Adenovirus-mediated silencing of Synaptotagmin 9 inhibits Ca2+-dependent insulin secretion in islets

The Zika virus infection remodels the expression of the synaptotagmin-9 secretory protein

The exact mechanisms involved in flaviviruses virions' release and the specific secretion of viral proteins, such as the Non Structural protein-1 (NS1), are still unclear. While these processes might involve vesicular transport to the cell membrane, NS1 from some flaviviruses was shown to participate in viral assembly and release. Here, we assessed the effect of the Zika virus (ZIKV) NS1 expression on the cellular proteome to identify trafficking-related targets that may be altered in the presence of the viral protein. We detected an increase in the synaptotagmin-9 (SYT9) secretory protein, which participates in the intracellular transport of protein-laden vesicles. We confirmed the effect of NS1 on SYT9 levels by transfection models while also detecting a significant subcellular redistribution of SYT9. We found that ZIKV prM-Env proteins, required for the viral particle release, also increased SYT9 levels and changed its localization. Finally, we demonstrated that ZIKV cellular infection raises SYT9 levels and promotes changes in its subcellular localization, together with a co-distribution with both Env and NS1. Altogether, the data suggest SYT9's implication in the vesicular transport of viral proteins or virions during ZIKV infection, showing for the first time the association of synaptotagmins with the flavivirus' life cycle.

Synaptotagmin 7 C2 domains induce membrane curvature stress via electrostatic interactions and the wedge mechanism

Synaptotagmin 7 (Syt-7) is part of the synaptotagmin protein family that regulates exocytotic lipid membrane fusion. Among the family, Syt-7 stands out by its membrane binding strength and stabilization of long-lived membrane fusion pores. Given that Syt-7 vesicles form long-lived fusion pores, we hypothesize that its interactions with the membrane stabilize the specific curvatures, thicknesses, and lipid compositions that support a metastable fusion pore. Using all-atom molecular dynamics simulations and FRET-based assays of Syt-7's membrane-binding C2 domains (C2A and C2B), we found that Syt-7 C2 domains sequester anionic lipids, are sensitive to cholesterol, thin membranes, and generate lipid membrane curvature by two competing, but related mechanisms. First, Syt-7 forms strong electrostatic contacts with the membrane, generating negative curvature stress. Second, Syt-7's calcium binding loops embed in the membrane surface, acting as a wedge to thin the membrane and induce positive curvature stress. These curvature mechanisms are linked by the protein insertion depth as well as the resulting protein tilt. Simplified quantitative models of the curvature-generating mechanisms link simulation observables to their membrane-reshaping effectiveness.

Synaptotagmin 9 Modulates Spontaneous Neurotransmitter Release in Striatal Neurons by Regulating Substance P Secretion

Synaptotagmin 9 (SYT9) is a tandem C2 domain Ca²⁺ sensor for exocytosis in neuroendocrine cells; its function in neurons remains unclear. Here, we show that, in mixed-sex cultures, SYT9 does not trigger rapid synaptic vesicle exocytosis in mouse cortical, hippocampal, or striatal neurons, unless it is massively overexpressed. In striatal neurons, loss of SYT9 reduced the frequency of spontaneous neurotransmitter release events (minis). We delved into the underlying mechanism and discovered that SYT9 was localized to dense-core vesicles that contain substance P (SP). Loss of SYT9 impaired SP release, causing the observed decrease in mini frequency. This model is further supported by loss of function mutants. Namely, Ca²⁺ binding to the C2A domain of SYT9 triggered membrane fusion in vitro, and mutations that disrupted this activity abolished the ability of SYT9 to regulate both SP release and mini frequency. We conclude that SYT9 indirectly regulates synaptic transmission in striatal neurons by controlling SP release.SIGNIFICANCE STATEMENT Synaptotagmin 9 (SYT9) has been described as a Ca²⁺ sensor for dense-core vesicle (DCV) exocytosis in neuroendocrine cells, but its role in neurons remains unclear, despite widespread expression in the brain. This article examines the role of SYT9 in synaptic transmission across cultured cortical, hippocampal, and striatal neuronal preparations. We found that SYT9 regulates spontaneous neurotransmitter release in striatal neurons by serving as a Ca²⁺ sensor for the release of the neuromodulator substance P from DCVs. This demonstrates a novel role for SYT9 in neurons and uncovers a new field of study into neuromodulation by SYT9, a protein that is widely expressed in the brain.

The highly expressed calcium-insensitive synaptotagmin-11 and synaptotagmin-13 modulate insulin secretion

AIM

SYT11 and SYT13, two calcium-insensitive synaptotagmins, are downregulated in islets from type-2 diabetic donors, but their function in insulin secretion is unknown. To address this, we investigated the physiological role of these two synaptotagmins in insulin secreting cells.

METHODS

Correlations between gene expression levels were performed using previously described RNA-seq data on islets from 188 human donors. SiRNA knockdown was performed in EndoC-βH1 and INS-1 832/13 cells. Insulin secretion was measured with ELISA. Patch clamp was used for single cell electrophysiology. Confocal microscopy was used to determine intra-cellular localization.

RESULTS

Human islet expression of the transcription factor PDX-1 was positively correlated with SYT11 (p = 2.4e^-10 ) and SYT13 (p<2.2 e^-16 ). Syt11 and Syt13 both co-localized with insulin, indicating their localization in insulin granules. Downregulation of Syt11 in INS-1 832/13 cells (siSYT11) resulted in increased basal and glucose-induced insulin secretion. Downregulation of Syt13 (siSYT13) decreased insulin secretion induced by glucose and K⁺ .Interestingly, the cAMP raising agent forskolin was unable to enhance insulin secretion in siSYT13 cells. There was no difference in insulin content, exocytosis, or voltage-gated Ca²⁺ currents in the two models. Double knockdown of Syt11 and Syt13 (DKD) resembled the results in siSYT13 cells.

