Control mechanisms of the oscillations of insulin secretion in vitro and in vivo

KCNH6 is essential for insulin secretion by regulating intracellular ER Ca2+ store

Appropriate Ca2+ concentration in the endoplasmic reticulum (ER), modulating cytosolic Ca2+ signal, serves significant roles in physiological function of pancreatic β cells. To maintaining ER homeostasis, Ca2+ movement across the ER membrane is always accompanied by a simultaneous K+ flux in the opposite direction. KCNH6 was proven to modulate insulin secretion by controlling plasma membrane action potential duration and intracellular Ca2+ influx. Meanwhile, the specific function of KCNH6 in pancreatic β-cells remains unclear. In this study, we found that KCNH6 exhibited mainly ER localization and Kcnh6 β-cell-specific knockout (βKO) mice suffered from abnormal glucose tolerance and impaired insulin secretion in adulthood. ER Ca2+ store was overloaded in islets of βKO mice, which contributed to ER stress and ER stress-induced apoptosis in β cells. Next, we verified that ethanol treatment induced increases in ER Ca2+ store and apoptosis in pancreatic β cells, whereas adenovirus-mediated KCNH6 overexpression in islets attenuated ethanol-induced ER stress and apoptosis. In addition, tail-vein injections of KCNH6 lentivirus rescued KCNH6 expression in βKO mice, restored ER Ca2+ overload and attenuated ER stress in β cells, which further confirms that KCNH6 protects islets from ER stress and apoptosis. These data suggest that KCNH6 on the ER membrane may help to stabilize intracellular ER Ca2+ stores and protect β cells from ER stress and apoptosis. In conclusion, our study reveals the protective potential of KCNH6-targeting drugs in ER stress-induced diabetes.

Periodic insulin stimulation of Akt: Dynamic steady states and robustness

The periodic secretion of insulin is a salient feature of the blood glucose control system in vivo. Insulin levels in the blood exhibit oscillations on multiple time scales - rapid, ultradian, and circadian - and the improved metabolic regulation resulting from pulsatile insulin release has been well established. Although numerous mathematical models investigating the causal mechanisms of insulin oscillations have appeared in the literature, to date there has been comparatively little attention given to the influence of periodic insulin stimulation on downstream components of the insulin signalling pathway. In this paper, we explore the effect of high frequency periodic insulin stimulation on Akt (also known as PKB), a crucial crosstalk node in the insulin signalling pathway that coordinates metabolic and mitogenic processes in the cell. We analyse a mathematical model of Akt translocation to the plasma membrane under both single step insulin perturbations and periodic insulin stimulation with an emphasis on - but not limited to - the physiological range of parameter values. It was shown that the system rapidly attains a robust dynamic steady state entrained to the periodic insulin stimulation. Moreover, the translocation of Akt to the plasma membrane in the model permits a sufficient level of phosphorylation to trigger downstream metabolic regulators. However, the modelling also indicated that further investigation of this activation process is required to determine whether the response of Akt is a key determinant of the enhanced metabolic control observed under periodic insulin stimulation.

Beta Cell Dysfunction in Youth- and Adult-Onset Type 2 Diabetes: An Extensive Narrative Review with a Special Focus on the Role of Nutrients

Traditionally a disease of adults, type 2 diabetes (T2D) has been increasingly diagnosed in youth, particularly among adolescents and young adults of minority ethnic groups. Especially, during the recent COVID-19 pandemic, obesity and prediabetes have surged not only in minority ethnic groups but also in the general population, further raising T2D risk. Regarding its pathogenesis, a gradually increasing insulin resistance due to central adiposity combined with a progressively defective β-cell function are the main culprits. Especially in youth-onset T2D, a rapid β-cell activity decline has been observed, leading to higher treatment failure rates, and early complications. In addition, it is well established that both the quantity and quality of food ingested by individuals play a key role in T2D pathogenesis. A chronic imbalance between caloric intake and expenditure together with impaired micronutrient intake can lead to obesity and insulin resistance on one hand, and β-cell failure and defective insulin production on the other. This review summarizes our evolving understanding of the pathophysiological mechanisms involved in defective insulin secretion by the pancreatic islets in youth- and adult-onset T2D and, further, of the role various micronutrients play in these pathomechanisms. This knowledge is essential if we are to curtail the serious long-term complications of T2D both in pediatric and adult populations.

Characterization of human islet function in a convection-driven intravascular bioartificial pancreas

Clinical islet transplantation for treatment of type 1 diabetes (T1D) is limited by the shortage of pancreas donors and need for lifelong immunosuppressive therapy. A convection-driven intravascular bioartificial pancreas (iBAP) based on highly permeable, yet immunologically protective, silicon nanopore membranes (SNM) holds promise to sustain islet function without the need for immunosuppressants. Here, we investigate short-term functionality of encapsulated human islets in an iBAP prototype. Using the finite element method (FEM), we calculated predicted oxygen profiles within islet scaffolds at normalized perifusion rates of 14-200 nl/min/IEQ. The modeling showed the need for minimum in vitro and in vivo islet perifusion rates of 28 and 100 nl/min/IEQ, respectively to support metabolic insulin production requirements in the iBAP. In vitro glucose-stimulated insulin secretion (GSIS) profiles revealed a first-phase response time of <15 min and comparable insulin production rates to standard perifusion systems (~10 pg/min/IEQ) for perifusion rates of 100-200 nl/min/IEQ. An intravenous glucose tolerance test (IVGTT), performed at a perifusion rate of 100-170 nl/min/IEQ in a non-diabetic pig, demonstrated a clinically relevant C-peptide production rate (1.0-2.8 pg/min/IEQ) with a response time of <5 min.

Cell Replacement Therapy for Type 1 Diabetes Patients: Potential Mechanisms Leading to Stem-Cell-Derived Pancreatic β-Cell Loss upon Transplant

Cell replacement therapy using stem-cell-derived insulin-producing β-like cells (sBCs) has been proposed as a practical cure for patients with type one diabetes (T1D). sBCs can correct diabetes in preclinical animal models, demonstrating the promise of this stem cell-based approach. However, in vivo studies have demonstrated that most sBCs, similarly to cadaveric human islets, are lost upon transplantation due to ischemia and other unknown mechanisms. Hence, there is a critical knowledge gap in the current field concerning the fate of sBCs upon engraftment. Here we review, discuss effects, and propose additional potential mechanisms that could contribute toward β-cell loss in vivo. We summarize and highlight some of the literature on phenotypic loss in β-cells under both steady, stressed, and diseased diabetic conditions. Specifically, we focus on β-cell death, dedifferentiation into progenitors, trans-differentiation into other hormone-expressing cells, and/or interconversion into less functional β-cell subtypes as potential mechanisms. While current cell replacement therapy efforts employing sBCs carry great promise as an abundant cell source, addressing the somewhat neglected aspect of β-cell loss in vivo will further accelerate sBC transplantation as a promising therapeutic modality that could significantly enhance the life quality of T1D patients.

Dorzagliatin, a Dual-Acting Glucokinase Activator, Increases Insulin Secretion and Glucose Sensitivity in Glucokinase Maturity-Onset Diabetes of the Young and Recent-Onset Type 2 Diabetes

Glucokinase (GK, gene symbol GCK) maturity-onset diabetes of the young (MODY) is caused by heterozygous inactivating mutations in GK and impaired glucose sensing. We investigated effects of dorzagliatin, a novel allosteric GK activator, on insulin secretion rates (ISRs) and β-cell glucose sensitivity (βCGS) in GCK-MODY and recent-onset type 2 diabetes. In a double-blind, randomized, crossover study, 8 participants with GCK-MODY and 10 participants with type 2 diabetes underwent 2-h 12 mmol/L hyperglycemic clamps following a single oral dose of dorzagliatin 75 mg or matched placebo. Effects of dorzagliatin on wild-type and mutant GK enzyme activity were investigated using an NADP+-coupled assay with glucose-6-phosphate dehydrogenase in vitro. In GCK-MODY, dorzagliatin significantly increased absolute and incremental second-phase ISRs versus placebo but not the acute insulin response. Dorzagliatin improved βCGS in GCK-MODY with an upward and leftward shift in ISR-glucose response. Dorzagliatin increased basal ISRs in type 2 diabetes, with smaller changes in second-phase ISRs versus GCK-MODY. In vitro, dorzagliatin directly reduced the glucose half saturation concentration of wild-type GK and selected GK mutants to varying degrees. Dorzagliatin directly restored enzyme activity of select GK mutants and enhanced wild-type GK activity, thereby correcting the primary defect of glucose sensing in GCK-MODY.

Loss of Slc12a2 specifically in pancreatic β-cells drives metabolic syndrome in mice

The risk of type-2 diabetes and cardiovascular disease is higher in subjects with metabolic syndrome, a cluster of clinical conditions characterized by obesity, impaired glucose metabolism, hyperinsulinemia, hyperlipidemia and hypertension. Diuretics are frequently used to treat hypertension in these patients, however, their use has long been associated with poor metabolic outcomes which cannot be fully explained by their diuretic effects. Here, we show that mice lacking the diuretic-sensitive Na+K+2Cl-cotransporter-1 Nkcc1 (Slc12a2) in insulin-secreting β-cells of the pancreatic islet (Nkcc1βKO) have reduced in vitro insulin responses to glucose. This is associated with islet hypoplasia at the expense of fewer and smaller β-cells. Remarkably, Nkcc1βKO mice excessively gain weight and progressive metabolic syndrome when fed a standard chow diet ad libitum. This is characterized by impaired hepatic insulin receptor activation and altered lipid metabolism. Indeed, overweight Nkcc1βKO but not lean mice had fasting and fed hyperglycemia, hypertriglyceridemia and non-alcoholic steatohepatitis. Notably, fasting hyperinsulinemia was detected earlier than hyperglycemia, insulin resistance, glucose intolerance and increased hepatic de novo gluconeogenesis. Therefore, our data provide evidence supporting the novel hypothesis that primary β-cell defects related to Nkcc1-regulated intracellular Cl-homeostasis and β-cell growth can result in the development of metabolic syndrome shedding light into additional potential mechanisms whereby chronic diuretic use may have adverse effects on metabolic homeostasis in susceptible individuals.

Harnessing conserved signaling and metabolic pathways to enhance the maturation of functional engineered tissues

The development of induced-pluripotent stem cell (iPSC)-derived cell types offers promise for basic science, drug testing, disease modeling, personalized medicine, and translatable cell therapies across many tissue types. However, in practice many iPSC-derived cells have presented as immature in physiological function, and despite efforts to recapitulate adult maturity, most have yet to meet the necessary benchmarks for the intended tissues. Here, we summarize the available state of knowledge surrounding the physiological mechanisms underlying cell maturation in several key tissues. Common signaling consolidators, as well as potential synergies between critical signaling pathways are explored. Finally, current practices in physiologically relevant tissue engineering and experimental design are critically examined, with the goal of integrating greater decision paradigms and frameworks towards achieving efficient maturation strategies, which in turn may produce higher-valued iPSC-derived tissues.

Contribution of Mitochondria to Insulin Secretion by Various Secretagogues

Significance: Mitochondria determine glucose-stimulated insulin secretion (GSIS) in pancreatic β-cells by elevating ATP synthesis. As the metabolic and redox hub, mitochondria provide numerous links to the plasma membrane channels, insulin granule vesicles (IGVs), cell redox, NADH, NADPH, and Ca²⁺ homeostasis, all affecting insulin secretion. Recent Advances: Mitochondrial redox signaling was implicated in several modes of insulin secretion (branched-chain ketoacid [BCKA]-, fatty acid [FA]-stimulated). Mitochondrial Ca²⁺ influx was found to enhance GSIS, reflecting cytosolic Ca²⁺ oscillations induced by action potential spikes (intermittent opening of voltage-dependent Ca²⁺ and K⁺ channels) or the superimposed Ca²⁺ release from the endoplasmic reticulum (ER). The ATPase inhibitory factor 1 (IF1) was reported to tune the glucose sensitivity range for GSIS. Mitochondrial protein kinase A was implicated in preventing the IF1-mediated inhibition of the ATP synthase. Critical Issues: It is unknown how the redox signal spreads up to the plasma membrane and what its targets are, what the differences in metabolic, redox, NADH/NADPH, and Ca²⁺ signaling, and homeostasis are between the first and second GSIS phase, and whether mitochondria can replace ER in the amplification of IGV exocytosis. Future Directions: Metabolomics studies performed to distinguish between the mitochondrial matrix and cytosolic metabolites will elucidate further details. Identifying the targets of cell signaling into mitochondria and of mitochondrial retrograde metabolic and redox signals to the cell will uncover further molecular mechanisms for insulin secretion stimulated by glucose, BCKAs, and FAs, and the amplification of secretion by glucagon-like peptide (GLP-1) and metabotropic receptors. They will identify the distinction between the hub β-cells and their followers in intact and diabetic states. Antioxid. Redox Signal. 36, 920-952.

Stimulatory responses in α- and β-cells by near-infrared (810 nm) photobiomodulation

Significant efforts have been committed to better understand and regulate insulin secretion as it has direct implications on diabetes. The first phase of biphasic insulin secretion in response to glucose lasts about 10 minutes, followed by a more sustained release persisting several hours. Attenuated insulin release in the first phase is typically associated with abnormal β-cells. While near-infrared photobiomodulation (PBM) demonstrates potential for multiple therapeutic applications, photostimulatory effects on α- and β-cells remain to be further elucidated. Herein, we demonstrate that 810 nm PBM exposure at fluence of 9 J/cm² can elevate the intracellular reactive oxygen species within 15 minutes following photostimulation. In addition, calcium spiking showed an approximately 3-fold increase in both ATC1 (α-cells) and BTC6 (β-cells) and correlates with hormone secretion in response to PBM stimulation. Our findings could lay a foundation for the development of non-biologic therapeutics that can augment islet transplantation.

The physiological role of β-cell heterogeneity in pancreatic islet function

Endocrine cells within the pancreatic islets of Langerhans are heterogeneous in terms of transcriptional profile, protein expression and the regulation of hormone release. Even though this heterogeneity has long been appreciated, only within the past 5 years have detailed molecular analyses led to an improved understanding of its basis. Although we are beginning to recognize why some subpopulations of endocrine cells are phenotypically different to others, arguably the most important consideration is how this heterogeneity affects the regulation of hormone release to control the homeostasis of glucose and other energy-rich nutrients. The focus of this Review is the description of how endocrine cell heterogeneity (and principally that of insulin-secreting β-cells) affects the regulation of hormone secretion within the islets of Langerhans. This discussion includes an overview of the functional characteristics of the different islet cell subpopulations and describes how they can communicate to influence islet function under basal and glucose-stimulated conditions. We further discuss how changes to the specific islet cell subpopulations or their numbers might underlie islet dysfunction in type 2 diabetes mellitus. We conclude with a discussion of several key open questions regarding the physiological role of islet cell heterogeneity.

Synchronisation of glycolytic activity in yeast cells

Glycolysis is the central metabolic pathway of almost every cell and organism. Under appropriate conditions, glycolytic oscillations may occur in individual cells as well as in entire cell populations or tissues. In many biological systems, glycolytic oscillations drive coherent oscillations of other metabolites, for instance in cardiomyocytes near anorexia, or in pancreas where they lead to a pulsatile release of insulin. Oscillations at the population or tissue level require the cells to synchronize their metabolism. We review the progress achieved in studying a model organism for glycolytic oscillations, namely yeast. Oscillations may occur on the level of individual cells as well as on the level of the cell population. In yeast, the cell-to-cell interaction is realized by diffusion-mediated intercellular communication via a messenger molecule. The present mini-review focuses on the synchronisation of glycolytic oscillations in yeast. Synchronisation is a quorum-sensing phenomenon because the collective oscillatory behaviour of a yeast cell population ceases when the cell density falls below a threshold. We review the question, under which conditions individual cells in a sparse population continue or cease to oscillate. Furthermore, we provide an overview of the pathway leading to the onset of synchronized oscillations. We also address the effects of spatial inhomogeneities (e.g., the formation of spatial clusters) on the collective dynamics, and also review the emergence of travelling waves of glycolytic activity. Finally, we briefly review the approaches used in numerical modelling of synchronized cell populations.

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.

Assessing Different Temporal Scales of Calcium Dynamics in Networks of Beta Cell Populations

Beta cells within the pancreatic islets of Langerhans respond to stimulation with coherent oscillations of membrane potential and intracellular calcium concentration that presumably drive the pulsatile exocytosis of insulin. Their rhythmic activity is multimodal, resulting from networked feedback interactions of various oscillatory subsystems, such as the glycolytic, mitochondrial, and electrical/calcium components. How these oscillatory modules interact and affect the collective cellular activity, which is a prerequisite for proper hormone release, is incompletely understood. In the present work, we combined advanced confocal Ca2+ imaging in fresh mouse pancreas tissue slices with time series analysis and network science approaches to unveil the glucose-dependent characteristics of different oscillatory components on both the intra- and inter-cellular level. Our results reveal an interrelationship between the metabolically driven low-frequency component and the electrically driven high-frequency component, with the latter exhibiting the highest bursting rates around the peaks of the slow component and the lowest around the nadirs. Moreover, the activity, as well as the average synchronicity of the fast component, considerably increased with increasing stimulatory glucose concentration, whereas the stimulation level did not affect any of these parameters in the slow component domain. Remarkably, in both dynamical components, the average correlation decreased similarly with intercellular distance, which implies that intercellular communication affects the synchronicity of both types of oscillations. To explore the intra-islet synchronization patterns in more detail, we constructed functional connectivity maps. The subsequent comparison of network characteristics of different oscillatory components showed more locally clustered and segregated networks of fast oscillatory activity, while the slow oscillations were more global, resulting in several long-range connections and a more cohesive structure. Besides the structural differences, we found a relatively weak relationship between the fast and slow network layer, which suggests that different synchronization mechanisms shape the collective cellular activity in islets, a finding which has to be kept in mind in future studies employing different oscillations for constructing networks.

The Pancreatic β-Cell: The Perfect Redox System

Pancreatic β-cell insulin secretion, which responds to various secretagogues and hormonal regulations, is reviewed here, emphasizing the fundamental redox signaling by NADPH oxidase 4- (NOX4-) mediated H₂O₂ production for glucose-stimulated insulin secretion (GSIS). There is a logical summation that integrates both metabolic plus redox homeostasis because the ATP-sensitive K⁺ channel (K_ATP) can only be closed when both ATP and H₂O₂ are elevated. Otherwise ATP would block K_ATP, while H₂O₂ would activate any of the redox-sensitive nonspecific calcium channels (NSCCs), such as TRPM2. Notably, a 100%-closed K_ATP ensemble is insufficient to reach the -50 mV threshold plasma membrane depolarization required for the activation of voltage-dependent Ca²⁺ channels. Open synergic NSCCs or Cl^- channels have to act simultaneously to reach this threshold. The resulting intermittent cytosolic Ca²⁺-increases lead to the pulsatile exocytosis of insulin granule vesicles (IGVs). The incretin (e.g., GLP-1) amplification of GSIS stems from receptor signaling leading to activating the phosphorylation of TRPM channels and effects on other channels to intensify integral Ca²⁺-influx (fortified by endoplasmic reticulum Ca²⁺). ATP plus H₂O₂ are also required for branched-chain ketoacids (BCKAs); and partly for fatty acids (FAs) to secrete insulin, while BCKA or FA β-oxidation provide redox signaling from mitochondria, which proceeds by H₂O₂ diffusion or hypothetical SH relay via peroxiredoxin "redox kiss" to target proteins.

In Vivo ZIMIR Imaging of Mouse Pancreatic Islet Cells Shows Oscillatory Insulin Secretion

Appropriate insulin secretion is essential for maintaining euglycemia, and impairment or loss of insulin release represents a causal event leading to diabetes. There have been extensive efforts of studying insulin secretion and its regulation using a variety of biological preparations, yet it remains challenging to monitor the dynamics of insulin secretion at the cellular level in the intact pancreas of living animals, where islet cells are supplied with physiological blood circulation and oxygenation, nerve innervation, and tissue support of surrounding exocrine cells. Herein we presented our pilot efforts of ZIMIR imaging in pancreatic islet cells in a living mouse. The imaging tracked insulin/Zn²⁺ release of individual islet β-cells in the intact pancreas with high spatiotemporal resolution, revealing a rhythmic secretion activity that appeared to be synchronized among islet β-cells. To facilitate probe delivery to islet cells, we also developed a chemogenetic approach by expressing the HaloTag protein on the cell surface. Finally, we demonstrated the application of a fluorescent granule zinc indicator, ZIGIR, as a selective and efficient islet cell marker in living animals through systemic delivery. We expect future optimization and integration of these approaches would enable longitudinal tracking of beta cell mass and function in vivo by optical imaging.

Integrin and autocrine IGF2 pathways control fasting insulin secretion in β-cells

Elevated levels of fasting insulin release and insufficient glucose-stimulated insulin secretion (GSIS) are hallmarks of diabetes. Studies have established cross-talk between integrin signaling and insulin activity, but more details of how integrin-dependent signaling impacts the pathophysiology of diabetes are needed. Here, we dissected integrin-dependent signaling pathways involved in the regulation of insulin secretion in β-cells and studied their link to the still debated autocrine regulation of insulin secretion by insulin/insulin-like growth factor (IGF) 2-AKT signaling. We observed for the first time a cooperation between different AKT isoforms and focal adhesion kinase (FAK)-dependent adhesion signaling, which either controlled GSIS or prevented insulin secretion under fasting conditions. Indeed, β-cells form integrin-containing adhesions, which provide anchorage to the pancreatic extracellular matrix and are the origin of intracellular signaling via FAK and paxillin. Under low-glucose conditions, β-cells adopt a starved adhesion phenotype consisting of actin stress fibers and large peripheral focal adhesion. In contrast, glucose stimulation induces cell spreading, actin remodeling, and point-like adhesions that contain phospho-FAK and phosphopaxillin, located in small protrusions. Rat primary β-cells and mouse insulinomas showed an adhesion remodeling during GSIS resulting from autocrine insulin/IGF2 and AKT1 signaling. However, under starving conditions, the maintenance of stress fibers and the large adhesion phenotype required autocrine IGF2-IGF1 receptor signaling mediated by AKT2 and elevated FAK-kinase activity and ROCK-RhoA levels but low levels of paxillin phosphorylation. This starved adhesion phenotype prevented excessive insulin granule release to maintain low insulin secretion during fasting. Thus, deregulation of the IGF2 and adhesion-mediated signaling may explain dysfunctions observed in diabetes.

