Role of syntaxin in mouse pancreatic beta cells

VAMP5 and distinct sets of cognate Q-SNAREs mediate exosome release

Small extracellular vesicles (sEVs) are largely classified into two types, plasma-membrane derived sEVs and endomembrane-derived sEVs. The latter type (referred to as exosomes herein) is originated from late endosomes or multivesicular bodies (MVBs). In order to release exosomes extracellularly, MVBs must fuse with the plasma membrane, not with lysosomes. In contrast to the mechanism responsible for MVB-lysosome fusion, the mechanism underlying the MVB-plasma membrane fusion is poorly understood. Here, we systematically analyze soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) family proteins and identify VAMP5 as an MVB-localized SNARE protein required for exosome release. Depletion of VAMP5 in HeLa cells impairs exosome release. Mechanistically, VAMP5 mediates exosome release by interacting with SNAP47 and plasma membrane SNARE Syntaxin 1 (STX1) or STX4 to release exosomes. VAMP5 is also found to mediate asymmetric exosome release from polarized Madin-Darby canine kidney (MDCK) epithelial cells through interaction with the distinct sets of Q-SNAREs, suggesting that VAMP5 is a general exosome regulator in both polarized cells and non-polarized cells.Key words: exosome, small extracellular vesicle (sEV), multivesicular body, SNARE, VAMP5.

Conventional and Unconventional Mechanisms by which Exocytosis Proteins Oversee β-cell Function and Protection

Type 2 diabetes (T2D) is one of the prominent causes of morbidity and mortality in the United States and beyond, reaching global pandemic proportions. One hallmark of T2D is dysfunctional glucose-stimulated insulin secretion from the pancreatic β-cell. Insulin is secreted via the recruitment of insulin secretory granules to the plasma membrane, where the soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs) and SNARE regulators work together to dock the secretory granules and release insulin into the circulation. SNARE proteins and their regulators include the Syntaxins, SNAPs, Sec1/Munc18, VAMPs, and double C2-domain proteins. Recent studies using genomics, proteomics, and biochemical approaches have linked deficiencies of exocytosis proteins with the onset and progression of T2D. Promising results are also emerging wherein restoration or enhancement of certain exocytosis proteins to β-cells improves whole-body glucose homeostasis, enhances β-cell function, and surprisingly, protection of β-cell mass. Intriguingly, overexpression and knockout studies have revealed novel functions of certain exocytosis proteins, like Syntaxin 4, suggesting that exocytosis proteins can impact a variety of pathways, including inflammatory signaling and aging. In this review, we present the conventional and unconventional functions of β-cell exocytosis proteins in normal physiology and T2D and describe how these insights might improve clinical care for T2D.

Chemical Biology Toolbox for Studying Pancreatic Islet Function - A Perspective

The islets of Langerhans represent one of the many complex endocrine organs in mammals. Traditionally, islet function is studied by a mixture of physiological, cell biological, and molecular biological methods. Recently, novel techniques stemming from the ever-increasing toolbox provided by chemical laboratories have been added to the repertoire. Many emerging techniques will soon be available to manipulate and monitor islet function at the single-cell level and potentially in intact model animals, as well as in isolated human islets. Here, we review the most current small-molecule-based and genetically encoded molecular tool sets available to study islet function. We provide an outlook regarding future tool developments that will impact islet research, with a special focus on the interplay between different islet cell types.

Reduced insulin secretion correlates with decreased expression of exocytotic genes in pancreatic islets from patients with type 2 diabetes

Reduced insulin release has been linked to defect exocytosis in β-cells. However, whether expression of genes suggested to be involved in the exocytotic process (exocytotic genes) is altered in pancreatic islets from patients with type 2 diabetes (T2D), and correlate to insulin secretion, needs to be further investigated. Analysing expression levels of 23 exocytotic genes using microarray revealed reduced expression of five genes in human T2D islets (χ(2)=13.25; p<0.001). Gene expression of STX1A, SYT4, SYT7, SYT11, SYT13, SNAP25 and STXBP1 correlated negatively to in vivo measurements of HbA1c levels and positively to glucose stimulated insulin secretion (GSIS) in vitro in human islets. STX1A, SYT4 and SYT11 protein levels correspondingly decreased in human T2D islets. Moreover, silencing of SYT4 and SYT13 reduced GSIS in INS1-832/13 cells. Our data support that reduced expression of exocytotic genes contributes to impaired insulin secretion, and suggest decreased expression of these genes as part of T2D pathogenesis.

Changes in microRNA expression contribute to pancreatic β-cell dysfunction in prediabetic NOD mice

During the initial phases of type 1 diabetes, pancreatic islets are invaded by immune cells, exposing β-cells to proinflammatory cytokines. This unfavorable environment results in gene expression modifications leading to loss of β-cell functions. To study the contribution of microRNAs (miRNAs) in this process, we used microarray analysis to search for changes in miRNA expression in prediabetic NOD mice islets. We found that the levels of miR-29a/b/c increased in islets of NOD mice during the phases preceding diabetes manifestation and in isolated mouse and human islets exposed to proinflammatory cytokines. Overexpression of miR-29a/b/c in MIN6 and dissociated islet cells led to impairment in glucose-induced insulin secretion. Defective insulin release was associated with diminished expression of the transcription factor Onecut2, and a consequent rise of granuphilin, an inhibitor of β-cell exocytosis. Overexpression of miR-29a/b/c also promoted apoptosis by decreasing the level of the antiapoptotic protein Mcl1. Indeed, a decoy molecule selectively masking the miR-29 binding site on Mcl1 mRNA protected insulin-secreting cells from apoptosis triggered by miR-29 or cytokines. Taken together, our findings suggest that changes in the level of miR-29 family members contribute to cytokine-mediated β-cell dysfunction occurring during the initial phases of type 1 diabetes.

Pancreatic islet cells: a model for calcium-dependent peptide release

In mammals the concentration of blood glucose is kept close to 5 mmol∕l. Different cell types in the islet of Langerhans participate in the control of glucose homeostasis. β-cells, the most frequent type in pancreatic islets, are responsible for the synthesis, storage, and release of insulin. Insulin, released with increases in blood glucose promotes glucose uptake into the cells. In response to glucose changes, pancreatic α-, β-, and δ-cells regulate their electrical activity and Ca(2+) signals to release glucagon, insulin, and somatostatin, respectively. While all these signaling steps are stimulated in hypoglycemic conditions in α-cells, the activation of these events require higher glucose concentrations in β and also in δ-cells. The stimulus-secretion coupling process and intracellular Ca(2+) ([Ca(2+)](i)) dynamics that allow β-cells to secrete is well-accepted. Conversely, the mechanisms that regulate α- and δ-cell secretion are still under study. Here, we will consider the glucose-induced signaling mechanisms in each cell type and the mathematical models that explain Ca(2+) dynamics.

Cell therapy for diabetes mellitus: an opportunity for stem cells?

