SNAP23 promotes insulin-dependent glucose uptake in 3T3-L1 adipocytes: possible interaction with cytoskeleton

A comprehensive view of muscle glucose uptake: regulation by insulin, contractile activity, and exercise

Skeletal muscle is the main site of glucose deposition in the body during meals and the major glucose utilizer during physical activity. Although in both instances the supply of glucose from the circulation to the muscle is of paramount importance, in most conditions the rate-limiting step in glucose uptake, storage, and utilization is the transport of glucose across the muscle cell membrane. This step is dependent upon the translocation of the insulin- and contraction-responsive glucose transporter GLUT4 from intracellular storage sites to the sarcolemma and T tubules. Here, we first analyze how glucose can traverse the capillary wall into the muscle interstitial space. We then review the molecular processes that regulate GLUT4 translocation in response to insulin and muscle contractions and the methodologies utilized to unravel them. We further discuss how physical activity and inactivity, respectively, lead to increased and decreased insulin action in muscle and touch upon sex differences in glucose metabolism. Although many key processes regulating glucose uptake in muscle are known, the advent of newer and bioinformatics tools has revealed further molecular signaling processes reaching a staggering level of complexity. Much of this molecular mapping has emerged from cellular and animal studies and more recently from application of a variety of -omics in human tissues. In the future, it will be imperative to validate the translatability of results drawn from experimental systems to human physiology.

Marine N-3 Fatty Acids Mitigate Hyperglycemia in Prediabetes by Improving Muscular Glucose Transporter 4 Translocation and Glucose Homeostasis

Docosahexaenoic acid (DHA) and eicosapentaenoic acid (EPA) have been proposed to benefit cardiometabolic health. However, the relationship between the intake of DHA and EPA and type 2 diabetes (T2D) risk remains equivocal, and the effects of DHA and EPA on skeletal muscle, the primary organ for glucose metabolism, merit further investigation. Here, we show that habitual fish oil supplementation was associated with a 9% lower T2D risk and significantly interacted with variants at GLUT4 in a prospective cohort of 48,358 people with prediabetes. Muscular metabolome analysis in the animal study revealed that DHA and EPA altered branched-chain amino acids, creatine, and glucose oxidation-related metabolites, concurrently with elevated muscular glycogen synthase and pyruvate dehydrogenase contents that promoted glucose disposal. Further myotube investigation revealed that DHA and EPA promoted muscular GLUT4 translocation by elevating Rab GTPases and target-SNARE expression. Together, DHA and EPA supplementation provides a promising approach for T2D prevention through targeting muscular glucose homeostasis, including enhancing GLUT4 translocation, glycogen synthesis, and aerobic glycolysis.

Breast density and estradiol are associated with distinct different expression patterns of metabolic proteins in normal human breast tissue in vivo

Background

Breast density and exposure to sex steroids are major risk factors for breast cancer. The local microenvironment plays an essential role in progression of breast cancer. Metabolic adaption is a major hallmark of cancer. Whether proteins from the extracellular space regulating metabolism are affected in breast cancer, dense breasts or by estrogen exposure are not yet fully elucidated.

Methods

Women with breast cancer, postmenopausal women with normal breast tissue with varying breast density or premenopausal women with breasts exposed to high levels of estradiol were included in the study. Microdialysis was used to collect proteins from the extracellular space in vivo in 73 women; 12 with breast cancer, 42 healthy postmenopausal women with different breast densities, and 19 healthy premenopausal women. Breast density was determined as lean tissue fraction (LTF) using magnetic resonance imaging. Data were evaluated in a murine breast cancer model. We quantified a panel of 92 key proteins regulating metabolism using proximity extension assay.

Results

We report that 29 proteins were upregulated in human breast cancer. In dense breasts 37 proteins were upregulated and 17 of these were similarly regulated as in breast cancer. 32 proteins correlated with LTF. In premenopausal breasts 19 proteins were up-regulated and 9 down-regulated. Of these, 27 correlated to estradiol, a result that was confirmed for most proteins in experimental breast cancer. Only two proteins, pro-cathepsin H and galanin peptide, were similarly regulated in breast cancer, dense- and estrogen exposed breasts.

Conclusions

Metabolic proteins may be targetable for breast cancer prevention. Depending on risk factor, this may, however, require different approaches as breast density and estradiol induce distinct different expression patterns in the breast. Additionally, metabolic proteins from the extracellular space may indeed be further explored as therapeutic targets for breast cancer treatment.

Exocytosis Proteins: Typical and Atypical Mechanisms of Action in Skeletal Muscle

Insulin-stimulated glucose uptake in skeletal muscle is of fundamental importance to prevent postprandial hyperglycemia, and long-term deficits in insulin-stimulated glucose uptake underlie insulin resistance and type 2 diabetes. Skeletal muscle is responsible for ~80% of the peripheral glucose uptake from circulation via the insulin-responsive glucose transporter GLUT4. GLUT4 is mainly sequestered in intracellular GLUT4 storage vesicles in the basal state. In response to insulin, the GLUT4 storage vesicles rapidly translocate to the plasma membrane, where they undergo vesicle docking, priming, and fusion via the high-affinity interactions among the soluble N-ethylmaleimide sensitive factor attachment protein receptor (SNARE) exocytosis proteins and their regulators. Numerous studies have elucidated that GLUT4 translocation is defective in insulin resistance and type 2 diabetes. Emerging evidence also links defects in several SNAREs and SNARE regulatory proteins to insulin resistance and type 2 diabetes in rodents and humans. Therefore, we highlight the latest research on the role of SNAREs and their regulatory proteins in insulin-stimulated GLUT4 translocation in skeletal muscle. Subsequently, we discuss the novel emerging role of SNARE proteins as interaction partners in pathways not typically thought to involve SNAREs and how these atypical functions reveal novel therapeutic targets for combating peripheral insulin resistance and diabetes.

Putative Role of Protein Palmitoylation in Cardiac Lipid-Induced Insulin Resistance

In the heart, inhibition of the insulin cascade following lipid overload is strongly associated with contractile dysfunction. The translocation of fatty acid transporter CD36 (SR-B2) from intracellular stores to the cell surface is a hallmark event in the lipid-overloaded heart, feeding forward to intracellular lipid accumulation. Yet, the molecular mechanisms by which intracellularly arrived lipids induce insulin resistance is ill-understood. Bioactive lipid metabolites (diacyl-glycerols, ceramides) are contributing factors but fail to correlate with the degree of cardiac insulin resistance in diabetic humans. This leaves room for other lipid-induced mechanisms involved in lipid-induced insulin resistance, including protein palmitoylation. Protein palmitoylation encompasses the reversible covalent attachment of palmitate moieties to cysteine residues and is governed by protein acyl-transferases and thioesterases. The function of palmitoylation is to provide proteins with proper spatiotemporal localization, thereby securing the correct unwinding of signaling pathways. In this review, we provide examples of palmitoylations of individual signaling proteins to discuss the emerging role of protein palmitoylation as a modulator of the insulin signaling cascade. Second, we speculate how protein hyper-palmitoylations (including that of CD36), as they occur during lipid oversupply, may lead to insulin resistance. Finally, we conclude that the protein palmitoylation machinery may offer novel targets to fight lipid-induced cardiomyopathy.

Role of Skeletal Muscle in Insulin Resistance and Glucose Uptake

The skeletal muscle is the largest organ in the body, by mass. It is also the regulator of glucose homeostasis, responsible for 80% of postprandial glucose uptake from the circulation. Skeletal muscle is essential for metabolism, both for its role in glucose uptake and its importance in exercise and metabolic disease. In this article, we give an overview of the importance of skeletal muscle in metabolism, describing its role in glucose uptake and the diseases that are associated with skeletal muscle metabolic dysregulation. We focus on the role of skeletal muscle in peripheral insulin resistance and the potential for skeletal muscle-targeted therapeutics to combat insulin resistance and diabetes, as well as other metabolic diseases like aging and obesity. In particular, we outline the possibilities and pitfalls of the quest for exercise mimetics, which are intended to target the molecular mechanisms underlying the beneficial effects of exercise on metabolic disease. We also provide a description of the molecular mechanisms that regulate skeletal muscle glucose uptake, including a focus on the SNARE proteins, which are essential regulators of glucose transport into the skeletal muscle. © 2020 American Physiological Society. Compr Physiol 10:785-809, 2020.

