Β-cell Dysfunction In Diabetes Research Articles

Glucose metabolism partially regulates β-cell function through epigenomic changes.

The β-cell relies predominantly on glucose utilization to generate adenosine triphosphate, which is crucial for both cell viability and insulin secretion. The β-cell has evolved remarkable metabolic flexibility to productively respond to shifts in environmental conditions and changes in glucose availability. Although these adaptive responses are important for maintaining optimal cellular function, there is emerging evidence that the resulting changes in cellular metabolites can impact the epigenome, causing transient and lasting alterations in gene expression. This review explores the intricate interplay between metabolism and the epigenome, providing valuable insights into the molecular mechanisms leading to β-cell dysfunction in diabetes. Understanding these mechanisms will be critical for developing targeted therapeutic strategies to preserve and enhance β-cell function, offering potential avenues for interventions to improve glycemic control in individuals with diabetes.

Genetic and pharmacologic inhibition of ALDH1A3 as a treatment of β-cell failure

Type 2 diabetes (T2D) is associated with β-cell dedifferentiation. Aldehyde dehydrogenase 1 isoform A3 (ALHD1A3) is a marker of β-cell dedifferentiation and correlates with T2D progression. However, it is unknown whether ALDH1A3 activity contributes to β-cell failure, and whether the decrease of ALDH1A3-positive β-cells (A+) following pair-feeding of diabetic animals is due to β-cell restoration. To tackle these questions, we (i) investigated the fate of A+ cells during pair-feeding by lineage-tracing, (ii) somatically ablated ALDH1A3 in diabetic β-cells, and (iii) used a novel selective ALDH1A3 inhibitor to treat diabetes. Lineage tracing and functional characterization show that A+ cells can be reconverted to functional, mature β-cells. Genetic or pharmacological inhibition of ALDH1A3 in diabetic mice lowers glycemia and increases insulin secretion. Characterization of β-cells following ALDH1A3 inhibition shows reactivation of differentiation as well as regeneration pathways. We conclude that ALDH1A3 inhibition offers a therapeutic strategy against β-cell dysfunction in diabetes.

359-OR: Imaging Pancreatic Blood Flow during Stimulation of Insulin Secretion in Humans

Non-invasive imaging of endocrine pancreatic function could provide valuable insights into the magnitude and temporal pattern of β-cell dysfunction in diabetes. Islets comprise 1-2% of pancreatic mass, but receive about 10-20% of pancreatic blood flow, which increases further during stimulation of insulin secretion. We examined the relationship between pancreatic blood flow (PBF) and insulin secretory dynamics in 12 healthy volunteers (6M / 6F, age = 29 ± 5 yrs, BMI = 24.0 ± 2.7 kg/m², HbA1c = 5.1 ± 0.3 %, FPG = 88 ± mg/dl, fasting insulin = 3.7 ± 2.3 μU/ml) . PBF was measured by pseudo-continuous arterial spin labeling (ASL) MRI, with the labeling plane positioned 5-6 cm superior to the imaging slice to label blood in the descending aorta. Fasting subjects were placed in a 3T scanner, and PBF was measured during 1) a 15 min basal period, 2) a +100 mg/dl hyperglycemic clamp for 40 min, followed by 3) a 5 gm bolus of arginine to assess maximal acute insulin response (AIRmax) . First phase (0-min) insulin response to hyperglycemia was 181 ± 96 μU/ml · min, and AIRmax after arginine + hyperglycemia (40-50 min) was 1097 ± 629 μU/ml · min. Basal PBF (188 ± 53 ml/100 gm/min) increased during first phase insulin release, peaking at 212 ± 66 at 12 min, subsequently returned to basal levels (176 ± 59) from 10-40 min during sustained hyperglycemia,and peaked again at 2± 62 at 2 min after arginine. Although there were no significant correlations between glucose, insulin or c-peptide levels and PBF, time-series Granger causality analysis demonstrated that the time-derivative of c-peptide and insulin concentrations, reflectingacute changes in insulin secretion, preceded the increase in PBF (p=0.03 for d[c-peptide]/dt, and p = 0.for d[insulin]/dt) . Conclusions: 1) Changes in PBF during stimulation of insulin secretion in humans can be imaged non-invasively by ASL, 2) Insulin secretion precedes changes in PBF, and 3) The rate of change in insulin and c-peptide release predicts subsequent modulation of PBF. Disclosure D.C.Simonson: Stock/Shareholder; GI Windows, Phase V Technologies, Inc. M.Taso: Research Support; GE Healthcare Systems. C.Mcgrath: None. S.A.Waldman: None. F.Papadopoulou: None. D.C.Alsop: Other Relationship; GE Healthcare Systems, Hitachi, Ltd., Philips Healthcare, Siemens, UIH America, Research Support; GE Healthcare Systems.