Diabetes is a chronic disease characterized by a deficit in beta cell mass and a failure of glucose homeostasis. Both circumstances result in a variety of severe complications and an overall shortened life expectancy. Thus, diabetes represents an attractive candidate for cell therapy. Reversal of diabetes can be achieved through pancreas and islet transplantation, but shortage of donor organs has prompted an intensive search for alternative sources of beta cells. This achievement has stimulated the search for appropriate stem cell sources. Both embryonic and adult stem cells have been used to generate surrogate beta cells or otherwise restore beta cell functioning. In this regard, several studies have reported the generation of insulin-secreting cells from embryonic and adult stem cells that normalized blood glucose values when transplanted into diabetic animal models. Due to beta cell complexity, insulin-producing cells generated from stem cells do not possess all beta cell attributes. This indicates the need for further development of methods for differentiation and selection of completely functional beta cells. While these problems are overcome, diabetic patients may benefit from therapeutic strategies based on autologous stem cell therapies addressing late diabetic complications. In this article, we discuss the recent progress in the generation of insulin-producing cells from embryonic and adult stem cells, together with the challenges for the clinical use of diabetes stem cell therapy.

Syntaxin 4 Facilitates Biphasic Glucose-Stimulated Insulin Secretion from Pancreatic β-Cells

Numerous overexpression studies have recently implicated Syntaxin 4 as an effector of insulin secretion, although its requirement in insulin granule exocytosis is unknown. To address this, islets from Syntaxin 4 heterozygous (−/+) knockout mice were isolated and compared with islets from wild-type mice. Under static incubation conditions, Syntaxin 4 (−/+) islets showed a 60% reduction in glucose-stimulated insulin secretion compared with wild-type islets. Perifusion analyses revealed that Syntaxin 4 (−/+) islets secreted 50% less insulin during the first phase of glucose-stimulated insulin secretion and that this defect could be fully restored by the specific replenishment of recombinant Syntaxin 4. This essential role for Syntaxin 4 in secretion from the islet was localized to the β-cells because small interfering RNA-mediated depletion of Syntaxin 4 in MIN6 β-cells abolished glucose-stimulated insulin secretion. Moreover, immunofluorescent confocal microscopy revealed that Syntaxin 4 was principally localized to the β-cells and not the α-cells of the mouse islet. Remarkably, islets isolated from transgenic mice that express 2.4-fold higher levels of Syntaxin 4 relative to wild-type mice secreted approximately 35% more insulin during both phases of insulin secretion, suggesting that increased Syntaxin 4 may be beneficial for enhancing biphasic insulin secretion in a regulated manner. Taken together, these data support the notion that Syntaxin 4-based SNARE complexes are essential for biphasic insulin granule fusion in pancreatic β-cells.

PKA-dependent and PKA-independent pathways for cAMP-regulated exocytosis

Stimulus-secretion coupling is an essential process in secretory cells in which regulated exocytosis occurs, including neuronal, neuroendocrine, endocrine, and exocrine cells. While an increase in intracellular Ca(2+) concentration ([Ca(2+)](i)) is the principal signal, other intracellular signals also are important in regulated exocytosis. In particular, the cAMP signaling system is well known to regulate and modulate exocytosis in a variety of secretory cells. Until recently, it was generally thought that the effects of cAMP in regulated exocytosis are mediated by activation of cAMP-dependent protein kinase (PKA), a major cAMP target, followed by phosphorylation of the relevant proteins. Although the involvement of PKA-independent mechanisms has been suggested in cAMP-regulated exocytosis by pharmacological approaches, the molecular mechanisms are unknown. Newly discovered cAMP-GEF/Epac, which belongs to the cAMP-binding protein family, exhibits guanine nucleotide exchange factor activities and exerts diverse effects on cellular functions including hormone/transmitter secretion, cell adhesion, and intracellular Ca(2+) mobilization. cAMP-GEF/Epac mediates the PKA-independent effects on cAMP-regulated exocytosis. Thus cAMP regulates and modulates exocytosis by coordinating both PKA-dependent and PKA-independent mechanisms. Localization of cAMP within intracellular compartments (cAMP compartmentation or compartmentalization) may be a key mechanism underlying the distinct effects of cAMP in different domains of the cell.

Ectodermal commitment of insulin‐producing cells derived from mouse embryonic stem cells

Embryonic stem cells possess the ability to differentiate in vitro into a variety of cell lineages, including insulin-producing cells. Pancreatic beta-cells derive from foregut endoderm during embryonic development. However, previous reports using transgenic mice strongly indicate that insulin-positive cells may be generated also through the neuroectoderm pathway. To analyze this point, a culture system was performed in which only ectoderm committed cells were present. Based on published work, we achieved this by maintaining transfected clonal R1 mouse embryonic stem cells in monolayer in the absence of LIF. Contrary to differentiation protocols via embryoid body formation, monolayer cultured cells displayed ectodermal fates according to the marker gene expression pattern. Under these particular conditions, neomycin was added in order to select insulin-expressing cells. The cell lineage obtained expressed Pdx1, Pax6, Isl1, AChE, MBP, TH, and GS genes, confirming ectodermal commitment, even though some of these factors are also expressed in endoderm. In addition these cells displayed excitatory properties similar to astrocytes. Co-expression of insulin II and nestin was observed in monolayer culture and in the presence of specific conditioned media. No expression of early endodermal markers was detected along monolayer cultures. Altogether, these observations suggest that cells with ectoderm fates could participate in vitro in the derivation of insulin-producing cells. These results have implications for insulin gene regulation and hormone secretion in order to generate insulin-producing cells for replacement protocols in the treatment of diabetes.

Correlation of syntaxin-1 and SNAP-25 clusters with docking and fusion of insulin granules analysed by total internal reflection fluorescence microscopy

AIMS/HYPOTHESIS

The interaction of syntaxin-1 and SNAP-25 with insulin exocytosis was examined using the diabetic Goto-Kakizaki (GK) rat and a total internal reflection fluorescence (TIRF) imaging system.

METHODS

Primary rat pancreatic beta cells were immunostained with anti-syntaxin-1A, anti-SNAP-25 and anti-insulin antibodies, and then observed by TIRF microscopy. The real-time image of GFP-labelled insulin granules motion was monitored by TIRF.

RESULTS

The number of syntaxin-1A and SNAP-25 clusters, and the number of docked insulin granules on the plasma membrane were reduced in GK beta cells. When GK rats were treated with daily insulin injection for 2 weeks, the number of syntaxin-1 and SNAP-25 clusters was restored, along with the number of docked insulin granules. The infection of GK beta cells with Adex1CA SNAP-25 increased the number of docked insulin granules. TIRF imaging analysis demonstrated that the decreased number of fusion events from previously docked insulin granules in GK beta cells was restored when the number of docked insulin granules increased by insulin treatment or Adex1CA SNAP-25 infection.

CONCLUSIONS/INTERPRETATION

There was a close correlation between the number of syntaxin-1 and SNAP-25 clusters and the number of docked insulin granules, which is associated with the fusion of insulin granules.

