Critical role of gap junction coupled KATP channel activity for regulated insulin secretion

Extracellular electrophysiology on clonal human β-cell spheroids

Background

Pancreatic islets are important in nutrient homeostasis and improved cellular models of clonal origin may very useful especially in view of relatively scarce primary material. Close 3D contact and coupling between β-cells are a hallmark of physiological function improving signal/noise ratios. Extracellular electrophysiology using micro-electrode arrays (MEA) is technically far more accessible than single cell patch clamp, enables dynamic monitoring of electrical activity in 3D organoids and recorded multicellular slow potentials (SP) provide unbiased insight in cell-cell coupling.

Objective

We have therefore asked whether 3D spheroids enhance clonal β-cell function such as electrical activity and hormone secretion using human EndoC-βH1, EndoC-βH5 and rodent INS-1 832/13 cells.

Methods

Spheroids were formed either by hanging drop or proprietary devices. Extracellular electrophysiology was conducted using multi-electrode arrays with appropriate signal extraction and hormone secretion measured by ELISA.

Results

EndoC-βH1 spheroids exhibited increased signals in terms of SP frequency and especially amplitude as compared to monolayers and even single cell action potentials (AP) were quantifiable. Enhanced electrical signature in spheroids was accompanied by an increase in the glucose stimulated insulin secretion index. EndoC-βH5 monolayers and spheroids gave electrophysiological profiles similar to EndoC-βH1, except for a higher electrical activity at 3 mM glucose, and exhibited moreover a biphasic profile. Again, physiological concentrations of GLP-1 increased AP frequency. Spheroids also exhibited a higher secretion index. INS-1 cells did not form stable spheroids, but overexpression of connexin 36, required for cell-cell coupling, increased glucose responsiveness, dampened basal activity and consequently augmented the stimulation index.

Conclusion

In conclusion, spheroid formation enhances physiological function of the human clonal β-cell lines and these models may provide surrogates for primary islets in extracellular electrophysiology.

Twenty years of islet-on-a-chip: microfluidic tools for dissecting islet metabolism and function

Pancreatic islets are metabolically active micron-sized tissues responsible for controlling blood glucose through the secretion of insulin and glucagon. A loss of functional islet mass results in type 1 and 2 diabetes. Islet-on-a-chip devices are powerful microfluidic tools used to trap and study living ex vivo human and murine pancreatic islets and potentially stem cell-derived islet organoids. Devices developed over the past twenty years offer the ability to treat islets with controlled and dynamic microenvironments to mimic in vivo conditions and facilitate diabetes research. In this review, we explore the various islet-on-a-chip devices used to immobilize islets, regulate the microenvironment, and dynamically detect islet metabolism and insulin secretion. We first describe and assess the various methods used to immobilize islets including chambers, dam-walls, and hydrodynamic traps. We subsequently describe the surrounding methods used to create glucose gradients, enhance the reaggregation of dispersed islets, and control the microenvironment of stem cell-derived islet organoids. We focus on the various methods used to measure insulin secretion including capillary electrophoresis, droplet microfluidics, off-chip ELISAs, and on-chip fluorescence anisotropy immunoassays. Additionally, we delve into the various multiparametric readouts (NAD(P)H, Ca2+-activity, and O2-consumption rate) achieved primarily by adopting a microscopy-compatible optical window into the devices. By critical assessment of these advancements, we aim to inspire the development of new devices by the microfluidics community and accelerate the adoption of islet-on-a-chip devices by the wider diabetes research and clinical communities.

Self-organized insulin-producing β-cells differentiated from human omentum-derived stem cells and their in vivo therapeutic potential

BACKGROUND

Human omentum-derived mesenchymal stem cells (hO-MSCs) possess great potential to differentiate into multiple lineages and have self-renewal capacity, allowing them to be utilized as patient-specific cell-based therapeutics. Although the use of various stem cell-derived β-cells has been proposed as a novel approach for treating diabetes mellitus, developing an efficient method to establish highly functional β-cells remains challenging.

METHODS

We aimed to develop a novel cell culture platform that utilizes a fibroblast growth factor 2 (FGF2)-immobilized matrix to regulate the adhesion and differentiation of hO-MSCs into insulin-producing β-cells via cell-matrix/cell-cell interactions. In our study, we evaluated the in vitro differentiation potential of hO-MSCs cultured on an FGF2-immobilized matrix and a round-bottom plate (RBP). Further, the in vivo therapeutic efficacy of the β-cells transplanted into kidney capsules was evaluated using animal models with streptozotocin (STZ)-induced diabetes.

RESULTS

Our findings demonstrated that cells cultured on an FGF2-immobilized matrix could self-organize into insulin-producing β-cell progenitors, as evident from the upregulation of pancreatic β-cell-specific markers (PDX-1, Insulin, and Glut-2). Moreover, we observed significant upregulation of heparan sulfate proteoglycan, gap junction proteins (Cx36 and Cx43), and cell adhesion molecules (E-cadherin and Ncam1) in cells cultured on the FGF2-immobilized matrix. In addition, in vivo transplantation of differentiated β-cells into animal models of STZ-induced diabetes revealed their survival and engraftment as well as glucose-sensitive production of insulin within the host microenvironment, at over 4 weeks after transplantation.

CONCLUSIONS

Our findings suggest that the FGF2-immobilized matrix can support initial cell adhesion, maturation, and glucose-stimulated insulin secretion within the host microenvironment. Such a cell culture platform can offer novel strategies to obtain functional pancreatic β-cells from patient-specific cell sources, ultimately enabling better treatment for diabetes mellitus.

KATP channels are necessary for glucose-dependent increases in amyloid-β and Alzheimer's disease-related pathology

Elevated blood glucose levels, or hyperglycemia, can increase brain excitability and amyloid-β (Aβ) release, offering a mechanistic link between type 2 diabetes and Alzheimer's disease (AD). Since the cellular mechanisms governing this relationship are poorly understood, we explored whether ATP-sensitive potassium (KATP) channels, which couple changes in energy availability with cellular excitability, play a role in AD pathogenesis. First, we demonstrate that KATP channel subunits Kir6.2/KCNJ11 and SUR1/ABCC8 were expressed on excitatory and inhibitory neurons in the human brain, and cortical expression of KCNJ11 and ABCC8 changed with AD pathology in humans and mice. Next, we explored whether eliminating neuronal KATP channel activity uncoupled the relationship between metabolism, excitability, and Aβ pathology in a potentially novel mouse model of cerebral amyloidosis and neuronal KATP channel ablation (i.e., amyloid precursor protein [APP]/PS1 Kir6.2-/- mouse). Using both acute and chronic paradigms, we demonstrate that Kir6.2-KATP channels are metabolic sensors that regulate hyperglycemia-dependent increases in interstitial fluid levels of Aβ, amyloidogenic processing of APP, and amyloid plaque formation, which may be dependent on lactate release. These studies identify a potentially new role for Kir6.2-KATP channels in AD and suggest that pharmacological manipulation of Kir6.2-KATP channels holds therapeutic promise in reducing Aβ pathology in patients with diabetes or prediabetes.

