Changes in pancreatic islets in aging Wistar and Zucker rats: a histochemical and ultrastructural morphometric study

Notch-mediated Ephrin signaling disrupts islet architecture and β cell function

Altered islet architecture is associated with β cell dysfunction and type 2 diabetes (T2D) progression, but molecular effectors of islet spatial organization remain mostly unknown. Although Notch signaling is known to regulate pancreatic development, we observed “reactivated” β cell Notch activity in obese mouse models. To test the repercussions and reversibility of Notch effects, we generated doxycycline-dependent, β cell–specific Notch gain-of-function mice. As predicted, we found that Notch activation in postnatal β cells impaired glucose-stimulated insulin secretion and glucose intolerance, but we observed a surprising remnant glucose intolerance after doxycycline withdrawal and cessation of Notch activity, associated with a marked disruption of normal islet architecture. Transcriptomic screening of Notch-active islets revealed increased Ephrin signaling. Commensurately, exposure to Ephrin ligands increased β cell repulsion and impaired murine and human pseudoislet formation. Consistent with our mouse data, Notch and Ephrin signaling were increased in metabolically inflexible β cells in patients with T2D. These studies suggest that β cell Notch/Ephrin signaling can permanently alter islet architecture during a morphogenetic window in early life.

Endocrine cell type sorting and mature architecture in the islets of Langerhans require expression of Roundabout receptors in β cells

Pancreatic islets of Langerhans display characteristic spatial architecture of their endocrine cell types. This architecture is critical for cell-cell communication and coordinated hormone secretion. Islet architecture is disrupted in type-2 diabetes. Moreover, the generation of architecturally correct islets in vitro remains a challenge in regenerative approaches to type-1 diabetes. Although the characteristic islet architecture is well documented, the mechanisms controlling its formation remain obscure. Here, we report that correct endocrine cell type sorting and the formation of mature islet architecture require the expression of Roundabout (Robo) receptors in β cells. Mice with whole-body deletion of Robo1 and conditional deletion of Robo2 either in all endocrine cells or selectively in β cells show complete loss of endocrine cell type sorting, highlighting the importance of β cells as the primary organizer of islet architecture. Conditional deletion of Robo in mature β cells subsequent to islet formation results in a similar phenotype. Finally, we provide evidence to suggest that the loss of islet architecture in Robo KO mice is not due to β cell transdifferentiation, cell death or loss of β cell differentiation or maturation.

Ageing is associated with molecular signatures of inflammation and type 2 diabetes in rat pancreatic islets

AIMS/HYPOTHESIS

Ageing is a major risk factor for development of metabolic diseases such as type 2 diabetes. Identification of the mechanisms underlying this association could help to elucidate the relationship between age-associated progressive loss of metabolic health and development of type 2 diabetes. We aimed to determine molecular signatures during ageing in the endocrine pancreas.

METHODS

Global gene transcription was measured in pancreatic islets isolated from young and old rats by Ilumina BeadChip arrays. Promoter DNA methylation was measured by Sequenom MassArray in 46 genes that showed differential expression with age, and correlations with expression were established. Alterations in morphological and cellular processes with age were determined by immunohistochemical methods.

RESULTS

Age-related changes in gene expression were found at 623 loci (>1.5-fold, false discovery rate [FDR] <5%), with a significant (FDR < 0.05) enrichment in genes previously implicated in islet-cell function (Enpp1, Abcc8), type 2 diabetes (Tspan8, Kcnq1), inflammatory processes (Cxcl9, Il33) and extracellular matrix organisation (Col3a1, Dpt). Age-associated transcriptional differences negatively correlated with promoter DNA methylation at several loci related to inflammation, glucose homeostasis, cell proliferation and cell-matrix interactions (Il33, Cxcl9, Gpr119, Fbp2, Col3a1, Dpt, Spp1).

CONCLUSIONS/INTERPRETATION

Our findings suggest that a significant proportion of pancreatic islets develop a low-grade 'chronic' inflammatory status with ageing and this may trigger altered functional plasticity. Furthermore, we identified changes in expression of genes previously linked to type 2 diabetes and associated changes in DNA methylation that could explain their age-associated dysregulation. These findings provide new insights into key (epi)genetic signatures of the ageing process in islets.

