Isocitrate-to-SENP1 signaling amplifies insulin secretion and rescues dysfunctional β cells

The role of anaplerotic metabolism of glucose and glutamine in insulin secretion: A model approach

We propose a detailed computational beta cell model that emphasizes the role of anaplerotic metabolism under glucose and glucose-glutamine stimulation. This model goes beyond the traditional focus on mitochondrial oxidative phosphorylation and ATP-sensitive K+ channels, highlighting the predominant generation of ATP from phosphoenolpyruvate in the vicinity of KATP channels. It also underlines the modulatory role of H2O2 as a signaling molecule in the first phase of glucose-stimulated insulin secretion. In the second phase, the model emphasizes the critical role of anaplerotic pathways, activated by glucose stimulation via pyruvate carboxylase and by glutamine via glutamate dehydrogenase. It particularly focuses on the production of NADPH and glutamate as key enhancers of insulin secretion. The predictions of the model are consistent with empirical data, highlighting the complex interplay of metabolic pathways and emphasizing the primary role of glucose and the facilitating role of glutamine in insulin secretion. By delineating these crucial metabolic pathways, the model provides valuable insights into potential therapeutic targets for diabetes.

What do Stimulated Beta Cells have in Common with Cancer Cells?

This study investigates the metabolic parallels between stimulated pancreatic beta cells and cancer cells, focusing on glucose and glutamine metabolism. Addressing the significant public health challenges of Type 2 Diabetes (T2D) and cancer, we aim to deepen our understanding of the mechanisms driving insulin secretion and cellular proliferation. Our analysis of anaplerotic cycles and the role of NADPH in biosynthesis elucidates their vital functions in both processes. Additionally, we point out that both cell types share an antioxidative response mediated by the Nrf2 signaling pathway, glutathione synthesis, and UCP2 upregulation. Notably, UCP2 facilitates the transfer of C4 metabolites, enhancing reductive TCA cycle metabolism. Furthermore, we observe that hypoxic responses are transient in beta cells post-stimulation but persistent in cancer cells. By synthesizing these insights, the research may suggest novel therapeutic targets for T2D, highlighting the shared metabolic strategies of stimulated beta cells and cancer cells. This comparative analysis not only illuminates the metabolic complexity of these conditions but also emphasizes the crucial role of metabolic pathways in cell function and survival, offering fresh perspectives for tackling T2D and cancer challenges.

Simultaneous TIRF imaging of subplasmalemmal Ca2+ dynamics and granule fusions in insulin-secreting INS-1 cells reveals coexistent synchronized and asynchronous release

The basal and glucose-induced insulin secretion from pancreatic beta cells is a tightly regulated process that is triggered in a Ca2+-dependent fashion and further positively modulated by substances that raise intracellular levels of adenosine 3',5'-cyclic monophosphate (cAMP) or by certain antidiabetic drugs. In a previous study, we have temporally resolved the subplasmalemmal [Ca2+]i dynamics in beta cells that are characterized by trains of sharply delimited spikes, reaching peak values up to 5 µM. Applying total internal reflection fluorescence (TIRF) microscopy and synaptopHluorin to visualize fusion events of individual granules, we found that several fusion events can coincide within 50 to 150 ms. To test whether subplasmalemmal [Ca2+]i microdomains around single or clustered Ca2+ channels may cause a synchronized release of insulin-containing vesicles, we applied simultaneous dual-color TIRF microscopy and monitored Ca2+ fluctuations and exocytotic events in INS-1 cells at high frame rates. The results indicate that fusions can be triggered by subplasmalemmal Ca2+ spiking. This, however, does account for a minority of fusion events. About 90 %-95 % of fusion events either happen between Ca2+ spikes or incidentally overlap with subplasmalemmal Ca2+ spikes. We conclude that only a fraction of exocytotic events in glucose-induced and tolbutamide- or forskolin-enhanced insulin release from INS-1 cells is tightly coupled to Ca2+ microdomains around voltage-gated Ca2+ channels.

Checkpoint kinase 2 controls insulin secretion and glucose homeostasis

After the discovery of insulin, a century ago, extensive work has been done to unravel the molecular network regulating insulin secretion. Here we performed a chemical screen and identified AZD7762, a compound that potentiates glucose-stimulated insulin secretion (GSIS) of a human β cell line, healthy and type 2 diabetic (T2D) human islets and primary cynomolgus macaque islets. In vivo studies in diabetic mouse models and cynomolgus macaques demonstrated that AZD7762 enhances GSIS and improves glucose tolerance. Furthermore, genetic manipulation confirmed that ablation of CHEK2 in human β cells results in increased insulin secretion. Consistently, high-fat-diet-fed Chk2-/- mice show elevated insulin secretion and improved glucose clearance. Finally, untargeted metabolic profiling demonstrated the key role of the CHEK2-PP2A-PLK1-G6PD-PPP pathway in insulin secretion. This study successfully identifies a previously unknown insulin secretion regulating pathway that is conserved across rodents, cynomolgus macaques and human β cells in both healthy and T2D conditions.

LDHB contributes to the regulation of lactate levels and basal insulin secretion in human pancreatic β cells

Using 13C6 glucose labeling coupled to gas chromatography-mass spectrometry and 2D 1H-13C heteronuclear single quantum coherence NMR spectroscopy, we have obtained a comparative high-resolution map of glucose fate underpinning β cell function. In both mouse and human islets, the contribution of glucose to the tricarboxylic acid (TCA) cycle is similar. Pyruvate fueling of the TCA cycle is primarily mediated by the activity of pyruvate dehydrogenase, with lower flux through pyruvate carboxylase. While the conversion of pyruvate to lactate by lactate dehydrogenase (LDH) can be detected in islets of both species, lactate accumulation is 6-fold higher in human islets. Human islets express LDH, with low-moderate LDHA expression and β cell-specific LDHB expression. LDHB inhibition amplifies LDHA-dependent lactate generation in mouse and human β cells and increases basal insulin release. Lastly, cis-instrument Mendelian randomization shows that low LDHB expression levels correlate with elevated fasting insulin in humans. Thus, LDHB limits lactate generation in β cells to maintain appropriate insulin release.

The Role of G Protein-Coupled Receptors and Receptor Kinases in Pancreatic β-Cell Function and Diabetes

Type 2 diabetes (T2D) mellitus has emerged as a major global health concern that has accelerated in recent years due to poor diet and lifestyle. Afflicted individuals have high blood glucose levels that stem from the inability of the pancreas to make enough insulin to meet demand. Although medication can help to maintain normal blood glucose levels in individuals with chronic disease, many of these medicines are outdated, have severe side effects, and often become less efficacious over time, necessitating the need for insulin therapy. G protein-coupled receptors (GPCRs) regulate many physiologic processes, including blood glucose levels. In pancreatic β cells, GPCRs regulate β-cell growth, apoptosis, and insulin secretion, which are all critical in maintaining sufficient β-cell mass and insulin output to ensure euglycemia. In recent years, new insights into the signaling of incretin receptors and other GPCRs have underscored the potential of these receptors as desirable targets in the treatment of diabetes. The signaling of these receptors is modulated by GPCR kinases (GRKs) that phosphorylate agonist-activated GPCRs, marking the receptor for arrestin binding and internalization. Interestingly, genome-wide association studies using diabetic patient cohorts link the GRKs and arrestins with T2D. Moreover, recent reports show that GRKs and arrestins expressed in the β cell serve a critical role in the regulation of β-cell function, including β-cell growth and insulin secretion in both GPCR-dependent and -independent pathways. In this review, we describe recent insights into GPCR signaling and the importance of GRK function in modulating β-cell physiology. SIGNIFICANCE STATEMENT: Pancreatic β cells contain a diverse array of G protein-coupled receptors (GPCRs) that have been shown to improve β-cell function and survival, yet only a handful have been successfully targeted in the treatment of diabetes. This review discusses recent advances in our understanding of β-cell GPCR pharmacology and regulation by GPCR kinases while also highlighting the necessity of investigating islet-enriched GPCRs that have largely been unexplored to unveil novel treatment strategies.