Cytochrome c Oxidase Activity as a Metabolic Regulator in Pancreatic Beta-Cells.

Pancreatic β-cells couple glucose-stimulated insulin secretion (GSIS) with oxidative phosphorylation via cytochrome c oxidase (COX), a mitochondrial respiratory-chain enzyme. The Cohen diabetic-sensitive (CDs) rats exhibit hyperglycemia when fed a diabetogenic diet but maintain normoglycemia on a regular diet. We have previously reported a decreased COX activity in CDs rats and explored its relevance for type 2 diabetes (T2D). In this study, we investigated the relation between COX activity in islets, peripheral-blood mononuclear cells (PBMCs), and GSIS during diabetes development in CDs rats fed a diabetogenic diet for 4, 11, 20, and 30 days and during reversion to normoglycemia in hyperglycemic CDs rats fed a reversion diet for 7, 11, and 20 days. An oral glucose-tolerance test was performed at different periods of the diets measuring blood glucose and insulin concentrations. COX activity was determined in islets and PBMCs isolated from rats at the different periods of the diets. We demonstrated a progressive reduction in COX activity in CDs-islets that correlated positively with the decreasing GSIS (R2 = 0.9691, p < 0.001) and inversely with the elevation in blood glucose levels (R2 = 0.8396, p < 0.001). Hyperglycemia was initiated when islet COX activity decreased below 46%. The reversion diet restored >46% of the islet COX activity and GSIS while re-establishing normoglycemia. Interestingly, COX activity in PBMCs correlated significantly with islet COX activity (R2 = 0.8944, p < 0.001). Our data support islet COX activity as a major metabolic regulator of β-cells function. The correlation between COX activity in PBMCs and islets may serve as a noninvasive biomarker to monitor β-cell dysfunction in diabetes.

β-Cell Pathophysiology: A Review of Advanced Optical Microscopy Applications.

β-cells convert glucose (input) resulting in the controlled release of insulin (output), which in turn has the role to maintain glucose homeostasis. β-cell function is regulated by a complex interplay between the metabolic processing of the input, its transformation into second-messenger signals, and final mobilization of insulin-containing granules towards secretion of the output. Failure at any level in this process marks β-cell dysfunction in diabetes, thus making β-cells obvious potential targets for therapeutic purposes. Addressing quantitatively β-cell (dys)function at the molecular level in living samples requires probing simultaneously the spatial and temporal dimensions at the proper resolution. To this aim, an increasing amount of research efforts are exploiting the potentiality of biophysical techniques. In particular, using excitation light in the visible/infrared range, a number of optical-microscopy-based approaches have been tailored to the study of β-cell-(dys)function at the molecular level, either in label-free mode (i.e., exploiting intrinsic autofluorescence of cells) or by the use of organic/genetically-encoded fluorescent probes. Here, relevant examples from the literature are reviewed and discussed. Based on this, new potential lines of development in the field are drawn.

Cigarette smoke exposure impairs β-cell function through activation of oxidative stress and ceramide accumulation

