The morphogenesis of the pancreatic mesenchyme is uncoupled from that of the pancreatic epithelium in IPF1/PDX1-deficient mice

Expression and localization of mesothelin in developing rat pancreas

To define a genetic network that regulates development of the pancreas, we used high-density microarray (Affymetrix) to generate transcriptional profiles of rat pancreas from five biologically significant stages of development: embryonic day 12.5 (E12.5), E15.5, E18.5, postnatal day 0 (P0) and adult. Many genes were notably highly expressed in the later gestation when islet architecture and function are gradually forming. The expression and localization of mesothelin, one of these genes, was further examined. Reverse transcription-polymerase chain reaction and Western blot analysis revealed that mRNA and protein levels of mesothelin were high from later gestation to 2-3 weeks after birth, and with relatively low but detectable expression levels in adult rat pancreas. Immunolocalization indicated that mesothelin localized not only in islet beta-cells but also in the mesenchyme of developing rat pancreas. Transient mesothelin expression was concomitant with the development of islets architecture formation, remodeling and maturation. These findings indicate that mesothelin is dynamically expressed in the developing rat pancreas and that mesothelin might be involved in some developmental events during development of rat pancreas.

Biphasic Ngn3 expression in the developing pancreas

Ngn3 is a bHLH transcription factor critical for the specification of endocrine cells in the pancreatic Islets of Langerhans. Previous studies in mouse embryos have reported transient expression of Ngn3 in scattered cells within the developing pancreatic epithelium during midgestation (Schwitzgebel et al. [2000] Development 127:3533-3542). Specifically, these Ngn3-expressing cells have been shown to be progenitor cells fated to give rise to islet endocrine cells (Gradwohl et al. [2000] Proc Natl Acad Sci USA 97:1607-1611). Here, we characterize the expression of Ngn3 transcripts and protein throughout pancreatic development. Interestingly, we identify and define a dramatic and previously unnoticed gap in developmental Ngn3 expression. We show that both Ngn3 transcript and protein expression occur in two distinct temporal waves, the first occurring early from approximately E8.5 to E11.0, and the second initiating at approximately E12.0. Strikingly, this observed biphasic expression correlates with the "first" and "second" transitions, which encompass two distinct waves of embryonic endocrine differentiation. In addition, our studies demonstrate that Ngn3 transcripts are markedly more widespread in the pancreatic epithelium than NGN3 protein, indicating that post-transcriptional regulation is likely to play a critical role during endocrine differentiation.

MFng is dispensable for mouse pancreas development and function

Notch signaling regulates pancreatic cell differentiation, and mutations of various Notch signaling components result in perturbed pancreas development. Members of the Fringe family of beta1,3-N-acetylglucosaminyltransferases, Manic Fringe (MFng), Lunatic Fringe (LFng), and Radical Fringe (RFng), modulate Notch signaling, and MFng has been suggested to regulate pancreatic endocrine cell differentiation. We have characterized the expression of the three mouse Fringe genes in the developing mouse pancreas between embryonic days 9 and 14 and show that the expression of MFng colocalized with the proendocrine transcription factor Ngn3. In contrast, the expression of LFng colocalized with the exocrine marker Ptf1a, whereas RFng was not expressed. Moreover, we show that expression of MFng is lost in Ngn3 mutant mice, providing evidence that MFng is genetically downstream of Ngn3. Gain- and loss-of-function analyses of MFng by the generation of mice that overexpress MFng in early pancreatic progenitor cells and mice with a targeted deletion of MFng provide, however, evidence that MFng is dispensable for pancreas development and function, since no pancreatic defects in these mice were observed.

Growth factor control of pancreatic islet regeneration and function

Type 1 and type 2 diabetes mellitus together are predicted to affect over 300 million people worldwide by the year 2020. A relative or absolute paucity of functional β-cells is a central feature of both types of disease, and identifying the pathways that mediate the embryonic origin of new β-cells and mechanisms that underlie the proliferation of existing β-cells are major efforts in the fields of developmental and islet biology. A poor secretory response of existing β-cells to nutrients and hormones and the defects in hormone processing also contribute to the hyperglycemia observed in type 2 diabetes and has prompted studies aimed at enhancing β-cell function. The factors that contribute to a greater susceptibility in aging individuals to develop diabetes is currently unclear and may be linked to a poor turnover of β-cells and/or enhanced susceptibility of β-cells to apoptosis. This review is an update on the recent work in the areas of islet/β-cell regeneration and hormone processing that are relevant to the pathophysiology of the endocrine pancreas in type 1, type 2 and obesity-associated diabetes.

Transcription factors as therapeutic targets for diabetes

BACKGROUND

Islet cell implantation and pancreas transplantation have been used as treatments for diabetes but are limited by the shortage of donors and the requirement for lifelong immunosuppression. As an alternative, the generation of surrogate insulin-producing cells has been an area of interest for many researchers. Understanding how pancreatic beta-cells are generated during pancreas development will provide information that can be applied to generating surrogate beta-cells.

OBJECTIVE

To outline the current knowledge of pancreas development and differentiation, with a focus on the regulatory network of pancreas-enriched transcription factors and their targets.

METHODS

A review of relevant literature.

CONCLUSIONS

Pancreatic and duodenal homeobox 1 (Pdx1), Neurogenin 3 (Ngn3), and musculoaponeurotic fibrosarcoma oncogene homolog A (MafA) have been shown to play essential roles in pancreas development and beta-cell differentiation, and gain-of-function approaches indicate the potency of these factors for inducing differentiation of non-beta-cells into insulin-producing cells, which could lead to a novel therapy to cure diabetes.

The homeodomain-interacting protein kinase 2 regulates insulin promoter factor-1/pancreatic duodenal homeobox-1 transcriptional activity

The homeodomain transcription factor insulin promoter factor (IPF)-1/pancreatic duodenal homeobox (PDX)-1 plays a crucial role in both pancreas development and maintenance of beta-cell function. Targeted disruption of the Ipf1/Pdx1 gene in beta-cells of mice leads to overt diabetes and reduced Ipf1/Pdx1 gene expression results in decreased insulin expression and secretion. In humans, mutations in the IPF1 gene have been linked to diabetes. Hence, the identification of molecular mechanisms regulating the transcriptional activity of this key transcription factor is of great interest. Herein we analyzed homeodomain-interacting protein kinase (Hipk) 2 expression in the embryonic and adult pancreas by in situ hybridization and RT-PCR. Moreover, we functionally characterized the role of HIPK2 in regulating IPF1/PDX1 transcriptional activity by performing transient transfection experiments and RNA interference. We show that Hipk2 is expressed in the developing pancreatic epithelium from embryonic d 12-15 but that the expression becomes preferentially confined to pancreatic endocrine cells at later developmental stages. Moreover, we show that HIPK2 positively influences IPF1/PDX1 transcriptional activity and that the kinase activity of HIPK2 is required for this effect. We also demonstrate that HIPK2 directly phosphorylates the C-terminal portion of IPF1/PDX1. Taken together, our data provide evidence for a new mechanism by which IPF1/PDX1 transcriptional activity, and thus possibly pancreas development and/or beta-cell function, is regulated.