CONCLUSION

SYT11 and SYT13 have similar localization and transcriptional regulation but they regulate insulin secretion differentially. While downregulation of SYT11 might be a compensatory mechanism in type-2 diabetes, downregulation of SYT13 reduces the insulin secretory response and overrules the compensatory regulation of SYT11 in a way that could aggravate the disease.

Beta-Cell Adaptation to Pregnancy - Role of Calcium Dynamics

During pregnancy, the mother develops insulin resistance to shunt nutrients to the growing fetus. As a result, the maternal islets of Langerhans undergo several changes to increase insulin secretion in order to maintain glucose homeostasis and prevent the development of gestational diabetes. These changes include an increase in β-cell proliferation and β-cell mass, upregulation of insulin synthesis and insulin content, enhanced cell-to-cell communication, and a lowering of the glucose threshold for insulin secretion, all of which resulting in an increase in glucose-stimulated insulin secretion. Emerging data suggests that a change in intracellular calcium dynamics occurs in the β-cell during pregnancy as part of the adaptive process. Influx of calcium into β-cells is crucial in the regulation of glucose-stimulated insulin secretion. Calcium fluxes into and out of the cytosol, endoplasmic reticulum, and mitochondria are also important in controlling β-cell function and survival. Here, we review calcium dynamics in islets in response to pregnancy-induced changes in hormones and signaling molecules, and how these changes may enhance insulin secretion to stave off gestational diabetes.

Regulatory effects of protein S-acylation on insulin secretion and insulin action

Post-translational modifications (PTMs) such as phosphorylation and ubiquitination are well-studied events with a recognized importance in all aspects of cellular function. By contrast, protein S-acylation, although a widespread PTM with important functions in most physiological systems, has received far less attention. Perturbations in S-acylation are linked to various disorders, including intellectual disability, cancer and diabetes, suggesting that this less-studied modification is likely to be of considerable biological importance. As an exemplar, in this review, we focus on the newly emerging links between S-acylation and the hormone insulin. Specifically, we examine how S-acylation regulates key components of the insulin secretion and insulin response pathways. The proteins discussed highlight the diverse array of proteins that are modified by S-acylation, including channels, transporters, receptors and trafficking proteins and also illustrate the diverse effects that S-acylation has on these proteins, from membrane binding and micro-localization to regulation of protein sorting and protein interactions.

Distinct insulin granule subpopulations implicated in the secretory pathology of diabetes types 1 and 2

Insulin secretion from β-cells is reduced at the onset of type-1 and during type-2 diabetes. Although inflammation and metabolic dysfunction of β-cells elicit secretory defects associated with type-1 or type-2 diabetes, accompanying changes to insulin granules have not been established. To address this, we performed detailed functional analyses of insulin granules purified from cells subjected to model treatments that mimic type-1 and type-2 diabetic conditions and discovered striking shifts in calcium affinities and fusion characteristics. We show that this behavior is correlated with two subpopulations of insulin granules whose relative abundance is differentially shifted depending on diabetic model condition. The two types of granules have different release characteristics, distinct lipid and protein compositions, and package different secretory contents alongside insulin. This complexity of β-cell secretory physiology establishes a direct link between granule subpopulation and type of diabetes and leads to a revised model of secretory changes in the diabetogenic process.

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.

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.

Identification of Candidate Gene Variants in Korean MODY Families by Whole-Exome Sequencing

AIMS

To date, 13 genes causing maturity-onset diabetes of the young (MODY) have been identified. However, there is a big discrepancy in the genetic locus between Asian and Caucasian patients with MODY. Thus, we conducted whole-exome sequencing in Korean MODY families to identify causative gene variants.

METHODS

Six MODY probands and their family members were included. Variants in the dbSNP135 and TIARA databases for Koreans and the variants with minor allele frequencies >0.5% of the 1000 Genomes database were excluded. We selected only the functional variants (gain of stop codon, frameshifts and nonsynonymous single-nucleotide variants) and conducted a case-control comparison in the family members. The selected variants were scanned for the previously introduced gene set implicated in glucose metabolism.

RESULTS

Three variants c.620C>T:p.Thr207Ile in PTPRD, c.559C>G:p.Gln187Glu in SYT9, and c.1526T>G:p.Val509Gly in WFS1 were respectively identified in 3 families. We could not find any disease-causative alleles of known MODY 1-13 genes. Based on the predictive program, Thr207Ile in PTPRD was considered pathogenic.

CONCLUSIONS

Whole-exome sequencing is a valuable method for the genetic diagnosis of MODY. Further evaluation is necessary about the role of PTPRD, SYT9 and WFS1 in normal insulin release from pancreatic beta cells.

Arabidopsis Synaptotagmin 2 Participates in Pollen Germination and Tube Growth and Is Delivered to Plasma Membrane via Conventional Secretion

Arabidopsis synaptotagmin 2 (SYT2) has been reported to participate in an unconventional secretory pathway in somatic cells. Our results showed that SYT2 was expressed mainly in the pollen of Arabidopsis thaliana. The pollen of syt2 T-DNA and RNA interference mutant lines exhibited reduced total germination and impeded pollen tube growth. Analysis of the expression of SYT2-GFP fusion protein in the pollen tube indicates that SYT2 was localized to distinct, patchy compartments but could co-localize with the Golgi markers, BODIPY TR C5 ceramide and GmMan1-mCherry. However, SYT2-DsRed-E5 was localized to the plasma membrane in Arabidopsis suspension cells, in addition to the Golgi apparatus. The localization of SYT2 at the plasma membrane was further supported by immunofluorescence staining in pollen tubes. Moreover, brefeldin A treatment inhibited the transport of SYT2 to the plasma membrane and caused SYT2 to aggregate and form enlarged compartments. Truncation of the SYT2-C2AB domains also resulted in retention of SYT2 in the Golgi apparatus. An in vitro phospholipid-binding assay showed that SYT2-C2AB domains bind to the phospholipid membrane in a calcium-dependent manner. Take together, our results indicated that SYT2 was required for pollen germination and pollen tube growth, and was involved in conventional exocytosis.