Visualizing hypoxic modulation of beta cell secretions via a sensor augmented oxygen gradient

One distinct advantage of microfluidic-based cell assays is their scalability for multiple concentrations or gradients. Microfluidic scaling can be extremely powerful when combining multiple parameters and modalities. Moreover, in situ stimulation and detection eliminates variability between individual bioassays. However, conventional microfluidics must combat diffusion, which limits the spatial distance and time for molecules traveling through microchannels. Here, we leveraged a multilayered microfluidic approach to integrate a novel oxygen gradient (0-20%) with an enhanced hydrogel sensor to study pancreatic beta cells. This enabled our microfluidics to achieve spatiotemporal detection that is difficult to achieve with traditional microfluidics. Using this device, we demonstrated the in situ detection of calcium, insulin, and ATP (adenosine triphosphate) in response to glucose and oxygen stimulation. Specifically, insulin was quantified at levels as low as 25 pg/mL using our imaging technique. Furthermore, by analyzing the spatial detection data dynamically over time, we uncovered a new relationship between oxygen and beta cell oscillations. We observed an optimum oxygen level between 10 and 12%, which is neither hypoxic nor normoxic in the conventional cell culture sense. These results provide evidence to support the current islet oscillator model. In future applications, this spatial microfluidic technique can be adapted for discrete protein detection in a robust platform to study numerous oxygen-dependent tissue dysfunctions.

Functional architecture of pancreatic islets identifies a population of first responder cells that drive the first-phase calcium response

Insulin-secreting β-cells are functionally heterogeneous. Whether there exist cells driving the first-phase calcium response in individual islets, has not been examined. We examine “first responder” cells, defined by the earliest [Ca²⁺] response during first-phase [Ca²⁺] elevation, distinct from previously identified “hub” and “leader” cells. We used islets isolated from Mip-Cre^ER; Rosa-Stop-Lox-Stop-GCamP6s mice (β-GCamP6s) that show β-cell-specific GCamP6s expression following tamoxifen-induced CreER-mediated recombination. First responder cells showed characteristics of high membrane excitability and lower electrical coupling to their neighbors. The first-phase response time of β-cells in the islet was spatially organized, dependent on the cell’s distance to the first responder cell, and consistent over time up to approximately 24 h. When first responder cells were laser ablated, the first-phase [Ca²⁺] was slowed down, diminished, and discoordinated compared to random cell ablation. Cells that were next earliest to respond often took over the role of the first responder upon ablation. In summary, we discover and characterize a distinct first responder β-cell state, critical for the islet first-phase response to glucose.

A hallmark of the early stages in diabetes is the disruption of the first-phase peak of insulin secretion. This study reveals a state of beta cells that drives the first-phase calcium response (a precursor to insulin secretion), challenging the existing paradigm that first-phase calcium response to glucose is not organized within the islet.

Design of a versatile microfluidic device for imaging precision-cut-tissue slices

Precision-cut-tissues (PCTs), which preserve many aspects of a tissue's microenvironment, are typically imaged using conventional sample dishes and chambers. These can require large amounts of reagent and, when used for flow-through experiments, the shear forces applied on the tissues are often ill-defined. Their physical design also makes it difficult to image large volumes and repetitively image smaller regions of interest in the living slice. We report here on the design of a versatile microfluidic device capable of holding mouse or human pancreas PCTs for 3D fluorescence imaging using confocal and selective plane illumination microscopy (SPIM). Our design positions PCTs within a 5 × 5 mm × 140µm deep chamber fitted with 150µm tall channels to facilitate media exchange. Shear stress in the device is localized to small regions on the surface of the tissue and can be easily controlled. This design allows for media exchange at flowrates ∼10-fold lower than those required for conventional chambers. Finally, this design allows for imaging the same immunofluorescently labeled PCT with high resolution on a confocal and with large field of view on a SPIM, without adversely affecting image quality.

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

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

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.

Intercellular Communication in the Islet of Langerhans in Health and Disease

Blood glucose homeostasis requires proper function of pancreatic islets, which secrete insulin, glucagon, and somatostatin from the β-, α-, and δ-cells, respectively. Each islet cell type is equipped with intrinsic mechanisms for glucose sensing and secretory actions, but these intrinsic mechanisms alone cannot explain the observed secretory profiles from intact islets. Regulation of secretion involves interconnected mechanisms among and between islet cell types. Islet cells lose their normal functional signatures and secretory behaviors upon dispersal as compared to intact islets and in vivo. In dispersed islet cells, the glucose response of insulin secretion is attenuated from that seen from whole islets, coordinated oscillations in membrane potential and intracellular Ca²⁺ activity, as well as the two-phase insulin secretion profile, are missing, and glucagon secretion displays higher basal secretion profile and a reverse glucose-dependent response from that of intact islets. These observations highlight the critical roles of intercellular communication within the pancreatic islet, and how these communication pathways are crucial for proper hormonal and nonhormonal secretion and glucose homeostasis. Further, misregulated secretions of islet secretory products that arise from defective intercellular islet communication are implicated in diabetes. Intercellular communication within the islet environment comprises multiple mechanisms, including electrical synapses from gap junctional coupling, paracrine interactions among neighboring cells, and direct cell-to-cell contacts in the form of juxtacrine signaling. In this article, we describe the various mechanisms that contribute to proper islet function for each islet cell type and how intercellular islet communications are coordinated among the same and different islet cell types. © 2021 American Physiological Society. Compr Physiol 11:2191-2225, 2021.

Pancreatic β-cell heterogeneity in health and diabetes: classes, sources, and subtypes

Pancreatic β-cells perform glucose-stimulated insulin secretion, a process at the center of type 2 diabetes etiology. Efforts to understand how β-cells behave in healthy and stressful conditions have revealed a wide degree of morphological, functional, and transcriptional heterogeneity. Sources of heterogeneity include β-cell topography, developmental origin, maturation state, and stress response. Advances in sequencing and imaging technologies have led to the identification of β-cell subtypes, which play distinct roles in the islet niche. This review examines β-cell heterogeneity from morphological, functional, and transcriptional perspectives, and considers the relevance of topography, maturation, development, and stress response. It also discusses how these factors have been used to identify β-cell subtypes, and how heterogeneity is impacted by diabetes. We examine open questions in the field and discuss recent technological innovations that could advance understanding of β-cell heterogeneity in health and disease.

