Differential ability of Ptf1a and Ptf1a-VP16 to convert stomach, duodenum and liver to pancreas

Direct Reprogramming of Somatic Cells into Induced β-Cells: An Overview

The persistent shortage of insulin-producing islet mass or β-cells for transplantation in the ever-growing diabetic population worldwide is a matter of concern. To date, permanent cure to this medical complication is not available and soon after the establishment of lineage-specific reprogramming, direct β-cell reprogramming became a viable alternative for β-cell regeneration. Direct reprogramming is a straightforward and powerful technique that can provide an unlimited supply of cells by transdifferentiating terminally differentiated cells toward the desired cell type. This approach has been extensively used by multiple groups to reprogram non-β-cells toward insulin-producing β-cells. The β-cell identity has been achieved by various studies via ectopic expression of one or more pancreatic-specific transcription factors in somatic cells, bypassing the pluripotent state. This work highlights the importance of the direct reprogramming approaches (both integrative and non-integrative) in generating autologous β-cells for various applications. An in-depth understanding of the strategies and cell sources could prove beneficial for the efficient generation of integration-free functional insulin-producing β-cells for diabetic patients lacking endogenous β-cells.

Direct Reprogramming of Different Cell Lineages into Pancreatic β-Like Cells

One major goal of regenerative medicine is the production of pancreatic endocrine islets to treat insulin-dependent diabetic patients. Among the different methods developed to achieve this goal, a particularly promising approach is direct lineage reprogramming, in which non-β-cells are directly converted to glucose-responsive, insulin-secreting β-like cells. Efforts by different research groups have led to critical insights in the inducing factors necessary and types of somatic tissues suitable for direct conversion to β-like cells. Nevertheless, there is limited understanding of the molecular mechanisms underlying direct cell fate conversion. Significant challenges also remain in translating discoveries into therapeutics that will eventually benefit diabetic patients. This review aims to cover the advances made in the direct reprogramming of somatic cells into β-like cells and discuss the remaining challenges.

Nutritional control of thyroid morphogenesis through gastrointestinal hormones

Developing animals absorb nutrients either through the placenta or from ingested food; however, the mechanisms by which embryos use external nutrients for individual organ morphogenesis remain to be elucidated. In this study, we assessed nutrient-dependent thyroid follicle morphogenesis in Xenopus laevis and investigated the role of secreted gastrointestinal (GI) hormones post-feeding. We found that feeding triggers thyroid follicle formation, and the thyroid cells showed transient inactivation of cell proliferation after feeding. In addition, the thyroid cells with multi-lumina were frequently observed in the fed tadpoles. The expression of the particular GI hormone incretin, glucose-dependent insulinotropic polypeptide (GIP), responded to feeding in the intestines of Xenopus tadpoles. Inhibition of dipeptidyl peptidase 4 (Dpp4), a degradative enzyme of incretin, increased the size of the thyroid follicles by facilitating follicular lumina connection, whereas inhibition of the sodium-glucose cotransporter (SGLT) reversed the effects of Dpp4 inhibition. Furthermore, injection of GIP peptide in unfed tadpoles initiated thyroid follicle formation-without requiring feeding-and injection of an incretin receptor antagonist suppressed follicle enlargement in the fed tadpoles. Lastly, GIP receptor knockout in neonatal mice showed smaller follicles in the thyroid, suggesting that the GI hormone-dependent thyroid morphogenesis is conserved in mammals. In conclusion, our study links external nutrients to thyroid morphogenesis and provides new insights into the function of GI hormone as a regulator of organ morphology in developing animals.

Single-cell transcriptomes of pancreatic preinvasive lesions and cancer reveal acinar metaplastic cells' heterogeneity

Acinar metaplasia is an initial step in a series of events that can lead to pancreatic cancer. Here we perform single-cell RNA-sequencing of mouse pancreas during the progression from preinvasive stages to tumor formation. Using a reporter gene, we identify metaplastic cells that originated from acinar cells and express two transcription factors, Onecut2 and Foxq1. Further analyses of metaplastic acinar cell heterogeneity define six acinar metaplastic cell types and states, including stomach-specific cell types. Localization of metaplastic cell types and mixture of different metaplastic cell types in the same pre-malignant lesion is shown. Finally, single-cell transcriptome analyses of tumor-associated stromal, immune, endothelial and fibroblast cells identify signals that may support tumor development, as well as the recruitment and education of immune cells. Our findings are consistent with the early, premalignant formation of an immunosuppressive environment mediated by interactions between acinar metaplastic cells and other cells in the microenvironment.

Loss of GATA4 causes ectopic pancreas in the stomach

Pancreatic heterotopia is defined as pancreatic tissue outside its normal location in the body and anatomically separated from the pancreas. In this work we have analyzed the stomach glandular epithelium of Gata4 ^flox/flox ; Pdx1-Cre mice (Gata4KO mice). We found that Gata4KO glandular epithelium displays an atypical morphology similar to the cornified squamous epithelium and exhibits upregulation of forestomach markers. The developing gastric units fail to form properly, and the glandular epithelial cells do not express markers of gastric gland in the absence of GATA4. Of interest, the developing glands of the Gata4KO stomach express pancreatic cell markers. Furthermore, a mass of pancreatic tissue located in the subserosa of the Gata4KO stomach is observed at adult stages. Heterotopic pancreas found in Gata4-deficient mice contains all three pancreatic cell lineages: ductal, acinar, and endocrine. Moreover, Gata4 expression is downregulated in ectopic pancreatic tissue of some human biopsy samples. © 2019 Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.

Expanding the genetic toolkit in Xenopus: Approaches and opportunities for human disease modeling

The amphibian model Xenopus, has been used extensively over the past century to study multiple aspects of cell and developmental biology. Xenopus offers advantages of a non-mammalian system, including high fecundity, external development, and simple housing requirements, with additional advantages of large embryos, highly conserved developmental processes, and close evolutionary relationship to higher vertebrates. There are two main species of Xenopus used in biomedical research, Xenopus laevis and Xenopus tropicalis; the common perception is that both species are excellent models for embryological and cell biological studies, but only Xenopus tropicalis is useful as a genetic model. The recent completion of the Xenopus laevis genome sequence combined with implementation of genome editing tools, such as TALENs (transcription activator-like effector nucleases) and CRISPR-Cas (clustered regularly interspaced short palindromic repeats-CRISPR associated nucleases), greatly facilitates the use of both Xenopus laevis and Xenopus tropicalis for understanding gene function in development and disease. In this paper, we review recent advances made in Xenopus laevis and Xenopus tropicalis with TALENs and CRISPR-Cas and discuss the various approaches that have been used to generate knockout and knock-in animals in both species. These advances show that both Xenopus species are useful for genetic approaches and in particular counters the notion that Xenopus laevis is not amenable to genetic manipulations.

