Synaptic vesicle traffic: rush hour in the nerve terminal

Exosome: A novel neurotransmission modulator or non-canonical neurotransmitter?

Neurotransmission is the electrical impulse-triggered propagation of signals between neurons or between neurons and other cell types such as skeletal muscle cells. Recent studies point out the involvement of exosomes, a type of small bilipid layer-enclosed extracellular vesicles, in regulating neurotransmission. Through horizontally transferring proteins, lipids, and nucleic acids, exosomes can modulate synaptic activities rapidly by controlling neurotransmitter release or progressively by regulating neural plasticity including synapse formation, neurite growth & removal, and axon guidance & elongation. In this review, we summarize the similarities and differences between exosomes and synaptic vesicles in their biogenesis, contents, and release. We also highlight the recent progress made in demonstrating the biological roles of exosome in regulating neurotransmission, and propose a modified model of neurotransmission, in which exosomes act as novel neurotransmitters. Lastly, we provide a comprehensive discussion of the enlightenment of the current knowledge on neurotransmission to the future directions of exosome research.

Isolation of Synaptic Vesicles from Mammalian Brain

Synaptic vesicles (SVs) store neurotransmitters and undergo a fine-tuned regulatory and dynamic cycle of exo- and endocytosis, which is essential for neurotransmission at chemical synapses. The development of protocols for isolating SVs from biological extracts was a fundamental accomplishment since it allowed for characterizing the molecular properties of SVs using biochemical methods. In this chapter, we describe a modified procedure for isolating SVs from a few g of rodent brain and that can be completed within ~12 h. The protocol involves the preparation of isolated nerve terminals from which SVs are released by osmotic shock and then enriched via various centrifugation steps, followed by size exclusion chromatography as final purification step. The final vesicle fraction is 22-fold enriched in SVs over the starting material, and the final yield of SVs obtained using this protocol is approximately 20 μg of protein per gram of mouse brain. The degree of contamination by other organelles and particles monitored by morphology and immunolabeling compares well with that of the classical protocols.

Impact of a multi-nutrient diet on cognition, brain metabolism, hemodynamics, and plasticity in apoE4 carrier and apoE knockout mice

Lipid metabolism and genetic background together strongly influence the development of both cardiovascular and neurodegenerative diseases like Alzheimer's disease (AD). A non-pharmacological way to prevent the genotype-induced occurrence of these pathologies is given by dietary behavior. In the present study, we tested the effects of long-term consumption of a specific multi-nutrient diet in two models for atherosclerosis and vascular risk factors in AD: the apolipoprotein ε4 (apoE4) and the apoE knockout (apoE ko) mice. This specific multi-nutrient diet was developed to support neuronal membrane synthesis and was expected to contribute to the maintenance of vascular health. At 12 months of age, both genotypes showed behavioral changes compared to control mice and we found increased neurogenesis in apoE ko mice. The specific multi-nutrient diet decreased anxiety-related behavior in the open field, influenced sterol composition in serum and brain tissue, and increased the concentration of omega-3 fatty acids in the brain. Furthermore, we found that wild-type and apoE ko mice fed with this multi-nutrient diet showed locally increased cerebral blood volume and decreased hippocampal glutamate levels. Taken together, these data suggest that a specific dietary intervention has beneficial effects on early pathological consequences of hypercholesterolemia and vascular risk factors for AD.

A longitudinal study of cognition, proton MR spectroscopy and synaptic and neuronal pathology in aging wild-type and AβPPswe-PS1dE9 mice

Proton magnetic resonance spectroscopy (¹H MRS) is a valuable tool in Alzheimer’s disease research, investigating the functional integrity of the brain. The present longitudinal study set out to characterize the neurochemical profile of the hippocampus, measured by single voxel ¹H MRS at 7 Tesla, in the brains of AβPPSswe-PS1dE9 and wild-type mice at 8 and 12 months of age. Furthermore, we wanted to determine whether alterations in hippocampal metabolite levels coincided with behavioral changes, cognitive decline and neuropathological features, to gain a better understanding of the underlying neurodegenerative processes. Moreover, correlation analyses were performed in the 12-month-old AβPP-PS1 animals with the hippocampal amyloid-β deposition, TBS-T soluble Aβ levels and high-molecular weight Aβ aggregate levels to gain a better understanding of the possible involvement of Aβ in neurochemical and behavioral changes, cognitive decline and neuropathological features in AβPP-PS1 transgenic mice. Our results show that at 8 months of age AβPPswe-PS1dE9 mice display behavioral and cognitive changes compared to age-matched wild-type mice, as determined in the open field and the (reverse) Morris water maze. However, there were no variations in hippocampal metabolite levels at this age. AβPP-PS1 mice at 12 months of age display more severe behavioral and cognitive impairment, which coincided with alterations in hippocampal metabolite levels that suggest reduced neuronal integrity. Furthermore, correlation analyses suggest a possible role of Aβ in inflammatory processes, synaptic dysfunction and impaired neurogenesis.

Monomeric synucleins generate membrane curvature

Synucleins are a family of presynaptic membrane binding proteins. α-Synuclein, the principal member of this family, is mutated in familial Parkinson disease. To gain insight into the molecular functions of synucleins, we performed an unbiased proteomic screen and identified synaptic protein changes in αβγ-synuclein knock-out brains. We observed increases in the levels of select membrane curvature sensing/generating proteins. One of the most prominent changes was for the N-BAR protein endophilin A1. Here we demonstrate that the levels of synucleins and endophilin A1 are reciprocally regulated and that they are functionally related. We show that all synucleins can robustly generate membrane curvature similar to endophilins. However, only monomeric but not tetrameric α-synuclein can bend membranes. Further, A30P α-synuclein, a Parkinson disease mutant that disrupts protein folding, is also deficient in this activity. This suggests that synucleins generate membrane curvature through the asymmetric insertion of their N-terminal amphipathic helix. Based on our findings, we propose to include synucleins in the class of amphipathic helix-containing proteins that sense and generate membrane curvature. These results advance our understanding of the physiological function of synucleins.

Longitudinal characterization of the brain proteomes for the tg2576 amyloid mouse model using shotgun based mass spectrometry

Neurodegenerative disorders are often defined pathologically by the presence of protein aggregates, such as amyloid plaques composed of β-amyloid (Aβ) peptide in Alzheimer's disease. Such aggregates are the result of abnormal protein accumulation and may lead to neuronal dysfunction and cell death. In this study, APPSWE transgenic mice (Tg2576), which overexpress the Swedish mutated form of human amyloid precursor protein (APP), were used to study the brain proteome associated with amyloid plaque deposition. The major aim of the study was to map and compare the Tg2576 model brain proteome profiles during pathology progression using a shotgun approach based on label free quantification with mass spectrometry. Overall, 1085 proteins were identified and longitudinally quantified. Principal component analysis (PCA) showed the appearance of the pathology onset between twelve and fifteen months, correlating with sharp amyloid plaque accumulation within the same ages. Cluster analysis followed by protein-protein interaction analysis revealed an age-dependent decrease in mitochondrial protein expression. We identified 57 significantly affected mitochondrial proteins, several of which have been reported to alter expression in neurological diseases. We also found ten proteins that are upregulated early in the amyloid driven pathology progression with high confidence, some of which are directly involved in the onset of mitochondrial apoptosis and may represent potential markers for use in human neurological diseases prognosis. Our results further contribute to identifying common pathological pathways involved in both aging and progressive neurodegenerative disorders enhancing the understanding of disease pathogenesis.

