Involvement of gap junctional communication in secretion

Expression of the gap junction protein connexin36 in small intensely fluorescent (SIF) cells in cardiac parasympathetic ganglia of rodents

In mammals, several endocrine cell types are electrically coupled by connexin36 (Cx36)-containing gap junctions, which mediate intercellular communication and allow regulated and synchronized cellular activity through exchange of ions and small metabolites via formation of intercellular channels that link plasma membranes of apposing cells. One cell type thought to be endocrine-like in nature are small intensely fluorescent (SIF) cells that store catecholamines in their dense-core vesicles and reside in autonomic ganglia. Here, using immunofluorescence approaches, we examined whether SIF cells located specifically in cardiac parasympathetic ganglia of adult and neonatal mice and adult rats follow patterns of Cx36 expression seen in other endocrine cells. In these ganglia, SIF cells were identified by their distinct small soma size, autofluorescence at 475 nm, and immunolabelling for their markers tyrosine hydroxylase and vesicular monoamine transporter-1. SIF cells were often found in pairs or clusters among principal cholinergic neurons. Immunofluorescence labelling of Cx36 occurred exclusively as fine puncta that appeared at contacts between SIF cell processes and somata or at somato-somatic appositions of SIF cells. These puncta were absent in cardiac parasympathetic ganglia of Cx36 null mice. Transgenic mice expressing enhanced green fluorescent protein reporter for Cx36 expression displayed labelling for the reporter in SIF cells. The results suggest that Cx36-containing gap junctions electrically couple SIF cells, which is consistent with previous suggestions that these may be classified as endocrine-type cells that secrete catecholamines into the bloodstream in a regulated manner.

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.

Calcium imaging in intact mouse acinar cells in acute pancreas tissue slices

The physiology and pathophysiology of the exocrine pancreas are in close connection to changes in intra-cellular Ca²⁺ concentration. Most of our knowledge is based on in vitro experiments on acinar cells or acini enzymatically isolated from their surroundings, which can alter their structure, physiology, and limit our understanding. Due to these limitations, the acute pancreas tissue slice technique was introduced almost two decades ago as a complementary approach to assess the morphology and physiology of both the endocrine and exocrine pancreas in a more conserved in situ setting. In this study, we extend previous work to functional multicellular calcium imaging on acinar cells in tissue slices. The viability and morphological characteristics of acinar cells within the tissue slice were assessed using the LIVE/DEAD assay, transmission electron microscopy, and immunofluorescence imaging. The main aim of our study was to characterize the responses of acinar cells to stimulation with acetylcholine and compare them with responses to cerulein in pancreatic tissue slices, with special emphasis on inter-cellular and inter-acinar heterogeneity and coupling. To this end, calcium imaging was performed employing confocal microscopy during stimulation with a wide range of acetylcholine concentrations and selected concentrations of cerulein. We show that various calcium oscillation parameters depend monotonically on the stimulus concentration and that the activity is rather well synchronized within acini, but not between acini. The acute pancreas tissue slice represents a viable and reliable experimental approach for the evaluation of both intra- and inter-cellular signaling characteristics of acinar cell calcium dynamics. It can be utilized to assess many cells simultaneously with a high spatiotemporal resolution, thus providing an efficient and high-yield platform for future studies of normal acinar cell biology, pathophysiology, and screening pharmacological substances.

Abnormal exocrine-endocrine cell cross-talk promotes β-cell dysfunction and loss in MODY8

MODY8 (maturity-onset diabetes of the young, type 8) is a dominantly inherited monogenic form of diabetes associated with mutations in the carboxyl ester lipase (CEL) gene expressed by pancreatic acinar cells. MODY8 patients develop childhood-onset exocrine pancreas dysfunction followed by diabetes during adulthood. However, it is unclear how CEL mutations cause diabetes. In the present study, we report the transfer of CEL proteins from acinar cells to β-cells as a form of cross-talk between exocrine and endocrine cells. Human β-cells show a relatively higher propensity for internalizing the mutant versus the wild-type CEL protein. After internalization, the mutant protein forms stable intracellular aggregates leading to β-cell secretory dysfunction. Analysis of pancreas sections from a MODY8 patient reveals the presence of CEL protein in the few extant β-cells. The present study provides compelling evidence for the mechanism by which a mutant gene expressed specifically in acinar cells promotes dysfunction and loss of β-cells to cause diabetes.

Cysteine residues in the C-terminal tail of connexin32 regulate its trafficking

Gap junctions (GJs) are formed by the assembly of constituent transmembrane proteins called connexins (Cxs). Aberrations in this assembly of Cxs are observed in several genetic diseases as well as in cancers. Hence it becomes imperative to understand the molecular mechanisms underlying such assembly defect. The polarized cells in the epithelia express Connexin32 (Cx32). The C-terminal tail (CT) of Cx32 orchestrates several aspects of GJ dynamics, function and growth. The study here was aimed at determining if post-translational modifications, specifically, palmitoylation of cysteine residues, present in the CT of Cx32, has any effect on GJ assembly. The CT of Cx32 was found to harbor three cysteine residues, which are likely to be modified by palmitoylation. The study here has revealed for the first time that Cx32 is palmitoylated at cysteine 217 (C217) in cell line derived from prostate tumors. However, it was found that mutating C217 to alanine affected neither the trafficking nor the ability of Cx32 to assemble into GJs. Intriguingly, it was discovered that mutating cysteine 280 and 283, only in combination, blocked the trafficking of Cx32 from the trans-Golgi network to the cell surface. The mutants showed reduced stability due to enhanced lysosomal degradation. Overall, the findings reveal the importance of the two C-terminal cysteine residues of Cx32 in regulating its trafficking and stability and hence its ability to assemble into GJs.

The roles of calcium and ATP in the physiology and pathology of the exocrine pancreas

This review deals with the roles of calcium ions and ATP in the control of the normal functions of the different cell types in the exocrine pancreas as well as the roles of these molecules in the pathophysiology of acute pancreatitis. Repetitive rises in the local cytosolic calcium ion concentration in the apical part of the acinar cells not only activate exocytosis but also, via an increase in the intramitochondrial calcium ion concentration, stimulate the ATP formation that is needed to fuel the energy-requiring secretion process. However, intracellular calcium overload, resulting in a global sustained elevation of the cytosolic calcium ion concentration, has the opposite effect of decreasing mitochondrial ATP production, and this initiates processes that lead to necrosis. In the last few years it has become possible to image calcium signaling events simultaneously in acinar, stellate, and immune cells in intact lobules of the exocrine pancreas. This has disclosed processes by which these cells interact with each other, particularly in relation to the initiation and development of acute pancreatitis. By unraveling the molecular mechanisms underlying this disease, several promising therapeutic intervention sites have been identified. This provides hope that we may soon be able to effectively treat this often fatal disease.