Mitochondrial bioenergetics, metabolism, and beyond in pancreatic β-cells and diabetes

In Type 1 and Type 2 diabetes, pancreatic β-cell survival and function are impaired. Additional etiologies of diabetes include dysfunction in insulin-sensing hepatic, muscle, and adipose tissues as well as immune cells. An important determinant of metabolic health across these various tissues is mitochondria function and structure. This review focuses on the role of mitochondria in diabetes pathogenesis, with a specific emphasis on pancreatic β-cells. These dynamic organelles are obligate for β-cell survival, function, replication, insulin production, and control over insulin release. Therefore, it is not surprising that mitochondria are severely defective in diabetic contexts. Mitochondrial dysfunction poses challenges to assess in cause-effect studies, prompting us to assemble and deliberate the evidence for mitochondria dysfunction as a cause or consequence of diabetes. Understanding the precise molecular mechanisms underlying mitochondrial dysfunction in diabetes and identifying therapeutic strategies to restore mitochondrial homeostasis and enhance β-cell function are active and expanding areas of research. In summary, this review examines the multidimensional role of mitochondria in diabetes, focusing on pancreatic β-cells and highlighting the significance of mitochondrial metabolism, bioenergetics, calcium, dynamics, and mitophagy in the pathophysiology of diabetes. We describe the effects of diabetes-related gluco/lipotoxic, oxidative and inflammation stress on β-cell mitochondria, as well as the role played by mitochondria on the pathologic outcomes of these stress paradigms. By examining these aspects, we provide updated insights and highlight areas where further research is required for a deeper molecular understanding of the role of mitochondria in β-cells and diabetes.

A role and mechanism for redox sensing by SENP1 in β-cell responses to high fat feeding

Pancreatic β-cells respond to metabolic stress by upregulating insulin secretion, however the underlying mechanisms remain unclear. Here we show, in β-cells from overweight humans without diabetes and mice fed a high-fat diet for 2 days, insulin exocytosis and secretion are enhanced without increased Ca2+ influx. RNA-seq of sorted β-cells suggests altered metabolic pathways early following high fat diet, where we find increased basal oxygen consumption and proton leak, but a more reduced cytosolic redox state. Increased β-cell exocytosis after 2-day high fat diet is dependent on this reduced intracellular redox state and requires the sentrin-specific SUMO-protease-1. Mice with either pancreas- or β-cell-specific deletion of this fail to up-regulate exocytosis and become rapidly glucose intolerant after 2-day high fat diet. Mechanistically, redox-sensing by the SUMO-protease requires a thiol group at C535 which together with Zn+-binding suppresses basal protease activity and unrestrained β-cell exocytosis, and increases enzyme sensitivity to regulation by redox signals.

Stem Cell Therapy for the Management of Type 1 Diabetes: Advances and Perspectives

Due to insulin resistance and excessive blood sugar levels, type 1 diabetes mellitus (T1DM) is characterized by pancreatic cell loss. This condition affects young people at a higher rate than any other chronic autoimmune disease. Regardless of the method, exogenous insulin cannot substitute for insulin produced by a healthy pancreas. An emerging area of medicine is pancreatic and islet transplantation for type 1 diabetics to restore normal blood sugar regulation. However, there are still obstacles standing in the way of the widespread use of these therapies, including very low availability of pancreatic and islets supplied from human organ donors, challenging transplantation conditions, high expenses, and a lack of easily accessible methods. Efforts to improve Type 1 Diabetes treatment have been conducted in response to the disease's increasing prevalence. Type 1 diabetes may one day be treated with stem cell treatment. Stem cell therapy has proven to be an effective treatment for type 1 diabetes. Recent progress in stem cell-based diabetes treatment is summarised, and the authors show how to isolate insulin-producing cells (IPCs) from a variety of progenitor cells.

NADPH Dynamics: Linking Insulin Resistance and β-Cells Ferroptosis in Diabetes Mellitus

This review offers an in-depth exploration of Nicotinamide Adenine Dinucleotide Phosphate (NADPH) in metabolic health. It delves into how NADPH affects insulin secretion, influences insulin resistance, and plays a role in ferroptosis. NADPH, a critical cofactor in cellular antioxidant systems and lipid synthesis, plays a central role in maintaining metabolic homeostasis. In adipocytes and skeletal muscle, NADPH influences the pathophysiology of insulin resistance, a hallmark of metabolic disorders such as type 2 diabetes and obesity. The review explores the mechanisms by which NADPH contributes to or mitigates insulin resistance, including its role in lipid and reactive oxygen species (ROS) metabolism. Parallelly, the paper investigates the dual nature of NADPH in the context of pancreatic β-cell health, particularly in its relation to ferroptosis, an iron-dependent form of programmed cell death. While NADPH's antioxidative properties are crucial for preventing oxidative damage in β-cells, its involvement in lipid metabolism can potentiate ferroptotic pathways under certain pathological conditions. This complex relationship underscores the delicate balance of NADPH homeostasis in pancreatic health and diabetes pathogenesis. By integrating findings from recent studies, this review aims to illuminate the nuanced roles of NADPH in different tissues and its potential as a therapeutic target. Understanding these dynamics offers vital insights into the development of more effective strategies for managing insulin resistance and preserving pancreatic β-cell function, thereby advancing the treatment of metabolic diseases.

Spotlight on iron and ferroptosis: research progress in diabetic retinopathy

Iron, as the most abundant metallic element within the human organism, is an indispensable ion for sustaining life and assumes a pivotal role in governing glucose and lipid metabolism, along with orchestrating inflammatory responses. The presence of diabetes mellitus (DM) can induce aberrant iron accumulation within the corporeal system. Consequentially, iron overload precipitates a sequence of important adversities, subsequently setting in motion a domino effect wherein ferroptosis emerges as the utmost pernicious outcome. Ferroptosis, an emerging variant of non-apoptotic regulated cell death, operates independently of caspases and GSDMD. It distinguishes itself from alternative forms of controlled cell death through distinctive morphological and biochemical attributes. Its principal hallmark resides in the pathological accrual of intracellular iron and the concomitant generation of iron-driven lipid peroxides. Diabetic retinopathy (DR), established as the predominant cause of adult blindness, wields profound influence over the well-being and psychosocial strain experienced by afflicted individuals. Presently, an abundance of research endeavors has ascertained the pervasive engagement of iron and ferroptosis in the microangiopathy inherent to DR. Evidently, judicious management of iron overload and ferroptosis in the early stages of DR bears the potential to considerably decelerate disease progression. Within this discourse, we undertake a comprehensive exploration of the regulatory mechanisms governing iron homeostasis and ferroptosis. Furthermore, we expound upon the subsequent detriments induced by their dysregulation. Concurrently, we elucidate the intricate interplay linking iron overload, ferroptosis, and DR. Delving deeper, we engage in a comprehensive deliberation regarding strategies to modulate their influence, thereby effecting prospective interventions in the trajectory of DR's advancement or employing them as therapeutic modalities.

