Syntaxin-4 mediates exocytosis of pre-docked and newcomer insulin granules underlying biphasic glucose-stimulated insulin secretion in human pancreatic beta cells

Exome sequencing to explore the possibility of predicting genetic susceptibility to the joint occurrence of polycystic ovary syndrome and Hashimoto's thyroiditis

A large body of evidence indicates that women with polycystic ovary syndrome (PCOS) have a higher risk of developing Hashimoto's thyroiditis (HT) than healthy individuals. Given the strong genetic impact on both diseases, common predisposing genetic factors are possibly involved but are not fully understood. Here, we performed whole-exome sequencing (WES) for 250 women with sporadic PCOS, HT, combined PCOS and HT (PCOS+HT), and healthy controls to explore the genetic background of the joint occurrence of PCOS and HT. Based on relevant comparative analyses, multivariate logistic regression prediction modeling, and the most informative feature selection using the Monte Carlo feature selection and interdependency discovery algorithm, 77 variants were selected for further validation by TaqMan genotyping in a group of 533 patients. In the allele frequency test, variants in RAB6A, GBP3, and FNDC7 genes were found to significantly (padjusted < 0.05) differentiated the PCOS+HT and PCOS groups, variant in HIF3A differentiated the PCOS+HT and HT groups, whereas variants in CDK20 and CCDC71 differentiated the PCOS+HT and both single disorder groups. TaqMan genotyping data were used to create final prediction models, which differentiated between PCOS+HT and PCOS or HT with a prediction accuracy of AUC = 0.78. Using a 70% cutoff of the prediction score improved the model parameters, increasing the AUC value to 0.87. In summary, we demonstrated the polygenic burden of both PCOS and HT, and many common and intersecting signaling pathways and biological processes whose disorders mutually predispose patients to the development of both diseases.

Rapid Quantification of First and Second Phase Insulin Secretion Dynamics using an In vitro Platform for Improving Insulin Therapy

High-throughput quantification of the first- and second-phase insulin secretion dynamics is intractable with current methods. The fact that independent secretion phases play distinct roles in metabolism necessitates partitioning them separately and performing high-throughput compound screening to target them individually. We developed an insulin-nanoluc luciferase reporter system to dissect the molecular and cellular pathways involved in the separate phases of insulin secretion. We validated this method through genetic studies, including knockdown and overexpression, as well as small-molecule screening and their effects on insulin secretion. Furthermore, we demonstrated that the results of this method are well correlated with those of single-vesicle exocytosis experiments conducted on live cells, providing a quantitative reference for the approach. Thus, we have developed a robust methodology for screening small molecules and cellular pathways that target specific phases of insulin secretion, resulting in a better understanding of insulin secretion, which in turn will result in a more effective insulin therapy through the stimulation of endogenous glucose-stimulated insulin secretion.

Epigallocatechin gallate inhibits SNARE-dependent membrane fusion by blocking trans-SNARE assembly

Insulin secretion is a signal-triggered process that requires membrane fusion between the secretory granules and plasma membrane in pancreatic β cells. The exocytosis of insulin is mediated by target-soluble N-ethylmaleimide sensitive factor attachment protein receptors (SNAREs) on the plasma membrane and vesicle-SNAREs on the vesicles, which assemble into a quaternary trans-SNARE complex to initiate the fusion. Expression of fusion proteins is reduced in the islets of patients with type II diabetes, indicating that SNARE-mediated fusion defect is closely related to insulin-based metabolic diseases. Previous studies have suggested that epigallocatechin gallate (EGCG) has an inhibitory effect on membrane fusion. In the present study, we performed in vitro reconstitution assays to unravel the molecular mechanisms of EGCG in SNARE-mediated insulin secretory vesicle fusion. Our data show that EGCG efficiently inhibits insulin secretory SNARE-mediated membrane fusion. Mechanistic studies indicated that EGCG blocks the formation of the trans-SNARE complex. Furthermore, calcium/synaptotagmin-7-stimulated fusion kinetics were largely reduced by EGCG, confirming that it is a potential regulator of SNARE-dependent insulin secretion. Our findings suggest that the trans-SNARE complex might be a promising target for controlling SNARE-dependent vesicle fusion.

Assembly-promoting protein Munc18c stimulates SNARE-dependent membrane fusion through its SNARE-like peptide

Intracellular vesicle fusion requires the soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs) and their cognate Sec1/Munc18 (SM) proteins. How SM proteins act in concert with trans-SNARE complexes to promote membrane fusion remains incompletely understood. Munc18c, a broadly distributed SM protein, selectively regulates multiple exocytotic pathways, including GLUT4 exocytosis. Here, using an in vitro reconstituted system, we discovered a SNARE-like peptide (SLP), conserved in Munc18-1 of synaptic exocytosis, is crucial to the stimulatory activity of Munc18c in vesicle fusion. The direct stimulation of the SNARE-mediated fusion reaction by SLP further supported the essential role of this fragment. Interestingly, we found SLP strongly accelerates the membrane fusion rate when anchored to the target membrane but not the vesicle membrane, suggesting it primarily interacts with t-SNAREs in cis to drive fusion. Furthermore, we determined the SLP fragment is competitive with the full-length Munc18c protein and specific to the cognate v-SNARE isoforms, supporting how it could resemble Munc18c’s activity in membrane fusion. Together, our findings demonstrate that Munc18c facilitates SNARE-dependent membrane fusion through SLP, revealing that the t-SNARE-SLP binding mode might be a conserved mechanism for the stimulatory function of SM proteins in vesicle fusion.

Imaging Insulin Granule Dynamics in Human Pancreatic β-Cells Using Total Internal Reflection Fluorescence (TIRF) Microscopy

Due to the ultra-thin optical sectioning capability of exclusively illuminating space at the interface where total internal reflection occurs, the TIRF microscope has been indispensable for monitoring biological processes adjacent to the plasma membrane with excellent signal-to-noise ratio. Insulin-containing granules fuse with the plasma membrane to release contents within hundreds of milliseconds, which involves well-orchestrated assembly of SNARE complex and associated proteins. A video-rate multiple-color TIRF microscope offers the unique opportunity to visualize single secretory granule docking and fusion dynamics and can also map its regulators with high spatiotemporal resolution. Here, we describe the basic principles and practical implementation of a fast dual-color TIRF microscope, detailing a how-to guide on imaging and analysis of insulin granule dynamics in human β-cells.

Proteomic and Bioinformatic Analysis of Decellularized Pancreatic Extracellular Matrices

Tissue microenvironments are rich in signaling molecules. However, factors in the tissue matrix that can serve as tissue-specific cues for engineering pancreatic tissues have not been thoroughly identified. In this study, we performed a comprehensive proteomic analysis of porcine decellularized pancreatic extracellular matrix (dpECM). By profiling dpECM collected from subjects of different ages and genders, we showed that the detergent-free decellularization method developed in this study permits the preservation of approximately 62.4% more proteins than a detergent-based method. In addition, we demonstrated that dpECM prepared from young pigs contained approximately 68.5% more extracellular matrix proteins than those prepared from adult pigs. Furthermore, we categorized dpECM proteins by biological process, molecular function, and cellular component through gene ontology analysis. Our study results also suggested that the protein composition of dpECM is significantly different between male and female animals while a KEGG enrichment pathway analysis revealed that dpECM protein profiling varies significantly depending on age. This study provides the proteome of pancreatic decellularized ECM in different animal ages and genders, which will help identify the bioactive molecules that are pivotal in creating tissue-specific cues for engineering tissues in vitro.

Non-glucose modulators of insulin secretion in healthy humans: (dis)similarities between islet and in vivo studies

Optimal metabolic homeostasis requires precise temporal and quantitative control of insulin secretion. Both in vivo and in vitro studies have often focused on the regulation by glucose although many additional factors including other nutrients, neurotransmitters, hormones and drugs, modulate the secretory function of pancreatic β-cells. This review is based on the analysis of clinical investigations characterizing the effects of non-glucose modulators of insulin secretion in healthy subjects, and of experimental studies testing the same modulators in islets isolated from normal human donors. The aim was to determine whether the information gathered in vitro can reliably be translated to the in vivo situation. The comparison evidenced both convincing similarities and areas of discordance. The lack of coherence generally stems from the use of exceedingly high concentrations of test agents at too high or too low glucose concentrations in vitro, which casts doubts on the physiological relevance of a number of observations made in isolated islets. Future projects resorting to human islets should avoid extreme experimental conditions, such as oversized stimulations or inhibitions of β-cells, which are unlikely to throw light on normal insulin secretion and contribute to the elucidation of its defects.

Microtubules and Gαo-signaling modulate the preferential secretion of young insulin secretory granules in islet β cells via independent pathways

For sustainable function, each pancreatic islet β cell maintains thousands of insulin secretory granules (SGs) at all times. Glucose stimulation induces the secretion of a small portion of these SGs and simultaneously boosts SG biosynthesis to sustain this stock. The failure of these processes, often induced by sustained high-insulin output, results in type 2 diabetes. Intriguingly, young insulin SGs are more likely secreted during glucose-stimulated insulin secretion (GSIS) for unknown reasons, while older SGs tend to lose releasability and be degraded. Here, we examine the roles of microtubule (MT) and Gαo-signaling in regulating the preferential secretion of young versus old SGs. We show that both MT-destabilization and Gαo inactivation results in more SGs localization near plasma membrane (PM) despite higher levels of GSIS and reduced SG biosynthesis. Intriguingly, MT-destabilization or Gαo-inactivation results in higher secretion probabilities of older SGs, while combining both having additive effects on boosting GSIS. Lastly, Gαo inactivation does not detectably destabilize the β-cell MT network. These findings suggest that Gαo and MT can modulate the preferential release of younger insulin SGs via largely parallel pathways.

