Genetic Reduction of Glucose Metabolism Preserves Functional β-Cell Mass in KATP-Induced Neonatal Diabetes

β-Cell failure and loss of β-cell mass are key events in diabetes progression. Although insulin hypersecretion in early stages has been implicated in β-cell exhaustion/failure, loss of β-cell mass still occurs in KATP gain-of-function (GOF) mouse models of human neonatal diabetes in the absence of insulin secretion. Thus, we hypothesize that hyperglycemia-induced increased β-cell metabolism is responsible for β-cell failure and that reducing glucose metabolism will prevent loss of β-cell mass. To test this, KATP-GOF mice were crossed with mice carrying β-cell-specific glucokinase haploinsufficiency (GCK+/-), to genetically reduce glucose metabolism. As expected, both KATP-GOF and KATP-GOF/GCK+/- mice showed lack of glucose-stimulated insulin secretion. However, KATP-GOF/GCK+/- mice demonstrated markedly reduced blood glucose, delayed diabetes progression, and improved glucose tolerance compared with KATP-GOF mice. In addition, decreased plasma insulin and content, increased proinsulin, and augmented plasma glucagon observed in KATP-GOF mice were normalized to control levels in KATP-GOF/GCK+/- mice. Strikingly, KATP-GOF/GCK+/- mice demonstrated preserved β-cell mass and identity compared with the marked decrease in β-cell identity and increased dedifferentiation observed in KATP-GOF mice. Moreover KATP-GOF/GCK+/- mice demonstrated restoration of body weight and liver and brown/white adipose tissue mass and function and normalization of physical activity and metabolic efficiency compared with KATP-GOF mice. These results demonstrate that decreasing β-cell glucose signaling can prevent glucotoxicity-induced loss of insulin content and β-cell failure independently of compensatory insulin hypersecretion and β-cell exhaustion.

SGLT2 inhibitors therapy protects glucotoxicity-induced β-cell failure in a mouse model of human KATP-induced diabetes through mitigation of oxidative and ER stress

Progressive loss of pancreatic β-cell functional mass and anti-diabetic drug responsivity are classic findings in diabetes, frequently attributed to compensatory insulin hypersecretion and β-cell exhaustion. However, loss of β-cell mass and identity still occurs in mouse models of human KATP-gain-of-function induced Neonatal Diabetes Mellitus (NDM), in the absence of insulin secretion. Here we studied the temporal progression and mechanisms underlying glucotoxicity-induced loss of functional β-cell mass in NDM mice, and the effects of sodium-glucose transporter 2 inhibitors (SGLT2i) therapy. Upon tamoxifen induction of transgene expression, NDM mice rapidly developed severe diabetes followed by an unexpected loss of insulin content, decreased proinsulin processing and increased proinsulin at 2-weeks of diabetes. These early events were accompanied by a marked increase in β-cell oxidative and ER stress, without changes in islet cell identity. Strikingly, treatment with the SGLT2 inhibitor dapagliflozin restored insulin content, decreased proinsulin:insulin ratio and reduced oxidative and ER stress. However, despite reduction of blood glucose, dapagliflozin therapy was ineffective in restoring β-cell function in NDM mice when it was initiated at >40 days of diabetes, when loss of β-cell mass and identity had already occurred. Our data from mouse models demonstrate that: i) hyperglycemia per se, and not insulin hypersecretion, drives β-cell failure in diabetes, ii) recovery of β-cell function by SGLT2 inhibitors is potentially through reduction of oxidative and ER stress, iii) SGLT2 inhibitors revert/prevent β-cell failure when used in early stages of diabetes, but not when loss of β-cell mass/identity already occurred, iv) common execution pathways may underlie loss and recovery of β-cell function in different forms of diabetes. These results may have important clinical implications for optimal therapeutic interventions in individuals with diabetes, particularly for those with long-standing diabetes.

