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

Hypoxia causes pancreatic β-cell dysfunction and impairs insulin secretion by activating the transcriptional repressor BHLHE40

Hypoxia can occur in pancreatic β-cells in type 2 diabetes. Although hypoxia exerts deleterious effects on β-cell function, the associated mechanisms are largely unknown. Here, we show that the transcriptional repressor basic helix-loop-helix family member e40 (BHLHE40) is highly induced in hypoxic mouse and human β-cells and suppresses insulin secretion. Conversely, BHLHE40 deficiency in hypoxic MIN6 cells or β-cells of ob/ob mice reverses defects in insulin secretion. Mechanistically, BHLHE40 represses the expression of Mafa, encoding the transcription factor musculoaponeurotic fibrosarcoma oncogene family A (MAFA), by attenuating the binding of pancreas/duodenum homeobox protein 1 (PDX1) to its enhancer region. Impaired insulin secretion in hypoxic β-cells was recovered by MAFA re-expression. Collectively, our work identifies BHLHE40 as a key hypoxia-induced transcriptional repressor in β-cells that inhibit insulin secretion by suppressing MAFA expression.

The Role of α-Cells in Islet Function and Glucose Homeostasis in Health and Type 2 Diabetes

Pancreatic α-cells are the major source of glucagon, a hormone that counteracts the hypoglycemic action of insulin and strongly contributes to the correction of acute hypoglycemia. The mechanisms by which glucose controls glucagon secretion are hotly debated, and it is still unclear to what extent this control results from a direct action of glucose on α-cells or is indirectly mediated by β- and/or δ-cells. Besides its hyperglycemic action, glucagon has many other effects, in particular on lipid and amino acid metabolism. Counterintuitively, glucagon seems also required for an optimal insulin secretion in response to glucose by acting on its cognate receptor and, even more importantly, on GLP-1 receptors. Patients with diabetes mellitus display two main alterations of glucagon secretion: a relative hyperglucagonemia that aggravates hyperglycemia, and an impaired glucagon response to hypoglycemia. Under metabolic stress states, such as diabetes, pancreatic α-cells also secrete GLP-1, a glucose-lowering hormone, whereas the gut can produce glucagon. The contribution of extrapancreatic glucagon to the abnormal glucose homeostasis is unclear. Here, I review the possible mechanisms of control of glucagon secretion and the role of α-cells on islet function in healthy state. I discuss the possible causes of the abnormal glucagonemia in diabetes, with particular emphasis on type 2 diabetes, and I briefly comment the current antidiabetic therapies affecting α-cells.

α-SNAP is expressed in mouse ovarian granulosa cells and plays a key role in folliculogenesis and female fertility

The balance between ovarian folliculogenesis and follicular atresia is critical for female fertility and is strictly regulated by a complex network of neuroendocrine and intra-ovarian signals. Despite the numerous functions executed by granulosa cells (GCs) in ovarian physiology, the role of multifunctional proteins able to simultaneously coordinate/modulate several cellular pathways is unclear. Soluble N-ethylmaleimide-sensitive factor (NSF) attachment protein (α-SNAP) is a multifunctional protein that participates in SNARE-mediated membrane fusion events. In addition, it regulates cell-to-cell adhesion, AMPK signaling, autophagy and apoptosis in different cell types. In this study we examined the expression pattern of α-SNAP in ovarian tissue and the consequences of α-SNAP (M105I) mutation (hyh mutation) in folliculogenesis and female fertility. Our results showed that α-SNAP protein is highly expressed in GCs and its expression is modulated by gonadotropin stimuli. On the other hand, α-SNAP-mutant mice show a reduction in α-SNAP protein levels. Moreover, increased apoptosis of GCs and follicular atresia, reduced ovulation rate, and a dramatic decline in fertility is observed in α-SNAP-mutant females. In conclusion, α-SNAP plays a critical role in the balance between follicular development and atresia. Consequently, a reduction in its expression/function (M105I mutation) causes early depletion of ovarian follicles and female subfertility.

Glucose intolerance and pancreatic β-cell dysfunction in the anorectic anx/anx mouse

Inflammation and impaired mitochondrial oxidative phosphorylation are considered key players in the development of several metabolic disorders, including diabetes. We have previously shown inflammation and mitochondrial dysfunction in the hypothalamus of an animal model for anorexia, the anx/anx mouse. Moreover, increased incidence of eating disorders, e.g., anorexia nervosa, has been observed in diabetic individuals. In the present investigation we evaluated whether impaired mitochondrial phosphorylation and inflammation also occur in endocrine pancreas of anorectic mice, and if glucose homeostasis is disturbed. We show that anx/anx mice exhibit marked glucose intolerance associated with reduced insulin release following an intraperitoneal injection of glucose. In contrast, insulin release from isolated anx/anx islets is increased after stimulation with glucose or KCl. In isolated anx/anx islets there is a strong downregulation of the mitochondrial complex I (CI) assembly factor, NADH dehydrogenase (ubiquinone) 1α subcomplex, assembly factor 1 (Ndufaf1), and a reduced CI activity. In addition, we show elevated concentrations of free fatty acids (FFAs) in anx/anx serum and increased macrophage infiltration (indicative of inflammation) in anx/anx islets. However, isolated islets from anx/anx mice cultured in the absence of FFAs do not exhibit increased inflammation. We conclude that the phenotype of the endocrine pancreas of the anx/anx mouse is characterized by increased levels of circulating FFAs, as well as inflammation, which can inhibit insulin secretion in vivo. The anx/anx mouse may represent a useful tool for studying molecular mechanisms underlying the association between diabetes and eating disorders.