ObjectivesEpidemiological studies indicate that first- and second-hand cigarette smoke (CS) exposure are important risk factors for the development of type 2 diabetes (T2D). Additionally, elevated diabetes risk has been reported to occur within a short period of time after smoking cessation, and health risks associated with smoking are increased when combined with obesity. At present, the mechanisms underlying these associations remain incompletely understood. The objective of this study was to test the impact of CS exposure on pancreatic β-cell function using rodent and in vitro models. MethodsBeginning at 8 weeks of age, C57BL/6 J mice were concurrently fed a high-fat diet (HFD) and exposed to CS for 11 weeks, followed by an additional 11 weeks of smoking cessation with continued HFD. Glucose tolerance testing was performed during CS exposure and during the cessation period. Cultured INS-1 β-cells and primary islets were exposed ex vivo to CS extract (CSE), and β-cell function and viability were tested. Since CS increases ceramide accumulation in the lung and these bioactive sphingolipids have been implicated in pancreatic β-cell dysfunction in diabetes, islet and β-cell sphingolipid levels were measured in islets from CS-exposed mice and in CSE-treated islets and INS-1 cells using liquid chromatography-tandem mass spectrometry. ResultsCompared to HFD-fed, ambient air-exposed mice, HFD-fed and CS-exposed mice had reduced weight gain and better glucose tolerance during the active smoking period. Following smoking cessation, CS-mice exhibited rapid weight gain and had accelerated worsening of their glucose tolerance. CS-exposed mice had higher serum proinsulin/insulin ratios, indicative of β-cell dysfunction, significantly lower β-cell mass (p = 0.017), reduced β-cell proliferation (p = 0.006), and increased islet ceramide content compared to non-smoking control mice. Ex vivo exposure of isolated islets to CSE was sufficient to increase islet ceramide levels, which was correlated with reduced insulin gene expression and glucose-stimulated insulin secretion, and increased β-cell oxidative and endoplasmic reticulum (ER) stress. Treatment with the antioxidant N-acetylcysteine markedly attenuated the effects of CSE on ceramide levels, restored β-cell function and survival, and increased cyclin D2 expression, while also reducing activation of β-cell ER and oxidative stress. ConclusionsOur results indicate that CS exposure leads to impaired insulin production, processing, secretion and reduced β-cell viability and proliferation. These effects were linked to increased β-cell oxidative and ER stress and ceramide accumulation. Mice fed HFD continued to experience detrimental effects of CS exposure even during smoking cessation. Elucidation of the mechanisms by which CS exposure impairs β-cell function in synergy with obesity will help design therapeutic and preventive interventions for both active and former smokers.

RNA binding protein HuD contributes to β-cell dysfunction by impairing mitochondria dynamics.

Imbalanced mitochondrial dynamics in pancreatic β-cells contributes to β-cell dysfunction in diabetes; however, the molecular mechanisms underlying mitochondrial dynamics in the pathology of diabetes are not fully elucidated. We previously reported the reduction of RNA binding protein HuD in pancreatic β-cells of diabetes. Herein, we demonstrate that HuD plays a novel role in the regulation of mitochondrial dynamics by promoting mitochondrial fusion. We show enhanced mitochondrial fragmentation in the pancreas of db/db mice and HuD KO mice. Downregulation of HuD increases the number of cells with fragmented mitochondria and reduces the mitochondrial activity determined by mitochondrial membrane potential and ATP production in mouse insulinoma βTC6 cells. HuD binds to 3'-untraslated region of mitofusin 2 (Mfn2) mRNA and positively regulates its expression. Ectopic expression of Mfn2 in βTC6 cells stably expressing short hairpin RNA against HuD (shHuD) restores HuD-mediated mitochondrial dysfunction. Taken together, our results suggest that HuD regulates mitochondrial dynamics by regulating Mfn2 level and its reduced expression leads to mitochondrial dysfunction in pancreatic β-cells.

2192-P: Extracellular Vesicles as Novel Mediators of Proinflammatory Cytokine-Induced ß-Cell Dysfunction

The presence of a proinflammatory islet microenviroment and consequent dysregulation of glucose-stimulated insulin secretion (GSIS) is a hallmark feature of both type 1 and type 2 diabetes. Recent evidence has implicated islet cell-derived extracellular vesicles (EVs) as novel mediators of cytokine-derived inflammatory stress in diabetes, however the mechanisms governing this process remain largely unknown. Therefore, we set out to test the hypothesis that cytokine-mediated β-cell dysfunction is mediated in part through β-cell autocrine release of proinflammatory EVs which promote inflammation and inhibit GSIS. Particle size analysis of EVs from conditioned media of cytokine treated (IL-1β, TNF-α, and IFNγ) Min6 β-cells (CytoEV) showed an overall increase in β-cell EV secretion (∼2 fold increase, P&amp;lt;0.05) and a decrease in the average size of secreted EVs (P&amp;lt;0.05) implicating a shift towards a more exosomal EV profile. Subsequently, functional assessment of isolated mouse islets treated (48h) with CytoEVs resulted in a significant suppression of GSIS (∼80%, P&amp;lt;0.05 vs. control). Moreover, acute exposure to CytoEVs also recapitulated the transcriptional signature of β-cell failure in diabetes characterized by increased expression of key proinflammatory genes (e.g., Cxcl10, Tlr1, Tlr4), and a decrease in Insulin and Pdx-1 expression (∼40%, P&amp;lt;0.05 for CytoEVs vs. control). Finally, proteomics analysis of CytoEVs from Min6 and INS-1 832/13 lines (compared to control EVs from each line) revealed a distinct proinflammatory profile consisting of proteins implicated in type 1 diabetes and dendritic cell maturation pathways (e.g., HLA-A, STAT1, INS, CPE etc., P&amp;lt;0.05). Taken together, this work provides a novel mechanism of β-cell dysfunction in diabetes mediated by the autocrine exchange of proinflammatory EVs which has implications for the detection and treatment of β-cell failure in diabetes. Disclosure N. Javeed: None. T.K. Her: None. P.M. Vanderboom: None. I.R. Lanza: None. A. Matveyenko: None.

