Electrophysiology of the pancreatic beta-cell

The impact of ATP-sensitive potassium channel modulation on mitochondria in a Parkinson's disease model using SH-SY5Y cells depends on their differentiation state

Inward rectifying potassium channels sensitive to ATP levels (KATP) have been the subject of investigation for several decades. Modulators of KATP channels are well-established treatments for metabolic as well as cardiovascular diseases. Experimental studies have also shown the potential of KATP modulation in neurodegenerative disorders. However, to date, data regarding the effects of KATP antagonists/agonists in experiments related to neurodegeneration remain inconsistent. The main source of confusion in evaluating available data seems to be the choice of experimental models. The present study aims to provide a comprehensive understanding of the effects of both opening and blocking KATP channels in two forms of SH-SY5Y cells. Our results offer valuable insights into the significance of metabolic differences between differentiated and non-differentiated SH-SY5Y cells, particularly in the context of glibenclamide and diazoxide effects under normal conditions and during the initiation of pathological events simulating Parkinson's disease in vitro. We emphasize the analysis of mitochondrial functions and changes in mitochondrial network morphology. The heightened protein expression of KATP channels identified in non-differentiated SH-SY5Y cells seems to be a platform for a more significant impact of KATP modulators in this cell type. The efficiency of rotenone treatment in inducing morphological changes in the mitochondrial network depends on the differentiation status of SH-SY5Y cells.

Current Position of Gliclazide and Sulfonylureas in the Contemporary Treatment Paradigm for Type 2 Diabetes: A Scoping Review

The increasing burden of type 2 diabetes (T2D), in relation to alarming rise in the prevalence; challenges in the diagnosis, prevention, and treatment; as well as the substantial impact of disease on longevity and quality of life, is a major concern in healthcare worldwide. Sulfonylureas (SUs) have been a cornerstone of T2D pharmacotherapy for over 60 years as oral antidiabetic drugs (OADs), while the newer generation SUs, such as gliclazide modified release (MR), are known to be associated with low risk of hypoglycemia in addition to the cardiovascular neutrality. This scoping review aimed to specifically address the current position of gliclazide MR among other SUs in the contemporary treatment paradigm for T2D and to provide a practical guidance document to assist clinicians in using gliclazide MR in real-life clinical practice. The main topics addressed in this paper include the role of early and sustained glycemic control and use of SUs in T2D management, the properties of gliclazide MR in relation to its effectiveness and safety, the use of gliclazide therapy in special populations, and the place of SUs as a class and gliclazide MR specifically in the current T2D treatment algorithm.

A microrheological examination of insulin-secreting β-cells in healthy and diabetic-like conditions

Pancreatic β-cells regulate glucose homeostasis through glucose-stimulated insulin secretion, which is hindered in type-2 diabetes. Transport of the insulin vesicles is expected to be affected by changes in the viscoelastic and transport properties of the cytoplasm. These are evaluated in situ through particle-tracking measurements using a rat insulinoma β-cell line. The use of inert probes assists in decoupling the material properties of the cytoplasm from the active transport through cellular processes. The effect of glucose-stimulated insulin secretion is examined, and the subsequent remodeling of the cytoskeleton, at constant effects of cell activity, is shown to result in reduced mobility of the tracer particles. Induction of diabetic-like conditions is identified to alter the mean-squared displacement of the passive particles in the cytoplasm and diminish its reaction to glucose stimulation.

Encyclopaedic Review of Glipizide Pre-clinical and Clinical Status

Glipizide is an oral glucose-lowering medication that is beneficial for the treatment of type 2 diabetes. This study compiles exhaustively all accessible information on glipizide, from preclinical to clinical studies. Glipizide may be used in concert with TRAIL to treat cancer cells; in vitro studies have shown that it suppresses angiogenesis and vasculogenesis while shielding cells from glycation-induced damage. Anticonvulsant effects and modifications in the pharmacokinetics of other medications, such as Divalproex Sodium, were seen in glipizide in vivo experiments. Propranolol amplifies glipizide's hypoglycemic effect briefly in normal animals but consistently enhances it in diabetic ones. In the treatment of cancer and neurodegenerative poly(Q) illnesses, glipizide has demonstrated to offer potential therapeutic advantages. It is ineffective in preventing DENA-induced liver cancer and may cause DNA damage over time. The way glipizide interacts with genetic variants may increase the risk of hypoglycemia. Combining Syzygium cumini and ARBE to glipizide may enhance glycemic and lipid control in type 2 diabetes. Individuals with coronary artery disease who take glipizide or glyburide have an increased risk of death. The risk of muscular responses and acute pancreatitis is minimal when glipizide and dulaglutide are combined. In conclusion, glipizide has shown promising therapeutic efficacy across a variety of disorders.

Glycaemic abnormalities induced by small molecule tryosine kinase inhibitors: a review

In light of the expected increase in the prevalence of diabetes mellitus due to an aging population, sedentary lifestyles, an increase in obesity, and unhealthy diets, there is a need to identify potential pharmacological agents that can heighten the risk of developing diabetes. Similarly, it is equally important to also identify those agents that show blood glucose-lowering properties. Amongst these agents are tyrosine kinase inhibitors used to treat certain types of cancers. Over the last two decades, there has been an increase in the use of targeted chemotherapy for cancers such as renal cell carcinoma, chronic leukaemia, and gastrointestinal stromal tumours. Small molecule tyrosine kinase inhibitors have been at the forefront of targeted chemotherapy. Studies have shown that small molecule tyrosine kinase inhibitors can alter glycaemic control and glucose metabolism, with some demonstrating hypoglycaemic activities whilst others showing hyperglycaemic properties. The mechanism by which small molecule tyrosine kinase inhibitors cause glycaemic dysregulation is not well understood, therefore, the clinical significance of these chemotherapeutic agents on glucose handling is also poorly documented. In this review, the effort is directed at mapping mechanistic insights into the effect of various small molecule tyrosine kinase inhibitors on glycaemic dysregulation envisaged to provide a deeper understanding of these chemotherapeutic agents on glucose metabolism. Small molecule tyrosine kinase inhibitors may elicit these observed glycaemic effects through preservation of β-cell function, improving insulin sensitivity and insulin secretion. These compounds bind to a spectrum of receptors and proteins implicated in glucose regulation for example, non-receptor tyrosine kinase SRC and ABL. Then receptor tyrosine kinase EGFR, PDGFR, and FGFR.

Identification of eight genomic protective alleles for mitochondrial diabetes by Kinship-graph convolutional network

AIMS

Nearly 85% of maternally inherited diabetes and deafness (MIDD) are caused by the m.3243A>G mutation in the mitochondrial DNA. However, the clinical phenotypes of MIDD may also be influenced by the nuclear genome, this study aimed to investigate nuclear genome variants that influence clinical phenotypes associated with m.3243A>G mutation in MIDD based on whole-genome sequencing of the patients belonging to pedigrees.

MATERIALS AND METHODS

We analyzed a whole-genome sequencing (WGS) dataset from blood samples of 38 MIDD patients with the m.3243A > G mutation belonging to 10 pedigrees, by developing a Kinship-graph convolutional network approach, called Ki-GCN, integrated with the conventional genome-wide association study (GWAS) methods.

RESULTS

We identified eight protective alleles in the nuclear genome that have protective effects against the onset of MIDD, related deafness, and also type 2 diabetes. Based on these eight protective alleles, we constructed an effective logistic regression model to predict the early or late onset of MIDD patients.

CONCLUSIONS

There are protective alleles in the nuclear genome that are associated with the onset-age of MIDD patients and might also provide protective effects on the deafness derived from MIDD patients.

