Reduced insulin exocytosis in human pancreatic β-cells with gene variants linked to type 2 diabetes

Kindlin-2 modulates MafA and β-catenin expression to regulate β-cell function and mass in mice

β-Cell dysfunction and reduction in β-cell mass are hallmark events of diabetes mellitus. Here we show that β-cells express abundant Kindlin-2 and deleting its expression causes severe diabetes-like phenotypes without markedly causing peripheral insulin resistance. Kindlin-2, through its C-terminal region, binds to and stabilizes MafA, which activates insulin expression. Kindlin-2 loss impairs insulin secretion in primary human and mouse islets in vitro and in mice by reducing, at least in part, Ca²⁺ release in β-cells. Kindlin-2 loss activates GSK-3β and downregulates β-catenin, leading to reduced β-cell proliferation and mass. Kindlin-2 loss reduces the percentage of β-cells and concomitantly increases that of α-cells during early pancreatic development. Genetic activation of β-catenin in β-cells restores the diabetes-like phenotypes induced by Kindlin-2 loss. Finally, the inducible deletion of β-cell Kindlin-2 causes diabetic phenotypes in adult mice. Collectively, our results establish an important function of Kindlin-2 and provide a potential therapeutic target for diabetes.

Beta cell dysfunction and reduction in beta cell mass are hallmark events in the pathogenesis of diabetes mellitus. We identify focal adhesion protein Kindlin-2 as a key factor that controls insulin synthesis and secretion and beta cell mass by modulating MafA and beta-catenin proteins in pancreatic beta cells.

Somatostatin secretion by Na+-dependent Ca2+-induced Ca2+ release in pancreatic delta-cells

Pancreatic islets are complex micro-organs consisting of at least three different cell types: glucagon-secreting α-, insulin-producing β- and somatostatin-releasing δ-cells¹. Somatostatin is a powerful paracrine inhibitor of insulin and glucagon secretion². In diabetes, increased somatostatinergic signalling leads to defective counter-regulatory glucagon secretion³. This increases the risk of severe hypoglycaemia, a dangerous complication of insulin therapy⁴. The regulation of somatostatin secretion involves both intrinsic and paracrine mechanisms⁵ but their relative contributions and whether they interact remains unclear. Here we show that dapagliflozin-sensitive glucose- and insulin-dependent sodium uptake stimulates somatostatin secretion by elevating the cytoplasmic Na⁺ concentration ([Na⁺]_i) and promoting intracellular Ca²⁺-induced Ca²⁺ release (CICR). This mechanism also becomes activated when [Na⁺]_i is elevated following the inhibition of the plasmalemmal Na⁺-K⁺ pump by reductions of the extracellular K⁺ concentration emulating those produced by exogenous insulin in vivo ⁶. Islets from some donors with type-2 diabetes hypersecrete somatostatin, leading to suppression of glucagon secretion that can be alleviated by a somatostatin receptor antagonist. Our data highlight the role of Na⁺ as an intracellular second messenger, illustrate the significance of the intraislet paracrine network and provide a mechanistic framework for pharmacological correction of the hormone secretion defects associated with diabetes that selectively target the δ-cells.

Selectively Bred Diabetes Models: GK Rats, NSY Mice, and ON Mice

The polygenic background of selectively bred diabetes models mimics the etiology of type 2 diabetes. So far, three different rodent models (Goto-Kakizaki rats, Nagoya-Shibata-Yasuda mice, and Oikawa-Nagao mice) have been established in the diabetes research field by continuous selective breeding for glucose tolerance from outbred rodent stocks. The origin of hyperglycemia in these rodents is mainly insulin secretion deficiency from the pancreatic β-cells and mild insulin resistance in insulin target organs. In this chapter, we summarize backgrounds and phenotypes of these rodent models to highlight their importance in diabetes research. Then, we introduce experimental methodologies to evaluate β-cell exocytosis as a putative common defect observed in these rodent models.

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

Background

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

Scope of review

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

Major Conclusions

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

Carriers of the TCF7L2 rs7903146, rs12255372 Risk Alleles in the South Tamil Nadu T2DM Patients Present with Early Incidence and Insulin Dependence

BACKGROUND AND AIMS

Metabolic abnormalities in T2DM (Type 2 diabetes mellitus) include classic manifestations such as impaired insulin secretion, synthesis and peripheral insulin resistance. The intronic variants rs7903146 and rs12255372 of the TCF7L2 (transcription factor 7-like 2) gene are strongly associated with risk of incidence of T2DM and impaired β-cell functions. Studies addressing the early T2DM onset, and early insulin dependence in T2DM patients of south Tamil Nadu are still lacking, and hence the present study focuses in determining the influence of the TCF7L2 polymorphisms in the incidence and disease course in the T2DM patients of south Tamil Nadu.

METHODS

Anthropometric measurements and biochemical parameters were carried out in early onset (Group A), early onset insulin dependent T2DM patients (Group B) and non-insulin dependent T2DM patients (Group C). PCR, allele specific PCR (ASP), PCR product sequencing strategies were utilized to determine the genotype and the impact of the TCF7L2 SNPs in the T2DM disease course.

RESULTS

Female T2DM patients with the CT/TT rs7903146 genotype (P = 0.005) and the rs12255372 GT/TT genotype (P = 0.036) exhibited a significantly low mean age for T2DM incidence. Correlation/regression analysis in the T2DM patients revealed that rs12255372 (P = 0.042) is associated with early onset in the Group C patients and the rs7903146 (P = 0.018), rs12255372 (P = 0.026) are associated with insulin dependence in the group B patients.

CONCLUSION

Screening for the TCF7L2 polymorphisms will prevent T2DM incidence and enable life style changes, appropriate therapeutic strategies that would help combat the accelerated disease course in the T2DM patients.

ATAC-seq reveals alterations in open chromatin in pancreatic islets from subjects with type 2 diabetes

Impaired insulin secretion from pancreatic islets is a hallmark of type 2 diabetes (T2D). Altered chromatin structure may contribute to the disease. We therefore studied the impact of T2D on open chromatin in human pancreatic islets. We used assay for transposase-accessible chromatin using sequencing (ATAC-seq) to profile open chromatin in islets from T2D and non-diabetic donors. We identified 57,105 and 53,284 ATAC-seq peaks representing open chromatin regions in islets of non-diabetic and diabetic donors, respectively. The majority of ATAC-seq peaks mapped near transcription start sites. Additionally, peaks were enriched in enhancer regions and in regions where islet-specific transcription factors (TFs), e.g. FOXA2, MAFB, NKX2.2, NKX6.1 and PDX1, bind. Islet ATAC-seq peaks overlap with 13 SNPs associated with T2D (e.g. rs7903146, rs2237897, rs757209, rs11708067 and rs878521 near TCF7L2, KCNQ1, HNF1B, ADCY5 and GCK, respectively) and with additional 67 SNPs in LD with known T2D SNPs (e.g. SNPs annotated to GIPR, KCNJ11, GLIS3, IGF2BP2, FTO and PPARG). There was enrichment of open chromatin regions near highly expressed genes in human islets. Moreover, 1,078 open chromatin peaks, annotated to 898 genes, differed in prevalence between diabetic and non-diabetic islet donors. Some of these peaks are annotated to candidate genes for T2D and islet dysfunction (e.g. HHEX, HMGA2, GLIS3, MTNR1B and PARK2) and some overlap with SNPs associated with T2D (e.g. rs3821943 near WFS1 and rs508419 near ANK1). Enhancer regions and motifs specific to key TFs including BACH2, FOXO1, FOXA2, NEUROD1, MAFA and PDX1 were enriched in differential islet ATAC-seq peaks of T2D versus non-diabetic donors. Our study provides new understanding into how T2D alters the chromatin landscape, and thereby accessibility for TFs and gene expression, in human pancreatic islets.

