Loss of connexin36 channels alters beta-cell coupling, islet synchronization of glucose-induced Ca2+ and insulin oscillations, and basal insulin release

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.

Lack of "hemichannel" activity in insulin-producing cells

Connexins and pannexins have been implicated in the formation of "hemichannels," which may account for the uptake and release of membrane-impermeant molecules in single cells. The in vivo existence of "hemichannels" and their protein composition is still debated. Investigations on these matters are complicated by the lack of adequate negative controls. In search for such essential controls, the authors have investigated transformed (MIN6 line) and primary insulin-producing cells. Here, the authors report that these cells, which express Cx36 and pannexin 1, cannot be shown to display functional "hemichannels," as evaluated by (1) uptake of the membrane-impermeant tracer ethidium bromide, whether in the presence or absence of extracellular Ca(2+), following stimulation of P2X(7) receptors, and after exposure to hypotonic medium; and (2) lack of exocytosis-independent release of endogenous ATP. Moreover, electrophysiological recordings indicated the absence of carbenoxolone-sensitive pannexin 1 currents evoked by membrane potentials above +30 mV. Thus, insulin-producing cells are expected to provide a useful tool in the further characterization of hemichannel composition, properties, and physiological relevance.

Gap junction coupling and calcium waves in the pancreatic islet

The pancreatic islet is a highly coupled, multicellular system that exhibits complex spatiotemporal electrical activity in response to elevated glucose levels. The emergent properties of islets, which differ from those arising in isolated islet cells, are believed to arise in part by gap junctional coupling, but the mechanisms through which this coupling occurs are poorly understood. To uncover these mechanisms, we have used both high-speed imaging and theoretical modeling of the electrical activity in pancreatic islets under a reduction in the gap junction mediated electrical coupling. Utilizing islets from a gap junction protein connexin 36 knockout mouse model together with chemical inhibitors, we can modulate the electrical coupling in the islet in a precise manner and quantify this modulation by electrophysiology measurements. We find that after a reduction in electrical coupling, calcium waves are slowed as well as disrupted, and the number of cells showing synchronous calcium oscillations is reduced. This behavior can be reproduced by computational modeling of a heterogeneous population of beta-cells with heterogeneous levels of electrical coupling. The resulting quantitative agreement between the data and analytical models of islet connectivity, using only a single free parameter, reveals the mechanistic underpinnings of the multicellular behavior of the islet.

Selection of in-phase or out-of-phase synchronization in a model based on global coupling of cells undergoing metabolic oscillations

On the basis of experimental observations, it has been suggested that glycolytic oscillations underlie the pulsatile secretion of insulin by pancreatic beta cells, with a periodicity of about 13 min. If beta cells within an islet are synchronized through gap junctions, the question arises as to how beta cells located in different islets of Langerhans synchronize to produce oscillations in plasma levels of insulin. We address this question by means of a minimal model that incorporates the secretion of insulin by cells undergoing glycolytic oscillations. Global coupling and synchronization result from the inhibition exerted by insulin on the production of glucose, which serves as the substrate for metabolic oscillations. Glycolytic oscillations are described by a simple two-variable model centered on the product-activated reaction catalyzed by the allosteric enzyme phosphofructokinase. We obtain bifurcation diagrams for the cases in which insulin secretion is controlled solely by the product or by the substrate of the metabolic oscillator. Remarkably, we find that the oscillating cells in these conditions synchronize, respectively, in phase or out of phase. Numerical simulations show that in-phase and out-of-phase synchronization can sometimes coexist when insulin release is controlled by both the substrate and the product of the metabolic oscillator. The results provide an example of a system in which the selection of in-phase or out-of-phase synchronization is governed by the nature of the coupling between the intracellular oscillations and the secretion of the biochemical signal through which the oscillating cells are globally coupled.

The hyperbolic effect of density and strength of inter beta-cell coupling on islet bursting: a theoretical investigation

BACKGROUND

Insulin, the principal regulating hormone of blood glucose, is released through the bursting of the pancreatic islets. Increasing evidence indicates the importance of islet morphostructure in its function, and the need of a quantitative investigation. Recently we have studied this problem from the perspective of islet bursting of insulin, utilizing a new 3D hexagonal closest packing (HCP) model of islet structure that we have developed. Quantitative non-linear dependence of islet function on its structure was found. In this study, we further investigate two key structural measures: the number of neighboring cells that each beta-cell is coupled to, nc, and the coupling strength, gc.

RESULTS

BETA-cell clusters of different sizes with number of beta-cells nbeta ranging from 1-343, nc from 0-12, and gc from 0-1000 pS, were simulated. Three functional measures of islet bursting characteristics--fraction of bursting beta-cells fb, synchronization index lambda, and bursting period Tb, were quantified. The results revealed a hyperbolic dependence on the combined effect of nc and gc. From this we propose to define a dimensionless cluster coupling index or CCI, as a composite measure for islet morphostructural integrity. We show that the robustness of islet oscillatory bursting depends on CCI, with all three functional measures fb, lambda and Tb increasing monotonically with CCI when it is small, and plateau around CCI = 1.

CONCLUSION

CCI is a good islet function predictor. It has the potential of linking islet structure and function, and providing insight to identify therapeutic targets for the preservation and restoration of islet beta-cell mass and function.

Wave-block due to a threshold gradient underlies limited coordination in pancreatic islets

Two models for coupled pancreatic beta cells are used to investigate excited wave propagation in spatially inhomogeneous islets of Langerhans. The application concerns spatial variation of glucose concentration across the islet. A comprehensive model of coupled cells shows that wave blocking occurs as the conductance of adenosine triphosphate regulated potassium channels increases, corresponding to spatially decreasing glucose concentration. A simplified model based on a perturbed version of Fisher's equation has been investigated using perturbation theory. We show that the perturbed Fisher's equation likewise can exhibit wave blocking.

