The glucose sensor protein glucokinase is expressed in glucagon-producing alpha-cells

Sugar-sensing swodkoreceptors and swodkocrine signaling

Sugars are one of the major metabolites and are essential for nucleic acid synthesis and energy production. In addition, sugars can act as signaling molecules. To study sugar signaling at the systemic level, there is an urgent need to systematically identify sugar-sensing proteins and nucleic acids. I propose the terms "swodkoreceptor" and "swodkocrine signaling," derived from the Polish word "słodki" meaning "sweet," to comprise all sugar-sensing proteins and signaling events, respectively, regardless of their cellular location and signaling domains. This proposal is intended to facilitate the inclusion of proteins such as the Escherichia coli LacI repressor as an allolactose receptor, human glucokinase regulatory protein (GCKR) as a fructose receptor, and other sugar-binding based allosterically regulated enzymes and transcription factors as sugar-sensing receptors. In addition, enzyme-interacting proteins whose interaction state is regulated by sugar binding have also been proposed as sugar receptors. The systemic study of protein- and nucleic-acid-based swodkoreceptors may help to identify organelle-specific swodkoreceptors and to also address receptor duality. The study of intra- and inter-organism swodkocrine signaling and its crosstalk with gasocrine signaling may help to understand the etiology of diseases due to dysregulation in sugar homeostasis and signaling.

BAD-Glucokinase Axis Regulates Platelet Activation and Thrombosis

BACKGROUND

BAD (Bcl2-associated death promoter), a member of the Bcl2 proapoptotic family, promotes cell apoptosis by sequestering the prosurvival proteins Bcl-XL and Bcl2 from the proapoptotic proteins BAK (Bcl2 homologous antagonist/killer) and BAX (Bcl2-associated X protein) in nucleated cells. BAD is also expressed in platelets, playing a role in regulating platelet lifespan, apoptosis, and clearance. However, whether BAD regulates platelet activation and arterial thrombosis remains unclear.

METHODS

The role of BAD in platelet activation and arterial thrombosis was investigated using BAD-deficient mice (Bad-/-), in vitro functional studies, and arterial thrombosis models. The regulatory effect of BAD on platelet energy metabolism was detected using a Seahorse Extracellular Flux Analyzer. The regulatory effect of BAD on glucokinase was investigated by coimmunoprecipitation and activity measurement. The glucokinase heterozygous knockout mice (Gck+/-) and activator were used to study its role in platelet activation.

RESULTS

BAD-deficient mice (Bad-/-) and wild-type mice transfused with Bad-/- platelets displayed prolonged tail bleeding and arterial occlusion times. Bad-/- platelets exhibited decreased aggregation in response to stimulations by proteinase-activated receptor 4-activating peptide, thrombin, and U46619. Furthermore, BAD ablation suppressed platelet integrin αIIbβ3 activation, granule secretion, and clot retraction induced by these agonists. Mechanistically, BAD interacted with glucokinase, and BAD deficiency resulted in decreased platelet glucokinase activity, mitochondrial oxidative phosphorylation, and mitochondrial ATP production. The partial loss of glucokinase (Gck+/-) phenocopied platelet function defects caused by BAD deficiency, and a glucokinase activator rescued the impaired mitochondrial ATP production and function of Bad-/- platelets. Additionally, the glucokinase activator enhanced human platelet activation.

CONCLUSIONS

Our findings demonstrate the critical role of the BAD-glucokinase axis in platelet activation and thrombosis, suggesting a potential target for antithrombotic therapy.

Unraveling the impaired incretin effect in obesity and type 2 diabetes: Key role of hyperglycemia-induced unscheduled glycolysis and glycolytic overload

Glucagon-like peptide-1 (GLP-1) agonists and GLP-1 and glucose-dependent insulinotropic polypeptide (GIP) co-agonists are major treatment options for subjects with obesity and patients with type 2 diabetes mellitus (T2DM). They counter without addressing the mechanistic cause of the impaired incretin effect associated with obesity and T2DM. Incretin effect impairment is characterized by decreased secretion of incretins from enteroendocrine cells and incretin resistance of pancreatic β-cells. It is linked to hyperglycemia. We present evidence that subversion of the gating of glucose entry into glycolysis, mainly by glucokinase (hexokinase-4), during persistent hyperglycemia in enteroendocrine cells, pancreatic β- and α-cells and appetite-regulating neurons contributes to the biochemical mechanism of the impaired incretin effect. Unscheduled glycolysis and glycolytic overload thereby produced decreases cell signalling of incretin secretion to glucose and other secretion stimuli and incretin receptor responses. This mechanism provides a guide for development of alternative therapies targeting recovery of the impaired incretin effect.

Glucokinase (GCK) in diabetes: from molecular mechanisms to disease pathogenesis

Glucokinase (GCK), a key enzyme in glucose metabolism, plays a central role in glucose sensing and insulin secretion in pancreatic β-cells, as well as glycogen synthesis in the liver. Mutations in the GCK gene have been associated with various monogenic diabetes (MD) disorders, including permanent neonatal diabetes mellitus (PNDM) and maturity-onset diabetes of the young (MODY), highlighting its importance in maintaining glucose homeostasis. Additionally, GCK gain-of-function mutations lead to a rare congenital form of hyperinsulinism known as hyperinsulinemic hypoglycemia (HH), characterized by increased enzymatic activity and increased glucose sensitivity in pancreatic β-cells. This review offers a comprehensive exploration of the critical role played by the GCK gene in diabetes development, shedding light on its expression patterns, regulatory mechanisms, and diverse forms of associated monogenic disorders. Structural and mechanistic insights into GCK's involvement in glucose metabolism are discussed, emphasizing its significance in insulin secretion and glycogen synthesis. Animal models have provided valuable insights into the physiological consequences of GCK mutations, although challenges remain in accurately recapitulating human disease phenotypes. In addition, the potential of human pluripotent stem cell (hPSC) technology in overcoming current model limitations is discussed, offering a promising avenue for studying GCK-related diseases at the molecular level. Ultimately, a deeper understanding of GCK's multifaceted role in glucose metabolism and its dysregulation in disease states holds implications for developing targeted therapeutic interventions for diabetes and related disorders.

β-cell neogenesis: A rising star to rescue diabetes mellitus

BACKGROUND

Diabetes Mellitus (DM), a chronic metabolic disease characterized by elevated blood glucose, is caused by various degrees of insulin resistance and dysfunctional insulin secretion, resulting in hyperglycemia. The loss and failure of functional β-cells are key mechanisms resulting in type 1 diabetes mellitus (T1DM) and type 2 diabetes mellitus (T2DM).

AIM OF REVIEW

Elucidating the underlying mechanisms of β-cell failure, and exploring approaches for β-cell neogenesis to reverse β-cell dysfunction may provide novel strategies for DM therapy.

KEY SCIENTIFIC CONCEPTS OF REVIEW

Emerging studies reveal that genetic susceptibility, endoplasmic reticulum (ER) stress, oxidative stress, islet inflammation, and protein modification linked to multiple signaling pathways contribute to DM pathogenesis. Over the past few years, replenishing functional β-cell by β-cell neogenesis to restore the number and function of pancreatic β-cells has remarkably exhibited a promising therapeutic approach for DM therapy. In this review, we provide a comprehensive overview of the underlying mechanisms of β-cell failure in DM, highlight the effective approaches for β-cell neogenesis, as well as discuss the current clinical and preclinical agents research advances of β-cell neogenesis. Insights into the challenges of translating β-cell neogenesis into clinical application for DM treatment are also offered.