The CCR4-NOT Complex Maintains ß-Cell Identity through the Repression of ß-Cell Disallowed Genes

Loss of β-cell identity, dedifferentiation and reprogramming are recognized as mechanisms of β-cell dysfunction in diabetes. Molecular β-cell identity is not only defined by the expression of signature functional β-cell specific genes but also the repression of several genes recently identified in β-cell research as β-cell disallowed genes. The molecular mechanisms involved in the repression of β-cell disallowed genes are largely unknown. Here, we show that the CCR4-NOT complex, a major deadenylase conserved in eukaryotes, is involved in post-transcriptional regulation of β-cell disallowed genes. β-cell specific depletion of CNOT3 (CNOT3βKO), a CCR4-NOT complex subunit, in mice promotes deceased insulin secretion, early impaired glucose tolerance and the development of overt diabetes at 12 weeks of age. CNOT3βKO islets display decreased insulin content and decreased glucose responsiveness. Furthermore, CNOT3 depleted ß cells exhibit ultrastructural abnormalities including: degranulated β cells, increased immature insulin granules and abnormal mitochondria. RNAseq and qRT-PCR analyses revealed that CNOT3βKO islets have decreased expression of β-cell functional genes: (Ins1, Ins2, MafA, Pcsk2 and Cpe) and increased expression of 9 of the 11 core β-cell disallowed genes: (Slc16A1, Ldha, Pdgfra, Cxcl12, Igfbp4, Oat, Maf, Smad3 and Cd302) and progenitor-like cells genes: (Nanog and Ngn3) supporting the loss of β-cell identity. Using the total RNAseq data, we assessed mRNA stability by calculating the relative amounts of introns and exons for a given mRNA. RNA immunoprecipitation revealed that the CCR4-NOT complex directly binds several upregulated stabilized β-cell disallowed genes: Slc16A1, Tgm2, Cat and Parp3 genes. The present data highlight deadenylation as a novel molecular mechanism regulating β-cell disallowed genes. Thus, we further propose that the CCR4-NOT complex is essential for maintaining β-cell identity by repressing β-cell disallowed genes. Disclosure D. Mostafa: None. A. Takahashi: None. A. Yanagiya: None. T. Yamamoto: None.

Interleukin-6 Reduces β-Cell Oxidative Stress by Linking Autophagy With the Antioxidant Response.

Production of reactive oxygen species (ROS) is a key instigator of β-cell dysfunction in diabetes. The pleiotropic cytokine interleukin 6 (IL-6) has previously been linked to β-cell autophagy but has not been studied in the context of β-cell antioxidant response. We used a combination of animal models of diabetes and analysis of cultured human islets and rodent β-cells to study how IL-6 influences antioxidant response. We show that IL-6 couples autophagy to antioxidant response and thereby reduces ROS in β-cells and human islets. β-Cell-specific loss of IL-6 signaling in vivo renders mice more susceptible to oxidative damage and cell death through the selective β-cell toxins streptozotocin and alloxan. IL-6-driven ROS reduction is associated with an increase in the master antioxidant factor NRF2, which rapidly translocates to the mitochondria to decrease mitochondrial activity and stimulate mitophagy. IL-6 also initiates a robust transient decrease in cellular cAMP levels, likely contributing to the stimulation of mitophagy to mitigate ROS. Our findings suggest that coupling autophagy to antioxidant response in β-cells leads to stress adaptation that can reduce cellular apoptosis. These findings have implications for β-cell survival under diabetogenic conditions and present novel targets for therapeutic intervention.

