Postnatal development of numbers and mean sizes of pancreatic islets and beta-cells in healthy mice and GIPR(dn) transgenic diabetic mice

Metabolic Responses to High-Fat Feeding and Chronic Psychological Stress Combination

INTRODUCTION

High-fat diet (HFD) consumption and being exposed to daily psychological stress, common environmental factors in modern lifestyle, play an important role on metabolic disorders such as glucose homeostasis impairment. The aim of this study was to investigate the effects of high-fat diet (HFD) and psychological stress combination on metabolic response to chronic psychological stress in male rats.

METHOD

Male Wistar rats were divided into HFD, and normal diet (ND) groups and then into stress and nonstress subgroups. The diets were applied for 5 weeks, and psychological stress was induced for 7 consecutive days. Then, blood samples were taken to measure glucose, insulin, free fatty acids (FFA), and leptin and corticosterone concentrations. Subsequently, glucose-stimulated insulin release from pancreatic isolated islets was assessed.

RESULTS

HFD did not significantly change fasting plasma glucose, insulin and corticosterone levels, whereas increased plasma leptin (7.05 ± 0.33) and FFA (p < 0.01) levels and impaired glucose tolerance. Additionally, HFD and stress combination induced more profound glucose intolerance associated with increased plasma corticosterone (p < 0.01) and leptin (8.63 ± 0.38) levels. However, insulin secretion from isolated islets did not change in the presence of high-fat diet and/or stress.

CONCLUSION

HFD should be considered as an intensified factor of metabolic impairments caused by chronic psychological stress.

Duodenal enteroendocrine cells and GIP as treatment targets for obesity and type 2 diabetes

The duodenum is an important source of endocrine and paracrine signals controlling digestion and nutrient disposition, notably including the main incretin hormone glucose-dependent insulinotropic polypeptide (GIP). Bariatric procedures that prevent nutrients from contact with the duodenal mucosa are particularly effective interventions to reduce body weight and improve glycaemic control in obesity and type 2 diabetes. These procedures take advantage of increased nutrient delivery to more distal regions of the intestine which enhances secretion of the other incretin hormone glucagon-like peptide-1 (GLP-1). Preclinical experiments have shown that either an increase or a decrease in the secretion or action of GIP can decrease body weight and blood glucose in obesity and non-insulin dependent hyperglycaemia, but clinical studies involving administration of GIP have been inconclusive. However, a synthetic dual agonist peptide (tirzepatide) that exerts agonism at receptors for GIP and GLP-1 has produced marked weight-lowering and glucose-lowering effects in people with obesity and type 2 diabetes. This appears to result from chronic biased agonism in which the novel conformation of the peptide triggers enhanced signalling by the GLP-1 receptor through reduced internalisation while reducing signalling by the GIP receptor directly or via functional antagonism through increased internalisation and degradation.

Prolonged insulin-induced hypoglycaemia reduces ß-cell activity rather than number in pancreatic islets in non-diabetic rats

Pancreatic β-cells have an extraordinary ability to adapt to acute fluctuations in glucose levels by rapid changing insulin production to meet metabolic needs. Although acute changes have been characterised, effects of prolonged metabolic stress on β-cell dynamics are still unclear. Here, the aim was to investigate pancreatic β-cell dynamics and function during and after prolonged hypoglycaemia. Hypoglycaemia was induced in male and female rats by infusion of human insulin for 8 weeks, followed by a 4-week infusion-free recovery period. Animals were euthanized after 4 or 8 weeks of infusion, and either 2 days and 4 weeks after infusion-stop. Total volumes of pancreatic islets and β-cell nuclei, islet insulin and glucagon content, and plasma c-peptide levels were quantified. Prolonged hypoglycaemia reduced c-peptide levels, islet volume and almost depleted islet insulin. Relative β-cell nuclei: total pancreas volume decreased, while being unchanged relative to islet volume. Glucagon: total pancreas volume decreased during hypoglycaemia, whereas glucagon: islet volume increased. Within two days after infusion-stop, plasma glucose and c-peptide levels normalised and all remaining parameters were fully reversed after 4 weeks. In conclusion, our findings indicate that prolonged hypoglycaemia inactivates β-cells, which can rapidly be reactivated when needed, demonstrating the high plasticity of β-cells even following prolonged suppression.

