Syntaxin-3 and syntaxin-1A inhibit L-type calcium channel activity, insulin biosynthesis and exocytosis in beta-cell lines

Artificial intelligence-enhanced electrocardiography derived body mass index as a predictor of future cardiometabolic disease

The electrocardiogram (ECG) can capture obesity-related cardiac changes. Artificial intelligence-enhanced ECG (AI-ECG) can identify subclinical disease. We trained an AI-ECG model to predict body mass index (BMI) from the ECG alone. Developed from 512,950 12-lead ECGs from the Beth Israel Deaconess Medical Center (BIDMC), a secondary care cohort, and validated on UK Biobank (UKB) (n = 42,386), the model achieved a Pearson correlation coefficient (r) of 0.65 and 0.62, and an R2 of 0.43 and 0.39 in the BIDMC cohort and UK Biobank, respectively for AI-ECG BMI vs. measured BMI. We found delta-BMI, the difference between measured BMI and AI-ECG-predicted BMI (AI-ECG-BMI), to be a biomarker of cardiometabolic health. The top tertile of delta-BMI showed increased risk of future cardiometabolic disease (BIDMC: HR 1.15, p < 0.001; UKB: HR 1.58, p < 0.001) and diabetes mellitus (BIDMC: HR 1.25, p < 0.001; UKB: HR 2.28, p < 0.001) after adjusting for covariates including measured BMI. Significant enhancements in model fit, reclassification and improvements in discriminatory power were observed with the inclusion of delta-BMI in both cohorts. Phenotypic profiling highlighted associations between delta-BMI and cardiometabolic diseases, anthropometric measures of truncal obesity, and pericardial fat mass. Metabolic and proteomic profiling associates delta-BMI positively with valine, lipids in small HDL, syntaxin-3, and carnosine dipeptidase 1, and inversely with glutamine, glycine, colipase, and adiponectin. A genome-wide association study revealed associations with regulators of cardiovascular/metabolic traits, including SCN10A, SCN5A, EXOG and RXRG. In summary, our AI-ECG-BMI model accurately predicts BMI and introduces delta-BMI as a non-invasive biomarker for cardiometabolic risk stratification.

Syntaxin1A overexpression and pain insensitivity in individuals with 7q11.23 duplication syndrome

Genetic modifications leading to pain insensitivity phenotypes, while rare, provide invaluable insights into the molecular biology of pain and reveal targets for analgesic drugs. Pain insensitivity typically results from Mendelian loss-of-function mutations in genes expressed in nociceptive (pain-sensing) dorsal root ganglion (DRG) neurons that connect the body to the spinal cord. We document a pain insensitivity mechanism arising from gene overexpression in individuals with the rare 7q11.23 duplication syndrome (Dup7), who have 3 copies of the approximately 1.5-megabase Williams syndrome (WS) critical region. Based on parental accounts and pain ratings, people with Dup7, mainly children in this study, are pain insensitive following serious injury to skin, bones, teeth, or viscera. In contrast, diploid siblings (2 copies of the WS critical region) and individuals with WS (1 copy) show standard reactions to painful events. A converging series of human assessments and cross-species cell biological and transcriptomic studies identified 1 likely candidate in the WS critical region, STX1A, as underlying the pain insensitivity phenotype. STX1A codes for the synaptic vesicle fusion protein syntaxin1A. Excess syntaxin1A was demonstrated to compromise neuropeptide exocytosis from nociceptive DRG neurons. Taken together, these data indicate a mechanism for producing "genetic analgesia" in Dup7 and offer previously untargeted routes to pain control.

The adipocyte-enriched secretory protein tetranectin exacerbates type 2 diabetes by inhibiting insulin secretion from β cells

Pancreatic β cell failure is a hallmark of diabetes. However, the causes of β cell failure remain incomplete. Here, we report the identification of tetranectin (TN), an adipose tissue-enriched secretory molecule, as a negative regulator of insulin secretion in β cells in diabetes. TN expression is stimulated by high glucose in adipocytes via the p38 MAPK/TXNIP/thioredoxin/OCT4 signaling pathway, and elevated serum TN levels are associated with diabetes. TN treatment greatly exacerbates hyperglycemia in mice and suppresses glucose-stimulated insulin secretion in islets. Conversely, knockout of TN or neutralization of TN function notably improves insulin secretion and glucose tolerance in high-fat diet-fed mice. Mechanistically, TN binds with high selectivity to β cells and inhibits insulin secretion by blocking L-type Ca²⁺ channels. Our study uncovers an adipocyte-β cell cross-talk that contributes to β cell dysfunction in diabetes and suggests that neutralization of TN levels may provide a new treatment strategy for type 2 diabetes.

Exocytosis Proteins: Typical and Atypical Mechanisms of Action in Skeletal Muscle

Insulin-stimulated glucose uptake in skeletal muscle is of fundamental importance to prevent postprandial hyperglycemia, and long-term deficits in insulin-stimulated glucose uptake underlie insulin resistance and type 2 diabetes. Skeletal muscle is responsible for ~80% of the peripheral glucose uptake from circulation via the insulin-responsive glucose transporter GLUT4. GLUT4 is mainly sequestered in intracellular GLUT4 storage vesicles in the basal state. In response to insulin, the GLUT4 storage vesicles rapidly translocate to the plasma membrane, where they undergo vesicle docking, priming, and fusion via the high-affinity interactions among the soluble N-ethylmaleimide sensitive factor attachment protein receptor (SNARE) exocytosis proteins and their regulators. Numerous studies have elucidated that GLUT4 translocation is defective in insulin resistance and type 2 diabetes. Emerging evidence also links defects in several SNAREs and SNARE regulatory proteins to insulin resistance and type 2 diabetes in rodents and humans. Therefore, we highlight the latest research on the role of SNAREs and their regulatory proteins in insulin-stimulated GLUT4 translocation in skeletal muscle. Subsequently, we discuss the novel emerging role of SNARE proteins as interaction partners in pathways not typically thought to involve SNAREs and how these atypical functions reveal novel therapeutic targets for combating peripheral insulin resistance and diabetes.

OUP accepted manuscript

Type 2 diabetes is a complex, systemic disease affected by both genetic and environmental factors. Previous research has identified genetic variants associated with type 2 diabetes risk; however, gene regulatory changes underlying progression to metabolic dysfunction are still largely unknown. We investigated RNA expression changes that occur during diabetes progression using a two-stage approach. In our discovery stage, we compared changes in gene expression using two longitudinally collected blood samples from subjects whose fasting blood glucose transitioned to a level consistent with type 2 diabetes diagnosis between the time points against those who did not with a novel analytical network approach. Our network methodology identified 17 networks, one of which was significantly associated with transition status. This 822-gene network harbors many genes novel to the type 2 diabetes literature but is also significantly enriched for genes previously associated with type 2 diabetes. In the validation stage, we queried associations of genetically determined expression with diabetes-related traits in a large biobank with linked electronic health records. We observed a significant enrichment of genes in our identified network whose genetically determined expression is associated with type 2 diabetes and other metabolic traits and validated 31 genes that are not near previously reported type 2 diabetes loci. Finally, we provide additional functional support, which suggests that the genes in this network are regulated by enhancers that operate in human pancreatic islet cells. We present an innovative and systematic approach that identified and validated key gene expression changes associated with type 2 diabetes transition status and demonstrated their translational relevance in a large clinical resource.

Gestational diabetes mellitus alters DNA methylation profiles in pancreas of the offspring mice

Gestational diabetes mellitus (GDM), which has an increasing global prevalence, contributes to the susceptibility to metabolic dysregulation and obesity in the offspring via epigenetic modifications. However, the underlying mechanism remains largely obscure. The current study established a GDM mice model to investigate the alternations in the metabolic phenotypes and genomic DNA methylation in the pancreas of the offspring. We found that in the GDM offspring, intrauterine hyperglycemia induced dyslipidemia, insulin resistance, and glucose intolerance. Meanwhile, altered DNA methylation patterns were exhibited in the pancreas and many differentially methylated regions (DMRs)-related genes were involved in glycolipids metabolism and related signaling pathways, including Agap2, Plcbr, Hnf1b, Gnas, Fbp2, Cdh13, Wnt2, Kcnq1, Lhcgr, Irx3, etc. Additionally, the overall hypermethylation of Agap2, verified by bisulfite sequencing PCR (BSP), was negatively correlated with its mRNA expression level. In conclusion, these findings suggest that the DNA methylation changes in the pancreatic genome of the GDM offspring may be associated with the glycolipid metabolism abnormalities, T2DM susceptibility, and obesity in the adult GDM offspring.