Hexokinase-linked glycolytic overload and unscheduled glycolysis in hyperglycemia-induced pathogenesis of insulin resistance, beta-cell glucotoxicity, and diabetic vascular complications

Hyperglycemia is a risk factor for the development of insulin resistance, beta-cell glucotoxicity, and vascular complications of diabetes. We propose the hypothesis, hexokinase-linked glycolytic overload and unscheduled glycolysis, in explanation. Hexokinases (HKs) catalyze the first step of glucose metabolism. Increased flux of glucose metabolism through glycolysis gated by HKs, when occurring without concomitant increased activity of glycolytic enzymes-unscheduled glycolysis-produces increased levels of glycolytic intermediates with overspill into effector pathways of cell dysfunction and pathogenesis. HK1 is saturated with glucose in euglycemia and, where it is the major HK, provides for basal glycolytic flux without glycolytic overload. HK2 has similar saturation characteristics, except that, in persistent hyperglycemia, it is stabilized to proteolysis by high intracellular glucose concentration, increasing HK activity and initiating glycolytic overload and unscheduled glycolysis. This drives the development of vascular complications of diabetes. Similar HK2-linked unscheduled glycolysis in skeletal muscle and adipose tissue in impaired fasting glucose drives the development of peripheral insulin resistance. Glucokinase (GCK or HK4)-linked glycolytic overload and unscheduled glycolysis occurs in persistent hyperglycemia in hepatocytes and beta-cells, contributing to hepatic insulin resistance and beta-cell glucotoxicity, leading to the development of type 2 diabetes. Downstream effector pathways of HK-linked unscheduled glycolysis are mitochondrial dysfunction and increased reactive oxygen species (ROS) formation; activation of hexosamine, protein kinase c, and dicarbonyl stress pathways; and increased Mlx/Mondo A signaling. Mitochondrial dysfunction and increased ROS was proposed as the initiator of metabolic dysfunction in hyperglycemia, but it is rather one of the multiple downstream effector pathways. Correction of HK2 dysregulation is proposed as a novel therapeutic target. Pharmacotherapy addressing it corrected insulin resistance in overweight and obese subjects in clinical trial. Overall, the damaging effects of hyperglycemia are a consequence of HK-gated increased flux of glucose metabolism without increased glycolytic enzyme activities to accommodate it.

Hexokinases in cancer and other pathologies

Glucose metabolism is indispensable for cell growth and survival. Hexokinases play pivotal roles in glucose metabolism through canonical functions of hexokinases as well as in immune response, cell stemness, autophagy, and other cellular activities through noncanonical functions. The aberrant regulation of hexokinases contributes to the development and progression of pathologies, including cancer and immune diseases.

Glucokinase activity in diabetes: too much of a good thing?

Type 2 diabetes (T2D) is a global health problem characterised by chronic hyperglycaemia due to inadequate insulin secretion. Because glucose must be metabolised to stimulate insulin release it was initially argued that drugs that stimulate glucokinase (the first enzyme in glucose metabolism) would enhance insulin secretion in diabetes. However, in the long term, glucokinase activators have been largely disappointing. Recent studies show it is hyperactivation of glucose metabolism, not glucose itself, that underlies the progressive decline in beta-cell function in diabetes. This perspective discusses if glucokinase activators exacerbate this decline (by promoting glucose metabolism) and, counterintuitively, if glucokinase inhibitors might be a better therapeutic strategy for preserving beta-cell function in T2D.

The past, present, and future physiology and pharmacology of glucagon

The evolution of glucagon has seen the transition from an impurity in the preparation of insulin to the development of glucagon receptor agonists for use in type 1 diabetes. In type 2 diabetes, glucagon receptor antagonists have been explored to reduce glycemia thought to be induced by hyperglucagonemia. However, the catabolic actions of glucagon are currently being leveraged to target the rise in obesity that paralleled that of diabetes, bringing the pharmacology of glucagon full circle. During this evolution, the physiological importance of glucagon advanced beyond the control of hepatic glucose production, incorporating critical roles for glucagon to regulate both lipid and amino acid metabolism. Thus, it is unsurprising that the study of glucagon has left several paradoxes that make it difficult to distill this hormone down to a simplified action. Here, we describe the history of glucagon from the past to the present and suggest some direction to the future of this field.

Dorzagliatin in drug-naïve patients with type 2 diabetes: a randomized, double-blind, placebo-controlled phase 3 trial

Improving glucose sensitivity remains an unmet medical need in treating type 2 diabetes (T2D). Dorzagliatin is a dual-acting, orally bioavailable glucokinase activator that enhances glucokinase activity in a glucose-dependent manner, improves glucose-stimulated insulin secretion and demonstrates effects on glycemic control in patients with T2D. We report the findings of a randomized, double-blind, placebo-controlled phase 3 clinical trial to evaluate the efficacy and safety of dorzagliatin in patients with T2D. Eligible drug-naïve patients with T2D (n = 463) were randomly assigned to the dorzagliatin or placebo group at a ratio of 2:1 for 24 weeks of double-blind treatment, followed by 28 weeks of open-label treatment with dorzagliatin for all patients. The primary efficacy endpoint was the change in glycated hemoglobin from baseline to week 24. Safety was assessed throughout the trial. At week 24, the least-squares mean change in glycated hemoglobin from baseline (95% confidence interval) was -1.07% (-1.19%, -0.95%) in the dorzagliatin group and -0.50% (-0.68%, -0.32%) in the placebo group (estimated treatment difference, -0.57%; 95% confidence interval: -0.79%, -0.36%; P < 0.001). The incidence of adverse events was similar between the two groups. There were no severe hypoglycemia events or drug-related serious adverse events in the dorzagliatin group. In summary, dorzagliatin improved glycemic control in drug-naïve patients with T2D and showed a good tolerability and safety profile.

Modeling the Amino Acid Effect on Glucagon Secretion from Pancreatic Alpha Cells

Type 2 Diabetes Mellitus (T2DM) is a burdensome problem in modern society, and intensive research is focused on better understanding the underlying cellular mechanisms of hormone secretion for blood glucose regulation. T2DM is a bi-hormonal disease, and in addition to 100 years of increasing knowledge about the importance of insulin, the second hormone glucagon, secreted by pancreatic alpha cells, is becoming increasingly important. We have developed a mathematical model for glucagon secretion that incorporates all major metabolic processes of glucose, fatty acids, and glutamine as the most abundant postprandial amino acid in blood. In addition, we consider cAMP signaling in alpha cells. The model predictions quantitatively estimate the relative importance of specific metabolic and signaling pathways and particularly emphasize the important role of glutamine in promoting glucagon secretion, which is in good agreement with known experimental data.

Identification and management of GCK-MODY complicating pregnancy in Chinese patients with gestational diabetes

Precise differentiation of glucokinase (GCK) monogenic diabetes from gestational diabetes mellitus (GDM) is critical for accurate management of the pregnancy outcome. We screened GCK-MODY complicating pregnancies in Chinese GDM patients, explored the pathogenesis of novel GCK mutations, and evaluated the patients' pregnancy outcome and management. The GCK gene from 411 GDM patients was screened with PCR-direct sequencing and multiplex ligation-dependent probe amplification (MLPA) and 15 GCK mutations were identified. We also retrospectively analyzed a total of 65 pregnancies from 21 GCK-MODY families, wherein 41 were from 15 maternal families and 24 were from six paternal families. Bioinformatic analysis and biochemical functional study were conducted to identify novel GCK mutations. In total, we identified 21 GCK mutations: 15 from the 411 GDM patients and six from 24 fathers. Of th Asp78Asn (GAC → AAC), Met87Arg (ATG → AGG), Leu451Val (CTT → GTT), Leu451Pro (CTG → CCG) and 1019 + 20G > A e mutations, five, i.e., were novel and deleterious, with markedly decreased enzyme activity and thermal stability. The unaffected offspring of GCK mutation-affected mothers were heavier than affected offspring (p < 0.001). Of 21 insulin-treated affected mothers, 10 had maternal hypoglycemia (47.6%) and seven had perinatal complications (33.3%), and the affected offspring of the insulin-treated affected mothers had significantly lower birth weights than that of the 20 diet-control affected mothers (p = 0.031). In this study, the prevalence of GCK-MODY complicating pregnancy in Chinese GDM patients was 3.6% (15/411). The defective GCK may contribute to the hyperglycemia in GCK-MODY. Insulin therapy is not beneficial for GCK-MODY complicating pregnancy and therefore should not be recommended.

