Dual role of VAMP8 in regulating insulin exocytosis and islet β cell growth

Beta-Cell Tipe1 Orchestrates Insulin Secretion and Cell Proliferation by Promoting Gαs/cAMP Signaling via USP5

Inadequate β-cell mass and insulin secretion are essential for the development of type 2 diabetes (T2D). TNF-α-induced protein 8-like 1 (Tipe1) plays a crucial role in multiple diseases, however, a specific role in T2D pathogenesis remains largely unexplored. Herein, Tipe1 as a key regulator in T2D, contributing to the maintenance of β cell homeostasis is identified. The results show that the β-cell-specific knockout of Tipe1 (termed Ins2-Tipe1BKO) aggravated diabetic phenotypes in db/db mice or in mice with high-fat diet-induced diabetes. Notably, Tipe1 improves β cell mass and function, a process that depends on Gαs, the α subunit of the G-stimulating protein. Mechanistically, Tipe1 inhibited the K48-linked ubiquitination degradation of Gαs by recruiting the deubiquitinase USP5. Consequently, Gαs or cAMP agonists almost completely restored the dysfunction of β cells observed in Ins2-Tipe1BKO mice. The findings characterize Tipe1 as a regulator of β cell function through the Gαs/cAMP pathway, suggesting that Tipe1 may emerge as a novel target for T2D intervention.

Gβγ-SNAP25 exocytotic brake removal enhances insulin action, promotes adipocyte browning, and protects against diet-induced obesity

Negative regulation of exocytosis from secretory cells is accomplished through inhibitory signals from Gi/o GPCRs by Gβγ subunit inhibition of 2 mechanisms: decreased calcium entry and direct interaction of Gβγ with soluble N-ethylmaleimide-sensitive factor attachment protein (SNAP) receptor (SNARE) plasma membrane fusion machinery. Previously, we disabled the second mechanism with a SNAP25 truncation (SNAP25Δ3) that decreased Gβγ affinity for the SNARE complex, leaving exocytotic fusion and modulation of calcium entry intact and removing GPCR-Gβγ inhibition of SNARE-mediated exocytosis. Here, we report substantial metabolic benefit in mice carrying this mutation. Snap25Δ3/Δ3 mice exhibited enhanced insulin sensitivity and beiging of white fat. Metabolic protection was amplified in Snap25Δ3/Δ3 mice challenged with a high-fat diet. Glucose homeostasis, whole-body insulin action, and insulin-mediated glucose uptake into white adipose tissue were improved along with resistance to diet-induced obesity. Metabolic protection in Snap25Δ3/Δ3 mice occurred without compromising the physiological response to fasting or cold. All metabolic phenotypes were reversed at thermoneutrality, suggesting that basal autonomic activity was required. Direct electrode stimulation of sympathetic neuron exocytosis from Snap25Δ3/Δ3 inguinal adipose depots resulted in enhanced and prolonged norepinephrine release. Thus, the Gβγ-SNARE interaction represents a cellular mechanism that deserves further exploration as an additional avenue for combating metabolic disease.

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.

Loss of ZNF148 enhances insulin secretion in human pancreatic β cells

Insulin secretion from pancreatic β cells is essential to the maintenance of glucose homeostasis. Defects in this process result in diabetes. Identifying genetic regulators that impair insulin secretion is crucial for the identification of novel therapeutic targets. Here, we show that reduction of ZNF148 in human islets, and its deletion in stem cell-derived β cells (SC-β cells), enhances insulin secretion. Transcriptomics of ZNF148-deficient SC-β cells identifies increased expression of annexin and S100 genes whose proteins form tetrameric complexes involved in regulation of insulin vesicle trafficking and exocytosis. ZNF148 in SC-β cells prevents translocation of annexin A2 from the nucleus to its functional place at the cell membrane via direct repression of S100A16 expression. These findings point to ZNF148 as a regulator of annexin-S100 complexes in human β cells and suggest that suppression of ZNF148 may provide a novel therapeutic strategy to enhance insulin secretion.

Gut microbiome modified by bariatric surgery improves insulin sensitivity and correlates with increased brown fat activity and energy expenditure

Alterations in the microbiome correlate with improved metabolism in patients following bariatric surgery. While fecal microbiota transplantation (FMT) from obese patients into germ-free (GF) mice has suggested a significant role of the gut microbiome in metabolic improvements following bariatric surgery, causality remains to be confirmed. Here, we perform paired FMT from the same obese patients (BMI > 40; four patients), pre- and 1 or 6 months post-Roux-en-Y gastric bypass (RYGB) surgery, into Western diet-fed GF mice. Mice colonized by FMT from patients' post-surgery stool exhibit significant changes in microbiota composition and metabolomic profiles and, most importantly, improved insulin sensitivity compared with pre-RYGB FMT mice. Mechanistically, mice harboring the post-RYGB microbiome show increased brown fat mass and activity and exhibit increased energy expenditure. Moreover, improvements in immune homeostasis within the white adipose tissue are also observed. Altogether, these findings point to a direct role for the gut microbiome in mediating improved metabolic health post-RYGB surgery.

