Expression of the synaptic vesicle proteins VAMPs/synaptobrevins 1 and 2 in non-neural tissues

Life, death, and self: Fundamental questions of primitive cognition viewed through the lens of body plasticity and synthetic organisms

Central to the study of cognition is being able to specify the Subject that is making decisions and owning memories and preferences. However, all real cognitive agents are made of parts (such as brains made of cells). The integration of many active subunits into a coherent Self appearing at a larger scale of organization is one of the fundamental questions of evolutionary cognitive science. Typical biological model systems, whether basal or advanced, have a static anatomical structure which obscures important aspects of the mind-body relationship. Recent advances in bioengineering now make it possible to assemble, disassemble, and recombine biological structures at the cell, organ, and whole organism levels. Regenerative biology and controlled chimerism reveal that studies of cognition in intact, "standard", evolved animal bodies are just a narrow slice of a much bigger and as-yet largely unexplored reality: the incredible plasticity of dynamic morphogenesis of biological forms that house and support diverse types of cognition. The ability to produce living organisms in novel configurations makes clear that traditional concepts, such as body, organism, genetic lineage, death, and memory are not as well-defined as commonly thought, and need considerable revision to account for the possible spectrum of living entities. Here, I review fascinating examples of experimental biology illustrating that the boundaries demarcating somatic and cognitive Selves are fluid, providing an opportunity to sharpen inquiries about how evolution exploits physical forces for multi-scale cognition. Developmental (pre-neural) bioelectricity contributes a novel perspective on how the dynamic control of growth and form of the body evolved into sophisticated cognitive capabilities. Most importantly, the development of functional biobots - synthetic living machines with behavioral capacity - provides a roadmap for greatly expanding our understanding of the origin and capacities of cognition in all of its possible material implementations, especially those that emerge de novo, with no lengthy evolutionary history of matching behavioral programs to bodyplan. Viewing fundamental questions through the lens of new, constructed living forms will have diverse impacts, not only in basic evolutionary biology and cognitive science, but also in regenerative medicine of the brain and in artificial intelligence.

The microRNA-127-3p directly targeting Vamp2 in C2C12 myoblasts

MicroRNAs (miRNAs) have been reported that can regulate skeletal muscle growth and development. Previously, we demonstrated that miR-127-3p were differently expressed in skeletal muscle and muscle cells. However, the molecular mechanism of miR-127-3p regulation of skeletal myogenesis are not well elucidated. In this study, we transfected miR-127-3p into C2C12 cells, and found miR-127-3p induces myogenesis by targeting Vamp2. Moreover, the regulatory mechanism of Vamp2 in myoblasts proliferation and differentiation was further confirmed. In conclusion, our data providedevidences that miR-127-3p reciprocally regulated myoblasts proliferation and differentiation through directly targeting Vamp2.

Exercise, GLUT4, and skeletal muscle glucose uptake

Glucose is an important fuel for contracting muscle, and normal glucose metabolism is vital for health. Glucose enters the muscle cell via facilitated diffusion through the GLUT4 glucose transporter which translocates from intracellular storage depots to the plasma membrane and T-tubules upon muscle contraction. Here we discuss the current understanding of how exercise-induced muscle glucose uptake is regulated. We briefly discuss the role of glucose supply and metabolism and concentrate on GLUT4 translocation and the molecular signaling that sets this in motion during muscle contractions. Contraction-induced molecular signaling is complex and involves a variety of signaling molecules including AMPK, Ca(2+), and NOS in the proximal part of the signaling cascade as well as GTPases, Rab, and SNARE proteins and cytoskeletal components in the distal part. While acute regulation of muscle glucose uptake relies on GLUT4 translocation, glucose uptake also depends on muscle GLUT4 expression which is increased following exercise. AMPK and CaMKII are key signaling kinases that appear to regulate GLUT4 expression via the HDAC4/5-MEF2 axis and MEF2-GEF interactions resulting in nuclear export of HDAC4/5 in turn leading to histone hyperacetylation on the GLUT4 promoter and increased GLUT4 transcription. Exercise training is the most potent stimulus to increase skeletal muscle GLUT4 expression, an effect that may partly contribute to improved insulin action and glucose disposal and enhanced muscle glycogen storage following exercise training in health and disease.