Mechanisms of embryonic stomach development

The stomach is a digestive organ that has important roles in human physiology and pathophysiology. The developmental origin of the stomach is the embryonic foregut, which also gives rise a number of other structures. There are several signaling pathways and transcription factors that are known to regulate stomach development at different stages, including foregut patterning, stomach specification, and gastric regionalization. These developmental events have important implications in later homeostasis and disease in the adult stomach. Here we will review the literature that has shaped our current understanding of the molecular mechanisms that coordinate gastric organogenesis. Further we will discuss how developmental paradigms have guided recent efforts to differentiate stomach tissue from pluripotent stem cells.

Transcriptional Maintenance of Pancreatic Acinar Identity, Differentiation, and Homeostasis by PTF1A

Maintenance of cell type identity is crucial for health, yet little is known of the regulation that sustains the long-term stability of differentiated phenotypes. To investigate the roles that key transcriptional regulators play in adult differentiated cells, we examined the effects of depletion of the developmental master regulator PTF1A on the specialized phenotype of the adult pancreatic acinar cell in vivo Transcriptome sequencing and chromatin immunoprecipitation sequencing results showed that PTF1A maintains the expression of genes for all cellular processes dedicated to the production of the secretory digestive enzymes, a highly attuned surveillance of unfolded proteins, and a heightened unfolded protein response (UPR). Control by PTF1A is direct on target genes and indirect through a ten-member transcription factor network. Depletion of PTF1A causes an imbalance that overwhelms the UPR, induces cellular injury, and provokes acinar metaplasia. Compromised cellular identity occurs by derepression of characteristic stomach genes, some of which are also associated with pancreatic ductal cells. The loss of acinar cell homeostasis, differentiation, and identity is directly relevant to the pathologies of pancreatitis and pancreatic adenocarcinoma.

Stomach development, stem cells and disease

The stomach, an organ derived from foregut endoderm, secretes acid and enzymes and plays a key role in digestion. During development, mesenchymal-epithelial interactions drive stomach specification, patterning, differentiation and growth through selected signaling pathways and transcription factors. After birth, the gastric epithelium is maintained by the activity of stem cells. Developmental signals are aberrantly activated and stem cell functions are disrupted in gastric cancer and other disorders. Therefore, a better understanding of stomach development and stem cells can inform approaches to treating these conditions. This Review highlights the molecular mechanisms of stomach development and discusses recent findings regarding stomach stem cells and organoid cultures, and their roles in investigating disease mechanisms.

Endocrine Pancreas Development and Regeneration: Noncanonical Ideas From Neural Stem Cell Biology

Loss of insulin-producing pancreatic islet β-cells is a hallmark of type 1 diabetes. Several experimental paradigms demonstrate that these cells can, in principle, be regenerated from multiple endogenous sources using signaling pathways that are also used during pancreas development. A thorough understanding of these pathways will provide improved opportunities for therapeutic intervention. It is now appreciated that signaling pathways should not be seen as "on" or "off" but that the degree of activity may result in wildly different cellular outcomes. In addition to the degree of operation of a signaling pathway, noncanonical branches also play important roles. Thus, a pathway, once considered as "off" or "low" may actually be highly operational but may be using noncanonical branches. Such branches are only now revealing themselves as new tools to assay them are being generated. A formidable source of noncanonical signal transduction concepts is neural stem cells because these cells appear to have acquired unusual signaling interpretations to allow them to maintain their unique dual properties (self-renewal and multipotency). We discuss how such findings from the neural field can provide a blueprint for the identification of new molecular mechanisms regulating pancreatic biology, with a focus on Notch, Hes/Hey, and hedgehog pathways.

Xenopus as a model system for studying pancreatic development and diabetes

Diabetes is a chronic disease caused by the loss or dysfunction of the insulin-producing β-cells in the pancreas. To date, much of our knowledge about β-cells in humans comes from studying rare monogenic forms of diabetes. Importantly, the majority of mutations so far associated to monogenic diabetes are in genes that exert a regulatory role in pancreatic development and/or β-cell function. Thus, the identification and study of novel mutations open an unprecedented window into human pancreatic development. In this review, we summarize major advances in the genetic dissection of different types of monogenic diabetes and the insights gained from a developmental perspective. We highlight future challenges to bridge the gap between the fast accumulation of genetic data through next-generation sequencing and the need of functional insights into disease mechanisms. Lastly, we discuss the relevance and advantages of studying candidate gene variants in vivo using the Xenopus as model system.

Xenopus as a Model for GI/Pancreas Disease

Diseases affecting endodermal organs like the pancreas, lung and gastrointestinal (GI) tract have a substantial impact on human welfare. Since many of these are congenital defects that arise as a result of defects during development broad efforts are focused on understanding the development of these organs so as to better identify risk factors, disease mechanisms and therapeutic targets. Studies implementing model systems, like the amphibian Xenopus, have contributed immensely to our understanding of signaling (e.g. Wnt, FGF, BMP, RA) pathways and gene regulation (e.g. hhex, ptf1a, ngn3) that underlie normal development as well as disease progression. Recent advances in genome engineering further enhance the capabilities of the Xenopus model system for pursuing biomedical research, and will undoubtedly result in a boom of new information underlying disease mechanisms ultimately leading to advancements in diagnosis and therapy.

Direct Reprogramming for Pancreatic Beta-Cells Using Key Developmental Genes

Direct reprogramming is a promising approach for regenerative medicine whereby one cell type is directly converted into another without going through a multipotent or pluripotent stage. This reprogramming approach has been extensively explored for the generation of functional insulin-secreting cells from non-beta-cells with the aim of developing novel cell therapies for the treatment of people with diabetes lacking sufficient endogenous beta-cells. A common approach for such conversion studies is the introduction of key regulators that are important in controlling beta-cell development and maintenance. In this review, we will summarize the recent advances in the field of beta-cell reprogramming and discuss the challenges of creating functional and long-lasting beta-cells.