Sardinian Infants of Diabetic Mothers: A Metabolomics Observational Study

Gestational diabetes mellitus (GDM) is a condition characterized by glucose intolerance, with hyperglycemia of varying severity with onset during pregnancy. An uncontrolled GDM can lead to an increased risk of morbidity in the fetus and newborn, and an increased risk of obesity or developing type 2 diabetes, hypertension or neurocognitive developmental impairment in adulthood. In this study, we used nuclear magnetic resonance (NMR) spectroscopy and gas chromatography-mass spectrometry (GS-MS) to analyze the urinary metabolomic profile of newborns of diabetic mothers (NDMs) with the aim of identifying biomarkers useful for the monitoring of NDMs and for early diagnosis of predisposition to develop related chronic diseases. A total of 26 newborns were recruited: 21 children of diabetic mothers, comprising 13 in diet therapy (NDM-diet) and 8 in insulin therapy (NDM-insulin), and 5 control children of non-diabetic mothers (CTR). Urine samples were collected at five time points: at birth (T1), on the third day of life (T2), one week (T3), one month (T4) and six months postpartum (T5). At T1, variations were observed in the levels of seven potential biomarkers (acetate, lactate, glycylproline/proline, isocitrate, N,N-dimethylglycine, N-acetylglucosamine and N-carbamoyl-aspartate) in NMD-insulin infants compared to NDM-diet and CTR infants. In particular, the altered metabolites were found to be involved in several metabolic pathways such as citrate metabolism, glycine, serine and threonine metabolism, arginine and proline metabolism, amino sugar and nucleotide sugar metabolism, and pyruvate metabolism. In contrast, these changes were not visible at subsequent sampling times. The impact of early nutrition (maternal and formula milk) on the metabolomic profile was considered as a potential contributing factor to this finding.

Aberrant metabolite trafficking and fuel sensitivity in human pluripotent stem cell-derived islets

Pancreatic islets regulate blood glucose homeostasis through the controlled release of insulin; however, current metabolic models of glucose-sensitive insulin secretion are incomplete. A comprehensive understanding of islet metabolism is integral to studies of endocrine cell development as well as diabetic islet dysfunction. Human pluripotent stem cell-derived islets (SC-islets) are a developmentally relevant model of human islet function that have great potential in providing a cure for type 1 diabetes. Using multiple 13C-labeled metabolic fuels, we demonstrate that SC-islets show numerous divergent patterns of metabolite trafficking in proposed insulin release pathways compared with primary human islets but are still reliant on mitochondrial aerobic metabolism to derive function. Furthermore, reductive tricarboxylic acid cycle activity and glycolytic metabolite cycling occur in SC-islets, suggesting that non-canonical coupling factors are also present. In aggregate, we show that many facets of SC-islet metabolism overlap with those of primary islets, albeit with a retained immature signature.

The emerging roles of SUMOylation in the tumor microenvironment and therapeutic implications

Tumor initiation, progression, and response to therapies depend to a great extent on interactions between malignant cells and the tumor microenvironment (TME), which denotes the cancerous/non-cancerous cells, cytokines, chemokines, and various other factors around tumors. Cancer cells as well as stroma cells can not only obtain adaption to the TME but also sculpt their microenvironment through a series of signaling pathways. The post-translational modification (PTM) of eukaryotic cells by small ubiquitin-related modifier (SUMO) proteins is now recognized as a key flexible pathway. Proteins involved in tumorigenesis guiding several biological processes including chromatin organization, DNA repair, transcription, protein trafficking, and signal conduction rely on SUMOylation. The purpose of this review is to explore the role that SUMOylation plays in the TME formation and reprogramming, emphasize the importance of targeting SUMOylation to intervene in the TME and discuss the potential of SUMOylation inhibitors (SUMOi) in ameliorating tumor prognosis.

Vitamin K-dependent carboxylation regulates Ca2+ flux and adaptation to metabolic stress in β cells

Vitamin K is a micronutrient necessary for γ-carboxylation of glutamic acids. This post-translational modification occurs in the endoplasmic reticulum (ER) and affects secreted proteins. Recent clinical studies implicate vitamin K in the pathophysiology of diabetes, but the underlying molecular mechanism remains unknown. Here, we show that mouse β cells lacking γ-carboxylation fail to adapt their insulin secretion in the context of age-related insulin resistance or diet-induced β cell stress. In human islets, γ-carboxylase expression positively correlates with improved insulin secretion in response to glucose. We identify endoplasmic reticulum Gla protein (ERGP) as a γ-carboxylated ER-resident Ca²⁺-binding protein expressed in β cells. Mechanistically, γ-carboxylation of ERGP protects cells against Ca²⁺ overfilling by diminishing STIM1 and Orai1 interaction and restraining store-operated Ca²⁺ entry. These results reveal a critical role of vitamin K-dependent carboxylation in regulation of Ca²⁺ flux in β cells and in their capacity to adapt to metabolic stress.

Iron and the Pathophysiology of Diabetes

High iron is a risk factor for type 2 diabetes mellitus (T2DM) and affects most of its cardinal features: decreased insulin secretion, insulin resistance, and increased hepatic gluconeogenesis. This is true across the normal range of tissue iron levels and in pathologic iron overload. Because of iron's central role in metabolic processes (e.g., fuel oxidation) and metabolic regulation (e.g., hypoxia sensing), iron levels participate in determining metabolic rates, gluconeogenesis, fuel choice, insulin action, and adipocyte phenotype. The risk of diabetes related to iron is evident in most or all tissues that determine diabetes phenotypes, with the adipocyte, beta cell, and liver playing central roles. Molecular mechanisms for these effects are diverse, although there may be integrative pathways at play. Elucidating these pathways has implications not only for diabetes prevention and treatment, but also for the pathogenesis of other diseases that are, like T2DM, associated with aging, nutrition, and iron.

ER Redox Homeostasis Regulates Proinsulin Trafficking and Insulin Granule Formation in the Pancreatic Islet β-Cell

Defects in the pancreatic β-cell's secretion system are well-described in type 2 diabetes (T2D) and include impaired proinsulin processing and a deficit in mature insulin-containing secretory granules; however, the cellular mechanisms underlying these defects remain poorly understood. To address this, we used an in situ fluorescent pulse-chase strategy to study proinsulin trafficking. We show that insulin granule formation and the appearance of nascent granules at the plasma membrane are decreased in rodent and cell culture models of prediabetes and hyperglycemia. Moreover, we link the defect in insulin granule formation to an early trafficking delay in endoplasmic reticulum (ER) export of proinsulin, which is independent of overt ER stress. Using a ratiometric redox sensor, we show that the ER becomes hyperoxidized in β-cells from a dietary model of rodent prediabetes and that addition of reducing equivalents restores ER export of proinsulin and insulin granule formation and partially restores β-cell function. Together, these data identify a critical role for the regulation of ER redox homeostasis in proinsulin trafficking and suggest that alterations in ER redox poise directly contribute to the decline in insulin granule production in T2D. This model highlights a critical link between alterations in ER redox and ER function with defects in proinsulin trafficking in T2D. Hyperoxidation of the ER lumen, shown as hydrogen peroxide, impairs proinsulin folding and disulfide bond formation that prevents efficient exit of proinsulin from the ER to the Golgi. This trafficking defect limits available proinsulin for the formation of insulin secretory granules during the development of T2D.