Hox, Cdx, and anteroposterior patterning in the mouse embryo

Cdx and Hox gene families descend from the same ProtoHox cluster, already present in the common ancestors of bilaterians and cnidarians, and thought to act by providing anteroposterior (A-P) positional identity to axial tissues in all bilaterians. Mouse Cdx and Hox genes still exhibit common features in their early expression and function. The initiation and early shaping of Hox and Cdx transcriptional domains in mouse embryos are very similar, in keeping with their common involvement in conveying A-P information to the nascent tissues during embryonic axial elongation. Considerations of the impact on axial patterning of the early expression phase of these genes that correlates with the temporally collinear expression of 3'-5'Hox genes suggest that it is concerned with the acquisition of A-P information by the three germ layers as the axis extends. This early A-P information acquired by all cells emerging from the primitive streak or tailbud and their neighbors in the caudal neural plate gets further modulated by the second phase of gene expression occurring later as the tissues mature and differentiate along the growing axis. We discuss the possibility that regulatory phase 1, common to all Cdx and Hox genes, is inherent to the concerted mechanism sequentially turning on 3'-5'Hox genes at early stages, and keeping expression of the initiated genes subsequently in the new materials added posteriorly at the axis extends. The posterior Hox gene expression domain would be subsequently complemented by Hox regulatory phase 2, consisting in a variety of gene-specific, region-specific, and/or tissue-specific gene expression controls. We also touch on the unanswered question whether vertebrate Cdx gene expression delivers A-P positional information in its own right, as Caudal does in Drosophila, or whether it does so exclusively by upregulating Hox genes.

Generation and regeneration of cells of the liver and pancreas

Liver and pancreas progenitors develop from endoderm cells in the embryonic foregut. Shortly after their specification, liver and pancreas progenitors rapidly acquire markedly different cellular functions and regenerative capacities. These changes are elicited by inductive signals and genetic regulatory factors that are highly conserved among vertebrates. Interest in the development and regeneration of the organs has been fueled by the intense need for hepatocytes and pancreatic beta cells in the therapeutic treatment of liver failure and type I diabetes. Studies in diverse model organisms have revealed evolutionarily conserved inductive signals and transcription factor networks that elicit the differentiation of liver and pancreatic cells and provide guidance for how to promote hepatocyte and beta cell differentiation from diverse stem and progenitor cell types.

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.

Beta-cell development: the role of intercellular signals

Understanding in detail how pancreatic endocrine cells develop is important for many reasons. From a scientific point of view, elucidation of such a complex process is a major challenge. From a more applied point of view, this may help us to better understand and treat specific forms of diabetes. Although a variety of therapeutic approaches are well validated, no cure for diabetes is available. Many arguments indicate that the development of new strategies to cure diabetic patients will require precise understanding of the way beta-cells form during development. This is obvious for a future cell therapy using beta-cells produced from embryonic stem cells. This also holds true for therapeutic approaches based on regenerative medicine. In this review, we summarize our current knowledge concerning pancreatic development and focus on the role of extracellular signals implicated in beta-cell development from pancreatic progenitors.

Developmental biology of the pancreas: a comprehensive review

Pancreatic development represents a fascinating process in which two morphologically distinct tissue types must derive from one simple epithelium. These two tissue types, exocrine (including acinar cells, centro-acinar cells, and ducts) and endocrine cells serve disparate functions, and have entirely different morphology. In addition, the endocrine tissue must become disconnected from the epithelial lining during its development. The pancreatic development field has exploded in recent years, and numerous published reviews have dealt specifically with only recent findings, or specifically with certain aspects of pancreatic development. Here I wish to present a more comprehensive review of all aspects of pancreatic development, though still there is not a room for discussion of stem cell differentiation to pancreas, nor for discussion of post-natal regeneration phenomena, two important fields closely related to pancreatic development.

Beta cells within single human islets originate from multiple progenitors

Background

In both humans and rodents, glucose homeostasis is controlled by micro-organs called islets of Langerhans composed of beta cells, associated with other endocrine cell types. Most of our understanding of islet cell differentiation and morphogenesis is derived from rodent developmental studies. However, little is known about human islet formation. The lack of adequate experimental models has restricted the study of human pancreatic development to the histological analysis of different stages of pancreatic development. Our objective was to develop a new experimental model to (i) transfer genes into developing human pancreatic cells and (ii) validate gene transfer by defining the clonality of developing human islets.

Methods and Findings

In this study, a unique model was developed combining ex vivo organogenesis from human fetal pancreatic tissue and cell type-specific lentivirus-mediated gene transfer. Human pancreatic progenitors were transduced with lentiviruses expressing GFP under the control of an insulin promoter and grafted to severe combined immunodeficient mice, allowing human beta cell differentiation and islet morphogenesis. By performing gene transfer at low multiplicity of infection, we created a chimeric graft with a subpopulation of human beta cells expressing GFP and found both GFP-positive and GFP-negative beta cells within single islets.

Conclusion

The detection of both labeled and unlabeled beta cells in single islets demonstrates that beta cells present in a human islet are derived from multiple progenitors thus providing the first dynamic analysis of human islet formation during development. This human transgenic-like tool can be widely used to elucidate dynamic genetic processes in human tissue formation.

Potential role of glucocorticoid signaling in the formation of pancreatic islets in the human fetus

Glucocorticoids have been suggested to play a role in programming late adult disorders like diabetes during fetal life. Recent work in rodents showed their role in pancreas development by modulating the expression of transcription factors. The aim of this work was to investigate their possible implication in human pancreas development. The ontogenesis of glucocorticoid receptor (GR) and several pancreatic transcription factors was studied by immunohistochemistry and RT-PCR on human fetal pancreatic specimens. At 6 wk of development (wd) insulin promoting factor 1 (IPF1) was expressed in the majority of epithelial cells forming tubular structures while GR was present in the mesenchyme, suggesting an early role of glucocorticoids, before endocrine and exocrine differentiation. Only GR alpha (active form) mRNA was expressed from 6 wk onwards while GR beta (inactive form) was never observed. The first insulin cells did not express IPF1 or GR. Islet formation occurred from 10 wd as IPF1-positive cells started to express simultaneously insulin and GR. This coexpression in beta cells persisted until adulthood. The mRNA expression profiles confirmed immunohistochemistry and showed the early expression of crucial transcription factors. In conclusion, the presence of the active GR isoform around islet formation supports the novel idea that glucocorticoids could modulate human pancreas development.

Glucose regulation of insulin gene expression in pancreatic β-cells

Production and secretion of insulin from the β-cells of the pancreas is very crucial in maintaining normoglycaemia. This is achieved by tight regulation of insulin synthesis and exocytosis from the β-cells in response to changes in blood glucose levels. The synthesis of insulin is regulated by blood glucose levels at the transcriptional and post-transcriptional levels. Although many transcription factors have been implicated in the regulation of insulin gene transcription, three β-cell-specific transcriptional regulators, Pdx-1 (pancreatic and duodenal homeobox-1), NeuroD1 (neurogenic differentiation 1) and MafA (V-maf musculoaponeurotic fibrosarcoma oncogene homologue A), have been demonstrated to play a crucial role in glucose induction of insulin gene transcription and pancreatic β-cell function. These three transcription factors activate insulin gene expression in a co-ordinated and synergistic manner in response to increasing glucose levels. It has been shown that changes in glucose concentrations modulate the function of these β-cell transcription factors at multiple levels. These include changes in expression levels, subcellular localization, DNA-binding activity, transactivation capability and interaction with other proteins. Furthermore, all three transcription factors are able to induce insulin gene expression when expressed in non-β-cells, including liver and intestinal cells. The present review summarizes the recent findings on how glucose modulates the function of the β-cell transcription factors Pdx-1, NeuroD1 and MafA, and thereby tightly regulates insulin synthesis in accordance with blood glucose levels.