Cardiac SNARE Expression in Health and Disease

SNARE proteins are integral to intracellular vesicular trafficking, which in turn is the process underlying the regulated expression of substrate transporters such as the glucose transporter GLUT4 at the cell surface of insulin target tissues. Impaired insulin stimulated GLUT4 trafficking is associated with reduced cardiac function in many disease states, most notably diabetes. Despite this, our understanding of the expression and regulation of SNARE proteins in cardiac tissue and how these may change in diabetes is limited. Here we characterize the array of SNARE proteins expressed in cardiac tissue, and quantify the levels of expression of VAMP2, SNAP23, and Syntaxin4-key proteins involved in insulin-stimulated GLUT4 translocation. We examined SNARE protein levels in cardiac tissue from two rodent models of insulin resistance, db/db mice and high-fat fed mice, and show alterations in patterns of expression are evident. Such changes may have implications for cardiac function.

SNAP23 suppresses cervical cancer progression via modulating the cell cycle

OBJECTIVE

Cervical cancer (CC) is one of the most common gynecologic tumors in women worldwide, with poor prognosis and low survival rate. In this study, we identified SNAP23 as a potential tumor suppressor gene in CC.

METHODS

The expression of SNAP23 in tissues and cell lines were measured by qRT-PCR, western blot and IHC. Knockdown of SNAP23 by siRNA and ectopic expression of SNAP23 by overexpression plasmid were performed to observe the biological function of SNAP23 in CC. Xenograft nude mice models were established to measure its function in vivo.

RESULTS

SNAP23 was downregulated in CC tissues and had a negative correlation with advanced clinical characteristics. Ectopic expression of SNAP23 suppressed malignant phonotype of CC while knockdown of SNAP23 promoted the progression of CC in vitro. The flow cytometry analysis revealed that SNAP23 exerted its tumor suppressor activity via inducing G2/M cell cycle arrest. Moreover, xenograft tumor models showed that SNAP23 suppresses tumor growth in vivo.

CONCLUSIONS

Our results revealed that SNAP23 suppressed progression of CC and induced cell cycle G2/M arrest via upregulating p21^cip1 and downregulating CyclinB1.

SNAP23 regulates BAX-dependent adipocyte programmed cell death independently of canonical macroautophagy

The t-SNARE protein SNAP23 conventionally functions as a component of the cellular machinery required for intracellular transport vesicle fusion with target membranes and has been implicated in the regulation of fasting glucose levels, BMI, and type 2 diabetes. Surprisingly, we observed that adipocyte-specific KO of SNAP23 in mice resulted in a temporal development of severe generalized lipodystrophy associated with adipose tissue inflammation, insulin resistance, hyperglycemia, liver steatosis, and early death. This resulted from adipocyte cell death associated with an inhibition of macroautophagy and lysosomal degradation of the proapoptotic regulator BAX, with increased BAX activation. BAX colocalized with LC3-positive autophagic vacuoles and was increased upon treatment with lysosome inhibitors. Moreover, BAX deficiency suppressed the lipodystrophic phenotype in the adipocyte-specific SNAP23-KO mice and prevented cell death. In addition, ATG9 deficiency phenocopied SNAP23 deficiency, whereas ATG7 deficiency had no effect on BAX protein levels, BAX activation, or apoptotic cell death. These data demonstrate a role for SNAP23 in the control of macroautophagy and programmed cell death through an ATG9-dependent, but ATG7-independent, pathway regulating BAX protein levels and BAX activation.

Postprandial gut microbiota-driven choline metabolism links dietary cues to adipose tissue dysfunction

The human body is an integrated circuit between microbial symbionts and our Homo sapien genome, which communicate bi-directionally to maintain homeostasis within the human meta-organism. There is now strong evidence that microbes resident in the human intestine can directly contribute to the pathogenesis of obesity and associated cardiometabolic disorders. In fact, gut microbes represent a filter of our greatest environmental exposure - the foods we consume. It is now clear that we each experience a given meal differently, based on our unique gut microbial communities. Biologically active gut microbe-derived metabolites, such as short chain fatty acids, secondary bile acids, and trimethylamine-N-oxide (TMAO), are now uniquely recognized as contributors to obesity and related cardiometabolic disorders. However, mechanistic insights into how microbe-derived metabolites promote obesity are largely unknown. Recent work has demonstrated that the meta-organismal production of the bacterial co-metabolite TMAO is linked to suppression of beiging of white adipose tissue in mice and humans. Furthermore, the TMAO pathway is becoming an increasingly attractive therapeutic target in obesity-associated diseases such as type 2 diabetes, kidney failure, and cardiovascular disease. In this commentary we discuss recent findings linking the TMAO pathway to obesity-associated disorders, and provide additional insights into potential mechanisms driving this microbe-host interaction.

Alternative Splicing of Four Trafficking Genes Regulates Myofiber Structure and Skeletal Muscle Physiology

During development, transcriptional and post-transcriptional networks are coordinately regulated to drive organ maturation. Alternative splicing contributes by producing temporal-specific protein isoforms. We previously found that genes undergoing splicing transitions during mouse postnatal heart development are enriched for vesicular trafficking and membrane dynamics functions. Here, we show that adult trafficking isoforms are also expressed in adult skeletal muscle and hypothesize that striated muscle utilizes alternative splicing to generate specific isoforms required for function of adult tissue. We deliver morpholinos into flexor digitorum brevis muscles in adult mice to redirect splicing of four trafficking genes to the fetal isoforms. The splicing switch results in multiple structural and functional defects, including transverse tubule (T-tubule) disruption and dihydropyridine receptor alpha (DHPR) and Ryr1 mislocalization, impairing excitation-contraction coupling, calcium handling, and force generation. The results demonstrate a previously unrecognized role for trafficking functions in adult muscle tissue homeostasis and a specific requirement for the adult splice variants.

Effect of Bisphenol-A (BPA) on insulin signal transduction and GLUT4 translocation in gastrocnemius muscle of adult male albino rat

Environmental estrogens bind to estrogen receptors, mimic estrogenic actions, and have adverse effects on human health like Bisphenol - A (BPA) which is used as a monomer in the production of polycarbonate plastics (PC) and epoxy resins which are used in variety of canned foods. Skeletal muscle plays an essential role in maintaining systemic glucose metabolism. In the present study, we investigated the possible effects of BPA on insulin signalling molecules and GLUT4 translocation in the gastrocnemius muscle of adult male rat. Rats were divided into four groups - Group I: Control (vehicle-corn oil treated), Group II, III and IV were administered with BPA (10, 100 and 400mg/kg b.wt/day, respectively) through oral gavage. Fasting blood glucose level of BPA treated groups showed a significant increase, oral glucose tolerance and insulin tolerance were also impaired in these animals. BPA significantly decreased the protein levels of insulin signalling molecules like IR, IRS-1, Akt, AS160 and its phosphorylated forms and blunts GLUT4 translocation by altering the levels of v- and t- SNARE proteins that assist the translocation process, thereby decreasing glucose uptake and oxidation in the gastrocnemius muscle. These results suggest that BPA has detrimental effects on insulin signalling molecules and GLUT4 translocation in the gastrocnemius muscle and thus impairs glucose homeostasis.