Dominant and context-specific control of endodermal organ allocation by Ptf1a

The timing and gene regulatory logic of organ-fate commitment from within the posterior foregut of the mammalian endoderm is largely unexplored. Transient misexpression of a presumed pancreatic-commitment transcription factor, Ptf1a, in embryonic mouse endoderm (Ptf1a(EDD)) dramatically expanded the pancreatic gene regulatory network within the foregut. Ptf1a(EDD) temporarily suppressed Sox2 broadly over the anterior endoderm. Pancreas-proximal organ territories underwent full tissue conversion. Early-stage Ptf1a(EDD) rapidly expanded the endogenous endodermal Pdx1-positive domain and recruited other pancreas-fate-instructive genes, thereby spatially enlarging the potential for pancreatic multipotency. Early Ptf1a(EDD) converted essentially the entire glandular stomach, rostral duodenum and extrahepatic biliary system to pancreas, with formation of many endocrine cell clusters of the type found in normal islets of Langerhans. Sliding the Ptf1a(EDD) expression window through embryogenesis revealed differential temporal competencies for stomach-pancreas respecification. The response to later-stage Ptf1a(EDD) changed radically towards unipotent, acinar-restricted conversion. We provide strong evidence, beyond previous Ptf1a inactivation or misexpression experiments in frog embryos, for spatiotemporally context-dependent activity of Ptf1a as a potent gain-of-function trigger of pro-pancreatic commitment.

Efficient RNA/Cas9-mediated genome editing in Xenopus tropicalis

For the emerging amphibian genetic model Xenopus tropicalis targeted gene disruption is dependent on zinc-finger nucleases (ZFNs) or transcription activator-like effector nucleases (TALENs), which require either complex design and selection or laborious construction. Thus, easy and efficient genome editing tools are still highly desirable for this species. Here, we report that RNA-guided Cas9 nuclease resulted in precise targeted gene disruption in all ten X. tropicalis genes that we analyzed, with efficiencies above 45% and readily up to 100%. Systematic point mutation analyses in two loci revealed that perfect matches between the spacer and the protospacer sequences proximal to the protospacer adjacent motif (PAM) were essential for Cas9 to cleave the target sites in the X. tropicalis genome. Further study showed that the Cas9 system could serve as an efficient tool for multiplexed genome engineering in Xenopus embryos. Analysis of the disruption of two genes, ptf1a/p48 and tyrosinase, indicated that Cas9-mediated gene targeting can facilitate direct phenotypic assessment in X. tropicalis embryos. Finally, five founder frogs from targeting of either elastase-T1, elastase-T2 or tyrosinase showed highly efficient transmission of targeted mutations into F1 embryos. Together, our data demonstrate that the Cas9 system is an easy, efficient and reliable tool for multiplex genome editing in X. tropicalis.

Pancreas organogenesis: from lineage determination to morphogenesis

The pancreas is an essential organ for proper nutrient metabolism and has both endocrine and exocrine function. In the past two decades, knowledge of how the pancreas develops during embryogenesis has significantly increased, largely from developmental studies in model organisms. Specifically, the molecular basis of pancreatic lineage decisions and cell differentiation is well studied. Still not well understood are the mechanisms governing three-dimensional morphogenesis of the organ. Strategies to derive transplantable β-cells in vitro for diabetes treatment have benefited from the accumulated knowledge of pancreas development. In this review, we provide an overview of the current understanding of pancreatic lineage determination and organogenesis, and we examine future implications of these findings for treatment of diabetes mellitus through cell replacement.

Retinoic acid-activated Ndrg1a represses Wnt/β-catenin signaling to allow Xenopus pancreas, oesophagus, stomach, and duodenum specification

How cells integrate multiple patterning signals to achieve early endoderm regionalization remains largely unknown. Between gastrulation and neurulation, retinoic acid (RA) signaling is required, while Wnt/β-catenin signaling has to be repressed for the specification of the pancreas, oesophagus, stomach, and duodenum primordia in Xenopus embryos. In attempt to screen for RA regulated genes in Xenopus endoderm, we identified a direct RA target gene, N-myc downstream regulated gene 1a (ndrg1a) that showed expression early in the archenteron roof endoderm and late in the developing pancreas, oesophagus, stomach, and duodenum. Both antisense morpholino oligonucleotide mediated knockdown of ndrg1a in Xenopus laevis and the transcription activator-like effector nucleases (TALEN) mediated disruption of ndrg1 in Xenopus tropicalis demonstrate that like RA signaling, Ndrg1a is specifically required for the specification of Xenopus pancreas, oesophagus, stomach, and duodenum primordia. Immunofluorescence data suggest that RA-activated Ndrg1a suppresses Wnt/β-catenin signaling in Xenopus archenteron roof endoderm cells. Blocking Wnt/β-catenin signaling rescued Ndrg1a knockdown phenotype. Furthermore, overexpression of the putative Wnt/β-catenin target gene Atf3 phenocopied knockdown of Ndrg1a or inhibition of RA signaling, while Atf3 knockdown can rescue Ndrg1a knockdown phenotype. Lastly, the pancreas/stomach/duodenum transcription factor Pdx1 was able to rescue Atf3 overexpression or Ndrg1a knockdown phenotype. Together, we conclude that RA activated Ndrg1a represses Wnt/β-catenin signaling to allow the specification of pancreas, oesophagus, stomach, and duodenum progenitor cells in Xenopus embryos.