Phenotype of a transient neonatal diabetes point mutation (SUR1-R1183W) in mice

Background: The K ATP channel plays a key role in glucose homeostasis by coupling metabolically generated changes in ATP to insulin secretion from pancreatic beta-cells.  Gain-of-function mutations in either the pore-forming (Kir6.2) or regulatory (SUR1) subunit of this channel are a common cause of transient neonatal diabetes mellitus (TNDM), in which diabetes presents shortly after birth but remits within the first few years of life, only to return in later life. The reasons behind this time dependence are unclear. Methods: In an attempt to understand the mechanism behind diabetes remission and relapse, we generated mice expressing the common TNDM mutation SUR1-R1183W. We employed Cre/LoxP technology for both inducible and constitutive expression of SUR1-R1183W specifically in mouse beta-cells, followed by investigation of their phenotype using glucose tolerance tests and insulin secretion from isolated islets.  Results: We found that the R1183W mutation impaired inhibition of K ATP channels by ATP when heterologously expressed in human embryonic kidney cells. However, neither induced nor constitutive expression of SUR1-R1183W in mice resulted in changes in blood glucose homeostasis, compared to littermate controls. When challenged with a high fat diet, female mice expressing SUR1-R1183W showed increased weight gain, elevated blood glucose and impaired glycaemic control, but glucose-stimulated insulin secretion from pancreatic islets appeared unchanged. Conclusions: The mouse model of TNDM did not recapitulate the human phenotype. We discuss multiple potential reasons why this might be the case. Based on our findings, we recommend future TNDM mouse models employing a gain-of-function SUR1 mutation should be created using the minimally invasive CRISPR/Cas technology, which avoids many potential pitfalls associated with the Cre/LoxP system.

Chemical Biology Toolbox for Studying Pancreatic Islet Function - A Perspective

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

"Take Me To Your Leader": An Electrophysiological Appraisal of the Role of Hub Cells in Pancreatic Islets

The coordinated electrical activity of β-cells within the pancreatic islet drives oscillatory insulin secretion. A recent hypothesis postulates that specially equipped "hub" or "leader" cells within the β-cell network drive islet oscillations and that electrically silencing or optically ablating these cells suppresses coordinated electrical activity (and thus insulin secretion) in the rest of the islet. In this Perspective, we discuss this hypothesis in relation to established principles of electrophysiological theory. We conclude that whereas electrical coupling between β-cells is sufficient for the propagation of excitation across the islet, there is no obvious electrophysiological mechanism that explains how hyperpolarizing a hub cell results in widespread inhibition of islet electrical activity and disruption of their coordination. Thus, intraislet diffusible factors should perhaps be considered as an alternate mechanism.

Phenotype of a transient neonatal diabetes point mutation (SUR1-R1183W) in mice

Background: The K ATP channel plays a key role in glucose homeostasis by coupling metabolically generated changes in ATP to insulin secretion from pancreatic beta-cells.  Gain-of-function mutations in either the pore-forming (Kir6.2) or regulatory (SUR1) subunit of this channel are a common cause of transient neonatal diabetes mellitus (TNDM), in which diabetes presents shortly after birth but remits within the first few years of life, only to return in later life. The reasons behind this time dependence are unclear. Methods: In an attempt to understand the mechanism behind diabetes remission and relapse, we generated mice expressing the common TNDM mutation SUR1-R1183W. We employed Cre/LoxP technology for both inducible and constitutive expression of SUR1-R1183W specifically in mouse beta-cells, followed by investigation of their phenotype using glucose tolerance tests and insulin secretion from isolated islets.  Results: We found that the R1183W mutation impaired inhibition of K ATP channels by ATP when heterologously expressed in human embryonic kidney cells. However, neither induced nor constitutive expression of SUR1-R1183W in mice resulted in changes in blood glucose homeostasis, compared to littermate controls. When challenged with a high fat diet, female mice expressing SUR1-R1183W showed increased weight gain, elevated blood glucose and impaired glycaemic control, but glucose-stimulated insulin secretion from pancreatic islets appeared unchanged. Conclusions: The mouse model of TNDM did not recapitulate the human phenotype. We discuss multiple potential reasons why this might be the case. Based on our findings, we recommend future TNDM mouse models employing a gain-of-function SUR1 mutation should be created using the minimally invasive CRISPR/Cas technology, which avoids many potential pitfalls associated with the Cre/LoxP system.

Cooperative maintenance of cellular identity in systems with intercellular communication defects

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

How Heterogeneity in Glucokinase and Gap-Junction Coupling Determines the Islet [Ca2+] Response

Understanding how cell subpopulations in a tissue impact overall system function is challenging. There is extensive heterogeneity among insulin-secreting β-cells within islets of Langerhans, including their insulin secretory response and gene expression profile, and this heterogeneity can be altered in diabetes. Several studies have identified variations in nutrient sensing between β-cells, including glucokinase (GK) levels, mitochondrial function, or expression of genes important for glucose metabolism. Subpopulations of β-cells with defined electrical properties can disproportionately influence islet-wide free-calcium activity ([Ca2+]) and insulin secretion via gap-junction electrical coupling. However, it is poorly understood how subpopulations of β-cells with altered glucose metabolism may impact islet function. To address this, we utilized a multicellular computational model of the islet in which a population of cells deficient in GK activity and glucose metabolism was imposed on the islet or in which β-cells were heterogeneous in glucose metabolism and GK kinetics were altered. This included simulating GK gene (GCK) mutations that cause monogenic diabetes. We combined these approaches with experimental models in which gck was genetically deleted in a population of cells or GK was pharmacologically inhibited. In each case, we modulated gap-junction electrical coupling. Both the simulated islet and the experimental system required 30-50% of the cells to have near-normal glucose metabolism, fewer than cells with normal KATP conductance. Below this number, the islet lacked any glucose-stimulated [Ca2+] elevations. In the absence of electrical coupling, the change in [Ca2+] was more gradual. As such, electrical coupling allows a large minority of cells with normal glucose metabolism to promote glucose-stimulated [Ca2+]. If insufficient numbers of cells are present, which we predict can be caused by a subset of GCK mutations that cause monogenic diabetes, electrical coupling exacerbates [Ca2+] suppression. This demonstrates precisely how metabolically heterogeneous β-cell populations interact to impact islet function.