A head-to-head comparison review of biological and toxicological studies of isomaltulose, d-tagatose, and trehalose on glycemic control

Diabetes mellitus is the most common metabolic disorder contributing to significant morbidity and mortality in humans. Different preventive and therapeutic agents, as well as various pharmacological strategies or non-pharmacological tools, improve the glycemic profile of diabetic patients. Isomaltulose, d-tagatose, and trehalose are naturally occurring, low glycemic sugars that are not synthesized by humans but widely used in food industries. Various studies have shown that these carbohydrates can regulate glucose metabolism and provide support in maintaining glucose homeostasis in patients with diabetes, but also can improve insulin response, subsequently leading to better control of hyperglycemia. In this review, we discussed the anti-hyperglycemic effects of isomaltulose, D-tagatose, and trehalose, comparing their properties with other known sweeteners, and highlighting their importance for the development of the pharmaceutical and food industries.

GIP analogues and the treatment of obesity-diabetes

The potential application of glucose-dependent insulinotropic polypeptide (gastric inhibitory polypeptide, GIP) in the management of obesity and type 2 diabetes has been controversial. Initial interest in the therapeutic use of GIP was dampened by evidence that its insulinotropic activity was reduced in type 2 diabetes and by reports that it increased glucagon secretion and adipose deposition in non-diabetic individuals. Also, attention was diverted away from GIP by the successful development of glucagon-like peptide-1 (GLP-1) receptor agonists, and a therapeutic strategy for GIP became uncertain when evidence emerged that both inhibition and enhancement of GIP action could prevent or reverse obese non-insulin dependent forms of diabetes in rodents. Species differences in GIP receptor responsiveness complicated the extrapolation of evidence from rodents to humans, but initial clinical studies are investigating the effect of a GIP antagonist in non-diabetic individuals. A therapeutic role for GIP agonists was reconsidered when clinical studies noted that the insulinotropic effect of GIP was increased if near-normal glycaemia was re-established, and GIP was found to have little effect on glucagon secretion or adipose deposition in obese type 2 diabetes patients. This encouraged the development of designer peptides that act as GIP receptor agonists, including chimeric peptides that mimic the incretin partnership of GIP with GLP-1, where the two agents exert complementary and often additive effects to improve glycaemic control and facilitate weight loss. Polyagonist peptides that exert agonism at GIP, GLP-1 and glucagon receptors are also under investigation as potential treatments for obese type 2 diabetes.

The antidiabetic and hepatoprotective effects of myricitrin on aged mice with D-galactose

AIM

The present study aims to evaluate the effects of antidiabetic and hepatoprotective of myricitrin in the aged mice induced by D-galactose (D-gal).

BACKGROUND

Aging occurs during a person's life; there has been no way to stop the aging process, but antioxidant and changing lifestyles can delay it.

METHODS

In this experimental study, 72 female adult mice (weighing30-35g) were randomly divided into six groups: 1: control, 2: D-gal at 500mg/kg/d, 3-5: D-gal+ Myricitrin at 5, 10 and 20mg/kg/d 6: D-gal+ Vitamin E at 100mg/kg/d. Aging induced by D-gal for 45 days via intraperitoneal. Myricitrin and Vitamin E administrated orally by gavage for the last 28 days. The blood glucose, insulin level, β-cell function, insulin resistance, hepatic enzymes, lipid profile, and histology of the liver, and pancreas were evaluated.

RESULTS

D-gal injection increased the glucose (p<0.001) and insulin levels (p<0.01) compared to control group. Myricitrin (p<0.01) and Vitamin E (p<0.001) increased insulin and decreased blood glucose levels compared to D-gal group. Myricitrin had a similar impact on insulin levels to vitamin E. Insulin resistance induced in the D-gal group (p<0.001). Myricitrin reduced insulin resistance and increased β-cell function (p<0.01) compared to D-gal group. D-gal elevated (p<0.01) cholesterol, LDL and triglyceride level, myricitrin (p<0.001), and Vitamin E (p<0.05) were reduced.

CONCLUSION

D-gal-induced aging causes the accumulation of RBCs, inflammation in the liver, and changes in the number and diameter of Langerhans islets in the pancreas. Myricitrin improved these D-gal effects. Myricitrin had the anti-diabetic and hepatoprotective effects on the aged mice induced by D-galactose.

