Beta cell coupling and connexin expression change during the functional maturation of rat pancreatic islets

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

BACKGROUND

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

Role of Delta/Notch-like EGF-related receptor in blood glucose homeostasis

Cell-cell interactions are necessary for optimal endocrine functions in the pancreas. β-cells, characterized by the expression and secretion of the hormone insulin, are a major constituent of functional micro-organs in the pancreas known as islets of Langerhans. Cell-cell contacts between β-cells are required to regulate insulin production and glucose-stimulated insulin secretion, which are key determinants of blood glucose homeostasis. Contact-dependent interactions between β-cells are mediated by gap junctions and cell adhesion molecules such as E-cadherin and N-CAM. Recent genome-wide studies have implicated Delta/Notch-like EGF-related receptor (Dner) as a potential susceptibility locus for Type 2 Diabetes in humans. DNER is a transmembrane protein and a proposed Notch ligand. DNER has been implicated in neuron-glia development and cell-cell interactions. Studies herein demonstrate that DNER is expressed in β-cells with an onset during early postnatal life and sustained throughout adulthood in mice. DNER loss in adult β-cells in mice (β-Dner cKO mice) disrupted islet architecture and decreased the expression of N-CAM and E-cadherin. β-Dner cKO mice also exhibited impaired glucose tolerance, defects in glucose- and KCl-induced insulin secretion, and decreased insulin sensitivity. Together, these studies suggest that DNER plays a crucial role in mediating islet cell-cell interactions and glucose homeostasis.

Human pluripotent stem cell-derived β cells: Truly immature islet β cells for type 1 diabetes therapy?

A century has passed since the Nobel Prize winning discovery of insulin, which still remains the mainstay treatment for type 1 diabetes mellitus (T1DM) to this day. True to the words of its discoverer Sir Frederick Banting, “insulin is not a cure for diabetes, it is a treatment”, millions of people with T1DM are dependent on daily insulin medications for life. Clinical donor islet transplantation has proven that T1DM is curable, however due to profound shortages of donor islets, it is not a mainstream treatment option for T1DM. Human pluripotent stem cell derived insulin-secreting cells, pervasively known as stem cell-derived β cells (SC-β cells), are a promising alternative source and have the potential to become a T1DM treatment through cell replacement therapy. Here we briefly review how islet β cells develop and mature in vivo and several types of reported SC-β cells produced using different ex vivo protocols in the last decade. Although some markers of maturation were expressed and glucose stimulated insulin secretion was shown, the SC-β cells have not been directly compared to their in vivo counterparts, generally have limited glucose response, and are not yet fully matured. Due to the presence of extra-pancreatic insulin-expressing cells, and ethical and technological issues, further clarification of the true nature of these SC-β cells is required.

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

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

Pancreatic β-Cell Development and Regeneration

The pancreatic β-cells are essential for regulating glucose homeostasis through the coordinated release of the insulin hormone. Dysfunction of the highly specialized β-cells results in diabetes mellitus, a growing global health epidemic. In this review, we describe the development and function of β-cells the emerging concept of heterogeneity within insulin-producing cells, and the potential of other cell types to assume β-cell functionality via transdifferentiation. We also discuss emerging routes to design cells with minimal β-cell properties and human stem cell differentiation efforts that carry the promise to restore normoglycemia in patients suffering from diabetes.

Persistent coxsackievirus B1 infection triggers extensive changes in the transcriptome of human pancreatic ductal cells

Enteroviruses, particularly the group B coxsackieviruses (CVBs), have been associated with the development of type 1 diabetes. Several CVB serotypes establish chronic infections in human cells in vivo and in vitro. However, the mechanisms leading to enterovirus persistency and, possibly, beta cell autoimmunity are not fully understood. We established a carrier-state-type persistent infection model in human pancreatic cell line PANC-1 using two distinct CVB1 strains and profiled the infection-induced changes in cellular transcriptome. In the current study, we observed clear changes in the gene expression of factors associated with the pancreatic microenvironment, the secretory pathway, and lysosomal biogenesis during persistent CVB1 infections. Moreover, we found that the antiviral response pathways were activated differently by the two CVB1 strains. Overall, our study reveals extensive transcriptional responses in persistently CVB1-infected pancreatic cells with strong opposite but also common changes between the two strains.

A Method for Encapsulation and Transplantation into Diabetic Mice of Human Induced Pluripotent Stem Cells (hiPSC)-Derived Pancreatic Progenitors

Pancreatic islet endocrine cells generated from patient-derived induced pluripotent stem cells represent a great strategy for both disease modeling and regenerative medicine. Nevertheless, these cells inherently miss the effects of the intricate network of systemic signals characterizing the living organisms. Xenotransplantation of in vitro differentiating cells into murine hosts substantially compensates for this drawback.Here we describe our transplantation strategy of encapsulated differentiating pancreatic progenitors into diabetic immunosuppressed (NSG) overtly diabetic mice generated by the total ablation of insulin-producing cells following diphtheria toxin administration. We will detail the differentiation protocol employed, the alginate encapsulation procedure, and the xenotransplantation steps required for a successful and reproducible experiment.

Differential Toxicological Outcome of Corn Oil Exposure in Rats and Mice as Assessed by Microbial Composition, Epithelial Permeability, and Ileal Mucosa-Associated Immune Status

Studies to evaluate the toxicity of xenobiotics on the human gut microbiome and related health effects require a diligent selection of (1) an appropriate animal model to facilitate toxicity assessment in predicting human exposure, and (2) an appropriate non-interfering vehicle for the administration of water insoluble compounds. In biomedical studies with water insoluble xenobiotics, corn oil is one of the most commonly used nonaqueous vehicles. This study evaluated the suitability of corn oil as a vehicle in adult female Sprague Dawley rats and adult CD-1 mice; the rodent models that are often utilized in toxicological studies. We studied the host response in terms of change in the intestinal microbiome and mRNA expression of intestinal permeability and immune response-related genes when water (control) and corn oil (2 ml/kg) were administered as a vehicle through oral gavage. The results showed that the use of corn oil as a vehicle has no adverse impact in rats for either the immune response or the intestinal microbial population. On the other hand, mice treated with corn oil showed changes in bacterial community adhered to the ileum, as well as changes in the mRNA expression of intestinal permeability-related and ileal mucosa-associated immune response genes. Overall, results of this study suggest that the type of rodent species and vehicle used in toxicological risk assessments of xenobiotics studies should be taken into consideration in the experimental setup and study design.