MiR-335 overexpression impairs insulin secretion through defective priming of insulin vesicles

MicroRNAs contribute to the maintenance of optimal cellular functions by fine‐tuning protein expression levels. In the pancreatic β‐cells, imbalances in the exocytotic machinery components lead to impaired insulin secretion and type 2 diabetes (T2D). We hypothesize that dysregulated miRNA expression exacerbates β‐cell dysfunction, and have earlier shown that islets from the diabetic GK‐rat model have increased expression of miRNAs, including miR‐335‐5p (miR‐335). Here, we aim to determine the specific role of miR‐335 during development of T2D, and the influence of this miRNA on glucose‐stimulated insulin secretion and Ca²⁺‐dependent exocytosis. We found that the expression of miR‐335 negatively correlated with secretion index in human islets of individuals with prediabetes. Overexpression of miR‐335 in human EndoC‐βH1 and in rat INS‐1 832/13 cells (OE335) resulted in decreased glucose‐stimulated insulin secretion, and OE335 cells showed concomitant reduction in three exocytotic proteins: SNAP25, Syntaxin‐binding protein 1 (STXBP1), and synaptotagmin 11 (SYT11). Single‐cell capacitance measurements, complemented with TIRF microscopy of the granule marker NPY‐mEGFP demonstrated a significant reduction in exocytosis in OE335 cells. The reduction was not associated with defective docking or decreased Ca²⁺ current. More likely, it is a direct consequence of impaired priming of already docked granules. Earlier reports have proposed reduced granular priming as the cause of reduced first‐phase insulin secretion during prediabetes. Here, we show a specific role of miR‐335 in regulating insulin secretion during this transition period. Moreover, we can conclude that miR‐335 has the capacity to modulate insulin secretion and Ca²⁺‐dependent exocytosis through effects on granular priming.

Ca2+ channel clustering with insulin-containing granules is disturbed in type 2 diabetes

Loss of first-phase insulin secretion is an early sign of developing type 2 diabetes (T2D). Ca²⁺ entry through voltage-gated L-type Ca²⁺ channels triggers exocytosis of insulin-containing granules in pancreatic β cells and is required for the postprandial spike in insulin secretion. Using high-resolution microscopy, we have identified a subset of docked insulin granules in human β cells and rat-derived clonal insulin 1 (INS1) cells for which localized Ca²⁺ influx triggers exocytosis with high probability and minimal latency. This immediately releasable pool (IRP) of granules, identified both structurally and functionally, was absent in β cells from human T2D donors and in INS1 cells cultured in fatty acids that mimic the diabetic state. Upon arrival at the plasma membrane, IRP granules slowly associated with 15 to 20 L-type channels. We determined that recruitment depended on a direct interaction with the synaptic protein Munc13, because expression of the II–III loop of the channel, the C2 domain of Munc13-1, or of Munc13-1 with a mutated C2 domain all disrupted L-type channel clustering at granules and ablated fast exocytosis. Thus, rapid insulin secretion requires Munc13-mediated recruitment of L-type Ca²⁺ channels in close proximity to insulin granules. Loss of this organization underlies disturbed insulin secretion kinetics in T2D.

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.

Modulation of Ionic Channels and Insulin Secretion by Drugs and Hormones in Pancreatic Beta Cells

Pancreatic beta cells, unique cells that secrete insulin in response to an increase in glucose levels, play a significant role in glucose homeostasis. Glucose-stimulated insulin secretion (GSIS) in pancreatic beta cells has been extensively explored. In this mechanism, glucose enters the cells and subsequently the metabolic cycle. During this process, the ATP/ADP ratio increases, leading to ATP-sensitive potassium (KATP) channel closure, which initiates depolarization that is also dependent on the activity of TRP nonselective ion channels. Depolarization leads to the opening of voltage-gated Na(+) channels (Nav) and subsequently voltage-dependent Ca(2+) channels (Cav). The increase in intracellular Ca(2+) triggers the exocytosis of insulin-containing vesicles. Thus, electrical activity of pancreatic beta cells plays a central role in GSIS. Moreover, many growth factors, incretins, neurotransmitters, and hormones can modulate GSIS, and the channels that participate in GSIS are highly regulated. In this review, we focus on the principal ionic channels (KATP, Nav, and Cav channels) involved in GSIS and how classic and new proteins, hormones, and drugs regulate it. Moreover, we also discuss advances on how metabolic disorders such as metabolic syndrome and diabetes mellitus change channel activity leading to changes in insulin secretion.

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.

The skeleton in the closet: actin cytoskeletal remodeling in β-cell function

Over the last few decades, biomedical research has considered not only the function of single cells but also the importance of the physical environment within a whole tissue, including cell-cell and cell-extracellular matrix interactions. Cytoskeleton organization and focal adhesions are crucial sensors for cells that enable them to rapidly communicate with the physical extracellular environment in response to extracellular stimuli, ensuring proper function and adaptation. The involvement of the microtubular-microfilamentous cytoskeleton in secretion mechanisms was proposed almost 50 years ago, since when the evolution of ever more sensitive and sophisticated methods in microscopy and in cell and molecular biology have led us to become aware of the importance of cytoskeleton remodeling for cell shape regulation and its crucial link with signaling pathways leading to β-cell function. Emerging evidence suggests that dysfunction of cytoskeletal components or extracellular matrix modification influences a number of disorders through potential actin cytoskeleton disruption that could be involved in the initiation of multiple cellular functions. Perturbation of β-cell actin cytoskeleton remodeling could arise secondarily to islet inflammation and fibrosis, possibly accounting in part for impaired β-cell function in type 2 diabetes. This review focuses on the role of actin remodeling in insulin secretion mechanisms and its close relationship with focal adhesions and myosin II.

Munc18-1 and Munc18-2 proteins modulate beta-cell Ca2+ sensitivity and kinetics of insulin exocytosis differently

Fast neurotransmission and slower hormone release share the same core fusion machinery consisting of SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor) proteins. In evoked neurotransmission, interactions between SNAREs and the Munc18-1 protein, a member of the Sec1/Munc18 (SM) protein family, are essential for exocytosis, whereas other SM proteins are dispensable. To address if the exclusivity of Munc18-1 demonstrated in neuroexocytosis also applied to fast insulin secretion, we characterized the presence and function of Munc18-1 and its closest homologue Munc18-2 in β-cell stimulus-secretion coupling. We show that pancreatic β-cells express both Munc18-1 and Munc18-2. The two Munc18 homologues exhibit different subcellular localization, and only Munc18-1 redistributes in response to glucose stimulation. However, both Munc18-1 and Munc18-2 augment glucose-stimulated hormone release. Ramp-like photorelease of caged Ca(2+) and high resolution whole-cell patch clamp recordings show that Munc18-1 and Munc18-2 overexpression shift the Ca(2+) sensitivity of the fastest phase of insulin exocytosis differently. In addition, we reveal that Ca(2+) sensitivity of exocytosis in β-cells depends on the phosphorylation status of the Munc18 proteins. Even though Munc18-1 emerges as the key SM-protein determining the Ca(2+) threshold for triggering secretory activity in a stimulated β-cell, Munc18-2 has the ability to increase Ca(2+) sensitivity and thus mediates the release of fusion-competent granules requiring a lower cytoplasmic-free Ca(2+) concentration, [Ca(2+)](i)(.) Hence, Munc18-1 and Munc18-2 display distinct subcellular compartmentalization and can coordinate the insulin exocytotic process differently as a consequence of the actual [Ca(2+)](i).