A glucose-dependent spatial patterning of exocytosis in human β-cells is disrupted in type 2 diabetes

Impaired insulin secretion in type 2 diabetes (T2D) is linked to reduced insulin granule docking, disorganization of the exocytotic site, and an impaired glucose-dependent facilitation of insulin exocytosis. We show in β-cells from 80 human donors that the glucose-dependent amplification of exocytosis is disrupted in T2D. Spatial analyses of granule fusion, visualized by total internal reflection fluorescence (TIRF) microscopy in 24 of these donors, demonstrate that these are non-random across the surface of β-cells from donors with no diabetes (ND). The compartmentalization of events occurs within regions defined by concurrent or recent membrane-resident secretory granules. This organization, and the number of membrane-associated granules, is glucose-dependent and notably impaired in T2D β-cells. Mechanistically, multi-channel Kv2.1 clusters contribute to maintaining the density of membrane-resident granules and the number of fusion 'hotspots', while SUMOylation sites at the channel N- (K145) and C-terminus (K470) determine the relative proportion of fusion events occurring within these regions. Thus, a glucose-dependent compartmentalization of fusion, regulated in part by a structural role for Kv2.1, is disrupted in β-cells from donors with type 2 diabetes.

Screening the RFX6-DNA binding domain for potential genetic variants in patients with type 2 diabetes

BACKGROUND

The regulatory factor X6 (RFX6), a member of regulatory factor X family, is known to play a key role in the development and differentiation of pancreatic beta cells as well as insulin production and secretion. However, the potential role of RFX6 in type 2 diabetes (T2D) is still unclear.

AIM

Recent studies have indicated that RFX6 binding to DNA could be disrupted in diabetes. Therefore, in this study we investigated whether genetic mutations are present in the DNA binding domain of RFX6 gene that could abrogate its function in T2D.

METHODS

A cohort of T2D patients was enrolled in this study, and the gene encoding the DNA binding domain of RFX6 was amplified by polymerase chain reaction and then analysed by direct DNA sequencing.

RESULTS

The DNA sequence analysis revealed the absence of any exonic mutation. However, we have identified a new heterozygous single nucleotide polymorphism (IVS6+31 C>T) in the intronic region of DNA binding domain gene that is present in 9.2% and 8.5% of diabetic and control people, respectively (P = 0.97).

CONCLUSION

We report the absence of any significant genetic variant that could affect the function of RFX6-DNA binding domain in T2D.

β Cell Dysfunction in Type 2 Diabetes: Drained of Energy?

Type 2 diabetes is a progressive disorder, but exactly how the progression occurs remains unknown. In this issue of Cell Metabolism, Zhang et al. (2019) present evidence that diabetes, via hyperglycemia, leads to aberrant insertion of a mitochondrial ion channel in the plasma membrane, rendering it leaky to key intracellular signaling molecules with resultant suppression of insulin secretion.

Type 2 diabetes genetic loci informed by multi-trait associations point to disease mechanisms and subtypes: A soft clustering analysis

BACKGROUND

Type 2 diabetes (T2D) is a heterogeneous disease for which (1) disease-causing pathways are incompletely understood and (2) subclassification may improve patient management. Unlike other biomarkers, germline genetic markers do not change with disease progression or treatment. In this paper, we test whether a germline genetic approach informed by physiology can be used to deconstruct T2D heterogeneity. First, we aimed to categorize genetic loci into groups representing likely disease mechanistic pathways. Second, we asked whether the novel clusters of genetic loci we identified have any broad clinical consequence, as assessed in four separate subsets of individuals with T2D.

METHODS AND FINDINGS

In an effort to identify mechanistic pathways driven by established T2D genetic loci, we applied Bayesian nonnegative matrix factorization (bNMF) clustering to genome-wide association study (GWAS) results for 94 independent T2D genetic variants and 47 diabetes-related traits. We identified five robust clusters of T2D loci and traits, each with distinct tissue-specific enhancer enrichment based on analysis of epigenomic data from 28 cell types. Two clusters contained variant-trait associations indicative of reduced beta cell function, differing from each other by high versus low proinsulin levels. The three other clusters displayed features of insulin resistance: obesity mediated (high body mass index [BMI] and waist circumference [WC]), "lipodystrophy-like" fat distribution (low BMI, adiponectin, and high-density lipoprotein [HDL] cholesterol, and high triglycerides), and disrupted liver lipid metabolism (low triglycerides). Increased cluster genetic risk scores were associated with distinct clinical outcomes, including increased blood pressure, coronary artery disease (CAD), and stroke. We evaluated the potential for clinical impact of these clusters in four studies containing individuals with T2D (Metabolic Syndrome in Men Study [METSIM], N = 487; Ashkenazi, N = 509; Partners Biobank, N = 2,065; UK Biobank [UKBB], N = 14,813). Individuals with T2D in the top genetic risk score decile for each cluster reproducibly exhibited the predicted cluster-associated phenotypes, with approximately 30% of all individuals assigned to just one cluster top decile. Limitations of this study include that the genetic variants used in the cluster analysis were restricted to those associated with T2D in populations of European ancestry.

CONCLUSION

Our approach identifies salient T2D genetically anchored and physiologically informed pathways, and supports the use of genetics to deconstruct T2D heterogeneity. Classification of patients by these genetic pathways may offer a step toward genetically informed T2D patient management.

Metabolic benefits of 1-(3-(4-(o-tolyl)piperazin-1-yl)propyl)pyrrolidin-2-one: a non-selective α-adrenoceptor antagonist

PURPOSE

Previous studies have shown that several components of the metabolic syndrome, such as hypertension, obesity or imbalanced lipid and carbohydrate homeostasis, are associated with the sympathetic nervous system overactivity. Therefore, the inhibition of the adrenergic nervous system seems to be a reasonable and appropriate therapeutic approach for the treatment of metabolic disturbances. It has been suggested that non-selective adrenoceptor antagonists could be particularly beneficial, since α₁-adrenoceptor antagonists can improve disrupted lipid and carbohydrate profiles, while the inhibition of the α₂-adrenoceptor may contribute to body weight reduction. The aim of the present study was to investigate the metabolic benefits deriving from administration of a non-selective α-adrenoceptor antagonist from the group of pyrrolidin-2-one derivatives. The aim of the present study was to investigate the potential metabolic benefits deriving from chronic administration of a non-selective α-adrenoceptor antagonist, from the group of pyrrolidin-2-one derivatives.