The RhoGAP Stard13 controls insulin secretion through F-actin remodeling

ObjectiveActin cytoskeleton remodeling is necessary for glucose-stimulated insulin secretion in pancreatic β-cells. A mechanistic understanding of actin dynamics in the islet is paramount to a better comprehension of β-cell dysfunction in diabetes. Here, we investigate the Rho GTPase regulator Stard13 and its role in F-actin cytoskeleton organization and islet function in adult mice. MethodsWe used Lifeact-EGFP transgenic animals to visualize actin cytoskeleton organization and dynamics in vivo in the mouse islets. Furthermore, we applied this model to study actin cytoskeleton and insulin secretion in mutant mice deleted for Stard13 selectively in pancreatic cells. We isolated transgenic islets for 3D-imaging and perifusion studies to measure insulin secretion dynamics. In parallel, we performed histological and morphometric analyses of the pancreas and used in vivo approaches to study glucose metabolism in the mouse. ResultsIn this study, we provide the first genetic evidence that Stard13 regulates insulin secretion in response to glucose. Postnatally, Stard13 expression became restricted to the mouse pancreatic islets. We showed that Stard13 deletion results in a marked increase in actin polymerization in islet cells, which is accompanied by severe reduction of insulin secretion in perifusion experiments. Consistently, Stard13-deleted mice displayed impaired glucose tolerance and reduced glucose-stimulated insulin secretion. ConclusionsTaken together, our results suggest a previously unappreciated role for the RhoGAP protein Stard13 in the interplay between actin cytoskeletal remodeling and insulin secretion.

The Na+/Ca2+ exchanger and the Plasma Membrane Ca2+-ATPase in β-cell function and diabetes

The rat pancreatic β-cell expresses 6 splice variants of the Plasma Membrane Ca2+-ATPase (PMCA) and two splice variants of the Na+/Ca2+ exchanger 1 (NCX1). In the β-cell Na+/Ca2+ exchange displays a high capacity, contributes to both Ca2+ outflow and influx and participates to the control of insulin release. Gain of function studies show that overexpression of PMCA2 or NCX1 leads to endoplasmic reticulum (ER) Ca2+ depletion with subsequent ER stress, decrease in β-cell proliferation and β-cell death by apoptosis. Loss of function studies show, on the contrary, that heterozygous inactivation of NCX1 (Ncx1+/-) leads to an increase in β-cell function and a 5 fold increase in both β-cell mass and proliferation. The mutation also increases β-cell resistance to hypoxia, and Ncx1+/- islets show a 2-4 times higher rate of diabetes cure than Ncx1+/+ islets when transplanted in diabetic animals. Thus, down-regulation of the Na+/Ca2+ exchanger leads to various changes in β-cell function that are opposite to the major abnormalities seen in diabetes. In addition, the β-cell includes the mutually exclusive exon B in the alternative splicing region of NCX1, which confers a high sensitivity of its NCX splice variants (NCX1.3 & 1.7) to the inhibitory action of compounds like KBR-7943. Heterozygous inactivation of PMCA2 leads to apparented, though not completely similar results.These provide 2 unique models for the prevention and treatment of β-cell dysfunction in diabetes and following islet transplantation.

Exploring the molecular mechanisms underlying α- and β-cell dysfunction in diabetes.

Pancreatic islet dysfunction, including impaired insulin secretion in β cells and dysregulated glucagon secretion in α cells, is the chief pathology of diabetes. In β cells, oxidative stress, evoked by chronic hyperglycemia, was found to induce dysfunction of a critical transcription factor, PDX1, caused by its nucleocytoplasmic translocation via interactions with the insulin and JNK signaling pathways and another transcription factor, FOXO1. The significance of α-cell insulin signaling in the physiological and pathological regulation of α-cell biology was demonstrated in α-cell-specific insulin receptor knockout mice, which exhibited dysregulated glucagon secretion. Moreover, a high-glucose load directly induced excessive glucagon secretion in a glucagon-secreting cell line and isolated islets, together with impairment of insulin signaling. These findings indicate that disordered insulin signaling is central to the pathophysiology of islet dysfunction in both α and β cells. On the other hand, certain beneficial effects of GLP-1 on dysfunctional α and β cells indicate that it has therapeutic potential for diabetes patients who exhibit insulin resistance in islets. These studies, involving basic medical research approaches, have-at least in part-clarified the molecular mechanisms underlying α- and β-cell dysfunction in diabetes, and offer important clues that should aid the development of future therapeutic approaches to the disease.