CD47 and thrombospondin-1 regulation of mitochondria, metabolism, and diabetes

Thrombospondin-1 (TSP1) is the prototypical member of a family of secreted proteins that modulate cell behavior by engaging with molecules in the extracellular matrix and with receptors on the cell surface. CD47 is widely displayed on many, if not all, cell types and is a high-affinity TSP1 receptor. CD47 is a marker of self that limits innate immune cell activities, a feature recently exploited to enhance cancer immunotherapy. Another major role for CD47 in health and disease is to mediate TSP1 signaling. TSP1 acting through CD47 contributes to mitochondrial, metabolic, and endocrine dysfunction. Studies in animal models found that elevated TSP1 expression, acting in part through CD47, causes mitochondrial and metabolic dysfunction. Clinical studies established that abnormal TSP1 expression positively correlates with obesity, fatty liver disease, and diabetes. The unabated increase in these conditions worldwide and the availability of CD47 targeting drugs justify a closer look into how TSP1 and CD47 disrupt metabolic balance and the potential for therapeutic intervention.

Genetic evidence for an inhibitory role of tomosyn in insulin-stimulated GLUT4 exocytosis

Exocytosis is a vesicle fusion process driven by soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs). A classic exocytic pathway is insulin-stimulated translocation of the glucose transporter type 4 (GLUT4) from intracellular vesicles to the plasma membrane in adipocytes and skeletal muscles. The GLUT4 exocytic pathway plays a central role in maintaining blood glucose homeostasis and is compromised in insulin resistance and type 2 diabetes. A candidate regulator of GLUT4 exocytosis is tomosyn, a soluble protein expressed in adipocytes. Tomosyn directly binds to GLUT4 exocytic SNAREs in vitro but its role in GLUT4 exocytosis was unknown. In this work, we used CRISPR-Cas9 genome editing to delete the two tomosyn-encoding genes in adipocytes. We observed that both basal and insulin-stimulated GLUT4 exocytosis was markedly elevated in the double knockout (DKO) cells. By contrast, adipocyte differentiation and insulin signaling remained intact in the DKO adipocytes. In a reconstituted liposome fusion assay, tomosyn inhibited all the SNARE complexes underlying GLUT4 exocytosis. The inhibitory activity of tomosyn was relieved by NSF and α-SNAP, which act in concert to remove tomosyn from GLUT4 exocytic SNAREs. Together, these studies revealed an inhibitory role for tomosyn in insulin-stimulated GLUT4 exocytosis in adipocytes. We suggest that tomosyn-arrested SNAREs represent a reservoir of fusion capacity that could be harnessed to treat patients with insulin resistance and type 2 diabetes.

Hyperglycemia-Induced Dysregulated Fusion Intermediates in Insulin-Secreting Cells Visualized by Super-Resolution Microscopy

Impaired insulin release is a hallmark of type 2 diabetes and is closely related to chronically elevated glucose concentrations, known as “glucotoxicity.” However, the molecular mechanisms by which glucotoxicity impairs insulin secretion remain poorly understood. In addition to known kiss-and-run and kiss-and-stay fusion events in INS-1 cells, ultrafast Hessian structured illumination microscopy (Hessian SIM) enables full fusion to be categorized according to the newly identified structures, such as ring fusion (those with enlarged pores) or dot fusion (those without apparent pores). In addition, we identified four fusion intermediates during insulin exocytosis: initial pore opening, vesicle collapse, enlarged pore formation, and final pore dilation. Long-term incubation in supraphysiological doses of glucose reduced exocytosis in general and increased the occurrence of kiss-and-run events at the expense of reduced full fusion. In addition, hyperglycemia delayed pore opening, vesicle collapse, and enlarged pore formation in full fusion events. It also reduced the size of apparently enlarged pores, all of which contributed to the compromised insulin secretion. These phenotypes were mostly due to the hyperglycemia-induced reduction in syntaxin-1A (Stx-1A) and SNAP-25 protein, since they could be recapitulated by the knockdown of endogenous Stx-1A and SNAP-25. These findings suggest essential roles for the vesicle fusion type and intermediates in regulating insulin secretion from pancreatic beta cells in normal and disease conditions.

Syntaxin 4 Mediates NF-κB Signaling and Chemokine Ligand Expression via Specific Interaction With IκBβ

Enrichment of human islets with syntaxin 4 (STX4) improves functional β-cell mass through a nuclear factor-κB (NF-κB)-dependent mechanism. However, the detailed mechanisms underlying the protective effect of STX4 are unknown. For determination of the signaling events linking STX4 enrichment and downregulation of NF-κB activity, STX4 was overexpressed in human islets, EndoC-βH1 and INS-1 832/13 cells in culture, and the cells were challenged with the proinflammatory cytokines interleukin-1β, tumor necrosis factor-α, and interferon-γ individually and in combination. STX4 expression suppressed cytokine-induced proteasomal degradation of IκBβ but not IκBα. Inhibition of IKKβ prevented IκBβ degradation, suggesting that IKKβ phosphorylates IκBβ. Moreover, the IKKβ inhibitor, as well as a proteosomal degradation inhibitor, prevented the loss of STX4 caused by cytokines. This suggests that STX4 may be phosphorylated by IKKβ in response to cytokines, targeting STX4 for proteosomal degradation. Expression of a stabilized form of STX4 further protected IκBβ from proteasomal degradation, and like wild-type STX4, stabilized STX4 coimmunoprecipitated with IκBβ and the p50-NF-κB. This work proposes a novel pathway wherein STX4 regulates cytokine-induced NF-κB signaling in β-cells via associating with and preventing IκBβ degradation, suppressing chemokine expression, and protecting islet β-cells from cytokine-mediated dysfunction and demise.

