The synaptophysin-synaptobrevin complex: a hallmark of synaptic vesicle maturation

Neural stem/progenitor cells transplanted to the hypoglossal nucleus integrates with the host CNS in adult rats and promotes motor neuron survival

Transplantation of neural stem cells and the mobilization of endogenous neuronal precursors in the adult brain have been proposed as therapeutic strategies for central nervous system disorders and injuries. The aim of the present study was to investigate the possible survival and integration of grafted neural progenitor cells (NPCs) from the subventricular zone (SVZ) in a hypoglossal nerve avulsion model with substantial neuronal loss. Adult neural progenitor cells (NPCs) from the subventricular zone (SVZ) were cultured from inbred transgenic eGFP Lewis rats and transplanted to the hypoglossal nucleus of inbred Lewis rat from the same family but that were not carrying the eGFP strain after avulsion of the hypoglossal nerve. Grafted cells survived in the host more than 3 months and differentiated into neurons [βIII tubulin (Tuj-1 staining)] with fine axon- and dendrite-like processes as well as astrocytes (GFAP) and oligodendrocytes (O4) with typical morphology. Staining for synaptic structures (synaptophysin and bassoon) indicated integration of differentiated cells from the graft with the host CNS. Furthermore, transplantation of NPCs increased the number of surviving motoneurons in the hypoglossal nucleus after nerve avulsion that, if untreated, result in substantial neuronal death. The NPCs used in this study expressed VEGF in vitro as well as in vivo following transplantation that may mediate the rescue effect of the axotomized motoneurons.

Neurotoxic phospholipases directly affect synaptic vesicle function

Snake neurotoxic phospholipases (SPAN) exclusively affect pre-synaptic nerve terminals where they lead to a block of neurotransmission by not fully understood mechanisms. Here, we report that the SPANs, taipoxin and paradoxin, in nanomolar concentrations directly dissociate the synaptophysin/synaptobrevin (Syp/Syb) complex on isolated synaptic vesicles in the presence of synaptosomal cytosol. The phospholipase activity of SPANs depends on Ca(2+) but the dissociation of the Syp/Syb complex does not require Ca(2+). Ca(2+) (100 μM free) alone also dissociates the Syp/Syb complex in the presence of cytosol. Treatment with SPANs disturbs the lipid raft association of synaptophysin and synaptobrevin comparable to cholesterol depletion by β-methyl-cyclodextrin while Ca(2+) alone has no effect. SPANs but not Ca(2+) directly inhibit vesicular uptake of serotonin and glutamate. It is concluded that SPANs directly affect vesicular properties independent from their Ca(2+) -dependent phospholipase activity. SPANs and Ca(2+) dissociate the Syp/Syb complex as a prerequisite for exocytosis. SPANs also prevent the filling of synaptic vesicles thereby adding to the inhibition of neurotransmission.

Normal gut microbiota modulates brain development and behavior

Microbial colonization of mammals is an evolution-driven process that modulate host physiology, many of which are associated with immunity and nutrient intake. Here, we report that colonization by gut microbiota impacts mammalian brain development and subsequent adult behavior. Using measures of motor activity and anxiety-like behavior, we demonstrate that germ free (GF) mice display increased motor activity and reduced anxiety, compared with specific pathogen free (SPF) mice with a normal gut microbiota. This behavioral phenotype is associated with altered expression of genes known to be involved in second messenger pathways and synaptic long-term potentiation in brain regions implicated in motor control and anxiety-like behavior. GF mice exposed to gut microbiota early in life display similar characteristics as SPF mice, including reduced expression of PSD-95 and synaptophysin in the striatum. Hence, our results suggest that the microbial colonization process initiates signaling mechanisms that affect neuronal circuits involved in motor control and anxiety behavior.

Neurorestorative treatments for traumatic brain injury

Traumatic brain injury (TBI) remains a major cause of death and permanent disability worldwide, especially in children and young adults. A total of 1.5 million people experience head trauma each year in the United States, with an annual economic cost exceeding $56 billion. Unfortunately, almost all Phase III TBI clinical trials have yet to yield a safe and effective neuroprotective treatment, raising questions regarding the use of neuroprotective strategies as the primary therapy for acute brain injuries. Recent preclinical data suggest that neurorestorative strategies that promote angiogenesis (formation of new blood vessels from pre-existing endothelial cells), axonal remodeling (axonal sprouting and pruning), neurogenesis (generation of new neurons) and synaptogenesis (formation of new synapses) provide promising opportunities for the treatment of TBI. This review discusses select cell-based and pharmacological therapies that activate and amplify these endogenous restorative brain plasticity processes to promote both repair and regeneration of injured brain tissue and functional recovery after TBI.

Research progress on the age-related changes in proteins of the synaptic active zone

Neurotransmitter release during synaptic transmission is mediated by the presynaptic active zone. Multiple protein components at the active zone region interact to regulate docking, priming and fusion of the synaptic vesicles with the presynaptic membrane to maintain normal neurotransmitter release. This review discusses research progress in questions of protein transcript and expression pattern changes at the synaptic active zone related to aging and whether these changes have the effects on learning and memory. We will specifically address normal synaptic structure and proteins; active zone structure and components; active zone functional regulation and age-related changes in active zone proteins.

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.

Protein expression is altered during spontaneous sleep in aged Sprague Dawley rats

Age-related changes in brain function include those affecting learning, memory, and sleep-wakefulness. Sleep-wakefulness is an essential behavior that results from the interaction of multiple brain regions, peptides, and neurotransmitters. The biological function(s) of sleep, however, remains unknown due to a paucity of information available at the cellular level. Aged rats exhibit alterations in the circadian and homeostatic influences associated with sleep-wake regulation. We recently showed that alterations in cortical profiles occur after timed bouts of spontaneous sleep in young rats. Examination of the cellular response to sleep-wake in old rats may thus provide insight(s) into the biological function(s) of sleep. To test this hypothesis, we monitored cortical profiles in the frontal cortex of young and old Sprague-Dawley rats after timed bouts of spontaneous sleep-wake behavior. Proteins were separated by two-dimensional electrophoresis (2-DE), visualized by fluorescent staining, imaged, and analyzed as a function of behavioral state and age. Old rats showed a 6-fold increase in total protein expression, independent of the behavioral state at sacrifice. When analyzed according to age and behavioral state, there was a decrease (approximately 46%) in the number of phospho-spots present during SWS in aged animals. SWS-associated spots present only in old animals were associated with multiple functions including vesicular transport, cell signaling, oxidation state, cytoskeletal support, and energy metabolism. These data suggest that the intracellular response to the signaling associated with spontaneous sleep is affected by age and is consistent with the idea that the ability of sleep to fulfill its function(s) may become diminished with age.

