The secretory deficit in islets from db/db mice is mainly due to a loss of responding beta cells

Ca2+ signaling and metabolic stress-induced pancreatic β-cell failure

Early in the development of Type 2 diabetes (T2D), metabolic stress brought on by insulin resistance and nutrient overload causes β-cell hyperstimulation. Herein we summarize recent studies that have explored the premise that an increase in the intracellular Ca2+ concentration ([Ca2+]i), brought on by persistent metabolic stimulation of β-cells, causes β-cell dysfunction and failure by adversely affecting β-cell function, structure, and identity. This mini-review builds on several recent reviews that also describe how excess [Ca2+]i impairs β-cell function.

Strategy for the Identification of Host-Defense Peptides in Frog Skin Secretions with Therapeutic Potential as Antidiabetic Agents

Several amphibian peptides that were first identified on the basis of their antimicrobial or cytotoxic properties have subsequently shown potential for development into agents for the treatment of patients with Type 2 diabetes. A strategy is presented for the isolation and characterization of such peptides that are present in norepinephrine-stimulated skin secretions from a range of frog species. The methodology involves (1) fractionation of the secretions by reversed-phase HPLC, (2) identification of fractions containing components that stimulate the rate of release of insulin from BRIN-BD11 clonal β-cells without simultaneously stimulating the release of lactate dehydrogenase, (3) identification of active peptides in the fractions in the mass range 1-6 kDa by MALDI-ToF mass spectrometry, (4) purification of the peptides to near homogeneity by further reversed-phase HPLC on various column matrices, and (5) structural characterization by automated Edman degradation. The effect of synthetic replicates of the active peptides on glucose homeostasis in vivo may be evaluated in appropriate animal models of Type 2 diabetes such as db/db mice and mice fed a high fat diet to produce obesity, glucose intolerance, and insulin resistance.

Deletion of Ascl1 in pancreatic β-cells improves insulin secretion, promotes parasympathetic innervation, and attenuates dedifferentiation during metabolic stress

OBJECTIVE

ASCL1, a pioneer transcription factor, is essential for neural cell differentiation and function. Previous studies have shown that Ascl1 expression is increased in pancreatic β-cells lacking functional KATP channels or after feeding of a high fat diet (HFD) suggesting that it may contribute to the metabolic stress response of β-cells.

METHODS

We generated β-cell-specific Ascl1 knockout mice (Ascl1βKO) and assessed their glucose homeostasis, islet morphology and gene expression after feeding either a normal diet or HFD for 12 weeks, or in combination with a genetic disruption of Abcc8, an essential KATP channel component.

RESULTS

Ascl1 expression is increased in response to both a HFD and membrane depolarization and requires CREB-dependent Ca2+ signaling. No differences in glucose homeostasis or islet morphology were observed in Ascl1βKO mice fed a normal diet or in the absence of KATP channels. However, male Ascl1βKO mice fed a HFD exhibited decreased blood glucose levels, improved glucose tolerance, and increased β-cell proliferation. Bulk RNA-seq analysis of islets from Ascl1βKO mice from three studied conditions showed alterations in genes associated with the secretory function. HFD-fed Ascl1βKO mice showed the most extensive changes with increased expression of genes necessary for glucose sensing, insulin secretion and β-cell proliferation, and a decrease in genes associated with β-cell dysfunction, inflammation and dedifferentiation. HFD-fed Ascl1βKO mice also displayed increased expression of parasympathetic neural markers and cholinergic receptors that was accompanied by increased insulin secretion in response to acetylcholine and an increase in islet innervation.

CONCLUSIONS

Ascl1 expression is induced by stimuli that cause Ca2+-signaling to the nucleus and contributes in a multifactorial manner to the loss of β-cell function by promoting the expression of genes associated with cellular dedifferentiation, attenuating β-cells proliferation, suppressing acetylcholine sensitivity, and repressing parasympathetic innervation of islets. Thus, the removal of Ascl1 from β-cells improves their function in response to metabolic stress.

BRSK2 in pancreatic β cells promotes hyperinsulinemia-coupled insulin resistance and its genetic variants are associated with human type 2 diabetes

Brain-specific serine/threonine-protein kinase 2 (BRSK2) plays critical roles in insulin secretion and β-cell biology. However, whether BRSK2 is associated with human type 2 diabetes mellitus (T2DM) has not been determined. Here, we report that BRSK2 genetic variants are closely related to worsening glucose metabolism due to hyperinsulinemia and insulin resistance in the Chinese population. BRSK2 protein levels are significantly elevated in β cells from T2DM patients and high-fat diet (HFD)-fed mice due to enhanced protein stability. Mice with inducible β-cell-specific Brsk2 knockout (βKO) exhibit normal metabolism with a high potential for insulin secretion under chow-diet conditions. Moreover, βKO mice are protected from HFD-induced hyperinsulinemia, obesity, insulin resistance, and glucose intolerance. Conversely, gain-of-function BRSK2 in mature β cells reversibly triggers hyperglycemia due to β-cell hypersecretion-coupled insulin resistance. Mechanistically, BRSK2 senses lipid signals and induces basal insulin secretion in a kinase-dependent manner. The enhanced basal insulin secretion drives insulin resistance and β-cell exhaustion and thus the onset of T2DM in mice fed an HFD or with gain-of-function BRSK2 in β cells. These findings reveal that BRSK2 links hyperinsulinemia to systematic insulin resistance via interplay between β cells and insulin-sensitive tissues in the populations carrying human genetic variants or under nutrient-overload conditions.

The effect of forskolin and the role of Epac2A during activation, activity, and deactivation of beta cell networks

Beta cells couple stimulation by glucose with insulin secretion and impairments in this coupling play a central role in diabetes mellitus. Cyclic adenosine monophosphate (cAMP) amplifies stimulus-secretion coupling via protein kinase A and guanine nucleotide exchange protein 2 (Epac2A). With the present research, we aimed to clarify the influence of cAMP-elevating diterpene forskolin on cytoplasmic calcium dynamics and intercellular network activity, which are two of the crucial elements of normal beta cell stimulus-secretion coupling, and the role of Epac2A under normal and stimulated conditions. To this end, we performed functional multicellular calcium imaging of beta cells in mouse pancreas tissue slices after stimulation with glucose and forskolin in wild-type and Epac2A knock-out mice. Forskolin evoked calcium signals in otherwise substimulatory glucose and beta cells from Epac2A knock-out mice displayed a faster activation. During the plateau phase, beta cells from Epac2A knock-out mice displayed a slightly higher active time in response to glucose compared with wild-type littermates, and stimulation with forskolin increased the active time via an increase in oscillation frequency and a decrease in oscillation duration in both Epac2A knock-out and wild-type mice. Functional network properties during stimulation with glucose did not differ in Epac2A knock-out mice, but the presence of Epac2A was crucial for the protective effect of stimulation with forskolin in preventing a decline in beta cell functional connectivity with time. Finally, stimulation with forskolin prolonged beta cell activity during deactivation, especially in Epac2A knock-out mice.