Do sodium-glucose co-transporter-2 inhibitors increase plasma glucagon by direct actions on the alpha cell? And does the increase matter for the associated increase in endogenous glucose production?

Sodium-glucose co-transporter-2 inhibitors (SGLT2is) lower blood glucose and are used for treatment of type 2 diabetes. However, SGLT2is have been associated with increases in endogenous glucose production (EGP) by mechanisms that have been proposed to result from SGLT2i-mediated increases in circulating glucagon concentrations, but the relative importance of this effect is debated, and mechanisms possibly coupling SGLT2is to increased plasma glucagon are unclear. A direct effect on alpha-cell activity has been proposed, but data on alpha-cell SGLT2 expression are inconsistent, and studies investigating the direct effects of SGLT2 inhibition on glucagon secretion are conflicting. By contrast, alpha-cell sodium-glucose co-transporter-1 (SGLT1) expression has been found more consistently and appears to be more prominent, pointing to an underappreciated role for this transporter. Nevertheless, the selectivity of most SGLT2is does not support interference with SGLT1 during therapy. Paracrine effects mediated by secretion of glucagonotropic/static molecules from beta and/or delta cells have also been suggested to be involved in SGLT2i-induced increase in plasma glucagon, but studies are few and arrive at different conclusions. It is also possible that the effect on glucagon is secondary to drug-induced increases in urinary glucose excretion and lowering of blood glucose, as shown in experiments with glucose clamping where SGLT2i-associated increases in plasma glucagon are prevented. However, regardless of the mechanisms involved, the current balance of evidence does not support that SGLT2 plays a crucial role for alpha-cell physiology or that SGLT2i-induced glucagon secretion is important for the associated increased EGP, particularly because the increase in EGP occurs before any rise in plasma glucagon.

Mini-review: Glucagon responses in type 1 diabetes - a matter of complexity

In recent years the role of altered alpha cell function and glucagon secretion in type 1 diabetes has attracted scientific attention. It is well established that glucagon responses to hypoglycemia are absent in type 1 diabetes, but more uncertain whether it is intact following other physiological and metabolic stimuli compared with nondiabetic individuals. The aim of this review is to (i) summarize current knowledge on glucagon responses during hypoglycemia in normal physiology and type 1 diabetes, and (ii) review human in vivo studies investigating glucagon responses after other stimuli in individuals with type 1 diabetes and nondiabetic individuals. Available data suggest that in type 1 diabetes the absence of glucagon secretion after hypoglycemia is irreversible. This is a scenario specific to hypoglycemia, since other stimuli, including administration of amino acids, insulin withdrawal, lipopolysaccharide exposure and exercise lead to substantial glucagon responses though attenuated compared to nondiabetic individuals in head-to-head studies. The derailed glucagon secretion is not confined to hypoglycemia as individuals with type 1 diabetes, as opposed to nondiabetic individuals display glucagon hypersecretion after meals, thereby potentially contributing to insulin resistance. The complexity of these phenomena may relate to activation of distinct regulatory pathways controlling glucagon secretion i.e., intra-islet paracrine signaling, direct and autonomic nervous signaling.

Regulation of α-cell glucagon secretion: The role of second messengers

Glucagon is a potent glucose‐elevating hormone that is secreted by pancreatic α‐cells. While well‐controlled glucagon secretion plays an important role in maintaining systemic glucose homeostasis and preventing hypoglycaemia, it is increasingly apparent that defects in the regulation of glucagon secretion contribute to impaired counter‐regulation and hyperglycaemia in diabetes. It has therefore been proposed that pharmacological interventions targeting glucagon secretion/signalling can have great potential in improving glycaemic control of patients with diabetes. However, despite decades of research, a consensus on the precise mechanisms of glucose regulation of glucagon secretion is yet to be reached. Second messengers are a group of small intracellular molecules that relay extracellular signals to the intracellular signalling cascade, modulating cellular functions. There is a growing body of evidence that second messengers, such as cAMP and Ca²⁺, play critical roles in α‐cell glucose‐sensing and glucagon secretion. In this review, we discuss the impact of second messengers on α‐cell electrical activity, intracellular Ca²⁺ dynamics and cell exocytosis. We highlight the possibility that the interaction between different second messengers may play a key role in the glucose‐regulation of glucagon secretion.

Intercellular Communication in the Islet of Langerhans in Health and Disease

Blood glucose homeostasis requires proper function of pancreatic islets, which secrete insulin, glucagon, and somatostatin from the β-, α-, and δ-cells, respectively. Each islet cell type is equipped with intrinsic mechanisms for glucose sensing and secretory actions, but these intrinsic mechanisms alone cannot explain the observed secretory profiles from intact islets. Regulation of secretion involves interconnected mechanisms among and between islet cell types. Islet cells lose their normal functional signatures and secretory behaviors upon dispersal as compared to intact islets and in vivo. In dispersed islet cells, the glucose response of insulin secretion is attenuated from that seen from whole islets, coordinated oscillations in membrane potential and intracellular Ca2+ activity, as well as the two-phase insulin secretion profile, are missing, and glucagon secretion displays higher basal secretion profile and a reverse glucose-dependent response from that of intact islets. These observations highlight the critical roles of intercellular communication within the pancreatic islet, and how these communication pathways are crucial for proper hormonal and nonhormonal secretion and glucose homeostasis. Further, misregulated secretions of islet secretory products that arise from defective intercellular islet communication are implicated in diabetes. Intercellular communication within the islet environment comprises multiple mechanisms, including electrical synapses from gap junctional coupling, paracrine interactions among neighboring cells, and direct cell-to-cell contacts in the form of juxtacrine signaling. In this article, we describe the various mechanisms that contribute to proper islet function for each islet cell type and how intercellular islet communications are coordinated among the same and different islet cell types. © 2021 American Physiological Society. Compr Physiol 11:2191-2225, 2021.

Role of cAMP in Double Switch of Glucagon Secretion

Glucose metabolism plays a crucial role in modulating glucagon secretion in pancreatic alpha cells. However, the downstream effects of glucose metabolism and the activated signaling pathways influencing glucagon granule exocytosis are still obscure. We developed a computational alpha cell model, implementing metabolic pathways of glucose and free fatty acids (FFA) catabolism and an intrinsically activated cAMP signaling pathway. According to the model predictions, increased catabolic activity is able to suppress the cAMP signaling pathway, reducing exocytosis in a Ca²⁺-dependent and Ca²⁺ independent manner. The effect is synergistic to the pathway involving ATP-dependent closure of K_ATP channels and consequent reduction of Ca²⁺. We analyze the contribution of each pathway to glucagon secretion and show that both play decisive roles, providing a kind of "secure double switch". The cAMP-driven signaling switch plays a dominant role, while the ATP-driven metabolic switch is less favored. The ratio is approximately 60:40, according to the most recent experimental evidence.

Mechanisms controlling pancreatic islet cell function in insulin secretion

Metabolic homeostasis in mammals is tightly regulated by the complementary actions of insulin and glucagon. The secretion of these hormones from pancreatic β-cells and α-cells, respectively, is controlled by metabolic, endocrine, and paracrine regulatory mechanisms and is essential for the control of blood levels of glucose. The deregulation of these mechanisms leads to various pathologies, most notably type 2 diabetes, which is driven by the combined lesions of impaired insulin action and a loss of the normal insulin secretion response to glucose. Glucose stimulates insulin secretion from β-cells in a bi-modal fashion, and new insights about the underlying mechanisms, particularly relating to the second or amplifying phase of this secretory response, have been recently gained. Other recent work highlights the importance of α-cell-produced proglucagon-derived peptides, incretin hormones from the gastrointestinal tract and other dietary components, including certain amino acids and fatty acids, in priming and potentiation of the β-cell glucose response. These advances provide a new perspective for the understanding of the β-cell failure that triggers type 2 diabetes.