Inositol 1, 4, 5-trisphosphate receptor interacts with the SNARE domain of syntaxin 1B

Inositol 1, 4, 5-trisphosphate receptors (IP(3)Rs) are intracellular ligand-gated Ca(2+) channels that mediate Ca(2+) release from the endoplasmic reticulum (ER) into the cytosol and function in diverse cellular processes including fertilization, muscle contraction, apoptosis, secretion, and synaptic plasticity. The Ca(2+) release activity of IP(3)Rs is tightly regulated by many factors including IP(3)R-binding proteins. We show that IP(3)Rs interact with syntaxin 1 (Syx1), a membrane trafficking protein that regulates various plasma-membrane ion channels including N-, P/Q, and L-type voltage-gated Ca(2+) channels, voltage-gated potassium channels, and an epithelial sodium channel. We found that a SNARE-domain of Syx1B, one of the two Syx1 isoforms, directly interacted with the type1 IP(3)R (IP(3)R1) internal coupling domain, a known modulator for channel opening. These results indicate that Syx1B is an IP(3)R-interacting protein and that its interaction may play a crucial role in regulating the channel activity of IP(3)Rs in Syx1B-expressing cells.

miRNAs control insulin content in pancreatic β-cells via downregulation of transcriptional repressors

MicroRNAs (miRNAs) were shown to be important for pancreas development, yet their roles in differentiated β-cells remain unclear. Here, we show that miRNA inactivation in β-cells of adult mice results in a striking diabetic phenotype. While islet architecture is intact and differentiation markers are maintained, Dicer1-deficient β-cells show a dramatic decrease in insulin content and insulin mRNA. As a consequence of the change in insulin content, the animals become diabetic. We provide evidence for involvement of a set of miRNAs in regulating insulin synthesis. The specific knockdown of miR-24, miR-26, miR-182 or miR-148 in cultured β-cells or in isolated primary islets downregulates insulin promoter activity and insulin mRNA levels. Further, miRNA-dependent regulation of insulin expression is associated with upregulation of transcriptional repressors, including Bhlhe22 and Sox6. Thus, miRNAs in the adult pancreas act in a new network that reinforces insulin expression by reducing the expression of insulin transcriptional repressors.

Syntaxin 1A regulates surface expression of beta-cell ATP-sensitive potassium channels

The pancreatic ATP-sensitive potassium (K(ATP)) channel consisting of four inwardly rectifying potassium channel 6.2 (Kir6.2) and four sulfonylurea receptor SUR1 subunits plays a key role in insulin secretion by linking glucose metabolism to membrane excitability. Syntaxin 1A (Syn-1A) is a plasma membrane protein important for membrane fusion during exocytosis of insulin granules. Here, we show that Syn-1A and K(ATP) channels endogenously expressed in the insulin-secreting cell INS-1 interact. Upregulation of Syn-1A by overexpression in INS-1 leads to a decrease, whereas downregulation of Syn-1A by small interfering RNA (siRNA) leads to an increase, in surface expression of K(ATP) channels. Using COSm6 cells as a heterologous expression system for mechanistic investigation, we found that Syn-1A interacts with SUR1 but not Kir6.2. Furthermore, Syn-1A decreases surface expression of K(ATP) channels via two mechanisms. One mechanism involves accelerated endocytosis of surface channels. The other involves decreased biogenesis and processing of channels in the early secretory pathway. This regulation is K(ATP) channel specific as Syn-1A has no effect on another inward rectifier potassium channel Kir3.1/3.4. Our results demonstrate that in addition to a previously documented role in modulating K(ATP) channel gating, Syn-1A also regulates K(ATP) channel expression in β-cells. We propose that physiological or pathological changes in Syn-1A expression may modulate insulin secretion by altering glucose-secretion coupling via changes in K(ATP) channel expression.

Hyperinsulinism and diabetes: genetic dissection of beta cell metabolism-excitation coupling in mice

The role of metabolism-excitation coupling in insulin secretion has long been apparent, but in recent years, in parallel with studies of human hyperinsulinism and diabetes, genetic manipulation of proteins involved in glucose transport, metabolism, and excitability in mice has brought the central importance of this pathway into sharp relief. We focus on these animal studies and how they provide important insights into not only metabolic and electrical regulation of insulin secretion, but also downstream consequences of alterations in this pathway and the etiology and treatment of insulin-secretion diseases in humans.

Mechanisms of biphasic insulin-granule exocytosis - roles of the cytoskeleton, small GTPases and SNARE proteins

The release of insulin from pancreatic islets requires negative regulation to ensure low levels of insulin release under resting conditions, as well as positive regulation to facilitate robust responsiveness to conditions of elevated fuel or glucose. The first phase of release involves the plasma-membrane fusion of a small pool of granules, termed the readily releasable pool; these granules are already at the membrane under basal conditions, and discharge their cargo in response to nutrient and also non-nutrient secretagogues. By contrast, second-phase secretion is evoked exclusively by nutrients, and involves the mobilization of intracellular granules to t-SNARE sites at the plasma membrane to enable the distal docking and fusion steps of insulin exocytosis. Nearly 40 years ago, the actin cytoskeleton was first recognized as a key mediator of biphasic insulin release, and was originally presumed to act as a barrier to block granule docking at the cell periphery. More recently, however, the discovery of cycling GTPases that are involved in F-actin reorganization in the islet beta-cell, combined with the availability of reagents that are more specific and tools with which to study the mechanisms that underlie granule movement, have contributed greatly to our understanding of the role of the cytoskeleton in regulating biphasic insulin secretion. Herein, we provide historical perspective and review recent progress that has been made towards integrating cytoskeletal reorganization and cycling of small Rho-, Rab- and Ras-family GTPases into our current models of stimulus-secretion coupling and second-phase insulin release.

Filamentous actin regulates insulin exocytosis through direct interaction with Syntaxin 4

Glucose-induced insulin exocytosis is coupled to associations between F-actin and SNARE proteins, although the nature and function of these interactions remains unknown. Toward this end we show here that both Syntaxin 1A and Syntaxin 4 associated with F-actin in MIN6 cells and that each interaction was rapidly and transiently diminished by stimulation of cells with d-glucose. Of the two isoforms, only Syntaxin 4 was capable of interacting directly with F-actin in an in vitro sedimentation assay, conferred by the N-terminal 39-112 residues of Syntaxin 4. The 39-112 fragment was capable of selective competitive inhibitory action, disrupting endogenous F-actin-Syntaxin 4 binding in MIN6 cells. Disruption of F-actin-Syntaxin 4 binding correlated with enhanced glucose-stimulated insulin secretion, mediated by increased granule accumulation at the plasma membrane and increased Syntaxin 4 accessibility under basal conditions. However, no increase in basal level Syntaxin 4-VAMP2 association occurred with either latrunculin treatment or expression of the 39-112 fragment. Taken together, these data disclose a new underlying mechanism by which F-actin negatively regulates exocytosis via binding and blocking Syntaxin 4 accessibility, but they also reveal the existence of additional signals and/or steps required to trigger the subsequent docking and fusion steps of exocytosis.

Comparative genomics of human-like Schistosoma japonicum genes indicates a putative mechanism for host-parasite relationship

Schistosoma japonicum causes schistosomiasis in humans and livestock in the Asia-Pacific region. We assembled more than 43,700 S. japonicum expressed sequence tags and conducted comparative genomic analyses between S. japonicum and its human host. Some schistosome genes showed exceptionally high similarity in nucleotide sequence to their human homologues, of which five exhibited anomalous phylogeny and human codon usage bias. The most plausible explanation for their presence is horizontal gene transfer from host to parasite. Functional evidence suggests that S. japonicum might exploit host endocrine and immune signals for cell development and maturation via these host-like genes.