Ca2+-activated K+ channels modulate membrane potential in the human parathyroid cell: Possible role in exocytosis

The relationships between parathyroid hormone (PTH) secretion and parathyroid cell membrane potential, including the identities and roles of K+ channels that regulate and/or modulate membrane potential are not well defined. Here we have used Western blot/immunohistochemistry as well as patch-clamp and perifusion techniques to identify and localize specific K+ channels in parathyroid cells and to investigate their roles in the control of membrane potential and PTH secretion. We also re-investigated the relationship between membrane potential and exocytosis. We showed that in single human parathyroid cells K+ current is dependent on at least two types of Ca2+-activated K+ channels: a small-conductance Ca2+-activated K+ channel (KSK) and a large-conductance voltage and Ca2+-activated K+ channel (KBK). These channels were sensitive to specific peptide blocking toxins including apamin, charybdotoxin, and iberiotoxin. These channels confer sensitivity of the membrane potential in single cells to high extracellular K+, TEA, and peptide toxins. Blocking of KBK potently inhibited K+ channel current, and KBK was shown to be expressed in the plasma membrane of parathyroid cells. In addition, when using the capacitance technique as an indicator of exocytosis, clamping the parathyroid cell at -60 mV prevented exocytosis, whereas holding the membrane potential at 0 mV facilitated it. Taken together, the results show that human parathyroid cells have functional KBK and KSK channels but the data presented herein suggest that KBK/KSK channels likely contribute to the maintenance of the membrane potential, and that membrane potential, per se, modulates exocytosis independently of [Ca2+]i.

KATP channel activity and slow oscillations in pancreatic beta cells are regulated by mitochondrial ATP production

Pancreatic beta cells secrete insulin in response to plasma glucose. The ATP-sensitive potassium channel (KATP ) links glucose metabolism to islet electrical activity in these cells by responding to increased cytosolic [ATP]/[ADP]. It was recently proposed that pyruvate kinase (PK) in close proximity to beta cell KATP locally produces the ATP that inhibits KATP activity. This proposal was largely based on the observation that applying phosphoenolpyruvate (PEP) and ADP to the cytoplasmic side of excised inside-out patches inhibited KATP . To test the relative contributions of local vs. mitochondrial ATP production, we recorded KATP activity using mouse beta cells and INS-1 832/13 cells. In contrast to prior reports, we could not replicate inhibition of KATP activity by PEP + ADP. However, when the pH of the PEP solutions was not corrected for the addition of PEP, strong channel inhibition was observed as a result of the well-known action of protons to inhibit KATP . In cell-attached recordings, perifusing either a PK activator or an inhibitor had little or no effect on KATP channel closure by glucose, further suggesting that PK is not an important regulator of KATP . In contrast, addition of mitochondrial inhibitors robustly increased KATP activity. Finally, by measuring the [ATP]/[ADP] responses to imposed calcium oscillations in mouse beta cells, we found that oxidative phosphorylation could raise [ATP]/[ADP] even when ADP was at its nadir during the burst silent phase, in agreement with our mathematical model. These results indicate that ATP produced by mitochondrial oxidative phosphorylation is the primary controller of KATP in pancreatic beta cells. KEY POINTS: Phosphoenolpyruvate (PEP) plus adenosine diphosphate does not inhibit KATP activity in excised patches. PEP solutions only inhibit KATP activity if the pH is unbalanced. Modulating pyruvate kinase has minimal effects on KATP activity. Mitochondrial inhibition, in contrast, robustly potentiates KATP activity in cell-attached patches. Although the ADP level falls during the silent phase of calcium oscillations, mitochondria can still produce enough ATP via oxidative phosphorylation to close KATP . Mitochondrial oxidative phosphorylation is therefore the main source of the ATP that inhibits the KATP activity of pancreatic beta cells.

Ca2+ release or Ca2+ entry, that is the question: what governs Ca2+ oscillations in pancreatic β cells?

The standard model for Ca2+ oscillations in insulin-secreting pancreatic β cells centers on Ca2+ entry through voltage-activated Ca2+ channels. These work in combination with ATP-dependent K+ channels, which are the bridge between the metabolic state of the cells and plasma membrane potential. This partnership underlies the ability of the β cells to secrete insulin appropriately on a minute-to-minute time scale to control whole body plasma glucose. Though this model, developed over more than 40 years through many cycles of experimentation and mathematical modeling, has been very successful, it has been challenged by a hypothesis that calcium-induced calcium release from the endoplasmic reticulum through ryanodine or inositol trisphosphate (IP3) receptors is instead the key driver of islet oscillations. We show here that the alternative model is in fact incompatible with a large body of established experimental data and that the new observations offered in support of it can be better explained by the standard model.

Metabolic Role of GABA in the Secretory Function of Pancreatic β-Cells: Its Hypothetical Implication in β-Cell Degradation in Type 2 Diabetes

The stimulus-secretion coupling of a glucose-induced release is generally attributed to the metabolism of the hexose in the β-cells in the glycolytic pathway and the citric acid cycle. Glucose metabolism generates an increased cytosolic concentration of ATP and of the ATP/ADP ratio that closes the ATP-dependent K⁺-channel at the plasma membrane. The resultant depolarization of the β-cells opens voltage-dependent Ca²⁺-channels at the plasma membrane that triggers the exocytosis of insulin secretory granules. The secretory response is biphasic with a first and transient peak followed by a sustained phase. The first phase is reproduced by a depolarization of the β-cells with high extracellular KCl maintaining the KATP-channels open with diazoxide (triggering phase); the sustained phase (amplifying phase) depends on the participation of metabolic signals that remain to be determined. Our group has been investigating for several years the participation of the β-cell GABA metabolism in the stimulation of insulin secretion by three different secretagogues (glucose, a mixture of L-leucine plus L-glutamine, and some branched chain alpha-ketoacids, BCKAs). They stimulate a biphasic secretion of insulin accompanied by a strong suppression of the intracellular islet content of gamma-aminobutyric acid (GABA). As the islet GABA release simultaneously decreased, it was concluded that this resulted from an increased GABA shunt metabolism. The entrance of GABA into the shunt is catalyzed by GABA transaminase (GABAT) that transfers an amino group between GABA and alpha-ketoglutarate, resulting in succinic acid semialdehyde (SSA) and L-glutamate. SSA is oxidized to succinic acid that is further oxidized in the citric acid cycle. Inhibitors of GABAT (gamma-vinyl GABA, gabaculine) or glutamic acid decarboxylating activity (GAD), allylglycine, partially suppress the secretory response as well as GABA metabolism and islet ATP content and the ATP/ADP ratio. It is concluded that the GABA shunt metabolism contributes together with the own metabolism of metabolic secretagogues to increase islet mitochondrial oxidative phosphorylation. These experimental findings emphasize that the GABA shunt metabolism is a previously unrecognized anaplerotic mitochondrial pathway feeding the citric acid cycle with a β-cell endogenous substrate. It is therefore a postulated alternative to the proposed mitochondrial cataplerotic pathway(s) responsible for the amplification phase of insulin secretion. It is concluded the new postulated alternative suggests a possible new mechanism of β-cell degradation in type 2 (perhaps also in type 1) diabetes.

Characterizing Oscillatory and Excitability Regimes in a Protein-Free Lipid Membrane

Observations of electric potential oscillations in artificial lipid bilayers near the order-disorder transition indicate the existence of a stable limit cycle and, therefore, the possibility of producing excitable signals close to the bifurcation. We present a theoretical investigation of membrane oscillatory and excitability regimes induced by an increase in ion permeability at the order-disorder transition. The model accounts for the coupled effects of state-dependent permeability, membrane charge density, and hydrogen ion adsorption. A bifurcation diagram shows a transition between fixed-point and limit cycle solutions, enabling both oscillatory and excitability responses at different values of the acid association parameter. Oscillations are identified in terms of the membrane state, electric potential difference, and ion concentration near the membrane. The emerging voltage and time scales agree with measurements. Excitability is demonstrated by applying an external electric current stimulus, and the emerging signals display a threshold response and the appearance of repetitive signals upon using a long-lasting stimulus. The approach highlights the important role of the order-disorder transition, enabling membrane excitability in the absence of specialized proteins.