Promoting ectopic pancreatic fates: pancreas development and future diabetes therapies

Diabetes is a disease that could be treated more effectively with a better understanding of pancreas development. This review examines the role of master regulator genes driving crucial steps in pancreas development, from foregut specification to differentiation of the five endocrine cell types. The roles of Pdx1, Ptf1a, and Ngn3 are particularly examined as they are both necessary and sufficient for promoting pancreatic cell fates (Pdx1, Ptf1a) and endocrine cell development (Ngn3). The roles of Arx and Pax4 are studied as they compose part of the regulatory mechanism balancing development of different types of endocrine cells within the iselts and promote the development of alpha/PP and beta/delta cell progenitors, respectively. The roles of the aforementioned genes, and the consequences of misexpression of them for functionality of the pancreas, are examined through recent studies in model organisms, particularly Xenopus and zebrafish. Recent developments in cell replacement therapy research are also covered, concentrating on stem cell research (coaxing both adult and embryonic stem cells toward a beta cell fate) and transdifferentiation (generating beta cells from other differentiated cell types).

Cholesterol accumulation and diabetes in pancreatic beta-cell-specific SREBP-2 transgenic mice: a new model for lipotoxicity

To determine the role of cholesterol synthesis in pancreatic beta-cells, a transgenic model of in vivo activation of sterol-regulatory element binding protein 2 (SREBP-2) specifically in beta-cells (TgRIP-SREBP-2) was developed and analyzed. Expression of nuclear human SREBP-2 in beta-cells resulted in severe diabetes as evidenced by greater than 5-fold elevations in glycohemoglobin compared with C57BL/6 controls. Diabetes in TgRIP-SREBP-2 mice was primarily due to defects in glucose- and potassium-stimulated insulin secretion as determined by glucose tolerance test. Isolated islets of TgSREBP-2 mice were fewer in number, smaller, deformed, and had decreased insulin content. SREBP-2-expressing islets also contained increased esterified cholesterol and unchanged triglycerides with reduced ATP levels. Consistently, these islets exhibited elevated expression of HMG-CoA synthase and reductase and LDL receptor, with suppression of endogenous SREBPs. Genes involved in beta-cell differentiation, such as PDX1 and BETA2, were suppressed, explaining loss of beta-cell mass, whereas IRS2 expression was not affected. These phenotypes were dependent on the transgene expression. Taken together, these results indicate that activation of SREBP-2 in beta-cells caused severe diabetes by loss of beta-cell mass with accumulation of cholesterol, providing a new lipotoxic model and a potential link of disturbed cholesterol metabolism to impairment of beta-cell function.

On the origin of the beta cell

The major forms of diabetes are characterized by pancreatic islet beta-cell dysfunction and decreased beta-cell numbers, raising hope for cell replacement therapy. Although human islet transplantation is a cell-based therapy under clinical investigation for the treatment of type 1 diabetes, the limited availability of human cadaveric islets for transplantation will preclude its widespread therapeutic application. The result has been an intense focus on the development of alternate sources of beta cells, such as through the guided differentiation of stem or precursor cell populations or the transdifferentiation of more plentiful mature cell populations. Realizing the potential for cell-based therapies, however, requires a thorough understanding of pancreas development and beta-cell formation. Pancreas development is coordinated by a complex interplay of signaling pathways and transcription factors that determine early pancreatic specification as well as the later differentiation of exocrine and endocrine lineages. This review describes the current knowledge of these factors as they relate specifically to the emergence of endocrine beta cells from pancreatic endoderm. Current therapeutic efforts to generate insulin-producing beta-like cells from embryonic stem cells have already capitalized on recent advances in our understanding of the embryonic signals and transcription factors that dictate lineage specification and will most certainly be further enhanced by a continuing emphasis on the identification of novel factors and regulatory relationships.

Disruption of Tsc2 in pancreatic beta cells induces beta cell mass expansion and improved glucose tolerance in a TORC1-dependent manner

Regulation of pancreatic beta cell mass and function is a major determinant for the development of diabetes. Growth factors and nutrients are important regulators of beta cell mass and function. The signaling pathways by which these growth signals modulate these processes have not been completely elucidated. Tsc2 is an attractive candidate to modulate these processes, because it is a converging point for growth factor and nutrient signals. In these experiments, we generated mice with conditional deletion of Tsc2 in beta cells (betaTsc2(-/-)). These mice exhibited decreased glucose levels and hyperinsulinemia in the fasting and fed state. Improved glucose tolerance in these mice was observed as early as 4 weeks of age and was still present in 52-week-old mice. Deletion of Tsc2 in beta cells induced expansion of beta cell mass by increased proliferation and cell size. Rapamycin treatment reversed the metabolic changes in betaTsc2(-/-) mice by induction of insulin resistance and reduction of beta cell mass. The reduction of beta cell mass in betaTsc2(-/-) mice by inhibition of the mTOR/Raptor (TORC1) complex with rapamycin treatment suggests that TORC1 mediates proliferative and growth signals induced by deletion of Tsc2 in beta cells. These studies uncover a critical role for the Tsc2/mTOR pathway in regulation of beta cell mass and carbohydrate metabolism in vivo.

PDX-1 and MafA play a crucial role in pancreatic beta-cell differentiation and maintenance of mature beta-cell function

Pancreatic and duodenal homeobox factor-1 (PDX-1) plays a crucial role in pancreas development, beta-cell differentiation, and maintenance of mature beta-cell function. PDX-1 expression is maintained in pancreatic precursor cells during pancreas development but becomes restricted to beta-cells in mature pancreas. In mature beta-cells, PDX-1 transactivates the insulin and other genes involved in glucose sensing and metabolism such as GLUT2 and glucokinase. MafA is a recently isolated beta-cell-specific transcription factor which functions as a potent activator of insulin gene transcription. Furthermore, these transcription factors play an important role in induction of insulin-producing cells in various non-beta-cells and thus could be therapeutic targets for diabetes. On the other hand, under diabetic conditions, expression and/or activities of PDX-1 and MafA in beta-cells are reduced, which leads to suppression of insulin biosynthesis and secretion. It is likely that alteration of such transcription factors explains, at least in part, the molecular mechanism for beta-cell glucose toxicity found in diabetes.

PAX4 enhances beta-cell differentiation of human embryonic stem cells

Background

Human embryonic stem cells (HESC) readily differentiate into an apparently haphazard array of cell types, corresponding to all three germ layers, when their culture conditions are altered, for example by growth in suspension as aggregates known as embryoid bodies (EBs). However, this diversity of differentiation means that the efficiency of producing any one particular cell type is inevitably low. Although pancreatic differentiation has been reported from HESC, practicable applications for the use of β-cells derived from HESC to treat diabetes will only be possible once techniques are developed to promote efficient differentiation along the pancreatic lineages.

Methods and Findings

Here, we have tested whether the transcription factor, Pax4 can be used to drive the differentiation of HESC to a β-cell fate in vitro. We constitutively over-expressed Pax4 in HESCs by stable transfection, and used Q-PCR analysis, immunocytochemistry, ELISA, Ca²⁺ microfluorimetry and cell imaging to assess the role of Pax4 in the differentiation and intracellular Ca²⁺ homeostasis of β-cells developing in embryoid bodies produced from such HESC. Cells expressing key β-cell markers were isolated by fluorescence-activated cell sorting after staining for high zinc content using the vital dye, Newport Green.

Conclusion

Constitutive expression of Pax4 in HESC substantially enhances their propensity to form putative β-cells. Our findings provide a novel foundation to study the mechanism of pancreatic β-cells differentiation during early human development and to help evaluate strategies for the generation of purified β-cells for future clinical applications.