The anterior pituitary gap junctions: potential targets for toxicants

The anterior pituitary regulates endocrine organs and physiological activities in the body. Environmental pollutants and drugs deleterious to the endocrine system may affect anterior pituitary activity through direct action on anterior pituitary cells. Within the gland, endocrine and folliculostellate cells are organized into and function as individual tridimensional networks, each network regulating its activity by coordinating the connected cells' responses to physiological or pathological cues. The gap junctions connecting endocrine cells and/or folliculostellate cells allow transmission of information among cells that is necessary for adequate network function. Toxicants may affect gap junctions as well as the physiology of the anterior pituitary. However, whether toxicants effects on anterior pituitary hormone secretion involve gap junctions is unknown. The folliculostellate cell gap junctions are sensitive to hormones, cytokines and growth factors. These cells may be an interesting experimental model for evaluating whether toxicants target anterior pituitary gap junctions.

Connexin36 localization to pinealocytes in the pineal gland of mouse and rat

Several cell types in the pineal gland are known to establish intercellular gap junctions, but the connexin constituents of those junctions have not been fully characterized. Specifically, the expression of connexin36 (Cx36) protein and mRNA has been examined in the pineal, but the identity of cells that produce Cx36 and that form Cx36-containing gap junctions has not been determined. We used immunofluorescence and freeze fracture replica immunogold labelling (FRIL) of Cx36 to investigate the cellular and subcellular localization of Cx36 in the pineal gland of adult mouse and rat. Immunofluorescence labelling of Cx36 was visualized exclusively as puncta or short immunopositive strands that were distributed throughout the pineal, and which were absent in pineal sections from Cx36 null mice. By double immunofluorescence labelling, Cx36 was localized to tryptophan hydroxylase-positive and 5-hydroxytryptamine-positive pinealocyte cell bodies and their large initial processes, including at intersections of those processes and at sites displaying a confluence of processes. Labelling for the cell junction marker zonula occludens-1 (ZO-1) either overlapped or was closely associated with labelling for Cx36. Pinealocytes thus form Cx36-containing gap junctions that also incorporate the scaffolding protein ZO-1. FRIL revealed labelling of Cx36 at ultrastructurally defined gap junctions between pinealocytes, most of which was at gap junctions having reticular, ribbon or string configurations. The results suggest that the endocrine functions of pinealocytes and their secretion of melatonin is supported by their intercellular communication via Cx36-containing gap junctions, which may now be tested by the use of Cx36 null mice.

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.

A Variant of GJD2, Encoding for Connexin 36, Alters the Function of Insulin Producing β-Cells

Signalling through gap junctions contributes to control insulin secretion and, thus, blood glucose levels. Gap junctions of the insulin-producing β-cells are made of connexin 36 (Cx36), which is encoded by the GJD2 gene. Cx36-null mice feature alterations mimicking those observed in type 2 diabetes (T2D). GJD2 is also expressed in neurons, which share a number of common features with pancreatic β-cells. Given that a synonymous exonic single nucleotide polymorphism of human Cx36 (SNP rs3743123) associates with altered function of central neurons in a subset of epileptic patients, we investigated whether this SNP also caused alterations of β-cell function. Transfection of rs3743123 cDNA in connexin-lacking HeLa cells resulted in altered formation of gap junction plaques and cell coupling, as compared to those induced by wild type (WT) GJD2 cDNA. Transgenic mice expressing the very same cDNAs under an insulin promoter revealed that SNP rs3743123 expression consistently lead to a post-natal reduction of islet Cx36 levels and β-cell survival, resulting in hyperglycemia in selected lines. These changes were not observed in sex- and age-matched controls expressing WT hCx36. The variant GJD2 only marginally associated to heterogeneous populations of diabetic patients. The data document that a silent polymorphism of GJD2 is associated with altered β-cell function, presumably contributing to T2D pathogenesis.

Adrenocortical Gap Junctions and Their Functions

Adrenal cortical steroidogenesis and proliferation are thought to be modulated by gap junction-mediated direct cell-cell communication of regulatory molecules between cells. Such communication is regulated by the number of gap junction channels between contacting cells, the rate at which information flows between these channels, and the rate of channel turnover. Knowledge of the factors regulating gap junction-mediated communication and the turnover process are critical to an understanding of adrenal cortical cell functions, including development, hormonal response to adrenocorticotropin, and neoplastic dedifferentiation. Here, we review what is known about gap junctions in the adrenal gland, with particular attention to their role in adrenocortical cell steroidogenesis and proliferation. Information and insight gained from electrophysiological, molecular biological, and imaging (immunocytochemical, freeze fracture, transmission electron microscopic, and live cell) techniques will be provided.

Defective goblet cell exocytosis contributes to murine cystic fibrosis-associated intestinal disease

Cystic fibrosis (CF) intestinal disease is associated with the pathological manifestation mucoviscidosis, which is the secretion of tenacious, viscid mucus that plugs ducts and glands of epithelial-lined organs. Goblet cells are the principal cell type involved in exocytosis of mucin granules; however, little is known about the exocytotic process of goblet cells in the CF intestine. Using intestinal organoids from a CF mouse model, we determined that CF goblet cells have altered exocytotic dynamics, which involved intrathecal granule swelling that was abruptly followed by incomplete release of partially decondensated mucus. Some CF goblet cells exhibited an ectopic granule location and distorted cellular morphology, a phenotype that is consistent with retrograde intracellular granule movement during exocytosis. Increasing the luminal concentration of bicarbonate, which mimics CF transmembrane conductance regulator-mediated anion secretion, increased spontaneous degranulation in WT goblet cells and improved exocytotic dynamics in CF goblet cells; however, there was still an apparent incoordination between granule decondensation and exocytosis in the CF goblet cells. Compared with those within WT goblet cells, mucin granules within CF goblet cells had an alkaline pH, which may adversely affect the polyionic composition of the mucins. Together, these findings indicate that goblet cell dysfunction is an epithelial-autonomous defect in the CF intestine that likely contributes to the pathology of mucoviscidosis and the intestinal manifestations of obstruction and inflammation.