Modulatory Effect of Medicinal Plants and Their Active Constituents on ATP-Sensitive Potassium Channels (KATP) in Diabetes

Hyperglycemia, which is a chronic metabolic condition caused by either a defect in insulin secretion or insulin resistance, is a hallmark of diabetes mellitus (DM). Sustained hyperglycemia leads to the onset and development of many health complications. Despite the number of available antidiabetic medications on the market, there is still a need for novel treatment agents with increased efficacy and fewer adverse effects. Many medicinal plants offer a rich supply of bioactive compounds that have remarkable pharmacological effects with less toxicity and side effects. According to published evidence, natural antidiabetic substances influence pancreatic β-cell development and proliferation, inhibit pancreatic β-cell death, and directly increase insulin output. Pancreatic ATP-sensitive potassium channels play an essential role in coupling glucose metabolism to the secretion of insulin. Although much of the literature is available on the antidiabetic effects of medicinal plants, very limited studies discuss their direct action on pancreatic KATP. The aim of this review is to focus on the modulatory effects of antidiabetic medicinal plants and their active constituents on pancreatic KATP. The KATP channel should be regarded as a key therapeutic milestone in the treatment of diabetes. Therefore, continuous research into the interaction of medicinal plants with the KATP channel is crucial.

Spatial distribution of heterogeneity as a modulator of collective dynamics in pancreatic beta-cell networks and beyond

We study the impact of spatial distribution of heterogeneity on collective dynamics in gap-junction coupled beta-cell networks comprised on cells from two populations that differ in their intrinsic excitability. Initially, these populations are uniformly and randomly distributed throughout the networks. We develop and apply an iterative algorithm for perturbing the arrangement of the network such that cells from the same population are increasingly likely to be adjacent to one another. We find that the global input strength, or network drive, necessary to transition the network from a state of quiescence to a state of synchronised and oscillatory activity decreases as network sortedness increases. Moreover, for weak coupling, we find that regimes of partial synchronisation and wave propagation arise, which depend both on network drive and network sortedness. We then demonstrate the utility of this algorithm for studying the distribution of heterogeneity in general networks, for which we use Watts-Strogatz networks as a case study. This work highlights the importance of heterogeneity in node dynamics in establishing collective rhythms in complex, excitable networks and has implications for a wide range of real-world systems that exhibit such heterogeneity.

The Mutation Spectrum of Rare Variants in the Gene of Adenosine Triphosphate (ATP)-Binding Cassette Subfamily C Member 8 in Patients with a MODY Phenotype in Western Siberia

During differential diagnosis of diabetes mellitus, the greatest difficulties are encountered with young patients because various types of diabetes can manifest themselves in this age group (type 1, type 2, and monogenic types of diabetes mellitus, including maturity-onset diabetes of the young (MODY)). The MODY phenotype is associated with gene mutations leading to pancreatic-β-cell dysfunction. Using next-generation sequencing technology, targeted sequencing of coding regions and adjacent splicing sites of MODY-associated genes (HNF4A, GCK, HNF1A, PDX1, HNF1B, NEUROD1, KLF11, CEL, PAX4, INS, BLK, KCNJ11, ABCC8, and APPL1) was carried out in 285 probands. Previously reported missense variants c.970G>A (p.Val324Met) and c.1562G>A (p.Arg521Gln) in the ABCC8 gene were found once each in different probands. Variant c.1562G>A (p.Arg521Gln) in ABCC8 was detected in a compound heterozygous state with a pathogenic variant of the HNF1A gene in a diabetes patient and his mother. Novel frameshift mutation c.4609_4610insC (p.His1537ProfsTer22) in this gene was found in one patient. All these variants were detected in available family members of the patients and cosegregated with diabetes mellitus. Thus, next-generation sequencing of MODY-associated genes is an important step in the diagnosis of rare MODY subtypes.

High-resolution analysis of the cytosolic Ca2+ events in β cell collectives in situ

The release of peptide hormones is predominantly regulated by a transient increase in cytosolic Ca2+ concentration ([Ca2+]c). To trigger exocytosis, Ca2+ ions enter the cytosol from intracellular Ca2+ stores or from the extracellular space. The molecular events of late stages of exocytosis, and their dependence on [Ca2+]c, were extensively described in isolated single cells from various endocrine glands. Notably, less work has been done on endocrine cells in situ to address the heterogeneity of [Ca2+]c events contributing to a collective functional response of a gland. For this, β cell collectives in a pancreatic islet are particularly well suited as they are the smallest, experimentally manageable functional unit, where [Ca2+]c dynamics can be simultaneously assessed on both cellular and collective level. Here, we measured [Ca2+]c transients across all relevant timescales, from a subsecond to a minute time range, using high-resolution imaging with a low-affinity Ca2+ sensor. We quantified the recordings with a novel computational framework for automatic image segmentation and [Ca2+]c event identification. Our results demonstrate that under physiological conditions the duration of [Ca2+]c events is variable, and segregated into three reproducible modes, subsecond, second, and tens of seconds time range, and are a result of a progressive temporal summation of the shortest events. Using pharmacological tools we show that activation of intracellular Ca2+ receptors is both sufficient and necessary for glucose-dependent [Ca2+]c oscillations in β cell collectives, and that a subset of [Ca2+]c events could be triggered even in the absence of Ca2+ influx across the plasma membrane. In aggregate, our experimental and analytical platform was able to readily address the involvement of intracellular Ca2+ receptors in shaping the heterogeneity of [Ca2+]c responses in collectives of endocrine cells in situ.NEW & NOTEWORTHY Physiological glucose or ryanodine stimulation of β cell collectives generates a large number of [Ca2+]c events, which can be rapidly assessed with our newly developed automatic image segmentation and [Ca2+]c event identification pipeline. The event durations segregate into three reproducible modes produced by a progressive temporal summation. Using pharmacological tools, we show that activation of ryanodine intracellular Ca2+ receptors is both sufficient and necessary for glucose-dependent [Ca2+]c oscillations in β cell collectives.

Genome-edited zebrafish model of ABCC8 loss-of-function disease

ATP-sensitive potassium channel (KATP)gain- (GOF) and loss-of-function (LOF) mutations underlie human neonatal diabetes mellitus (NDM) and hyperinsulinism (HI), respectively. While transgenic mice expressing incomplete KATP LOF do reiterate mild hyperinsulinism, KATP knockout animals do not exhibit persistent hyperinsulinism. We have shown that islet excitability and glucose homeostasis are regulated by identical KATP channels in zebrafish. SUR1 truncation mutation (K499X) was introduced into the abcc8 gene to explore the possibility of using zebrafish for modeling human HI. Patch-clamp analysis confirmed the complete absence of channel activity in β-cells from K499X (SUR1-/-) fish. No difference in random blood glucose was detected in heterozygous SUR1+/- fish nor in homozygous SUR1-/- fish, mimicking findings in SUR1 knockout mice. Mutant fish did, however, demonstrate impaired glucose tolerance, similar to partial LOF mouse models. In paralleling features of mammalian diabetes and hyperinsulinism resulting from equivalent LOF mutations, these gene-edited animals provide valid zebrafish models of KATP -dependent pancreatic diseases.

Polymorphisms in glucose homeostasis genes are associated with cardiovascular and renal parameters in patients with diabetic nephropathy

BACKGROUND

Diabetic nephropathy (DN) has become the major cause of end-stage kidney disease and is associated to an extremely high cardiovascular (CV) risk.

METHODS

We screened 318 DN patients for 23 SNPs in four glucose transporters (SLC2A1, SLC2A2, SLC5A1 and SLC5A2) and in KCNJ11 and ABCC8, which participate in insulin secretion. Regression models were utilised to identify associations with renal parameters, atherosclerosis measurements and CV events. In addition, 506 individuals with normal renal function were also genotyped as a control group.

