Adult insulin- and glucagon-producing cells differentiate from two independent cell lineages

Cyclic GMP kinase I modulates glucagon release from pancreatic α-cells

OBJECTIVE

The physiologic significance of the nitric oxide (NO)/cGMP signaling pathway in islets is unclear. We hypothesized that cGMP-dependent protein kinase type I (cGKI) is directly involved in the secretion of islet hormones and glucose homeostasis.

RESEARCH DESIGN AND METHODS

Gene-targeted mice that lack cGKI in islets (conventional cGKI mutants and cGKIα and Iβ rescue mice [α/βRM] that express cGKI only in smooth muscle) were studied in comparison to control (CTR) mice. cGKI expression was mapped in the endocrine pancreas by Western blot, immuno-histochemistry, and islet-specific recombination analysis. Insulin, glucagon secretion, and cytosolic Ca²(+) ([Ca²(+)](i)) were assayed by radioimmunoassay and FURA-2 measurements, respectively. Serum levels of islet hormones were analyzed at fasting and upon glucose challenge (2 g/kg) in vivo.

RESULTS

Immunohistochemistry showed that cGKI is present in α- but not in β-cells in islets of Langerhans. Mice that lack α-cell cGKI had significantly elevated fasting glucose and glucagon levels, whereas serum insulin levels were unchanged. High glucose concentrations strongly suppressed the glucagon release in CTR mice, but had only a moderate effect on islets that lacked cGKI. 8-Br-cGMP reduced stimulated [Ca²(+)](i) levels and glucagon release rates of CTR islets at 0.5 mmol/l glucose, but was without effect on [Ca²(+)](i) or hormone release in cGKI-deficient islets.

CONCLUSIONS

We propose that cGKI modulates glucagon release by suppression of [Ca²(+)](i) in α-cells.

Conditional gene targeting in mouse pancreatic ß-Cells: analysis of ectopic Cre transgene expression in the brain

OBJECTIVE

Conditional gene targeting has been extensively used for in vivo analysis of gene function in β-cell biology. The objective of this study was to examine whether mouse transgenic Cre lines, used to mediate β-cell- or pancreas-specific recombination, also drive Cre expression in the brain.

RESEARCH DESIGN AND METHODS

Transgenic Cre lines driven by Ins1, Ins2, and Pdx1 promoters were bred to R26R reporter strains. Cre activity was assessed by β-galactosidase or yellow fluorescent protein expression in the pancreas and the brain. Endogenous Pdx1 gene expression was monitored using Pdx1(tm1Cvw) lacZ knock-in mice. Cre expression in β-cells and co-localization of Cre activity with orexin-expressing and leptin-responsive neurons within the brain was assessed by immunohistochemistry.

RESULTS

All transgenic Cre lines examined that used the Ins2 promoter to drive Cre expression showed widespread Cre activity in the brain, whereas Cre lines that used Pdx1 promoter fragments showed more restricted Cre activity primarily within the hypothalamus. Immunohistochemical analysis of the hypothalamus from Tg(Pdx1-cre)(89.1Dam) mice revealed Cre activity in neurons expressing orexin and in neurons activated by leptin. Tg(Ins1-Cre/ERT)(1Lphi) mice were the only line that lacked Cre activity in the brain.

CONCLUSIONS

Cre-mediated gene manipulation using transgenic lines that express Cre under the control of the Ins2 and Pdx1 promoters are likely to alter gene expression in nutrient-sensing neurons. Therefore, data arising from the use of these transgenic Cre lines must be interpreted carefully to assess whether the resultant phenotype is solely attributable to alterations in the islet β-cells.

Harnessing the pancreatic stem cell

Building on the elaborate research studies that have helped map out key decision points in the process of pancreas development, reprogramming of pluripotent embryonic stem cells or induced pluripotent stem cells offers the possibility of overcoming restrictions on tissue supply associated with transplantation of donor islets. In a healthy pancreas, the beta-cell mass can exhibit significant plasticity, as reflected in the normal adaptive response in beta-cell mass to offset the metabolic challenge associated with pregnancy and obesity. In this article, alternative strategies and potential sources of pancreatic stem cells are considered.

Conditional ablation of Gsk-3β in islet beta cells results in expanded mass and resistance to fat feeding-induced diabetes in mice

AIMS/HYPOTHESIS

Glycogen synthase kinase 3β (GSK-3β) is an enzyme that is suppressed by insulin and when elevated results in insulin resistance in skeletal muscle and diabetes. Its role in beta cell development and function is little known. Because of the enzyme's anti-proliferative and pro-apoptotic properties, the hypothesis to be tested here was that beta cell specific deficiency of GSK-3β in mice would result in enhanced beta cell mass and function.

METHODS

Mice with beta cell deficiency of GSK-3β (β-Gsk-3β [also known as Gsk3b](-/-)) were generated by breeding Gsk-3β (flox/flox) mice with mice overexpressing the Cre recombinase gene under the control of the rat insulin 2 gene promoter (RIP-Cre mice), and glucose tolerance, insulin secretion, islet mass, proliferation and apoptosis were measured. Changes in islet proteins were investigated by western blotting.

RESULTS

On a normal diet β-Gsk-3β ( -/- ) mice were found to have mild improvement of glucose tolerance and glucose-induced insulin secretion, and increased beta cell mass accompanied by increased proliferation and decreased apoptosis. On a high-fat diet β-Gsk-3β (-/-) mice exhibited improved glucose tolerance and expanded beta cell mass with increased proliferation relative to that in control mice, resisting fat-fed diabetes. Molecular mechanisms accounting for these phenotypic changes included increased levels of islet IRS1 and IRS2 proteins and phospho-Akt, suggesting enhanced signalling through the phosphatidylinositol 3-kinase (PI3K)/Akt pathway, and increased islet levels of pancreas/duodenum homeobox protein 1 (PDX1). Inhibition of GSK3 in MIN6 cells in vitro led to increased IRS1 and IRS2 protein levels through inhibition of proteosomal degradation.

CONCLUSIONS/INTERPRETATION

These results are consistent with a mechanism whereby endogenous GSK-3β activity controls islet beta cell growth by feedback inhibition of the insulin receptor/PI3K/Akt signalling pathway.

A factor(s) secreted from MIN-6 beta-cells stimulates differentiation of definitive endoderm enriched embryonic stem cells towards a pancreatic lineage

In the mouse the developing pancreas is controlled by contact with, and signalling molecules secreted from, surrounding cells. These factors are best studied using explant cultures of embryonic tissue. The present study was undertaken to determine whether embryonic stem (ES) cells could be used as an alternative model in vitro system to investigate the role of cell-cell interactions in the developing pancreas. Transwell culture experiments showed that MIN-6 beta-cells secreted a factor or factors that promoted differentiation of ES cell derived definitive endoderm enriched cells towards a pancreatic fate. Further studies using MIN-6 condition medium showed that the factor(s) involved was restricted to MIN-6 cells, could be concentrated with ammonium sulphate, and was sensitive to heat treatment, suggesting that it was a protein or peptide. Further analyses showed that insulin or proinsulin failed to mimic the effects of the conditioned media. Collectively, these results suggest that beta-cells secrete a factor(s) capable of controlling their own differentiation and maturation. The culture system described here presents unique advantages in the identification and characterisation of these factors.