Immune Protection of Stem Cell-Derived Islet Cell Therapy for Treating Diabetes

Insulin injection is currently the main therapy for type 1 diabetes (T1D) or late stage of severe type 2 diabetes (T2D). Human pancreatic islet transplantation confers a significant improvement in glycemic control and prevents life-threatening severe hypoglycemia in T1D patients. However, the shortage of cadaveric human islets limits their therapeutic potential. In addition, chronic immunosuppression, which is required to avoid rejection of transplanted islets, is associated with severe complications, such as an increased risk of malignancies and infections. Thus, there is a significant need for novel approaches to the large-scale generation of functional human islets protected from autoimmune rejection in order to ensure durable graft acceptance without immunosuppression. An important step in addressing this need is to strengthen our understanding of transplant immune tolerance mechanisms for both graft rejection and autoimmune rejection. Engineering of functional human pancreatic islets that can avoid attacks from host immune cells would provide an alternative safe resource for transplantation therapy. Human pluripotent stem cells (hPSCs) offer a potentially limitless supply of cells because of their self-renewal ability and pluripotency. Therefore, studying immune tolerance induction in hPSC-derived human pancreatic islets will directly contribute toward the goal of generating a functional cure for insulin-dependent diabetes. In this review, we will discuss the current progress in the immune protection of stem cell-derived islet cell therapy for treating diabetes.

Transcriptomic and Quantitative Proteomic Profiling Reveals Signaling Pathways Critical for Pancreatic Islet Maturation

Pancreatic β-cell dysfunction and reduced insulin secretion play a key role in the pathogenesis of diabetes. Fetal and neonatal islets are functionally immature and have blunted glucose responsiveness and decreased insulin secretion in response to stimuli and are far more proliferative. However, the mechanisms underlying functional immaturity are not well understood. Pancreatic islets are composed of a mixture of different cell types, and the microenvironment of islets and interactions between these cell types are critical for β-cell development and maturation. RNA sequencing and quantitative proteomic data from intact islets isolated from fetal (embryonic day 19) and 2-week-old Sprague-Dawley rats were integrated to compare their gene and protein expression profiles. Ingenuity Pathway Analysis (IPA) was also applied to elucidate pathways and upstream regulators modulating functional maturation of islets. By integrating transcriptome and proteomic data, 917 differentially expressed genes/proteins were identified with a false discovery rate of less than 0.05. A total of 411 and 506 of them were upregulated and downregulated in the 2-week-old islets, respectively. IPA revealed novel critical pathways associated with functional maturation of islets, such as AMPK (adenosine monophosphate-activated protein kinase) and aryl hydrocarbon receptor signaling, as well as the importance of lipid homeostasis/signaling and neuronal function. Furthermore, we also identified many proteins enriched either in fetal or 2-week-old islets related to extracellular matrix and cell communication, suggesting that these pathways play critical roles in islet maturation. Our present study identified novel pathways for mature islet function in addition to confirming previously reported mechanisms, and provided new mechanistic insights for future research on diabetes prevention and treatment.

Enhanced differentiation of human pluripotent stem cells into pancreatic endocrine cells in 3D culture by inhibition of focal adhesion kinase

Background

Generation of insulin-producing cells from human pluripotent stem cells (hPSCs) in vitro would be useful for drug discovery and cell therapy in diabetes. Three-dimensional (3D) culture is important for the acquisition of mature insulin-producing cells from hPSCs, but the mechanism by which it promotes β cell maturation is poorly understood.

Methods

We established a stepwise method to induce high-efficiency differentiation of human embryonic stem cells (hESCs) into mature monohormonal pancreatic endocrine cells (PECs), with the last maturation stage in 3D culture. To comprehensively compare two-dimensional (2D) and 3D cultures, we examined gene expression, pancreas-specific markers, and functional characteristics in 2D culture-induced PECs and 3D culture-induced PECs. The mechanisms were considered from the perspectives of cell–cell and cell–extracellular matrix interactions which are fundamentally different between 2D and 3D cultures.

Results

The expression of the pancreatic endocrine-specific transcription factors PDX1, NKX6.1, NGN3, ISL1, and PAX6 and the hormones INS, GCG, and SST was significantly increased in 3D culture-induced PECs. 3D culture yielded monohormonal endocrine cells, while 2D culture-induced PECs co-expressed INS and GCG or INS and SST or even expressed all three hormones. We found that focal adhesion kinase (FAK) phosphorylation was significantly downregulated in 3D culture-induced PECs, and treatment with the selective FAK inhibitor PF-228 improved the expression of β cell-specific transcription factors in 2D culture-induced PECs. We further demonstrated that 3D culture may promote endocrine commitment by limiting FAK-dependent activation of the SMAD2/3 pathway. Moreover, the expression of the gap junction protein Connexin 36 was much higher in 3D culture-induced PECs than in 2D culture-induced PECs, and inhibition of the FAK pathway in 2D culture increased Connexin 36 expression.

Conclusion

We developed a strategy to induce differentiation of monohormonal mature PECs from hPSCs and found limited FAK-dependent activation of the SMAD2/3 pathway and unregulated expression of Connexin 36 in 3D culture-induced PECs. This study has important implications for the generation of mature, functional β cells for drug discovery and cell transplantation therapy for diabetes and sheds new light on the signaling events that regulate endocrine specification.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13287-020-02003-z.

