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Ngn3 is recognized as a regulator of pancreatic endocrine formation, and Notch signaling as an important negative regulator Ngn3 gene expression. By conditionally controlling expression of Ngn3 in the pancreas, we find that these two signaling components are dynamically linked. This connection involves transcriptional repression as previously shown, but also incorporates a novel post-translational mechanism. In addition to its ability to promote endocrine fate, we provide evidence of a competing ability of Ngn3 in the patterning of multipotent progenitor cells in turn controlling the formation of ducts. On one hand, Ngn3 cell-intrinsically activates endocrine target genes; on the other, Ngn3 cell-extrinsically promotes lateral signaling via the Dll1>Notch>Hes1 pathway which substantially limits its ability to sustain endocrine formation. Prior to endocrine commitment, the Ngn3-mediated activation of the Notch>Hes1 pathway impacts formation of the trunk domain in the pancreas causing multipotent progenitors to lose acinar, while gaining endocrine and ductal, competence. The subsequent selection of fate from such bipotential progenitors is then governed by lateral inhibition, where Notch>Hes1-mediated Ngn3 protein destabilization serves to limit endocrine differentiation by reducing cellular levels of Ngn3. This system thus allows for rapid dynamic changes between opposing bHLH proteins in cells approaching a terminal differentiation event. Inhibition of Notch signaling leads to Ngn3 protein stabilization in the normal mouse pancreas explants. We conclude that the mutually exclusive expression pattern of Ngn3/Hes1 proteins in the mammalian pancreas is partially controlled through Notch-mediated post-translational regulation and we demonstrate that the formation of insulin-producing beta-cells can be significantly enhanced upon induction of a pro-endocrine drive combined with the inhibition of Notch processing.

During early pancreatic development, Notch signaling represses differentiation of endocrine cells and promotes proliferation of Nkx6-1 + Ptf1a + multipotent progenitor cells (MPCs). Later, antagonistic interactions between Nkx6 transcription factors and Ptf1a function to segregate MPCs into distal Nkx6-1 − Ptf1a + acinar progenitors and proximal Nkx6-1 + Ptf1a − duct and β-cell progenitors. Distal cells are initially multipotent, but evolve into unipotent, acinar cell progenitors. Conversely, proximal cells are bipotent and give rise to duct cells and late-born endocrine cells, including the insulin producing β-cells. However, signals that regulate proximodistal (P-D) patterning and thus formation of β-cell progenitors are unknown. Here we show that Mind bomb 1 ( Mib1 ) is required for correct P-D patterning of the developing pancreas and β-cell formation. We found that endoderm-specific inactivation of Mib1 caused a loss of Nkx6-1 + Ptf1a − and Hnf1β + cells and a corresponding loss of Neurog3 + endocrine progenitors and β-cells. An accompanying increase in Nkx6-1 − Ptf1a + and amylase + cells, occupying the proximal domain, suggests that proximal cells adopt a distal fate in the absence of Mib1 activity. Impeding Notch-mediated transcriptional activation by conditional expression of dominant negative Mastermind-like 1 (Maml1) resulted in a similarly distorted P-D patterning and suppressed β-cell formation, as did conditional inactivation of the Notch target gene Hes1 . Our results reveal iterative use of Notch in pancreatic development to ensure correct P-D patterning and adequate β-cell formation.

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.

Maintenance of progenitor cell properties in development is required for proper organogenesis of most organs, including those derived from the endoderm. FGF10 has been shown to play a role in both lung and pancreatic development. Here we find that FGF10 signaling controls stomach progenitor maintenance, morphogenesis and cellular differentiation. Through a characterization of the initiation of terminal differentiation of the three major gastric regions in the mouse, forestomach, corpus and antrum, we first describe the existence of a “secondary transition” event occurring in mouse stomach between E15.5 and E16.5. This includes the formation of terminally differentiated squamous cells, parietal, chief and gastric endocrine cells from a pre-patterned gastric progenitor epithelium. Expression analysis of both FGF and Notch signaling components suggested a role of these networks in such progenitors, which was tested through ectopically expressing FGF10 in the developing posterior stomach. These data provide evidence that gastric gland specification and progenitor cell maintenance is controlled by FGF10. The glandular proliferative niche was disrupted in pPDX-FGF10FLAG mice leading to aberrant gland formation, and endocrine and parietal cell differentiation was attenuated. These effects were paralleled by changes in Hes1, Shh and Wnt6 expression, suggesting that FGF10 acts in concert with multiple morphogenetic signaling systems during gastric development.

Spatio-temporal regulation of the balance between cell renewal and cell differentiation is of vital importance for embryonic development and adult homeostasis. Fibroblast growth factor signaling relayed from the mesenchyme to the epithelium is necessary for progenitor maintenance during organogenesis of most endoderm-derived organs, but it is still ambiguous whether the signal is exclusively mitogenic. Furthermore, the downstream mechanisms are largely unknown. In order to elucidate these questions we performed a complementary analysis of fibroblast growth factor 10 (Fgf10), gain-of-function and loss-of-function in the embryonic mouse duodenum, where the progenitor niche is clearly defined and differentiation proceeds in a spatially organized manner. In agreement with a role in progenitor maintenance, FGF10 is expressed in the duodenal mesenchyme during early development while the cognate receptor FGFR2b is expressed in the epithelial progenitor niche. Fgf10 gain-of-function in the epithelium leads to spatial expansion of the progenitor niche and repression of cell differentiation, while loss-of-function results in premature cell differentiation and subsequent epithelial hypoplasia. We conclude that FGF10 mediated mesenchymal-to-epithelial signaling maintains the progenitor niche in the embryonic duodenum primarily by repressing cell differentiation, rather than through mitogenic signaling. Furthermore, we demonstrate that FGF10-signaling targets include ETS-family transcription factors, which have previously been shown to regulate epithelial maturation and tumor progression.