Dual effect of cell-cell contact disruption on cytosolic calcium and insulin secretion

Cell-to-cell interactions play an important role in insulin secretion. Compared with intact islets, dispersed pancreatic beta-cells show increased basal and decreased glucose-stimulated insulin secretion. In this study, we used mouse MIN6B1 cells to investigate the mechanisms that control insulin secretion when cells are in contact with each other or not. RNAi-mediated silencing of the adhesion molecule E-cadherin in confluent cells reduced glucose-stimulated secretion to the levels observed in isolated cells but had no impact on basal secretion. Dispersed cells presented high cytosolic Ca(2+) activity, depolymerized cytoskeleton and ERK1/2 activation in low glucose conditions. Both the increased basal secretion and the spontaneous Ca(2+) activity were corrected by transient removal of Ca(2+) or prolonged incubation of cells in low glucose, a procedure that restored the ability of dispersed cells to respond to glucose (11-fold stimulation). In conclusion, we show that dispersed pancreatic beta-cells can respond robustly to glucose once their elevated basal secretion has been corrected. The increased basal insulin secretion of dispersed cells is due to spontaneous Ca(2+) transients that activate downstream Ca(2+) effectors, whereas engagement of cell adhesion molecules including E-cadherin contributes to the greater secretory response to glucose seen in cells with normal intercellular contacts.

Cell adhesion molecule 1 is a novel pancreatic-islet cell adhesion molecule that mediates nerve-islet cell interactions

BACKGROUND & AIMS

Cell adhesion molecule 1 (CADM1), mediates nerve-mast cell attachment and communication through homophilic binding. An immunohistochemical screen showed that CADM1 is expressed in pancreatic islets. Here, we determined the cell types expressing CADM1 and examined its role in nerve-islet cell interactions.

METHODS

Immunohistochemistry and double-staining immunofluorescence were performed on murine and human pancreases and on islet cell tumors (ICTs). alphaTC6 cells, a murine alpha cell line, were cultured on neurite networks of superior cervical ganglia. Neurite-alphaTC6 cell attachment and communication were examined after nerves were activated specifically by scorpion venom.

RESULTS

CADM1 was expressed on the plasma membrane in all 4 major types of islet cells, alpha, beta, D, and pancreatic polypeptide in human beings, but primarily in alpha cells in mice. In cocultures, alphaTC6 cell to neurite attachment was inhibited dose-dependently by an anti-CADM1 function-blocking antibody. In response to scorpion venom-evoked nerve activation, 36% of the alphaTC6 cells mobilized Ca(2+), and introduction of a CADM1-targeting small interfering RNA reduced the fraction of responding cells to 7%. In 21 human ICTs, CADM1 was present in the plasma membrane of 7, and the others were negative for CADM1. Six of the CADM1-expressing tumors were functional hormonally, whereas all but 2 of the CADM1-negative tumors were nonfunctional (P = .0032).

CONCLUSIONS

CADM1 is a novel islet cell adhesion molecule mediating nerve-islet cell interactions. The strong correlation between CADM1 expression and hormonally functional phenotypes suggests that CADM1 is involved in hormone secretion from ICTs.

A new conditional mouse mutant reveals specific expression and functions of connexin36 in neurons and pancreatic beta-cells

Connexin36 (Cx36) is the main connexin isoform expressed in neurons of the central nervous system (CNS) and in pancreatic beta-cells, i.e. two types of excitable cells that share - in spite of their different origins - a number of common features. Previous studies on Cx36 deficient mice have documented that loss of Cx36 resulted in phenotypic abnormalities in both the CNS and the pancreas which, however, could not be attributed to specific cell types due to the general deletion nature of the animal model used. Attempts to address this limitation using cell type specific deletions generated by the Cre/loxP strategy have so far been complicated by the lack of Cx36 expression from the floxed allele. We have now generated a conditional Cx36 deficient mouse mutant in which the coding region of Cx36 is flanked by loxP sites, followed by a cyan fluorescent protein (CFP) reporter gene. Here we show that Cx36 was still expressed from the floxed allele in neurons and pancreatic beta-cells. In these cells, a 30-60% decrease of this protein, relative to the expression level of the wildtype allele, did not significantly perturb cell coupling. The deletion of Cx36 by ubiquitously and cell type specifically expressed Cre recombinases revealed that CFP functions as a reliable reporter for Cx36 expression in brain neurons and to some extent in retina neurons, but not in pancreas. Loss of Cx36 by Cre-mediated recombination was documented at transcript and protein levels. Cell type specific deletion of Cx36 in the endocrine pancreas revealed major alterations in the basal as well as the glucose-induced insulin secretion, hence specifically attributing to pancreatic Cx36 an important regulatory role in the control of beta-cell function. Cell type specific deletion of Cx36 in the CNS by suitable Cre recombinases should also help to elucidate the functional role of Cx36 in different neuronal subtypes.

Presence of functional cannabinoid receptors in human endocrine pancreas

AIMS/HYPOTHESIS

We examined the presence of functional cannabinoid receptors 1 and 2 (CB1, CB2) in isolated human islets, phenotyped the cells producing cannabinoid receptors and analysed the actions of selective cannabinoid receptor agonists on insulin, glucagon and somatostatin secretion in vitro. We also described the localisation on islet cells of: (1) the endocannabinoid-producing enzymes N-acyl-phosphatidyl ethanolamine-hydrolysing phospholipase D and diacylglycerol lipase; and (2) the endocannabinoid-degrading enzymes fatty acid amidohydrolase and monoacyl glycerol lipase.

METHODS

Real-time PCR, western blotting and immunocytochemistry were used to analyse the presence of endocannabinoid-related proteins and genes. Static secretion experiments were used to examine the effects of activating CB1 or CB2 on insulin, glucagon and somatostatin secretion and to measure changes in 2-arachidonoylglycerol (2-AG) levels within islets. Analyses were performed in isolated human islets and in paraffin-embedded sections of human pancreas.