Role of Reactive Oxygen Species in Glucose Metabolism Disorder in Diabetic Pancreatic β-Cells

The dysfunction of pancreatic β-cells plays a central role in the onset and progression of type 2 diabetes mellitus (T2DM). Insulin secretory defects in β-cells are characterized by a selective impairment of glucose stimulation, and a reduction in glucose-induced ATP production, which is essential for insulin secretion. High glucose metabolism for insulin secretion generates reactive oxygen species (ROS) in mitochondria. In addition, the expression of antioxidant enzymes is very low in β-cells. Therefore, β-cells are easily exposed to oxidative stress. In islet studies using a nonobese T2DM animal model that exhibits selective impairment of glucose-induced insulin secretion (GSIS), quenching ROS generated by glucose stimulation and accumulated under glucose toxicity can improve impaired GSIS. Acute ROS generation and toxicity cause glucose metabolism disorders through different molecular mechanisms. Nuclear factor erythroid 2-related factor 2 (Nrf2), a transcription factor, is a master regulator of antioxidant defense and a potential therapeutic target in oxidative stress-related diseases, suggesting the possible involvement of Nrf2 in β-cell dysfunction caused by ROS. In this review, we describe the mechanisms of insulin secretory defects induced by oxidative stress in diabetic β-cells.

The loss of pancreatic islet NADPH oxidase (NOX)2 improves islet transplantation

Islet transplantation is a promising treatment strategy for type 1 diabetes mellitus (T1DM) patients. However, oxidative stress-induced graft failure due to an insufficient revascularization is a major problem of this therapeutic approach. NADPH oxidase (NOX)2 is an important producer of reactive oxygen species (ROS) and several studies have already reported that this enzyme plays a crucial role in the endocrine function and viability of β-cells. Therefore, we hypothesized that targeting islet NOX2 improves the outcome of islet transplantation. To test this, we analyzed the cellular composition and viability of isolated wild-type (WT) and Nox2-/- islets by immunohistochemistry as well as different viability assays. Ex vivo, the effect of Nox2 deficiency on superoxide production, endocrine function and anti-oxidant protein expression was studied under hypoxic conditions. In vivo, we transplanted WT and Nox2-/- islets into mouse dorsal skinfold chambers and under the kidney capsule of diabetic mice to assess their revascularization and endocrine function, respectively. We found that the loss of NOX2 does not affect the cellular composition and viability of isolated islets. However, decreased superoxide production, higher glucose-stimulated insulin secretion as well as expression of nuclear factor erythroid 2-related factor (Nrf)2, heme oxygenase (HO)-1 and superoxide dismutase 1 (SOD1) was detected in hypoxic Nox2-/- islets when compared to WT islets. Moreover, we detected an early revascularization, a higher take rate and restoration of normoglycemia in diabetic mice transplanted with Nox2-/- islets. These findings indicate that the suppression of NOX2 activity represents a promising therapeutic strategy to improve engraftment and function of isolated islets.

Genome-wide CRISPR screen identified a role for commander complex mediated ITGB1 recycling in basal insulin secretion

OBJECTIVES

Pancreatic beta cells secrete insulin postprandially and during fasting to maintain glucose homeostasis. Although glucose-stimulated insulin secretion (GSIS) has been extensively studied, much less is known about basal insulin secretion. Here, we performed a genome-wide CRISPR/Cas9 knockout screen to identify novel regulators of insulin secretion.

METHODS

To identify genes that cell autonomously regulate insulin secretion, we engineered a Cas9-expressing MIN6 subclone that permits irreversible fluorescence labeling of exocytic insulin granules. Using a fluorescence-activated cell sorting assay of exocytosis in low glucose and high glucose conditions in individual cells, we performed a genome-wide CRISPR/Cas9 knockout screen.

RESULTS

We identified several members of the COMMD family, a conserved family of proteins with central roles in intracellular membrane trafficking, as positive regulators of basal insulin secretion, but not GSIS. Mechanistically, we show that the Commander complex promotes insulin granules docking in basal state. This is mediated, at least in part, by its function in ITGB1 recycling. Defective ITGB1 recycling reduces its membrane distribution, the number of focal adhesions and cortical ELKS-containing complexes.

CONCLUSIONS

We demonstrated a previously unknown function of the Commander complex in basal insulin secretion. We showed that by ITGB1 recycling, Commander complex increases cortical adhesions, which enhances the assembly of the ELKS-containing complexes. The resulting increase in the number of insulin granules near the plasma membrane strengthens basal insulin secretion.

Metabolic cycles and signals for insulin secretion

In this review, we focus on recent developments in our understanding of nutrient-induced insulin secretion that challenge a key aspect of the "canonical" model, in which an oxidative phosphorylation-driven rise in ATP production closes K_ATP channels. We discuss the importance of intrinsic β cell metabolic oscillations; the phasic alignment of relevant metabolic cycles, shuttles, and shunts; and how their temporal and compartmental relationships align with the triggering phase or the secretory phase of pulsatile insulin secretion. Metabolic signaling components are assigned regulatory, effectory, and/or homeostatic roles vis-à-vis their contribution to glucose sensing, signal transmission, and resetting the system. Taken together, these functions provide a framework for understanding how allostery, anaplerosis, and oxidative metabolism are integrated into the oscillatory behavior of the secretory pathway. By incorporating these temporal as well as newly discovered spatial aspects of β cell metabolism, we propose a much-refined Mito_Cat-Mito_Ox model of the signaling process for the field to evaluate.

Pulsatile Basal Insulin Secretion Is Driven by Glycolytic Oscillations

In fasted and fed states, blood insulin levels are oscillatory. While this phenomenon is well studied at high glucose levels, comparatively little is known about its origin under basal conditions. We propose a possible mechanism for basal insulin oscillations based on oscillations in glycolysis, demonstrated using an established mathematical model. At high glucose, this is superseded by a calcium-dependent mechanism.

Metabolism-secretion coupling in glucose-stimulated insulin secretion

Pancreatic β-cells in the islets of Langerhans secrete insulin in response to blood glucose levels. Precise control of the amount of insulin secreted is of critical importance for maintaining systemic carbohydrate homeostasis. It is now well established that glucose induced production of ATP from ADP and the K_ATP channel closure elevate cytosolic Ca²⁺, triggering insulin exocytosis in β-cells. However, for full activation of insulin secretion by glucose, other mechanisms besides Ca²⁺ elevation are needed. These mechanisms are the targets of current research and include intracellular metabolic pathways branching from glycolysis. They are metabolic pathways originating from the TCA cycle intermediates, the glycerolipid/free fatty acid cycle and the pentose phosphate pathway. Signaling effects of these pathways including degradation (removal) of protein SUMOylation, modulation of insulin vesicular energetics, and lipid modulation of exocytotic machinery may converge to fulfill insulin secretion, though the precise mechanisms have yet to be elucidated. This mini-review summarize recent advances in research on metabolic coupling mechanisms functioning in insulin secretion.

Maturation of beta cells: lessons from in vivo and in vitro models

The ability to maintain normoglycaemia, through glucose-sensitive insulin release, is a key aspect of postnatal beta cell function. However, terminally differentiated beta cell identity does not necessarily imply functional maturity. Beta cell maturation is therefore a continuation of beta cell development, albeit a process that occurs postnatally in mammals. Although many important features have been identified in the study of beta cell maturation, as of yet no unified mechanistic model of beta cell functional maturity exists. Here, we review recent findings about the underlying mechanisms of beta cell functional maturation. These findings include systemic hormonal and nutritional triggers that operate through energy-sensing machinery shifts within beta cells, resulting in primed metabolic states that allow for appropriate glucose trafficking and, ultimately, insulin release. We also draw attention to the expansive synergistic nature of these pathways and emphasise that beta cell maturation is dependent on overlapping regulatory and metabolic networks.