RESULTS

In the patient's cohort, common carotid intima media thickness values were higher in carriers of ABCC8 rs3758953 and rs2188966 vs. non-carriers [0.78(0.25) vs. 0.72(0.22) mm, p < 0.05 and 0.79(0.26) vs. 0.72(0.22) mm, p < 0.05], respectively. Furthermore, ABCC8 rs1799859 was linked to presence of plaque in these patients [1.89(1.03-3.46), p < 0.05]. Two variants, SLC2A2 rs8192675 and SLC5A2 rs9924771, were associated with better [OR = 0.49 (0.30-0.81), p < 0.01] and worse [OR = 1.92 (1.15-3.21), p < 0.05] CV event-free survival, respectively. With regard to renal variables, rs841848 and rs710218 in SLC2A1, as well as rs3813008 in SLC5A2, significantly altered estimated glomerular filtration rate values [carriers vs. non-carriers: 30.41(22.57) vs. 28.25(20.10), p < 0.05; 28.95(21.11) vs. 29.52(21.66), p < 0.05 and 32.03(18.06) vs. 28.14(23.06) ml/min/1.73 m², p < 0.05]. In addition, ABCC8 rs3758947 was associated with higher albumin-to-creatinine ratios [193.5(1139.91) vs. 160(652.90) mg/g, p < 0.05]. The epistasis analysis of SNP-pairs interactions showed that ABCC8 rs3758947 interacted with several SNPs in SLC2A2 to significantly affect CV events (p < 0.01). No SNPs were associated with DN risk.

CONCLUSIONS

Polymorphisms in genes determining glucose homeostasis may play a relevant role in renal parameters and CV-related outcomes of DN patients.

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.

Structural Insights Into the High Selectivity of the Anti-Diabetic Drug Mitiglinide

Mitiglinide is a highly selective fast-acting anti-diabetic drug that induces insulin secretion by inhibiting pancreatic K_ATP channels. However, how mitiglinide binds K_ATP channels remains unknown. Here, we show the cryo-EM structure of the SUR1 subunit complexed with mitiglinide. The structure reveals that mitiglinide binds inside the common insulin secretagogue-binding site of SUR1, which is surrounded by TM7, TM8, TM16, and TM17. Mitiglinide locks SUR1 in the NBD-separated inward-facing conformation. The detailed structural analysis of the mitiglinide-binding site uncovers the molecular basis of its high selectivity.

ATP-Sensitive Potassium Channels in Hyperinsulinism and Type 2 Diabetes: Inconvenient Paradox or New Paradigm?

Secretion of insulin from pancreatic β-cells is complex, but physiological glucose-dependent secretion is dominated by electrical activity, in turn controlled by ATP-sensitive potassium (KATP) channel activity. Accordingly, loss-of-function mutations of the KATP channel Kir6.2 (KCNJ11) or SUR1 (ABCC8) subunit increase electrical excitability and secretion, resulting in congenital hyperinsulinism (CHI), whereas gain-of-function mutations cause underexcitability and undersecretion, resulting in neonatal diabetes mellitus (NDM). Thus, diazoxide, which activates KATP channels, and sulfonylureas, which inhibit KATP channels, have dramatically improved therapies for CHI and NDM, respectively. However, key findings do not fit within this simple paradigm: mice with complete absence of β-cell KATP activity are not hyperinsulinemic; instead, they are paradoxically glucose intolerant and prone to diabetes, as are older human CHI patients. Critically, despite these advances, there has been little insight into any role of KATP channel activity changes in the development of type 2 diabetes (T2D). Intriguingly, the CHI progression from hypersecretion to undersecretion actually mirrors the classical response to insulin resistance in the progression of T2D. In seeking to explain the progression of CHI, multiple lines of evidence lead us to propose that underlying mechanisms are also similar and that development of T2D may involve loss of KATP activity.

An Immersible Microgripper for Pancreatic Islet and Organoid Research

To improve the predictive value of in vitro experimentation, the use of 3D cell culture models, or organoids, is becoming increasingly popular. However, the current equipment of life science laboratories has been developed to deal with cell monolayers or cell suspensions. To handle 3D cell aggregates and organoids in a well-controlled manner, without causing structural damage or disturbing the function of interest, new instrumentation is needed. In particular, the precise and stable positioning in a cell bath with flow rates sufficient to characterize the kinetic responses to physiological or pharmacological stimuli can be a demanding task. Here, we present data that demonstrate that microgrippers are well suited to this task. The current version is able to work in aqueous solutions and was shown to position isolated pancreatic islets and 3D aggregates of insulin-secreting MIN6-cells. A stable hold required a gripping force of less than 30 μN and did not affect the cellular integrity. It was maintained even with high flow rates of the bath perfusion, and it was precise enough to permit the simultaneous microfluorimetric measurements and membrane potential measurements of the single cells within the islet through the use of patch-clamp electrodes.

Isoform-specific Roles of Prolyl Hydroxylases in the Regulation of Pancreatic β-Cell Function

Pancreatic β-cells can secrete insulin via 2 pathways characterized as KATP channel -dependent and -independent. The KATP channel-independent pathway is characterized by a rise in several potential metabolic signaling molecules, including the NADPH/NADP+ ratio and α-ketoglutarate (αKG). Prolyl hydroxylases (PHDs), which belong to the αKG-dependent dioxygenase superfamily, are known to regulate the stability of hypoxia-inducible factor α. In the current study, we assess the role of PHDs in vivo using the pharmacological inhibitor dimethyloxalylglycine (DMOG) and generated β-cell-specific knockout (KO) mice for all 3 isoforms of PHD (β-PHD1 KO, β-PHD2 KO, and β-PHD3 KO mice). DMOG inhibited in vivo insulin secretion in response to glucose challenge and inhibited the first phase of insulin secretion but enhanced the second phase of insulin secretion in isolated islets. None of the β-PHD KO mice showed any significant in vivo defects associated with glucose tolerance and insulin resistance except for β-PHD2 KO mice which had significantly increased plasma insulin during a glucose challenge. Islets from both β-PHD1 KO and β-PHD3 KO had elevated β-cell apoptosis and reduced β-cell mass. Isolated islets from β-PHD1 KO and β-PHD3 KO had impaired glucose-stimulated insulin secretion and glucose-stimulated increases in the ATP/ADP and NADPH/NADP+ ratio. All 3 PHD isoforms are expressed in β-cells, with PHD3 showing the most distinct expression pattern. The lack of each PHD protein did not significantly impair in vivo glucose homeostasis. However, β-PHD1 KO and β-PHD3 KO mice had defective β-cell mass and islet insulin secretion, suggesting that these mice may be predisposed to developing diabetes.

Photo-Switchable Sulfonylureas Binding to ATP-Sensitive Potassium Channel Reveal the Mechanism of Light-Controlled Insulin Release

ATP-sensitive potassium (KATP) channels are present in numerous organs, including the heart, brain, and pancreas. Physiological opening and closing of KATPs present in pancreatic β-cells, in response to changes in the ATP/ADP concentration ratio, are correlated with insulin release into the bloodstream. Sulfonylurea drugs, commonly used in type 2 diabetes mellitus treatment, bind to the octamer KATP channels composed of four pore-forming Kir6.2 and four SUR1 subunits and increase the probability of insulin release. Azobenzene-based derivatives of sulfonylureas, such as JB253 inspired by well-established antidiabetic drug glimepiride, allow for control of this process by light. The mechanism of that phenomenon was not known until now. In this paper, we use molecular docking, molecular dynamics, and metadynamics to reveal structural determinants explaining light-controlled insulin release. We show that both trans- and cis-JB253 bind to the same SUR1 cavity as antidiabetic sulfonylurea glibenclamide (GBM). Simulations indicate that, in contrast to trans-JB253, the cis-JB253 structure generated by blue light absorption promotes open structures of SUR1, in close similarity to the GBM effect. We postulate that in the open SUR1 structures, the N-terminal tail from Kir6.2 protruding into the SUR1 pocket is stabilized by flexible enough sulfonylureas. Therefore, the adjacent Kir6.2 pore is more often closed, which in turn facilitates insulin release. Thus, KATP conductance is regulated by peptide linkers between its Kir6.2 and SUR1 subunits, a phenomenon present in other biological signaling pathways. Our data explain the observed light-modulated activity of photoactive sulfonylureas and widen a way to develop new antidiabetic drugs having reduced adverse effects.

