cAMP signalling in insulin and glucagon secretion

Integrated human pseudoislet system and microfluidic platform demonstrate differences in GPCR signaling in islet cells

Pancreatic islets secrete insulin from β cells and glucagon from α cells, and dysregulated secretion of these hormones is a central component of diabetes. Thus, an improved understanding of the pathways governing coordinated β and α cell hormone secretion will provide insight into islet dysfunction in diabetes. However, the 3D multicellular islet architecture, essential for coordinated islet function, presents experimental challenges for mechanistic studies of intracellular signaling pathways in primary islet cells. Here, we developed an integrated approach to study the function of primary human islet cells using genetically modified pseudoislets that resemble native islets across multiple parameters. Further, we developed a microperifusion system that allowed synchronous acquisition of GCaMP6f biosensor signal and hormone secretory profiles. We demonstrate the utility of this experimental approach by studying the effects of Gi and Gq GPCR pathways on insulin and glucagon secretion by expressing the designer receptors exclusively activated by designer drugs (DREADDs) hM4Di or hM3Dq. Activation of Gi signaling reduced insulin and glucagon secretion, while activation of Gq signaling stimulated glucagon secretion but had both stimulatory and inhibitory effects on insulin secretion, which occur through changes in intracellular Ca2+. The experimental approach of combining pseudoislets with a microfluidic system allowed the coregistration of intracellular signaling dynamics and hormone secretion and demonstrated differences in GPCR signaling pathways between human β and α cells.

Reduced somatostatin signalling leads to hypersecretion of glucagon in mice fed a high-fat diet

Objectives

Elevated plasma glucagon is an early symptom of diabetes, occurring in subjects with impaired glucose regulation. Here, we explored alpha-cell function in female mice fed a high-fat diet (HFD).

Methods

Female mice expressing the Ca²⁺ indicator GCaMP3 specifically in alpha-cells were fed a high-fat or control (CTL) diet. We then conducted in vivo phenotyping of these mice, as well as experiments on isolated (ex vivo) islets and in the in situ perfused pancreas.

Results

In HFD-fed mice, fed plasma glucagon levels were increased and glucagon secretion from isolated islets and in the perfused mouse pancreas was also elevated. In mice fed a CTL diet, increasing glucose reduced intracellular Ca²⁺ ([Ca²⁺]_i) oscillation frequency and amplitude. This effect was also observed in HFD mice; however, both the frequency and amplitude of the [Ca²⁺]_i oscillations were higher than those in CTL alpha-cells. Given that alpha-cells are under strong paracrine control from neighbouring somatostatin-secreting delta-cells, we hypothesised that this elevation of alpha-cell output was due to a lack of somatostatin (SST) secretion. Indeed, SST secretion in isolated islets from HFD-fed mice was reduced but exogenous SST also failed to suppress glucagon secretion and [Ca²⁺]_i activity from HFD alpha-cells, in contrast to observations in CTL mice.

Conclusions

These findings suggest that reduced delta-cell function, combined with intrinsic changes in alpha-cells including sensitivity to somatostatin, accounts for the hyperglucagonaemia in mice fed a HFD.

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HFD feeding causes hyperglucagonaemia in vivo.

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Glucagon is inadequately suppressed by glucose in HFD-fed mice.

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Alpha-cell [Ca²⁺]_i oscillations and glucagon output are elevated ex vivo in response to HFD feeding.

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SST secretion from HFD islets is reduced.

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Alpha-cells from HFD-fed mice become ‘resistant’ to SST.

Clinical significance of neutrophil-lymphocyte ratio and monocyte-lymphocyte ratio in women with hyperglycemia

Objective: Abnormal pro-inflammatory regulation of the immune system might contribute to the pathogenesis of hyperglycemia during pregnancy. We examined the correlations of neutrophil-lymphocyte ratio (NLR) and monocyte-lymphocyte ratio (MLR) with disease severity and assessed their predictive values. Methods: This retrospective case-control study included 311 cases of hyperglycemia first detected during pregnancy (HFDP) [153 with gestational diabetes mellitus (GDM) and 158 with diabetes in pregnancy (DIP)] and, as a control group, 172 pregnant women with normal glucose tolerance. The NLRs and MLRs were calculated from the blood test data. Results: The absolute leukocyte, neutrophil, monocyte, and lymphocyte counts as well as the NLR and MLR values of HFDP patients significantly differed from control values, but no significant differences were detected in the leukocyte, neutrophil, and monocyte counts of the GDM and DIP groups. Significantly different metrics were selected, binary analysis performed, and odds ratios calculated to identify risk factors. Age, BMI, NLR, and MLR were found to be risk factors for HFDP, and high systolic blood pressure (SBP) at triage and MLR related to the occurrence of DIP. Receiver operating characteristics curve analysis showed that NLR and MLR had better diagnostic accuracy in distinguishing HFDP from controls [NLR area under the curve (AUC) = 0.78; MLR AUC = 0.72] than age and BMI. Values for NLR > 4.394 or MLR > 0.309 correlated with the severity of maternal clinical symptoms and perinatal infant outcomes. MLR was the best predictor of DIP (AUC = 0.72) and MLR values > 0.299 could identify patients at risk for developing DIP and having poor fetal outcomes. Conclusion: Metrics derived from peripheral blood neutrophil, monocyte, and lymphocyte counts are thought to reflect systemic immune-inflammation. Elevated MLR and NLR may be unfavorable prognostic factors for clinical outcomes in patients with hyperglycemia during pregnancy.