Exploring the molecular mechanisms underlying α- and β-cell dysfunction in diabetes

Exploring the molecular mechanisms underlying α- and β-cell dysfunction in diabetes

Stimulation of β-adrenergic receptors plays a protective role via increased expression of RAF-1 and PDX-1 in hyperglycemic rat pancreatic islet (RIN-m5F) cells.

IntroductionIt is a widely held view that a progressive reduction of beta-cell mass occurs in the progression of diabetes. RAF-1 kinase and pancreas duodenal homeobox 1 (PDX-1) are major factors that promote survival of cells and maintain normal insulin functions. In this study we investigated the effect of a β-adrenergic receptor agonist and antagonist on RAF-1 and PDX-1, and their respective effects on apoptosis and insulin release in RIN-m5F cells.Material and methodsRIN-m5F cells were cultured in normal (5 mM) and high (25 mM) glucose to mimic diabetic conditions, followed by treatment with 5 µM, 10 µM and 20 µM of isoproterenol and isoproterenol + propranolol for 6, 12 and 24 h. Western blotting and reverse transcription analysis were performed to examine the expression of RAF-1 and PDX-1. Annexin-V-FITC and terminal deoxynucleotidyl transferase dUTP nick-end labeling (TUNEL) assays were used to investigate apoptosis. ELISA was used to measure insulin levels. Reverse transcription polymerase chain reaction was conducted to investigate the expression of genes.ResultsStimulation of β-adrenergic receptors with isoproterenol significantly induced RAF-1 and PDX-1 genes in a concentration-dependent and time-independent manner. Changes were significant both at protein and mRNA levels. Up-regulation of RAF-1 and PDX-1 was accompanied by improved insulin levels and reduced apoptosis. Concentrations of 10 µM and 20 µM for 12 and 24 h were more effective in achieving significant differences in the experimental and control groups. Propranolol reversed the effect of isoproterenol mostly at maximum concentrations and time periods.ConclusionsA positive effect of a β-adrenergic agonist on RAF-1 and PDX-1, reduction in β-cell apoptosis and improved insulin contents can help to understand the pathogenesis of diabetes and to develop novel approaches for the β-cell dysfunction in diabetes.

β-Cell dysfunction in diabetes: a crisis of identity?

Type 2 diabetes is characterized by insulin resistance and a progressive loss of β-cell function induced by a combination of both β-cell loss and impaired insulin secretion from remaining β-cells. Here, we review the fate of the β-cell under chronic hyperglycaemic conditions with regard to β-cell mass, gene expression, hormone content, secretory capacity and the ability to de- or transdifferentiate into other cell types. We compare data from various in vivo and in vitro models of diabetes with a novel mouse model of inducible, reversible hyperglycaemia (βV59M mice). We suggest that insulin staining using standard histological methods may not always provide an accurate estimation of β-cell mass or number. We consider how β-cell identity is best defined, and whether expression of transcription factors normally found in islet progenitor cells, or in α-cells, implies that mature β-cells have undergone dedifferentiation or transdifferentiation. We propose that even in long-standing diabetes, β-cells predominantly remain β-cells-but not as we know them.

Extracellular CADM1 interactions influence insulin secretion by rat and human islet β-cells and promote clustering of syntaxin-1.

Contact between β-cells is necessary for their normal function. Identification of the proteins mediating the effects of β-cell-to-β-cell contact is a necessary step toward gaining a full understanding of the determinants of β-cell function and insulin secretion. The secretory machinery of the β-cells is nearly identical to that of central nervous system (CNS) synapses, and we hypothesize that the transcellular protein interactions that drive maturation of the two secretory machineries upon contact of one cell (or neural process) with another are also highly similar. Two such transcellular interactions, important for both synaptic and β-cell function, have been identified: EphA/ephrin-A and neuroligin/neurexin. Here, we tested the role of another synaptic cleft protein, CADM1, in insulinoma cells and in rat and human islet β-cells. We found that CADM1 is a predominant CADM isoform in β-cells. In INS-1 cells and primary β-cells, CADM1 constrains insulin secretion, and its expression decreases after prolonged glucose stimulation. Using a coculture model, we found that CADM1 also influences insulin secretion in a transcellular manner. We asked whether extracellular CADM1 interactions exert their influence via the same mechanisms by which they influence neurotransmitter exocytosis. Our results suggest that, as in the CNS, CADM1 interactions drive exocytic site assembly and promote actin network formation. These results support the broader hypothesis that the effects of cell-cell contact on β-cell maturation and function are mediated by the same extracellular protein interactions that drive the formation of the presynaptic exocytic machinery. These interactions may be therapeutic targets for reversing β-cell dysfunction in diabetes.