RESULTS

Human islets of Langerhans expressed CB1 and CB2 (also known as CNR1 and CNR2) mRNA and CB1 and CB2 proteins, and also the machinery involved in synthesis and degradation of 2-AG (the most abundant endocannabinoid, levels of which were modulated by glucose). Immunofluorescence revealed that CB1 was densely located in glucagon-secreting alpha cells and less so in insulin-secreting beta cells. CB2 was densely present in somatostatin-secreting delta cells, but absent in alpha and beta cells. In vitro experiments revealed that CB1 stimulation enhanced insulin and glucagon secretion, while CB2 agonism lowered glucose-dependent insulin secretion, showing these cannabinoid receptors to be functional.

CONCLUSIONS/INTERPRETATION

Together, these results suggest a role for endogenous endocannabinoid signalling in regulation of endocrine secretion in the human pancreas.

ICER-1gamma overexpression drives palmitate-mediated connexin36 down-regulation in insulin-secreting cells

Channels formed by the gap junction protein connexin36 (Cx36) contribute to the proper control of insulin secretion. We investigated the impact of chronic hyperlipidemia on Cx36 expression in pancreatic beta-cells. Prolonged exposure to the saturated free fatty acid palmitate reduced the expression of Cx36 in several insulin-secreting cell lines and isolated mouse islets. The effect of palmitate was fully blocked upon protein kinase A (PKA) inhibition by H89 and (Rp)-cAMP, indicating that the cAMP/PKA pathway is involved in the control of Cx36 expression. Palmitate treatment led to overexpression of the inducible cAMP early repressor (ICER-1gamma), which bound to a functional cAMP-response element located in the promoter of the CX36 gene. Inhibition of ICER-1gamma overexpression prevented the Cx36 decrease, as well as the palmitate-induced beta-cell secretory dysfunction. Finally, freshly isolated islets from mice undergoing a long term high fat diet expressed reduced Cx36 levels and increased ICER-1gamma levels. Taken together, these data demonstrate that chronic exposure to palmitate inhibits the Cx36 expression through PKA-mediated ICER-1gamma overexpression. This Cx36 down-regulation may contribute to the reduced glucose sensitivity and altered insulin secretion observed during the pre-diabetic stage and in the metabolic syndrome.

Purinergic receptors in the endocrine and exocrine pancreas

The pancreas is a complex gland performing both endocrine and exocrine functions. In recent years there has been increasing evidence that both endocrine and exocrine cells possess purinergic receptors, which influence processes such as insulin secretion and epithelial ion transport. Most commonly, these processes have been viewed separately. In beta cells, stimulation of P2Y(1) receptors amplifies secretion of insulin in the presence of glucose. Nucleotides released from secretory granules could also contribute to autocrine/paracrine regulation in pancreatic islets. In addition to P2Y(1) receptors, there is also evidence for other P2 and adenosine receptors in beta cells (P2Y(2), P2Y(4), P2Y(6), P2X subtypes and A(1) receptors) and in glucagon-secreting alpha cells (P2X(7), A(2) receptors). In the exocrine pancreas, acini release ATP and ATP-hydrolysing and ATP-generating enzymes. P2 receptors are prominent in pancreatic ducts, and several studies indicate that P2Y(2), P2Y(4), P2Y(11), P2X(4) and P2X(7) receptors could regulate secretion, primarily by affecting Cl(-) and K(+) channels and intracellular Ca(2+) signalling. In order to understand the physiology of the whole organ, it is necessary to consider the full complement of purinergic receptors on different cells as well as the structural and functional relation between various cells within the whole organ. In addition to the possible physiological function of purinergic receptors, this review analyses whether the receptors could be potential therapeutic targets for drug design aimed at treatment of pancreatic diseases.

Beta cells preferentially exchange cationic molecules via connexin 36 gap junction channels

AIMS/HYPOTHESIS

Pancreatic beta cells are connected by gap junction channels made of connexin 36 (Cx36), which permit intercellular exchanges of current-carrying ions (ionic coupling) and other molecules (metabolic coupling). Previous studies have suggested that ionic coupling may extend to larger regions of pancreatic islets than metabolic coupling. The aim of the present study was to investigate whether this apparent discrepancy reflects a difference in the sensitivity of the techniques used to evaluate beta cell communication or a specific characteristic of the Cx36 channels themselves.

METHODS

We microinjected several gap junction tracers, differing in size and charge, into individual insulin-producing cells and evaluated their intercellular exchange either within intact islets of control, knockout and transgenic mice featuring beta cells with various levels of Cx36, or in cultures of wild-type and Cx36-transfected MIN6 cells.

RESULTS

We found that (1) Cx36 channels favour the exchange of cations and larger positively charged molecules between beta cells at the expense of anionic molecules; (2) this exchange occurs across sizable portions of pancreatic islets; and (3) during glibenclamide (known as glyburide in the USA and Canada) stimulation beta cell coupling increases to an extent that varies for different gap junction-permeant molecules.

CONCLUSIONS/INTERPRETATION

The data show that beta cells are extensively coupled within pancreatic islets via exchanges of mostly positively charged molecules across Cx36 channels. These exchanges selectively increase during stimulation of insulin secretion. The identification of this permselectivity is expected to facilitate the identification of endogenous permeant molecules and of the mechanism whereby Cx36 signalling significantly contributes to the modulation of insulin secretion.

Islet-cell-to-cell communication as basis for normal insulin secretion

The emergence of pancreatic islets has necessitated the development of a signalling system for the intra- and inter-islet coordination of beta cells. With evolution, this system has evolved into a complex regulatory network of partially cross-talking pathways, whereby individual cells sense the state of activity of their neighbours and, accordingly, regulate their own level of functioning. A consistent feature of this network in vertebrates is the expression of connexin (Cx)-36-made cell-to-cell channels, which cluster at gap junction domains of the cell membrane, and which adjacent beta cells use to share cytoplasmic ions and small metabolites within individual islets. This chapter reviews what is known about Cx36, and the mechanism whereby this beta-cell connexin significantly regulates insulin secretion. It further outlines other less established functions of the protein and evaluates its potential relevance for the development of novel therapeutic approaches to diabetes.

Investigating the role of islet cytoarchitecture in its oscillation using a new beta-cell cluster model

The oscillatory insulin release is fundamental to normal glycemic control. The basis of the oscillation is the intercellular coupling and bursting synchronization of β cells in each islet. The functional role of islet β cell mass organization with respect to its oscillatory bursting is not well understood. This is of special interest in view of the recent finding of islet cytoarchitectural differences between human and animal models. In this study we developed a new hexagonal closest packing (HCP) cell cluster model. The model captures more accurately the real islet cell organization than the simple cubic packing (SCP) cluster that is conventionally used. Using our new model we investigated the functional characteristics of β-cell clusters, including the fraction of cells able to burst f_b, the synchronization index λ of the bursting β cells, the bursting period T_b, the plateau fraction p_f, and the amplitude of intracellular calcium oscillation [Ca]. We determined their dependence on cluster architectural parameters including number of cells n_β, number of inter-β cell couplings of each β cell n_c, and the coupling strength g_c. We found that at low values of n_β, n_c and g_c, the oscillation regularity improves with their increasing values. This functional gain plateaus around their physiological values in real islets, at n_β∼100, n_c∼6 and g_c∼200 pS. In addition, normal β-cell clusters are robust against significant perturbation to their architecture, including the presence of non-β cells or dead β cells. In clusters with n_β>∼100, coordinated β-cell bursting can be maintained at up to 70% of β-cell loss, which is consistent with laboratory and clinical findings of islets. Our results suggest that the bursting characteristics of a β-cell cluster depend quantitatively on its architecture in a non-linear fashion. These findings are important to understand the islet bursting phenomenon and the regulation of insulin secretion, under both physiological and pathological conditions.

Connexin40 regulates renin production and blood pressure

Renin secretion is regulated by coordinated signaling between the various cells of the juxtaglomerular apparatus. The renin-secreting cells (RSC), which play a major role in the control of blood pressure, are coupled to each other and to endothelial cells by Connexin40 (Cx40)-containing channels. In this study, we show that Cx40 knockout (Cx40-/-) mice, but not their heterozygous littermates, are hypertensive due to the increase in the number of RSC, renin biosynthesis, and plasma renin. Treatment with the angiotensin II receptor AT1 antagonist candesartan or the angiotensin II-converting enzyme inhibitor ramipril reduced the blood pressure of the Cx40-/- mice to the same levels seen in wild-type (WT) mice. The elevated blood pressure of the knockout mice was not affected by clipping one renal artery (2K1C, renin-dependent model of hypertension) or after a high salt diet. Under these conditions, however, Cx40-/- mice showed an altered production and release of renin. The renin mRNA ratio between the clipped and the non-clipped kidney was lower in the knockout than in the WT 2K1C mice. This indicates that the response to a change in blood pressure was altered. The RSC of the Cx40-/- mice did not have a compensatory increase in the levels of either Cx43 or Cx37. Our data show that renin secretion is dependent on Cx40 and suggest the Cx40-/- mice may be a genetic model of renin-dependent hypertension.

Phospholipase A2 is important for glucose induction of rhythmic Ca2+ signals in pancreatic beta cells

OBJECTIVES

Pancreatic beta cells respond to glucose stimulation with pulses of insulin release generated by oscillatory rises of the cytoplasmic Ca2+ concentration ([Ca2+]i). The observation that exposure to external ATP and other activators of cytoplasmic phospholipase A2 (cPLA2) rapidly induces rises of [Ca2+]i similar to ordinary oscillations made it important to analyze whether suppression of the cPLA2 activity affects glucose-induced [Ca2+]i rhythmicity in pancreatic beta cells.

METHODS

Ratiometric fura-2 technique was used for measuring [Ca2+]i in single beta cells and small aggregates prepared from ob/ob mouse islets.

RESULTS

Testing the effects of different inhibitors of cPLA2 in the presence of 20 mM glucose, it was found that N-(p-amylcinnamoyl)anthranilic acid (ACA) removed the oscillations at a concentration of 25 microM, arachidonyl trifluoromethyl ketone (AACOCF3) at 10 microM, and bromoenol lactone (BEL) at 10 to 15 microM. Withdrawal of ACA and BEL resulted in reappearance of the oscillations. Suppression of the arachidonic acid production by addition of 5 microM of the diacylglycerol lipase inhibitor 1,6-bis-(cyclohexyloximinocarbonylamino)-hexane (RHC 80267) effectively removed the [Ca2+]i oscillations, an effect reversed by removal of the inhibitor or addition of 100 microM tolbutamide. Suppression of the arachidonic acid production had a restrictive influence also on the transients of [Ca2+]i supposed to synchronize the beta-cell rhythmicity. Although less sensitive than the oscillations, most transients disappeared during exposure to 50 microM ACA or 35 microM RHC 80267.

CONCLUSIONS

The results support the idea that cyclic variations of cPLA2 activity are important for the generation and synchronization of the beta-cell [Ca2+]i oscillations responsible for pulsatile release of insulin.