Immunocytochemical detection of synaptophysin in C57BL/6 mice cochlea during aging process

Aged mammals frequently exhibit a bilateral, progressive, and symmetric deafness related to the degeneration of auditory receptor. However, little is still known about aging effects on synapses in this receptor. Synaptophysin (Syp) is a 38 kDa Ca2+ binding glycoprotein widely found in presynaptic membrane and vesicles. The Syp has been found in presynaptic buttons of efferent auditory fibers, within the developing and adult auditory receptor. The detection of Syp in aged cochleae could provide relevant information about synaptic changes and receptor degeneration process observed in old animals. This paper focuses on aging linked changes related to the presence of Syp in cochleae of C57BL/6J mice (from 1 to 24 months old). Results showed that during the first months of age, no significant changes were observed in the Syp distribution under the basal pole of inner (IHCs) neither the outer (OHCs) hair cells. At six months of age, a significant decrease of Syp immunocytochemical detection appeared in fibers under the most external row of OHCs, but restricted to the cochlear basal coil. Only a very scarce reduction of Syp was noted under the IHC and the other OHC rows, also at the basal coil. From mice 9 months old on, a progressive decrease of the presence of Syp was found under IHC and all OHC rows starting at the basal coil and reaching the apical coil in the oldest mice. All these data could indicate that the cochlea aging process early affects to presynaptic membrane proteins of efferent endings fibers. This early alteration of cochleae efferent synapses could be involved in the whole degeneration of the Corti's organ.

Synaptic plasticity: the new explanation of visceral hypersensitivity in rats with Trichinella spiralis infection?

BACKGROUND AND AIM

Synaptic plasticity plays an important role in affecting the intensity of visceral reflex. It may also be involved in the development of visceral hypersensitivity. The aim of this study was to investigate the role of synaptic plasticity on visceral hypersensitivity of rats infected by Trichinella spiralis.

METHODS

Thirty male Sprague-Dawley (SD) rats were randomly divided into control, acute, and chronic infection groups, and were investigated at 1 week after adaptive feeding and at 2 and 8 weeks post infection (PI) by oral administration of 1 ml phosphate-buffered saline (PBS) containing 8,000 Trichinella spiralis larvae. Visceral sensitivity was evaluated by electromyography (EMG) recording during colorectal distension. Intestinal inflammation was observed by hematoxylin-eosin (HE) staining. Synaptic ultrastructure parameters, such as postsynaptic density (PSD) length, synaptic cleft, and number of synaptic vesicles, were examined by transmission electron microscopy (TEM). The expression of protein associated with synaptic plasticity, including postsynaptic density-95 (PSD-95), synaptophysin, calbindin-28 K, N-methyl-D-aspartate receptor-1 (NMDA-R1), alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid receptor (AMPA-R), and glial cell line-derived neurotrophic factor (GDNF), were analyzed by Western blot.

RESULTS

(1) Visceral hypersensitivity was noted in the chronic infection group, although the inflammation was nearly eliminated (P<0.05). Severe inflammation and downregulation of visceral sensitivity were observed in the acute infection group (P<0.05). (2) There were many more synaptic vesicles and longer PSD in the chronic infection group than in the control group (P<0.05, respectively). However, in comparison with control rats, disappearance of mitochondria cristae in the synapses, and decrease of synaptic vesicles and length of PSD were observed in the acute infection group. There was no significant difference in width of synaptic cleft among the three groups. (3) Compared with the control, the expression of proteins associated with synaptic plasticity was significantly upregulated during chronic infection phase (P<0.05), and downregulated during acute infection phase.

CONCLUSION

Synaptic plasticity was observed in SD rats infected by Trichinella spiralis and was associated with visceral sensitivity, which suggests that it may play an important role in the formation of visceral hypersensitivity.

Detection of behavioral alterations and learning deficits in mice lacking synaptophysin

The integral membrane protein synaptophysin is one of the most abundant polypeptide components of synaptic vesicles. It is not essential for neurotransmission despite its abundance but is believed to modulate the efficiency of the synaptic vesicle cycle. Detailed behavioral analyses were therefore performed on synaptophysin knockout mice to test whether synaptophysin affects higher brain functions. We find that these animals are more exploratory than their wild type counterparts examining novel objects more closely and intensely in an enriched open field arena. We also detect impairments in learning and memory, most notably reduced object novelty recognition and reduced spatial learning. These deficits are unlikely caused by impaired vision, since all electroretinographic parameters measured were indistinguishable from those in wild type controls although an inverse optomotor reaction was observed. Taken together, our observations demonstrate functional consequences of synaptophysin depletion in a living organism.

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.