Secretory autophagy promotes RAB37-mediated insulin secretion under glucose stimulation both in vitro and in vivo

High blood glucose is one of the risk factors for metabolic disease and INS (insulin) is the key regulatory hormone for glucose homeostasis. Hypoinsulinemia accompanied with hyperglycemia was diagnosed in mice with pancreatic β-cells exhibiting autophagy deficiency; however, the underlying mechanism remains elusive. The role of secretory autophagy in the regulation of metabolic syndrome is gaining more attention. Our data demonstrated that increased macroautophagic/autophagic activity leads to induction of insulin secretion in β-cells both in vivo and in vitro under high-glucose conditions. Moreover, proteomic analysis of purified autophagosomes from β-cells identified a group of vesicular transport proteins participating in insulin secretion, implying that secretory autophagy regulates insulin exocytosis. RAB37, a small GTPase, regulates vesicle biogenesis, trafficking, and cargo release. We demonstrated that the active form of RAB37 increased MAP1LC3/LC3 lipidation (LC3-II) and is essential for the promotion of insulin secretion by autophagy, but these phenomena were not observed in rab37 knockout (rab37^-/-) cells and mice. Unbalanced insulin and glucose concentration in the blood was improved by manipulating autophagic activity using a novel autophagy inducer niclosamide (an antihelminthic drug) in a high-fat diet (HFD)-obesity mouse model. In summary, we reveal that secretory autophagy promotes RAB37-mediated insulin secretion to maintain the homeostasis of insulin and glucose both in vitro and in vivo.

Plasma membrane flipping of Syntaxin-2 regulates its inhibitory action on insulin granule exocytosis

Enhancing pancreatic β-cell secretion is a primary therapeutic target for type-2 diabetes (T2D). Syntaxin-2 (Stx2) has just been identified to be an inhibitory SNARE for insulin granule exocytosis, holding potential as a treatment for T2D, yet its molecular underpinnings remain unclear. We show that excessive Stx2 recruitment to raft-like granule docking sites at higher binding affinity than pro-fusion syntaxin-1A effectively competes for and inhibits fusogenic SNARE machineries. Depletion of Stx2 in human β-cells improves insulin secretion by enhancing trans-SNARE complex assembly and cis-SNARE disassembly. Using a genetically-encoded reporter, glucose stimulation is shown to induce Stx2 flipping across the plasma membrane, which relieves its suppression of cytoplasmic fusogenic SNARE complexes to promote insulin secretion. Targeting the flipping efficiency of Stx2 profoundly modulates secretion, which could restore the impaired insulin secretion in diabetes. Here, we show that Stx2 acts to assist this precise tuning of insulin secretion in β-cells, including in diabetes.

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.

Novel roles of mTORC2 in regulation of insulin secretion by actin filament remodeling

Mammalian target of rapamycin (mTOR) kinase is an essential hub where nutrients and growth factors converge to control cellular metabolism. mTOR interacts with different accessory proteins to form complexes 1 and 2 (mTORC), and each complex has different intracellular targets. Although mTORC1's role in β-cells has been extensively studied, less is known about mTORC2's function in β-cells. Here, we show that mice with constitutive and inducible β-cell-specific deletion of RICTOR (βRicKO and iβRicKO mice, respectively) are glucose intolerant due to impaired insulin secretion when glucose is injected intraperitoneally. Decreased insulin secretion in βRicKO islets was caused by abnormal actin polymerization. Interestingly, when glucose was administered orally, no difference in glucose homeostasis and insulin secretion were observed, suggesting that incretins are counteracting the mTORC2 deficiency. Mechanistically, glucagon-like peptide-1 (GLP-1), but not gastric inhibitory polypeptide (GIP), rescued insulin secretion in vivo and in vitro by improving actin polymerization in βRicKO islets. In conclusion, mTORC2 regulates glucose-stimulated insulin secretion by promoting actin filament remodeling.NEW & NOTEWORTHY The current studies uncover a novel mechanism linking mTORC2 signaling to glucose-stimulated insulin secretion by modulation of the actin filaments. This work also underscores the important role of GLP-1 in rescuing defects in insulin secretion by modulating actin polymerization and suggests that this effect is independent of mTORC2 signaling.

Snazarus and its human ortholog SNX25 modulate autophagic flux

Macroautophagy, the degradation and recycling of cytosolic components in the lysosome, is an important cellular mechanism. It is a membrane-mediated process that is linked to vesicular trafficking events. The sorting nexin (SNX) protein family controls the sorting of a large array of cargoes, and various SNXs impact autophagy. To improve our understanding of their functions in vivo, we screened all Drosophila SNXs using inducible RNA interference in the fat body. Significantly, depletion of Snazarus (Snz) led to decreased autophagic flux. Interestingly, we observed altered distribution of Vamp7-positive vesicles with Snz depletion, and the roles of Snz were conserved in human cells. SNX25, the closest human ortholog to Snz, regulates both VAMP8 endocytosis and lipid metabolism. Through knockout-rescue experiments, we demonstrate that these activities are dependent on specific SNX25 domains and that the autophagic defects seen upon SNX25 loss can be rescued by ethanolamine addition. We also demonstrate the presence of differentially spliced forms of SNX14 and SNX25 in cancer cells. This work identifies a conserved role for Snz/SNX25 as a regulator of autophagic flux and reveals differential isoform expression between paralogs.

Integrative structural modelling and visualisation of a cellular organelle

Models of insulin secretory vesicles from pancreatic beta cells have been created using the cellPACK suite of tools to research, curate, construct and visualise the current state of knowledge. The model integrates experimental information from proteomics, structural biology, cryoelectron microscopy and X-ray tomography, and is used to generate models of mature and immature vesicles. A new method was developed to generate a confidence score that reconciles inconsistencies between three available proteomes using expert annotations of cellular localisation. The models are used to simulate soft X-ray tomograms, allowing quantification of features that are observed in experimental tomograms, and in turn, allowing interpretation of X-ray tomograms at the molecular level.