The Connexin 43 Regulator Rotigaptide Reduces Cytokine-Induced Cell Death in Human Islets

Background: Intercellular communication mediated by cationic fluxes through the Connexin family of gap junctions regulates glucose-stimulated insulin secretion and beta cell defense against inflammatory stress. Rotigaptide (RG, ZP123) is a peptide analog that increases intercellular conductance in cardiac muscle cells by the prevention of dephosphorylation and thereby uncoupling of Connexin-43 (Cx43), possibly via action on unidentified protein phosphatases. For this reason, it is being studied in human arrhythmias. It is unknown if RG protects islet cell function and viability against inflammatory or metabolic stress, a question of considerable translational interest for the treatment of diabetes. Methods: Apoptosis was measured in human islets shown to express Cx43, treated with RG or the control peptide ZP119 and exposed to glucolipotoxicity or IL-1β + IFNɣ. INS-1 cells shown to lack Cx43 were used to examine if RG protected human islet cells via Cx43 coupling. To study the mechanisms of action of Cx43-independent effects of RG, NO, IkBα degradation, mitochondrial activity, ROS, and insulin mRNA levels were determined. Results: RG reduced cytokine-induced apoptosis ~40% in human islets. In Cx43-deficient INS-1 cells, this protective effect was markedly blunted as expected, but unexpectedly, RG still modestly reduced apoptosis, and improved mitochondrial function, insulin-2 gene levels, and accumulated insulin release. RG reduced NO production in Cx43-deficient INS-1 cells associated with reduced iNOS expression, suggesting that RG blunts cytokine-induced NF-κB signaling in insulin-producing cells in a Cx43-independent manner. Conclusion: RG reduces cytokine-induced cell death in human islets. The protective action in Cx43-deficient INS-1 cells suggests a novel inhibitory mechanism of action of RG on NF-κB signaling.

Encapsulation boosts islet-cell signature in differentiating human induced pluripotent stem cells via integrin signalling

Cell replacement therapies hold great therapeutic potential. Nevertheless, our knowledge of the mechanisms governing the developmental processes is limited, impeding the quality of differentiation protocols. Generating insulin-expressing cells in vitro is no exception, with the guided series of differentiation events producing heterogeneous cell populations that display mixed pancreatic islet phenotypes and immaturity. The achievement of terminal differentiation ultimately requires the in vivo transplantation of, usually, encapsulated cells. Here we show the impact of cell confinement on the pancreatic islet signature during the guided differentiation of alginate encapsulated human induced pluripotent stem cells (hiPSCs). Our results show that encapsulation improves differentiation by significantly reshaping the proteome landscape of the cells towards an islet-like signature. Pathway analysis is suggestive of integrins transducing the encapsulation effect into intracellular signalling cascades promoting differentiation. These analyses provide a molecular framework for understanding the confinement effects on hiPSCs differentiation while confirming its importance for this process.

ARHGAP21 Acts as an Inhibitor of the Glucose-Stimulated Insulin Secretion Process

ARHGAP21 is a RhoGAP protein implicated in the modulation of insulin secretion and energy metabolism. ARHGAP21 transient-inhibition increase glucose-stimulated insulin secretion (GSIS) in neonatal islets; however, ARHGAP21 heterozygote mice have a reduced insulin secretion. These discrepancies are not totally understood, and it might be related to functional maturation of beta cells and peripheral sensitivity. Here, we investigated the real ARHGAP21 role in the insulin secretion process using an adult mouse model of acute ARHGAP21 inhibition, induced by antisense. After ARHGAP21 knockdown induction by antisense injection in 60-day old male mice, we investigated glucose and insulin tolerance test, glucose-induced insulin secretion, glucose-induced intracellular calcium dynamics, and gene expression. Our results showed that ARHGAP21 acts negatively in the GSIS of adult islet. This effect seems to be due to the modulation of important points of insulin secretion process, such as the energy metabolism (PGC1α), Ca²⁺ signalization (SYTVII), granule-extrusion (SNAP25), and cell-cell interaction (CX36). Therefore, based on these finds, ARHGAP21 may be an important target in Diabetes Mellitus (DM) treatment.

Connexin 43 Functions as a Positive Regulator of Stem Cell Differentiation into Definitive Endoderm and Pancreatic Progenitors

Efficient stem cell differentiation into pancreatic islet cells is of critical importance for the development of cell replacement therapies for diabetes. Here, we identify the expression pattern of connexin 43 (Cx43), a gap junction (GJ) channel protein, in human embryonic stem cell (hESC)-derived definitive endoderm (DE) and primitive gut tube cells, representing early lineages for posterior foregut (PF), pancreatic progenitors (PP), pancreatic endocrine progenitors (PE), and islet cells. As the function of GJ channels is dependent on their gating status, we tested the impact of supplementing hESC-derived PP cell cultures with AAP10, a peptide that promotes Cx43 GJ channel opening. We found that this treatment promotes the expression of DE markers FoxA2 and Sox17, leads to a more efficient derivation of DE, and improves the yield of PF, PP, and PE cells. These results demonstrate a functional involvement of GJ channels in the differentiation of embryonic stem cells into pancreatic cell lineages.

In vivo imaging of β-cell function reveals glucose-mediated heterogeneity of β-cell functional development

How pancreatic β-cells acquire function in vivo is a long-standing mystery due to the lack of technology to visualize β-cell function in living animals. Here, we applied a high-resolution two-photon light-sheet microscope for the first in vivo imaging of Ca²⁺activity of every β-cell in Tg (ins:Rcamp1.07) zebrafish. We reveal that the heterogeneity of β-cell functional development in vivo occurred as two waves propagating from the islet mantle to the core, coordinated by islet vascularization. Increasing amounts of glucose induced functional acquisition and enhancement of β-cells via activating calcineurin/nuclear factor of activated T-cells (NFAT) signaling. Conserved in mammalians, calcineurin/NFAT prompted high-glucose-stimulated insulin secretion of neonatal mouse islets cultured in vitro. However, the reduction in low-glucose-stimulated insulin secretion was dependent on optimal glucose but independent of calcineurin/NFAT. Thus, combination of optimal glucose and calcineurin activation represents a previously unexplored strategy for promoting functional maturation of stem cell-derived β-like cells in vitro.