Tentonin 3/TMEM150C regulates glucose-stimulated insulin secretion in pancreatic β-cells

Glucose homeostasis is initially regulated by the pancreatic hormone insulin. Glucose-stimulated insulin secretion in β-cells is composed of two cellular mechanisms: a high glucose concentration not only depolarizes the membrane potential of the β-cells by ATP-sensitive K⁺ channels but also induces cell inflation, which is sufficient to release insulin granules. However, the molecular identity of the stretch-activated cation channel responsible for the latter pathway remains unknown. Here, we demonstrate that Tentonin 3/TMEM150C (TTN3), a mechanosensitive channel, contributes to glucose-stimulated insulin secretion by mediating cation influx. TTN3 is expressed specifically in β-cells and mediates cation currents to glucose and hypotonic stimulations. The glucose-induced depolarization, firing activity, and Ca²⁺ influx of β-cells were significantly lower in Ttn3^-/- mice. More importantly, Ttn3^-/- mice show impaired glucose tolerance with decreased insulin secretion in vivo. We propose that TTN3, as a stretch-activated cation channel, contributes to glucose-stimulated insulin secretion.

Novel association between a transient receptor potential cation channel subfamily M member 5 expression quantitative trait locus rs35197079 and decreased susceptibility of gestational diabetes mellitus in a Chinese population

AIMS/INTRODUCTION

Emerging evidence suggests that expression quantitative trait loci (eQTLs) are more likely to associate with complex diseases. Transient receptor potential cation channel subfamily M member 5 (TRPM5) is a ubiquitously expressed voltage-gated cation channel that acts indispensably to trigger insulin secretion in pancreatic β-cells. The present study evaluated the association between TRPM5 eQTL single-nucleotide polymorphisms and the risk of gestational diabetes mellitus (GDM) in a Chinese population.

MATERIALS AND METHODS

A total of 380 unrelated Chinese pregnant women including 241 GDM patients and 139 controls were included in this study. The eQTL single-nucleotide polymorphisms of TRPM5 were obtained from the GTEx eQTL Browser, and were subsequently genotyped using the Agena MassARRAY iPLEX platform.

RESULTS

Logistic regression analysis and linear regression analysis showed that rs35197079 and rs74848824 were significantly associated with reduced GDM risk and lower fasting plasma glucose levels after adjusting confounder factors in dominant genetic models. Stratification analysis based on pre-pregnancy body mass index validated a strong association between rs35197079 and GDM susceptibility in underweight and normal weight individuals. Luciferase and electrophoretic mobility shift assays carried out in rat pancreatic β-cells showed that rs35197079 was functional.

CONCLUSIONS

The TRPM5 eQTL single-nucleotide polymorphism rs35197079 was associated with decreased GDM susceptibility in a Chinese population, especially in underweight and normal weight pregnant women, and it was functional in modulating gene transcription.

Cavβ3 Regulates Ca2+ Signaling and Insulin Expression in Pancreatic β-Cells in a Cell-Autonomous Manner

Voltage-gated Ca²⁺ (Cav) channels consist of a pore-forming Cavα1 subunit and auxiliary Cavα2-δ and Cavβ subunits. In fibroblasts, Cavβ3, independent of its role as a Cav subunit, reduces the sensitivity to low concentrations of inositol-1,4,5-trisphosphate (IP₃). Similarly, Cavβ3 could affect cytosolic calcium concentration ([Ca^{2 +}]) in pancreatic β-cells. In this study, we deleted the Cavβ3-encoding gene Cacnb3 in insulin-secreting rat β-(Ins-1) cells using CRISPR/Cas9. These cells were used as controls to investigate the role of Cavβ3 on Ca²⁺ signaling, glucose-induced insulin secretion (GIIS), Cav channel activity, and gene expression in wild-type cells in which Cavβ3 and the IP₃ receptor were coimmunoprecipitated. Transcript and protein profiling revealed significantly increased levels of insulin transcription factor Mafa, CaMKIV, proprotein convertase subtilisin/kexin type-1, and nitric oxide synthase-1 in Cavβ3-knockout cells. In the absence of Cavβ3, Cav currents were not altered. In contrast, CREB activity, the amount of MAFA protein and GIIS, the extent of IP₃-dependent Ca²⁺ release and the frequency of Ca²⁺ oscillations were increased. These processes were decreased by the Cavβ3 protein in a concentration-dependent manner. Our study shows that Cavβ3 interacts with the IP₃ receptor in isolated β-cells, controls IP₃-dependent Ca²⁺-signaling independently of Cav channel functions, and thereby regulates insulin expression and its glucose-dependent release in a cell-autonomous manner.

Potential Therapeutic Applications of Synthetic Conotoxin s-cal14.2b, Derived from Californiconus californicus, for Treating Type 2 Diabetes

The FDA’s approval of peptide drugs such as Ziconotide or Exendin for pain relief and diabetes treatment, respectively, enhanced the interest to explore novel conotoxins from Conus species venom. In general, conotoxins can be used in pathologies where voltage-gated channels, membrane receptors, or ligands alter normal physiological functions, as in metabolic diseases such as Type 2 diabetes. In this study, the synthetic cal14.2b (s-cal14.2b) from the unusual Californiconus californicus demonstrated bioactivity on NIT-1 insulinoma cell lines stimulating insulin secretion detecting by high performance liquid chromatography (HPLC). Accordingly, s-cal14.2b increased the Ca_V1.2/1.3 channel-current by 35 ± 4% with a recovery τ of 10.3 ± 4 s in primary cell culture of rat pancreatic β-cells. The in vivo results indicated a similar effect of insulin secretion on mice in the glucose tolerance curve model by reducing the glucose from 500 mg/dL to 106 mg/dL in 60 min, compared to the negative control of 325 mg/dL at the same time. The PET-SCAN with radiolabeling 99mTc-s-cal14.2b demonstrated biodistribution and accumulation in rat pancreas with complete depuration in 24 h. These findings show the potential therapeutic use of s-cal14.2b in endocrinal pathologies such as early stages of Type 2 Diabetes where the pancreas’s capability to produce insulin is still effective.

Association of Combined TCF7L2 and KCNQ1 Gene Polymorphisms with Diabetic Micro- and Macrovascular Complications in Type 2 Diabetes Mellitus

BACKGROUND

Vascular complications are the major morbid consequences of type 2 diabetes mellitus (T2DM). The transcription factor 7-like 2 (TCF7L2), potassium voltage-gated channel subfamily Q member 1 (KCNQ1), and inwardly-rectifying potassium channel, subfamily J, member 11 gene (KCNJ11) are common T2DM susceptibility genes in various populations. However, the associations between polymorphisms in these genes and diabetic complications are controversial. This study aimed to investigate the effects of combined gene-polymorphisms within TCF7L2, KCNQ1, and KCNJ11 on vascular complications in Thai subjects with T2DM.

METHODS

We conducted a case-control study comprising 960 T2DM patients and 740 non-diabetes controls. Single nucleotide polymorphisms in TCF7L2, KCNQ1, and KCNJ11 were genotyped and evaluated for their association with diabetic vascular complications.