Therapeutic potential of mesenchymal stem cells in treating both types of diabetes mellitus and associated diseases

Diabetes mellitus is a common lifestyle disease which can be classified into type 1 diabetes mellitus and type 2 diabetes mellitus. While both result in hyperglycemia due to lack of insulin action and further associated chronic ailments, there is a marked distinction in the cause for each type due to which both require a different prophylaxis. As observed, type 1 diabetes is caused due to the autoimmune action of the body resulting in the destruction of pancreatic islet cells. On the other hand, type 2 diabetes is caused either due to insulin resistance of target cells or lack of insulin production as per physiological requirements. Attempts to cure the disease have been made by bringing drastic changes in the patients' lifestyle; parenteral administration of insulin; prescription of drugs such as biguanides, meglitinides, and amylin; pancreatic transplantation; and immunotherapy. While these attempts cause a certain degree of relief to the patient, none of these can cure diabetes mellitus. However, a new treatment strategy led by the discovery of mesenchymal stem cells and their unique immunomodulatory and multipotent properties has inspired therapies to treat diabetes by essentially reversing the conditions causing the disease. The current review aims to enumerate the role of various mesenchymal stem cells and the different approaches to treat both types of diabetes and its associated diseases as well.

Glucokinase intrinsically regulates glucose sensing and glucagon secretion in pancreatic alpha cells

The secretion of glucagon by pancreatic alpha cells is regulated by a number of external and intrinsic factors. While the electrophysiological processes linking a lowering of glucose concentrations to an increased glucagon release are well characterized, the evidence for the identity and function of the glucose sensor is still incomplete. In the present study we aimed to address two unsolved problems: (1) do individual alpha cells have the intrinsic capability to regulate glucagon secretion by glucose, and (2) is glucokinase the alpha cell glucose sensor in this scenario. Single cell RT-PCR was used to confirm that glucokinase is the main glucose-phosphorylating enzyme expressed in rat pancreatic alpha cells. Modulation of glucokinase activity by pharmacological activators and inhibitors led to a lowering or an increase of the glucose threshold of glucagon release from single alpha cells, measured by TIRF microscopy, respectively. Knockdown of glucokinase expression resulted in a loss of glucose control of glucagon secretion. Taken together this study provides evidence for a crucial role of glucokinase in intrinsic glucose regulation of glucagon release in rat alpha cells.

Modelling of energy-driven switch for glucagon and insulin secretion

We present a mathematical model of the energy-driven metabolic switch for glucagon and insulin secretion from pancreatic alpha and beta cells, respectively. The energy status related to hormone secretion is studied for various glucose concentrations. Additionally, the physiological response is studied with regards to the presence of other metabolites, particularly the free-fatty acids. At low glucose, the ATP production in alpha cells is high due to free-fatty acids oxidation in mitochondria, which enables glucagon secretion. When the glucose concentration is elevated above the threshold value, the glucagon secretion is switched off due to the contribution of glycolytic ATP production, representing an "anaerobic switch". On the other hand, during hypoglycemia, the ATP production in beta cells is low, reflecting a "waiting state" for glucose as the main metabolite. When glucose is elevated above the threshold value, the oxidative fate of glucose in mitochondria is the main source of energy required for effective insulin secretion, i.e. the "aerobic switch". Our results show the importance of well-regulated and fine-tuned energetic processes in pancreatic alpha and beta cells required for efficient hormone secretion and hence effective blood glucose regulation. These energetic processes have to be appropriately switched on and off based on the sensing of different metabolites by alpha and beta cells. Our computational results indicate that disturbances in cell energetics (e.g. mitochondrial dysfunction), and dysfunctional metabolite sensing and distribution throughout the cell might be related to pathologies such as metabolic syndrome and diabetes.

Modelling of dysregulated glucagon secretion in type 2 diabetes by considering mitochondrial alterations in pancreatic α-cells

Type 2 diabetes mellitus (T2DM) has been associated with insulin resistance and the failure of β-cells to produce and secrete enough insulin as the disease progresses. However, clinical treatments based solely on insulin secretion and action have had limited success. The focus is therefore shifting towards α-cells, in particular to the dysregulated secretion of glucagon. Our qualitative electron-microscopy-based observations gave an indication that mitochondria in α-cells are altered in Western-diet-induced T2DM. In particular, α-cells extracted from mouse pancreatic tissue showed a lower density of mitochondria, a less expressed matrix and a lower number of cristae. These deformities in mitochondrial ultrastructure imply a decreased efficiency in mitochondrial ATP production, which prompted us to theoretically explore and clarify one of the most challenging problems associated with T2DM, namely the lack of glucagon secretion in hypoglycaemia and its oversecretion at high blood glucose concentrations. To this purpose, we constructed a novel computational model that links α-cell metabolism with their electrical activity and glucagon secretion. Our results show that defective mitochondrial metabolism in α-cells can account for dysregulated glucagon secretion in T2DM, thus improving our understanding of T2DM pathophysiology and indicating possibilities for new clinical treatments.

Modelling of dysregulated glucagon secretion in type 2 diabetes by considering mitochondrial alterations in pancreatic α-cells

Type 2 diabetes mellitus (T2DM) has been associated with insulin resistance and the failure of β-cells to produce and secrete enough insulin as the disease progresses. However, clinical treatments based solely on insulin secretion and action have had limited success. The focus is therefore shifting towards α-cells, in particular to the dysregulated secretion of glucagon. Our qualitative electron-microscopy-based observations gave an indication that mitochondria in α-cells are altered in Western-diet-induced T2DM. In particular, α-cells extracted from mouse pancreatic tissue showed a lower density of mitochondria, a less expressed matrix and a lower number of cristae. These deformities in mitochondrial ultrastructure imply a decreased efficiency in mitochondrial ATP production, which prompted us to theoretically explore and clarify one of the most challenging problems associated with T2DM, namely the lack of glucagon secretion in hypoglycaemia and its oversecretion at high blood glucose concentrations. To this purpose, we constructed a novel computational model that links α-cell metabolism with their electrical activity and glucagon secretion. Our results show that defective mitochondrial metabolism in α-cells can account for dysregulated glucagon secretion in T2DM, thus improving our understanding of T2DM pathophysiology and indicating possibilities for new clinical treatments.

CPT1a-Dependent Long-Chain Fatty Acid Oxidation Contributes to Maintaining Glucagon Secretion from Pancreatic Islets

Glucagon, the principal hyperglycemic hormone, is secreted from pancreatic islet α cells as part of the counter-regulatory response to hypoglycemia. Hence, secretory output from α cells is under high demand in conditions of low glucose supply. Many tissues oxidize fat as an alternate energy substrate. Here, we show that glucagon secretion in low glucose conditions is maintained by fatty acid metabolism in both mouse and human islets, and that inhibiting this metabolic pathway profoundly decreases glucagon output by depolarizing α cell membrane potential and decreasing action potential amplitude. We demonstrate, by using experimental and computational approaches, that this is not mediated by the K_ATP channel, but instead due to reduced operation of the Na⁺-K⁺ pump. These data suggest that counter-regulatory secretion of glucagon is driven by fatty acid metabolism, and that the Na⁺-K⁺ pump is an important ATP-dependent regulator of α cell function.

