Pancreatic Development and Disease

Abstract

The last decade has seen tremendous advances in understanding the molecular mechanisms that control vertebrate organogenesis. Transgenic animal models, mainly in mouse and zebrafish, have allowed the mapping of endodermal cell specification toward pancreas fate and the subsequent differentiation into distinct endocrine and exocrine cell types. Using these sophisticated approaches, cell-type–specific activation or deactivation of genes has become a reality. In some cases, these manipulations have provided crucial information about human pancreatic disorders. Here, we review the role of key signaling factors during pancreas development and how they relate to human congenital anomalies. The mammalian pancreas is a mixed exocrine and endocrine gland that plays a central role in glucose homeostasis and food digestion. The endocrine portion includes 5 distinct hormone-producing cell types organized into the islets of Langerhans. The exocrine pancreas consists of acinar cells that produce and secrete digestive enzymes into an elaborate ductal system formed by ductal cells. The ductal system transports the digestive enzymes to the intestine where they ensure nutrient digestion and absorption. In mice, the pancreas arises from 3 spatially distinct primordia (1 dorsal and 2 ventral buds) of the foregut epithelium as early as embryonic day 9 (E9; Figure 1).1Slack J.M.W. Developmental biology of the pancreas.Development. 1995; 121: 1569-1580Crossref Google Scholar The pancreatic buds develop in response to signals from the adjacent mesodermal tissues, including notochord, aorta, and cardiac mesoderm. The epithelial part of the pancreatic primordia undergoes extensive branching. One of the ventral buds is lost and the other buds fuse during gut rotation by E12.5. The pancreatic ductal epithelium formed contains the progenitor cells that begin to differentiate into all mature pancreatic cell types by E13.5. At this time, endocrine progenitor cells migrate into the surrounding mesenchyme and aggregate into cell clusters, while exocrine cells organize into acini. Between E15.5 and birth, the differentiated cells undergo additional growth and maturation. Embryonic development of the pancreas appears to be very similar in other species, including humans. Early stages of pancreas formation. The study of the signaling pathways involved in the different stages of pancreatic development is a field of great interest. The prospect of generating insulin-producing cells from either adult or embryonic stem cells for transplantation-based therapy of diabetes has fueled the research on this subject and significant advances have been made in recent years. We next discuss how embryonic signaling pathways regulate the distinct stages of pancreas development. During gastrulation, the epiblast, consisting of multipotential cells, generates the 3 embryo layers: endoderm, mesoderm, and ectoderm. The gastrointestinal organs, including the pancreas, are derived from the endodermal layer. The induction of endoderm appears to be governed by nodal/transforming growth factor (TGF)-β signaling from adjacent ectoderm and mesoderm within the primitive streak and the node.2Lewis S.L. Tam P.P. Definitive endoderm of the mouse embryo: formation, cell fates, and morphogenetic function.Dev Dyn. 2006; 235: 2315-2329Google Scholar, 3Wells J.M. Melton D.A. Vertebrate endoderm development.Annu Rev Cell Dev Biol. 1999; 15: 393-410Google Scholar After gastrulation, the endoderm is patterned in an anterior–posterior fashion in response to fibroblast growth factor (FGF)-4 signals from adjacent mesoderm.4Dessimoz J. Opoka R. Kordich J.J. Grapin-Botton A. Wells J.M. FGF signaling is necessary for establishing gut tube domains along the anterior-posterior axis in vivo.Mech Dev. 2006; 123: 42-55Google Scholar, 5Wells J.M. Melton D.A. Early mouse endoderm is patterned by soluble factors from adjacent germ layers.Development. 2000; 127: 1563-1572Google Scholar Subsequently, cells within the specified areas of the endoderm respond to inductive signals that direct the differentiation of the distinct endodermal organs.3Wells J.M. Melton D.A. Vertebrate endoderm development.Annu Rev Cell Dev Biol. 1999; 15: 393-410Google Scholar, 6Tam P.P. Kanai-Azuma M. Kanai Y. Early endoderm development in vertebrates: lineage differentiation and morphogenetic function.Curr Opin Genet Dev. 2003; 13: 393-400Google Scholar, 7Fukuda K. