A new beta cell-specific mitophagy reporter mouse shows that metabolic stress leads to accumulation of dysfunctional mitochondria despite increased mitophagy.

AIMS/HYPOTHESIS: Mitophagy, the selective autophagy of mitochondria, is essential for maintenance of mitochondrial function. Recent studies suggested that defective mitophagy in beta cells caused diabetes. However, because of technical difficulties, the development of a convenient and reliable method to evaluate mitophagy in beta cells in vivo is needed. The aim of this study was to establish beta cell-specific mitophagy reporter mice and elucidate the role of mitophagy in beta cell function under metabolically stressed conditions induced by a high-fat diet (HFD). METHODS: Mitophagy was assessed using newly generated conditional mitochondrial matrix targeting mitophagy reporter (CMMR) mice, in which mitophagy can be visualised specifically in beta cells in vivo using a fluorescent probe sensitive to lysosomal pH and degradation. Metabolic stress was induced in mice by exposure to the HFD for 20 weeks. The accumulation of dysfunctional mitochondria was examined by staining for functional/total mitochondria and reactive oxygen species (ROS) using specific fluorescent dyes and antibodies. To investigate the molecular mechanism underlying mitophagy in beta cells, overexpression and knockdown experiments were performed. HFD-fed mice were examined to determine whether chronic insulin treatment for 6 weeks could ameliorate mitophagy, mitochondrial function and impaired insulin secretion. RESULTS: Exposure to the HFD increased the number of enlarged (HFD-G) islets with markedly elevated mitophagy. Mechanistically, HFD feeding induced severe hypoxia in HFD-G islets, which upregulated mitophagy through the hypoxia-inducible factor 1-ɑ (Hif-1ɑ)/BCL2 interacting protein 3 (BNIP3) axis in beta cells. However, HFD-G islets unexpectedly showed the accumulation of dysfunctional mitochondria due to excessive ROS production, suggesting an insufficient capacity of mitophagy for the degradation of dysfunctional mitochondria. Chronic administration of insulin ameliorated hypoxia and reduced ROS production and dysfunctional mitochondria, leading to decreased mitophagy and restored insulin secretion. CONCLUSIONS/INTERPRETATION: We demonstrated that CMMR mice enabled the evaluation of mitophagy in beta cells. Our results suggested that metabolic stress induced by the HFD caused the aberrant accumulation of dysfunctional mitochondria, which overwhelmed the mitophagic capacity and was associated with defective maintenance of mitochondrial function and impaired insulin secretion.

Serotonin regulates glucose-stimulated insulin secretion from pancreatic ß cells during pregnancy.

In preparation for the metabolic demands of pregnancy, ß cells in the maternal pancreatic islets increase both in number and in glucose-stimulated insulin secretion (GSIS) per cell. Mechanisms have been proposed for the increased ß cell mass, but not for the increased GSIS. Because serotonin production increases dramatically during pregnancy, we tested whether flux through the ionotropic 5-HT3 receptor (Htr3) affects GSIS during pregnancy. Pregnant Htr3a(-/-) mice exhibited impaired glucose tolerance despite normally increased ß cell mass, and their islets lacked the increase in GSIS seen in islets from pregnant wild-type mice. Electrophysiological studies showed that activation of Htr3 decreased the resting membrane potential in ß cells, which increased Ca(2+) uptake and insulin exocytosis in response to glucose. Thus, our data indicate that serotonin, acting in a paracrine/autocrine manner through Htr3, lowers the ß cell threshold for glucose and plays an essential role in the increased GSIS of pregnancy.

Imeglimin mitigates the accumulation of dysfunctional mitochondria to restore insulin secretion and suppress apoptosis of pancreatic ß-cells from db/db mice.

Mitochondrial dysfunction in pancreatic ß-cells leads to impaired glucose-stimulated insulin secretion (GSIS) and type 2 diabetes (T2D), highlighting the importance of autophagic elimination of dysfunctional mitochondria (mitophagy) in mitochondrial quality control (mQC). Imeglimin, a new oral anti-diabetic drug that improves hyperglycemia and GSIS, may enhance mitochondrial activity. However, chronic imeglimin treatment's effects on mQC in diabetic ß-cells are unknown. Here, we compared imeglimin, structurally similar anti-diabetic drug metformin, and insulin for their effects on clearance of dysfunctional mitochondria through mitophagy in pancreatic ß-cells from diabetic model db/db mice and mitophagy reporter (CMMR) mice. Pancreatic islets from db/db mice showed aberrant accumulation of dysfunctional mitochondria and excessive production of reactive oxygen species (ROS) along with markedly elevated mitophagy, suggesting that the generation of dysfunctional mitochondria overwhelmed the mitophagic capacity in db/db ß-cells. Treatment with imeglimin or insulin, but not metformin, reduced ROS production and the numbers of dysfunctional mitochondria, and normalized mitophagic activity in db/db ß-cells. Concomitantly, imeglimin and insulin, but not metformin, restored the secreted insulin level and reduced ß-cell apoptosis in db/db mice. In conclusion, imeglimin mitigated accumulation of dysfunctional mitochondria through mitophagy in diabetic mice, and may contribute to preserving ß-cell function and effective glycemic control in T2D.

DPP-4 inhibitor des-F-sitagliptin treatment increased insulin exocytosis from db/db mice ß 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 ß-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.

Insulin/phosphoinositide 3-kinase pathway accelerates the glucose-induced first-phase insulin secretion through TrpV2 recruitment in pancreatic ß-cells.

Functional insulin receptor and its downstream effector PI3K (phosphoinositide 3-kinase) have been identified in pancreatic ß-cells, but their involvement in the regulation of insulin secretion from ß-cells remains unclear. In the present study, we investigated the physiological role of insulin and PI3K in glucose-induced biphasic insulin exocytosis in primary cultured ß-cells and insulinoma Min6 cells using total internal reflection fluorescent microscopy. The pretreatment of ß-cells with insulin induced the rapid increase in intracellular Ca2+ levels and accelerated the exocytotic response without affecting the second-phase insulin secretion. The inhibition of PI3K not only abolished the insulin-induced rapid development of the exocytotic response, but also potentiated the second-phase insulin secretion. The rapid development of Ca2+ and accelerated exocytotic response induced by insulin were accompanied by the translocation of the Ca2+-permeable channel TrpV2 (transient receptor potential V2) in a PI3K-dependent manner. Inhibition of TrpV2 by the selective blocker tranilast, or the expression of shRNA (short-hairpin RNA) against TrpV2 suppressed the effect of insulin in the first phase, but the second phase was not affected. Thus our results demonstrate that insulin treatment induced the acceleration of the exocytotic response during the glucose-induced first-phase response by the insertion of TrpV2 into the plasma membrane in a PI3K-dependent manner.

Pattern of rise in subplasma membrane Ca2+ concentration determines type of fusing insulin granules in pancreatic beta cells.

We simultaneously analyzed insulin granule fusion with insulin fused to green fluorescent protein and the subplasma membrane Ca2+ concentration ([Ca2+](PM)) with the Ca2+ indicator Fura Red in rat beta cells by dual-color total internal reflection fluorescence microscopy. We found that rapid and marked elevation in [Ca2+](PM) caused insulin granule fusion mostly from previously docked granules during the high KCl-evoked release and high glucose-evoked first phase release. In contrast, the slow and sustained elevation in [Ca2+](PM) induced fusion from newcomers translocated from the internal pool during the low KCl-evoked release and glucose-evoked second phase release. These data suggest that the pattern of the [Ca2+](PM) rise directly determines the types of fusing granules.

Imaging exocytosis of single glucagon-like peptide-1 containing granules in a murine enteroendocrine cell line with total internal reflection fluorescent microscopy.

To analyze the exocytosis of glucagon-like peptide-1 (GLP-1) granules, we imaged the motion of GLP-1 granules labeled with enhanced yellow fluorescent protein (Venus) fused to human growth hormone (hGH-Venus) in an enteroendocrine cell line, STC-1 cells, by total internal reflection fluorescent (TIRF) microscopy. We found glucose stimulation caused biphasic GLP-1 granule exocytosis: during the first phase, fusion events occurred from two types of granules (previously docked granules and newcomers), and thereafter continuous fusion was observed mostly from newcomers during the second phase. Closely similar to the insulin granule fusion from pancreatic beta cells, the regulated biphasic exocytosis from two types of granules may be a common mechanism in glucose-evoked hormone release from endocrine cells.

ELKS/Voltage-Dependent Ca2+ Channel-ß Subunit Module Regulates Polarized Ca2+ Influx in Pancreatic ß Cells.

Pancreatic ß cells secrete insulin by Ca2+-triggered exocytosis. However, there is no apparent secretory site similar to the neuronal active zones, and the cellular and molecular localization mechanism underlying polarized exocytosis remains elusive. Here, we report that ELKS, a vertebrate active zone protein, is used in ß cells to regulate Ca2+ influx for insulin secretion. ß cell-specific ELKS-knockout (KO) mice showed impaired glucose-stimulated first-phase insulin secretion and reduced L-type voltage-dependent Ca2+ channel (VDCC) current density. In situ Ca2+ imaging of ß cells within islets expressing a membrane-bound G-CaMP8b Ca2+ sensor demonstrated initial local Ca2+ signals at the ELKS-localized vascular side of the ß cell plasma membrane, which were markedly decreased in ELKS-KO ß cells. Mechanistically, ELKS directly interacted with the VDCC-ß subunit via the GK domain. These findings suggest that ELKS and VDCCs form a potent insulin secretion complex at the vascular side of the ß cell plasma membrane for polarized Ca2+ influx and first-phase insulin secretion from pancreatic islets.

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.

VAMP7 Regulates Autophagosome Formation by Supporting Atg9a Functions in Pancreatic ß-Cells From Male Mice.

Dysfunctional mitochondria are observed in ß-cells of diabetic patients, which are eventually removed by autophagy. Vesicle-associated membrane protein (VAMP)7, a vesicular SNARE protein, regulates autophagosome formation to maintain mitochondrial homeostasis and control insulin secretion in pancreatic ß-cells. However, its molecular mechanism is largely unknown. In this study, we investigated the molecular mechanism of VAMP7-dependent autophagosome formation using VAMP7-deficient ß-cells and ß-cell-derived Min6 cells. VAMP7 localized in autophagy-related (Atg)9a-resident vesicles of recycling endosomes (REs), which contributed to autophagosome formation, and it interacted with Hrb, Syntaxin16, and SNAP-47. Hrb recruited VAMP7 and Atg9a from the plasma membrane to REs. Syntaxin16 and SNAP-47 mediated autophagosome formation at a step later than the proper localization of VAMP7 to Atg9a-resident vesicles. Knockdown of Hrb, Syntaxin16, and SNAP-47 resulted in defective autophagosome formation, accumulation of dysfunctional mitochondria, and impairment of glucose-stimulated insulin secretion. Our data indicate that VAMP7 and Atg9a are initially recruited to REs to organize VAMP7 and Atg9a-resident vesicles in an Hrb-dependent manner. Additionally, VAMP7 forms a SNARE complex with Syntaxin16 and SNAP-47, which may cause fusions of Atg9a-resident vesicles during autophagosome formation. Thus, VAMP7 participates in autophagosome formation by supporting Atg9a functions that contribute to maintenance of mitochondrial quality.

Imaging docking and fusion of insulin granules induced by antidiabetes agents: sulfonylurea and glinide drugs preferentially mediate the fusion of newcomer, but not previously docked, insulin granules.

Sulfonylurea and glinide drugs, commonly used for antidiabetes therapies, are known to stimulate insulin release from pancreatic beta-cells by closing ATP-sensitive K+ channels. However, the specific actions of these drugs on insulin granule motion are largely unknown. Here, we used total internal reflection fluorescence (TIRF) microscopy to analyze the docking and fusion of single insulin granules in live beta-cells exposed to either the sulfonylurea drug glibenclamide or the glinide drug mitiglinide. TIRF images showed that both agents caused rapid fusion of newcomer insulin granules with the cell membrane in both control and diabetic Goto-Kakizaki (GK) rat pancreatic beta-cells. However, in the context of beta-cells from sulfonylurea receptor 1 (SUR1) knockout mice, TIRF images showed that only mitiglinide, but not glibenclamide, caused fusion of newcomer insulin granules. Compositely, our data indicate that 1) the mechanism by which both sulfonylurea and glinide drugs promote insulin release entails the preferential fusion of newcomer, rather than previously docked, insulin granules, and that 2) mitiglinide can induce insulin release by a mechanism independent of mitiglinide binding to SUR1.

VAMP7 Regulates Autophagy to Maintain Mitochondrial Homeostasis and to Control Insulin Secretion in Pancreatic ß-Cells.

VAMP7 is a SNARE protein that mediates specific membrane fusions in intracellular trafficking and was recently reported to regulate autophagosome formation. However, its function in pancreatic ß-cells is largely unknown. To elucidate the physiological role of VAMP7 in ß-cells, we generated pancreatic ß-cell-specific VAMP7 knockout (Vamp7(flox/Y);Cre) mice. VAMP7 deletion impaired glucose-stimulated ATP production and insulin secretion, though VAMP7 was not localized to insulin granules. VAMP7-deficient ß-cells showed defective autophagosome formation and reduced mitochondrial function. p62/SQSTM1, a marker protein for defective autophagy, was selectively accumulated on mitochondria in VAMP7-deficient ß-cells. These findings suggest that accumulation of dysfunctional mitochondria that are degraded by autophagy caused impairment of glucose-stimulated ATP production and insulin secretion in Vamp7(flox/Y);Cre ß-cells. Feeding a high-fat diet to Vamp7(flox/Y);Cre mice exacerbated mitochondrial dysfunction, further decreased ATP production and insulin secretion, and consequently induced glucose intolerance. Moreover, we found upregulated VAMP7 expression in wild-type mice fed a high-fat diet and in db/db mice, a model for diabetes. Thus our data indicate that VAMP7 regulates autophagy to maintain mitochondrial quality and insulin secretion in response to pathological stress in ß-cells.

Docking and fusion of insulin secretory granules in SUR1 knock out mouse beta-cells observed by total internal reflection fluorescence microscopy.

To explore how the sulfonylurea receptor (SUR1) is involved in docking and fusion of insulin granules, dynamic motion of single insulin secretory granules near the plasma membrane was examined in SUR1 knock-out (Sur1KO) beta-cells by total internal reflection fluorescence microscopy. Sur1KO beta-cells exhibited a marked reduction in the number of fusion events from previously docked granules. However, the number of docked granules declined during stimulation as a consequence of the release of docked granules into the cytoplasm vs. fusion with the plasma membrane. Thus, the impaired docking and fusion results in decreased insulin exocytosis from Sur1KO beta-cells.

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.

Acute inhibition of PI3K-PDK1-Akt pathway potentiates insulin secretion through upregulation of newcomer granule fusions in pancreatic ß-cells.

In glucose-induced insulin secretion from pancreatic ß-cells, a population of insulin granules fuses with the plasma membrane without the typical docking process (newcomer granule fusions), however, its mechanism is unclear. In this study, we investigated the PI3K signaling pathways involved in the upregulation of newcomer granule fusions. Acute treatment with the class IA-selective PI3K inhibitors, PIK-75 and PI-103, enhanced the glucose-induced insulin secretion. Total internal reflection fluorescent microscopy revealed that the PI3K inhibitors increased the fusion events from newcomer granules. We developed a new system for transfection into pancreatic islets and demonstrated the usefulness of this system in order for evaluating the effect of transfected genes on the glucose-induced secretion in primary cultured pancreatic islets. Using this transfection system together with a series of constitutive active mutants, we showed that the PI3K-3-phosphoinositide dependent kinase-1 (PDK1)-Akt pathway mediated the potentiation of insulin secretion. The Akt inhibitor also enhanced the glucose-induced insulin secretion in parallel with the upregulation of newcomer granule fusions, probably via increased motility of intracellular insulin granules. These data suggest that the PI3K-PDK1-Akt pathway plays a significant role in newcomer granule fusions, probably through an alteration of the dynamics of the intracellular insulin granules.

Deletion of CDKAL1 affects mitochondrial ATP generation and first-phase insulin exocytosis.

BACKGROUND: A variant of the CDKAL1 gene was reported to be associated with type 2 diabetes and reduced insulin release in humans; however, the role of CDKAL1 in ß cells is largely unknown. Therefore, to determine the role of CDKAL1 in insulin release from ß cells, we studied insulin release profiles in CDKAL1 gene knockout (CDKAL1 KO) mice. PRINCIPAL FINDINGS: Total internal reflection fluorescence imaging of CDKAL1 KO ß cells showed that the number of fusion events during first-phase insulin release was reduced. However, there was no significant difference in the number of fusion events during second-phase release or high K(+)-induced release between WT and KO cells. CDKAL1 deletion resulted in a delayed and slow increase in cytosolic free Ca(2+) concentration during high glucose stimulation. Patch-clamp experiments revealed that the responsiveness of ATP-sensitive K(+) (K(ATP)) channels to glucose was blunted in KO cells. In addition, glucose-induced ATP generation was impaired. Although CDKAL1 is homologous to cyclin-dependent kinase 5 (CDK5) regulatory subunit-associated protein 1, there was no difference in the kinase activity of CDK5 between WT and CDKAL1 KO islets. CONCLUSIONS/SIGNIFICANCE: We provide the first report describing the function of CDKAL1 in ß cells. Our results indicate that CDKAL1 controls first-phase insulin exocytosis in ß cells by facilitating ATP generation, K(ATP) channel responsiveness and the subsequent activity of Ca(2+) channels through pathways other than CDK5-mediated regulation.

Glinide, but not sulfonylurea, can evoke insulin exocytosis by repetitive stimulation: imaging analysis of insulin exocytosis by secretagogue-induced repetitive stimulations.

To investigate the different effects between sulfonylurea (SU) and glinide drugs in insulin secretion, pancreatic beta-cells were repeatedly stimulated with SU (glimepiride) or glinide (mitiglinide). Total internal reflection fluorescent (TIRF) microscopy revealed that secondary stimulation with glimepiride, but not glucose and mitiglinide, failed to evoke fusions of insulin granules although primary stimulation with glucose, glimepiride, and mitiglinide induced equivalent numbers of exocytotic responses. Glimepiride, but not glucose and mitiglinide, induced abnormally sustained [Ca(2+)](i) elevations and reductions of docked insulin granules on the plasma membrane. Our data suggest that the effect of glinide on insulin secretory mechanisms is similar to that of glucose.

Elevation of the post-translational modification of proteins by O-linked N-acetylglucosamine leads to deterioration of the glucose-stimulated insulin secretion in the pancreas of diabetic Goto-Kakizaki rats.

Many nuclear and cytoplasmic proteins are O-glycosylated on serine or threonine residues with the monosaccharide beta-N-acetylglucosamine, which is then termed O-linked N-acetylglucosamine (O-GlcNAc). It has been shown that abnormal O-GlcNAc modification (O-GlcNAcylation) of proteins is one of the causes of insulin resistance and diabetic complications. In this study, in order to examine the relationship between O-GlcNAcylation of proteins and glucose-stimulated insulin secretion in noninsulin-dependent type (type 2) diabetes, we investigated the level of O-GlcNAcylation of proteins, especially that of PDX-1, and the expression of O-GlcNAc transferase in Goto-Kakizaki (GK) rats, which are an animal model of type-2 diabetes. By immunoblot and immunohistochemical analyses, the expression of O-GlcNAc transferase protein and O-GlcNAc-modified proteins in whole pancreas and islets of Langerhans of 15-week-old diabetic GK rats and nondiabetic Wistar rats was examined. The expression of O-GlcNAc transferase at the protein level and O-GlcNAc transferase activity were increased significantly in the diabetic pancreas and islets. The diabetic pancreas and islets also showed an increase in total cellular O-GlcNAc-modified proteins. O-GlcNAcylation of PDX-1 was also increased. In the diabetic GK rats, significant increases in the immunoreactivities of both O-GlcNAc and O-GlcNAc transferase were observed. PUGNAc, an inhibitor of O-GlcNAcase, induced an elevation of O-GlcNAc level and a decrease of glucose-stimulated insulin secretion in isolated islets. These results indicate that elevation of the O-GlcNAcylation of proteins leads to deterioration of insulin secretion in the pancreas of diabetic GK rats, further providing evidence for the role of O-GlcNAc in the insulin secretion.

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.

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.