Sex-specific regulation of follicle-stimulating hormone secretion by synaptotagmin 9

The anterior pituitary releases six different hormones that control virtually all aspects of vertebrate physiology, yet the molecular mechanisms underlying their Ca(2+)-triggered release remain unknown. A subset of the synaptotagmin (syt) family of proteins serve as Ca(2+) sensors for exocytosis in neurons and neuroendocrine cells, and are thus likely to regulate pituitary hormone secretion. Here we show that numerous syt isoforms are highly expressed in the pituitary gland in a lobe, and sex-specific manner. We further investigated a Ca(2+)-activated isoform, syt-9, and found that it is expressed in a subpopulation of anterior pituitary cells, the gonadotropes. Follicle-stimulating hormone (FSH) and syt-9 are highly co-localized in female, but not male, mice. Loss of syt-9 results in diminished basal and stimulated FSH secretion only in females, resulting in alterations in the oestrus cycle. This work uncovers a new function for syt-9 and reveals a novel sex difference in reproductive hormone secretion.

Peptidergic cell-specific synaptotagmins in Drosophila: localization to dense-core granules and regulation by the bHLH protein DIMMED

Bioactive peptides are packaged in large dense-core secretory vesicles, which mediate regulated secretion by exocytosis. In a variety of tissues, the regulated release of neurotransmitters and hormones is dependent on calcium levels and controlled by vesicle-associated synaptotagmin (SYT) proteins. Drosophila express seven SYT isoforms, of which two (SYT-α and SYT-β) were previously found to be enriched in neuroendocrine cells. Here we show that SYT-α and SYT-β tissue expression patterns are similar, though not identical. Furthermore, both display significant overlap with the bHLH transcription factor DIMM, a known neuroendocrine (NE) regulator. RNAi-mediated knockdown indicates that both SYT-α and SYT-β functions are essential in identified NE cells as these manipulations phenocopy loss-of-function states for the indicated peptide hormones. In Drosophila cell culture, both SYT-α and neuropeptide cargo form DIMM-dependent fluorescent puncta that are coassociated by super-resolution microscopy. DIMM is required to maintain SYT-α and SYT-β protein levels in DIMM-expressing cells in vivo. In neurons normally lacking all three proteins (DIMM(-)/SYT-α(-)/SYT-β(-)), DIMM misexpression conferred accumulation of endogenous SYT-α and SYT-β proteins. Furthermore transgenic SYT-α does not appreciably accumulate in nonpeptidergic neurons in vivo but does so if DIMM is comisexpressed. Among Drosophila syt genes, only syt-α and syt-β RNA levels are upregulated by DIMM overexpression. Together, these data suggest that SYT-α and SYT-β are important for NE cell physiology, that one or both are integral membrane components of the large dense-core vesicles, and that they are closely regulated by DIMM at a post-transcriptional level.

The functional significance of synaptotagmin diversity in neuroendocrine secretion

Synaptotagmins (syts) are abundant, evolutionarily conserved integral membrane proteins that play essential roles in regulated exocytosis in nervous and endocrine systems. There are at least 17 syt isoforms in mammals, all with tandem C-terminal C2 domains with highly variable capacities for Ca(2+) binding. Many syts play roles in neurotransmitter release or hormone secretion or both, and a growing body of work supports a role for some syts as Ca(2+) sensors of exocytosis. Work in many types of endocrine cells has documented the presence of a number of syt isoforms on dense-core vesicles containing various hormones. Syts can influence the kinetics of exocytotic fusion pores and the choice of release mode between kiss-and-run and full-fusion. Vesicles harboring different syt isoforms can preferentially undergo distinct modes of exocytosis with different forms of stimulation. The diverse properties of syt isoforms enable these proteins to shape Ca(2+) sensing in endocrine cells, thus contributing to the regulation of hormone release and the organization of complex endocrine functions.

Chromatin structure, epigenetic mechanisms and long-range interactions in the human insulin locus

Regulation of gene expression in eukaryotes is largely dependent on variations in chromatin structure. More recently, it has become clear that this may involve not only local chromatin organization but also distant regulatory elements that participate in large-scale chromatin architecture within the nucleus. We describe recent methods that make possible the detection of such structures and apply them to analysis of the human insulin (INS) locus in pancreatic islets. We show that the INS gene is part of an extended 'open' chromatin domain that includes adjacent genes as well. We also find that in islets, the INS promoter is in physical contact with distant sites on the same human chromosome and notably, with the SYT8 gene, located nearly 300 kb away. The strength of the contact between INS and SYT8 is increased by glucose, and this results in stimulation of SYT8 expression. Inhibition of INS transcription decreases SYT8 expression. Furthermore, downregulation of SYT8 results in decreased secretion of insulin. Our results thus establish the existence of a regulatory network between the INS gene and other distant genes through long-range physical interactions, and suggest that such networks may have general importance for insulin biology and diabetes.

Calcium sensing in exocytosis

Neurotransmitters, neuropeptides and hormones are released through regulated exocytosis of synaptic vesicles and large dense core vesicles. This complex and highly regulated process is orchestrated by SNAREs and their associated proteins. The triggering signal for regulated exocytosis is usually an increase in intracellular calcium levels. Besides the triggering role, calcium signaling modulates the precise amount and kinetics of vesicle release. Thus, it is a central question to understand the molecular machineries responsible for calcium sensing in exocytosis. Here we provide an overview of our current understanding of calcium sensing in neurotransmitter release and hormone secretion.

