Lineage-specific morphogenesis in the developing pancreas: role of mesenchymal factors

Pancreatic mesenchyme regulates epithelial organogenesis throughout development

Genetic disruption of the pancreatic mesenchyme reveals that it is critical for the expansion of epithelial progenitors and for the proliferation of insulin-producing beta cells.

The developing pancreatic epithelium gives rise to all endocrine and exocrine cells of the mature organ. During organogenesis, the epithelial cells receive essential signals from the overlying mesenchyme. Previous studies, focusing on ex vivo tissue explants or complete knockout mice, have identified an important role for the mesenchyme in regulating the expansion of progenitor cells in the early pancreas epithelium. However, due to the lack of genetic tools directing expression specifically to the mesenchyme, the potential roles of this supporting tissue in vivo, especially in guiding later stages of pancreas organogenesis, have not been elucidated. We employed transgenic tools and fetal surgical techniques to ablate mesenchyme via Cre-mediated mesenchymal expression of Diphtheria Toxin (DT) at the onset of pancreas formation, and at later developmental stages via in utero injection of DT into transgenic mice expressing the Diphtheria Toxin receptor (DTR) in this tissue. Our results demonstrate that mesenchymal cells regulate pancreatic growth and branching at both early and late developmental stages by supporting proliferation of precursors and differentiated cells, respectively. Interestingly, while cell differentiation was not affected, the expansion of both the endocrine and exocrine compartments was equally impaired. To further elucidate signals required for mesenchymal cell function, we eliminated β-catenin signaling and determined that it is a critical pathway in regulating mesenchyme survival and growth. Our study presents the first in vivo evidence that the embryonic mesenchyme provides critical signals to the epithelium throughout pancreas organogenesis. The findings are novel and relevant as they indicate a critical role for the mesenchyme during late expansion of endocrine and exocrine compartments. In addition, our results provide a molecular mechanism for mesenchymal expansion and survival by identifying β-catenin signaling as an essential mediator of this process. These results have implications for developing strategies to expand pancreas progenitors and β-cells for clinical transplantation.

Embryonic development is a highly complex process that requires tight orchestration of cellular proliferation, differentiation, and migration as cells grow within loosely aggregated mesenchyme and more organized epithelial sheets to form organs and tissues. In addition to intrinsic cell-autonomous signals, these events are further regulated by environmental cues provided by neighboring cells. Prior work demonstrated a critical role for the surrounding mesenchyme in guiding epithelial growth during the early stages of pancreas development. However, it remained unclear whether the mesenchyme also guided the later stages of pancreas organogenesis when the functional exocrine and endocrine cells are formed. Here, we show that specific genetic ablation of the mesenchyme at distinct developmental stages in vivo results in the formation of a smaller, misshapen pancreas. Loss of the mesenchyme profoundly impairs the expansion of both endocrine and exocrine pancreatic progenitors, as well as the proliferative capacity of maturing cells, including insulin-producing beta-cells. Thus, our studies reveal unappreciated roles for the mesenchyme in guiding the formation of the epithelial pancreas throughout development. The results suggest that identifying the specific mesenchymal signals might help to optimize cell culture protocols that aim to achieve the differentiation of stem cells into insulin-producing beta cells.

Pancreas organogenesis: from bud to plexus to gland

Pancreas oganogenesis comprises a coordinated and highly complex interplay of signaling events and transcriptional networks that guide a step-wise process of organ development from early bud specification all the way to the final mature organ state. Extensive research on pancreas development over the last few years, largely driven by a translational potential for pancreatic diseases (diabetes, pancreatic cancer, and so on), is markedly advancing our knowledge of these processes. It is a tenable goal that we will one day have a clear, complete picture of the transcriptional and signaling codes that control the entire organogenetic process, allowing us to apply this knowledge in a therapeutic context, by generating replacement cells in vitro, or perhaps one day to the whole organ in vivo. This review summarizes findings in the past 5 years that we feel are amongst the most significant in contributing to the deeper understanding of pancreas development. Rather than try to cover all aspects comprehensively, we have chosen to highlight interesting new concepts, and to discuss provocatively some of the more controversial findings or proposals. At the end of the review, we include a perspective section on how the whole pancreas differentiation process might be able to be unwound in a regulated fashion, or redirected, and suggest linkages to the possible reprogramming of other pancreatic cell-types in vivo, and to the optimization of the forward-directed-differentiation of human embryonic stem cells (hESC), or induced pluripotential cells (iPSC), towards mature β-cells.

The Cdk4-E2f1 pathway regulates early pancreas development by targeting Pdx1+ progenitors and Ngn3+ endocrine precursors

Cell division and cell differentiation are intricately regulated processes vital to organ development. Cyclin-dependent kinases (Cdks) are master regulators of the cell cycle that orchestrate the cell division and differentiation programs. Cdk1 is essential to drive cell division and is required for the first embryonic divisions, whereas Cdks 2, 4 and 6 are dispensable for organogenesis but vital for tissue-specific cell development. Here, we illustrate an important role for Cdk4 in regulating early pancreas development. Pancreatic development involves extensive morphogenesis, proliferation and differentiation of the epithelium to give rise to the distinct cell lineages of the adult pancreas. The cell cycle molecules that specify lineage commitment within the early pancreas are unknown. We show that Cdk4 and its downstream transcription factor E2f1 regulate mouse pancreas development prior to and during the secondary transition. Cdk4 deficiency reduces embryonic pancreas size owing to impaired mesenchyme development and fewer Pdx1(+) pancreatic progenitor cells. Expression of activated Cdk4(R24C) kinase leads to increased Nkx2.2(+) and Nkx6.1(+) cells and a rise in the number and proliferation of Ngn3(+) endocrine precursors, resulting in expansion of the β cell lineage. We show that E2f1 binds and activates the Ngn3 promoter to modulate Ngn3 expression levels in the embryonic pancreas in a Cdk4-dependent manner. These results suggest that Cdk4 promotes β cell development by directing E2f1-mediated activation of Ngn3 and increasing the pool of endocrine precursors, and identify Cdk4 as an important regulator of early pancreas development that modulates the proliferation potential of pancreatic progenitors and endocrine precursors.

