Effects of CaMKII-mediated phosphorylation of ryanodine receptor type 2 on islet calcium handling, insulin secretion, and glucose tolerance

Disrupted Endoplasmic Reticulum Ca2+ Handling: A Harβinger of β-Cell Failure

The β-cell workload increases in the setting of insulin resistance and reduced β-cell mass, which occurs in type 2 and type 1 diabetes, respectively. The prolonged elevation of insulin production and secretion during the pathogenesis of diabetes results in β-cell ER stress. The depletion of β-cell Ca2+ER during ER stress activates the unfolded protein response, leading to β-cell dysfunction. Ca2+ER is involved in many pathways that are critical to β-cell function, such as protein processing, tuning organelle and cytosolic Ca2+ handling, and modulating lipid homeostasis. Mutations that promote β-cell ER stress and deplete Ca2+ER stores are associated with or cause diabetes (e.g., mutations in ryanodine receptors and insulin). Thus, improving β-cell Ca2+ER handling and reducing ER stress under diabetogenic conditions could preserve β-cell function and delay or prevent the onset of diabetes. This review focuses on how mechanisms that control β-cell Ca2+ER are perturbed during the pathogenesis of diabetes and contribute to β-cell failure.

Whole-exome sequencing in familial type 2 diabetes identifies an atypical missense variant in the RyR2 gene

Genome-wide association studies have identified several hundred loci associated with type 2 diabetes mellitus (T2DM). Additionally, pathogenic variants in several genes are known to cause monogenic diabetes that overlaps clinically with T2DM. Whole-exome sequencing of related individuals with T2DM is a powerful approach to identify novel high-penetrance disease variants in coding regions of the genome. We performed whole-exome sequencing on four related individuals with T2DM - including one individual diagnosed at the age of 33 years. The individuals were negative for mutations in monogenic diabetes genes, had a strong family history of T2DM, and presented with several characteristics of metabolic syndrome. A missense variant (p.N2291D) in the type 2 ryanodine receptor (RyR2) gene was one of eight rare coding variants shared by all individuals. The variant was absent in large population databases and affects a highly conserved amino acid located in a mutational hotspot for pathogenic variants in Catecholaminergic polymorphic ventricular tachycardia (CPVT). Electrocardiogram data did not reveal any cardiac abnormalities except a lower-than-normal resting heart rate (< 60 bpm) in two individuals - a phenotype observed in CPVT individuals with RyR2 mutations. RyR2-mediated Ca2+ release contributes to glucose-mediated insulin secretion and pathogenic RyR2 mutations cause glucose intolerance in humans and mice. Analysis of glucose tolerance testing data revealed that missense mutations in a CPVT mutation hotspot region - overlapping the p.N2291D variant - are associated with complete penetrance for glucose intolerance. In conclusion, we have identified an atypical missense variant in the RyR2 gene that co-segregates with diabetes in the absence of overt CPVT.

Regulation of insulin secretion by the post-translational modifications

Post-translational modification (PTM) has a significant impact on cellular signaling and function regulation. In pancreatic β cells, PTMs are involved in insulin secretion, cell development, and viability. The dysregulation of PTM in β cells is clinically associated with the development of diabetes mellitus. Here, we summarized current findings on major PTMs occurring in β cells and their roles in insulin secretion. Our work provides comprehensive insight into understanding the mechanisms of insulin secretion and potential therapeutic targets for diabetes from the perspective of protein PTMs.

Ca2+-Mediated Signaling Pathways: A Promising Target for the Successful Generation of Mature and Functional Stem Cell-Derived Pancreatic Beta Cells In Vitro

Diabetes mellitus is a chronic disease affecting over 500 million adults globally and is mainly categorized as type 1 diabetes mellitus (T1DM), where pancreatic beta cells are destroyed, and type 2 diabetes mellitus (T2DM), characterized by beta cell dysfunction. This review highlights the importance of the divalent cation calcium (Ca²⁺) and its associated signaling pathways in the proper functioning of beta cells and underlines the effects of Ca²⁺ dysfunction on beta cell function and its implications for the onset of diabetes. Great interest and promise are held by human pluripotent stem cell (hPSC) technology to generate functional pancreatic beta cells from diabetic patient-derived stem cells to replace the dysfunctional cells, thereby compensating for insulin deficiency and reducing the comorbidities of the disease and its associated financial and social burden on the patient and society. Beta-like cells generated by most current differentiation protocols have blunted functionality compared to their adult human counterparts. The Ca²⁺ dynamics in stem cell-derived beta-like cells and adult beta cells are summarized in this review, revealing the importance of proper Ca²⁺ homeostasis in beta-cell function. Consequently, the importance of targeting Ca²⁺ function in differentiation protocols is suggested to improve current strategies to use hPSCs to generate mature and functional beta-like cells with a comparable glucose-stimulated insulin secretion (GSIS) profile to adult beta cells.

RyR2 regulates store-operated Ca2+ entry, phospholipase C activity, and electrical excitability in the insulinoma cell line INS-1

The ER Ca2+ channel ryanodine receptor 2 (RyR2) is required for maintenance of insulin content and glucose-stimulated insulin secretion, in part, via regulation of the protein IRBIT in the insulinoma cell line INS-1. Here, we examined store-operated and depolarization-dependent Ca2+entry using INS-1 cells in which either RyR2 or IRBIT were deleted. Store-operated Ca2+ entry (SOCE) stimulated with thapsigargin was reduced in RyR2KO cells compared to controls, but was unchanged in IRBITKO cells. STIM1 protein levels were not different between the three cell lines. Basal and stimulated (500 μM carbachol) phospholipase C (PLC) activity was also reduced specifically in RyR2KO cells. Insulin secretion stimulated by tolbutamide was reduced in RyR2KO and IRBITKO cells compared to controls, but was potentiated by an EPAC-selective cAMP analog in all three cell lines. Cellular PIP2 levels were increased and cortical f-actin levels were reduced in RyR2KO cells compared to controls. Whole-cell Cav channel current density was increased in RyR2KO cells compared to controls, and barium current was reduced by acute activation of the lipid phosphatase pseudojanin preferentially in RyR2KO cells over control INS-1 cells. Action potentials stimulated by 18 mM glucose were more frequent in RyR2KO cells compared to controls, and insensitive to the SK channel inhibitor apamin. Taken together, these results suggest that RyR2 plays a critical role in regulating PLC activity and PIP2 levels via regulation of SOCE. RyR2 also regulates β-cell electrical activity by controlling Cav current density and SK channel activation.