VAMP2 marks quiescent satellite cells and myotubes, but not activated myoblasts

We examined the expression and intracellular localization of vesicle-associated membrane protein 2 (VAMP2) during the differentiation of skeletal muscle cells by immunofluorescence microscopy. In isolated single myofibers, VAMP2 was expressed in quiescent satellite cells, downregulated in proliferating myoblastic cells, and re-expressed with differentiation. In the myoblastic cell line C2C12, VAMP2 was expressed at a low level in the proliferating stage, and then increased after differentiation into myotubes. Based on these results, we propose that VAMP2 can be used as a molecular marker for both quiescent satellite cells and myotubes, but not for proliferating myoblasts. We also found the partial colocalization of VAMP2 with transferrin- or Rab11-labeled vesicles in myotubes, suggesting a role of VAMP2 in the trafficking of recycling endosomes.

Effects of contraction on localization of GLUT4 and v-SNARE isoforms in rat skeletal muscle

In skeletal muscle, contractions increase glucose uptake due to a translocation of GLUT4 glucose transporters from intracellular storage sites to the surface membrane. Vesicle-associated membrane proteins (VAMPs) are believed to play an important role in docking and fusion of the GLUT4 transporters at the surface membrane. However, knowledge about which VAMP isoforms colocalize with GLUT4 vesicles in mature skeletal muscle and whether they translocate during muscle contractions is incomplete. The aim of the present study was to further identify VAMP isoforms, which are associated with GLUT4 vesicles and examine which VAMP isoforms translocate to surface membranes in skeletal muscles undergoing contractions. VAMP2, VAMP3, VAMP5, and VAMP7 were enriched in immunoprecipitated GLUT4 vesicles. In response to 20 min of in situ contractions, there was a redistribution of GLUT4 (+64 +/- 13%), transferrin receptor (TfR; +75 +/- 22%), and insulin-regulated aminopeptidase (IRAP; +70 +/- 13%) to fractions enriched in heavy membranes away from low-density membranes (-32 +/- 7%; -18 +/- 12%; -33 +/- 9%; respectively), when compared with the resting contralateral muscle. Similarly, there was a redistribution of VAMP2 (+240 +/- 40%), VAMP5 (+79 +/- 9%), and VAMP7 (+79 +/- 29%), but not VAMP3, to fractions enriched in heavy membranes away from low-density membranes (-49 +/- 10%, -54 +/- 9%, -14 +/- 11%, respectively) in contracted vs. resting muscle. In summary, VAMP2, VAMP3, VAMP5, and VAMP7 coimmunoprecipitate with intracellular GLUT4 vesicles in muscle, and VAMP2, VAMP5, VAMP7, but not VAMP3, translocate to the cell surface membranes similar to GLUT4, TfR, and IRAP in response to muscle contractions. These findings suggest that VAMP2, VAMP5, and VAMP7 may be involved in translocation of GLUT4 during muscle contractions.

v-SNARE cellubrevin is required for basolateral sorting of AP-1B-dependent cargo in polarized epithelial cells

The epithelial cell–specific adaptor complex AP-1B is crucial for correct delivery of many transmembrane proteins from recycling endosomes to the basolateral plasma membrane. Subsequently, membrane fusion is dependent on the formation of complexes between SNARE proteins located at the target membrane and on transport vesicles. Although the t-SNARE syntaxin 4 has been localized to the basolateral membrane, the v-SNARE operative in the AP-1B pathway remained unknown. We show that the ubiquitously expressed v-SNARE cellubrevin localizes to the basolateral membrane and to recycling endosomes, where it colocalizes with AP-1B. Furthermore, we demonstrate that cellubrevin coimmunoprecipitates preferentially with syntaxin 4, implicating this v-SNARE in basolateral fusion events. Cleavage of cellubrevin with tetanus neurotoxin (TeNT) results in scattering of AP-1B localization and missorting of AP-1B–dependent cargos, such as transferrin receptor and a truncated low-density lipoprotein receptor, LDLR-CT27. These data suggest that cellubrevin and AP-1B cooperate in basolateral membrane trafficking.

Characterization of VAMP-2 gene from marine teleostean, Lateolabrax japonicus

The whole length SPV2 gene of 715 bp, encoding VAMP-2 protein of 110 amino acids from Japanese sea perch, Lateolabrax japonicus, was obtained by using both RT-PCR and anchored PCR strategies while we initiated the structural and functional study on SNARE proteins in marine teleostean. Analysis of the deduced amino acid sequence indicated that SPV2 has its core arginine residue, a potential N-linked glycosylation site near its N-terminal, and one transmembrane domain in its C-terminal. Advanced structural analysis of bioinformatics approach predicts a coiled-coil alpha-helix backbone as the characteristic of SPV2 main conformational structure, identical to the structure of rat VAMP-2 obtained by crystallography. Semi-quantitative RT-PCR revealed that SPV2 was generally expressed in 10 neural and non-neural tissues, with the highest concentration in brain and the least in muscle.