Spatially compartmentalized phase regulation of a Ca2+-cAMP-PKA oscillatory circuit

Signaling networks are spatiotemporally organized to sense diverse inputs, process information, and carry out specific cellular tasks. In β cells, Ca2+, cyclic adenosine monophosphate (cAMP), and Protein Kinase A (PKA) exist in an oscillatory circuit characterized by a high degree of feedback. Here, we describe a mode of regulation within this circuit involving a spatial dependence of the relative phase between cAMP, PKA, and Ca2+. We show that in mouse MIN6 β cells, nanodomain clustering of Ca2+-sensitive adenylyl cyclases (ACs) drives oscillations of local cAMP levels to be precisely in-phase with Ca2+ oscillations, whereas Ca2+-sensitive phosphodiesterases maintain out-of-phase oscillations outside of the nanodomain. Disruption of this precise phase relationship perturbs Ca2+ oscillations, suggesting the relative phase within an oscillatory circuit can encode specific functional information. This work unveils a novel mechanism of cAMP compartmentation utilized for localized tuning of an oscillatory circuit and has broad implications for the spatiotemporal regulation of signaling networks.

A direct look at the dysfunction and pathology of the β cells in human type 2 diabetes

β cells uniquely produce and secrete insulin under the control of several, integrated signals, to maintain blood glucose concentrations within a narrow physiological interval. β cell failure is key to the onset and progression of type 2 diabetes, due to impaired function and reduced mass. In this review we focus on several features of human β cell dysfunction and pathology in type 2 diabetes, as revealed by direct assessment of isolated islet traits and examination of pancreatic tissue from organ donors, surgical samples or autoptic specimens. Insulin secretion defects and pathology findings are discussed in relation to some of the major underlying mechanisms, to also provide clues for conceiving better prevention and treatment of type 2 diabetes by targeting the pancreatic β cells.

ER stress increases store-operated Ca2+ entry (SOCE) and augments basal insulin secretion in pancreatic beta cells

Type 2 diabetes mellitus (T2DM) is characterized by impaired glucose-stimulated insulin secretion and increased peripheral insulin resistance. Unremitting endoplasmic reticulum (ER) stress can lead to beta-cell apoptosis and has been linked to type 2 diabetes. Although many studies have attempted to link ER stress and T2DM, the specific effects of ER stress on beta-cell function remain incompletely understood. To determine the interrelationship between ER stress and beta-cell function, here we treated insulin-secreting INS-1(832/13) cells or isolated mouse islets with the ER stress-inducer tunicamycin (TM). TM induced ER stress as expected, as evidenced by activation of the unfolded protein response. Beta cells treated with TM also exhibited concomitant alterations in their electrical activity and cytosolic free Ca2+ oscillations. As ER stress is known to reduce ER Ca2+ levels, we tested the hypothesis that the observed increase in Ca2+ oscillations occurred because of reduced ER Ca2+ levels and, in turn, increased store-operated Ca2+ entry. TM-induced cytosolic Ca2+ and membrane electrical oscillations were acutely inhibited by YM58483, which blocks store-operated Ca2+ channels. Significantly, TM-treated cells secreted increased insulin under conditions normally associated with only minimal release, e.g. 5 mm glucose, and YM58483 blocked this secretion. Taken together, these results support a critical role for ER Ca2+ depletion-activated Ca2+ current in mediating Ca2+-induced insulin secretion in response to ER stress.

ATP-sensitive K+ channels and mitochondrial permeability transition pore mediate effects of hydrogen sulfide on cytosolic Ca2+ homeostasis and insulin secretion in β-cells

Hydrogen sulfide (H₂S) is endogenously produced in pancreatic ß cells and its level is elevated in diabetes. Here, we report that H₂S affects insulin secretion via two mechanisms that converge on cytosolic free Ca²⁺ ([Ca²⁺]_i), a key mediator of insulin exocytosis. Cellular calcium imaging, using Fura-2 or Fluo-4, showed that exposure of INS-1E cells to H₂S (30-100 μM) reduced both [Ca²⁺]_i levels (by 21.7 ± 2.3%) and oscillation frequency (p < 0.01, n = 4). Consistent with a role of plasma membrane K_ATP channels (plasma-K_ATP), the effects of H₂S on [Ca²⁺]_i were blocked by gliclazide (a blocker of plasma-K_ATP channels), but were mimicked by diazoxide (an activator of plasma-K_ATP channels). Surprisingly, when Ca²⁺ entry via plasma membrane was inhibited using Ca²⁺-free external solutions, H₂S increased [Ca²⁺]_i by 39.7 ± 3.6% suggesting Ca²⁺ release from intracellular stores. H₂S-induced [Ca²⁺]_i increases were abolished by either FCCP (which depletes Ca²⁺ stored in mitochondria) or cyclosporine A (an inhibitor of mitochondrial permeability transition pore, mPTP) suggesting that H₂S induces Ca²⁺ release from mitochondria. Measurement of mitochondrial membrane potential (MMP) suggested that H₂S causes MMP depolarization, which was blocked by cyclosporine A. Finally, insulin measurements by ELISA indicated that H₂S decreased insulin release from INS-1E cells, but after plasma membrane Ca²⁺ entry was blocked by nifedipine, H₂S-induced mitochondrial Ca²⁺ release is able to increase insulin release. Together, our results indicate that H₂S has dual effects on insulin release suggesting that, with different metabolic conditions, H₂S may differentially modulate the insulin release from pancreatic ß cells and play a role in ß cell dysfunction.

Intact pancreatic islets and dispersed beta-cells both generate intracellular calcium oscillations but differ in their responsiveness to glucose

Pancreatic islets produce pulses of insulin and other hormones that maintain normal glucose homeostasis. These micro-organs possess exquisite glucose-sensing capabilities, allowing for precise changes in pulsatile insulin secretion in response to small changes in glucose. When communication among these cells is disrupted, precision glucose sensing falters. We measured intracellular calcium patterns in 6-mM-steps between 0 and 16 mM glucose, and also more finely in 2-mM-steps from 8 to 12 mM glucose, to compare glucose sensing systematically among intact islets and dispersed islet cells derived from the same mouse pancreas in vitro. The calcium activity of intact islets was uniformly low (quiescent) below 4 mM glucose and active above 8 mM glucose, whereas dispersed beta-cells displayed a broader activation range (2-to-10 mM). Intact islets exhibited calcium oscillations with 2-to-5-min periods, yet beta-cells exhibited longer 7-10 min periods. In every case, intact islets showed changes in activity with each 6-mM-glucose step, whereas dispersed islet cells displayed a continuum of calcium responses ranging from islet-like patterns to stable oscillations unaffected by changes in glucose concentration. These differences were also observed for 2-mM-glucose steps. Despite the diversity of dispersed beta-cell responses to glucose, the sum of all activity produced a glucose dose-response curve that was surprisingly similar to the curve for intact islets, arguing against the importance of "hub cells" for function. Beta-cells thus retain many of the features of islets, but some are more islet-like than others. Determining the molecular underpinnings of these variations could be valuable for future studies of stem-cell-derived beta-cell therapies.