RyR2/IRBIT regulates insulin gene transcript, insulin content, and secretion in the insulinoma cell line INS-1

The role of ER Ca²⁺ release via ryanodine receptors (RyR) in pancreatic β-cell function is not well defined. Deletion of RyR2 from the rat insulinoma INS-1 (RyR2^KO) enhanced IP₃ receptor activity stimulated by 7.5 mM glucose, coincident with reduced levels of the protein IP₃Receptor Binding protein released with Inositol 1,4,5 Trisphosphate (IRBIT). Insulin content, basal (2.5 mM glucose) and 7.5 mM glucose-stimulated insulin secretion were reduced in RyR2^KO and IRBIT^KO cells compared to controls. INS2 mRNA levels were reduced in both RyR2^KO and IRBIT^KO cells, but INS1 mRNA levels were specifically decreased in RyR2^KO cells. Nuclear localization of S-adenosylhomocysteinase (AHCY) was increased in RyR2^KO and IRBIT^KO cells. DNA methylation of the INS1 and INS2 gene promotor regions was very low, and not different among RyR2^KO, IRBIT^KO, and controls, but exon 2 of the INS1 and INS2 genes was more extensively methylated in RyR2^KO and IRBIT^KO cells. Exploratory proteomic analysis revealed that deletion of RyR2 or IRBIT resulted in differential regulation of 314 and 137 proteins, respectively, with 41 in common. These results suggest that RyR2 regulates IRBIT levels and activity in INS-1 cells, and together maintain insulin content and secretion, and regulate the proteome, perhaps via DNA methylation.

The manifold roles of protein S-nitrosylation in the life of insulin

Insulin, which is released by pancreatic islet β-cells in response to elevated levels of glucose in the blood, is a critical regulator of metabolism. Insulin triggers the uptake of glucose and fatty acids into the liver, adipose tissue and muscle, and promotes the storage of these nutrients in the form of glycogen and lipids. Dysregulation of insulin synthesis, secretion, transport, degradation or signal transduction all cause failure to take up and store nutrients, resulting in type 1 diabetes mellitus, type 2 diabetes mellitus and metabolic dysfunction. In this Review, we make the case that insulin signalling is intimately coupled to protein S-nitrosylation, in which nitric oxide groups are conjugated to cysteine thiols to form S-nitrosothiols, within effectors of insulin action. We discuss the role of S-nitrosylation in the life cycle of insulin, from its synthesis and secretion in pancreatic β-cells, to its signalling and degradation in target tissues. Finally, we consider how aberrant S-nitrosylation contributes to metabolic diseases, including the roles of human genetic mutations and cellular events that alter S-nitrosylation of insulin-regulating proteins. Given the growing influence of S-nitrosylation in cellular metabolism, the field of metabolic signalling could benefit from renewed focus on S-nitrosylation in type 2 diabetes mellitus and insulin-related disorders.

"Ryanopathies" and RyR2 dysfunctions: can we further decipher them using in vitro human disease models?

The regulation of intracellular calcium (Ca²⁺) homeostasis is fundamental to maintain normal functions in many cell types. The ryanodine receptor (RyR), the largest intracellular calcium release channel located on the sarco/endoplasmic reticulum (SR/ER), plays a key role in the intracellular Ca²⁺ handling. Abnormal type 2 ryanodine receptor (RyR2) function, associated to mutations (ryanopathies) or pathological remodeling, has been reported, not only in cardiac diseases, but also in neuronal and pancreatic disorders. While animal models and in vitro studies provided valuable contributions to our knowledge on RyR2 dysfunctions, the human cell models derived from patients’ cells offer new hope for improving our understanding of human clinical diseases and enrich the development of great medical advances. We here discuss the current knowledge on RyR2 dysfunctions associated with mutations and post-translational remodeling. We then reviewed the novel human cellular technologies allowing the correlation of patient’s genome with their cellular environment and providing approaches for personalized RyR-targeted therapeutics.

Bayesian variable selection for high dimensional predictors and self-reported outcomes

Background

The onset of silent diseases such as type 2 diabetes is often registered through self-report in large prospective cohorts. Self-reported outcomes are cost-effective; however, they are subject to error. Diagnosis of silent events may also occur through the use of imperfect laboratory-based diagnostic tests. In this paper, we describe an approach for variable selection in high dimensional datasets for settings in which the outcome is observed with error.

Methods

We adapt the spike and slab Bayesian Variable Selection approach in the context of error-prone, self-reported outcomes. The performance of the proposed approach is studied through simulation studies. An illustrative application is included using data from the Women’s Health Initiative SNP Health Association Resource, which includes extensive genotypic (>900,000 SNPs) and phenotypic data on 9,873 African American and Hispanic American women.

Results

Simulation studies show improved sensitivity of our proposed method when compared to a naive approach that ignores error in the self-reported outcomes. Application of the proposed method resulted in discovery of several single nucleotide polymorphisms (SNPs) that are associated with risk of type 2 diabetes in a dataset of 9,873 African American and Hispanic participants in the Women’s Health Initiative. There was little overlap among the top ranking SNPs associated with type 2 diabetes risk between the racial groups, adding support to previous observations in the literature of disease associated genetic loci that are often not generalizable across race/ethnicity populations. The adapted Bayesian variable selection algorithm is implemented in R. The source code for the simulations are available in the Supplement.

Conclusions

Variable selection accuracy is reduced when the outcome is ascertained by error-prone self-reports. For this setting, our proposed algorithm has improved variable selection performance when compared to approaches that neglect to account for the error-prone nature of self-reports.