The role of membrane excitability in pancreatic β-cell glucotoxicity

Persistent hyperglycemia is causally associated with pancreatic β-cell dysfunction and loss of pancreatic insulin. Glucose normally enhances β-cell excitability through inhibition of K_ATP channels, opening of voltage-dependent calcium channels, increased [Ca²⁺]_i, which triggers insulin secretion. Glucose-dependent excitability is lost in islets from K_ATP-knockout (K_ATP-KO) mice, in which β-cells are permanently hyperexcited, [Ca²⁺]_i, is chronically elevated and insulin is constantly secreted. Mouse models of human neonatal diabetes in which K_ATP gain-of-function mutations are expressed in β-cells (K_ATP-GOF) also lose the link between glucose metabolism and excitation-induced insulin secretion, but in this case K_ATP-GOF β-cells are chronically underexcited, with permanently low [Ca²⁺]_i and lack of glucose-dependent insulin secretion. We used K_ATP-GOF and K_ATP-KO islets to examine the role of altered-excitability in glucotoxicity. Wild-type islets showed rapid loss of insulin content when chronically incubated in high-glucose, an effect that was reversed by subsequently switching to low glucose media. In contrast, hyperexcitable K_ATP-KO islets lost insulin content in both low- and high-glucose, while underexcitable K_ATP-GOF islets maintained insulin content in both conditions. Loss of insulin content in chronic excitability was replicated by pharmacological inhibition of K_ATP by glibenclamide, The effects of hyperexcitable and underexcitable islets on glucotoxicity observed in in vivo animal models are directly opposite to the effects observed in vitro: we clearly demonstrate here that in vitro, hyperexcitability is detrimental to islets whereas underexcitability is protective.

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

KEY POINTS

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

ABSTRACT

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

New Understanding of β-Cell Heterogeneity and In Situ Islet Function

Insulin-secreting β-cells are heterogeneous in their regulation of hormone release. While long known, recent technological advances and new markers have allowed the identification of novel subpopulations, improving our understanding of the molecular basis for heterogeneity. This includes specific subpopulations with distinct functional characteristics, developmental programs, abilities to proliferate in response to metabolic or developmental cues, and resistance to immune-mediated damage. Importantly, these subpopulations change in disease or aging, including in human disease. Although discovering new β-cell subpopulations has substantially advanced our understanding of islet biology, a point of caution is that these characteristics have often necessarily been identified in single β-cells dissociated from the islet. β-Cells in the islet show extensive communication with each other via gap junctions and with other cell types via diffusible chemical messengers. As such, how these different subpopulations contribute to in situ islet function, including during plasticity, is not well understood. We will discuss recent findings revealing functional β-cell subpopulations in the intact islet, the underlying basis for these identified subpopulations, and how these subpopulations may influence in situ islet function. Furthermore, we will discuss the outlook for emerging technologies to gain further insight into the role of subpopulations in in situ islet function.

Pancreatic β-Cell Electrical Activity and Insulin Secretion: Of Mice and Men

The pancreatic β-cell plays a key role in glucose homeostasis by secreting insulin, the only hormone capable of lowering the blood glucose concentration. Impaired insulin secretion results in the chronic hyperglycemia that characterizes type 2 diabetes (T2DM), which currently afflicts >450 million people worldwide. The healthy β-cell acts as a glucose sensor matching its output to the circulating glucose concentration. It does so via metabolically induced changes in electrical activity, which culminate in an increase in the cytoplasmic Ca2+ concentration and initiation of Ca2+-dependent exocytosis of insulin-containing secretory granules. Here, we review recent advances in our understanding of the β-cell transcriptome, electrical activity, and insulin exocytosis. We highlight salient differences between mouse and human β-cells, provide models of how the different ion channels contribute to their electrical activity and insulin secretion, and conclude by discussing how these processes become perturbed in T2DM.

Spatially Organized β-Cell Subpopulations Control Electrical Dynamics across Islets of Langerhans

Understanding how heterogeneous cells within a multicellular system interact and affect overall function is difficult without a means of perturbing individual cells or subpopulations. Here we apply optogenetics to understand how subpopulations of β-cells control the overall [Ca²⁺]_i response and insulin secretion dynamics of the islets of Langerhans. We spatiotemporally perturbed electrical activity in β-cells of channelrhodopsin2-expressing islets, mapped the [Ca²⁺]_i response, and correlated this with the cellular metabolic activity and an in silico electrophysiology model. We discovered organized regions of metabolic activity across the islet, and these affect the way in which β-cells electrically interact. Specific regions acted as pacemakers by initiating calcium wave propagation. Our findings reveal the functional architecture of the islet, and show how distinct subpopulations of cells can disproportionality affect function. These results also suggest ways in which other neuroendocrine systems can be regulated, and demonstrate how optogenetic tools can discern their functional architecture.

Gap-junction coupling and ATP-sensitive potassium channels in human β-cell clusters: Effects on emergent dynamics

The importance of gap-junction coupling between β cells in pancreatic islets is well established in mouse. Such ultrastructural connections synchronize cellular activity, confine biological heterogeneity, and enhance insulin pulsatility. Dysfunction of coupling has been associated with diabetes and altered β-cell function. However, the role of gap junctions between human β cells is still largely unexplored. By using patch-clamp recordings of β cells from human donors, we previously estimated electrical properties of these channels by mathematical modeling of pairs of human β cells. In this work we revise our estimate by modeling triplet configurations and larger heterogeneous clusters. We find that a coupling conductance in the range 0.005-0.020 nS/pF can reproduce experiments in almost all the simulated arrangements. We finally explore the consequence of gap-junction coupling of this magnitude between β cells with mutant variants of the ATP-sensitive potassium channels involved in some metabolic disorders and diabetic conditions, translating studies performed on rodents to the human case. Our results are finally discussed from the perspective of therapeutic strategies. In summary, modeling of more realistic clusters with more than two β cells slightly lowers our previous estimate of gap-junction conductance and gives rise to patterns that more closely resemble experimental traces.

All mixed up: defining roles for β-cell subtypes in mature islets

Following differentiation during fetal development, β cells further adapt to their postnatal role through functional maturation. While adult islets are thought to contain functionally mature β cells, recent analyses of transgenic rodent and human pancreata reveal a number of novel heterogeneity markers in mammalian β cells. The marked heterogeneity long after maturation raises the prospect that diverse populations harbor distinct roles aside from glucose-stimulated insulin secretion. In this review, we outline our current understanding of the β-cell maturation process, emphasize recent literature on novel heterogeneity markers, and offer perspectives on reconciling the findings from these two areas.

Gap junction proteins are key drivers of endocrine function

It has long been known that the main secretory cells of exocrine and endocrine glands are connected by gap junctions, made by a variety of connexin species that ensure their electrical and metabolic coupling. Experiments in culture systems and animal models have since provided increasing evidence that connexin signaling contributes to control the biosynthesis and release of secretory products, as well as to the life and death of secretory cells. More recently, genetic studies have further provided the first lines of evidence that connexins also control the function of human glands, which are central to the pathogenesis of major endocrine diseases. Here, we summarize the recent information gathered on connexin signaling in these systems, since the last reviews on the topic, with particular regard to the pancreatic beta cells which produce insulin, and the renal cells which produce renin. These cells are keys to the development of various forms of diabetes and hypertension, respectively, and combine to account for the exploding, worldwide prevalence of the metabolic syndrome. This article is part of a Special Issue entitled: Gap Junction Proteins edited by Jean Claude Herve.