Synchronization of pancreatic islets by periodic or non-periodic muscarinic agonist pulse trains

Pulsatile insulin secretion into the portal vein from the many pancreatic islets of Langerhans is critical for efficient glucose homeostasis. The islets are themselves endogenous oscillators, but since they are not physically coupled it is not obvious how their oscillations are synchronized across the pancreas. It has been proposed that synchronization of islets is achieved through periodic activity of intrapancreatic ganglia, and indeed there are data supporting this proposal. Postganglionic nerves are cholinergic, and their product, acetylcholine, can influence islet β-cells through actions on M₃ muscarinic receptors which are coupled to G_q type G-proteins. In addition, the neurons secrete several peptide hormones that act on β-cell receptors. The data supporting synchronization via intrapancreatic ganglia are, however, limited. In particular, it has not been shown that trains of muscarinic pulses are effective at synchronizing islets in vitro. Also, if as has been suggested, there is a ganglionic pacemaker driving islets to a preferred frequency, no neural circuitry for this pacemaker has been identified. In this study, both points are addressed using a microfluidic system that allows for the pulsed application of the muscarinic agonist carbachol. We find that murine islets are entrained and synchronized over a wide range of frequencies when the carbachol pulsing is periodic, adding support to the hypothesis that ganglia can synchronize islets in vivo. We also find that islet synchronization is very effective even if the carbachol pulses are applied at random times. This suggests that a neural pacemaker is not needed; all that is required is that islets receive occasional coordinated input from postganglionic neurons. The endogenous rhythmic activity of the islets then sets the frequency of the islet population rhythm, while the input from ganglia acts only to keep the islet oscillators in phase.

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.

Automated microfluidic droplet sampling with integrated, mix-and-read immunoassays to resolve endocrine tissue secretion dynamics

A fully automated droplet generation and analysis device based on pressure driven push-up valves for precise pumping of fluid and volumetric metering has been developed for high resolution hormone secretion sampling and measurement. The device consists of a 3D-printer templated reservoir for single cells or single tissue culturing, a Y-shaped channel for reagents and sample mixing, a T-junction channel for droplet formation, a reference channel to overcome drifts in fluorescence signal, and a long droplet storage channel allowing incubation for homogeneous immunoassays. The droplets were made by alternating peristaltic pumping of aqueous and oil phases. Device operation was automated, giving precise control over several droplet parameters such as size, oil spacing, and ratio of sample and reference droplets. By integrating an antibody-oligonucleotide based homogeneous immunoassay on-chip, high resolution temporal sampling into droplets was combined with separation-free quantification of insulin secretion from single islets of Langerhans using direct optical readout from the droplets. Quantitative assays of glucose-stimulated insulin secretion were demonstrated at 15 second temporal resolution while detecting as low as 10 amol per droplet, revealing fast insulin oscillations that mirror well-known intracellular calcium signals. This droplet sampling and direct optical analysis approach effectively digitizes the secretory time record from cells into droplets, and the system should be generalizable to a variety of cells and tissue types.

Gap-junction coupling can prolong beta-cell burst period by an order of magnitude via phantom bursting

Pancreatic β-cells show multiple intrinsic modes of oscillation with bursting electrical activity playing a crucial role. Bursting is seen both in experimentally isolated β-cells as well as in electrically coupled cells in the pancreatic islets, but the burst period is typically an order of magnitude greater in coupled cells. This difference has previously been attributed to noisier dynamics, or perturbed electrophysiological properties, in isolated β-cells. Here, we show that diffusive coupling alone can extend the period more than ten-fold in bursting oscillators modeled with a so-called phantom burster model and analyze this result with slow-fast bifurcation analysis of an electrically coupled pair of cells. Our results should be applicable to other scenarios where coupling of bursting units, e.g., neurons, may increase the oscillation period drastically.

Insulin modulates the frequency of Ca2+ oscillations in mouse pancreatic islets

Pancreatic islets can adapt to oscillatory glucose to produce synchronous insulin pulses. Can islets adapt to other oscillatory stimuli, specifically insulin? To answer this question, we stimulated islets with pulses of exogenous insulin and measured their Ca²⁺ oscillations. We observed that sufficiently high insulin (> 500 nM) with an optimal pulse period (~ 4 min) could make islets to produce synchronous Ca²⁺ oscillations. Glucose and insulin, which are key stimulatory factors of islets, modulate islet Ca²⁺ oscillations differently. Glucose increases the active-to-silent ratio of phases, whereas insulin increases the period of the oscillation. To examine the dual modulation, we adopted a phase oscillator model that incorporated the phase and frequency modulations. This mathematical model showed that out-of-phase oscillations of glucose and insulin were more effective at synchronizing islet Ca²⁺ oscillations than in-phase stimuli. This finding suggests that a phase shift in glucose and insulin oscillations can enhance inter-islet synchronization.

COEXISTENCE OF THREE OSCILLATORY MODES OF INSULIN SECRETION: MATHEMATICAL MODELING AND RELEVANCE TO GLUCOSE REGULATION

Insulin secretion in pancreatic [Formula: see text]-cells exhibits three oscillatory modes with distinct period ranges, called fast, slow, and ultradian modes. To unveil the mechanism underlying such oscillatory behaviors and their roles in blood glucose regulation, we propose a combined model for the glucose–insulin regulation system, incorporating both the cell-level insulin secretion mechanism and inter-organ interactions in the blood glucose regulation. Special emphasis is placed on the identification of the mechanism of the slow oscillation and its role associated with the whole-body glucose regulation. Via extensive numerical simulations, we obtain macroscopic behaviors of the three types of insulin/glucose oscillations in the whole-body as well as microscopic behaviors of the membrane potential and the calcium concentration in the [Formula: see text]-cell. Finally, optimal regulatory strategies for the blood glucose level are discussed on the basis of the quantitative information obtained from the mathematical modeling and numerical simulations.

Molecular mechanisms redirecting the GLP-1 receptor signalling profile in pancreatic β-cells during type 2 diabetes

Treatments with β-cell preserving properties are essential for the management of type 2 diabetes (T2D), and the new therapeutic avenues, developed over the last years, rely on the physiological role of glucagon-like peptide-1 (GLP-1). Sustained pharmacological levels of GLP-1 are achieved by subcutaneous administration of GLP-1 analogues, while transient and lower physiological levels of GLP-1 are attained following treatment with inhibitors of dipeptidylpeptidase 4 (DPP4), an endoprotease which degrades the peptide. Both therapeutic classes display a sustained and durable hypoglycaemic action in patients with T2D. However, the GLP-1 incretin effect is known to be reduced in patients with T2D, and GLP-1 analogues and DPP4 inhibitors were shown to lose their effectiveness over time in some patients. The pathological mechanisms behind these observations can be either a decrease in GLP-1 secretion from intestinal L-cells and, as a consequence, a reduction in GLP-1 plasma concentrations, combined or not with a reduced action of GLP-1 in the β-cell, the so-called GLP-1 resistance. Much evidence for a GLP-1 resistance of the β-cell in subjects with T2D have emerged. Here, we review the potential roles of the genetic background, the hyperglycaemia, the hyperlipidaemia, the prostaglandin E receptor 3, the nuclear glucocorticoid receptor, the GLP-1R desensitization and internalisation processes, and the β-arrestin-1 expression levels on GLP-1 resistance in β-cells during T2D.