Depolarization-evoked secretion requires two vicinal transmembrane cysteines of syntaxin 1A

Background

The interactions of the voltage-gated Ca²⁺ channel (VGCC) with syntaxin 1A (Sx 1A), Synaptosome-associated protein of 25 kD (SNAP-25), and synaptotagmin, couple electrical excitation to evoked secretion. Two vicinal Cys residues, Cys 271 and Cys 272 in the Sx 1A transmembrane domain, are highly conserved and participate in modulating channel kinetics. Each of the Sx1A Cys mutants, differently modify the kinetics of Cav1.2, and neuronal Cav2.2 calcium channel.

Methodology/Principle Findings

We examined the effects of various Sx1A Cys mutants and the syntaxin isoforms 2, 3, and 4 each of which lack vicinal Cys residues, on evoked secretion, monitoring capacitance transients in a functional release assay. Membrane capacitance in Xenopus oocytes co-expressing Cav1.2, Sx1A, SNAP-25 and synaptotagmin, which is Bot C- and Bot A-sensitive, was elicited by a double 500 ms depolarizing pulse to 0 mV. The evoked-release was obliterated when a single Cys Sx1A mutant or either one of the Sx isoforms were substituted for Sx 1A, demonstrating the essential role of vicinal Cys residues in the depolarization mediated process. Protein expression and confocal imaging established the level of the mutated proteins in the cell and their targeting to the plasma membrane.

Conclusions/Significance

We propose a model whereby the two adjacent transmembranal Cys residues of Sx 1A, lash two calcium channels. Consistent with the necessity of a minimal fusion complex termed the excitosome, each Sx1A is in a complex with SNAP-25, Syt1, and the Ca²⁺ channel. A Hill coefficient >2 imply that at least three excitosome complexes are required for generating a secreting hetero-oligomer protein complex. This working model suggests that a fusion pore that opens during membrane depolarization could be lined by alternating transmembrane segments of Sx1A and VGCC. The functional coupling of distinct amino acids of Sx 1A with VGCC appears to be essential for depolarization-evoked secretion.

SNAREing voltage-gated K+ and ATP-sensitive K+ channels: tuning beta-cell excitability with syntaxin-1A and other exocytotic proteins

The three SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor) proteins, syntaxin, SNAP25 (synaptosome-associated protein of 25 kDa), and synaptobrevin, constitute the minimal machinery for exocytosis in secretory cells such as neurons and neuroendocrine cells by forming a series of complexes prior to and during vesicle fusion. It was subsequently found that these SNARE proteins not only participate in vesicle fusion, but also tether with voltage-dependent Ca(2+) channels to form an excitosome that precisely regulates calcium entry at the site of exocytosis. In pancreatic islet beta-cells, ATP-sensitive K(+) (K(ATP)) channel closure by high ATP concentration leads to membrane depolarization, voltage-dependent Ca(2+) channel opening, and insulin secretion, whereas subsequent opening of voltage-gated K(+) (Kv) channels repolarizes the cell to terminate exocytosis. We have obtained evidence that syntaxin-1A physically interacts with Kv2.1 (the predominant Kv in beta-cells) and the sulfonylurea receptor subunit of beta-cell K(ATP) channel to modify their gating behaviors. A model has proposed that the conformational changes of syntaxin-1A during exocytosis induce distinct functional modulations of K(ATP) and Kv2.1 channels in a manner that optimally regulates cell excitability and insulin secretion. Other proteins involved in exocytosis, such as Munc-13, tomosyn, rab3a-interacting molecule, and guanyl nucleotide exchange factor II, have also been implicated in direct or indirect regulation of beta-cell ion channel activities and excitability. This review discusses this interesting aspect that exocytotic proteins not only promote secretion per se, but also fine-tune beta-cell excitability via modulation of ion channel gating.

Interaction of Munc18 and Syntaxin in the regulation of insulin secretion

Syntaxin1A and Munc18-1 play essential roles in exocytosis. However, the molecular mechanism and the functional roles of their interaction in insulin secretion remain to be explored. Using membrane capacitance measurement, we examine effect of overexpressing Munc18-1 on exocytosis in pancreatic beta cells. The results show that Munc18-1 negatively regulates vesicle fusion. To probe the interaction between Munc18-1 and Syntaxin1A, Munc18-1-Tdimer2 and EGFP-Syntaxin1A were co-transfected into INS-1 cells. FRET measurement confirmed that Munc18-1 interacted with wild type Syntaxin 1A, but not the constitutively open form (DM) of Syntaxin1A. Overexpressing DM in primary pancreatic beta cells augmented insulin secretion, and this effect can overcome the inhibitory effect of Munc18-1 overexpression. We propose that Munc18-1 inhibitis the SNARE complex assembly by stabilizing Syntaxin1A in a closed conformation in vesicle priming process, therefore negatively regulates insulin secretion.

Imaging analysis reveals mechanistic differences between first- and second-phase insulin exocytosis

The mechanism of glucose-induced biphasic insulin release is unknown. We used total internal reflection fluorescence (TIRF) imaging analysis to reveal the process of first- and second-phase insulin exocytosis in pancreatic β cells. This analysis showed that previously docked insulin granules fused at the site of syntaxin (Synt)1A clusters during the first phase; however, the newcomers fused during the second phase external to the Synt1A clusters. To reveal the function of Synt1A in phasic insulin exocytosis, we generated Synt1A-knockout (Synt1A^−/−) mice. Synt1A^−/− β cells showed fewer previously docked granules with no fusion during the first phase; second-phase fusion from newcomers was preserved. Rescue experiments restoring Synt1A expression demonstrated restoration of granule docking status and fusion events. Inhibition of other syntaxins, Synt3 and Synt4, did not affect second-phase insulin exocytosis. We conclude that the first phase is Synt1A dependent but the second phase is not. This indicates that the two phases of insulin exocytosis differ spatially and mechanistically.

Distinct modulation of Kv1.2 channel gating by wild type, but not open form, of syntaxin-1A

SNARE proteins, syntaxin-1A (Syn-1A) and SNAP-25, inhibit delayed rectifier K(+) channels, K(v)1.1 and K(v)2.1, in secretory cells. We showed previously that the mutant open conformation of Syn-1A (Syn-1A L165A/E166A) inhibits K(v)2.1 channels more optimally than wild-type Syn-1A. In this report we examined whether Syn-1A in its wild-type and open conformations would exhibit similar differential actions on the gating of K(v)1.2, a major delayed rectifier K(+) channel in nonsecretory smooth muscle cells and some neuronal tissues. In coexpression and acute dialysis studies, wild-type Syn-1A inhibited K(v)1.2 current magnitude. Of interest, wild-type Syn-1A caused a right shift in the activation curves of K(v)1.2 without affecting its steady-state availability, an inhibition profile opposite to its effects on K(v)2.1 (steady-state availability reduction without changes in voltage dependence of activation). Also, although both wild-type and open-form Syn-1A bound equally well to K(v)1.2 in an expression system, open-form Syn-1A failed to reduce K(v)1.2 current magnitude or affect its gating. This is in contrast to the reported more potent effect of open-form Syn-1A on K(v)2.1 channels in secretory cells. This finding together with the absence of Munc18 and/or 13-1 in smooth muscles suggested that a change to an open conformation Syn-1A, normally facilitated by Munc18/13-1, is not required in nonsecretory smooth muscle cells. Taken together with previous reports, our results demonstrate the multiplicity of gating inhibition of different K(v) channels by Syn-1A and is compatible with versatility of Syn-1A modulation of repolarization in various secretory and nonsecretory (smooth muscle) cell types.

Effect of glucolipotoxicity and rosiglitazone upon insulin secretion

Type 2 diabetes is characterised by elevated blood glucose and fatty acid concentrations, and aberrant expression of exocytotic soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) proteins. Restoration of normoglycaemia is often accomplished through use of the thiazolidinedione drug rosiglitazone (RSG), although little is known of its actions on the pancreas. Here we report that high glucose resulted in 96.6+/-0.2% inhibition of secretagogue-stimulated insulin secretion and 44.9+/-6.2% reduction in beta-cell insulin content. High glucose and lipid resulted in altered target-SNARE expression, syntaxin 1 becoming barely detectable whilst SNAP-25 was greatly up-regulated. RSG intervention further increased the expression of SNAP-25, but did not up-regulate syntaxin 1 expression. In summary, high glucose results in almost total attenuation of stimulated insulin secretion, partial depletion of beta-cell insulin stores and dysregulation of SNARE protein expression. RSG up-regulates SNAP-25 expression, but crucially not syntaxin 1 and hence fails to enhance insulin secretion.

