Development of the insulin secretion mechanism in fetal and neonatal rat pancreatic B-cells: response to glucose, K+, theophylline, and carbamylcholine

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.

Maternal high-fat diet intensifies the metabolic response to stress in male rat offspring

BACKGROUND

The mother's consumption of high-fat food can affect glucose metabolism and the hypothalamic-pituitary-adrenal axis responsiveness in the offspring and potentially affect the metabolic responses to stress as well. This study examines the effect of maternal high-fat diet on the expression of pancreatic glucose transporter 2 and the secretion of insulin in response to stress in offspring.

METHODS

Female rats were randomly divided into normal and high-fat diet groups and were fed in accordance with their given diets from pre-pregnancy to the end of lactation. The offspring were divided into control (NC and HFC) and stress (NS and HFS) groups based on their mothers' diet and exposure to stress in adulthood. After the two-week stress induction period was over, an intraperitoneal glucose tolerance test (IPGTT) was performed and plasma glucose and insulin levels were assessed. The pancreas was then removed for measuring insulin secretion from the isolated islets as well as glucose transporter 2 mRNA expression and protein levels.

RESULTS

According to the results obtained, plasma corticosterone concentrations increased significantly on days 1 and 14 of the stress induction period and were lower on the last day compared to on the first day. In both the NS and HFS groups, stress reduced plasma insulin concentration in the IPGTT without changing the plasma glucose concentration, suggesting an increased insulin sensitivity in the NS and HFS groups, although more markedly in the latter. Stress reduced insulin secretion (at high glucose concentrations) and increased glucose transporter 2 mRNA and protein expression, especially in the HFS group.

CONCLUSION

Mothers' high-fat diet appears to intensify the stress response by changing the programming of the neuroendocrine system in the offspring.

Perinatal programming of metabolic diseases: role of insulin in the development of hypothalamic neurocircuits

It is increasingly accepted that the metabolic future of an individual can be programmed as early as at developmental stages. For instance, offspring of diabetic mothers have a greater risk of becoming obese and diabetic later in life. Animal studies have demonstrated that hyperinsulinemia and/or hyperglycemia during perinatal life permanently impair the organization and long-term function of hypothalamic networks that control appetite and glucose homeostasis. This review summarizes the main findings regarding the key regulatory roles of perinatal insulin and glucose levels on hypothalamic development and on long-term programming of metabolic diseases reported in different rodent models.

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.

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

AIMS/HYPOTHESIS

Cell-cell coupling mediated by gap junctions formed from connexin (CX) contributes to the control of insulin secretion in the endocrine pancreas. We investigated the cellular production and localisation of CX36 and CX43, and gap junction-mediated beta cell coupling in pancreatic islets from rats of different ages, displaying different degrees of maturation of insulin secretion.

METHODS

The presence and distribution of islet connexins were assessed by immunoblotting and immunofluorescence. The expression of connexin genes was evaluated by RT-PCR and quantitative real-time PCR. The ultrastructure of gap junctions and the function of connexin channels were assessed by freeze-fracture electron microscopy and tracer microinjection, respectively.

RESULTS

Young and adult beta cells, which respond to glucose, expressed significantly higher levels of Cx36 (also known as Gjd2) than fetal and newborn beta cells, which respond poorly to the sugar. Accordingly, adult beta cells also showed a significantly higher membrane density of gap junctions and greater intercellular exchange of ethidium bromide than newborn beta cells. Cx43 (also known as Gja1) was not expressed by beta cells, but was located in various cell types at the periphery of fetal and newborn islets.

CONCLUSIONS/INTERPRETATION

These findings show that the pattern of connexins, gap junction membrane density and coupling changes in islets during the functional maturation of beta cells.

Breast- v. formula-feeding: impacts on the digestive tract and immediate and long-term health effects