Coupling exo- and endocytosis: an essential role for PIP₂ at the synapse

Chemical synapses are specialist points of contact between two neurons, where information transfer takes place. Communication occurs through the release of neurotransmitter substances from small synaptic vesicles in the presynaptic terminal, which fuse with the presynaptic plasma membrane in response to neuronal stimulation. However, as neurons in the central nervous system typically only possess ~200 vesicles, high levels of release would quickly lead to a depletion in the number of vesicles, as well as leading to an increase in the area of the presynaptic plasma membrane (and possible misalignment with postsynaptic structures). Hence, synaptic vesicle fusion is tightly coupled to a local recycling of synaptic vesicles. For a long time, however, the exact molecular mechanisms coupling fusion and subsequent recycling remained unclear. Recent work now indicates a unique role for the plasma membrane lipid phosphatidylinositol 4,5-bisphosphate (PIP(2)), acting together with the vesicular protein synaptotagmin, in coupling these two processes. In this work, we review the evidence for such a mechanism and discuss both the possible advantages and disadvantages for vesicle recycling (and hence signal transduction) in the nervous system. This article is part of a Special Issue entitled Lipids and Vesicular Transport.

Huperzine A alleviates synaptic deficits and modulates amyloidogenic and nonamyloidogenic pathways in APPswe/PS1dE9 transgenic mice

Huperzine A (HupA) is a potent acetylcholinesterase inhibitor (AChEI) used in the treatment of Alzheimer's disease (AD). Recently, HupA was shown to be active in modulating the nonamyloidogenic metabolism of β-amyloid precursor protein (APP) in APP-transfected human embryonic kidney cell line (HEK293swe). However, in vivo research concerning the mechanism of HupA in APP transgenic mice has not yet been fully elucidated. The present study indicates that the loss of dendritic spine density and synaptotagmin levels in the brain of APPswe/presenilin-1 (PS1) transgenic mice was significantly ameliorated by chronic HupA treatment and provides evidence that this neuroprotection was associated with reduced amyloid plaque burden and oligomeric β-amyloid (Aβ) levels in the cortex and hippocampus of APPswe/PS1dE9 transgenic mice. Our findings further demonstrate that the amelioration effect of HupA on Aβ deposits may be mediated, at least in part, by regulation of the compromised expression of a disintegrin and metalloprotease 10 (ADAM10) and excessive membrane trafficking of β-site APP cleavage enzyme 1 (BACE1) in these transgenic mice. In addition, extracellular signal-regulated kinases 1/2 (Erk1/2) phosphorylation may also be partially involved in the effect of HupA on APP processing. In conclusion, our work for the first time demonstrates the neuroprotective effect of HupA on synaptic deficits in APPswe/PS1dE9 transgenic mice and further clarifies the potential pharmacological targets for this protective effect, in which modulation of nonamyloidogenic and amyloidogenic APP processing pathways may be both involved. These findings may provide adequate evidence for the clinical and experimental benefits gained from HupA treatment.

CSPα promotes SNARE-complex assembly by chaperoning SNAP-25 during synaptic activity

A neuron forms thousands of presynaptic nerve terminals on its axons, far removed from the cell body. The protein CSPα resides in presynaptic terminals, where it forms a chaperone complex with Hsc70 and SGT. Deletion of CSPα results in massive neurodegeneration that impairs survival in mice and flies. In CSPα-knockout mice, levels of presynaptic SNARE complexes and the SNARE protein SNAP-25 are reduced, suggesting that CSPα may chaperone SNARE proteins, which catalyse synaptic vesicle fusion. Here, we show that the CSPα-Hsc70-SGT complex binds directly to monomeric SNAP-25 to prevent its aggregation, enabling SNARE-complex formation. Deletion of CSPα produces an abnormal SNAP-25 conformer that inhibits SNARE-complex formation, and is subject to ubiquitylation and proteasomal degradation. Even in wild-type mouse terminals, SNAP-25 degradation is regulated by synaptic activity; this degradation is decreased by CSPα overexpression, and enhanced by CSPα deletion. Thus, SNAP-25 function is maintained during rapid SNARE cycles by equilibrium between CSPα-dependent chaperoning and ubiquitin-dependent degradation, revealing unique protein quality-control machinery within the presynaptic compartment.

Isolation of synaptic vesicles

Synaptic vesicles are the most abundant secretory organelle in eukaryotic neural cells. Synaptic vesicles are physically distinct from other membrane-bound organelles because they are small, spherical, and highly uniform in size with a diameter of about 40 nm. Synaptic vesicles also have a characteristically low density because water and phospholipids account for 88% of an individual synaptic vesicle's weight. The homogeneous size and density of the synaptic vesicle are characteristics that are exploited in most synaptic vesicle isolation procedures. Synaptic vesicles are purified by isopycnic/velocity sedimentation and size-based purification schemes. However, protocols differ in the tissue source of vesicles, the way the tissue is homogenized, and the way the vesicles are fractionated. This unit describes two well-characterized and widely used synaptic vesicle isolation procedures that are capable of generating membrane fractions that are 20 to 30 times enriched in synaptic vesicles.

Synaptic proteins as multi-sensor devices of neurotransmission

Neuronal communication is tightly regulated in time and space. Following neuronal activation, an electrical signal triggers neurotransmitter (NT) release at the active zone. The process starts by the signal reaching the synapse followed by a fusion of the synaptic vesicle (SV) and diffusion of the released NT in the synaptic cleft. The NT then binds to the appropriate receptor and induces a membrane potential change at the target cell membrane. The entire process is controlled by a fairly small set of synaptic proteins, collectively called SYCONs. The biochemical features of SYCONs underlie the properties of NT release. SYCONs are characterized by their ability to detect and respond to changes in environmental signals. For example, consider synaptotagmin I (Syt1), a prototype of a protein family with over 20 gene and variants in mammals. Syt1 is a specific example of a multi-sensor device with a large repertoire of discrete states. Several of these states are stimulated by a local concentration of signaling molecules such as Ca2+. The ability of this protein to sense signaling molecules and to adopt multiple biochemical states is shared by other SYCONs such as the synapsins (Syns). Specific biochemical states of Syns determine the accessibility of SV for NT release. Each of these states is defined by a specific alternative spliced variant with a unique profile of phosphorylation modified sites. The plasticity of the synapse is a direct reflection of SYCON's multiple biochemical states. State transitions occurs in a wide range of time scales, and therefore these molecules need to cope with events that last milliseconds (i.e., exocytosis in fast responding synapses) and with events that can carry on for many minutes (i.e., organization of SV pools). We suggest that SYCONs are optimized throughout evolution as multi-sensor devices. A full repertoire of the switches leading to alternation of protein states and a detailed characterization of protein-protein network within the synapse is critical for the development of a dynamic model of synaptic transmission.