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Glucagon secretion in low glucose is maintained by CPT1a-dependent FAO

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Loss of CPT1a-dependent FAO in mouse and human islets decreases glucagon secretion

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CPT1a-dependent FAO maintains glucagon secretion by supplying ATP to the Na⁺-K⁺-ATPase

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CPT1a-dependent FAO contributes to the counter-regulatory secretion of glucagon

The Central Role of Glucokinase in Glucose Homeostasis: A Perspective 50 Years After Demonstrating the Presence of the Enzyme in Islets of Langerhans

It is hypothesized that glucokinase (GCK) is the glucose sensor not only for regulation of insulin release by pancreatic β-cells, but also for the rest of the cells that contribute to glucose homeostasis in mammals. This includes other cells in endocrine pancreas (α- and δ-cells), adrenal gland, glucose sensitive neurons, entero-endocrine cells, and cells in the anterior pituitary. Glucose transport is by facilitated diffusion and is not rate limiting. Once inside, glucose is phosphorylated to glucose-6-phosphate by GCK in a reaction that is dependent on glucose throughout the physiological range of concentrations, is irreversible, and not product inhibited. High glycerol phosphate shuttle, pyruvate dehydrogenase, and pyruvate carboxylase activities, combined with low pentose-P shunt, lactate dehydrogenase, plasma membrane monocarboxylate transport, and glycogen synthase activities constrain glucose-6-phosphate to being metabolized through glycolysis. Under these conditions, glycolysis produces mostly pyruvate and little lactate. Pyruvate either enters the citric acid cycle through pyruvate dehydrogenase or is carboxylated by pyruvate carboxylase. Reducing equivalents from glycolysis enter oxidative phosphorylation through both the glycerol phosphate shuttle and citric acid cycle. Raising glucose concentration increases intramitochondrial [NADH]/[NAD⁺] and thereby the energy state ([ATP]/[ADP][Pi]), decreasing [Mg²⁺ADP] and [AMP]. [Mg²⁺ADP] acts through control of K_ATP channel conductance, whereas [AMP] acts through regulation of AMP-dependent protein kinase. Specific roles of different cell types are determined by the diverse molecular mechanisms used to couple energy state to cell specific responses. Having a common glucose sensor couples complementary regulatory mechanisms into a tightly regulated and stable glucose homeostatic network.

The α-cell in diabetes mellitus

Findings from the past 10 years have placed the glucagon-secreting pancreatic α-cell centre stage in the development of diabetes mellitus, a disease affecting almost one in every ten adults worldwide. Glucagon secretion is reduced in patients with type 1 diabetes mellitus, increasing the risk of insulin-induced hypoglycaemia, but is enhanced in type 2 diabetes mellitus, exacerbating the effects of diminished insulin release and action on blood levels of glucose. A better understanding of the mechanisms underlying these changes is therefore an important goal. RNA sequencing reveals that, despite their opposing roles in the control of blood levels of glucose, α-cells and β-cells have remarkably similar patterns of gene expression. This similarity might explain the fairly facile interconversion between these cells and the ability of the α-cell compartment to serve as a source of new β-cells in models of extreme β-cell loss that mimic type 1 diabetes mellitus. Emerging data suggest that GABA might facilitate this interconversion, whereas the amino acid glutamine serves as a liver-derived factor to promote α-cell replication and maintenance of α-cell mass. Here, we survey these developments and their therapeutic implications for patients with diabetes mellitus.

Molecular reductions in glucokinase activity increase counter-regulatory responses to hypoglycemia in mice and humans with diabetes

OBJECTIVE

Appropriate glucose levels are essential for survival; thus, the detection and correction of low blood glucose is of paramount importance. Hypoglycemia prompts an integrated response involving reduction in insulin release and secretion of key counter-regulatory hormones glucagon and epinephrine that together promote endogenous glucose production to restore normoglycemia. However, specifically how this response is orchestrated remains to be fully clarified. The low affinity hexokinase glucokinase is found in glucose-sensing cells involved in glucose homeostasis including pancreatic β-cells and in certain brain areas. Here, we aimed to examine the role of glucokinase in triggering counter-regulatory hormonal responses to hypoglycemia, hypothesizing that reduced glucokinase activity would lead to increased and/or earlier triggering of responses.

METHODS

Hyperinsulinemic glucose clamps were performed to examine counter-regulatory responses to controlled hypoglycemic challenges created in humans with monogenic diabetes resulting from heterozygous glucokinase mutations (GCK-MODY). To examine the relative importance of glucokinase in different sensing areas, we then examined responses to clamped hypoglycemia in mice with molecularly defined disruption of whole body and/or brain glucokinase.

RESULTS

GCK-MODY patients displayed increased and earlier glucagon responses during hypoglycemia compared with a group of glycemia-matched patients with type 2 diabetes. Consistent with this, glucagon responses to hypoglycemia were also increased in I366F mice with mutated glucokinase and in streptozotocin-treated β-cell ablated diabetic I366F mice. Glucagon responses were normal in conditional brain glucokinase-knockout mice, suggesting that glucagon release during hypoglycemia is controlled by glucokinase-mediated glucose sensing outside the brain but not in β-cells. For epinephrine, we found increased responses in GCK-MODY patients, in β-cell ablated diabetic I366F mice and in conditional (nestin lineage) brain glucokinase-knockout mice, supporting a role for brain glucokinase in triggering epinephrine release.

CONCLUSIONS

Our data suggest that glucokinase in brain and other non β-cell peripheral hypoglycemia sensors is important in glucose homeostasis, allowing the body to detect and respond to a falling blood glucose.

α-cell glucokinase suppresses glucose-regulated glucagon secretion

Glucagon secretion by pancreatic α-cells is triggered by hypoglycemia and suppressed by high glucose levels; impaired suppression of glucagon secretion is a hallmark of both type 1 and type 2 diabetes. Here, we show that α-cell glucokinase (Gck) plays a role in the control of glucagon secretion. Using mice with α-cell-specific inactivation of Gck (αGckKO mice), we find that glucokinase is required for the glucose-dependent increase in intracellular ATP/ADP ratio and the closure of KATP channels in α-cells and the suppression of glucagon secretion at euglycemic and hyperglycemic levels. αGckKO mice display hyperglucagonemia in the fed state, which is associated with increased hepatic gluconeogenic gene expression and hepatic glucose output capacity. In adult mice, fed hyperglucagonemia is further increased and glucose intolerance develops. Thus, glucokinase governs an α-cell metabolic pathway that suppresses secretion at or above normoglycemic levels; abnormal suppression of glucagon secretion deregulates hepatic glucose metabolism and, over time, induces a pre-diabetic phenotype.

Comparison of Fetal Porcine Aggregates of Purified β-Cells versus Islet-Like Cell Clusters as a Treatment of Diabetes

Fetal pig islet-like cell clusters (ICCs) have the potential to reverse diabetes 1–5 months after transplantation. In a fetal ICC, however, β-cells constitute only 6–8% of the cells, in contrast to 65% in an adult pig islet. Attempts to purify fetal β-cells from cell clusters and compare their function to that of ICCs have not been shown previously. β-Cells were purified from ICCs isolated from the fetal pig pancreas. These were then aggregated and maintained in culture for 3 days. ICCs were isolated from fetal pig pancreas and allowed to round up in culture for 3 days. Transplantation of aggregates and ICCs (10,000 and 12,600, respectively) into diabetic immunoincompetent mice resulted in normoglycemia at 18 ± 2 and 8 ± 1 weeks, respectively (p = 0.0006). Removal of grafts after normalization of blood glucose levels resulted in rapid return of hyperglycemia in both groups. In conclusion, a purified population of immature β-cells can be produced from the fetal pig pancreas. The reason these cells take longer than ICCs to reverse diabetes when transplanted is postulated to be because of the relative lack of precursor cells from which β-cells differentiate. This finding may have implications for stem cell therapy, as other cell types, other than purified β-cells, may be necessary for appropriate function in vivo.

