Origin of slow and fast oscillations of Ca2+ in mouse pancreatic islets

Exploring pancreatic beta-cell subgroups and their connectivity

Functional pancreatic islet beta cells are essential to ensure glucose homeostasis across species from zebrafish to humans. These cells show significant heterogeneity, and emerging studies have revealed that connectivity across a hierarchical network is required for normal insulin release. Here, we discuss current thinking and areas of debate around intra-islet connectivity, cellular hierarchies and potential "controlling" beta-cell populations. We focus on methodologies, including comparisons of different cell preparations as well as in vitro and in vivo approaches to imaging and controlling the activity of human and rodent islet preparations. We also discuss the analytical approaches that can be applied to live-cell data to identify and study critical subgroups of cells with a disproportionate role in control Ca2+ dynamics and thus insulin secretion (such as "first responders", "leaders" and "hubs", as defined by Ca2+ responses to glucose stimulation). Possible mechanisms by which this hierarchy is achieved, its physiological relevance and how its loss may contribute to islet failure in diabetes mellitus are also considered. A glossary of terms and links to computational resources are provided.

Network representation of multicellular activity in pancreatic islets: Technical considerations for functional connectivity analysis

Within the islets of Langerhans, beta cells orchestrate synchronized insulin secretion, a pivotal aspect of metabolic homeostasis. Despite the inherent heterogeneity and multimodal activity of individual cells, intercellular coupling acts as a homogenizing force, enabling coordinated responses through the propagation of intercellular waves. Disruptions in this coordination are implicated in irregular insulin secretion, a hallmark of diabetes. Recently, innovative approaches, such as integrating multicellular calcium imaging with network analysis, have emerged for a quantitative assessment of the cellular activity in islets. However, different groups use distinct experimental preparations, microscopic techniques, apply different methods to process the measured signals and use various methods to derive functional connectivity patterns. This makes comparisons between findings and their integration into a bigger picture difficult and has led to disputes in functional connectivity interpretations. To address these issues, we present here a systematic analysis of how different approaches influence the network representation of islet activity. Our findings show that the choice of methods used to construct networks is not crucial, although care is needed when combining data from different islets. Conversely, the conclusions drawn from network analysis can be heavily affected by the pre-processing of the time series, the type of the oscillatory component in the signals, and by the experimental preparation. Our tutorial-like investigation aims to resolve interpretational issues, reconcile conflicting views, advance functional implications, and encourage researchers to adopt connectivity analysis. As we conclude, we outline challenges for future research, emphasizing the broader applicability of our conclusions to other tissues exhibiting complex multicellular dynamics.

β-cell intrinsic dynamics rather than gap junction structure dictates subpopulations in the islet functional network

Diabetes is caused by the inability of electrically coupled, functionally heterogeneous β-cells within the pancreatic islet to provide adequate insulin secretion. Functional networks have been used to represent synchronized oscillatory [Ca2+] dynamics and to study β-cell subpopulations, which play an important role in driving islet function. The mechanism by which highly synchronized β-cell subpopulations drive islet function is unclear. We used experimental and computational techniques to investigate the relationship between functional networks, structural (gap junction) networks, and intrinsic β-cell dynamics in slow and fast oscillating islets. Highly synchronized subpopulations in the functional network were differentiated by intrinsic dynamics, including metabolic activity and KATP channel conductance, more than structural coupling. Consistent with this, intrinsic dynamics were more predictive of high synchronization in the islet functional network as compared to high levels of structural coupling. Finally, dysfunction of gap junctions, which can occur in diabetes, caused decreases in the efficiency and clustering of the functional network. These results indicate that intrinsic dynamics rather than structure drive connections in the functional network and highly synchronized subpopulations, but gap junctions are still essential for overall network efficiency. These findings deepen our interpretation of functional networks and the formation of functional subpopulations in dynamic tissues such as the islet.

Bright and sensitive red voltage indicators for imaging action potentials in brain slices and pancreatic islets

Genetically encoded voltage indicators (GEVIs) allow the direct visualization of cellular membrane potential at the millisecond time scale. Among these, red-emitting GEVIs have been reported to support multichannel recordings and manipulation of cellular activities with reduced autofluorescence background. However, the limited sensitivity and dimness of existing red GEVIs have restricted their applications in neuroscience. Here, we report a pair of red-shifted opsin-based GEVIs, Cepheid1b and Cepheid1s, with improved dynamic range, brightness, and photostability. The improved dynamic range is achieved by a rational design to raise the electrochromic Förster resonance energy transfer efficiency, and the higher brightness and photostability are approached with separately engineered red fluorescent proteins. With Cepheid1 indicators, we recorded complex firings and subthreshold activities of neurons on acute brain slices and observed heterogeneity in the voltage‑calcium coupling on pancreatic islets. Overall, Cepheid1 indicators provide a strong tool to investigate excitable cells in various sophisticated biological systems.

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

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

Molecular phenotyping of single pancreatic islet leader beta cells by "Flash-Seq"

AIMS

Spatially-organized increases in cytosolic Ca²⁺ within pancreatic beta cells in the pancreatic islet underlie the stimulation of insulin secretion by high glucose. Recent data have revealed the existence of subpopulations of beta cells including "leaders" which initiate Ca²⁺ waves. Whether leader cells possess unique molecular features, or localisation, is unknown.

MAIN METHODS

High speed confocal Ca²⁺ imaging was used to identify leader cells and connectivity analysis, running under MATLAB and Python, to identify highly connected "hub" cells. To explore transcriptomic differences between beta cell sub-groups, individual leaders or followers were labelled by photo-activation of the cryptic fluorescent protein PA-mCherry and subjected to single cell RNA sequencing ("Flash-Seq").

KEY FINDINGS

Distinct Ca²⁺ wave types were identified in individual islets, with leader cells present in 73 % (28 of 38 islets imaged). Scale-free, power law-adherent behaviour was also observed in 29 % of islets, though "hub" cells in these islets did not overlap with leaders. Transcripts differentially expressed (295; padj < 0.05) between leader and follower cells included genes involved in cilium biogenesis and transcriptional regulation. Providing some support for these findings, ADCY6 immunoreactivity tended to be higher in leader than follower cells, whereas cilia number and length tended to be lower in the former. Finally, leader cells were located significantly closer to delta, but not alpha, cells in Euclidian space than were follower cells.

SIGNIFICANCE

The existence of both a discrete transcriptome and unique localisation implies a role for these features in defining the specialized function of leaders. These data also raise the possibility that localised signalling between delta and leader cells contributes to the initiation and propagation of islet Ca²⁺ waves.