The vasculature as a neural stem cell niche

Neural stem cells (NSCs) are multipotent, self-renewing progenitors that generate progeny that differentiate into neurons and glia. NSCs in the adult mammalian brain are generally quiescent. Environmental stimuli such as learning or exercise can activate quiescent NSCs, inducing them to proliferate and produce new neurons and glia. How are these behaviours coordinated? The neurovasculature, the circulatory system of the brain, is a key component of the NSC microenvironment, or 'niche'. Instructive signals from the neurovasculature direct NSC quiescence, proliferation, self-renewal and differentiation. During ageing, a breakdown in the niche accompanies NSC dysfunction and cognitive decline. There is much interest in reversing these changes and enhancing NSC activity by targeting the neurovasculature therapeutically. Here we discuss principles of neurovasculature-NSC crosstalk, and the implications for the design of NSC-based therapies. We also consider the emerging contributions to this field of the model organism Drosophila melanogaster.

Rpl13a small nucleolar RNAs regulate systemic glucose metabolism

Small nucleolar RNAs (snoRNAs) are non-coding RNAs that form ribonucleoproteins to guide covalent modifications of ribosomal and small nuclear RNAs in the nucleus. Recent studies have also uncovered additional non-canonical roles for snoRNAs. However, the physiological contributions of these small RNAs are largely unknown. Here, we selectively deleted four snoRNAs encoded within the introns of the ribosomal protein L13a (Rpl13a) locus in a mouse model. Loss of Rpl13a snoRNAs altered mitochondrial metabolism and lowered reactive oxygen species tone, leading to increased glucose-stimulated insulin secretion from pancreatic islets and enhanced systemic glucose tolerance. Islets from mice lacking Rpl13a snoRNAs demonstrated blunted oxidative stress responses. Furthermore, these mice were protected against diabetogenic stimuli that cause oxidative stress damage to islets. Our study illuminates a previously unrecognized role for snoRNAs in metabolic regulation.

Decreases in Gap Junction Coupling Recovers Ca2+ and Insulin Secretion in Neonatal Diabetes Mellitus, Dependent on Beta Cell Heterogeneity and Noise

Diabetes is caused by dysfunction to β-cells in the islets of Langerhans, disrupting insulin secretion and glucose homeostasis. Gap junction-mediated electrical coupling between β-cells in the islet plays a major role in coordinating a pulsatile secretory response at elevated glucose and suppressing insulin secretion at basal glucose. Previously, we demonstrated that a critical number of inexcitable cells can rapidly suppress the overall islet response, as a result of gap junction coupling. This was demonstrated in a murine model of Neonatal Diabetes Mellitus (NDM) involving expression of ATP-insensitive K_ATP channels, and by a multi-cellular computational model of islet electrical activity. Here we examined the mechanisms by which gap junction coupling contributes to islet dysfunction in NDM. We first verified the computational model against [Ca²⁺] and insulin secretion measurements in islets expressing ATP-insensitive K_ATP channels under different levels of gap junction coupling. We then applied this model to predict how different K_ATP channel mutations found in NDM suppress [Ca²⁺], and the role of gap junction coupling in this suppression. We further extended the model to account for stochastic noise and insulin secretion dynamics. We found experimentally and in the islet model that reductions in gap junction coupling allow progressively greater glucose-stimulated [Ca²⁺] and insulin secretion following expression of ATP-insensitive K_ATP channels. The model demonstrated good correspondence between suppression of [Ca²⁺] and clinical presentation of different NDM mutations. Significant recoveries in [Ca²⁺] and insulin secretion were predicted for many mutations upon reductions in gap junction coupling, where stochastic noise played a significant role in the recoveries. These findings provide new understanding how the islet functions as a multicellular system and for the role of gap junction channels in exacerbating the effects of decreased cellular excitability. They further suggest novel therapeutic options for NDM and other monogenic forms of diabetes.

Diabetes is a disease reaching a global epidemic, which results from dysfunction to the islets of Langerhans in the pancreas and their ability to secrete the hormone insulin to regulate glucose homeostasis. Islets are multicellular structures that show extensive coupling between heterogeneous cellular units; and central to the causes of diabetes is a dysfunction to these cellular units and their interactions. Understanding the inter-relationship between structure and function is challenging in biological systems, but is crucial to the cause of disease and discovering therapeutic targets. With the goal of further characterizing the islet of Langerhans and its excitable behavior, we examined the role of important channels in the islet where dysfunction is linked to or causes diabetes. Advances in our ability to computationally model perturbations in physiological systems has allowed for the testing of hypothesis quickly, in systems that are not experimentally accessible. Using an experimentally validated model and modeling human mutations, we discover that monogenic forms of diabetes may be remedied by a reduction in electrical coupling between cells; either alone or in conjunction with pharmacological intervention. Knowledge of biological systems in general is also helped by these findings, in that small changes to cellular elements may lead to major disruptions in the overall system. This may then be overcome by allowing the system components to function independently in the presence of dysfunction to individual cells.

Highly efficient adenoviral transduction of pancreatic islets using a microfluidic device

Tissues are challenging to genetically manipulate due to limited penetration of viral particles resulting in low transduction efficiency. We are particularly interested in expressing genetically-encoded sensors in ex vivo pancreatic islets to measure glucose-stimulated metabolism, however poor viral penetration biases these measurements to only a subset of cells at the periphery. To increase mass transfer of viral particles, we designed a microfluidic device that holds islets in parallel hydrodynamic traps connected by an expanding by-pass channel. We modeled viral particle flow into the tissue using fluorescently-labelled gold nanoparticles of varying sizes and showed a penetration threshold of only ∼5 nm. To increase this threshold, we used EDTA to transiently reduce cell-cell adhesion and expand intercellular space. Ultimately, a combination of media flow and ETDA treatment significantly increased adenoviral transduction to the core of the islet. As proof-of-principle, we used this protocol to transduce an ER-targeted redox sensitive sensor (eroGFP), and revealed significantly greater ER redox capacity at core islet cells. Overall, these data demonstrate a robust method to enhance transduction efficiency of islets, and potentially other tissues, by using a combination of microfluidic flow and transient tissue expansion.

ATP-sensitive K(+) channels (Kir6.1/SUR1) regulate gap junctional coupling in cochlear-supporting cells

Using the double whole-cell patch-clamp technique, we found that the absence of intracellular ATP led to gap junction uncoupling in cochlear-supporting Hensen cells. The uncoupling was observed as a progressive reduction of the gap junctional electrical conductance from a starting value of approximately 40 nS to less than 0.04 nS within 10-20 min. The conductance rundown was partly avoided by at least 3 mM ATP and completely suppressed by 5 mM ATP or 5'-adenylyl-imidodiphosphate (AMP-PNP), the non-hydrolysable ATP analog, in the pipette filling solution, suggesting that ATP was needed as ligand and not as a hydrolysable energy supplier or substrate for enzymatic reactions. The effect of intracellular ATP was mimicked by the external application of barium, a nonselective blocker of inwardly rectifying K(+) (Kir) channels, and glibenclamide, an inhibitor of the ATP-sensitive Kir channels (KATP). Moreover a Ba(2+)-sensitive whole-cell inward current was observed in absence of internal ATP. We propose that the internal ATP kept the KATP channels in a closed state, thereby maintaining the gap junction coupling of Hensen cells. The immunostaining of guinea pig cochlear tissue revealed for the first time the expression of the KATP channel subunits Kir6.1 and SUR1 in Hensen cells and supported the proposed hypothesis. The results suggest that KATP channels, as regulator of the gap junction coupling in Hensen cells, could be the physiological link between the metabolic state of the supporting cells and K(+) recycling in the organ of Corti.