A Nutrient-Sensing Transition at Birth Triggers Glucose-Responsive Insulin Secretion

A drastic transition at birth, from constant maternal nutrient supply in utero to intermittent postnatal feeding, requires changes in the metabolic system of the neonate. Despite their central role in metabolic homeostasis, little is known about how pancreatic β cells adjust to the new nutritional challenge. Here, we find that after birth β cell function shifts from amino acid- to glucose-stimulated insulin secretion in correlation with the change in the nutritional environment. This adaptation is mediated by a transition in nutrient sensitivity of the mTORC1 pathway, which leads to intermittent mTORC1 activity. Disrupting nutrient sensitivity of mTORC1 in mature β cells reverts insulin secretion to a functionally immature state. Finally, manipulating nutrient sensitivity of mTORC1 in stem cell-derived β cells in vitro strongly enhances their glucose-responsive insulin secretion. These results reveal a mechanism by which nutrients regulate β cell function, thereby enabling a metabolic adaptation for the newborn.

PIE-FLIM Measurements of Two Different FRET-Based Biosensor Activities in the Same Living Cells

We report the use of pulsed interleaved excitation (PIE)-fluorescence lifetime imaging microscopy (FLIM) to measure the activities of two different biosensor probes simultaneously in single living cells. Many genetically encoded biosensors rely on the measurement of Förster resonance energy transfer (FRET) to detect changes in biosensor conformation that accompany the targeted cell signaling event. One of the most robust ways of quantifying FRET is to measure changes in the fluorescence lifetime of the donor fluorophore using FLIM. The study of complex signaling networks in living cells demands the ability to track more than one of these cellular events at the same time. Here, we demonstrate how PIE-FLIM can separate and quantify the signals from different FRET-based biosensors to simultaneously measure changes in the activity of two cell signaling pathways in the same living cells in tissues. The imaging system described here uses selectable laser wavelengths and synchronized detection gating that can be tailored and optimized for each FRET pair. Proof-of-principle studies showing simultaneous measurement of cytosolic calcium and protein kinase A activity are shown, but the PIE-FLIM approach is broadly applicable to other signaling pathways.

Potential role of adenylyl cyclase 8 signaling complexes in regulating insulin secretion from pancreatic beta cells

Glucose-stimulated insulin secretion from pancreatic β cells is mediated by Ca²⁺ influx and amplified by stimulation of GLP-1-receptors through cAMP-based signaling pathways. Interestingly, it has been found that glucose-induced Ca²⁺ signals can induce concurrent adenylyl cyclase isoform 8 (AC8)-mediated cAMP signals and, conversely, that GLP-1-receptor-mediated cAMP signals are able to induce Ca²⁺ signals. In this review, we explore the signaling complexes revolving around AC8 in modulating insulin release, from the initial discovery of the importance of this AC isoform to recent investigations of its interacting molecular partners. We suggest that investigating the structural assembly of the proteins associated with AC8 in β cells might reveal how this particular protein complex could be targeted to modify insulin secretion. Specifically, we suggest that disrupting the protein-protein interaction between A-kinase anchoring protein 79 (AKAP79) and AC8 could lead to disinhibition of AC8 activity and increased insulin secretion. Potentially, AC8 protein interactions could become a future target in type 2 diabetic patients with dysfunction of insulin secretion.

Involvement of the Adhesion GPCRs Latrophilins in the Regulation of Insulin Release

Insulin secretion from pancreatic β cells is a highly complex and tightly regulated process. Its dysregulation is one characteristic of type 2 diabetes, and thus, an in-depth understanding of the mechanisms controlling insulin secretion is essential for rational therapeutic intervention. G-protein-coupled receptors (GPCRs) have been established as major regulators of insulin exocytosis. Recent studies also suggest the involvement of adhesion GPCRs, a non-prototypical class of GPCRs. Here, we identify latrophilins, which belong to the class of adhesion GPCRs, to be highly expressed in different cell types of pancreatic islets. In vitro and ex vivo analyses show that distinct splice variants of the latrophilin LPHN3/ADGRL3 decrease insulin secretion from pancreatic β cells by reducing intracellular cyclic AMP levels via the G_i-mediated pathway. Our data highlight the key role of LPHN3 in modulating insulin secretion and its potential as therapeutic target for type 2 diabetes.

Molecular and genetic regulation of pig pancreatic islet cell development

Reliance on rodents for understanding pancreatic genetics, development and islet function could limit progress in developing interventions for human diseases such as diabetes mellitus. Similarities of pancreas morphology and function suggest that porcine and human pancreas developmental biology may have useful homologies. However, little is known about pig pancreas development. To fill this knowledge gap, we investigated fetal and neonatal pig pancreas at multiple, crucial developmental stages using modern experimental approaches. Purification of islet β-, α- and δ-cells followed by transcriptome analysis (RNA-seq) and immunohistology identified cell- and stage-specific regulation, and revealed that pig and human islet cells share characteristic features that are not observed in mice. Morphometric analysis also revealed endocrine cell allocation and architectural similarities between pig and human islets. Our analysis unveiled scores of signaling pathways linked to native islet β-cell functional maturation, including evidence of fetal α-cell GLP-1 production and signaling to β-cells. Thus, the findings and resources detailed here show how pig pancreatic islet studies complement other systems for understanding the developmental programs that generate functional islet cells, and that are relevant to human pancreatic diseases.