Differential expression pattern of ZAC in developing mouse and human pancreas

ZAC is a transcription factor and cofactor, a strong candidate for transient neonatal diabetes mellitus (TNDM). TNDM involves impaired beta-cell development and is probably due to a double dose of ZAC, which is normally expressed only from the paternal copy. ZAC and Zac1 (its mouse orthologue) are strongly expressed in the proliferating progenitor/stem cells in many systems and also in some differentiated sites in human and mouse, suggesting a dual role in cell proliferation and differentiation control. Little is known about its expression in developing pancreas, the organ affected in TNDM. In this study, we examined ZAC/Zac1 expression in developing mouse and human pancreas by real-time PCR and dual in situ hybridization and immunofluorescence. Overall pancreatic expression drastically declined during gestation and early post-natal life in the mouse, and between the second trimester and adult in the human. Zac1 was predominantly expressed in mesenchyme in the mouse embryo, while ZAC was specifically expressed in islets of the human fetus. Thus, ZAC/Zac1 may play different roles in mouse and human pancreas development. The specific expression of ZAC in the human fetal beta-cells supports it as the gene involved in TNDM and the different expression pattern of Zac1 in mice from human may explain the much milder phenotype in the mouse model of ZAC double dose.

The expression and function of glucose-dependent insulinotropic polypeptide in the embryonic mouse pancreas

OBJECTIVE

Glucose-dependent insulinotropic polypeptide (GIP) is a member of a structurally related group of hormones that also includes glucagon, glucagon-like peptides, and secretin. GIP is an incretin, known to modulate glucose-induced insulin secretion. Recent studies have shown that glucagon is necessary for early insulin-positive differentiation, and a similar role for incretins in regulating embryonic insulin-positive differentiation seems probable. Here we studied the role of GIP signaling in insulin-positive differentiation in the embryonic mouse pancreas.

RESEARCH DESIGN AND METHODS

The ontogeny of the GIP ligand and GIP receptor in the embryonic pancreas was investigated by immunohistochemistry and RT-PCR. GIP signaling was inhibited in cultured embryonic pancreata using morpholine-ring antisense against GIP ligand and receptor, or small interfering RNA (siRNA) for GIP ligand and receptor. Markers of endocrine cells and their progenitors were studied by immunohistochemistry and RT-PCR.

RESULTS

GIP and GIP receptor mRNA were both detected in the embryonic pancreas by embryonic day 9.5 and then persisted throughout gestation. GIP was generally coexpressed with glucagon by immunostaining. The GIP receptor was typically coexpressed with insulin. Morpholine-ring antisense or siRNA against either GIP ligand or GIP receptor both inhibited the differentiation of insulin-positive cells. Inhibition of GIP or its receptor also led to a decrease in the number of Pdx-1-positive and sox9-positive cells in the cultured embryonic pancreas. The number of Pax6- and Nkx2.2-positive cells, representative of developing pancreatic endocrine cells and β-cells, respectively, was also decreased.

CONCLUSIONS

GIP signaling may play a role in early embryonic pancreas differentiation to form insulin-positive cells or β-cells.

Inhibition of gamma-secretase activity promotes differentiation of embryonic pancreatic precursor cells into functional islet-like clusters in poly(ethylene glycol) hydrogel culture

We assessed the ability of a gamma-secretase inhibitor to promote the in vitro differentiation of induced embryonic pancreatic precursor cell aggregates into functional islet-like clusters when encapsulated within a three-dimensional hydrogel. Undifferentiated pancreatic precursor cells were isolated from E.15 rat embryos, dissociated into single cells, and aggregated in suspension-rotation culture. Aggregates were photoencapsulated into poly(ethylene glycol) hydrogels with entrapped collagen type 1 and cultured for 14 days with or without a gamma-secretase inhibitor. Gene expression, proinsulin content, and C-peptide release were measured to determine differentiation and maturation of encapsulated precursor cell aggregates. In the control medium, scattered breakthrough beta cell differentiation was observed; however, cells remained largely insulin negative. Upon addition of a gamma-secretase inhibitor the majority of cells in clusters became insulin positive, and insulin per DNA and glucose-stimulated insulin release measurements for these cultures were comparable with those for adult rat islets. Cluster counts after culture day 14 were 88% of those initially encapsulated, demonstrating excellent cluster survival in hydrogel culture. These results indicate that concerted differentiation of pancreatic precursor cell aggregates into functionally mature islet-like clusters can be achieved in poly(ethylene glycol)-based hydrogel cultures by blocking cell contact-mediated Notch signaling with a gamma-secretase inhibitor.