A low-protein diet during pregnancy prevents modifications in intercellular communication proteins in rat islets

BACKGROUND

Gap junctions between β-cells participate in the precise regulation of insulin secretion. Adherens junctions and their associated proteins are required for the formation, function and structural maintenance of gap junctions. Increases in the number of the gap junctions between β-cells and enhanced glucose-stimulated insulin secretion are observed during pregnancy. In contrast, protein restriction produces structural and functional alterations that result in poor insulin secretion in response to glucose. We investigated whether protein restriction during pregnancy affects the expression of mRNA and proteins involved in gap and adherens junctions in pancreatic islets. An isoenergetic low-protein diet (6% protein) was fed to non-pregnant or pregnant rats from day 1-15 of pregnancy, and rats fed an isocaloric normal-protein diet (17% protein) were used as controls.

RESULTS

The low-protein diet reduced the levels of connexin 36 and β-catenin protein in pancreatic islets. In rats fed the control diet, pregnancy increased the levels of phospho-[Ser(279/282)]-connexin 43, and it decreased the levels of connexin 36, β-catenin and beta-actin mRNA as well as the levels of connexin 36 and β-catenin protein in islets. The low-protein diet during pregnancy did not alter these mRNA and protein levels, but avoided the increase of levels of phospho-[Ser(279/282)]-connexin 43 in islets. Insulin secretion in response to 8.3 mmol/L glucose was higher in pregnant rats than in non-pregnant rats, independently of the nutritional status.

CONCLUSION

Short-term protein restriction during pregnancy prevented the Cx43 phosphorylation, but this event did not interfer in the insulin secretion.

Involvement of RhoA/ROCK in insulin secretion of pancreatic β-cells in 3D culture

Cell-cell contacts and interactions between pancreatic β-cells and/or other cell populations within islets are essential for cell survival, insulin secretion, and functional synchronization. Three-dimensional (3D) culture systems supply the ideal microenvironment for islet-like cluster formation and functional maintenance. However, the underlying mechanisms remain unclear. In this study, mouse insulinoma 6 (MIN6) cells were cultured in a rotating 3D culture system to form islet-like aggregates. Glucose-stimulated insulin secretion (GSIS) and the RhoA/ROCK pathway were investigated. In the 3D-cultured MIN6 cells, more endocrine-specific genes were up-regulated, and GSIS was increased to a greater extent than in cells grown in monolayers. RhoA/ROCK inactivation led to F-actin remodeling in the MIN6 cell aggregates and greater insulin exocytosis. The gap junction protein, connexin 36 (Cx36), was up-regulated in MIN6 cell aggregates and RhoA/ROCK-inactivated monolayer cells. GSIS dramatically decreased when Cx36 was knocked down by short interfering RNA and could not be reversed by RhoA/ROCK inactivation. Thus, the RhoA/ROCK signaling pathway is involved in insulin release through the up-regulation of Cx36 expression in 3D-cultured MIN6 cells.

Adipocytes in both brown and white adipose tissue of adult mice are functionally connected via gap junctions: implications for Chagas disease

Adipose tissue serves as a host reservoir for the protozoan Trypanosoma cruzi, the causative organism in Chagas disease. Gap junctions interconnect cells of most tissues, serving to synchronize cell activities including secretion in glandular tissue, and we have previously demonstrated that gap junctions are altered in various tissues and cells infected with T. cruzi. Herein, we examined the gap junction protein connexin 43 (Cx43) expression in infected adipose tissues. Adipose tissue is the largest endocrine organ of the body and is also involved in other physiological functions. In mammals, it is primarily composed of white adipocytes. Although gap junctions are a prominent feature of brown adipocytes, they have not been explored extensively in white adipocytes, especially in the setting of infection. Thus, we examined functional coupling in both white and brown adipocytes in mice. Injection of electrical current or the dye Lucifer Yellow into adipocytes within fat tissue spread to adjacent cells, which was reduced by treatment with agents known to block gap junctions. Moreover, Cx43 was detected in both brown and white fat tissue. At thirty and ninety days post-infection, Cx43 was downregulated in brown adipocytes and upregulated in white adipocytes. Gap junction-mediated intercellular communication likely contributes to hormone secretion and other functions in white adipose tissue and to nonshivering thermogenesis in brown fat, and modulation of the coupling by T. cruzi infection is expected to impact these functions.

Mammary gland specific knockdown of the physiological surge in Cx26 during lactation retains normal mammary gland development and function

Connexin26 (Cx26) is the major Cx protein expressed in the human mammary gland and is up-regulated during pregnancy while remaining elevated throughout lactation. It is currently unknown if patients with loss-of-function Cx26 mutations that result in hearing loss and skin diseases have a greater susceptibility to impaired breast development. To investigate if Cx26 plays a critical role in mammary gland development and differentiation, a novel Cx26 conditional knockout mouse model was generated by crossing Cx26^fl/fl mice with mice expressing Cre under the β-Lactoglobulin promoter. Conditional knockdown of Cx26 from the mammary gland resulted in a dramatic reduction in detectable gap junction plaques confirmed by a significant ∼65-70% reduction in Cx26 mRNA and protein throughout parturition and lactation. Interestingly, this reduction was accompanied by a decrease in mammary gland Cx30 gap junction plaques at parturition, while no change was observed for Cx32 or Cx43. Whole mount, histological and immunofluorescent assessment of breast tissue revealed comparatively normal lobuloalveolar development following pregnancy in the conditionally knockdown mice compared to control mice. In addition, glands from genetically-modified mice were capable of producing milk proteins that were evident in the lumen of alveoli and ducts at similar levels as controls, suggesting normal gland function. Together, our results suggest that low levels of Cx26 expression throughout pregnancy and lactation, and not the physiological surge in Cx26, is sufficient for normal gland development and function.

Neurobiological study of fish brains gives insights into the nature of gonadotropin-releasing hormone 1-3 neurons

Accumulating evidence suggests that up to three different molecular species of GnRH peptides encoded by different paralogs of gnrh genes are expressed by anatomically distinct groups of GnRH neurons in the brain of one vertebrate species. They are called gnrh1, gnrh2, and gnrh3. Recent evidence from molecular, anatomical, and physiological experiments strongly suggests that each GnRH system functions differently. Here, we review recent advancement in the functional studies of the three different GnRH neuron systems, mainly focusing on the electrophysiological analysis of the GnRH-green fluorescent protein (GFP) transgenic animals. The introduction of GFP-transgenic animals for the electrophysiological analysis of GnRH neurons greatly advanced our knowledge on their anatomy and electrophysiology, especially of gnrh1 neurons, which has long defied detailed electrophysiological analysis of single neurons because of their small size and scattered distribution. Based on the results of recent studies, we propose that different electrophysiological properties, especially the spontaneous patterns of electrical activities and their time-dependent changes, and the axonal projections characterize the different functions of GnRH1-3 neurons; GnRH1 neurons act as hypophysiotropic neuroendocrine regulators, and GnRH2 and GnRH3 neurons act as neuromodulators in wide areas of the brain.

FSH induces the development of circadian clockwork in rat granulosa cells via a gap junction protein Cx43-dependent pathway

The present study was designed to assess the relationship between gap junctions and the maturation of a clock system in rat granulosa cells stimulated by follicle-stimulating hormone (FSH). Immature and mature granulosa cells were prepared by puncturing the ovaries of diethylstilbestrol- and equine chorionic gonadotropin (eCG)-treated mouse Period2 (Per2)-dLuc reporter gene transgenic rats, respectively. Mature granulosa cells exposed to dexamethasone (DXM) synchronization displayed several Per2-dLuc oscillations and a rhythmic expression of clock genes. Intriguingly, we observed clear evidence that the FSH stimulation significantly increased the amplitude of Per2 oscillations in the granulosa cells, which was confirmed by the elevation of the Per2 and Rev-erbα (Nr1d1) mRNA levels. FSH also induced a major phase-advance shift of Per2 oscillations. The mature granulosa cells cultured for 2 days with FSH expressed higher mRNA levels of Per2, Rev-erbα, Bmal1 (Arnt1), Lhcgr, and connexin (Cx) 43 (Gja1) compared with the immature granulosa cells. Consistently, our immunofluorescence results revealed abundant Cx43 protein in antral follicles stimulated with eCG and weak or no fluorescence signal of Cx43 in primary and preantral follicles. Similar results were confirmed by Western blotting analysis. Two gap junction blockers, lindane and carbenoxolone (CBX), significantly decreased the amplitude of Per2 oscillations, which further adhered significant decreases in Per2 and Rev-erbα transcript levels. In addition, both lindane and CBX induced a clear phase-delay shift of Per2 oscillations. These findings suggest that FSH induces the development of the clock system by increasing the expression of Cx43.

A stochastic mathematical model to study the autoimmune progression towards type 1 diabetes

BACKGROUND

The integrity of the interactions and the 3D architecture among beta cell populations in pancreatic islets is critical for proper biosynthesis, storage and release of insulin. The aim of this study was to evaluate the effect on electrophysiological signalling of beta cells that is produced by progressive lymphocytic islet cell infiltration (insulitis), by modelling the disruption of pancreatic islet anatomy as a consequence of insulitis and altered glucose concentrations.

METHODS

On the basis of histopathological images of murine islets from non-obese diabetic mice, we simulated the electrophysiological dynamics of a 3D cluster of mouse beta cells via a stochastic model. Progressive damage was modelled at different glucose concentrations, representing the different glycaemic states in the autoimmune progression towards type 1 diabetes.

RESULTS

At 31% of dead beta cells (normoglycaemia) and 69% (hyperglycaemia), the system appeared to be biologically robust to maintain regular Ca(2+) ion oscillations guaranteeing an effective insulin release. Simulations at 84%, 94% and 98% grades (severe hyperglycemia) showed that intracellular calcium oscillations were absent. In such conditions, insulin pulsatility is not expected to occur.

CONCLUSIONS

Our results suggest that the islet tissue is biophysically robust enough to compensate for high rates of beta cell loss. These predictions can be experimentally tested in vitro by quantifying space and time electrophysiological dynamics of animal islets kept at different glucose gradients. The model indicates the necessity of maintaining glycaemia within the physiological range as soon as possible after diabetes onset to avoid a dramatic interruption of Ca(2+) pulsatility and the consequent drop of insulin release.

Investigating the suspension culture on aggregation and function of mouse pancreatic β-cells

The integrity and hierarchical structure of islet influence β-cells physiology dramatically. A culture substrate which can maintain or improve β-cells aggregation shall benefit cell therapy for diabetics. In this study, nontreated, type IV collagen, Lipidure, and ultralow attachment dishes were used to culture a murine β-cell line, MIN-6. The formation and biological performances of pseudoislets were investigated. Results showed that β-cells formed loose and irregular aggregates on nontreated dishes. Oppositely, pseudoislets formed on other three substrates. Most pseudoislets on Lipidure and type IV collagen dishes had a diameter between 100-150 μm with high survival rate, while large pseudoislets (>250 μm) with seriously central necrosis were found on ultralow attachment dishes. Western blot analysis revealed that pseudoislets had relatively higher connexin 36 protein productions relative to single cells. The glucose-stimulated insulin secretion test showed pseudoislets on type IV collagen have high stimulation index. Monolayers from TCPS dishes and pseudoislets from type IV collagen or Lipidure dishes were further transplanted into diabetic mice. Animals received both single cells and pseudoislets had decreasing blood glucose level and regained body weight. Histologic examination revealed that all implants successfully engrafted with positive insulin staining. Interestingly, the area under curve for the intraperitoneal glucose tolerance test showed pseudoislets had superior glucose disappearance rate. This study reveals that isolated islets or insulin-producing cells can be cultured on type IV collagen or Lipidure dishes to improve/maintain integrity prior to transplantation.