Glucose-Dependent Insulinotropic Polypeptide Receptor Therapies for the Treatment of Obesity, Do Agonists = Antagonists?

Glucose-dependent insulinotropic polypeptide receptor (GIPR) is associated with obesity in human genome-wide association studies. Similarly, mouse genetic studies indicate that loss of function alleles and glucose-dependent insulinotropic polypeptide overexpression both protect from high-fat diet-induced weight gain. Together, these data provide compelling evidence to develop therapies targeting GIPR for the treatment of obesity. Further, both antagonists and agonists alone prevent weight gain, but result in remarkable weight loss when codosed or molecularly combined with glucagon-like peptide-1 analogs preclinically. Here, we review the current literature on GIPR, including biology, human and mouse genetics, and pharmacology of both agonists and antagonists, discussing the similarities and differences between the 2 approaches. Despite opposite approaches being investigated preclinically and clinically, there may be viability of both agonists and antagonists for the treatment of obesity, and we expect this area to continue to evolve with new clinical data and molecular and pharmacological analyses of GIPR function.

Early postnatal overnutrition accelerates aging-associated epigenetic drift in pancreatic islets

Pancreatic islets of type 2 diabetes patients have altered DNA methylation, contributing to islet dysfunction and the onset of type 2 diabetes. The cause of these epigenetic alterations is largely unknown. We set out to test whether (i) islet DNA methylation would change with aging and (ii) early postnatal overnutrition would persistently alter DNA methylation. We performed genome-scale DNA methylation profiling in islets from postnatally over-nourished (suckled in a small litter) and control male mice at both postnatal day 21 and postnatal day 180. DNA methylation differences were validated using quantitative bisulfite pyrosequencing, and associations with expression were assessed by RT-PCR. We discovered that genomic regions that are hypermethylated in exocrine relative to endocrine pancreas tend to gain methylation in islets during aging (R ² = 0.33, P < 0.0001). These methylation differences were inversely correlated with mRNA expression of genes relevant to β cell function [including Rab3b (Ras-related protein Rab-3B), Cacnb3 (voltage-dependent L-type calcium channel subunit 3), Atp2a3 (sarcoplasmic/endoplasmic reticulum calcium ATPase 3) and Ins2 (insulin 2)]. Relative to control, small litter islets showed DNA methylation differences directly after weaning and in adulthood, but few of these were present at both ages. Surprisingly, we found substantial overlap of methylated loci caused by aging and small litter feeding, suggesting that the age-associated gain of DNA methylation happened much earlier in small litter islets than control islets. Our results provide the novel insights that aging-associated DNA methylation increases reflect an epigenetic drift toward the exocrine pancreas epigenome, and that early postnatal overnutrition may accelerate this process.

The Effect of the Taste Receptor Protein T1R3 on the Development of Islet Tissue of the Murine Pancreas

T1R3 protein, the main subunit of the sweet taste receptor and receptor of amino acid taste, is expressed in the epithelium of the tongue and gastrointestinal tract, in β cells of the pancreas, hypothalamus, and numerous other organs. Recently, convincing evidences on the involvement of T1R3 in the control of carbohydrate and lipid metabolism, and the control of incretin and insulin production were obtained. In the study on Tas1r3-gene knockout mouse strain and parent C57BL/6J strain as a control, the data on the effect of T1R3 on morphological characteristics of Langerhans islets in the pancreas were obtained. In Tas1r3 knockout animals, we found a reduction in the size of islets and their density in pancreatic tissue as compared to the parent strain. In addition, a decrease in the expression of active caspase-3 in the islets of gene-knockout mice was demonstrated. The data obtained indicate that the lack of functioning gene encoding sweet taste receptor protein causes a dystrophy of the islet tissue and is associated with the development of pathological changes in the pancreas specific to type 2 diabetes mellitus and obesity in humans.

Inferring population dynamics from single-cell RNA-sequencing time series data

Recent single-cell RNA-sequencing studies have suggested that cells follow continuous transcriptomic trajectories in an asynchronous fashion during development. However, observations of cell flux along trajectories are confounded with population size effects in snapshot experiments and are therefore hard to interpret. In particular, changes in proliferation and death rates can be mistaken for cell flux. Here we present pseudodynamics, a mathematical framework that reconciles population dynamics with the concepts underlying developmental trajectories inferred from time-series single-cell data. Pseudodynamics models population distribution shifts across trajectories to quantify selection pressure, population expansion, and developmental potentials. Applying this model to time-resolved single-cell RNA-sequencing of T-cell and pancreatic beta cell maturation, we characterize proliferation and apoptosis rates and identify key developmental checkpoints, data inaccessible to existing approaches.

Inhibition of mTORC1 by ER stress impairs neonatal β-cell expansion and predisposes to diabetes in the Akita mouse

Unresolved ER stress followed by cell death is recognized as the main cause of a multitude of pathologies including neonatal diabetes. A systematic analysis of the mechanisms of β-cell loss and dysfunction in Akita mice, in which a mutation in the proinsulin gene causes a severe form of permanent neonatal diabetes, showed no increase in β-cell apoptosis throughout life. Surprisingly, we found that the main mechanism leading to β-cell dysfunction is marked impairment of β-cell growth during the early postnatal life due to transient inhibition of mTORC1, which governs postnatal β-cell growth and differentiation. Importantly, restoration of mTORC1 activity in neonate β-cells was sufficient to rescue postnatal β-cell growth, and to improve diabetes. We propose a scenario for the development of permanent neonatal diabetes, possibly also common forms of diabetes, where early-life events inducing ER stress affect β-cell mass expansion due to mTOR inhibition.

20-HETE promotes glucose-stimulated insulin secretion in an autocrine manner through FFAR1

The long-chain fatty acid receptor FFAR1 is highly expressed in pancreatic β-cells. Synthetic FFAR1 agonists can be used as antidiabetic drugs to promote glucose-stimulated insulin secretion (GSIS). However, the physiological role of FFAR1 in β-cells remains poorly understood. Here we show that 20-HETE activates FFAR1 and promotes GSIS via FFAR1 with higher potency and efficacy than dietary fatty acids such as palmitic, linoleic, and α-linolenic acid. Murine and human β-cells produce 20-HETE, and the ω-hydroxylase-mediated formation and release of 20-HETE is strongly stimulated by glucose. Pharmacological inhibition of 20-HETE formation and blockade of FFAR1 in islets inhibits GSIS. In islets from type-2 diabetic humans and mice, glucose-stimulated 20-HETE formation and 20-HETE-dependent stimulation of GSIS are strongly reduced. We show that 20-HETE is an FFAR1 agonist, which functions as an autocrine positive feed-forward regulator of GSIS, and that a reduced glucose-induced 20-HETE formation contributes to inefficient GSIS in type-2 diabetes.

FFAR1 receptor is highly expressed in beta cells and its activation has been suggested as therapy against type-2 diabetes. Here, Tunaru et al. show that 20-hydroxyeicosatetraenoic acid, produced within the islets upon glucose stimulation, acts in an autocrine manner to stimulate insulin secretion via FFAR1 activation.

Insulin sensitivity, leptin, adiponectin, resistin, and testosterone in adult male and female rats after maternal-neonatal separation and environmental stress

Care of premature infants often requires parental and caregiver separation, particularly during hypoxic and hypothermic episodes. We have established a neonatal rat model of human prematurity involving maternal-neonatal separation and hypoxia with spontaneous hypothermia prevented by external heat. Adults previously exposed to these neonatal stressors show a sex difference in the insulin and glucose response to arginine stimulation suggesting a state of insulin resistance. The current study used this cohort of adult rats to evaluate insulin resistance [homeostatic model assessment of insulin resistance (HOMA-IR)], plasma adipokines (reflecting insulin resistance states), and testosterone. The major findings were that daily maternal-neonatal separation led to an increase in body weight and HOMA-IR in adult male and female rats and increased plasma leptin in adult male rats only; neither prior neonatal hypoxia (without or with body temperature control) nor neonatal hypothermia altered subsequent adult HOMA-IR or plasma adiponectin. Adult male-female differences in plasma leptin were lost with prior exposure to neonatal hypoxia or hypothermia; male-female differences in resistin were lost in the adults that were exposed to hypoxia and spontaneous hypothermia as neonates. Exposure of neonates to daily hypoxia without spontaneous hypothermia led to a decrease in plasma testosterone in adult male rats. We conclude that neonatal stressors result in subsequent adult sex-dependent increases in insulin resistance and adipokines and that our rat model of prematurity with hypoxia without hypothermia alters adult testosterone dynamics.