Paracrine interactions within islets of Langerhans

Glucose supply fluctuates between meal and fasting periods and its consumption by the body varies greatly depending on bodily metabolism. Pancreatic islets of Langerhans secrete various endocrine hormones including insulin and glucagon to keep blood glucose level relatively constant. Additionally, islet hormones regulate activity of neighboring cells as local autocrine or paracrine modulators. Moreover, islet cells release neurotransmitters such as glutamate and γ-aminobutyric acid (GABA) to gain more precise regulation of hormones release kinetics. Excitatory glutamate is co-released with glucagon from α-cells and activates glutamate receptors in the neighboring cells. GABA released from β-cells was shown to inhibit α-cells but to activate β-cells by acting GABA(A) receptors. This review summarizes the recent progress in understanding the paracrine/autocrine interactions in islets.

DPP-4 inhibitor des-F-sitagliptin treatment increased insulin exocytosis from db/db mice β cells

Incretin promotes insulin secretion acutely. Recently, orally-administered DPP-4 inhibitors represent a new class of anti-hyperglycemic agents. Indeed, inhibitors of dipeptidyl peptidase-IV (DPP-4), sitagliptin, has just begun to be widely used as therapeutics for type 2 diabetes. However, the effects of sitagliptin-treatment on insulin exocytosis from single β-cells are yet unknown. We therefore investigated how sitagliptin-treatment in db/db mice affects insulin exocytosis by treating db/db mice with des-F-sitagliptin for 2 weeks. Perfusion studies showed that 2 weeks-sitagliptin treatment potentiated insulin secretion. We then analyzed insulin granule motion and SNARE protein, syntaxin 1, by TIRF imaging system. TIRF imaging of insulin exocytosis showed the increased number of docked insulin granules and increased fusion events from them during first-phase release. In accord with insulin exocytosis data, des-F-sitagliptin-treatment increased the number of syntaxin 1 clusters on the plasma membrane. Thus, our data demonstrated that 2-weeks des-F-sitagliptin-treatment increased the fusion events of insulin granules, probably via increased number of docked insulin granules and that of syntaxin 1 clusters.

A simple PCR-based genotyping method for M105I mutation of alpha-SNAP enhances the study of early pathological changes in hyh phenotype

alpha-SNAP is an essential component of the protein machinery responsible for membrane fusion events in different cell types. The hyh (hydrocephalus with hop gait) mouse carries a missense mutation in Napa gene that results in a point mutation (M105I) in alpha-SNAP protein. Homozygous animals for the mutant allele have been identified by the clinical and/or neuropathological phenotype, or by direct sequencing of PCR products. The aims of the present study were (i) to develop a high-throughput technique to genotype hyh mice, (ii) to correlate genotype-phenotype, and (iii) to analyze the earliest pathological changes of hyh mutant mice. As no restriction sites are affected by the hyh mutation, we resolved this problem by creating a BspHI restriction site with a modified (mismatch) polymerase chain reaction (PCR) primer in wild-type allele. This artificially created restriction site (ACRS)-PCR technique is a simple, rapid and reliable method to genotype hyh mice in a day-work procedure. Biochemical and histological analysis of genotyped hyh embryos at different developmental stages allowed us to identify and characterize the earliest brain pathological changes of the hyh phenotype, including the first signs of neuroepithelial disruption and neuronal ectopia. In addition, genotype-phenotype analysis of 327 animals confirmed that (i) hyh is a single-gene autosomal recessive disorder, and (ii) the disorder has 100% penetrance (i.e., the mutation was only present in affected mice). The genotyping method described here enhances the potentiality of hyh mouse as a unique in vivo model to study the role of membrane trafficking in different developmental and physiological processes.

Insulin exocytosis in Goto-Kakizaki rat beta-cells subjected to long-term glinide or sulfonylurea treatment

Sulfonylurea and glinide drugs display different effects on insulin granule motion in single beta-cells in vitro. We therefore investigated the different effects that these drugs manifest towards insulin release in an in vivo long-term treatment model. Diabetic GK (Goto-Kakizaki) rats were treated with nateglinide, glibenclamide or insulin for 6 weeks. Insulin granule motion in single beta-cells and the expression of SNARE (soluble N-ethylmaleimide-sensitive factor-attachment protein receptor) proteins were then analysed. Perifusion studies showed that decreased first-phase insulin release was partially recovered when GK rats were treated with nateglinide or insulin for 6 weeks, whereas no first-phase release occurred with glibenclamide treatment. In accord with the perifusion results, TIRF (total internal reflection fluorescence) imaging of insulin exocytosis showed restoration of the decreased number of docked insulin granules and the fusion events from them during first-phase release for nateglinide or insulin, but not glibenclamide, treatment; electron microscopy results confirmed the TIRF microscopy data. Relative to vehicle-treated GK beta-cells, an increased number of SNARE clusters were evident in nateglinide- or insulin-treated cells; a lesser increase was observed in glibenclamide-treated cells. Immunostaining for insulin showed that nateglinide treatment better preserved pancreatic islet morphology than did glibenclamide treatment. However, direct exposure of GK beta-cells to these drugs could not restore the decreased first-phase insulin release nor the reduced numbers of docked insulin granules. We conclude that treatment of GK rats with nateglinide and glibenclamide varies in long-term effects on beta-cell functions; nateglinide treatment appears overall to be more beneficial.