Pulses of somatostatin release are slightly delayed compared with insulin and antisynchronous to glucagon

It was early proposed that somatostatin-producing delta-cells in pancreatic islets have local inhibitory effects on the release of insulin and glucagon. Recent observations that pulses of insulin and glucagon are antisynchronous make it important to examine the temporal characteristics of glucose-induced somatostatin release. Analysis of 30 s fractions from the perfused rat pancreas indicated that increase of glucose from 3 to 20 mmol/l results in initial suppression of somatostatin release followed by regular 4-5 min pulses. During continued exposure to 20 mmol/l glucose, the pulses of somatostatin overlapped those of insulin with a delay of 30 s. Somatostatin and glucagon pulses were coupled in antisynchronous fashion (phase shift 2.4+/-0.2 min), supporting the idea that the delta-cells have a local inhibitory effect on glucagon release. It was possible to remove the pulses of somatostatin and glucagon with maintenance of the insulin rhythmicity by addition of 1 micromol/l of the P2Y(1) receptor antagonist MRS 2179.

EphA-Ephrin-A-mediated beta cell communication regulates insulin secretion from pancreatic islets

In vertebrates, beta cells are aggregated in the form of pancreatic islets. Within these islets, communication between beta cells inhibits basal insulin secretion and enhances glucose-stimulated insulin secretion, thus contributing to glucose homeostasis during fasting and feeding. In the search for the underlying molecular mechanism, we have discovered that beta cells communicate via ephrin-As and EphAs. We provide evidence that ephrin-A5 is required for glucose-stimulated insulin secretion. We further show that EphA-ephrin-A-mediated beta cell communication is bidirectional: EphA forward signaling inhibits insulin secretion, whereas ephrin-A reverse signaling stimulates insulin secretion. EphA forward signaling is downregulated in response to glucose, which indicates that, under basal conditions, beta cells use EphA forward signaling to suppress insulin secretion and that, under stimulatory conditions, they shift to ephrin-A reverse signaling to enhance insulin secretion. Thus, we explain how beta cell communication in pancreatic islets conversely affects basal and glucose-stimulated insulin secretion to improve glucose homeostasis.

Glucose induces synchronous mitochondrial calcium oscillations in intact pancreatic islets

Mitochondria shape Ca(2+) signaling and exocytosis by taking up calcium during cell activation. In addition, mitochondrial Ca(2+) ([Ca(2+)](M)) stimulates respiration and ATP synthesis. Insulin secretion by pancreatic beta-cells is coded mainly by oscillations of cytosolic Ca(2+) ([Ca(2+)](C)), but mitochondria are also important in excitation-secretion coupling. Here, we have monitored [Ca(2+)](M) in single beta-cells within intact mouse islets by imaging bioluminescence of targeted aequorins. We find an increase of [Ca(2+)](M) in islet-cells in response to stimuli that induce either Ca(2+) entry, such as extracellular glucose, tolbutamide or high K(+), or Ca(2+) mobilization from the intracellular stores, such as ATP or carbamylcholine. Many cells responded to glucose with synchronous [Ca(2+)](M) oscillations, indicating that mitochondrial function is coordinated at the whole islet level. Mitochondrial Ca(2+) uptake in permeabilized beta-cells increased exponentially with increasing [Ca(2+)], and, particularly, it became much faster at [Ca(2+)](C)>2 microM. Since the bulk [Ca(2+)](C) signals during stimulation with glucose are smaller than 2 microM, mitochondrial Ca(2+) uptake could be not uniform, but to take place preferentially from high [Ca(2+)](C) microdomains formed near the mouth of the plasma membrane Ca(2+) channels. Measurements of mitochondrial NAD(P)H fluorescence in stimulated islets indicated that the [Ca(2+)](M) changes evidenced here activated mitochondrial dehydrogenases and therefore they may modulate the function of beta-cell mitochondria. Diazoxide, an activator of K(ATP), did not modify mitochondrial Ca(2+) uptake.

Dexamethasone-induced insulin resistance is associated with increased connexin 36 mRNA and protein expression in pancreatic rat islets

Augmented glucose-stimulated insulin secretion (GSIS) is an adaptive mechanism exhibited by pancreatic islets from insulin-resistant animal models. Gap junction proteins have been proposed to contribute to islet function. As such, we investigated the expression of connexin 36 (Cx36), connexin 43 (Cx43), and the glucose transporter Glut2 at mRNA and protein levels in pancreatic islets of dexamethasone (DEX)-induced insulin-resistant rats. Study rats received daily injections of DEX (1 mg/kg body mass, i.p.) for 5 days, whereas control rats (CTL) received saline solution. DEX rats exhibited peripheral insulin resistance, as indicated by the significant postabsorptive insulin levels and by the constant rate for glucose disappearance (KITT). GSIS was significantly higher in DEX islets (1.8-fold in 16.7 mmol/L glucose vs. CTL, p < 0.05). A significant increase of 2.25-fold in islet area was observed in DEX vs. CTL islets (p < 0.05). Cx36 mRNA expression was significantly augmented, Cx43 diminished, and Glut2 mRNA was unaltered in islets of DEX vs. CTL (p < 0.05). Cx36 protein expression was 1.6-fold higher than that of CTL islets (p < 0.05). Glut2 protein expression was unaltered and Cx43 was not detected at the protein level. We conclude that DEX-induced insulin resistance is accompanied by increased GSIS and this may be associated with increase of Cx36 protein expression.

Cx36-mediated coupling reduces beta-cell heterogeneity, confines the stimulating glucose concentration range, and affects insulin release kinetics

We studied the effect of gap junctional coupling on the excitability of beta-cells in slices of pancreas, which provide a normal environment for islet cells. The electrophysiological properties of beta-cells from mice (C57Bl/6 background) lacking the gap junction protein connexin36 (Cx36(-/-)) were compared with heterozygous (Cx36(+/-)) and wild-type littermates (Cx36(+/+)) and with frequently used wild-type NMRI mice. Most electrophysiological characteristics of beta-cells were found to be unchanged after the knockout of Cx36, except the density of Ca(2+) channels, which was increased in uncoupled cells. With closed ATP-sensitive K(+) (K(ATP)) channels, the electrically coupled beta-cells of Cx36(+/+) and Cx36(+/-) mice were hyperpolarized by the membrane potential of adjacent, inactive cells. Additionally, the hyperpolarization of one beta-cell could attenuate or even stop the electrical activity of nearby coupled cells. In contrast, beta-cells of Cx36(-/-) littermates with blocked K(ATP) channels rapidly depolarized and exhibited a continuous electrical activity. Absence of electrical coupling modified the electrophysiological properties of beta-cells consistent with the reported increase in basal insulin release and altered the switch on/off response of beta-cells during an acute drop of the glucose concentration. Our data indicate an important role for Cx36-gap junctions in modulating stimulation threshold and kinetics of insulin release.