Fgf9 signalling stimulates Spred and Sprouty expression in embryonic mouse pancreas mesenchyme

Epithelial-mesenchymal interactions are critical for normal pancreas development. Fibroblast growth factor (Fgf)-10 is expressed in the pancreatic mesenchyme and its signalling is required for normal growth and regulation of gene expression in the pancreatic epithelium. However, little is known about putative Fgf signalling to the mesenchyme. Here we have examined the embryonic pancreas expression of differentially spliced Fgf receptor isoforms and their targets; the Sprouty (Spry) and Spred family genes which are induced by Fgf signalling. Using qPCR to quantify mRNA levels in microdissected pancreatic epithelium and mesenchyme as well as in FACS isolated Pdx1-GFP(+) and -GFP(-) cell populations we demonstrate that several members of the Spred and Sprouty families are expressed in embryonic mouse pancreas and find Spred1 and -2 as well as Spry2 and -4 to be predominantly expressed in pancreatic mesenchyme. Using embryonic pancreas explant cultures we demonstrate that Spred1/2 and Spry2/4 expression is regulated by Fgf receptor signalling and is increased by treatment with Fgf9, but not by Fgf7 or Fgf10. We extend previous work showing that Fgf9 is expressed in pancreatic mesenchyme, and since Fgf9 is known to activate the mesenchyme-specific "c"-splice forms of Fgf receptors, while Fgf7 and -10 both activate the epithelium-specific "b"-splice forms of Fgf receptors, these results suggest that Fgf signalling is active in the pancreatic mesenchyme, where expression of Spred1/2 and Spry2/4 appear downstream of Fgf9 signalling.

Pancreatic beta-cells: from generation to regeneration

The pancreas is composed of two main compartments consisting of endocrine and exocrine tissues. The majority of the organ is exocrine and responsible for the synthesis of digestive enzymes and for their transport via an intricate ductal system into the duodenum. The endocrine tissue represents less than 2% of the organ and is organized into functional units called islets of Langerhans, comprising alpha-, beta-, delta-, epsilon- and PP-cells, producing the hormones glucagon, insulin, somatostatin, ghrelin and pancreatic polypeptide (PP), respectively. Insulin-producing beta-cells play a central role in the control of the glucose homeostasis. Accordingly, absolute or relative deficiency in beta-cells may ultimately lead to type 1 and/or type 2 diabetes, respectively. One major goal of diabetes research is therefore to understand the molecular mechanisms controlling the development of beta-cells during pancreas morphogenesis, but also those underlying the regeneration of adult injured pancreas, and assess their significance for future cell-based therapy. In this review, we will therefore present new insights into beta-cell development with focus on beta-cell regeneration.

Increased beta-cell mass by islet transplantation and PLAG1 overexpression causes hyperinsulinemic normoglycemia and hepatic insulin resistance in mice

OBJECTIVE

It is believed that an organism remains normoglycemic despite an increase in the beta-cell mass because of decreased insulin production by beta-cells on a per-cell basis. However, some transgenic mouse models with beta-cell hyperplasia suggest that insulin production remains excessive and that normoglycemia is maintained by insulin resistance.

METHODS

Here, we investigated the effect of an increased beta-cell mass on glycemia and insulin resistance by grafting excess normal islets in normoglycemic mice, as well as using targeted PLAG1 expression in beta-cells, which leads to beta-cell expansion.

RESULTS

In both models, fasting plasma insulin levels were increased, even though animals were normoglycemic. After an intraperitoneal glucose tolerance test, plasma insulin levels increased, which was associated with improved glucose clearing. Under these conditions, normoglycemia is maintained by hepatic insulin resistance as demonstrated by hyperinsulinemic euglycemic clamp experiments.

CONCLUSIONS

In conclusion, we demonstrate that when excess beta-cells are grafted, insulin production on a per beta-cell basis is not sufficiently decreased, leading to hyperinsulinemia and hepatic insulin resistance. This observation might be important for the design of stem cell-based islet replacement therapies.

Multiple endocrine neoplasia type 1 deletion in pancreatic alpha-cells leads to development of insulinomas in mice

The pancreatic alpha- and beta-cells are critical components in regulating blood glucose homeostasis via secretion of glucagon and insulin, respectively. Both cell types are typically localized in the islets of Langerhans. However, little is known about the roles of paracrine interactions that contribute to their physiological functions. The lack of suitable cell lines to study alpha- and beta-cells interactions have led us to develop an alpha-cell-specific Cre-expressing transgenic line utilizing a glucagon promoter sequence, the Glu-Cre transgenic mouse. Here, we demonstrate that the Glu-Cre could specifically and efficiently excise floxed target genes in adult islet alpha-cells. We further showed that deletion of the tumor suppressor gene, multiple endocrine neoplasia type 1 (Men1), in alpha-cells led to tumorigenesis. However, to our surprise, the lack of Men1 in alpha-cells did not result in glucagonomas but rather beta-cell insulinomas. Because deletion of the Men1 alleles was only present in alpha-cells, our data suggested that cross communication between alpha- and beta-cells contributes to tumorigenesis in the absence of Men1. Together, we believed that the new model systems described here will allow future studies to decipher cellular interactions between islet alpha- and beta-cells in a physiological context.

Glucagon deficiency reduces hepatic glucose production and improves glucose tolerance in adult mice

The major role of glucagon is to promote hepatic gluconeogenesis and glycogenolysis to raise blood glucose levels during hypoglycemic conditions. Several animal models have been established to examine the in vivo function of glucagon in the liver through attenuation of glucagon via glucagon receptor knockout animals and pharmacological interventions. To investigate the consequences of glucagon loss to hepatic glucose production and glucose homeostasis, we derived mice with a pancreas specific ablation of the alpha-cell transcription factor, Arx, resulting in a complete loss of the glucagon-producing pancreatic alpha-cell. Using this model, we found that glucagon is not required for the general health of mice but is essential for total hepatic glucose production. Our data clarifies the importance of glucagon during the regulation of fasting and postprandial glucose homeostasis.

The transcription factor Rfx3 regulates beta-cell differentiation, function, and glucokinase expression

OBJECTIVE

Pancreatic islets of perinatal mice lacking the transcription factor Rfx3 exhibit a marked reduction in insulin-producing beta-cells. The objective of this work was to unravel the cellular and molecular mechanisms underlying this deficiency.

RESEARCH DESIGN AND METHODS

Immunofluorescence studies and quantitative RT-PCR experiments were used to study the emergence of insulin-positive cells, the expression of transcription factors implicated in the differentiation of beta-cells from endocrine progenitors, and the expression of mature beta-cell markers during development in Rfx3(-/-) and pancreas-specific Rfx3-knockout mice. RNA interference experiments were performed to document the consequences of downregulating Rfx3 expression in Min6 beta-cells. Quantitative chromatin immunoprecipitation (ChIP), ChIP sequencing, and bandshift experiments were used to identify Rfx3 target genes.