Ginseng berry extract supplementation improves age-related decline of insulin signaling in mice

The aim of this study was to evaluate the effects of ginseng berry extract on insulin sensitivity and associated molecular mechanisms in aged mice. C57BL/6 mice (15 months old) were maintained on a regular diet (CON) or a regular diet supplemented with 0.05% ginseng berry extract (GBD) for 24 or 32 weeks. GBD-fed mice showed significantly lower serum insulin levels (p = 0.016) and insulin resistance scores (HOMA-IR) (p = 0.012), suggesting that GBD improved insulin sensitivity. Pancreatic islet hypertrophy was also ameliorated in GBD-fed mice (p = 0.007). Protein levels of tyrosine phosphorylated insulin receptor substrate (IRS)-1 (p = 0.047), and protein kinase B (AKT) (p = 0.037), were up-regulated in the muscle of insulin-injected GBD-fed mice compared with CON-fed mice. The expressions of forkhead box protein O1 (FOXO1) (p = 0.036) and peroxisome proliferator-activated receptor gamma (PPARγ) (p = 0.032), which are known as aging- and insulin resistance-related genes, were also increased in the muscle of GBD-fed mice. We conclude that ginseng berry extract consumption might increase activation of IRS-1 and AKT, contributing to the improvement of insulin sensitivity in aged mice.

Islet adaptation to obesity and insulin resistance in WNIN/GR-Ob rats

WNIN/GR-Ob mutant rat is a novel animal model to study metabolic syndrome (obesity, insulin resistance, hyperinsulinemia, impaired glucose tolerance and cardiovascular diseases). We have investigated the islet characteristics of obese mutants at different age groups (1, 6 and 12 months) to assess the islet changes in response to early and chronic metabolic stress. Our data demonstrates altered islet cell morphology and function (hypertrophy, fibrotic lesions, vacuolation, decreased stimulation index, increased TNFα, ROS and TBARS levels) in mutants as compared to controls. Furthermore, network analysis (gene-gene interaction) studied in pancreas demonstrated increased inflammation as a key factor underlying obesity/metabolic syndrome in mutants. These observations pave way to explore this model to understand islet adaptation in response to metabolic syndrome.

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.

Progression of pancreatitis prior to diabetes onset in WBN/Kob-Lepr(fa) rats

We established the WBN/Kob-Lepr(fa) rat as a new congenic strain for the fa allele of the leptin receptor gene (Lepr). Homozygous (fa/fa) WBN/Kob-Lepr(fa) rats provide a model of non-insulin-dependent diabetes, although its onset is secondary to pancreatitis. In the present study, we compared histopathological observations of pancreatitis in each genotype of this rat, to examine its suitability as a model of pancreatitis. The histopathological findings of the pancreatitis revealed intense changes dependent on age, such as hemorrhage or hemosiderin deposition. The pancreatitis in homozygous (fa/fa) WBN/Kob-Lepr(fa) rats were more severe than those of WBN/Kob rats.

The influence of dietary restriction on the development of diabetes and pancreatitis in female WBN/Kob-fatty rats

Original WBN/Kob male rats commonly develop chronic pancreatitis by the age of 3 months, while diabetes mellitus occurs at 9 months. In contrast, female rats of this strain do not show pancreatitis or diabetes. The WBN/Kob-fatty rat is a homozygous (fa/fa) congenic strain for the fa allele of the leptin receptor gene (Lepr). In WBN/Kob-fatty rats, both females and males provide a model of non-insulin-dependent diabetes with obesity. The leptin receptor fatty gene (Lepr(fa)) induces obesity and hyperphagia. In the present study, we examined the effect of dietary restriction on pancreatitis and diabetes in female WBN/Kob-fatty rats. Five female fatty rats comprised a restricted feeding group with paired-feeding from 3 to 13 weeks of age, and five female lean rats comprised a control group with paired-feeding. At 13 weeks of age, two of the five female fatty rats of the control group developed diabetes mellitus, while no female fatty rats of the restricted feeding group developed diabetes mellitus. At this stage, pathological changes of the pancreas were observed in female fatty rats. All female fatty rats showed severe interlobular, intra-lobular and intra-islet fibrosis. In female fatty rats of the restricted feeding group, pathological changes of the pancreas were milder those of the free-feeding fatty group. Although dietary restriction could not completely prevent pancreatitis in female fatty rats, the development of diabetes was inhibited by its reduction of the severity of pancreatitis.