Low Level Pro-inflammatory Cytokines Decrease Connexin36 Gap Junction Coupling in Mouse and Human Islets through Nitric Oxide-mediated Protein Kinase Cδ

Pro-inflammatory cytokines contribute to the decline in islet function during the development of diabetes. Cytokines can disrupt insulin secretion and calcium dynamics; however, the mechanisms underlying this are poorly understood. Connexin36 gap junctions coordinate glucose-induced calcium oscillations and pulsatile insulin secretion across the islet. Loss of gap junction coupling disrupts these dynamics, similar to that observed during the development of diabetes. This study investigates the mechanisms by which pro-inflammatory cytokines mediate gap junction coupling. Specifically, as cytokine-induced NO can activate PKCδ, we aimed to understand the role of PKCδ in modulating cytokine-induced changes in gap junction coupling. Isolated mouse and human islets were treated with varying levels of a cytokine mixture containing TNF-α, IL-1β, and IFN-γ. Islet dysfunction was measured by insulin secretion, calcium dynamics, and gap junction coupling. Modulators of PKCδ and NO were applied to determine their respective roles in modulating gap junction coupling. High levels of cytokines caused cell death and decreased insulin secretion. Low levels of cytokine treatment disrupted calcium dynamics and decreased gap junction coupling, in the absence of disruptions to insulin secretion. Decreases in gap junction coupling were dependent on NO-regulated PKCδ, and altered membrane organization of connexin36. This study defines several mechanisms underlying the disruption to gap junction coupling under conditions associated with the development of diabetes. These mechanisms will allow for greater understanding of islet dysfunction and suggest ways to ameliorate this dysfunction during the development of diabetes.

Microfluidic perfusion systems for secretion fingerprint analysis of pancreatic islets: applications, challenges and opportunities

A secretome signature is a heterogeneous profile of secretions present in a single cell type. From the secretome signature a smaller panel of proteins, namely a secretion fingerprint, can be chosen to feasibly monitor specific cellular activity. Based on a thorough appraisal of the literature, this review explores the possibility of defining and using a secretion fingerprint to gauge the functionality of pancreatic islets of Langerhans. It covers the state of the art regarding microfluidic perfusion systems used in pancreatic islet research. Candidate analytical tools to be integrated within microfluidic perfusion systems for dynamic secretory fingerprint monitoring were identified. These analytical tools include patch clamp, amperometry/voltametry, impedance spectroscopy, field effect transistors and surface plasmon resonance. Coupled with these tools, microfluidic devices can ultimately find applications in determining islet quality for transplantation, islet regeneration and drug screening of therapeutic agents for the treatment of diabetes.

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.

Concerted Trafficking Regulation of Kv2.1 and KATP Channels by Leptin in Pancreatic β-Cells

In pancreatic β-cells, voltage-gated potassium 2.1 (Kv2.1) channels are the dominant delayed rectifier potassium channels responsible for action potential repolarization. Here, we report that leptin, a hormone secreted by adipocytes known to inhibit insulin secretion, causes a transient increase in surface expression of Kv2.1 channels in rodent and human β-cells. The effect of leptin on Kv2.1 surface expression is mediated by the AMP-activated protein kinase (AMPK). Activation of AMPK mimics whereas inhibition of AMPK occludes the effect of leptin. Inhibition of Ca(2+)/calmodulin-dependent protein kinase kinase β, a known upstream kinase of AMPK, also blocks the effect of leptin. In addition, the cAMP-dependent protein kinase (PKA) is involved in Kv2.1 channel trafficking regulation. Inhibition of PKA prevents leptin or AMPK activators from increasing Kv2.1 channel density, whereas stimulation of PKA is sufficient to promote Kv2.1 channel surface expression. The increased Kv2.1 surface expression by leptin is dependent on actin depolymerization, and pharmacologically induced actin depolymerization is sufficient to enhance Kv2.1 surface expression. The signaling and cellular mechanisms underlying Kv2.1 channel trafficking regulation by leptin mirror those reported recently for ATP-sensitive potassium (KATP) channels, which are critical for coupling glucose stimulation with membrane depolarization. We show that the leptin-induced increase in surface KATP channels results in more hyperpolarized membrane potentials than control cells at stimulating glucose concentrations, and the increase in Kv2.1 channels leads to a more rapid repolarization of membrane potential in cells firing action potentials. This study supports a model in which leptin exerts concerted trafficking regulation of KATP and Kv2.1 channels to coordinately inhibit insulin secretion.

Modeling K,ATP--dependent excitability in pancreatic islets

In pancreatic ?-cells, K,ATP channels respond to changes in glucose to regulate cell excitability and insulin release. Confirming a high sensitivity of electrical activity to K,ATP activity, mutations that cause gain of K,ATP function cause neonatal diabetes. Our aim was to quantitatively assess the contribution of K,ATP current to the regulation of glucose-dependent bursting by reproducing experimentally observed changes in excitability when K,ATP conductance is altered by genetic manipulation. A recent detailed computational model of single cell pancreatic ?-cell excitability reproduces the ?-cell response to varying glucose concentrations. However, initial simulations showed that the model underrepresents the significance of K,ATP activity and was unable to reproduce K,ATP conductance-dependent changes in excitability. By altering the ATP and glucose dependence of the L-type Ca(2+) channel and the Na-K ATPase to better fit experiment, appropriate dependence of excitability on K,ATP conductance was reproduced. Because experiments were conducted in islets, which contain cell-to-cell variability, we extended the model from a single cell to a three-dimensional model (10×10×10 cell) islet with 1000 cells. For each cell, the conductance of the major currents was allowed to vary as was the gap junction conductance between cells. This showed that single cell glucose-dependent behavior was then highly variable, but was uniform in coupled islets. The study highlights the importance of parameterization of detailed models of ?-cell excitability and suggests future experiments that will lead to improved characterization of ?-cell excitability and the control of insulin secretion.

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.

Beta cell connectivity in pancreatic islets: a type 2 diabetes target?