METHODS

The α₁- and α₂-adrenoreceptor affinities of the tested compound-1-(3-(4-(o-tolyl)piperazin-1-yl)propyl)pyrrolidin-2-one had been investigated previously by means of the radioligand binding assay. In the present study, we extended the pharmacological profile characteristics of the selected molecule by additional intrinistic activity assays. Next, we investigated the influence of the tested compound on body weight, hyperglycemia, hypertriglyceridemia, blood pressure in the animal model of obesity induced by a high-fat diet, and additionally we measured the spontaneous activity and body temperature.

RESULTS

The intrinistic activity studies revealed that the tested compound is a potent, non-selective antagonist of α_1B and α_2A-adrenoceptors. After the chronic administration of the tested compound, we observed reduced level of triglycerides and glucose in the rat plasma. Interestingly, the tested did not reduce the body weight and did not influence the blood pressure in normotensive animals. Additionally, the administration of the tested compound did not change the animals' spontaneous activity and body temperature.

CONCLUSION

Non-selective α-adrenoceptor antagonist seems to carry potential benefits in the improvement of the reduction of elevated glucose and triglyceride level. The lack of influence on blood pressure suggests that compounds with such a pharmacological profile may be particulary beneficial for the patients with disturbed lipid and carbohydrate profile, who do not suffer from hypertension. These results are particulary valuable, since currently there are no safe α_2A-adrenoceptor antagonist drugs available in clinical use with the ability to modulate hyperglycemia that would not affect blood pressure.

Inositol hexakisphosphate kinase 1 is a metabolic sensor in pancreatic β-cells

Diphosphoinositol pentakisphosphate (IP₇) is critical for the exocytotic capacity of the pancreatic β-cell, but its regulation by the primary instigator of β-cell exocytosis, glucose, is unknown. The high K_m for ATP of the IP₇-generating enzymes, the inositol hexakisphosphate kinases (IP6K1 and 2) suggests that these enzymes might serve as metabolic sensors in insulin secreting β-cells and act as translators of disrupted metabolism in diabetes. We investigated this hypothesis and now show that glucose stimulation, which increases the ATP/ADP ratio, leads to an early rise in IP₇ concentration in β-cells. RNAi mediated knock down of the IP6K1 isoform inhibits both glucose-mediated increase in IP₇ and first phase insulin secretion, demonstrating that IP6K1 integrates glucose metabolism and insulin exocytosis. In diabetic mouse islets the deranged ATP/ADP levels under both basal and glucose-stimulated conditions are mirrored in both disrupted IP₇ generation and insulin release. Thus the unique metabolic sensing properties of IP6K1 guarantees appropriate concentrations of IP₇ and thereby both correct basal insulin secretion and intact first phase insulin release. In addition, our data suggest that a specific cell signaling defect, namely, inappropriate IP₇ generation may be an essential convergence point integrating multiple metabolic defects into the commonly observed phenotype in diabetes.

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Glucose increases IP7 levels transiently through IP6K1 in pancreatic β-cells.

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IP6K1 decodes glucose-driven increases in ATP/ADP ratio into 1st phase insulin release.

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IP7 production and insulin release mirror perturbed metabolism in diabetic islets.

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IP6K1 acts as a β-cell metabolic sensor under normal and pathological conditions.

Glucose-Dependent Granule Docking Limits Insulin Secretion and Is Decreased in Human Type 2 Diabetes

Glucose-stimulated insulin secretion is biphasic, with a rapid first phase and a slowly developing sustained second phase; both are disturbed in type 2 diabetes (T2D). Biphasic secretion results from vastly different release probabilities of individual insulin granules, but the morphological and molecular basis for this is unclear. Here, we show that human insulin secretion and exocytosis critically depend on the availability of membrane-docked granules and that T2D is associated with a strong reduction in granule docking. Glucose accelerated granule docking, and this effect was absent in T2D. Newly docked granules only slowly acquired release competence; this was regulated by major signaling pathways, but not glucose. Gene expression analysis indicated that key proteins involved in granule docking are downregulated in T2D, and overexpression of these proteins increased granule docking. The findings establish granule docking as an important glucose-dependent step in human insulin secretion that is dysregulated in T2D.

SWELL1 is a glucose sensor regulating β-cell excitability and systemic glycaemia

Insulin secretion is initiated by activation of voltage-gated Ca²⁺ channels (VGCC) to trigger Ca²⁺-mediated insulin vesicle fusion with the β-cell plasma membrane. The firing of VGCC requires β-cell membrane depolarization, which is regulated by a balance of depolarizing and hyperpolarizing ionic currents. Here, we show that SWELL1 mediates a swell-activated, depolarizing chloride current (I_Cl,SWELL) in both murine and human β-cells. Hypotonic and glucose-stimulated β-cell swelling activates SWELL1-mediated I_Cl,SWELL and this contributes to membrane depolarization and activation of VGCC-dependent intracellular calcium signaling. SWELL1 depletion in MIN6 cells and islets significantly impairs glucose-stimulated insulin secretion. Tamoxifen-inducible β-cell-targeted Swell1 KO mice have normal fasting serum glucose and insulin levels but impaired glucose-stimulated insulin secretion and glucose tolerance; and this is further exacerbated in mild obesity. Our results reveal that β-cell SWELL1 modulates insulin secretion and systemic glycaemia by linking glucose-mediated β-cell swelling to membrane depolarization and activation of VGCC-triggered calcium signaling.

SWELL1 is a glucose sensor regulating β-cell excitability and systemic glycaemia

Insulin secretion is initiated by activation of voltage-gated Ca2+ channels (VGCC) to trigger Ca2+-mediated insulin vesicle fusion with the β-cell plasma membrane. The firing of VGCC requires β-cell membrane depolarization, which is regulated by a balance of depolarizing and hyperpolarizing ionic currents. Here, we show that SWELL1 mediates a swell-activated, depolarizing chloride current (ICl,SWELL) in both murine and human β-cells. Hypotonic and glucose-stimulated β-cell swelling activates SWELL1-mediated ICl,SWELL and this contributes to membrane depolarization and activation of VGCC-dependent intracellular calcium signaling. SWELL1 depletion in MIN6 cells and islets significantly impairs glucose-stimulated insulin secretion. Tamoxifen-inducible β-cell-targeted Swell1 KO mice have normal fasting serum glucose and insulin levels but impaired glucose-stimulated insulin secretion and glucose tolerance; and this is further exacerbated in mild obesity. Our results reveal that β-cell SWELL1 modulates insulin secretion and systemic glycaemia by linking glucose-mediated β-cell swelling to membrane depolarization and activation of VGCC-triggered calcium signaling.