A synopsis of factors regulating beta cell development and beta cell mass

The insulin-secreting beta cells in the endocrine pancreas regulate blood glucose levels, and loss of functional beta cells leads to insulin deficiency, hyperglycemia (high blood glucose) and diabetes mellitus. Current treatment strategies for type-1 (autoimmune) diabetes are islet transplantation, which has significant risks and limitations, or normalization of blood glucose with insulin injections, which is clearly not ideal. The type-1 patients can lack insulin counter-regulatory mechanism; therefore, hypoglycemia is a potential risk. Hence, a cell-based therapy offers a better alternative for the treatment of diabetes. Past research was focused on attempting to generate replacement beta cells from stem cells; however, recently there has been an increasing interest in identifying mechanisms that will lead to the conversion of pre-existing differentiated endocrine cells into beta cells. The goal of this review is to provide an overview of several of the key factors that regulate new beta cell formation (neogenesis) and beta cell proliferation.

Growth hormone (GH)-transgenic insulin-like growth factor 1 (IGF1)-deficient mice allow dissociation of excess GH and IGF1 effects on glomerular and tubular growth

Growth hormone (GH)‐transgenic mice with permanently elevated systemic levels of GH and insulin‐like growth factor 1 (IGF1) reproducibly develop renal and glomerular hypertrophy and subsequent progressive glomerulosclerosis, finally leading to terminal renal failure. To dissociate IGF1‐dependent and ‐independent effects of GH excess on renal growth and lesion development in vivo, the kidneys of 75 days old IGF1‐deficient (I^−/−) and of IGF1‐deficient GH‐transgenic mice (I^−/−/G), as well as of GH‐transgenic (G) and nontransgenic wild‐type control mice (I^+/+) were examined by quantitative stereological and functional analyses. Both G and I^−/−/G mice developed glomerular hypertrophy, hyperplasia of glomerular mesangial and endothelial cells, podocyte hypertrophy and foot process effacement, albuminuria, and glomerulosclerosis. However, I^−/−/G mice exhibited less severe glomerular alterations, as compared to G mice. Compared to I^+/+ mice, G mice exhibited renal hypertrophy with a significant increase in the number without a change in the size of proximal tubular epithelial (PTE) cells. In contrast, I^−/−/G mice did not display significant PTE cell hyperplasia, as compared to I^−/− mice. These findings indicate that GH excess stimulates glomerular growth and induces lesions progressing to glomerulosclerosis in the absence of IGF1. In contrast, IGF1 represents an important mediator of GH‐dependent proximal tubular growth in GH‐transgenic mice.

Missense Mutation of POU Domain Class 3 Transcription Factor 3 in Pou3f3L423P Mice Causes Reduced Nephron Number and Impaired Development of the Thick Ascending Limb of the Loop of Henle

During nephrogenesis, POU domain class 3 transcription factor 3 (POU3F3 aka BRN1) is critically involved in development of distinct nephron segments, including the thick ascending limb of the loop of Henle (TAL). Deficiency of POU3F3 in knock-out mice leads to underdevelopment of the TAL, lack of differentiation of TAL cells, and perinatal death due to renal failure. Pou3f3L423P mutant mice, which were established in the Munich ENU Mouse Mutagenesis Project, carry a recessive point mutation in the homeobox domain of POU3F3. Homozygous Pou3f3L423P mutants are viable and fertile. The present study used functional, as well as qualitative and quantitative morphological analyses to characterize the renal phenotype of juvenile (12 days) and aged (60 weeks) homo- and heterozygous Pou3f3L423P mutant mice and age-matched wild-type controls. In both age groups, homozygous mutants vs. control mice displayed significantly smaller kidney volumes, decreased nephron numbers and mean glomerular volumes, smaller TAL volumes, as well as lower volume densities of the TAL in the kidney. No histological or ultrastructural lesions of TAL cells or glomerular cells were observed in homozygous mutant mice. Aged homozygous mutants displayed increased serum urea concentrations and reduced specific urine gravity, but no evidence of glomerular dysfunction. These results confirm the role of POU3F3 in development and function of the TAL and provide new evidence for its involvement in regulation of the nephron number in the kidney. Therefore, Pou3f3L423P mutant mice represent a valuable research model for further analyses of POU3F3 functions, or for nephrological studies examining the role of congenital low nephron numbers.