Enhancement of Sphingomyelinase-Induced Endothelial Nitric Oxide Synthase-Mediated Vasorelaxation in a Murine Model of Type 2 Diabetes

Sphingolipids are important biological mediators both in health and disease. We investigated the vascular effects of enhanced sphingomyelinase (SMase) activity in a mouse model of type 2 diabetes mellitus (T2DM) to gain an understanding of the signaling pathways involved. Myography was used to measure changes in the tone of the thoracic aorta after administration of 0.2 U/mL neutral SMase in the presence or absence of the thromboxane prostanoid (TP) receptor antagonist SQ 29,548 and the nitric oxide synthase (NOS) inhibitor L-NAME. In precontracted aortic segments of non-diabetic mice, SMase induced transient contraction and subsequent weak relaxation, whereas vessels of diabetic (Lepr^db/Lepr^db, referred to as db/db) mice showed marked relaxation. In the presence of the TP receptor antagonist, SMase induced enhanced relaxation in both groups, which was 3-fold stronger in the vessels of db/db mice as compared to controls and could not be abolished by ceramidase or sphingosine-kinase inhibitors. Co-administration of the NOS inhibitor L-NAME abolished vasorelaxation in both groups. Our results indicate dual vasoactive effects of SMase: TP-mediated vasoconstriction and NO-mediated vasorelaxation. Surprisingly, in spite of the general endothelial dysfunction in T2DM, the endothelial NOS-mediated vasorelaxant effect of SMase was markedly enhanced.

pH-Dependence of Glucose-Dependent Activity of Beta Cell Networks in Acute Mouse Pancreatic Tissue Slice

Extracellular pH has the potential to affect various aspects of the pancreatic beta cell function. To explain this effect, a number of mechanisms was proposed involving both extracellular and intracellular targets and pathways. Here, we focus on reassessing the influence of extracellular pH on glucose-dependent beta cell activation and collective activity in physiological conditions. To this end we employed mouse pancreatic tissue slices to perform high-temporally resolved functional imaging of cytosolic Ca²⁺ oscillations. We investigated the effect of either physiological H⁺ excess or depletion on the activation properties as well as on the collective activity of beta cell in an islet. Our results indicate that lowered pH invokes activation of a subset of beta cells in substimulatory glucose concentrations, enhances the average activity of beta cells, and alters the beta cell network properties in an islet. The enhanced average activity of beta cells was determined indirectly utilizing cytosolic Ca²⁺ imaging, while direct measuring of insulin secretion confirmed that this enhanced activity is accompanied by a higher insulin release. Furthermore, reduced functional connectivity and higher functional segregation at lower pH, both signs of a reduced intercellular communication, do not necessary result in an impaired insulin release.

Glucose-dependent activation, activity, and deactivation of beta cell networks in acute mouse pancreas tissue slices

Many details of glucose-stimulated intracellular calcium changes in β cells during activation, activity, and deactivation, as well as their concentration-dependence, remain to be analyzed. Classical physiological experiments indicated that in islets, functional differences between individual cells are largely attenuated, but recent findings suggest considerable intercellular heterogeneity, with some cells possibly coordinating the collective responses. To address the above with an emphasis on heterogeneity and describing the relations between classical physiological and functional network properties, we performed functional multicellular calcium imaging in mouse pancreas tissue slices over a wide range of glucose concentrations. During activation, delays to activation of cells and any-cell-to-first-responder delays are shortened, and the sizes of simultaneously responding clusters increased with increasing glucose concentrations. Exactly the opposite characterized deactivation. The frequency of fast calcium oscillations during activity increased with increasing glucose up to 12 mM glucose concentration, beyond which oscillation duration became longer, resulting in a homogenous increase in active time. In terms of functional connectivity, islets progressed from a very segregated network to a single large functional unit with increasing glucose concentration. A comparison between classical physiological and network parameters revealed that the first-responders during activation had longer active times during plateau and the most active cells during the plateau tended to deactivate later. Cells with the most functional connections tended to activate sooner, have longer active times, and deactivate later. Our findings provide a common ground for recent differing views on β cell heterogeneity and an important baseline for future studies of stimulus-secretion and intercellular coupling.NEW & NOTEWORTHY We assessed concentration-dependence in coupled β cells, degree of functional heterogeneity, and uncovered possible specialized subpopulations during the different phases of the response to glucose at the level of many individual cells. To this aim, we combined acute mouse pancreas tissue slices with functional multicellular calcium imaging over a wide range from threshold (7 mM) and physiological (8 and 9 mM) to supraphysiological (12 and 16 mM) glucose concentrations, classical physiological, and advanced network analyses.

Beneficial actions of the [A14K] analog of the frog skin peptide PGLa-AM1 in mice with obesity and degenerative diabetes: A mechanistic study

The antidiabetic actions of [A14K]PGLa-AM1, an analog of peptide glycine-leucine-amide-AM1 isolated from skin secretions of the octoploid frog Xenopus amieti, were investigated in genetically diabetic-obese db/db mice. Twice daily administration of [A14K]PGLa-AM1 (75 nmol/kg body weight) for 28 days significantly (P < 0.05) decreased circulating blood glucose and HbA1c and increased plasma insulin concentrations leading to improvements in glucose tolerance. The elevated levels of triglycerides, LDL and cholesterol associated with the db/db phenotype were significantly reduced by peptide administration. Elevated plasma alanine transaminase, aspartic acid transaminase, and alkaline phosphatase activities and creatinine concentrations were also significantly decreased. Peptide treatment increased pancreatic insulin content and improved the responses of isolated islets to established insulin secretagogues. No significant changes in islet β-cell and α-cell areas were observed in [A14K]PGLa-AM1 treated mice but the loss of large and medium-size islets was prevented. Peptide administration resulted in a significant (P < 0.01) increase in islet expression of the gene encoding Pdx-1, a major transcription factor in islet cells determining β-cell survival and function, resulting in increased expression of genes involved with insulin secretion (Abcc8, Kcnj11, Slc2a2, Cacn1c) together with the genes encoding the incretin receptors Glp1r and Gipr. In addition, the elevated expression of insulin signalling genes (Slc2a4, Insr, Irs1, Akt1, Pik3ca, Ppm1b) in skeletal muscle associated with the db/db phenotype was downregulated by peptide treatment These data suggest that the anti-diabetic properties of [A14K]PGLa-AM1 are mediated by molecular changes that enhance both the secretion and action of insulin.

Inhibition of miR-153, an IL-1β-responsive miRNA, prevents beta cell failure and inflammation-associated diabetes

OBJECTIVE

Systemic levels of up-regulated IL-1β and IL-1 receptors promote the pathogenesis of inflammation-associated diabetes. IL-1 receptor antagonist (IL-Ra) has shown slightly elevated beta cell function in patients with type 2 diabetes without significant improvement of hyperglycaemia. We investigated whether miR-153, an IL-1β responsive miRNA, could mimic IL-1β effects and whether its interruption would improve blood glucose control then offer an assistant curative approach to inflammation-associated diabetes.

MATERIALS/METHODS

Antago-miR-153 and Ago-miR-153 were injected into the abdominal aorta of leptin receptor-mutant db/db mice and C57BL/6 J mice, respectively. Blood glucose levels, glucose tolerance tests, insulin tolerance tests and insulin levels were regularly checked. Proteomic profiling combined with unbiased bioinformatics analysis, as well as experimental techniques, were utilized to identify target genes of miR-153. Anti-miR-153 and plasmid-based recovery assays were also performed using primary mouse islets and beta cell lines.

RESULTS

The miR-153 expression level was increased in IL-1β-treated beta cells and primary islets from the diabetic rodents. Pancreas overexpression of miR-153 caused glucose intolerance in C57BL/6 J mice but no alterations in body weight or insulin sensitivity. The inhibition of miR-153 temporarily reduced hyperglycaemia of db/db mice due to enhanced insulin secretion. Antago-miR-153 treatment ameliorated glucose intolerance in db/db mice during our observation period but did not improve insulin sensitivity. Mechanistically, miR-153 targeted three members of SNAREs to disturb insulin granule docking, thereby decreasing basal insulin secretion. Overexpression of anti-miR-153 or SNARE rescued the IL-1β-induced basal insulin secretion defect. Furthermore, miR-153 targeted beta cell-specific transcriptional factors and survival molecules to inhibit insulin biosynthesis and cell viability.

CONCLUSIONS

The IL-1β-responsive miR-153 targets SNAREs, beta cell specific TFs and other key factors to eventually causes beta cell failure. Inhibiting miR-153 with Antago-miR-153 prevents hyperglycaemia in db/db mice, indicating that miR-153 may be a promising therapeutic target for the treatment of inflammation-associated diabetes.

Caloric restriction recovers impaired β-cell-β-cell gap junction coupling, calcium oscillation coordination, and insulin secretion in prediabetic mice

Caloric restriction can decrease the incidence of metabolic diseases, such as obesity and Type 2 diabetes mellitus. The mechanisms underlying the benefits of caloric restriction involved in insulin secretion and glucose homeostasis are not fully understood. Intercellular communication within the islets of Langerhans, mediated by Connexin36 (Cx36) gap junctions, regulates insulin secretion dynamics and glucose homeostasis. The goal of this study was to determine whether caloric restriction can protect against decreases in Cx36 gap junction coupling and altered islet function induced in models of obesity and prediabetes. C57BL6 mice were fed with a high-fat diet (HFD), showing indications of prediabetes after 2 mo, including weight gain, insulin resistance, and elevated fasting glucose and insulin levels. Subsequently, mice were submitted to 1 mo of 40% caloric restriction (2 g/day of HFD). Mice under 40% caloric restriction showed reversal in weight gain and recovered insulin sensitivity, fasting glucose, and insulin levels. In islets of mice fed the HFD, caloric restriction protected against obesity-induced decreases in gap junction coupling and preserved glucose-stimulated calcium signaling, including Ca2+ oscillation coordination and oscillation amplitude. Caloric restriction also promoted a slight increase in glucose metabolism, as measured by increased NAD(P)H autofluorescence, as well as recovering glucose-stimulated insulin secretion. We conclude that declines in Cx36 gap junction coupling that occur in obesity can be completely recovered by caloric restriction and obesity reversal, improving Ca2+ dynamics and insulin secretion regulation. This suggests a critical role for caloric restriction in the context of obesity to prevent islet dysfunction.

Acute insulin deprivation results in altered mitochondrial substrate sensitivity conducive to greater fatty acid transport

Type 1 and type 2 diabetes are both tightly associated with impaired glucose control. Although both pathologies stem from different mechanisms, a reduction in insulin action coincides with drastic metabolic dysfunction in skeletal muscle and metabolic inflexibility. However, the underlying explanation for this response remains poorly understood, particularly since it is difficult to distinguish the role of attenuated insulin action from the detrimental effects of reactive lipid accumulation, which impairs mitochondrial function and promotes reactive oxygen species (ROS) emission. We therefore utilized streptozotocin to examine the effects of acute insulin deprivation, in the absence of a high-lipid/nutrient excess environment, on the regulation of mitochondrial substrate sensitivity and ROS emission. The ablation of insulin resulted in reductions in absolute mitochondrial oxidative capacity and ADP-supported respiration and reduced the ability for malonyl-CoA to inhibit carnitine palmitoyltransferase I (CPT-I) and suppress fatty acid-supported respiration. These bioenergetic responses coincided with increased mitochondrial-derived H₂O₂ emission and lipid transporter content, independent of major mitochondrial substrate transporter proteins and enzymes involved in fatty acid oxidation. Together, these data suggest that attenuated/ablated insulin signaling does not affect mitochondrial ADP sensitivity, whereas the increased reliance on fatty acid oxidation in situations where insulin action is reduced may occur as a result of altered regulation of mitochondrial fatty acid transport through CPT-I.

Excitotoxicity and Overnutrition Additively Impair Metabolic Function and Identity of Pancreatic β-Cells

A sustained increase in intracellular Ca²⁺ concentration (referred to hereafter as excitotoxicity), brought on by chronic metabolic stress, may contribute to pancreatic β-cell failure. To determine the additive effects of excitotoxicity and overnutrition on β-cell function and gene expression, we analyzed the impact of a high-fat diet (HFD) on Abcc8 knockout mice. Excitotoxicity caused β-cells to be more susceptible to HFD-induced impairment of glucose homeostasis, and these effects were mitigated by verapamil, a Ca²⁺ channel blocker. Excitotoxicity, overnutrition, and the combination of both stresses caused similar but distinct alterations in the β-cell transcriptome, including additive increases in genes associated with mitochondrial energy metabolism, fatty acid β-oxidation, and mitochondrial biogenesis and their key regulator Ppargc1a Overnutrition worsened excitotoxicity-induced mitochondrial dysfunction, increasing metabolic inflexibility and mitochondrial damage. In addition, excitotoxicity and overnutrition, individually and together, impaired both β-cell function and identity by reducing expression of genes important for insulin secretion, cell polarity, cell junction, cilia, cytoskeleton, vesicular trafficking, and regulation of β-cell epigenetic and transcriptional program. Sex had an impact on all β-cell responses, with male animals exhibiting greater metabolic stress-induced impairments than females. Together, these findings indicate that a sustained increase in intracellular Ca²⁺, by altering mitochondrial function and impairing β-cell identity, augments overnutrition-induced β-cell failure.

Role of the active zone protein, ELKS, in insulin secretion from pancreatic β-cells

Background

Insulin is stored within large dense-core granules in pancreatic beta (β)-cells and is released by Ca²⁺-triggered exocytosis with increasing blood glucose levels. Polarized and targeted secretion of insulin from β-cells in pancreatic islets into the vasculature has been proposed; however, the mechanisms related to cellular and molecular localization remain largely unknown. Within nerve terminals, the Ca²⁺-dependent release of a polarized transmitter is limited to the active zone, a highly specialized area of the presynaptic membrane. Several active zone-specific proteins have been characterized; among them, the CAST/ELKS protein family members have the ability to form large protein complexes with other active zone proteins to control the structure and function of the active zone for tight regulation of neurotransmitter release. Notably, ELKS but not CAST is also expressed in β-cells, implying that ELKS may be involved in polarized insulin secretion from β-cells.

Scope of review

This review provides an overview of the current findings regarding the role(s) of ELKS and other active zone proteins in β-cells and focuses on the molecular mechanism underlying ELKS regulation within polarized insulin secretion from islets.

Major conclusions

ELKS localizes at the vascular-facing plasma membrane of β-cells in mouse pancreatic islets. ELKS forms a potent insulin secretion complex with L-type voltage-dependent Ca²⁺ channels on the vascular-facing plasma membrane of β-cells, enabling polarized Ca²⁺ influx and first-phase insulin secretion from islets. This model provides novel insights into the functional polarity observed during insulin secretion from β-cells within islets at the molecular level. This active zone-like region formed by ELKS at the vascular side of the plasma membrane is essential for coordinating physiological insulin secretion and may be disrupted in diabetes.

Metabolic regulation of calcium signaling in beta cells

The cytoplasmic Ca²⁺ concentration ([Ca²⁺]_cyt) regulates a vast number of cellular functions, including insulin secretion from beta cells. The major physiological insulin secretagogue, glucose, triggers [Ca²⁺]_cyt oscillations in beta cells. Synchronization of the oscillations among the beta cells within an islet underlies the generation of pulsatile insulin secretion. This review describes the mechanisms generating [Ca²⁺]_cyt oscillations, the interactions between [Ca²⁺]_cyt and cell metabolism, as well as the contribution of various organelles to the shaping of [Ca²⁺]_cyt signals and insulin secretion. It also discusses how Ca²⁺ signals are coordinated and spread throughout the islets and data indicating that altered Ca²⁺ signaling is associated with beta cell dysfunction and development of type 2 diabetes.

Long-term c-Kit overexpression in beta cells compromises their function in ageing mice

AIMS/HYPOTHESIS

c-Kit signalling regulates intracellular pathways that enhance beta cell proliferation, insulin secretion and islet vascularisation in mice up to 28 weeks of age and on short-term high-fat diet. However, long-term c-Kit activation in ageing mouse islets has yet to be examined. This study utilises beta cell-specific c-Kit-overexpressing transgenic (c-KitβTg) ageing mice (~60 weeks) to determine the effect of its activation on beta cell dysfunction and insulin secretion.

METHODS

Wild-type and c-KitβTg mice, aged 60 weeks, were examined using metabolic tests to determine glucose tolerance and insulin secretion. Pancreas histology and proteins in isolated islets were examined to determine the expression of beta cell transcription factors, proliferation and intracellular signalling. To determine the role of insulin receptor signalling in ageing c-KitβTg mice, we generated beta cell-specific inducible insulin receptor knockout in ageing c-KitβTg mice (c-KitβTg;βIRKO mice) and examined the ageing mice for glucose tolerance and islet histology.

RESULTS

Ageing c-KitβTg mice progressively developed glucose intolerance, compared with age-matched wild-type littermates, due to impaired insulin secretion. Increased beta cell mass, proliferation and nuclear forkhead box transcription factor O1 (FOXO1) expression and reduced exocytotic protein levels were detected in ageing c-KitβTg mouse islets. Protein analyses of isolated islets showed increased insulin receptor, phosphorylated IRS-1^Ser612 and cleaved poly(ADP-ribose) polymerase levels in ageing c-KitβTg mice. Ageing c-KitβTg mouse islets treated ex vivo with insulin demonstrated reduced Akt phosphorylation, indicating that prolonged c-Kit induced beta cell insulin insensitivity. Ageing c-KitβTg;βIRKO mice displayed improved glucose tolerance and beta cell function compared with ageing c-KitβTg mice.

CONCLUSIONS/INTERPRETATION

These findings indicate that long-term c-Kit overexpression in beta cells has a negative impact on insulin exocytosis and that temporally dependent regulation of c-Kit-insulin receptor signalling is important for optimal beta cell function.

Phosphoproteomics Reveals the GSK3-PDX1 Axis as a Key Pathogenic Signaling Node in Diabetic Islets

Progressive decline of pancreatic beta cell function is central to the pathogenesis of type 2 diabetes. Protein phosphorylation regulates glucose-stimulated insulin secretion from beta cells, but how signaling networks are remodeled in diabetic islets in vivo remains unknown. Using high-sensitivity mass spectrometry-based proteomics, we quantified 6,500 proteins and 13,000 phosphopeptides in islets of obese diabetic mice and matched controls, revealing drastic remodeling of key kinase hubs and signaling pathways. Integration with a literature-derived signaling network implicated GSK3 kinase in the control of the beta cell-specific transcription factor PDX1. Deep phosphoproteomic analysis of human islets chronically treated with high glucose demonstrated a conserved glucotoxicity-dependent role of GSK3 kinase in regulating insulin secretion. Remarkably, the ability of beta cells to secrete insulin in response to glucose was rescued almost completely by pharmacological inhibition of GSK3. Thus, our resource enables investigation of mechanisms and drug targets in type 2 diabetes.

Friend and foe: β-cell Ca2+ signaling and the development of diabetes

BACKGROUND

The divalent cation Calcium (Ca2+) regulates a wide range of processes in disparate cell types. Within insulin-producing β-cells, increases in cytosolic Ca2+ directly stimulate insulin vesicle exocytosis, but also initiate multiple signaling pathways. Mediated through activation of downstream kinases and transcription factors, Ca2+-regulated signaling pathways leverage substantial influence on a number of critical cellular processes within the β-cell. Additionally, there is evidence that prolonged activation of these same pathways is detrimental to β-cell health and may contribute to Type 2 Diabetes pathogenesis.

SCOPE OF REVIEW

This review aims to briefly highlight canonical Ca2+ signaling pathways in β-cells and how β-cells regulate the movement of Ca2+ across numerous organelles and microdomains. As a main focus, this review synthesizes experimental data from in vitro and in vivo models on both the beneficial and detrimental effects of Ca2+ signaling pathways for β-cell function and health.

MAJOR CONCLUSIONS

Acute increases in intracellular Ca2+ stimulate a number of signaling cascades, resulting in (de-)phosphorylation events and activation of downstream transcription factors. The short-term stimulation of these Ca2+ signaling pathways promotes numerous cellular processes critical to β-cell function, including increased viability, replication, and insulin production and secretion. Conversely, chronic stimulation of Ca2+ signaling pathways increases β-cell ER stress and results in the loss of β-cell differentiation status. Together, decades of study demonstrate that Ca2+ movement is tightly regulated within the β-cell, which is at least partially due to its dual roles as a potent signaling molecule.

Optical tools for understanding the complexity of β-cell signalling and insulin release

Following stimulation, pancreatic β-cells must orchestrate a plethora of signalling events to ensure the appropriate release of insulin and maintenance of normal glucose homeostasis. Failure at any point in this cascade leads to impaired insulin secretion, elevated blood levels of glucose and eventually type 2 diabetes mellitus. Likewise, β-cell replacement or regeneration strategies for the treatment of both type 1 and type 2 diabetes mellitus might fail if the correct cell signalling phenotype cannot be faithfully recreated. However, current understanding of β-cell function is complicated because of the highly dynamic nature of their intracellular and intercellular signalling as well as insulin release itself. β-Cells must precisely integrate multiple signals stemming from multiple cues, often with differing intensities, frequencies and cellular and subcellular localizations, before converging these signals onto insulin exocytosis. In this respect, optical approaches with high resolution in space and time are extremely useful for properly deciphering the complexity of β-cell signalling. An increased understanding of β-cell signalling might identify new mechanisms underlying insulin release, with relevance for future drug therapy and de novo stem cell engineering of functional islets.

Pancreatic fibroblast growth factor 21 protects against type 2 diabetes in mice by promoting insulin expression and secretion in a PI3K/Akt signaling-dependent manner

Fibroblast growth factor 21 (FGF21) is important in glucose, lipid homeostasis and insulin sensitivity. However, it remains unknown whether FGF21 is involved in insulin expression and secretion that are dysregulated in type 2 diabetes mellitus (T2DM). In this study, we found that FGF21 was down-regulated in pancreatic islets of db/db mice, a mouse model of T2DM, along with decreased insulin expression, suggesting the possible involvement of FGF21 in maintaining insulin homeostasis and islet β-cell function. Importantly, FGF21 knockout exacerbated palmitate-induced islet β-cell failure and suppression of glucose-stimulated insulin secretion (GSIS). Pancreatic FGF21 overexpression significantly increased insulin expression, enhanced GSIS, improved islet morphology and reduced β-cell apoptosis in db/db mice. Mechanistically, FGF21 promoted expression of insulin gene transcription factors and soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) proteins, the major regulators of insulin secretion, as well as activating phosphatidylinositol 3-kinase (PI3K)/Akt signaling in islets of db/db mice. In addition, pharmaceutical inhibition of PI3K/Akt signaling effectively suppressed FGF21-induced expression of insulin gene transcription factors and SNARE proteins, suggesting an essential role of PI3K/Akt signaling in FGF21-induced insulin expression and secretion. Taken together, our results demonstrate a protective role of pancreatic FGF21 in T2DM mice through inducing PI3K/Akt signaling-dependent insulin expression and secretion.

The role of beta cell heterogeneity in islet function and insulin release

It is becoming increasingly apparent that not all insulin-secreting beta cells are equal. Subtle differences exist at the transcriptomic and protein expression levels, with repercussions for beta cell survival/proliferation, calcium signalling and insulin release. Notably, beta cell heterogeneity displays plasticity during development, metabolic stress and type 2 diabetes mellitus (T2DM). Thus, heterogeneity or lack thereof may be an important contributor to beta cell failure during T2DM in both rodents and humans. The present review will discuss the molecular and cellular features of beta cell heterogeneity at both the single-cell and islet level, explore how this influences islet function and insulin release and look into the alterations that may occur during obesity and T2DM.

Synaptotagmin 4 Regulates Pancreatic β Cell Maturation by Modulating the Ca2+ Sensitivity of Insulin Secretion Vesicles

Islet β cells from newborn mammals exhibit high basal insulin secretion and poor glucose-stimulated insulin secretion (GSIS). Here we show that β cells of newborns secrete more insulin than adults in response to similar intracellular Ca2+ concentrations, suggesting differences in the Ca2+ sensitivity of insulin secretion. Synaptotagmin 4 (Syt4), a non-Ca2+ binding paralog of the β cell Ca2+ sensor Syt7, increased by ∼8-fold during β cell maturation. Syt4 ablation increased basal insulin secretion and compromised GSIS. Precocious Syt4 expression repressed basal insulin secretion but also impaired islet morphogenesis and GSIS. Syt4 was localized on insulin granules and Syt4 levels inversely related to the number of readily releasable vesicles. Thus, transcriptional regulation of Syt4 affects insulin secretion; Syt4 expression is regulated in part by Myt transcription factors, which repress Syt4 transcription. Finally, human SYT4 regulated GSIS in EndoC-βH1 cells, a human β cell line. These findings reveal the role that altered Ca2+ sensing plays in regulating β cell maturation.