Cavβ3 Regulates Ca2+ Signaling and Insulin Expression in Pancreatic β-Cells in a Cell-Autonomous Manner

Voltage-gated Ca²⁺ (Cav) channels consist of a pore-forming Cavα1 subunit and auxiliary Cavα2-δ and Cavβ subunits. In fibroblasts, Cavβ3, independent of its role as a Cav subunit, reduces the sensitivity to low concentrations of inositol-1,4,5-trisphosphate (IP₃). Similarly, Cavβ3 could affect cytosolic calcium concentration ([Ca^{2 +}]) in pancreatic β-cells. In this study, we deleted the Cavβ3-encoding gene Cacnb3 in insulin-secreting rat β-(Ins-1) cells using CRISPR/Cas9. These cells were used as controls to investigate the role of Cavβ3 on Ca²⁺ signaling, glucose-induced insulin secretion (GIIS), Cav channel activity, and gene expression in wild-type cells in which Cavβ3 and the IP₃ receptor were coimmunoprecipitated. Transcript and protein profiling revealed significantly increased levels of insulin transcription factor Mafa, CaMKIV, proprotein convertase subtilisin/kexin type-1, and nitric oxide synthase-1 in Cavβ3-knockout cells. In the absence of Cavβ3, Cav currents were not altered. In contrast, CREB activity, the amount of MAFA protein and GIIS, the extent of IP₃-dependent Ca²⁺ release and the frequency of Ca²⁺ oscillations were increased. These processes were decreased by the Cavβ3 protein in a concentration-dependent manner. Our study shows that Cavβ3 interacts with the IP₃ receptor in isolated β-cells, controls IP₃-dependent Ca²⁺-signaling independently of Cav channel functions, and thereby regulates insulin expression and its glucose-dependent release in a cell-autonomous manner.

Pseudotime Ordering Single-Cell Transcriptomic of β Cells Pancreatic Islets in Health and Type 2 Diabetes

β cells are defined by the ability to produce and secret insulin. Recent studies have evaluated that human pancreatic β cells are heterogeneous and demonstrated the transcript alterations of β cell subpopulation in diabetes. Single-cell RNA sequence (scRNA-seq) analysis helps us to refine the cell types signatures and understand the role of the β cells during metabolic challenges and diseases. Here, we construct the pseudotime trajectory of β cells from publicly available scRNA-seq data in health and type 2 diabetes (T2D) based on highly dispersed and highly expressed genes using Monocle2. We identified three major states including 1) Normal branch, 2) Obesity-like branch and 3) T2D-like branch based on biomarker genes and genes that give rise to bifurcation in the trajectory. β cell function-maintain-related genes, insulin expression-related genes, and T2D-related genes enriched in three branches, respectively. Continuous pseudotime spectrum might suggest that β cells transition among different states. The application of pseudotime analysis is conducted to clarify the different cell states, providing novel insights into the pathology of β cells in T2D.

Supplementary Information

The online version contains supplementary material is available at 10.1007/s43657-021-00024-z.

Dynamin deficiency causes insulin secretion failure and hyperglycemia

Pancreatic β cells operate with a high rate of membrane recycling for insulin secretion, yet endocytosis in these cells is not fully understood. We investigate this process in mature mouse β cells by genetically deleting dynamin GTPase, the membrane fission machinery essential for clathrin-mediated endocytosis. Unexpectedly, the mice lacking all three dynamin genes (DNM1, DNM2, DNM3) in their β cells are viable, and their β cells still contain numerous insulin granules. Endocytosis in these β cells is severely impaired, resulting in abnormal endocytic intermediates on the plasma membrane. Although insulin granules are abundant, their release upon glucose stimulation is blunted in both the first and second phases, leading to hyperglycemia and glucose intolerance in mice. Dynamin triple deletion impairs insulin granule exocytosis and decreases intracellular Ca2+ responses and granule docking. The docking defect is correlated with reduced expression of Munc13-1 and RIM1 and reorganization of cortical F-actin in β cells. Collectively, these findings uncover the role of dynamin in dense-core vesicle endocytosis and secretory capacity. Insulin secretion deficiency in the absence of dynamin-mediated endocytosis highlights the risk of impaired membrane trafficking in endocrine failure and diabetes pathogenesis.

Clearance of intracellular tau protein from neuronal cells via VAMP8-induced secretion

In Alzheimer's disease (AD), tau, a microtubule-associated protein (MAP), becomes hyperphosphorylated, aggregates, and accumulates in the somato-dendritic compartment of neurons. In parallel to its intracellular accumulation in AD, tau is also released in the extracellular space, as revealed by its increased presence in cerebrospinal fluid (CSF). Consistent with this, recent studies, including ours, have reported that neurons secrete tau, and several therapeutic strategies aim to prevent the intracellular tau accumulation. Previously, we reported that late endosomes were implicated in tau secretion. Here, we explore the possibility of preventing intracellular tau accumulation by increasing tau secretion. Using neuronal models, we investigated whether overexpression of the vesicle-associated membrane protein 8 (VAMP8), an R-SNARE found on late endosomes, could increase tau secretion. The overexpression of VAMP8 significantly increased tau secretion, decreasing its intracellular levels in the neuroblastoma (N2a) cell line. Increased tau secretion by VAMP8 was also observed in murine hippocampal slices. The intracellular reduction of tau by VAMP8 overexpression correlated to a decrease of acetylated tubulin induced by tau overexpression in N2a cells. VAMP8 staining was preferentially found on late endosomes in N2a cells. Using total internal reflection fluorescence (TIRF) microscopy, the fusion of VAMP8-positive vesicles with the plasma membrane was correlated to the depletion of tau in the cytoplasm. Finally, overexpression of VAMP8 reduced the intracellular accumulation of tau mutants linked to frontotemporal dementia with parkinsonism and α-synuclein by increasing their secretion. Collectively, the present data indicate that VAMP8 could be used to increase tau and α-synuclein clearance to prevent their intracellular accumulation.

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.