Pdx-1 and Ptf1a concurrently determine fate specification of pancreatic multipotent progenitor cells

The pancreas is derived from a pool of multipotent progenitor cells (MPCs) that co-express Pdx-1 and Ptf1a. To more precisely define how the individual and combined loss of Pdx-1 and Ptf1a affects pancreatic MPC specification and differentiation we derived and studied mice bearing a novel Ptf1a(YFP) allele. While the expression of Pdx-1 and Ptf1a in pancreatic MPCs coincides between E9.5 and 12.5 the developmental phenotypes of Pdx-1 null and Pdx-1; Ptf1a double null mice are indistinguishable, and an early pancreatic bud is formed in both cases. This finding indicates that Pdx-1 is required in the foregut endoderm prior to Ptf1a for pancreatic MPC specification. We also found that Ptf1a is neither required for specification of Ngn3-positive endocrine progenitors nor differentiation of mature beta-cells. In the absence of Pdx-1 Ngn3-positive cells were not observed after E9.5. Thus, in contrast to the deletion of Ptf1a, the loss of Pdx-1 precludes the sustained Ngn3-based derivation of endocrine progenitors from pancreatic MPCs. Taken together, these studies indicate that Pdx-1 and Ptf1a have distinct but interdependent functions during pancreatic MPC specification.

MicroRNA expression is required for pancreatic islet cell genesis in the mouse

OBJECTIVE

The generation of distinct cell types during the development of the pancreas depends on sequential changes in gene expression. We tested the hypothesis that microRNAs (miRNAs), which limit gene expression through posttranscriptional silencing, modulate the gene expression cascades involved in pancreas development.

RESEARCH DESIGN AND METHODS

miRNAs were cloned and sequenced from developing pancreata, and expression of a subset of these genes was tested using locked nucleic acid in situ analyses. To assess the overall contribution of miRNAs to pancreatic development, Dicer1, an enzyme required for miRNA processing, was conditionally deleted from the developing pancreas.

RESULTS

Sequencing of small RNAs identified over 125 miRNAs, including 18 novel sequences, with distinct expression domains within the developing pancreas. To test the developmental contribution of these miRNAs, we conditionally deleted the miRNA processing enzyme Dicer1 early in pancreas development. Dicer-null animals displayed gross defects in all pancreatic lineages, although the endocrine cells, and especially the insulin-producing beta-cells, were most dramatically reduced. The endocrine defect was associated with an increase in the notch-signaling target Hes1 and a reduction in the formation of endocrine cell progenitors expressing the Hes1 target gene neurogenin3.

CONCLUSIONS

The expression of a unique profile of miRNAs is required during pancreas development and is necessary for beta-cell formation.

Gene array identification of Ipf1/Pdx1-/- regulated genes in pancreatic progenitor cells

Background

The homeodomain transcription factor IPF1/PDX1 exerts a dual role in the pancreas; Ipf1/Pdx1 global null mutants fail to develop a pancreas whereas conditional inactivation of Ipf1/Pdx1 in β-cells leads to impaired β-cell function and diabetes. Although several putative target genes have been linked to the β-cell function of Ipf1/Pdx1, relatively little is known with respect to genes regulated by IPF1/PDX1 in early pancreatic progenitor cells.

Results

Microarray analyses identified a total of 111 genes that were differentially expressed in e10.5 pancreatic buds of Ipf1/Pdx1^-/- embryos. The expression of one of these, Spondin 1, which encodes an extracellular matrix protein, has not previously been described in the pancreas. Quantitative real-time RT-PCR analyses and immunohistochemical analyses also revealed that the expression of FgfR2IIIb, that encodes the receptor for FGF10, was down-regulated in Ipf1/Pdx1^-/- pancreatic progenitor cells.

Conclusion

This microarray analysis has identified a number of candidate genes that are differentially expressed in Ipf1/Pdx1^-/- pancreatic buds. Several of the differentially expressed genes were known to be important for pancreatic progenitor cell proliferation and differentiation whereas others have not previously been associated with pancreatic development.

Early pancreatic development requires the vertebrate Suppressor of Hairless (RBPJ) in the PTF1 bHLH complex

PTF1a is an unusual basic helix-loop-helix (bHLH) transcription factor that is required for the development of the pancreas. We show that early in pancreatic development, active PTF1a requires interaction with RBPJ, the vertebrate Suppressor of Hairless, within a stable trimeric DNA-binding complex (PTF1). Later, as acinar cell development begins, RBPJ is swapped for RBPJL, the constitutively active, pancreas-restricted paralog of RBPJ. Moreover, the Rbpjl gene is a direct target of the PTF1 complex: At the onset of acinar cell development when the Rbpjl gene is first induced, a PTF1 complex containing RBPJ is bound to the Rbpjl promoter. As development proceeds, RBPJL gradually replaces RBPJ in the PTF1 complex bound to Rbpjl and appears on the binding sites for the complex in the promoters of other acinar-specific genes, including those for the secretory digestive enzymes. A single amino acid change in PTF1a that eliminates its ability to bind RBPJ (but does not affect its binding to RBPJL) causes pancreatic development to truncate at an immature stage, without the formation of acini or islets. These results indicate that the interaction between PTF1a and RBPJ is required for the early stage of pancreatic growth, morphogenesis, and lineage fate decisions. The defects in pancreatic development phenocopy those of Ptf1a-null embryos; thus, the first critical function of PTF1a is in the context of the PTF1 complex containing RBPJ. Action within an organ-specific transcription factor is a previously unknown function for RBPJ and is independent of its role in Notch signaling.

pdx-1 function is specifically required in embryonic beta cells to generate appropriate numbers of endocrine cell types and maintain glucose homeostasis

The pdx1 gene is essential for pancreatic organogenesis in humans and mice; pdx1 mutations have been identified in human diabetic patients. Specific inactivation of pdx1 in adult beta cells revealed that this gene is required for maintenance of mature beta cell function. In the following study, a Cre-lox strategy was used to remove pdx1 function specifically from embryonic beta cells beginning at late-gestation, prior to islet formation. Animals in which pdx1 is lost in insulin-producing cells during embryogenesis had elevated blood glucose levels at birth and were overtly diabetic by weaning. Neonatal and adult mutant islets showed a dramatic reduction in the number of insulin(+) cells and an increase in both glucagon(+) and somatostatin(+) cells. Lineage tracing revealed that excess glucagon(+) and somatostatin(+) cells did not arise by interconversion of endocrine cell types. Examination of mutant islets revealed a decrease in proliferation of insulin-producing cells just before birth and a concomitant increase in proliferation of glucagon-producing cells. We propose that pdx1 is required for proliferation and function of the beta cells generated at late gestation, and that one function of normal beta cells is to inhibit the proliferation of other islet cell types, resulting in the appropriate numbers of the different endocrine cell types.

c-Kit in early onset of diabetes: a morphological and functional analysis of pancreatic beta-cells in c-KitW-v mutant mice

c-Kit tyrosine receptor kinase, a well-established stem cell marker, is expressed in a variety of tissues including the pancreas. The involvement of c-Kit in fetal rat and human endocrine pancreatic development, survival, and function has been well characterized but primarily using in vitro experimental approaches. Therefore, the aim of the current study was to examine whether deficiency of a functional c-Kit receptor would have physiological and functional implications in vivo. We characterized the c-Kit mutant mouse, c-Kit(W-v/+), to evaluate the in vivo role of c-Kit in beta-cell growth and function. Here we report that male c-Kit(W-v/+) mice, at 8 wk of age, showed high fasting blood glucose levels and impaired glucose tolerance, which was associated with low levels of insulin secretion after glucose stimulation in vivo and in isolated islets. Morphometric analysis revealed that beta-cell mass was significantly reduced (50%) in male c-Kit(W-v/+) mice when compared with controls (c-Kit(+/+)) (P < 0.05). In parallel, a reduction in pancreatic duodenal homeobox-1 and insulin gene expression in whole pancreas as well as isolated islets of c-Kit(W-v/+) male mice was noted along with a decrease in pancreatic insulin content. Furthermore, the reduction in beta-cell mass in male c-Kit(W-v/+) mice was associated with a decrease in beta-cell proliferation. Interestingly, these changes were not observed in female c-Kit(W-v/+) mice until 40 wk of age. Our results clearly demonstrate that the c-Kit receptor is involved in the regulation of glucose metabolism, likely through an important role in beta-cell development and function.