Local activation of focal adhesion kinase orchestrates the positioning of presynaptic scaffold proteins and Ca2+ signalling to control glucose dependent insulin secretion

A developing understanding suggests that spatial compartmentalisation in pancreatic β cells is critical in controlling insulin secretion. To investigate the mechanisms, we have developed live-cell sub-cellular imaging methods using the mouse organotypic pancreatic slice. We demonstrate that the organotypic pancreatic slice, when compared with isolated islets, preserves intact β cell structure, and enhances glucose dependent Ca²⁺ responses and insulin secretion. Using the slice technique, we have discovered the essential role of local activation of integrins and the downstream component, focal adhesion kinase, in regulating β cells. Integrins and focal adhesion kinase are exclusively activated at the β cell capillary interface and using in situ and in vitro models we show their activation both positions presynaptic scaffold proteins, like ELKS and liprin, and regulates glucose dependent Ca²⁺ responses and insulin secretion. We conclude that focal adhesion kinase orchestrates the final steps of glucose dependent insulin secretion within the restricted domain where β cells contact the islet capillaries.

Metabolism and Endocrine Disorders: What Wnt Wrong?

A fundamental question in cell biology underlies how nutrients are regenerated to maintain and renew tissues. Physiologically, the canonical Wnt signaling is a vital pathway for cell growth, tissue remodeling, and organ formation; pathologically, Wnt signaling contributes to the development of myriad human diseases such as cancer. Despite being the focus of intense research, how Wnt intersects with the metabolic networks to promote tissue growth and remodeling has remained mysterious. Our understanding of metabolism has been revolutionized by technological advances in the fields of chemical biology, metabolomics, and live microscopy that have now made it possible to visualize and manipulate metabolism in living cells and tissues. The application of these toolsets to innovative model systems have propelled the Wnt field into new realms at the forefront answering the most pressing paradigms of cell metabolism in health and disease states. Elucidating the basis of Wnt signaling and metabolism in a cell-type and tissue-specific manner will provide a powerful base of knowledge for both basic biomedical fields and clinician scientists, and has the promise to generate new, transformative therapies in disease and even processes of aging.

More than meets the islet: aligning nutrient and paracrine inputs with hormone secretion in health and disease

The pancreatic islet is responsive to an array of endocrine, paracrine, and nutritional inputs that adjust hormone secretion to ensure accurate control of glucose homeostasis. Although the mechanisms governing glucose-coupled insulin secretion have received the most attention, there is emerging evidence for a multitude of physiological signaling pathways and paracrine networks that collectively regulate insulin, glucagon, and somatostatin release. Moreover, the modulation of these pathways in conditions of glucotoxicity or lipotoxicity are areas of both growing interest and controversy. In this review, the contributions of external, intrinsic, and paracrine factors in pancreatic β-, α-, and δ-cell secretion across the full spectrum of physiological (i.e., fasting and fed) and pathophysiological (gluco- and lipotoxicity; diabetes) environments will be critically discussed.

SENP1-Sirt3 signaling promotes α-ketoglutarate production during M2 macrophage polarization

The metabolic program is altered during macrophage activation and influences macrophage polarization. Glutaminolysis promotes accumulation of α-ketoglutarate (αKG), leading to Jumonji domain-containing protein D3 (Jmjd3)-dependent demethylation at H3K27me3 during M2 polarization of macrophages. However, it remains unclear how αKG accumulation is regulated during M2 polarization of macrophages. This study shows that SENP1-Sirt3 signaling controls glutaminolysis, leading to αKG accumulation during IL-4-stimulated M2 polarization. Activation of the SENP1-Sirt3 axis augments M2 macrophage polarization through the accumulation of αKG via glutaminolysis. We also identify glutamate dehydrogenase 1 (GLUD1) as an acetylated protein in mitochondria. The SENP1-Sirt3 axis deacetylates GLUD1 and increases its activity in glutaminolysis to promote αKG production, leading to M2 polarization of macrophages. Therefore, SENP1-Sirt3 signaling plays a critical role in αKG accumulation via glutaminolysis to promote M2 polarization.

The glucose-lowering effect of low-dose diacerein and its responsiveness metabolic markers in uncontrolled diabetes

Objective

Diacerein inhibits the synthesis and activity of pro-inflammatory cytokines, decreases macrophage infiltration in adipose tissue and thus increases insulin sensitivity and signalling. We conducted this study to determine the efficacy of low-dose diacerein in improving glycaemic control in type 2 diabetes mellitus (T2DM) patients with inadequate glycaemic control and to identify the metabolic determinants for such improvement. We randomised 25 T2DM patients with poor glycaemic control, despite being treated with at least three glucose-lowering agents, to receive diacerein 50 mg once-daily (n = 18) or placebo (n = 17) for 12 weeks. Changes in glycated haemoglobin (HbA1c) were evaluated at the 4th and 12th weeks. Metabolic profiling was performed using liquid chromatography electrospray ionisation quadrupole time-of-flight mass spectrometry.

Results

HbA1c levels were significantly reduced from baseline in the diacerein group at 12 weeks (− 0.6%, p < 0.05), whereas fasting plasma glucose (FPG) levels were not significantly decreased (− 18.9 mg/dl, p = 0.06). Partial least squares-discriminant analysis demonstrated an association between the serum abundance of threo-isocitric acid (ICA) and HbA1c response in the diacerein group. After adjusting for serum high-sensitivity C-reactive protein, ICA was still significantly related to the change in HbA1c.

Retrospective trial registration Current Controlled Trials TCTR20200820004, 20 August 2020.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13104-022-05974-9.

Functional, metabolic and transcriptional maturation of human pancreatic islets derived from stem cells

Transplantation of pancreatic islet cells derived from human pluripotent stem cells is a promising treatment for diabetes. Despite progress in the generation of stem-cell-derived islets (SC-islets), no detailed characterization of their functional properties has been conducted. Here, we generated functionally mature SC-islets using an optimized protocol and benchmarked them comprehensively against primary adult islets. Biphasic glucose-stimulated insulin secretion developed during in vitro maturation, associated with cytoarchitectural reorganization and the increasing presence of alpha cells. Electrophysiology, signaling and exocytosis of SC-islets were similar to those of adult islets. Glucose-responsive insulin secretion was achieved despite differences in glycolytic and mitochondrial glucose metabolism. Single-cell transcriptomics of SC-islets in vitro and throughout 6 months of engraftment in mice revealed a continuous maturation trajectory culminating in a transcriptional landscape closely resembling that of primary islets. Our thorough evaluation of SC-islet maturation highlights their advanced degree of functionality and supports their use in further efforts to understand and combat diabetes.

Pancreatic islets derived from stem cells are benchmarked against primary cells.

Nutrient Regulation of Pancreatic Islet β-Cell Secretory Capacity and Insulin Production

Pancreatic islet β-cells exhibit tremendous plasticity for secretory adaptations that coordinate insulin production and release with nutritional demands. This essential feature of the β-cell can allow for compensatory changes that increase secretory output to overcome insulin resistance early in Type 2 diabetes (T2D). Nutrient-stimulated increases in proinsulin biosynthesis may initiate this β-cell adaptive compensation; however, the molecular regulators of secretory expansion that accommodate the increased biosynthetic burden of packaging and producing additional insulin granules, such as enhanced ER and Golgi functions, remain poorly defined. As these adaptive mechanisms fail and T2D progresses, the β-cell succumbs to metabolic defects resulting in alterations to glucose metabolism and a decline in nutrient-regulated secretory functions, including impaired proinsulin processing and a deficit in mature insulin-containing secretory granules. In this review, we will discuss how the adaptative plasticity of the pancreatic islet β-cell's secretory program allows insulin production to be carefully matched with nutrient availability and peripheral cues for insulin signaling. Furthermore, we will highlight potential defects in the secretory pathway that limit or delay insulin granule biosynthesis, which may contribute to the decline in β-cell function during the pathogenesis of T2D.