Development, growth and maintenance of β-cell mass: models are also part of the story

Pancreatic β-cells in the islets of Langerhans play a crucial role in regulating glucose homeostasis in the circulation. Loss of β-cell mass or function due to environmental, genetic and immunological factors leads to the manifestation of diabetes mellitus. The mechanisms regulating the dynamics of pancreatic β-cell mass during normal development and diabetes progression are complex. To fully unravel such complexity, experimental and clinical approaches need to be combined with mathematical and computational models. In the natural sciences, mathematical and computational models have aided the identification of key mechanisms underlying the behavior of systems comprising multiple interacting components. A number of mathematical and computational models have been proposed to explain the development, growth and death of pancreatic β-cells. In this review, we discuss some of these models and how their predictions provide novel insight into the mechanisms controlling β-cell mass during normal development and diabetes progression. Lastly, we discuss a handful of the major open questions in the field.

Stereological estimation and morphological assessment of the endocrine pancreatic components in relation to sex in hen

In pancreatic survey, quantitative and morphological characteristics could be fundamental variables for the evaluation of some structures. The aim of this study was the application of stereological methods for estimation of quantitative parameters and morphological evaluation of the pancreas in chickens. Ten adult male and ten adult female native chickens were used in this study. Routine tissue processing was carried out for all samples. The sections were stained with hematoxylin and eosin and Gomori's aldehyde-fuchsin. Stereological examinations were performed according to systemic uniform random sampling as well as Cavalieri and point counting method. The morphological results showed that in both sexes, pancreas of chickens was composed of four main pancreatic lobes and contained two distinct islet types. Alpha islets consisted of alpha and a few delta cells and beta islets contained beta and seldom delta cells. Stereological results showed that in both sexes, there were significantly distinctive regional differences in the volume of islet and the numerical density of cells among some lobes with highest values in third and splenic lobes, respectively (p < 0.05). The nuclear volume was lowest in ventral lobe either males (80.60 ± 33.50) or females (84.20 ± 44.10). Beside, in the islets diameter of splenic lobe, there were significant differences in males as compared with females (p < 0.05). In splenic lobe, a significant difference was observed in the nuclear diameter in males than to females (p < 0.05). The results indicated that although there were not morphological changes between sexes in pancreatic lobes, however, some stereological parameters could be affected by sex.

Chronic reduction of GIP secretion alleviates obesity and insulin resistance under high-fat diet conditions

Gastric inhibitory polypeptide (GIP) exhibits potent insulinotropic effects on β-cells and anabolic effects on bone formation and fat accumulation. We explored the impact of reduced GIP levels in vivo on glucose homeostasis, bone formation, and fat accumulation in a novel GIP-GFP knock-in (KI) mouse. We generated GIP-GFP KI mice with a truncated prepro-GIP gene. The phenotype was assessed in heterozygous and homozygous states in mice on a control fat diet and a high-fat diet (HFD) in vivo and in vitro. Heterozygous GIP-GFP KI mice (GIP-reduced mice [GIP(gfp/+)]) exhibited reduced GIP secretion; in the homozygous state (GIP-lacking mice [GIP(gfp/gfp)]), GIP secretion was undetectable. When fed standard chow, GIP(gfp/+) and GIP(gfp/gfp) mice showed mild glucose intolerance with decreased insulin levels; bone volume was decreased in GIP(gfp/gfp) mice and preserved in GIP(gfp/+) mice. Under an HFD, glucose levels during an oral glucose tolerance test were similar in wild-type, GIP(gfp/+), and GIP(gfp/gfp) mice, while insulin secretion remained lower. GIP(gfp/+) and GIP(gfp/gfp) mice showed reduced obesity and reduced insulin resistance, accompanied by higher fat oxidation and energy expenditure. GIP-reduced mice demonstrate that partial reduction of GIP does not extensively alter glucose tolerance, but it alleviates obesity and lessens the degree of insulin resistance under HFD conditions, suggesting a potential therapeutic value.

