD) decreased the solubility of synaptophysin in Triton X-100 with less effects on synaptobrevin. In small synaptic vesicles from rat brain the synaptophysin/synaptobrevin complex was diminished upon beta-MCD treatment as revealed by chemical cross-linking. Mice with a genetic mutation in the Niemann-Pick C1 gene developing a defect in cholesterol sorting showed significantly reduced amounts of the synaptophysin/synaptobrevin complex compared to their homo- or heterozygous littermates. Finally when using primary cultures of mouse hippocampus the synaptophysin/synaptobrevin complex was down-regulated after depleting the endogenous cholesterol content by the HMG-CoA-reductase inhibitor lovastatin. Alternatively, treatment with cholesterol up-regulated the synaptophysin/synaptobrevin interaction in these cultures. These data indicate that the synaptophysin/synaptobrevin interaction critically depends on a high cholesterol content in the membrane of synaptic vesicles. Variations in the availability of cholesterol may promote or impair synaptic efficiency by interfering with this complex.

Activity-dependent changes of the presynaptic synaptophysin-synaptobrevin complex in adult rat brain

The vesicular protein synaptobrevin contributes to two mutually exclusive complexes in mature synapses. Synaptobrevin tightly interacts with the plasma membrane proteins syntaxin and SNAP 25 forming the SNARE complex as a prerequisite for exocytotic membrane fusion. Alternatively, synaptobrevin binds to the vesicular protein synaptophysin. It is unclear whether SNARE complex formation is diminished or facilitated when synaptobrevin is bound to synaptophysin. Here we show that the synaptophysin-synaptobrevin complex is increased in adult rat brain after repeated synaptic hyperactivity in the kindling model of epilepsy. Two days after the last kindling-induced stage V seizure the relative amount of synaptophysin-synaptobrevin complex obtained by co-immunoprecipitation from cortical and hippocampal membranes was increased twofold compared to controls. By contrast the relative amounts of various synaptic proteins as well as that of the SNARE complex did not change in membrane preparations from kindled rats compared to controls. The increased amount of synaptophysin-synaptobrevin complex in kindled rats supports the idea that this complex represents a reserve pool for synaptobrevin enabling synaptic vesicles to adjust to an increased demand for synaptic efficiency. We conclude that the synaptophysin-synaptobrevin interaction is involved in activity-dependent plastic changes in adult rat brain.

Chronic blockade of glutamate receptors enhances presynaptic release and downregulates the interaction between synaptophysin-synaptobrevin-vesicle-associated membrane protein 2

During development of neuronal circuits, presynaptic and postsynaptic functions are adjusted in concert, to optimize interneuronal signaling. We have investigated whether activation of glutamate receptors affects presynaptic function during synapse formation, when constitutive synaptic vesicle recycling is downregulated. Using primary cultures of hippocampal neurons as a model system, we have found that chronic exposure to both NMDA and non-NMDA glutamate receptor blockers during synaptogenesis produces an increase in miniature EPSC (mEPSC) frequency, with no significant changes in mEPSC amplitude or in the number of synapses. Enhanced synaptic vesicle recycling, selectively in glutamatergic nerve terminals, was confirmed by the increased uptake of antibodies directed against the lumenal domain of synaptotagmin. No increased uptake was detected in neuronal cultures grown in the chronic presence of TTX, speaking against an indirect effect caused by decreased electrical activity. Enhanced mEPSC frequency correlated with a reduction of synaptophysin-synaptobrevin-vesicle-associated membrane protein 2 (VAMP2) complexes detectable by immunoprecipitation. Intracellular perfusion with a peptide that inhibits the binding of synaptophysin to synaptobrevin-VAMP2 induced a remarkable increase of mEPSC frequency in control but not in glutamate receptor blocker-treated neurons. These findings suggest that activation of glutamate receptors plays a role in the downregulation of the basal rate of synaptic vesicle recycling that accompanies synapse formation. They also suggest that one of the mechanisms through which this downregulation is achieved is an increased interaction of synaptophysin with synaptobrevin-VAMP2.