Novel players in pancreatic islet signaling: from membrane receptors to nuclear channels

Glucose and other nutrients regulate many aspects of pancreatic islet physiology. This includes not only insulin release, but also insulin synthesis and storage and other aspects of beta-cell biology, including cell proliferation, apoptosis, differentiation, and gene expression. This implies that in addition to the well-described signals for insulin release, other intracellular signaling mechanisms are needed. Here we describe the role of global and local Ca(2+) signals in insulin release, the regulation of these signals by new membrane receptors, and the generation of nuclear Ca(2+) signals involved in gene expression. An integrated view of these pathways should improve the present description of the beta-cell biology and provide new targets for novel drugs.

Insulin granule dynamics in pancreatic beta cells

Glucose-induced insulin secretion in response to a step increase in blood glucose concentrations follows a biphasic time course consisting of a rapid and transient first phase followed by a slowly developing and sustained second phase. Because Type 2 diabetes involves defects of insulin secretion, manifested as a loss of first phase and a reduction of second phase, it is important to understand the cellular mechanisms underlying biphasic insulin secretion. Insulin release involves the packaging of insulin in small (diameter approximately 0.3 micro m) secretory granules, the trafficking of these granules to the plasma membrane, the exocytotic fusion of the granules with the plasma membrane and eventually the retrieval of the secreted membranes by endocytosis. Until recently, studies on insulin secretion have been confined to the appearance of insulin in the extracellular space and the cellular events preceding exocytosis have been inaccessible to more detailed analysis. Evidence from a variety of secretory tissues, including pancreatic islet cells suggests, however, that the secretory granules can be functionally divided into distinct pools that are distinguished by their release competence and/or proximity to the plasma membrane. The introduction of fluorescent proteins that can be targeted to the secretory granules, in combination with the advent of new techniques that allow real-time imaging of granule trafficking in living cells (granule dynamics), has led to an explosion of our knowledge of the pre-exocytotic and post-exocytotic processes in the beta cell. Here we discuss these observations in relation to previous functional and ultra-structural data as well as the secretory defects of Type 2 diabetes.

Transduction of MIN6 beta cells with TAT-syntaxin SNARE motif inhibits insulin exocytosis in biphasic insulin release in a distinct mechanism analyzed by evanescent wave microscopy

To investigate the in vivo interaction of syntaxin-mediated soluble N-ethylmaleimide-sensitive fusion protein attachment protein receptor (SNARE) assembly and insulin exocytosis in biphasic release, we examined the dynamics of insulin granule motion such as docking and fusion with the plasma membrane when the syntaxin SNARE motif (H3 domain) was transduced into living MIN6 beta cells. TAT-H3, produced by fusion of the protein transduction domain of human immunodeficiency virus-1 TAT to the syntaxin-H3 domain, was rapidly transduced into the subplasmalemmal region in living MIN6 cells. Immunoblotting analysis followed by immunoprecipitation on TAT-H3-treated MIN6 cells showed that TAT-H3 binds SNAP-25 and VAMP-2 in vivo. Transduction of MIN6 cells with TAT-H3 caused a decrease in both the first and second phase of insulin release. We therefore quantitatively analyzed approaching, docking, and fusing of green fluorescent protein-labeled single insulin granules in TAT-H3-transduced MIN6 cells by evanescent wave microscopy. Under high glucose stimulation, TAT-H3 treatment not only reduced the fusion events from previously docked granules for the first 120 s (first phase of release) but also strongly inhibited the docking and fusion from newly recruited insulin granules after this point (second phase of release). During the second phase of release we observed a marked reduction in the accumulation of newly docked insulin granules; subsequently, fusion events were significantly decreased. TAT-H3 treatment by itself, however, did not alter the number of previously docked granules without stimulation. We conclude that introduction of the H3 domain into MIN6 cells inhibits biphasic insulin release by two mechanisms. 1) In the first phase of insulin release, the H3 domain interferes with previously docked granules to be fused, and 2) in the second phase of insulin release reduced fusion events result from a marked decline of newly docked granules. Thus, syntaxin-mediated SNARE assembly modulates insulin exocytosis in biphasic insulin release in a distinct way.

The hexosamine biosynthesis pathway regulates insulin secretion via protein glycosylation in mouse islets

The hexosamine biosynthesis pathway plays a role in the modification of cellular proteins via the provision of substrate for addition of O-linked N-acetylglucosamine (GlcNAc). The relative importance of the GlcNAc modification of proteins to insulin secretion from pancreatic beta-cells has not been investigated and so remains unclear. In the present study, we show that inhibition of the hexosamine biosynthesis pathway decreases insulin secretion from mouse islets in response to a number of secretagogues, including glucose. This impairment in beta-cell function could not be attributed to reduced islet insulin content, altered ATP levels, or cell death and was restored with the addition of N-acetylglucosamine, a substrate that enters the pathway below the point of inhibition. Western blot analysis revealed that decreased islet protein glycosylation paralleled the decrease in insulin secretion following inhibition of the pathway. In conclusion, the data suggest a role for the hexosamine biosynthesis pathway in regulating the secretion of insulin by altering protein glycosylation. This finding may have implications for the development of type 2 diabetes, as chronic increase in flux through the hexosamine biosynthesis pathway may lead to the deterioration of beta-cell function via abnormal protein glycosylation.

Nuclear KATP channels trigger nuclear Ca(2+) transients that modulate nuclear function

Glucose, the principal regulator of endocrine pancreas, has several effects on pancreatic beta cells, including the regulation of insulin release, cell proliferation, apoptosis, differentiation, and gene expression. Although the sequence of events linking glycemia with insulin release is well described, the mechanism whereby glucose regulates nuclear function is still largely unknown. Here, we have shown that an ATP-sensitive K(+) channel (K(ATP)) with similar properties to that found on the plasma membrane is also present on the nuclear envelope of pancreatic beta cells. In isolated nuclei, blockade of the K(ATP) channel with tolbutamide or diadenosine polyphosphates triggers nuclear Ca(2+) transients and induces phosphorylation of the transcription factor cAMP response element binding protein. In whole cells, fluorescence in situ hybridization revealed that these Ca(2+) signals may trigger c-myc expression. These results demonstrate a functional K(ATP) channel in nuclei linking glucose metabolism, nuclear Ca(2+) signals, and nuclear function.

Desensitization of insulin secretion

Desensitization of insulin secretion describes a reversible state of decreased secretory responsiveness of the pancreatic beta-cell, induced by a prolonged exposure to a multitude of stimuli. These include the main physiological stimulator, glucose, but also other nutrients like free fatty acids and practically all pharmacological stimulators acting by depolarization and Ca2+ influx into the beta-cell. Desensitization of insulin secretion appears to be an important step in the manifestation of type 2 diabetes and in the secondary failure of oral antidiabetic treatment. In this commentary, the basic concepts and the controversial issues in the field will be outlined. With regard to glucose-induced desensitization, two fundamentally opposing concepts have emerged. The first is that desensitization is the consequence of functional changes in the beta-cell that impair glucose-recognition. The second is that long-term increased secretory activity leads to a depletion of releasable insulin, often in spite of increased insulin synthesis. The latter concept is more appropriately termed beta-cell exhaustion. The same dichotomy applies to the desensitization evoked by pharmacological stimuli: again the relative contributions of a decreased insulin content versus alterations in signal transduction are in dispute. The action of tolbutamide on beta-cells may be an example of desensitization caused by a lack of releasable insulin since the signaling mechanisms are nearly unchanged, whereas the action of phentolamine, an imidazoline, induces a strong desensitization without reducing insulin content or secretory granules, apparently by abolishing Ca2+ influx. With pharmacological agents it seems that both, alterations in signal transduction and decreased availability of releasable insulin, can contribute to the desensitized state of the beta-cell, the relative contribution being variable depending upon the exact nature of the secretory stimulus.