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.

The ARFRP1-dependent Golgi scaffolding protein GOPC is required for insulin secretion from pancreatic β-cells

Objective

Hormone secretion from metabolically active tissues, such as pancreatic islets, is governed by specific and highly regulated signaling pathways. Defects in insulin secretion are among the major causes of diabetes. The molecular mechanisms underlying regulated insulin secretion are, however, not yet completely understood. In this work, we studied the role of the GTPase ARFRP1 on insulin secretion from pancreatic β-cells.

Methods

A β-cell-specific Arfrp1 knockout mouse was phenotypically characterized. Pulldown experiments and mass spectrometry analysis were employed to screen for new ARFRP1-interacting proteins. Co-immunoprecipitation assays as well as super-resolution microscopy were applied for validation.

Results

The GTPase ARFRP1 interacts with the Golgi-associated PDZ and coiled-coil motif-containing protein (GOPC). Both proteins are co-localized at the trans-Golgi network and regulate the first and second phase of insulin secretion by controlling the plasma membrane localization of the SNARE protein SNAP25. Downregulation of both GOPC and ARFRP1 in Min6 cells interferes with the plasma membrane localization of SNAP25 and enhances its degradation, thereby impairing glucose-stimulated insulin release from β-cells. In turn, overexpression of SNAP25 as well as GOPC restores insulin secretion in islets from β-cell-specific Arfrp1 knockout mice.

Conclusion

Our results identify a hitherto unrecognized pathway required for insulin secretion at the level of trans-Golgi sorting.

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β-cell specific deletion of the trans-Golgi residing small GTPase ARFRP1 leads to elevated blood glucose levels in mice.

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GOPC is a newly identified ARFRP1 dependent scaffolding protein.

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ARFRP1 and GOPC are required for glucose-stimulated insulin secretion from pancreatic β-cells.

Melanophilin Accelerates Insulin Granule Fusion without Predocking to the Plasma Membrane

Direct observation of fluorescence-labeled secretory granule exocytosis in living pancreatic β-cells has revealed heterogeneous prefusion behaviors: some granules dwell beneath the plasma membrane before fusion, while others fuse immediately once they are recruited to the plasma membrane. Although the former mode seems to follow sequential docking-priming-fusion steps as found in synaptic vesicle exocytosis, the latter mode, which is unique to secretory granule exocytosis, has not been explored well. Here, we show that melanophilin, one of the effectors of the monomeric guanosine-5'-triphosphatase Rab27 on the granule membrane, is involved in such an accelerated mode of exocytosis. Melanophilin-mutated leaden mouse and melanophilin-downregulated human pancreatic β-cells both exhibit impaired glucose-stimulated insulin secretion, with a specific reduction in fusion events that bypass stable docking to the plasma membrane. Upon stimulus-induced [Ca²⁺]_i rise, melanophilin mediates this type of fusion by dissociating granules from myosin-Va and actin in the actin cortex and by associating them with a fusion-competent, open form of syntaxin-4 on the plasma membrane. These findings provide the hitherto unknown mechanism to support sustainable exocytosis by which granules are recruited from the cell interior and fuse promptly without stable predocking to the plasma membrane.

Recent Insights into Beta-cell Exocytosis in Type 2 Diabetes

As one of the leading causes of morbidity and mortality worldwide, diabetes affects an estimated 422 million adults, and it is expected to continue expanding such that by 2050, 30% of the U.S. population will become diabetic within their lifetime. Out of the estimated 422 million people currently afflicted with diabetes worldwide, about 5% have type 1 diabetes (T1D), while the remaining ~95% of diabetics have type 2 diabetes (T2D). Type 1 diabetes results from the autoimmune-mediated destruction of functional β-cell mass, whereas T2D results from combinatorial defects in functional β-cell mass plus peripheral glucose uptake. Both types of diabetes are now believed to be preceded by β-cell dysfunction. T2D is increasingly associated with numerous reports of deficiencies in the exocytosis proteins that regulate insulin release from β-cells, specifically the soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) proteins. SNARE protein's functionality is further regulated by a variety of accessory factors such as Sec1/Munc18 (SM), double C2-domain proteins (DOC2), and additional interacting proteins at the cell surface that influence the fidelity of insulin release. As new evidence emerges about the detailed mechanisms of exocytosis, new questions and controversies have come to light. This emerging information is also contributing to dialogue in the islet biology field focused on how to correct the defects in insulin exocytosis. Herein we present a balanced review of the role of exocytosis proteins in T2D, with thoughts on novel strategies to protect functional β-cell mass.

SNAP23 depletion enables more SNAP25/calcium channel excitosome formation to increase insulin exocytosis in type 2 diabetes

SNAP23 is the ubiquitous SNAP25 isoform that mediates secretion in non-neuronal cells, similar to SNAP25 in neurons. However, some secretory cells like pancreatic islet β cells contain an abundance of both SNAP25 and SNAP23, where SNAP23 is believed to play a redundant role to SNAP25. We show that SNAP23, when depleted in mouse β cells in vivo and human β cells (normal and type 2 diabetes [T2D] patients) in vitro, paradoxically increased biphasic glucose-stimulated insulin secretion corresponding to increased exocytosis of predocked and newcomer insulin granules. Such effects on T2D Goto-Kakizaki rats improved glucose homeostasis that was superior to conventional treatment with sulfonylurea glybenclamide. SNAP23, although fusion competent in slower secretory cells, in the context of β cells acts as a weak partial fusion agonist or inhibitory SNARE. Here, SNAP23 depletion promotes SNAP25 to bind calcium channels more quickly and longer where granule fusion occurs to increase exocytosis efficiency. β Cell SNAP23 antagonism is a strategy to treat diabetes.