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.

Identification of key gene pathways and coexpression networks of islets in human type 2 diabetes

PURPOSE

The number of people with type 2 diabetes (T2D) is growing rapidly worldwide. Islet β-cell dysfunction and failure are the main causes of T2D pathological processes. The aim of this study was to elucidate the underlying pathways and coexpression networks in T2D islets.

MATERIALS AND METHODS

We analyzed the differentially expressed genes (DEGs) in the data set GSE41762, which contained 57 nondiabetic and 20 diabetic samples, and developed Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway enrichment analyses. Protein-protein interaction (PPI) network, the modules from the PPI network, and the gene annotation enrichment of modules were analyzed as well. Moreover, a weighted correlation network analysis (WGCNA) was applied to screen critical gene modules and coexpression networks and explore the biological significance.

RESULTS

We filtered 957 DEGs in T2D islets. Then GO and KEGG analyses identified that key pathways like inflammatory response, type B pancreatic cell differentiation, and calcium ion-dependent exocytosis were involved in human T2D. Three significant modules were filtered from the PPI network. Ribosome biogenesis, extrinsic apoptotic signaling pathway, and membrane depolarization during action potential were associated with the modules, respectively. Furthermore, coexpression network analysis by WGCNA identified 13 distinct gene modules of T2D islets and revealed four modules, which were strongly correlated with T2D and T2D biomarker hemoglobin A1c (HbA1c). Functional annotation showed that these modules mainly enriched KEGG pathways such as NF-kappa B signaling pathway, tumor necrosis factor signaling pathway, cyclic adenosine monophosphate signaling pathway, and peroxisome proliferators-activated receptor signaling pathway.

CONCLUSION

The results provide potential gene pathways and underlying molecular mechanisms for the prevention, diagnosis, and treatment of T2D.

The Kv7 Channel and Cardiovascular Risk Factors

Potassium channels play a pivotal role in the regulation of excitability in cells such as neurons, cardiac myocytes, and vascular smooth muscle cells. The KCNQ (Kv7) family of voltage-activated K⁺ channels hyperpolarizes the cell and stabilizes the membrane potential. Here, we outline how Kv7 channel activity may contribute to the development of the cardiovascular risk factors such as hypertension, diabetes, and obesity. Questions and hypotheses regarding previous and future research have been raised. Alterations in the Kv7 channel may contribute to the development of cardiovascular disease (CVD). Pharmacological modification of Kv7 channels may represent a possible treatment for CVD in the future.

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.

Two missense mutations in KCNQ1 cause pituitary hormone deficiency and maternally inherited gingival fibromatosis

Familial growth hormone deficiency provides an opportunity to identify new genetic causes of short stature. Here we combine linkage analysis with whole-genome resequencing in patients with growth hormone deficiency and maternally inherited gingival fibromatosis. We report that patients from three unrelated families harbor either of two missense mutations, c.347G>T p.(Arg116Leu) or c.1106C>T p.(Pro369Leu), in KCNQ1, a gene previously implicated in the long QT interval syndrome. Kcnq1 is expressed in hypothalamic GHRH neurons and pituitary somatotropes. Co-expressing KCNQ1 with the KCNE2 β-subunit shows that both KCNQ1 mutants increase current levels in patch clamp analyses and are associated with reduced pituitary hormone secretion from AtT-20 cells. In conclusion, our results reveal a role for the KCNQ1 potassium channel in the regulation of human growth, and show that growth hormone deficiency associated with maternally inherited gingival fibromatosis is an allelic disorder with cardiac arrhythmia syndromes caused by KCNQ1 mutations.

Obstacles to Translating Genotype-Phenotype Correlates in Metabolic Disease

Type 2 diabetes mellitus is a polygenic disease with a variable phenotype. Many genetic associations have been described; however, understanding their underlying pathophysiological role in Type 2 diabetes mellitus is important for development of future therapeutic targets. Here, we review the physiological mechanisms of diabetes-associated variants that affect glycemia.

A Type 2 Diabetes-Associated Functional Regulatory Variant in a Pancreatic Islet Enhancer at the ADCY5 Locus

Molecular mechanisms remain unknown for most type 2 diabetes genome-wide association study identified loci. Variants associated with type 2 diabetes and fasting glucose levels reside in introns of ADCY5, a gene that encodes adenylate cyclase 5. Adenylate cyclase 5 catalyzes the production of cyclic AMP, which is a second messenger molecule involved in cell signaling and pancreatic β-cell insulin secretion. We demonstrated that type 2 diabetes risk alleles are associated with decreased ADCY5 expression in human islets and examined candidate variants for regulatory function. rs11708067 overlaps a predicted enhancer region in pancreatic islets. The type 2 diabetes risk rs11708067-A allele showed fewer H3K27ac ChIP-seq reads in human islets, lower transcriptional activity in reporter assays in rodent β-cells (rat 832/13 and mouse MIN6), and increased nuclear protein binding compared with the rs11708067-G allele. Homozygous deletion of the orthologous enhancer region in 832/13 cells resulted in a 64% reduction in expression level of Adcy5, but not adjacent gene Sec22a, and a 39% reduction in insulin secretion. Together, these data suggest that rs11708067-A risk allele contributes to type 2 diabetes by disrupting an islet enhancer, which results in reduced ADCY5 expression and impaired insulin secretion.

KCNQ1 gene polymorphism is associated with glycaemic response to treatment with DPP-4 inhibitors

AIMS

Only afew gene variants were associated with the response to dipeptidylpeptidase-4 inhibitors (DPP4I). KCNQ1 gene variants were previously related both to type 2 diabetes (T2D) and incretin effect. We hypothesized that T2D related KCNQ1 variants would be associated with smaller glucose-lowering effect of DDP4I.

METHODS

We performed a retrospective study in 137 Caucasian subjects with T2D who were followed for 6months after initiation of DPP4I treatment. Genotyping for KCNQ1 rs163184 and rs151290 was performed using PCR-HRMA and PCR-RFLP methods, respectively. The main clinical outcome was reduction in HbA1c (ΔHbA1c) after 6-month DPP4I treatment.

RESULTS

KCNQ1 rs163184 T>G variant was associated with the response to DPP4I treatment in genetic additive model (β=-0.30, p=0.022). For each G allele in the rs163184 genotype, we observed a 0.3% (3.3mmol/mol) less reduction in HbA1c during treatment with a DPP4I. Both the GG homozygotes and G-allele carriers had significantly smaller HbA1c reduction in comparison with the TT homozygotes.

CONCLUSIONS

KCNQ1 rs163184 T>G variant was associated with a reduced glycaemic response to DPP4I. The difference of 0.6% (6.5mmol/mol) in HbA1c reduction between the TT and GG homozygotes might be of clinical significance if replicated in further studies.