Mechanisms of β-cell dedifferentiation in diabetes: recent findings and future research directions

Like all the cells of an organism, pancreatic β-cells originate from embryonic stem cells through a complex cellular process termed differentiation. Differentiation involves the coordinated and tightly controlled activation/repression of specific effectors and gene clusters in a time-dependent fashion thereby giving rise to particular morphological and functional cellular features. Interestingly, cellular differentiation is not a unidirectional process. Indeed, growing evidence suggests that under certain conditions, mature β-cells can lose, to various degrees, their differentiated phenotype and cellular identity and regress to a less differentiated or a precursor-like state. This concept is termed dedifferentiation and has been proposed, besides cell death, as a contributing factor to the loss of functional β-cell mass in diabetes. β-cell dedifferentiation involves: (1) the downregulation of β-cell-enriched genes, including key transcription factors, insulin, glucose metabolism genes, protein processing and secretory pathway genes; (2) the concomitant upregulation of genes suppressed or expressed at very low levels in normal β-cells, the β-cell forbidden genes; and (3) the likely upregulation of progenitor cell genes. These alterations lead to phenotypic reconfiguration of β-cells and ultimately defective insulin secretion. While the major role of glucotoxicity in β-cell dedifferentiation is well established, the precise mechanisms involved are still under investigation. This review highlights the identified molecular mechanisms implicated in β-cell dedifferentiation including oxidative stress, endoplasmic reticulum (ER) stress, inflammation and hypoxia. It discusses the role of Foxo1, Myc and inhibitor of differentiation proteins and underscores the emerging role of non-coding RNAs. Finally, it proposes a novel hypothesis of β-cell dedifferentiation as a potential adaptive mechanism to escape cell death under stress conditions.

The triggering pathway to insulin secretion: Functional similarities and differences between the human and the mouse β cells and their translational relevance

In β cells, stimulation by metabolic, hormonal, neuronal, and pharmacological factors is coupled to secretion of insulin through different intracellular signaling pathways. Our knowledge about the molecular machinery supporting these pathways and the patterns of signals it generates comes mostly from rodent models, especially the laboratory mouse. The increased availability of human islets for research during the last few decades has yielded new insights into the specifics in signaling pathways leading to insulin secretion in humans. In this review, we follow the most central triggering pathway to insulin secretion from its very beginning when glucose enters the β cell to the calcium oscillations it produces to trigger fusion of insulin containing granules with the plasma membrane. Along the way, we describe the crucial building blocks that contribute to the flow of information and focus on their functional role in mice and humans and on their translational implications.

Pancreatic islet inflammation: an emerging role for chemokines

Both type 1 and type 2 diabetes exhibit features of inflammation associated with alterations in pancreatic islet function and mass. These immunological disruptions, if unresolved, contribute to the overall pathogenesis of disease onset. This review presents the emerging role of pancreatic islet chemokine production as a critical factor regulating immune cell entry into pancreatic tissue as well as an important facilitator of changes in tissue resident leukocyte activity. Signaling through two specific chemokine receptors (i.e., CXCR2 and CXCR3) is presented to illustrate key points regarding ligand-mediated regulation of innate and adaptive immune cell responses. The prospective roles of chemokine ligands and their corresponding chemokine receptors to influence the onset and progression of autoimmune- and obesity-associated forms of diabetes are discussed.

An optimized protocol for purification of functional islets of Langerhans

Islets of Langerhans and β-cell isolation constitute routinely used cell models for diabetic research, and refining islet isolation protocols and cell quality assessment is a high priority. Numerous protocols have been published describing isolate of islets, but often rigorous and systematic assessment of their integrity is lacking. Herein, we propose a new protocol for optimal generation of islets. Pancreases from mice and rats were excised and digested using a low-activity collagenase solution and islets were then purified by a series of sedimentations and a Percoll gradient. Islets were maintained in culture for 5 days, during which viability, pro/antiapoptotic, and islet-specific genes, glucose-stimulated calcium entry, glucose uptake, and insulin secretion were assessed. The commonly used islet isolation technique by collagenase injection through the common bile duct (CBD) was also performed and compared with the present approach. This new protocol produced islets that retained a healthy status as demonstrated by the yield of stable living cells. Furthermore, calcium oscillation, glucose uptake, and insulin secretion remained intact in the islet cultures. This was reproducible when many rodent species were used, and neither sex nor age affected the cells behavior. When compared with the CBD technique, islet physiology was similar. Finally, this approach was used to uncover new ion channel candidates implicated in insulin secretion. In conclusion, this study outlines an efficient protocol for islet preparation that may support research into new therapeutic targets in diabetes research.

Pancreatic Islet Responses to Metabolic Trauma

Carbohydrate, lipid, and protein metabolism are largely controlled by the interplay of various hormones, which includes those secreted by the pancreatic islets of Langerhans. While typically representing only 1% to 2% of the total pancreatic mass, the islets have a remarkable ability to adapt to disparate situations demanding a change in hormone release, such as peripheral insulin resistance. There are many different routes to the onset of insulin resistance, including obesity, lipodystrophy, glucocorticoid excess, and the chronic usage of atypical antipsychotic drugs. All of these situations are coupled to an increase in pancreatic islet size, often with a corresponding increase in insulin production. These adaptive responses within the islets are ultimately intended to maintain glycemic control and to promote macronutrient homeostasis during times of stress. Herein, we review the consequences of specific metabolic trauma that lead to insulin resistance and the corresponding adaptive alterations within the pancreatic islets.