RESULTS

Reduced development of insulin-positive cells in Rfx3(-/-) mice was not due to deficiencies in endocrine progenitors or beta-lineage specification, but reflected the accumulation of insulin-positive beta-cell precursors and defective beta-cells exhibiting reduced insulin, Glut-2, and Gck expression. Similar incompletely differentiated beta-cells developed in pancreas-specific Rfx3-deficient embryos. Defective beta-cells lacking Glut-2 and Gck expression dominate in Rfx3-deficent adults, leading to glucose intolerance. Attenuated Glut-2 and glucokinase expression, and impaired glucose-stimulated insulin secretion, were also induced by RNA interference-mediated inhibition of Rfx3 expression in Min6 cells. Finally, Rfx3 was found to bind in Min6 cells and human islets to two well-known regulatory sequences, Pal-1 and Pal-2, in the neuroendocrine promoter of the glucokinase gene.

CONCLUSIONS

Our results show that Rfx3 is required for the differentiation and function of mature beta-cells and regulates the beta-cell promoter of the glucokinase gene.

Epistasis of transcriptomes reveals synergism between transcriptional activators Hnf1alpha and Hnf4alpha

The transcription of individual genes is determined by combinatorial interactions between DNA-binding transcription factors. The current challenge is to understand how such combinatorial interactions regulate broad genetic programs that underlie cellular functions and disease. The transcription factors Hnf1alpha and Hnf4alpha control pancreatic islet beta-cell function and growth, and mutations in their genes cause closely related forms of diabetes. We have now exploited genetic epistasis to examine how Hnf1alpha and Hnf4alpha functionally interact in pancreatic islets. Expression profiling in islets from either Hnf1a(+/-) or pancreas-specific Hnf4a mutant mice showed that the two transcription factors regulate a strikingly similar set of genes. We integrated expression and genomic binding studies and show that the shared transcriptional phenotype of these two mutant models is linked to common direct targets, rather than to known effects of Hnf1alpha on Hnf4a gene transcription. Epistasis analysis with transcriptomes of single- and double-mutant islets revealed that Hnf1alpha and Hnf4alpha regulate common targets synergistically. Hnf1alpha binding in Hnf4a-deficient islets was decreased in selected targets, but remained unaltered in others, thus suggesting that the mechanisms for synergistic regulation are gene-specific. These findings provide an in vivo strategy to study combinatorial gene regulation and reveal how Hnf1alpha and Hnf4alpha control a common islet-cell regulatory program that is defective in human monogenic diabetes.

Alpha cell-specific Men1 ablation triggers the transdifferentiation of glucagon-expressing cells and insulinoma development

BACKGROUND & AIMS

The tumor suppressor menin is recognized as a key regulator of pancreatic islet development, proliferation, and beta-cell function, whereas its role in alpha cells remains poorly understood. The purpose of the current study was to address this issue in relation to islet tumor histogenesis.

METHODS

We generated alpha cell-specific Men1 mutant mice with Cre/loxP technology and carried out analyses of pancreatic lesions developed in the mutant mice during aging.

RESULTS

We showed that, despite the alpha-cell specificity of the GluCre transgene, both glucagonomas and a large amount of insulinomas developed in mutant mice older than 6 months, accompanied by mixed islet tumors. Interestingly, the cells sharing characteristics of both alpha and beta cells were identified shortly after the appearance of menin-deficient alpha cells but well before the tumor onset. Using a genetic cell lineage tracing analysis, we demonstrated that insulinoma cells were directly derived from transdifferentiating glucagon-expressing cells. Furthermore, our data indicated that the expression of Pdx1, MafA, Pax4, and Ngn3 did not seem to be required for the initiation of this transdifferentiation.

CONCLUSIONS

Our work shows cell transdifferentiation as a novel mechanism involved in islet tumor development and provides evidence showing that menin regulates the plasticity of differentiated pancreatic alpha cells in vivo, shedding new light on the mechanisms of islet tumorigenesis.

Conversion of adult pancreatic alpha-cells to beta-cells after extreme beta-cell loss

Pancreatic insulin-producing beta-cells have a long lifespan, such that in healthy conditions they replicate little during a lifetime. Nevertheless, they show increased self-duplication after increased metabolic demand or after injury (that is, beta-cell loss). It is not known whether adult mammals can differentiate (regenerate) new beta-cells after extreme, total beta-cell loss, as in diabetes. This would indicate differentiation from precursors or another heterologous (non-beta-cell) source. Here we show beta-cell regeneration in a transgenic model of diphtheria-toxin-induced acute selective near-total beta-cell ablation. If given insulin, the mice survived and showed beta-cell mass augmentation with time. Lineage-tracing to label the glucagon-producing alpha-cells before beta-cell ablation tracked large fractions of regenerated beta-cells as deriving from alpha-cells, revealing a previously disregarded degree of pancreatic cell plasticity. Such inter-endocrine spontaneous adult cell conversion could be harnessed towards methods of producing beta-cells for diabetes therapies, either in differentiation settings in vitro or in induced regeneration.

Pancreatic beta cells require NeuroD to achieve and maintain functional maturity

NeuroD, a transactivator of the insulin gene, is critical for development of the endocrine pancreas, and NeuroD mutations cause MODY6 in humans. To investigate the role of NeuroD in differentiated beta cells, we generated mice in which neuroD is deleted in insulin-expressing cells. These mice exhibit severe glucose intolerance. Islets lacking NeuroD respond poorly to glucose and display a glucose metabolic profile similar to immature beta cells, featuring increased expression of glycolytic genes and LDHA, elevated basal insulin secretion and O2 consumption, and overexpression of NPY. Moreover, the mutant islets appear to have defective K(ATP) channel-mediated insulin secretion. Unexpectedly, virtually all insulin in the mutant mice is derived from ins2, whereas ins1 expression is almost extinguished. Overall, these results indicate that NeuroD is required for beta cell maturation and demonstrate the importance of NeuroD in the acquisition and maintenance of fully functional glucose-responsive beta cells.

Expression of Pdx1 mediates differentiation from mesenchymal stem cells into insulin-producing cells

The transplantation of insulin-producing cells is a promising approach for the treatment of insulin-dependent diabetes mellitus; however, lack of pancreas donors limits its application. Pancreatic duodenal homeobox 1 (Pdx1) plays a key role in the differentiation of various non-β-cells into insulin-producing cells, but the potential mechanism remains to be clarified. The purpose of this study was to confirm that the expression of Pdx1 could mediate the differentiation of rat mesenchymal stem cells (MSCs) into insulin-producing cells and evaluate the potential molecular mechanisms in the process that Pdx1 activates transcription of insulin gene. In this study, glucose-stimulated insulin secretion was obviously detected in MSCs transfected with Pdx1 cDNA by insulin release assay and the islet-like structure formed in Pdx1-expressing MSCs was stained into black-red by dithizone, while the native MSCs were opposite. In addition, we uncovered the close relationships among the expression of Pdx1, insulin and Ngn3 genes, whose expression indicated parallel changes after high glucose challenge, and the fluctuation of Pdx1 and Ngn3 partly resulted in the unstable release of insulin. Taken together, these findings demonstrated that the effective role of Pdx1 gene in inducing insulin-producing cells, which may shuttle to the nucleoplasm of MSCs under high glucose, then initiate the expression of native transcription factors Ngn3 and recruit other proteins, resulting in transactivation of the relevant genes including insulin and generation of β cell phenotype. Accordingly, these results would provide new insights that may be applicable to improve β cell replacement strategies and enhance diabetes therapy in the future.