The health benefits of breast-feeding have been recognised for a long time. In particular, breast-feeding is associated with lower incidence of necrotising enterocolitis and diarrhoea during the early period of life and with lower incidence of inflammatory bowel diseases, type 2 diabetes and obesity later in life. The higher nutritional and protective degree of human milk is related to its nutritional composition that changes over the lactation period and to the biological activities of specific components while lower growth rate of breast-fed infants may be attributed to their self-regulation of milk intake at a lower level than formula-fed infants. Many results now suggest that the developmental changes in intestinal and pancreatic function that occur postnatally are modulated by the diet. Indeed, formula-feeding induces intestinal hypertrophy and accelerates maturation of hydrolysis capacities; it increases intestinal permeability and bacterial translocation, but does not induce evident differences in microbiota composition. Whether these changes would be beneficial for enhancing absorptive capacities and for educating the gut-associated immune system remains to be further studied. Moreover, it is evident that formula-feeding increases basal blood glucose and decreases plasma ketone body concentrations, while discrepancies on postprandial glycaemia, insulin and incretin responses in both human studies and experimental studies are inconclusive. Manipulating the composition of formula, by reducing protein content, adding prebiotics, growth factors or secretory IgA can modulate intestinal and pancreatic function development, and thereby may reduce the differential responses between breast-fed and formula-fed neonates. However, the developmental responses of the digestive tract to different feeding strategies must be elucidated in terms of sensitivity to developing diseases, taking into account the major role of the intestinal microbiota.

Physiological development of insulin secretion, calcium channels, and GLUT2 expression of pancreatic rat beta-cells

Insulin secretion in mature beta-cells increases vigorously when extracellular glucose concentration rises. Glucose-stimulated insulin secretion depends on Ca(2+) influx through voltage-gated Ca(2+) channels. During fetal development, this structured response is not well established, and it is after birth that beta-cells acquire glucose sensitivity and a robust secretion. We compared some elements of glucose-induced insulin secretion coupling in beta-cells obtained from neonatal and adult rats and found that neonatal cells are functionally immature compared with adult cells. We observed that neonatal cells secrete less insulin and cannot sense changes in extracellular glucose concentrations. This could be partially explained because in neonates Ca(2+) current density and synthesis of mRNA alpha1 subunit Ca(2+) channel are lower than in adult cells. Interestingly, immunostaining for alpha1B, alpha1C, and alpha1D subunits in neonatal cells is similar in cytoplasm and plasma membrane, whereas it occurs predominantly in the plasma membrane in adult cells. We also observed that GLUT2 expression in adult beta-cells is mostly located in the membrane, whereas in neonatal cells glucose transporters are predominantly in the cytoplasm. This could explain, in part, the insensitivity to extracellular glucose in neonatal beta-cells. Understanding neonatal beta-cell physiology and maturation contributes toward a better comprehension of type 2 diabetes physiopathology, where alterations in beta-cells include diminished L-type Ca(2+) channels and GLUT2 expression that results in an insufficient insulin secretion.

Histomorphology and ultrastructure of pancreatic islet tissue during in vivo maturation of rat pancreas

In this study, we have investigated the structural and ultrastructural features of pancreatic islet tissue during rat postnatal development. For this purpose, we used neonatal (1-2 days old), young (21 days old) and adult (3-4 months old) rats. From a functional point of view, neonatal islet tissue displayed a relatively poor insulin secretory response to glucose stimulation in comparison with the adult ones. Histological analysis showed that neonatal islet cells display a less organized morphology in comparison with the young and adult ones, characterized by a less defined form and the presence of ductal structures within or nearby the islet. Regarding the islet cytoarchitecture, no differences were observed among all animal groups studied. B-cells were always typically detected within the islet core while A-cells occupied the islet periphery area. No marked differences were found during postnatal animal development regarding the ultrastructural aspect of the endocrine cells and their secretory granules. Nevertheless, quantitative analysis showed a lower B-cell/non-B-cell ratio, a higher association with ducts and an increased immunoreaction for proliferating cell nuclear antigen (PCNA) in neonatal islets as compared to young and adults. In conclusion, the acquisition of an adult pattern of insulin secretion may require an appropriate histoarchitecture and B-cell/non-B-cell proportion that may affect crucial regulatory events such as the paracrine and/or the cell-cell interaction or communication within the islet.