Glucagon-like peptide-1 stimulates GABA formation by pancreatic beta-cells at the level of glutamate decarboxylase

Pancreatic beta-cells are the major extraneural site of glutamate decarboxylase expression (GAD). During culture of isolated beta-cells, the GAD product gamma-aminobutyrate (GABA) is rapidly released in the medium, independently of insulin. It is considered as a possible mediator of beta-cell influences on alpha-cells, acinar cells, and/or infiltrating lymphocytes. In this perspective, we investigated the regulation of GABA release by rat beta-cells during a 24-h culture period. Glucose was previously reported to inhibit GABA release by diverting cellular GABA to mitochondrial breakdown through activation of GABA transferase (GABA-T). In the present study, glucagon-like peptide-1 (GLP-1) was shown to stimulate GABA formation at the level of GAD, its effect being suppressed by the GAD inhibitor allylglycine and remaining unaltered by the GABA-T inhibitor gamma-vinyl-GABA. The stimulatory action of GLP-1 is cAMP dependent, being reproduced by the adenylate cyclase activator forskolin and the cAMP analog N(6)-benzoyladenosine-3',5'-cAMP and inhibited by a PKA inhibitor. It is dependent on protein synthesis and associated with an increased expression of GAD67 but not GAD65. The GLP-1-induced stimulation of GAD activity in beta-cells can elevate medium GABA levels in conditions of glucose-driven intracellular GABA breakdown and thus maintain GABA-mediated beta-cell influences on neighboring cells.

A ubiquitous membrane fusion protein αSNAP: a potential therapeutic target for cancer, diabetes and neurological disorders?

Alpha soluble NSF attachment protein (alphaSNAP) is a ubiquitous and indispensable component of membrane fusion machinery. Deletion of alphaSNAP is embryonically lethal. Yet, there is accumulating evidence that milder alterations in expression levels of alphaSNAP may be associated with a number of specific pathological conditions, such as several neurological disorders, Type 2 diabetes and aggressive neuroendocrine tumours. Here, the authors review the evidence available for animal models and for humans, and discuss possible therapeutic approaches that may target alphaSNAP.

Deletion of SERP1/RAMP4, a component of the endoplasmic reticulum (ER) translocation sites, leads to ER stress

Stress-associated endoplasmic reticulum (ER) protein 1 (SERP1), also known as ribosome-associated membrane protein 4 (RAMP4), is a Sec61-associated polypeptide that is induced by ER stress. SERP1-/- mice, made by targeted gene disruption, demonstrated growth retardation, increased mortality, and impaired glucose tolerance. Consistent with high levels of SERP1 expression in pancreas, pancreatic islets from SERP1-/- mice failed to rapidly synthesize proinsulin in response to a glucose load. In addition, reduced size and enhanced ER stress were observed in the anterior pituitary of SERP1-/- mice, and growth hormone production was slowed in SERP1-/- pituitary after insulin stimulation. Experiments using pancreatic microsomes revealed aberrant association of ribosomes and the Sec61 complex and enhanced ER stress in SERP1-/- pancreas. In basal conditions, the Sec61 complex in SERP1-/- microsomes was more cofractionated with ribosomes, compared with SERP1+/+ counterparts, in high-salt conditions. In contrast, after glucose stimulation, the complex showed less cofractionation at an early phase (45 min) but more at a later phase (120 min). Although intracellular insulin/proinsulin levels were not significantly changed in both genotypes, these results suggest that subtle changes in translocation efficiency play an important role in the regulation of ER stress and rapid polypeptide synthesis.

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.

Complexin II facilitates exocytotic release in mast cells by enhancing Ca2+ sensitivity of the fusion process

Recent studies have shown that soluble N-ethyl maleimide-sensitive factor attachment protein receptor (SNARE) proteins are involved in exocytotic release in mast cells as in neurotransmitter release. However, the roles of the proteins that regulate the structure and activity of SNARE proteins are poorly understood. Complexin is one such regulatory protein and is involved in neurotransmitter release, although ideas about its role are still controversial. In this study, we investigated the expression and role of complexin in the regulation of exocytotic release (degranulation) in mast cells. We found that complexin II, but not complexin I, is expressed in mast cells. We obtained RBL-2H3 cells that expressed a low level of complexin II and found that antigen-induced degranulation was suppressed in these cells. No significant changes in the Ca2+ response or expression levels of syntaxins and synaptotagmin were observed in knockdown cells. An immunocytochemical study revealed that complexin II was distributed throughout the cytoplasm before antigen stimulation. However, the distribution of complexin II changed dramatically with stimulation and it became localized on the plasma membrane. This change in the intracellular distribution was observed even in the absence of extracellular Ca2+, while exocytotic release was inhibited almost completely under this condition. The degranulation induced by phorbol 12-myristate 13-acetate and A23187 depended on the extracellular Ca2+ concentration, and its sensitivity to Ca2+ was decreased in knockdown cells. These results suggest that complexin II regulates exocytosis positively by translocating to the plasma membrane and enhancing the Ca2+ sensitivity of fusion machinery, although this translocation to the plasma membrane is not sufficient to trigger exocytotic membrane fusion.