Glucose suppression of glucagon secretion: metabolic and calcium responses from alpha-cells in intact mouse pancreatic islets

Glucagon is released from alpha-cells present in intact pancreatic islets at glucose concentrations below 4 mm, whereas higher glucose levels inhibit its secretion. The mechanisms underlying the suppression of alpha-cell secretory activity are poorly understood, but two general types of models have been proposed as follows: direct inhibition by glucose or paracrine inhibition from non-alpha-cells within the islet of Langerhans. To identify alpha-cells for analysis, we utilized transgenic mice expressing fluorescent proteins targeted specifically to these cells. Measurements of glucagon secretion from pure populations of flow-sorted alpha-cells show that contrary to its effect on intact islets, glucose does stimulate glucagon secretion from isolated alpha-cells. This observation argues against a direct inhibition of glucagon secretion by glucose and supports the paracrine inhibition model. Imaging of cellular metabolism by two-photon excitation of NAD(P)H autofluorescence indicates that glucose is metabolized in alpha-cells and that glucokinase is the likely rate-limiting step in this process. Imaging calcium dynamics of alpha-cells in intact islets reveals that inhibiting concentrations of glucose increase the intracellular calcium concentration and the frequency of alpha-cell calcium oscillations. Application of candidate paracrine inhibitors leads to reduced glucagon secretion but did not decrease the alpha-cell calcium activity. Taken together, the data suggest that suppression occurs downstream from alpha-cell calcium signaling, presumably at the level of vesicle trafficking or exocytotic machinery.

Single pancreatic beta cells co-express multiple islet hormone genes in mice

AIMS/HYPOTHESIS

It is widely accepted that production of insulin, glucagon, somatostatin and pancreatic polypeptide in islet cells is specific to beta, alpha, delta and pancreatic polypeptide cells, respectively. We examined whether beta cells express other genes encoding islet hormones.

METHODS

Nested RT-PCR was performed on single beta cells of transgenic mice with green fluorescent protein (GFP) driven by mouse insulin I promoter (MIP-GFP).

RESULTS

Only 55% of adult beta cells expressed the insulin gene alone, while others expressed two or more islet hormone genes; 4% expressed all four hormone genes. In embryonic and neonatal cells, 60% to 80% of GFP(+) cells co-expressed pancreatic polypeptide and insulin genes in contrast to 29% in adult. To clarify cell fate, we conducted lineage tracing using rat insulin II promoter-cre mice crossed with reporter mice Gt(ROSA)26Sor-loxP-flanked STOP-cassette-GFP. All GFP(+) cells expressed insulin I and II genes, and showed similar heterogeneity of co-expression to that seen in MIP-GFP mice. Although we report expression of other hormone genes in a significant proportion of beta cells, our lineage tracing results demonstrate that after inducing InsII (also known as Ins2) expression, beta cell progenitors do not redifferentiate to non-beta cells.

CONCLUSIONS/INTERPRETATION

This study shows co-expression of multiple hormone genes in beta cells of adult mice as well as in embryos and neonates. This finding could: (1) represent residual expression from beta cell precursors; (2) result from alternative developmental pathways for beta cells; or (3) denote the differentiation potential of these cells. It may be linked to functional heterogeneity. This heterogeneity in gene expression may provide a means to characterise the functional, cellular and developmental heterogeneity seen in beta cells.

Notch signaling in pancreatic endocrine cell and diabetes

Recent studies have improved our understanding of the physiological function of Notch signaling pathway and now there is compelling evidence demonstrating that Notch is a key regulator of embryonic development and tissue homeostasis. Although further extensive studies are necessary to illustrate the molecular mechanisms, new insights into the role of Notch signaling in pancreas development and diabetes have been achieved. Importantly, the ability to regulate Notch signaling intensity both positively and negatively may have therapeutic relevance for diabetes. Thus, this paper reviews the current knowledge of the roles of Notch signaling in the pancreatic endocrine cell system.

Neurog3 gene dosage regulates allocation of endocrine and exocrine cell fates in the developing mouse pancreas

The basic helix-loop-helix transcription factor Neurog3 (Neurogenin3 or Ngn3) actively drives endodermal progenitor cells towards endocrine islet cell differentiation during embryogenesis. Here, we manipulate Neurog3 expression levels in endocrine progenitor cells without altering its expression pattern using heterozygosity and a hypomorph. Lowered Neurog3 gene dosage in the developing pancreatic epithelium reduces the overall production of endocrine islet cells without significantly affecting the proportions of various islet cell types that do form. A reduced Neurog3 production level in the endocrine-directed pancreatic progenitor population activates the expression of Neurog3 in an increased number of epithelial progenitors. Yet a significant number of these Neurog3+ cells detected in heterozygous and hypomorphic pancreata, possibly those that express low levels of Neurog3, move on to adopt pancreatic ductal or acinar fates. These data directly demonstrate that achieving high levels of Neurog3 expression is a critical step for endocrine commitment from multipotent pancreatic progenitors. These findings also suggest that a high level of Neurog3 expression could mediate lateral inhibition or other unknown feedback mechanisms to regulate the number of cells that initiate Neurog3 transcription and protein production. The control of Neurog3+ cell number and the Neurog3 threshold-dependent endocrine differentiation mechanism combine to select a specific proportion of pancreatic progenitor cells to adopt the islet cell fate.

Pancreas cell fate

Diabetes is characterized by decreased function of insulin-producing beta cells and insufficient insulin output resulting from an absolute (Type 1) or relative (Type 2) inadequate functional beta cell mass. Both forms of the disease would greatly benefit from treatment strategies that could enhance beta cell regeneration and/or function. Successful and reliable methods of generating beta cells or whole islets from progenitor cells in vivo or in vitro could lead to restoration of beta cell mass in individuals with Type 1 diabetes and enhanced beta cell compensation in Type 2 patients. A thorough understanding of the normal developmental processes that occur during pancreatic organogenesis, for example, transcription factors, cell signaling molecules, and cell-cell interactions that regulate endocrine differentiation from the embryonic pancreatic epithelium, is required in order to successfully reach these goals. This review summarizes our current understanding of pancreas development, with particular emphasis on factors intrinsic or extrinsic to the pancreatic epithelium that are involved in regulating the development and differentiation of the various pancreatic cell types. We also discuss the recent progress in generating insulin-producing cells from progenitor sources.