Exclusion of synaptotagmin V at the phagocytic cup by Leishmania donovani lipophosphoglycan results in decreased promastigote internalization

Regulators of membrane fusion play an important role in phagocytosis, as they regulate the focal delivery of endomembrane that is required for optimal internalization of large particles. During internalization of Leishmania promastigotes, the surface glycolipid lipophosphoglycan (LPG) is transferred to the macrophage membrane and modifies its fusogenic properties. In this study, we investigated the impact of LPG on the recruitment of the exocytosis regulator synaptotagmin V (Syt V) at the area of internalization and on the early steps of phagocytosis. Using Leishmania donovani LPG-defective mutants and LPG-coated particles, we established that LPG reduces the phagocytic capacity of macrophages and showed that it causes exclusion of Syt V from the nascent phagosome. Silencing of Syt V inhibited phagocytosis to the same extent as LPG, and these effects were not cumulative, consistent with a Syt V-dependent mechanism for the inhibition of phagocytosis by LPG. Previous work has revealed that LPG-mediated exclusion of Syt V from phagosomes prevents the recruitment of the vacuolar ATPase and acidification. Thus, whereas exclusion of Syt V from phagosomes in the process of formation may be beneficial for the creation of a hospitable intracellular niche, it reduces the phagocytic capacity of macrophages. We propose that the cost associated with a reduced internalization rate may be compensated by increased survival, and could lead to a greater overall parasite fitness.

Neuropeptide exocytosis involving synaptotagmin-4 and oxytocin in hypothalamic programming of body weight and energy balance

Hypothalamic neuropeptides play essential roles in regulating energy and body weight balance. Energy imbalance and obesity have been linked to hypothalamic signaling defects in regulating neuropeptide genes; however, it is unknown whether dysregulation of neuropeptide exocytosis could be critically involved. This study discovered that synaptotagmin-4, an atypical modulator of synaptic exocytosis, is expressed most abundantly in oxytocin neurons of the hypothalamus. Synaptotagmin-4 negatively regulates oxytocin exocytosis, and dietary obesity is associated with increased vesicle binding of synaptotagmin-4 and thus enhanced negative regulation of oxytocin release. Overexpressing synaptotagmin-4 in hypothalamic oxytocin neurons and centrally antagonizing oxytocin in mice are similarly obesogenic. Synaptotagmin-4 inhibition prevents against dietary obesity by normalizing oxytocin release and energy balance under chronic nutritional excess. In conclusion, the negative regulation of synaptotagmin-4 on oxytocin release represents a hypothalamic basis of neuropeptide exocytosis in controlling obesity and related diseases.

SUMOylation regulates insulin exocytosis downstream of secretory granule docking in rodents and humans

OBJECTIVE

The reversible attachment of small ubiquitin-like modifier (SUMO) proteins controls target localization and function. We examined an acute role for the SUMOylation pathway in downstream events mediating insulin secretion.

RESEARCH DESIGN AND METHODS

We studied islets and β-cells from mice and human donors, as well as INS-1 832/13 cells. Insulin secretion, intracellular Ca(2+), and β-cell exocytosis were monitored after manipulation of the SUMOylation machinery. Granule localization was imaged by total internal reflection fluorescence and electron microscopy; immunoprecipitation and Western blotting were used to examine the soluble NSF attachment receptor (SNARE) complex formation and SUMO1 interaction with synaptotagmin VII.

RESULTS

SUMO1 impairs glucose-stimulated insulin secretion by blunting the β-cell exocytotic response to Ca(2+). The effect of SUMO1 to impair insulin secretion and β-cell exocytosis is rapid and does not require altered gene expression or insulin content, is downstream of granule docking at the plasma membrane, and is dependent on SUMO-conjugation because the deSUMOylating enzyme, sentrin/SUMO-specific protease (SENP)-1, rescues exocytosis. SUMO1 coimmunoprecipitates with the Ca(2+) sensor synaptotagmin VII, and this is transiently lost upon glucose stimulation. SENP1 overexpression also disrupts the association of SUMO1 with synaptotagmin VII and mimics the effect of glucose to enhance exocytosis. Conversely, SENP1 knockdown impairs exocytosis at stimulatory glucose levels and blunts glucose-dependent insulin secretion from mouse and human islets.

CONCLUSIONS

SUMOylation acutely regulates insulin secretion by the direct and reversible inhibition of β-cell exocytosis in response to intracellular Ca(2+) elevation. The SUMO protease, SENP1, is required for glucose-dependent insulin secretion.

Mapping of INS promoter interactions reveals its role in long-range regulation of SYT8 transcription

Insulin (INS) synthesis and secretion from pancreatic β cells are tightly regulated; their deregulation causes diabetes. Here we map INS-associated loci in human pancreatic islets by 4C and 3C techniques and show that the INS gene physically interacts with the SYT8 gene, located over 300 kb away. This interaction is elevated by glucose and accompanied by increases in SYT8 expression. Inactivation of the INS promoter by promoter-targeting siRNA reduces SYT8 gene expression. SYT8-INS interaction and SYT8 transcription are attenuated by CTCF depletion. Furthermore, SYT8 knockdown decreases insulin secretion in islets. These results reveal a non-redundant role for SYT8 in insulin secretion and indicate that the INS promoter acts from a distance to stimulate SYT8 transcription. This suggests a function for the INS promoter in coordinating insulin transcription and secretion through long-range regulation of SYT8 expression in human islets.