Crossreactivity of an Antiserum Directed to the Gram-Negative Bacterium Neisseria gonorrhoeae with the SNARE-Complex Protein Snap23 Correlates to Impaired Exocytosis in SH-SY5Y Cells

Early maternal infections with Neisseria gonorrhoeae (NG) correlate to an increased lifetime schizophrenia risk for the offspring, which might be due to an immune-mediated mechanism. Here, we investigated the interactions of polyclonal antisera to NG (α-NG) with a first trimester prenatal brain multiprotein array, revealing among others the SNARE-complex protein Snap23 as a target antigen for α-NG. This interaction was confirmed by Western blot analysis with a recombinant Snap23 protein, whereas the closely related Snap25 failed to interact with α-NG. Furthermore, a polyclonal antiserum to the closely related bacterium Neisseria meningitidis (α-NM) failed to interact with both proteins. Functionally, in SH-SY5Y cells, α-NG pretreatment interfered with both insulin-induced vesicle recycling, as revealed by uptake of the fluorescent endocytosis marker FM1-43, and insulin-dependent membrane translocation of the glucose transporter GluT4. Similar effects could be observed for an antiserum raised directly to Snap23, whereas a serum to Snap25 failed to do so. In conclusion, Snap23 seems to be a possible immune target for anti-gonococcal antibodies, the interactions of which seem at least in vitro to interfere with vesicle-associated exocytosis. Whether these changes contribute to the correlation between maternal gonococcal infections and psychosis in vivo remains still to be clarified.

Immunofluorescence microscopy of SNAP23 in human skeletal muscle reveals colocalization with plasma membrane, lipid droplets, and mitochondria

Synaptosomal-associated protein 23 (SNAP23) is a SNARE protein expressed abundantly in human skeletal muscle. Its established role is to mediate insulin-stimulated docking and fusion of glucose transporter 4 (GLUT4) with the plasma membrane. Recent in vitro research has proposed that SNAP23 may also play a role in the fusion of growing lipid droplets (LDs) and the channeling of LD-derived fatty acids (FAs) into neighboring mitochondria for β-oxidation. This study investigates the subcellular distribution of SNAP23 in human skeletal muscle using immunofluorescence microscopy to confirm that SNAP23 localization supports the three proposed metabolic roles. Percutaneous biopsies were obtained from the m. vastus lateralis of six lean, healthy males in the rested, overnight fasted state. Cryosections were stained with antibodies targeting SNAP23, the mitochondrial marker cytochrome c oxidase and the plasma membrane marker dystrophin, whereas intramuscular LDs were stained using the neutral lipid dye oil red O. SNAP23 displayed areas of intense punctate staining in the intracellular regions of all muscle fibers and continuous intense staining in peripheral regions of the cell. Quantitation of confocal microscopy images showed colocalization of SNAP23 with the plasma membrane marker dystrophin (Pearson's correlation coefficient r = 0.50 ± 0.01). The intense punctate intracellular staining colocalized primarily with the mitochondrial marker cytochrome C oxidase (r = 0.50 ± 0.012) and to a lesser extent with LDs (r = 0.21 ± 0.01) visualized with oil red O. We conclude that the observed subcellular distribution of SNAP23 in human skeletal muscle supports the three aforementioned metabolic roles.

SNAP23 is selectively expressed in airway secretory cells and mediates baseline and stimulated mucin secretion

Airway mucin secretion is important pathophysiologically and as a model of polarized epithelial regulated exocytosis. We find the trafficking protein, SNAP23 (23-kDa paralogue of synaptosome-associated protein of 25 kDa), selectively expressed in secretory cells compared with ciliated and basal cells of airway epithelium by immunohistochemistry and FACS, suggesting that SNAP23 functions in regulated but not constitutive epithelial secretion. Heterozygous SNAP23 deletant mutant mice show spontaneous accumulation of intracellular mucin, indicating a defect in baseline secretion. However mucins are released from perfused tracheas of mutant and wild-type (WT) mice at the same rate, suggesting that increased intracellular stores balance reduced release efficiency to yield a fully compensated baseline steady state. In contrast, acute stimulated release of intracellular mucin from mutant mice is impaired whether measured by a static imaging assay 5 min after exposure to the secretagogue ATP or by kinetic analysis of mucins released from perfused tracheas during the first 10 min of ATP exposure. Together, these data indicate that increased intracellular stores cannot fully compensate for the defect in release efficiency during intense stimulation. The lungs of mutant mice develop normally and clear bacteria and instilled polystyrene beads comparable to WT mice, consistent with these functions depending on baseline secretion that is fully compensated.

Expression of the SNARE protein SNAP-23 is essential for cell survival

Members of the SNARE-family of proteins are known to be key regulators of the membrane-membrane fusion events required for intracellular membrane traffic. The ubiquitously expressed SNARE protein SNAP-23 regulates a wide variety of exocytosis events and is essential for mouse development. Germline deletion of SNAP-23 results in early embryonic lethality in mice, and for this reason we now describe mice and cell lines in which SNAP-23 can be conditionally-deleted using Cre-lox technology. Deletion of SNAP-23 in CD19-Cre expressing mice prevents B lymphocyte development and deletion of SNAP-23 using a variety of T lymphocyte-specific Cre mice prevents T lymphocyte development. Acute depletion of SNAP-23 in mouse fibroblasts leads to rapid apoptotic cell death. These data highlight the importance of SNAP-23 for cell survival and describe a mouse in which specific cell types can be eliminated by expression of tissue-specific Cre-recombinase.

Drosophila SNAP-29 is an essential SNARE that binds multiple proteins involved in membrane traffic

Each membrane fusion event along the secretory and endocytic pathways requires a specific set of SNAREs to assemble into a 4-helical coiled-coil, the so-called trans-SNARE complex. Although most SNAREs contribute one helix to the trans-SNARE complex, members of the SNAP-25 family contribute two helixes. We report the characterization of the Drosophila homologue of SNAP-29 (dSNAP-29), which is expressed throughout development. Unlike the other SNAP-25 like proteins in fruit fly (i.e., dSNAP-25 and dSNAP-24), which form SDS-resistant SNARE complexes with their cognate SNAREs, dSNAP-29 does not participate in any SDS-resistant complexes, despite its interaction with dsyntaxin1 and dsyntaxin16 in vitro. Immunofluorescence studies indicated that dSNAP-29 is distributed in various tissues, locating in small intracellular puncta and on the plasma membrane, where it associates with EH domain-containing proteins implicated in the endocytic pathway. Overexpression and RNAi studies suggested that dSNAP-29 mediates an essential process in Drosophila development.

Adipose Tissue Transcriptome by Serial Analysis of Gene Expression

OBJECTIVE

To describe the genomic expression profile or transcriptome of adipose tissue using the serial analysis of gene expression method.

RESEARCH METHODS AND PROCEDURES

The serial analysis of gene expression strategy is based on isolation of short sequences (tags), which usually correspond to unique transcripts, and on their concatenation into long DNA molecules, which are then cloned and sequenced. Experiments were performed with mRNA from retroperitoneal adipose tissue of male C57BL6 mice.

RESULTS

We isolated 45,996 tags corresponding to more than 17,000 different genes. Eighty-eight genes were expressed at more than 0.1% of the total population and represented 26% of the mRNA population identified. The most expressed genes were: carbonic anhydrase 3 (1.97%), cytochrome c oxidase (COX) 1 (1.47%), COX2 (1.25%), diazepam binding inhibitor (1.04%), a novel transcript (0.87%), COX3 (0.55%), fatty acid-binding protein 4 (0.55%), and NADH dehydrogenase 4 (0.52%). Other genes known to be expressed in adipose tissue, such as uncoupling protein 2, angiotensinogen, adipsin, and insulin-like growth factor 1, were found at a lower level. Several tags corresponding to novel transcripts were also found.

DISCUSSION

To our knowledge, the present results provide for the first time a quantitative description of the transcriptome in adipose tissue.