GPCRs, G Proteins, and Their Impact on β-cell Function

Glucose-induced (physiological) insulin secretion from the islet β-cell involves interplay between cationic (i.e., changes in intracellular calcium) and metabolic (i.e., generation of hydrophobic and hydrophilic second messengers) events. A large body of evidence affirms support for novel regulation, by G proteins, of specific intracellular signaling events, including actin cytoskeletal remodeling, transport of insulin-containing granules to the plasma membrane for fusion, and secretion of insulin into the circulation. This article highlights the following aspects of GPCR-G protein biology of the islet. First, it overviews our current understanding of the identity of a wide variety of G protein regulators and their modulatory roles in GPCR-G protein-effector coupling, which is requisite for optimal β-cell function under physiological conditions. Second, it describes evidence in support of novel, noncanonical, GPCR-independent mechanisms of activation of G proteins in the islet. Third, it highlights the evidence indicating that abnormalities in G protein function lead to islet β-cell dysregulation and demise under the duress of metabolic stress and diabetes. Fourth, it summarizes observations of potential beneficial effects of GPCR agonists in preventing/halting metabolic defects in the islet β-cell under various pathological conditions (e.g., metabolic stress and inflammation). Lastly, it identifies knowledge gaps and potential avenues for future research in this evolving field of translational islet biology. Published 2020. Compr Physiol 10:453-490, 2020.

Pancreatic α-cells - The unsung heroes in islet function

The pancreatic islets of Langerhans consist of several hormone-secreting cell types important for blood glucose control. The insulin secreting β-cells are the best studied of these cell types, but less is known about the glucagon secreting α-cells. The α-cells secrete glucagon as a response to low blood glucose. The major function of glucagon is to release glucose from the glycogen stores in the liver. In both type 1 and type 2 diabetes, glucagon secretion is dysregulated further exaggerating the hyperglycaemia, and in type 1 diabetes α-cells fail to counter regulate hypoglycaemia. Although glucagon has been recognized for almost 100 years, the understanding of how glucagon secretion is regulated and how glucagon act within the islet is far from complete. However, α-cell research has taken off lately which is promising for future knowledge. In this review we aim to highlight α-cell regulation and glucagon secretion with a special focus on recent discoveries from human islets. We will present some novel aspects of glucagon function and effects of selected glucose lowering agents on glucagon secretion.

D2-Like Receptors Mediate Dopamine-Inhibited Insulin Secretion via Ion Channels in Rat Pancreatic β-Cells

Dopamine (DA) has a vital role in the central nervous system and also modulates lipid and glucose metabolism. The present study aimed to investigate the effect of dopamine on insulin secretion and the underlying mechanisms in rat pancreatic β-cells. Data from the radioimmunoassay indicated that dopamine inhibited insulin secretion in a glucose- and dose-dependent manner. This inhibitory effect of dopamine was mediated mainly by D2-like receptors, but not D1-like receptors. Whole-cell patch-clamp recordings showed that dopamine decreased voltage-dependent Ca²⁺ channel currents, which could be reversed by inhibition of the D2-like receptor. Dopamine increased voltage-dependent potassium (K_V) channel currents and shortened action potential duration, which was antagonized by inhibition of D2-like receptors. Further experiments showed that D2-like receptor activation by quinpirole increased K_V channel currents. In addition, using calcium imaging techniques, we found that dopamine reduced intracellular Ca²⁺ concentration, which was also reversed by D2-like receptor antagonists. Similarly, quinpirole was found to decrease intracellular Ca²⁺ levels. Taken together, these findings demonstrate that dopamine inhibits insulin secretion mainly by acting on D2-like receptors, inhibiting Ca²⁺ channels, and activating Kv channels. This process results in shortened action potential duration and decreased intracellular Ca²⁺ levels in β-cells. This work offers new insights into a glucose-dependent mechanism whereby dopamine regulates insulin secretion.

A Membrane Permeable Prodrug of S223 for Selective Epac2 Activation in Living Cells

Signalling by cyclic adenosine monophosphate (cAMP) occurs via various effector proteins, notably protein kinase A and the guanine nucleotide exchange factors Epac1 and Epac2. These proteins are activated by cAMP binding to conserved cyclic nucleotide binding domains. The specific roles of the effector proteins in various processes in different types of cells are still not well defined, but investigations have been facilitated by the development of cyclic nucleotide analogues with distinct selectivity profiles towards a single effector protein. A remaining challenge in the development of such analogues is the poor membrane permeability of nucleotides, which limits their applicability in intact living cells. Here, we report the synthesis and characterisation of S223-AM, a cAMP analogue designed as an acetoxymethyl ester prodrug to overcome limitations of permeability. Using total internal reflection imaging with various fluorescent reporters, we show that S223-AM selectively activates Epac2, but not Epac1 or protein kinase A, in intact insulin-secreting β-cells, and that this effect was associated with pronounced activation of the small G-protein Rap. A comparison of the effects of different cAMP analogues in pancreatic islet cells deficient in Epac1 and Epac2 demonstrates that cAMP-dependent Rap activity at the β-cell plasma membrane is exclusively dependent on Epac2. With its excellent selectivity and permeability properties, S223-AM should get broad utility in investigations of cAMP effector involvement in many different types of cells.