Insulin granule biogenesis and exocytosis

Insulin is produced by pancreatic β-cells, and once released to the blood, the hormone stimulates glucose uptake and suppresses glucose production. Defects in both the availability and action of insulin lead to elevated plasma glucose levels and are major hallmarks of type-2 diabetes. Insulin is stored in secretory granules that form at the trans-Golgi network. The granules undergo extensive modifications en route to their release sites at the plasma membrane, including changes in both protein and lipid composition of the granule membrane and lumen. In parallel, the insulin molecules also undergo extensive modifications that render the hormone biologically active. In this review, we summarize current understanding of insulin secretory granule biogenesis, maturation, transport, docking, priming and eventual fusion with the plasma membrane. We discuss how different pools of granules form and how these pools contribute to insulin secretion under different conditions. We also highlight the role of the β-cell in the development of type-2 diabetes and discuss how dysregulation of one or several steps in the insulin granule life cycle may contribute to disease development or progression.

Association of KLK3, VAMP8 and MDM4 Genetic Variants within microRNA Binding Sites with Prostate Cancer: Evidence from Serbian Population

A growing number of studies have suggested that genetic variants affecting the micro-RNA- binding mechanisms (miRSNPs) constitute a promising novel class of biomarkers for prostate cancer (PCa) biology. Among the most extensively studied miRSNPs in the context of cancer is the variation rs4245739 in the MDM4 gene, while a recent large-scale analysis revealed significant differences in genotype distributions between aggressive and non-aggressive disease for rs1058205 in KLK3 and rs1010 in VAMP8. In this study, we examined a total of 1083 subjects for these three variants using Taqman® SNP Genotyping Assays. Three hundred and fifty-five samples of peripheral blood were obtained from patients with PCa and 358 samples from patients with benign prostatic hyperplasia (BPH). The control group consisted of 370 healthy volunteers. Comparisons of genotype distributions among PCa and BPH patients, as well as between PCa patients and healthy controls, yielded no evidence of association between the analyzed genetic variants and the risk of developing PCa. However, all three tested genetic variants have shown the association with the parameters of PCa progression. For KLK3 variant rs1058205, minor allele C was found to associate with the lower serum PSA score in PCa patients (PSA > 20 ng/ml vs. PSA < 10 ng/ml comparison, Prec = 0.038; ORrec = 0.20, 95%CI 0.04-1.05). The obtained results point out the potential relevance of the tested genetic variants for the disease aggressiveness assessment.

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.

Spatiotemporal organization and protein dynamics involved in regulated exocytosis of MMP-9 in breast cancer cells

This paper describes the dynamics of proteins and lipids during exocytosis of MMP-9 from cancer cells in real time using fluorescence microscopy. Stephens et al. find that core exocytic proteins, accessory proteins, and lipids are involved at sites of secretory vesicle fusion.

Altered regulation of exocytosis is an important mechanism controlling many diseases, including cancer. Defects in exocytosis have been implicated in many cancer cell types and are generally attributed to mutations in cellular transport, trafficking, and assembly of machinery necessary for exocytosis of secretory vesicle cargo. In these cancers, up-regulation of trafficking and secretion of matrix metalloproteinase-9 (MMP-9), a proteolytic enzyme, is responsible for degrading the extracellular matrix, a necessary step in tumor progression. Using TIRF microscopy, we identified proteins associated with secretory vesicles containing MMP-9 and imaged the local dynamics of these proteins at fusion sites during regulated exocytosis of MMP-9 from MCF-7 breast cancer cells. We found that many regulators of exocytosis, including several Rab GTPases, Rab effector proteins, and SNARE/SNARE modulator proteins, are stably assembled on docked secretory vesicles before exocytosis. At the moment of fusion, many of these components are quickly lost from the vesicle, while several endocytic proteins and lipids are simultaneously recruited to exocytic sites at precisely that moment. Our findings provide insight into the dynamic behavior of key core exocytic proteins, accessory proteins, lipids, and some endocytic proteins at single sites of secretory vesicle fusion in breast cancer cells.

Significance of Vesicle-Associated Membrane Protein 8 Expression in Predicting Survival in Breast Cancer

Purpose

Vesicle-associated membrane protein 8 (VAMP8) is a soluble N-ethylmaleimide-sensitive factor receptor protein that participates in autophagy by directly regulating autophagosome membrane fusion and has been reported to be involved in tumor progression. Nevertheless, the expression and prognostic value of VAMP8 in breast cancer (BC) remain unknown. This study aimed to evaluate the clinical significance and biological function of VAMP8 in BC.

Methods

A total of 112 BC samples and 30 normal mammary gland samples were collected. The expression of VAMP8 was assessed in both BC tissues and normal mammary gland tissues via a two-step immunohistochemical detection method.

Results

The expression of VAMP8 in BC tissues was significantly higher than that in normal breast tissues. Furthermore, increased VAMP8 expression was significantly correlated with tumor size (p=0.007), lymph node metastasis (p=0.024) and recurrence (p=0.001). Patients with high VAMP8 expression had significantly lower cumulative recurrence-free survival and overall survival (p<0.001 for both) than patients with low VAMP8 expression. In multivariate logistic regression and Cox regression analyses, lymph node metastasis and VAMP8 expression were independent prognostic factors for BC.

Conclusion

VAMP8 is significantly upregulated in human BC tissues and can thus be a practical and potentially effective surrogate marker for survival in BC patients.

VAMP8, a vesicle-SNARE required for RAB37-mediated exocytosis, possesses a tumor metastasis suppressor function

We previously identified a metastasis suppressor RAB37 small GTPase that regulated exocytosis of tissue inhibitor of metalloproteinases 1 (TIMP1) to suppress lung cancer metastasis. Here, we show that vesicle-associated membrane protein 8 (VAMP8), a v-SNARE (vesicle soluble N-ethylmaleimide-sensitive factor activating protein receptor), interacts with RAB37 and drives the secretion of TIMP1 to inhibit tumor metastases. Confocal and total internal reflection fluorescence microscopic images demonstrated that VAMP8 co-localized with RAB37 and facilitated trafficking of RAB37-TIMP1 vesicles. Reconstitution experiments using tail-vein injection and lung-to-lung metastasis in mice showed that VAMP8 was essential for RAB37-regulated vesicle trafficking of TIMP1 to suppress cancer metastasis. Lung cancer patients with low VAMP8 showed distant metastasis, poor overall survival and progression-free survival. Importantly, multivariate Cox regression analysis indicated that patients with low VAMP8/low RAB37 expression profile showed significantly high risk of death (hazard ratio = 3.42, P < 0.001) even after adjusting for tumor metastasis parameter. Our findings reveal that VAMP8 is a novel v-SNARE crucial for RAB37-mediated exocytic transport of TIMP1 to suppress lung tumor metastasis. VAMP8 possesses a tumor metastasis suppressor function with a prognostic value in lung cancer.