Restoring insulin production for type 1 diabetes

Current therapies for the treatment of type 1 diabetes include daily administration of exogenous insulin and, less frequently, whole-pancreas or islet transplantation. Insulin injections often result in inaccurate insulin doses, exposing the patient to hypo- and/or hyperglycemic episodes that lead to long-term complications. Islet transplantation is also limited by lack of high-quality islet donors, early graft failure, and chronic post-transplant immunosuppressive treatment. These barriers could be circumvented by designing a safe and efficient strategy to restore insulin production within the patient's body. Porcine islets have been considered as a possible alternative source of transplantable insulin-producing cells to replace human cadaveric islets. More recently, embryonic or induced pluripotent stem cells have also been examined for their ability to differentiate in vitro into pancreatic endocrine cells. Alternatively, it may be feasible to generate new β-cells by ectopic expression of key transcription factors in endogenous non-β-cells. Finally, engineering surrogate β-cells by in vivo delivery of the insulin gene to specific tissues is also being studied as a possible therapy for type 1 diabetes. In the present review, we discuss these different approaches to restore insulin production.

Prolonged FGF signaling is necessary for lung and liver induction in Xenopus

BACKGROUND

FGF signaling plays numerous roles during organogenesis of the embryonic gut tube. Mouse explant studies suggest that different thresholds of FGF signaling from the cardiogenic mesoderm induce lung, liver, and pancreas lineages from the ventral foregut progenitor cells. The mechanisms that regulate FGF dose in vivo are unknown. Here we use Xenopus embryos to examine the hypothesis that a prolonged duration of FGF signaling from the mesoderm is required to induce foregut organs.

RESULTS

We show that both mesoderm and FGF signaling are required for liver and lung development in Xenopus; formally demonstrating that this important step in organ induction is conserved with other vertebrate species. Prolonged contact with the mesoderm and persistent FGF signaling through both MEK and PI3K over an extended period of time are required for liver and lung specification. Inhibition of FGF signaling results in reduced liver and lung development, with a modest expansion of the pancreas/duodenum progenitor domain. Hyper-activation of FGF signaling has the opposite effect expanding liver and lung gene expression and repressing pancreatic markers. We show that FGF signaling is cell autonomously required in the endoderm and that a dominant negative FGF receptor decreases the ability of ventral foregut progenitor cells to contribute to the lung and liver buds.

CONCLUSIONS

These results suggest that the liver and lungs are specified at progressively later times in development requiring mesoderm contact for different lengths of time. Our data suggest that this is achieved at least in part through prolonged FGF signaling. In addition to providing a foundation for further mechanistic studies on foregut organogenesis using the experimental advantages of the Xenopus system, these data have implications for the directed differentiation of stem cells into foregut lineages.

Microarray analysis of Xenopus endoderm expressing Ptf1a

Pancreas-specific transcription factor 1a (Ptf1a), a bHLH transcription factor, has two temporally distinct functions during pancreas development; initially it is required for early specification of the entire pancreas, while later it is required for proper differentiation and maintenance of only acinar cells. The importance of Ptf1a function was revealed by the fact that loss of Ptf1a leads to pancreas agenesis in humans. While Ptf1a is one of the most important pancreatic transcription factors, little is known about the differences between the regulatory networks it controls during initial specification of the pancreas as opposed to acinar cell development, and to date no comprehensive analysis of its downstream targets has been published. In this article, we use Xenopus embryos to identify putative downstream targets of Ptf1a. We isolated anterior endoderm tissue overexpressing Ptf1a at two early stages, NF32 and NF36, and compared their gene expression profiles using microarrays. Our results revealed that Ptf1a regulates genes with a wide variety of functions, providing insight into the complexity of the regulatory network required for pancreas specification.

Homeoprotein hhex-induced conversion of intestinal to ventral pancreatic precursors results in the formation of giant pancreata in Xenopus embryos

Liver and ventral pancreas develop from neighboring territories within the endoderm of gastrulae. ventral pancreatic precursor 1 (vpp1) is a marker gene that is differentially expressed in a cell population within the dorsal endoderm in a pattern partially overlapping with that of hematopoietically expressed homeobox (hhex) during gastrulation. In tail bud embryos, vpp1 expression specifically demarcates two ventral pancreatic buds, whereas hhex expression is mainly restricted to the liver diverticulum. Ectopic expression of a critical dose of hhex led to a greatly enlarged vpp1-positive domain and, subsequently, to the formation of giant ventral pancreata, putatively by conversion of intestinal to ventral pancreatic precursor cells. Conversely, antisense morpholino oligonucleotide-mediated knockdown of hhex resulted in a down-regulation of vpp1 expression and a specific loss of the ventral pancreas. Furthermore, titration of hhex with a dexamethasone-inducible hhex-VP16GR fusion construct suggested that endogenous hhex activity during gastrulation is essential for the formation of ventral pancreatic progenitor cells. These observations suggest that, beyond its role in liver development, hhex controls specification of a vpp1-positive endodermal cell population during gastrulation that is required for the formation of the ventral pancreas.

Development of Xenopus resource centers: the National Xenopus Resource and the European Xenopus Resource Center

Xenopus is an essential vertebrate model system for biomedical research that has contributed to important discoveries in many disciplines, including cell biology, molecular biology, physiology, developmental biology, and neurobiology. However, unlike other model systems no central repository/stock center for Xenopus had been established until recently. Similar to mouse, zebrafish, and fly communities, which have established stock centers, Xenopus researchers need to maintain and distribute rapidly growing numbers of inbred, mutant, and transgenic frog strains, along with DNA and protein resources, and individual laboratories struggle to accomplish this efficiently. In the last 5 years, two resource centers were founded to address this need: the European Xenopus Resource Center (EXRC) at the University of Portsmouth in England, and the National Xenopus Resource (NXR) at the Marine Biological Laboratory in Woods Hole, MA. These two centers work together to provide resources and support to the Xenopus research community. The EXRC and NXR serve as stock centers and acquire, produce, maintain and distribute mutant, inbred and transgenic Xenopus laevis and Xenopus tropicalis lines. Independently, the EXRC is a repository for Xenopus cDNAs, fosmids, and antibodies; it also provides oocytes and wild-type frogs within the United Kingdom. The NXR will complement these services by providing research training and promoting intellectual interchange through hosting mini-courses and workshops and offering space for researchers to perform short-term projects at the Marine Biological Laboratory. Together the EXRC and NXR will enable researchers to improve productivity by providing resources and expertise to all levels, from graduate students to experienced PIs. These two centers will also enable investigators that use other animal systems to take advantage of Xenopus' unique experimental features to complement their studies.