An illustrated review of early pancreas development in the mouse

Pancreas morphogenesis and cell differentiation are highly conserved among vertebrates during fetal development. The pancreas develops through simple budlike structures on the primitive gut tube to a highly branched organ containing many specialized cell types. This review presents an overview of key molecular components and important signaling sources illustrated by an extensive three-dimensional (3D) imaging of the developing mouse pancreas at single cell resolution. The 3D documentation covers the time window between embryonic days 8.5 and 14.5 in which all the pancreatic cell types become specified and therefore includes gene expression patterns of pancreatic endocrine hormones, exocrine gene products, and essential transcription factors. The 3D perspective provides valuable insight into how a complex organ like the pancreas is formed and a perception of ventral and dorsal pancreatic growth that is otherwise difficult to uncover. We further discuss how this global analysis of the developing pancreas confirms and extends previous studies, and we envisage that this type of analysis can be instrumental for evaluating mutant phenotypes in the future.

Role of PDX-1 and MafA as a potential therapeutic target for diabetes

Pancreatic and duodenal homeobox factor-1 (PDX-1) plays a crucial role in pancreas development, beta-cell differentiation, and maintaining mature beta-cell function. During pancreas development, PDX-1 expression is maintained in precursor cells, and later it becomes restricted to beta-cells. In mature beta-cells, PDX-1 regulates gene expression of various beta-cell-related factors including insulin. Also, PDX-1 has potency to induce insulin-producing cells from non-beta-cells in various tissues, and PDX-1-VP16 fusion protein more efficiently induces insulin-producing cells, especially in the presence of NeuroD or Ngn3. MafA is a recently isolated beta-cell-specific transcription factor which functions as a potent activator of insulin gene transcription. During pancreas development, MafA expression is first detected at the beginning of the principal phase of insulin-producing cell production. Furthermore, MafA markedly enhances insulin gene promoter activity and ameliorates glucose tolerance in diabetic mice, especially in the presence of PDX-1 and NeuroD. Taken together, PDX-1 and MafA play a crucial role in inducing surrogate beta-cells and could be a therapeutic target for diabetes.

A feat of metabolic proportions: Pdx1 orchestrates islet development and function in the maintenance of glucose homeostasis

Emerging evidence over the past decade indicates a central role for transcription factors in the embryonic development of pancreatic islets and the consequent maintenance of normal glucose homeostasis. Pancreatic and duodenal homeobox 1 (Pdx1) is the best studied and perhaps most important of these factors. Whereas deletion or inactivating mutations of the Pdx1 gene causes whole pancreas agenesis in both mice and humans, even haploinsufficiency of the gene or alterations in its expression in mature islet cells causes substantial impairments in glucose tolerance and the development of a late-onset form of diabetes known as maturity onset diabetes of the young. The study of Pdx1 has revealed crucial phenotypic interrelationships of the varied cell types within the pancreas, particularly as these impinge upon cellular differentiation in the embryo and neogenesis and regeneration in the adult. In this review, we describe the actions of Pdx1 in the developing and mature pancreas and attempt to unify these actions with its known roles in modulating transcriptional complex formation and chromatin structure at the molecular genetic level.

Key mechanisms of early lung development

Lung morphogenesis requires the integration of multiple regulatory factors, which results in a functional air-blood interface required for gas exchange at birth. The respiratory tract is composed of endodermally derived epithelium surrounded by cells of mesodermal origin. Inductive signaling between these 2 tissue compartments plays a critical role in formation and differentiation of the lung, which is mediated by evolutionarily conserved signaling families used reiteratively during lung formation, including the fibroblast growth factor, hedgehog, retinoic acid, bone morphogenetic protein, and Wnt signaling pathways. Cells coordinate their response to these signaling proteins largely through transcription factors, which determine respiratory cell fate and pattern formation via the activation and repression of downstream target genes. Gain- and loss-of-function studies in null mutant and transgenic mice models have greatly facilitated the identification and hierarchical classification of these molecular programs. In this review, we highlight select molecular events that drive key phases of pulmonary development, including specification of a lung cell fate, primary lung bud formation, tracheoesophageal septation, branching morphogenesis, and proximal-distal epithelial patterning. Understanding the genetic pathways that regulate respiratory tract development is essential to provide insight into the pathogenesis of congenital anomalies and to develop innovative strategies to treat inherited and acquired lung disease.

Beneficial effects of L-arginine nitric oxide-producing pathway in rats treated with alloxan

In an attempt to elucidate molecular mechanisms and factors involved in beta cell regeneration, we evaluated a possible role of the L-arginine-nitric oxide (NO)-producing pathway in alloxan-induced diabetes mellitus. Diabetes was induced in male Mill Hill rats with a single alloxan dose (120 mg kg(-1)). Both non-diabetic and diabetic groups were additionally separated into three subgroups: (i) receiving L-arginine . HCl (2.25%), (ii) receiving L-NAME . HCl (0.01%) for 12 days as drinking liquids, and (iii) control. Treatment of diabetic animals started after diabetes induction (glucose level > or = 12 mmol l(-1)). We found that disturbed glucose homeostasis, i.e. blood insulin and glucose levels in diabetic rats was restored after L-arginine treatment. Immunohistochemical findings revealed that L-arginine had a favourable effect on beta cell neogenesis, i.e. it increased the area of insulin-immunopositive cells. Moreover, confocal microscopy showed colocalization of insulin and pancreas duodenum homeobox-1 (PDX-1) in both endocrine and exocrine pancreas. This increase in insulin-expressing cells was accompanied by increased cell proliferation (observed by proliferating cell nuclear antigen-PCNA immunopositivity) which occurred in a regulated manner since it was associated with increased apoptosis (detected by the TUNEL method). Furthermore, L-arginine enhanced both nuclear factor-kB (NF-kB) and neuronal nitric oxide synthase (nNOS) immunopositivities. The effect of L-arginine on antioxidative defence was observed especially in restoring to control level the diabetes-induced increase in glutathione peroxidase activity. In contrast to L-arginine, diabetic pancreas was not affected by L-NAME supplementation. In conclusion, the results suggest beneficial L-arginine effects on alloxan-induced diabetes resulting from the stimulation of beta cell neogenesis, including complex mechanisms of transcriptional and redox regulation.

Endocrine cells develop within pancreatic bud-like structures derived from mouse ES cells differentiated in response to BMP4 and retinoic acid

We have examined factors affecting the in vitro differentiation of Pdx1(GFP/w) ESCs to pancreatic endocrine cells. Inclusion of Bone Morphogenetic Protein 4 (BMP4) during the first four days of differentiation followed by a 24-hour pulse of retinoic acid (RA) induced the formation of GFP(+) embryoid bodies (EBs). GFP expression was restricted to E-cadherin(+) tubes and GFP bright (GFP(br)) buds, reminiscent of GFP(+) early foregut endoderm and GFP(br) pancreatic buds observed in Pdx1(GFP/w) embryos. These organoid structures developed without further addition of exogenous factors between days 5 and 12, suggesting that day 5 EBs contained a template for the subsequent phase of development. EBs treated with nicotinamide after day 12 of differentiation expressed markers of endocrine and exocrine differentiation, but only in cells within the GFP(br) buds. Analysis of Pdx1(GFP/w) ESCs modified by targeting a dsRed1 gene to the Ins1 locus (Pdx1(GFP/w)Ins1(RFP/w) ESCs) provided corroborating evidence that insulin positive cells arose from GFP(br) buds, mirroring the temporal relationship between pancreatic bud development and the formation of endocrine cells in the developing embryo. The readily detectable co-expression of GFP and RFP in grafts derived from transplanted EBs demonstrated the utility of Pdx1(GFP/w)Ins1(RFP/w) ESCs for investigating pancreatic differentiation in vitro and in vivo.