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.

Kv2.1 clusters on β-cell plasma membrane act as reservoirs that replenish pools of newcomer insulin granule through their interaction with syntaxin-3

The voltage-dependent K⁺ (K_v) channel K_v2.1 is a major delayed rectifier in many secretory cells, including pancreatic β cells. In addition, K_v2.1 has a direct role in exocytosis at an undefined step, involving SNARE proteins, that is independent of its ion-conducting pore function. Here, we elucidated the precise step in exocytosis. We previously reported that syntaxin-3 (Syn-3) is the key syntaxin that mediates exocytosis of newcomer secretory granules that spend minimal residence time on the plasma membrane before fusion. Using high-resolution total internal reflection fluorescence microscopy, we now show that K_v2.1 forms reservoir clusters on the β-cell plasma membrane and binds Syn-3 via its C-terminal C1b domain, which recruits newcomer insulin secretory granules into this large reservoir. Upon glucose stimulation, secretory granules were released from this reservoir to replenish the pool of newcomer secretory granules for subsequent fusion, occurring just adjacent to the plasma membrane K_v2.1 clusters. C1b deletion blocked the aforementioned K_v2.1-Syn-3-mediated events and reduced fusion of newcomer secretory granules. These insights have therapeutic implications, as K_v2.1 overexpression in type-2 diabetes rat islets restored biphasic insulin secretion.

Pancreatic β-Cell Electrical Activity and Insulin Secretion: Of Mice and Men

The pancreatic β-cell plays a key role in glucose homeostasis by secreting insulin, the only hormone capable of lowering the blood glucose concentration. Impaired insulin secretion results in the chronic hyperglycemia that characterizes type 2 diabetes (T2DM), which currently afflicts >450 million people worldwide. The healthy β-cell acts as a glucose sensor matching its output to the circulating glucose concentration. It does so via metabolically induced changes in electrical activity, which culminate in an increase in the cytoplasmic Ca2+ concentration and initiation of Ca2+-dependent exocytosis of insulin-containing secretory granules. Here, we review recent advances in our understanding of the β-cell transcriptome, electrical activity, and insulin exocytosis. We highlight salient differences between mouse and human β-cells, provide models of how the different ion channels contribute to their electrical activity and insulin secretion, and conclude by discussing how these processes become perturbed in T2DM.

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.

Molecular regulation of insulin granule biogenesis and exocytosis

Type 2 diabetes mellitus (T2DM) is a metabolic disorder characterized by hyperglycemia, insulin resistance and hyperinsulinemia in early disease stages but a relative insulin insufficiency in later stages. Insulin, a peptide hormone, is produced in and secreted from pancreatic β-cells following elevated blood glucose levels. Upon its release, insulin induces the removal of excessive exogenous glucose from the bloodstream primarily by stimulating glucose uptake into insulin-dependent tissues as well as promoting hepatic glycogenesis. Given the increasing prevalence of T2DM worldwide, elucidating the underlying mechanisms and identifying the various players involved in the synthesis and exocytosis of insulin from β-cells is of utmost importance. This review summarizes our current understanding of the route insulin takes through the cell after its synthesis in the endoplasmic reticulum as well as our knowledge of the highly elaborate network that controls insulin release from the β-cell. This network harbors potential targets for anti-diabetic drugs and is regulated by signaling cascades from several endocrine systems.

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.

Exocytosis proteins as novel targets for diabetes prevention and/or remediation?

Diabetes remains one of the leading causes of morbidity and mortality worldwide, affecting an estimated 422 million adults. In the US, it is predicted that one in every three children born as of 2000 will suffer from diabetes in their lifetime. Type 2 diabetes results from combinatorial defects in pancreatic β-cell glucose-stimulated insulin secretion and in peripheral glucose uptake. Both processes, insulin secretion and glucose uptake, are mediated by exocytosis proteins, SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor) complexes, Sec1/Munc18 (SM), and double C2-domain protein B (DOC2B). Increasing evidence links deficiencies in these exocytosis proteins to diabetes in rodents and humans. Given this, emerging studies aimed at restoring and/or enhancing cellular levels of certain exocytosis proteins point to promising outcomes in maintaining functional β-cell mass and enhancing insulin sensitivity. In doing so, new evidence also shows that enhancing exocytosis protein levels may promote health span and longevity and may also harbor anti-cancer and anti-Alzheimer's disease capabilities. Herein, we present a comprehensive review of the described capabilities of certain exocytosis proteins and how these might be targeted for improving metabolic dysregulation.

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.

Key proteins involved in insulin vesicle exocytosis and secretion

In vivo insulin secretion is predominantly affected by blood glucose concentration, blood concentration of amino acids, gastrointestinal hormones and free nerve functional status, in addition to other factors. Insulin is one of the most important hormones in the body, and its secretion is precisely controlled by nutrients, neurotransmitters and hormones. The insulin exocytosis process is similar to the neurotransmitter release mechanism. There are various types of proteins and lipids that participate in the insulin secretory vesicle fusion process, such as soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) protein, Ras-related proteins and vacuolar-type H⁺-ATPase (V-ATPase). Notably, the SNARE protein is the molecular basis of exocytotic activity. In the current review, the role of the vesicle membrane proteins (synaptobrevins, vesicle associated membrane proteins and target membrane proteins) and auxiliary proteins (Rab proteins and Munc-18 proteins) in vesicle fusion activity were summarized. A summary of these key proteins involved in insulin granule secretion will facilitate understanding of the pathogenesis of diabetes.