Xenopus staufen2 is required for anterior endodermal organ formation

Defining the regulatory molecular networks involved in patterning the developing anterior endoderm is essential to understand how the pancreas, liver, stomach, and duodenum are discretely specified from each other. In this study, we analyzed the expression and function of the double-stranded RNA-binding protein Staufen2 in Xenopus laevis endoderm. We found that staufen2 was broadly expressed within the developing endoderm beginning at gastrulation becoming localized to the anterior endoderm at later stages. Through morpholino-mediated knockdown, we demonstrate that Staufen2 function is required for proper formation of the stomach, liver, and pancreas. We define that its function is required during gastrulation for proper patterning of the dorsal-ventral axis and that it acts to regulate expression of BMP signaling components.

Transient expression of Ngn3 in Xenopus endoderm promotes early and ectopic development of pancreatic beta and delta cells

Promoting ectopic development of pancreatic beta cells from other cell types is one of the strategies being pursued for the treatment of diabetes. To achieve this, a detailed outline of the molecular lineage that operates in pancreatic progenitor cells to generate beta cells over other endocrine cell types is necessary. Here, we demonstrate that early transient expression of the endocrine progenitor bHLH protein Neurogenin 3 (Ngn3) favors the promotion of pancreatic beta and delta cell fates over an alpha cell fate, while later transient expression promotes ectopic development of all three endocrine cell fates. We found that short-term activation of Ngn3 in Xenopus laevis endoderm just after gastrulation was sufficient to promote both early and ectopic development of beta and delta cells. By examining gene expression changes 4 h after Ngn3 activation we identified several new downstream targets of Ngn3. We show that several of these are required for the promotion of ectopic beta cells by Ngn3 as well as for normal beta cell development. These results provide new detail regarding the Ngn3 transcriptional network operating in endocrine progenitor cells to specify a beta cell phenotype and should help define new approaches to promote ectopic development of beta cells for diabetes therapy.

MiR-18a regulates expression of the pancreatic transcription factor Ptf1a in pancreatic progenitor and acinar cells

The basic helix-loop-helix (bHLH) transcription factor Ptf1a plays stage-specific roles in the developing pancreas. During early pancreatic development, low levels of Ptf1a preferentially promote the differentiation of pancreatic progenitor cells into endocrine cells, whereas high levels of Ptf1a shift pancreatic progenitors towards an exocrine cell fate. In adults, Ptf1a is essential for the production of exocrine enzymes by pancreatic acinar cells. In this paper, we show that Ptf1a expression is repressed by miR-18a in pancreatic progenitors and acinar cells via its binding to the 3'UTR of Ptf1a mRNA. Furthermore, overexpression of miR-18a exerts little effect on pancreatic progenitors and acinar cells. These results indicate that miR-18a plays a fine-tuning role in regulating pancreatic progenitors and exocrine cells through the repression of Ptf1a expression.

RNA profiling and chromatin immunoprecipitation-sequencing reveal that PTF1a stabilizes pancreas progenitor identity via the control of MNX1/HLXB9 and a network of other transcription factors

Pancreas development is initiated by the specification and expansion of a small group of endodermal cells. Several transcription factors are crucial for progenitor maintenance and expansion, but their interactions and the downstream targets mediating their activity are poorly understood. Among those factors, PTF1a, a basic helix-loop-helix (bHLH) transcription factor which controls pancreas exocrine cell differentiation, maintenance, and functionality, is also needed for the early specification of pancreas progenitors. We used RNA profiling and chromatin immunoprecipitation (ChIP) sequencing to identify a set of targets in pancreas progenitors. We demonstrate that Mnx1, a gene that is absolutely required in pancreas progenitors, is a major direct target of PTF1a and is regulated by a distant enhancer element. Pdx1, Nkx6.1, and Onecut1 are also direct PTF1a targets whose expression is promoted by PTF1a. These proteins, most of which were previously shown to be necessary for pancreas bud maintenance or formation, form a transcription factor network that allows the maintenance of pancreas progenitors. In addition, we identify Bmp7, Nr5a2, RhoV, and P2rx1 as new targets of PTF1a in pancreas progenitors.

Pioneer transcription factors: establishing competence for gene expression

Transcription factors are adaptor molecules that detect regulatory sequences in the DNA and target the assembly of protein complexes that control gene expression. Yet much of the DNA in the eukaryotic cell is in nucleosomes and thereby occluded by histones, and can be further occluded by higher-order chromatin structures and repressor complexes. Indeed, genome-wide location analyses have revealed that, for all transcription factors tested, the vast majority of potential DNA-binding sites are unoccupied, demonstrating the inaccessibility of most of the nuclear DNA. This raises the question of how target sites at silent genes become bound de novo by transcription factors, thereby initiating regulatory events in chromatin. Binding cooperativity can be sufficient for many kinds of factors to simultaneously engage a target site in chromatin and activate gene expression. However, in cases in which the binding of a series of factors is sequential in time and thus not initially cooperative, special "pioneer transcription factors" can be the first to engage target sites in chromatin. Such initial binding can passively enhance transcription by reducing the number of additional factors that are needed to bind the DNA, culminating in activation. In addition, pioneer factor binding can actively open up the local chromatin and directly make it competent for other factors to bind. Passive and active roles for the pioneer factor FoxA occur in embryonic development, steroid hormone induction, and human cancers. Herein we review the field and describe how pioneer factors may enable cellular reprogramming.

Xenopus research: metamorphosed by genetics and genomics

Research using Xenopus takes advantage of large, abundant eggs and readily manipulated embryos in addition to conserved cellular, developmental and genomic organization with mammals. Research on Xenopus has defined key principles of gene regulation and signal transduction, embryonic induction, morphogenesis and patterning as well as cell cycle regulation. Genomic and genetic advances in this system, including the development of Xenopus tropicalis as a genetically tractable complement to the widely used Xenopus laevis, capitalize on the classical strengths and wealth of achievements. These attributes provide the tools to tackle the complex biological problems of the new century, including cellular reprogramming, organogenesis, regeneration, gene regulatory networks and protein interactions controlling growth and development, all of which provide insights into a multitude of human diseases and their potential treatments.