Down-regulated expression of exocytotic proteins in pancreatic islets of diabetic GK rats

Exocytosis is regulated by exocytotic proteins, which are present in insulin-secreting beta-cells and play regulatory roles in insulin secretion. Non-insulin dependent diabetes mellitus (type 2 diabetes) is a disease characterized by impaired insulin secretion and insulin resistance. Exocytotic protein immunoreactivities were studied in pancreatic islets of type 2 diabetic Goto-Kakizaki (GK) rats using immunofluorescence histochemistry. The immunoreactivities for vesicle-associated membrane protein-2 (VAMP-2), synaptotagmin III, cysteine string protein (CSP), mammalian homologue of the unc-18 gene (Munc-18), alpha-soluble N-ethylmaleimide-sensitive attachment protein (alpha-SNAP), N-ethylmaleimide-sensitive factor (NSF) and synaptosomal-associated protein of 25 kDa (SNAP-25) exhibited weaker immunofluorescence intensity in islets of GK rats as compared to control Wistar rats. Insulin immunoreactivity was also decreased in GK rat beta-cells, whereas no detectable alterations in the expression of actin immunoreactivity could be detected. The data suggest that reduced expression of exocytotic proteins and decreased insulin content may contribute to the diabetic syndrome in the GK rat.

Triggering and augmentation mechanisms, granule pools, and biphasic insulin secretion

The insulin secretory response by pancreatic beta-cells to an acute "square wave" stimulation by glucose is characterized by a first phase that occurs promptly after exposure to glucose, followed by a decrease to a nadir, and a prolonged second phase. The first phase of release is due to the ATP-sensitive K(+) (K(ATP)) channel-dependent (triggering) pathway that increases [Ca(2+)](i) and has been thought to discharge the granules from a "readily releasable pool." It follows that the second phase entails the preparation of granules for release, perhaps including translocation and priming for fusion competency before exocytosis. The pathways responsible for the second phase include the K(ATP) channel-dependent pathway because of the need for elevated [Ca(2+)](i) and additional signals from K(ATP) channel-independent pathways. The mechanisms underlying these additional signals are unknown. Current hypotheses include increased cytosolic long-chain acyl-CoA, the pyruvate-malate shuttle, glutamate export from mitochondria, and an increased ATP/ADP ratio. In mouse islets, the beta-cell contains some 13,000 granules, of which approximately 100 are in a "readily releasable" pool. Rates of granule release are slow, e.g., one every 3 s, even at the peak of the first phase of glucose-stimulated release. As both phases of glucose-stimulated insulin secretion can be enhanced by agents such as glucagon-like peptide 1, which increases cyclic AMP levels and protein kinase A activity, or acetylcholine, which increases diacylglycerol levels and protein kinase C activity, a single "readily releasable pool" hypothesis is an inadequate explanation for insulin secretion. Multiple pools available for rapid release or rapid conversion of granules to a rapidly releasable state are required.

Expression and localisation of synaptotagmin isoforms in endocrine (β)-cells: their function in insulin exocytosis

Exocytosis of insulin containing Large Dense Core Vesicles (LDCVs) from pancreatic (β)-cells and derived cell lines is mainly controlled by Ca(2+). Several lines of evidence have demonstrated a role of the Ca(2+)- and phospholipid-binding protein synaptotagmin (syt) in this event. Synaptotagmins form a large protein family with distinct affinities for Ca(2+) determined by their two C(2) domains (C(2)A/B). Except for the well-characterized isoforms I and II, their role is still unclear. We have used here insulin-secreting cells as a model system for LDCV exocytosis to gain insight into the function of synaptotagmins. Immunocytochemical analysis revealed that of the candidate Ca(2+) sensors in LDCV exocytosis, syt III was not expressed in primary (β)-cells, whereas syt IV was only found adjacent to the TGN. However, syt V-VIII isoforms were expressed at different levels in various insulin-secreting cells and in pancreatic islet preparations. In streptolysin-O permeabilized primary (β)-cells the introduction of recombinant peptides (100 nM) corresponding to the C(2) domains of syt V, VII and VIII, but not of syt III, IV or VI, inhibited Ca(2+)-evoked insulin exocytosis by 30% without altering GTP*S-induced release. Our observations demonstrate that syt III and IV are not involved in the exocytosis of LDCVs from primary (β)-cells whereas V, VII and VIII may mediate Ca(2+)-regulation of exocytosis.

Hexamminecobalt(III) chloride inhibits glucose-induced insulin secretion at the exocytotic process

Hexamminecobalt(III) (HAC) chloride was found to have a potent inhibitory effect on glucose-induced insulin secretion from pancreatic islets. HAC at 2 mm inhibited the secretion in response to 22.2 mm glucose by 90% in mouse islets. Perifusion experiments revealed that the first phase of insulin secretion was severely suppressed and that the second phase of secretion was completely abrogated. Removal of HAC from the perifusate immediately restored insulin secretion with a transient overshooting above the normal level. However, HAC failed to affect glucose-induced changes in d-[6-(14)C]glucose oxidation, levels of reduced forms of NAD and NADP, mitochondrial membrane potential, ATP content, cytosolic calcium concentration, or calcium influx into mitochondria. Furthermore, HAC inhibited 50 mm potassium-stimulated insulin secretion by 77% and 10 microm mastoparan-stimulated insulin secretion in the absence of extracellular Ca(2+) by 80%. The results of a co-immunoprecipitation study of lysates from insulin-secreting betaHC9 cells using anti-syntaxin and anti-vesicle-associated membrane protein antibodies for immunoprecipitation or Western blotting suggested that HAC inhibited disruption of the SNARE complex, which is normally observed upon glucose challenge. These results suggest that the inhibitory effect of HAC on glucose-induced insulin secretion is exerted at a site(s) distal to the elevation of cytosolic [Ca(2+)], possibly in the exocytotic machinery per se; and thus, HAC may serve as a useful tool for dissecting the molecular mechanism of insulin exocytotic processes.