Overnight culture unmasks glucose-induced insulin secretion in mouse islets lacking ATP-sensitive K+ channels by improving the triggering Ca2+ signal

A current model ascribes glucose-induced insulin secretion to the interaction of a triggering pathway (K(ATP) channel-dependent Ca(2+) influx and rise in cytosolic [Ca(2+)](c)) and an amplifying pathway (K(ATP) channel-independent augmentation of secretion without further increase of [Ca(2+)](c)). However, several studies of sulfonylurea receptor 1 null mice (Sur1KO) failed to measure significant effects of glucose in their islets lacking K(ATP) channels. We addressed this issue that challenges the model. Compared with controls, fresh Sur1KO islets showed slightly elevated basal [Ca(2+)](c) and insulin secretion. In 15 mm glucose, the absolute rate of secretion was approximately 3-fold lower in Sur1KO than control islets, with only poor increase above base line. Overnight culture of Sur1KO islets in 10 mm glucose (not in 5 mm) augmented basal insulin secretion and considerably improved the response to 15 mm glucose, which reached higher values than in control islets, in which culture had little impact. Glucose stimulation during KCl depolarization showed that the amplifying pathway is functional in fresh and cultured Sur1KO islets. The differences in insulin secretion between fresh and cultured Sur1KO islets and between Sur1KO and control islets were not attributable to differences in insulin content, glucose oxidation rate, or synchronization of [Ca(2+)](c) oscillations. The unmasking of glucose-induced insulin secretion in beta-cells lacking K(ATP) channels is paradoxically due to improvement in the production of a triggering signal (elevated [Ca(2+)](c)). The results show that K(ATP) channels are not the only transducer of glucose effects on [Ca(2+)](c) in beta-cells. They explain controversies in the literature and refute arguments raised against the model implicating an amplifying pathway in glucose-induced insulin secretion.

Connexin channel permeability to cytoplasmic molecules

Connexin channels are known to be permeable to a variety of cytoplasmic molecules. The first observation of second messenger junctional permeability, made approximately 30 years ago, sparked broad interest in gap junction channels as mediators of intercellular molecular signaling. Since then, much has been learned about the diversity of connexin channels with regard to isoform diversity, tissue and developmental distribution, modes of channel regulation, assembly, expression, biochemical modification and permeability, all of which appear to be dynamically regulated. This information has expanded the potential roles of connexin channels in development, physiology and disease, and made their elucidation much more complex--30 years ago such an orchestra of junctional dynamics was unanticipated. Only recently, however, have investigators been able to directly address, in this more complex framework, the key issue: what specific biological molecules, second messengers and others, are able to permeate the various types of connexin channels, and how well? An important related issue, given the ever-growing list of connexin-related pathologies, is how these permeabilities are altered by disease-causing connexin mutations. Together, many studies show that a variety of cytoplasmic molecules can permeate the different types of connexin channels. A few studies reveal differences in permeation by different molecules through a particular type of connexin channel, and differences in permeation by a particular molecule through different types of connexin channels. This article describes and evaluates the various methods used to obtain these data, presents an annotated compilation of the results, and discusses the findings in the context of what can be inferred about mechanism of selectivity and potential relevance to signaling. The data strongly suggest that highly specific interactions take place between connexin pores and specific biological molecular permeants, and that those interactions determine which cytoplasmic molecules can permeate and how well. At this time, the nature of those interactions is unclear. One hopes that with more detailed permeability and structural information, the specific molecular mechanisms of the selectivity can be elucidated.

Interactions of connexins with other membrane channels and transporters

Cell-to-cell communication through gap junctions exists in most animal cells and is essential for many important biological processes including rapid transmission of electric signals to coordinate contraction of cardiac and smooth muscle, the intercellular propagation of Ca(2+) waves and synchronization of physiological processes between adjacent cells within a tissue. Recent studies have shown that connexins (Cx) can have either direct or indirect interactions with other plasma membrane ion channels or membrane transport proteins with important functional consequences. For example, in tissues most severely affected by cystic fibrosis (CF), activation of the CF Transmembrane Conductance Regulator (CFTR) has been shown to influence connexin function. Moreover, a direct interaction between Cx45.6 and the Major Intrinsic Protein/AQP0 in lens appears to influence the process of cell differentiation whereas interactions between aquaporin 4 (AQP4) and Cx43 in mouse astrocytes may coordinate the intercellular movement of ions and water between astrocytes. In this review, we discuss evidence supporting interactions between Cx and membrane channels/transporters including CFTR, aquaporins, ionotropic glutamate receptors, and between pannexin1, another class of putative gap-junction-forming proteins, and Kvbeta3, a regulatory beta-subunit of voltage gated potassium channels. Although the precise molecular nature of these interactions has yet to be defined, their consequences may be critical for normal tissue homeostasis.

Pancreatic islet production of vascular endothelial growth factor--a is essential for islet vascularization, revascularization, and function

To investigate molecular mechanisms controlling islet vascularization and revascularization after transplantation, we examined pancreatic expression of three families of angiogenic factors and their receptors in differentiating endocrine cells and adult islets. Using intravital lectin labeling, we demonstrated that development of islet microvasculature and establishment of islet blood flow occur concomitantly with islet morphogenesis. Our genetic data indicate that vascular endothelial growth factor (VEGF)-A is a major regulator of islet vascularization and revascularization of transplanted islets. In spite of normal pancreatic insulin content and beta-cell mass, mice with beta-cell-reduced VEGF-A expression had impaired glucose-stimulated insulin secretion. By vascular or diffusion delivery of beta-cell secretagogues to islets, we showed that reduced insulin output is not a result of beta-cell dysfunction but rather caused by vascular alterations in islets. Taken together, our data indicate that the microvasculature plays an integral role in islet function. Factors modulating VEGF-A expression may influence islet vascularity and, consequently, the amount of insulin delivered into the systemic circulation.