K+ channel facilitation of exocytosis by dynamic interaction with syntaxin

Kv channels inhibit release indirectly by hyperpolarizing membrane potential, but the significance of Kv channel interaction with the secretory apparatus is not known. The Kv2.1 channel is commonly expressed in the soma and dendrites of neurons, where it could influence the release of neuropeptides and neurotrophins, and in neuroendocrine cells, where it could influence hormone release. Here we show that Kv2.1 channels increase dense-core vesicle (DCV)-mediated release after elevation of cytoplasmic Ca2+. This facilitation occurs even after disruption of pore function and cannot be explained by changes in membrane potential and cytoplasmic Ca2+. However, triggering release increases channel binding to syntaxin, a secretory apparatus protein. Disrupting this interaction with competing peptides or by deleting the syntaxin association domain of the channel at the C terminus blocks facilitation of release. Thus, direct association of Kv2.1 with syntaxin promotes exocytosis. The dual functioning of the Kv channel to influence release, through its pore to hyperpolarize the membrane potential and through its C-terminal association with syntaxin to directly facilitate release, reinforces the requirements for repetitive firing for exocytosis of DCVs in neuroendocrine cells and in dendrites.

Distinct domain-dependent effect of syntaxin1A on amiloride-sensitive sodium channel (ENaC) currents in HT-29 colonic epithelial cells

The amiloride-sensitive epithelial sodium channel (ENaC), a plasma membrane protein mediates sodium reabsorption in epithelial tissues, including the distal nephron and colon. Syntaxin1A, a trafficking protein of the t-SNARE family has been reported to inhibit ENaC in the Xenopus oocyte expression and artificial lipid bilayer systems. The present report describes the regulation of the epithelial sodium channel by syntaxin1A in a human cell line that is physiologically relevant as it expresses both components and also responds to aldosterone stimulation. In order to evaluate the physiological significance of syntaxin1A interaction with natively expressed ENaC, we over-expressed HT-29 with syntaxin1A constructs comprising various motifs. Unexpectedly, we observed the augmentation of amiloride-sensitive currents with wild-type syntaxin1A full-length construct (1-288) in this cell line. Both γENaC and neutralizing syntaxin1A antibodies blocked native expression as amiloride-sensitive sodium currents were inhibited while munc18-1 antibody reversed this effect. The coiled-coiled domain H3 (194-266) of syntaxin1A inhibited, however the inclusion of the transmembrane domain to this motif (194-288) augmented amiloride sensitive currents. More so, data suggest that ENaC interacts with multiple syntaxin1A domains, which differentially regulate channel function. This functional modulation is the consequence of the physical enhancement of ENaC at the cell surface in cells over-expressed with syntaxin(s). Our data further suggest that syntaxin1A up-regulates ENaC function by multiple mechanisms that include PKA, PLC, PI3 and MAP Kinase (p42/44) signaling systems. We propose that syntaxin1A possesses distinct inhibitory and stimulatory domains that interact with ENaC subunits, which critically determines the overall ENaC functionality/regulation under distinct physiological conditions.

The role of voltage-gated calcium channels in pancreatic beta-cell physiology and pathophysiology

Voltage-gated calcium (CaV) channels are ubiquitously expressed in various cell types throughout the body. In principle, the molecular identity, biophysical profile, and pharmacological property of CaV channels are independent of the cell type where they reside, whereas these channels execute unique functions in different cell types, such as muscle contraction, neurotransmitter release, and hormone secretion. At least six CaValpha1 subunits, including CaV1.2, CaV1.3, CaV2.1, CaV2.2, CaV2.3, and CaV3.1, have been identified in pancreatic beta-cells. These pore-forming subunits complex with certain auxiliary subunits to conduct L-, P/Q-, N-, R-, and T-type CaV currents, respectively. beta-Cell CaV channels take center stage in insulin secretion and play an important role in beta-cell physiology and pathophysiology. CaV3 channels become expressed in diabetes-prone mouse beta-cells. Point mutation in the human CaV1.2 gene results in excessive insulin secretion. Trinucleotide expansion in the human CaV1.3 and CaV2.1 gene is revealed in a subgroup of patients with type 2 diabetes. beta-Cell CaV channels are regulated by a wide range of mechanisms, either shared by other cell types or specific to beta-cells, to always guarantee a satisfactory concentration of Ca2+. Inappropriate regulation of beta-cell CaV channels causes beta-cell dysfunction and even death manifested in both type 1 and type 2 diabetes. This review summarizes current knowledge of CaV channels in beta-cell physiology and pathophysiology.

Syntaxin-1A actions on sulfonylurea receptor 2A can block acidic pH-induced cardiac K(ATP) channel activation

During cardiac ischemia, ATP stores are depleted, and cardiomyocyte intracellular pH lowers to <7.0. The acidic pH acts on the Kir6.2 subunit of K(ATP) channels to reduce its sensitivity to ATP, causing channel opening. We recently reported that syntaxin-1A (Syn-1A) binds nucleotide binding folds (NBF)-1 and NBF2 of sulfonylurea receptor 2A (SUR2A) to inhibit channel activity (Kang, Y., Leung, Y. M., Manning-Fox, J. E., Xia, F., Xie, H., Sheu, L., Tsushima, R. G., Light, P. E., and Gaisano, H. Y. (2004) J. Biol. Chem. 279, 47125-47131). Here, we examined Syn-1A actions on SUR2A to influence the pH regulation of cardiac K(ATP) channels. K(ATP) channel currents from inside-out patches excised from Kir6.2/SUR2A expressing HEK293 cells and freshly isolated cardiac myocytes were increased by reducing intracellular pH from 7.4 to 6.8, which could be blocked by increasing concentrations of Syn-1A added to the cytoplasmic surface. Syn-1A had no effect on C-terminal truncated Kir6.2 (Kir6.2-deltaC26) channels expressed in TSA cells without the SUR subunit. In vitro binding and co-immunoprecipitation studies show that Syn-1A binding to SUR2A or its NBF-1 and NBF-2 domain proteins increased progressively as pH was reduced from 7.4 to 6.0. The enhancement of Syn-1A binding to SUR2A by acidic pH was further regulated by Mg2+ and ATP. Therefore, pH regulates Kir.6.2/SUR2A channels not only by its direct actions on the Kir6.2 subunit but also by modulation of Syn-1A binding to SUR2A. The increased Syn-1A binding to the SUR2A at acidic pH would assert some inhibition of the K(ATP) channels, which may serve as a "brake" to temper the fluctuation of low pH-induced K(ATP) channel opening that could induce fatal reentrant arrhythmias.

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

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

Synaptic transmission at retinal ribbon synapses

The molecular organization of ribbon synapses in photoreceptors and ON bipolar cells is reviewed in relation to the process of neurotransmitter release. The interactions between ribbon synapse-associated proteins, synaptic vesicle fusion machinery and the voltage-gated calcium channels that gate transmitter release at ribbon synapses are discussed in relation to the process of synaptic vesicle exocytosis. We describe structural and mechanistic specializations that permit the ON bipolar cell to release transmitter at a much higher rate than the photoreceptor does, under in vivo conditions. We also consider the modulation of exocytosis at photoreceptor synapses, with an emphasis on the regulation of calcium channels.