Accurate quantification of functional analogy among close homologs

Correctly evaluating functional similarities among homologous proteins is necessary for accurate transfer of experimental knowledge from one organism to another, and is of particular importance for the development of animal models of human disease. While the fact that sequence similarity implies functional similarity is a fundamental paradigm of molecular biology, sequence comparison does not directly assess the extent to which two proteins participate in the same biological processes, and has limited utility for analyzing families with several parologous members. Nevertheless, we show that it is possible to provide a cross-organism functional similarity measure in an unbiased way through the exclusive use of high-throughput gene-expression data. Our methodology is based on probabilistic cross-species mapping of functionally analogous proteins based on Bayesian integrative analysis of gene expression compendia. We demonstrate that even among closely related genes, our method is able to predict functionally analogous homolog pairs better than relying on sequence comparison alone. We also demonstrate that the landscape of functional similarity is often complex and that definitive “functional orthologs” do not always exist. Even in these cases, our method and the online interface we provide are designed to allow detailed exploration of sources of inferred functional similarity that can be evaluated by the user.

Common ancestry is a central tenet of modern biology, as genes from different species often show a high degree of sequence similarity, making it possible to study analogous processes across model organisms. However, many genes belong to large families with several duplicates and the relationship between genes from different species is often not one-to-one, complicating the transfer of experimental knowledge. We present a method that uses a large compendia of high-throughput expression data, that covers many genes that have not been analyzed in any other way, to systematically predict which genes are most likely to participate in the same biological process and thus have analogous function in different organisms. We show that our method agrees well with current experimental knowledge and we use it to investigate several families of genes that demonstrate the complexity of functional analogy.

VAMP-1, VAMP-2, and syntaxin-4 regulate ANP release from cardiac myocytes

ANP is a peptide released by cardiac myocytes that regulates blood pressure and natriuresis. However, the molecular mechanisms controlling ANP release from cardiac myocytes are not defined. We now identify three components of the exocytic machinery that regulate ANP release from atrial myocytes. We found that cardiac myocytes express N-ethylmaleimide sensitive factor (NSF), soluble NSF attachment protein (α-SNAP), and SNAP receptors (SNAREs). Additionally we found that specific SNARE molecules, VAMP-1 and VAMP-2, both co-sediment and co-localize with ANP. Also, one SNARE molecule, syntaxin-4, partially co-sediments and partially co-localizes with ANP. Furthermore, these three SNAREs, syntaxin-4 and VAMP-1 and VAMP-2, form a SNARE complex inside cardiac myocytes. Finally, knockdown of VAMP-1, VAMP-2, or syntaxin-4 blocks regulated release of ANP. In contrast, silencing of VAMP-3 did not have an effect on ANP release. Our data suggest that three specific SNAREs regulate cardiac myocyte exocytosis of ANP. Pathways that modify the exocytic machinery may influence natriuresis and blood pressure.

The SNARE protein SNAP23 and the SNARE-interacting protein Munc18c in human skeletal muscle are implicated in insulin resistance/type 2 diabetes

OBJECTIVE

Our previous studies suggest that the SNARE protein synaptosomal-associated protein of 23 kDa (SNAP23) is involved in the link between increased lipid levels and insulin resistance in cardiomyocytes. The objective was to determine whether SNAP23 may also be involved in the known association between lipid accumulation in skeletal muscle and insulin resistance/type 2 diabetes in humans, as well as to identify a potential regulator of SNAP23.

RESEARCH DESIGN AND METHODS

We analyzed skeletal muscle biopsies from patients with type 2 diabetes and healthy, insulin-sensitive control subjects for expression (mRNA and protein) and intracellular localization (subcellular fractionation and immunohistochemistry) of SNAP23, and for expression of proteins known to interact with SNARE proteins. Insulin resistance was determined by a euglycemic hyperinsulinemic clamp. Potential mechanisms for regulation of SNAP23 were also investigated in the skeletal muscle cell line L6.

RESULTS

We showed increased SNAP23 levels in skeletal muscle from patients with type 2 diabetes compared with that from lean control subjects. Moreover, SNAP23 was redistributed from the plasma membrane to the microsomal/cytosolic compartment in the patients with the type 2 diabetes. Expression of the SNARE-interacting protein Munc18c was higher in skeletal muscle from patients with type 2 diabetes. Studies in L6 cells showed that Munc18c promoted the expression of SNAP23.

CONCLUSIONS

We have translated our previous in vitro results into humans by showing that there is a change in the distribution of SNAP23 to the interior of the cell in skeletal muscle from patients with type 2 diabetes. We also showed that Munc18c is a potential regulator of SNAP23.

A neuronal role for SNAP-23 in postsynaptic glutamate receptor trafficking

Regulated exocytosis is essential for many biological processes and many components of the protein trafficking machinery are ubiquitous. However, there are also exceptions, such as SNAP-25, a neuron-specific SNARE protein that is essential for synaptic vesicle release from presynaptic nerve terminals. In contrast, SNAP-23 is a ubiquitously expressed SNAP-25 homolog that is critical for regulated exocytosis in non-neuronal cells. However, the role of SNAP-23 in neurons has not been elucidated. We found that SNAP-23 was enriched in dendritic spines and colocalized with constituents of the postsynaptic density, whereas SNAP-25 was restricted to axons. In addition, loss of SNAP-23 using genetically altered mice or shRNA targeted to SNAP-23 led to a marked decrease in NMDA receptor surface expression and NMDA receptor currents, whereas loss of SNAP-25 did not. SNAP-23 is therefore important for the functional regulation of postsynaptic glutamate receptors.

Involvement of the p66Shc protein in glucose transport regulation in skeletal muscle myoblasts

The p66(Shc) protein isoform regulates MAP kinase activity and the actin cytoskeleton turnover, which are both required for normal glucose transport responses. To investigate the role of p66(Shc) in glucose transport regulation in skeletal muscle cells, L6 myoblasts with antisense-mediated reduction (L6/p66(Shc)as) or adenovirus-mediated overexpression (L6/p66(Shc)adv) of the p66(Shc) protein were examined. L6/(Shc)as myoblasts showed constitutive activation of ERK-1/2 and disruption of the actin network, associated with an 11-fold increase in basal glucose transport. GLUT1 and GLUT3 transporter proteins were sevenfold and fourfold more abundant, respectively, and were localized throughout the cytoplasm. Conversely, in L6 myoblasts overexpressing p66(Shc), basal glucose uptake rates were reduced by 30% in parallel with a approximately 50% reduction in total GLUT1 and GLUT3 transporter levels. Inhibition of the increased ERK-1/2 activity with PD98059 in L6/(Shc)as cells had a minimal effect on increased GLUT1 and GLUT3 protein levels, but restored the actin cytoskeleton, and reduced the abnormally high basal glucose uptake by 70%. In conclusion, p66(Shc) appears to regulate the glucose transport system in skeletal muscle myoblasts by controlling, via MAP kinase, the integrity of the actin cytoskeleton and by modulating cellular expression of GLUT1 and GLUT3 transporter proteins via ERK-independent pathways.

GLUT4 vesicle recruitment and fusion are differentially regulated by Rac, AS160, and Rab8A in muscle cells

Insulin increases glucose uptake into muscle by enhancing the surface recycling of GLUT4 transporters. In myoblasts, insulin signals bifurcate downstream of phosphatidylinositol 3-kinase into separate Akt and Rac/actin arms. Akt-mediated Rab-GAP AS160 phosphorylation and Rac/actin are required for net insulin gain of GLUT4, but the specific steps (vesicle recruitment, docking or fusion) regulated by Rac, actin dynamics, and AS160 target Rab8A are unknown. In L6 myoblasts expressing GLUT4myc, blocking vesicle fusion by tetanus toxin cleavage of VAMP2 impeded GLUT4myc membrane insertion without diminishing its build-up at the cell periphery. Conversely, actin disruption by dominant negative Rac or Latrunculin B abolished insulin-induced surface and submembrane GLUT4myc accumulation. Expression of non-phosphorylatable AS160 (AS160-4P) abrogated membrane insertion of GLUT4myc and partially reduced its cortical build-up, an effect magnified by selective Rab8A knockdown. We propose that insulin-induced actin dynamics participates in GLUT4myc vesicle retention beneath the membrane, whereas AS160 phosphorylation is essential for GLUT4myc vesicle-membrane docking/fusion and also contributes to GLUT4myc cortical availability through Rab8A.