RESULTS

The gene variants TCF7L2 rs290487-T, KCNQ1 rs2237892-C, and KCNQ1 rs2237897-C were associated with increased risk of T2DM. TCF7L2 rs7903146-C, TCF7L2 rs290487-C, KCNQ1 rs2237892-T, and KCNQ1 rs2237897-T revealed an association with hypertension. The specific combination of risk-alleles that have effects on T2DM and hypertension, TCF7L2 rs7903146-C, KCNQ1 rs2237892-C, and KCNQ1 rs2237897-T, as genetic risk score (GRS), pronounced significant association with coronary artery disease (CAD), cumulative nephropathy and CAD, and cumulative microvascular and macrovascular complications (respective odds ratios [ORs] with 95% confidence interval [95% CI], comparing between GRS 2-3 and GRS 5-6, were 7.31 [2.03 to 26.35], 3.92 [1.75 to 8.76], and 2.33 [1.13 to 4.79]).

CONCLUSION

This study demonstrated, for the first time, the effect conferred by specific combined genetic variants in TCF7L2 and KCNQ1 on diabetic vascular complications, predominantly with nephropathy and CAD. Such a specific pattern of gene variant combination may implicate in the progression of T2DM and life-threatening vascular complications.

New Aspects of Diabetes Research and Therapeutic Development

Both type 1 and type 2 diabetes mellitus are advancing at exponential rates, placing significant burdens on health care networks worldwide. Although traditional pharmacologic therapies such as insulin and oral antidiabetic stalwarts like metformin and the sulfonylureas continue to be used, newer drugs are now on the market targeting novel blood glucose-lowering pathways. Furthermore, exciting new developments in the understanding of beta cell and islet biology are driving the potential for treatments targeting incretin action, islet transplantation with new methods for immunologic protection, and the generation of functional beta cells from stem cells. Here we discuss the mechanistic details underlying past, present, and future diabetes therapies and evaluate their potential to treat and possibly reverse type 1 and 2 diabetes in humans. SIGNIFICANCE STATEMENT: Diabetes mellitus has reached epidemic proportions in the developed and developing world alike. As the last several years have seen many new developments in the field, a new and up to date review of these advances and their careful evaluation will help both clinical and research diabetologists to better understand where the field is currently heading.

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.

Dual Mode of Action of Acetylcholine on Cytosolic Calcium Oscillations in Pancreatic Beta and Acinar Cells In Situ

Cholinergic innervation in the pancreas controls both the release of digestive enzymes to support the intestinal digestion and absorption, as well as insulin release to promote nutrient use in the cells of the body. The effects of muscarinic receptor stimulation are described in detail for endocrine beta cells and exocrine acinar cells separately. Here we describe morphological and functional criteria to separate these two cell types in situ in tissue slices and simultaneously measure their response to ACh stimulation on cytosolic Ca2+ oscillations [Ca2+]c in stimulatory glucose conditions. Our results show that both cell types respond to glucose directly in the concentration range compatible with the glucose transporters they express. The physiological ACh concentration increases the frequency of glucose stimulated [Ca2+]c oscillations in both cell types and synchronizes [Ca2+]c oscillations in acinar cells. The supraphysiological ACh concentration further increases the oscillation frequency on the level of individual beta cells, inhibits the synchronization between these cells, and abolishes oscillatory activity in acinar cells. We discuss possible mechanisms leading to the observed phenomena.

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.

Subtype-dependent regulation of Gβγ signalling

G protein-coupled receptors (GPCRs) transmit information to the cell interior by transducing external signals to heterotrimeric G protein subunits, Gα and Gβγ subunits, localized on the inner leaflet of the plasma membrane. Though the initial focus was mainly on Gα-mediated events, Gβγ subunits were later identified as major contributors to GPCR-G protein signalling. A broad functional array of Gβγ signalling has recently been attributed to Gβ and Gγ subtype diversity, comprising 5 Gβ and 12 Gγ subtypes, respectively. In addition to displaying selectivity towards each other to form the Gβγ dimer, numerous studies have identified preferences of distinct Gβγ combinations for specific GPCRs, Gα subtypes and effector molecules. Importantly, Gβ and Gγ subtype-dependent regulation of downstream effectors, representing a diverse range of signalling pathways and physiological functions have been found. Here, we review the literature on the repercussions of Gβ and Gγ subtype diversity on direct and indirect regulation of GPCR/G protein signalling events and their physiological outcomes. Our discussion additionally provides perspective in understanding the intricacies underlying molecular regulation of subtype-specific roles of Gβγ signalling and associated diseases.

GRK2 contributes to glucose mediated calcium responses and insulin secretion in pancreatic islet cells

Diabetes is a metabolic syndrome rooted in impaired insulin and/or glucagon secretory responses within the pancreatic islets of Langerhans (islets). Insulin secretion is primarily regulated by two key factors: glucose-mediated ATP production and G-protein coupled receptors (GPCRs) signaling. GPCR kinase 2 (GRK2), a key regulator of GPCRs, is reported to be downregulated in the pancreas of spontaneously obesogenic and diabetogenic mice (ob/ob). Moreover, recent studies have shown that GRK2 non-canonically localizes to the cardiac mitochondrion, where it can contribute to glucose metabolism. Thus, islet GRK2 may impact insulin secretion through either mechanism. Utilizing Min6 cells, a pancreatic ß-cell model, we knocked down GRK2 and measured glucose-mediated intracellular calcium responses and insulin secretion. Silencing of GRK2 attenuated calcium responses, which were rescued by pertussis toxin pre-treatment, suggesting a Gαi/o-dependent mechanism. Pancreatic deletion of GRK2 in mice resulted in glucose intolerance with diminished insulin secretion. These differences were due to diminished insulin release rather than decreased insulin content or gross differences in islet architecture. Furthermore, a high fat diet feeding regimen exacerbated the metabolic phenotype in this model. These results suggest a new role for pancreatic islet GRK2 in glucose-mediated insulin responses that is relevant to type 2 diabetes disease progression.

Small subpopulations of β-cells do not drive islet oscillatory [Ca2+] dynamics via gap junction communication

The islets of Langerhans exist as multicellular networks that regulate blood glucose levels. The majority of cells in the islet are excitable, insulin-producing β-cells that are electrically coupled via gap junction channels. β-cells are known to display heterogeneous functionality. However, due to gap junction coupling, β-cells show coordinated [Ca²⁺] oscillations when stimulated with glucose, and global quiescence when unstimulated. Small subpopulations of highly functional β-cells have been suggested to control [Ca²⁺] dynamics across the islet. When these populations were targeted by optogenetic silencing or photoablation, [Ca²⁺] dynamics across the islet were largely disrupted. In this study, we investigated the theoretical basis of these experiments and how small populations can disproportionality control islet [Ca²⁺] dynamics. Using a multicellular islet model, we generated normal, skewed or bimodal distributions of β-cell heterogeneity. We examined how islet [Ca²⁺] dynamics were disrupted when cells were targeted via hyperpolarization or populations were removed; to mimic optogenetic silencing or photoablation, respectively. Targeted cell populations were chosen based on characteristics linked to functional subpopulation, including metabolic rate of glucose oxidation or [Ca²⁺] oscillation frequency. Islets were susceptible to marked suppression of [Ca²⁺] when ~10% of cells with high metabolic activity were hyperpolarized; where hyperpolarizing cells with normal metabolic activity had little effect. However, when highly metabolic cells were removed from the model, [Ca²⁺] oscillations remained. Similarly, when ~10% of cells with either the highest frequency or earliest elevations in [Ca²⁺] were removed from the islet, the [Ca²⁺] oscillation frequency remained largely unchanged. Overall, these results indicate small populations of β-cells with either increased metabolic activity or increased frequency are unable to disproportionately control islet-wide [Ca²⁺] via gap junction coupling. Therefore, we need to reconsider the physiological basis for such small β-cell populations or the mechanism by which they may be acting to control normal islet function.