The challenge of correctly reporting hormones content and secretion in isolated human islets

Background

An increased access of research laboratories to isolated human islets has improved our understanding of the biology of the endocrine pancreas and hence the mechanisms causing diabetes. However, in vitro studies of human islets remain technically challenging, and optimal use of such precious material requires a minimum of rigor and coordination to optimize the reliability and share of the information. A detailed report of the demographics of pancreas donors and of the procedures of islet handling after isolation is important but insufficient. Correct characterization of islet basic functions (a token of quality) at the time of experimentation is also crucial.

Scope of review

I have analyzed the literature reporting measurements of insulin and glucagon in the human pancreas or isolated human islets. The published information is often fragmentary. Elementary features such as islet size, insulin content, or rate of hormone secretion are either unreported or incorrectly reported in many papers. Although internal comparisons between control and test groups may remain valid, comparisons with data from other laboratories are problematic. The drawbacks, pitfalls and errors of common ways of expressing hormone content or secretion rates are discussed and alternatives to harmonize data presentation are proposed.

Major Conclusions

Greater coherence and rigor in the report of in vitro studies using human islets are necessary to ensure optimal progress in our understanding of the pathogenesis of diabetes.

Doc2b Protects β-Cells Against Inflammatory Damage and Enhances Function

Loss of functional β-cell mass is an early feature of type 1 diabetes. To release insulin, β-cells require soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) complexes, as well as SNARE complex regulatory proteins like double C2 domain-containing protein β (Doc2b). We hypothesized that Doc2b deficiency or overabundance may confer susceptibility or protection, respectively, to the functional β-cell mass. Indeed, Doc2b^+/- knockout mice show an unusually severe response to multiple-low-dose streptozotocin (MLD-STZ), resulting in more apoptotic β-cells and a smaller β-cell mass. In addition, inducible β-cell-specific Doc2b-overexpressing transgenic (βDoc2b-dTg) mice show improved glucose tolerance and resist MLD-STZ-induced disruption of glucose tolerance, fasting hyperglycemia, β-cell apoptosis, and loss of β-cell mass. Mechanistically, Doc2b enrichment enhances glucose-stimulated insulin secretion (GSIS) and SNARE activation and prevents the appearance of apoptotic markers in response to cytokine stress and thapsigargin. Furthermore, expression of a peptide containing the Doc2b tandem C2A and C2B domains is sufficient to confer the beneficial effects of Doc2b enrichment on GSIS, SNARE activation, and apoptosis. These studies demonstrate that Doc2b enrichment in the β-cell protects against diabetogenic and proapoptotic stress. Furthermore, they identify a Doc2b peptide that confers the beneficial effects of Doc2b and may be a therapeutic candidate for protecting functional β-cell mass.

Fusion pore in exocytosis: More than an exit gate? A β-cell perspective

Secretory vesicle exocytosis is a fundamental biological event and the process by which hormones (like insulin) are released into the blood. Considerable progress has been made in understanding this precisely orchestrated sequence of events from secretory vesicle docked at the cell membrane, hemifusion, to the opening of a membrane fusion pore. The exact biophysical and physiological regulation of these events implies a close interaction between membrane proteins and lipids in a confined space and constrained geometry to ensure appropriate delivery of cargo. We consider some of the still open questions such as the nature of the initiation of the fusion pore, the structure and the role of the Soluble N-ethylmaleimide-sensitive-factor Attachment protein REceptor (SNARE) transmembrane domains and their influence on the dynamics and regulation of exocytosis. We discuss how the membrane composition and protein-lipid interactions influence the likelihood of the nascent fusion pore forming. We relate these factors to the hypothesis that fusion pore expansion could be affected in type-2 diabetes via changes in disease-related gene transcription and alterations in the circulating lipid profile. Detailed characterisation of the dynamics of the fusion pore in vitro will contribute to understanding the larger issue of insulin secretory defects in diabetes.

Recent new insights into the role of SNARE and associated proteins in insulin granule exocytosis

Initial work on the exocytotic machinery of predocked insulin secretory granules (SGs) in pancreatic β-cells mimicked the SNARE hypothesis work in neurons, which includes SM/SNARE complex and associated priming proteins, fusion clamps and Ca²⁺ sensors. However, β-cell SGs, unlike neuronal synaptic vesicles, exhibit a biphasic secretory response that requires additional distinct features in exocytosis including newcomer SGs that undergo minimal docking time at the plasma membrane (PM) before fusion and multi-SG (compound) fusion. These exocytotic events are mediated by Munc18/SNARE complexes distinct from that which mediates predocked SG fusion. We review some recent insights in SNARE complex assembly and the promiscuity in SM/SNARE complex formation, whereby both contribute to conferring different insulin SG fusion kinetics. Some SNARE and associated proteins play non-fusion roles, including tethering SGs to Ca²⁺ channels, SG recruitment from cell interior to PM, and inhibitory SNAREs that block the action of profusion SNAREs. We discuss new insights into how sub-PM cytoskeletal mesh gates SG access to the PM and the targeting of SG exocytosis to PM domains in functionally polarized β-cells within intact islets. These recent developments have major implications on devising clever SNARE replacement therapies that could restore the deficient insulin secretion in diabetic islet β-cells.