Influence and timing of arrival of murine neural crest on pancreatic beta cell development and maturation

Interactions between cells from the ectoderm and mesoderm influence development of the endodermally-derived pancreas. While much is known about how mesoderm regulates pancreatic development, relatively little is understood about how and when the ectodermally-derived neural crest regulates pancreatic development and specifically, beta cell maturation. A previous study demonstrated that signals from the neural crest regulate beta cell proliferation and ultimately, beta cell mass. Here, we expand on that work to describe timing of neural crest arrival at the developing pancreatic bud and extend our knowledge of the non-cell autonomous role for neural crest derivatives in the process of beta cell maturation. We demonstrated that murine neural crest entered the pancreatic mesenchyme between the 26 and 27 somite stages (approximately 10.0 dpc) and became intermingled with pancreatic progenitors as the epithelium branched into the surrounding mesenchyme. Using a neural crest-specific deletion of the Forkhead transcription factor Foxd3, we ablated neural crest cells that migrate to the pancreatic primordium. Consistent with previous data, in the absence of Foxd3, and therefore the absence of neural crest cells, proliferation of insulin-expressing cells and insulin-positive area are increased. Analysis of endocrine cell gene expression in the absence of neural crest demonstrated that, although the number of insulin-expressing cells was increased, beta cell maturation was significantly impaired. Decreased MafA and Pdx1 expression illustrated the defect in beta cell maturation; we discovered that without neural crest, there was a reduction in the percentage of insulin-positive cells that co-expressed Glut2 and Pdx1 compared to controls. In addition, transmission electron microscopy analyses revealed decreased numbers of characteristic insulin granules and the presence of abnormal granules in insulin-expressing cells from mutant embryos. Together, these data demonstrate that the neural crest is a critical regulator of beta cell development on two levels: by negatively regulating beta cell proliferation and by promoting beta cell maturation.

Recent advances and prospects in the differentiation of pancreatic cells from human embryonic stem cells

Recent studies with human embryonic stem (hES) cells have established new protocols for substantial generation of pancreatic progenitors from definitive endoderm. These findings add to the efficient derivation of definitive endoderm, which is controlled by Wnt and Nodal pathways, and delineate a step forward in the quest for alternative beta-cell sources. It also indicates that critical refining of the available strategies might help define a universal protocol for pancreatic differentiation applicable to several cell lines, therefore offering the possibility for transplantation of immune-matched or patient-specific hES-derived beta-cells. We appraise here the fundamental role that bone morphogenetic protein, fibroblast growth factor, and retinoid signaling play during pancreas development, and describe a fundamental emergence of their combination in recent studies that generated pancreatic cells from hES cells. We finally enumerate some prospects that might improve further differentiation of the progenitor cells into functional beta-cells needed in diabetes cell therapy.

Hedgehog signaling in pancreas epithelium regulates embryonic organ formation and adult beta-cell function

OBJECTIVE

Current studies indicate that Hedgehog (Hh) signaling must be excluded during early stages of pancreas formation. However, conflicting evidence suggests that Hh signaling may be active later during pancreas formation and that it is required for insulin production and secretion in cultured beta-cell lines. The objective of this study was to address these discrepancies by assessing the in vivo role of epithelial Hh signaling in the pancreas.

RESEARCH DESIGN AND METHODS

To identify Hh-active cells in the developing and adult pancreas epithelium, we characterized transgenic reporter Patched1-LacZ mice. To determine the requirement for epithelial Hh signaling in the pancreas, we eliminated an essential Hh signaling component, Smoothened (Smo), in the pancreatic epithelium, and assessed pancreatic development and adult beta-cell physiology phenotypes.

RESULTS

Characterization of Patched1-LacZ reporter mice revealed low-level LacZ expression in pancreatic epithelial cells throughout development until birth, when LacZ activity increases in intensity specifically in endocrine and ductal cells. In the absence of Hh signaling, Smo-deficient mice have delayed pancreas formation leading to a temporary reduction in pancreatic epithelium and beta-cell numbers. Although beta-cell numbers recover by birth, adult Smo-deficient mice display glucose intolerance, increased insulin sensitivity, and reduced total insulin production.

CONCLUSIONS

These data show that Hh signaling functions early during pancreas morphogenesis to regulate epithelial and beta-cell expansion and to modulate glucose metabolism by regulating insulin production in adult mice.

Cellular plasticity within the pancreas--lessons learned from development

The pancreas has been the subject of intense research due to the debilitating diseases that result from its dysfunction. In this review, we summarize current understanding of the critical tissue interactions and intracellular regulatory events that take place during formation of the pancreas from a small cluster of cells in the foregut domain of the mouse embryo. Importantly, an understanding of principles that govern the development of this organ has equipped us with the means to manipulate both embryonic and differentiated adult cells in the context of regenerative medicine. The emerging area of lineage modulation within the adult pancreas is of particular interest, and this review summarizes recent findings that exemplify how lessons learned from development are being applied to reveal the potential of fully differentiated cells to change fate.

Oxygen tension regulates pancreatic beta-cell differentiation through hypoxia-inducible factor 1alpha

OBJECTIVE

Recent evidence indicates that low oxygen tension (pO2) or hypoxia controls the differentiation of several cell types during development. Variations of pO2 are mediated through the hypoxia-inducible factor (HIF), a crucial mediator of the adaptative response of cells to hypoxia. The aim of this study was to investigate the role of pO2 in beta-cell differentiation.

RESEARCH DESIGN AND METHODS

We analyzed the capacity of beta-cell differentiation in the rat embryonic pancreas using two in vitro assays. Pancreata were cultured either in collagen or on a filter at the air/liquid interface with various pO2. An inhibitor of the prolyl hydroxylases, dimethyloxaloylglycine (DMOG), was used to stabilize HIF1alpha protein in normoxia.

RESULTS

When cultured in collagen, embryonic pancreatic cells were hypoxic and expressed HIF1alpha and rare beta-cells differentiated. In pancreata cultured on filter (normoxia), HIF1alpha expression decreased and numerous beta-cells developed. During pancreas development, HIF1alpha levels were elevated at early stages and decreased with time. To determine the effect of pO2 on beta-cell differentiation, pancreata were cultured in collagen at increasing concentrations of O2. Such conditions repressed HIF1alpha expression, fostered development of Ngn3-positive endocrine progenitors, and induced beta-cell differentiation by O2 in a dose-dependent manner. By contrast, forced expression of HIF1alpha in normoxia using DMOG repressed Ngn3 expression and blocked beta-cell development. Finally, hypoxia requires hairy and enhancer of split (HES)1 expression to repress beta-cell differentiation.

CONCLUSIONS

These data demonstrate that beta-cell differentiation is controlled by pO2 through HIF1alpha. Modifying pO2 should now be tested in protocols aiming to differentiate beta-cells from embryonic stem cells.

Stem cells: The therapeutic role in the treatment of diabetes mellitus

The unlimited proliferative ability and plasticity to generate other cell types ensures that stem cells represent a dynamic system apposite for the identification of new molecular targets and the production and development of novel drugs. These cell lines derived from embryos could be used as a model for the study of basic and applied aspects in medical therapeutics, environmental mutagenesis and disease management. As a consequence, these can be tested for safety or to predict or anticipate potential toxicity in humans. Human ES cell lines may, therefore, prove clinically relevant to the development of safer and more effective drugs for patients presenting with diabetes mellitus.

Notch signaling in pancreatic endocrine cell and diabetes

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

Mechanism of primitive duct formation in the pancreas and submandibular glands: a role for SDF-1

Background

The exocrine pancreas is composed of a branched network of ducts connected to acini. They are lined by a monolayered epithelium that derives from the endoderm and is surrounded by mesoderm-derived mesenchyme. The morphogenic mechanisms by which the ductal network is established as well as the signaling pathways involved in this process are poorly understood.

Results

By morphological analyzis of wild-type and mutant mouse embryos and using cultured embryonic explants we investigated how epithelial morphogenesis takes place and is regulated by chemokine signaling. Pancreas ontogenesis displayed a sequence of two opposite epithelial transitions. During the first transition, the monolayered and polarized endodermal cells give rise to tissue buds composed of a mass of non polarized epithelial cells. During the second transition the buds reorganize into branched and polarized epithelial monolayers that further differentiate into tubulo-acinar glands. We found that the second epithelial transition is controlled by the chemokine Stromal cell-Derived Factor (SDF)-1. The latter is expressed by the mesenchyme, whereas its receptor CXCR4 is expressed by the epithelium. Reorganization of cultured pancreatic buds into monolayered epithelia was blocked in the presence of AMD3100, a SDF-1 antagonist. Analyzis of sdf1 and cxcr4 knockout embryos at the stage of the second epithelial transition revealed transient defective morphogenesis of the ventral and dorsal pancreas. Reorganization of a globular mass of epithelial cells in polarized monolayers is also observed during submandibular glands development. We found that SDF-1 and CXCR4 are expressed in this organ and that AMD3100 treatment of submandibular gland explants blocks its branching morphogenesis.

Conclusion

In conclusion, our data show that the primitive pancreatic ductal network, which is lined by a monolayered and polarized epithelium, forms by remodeling of a globular mass of non polarized epithelial cells. Our data also suggest that SDF-1 controls the branching morphogenesis of several exocrine tissues.

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.

Conditional control of the differentiation competence of pancreatic endocrine and ductal cells by Fgf10

Fgf10 is a critical component of mesenchymal-to-epithelial signaling during endodermal development. In the Fgf10 null pancreas, the embryonic progenitor population fails to expand, while ectopic Fgf10 expression forces progenitor arrest and organ hyperplasia. Using a conditional Fgf10 gain-of-function model, we observed that the timing of Fgf10 expression affected the cellular competence of the arrested pancreatic progenitors. We present evidence that the Fgf10-arrested progenitor state is reversible and that terminal differentiation resumes upon cessation of Fgf10 production. However, competence towards the individual pancreatic cell lineages depended upon the gestational time of when Fgf10 expression was attenuated. This revealed a competence window of endocrine and ductal cell formation that coincided with the pancreatic secondary transition between E13.5 and E15.5. We demonstrate that maintaining the Fgf10-arrested state during this period leads to permanent loss of competence for the endocrine and ductal cell fates. However, competence of the arrested progenitors towards the exocrine cell fate was retained throughout the secondary transition. Sustained Fgf10 expression caused irreversible loss of Ngn3 expression, which may underlie the loss of endocrine competence. Maintenance of exocrine competence may be attributable to continuous Ptf1a expression in the Fgf10-arrested progenitors. This may explain the rapid induction of Bhlhb8, a normally distalized cell intrinsic marker, following loss of ectopic Fgf10 expression. We conclude that the window for endocrine and ductal cell competence ceases during the secondary transition in pancreatic development.