ELKS, a protein structurally related to the active zone-associated protein CAST, is expressed in pancreatic beta cells and functions in insulin exocytosis: interaction of ELKS with exocytotic machinery analyzed by total internal reflection fluorescence microscopy

The cytomatrix at the active zone (CAZ) has been implicated in defining the site of Ca2+-dependent exocytosis of neurotransmitters. Here, we demonstrate the expression and function of ELKS, a protein structurally related to the CAZ protein CAST, in insulin exocytosis. The results of confocal and immunoelectron microscopic analysis showed that ELKS is present in pancreatic beta cells and is localized close to insulin granules docked on the plasma membrane-facing blood vessels. Total internal reflection fluorescence microscopy imaging in insulin-producing clonal cells revealed that the ELKS clusters are less dense and unevenly distributed than syntaxin 1 clusters, which are enriched in the plasma membrane. Most of the ELKS clusters were on the docking sites of insulin granules that were colocalized with syntaxin 1 clusters. Total internal reflection fluorescence images of single-granule motion showed that the fusion events of insulin granules mostly occurred on the ELKS cluster, where repeated fusion was sometimes observed. When the Bassoon-binding region of ELKS was introduced into the cells, the docking and fusion of insulin granules were markedly reduced. Moreover, attenuation of ELKS expression by small interfering RNA reduced the glucose-evoked insulin release. These data suggest that the CAZ-related protein ELKS functions in insulin exocytosis from pancreatic beta cells.

Regulated exocytosis and kiss-and-run of synaptic-like microvesicles in INS-1 and primary rat beta-cells

We have applied cell-attached capacitance measurements to investigate whether synaptic-like microvesicles (SLMVs) undergo regulated exocytosis in insulinoma and primary pancreatic beta-cells. SLMV and large dense-core vesicle (LDCV) exocytosis was increased 1.6- and 2.4-fold upon stimulation with 10 mmol/l glucose in INS-1 cells. Exocytosis of both types of vesicles was coupled to Ca(2+) entry through l-type channels. Thirty percent of SLMV exocytosis in INS-1 and rat beta-cells was associated with transient capacitance increases consistent with kiss-and-run. Elevation of intracellular cAMP (5 micromol/l forskolin) increased SLMV exocytosis 1.6-fold and lengthened the duration of kiss-and-run events in rat beta-cells. Experiments using isolated inside-out patches of INS-1 cells revealed that the readily releasable pool (RRP) of SLMVs preferentially undergoes kiss-and-run exocytosis (67%), is proportionally larger than the LDCV RRP, and is depleted more quickly upon Ca(2+) stimulation. We conclude that SLMVs undergo glucose-regulated exocytosis and are capable of high turnover. Following kiss-and-run exocytosis, the SLMV RRP may be reloaded with gamma-aminobutyric acid and undergo several cycles of exo- and endocytosis. Our observations support a role for beta-cell SLMVs in a synaptic-like function of rapid intra-islet signaling.

Comparison of cellular and medium insulin and GABA content as markers for living beta-cells

Experimental and therapeutic use of islet cell preparations could benefit from assays that measure variations in the mass of living beta-cells. Because processes of cell death can be followed by depletion and/or discharge of cell-specific substances, we examined whether in vitro conditions of beta-cell death resulted in changes in tissue and medium content of insulin and of gamma-aminobutyric acid (GABA), two beta-cell-specific compounds with different cellular localization and turnover. Exposure of rat purified beta-cells to streptozotocin (5 mM, 120 min) or to the nitric oxide donor GEA-3162 (GEA; 50 microM, 120 min) caused 80% necrosis within 24 h; at the end of this period, cellular insulin content was not significantly decreased, but cellular GABA content was reduced by 70%; when cultured at basal glucose (6 mM), the toxin-exposed cells did not discharge less insulin but released 80% less GABA in the period 8-24 h. As in rat beta-cell purification, GABA comigrated with insulin during human islet cell isolation. Twenty-four hours after GEA (500 microM, 120 min), human islet cell preparations exhibited 90% dead cells and a 45 and 90% reduction, respectively, in tissue insulin and GABA content; in the period 9-24 h, insulin discharge in the medium was not reduced, but GABA release was decreased by 90%. When rat beta-cells were cultured for 24 h with nontoxic interleukin (IL)-1beta concentrations that suppressed glucose-induced insulin release, cellular GABA content was not decreased and GABA release increased by 90% in the period 8-24 h. These data indicate that a reduction in cellular and medium GABA levels is more sensitive than insulin as a marker for the presence of dead beta-cells in isolated preparations. Pancreatic GABA content also rapidly decreased after streptozotocin injection and remained unaffected by 12 h of hyperglycemia. At further variance with insulin, GABA release from living beta-cells depends little on its cellular content but increases with IL-1beta-induced alterations in beta-cell phenotype.