Munc-18 associates with syntaxin and serves as a negative regulator of exocytosis in the pancreatic beta -cell

The Munc-18 protein (mammalian homologue of the unc-18 gene; also called nSec1 or rbSec1) has been identified as an essential component of the synaptic vesicle fusion protein complex. The cellular and subcellular localization and functional role of Munc-18 protein in pancreatic beta-cells was investigated. Subcellular fractionation of insulin-secreting HIT-T15 cells revealed a 67-kDa protein in both cytosol and membrane fractions. Immunohistochemistry showed punctate Munc-18 immunoreactivity in the cytoplasm of rat pancreatic islet cells. Direct double-labeling immunofluorescence histochemistry combined with confocal laser microscopy revealed the presence of Munc-18 immunoreactivity in insulin-, glucagon-, pancreatic polypeptide-, and somatostatin-containing cells. Syntaxin 1 immunoreactivity was detected in extracts of HIT-T15 cells, which were immunoprecipitated using Munc-18 antiserum, suggesting an intimate association of Munc-18 with syntaxin 1. Administration of Munc-18 peptide or Munc-18 antiserum to streptolysin O-permeabilized HIT-T15 cells resulted in significantly increased insulin release, but did not have any significant effect on voltage-gated Ca(2+) channel activity. The findings taken together show that the Munc-18 protein is present in insulin-secreting beta-cells and implicate Munc-18 as a negative regulator of the insulin secretory machinery via a mechanism that does not involve syntaxin-associated Ca(2+) channels.

Membrane localization and biological activity of SNAP-25 cysteine mutants in insulin-secreting cells

The tSNARE SNAP-25 is expressed in pancreatic (beta)-cells and is involved in the regulated release of insulin. It has been shown previously that SNAP-25 associates with the plasma membrane consequent to palmitoylation of one or more cysteines in the central region of the molecule. The importance of palmitolyation in the biological function of SNAP-25 in exocytosis was not addressed. Furthermore, studies on both SNAP-25 and its non-palmitoylated homologues SNAP-29 and sec9, have suggested an alternative or complementary mechanism for membrane association involving interaction with syntaxin. To address these issues, we have now studied the behavior and biological activity of cysteine mutant SNAP-25 in insulin-secreting (HIT) cells. While 91% of native SNAP-25 was associated with the membrane, this value decreased to 56% for the single cysteine mutant C85/A and to 10% for the double (C85,88/A) and quadruple (C85,88,90,92/A) mutants. The mutant SNAP-25 forms were all found to bind syntaxin 1A with equal efficacy. Over-expression of syntaxin 1A in HIT cells allowed for partial relocalization of both the double and quadruple SNAP-25 cys mutants to the membrane. By introducing a further mutation to the SNAP-25 molecules to render them resistant to botulinum neurotoxin E, it was possible to study their ability to reconstitute regulated insulin secretion in toxin-treated HIT cells. Native SNAP-25 was able to fully reconstitute secretory activity in such cells. Despite the fact that the single cysteine mutant was significantly displaced to the cytosol, it still displayed 82% activity in the secretion reconstitution assay, and a similar discrepancy was seen for the double mutant. Even the quadruple mutant with no remaining cysteines was able to support a minimal level of secretion. It is concluded that both palmitoylation and binding to syntaxin are implicated in membrane association of SNAP-25. This as well as the discrepancy between membrane localization and biological activity of the cysteine mutants, suggests a complex, multi-component process for association of SNAP-25 with the membrane and its recruitment to a biologically productive state.

Beta-granule transport and exocytosis

Regulated beta -granule exocytosis is critical for the ability of the beta -cell to finely control body glucose homeostasis. This is now understood to be a multistage process whereby beta -granules are transported from biosynthetic/storage sites in the cell cytoplasm and targeted to specific regions of the plasma membrane. Exocytosis is achieved when these granules are triggered to fuse with the membrane by an elevated cytosolic Ca(2+). Dramatic advances have been made recently in our understanding of the protein-protein interactions and regulatory signals that govern intracellular transport and fusion. Although best understood for exocytosis from neurons and neuroendocrine cells, similar processes are thought to be conserved in the beta -cell.

Syntaxin 1 interacts with the L(D) subtype of voltage-gated Ca(2+) channels in pancreatic beta cells

Interaction of syntaxin 1 with the alpha(1D) subunit of the voltage-gated L type Ca(2+) channel was investigated in the pancreatic beta cell. Coexpression of the enhanced green fluorescent protein-linked alpha(1D) subunit with the enhanced blue fluorescent protein-linked syntaxin 1 and Western blot analysis together with subcellular fractionation demonstrated that the alpha(1D) subunit and syntaxin 1 were colocalized in the plasma membrane. Furthermore, the alpha(1D) subunit was coimmunoprecipitated efficiently by a polyclonal antibody against syntaxin 1. Syntaxin 1 also played a central role in the modulation of L type Ca(2+) channel activity because there was a faster Ca(2+) current run-down in cells incubated with antisyntaxin 1 compared with controls. In parallel, antisyntaxin 1 markedly reduced insulin release in both intact and permeabilized cells, subsequent to depolarization with K(+) or exposure to high Ca(2+). Exchanging Ca(2+) for Ba(2+) abolished the effect of antisyntaxin 1 on both Ca(2+) channel activity and insulin exocytosis. Moreover, antisyntaxin 1 had no significant effects on Ca(2+)-independent insulin release trigged by hypertonic stimulation. This suggests that there is a structure-function relationship between the alpha(1D) subunit of the L type Ca(2+) channel and the exocytotic machinery in the pancreatic beta cell.

Alpha-SNAP functions in insulin exocytosis from mature, but not immature secretory granules in pancreatic beta cells

To explore alpha-SNAP function in insulin exocytosis from either immature or mature secretory granules in pancreatic beta cells, we studied the effects of overexpression of adenovirus-mediated wild-type alpha-SNAP and C-terminally deleted alpha-SNAP mutant (1-285) on newly synthesized proinsulin and insulin release by rat islets and MIN6 cells. Rat islets overexpressing alpha-SNAP and mutant alpha-SNAP were pulse-chased. Exocytosis from immature and mature insulin secretory granules was measured as fractional (%) labeled-proinsulin release immediately after the pulse-labeling and percentage labeled-insulin release after a 3-h chase period, respectively. There was no difference in percentage labeled-proinsulin release between the control and alpha-SNAP or mutant alpha-SNAP-overexpressed islets. Although percentage labeled-insulin release after a 3-h chase period was significantly increased in alpha-SNAP-overexpressed islets, it was decreased in mutant alpha-SNAP-overexpressed islets. Thus, the results demonstrated that alpha-SNAP overexpression in rat islets primarily increased exocytosis from mature, but not immature insulin secretory granules. On the other hand, in MIN6 cells, alpha-SNAP overexpression scarcely affected glucose-stimulated insulin release; therefore, we examined the effect of mutant alpha-SNAP overexpression as the dominant-negative inhibitor on the newly synthesized proinsulin/insulin release using the same protocol as in the rat islet experiments. alpha-SNAP mutant (1-285) overexpression in MIN6 cells decreased the percentage labeled insulin release from mature secretory granules, but not percentage labeled proinsulin release from immature secretory granules. Thus, our data demonstrate that alpha-SNAP functions mainly in the mature insulin secretory granules in pancreatic beta cells.