Many biological systems can be studied using network theory. How heterogeneous cell subpopulations come together to create complex multicellular behavior is of great value in understanding function and dysfunction in tissues. The pancreatic islet of Langerhans is a highly coupled structure that is important for maintaining blood glucose homeostasis. β-cell electrical activity is coordinated via gap junction communication. The function of the insulin-producing β-cell within the islet is disrupted in diabetes. As such, to understand the causes of islet dysfunction we need to understand how different cells within the islet contribute to its overall function via gap junction coupling. Using a computational model of β-cell electrophysiology, we investigated how small highly functional β-cell populations within the islet contribute to its function. We found that when small populations with greater functionality were introduced into the islet, they displayed signatures of this enhanced functionality. However, when these cells were removed, the islet, retained near-normal function. Thus, in a highly coupled system, such as an islet, the heterogeneity of cells allows small subpopulations to be dispensable, and thus their absence is unable to disrupt the larger cellular network. These findings can be applied to other electrical systems that have heterogeneous cell populations.

Therapeutic Potential of Peptides Derived from Animal Venoms: Current Views and Emerging Drugs for Diabetes

The therapeutic potential of venom-derived drugs is evident today. Currently, several significant drugs are FDA approved for human use that descend directly from animal venom products, with others having undergone, or progressing through, clinical trials. In addition, there is growing awareness of the important cosmeceutical application of venom-derived products. The success of venom-derived compounds is linked to their increased bioactivity, specificity and stability when compared to synthetically engineered compounds. This review highlights advancements in venom-derived compounds for the treatment of diabetes and related disorders. Exendin-4, originating from the saliva of Gila monster lizard, represents proof-of-concept for this drug discovery pathway in diabetes. More recent evidence emphasises the potential of venom-derived compounds from bees, cone snails, sea anemones, scorpions, snakes and spiders to effectively manage glycaemic control. Such compounds could represent exciting exploitable scaffolds for future drug discovery in diabetes, as well as providing tools to allow for a better understanding of cell signalling pathways linked to insulin secretion and metabolism.

Physiology of pancreatic β-cells: Ion channels and molecular mechanisms implicated in stimulus-secretion coupling

The human and mouse islet of Langerhans is an endocrine organ composed of five different cells types; insulin-secreting β-cells, glucagon-producing α-cells, somatostatin-producing δ-cells, pancreatic polypeptide-secreting PP cells and ɛ-cells that secretes ghrelin. The most important cells are the pancreatic β-cells that comprise around 45-50% of human islets and 75-80% in the mouse. Pancreatic β-cells secrete insulin at high glucose concentration, thereby finely regulating glycaemia by the hypoglycaemic effects of this hormone. Different ion channels are implicated in the stimulus-secretion coupling of insulin. An increase in the intracellular ATP concentration leads to closure K_ATP channels, depolarizing the cell and opening voltage-gated calcium channels. The increase of intracellular calcium concentration induced by calcium entry through voltage-gated calcium channels promotes insulin secretion. Here, we briefly describe the diversity of ion channels present in pancreatic β-cells and the different mechanisms that are responsible to induce insulin secretion in human and mouse cells. Moreover, we described the pathophysiology due to alterations in the physiology of the main ion channels present in pancreatic β-cell and its implication to predispose metabolic disorders as type 2 diabetes mellitus.

A Parallel Perifusion Slide From Glass for the Functional and Morphological Analysis of Pancreatic Islets

An islet-on-chip system in the form of a completely transparent microscope slide optically accessible from both sides was developed. It is made from laser-structured borosilicate glass and enables the parallel perifusion of five microchannels, each containing one islet precisely immobilized in a pyramidal well. The islets can be in inserted via separate loading windows above each pyramidal well. This design enables a gentle, fast and targeted insertion of the islets and a reliable retention in the well while at the same time permitting a sufficiently fast exchange of the media. In addition to the measurement of the hormone content in the fractionated efflux, parallel live cell imaging of the islet is possible. By programmable movement of the microscopic stage imaging of five wells can be performed. The current chip design ensures sufficient time resolution to characterize typical parameters of stimulus-secretion coupling. This was demonstrated by measuring the reaction of the islets to stimulation by glucose and potassium depolarization. After the perifusion experiment islets can be removed for further analysis. The live-dead assay of the removed islets confirmed that the process of insertion and removal was not detrimental to islet structure and viability. In conclusion, the present islet-on-chip design permits the practical implementation of parallel perifusion experiments on a single and easy to load glass slide. For each immobilized islet the correlation between secretion, signal transduction and morphology is possible. The slide concept allows the scale-up to even higher degrees of parallelization.

Preferential Gq signaling in diabetes: an electrical switch in incretin action and in diabetes progression?

Patients with type 2 diabetes (T2D) fail to secrete insulin in response to increased glucose levels that occur with eating. Glucagon-like peptide-1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP) are two incretins secreted from gastrointestinal cells that amplify insulin secretion when glucose is high. In this issue of the JCI, Oduori et al. explore the role of ATP-sensitive K+ (KATP) channels in maintaining glucose homeostasis. In persistently depolarized β cells from KATP channel knockout (KO) mice, the researchers revealed a shift in G protein signaling from the Gs family to the Gq family. This shift explains why GLP-1, which signals via Gq, but not GIP, which signals preferentially via Gs, can effectively potentiate secretion in islets from the KATP channel-deficient mice and in other models of KATP deficiency, including diabetic KK-Ay mice. Their results provide one explanation for differential insulinotropic potential of incretins in human T2D and point to a potentially unifying model for T2D progression itself.

Regulation of Postabsorptive and Postprandial Glucose Metabolism by Insulin-Dependent and Insulin-Independent Mechanisms: An Integrative Approach

Glucose levels in blood must be constantly maintained within a tight physiological range to sustain anabolism. Insulin regulates glucose homeostasis via its effects on glucose production from the liver and kidneys and glucose disposal in peripheral tissues (mainly skeletal muscle). Blood levels of glucose are regulated simultaneously by insulin-mediated rates of glucose production from the liver (and kidneys) and removal from muscle; adipose tissue is a key partner in this scenario, providing nonesterified fatty acids (NEFA) as an alternative fuel for skeletal muscle and liver when blood glucose levels are depleted. During sleep at night, the gradual development of insulin resistance, due to growth hormone and cortisol surges, ensures that blood glucose levels will be maintained within normal levels by: (a) switching from glucose to NEFA oxidation in muscle; (b) modulating glucose production from the liver/kidneys. After meals, several mechanisms (sequence/composition of meals, gastric emptying/intestinal glucose absorption, gastrointestinal hormones, hyperglycemia mass action effects, insulin/glucagon secretion/action, de novo lipogenesis and glucose disposal) operate in concert for optimal regulation of postprandial glucose fluctuations. The contribution of the liver in postprandial glucose homeostasis is critical. The liver is preferentially used to dispose over 50% of the ingested glucose and restrict the acute increases of glucose and insulin in the bloodstream after meals, thus protecting the circulation and tissues from the adverse effects of marked hyperglycemia and hyperinsulinemia.

Cellular and systemic mechanisms for glucose sensing and homeostasis

Glucose is a major source of energy in animals. Maintaining blood glucose levels within a physiological range is important for facilitating glucose uptake by cells, as required for optimal functioning. Glucose homeostasis relies on multiple glucose-sensing cells in the body that constantly monitor blood glucose levels and respond accordingly to adjust its glycemia. These include not only pancreatic β-cells and α-cells that secrete insulin and glucagon, but also central and peripheral neurons regulating pancreatic endocrine function. Different types of cells respond distinctively to changes in blood glucose levels, and the mechanisms involved in glucose sensing are diverse. Notably, recent studies have challenged the currently held views regarding glucose-sensing mechanisms. Furthermore, peripheral and central glucose-sensing cells appear to work in concert to control blood glucose level and maintain glucose and energy homeostasis in organisms. In this review, we summarize the established concepts and recent advances in the understanding of cellular and systemic mechanisms that regulate glucose sensing and its homeostasis.