Glucose induces glucagon release pulses antisynchronous with insulin and sensitive to purinoceptor inhibition

Both increase of the glucose concentration and activation of purinoceptors are known to affect pancreatic alpha-cells. Effects obtained with various purino derivatives at 2.8 and 8.3 mmol/liter glucose have been taken to indicate that external ATP is less potent than adenosine as a stimulator of glucagon release. However, when making a corresponding comparison at 20 mmol/liter glucose, we observed marked stimulation of glucagon release from isolated rat islets with 100 micromol/liter adenosine-5-O-2-thiodiphosphate but inhibition with 10 micromol/liter adenosine. Analyses of 30-sec samples of perfusate from rat pancreas indicated that a rise of the glucose concentration from 3 to 20 mmol/liter rapidly induces a glucagon peak followed by regular 4- to 5-min pulses. The glucagon pulses preceded those of insulin with a phase shift (1.8 +/- 0.1 min) near half the interpeak interval. Because of the antisynchrony, the maximal glucagon effect on liver cells will be manifested during periods with low concentrations of insulin. In support for the idea that neural P2Y(1) receptors are important for coordinating the secretory activity of the islets, both the insulin and glucagon pulses disappeared in the presence of the purinoceptor inhibitor MRS 2179 (10 micromol/liter). However, in contrast to what was observed for insulin, MRS 2179 lowered average glucagon release to the level of the oscillatory nadirs.

Involvement of the Pancreatic Renin-Angiotensin System in Insulin Resistance and the Metabolic Syndrome

The cardiometabolic syndrome consists of several major components: hypertension, hyperinsulinemia, hyperlipidemia, and hyperglycemia. Central to this syndrome are insulin resistance and generation of reactive oxygen species; these features are particularly prominent in patients with type 2 diabetes mellitus. In this context, large clinical trials have shown that blockade of the renin-angiotensin system (RAS) is protective against type 2 diabetes. In spite of these solid clinical data, the mechanistic pathways by which RAS blockade achieves these protective effects have yet to be resolved. A recently identified local pancreatic islet RAS has, however, been implicated in this regard. Furthermore, RAS blockade was recently shown to enhance islet blood flow, oxygen tension, and insulin biosynthesis, thus improving beta-cell function and glucose tolerance. Meanwhile, RAS activation may also influence islet cell inflammatory responses, apoptosis, fibrosis, and superoxide anion production. This RAS-associated oxidative stress can induce islet cell dysfunction in the pancreas and insulin resistance in peripheral tissues.

Connexin36 and pancreatic beta-cell functions

Most cell types are functionally coupled by connexin (Cx) channels, i.e. exchange cytoplasmic ions and small metabolites through gap junction domains of their membrane. This form of direct cell-to-cell communication occurs in all existing animals, whatever their position in the phylogenetic scale, and up to humans. Pancreatic beta-cells are no exception, and normally cross-talk with their neighbors via channels made of Cx36. These exchanges importantly contribute to coordinate and synchronize the function of individual cells within pancreatic islets, particularly in the context of glucose-induced insulin secretion. Compelling evidence now indicates that Cx36-mediated coupling, and/or the Cx36 protein per se, play significant regulatory roles in various beta-cell functions, ranging from the biosynthesis, storage and release of insulin. Recent preliminary data further suggest that the protein may also be implicated in the balance of beta-cell growth versus necrosis and apoptosis, and in the regulated expression of specific genes. Here, we review this evidence, discuss the possible involvement of Cx36 in the pathophysiology of diabetes, and evaluate the relevance of this connexin in the therapeutic approaches to the disease.

Diffusion of calcium and metabolites in pancreatic islets: killing oscillations with a pitchfork

Cell coupling is important for the normal function of the beta-cells of the pancreatic islet of Langerhans, which secrete insulin in response to elevated plasma glucose. In the islets, electrical and metabolic communications are mediated by gap junctions. Although electrical coupling is believed to account for synchronization of the islets, the role and significance of diffusion of calcium and metabolites are not clear. To address these questions we analyze two different mathematical models of islet calcium and electrical dynamics. To study diffusion of calcium, we use a modified Morris-Lecar model. Based on our analysis, we conclude that intercellular diffusion of calcium is not necessary for islet synchronization, at most supplementing electrical coupling. Metabolic coupling is investigated with a recent mathematical model incorporating glycolytic oscillations. Bifurcation analysis of the coupled system reveals several modes of behavior, depending on the relative strength of electrical and metabolic coupling. We find that whereas electrical coupling always produces synchrony, metabolic coupling can abolish both oscillations and synchrony, explaining some puzzling experimental observations. We suggest that these modes are generic features of square-wave bursters and relaxation oscillators coupled through either the activation or recovery variable.

Oscillations, intercellular coupling, and insulin secretion in pancreatic beta cells

Insulin is a potent metabolic regulator that is released by pancreatic beta-cells, which respond to body glucose concentrations. Here the authors explain the physiological basis of insulin release.