alpha-soluble N-ethylmaleimide-sensitive factor attachment protein is expressed in pancreatic beta cells and functions in insulin but not gamma-aminobutyric acid secretion

The function of soluble N-ethylmaleimide-sensitive attachment protein-alpha (alpha-SNAP) in exocytosis still remains obscure. This study was conducted to determine the physiological role of alpha-SNAP in the secretion of insulin and gamma-aminobutryric acid (GABA) from pancreatic beta cells. Reverse transcriptase-polymerase chain reaction analysis of total RNA isolated from rat islets disclosed alpha-SNAP, but not beta-SNAP, mRNA expression, and an immunofluorescence study of rat pancreas showed that alpha-SNAP was present predominantly in the cytoplasm of the islets of Langerhans. alpha-SNAP overexpression in rat islets enhanced insulin release relative to the control levels. An in vitro binding study showed that both wild-type alpha-SNAP and C-terminal-deleted alpha-SNAP mutant (1-285) can bind to syntaxin 1A. alpha-SNAP mutant (1-285) was overexpressed to evaluate its activity as dominant-negative effector on insulin release. Overexpression of alpha-SNAP mutant (1-285) in rat islets and MIN6 cells decreased glucose-stimulated insulin release to about 50% of the control levels. Suppression of endogeneous alpha-SNAP in MIN6 cells by treatment with an antisense phosphorothioate oligonucleotide resulted in inhibition of insulin release. In order to examine if alpha-SNAP functions in exocytosis from synaptic-like microvesicles in pancreatic beta cells, the functional role of alpha-SNAP in GABA release from MIN6 cells was studied. The data showed no effect of alpha-SNAP mutant (1-285) overexpression on GABA release. We conclude that 1) alpha-SNAP plays a crucial role in insulin exocytosis via large dense core vesicles, but not GABA released via synaptic-like microvesicles, in pancreatic beta cells; and 2) the interaction of alpha-SNAP and syntaxin 1A may play an important role in the insulin exocytotic process.

Subcellular distribution and function of Rab3A, B, C, and D isoforms in insulin-secreting cells

Insulin-secreting cells express four GTPases of the Rab3 family. After separation of extracts of INS-1 cells on a sucrose density gradient, the bulk of the A, B, and C isoforms was recovered in the fractions enriched in insulin-containing secretory granules. Rab3D was also mainly associated with secretory granules, but a fraction of this isoform was localized on lighter organelles. Analyses by confocal microscopy of immunostained HIT-T15 cells transfected with epitope-tagged constructs confirmed the distribution of the Rab3 isoforms. Transfection of HIT-T15 cells with GTPase-deficient mutants of the Rab3 isoforms decreased nutrient-induced insulin release to different degrees (D>B>A>>C), while overexpression of Rab3 wild types had minor or no effects. Expression of the same Rab3 mutants in PC12 cells provoked an inhibition of K+-stimulated secretion of dense core vesicles, indicating that, in beta-cells and neuroendocrine cells, the four Rab3 isoforms play a similar role in exocytosis. A Rab3A/C chimera in which the carboxyterminal domain of A was replaced with the corresponding region of C inhibited insulin secretion as Rab3A. In contrast, a Rab3C/A chimera containing the amino-terminal domain of C was less potent and reduced exocytosis as Rab3C. This suggests that the degree of inhibition obtained after transfection of the Rab3 isoforms is determined by differences in the variable amino-terminal region.

Syntaxin 1A and 1B display distinct distribution patterns in the rat peripheral nervous system

Syntaxin 1 has been shown to play an outstanding role in synaptic vesicle exocytosis. Two isoforms of this protein are expressed in neurons, syntaxin 1A and 1B. However, the physiological significance of the occurrence of such closely related isoforms is not still understood. Here, by means of isoform-specific immunocytochemistry, we show that syntaxin 1A and 1B display different patterns of expression in the rat peripheral nervous system. Nerve terminals of sensory neurons reaching the spinal cord were clearly enriched in immunoreactive syntaxin 1A. Both isoforms were detected in cell bodies of sensory neurons at the dorsal root ganglia, although specific immunolabelling displayed very different patterns at the cellular level. Motor endplates and muscle spindles were only immunostained for syntaxin 1B. Syntaxin 1A was mainly associated with nerve fibres reaching small blood vessels. In addition, nerve plexuses of the enteric nervous system showed immunostaining for both syntaxin isoforms. The different distribution pattern of the two neuronal syntaxin isoforms in the rat peripheral nervous system could be related to isoform-specific biochemical properties involved in the exocytotic process.

Molecular mechanisms and regulation of insulin exocytosis as a paradigm of endocrine secretion

Secretion of the peptide hormone insulin from pancreatic beta cells constitutes an important step in the regulation of body homeostasis. Insulin is stored in large dense core vesicles and released by exocytosis, a multistage process involving transport of vesicles to the plasma membrane, their docking, priming and finally their fusion with the plasma membrane. Some of the protein components necessary for this process have been identified in beta cells. The export of potent and potentially harmful substances has to be tightly controlled. The secretory response in pancreatic beta cells requires the concerted action of nutrients together with enteric hormones and neurotransmitters acting on G-protein coupled receptors. It is well established that glucose and other metabolizable nutrients depolarize the beta-cell membrane and the ensuing Ca2+ influx through voltage-dependent channels constitutes a main stimulus for insulin exocytosis. Theoretical considerations and recent observations suggest in addition an organizing role for the Ca2+ channel similar to neurotransmission. A second regulatory control on exocytosis is exerted by monomeric and heterotrimeric G-proteins. The monomeric GTPase Rab3A controls insulin secretion through cycling between a guanosine triphosphate liganded vesicle-bound form and a guanosine diphosphate liganded, cytosolic form. The effect of neurohormones is transduced by the heterotrimeric GTPases. Whereas pertussis-toxin sensitive alpha-subunits exert direct inhibition at the level of exocytosis, the Gbeta gamma-subunits are required for stimulation. It is possible that these GTPases exert immediate regulation, while protein kinases and phosphatases may modulate long-term adaptation at the exocytotic machinery itself. The molecular nature of their activators and effectors still await identification. Insights into the progression of the exocytotic vesicle from docking to fusion and how these processes are precisely regulated by proteins and second messengers may provide the basis for new therapeutic principles.

Cysteine-string proteins regulate exocytosis of insulin independent from transmembrane ion fluxes

Cysteine-string proteins (Csps) are vesicle proteins involved in exocytosis of synaptic vesicles in Drosophila and modulation of presynaptic calcium influx. As both the contribution of calcium channel regulation to the role of Csp in exocytosis and a function of Csp outside the nervous system are unknown, we studied its function in endocrine exocytosis from large dense core vesicles (LDCVs) using insulin-secreting pancreatic beta-cells. Csps were expressed in primary and derived beta-cell lines on insulin-containing LDCVs. Suppression of Csp expression reduced not only depolarisation induced insulin release but also exocytosis in permeabilised cells directly stimulated by Ca2+. Thus, Csp is a secretory granule protein and is required for endocrine exocytosis independent of the modulation of transmembrane calcium fluxes.