VAMP2 is expressed in muscle satellite cells and up-regulated during muscle regeneration

Membrane trafficking is one of the most important mechanisms involved in the establishment and maintenance of the forms and functions of the cell. However, it is poorly understood in skeletal muscle cells. In this study, we have focused on vesicle-associated membrane proteins (VAMPs), which are components of the vesicle docking and fusion complex, and have performed immunostaining to investigate the expression of VAMPs in rat skeletal muscle tissue. We have found that VAMP2, but not VAMP1 or VAMP3, is expressed in satellite cells. VAMP2 is also expressed in myofibers in the soleus muscle and nerve endings. This is consistent with previous studies in which VAMP2 has been shown to regulate GLUT4 trafficking in slow-twitch myofibers in soleus muscle and neurotransmitter release in nerve endings. As satellite cells are quiescent myogenic cells, the expression of VAMP2 has further been examined in regenerating muscles after injury by the snake venom, cardiotoxin; we have observed enhanced expression of VAMP2 in immature myotubes with a peak at 3 days after injury. Our findings suggest that VAMP2 plays roles in quiescent satellite cells and is involved in muscle regeneration. The nature of the material transported in the VAMP2-bearing vesicles in satellite cells and myotubes is still under investigation.

Deletion of the SNARE vti1b in mice results in the loss of a single SNARE partner, syntaxin 8

SNARE proteins participate in recognition and fusion of membranes. A SNARE complex consisting of vti1b, syntaxin 8, syntaxin 7, and endobrevin/VAMP-8 which is required for fusion of late endosomes in vitro has been identified recently. Here, we generated mice deficient in vti1b to study the function of this protein in vivo. vti1b-deficient mice had reduced amounts of syntaxin 8 due to degradation of the syntaxin 8 protein, while the amounts of syntaxin 7 and endobrevin did not change. These data indicate that vti1b is specifically required for the stability of a single SNARE partner. vti1b-deficient mice were viable and fertile. Most vti1b-deficient mice were indistinguishable from wild-type mice and did not display defects in transport to the lysosome. However, 20% of the vti1b-deficient mice were smaller. Lysosomal degradation of an endocytosed protein was slightly delayed in hepatocytes derived from these mice. Multivesicular bodies and autophagic vacuoles accumulated in hepatocytes of some smaller vti1b-deficient mice. This suggests that other SNAREs can compensate for the reduction in syntaxin 8 and for the loss of vti1b in most mice even though vti1b shows only 30% amino acid identity with its closest relative.

Identification of SNARE and cell trafficking regulatory proteins in the salivary glands of the lone star tick, Amblyomma americanum (L.)

Prostaglandin E(2) (PGE(2)) stimulates secretion of tick salivary gland proteins via a phosphoinositide signaling pathway and mobilization of intracellular Ca(2+) (). Highly conserved intracellular SNARE (soluble NSF attachment protein receptors) complex proteins are associated with the mechanism of protein secretion in vertebrate and invertebrate neuronal and non-neuronal cells. Proteins in the salivary glands of partially fed female lone star ticks cross-react individually with antibodies to synaptobrevin-2 (vesicle (v)-SNARE), syntaxin-1A, syntaxin-2 and SNAP-25 (target (t)-SNAREs), cytosolic alpha/beta SNAP and NSF (N-ethylmaleimide-sensitive fusion protein), Ca(2+) sensitive synaptotagmin, vesicle associated synaptophysin, and regulatory cell trafficking GTPases Rab3A and nSec1. V-SNARE and t-SNARE proteins form an SDS-resistant, boiling sensitive core complex in the salivary glands. Antibodies to SNARE complex proteins inhibit PGE(2)-stimulated secretion of anticoagulant protein in permeabilized tick salivary glands. We conclude that SNARE and cell trafficking regulatory proteins are present and functioning in the process of PGE(2)-stimulated Ca(2+) regulated protein secretion in tick salivary glands.