RNA interference for improving the outcome of islet transplantation

Islet transplantation has the potential to cure type 1 diabetes. Despite recent therapeutic success, it is still not common because a large number of transplanted islets get damaged by multiple challenges including instant blood mediated inflammatory reaction, hypoxia/reperfusion injury, inflammatory cytokines, and immune rejection. RNA interference (RNAi) is a novel strategy to selectively degrade target mRNA. The use of RNAi technologies to downregulate the expression of harmful genes has the potential to improve the outcome of islet transplantation. The aim of this review is to gain a thorough understanding of biological obstacles to islet transplantation and discuss how to overcome these barriers using different RNAi technologies. This eventually will help improve islet survival and function post transplantation. Chemically synthesized small interferring RNA (siRNA), vector based short hairpin RNA (shRNA), and their critical design elements (such as sequences, promoters, and backbone) are discussed. The application of combinatorial RNAi in islet transplantation is also discussed. Last but not the least, several delivery strategies for enhanced gene silencing are discussed, including chemical modification of siRNA, complex formation, bioconjugation, and viral vectors.

Neuronal calcium sensor synaptotagmin-9 is not involved in the regulation of glucose homeostasis or insulin secretion

BACKGROUND

Insulin secretion is a complex and highly regulated process. It is well established that cytoplasmic calcium is a key regulator of insulin secretion, but how elevated intracellular calcium triggers insulin granule exocytosis remains unclear, and we have only begun to define the identities of proteins that are responsible for sensing calcium changes and for transmitting the calcium signal to release machineries. Synaptotagmins are primarily expressed in brain and endocrine cells and exhibit diverse calcium binding properties. Synaptotagmin-1, -2 and -9 are calcium sensors for fast neurotransmitter release in respective brain regions, while synaptotagmin-7 is a positive regulator of calcium-dependent insulin release. Unlike the three neuronal calcium sensors, whose deletion abolished fast neurotransmitter release, synaptotagmin-7 deletion resulted in only partial loss of calcium-dependent insulin secretion, thus suggesting that other calcium-sensors must participate in the regulation of insulin secretion. Of the other synaptotagmin isoforms that are present in pancreatic islets, the neuronal calcium sensor synaptotagmin-9 is expressed at the highest level after synaptotagmin-7.

METHODOLOGY/PRINCIPAL FINDINGS

In this study we tested whether synaptotagmin-9 participates in the regulation of glucose-stimulated insulin release by using pancreas-specific synaptotagmin-9 knockout (p-S9X) mice. Deletion of synaptotagmin-9 in the pancreas resulted in no changes in glucose homeostasis or body weight. Glucose tolerance, and insulin secretion in vivo and from isolated islets were not affected in the p-S9X mice. Single-cell capacitance measurements showed no difference in insulin granule exocytosis between p-S9X and control mice.

CONCLUSIONS

Thus, synaptotagmin-9, although a major calcium sensor in the brain, is not involved in the regulation of glucose-stimulated insulin release from pancreatic β-cells.

Calcium-sensing beyond neurotransmitters: functions of synaptotagmins in neuroendocrine and endocrine secretion

Neurotransmitters, neuropeptides and hormones are released through the regulated exocytosis of SVs (synaptic vesicles) and LDCVs (large dense-core vesicles), a process that is controlled by calcium. Synaptotagmins are a family of type 1 membrane proteins that share a common domain structure. Most synaptotagmins are located in brain and endocrine cells, and some of these synaptotagmins bind to phospholipids and calcium at levels that trigger regulated exocytosis of SVs and LDCVs. This led to the proposed synaptotagmin-calcium-sensor paradigm, that is, members of the synaptotagmin family function as calcium sensors for the regulated exocytosis of neurotransmitters, neuropeptides and hormones. Here, we provide an overview of the synaptotagmin family, and review the recent mouse genetic studies aimed at understanding the functions of synaptotagmins in neurotransmission and endocrine-hormone secretion. Also, we discuss potential roles of synaptotagmins in non-traditional endocrine systems.

Proteins associated with immunopurified granules from a model pancreatic islet beta-cell system: proteomic snapshot of an endocrine secretory granule

beta-Cell granules contain proteins involved in fuel regulation, which when altered, contribute to metabolic disorders including diabetes mellitus. We analyzed proteins present in purified granules from the INS-1E beta-cell model. Fifty-one component proteins were identified by LC-MS/MS including hormones, granins, protein processing components, cellular trafficking components, enzymes implicated in cellular metabolism and chaperone proteins. These findings may increase understanding of granule secretion and the processes leading to protein aggregation and beta-cell death in type-2 diabetes.

Exocytotic properties of human pancreatic beta-cells

Pancreatic beta-cells secrete insulin in response to elevated blood glucose via Ca(2+)-dependent fusion of secretory granules with the plasma membrane (regulated exocytosis). While exocytosis has been extensively investigated in rodent beta-cells, studies on human beta-cells are scarce. We have characterized the exocytotic properties of human beta-cells by insulin release measurements, carbon fiber amperometry, and capacitance measurements using the patch-clamp technique. Voltage-clamp depolarizations evoked capacitance increases in single beta-cells in a time- and voltage-dependent manner. The capacitance responses as well as insulin release from intact islets were strongly amplified by elevation of intracellular cAMP levels. Exocytosis was more dependent on Ca(2+) influx through P/Q-type than L-type Ca(2+) channels, reflecting the relative contribution of these channels to the total Ca(2+) current. Exocytosis (as monitored by capacitance or amperometric measurements) decreased during repetitive stimulation as a result of inactivation of Ca(2+) channels as well as depletion of a readily releasable pool of granules. These results reveal both similarities and differences between human and rodent beta-cells.