Role of FoxO Proteins in Pancreatic beta Cells

Forkhead transcription factors of the FoxO family have important roles in cellular proliferation, apoptosis, differentiation and stress resistance. FoxO proteins also play important roles in metabolism of complex organisms. FoxO1 regulates glucose and lipid metabolism in liver, as well as preadipocyte, myoblast and vascular endothelial cell differentiation. In the hypothalamus, FoxO controls food intake. In this chapter, we review the role of FoxO in pancreatic beta cells. Pancreatic beta cells secrete insulin to maintain the plasma glucose levels in a strict physiological range. Defects of beta cell function cause diabetes. The expression pattern of FoxO1 during pancreatic organogenesis is similar to that of Pdx1, Nkx2.2 and Pax4, transcription factors known to be critical for beta cell development. FoxO1 is expressed in a subset of pancreatic duct cells, in which insulin and/or Pdx1 are occasionally expressed. FoxO1 inhibits beta cell proliferation through suppression of Pdx1 by competing with FoxA2 and protects against beta cell failure induced by oxidative stress through NeuroD and MafA induction. Thus, a series of FoxO1 studies in pancreas suggested that FoxO1 plays important roles in pancreatic beta cell differentiation, neogenesis, proliferation and stress resistance. Genetic or pharmacological manipulation of FoxO can be used to prevent beta cell failure or aid in the differentiation of uncommitted endocrine progenitors into beta cells for transplantation.

Glucagon-like peptide-1 and its receptor agonist exendin-4 modulate cholangiocyte adaptive response to cholestasis

BACKGROUND & AIMS

Cholangiopathies are characterized by progressive dysregulation of the balance between proliferation and death of cholangiocytes. In the course of cholestasis, cholangiocytes undergo a neuroendocrine transdifferentiation and their biology is regulated by neuroendocrine hormones. Glucagon-like peptide-1 (GLP-1), secreted by neuroendocrine cells, sustains beta-cell survival in experimental diabetes and induces the neuroendocrine transdifferentiation of pancreatic ductal cells. GLP-1 receptor (GLP-1R) selective agonist exendin-4 is used in humans as a novel therapeutic tool for diabetes. The aim of this study was to define if GLP-1 modulates cholangiocyte biologic response to cholestasis.

METHODS

Expression of GLP-1R in cholangiocytes was determined. Effects on cholangiocyte proliferation of the in vitro and in vivo exposure to GLP-1 or exendin-4, together with the intracellular signals, were then studied. Synthesis of GLP-1 by cholangiocytes and the effects of GLP-1R blockage on their growth were also determined.

RESULTS

Cholangiocytes express the GLP-1 receptor, which is up-regulated in the course of cholestasis. GLP-1 and exendin-4 increase cholangiocyte growth both in vitro and in vivo. The GLP-1R signal is mediated by the phosphatidyl-inositol-3-kinase, cAMP/Protein Kinase A, and Ca(2+)-CamKIIalpha but not by the ERK1/2 and PKCalpha pathways. Proliferating cholangiocytes synthesize GLP-1: neutralization of its action by GLP-1R antagonist blunts cholangiocyte response to cholestasis.

CONCLUSIONS

GLP-1 is required for the cholangiocyte adaptive response to cholestasis. Cholangiocytes are susceptible to the activation of GLP-1R and respond with increased proliferation and functional activity. Exendin-4 availability for employment in humans and these data may open novel perspectives for the medical treatment of cholangiopathies.

Sox9 coordinates a transcriptional network in pancreatic progenitor cells

During pancreas development, both the exocrine and endocrine lineages differentiate from a common pool of progenitor cells with similarities to mature pancreatic duct cells. A small set of transcription factors, including Tcf2, Onecut1, and Foxa2, has been identified in these pancreatic progenitor cells. The Sry/HMG box transcription factor Sox9 is also expressed in the early pancreatic epithelium and is required for normal pancreatic exocrine and endocrine development in humans. In this study, we found Sox9 in mice specifically expressed with the other progenitor transcription factors in both pancreatic progenitor cells and duct cells in the adult pancreas. Sox9 directly bound to all three genes in vitro and in intact cells, and regulated their expression. In turn, both Foxa2 and Tcf2 regulated Sox9 expression, demonstrating feedback circuits between these genes. Furthermore, Sox9 activated the expression of the proendocrine factor Neurogenin3, which also depends on the other members of the progenitor transcription network. These studies indicate that Sox9 plays a dual role in pancreatic progenitor cells: both maintaining a stable transcriptional network and supporting the programs by which these cells differentiate into distinct lineages.

Embryonic endocrine pancreas and mature beta cells acquire alpha and PP cell phenotypes upon Arx misexpression

Aristaless-related homeobox (Arx) was recently demonstrated to be involved in pancreatic alpha cell fate specification while simultaneously repressing the beta and delta cell lineages. To establish whether Arx is not only necessary, but also sufficient to instruct the alpha cell fate in endocrine progenitors, we used a gain-of-function approach to generate mice conditionally misexpressing this factor. Mice with forced Arx expression in the embryonic pancreas or in developing islet cells developed a dramatic hyperglycemia and eventually died. Further analysis demonstrated a drastic loss of beta and delta cells. Concurrently, a remarkable increase in the number of cells displaying alpha cell or, strikingly, pancreatic polypeptide (PP) cell features was observed. Notably, the ectopic expression of Arx induced in embryonic or adult beta cells led to a loss of the beta cell phenotype and a concomitant increase in a number of cells with alpha or PP cell characteristics. Combining quantitative real-time PCR and lineage-tracing experiments, we demonstrate that, in adult mice, the misexpression of Arx, rather than its overexpression, promotes a conversion of beta cells into glucagon- or PP-producing cells in vivo. These results provide important insights into the complex mechanisms underlying proper pancreatic endocrine cell allocation and cell identity acquisition.

Control of beta-cell differentiation by the pancreatic mesenchyme

The importance of mesenchymal-epithelial interactions for normal development of the pancreas was recognized in the early 1960s, and mesenchymal signals have been shown to control the proliferation of early pancreatic progenitor cells. The mechanisms by which the mesenchyme coordinates cell proliferation and differentiation to produce the normal number of differentiated pancreatic cells are not fully understood. Here, we demonstrate that the mesenchyme positively controls the final number of beta-cells that develop from early pancreatic progenitor cells. In vitro, the number of beta-cells that developed from rat embryonic pancreatic epithelia was larger in cultures with mesenchyme than without mesenchyme. The effect of mesenchyme was not due to an increase in beta-cell proliferation but was due to increased proliferation of early pancreatic duodenal homeobox-1 (PDX1)-positive progenitor cells, as confirmed by bromodeoxyuridine incorporation. Consequently, the window during which early PDX1(+) pancreatic progenitor cells differentiated into endocrine progenitor cells expressing Ngn3 was extended. Fibroblast growth factor 10 mimicked mesenchyme effects on proliferation of early PDX1(+) progenitor cells and induction of Ngn3 expression. Taken together, our results indicate that expansion of early PDX1(+) pancreatic progenitor cells represents a way to increase the final number of beta-cells developing from early embryonic pancreas.