Ca2+-dependent activator proteins of secretion promote vesicular monoamine uptake

Ca(2+)-dependent activator proteins of secretion (CAPS) 1 and 2 are essential regulators of synaptic vesicle and large dense core vesicle priming in mammalian neurons and neuroendocrine cells. CAPS1 appears to have an additional and as yet unexplained function in vesicular catecholamine uptake or storage as CAPS1-deficient chromaffin cells exhibit strongly reduced vesicular catecholamine levels. Here we describe a role of CAPS proteins in vesicular monoamine uptake. Both CAPS1 and CAPS2 promote monoamine uptake and storage mediated by the vesicular monoamine transporters VMAT1 and VMAT2. Monoamine uptake of vesicular preparations from embryonic brains of CAPS1 deletion mutants is decreased as compared with corresponding preparations from wild type littermates, and anti-CAPS1 or anti-CAPS2 antibodies inhibit monoamine sequestration by synaptic vesicles from adult mouse brain. In addition, overexpression of CAPS1 or CAPS2 enhances vesicular monoamine uptake in Chinese hamster ovary cells that stably express VMAT1 or VMAT2. CAPS function has been linked to the heterotrimeric GTPase G(o), which modulates vesicular monoamine uptake. We found that the expression of CAPS1 is decreased in brain membrane preparations from mice lacking G(o2)alpha, which may explain the reduced monoamine uptake by G(o2)alpha-deficient synaptic vesicles. Accordingly, anti-CAPS1 antibodies do not further reduce monoamine uptake by G(o2)alpha-deficient synaptic vesicles, whereas antibodies directed against CAPS2, whose expression is not altered in G(o2)alpha-deficient brain, still reduce monoamine uptake into G(o2)alpha-deficient vesicles. We conclude that CAPS proteins are involved in optimizing vesicular monoamine uptake and storage mediated by VMAT1 and VMAT2.

Interaction assays in yeast and cultured cells confirm known and identify novel partners of the synaptic vesicle protein synaptophysin

Synaptophysin (SYP) is a major protein of neurotransmitter-containing vesicles spanning the membrane four times and contributing to various aspects of the synaptic vesicle cycle. The split-ubiquitin yeast two-hybrid system was used to characterize molecular interactions of membrane-bound, full-length murine SYP. In this way, the known homophilic SYP-SYP association could be confirmed and heterophilic binding of SYP to other tetraspan vesicle membrane proteins of the secretory carrier-associated membrane- and synaptogyrin-type could be detected for the first time. SYP-binding was also observed for the vSNARE synaptobrevin2 and various membrane and membrane-associated proteins. Double labeling immunofluorescence microscopy of murine retina, co-immunoprecipitation experiments and fluorescence energy resonance transfer (FRET) analyses between fluorescent protein-tagged polypeptides were carried out to validate and further characterize the association of SYP with the tetraspan vesicle membrane proteins secretory carrier-associated membrane protein 1 and synaptogyrin3, with synaptobrevin2, and the newly identified binding partners phospholipase D4, stathmin-like3, Rho family GTPase2 and ADP-ribosylation factor interacting protein2. It was observed that the carboxyterminus of SYP is dispensable for association with integral membrane proteins while it is needed for binding to membrane-associated polypeptides. The latter appears to be regulated by phosphorylation, since src homology 2-domains were shown to attach to the multiple carboxyterminal phosphotyrosine residues of SYP. In conclusion, the association of SYP with different tetraspan vesicle membrane proteins suggests shared functions and the multiple other interactions identify SYP as part of a membrane platform acting as a facilitator of various steps of the synaptic vesicle cycle.

Deletion of Go2alpha abolishes cocaine-induced behavioral sensitization by disturbing the striatal dopamine system

The alpha-subunits of the trimeric Go class of GTPases, comprising the splice variants Go1alpha and Go2alpha, are abundantly expressed in brain and reside on both plasma membrane and synaptic vesicles. Go2alpha is involved in the vesicular storage of monoamines but its physiological relevance is still obscure. We now show that genetic depletion of Go2alpha reduces motor activity induced by dopamine-enhancing drugs like cocaine, as repeated injections of cocaine fail to provoke behavioral sensitization in Go2alpha(-/-) mice. In Go2alpha(-/-) mice, D1 receptor signaling in the striatum is attenuated due to a reduced expression of Golf alpha and Gs alpha. Following cocaine treatment, Go2alpha(-/-) mice have lower D1 and higher D2 receptor amounts compared to wild-type mice. The lack of behavioral sensitization correlates with reduced dopamine levels in the striatum and decreased expression of tyrosine hydroxylase. One reason for the neurochemical changes may be a reduced uptake of monoamines by synaptic vesicles from Go2alpha(-/-) mice as a consequence of a lowered set point for filling. We conclude that Go2alpha optimizes vesicular filling which is instrumental for normal dopamine functioning and for the development of drug-induced behavioral sensitization.

Differential effects of prostate cancer therapeutics on neuroendocrine transdifferentiation

Androgen ablation therapy is widely used for the treatment of advanced prostate cancer. However, the effectiveness of this intervention strategy is generally short-lived as the disease ultimately progresses to a hormone-refractory state. In recent years, it has become clear that even in antiandrogen-resistant cancers the androgen receptor (AR) signaling axis is intact and is required for prostate cancer growth. Thus, there is a heightened interest in developing small molecules that function in part by down-regulating AR expression in tumors. Paradoxically, AR expression has been shown to be important in preventing the transdifferentiation of epithelial prostate cancer cells toward a neuroendocrine phenotype associated with tumor progression. Consequently, we have evaluated the relative effect of prostate cancer therapeutics that function in part by depleting AR levels on neuroendocrine differentiation in established cellular models of prostate cancer. These studies reveal that although histone deacetylase inhibitors can down-regulate AR expression they increase the expression of neuroendocrine markers and alter cellular morphology. Inhibition of AR signaling using classic AR antagonists or small interfering RNA-mediated AR ablation induces incomplete neuroendocrine differentiation. Importantly, the Hsp90 inhibitor geldanamycin effectively down-regulates AR expression while having no effect on neuroendocrine differentiation. Taken together, these data show that the phenotypic responses to pharmacologic agents used in the clinic to prevent the progression of prostate cancer are not equivalent, a finding of significant therapeutic importance.