When the amount of sugar in our body rises, specialised cells known as β-cells respond by releasing insulin, a hormone that acts on various organs to keep blood sugar levels within a healthy range. These cells cluster in small ‘islets’ inside our pancreas. If the number of working β-cells declines, diseases such as diabetes may appear and it becomes difficult to regulate the amount of sugar in our bodies. Understanding how β-cells normally develop and mature in the embryo could help us learn how to make new ones in the laboratory. In particular, researchers are interested in studying how different body signals, such as blood sugar levels, turn immature β-cells into fully productive cells. However, in mammals, the pancreas and its islets are buried deep inside the embryo and they cannot be observed easily.

Here, Zhao et al. circumvented this problem by doing experiments on zebrafish embryos, which are transparent, grow outside their mother’s body, and have pancreatic islets that are similar to the ones found in mammals. A three-dimensional microscopy technique was used to watch individual β-cells activity over long periods, which revealed that the cells start being able to produce insulin at different times. The β-cells around the edge of each islet were the first to have access to blood sugar signals: they gained their hormone-producing role earlier than the cells in the core of an islet, which only sensed the information later on.

Zhao et al. then exposed the zebrafish embryos to different amounts of sugar. This showed that there is an optimal concentration of sugar which helps β-cells develop by kick-starting a cascade of events inside the cell. Further experiments confirmed that the same pathway and optimal sugar concentration exist for mammalian islets grown in the laboratory.

These findings may help researchers find better ways of making new β-cells to treat diabetic patients. In the future, using the three-dimensional imaging technique in zebrafish embryos may lead to more discoveries on how the pancreas matures.

Whole body ARHGAP21 reduction improves glucose homeostasis in high-fat diet obese mice

GTPase activating proteins (GAPs) are ubiquitously expressed, and their role in cellular adhesion and membrane traffic processes have been well described. TBC1D1, which is a Rab-GAP, is necessary for adequate glucose uptake by muscle cells, whereas increased TCGAP, which is a Rho-GAP, decreases GLUT4 translocation, and consequently glucose uptake in adipocytes. Here, we assessed the possible involvement of ARHGAP21, a Rho-GAP protein, in glucose homeostasis. For this purpose, wild type mice and ARHGAP21 transgenic whole-body gene-deficiency mice (heterozygous mice, expressing approximately 50% of ARHGAP21) were fed either chow (Ctl and Het) or high-fat diet (Ctl-HFD and Het-HFD). Het-HFD mice showed a reduction in white fat storage, reflected in a lower body weight gain. These mice also displayed an improvement in insulin sensitivity and glucose tolerance, which likely contributed to reduced insulin secretion and pancreatic beta cell area. The reduction of body weight was also observed in Het mice and this phenomenon was associated with an increase in brown adipose tissue and reduced muscle weight, without alteration in glucose-insulin homeostasis. In conclusion, the whole body ARHGAP21 reduction improved glucose homeostasis and protected against diet-induced obesity specifically in Het-HFD mice. However, the mechanism by which ARHGAP21 leads to these outcomes requires further investigation.

Age-Dependent Decline in the Coordinated [Ca2+] and Insulin Secretory Dynamics in Human Pancreatic Islets

Aging is associated with increased risk for type 2 diabetes, resulting from reduced insulin sensitivity and secretion. Reduced insulin secretion can result from reduced proliferative capacity and reduced islet function. Mechanisms underlying altered β-cell function in aging are poorly understood in mouse and human islets, and the impact of aging on intraislet communication has not been characterized. Here, we examine how β-cell [Ca2+] and electrical communication are impacted during aging in mouse and human islets. Islets from human donors and from mice were studied using [Ca2+] imaging, static and perifusion insulin secretion assays, and gap junction permeability measurements. In human islets, [Ca2+] dynamics were coordinated within distinct subregions of the islet, invariant with islet size. There was a marked decline in the coordination of [Ca2+] dynamics, gap junction coupling, and insulin secretion dynamics with age. These age-dependent declines were reversed by pharmacological gap junction activation. These results show that human islet function declines with aging, which can reduce insulin action and may contribute to increased risk of type 2 diabetes.

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.

Reduced insulin secretion function is associated with pancreatic islet redistribution of cell adhesion molecules (CAMS) in diabetic mice after prolonged high-fat diet

Intercellular junctions play a role in regulating islet cytoarchitecture, insulin biosynthesis and secretion. In this study, we investigated the animal metabolic state as well as islet histology and cellular distribution/expression of CAMs and F-actin in the endocrine pancreas of C57BL/6/JUnib mice fed a high-fat diet (HFd) for a prolonged time period (8 months). Mice fed a HFd became obese and type 2 diabetic, displaying significant peripheral insulin resistance, hyperglycemia and moderate hyperinsulinemia. Isolated islets of HFd-fed mice displayed a significant impairment of glucose-induced insulin secretion associated with a diminished frequency of intracellular calcium oscillations compared with control islets. No marked change in islet morphology and cytoarchitecture was observed; however, HFd-fed mice showed higher beta cell relative area in comparison with controls. As shown by immunohistochemistry, ZO-1, E-, N-cadherins, α- and β-catenins were expressed at the intercellular contact site of endocrine cells, while VE-cadherin, as well as ZO-1, was found at islet vascular compartment. Redistribution of N-, E-cadherins and α-catenin (from the contact region to the cytoplasm in endocrine cells) associated with increased submembranous F-actin cell level as well as increased VE-cadherin islet immunolabeling was observed in diabetic mice. Increased gene expression of VE-cadherin and ZO-1, but no change for the other proteins, was observed in islets of diabetic mice. Only in the case of VE-cadherin, a significant increase in islet content of this CAM was detected by immunoblotting in diabetic mice. In conclusion, CAMs are expressed by endocrine and endothelial cells of pancreatic islets. The distribution/expression of N-, E- and VE-cadherins as well as α-catenin and F-actin is significantly altered in islet cells of obese and diabetic mice.

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.