A method for high-throughput functional imaging of single cells within heterogeneous cell preparations

Functional characterization of individual cells within heterogeneous tissue preparations is challenging. Here, we report the development of a versatile imaging method that assesses single cell responses of various endpoints in real time, while identifying the individual cell types. Endpoints that can be measured include (but are not limited to) ionic flux (calcium, sodium, potassium and hydrogen), metabolic responsiveness (NAD(P)H, mitochondrial membrane potential), and signal transduction (H₂O₂ and cAMP). Subsequent to fluorescent imaging, identification of cell types using immunohistochemistry allows for mapping of cell type to their respective functional real time responses. To validate the utility of this method, NAD(P)H responses to glucose of islet alpha versus beta cells generated from dispersed pancreatic islets, followed by the construction of frequency distributions characterizing the variability in the magnitude of each individual cell responses were compared. As expected, no overlap between the glucose response frequency distributions for beta cells versus alpha cells was observed, thereby establishing both the high degree of fidelity and low rate of both false-negatives and false-positives in this approach. This novel method has the ability not only to resolve single cell level functional differences between cell types, but also to characterize functional heterogeneity within a given cell type.

Glucosensing in the gastrointestinal tract: Impact on glucose metabolism

The gastrointestinal tract is an important interface of exchange between ingested food and the body. Glucose is one of the major dietary sources of energy. All along the gastrointestinal tube, e.g., the oral cavity, small intestine, pancreas, and portal vein, specialized cells referred to as glucosensors detect variations in glucose levels. In response to this glucose detection, these cells send hormonal and neuronal messages to tissues involved in glucose metabolism to regulate glycemia. The gastrointestinal tract continuously communicates with the brain, especially with the hypothalamus, via the gut-brain axis. It is now well established that the cross talk between the gut and the brain is of crucial importance in the control of glucose homeostasis. In addition to receiving glucosensing information from the gut, the hypothalamus may also directly sense glucose. Indeed, the hypothalamus contains glucose-sensitive cells that regulate glucose homeostasis by sending signals to peripheral tissues via the autonomous nervous system. This review summarizes the mechanisms by which glucosensors along the gastrointestinal tract detect glucose, as well as the results of such detection in the whole body, including the hypothalamus. We also highlight how disturbances in the glucosensing process may lead to metabolic disorders such as type 2 diabetes. A better understanding of the pathways regulating glucose homeostasis will further facilitate the development of novel therapeutic strategies for the treatment of metabolic diseases.

Glucose control of glucagon secretion-'There's a brand-new gimmick every year'

Glucagon from the pancreatic α-cells is a major blood glucose-regulating hormone whose most important role is to prevent hypoglycaemia that can be life-threatening due to the brain's strong dependence on glucose as energy source. Lack of blood glucose-lowering insulin after malfunction or autoimmune destruction of the pancreatic β-cells is the recognized cause of diabetes, but recent evidence indicates that diabetic hyperglycaemia would not develop unless lack of insulin was accompanied by hypersecretion of glucagon. Glucagon release has therefore become an increasingly important target in diabetes management. Despite decades of research, an understanding of how glucagon secretion is regulated remains elusive, and fundamentally different mechanisms continue to be proposed. The autonomous nervous system is an important determinant of glucagon release, but it is clear that secretion is also directly regulated within the pancreatic islets. The present review focuses on pancreatic islet mechanisms involved in glucose regulation of glucagon release. It will be argued that α-cell-intrinsic processes are most important for regulation of glucagon release during recovery from hypoglycaemia and that paracrine inhibition by somatostatin from the δ-cells shapes pulsatile glucagon release in hyperglycaemia. The electrically coupled β-cells ultimately determine islet hormone pulsatility by releasing synchronizing factors that affect the α- and δ-cells.

ATP-regulated potassium channels and voltage-gated calcium channels in pancreatic alpha and beta cells: similar functions but reciprocal effects on secretion

Closure of ATP-regulated K(+) channels (K(ATP) channels) plays a central role in glucose-stimulated insulin secretion in beta cells. K(ATP) channels are also highly expressed in glucagon-producing alpha cells, where their function remains unresolved. Under hypoglycaemic conditions, K(ATP) channels are open in alpha cells but their activity is low and only ~1% of that in beta cells. Like beta cells, alpha cells respond to hyperglycaemia with K(ATP) channel closure, membrane depolarisation and stimulation of action potential firing. Yet, hyperglycaemia reciprocally regulates glucagon (inhibition) and insulin secretion (stimulation). Here we discuss how this conundrum can be resolved and how reduced K(ATP) channel activity, via membrane depolarisation, paradoxically reduces alpha cell Ca(2+) entry and glucagon exocytosis. Finally, we consider whether the glucagon secretory defects associated with diabetes can be attributed to impaired K(ATP) channel regulation and discuss the potential for remedial pharmacological intervention using sulfonylureas.

Hypoxia lowers SLC30A8/ZnT8 expression and free cytosolic Zn2+ in pancreatic beta cells

AIMS/HYPOTHESIS

Hypoxic damage complicates islet isolation for transplantation and may contribute to beta cell failure in type 2 diabetes. Polymorphisms in the SLC30A8 gene, encoding the secretory granule zinc transporter 8 (ZnT8), influence type 2 diabetes risk, conceivably by modulating cytosolic Zn(2+) levels. We have therefore explored the role of ZnT8 and cytosolic Zn(2+) in the response to hypoxia of pancreatic islet cells.

METHODS

Human, mouse or rat islets were isolated and exposed to varying O2 tensions. Cytosolic free zinc was measured using the adenovirally expressed recombinant targeted zinc probe eCALWY4. Gene expression was measured using quantitative (q)RT-PCR, western (immuno-) blotting or immunocytochemistry. Beta cells were identified by insulin immunoreactivity.

RESULTS

Deprivation of O2 (1% vs 5% or 21%) for 24 h lowered free cytosolic Zn(2+) concentrations by ~40% (p < 0.05) and ~30% (p < 0.05) in mouse and human islet cells, respectively. Hypoxia similarly decreased SLC30A8 mRNA expression in islets, and immunoreactivity in beta cells. Implicating lowered ZnT8 levels in the hypoxia-induced fall in cytosolic Zn(2+), genetic ablation of Slc30a8 from mouse islets lowered cytosolic Zn(2+) by ~40% (p < 0.05) and decreased the induction of metallothionein (Mt1, Mt2) genes. Cell survival in the face of hypoxia was enhanced in small islets of older (>12 weeks) Slc30a8 null mice vs controls, but not younger animals.

CONCLUSIONS/INTERPRETATION

The response of pancreatic beta cells to hypoxia is characterised by decreased SLC30A8 expression and lowered cytosolic Zn(2+) concentrations. The dependence on ZnT8 of hypoxia-induced changes in cell survival may contribute to the actions of SLC30A8 variants on diabetes risk in humans.

Glucokinase activators

In this review we highlight recently disclosed progress in the field of small-molecule activators of the human glucokinase enzyme. Several of the reported chemotypes possess structural features that diverge from known leads; some of these modifications appear to be specifically designed to modulate tissue selectivity or discrete parameters of enzyme function (e.g., S0.5 v Vmax). This review will inform the reader of the extent of continued effort being directed toward discovery of a first-in-class drug for Type II diabetes mellitus that functions through this target. Patents were selected from those published in December 2009 up to November 2011; foreign filings were translated where possible to understand the claims and biological techniques utilized to characterize the reported glucokinase activators. Overall, there appears to be a recent trend leading to reduced patent filings for small-molecule glucokinase activators. There are many possible explanations for this trend; however, it is likely that the field has reached maturity and that the downturn of new disclosures represents the transition of many of these programs to the clinic.