Pharmacogenetics of Type 2 Diabetes-Progress and Prospects

Type 2 diabetes mellitus (T2D) is a chronic metabolic disease resulting from insulin resistance and progressively reduced insulin secretion, which leads to impaired glucose utilization, dyslipidemia and hyperinsulinemia and progressive pancreatic beta cell dysfunction. The incidence of type 2 diabetes mellitus is increasing worldwide and nowadays T2D already became a global epidemic. The well-known interindividual variability of T2D drug actions such as biguanides, sulfonylureas/meglitinides, DPP-4 inhibitors/GLP1R agonists and SGLT-2 inhibitors may be caused, among other things, by genetic factors. Pharmacogenetic findings may aid in identifying new drug targets and obtaining in-depth knowledge of the causes of disease and its physiological processes, thereby, providing an opportunity to elaborate an algorithm for tailor or precision treatment. The aim of this article is to summarize recent progress and discoveries for T2D pharmacogenetics and to discuss the factors which limit the furthering accumulation of genetic variability knowledge in patient response to therapy that will allow improvement the personalized treatment of T2D.

The role of α-ketoglutarate and the hypoxia sensing pathway in the regulation of pancreatic β-cell function

Anaplerosis and the associated mitochondrial metabolite transporters generate unique cytosolic metabolic signaling molecules that can regulate insulin release from pancreatic β-cells. It has been shown that mitochondrial metabolites, transported by the citrate carrier (CIC), dicarboxylate carrier (DIC), oxoglutarate carrier (OGC), and mitochondrial pyruvate carrier (MPC) play a vital role in the regulation of glucose-stimulated insulin secretion (GSIS). Metabolomic studies on static and biphasic insulin secretion, suggests that several anaplerotic derived metabolites, including α-ketoglutarate (αKG), are strongly associated with nutrient regulated insulin secretion. Support for a role of αKG in the regulation of insulin secretion comes from studies looking at αKG dependent enzymes, including hypoxia-inducible factor-prolyl hydroxylases (PHDs) in clonal β-cells, and rodent and human islets. This review will focus on the possible link between defective anaplerotic-derived αKG, PHDs, and the development of type 2 diabetes (T2D).

Chemical Biology Toolbox for Studying Pancreatic Islet Function - A Perspective

The islets of Langerhans represent one of the many complex endocrine organs in mammals. Traditionally, islet function is studied by a mixture of physiological, cell biological, and molecular biological methods. Recently, novel techniques stemming from the ever-increasing toolbox provided by chemical laboratories have been added to the repertoire. Many emerging techniques will soon be available to manipulate and monitor islet function at the single-cell level and potentially in intact model animals, as well as in isolated human islets. Here, we review the most current small-molecule-based and genetically encoded molecular tool sets available to study islet function. We provide an outlook regarding future tool developments that will impact islet research, with a special focus on the interplay between different islet cell types.

Adaptive short-term associative conditioning in the pancreatic β-cell

This study associates cholinergic stimulation of the pancreatic β-cell electrical activity with a short-term memory phenomenon. Glucose pulses applied to a basal glucose concentration induce depolarizing waves which are used to estimate the evolution of the β-cell glucose sensitivity. Exposure to carbamoylcholine (carbachol) increases the size of the glucose-induced depolarizing waves. This change appears after carbachol withdrawal and implies a temporal potentiation of sensitivity (TPS) lasting up to one hour. TPS induction requires the simultaneous action of carbachol and glucose. The substitution of glucose with the secretagogues glyceraldehyde or 2-ketoisocaproate mimics glucose-induced TPS, while palmitate does not. TPS is not produced if the membrane is kept hyperpolarized by diazoxide. Glucose can be replaced by tolbutamide, suggesting a role of depolarization and a subsequent increase in intracellular calcium concentration. A role for kinases is suggested because staurosporine prevents TPS induction. Cycloheximide does not impair TPS induction, indicating that de novo protein synthesis is not required. The fact that the two inputs acting simultaneously produce an effect that lasts up to one hour without requiring de novo protein synthesis suggests that TPS constitutes a case of short-term associative conditioning in non-neural tissue. The convergence of basal glucose levels and muscarinic activation happens physiologically during the cephalic phase of digestion, in order to later absorb incoming fuels. Our data reveals that the role of the cephalic phase may be extended, increasing nutrient sensitivity during meals while remaining low between them.

Chloride transporters and channels in β-cell physiology: revisiting a 40-year-old model

It is accepted that insulin-secreting β-cells release insulin in response to glucose even in the absence of functional ATP-sensitive K+ (KATP)-channels, which play a central role in a 'consensus model' of secretion broadly accepted and widely reproduced in textbooks. A major shortcoming of this consensus model is that it ignores any and all anionic mechanisms, known for more than 40 years, to modulate β-cell electrical activity and therefore insulin secretion. It is now clear that, in addition to metabolically regulated KATP-channels, β-cells are equipped with volume-regulated anion (Cl-) channels (VRAC) responsive to glucose concentrations in the range known to promote electrical activity and insulin secretion. In this context, the electrogenic efflux of Cl- through VRAC and other Cl- channels known to be expressed in β-cells results in depolarization because of an outwardly directed Cl- gradient established, maintained and regulated by the balance between Cl- transporters and channels. This review will provide a succinct historical perspective on the development of a complex hypothesis: Cl- transporters and channels modulate insulin secretion in response to nutrients.

Toxicity of Metformin and Hypoglycemic Therapies

Metformin along with other antidiabetic medications provide benefit to patients in the treatment of type 2 diabetes mellitus, but caution is advised in certain scenarios to avoid toxicity in kidney disease. Renal dosing, monitoring of kidney function, and evaluating the risk of developing serious side effects are warranted with some agents. The available literature with regard to incidence of adverse events and toxicity of hypoglycemic therapies is reviewed.

Cooperative maintenance of cellular identity in systems with intercellular communication defects

The cooperative dynamics of cellular populations emerging from the underlying interactions determines cellular functions and thereby their identity in tissues. Global deviations from this dynamics, on the other hand, reflect pathological conditions. However, how these conditions are stabilized from dysregulation on the level of the single entities is still unclear. Here, we tackle this question using the generic Hodgkin-Huxley type of models that describe physiological bursting dynamics of pancreatic β-cells and introduce channel dysfunction to mimic pathological silent dynamics. The probability for pathological behavior in β-cell populations is ∼100% when all cells have these defects, despite the negligible size of the silent state basin of attraction for single cells. In stark contrast, in a more realistic scenario for a mixed population, stabilization of the pathological state depends on the size of the subpopulation which acquired the defects. However, the probability to exhibit stable pathological dynamics in this case is less than 10%. These results, therefore, suggest that the physiological bursting dynamics of a population of β-cells is cooperatively maintained, even under intercellular communication defects induced by dysfunctional channels of single cells.

Death versus dedifferentiation: The molecular bases of beta cell mass reduction in type 2 diabetes

Diabetes Mellitus is currently affecting more than 425 million people worldwide, among which over 90 % of the cases belong to type 2 diabetes. The number is growing quickly every year. Together with its many complications, the disease is causing tremendous social and economic burden and is classified as one of the leading causes of high morbidity and mortality rate. Residing in the islets of Langerhans, pancreatic beta cell serves as a central mediator in glucose homeostasis through secreting insulin, the only hormone that could reduce glucose level in the body, into the blood. Abnormality in pancreatic beta cell is generally considered as the fundamental reason which is responsible for the development of diabetes. Evidence shows that beta cell mass is greatly reduced in the biopsy of type 2 diabetic patients. Since then, large amount of research was conducted in order to decipher the molecular mechanisms behind the phenotype above and enormous progression has been made. The aim of this review is to summarize and provide a rudimentary molecular road map for beta cell mass reduction from the aspects of apoptosis and dedifferentiation based on recent research advances. Hopefully, this review could give the community some enlightenment for the next-step research and, more importantly, could provide avenues for developing novel and effective therapies to restrain or reverse beta cell loss in type 2 diabetes in the clinic.