Recent Advances in EPAC-Targeted Therapies: A Biophysical Perspective

The universal second messenger cAMP regulates diverse intracellular processes by interacting with ubiquitously expressed proteins, such as Protein Kinase A (PKA) and the Exchange Protein directly Activated by cAMP (EPAC). EPAC is implicated in multiple pathologies, thus several EPAC-specific inhibitors have been identified in recent years. However, the mechanisms and molecular interactions underlying the EPAC inhibition elicited by such compounds are still poorly understood. Additionally, being hydrophobic low molecular weight species, EPAC-specific inhibitors are prone to forming colloidal aggregates, which result in non-specific aggregation-based inhibition (ABI) in aqueous systems. Here, we review from a biophysical perspective the molecular basis of the specific and non-specific interactions of two EPAC antagonists—CE3F4R, a non-competitive inhibitor, and ESI-09, a competitive inhibitor of EPAC. Additionally, we discuss the value of common ABI attenuators (e.g., TX and HSA) to reduce false positives at the expense of introducing false negatives when screening aggregation-prone compounds. We hope this review provides the EPAC community effective criteria to evaluate similar compounds, aiding in the optimization of existing drug leads, and informing the development of the next generation of EPAC-specific inhibitors.

Neurotrophic and neuroprotective effects of a monomeric GLP-1/GIP/Gcg receptor triagonist in cellular and rodent models of mild traumatic brain injury

A synthetic monomeric peptide triple receptor agonist, termed "Triagonist" that incorporates glucagon-like peptide-1 (GLP-1), glucose-dependent insulinotropic polypeptide (GIP) and glucagon (Gcg) actions, was previously developed to improve upon metabolic and glucose regulatory benefits of single and dual receptor agonists in rodent models of diet-induced obesity and type 2 diabetes. In the current study, the neurotrophic and neuroprotective actions of this Triagonist were probed in cellular and mouse models of mild traumatic brain injury (mTBI), a prevalent cause of neurodegeneration in both the young and elderly. Triagonist dose- and time-dependently elevated cyclic AMP levels in cultured human SH-SY5Y neuronal cells, and induced neurotrophic and neuroprotective actions, mitigating oxidative stress and glutamate excitotoxicity. These actions were inhibited only by the co-administration of antagonists for all three receptor types, indicating the balanced co-involvement of GLP-1, GIP and Gcg receptors. To evaluate physiological relevance, a clinically translatable dose of Triagonist was administered subcutaneously, once daily for 7 days, to mice following a 30 g weight drop close head injury. Triagonist fully mitigated mTBI-induced visual and spatial memory deficits, evaluated at 7 and 30 days post injury. These results establish Triagonist as a novel neurotrophic/protective agent worthy of further evaluation as a TBI treatment strategy.

Amelioration of Diabetes in a Murine Model upon Transplantation of Pancreatic β-Cells with Optogenetic Control of Cyclic Adenosine Monophosphate

Pharmacological augmentation of glucose-stimulated insulin secretion (GSIS), for example, to overcome insulin resistance in type 2 diabetes is linked to suboptimal regulation of blood sugar. Cultured β-cells and islets expressing a photoactivatable adenylyl cyclase (PAC) are amenable to GSIS potentiation with light. However, whether PAC-mediated enhancement of GSIS can improve the diabetic state remains unknown. To this end, β-cells were engineered with stable PAC expression that led to over 2-fold greater GSIS upon exposure to blue light while there were no changes in the absence of glucose. Moreover, the rate of oxygen consumption was unaltered despite the photoinduced elevation of GSIS. Transplantation of these cells into streptozotocin-treated mice resulted in improved glucose tolerance, lower hyperglycemia, and higher plasma insulin when subjected to illumination. Embedding optogenetic networks in β-cells for physiologically relevant control of GSIS will enable novel solutions potentially overcoming the shortcomings of current treatments for diabetes.

Heterogeneous alpha-cell population modeling of glucose-induced inhibition of electrical activity

Glucagon release from the pancreatic alpha-cells is regulated by glucose, but the underlying mechanisms are far from understood. It is known that the alpha-cell population is very heterogeneous, but - compared to the insulin-secreting beta-cells - the consequences of this cell-to-cell variation are much less studied. Since the alpha-cells are not electrically coupled, large differences in the single cell responses are to be expected, and this variation may contribute to the confusion regarding the mechanisms of glucose-induced suppression of glucagon release. Using mathematical modeling of alpha-cells with realistic cell-to-cell parameter variation based on recent experimental results, we show that the simulated alpha-cells exhibit great diversity in their electrophysiological behavior. To robustly reproduce experimental recordings from alpha-cell exposed to a rise in glucose levels, we must assume that both intrinsic mechanisms and paracrine signals contribute to glucose-induced changes in electrical activity. Our simulations suggest that the sum of different electrophysiological responses due to alpha-cell heterogeneity is involved in glucose-suppressed glucagon secretion, and that more than one mechanism contribute to control the alpha-cell populations' behavior. Finally, we apply regression analysis to our synthetic alpha-cell population to infer which membrane currents influence electrical activity in alpha-cells at different glucose levels. The results from such statistical modeling suggest possible disturbances underlying defect regulation of alpha-cell electrical behavior in diabetics. Thus, although alpha-cells appear to be inherently complex and heterogeneous as reflected in published data, realistic modeling of the alpha-cells at the population level provides insight into the mechanisms of glucagon release.

New Insights into Beta-Cell GLP-1 Receptor and cAMP Signaling

Harnessing the translational potential of the GLP-1/GLP-1R system in pancreatic beta cells has led to the development of established GLP-1R-based therapies for the long-term preservation of beta cell function. In this review, we discuss recent advances in the current research on the GLP-1/GLP-1R system in beta cells, including the regulation of signaling by endocytic trafficking as well as the application of concepts such as signal bias, allosteric modulation, dual agonism, polymorphic receptor variants, spatial compartmentalization of cAMP signaling and new downstream signaling targets involved in the control of beta cell function.