Reduction of endoplasmic reticulum- mitochondria interactions in beta cells from patients with type 2 diabetes

Type 2 diabetes develops when beta cells are not able to fulfill insulin needs. The role of the endoplasmic reticulum-mitochondria junction in coordinating the functions of these two organelles throughout the natural history of type 2 diabetes is determinant and may explain the alterations of insulin biosynthesis. Our goal was to study endoplasmic reticulum and mitochondrial interactions in human beta cells from organ donors with type 2 diabetes. Pancreas samples were obtained via the network for pancreatic organ donors with diabetes (nPOD) based on disease status with 12 subjects with type 2 diabetes and 9 non-diabetic controls. We examined pancreatic specimens by immunofluorescence, in situ hybridization and in situ proximity ligation assay and compared the results to an in vitro model of beta-cell dysfunction. Expression of proteins that enable tethering and exchanges between endoplasmic reticulum (ER) and mitochondria and quantification of interconnection through mitochondria associated membranes (MAM) was investigated. In beta cells from type 2 diabetic cases as compared to controls, there was a significant increase in reticular expression of inositol triphosphate receptor-2 (IP3R2) both at the protein and mRNA levels, no difference in mitochondrial transit peptide receptor TOM20 and mitofusin-2 expressions, and a decrease in the expression of voltage-dependent anion channel-1 (VDAC-1). The number of IP3R2-VDAC-1 complexes identified by in situ proximity ligation assay was significantly lower in diabetic islets and in beta cells of diabetics as compared to controls. Treatment of Min6-B1 cells with palmitate altered glucose-stimulated insulin secretion, increased ER stress and significantly reduced ER-mitochondrial interactions. We can conclude that specific changes in reticular and mitochondrial beta cell proteins characterize human type 2 diabetes with reduction in organelle interactions. This finding opens new targets of intervention.

Kv2.1 Clustering Contributes to Insulin Exocytosis and Rescues Human β-Cell Dysfunction

Insulin exocytosis is regulated by ion channels that control excitability and Ca²⁺ influx. Channels also play an increasingly appreciated role in microdomain structure. In this study, we examine the mechanism by which the voltage-dependent K⁺ (Kv) channel Kv2.1 (KCNB1) facilitates depolarization-induced exocytosis in INS 832/13 cells and β-cells from human donors with and without type 2 diabetes (T2D). We find that Kv2.1, but not Kv2.2 (KCNB2), forms clusters of 6-12 tetrameric channels at the plasma membrane and facilitates insulin exocytosis. Knockdown of Kv2.1 expression reduces secretory granule targeting to the plasma membrane. Expression of the full-length channel (Kv2.1-wild-type) supports the glucose-dependent recruitment of secretory granules. However, a truncated channel (Kv2.1-ΔC318) that retains electrical function and syntaxin 1A binding, but lacks the ability to form clusters, does not enhance granule recruitment or exocytosis. Expression of KCNB1 appears reduced in T2D islets, and further knockdown of KCNB1 does not inhibit Kv current in T2D β-cells. Upregulation of Kv2.1-wild-type, but not Kv2.1-ΔC318, rescues the exocytotic phenotype in T2D β-cells and increases insulin secretion from T2D islets. Thus, the ability of Kv2.1 to directly facilitate insulin exocytosis depends on channel clustering. Loss of this structural role for the channel might contribute to impaired insulin secretion in diabetes.

Sox5 regulates beta-cell phenotype and is reduced in type 2 diabetes

Type 2 diabetes (T2D) is characterized by insulin resistance and impaired insulin secretion, but the mechanisms underlying insulin secretion failure are not completely understood. Here, we show that a set of co-expressed genes, which is enriched for genes with islet-selective open chromatin, is associated with T2D. These genes are perturbed in T2D and have a similar expression pattern to that of dedifferentiated islets. We identify Sox5 as a regulator of the module. Sox5 knockdown induces gene expression changes similar to those observed in T2D and diabetic animals and has profound effects on insulin secretion, including reduced depolarization-evoked Ca²⁺-influx and β-cell exocytosis. SOX5 overexpression reverses the expression perturbations observed in a mouse model of T2D, increases the expression of key β-cell genes and improves glucose-stimulated insulin secretion in human islets from donors with T2D. We suggest that human islets in T2D display changes reminiscent of dedifferentiation and highlight SOX5 as a regulator of β-cell phenotype and function.

Using genetics to inform new therapeutics for diabetes

The genetic architecture of diabetes has been extensively studied. Numerous genetic markers for diabetes have been reported. However, the translation of such knowledge into clinical interventions has been inadequate. Areas covered: We performed a literature search on various frontiers in diabetes treatment that could be improved using genetic information: (1) understanding the mechanisms of existing antidiabetic drugs, (2) repurposing existing drugs for the treatment of diabetes, (3) complementing clinical trial findings; (4) finding novel treatment approaches; (5) better estimation of the efficacy of metabolic surgery. Expert commentary: The translation of genetic information to clinical intervention requires further study, including the development of an appropriate genetic risk score algorithm for type 2 diabetes. Genomic studies provide empirical explanations for clinical trial findings. Moreover, the mechanisms of antidiabetic drugs should be thoroughly investigated to enable clinical trials and pharmacogenomics studies of these drugs. As metabolic surgery becomes more prevalent for the treatment of diabetes, genetic approaches may improve patient prioritization.

KCNQ1 variants associate with hypertension in type 2 diabetes and affect smooth muscle contractility in vitro

KCNQ1 encodes a potassium voltage-gated channel and represents a susceptibility locus for type 2 diabetes mellitus (T2DM). Here, we explored the association between KCNQ1 polymorphisms and hypertension risk in individuals with T2DM, as well as the role of KCNQ1 in vascular smooth muscle cell contraction in vitro. To investigate the relationship between KCNQ1 and the risk of developing hypertension in patients with T2DM, we divided the T2DM cohort into hypertension (n = 452) and non-hypertension (n = 541) groups. The Mann-Whitney U test, chi-square test, and multivariate regression analyses were used to assess the clinical characteristics and genotypic frequencies. In vitro studies utilized the rat aortic smooth muscle A10 cell line. Patients in the hypertension group were significantly older at the time of enrollment and had higher levels of body mass index, waist-to-hip ratio, and triglyceride than those in the non-hypertension group. The KCNQ1 rs3864884 and rs12576239 genetic variants were associated with hypertension in T2DM. KCNQ1 expression was lower in the individuals with the CC versus the CT and TT genotypes. Smooth muscle cell contractility was inhibited by treatment with a KCNQ1 inhibitor. These results suggest that KCNQ1 might be associated with hypertension in individuals with T2DM.