Potential contribution of ryanodine receptor 2 upregulation to cGMP/PKG signaling-induced cone degeneration in cyclic nucleotide-gated channel deficiency

Photoreceptor cyclic nucleotide-gated (CNG) channels regulate Ca2+ influx in rod and cone photoreceptors. Mutations in cone CNG channel subunits CNGA3 and CNGB3 are associated with achromatopsia and cone dystrophies. Mice lacking functional cone CNG channel show endoplasmic reticulum (ER) stress-associated cone degeneration. The elevated cyclic guanosine monophosphate (cGMP)/cGMP-dependent protein kinase (PKG) signaling and upregulation of the ER Ca2+ channel ryanodine receptor 2 (RyR2) have been implicated in cone degeneration. This work investigates the potential contribution of RyR2 to cGMP/PKG signaling-induced ER stress and cone degeneration. We demonstrated that the expression and activity of RyR2 were highly regulated by cGMP/PKG signaling. Depletion of cGMP by deleting retinal guanylate cyclase 1 or inhibition of PKG using chemical inhibitors suppressed the upregulation of RyR2 in CNG channel deficiency. Depletion of cGMP or deletion of Ryr2 equivalently inhibited unfolded protein response/ER stress, activation of the CCAAT-enhancer-binding protein homologous protein, and activation of the cyclic adenosine monophosphate response element-binding protein, leading to early-onset cone protection. In addition, treatment with cGMP significantly enhanced Ryr2 expression in cultured photoreceptor-derived Weri-Rb1 cells. Findings from this work demonstrate the regulation of cGMP/PKG signaling on RyR2 in the retina and support the role of RyR2 upregulation in cGMP/PKG signaling-induced ER stress and photoreceptor degeneration.

The Endoplasmic Reticulum and Calcium Homeostasis in Pancreatic Beta Cells

The endoplasmic reticulum (ER) mediates the first steps of protein assembly within the secretory pathway and is the site where protein folding and quality control are initiated. The storage and release of Ca2+ are critical physiological functions of the ER. Disrupted ER homeostasis activates the unfolded protein response (UPR), a pathway which attempts to restore cellular equilibrium in the face of ER stress. Unremitting ER stress, and insufficient compensation for it results in beta-cell apoptosis, a process that has been linked to both type 1 diabetes (T1D) and type 2 diabetes (T2D). Both types are characterized by progressive beta-cell failure and a loss of beta-cell mass, although the underlying causes are different. The reduction of mass occurs secondary to apoptosis in the case of T2D, while beta cells undergo autoimmune destruction in T1D. In this review, we examine recent findings that link the UPR pathway and ER Ca2+ to beta cell dysfunction. We also discuss how UPR activation in beta cells favors cell survival versus apoptosis and death, and how ER protein chaperones are involved in regulating ER Ca2+ levels. Abbreviations: BiP, Binding immunoglobulin Protein ER; endoplasmic reticulum; ERAD, ER-associated protein degradation; IFN, interferon; IL, interleukin; JNK, c-Jun N-terminal kinase; KHE, proton-K+ exchanger; MODY, maturity-onset diabetes of young; PERK, PRKR-like ER kinase; SERCA, Sarco/Endoplasmic Reticulum Ca2+-ATPases; T1D, type 1 diabetes; T2D, type 2 diabetes; TNF, tumor necrosis factor; UPR, unfolded protein response; WRS, Wolcott-Rallison syndrome.

Arsenic is more potent than cadmium or manganese in disrupting the INS-1 beta cell microRNA landscape

Diabetes is a metabolic disorder characterized by fasting hyperglycemia and impaired glucose tolerance. Laboratory and population studies have shown that inorganic arsenic (iAs) can impair these pathways. Other metals including cadmium (Cd) and manganese (Mn) have also been linked to diabetes phenotypes. MicroRNAs, short non-coding RNAs that regulate gene expression, have emerged as potential drivers of metabolic dysfunction. MicroRNAs responsive to metal exposures in vitro have also been reported in independent studies to regulate insulin secretion in vivo. We hypothesize that microRNA dysregulation may associate with and possibly contribute to insulin secretion impairment upon exposure to iAs, Cd, or Mn. We exposed insulin secreting rat insulinoma cells to non-cytotoxic concentrations of iAs (1 µM), Cd (5 µM), and Mn (25 µM) for 24 h followed by small RNA sequencing to identify dysregulated microRNAs. RNA sequencing was then performed to further investigate changes in gene expression caused by iAs exposure. While all three metals significantly inhibited glucose-stimulated insulin secretion, high-throughput sequencing revealed distinct microRNA profiles specific to each exposure. One of the most significantly upregulated microRNAs post-iAs treatment is miR-146a (~ + 2-fold), which is known to be activated by nuclear factor κB (NF-κB) signaling. Accordingly, we found by RNA-seq analysis that genes upregulated by iAs exposure are enriched in the NF-κB signaling pathway and genes down-regulated by iAs exposure are enriched in miR-146a binding sites and are involved in regulating beta cell function. Notably, iAs exposure caused a significant decrease in the expression of Camk2a, a calcium-dependent protein kinase that regulates insulin secretion, has been implicated in type 2 diabetes, and is a likely target of miR-146a. Further studies are needed to elucidate potential interactions among NF-kB, miR-146a, and Camk2a in the context of iAs exposure.

The Cardiac Ryanodine Receptor Phosphorylation Hotspot Embraces PKA in a Phosphorylation-Dependent Manner

Ryanodine receptors (RyRs) are intracellular Ca²⁺ release channels controlling essential cellular functions. RyRs are targeted by cyclic AMP (cAMP)-dependent protein kinase A (PKA), a controversial regulation implicated in disorders ranging from heart failure to Alzheimer's. Using crystal structures, we show that the phosphorylation hotspot domain of RyR2 embraces the PKA catalytic subunit, with an extensive interface not seen in PKA complexes with peptides. We trapped an intermediary open-form PKA bound to the RyR2 domain and an ATP analog, showing that PKA can engage substrates in an open form. Phosphomimetics or prior phosphorylation at nearby sites in RyR2 either enhance or reduce the activity of PKA. Finally, we show that a phosphomimetic at S2813, a well-known target site for calmodulin-dependent kinase II, induces the formation of an alpha helix in the phosphorylation domain, resulting in increased interactions and PKA activity. This shows that the different phosphorylation sites in RyR2 are not independent.