Synaptotagmin 2 couples mucin granule exocytosis to Ca2+ signaling from endoplasmic reticulum

Synaptotagmin 2 (Syt2) functions as a low affinity, fast exocytic Ca(2+) sensor in neurons, where it is activated by Ca(2+) influx through voltage-gated channels. Targeted insertion of lacZ into the mouse syt2 locus reveals expression in mucin-secreting goblet cells of the airways. In these cells, rapid Ca(2+) entry from the extracellular medium does not contribute significantly to stimulated secretion (Davis, C. W., and Dickey, B. F. (2008) Annu. Rev. Physiol. 70, 487-512). Nonetheless, Syt2(-/-) mice show a severe defect in acute agonist-stimulated airway mucin secretion, and Syt2(+/-) mice show a partial defect. In contrast to Munc13-2(-/-) mice (Zhu, Y., Ehre, C., Abdullah, L. H., Sheehan, J. K., Roy, M., Evans, C. M., Dickey, B. F., and Davis, C. W. (2008) J. Physiol. (Lond.) 586, 1977-1992), Syt2(-/-) mice show no spontaneous mucin accumulation, consistent with the inhibitory action of Syt2 at resting cytoplasmic Ca(2+) in neurons. In human airway goblet cells, inositol trisphosphate receptors are found in rough endoplasmic reticulum that closely invests apical mucin granules, consistent with the known dependence of exocytic Ca(2+) signaling on intracellular stores in these cells. Hence, Syt2 can serve as an exocytic sensor for diverse Ca(2+) signaling systems, and its levels are limiting for stimulated secretory function in airway goblet cells.

Synaptotagmins bind calcium to release insulin

Plasma insulin levels are determined mainly by the rate of exocytosis of the insulin-containing large dense core vesicles (LDCVs) of pancreatic islet beta-cells. This process involves the recruitment of LDCVs to the plasma membrane, where they are docked by the assembly of multiprotein SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor) complexes. However, fusion of the two membranes will proceed only in the presence of Ca(2+) ions, implicating a Ca(2+) sensor protein. The synaptotagmin gene family, comprising 15 members, was proposed to act as such Ca(2+) sensor in regulated exocytosis in neurons and neuroendocrine and endocrine cells. Herein, we review the physiological function of the various synaptotagmins with reference to their impact on insulin exocytosis. Cumulating evidence emphasizes the crucial role of synaptotagmin VII and IX as mediators of glucose-induced insulin secretion.

Synaptotagmin-1 utilizes membrane bending and SNARE binding to drive fusion pore expansion

In regulated vesicle exocytosis, SNARE protein complexes drive membrane fusion to connect the vesicle lumen with the extracellular space. The triggering of fusion pore formation by Ca(2+) is mediated by specific isoforms of synaptotagmin (Syt), which employ both SNARE complex and membrane binding. Ca(2+) also promotes fusion pore expansion and Syts have been implicated in this process but the mechanisms involved are unclear. We determined the role of Ca(2+)-dependent Syt-effector interactions in fusion pore expansion by expressing Syt-1 mutants selectively altered in Ca(2+)-dependent SNARE binding or in Ca(2+)-dependent membrane insertion in PC12 cells that lack vesicle Syts. The release of different-sized fluorescent peptide-EGFP vesicle cargo or the vesicle capture of different-sized external fluorescent probes was used to assess the extent of fusion pore dilation. We found that PC12 cells expressing partial loss-of-function Syt-1 mutants impaired in Ca(2+)-dependent SNARE binding exhibited reduced fusion pore opening probabilities and reduced fusion pore expansion. Cells with gain-of-function Syt-1 mutants for Ca(2+)-dependent membrane insertion exhibited normal fusion pore opening probabilities but the fusion pores dilated extensively. The results indicate that Syt-1 uses both Ca(2+)-dependent membrane insertion and SNARE binding to drive fusion pore expansion.

Novel aspects of the molecular mechanisms controlling insulin secretion

Pancreatic beta-cells secrete insulin by Ca(2+)-dependent exocytosis of secretory granules. beta-cell exocytosis involves SNARE (soluble NSF-attachment protein receptor) proteins similar to those controlling neurotransmitter release and depends on the close association of L-type Ca(2+) channels and granules. In most cases, the secretory granules fuse individually but there is ultrastructural and biophysical evidence of multivesicular exocytosis. Estimates of the secretory rate in beta-cells in intact islets indicate a release rate of approximately 15 granules per beta-cell per second, 100-fold higher than that observed in biochemical assays. Single-vesicle capacitance measurements reveal that the diameter of the fusion pore connecting the granule lumen with the exterior is approximately 1.4 nm. This is considerably smaller than the size of insulin and membrane fusion is therefore not obligatorily associated with release of the cargo, a feature that may contribute to the different rates of secretion detected by the biochemical and biophysical measurements. However, small molecules like ATP and GABA, which are stored together with insulin in the granules, are small enough to be released via the narrow fusion pore, which accordingly functions as a molecular sieve. We finally consider the possibility that defective fusion pore expansion accounts for the decrease in insulin secretion observed in pathophysiological states including long-term exposure to lipids.

Impaired insulin secretion and glucose intolerance in synaptotagmin-7 null mutant mice

Vertebrates express at least 15 different synaptotagmins with the same domain structure but diverse localizations and tissue distributions. Synaptotagmin-1,-2, and -9 act as calcium sensors for the fast phrase of neurotransmitter release, and synaptotagmin-12 acts as a calcium-independent modulator of release. The exact functions of the remaining 11 synaptotagmins, however, have not been established. By analogy to the role of synaptotagmin-1, -2, and -9 in neurotransmission, these other synaptotagmins may serve as Ca(2+) transducers regulating other Ca(2+)-dependent membrane processes, such as insulin secretion in pancreatic beta-cells. Of these other synaptotagmins, synaptotagmin-7 is one of the most abundant and is present in pancreatic beta-cells. To determine whether synaptotagmin-7 regulates Ca(2+)-dependent insulin secretion, we analyzed synaptotagmin-7 null mutant mice for glucose tolerance and insulin release. Here, we show that synaptotagmin-7 is required for the maintenance of systemic glucose tolerance and glucose-stimulated insulin secretion. Mutant mice have normal insulin sensitivity, insulin production, islet architecture and ultrastructural organization, and metabolic and calcium responses but exhibit impaired glucose-induced insulin secretion, indicating a calcium-sensing defect during insulin-containing secretory granule exocytosis. Taken together, our findings show that synaptotagmin-7 functions as a positive regulator of insulin secretion and may serve as a calcium sensor controlling insulin secretion in pancreatic beta cells.