PPAR-gamma overexpression selectively suppresses insulin secretory capacity in isolated pancreatic islets through induction of UCP-2 protein

Peroxisome proliferator-activated receptor-gamma (PPAR-gamma) regulates several cellular functions, but its physiological role in pancreatic islet cells remains to be investigated. In this study, we confirmed the presence of PPAR-gamma in rat isolated islets and examined its role on insulin and glucagon secretion by using PPAR-gamma-overexpressed islets. PPAR-gamma overexpression significantly suppressed insulin secretion induced by stimulatory concentration of glucose (p<0.05). In addition, insulin secretion evoked by high potassium depolarization also was significantly decreased from PPAR-gamma-overexpressed islets (p<0.05). On the other hand, no significant change in glucagon release was observed after high potassium depolarization between PPAR-gamma-overexpressed and control islets. Insulin and glucagon content in islets was not statistically different between the two groups. In addition, the expression of uncoupling protein-2 (UCP-2) was found to be induced in PPAR-gamma-overexpressed islets. This result clearly indicates that the deteriorative effect of PPAR-gamma overexpression on the secretory machinery is selective for pancreatic beta-cells. And it is possible that its site of action can be located in the energy-consuming exocytotic process of insulin secretory granules, and that the reduction of ATP production through increased UCP-2 reduces insulin exocytosis.

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.

alpha-SNAP and NSF are required in a priming step during the human sperm acrosome reaction

The acrosome is a membrane-limited granule that overlies the nucleus of the mature spermatozoon. In response to physiological or pharmacological stimuli it undergoes a special type of Ca2+-dependent exocytosis termed the acrosome reaction (AR), which is an absolute prerequisite for fertilization. Aided by a streptolysin-O permeabilization protocol developed in our laboratory, we have previously demonstrated requirements for Rab3A, N-ethylmaleimide-sensitive factor (NSF), several soluble NSF-attachment protein receptor (SNARE) proteins, and synaptotagmin VI in the human sperm AR. Here, we show that alpha-soluble NSF-attachment protein (alpha-SNAP), a protein essential for most fusion events through its interaction with NSF and the SNARE complex, exhibits a direct role in the AR. First, the presence of alpha-SNAP is demonstrated by the Western blot of human sperm protein extracts. Immunostaining experiments reveal an acrosomal localization for this protein. Second, the Ca2+ and Rab3A-triggered ARs are inhibited by anti-alpha-SNAP antibodies. Third, bacterially expressed alpha-SNAP abolishes exocytosis in a fashion that depends on its interaction with NSF. Fourth, we show a requirement for alpha-SNAP/NSF in a prefusion step early in the exocytotic pathway, after the tethering of the acrosome to the plasma membrane and before the efflux of intra-acrosomal Ca2+. These results suggest a key role for alpha-SNAP/NSF in the AR, and strengthen our understanding of the molecular players involved in the vesicle-to-plasma membrane fusion taking place during exocytosis.

TIRF imaging of docking and fusion of single insulin granule motion in primary rat pancreatic beta-cells: different behaviour of granule motion between normal and Goto-Kakizaki diabetic rat beta-cells

We imaged and analysed the motion of single insulin secretory granules near the plasma membrane in live pancreatic beta-cells, from normal and diabetic Goto-Kakizaki (GK) rats, using total internal reflection fluorescence microscopy (TIRFM). In normal rat primary beta-cells, the granules that were fusing during the first phase originate from previously docked granules, and those during the second phase originate from 'newcomers'. In diabetic GK rat beta-cells, the number of fusion events from previously docked granules were markedly reduced, and, in contrast, the fusion from newcomers was still preserved. The dynamic change in the number of docked insulin granules showed that, in GK rat beta-cells, the total number of docked insulin granules was markedly decreased to 35% of the initial number after glucose stimulation. Immunohistochemistry with anti-insulin antibody observed by TIRFM showed that GK rat beta-cells had a marked decline of endogenous insulin granules docked to the plasma membrane. Thus our results indicate that the decreased number of docked insulin granules accounts for the impaired insulin release during the first phase of insulin release in diabetic GK rat beta-cells.

Functions of pancreatic β cells and adipocytes in bombesin receptor subtype-3-deficient mice

We previously reported that mice lacking bombesin receptor subtype-3 (BRS-3) exhibit mild late-onset obesity and glucose intolerance [Nature 390 (1997) 160]. To examine the mechanism by which glucose intolerance is developed in these mice, we studied insulin release and proinsulin biosynthesis in isolated pancreatic islets and glucose uptake and facilitative glucose transporter (GLUT)-4 translocation in adipose tissues. Although islet insulin contents and the size and number of islets of Langerhans in BRS-3-deficient mice decreased, there was no difference in glucose-stimulated insulin release and proinsulin biosynthesis between BRS-3-deficient and wild-type control mice. In contrast, adipose tissues exhibited a marked difference: the uptake of [(14)C]2-deoxy-D-glucose by adipocytes isolated from BRS-3-deficient mice was not stimulated by 10(-7)M insulin addition, and membrane fractionation analysis showed that GLUT4 was barely detected in the fraction of plasma membrane in BRS-3-deficient mice in the presence of 10(-7)M insulin. Quantitative reverse transcription-PCR (RT-PCR) showed that mRNA levels of GLUT4, insulin receptor, insulin receptor substrate (IRS)-1 and IRS-2, syntaxin 4, SNAP23, and VAMP-2 in adipose tissues of BRS-3-deficient mice were unchanged compared with those in wild-type control mice. We concluded that impaired glucose metabolism observed in BRS-3-deficient mice was mainly caused by impaired GLUT4 translocation in adipocytes.