Reprogramming gut and pancreas endocrine cells to treat diabetes

Multiple approaches have been investigated with the ultimate goal of providing insulin independence to patients with either type 1 or type 2 diabetes. Approaches to produce insulin-secreting cells in culture, convert non-β-cells into functional β-cells or engineer autologous cells to express and secrete insulin in a meal-responsive manner have all been described. This research has been facilitated by significant improvements in both viral and non-viral gene delivery approaches that have enabled new experimental strategies. Many studies have examined possible avenues to confer islet cytoprotection against immune rejection, inflammation and apoptosis by genetic manipulation of islet cells prior to islet transplantation. Here we review several reports based on the reprogramming of pancreas and gut endocrine cells to treat diabetes.

Restriction of hepatic competence by Fgf signaling

Hepatic competence, or the ability to respond to hepatic-inducing signals, is regulated by a number of transcription factors broadly expressed in the endoderm. However, extrinsic signals might also regulate hepatic competence, as suggested by tissue explant studies. Here, we present genetic evidence that Fgf signaling regulates hepatic competence in zebrafish. We first show that the endoderm posterior to the liver-forming region retains hepatic competence: using transgenic lines that overexpress hepatic inducing signals following heat-shock, we found that at late somitogenesis stages Wnt8a, but not Bmp2b, overexpression could induce liver gene expression in pancreatic and intestinal bulb cells. These manipulations resulted in the appearance of ectopic hepatocytes in the intestinal bulb. Second, by overexpressing Wnt8a at various stages, we found that as embryos develop, the extent of the endodermal region retaining hepatic competence is gradually reduced. Most significantly, we found, using gain- and loss-of-function approaches, that Fgf10a signaling regulates this gradual reduction of the hepatic-competent domain. These data provide in vivo evidence that endodermal cells outside the liver-forming region retain hepatic competence and show that an extrinsic signal, Fgf10a, negatively regulates hepatic competence.

Functional analysis of Rfx6 and mutant variants associated with neonatal diabetes

Mutations in rfx6 were recently associated with Mitchell-Riley syndrome, which involves neonatal diabetes, and other digestive system defects. To better define the function of Rfx6 in early endoderm development we cloned the Xenopus homologue. Expression of rfx6 begins early, showing broad expression throughout the anterior endoderm; at later stages rfx6 expression becomes restricted to the endocrine cells of the gut and pancreas. Morpholino knockdown of rfx6 caused a loss of pancreas marker expression, as well as other abnormalities. Co-injection of exogenous wild-type rfx6 rescued the morpholino phenotype in Xenopus tadpoles, whereas attempts to rescue the loss-of-function phenotype using mutant rfx6 based on Mitchell-Riley patients were unsuccessful. To better define the pleiotropic effects, we performed microarray analyses of gene expression in knockdown foregut tissue. In addition to pancreatic defects, the microarray analyses revealed downregulation of lung, stomach and heart markers and an upregulation of kidney markers. We verified these results using RT-PCR and in situ hybridization. Based on the different rfx6 expression patterns and our functional analyses, we propose that rfx6 has both early and late functions. In early development Rfx6 plays a broad role, being essential for development of most anterior endodermal organs. At later stages however, Rfx6 function is restricted to endocrine cells.

Appl1 is essential for the survival of Xenopus pancreas, duodenum, and stomach progenitor cells

An understanding of the molecular mechanisms governing the survival of organ progenitor cells in vivo is crucial for in vitro tissue regeneration. Here, we have found that Xenopus appl1 and akt2 share a similar embryonic expression pattern, showing characteristic expression in the central nervous system as well as in the pancreas and part of the stomach/duodenum (SD) at tadpole stages of development. Specific knockdown of appl1 in endoderm or inhibition of akt activity did not affect the formation of endodermal organ primordia at tail bud stages of development, but led to a gut-coiling defect, strong apoptosis in endodermal organs, and pancreas and SD hypoplasia or even aplasia at tadpole stages of development. Furthermore, appl1 is required for akt phosphorylation and akt2 in turn can rescue appl1 knockdown phenotypes. Together, our data suggest that appl1-akt signaling is specifically required for the survival of pancreas and SD progenitor cells in Xenopus laevis embryos.

Spatiotemporal expression of Pax genes in amphioxus: insights into Pax-related organogenesis and evolution

The expression of four AmphiPax genes in 16 developmental stages and different organs in amphioxus (Branchiostoma belcheri) was investigated, finding those genes expressed throughout amphioxus life with temporal-specific (especially during embryogenesis and metamorphosis) and spatial-specific patterns. This study suggests that duplicated Pax genes in vertebrates might maintain most of their ancestral functions and also expand their expression patterns after the divergence of protochordates and vertebrates.

BrunoL1 regulates endoderm proliferation through translational enhancement of cyclin A2 mRNA

Developmental control of proliferation relies on tight regulation of protein expression. Although this has been well studied in early embryogenesis, how the cell cycle is regulated during organogenesis is not well understood. Bruno-Like RNA binding proteins bind to consensus sequences in the 3'UTR of specific mRNAs and repress protein translation, but much of this functional information is derived from studies on mainly two members, Drosophila Bruno and vertebrate BrunoL2 (CUGBP1). There are however, six vertebrate and three Drosophila Bruno family members, but less is known about these other family members, and none have been shown to function in the endoderm. We recently identified BrunoL1 as a dorsal pancreas enriched gene, and in this paper we define BrunoL1 function in Xenopus endoderm development. We find that, in contrast to other Bruno-Like proteins, BrunoL1 acts to enhance rather than repress translation. We demonstrate that BrunoL1 regulates proliferation of endoderm cells through translational control of cyclin A2 mRNA. Specifically BrunoL1 enhanced translation of cyclin A2 through binding consensus Bruno Response Elements (BREs) in its 3'UTR. We compared the ability of other Bruno-Like proteins, both vertebrate and invertebrate, to stimulate translation via the cyclin A2 3'UTR and found that only Drosophila Bru-3 had similar activity. In addition, we also found that both BrunoL1 and Bru-3 enhanced translation of mRNAs containing the 3'UTRs of Drosophila oskar or cyclin A, which have been well characterized to mediate repression. Lastly, we show that it is the Linker region of BrunoL1 that is both necessary and sufficient for this activity. These results are the first example of BRE-dependent translational enhancement and are the first demonstration in vertebrates of Bruno-Like proteins regulating translation through BREs.