Nutritional regulation of glucokinase: a cross-species story

The glucokinase (GK) enzyme (EC 2.7.1.1.) is essential for the use of dietary glucose because it is the first enzyme to phosphorylate glucose in excess in different key tissues such as the pancreas and liver. The objective of the present review is not to fully describe the biochemical characteristics and the genetics of this enzyme but to detail its nutritional regulation in different vertebrates from fish to human. Indeed, the present review will describe the existence of the GK enzyme in different animal species that have naturally different levels of carbohydrate in their diets. Thus, some studies have been performed to analyse the nutritional regulation of the GK enzyme in humans and rodents (having high levels of dietary carbohydrates in their diets), in the chicken (moderate level of carbohydrates in its diet) and rainbow trout (no carbohydrate intake in its diet). All these data illustrate the nutritional importance of the GK enzyme irrespective of feeding habits, even in animals known to poorly use dietary carbohydrates (carnivorous species).

A fresh view of glycolysis and glucokinase regulation: history and current status

This minireview looks back at a century of glycolysis research with a focus on the mechanisms of flux regulation. Traditionally, glycolysis is regarded as a feeder pathway that prepares glucose for further catabolism and energy production. However, glycolysis is much more than that, in particular in those tissues that express the low affinity glucose-phosphorylating enzyme glucokinase. This enzyme equips the glycolytic pathway with a special steering function for the regulation of intermediary metabolism. In beta cells, glycolysis acts as a transducer for triggering and amplifying physiological glucose-induced insulin secretion. On the basis of these considerations, I have defined a glycolytic flux regulatory unit composed of the two fructose ester steps of this pathway with various enzymes and metabolites that regulate glycolysis.

Glucose regulation of glucagon secretion

Glucagon secreted by pancreatic α-cells is the major hyperglycemic hormone correcting acute hypoglycaemia (glucose counterregulation). In diabetes the glucagon response to hypoglycaemia becomes compromised and chronic hyperglucagonemia appears. There is increasing awareness that glucagon excess may underlie important manifestations of diabetes. However opinions differ widely how glucose controls glucagon secretion. The autonomous nervous system plays an important role in the glucagon response to hypoglycaemia. But it is clear that glucose controls glucagon secretion also by mechanisms involving direct effects on α-cells or indirect effects via paracrine factors released from non-α-cells within the pancreatic islets. The present review discusses these mechanisms and argues that different regulatory processes are involved in a glucose concentration-dependent manner. Direct glucose effects on the α-cell and autocrine mechanisms are probably most significant for the glucagon response to hypoglycaemia. During hyperglycaemia, when secretion from β- and δ-cells is stimulated, paracrine inhibitory factors generate pulsatile glucagon release in opposite phase to pulsatile release of insulin and somatostatin. High concentrations of glucose have also stimulatory effects on glucagon secretion that tend to balance and even exceed the inhibitory influence. The latter actions might underlie the paradoxical hyperglucagonemia that aggravates hyperglycaemia in persons with diabetes.

Tolbutamide controls glucagon release from mouse islets differently than glucose: involvement of K(ATP) channels from both α-cells and δ-cells

We evaluated the role of ATP-sensitive K⁺ (K(ATP)) channels, somatostatin, and Zn²⁺ in the control of glucagon secretion from mouse islets. Switching from 1 to 7 mmol/L glucose inhibited glucagon release. Diazoxide did not reverse the glucagonostatic effect of glucose. Tolbutamide decreased glucagon secretion at 1 mmol/L glucose (G1) but stimulated it at 7 mmol/L glucose (G7). The reduced glucagon secretion produced by high concentrations of tolbutamide or diazoxide, or disruption of K(ATP) channels (Sur1(-/-) mice) at G1 could be inhibited further by G7. Removal of the somatostatin paracrine influence (Sst(-/-) mice or pretreatement with pertussis toxin) strongly increased glucagon release, did not prevent the glucagonostatic effect of G7, and unmasked a marked glucagonotropic effect of tolbutamide. Glucose inhibited glucagon release in the absence of functional K(ATP) channels and somatostatin signaling. Knockout of the Zn²⁺ transporter ZnT8 (ZnT8(-/-) mice) did not prevent the glucagonostatic effect of glucose. In conclusion, glucose can inhibit glucagon release independently of Zn²⁺, K(ATP) channels, and somatostatin. Closure of K(ATP) channels controls glucagon secretion by two mechanisms, a direct stimulation of α-cells and an indirect inhibition via somatostatin released from δ-cells. The net effect on glucagon release results from a balance between both effects.

Strategies for the design of hepatoselective glucokinase activators to treat type 2 diabetes

INTRODUCTION

Type 2 diabetes mellitus (T2DM) represents a rapidly expanding healthcare challenge. There is a significant need for novel therapies to help patients achieve and maintain glycemic control in order to avoid the long-term microvascular and macrovascular complications associated with the disease. Small molecule allosteric activators of the glucokinase enzyme, an important regulator of glucose homeostasis, have emerged as a potential new class of therapeutics. Glucokinase activators have been shown to effectively lower fasting and postprandial glucose in T2DM patients; however, hypoglycemia emerged as a potential risk limiting their therapeutic potential. To mitigate this risk, recent efforts have focused on the design of liver-specific activators that seek to normalize hepatic glucose uptake and production without potentiating glucose-stimulated insulin secretion.

AREAS COVERED

The article reviews the various drug discovery strategies that have emerged for the development of candidates that selectively activate glucokinase in the liver. Literature from 2000 to 2012 is surveyed including scientific publications, patent applications, conferences and clinical trials.

EXPERT OPINION

Liver selective agents have proven to be an effective strategy for mitigating the hypoglycemia risk that has been historically associated with this mechanism. The ultimate therapeutic potential of this approach will depend on the results of longer patient studies which are currently being conducted with several clinical candidates. The discovery of these liver-specific activators has highlighted several challenges in the design of tissue-selective therapeutics, which will need to be overcome in the future.

High-fat diet consumption during pregnancy and the early post-natal period leads to decreased α cell plasticity in the nonhuman primate

We investigated the impact of poor maternal nutrition and metabolic health on the development of islets of the nonhuman primate (NHP). Interestingly, fetal offspring of high fat diet (HFD) fed animals had normal total islet and β cell mass; however, there was a significant reduction in α cell mass, and decreased expression of transcription factors involved in α cell differentiation. In juvenile animals all offspring maintained on a HFD during the postweaning period demonstrated increases in total islet mass, however, the control offspring displaying increased islet number, and HFD offspring displayed increased islet size. Finally, while control offspring had increases in α and β cells, the HFD offspring had increases only in β cell number. These studies indicate that consumption of a HFD diet during pregnancy in the NHP, independent of maternal metabolic health, causes long-term abnormalities in α cell plasticity that may contribute to chronic disease susceptibility.

Repair of diverse diabetic defects of β-cells in man and mouse by pharmacological glucokinase activation

Glucokinase activators (GKAs) are being developed and clinically tested for potential antidiabetic therapy. The potential benefits and limitations of this approach continue to be intensively debated. To contribute to the understanding of experimental pharmacology and therapeutics of GKAs, we have tested the efficacy of one of these agents (Piragliatin) in isolated islets from humans with type 2 diabetes mellitus (T2DM), from mice with glucokinase (GK) mutations induced by ethyl-nitroso-urea (ENU) as models of Maturity Onset Diabetes of the Young linked to GK and Permanent Neonatal Diabetes Mellitus linked to GK (PNDM-GK) and finally of islets rendered glucose insensitive by treatment with the sulphonyl urea compound glyburide in organ culture. We found that the GKA repaired the defect in all three instances as manifest in increased glucose-induced insulin release and elevated intracellular calcium responses. The results show the remarkable fact that acute pharmacological activation of GK reverses secretion defects of β-cells caused by molecular mechanism that differ vastly in nature, including the little understood multifactorial lesion of β-cells in T2DM of man, the complex GK mutations in mice resembling GK disease and acute sulphonylurea failure of mouse β-cells in tissue culture. The implications of these results are to be discussed on the theoretical basis underpinning the strategy of developing these drugs and in light of recent results of clinical trials with GKAs that failed for little understood reasons.