Beta cell connectivity describes the phenomenon whereby the islet context improves insulin secretion by providing a three-dimensional platform for intercellular signaling processes. Thus, the precise flow of information through homotypically interconnected beta cells leads to the large-scale organization of hormone release activities, influencing cell responses to glucose and other secretagogues. Although a phenomenon whose importance has arguably been underappreciated in islet biology until recently, a growing number of studies suggest that such cell-cell communication is a fundamental property of this micro-organ. Hence, connectivity may plausibly be targeted by both environmental and genetic factors in type 2 diabetes mellitus (T2DM) to perturb normal beta cell function and insulin release. Here, we review the mechanisms that contribute to beta cell connectivity, discuss how these may fail during T2DM, and examine approaches to restore insulin secretion by boosting cell communication.

Intrinsic islet heterogeneity and gap junction coupling determine spatiotemporal Ca²⁺ wave dynamics

Insulin is released from the islets of Langerhans in discrete pulses that are linked to synchronized oscillations of intracellular free calcium ([Ca(2+)]i). Associated with each synchronized oscillation is a propagating calcium wave mediated by Connexin36 (Cx36) gap junctions. A computational islet model predicted that waves emerge due to heterogeneity in β-cell function throughout the islet. To test this, we applied defined patterns of glucose stimulation across the islet using a microfluidic device and measured how these perturbations affect calcium wave propagation. We further investigated how gap junction coupling regulates spatiotemporal [Ca(2+)]i dynamics in the face of heterogeneous glucose stimulation. Calcium waves were found to originate in regions of the islet having elevated excitability, and this heterogeneity is an intrinsic property of islet β-cells. The extent of [Ca(2+)]i elevation across the islet in the presence of heterogeneity is gap-junction dependent, which reveals a glucose dependence of gap junction coupling. To better describe these observations, we had to modify the computational islet model to consider the electrochemical gradient between neighboring β-cells. These results reveal how the spatiotemporal [Ca(2+)]i dynamics of the islet depend on β-cell heterogeneity and cell-cell coupling, and are important for understanding the regulation of coordinated insulin release across the islet.

Fluorescence recovery after photobleaching reveals regulation and distribution of connexin36 gap junction coupling within mouse islets of Langerhans

The pancreatic islets are central to the maintenance of glucose homeostasis through insulin secretion. Glucose‐stimulated insulin secretion is tightly linked to electrical activity in β cells within the islet. Gap junctions, composed of connexin36 (Cx36), form intercellular channels between β cells, synchronizing electrical activity and insulin secretion. Loss of gap junction coupling leads to altered insulin secretion dynamics and disrupted glucose homeostasis. Gap junction coupling is known to be disrupted in mouse models of pre‐diabetes. Although approaches to measure gap junction coupling have been devised, they either lack cell specificity, suitable quantification of coupling or spatial resolution, or are invasive. The purpose of this study was to develop fluorescence recovery after photobleaching (FRAP) as a technique to accurately and robustly measure gap junction coupling in the islet. The cationic dye Rhodamine 123 was used with FRAP to quantify dye diffusion between islet β cells as a measure of Cx36 gap junction coupling. Measurements in islets with reduced Cx36 verified the accuracy of this technique in distinguishing between distinct levels of gap junction coupling. Analysis of individual cells revealed that the distribution of coupling across the islet is highly heterogeneous. Analysis of several modulators of gap junction coupling revealed glucose‐ and cAMP‐dependent modulation of gap junction coupling in islets. Finally, FRAP was used to determine cell population specific coupling, where no functional gap junction coupling was observed between α cells and β cells in the islet. The results of this study show FRAP to be a robust technique which provides the cellular resolution to quantify the distribution and regulation of Cx36 gap junction coupling in specific cell populations within the islet. Future studies utilizing this technique may elucidate the role of gap junction coupling in the progression of diabetes and identify mechanisms of gap junction regulation for potential therapies.

Phase transitions in the multi-cellular regulatory behavior of pancreatic islet excitability

The pancreatic islets of Langerhans are multicellular micro-organs integral to maintaining glucose homeostasis through secretion of the hormone insulin. β-cells within the islet exist as a highly coupled electrical network which coordinates electrical activity and insulin release at high glucose, but leads to global suppression at basal glucose. Despite its importance, how network dynamics generate this emergent binary on/off behavior remains to be elucidated. Previous work has suggested that a small threshold of quiescent cells is able to suppress the entire network. By modeling the islet as a Boolean network, we predicted a phase-transition between globally active and inactive states would emerge near this threshold number of cells, indicative of critical behavior. This was tested using islets with an inducible-expression mutation which renders defined numbers of cells electrically inactive, together with pharmacological modulation of electrical activity. This was combined with real-time imaging of intracellular free-calcium activity [Ca²⁺]_i and measurement of physiological parameters in mice. As the number of inexcitable cells was increased beyond ∼15%, a phase-transition in islet activity occurred, switching from globally active wild-type behavior to global quiescence. This phase-transition was also seen in insulin secretion and blood glucose, indicating physiological impact. This behavior was reproduced in a multicellular dynamical model suggesting critical behavior in the islet may obey general properties of coupled heterogeneous networks. This study represents the first detailed explanation for how the islet facilitates inhibitory activity in spite of a heterogeneous cell population, as well as the role this plays in diabetes and its reversal. We further explain how islets utilize this critical behavior to leverage cellular heterogeneity and coordinate a robust insulin response with high dynamic range. These findings also give new insight into emergent multicellular dynamics in general which are applicable to many coupled physiological systems, specifically where inhibitory dynamics result from coupled networks.

As science has successfully broken down the elements of many biological systems, the network dynamics of large-scale cellular interactions has emerged as a new frontier. One way to understand how dynamical elements within large networks behave collectively is via mathematical modeling. Diabetes, which is of increasing international concern, is commonly caused by a deterioration of these complex dynamics in a highly coupled micro-organ called the islet of Langerhans. Therefore, if we are to understand diabetes and how to treat it, we must understand how coupling affects ensemble dynamics. While the role of network connectivity in islet excitation under stimulatory conditions has been well studied, how connectivity also suppresses activity under fasting conditions remains to be elucidated. Here we use two network models of islet connectivity to investigate this process. Using genetically altered islets and pharmacological treatments, we show how suppression of islet activity is solely dependent on a threshold number of inactive cells. We found that the islet exhibits critical behavior in the threshold region, rapidly transitioning from global activity to inactivity. We therefore propose how the islet and multicellular systems in general can generate a robust stimulated response from a heterogeneous cell population.