Drug combination therapy for emerging viral diseases

Effective therapeutics to combat emerging viral infections are an unmet need. Historically, treatments for chronic viral infections with single drugs have not been successful, as exemplified by human immunodeficiency virus (HIV) and hepatitis C virus (HCV) infections. Combination therapy for these diseases has led to improved clinical outcomes with dramatic reductions in viral load, morbidity, and mortality. Drug combinations can enhance therapeutic efficacy through additive, and ideally synergistic, effects for emerging and re-emerging viruses, such as influenza, severe acute respiratory syndrome-coronavirus (SARS-CoV), Middle East respiratory syndrome (MERS)-CoV, Ebola, Zika, and SARS-coronavirus 2 (CoV-2). Although novel drug development through traditional pipelines remains a priority, in the interim, effective synergistic drug candidates could be rapidly identified by drug-repurposing screens, facilitating accelerated paths to clinical testing and potential emergency use authorizations.

Drug Discovery Strategies for SARS-CoV-2

Coronavirus disease 2019 (COVID-19) is a novel disease caused by the severe acute respiratory syndrome coronavirus (SARS-CoV)-2 virus that was first detected in December of 2019 in Wuhan, China, and has rapidly spread worldwide. The search for a suitable vaccine as well as effective therapeutics for the treatment of COVID-19 is underway. Drug repurposing screens provide a useful and effective solution for identifying potential therapeutics against SARS-CoV-2. For example, the experimental drug remdesivir, originally developed for Ebola virus infections, has been approved by the US Food and Drug Administration as an emergency use treatment of COVID-19. However, the efficacy and toxicity of this drug need further improvements. In this review, we discuss recent findings on the pathology of coronaviruses and the drug targets for the treatment of COVID-19. Both SARS-CoV-2–specific inhibitors and broad-spectrum anticoronavirus drugs against SARS-CoV, Middle East respiratory syndrome coronavirus, and SARS-CoV-2 will be valuable additions to the anti–SARS-CoV-2 armament. A multitarget treatment approach with synergistic drug combinations containing different mechanisms of action may be a practical therapeutic strategy for the treatment of severe COVID-19.

Alterations in pancreatic islet cell function in response to small bowel resection

After 50% proximal small bowel resection (SBR) in mice, we have demonstrated hepatic steatosis, impaired glucose metabolism without insulin resistance, and increased pancreatic islet area. We sought to determine the consequences of SBR on pancreatic β-cell morphology, proliferation, and expression of a key regulatory hormone, glucagon-like peptide-1 (GLP-1). C57BL/6 mice underwent 50% SBR or sham operation. At 10 wk, pancreatic insulin content and secretion was measured by ELISA. Immunohistochemistry was performed to determine structural alterations in pancreatic α-and β-cells. Western blot analysis was used to measure GLP-1R expression, and immunoassay was used to measure plasma insulin and GLP-1. Experiments were repeated by administering a GLP-1 agonist (exendin-4) to a cohort of mice following SBR. After SBR, there was pancreatic islet hypertrophy and impaired glucose tolerance. The proportion of α and β cells was not grossly altered. Whole pancreas and pancreatic islet insulin content was not significantly different; however, SBR mice demonstrated decreased insulin secretion in both static incubation and islet perfusion experiments. The expression of pancreatic GLP-1R was decreased approximately twofold after SBR, compared with sham and serum GLP-1, was decreased. These metabolic derangements were mitigated after administration of the GLP-1 agonist. Following massive SBR, there is significant hypertrophy of pancreatic islet cells with morphologically intact α- and β-cells. Significantly reduced pancreatic insulin release in both static and dynamic conditions demonstrate a perturbed second phase of insulin secretion. GLP-1 is a key mediator of this amplification pathway. Decreased expression of serum GLP-1 and pancreatic GLP-1R in face of no change in insulin content presents a novel pathway for enteropancreatic glucose regulation following SBR.NEW & NOTEWORTHY Metabolic changes occur following intestinal resection; however, the effects on pancreatic function are unknown. Prior studies have demonstrated that glucagon-like protein-1 (GLP-1) signaling is a crucial player in the improved insulin sensitivity after bariatric surgery. In this study, we explore the effect of massive small bowel resection on gut hormone physiology and provide novel insights into the enteropancreatic axis.