Pancreatic β-cell proliferation in obesity

Because obesity rates have increased dramatically over the past 3 decades, type 2 diabetes has become increasingly prevalent as well. Type 2 diabetes is associated with decreased pancreatic β-cell mass and function, resulting in inadequate insulin production. Conversely, in nondiabetic obesity, an expansion in β-cell mass occurs to provide sufficient insulin and to prevent hyperglycemia. This expansion is at least in part due to β-cell proliferation. This review focuses on the mechanisms regulating obesity-induced β-cell proliferation in humans and mice. Many factors have potential roles in the regulation of obesity-driven β-cell proliferation, including nutrients, insulin, incretins, hepatocyte growth factor, and recently identified liver-derived secreted factors. Much is still unknown about the regulation of β-cell replication, especially in humans. The extracellular signals that activate proliferative pathways in obesity, the relative importance of each of these pathways, and the extent of cross-talk between these pathways are important areas of future study.

The ductal origin of structural and functional heterogeneity between pancreatic islets

Islets form in the pancreas after the first endocrine cells have arisen as either single cells or small cell clusters in the epithelial cords. These cords constitute the developing pancreas in one of its earliest recognizable stages. Islet formation begins at the time the cords transform into a branching ductal system, continues while the ductal system expands, and finally stops before the exocrine tissue of ducts and acini reaches its final expansion. Thus, islets continuously arise from founder cells located in the branching and ramifying ducts. Islets arising from proximal duct cells locate between the exocrine lobules, develop strong autonomic and sensory innervations, and pass their blood to efferent veins (insulo-venous efferent system). Islets arising from cells of more distal ducts locate within the exocrine lobules, respond to nerve impulses ending at neighbouring blood vessels, and pass their blood to the surrounding acini (insulo-acinar portal system). Consequently, the section of the ductal system from which an islet arises determines to a large extent its future neighbouring tissue, architecture, properties, and functions. We note that islets interlobular in position are frequently found in rodents (rats and mice), whereas intralobularly-located, peripheral duct islets prevail in humans and cattle. Also, we expound on bovine foetal Laguesse islets as a prominent foetal type of type 1 interlobular neuro-insular complexes, similar to neuro-insular associations frequently found in rodents. Finally, we consider the probable physiological and pathophysiological implications of the different islet positions within and between species.

[Pathogenesis of diabetes mellitus and diabetic complications. Studies on diabetic mouse models]

Diabetic mouse models created via random mutagenesis or genetic modification are essential tools to unravel the mechanisms involved in the development of diabetes mellitus and associated diseases. Three diabetic mutant mouse lines derived from the Munich N-ethyl-N-nitrosourea (ENU) mouse mutagenesis project and one transgenic mouse line were analyzed with respect to diabetes-relevant clinical, pathomorphological and therapeutic aspects. An Ins2 mutation and two Gck mutations were identified as the cause of diabetes mellitus in the mutant lines. Heterozygous Ins2 and homozygous Gck mutants serve as model for permanent neonatal diabetes mellitus (PNDM) and heterozygous Gck mutants develop maturity onset diabetes of the young type 2. Dominant-negative glucose-dependent insulinotropic polypeptide receptor (GIPR(dn)) transgenic mice exhibit defective postnatal islet growth, develop PNDM and progressive diabetes-associated kidney lesions. The mutant and transgenic diabetic mouse models analyzed in the study were shown to represent valuable models to study the pathogenesis of monogenic diabetes and to establish novel treatment strategies.

Subpopulations of GFP-marked mouse pancreatic β-cells differ in size, granularity, and insulin secretion

There is growing information about the heterogeneity of pancreatic β-cells and how it relates to insulin secretion. This study used the approach of flow cytometry to sort and analyze β-cells from transgenic mice expressing green fluorescent protein (GFP) under the control of the mouse insulin I gene promoter. Three populations of β-cells with differing GFP brightness could be identified, which were classified as GFP-low, GFP-medium, and GFP-bright. The GFP-medium population comprised about 70% of the total. The GFP-low population had less insulin secretion as determined by the reverse hemolytic plaque assay and reduced insulin gene expression. Additionally, all three subpopulations of β-cells were found in mice of varying ages (embryonic d 15.5 and postnatal wk 1-9). The three populations from the youngest had larger cells (forward scatter) and less granularity (side scatter) than those from the adults. This approach opens up new ways to advance knowledge about β-cell heterogeneity.