Role of the Onecut transcription factors in pancreas morphogenesis and in pancreatic and enteric endocrine differentiation

The Onecut (OC) transcription factor HNF-6 (OC-1) is required during embryogenesis for pancreatic specification, morphogenesis and endocrine differentiation. In mammals, HNF-6 has two paralogs, OC-2 and OC-3, which share DNA-binding and transcriptional activation properties and have expression patterns that overlap with that of HNF-6. This suggested that OC-2 and OC-3 play redundant roles with HNF-6 in pancreas development. Here, we have addressed this hypothesis by analyzing the phenotype of mice knockout for the Onecut factors. We found that neither OC-2 nor OC-3 is required for pancreas specification. However, OC-2 plays partially redundant roles with HNF-6 in pancreas morphogenesis and in the differentiation of endocrine precursors. As similar molecular events drive endocrine differentiation in the pancreas and gastrointestinal tract, we also investigated if Onecut factors are involved in enteroendocrine differentiation. OC-2 and OC-3 were found to delineate specific antero-posterior regions of the gut around embryonic day 12.5. Later on, OC2 was expressed in several gut cell types, whereas OC-3 behaved as a specific marker of the enteroendocrine lineage. However, OC-2 and OC-3, alone or in combination, were dispensable for gut development and enteroendocrine differentiation. In conclusion, our data reveal partially redundant roles for HNF-6 and OC-2 in developing pancreas and identify new markers for antero-posterior patterning of the gut and for enteroendocrine differentiation.

Temporal control of neurogenin3 activity in pancreas progenitors reveals competence windows for the generation of different endocrine cell types

All pancreatic endocrine cells, producing glucagon, insulin, somatostatin, or PP, differentiate from Pdx1+ progenitors that transiently express Neurogenin3. To understand whether the competence of pancreatic progenitors changes over time, we generated transgenic mice expressing a tamoxifen-inducible Ngn3 fusion protein under the control of the pdx1 promoter and backcrossed the transgene into the ngn3(-/-) background, devoid of endogenous endocrine cells. Early activation of Ngn3-ER(TM) almost exclusively induced glucagon+ cells, while depleting the pool of pancreas progenitors. As from E11.5, Pdx1+ progenitors became competent to differentiate into insulin+ and PP+ cells. Somatostatin+ cells were generated from E14.5, while the competence to make glucagon+ cells was dramatically decreased. Hence, pancreas progenitors, similar to retinal or cortical progenitors, go through competence states that each allow the generation of a subset of cell types. We further show that the progenitors acquire competence to generate late-born cells in a mechanism that is intrinsic to the epithelium.

Glucose is necessary for embryonic pancreatic endocrine cell differentiation

Mature pancreatic cells develop during embryonic life from endodermal progenitors, and this developmental process depends on activation of a hierarchy of transcription factors. While information is available on mesodermal signals controlling pancreas development, little is known about environmental factors, such as the levels of nutrients including glucose, that may control this process. Here, we studied the effects of glucose on pancreatic cells development. We used an in vitro model where both endocrine and acinar cells develop from early pancreatic and duodenal homeobox-1 (PDX1)-positive embryonic pancreatic progenitors. We first showed that glucose does not have a major effect on global pancreatic cell proliferation, survival, and acinar cell development. On the other hand, glucose controlled both alpha and beta cell development. Specifically, the surface occupied by insulin-positive cells was 20-fold higher in pancreases cultured in presence than in absence of glucose, and this effect was dose-dependent over the range 0.5-10 mm. Glucose did not appear to control beta cell development by activating the proliferation of early progenitors or beta cells themselves but instead tightly regulated cell differentiation. Thus, glucose did not modify the pattern of expression of Neurogenin3, the earliest marker of endocrine progenitor cells, but was necessary for the expression of the transcription factor NeuroD, a direct target of Neurogenin3 known to be important for proper pancreatic endocrine cell development. We conclude that glucose interferes with the pancreatic endocrine cells development by regulating the transition between Ngn3 and upstream NeuroD.

Development and function of the mammalian spleen

The vertebrate spleen has important functions in immunity and haematopoiesis, many of which have been well studied. In contrast, we know much less about the mechanisms governing its early embryonic development. However, as a result of work over the past decade-mostly using knockout mice--significant progress has been made in unravelling the genetic processes governing the spleen's early development. Key genetic regulators, such as Tlx1 and Pbx1, have been identified, and we know some of the early transcriptional hierarchies that control the early patterning and proliferation of the splenic primordium. In mouse and humans, asplenia can arise as a result of laterality defects, or the spleen can be absent with no other discernible abnormalities. Surprisingly, given the spleen's diverse functions, asplenic individuals suffer no major haematopoietic or immune defects apart from a susceptibility to infection with encapsulated bacteria. Recent evidence has shed light on a previously unknown role of the spleen in the development and maintenance of specific B cell populations that are involved in the initial response to infection caused by encapsulated bacteria. The lack of these populations in asplenic mice and humans may go some way to explaining this susceptibility.

Regulation of pancreatic endocrine cell differentiation by sulphated proteoglycans

AIMS/HYPOTHESIS

Epithelium-mesenchyme interactions play a major role in pancreas development. Recently, we demonstrated that embryonic pancreatic mesenchyme enhanced progenitor cell proliferation but inhibited endocrine cell differentiation. Here, we investigated the role played by sulphated proteoglycans, which are known to be essential to embryonic development, in this inhibitory effect.

MATERIALS AND METHODS

We first determined the expression of the genes encoding glypicans, syndecans and the main glycosaminoglycan chain-modifying enzymes in immature embryonic day (E) 13.5 and more differentiated E17.5 rat pancreases. Next, using an in vitro model of pancreas development, we blocked the action of endogenous sulphated proteoglycans by treating embryonic pancreases in culture with chlorate, an inhibitor of proteoglycan sulphation, and examined the effects on pancreatic endocrine cell differentiation.

RESULTS

We first showed that expression of the genes encoding glypicans 1, 2, 3 and 5 and heparan sulphate 2-sulfotransferase decreased between E13.5 and E17.5. We next found that alteration of proteoglycan action by chlorate blocked the inhibitory effect of the mesenchyme on endocrine differentiation. Chlorate-treated pancreases exhibited a dramatic increase in beta cell number in a dose-dependent manner (169-and 375-fold increase with 30 mmol/l and 40 mmol/l chlorate, respectively) and in alpha cell development. Insulin-positive cells that developed in the presence of chlorate exhibited a phenotype of mature cells with regard to the expression of the following genes: pancreatic and duodenal homeobox gene 1 (Pdx1), proprotein convertase subtilisin/kexin type 1 (Pcsk1; previously known as pro-hormone convertase 1/3), proprotein convertase subtilisin/kexin type 2 (Pcsk2; previously known as pro-hormone convertase 2) and solute carrier family 2 (facilitated glucose transporter), member 2 (Slc2a1; previously known as glucose transporter 2). Finally, we showed that chlorate activated endocrine cell development by inducing neurogenin 3 (Neurog3) expression in early endocrine progenitor cells.

CONCLUSIONS/INTERPRETATION

We demonstrated that sulphated proteoglycans control pancreatic endocrine cell differentiation. Understanding the mechanism by which sulphated proteoglycans affect beta cell development could be useful in the generation of beta cells from embryonic stem cells.

Impaired insulin secretion in vivo but enhanced insulin secretion from isolated islets in pancreatic beta cell-specific vascular endothelial growth factor-A knock-out mice

AIMS/HYPOTHESIS

Endothelial cells are considered to be essential for normal pancreatic beta cell function. However, there have been no reports showing their importance for beta cell function.

MATERIALS AND METHODS

Using mice with disrupted vascular endothelial growth factor-A gene specifically in beta cells, we investigated the relation between islet vascular structure and beta cell function.