MIN6 beta-cell-beta-cell interactions influence insulin secretory responses to nutrients and non-nutrients

Insulin-secreting MIN6 cells show greatly enhanced secretory responsiveness to nutrients when grown as islet-like structures (pseudoislets). Since beta-cells use different mechanisms to respond to nutrient and non-nutrient stimuli, we have now investigated the role of homotypic beta-cell interactions in secretory responses to pharmacological or receptor-operated non-nutrient stimuli in MIN6 pseudoislets. In addition to an enhanced secretory responsiveness to glucose, insulin secretion from MIN6 pseudoislets was also enhanced by non-nutrients, including carbachol, tolbutamide, PMA, and forskolin. The improved secretory responsiveness was dependent on the cells being configured as pseudoislets and was lost on dispersal of the pseudoislets into single cells and regained on the re-formation of pseudoislet structures. These observations emphasise the importance of islet anatomy on secretory responsiveness, and demonstrate that homotypic beta-cell interactions play an important role in generating physiologically appropriate insulin secretory responses to both nutrient and non-nutrient stimuli.

The unique cytoarchitecture of human pancreatic islets has implications for islet cell function

The cytoarchitecture of human islets has been examined, focusing on cellular associations that provide the anatomical framework for paracrine interactions. By using confocal microscopy and multiple immunofluorescence, we found that, contrary to descriptions of prototypical islets in textbooks and in the literature, human islets did not show anatomical subdivisions. Insulin-immunoreactive beta cells, glucagon-immunoreactive alpha cells, and somatostatin-containing delta cells were found scattered throughout the human islet. Human beta cells were not clustered, and most (71%) showed associations with other endocrine cells, suggesting unique paracrine interactions in human islets. Human islets contained proportionally fewer beta cells and more alpha cells than did mouse islets. In human islets, most beta, alpha, and delta cells were aligned along blood vessels with no particular order or arrangement, indicating that islet microcirculation likely does not determine the order of paracrine interactions. We further investigated whether the unique human islet cytoarchitecture had functional implications. Applying imaging of cytoplasmic free Ca2+ concentration, [Ca2+]i, we found that beta cell oscillatory activity was not coordinated throughout the human islet as it was in mouse islets. Furthermore, human islets responded with an increase in [Ca2+]i when lowering the glucose concentration to 1 mM, which can be attributed to the large contribution of alpha cells to the islet composition. We conclude that the unique cellular arrangement of human islets has functional implications for islet cell function.

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

Pancreatic beta-cells secrete insulin in response to closure of ATP-sensitive K+ (KATP) channels, which causes membrane depolarization and a concomitant rise in intracellular Ca2+ (Cai). In intact islets, beta-cells are coupled by gap junctions, which are proposed to synchronize electrical activity and Cai oscillations after exposure to stimulatory glucose (>7 mM). To determine the significance of this coupling in regulating insulin secretion, we examined islets and beta-cells from transgenic mice that express zero functional KATP channels in approximately 70% of their beta-cells, but normal KATP channel density in the remainder. We found that KATP channel activity from approximately 30% of the beta-cells is sufficient to maintain strong glucose dependence of metabolism, Cai, membrane potential, and insulin secretion from intact islets, but that glucose dependence is lost in isolated transgenic cells. Further, inhibition of gap junctions caused loss of glucose sensitivity specifically in transgenic islets. These data demonstrate a critical role of gap junctional coupling of KATP channel activity in control of membrane potential across the islet. Control via coupling lessens the effects of cell-cell variation and provides resistance to defects in excitability that would otherwise lead to a profound diabetic state, such as occurs in persistent neonatal diabetes mellitus.

In vivo and in vitro glucose-induced biphasic insulin secretion in the mouse: pattern and role of cytoplasmic Ca2+ and amplification signals in beta-cells

The mechanisms underlying biphasic insulin secretion have not been completely elucidated. We compared the pattern of plasma insulin changes during hyperglycemic clamps in mice to that of glucose-induced insulin secretion and cytosolic calcium concentration ([Ca(2+)](c)) changes in perifused mouse islets. Anesthetized mice were infused with glucose to clamp blood glucose at 8.5 (baseline), 11.1, 16.7, or 30 mmol/l. A first-phase insulin response consistently peaked at 1 min, and a slowly ascending second phase occurred at 16.7 and 30 mmol/l glucose. Glucose-induced insulin secretion in vivo is thus biphasic, with a similarly increasing second phase in the mouse as in humans. In vitro, square-wave stimulation from a baseline of 3 mmol/l glucose induced similar biphasic insulin secretion and [Ca(2+)](c) increases, with sustained and flat second phases. The glucose dependency (3-30 mmol/l) of both changes was sigmoidal with, however, a shift to the right of the relation for insulin secretion compared with that for [Ca(2+)](c). The maximum [Ca(2+)](c) increase was achieved by glucose concentrations, causing half-maximum insulin secretion. Because this was true for both phases, we propose that contrary to current concepts, amplifying signals are also implicated in first-phase glucose-induced insulin secretion. To mimic in vivo conditions, islets were stimulated with high glucose after being initially perifused with 8.5 instead of 3.0 mmol/l glucose. First-phase insulin secretion induced by glucose at 11.1, 16.7, and 30 mmol/l was decreased by approximately 50%, an inhibition that could not be explained by commensurate decreases in [Ca(2+)](c) or in the pool of readily releasable granules. Also unexpected was the gradually ascending pattern of the second phase, now similar to that in vivo. These observations indicated that variations in prestimulatory glucose can secondarily affect the magnitude and pattern of subsequent glucose-induced insulin secretion.

Involvement of gap junctional communication in secretion

Glands were the first type of tissues in which the permissive role of gap junctions in the cell-to-cell transfer of membrane-impermeant molecules was shown. During the 40 years that have followed this seminal finding, gap junctions have been documented in all types of multicellular secretory systems, whether of the exocrine, endocrine or pheromonal nature. Also, compelling evidence now indicates that gap junction-mediated coupling, and/or the connexin proteins per se, play significant regulatory roles in various aspects of gland functions, ranging from the biosynthesis, storage and release of a variety of secretory products, to the control of the growth and differentiation of secretory cells, and to the regulation of gland morphogenesis. This review summarizes this evidence in the light of recent reports.