Single-molecule studies of synaptotagmin and complexin binding to the SNARE complex

The assembly of multiprotein complexes at the membrane interface governs many signaling processes in cells. However, very few methods exist for obtaining biophysical information about protein complex formation at the membrane. We used single molecule fluorescence resonance energy transfer to study complexin and synaptotagmin interactions with the SNARE complex in deposited lipid bilayers. Using total internal reflectance microscopy, individual binding events at the membrane could be resolved despite an excess of unbound protein in solution. Fluorescence resonance energy transfer (FRET)-efficiency derived distances for the complexin-SNARE interaction were consistent with the crystal structure of the complexin-SNARE complex. The unstructured N-terminal region of complexin showed broad distributions of FRET efficiencies to the SNARE complex, suggesting that information on conformational variability can be obtained from FRET efficiency distributions. The low-affinity interaction of synaptotagmin with the SNARE complex changed dramatically upon addition of Ca2+ with high FRET efficiency interactions appearing between the C2B domain and linker domains of synaptotagmin and the membrane proximal portion of the SNARE complex. These results demonstrate that single molecule FRET can be used as a "spectroscopic ruler" to simultaneously gain structural and kinetic information about transient multiprotein complexes at the membrane interface.

Cellugyrin induces biogenesis of synaptic-like microvesicles in PC12 cells

The four-transmembrane domain proteins synaptophysin and synaptogyrin represent the major constituents of synaptic vesicles. Our previous studies in PC12 cells demonstrated that synaptogyrin or its nonneuronal paralog cellugyrin targets efficiently to synaptic-like microvesicles (SLMVs) and dramatically increases the synaptophysin content of SLMVs (Belfort, G. M., and Kandror, K. V. (2003) J. Biol. Chem. 278, 47971-47978). Here, we explored the mechanism of these phenomena and found that ectopic expression of cellugyrin increases the number of SLMVs in PC12 cells. Mutagenesis studies revealed that cellugyrin's hydrophilic cytoplasmic domains are not involved in vesicle biogenesis, whereas small conserved hydrophobic hairpins in the first luminal loop and the carboxyl terminus of cellugyrin were found to be critical for the formation of SLMVs. In addition, the length but not the primary sequence of the second luminal loop was essential for SLMV biogenesis. We suggest that changing the length of this loop similar to disruption of the short hydrophobic hairpins alters the position of the vicinal transmembrane domains that may be crucial for protein function.

Differential expression pattern and steroid hormone sensitivity of SNAP-25 and synaptoporin mRNA in the telencephalic song control nucleus HVC of the zebra finch

Gonadal steroid hormones play an important role in the process of sexual differentiation of brain areas and behavior such as singing and song learning in songbirds. These hormones affect behavior controlling circuits on both the gross morphological and ultrastructural levels. Here we study whether the expression of genes coding for synaptic proteins is sensitive to gonadal steroid hormones and whether such altered expression coincides with changes in brain area size. We treated adult male zebra finches with the aromatase inhibitor fadrozole, to reduce estrogen synthesis and analyzed the mRNA expression of the synaptic proteins synaptoporin (SPO) and synaptosomal-associated protein 25 kDa (SNAP-25) in song control areas and surrounding tissues of adult male zebra finches. SPO and SNAP-25 are differently expressed throughout the song system. Generally, the telencephalic song nuclei expressed SNAP-25 at high intensity whereas SPO expression was area-specific. Elevated levels of SNAP-25 mRNA were present in the nucleus hyperstriatalis ventrale pars caudale (HVC) and in the robust nucleus of the archistriatum (RA). SPO mRNA was found in moderate levels in the HVC, in low levels in the lateral nucleus magnocellularis (lMAN) and Area X, and was absent in the RA. The treatment significantly increased the mRNA level of SPO in the HVC, whereas SNAP-25 expression level was not affected. These expression patterns are not explained by the decrease of HVC volume after treatment. The decreased HVC size is not area-specific but correlates with an overall reduction in size and an overall increase in cell density of the forebrain.

Phosphorylation of proteins in neuron terminals: specificity depends on coincidental signaling

We investigate the role of neuronal coincidental signaling mediated by the second messengers, on phosphorylation of three major proteins of neurosecretory vesicles. Our data show that different combinations of coincidental signaling generate specific pattern of phosphoproteins and not strictly additional effects. This suggests that an added phosphate on a site might 'mask' or 'unmask' the next sites for specific kinases and phosphatases action by inducing conformation change or protein association. We show that a function of vesicles such as the uptake of glutamate is highly regulated by coincidental signaling.

Identification of the differentiation-associated Na+/PI transporter as a novel vesicular glutamate transporter expressed in a distinct set of glutamatergic synapses

Glutamate transport into synaptic vesicles is a prerequisite for its regulated neurosecretion. Here we functionally identify a second isoform of the vesicular glutamate transporter (VGLUT2) that was previously identified as a plasma membrane Na+-dependent inorganic phosphate transporter (differentiation-associated Na+/P(I) transporter). Studies using intracellular vesicles from transiently transfected PC12 cells indicate that uptake by VGLUT2 is highly selective for glutamate, is H+ dependent, and requires Cl- ion. Both the vesicular membrane potential (Deltapsi) and the proton gradient (DeltapH) are important driving forces for vesicular glutamate accumulation under physiological Cl- concentrations. Using an antibody specific for VGLUT2, we also find that this protein is enriched on synaptic vesicles and selective for a distinct class of glutamatergic nerve terminals. The pathway-specific, complementary expression of two different vesicular glutamate transporters suggests functional diversity in the regulation of vesicular release at excitatory synapses. Together, the two isoforms may account for the uptake of glutamate by synaptic vesicles from all central glutamatergic neurons.

Access of a membrane protein to secretory granules is facilitated by phosphorylation

Peptidylglycine alpha-amidating monooxygenase (PAM), an integral membrane protein essential for the biosynthesis of amidated peptides, was used to assess the role of cytosolic acidic clusters in trafficking to regulated secretory granules. Casein kinase II phosphorylates Ser(949) and Thr(946) of PAM, generating a short, cytosolic acidic cluster. P-CIP2, a protein kinase identified by its ability to interact with several juxtamembrane determinants in the PAM cytosolic domain, also phosphorylates Ser(949). Antibody specific for phospho-Ser(949)-PAM-CD demonstrates that a small fraction of the PAM-1 localized to the perinuclear region bears this modification. Pituitary cell lines expressing PAM-1 mutants that mimic (TS/DD) or prevent (TS/AA) phosphorylation at these sites were studied. PAM-1 TS/AA yields a lumenal monooxygenase domain that enters secretory granules inefficiently and is rapidly degraded. In contrast, PAM-1 TS/DD is routed to regulated secretory granules more efficiently than wild-type PAM-1 and monooxygenase release is more responsive to secretagogue. Furthermore, this acidic cluster affects exit of internalized PAM-antibody complexes from late endosomes; internalized PAM-1 TS/DD accumulates in a late endocytic compartment instead of the trans-Golgi network. The increased ability of solubilized PAM-1 TS/DD to aggregate at neutral pH may play an important role in its altered trafficking.

A conserved mechanism of synaptogyrin localization

We have studied the localization of synaptogyrin family members in vivo. Both native and green fluorescent protein (GFP)-tagged Caenorhabditis elegans synaptogyrin (SNG-1) are expressed in neurons and synaptically localized. Deletion and mutational analysis with the use of GFP-tagged SNG-1 has defined a 38 amino acid sequence within the C terminus of SNG-1 and a single arginine in the cytoplasmic loop between transmembrane domain 2 and 3 that are required for SNG-1 localization. These domains may represent components of signals that target synaptogyrin for endocytosis from the plasma membrane and direct synaptogyrin to synaptic vesicles, respectively. In chimeric studies, these regions were sufficient to relocalize cellugyrin, a nonneuronal form of synaptogyrin, from nonsynaptic regions such as the sensory dendrites and the cell body to synaptic vesicles. Furthermore, GFP-tagged rat synaptogyrin is synaptically localized in neurons of C. elegans and in cultured hippocampal neurons. Similarly, the C-terminal domain of rat synaptogyrin is necessary for localization in hippocampal neurons. Our study suggests that the mechanisms for synaptogyrin localization are likely to be conserved from C. elegans to vertebrates.