Phase modulation of insulin pulses enhances glucose regulation and enables inter-islet synchronization

Insulin is secreted in a pulsatile manner from multiple micro-organs called the islets of Langerhans. The amplitude and phase (shape) of insulin secretion are modulated by numerous factors including glucose. The role of phase modulation in glucose homeostasis is not well understood compared to the obvious contribution of amplitude modulation. In the present study, we measured Ca²⁺ oscillations in islets as a proxy for insulin pulses, and we observed their frequency and shape changes under constant/alternating glucose stimuli. Here we asked how the phase modulation of insulin pulses contributes to glucose regulation. To directly answer this question, we developed a phenomenological oscillator model that drastically simplifies insulin secretion, but precisely incorporates the observed phase modulation of insulin pulses in response to glucose stimuli. Then, we mathematically modeled how insulin pulses regulate the glucose concentration in the body. The model of insulin oscillation and glucose regulation describes the glucose-insulin feedback loop. The data-based model demonstrates that the existence of phase modulation narrows the range within which the glucose concentration is maintained through the suppression/enhancement of insulin secretion in conjunction with the amplitude modulation of this secretion. The phase modulation is the response of islets to glucose perturbations. When multiple islets are exposed to the same glucose stimuli, they can be entrained to generate synchronous insulin pulses. Thus, we conclude that the phase modulation of insulin pulses is essential for glucose regulation and inter-islet synchronization.

Insulinotropic effects of GPR120 agonists are altered in obese diabetic and obese non-diabetic states

G-protein-coupled receptor 120 (GPR120) is a putative target for obesity and diabetes therapies. However, it remains controversial whether resident GPR120 plays a direct regulatory role in islet β-cell insulin secretion. The present study examined this issue in isolated rodent islets and rat β-cell line INS-1E, and assessed the role of GPR120 in islet insulin secretion in obese non-diabetic (OND) and diabetic states. GPR120 expression was detected in rodent islet β-cells. Docosahexaenoic acid (DHA) and synthetic GPR120 agonist GSK137647 (GSK) augmented insulin release from rat/mouse islets and INS-1E; DHA effects were partially mediated by GPR40. GPR120 knockdown and overexpression attenuated and enhanced DHA effects in INS-1E respectively. DHA and GSK improved postprandial hyperglycaemia of diabetic mice. Inhibition of calcium signalling in INS-1E reduced GPR120 activation-induced insulinotropic effects. The insulinotropic effects of DHA/GSK were amplified in OND rat islets, but diminished in diabetic rat islets. GPR120 and peroxisome proliferator-activated receptor γ (PPARγ) expression were elevated in OND islets and palmitic acid (PA)-treated INS-1E, but reduced in diabetic islets and high glucose-treated INS-1E. PPARγ activation increased GPR120 expression in rat islets and INS-1E. DHA and GSK induced protein kinase B (Akt)/extracellular signal-regulated kinase (ERK) phosphorylation in rodent islets and INS-1E, and these effects were altered in OND and diabetic states. Taken together, the present study indicates that (i) GPR120 activation has an insulinotropic influence on β-cells with the involvement of calcium signalling; (ii) GPR120 expression in β-cells and GPR120-mediated insulinotropic effects are altered in OND and diabetic states in distinct ways, and these alterations may be mediated by PPARγ.

PAX4 preserves endoplasmic reticulum integrity preventing beta cell degeneration in a mouse model of type 1 diabetes mellitus

AIMS/HYPOTHESIS

A strategy to enhance pancreatic islet functional beta cell mass (BCM) while restraining inflammation, through the manipulation of molecular and cellular targets, would provide a means to counteract the deteriorating glycaemic control associated with diabetes mellitus. The aims of the current study were to investigate the therapeutic potential of such a target, the islet-enriched and diabetes-linked transcription factor paired box 4 (PAX4), to restrain experimental autoimmune diabetes (EAD) in the RIP-B7.1 mouse model background and to characterise putative cellular mechanisms associated with preserved BCM.

METHODS

Two groups of RIP-B7.1 mice were genetically engineered to: (1) conditionally express either PAX4 (BPTL) or its diabetes-linked mutant variant R129W (mutBPTL) using doxycycline (DOX); and (2) constitutively express luciferase in beta cells through the use of RIP. Mice were treated or not with DOX, and EAD was induced by immunisation with a murine preproinsulin II cDNA expression plasmid. The development of hyperglycaemia was monitored for up to 4 weeks following immunisation and alterations in the BCM were assessed weekly by non-invasive in vivo bioluminescence intensity (BLI). In parallel, BCM, islet cell proliferation and apoptosis were evaluated by immunocytochemistry. Alterations in PAX4- and PAX4R129W-mediated islet gene expression were investigated by microarray profiling. PAX4 preservation of endoplasmic reticulum (ER) homeostasis was assessed using thapsigargin, electron microscopy and intracellular calcium measurements.

RESULTS

PAX4 overexpression blunted EAD, whereas the diabetes-linked mutant variant PAX4R129W did not convey protection. PAX4-expressing islets exhibited reduced insulitis and decreased beta cell apoptosis, correlating with diminished DNA damage and increased islet cell proliferation. Microarray profiling revealed that PAX4 but not PAX4R129W targeted expression of genes implicated in cell cycle and ER homeostasis. Consistent with the latter, islets overexpressing PAX4 were protected against thapsigargin-mediated ER-stress-related apoptosis. Luminal swelling associated with ER stress induced by thapsigargin was rescued in PAX4-overexpressing beta cells, correlating with preserved cytosolic calcium oscillations in response to glucose. In contrast, RNA interference mediated repression of PAX4-sensitised MIN6 cells to thapsigargin cell death.

CONCLUSIONS/INTERPRETATION

The coordinated regulation of distinct cellular pathways particularly related to ER homeostasis by PAX4 not achieved by the mutant variant PAX4R129W alleviates beta cell degeneration and protects against diabetes mellitus. The raw data for the RNA microarray described herein are accessible in the Gene Expression Omnibus database under accession number GSE62846.

Modeling analysis of inositol 1,4,5-trisphosphate receptor-mediated Ca2+ mobilization under the control of glucagon-like peptide-1 in mouse pancreatic β-cells

Glucagon-like peptide-1 (GLP-1) is an intestinally derived blood glucose-lowering hormone that potentiates glucose-stimulated insulin secretion from pancreatic β-cells. The secretagogue action of GLP-1 is explained, at least in part, by its ability to stimulate cAMP production so that cAMP may facilitate the release of Ca(2+) from inositol trisphosphate receptor (IP3R)-regulated Ca(2+) stores. However, a quantitative model has yet to be provided that explains the molecular mechanisms and dynamic processes linking GLP-1-stimulated cAMP production to Ca(2+) mobilization. Here, we performed simulation studies to investigate how GLP-1 alters the abilities of Ca(2+) and IP3 to act as coagonists at IP3R Ca(2+) release channels. A new dynamic model was constructed based on the Kaftan model, which demonstrates dual steady-state allosteric regulation of the IP3R by Ca(2+) and IP3. Data obtained from β-cells were then analyzed to understand how GLP-1 facilitates IP3R-mediated Ca(2+) mobilization when UV flash photolysis is used to uncage Ca(2+) and IP3 intracellularly. When the dynamic model for IP3R activation was incorporated into a minimal cell model, the Ca(2+) transients and oscillations induced by GLP-1 were successfully reconstructed. Simulation studies indicated that transient and oscillatory responses to GLP-1 were produced by sequential positive and negative feedback regulation due to fast activation and slow inhibition of the IP3R by Ca(2+). The slow rate of Ca(2+)-dependent inhibition was revealed to provide a remarkable contribution to the time course of the decay of cytosolic Ca(2+) transients. It also served to drive and pace Ca(2+) oscillations that are significant when evaluating how GLP-1 stimulates insulin secretion.