The Dynamics of Calcium Signaling in Beta Cells-A Discussion on the Comparison of Experimental and Modelling Data

The stimulus-secretion coupling of the pancreatic beta cell is particularly complex, as it integrates the availability of glucose and other nutrients with the neuronal and hormonal input to generate rates of insulin secretion that are appropriate for the entire organism. It is beyond dispute however, that the cytosolic Ca²⁺ concentration plays a particularly prominent role in this process, as it not only triggers the fusion of insulin granules with the plasma membrane, but also regulates the metabolism of nutrient secretagogues and affects the function of ion channels and transporters. In order to obtain a better understanding of the interdependence of these processes and, ultimately, of the entire beta cell as a working system, models have been developed based on a set of nonlinear ordinary differential equations, and were tested and parametrized on a limited set of experiments. In the present investigation, we have used a recently published version of the beta cell model to test its ability to describe further measurements from our own experimentation and from the literature. The sensitivity of the parameters is quantified and discussed; furthermore, the possible influence of the measuring technique is taken into account. The model proved to be powerful in correctly describing the depolarization pattern in response to glucose and the reaction of the cytosolic Ca²⁺ concentration to stepwise increases of the extracellular K⁺ concentration. Additionally, the membrane potential during a KATP channel block combined with a high extracellular K⁺ concentration could be reproduced. In some cases, however, a slight change of a single parameter led to an abrupt change in the cellular response, such as the generation of a Ca²⁺ oscillation with high amplitude and high frequency. This raises the question as to whether the beta cell may be a partially unstable system or whether further developments in modeling are needed to achieve a generally valid description of the stimulus-secretion coupling of the beta cell.

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

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

Liraglutide increases islet Ca2+ oscillation frequency and insulin secretion by activating hyperpolarization-activated cyclic nucleotide-gated channels

AIM

To determine whether hyperpolarization-activated cyclic nucleotide-gated (HCN) channels impact glucagon-like peptide-1 (GLP-1) receptor (GLP-1R) modulation of islet Ca²⁺ handling and insulin secretion.

METHODS

The impact of liraglutide (GLP-1 analogue) on islet Ca²⁺ handling, HCN currents and insulin secretion was monitored with fluorescence microscopy, electrophysiology and enzyme immunoassays, respectively. Furthermore, liraglutide-mediated β-to-δ-cell cross-communication was assessed following selective ablation of either mouse islet δ or β cells.

RESULTS

Liraglutide increased β-cell Ca²⁺ oscillation frequency in mouse and human islets under stimulatory glucose conditions. This was dependent in part on liraglutide activation of HCN channels, which also enhanced insulin secretion. Similarly, liraglutide activation of HCN channels also increased β-cell Ca²⁺ oscillation frequency in islets from rodents exposed to a diabetogenic diet. Interestingly, liraglutide accelerated Ca²⁺ oscillations in a majority of islet δ cells, which showed synchronized Ca²⁺ oscillations equivalent to β cells; therefore, we assessed if either cell type was driving this liraglutide-mediated islet Ca²⁺ response. Although δ-cell loss did not impact liraglutide-mediated increase in β-cell Ca²⁺ oscillation frequency, β-cell ablation attenuated liraglutide-facilitated acceleration of δ-cell Ca²⁺ oscillations.

CONCLUSION

The data presented here show that liraglutide-induced stimulation of islet HCN channels augments Ca²⁺ oscillation frequency. As insulin secretion oscillates with β-cell Ca²⁺ , these findings have important implications for pulsatile insulin secretion that is probably enhanced by liraglutide activation of HCN channels and therapeutics that target GLP-1Rs for treating diabetes. Furthermore, these studies suggest that liraglutide as well as GLP-1-based therapies enhance δ-cell Ca²⁺ oscillation frequency and somatostatin secretion kinetics in a β-cell-dependent manner.

Similarities in Calcium Oscillations Between Neonatal Mouse Islets and Mature Islets Exposed to Chronic Hyperglycemia

Pulsatility is important to islet function. As islets mature into fully developed insulin-secreting micro-organs, their ability to produce oscillatory intracellular calcium ([Ca2+]i) patterns in response to glucose also matures. In this study, we measured [Ca2+]i using fluorescence imaging to characterize oscillations from neonatal mice on postnatal (PN) days 0, 4, and 12 in comparison to adult islets. Under substimulatory (3-mM) glucose levels, [Ca2+]i was low and quiescent for adult islets as expected, as well as for PN day 12 islets. In contrast, one-third of islets on PN day 0 and 4 displayed robust [Ca2+]i oscillations in low glucose. In stimulatory glucose (11 mM) conditions, oscillations were present on all neonatal days but differed from patterns in adults. By PN day 12, [Ca2+]i oscillations were approaching characteristics of fully developed islets. The immature response pattern of neonatal islets was due, at least in part, to differences in adenosine 5'-triphosphate (ATP)-sensitive K+-channel activity estimated by [Ca2+]i responses to KATP channel agents diazoxide and tolbutamide. Neonatal [Ca2+]i patterns were also strikingly similar to patterns observed in mature islets exposed to hyperglycemic conditions (20 mM glucose for 48 hours): elevated [Ca2+]i and oscillations in low glucose along with reduced pulse mass in high glucose. Since a hallmark of diabetic islets is dedifferentiation, we propose that diabetic islets display features of "reverse maturation," demonstrating similar [Ca2+]i dynamics as neonatal islets. Pulsatility is thus an important emergent feature of neonatal islets. Our findings may provide insight into reversing β-cell dedifferentiation and to producing better functioning β cells from pluripotent stem cells.

Pancreatic α and β cells are globally phase-locked

The Ca²⁺ modulated pulsatile glucagon and insulin secretions by pancreatic α and β cells play a crucial role in glucose homeostasis. However, how α and β cells coordinate to produce various Ca²⁺ oscillation patterns is still elusive. Using a microfluidic device and transgenic mice, we recorded Ca²⁺ signals from islet α and β cells, and observed heterogeneous Ca²⁺ oscillation patterns intrinsic to each islet. After a brief period of glucose stimulation, α and β cells’ oscillations were globally phase-locked. While the activation of α cells displayed a fixed time delay of ~20 s to that of β cells, β cells activated with a tunable period. Moreover, islet α cell number correlated with oscillation frequency. We built a mathematical model of islet Ca²⁺ oscillation incorporating paracrine interactions, which quantitatively agreed with the experimental data. Our study highlights the importance of cell-cell interaction in generating stable but tunable islet oscillation patterns.

The Ca²⁺ modulated pulsatile glucagon and insulin secretions by pancreatic α and β cells are critical in glucose homeostasis. Here the authors show that the Ca²⁺ oscillations of α and β cells are phase-locked, and that the oscillation pattern is tuned by paracrine interactions between α and β cells.

From Isles of Königsberg to Islets of Langerhans: Examining the Function of the Endocrine Pancreas Through Network Science

Islets of Langerhans are multicellular microorgans located in the pancreas that play a central role in whole-body energy homeostasis. Through secretion of insulin and other hormones they regulate postprandial storage and interprandial usage of energy-rich nutrients. In these clusters of hormone-secreting endocrine cells, intricate cell-cell communication is essential for proper function. Electrical coupling between the insulin-secreting beta cells through gap junctions composed of connexin36 is particularly important, as it provides the required, most important, basis for coordinated responses of the beta cell population. The increasing evidence that gap-junctional communication and its modulation are vital to well-regulated secretion of insulin has stimulated immense interest in how subpopulations of heterogeneous beta cells are functionally arranged throughout the islets and how they mediate intercellular signals. In the last decade, several novel techniques have been proposed to assess cooperation between cells in islets, including the prosperous combination of multicellular imaging and network science. In the present contribution, we review recent advances related to the application of complex network approaches to uncover the functional connectivity patterns among cells within the islets. We first provide an accessible introduction to the basic principles of network theory, enumerating the measures characterizing the intercellular interactions and quantifying the functional integration and segregation of a multicellular system. Then we describe methodological approaches to construct functional beta cell networks, point out possible pitfalls, and specify the functional implications of beta cell network examinations. We continue by highlighting the recent findings obtained through advanced multicellular imaging techniques supported by network-based analyses, giving special emphasis to the current developments in both mouse and human islets, as well as outlining challenges offered by the multilayer network formalism in exploring the collective activity of islet cell populations. Finally, we emphasize that the combination of these imaging techniques and network-based analyses does not only represent an innovative concept that can be used to describe and interpret the physiology of islets, but also provides fertile ground for delineating normal from pathological function and for quantifying the changes in islet communication networks associated with the development of diabetes mellitus.