Identification of Human T Cell Leukemia Virus Type 1 Tax Amino Acid Signals and Cellular Factors Involved in Secretion of the Viral Oncoprotein

Human T cell leukemia virus type 1 (HTLV-1) is the etiologic agent of a number of pathologic abnormalities, including adult T cell leukemia (ATL) and HTLV-1-associated myelopathy/tropical spastic paraparesis (HAM/TSP). The viral oncoprotein Tax has been implicated in the pathogenesis of these diseases. Recently, cell-free Tax was detected in the cerebrospinal fluid of HAM/TSP patients, implying that extracellular Tax may be relevant to neurologic disease. Additionally, the presence of a nuclear export signal within Tax and its active secretion has been demonstrated in vitro. However, the mechanism of Tax secretion remains to be established. Studies reported herein elucidate the process of Tax secretion and identify domains of Tax critical to its subcellular localization and secretion. Tax was shown to interact with a number of cellular secretory pathway proteins in both the model cell line BHK (baby hamster kidney)-21 and an HTLV-1-infected T cell line, C8166, physiologically relevant to HTLV-1-induced disease. Silencing of selected components of the secretory pathway affected Tax secretion, further confirming regulated secretion of Tax. Additionally, mutations in two putative secretory signals within Tax DHE and YTNI resulted in aberrant subcellular localization of Tax and significantly altered protein secretion. Together, these studies demonstrate that Tax secretion is a regulated event facilitated by its interactions with proteins of the cellular secretory pathway and the presence of secretory signals within the carboxyl-terminal domain of the protein.

SNARE proteins mediate fusion between cytosolic lipid droplets and are implicated in insulin sensitivity

The accumulation of cytosolic lipid droplets in muscle and liver cells has been linked to the development of insulin resistance and type 2 diabetes. Such droplets are formed as small structures that increase in size through fusion, a process that is dependent on intact microtubules and the motor protein dynein. Approximately 15% of all droplets are involved in fusion processes at a given time. Here, we show that lipid droplets are associated with proteins involved in fusion processes in the cell: NSF (N-ethylmaleimide-sensitive-factor), alpha-SNAP (soluble NSF attachment protein) and the SNAREs (SNAP receptors), SNAP23 (synaptosomal-associated protein of 23 kDa), syntaxin-5 and VAMP4 (vesicle-associated membrane protein 4). Knockdown of the genes for SNAP23, syntaxin-5 or VAMP4, or microinjection of a dominant-negative mutant of alpha-SNAP, decreases the rate of fusion and the size of the lipid droplets. Thus, the SNARE system seems to have an important role in lipid droplet fusion. We also show that oleic acid treatment decreases the insulin sensitivity of heart muscle cells, and this sensitivity is completely restored by transfection with SNAP23. Thus, SNAP23 might be a link between insulin sensitivity and the inflow of fatty acids to the cell.

Protein kinase Czeta and glucose uptake

Protein kinase Czeta (PKCzeta) is a member of the PKC family, serving downstream of insulin receptor and phosphatidylinositol (PI) 3-kinase. Many evidences suggest that PKCzeta plays a very important role in activating glucose transport response. Not only insulin but also glucose and exercise can activate PKCzeta through diverse pathways. PKCzeta activation and activity are impaired with insulin resistance in muscle and adipose tissues of type II diabetes individuals, but heightened in liver tissue, wherein it also increases lipid synthesis mediated by SREBP-1c (sterol-regulatory element-binding protein). Many studies have focused on linkage between PKCzeta and GLUT4 translocation and activation. Exploring the molecular mechanisms and pathways by which PKCzeta mediates glucose transport will highlight the insulin-signaling pathway.

How many signals impinge on GLUT4 activation by insulin?

GLUT4 is the main glucose transporter activated by insulin in skeletal muscle cells and adipocytes. GLUT4 storage vesicles (GSVs) traffic in endocytic and exocytic compartments. In the basal state, GLUT4 compartments are preferentially sequestered in perinuclear deposits wherein stimuli including insulin and non-insulin factors can increase GLUT4 vesicle formation, its exocytosis, and fusion to plasma membrane. In addition to well-established effectors of insulin signaling pathway, such as PKCzeta and Akt, the cytoskeletal network is implicated in GLUT4 translocation. This review will discuss the mechanisms and activation of GLUT4 trafficking and incorporating to PM from three aspects: known molecules of the insulin signaling pathway; Rho and Rab family proteins and cytoskeletal molecules.

The subcellular fractionation properties and function of insulin receptor substrate-1 (IRS-1) are independent of cytoskeletal integrity

Efficient insulin action requires spatial and temporal coordination of signaling cascades. The prototypical insulin receptor substrate, IRS-1 plays a central role in insulin signaling. By subcellular fractionation IRS-1 is enriched in a particulate fraction, termed the high speed pellet (HSP), and its redistribution from this fraction is associated with signal attenuation and insulin resistance. Anecdotal evidence suggests the cytoskeleton may underpin the localization of IRS-1 to the HSP. In the present study we have taken a systematic approach to examine whether the cytoskeleton contributes to the subcellular fractionation properties and function of IRS-1. By standard microscopy or immunoprecipitation we were unable to detect evidence to support a specific interaction between IRS-1 and the major cytoskeletal components actin (microfilaments), vimentin (intermediate filaments), and tubulin (microtubules) in 3T3-L1 adipocytes or in CHO.IR.IRS-1 cells. Pharmacological disruption of microfilaments and microtubules, individually or in combination, was without effect on the subcellular distribution of IRS-1 or insulin-stimulated tyrosine phosphorylation in either cell type. Phosphorylation of Akt was modestly reduced (20-35%) in 3T3-L1 adipocytes but not in CHO.IR.IRS-1 cells. In cells lacking intermediate filaments (Vim(-/-)) IRS-1 expression, distribution and insulin-stimulated phosphorylation appeared normal. Even after depolymerisation of microfilaments and microtubules, insulin-stimulated phosphorylation of IRS-1 and Akt were maintained in Vim(-/-) cells. Taken together these data indicate that the characteristic subcellular fractionation properties and function of IRS-1 are unlikely to be mediated by cytoskeletal networks and that proximal insulin signaling does not require an intact cytoskeleton.

Syntaxin 2 and SNAP-23 are required for regulated surfactant secretion

The secretion of lung surfactant in alveolar type II cells is a complex process involving the fusion of lamellar bodies with the plasma membrane. This process is somewhat different from the exocytosis of hormones and neurotransmitters. For example, it is a relatively slower process, and lamellar bodies are very large vesicles with a diameter of approximately 1 microm. SNARE proteins are the conserved molecular machinery of exocytosis in the majority of secretory cells. However, their involvement in surfactant secretion has not been reported. Here, we showed that syntaxin 2 and SNAP-23 are expressed in alveolar type II cells. Both proteins are associated with the plasma membrane, and to some degree with lamellar bodies. An antisense oligonucleotide complementary to syntaxin 2 decreased its mRNA and protein levels. The same oligonucleotide also inhibited surfactant secretion, independent of secretagogues. A peptide derived from the N-terminus of syntaxin 2 or the C-terminus of SNAP-23 significantly inhibited Ca(2+)- and GTPgammaS-stimulated surfactant secretion from permeabilized type II cells in a dose-dependent manner. Furthermore, introduction of anti-syntaxin 2 or anti-SNAP-23 antibodies into permeabilized type II cells also inhibited surfactant release. Our results suggest that syntaxin 2 and SNAP-23 are required for regulated surfactant secretion.