Spatial and temporal coordination of insulin granule exocytosis in intact human pancreatic islets

AIMS/HYPOTHESIS

Insulin secretion is widely studied because it plays a central role in glucose homeostasis and diabetes. Processes from insulin granule fusion in beta cells to in vivo insulin secretion have been elucidated, but data at the cellular level do not fully account for several aspects of the macroscopic secretory pattern. Here we investigated how individual secretory events are coordinated spatially and temporally within intact human islets.

METHODS

We used the fluorescent probe neuropeptide Y (NPY)-pHluorin to visualise insulin granule secretion in isolated intact human islets.

RESULTS

We found that individual beta cells respond to increases in glucose concentration by releasing insulin granules in very discrete bursts with periods consistent with in vivo pulsatile insulin secretion. In successive secretory bursts during prolonged exposure to high glucose levels, secretory events progressively localised to preferential release sites, coinciding with the transition to second phase insulin secretion. Granule secretion was very synchronised in neighbouring beta cells, forming discrete regional clusters of activity.

CONCLUSIONS/INTERPRETATION

These results reveal how individual secretory events are coordinated to produce pulsatile insulin secretion from human islets.

Insulin secretion in health and disease: nutrients dictate the pace

Insulin is a key hormone controlling metabolic homeostasis. Loss or dysfunction of pancreatic β-cells lead to the release of insufficient insulin to cover the organism needs, promoting diabetes development. Since dietary nutrients influence the activity of β-cells, their inadequate intake, absorption and/or utilisation can be detrimental. This review will highlight the physiological and pathological effects of nutrients on insulin secretion and discuss the underlying mechanisms. Glucose uptake and metabolism in β-cells trigger insulin secretion. This effect of glucose is potentiated by amino acids and fatty acids, as well as by entero-endocrine hormones and neuropeptides released by the digestive tract in response to nutrients. Glucose controls also basal and compensatory β-cell proliferation and, along with fatty acids, regulates insulin biosynthesis. If in the short-term nutrients promote β-cell activities, chronic exposure to nutrients can be detrimental to β-cells and causes reduced insulin transcription, increased basal secretion and impaired insulin release in response to stimulatory glucose concentrations, with a consequent increase in diabetes risk. Likewise, suboptimal early-life nutrition (e.g. parental high-fat or low-protein diet) causes altered β-cell mass and function in adulthood. The mechanisms mediating nutrient-induced β-cell dysfunction include transcriptional, post-transcriptional and translational modifications of genes involved in insulin biosynthesis and secretion, carbohydrate and lipid metabolism, cell differentiation, proliferation and survival. Altered expression of these genes is partly caused by changes in non-coding RNA transcripts induced by unbalanced nutrient uptake. A better understanding of the mechanisms leading to β-cell dysfunction will be critical to improve treatment and find a cure for diabetes.

Co-Culture of α TC-6 Cells and β TC-1 Cells: Morphology and Function

BACKGROUND

In vitro experiments using only β-cell lines instead of islets are limited because pancreatic islets are composed of four different types of endocrine cells. Several recent studies have focused on cellular interactions among these cell types, especially α- and β-cells. Because islet isolation needs time and experience, we tested a simple co-culture system with α- and β-cells. Their morphology and function were assessed by comparison to each single cell culture and pancreatic islets.

METHODS

α TC-6 cells and β TC-1 cells were maintained in Dulbecco's Minimal Essential Medium containing 5 mM glucose and 10% fetal bovine serum. Cells were mixed at a 1:1 ratio (5×10⁵) in 6-well plates and cultured for 24, 48, and 72 hours. After culture, cells were used for insulin and glucagon immunoassays and tested for glucose-stimulated insulin secretion (GSIS).

RESULTS

α TC-6 and β TC-1 cells became condensed by 24 hours and were more strongly compacted after 48 hours. β TC-1 cells showed both β-β and β-α cell contacts. GSIS increased with increasing glucose concentration in co-cultured cells, which showed lower secreted insulin levels than β TC-1 cells alone. The increase in the secreted insulin/insulin content ratio was significantly lower for co-cultured cells than for β-cells alone (P=0.04). Compared to islets, the α-/β-cell co-culture showed a higher ratio of GSIS to insulin content, but the difference was not statistically significant (P=0.09).

CONCLUSION

α TC-6 and β TC-1 cells in the co-culture system showed cell-to-cell contacts and a similar stimulated insulin secretion pattern to islets. The co-culture system may be used to better mimic pancreatic islets in in vitro assessments.

Integrated perfusion and separation systems for entrainment of insulin secretion from islets of Langerhans

A microfluidic system was developed to investigate the entrainment of insulin secretion from islets of Langerhans to oscillatory glucose levels. A gravity-driven perfusion system was integrated with a microfluidic system to deliver sinusoidal glucose waveforms to the islet chamber. Automated manipulation of the height of the perfusion syringes allowed precise control of the ratio of two perfusion solutions into a chamber containing 1-10 islets. Insulin levels in the perfusate were measured using an online competitive electrophoretic immunoassay with a sampling period of 10 s. The insulin immunoassay had a detection limit of 3 nM with RSDs of calibration points ranging from 2-8%. At 11 mM glucose, insulin secretion from single islets was oscillatory with a period ranging from 3-6 min. Application of a small amplitude sinusoidal wave of glucose with a period of 5 or 10 min, shifted the period of the insulin oscillations to this forcing period. Exposing groups of 6-10 islets to a sinusoidal glucose wave synchronized their behavior, producing a coherent pulsatile insulin response from the population. These results demonstrate the feasibility of the developed system for the study of oscillatory insulin secretion and can be easily modified for investigating the dynamic nature of other hormones released from different cell types.

Pulsatile insulin secretion, impaired glucose tolerance and type 2 diabetes

Type 2 diabetes (T2DM) results when increases in beta cell function and/or mass cannot compensate for rising insulin resistance. Numerous studies have documented the longitudinal changes in metabolism that occur during the development of glucose intolerance and lead to T2DM. However, the role of changes in insulin secretion, both amount and temporal pattern, has been understudied. Most of the insulin secreted from pancreatic beta cells of the pancreas is released in a pulsatile pattern, which is disrupted in T2DM. Here we review the evidence that changes in beta cell pulsatility occur during the progression from glucose intolerance to T2DM in humans, and contribute significantly to the etiology of the disease. We review the evidence that insulin pulsatility improves the efficacy of secreted insulin on its targets, particularly hepatic glucose production, but also examine evidence that pulsatility alters or is altered by changes in peripheral glucose uptake. Finally, we summarize our current understanding of the biophysical mechanisms responsible for oscillatory insulin secretion. Understanding how insulin pulsatility contributes to normal glucose homeostasis and is altered in metabolic disease states may help improve the treatment of T2DM.

Negative feedback synchronizes islets of Langerhans

Insulin is released from the pancreas in pulses with a period of ~ 5 min. These oscillatory insulin levels are essential for proper liver utilization and perturbed pulsatility is observed in type 2 diabetes. What coordinates the many islets of Langerhans throughout the pancreas to produce unified oscillations of insulin secretion? One hypothesis is that coordination is achieved through an insulin-dependent negative feedback action of the liver onto the glucose level. This hypothesis was tested in an in vitro setting using a microfluidic system where the population response from a group of islets was input to a model of hepatic glucose uptake, which provided a negative feedback to the glucose level. This modified glucose level was then delivered back to the islet chamber where the population response was again monitored and used to update the glucose concentration delivered to the islets. We found that, with appropriate parameters for the model, oscillations in islet activity were synchronized. This approach demonstrates that rhythmic activity of a population of physically uncoupled islets can be coordinated by a downstream system that senses islet activity and supplies negative feedback. In the intact animal, the liver can play this role of the coordinator of islet activity.