Development of a quantitative methylation-specific polymerase chain reaction method for monitoring beta cell death in type 1 diabetes

DNA methylation is a mechanism by which cells control gene expression, and cell-specific genes often exhibit unique patterns of DNA methylation. We previously reported that the mouse insulin-2 gene (Ins2) promoter has three potential methylation (CpG) sites, all of which are unmethylated in insulin-producing cells but methylated in other tissues. In this study we examined Ins2 exon 2 and found a similar tissue-specific methylation pattern. These methylation patterns can differentiate between DNA from insulin-producing beta cells and other tissues. We hypothesized that damaged beta cells release their DNA into circulation at the onset of type 1 diabetes mellitus (T1DM) and sought to develop a quantitative methylation-specific polymerase chain reaction (qMSP) assay for circulating beta cell DNA to monitor the loss of beta cells. Methylation-specific primers were designed to interrogate two or more CpG in the same assay. The cloned mouse Ins2 gene was methylated in vitro and used for development of the qMSP assay. We found the qMSP method to be sensitive and specific to differentiate between insulin-producing cells and other tissues with a detection limit of 10 copies in the presence of non-specific genomic DNA background. We also compared different methods for data analysis and found that the Relative Expression Ratio method is the most robust method since it incorporates both a reference value to normalize day-to-day variability as well as PCR reaction efficiencies to normalize between the methylation-specific and bisulfite-specific components of the calculations. The assay was applied in the streptozotocin-treated diabetic mouse model and detected a significant increase in circulating beta cell DNA before the rise in blood glucose level. These results demonstrate that this qMSP assay can be used for monitoring circulating DNA from insulin-producing cells, which will provide the basis for development of assays to detect beta cell destruction in early T1DM.

Enhanced insulin secretion and improved glucose tolerance in mice with homozygous inactivation of the Na(+)K(+)2Cl(-) co-transporter 1

The intracellular chloride concentration ([Cl(-)](i)) in β-cells plays an important role in glucose-stimulated plasma membrane depolarisation and insulin secretion. [Cl(-)](i) is maintained above equilibrium in β-cells by the action of Cl(-) co-transporters of the solute carrier family 12 group A (Slc12a). β-Cells express Slc12a1 and Slc12a2, which are known as the bumetanide (BTD)-sensitive Na(+)-dependent K(+)2Cl(-) co-transporters 2 and 1 respectively. We show that mice lacking functional alleles of the Slc12a2 gene exhibit better fasting glycaemia, increased insulin secretion in response to glucose, and improved glucose tolerance when compared with wild-type (WT). This phenomenon correlated with increased sensitivity of β-cells to glucose in vitro and with increased β-cell mass. Further, administration of low doses of BTD to mice deficient in Slc12a2 worsened their glucose tolerance, and low concentrations of BTD directly inhibited glucose-stimulated insulin secretion from β-cells deficient in Slc12a2 but expressing intact Slc12a1 genes. Together, our results suggest for the first time that the Slc12a2 gene is not necessary for insulin secretion and that its absence increases β-cell secretory capacity. Further, impairment of insulin secretion with BTD in vivo and in vitro in islets lacking Slc12a2 genes unmasks a potential new role for Slc12a1 in β-cell physiology.

Islet β-Cell Mass Preservation and Regeneration in Diabetes Mellitus: Four Factors with Potential Therapeutic Interest

Islet β-cell replacement and regeneration are two promising approaches for the treatment of Type 1 Diabetes Mellitus. Indeed, the success of islet transplantation in normalizing blood glucose in diabetic patients has provided the proof of principle that cell replacement can be employed as a safe and efficacious treatment. Nonetheless, shortage of organ donors has hampered expansion of this approach. Alternative sources of insulin-producing cells are mandatory to fill this gap. Although great advances have been achieved in generating surrogate β-cells from stem cells, current protocols have yet to produce functionally mature insulin-secreting cells. Recently, the concept of islet regeneration in which new β-cells are formed from either residual β-cell proliferation or transdifferentiation of other endocrine islet cells has gained much interest as an attractive therapeutic alternative to restore β-cell mass. Complementary approaches to cell replacement and regeneration could aim at enhancing β-cell survival and function. Herein, we discuss the value of Hepatocyte Growth Factor (HGF), Glucose-Dependent Insulinotropic Peptide (GIP), Paired box gene 4 (Pax4) and Liver Receptor Homolog-1 (LRH-1) as key players for β-cell replacement and regeneration therapies. These factors convey β-cell protection and enhanced function as well as facilitating proliferation and transdifferentiation of other pancreatic cell types to β-cells, under stressful conditions.