Glucose, adrenaline and palmitate antagonistically regulate insulin and glucagon secretion in human pseudoislets

Isolated human islets do not always meet the quality standards required for transplant survival and reliable functional in vitro studies. The formation of pseudoislets, i.e. the reaggregation of a defined number of islet cells after dissociation, improves insulin secretion. We present a simple method of pseudoislet formation from human islet cells and assess the transcriptome and function of isolated human islets and pseudoislets from the same organ donors. Following pseudoislet formation, insulin content/DNA and mRNA/RPS13 resembled that of islets. In pseudoislets, glucose-stimulated insulin secretion (GSIS) was significantly higher (8–13-fold) than in islets (2–4-fold). GSIS of pseudoislets was partly inhibited by the glucagon-like peptide-1 receptor (GLP-1R) antagonist exendin-9. The stimulatory effects of palmitate and forskolin at 12 mM glucose were also significantly higher in pseudoislets than in islets. Further analysis of pseudoislets revealed that regulation of secretion and insulin and glucagon content was maintained over a longer culture period (6–14 d). While adrenaline inhibited GSIS, adrenaline together with palmitate stimulated glucagon secretion 2-fold at low glucose, an effect suppressed by high glucose. Transcriptome analysis revealed that, unlike islets, pseudoislets were deprived of exocrine and endothelial cells. In conclusion, pseudoislet formation restores functional integrity of human islet cells and allows long-term in vitro testing.

Insulin Release Mechanism Modulated by Toxins Isolated from Animal Venoms: From Basic Research to Drug Development Prospects

Venom from mammals, amphibians, snakes, arachnids, sea anemones and insects provides diverse sources of peptides with different potential medical applications. Several of these peptides have already been converted into drugs and some are still in the clinical phase. Diabetes type 2 is one of the diseases with the highest mortality rate worldwide, requiring specific attention. Diverse drugs are available (e.g., Sulfonylureas) for effective treatment, but with several adverse secondary effects, most of them related to the low specificity of these compounds to the target. In this context, the search for specific and high-affinity compounds for the management of this metabolic disease is growing. Toxins isolated from animal venom have high specificity and affinity for different molecular targets, of which the most important are ion channels. This review will present an overview about the electrical activity of the ion channels present in pancreatic β cells that are involved in the insulin secretion process, in addition to the diversity of peptides that can interact and modulate the electrical activity of pancreatic β cells. The importance of prospecting bioactive peptides for therapeutic use is also reinforced.

Incretin Mimetics as Rational Candidates for the Treatment of Traumatic Brain Injury

Traumatic brain injury (TBI) is becoming an increasing public health issue. With an annually estimated 1.7 million TBIs in the United States (U.S) and nearly 70 million worldwide, the injury, isolated or compounded with others, is a major cause of short- and long-term disability and mortality. This, along with no specific treatment, has made exploration of TBI therapies a priority of the health system. Age and sex differences create a spectrum of vulnerability to TBI, with highest prevalence among younger and older populations. Increased public interest in the long-term effects and prevention of TBI have recently reached peaks, with media attention bringing heightened awareness to sport and war related head injuries. Along with short-term issues, TBI can increase the likelihood for development of long-term neurodegenerative disorders. A growing body of literature supports the use of glucagon-like peptide-1 (GLP-1), glucose-dependent insulinotropic peptide (GIP), and glucagon (Gcg) receptor (R) agonists, along with unimolecular combinations of these therapies, for their potent neurotrophic/neuroprotective activities across a variety of cellular and animal models of chronic neurodegenerative diseases (Alzheimer's and Parkinson's diseases) and acute cerebrovascular disorders (stroke). Mild or moderate TBI shares many of the hallmarks of these conditions; recent work provides evidence that use of these compounds is an effective strategy for its treatment. Safety and efficacy of many incretin-based therapies (GLP-1 and GIP) have been demonstrated in humans for the treatment of type 2 diabetes mellitus (T2DM), making these compounds ideal for rapid evaluation in clinical trials of mild and moderate TBI.

Insulin/IGF1 signalling mediates the effects of β2 -adrenergic agonist on muscle proteostasis and growth

BACKGROUND

Stimulation of β₂ -adrenoceptors can promote muscle hypertrophy and fibre type shift, and it can counteract atrophy and weakness. The underlying mechanisms remain elusive.

METHODS

Fed wild type (WT), 2-day fasted WT, muscle-specific insulin (INS) receptor (IR) knockout (M-IR^-/- ), and MKR mice were studied with regard to acute effects of the β₂ -agonist formoterol (FOR) on protein metabolism and signalling events. MKR mice express a dominant negative IGF1 receptor, which blocks both INS/IGF1 signalling. All received one injection of FOR (300 μg kg^-1 subcutaneously) or saline. Skeletal muscles and serum samples were analysed from 30 to 240 min. For the study of chronic effects of FOR on muscle plasticity and function as well as intracellular signalling pathways, fed WT and MKR mice were treated with formoterol (300 μg kg^-1  day^-1 ) for 30 days.