Friend and foe: β-cell Ca2+ signaling and the development of diabetes

BACKGROUND

The divalent cation Calcium (Ca2+) regulates a wide range of processes in disparate cell types. Within insulin-producing β-cells, increases in cytosolic Ca2+ directly stimulate insulin vesicle exocytosis, but also initiate multiple signaling pathways. Mediated through activation of downstream kinases and transcription factors, Ca2+-regulated signaling pathways leverage substantial influence on a number of critical cellular processes within the β-cell. Additionally, there is evidence that prolonged activation of these same pathways is detrimental to β-cell health and may contribute to Type 2 Diabetes pathogenesis.

SCOPE OF REVIEW

This review aims to briefly highlight canonical Ca2+ signaling pathways in β-cells and how β-cells regulate the movement of Ca2+ across numerous organelles and microdomains. As a main focus, this review synthesizes experimental data from in vitro and in vivo models on both the beneficial and detrimental effects of Ca2+ signaling pathways for β-cell function and health.

MAJOR CONCLUSIONS

Acute increases in intracellular Ca2+ stimulate a number of signaling cascades, resulting in (de-)phosphorylation events and activation of downstream transcription factors. The short-term stimulation of these Ca2+ signaling pathways promotes numerous cellular processes critical to β-cell function, including increased viability, replication, and insulin production and secretion. Conversely, chronic stimulation of Ca2+ signaling pathways increases β-cell ER stress and results in the loss of β-cell differentiation status. Together, decades of study demonstrate that Ca2+ movement is tightly regulated within the β-cell, which is at least partially due to its dual roles as a potent signaling molecule.

Critical Roles of Carbon Monoxide and Nitric Oxide in Ca2+ Signaling for Insulin Secretion in Pancreatic Islets

AIMS

Glucagon-like peptide-1 (GLP-1) increases intracellular Ca²⁺ concentrations, resulting in insulin secretion from pancreatic β-cells through the sequential production of Ca²⁺ mobilizing messengers nicotinic acid adenine dinucleotide phosphate (NAADP) and cyclic ADP-ribose (cADPR). We previously found that NAADP activates the neuronal type of nitric oxide (NO) synthase (nNOS), the product of which, NO, activates guanylyl cyclase to produce cyclic guanosine monophosphate (cGMP), which, in turn, induces cADPR formation. Our aim was to explore the relationship between Ca²⁺ signals and gasotransmitters formation in insulin secretion in β-cells upon GLP-1 stimulation.

RESULTS

We show that NAADP-induced cGMP production by nNOS activation is dependent on carbon monoxide (CO) formation by heme oxygenase-2 (HO-2). Treatment with exogenous NO and CO amplifies cGMP formation, Ca²⁺ signal strength, and insulin secretion, whereas this signal is impeded when exposed to combined treatment with NO and CO. Furthermore, CO potentiates cGMP formation in a dose-dependent manner, but higher doses of CO inhibited cGMP formation. Our data with regard to zinc protoporphyrin, a HO inhibitor, and HO-2 knockdown, revealed that NO-induced cADPR formation and insulin secretion are dependent on HO-2. Consistent with this observation, the administration of NO or CO donors to type 2 diabetic mice improved glucose tolerance, but the same did not hold true when both were administered concurrently.

INNOVATION

Our research reveals the role of two gas transmitters, CO and NO, for Ca²⁺ second messengers formation in pancreatic β-cells.

CONCLUSION

These results demonstrate that CO, the downstream regulator of NO, plays a role in bridging the gap between the Ca²⁺ signaling messengers during insulin secretion in pancreatic β-cells.

CaMKII signaling in heart diseases: Emerging role in diabetic cardiomyopathy

Calcium/calmodulin-dependent protein kinase II (CaMKII) is upregulated in diabetes and significantly contributes to cardiac remodeling with increased risk of cardiac arrhythmias. Diabetes is frequently associated with atrial fibrillation, coronary artery disease, and heart failure, which may further enhance CaMKII. Activation of CaMKII occurs downstream of neurohormonal stimulation (e.g. via G-protein coupled receptors) and involve various posttranslational modifications including autophosphorylation, oxidation, S-nitrosylation and O-GlcNAcylation. CaMKII signaling regulates diverse cellular processes in a spatiotemporal manner including excitation-contraction and excitation-transcription coupling, mechanics and energetics in cardiac myocytes. Chronic activation of CaMKII results in cellular remodeling and ultimately arrhythmogenic alterations in Ca²⁺ handling, ion channels, cell-to-cell coupling and metabolism. This review addresses the detrimental effects of the upregulated CaMKII signaling to enhance the arrhythmogenic substrate and trigger mechanisms in the heart. We also briefly summarize preclinical studies using kinase inhibitors and genetically modified mice targeting CaMKII in diabetes. The mechanistic understanding of CaMKII signaling, cardiac remodeling and arrhythmia mechanisms may reveal new therapeutic targets and ultimately better treatment in diabetes and heart disease in general.

Endoplasmic reticulum stress alters ryanodine receptor function in the murine pancreatic β cell

Alterations in endoplasmic reticulum (ER) calcium (Ca2+) levels diminish insulin secretion and reduce β-cell survival in both major forms of diabetes. The mechanisms responsible for ER Ca2+ loss in β cells remain incompletely understood. Moreover, a specific role for either ryanodine receptor (RyR) or inositol 1,4,5-triphosphate receptor (IP3R) dysfunction in the pathophysiology of diabetes remains largely untested. To this end, here we applied intracellular and ER Ca2+ imaging techniques in INS-1 β cells and isolated islets to determine whether diabetogenic stressors alter RyR or IP3R function. Our results revealed that the RyR is sensitive mainly to ER stress-induced dysfunction, whereas cytokine stress specifically alters IP3R activity. Consistent with this observation, pharmacological inhibition of the RyR with ryanodine and inhibition of the IP3R with xestospongin C prevented ER Ca2+ loss under ER and cytokine stress conditions, respectively. However, RyR blockade distinctly prevented β-cell death, propagation of the unfolded protein response (UPR), and dysfunctional glucose-induced Ca2+ oscillations in tunicamycin-treated INS-1 β cells and mouse islets and Akita islets. Monitoring at the single-cell level revealed that ER stress acutely increases the frequency of intracellular Ca2+ transients that depend on both ER Ca2+ leakage from the RyR and plasma membrane depolarization. Collectively, these findings indicate that RyR dysfunction shapes ER Ca2+ dynamics in β cells and regulates both UPR activation and cell death, suggesting that RyR-mediated loss of ER Ca2+ may be an early pathogenic event in diabetes.