Ablation of C/EBPbeta alleviates ER stress and pancreatic beta cell failure through the GRP78 chaperone in mice

Pancreatic beta cell failure is thought to underlie the progression from glucose intolerance to overt diabetes, and ER stress is implicated in such beta cell dysfunction. We have now shown that the transcription factor CCAAT/enhancer-binding protein beta (C/EBPbeta) accumulated in the islets of diabetic animal models as a result of ER stress before the onset of hyperglycemia. Transgenic overexpression of C/EBPbeta specifically in beta cells of mice reduced beta cell mass and lowered plasma insulin levels, resulting in the development of diabetes. Conversely, genetic ablation of C/EBPbeta in the beta cells of mouse models of diabetes, including Akita mice, which harbor a heterozygous mutation in Ins2 (Ins2WT/C96Y), and leptin receptor-deficient (Lepr-/-) mice, resulted in an increase in beta cell mass and ameliorated hyperglycemia. The accumulation of C/EBPbeta in pancreatic beta cells reduced the abundance of the molecular chaperone glucose-regulated protein of 78 kDa (GRP78) as a result of suppression of the transactivation activity of the transcription factor ATF6alpha, thereby increasing the vulnerability of these cells to excess ER stress. Our results thus indicate that the accumulation of C/EBPbeta in pancreatic beta cells contributes to beta cell failure in mice by enhancing susceptibility to ER stress.

Pancreatic exocrine duct cells give rise to insulin-producing beta cells during embryogenesis but not after birth

A longstanding unsettled question is whether pancreatic beta cells originate from exocrine duct cells. We have now used genetic labeling to fate map embryonic and adult pancreatic duct cells. We show that Hnf1beta+ cells of the trunk compartment of the early branching pancreas are precursors of acinar, duct, and endocrine lineages. Hnf1beta+ cells subsequent form the embryonic duct epithelium, which gives rise to both ductal and endocrine lineages, but not to acinar cells. By the end of gestation, the fate of Hnf1beta+ duct cells is further restrained. We provide compelling evidence that the ductal epithelium does not make a significant contribution to acinar or endocrine cells during neonatal growth, during a 6 month observation period, or during beta cell growth triggered by ligation of the pancreatic duct or by cell-specific ablation with alloxan followed by EGF/gastrin treatment. Thus, once the ductal epithelium differentiates it has a restricted plasticity, even under regenerative settings.

Cellular origins of beta-cell regeneration: a legacy view of historical controversies

Beta-cell regeneration represents a major goal of therapy for diabetes. Unravelling the origin of beta cells during pancreatic regeneration could help restore a functional beta-cell mass in diabetes patients. This scientific question has represented a longstanding interest still intensively investigated today. This review focuses on pioneering observations and subsequent theories made 100 years ago and describes how technical innovation helped resolve some, but not all, of the controversies generated by these early investigators. At the end of the 19th century, complete pancreatectomy demonstrated the crucial physiological role of the pancreas and its link with diabetes. Pancreatic injury models, including pancreatectomy and ductal ligation, allowed investigators to describe islet function and to assess the regenerative capacity of the pancreas. Three main theories were proposed to explain the origins of newly formed islets: (i) transdifferentiation of acinar cells into islets, (ii) islet neogenesis, a process reminiscent of islet formation during embryonic development, and (iii) replication of preexisting islet cells. Despite considerable technical innovation in the last 50 years, the origin of new adult beta cells remains highly controversial and the same three theories are still debated today.

Islet-1 is required for the maturation, proliferation, and survival of the endocrine pancreas

OBJECTIVE

The generation of mature cell types during pancreatic development depends on the expression of many regulatory and signaling proteins. In this study, we tested the hypothesis that the transcriptional regulator Islet-1 (Isl-1), whose expression is first detected in the mesenchyme and epithelium of the developing pancreas and is later restricted to mature islet cells, is involved in the terminal differentiation of islet cells and maintenance of islet mass.

RESEARCH DESIGN AND METHODS

To investigate the role of Isl-1 in the pancreatic epithelium during the secondary transition, Isl-1 was conditionally and specifically deleted from embryonic day 13.5 onward using Cre/LoxP technology.

RESULTS

Isl-1-deficient endocrine precursors failed to mature into functional islet cells. The postnatal expansion of endocrine cell mass was impaired, and consequently Isl-1 deficient mice were diabetic. In addition, MafA, a potent regulator of the Insulin gene and beta-cell function, was identified as a direct transcriptional target of Isl-1.

CONCLUSIONS

These results demonstrate the requirement for Isl-1 in the maturation, proliferation, and survival of the second wave of hormone-producing islet cells.

Distinct populations of quiescent and proliferative pancreatic beta-cells identified by HOTcre mediated labeling

Pancreatic beta-cells are critical regulators of glucose homeostasis, and they vary dramatically in their glucose stimulated metabolic response and levels of insulin secretion. It is unclear whether these parameters are influenced by the developmental origin of individual beta-cells. Using HOTcre, a Cre-based genetic switch that uses heat-induction to precisely control the temporal expression of transgenes, we labeled two populations of beta-cells within the developing zebrafish pancreas. These populations originate in distinct pancreatic buds and exhibit gene expression profiles suggesting distinct functions during development. We find that the dorsal bud derived beta-cells are quiescent and exhibit a marked decrease in insulin expression postembryonically. In contrast, ventral bud derived beta-cells proliferate actively, and maintain high levels of insulin expression compared with dorsal bud derived beta-cells. Therapeutic strategies to regulate beta-cell proliferation and function are required to cure pathological states that result from excessive beta-cell proliferation (e.g., insulinoma) or insufficient beta-cell mass (e.g., diabetes mellitus). Our data reveal the existence of distinct populations of beta-cells in vivo and should help develop better strategies to regulate beta-cell differentiation and proliferation.

The ectopic expression of Pax4 in the mouse pancreas converts progenitor cells into alpha and subsequently beta cells

We have previously reported that the loss of Arx and/or Pax4 gene activity leads to a shift in the fate of the different endocrine cell subtypes in the mouse pancreas, without affecting the total endocrine cell numbers. Here, we conditionally and ectopically express Pax4 using different cell-specific promoters and demonstrate that Pax4 forces endocrine precursor cells, as well as mature alpha cells, to adopt a beta cell destiny. This results in a glucagon deficiency that provokes a compensatory and continuous glucagon+ cell neogenesis requiring the re-expression of the proendocrine gene Ngn3. However, the newly formed alpha cells fail to correct the hypoglucagonemia since they subsequently acquire a beta cell phenotype upon Pax4 ectopic expression. Notably, this cycle of neogenesis and redifferentiation caused by ectopic expression of Pax4 in alpha cells is capable of restoring a functional beta cell mass and curing diabetes in animals that have been chemically depleted of beta cells.