Regenerating pancreatic beta-cells: plasticity of adult pancreatic cells and the feasibility of in-vivo neogenesis

PURPOSE OF REVIEW

Diabetes results from inadequate functional mass of pancreatic beta-cells and therefore replenishing with new glucose-responsive beta-cells is an important therapeutic option. In addition to replication of pre-existing beta-cells, new beta-cells can be produced from differentiated adult cells using in-vitro or in-vivo approaches. This review will summarize recent advances in in-vivo generation of beta-cells from cells that are not beta-cells (neogenesis) and discuss ways to overcome the limitations of this process.

RECENT FINDINGS

Multiple groups have shown that adult pancreatic ducts, acinar and even endocrine cells exhibit cellular plasticity and can differentiate into beta-cells in vivo. Several different approaches, including misexpression of transcription factors and tissue injury, have induced neogenesis of insulin-expressing cells in vivo and ameliorated diabetes.

SUMMARY

Recent breakthroughs demonstrating cellular plasticity of adult pancreatic cells to form new beta-cells are a positive first step towards developing in-vivo regeneration-based therapy for diabetes. Currently, neogenesis processes are inefficient and do not generate sufficient amounts of beta-cells required to normalize hyperglycemia. However, an improved understanding of mechanisms regulating neogenesis of beta-cells from adult pancreatic cells and of their maturation into functional glucose-responsive beta-cells can make therapies based on in-vivo regeneration a reality.

Xenopus insm1 is essential for gastrointestinal and pancreatic endocrine cell development

In mammals, it has been well established that gastrointestinal and pancreatic endocrine cells are specified by a cascade of different transcription factors, but whether these same pathways (or linear relationships) operate in Xenopus is currently unknown. We recently identified the endocrine-specific zinc finger transcription factor insulinoma associated protein 1 (insm1) as a dorsal-enriched gene. We found that insm1 is expressed in the dorsal pancreas as early as NF28, making it one of the earliest markers to be localized to the dorsal pancreas. Through morpholino-mediated knockdown, we demonstrate that insm1 is essential for proper specification of both gastrointestinal and pancreatic endocrine cells. In addition, we place insm1 downstream of ngn3 and upstream of pax6 and neuroD in the endocrine cell transcription factor cascade. These are the first results showing that the endodermal endocrine cell development in Xenopus uses the same transcriptional cascade as in mammals.

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.

Xenopus pancreas development

Understanding how the pancreas develops is vital to finding new treatments for a range of pancreatic diseases, including diabetes and pancreatic cancer. Xenopus is a relatively new model organism for the elucidation of pancreas development, and has already made contributions to the field. Recent studies have shown benefits of using Xenopus for understanding both early patterning and lineage specification aspects of pancreas organogenesis. This review focuses specifically on Xenopus pancreas development, and covers events from the end of gastrulation, when regional specification of the endoderm is occurring, right through metamorphosis, when the mature pancreas is fully formed. We have attempted to cover pancreas development in Xenopus comprehensively enough to assist newcomers to the field and also to enable those studying pancreas development in other model organisms to better place the results from Xenopus research into the context of the field in general and their studies specifically. Developmental Dynamics 238:1271-1286, 2009. (c) 2009 Wiley-Liss, Inc.

The tetraspanin Tm4sf3 is localized to the ventral pancreas and regulates fusion of the dorsal and ventral pancreatic buds

During embryogenesis, the pancreas develops from separate dorsal and ventral buds, which fuse to form the mature pancreas. Little is known about the functional differences between these two buds or the relative contribution of cells derived from each region to the pancreas after fusion. To follow the fate of dorsal or ventral bud derived cells in the pancreas after fusion, we produced chimeric Elas-GFP transgenic/wild-type embryos in which either dorsal or ventral pancreatic bud cells expressed GFP. We found that ventral pancreatic cells migrate extensively into the dorsal pancreas after fusion, whereas the converse does not occur. Moreover, we found that annular pancreatic tissue is composed exclusively of ventral pancreas-derived cells. To identify ventral pancreas-specific genes that may play a role in pancreatic bud fusion, we isolated individual dorsal and ventral pancreatic buds, prior to fusion, from NF38/39 Xenopus laevis tadpoles and compared their gene expression profiles (NF refers to the specific stage of Xenopus development). As a result of this screen, we have identified several new ventral pancreas-specific genes, all of which are expressed in the same location within the ventral pancreas at the junction where the two ventral pancreatic buds fuse. Morpholino-mediated knockdown of one of these ventral-specific genes, transmembrane 4 superfamily member 3 (tm4sf3), inhibited dorsal-ventral pancreatic bud fusion, as well as acinar cell differentiation. Conversely, overexpression of tm4sf3 promoted development of annular pancreas. Our results are the first to define molecular and behavioral differences between the dorsal and ventral pancreas, and suggest an unexpected role for the ventral pancreas in pancreatic bud fusion.

On the role of FOX transcription factors in adipocyte differentiation and insulin-stimulated glucose uptake

In this study, we explore the effects of several FOX and mutant FOX transcription factors on adipocyte determination, differentiation, and metabolism. In addition to Foxc2 and Foxo1, we report that Foxf2, Foxp1, and Foxa1 are other members of the Fox family that show regulated expression during adipogenesis. Although enforced expression of FOXC2 inhibits adipogenesis, Foxf2 slightly enhances the rate of differentiation. Constitutively active FOXC2-VP16 inhibits adipogenesis through multiple mechanisms. FOXC2-VP16 impairs the transient induction of C/EBPbeta during adipogenesis and induces expression of the transcriptional repressor Hey1 as well as the activator of Wnt/beta-catenin signaling, Wnt10b. The constitutive transcriptional repressor, FOXC2-Eng, enhances adipogenesis of preadipocytes and multipotent mesenchymal precursors and determines NIH-3T3 and C2C12 cells to the adipocyte lineage. Although PPARgamma ligand or C/EBPalpha are not necessary for stimulation of adipogenesis by FOXC2-Eng, at least low levels of PPARgamma protein are absolutely required. Finally, expression of FOXC2-Eng in adipocytes increases insulin-stimulated glucose uptake, further expanding the profound and pleiotropic effects of FOX transcription factors on adipocyte biology.