SNAP-23 functions in docking/fusion of granules at low Ca2+

Ca(2+)-triggered exocytosis of secretory granules mediates the release of hormones from endocrine cells and neurons. The plasma membrane protein synaptosome-associated protein of 25 kDa (SNAP-25) is thought to be a key component of the membrane fusion apparatus that mediates exocytosis in neurons. Recently, homologues of SNAP-25 have been identified, including SNAP-23, which is expressed in many tissues, albeit at different levels. At present, little is known concerning functional differences among members of this family of proteins. Using an in vitro assay, we show here that SNAP-25 and SNAP-23 mediate the docking of secretory granules with the plasma membrane at high (1 microM) and low (100 nM) Ca(2+) levels, respectively, by interacting with different members of the synaptotagmin family. In intact endocrine cells, expression of exogenous SNAP-23 leads to high levels of hormone secretion under basal conditions. Thus, the relative expression levels of SNAP-25 and SNAP-23 might control the mode (regulated vs. basal) of granule release by forming docking complexes at different Ca(2+) thresholds.

Increased expression of the SNARE accessory protein Munc18c in lipid-mediated insulin resistance

Fatty acids inhibit insulin-mediated glucose metabolism in skeletal muscle, an effect largely attributed to defects in insulin-mediated glucose transport. Insulin-resistant mice transgenic for the overexpression of lipoprotein lipase (LPL) in skeletal muscle were used to examine the molecular mechanism(s) in more detail. Using DNA gene chip array technology, and confirmation by RT-PCR and Western analysis, increases in the yeast Sec1p homolog Munc18c mRNA and protein were found in the gastrocnemius muscle of transgenic mice, but not other tissues. Munc18c has been previously demonstrated to impair insulin-mediated glucose transport in mammalian cells in vitro. Of interest, stably transfected C2C12 cells overexpressing LPL not only demonstrated increases in Munc18c mRNA and protein but also in transcription rates of the Munc18c gene. To confirm the relevance of fatty acid metabolism and insulin resistance to the expression of Munc18c in vivo, a 2-fold increase in Munc18c protein was demonstrated in mice fed a high-fat diet for 4 weeks. Together, these data are the first to implicate in vivo increases in Munc18c as a potential contributing mechanism to fatty acid-induced insulin resistance.

Unbiased quantitative proteomics of lipid rafts reveals high specificity for signaling factors

Membrane lipids were once thought to be homogenously distributed in the 2D surface of a membrane, but the lipid raft theory suggests that cholesterol and sphingolipids partition away from other membrane lipids. Lipid raft theory further implicates these cholesterol-rich domains in many processes such as signaling and vesicle traffic. However, direct characterization of rafts has been difficult, because they cannot be isolated in pure form. In the first functional proteomic analysis of rafts, we use quantitative high-resolution MS to specifically detect proteins depleted from rafts by cholesterol-disrupting drugs, resulting in a set of 241 authentic lipid raft components. We detect a large proportion of signaling molecules, highly enriched versus total membranes and detergent-resistant fractions, which thus far biochemically defined rafts. Our results provide the first large-scale and unbiased evidence, to our knowledge, for the connection of rafts with signaling and place limits on the fraction of plasma membrane composed by rafts.

Patterns of global gene expression in rat skeletal muscle during unloading and low-intensity ambulatory activity

Physical inactivity and unloading lead to diverse skeletal muscle alterations. Our goal was to identify the genes in skeletal muscle whose expression is most sensitive to periods of unloading/reduced physical activity and that may be involved in triggering initial responses before phenotypic changes are evident. The ability of short periods of physical activity/loading as an effective countermeasure against changes in gene expression mediated by inactivity was also tested. Affymetrix microarrays were used to compare mRNA levels in the soleus muscle under three experimental treatments (n = 20-29 rats each): 12-h hindlimb unloading (HU), 12-h HU followed by 4 h of intermittent low-intensity ambulatory and postural activity (4-h reloading), and control (with ambulatory and postural activity). Using a combination of criteria, we identified a small set of genes (approximately 1% of 8,738 genes on the array or 4% of significant expressed genes) with the most reproducible and largest responses to altered activity. Analysis revealed a coordinated regulation of transcription for a large number of key signaling proteins and transcription factors involved in protein synthesis/degradation and energy metabolism. Most (21 of 25) of the gene expression changes that were downregulated during HU returned at least to control levels during the reloading. In surprising contrast, 27 of 38 of the genes upregulated during HU remained significantly above control, but most showed trends toward reversal. This introduces a new concept that, in general, genes that are upregulated during unloading/inactivity will be more resistant to periodic reloading than those genes that are downregulated. This study reveals genes that are the most sensitive to loading/activity in rat skeletal muscle and indicates new targets that may initiate muscle alterations during inactivity.

The vesicle- and target-SNARE proteins that mediate Glut4 vesicle fusion are localized in detergent-insoluble lipid rafts present on distinct intracellular membranes

Insulin stimulates the fusion of intracellular vesicles containing the glucose transporter Glut4 with the plasma membrane in adipocytes and muscle cells. Glut4 vesicle fusion is thought to be catalyzed by the interaction of the vesicle soluble N-ethyl-maleimide-sensitive fusion protein attachment protein receptor VAMP2 with the target soluble N-ethyl-maleimide-sensitive fusion protein attachment protein receptors SNAP-23 and syntaxin 4. Here, we use combined membrane fractionation, detergent solubility, and sucrose gradient flotation to demonstrate that the large majority (>70%) of SNAP-23 and a significant proportion of syntaxin 4 ( approximately 35%) are associated with plasma membrane lipid rafts in 3T3-L1 adipocytes. Furthermore, VAMP2 is shown to be concentrated in lipid rafts isolated from intracellular membranes. Insulin stimulation had no effect on the plasma membrane raft association of SNAP-23 or syntaxin 4 but promoted VAMP2 insertion into plasma membrane rafts. Immunofluorescence analysis revealed that SNAP-23 was clustered at the plasma membrane and almost completely segregated from the transferrin receptor. SNAP-23 distribution seemed to be distinct from caveolin-1, and clusters of SNAP-23 were dispersed after cholesterol extraction with methyl-beta-cyclodextrin, suggesting that the majority of SNAP-23 is associated with non-caveolar, cholesterol-rich lipid rafts. The results described implicate lipid rafts as important platforms for Glut4 vesicle fusion and suggest the hypothesis that such rafts may represent a spatial integration point of insulin signaling and membrane traffic.

SNARE expression and distribution during 3T3-L1 adipocyte differentiation

Differentiation of 3T3-L1 cells into adipocytes presupposes the expression of the glucose transporter isoform GLUT4 and the acquisition of insulin-dependent GLUT4 translocation from intracellular storage vesicles to plasma membrane. This ability to translocate GLUT4 depends on the presence of a set of proteins of the SNARE category that are essential in the fusion step. The expression and levels of some of these SNARE proteins are altered during 3T3-L1 differentiation. Levels of the v-SNARE protein cellubrevin and of the t-SNARE protein syntaxin 4 were increased in this process in parallel to GLUT4. However, the levels of SNAP-23, another t-SNARE, were maintained during differentiation. Immunofluorescence images of SNAP-23 showed the initial distribution of this protein in a perinuclear region before differentiation and its redistribution towards plasma membrane in the adipocyte form. These results suggest a capital role in the expression levels and cellular distribution, during 3T3-L1 differentiation, of SNARE proteins involved in the late steps of GLUT4 translocation.

Insulin acutely regulates Munc18-c subcellular trafficking: altered response in insulin-resistant 3T3-L1 adipocytes

Preincubation of 3T3-L1 adipocytes in high glucose or glucosamine decreases acute insulin (100 nm)-stimulated glucose transport provided that insulin (0.6 nm) is included during preincubation. GLUT4 expression is unchanged (Nelson, B. A., Robinson, K. A., and Buse, M. G. (2000) Diabetes 49, 981-991). Munc18-c, a Syntaxin 4-binding protein, is a proposed regulator of the docking/fusion of GLUT4-containing vesicles with the plasma membrane. We examined the subcellular distribution of Munc18-c in response to acute (15-min) insulin (100 nm) stimulation after preincubation in 5 or 25 mm glucose +/- 0.6 nm insulin. Immunoblotting detected Munc18-c mainly in the Triton X-100-soluble plasma membrane (TS-PM) and the Triton X-100-insoluble low density microsomal (TI-LDM) fraction. Under each condition except high glucose + insulin preincubation, acute insulin increased Munc18-c (50-200%) in TS-PM and decreased Munc18-c (60%) in TI-LDM. Munc18-c traffic was time-dependent with a lag time of 3 min compared with GLUT4. Preincubation with high glucose + 0.6 nm insulin significantly impaired acute insulin-stimulated Munc18-c trafficking and decreased basal Munc18-c in the TI-LDM. Preincubation with glucosamine + insulin had similar effects. Total cellular Munc18-c remained unchanged. In conclusion, acute insulin stimulation promotes the translocation of Munc18-c, apparently from a TI-LDM-associated compartment to the TS-PM. Chronically increased glucose flux or exposure to glucosamine disrupts this process, which may negatively impact the fusion of GLUT4-containing vesicles with the plasma membrane.