Role of Connexins and Pannexins in the Pancreas

The pancreas produces enzymes with a digestive function and hormones with a metabolic function, which are produced by distinct cell types of acini and islets, respectively. Within these units, secretory cells coordinate their functioning by exchanging information via signals that flow in the intercellular spaces and are generated either at distance (several neural and hormonal inputs) or nearby the pancreatic cells themselves (inputs mediated by membrane ionic-specific channels and by ionic- and metabolite-permeant pannexin channels and connexin "hemichannels"). Pancreatic secretory cells further interact via the extracellular matrix of the pancreas (inputs mediated by integrins) and directly with neighboring cells, by mechanisms that do not require extracellular mediators (inputs mediated by gap and tight junction channels). Here, we review the expression and function of the connexins and pannexins that are expressed by the main secretory cells of the exocrine and endocrine pancreatic cells. Available data show that the patterns of expression of these proteins differ in acini and islets, supporting distinct functions in the physiological secretion of pancreatic enzymes and hormones. Circumstantial evidence further suggests that alterations in the signaling provided by these proteins are involved in pancreatic diseases.

The Beta Cell in Its Cluster: Stochastic Graphs of Beta Cell Connectivity in the Islets of Langerhans

Pancreatic islets of Langerhans consist of endocrine cells, primarily α, β and δ cells, which secrete glucagon, insulin, and somatostatin, respectively, to regulate plasma glucose. β cells form irregular locally connected clusters within islets that act in concert to secrete insulin upon glucose stimulation. Due to the central functional significance of this local connectivity in the placement of β cells in an islet, it is important to characterize it quantitatively. However, quantification of the seemingly stochastic cytoarchitecture of β cells in an islet requires mathematical methods that can capture topological connectivity in the entire β-cell population in an islet. Graph theory provides such a framework. Using large-scale imaging data for thousands of islets containing hundreds of thousands of cells in human organ donor pancreata, we show that quantitative graph characteristics differ between control and type 2 diabetic islets. Further insight into the processes that shape and maintain this architecture is obtained by formulating a stochastic theory of β-cell rearrangement in whole islets, just as the normal equilibrium distribution of the Ornstein-Uhlenbeck process can be viewed as the result of the interplay between a random walk and a linear restoring force. Requiring that rearrangements maintain the observed quantitative topological graph characteristics strongly constrained possible processes. Our results suggest that β-cell rearrangement is dependent on its connectivity in order to maintain an optimal cluster size in both normal and T2D islets.

High or low blood glucose levels are detrimental to human health. The hormone-secreting cells primarily responsible for maintaining glucose at physiologically appropriate levels are embedded in small clusters within the pancreas, the so-called islets of Langerhans. These islets have an irregular arrangement of cells, β cells that secrete insulin, α cells that secrete glucagon, and other cells with less well-understood functions. While the arrangement of β cells is irregular, these cells need to be touching for the islet to respond to glucose with insulin secretion. We first use a mathematical formalism called graph theory to show that cell arrangements in islets from diabetic and control donors are significantly different. The question we then address is: Is there some set of moves of islet cells that will preserve the observed arrangement? The aim is to gain insight into the biological processes by which islets are formed and maintained. We find moves on β-cell graphs that leave the same significant aspects of cell arrangements unchanged. These moves turn out to be severely restricted, and suggest that β cells may prefer to move from larger clusters but can move to a cluster of any size, possibly to maximize their exposure to blood vessels.

Slow potentials encode intercellular coupling and insulin demand in pancreatic beta cells

AIMS/HYPOTHESIS

Ion fluxes constitute a major integrative signal in beta cells that leads to insulin secretion and regulation of gene expression. Understanding these electrical signals is important for deciphering the endogenous algorithms used by islets to attain homeostasis and for the design of new sensors for monitoring beta cell function.

METHODS

Mouse and human islets were cultured on multielectrode arrays (MEAs) for 3-13 days. Extracellular electrical activities received on each electrode were continuously amplified and recorded for offline characterisation.

RESULTS

Differential band-pass filtering of MEA recordings of mouse islets showed two extracellular voltage waveforms: action potentials (lasting 40-60 ms) and very robust slow potentials (SPs, lasting 800-1,500 ms), the latter of which have not been described previously. The frequency of SPs directly correlated with glucose concentration, peaked at 10 mmol/l glucose and was further augmented by picomolar concentrations of glucagon-like peptide-1. SPs required the closure of ATP-dependent potassium channels as they were induced by glucose or glibenclamide but were not elicited by KCl-induced depolarisation. Pharmacological tools and the use of beta cell specific knockout mice showed that SPs reflected cell coupling via connexin 36. Moreover, increasing and decreasing glucose ramps showed hysteresis with reduced glucose sensitivity during the decreasing phase. SPs were also observed in human islets and could be continuously recorded over 24 h.

CONCLUSIONS/INTERPRETATION

This novel electrical signature reflects the syncytial function of the islets and is specific to beta cells. Moreover, the observed hysteresis provides evidence for an endogenous algorithm naturally present in islets to protect against hypoglycaemia.

Islet formation in mice and men: lessons for the generation of functional insulin-producing β-cells from human pluripotent stem cells

The Islets of Langerhans are crucial 'micro-organs' embedded in the glandular exocrine pancreas that regulate nutrient metabolism. They not only synthesize, but also secrete endocrine hormones in a modulated fashion in response to physiologic metabolic demand. These highly sophisticated structures with intricate organization of multiple cell types, namely endocrine, vascular, neuronal and mesenchymal cells, have evolved to perform this task to perfection over time. Not surprisingly, islet architecture and function are dissimilar between humans and typically studied model organisms, such as rodents and zebrafish. Further, recent findings also suggest noteworthy differences in human islet development from that in mouse, including delayed appearance and gradual resolution of key differentiation markers, a single-phase of endocrine differentiation, and prenatal association of developing islets with neurovascular milieu. In light of these findings, it is imperative that a systematic study is undertaken to compare islet development between human and mouse. Illuminating inter-species differences in islet development will likely be critical in furthering our pursuit to generate an unlimited supply of truly functional and fully mature β-cells from human pluripotent stem cell (hPSC) sources for therapeutic purposes.