Prostaglandins and other lipid mediators in Alzheimer's disease

In the central nervous system (CNS), prostaglandin (PG) and other bioactive lipids regulate vital aspects of neural membrane biology, including protein-lipid interactions, trans-membrane and trans-synaptic signaling. However, a series of highly reactive PGs, free fatty acids, lysophospolipids, eicosanoids, platelet-activating factor, and reactive oxygen species (ROS), all generated by enhanced phospholipase A2 (PLA2) activity and arachidonic acid (AA) release, participate in cellular injury, particularly in neurodegeneration. PLA2 activation and PG production are among the earliest initiating events in triggering brain-damage pathways, which can lead to long-term neurologic deficits. Altered membrane-associated PLA2 activities have been correlated with several forms of acute and chronic brain injury, including cerebral trauma, ischemic damage, induced seizures in the brain and epilepsy, schizophrenia, and in particular, Alzheimer's disease (AD). Biochemical mechanisms of PLA2 overactivation and its pathophysiological consequences on CNS structure and function have been extensively studied using animal models and brain cells in culture triggered with PLA2 inducers, PGs, cytokines, and related lipid mediators. Moreover, the expression of both COX-2 and PLA2 appears to be strongly activated during Alzheimer's disease (AD), indicating the importance of inflammatory gene pathways as a response to brain injury. This review addresses some current ideas concerning how brain PLA2 and brain PGs are early and key players in acute neural trauma and in brain-cell damage associated with chronic neurodegenerative diseases such as AD.

Syntaxin 4 heterozygous knockout mice develop muscle insulin resistance

To investigate the physiological function of syntaxin 4 in the regulation of GLUT4 vesicle trafficking, we used homologous recombination to generate syntaxin 4-knockout mice. Homozygotic disruption of the syntaxin 4 gene results in early embryonic lethality, whereas heterozygous knockout mice, Syn4(+/-), had normal viability with no significant impairment in growth, development, or reproduction. However, the Syn4(+/-) mice manifested impaired glucose tolerance with a 50% reduction in whole-body glucose uptake. This defect was attributed to a 50% reduction in skeletal muscle glucose transport determined by 2-deoxyglucose uptake during hyperinsulinemic-euglycemic clamp procedures. In parallel, insulin-stimulated GLUT4 translocation in skeletal muscle was also significantly reduced in these mice. In contrast, Syn4(+/-) mice displayed normal insulin-stimulated glucose uptake and metabolism in adipose tissue and liver. Together, these data demonstrate that syntaxin 4 plays a critical physiological role in insulin-stimulated glucose uptake in skeletal muscle. Furthermore, reduction in syntaxin 4 protein levels in this tissue can account for the impairment in whole-body insulin-stimulated glucose metabolism in this animal model.

Mechanism and regulation of GLUT-4 vesicle fusion in muscle and fat cells

Twenty years ago it was shown that recruitment of glucose transporters from an internal membrane compartment to the plasma membrane led to increased glucose uptake into fat and muscle cells stimulated by insulin. The final step of this process is the fusion of glucose transporter 4 (GLUT-4)-containing vesicles with the plasma membrane. The identification of a neuronal soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) complex as a requirement for synaptic vesicle-plasma membrane fusion led to the search for homologous complexes outside the nervous system. Indeed, isoforms of the neuronal SNAREs were identified in muscle and fat cells and were shown to be required for GLUT-4 incorporation into the cell membrane. In addition, proteins that bind to nonneuronal SNAREs were cloned and proposed to regulate vesicle fusion. We have summarized the molecular mechanisms leading to membrane fusion in nonneuronal systems, focusing on the role of SNAREs and accessory proteins (Munc18c, synip, Rab4, and VAP-33) in incorporation of GLUT-4 into the plasma membrane. Potential modes of regulation of this process are discussed, including SNARE phosphorylation and interaction with the cytoskeleton.

Role of SNARE's in the GLUT4 translocation response to insulin in adipose cells and muscle

Insulin stimulates glucose transport in skeletal muscle, heart, and adipose tissue by promoting the appearance of GLUT4, the major glucose transporter isoform present in these tissues, on the cell surface. This is achieved by differentially modulating GLUT4 exocytosis and endocytosis, between a specialized intracellular compartment and the plasma membrane. Ligands which activate the heterotrimeric GTP-binding proteins Gs and Gi appear to modulate insulin-stimulated glucose transport through effects on the fusion of docked GLUT4-containing vesicles with the plasma membrane. In insulin resistance states, reduced cellular GLUT4 levels in adipose cells fully account for the decreased glucose transport response to insulin in these cells. In contrast, although insulin-stimulated GLUT4 translocation is also impaired in muscle, total cellular levels of GLUT4 are not altered. The defect in muscle has been attributed to a GLUT4 trafficking problem and thus studies of this mechanism could provide clues as to the nature of the impairment. The movement of GLUT4-containing vesicles from an intracellular storage site to the plasma membrane and the fusion of docked GLUT4-containing vesicles with the plasma membrane are conceptually similar to some secretory processes. A general hypothesis called the SNARE hypothesis (soluble NSF attachment protein receptors where NSF stands for N-ethylmaleimide-sensitive fusion protein) postulates that the specificity of secretory vesicle targeting is generated by complexes that form between membrane proteins on the transport vesicle (v-SNARE's) and membrane proteins located on the target membrane (t-SNARE's). Several v- and t-SNARE's have been identified in adipose cells and muscle. VAMP2 and VAMP3/cellubrevin (v-SNARE's) have been shown to interact with the t-SNARE's syntaxin 4 and SNAP-23. The cytosolic protein NSF has the characteristic of binding to the v-/t-SNARE complex through its interaction with alpha-SNAP, another soluble factor. Furthermore, recent studies have demonstrated that VAMP2/3, syntaxin 4, SNAP-23, and NSF are functionally involved in insulin-stimulated GLUT4 translocation in adipose cells and thus are likely to be involved in the Gs- and Gi-mediated modulation of the glucose transport response to insulin as well. This review summarizes recent advances on the normal mechanism of GLUT4 translocation and discusses how this process could be affected in insulin resistant states such as type II diabetes.