The diabetes gene Pdx1 regulates the transcriptional network of pancreatic endocrine progenitor cells in mice

Heterozygous mutations in the gene encoding the pancreatic homeodomain transcription factor pancreatic duodenal homeobox 1 (PDX1) are associated with maturity onset diabetes of the young, type 4 (MODY4) and type 2 diabetes. Pdx1 governs the early embryonic development of the pancreas and the later differentiation of the insulin-producing islet beta cells of the endocrine compartment. We derived a Pdx1 hypomorphic allele that reveals a role for Pdx1 in the specification of endocrine progenitors. Mice homozygous for this allele displayed a selective reduction in endocrine lineages associated with decreased numbers of endocrine progenitors and a marked reduction in levels of mRNA encoding the proendocrine transcription factor neurogenin 3 (Ngn3). During development, Pdx1 occupies an evolutionarily conserved enhancer region of Ngn3 and interacts with the transcription factor one cut homeobox 1 (Hnf6) to activate this enhancer. Furthermore, mRNA levels of all 4 members of the transcription factor network that regulates Ngn3 expression, SRY-box containing gene 9 (Sox9), Hnf6, Hnf1b, and forkhead box A2 (Foxa2), were decreased in homozygous mice. Pdx1 also occupied regulatory sequences in Foxa2 and Hnf1b. Thus, Pdx1 contributes to specification of endocrine progenitors both by regulating expression of Ngn3 directly and by participating in a cross-regulatory transcription factor network during early pancreas development. These results provide insights that may be applicable to beta cell replacement strategies involving the guided differentiation of ES cells or other progenitor cell types into the beta cell lineage, and they suggest a molecular mechanism whereby human PDX1 mutations cause diabetes.

Notch-responsive cells initiate the secondary transition in larval zebrafish pancreas

Zebrafish provide a highly versatile model in which to study vertebrate development. Many recent studies have elucidated early events in the organogenesis of the zebrafish pancreas; however, several aspects of early endocrine pancreas formation in the zebrafish are not homologous to the mammalian system. To better identify mechanisms of islet formation in the zebrafish, with true homology to those observed in mammals, we have temporally and spatially characterized zebrafish secondary islet formation. As is the case in the mouse, we show that Notch inhibition leads to precocious differentiation of endocrine tissues. Furthermore, we have used transgenic fish expressing fluorescent markers under the control of a Notch-responsive element to observe the precursors of these induced endocrine cells. These pancreatic Notch-responsive cells represent a novel population of putative progenitors that are associated with larval pancreatic ductal epithelium, suggesting functional homology between secondary islet formation in zebrafish and the secondary transition in mammals. We also show that Notch-responsive cells persist in the adult pancreas and possess the classical characteristics of centroacinar cells, a cell type believed to be a multipotent progenitor cell in adult mammalian pancreas.

Regulation of pancreatic juxtaductal endocrine cell formation by FoxO1

An understanding of the mechanisms that govern pancreatic endocrine cell ontogeny may offer strategies for their somatic replacement in diabetic patients. During embryogenesis, transcription factor FoxO1 is expressed in pancreatic progenitor cells. Subsequently, it becomes restricted to beta cells and to a rare population of insulin-negative juxtaductal cells (FoxO1+ Ins(-)). It is unclear whether FoxO1+ Ins(-) cells give rise to endocrine cells. To address this question, we first evaluated FoxO1's role in pancreas development using gain- and loss-of-function alleles in mice. Premature FoxO1 activation in pancreatic progenitors promoted alpha-cell formation but curtailed exocrine development. Conversely, FoxO1 ablation in pancreatic progenitor cells, but not in committed endocrine progenitors or terminally differentiated beta cells, selectively increased juxtaductal beta cells. As these data indicate an involvement of FoxO1 in pancreatic lineage determination, FoxO1+ Ins(-) cells were clonally isolated and assayed for their capacity to undergo endocrine differentiation. Upon FoxO1 activation, FoxO1+ Ins(-) cultures converted into glucagon-producing cells. We conclude that FoxO1+ Ins(-) juxtaductal cells represent a hitherto-unrecognized pancreatic cell population with in vitro capability of endocrine differentiation.

Stem cells to pancreatic beta-cells: new sources for diabetes cell therapy

The number of patients worldwide suffering from the chronic disease diabetes mellitus is growing at an alarming rate. Insulin-secreting beta-cells in the islet of Langerhans are damaged to different extents in diabetic patients, either through an autoimmune reaction present in type 1 diabetic patients or through inherent changes within beta-cells that affect their function in patients suffering from type 2 diabetes. Cell replacement strategies via islet transplantation offer potential therapeutic options for diabetic patients. However, the discrepancy between the limited number of donor islets and the high number of patients who could benefit from such a treatment reflects the dire need for renewable sources of high-quality beta-cells. Human embryonic stem cells (hESCs) are capable of self-renewal and can differentiate into components of all three germ layers, including all pancreatic lineages. The ability to differentiate hESCs into beta-cells highlights a promising strategy to meet the shortage of beta-cells. Here, we review the different approaches that have been used to direct differentiation of hESCs into pancreatic and beta-cells. We will focus on recent progress in the understanding of signaling pathways and transcription factors during embryonic pancreas development and how this knowledge has helped to improve the methodology for high-efficiency beta-cell differentiation in vitro.

Polycomb protein Ezh2 regulates pancreatic beta-cell Ink4a/Arf expression and regeneration in diabetes mellitus

Proliferation of pancreatic islet beta cells is an important mechanism for self-renewal and for adaptive islet expansion. Increased expression of the Ink4a/Arf locus, which encodes the cyclin-dependent kinase inhibitor p16(INK4a) and tumor suppressor p19(Arf), limits beta-cell regeneration in aging mice, but the basis of beta-cell Ink4a/Arf regulation is poorly understood. Here we show that Enhancer of zeste homolog 2 (Ezh2), a histone methyltransferase and component of a Polycomb group (PcG) protein complex, represses Ink4a/Arf in islet beta cells. Ezh2 levels decline in aging islet beta cells, and this attrition coincides with reduced histone H3 trimethylation at Ink4a/Arf, and increased levels of p16(INK4a) and p19(Arf). Conditional deletion of beta-cell Ezh2 in juvenile mice also reduced H3 trimethylation at the Ink4a/Arf locus, leading to precocious increases of p16(INK4a) and p19(Arf). These mutant mice had reduced beta-cell proliferation and mass, hypoinsulinemia, and mild diabetes, phenotypes rescued by germline deletion of Ink4a/Arf. beta-Cell destruction with streptozotocin in controls led to increased Ezh2 expression that accompanied adaptive beta-cell proliferation and re-establishment of beta-cell mass; in contrast, mutant mice treated similarly failed to regenerate beta cells, resulting in lethal diabetes. Our discovery of Ezh2-dependent beta-cell proliferation revealed unique epigenetic mechanisms underlying normal beta-cell expansion and beta-cell regenerative failure in diabetes pathogenesis.

Developmental biology of the pancreas

In this review, I summarize some aspects of murine pancreas development, with particular emphasis on the analysis of the ontogenetic relationships between different pancreatic cell types. Lineage analyses allow the identification of the progenitor cells from which mature cell types arise. The identification and successful in vitro culture of putative pancreatic stem cells is highly relevant for future cell replacement therapies in diabetic patients.

Insulin signaling in alpha cells modulates glucagon secretion in vivo

Glucagon plays an important role in glucose homeostasis by regulating hepatic glucose output in both normo- and hypoglycemic conditions. In this study, we created and characterized alpha cell-specific insulin receptor knockout (alphaIRKO) mice to directly explore the role of insulin signaling in the regulation of glucagon secretion in vivo. Adult male alphaIRKO mice exhibited mild glucose intolerance, hyperglycemia, and hyperglucagonemia in the fed state and enhanced glucagon secretion in response to L-arginine stimulation. Hyperinsulinemic-hypoglycemic clamp studies revealed an enhanced glucagon secretory response and an abnormal norepinephrine response to hypoglycemia in alphaIRKO mice. The mutants also exhibited an age-dependent increase in beta cell mass. Furthermore, siRNA-mediated knockdown of insulin receptor in glucagon-secreting InR1G cells promoted enhanced glucagon secretion and complemented our in vivo findings. Together, these data indicate a significant role for intraislet insulin signaling in the regulation of alpha cell function in both normo- and hypoglycemic conditions.