Insm1 cooperates with Neurod1 and Foxa2 to maintain mature pancreatic β-cell function

Key transcription factors control the gene expression program in mature pancreatic β-cells, but their integration into regulatory networks is little understood. Here, we show that Insm1, Neurod1 and Foxa2 directly interact and together bind regulatory sequences in the genome of mature pancreatic β-cells. We used Insm1 ablation in mature β-cells in mice and found pronounced deficits in insulin secretion and gene expression. Insm1-dependent genes identified previously in developing β-cells markedly differ from the ones identified in the adult. In particular, adult mutant β-cells resemble immature β-cells of newborn mice in gene expression and functional properties. We defined Insm1, Neurod1 and Foxa2 binding sites associated with genes deregulated in Insm1 mutant β-cells. Remarkably, combinatorial binding of Insm1, Neurod1 and Foxa2 but not binding of Insm1 alone explained a significant fraction of gene expression changes. Human genomic sequences corresponding to the murine sites occupied by Insm1/Neurod1/Foxa2 were enriched in single nucleotide polymorphisms associated with glycolytic traits. Thus, our data explain part of the mechanisms by which β-cells maintain maturity: Combinatorial Insm1/Neurod1/Foxa2 binding identifies regulatory sequences that maintain the mature gene expression program in β-cells, and disruption of this network results in functional failure.

ARHGAP21 prevents abnormal insulin release through actin rearrangement in pancreatic islets from neonatal mice

AIMS

ARHGAP21 is a Rho GTPase-activating protein (RhoGAP) that associates with many proteins and modulates several cellular functions, including actin cytoskeleton rearrangement in different tissues. However, it is unknown whether ARHGAP21 is expressed in pancreatic beta cells and its function in these cells. Herein, we assess the participation of ARHGAP21 in insulin secretion.

MAIN METHODS

Neonatal mice were treated with anti-sense oligonucleotide against ARHG AP21 (AS) for 2 days, resulting in a reduction of the protein's expression of about 60% in the islets. F-actin depolimerization, insulin secretion,mRNA level of genes involved in insulin secretion, maturation and proliferation were evaluated in islets from both control and AS-treated mice.

KEY FINDINGS

ARHGAP21 co-localized with actin inMIN6 beta cells and with insulin in neonatal pancreatic islets. F-actin was reduced in AS-islets, as judged by lower phalloidin intensity. Insulin secretion was increased in islets from AS-treated mice, however no differences were observed in the GSIS (glucose-stimulated insulin secretion). In these islets, the pERK1/2 was increased, as well as the gene expressions of VAMP2 and SNAP25, proteins that are present in the secretory machinery. Maturation and cell proliferation were not affected in islets from AS-treated mice.

SIGNIFICANCE

In conclusion, our data show, for the first time, that ARHGAP21 is expressed and participates in the secretory process of pancreatic beta cells. Its effect is probably via pERK1/2, which modulates the rearrangement of the cytoskeleton. ARHGAP21 also controls the expression of genes that encodes proteins of the secretory machinery.

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

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

Impaired compensatory beta-cell function and growth in response to high-fat diet in LDL receptor knockout mice

In this study, we investigated the effect of low density lipoprotein receptor (LDLr) deficiency on gap junctional connexin 36 (Cx36) islet content and on the functional and growth response of pancreatic beta-cells in C57BL/6 mice fed a high-fat (HF) diet. After 60 days on regular or HF diet, the metabolic state and morphometric islet parameters of wild-type (WT) and LDLr-/- mice were assessed. HF diet-fed WT animals became obese and hypercholesterolaemic as well as hyperglycaemic, hyperinsulinaemic, glucose intolerant and insulin resistant, characterizing them as prediabetic. Also they showed a significant decrease in beta-cell secretory response to glucose. Overall, LDLr-/- mice displayed greater susceptibility to HF diet as judged by their marked cholesterolaemia, intolerance to glucose and pronounced decrease in glucose-stimulated insulin secretion. HF diet induced similarly in WT and LDLr-/- mice, a significant decrease in Cx36 beta-cell content as revealed by immunoblotting. Prediabetic WT mice displayed marked increase in beta-cell mass mainly due to beta-cell hypertrophy/replication. Nevertheless, HF diet-fed LDLr-/- mice showed no significant changes in beta-cell mass, but lower islet-duct association (neogenesis) and higher beta-cell apoptosis index were seen as compared to controls. The higher metabolic susceptibility to HF diet of LDLr-/- mice may be explained by a deficiency in insulin secretory response to glucose associated with lack of compensatory beta-cell expansion.

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

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

Endothelial progenitor cells enhance islet engraftment, influence β-cell function, and modulate islet connexin 36 expression

The success of pancreatic islet transplantation is limited by delayed engraftment and suboptimal function in the longer term. Endothelial progenitor cells (EPCs) represent a potential cellular therapy that may improve the engraftment of transplanted pancreatic islets. In addition, EPCs may directly affect the function of pancreatic β-cells. The objective of this study was to examine the ability of EPCs to enhance pancreatic islet transplantation in a murine syngeneic marginal mass transplant model and to examine the mechanisms through which this occurs. We found that cotransplanted EPCs improved the cure rate and initial glycemic control of transplanted islets. Gene expression data indicate that EPCs, or their soluble products, modulate the expression of the β-cell surface molecule connexin 36 and affect glucose-stimulated insulin release in vitro. In conclusion, EPCs are a promising candidate for improving outcomes in islet transplantation, and their mechanisms of action warrant further study.

Unraveling the effects of 1,25OH2D3 on global gene expression in pancreatic islets

INTRODUCTION

Vitamin D deficiency has been linked to type 1 and 2 diabetes, whereas supplementation may prevent both diseases. However, the extent of the effects of vitamin D or its metabolites directly on pancreatic islets is still largely unknown. The aim of the present study was to investigate how active vitamin D, 1,25(OH)2D3, affects beta cells directly by establishing its effects on global gene expression in healthy murine islets.

MATERIALS AND METHODS

Pancreatic islets were isolated from 2 to 3 week old C57BL/6 mice and cultured in vitro with 1,25(OH)2D3 or vehicle for 6 and 24h. Total RNA was extracted from the islets and the effects on global gene expression were analyzed using Affymetrix microarrays.