Dieta hiperlipídica e capacidade secretória de insulina em ratos

OBJETIVOS: Este estudo investigou, em ratos, os efeitos da administração crônica de uma dieta hiperlipídica palatável sobre: ganho de peso, adiposidade, conteúdos de glicogênio hepático e muscular, glicemia e insulinemia, morfologia do pâncreas e secreção de insulina por ilhotas isoladas, incubadas in vitro. MÉTODOS: Ratos Wistar machos (21 dias de idade) foram alimentados com dieta hiperlipídica palatável ou com dieta padrão, durante 15 semanas. Peso corporal e consumo de ração foram avaliados diariamente, glicose e insulina plasmática foram avaliadas semanalmente. Após o sacrifício, pâncreas, fígado, gastrocnêmio e tecidos adiposos foram coletados e pesados. Cortes do pâncreas foram analisados por microscopia ótica comum. Insulina plasmática e a secretada por ilhotas isoladas, após incubação na presença de diferentes concentrações de glicose, foram avaliadas por radioimunoensaio. RESULTADOS: A dieta hiperlipídica palatável aumentou a adiposidade, a percentagem do ganho de peso corporal e o conteúdo do glicogênio hepático, quando comparada à dos animais alimentados com dieta padrão. Glicemias e insulinemias de jejum não diferiram entre os grupos. A secreção de insulina das ilhotas isoladas dos ratos aumentou, nos tratados com dieta hiperlipídica, apenas em presença de concentrações fisiológicas de glicose (G= 8,3mM). A dieta hiperlipídica reduziu o tamanho do pâncreas, mas aumentou o número de células beta. Além disso, o lúmen dos vasos sangüíneos pancreáticos apresentou-se reduzido, quando comparado aos controles. CONCLUSÃO: A obesidade provocada pela dieta hiperlipídica não alterou os níveis de glicose e insulina de jejum desses animais. Apesar das alterações morfológicas do pâncreas, a manutenção da normoglicemia dos ratos tratados com dieta hiperlipídica, provavelmente, deveu-se à capacidade preservada de suas ilhotas em secretar insulina.

Effect of thawing rate and post-thaw culture on the cryopreserved fetal rat islets: Functional and morphological correlation

The ability of the fetal pancreatic islet cells to multiply rendered them a potential tissue for transplantation studies to cure diabetes. A bank of fetal islets could be created with proper storage in liquid nitrogen. The aim of this study is to evaluate the effect of thawing rate and post-thaw culture on the structural and functional integrity of isolated cryopreserved islets of rat fetuses. Fetal rat islets were isolated by the collagenase digestion, cultured for three days, and then cryopreserved using dimethylsulphoxide as cryoprotectant and the step-rate cooling to -40 degrees C before immersing them in liquid nitrogen. The islets were thawed by the slow or fast warming rates using hyperosmolar sucrose solution and then cultured for 1 or 2 days. Insulin and C-peptide contents of the slow thawed islets were higher than those of the control. In the fast thawed islets the contents were similar to those of the control. Insulin and C-peptide release in response to glucose for the slow thawed islets were lower than those of the control and in the fast thawed islets they were similar to that of the control. Histological examination showed irregular periphery and fragmented central part of the large slowly thawed islets, which showed also variable immunohistochemical reaction to anti-insulin serum, ranging from strongly positive reaction to markedly weak reaction. Fast thawed islets showed mostly regular periphery and their reaction to the anti-insulin serum was slightly weaker than that of the control islets. It was concluded that fast thawing and post-thaw culture is much better than slow thawing, as indicated by nearly normal insulin and C-peptide content and release and intact structural integrity.

Manipulation of pancreatic stem cells for cell replacement therapy

The demonstration of the existence of tissue-specific adult stem cells has had a great impact on our understanding of stem cell biology and its application in clinical medicine. Their existence has revolutionized the implications for the treatment of many degenerative diseases characterized by either the loss or malfunction of discrete cell types. However, successful exploitation of this opportunity requires that we have sufficient know-how of stem cell manipulation. Because stem cells are the founders of virtually all tissues during embryonic development, we believe that understanding the cellular and molecular mechanisms of embryogenesis and organogenesis will ultimately serve as a platform to identify factors and conditions that regulate stem cell behavior. Discovery of stem cell regulatory factors will create potential pharmaceutical opportunities for treatment of degenerative diseases, as well as providing critical knowledge of the processes by which stem cells can be expanded in vitro, differentiated, and matured into desired functional cells for implantation into humans. A well-characterized example of this is the hematopoietic system where the discovery of erythropoietin (EPO) and granulocyte-colony stimulating factor (G-CSF), which regulate hematopoietic progenitor cell behavior, have provided significant clinical success in disease treatment as well as providing important insights into hematopoiesis. In contrast, little is known about the identity of pancreatic stem cells, the focus of this review. Recent reports of the potential existence of pancreatic stem cells and their utility in rescuing the diabetic state now raise the same possibilities of generating insulin-producing beta cells as well as other cell types of the pancreatic islet from a stem cell. In this review, we will focus on the potential of these new developments and how our understanding of pancreas development can help design strategies and approaches by which a cell replacement therapy can be implemented for the treatment of insulin-dependent diabetes which is manifested by the loss of beta cells in the pancreas.