Insulin-induced cortical actin remodeling promotes GLUT4 insertion at muscle cell membrane ruffles

Insulin stimulates glucose uptake by recruiting glucose transporter 4 (GLUT4) from an intracellular compartment to the cell surface; this phenomenon is defective in type 2 diabetes. Here we examine the involvement of actin filaments in GLUT4 translocation and their possible defects in insulin resistance, using L6 myotubes expressing myc-tagged GLUT4. Insulin caused membrane ruffling, a dynamic distortion of the myotube dorsal surface. Fluorescence microscopy and immunogold staining of surface GLUT4myc coupled to backscatter electron microscopy revealed a high density of this protein in membrane ruffles. The t-SNAREs syntaxin4 and SNAP-23 were also abundant in these regions. Below the membrane, GLUT4 and the vesicular protein VAMP2, but not VAMP3, colocalized with the actin structures supporting the membrane ruffles. GLUT4myc externalization and membrane ruffles were reduced by jasplakinolide and by swinholide-A, drugs that affect actin filament stability and prevent actin branching, respectively. Insulin resistance generated by prolonged (24 hours) exposure of myotubes to high glucose and insulin diminished the acute insulin-dependent remodeling of cortical actin and GLUT4myc translocation, reminiscent of the effect of swinholide-A. We propose that GLUT4 vesicle incorporation into the plasma membrane involves insulin-dependent cortical actin remodeling and that defective actin remodeling contributes to insulin resistance.

Hyperosmolarity reduces GLUT4 endocytosis and increases its exocytosis from a VAMP2-independent pool in l6 muscle cells

The intracellular traffic of the glucose transporter 4 (GLUT4) in muscle cells remains largely unexplored. Here we make use of L6 myoblasts stably expressing GLUT4 with an exofacially directed Myc-tag (GLUT4myc) to determine the exocytic and endocytic rates of the transporter. Insulin caused a rapid (t(12) = 4 min) gain, whereas hyperosmolarity (0.45 m sucrose) caused a slow (t(12) = 20 min) gain in surface GLUT4myc molecules. With prior insulin stimulation followed by addition of hypertonic sucrose, the increase in surface GLUT4myc was partly additive. Unlike the effect of insulin, the GLUT4myc gain caused by hyperosmolarity was insensitive to wortmannin or to tetanus toxin cleavage of VAMP2 and VAMP3. Disappearance of GLUT4myc from the cell surface was rapid (t(12) = 1.5 min). Insulin had no effect on the initial rate of GLUT4myc internalization. In contrast, hyperosmolarity almost completely abolished GLUT4myc internalization. Surface GLUT4myc accumulation in response to hyperosmolarity was only partially blocked by inhibition of tyrosine kinases with erbstatin analog (erbstatin A) and genistein. However, neither inhibitor interfered with the ability of hyperosmolarity to block GLUT4myc internalization. We propose that hyperosmolarity increases surface GLUT4myc by preventing GLUT4 endocytosis and stimulating its exocytosis via a pathway independent of phosphatidylinositol 3-kinase activity and of VAMP2 or VAMP3. A tetanus toxin-insensitive v-SNARE such as TI-VAMP detected in these cells, might mediate membrane fusion of the hyperosmolarity-sensitive pool.

Biochemical, electron microscopic and immunohistological observations of cationic detergent-extracted cells: detection and improved preservation of microextensions and ultramicroextensions

BACKGROUND

Filopodia, retraction fibers and microvilli, are fragile microextensions of the plasma membrane that are easily damaged by mechanical force during specimen preparation for microscopy. To preserve these structures for electron microscopy glutaraldehyde is generally used, but it often causes antigen masking. By contrast, formaldehyde is generally used for immunofluorescence light microscopy, but few studies have been concerned with the loss of microextensions.

RESULTS

We demonstrate in biochemical experiments that cultured cells needed to be kept in 4% formaldehyde for at least 60 min at room temperature or for 20 min at 37 degrees C to irreversibly crosslink most of the polypeptides. Also, fragmentation of fragile microextensions was observed after Triton X-100 extraction depending on concentration and extent of crosslinking. We also report on a novel fixation procedure that includes the cationic detergent dodecyltrimethylammonium chloride (DOTMAC). Treatment of NIH3T3 cells with DOTMAC resulted in complete removal of membrane lipids and in good preservation of the cytoskeleton in microextensions as well as preservation of ultramicroextensions of <0.05 microm in diameter that have not been observed previously unless glutaraldehyde was used. Stress fibers and microextensions of DOTMAC-extracted cells were readily stained with anti-beta-actin antibodies, and antibodies to vinculin and moesin stained focal contacts and microextensions, respectively.

CONCLUSIONS

Some microextensions were fragmented by the standard Triton X-100 permeabilization method. By contrast, DOTMAC completely extracted membrane lipids while maintaining the cytoskeleton of microextensions. Thus, DOTMAC treatment may provide a valuable new tool for the reliable visualization of previously undetectable or poorly detectable antigens while preserving the actin cytoskeleton of microextensions.

SNAP-24, a Drosophila SNAP-25 homologue on granule membranes, is a putative mediator of secretion and granule-granule fusion in salivary glands

Fusion of vesicles with target membranes is dependent on the interaction of target (t) and vesicle (v) SNARE (soluble NSF (N-ethylmaleimide-sensitive fusion protein) attachment protein receptor) proteins located on opposing membranes. For fusion at the plasma membrane, the t-SNARE SNAP-25 is essential. In Drosophila, the only known SNAP-25 isoform is specific to neuronal axons and synapses and additional t-SNAREs must exist that mediate both non-synaptic fusion in neurons and constitutive and regulated fusion in other cells. Here we report the identification and characterization of SNAP-24, a closely related Drosophila SNAP-25 homologue, that is expressed throughout development. The spatial distribution of SNAP-24 in the nervous system is punctate and, unlike SNAP-25, is not concentrated in synaptic regions. In vitro studies, however, show that SNAP-24 can form core complexes with syntaxin and both synaptic and non-synaptic v-SNAREs. High levels of SNAP-24 are found in larval salivary glands, where SNAP-24 localizes mainly to granule membranes rather than the plasma membrane. During glue secretion, the massive exocytotic event of these glands, SNAP-24 containing granules fuse with one another and the apical membrane, suggesting that glue secretion utilizes compound exocytosis and that SNAP-24 mediates secretion.

Vimentin filaments in fibroblasts are a reservoir for SNAP23, a component of the membrane fusion machinery

Soluble N-ethyl maleimide-sensitive fusion protein attachment protein receptors (SNAREs) are core machinery for membrane fusion during intracellular vesicular transport. Synaptosome-associated protein of 23 kDa (SNAP23) is a target SNARE previously identified at the plasma membrane, where it is involved in exocytotic membrane fusion. Here we show that SNAP23 associates with vimentin filaments in a Triton X-100 insoluble fraction in fibroblasts in primary culture and HeLa cells. Upon treatment of human fibroblasts with N-ethyl-maleimide, SNAP23 dissociates from vimentin filaments and forms a protein complex with syntaxin 4, a plasma membrane SNARE. The vimentin-associated pool of SNAP23 can therefore be a reservoir, which would supply the plasma membrane fusion machinery, in fibroblasts. Our observation points to a yet unexplored role of intermediate filaments.