Selective beta-cell differentiation of dissociated embryonic pancreatic precursor cells cultured in synthetic polyethylene glycol hydrogels

Continuing advances in islet cell transplantation have been promising; however, several limitations, including severe shortage of transplantable islets, hinder the widespread use of this therapy. Pancreatic precursor cells are one alternative to cadaveric donor islets. These cells found in the developing pancreatic buds are capable of self-renewal and also have the innate ability to become insulin-producing beta-cells. For this work, bioinert polyethylene glycol (PEG) hydrogels were chosen as the supportive three-dimensional matrix for encapsulation of dissociated pancreatic precursor cells obtained from the dorsal pancreatic bud of day-15 rat embryos. This culture system was selected in order to eliminate cell-extracellular matrix and cell-cell signal heterogeneity present when intact pancreatic buds are embedded in protein-based gels, the typical in vitro culture conditions used to study this cell population. In this study it was found that (1) dissociated precursor cells maintain a robust viability for 7 days in PEG hydrogel culture, (2) encapsulated cells selectively differentiate into insulin-expressing beta-cells, and (3) differentiated beta-cells have releasable insulin stores, but are not achieving a mature, glucose responsive phenotype. These findings suggest that encapsulating dissociated pancreatic precursor cells in an environment designed to minimize the heterogeneous signaling cues present during development or in standard culture conditions generates a population highly enriched in pancreatic beta-cells; however, future efforts must focus on achieving glucose responsiveness in this cell population. Further, these results indicate that differentiation down a beta-cell lineage may be the default pathway in pancreatic development.

[Neuroendocrine system of the pancreas and gastrointestinal tract: origin and development]

Gastroenteropancreatic neuroendocrine tumours (GEP NETs) originate from the neuroendocrine cells through the gastrointestinal tract and endocrine pancreas. The embryologic development of the pancreas is a complex process that begins with the "stem cell" that come from the endodermus. These cells go through two phases: in the first transition the "stem cell" differentiates in exocrine and endocrine cells. This process is regulated by transcription factors such as Pdx1 ("insulin promoter factor 1"), Hlxb6 and SOX9. In the second transition the neuroendocrine cell differentiates in the 5 cell types (alpha, beta, delta, PP y epsilon.). This process is regulated through the balance between factors favoring differentiation (mainly neurogenin 3) and inhibitor factors which depend on Notch signals. The existence of a third transition in postnatal pancreas is hypothesized. The "stem cell" from pancreatic ducts would become adult beta cells, through autoduplication and neogenesis. In the small gut of the adult the stem cell are placed in the intestinal crypts and develop to villi in secretor lines (enterocytes, globet and Paneths cells) or neuroendocrine cells from which at least 10 cell types depend. This process is regulated by transcription factors: Math1, neurogenina 3 and NeuroD.

Regulation of pancreatic juxtaductal endocrine cell formation by FoxO1

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

Developmental biology of the pancreas

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

Mesenchymal epimorphin is important for pancreatic duct morphogenesis

Epithelial-mesenchymal interactions are crucial for the proper development of many organs, including the pancreas. Within the pancreas, the ducts are thought to harbor stem/progenitor cells, and possibly to give rise to pancreatic ductal carcinoma. Little is known about the mechanism of formation of pancreatic ducts in the embryo. Pancreatic mesenchyme contains numerous soluble factors which help to sustain the growth and differentiation of exocrine and endocrine structures. Here, we report that one such morphoregulatory mesenchymal protein, epimorphin, plays an important role during pancreatic ductal proliferation and differentiation. We found that epimorphin is expressed in pancreatic mesenchyme during early stages of development, and at mesenchymal-epithelial interfaces surrounding the ducts at later stages. Strong upregulation of epimorphin expression was seen during in vitro pancreatic duct differentiation. Similarly, in vitro pancreatic duct formation was inhibited by a neutralizing antibody against epimorphin, whereas addition of recombinant epimorphin partially rescued duct formation. Together, our study demonstrates the role of epimorphin in pancreatic ductal morphogenesis.

PBX1 is dispensable for neural commitment of RA-treated murine ES cells

Experimentation with PBX1 knockout mice has shown that PBX1 is necessary for early embryogenesis. Despite broad insight into PBX1 function, little is known about the underlying target gene regulation. Utilizing the Cre–loxP system, we targeted a functionally important part of the homeodomain of PBX1 through homozygous deletion of exon-6 and flanking intronic regions leading to exon 7 skipping in embryonic stem (ES) cells. We induced in vitro differentiation of wild-type and PBX1 mutant ES cells by aggregation and retinoic acid (RA) treatment and compared their profiles of gene expression at the ninth day post-reattachment to adhesive media. Our results indicate that PBX1 interactions with HOX proteins and DNA are dispensable for RA-induced ability of ES to express neural genes and point to a possible involvement of PBX1 in the regulation of imprinted genes.

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.

Diabetes mellitus: an opportunity for therapy with stem cells?

In both Type 1 and 2 diabetes, insufficient numbers of insulin-producing beta-cells are a major cause of defective control of blood glucose and its complications. Restoration of damaged beta-cells by endocrine pancreas regeneration would be an ideal therapeutic option. The possibility of generating insulin-secreting cells with adult pancreatic stem or progenitor cells has been investigated extensively. The conversion of differentiated cells such as hepatocytes into beta-cells is being attempted using molecular insights into the transcriptional make-up of beta-cells. Additionally, the enhanced proliferation of beta-cells in vivo or in vitro is being pursued as a strategy for regenerative medicine for diabetes. Advances have also been made in directing the differentiation of embryonic stem cells into beta-cells. Although progress is encouraging, major gaps in our understanding of developmental biology of the pancreas and adult beta-cell dynamics remain to be bridged before a therapeutic application is made possible.