Core proteins of the secretory machinery

Members of the Rab, SM- and SNARE-protein families play key roles in all intracellular membrane trafficking steps. While SM- and SNARE-proteins become directly involved in the fusion reaction at a late stage, Rabs and their effectors mediate upstream steps such as vesicle budding, delivery, tethering, and transport. Exocytosis of synaptic vesicles and regulated secretory granules are among the best-studied fusion events and involve the Rab3 isoforms Rab3A-D, the SM protein munc18-1, and the SNAREs syntaxin 1A, SNAP-25, and synaptobrevin 2. According to the current view, syntaxin 1A and SNAP-25 at the presynaptic membrane form a complex with synaptic vesicle-associated synaptobrevin 2. As complex formation proceeds, the opposed membranes are pulled tightly together, enforcing the fusion reaction. Munc18-1 is essential for regulated exocytosis and interacts with syntaxin 1A alone or with SNARE complexes, suggesting a role for munc18-1 in controlling the SNARE-assembly reaction. Compared to other intracellular fusion steps, special adaptations evolved in the synapse to allow for the tight regulation and high membrane turnover rates required for synaptic transmission. Synaptic vesicle fusion is triggered by the intracellular second messenger calcium, with members of the synaptotagmin protein family being prime candidates for linking calcium influx to fusion in the fast phase of exocytosis. To compensate for the massive incorporation of synaptic vesicles into the plasma membrane during exocytosis, special adaptations to endocytic mechanisms have evolved at the synapse to allow for efficient vesicle recycling.

Glucagon-like peptide-1 (GLP-1) receptors expressed on nerve terminals in the portal vein mediate the effects of endogenous GLP-1 on glucose tolerance in rats

Glucagon-like peptide-1 (GLP-1) is an intestinal hormone that is secreted during meal absorption and is essential for normal glucose homeostasis. However, the relatively low plasma levels and rapid metabolism of GLP-1 raise questions as to whether direct endocrine action on target organs, such as islet cells, account for all of its effects on glucose tolerance. Recently, an alternative neural pathway initiated by sensors in the hepatic portal region has been proposed to mediate GLP-1 activity. We hypothesized that visceral afferent neurons in the portal bed express the GLP-1 receptor (GLP-1r) and regulate glucose tolerance. Consistent with this hypothesis, GLP-1r mRNA was present in the nodose ganglia, and nerve terminals innervating the portal vein contained the GLP-1r. Rats given an intraportal infusion of the GLP-1r antagonist, [des-His(1),Glu(9)] exendin-4, in a low dose, had glucose intolerance, with a 53% higher glucose excursion compared with a vehicle-infused control group. Infusion of [des-His(1),Glu(9)] exendin-4 at an identical rate into the jugular vein had no effect on glucose tolerance, demonstrating that this dose of GLP-1r antagonist did not affect blood glucose due to spillover into the systemic circulation. These studies demonstrate that GLP-1r are present on nerve terminals in the hepatic portal bed and that GLP-1 antagonism localized to this region impairs glucose tolerance. These data are consistent with an important component of neural mediation of GLP-1 action.

Structure of synaptophysin: a hexameric MARVEL-domain channel protein

Synaptophysin I (SypI) is an archetypal member of the MARVEL-domain family of integral membrane proteins and one of the first synaptic vesicle proteins to be identified and cloned. Most all MARVEL-domain proteins are involved in membrane apposition and vesicle-trafficking events, but their precise role in these processes is unclear. We have purified mammalian SypI and determined its three-dimensional (3D) structure by using electron microscopy and single-particle 3D reconstruction. The hexameric structure resembles an open basket with a large pore and tenuous interactions within the cytosolic domain. The structure suggests a model for Synaptophysin's role in fusion and recycling that is regulated by known interactions with the SNARE machinery. This 3D structure of a MARVEL-domain protein provides a structural foundation for understanding the role of these important proteins in a variety of biological processes.

Matrix metalloproteinase-7 modulates synaptic vesicle recycling and induces atrophy of neuronal synapses

Matrix metalloproteinase-7 (MMP-7) belongs to a family of zinc dependent endopeptidases that are expressed in a variety of tissues including the brain. MMPs are known to be potent mediators of pericellular proteolysis and likely mediators of dynamic remodelling of neuronal connections. While an association between proteases and the neuronal synapse is emerging, a full understanding of this relationship is lacking. Here, we show that MMP-7 alters the structure and function of presynaptic terminals without affecting neuronal survival. Bath application of recombinant MMP-7 to cultured rat neurons induced long-lasting inhibition of vesicular recycling as measured by synaptotagmin 1 antibody uptake assays and FM4-64 optical imaging. MMP-7 application resulted in reduced abundance of vesicular and active zone proteins locally within synaptic terminals although their general levels remained unaltered. Finally, chronic application of the protease resulted in synaptic atrophy, including smaller terminals and fewer synaptic vesicles, as determined by electron microscopy. Together these results suggest that MMP-7 is a potent modulator of synaptic vesicle recycling and synaptic ultrastructure and that elevated levels of the enzyme, as may occur with brain inflammation, may adversely influence neurotransmission.

Long-lasting effects of elevated neonatal leptin on rat hippocampal function, synaptic proteins and NMDA receptor subunits

The high circulating levels of leptin in neonatal rodents do not seem to be regulating energy balance at this age, but rather may play an important role for brain development. We tested the hypothesis that high neonatal leptin levels modify hippocampal function and production of synaptic proteins with possible long-term consequences on long-term potentiation (LTP) in adulthood. We first showed that in postnatal day (PND) 10 neonates, acute leptin treatment functionally activated leptin receptors (ObR) in the CA1 and DG regions of the hippocampus through the induction of phosphoERK1/2, but not phosphoSTAT3 protein although both phospho-proteins were induced in the arcuate nucleus. We next examined whether chronic leptin administration (3 mg/kg BW, intraperitoneally) during the first 2 weeks of life (postnatal day, PND 2-14) produces a functional signal in the hippocampus that alters the expression of NMDA receptor subunits (NR1, NR2A, NR2B), synaptic proteins and LTP in the short and long-term. In PND 10 as in adults (PND 70) rats, chronic leptin treatment increased NR1 expression in the hippocampus while reducing NR2B protein levels. Elevated hippocampal concentrations of synapsin2A and synaptophysin were detected during leptin treatment on PND 10 suggesting increased neurotransmitter release. In adults, only SNAP-25 expression was increased after neonatal leptin treatment. LTP was reduced dramatically by leptin treatment in preweaning rats although the changes did not persist until adulthood. Elevated exposure to leptin during a critical period of neonatal hippocampal development might serve to enhance NMDA-dependent functions other than LTP and have important effects on synaptogenesis and neurotransmitter release.