RESULTS

Mice with disrupted vascular endothelial growth factor-A gene specifically in beta cells had reduced islet vascular density with impaired formation of endothelial fenestration. While their fasting glucose and body weight were comparable with control mice, their glucose- and tolbutamide-induced rapid insulin release were impaired, thus resulting in glucose intolerance. On the other hand, glucose and KCl enhanced the levels of insulin secreted from islets isolated from these mice. In addition, the production of soluble N-ethylmaleimide-sensitive factor attachment protein receptors in the islets was increased. Insulin content and expression of insulin I and pancreas duodenum homeobox 1 mRNA in the islets were also increased.

CONCLUSIONS/INTERPRETATION

Our results indicate that an abnormal quality and quantity of blood vessels due to reduced expression of vascular endothelial growth factor-A in beta cells could be a cause of impaired insulin secretion without impairment of beta cell function.

Activation of Notch signaling in tumorigenesis of experimental pancreatic cancer induced by dimethylbenzanthracene in mice

To establish pancreatic cancer in mice, dimethylbenzanthracene (DMBA) was administered into mice pancreata. The formation of tubular complex lesions was found in the pancreatic sections from 2 weeks after DMBA treatment. Abnormal tubular complex formations with ductal metaplasia were found from 1 month after the administration. By 3 months after DMBA injection into the pancreas, 6 of 10 mice showed visually recognizable tumors with precursor lesions of various types of cell atypia. In contrast, there were no visually or histologically detectable tumors in the placebo-treated animals. The expression profiles of smad 4, cyclin D1 and p53 in the DMBA-induced tumors were similar to those of human pancreatic cancer, suggesting that this would be a useful mouse model for studying the morphological and molecular mechanisms involved in pancreatic carcinogenesis. Immunohistochemical study using specific antibodies revealed that Notch-1 and Hes-1 were expressed in lesions ranging from tubular complexes to carcinoma in these chemically induced pancreatic tumors. Semiquantitative reverse transcription-polymerase chain reaction with microdissection demonstrated that Notch-1 expression was continuous from precursor lesions to carcinoma cells, whereas Pdx-1 expression was attenuated in carcinoma cells compared to precursor lesions. In addition, inhibition of the Notch signaling pathway by the gamma-secretase inhibitor N-(N-[3,5-difluorophenacetyl]-L-alanyl)-S-phenylglycine t-butyl ester reduced pancreatic cancer cell growth. Therefore, Notch signaling is required to form the tubular complexes and its continuous activation might lead to the transition from tubular complexes to premalignant or malignant lesions and carcinoma cell development in the pancreas.

Understanding the extrinsic and intrinsic signals involved in pancreas and β-cell development: from endoderm to β cells

PURPOSE OF REVIEW

To summarize recent progress in understanding of the extrinsic and intrinsic signals directing pancreas development from early endoderm.

RECENT FINDINGS

The pancreatic mesoderm was shown not only to play a permissive role in pancreas determination but also to control endocrine commitment and maturation through the interplay between Notch and fibroblast growth factor signaling. The requirement of Wnt (wingless-type)/β-catenin signaling in the expansion of the acinar cell lineage, and the spatial-temporal specificity of PDX1 (pancreatic and duodenal homeobox) activity, which is needed for proper acinar development, were also demonstrated. A novel factor, IA1 (insulinoma-associated 1), was identified as an endocrine marker downstream of Ngn3 (neurogenin); MAFB (musculo-aponeurotic fibrosarcoma) was shown to be a marker of α-cell and β-cell precursors, and ARX (aristaless-related homeobox), a marker of α-cell progenitors, was revealed to directly antagonize PAX4 (paired homeobox) in determining α-cell and β-cell lineages.

SUMMARY

Cell fate specification results from combined effects of extrinsic and intrinsic regulators and sensitivity of target cells to them, which vary depending on the precise stage of cell commitment or differentiation. Knowledge of the hierarchy of the different factors influencing pancreas development will aid in developing new cell therapies to treat diabetes.

Organ size is limited by the number of embryonic progenitor cells in the pancreas but not the liver

The determinants of vertebrate organ size are poorly understood, but the process is thought to depend heavily on growth factors and other environmental cues. In the blood and central nervous system, for example, organ mass is determined primarily by growth-factor-regulated cell proliferation and apoptosis to achieve a final target size. Here, we report that the size of the mouse pancreas is constrained by an intrinsic programme established early in development, one that is essentially not subject to growth compensation. Specifically, final pancreas size is limited by the size of the progenitor cell pool that is set aside in the developing pancreatic bud. By contrast, the size of the liver is not constrained by reductions in the progenitor cell pool. These findings show that progenitor cell number, independently of regulation by growth factors, can be a key determinant of organ size.

Calsenilin is required for endocrine pancreas development in zebrafish

Calsenilin/DREAM/Kchip3 is a neuronal calcium-binding protein. It is a multifunctional protein, mainly expressed in neural tissues and implicated in regulation of presenilin processing, repression of transcription, and modulation of A-type potassium channels. Here, we performed a search for new genes expressed during pancreatic development and have studied the spatiotemporal expression pattern and possible role of calsenilin in pancreatic development in zebrafish. We detected calsenilin transcripts in the pancreas from 21 somites to 39 hours postfertilization stages. Using double in situ hybridization, we found that the calsenilin gene was expressed in pancreatic endocrine cells. Loss-of-function experiments with anti-calsenilin morpholinos demonstrated that injected morphants have a significant decrease in the number of pancreatic endocrine cells. Furthermore, the knockdown of calsenilin leads to perturbation in islet morphogenesis, suggesting that calsenilin is required for early islet cell migration. Taken together, our results show that zebrafish calsenilin is involved in endocrine cell differentiation and morphogenesis within the pancreas.

Specific targeting of pancreatic islet cells in vivo by insulin-promoter-driven adenoviral conjugated reporter genes

BACKGROUND

The objective of this study was to determine whether rat insulin promoter (RIP) could, in a mouse model, direct expression of an adenovirus-mediated reporter gene specifically into pancreatic islets via systemic delivery.

METHODS

Five hundred and eight base pairs of the RIP DNA sequence were constructed into an adenoviral vector containing a lacZ reporter gene (Adeno-RIP-lacZ). The cytomegalovirus (CMV) promoter was constructed to drive lacZ reporter-gene expression (Adeno-CMV-lacZ) and used as controls. In vitro transient transfection assays were performed to determine levels of reporter-gene expression and compared with that of liposome-mediated plasmid transfection. SCID mice were bred and housed in the barrier BL-4 animal facility. At 2 months of age, the human pancreatic cancer cell PANC-1 was intraperitoneally injected into male mice. Two months after the tumor cell inoculation, mice were injected with 10(7) adenoviral particles via tail veins. After gene delivery, mice were sacrificed at different time points to determine transgene expression levels. Complete necropsies were performed. Morphological alterations were determined using hematoxylin and eosin (H&E) staining, and distribution of the reporter lacZ gene was determined by immunohistochemistry analyses.

RESULTS

Adenoviral-driven reporter-gene expression resulted in more than 5 times higher transgene expression compared with conventional plasmid transfections. In Adeno-RIP-lacZ-injected mice, lacZ expression was specifically detected in pancreatic islets. By contrast, in Adeno-CMV-lacZ-injected mice, lacZ gene expression was observed in multiple organs and tissues. Mononuclear cell infiltration and liver cell inflammation were found in Adeno-CMV-lacZ-treated mice. Similar phenomena were observed in islet cells of Adeno-RIP-lacZ-treated mice. A significantly higher level of reporter-gene expression was also found at the edge of in-vivo-inoculated human pancreatic tumors.

CONCLUSION

These results demonstrate that RIP-directed reporter-gene expression was found specifically in mouse pancreatic islets and implanted human pancreatic cancer cells. These data thus demonstrate that the combination of an adenoviral vector and a tissue-specific promoter could lead to an enhanced and more specific transgene expression in vivo.