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.

Embryonic stem cells to beta-cells by understanding pancreas development

Insulin injections treat but do not cure Type 1 diabetes (T1DM). The success of islet transplantation suggests cell replacement therapies may offer a curative strategy. However, cadaver islets are of insufficient number for this to become a widespread treatment. To address this deficiency, the production of beta-cells from pluripotent stem cells offers an ambitious far-sighted opportunity. Recent progress in generating insulin-producing cells from embryonic stem cells has shown promise, highlighting the potential of trying to mimic normal developmental pathways. Here, we provide an overview of the current methodology that has been used to differentiate stem cells toward a beta-cell fate. Parallels are drawn with what is known about normal development, especially regarding the human pancreas.

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.

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.

All-trans retinoic acid suppresses exocrine differentiation and branching morphogenesis in the embryonic pancreas

Recent evidence has shown that retinoic acid (RA) signalling is required for early pancreatic development in zebrafish and frog but its role in later development in mammals is less clear cut. In the present study, we determined the effects of RA on the differentiation of the mouse embryonic pancreas. Addition of all-trans retinoic acid (atRA) to embryonic pancreatic cultures induced a number of changes. Branching morphogenesis and exocrine differentiation were suppressed and there was premature formation of endocrine cell clusters (although the total area of β cells was not different in control and atRA-treated buds). We investigated the mechanism of these changes and found that the premature formation of β cells was associated with the early expression of high-level Pdx1 in the endocrine cell clusters. In contrast, the suppressive effect of RA on exocrine differentiation may be due to a combination of two mechanisms (i) up-regulation of the extracellular matrix component laminin and (ii) enhancement of apoptosis. We also demonstrate that addition of fibroblast growth factor (FGF)-10 is able to partially prevent apoptosis and rescue exocrine differentiation and branching morphogenesis in atRA-treated cultures but not in mice lacking the FGF receptor 2-IIIb, suggesting the effects of FGF-10 are mediated through this receptor.

Insulin-expressing colonies developed from murine embryonic stem cell-derived progenitors

Previous studies describe a unique culture method for the commitment of murine embryonic stem cells to early endocrine pancreata. In this report, early pancreatic-like beta-cell progenitors were enriched and a colony assay devised to allow these progenitors to differentiate into insulin-expressing colonies in vitro. An embryonic stem cell line with enhanced green fluorescent protein (EGFP) inserted into one allele of neurogenin 3 (Ngn3), a marker for pancreatic endocrine progenitors, was differentiated. During the late stage of culture, 20-30% of cells were Ngn3-EGFP(+). Gene expression profiling using the PancChip microarray platform demonstrated that Ngn3-EGFP(+) cells differentially express endocrine-related genes. A novel semisolid culture method was developed to support the formation of individual insulin/C-peptide-expressing colonies from dissociated single cells. Approximately 0.1-0.6% of Ngn3-EGFP(+) cells gave rise to insulin-expressing colonies, a three- to fivefold enrichment of beta-cell-like progenitors, or insulin-expressing colony-forming units (ICFUs), compared with nonsorted cells. All of the single colonies expressed insulin II, while 69% coexpressed insulin I and 44% coexpressed glucagon. Some single colonies expressed insulin I, insulin II, and Pdx-1 (pancreatic duodenal homeobox-1), but not glucagon. In other colonies, glucagon expression overlapped with C-peptide II in double immunostaining analysis, suggesting heterogeneity among the ICFUs and their resulting colonies. Together, these results demonstrate that progenitors that have the potential to give rise to insulin-expressing cells can be derived from murine embryonic stem cells.

Dynamics of embryonic pancreas development using real-time imaging

Current knowledge about developmental processes in complex organisms has relied almost exclusively on analyses of fixed specimens. However, organ growth is highly dynamic, and visualization of such dynamic processes, e.g., real-time tracking of cell movement and tissue morphogenesis, is becoming increasingly important. Here, we use live imaging to investigate expansion of the embryonic pancreatic epithelium in mouse. Using time-lapse imaging of tissue explants in culture, fluorescently labeled pancreatic epithelium was found to undergo significant expansion accompanied by branching. Quantification of the real-time imaging data revealed lateral branching as the predominant mode of morphogenesis during epithelial expansion. Live imaging also allowed documentation of dynamic beta-cell formation and migration. During in vitro growth, appearance of newly formed beta-cells was visualized using pancreatic explants from MIP-GFP transgenic animals. Migration and clustering of beta-cells were recorded for the first time using live imaging. Total beta-cell mass and concordant aggregation increased during the time of imaging, demonstrating that cells were clustering to form "pre-islets". Finally, inhibition of Hedgehog signaling in explant cultures led to a dramatic increase in total beta-cell mass, demonstrating application of the system in investigating roles of critical embryonic signaling pathways in pancreas development including beta-cell expansion. Thus, pancreas growth in vitro can be documented by live imaging, allowing visualization of the developing pancreas in real-time.