Tetanus toxin impairs accessory and secretory functions in interferon-gamma-treated murine macrophages

Tetanus neurotoxin (TT), a product of microbial origin, acts as a zinc endopeptidase on vesicle-associated membrane proteins (VAMP). We have demonstrated that TT displays inhibitory effects on secretory and accessory functions in the murine macrophage (Mphi) cell line GG2EE. Nitric oxide (NO) secretion was decreased when interferon (IFN)-gamma-pretreated GG2EE Mphis were coincubated with a fungal costimulus (SMP200) and TT. When heat-inactivated TT was used this effect was not evident. The TT-mediated phenomenon was dose-dependent and specific since, under the same experimental conditions, it did not affect interleukin-6 or tumor necrosis factor-alpha secretion. Furthermore, IFN-gamma-induced major histocompatibility complex class II molecule expression and GG2EE accessory function, assessed by SMP200-stimulated lymphoproliferation, were also inhibited by TT. Such inhibition was incomplete, in line with our previous results showing that TT partially cleaves VAMP proteins in murine Mφ.

A novel synaptobrevin/VAMP homologous protein (VAMP5) is increased during in vitro myogenesis and present in the plasma membrane

cDNA clones encoding a novel protein (VAMP5) homologous to synaptobrevins/VAMPs are detected during database searches. The predicted 102-amino acid VAMP5 harbors a 23-residue hydrophobic region near the carboxyl terminus and exhibits an overall amino acid identity of 33% with synaptobrevin/VAMP1 and 2 and cellubrevin. Northern blot analysis reveals that the mRNA for VAMP5 is preferentially expressed in the skeletal muscle and heart, whereas significantly lower levels are detected in several other tissues but not in the brain. During in vitro differentiation (myogenesis) of C2C12 myoblasts into myotubes, the mRNA level for VAMP5 is increased approximately 8- to 10-fold. Immunoblot analysis using antibodies specific for VAMP5 shows that the protein levels are also elevated approximately 6-fold during in vitro myogenesis of C2C12 cells. Indirect immunofluorescence microscopy and immunoelectron microscopy reveal that VAMP5 is associated with the plasma membrane as well as intracellular perinuclear and peripheral vesicular structures of myotubes. Epitope-tagged versions of VAMP5 are similarly targeted to the plasma membrane.

Divergence and convergence in regulated exocytosis: the characteristics of cAMP-dependent enzyme secretion of parotid salivary acinar cells

The process of membrane fusion is separated into three steps: docking, priming and fusion. The last fusion step of most regulated exocytosis is triggered by cytosolic free calcium (Ca2+). However, enzyme secretion from the parotid salivary glands is regulated by the accumulation of intracellular cAMP, although Ca2+ does augment the cAMP-induced secretion. The difference of the regulatory mechanisms is thought to be due to the brake points that will be passed upon stimulations. Vesicles of Ca2+-regulated exocytosis such as neurotransmission and norepinephrine release from chromaffin cells are waiting for the stimulation docked to the plasma membrane, and Ca2+ triggers the membrane fusion after the priming. In contrast, secretory granules of parotid acinar cells begin exocytosis with the docking step that may be regulated by cAMP. After the start of the docking, the exocytotic process of enzyme release runs a similar course to that of the neurotransmission: the priming and the Ca2+-enhanced fusion steps. Therefore, there are probably some common mechanisms involving the SNARE proteins both in Ca2+-regulated exocytosis and cAMP-dependent secretion.