Glucose-6-phosphatase catalytic subunit 2 negatively regulates glucose oxidation and insulin secretion in pancreatic β-cells

Elevated fasting blood glucose (FBG) is associated with increased risks of developing type 2 diabetes (T2D) and cardiovascular-associated mortality. G6PC2 is predominantly expressed in islets, encodes a glucose-6-phosphatase catalytic subunit that converts glucose-6-phosphate (G6P) to glucose, and has been linked with variations in FBG in genome-wide association studies. Deletion of G6pc2 in mice has been shown to lower FBG without affecting fasting plasma insulin levels in vivo. At 5 mM glucose, pancreatic islets from G6pc2 knockout (KO) mice exhibit no glucose cycling, increased glycolytic flux, and enhanced glucose-stimulated insulin secretion (GSIS). However, the broader effects of G6pc2 KO on β-cell metabolism and redox regulation are unknown. Here we used CRISPR/Cas9 gene editing and metabolic flux analysis in βTC3 cells, a murine pancreatic β-cell line, to examine the role of G6pc2 in regulating glycolytic and mitochondrial fluxes. We found that deletion of G6pc2 led to ∼60% increases in glycolytic and citric acid cycle (CAC) fluxes at both 5 and 11 mM glucose concentrations. Furthermore, intracellular insulin content and GSIS were enhanced by approximately two-fold, along with increased cytosolic redox potential and reductive carboxylation flux. Normalization of fluxes relative to net glucose uptake revealed upregulation in two NADPH-producing pathways in the CAC. These results demonstrate that G6pc2 regulates GSIS by modulating not only glycolysis but also, independently, citric acid cycle activity in β-cells. Overall, our findings implicate G6PC2 as a potential therapeutic target for enhancing insulin secretion and lowering FBG, which could benefit individuals with prediabetes, T2D, and obesity.

Heterogenous impairment of α cell function in type 2 diabetes is linked to cell maturation state

In diabetes, glucagon secretion from pancreatic α cells is dysregulated. The underlying mechanisms, and whether dysfunction occurs uniformly among cells, remain unclear. We examined α cells from human donors and mice using electrophysiological, transcriptomic, and computational approaches. Rising glucose suppresses α cell exocytosis by reducing P/Q-type Ca2+ channel activity, and this is disrupted in type 2 diabetes (T2D). Upon high-fat feeding of mice, α cells shift toward a "β cell-like" electrophysiological profile in concert with indications of impaired identity. In human α cells we identified links between cell membrane properties and cell surface signaling receptors, mitochondrial respiratory chain complex assembly, and cell maturation. Cell-type classification using machine learning of electrophysiology data demonstrated a heterogenous loss of "electrophysiologic identity" in α cells from donors with type 2 diabetes. Indeed, a subset of α cells with impaired exocytosis is defined by an enrichment in progenitor and lineage markers and upregulation of an immature transcriptomic phenotype, suggesting important links between α cell maturation state and dysfunction.

Cryopreservation and post-thaw characterization of dissociated human islet cells

The objective of this study is to optimize the cryopreservation of dissociated islet cells and obtain functional cells that can be used in single-cell transcriptome studies on the pathology and treatment of diabetes. Using an iterative graded freezing approach we obtained viable cells after cooling in 10% dimethyl sulfoxide and 6% hydroxyethyl starch at 1°C/min to -40°C, storage in liquid nitrogen, rapid thaw, and removal of cryoprotectants by serial dilution. The expression of epithelial cell adhesion molecule declined immediately after thaw, but recovered after overnight incubation, while that of an endocrine cell marker (HPi2) remained high after cryopreservation. Patch-clamp electrophysiology revealed differences in channel activities and exocytosis of various islet cell types; however, exocytotic responses, and the biophysical properties of voltage-gated Na+ and Ca2+ channels, are sustained after cryopreservation. Single-cell RNA sequencing indicates that overall transcriptome and crucial exocytosis genes are comparable between fresh and cryopreserved dispersed human islet cells. Thus, we report an optimized procedure for cryopreserving dispersed islet cells that maintained their membrane integrity, along with their molecular and functional phenotypes. Our findings will not only provide a ready source of cells for investigating cellular mechanisms in diabetes but also for bio-engineering pseudo-islets and islet sheets for modeling studies and potential transplant applications.

SENP2 regulates mitochondrial function and insulin secretion in pancreatic β cells

Increasing evidence has shown that small ubiquitin-like modifier (SUMO) modification plays an important role in metabolic regulation. We previously demonstrated that SUMO-specific protease 2 (SENP2) is involved in lipid metabolism in skeletal muscle and adipogenesis. In this study, we investigated the function of SENP2 in pancreatic β cells by generating a β cell-specific knockout (Senp2-βKO) mouse model. Glucose tolerance and insulin secretion were significantly impaired in the Senp2-βKO mice. In addition, glucose-stimulated insulin secretion (GSIS) was decreased in the islets of the Senp2-βKO mice without a significant change in insulin synthesis. Furthermore, islets of the Senp2-βKO mice exhibited enlarged mitochondria and lower oxygen consumption rates, accompanied by lower levels of S616 phosphorylated DRP1 (an active form of DRP1), a mitochondrial fission protein. Using a cell culture system of NIT-1, an islet β cell line, we found that increased SUMO2/3 conjugation to DRP1 due to SENP2 deficiency suppresses the phosphorylation of DRP1, which possibly induces mitochondrial dysfunction. In addition, SENP2 overexpression restored GSIS impairment induced by DRP1 knockdown and increased DRP1 phosphorylation. Furthermore, palmitate treatment decreased phosphorylated DRP1 and GSIS in β cells, which was rescued by SENP2 overexpression. These results suggest that SENP2 regulates mitochondrial function and insulin secretion at least in part by modulating the phosphorylation of DRP1 in pancreatic β cells.

SENP2 is vital for optimal insulin signaling and insulin-stimulated glycogen synthesis in human skeletal muscle cells

Sentrin-specific protease (SENP) 2 has been suggested as a possible novel drug target for the treatment of obesity and type 2 diabetes mellitus after observations of a palmitate-induced increase in SENP2 that lead to increased fatty acid oxidation and improved insulin sensitivity in skeletal muscle cells from mice. However, no precedent research has examined the role of SENP2 in human skeletal muscle cells. In the present work, we have investigated the impact of SENP2 on fatty acid and glucose metabolism as well as insulin sensitivity in human skeletal muscle using cultured primary human myotubes. Acute (4 ​h) oleic acid oxidation was reduced in SENP2-knockdown (SENP2-KD) cells compared to control cells, with no difference in uptake. After prelabeling (24 ​h) with oleic acid, total lipid content and incorporation into triacylglycerol was decreased, while incorporation into other lipids, as well as complete oxidation and β-oxidation was increased in SENP2-KD cells. Basal glucose uptake (i.e., not under insulin-stimulated conditions) was higher in SENP2-KD cells, whereas oxidation was similar to control myotubes. Further, basal glycogen synthesis was not different in SENP2-KD myotubes, but both insulin-stimulated glycogen synthesis and Akt^Ser473 phosphorylation was completely blunted in SENP2-KD cells. In conclusion, SENP2 plays an important role in fatty acid and glucose metabolism in human myotubes. Interestingly, it also appears to have a pivotal role in regulating myotube insulin sensitivity. Future studies should examine the role of SENP2 in regulation of insulin sensitivity in other tissues and in vivo, defining the potential for SENP2 as a drug target.