Glucose ingestion causes cardiac repolarization disturbances in type 1 long QT syndrome patients and healthy subjects

BACKGROUND

Both hypoglycemia and severe hyperglycemia constitute known risk factors for cardiac repolarization changes potentially leading to malignant arrhythmias. Patients with loss of function mutations in KCNQ1 are characterized by long QT syndrome (LQTS) and may be at increased risk for glucose-induced repolarization disturbances.

OBJECTIVE

The purpose of this study was to test the hypothesis that KCNQ1 LQTS patients are at particular risk for cardiac repolarization changes during the relative hyperglycemia that occurs after an oral glucose load.

METHODS

Fourteen KCNQ1 LQTS patients and 28 control participants matched for gender, body mass index, and age underwent a 3-hour oral 75-g glucose tolerance test with ECGs obtained at 7 time points. Fridericia corrected QT interval (QTcF), Bazett corrected QT interval (QTcB), and the Morphology Combination Score (MCS) were calculated.

RESULTS

QTc and MCS increased in both groups. MCS remained elevated until 150 minutes after glucose ingestion, and the maximal change from baseline was larger among KCNQ1 LQTS patients compared with control subjects (0.28 ± 0.27 vs 0.15 ± 0.13; P <.05).

CONCLUSION

Relative hyperglycemia induced by ingestion of 75-g glucose caused cardiac repolarization disturbances that were more severe in KCNQ1 LQTS patients compared with control subjects.

Whole-Genome Bisulfite Sequencing of Human Pancreatic Islets Reveals Novel Differentially Methylated Regions in Type 2 Diabetes Pathogenesis

Current knowledge about the role of epigenetics in type 2 diabetes (T2D) remains limited. Only a few studies have investigated DNA methylation of selected candidate genes or a very small fraction of genomic CpG sites in human pancreatic islets, the tissue of primary pathogenic importance for diabetes. Our aim was to characterize the whole-genome DNA methylation landscape in human pancreatic islets, to identify differentially methylated regions (DMRs) in diabetic islets, and to investigate the function of DMRs in islet biology. Here, we performed whole-genome bisulfite sequencing, which is a comprehensive and unbiased method to study DNA methylation throughout the genome at a single nucleotide resolution, in pancreatic islets from donors with T2D and control subjects without diabetes. We identified 25,820 DMRs in islets from individuals with T2D. These DMRs cover loci with known islet function, e.g., PDX1, TCF7L2, and ADCY5 Importantly, binding sites previously identified by ChIP-seq for islet-specific transcription factors, enhancer regions, and different histone marks were enriched in the T2D-associated DMRs. We also identified 457 genes, including NR4A3, PARK2, PID1, SLC2A2, and SOCS2, that had both DMRs and significant expression changes in T2D islets. To mimic the situation in T2D islets, candidate genes were overexpressed or silenced in cultured β-cells. This resulted in impaired insulin secretion, thereby connecting differential methylation to islet dysfunction. We further explored the islet methylome and found a strong link between methylation levels and histone marks. Additionally, DNA methylation in different genomic regions and of different transcript types (i.e., protein coding, noncoding, and pseudogenes) was associated with islet expression levels. Our study provides a comprehensive picture of the islet DNA methylome in individuals with and without diabetes and highlights the importance of epigenetic dysregulation in pancreatic islets and T2D pathogenesis.

Preliminary evidence for a role of the adrenergic nervous system in generalized anxiety disorder

Generalized anxiety disorder (GAD) is a common chronic condition that is understudied compared to other psychiatric disorders. An altered adrenergic function has been reported in GAD, however direct evidence for genetic susceptibility is missing. This study evaluated the associations of gene variants in adrenergic receptors (ADRs) with GAD, with the involvement of stressful events. Data were obtained from 844 French community-dwelling elderly aged 65 or over. Anxiety disorders were assessed using the Mini-International Neuropsychiatry Interview, according to DSM-IV criteria. Eight single-nucleotide polymorphisms (SNPs) involved with adrenergic function were genotyped; adrenergic receptors alpha(1A) (ADRA1A), alpha(2A) (ADRA2A), and beta2 (ADRB2) and transcription factor TCF7L2. Questionnaires evaluated recent stressful life events as well as early environment during childhood and adolescence. Using multivariate logistic regression analyses four SNPs were significantly associated with GAD. A 4-fold modified risk was found with ADRA1A rs17426222 and rs573514, and ADRB2 rs1042713 which remained significant after Bonferroni correction. Certain variants may moderate the effect of adverse life events on the risk of GAD. Replication in larger samples is needed due to the small case number. This is the first study showing that ADR variants are susceptibility factors for GAD, further highlighting the critical role of the adrenergic nervous system in this disorder.

Islet adaptations in fetal sheep persist following chronic exposure to high norepinephrine

Complications in pregnancy elevate fetal norepinephrine (NE) concentrations. Previous studies in NE-infused sheep fetuses revealed that sustained exposure to high NE resulted in lower expression of α₂-adrenergic receptors in islets and increased insulin secretion responsiveness after acutely terminating the NE infusion. In this study, we determined if the compensatory increase in insulin secretion after chronic elevation of NE is independent of hyperglycemia in sheep fetuses and whether it is persistent in conjunction with islet desensitization to NE. After an initial assessment of glucose-stimulated insulin secretion (GSIS) at 129 ± 1 days of gestation, fetuses were continuously infused for seven days with NE and maintained at euglycemia with a maternal insulin infusion. Fetal GSIS studies were performed again on days 8 and 12. Adrenergic sensitivity was determined in pancreatic islets collected at day 12. NE infusion increased (P < 0.01) fetal plasma NE concentrations and lowered (P < 0.01) basal insulin concentrations compared to vehicle-infused controls. GSIS was 1.8-fold greater (P < 0.05) in NE-infused fetuses compared to controls at both one and five days after discontinuing the infusion. Glucose-potentiated arginine-induced insulin secretion was also enhanced (P < 0.01) in NE-infused fetuses. Maximum GSIS in islets isolated from NE-infused fetuses was 1.6-fold greater (P < 0.05) than controls, but islet insulin content and intracellular calcium signaling were not different between treatments. The half-maximal inhibitory concentration for NE was 2.6-fold greater (P < 0.05) in NE-infused islets compared to controls. These findings show that chronic NE exposure and not hyperglycemia produce persistent adaptations in pancreatic islets that augment β-cell responsiveness in part through decreased adrenergic sensitivity.

Lessons from basic pancreatic beta cell research in type-2 diabetes and vascular complications

The changes in life-style with increased access of food and reduced physical activity have resulted in the global epidemic of obesity. Consequently, individuals with type 2 diabetes and cardiovascular disease have also escalated. A central organ in the development of diabetes is the pancreas, and more specifically the pancreatic beta cells within the islets of Langerhans. Beta cells have been assigned the important task of secreting insulin when blood glucose is increased to lower the glucose level. An early sign of diabetes pathogenesis is lack of first phase insulin response and reduced second phase secretion. In this review, which is based on the foreign investigator award lecture given at the JSDC meeting in Sendai in October 2016, we discuss a possible cellular explanation for the reduced first phase insulin response and how this can be influenced by lipids. Moreover, since patients with cardiovascular disease and high levels of cholesterol are often treated with statins, we summarize recent data regarding effects on statins on glucose homeostasis and insulin secretion. Finally, we suggest microRNAs (miRNAs) as central players in the adjustment of beta cell function during the development of diabetes. We specifically discuss miRNAs regarding their involvement in insulin secretion regulation, differential expression in type 2 diabetes, and potential as biomarkers for prediction of diabetes and cardiovascular complications.