High glucose predisposes gene expression and ERK phosphorylation to apoptosis and impaired glucose-stimulated insulin secretion via the cytoskeleton

Chronic high glucose (HG) inflicts glucotoxicity on vulnerable cell types such as pancreatic β cells and contributes to insulin resistance and impaired insulin secretion in diabetic patients. To identify HG-induced cellular aberrations that are candidate mediators of glucotoxicity in pancreatic β cells, we analyzed gene expression in ERoSHK6, a mouse insulin-secreting cell line after chronic HG exposure (six-day exposure to 33.3 mM glucose). Chronic HG exposure which reduced glucose-stimulated insulin secretion (GSIS) increased transcript levels of 185 genes that clustered primarily in 5 processes namely cellular growth and proliferation; cell death; cellular assembly and organization; cell morphology; and cell-to-cell signaling and interaction. The former two were validated by increased apoptosis of ERoSHK6 cells after chronic HG exposure and reaffirmed the vulnerability of β cells to glucotoxicity. The three remaining processes were partially substantiated by changes in cellular morphology and structure, and instigated an investigation of the cytoskeleton and cell-cell adhesion. These studies revealed a depolymerized actin cytoskeleton that lacked actin stress fibers anchored at vinculin-containing focal adhesion sites as well as loss of E-cadherin-mediated cell-cell adherence after exposure to chronic HG, and were concomitant with constitutive ERK1/2 phosphorylation that was refractory to serum and glucose deprivation. Although inhibition of ERK phosphorylation by PD98059 promoted actin polymerization, it increased apoptosis and GSIS impairment. These findings suggest that ERK phosphorylation is a proximate regulator of cellular processes targeted by chronic HG-induced gene expression and that dynamic actin polymerization and depolymerization is important in β cell survival and function. Therefore, chronic HG alters gene expression and signal transduction to predispose the cytoskeleton towards apoptosis and GSIS impairment.

The anti-ulcer agent, irsogladine, increases insulin secretion by MIN6 cells

Insulin secretion by pancreatic islets is a multicellular process. In addition to other essential systems, gap junctions are an important component of cell-to-cell communication in pancreatic islets. It is well known that dysfunction of gap junctions causes inappropriate insulin secretion. The anti-ulcer agent, irsogladine, increases gap junctions in some cell types. To examine the effect of irsogladine on insulin secretion, we investigated insulin secretion by MIN6 cells treated with or without irsogladine. The expression of connexin 36 proteins and intracellular cAMP levels were also determined using immunoblotting and ELISA assays, respectively. Irsogladine had no effect on insulin secretion under 5.6mM glucose conditions. However, under 16.7 mM glucose conditions, irsogladine (1.0 × 10(-5)M) induced a 1.7 ± 0.20 fold increase in insulin secretion compared to the control (P<0.05). This effect of irsogladine on insulin secretion was inhibited by the addition of the gap junction inhibitor 18-beta-glycyrrhetinic acid. Irsogladine treatment increased the protein level of connexin 36 in the plasma membrane fraction. The intracellular cAMP level in MIN6 cells was significantly, but mildly, increased by irsogladine treatment. Furthermore, Rp-cAMP and H89 inhibited the effects of irsogladine on insulin secretion under high glucose conditions. Irsogladine increases insulin secretion under high glucose conditions. The up-regulation of gap junction channels and the increased level of intracellular cAMP induced by irsogladine treatment suggest that these phenomena are involved in irsogladine-induced increased insulin secretion.

Connexins protect mouse pancreatic β cells against apoptosis

Type 1 diabetes develops when most insulin-producing β cells of the pancreas are killed by an autoimmune attack. The in vivo conditions modulating the sensitivity and resistance of β cells to this attack remain largely obscure. Here, we show that connexin 36 (Cx36), a trans-membrane protein that forms gap junctions between β cells in the pancreatic islets, protects mouse β cells against both cytotoxic drugs and cytokines that prevail in the islet environment at the onset of type 1 diabetes. We documented that this protection was at least partially dependent on intercellular communication, which Cx36 and other types of connexin channels establish within pancreatic islets. We further found that proinflammatory cytokines decreased expression of Cx36 and that experimental reduction or augmentation of Cx36 levels increased or decreased β cell apoptosis, respectively. Thus, we conclude that Cx36 is central to β cell protection from toxic insults.

Altered expression of the gap junction protein connexin43 is associated with papillary thyroid carcinomas when compared with other noncancer pathologies of the thyroid

BACKGROUND

Gap junctions are membrane structures composed of connexins (Cx) that allow diffusion of small molecules between cells. They are involved in tissue homeostasis, and various organ dysfunctions have been associated with gap junction defects. To verify their possible involvement in thyroid pathologies, the expression of connexin43 (Cx43), the major Cx in the human thyroid, was evaluated in a variety of diseases including cancer.

METHODS

There were 122 samples from various thyroid pathologies that were collected to analyze the presence of Cx43 by immunofluorescence. Through confocal microscopy, different patterns of Cx43 localization were identified as normal (membrane) or abnormal (cytoplasmic or lack of detection). The analysis of Cx43 expression was further performed by quantitative reverse transcriptase-polymerase chain reaction and immunohistochemistry in a subset of 25 papillary carcinomas and compared with nontumoral thyroid tissues.

RESULTS

The presence of Cx43 was commonly altered in thyroid cancer, as abnormal Cx43 staining was detected in 94.1% of cancer, 47.4% of adenomas, 45.7% of multinodular goiter, 16.7% of Graves' disease, and 25% of thyroiditis. In papillary carcinoma samples, the deregulation of Cx43 expression was mostly the consequence of a decrease of Cx43 mRNA (68% of cases) when compared with normal tissue. When Cx43 mRNA was not downregulated (32% of cases), both loss of membrane staining and aberrant cytoplasmic distribution of the protein were observed.

CONCLUSIONS

These results show that aberrations of Cx43 expression are associated with thyroid papillary carcinoma.

Connexin-dependent signaling in neuro-hormonal systems

The advent of multicellular organisms was accompanied by the development of short- and long-range chemical signalling systems, including those provided by the nervous and endocrine systems. In turn, the cells of these two systems have developed mechanisms for interacting with both adjacent and distant cells. With evolution, such mechanisms have diversified to become integrated in a complex regulatory network, whereby individual endocrine and neuro-endocrine cells sense the state of activity of their neighbors and, accordingly, regulate their own level of functioning. A consistent feature of this network is the expression of connexin-made channels between the (neuro)hormone-producing cells of all endocrine glands and secretory regions of the central nervous system so far investigated in vertebrates. This review summarizes the distribution of connexins in the mammalian (neuro)endocrine systems, and what we know about the participation of these proteins on hormone secretion, the life of the producing cells, and the action of (neuro)hormones on specific targets. The data gathered since the last reviews on the topic are summarized, with particular emphasis on the roles of Cx36 in the function of the insulin-producing beta cells of the endocrine pancreas, and of Cx40 in that of the renin-producing juxta-glomerular epithelioid cells of the kidney cortex. This article is part of a Special Issue entitled: The Communicating junctions, composition, structure and characteristics.