Ionic mechanisms in pancreatic β cell signaling

The function and survival of pancreatic β cells critically rely on complex electrical signaling systems composed of a series of ionic events, namely fluxes of K(+), Na(+), Ca(2+) and Cl(-) across the β cell membranes. These electrical signaling systems not only sense events occurring in the extracellular space and intracellular milieu of pancreatic islet cells, but also control different β cell activities, most notably glucose-stimulated insulin secretion. Three major ion fluxes including K(+) efflux through ATP-sensitive K(+) (KATP) channels, the voltage-gated Ca(2+) (CaV) channel-mediated Ca(2+) influx and K(+) efflux through voltage-gated K(+) (KV) channels operate in the β cell. These ion fluxes set the resting membrane potential and the shape, rate and pattern of firing of action potentials under different metabolic conditions. The KATP channel-mediated K(+) efflux determines the resting membrane potential and keeps the excitability of the β cell at low levels. Ca(2+) influx through CaV1 channels, a major type of β cell CaV channels, causes the upstroke or depolarization phase of the action potential and regulates a wide range of β cell functions including the most elementary β cell function, insulin secretion. K(+) efflux mediated by KV2.1 delayed rectifier K(+) channels, a predominant form of β cell KV channels, brings about the downstroke or repolarization phase of the action potential, which acts as a brake for insulin secretion owing to shutting down the CaV channel-mediated Ca(2+) entry. These three ion channel-mediated ion fluxes are the most important ionic events in β cell signaling. This review concisely discusses various ionic mechanisms in β cell signaling and highlights KATP channel-, CaV1 channel- and KV2.1 channel-mediated ion fluxes.

Changes in Components, Glycyrrhizin and Glycyrrhetinic Acid, in RawGlycyrrhiza uralensisFisch, Modify Insulin Sensitizing and Insulinotropic Actions

We hypothesized that roasted Glycyrrhizae Radix (Glycyrrhizin Radix Praeparata, GRP) might modify anti-diabetic action due to compositional changes. Then we examined the anti-diabetic effect and mechanism of raw Glycyrrhizae Radix (GR) and GRP extracts and their major respective components, glycyrrhizin and glycyrrhetinic acid. In partial pancreatectomized (Px) diabetic mice, both GR and GRP improved glucose tolerance, but only GRP enhanced glucose-stimulated insulin secretion as much as exendin-4. Both GR and GRP extracts enhanced insulin-stimulated glucose uptake through peroxisome proliferation-activated receptor (PPAR)-gamma activation in 3T3-L1 adipocytes. Consistently with the results of the mice study, only GRP and glycyrrhetinic acid enhanced glucose-stimulated insulin secretion in isolated islets. In addition, they induced mRNA levels of insulin receptor substrate-2, pancreas duodenum homeobox-1, and glucokinase in the islets, which contributed to improving beta-cell viability. In conclusion, GRP extract containing glycyrrhetinic acid improved glucose tolerance better than GR extract by enhancing insulinotropic action. Thus, GRP had better anti-diabetic action than GR.

Spontaneous and nicotine-induced Ca2+ oscillations mediated by Ca2+ influx in rat pinealocytes

The pineal gland regulates circadian rhythm through the synthesis and secretion of melatonin. The rise of intracellular Ca(2+) concentration ([Ca(2+)]i) following nicotinic acetylcholine receptor (nAChR) stimulation due to parasympathetic nerve activity downregulates melatonin production. Important characteristics and roles of Ca(2+) mobilization due to nAChR stimulation remain to be clarified. We report here that spontaneous Ca(2+) oscillations can be observed in ∼15% of the pinealocytes in slice preparations from rat pineal glands when this dissociation procedure is done within 6 h from a dark-to-light change. The frequency and half-life of [Ca(2+)]i rise were 0.86 min(-1) and 19 s, respectively. Similar spontaneous Ca(2+) oscillations were recorded in 17% of rat pinealocytes that were primary cultured for several days. Simultaneous measurement of [Ca(2+)]i and membrane potential revealed that spontaneous Ca(2+) oscillations were triggered by periodic membrane depolarizations. Spontaneous Ca(2+) oscillations in cultured pinealocytes were abolished by extracellular Ca(2+) removal or application of nifedipine, a blocker of voltage-dependent Ca(2+) channel (VDCC). In contrast, blockers of intracellular Ca(2+)-release channels, 2-aminoethoxydiphenylborate and ryanodine, have no effect. Our results also reveal that, in 23% quiescent pinealocytes, Ca(2+) oscillations were observed following the withdrawal of nicotine. Norepinephrine-induced melatonin secretion from whole pineal glands was significantly decreased by the coapplication of acetylcholine (ACh). This inhibitory effect of ACh was attenuated by nifedipine. In conclusion, both spontaneous and evoked Ca(2+) oscillations are due to membrane depolarization following activation of VDCCs. This consists of VDCC α1F subunit, and the associated Ca(2+) influx can strongly regulate melatonin secretion in pineal glands.

Connexin 36 mediates blood cell flow in mouse pancreatic islets

The insulin-secreting β-cells are contained within islets of Langerhans, which are highly vascularized. Blood cell flow rates through islets are glucose-dependent, even though there are no changes in blood cell flow within in the surrounding exocrine pancreas. This suggests a specific mechanism of glucose-regulated blood flow in the islet. Pancreatic islets respond to elevated glucose with synchronous pulses of electrical activity and insulin secretion across all β-cells in the islet. Connexin 36 (Cx36) gap junctions between islet β-cells mediate this synchronization, which is lost in Cx36 knockout mice (Cx36(-/-)). This leads to glucose intolerance in these mice, despite normal plasma insulin levels and insulin sensitivity. Thus, we sought to investigate whether the glucose-dependent changes in intraislet blood cell flow are also dependent on coordinated pulsatile electrical activity. We visualized and quantified blood cell flow using high-speed in vivo fluorescence imaging of labeled red blood cells and plasma. With the use of a live animal glucose clamp, blood cell flow was measured during either hypoglycemia (∼50 mg/dl) or hyperglycemia (∼300 mg/dl). In contrast to the large glucose-dependent islet blood velocity changes observed in wild-type mice, only minimal differences are observed in both Cx36(+/-) and Cx36(-/-) mice. This observation supports a novel model where intraislet blood cell flow is regulated by the coordinated electrical activity in the islet β-cells. Because Cx36 expression and function is reduced in type 2 diabetes, the resulting defect in intraislet blood cell flow regulation may also play a significant role in diabetic pathology.

Mathematical models of electrical activity of the pancreatic β-cell: a physiological review

Mathematical modeling of the electrical activity of the pancreatic β-cell has been extremely important for understanding the cellular mechanisms involved in glucose-stimulated insulin secretion. Several models have been proposed over the last 30 y, growing in complexity as experimental evidence of the cellular mechanisms involved has become available. Almost all the models have been developed based on experimental data from rodents. However, given the many important differences between species, models of human β-cells have recently been developed. This review summarizes how modeling of β-cells has evolved, highlighting the proposed physiological mechanisms underlying β-cell electrical activity.