TGF-beta isoform signaling regulates secondary transition and mesenchymal-induced endocrine development in the embryonic mouse pancreas

Transforming growth factor-beta (TGF-beta) superfamily signaling has been implicated in many developmental processes, including pancreatic development. Previous studies are conflicting with regard to an exact role for TGF-beta signaling in various aspects of pancreatic organogenesis. Here we have investigated the role of TGF-beta isoform signaling in embryonic pancreas differentiation and lineage selection. The TGF-beta isoform receptors (RI, RII and ALK1) were localized mainly to both the pancreatic epithelium and mesenchyme at early stages of development, but then with increasing age localized to the pancreatic islets and ducts. To determine the specific role of TGF-beta isoforms, we functionally inactivated TGF-beta signaling at different points in the signaling cascade. Disruption of TGF-beta signaling at the receptor level using mice overexpressing the dominant-negative TGF-beta type II receptor showed an increase in endocrine precursors and proliferating endocrine cells, with an abnormal accumulation of endocrine cells around the developing ducts of mid-late stage embryonic pancreas. This pattern suggested that TGF-beta isoform signaling may suppress the origination of secondary transition endocrine cells from the ducts. Secondly, TGF-beta isoform ligand inhibition with neutralizing antibody in pancreatic organ culture also led to an increase in the number of endocrine-positive cells. Thirdly, hybrid mix-and-match in vitro recombinations of transgenic pancreatic mesenchyme and wild-type epithelium also led to increased endocrine cell differentiation, but with different patterns depending on the directionality of the epithelial-mesenchymal signaling. Together these results suggest that TGF-beta signaling is important for restraining the growth and differentiation of pancreatic epithelial cells, particularly away from the endocrine lineage. Inhibition of TGF-beta signaling in the embryonic period may thus allow pancreatic epithelial cells to progress towards the endocrine lineage unchecked, particularly as part of the secondary transition of pancreatic endocrine cell development. TGF-beta RII in the ducts and islets may normally serve to downregulate the production of beta cells from embryonic ducts.

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.

Tyrosine kinase receptors are crucial for normal β-cell development and function

Signaling pathways play critical roles in most physiological and pathological processes and convert an extracellular stimulus into a change of function in the recipient cell. Intracellular messages originate from the activation of membrane receptors by a variety of ligands, such as hormones, nutrients or growth factors. The receptors subsequently interact with specific intracellular cascades, triggering the phosphorylation of cell effectors. In the pancreas, these processes control the organogenesis, maintenance and function of endocrine cells within the islets. Growth factors acting through tyrosine kinase receptors play a prominent role among the multitude of signaling pathways active in pancreatic β cells. Deregulation of these processes leads to the development of disorders such as hypoglycemia or diabetes. This review will describe recent advances made on the understanding of the roles of major tyrosine kinase receptors in pancreatic β-cell physiology.

FGFR3 is a negative regulator of the expansion of pancreatic epithelial cells

Fibroblast growth factors (FGFs) and their receptors (FGFRs) are key signaling molecules for pancreas development. Although FGFR3 is a crucial developmental gene, acting as a negative regulator of bone formation, its participation remains unexplored in pancreatic organogenesis. We found that FGFR3 was expressed in the epithelia in both mouse embryonic and adult regenerating pancreata but was absent in normal adult islets. In FGFR3 knockout mice, we observed an increase in the proliferation of epithelial cells in neonates, leading to a marked increase in islet areas in adults. In vitro studies showed that FGF9 is a very potent ligand for FGFR3 and activates extracellular signal-related kinases (ERKs) in pancreatic cell lines. Moreover, FGFR3 blockade or FGFR3 deficiency led to increased proliferation of pancreatic epithelial cells in vivo. This was accompanied by an increase in the proportion of potential islet progenitor cells. Thus, our results show that FGFR3 signaling inhibits the expansion of the immature pancreatic epithelium. Consequently, this study suggests that FGFR3 participates in regulating pancreatic growth during the emergence of mature islet cells.

Effects of Growth Factors on Development of Fetal Islet B-Cells In Vitro

To investigate the role of growth factors (epidermal growth factor [EGF], betacellulin, and activin A) in the development of islet B cells of rat fetal pancreatic explants in vitro, pancreases from rat fetuses at day 18 of gestation were cultured for 96 hr, with or without these growth factors. Culture medium was changed every 24 hr, and the level of insulin released in the culture medium was measured. After 72 hr of culture, pancreases were examined histologically. As a result, EGF promoted cell proliferation, but reduced B cell volume. Whereas, betacellulin and activin A inhibited cell division, but promoted increased B cell volume and insulin secretion, especially activin A, which stimulated insulin release in a time dependent manner. These results suggest that EGF, betacellulin, and activin A promote pancreatic cell proliferation, islet B-cell differentiation, and islet B-cell differentiation and functional maturation, respectively, and that EGF, betacellulin, and activin A, in this order, regulate islet B-cell neogenesis.

Role of FGF1, FGF2 and FGF7 in the development of the pancreas from control and streptozotocin-treated hamsters

Although progress has been made with respect to the growth and transcription factors implicated in pancreatic development, many questions remain unsolved. It has been established that during embryonic life, both endocrine and acinar cells are derived from pancreatic epithelial precursor cells. Growth factors control the proliferation of precursor cells and their ability to differentiate into mature cells, both in pre-natal and in early post-natal life. Pancreatic development during the early post-natal period is an area of great interest for many scientists. In this study we have examined the structure characteristics, functional and proliferative activity of control and diabetic hamster pancreatic ductal, exocrine and beta cells, following treatment with FGFs 1, 2 and 7 in vitro. Light and electron microscopic studies indicated active synthetic processes in these cells under the influence of the investigated FGFs. In our experimental model of diabetes, the labelling index of the cells was significantly higher than in corresponding control groups of hamsters. We established that FGF2 at a concentration of 10 ng/l was responsible for the most prominent effect on ductal cells and beta cells in the diabetic groups. FGF1 at a concentration of 10 ng/l displayed the highest stimulatory effect on exocrine cells in the diabetic groups at post-natal day 10. Taken together these data strongly suggest that FGF1 and FGF2 induce proliferation of pancreatic epithelial cells during the early post-natal period whereas FGF7 is not strictly specific for pancreatic cell proliferation.