RESULTS AND DISCUSSION

Exposure to 1,25(OH)2D3 compared to vehicle resulted in 306 and 151 differentially expressed genes after 6 and 24h, respectively (n=4, >1.3-fold, p<0.02). Of these 220 were up-regulated, whereas 86 displayed a decreased expression after 6h. Furthermore, expression levels were increased for 124 and decreased for 27 genes following 24h of exposure. Formation of intercellular junctions, cytoskeletal organization, and intracellular trafficking as well as lipid metabolism and ion transport were among the most affected gene classes. Effects on several genes already identified as being part of vitamin D signaling in other cell types were observed along with genes known to affect insulin release, although with our assay we were not able to detect any effects of 1,25(OH)2D3 on glucose-stimulated insulin release from healthy pancreatic islets.

CONCLUSION

The effects of 1,25(OH)2D3 on the expression of cytoskeletal and intracellular trafficking genes along with genes involved in ion transport may influence insulin exocytosis. However, an effect of 1,25(OH)2D3 on insulin release could not be detected for healthy islets in contrast to islets subjected to pathological conditions such as cytokine exposure and vitamin D deficiency as suggested by other studies. Thus, in addition to previously identified tolerogenic effects on the immune system, 1,25(OH)2D3 may affect basic functions of pancreatic beta cells, with the potential to render them more resistant to the detrimental conditions encountered during type 1 and 2 diabetes. This article is part of a Special Issue entitled 'Vitamin D Workshop'.

The fractalkine/CX3CR1 system regulates β cell function and insulin secretion

Here, we demonstrate that the fractalkine (FKN)/CX3CR1 system represents a regulatory mechanism for pancreatic islet β cell function and insulin secretion. CX3CR1 knockout (KO) mice exhibited a marked defect in glucose and GLP1-stimulated insulin secretion, and this defect was also observed in vitro in isolated islets from CX3CR1 KO mice. In vivo administration of FKN improved glucose tolerance with an increase in insulin secretion. In vitro treatment of islets with FKN increased intracellular Ca(2+) and potentiated insulin secretion in both mouse and human islets. The KO islets exhibited reduced expression of a set of genes necessary for the fully functional, differentiated β cell state, whereas treatment of wild-type (WT) islets with FKN led to increased expression of these genes. Lastly, expression of FKN in islets was decreased by aging and high-fat diet/obesity, suggesting that decreased FKN/CX3CR1 signaling could be a mechanism underlying β cell dysfunction in type 2 diabetes.

Connexins and β-cell functions

Proper functioning of pancreatic islets requires that numerous β-cells are properly coordinated. With evolution, many mechanisms have converged, which now allow individual β-cells to sense the state of activity of their neighbors as well as the changes taking place in the extracellular medium, and to regulate accordingly their own function. Here, we review one such mechanism for intercellular coordination, which depends on connexins. These integral membrane proteins accumulate at sites of close apposition between adjacent islet cell membranes, referred to as gap junctions. Recent evidence demonstrates that connexin-dependent signaling is relevant for the in vivo control of insulin biosynthesis and release, as well as for the survival of β-cells under stressing conditions. The data suggest that alterations of this signaling may be implicated in the β-cell alterations which characterize most forms of diabetes, raising the tantalizing possibility that targeting of the direct intercellular communications β-cells establish within each pancreatic islet may provide a novel, therapeutically useful strategy.

Protein-mediated interactions of pancreatic islet cells

The islets of Langerhans collectively form the endocrine pancreas, the organ that is soley responsible for insulin secretion in mammals, and which plays a prominent role in the control of circulating glucose and metabolism. Normal function of these islets implies the coordination of different types of endocrine cells, noticeably of the beta cells which produce insulin. Given that an appropriate secretion of this hormone is vital to the organism, a number of mechanisms have been selected during evolution, which now converge to coordinate beta cell functions. Among these, several mechanisms depend on different families of integral membrane proteins, which ensure direct (cadherins, N-CAM, occludin, and claudins) and paracrine communications (pannexins) between beta cells, and between these cells and the other islet cell types. Also, other proteins (integrins) provide communication of the different islet cell types with the materials that form the islet basal laminae and extracellular matrix. Here, we review what is known about these proteins and their signaling in pancreatic β -cells, with particular emphasis on the signaling provided by Cx36, given that this is the integral membrane protein involved in cell-to-cell communication, which has so far been mostly investigated for effects on beta cell functions.

Cell-to-cell contact dependence and junctional protein content are correlated with in vivo maturation of pancreatic beta cells

In this study, we investigated the cellular distribution of junctional proteins and the dependence on cell–cell contacts of pancreatic beta cells during animal development. Fetus and newborn rat islets, which display a relatively poor insulin secretory response to glucose, present an immature morphology and cytoarchitecture when compared with young and adult islets that are responsive to glucose. At the perinatal stage, beta cells display a low junctional content of neural cell adhesion molecule (N-CAM), α- and β-catenins, ZO-1, and F-actin, while a differential distribution of N-CAM and Pan-cadherin was seen in beta cells and nonbeta cells only from young and adult islets. In the absence of intercellular contacts, the glucose-stimulated insulin secretion was completely blocked in adult beta cells, but after reaggregation they partially reestablished the secretory response to glucose. By contrast, neonatal beta cells were poorly responsive to sugar, regardless of whether they were arranged as intact islets or as isolated cells. Interestingly, after 10 days of culturing, neonatal beta cells, known to display increased junctional protein content in vitro, became responsive to glucose and concomitantly dependent on cell–cell contacts. Therefore, our data suggest that the developmental acquisition of an adult-like insulin secretory pattern is paralleled by a dependence on direct cell–cell interactions.

Impaired β-cell-β-cell coupling mediated by Cx36 gap junctions in prediabetic mice

Gap junctional intercellular communication between β-cells is crucial for proper insulin biosynthesis and secretion. The aim of this work was to investigate the expression of connexin (Cx)36 at the protein level as well as the function and structure of gap junctions (GJ) made by this protein in the endocrine pancreas of prediabetic mice. C57BL/6 mice were fed a high-fat (HF) or regular chow diet for 60 days. HF-fed mice became obese and prediabetic, as shown by peripheral insulin resistance, moderate hyperglycemia, hyperinsulinemia, and compensatory increase in endocrine pancreas mass. Compared with control mice, prediabetic animals showed a significant decrease in insulin-secretory response to glucose and displayed a significant reduction in islet Cx36 protein. Ultrastructural analysis further showed that prediabetic mice had GJ plaques about one-half the size of those of the control group. Microinjection of isolated pancreatic islets with ethidium bromide revealed that prediabetic mice featured a β-cell-β-cell coupling 30% lower than that of control animals. We conclude that β-cell-β-cell coupling mediated by Cx36 made-channels is impaired in prediabetic mice, suggesting a role of Cx36-dependent cell-to-cell communication in the pathogenesis of the early β-cell dysfunctions that lead to type 2-diabetes.