The SNARE Protein Syntaxin-1a Plays an Essential Role in Biphasic Exocytosis of the Incretin Hormone Glucagon-Like Peptide 1

Exocytosis of the hormone glucagon-like peptide 1 (GLP-1) by the intestinal L cell is essential for the incretin effect after nutrient ingestion and is critical for the actions of dipeptidyl peptidase 4 inhibitors that enhance GLP-1 levels in patients with type 2 diabetes. Two-photon microscopy revealed that exocytosis of GLP-1 is biphasic, with a first peak at 1-6 min and a second peak at 7-12 min after stimulation with forskolin. Approximately 75% of the exocytotic events were represented by compound granule fusion, and the remainder were accounted for by full fusion of single granules under basal and stimulated conditions. The core SNARE protein syntaxin-1a (syn1a) was expressed by murine ileal L cells. At the single L-cell level, first-phase forskolin-induced exocytosis was reduced to basal (P < 0.05) and second-phase exocytosis abolished (P < 0.05) by syn1a knockout. L cells from intestinal-epithelial syn1a-deficient mice demonstrated a 63% reduction in forskolin-induced GLP-1 release in vitro (P < 0.001) and a 23% reduction in oral glucose-stimulated GLP-1 secretion (P < 0.05) in association with impairments in glucose-stimulated insulin release (by 60%; P < 0.01) and glucose tolerance (by 20%; P < 0.01). The findings identify an exquisite mechanism of metered secretory output that precisely regulates release of the incretin hormone GLP-1 and hence insulin secretion after a meal.

Syntaxin 2 Acts as Inhibitory SNARE for Insulin Granule Exocytosis

Of the four syntaxins specialized for exocytosis, syntaxin (Syn)-2 is the least understood. In this study, we used Syn-2/epimorphin knockout mice to examine the role of Syn-2 in insulin secretory granule (SG) exocytosis. Unexpectedly, Syn-2 knockout mice exhibited paradoxical superior glucose homeostasis resulting from an enhanced insulin secretion. This was confirmed in vitro by pancreatic islet perifusion showing an amplified biphasic glucose-stimulated insulin secretion arising from an increase in size of the readily releasable pool of insulin SGs and enhanced SG pool refilling. The increase in insulin exocytosis was attributed mainly to an enhanced recruitment of the larger pool of newcomer SGs that undergoes no residence time on plasma membrane before fusion and, to a lesser extent, also the predocked SGs. Consistently, Syn-2 depletion resulted in a stimulation-induced increase in abundance of exocytotic complexes we previously demonstrated as mediating the fusion of newcomer SGs (Syn-3/VAMP8/SNAP25/Munc18b) and predocked SGs (Syn-1A/VAMP2/SNAP25/Muncn18a). This work is the first to show in mammals that Syn-2 could function as an inhibitory SNARE protein that, when relieved, could promote exocytosis in pancreatic islet β-cells. Thus, Syn-2 may serve as a potential target to treat diabetes.

4μ8C Inhibits Insulin Secretion Independent of IRE1α RNase Activity

IRE1α plays an important role in the unfolded protein response (UPR), which is activated by the accumulation of unfolded proteins in the endoplasmic reticulum. 4μ8C, a well-known inhibitor of IRE1α RNase activity, is commonly used to analyze IRE1α function during ER stress in cultured mammalian cells. However, the off-target effects of 4μ8C remain elusive. Pancreatic β-cells synthesize a large amount of insulin in response to high glucose stimulation, and IRE1α plays an important role in insulin secretion from pancreatic β-cells. Here, to analyze the role of IRE1α in pancreatic β-cells, we examined insulin secretion after 4μ8C treatment. Although 4μ8C inhibited insulin secretion within 2 hr, neither insulin synthesis nor maturation was inhibited by 4μ8C under the same conditions. This result prompted us to examine the precise effects of 4μ8C on insulin secretion in pancreatic β-cells. Unexpectedly, with just 5 min of treatment, 4μ8C blocked insulin secretion in cultured pancreatic β-cells as well as in pancreatic islets. Furthermore, insulin secretion was prevented by 4μ8C, even in pancreatic β-cells lacking the IRE1α RNase domain, suggesting that 4μ8C blocked the late stage of the insulin secretory process, independent of the IRE1α-XBP1 pathway. Our results indicate that 4μ8C has an off-target effect on insulin secretion in pancreatic β-cells. These findings inform the researchers in the field that the use of 4μ8C requires the special consideration for the future studies.Key words: 4μ8C, XBP1, insulin, IRE1α, pancreatic β-cells.