β Cell-specific deletion of Zfp148 improves nutrient-stimulated β cell Ca2+ responses

Insulin secretion from pancreatic β cells is essential for glucose homeostasis. An insufficient response to the demand for insulin results in diabetes. We previously showed that β cell-specific deletion of Zfp148 (β-Zfp148KO) improves glucose tolerance and insulin secretion in mice. Here, we performed Ca2+ imaging of islets from β‑Zfp148KO and control mice fed both a chow and a Western-style diet. β-Zfp148KO islets demonstrated improved sensitivity and sustained Ca2+ oscillations in response to elevated glucose levels. β-Zfp148KO islets also exhibited elevated sensitivity to amino acid-induced Ca2+ influx under low glucose conditions, suggesting enhanced mitochondrial phosphoenolpyruvate-dependent (PEP-dependent), ATP-sensitive K+ channel closure, independent of glycolysis. RNA-Seq and proteomics of β-Zfp148KO islets revealed altered levels of enzymes involved in amino acid metabolism (specifically, SLC3A2, SLC7A8, GLS, GLS2, PSPH, PHGDH, and PSAT1) and intermediary metabolism (namely, GOT1 and PCK2), consistent with altered PEP cycling. In agreement with this, β-Zfp148KO islets displayed enhanced insulin secretion in response to l-glutamine and activation of glutamate dehydrogenase. Understanding pathways controlled by ZFP148 may provide promising strategies for improving β cell function that are robust to the metabolic challenge imposed by a Western diet.

Dynamic changes in β-cell [Ca2+] regulate NFAT activation, gene transcription, and islet gap junction communication

OBJECTIVE

Diabetes occurs because of insufficient insulin secretion due to β-cell dysfunction within the islet of Langerhans. Elevated glucose levels trigger β-cell membrane depolarization, action potential generation, and slow sustained free-Ca2+ ([Ca2+]) oscillations, which trigger insulin release. Nuclear factor of activated T-cell (NFAT) is a transcription factor, which is regulated by the increases in [Ca2+] and calceineurin (CaN) activation. NFAT regulation links cell activity with gene transcription in many systems and regulates proliferation and insulin granule biogenesis within the β-cell. However, the link between the regulation of β-cell electrical activity and oscillatory [Ca2+] dynamics with NFAT activation and downstream transcription is poorly understood. Here, we tested whether dynamic changes to β-cell electrical activity and [Ca2+] regulate NFAT activation and downstream transcription.

METHODS

In cell lines, mouse islets, and human islets, including those from donors with type 2 diabetes, we applied both agonists/antagonists of ion channels together with optogenetics to modulate β-cell electrical activity. We measured the dynamics of [Ca2+] and NFAT activation as well as performed whole transcriptome and functional analyses.

RESULTS

Both glucose-induced membrane depolarization and optogenetic stimulation triggered NFAT activation as well as increased the transcription of NFAT targets and intermediate early genes (IEGs). Importantly, slow, sustained [Ca2+] oscillation conditions led to NFAT activation and downstream transcription. In contrast, in human islets from donors with type2 diabetes, NFAT activation by glucose was diminished, but rescued upon pharmacological stimulation of electrical activity. NFAT activation regulated GJD2 expression and increased Cx36 gap junction permeability upon elevated oscillatory [Ca2+] dynamics. However, it is unclear if NFAT directly binds the GJD2 gene to regulate expression.

CONCLUSIONS

This study provides an insight into the specific patterns of electrical activity that regulate NFAT activation, gene transcription, and islet function. In addition, it provides information on how these factors are disrupted in diabetes.

Sensing Intra‐ and Extra‐Cellular Ca2+ in the Islet of Langerhans

Calcium ions (Ca2+) take part in intra‐ and inter‐cellular signaling to mediate cellular functions. Sensing this ubiquitous messenger is instrumental in disentangling the specific functions of cellular sub‐compartments and/or intercellular communications. In this review, the authors first describe intra‐ and inter‐cellular Ca2+ signaling in relation to insulin secretion from the pancreatic islets, and then outline the development of diverse sensors, for example, chemically synthesized indicators, genetically encoded proteins, and ion‐selective microelectrodes, for intra‐ and extra‐cellular sensing of Ca2+. Particular emphasis is placed on emerging approaches in this field, such as low‐affinity Ca2+ indicators and unique Ca2+‐responsive composite materials. The authors conclude by remarking on the challenges and opportunities for further developments in this field, which may facilitate a more comprehensive understanding of Ca2+ signaling within and outside the islets, and its relevance in health and disease.

The Role of cAMP in Beta Cell Stimulus-Secretion and Intercellular Coupling

Pancreatic beta cells secrete insulin in response to stimulation with glucose and other nutrients, and impaired insulin secretion plays a central role in development of diabetes mellitus. Pharmacological management of diabetes includes various antidiabetic drugs, including incretins. The incretin hormones, glucagon-like peptide-1 and gastric inhibitory polypeptide, potentiate glucose-stimulated insulin secretion by binding to G protein-coupled receptors, resulting in stimulation of adenylate cyclase and production of the secondary messenger cAMP, which exerts its intracellular effects through activation of protein kinase A or the guanine nucleotide exchange protein 2A. The molecular mechanisms behind these two downstream signaling arms are still not fully elucidated and involve many steps in the stimulus-secretion coupling cascade, ranging from the proximal regulation of ion channel activity to the central Ca2+ signal and the most distal exocytosis. In addition to modifying intracellular coupling, the effect of cAMP on insulin secretion could also be at least partly explained by the impact on intercellular coupling. In this review, we systematically describe the possible roles of cAMP at these intra- and inter-cellular signaling nodes, keeping in mind the relevance for the whole organism and translation to humans.

NMDA receptor inhibition increases, synchronizes, and stabilizes the collective pancreatic beta cell activity: Insights through multilayer network analysis

NMDA receptors promote repolarization in pancreatic beta cells and thereby reduce glucose-stimulated insulin secretion. Therefore, NMDA receptors are a potential therapeutic target for diabetes. While the mechanism of NMDA receptor inhibition in beta cells is rather well understood at the molecular level, its possible effects on the collective cellular activity have not been addressed to date, even though proper insulin secretion patterns result from well-synchronized beta cell behavior. The latter is enabled by strong intercellular connectivity, which governs propagating calcium waves across the islets and makes the heterogeneous beta cell population work in synchrony. Since a disrupted collective activity is an important and possibly early contributor to impaired insulin secretion and glucose intolerance, it is of utmost importance to understand possible effects of NMDA receptor inhibition on beta cell functional connectivity. To address this issue, we combined confocal functional multicellular calcium imaging in mouse tissue slices with network science approaches. Our results revealed that NMDA receptor inhibition increases, synchronizes, and stabilizes beta cell activity without affecting the velocity or size of calcium waves. To explore intercellular interactions more precisely, we made use of the multilayer network formalism by regarding each calcium wave as an individual network layer, with weighted directed connections portraying the intercellular propagation. NMDA receptor inhibition stabilized both the role of wave initiators and the course of waves. The findings obtained with the experimental antagonist of NMDA receptors, MK-801, were additionally validated with dextrorphan, the active metabolite of the approved drug dextromethorphan, as well as with experiments on NMDA receptor KO mice. In sum, our results provide additional and new evidence for a possible role of NMDA receptor inhibition in treatment of type 2 diabetes and introduce the multilayer network paradigm as a general strategy to examine effects of drugs on connectivity in multicellular systems.