Alterations in β-Cell Calcium Dynamics and Efficacy Outweigh Islet Mass Adaptation in Compensation of Insulin Resistance and Prediabetes Onset

Emerging insulin resistance is normally compensated by increased insulin production of pancreatic β-cells, thereby maintaining normoglycemia. However, it is unclear whether this is achieved by adaptation of β-cell function, mass, or both. Most importantly, it is still unknown which of these adaptive mechanisms fail when type 2 diabetes develops. We performed longitudinal in vivo imaging of β-cell calcium dynamics and islet mass of transplanted islets of Langerhans throughout diet-induced progression from normal glucose homeostasis, through compensation of insulin resistance, to prediabetes. The results show that compensation of insulin resistance is predominated by alterations of β-cell function, while islet mass only gradually expands. Hereby, functional adaptation is mediated by increased calcium efficacy, which involves Epac signaling. Prior to prediabetes, β-cell function displays decreased stimulated calcium dynamics, whereas islet mass continues to increase through prediabetes onset. Thus, our data reveal a predominant role of islet function with distinct contributions of triggering and amplifying pathway in the in vivo processes preceding diabetes onset. These findings support protection and recovery of β-cell function as primary goals for prevention and treatment of diabetes and provide insight into potential therapeutic targets.

Changes in beta cell function occur in prediabetes and early disease in the Lepr (db) mouse model of diabetes

AIMS/HYPOTHESIS

Type 2 diabetes is a progressive disease that increases morbidity and the risk of premature death. Glucose dysregulation, such as elevated fasting blood glucose, is observed prior to diabetes onset. A decline in beta cell insulin secretion contributes to the later stages of diabetes, but it is not known what, if any, functional beta cell changes occur in prediabetes and early disease.

METHODS

The Lepr (db) mouse (age 13-18 weeks) was used as a model of type 2 diabetes and a two-photon granule fusion assay was used to characterise the secretory response of pancreatic beta cells.

RESULTS

We identified a prediabetic state in db/db mice where the animals responded normally to a glucose challenge but have elevated fasting blood glucose. Isolated islets from prediabetic animals secreted more and were bigger. Insulin secretion, normalised to insulin content, was similar to wild type but basal insulin secretion was elevated. There was increased glucose-induced granule fusion with a high prevalence of granule-granule fusion. The glucose-induced calcium response was not changed but there was altered expression of the exocytic machinery. db/db animals at the next stage of disease had overt glucose intolerance. Isolated islets from these animals had reduced insulin secretion, reduced glucose-induced granule fusion events and decreased calcium responses to glucose.

CONCLUSIONS/INTERPRETATION

Beta cell function is altered in prediabetes and there are further changes in the progression to early disease.

Angular Approach Scanning Ion Conductance Microscopy

Scanning ion conductance microscopy (SICM) is a super-resolution live imaging technique that uses a glass nanopipette as an imaging probe to produce three-dimensional (3D) images of cell surface. SICM can be used to analyze cell morphology at nanoscale, follow membrane dynamics, precisely position an imaging nanopipette close to a structure of interest, and use it to obtain ion channel recordings or locally apply stimuli or drugs. Practical implementations of these SICM advantages, however, are often complicated due to the limitations of currently available SICM systems that inherited their design from other scanning probe microscopes in which the scan assembly is placed right above the specimen. Such arrangement makes the setting of optimal illumination necessary for phase contrast or the use of high magnification upright optics difficult. Here, we describe the designs that allow mounting SICM scan head on a standard patch-clamp micromanipulator and imaging the sample at an adjustable approach angle. This angle could be as shallow as the approach angle of a patch-clamp pipette between a water immersion objective and the specimen. Using this angular approach SICM, we obtained topographical images of cells grown on nontransparent nanoneedle arrays, of islets of Langerhans, and of hippocampal neurons under upright optical microscope. We also imaged previously inaccessible areas of cells such as the side surfaces of the hair cell stereocilia and the intercalated disks of isolated cardiac myocytes, and performed targeted patch-clamp recordings from the latter. Thus, our new, to our knowledge, angular approach SICM allows imaging of living cells on nontransparent substrates and a seamless integration with most patch-clamp setups on either inverted or upright microscopes, which would facilitate research in cell biophysics and physiology.

An optical method to evaluate both mass and functional competence of pancreatic α- and β-cells

Imbalanced glucagon and insulin release leads to the onset of type 2 diabetes. To pinpoint the underlying primary driving force, here we have developed a fast, non-biased optical method to measure ratios of pancreatic α- and β-cell mass and function simultaneously. We firstly label both primary α- and β-cells with the red fluorescent probe ZinRhodaLactam-1 (ZRL1), and then highlight α-cells by selectively quenching the ZRL1 signal from β-cells. Based on the signals before and after quenching, we calculate the ratio of the α-cell to β-cell mass within live islets, which we found matched the results from immunohistochemistry. From the same islets, glucagon and insulin release capability can be concomitantly measured. Thus, we were able to measure the ratio of α-cell to β-cell mass and their function in wild-type and diabetic Lepr(db)/Lepr(db) (denoted db/db) mice at different ages. We find that the initial glucose intolerance that appears in 10-week-old db/db mice is associated with further expansion of α-cell mass prior to deterioration in functional β-cell mass. Our method is extendable to studies of islet mass and function in other type 2 diabetes animal models, which shall benefit mechanistic studies of imbalanced hormone secretion during type 2 diabetes progression.

Serotonin- and Dopamine-Related Gene Expression in db/db Mice Islets and in MIN6 β-Cells Treated with Palmitate and Oleate

High circulating nonesterified fatty acids (NEFAs) concentration, often reported in diabetes, leads to impaired glucose-stimulated insulin secretion (GSIS) through not yet well-defined mechanisms. Serotonin and dopamine might contribute to NEFA-dependent β-cell dysfunction, since extracellular signal of these monoamines decreases GSIS. Moreover, palmitate-treated β-cells may enhance the expression of the serotonin receptor Htr2c, affecting insulin secretion. Additionally, the expression of monoamine-oxidase type B (Maob) seems to be lower in islets from humans and mice with diabetes compared to nondiabetic islets, which may lead to increased monoamine concentrations. We assessed the expression of serotonin- and dopamine-related genes in islets from db/db and wild-type (WT) mice. In addition, the effect of palmitate and oleate on the expression of such genes, 5HT content, and GSIS in MIN6 β-cell was determined. Lower Maob expression was found in islets from db/db versus WT mice and in MIN6 β-cells in response to palmitate and oleate treatment compared to vehicle. Reduced 5HT content and impaired GSIS in response to palmitate (-25%; p < 0.0001) and oleate (-43%; p < 0.0001) were detected in MIN6 β-cells. In conclusion, known defects of GSIS in islets from db/db mice and MIN6 β-cells treated with NEFAs are accompanied by reduced Maob expression and reduced 5HT content.