Connexins: key mediators of endocrine function

The appearance of multicellular organisms imposed the development of several mechanisms for cell-to-cell communication, whereby different types of cells coordinate their function. Some of these mechanisms depend on the intercellular diffusion of signal molecules in the extracellular spaces, whereas others require cell-to-cell contact. Among the latter mechanisms, those provided by the proteins of the connexin family are widespread in most tissues. Connexin signaling is achieved via direct exchanges of cytosolic molecules between adjacent cells at gap junctions, for cell-to-cell coupling, and possibly also involves the formation of membrane "hemi-channels," for the extracellular release of cytosolic signals, direct interactions between connexins and other cell proteins, and coordinated influence on the expression of multiple genes. Connexin signaling appears to be an obligatory attribute of all multicellular exocrine and endocrine glands. Specifically, the experimental evidence we review here points to a direct participation of the Cx36 isoform in the function of the insulin-producing β-cells of the endocrine pancreas, and of the Cx40 isoform in the function of the renin-producing juxtaglomerular epithelioid cells of the kidney cortex.

Connexin implication in the control of the murine beta-cell mass

Diabetes develops when the insulin needs of peripheral cells exceed the availability or action of the hormone. This situation results from the death of most beta-cells in type 1 diabetes, and from an inability of the beta-cell mass to adapt to increasing insulin needs in type 2 and gestational diabetes. We analyzed several lines of transgenic mice and showed that connexins (Cxs), the transmembrane proteins that form gap junctions, are implicated in the modulation of the beta-cell mass. Specifically, we found that the native Cx36 does not alter islet size or insulin content, whereas the Cx43 isoform increases both parameters, and Cx32 has a similar effect only when combined with GH. These findings open interesting perspectives for the in vitro and in vivo regulation of the beta-cell mass.

The spatiotemporal expression of multiple coho salmon ovarian connexin genes and their hormonal regulation in vitro during oogenesis

BACKGROUND

Throughout oogenesis, cell-cell communication via gap junctions (GJs) between oocytes and surrounding follicle cells (theca and granulosa cells), and/or amongst follicle cells is required for successful follicular development. To gain a fundamental understanding of ovarian GJs in teleosts, gene transcripts encoding GJ proteins, connexins (cx), were identified in the coho salmon, Oncorhynchus kisutch, ovary. The spatiotemporal expression of four ovarian cx transcripts was assessed, as well as their potential regulation by follicle-stimulating hormone (FSH), luteinizing hormone (LH) and insulin-like growth factor 1 (IGF1).

METHODS

Salmonid ovarian transcriptomes were mined for cx genes. Four gene transcripts designated cx30.9, cx34.3, cx43.2, and cx44.9 were identified. Changes in gene expression across major stages of oogenesis were determined with real-time, quantitative RT-PCR (qPCR) and cx transcripts were localized to specific ovary cell-types by in situ hybridization. Further, salmon ovarian follicles were cultured with various concentrations of FSH, LH and IGF1 and effects of each hormone on cx gene expression were determined by qPCR.

RESULTS

Transcripts for cx30.9 and cx44.9 were highly expressed at the perinucleolus (PN)-stage and decreased thereafter. In contrast, transcripts for cx34.3 and cx43.2 were low at the PN-stage and increased during later stages of oogenesis, peaking at the mid vitellogenic (VIT)-stage and maturing (MAT)-stage, respectively. In situ hybridization revealed that transcripts for cx34.3 were only detected in granulosa cells, but other cx transcripts were detected in both oocytes and follicle cells. Transcripts for cx30.9 and cx44.9 were down-regulated by FSH and IGF1 at the lipid droplet (LD)-stage, whereas transcripts for cx34.3 were up-regulated by FSH and IGF1 at the LD-stage, and LH and IGF1 at the late VIT-stage. Transcripts for cx43.2 were down-regulated by IGF1 at the late VIT-stage and showed no response to gonadotropins.

CONCLUSION

Our findings demonstrate the presence and hormonal regulation of four different cx transcripts in the salmon ovary. Differences in the spatiotemporal expression profile and hormonal regulation of these cx transcripts likely relate to their different roles during ovarian follicle differentiation and development.

Connexin32 protects against vascular inflammation by modulating inflammatory cytokine expression by endothelial cells

Gap junctions (GJs) play an important role in vascular function, stability, and homeostasis in endothelial cells (ECs), and GJs are comprised of members of the connexin (Cx) family. GJs of vascular ECs are assembled from Cx37, Cx40, and Cx43, and we showed that ECs also express Cx32. In this study, we investigated a potential role for Cx32 during vascular inflammation. Expression of Cx32 mRNA and protein by human umbilical venous ECs (HUVECs) decreased following treatment with tumor necrosis factor (TNF)-α, but lipopolysaccharide (LPS) and interleukin (IL)-1β did not affect Cx32 expression. Intracellular transfer of an inhibitory anti-Cx32 monoclonal antibody significantly enhanced TNF-α-induced monocyte chemotactic protein (MCP)-1 and IL-6 expression, but overexpression of Cx32 abrogated TNF-α-induced MCP-1 and IL-6 expression. LPS treatment of Cx32 knock-out mice significantly increased the serum concentrations of TNF-α, interferon-γ, IL-6 and MCP-1, compared to wild-type littermate mice. These data suggest that Cx32 protects ECs from inflammation by regulating cytokine expression and plays an important role in the maintenance of vascular function.