The lack of functional nicotinamide nucleotide transhydrogenase only moderately contributes to the impairment of glucose tolerance and glucose-stimulated insulin secretion in C57BL/6J vs C57BL/6N mice

AIMS/HYPOTHESIS

Nicotinamide nucleotide transhydrogenase (NNT) is involved in mitochondrial NADPH production and its spontaneous inactivating mutation (Nnt^Tr [Tr, truncated]) is usually considered to be the main cause of the lower glucose tolerance of C57BL/6J vs C57BL/6N mice. However, the impact of this mutation on glucose tolerance remains disputed. Here, we singled out the impact of Nnt^Tr from that of other genetic variants between C57BL/6J and C57BL/6N mice on mitochondrial glutathione redox state (E_GSH), glucose-stimulated insulin secretion (GSIS) and glucose tolerance.

METHODS

Male and female N5BL/6J mice that express wild-type Nnt (Nnt^WT) or Nnt^Tr (N5-WT and N5-Tr mice) on the C57BL/6J genetic background were obtained by crossing N5BL/6J Nnt^WT/Tr heterozygous mice. C57BL/6J and C57BL/6N mice were from Janvier Labs. The Nnt genotype was confirmed by PCR and the genetic background by whole genome sequencing of one mouse of each type. Glucose tolerance was assessed by IPGTT, ITT and fasting/refeeding tests. Stimulus-secretion coupling events and GSIS were measured in isolated pancreatic islets. Cytosolic and mitochondrial E_GSH were measured using the fluorescent redox probe GRX1-roGFP2 (glutaredoxin 1 fused to redox-sensitive enhanced GFP).

RESULTS

The Nnt genotype and genetic background of each type of mouse were confirmed. As reported previously in C57BL/6N vs C57BL/6J islets, the glucose regulation of mitochondrial (but not cytosolic) E_GSH and of NAD(P)H autofluorescence was markedly improved in N5-WT vs N5-Tr islets, confirming the role of NNT in mitochondrial redox regulation. However, ex vivo GSIS was only 1.2-1.4-times higher in N5-WT vs N5-Tr islets, while it was 2.4-times larger in C57BL/6N vs N5-WT islets, questioning the role of NNT in GSIS. In vivo, the ITT results did not differ between N5-WT and N5-Tr or C57BL/6N mice. However, the glucose excursion during an IPGTT was only 15-20% lower in female N5-WT mice than in N5-Tr and C57BL/6J mice and remained 3.5-times larger than in female C57BL/6N mice. Similar observations were made during a fasting/refeeding test. A slightly larger (~30%) impact of NNT on glucose tolerance was found in males.

CONCLUSIONS/INTERPRETATION

Although our results confirm the importance of NNT in the regulation of mitochondrial redox state by glucose, they markedly downsize the role of NNT in the alteration of GSIS and glucose tolerance in C57BL/6J vs C57BL/6N mice. Therefore, documenting an Nnt^WT genotype in C57BL/6 mice does not provide proof that their glucose tolerance is as good as in C57BL/6N mice.

The Pancreatic ß-cell Response to Secretory Demands and Adaption to Stress

Pancreatic β cells dedicate much of their protein translation capacity to producing insulin to maintain glucose homeostasis. In response to increased secretory demand, β cells can compensate by increasing insulin production capability even in the face of protracted peripheral insulin resistance. The ability to amplify insulin secretion in response to hyperglycemia is a critical facet of β-cell function, and the exact mechanisms by which this occurs have been studied for decades. To adapt to the constant and fast-changing demands for insulin production, β cells use the unfolded protein response of the endoplasmic reticulum. Failure of these compensatory mechanisms contributes to both type 1 and 2 diabetes. Additionally, studies in which β cells are "rested" by reducing endogenous insulin demand have shown promise as a therapeutic strategy that could be applied more broadly. Here, we review recent findings in β cells pertaining to the metabolic amplifying pathway, the unfolded protein response, and potential advances in therapeutics based on β-cell rest.

The β-cell glucose toxicity hypothesis: Attractive but difficult to prove

β cells in the hyperglycemic environment of diabetes have marked changes in phenotype and function that are largely reversible if glucose levels can be returned to normal. A leading hypothesis is that these changes are caused by the elevated glucose levels leading to the concept of glucose toxicity. Support for the glucose toxicity hypothesis is largely circumstantial, but little progress has been made in defining the responsible mechanisms. Then questions emerge that are difficult to answer. In the very earliest stages of diabetes development, there is a dramatic loss of glucose-induced first-phase insulin release (FPIR) with only trivial elevations of blood glucose levels. A related question is how impaired insulin action on target tissues such as liver, muscle and fat can cause increased insulin secretion. The existence of a sophisticated feedback mechanism between insulin secretion and insulin action on peripheral tissues driven by glucose has been postulated, but it has been difficult to measure increases in blood glucose levels that might have been expected. These complexities force us to challenge the simplicity of the glucose toxicity hypothesis and feedback mechanisms. It may turn out that glucose is somehow driving all of these changes, but we must develop new questions and experimental approaches to test the hypothesis.

β-Cell Knockout of SENP1 Reduces Responses to Incretins and Worsens Oral Glucose Tolerance in High-Fat Diet-Fed Mice

SUMOylation reduces oxidative stress and preserves islet mass at the expense of robust insulin secretion. To investigate a role for the deSUMOylating enzyme sentrin-specific protease 1 (SENP1) following metabolic stress, we put pancreas/gut-specific SENP1 knockout (pSENP1-KO) mice on a high-fat diet (HFD). Male pSENP1-KO mice were more glucose intolerant following HFD than littermate controls but only in response to oral glucose. A similar phenotype was observed in females. Plasma glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide 1 (GLP-1) responses were identical in pSENP1-KO and wild-type littermates, including the HFD-induced upregulation of GIP responses. Islet mass was not different, but insulin secretion and β-cell exocytotic responses to the GLP-1 receptor agonist exendin-4 (Ex4) and GIP were impaired in islets lacking SENP1. Glucagon secretion from pSENP1-KO islets was also reduced, so we generated β-cell-specific SENP1 KO mice. These phenocopied the pSENP1-KO mice with selective impairment in oral glucose tolerance following HFD, preserved islet mass expansion, and impaired β-cell exocytosis and insulin secretion to Ex4 and GIP without changes in cAMP or Ca²⁺ levels. Thus, β-cell SENP1 limits oral glucose intolerance following HFD by ensuring robust insulin secretion at a point downstream of incretin signaling.

The Human Islet: Mini-Organ With Mega-Impact

This review focuses on the human pancreatic islet-including its structure, cell composition, development, function, and dysfunction. After providing a historical timeline of key discoveries about human islets over the past century, we describe new research approaches and technologies that are being used to study human islets and how these are providing insight into human islet physiology and pathophysiology. We also describe changes or adaptations in human islets in response to physiologic challenges such as pregnancy, aging, and insulin resistance and discuss islet changes in human diabetes of many forms. We outline current and future interventions being developed to protect, restore, or replace human islets. The review also highlights unresolved questions about human islets and proposes areas where additional research on human islets is needed.