The mesenchyme controls the timing of pancreatic beta-cell differentiation

The importance of mesenchymal-epithelial interactions in the proliferation of pancreatic progenitor cells is well established. Here, we provide evidence that the mesenchyme also controls the timing of beta-cell differentiation. When rat embryonic pancreatic epithelium was cultured without mesenchyme, we found first rapid induction in epithelial progenitor cells of the transcription factor neurogenin3 (Ngn3), a master gene controlling endocrine cell-fate decisions in progenitor cells; then beta-cell differentiation occurred. In the presence of mesenchyme, Ngn3 induction was delayed, and few beta-cells developed. This effect of the mesenchyme on Ngn3 induction was mediated by cell-cell contacts and required a functional Notch pathway. We then showed that associating Ngn3-expressing epithelial cells with mesenchyme resulted in poor beta-cell development via a mechanism mediated by soluble factors. Thus, in addition to its effect upstream of Ngn3, the mesenchyme regulated beta-cell differentiation downstream of Ngn3. In conclusion, these data indicate that the mesenchyme controls the timing of beta-cell differentiation both upstream and downstream of Ngn3.

Effects of high dose retinoic acid on TGF-β2 expression during pancreatic organogenesis

The aim of this study was to investigate the effects of excess all-trans retinoic acid, a vitamin A metabolite, on pancreatic organogenesis and TGF-beta2 expression during prenatal development in rats. First group of animals used as control while a single dose of 60 mg/kg all-trans retinoic acid was ingested by the mothers, at day 8 of gestation (before the neurulation period) in group II and at day 12 of gestation (after the neurulation period) in group III, and all embryos were sacrificed at day 18 of gestation. TGF-beta2 expression was detected in the capsule, acini and Langerhans islets in the control group. In the pancreas of group II, dilatation and congestion of interlobular vessels were observed. Langerhans islet structures were completely absent. Moreover acinar TGF-beta2 immune reactivity was not determined. In group III, acinar expression of TGF-beta2 in acid was similar to that in the controls but their Langerhans islets TGF-beta2 immune reactivity was significantly less than the controls. In view of the present findings we suggest that TGF-beta2 plays important role in pancreatic morphogenesis and administration of excess all-trans retinoic acid before neurulation inhibit TGF-beta2 expression disrupted pancreatic morphogenesis particularly Langerhans islets. However, its administration after neurulation had less adverse affect on pancreatic organogenesis and TGF-beta2 immune reactivity.

Activin A-induced expression of PAX4 in AR42J-B13 cells involves the increase in transactivation of E47/E12

Pax4 is a paired-homeodomain containing transcriptional factor that controls the differentiation of pancreatic beta cells. The aim of this study was to investigate the mechanism of PAX4 expression by activin A. By reporter gene analysis using AR42J-B13 cells, in which treatment with activin A induced PAX4 mRNA expression, we identified that a short sequence located approximately 1930 bp upstream of the transcriptional start site is essential for activin A induced PAX4 promoter activation. This region contains an E box and binding sites for hepatocyte nuclear factor (HNF)-1alpha. Mutation introduced in each binding site markedly reduced activin A responsiveness. It has been reported that HNF-1alpha synergizes with basic helix-loop-helix (bHLH) proteins in activating the PAX4 promoter, and we demonstrated that activin A strongly enhanced the functional activity of E47/E12 without the increase in its binding ability. In addition, suppression of E47/E12 expression in AR42J-B13 cells using siRNA oligonucleotides results in the significant decrease in the intrinsic activin A-induced PAX4 expression. Our results suggest that activin A enhances PAX4 expression by enhanced transactivation of E47/E12 proteins and might result in a cumulative transactivation of the promoter.

Exocrine pancreas development in zebrafish

Although many of the genes that regulate development of the endocrine pancreas have been identified, comparatively little is known about how the exocrine pancreas forms. Previous studies have shown that exocrine pancreas development may be modeled in zebrafish. However, the timing and mechanism of acinar and ductal differentiation and morphogenesis have not been described. Here, we characterize zebrafish exocrine pancreas development in wild type and mutant larvae using histological, immunohistochemical and ultrastructural analyses. These data allow us to identify two stages of zebrafish exocrine development. During the first stage, the exocrine anlage forms from rostral endodermal cells. During the second stage, proto-differentiated progenitor cells undergo terminal differentiation followed by acinar gland and duct morphogenesis. Immunohistochemical analyses support a model in which the intrapancreatic ductal system develops from progenitors that join to form a contiguous network rather than by branching morphogenesis of the pancreatic epithelium, as described for mammals. Contemporaneous appearance of acinar glands and ducts in developing larvae and their disruption in pancreatic mutants suggest that common molecular pathways may regulate gland and duct morphogenesis and differentiation of their constituent cells. By contrast, analyses of mind bomb mutants and jagged morpholino-injected larvae suggest that Notch signaling principally regulates ductal differentiation of bipotential exocrine progenitors.