Oxidative and endoplasmic reticulum stress in β-cell dysfunction in diabetes

The inability of pancreatic β-cells to make sufficient insulin to control blood sugar is a central feature of the aetiology of most forms of diabetes. In this review we focus on the deleterious effects of oxidative stress and endoplasmic reticulum (ER) stress on β-cell insulin biosynthesis and secretion and on inflammatory signalling and apoptosis with a particular emphasis on type 2 diabetes (T2D). We argue that oxidative stress and ER stress are closely entwined phenomena fundamentally involved in β-cell dysfunction by direct effects on insulin biosynthesis and due to consequences of the ER stress-induced unfolded protein response. We summarise evidence that, although these phenomenon can be driven by intrinsic β-cell defects in rare forms of diabetes, in T2D β-cell stress is driven by a range of local environmental factors including increased drivers of insulin biosynthesis, glucolipotoxicity and inflammatory cytokines. We describe our recent findings that a range of inflammatory cytokines contribute to β-cell stress in diabetes and our discovery that interleukin 22 protects β-cells from oxidative stress regardless of the environmental triggers and can correct much of diabetes pathophysiology in animal models. Finally we summarise evidence that β-cell dysfunction is reversible in T2D and discuss therapeutic opportunities for relieving oxidative and ER stress and restoring glycaemic control.

Minireview: 12-Lipoxygenase and Islet β-Cell Dysfunction in Diabetes.

The insulin producing islet β-cells have increasingly gained attention for their role in the pathogeneses of virtually all forms of diabetes. Dysfunction, de-differentiation, and/or death of β-cells are pivotal features in the transition from normoglycemia to hyperglycemia in both animal models of metabolic disease and humans. In both type 1 and type 2 diabetes, inflammation appears to be a central cause of β-cell derangements, and molecular pathways that modulate inflammation or the inflammatory response are felt to be prime targets of future diabetes therapy. The lipoxygenases (LOs) represent a class of enzymes that oxygenate cellular polyunsaturated fatty acids to produce inflammatory lipid intermediates that directly and indirectly affect cellular function and survival. The enzyme 12-LO is expressed in all metabolically active tissues, including pancreatic islets, and has received increasing attention for its role in promoting cellular inflammation in the setting of diabetes. Genetic deletion models of 12-LO in mice reveal striking protection from metabolic disease and its complications and an emerging body of literature has implicated its role in human disease. This review focuses on the evidence supporting the proinflammatory role of 12-LO as it relates to islet β-cells, and the potential for 12-LO inhibition as a future avenue for the prevention and treatment of metabolic disease.

Legacy Effect of Foxo1 in Pancreatic Endocrine Progenitors on Adult β-Cell Mass and Function.

β-Cell dysfunction in diabetes results from abnormalities of insulin production, secretion, and cell number. These abnormalities may partly arise from altered developmental programming of β-cells. Foxo1 is important to maintain adult β-cells, but little is known about its role in pancreatic progenitor cells as determinants of future β-cell function. We addressed this question by generating an allelic series of somatic Foxo1 knockouts at different stages of pancreatic development in mice. Surprisingly, ablation of Foxo1 in pancreatic progenitors resulted in delayed appearance of Neurogenin3+ progenitors and their persistence into adulthood as a self-replicating pool, causing a fourfold increase of β-cell mass. Similarly, Foxo1 ablation in endocrine progenitors increased their numbers, extended their survival, and expanded β-cell mass. In contrast, ablation of Foxo1 in terminally differentiated β-cells did not increase β-cell mass nor did it affect Neurogenin3 expression. Despite the increased β-cell mass, islets from mice lacking Foxo1 in pancreatic or endocrine progenitors responded poorly to glucose, resulting in glucose intolerance. We conclude that Foxo1 integrates cues that determine developmental timing, pool size, and functional features of endocrine progenitor cells, resulting in a legacy effect on adult β-cell mass and function. Our results illustrate how developmental programming predisposes to β-cell dysfunction in adults and raise questions on the desirability of increasing β-cell mass for therapeutic purposes in type 2 diabetes.