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.

Effect of delay in transportation of extracellular glucose into cardiomyocytes under diabetic condition: a study through mathematical model

A four-dimensional model was built to mimic the cross-talk among plasma glucose, plasma insulin, intracellular glucose and cytoplasmic calcium of a cardiomyocyte. A time delay was considered to represent the time required for performing various cellular mechanisms between activation of insulin receptor and subsequent glucose entry from extracellular region into intracellular region of a cardiac cell. We analysed the delay-induced model and deciphered conditions for stability and bifurcation. Extensive numerical computations were performed to validate the analytical results and give further insights. Sensitivity study of the system parameters using LHS-PRCC method reveals that some rate parameters, which represent the input of plasma glucose, absorption of glucose by noncardiac cells and insulin production, are sensitive and may cause significant change in the system dynamics. It was observed that the time taken for transportation of extracellular glucose into the cell through GLUT4 plays an important role in maintaining physiological oscillations of the state variables. Parameter recalibration exercise showed that reduced input rate of glucose in the blood plasma or an alteration in transportation delay may be used for therapeutic targets in diabetic-like condition for maintaining normal cardiac function.

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.

Biophysical modeling of β-cells networks: Realistic architectures and heterogeneity effects

The β-cells dynamics is the regulator of insulin secretion in the pancreas, and its investigation is a central aspect in designing effective treatment strategies for diabetes. Despite great efforts, much is still unknown about the complex organization of such endocrine cells and realistic mathematical modeling represents a useful tool to elucidate key aspects of glucose control in humans. In this contribution, we study the human β-cells collective behaviour, by modeling their electric and metabolic coupling in a cluster, of size and architecture similar to human islets of Langerhans. We focus on the effect of coupling on various dynamics regimes observed in the islets, that are spiking and bursting on multiple timescales. In particular, we test the effect of hubs, that are highly glucose-sensitive β-cells, on the overall network dynamics, observing different modulation depending on the timescale of the dynamics. By properly taking into account the role of cells heterogeneity, recently emerged, our model effectively describes the effect of hubs on the synchronization of the islet response and the correlation of β-cells activity.

Using Electrophoretic Immunoassay to Monitor Hormone Secretion

It has been demonstrated that microfluidic systems allow integration of sampling, reagent mixing, and rapid electrophoretic analysis. They have also proven useful for culturing cells wherein control over the environment allows novel and automated experiments. Here, we describe a microchip-based electrophoresis assay that allows cell culture and hormone monitoring. An online gradient generator can control cell culture condition precisely. This system has been applied for Pancreas islets' glucose sensitivity studies.

CART is overexpressed in human type 2 diabetic islets and inhibits glucagon secretion and increases insulin secretion

AIMS/HYPOTHESIS

Insufficient insulin release and hyperglucagonaemia are culprits in type 2 diabetes. Cocaine- and amphetamine-regulated transcript (CART, encoded by Cartpt) affects islet hormone secretion and beta cell survival in vitro in rats, and Cart (-/-) mice have diminished insulin secretion. We aimed to test if CART is differentially regulated in human type 2 diabetic islets and if CART affects insulin and glucagon secretion in vitro in humans and in vivo in mice.

METHODS

CART expression was assessed in human type 2 diabetic and non-diabetic control pancreases and rodent models of diabetes. Insulin and glucagon secretion was examined in isolated islets and in vivo in mice. Ca(2+) oscillation patterns and exocytosis were studied in mouse islets.

RESULTS

We report an important role of CART in human islet function and glucose homeostasis in mice. CART was found to be expressed in human alpha and beta cells and in a subpopulation of mouse beta cells. Notably, CART expression was several fold higher in islets of type 2 diabetic humans and rodents. CART increased insulin secretion in vivo in mice and in human and mouse islets. Furthermore, CART increased beta cell exocytosis, altered the glucose-induced Ca(2+) signalling pattern in mouse islets from fast to slow oscillations and improved synchronisation of the oscillations between different islet regions. Finally, CART reduced glucagon secretion in human and mouse islets, as well as in vivo in mice via diminished alpha cell exocytosis.

CONCLUSIONS/INTERPRETATION

We conclude that CART is a regulator of glucose homeostasis and could play an important role in the pathophysiology of type 2 diabetes. Based on the ability of CART to increase insulin secretion and reduce glucagon secretion, CART-based agents could be a therapeutic modality in type 2 diabetes.

Fasiglifam (TAK-875) has dual potentiating mechanisms via Gαq-GPR40/FFAR1 signaling branches on glucose-dependent insulin secretion

Fasiglifam (TAK‐875) is a free fatty acid receptor 1 (FFAR1)/G‐protein–coupled receptor 40 (GPR40) agonist that improves glycemic control in type 2 diabetes with minimum risk of hypoglycemia. Fasiglifam potentiates glucose‐stimulated insulin secretion (GSIS) from pancreatic β‐cells glucose dependently, although the precise mechanism underlying the glucose dependency still remains unknown. Here, we investigated key cross‐talk between the GSIS pathway and FFAR1 signaling, and Ca²⁺ dynamics using mouse insulinoma MIN6 cells. We demonstrated that the glucose‐dependent insulinotropic effect of fasiglifam required membrane depolarization and that fasiglifam induced a glucose‐dependent increase in intracellular Ca²⁺ level and amplification of Ca²⁺ oscillations. This differed from the sulfonylurea glimepiride that induced changes in Ca²⁺ dynamics glucose independently. Stimulation with cell‐permeable analogs of IP₃ or diacylglycerol (DAG), downstream second messengers of Gαq‐FFAR1, augmented GSIS similar to fasiglifam, indicating their individual roles in the potentiation of GSIS pathway. Intriguingly, the IP₃ analog triggered similar Ca²⁺ dynamics to fasiglifam, whereas the DAG analog had no effect. Despite the lack of an effect on Ca²⁺ dynamics, the DAG analog elicited synergistic effects on insulin secretion with Ca²⁺ influx evoked by an L‐type voltage‐dependent calcium channel opener that mimics glucose‐dependent Ca²⁺ dynamics. These results indicate that the Gαq signaling activated by fasiglifam enhances GSIS pathway via dual potentiating mechanisms in which IP₃ amplifies glucose‐induced Ca²⁺ oscillations and DAG/protein kinase C (PKC) augments downstream secretory mechanisms independent of Ca²⁺ oscillations.

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.