Therapeutic RNA silencing of Cys-X3-Cys chemokine ligand 1 gene prevents mice from adenovirus vector-induced acute liver injury

Gene therapy using adenovirus vectors may induce acute liver injury. Tissue injury induced by an adenovirus is likely associated with elevated expression of the Cys-X3-Cys chemokine ligand 1 (CX(3)CL1)/fractalkine (FKN) protein at the site of inflammation. However, the extent to which the actions of FKN contribute to liver injury remains unclear. We induced acute liver injury in mice by a hydrodynamics-based injection of adenovirus vector, which was confirmed to depend on the presence of natural killer (NK) cells and NK-dependent interferon-gamma (IFN-gamma). When the transferred adenovirus vector was inserted with the FKN gene, the severity of liver injury increased with much more Cys-X3-Cys chemokine receptor 1 (CX(3)CR1)-positive NK cell recruitment into the liver because of exogenous overproduction of FKN protein. Moreover, when production of endogenous FKN protein was silenced by inserting FKN-small interfering RNA into the adenovirus vector or was neutralized by an FKN-specific antibody, the adenovirus-induced acute severe liver injury was notably prevented with much lower hepatic NK cell infiltration and a significant reduction in the serum levels of IFN-gamma.

CONCLUSION

Our findings suggest a strategy to prevent or alleviate adenovirus vector-induced acute liver injury by blocking FKN-CX(3)CR1 interaction in adenovirus vector-based gene therapy.

Synaptotagmin VII splice variants alpha, beta, and delta are expressed in pancreatic beta-cells and regulate insulin exocytosis

Synaptotagmins (SYT) are calcium-binding proteins that participate in regulated exocytosis. Although SYTI to IX isoforms are expressed in insulin-producing cell lines, hitherto only SYTIX has been associated with native beta-cell insulin granules and implicated in exocytosis. SYTVII was also proposed to regulate insulin exocytosis, but its subcellular location and number of alternative splice variants produced remain controversial. Only transcripts of SYTVII alpha, beta, and a novel splice variant delta are expressed in beta-cells and INS-1E cells. Western blotting revealed that INS-1E cells predominantly produced SYTVII alpha and low levels of SYTVII beta, whereas SYTVII delta was undetectable. The protein colocalized with insulin granules but not with synaptic-like microvesicles. Overexpression of SYTVII alpha resulted in decreased insulin granule content with a concomitant translocation of the variant to the plasma membrane, while SYTVII beta retained largely a granular pattern. Overexpressed SYTVII delta exhibited a distribution different to that of insulin granules and inhibited exocytosis when assessed by whole cell patch clamp capacitance recording. Silencing of SYTVII alpha by targeted RNA interference suppressed secretion, while repression of beta slightly increased release. Our results demonstrate that SYTVII is expressed on insulin granules and that only SYTVII alpha is implicated in exocytosis under physiological conditions.

The mast cell: where endocytosis and regulated exocytosis meet

We have investigated whether Ca(2+)-binding proteins, which have been implicated in the control of neurons and neuroendocrine secretion, play a role in controlling mast cell function. These studies have identified synaptotagmins (Syts) II, III, and IX as well as neuronal Ca(2+) sensor 1 (NCS-1) as important regulators of mast cell function. Strikingly, we find that these Ca(2+)-binding proteins contribute to mast cell function by regulating specific endocytic pathways. Syt II, the most abundant Syt homologue in mast cells, resides in an amine-free lysosomal compartment. Studying the function of Syt II-knocked down rat basophilic leukemia cells has shown a dual function of this homologue. Syt II is required for the downregulation of protein kinase Calpha, but it negatively regulates lysosomal exocytosis. Syt III, the next most abundant homologue, localizes to early endosomes and is required for the formation of the endocytic recycling compartment (ERC). Syt IX and NCS-1 localize to the ERC and regulate ERC export, NCS-1 by activating phosphatidylinositol 4-kinase beta. Finally, we show that recycling through the ERC is needed for secretory granule protein sorting as well as for the activation of the mitogen-activated protein kinases, extracellular signal-regulated kinase 1 and 2. Accordingly, NCS-1 stimulates Fc epsilon RI-triggered exocytosis and release of arachidonic acid metabolites.

Distinct roles of the C2A and the C2B domain of the vesicular Ca2+ sensor synaptotagmin 9 in endocrine beta-cells

Synaptotagmins form a family of calcium-sensor proteins implicated in exocytosis, and these vesicular transmembrane proteins are endowed with two cytosolic calcium-binding C2 domains, C2A and C2B. Whereas the isoforms syt1 and syt2 have been studied in detail, less is known about syt9, the calcium sensor involved in endocrine secretion such as insulin release from large dense core vesicles in pancreatic beta-cells. Using cell-based assays to closely mimic physiological conditions, we observed SNARE (soluble N-ethylmaleimide-sensitive fusion protein-attachment protein receptor)-independent translocation of syt9C2AB to the plasma membrane at calcium levels corresponding to endocrine exocytosis, followed by internalization to endosomes. The use of point mutants and truncations revealed that initial translocation required only the C2A domain, whereas the C2B domain ensured partial pre-binding of syt9C2AB to the membrane and post-stimulatory localization to endosomes. In contrast with the known properties of neuronal and neuroendocrine syt1 or syt2, the C2B domain of syt9 did not undergo calcium-dependent membrane binding despite a high degree of structural homology as observed through molecular modelling. The present study demonstrates distinct intracellular properties of syt9 with different roles for each C2 domain in endocrine cells.