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.

VAMP2, but not VAMP3/cellubrevin, mediates insulin-dependent incorporation of GLUT4 into the plasma membrane of L6 myoblasts

Like neuronal synaptic vesicles, intracellular GLUT4-containing vesicles must dock and fuse with the plasma membrane, thereby facilitating insulin-regulated glucose uptake into muscle and fat cells. GLUT4 colocalizes in part with the vesicle SNAREs VAMP2 and VAMP3. In this study, we used a single-cell fluorescence-based assay to compare the functional involvement of VAMP2 and VAMP3 in GLUT4 translocation. Transient transfection of proteolytically active tetanus toxin light chain cleaved both VAMP2 and VAMP3 proteins in L6 myoblasts stably expressing exofacially myc-tagged GLUT4 protein and inhibited insulin-stimulated GLUT4 translocation. Tetanus toxin also caused accumulation of the remaining C-terminal VAMP2 and VAMP3 portions in Golgi elements. This behavior was exclusive to these proteins, because the localization of intracellular myc-tagged GLUT4 protein was not affected by the toxin. Upon cotransfection of tetanus toxin with individual vesicle SNARE constructs, only toxin-resistant VAMP2 rescued the inhibition of insulin-dependent GLUT4 translocation by tetanus toxin. Moreover, insulin caused a cortical actin filament reorganization in which GLUT4 and VAMP2, but not VAMP3, were clustered. We propose that VAMP2 is a resident protein of the insulin-sensitive GLUT4 compartment and that the integrity of this protein is required for GLUT4 vesicle incorporation into the cell surface in response to insulin.

A functional role for VAP-33 in insulin-stimulated GLUT4 traffic

Soluble N-ethylmaleimide sensitive factor attachment protein receptors (SNAREs) are critical proteins in membrane fusion, in both regulated and constitutive vesicular traffic. In addition, proteins that interact with the SNAREs are thought to regulate fusion. Vesicle-associated membrane protein-2 (VAMP-2) is a SNARE protein involved in insulin-dependent glucose transporter 4 (GLUT4) traffic. VAMP-2 is required for productive GLUT4 incorporation into the plasma membrane. VAMP-associated protein of 33 kDa (VAP-33) is an integral membrane protein that binds VAMPs in vitro, and is hypothesized to be a regulator of VAMPs. In L6 skeletal myoblasts, which display insulin-dependent traffic of GLUT4, we show that VAP-33 colocalized significantly with VAMP-2 using indirect confocal immunofluorescence and biochemical cosegregation. Overexpression of wild-type VAP-33 in L6 myoblasts attenuated the insulin-dependent incorporation of myc-tagged GLUT4 into the plasma membrane, and this response was restored by co-overexpression of VAMP-2 linked to green fluorescent protein. Antibodies to VAP-33 microinjected into 3T3-L1 adipocytes abrogated the insulin-stimulated translocation of GLUT4 to the plasma membrane, as measured in adhered plasma membrane lawns. Immunopurified VAMP-2-containing compartments from L6 myotubes and 3T3-L1 adipocytes showed significant levels of VAP-33. We propose that VAP-33 may be a regulator of VAMP-2 availability for GLUT4 traffic and other vesicle fusion events.

Quantification of SNARE protein levels in 3T3-L1 adipocytes: implications for insulin-stimulated glucose transport

Insulin-stimulates glucose transport in peripheral tissues by stimulating the movement ('translocation') of a pool of intracellular vesicles containing the glucose transporter Glut4 to the cell surface. The fusion of these vesicles with the plasma membrane results in a large increase in the numbers of Glut4 molecules at the cell surface and a concomitant enhancement of glucose uptake. It is well established that proteins of the VAMP- (synaptobrevin) and syntaxin-families play a fundamental role in the insulin-stimulated fusion of Glut4-containing vesicles with the plasma membrane. Studies have identified key roles for vesicle associated membrane protein-2 (VAMP2) and syntaxin-4 in this event, and more recently have also implicated SNAP-23 and Munc18c in this process. In this study, we have quantified the absolute levels of expression of these proteins in murine 3T3-L1 adipocytes, with the objective of determining the stoichiometry of these proteins both relative to each other and also in comparison with previous estimates of Glut4 levels within these cells. To achieve this, we performed quantitative immunoblot analysis of these proteins in 3T3-L1 membranes compared to known amounts of purified recombinant proteins. Such analyses suggest that in 3T3-L1 adipocytes there are approximately 374,000 copies of syntaxin 4, 1.15 x 10(6) copies of SNAP23, 495,000 copies of VAMP2, 4.3 x 10(6) copies of cellubrevin and 452,000 copies of Munc18c per cell, compared to previous estimates of 280,000 copies of Glut4. Thus, the main SNARE proteins involved in insulin-stimulated Glut4 exocytosis (syntaxin 4 and VAMP2) are expressed in approximately equimolar amounts in adipocytes, whereas by contrast the endosomal v-SNARE cellubrevin is present at approximately 10-fold higher levels and the t-SNARE SNAP-23 is also present in an approximately 3-fold molar excess. The implications of this quantification for the mechanism of insulin-stimulated Glut4 translocation are discussed.

Molecular mechanisms of platelet exocytosis: role of SNAP-23 and syntaxin 2 in dense core granule release

To characterize the molecular mechanisms of platelet secretion, we focused on the calcium-induced exocytosis of dense core granules. Platelets contain several known t-SNAREs (soluble N-ethylmaleimide sensitive factor [NSF] attachment protein receptors) such as syntaxins 2, 4, and 7 and SNAP-23 (synaptosomal associated protein 23). By using an in vitro exocytosis assay, we have been able to assign roles for some of these t-SNAREs in dense core granule release. This calcium-induced secretion relies on the SNARE proteins because it is stimulated by the addition of recombinant -SNAP and inhibited by a dominant negative -SNAP–L294A mutant or by anti–-SNAP and anti-NSF antibodies. SNAP-23 antibodies and an inhibitory C-terminal SNAP-23 peptide both blocked dense core granule release, demonstrating a role for SNAP-23. Unlike other cell types, platelets contain a significant pool of soluble SNAP-23, which does not partition into Triton X-114. Of the anti-syntaxin antibodies tested, only anti–syntaxin 2 antibody inhibited dense core granule release. Immunoprecipitation studies showed that the 2 t-SNAREs syntaxin 2 and SNAP-23 do form a complex in vivo. These data clearly show that SNAPs, NSF, and specific t-SNAREs are used for dense core granule release; these data provide a greater understanding of regulated exocytosis in platelets.

SNIP, a novel SNAP-25-interacting protein implicated in regulated exocytosis

Synaptosome-associated protein of 25 kDa (SNAP-25) is a presynaptic membrane protein that has been clearly implicated in membrane fusion in both developing and mature neurons, although its mechanisms of action are unclear. We have now identified a novel SNAP-25-interacting protein named SNIP. SNIP is a hydrophilic, 145-kDa protein that comprises two predicted coiled-coil domains, two highly charged regions, and two proline-rich domains with multiple PPXY and PXXP motifs. SNIP is selectively expressed in brain where it co-distributes with SNAP-25 in most brain regions. Biochemical studies have revealed that SNIP is tightly associated with the brain cytoskeleton. Subcellular fractionation and immunofluorescence localization studies have demonstrated that SNIP co-localizes with SNAP-25 as well as the cortical actin cytoskeleton, suggesting that SNIP serves as a linker protein connecting SNAP-25 to the submembranous cytoskeleton. By using deletion analysis, we have mapped the binding domains of SNIP and SNAP-25, and we have demonstrated that the SNIP-SNAP-25 association is mediated via coiled-coil interactions. Moreover, we have shown that overexpression of SNIP or its SNAP-25-interacting domain inhibits Ca(2+)-dependent exocytosis from PC12 cells. These results indicate that SNIP is involved in regulation of neurosecretion, perhaps via its interaction with SNAP-25 and the cytoskeleton.