Identification of an early transcriptomic signature of insulin resistance and related diseases in lymphomonocytes of healthy subjects

Insulin resistance is considered to be a pathogenetic mechanism in several and diverse diseases (e.g. type 2 diabetes, atherosclerosis) often antedating them in apparently healthy subjects. The aim of this study is to investigate with a microarray based approach whether IR per se is characterized by a specific pattern of gene expression. For this purpose we analyzed the transcriptomic profile of peripheral blood mononuclear cells in two groups (10 subjects each) of healthy individuals, with extreme insulin resistance or sensitivity, matched for BMI, age and gender, selected within the MultiKnowledge Study cohort (n = 148). Data were analyzed with an ad-hoc rank-based classification method. 321 genes composed the gene set distinguishing the insulin resistant and sensitive groups, within which the "Adrenergic signaling in cardiomyocytes" KEGG pathway was significantly represented, suggesting a pattern of increased intracellular cAMP and Ca2+, and apoptosis in the IR group. The same pathway allowed to discriminate between insulin resistance and insulin sensitive subjects with BMI >25, supporting his role as a biomarker of IR. Moreover, ASCM pathway harbored biomarkers able to distinguish healthy and diseased subjects (from publicly available data sets) in IR-related diseases involving excitable cells: type 2 diabetes, chronic heart failure, and Alzheimer's disease. The altered gene expression profile of the ASCM pathway is an early molecular signature of IR and could provide a common molecular pathogenetic platform for IR-related disorders, possibly representing an important aid in the efforts aiming at preventing, early detecting and optimally treating IR-related diseases.

From insulin synthesis to secretion: Alternative splicing of type 2 ryanodine receptor gene is essential for insulin secretion in pancreatic β cells

Increases in the intracellular Ca²⁺ concentration in pancreatic islets, resulting from the Ca²⁺ mobilization from the intracellular source through the ryanodine receptor, are essential for insulin secretion by glucose. Cyclic ADP-ribose, a potent Ca²⁺ mobilizing second messenger synthesized from NAD⁺ by CD38, regulates the opening of ryanodine receptor. A novel ryanodine receptor mRNA (the islet-type ryanodine receptor) was found to be generated from the type 2 ryanodine receptor gene by the alternative splicing of exons 4 and 75. The islet-type ryanodine receptor mRNA is expressed in a variety of tissues such as pancreatic islets, cerebrum, cerebellum, and other neuro-endocrine cells, whereas the authentic type 2 ryanodine receptor mRNA (the heart-type ryanodine receptor) was found to be generated using GG/AG splicing of intron 75 and is expressed in the heart and the blood vessel. The islet-type ryanodine receptor caused a greater increase in the Ca²⁺ release by caffeine when expressed in HEK293 cells pre-treated with cyclic ADP-ribose, suggesting that the novel ryanodine receptor is an intracellular target for the CD38-cyclic ADP-ribose signal system in mammalian cells and that the tissue-specific alternative splicing of type 2 ryanodine receptor mRNA plays an important role in the functioning of the cyclic ADP-ribose-sensitive Ca²⁺ release.

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.

Conformational Dynamics in FKBP Domains: Relevance to Molecular Signaling and Drug Design

Among the 22 FKBP domains in the human genome, FKBP12.6 and the first FKBP domains (FK1) of FKBP51 and FKBP52 are evolutionarily and structurally most similar to the archetypical FKBP12. As such, the development of inhibitors with selectivity among these four FKBP domains poses a significant challenge for structure-based design. The pleiotropic effects of these FKBP domains in a range of signaling processes such as the regulation of ryanodine receptor calcium channels by FKBP12 and FKBP12.6 and steroid receptor regulation by the FK1 domains of FKBP51 and FKBP52 amply justify the efforts to develop selective therapies. In contrast to their close structural similarities, these four FKBP domains exhibit a substantial diversity in their conformational flexibility. A number of distinct conformational transitions have been characterized for FKBP12 spanning timeframes from 20 s to 10 ns and in each case these dynamics have been shown to markedly differ from the conformational behavior for one or more of the other three FKBP domains. Protein flexibilitybased inhibitor design could draw upon the transitions that are significantly populated in only one of the targeted proteins. Both the similarities and differences among these four proteins valuably inform the understanding of how dynamical effects propagate across the FKBP domains as well as potentially how such intramolecular transitions might couple to the larger scale transitions that are central to the signaling complexes in which these FKBP domains function.

Junctophilin 3 expresses in pancreatic beta cells and is required for glucose-stimulated insulin secretion

It is well accepted that junctophilin (JPHs) isoforms act as a physical bridge linking plasma membrane and endoplasmic reticulum (ER) for channel crosstalk in excitable cells. Our purpose is to investigate whether JPHs are involved in the proper communication between Ca(2+) influx and subsequent Ca(2+) amplification in pancreatic beta cells, thereby participating in regulating insulin secretion. The expression of JPH isoforms was examined in human and mouse pancreatic tissues, and JPH3 expression was found in both the beta cells. In mice, knockdown of Jph3 (si-Jph3) in islets decreased glucose-stimulated insulin secretion (GSIS) accompanied by mitochondrial function impairment. Si-Jph3 lowered the insulin secretory response to Ca(2+) signaling in the presence of glucose, and reduced [Ca(2+)]c transient amplitude triggered by caffeine. Si-Jph3 also attenuated mitofusin 2 expression, thereby disturbing the spatial organization of ER-mitochondria contact in islets. These results suggest that the regulation of GSIS by the KATP channel-independent pathways is partly impaired due to decrease of JPH3 expression in mouse islets. JPH3 also binds to type 2 ryanodine receptors (RyR2) in mouse and human pancreatic tissues, which might contribute to Ca(2+) release amplification in GSIS. This study demonstrates some previously unrecognized findings in pancreatic tissues: (1) JPH3 expresses in mouse and human beta cells; (2) si-Jph3 in mouse primary islets impairs GSIS in vitro; (3) impairment in GSIS in si-Jph3 islets is due to changes in RyR2-[Ca(2+)]c transient amplitude and ER-mitochondria contact.