Regulation of renin release by connexin 43 in As 4.1 cell line

Gap junction channels facilitate chemical and electrical communication between adjacent cells. Gap junction protein, connexin (Cx), is expressed in the endothelial cells of vessels, glomerulus, and renin-secreting cells of the kidney. The purpose of this study was to investigate the role of Cx in renin release using Cx-overexpressing As 4.1 cells. The adenovirus-induced Cx overexpression was conducted by using recombinant adenovirus containing the cDNA encoding Cx37, Cx40, Cx43 (Ad-Cx), and beta-galactosidase (Ad-beta-gal). In 40-overexpressing cells, basal renin release increased in a time-dependent manner but it was significantly lower than that in Ad-beta-gal-treated cells. In Cx37- and Cx43-overexpressing cells, basal renin release was increased in a time-dependent manner, which was not different from control cells. 18-beta glycyrrhetinic acid (GA), a gap junction blocker, stimulated renin release dose-dependently and increased intracellular Ca(2+) in both Cx43-overexpressing cells and control cells. However, no significant differences were observed. An increase in renin release by 3,4,5-trimethoxybenzoic acid 8-(diethylamino)-octyl ester, a putative antagonist of Ca(2+) release from intracellular sequestration sites, was also similar between two groups. These results suggest that Cx43 may unlikely alter the regulation of renin release and intracellular Ca(2+) by gap junction blocker in As 4.1 cells.

Pass the bicarb: the importance of HCO3- for mucin release

Accumulation of thick, sticky mucus is a hallmark of the genetic disease cystic fibrosis (CF) and has a central role in CF pathophysiology. Mutations in the CF transmembrane regulator (CFTR) ion channel are known to result in abnormally thick and sticky mucus; however, why mucus accumulates in CF is still not completely understood. In this issue of the JCI, Garcia and colleagues show that mucin--the heavily glycosylated protein contained within mucus--requires CFTR and bicarbonate in order to be released from mouse intestine (see the related article beginning on page 2613). The authors propose a model whereby CFTR-mediated bicarbonate secretion must be concurrent with mucin exocytosis for proper mucin release.

Exogenous Cx43 expression decrease cell proliferation rate in rat hepatocarcinoma cells independently of functional gap junction

BACKGROUND

Gap junction intercellular communication (GJIC) is considered to play a role in the regulation of homeostasis because it regulates important processes, such as cell proliferation and cell differentiation. A reduced or lost GJIC capacity has been observed in solid tumors and studies have demonstrated that GJIC restoration in tumor cells contribute to reversion of the transformed phenotype. This observation supports the idea that restoration of the functional channel is essential in this process. However, in the last years, reports have proposed that just the increase in the expression of specific connexins can contribute to reversion of the malign phenotype in some tumor cells. In the present work, we studied the effects of exogenous Connexin 43 (Cx43) expression on the proliferative behavior and phenotype of rat hepatocarcinoma cells.

RESULTS

The exogenous Cx43 did not increase GJIC capacity of transfected cells, but it was critical to decrease the cell proliferation rate as well as reorganization of the actin filaments and cell flattening. We also observed more adhesion capacity to substrate after Cx43 transfection.

CONCLUSION

Cx43 expression leads to a decrease of the growth of the rat hepatocellular carcinoma cells and it contributes to the reversion of the transformed phenotype. These effects were independent of the GJIC and were probably associated with the phosphorylation pattern changes and redistribution of the Cx43 protein.

Colocalization of connexin 36 and corticotropin-releasing hormone in the mouse brain

BACKGROUND

Gap junction proteins, connexins, are expressed in most endocrine and exocrine glands in the body and are at least in some glands crucial for the hormonal secretion. To what extent connexins are expressed in neurons releasing hormones or neuropeptides from or within the central nervous system is, however, unknown. Previous studies provide indirect evidence for gap junction coupling between subsets of neuropeptide-containing neurons in the paraventricular nucleus (PVN) of the hypothalamus. Here we employ double labeling and retrograde tracing methods to investigate to what extent neuroendocrine and neuropeptide-containing neurons of the hypothalamus and brainstem express the neuronal gap junction protein connexin 36.

RESULTS

Western blot analysis showed that connexin 36 is expressed in the PVN. In bacterial artificial chromosome transgenic mice, which specifically express the reporter gene Enhanced Green Fluorescent Protein (EGFP) under the control of the connexin 36 gene promoter, EGFP expression was detected in magnocellular (neuroendocrine) and in parvocellular neurons of the PVN. Although no EGFP/connexin36 expression was seen in neurons containing oxytocin or vasopressin, EGFP/connexin36 was found in subsets of PVN neurons containing corticotropin-releasing hormone (CRH), and in somatostatin neurons located along the third ventricle. Moreover, CRH neurons in brainstem areas, including the lateral parabrachial nucleus, also expressed EGFP/connexin 36.

CONCLUSION

Our data indicate that connexin 36 is expressed in subsets of neuroendocrine and CRH neurons in specific nuclei of the hypothalamus and brainstem.

Connexins within the rat larynx

OBJECTIVES/HYPOTHESIS

To determine the types and localization of connexins within the rat larynx.

STUDY DESIGN

Quantitative real time polymerase chain reaction (qRT-PCR) of the epiglottis and laryngeal mucosa was used to identify connexins (Cx). Immunohistochemical labeling was then used to localize the Cxs within the larynx.

METHODS

Twelve larynges from 3 to 4 month old Fisher-344 rats were used. RNA was extracted (N = 8) and cDNA produced. Primers for Cx26, Cx30, Cx32, Cx37, Cx40, and Cx43 were added and qRT-PCR performed. Others larynges were serially sectioned for immunohistochemistry.

RESULTS

qRT-PCR revealed Cx43, Cx32, and Cx30 within the epiglottis and Cx43 in the vocal folds and Cx43 and Cx32 within the subglottic mucosa. Immunohistochemical staining confirmed these results.

CONCLUSION

The rat epiglottis is rich in Cx43, Cx32, and Cx30 whereas the vocal folds contain Cx43 and the subglottic mucosa Cx43 and Cx32. Their localizations suggest involvement in secretion for protective purposes and they may play a key role in laryngeal pathoses.