Dimensionality and size scaling of coordinated Ca(2+) dynamics in MIN6 β-cell clusters

Pancreatic islets of Langerhans regulate blood glucose homeostasis by the secretion of the hormone insulin. Like many neuroendocrine cells, the coupling between insulin-secreting β-cells in the islet is critical for the dynamics of hormone secretion. We have examined how this coupling architecture regulates the electrical dynamics that underlie insulin secretion by utilizing a microwell-based aggregation method to generate clusters of a β-cell line with defined sizes and dimensions. We measured the dynamics of free-calcium activity ([Ca(2+)]i) and insulin secretion and compared these measurements with a percolating network model. We observed that the coupling dimension was critical for regulating [Ca(2+)]i dynamics and insulin secretion. Three-dimensional coupling led to size-invariant suppression of [Ca(2+)]i at low glucose and robust synchronized [Ca(2+)]i oscillations at elevated glucose, whereas two-dimensional coupling showed poor suppression and less robust synchronization, with significant size-dependence. The dimension- and size-scaling of [Ca(2+)]i at high and low glucose could be accurately described with the percolating network model, using similar network connectivity. As such this could explain the fundamentally different behavior and size-scaling observed under each coupling dimension. This study highlights the dependence of proper β-cell function on the coupling architecture that will be important for developing therapeutic treatments for diabetes such as islet transplantation techniques. Furthermore, this will be vital to gain a better understanding of the general features by which cellular interactions regulate coupled multicellular systems.

Na+ current properties in islet α- and β-cells reflect cell-specific Scn3a and Scn9a expression

Mouse pancreatic β- and α-cells are equipped with voltage-gated Na⁺ currents that inactivate over widely different membrane potentials (half-maximal inactivation (V_0.5) at −100 mV and −50 mV in β- and α-cells, respectively). Single-cell PCR analyses show that both α- and β-cells have Na_v1.3 (Scn3) and Na_v1.7 (Scn9a) α subunits, but their relative proportions differ: β-cells principally express Na_v1.7 and α-cells Na_v1.3. In α-cells, genetically ablating Scn3a reduces the Na⁺ current by 80%. In β-cells, knockout of Scn9a lowers the Na⁺ current by >85%, unveiling a small Scn3a-dependent component. Glucagon and insulin secretion are inhibited in Scn3a^−/− islets but unaffected in Scn9a-deficient islets. Thus, Na_v1.3 is the functionally important Na⁺ channel α subunit in both α- and β-cells because Na_v1.7 is largely inactive at physiological membrane potentials due to its unusually negative voltage dependence of inactivation. Interestingly, the Na_v1.7 sequence in brain and islets is identical and yet the V_0.5 for inactivation is >30 mV more negative in β-cells. This may indicate the presence of an intracellular factor that modulates the voltage dependence of inactivation.

Cellular communication and heterogeneity in pancreatic islet insulin secretion dynamics

Coordinated pulses of electrical activity and insulin secretion are a hallmark of the islet of Langerhans. These coordinated behaviors are lost when β cells are dissociated, which also leads to increased insulin secretion at low glucose levels. Islets without gap junctions exhibit asynchronous electrical activity similar to dispersed cells, but their secretion at low glucose levels is still clamped off, putatively by a juxtacrine mechanism. Mice lacking β cell gap junctions have near-normal average insulin levels, but are glucose intolerant due to reduced first-phase and pulsatile insulin secretion, illustrating the importance of temporal dynamics. Here, we review the quantitative data on islet synchronization and the current mathematical models that have been developed to explain these behaviors and generate greater understanding of the underlying mechanisms.

The MafA transcription factor becomes essential to islet β-cells soon after birth

The large Maf transcription factors, MafA and MafB, are expressed with distinct spatial-temporal patterns in rodent islet cells. Analysis of Mafa(-/-) and pancreas-specific Mafa(∆panc) deletion mutant mice demonstrated a primary role for MafA in adult β-cell activity, different from the embryonic importance of MafB. Our interests here were to precisely define when MafA became functionally significant to β-cells, to determine how this was affected by the brief period of postnatal MafB production, and to identify genes regulated by MafA during this period. We found that islet cell organization, β-cell mass, and β-cell function were influenced by 3 weeks of age in Mafa(Δpanc) mice and compromised earlier in Mafa(Δpanc);Mafb(+/-) mice. A combination of genome-wide microarray profiling, electron microscopy, and metabolic assays were used to reveal mechanisms of MafA control. For example, β-cell replication was produced by actions on cyclin D2 regulation, while effects on granule docking affected first-phase insulin secretion. Moreover, notable differences in the genes regulated by embryonic MafB and postnatal MafA gene expression were found. These results not only clearly define why MafA is an essential transcriptional regulator of islet β-cells, but also why cell maturation involves coordinated actions with MafB.

Decreasing cx36 gap junction coupling compensates for overactive KATP channels to restore insulin secretion and prevent hyperglycemia in a mouse model of neonatal diabetes

Mutations to the ATP-sensitive K(+) channel (KATP channel) that reduce the sensitivity of ATP inhibition cause neonatal diabetes mellitus via suppression of β-cell glucose-stimulated free calcium activity ([Ca(2+)]i) and insulin secretion. Connexin-36 (Cx36) gap junctions also regulate islet electrical activity; upon knockout of Cx36, β-cells show [Ca(2+)]i elevations at basal glucose. We hypothesized that in the presence of overactive ATP-insensitive KATP channels, a reduction in Cx36 would allow elevations in glucose-stimulated [Ca(2+)]i and insulin secretion to improve glucose homeostasis. To test this, we introduced a genetic knockout of Cx36 into mice that express ATP-insensitive KATP channels and measured glucose homeostasis and islet metabolic, electrical, and insulin secretion responses. In the normal presence of Cx36, after expression of ATP-insensitive KATP channels, blood glucose levels rapidly rose to >500 mg/dL. Islets from these mice showed reduced glucose-stimulated [Ca(2+)]i and no insulin secretion. In mice lacking Cx36 after expression of ATP-insensitive KATP channels, normal glucose levels were maintained. Islets from these mice had near-normal glucose-stimulated [Ca(2+)]i and insulin secretion. We therefore demonstrate a novel mechanism by which islet function can be recovered in a monogenic model of diabetes. A reduction of gap junction coupling allows sufficient glucose-stimulated [Ca(2+)]i and insulin secretion to prevent the emergence of diabetes.

New insights into the role of connexins in pancreatic islet function and diabetes

Multi-cellular systems require complex signaling mechanisms for proper tissue function, to mediate signaling between cells in close proximity and at distances. This holds true for the islets of Langerhans, which are multicellular micro-organs located in the pancreas responsible for glycemic control, through secretion of insulin and other hormones. Coupling of electrical and metabolic signaling between islet β-cells is required for proper insulin secretion and effective glycemic control. β-cell specific coupling is established through gap junctions composed of connexin36, which results in coordinated insulin release across the islet. Islet connexins have been implicated in both Type-1 and Type-2 diabetes; however a clear link remains to be determined. The goal of this review is to discuss recent discoveries regarding the role of connexins in regulating insulin secretion, the regulation of connexins within the islet, and recent studies which support a role for connexins in diabetes. Further studies which investigate the regulation of connexins in the islet and their role in diabetes may lead to novel diabetes therapies which regulate islet function and β-cell survival through modulation of gap junction coupling.

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.