Transient increase in rab 3A and synaptobrevin immunoreactivity after mild hypoxia in neonatal rats

1. In the present work we describe the short term effects of mild neonatal hypoxia on the synapse as assessed by the immunoreactivity (IR) of two synaptic proteins: rab 3A and synaptobrevin (VAMP). 2. Using the sensitive methodology of immunoblotting, we measured rab 3A and VAMP-IR in homogenates from the cerebral cortex, hippocampus, and corpus striatum of control (breathing room air) and hypoxiated (breathing 95.5% N2-6.5% O2 for 70 min) 4-day-old rats at 1, 2, and 6 h after the end of the hypoxia. Immunostaining with examination by light microscopy was performed using the synaptic protein-specific antibodies on fixed brain sections from animals belonging to the same litter and submitted to hypoxia. 3. A transient increase of VAMP-IR was observed in the hippocampus and corpus striatum, and for rab 3A in the striatum, 1 h after initiating reoxygenation. At the following time points the values returned to control levels. This effect was less clearly observed in the immunostained sections. 4. Mild hypoxia has an effect on sensitive brain regions, eliciting an increase in the IR of at least two proteins involved in the synaptic vesicle cycle. The transient nature of this effect possibly indicates the activation of endogenous neuroprotective mechanisms.

Facilitation, augmentation and potentiation at central synapses

Release probability (P) appears to be a major factor that influences the pattern of transmitter release. At cortical pyramidal axon inputs onto different classes of target cells, very different release patterns are observed, patterns that correlate with release probability. Simplistically, 'low P' synapses display facilitation and augmentation, whereas 'high P' synapses supplied by the same axon exhibit paired-pulse and frequency-dependent depression. Different combinations of factors probably contribute to release probability at different terminals, during development and under different experimental conditions. The recent advances made by molecular biological studies of the release machinery do, however, provide candidate proteins and protein-protein interactions whose differential distributions might be important factors in determining the patterns of transmitter release.

Role of synaptophysin in exocytotic release of dopamine from Xenopus oocytes injected with rat brain mRNA

1. The role of synaptophysin in the exocytotic release of dopamine (DA) was examined in Xenopus laevis oocytes injected with rat brain mRNA. 2. The mRNA-injected oocytes showed DA uptake which depended on the incubation time and external DA concentrations. 3. Stimulation with KCl (10-50 mM) of mRNA-injected oocytes preloaded with DA evoked external Ca2+ -dependent release of DA. The noninjected and water-injected oocytes did not produce uptake of DA and stimulation-evoked release of DA. 4. The high-KCl (50 mM)-stimulated release of DA decreased in the oocytes injected with rat brain mRNA together with antibody to synaptophysin. 5. Immunoblot analysis demonstrated that synaptophysin was expressed in the brain mRNA-injected oocytes but not in the noninjected and water-injected oocytes. 6. Thus, uptake and release machinery similar to native dopaminergic nerve terminals was expressed in Xenopus oocytes by injecting mRNA-extracted from the rat brain, and synaptophysin may play a role in the exocytotic release of DA.

Synaptic vesicle biogenesis

Synaptic vesicles, which have been a paradigm for the fusion of a vesicle with its target membrane, also serve as a model for understanding the formation of a vesicle from its donor membrane. Synaptic vesicles, which are formed and recycled at the periphery of the neuron, contain a highly restricted set of neuronal proteins. Insight into the trafficking of synaptic vesicle proteins has come from studying not only neurons but also neuroendocrine cells, which form synaptic-like microvesicles (SLMVs). Formation and recycling of synaptic vesicles/SLMVs takes place from the early endosome and the plasma membrane. The cytoplasmic machinery of synaptic vesicle/SLMV formation and recycling has been studied by a variety of experimental approaches, in particular using cell-free systems. This has revealed distinct machineries for membrane budding and fission. Budding is mediated by clathrin and clathrin adaptors, whereas fission is mediated by dynamin and its interacting protein SH3p4, a lysophosphatidic acid acyl transferase.

The synaptic-vesicle-specific proteins rab3a and synaptophysin are reduced in thalamus and related cortical brain regions in schizophrenic brains

Two synaptic-vesicle proteins, rab3a and synaptophysin, have been studied on post-mortem brain tissues of schizophrenics and healthy controls. We found significantly reduced levels of rab3a in thalamus (p<0.001); for both proteins in gyrus cinguli and hippocampus (p<0.0001); for rab3a in frontal and parietal cortex (p<0.05); and no differences in temporal cortex or cerebellum in schizophrenics compared with controls. Reduced synaptic density may be a prominent feature of the molecular neuropathology of schizophrenia.

Essential roles in synaptic plasticity for synaptogyrin I and synaptophysin I

We have generated mice lacking synaptogyrin I and synaptophysin I to explore the functions of these abundant tyrosine-phosphorylated proteins of synaptic vesicles. Single and double knockout mice were alive and fertile without significant morphological or biochemical changes. Electrophysiological recordings in the hippocampal CA1 region revealed that short-term and long-term synaptic plasticity were severely reduced in the synaptophysin/synaptogyrin double knockout mice. LTP was decreased independent of the induction protocol, suggesting that the defect in LTP was not caused by insufficient induction. Our data show that synaptogyrin I and synaptophysin I perform redundant and essential functions in synaptic plasticity without being required for neurotransmitter release itself.

Differences in the ways sympathetic neurons and endocrine cells process, store, and secrete exogenous neuropeptides and peptide-processing enzymes

Most neurons store peptides in large dense core vesicles (LDCVs) and release the neuropeptides in a regulated manner. Although LDCVs have been studied in endocrine cells, less is known about these storage organelles in neurons. In this study we use the endogenous peptide NPY (neuropeptide Y) and the endogenous peptide-processing enzyme PAM (peptidylglycine alpha-amidating monooxygenase) as tools to study the peptidergic system in cultured neurons from the superior cervical ganglion (SCG). Once mature, SCG neurons devote as much of their biosynthetic capabilities to neurotransmitter production as endocrine cells devote to hormone production. Unlike pituitary and atrium, SCG neurons cleave almost all of the bifunctional PAM protein they produce into soluble monofunctional enzymes. Very little PAM or NPY is secreted under basal conditions, and the addition of secretagogue dramatically stimulates the secretion of PAM and NPY to a similar extent. Although endocrine cells typically package "foreign" secretory products together with endogenous products, pro-opiomelanocortin- and PAM-derived products encoded by adenovirus in large part were excluded from the LDCVs of SCG neurons. When expressed in corticotrope tumor cells and primary anterior pituitary cultures, the same virally encoded products were metabolized normally. The differences that were observed could reflect differences in the properties of neuronal and endocrine peptidergic systems or differences in the ability of neurons and endocrine cells to express viral transcripts.