RESULTS

In fed and fasted mice, one injection of FOR inhibited autophagosome formation (LC3-II content, 65%, P ≤ 0.05) that was paralleled by an increase in serum INS levels (4-fold to 25-fold, P ≤ 0.05) and the phosphorylation of Akt (4.4-fold to 6.5-fold, P ≤ 0.05) and ERK1/2 (50% to two-fold, P ≤ 0.05). This led to the suppression (40-70%, P ≤ 0.05) of the master regulators of atrophy, FoxOs, and the mRNA levels of their target genes. FOR enhanced (41%, P ≤ 0.05) protein synthesis only in fed condition and stimulated (4.4-fold to 35-fold, P ≤ 0.05) the prosynthetic Akt/mTOR/p70S6K pathway in both fed and fasted states. FOR effects on Akt signalling during fasting were blunted in both M-IR^-/- and MKR mice. Inhibition of proteolysis markers by FOR was prevented only in MKR mice. Blockade of PI3K/Akt axis and mTORC1, but not ERK1/2, in fasted mice also suppressed the acute FOR effects on proteolysis and autophagy. Chronic stimulation of β₂ -adrenoceptors in fed WT mice increased body (11%, P ≤ 0.05) and muscle (15%, P ≤ 0.05) growth and downregulated atrophy-related genes (30-40%, P ≤ 0.05), but these effects were abolished in MKR mice. Increases in muscle force caused by FOR (WT, 24%, P ≤ 0.05) were only partially impaired in MKR mice (12%, P ≤ 0.05), and FOR-induced slow-to-fast fibre type shift was not blocked at all in these animals. In MKR mice, FOR also restored the lower levels of muscle SDH activity to basal WT values and caused a marked reduction (57%, P ≤ 0.05) in the number of centrally nucleated fibers.

CONCLUSIONS

NS/IGF1 signalling is necessary for the anti-proteolytic and hypertrophic effects of in vivo β₂ -adrenergic stimulation and appears to mediate FOR-induced enhancement of protein synthesis. INS/IGF1 signalling only partially contributes to gain in strength and does not mediate fibre type transition induced by FOR.

Glucose-mediated inhibition of calcium-activated potassium channels limits α-cell calcium influx and glucagon secretion

Pancreatic α-cells exhibit oscillations in cytosolic Ca²⁺ (Ca²⁺_c), which control pulsatile glucagon (GCG) secretion. However, the mechanisms that modulate α-cell Ca²⁺_c oscillations have not been elucidated. As β-cell Ca²⁺_c oscillations are regulated in part by Ca²⁺-activated K⁺ (K_slow) currents, this work investigated the role of K_slow in α-cell Ca²⁺ handling and GCG secretion. α-Cells displayed K_slow currents that were dependent on Ca²⁺ influx through L- and P/Q-type voltage-dependent Ca²⁺ channels (VDCCs) as well as Ca²⁺ released from endoplasmic reticulum stores. α-Cell K_slow was decreased by small-conductance Ca²⁺-activated K⁺ (SK) channel inhibitors apamin and UCL 1684, large-conductance Ca²⁺-activated K⁺ (BK) channel inhibitor iberiotoxin (IbTx), and intermediate-conductance Ca²⁺-activated K⁺ (IK) channel inhibitor TRAM 34. Moreover, partial inhibition of α-cell K_slow with apamin depolarized membrane potential ( V_m) (3.8 ± 0.7 mV) and reduced action potential (AP) amplitude (10.4 ± 1.9 mV). Although apamin transiently increased Ca²⁺ influx into α-cells at low glucose (42.9 ± 10.6%), sustained SK (38.5 ± 10.4%) or BK channel inhibition (31.0 ± 11.7%) decreased α-cell Ca²⁺ influx. Total α-cell Ca²⁺_c was similarly reduced (28.3 ± 11.1%) following prolonged treatment with high glucose, but it was not decreased further by SK or BK channel inhibition. Consistent with reduced α-cell Ca²⁺_c following prolonged K_slow inhibition, apamin decreased GCG secretion from mouse (20.4 ± 4.2%) and human (27.7 ± 13.1%) islets at low glucose. These data demonstrate that K_slow activation provides a hyperpolarizing influence on α-cell V_m that sustains Ca²⁺ entry during hypoglycemic conditions, presumably by preventing voltage-dependent inactivation of P/Q-type VDCCs. Thus, when α-cell Ca²⁺_c is elevated during secretagogue stimulation, K_slow activation helps to preserve GCG secretion.

Exendin-4 overcomes cytokine-induced decreases in gap junction coupling via protein kinase A and Epac2 in mouse and human islets

KEY POINTS

The pancreatic islets of Langerhans maintain glucose homeostasis through insulin secretion, where insulin secretion dynamics are regulated by intracellular Ca2+ signalling and electrical coupling of the insulin producing β-cells in the islet. We have previously shown that cytokines decrease β-cell coupling and that compounds which increase cAMP can increase coupling. In both mouse and human islets exendin-4, which increases cAMP, protected against cytokine-induced decreases in coupling and in mouse islets preserved glucose-stimulated calcium signalling by increasing connexin36 gap junction levels on the plasma membrane. Our data indicate that protein kinase A regulates β-cell coupling through a fast mechanism, such as channel gating or membrane organization, while Epac2 regulates slower mechanisms of regulation, such as gap junction turnover. Increases in β-cell coupling with exendin-4 may protect against cytokine-mediated β-cell death as well as preserve insulin secretion dynamics during the development of diabetes.