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.

VAMP2 is implicated in the secretion of antibodies by human plasma cells and can be replaced by other synaptobrevins

The production and secretion of antibodies by human plasma cells (PCs) are two essential processes of humoral immunity. The secretion process relies on a group of proteins known as soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs), which are located in the plasma membrane (t-SNAREs) and in the antibody-carrying vesicle membrane (v-SNARE), and mediate the fusion of both membranes. We have previously shown that SNAP23 and STX4 are the t-SNAREs responsible for antibody secretion. Here, using human PCs and antibody-secreting cell lines, we studied and characterized the expression and subcellular distribution of vesicle associated membrane protein (VAMP) isoforms, demonstrating that all isoforms (with the exception of VAMP1) are expressed by the referenced cells. Furthermore, the functional role in antibody secretion of each expressed VAMP isoform was tested using siRNA. Our results show that VAMP2 may be the v-SNARE involved in vesicular antibody release. To further support this conclusion, we used tetanus toxin light chain to cleave VAMP2, conducted experiments to verify co-localization of VAMP2 in antibody-carrying vesicles, and demonstrated the coimmunoprecipitation of VAMP2 with STX4 and SNAP23 and the in situ interaction of VAMP2 with STX4. Taken together, these findings implicate VAMP2 as the main VAMP isoform functionally involved in antibody secretion.

Synaptotagmin-7 Functions to Replenish Insulin Granules for Exocytosis in Human Islet β-Cells

Synaptotagmin (Syt)-7, a major component of the exocytotic machinery in neurons, is also the major Syt in rodent pancreatic β-cells shown to mediate glucose-stimulated insulin secretion (GSIS). However, Syt-7's precise exocytotic actions in β-cells remain unknown. We show that Syt-7 is abundant in human β-cells. Adenovirus-short hairpin RNA knockdown (KD) of Syt-7 in human islets reduced first- and second-phase GSIS attributed to the reduction of exocytosis of predocked and newcomer insulin secretory granules (SGs). Glucose stimulation expectedly induced Syt-7 association in a Ca(2+)-dependent manner with syntaxin-3 and syntaxin-1A soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) complexes known to mediate exocytosis of newcomer and predocked SGs, respectively. However, Syt-7-KD did not disrupt SNARE complex assembly. Instead, electron microscopy analysis showed that Syt-7-KD reduced the recruitment of SGs to the plasma membrane after glucose-stimulated depletion, which could not be rescued by glucagon-like peptide 1 pretreatment. To assess the possibility that this new action of Syt-7 on SG recruitment may involve calmodulin (CaM), pretreatment of islets with CaM blocker calmidazolium showed effects very similar to those of Syt-7-KD. Syt-7 therefore plays a novel more dominant function in the replenishment of releasable SG pools in human β-cells than its previously purported role in exocytotic fusion per se.

VAMP7 Regulates Autophagy to Maintain Mitochondrial Homeostasis and to Control Insulin Secretion in Pancreatic β-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 β-cells is largely unknown. To elucidate the physiological role of VAMP7 in β-cells, we generated pancreatic β-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 β-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 β-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 β-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 β-cells.

Imaging analysis of insulin secretion with two-photon microscopy

High-resolution deep tissue imaging is possible with two-photon excitation microscopy. With the combined application of two-photon imaging and perfusion with a polar fluorescent tracer, we have established a method to detect exocytic events inside secretory tissues. This method displays the spatiotemporal distribution of exocytic sites, dynamics of fusion pores, and modes of exocytosis. In glucose-stimulated pancreatic islets, exocytic events were observed to be synchronized with an increase in cytosolic Ca(2+) concentrations. Full fusion of a single secretory granule is the typical mode of exocytosis and compound exocytosis is inhibited. Because two-photon excitation enables simultaneous multicolor imaging due to the broadened excitation spectra, the distributions and conformational changes in fluorescent-labeled molecules can be simultaneously visualized with exocytic events. Therefore, we can analyze the dynamics of the molecules involved in membrane fusion and their association with exocytosis in living tissues.

Pancreatic regulation of glucose homeostasis

In order to ensure normal body function, the human body is dependent on a tight control of its blood glucose levels. This is accomplished by a highly sophisticated network of various hormones and neuropeptides released mainly from the brain, pancreas, liver, intestine as well as adipose and muscle tissue. Within this network, the pancreas represents a key player by secreting the blood sugar-lowering hormone insulin and its opponent glucagon. However, disturbances in the interplay of the hormones and peptides involved may lead to metabolic disorders such as type 2 diabetes mellitus (T2DM) whose prevalence, comorbidities and medical costs take on a dramatic scale. Therefore, it is of utmost importance to uncover and understand the mechanisms underlying the various interactions to improve existing anti-diabetic therapies and drugs on the one hand and to develop new therapeutic approaches on the other. This review summarizes the interplay of the pancreas with various other organs and tissues that maintain glucose homeostasis. Furthermore, anti-diabetic drugs and their impact on signaling pathways underlying the network will be discussed.

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.

Spatial and temporal coordination of insulin granule exocytosis in intact human pancreatic islets

AIMS/HYPOTHESIS

Insulin secretion is widely studied because it plays a central role in glucose homeostasis and diabetes. Processes from insulin granule fusion in beta cells to in vivo insulin secretion have been elucidated, but data at the cellular level do not fully account for several aspects of the macroscopic secretory pattern. Here we investigated how individual secretory events are coordinated spatially and temporally within intact human islets.

METHODS

We used the fluorescent probe neuropeptide Y (NPY)-pHluorin to visualise insulin granule secretion in isolated intact human islets.