PRELIMINARY ANALYSIS OF INTERLEUKIN-6 CHANGES IN PRE- AND POSTOPERATIVE IN DIABETIC PATIENTS WITH BMI<35 SUBMITTED TO PARTIAL DUODENAL SWITCH

Background:

Studies related to obesity have shown association with metabolic syndrome. Data showing that obesity is capable to cause low grade chronic inflammation, without its classic signs and symptoms, call attention to researches to study different cells types and the mechanism of the inflammatory process.

Aim:

To evaluate the variation of glycated hemoglobin (HbA1c) and the pro-inflammatory cytokine interleukin-6 (IL6) in diabetic patients with BMI <35 kg/m² in the pre and postoperative of partial duodenal switch.

Method:

Nine patients were studied before and one year after the operation and the variation of the serum IL6 was measured by Elisa. The changes of HbA1c were also registered.

Results:

The pre-operative IL6 levels reached 65,50436±2,911993 pg/ml and one year after de operation 39,47739±3,410057 and the HbA1c average of 10,67 and 5.8 in the same period.

Conclusion:

The partial duodenal switch was efficient to control one year after the procedure the chronic inflammatory process caused by the diabetes mellitus type 2 with BMI <35 by dropping the IL6 levels and bringing the HbA1c to normal.

HDAC7 is overexpressed in human diabetic islets and impairs insulin secretion in rat islets and clonal beta cells

AIMS/HYPOTHESIS

Pancreatic beta cell dysfunction is a prerequisite for the development of type 2 diabetes. Histone deacetylases (HDACs) may affect pancreatic endocrine function and glucose homeostasis through alterations in gene regulation. Our aim was to investigate the role of HDAC7 in human and rat pancreatic islets and clonal INS-1 beta cells (INS-1 832/13).

METHODS

To explore the role of HDAC7 in pancreatic islets and clonal beta cells, we used RNA sequencing, mitochondrial functional analyses, microarray techniques, and HDAC inhibitors MC1568 and trichostatin A.

RESULTS

Using RNA sequencing, we found increased HDAC7 expression in human pancreatic islets from type 2 diabetic compared with non-diabetic donors. HDAC7 expression correlated negatively with insulin secretion in human islets. To mimic the situation in type 2 diabetic islets, we overexpressed Hdac7 in rat islets and clonal beta cells. In both, Hdac7 overexpression resulted in impaired glucose-stimulated insulin secretion. Furthermore, it reduced insulin content, mitochondrial respiration and cellular ATP levels in clonal beta cells. Overexpression of Hdac7 also led to changes in the genome-wide gene expression pattern, including increased expression of Tcf7l2 and decreased expression of gene sets regulating DNA replication and repair as well as nucleotide metabolism. In accordance, Hdac7 overexpression reduced the number of beta cells owing to enhanced apoptosis. Finally, we found that inhibiting HDAC7 activity with pharmacological inhibitors or small interfering RNA-mediated knockdown restored glucose-stimulated insulin secretion in beta cells that were overexpressing Hdac7.

CONCLUSIONS/INTERPRETATION

Taken together, these results indicate that increased HDAC7 levels caused beta cell dysfunction and may thereby contribute to defects seen in type 2 diabetic islets. Our study supports HDAC7 inhibitors as a therapeutic option for the treatment of type 2 diabetes.

Toward Connecting Metabolism to the Exocytotic Site

Within cells the regulated exocytosis of secretory granules controls multiple physiological functions, including endocrine hormone secretion. Release of the glucose-regulating hormone insulin from pancreatic islet β cells is critical for whole-body metabolic homeostasis. Impaired insulin secretion appears early in the progression to type 2 diabetes (T2D). Key mechanisms that control the β-cell exocytotic response, mediating the long-known but little understood metabolic amplification of insulin secretion, are becoming clearer. Recent insights indicate a convergence of metabolism-driven signals, such as lipid-derived messengers and redox-dependent deSUMOylation, at the plasma membrane to augment Ca²⁺-dependent insulin exocytosis. These pathways have important implications for the metabolic control of hormone secretion, for the functional compensation that occurs in obesity, and for impaired insulin secretion in diabetes.

A variation in KCNQ1 gene is associated with repaglinide efficacy on insulin resistance in Chinese Type 2 Diabetes Mellitus Patients

Repaglinide is an insulin secretagogue that often exhibits considerable interindividual variability in therapeutic efficacy. The current study was designed to investigate the impact of KCNQ1 genetic polymorphism on the efficacy of repaglinide and furthermore to identify the potential mechanism of action in patients with type 2 diabetes. A total of 305 patients and 200 healthy subjects were genotyped for the KCNQ1 rs2237892 polymorphism, and 82 patients with T2DM were randomized for the oral administration of repaglinide for 8 weeks. HepG2 cells were incubated with repaglinide in the absence or presence of a KCNQ1 inhibitor or the pcDNA3.1-hKCNQ1 plasmid, after which the levels of Akt, IRS-2 and PI(3)K were determined. Our data showed that repaglinide significantly decreased HOMA-IR in patients with T2DM. Furthermore, the level of HOMA-IR was significantly reduced in those patients with CT or TT genotypes than CC homozygotes. The KCNQ1 inhibitor enhanced repaglinide efficacy on insulin resistance, with IRS-2/PI(3)K/Akt signaling being up-regulated markedly. As in our clinical experiment, these data strongly suggest that KCNQ1 genetic polymorphism influences repaglinide response due to the pivotal role of KCNQ1 in regulating insulin resistance through the IRS-2/PI(3)K/Akt signaling pathway. This study was registered in the Chinese Clinical Trial Register on May 14, 2013. (No. ChiCTR-CCC13003536).

Alterations in β-Cell Calcium Dynamics and Efficacy Outweigh Islet Mass Adaptation in Compensation of Insulin Resistance and Prediabetes Onset

Emerging insulin resistance is normally compensated by increased insulin production of pancreatic β-cells, thereby maintaining normoglycemia. However, it is unclear whether this is achieved by adaptation of β-cell function, mass, or both. Most importantly, it is still unknown which of these adaptive mechanisms fail when type 2 diabetes develops. We performed longitudinal in vivo imaging of β-cell calcium dynamics and islet mass of transplanted islets of Langerhans throughout diet-induced progression from normal glucose homeostasis, through compensation of insulin resistance, to prediabetes. The results show that compensation of insulin resistance is predominated by alterations of β-cell function, while islet mass only gradually expands. Hereby, functional adaptation is mediated by increased calcium efficacy, which involves Epac signaling. Prior to prediabetes, β-cell function displays decreased stimulated calcium dynamics, whereas islet mass continues to increase through prediabetes onset. Thus, our data reveal a predominant role of islet function with distinct contributions of triggering and amplifying pathway in the in vivo processes preceding diabetes onset. These findings support protection and recovery of β-cell function as primary goals for prevention and treatment of diabetes and provide insight into potential therapeutic targets.