Chronic exposure to estrogen and tamoxifen regulates synaptophysin and phosphorylated cAMP response element-binding (CREB) protein expression in CA1 of ovariectomized rat hippocampus

We report here the in vivo effects of estrogen (E2) on modulation of synaptic plasticity and the agonistic (estrogen-like) role of selective estrogen receptor modulator (SERM), tamoxifen (TAM) in the CA1 of the rat hippocampus. Effects on synaptophysin (SYP), a presynaptic vesicular protein, and phosphorylated cyclic AMP responsive element-binding (p-CREB) protein, a signal transduction pathway molecule, were studied using the ovariectomized (OVX) experimental rat model. Bilateral ovariectomy was performed on 40 rats and these were divided into 4 groups based on the treatment they received (at 2 weeks post-ovariectomy, a subcutaneous injection daily for 4 weeks) viz., OVX+E2 (0.1 mg/kg body weight), OVX+TAM (0.05 mg/kg body weight), OVX+vehicle and one group served as OVX control. An additional 10 animals served as the ovary intact control group. At the end of the treatment schedule, five animals/group were used for immunohistochemical staining of SYP and p-CREB using specific antibodies with peroxidase anti-peroxidase technique on paraformaldehyde-fixed cryostat sections. Protein estimation and Western blot analysis coupled with densitometric analysis (using gel-documentation system and image analysis software) were performed on unfixed hippocampus collected from rest of the five animals/group. Serum estradiol levels were estimated with radioimmunoassay prior to sacrifice. The results revealed that ovariectomy reduced SYP and p-CREB expression whereas E2 or TAM administration resulted in their upregulation. Serum estradiol levels of E2 administered animals were comparable with the ovary intact group whereas those of TAM administered group persisted in the range of OVX controls. To conclude, long-term estrogen therapy modulates the synaptic plasticity of hippocampal neurons and presumably, the agonist biocharacter of TAM as observed in the present investigations, may in the long run have a potential in the treatment and prevention of various estrogen-related disorders.

GLT-1 down-regulation induced by clozapine in rat frontal cortex is associated with synaptophysin up-regulation

In rat frontal cortex, extracellular levels of glutamate are raised by the anti-psychotic drug clozapine. We have recently shown that a significant reduction in the levels of the glutamate transporter GLT-1 may be one of the mechanisms responsible for this elevation. Here we studied whether GLT-1 down-regulation induced by chronic clozapine treatment is associated with changes in the expression of synaptophysin, synaptosome-associated protein of 25 kDa (SNAP-25) and vesicular glutamate transporter 1 (VGLUT1), three major presynaptic proteins involved in neurotransmitter release. Quantitative high-resolution confocal microscopy studies in vivo showed that GLT-1 down-regulation is closely associated with a significant increase in synaptophysin, but not SNAP-25 and VGLUT1, expression. This was confirmed in vitro studies, and in western blotting studies of synaptophysin, SNAP-25 and VGLUT1. In addition, our results show that, following clozapine treatment, synaptophysin expression increases in the very cortical regions in which GLT-1 expression is down-regulated. These findings suggest that part of the effects of clozapine may be exerted via an action on the presynaptic machinery involved in neurotransmitter release.

Tactile stimulation-induced rapid elevation of the synaptophysin mRNA expression level in rat somatosensory cortex

Synaptophysin is an integral membrane protein abundant in the synaptic vesicle and is found in nerve terminals throughout the brain. It was recently suggested that synaptophysin is also involved in the modulation of activity-dependent synapse formation. In this study, we examined at the individual level whether tactile stimulation selectively influenced the synaptophysin mRNA expression level in the somatosensory cortex of rats. Anesthetized rats were caressed on the back by an experimenter's palms for 20 min and the mRNA expression levels in the somatosensory and the visual cortices 5 min afterwards were determined using quantitative PCR methodology. The synaptophysin mRNA expression level was selectively higher in the experimental group than in the control group in the somatosensory cortex but not in the visual cortex. This suggests that the mRNA expression level of synaptophysin induced by neuronal activity is related to the regulation of synapse formation or remodeling or both.

Vesicular roundness and compound release in PC-12 cells

The principal goals of this study were to establish a quantitative morphological analysis of spatial and regional properties of dense core vesicles, and to use this analysis to assess whether homotypic fusion is prominent in chronically treated PC-12 cells at elevated release levels. Simple computerized image processing of electron-micrographs provided the binary images of vesicular dense cores, whilst the artificial intelligence methods were needed to determine the vesicular membranes. As in the past, the presence of large, highly irregular vesicles, provided the morphological evidence of fused vesicles, but the irregularity of vesicular shape was assessed quantitatively-from its roundness. Free space of each vesicle was determined from the distance to its nearest-neighbor, or from the size of its Voronoi polygon. Within a Voronoi polygon, each point is closer to that vesicle than to any other vesicle. Large vesicles were not less round and did not have larger free space, as expected if they result from fusion of several smaller vesicles. In conclusion, we present a novel and rigorous morphological analysis of spatial and regional properties of dense core vesicles. The results demonstrate that the homotypic fusion is not prominent in PC-12 cells, before or following a chronic treatment that enhances release.

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.

Marine Toxins Targeting Ion Channels

This introductory minireview points out the importance of ion channels for cell communication. The basic concepts on the structure and function of ion channels triggered by membrane voltage changes, the so-called voltage-gated ion channels (VGICs), as well as those activated by neurotransmitters, the so-called ligand-gated ion channel (LGICs), are introduced. Among the most important VGIC superfamiles, we can name the voltage-gated Na⁺ (Na_V), Ca²⁺ (Ca_V), and K⁺ (K_V) channels. Among the most important LGIC super families, we can include the Cys-loop or nicotinicoid, the glutamate-activated (GluR), and the ATP-activated (P2X_nR) receptor superfamilies. Ion channels are transmembrane proteins that allow the passage of different ions in a specific or unspecific manner. For instance, the activation of Na_V, Ca_V, or K_V channels opens a pore that is specific for Na⁺, Ca²⁺, or K⁺, respectively. On the other hand, the activation of certain LGICs such as nicotinic acetylcholine receptors, GluRs, and P2X_nRs allows the passage of cations (e.g., Na⁺, K⁺, and/or Ca²⁺), whereas the activation of other LGICs such as type A γ-butyric acid and glycine receptors allows the passage of anions (e.g., Cl⁻ and/or HCO₃⁻). In this regard, the activation of Na_V and Ca_V as well as ligand-gated cation channels produce membrane depolarization, which finally leads to stimulatory effects in the cell, whereas the activation of K_V as well as ligand-gated anion channels induce membrane hyperpolarization that finally leads to inhibitory effects in the cell. The importance of these ion channel superfamilies is emphasized by considering their physiological functions throughout the body as well as their pathophysiological implicance in several neuronal diseases. In this regard, natural molecules, and especially marine toxins, can be potentially used as modulators (e.g., inhibitors or prolongers) of ion channel functions to treat or to alleviate a specific ion channel-linked disease (e.g., channelopaties).