Early Cytokine-Induced Transient NOX2 Activity Is ER Stress-Dependent and Impacts β-Cell Function and Survival

In type 1 diabetes (T1D) development, proinflammatory cytokines (PIC) released by immune cells lead to increased reactive oxygen species (ROS) production in β-cells. Nonetheless, the temporality of the events triggered and the role of different ROS sources remain unclear. Isolated islets from C57BL/6J wild-type (WT), NOX1 KO and NOX2 KO mice were exposed to a PIC combination. We show that cytokines increase O₂^•− production after 2 h in WT and NOX1 KO but not in NOX2 KO islets. Using transgenic mice constitutively expressing a genetically encoded compartment specific H₂O₂ sensor, we show, for the first time, a transient increase of cytosolic/nuclear H₂O₂ in islet cells between 4 and 5 h during cytokine exposure. The H₂O₂ increase coincides with the intracellular NAD(P)H decrease and is absent in NOX2 KO islets. NOX2 KO confers better glucose tolerance and protects against cytokine-induced islet secretory dysfunction and death. However, NOX2 absence does not counteract the cytokine effects in ER Ca²⁺ depletion, Store-Operated Calcium Entry (SOCE) increase and ER stress. Instead, the activation of ER stress precedes H₂O₂ production. As early NOX2-driven ROS production impacts β-cells’ function and survival during insulitis, NOX2 might be a potential target for designing therapies against early β-cell dysfunction in the context of T1D onset.

Dynamin deficiency causes insulin secretion failure and hyperglycemia

Pancreatic β cells operate with a high rate of membrane recycling for insulin secretion, yet endocytosis in these cells is not fully understood. We investigate this process in mature mouse β cells by genetically deleting dynamin GTPase, the membrane fission machinery essential for clathrin-mediated endocytosis. Unexpectedly, the mice lacking all three dynamin genes (DNM1, DNM2, DNM3) in their β cells are viable, and their β cells still contain numerous insulin granules. Endocytosis in these β cells is severely impaired, resulting in abnormal endocytic intermediates on the plasma membrane. Although insulin granules are abundant, their release upon glucose stimulation is blunted in both the first and second phases, leading to hyperglycemia and glucose intolerance in mice. Dynamin triple deletion impairs insulin granule exocytosis and decreases intracellular Ca2+ responses and granule docking. The docking defect is correlated with reduced expression of Munc13-1 and RIM1 and reorganization of cortical F-actin in β cells. Collectively, these findings uncover the role of dynamin in dense-core vesicle endocytosis and secretory capacity. Insulin secretion deficiency in the absence of dynamin-mediated endocytosis highlights the risk of impaired membrane trafficking in endocrine failure and diabetes pathogenesis.

Mitochondrial Efflux of Citrate and Isocitrate Is Fully Dispensable for Glucose-Stimulated Insulin Secretion and Pancreatic Islet β-Cell Function

The defining feature of pancreatic islet β-cell function is the precise coordination of changes in blood glucose levels with insulin secretion to regulate systemic glucose homeostasis. While ATP has long been heralded as a critical metabolic coupling factor to trigger insulin release, glucose-derived metabolites have been suggested to further amplify fuel-stimulated insulin secretion. The mitochondrial export of citrate and isocitrate through the citrate-isocitrate carrier (CIC) has been suggested to initiate a key pathway that amplifies glucose-stimulated insulin secretion, though the physiological significance of β-cell CIC-to-glucose homeostasis has not been established. Here, we generated constitutive and adult CIC β-cell knockout (KO) mice and demonstrate that these animals have normal glucose tolerance, similar responses to diet-induced obesity, and identical insulin secretion responses to various fuel secretagogues. Glucose-stimulated NADPH production was impaired in β-cell CIC KO islets, whereas glutathione reduction was retained. Furthermore, suppression of the downstream enzyme cytosolic isocitrate dehydrogenase (Idh1) inhibited insulin secretion in wild-type islets but failed to impact β-cell function in β-cell CIC KO islets. Our data demonstrate that the mitochondrial CIC is not required for glucose-stimulated insulin secretion and that additional complexities exist for the role of Idh1 and NADPH in the regulation of β-cell function.

Glucose limitation activates AMPK coupled SENP1-Sirt3 signalling in mitochondria for T cell memory development

Metabolic programming and mitochondrial dynamics along with T cell differentiation affect T cell fate and memory development; however, how to control metabolic reprogramming and mitochondrial dynamics in T cell memory development is unclear. Here, we provide evidence that the SUMO protease SENP1 promotes T cell memory development via Sirt3 deSUMOylation. SENP1-Sirt3 signalling augments the deacetylase activity of Sirt3, promoting both OXPHOS and mitochondrial fusion. Mechanistically, SENP1 activates Sirt3 deacetylase activity in T cell mitochondria, leading to reduction of the acetylation of mitochondrial metalloprotease YME1L1. Consequently, deacetylation of YME1L1 suppresses its activity on OPA1 cleavage to facilitate mitochondrial fusion, which results in T cell survival and promotes T cell memory development. We also show that the glycolytic intermediate fructose-1,6-bisphosphate (FBP) as a negative regulator suppresses AMPK-mediated activation of the SENP1-Sirt3 axis and reduces memory development. Moreover, glucose limitation reduces FBP production and activates AMPK during T cell memory development. These data show that glucose limitation activates AMPK and the subsequent SENP1-Sirt3 signalling for T cell memory development.

What Is the Metabolic Amplification of Insulin Secretion and Is It (Still) Relevant?

The pancreatic beta-cell transduces the availability of nutrients into the secretion of insulin. While this process is extensively modified by hormones and neurotransmitters, it is the availability of nutrients, above all glucose, which sets the process of insulin synthesis and secretion in motion. The central role of the mitochondria in this process was identified decades ago, but how changes in mitochondrial activity are coupled to the exocytosis of insulin granules is still incompletely understood. The identification of ATP-sensitive K⁺-channels provided the link between the level of adenine nucleotides and the electrical activity of the beta cell, but the depolarization-induced Ca²⁺-influx into the beta cells, although necessary for stimulated secretion, is not sufficient to generate the secretion pattern as produced by glucose and other nutrient secretagogues. The metabolic amplification of insulin secretion is thus the sequence of events that enables the secretory response to a nutrient secretagogue to exceed the secretory response to a purely depolarizing stimulus and is thus of prime importance. Since the cataplerotic export of mitochondrial metabolites is involved in this signaling, an orienting overview on the topic of nutrient secretagogues beyond glucose is included. Their judicious use may help to define better the nature of the signals and their mechanism of action.

Reductive TCA cycle metabolism fuels glutamine- and glucose-stimulated insulin secretion

Metabolic fuels regulate insulin secretion by generating second messengers that drive insulin granule exocytosis, but the biochemical pathways involved are incompletely understood. Here we demonstrate that stimulation of rat insulinoma cells or primary rat islets with glucose or glutamine + 2-aminobicyclo-(2,2,1)-heptane-2-carboxylic acid (Gln + BCH) induces reductive, "counter-clockwise" tricarboxylic acid (TCA) cycle flux of glutamine to citrate. Molecular or pharmacologic suppression of isocitrate dehydrogenase-2 (IDH2), which catalyzes reductive carboxylation of 2-ketoglutarate to isocitrate, results in impairment of glucose- and Gln + BCH-stimulated reductive TCA cycle flux, lowering of NADPH levels, and inhibition of insulin secretion. Pharmacologic suppression of IDH2 also inhibits insulin secretion in living mice. Reductive TCA cycle flux has been proposed as a mechanism for generation of biomass in cancer cells. Here we demonstrate that reductive TCA cycle flux also produces stimulus-secretion coupling factors that regulate insulin secretion, including in non-dividing cells.