An Isogenic Human ESC Platform for Functional Evaluation of Genome-wide-Association-Study-Identified Diabetes Genes and Drug Discovery

Genome-wide association studies (GWASs) have increased our knowledge of loci associated with a range of human diseases. However, applying such findings to elucidate pathophysiology and promote drug discovery remains challenging. Here, we created isogenic human ESCs (hESCs) with mutations in GWAS-identified susceptibility genes for type 2 diabetes. In pancreatic beta-like cells differentiated from these lines, we found that mutations in CDKAL1, KCNQ1, and KCNJ11 led to impaired glucose secretion in vitro and in vivo, coinciding with defective glucose homeostasis. CDKAL1 mutant insulin+ cells were also hypersensitive to glucolipotoxicity. A high-content chemical screen identified a candidate drug that rescued CDKAL1-specific defects in vitro and in vivo by inhibiting the FOS/JUN pathway. Our approach of a proof-of-principle platform, which uses isogenic hESCs for functional evaluation of GWAS-identified loci and identification of a drug candidate that rescues gene-specific defects, paves the way for precision therapy of metabolic diseases.

Alpha2A adrenergic receptor genetic variation contributes to hyperglycemia after myocardial infarction

BACKGROUND

Acute myocardial infarction (AMI) is frequently associated with transient hyperglycemia even in patients without pre-existing diabetes. Acute stress can lead to increased blood glucose through the effect of catecholamines on alpha2A-adrenergic receptors (α2A-ARs) present in pancreatic islet β-cells. Variation in the gene (ADRA2A) that encodes the α2A-AR affects insulin release and glucose control and may play a particularly important role during times of stress.

METHODS

We performed a retrospective cohort study using de-identified electronic medical records linked to a DNA repository in 521 Caucasians and 55 African-American non-diabetic patients with AMI. We examined the association between admission blood glucose concentrations and ten selected ADRA2A SNPs in Caucasians.

RESULTS

Three ADRA2A SNPS were associated with stress-induced hyperglycemia in Caucasians. Individuals homozygous for the rs10885122 variant (n=9) had a 23% lower admission glucose (geometric mean [95% CI], 99 [83-118]mg/dl) compared with non-carriers (121 [118-125] mg/dl; n=401; P=0.001). Admission glucose was 14% higher in rs1800544 variant homozygotes (134 [119-150]mg/dl; n=36) compared to non-carriers (118 [115-121]mg/dl; n=290, P=0.046). Furthermore, homozygotes of the rs553668 variant (n=13) had a 13% higher glucose (133 [110-160]mg/dl) compared to non-carriers (118 [115-122]mg/dl; n=366; P=0.056). Haplotypes including these ADRA2A SNPs were associated with higher admission glucose levels.

CONCLUSIONS

Three ADRA2A genetic variants are associated with blood glucose and stress-induced hyperglycemia after AMI in Caucasians.

Type 2 diabetes: genetic data sharing to advance complex disease research

As with other complex diseases, unbiased association studies followed by physiological and experimental characterization have for years formed a paradigm for identifying genes or processes of relevance to type 2 diabetes mellitus (T2D). Recent large-scale common and rare variant genome-wide association studies (GWAS) suggest that substantially larger association studies are needed to identify most T2D loci in the population. To hasten clinical translation of genetic discoveries, new paradigms are also required to aid specialized investigation of nascent hypotheses. We argue for an integrated T2D knowledgebase, designed for a worldwide community to access aggregated large-scale genetic data sets, as one paradigm to catalyse convergence of these efforts.

Increased Expression of the Diabetes Gene SOX4 Reduces Insulin Secretion by Impaired Fusion Pore Expansion

The transcription factor Sox4 has been proposed to underlie the increased type 2 diabetes risk linked to an intronic single nucleotide polymorphism in CDKAL1 In a mouse model expressing a mutant form of Sox4, glucose-induced insulin secretion is reduced by 40% despite normal intracellular Ca(2+) signaling and depolarization-evoked exocytosis. This paradox is explained by a fourfold increase in kiss-and-run exocytosis (as determined by single-granule exocytosis measurements) in which the fusion pore connecting the granule lumen to the exterior expands to a diameter of only 2 nm, which does not allow the exit of insulin. Microarray analysis indicated that this correlated with an increased expression of the exocytosis-regulating protein Stxbp6. In a large collection of human islet preparations (n = 63), STXBP6 expression and glucose-induced insulin secretion correlated positively and negatively with SOX4 expression, respectively. Overexpression of SOX4 in the human insulin-secreting cell EndoC-βH2 interfered with granule emptying and inhibited hormone release, the latter effect reversed by silencing STXBP6 These data suggest that increased SOX4 expression inhibits insulin secretion and increased diabetes risk by the upregulation of STXBP6 and an increase in kiss-and-run exocytosis at the expense of full fusion. We propose that pharmacological interventions promoting fusion pore expansion may be effective in diabetes therapy.

Changes in beta cell function occur in prediabetes and early disease in the Lepr (db) mouse model of diabetes

AIMS/HYPOTHESIS

Type 2 diabetes is a progressive disease that increases morbidity and the risk of premature death. Glucose dysregulation, such as elevated fasting blood glucose, is observed prior to diabetes onset. A decline in beta cell insulin secretion contributes to the later stages of diabetes, but it is not known what, if any, functional beta cell changes occur in prediabetes and early disease.

METHODS

The Lepr (db) mouse (age 13-18 weeks) was used as a model of type 2 diabetes and a two-photon granule fusion assay was used to characterise the secretory response of pancreatic beta cells.

RESULTS

We identified a prediabetic state in db/db mice where the animals responded normally to a glucose challenge but have elevated fasting blood glucose. Isolated islets from prediabetic animals secreted more and were bigger. Insulin secretion, normalised to insulin content, was similar to wild type but basal insulin secretion was elevated. There was increased glucose-induced granule fusion with a high prevalence of granule-granule fusion. The glucose-induced calcium response was not changed but there was altered expression of the exocytic machinery. db/db animals at the next stage of disease had overt glucose intolerance. Isolated islets from these animals had reduced insulin secretion, reduced glucose-induced granule fusion events and decreased calcium responses to glucose.

CONCLUSIONS/INTERPRETATION

Beta cell function is altered in prediabetes and there are further changes in the progression to early disease.