Exocytosis in chromaffin cells of the adrenal medulla

The chromaffin cell has been used as a model to characterize releasable components present in secretory granules and to understand the cellular mechanisms involved in catecholamine release. Recent physiological and biochemical developments have revealed that molecular mechanisms implicated in granule trafficking are conserved in all eukaryotic species: a rise in intracellular calcium triggers regulated exocytosis, and highly conserved proteins are essential elements which interact with each other to form a molecular scaffolding, ensuring the docking of granules at the plasma membrane, and perhaps membrane fusion. However, the mechanisms regulating secretion are multiple and cell specific. They operate at different steps along the life of a granule, from the time of granule biosynthesis up to the last step of exocytosis. With regard to cell specificity, noradrenaline and adrenaline chromaffin cells display different receptor and signaling characteristics that may be important to exocytosis. Characterization of regulated exocytosis in chromaffin cells provides not only fundamental knowledge of neurosecretion but is of additional importance as these cells are used for therapeutic purposes.

Botulinum E toxin light chain does not cleave SNAP-23 and only partially impairs insulin stimulation of GLUT4 translocation in 3T3-L1 cells

The stimulation of glucose uptake into fat and muscle by insulin results predominantly from the translocation of the glucose transporter, GLUT4, from an intracellular vesicle pool to the cell surface. Homologues of several key proteins known to be involved in the process of synaptic vesicle fusion have been identified on GLUT4 vesicles, including VAMP2 and cellubrevin. Syntaxin 4, SNAP-23 and/or SNAP-25 are also implicated in this process. Bacterial toxins that specifically cleave these proteins have been utilised to assess their involvement in cell function. We aimed to distinguish which of the SNAP isoforms are specifically involved in GLUT4 translocation. Here we show that both human (h) and mouse (m) SNAP-23, unlike SNAP-25, are not substrates for Botulinum E toxin light chain (BoNT/E). Furthermore, we demonstrate that microinjection of differentiated 3T3-L1 cells with BoNT/E inhibited insulin stimulation of GLUT4 translocation only slightly, 27%, whereas tetanus toxin light chain, that cleaves VAMP2, inhibited insulin stimulation of GLUT4 translocation by 80%. These studies therefore do not support a major role for SNAP-25 in insulin stimulation of GLUT4 translocation and place SNAP-23 as a prime candidate for a role in this process.

Immunocytochemical evidence that GLUT4 resides in a specialized translocation post-endosomal VAMP2-positive compartment in rat adipose cells in the absence of insulin

Insulin stimulates glucose transport in rat adipose cells through the translocation of GLUT4 from a poorly defined intracellular compartment to the cell surface. We employed confocal microscopy to determine the in situ localization of GLUT4 relative to vesicle, Golgi, and endosomal proteins in these physiological insulin target cells. Three-dimensional analyses of GLUT4 immunostaining in basal cells revealed an intracellular punctate, patchy distribution both in the perinuclear region and scattered throughout the cytoplasm. VAMP2 closely associates with GLUT4 in many punctate vesicle-like structures. A small fraction of GLUT4 overlaps with TGN38-mannosidase II, gamma-adaptin, and mannose-6-phosphate receptors in the perinuclear region, presumably corresponding to late endosome and trans-Golgi network structures. GLUT4 does not co-localize with transferrin receptors, clathrin, and Igp-120. After insulin treatment, GLUT4 partially redistributes to the cell surface and decreases in the perinuclear area. However, GLUT4 remains co-localized with TGN38-mannosidase II and gamma-adaptin. Therefore, the basal compartment from which GLUT4 is translocated in response to insulin comprises specialized post-endosomal VAMP2-positive vesicles, distinct from the constitutively recycling endosomes. These results are consistent with a kinetic model in which GLUT4 is sequestered through two or more intracellular pools in series.

Characterization of Munc-18c and syntaxin-4 in 3T3-L1 adipocytes. Putative role in insulin-dependent movement of GLUT-4

We have previously identified three mammalian Sec1/Munc-18 homologues in adipocytes (Tellam, J. T., McIntosh, S., and James, D. E. (1995) J. Biol. Chem. 270, 5857-5863). These proteins are thought to modulate the interaction between vesicle membrane and target membrane soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs) and thus regulate intracellular vesicular transport. This study aimed to further characterize these Munc-18 isoforms and to define their potential role in the trafficking of GLUT-4 in adipocytes, a process reported to involve the vesicle membrane SNARE, VAMP-2. Using an in vitro binding assay with recombinant fusion proteins, we show that Munc-18a and Munc-18b bind to syntaxin-1A, -2, and -3, while Munc-18c binds only to syntaxin-2 and -4. The specific interaction between Munc-18c and syntaxin-4 is of interest because aside from syntaxin-1A, which is not expressed in adipocytes, syntaxin-4 is the only syntaxin that binds to VAMP-2. Using a three-way binding assay, it was shown that Munc-18c inhibits the binding of syntaxin-4 to VAMP-2. The subcellular distribution of syntaxin-4 and Munc-18c was almost identical, both being enriched in the plasma membrane, and both exhibiting an insulin-dependent movement out of an intracellular membrane fraction similar to that observed for GLUT-4. Munc-18b had a similar distribution to Munc-18c and so may also be involved in vesicle transport to the cell surface, whereas Munc-18a was undetectable by immunoblotting in adipocytes. Microinjection of a syntaxin-4 antibody into 3T3-L1 adipocytes blocked the insulin-dependent recruitment of GLUT-4 to the cell surface. These data suggest that syntaxin-4/Munc-18c/VAMP-2 may play a role in the docking/fusion of intracellular GLUT-4-containing vesicles with the cell surface in adipocytes.