RESULTS

We found that individual beta cells respond to increases in glucose concentration by releasing insulin granules in very discrete bursts with periods consistent with in vivo pulsatile insulin secretion. In successive secretory bursts during prolonged exposure to high glucose levels, secretory events progressively localised to preferential release sites, coinciding with the transition to second phase insulin secretion. Granule secretion was very synchronised in neighbouring beta cells, forming discrete regional clusters of activity.

CONCLUSIONS/INTERPRETATION

These results reveal how individual secretory events are coordinated to produce pulsatile insulin secretion from human islets.

Spatiotemporal detection and analysis of exocytosis reveal fusion "hotspots" organized by the cytoskeleton in endocrine cells

Total internal reflection fluorescence microscope has often been used to study the molecular mechanisms underlying vesicle exocytosis. However, the spatial occurrence of the fusion events within a single cell is not frequently explored due to the lack of sensitive and accurate computer-assisted programs to analyze large image data sets. Here, we have developed an image analysis platform for the nonbiased identification of different types of vesicle fusion events with high accuracy in different cell types. By performing spatiotemporal analysis of stimulus-evoked exocytosis in insulin-secreting INS-1 cells, we statistically prove that individual vesicle fusion events are clustered at hotspots. This spatial pattern disappears upon the disruption of either the actin or the microtubule network; this disruption also severely inhibits evoked exocytosis. By demonstrating that newcomer vesicles are delivered from the cell interior to the surface membrane for exocytosis, we highlight a previously unappreciated mechanism in which the cytoskeleton-dependent transportation of secretory vesicles organizes exocytosis hotspots in endocrine cells.

Dynamin 2 regulates biphasic insulin secretion and plasma glucose homeostasis

Alterations in insulin granule exocytosis and endocytosis are paramount to pancreatic β cell dysfunction in diabetes mellitus. Here, using temporally controlled gene ablation specifically in β cells in mice, we identified an essential role of dynamin 2 GTPase in preserving normal biphasic insulin secretion and blood glucose homeostasis. Dynamin 2 deletion in β cells caused glucose intolerance and substantial reduction of the second phase of glucose-stimulated insulin secretion (GSIS); however, mutant β cells still maintained abundant insulin granules, with no signs of cell surface expansion. Compared with control β cells, real-time capacitance measurements demonstrated that exocytosis-endocytosis coupling was less efficient but not abolished; clathrin-mediated endocytosis (CME) was severely impaired at the step of membrane fission, which resulted in accumulation of clathrin-coated endocytic intermediates on the plasma membrane. Moreover, dynamin 2 ablation in β cells led to striking reorganization and enhancement of actin filaments, and insulin granule recruitment and mobilization were impaired at the later stage of GSIS. Together, our results demonstrate that dynamin 2 regulates insulin secretory capacity and dynamics in vivo through a mechanism depending on CME and F-actin remodeling. Moreover, this study indicates a potential pathophysiological link between endocytosis and diabetes mellitus.

Synaptotagmin-7 phosphorylation mediates GLP-1-dependent potentiation of insulin secretion from β-cells

Glucose stimulates insulin secretion from β-cells by increasing intracellular Ca(2+). Ca(2+) then binds to synaptotagmin-7 as a major Ca(2+) sensor for exocytosis, triggering secretory granule fusion and insulin secretion. In type-2 diabetes, insulin secretion is impaired; this impairment is ameliorated by glucagon-like peptide-1 (GLP-1) or by GLP-1 receptor agonists, which improve glucose homeostasis. However, the mechanism by which GLP-1 receptor agonists boost insulin secretion remains unclear. Here, we report that GLP-1 stimulates protein kinase A (PKA)-dependent phosphorylation of synaptotagmin-7 at serine-103, which enhances glucose- and Ca(2+)-stimulated insulin secretion and accounts for the improvement of glucose homeostasis by GLP-1. A phospho-mimetic synaptotagmin-7 mutant enhances Ca(2+)-triggered exocytosis, whereas a phospho-inactive synaptotagmin-7 mutant disrupts GLP-1 potentiation of insulin secretion. Our findings thus suggest that synaptotagmin-7 is directly activated by GLP-1 signaling and may serve as a drug target for boosting insulin secretion. Moreover, our data reveal, to our knowledge, the first physiological modulation of Ca(2+)-triggered exocytosis by direct phosphorylation of a synaptotagmin.

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

AIMS/HYPOTHESIS

Of the four exocytotic syntaxins (Syns), much is now known about the role of Syn-1A (pre-docked secretory granules [SGs]) and Syn-3 (newcomer SGs) in insulin exocytosis. Some work was reported on Syn-4's role in biphasic glucose-stimulated insulin secretion (GSIS), but its precise role in insulin SG exocytosis remains unclear. In this paper we examine this role in human beta cells.

METHODS

Endogenous function of Syn-4 in human islets was assessed by knocking down its expression with lentiviral single hairpin RNA (lenti-shRNA)-RFP. Biphasic GSIS was determined by islet perifusion assay. Single-cell analysis of exocytosis of red fluorescent protein (RFP)-positive beta cells (exhibiting near-total depletion of Syn-4) was by patch clamp capacitance measurements (Cm) and total internal reflection fluorescence microscopy (TIRFM), the latter to further assess single SG behaviour. Co-immunoprecipitations were conducted on INS-1 cells to assess exocytotic complexes.