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.

Murine embryonic stem cell-derived pancreatic acinar cells recapitulate features of early pancreatic differentiation

BACKGROUND & AIMS

Acinar cells constitute 90% of the pancreas epithelium, are polarized, and secrete digestive enzymes. These cells play a crucial role in pancreatitis and pancreatic cancer. However, there are limited models to study normal acinar cell differentiation in vitro. The aim of this work was to generate and characterize purified populations of pancreatic acinar cells from embryonic stem (ES) cells.

METHODS

Reporter ES cells (Ela-pur) were generated that stably expressed both beta-galactosidase and puromycin resistance genes under the control of the elastase I promoter. Directed differentiation was achieved by incubation with conditioned media of cultured fetal pancreatic rudiments and adenoviral-mediated co-expression of p48/Ptf1a and Mist1, 2 basic helix-loop-helix transcription factors crucial for normal pancreatic acinar development and differentiation.

RESULTS

Selected cells expressed multiple markers of acinar cells, including digestive enzymes and proteins of the secretory pathway, indicating activation of a coordinated differentiation program. The genes coding for digestive enzymes were not regulated as a single module, thus recapitulating what occurs during in vivo pancreatic development. The generated cells displayed transient agonist-induced Ca(2+) mobilization and showed a typical response to physiologic concentrations of secretagogues, including enzyme synthesis and secretion. Importantly, these effects did not imply the acquisition of a mixed acinar-ductal phenotype.

CONCLUSIONS

These studies allow the generation of almost pure acinar-like cells from ES cells, providing a normal cell-based model for the study of the acinar differentiation program in vitro.

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).

Reduction of Ptf1a gene dosage causes pancreatic hypoplasia and diabetes in mice

OBJECTIVE

Most pancreatic endocrine cells derive from Ptf1a-expressing progenitor cells. In humans, nonsense mutations in Ptf1a have recently been identified as a cause of permanent neonatal diabetes associated with pancreatic agenesis. The death of Ptf1a-null mice soon after birth has not allowed further insight into the pathogenesis of the disease; it is therefore unclear how much pancreatic endocrine function is dependent on Ptf1a in mammals. This study aims to investigate gene-dosage effects of Ptf1a on pancreas development and function in mice.

RESEARCH DESIGN AND METHODS

Combining hypomorphic and null alleles of Ptf1a and Cre-mediated lineage tracing, we followed the cell fate of reduced Ptf1a-expressing progenitors and analyzed pancreas development and function in mice.

RESULTS

Reduced Ptf1a dosage resulted in pancreatic hypoplasia and glucose intolerance with insufficient insulin secretion in a dosage-dependent manner. In hypomorphic mutant mice, pancreatic bud size was small and substantial proportions of pancreatic progenitors were misspecified to the common bile duct and duodenal cells. Growth with branching morphogenesis and subsequent exocrine cytodifferentiation was reduced and delayed. Total beta-cell number was decreased, proportion of non-beta islet cells was increased, and alpha-cells were abnormally intermingled with beta-cells. Interestingly, Pdx1 expression was decreased in early pancreatic progenitors but elevated to normal level at the mid-to-late stages of pancreatogenesis. CONCLUSIONS-The dosage of Ptf1a is crucial for pancreas specification, growth, total beta-cell number, islet morphogenesis, and endocrine function. Some neonatal diabetes may be caused by mutation or single nucleotide polymorphisms in the Ptf1a gene that reduce gene expression levels.

Remodeling the exocrine pancreas at metamorphosis in Xenopus laevis

At metamorphosis the Xenopus laevis tadpole exocrine pancreas remodels in two stages. At the climax of metamorphosis thyroid hormone (TH) induces dedifferentiation of the entire exocrine pancreas to a progenitor state. The organ shrinks to 20% of its size, and approximately 40% of its cells die. The acinar cells lose their zymogen granules and approximately 75% of their RNA. The mRNAs that encode exocrine-specific proteins (including the transcription factor Ptf1a) undergo almost complete extinction at climax, whereas PDX-1, Notch-1, and Hes-1, genes implicated in differentiation of the progenitor cells, are activated. At the end of spontaneous metamorphosis when the endogenous TH has reached a low level, the pancreas begins to redifferentiate. Exogenous TH induces the dedifferentiation phase but not the redifferentation phase. The tadpole pancreas lacks the mature ductal system that is found in adult vertebrate pancreases, including the frog. Exocrine pancreases of transgenic tadpoles expressing a dominant negative form of the TH receptor controlled by the elastase promoter are resistant to TH. They do not shrink when subjected to TH. Their acinar cells do not dedifferentiate at climax, nor do they down-regulate exocrine-specific genes or activate Notch-1 and Hes-1. Even 2 months after metamorphosis these frogs have not developed a mature ductal system and the acinar cells are abnormally arranged. The TH-dependent dedifferentiation of the tadpole acinar cells at climax is a necessary step in the formation of a mature frog pancreas.

Repression of Wnt/beta-catenin signaling in the anterior endoderm is essential for liver and pancreas development

The liver and pancreas are specified from the foregut endoderm through an interaction with the adjacent mesoderm. However, the earlier molecular mechanisms that establish the foregut precursors are largely unknown. In this study, we have identified a molecular pathway linking gastrula-stage endoderm patterning to organ specification. We show that in gastrula and early-somite stage Xenopus embryos, Wnt/beta-catenin activity must be repressed in the anterior endoderm to maintain foregut identity and to allow liver and pancreas development. By contrast, high beta-catenin activity in the posterior endoderm inhibits foregut fate while promoting intestinal development. Experimentally repressing beta-catenin activity in the posterior endoderm was sufficient to induce ectopic organ buds that express early liver and pancreas markers. beta-catenin acts in part by inhibiting expression of the homeobox gene hhex, which is one of the earliest foregut markers and is essential for liver and pancreas development. Promoter analysis indicates that beta-catenin represses hhex transcription indirectly via the homeodomain repressor Vent2. Later in development, beta-catenin activity has the opposite effect and enhances liver development. These results illustrate that turning Wnt signaling off and on in the correct temporal sequence is essential for organ formation, a finding that might directly impact efforts to differentiate liver and pancreas tissue from stem cells.