Pancreatic β Cell Mass Death

Type two diabetes (T2D) is a challenging metabolic disorder for which a cure has not yet been found. Its etiology is associated with several phenomena, including significant loss of insulin-producing, beta cell (β cell) mass via progressive programmed cell death and disrupted cellular autophagy. In diabetes, the etiology of β cell death and the role of mitochondria are complex and involve several layers of mechanisms. Understanding the dynamics of those mechanisms could permit researchers to develop an intervention for the progressive loss of β cells. Currently, diabetes research has shifted toward rejuvenation and plasticity technology and away from the simplified approach of hormonal compensation. Diabetes research is currently challenged by questions such as how to enhance cell survival, decrease apoptosis and replenish β cell mass in diabetic patients. In this review, we discuss evidence that β cell development and mass formation are guided by specific signaling systems, particularly hormones, transcription factors, and growth factors, all of which could be manipulated to enhance mass growth. There is also strong evidence that β cells are dynamically active cells, which, under specific conditions such as obesity, can increase in size and subsequently increase insulin secretion. In certain cases of aggressive or advanced forms of T2D, β cells become markedly impaired, and the only alternatives for maintaining glucose homeostasis are through partial or complete cell grafting (the Edmonton protocol). In these cases, the harvesting of an enriched population of viable β cells is required for transplantation. This task necessitates a deep understanding of the pharmacological agents that affect β cell survival, mass, and function. The aim of this review is to initiate discussion about the important signals in pancreatic β cell development and mass formation and to highlight the process by which cell death occurs in diabetes. This review also examines the attempts that have been made to recover or increase cell mass in diabetic patients by using various pharmacological agents.

Sorcin Links Pancreatic β-Cell Lipotoxicity to ER Ca2+ Stores

Preserving β-cell function during the development of obesity and insulin resistance would limit the worldwide epidemic of type 2 diabetes. Endoplasmic reticulum (ER) calcium (Ca(2+)) depletion induced by saturated free fatty acids and cytokines causes β-cell ER stress and apoptosis, but the molecular mechanisms behind these phenomena are still poorly understood. Here, we demonstrate that palmitate-induced sorcin downregulation and subsequent increases in glucose-6-phosphatase catalytic subunit-2 (G6PC2) levels contribute to lipotoxicity. Sorcin is a calcium sensor protein involved in maintaining ER Ca(2+) by inhibiting ryanodine receptor activity and playing a role in terminating Ca(2+)-induced Ca(2+) release. G6PC2, a genome-wide association study gene associated with fasting blood glucose, is a negative regulator of glucose-stimulated insulin secretion (GSIS). High-fat feeding in mice and chronic exposure of human islets to palmitate decreases endogenous sorcin expression while levels of G6PC2 mRNA increase. Sorcin-null mice are glucose intolerant, with markedly impaired GSIS and increased expression of G6pc2 Under high-fat diet, mice overexpressing sorcin in the β-cell display improved glucose tolerance, fasting blood glucose, and GSIS, whereas G6PC2 levels are decreased and cytosolic and ER Ca(2+) are increased in transgenic islets. Sorcin may thus provide a target for intervention in type 2 diabetes.

Pancreatic regulation of glucose homeostasis

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

Npas4 Transcription Factor Expression Is Regulated by Calcium Signaling Pathways and Prevents Tacrolimus-induced Cytotoxicity in Pancreatic Beta Cells

Cytosolic calcium influx activates signaling pathways known to support pancreatic beta cell function and survival by modulating gene expression. Impaired calcium signaling leads to decreased beta cell mass and diabetes. To appreciate the causes of these cytotoxic perturbations, a more detailed understanding of the relevant signaling pathways and their respective gene targets is required. In this study, we examined the calcium-induced expression of the cytoprotective beta cell transcription factor Npas4. Pharmacological inhibition implicated the calcineurin, Akt/protein kinase B, and Ca(2+)/calmodulin-dependent protein kinase signaling pathways in the regulation of Npas4 transcription and translation. Both Npas4 mRNA and protein had high turnover rates, and, at the protein level, degradation was mediated via the ubiquitin-proteasome pathway. Finally, beta cell cytotoxicity of the calcineurin inhibitor and immunosuppressant tacrolimus (FK-506) was prevented by Npas4 overexpression. These results delineate the pathways regulating Npas4 expression and stability and demonstrate its importance in clinical settings such as islet transplantation.

Possible abilities of dietary factors to prevent and treat diabetes via the stimulation of glucagon-like peptide-1 secretion

There is a pressing need for countermeasures against diabetes, which has increased in incidence, becoming a global issue. Glucagon-like peptide-1 (GLP-1), a molecule secreted in enteroendocrine L cells in the lower small and large intestines, is thought to be one of the most important molecular targets for the prevention and treatment of diabetes. There has been increasing interest in the possible ability of dietary factors to treat diabetes via modulating GLP-1 secretion. There is thought to be a close relationship between incretin and diet, and the purported best approach for using dietary factors to increase GLP-1 activity is promotion of secretion of endogenous GLP-1. It have been reported that nutrients as well as various non-nutrient dietary factors can function as GLP-1 secretogogues. Here, we present our findings on the GLP-1 secretion-stimulating functions of two dietary factors, curcumin and extract of edible sweet potato leaves, which contain caffeoylquinic acid derivatives. However, it is necessary to reveal in greater detail the stimulation of GLP-1 secretion by dietary factors for preventing and treating diabetes. It is desirable to clarify the exact GLP-1 secretory pathway, the effect of metabolites derived from dietary factors in gut lumen, and the relationship between incretin and meal.