Submembrane ATP and Ca2+ kinetics in α-cells: unexpected signaling for glucagon secretion

Cytoplasmic ATP and Ca(2+) are implicated in current models of glucose's control of glucagon and insulin secretion from pancreatic α- and β-cells, respectively, but little is known about ATP and its relation to Ca(2+) in α-cells. We therefore expressed the fluorescent ATP biosensor Perceval in mouse pancreatic islets and loaded them with a Ca(2+) indicator. With total internal reflection fluorescence microscopy, we recorded subplasma membrane concentrations of Ca(2+) and ATP ([Ca(2+)]pm; [ATP]pm) in superficial α- and β-cells of intact islets and related signaling to glucagon and insulin secretion by immunoassay. Consistent with ATP's controlling glucagon and insulin secretion during hypo- and hyperglycemia, respectively, the dose-response relationship for glucose-induced [ATP]pm generation was left shifted in α-cells compared to β-cells. Both cell types showed [Ca(2+)]pm and [ATP]pm oscillations in opposite phase, probably reflecting energy-consuming Ca(2+) transport. Although pulsatile insulin and glucagon release are in opposite phase, [Ca(2+)]pm synchronized in the same phase between α- and β-cells. This paradox can be explained by the overriding of Ca(2+) stimulation by paracrine inhibition, because somatostatin receptor blockade potently stimulated glucagon release with little effect on Ca(2+). The data indicate that an α-cell-intrinsic mechanism controls glucagon in hypoglycemia and that paracrine factors shape pulsatile secretion in hyperglycemia.

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.

Reactive oxygen and nitrogen species disturb Ca(2+) oscillations in insulin-secreting MIN6 β-cells

Disturbances in pulsatile insulin secretion and Ca(2+) oscillations in pancreatic β-cells are early markers of diabetes, but the underlying mechanisms are still incompletely understood. Reactive oxygen/nitrogen species (ROS/RNS) are implicated in reduced β-cell function, and ROS/RNS target several Ca(2+) pumps and channels. Thus, we hypothesized that ROS/RNS could disturb Ca(2+) oscillations and downstream insulin pulsatility. We show that ROS/RNS production by photoactivation of aluminum phthalocyanine chloride (AlClPc) abolish or accelerate Ca(2+) oscillations in the MIN6 β-cell line, depending on the amount of ROS/RNS. Application of the sarcoplasmic/endoplasmic reticulum Ca(2+) ATPase (SERCA) inhibitor thapsigargin modifies the Ca(2+) response to high concentrations of ROS/RNS. Further, thapsigargin produces effects that resemble those elicited by moderate ROS/RNS production. These results indicate that ROS/RNS interfere with endoplasmic reticulum Ca(2+) handling. This idea is supported by theoretical studies using a mathematical model of Ca(2+) handling adapted to MIN6 cells. Our results suggest a putative link between ROS/RNS and disturbed pulsatile insulin secretion.

Defects in β-cell Ca2+ dynamics in age-induced diabetes

Little is known about the molecular mechanisms underlying age-dependent deterioration in β-cell function. We now demonstrate that age-dependent impairment in insulin release, and thereby glucose homeostasis, is associated with subtle changes in Ca(2+) dynamics in mouse β-cells. We show that these changes are likely to be accounted for by impaired mitochondrial function and to involve phospholipase C/inositol 1,4,5-trisphosphate-mediated Ca(2+) mobilization from intracellular stores as well as decreased β-cell Ca(2+) influx over the plasma membrane. We use three mouse models, namely, a premature aging phenotype, a mature aging phenotype, and an aging-resistant phenotype. Premature aging is studied in a genetically modified mouse model with an age-dependent accumulation of mitochondrial DNA mutations. Mature aging is studied in the C57BL/6 mouse, whereas the 129 mouse represents a model that is more resistant to age-induced deterioration. Our data suggest that aging is associated with a progressive decline in β-cell mitochondrial function that negatively impacts on the fine tuning of Ca(2+) dynamics. This is conceptually important since it emphasizes that even relatively modest changes in β-cell signal transduction over time lead to compromised insulin release and a diabetic phenotype.

β-Cell Ca(2+) dynamics and function are compromised in aging

Defects in pancreatic β-cell function and survival are key components in type 2 diabetes (T2D). An age-dependent deterioration in β-cell function has also been observed, but little is known about the molecular mechanisms behind this phenomenon. Our previous studies indicate that the regulation of cytoplasmic free Ca(2+) concentration ([Ca(2+)]i) may be critical and that this is dependent on the proper function of the mitochondria. The [Ca(2+)]i dynamics of the pancreatic β-cell are driven by an interplay between glucose-induced influx of extracellular Ca(2+) via voltage-dependent Ca(2+) channels and the inositol 1,4,5-trisphosphate (Ins(1,4,5)P3)-mediated liberation of Ca(2+) from intracellular stores. Our previous work has indicated a direct relationship between disruption of Ins(1,4,5)P3-mediated Ca(2+) regulation and loss of β-cell function, including disturbed [Ca(2+)]i dynamics and compromised insulin secretion. To investigate these processes in aging we used three mouse models, a premature aging mitochondrial mutator mouse, a mature aging phenotype (C57BL/6) and an aging-resistant phenotype (129). Our data suggest that age-dependent impairment in mitochondrial function leads to modest changes in [Ca(2+)]i dynamics in mouse β-cells, particularly in the pattern of [Ca(2+)]i oscillations. These changes are driven by modifications in both PLC/Ins(1,4,5)P3-mediated Ca(2+) mobilization from intracellular stores and decreased β-cell Ca(2+) influx over the plasma membrane. Our findings underscore an important concept, namely that even relatively small, time-dependent changes in β-cell signal-transduction result in compromised insulin release and in a diabetic phenotype.

The effect of unhealthy β-cells on insulin secretion in pancreatic islets

Background

Insulin secreted by pancreatic islet β-cells is the principal regulating hormone of glucose metabolism and plays a key role in controlling glucose level in blood. Impairment of the pancreatic islet function may cause glucose to accumulate in blood, and result in diabetes mellitus. Recent studies have shown that mitochondrial dysfunction has a strong negative effect on insulin secretion.

Methods

In order to study the cause of dysfunction of pancreatic islets, a multiple cell model containing healthy and unhealthy cells is proposed based on an existing single cell model. A parameter that represents the function of mitochondria is modified for unhealthy cells. A 3-D hexagonal lattice structure is used to model the spatial differences among β-cells in a pancreatic islet. The β-cells in the model are connected through direct electrical connections between neighboring β-cells.

Results

The simulation results show that the low ratio of total mitochondrial volume over cytoplasm volume per β-cell is a main reason that causes some mitochondria to lose their function. The results also show that the overall insulin secretion will be seriously disrupted when more than 15% of the β-cells in pancreatic islets become unhealthy.

Conclusion

Analysis of the model shows that the insulin secretion can be reinstated by increasing the glucokinase level. This new discovery sheds light on antidiabetic medication.

The relationship between membrane potential and calcium dynamics in glucose-stimulated beta cell syncytium in acute mouse pancreas tissue slices

Oscillatory electrical activity is regarded as a hallmark of the pancreatic beta cell glucose-dependent excitability pattern. Electrophysiologically recorded membrane potential oscillations in beta cells are associated with in-phase oscillatory cytosolic calcium activity ([Ca²⁺]_i) measured with fluorescent probes. Recent high spatial and temporal resolution confocal imaging revealed that glucose stimulation of beta cells in intact islets within acute tissue slices produces a [Ca²⁺]_i change with initial transient phase followed by a plateau phase with highly synchronized [Ca²⁺]_i oscillations. Here, we aimed to correlate the plateau [Ca²⁺]_i oscillations with the oscillations of membrane potential using patch-clamp and for the first time high resolution voltage-sensitive dye based confocal imaging. Our results demonstrated that the glucose-evoked membrane potential oscillations spread over the islet in a wave-like manner, their durations and wave velocities being comparable to the ones for [Ca²⁺]_i oscillations and waves. High temporal resolution simultaneous records of membrane potential and [Ca²⁺]_i confirmed tight but nevertheless limited coupling of the two processes, with membrane depolarization preceding the [Ca²⁺]_i increase. The potassium channel blocker tetraethylammonium increased the velocity at which oscillations advanced over the islet by several-fold while, at the same time, emphasized differences in kinetics of the membrane potential and the [Ca²⁺]_i. The combination of both imaging techniques provides a powerful tool that will help us attain deeper knowledge of the beta cell network.

Functional connectivity in islets of Langerhans from mouse pancreas tissue slices

We propose a network representation of electrically coupled beta cells in islets of Langerhans. Beta cells are functionally connected on the basis of correlations between calcium dynamics of individual cells, obtained by means of confocal laser-scanning calcium imaging in islets from acute mouse pancreas tissue slices. Obtained functional networks are analyzed in the light of known structural and physiological properties of islets. Focusing on the temporal evolution of the network under stimulation with glucose, we show that the dynamics are more correlated under stimulation than under non-stimulated conditions and that the highest overall correlation, largely independent of Euclidean distances between cells, is observed in the activation and deactivation phases when cells are driven by the external stimulus. Moreover, we find that the range of interactions in networks during activity shows a clear dependence on the Euclidean distance, lending support to previous observations that beta cells are synchronized via calcium waves spreading throughout islets. Most interestingly, the functional connectivity patterns between beta cells exhibit small-world properties, suggesting that beta cells do not form a homogeneous geometric network but are connected in a functionally more efficient way. Presented results provide support for the existing knowledge of beta cell physiology from a network perspective and shed important new light on the functional organization of beta cell syncitia whose structural topology is probably not as trivial as believed so far.

Glucose-stimulated calcium dynamics in islets of Langerhans in acute mouse pancreas tissue slices

In endocrine cells within islets of Langerhans calcium ions couple cell stimulation to hormone secretion. Since the advent of modern fluorimetry, numerous in vitro studies employing primarily isolated mouse islets have investigated the effects of various secretagogues on cytoplasmic calcium, predominantly in insulin-secreting beta cells. Due to technical limitations, insights of these studies are inherently limited to a rather small subpopulation of outermost cells. The results also seem to depend on various factors, like culture conditions and duration, and are not always easily reconcilable with findings in vivo. The main controversies regard the types of calcium oscillations, presence of calcium waves, and the level of synchronized activity. Here, we set out to combine the in situ acute mouse pancreas tissue slice preparation with noninvasive fluorescent calcium labeling and subsequent confocal laser scanning microscopy to shed new light on the existing controversies utilizing an innovative approach enabling the characterization of responses in many cells from all layers of islets. Our experiments reproducibly showed stable fast calcium oscillations on a sustained plateau rather than slow oscillations as the predominant type of response in acute tissue slices, and that calcium waves are the mechanistic substrate for synchronization of oscillations. We also found indirect evidence that even a large amplitude calcium signal was not sufficient and that metabolic activation was necessary to ensure cell synchronization upon stimulation with glucose. Our novel method helped resolve existing controversies and showed the potential to help answer important physiological questions, making it one of the methods of choice for the foreseeable future.

Mathematical models for insulin secretion in pancreatic β-cells

Insulin secretion is one of the most characteristic features of β-cell physiology. As it plays a central role in glucose regulation, a number of experimental and theoretical studies have been performed since the discovery of the pancreatic β-cell. This review article aims to give an overview of the mathematical approaches to insulin secretion. Beginning with the bursting electrical activity in pancreatic β-cells, we describe effects of the gap-junction coupling between β-cells on the dynamics of insulin secretion. Then, implications of paracrine interactions among such islet cells as α-, β-, and δ-cells are discussed. Finally, we present mathematical models which incorporate effects of glycolysis and mitochondrial glucose metabolism on the control of insulin secretion.

The neurotransmitter ATP triggers Ca2+ responses promoting coordination of pancreatic islet oscillations

OBJECTIVES

Pulsatile insulin release into the portal vein is critically dependent on entrainment of the islets in the pancreas into a common oscillatory phase. Because the pulses reflect periodic variations of the cytoplasmic Ca concentration ([Ca]i), we studied whether the neurotransmitters adenosine triphosphate (ATP) and acetylcholine promote synchronization of [Ca]i oscillations between islets lacking contact.

METHODS

Medium-sized and small mouse islets and cell aggregates were used for measuring [Ca]i with the indicator fura-2.

RESULTS

Exposure to acetylcholine resulted in an initial [Ca]i peak followed by disappearance of the [Ca]i oscillations induced by 11-mmol/L glucose. The effect of ATP was often restricted to an elusive [Ca]i peak. The incidence of distinct [Ca]i responses to ATP increased under conditions (accelerated superfusion, small islets, or cell aggregates) intended to counteract purinoceptor desensitization owing to intercellular accumulation of ATP. Attempts to imitate neural activity by brief (15 seconds) exposure to ATP or acetylcholine resulted in temporary synchronization of the glucose-induced [Ca]i oscillations between islets lacking contact.

CONCLUSIONS

The data support the idea that purinergic signaling has a key role for coordinating the oscillatory activity of the islets in the pancreas, reinforcing previous arguments for the involvement of nonadrenergic, noncholinergic neurons.

Mechanisms of control of the free Ca2+ concentration in the endoplasmic reticulum of mouse pancreatic β-cells: interplay with cell metabolism and [Ca2+]c and role of SERCA2b and SERCA3

OBJECTIVE

Sarco-endoplasmic reticulum Ca(2+)-ATPase 2b (SERCA2b) and SERCA3 pump Ca(2+) in the endoplasmic reticulum (ER) of pancreatic β-cells. We studied their role in the control of the free ER Ca(2+) concentration ([Ca(2+)](ER)) and the role of SERCA3 in the control of insulin secretion and ER stress.

RESEARCH DESIGN AND METHODS

β-Cell [Ca(2+)](ER) of SERCA3(+/+) and SERCA3(-/-) mice was monitored with an adenovirus encoding the low Ca(2+)-affinity sensor D4 addressed to the ER (D4ER) under the control of the insulin promoter. Free cytosolic Ca(2+) concentration ([Ca(2+)](c)) and [Ca(2+)](ER) were simultaneously recorded. Insulin secretion and mRNA levels of ER stress genes were studied.

RESULTS

Glucose elicited synchronized [Ca(2+)](ER) and [Ca(2+)](c) oscillations. [Ca(2+)](ER) oscillations were smaller in SERCA3(-/-) than in SERCA3(+/+) β-cells. Stimulating cell metabolism with various [glucose] in the presence of diazoxide induced a similar dose-dependent [Ca(2+)](ER) rise in SERCA3(+/+) and SERCA3(-/-) β-cells. In a Ca(2+)-free medium, glucose moderately raised [Ca(2+)](ER) from a highly buffered cytosolic Ca(2+) pool. Increasing [Ca(2+)](c) with high [K] elicited a [Ca(2+)](ER) rise that was larger but more transient in SERCA3(+/+) than SERCA3(-/-) β-cells because of the activation of a Ca(2+) release from the ER in SERCA3(+/+) β-cells. Glucose-induced insulin release was larger in SERCA3(-/-) than SERCA3(+/+) islets. SERCA3 ablation did not induce ER stress.