RESULTS

Syn-4 knockdown (KD) of 77% in human islets caused a concomitant reduction in cognate Munc18c expression (46%) without affecting expression of other exocytotic proteins; this resulted in reduction of GSIS in the first phase (by 42%) and the second phase (by 40%). Cm of RFP-tagged Syn-4-KD beta cells showed severe inhibition in the readily releasable pool (by 71%) and mobilisation from reserve pools (by 63%). TIRFM showed that Syn-4-KD-induced inhibition of first-phase GSIS was attributed to reduction in exocytosis of both pre-docked and newcomer SGs (which undergo minimal residence or docking time at the plasma membrane before fusion). Second-phase inhibition was attributed to reduction in newcomer SGs. Stx-4 co-immunoprecipitated Munc18c, VAMP2 and VAMP8, suggesting that these exocytotic complexes may be involved in exocytosis of pre-docked and newcomer SGs.

CONCLUSIONS/INTERPRETATION

Syn-4 is involved in distinct molecular machineries that influence exocytosis of both pre-docked and newcomer SGs in a manner functionally redundant to Syn-1A and Syn-3, respectively; this underlies Syn-4's role in mediating portions of first-phase and second-phase GSIS.

Characterization of Zinc Influx Transporters (ZIPs) in Pancreatic β Cells: ROLES IN REGULATING CYTOSOLIC ZINC HOMEOSTASIS AND INSULIN SECRETION

Zinc plays an essential role in the regulation of pancreatic β cell function, affecting important processes including insulin biosynthesis, glucose-stimulated insulin secretion, and cell viability. Mutations in the zinc efflux transport protein ZnT8 have been linked with both type 1 and type 2 diabetes, further supporting an important role for zinc in glucose homeostasis. However, very little is known about how cytosolic zinc is controlled by zinc influx transporters (ZIPs). In this study, we examined the β cell and islet ZIP transcriptome and show consistent high expression of ZIP6 (Slc39a6) and ZIP7 (Slc39a7) genes across human and mouse islets and MIN6 β cells. Modulation of ZIP6 and ZIP7 expression significantly altered cytosolic zinc influx in pancreatic β cells, indicating an important role for ZIP6 and ZIP7 in regulating cellular zinc homeostasis. Functionally, this dysregulated cytosolic zinc homeostasis led to impaired insulin secretion. In parallel studies, we identified both ZIP6 and ZIP7 as potential interacting proteins with GLP-1R by a membrane yeast two-hybrid assay. Knock-down of ZIP6 but not ZIP7 in MIN6 β cells impaired the protective effects of GLP-1 on fatty acid-induced cell apoptosis, possibly via reduced activation of the p-ERK pathway. Therefore, our data suggest that ZIP6 and ZIP7 function as two important zinc influx transporters to regulate cytosolic zinc concentrations and insulin secretion in β cells. In particular, ZIP6 is also capable of directly interacting with GLP-1R to facilitate the protective effect of GLP-1 on β cell survival.

A Large-Scale Analysis of Genetic Variants within Putative miRNA Binding Sites in Prostate Cancer

Prostate cancer is the second most common malignancy among men worldwide. Genome-wide association studies have identified 100 risk variants for prostate cancer, which can explain approximately 33% of the familial risk of the disease. We hypothesized that a comprehensive analysis of genetic variations found within the 3' untranslated region of genes predicted to affect miRNA binding (miRSNP) can identify additional prostate cancer risk variants. We investigated the association between 2,169 miRSNPs and prostate cancer risk in a large-scale analysis of 22,301 cases and 22,320 controls of European ancestry from 23 participating studies. Twenty-two miRSNPs were associated (P<2.3×10(-5)) with risk of prostate cancer, 10 of which were within 7 genes previously not mapped by GWAS studies. Further, using miRNA mimics and reporter gene assays, we showed that miR-3162-5p has specific affinity for the KLK3 rs1058205 miRSNP T-allele, whereas miR-370 has greater affinity for the VAMP8 rs1010 miRSNP A-allele, validating their functional role.

SIGNIFICANCE

Findings from this large association study suggest that a focus on miRSNPs, including functional evaluation, can identify candidate risk loci below currently accepted statistical levels of genome-wide significance. Studies of miRNAs and their interactions with SNPs could provide further insights into the mechanisms of prostate cancer risk.

Munc18c mediates exocytosis of pre-docked and newcomer insulin granules underlying biphasic glucose stimulated insulin secretion in human pancreatic beta-cells

Objective

Pancreatic beta-cells express three Munc18 isoforms. Much is known about the roles of Munc18a (pre-docked secretory granules-SGs) and Munc18b (newcomer SGs and SG–SG fusion) in insulin exocytosis. Although shown to influence glucose-stimulated insulin secretion (GSIS) in rodents the precise role of Munc18c in insulin SG exocytosis has not been elucidated. We here examined the role of Munc18c in human pancreatic beta-cells.

Methods

Munc18c-shRNA/RFP lenti-virus (versus control virus) was used to knock down the expression level of Munc18c in human islets or single beta-cells. Insulin secretion and granule exocytosis were measured by performing islets perifusion, single-cell patch clamp capacitance measurements and total internal reflection fluorescence microscopy (TIRFM).

Results

Munc18c is most abundant in the cytosol of human beta-cells. Endogenous function of Munc18c was assessed by knocking down (KD) its islet expression by 70% employing lenti-shRNA virus. Munc18c-KD caused reduction in cognate syntaxin-4 islet expression but not in other exocytotic proteins, resulting in the reduction in GSIS in first- (by 42%) and second phases (by 35%). Single cell analyses of RFP-tagged Munc18c-KD beta-cells by patch clamp capacitance measurements showed inhibition in both readily-releasable pool (by 52%) and mobilization from the reserve pool (by 57%). TIRFM to assess single SG behavior showed that Munc18c-KD inhibition of first phase GSIS was attributed to reduction in exocytosis of pre-docked and newcomer SGs, and second phase inhibition attributed entirely to reduction in newcomer SG fusion (SGs that undergo minimal residence or docking time at the plasma membrane before fusion).

Conclusion

Munc18c is involved in the distinct molecular machineries that affect exocytosis of both predocked and newcomer SG pools that underlie Munc18c's role in first and second phases of GSIS, respectively.