Analyses of the co-localization of cellubrevin and the GLUT4 glucose transporter in rat and human insulin-responsive tissues

We have investigated the subcellular distribution and association of cellubrevin, a low molecular weight protein implicated in the process of membrane fusion, with intracellular membranes containing the insulin-sensitive GLUT4 glucose transporter from rat adipocytes, rat skeletal muscle and human skeletal muscle. SDS-PAGE and immunoblot analyses of subcellular fractions of adipocytes and skeletal muscle indicated a positive correlation between the distribution of GLUT4 and cellubrevin in intracellular membrane fractions tested from all tissues. The identity of the polypeptide reacting with antiserum against cellubrevin was further confirmed on the basis of its susceptibility to proteolysis by tetanus toxin. Immunoisolation of GLUT4-containing vesicles from a microsomal fraction enriched with GLUT4 and cellubrevin revealed that cellubrevin could be coprecipitated with GLUT4 vesicles from adipocytes. In contrast, intracellular GLUT4 vesicles isolated from both rat and human skeletal muscle were devoid of any detectable immunoreactivity towards cellubrevin. The observation that cellubrevin does not colocalise with intracellular GLUT4 in skeletal muscle from two different species, rat and human, would strongly suggest that it is unlikely to participate in the insulin-induced delivery of GLUT4 to the cell surface in skeletal muscle.

Tetanus toxin inhibits spontaneous quantal release and cleaves VAMP/synaptobrevin

Tetanus toxin decreased the frequency of spontaneous events at the electric organ of Torpedo marmorata. This reduction was up to 70% in poisoned electric organ. According to distribution analysis of miniature end plate currents, only a subpopulation of events which have small amplitudes were recorded after poisoning. Furthermore, isolated cholinergic nerve terminals showed a decrease in VAMP/synaptobrevin when poisoned with tetanus toxin under similar conditions. The relationship between the two effects of the toxin, i.e. inhibition of vesicle exocytosis and peptidase activity on synaptobrevin, is discussed.

v- and t-SNAREs in neuronal exocytosis: a need for additional components to define sites of release

Synaptic vesicle recycling is a specialized form of membrane recycling which takes place in all cells between early endosomes and the plasmalemma. Synaptic vesicles exocytosis is highly regulated and occurs only at presynaptic active zones. In contrast, exocytosis of endosome-derived vesicles of the housekeeping recycling pathway takes place constitutively and throughout the cell surface. Since v- and t-SNAREs play a key role in membrane interactions leading to fusion, unique v- and t-SNAREs may be implicated in synaptic vesicle exocytosis. It was found, however, that the same v-SNAREs of the synaptobrevin family are found both on synaptic vesicles and on endosome-derived vesicles which undergo constitutive fusion. Likewise, t-SNAREs which act as plasmalemmal receptors for synaptic vesicles are not restricted to synaptic active zones. Thus, v- and t-SNAREs interactions may define which organelles can fuse with the plasmalemma, but require additional components to define properties of the exocytotic reaction which are specific for distinct classes of secretory organelles.

Structure and function of tetanus and botulinum neurotoxins

Tetanus and botulinum neurotoxins are produced by Clostridia and cause the neuroparalytic syndromes of tetanus and botulism. Tetanus neurotoxin acts mainly at the CNS synapse, while the seven botulinum neurotoxins act peripherally. Clostridial neurotoxins share a similar mechanism of cell intoxication: they block the release of neurotransmitters. They are composed of two disulfide-linked polypeptide chains. The larger subunit is responsible for neurospecific binding and cell penetration. Reduction releases the smaller chain in the neuronal cytosol, where it displays its zinc-endopeptidase activity specific for protein components of the neuroexocytosis apparatus. Tetanus neurotoxin and botulinum neurotoxins B, D, F and G recognize specifically VAMP/ synaptobrevin. This integral protein of the synaptic vesicle membrane is cleaved at single peptide bonds, which differ for each neurotoxin. Botulinum A, and E neurotoxins recognize and cleave specifically SNAP-25, a protein of the presynaptic membrane, at two different sites within the carboxyl-terminus. Botulinum neurotoxin type C cleaves syntaxin, another protein of the nerve plasmalemma. These results indicate that VAMP, SNAP-25 and syntaxin play a central role in neuroexocytosis. These three proteins are conserved from yeast to humans and are essential in a variety of docking and fusion events in every cell. Tetanus and botulinum neurotoxins form a new group of zinc-endopeptidases with characteristic sequence, mode of zinc coordination, mechanism of activation and target recognition. They will be of great value in the unravelling of the mechanisms of exocytosis and endocytosis, as they are in the clinical treatment of dystonias.