ABSTRACT

The pancreatic islets of Langerhans maintain glucose homeostasis. Insulin secretion from islet β-cells is driven by glucose metabolism, depolarization of the cell membrane and an influx of calcium, which initiates the release of insulin. Gap junctions composed of connexin36 (Cx36) electrically couple β-cells, regulating calcium signalling and insulin secretion dynamics. Cx36 coupling is decreased in pre-diabetic mice, suggesting a role for altered coupling in diabetes. Our previous work has shown that pro-inflammatory cytokines decrease Cx36 coupling and that compounds which increase cAMP can increase Cx36 coupling. The goal of this study was to determine if exendin-4, which increases cAMP, can protect against cytokine-induced decreases in Cx36 coupling and altered islet function. In both mouse and human islets, exendin-4 protected against cytokine-induced decreases in coupling and preserved glucose-stimulated calcium signalling. Exendin-4 also protected against protein kinase Cδ-mediated decreases in Cx36 coupling. Exendin-4 preserved coupling in mouse islets by preserving Cx36 levels on the plasma membrane. Exendin-4 regulated Cx36 coupling via both protein kinase A (PKA)- and Epac2-mediated mechanisms in cytokine-treated islets. In mouse islets, modulating Epac2 had a greater impact in mediating Cx36 coupling, while in human islets modulating PKA had a greater impact on Cx36 coupling. Our data indicate that PKA regulates Cx36 coupling through a fast mechanism, such as channel gating, while Epac2 regulates slower mechanisms of regulation, such as Cx36 turnover in the membrane. Increases in Cx36 coupling with exendin-4 may protect against cytokine-mediated β-cell dysfunction to insulin secretion dynamics during the development of diabetes.

AWRK6, a Novel GLP-1 Receptor Agonist, Attenuates Diabetes by Stimulating Insulin Secretion

Diabetes is a metabolic disorder leading to many complications. The treatment of diabetes mainly depends on hypoglycemic drugs, often with side effects, which drive us to develop novel agents. AWRK6 was a peptide developed from the antimicrobial peptide Dybowskin-2CDYa in our previous study, and the availability of AWRK6 on diabetes intervention was unknown. Here, in vivo and in vitro experiments were carried out to investigate the effects of AWRK6 against diabetes. In diabetic mice, induced by high-fat diet followed by streptozocin (STZ) administration, the daily administration of AWRK6 presented acute and sustained hypoglycemic effects. The plasma insulin was significantly elevated by AWRK6 during an oral glucose tolerance test (OGTT). The relative β cell mass in diabetic mice was increased by AWRK6 treatment. The body weight and food intake were remarkably reduced by AWRK6 administration. In the mouse pancreatic β cell line Min6 cells, the intracellular calcium concentration was found to be enhanced under the treatment with AWRK6, and protein kinase A (PKA) inhibitor H-89 and Epac2 inhibitor HJC0350 represented inhibitory effects of the insulinotropic function of AWRK6. By FITC-AWRK6 incubation and GLP-1 receptor (GLP-1R) knockdown, AWRK6 proved to be a novel GLP-1R agonist. In addition, AWRK6 showed no toxicity in cell viability and membrane integrity in Min6 cells, and no hypoglycemia risk and no lethal toxicity in mice. In summary, AWRK6 was found as a novel agonist of GLP-1R, which could stimulate insulin secretion to regulate blood glucose and energy metabolism, via cAMP-calcium signaling pathway, without significant toxicity. The peptide AWRK6 might become a novel candidate for diabetes treatment.

GLP-1 suppresses glucagon secretion in human pancreatic alpha-cells by inhibition of P/Q-type Ca2+ channels

Glucagon is the body's main hyperglycemic hormone, and its secretion is dysregulated in type 2 diabetes mellitus (T2DM). The incretin hormone glucagon-like peptide-1 (GLP-1) is released from the gut and is used in T2DM therapy. Uniquely, it both stimulates insulin and inhibits glucagon secretion and thereby lowers plasma glucose levels. In this study, we have investigated the action of GLP-1 on glucagon release from human pancreatic islets. Immunocytochemistry revealed that only <0.5% of the α-cells possess detectable GLP-1R immunoreactivity. Despite this, GLP-1 inhibited glucagon secretion by 50-70%. This was due to a direct effect on α-cells, rather than paracrine signaling, because the inhibition was not reversed by the insulin receptor antagonist S961 or the somatostatin receptor-2 antagonist CYN154806. The inhibitory effect of GLP-1 on glucagon secretion was prevented by the PKA-inhibitor Rp-cAMPS and mimicked by the adenylate cyclase activator forskolin. Electrophysiological measurements revealed that GLP-1 decreased action potential height and depolarized interspike membrane potential. Mathematical modeling suggests both effects could result from inhibition of P/Q-type Ca2+ channels. In agreement with this, GLP-1 and ω-agatoxin (a blocker of P/Q-type channels) inhibited glucagon secretion in islets depolarized by 70 mmol/L [K+ ]o , and these effects were not additive. Intracellular application of cAMP inhibited depolarization-evoked exocytosis in individual α-cells by a PKA-dependent (Rp-cAMPS-sensitive) mechanism. We propose that inhibition of glucagon secretion by GLP-1 involves activation of the few GLP-1 receptors present in the α-cell membrane. The resulting small elevation of cAMP leads to PKA-dependent inhibition of P/Q-type Ca2+ channels and suppression of glucagon exocytosis.