Beta-cell excitability and excitability-driven diabetes in adult Zebrafish islets

Islet β-cell membrane excitability is a well-established regulator of mammalian insulin secretion, and defects in β-cell excitability are linked to multiple forms of diabetes. Evolutionary conservation of islet excitability in lower organisms is largely unexplored. Here we show that adult zebrafish islet calcium levels rise in response to elevated extracellular [glucose], with similar concentration-response relationship to mammalian β-cells. However, zebrafish islet calcium transients are nor well coupled, with a shallower glucose-dependence of cytoplasmic calcium concentration. We have also generated transgenic zebrafish that conditionally express gain-of-function mutations in ATP-sensitive K+ channels (KATP -GOF) in β-cells. Following induction, these fish become profoundly diabetic, paralleling features of mammalian diabetes resulting from equivalent mutations. KATP -GOF fish become severely hyperglycemic, with slowed growth, and their islets lose glucose-induced calcium responses. These results indicate that, although lacking tight cell-cell coupling of intracellular Ca2+ , adult zebrafish islets recapitulate similar excitability-driven β-cell glucose responsiveness to those in mammals, and exhibit profound susceptibility to diabetes as a result of inexcitability. While illustrating evolutionary conservation of islet excitability in lower vertebrates, these results also provide important validation of zebrafish as a suitable animal model in which to identify modulators of islet excitability and diabetes.

The role of membrane excitability in pancreatic β-cell glucotoxicity

Persistent hyperglycemia is causally associated with pancreatic β-cell dysfunction and loss of pancreatic insulin. Glucose normally enhances β-cell excitability through inhibition of K_ATP channels, opening of voltage-dependent calcium channels, increased [Ca²⁺]_i, which triggers insulin secretion. Glucose-dependent excitability is lost in islets from K_ATP-knockout (K_ATP-KO) mice, in which β-cells are permanently hyperexcited, [Ca²⁺]_i, is chronically elevated and insulin is constantly secreted. Mouse models of human neonatal diabetes in which K_ATP gain-of-function mutations are expressed in β-cells (K_ATP-GOF) also lose the link between glucose metabolism and excitation-induced insulin secretion, but in this case K_ATP-GOF β-cells are chronically underexcited, with permanently low [Ca²⁺]_i and lack of glucose-dependent insulin secretion. We used K_ATP-GOF and K_ATP-KO islets to examine the role of altered-excitability in glucotoxicity. Wild-type islets showed rapid loss of insulin content when chronically incubated in high-glucose, an effect that was reversed by subsequently switching to low glucose media. In contrast, hyperexcitable K_ATP-KO islets lost insulin content in both low- and high-glucose, while underexcitable K_ATP-GOF islets maintained insulin content in both conditions. Loss of insulin content in chronic excitability was replicated by pharmacological inhibition of K_ATP by glibenclamide, The effects of hyperexcitable and underexcitable islets on glucotoxicity observed in in vivo animal models are directly opposite to the effects observed in vitro: we clearly demonstrate here that in vitro, hyperexcitability is detrimental to islets whereas underexcitability is protective.

High-fat-diet-induced remission of diabetes in a subset of KATP -GOF insulin-secretory-deficient mice

AIMS

To examine the effects of a high-fat-diet (HFD) on monogenic neonatal diabetes, without the confounding effects of compensatory hyperinsulinaemia.

METHODS

Mice expressing K_ATP channel gain-of-function (K_ATP -GOF) mutations, which models human neonatal diabetes, were fed an HFD.

RESULTS

Surprisingly, K_ATP -GOF mice exhibited resistance to HFD-induced obesity, accompanied by markedly divergent blood glucose control, with some K_ATP -GOF mice showing persistent diabetes (K_ATP -GOF-non-remitter [NR] mice) and others showing remission of diabetes (K_ATP -GOF-remitter [R] mice). Compared with the severely diabetic and insulin-resistant K_ATP -GOF-NR mice, HFD-fed K_ATP -GOF-R mice had lower blood glucose, improved insulin sensitivity, and increased circulating plasma insulin and glucagon-like peptide-1 concentrations. Strikingly, while HFD-fed K_ATP -GOF-NR mice showed increased food intake and decreased physical activity, reduced whole body fat mass and increased plasma lipids, K_ATP -GOF-R mice showed similar features to those of control littermates. Importantly, K_ATP -GOF-R mice had restored insulin content and β-cell mass compared with the marked loss observed in both HFD-fed K_ATP -GOF-NR and chow-fed K_ATP -GOF mice.

CONCLUSION

Together, our results suggest that restriction of dietary carbohydrates and caloric replacement by fat can induce metabolic changes that are beneficial in reducing glucotoxicity and secondary consequences of diabetes in a mouse model of insulin-secretory deficiency.