Munc18b Increases Insulin Granule Fusion, Restoring Deficient Insulin Secretion in Type-2 Diabetes Human and Goto-Kakizaki Rat Islets with Improvement in Glucose Homeostasis

Reduced pancreatic islet levels of Munc18a/SNARE complex proteins have been postulated to contribute to the deficient glucose-stimulated insulin secretion (GSIS) in type-2 diabetes (T2D). Whereas much previous work has purported Munc18a/SNARE complex (Syntaxin-1A/VAMP-2/SNAP25) to be primarily involved in predocked secretory granule (SG) fusion, less is known about newcomer SGs that undergo minimal docking time at the plasma membrane before fusion. Newcomer SG fusion has been postulated to involve a distinct SM/SNARE complex (Munc18b/Syntaxin-3/VAMP8/SNAP25), whose levels we find also reduced in islets of T2D humans and T2D Goto-Kakizaki (GK) rats. Munc18b overexpression by adenovirus infection (Ad-Munc18b), by increasing assembly of Munc18b/SNARE complexes, mediated increased fusion of not only newcomer SGs but also predocked SGs in T2D human and GK rat islets, resulting in rescue of the deficient biphasic GSIS.

Infusion of Ad-Munc18b into GK rat pancreas led to sustained improvement in glucose homeostasis. However, Munc18b overexpression in normal islets increased only newcomer SG fusion. Therefore, Munc18b could potentially be deployed in human T2D to rescue the deficient GSIS.

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Human T2D islet β-cells exhibit reduced fusion of predocked & newcomer secretory granules (SGs).

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Munc18b increases SNARE complexes involved in fusions of both newcomer & predocked SGs.

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Munc18b rescue of newcomer & predocked SGs increased biphasic secretion in human T2D β-cells.

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Munc18b rescue of T2D Goto-Kakizaki rat β-cell secretion improves glucose homeostasis.

Deficient insulin secretion from pancreatic islet β-cells in type-2 diabetes (T2D) is partly due to reduced expression of many proteins that assemble into specific complexes that mediate fusion of insulin secretory granules (SGs) with plasma membrane, termed exocytosis. We here show we can infuse a virus that contains the construct of one of the SG fusion proteins, Munc18b, into pancreatic ducts of T2D rats to reach the islets, which restored insulin secretion and improved glycemic control. Munc18b acts to promote the assembly of SG fusion complexes. This strategy could potentially be applied to treat human T2D by endoscopic infusion.

New Roles of Syntaxin-1A in Insulin Granule Exocytosis and Replenishment

In type-2 diabetes (T2D), severely reduced islet syntaxin-1A (Syn-1A) levels contribute to insulin secretory deficiency. We generated β-cell-specific Syn-1A-KO (Syn-1A-βKO) mice to mimic β-cell Syn-1A deficiency in T2D. Glucose tolerance tests showed that Syn-1A-βKO mice exhibited blood glucose elevation corresponding to reduced blood insulin levels. Perifusion of Syn-1A-βKO islets showed impaired first- and second-phase glucose-stimulated insulin secretion (GSIS) resulting from reduction in readily releasable pool and granule pool refilling. To unequivocally determine the β-cell exocytotic defects caused by Syn-1A deletion, EM and total internal reflection fluorescence microscopy showed that Syn-1A-KO β-cells had a severe reduction in the number of secretory granules (SGs) docked onto the plasma membrane (PM) at rest and reduced SG recruitment to the PM after glucose stimulation, the latter indicating defects in replenishment of releasable pools required to sustain second-phase GSIS. Whereas reduced predocked SG fusion accounted for reduced first-phase GSIS, selective reduction of exocytosis of short-dock (but not no-dock) newcomer SGs accounted for the reduced second-phase GSIS. These Syn-1A actions on newcomer SGs were partly mediated by Syn-1A interactions with newcomer SG VAMP8.

Opposing roles for SNAP23 in secretion in exocrine and endocrine pancreatic cells

Kunii et al. reveal that the SNARE protein SNAP23 plays distinct roles in the secretion of amylase in exocrine cells and of insulin in endocrine cells the pancreas and show that MF286, a novel inhibitor of SNAP23, may be a new drug candidate for diabetes.

The membrane fusion of secretory granules with plasma membranes is crucial for the exocytosis of hormones and enzymes. Secretion disorders can cause various diseases such as diabetes or pancreatitis. Synaptosomal-associated protein 23 (SNAP23), a soluble N-ethyl-maleimide sensitive fusion protein attachment protein receptor (SNARE) molecule, is essential for secretory granule fusion in several cell lines. However, the in vivo functions of SNAP23 in endocrine and exocrine tissues remain unclear. In this study, we show opposing roles for SNAP23 in secretion in pancreatic exocrine and endocrine cells. The loss of SNAP23 in the exocrine and endocrine pancreas resulted in decreased and increased fusion of granules to the plasma membrane after stimulation, respectively. Furthermore, we identified a low molecular weight compound, MF286, that binds specifically to SNAP23 and promotes insulin secretion in mice. Our results demonstrate opposing roles for SNAP23 in the secretion mechanisms of the endocrine and exocrine pancreas and reveal that the SNAP23-binding compound MF286 may be a promising drug for diabetes treatment.