Regulation of pancreas plasticity and malignant transformation by Akt signaling

BACKGROUND & AIMS

Extensive evidence suggests that Akt signaling plays an important role in beta-cell mass and function, although its function in the regulation of the different pancreatic fates has not been adequately investigated. The goal of these studies was to assess the role of Akt signaling in the pancreatic differentiation programs.

METHODS

For these experiments, we have generated a double reporter mouse model that provides activation of Akt signaling in a cell type-specific manner. This mouse model conditionally overexpresses a constitutively active form of Akt upon Cre-mediated recombination. Activation of Akt signaling in pancreatic progenitors and acinar and beta-cells was achieved by crossing this animal model to specific Cre-lines.

RESULTS

We showed that overexpression of a constitutively active Akt in pancreatic and duodenal homeobox 1 (Pdx1) progenitors induced expansion of ductal structures expressing progenitor markers. This expansion resulted in part from increased proliferation of the ductal epithelium. Lineage-tracing experiments in mice with activation of Akt signaling in mature acinar and beta-cells suggested that acinar-to-ductal and beta-cell-to-acinar/ductal transdifferentiation also contributed to the expansion of the ductal compartment. In addition to the changes in cell plasticity, these studies demonstrated that chronic activation of Akt signaling in Pdx1 progenitors induced the development of premalignant lesions and malignant transformation in old mice.

CONCLUSIONS

The current work unravels some of the molecular mechanisms of cellular plasticity and reprogramming, and demonstrates for the first time that activation of Akt signaling regulates the fate of differentiated pancreatic cells in vivo.

Recovery of endogenous beta-cell function in nonhuman primates after chemical diabetes induction and islet transplantation

OBJECTIVE

To describe the ability of nonhuman primate endocrine pancreata to reestablish endogenous insulin production after chemical beta-cell destruction.

RESEARCH DESIGN AND METHODS

Eleven monkeys (Macaca fascicularis) were rendered diabetic with streptozotocin. Eight diabetic monkeys received intraportal porcine islet transplantation.

RESULTS

Two monkeys transplanted after 75 days of type 1 diabetes showed recovery of endogenous C-peptide production a few weeks after transplantation, concomitant with graft failure. Histological analysis of the pancreas of these monkeys showed insulin-positive cells, single or in small aggregates, scattered in the pancreas and adjacent to ducts. Interestingly, numerous CK19(+) cells costained with proinsulin and PDX-1 antibodies. Furthermore, the peculiar double phenotype glucagon-positive/GLUT2(+) was observed. In these monkeys as well as in all others, the original islets showed no insulin staining.

CONCLUSIONS

Our data provide evidence that, in nonhuman primates, the pancreas can reestablish endogenous insulin production after chemical beta-cell destruction. This seems to be a nongeneralizable event with only 2 out of 11 monkeys recovering beta-cell function. In these two monkeys, younger age and islet graft behavior might have played a role in triggering endogenous beta-cell recovery.

A role for von Hippel-Lindau protein in pancreatic beta-cell function

OBJECTIVE

The Vhlh gene codes for the von Hippel-Lindau protein (VHL), a tumor suppressor that is a key player in the cellular response to oxygen sensing. In humans, a germline mutation in the VHL gene leads to the von Hippel-Lindau disease, a familial syndrome characterized by benign and malignant tumors of the kidney, central nervous system, and pancreas.

RESEARCH DESIGN AND METHODS

We use Cre-lox recombination to eliminate Vhlh in adult mouse pancreatic beta-cells. Morphology of mutant islets is assessed by immunofluorescence analysis. To determine the functional state of Vhlh(-/-) islets, insulin secretion is measured in vivo and in vitro, and quantitative PCR is used to identify changes in gene expression.

RESULTS

Loss of VHL in beta-cells leads to a severe glucose-intolerant phenotype in adult animals. Although VHL is not required for beta-cell specification and development, it is critical for beta-cell function. Insulin production is normal in beta-cells lacking VHL; however, insulin secretion in the presence of high concentrations of glucose is impaired. Furthermore, the loss of VHL leads to dysregulation of glycolytic enzymes, pointing to a perturbation of the intracellular energy homeostasis.

CONCLUSIONS

We show that loss of VHL in beta-cells leads to defects in glucose homeostasis, indicating an important and previously unappreciated role for VHL in beta-cell function. We believe that the beta-cell-specific Vhlh-deficient mice might be a useful tool as a "genetic hypoxia" model, to unravel the possible link between hypoxia signaling and impairment of beta-cell function.

Expression of an activating mutation in the gene encoding the KATP channel subunit Kir6.2 in mouse pancreatic beta cells recapitulates neonatal diabetes

Neonatal diabetes is a rare monogenic form of diabetes that usually presents within the first six months of life. It is commonly caused by gain-of-function mutations in the genes encoding the Kir6.2 and SUR1 subunits of the plasmalemmal ATP-sensitive K+ (KATP) channel. To better understand this disease, we generated a mouse expressing a Kir6.2 mutation (V59M) that causes neonatal diabetes in humans and we used Cre-lox technology to express the mutation specifically in pancreatic beta cells. These beta-V59M mice developed severe diabetes soon after birth, and by 5 weeks of age, blood glucose levels were markedly increased and insulin was undetectable. Islets isolated from beta-V59M mice secreted substantially less insulin and showed a smaller increase in intracellular calcium in response to glucose. This was due to a reduced sensitivity of KATP channels in pancreatic beta cells to inhibition by ATP or glucose. In contrast, the sulfonylurea tolbutamide, a specific blocker of KATP channels, closed KATP channels, elevated intracellular calcium levels, and stimulated insulin release in beta-V59M beta cells, indicating that events downstream of KATP channel closure remained intact. Expression of the V59M Kir6.2 mutation in pancreatic beta cells alone is thus sufficient to recapitulate the neonatal diabetes observed in humans. beta-V59M islets also displayed a reduced percentage of beta cells, abnormal morphology, lower insulin content, and decreased expression of Kir6.2, SUR1, and insulin mRNA. All these changes are expected to contribute to the diabetes of beta-V59M mice. Their cause requires further investigation.

Pancreatic inactivation of c-Myc decreases acinar mass and transdifferentiates acinar cells into adipocytes in mice

BACKGROUND & AIMS

The pancreatic mass is determined by the coordinated expansion and differentiation of progenitor cells and is maintained via tight control of cell replacement rates. The basic helix-loop-helix transcription factor c-Myc is one of the main regulators of these processes in many organs. We studied the requirement of c-Myc in controlling the generation and maintenance of pancreatic mass.