Differential sorting of the vesicular glutamate transporter 1 into a defined vesicular pool is regulated by light signaling involving the clock gene Period2

Synaptic strength depends on the amount of neurotransmitter stored in synaptic vesicles. The vesicular transmitter content has recently been shown to be directly dependent on the expression levels of vesicular neurotransmitter transporters indicating that the transport capacity of synaptic vesicles is a critical determinant for synaptic efficacy. Using synaptic vesicles prepared from whole brain at different times of the day we now show that the amount of vesicular glutamate transporter (VGLUT) 1 undergoes strong diurnal cycling. VGLUT1 protein levels are high before the start of the light period, decline at noon, increase again before start of the dark period, and decline again at midnight. Mice kept in complete darkness showed within a 24-h period only a single peak of VGLUT1 expression in the middle of the rest phase. In contrast, mice lacking the period gene Period 2, a core component of the circadian clock, did not show any light-cycle-dependent changes of VGLUT1 levels. No other of several synaptic vesicle proteins examined underwent circadian cycling. Circadian cycling of VGLUT1 was not seen when analyzing homogenate or synaptosomes, the starting fraction for vesicle preparation. Circadian cycling of VGLUT1 was also not reflected at the mRNA level. We conclude that nerve terminals are endowed with mechanisms that regulate quantal size by changing the copy number of transporters in synaptic vesicles. A reduced amount of VGLUT1 per vesicle is probably achieved by means of selective sorting controlled by clock genes.

Increased levels of SNAP-25 and synaptophysin in the dorsolateral prefrontal cortex in bipolar I disorder

OBJECTIVE

In order to identify whether the mechanisms associated with neurotransmitter release are involved in the pathologies of bipolar disorder and schizophrenia, levels of presynaptic [synaptosomal-associated protein-25 (SNAP-25), syntaxin, synaptophysin, vesicle-associated membrane protein, dynamin I] and structural (neuronal cell adhesion molecule and alpha-synuclein) neuronal markers were measured in Brodmann's area 9 obtained postmortem from eight subjects with bipolar I disorder (BPDI), 20 with schizophrenia and 20 controls.

METHODS

Determinations of protein levels were carried out using Western blot techniques with specific antibodies. Levels of mRNA were measured using real-time polymerase chain reaction.

RESULTS

In BPDI, levels of SNAP-25 (p < 0.01) and synaptophysin (p < 0.05) increased. There were no changes in schizophrenia or any other changes in BPDI. Levels of mRNA for SNAP-25 were decreased in BPDI (p < 0.05).

CONCLUSION

Changes in SNAP-25 and synaptophysin in BPDI suggest that changes in specific neuronal functions could be linked to the pathology of the disorder.

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.

Effects of restraint stress on the expression of proteins involved in synaptic vesicle exocytosis in the hippocampus

Chronic restraint stress has been associated with induction of morphological changes in the hippocampus. Postsynaptically, these changes include decreased length and branching of apical dendrites from CA3 pyramidal neurons, while presynaptically, depletion and clustering of synaptic vesicles have been observed. However, the molecular correlates of these changes remain poorly defined; while some studies have identified changes in the levels of some presynaptic proteins, none have assessed the coordinate expression of components of the membrane fusion complex, including synaptobrevin, syntaxin, and synaptosomal-associated protein 25 kDa, and their major regulatory molecules synaptotagmin, synaptophysin, and synapsin. Therefore, we undertook to assess the immunoreactivity of these proteins in hippocampal slices obtained from rats subjected to either acute (one 6 h session) or chronic (21 days at 6 h per day) of restraint stress. Specifically, we observed a significant increase in synaptobrevin immunoreactivity in the inner molecular layer of the dentate gyrus (54.2%; P=0.005), the stratum radiatum in the CA1 subfield (55.5%; P=0.007), and a region including the stratum lucidum and the proximal portion of the stratum radiatum in the CA3 subfield (52.7%; P=0.002); we also observed a trend toward increased synaptophysin levels in the stratum lucidum/radiatum of the CA3 subfield (8.0%; P=0.051) following chronic, but not acute, restraint stress. In that synaptobrevin has been associated with replenishment of the "readily-releasable" pool of synaptic vesicles and the efficiency of neurotransmitter release, the present results suggest that stress-induced changes in synaptobrevin may at least in part underlie the previously observed changes in synaptic and neuronal morphology.

Differential expression of synaptoporin and synaptophysin in primary sensory neurons and up-regulation of synaptoporin after peripheral nerve injury

Synaptoporin and synaptophysin are integral membrane components of synaptic vesicles. The distribution of synaptoporin and its relationship with synaptophysin in sensory afferent fibers remain unclear. In the present study, we showed that in the rat dorsal root ganglia synaptoporin was expressed in subsets of small neurons that contain either calcitonin gene-related peptide or isolectin B4, and was distributed in their afferent terminals in laminae I-II of the spinal cord. Synaptophysin was expressed in 57% of synaptoporin-containing small dorsal root ganglion neurons and in large dorsal root ganglion neurons. In the spinal dorsal horn, synaptophysin-immunolabeling was weak in the afferent fibers in lamina I, outer lamina II and the dorsal part of inner lamina II, but strong in the afferent fibers in laminae III-IV. However, a subpopulation of isolectin B4-positive small dorsal root ganglion neurons expressed both synaptoporin and synaptophysin, and their afferent fibers were mainly distributed in the ventral part of inner lamina II. After peripheral nerve injury, synaptoporin expression was up-regulated in small dorsal root ganglion neurons, and synaptoporin level was increased in their afferent terminals. Thus, synaptoporin and synaptophysin have topographically distinct distributions in afferent fibers. Synaptoporin is a major synaptic vesicle protein in Adelta- and C-fibers in both physiological and neuropathic pain states.