Transgenic mouse overexpressing syntaxin-1A as a diabetes model

Soluble N-ethylmaleimide-sensitive factor (NSF) attachment protein receptor (SNARE) protein syntaxin-1A (STX-1A) plays a role not only in exocytosis, but also binds and regulates Ca(2+) and K(+) (voltage-gated K(+) and ATP-sensitive K(+) channels) to influence the sequence of events leading to secretion. Islet levels of STX-1A and cognate SNARE proteins are reduced in type 2 diabetic rodents, suggesting their role in dysregulated insulin secretion contributing to the abnormal glucose homeostasis. We investigated the specific role of STX-1A in pancreatic beta-cells by generating transgenic mice, which express a moderately increased level ( approximately 30% higher) of STX-1A in pancreatic islets (hereafter called STX-1A mice). The STX-1A mice displayed fasting hyperglycemia and a more sustained elevation of plasma glucose levels after an intraperitoneal glucose tolerance test, with correspondingly reduced plasma insulin levels. Surprisingly, beta-cells from the STX-1A male mice also exhibited abnormal insulin tolerance. To unequivocally determine the beta-cell secretory defects, we used single-cell analyses of exocytosis by patch clamp membrane capacitance measurements and ion channel recordings. Depolarization-evoked membrane capacitance increases were reduced in the STX-1A mouse islet beta-cells. The STX-1A mouse also exhibited reduced currents through the Ca(2+) channels but little change in the voltage-gated K(+) channel or ATP-sensitive K(+) channel. These results suggest that fluctuation of islet STX-1A levels in diabetes could influence the pathological and differential regulation of beta-cell ion channels and the exocytotic machinery, collectively contributing to the impaired insulin secretion.

Open form of syntaxin-1A is a more potent inhibitor than wild-type syntaxin-1A of Kv2.1 channels

We have shown that SNARE (soluble N-ethylmaleimide-sensitive fusion protein attachment protein receptor) proteins not only participate directly in exocytosis, but also regulate the dominant membrane-repolarizing Kv channels (voltage-gated K+ channels), such as Kv2.1, in pancreatic beta-cells. In a recent report, we demonstrated that WT (wild-type) Syn-1A (syntaxin-1A) inhibits Kv2.1 channel trafficking and gating through binding to the cytoplasmic C-terminus of Kv2.1. During beta-cell exocytosis, Syn-1A converts from a closed form into an open form which reveals its active H3 domain to bind its SNARE partners SNAP-25 (synaptosome-associated protein of 25 kDa) and synaptobrevin. In the present study, we compared the effects of the WT Syn-1A and a mutant open form Syn-1A (L165A, E166A) on Kv2.1 channel trafficking and gating. When co-expressed in HEK-293 cells (human embryonic kidney-293 cells), the open form Syn-1A decreased Kv2.1 current density more than (P<0.05) the WT Syn-1A (166+/-35 and 371+/-93 pA/pF respectively; control=911+/-91 pA/pF). Confocal microscopy and biotinylation experiments showed that both the WT and open form Syn-1A inhibited Kv2.1 expression at the plasma membrane to a similar extent, suggesting that the stronger reduction of Kv2.1 current density by the open form compared with the WT Syn-1A is probably due to a stronger direct inhibition of channel activity. Consistently, dialysis of the recombinant open form Syn-1A protein into Kv2.1-expressing HEK-293 cells caused stronger inhibition of Kv2.1 current amplitude (P<0.05) than the WT Syn-1A protein (73+/-2 and 82+/-3% of the control respectively). We found that the H3 but not H(ABC) domain is the putative active domain of Syn-1A, which bound to and inhibited the Kv2.1 channel. When co-expressed in HEK-293 cells, the open-form Syn-1A slowed down Kv2.1 channel activation (tau=12.3+/-0.8 ms) much more than (P<0.05) WT Syn-1A (tau=7.9+/-0.8 ms; control tau=5.5+/-0.6 ms). In addition, only the open form Syn-1A, but not the WT Syn-1A, caused a significant (P<0.05) left-shift in the steady-state inactivation curve (V(1/2)=33.1+/-1.3 and -29.4+/-1.1 mV respectively; control V(1/2)=-24.8+/-2 mV). The present study therefore indicates that the open form of Syn-1A is more potent than the WT Syn-1A in inhibiting the Kv2.1 channel. Such stronger inhibition by the open form of Syn-1A may limit K+ efflux and thus decelerate membrane repolarization during exocytosis, leading to optimization of insulin release.

Beta-cell CaV channel regulation in physiology and pathophysiology

The beta-cell is equipped with at least six voltage-gated Ca2+ (CaV) channel alpha1-subunits designated CaV1.2, CaV1.3, CaV2.1, CaV2.2, CaV2.3, and CaV3.1. These principal subunits, together with certain auxiliary subunits, assemble into different types of CaV channels conducting L-, P/Q-, N-, R-, and T-type Ca2+ currents, respectively. The beta-cell shares customary mechanisms of CaV channel regulation with other excitable cells, such as protein phosphorylation, Ca2+-dependent inactivation, and G protein modulation. However, the beta-cell displays some characteristic features to bring these mechanisms into play. In islet beta-cells, CaV channels can be highly phosphorylated under basal conditions and thus marginally respond to further phosphorylation. In beta-cell lines, CaV channels can be surrounded by tonically activated protein phosphatases dominating over protein kinases; thus their activity is dramatically enhanced by inhibition of protein phosphatases. During the last 10 years, we have revealed some novel mechanisms of beta-cell CaV channel regulation under physiological and pathophysiological conditions, including the involvement of exocytotic proteins, inositol hexakisphosphate, and type 1 diabetic serum. This minireview highlights characteristic features of customary mechanisms of CaV channel regulation in beta-cells and also reviews our studies on newly identified mechanisms of beta-cell CaV channel regulation.

Protease-activated receptor-1 activation of endothelial cells induces protein kinase Calpha-dependent phosphorylation of syntaxin 4 and Munc18c: role in signaling p-selectin expression

Endothelial cells exhibit regulated exocytosis in response to inflammatory mediators such as thrombin and histamine. The exocytosis of Weibel-Palade bodies (WPBs) containing von Willebrand factor, P-selectin, and interleukin-8 within minutes after stimulation is important for vascular homeostasis. SNARE proteins are key components of the exocytic machinery in neurons and some secretory cells, but their role in regulating exocytosis in endothelial cells is not well understood. We examined the function of SNARE proteins in mediating exocytosis of WPBs in endothelial cells. We identified the presence of syntaxin 4, syntaxin 3, and the high affinity syntaxin 4-regulatory protein Munc18c in human lung microvascular endothelial cells. Small interfering RNA-induced knockdown of syntaxin 4 (but not of syntaxin 3) inhibited exocytosis of WPBs as detected by the reduction in thrombin-induced cell surface P-selectin expression. Thrombin ligation of protease-activated receptor-1 activated the phosphorylation of syntaxin 4 and Munc18c, which, in turn, disrupted the interaction between syntaxin 4 and Munc18. Protein kinase Calpha activation was required for the phosphorylation of syntaxin 4 and Munc18c as well as the cell surface expression of P-selectin. We also observed that syntaxin 4 knockdown inhibited the adhesion of neutrophils to thrombin-activated endothelial cells, demonstrating the functional role of syntaxin 4 in promoting endothelial adhesivity. Thus, protease-activated receptor-1-induced protein kinase Calpha activation and phosphorylation of syntaxin 4 and Munc18c are required for the cell surface expression of P-selectin and the consequent binding of neutrophils to endothelial cells.