Glucagon secretion and signaling in the development of diabetes

Normal release of glucagon from pancreatic islet α-cells promotes glucose mobilization, which counteracts the hypoglycemic actions of insulin, thereby ensuring glucose homeostasis. In treatment of diabetes aimed at rigorously reducing hyperglycemia to avoid chronic complications, the resulting hypoglycemia triggering glucagon release from α-cells is frequently impaired, with ensuing hypoglycemic complications. This review integrates the physiology of glucagon secretion regulating glucose homeostasis in vivo to single α-cell signaling, and how both become perturbed in diabetes. α-cells within the social milieu of the islet micro-organ are regulated not only by intrinsic signaling events but also by paracrine regulation, particularly by adjacent insulin-secreting β-cells and somatostatin-secreting δ-cells. We discuss the intrinsic α-cell signaling events, including glucose sensing and ion channel regulation leading to glucagon secretion. We then discuss the complex crosstalk between the islet cells and the breakdown of this crosstalk in diabetes contributing to the dysregulated glucagon secretion. Whereas, there are many secretory products released by β- and δ-cells that become deficient or excess in diabetes, we discuss the major ones, including the better known insulin and lesser known somatostatin, which act as putative paracrine on/off switches that very finely regulate α-cell secretory responses in health and diabetes. Of note in several type 1 diabetes (T1D) rodent models, blockade of excess somatostatin actions on α-cell could normalize glucagon secretion sufficient to attain normoglycemia in response to hypoglycemic assaults. There has been slow progress in fully elucidating the pathophysiology of the α-cell in diabetes because of the small number of α-cells within an islet and the islet mass becomes severely reduced and inflamed in diabetes. These limitations are just now being surmounted by new approaches.

A computational systems analysis of factors regulating α cell glucagon secretion

Glucagon, a peptide hormone secreted from the α-cells of the pancreatic islets, is critical for blood glucose homeostasis. We reviewed the literature and employed a computational systems analysis of intracellular metabolic and electrical regulation of glucagon secretion to better understand these processes. The mathematical model of α-cell metabolic parameters is based on our previous model for pancreatic β-cells. We also formulated an ionic model for action potentials that incorporates Ca ( 2+) , K (+) , Na (+) and Cl (-) currents. Metabolic and ionic models are coupled to the equations describing Ca ( 2+) homeostasis and glucagon secretion that depends on activation of specific voltage-gated Ca ( 2+) channels. Paracrine and endocrine regulations were analyzed with an emphasis on their effects on a hyperpolarization of membrane potential. This general model simulates and gives insight into the mechanisms of regulation of glucagon secretion under a wide range of experimental conditions. We also reviewed and analyzed dysfunctional mechanisms in α-cells to determine key pharmacological targets for modulating glucagon secretion in type 1 and 2 diabetes.

One-step purification of functional human and rat pancreatic alpha cells

Pancreatic alpha cells contribute to glucose homeostasis by the regulated secretion of glucagon, which increases glycogenolysis and hepatic gluconeogenesis in response to hypoglycemia. Alterations of glucagon secretion are observed in diabetic patients and exacerbate the disease. The restricted availability of purified primary alpha cells has limited our understanding of their function in health and disease. This study was designed to establish convenient protocols for the purification of viable alpha cells from rat and human pancreatic islets by FACS, using intrinsic cellular properties. Islets were isolated from the pancreata of Wistar rats or deceased human organ donors. Dispersed islet cells were separated by FACS based on light scatter and autofluorescence. Purity of sorted cells was evaluated by immunocytochemistry using hormone specific antibodies. Relative hormone expression was further determined by quantitative RT-PCR. Viability was determined by Annexin V and propidium iodide staining and function was assessed by monitoring cytoplasmic free Ca(2+) concentration ([Ca(2+)](i)) using Fura-2/AM. We developed species-specific FACS gating strategies that resulted in populations consisting mainly of alpha cells (96.6 ± 1.4%, n = 3 for rat; 95.4 ± 1.7%, n = 4 for human, mean ± SEM). These cell fractions showed ~5-fold and ~4-fold enrichment (rat and human, respectively) of glucagon mRNA expression compared to total ungated islet cells. Most of the sorted cells were viable and functional, as they responded with an increase in [Ca(2+)](i) upon stimulation with L-arginine (10 mM). The majority of the sorted human alpha cells responded also to stimulation with kainate (100 μM), whereas this response was infrequent in rat alpha cells. Using the same sample preparation, but a different gating strategy, we were also able to sort rat and human populations enriched in beta cells. In conclusion, we have simplified and optimized a method for the purification of rat alpha cells, as well as established a novel approach to separate human alpha cells using neither antibodies nor dyes possibly interfering with cellular functions.

Models of glucagon secretion, their application to the analysis of the defects in glucagon counterregulation and potential extension to approximate glucagon action

This review analyzes an interdisciplinary approach to the pancreatic endocrine network-like relationships that control glucagon secretion and glucagon counterregulation (GCR). Using in silico studies, we show that a pancreatic feedback network that brings together several explicit interactions between islet peptides and blood glucose reproduces the normal GCR axis and explains its impairment in diabetes. An α-cell auto-feedback loop drives glucagon pulsatility and mediates triggering of GCR by hypoglycemia by a rapid switch-off of β-cell signals. The auto-feedback explains the enhancement of defective GCR in β-cell deficiency by a switch-off of signals in the pancreas that suppress α cells. Our models also predict that reduced β-cell activity decreases and delays the GCR. A key application of our models is the in silico simulation and testing of possible scenarios to repair defective GCR in β-cell deficiency. In particular, we predict that partial suppression of hyperglucagonemia may repair the impaired GCR. We also outline how the models can be extended and tested using human data to become a part of a larger construct including the regulation of the hepatic glucose output by the pancreas, circulating glucose, and incretins. In conclusion, a model of the normal GCR control mechanisms and their dysregulation in insulin-deficient diabetes is proposed and partially validated. The model components are clinically measurable, which permits its application to the study of the abnormalities of the human endocrine pancreas and their role in the progression of many diseases, including diabetes, metabolic syndrome, polycystic ovary syndrome, and others. It may also be used to examine therapeutic responses.

The glucagon-producing alpha cell: an electrophysiologically exceptional cell

Activation of potassium channels normally serves to reduce cellular activity but recent data indicate that the glucagon-secreting alpha cells are different in this respect and that inhibition of voltage-gated potassium channels results in a paradoxical inhibition of glucagon secretion. Here we discuss these findings and attempt to provide a model for the regulation of glucagon secretion that incorporates these observations.

Conversion of adult pancreatic alpha-cells to beta-cells after extreme beta-cell loss

Pancreatic insulin-producing beta-cells have a long lifespan, such that in healthy conditions they replicate little during a lifetime. Nevertheless, they show increased self-duplication after increased metabolic demand or after injury (that is, beta-cell loss). It is not known whether adult mammals can differentiate (regenerate) new beta-cells after extreme, total beta-cell loss, as in diabetes. This would indicate differentiation from precursors or another heterologous (non-beta-cell) source. Here we show beta-cell regeneration in a transgenic model of diphtheria-toxin-induced acute selective near-total beta-cell ablation. If given insulin, the mice survived and showed beta-cell mass augmentation with time. Lineage-tracing to label the glucagon-producing alpha-cells before beta-cell ablation tracked large fractions of regenerated beta-cells as deriving from alpha-cells, revealing a previously disregarded degree of pancreatic cell plasticity. Such inter-endocrine spontaneous adult cell conversion could be harnessed towards methods of producing beta-cells for diabetes therapies, either in differentiation settings in vitro or in induced regeneration.