Mechanism of 1-methyl-4-phenylpyridinium-induced dopamine release from PC12 cells

The molecular mechanism of 1-methyl-4-phenylpyridinium (MPP+), a Parkinsonism-inducing neurotoxin, has been studied in PC12 cells. The cells treated with MPP+ (100 microM) induced a rapid increase in phosphorylation of tyrosine residues of several proteins, including synaptophysin, a major 38 kDa synaptic vesicle protein implicated in exocytosis. An accelerated release of dopamine by MPP+ correlated with phosphorylation of synaptophysin. Exposing the cells to MPP+ triggered reactive oxygen species (ROS) generation within 60 min of treatment and the said effect was blocked by mazindol, a dopamine uptake blocker. In addition, pretreatment with 50-100 microM of selegiline, a selective MAO-B inhibitor, significantly suppressed MPP+-mediated ROS generation. These effects of MPP+ result in the generation of ROS, which may be involved in neuronal degeneration seen in Parkinson's disease.

Synaptophysin enhances the neuroprotection of VMAT2 in MPP+-induced toxicity in MN9D cells

The use of the potent neurotoxin MPTP in producing a model for Parkinson's disease (PD) has allowed us to dissect the cellular processes responsible for both selective neuronal vulnerability and neuroprotection in idiopathic PD. It has been suggested that vesicular monoamine transporters (VMATs) play a critical neuroprotective role in MPP+ toxicity. However, little is known about how this detoxificative sequestration in dopaminergic (DAergic) neurons is regulated at the molecular and cellular levels. Using the DAergic cell line MN9D as an in vitro model, we found that overexpression of VMAT2 (a neuronal isoform of VMATs) protects the transformants from MPP+-induced toxicity, consistent with the previous work on fibroblastic CHO cells. We further found that the MN9D cells displayed lower expression levels of secretory vesicle proteins such as synaptophysin. Overexpression of synaptophysin in MN9D cells can significantly increase the resistance of the transformants to MPP+ toxicity. The co-expression of VMAT2 and synaptophysin has shown synergistic protection for the transformants, suggesting a role of synaptophysin in the biogenesis of secretory vesicles and in influencing the targeting of VMAT2 to these vesicles. Our work indicates that both the expression level of VMAT2 and capacity of vesicular packaging of DA are important in protecting DAergic cells from MPP+ toxicity.

Extrasynaptic localization of glycine receptors in the rat supraoptic nucleus: further evidence for their involvement in glia-to-neuron communication

Neurons of the rat supraoptic nucleus (SON) express glycine receptors (GlyRs), which are implicated in the osmoregulation of neuronal activity. The endogenous agonist of the receptors has been postulated to be taurine, shown to be released from astrocytes. We here provide additional pieces of evidence supporting the absence of functional glycinergic synapses in the SON. First, we show that blockade of GlyRs with strychnine has no effect on either the amplitude or frequency of miniature inhibitory postsynaptic currents recorded in SON neurons, whereas they were all suppressed by the GABA(A) antagonist gabazine. Then, double immunostaining of sections with presynaptic markers and either GlyR or GABA(A) receptor (GABA(A)R) antibodies indicates that, in contrast with GABA(A)Rs, most GlyR membrane clusters are not localized facing presynaptic terminals, indicative of their extrasynaptic localization. Moreover, we found a striking anatomical association between SON GlyR clusters and glial fibrillary acidic protein (GFAP)-positive astroglial processes, which contain high levels of taurine. This type of correlation is specific to GlyRs, since GABA(A)R clusters show no association with GFAP-positive structures. These results substantiate and strengthen the concept of extrasynaptic GlyRs mediating a paracrine communication between astrocytes and neurons in the SON.

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.

Area-specific effects of brain-derived neurotrophic factor (BDNF) genetic ablation on various neuronal subtypes of the mouse brain

The effects of brain-derived neurotrophic factor (BDNF) on the development of presynaptic terminals and of neuronal subtypes in various brain areas were studied in BDNF-knockout (BDNF-/-) mice at postnatal days 15-17. Western analysis revealed no changes in the overall amount of a variety of synaptic proteins in BDNF-/- mice as compared to wild type mice. In addition, the complex between the vesicular proteins, synaptophysin and synaptobrevin, as well as their respective homodimers were unaltered. Moreover, no changes in the density of neurons were found in, e.g., the CA3 region of the hippocampus and the nucleus nervi facialis of BDNF-/- mice. However, cholinergic cells were reduced by 20% in the medial septum of BDNF-/- mice associated with a decrease in the activity of choline acetyltransferase and protein levels of nerve growth factor in the hippocampus by 16% and 44%, respectively. In the striatum, however, the total number of cholinergic cells were comparable in both groups, although the activity of choline acetyltransferase was decreased by 46%. In GABAergic interneurons, the expression of neuropeptides in various brain areas was differentially affected by BDNF deletion as revealed by immunohistochemistry. In the hippocampus and cortex of BDNF-/- mice, the density of neuropeptide Y-, somatostatin-, and parvalbumin-immunoreactive cells was drastically reduced, whereas the density of calretinin-positive cells was increased. The extent of these changes in neuropeptide-containing cells varied among hippocampal subregions. In the striatum, only the density of parvalbumin-immunoreactive cells was decreased by approximately 45%. In conclusion, BDNF deficiency is accompanied by a differential dysregulation in the expression of neuropeptides and calcium-binding proteins in otherwise intact GABAergic and glutamatergic neurons in a region-specific manner.

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.