CONCLUSIONS

[Ca(2+)](c) and [Ca(2+)](ER) oscillate in phase in response to glucose. Upon [Ca(2+)](c) increase, Ca(2+) is taken up by SERCA2b and SERCA3. Strong Ca(2+) influx triggers a Ca(2+) release from the ER that depends on SERCA3. SERCA3 deficiency neither impairs Ca(2+) uptake by the ER upon cell metabolism acceleration and insulin release nor induces ER stress.

Bursting and calcium oscillations in pancreatic beta-cells: specific pacemakers for specific mechanisms

Oscillatory phenomenon in electrical activity and cytoplasmic calcium concentration in response to glucose are intimately connected to multiple key aspects of pancreatic β-cell physiology. However, there is no single model for oscillatory mechanisms in these cells. We set out to identify possible pacemaker candidates for burst activity and cytoplasmic Ca(2+) oscillations in these cells by analyzing published hypotheses, their corresponding mathematical models, and relevant experimental data. We found that although no single pacemaker can account for the variety of oscillatory phenomena in β-cells, at least several separate mechanisms can underlie specific kinds of oscillations. According to our analysis, slowly activating Ca(2+)-sensitive K(+) channels can be responsible for very fast Ca(2+) oscillations; changes in the ATP/ADP ratio and in the endoplasmic reticulum calcium concentration can be pacemakers for both fast bursts and cytoplasmic calcium oscillations, and cyclical cytoplasmic Na(+) changes may underlie patterning of slow calcium oscillations. However, these mechanisms still lack direct confirmation, and their potential interactions raises new issues. Further studies supported by improved mathematical models are necessary to understand oscillatory phenomena in β-cell physiology.

Targeted disruption of pancreatic-derived factor (PANDER, FAM3B) impairs pancreatic beta-cell function

OBJECTIVE

Pancreatic-derived factor (PANDER, FAM3B) is a pancreatic islet-specific cytokine-like protein that is secreted from beta-cells upon glucose stimulation. The biological function of PANDER is unknown, and to address this we generated and characterized a PANDER knockout mouse.

RESEARCH DESIGN AND METHODS

To generate the PANDER knockout mouse, the PANDER gene was disrupted and its expression was inhibited by homologous recombination via replacement of the first two exons, secretion signal peptide and transcriptional start site, with the neomycin gene. PANDER(-/-) mice were then phenotyped by a number of in vitro and in vivo tests to evaluate potential effects on glucose regulation, insulin sensitivity, and beta-cell morphology and function.

RESULTS

Glucose tolerance tests demonstrated significantly higher blood glucose levels in PANDER(-/-) versus wild-type male mice. To identify the mechanism of the glucose intolerance, insulin sensitivity and pancreatic beta-cell function were examined. Hyperinsulinemic-euglycemic clamps and insulin tolerance testing showed similar insulin sensitivity for both the PANDER(-/-) and wild-type mice. The in vivo insulin response following intraperitoneal glucose injection surprisingly produced significantly higher insulin levels in the PANDER(-/-) mice, whereas insulin release was blunted with arginine administration. Islet perifusion and calcium imaging studies showed abnormal responses of the PANDER(-/-) islets to glucose stimulation. In contrast, neither islet architecture nor insulin content was impacted by the loss of PANDER. Interestingly, the elevated insulin levels identified in vivo were attributed to decreased hepatic insulin clearance in the PANDER(-/-) islets. Taken together, these results demonstrated decreased pancreatic beta-cell function in the PANDER(-/-) mouse.

CONCLUSIONS

These results support a potential role of PANDER in the pancreatic beta-cell for regulation or facilitation of insulin secretion.

Loss of high-frequency glucose-induced Ca2+ oscillations in pancreatic islets correlates with impaired glucose tolerance in Trpm5-/- mice

Glucose homeostasis is critically dependent on insulin release from pancreatic beta-cells, which is strictly regulated by glucose-induced oscillations in membrane potential (V(m)) and the cytosolic calcium level ([Ca(2+)](cyt)). We propose that TRPM5, a Ca(2+)-activated monovalent cation channel, is a positive regulator of glucose-induced insulin release. Immunofluorescence revealed expression of TRPM5 in pancreatic islets. A Ca(2+)-activated nonselective cation current with TRPM5-like properties is significantly reduced in Trpm5(-/-) cells. Ca(2+)-imaging and electrophysiological analysis show that glucose-induced oscillations of V(m) and [Ca(2+)](cyt) have on average a reduced frequency in Trpm5(-/-) islets, specifically due to a lack of fast oscillations. As a consequence, glucose-induced insulin release from Trpm5(-/-) pancreatic islets is significantly reduced, resulting in an impaired glucose tolerance in Trpm5(-/-) mice.

Glucose metabolism, islet architecture, and genetic homogeneity in imprinting of [Ca2+](i) and insulin rhythms in mouse islets

We reported previously that islets isolated from individual, outbred Swiss-Webster mice displayed oscillations in intracellular calcium ([Ca²⁺]_i) that varied little between islets of a single mouse but considerably between mice, a phenomenon we termed “islet imprinting.” We have now confirmed and extended these findings in several respects. First, imprinting occurs in both inbred (C57BL/6J) as well as outbred mouse strains (Swiss-Webster; CD1). Second, imprinting was observed in NAD(P)H oscillations, indicating a metabolic component. Further, short-term exposure to a glucose-free solution, which transiently silenced [Ca²⁺]_i oscillations, reset the oscillatory patterns to a higher frequency. This suggests a key role for glucose metabolism in maintaining imprinting, as transiently suppressing the oscillations with diazoxide, a K_ATP-channel opener that blocks [Ca²⁺]_i influx downstream of glucose metabolism, did not change the imprinted patterns. Third, imprinting was not as readily observed at the level of single beta cells, as the [Ca²⁺]_i oscillations of single cells isolated from imprinted islets exhibited highly variable, and typically slower [Ca²⁺]_i oscillations. Lastly, to test whether the imprinted [Ca²⁺]_i patterns were of functional significance, a novel microchip platform was used to monitor insulin release from multiple islets in real time. Insulin release patterns correlated closely with [Ca²⁺]_i oscillations and showed significant mouse-to-mouse differences, indicating imprinting. These results indicate that islet imprinting is a general feature of islets and is likely to be of physiological significance. While islet imprinting did not depend on the genetic background of the mice, glucose metabolism and intact islet architecture may be important for the imprinting phenomenon.