Synaptotagmin-1, -2, and -9: Ca2+ Sensors for Fast Release that Specify Distinct Presynaptic Properties in Subsets of Neurons

Synaptotagmin-1 and -2 are known Ca(2+) sensors for fast synchronous neurotransmitter release, but the potential Ca(2+)-sensor functions of other synaptotagmins in release remain uncharacterized. We now show that besides synaptotagmin-1 and -2, only synaptotagmin-9 (also called synaptotagmin-5) mediates fast Ca(2+) triggering of release. Release induced by the three different synaptotagmin Ca(2+) sensors exhibits distinct kinetics and apparent Ca(2+) sensitivities, suggesting that the synaptotagmin isoform expressed by a neuron determines the release properties of its synapses. Conditional knockout mice producing GFP-tagged synaptotagmin-9 revealed that synaptotagmin-9 is primarily expressed in the limbic system and striatum. Acute deletion of synaptotagmin-9 in striatal neurons severely impaired fast synchronous release without changing the size of the readily-releasable vesicle pool. These data show that in mammalian brain, only synaptotagmin-1, -2, and -9 function as Ca(2+) sensors for fast release, and that these synaptotagmins are differentially expressed to confer distinct release properties onto synapses formed by defined subsets of neurons.

Proteomics analysis of insulin secretory granules

Insulin secretory granules (ISGs) are cytoplasmic organelles of pancreatic beta-cells. They are responsible for the storage and secretion of insulin. To date, only about 30 different proteins have been clearly described to be associated with these organelles. However, data from two-dimensional gel electrophoresis analyses suggested that almost 150 different polypeptides might be present within ISGs. The elucidation of the identity and function of the ISG proteins by proteomics strategies would be of considerable help to further understand some of the underlying mechanisms implicated in ISG biogenesis and trafficking. Furthermore it should give the bases to the comprehension of impaired insulin secretion observed during diabetes. A proteomics analysis of an enriched insulin granule fraction from the rat insulin-secreting cell line INS-1E was performed. The efficacy of the fractionation procedure was assessed by Western blot and electron microscopy. Proteins of the ISG fraction were separated by SDS-PAGE, excised from consecutive gel slices, and tryptically digested. Peptides were analyzed by nano-LC-ESI-MS/MS. This strategy identified 130 different proteins that were classified into four structural groups including intravesicular proteins, membrane proteins, novel proteins, and other proteins. Confocal microscopy analysis demonstrated the association of Rab37 and VAMP8 with ISGs in INS-1E cells. In conclusion, the present study identified 130 proteins from which 110 are new proteins associated with ISGs. The elucidation of their role will further help in the understanding of the mechanisms governing impaired insulin secretion during diabetes.

Synaptotagmins I and IX function redundantly in regulated exocytosis but not endocytosis in PC12 cells

Synaptotagmin I is considered to be a Ca2+ sensor for fast vesicle exocytosis. Because Ca2+-dependent vesicle exocytosis persists in synaptotagmin I mutants, there must be additional Ca2+ sensors. Multiple synaptotagmin isoforms co-reside on vesicles, which suggests that other isoforms complement synaptotagmin I function. We found that full downregulation of synaptotagmins I and IX, which co-reside on vesicles in PC12 cells, completely abolished Ca2+-dependent vesicle exocytosis. By contrast, Ca2+-dependent exocytosis persisted in cells expressing only synaptotagmin I or only synaptotagmin IX, which indicated a redundancy in function for these isoforms. Although either isoform was sufficient to confer Ca2+ regulation on vesicle exocytosis, synaptotagmins I and IX conferred faster and slower release rates, respectively, indicating that individual isoforms impart distinct kinetic properties to vesicle exocytosis. The downregulation of synaptotagmin I but not synaptotagmin IX impaired compensatory vesicle endocytosis, which revealed a lack of isoform redundancy and functional specialization of synaptotagmin I for endocytic retrieval.

Silence of synaptotagmin I in INS-1 cells inhibits fast exocytosis and fast endocytosis

Synaptotagmin I (Syt I) is a Ca(2+) sensor for triggering fast synchronized release of neurotransmitters. However, controversy remains whether Syt I is also obligatory for the exocytosis and endocytosis of larger dense core vesicles (LDCVs) in endocrine cells. In this study, we used a short hairpin RNA (shRNA) to silence the expression of Syt I and investigated the roles of Syt I on exocytosis and endocytosis in INS-1 cells. Our results demonstrated that expression of Syt I is remarkably reduced by the Syt I gene targeting shRNA. Using high-time resolution capacitance measurement, we found that the silence of Syt I decreased the calcium sensitivity of fusion of insulin granules and therefore reduced the exocytotic burst triggered by step-like [Ca(2+)](i) elevation. In addition, the occurrence frequency and amplitude of fast endocytosis were remarkably reduced in the silenced cells. We conclude that Syt I not only participates in the Ca(2+)-sensing of LDCV fusion with plasmalemma, but also plays a crucial role in fast endocytosis in INS-1 cells.

Distinct developmental expression of synaptotagmin I and IX in the mouse brain

Synaptotagmin IX has been postulated to function as a major Ca2+ sensor for dense-core vesicle exocytosis in neuroendocrine cells. In this study, we investigated the subcellular localization and developmental expression profile of synaptotagmin IX in the mouse brain and found that it is mainly present in the dense-core vesicle fraction, which is devoid of synaptotagmin I and synaptophysin. We also found that the synaptotagmin IX expression level is constant throughout the postnatal development of the mouse brain, whereas the synaptotagmins I, II, III, VI, and XII are upregulated in parallel with synaptogenesis. These findings suggest that synaptotagmin IX regulates the transport of certain vesicles in the brain other than synaptic vesicles.