Insulin secretion from beta cells within intact islets: location matters

The control of hormone secretion is central to body homeostasis, and its dysfunction is important in many diseases. The key cellular steps that lead to hormone secretion have been identified, and the stimulus-secretion pathway is understood in outline for many endocrine cells. In the case of insulin secretion from pancreatic beta cells, this pathway involves the uptake of glucose, cell depolarization, calcium entry, and the triggering of the fusion of insulin-containing granules with the cell membrane. The wealth of information on the control of insulin secretion has largely been obtained from isolated single-cell studies. However, physiologically, beta cells exist within the islets of Langerhans, with structural and functional specializations that are not preserved in single-cell cultures. This review focuses on recent work that is revealing distinct aspects of insulin secretion from beta cells within the islet.

Membrane Potential and Calcium Dynamics in Beta Cells from Mouse Pancreas Tissue Slices: Theory, Experimentation, and Analysis

Beta cells in the pancreatic islets of Langerhans are precise biological sensors for glucose and play a central role in balancing the organism between catabolic and anabolic needs. A hallmark of the beta cell response to glucose are oscillatory changes of membrane potential that are tightly coupled with oscillatory changes in intracellular calcium concentration which, in turn, elicit oscillations of insulin secretion. Both membrane potential and calcium changes spread from one beta cell to the other in a wave-like manner. In order to assess the properties of the abovementioned responses to physiological and pathological stimuli, the main challenge remains how to effectively measure membrane potential and calcium changes at the same time with high spatial and temporal resolution, and also in as many cells as possible. To date, the most wide-spread approach has employed the electrophysiological patch-clamp method to monitor membrane potential changes. Inherently, this technique has many advantages, such as a direct contact with the cell and a high temporal resolution. However, it allows one to assess information from a single cell only. In some instances, this technique has been used in conjunction with CCD camera-based imaging, offering the opportunity to simultaneously monitor membrane potential and calcium changes, but not in the same cells and not with a reliable cellular or subcellular spatial resolution. Recently, a novel family of highly-sensitive membrane potential reporter dyes in combination with high temporal and spatial confocal calcium imaging allows for simultaneously detecting membrane potential and calcium changes in many cells at a time. Since the signals yielded from both types of reporter dyes are inherently noisy, we have developed complex methods of data denoising that permit for visualization and pixel-wise analysis of signals. Combining the experimental approach of high-resolution imaging with the advanced analysis of noisy data enables novel physiological insights and reassessment of current concepts in unprecedented detail.

miR-375 gene dosage in pancreatic β-cells: implications for regulation of β-cell mass and biomarker development

Abstract

MicroRNAs play a crucial role in the regulation of cell growth and differentiation. Mice with genetic deletion of miR-375 exhibit impaired glycemic control due to decreased β-cell and increased α-cell mass and function. The relative importance of these processes for the overall phenotype of miR-375KO mice is unknown. Here, we show that mice overexpressing miR-375 exhibit normal β-cell mass and function. Selective re-expression of miR-375 in β-cells of miR-375KO mice normalizes both, α- and β-cell phenotypes as well as glucose metabolism. Using this model, we also analyzed the contribution of β-cells to the total plasma miR-375 levels. Only a small proportion (≈1 %) of circulating miR-375 originates from β-cells. Furthermore, acute and profound β-cell destruction is sufficient to detect elevations of miR-375 levels in the blood. These findings are supported by higher miR-375 levels in the circulation of type 1 diabetes (T1D) subjects but not mature onset diabetes of the young (MODY) and type 2 diabetes (T2D) patients. Together, our data support an essential role for miR-375 in the maintenance of β-cell mass and provide in vivo evidence for release of miRNAs from pancreatic β-cells. The small contribution of β-cells to total plasma miR-375 levels make this miRNA an unlikely biomarker for β-cell function but suggests a utility for the detection of acute β-cell death for autoimmune diabetes.

Key messages

Overexpression of miR-375 in β-cells does not influence β-cell mass and function.

Increased α-cell mass in miR-375KO arises secondarily to loss of miR-375 in β-cells.

Only a small proportion of circulating miR-375 levels originates from β-cells.

Acute β-cell destruction results in measurable increases of miR-375 in the blood.

Circulating miR-375 levels are not a biomarker for pancreatic β-cell function.

Electronic supplementary material

The online version of this article (doi:10.1007/s00109-015-1296-9) contains supplementary material, which is available to authorized users.

Lepr(db) mouse model of type 2 diabetes: pancreatic islet isolation and live-cell 2-photon imaging of intact islets

Type 2 diabetes is a chronic disease affecting 382 million people in 2013, and is expected to rise to 592 million by 2035 (1). During the past 2 decades, the role of beta-cell dysfunction in type 2 diabetes has been clearly established (2). Research progress has required methods for the isolation of pancreatic islets. The protocol of the islet isolation presented here shares many common steps with protocols from other groups, with some modifications to improve the yield and quality of isolated islets from both the wild type and diabetic Lepr(db) (db/db) mice. A live-cell 2-photon imaging method is then presented that can be used to investigate the control of insulin secretion within islets.

Progressive glucose stimulation of islet beta cells reveals a transition from segregated to integrated modular functional connectivity patterns

Collective beta cell activity in islets of Langerhans is critical for the supply of insulin within an organism. Even though individual beta cells are intrinsically heterogeneous, the presence of intercellular coupling mechanisms ensures coordinated activity and a well-regulated exocytosis of insulin. In order to get a detailed insight into the functional organization of the syncytium, we applied advanced analytical tools from the realm of complex network theory to uncover the functional connectivity pattern among cells composing the intact islet. The procedure is based on the determination of correlations between long temporal traces obtained from confocal functional multicellular calcium imaging of beta cells stimulated in a stepwise manner with a range of physiological glucose concentrations. Our results revealed that the extracted connectivity networks are sparse for low glucose concentrations, whereas for higher stimulatory levels they become more densely connected. Most importantly, for all ranges of glucose concentration beta cells within the islets form locally clustered functional sub-compartments, thereby indicating that their collective activity profiles exhibit a modular nature. Moreover, we show that the observed non-linear functional relationship between different network metrics and glucose concentration represents a well-balanced setup that parallels physiological insulin release.