Substitution of connexin40 with connexin45 prevents hyperreninemia and attenuates hypertension

Connexins (Cxs) are a family of transmembrane proteins that form gap junctions with unique and redundant biophysical functions. Juxtaglomerular cells express Cx40, which is crucial to the control of renin secretion by blood pressure and angiotensin II, and mice that lack Cx40 have high plasma renin and hypertension. To examine whether normal juxtaglomerular cell function depends on the unique properties of Cx40, we measured renin release in mice where the coding sequence for Cx40 was replaced by that for Cx45, using the knock-in method. We first found that the knock-in strategy indeed resulted in expression of Cx45 but not Cx40 in the juxtaglomerular cells of these mice. The plasma renin concentration of the knock-in mice was similar to that in wild-type mice. The high blood pressure of the Cx40 knockout mice was significantly reduced when Cx45 was knocked into the locus but remained mildly elevated compared to wild-type mice. Blockade of angiotensin II formation by enalapril increased the plasma renin concentration in wild-type and the Cx45 knock-in mice but not in the Cx40 knockout mice. Infusion of angiotensin II into isolated perfused kidneys results in decreased renin release, a phenomenon that was attenuated in the Cx40 knockout mice. However, in the Cx45 knock-in mice, angiotensin II suppressed renin release similar to its effect in wild type mice. Unilateral renal artery stenosis increased the plasma renin concentration and blood pressure in both the wild-type and the Cx45 knock-in mice but not in the Cx40 knockout mice. Since Cx40 can be replaced by Cx45, a connexin with a significantly lower conductivity, we suggest that the regulation of renin release is not dependent on the unique electrical properties of these channel proteins.

Cx36 makes channels coupling human pancreatic beta-cells, and correlates with insulin expression

Previous studies have documented that the insulin-producing beta-cells of laboratory rodents are coupled by gap junction channels made solely of the connexin36 (Cx36) protein, and have shown that loss of this protein desynchronizes beta-cells, leading to secretory defects reminiscent of those observed in type 2 diabetes. Since human islets differ in several respects from those of laboratory rodents, we have now screened human pancreas, and islets isolated thereof, for expression of a variety of connexin genes, tested whether the cognate proteins form functional channels for islet cell exchanges, and assessed whether this expression changes with beta-cell function in islets of control and type 2 diabetics. Here, we show that (i) different connexin isoforms are differentially distributed in the exocrine and endocrine parts of the human pancreas; (ii) human islets express at the transcript level different connexin isoforms; (iii) the membrane of beta-cells harbors detectable levels of gap junctions made of Cx36; (iv) this protein is concentrated in lipid raft domains of the beta-cell membrane where it forms gap junctions; (v) Cx36 channels allow for the preferential exchange of cationic molecules between human beta-cells; (vi) the levels of Cx36 mRNA correlated with the expression of the insulin gene in the islets of both control and type 2 diabetics. The data show that Cx36 is a native protein of human pancreatic islets, which mediates the coupling of the insulin-producing beta-cells, and contributes to control beta-cell function by modulating gene expression.

Coordinated synchronization in the electrically coupled network of terminal nerve gonadotropin-releasing hormone neurons as demonstrated by double patch-clamp study

The peptidergic neurons play important roles such as neuromodulatory and neuroendocrine functions in the central nervous system. However, our knowledge about the organization and the function of the peptidergic neuromodulator systems is still very poor. The terminal nerve GnRH peptidergic neurons of a teleost, the dwarf gourami (Colisa lalia), serve as an excellent model system for such study. The cell bodies are large and make up a tight cell cluster, and the easy access to the cell bodies on the ventral surface of the brain makes the electrophysiological measurements in a precisely controlled manner. Here we show direct evidence to demonstrate the electrical coupling and the synchronization of the neural firing activity among the terminal nerve GnRH neurons by using the double patch-clamp recording technique. The electrical coupling coefficient was strong enough (ranged from 0.083 to 0.370) to synchronize spontaneous firings of GnRH neurons in the cluster. A model, in which the firings in the cluster occur within a small time window (dozens of milliseconds), was verified by using the serial loose-seal extracellular patch-clamp recordings and the cross-correlogram analysis. The present findings provide several insights for understanding the physiological mechanisms and functional significance of synchronized activities in the peptidergic and/or aminergic neuromodulator system as well as in the peptidergic neuroendocrine cells.

Functional remodeling of gap junction-mediated electrical communication between adrenal chromaffin cells in stressed rats

An increase in circulating catecholamine levels represents one of the mechanisms whereby organisms cope with stress. In the periphery, catecholamines mainly originate from the sympathoadrenal system. As we reported, in addition to the central control through cholinergic innervation, a local gap junction-delineated route between adrenal chromaffin cells contributes to catecholamine exocytosis. Here, we investigated whether this intercellular communication is modified when the hormonal demand is increased as observed during cold stress. Our results show that in cold exposed rats, gap-junctional communication undergoes a functional plasticity, as evidenced by an increased number of dye-coupled cells. Of a physiological interest is that this upregulation of gap-junctional coupling results in the appearance of a robust electrical coupling between chromaffin cells that allows the transmission of action potentials between coupled cells. This enhancement of gap-junctional communication parallels an increase in expression levels of connexin36 (Cx36) and connexin43 (Cx43) proteins. Both transcriptional and posttranslational mechanisms are involved because Cx36 transcripts are increased in stressed rats and the expression of the scaffolding protein zonula occludens-1, known to interact with both Cx36 and Cx43, is also upregulated. Consistent with an upregulated coupling extent in stressed rats, the cytosolic Ca(2+) concentration rises triggered in a single cell by an iontophoretic application of nicotine occur simultaneously in several neighboring cells. These results describe for the first time a functional plasticity of junctional coupling between adult chromaffin cells that should be crucial for adaptation to stress or sensitization to subsequent stressors.

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.

Gap junctions in the ovary: expression, localization and function

Gap junctions that allow the direct communication between cytoplasmic compartments of neighboring cells are present in a variety of tissues and organs and play pivotal roles in a wide range of physiological processes. In the ovary, gap junctions consist mainly of connexin (Cx) 43 and Cx37, and their indispensable role in regulating folliculogenesis and oogenesis is well established. The ovarian Cx43 is regulated by gonadotropins at the transcriptional, translational and post-translational levels whereas the regulation of the ovarian Cx37 is yet unknown. In addition to their involvement in normal ovarian functions, gap junction proteins, particularly Cx43, seem to act as cancer suppressors. A summary of our present knowledge regarding gap junctional communication (GJC) and the ovarian gap junction proteins in normally developing ovaries and under pathological conditions is presented in this review.

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.

Hypertension in connexin40-null mice: a renin disorder

Studies described in this issue indicate that the gap junction protein connexin40 (Cx40) appears to play an unexpected role in blood pressure regulation. In mice lacking this gap junction protein, renin secretion is high and not regulated by arteriolar pressure.