SummaryUnderstanding how cells polarize and coordinate tubulogenesis during organ formation is a central question in biology. Tubulogenesis often coincides with cell-lineage specification during organ development. Hence, an elementary question is whether these two processes are independently controlled, or whether proper cell specification depends on formation of tubes. To address these fundamental questions, we have studied the functional role of Cdc42 in pancreatic tubulogenesis. We present evidence that Cdc42 is essential for tube formation, specifically for initiating microlumen formation and later for maintaining apical cell polarity. Finally, we show that Cdc42 controls cell specification non-cell-autonomously by providing the correct microenvironment for proper control of cell-fate choices of multipotent progenitors.For a video summary of this article, see the PaperFlick file with the Supplemental Data available online.

Background Interaction with the surrounding mesenchyme is necessary for development of endodermal organs, and Fibroblast growth factors have recently emerged as mesenchymal-expressed morphogens that direct endodermal morphogenesis. The fibroblast growth factor 10 (Fgf10) null mouse is characterized by the absence of lung bud development. Previous studies have shown that this requirement for Fgf10 is due in part to its role as a chemotactic factor during branching morphogenesis. In other endodermal organs Fgf10 also plays a role in regulating differentiation. Results Through gain-of-function analysis, we here find that FGF10 inhibits differentiation of the lung epithelium and promotes distalization of the embryonic lung. Ectopic expression of FGF10 in the lung epithelium caused impaired lung development and perinatal lethality in a transgenic mouse model. Lung lobes were enlarged due to increased interlobular distance and hyperplasia of the airway epithelium. Differentiation of bronchial and alveolar cell lineages was inhibited. The transgenic epithelium consisted predominantly of proliferating progenitor-like cells expressing Pro-surfactant protein C, TTF1, PEA3 and Clusterin similarly to immature distal tip cells. Strikingly, goblet cells developed within this arrested epithelium leading to goblet cell hyperplasia. Conclusion We conclude that FGF10 inhibits terminal differentiation in the embryonic lung and maintains the distal epithelium, and that excessive levels of FGF10 leads to metaplastic differentiation of goblet cells similar to that seen in chronic inflammatory diseases.

ETS-family factors play major roles in development and cancer, notably as critical targets for extra-cellular signaling pathways, including MAPK-signaling. Given the presently limited knowledge on the role of ETS-factors in pancreatic development, we here sought to characterize all 26 individual members of the ETS-family in relation to pancreatic development using a combination of genomics, RT-PCR, and histological techniques. This analysis uncovers 22 ETS family genes displaying select spatial and temporal expression patterns in the developing pancreas. Highly specific expression of ETS-family components is observed in pancreatic progenitor cells or the associated embryonic mesenchyme. Other members are linked to the differentiation of more mature pancreatic cells, including exocrine and endocrine cell types. We find that two members of the Etv subfamily, Etv4 and Etv5, are expressed in cells proximal to pancreatic mesenchyme, and, furthermore, induced in FGF10-arrested pancreatic progenitors suggesting that these factors mediate mesenchymal-to-epithelial signaling. Developmental Dynamics 236:3100–3110, 2007. © 2007 Wiley-Liss, Inc.

An immunohistochemical and genomic analysis of human pancreatic development from 9–23 weeks of fetal age was undertaken to provide a comparative analysis of human and murine islet development. Human fetal pancreases obtained at gestational ages 9–23 weeks were processed in parallel for immunohistochemistry and gene expression profiling by Affymetrix microarrays. By 9–11 weeks, the pancreas was made up principally of mesenchymal tissue infiltrated by branched epithelial structures containing scattered hormone-negative neurogenin3-positive endocrine cells. Protoacinar structures emerged by 15–19 weeks, along with clusters of endocrine cells producing either glucagon or insulin. By 20–23 weeks, vascularised islet-like structures appeared. More than 70% of endocrine cells produced a single hormone at any age. Analysis of Ki67 immunoreactivity showed that the replicative rate of endocrine cells was low and suggested that the endocrine expansion was derived from hormone-negative precursors. Insulin, glucagon, somatostatin, ghrelin and pancreatic polypeptide transcripts were present at 9–10 weeks and increased progressively, commensurate with the expansion of endocrine cell volume. The human equivalent of a mouse endocrine secondary transition was not evident, neither in terms of morphology nor in dramatic changes in endocrine-specific transcriptional regulators. By contrast, exocrine genes showed a marked transition at around 11 weeks, associated with a greater than sixfold increase in exocrine gene transcripts. The observed extension of terminal differentiation of human endocrine tissue into late gestation is in contrast with findings in the mouse. It indicates that the human fetal pancreas could provide an abundant islet precursor cell population that could be expanded ex vivo for therapeutic transplantation.