Genetics of synaptic vesicle function: toward the complete functional anatomy of an organelle

Synaptic transmission starts with the release of neurotransmitters by exocytosis of synaptic vesicles. As a relatively simple organelle with a limited number of components, synaptic vesicles are in principle accessible to complete structural and functional genetic analysis. At present, the majority of synaptic vesicle proteins has been characterized, and many have been genetically analyzed in mice, Drosophila, and Caenorhabditis elegans. These studies have shown that synaptic vesicles contain proteins with diverse structures and functions. Although the genetic studies are as yet unfinished, they promise to lead to a full description of synaptic vesicles as macromolecular machines involved in all aspects of presynaptic neurotransmitter release.

Identification of synaptic vesicle, pre- and postsynaptic proteins in human cerebrospinal fluid using liquid-phase isoelectric focusing

Synaptic pathology is central in the pathogenesis of several psychiatric disorders, for example in Alzheimer's disease (AD) and schizophrenia. Quantification of specific synaptic proteins has proved to be a useful method to estimate synapitc density in the brain. Using this approach, several synaptic proteins have been demonstrated to be altered in both AD and schizophrenia. Until recently, the analysis of synaptic pathology has been limited to postmortem tissue. In living subjects, these synaptic proteins may be studied through analysis of cerebrospinal fluid (CSF). In an earlier study performed by us, one synaptic vesicle specific protein, synaptotagmin, was detected in CSF for the first time using a procedure based on affinity chromatography, reversed-phase chromatography, sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) and chemiluminescence immunoblotting. However, other synaptic proteins were not detectable with this procedure. Therefore, we have developed a procedure including precipitation of CSF proteins with trichloroacetic acid, followed by liquid-phase isoelectric focusing using the Rotofor Cell, and finally analysis of Rotofor fractions by Western blotting for identification of synaptic proteins in CSF. Five synaptic proteins, rab3a, synaptotagmin, growth-associated protein (GAP-43), synaptosomal-associated protein (SNAP-25) and neurogranin, have been demonstrated in CSF using this method. The major advantage of liquid-phase isoelectric focusing (IEF) using the Rotofor cell is that it provides synaptic proteins from CSF in sufficient quantities for identification. This method may also be suitable for identification of other types of trace amounts of brain-specific proteins in CSF. These results demonstrate that several synaptic proteins can be identified and measured in CSF to study synaptic function and pathology in degenerative disorders.

Effects of calyculin A and okadaic acid on acetylcholine release and subcellular distribution in rat hippocampal formation

The mechanisms regulating the compartmentation of acetylcholine (ACh) and the relationship between transmitter release and ACh stores are not fully understood. In the present experiments, we investigated whether the inhibitors of serine/threonine phosphatases 1 and 2A, calyculin A and okadaic acid, alter subcellular distribution and the release of ACh in rat hippocampal slices. Calyculin A and okadaic acid significantly (p < 0.05) depleted the occluded ACh of the vesicular P3 fraction, but cytoplasmic ACh contained in the S3 fraction was not significantly affected. The P3 fraction is known to be heterogeneous; calyculin A and okadaic acid reduced significantly (p < 0.05) the amount of ACh recovered with a monodispersed fraction (D) of synaptic vesicles, but the other nerve terminal bound pools (E-F and G-H) were not so affected. K+-evoked ACh release decreased significantly (p < 0.01) in the presence of calyculin A and okadaic acid, suggesting that fraction D's vesicular store of ACh contributes to transmitter release. The loss of ACh from synaptic vesicle fractions prepared from tissue exposed to phosphatase inhibitors appeared not to result from a reduced ability to take up ACh. Thus, when tissue was allowed to synthesize [3H]ACh from [3H]choline, the ratio of [3H]ACh in the S3 to P3 fractions was not much changed by exposure of tissue to calyculin A or okadaic acid; furthermore, the specific activity of ACh recovered from the D fraction was not reduced disproportionately to that of cytosolic ACh. The changes are considered to reflect reduced synthesis of ACh by tissue treated with the phosphatase inhibitors, rather than an effect on vesicle uptake mechanisms. Thus, exposure of tissue to calyculin A or okadaic acid appears to produce selective depletion of tissue ACh content in a subpopulation of synaptic vesicles, suggesting that phosphatases play a role in ACh compartmentation.

SVOP, an evolutionarily conserved synaptic vesicle protein, suggests novel transport functions of synaptic vesicles

We describe a novel synaptic vesicle protein called SVOP that is distantly related to the synaptic vesicle proteins SV2A, SV2B, and SV2C (20-22% sequence identity). Both SVOP and SV2 contain 12 transmembrane regions. However, SV2 is highly glycosylated, whereas SVOP is not. Databank searches revealed that closely related homologs of SVOP are present in Caenorhabditis elegans and Drosophila (48% sequence identity), suggesting that SVOP is evolutionarily ancient. In contrast, no invertebrate orthologs of SV2 were detected. The sequences of SVOP and SV2 exhibit homology with transport proteins, in particular with mammalian organic cation and anion transporters. SVOP and SV2 are more distantly related to eukaryotic and bacterial phosphate, sugar, and organic acid transporters. SVOP is expressed at detectable levels only in brain and endocrine cells where it is primarily localized to synaptic vesicles and microvesicles. SVOP is present in all brain regions, with particularly high levels in large pyramidal neurons of the cerebral cortex. Immunocytochemical staining of adjacent rat brain sections for SVOP and SV2 demonstrated that SVOP and SV2 are probably coexpressed in most neurons. Although the functions of SV2 and SVOP remain obscure, the evolutionary conservation of SVOP, its hydrophobic nature, and its homology to transporters strongly support a role in the uptake of a novel, as yet unidentified component of synaptic vesicles. Thus synaptic vesicles contain two classes of abundant proteins with 12 transmembrane regions that are related to transporters, nonglycosylated SVOP and highly glycosylated SV2, suggesting that the transport functions of synaptic vesicles may be more complex than currently envisioned.

The rate zonal separation of organelles from dilute suspensions: the problem of a large sample volume

Transport vesicles that deliver proteins to the cell surface can be isolated by incubating cells that have been permeabilized by mechanical or chemical techniques in a high-K medium containing an ATP regenerating system. The vesicles released from permeabilized cells are, however, obtained as a very dilute suspension in the incubation solution. This presents a problem for the preparative separation of specific populations of vesicles by velocity sedimentation, because the small sample volume capacity of traditional glycerol or sucrose velocity gradients requires that the vesicles first be concentrated by sedimentation or that very small amounts of vesicles be loaded onto a gradient. We have addressed the problem of the loss of zonal resolution produced by the loading of large sample volumes, and we propose that high-viscosity Ficoll gradients can be used effectively to restore the resolution of zones when substantially larger sample volumes of dilute suspensions must be loaded onto velocity gradients.

Vesicular amine transporter expression and isoform selection in developing brain, peripheral nervous system and gut

The vesicular monoamine transporters VMAT1 and VMAT2 are essential components of monoaminergic neurons and endocrine cells whose expression in development may provide insight into lineage pathways for chemical coding in the diffuse neuroendocrine system. Thus, the brain is a compartment in which only monoaminergic neurons are generated, the gut epithelium generates only endocrine monoamine-containing cells, and the neural crest produces both autonomic monoaminergic neurons and endocrine/paracrine monoaminergic cells. Selection of either the VMAT1 or VMAT2 isoform was examined in these three compartments during development. In the central nervous system VMAT2, but not VMAT1, was expressed in neuroepithelial cells by embryonic day 12 (E12), and all major monoaminergic cell groups by E14. Thalamocortical and hypothalamic neurons that do not express VMAT2 in adulthood were transiently VMAT2-positive from E16 to postnatal day 6 (P6). EC cells of the gut expressed exclusively VMAT1 from E19 on, while histamine-containing enterochromaffin-like (ECL) cells of the stomach expressed only VMAT2 by E19 and throughout postnatal development. VMAT2 and the vesicular acetylcholine transporter VAChT were co-expressed in early development of the primary sympathetic chain as well as in the cranial parasympathetic ganglia. VAChT was progressively restricted to a small population of VMAT2-negative post-ganglionic neurons in the adult sympathetic chain, while VMAT2 expression persisted in sympathetic principal ganglion and SIF cells but was eventually extinguished in cranial parasympathetic ganglia. VMAT1 was co-expressed with VAChT and VMAT2 mRNA in the primary sympathetic chain on E12, but progressively restricted to small intensely fluorescent (SIF) and chromaffin cells thereafter. Thus, expression of the vesicular amine transporters appropriate for chemical coding of brain neurons and gut endocrine cells are pre-determined developmentally. In contrast, the neural crest-derived sympathoadrenal and neural crest-derived parasympathetic cell groups examined here initially co-express two or more vesicular amine transporters, followed by extinction of the inappropriate transporter(s) later in development. Some neural crest-derived neuroendocrine cell populations continue to express both isoforms of VMAT even in adulthood. Lineage distinctions in ontogeny of vesicular amine transporter expression in brain, gut and autonomic nervous system make it likely that the same genes are regulated differently in the autonomic nervous system compared to brain and gut.

Vesicular neurotransmitter transporters. Potential sites for the regulation of synaptic function

Neurotransmission depends on the regulated release of chemical transmitter molecules. This requires the packaging of these substances into the specialized secretory vesicles of neurons and neuroendocrine cells, a process mediated by specific vesicular transporters. The family of genes encoding the vesicular transporters for biogenic amines and acetylcholine have recently been cloned. Direct comparison of their transport characteristics and pharmacology provides information about vesicular transport bioenergetics, substrate feature recognition by each transporter, and the role of vesicular amine storage in the mechanism of action of psychopharmacologic and neurotoxic agents. Regulation of vesicular transport activity may affect levels of neurotransmitter available for neurosecretion and be an important site for the regulation of synaptic function. Gene knockout studies have determined vesicular transport function is critical for survival and have enabled further evaluation of the role of vesicular neurotransmitter transporters in behavior and neurotoxicity. Molecular analysis is beginning to reveal the sites involved in vesicular transporter function and the sites that determine substrate specificity. In addition, the molecular basis for the selective targeting of these transporters to specific vesicle populations and the biogenesis of monoaminergic and cholinergic synaptic vesicles are areas of research that are currently being explored. This information provides new insights into the pharmacology and physiology of biogenic amine and acetylcholine vesicular storage in cardiovascular, endocrine, and central nervous system function and has important implications for neurodegenerative disease.

Regulated Secretion in Platelets: Identification of Elements of the Platelet Exocytosis Machinery

To further characterize the molecular mechanisms of platelet function, we have sought to identify some of the proteins that mediate the secretory events of the platelet release reaction. We report that platelets contain the general elements of the membrane transport apparatus: N-ethylmaleimide sensitive fusion protein (NSF ), p115/transcytosis-associated protein (p115/TAP), and the soluble NSF attachment proteins (α- and, γ-SNAP). The cDNAs encoding two of these proteins, α- and γ-SNAP, have been cloned from a human platelet-derived cDNA library. Platelet membrane extracts possess SNAPreceptor (SNARE) activity, suggesting that the class of proteins (SNAREs) proposed to provide the specificity for vesicle docking and membrane fusion are present in platelets. To identify these proteins, we have used specific antibodies against known SNAREs to probe platelet extracts. Syntaxin 2 and 4 can be readily detected in platelet membrane preparations and are shown to participate in 20 S complex formation. Syntaxin 1, 3, and 5 could not be detected. Other known SNARE and SNARE-associated proteins such as vesicle-associated membrane protein (VAMP)/synaptobrevin 2, SNAP-25, synaptophysin, or synaptotagmin I could not be immunochemically detected in platelet membrane preparations. The presence of both the general transport proteins (NSF and SNAPs) and specific transport proteins (syntaxin 2 and 4) indicates that platelet exocytosis uses a molecular mechanism similar to other secretory cells such as neurons. However, the subcellular concentrations of these proteins suggest that, unlike neuronal secretion, granule-to plasma membrane docking may be the limiting step in platelet exocytosis.

Regulated Secretion in Platelets: Identification of Elements of the Platelet Exocytosis Machinery

To further characterize the molecular mechanisms of platelet function, we have sought to identify some of the proteins that mediate the secretory events of the platelet release reaction. We report that platelets contain the general elements of the membrane transport apparatus: N-ethylmaleimide sensitive fusion protein (NSF ), p115/transcytosis-associated protein (p115/TAP), and the soluble NSF attachment proteins (α- and, γ-SNAP). The cDNAs encoding two of these proteins, α- and γ-SNAP, have been cloned from a human platelet-derived cDNA library. Platelet membrane extracts possess SNAPreceptor (SNARE) activity, suggesting that the class of proteins (SNAREs) proposed to provide the specificity for vesicle docking and membrane fusion are present in platelets. To identify these proteins, we have used specific antibodies against known SNAREs to probe platelet extracts. Syntaxin 2 and 4 can be readily detected in platelet membrane preparations and are shown to participate in 20 S complex formation. Syntaxin 1, 3, and 5 could not be detected. Other known SNARE and SNARE-associated proteins such as vesicle-associated membrane protein (VAMP)/synaptobrevin 2, SNAP-25, synaptophysin, or synaptotagmin I could not be immunochemically detected in platelet membrane preparations. The presence of both the general transport proteins (NSF and SNAPs) and specific transport proteins (syntaxin 2 and 4) indicates that platelet exocytosis uses a molecular mechanism similar to other secretory cells such as neurons. However, the subcellular concentrations of these proteins suggest that, unlike neuronal secretion, granule-to plasma membrane docking may be the limiting step in platelet exocytosis.

Biochemical evidence for the presence of NAP-22, a novel acidic calmodulin binding protein, in the synaptic vesicles of rat brain

NAP-22 is a neuronal tissue-enriched acidic protein which is expressed predominantly in rat brain. In the present study, we quantitated the amount of NAP-22 in a highly purified synaptic vesicle fraction. NAP-22 comprised 1.3 +/- 0.15% of the total protein in the fraction, and NAP-22 was located on the external surface of the synaptic vesicle membrane. The results suggest NAP-22 may be involved in the synaptic vesicle cycling.