Altered interaction and expression of proteins involved in neurosecretion in scrapie-infected GT1-1 cells

Prions cause transmissible and fatal diseases that are associated with spongiform degeneration, astrogliosis, and loss of axon terminals in the brains. To determine the expression of proteins involved in neurosecretion and synaptic functions after prion infection, gonadotropin-releasing hormone neuronal cell line subclone (GT1-1) was infected with the RML scrapie strain and analyzed by Western blotting, real time PCR, and immunohistochemistry. As revealed by Western blotting of lysates exposed to different temperatures, the levels of complexed SNAP-25, syntaxin 1A, and synaptophysin were decreased in scrapie-infected GT1-1 cells (ScGT1-1), whereas the level of monomeric forms of these proteins was increased and correlated to the level of scrapie prion protein (PrPSc). However, when complex formation was prevented by prolonged heating of samples in SDS, the levels of monomeric SNAP-25, syntaxin 1A and synaptophysin in ScGT1-1 cells were decreased in comparison to GT1-1 cells. The reduced level of SNAP-25 was observed as early as 32 days postinfection. Increased mRNA levels of both splice variants SNAP-25a and -b in ScGT1-1 cells were seen. No difference in the morphology, neuritic outgrowth or distribution of SNAP-25, syntaxin 1A, or synaptophysin could be observed in ScGT1-1 cells. Treatment with quinacrine or pentosan polysulfate cleared the PrPSc from the ScGT1-1 cell cultures, and the increase in levels of monomeric SNAP-25 and synaptophysin was reversible. These results indicate that a scrapie infection can cause changes in the expression of proteins involved in neuronal secretion, which may be of pathogenetic relevance for the axon terminal changes seen in prion-infected brains.

The synaptic vesicle cycle

Neurotransmitter release is mediated by exocytosis of synaptic vesicles at the presynaptic active zone of nerve terminals. To support rapid and repeated rounds of release, synaptic vesicles undergo a trafficking cycle. The focal point of the vesicle cycle is Ca2+-triggered exocytosis that is followed by different routes of endocytosis and recycling. Recycling then leads to the docking and priming of the vesicles for another round of exo- and endocytosis. Recent studies have led to a better definition than previously available of how Ca2+ triggers exocytosis and how vesicles recycle. In particular, insight into how Munc18-1 collaborates with SNARE proteins in fusion, how the vesicular Ca2+ sensor synaptotagmin 1 triggers fast release, and how the vesicular Rab3 protein regulates release by binding to the active zone proteins RIM1 alpha and RIM2 alpha has advanced our understanding of neurotransmitter release. The present review attempts to relate these molecular data with physiological results in an emerging view of nerve terminals as macromolecular machines.

Ventilatory effects of gap junction blockade in the NTS in awake rats

We tested the hypothesis that focally perfusing carbenoxolone, which blocks gap junctions, into the nucleus tractus solitarius (NTS) would reduce the ventilatory response to CO(2). We measured minute ventilation (V(E)), tidal volume (V(T)) and respiratory frequency (F(R)) responses to increasing concentrations of inspired CO(2) (F(I)(CO(2) = 0-8%) in rats during wakefulness. Focal perfusion of acetazolamide (10 microM) into the NTS increased V(E) and V(T) during exposure to room air. Carbenoxolone (300 microM) decreased the V(E) and V(T) response to CO(2) when perfused within, but not adjacent to the NTS in animals less than 10 weeks of age. F(R) was decreased at F(I)(CO(2) = 4% in these animals. Carbenoxolone did not decrease V(E), V(T) or F(R) in animals 10 weeks of age and older. Carbenoxolone did not decrease V(E), V(T) or F(R) when focally perfused outside the NTS at any age tested. The NTS is an important CO(2) chemosensory site at all ages, and gap junctions amplify the ventilatory response to CO(2) in animals less than 10 weeks of age.

Ventilatory effects of gap junction blockade in the RTN in awake rats

We tested the hypothesis that carbenoxolone, a pharmacological inhibitor of gap junctions, would reduce the ventilatory response to CO(2) when focally perfused within the retrotrapezoid nucleus (RTN). We tested this hypothesis by measuring minute ventilation (V(E)), tidal volume (V(T)), and respiratory frequency (F(R)) responses to increasing concentrations of inspired CO(2) (Fi(CO(2)) = 0-8%) in rats during wakefulness. We confirmed that the RTN was chemosensitive by perfusing the RTN unilaterally with either acetazolamide (AZ; 10 microM) or hypercapnic artificial cerebrospinal fluid equilibrated with 50% CO(2) (pH approximately 6.5). Focal perfusion of AZ or hypercapnic aCSF increased V(E), V(T), and F(R) during exposure to room air. Carbenoxolone (300 microM) focally perfused into the RTN decreased V(E) and V(T) in animals <11 wk of age, but V(E) and V(T) were increased in animals >12 wk of age. Glyzyrrhizic acid, a congener of carbenoxolone, did not change V(E), V(T), or F(R) when focally perfused into the RTN. Carbenoxolone binds to the mineralocorticoid receptor, but spironolactone (10 microM) did not block the disinhibition of V(E) or V(T) in older animals when combined with carbenoxolone. Thus the RTN is a CO(2) chemosensory site in all ages tested, but the function of gap junctions in the chemosensory process varies substantially among animals of different ages: gap junctions amplify the ventilatory response to CO(2) in younger animals, but appear to inhibit the ventilatory response to CO(2) in older animals.

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: leading actor or walk-on role in synaptic vesicle exocytosis?

Synaptophysin (Syp) was the first synaptic vesicle (SV) protein to be cloned. Since its discovery in 1985, it has been used by us and by many laboratories around the world as an invaluable marker to study the distribution of synapses in the brain and to uncover the basic features of the life cycle of SVs. Although single gene ablation of Syp does not lead to an overt phenotype, a large body of experimental data both in vitro and in vivo indicate that Syp (alone or in association with homologous proteins) is involved in multiple, important aspects of SV exo-endocytosis, including regulation of SNARE assembly into the fusion core complex, formation of the fusion pore initiating neurotransmitter release, activation of SV endocytosis and SV biogenesis. In this article, we summarise the main results of the studies on Syp carried out by our and other laboratories, and explain why we believe that Syp plays a major role in SV trafficking.