Quantitative monitoring of insulin secretion from single islets of Langerhans in parallel on a microfluidic chip

Quantification of insulin release from pancreatic islets of Langerhans is of interest for diabetes research. Typical insulin secretion experiments are performed using offline techniques that are expensive, slow, have low-throughput, and require multiple islets. We have developed a microfluidic device for high-throughput, automated, and online monitoring of insulin secretion from individual islets in parallel. This chip consists of 15 channel networks each capable of superfusing a single islet and mixing superfusate from each islet online with fluorescein isothiocyanate-labeled insulin and anti-insulin antibody for a competitive immunoassay. The resulting continuous reaction streams are periodically injected onto parallel electrophoresis channels where the mixtures are separated. The resulting traces are used to quantify relative insulin released from islets. Serial immunoassays were performed at 10 s intervals on all 15 channels, corresponding to 5400 immunoassays per hour, to create temporally resolved insulin release profiles that captured single islet secretion dynamics. The chip was used to demonstrate that free fatty acid induced lipotoxicity in islets eliminates pulsatile insulin secretion.

Glucose-dependent and -independent electrical activity in islets of Langerhans of Psammomys obesus, an animal model of nutritionally induced obesity and diabetes

Glucose-induced insulin secretion from pancreatic beta-cells involves metabolism-induced membrane depolarization and voltage-dependent Ca(2+) influx. The electrical events in beta-cell glucose sensing have been studied intensely using mouse islets of Langerhans, but data from other species, including models of type 2 diabetes mellitus (T2DM), are lacking. In this work, we made intracellular recordings of electrical activity from cells within islets of the gerbil Psammomys obesus (fat sand rat), a model of dietary-induced T2DM. Most islet cells from lean, non-diabetic sand rats displayed glucose-induced, K(ATP) channel-dependent, oscillatory electrical activity that was similar to the classic "bursting" pattern of mouse beta-cells. However, the oscillations were slower in sand rat islets, and the dose-response curve of electrical activity versus glucose concentration was left-shifted. Of the non-bursting cells, some produced action potentials continuously, while others displayed electrical activity that was largely independent of glucose. The latter activity consisted of continuous or intermittent action potential firing, and persisted for long periods in the absence of glucose. The glucose-insensitive activity was suppressed by diazoxide, indicating that the cells expressed K(ATP) channels. Sand rat islets produced intracellular Ca(2+) oscillations reminiscent of the oscillatory electrical pattern observed in most cells, albeit with a longer period. Finally, we found that the glucose dependence of insulin secretion from sand rat islets closely paralleled that of the bursting electrical activity. We conclude that while subpopulations of K(ATP)-expressing cells in sand rat islets display heterogeneous electrical responses to glucose, insulin secretion most closely follows the oscillatory activity. The ease of recording membrane potential from sand rat islets makes this a useful model for studies of beta-cell electrical signaling during the development of T2DM.

Evidence of diminished glucose stimulation and endoplasmic reticulum function in nonoscillatory pancreatic islets

Pulsatility is a fundamental feature of pancreatic islets and a hallmark of hormone secretion. Isolated pancreatic islets endogenously generate rhythms in secretion, metabolic activity, and intracellular calcium ([Ca(2+)](i)) that are important to normal physiological function. Few studies have directly compared oscillatory and nonoscillatory islets to identify possible differences in function. We investigated the hypothesis that the loss of these oscillations is a leading indicator of islet dysfunction by comparing oscillatory and nonoscillatory mouse islets for multiple parameters of function. Nonoscillatory islets displayed elevated basal [Ca(2+)](i) and diminished [Ca(2+)](i) response and insulin secretory response to 3-28 mm glucose stimulation compared with oscillatory islets, suggesting diminished glucose sensitivity. We investigated several possible mechanisms to explain these differences. No differences were observed in mitochondrial membrane potential, estimated ATP-sensitive potassium channel and L-type calcium channel activity, or cell death rates. Nonoscillatory islets, however, showed a reduced response to the sarco(endo)plasmic reticulum calcium ATPase inhibitor thapsigargin, suggesting a disruption in calcium homeostasis in the endoplasmic reticulum (ER) compared with oscillatory islets. The diminished ER calcium homeostasis among nonoscillatory islets was also consistent with the higher cytosolic calcium levels observed in 3 mm glucose. Inducing mild damage with low-dose proinflammatory cytokines reduced islet oscillatory capacity and produced similar effects on glucose-stimulated [Ca(2+)](i), basal [Ca(2+)](i), and thapsigargin response observed among untreated nonoscillatory islets. Our data suggest the loss of oscillatory capacity may be an early indicator of diminished islet glucose sensitivity and ER dysfunction, suggesting targets to improve islet assessment.

Oscillatory control of insulin secretion

Pancreatic beta-cells possess an inherent ability to generate oscillatory signals that trigger insulin release. Coordination of the secretory activity among beta-cells results in pulsatile insulin secretion from the pancreas, which is considered important for the action of the hormone in the target tissues. This review focuses on the mechanisms underlying oscillatory control of insulin secretion at the level of the individual beta-cell. Recent studies have demonstrated that oscillations of the cytoplasmic Ca(2+) concentration are synchronized with oscillations in beta-cell metabolism, intracellular cAMP concentration, phospholipase C activity and plasma membrane phosphoinositide lipid concentrations. There are complex interdependencies between the different messengers and signalling pathways that contribute to amplitude regulation and shaping of the insulin secretory response to nutrient stimuli and neurohormonal modulators. Several of these pathways may be important pharmacological targets for improving pulsatile insulin secretion in type 2 diabetes.

A subcellular model of glucose-stimulated pancreatic insulin secretion

When glucose is raised from a basal to stimulating level, the pancreatic islets respond with a typical biphasic insulin secretion pattern. Moreover, the pancreas is able to recognize the rate of change of the glucose concentration. We present a relatively simple model of insulin secretion from pancreatic beta-cells, yet founded on solid physiological grounds and capable of reproducing a series of secretion patterns from perfused pancreases as well as from stimulated islets. The model includes the notion of distinct pools of granules as well as mechanisms such as mobilization, priming, exocytosis and kiss-and-run. Based on experimental data, we suggest that the individual beta-cells activate at different glucose concentrations. The model reproduces most of the data it was tested against very well, and can therefore serve as a general model of glucose-stimulated insulin secretion. Simulations predict that the effect of an increased frequency of kiss-and-run exocytotic events is a reduction in insulin secretion without modification of the qualitative pattern. Our model also appears to be the first physiology-based one to reproduce the staircase experiment, which underlies 'derivative control', i.e. the pancreatic capacity of measuring the rate of change of the glucose concentration.

Glucose-induced cyclic AMP oscillations regulate pulsatile insulin secretion

Cyclic AMP (cAMP) and Ca(2+) are key regulators of exocytosis in many cells, including insulin-secreting beta cells. Glucose-stimulated insulin secretion from beta cells is pulsatile and involves oscillations of the cytoplasmic Ca(2+) concentration ([Ca(2+)](i)), but little is known about the detailed kinetics of cAMP signaling. Using evanescent-wave fluorescence imaging we found that glucose induces pronounced oscillations of cAMP in the submembrane space of single MIN6 cells and primary mouse beta cells. These oscillations were preceded and enhanced by elevations of [Ca(2+)](i). However, conditions raising cytoplasmic ATP could trigger cAMP elevations without accompanying [Ca(2+)](i) rise, indicating that adenylyl cyclase activity may be controlled also by the substrate concentration. The cAMP oscillations correlated with pulsatile insulin release. Whereas elevation of cAMP enhanced secretion, inhibition of adenylyl cyclases suppressed both cAMP oscillations and pulsatile insulin release. We conclude that cell metabolism directly controls cAMP and that glucose-induced cAMP oscillations regulate the magnitude and kinetics of insulin exocytosis.