Chronological characterization of diabetes development in male Spontaneously Diabetic Torii rats

To characterize the underlying mechanisms of diabetes development in males of the Spontaneously Diabetic Torii (SDT) rat, a novel spontaneous model for diabetes, we chronologically examined them, focusing on their diabetic features and the pathological changes in the pancreatic islets. Male SDT rats exhibited glucose intolerance with impaired insulin secretion after 14 weeks and developed diabetes with remarkable hyperglycemia and marked hypoinsulinemia after 20 weeks. At prediabetic stage (10-20 weeks), they were normoglycemic, but had significantly lower insulin levels of plasma and pancreas than the normal rats. Their beta-cell volume was already smaller significantly at 10 weeks than that of normal rats. The primary changes of the pancreatic islets were microvascular events such as congestion and hemorrhage at 8-10 weeks. Thereafter, the SDT rat islets were affected with inflammation and progressive fibrosis (at 10-20 weeks), and eventually atrophied with a loss of beta-cells (at 38 weeks). These results indicate that the male SDT rats develop spontaneous diabetes with an absolute decrease in the insulin secretory capacity of the islets.

Pioglitazone preserves pancreatic islet structure and insulin secretory function in three murine models of type 2 diabetes

Thiazolidinediones may slow the progression of type 2 diabetes by preserving pancreatic beta-cells. The effects of pioglitazone (PIO) on structure and function of beta-cells in KKA(y), C57BL/6J ob/ob, and C57BL/KsJ db/db mice (genetic models of type 2 diabetes) were examined. ob/ob (n = 7) and db/db (n = 9) mice were randomly assigned to 50-125 mg.kg body wt-1.day-1 of PIO in chow beginning at 6-10 wk of age. Control ob/ob (n = 7) and db/db mice (n = 9) were fed chow without PIO. KKA(y) mice (n = 15) were fed PIO daily at doses of 62-144 mg.kg body wt-1.day-1. Control KKA(y) mice (n = 10) received chow without PIO. Treatment continued until euthanasia at 14-26 wk of age. Blood was collected at baseline (before treatment) and just before euthanasia and was analyzed for glucose, glycosylated hemoglobin, and plasma insulin. Some of the splenic pancreas of each animal was resected and partially sectioned for light or electron microscopy. The remainder of the pancreas was assayed for insulin content. Compared with baseline and control groups, PIO treatment significantly reduced blood glucose and glycosylated hemoglobin levels. Plasma insulin levels decreased significantly in ob/ob mice treated with PIO. All groups treated with PIO exhibited significantly greater beta-cell granulation, evidence of reduced beta-cell stress, and 1.5- to 15-fold higher levels of pancreatic insulin. The data from these studies suggest that comparable effects would be expected to slow the progression of type 2 diabetes, either delaying or possibly preventing progression to an insulin-dependent state.

A comparative immunohistochemical study of pancreatic islets in laboratory animals (rats, dogs, minipigs, nonhuman primates)

The aim of the present study was to distinguish and describe the patterns of distribution of pancreatic islets within the pancreas of four species of laboratory animals, including rats, dogs, minipigs and monkeys, and furthermore, to identify immunohistochemically various islet cell types and characterize their content. Histopathological examinations were performed on sections stained with hematoxylin and eosin (H&E) and immunostained using rabbit polyclonal antibodies (pAb) against insulin, glucagon, pancreatic polypeptide (PP), somatostatin, chromogranin A, keratin, bombesin and gastrin, or mouse monoclonal antibodies (mAb) against synaptophysin, Leu-7 and proliferating cell nuclear antigen (PCNA) in three-step rabbit immunoperoxidase (PAP) and streptavidin/peroxidase (StreptABC/HRP) reactions. Positive immunohistochemical reactions were observed in the pancreatic islets of all animal species with all antibodies, except with anti-bombesin and anti-gastrin antibodies. Our results revealed that: 1) there is species specific regional arrangement of islets in the pancreas, 2) each species presents a characteristic distribution of cells producing different hormones. 3) immunoreactivity with immunohistochemical markers varies between species and/or age. The present comparative immunohistochemical study could be helpful for answering questions which are important for understanding some of the intricate mechanisms that govern the integrated function of the endocrine pancreas.