Characterization of pancreatic NMDA receptors as possible drug targets for diabetes treatment

In the nervous system, NMDA receptors (NMDARs) participate in neurotransmission and modulate the viability of neurons. In contrast, little is known about the role of NMDARs in pancreatic islets and the insulin-secreting beta cells whose functional impairment contributes to diabetes mellitus. Here we found that inhibition of NMDARs in mouse and human islets enhanced their glucose-stimulated insulin secretion (GSIS) and survival of islet cells. Further, NMDAR inhibition prolonged the amount of time that glucose-stimulated beta cells spent in a depolarized state with high cytosolic Ca(2+) concentrations. We also noticed that, in vivo, the NMDAR antagonist dextromethorphan (DXM) enhanced glucose tolerance in mice, and that in vitro dextrorphan, the main metabolite of DXM, amplified the stimulatory effect of exendin-4 on GSIS. In a mouse model of type 2 diabetes mellitus (T2DM), long-term treatment with DXM improved islet insulin content, islet cell mass and blood glucose control. Further, in a small clinical trial we found that individuals with T2DM treated with DXM showed enhanced serum insulin concentrations and glucose tolerance. Our data highlight the possibility that antagonists of NMDARs may provide a useful adjunct treatment for diabetes.

Shared molecular pathways and gene networks for cardiovascular disease and type 2 diabetes mellitus in women across diverse ethnicities

BACKGROUND

Although cardiovascular disease (CVD) and type 2 diabetes mellitus (T2D) share many common risk factors, potential molecular mechanisms that may also be shared for these 2 disorders remain unknown.

METHODS AND RESULTS

Using an integrative pathway and network analysis, we performed genome-wide association studies in 8155 blacks, 3494 Hispanic American, and 3697 Caucasian American women who participated in the national Women's Health Initiative single-nucleotide polymorphism (SNP) Health Association Resource and the Genomics and Randomized Trials Network. Eight top pathways and gene networks related to cardiomyopathy, calcium signaling, axon guidance, cell adhesion, and extracellular matrix seemed to be commonly shared between CVD and T2D across all 3 ethnic groups. We also identified ethnicity-specific pathways, such as cell cycle (specific for Hispanic American and Caucasian American) and tight junction (CVD and combined CVD and T2D in Hispanic American). In network analysis of gene-gene or protein-protein interactions, we identified key drivers that included COL1A1, COL3A1, and ELN in the shared pathways for both CVD and T2D. These key driver genes were cross-validated in multiple mouse models of diabetes mellitus and atherosclerosis.

CONCLUSIONS

Our integrative analysis of American women of 3 ethnicities identified multiple shared biological pathways and key regulatory genes for the development of CVD and T2D. These prospective findings also support the notion that ethnicity-specific susceptibility genes and process are involved in the pathogenesis of CVD and T2D.

The ryanodine receptor is expressed in human pancreatic acinar cells and contributes to acinar cell injury

Physiological calcium (Ca(2+)) signals within the pancreatic acinar cell regulate enzyme secretion, whereas aberrant Ca(2+) signals are associated with acinar cell injury. We have previously identified the ryanodine receptor (RyR), a Ca(2+) release channel on the endoplasmic reticulum, as a modulator of these pathological signals. In the present study, we establish that the RyR is expressed in human acinar cells and mediates acinar cell injury. We obtained pancreatic tissue from cadaveric donors and identified isoforms of RyR1 and RyR2 by qPCR. Immunofluorescence staining of the pancreas showed that the RyR is localized to the basal region of the acinar cell. Furthermore, the presence of RyR was confirmed from isolated human acinar cells by tritiated ryanodine binding. To determine whether the RyR is functionally active, mouse or human acinar cells were loaded with the high-affinity Ca(2+) dye (Fluo-4 AM) and stimulated with taurolithocholic acid 3-sulfate (TLCS) (500 μM) or carbachol (1 mM). Ryanodine (100 μM) pretreatment reduced the magnitude of the Ca(2+) signal and the area under the curve. To determine the effect of RyR blockade on injury, human acinar cells were stimulated with pathological stimuli, the bile acid TLCS (500 μM) or the muscarinic agonist carbachol (1 mM) in the presence or absence of the RyR inhibitor ryanodine. Ryanodine (100 μM) caused an 81% and 47% reduction in acinar cell injury, respectively, as measured by lactate dehydrogenase leakage (P < 0.05). Taken together, these data establish that the RyR is expressed in human acinar cells and that it modulates acinar Ca(2+) signals and cell injury.

Endoplasmic reticulum stress in insulin resistance and diabetes

The endoplasmic reticulum is the main intracellular Ca(2+) store for Ca(2+) release during cell signaling. There are different strategies to avoid ER Ca(2+) depletion. Release channels utilize first Ca(2+)-bound to proteins and this minimizes the reduction of the free luminal [Ca(2+)]. However, if release channels stay open after exhaustion of Ca(2+)-bound to proteins, then the reduction of the free luminal ER [Ca(2+)] (via STIM proteins) activates Ca(2+) entry at the plasma membrane to restore the ER Ca(2+) load, which will work provided that SERCA pump is active. Nevertheless, there are several noxious conditions that result in decreased activity of the SERCA pump such as oxidative stress, inflammatory cytokines, and saturated fatty acids, among others. These conditions result in a deficient restoration of the ER [Ca(2+)] and lead to the ER stress response that should facilitate recovery of the ER. However, if the stressful condition persists then ER stress ends up triggering cell death and the ensuing degenerative process leads to diverse pathologies; particularly insulin resistance, diabetes and several of the complications associated with diabetes. This scenario suggests that limiting ER stress should decrease the incidence of diabetes and the mobility and mortality associated with this illness.

Calcium-signaling components in rat insulinoma β-cells (INS-1) and pancreatic islets are differentially influenced by melatonin

The pineal secretory product melatonin exerts its influence on the insulin secretion of pancreatic islets by different signaling pathways. The purpose of this study was to analyze the impact of melatonin on calcium-signaling components under different conditions. In a transfected INS-1 cell line overexpressing the human MT2 receptor (hMT2-INS-1), melatonin treatment induced even stronger depressive effects on calcium/calmodulin-dependent kinase 2d and IV (Camk2d, CamkIV) transcripts during 3-isobutyl-1-methylxanthine (IBMX) treatment than in normal INS-1 cells, indicating a crucial influence of melatonin receptor density on transcript-level regulation. In addition, melatonin induced a significant downregulation of calmodulin (Calm1) in IBMX-treated hMT2-INS-1 cells. Long-term administration of melatonin alone reduced CamkIV transcript levels in INS-1 cells; however, transcript levels of Camk2d remained unchanged. The release of insulin was diminished under long-term melatonin treatment. The impact of melatonin also involved reductions in CAMK2D protein during IBMX or forskolin treatments in INS-1 cells, as measured by an enzyme-linked immunosorbent assay, indicating a functional significance of transcriptional changes in pancreatic islets. Furthermore, analysis of melatonin receptor knockout mice showed that the transcript levels of Camk2d, CamkIV, and Calm1 were differentially influenced according to the melatonin receptor subtype deleted. In conclusion, this study provides evidence that melatonin has different impacts on the regulation of Calm1 and Camk. These calcium-signaling components are known as participants in the calcium/calmodulin pathway, which plays an important functional role in the modulation of the β-cell signaling pathways leading to insulin secretion.