Effect of alpha-lipoic acid supplementation on lipid profile: A systematic review and meta-analysis of controlled clinical trials

Several studies have shown the effect of alpha-lipoic acid (ALA) on lipid profile. However, findings remain controversial. This systematic review and meta-analysis was conducted to systematically summarize the available clinical trials that examined the effects ALA supplementation on the lipid profile of adults. A systematic search through PubMed and Scopus was done for studies published in English up to April 2017. Effect sizes were combined with fixed- or random-effects analysis, where appropriate. Between-study heterogeneity was evaluated by Cochran's Q test and I². Eleven clinical trials with 452 adults (51.5% women, 48.5% men) were included in this meta-analysis. Combining effect sizes of 10 studies on serum triacylglycerol (TG) concentrations revealed a significant effect of ALA supplementation on serum TG compared with the placebo group (weighted mean difference [WMD], -29.185 mg/dL; 95% confidence interval [CI], -51.454 to -6.916; P = 0.010). We also found significant changes in serum total cholesterol and low-density lipoprotein (WMD, -10.683 mg/dL; 95% CI, -19.816 to -1.550; P = 0.022, WMD, -12.906 mg/dL; 95% CI, -22.133 to -3.679; P = 0.006, respectively). Significant changes were not observed in serum high-density lipoprotein (WMD, -0.092 mg/dL; 95% CI, -3.014 to 2.831; P = 0.025). Supplementation dosage and body mass index were potential sources of heterogeneity, in which those with body mass index >30 kg/m² who received >600 mg/d ALA showed better improvements in lipid profile. Our findings showed that supplementation with ALA significantly decreased the serum concentrations of TG, total cholesterol, and low-density lipoprotein but did not affect serum levels of high-density lipoprotein in adults.

Regulation of hepatic gluconeogenesis by nuclear factor Y transcription factor in mice

Hepatic gluconeogenesis is essential to maintain blood glucose levels, and its abnormal activation leads to hyperglycemia and type 2 diabetes. However, the molecular mechanisms in the regulation of hepatic gluconeogenesis remain to be fully defined. In this study, using murine hepatocytes and a liver-specific knockout mouse model, we explored the physiological role of nuclear factor Y (NF-Y) in regulating hepatic glucose metabolism and the underlying mechanism. We found that NF-Y targets the gluconeogenesis pathway in the liver. Hepatic NF-Y expression was effectively induced by cAMP, glucagon, and fasting in vivo Lentivirus-mediated NF-Y overexpression in Hepa1-6 hepatocytes markedly raised the gluconeogenic gene expression and cellular glucose production compared with empty vector control cells. Conversely, CRISPR/Cas9-mediated knockdown of NF-Y subunit A (NF-YA) attenuated gluconeogenic gene expression and glucose production. We also provide evidence indicating that CRE-loxP-mediated, liver-specific NF-YA knockout compromises hepatic glucose production. Mechanistically, luciferase reporter gene assays and ChIP analysis indicated that NF-Y activates transcription of the gluconeogenic genes Pck1 and G6pc, by encoding phosphoenolpyruvate carboxykinase (PEPCK) and the glucose-6-phosphatase catalytic subunit (G6Pase), respectively, via directly binding to the CCAAT regulatory sequence motif in their promoters. Of note, NF-Y enhanced gluconeogenesis by interacting with cAMP-responsive element-binding protein (CREB). Overall, our results reveal a previously unrecognized physiological function of NF-Y in controlling glucose metabolism by up-regulating the gluconeogenic genes Pck1 and G6pc Modulation of hepatic NF-Y expression may therefore offer an attractive therapeutic approach to manage type 2 diabetes.

TALK-1 reduces delta-cell endoplasmic reticulum and cytoplasmic calcium levels limiting somatostatin secretion

OBJECTIVE

Single-cell RNA sequencing studies have revealed that the type-2 diabetes associated two-pore domain K+ (K2P) channel TALK-1 is abundantly expressed in somatostatin-secreting δ-cells. However, a physiological role for TALK-1 in δ-cells remains unknown. We previously determined that in β-cells, K+ flux through endoplasmic reticulum (ER)-localized TALK-1 channels enhances ER Ca2+ leak, modulating Ca2+ handling and insulin secretion. As glucose amplification of islet somatostatin release relies on Ca2+-induced Ca2+ release (CICR) from the δ-cell ER, we investigated whether TALK-1 modulates δ-cell Ca2+ handling and somatostatin secretion.

METHODS

To define the functions of islet δ-cell TALK-1 channels, we generated control and TALK-1 channel-deficient (TALK-1 KO) mice expressing fluorescent reporters specifically in δ- and α-cells to facilitate cell type identification. Using immunofluorescence, patch clamp electrophysiology, Ca2+ imaging, and hormone secretion assays, we assessed how TALK-1 channel activity impacts δ- and α-cell function.

RESULTS

TALK-1 channels are expressed in both mouse and human δ-cells, where they modulate glucose-stimulated changes in cytosolic Ca2+ and somatostatin secretion. Measurement of cytosolic Ca2+ levels in response to membrane potential depolarization revealed enhanced CICR in TALK-1 KO δ-cells that could be abolished by depleting ER Ca2+ with sarco/endoplasmic reticulum Ca2+ ATPase (SERCA) inhibitors. Consistent with elevated somatostatin inhibitory tone, we observed significantly reduced glucagon secretion and α-cell Ca2+ oscillations in TALK-1 KO islets, and found that blockade of α-cell somatostatin signaling with a somatostatin receptor 2 (SSTR2) antagonist restored glucagon secretion in TALK-1 KO islets.

CONCLUSIONS

These data indicate that TALK-1 reduces δ-cell cytosolic Ca2+ elevations and somatostatin release by limiting δ-cell CICR, modulating the intraislet paracrine signaling mechanisms that control glucagon secretion.