Ultrastructural localization of the vesicular monoamine transporter-2 in midbrain dopaminergic neurons: potential sites for somatodendritic storage and release of dopamine

Midbrain dopaminergic neurons are known to release dopamine from somata and/or dendrites located in the substantia nigra (SN) and the ventral tegmental area (VTA). There is considerable controversy, however, about the subcellular sites for somatodendritic dopamine storage in these regions. In the present study, we used dual-labeling electron microscopic immunocytochemistry to localize the vesicular monoamine transporter-2 (VMAT2), a novel marker for sites of intracellular monoamine storage, within identified dopaminergic (tyrosine hydroxylase-containing) neurons in the rat SN and VTA. In dopaminergic perikarya, immunogold labeling for VMAT2 was localized to the Golgi apparatus, tubulovesicles that resembled smooth endoplasmic reticulum (SER), and the limiting membranes of multivesicular bodies. In dopaminergic dendrites, VMAT2 was extensively localized to tubulovesicles that resembled saccules of SER, and less frequently localized to isolated small synaptic vesicles (SSVs) or large dense-core vesicles (DCVs). In rare cases, VMAT2-immunoreactive SSVs were clustered within the cytoplasm of an SN or a VTA dendrite. Dopaminergic dendrites in the VTA contained a significantly higher number of immunogold particles for VMAT2 per unit than those in the SN. Together, these observations support the proposal that dopamine is stored in and may be released from dendritic SSVs and DCVs, but suggest that the SER is the major site of dopamine storage within midbrain dopaminergic neurons. In addition, they provide new evidence that dopaminergic dendrites in the VTA may have greater potential for reserpine-sensitive storage and release of dopamine than those in the SN.

Novel proteins that interact with the COOH-terminal cytosolic routing determinants of an integral membrane peptide-processing enzyme

The steady state distribution of membrane forms of peptidylglycine alpha-amidating monooxygenase (PAM) in the secretory pathway of neurons and endocrine cells depends on signals in its cytosolic COOH-terminal domain (CD). Mutagenesis studies yielded catalytically active PAM proteins that are not properly localized or internalized. Employing the yeast two-hybrid system, we isolated two distinct cDNAs whose protein products showed a strong interaction with the CD of PAM. The interaction of these novel PAM COOH-terminal interactor proteins (P-CIPs) did not occur with a misrouted CD mutant as bait in the yeast system. Both proteins, P-CIP2 and P-CIP10, were expressed as fusion proteins that interacted in vitro with solubilized integral membrane PAM. P-CIP2 was homologous to several serine/threonine and dual specificity protein kinases, while P-CIP10 contained spectrin-like repeats. Endogenous P-CIP2 was localized to the Golgi region of AtT-20 corticotrope tumor cells, and expression of integral membrane PAM disrupted the distribution of endogenous P-CIP2. Both P-CIP2 and P-CIP10 mRNAs were found to be expressed in rat brain neurons also expressing PAM proteins. P-CIP2 and P-CIP10 may be members of a family of cytosolic proteins involved in the routing of membrane proteins that function in the regulated secretory pathway.

The synaptic protein UNC-18 is phosphorylated by protein kinase C

The C. elegans unc-18 encoded protein UNC-18 is implicated in the interactions between synaptic vesicles and presynaptic plasma membrane. To further characterize the neural protein, we investigated the phosphorylation in vitro of the protein expressed in Spodoptera frugiperda Sf21 cells. The UNC-18 protein is selectively phosphorylated by protein kinase C (PKC) but not by casein kinase II and cyclic AMP-dependent protein kinase. The presumed phosphorylation sites determined by manual Edman degradation were serine-2, serine-322, threonine-462 and serine-515, of which the last is highly conserved as a consensus phosphorylation site for PKC in Drosophila and the mammalian homologue. Phosphorylated UNC-18 extracted from C. elegans was also detected, indicating that it has a physiological role in intact nerve terminals. Therefore, the phosphorylation by PKC may play a physiological role in the regulation.

Synaptic organization of an organotypic slice culture of the mammalian retina

Vertical slices of postnatal day 6 (P6) rat retina were cut and cultured using the roller-tube technique. The organotypic differentiation during a culture period of up to 30 days has been described in a previous study (Feigenspan et al., 1993a). Here we concentrated on the synaptic organization in the retinal slice culture. Electron microscopy revealed the presence of ribbon synapses in the outer plexiform layer and conventional and ribbon synapses in the inner plexiform layer. Immunofluorescence with antibodies that recognize specific subunits of GABAA or glycine receptors revealed a punctate distribution of the receptors. They were aggregated in "hot spots" that correspond to a concentration of receptors at postsynaptic sites. Different isoforms of GABAA and glycine receptors occurred in the slice cultures. The experiments show that there is a differentiation of synapses and a diversity of transmitter receptors in the slice cultures that is comparable to the in vivo retina.

Distributions of two homologous synaptic vesicle proteins, synaptoporin and synaptophysin, in the mammalian retina

Synaptophysin and synaptoporin are homologous proteins that are among the most abundant synaptic vesicle proteins. Despite their high degree of sequence similarity, they are differentially distributed in the brain. The distribution of synaptophysin and synaptoporin was examined in the adult rat and rabbit retina by using single- and double- labeling immunocytochemistry with conventional light microscopy and confocal laser scanning microscopy. In the rat retina, synaptophysin immunoreactivity was found in the outer plexiform layer in terminals of photoreceptors and was homogeneously distributed throughout the inner plexiform layer. Synaptoporin immunoreactivity, however, was restricted to the inner plexiform layer. Labeling was most prominent in three distinct bands of the inner plexiform layer separated by two bands of very low synaptoporin immunoreactivity. In the rabbit retina, synaptophysin and synaptoporin immunoreactivity were found in the inner and outer plexiform layers. In the inner plexiform layer, labeling for both vesicle proteins was homogeneous, with no detectable stratification. In the outer plexiform layer, synaptophysin was present in photoreceptor terminals, and synaptoporin was present in horizontal cells. Staining of isolated rabbit retinal cells confirmed that both the axonless A type and the axon-bearing B type horizontal cells are immunoreactive for synaptoporin. In addition, electron microscopy of synaptoporin-immunostained rabbit retinas revealed no labeling of photoreceptor terminals but of putative synaptic sites in horizontal cells in the outer plexiform layer. No functional correlation was found in the expression of either synaptic vesicle protein with the type of neuron or synapse (ribbon or conventional).

Synaptic pathology in Alzheimer's disease: relation to severity of dementia, but not to senile plaques, neurofibrillary tangles, or the ApoE4 allele

Alzheimer's disease (AD) is characterised by an increased number of senile plaques (SP) and neurofibrillary tangles (NFT) as compared with that found in non-demented individuals of the same age, and a marked degeneration and loss of synapses. One of the main risk-factors for the disorder is inheritance of the apolipoprotein E4 (ApoE4) allele. To further study the relation between these pathogenetic substrates for AD, we quantified the synaptic vesicle membrane protein rab3a in brain tissue from 19 patients with AD and 9 age-matched control subjects. Rab3a levels were reduced in AD, both in the hippocampus (60% of control level, p < 0.0001), and in the frontal cortex (68% of control level, p < 0.01), but not in the cerebellum (92% of control level). Within the AD group, lower rab3a levels were found both with increasing duration and severity of dementia. These findings further support that synaptic pathology is closely correlated to the clinical dementia in AD. In contrast, no significant correlations were found between SP counts and duration or severity of dementia, while higher NFT counts in the frontal cortex were found with increasing severity of dementia (r = 0.54, p < 0.05). There were no significant correlations between the rab3a level and SP or NFT counts, and by immunohistochemistry, reduced rab3a immunostaining was found throughout the neuropil in AD brain, without relation to SP or NFT. These findings suggest that the synaptic pathology in AD is not closely related to the presence of SP and NFT. No significant differences in rab3a levels were found in any brain region between AD patients possessing different numbers of the ApoE4 allele, suggesting that, although ApoE4 is A risk factor for earlier development of AD, the degree of synaptic pathology does not differ between patients with or without the ApoE4 allele.