Mechanisms of CaMKII Activation in the Heart

Calcium/calmodulin (Ca²⁺/CaM) dependent protein kinase II (CaMKII) has emerged as a key nodal protein in the regulation of cardiac physiology and pathology. Due to the particularly elegant relationship between the structure and function of the kinase, CaMKII is able to translate a diverse set of signaling events into downstream physiological effects. While CaMKII is typically autoinhibited at basal conditions, prolonged rapid Ca²⁺ cycling can activate the kinase and allow post-translational modifications that depend critically on the biochemical environment of the heart. These modifications result in sustained, autonomous CaMKII activation and have been associated with pathological cardiac signaling. Indeed, improved understanding of CaMKII activation mechanisms could potentially lead to new clinical therapies for the treatment or prevention of cardiovascular disease. Here we review the known mechanisms of CaMKII activation and discuss some of the pathological signaling pathways in which they play a role.

Inhibition of pancreatic β-cell Ca2+/calmodulin-dependent protein kinase II reduces glucose-stimulated calcium influx and insulin secretion, impairing glucose tolerance

Glucose-stimulated insulin secretion (GSIS) from pancreatic β-cells is caused by Ca(2+) entry via voltage-dependent Ca(2+) channels. CaMKII is a key mediator and feedback regulator of Ca(2+) signaling in many tissues, but its role in β-cells is poorly understood, especially in vivo. Here, we report that mice with conditional inhibition of CaMKII in β-cells show significantly impaired glucose tolerance due to decreased GSIS. Moreover, β-cell CaMKII inhibition dramatically exacerbates glucose intolerance following exposure to a high fat diet. The impairment of islet GSIS by β-cell CaMKII inhibition is not accompanied by changes in either glucose metabolism or the activities of KATP and voltage-gated potassium channels. However, glucose-stimulated Ca(2+) entry via voltage-dependent Ca(2+) channels is reduced in islet β-cells with CaMKII inhibition, as well as in primary wild-type β-cells treated with a peptide inhibitor of CaMKII. The levels of basal β-cell cytoplasmic Ca(2+) and of endoplasmic reticulum Ca(2+) stores are also decreased by CaMKII inhibition. In addition, CaMKII inhibition suppresses glucose-stimulated action potential firing frequency. These results reveal that CaMKII is a Ca(2+) sensor with a key role as a feed-forward stimulator of β-cell Ca(2+) signals that enhance GSIS under physiological and pathological conditions.

Uncoupling of Cav1.2 from Ca(2+)-induced Ca(2+) release and SK channel regulation in pancreatic β-cells

We investigated the role of Cav1.2 in pancreatic β-cell function by expressing a Cav1.2 II-III loop/green fluorescent protein fusion in INS-1 cells (Cav1.2/II-III cells) to disrupt channel-protein interactions. Neither block of KATP channels nor stimulation of membrane depolarization by tolbutamide was different in INS-1 cells compared with Cav1.2/II-III cells, but whole-cell Cav current density was significantly increased in Cav1.2/II-III cells. Tolbutamide (200 μM) stimulated insulin secretion and Ca(2+) transients in INS-1 cells, and Cav1.2/II-III cells were completely blocked by nicardipine (2 μM), but thapsigargin (1 μM) blocked tolbutamide-stimulated secretion and Ca(2+) transients only in INS-1 cells. Tolbutamide-stimulated endoplasmic reticulum [Ca(2+)] decrease was reduced in Cav1.2/II-III cells compared with INS-1 cells. However, Ca(2+) transients in both INS-1 cells and Cav1.2/II-III cells were significantly potentiated by 8-pCPT-2'-O-Me-cAMP (5 μM), FPL-64176 (0.5 μM), or replacement of extracellular Ca(2+) with Sr(2+). Glucose (10 mM) + glucagon-like peptide-1 (10 nM) stimulated discrete spikes in [Ca(2+)]i in the presence of verapamil at a higher frequency in INS-1 cells than in Cav1.2/II-II cells. Glucose (18 mM) stimulated more frequent action potentials in Cav1.2/II-III cells and primary rat β-cells expressing the Cav1.2/II-II loop than in control cells. Further, apamin (1 μM) increased glucose-stimulated action potential frequency in INS-1 cells, but not Cav1.2/II-III cells, suggesting that SK channels were not activated under these conditions in Cav1.2/II-III loop-expressing cells. We propose the II-III loop of Cav1.2 as a key molecular determinant that couples the channel to Ca(2+)-induced Ca(2+) release and activation of SK channels in pancreatic β-cells.

Impaired glucose tolerance in a mouse model of sidt2 deficiency

Sidt2 was identified as a novel integral lysosomal membrane protein recently. We generated global Sidt2 knockout mice by gene targeting. These mice have a comparatively higher random and fasting glucose concentration. Intraperitoneal and oral glucose tolerance tests in Sidt2 knockout mice indicated glucose intolerance and decreased serum insulin level. Notably, the Sidt2(-/-) mice had hypertrophic islets compared with control mice. By Western blot and immunofluorescence, Sidt2(-/-) mouse islets were shown to have increased insulin protein, which actually contained more insulin secretory granules than their controls, demonstrated by electromicroscopy. Consistent with the in vivo study, isolated islet culture from the Sidt2(-/-) mice produced less insulin when stimulated by a high concentration of glucose or a depolarizing concentration of KCl. Under electromicroscope less empty vesicles and more mature ones in Sidt2(-/-) mice islets were observed, supporting impaired insulin secretory granule release. In conclusion, Sidt2 may play a critical role in the regulation of mouse insulin secretory granule secretion.