METHODS

We conditionally inactivated c-Myc in Pdx1+ pancreatic progenitor cells. Pancreata of mice lacking c-Myc (c-Myc(P-/-) mice) were analyzed during development and ageing.

RESULTS

Pancreatic growth in c-Myc(P-/-) mice was impaired starting on E12.5, in early primordia, because of decreased proliferation and altered differentiation of exocrine progenitors; islet progenitors were spared. Acinar cell maturation was defective in the adult hypotrophic pancreas, which hampered exocrine mass maintenance in aged animals. From 2 to 10 months of age, the c-Myc(P-/-) pancreas was progressively remodeled without inflammatory injury. Loss of acinar cells increased with time, concomitantly with adipose tissue accumulation. Using a genetic cell lineage tracing analysis, we demonstrated that pancreatic adipose cells were derived directly from transdifferentiating acinar cells. This epithelial-to-mesenchyme transition was also observed in normal aged specimens and in pancreatitis.

CONCLUSIONS

These results provide evidence indicating that c-Myc activity is required for growth and maturation of the exocrine pancreas, and sheds new light on the ontogeny of pancreatic adipose cells in processes of organ degenerescence and tissue involution.

Deletion of the von Hippel-Lindau gene in pancreatic beta cells impairs glucose homeostasis in mice

Defective insulin secretion in response to glucose is an important component of the beta cell dysfunction seen in type 2 diabetes. As mitochondrial oxidative phosphorylation plays a key role in glucose-stimulated insulin secretion (GSIS), oxygen-sensing pathways may modulate insulin release. The von Hippel-Lindau (VHL) protein controls the degradation of hypoxia-inducible factor (HIF) to coordinate cellular and organismal responses to altered oxygenation. To determine the role of this pathway in controlling glucose-stimulated insulin release from pancreatic beta cells, we generated mice lacking Vhl in pancreatic beta cells (betaVhlKO mice) and mice lacking Vhl in the pancreas (PVhlKO mice). Both mouse strains developed glucose intolerance with impaired insulin secretion. Furthermore, deletion of Vhl in beta cells or the pancreas altered expression of genes involved in beta cell function, including those involved in glucose transport and glycolysis, and isolated betaVhlKO and PVhlKO islets displayed impaired glucose uptake and defective glucose metabolism. The abnormal glucose homeostasis was dependent on upregulation of Hif-1alpha expression, and deletion of Hif1a in Vhl-deficient beta cells restored GSIS. Consistent with this, expression of activated Hif-1alpha in a mouse beta cell line impaired GSIS. These data suggest that VHL/HIF oxygen-sensing mechanisms play a critical role in glucose homeostasis and that activation of this pathway in response to decreased islet oxygenation may contribute to beta cell dysfunction.

Four-dimensional realistic modeling of pancreatic organogenesis

Organogenesis, the process by which organs develop from individual precursor stem cells, requires that the precursor cells proliferate, differentiate, and aggregate to form a functioning structure. This process progresses through changes in 4 dimensions: time and 3 dimensions of space-4D. Experimental analysis of organogenesis, by its nature, cuts the 4D developmental process into static, 2D histological images or into molecular or cellular markers and interactions with little or no spatial dimensionality and minimal dynamics. Understanding organogenesis requires integration of the piecemeal experimental data into a running, realistic and interactive 4D simulation that allows experimentation and hypothesis testing in silico. Here, we describe a fully executable, interactive, visual model for 4D simulation of organogenic development using the mouse pancreas as a representative case. Execution of the model provided a dynamic description of pancreas development, culminating in a structure that remarkably recapitulated morphologic features seen in the embryonic pancreas. In silico mutations in key signaling molecules resulted in altered patterning of the developing pancreas that were in general agreement with in vivo data. The modeling approach described here thus typifies a useful platform for studying organogenesis as a phenomenon in 4 dimensions.

Deletion of glutamate dehydrogenase in beta-cells abolishes part of the insulin secretory response not required for glucose homeostasis

Insulin exocytosis is regulated in pancreatic ss-cells by a cascade of intracellular signals translating glucose levels into corresponding secretory responses. The mitochondrial enzyme glutamate dehydrogenase (GDH) is regarded as a major player in this process, although its abrogation has not been tested yet in animal models. Here, we generated transgenic mice, named betaGlud1(-/-), with ss-cell-specific GDH deletion. Our results show that GDH plays an essential role in the full development of the insulin secretory response. In situ pancreatic perfusion revealed that glucose-stimulated insulin secretion was reduced by 37% in betaGlud1(-/-). Furthermore, isolated islets with either constitutive or acute adenovirus-mediated knock-out of GDH showed a 49 and 38% reduction in glucose-induced insulin release, respectively. Adenovirus-mediated re-expression of GDH in betaGlud1(-/-) islets fully restored glucose-induced insulin release. Thus, GDH appears to account for about 40% of glucose-stimulated insulin secretion and to lack redundant mechanisms. In betaGlud1(-/-) mice, the reduced secretory capacity resulted in lower plasma insulin levels in response to both feeding and glucose load, while body weight gain was preserved. The results demonstrate that GDH is essential for the full development of the secretory response in beta-cells. However, maximal secretory capacity is not required for maintenance of glucose homeostasis in normo-caloric conditions.

PVHL is a regulator of glucose metabolism and insulin secretion in pancreatic beta cells

Insulin secretion from pancreatic beta cells is stimulated by glucose metabolism. However, the relative importance of metabolizing glucose via mitochondrial oxidative phosphorylation versus glycolysis for insulin secretion remains unclear. von Hippel-Lindau (VHL) tumor suppressor protein, pVHL, negatively regulates hypoxia-inducible factor HIF1alpha, a transcription factor implicated in promoting a glycolytic form of metabolism. Here we report a central role for the pVHL-HIF1alpha pathway in the control of beta-cell glucose utilization, insulin secretion, and glucose homeostasis. Conditional inactivation of Vhlh in beta cells promoted a diversion of glucose away from mitochondria into lactate production, causing cells to produce high levels of glycolytically derived ATP and to secrete elevated levels of insulin at low glucose concentrations. Vhlh-deficient mice exhibited diminished glucose-stimulated changes in cytoplasmic Ca(2+) concentration, electrical activity, and insulin secretion, which culminate in impaired systemic glucose tolerance. Importantly, combined deletion of Vhlh and Hif1alpha rescued these phenotypes, implying that they are the result of HIF1alpha activation. Together, these results identify pVHL and HIF1alpha as key regulators of insulin secretion from pancreatic beta cells. They further suggest that changes in the metabolic strategy of glucose metabolism in beta cells have profound effects on whole-body glucose homeostasis.

Control of pancreatic development by intercellular signals

Understanding pancreatic development is important for at least three reasons: first, from a cognitive point of view, to understand the development of a complex organ, the pancreas; next, because it is now clear that abnormal pancreatic development can give rise to specific forms of diabetes in humans; and finally, because, if we want to define new treatments for diabetes based on cell therapy or regenerative medicine, we will have to understand in detail how beta-cells develop. In the present paper, we summarize what we currently know concerning pancreatic development and concentrate on some intercellular and environmental signals controlling pancreatic development.