Endocytosis of VAMP is facilitated by a synaptic vesicle targeting signal

After synaptic vesicles fuse with the plasma membrane and release their contents, vesicle membrane proteins recycle by endocytosis and are targeted to newly formed synaptic vesicles. The membrane traffic of an epitope-tagged form of VAMP-2 (VAMP-TAg) was observed in transfected cells to identify sequence requirements for recycling of a synaptic vesicle membrane protein. In the neuroendocrine PC12 cell line VAMP-TAg is found not only in synaptic vesicles, but also in endosomes and on the plasma membrane. Endocytosis of VAMP-TAg is a rapid and saturable process. At high expression levels VAMP-TAg accumulates at the cell surface. Rapid endocytosis of VAMP-TAg also occurs in transfected CHO cells and is therefore independent of other synaptic proteins. The majority of the measured endocytosis is not directly into synaptic vesicles since mutations in VAMP-TAg that enhance synaptic vesicle targeting did not affect endocytosis. Nonetheless, mutations that inhibited synaptic vesicle targeting, in particular replacement of methionine-46 by alanine, inhibited endocytosis by 85% in PC12 cells and by 35% in CHO cells. These results demonstrate that the synaptic vesicle targeting signal is also used for endocytosis and can be recognized in cells lacking synaptic vesicles.

Temporally resolved, independent stages of individual exocytotic secretion events

The stages of the complex events involved in exocytotic secretion after vesicle-cell membrane fusion have been examined at the level of individual vesicles. Catecholamine flux from single bovine adrenal medullary cells was measured with carbon-fiber microelectrodes firmly touching the cell surface. The data reveal that secretion during exocytotic events has three distinct stages: a small increase in catecholamine flux, a rapid, but not instantaneous, rise to a maximum, followed by an exponential decrease in the flux. These stages are interpreted in the following ways. The initial stage corresponds to catecholamine secretion through a fusion pore. The rate of pore expansion appears to control the rise time of the flux to its maximum value. The final exponential stage is consistent with chemical dissociation of the intravesicular matrix or gel.

Structure of synaptogyrin (p29) defines novel synaptic vesicle protein

Synaptogyrin (p29) is a synaptic vesicle protein that is uniformly distributed in the nervous system (Baumert et al., 1990). We have cloned and sequenced the cDNA encoding synaptogyrin, and the sequence predicts a protein with a molecular mass of 25,900 D with four membrane-spanning domains. The topology of the protein was confirmed by limited proteolysis using domain-specific antibodies. Database searches revealed several cDNA sequences coding polypeptides with sequence identities ranging from 32 to 46%, suggesting that synaptogyrin is a member of a multigene family. When the synaptogyrin cDNA is expressed in COS cells, the generated protein is indistinguishable from native synaptogyrin. To study intracellular sorting, synaptogyrin was expressed in CHO cells that revealed a punctate staining that was very similar to that of synaptophysin and endogenously expressed cellubrevin. Significant overlap with transferrin staining was also observed, suggesting that synaptogyrin is targeted to a recycling compartment involved in membrane traffic to and from the plasma membrane.

A targeting signal in VAMP regulating transport to synaptic vesicles

VAMP is a synaptic vesicle membrane protein required for fusion. Synaptic vesicle targeting was measured for mutants of an epitope-tagged form of VAMP in transfected PC12 cells. A signal within a predicted amphipathic alpha helix is essential for targeting to synaptic vesicles. Cellubrevin, a nonneural VAMP homolog, contains this signal and is also targeted to synaptic vesicles. Amino acid substitutions within the synaptic vesicle targeting signal either enhance or inhibit sorting of VAMP to synaptic vesicles, but do not affect the ability of VAMP to form complexes with syntaxin and SNAP-25.

Huntingtin is a cytoplasmic protein associated with vesicles in human and rat brain neurons

The gene defective in Huntington's disease encodes a protein, huntingtin, with unknown function. Antisera generated against three separate regions of huntingtin identified a single high molecular weight protein of approximately 320 kDa on immunoblots of human neuroblastoma extracts. The same protein species was detected in human and rat cortex synaptosomes and in sucrose density gradients of vesicle-enriched fractions, where huntingtin immunoreactivity overlapped with the distribution of vesicle membrane proteins (SV2, transferrin receptor, and synaptophysin). Immunohistochemistry in human and rat brain revealed widespread cytoplasmic labeling of huntingtin within neurons, particularly cell bodies and dendrites, rather than the more selective pattern of axon terminal labeling characteristic of many vesicle-associated proteins. At the ultrastructural level, immunoreactivity in cortical neurons was detected in the matrix of the cytoplasm and around the membranes of the vesicles. The ubiquitous cytoplasmic distribution of huntingtin in neurons and its association with vesicles suggest that huntingtin may have a role in vesicle trafficking.

Synaptic vesicles and release of transmitters: new insights at the molecular level

Neurotransmitter release from transmitter storage vesicles is a regulated signalling event that has properties in common with other secretory systems. Biochemical characterization of mammalian synaptic vesicle proteins has recently converged with studies of protein traffic in non-neuronal cells and the genetic dissection of the yeast secretory pathway to give us a considerable amount of new data. Many new synaptic vesicle proteins have been characterized together with plasma membrane proteins with which they interact, and it appears that many of the participating components may be part of a general machinery for secretion. The new results significantly improve our understanding of the molecular mechanisms governing transmitter release. This review discusses the recent progress in terms of synaptic vesicle components and the proposed mechanisms for exocytosis.

The t-SNAREs syntaxin 1 and SNAP-25 are present on organelles that participate in synaptic vesicle recycling

Syntaxin 1 and synaptosome-associated protein of 25 kD (SNAP-25) are neuronal plasmalemma proteins that appear to be essential for exocytosis of synaptic vesicles (SVs). Both proteins form a complex with synaptobrevin, an intrinsic membrane protein of SVs. This binding is thought to be responsible for vesicle docking and apparently precedes membrane fusion. According to the current concept, syntaxin 1 and SNAP-25 are members of larger protein families, collectively designated as target-SNAP receptors (t-SNAREs), whose specific localization to subcellular membranes define where transport vesicles bind and fuse. Here we demonstrate that major pools of syntaxin 1 and SNAP-25 recycle with SVs. Both proteins cofractionate with SVs and clathrin-coated vesicles upon subcellular fractionation. Using recombinant proteins as standards for quantitation, we found that syntaxin 1 and SNAP-25 each comprise approximately 3% of the total protein in highly purified SVs. Thus, both proteins are significant components of SVs although less abundant than synaptobrevin (8.7% of the total protein). Immunoisolation of vesicles using synaptophysin and syntaxin specific antibodies revealed that most SVs contain syntaxin 1. The widespread distribution of both syntaxin 1 and SNAP-25 on SVs was further confirmed by immunogold electron microscopy. Botulinum neurotoxin C1, a toxin that blocks exocytosis by proteolyzing syntaxin 1, preferentially cleaves vesicular syntaxin 1. We conclude that t-SNAREs participate in SV recycling in what may be functionally distinct forms.