Cysteine string protein (CSP) is an insulin secretory granule-associated protein regulating beta-cell exocytosis

Cysteine string proteins (CSPs) are novel synaptic vesicle-associated protein components characterized by an N-terminal J-domain and a central palmitoylated string of cysteine residues. The cellular localization and functional role of CSP was studied in pancreatic endocrine cells. In situ hybridization and RT-PCR analysis demonstrated CSP mRNA expression in insulin-producing cells. CSP1 mRNA was present in pancreatic islets; both CSP1 and CSP2 mRNAs were seen in insulin-secreting cell lines. Punctate CSP-like immunoreactivity (CSP-LI) was demonstrated in most islets of Langerhans cells, acinar cells and nerve fibers of the rat pancreas. Ultrastructural analysis showed CSP-LI in close association with membranes of secretory granules of cells in the endo- and exocrine pancreas. Subcellular fractionation of insulinoma cells showed CSP1 (34/36 kDa) in granular fractions; the membrane and cytosol fractions contained predominantly CSP2 (27 kDa). The fractions also contained proteins of 72 and 70 kDa, presumably CSP dimers. CSP1 overexpression in INS-1 cells or intracellular administration of CSP antibodies into mouse ob/ob beta-cells did not affect voltage-dependent Ca2+-channel activity. Amperometric measurements showed a significant decrease in insulin exocytosis in individual INS-1 cells after CSP1 overexpression. We conclude that CSP is associated with insulin secretory granules and that CSP participates in the molecular regulation of insulin exocytosis by mechanisms not involving changes in the activity of voltage-gated Ca2+-channels.

Engineered peptides corresponding to segments of the H3 domain of syntaxin inhibit insulin release both in intact and permeabilized mouse pancreatic beta cells

Syntaxin is one of the proteins involved in the exocytotic event through sequential binding to specific proteins, including SNAP25 and synaptobrevin. In a previous work in digitonin-permeabilized beta cells, we characterized the functional role of two segments: synA and synB of the H3 domain of syntaxin. As a continuation of these experiments in the present study we have initially outlined a zone of 17 residues as the very effective uncoupling element of the synA segment. Further functional studies have been accomplished in intact pancreatic beta cells with a specific myristoylated (myr) 13-mer peptide comprised in this active zone. These experiments showed a concentration-dependent inhibition of glucose-induced insulin release (IC50 = 4 microM) of this engineered peptide that was specific since a myristoylated random peptide with the same composition was ineffective. A second myristoylated 13-mer peptide comprised into the synB segment was shown to be even more potent promoting a selective inhibition of insulin release. These data show for the first time, that nutrient-induced secretory process can be specifically uncoupled in intact beta cells demonstrating at the same time that syntaxin plays a central role in this mechanism.

Ca2+-independent insulin exocytosis induced by alpha-latrotoxin requires latrophilin, a G protein-coupled receptor

alpha-Latrotoxin (alpha-LTX) induces exocytosis of small synaptic vesicles (SSVs) in neuronal cells both by a calcium-independent mechanism and by opening cation-permeable pores. Since the basic molecular events regulating exocytosis in neurons and endocrine cells may be similar, we have used the exocytosis of insulin-containing large dense core vesicles (LDCVs) as a model system. In primary pancreatic beta-cells and in the derived cell lines INS-1 and MIN6, alpha-LTX increased insulin release in the absence of extracellular calcium, but the insulin-secreting cell lines HIT-T15 and RINm5F were unresponsive. alpha-LTX did not alter membrane potential or cytosolic calcium, and its stimulatory effect on exocytosis was still observed in pre-permeabilized INS-1 cells kept at 0.1 microM Ca2+. Consequently, pore formation or ion fluxes induced by alpha-LTX could be excluded. The Ca2+-independent alpha-LTX-binding protein, latrophilin, is a novel member of the secretin family of G protein-coupled receptors (GPCR). Sensitivity to alpha-LTX correlated with expression of latrophilin, but not with synaptotagmin I or neurexin Ialpha expression. Moreover, transient expression of latrophilin in HIT-T15 cells conferred alpha-LTX-induced exocytosis. Our results indicate that direct stimulation of exocytosis by a GPCR mediates the Ca2+-independent effects of alpha-LTX in the absence of altered ion fluxes. Therefore, direct regulation by receptor-activated heterotrimeric G proteins constitutes an important feature of the endocrine exocytosis of insulin-containing LDCVs and may also apply to SSV exocytosis in neurons.

SNAP-23 is not cleaved by botulinum neurotoxin E and can replace SNAP-25 in the process of insulin secretion

The synaptosomal-associated protein of 25 kDa (SNAP-25) is expressed in neurons and endocrine cells. It has been shown to play an important role in the release mechanism of neurotransmitters and peptide hormones, including insulin. Thus, when insulin-secreting cells are permeabilized and treated with botulinum neurotoxin E (BoNT/E), SNAP-25 is hydrolyzed, and insulin secretion is inhibited. Recently SNAP-23, a more generally expressed isoform of SNAP-25, has been described. The functional role of SNAP-23 has not been investigated to date. It is now shown that SNAP-23 is resistant to cleavage by BoNT/E. It was therefore possible to test whether transfection of HIT (transformed pancreatic B-) cells with SNAP-23 reconstitutes insulin release from BoNT/E treated cells, in which SNAP-25 is inactivated by the toxin. The results show that SNAP-23 is able to replace SNAP-25 when it is overexpressed. While these results demonstrate that SNAP-23 is a functional homologue of SNAP-25, able to function in regulated exocytosis, they indicate that SNAP-23 may be inefficient in this process. This suggests that both isoforms may have their own specific binding partners and discrete, albeit mechanistically similar, functional roles within the cell.

Transient expression of botulinum neurotoxin C1 light chain differentially inhibits calcium and glucose induced insulin secretion in clonal beta-cells

We have investigated the effect of botulinum neurotoxin (BoNT) C1 light chain (LC) on insulin exocytosis from the clonal beta-cell line HIT-T15. In streptolysin-O permeabilized cells, the beta-cell impermeant BoNT C1 cleaved mainly syntaxin 1 and inhibited Ca2+ as well as GTPgammaS induced exocytosis. To study the effect of BoNTs in intact cells, we transiently coexpressed the BoNT LC together with a reporter gene for insulin release. BoNT C1 inhibited K+ induced insulin secretion by 95% but reduced insulin release stimulated by glucose only by 25%. Thus a component of glucose stimulated insulin release is insensitive to BoNT C1.