Connexin 36 is expressed in beta and connexins 26 and 32 in acinar cells at the end of the secondary transition of mouse pancreatic development and increase during fetal and perinatal life

To identify when during fetal development connexins (Cxs) 26 (Cx26) 32 (Cx32), and 36 (Cx36) begin to be expressed, as well as to characterize their spatial distribution, real time polymerase chain reaction and immunolabeling studies were performed. Total RNA from mouse pancreases at 13 and 18 days postcoitum (dpc) and 3 days postpartum (dpp) was analyzed. In addition, pancreatic sections of mouse at 13, 14, 15, 16, 18 dpc and 3 dpp and of rat at term were double labeled with either anti-insulin or anti-α-amylase and anti-Cx26 or -Cx32 or -Cx36 antibodies and studied with confocal microscopy. From day 13 dpc, Cxs 26, 32, and 36 transcripts were identified and their levels increased with age. At 13-14 dpc, Cxs 26 and 32 were localized in few acinar cells, whereas Cx36 was distributed in small beta cell clumps. From day 14 dpc onwards, the number of labeled cells and relative immunofluorescent reactivity of all three Cxs at junctional membranes of the respective cell types increased. Cxs 26 and 32 colocalized in fetal acinar cells. In rat pancreas at term, a similar connexin distribution was found. Relative Cxs levels evaluated by immunoblotting also increased (two-fold) in pancreas homogenates from day 18 dpc to 3 dpp. The early cell specific, wide distribution, and age dependent expression of Cxs 26, 32, and 36 during fetal pancreas ontogeny suggests their possible involvement in pancreas differentiation and prenatal maturation.

Cx36 is a target of Beta2/NeuroD1, which associates with prenatal differentiation of insulin-producing β cells

The insulin-producing β cells of pancreatic islets are coupled by connexin36 (Cx36) channels. To investigate what controls the expression of this connexin, we have investigated its pattern during mouse pancreas development, and the influence of three transcription factors that are critical for β-cell development and differentiation. We show that (1) the Cx36 gene (Gjd2) is activated early in pancreas development and is markedly induced at the time of the surge of the transcription factors that determine β-cell differentiation; (2) the cognate protein is detected about a week later and is selectively expressed by β cells throughout the prenatal development of mouse pancreas; (3) a 2-kbp fragment of the Gjd2 promoter, which contains three E boxes for the binding of the bHLH factor Beta2/NeuroD1, ensures the expression of Cx36 by β cells; and (4) Beta2/NeuroD1 binds to these E boxes and, in the presence of the E47 ubiquitous cofactor, transactivates the Gjd2 promoter. The data identify Cx36 as a novel early marker of β cells and as a target of Beta2/NeuroD1, which is essential for β-cell development and differentiation.

Implication of mitochondrial cytoprotection in human islet isolation and transplantation

Islet transplantation is a promising therapy for type 1 diabetes mellitus; however, success rates in achieving both short- and long-term insulin independence are not consistent, due in part to inconsistent islet quality and quantity caused by the complex nature and multistep process of islet isolation and transplantation. Since the introduction of the Edmonton Protocol in 2000, more attention has been placed on preserving mitochondrial function as increasing evidences suggest that impaired mitochondrial integrity can adversely affect clinical outcomes. Some recent studies have demonstrated that it is possible to achieve islet cytoprotection by maintaining mitochondrial function and subsequently to improve islet transplantation outcomes. However, the benefits of mitoprotection in many cases are controversial and the underlying mechanisms are unclear. This article summarizes the recent progress associated with mitochondrial cytoprotection in each step of the islet isolation and transplantation process, as well as islet potency and viability assays based on the measurement of mitochondrial integrity. In addition, we briefly discuss immunosuppression side effects on islet graft function and how transplant site selection affects islet engraftment and clinical outcomes.

Functional beta-cell maturation is marked by an increased glucose threshold and by expression of urocortin 3

Insulin-expressing cells that have been differentiated from human pluripotent stem cells in vitro lack the glucose responsiveness characteristic of mature beta cells. Beta-cell maturation in mice was studied to find genetic markers that enable screens for factors that induce bona fide beta cells in vitro. We find that functional beta-cell maturation is marked by an increase in the glucose threshold for insulin secretion and by expression of the gene urocortin 3.

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.

TAT-mediated transduction of MafA protein in utero results in enhanced pancreatic insulin expression and changes in islet morphology

Alongside Pdx1 and Beta2/NeuroD, the transcription factor MafA has been shown to be instrumental in the maintenance of the beta cell phenotype. Indeed, a combination of MafA, Pdx1 and Ngn3 (an upstream regulator of Beta2/NeuroD) was recently reported to lead to the effective reprogramming of acinar cells into insulin-producing beta cells. These experiments set the stage for the development of new strategies to address the impairment of glycemic control in diabetic patients. However, the clinical applicability of reprogramming in this context is deemed to be poor due to the need to use viral vehicles for the delivery of the above factors. Here we describe a recombinant transducible version of the MafA protein (TAT-MafA) that penetrates across cell membranes with an efficiency of 100% and binds to the insulin promoter in vitro. When injected in utero into living mouse embryos, TAT-MafA significantly up-regulates target genes and induces enhanced insulin production as well as cytoarchitectural changes consistent with faster islet maturation. As the latest addition to our armamentarium of transducible proteins (which already includes Pdx1 and Ngn3), the purification and characterization of a functional TAT-MafA protein opens the door to prospective therapeutic uses that circumvent the use of viral delivery. To our knowledge, this is also the first report on the use of protein transduction in utero.