H3 domain of syntaxin 1A inhibits KATP channels by its actions on the sulfonylurea receptor 1 nucleotide-binding folds-1 and -2

The ATP-sensitive potassium (K(ATP)) channel in pancreatic islet beta cells consists of four pore-forming (Kir6.2) subunits and four regulatory sulfonylurea receptor (SUR1) subunits. In beta cells, the K(ATP) channel links intracellular metabolism to the dynamic regulation of the cell membrane potential that triggers insulin secretion. Syntaxin 1A (Syn-1A) is a SNARE protein that not only plays a direct role in exocytosis, but also binds and modulates voltage-gated K(+) and Ca(2+) channels to fine tune exocytosis. We recently reported that wild type Syn-1A inhibits rat islet beta cell K(ATP) channels and binds both nucleotide-binding folds (NBF-1 and NBF-2) of SUR1. However, wild type Syn-1A inhibition of rat islet beta cell K(ATP) channels seems to be mediated primarily via NBF-1. During exocytosis, Syn-1A undergoes a conformational change from a closed form to an open form, which would fully expose its active domain, the C-terminal H3 domain. Here, we show that the constitutively open form Syn-1A mutant (L165A/E166A) has a similar affinity to NBF-1 and NBF-2 as wild type Syn-1A and was equally effective in inhibiting the K(ATP) channels of rat pancreatic beta cells and a cell line (BA8) stably expressing SUR1/Kir6.2. Although dialysis of NBF-1 into BA8 and islet beta cells effectively blocked wild type and open form Syn-1A inhibition of the K(ATP) current, NBF-2 was also effective in blocking the open form Syn-1A inhibition. This prompted us to examine the specific domains within Syn-1A that would mediate its action on the K(ATP) channels. The C-terminal H3 domain of Syn-1A (Syn-1A-H3), but not the N-terminal H(ABC) domain (Syn-1A-H(ABC)), binds the SUR1 protein of BA8 cells, causing an inhibition of K(ATP) currents, and this inhibition was mediated via both NBF-1 and NBF-2. It therefore appears that the H3 domain of Syn-1A is the putative domain, which binds SUR1, but its distinct actions on the NBFs may depend on the conformation of Syn-1A occurring during exocytosis.

Syntaxin 4 transgenic mice exhibit enhanced insulin-mediated glucose uptake in skeletal muscle

Insulin-stimulated translocation of GLUT4 vesicles from an intracellular compartment to the plasma membrane in 3T3L1 adipocytes is mediated through a syntaxin 4 (Syn4)- and Munc18c-dependent mechanism. To investigate the impact of increasing Syn4 protein abundance on glucose homeostasis in vivo, we engineered tetracycline-repressible transgenic mice to overexpress Syn4 by fivefold in skeletal muscle and pancreas and threefold in adipose tissue. Increases in Syn4 caused increases in Munc18c protein, indicating that Syn4 regulates Munc18c expression in vivo. An important finding was that female Syn4 transgenic mice exhibited an increased rate of glucose clearance during glucose tolerance tests that was repressible by the administration of tetracycline. Insulin-stimulated glucose uptake in skeletal muscle was increased by twofold in Syn4 transgenic mice compared with wild-type mice as assessed by hyperinsulinemic-euglycemic clamp analysis, consistent with a twofold increase in insulin-stimulated GLUT4 translocation in skeletal muscle. Hepatic insulin action was unaffected. Moreover, insulin content and glucose-stimulated insulin secretion by islets isolated from Syn4 transgenic mice did not differ from that of wild-type mice. In sum, these data suggest that increasing the number of Syn4-Munc18c "fusion sites" at the plasma membrane of skeletal muscle increases the amount of GLUT4 available to increase the overall rate of insulin-mediated glucose uptake in vivo.

Regulation of Insulin Secretion in Islets of Langerhans by Ca2+Channels

Insulin secretion from beta-cells of the pancreatic islets of Langerhans is triggered by Ca(2+) influx through voltage-dependent Ca(2+) channels. Electrophysiological and molecular studies indicate that beta-cells express several subtypes of these channels. This review discusses their roles in regulating insulin secretion, focusing on recent studies using beta-cells, exogenous expression systems, and Ca(2+) channel knockout mice. These investigations reveal that L-type Ca(2+) channels in the beta-cell physically interact with the secretory apparatus by binding to synaptic proteins on the plasma membrane and insulin granule. As a result, Ca(2+) influx through L-type channels efficiently and rapidly stimulates release of a pool of insulin granules in close contact with the channels. Thus, L-type Ca(2+) channel activity is preferentially coupled to exocytosis in the beta-cell, and plays a critical role in regulating the dynamics of insulin secretion. Non-L-type channels carry a significant portion of the total voltage-dependent Ca(2+) current in beta-cells and cell lines from some species, but nevertheless account for only a small fraction of insulin secretion. These channels may regulate exocytosis indirectly by affecting membrane potential or second messenger signaling pathways. Finally, voltage-independent Ca(2+) entry pathways and their potential roles in beta-cell function are discussed. The emerging picture is that Ca(2+) channels regulate insulin secretion at multiple sites in the stimulus-secretion coupling pathway, with the specific role of each channel determined by its biophysical and structural properties.

Disruption of Pancreatic β-Cell Lipid Rafts Modifies Kv2.1 Channel Gating and Insulin Exocytosis

In pancreatic beta-cells, the predominant voltage-gated Ca(2+) channel (Ca(V)1.2) and K(+) channel (K(V)2.1) are directly coupled to SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein (SNAP) receptor) proteins. These SNARE proteins modulate channel expression and gating and closely associate these channels with the insulin secretory vesicles. We show that K(V)2.1 and Ca(V)1.2, but not K(V)1.4, SUR1, or Kir6.2, target to specialized cholesterol-rich lipid raft domains on beta-cell plasma membranes. Similarly, the SNARE proteins syntaxin 1A, SNAP-25, and VAMP-2, but not Munc-13-1 or n-Sec1, are associated with lipid rafts. Disruption of the lipid rafts by depleting membrane cholesterol with methyl-beta-cyclodextrin shunts K(V)2.1, Ca(V)1.2, and SNARE proteins out of lipid rafts. Furthermore, methyl-beta-cyclodextrin inhibits K(V)2.1 but not Ca(V)1.2 channel activity and enhances single-cell exocytic events and insulin secretion. Membrane compartmentalization of ion channels and SNARE proteins in lipid rafts may be critical for the temporal and spatial coordination of insulin release, forming what has been described as the excitosome complex.

Syntaxin-1A binds the nucleotide-binding folds of sulphonylurea receptor 1 to regulate the KATP channel

ATP-sensitive potassium (KATP) channels in neuron and neuroendocrine cells consist of a pore-forming Kir6.2 and regulatory sulfonylurea receptor (SUR1) subunits, which are regulated by ATP and ADP. SNARE protein syntaxin 1A (Syn-1A) is known to mediate exocytic fusion, and more recently, to also bind and modulate membrane-repolarizing voltage-gated K+ channels. Here we show that Syn-1A acts as an endogenous regulator of KATP channels capable of closing these channels when cytosolic ATP concentrations were lowered. Botulinum neurotoxin C1 cleavage of endogenous Syn-1A in insulinoma HIT-T15 cells resulted in the increase in KATP currents, which could be subsequently inhibited by recombinant Syn-1A. Whereas Syn-1A binds both nucleotide-binding folds (NBF-1 and NBF-2) of SUR1, the functional inhibition of KATP channels in rat islet beta-cells by Syn-1A seems to be mediated primarily by its interactions with NBF-1. These inhibitory actions of Syn-1A can be reversed by physiologic concentrations of ADP and by diazoxide. Syn-1A therefore acts to fine-tune the regulation of KATP channels during dynamic changes in cytosolic ATP and ADP concentrations. These actions of Syn-1A on KATP channels contribute to the role of Syn-1A in coordinating the sequence of ionic and exocytic events leading to secretion.

Connexin-36 contributes to control function of insulin-producing cells

Connexin-36 (Cx36) is a gap junction protein expressed by the insulin-producing beta-cells. We investigated the contribution of this protein in normal beta-cell function by using a viral gene transfer approach to alter Cx36 content in the insulin-producing line of INS-1E cells and rat pancreatic islets. Transcripts for Cx43, Cx45, and Cx36 were detected by reverse transcriptase-PCR in freshly isolated pancreatic islets, whereas only a transcript for Cx36 was detected in INS-1E cells. After infection with a sense viral vector, which induced de novo Cx36 expression in the Cx-defective HeLa cells we used to control the transgene expression, Western blot, immunofluorescence, and freeze-fracture analysis showed a large increase of Cx36 within INS-1E cell membranes. In contrast, after infection with an antisense vector, Cx36 content was decreased by 80%. Glucose-induced insulin release and insulin content were decreased, whether infected INS-1E cells expressed Cx36 levels that were largely higher or lower than those observed in wild-type control cells. In both cases, basal insulin secretion was unaffected. Comparable observations on basal secretion and insulin content were made in freshly isolated rat pancreatic islets. The data indicate that large changes in Cx36 alter insulin content and, at least in INS-1E cells, also affect glucose-induced insulin release.