Water channel-carrying vesicles in the rat IMCD contain cellubrevin

Antidiuretic hormone (arginine vasopressin) induces a cyclic process of docking, fusion, and endocytosis of water channel-containing vesicles in the collecting duct. There is now evidence that docking and endocytosis are mediated by an array of proteins associated with vesicles and target membranes. In recent studies, we have shown that cellubrevin, a member of the vesicle-associated membrane protein family, as well as other docking proteins, are expressed in the rat inner medullary collecting duct. We now show by immunogold electron microscopy that cellubrevin is present on vesicles containing water channels, that it is associated with both coated and uncoated vesicles, and that it is present on the apical membrane. Cellubrevin, therefore, is in a position to mediate one or more steps in arginine vasopressin-induced water channel cycling.

Endothelial caveolae have the molecular transport machinery for vesicle budding, docking, and fusion including VAMP, NSF, SNAP, annexins, and GTPases

Transport by discrete vesicular carriers is well established at least in part because of recent discoveries identifying key protein mediators of vesicle formation, docking, and fusion. A general mechanism sensitive to N-ethylmaleimide (NEM) is required for the transport of a divergent group of vesicular carriers in all eukaryotes. Many endothelia have an abundant population of non-coated plasmalemmal vesicles or caveolae, which have been reported with considerable controversy to function in transport. We recently have shown that like other vesicular transport systems, caveolae-mediated endocytosis and transcytosis are inhibited by NEM (Schnitzer, J. E., Allard, J., and Oh, P. (1995) Am. J. Physiol. 268, H48-H55). Here, we continue this work by utilizing our recently developed method for purifying endothelial caveolae from rat lung tissue (Schnitzer, J. E., Oh, P., Jacobson, B. S., and Dvorak, A. M. (1995) Proc. Natl. Acad. Sci. U. S. A. 92, 1759-1763) to show that these caveolae contain key proteins known to mediate different aspects of vesicle formation, docking, and/or fusion including the vSNARE VAMP-2, monomeric and trimeric GTPases, annexins II and VI, and the NEM-sensitive fusion factor NSF along with its attachment protein SNAP. Like neuronal VAMPs, this endothelial VAMP is sensitive to cleavage by botulinum B and tetanus neurotoxins. Caveolae in endothelium are indeed like other carrier vesicles and contain similar NEM-sensitive molecular machinery for transport.

Vesicle-associated membrane protein (VAMP)/synaptobrevin-2 is associated with dense core secretory granules in PC12 neuroendocrine cells

The presence and intracellular distribution of vesicle-associated membrane protein-1 (VAMP-1) and VAMP-2 were investigated in the PC12 neuroendocrine cell line using isotype-specific polyclonal antibodies. VAMP-2 was detected in the total membrane fraction, while VAMP-1 was undetectable. Subcellular fractionation demonstrates that a substantial amount of the VAMP-2 (24-36%) is associated with dense core, catecholamine-containing granules (DCGs). This was confirmed by immunofluorescence microscopy. The L chain of tetanus neurotoxin, known to inhibit granule mediated secretion in permeabilized PC12 cells, as well as botulinum neurotoxins F and G, effectively cleaved DCG-associated VAMP-2. These data demonstrate that VAMP-2 is present on the secretory granules of PC12 cells.

The molecular machinery for fast and slow neurosecretion

Recent studies indicate that the molecular machinery for synaptic vesicle docking and fusion consists of a triad of botulinum/tetanus neurotoxin substrates (synaptobrevin, syntaxin, SNAP-25) that are homologues of proteins required for constitutive secretion. Proposed low-affinity Ca2+ sensors that regulate exocytosis remain to be identified, although recent studies on synaptotagmin suggest that it, along with other proteins, could play this role. Regulated peptide secretion from dense-core granules has been found to utilize a similar machinery for docking/fusion, and recent studies indicate that this pathway involves a pre-docking step that is regulated by a higher affinity Ca2+ sensor.