Regulated exocytosis of GABA-containing synaptic-like microvesicles in pancreatic beta-cells

We have explored whether γ-aminobutyric acid (GABA) is released by regulated exocytosis of GABA-containing synaptic-like microvesicles (SLMVs) in insulin-releasing rat pancreatic β-cells. To this end, β-cells were engineered to express GABA_A-receptor Cl⁻-channels at high density using adenoviral infection. Electron microscopy indicated that the average diameter of the SLMVs is 90 nm, that every β-cell contains ∼3,500 such vesicles, and that insulin-containing large dense core vesicles exclude GABA. Quantal release of GABA, seen as rapidly activating and deactivating Cl⁻-currents, was observed during membrane depolarizations from −70 mV to voltages beyond −40 mV or when Ca²⁺ was dialysed into the cell interior. Depolarization-evoked GABA release was suppressed when Ca²⁺ entry was inhibited using Cd²⁺. Analysis of the kinetics of GABA release revealed that GABA-containing vesicles can be divided into a readily releasable pool and a reserve pool. Simultaneous measurements of GABA release and cell capacitance indicated that exocytosis of SLMVs contributes ∼1% of the capacitance signal. Mathematical analysis of the release events suggests that every SLMV contains 0.36 amol of GABA. We conclude that there are two parallel pathways of exocytosis in pancreatic β-cells and that release of GABA may accordingly be temporally and spatially separated from insulin secretion. This provides a basis for paracrine GABAergic signaling within the islet.

Site of docking and fusion of insulin secretory granules in live MIN6 beta cells analyzed by TAT-conjugated anti-syntaxin 1 antibody and total internal reflection fluorescence microscopy

To determine the site of insulin exocytosis in the pancreatic beta cell plasma membrane, we analyzed the interaction between the docking/fusion of green fluorescent protein-tagged insulin granules and syntaxin 1 labeled by TAT-conjugated Cy3-labeled antibody (Ab) using total internal reflection fluorescence microscopy (TIRFM). Monoclonal Ab against syntaxin 1 was labeled with Cy3 then conjugated with the protein transduction domain of HIV-1 TAT. TAT-conjugated Cy3-labeled anti-syntaxin 1 Ab was transduced rapidly into the subplasmalemmal region in live MIN6 beta cells, which enabled us to observe the spatial organization and distribution of endogenous syntaxin 1. TIRFM imaging revealed that syntaxin 1 is distributed in numerous separate clusters in the intact plasma membrane, where insulin secretory granules were docked preferentially to the sites of syntaxin 1 clusters, colocalizing with synaptosomal-associated protein of 25 kDa (SNAP-25) clusters. TIRFM imaging analysis of the motion of single insulin granules demonstrated that the fusion of insulin secretory granules stimulated by 50 mm KCl occurred exclusively at the sites of the syntaxin 1 clusters. Cholesterol depletion by methyl-beta-cyclodextrin treatment, in which the syntaxin 1 clusters were disintegrated, decreased the number of docked insulin granules, and, eventually the number of fusion events was significantly reduced. Our results indicate that 1) insulin exocytosis occurs at the site of syntaxin 1 clusters; 2) syntaxin 1 clusters are essential for the docking and fusion of insulin granules in MIN6 beta cells; and 3) the sites of syntaxin 1 clusters are distinct from flotillin-1 lipid rafts.

Characterization of alpha-soluble N-ethylmaleimide-sensitive fusion attachment protein in alveolar type II cells: implications in lung surfactant secretion

N-ethylmaleimide-sensitive fusion protein (NSF) and soluble NSF attachment protein (alpha-SNAP) are thought to be soluble factors that transiently bind and disassemble SNAP receptor complex during exocytosis in neuronal and endocrine cells. Lung surfactant is secreted via exocytosis of lamellar bodies from alveolar epithelial type II cells. However, the secretion of lung surfactant is a relatively slow process, and involvement of SNAP receptor and its cofactors (NSF and alpha-SNAP) in this process has not been demonstrated. In this study, we investigated a possible role of alpha-SNAP in surfactant secretion. alpha-SNAP was predominantly associated with the membranes in alveolar type II cells as determined by Western blot and immunocytochemical analysis using confocal microscope. Membrane-associated alpha-SNAP was not released from the membrane fraction when the cells were lyzed in the presence of Ca2+ or Mg2+ATP. The alkaline condition (0.1 M Na2CO3, pH 12), known to extract peripheral membrane proteins also failed to release it from the membrane. Phase separation using Triton X-114 showed that alpha-SNAP partitioned into both aqueous and detergent phases. NSF had membrane-bound characteristics similar to alpha-SNAP in type II cells. Permeabilization of type II cells with beta-escin resulted in a partial loss of alpha-SNAP from the cells, but cellular NSF was relatively unchanged. Addition of exogenous alpha-SNAP to the permeabilized cells increased surfactant secretion in a dose-dependent manner, whereas exogenous NSF has much less effects. An alpha-SNAP antisense oligonucleotide decreased its protein level and inhibited surfactant secretion. Our results suggest a role of alpha-SNAP in lung surfactant secretion.

PPAR-gamma overexpression suppresses glucose-induced proinsulin biosynthesis and insulin release synergistically with pioglitazone in MIN6 cells

Peroxisome proliferator-activated receptor-gamma (PPAR-gamma) regulates several cellular functions; however, its physiological role in pancreatic beta cell functions remains to be determined. In the present study, we investigated the synergistic effect of PPAR-gamma and its agonist, pioglitazone, on proinsulin biosynthesis and insulin release in a glucose-responsible insulinoma cell line, MIN6 cells. Expression of PPAR-gamma in MIN6 cells was not detectable by RT-PCR and immunoblot analysis. When PPAR-gamma-1 was overexpressed adenovirally in MIN6 cells, glucose-stimulated proinsulin biosynthesis and insulin release were inhibited. Pioglitazone treatment alone had no effects on these parameters of beta cell function in control MIN6 cells, although pioglitazone synergistically augmented the inhibitory effect of PPAR-gamma on proinsulin biosynthesis and insulin release under the condition of PPAR-gamma overexpression. Our results demonstrate that PPAR-gamma plays a negative role in pancreatic beta cells.