Regulation of insulin exocytosis by Munc13-1

The slower kinetics of insulin release from pancreatic islet beta cells, as compared with other regulated secretory processes such as chromaffin granule secretion, can in part be explained by the small number of the insulin granules that are docked to the plasma membrane and readily releasable. In type-2 diabetes, the kinetics of insulin secretion become grossly distorted, and, to therapeutically correct this, it is imperative to elucidate the mechanisms that regulate priming and secretion of insulin secretory granules. Munc13-1, a synaptic protein that regulates SNARE complex assembly, is the major protein determining the priming of synaptic vesicles. Here, we demonstrate the presence of Munc13-1 in human, rat, and mouse pancreatic islet beta cells. Expression of Munc13-1, along with its cognate partners, syntaxin 1a and Munc18a, is reduced in the pancreatic islets of type-2 diabetes non-obese Goto-Kakizaki and obese Zucker fa/fa rats. In insulinoma cells, overexpressed Munc13-1-enhanced green fluorescent protein is translocated to the plasma membrane in a temperature-dependent manner. This, in turn, greatly amplifies insulin exocytosis as determined by patch clamp capacitance measurements and radioimmunoassay of the insulin released. The potentiation of exocytosis by Munc13-1 is dependent on endogenously produced diacylglycerol acting on the overexpressed Munc13-1 because it is blocked by a phospholipase C inhibitor (U73122) and abrogated when the diacylglycerol binding-deficient Munc13-1H567K mutant is expressed instead of the wild type protein. Our data demonstrate that Munc13-mediated vesicle priming is not restricted to neurotransmitter release but is also functional in insulin secretion, where it is subject to regulation by the diacylglycerol second messenger pathway. In view of our findings, Munc13-1 is a potential drug target for therapeutic optimization of insulin secretion in diabetes.

Syntaxin 1A binds to the cytoplasmic C terminus of Kv2.1 to regulate channel gating and trafficking

Voltage-gated K(+) (Kv) 2.1 is the dominant Kv channel that controls membrane repolarization in rat islet beta-cells and downstream insulin exocytosis. We recently showed that exocytotic SNARE protein SNAP-25 directly binds and modulates rat islet beta-cell Kv 2.1 channel protein at the cytoplasmic N terminus. We now show that SNARE protein syntaxin 1A (Syn-1A) binds and modulates rat islet beta-cell Kv2.1 at its cytoplasmic C terminus (Kv2.1C). In HEK293 cells overexpressing Kv2.1, we observed identical effects of channel inhibition by dialyzed GST-Syn-1A, which could be blocked by Kv2.1C domain proteins (C1: amino acids 412-633, C2: amino acids 634-853), but not the Kv2.1 cytoplasmic N terminus (amino acids 1-182). This was confirmed by direct binding of GST-Syn-1A to the Kv2.1C1 and C2 domains proteins. These findings are in contrast to our recent report showing that Syn-1A binds and modulates the cytoplasmic N terminus of neuronal Kv1.1 and not by its C terminus. Co-expression of Syn-1A in Kv2.1-expressing HEK293 cells inhibited Kv2.1 surfacing, which caused a reduction of Kv2.1 current density. In addition, Syn-1A caused a slowing of Kv2.1 current activation and reduction in the slope factor of steady-state inactivation, but had no affect on inactivation kinetics or voltage dependence of activation. Taken together, SNAP-25 and Syn-1A mediate secretion not only through its participation in the exocytotic SNARE complex, but also by regulating membrane potential and calcium entry through their interaction with Kv and Ca(2+) channels. In contrast to Ca(2+) channels, where these SNARE proteins act on a common synprint site, the SNARE proteins act not only on distinct sites within a Kv channel, but also on distinct sites between different Kv channel families.

Effects of selective endocrine or exocrine induction of AR42J on SNARE and Munc18 protein expression

INTRODUCTION AND AIM

We used the amphicrine AR42J as an excellent model to study the differentiation of the secretory machinery of pancreatic endocrine and exocrine cells. Dexamethasone treatment induced the AR42J to differentiate towards the exocrine phenotype capable of secreting amylase in response to cholecystokinin. In contrast, activin A plus hepatocyte growth factor treatment of a subclone of AR42J, AR42J-B13, induced this cell to differentiate morphologically and functionally toward an insulin-containing and insulin-secreting endocrine phenotype. We took advantage of these unique properties of selective exocrine and endocrine induction of the AR42J to reveal which distinct combinations of exocytic SNARE complex proteins (syntaxin, SNAP-25 and VAMP) and associated Munc18 proteins were preferentially expressed to play a role in enzyme and insulin secretion.

RESULTS AND CONCLUSION

To our surprise, both endocrine and exocrine induction of AR42J and AR42J-B13 caused very similar upregulation in the expression of the exocytic member isoforms of the syntaxin, SNAP-25, VAMP, and Munc18 families. We conclude that whereas the differentiation of the proximal components of the secretory machinery of the exocrine acinar and endocrine islet beta-cells is distinct, the differentiation of the distal components of exocytosis between these two cell types is very similar.

Distinct regional expression of SNARE proteins in the feline oesophagus

Abstract Soluble N-ethylmaleimide-sensitive factors attachment protein receptors (SNAREs), initially found to mediate membrane fusion, have now been shown to also bind and regulate a number of membrane ion channels in neurones and neuroendocrine cells. We recently reported that the SNARE protein SNAP-25 regulates Ca(2+)- activated (K(Ca)) and delays rectifier K(+) channels (K(V)) in oesophageal smooth muscle cells. This raised the possibility that cognate and other SNARE proteins could also be present in the oesophageal smooth muscle cell to regulate these and other functions. Circular muscle tissue sections and single freshly isolated muscle cells from the oesophageal body circular and longitudinal layers, and from lower oesophageal sphincter clasp and sling regions were studied. The subcellular location of SNAP-23, SNAP-25, syntaxins 1 to 4, and vesicle-associated membrane protein (VAMP)-2 were explored using a laser scanning confocal imaging system. Feline oesophageal smooth muscle of all regions examined demonstrated the presence of SNAP-23, SNAP-25, syntaxins 1 to 4, and VAMP-2 on the plasma membrane. The intensity of these syntaxins and SNAP-25/-23 proteins varied between the different muscle groups of the oesophagus. In some regions, some SNARE proteins were also noted in the muscle cell cytoplasm. No differential expression was found for VAMP-2. The differential expression of SNAP-25 and its regulation of K(+) channels indicate the important role of SNAP-25 in regulating the distinct membrane excitability and contractility along the smooth muscle of the oesophagus. This is further contributed by its interactions with the cognate syntaxins, which are also differentially expressed in the muscle groups of the oesophageal body and lower oesophageal sphincter (LOS). These SNARE proteins probably have other functions in the smooth muscle cell, such as regulating vesicular transport processes.

Visualization of sequential exocytosis in rat pancreatic islet beta cells

The examination of insulin exocytosis at the single cell level by conventional electrophysiologic and amperometric methods possesses inherent limitations, and may not accurately reflect the morphologic events of exocytosis of the insulin granule. To overcome some of these limitations, we show by epifluorescent microscopy of a fluorescent dye, FM1-43, its incorporation into the plasma membrane and oncoming insulin granules undergoing exocytosis, and their core proteins. Using this method, we tracked exocytosis in real-time in insulinoma INS-1 and single rat islet beta cells in response to KCl and glucose. We observed both single transient and multi-stepwise increases in membrane FM1-43 fluorescence, suggesting single granule exocytosis as well as sequential and compound exocytosis, respectively. Confocal microscopy of nonpermeabilized cells shows that some of the exocytosed insulin granules labeled by the FM1-43 dye could also be labeled with insulin antibodies, suggesting prolonged openings of the fusion pores and slow dissolution of the granule core proteins on the membrane surface.