Non-functional role of syntaxin 2 in insulin exocytosis by pancreatic beta cells

This study was designed in order to examine the expression and functional role of syntaxin 2/epimorphin in pancreatic beta cells. Northern blot analysis revealed that syntaxin 2 mRNA was able to be detected in mouse beta TC3 cells, but not in isolated mouse islets. In agreement with this result, immunoblot analysis detected an appreciable amount of syntaxin 2 protein in beta TC3 cells, but not in mouse islets. Immunohistochemistry of the mouse pancreas demonstrated that syntaxin 2 was little evident in islet cells of Langerhans, and somewhat predominant in exocrine tissues. In order to examine whether syntaxin 2 is anchored to cell surfaces in beta TC3 cells, living cells were incubated with a monoclonal antibody against syntaxin 2 (MC-1). The antibody bound to their surfaces, indicating that syntaxin 2 was localized on cell surfaces. The addition of MC-1 to the culture medium of beta TC3 cells did not affect insulin release under the presence or absence of 11 mM glucose, indicating that syntaxin 2 is not associated with insulin exocytosis. Thus, the expression of syntaxin 2 in islets of Langerhans is very low and the function of this protein is probably unrelated to the insulin exocytosis pathway.

The first C2 domain of synaptotagmin is required for exocytosis of insulin from pancreatic beta-cells: action of synaptotagmin at low micromolar calcium

The Ca2+- and phospholipid-binding protein synaptotagmin is involved in neuroexocytosis. Its precise role and Ca2+-affinity in vivo are unclear. We investigated its putative function in insulin secretion which is maximally stimulated by 10 microM cytosolic free Ca2+. The well-characterized synaptotagmin isoforms I and II are present in pancreatic beta-cell lines RINm5F, INS-1 and HIT-T15 as shown by Northern and Western blots. Subcellular fractionation and confocal microscopy revealed their presence mainly on insulin-containing secretory granules whereas only minor amounts were found on synaptic vesicle-like microvesicles. Antibodies or Fab-fragments directed against the Ca2+-dependent phospholipid binding site of the first C2 domain of synaptotagmin I or II inhibited Ca2+-stimulated, but not GTPgammaS-induced exocytosis from streptolysin-O-permeabilized INS-1 and HIT-T15 cells. Transient expression of wild-type synaptotagmin II did not alter exocytosis in HIT-T15 cells. However, mutations in the Ca2+-dependent phospholipid binding site of the first C2 domain (Delta180-183, D231S) again inhibited only Ca2+-, but not GTPgammaS-evoked exocytosis. In contrast, mutations in the IP4-binding sites of the second C2 domain (Delta325-341; K327,328, 332Q) did not alter exocytosis. Synaptotagmin II mutated in both C2 domains (Delta180-183/K327,328,332Q) induced greater inhibition than mutant Delta180-183, suggesting a discrete requirement for the second C2 domain. Thus, synaptotagmin isoforms regulate exocytotic events occurring at low micromolar Ca2+.

Overexpressed syntaxin 1A/HPC-1 inhibits insulin secretion via a regulated pathway, but does not influence glucose metabolism and intracellular Ca2+ in insulinoma cell line beta TC3 cells

We have previously established a stable beta TC3 cell line that overexpresses syntaxin 1A, designated beta TC-hpc1 cells, in which glucose-stimulated insulin release was decreased. Using beta TC-hpc1 cells, we aimed to determine whether syntaxin 1A functions in the regulatory or constitutive pathway of insulin release. We therefore examined the secretion of phorbol-12-myristate-13-acetate (TPA)-stimulated newly synthesized proinsulin/insulin and total immunoreactive insulin. beta TC3 and beta TC-hpc1 cells were simultaneously pulse-labeled with 3H-leucine for 30 min in 11 mM glucose and chased for 1 h in one of a number of different concentrations of TPA in 11 mM glucose. Total immunoreactive insulin release (IRI) by both cell types during the chase period was markedly increased by the addition of TPA in a dose-dependent manner; however, the IRI from beta TC-hpc1 cells was lower than that from beta TC3 cells. The secretion of newly synthesized proinsulin/insulin from both cell types, which in beta TC3 cells is thought to occur via a constitutive pathway, was in the same range under any condition. Thus, the evidence indicates that syntaxin 1A preferentially functions in the regulated insulin release pathway in beta TC3 cells. In order to clarify the effect of overexpressed syntaxin 1A on glucose metabolism and intracellular Ca2+ we analyzed the glucose transport system, glucose phosphorylation activity, and cytosolic concentration of free Ca2+ ([Ca2+]i). 2-Deoxy-glucose uptake and the content of GLUT1 protein in the plasma membrane fractions of beta TC-hpc1 cells were not different from those of beta TC3 cells. Radiometric assays of glucose phosphorylation activity showed that there were no differences in hexokinase activity and glucokinase activity between beta TC3 and beta TC-hpc1 cells. [Ca2+]i measured by using fura 2 demonstrated that there was no difference in [Ca2+]i between beta TC3 and beta TC-hpc 1 cells under glucose-stimulated conditions. The present experiments indicate that syntaxin 1A plays a central role in a late step of the regulatory insulin release pathway without a change in glucose metabolism and [Ca2+]i in beta TC3 cells.

Differential distribution of syntaxin isoforms 1A and 1B in the rat central nervous system

Syntaxin 1 binds to several proteins of the synaptic terminal and is a central component in the pathway of protein-protein interactions that underlies docking and fusion of synaptic vesicles. Molecular studies revealed the occurrence of two isoforms, syntaxin 1A and syntaxin 1B, which coexpress in neural tissues. However, they display differential expression patterns in endocrine cell types. We generated isoform-specific antibodies that were used in Western blotting and immunocytochemical studies. First, we confirmed the sole presence of syntaxin 1A in endocrine pituitary cells. Second, we found distinctive immunolabelling patterns of each isoform in the rat olfactory system, hippocampus, striatum, thalamus and spinal cord. In addition, the principal white matter commissures displayed distinct immunoreactivity for each isoform. This report shows, for the first time, major differences between the distributions of syntaxin 1A and syntaxin 1B isoforms in the rat central nervous system.

Inhibition of insulin release by synthetic peptides shows that the H3 region at the C-terminal domain of syntaxin-1 is crucial for Ca(2+)- but not for guanosine 5'-[gamma-thio]triphosphate-induced secretion

Recently, we have described the presence and possible role of syntaxin in pancreatic beta-cells by using monoclonal antibodies [F. Martin, F. Moya, L. M. Gutierrez, J.A. Reig, B. Soria (1995) Diabetologia 38, 860-863]. In order to characterize further the importance of specific domains of this protein, the functional role of a particular region of the syntaxin-1 molecule has now been investigated by using two synthetic peptides, SynA and SynB, corresponding to two portions of the H3 region at the C-terminal domain of the protein, residues 229-251 and 197-219 respectively. Functional experiments carried out in permeabilized pancreatic beta-cells demonstrate that these peptides inhibit Ca(2+)-dependent insulin release in a dose-dependent manner. This effect is specific because peptides of the same composition but random sequence do not show the same effect. In contrast with this inhibitory effect on Ca(2+)-induced secretion, both peptides increase basal release. However, under the same conditions, SynA and SynB do not affect guanosine 5'-[gamma-thio]triphosphate-induced insulin release. These results demonstrate that specific portions of the H3 region of syntaxin-1 are involved in critical protein-protein interactions specifically during Ca(2+)-induced insulin secretion.