Syntaxin-3 Binds and Regulates Both R- and L-Type Calcium Channels in Insulin-Secreting INS-1 832/13 Cells

Syntaxin (Syn)-1A mediates exocytosis of predocked insulin-containing secretory granules (SGs) during first-phase glucose-stimulated insulin secretion (GSIS) in part via its interaction with plasma membrane (PM)-bound L-type voltage-gated calcium channels (Cav). In contrast, Syn-3 mediates exocytosis of newcomer SGs that accounts for second-phase GSIS. We now hypothesize that the newcomer SG Syn-3 preferentially binds and modulates R-type Cav opening, which was postulated to mediate second-phase GSIS. Indeed, glucose-stimulation of pancreatic islet β-cell line INS-1 induced a predominant increase in interaction between Syn-3 and Cavα1 pore-forming subunits of R-type Cav2.3 and to lesser extent L-type Cavs, while confirming the preferential interactions between Syn-1A with L-type (Cav1.2, Cav1.3) Cavs. Consistently, direct binding studies employing heterologous HEK cells confirmed that Syn-3 preferentially binds Cav2.3, whereas Syn-1A prefers L-type Cavs. We then used siRNA knockdown (KD) of Syn-3 in INS-1 to study the endogenous modulatory actions of Syn-3 on Cav channels. Syn-3 KD enhanced Ca2+ currents by 46% attributed mostly to R- and L-type Cavs. Interestingly, while the transmembrane domain of Syn-1A is the putative functional domain modulating Cav activity, it is the cytoplasmic domain of Syn-3 that appears to modulate Cav activity. We conclude that Syn-3 may mimic Syn-1A in the ability to bind and modulate Cavs, but preferring Cav2.3 to perhaps participate in triggering fusion of newcomer insulin SGs during second-phase GSIS.

Research-Focused Isolation of Human Islets From Donors With and Without Diabetes at the Alberta Diabetes Institute IsletCore

Recent years have seen an increased focus on human islet biology, and exciting findings in the stem cell and genomic arenas highlight the need to define the key features of mature human islets and β-cells. Donor and organ procurement parameters impact human islet yield, although for research purposes islet yield may be secondary in importance to islet function. We examined the feasibility of a research-only human islet isolation, distribution, and biobanking program and whether key criteria such as cold ischemia time (CIT) and metabolic status may be relaxed and still allow successful research-focused isolations, including from donors with type 1 diabetes and type 2 diabetes. Through 142 isolations over approximately 5 years, we confirm that CIT and glycated hemoglobin each have a weak negative impacts on isolation purity and yield, and extending CIT beyond the typical clinical isolation cutoff of 12 hours (to ≥ 18 h) had only a modest impact on islet function. Age and glycated hemoglobin/type 2 diabetes status negatively impacted secretory function; however, these and other biological (sex, body mass index) and procurement/isolation variables (CIT, time in culture) appear to make only a small contribution to the heterogeneity of human islet function. This work demonstrates the feasibility of extending acceptable CIT for research-focused human islet isolation and highlights the biological variation in function of human islets from donors with and without diabetes.

Syntaxin 4 regulates the surface localization of a promyogenic receptor Cdo thereby promoting myogenic differentiation

Background

Syntaxins are a family of membrane proteins involved in vesicle trafficking, such as synaptic vesicle exocytosis. Syntaxin 4 (Stx4) is expressed highly in skeletal muscle and plays a critical role in insulin-stimulated glucose uptake by promoting translocation of glucose transporter 4 (GLUT4) to the cell surface. A cell surface receptor cell adhesion molecule-related, down-regulated by oncogenes (Cdo) is a component of cell adhesion complexes and promotes myoblast differentiation via activation of key signalings, including p38MAPK and AKT. In this study, we investigate the function of Stx4 in myoblast differentiation and the crosstalk between Stx4 and Cdo in myoblast differentiation.

Methods

The effects of overexpression or shRNA-based depletion of Stx4 and Cdo genes on C2C12 myoblast differentiation are assessed by Western blotting and immunofluorescence approaches. The interaction between Cdo and Stx4 and the responsible domain mapping are assessed by coimmunoprecipitation or pulldown assays. The effect of Stx4 depletion on cell surface localization of Cdo and GLUT4 in C2C12 myoblasts is assessed by surface biotinylation and Western blotting.

Results

Overexpression or knockdown of Stx4 enhances or inhibits myogenic differentiation, respectively. Stx4 binds to the cytoplasmic tail of Cdo, and this interaction seems to be critical for induction of p38MAPK activation and myotube formation. Stx4 depletion decreases specifically the cell surface localization of Cdo without changes in surface N-Cadherin levels. Interestingly, Cdo depletion reduces the level of GLUT4 and Stx4 at cell surface. Consistently, overexpression of Cdo in C2C12 myoblasts generally increases glucose uptake, while Cdo depletion reduces it.

Conclusions

Stx4 promotes myoblast differentiation through interaction with Cdo and stimulation of its surface translocation. Both Cdo and Stx4 are required for GLUT4 translocation to cell surface and glucose uptake in myoblast differentiation.

Electronic supplementary material

The online version of this article (doi:10.1186/s13395-015-0052-8) contains supplementary material, which is available to authorized users.