Secretory granule exocytosis

Regulated exocytosis of secretory granules or dense-core granules has been examined in many well-characterized cell types including neurons, neuroendocrine, endocrine, exocrine, and hemopoietic cells and also in other less well-studied cell types. Secretory granule exocytosis occurs through mechanisms with many aspects in common with synaptic vesicle exocytosis and most likely uses the same basic protein components. Despite the widespread expression and conservation of a core exocytotic machinery, many variations occur in the control of secretory granule exocytosis that are related to the specialized physiological role of particular cell types. In this review we describe the wide range of cell types in which regulated secretory granule exocytosis occurs and assess the evidence for the expression of the conserved fusion machinery in these cells. The signals that trigger and regulate exocytosis are reviewed. Aspects of the control of exocytosis that are specific for secretory granules compared with synaptic vesicles or for particular cell types are described and compared to define the range of accessory control mechanisms that exert their effects on the core exocytotic machinery.

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.

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.

Imaging exocytosis of single insulin secretory granules with evanescent wave microscopy: distinct behavior of granule motion in biphasic insulin release

To study insulin exocytosis by monitoring the single insulin secretory granule motion, evanescent wave microscopy was used to quantitatively analyze the final stage of insulin exocytosis with biphasic release. Green fluorescent protein-tagged insulin transfected in MIN6 beta cells was packed in insulin secretory granules, which appeared to preferentially dock to the plasma membrane. Upon fusion evoked by secretagogues, evanescent wave microscopy revealed that fluorescence of green fluorescent protein-tagged insulin brightened, spread (within 300 ms), and then vanished. Under KCl stimulation, which represents the 1st phase of release, the successive fusion events were seen mostly from previously docked granules for the first minute, followed by the recruitment of new granules to the plasmalemmal docking sites. Stimulation with glucose, in contrast, caused the fusion events from previously docked granules for the first 120 s, thereafter a continuous fusion (2nd phase of release) was observed over 10 min mostly from newly recruited granules that progressively accumulated on the plasma membrane. Thus, our data revealed the distinct behavior of the insulin granule motion during the 1st and 2nd phase of release.

Differential regulation of exocytosis by alpha- and beta-SNAPs

We examined the role of SNAPs, soluble proteins that attach N-ethylmaleimide-sensitive factor (NSF), in regulating exocytosis in single rat adrenal chromaffin cells. Whole-cell dialysis of Ca2+-buffered solution or photolysis of caged-Ca2+ was used to manipulate cytosolic Ca2+ concentration ([Ca2+]i), whereas exocytosis was measured via carbon fiber amperometry or membrane capacitance. Buffering [Ca2+]i to approximately 170 nm produced a mean rate of exocytosis of approximately one amperometric event per minute. Including alpha-SNAP (60 or 500 nm) in the intracellular solution dramatically increased the mean rate of exocytosis. The stimulatory action of alpha-SNAP requires ATP hydrolysis mediated via NSF, because this action was blocked by intracellular dialysis of ATP-gamma-S (2 mm) and could not be mimicked by a mutant alpha-SNAP that does not stimulate the ATPase activity of NSF. This action of alpha-SNAP was significant only at [Ca2+]i between 100 and 300 nm and was not shared by beta-SNAP (500 nm), suggesting that alpha-SNAP enhanced a component of exocytosis that is regulated by a high-affinity Ca2+ sensor. In cells dialyzed with both alpha- and beta-SNAP, the rate of exocytosis was smaller than that produced by alpha-SNAP alone, suggesting that alpha- and beta-SNAP interact competitively. Although only alpha-SNAP stimulated exocytosis at [Ca2+]i between 100 and 300 nm, both alpha- and beta-SNAP isoforms equally slowed the time-dependent rundown of the exocytic response. Our results indicate that alpha- and beta-SNAP have different actions in exocytosis. Thus, the ratio of different isoforms of SNAPs can determine release probability at the levels of [Ca2+]i that are involved in regulation of exocytosis.

Mastoparan stimulates GABA release from MIN6 cells: relationship between SNARE proteins and mastoparan action

We examined the action of mastoparan on beta cell exocytosis. Mastoparan stimulated GABA and insulin release from MIN6 beta cells. On the other hand, mastopraran-induced GABA release was decreased by expressing the tetanus toxin C1 light chain in MIN6 cells. We have then investigated the relationship between SNARE proteins and mastoparan action using adenovirus-mediated gene transfer system. Overexpression of t-SNAREs, syntaxin 1A, and SNAP-25 inhibited the mastoparan-induced insulin release by approximately half-fold of control levels, however, the mastoparan-induced GABA release was not affected by these t-SNAREs overexpression. The overexpression of mutant alpha-SNAP (1-285), which inhibits the wild-type alpha-SNAP function in a dominant negative manner, did not influence either mastoparan-induced GABA or insulin release in spite of its marked inhibition of glucose-stimulated insulin release. Our data indicate that mastoparan stimulates GABA exocytosis via vesicular transport; however, SNARE proteins are differently involved in the exocytosis of insulin and GABA.

An osmotic-sensitive taurine pool is localized in rat pancreatic islet cells containing glucagon and somatostatin

Previous reports have dealt with the hypoglycemic properties of taurine and its effects on insulin secretion by adult and fetal isolated islets. We have studied the presence and cellular distribution of taurine in rat islets, the conditions to evoke its release, and its possible modulatory action on insulin secretion. We localized taurine by techniques of double immunolabeling in most glucagon-positive cells and in some somatostatin-positive cells, whereas insulin-positive cells were not labeled with the taurine antibody. Although high-glucose stimulation did not evoke any taurine release, a hyposmotic solution (17% osmolarity reduction) induced a specific phasic release of taurine and GABA (34 and 52% increase on their basal release rate). On the other hand, taurine (10 mmol/l) application slightly reduced the second phase of insulin secretion induced by glucose stimulation. In conclusion, taurine is highly concentrated in glucagon-containing cells of the islet periphery. It is not liberated by glucose stimulation but is strongly released under hyposmotic conditions. All of these data suggest that taurine plays an osmoregulatory role in alpha-cells.

Adenovirus-mediated preproinsulin gene transfer into adipose tissues ameliorates hyperglycemia in obese diabetic KKA(y) mice

We investigated whether adenovirus-mediated preproinsulin gene transfer into insulin target tissues (adipocytes) ameliorates hyperglycemia in diabetic mice. KKA(y) mice, a genetically obese type 2 diabetic animal model, were treated with a single subcutaneous injection of recombinant adenovirus, Adex1CA-human preproinsulin (Adex1CA-pchi), into the epididymal fat pads. pchi mRNA was expressed only in adipose tissue in which mature insulin was produced. Three days after virus injection these mice showed a marked decrease of blood glucose levels (from about 400 to 200 mg/dl), and an intraperitoneal glucose tolerance test revealed the markedly improved glucose tolerance. There was no significant difference in serum insulin levels between control and recombinant adenovirus-treated KKA(y) mice. The normalized glucose levels in diabetic mice were maintained for at least 2 weeks after the virus injection. This strategy could provide a novel and, most importantly, a simple and convenient gene therapy for obese type 2 diabetes patients.

Localization of cellubrevin-related peptide, endobrevin, in the early endosome in pancreatic beta cells and its physiological function in exo-endocytosis of secretory granules

Cellubrevins are integral membrane proteins expressed in a wide variety of tissues and usually localized in recycling vesicles. Here, we investigated the cellular localization of a cellubrevin-related peptide, endobrevin, in pancreatic (beta) cells and its implication in the exo-endocytosis of insulin and (gamma)-amino butyric acid (GABA). Immunocytochemistry showed that endobrevin is associated with tubulo-vesicular structures, which are colocalized with early endosomes labeled by early endosome antigen (EEA)-1 in insulinoma MIN6 cells. To determine the cellular localization of endobrevin, we appended the green fluorescent protein (GFP) to endobrevin and the fusion protein was introduced into MIN6 cells. The subcellular localization of GFP-endobrevin was visualized by confocal laser microscopy. Colocalization study based on the expressed GFP-endobrevin and endocytosed Texas-Red(Tx-R) labeled transferrin receptor and immunocytochemistry with anti-EEA1 antibody revealed that endobrevin was preferentially localized in the early endosome. Then, we examined the functional role of endobrevin in the exocytosis of insulin and GABA from pancreatic (beta) cells. Endobrevin overexpression increased the amount of GABA released from MIN6 cells; in contrast, it decreased the glucose-stimulated insulin release from rat islets, MIN6 and INS1-D cells to approximately 50% of the control levels. Both in vitro and in vivo binding studies showed that endobrevin binds to syntaxin 1. Finally, using the fluorescent probe FM4-64, it was revealed that endobrevin overexpression accelerates vesicle recycling. We conclude that (1) endobrevin is localized in the early endosome in pancreatic (beta) cells and (2) endobrevin plays a physiological role in the exo-endocytosis of insulin and GABA from pancreatic (beta) cells, probably via an interaction between endocytic vesicles and the endosome.

Increased expression of GAD65 and GABA in pancreatic beta-cells impairs first-phase insulin secretion

The functional role of glutamate decarboxylase (GAD) and its product GABA in pancreatic islets has remained elusive. Mouse beta-cells express the larger isoform GAD67, whereas human islets express only the smaller isoform GAD65. We have generated two lines of transgenic mice expressing human GAD65 in pancreatic beta-cells (RIP7-hGAD65, Lines 1 and 2) to study the effect that GABA generated by this isoform has on islet cell function. The ascending order of hGAD65 expression and/or activity in beta-cells was Line 1 heterozygotes < Line 2 heterozygotes < Line 1 homozygotes. Line 1 heterozygotes have normal glucose tolerance, whereas Line 1 homozygotes and Line 2 heterozygotes exhibit impaired glucose tolerance and inhibition of insulin secretion in vivo in response to glucose. In addition, fasting levels of blood glucose are elevated and insulin is decreased in Line 1 homozygotes. Pancreas perfusion experiments suggest that GABA generated by GAD65 may function as a negative regulator of first-phase insulin secretion in response to glucose by affecting a step proximal to or at the K(ATP)(+) channel.

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.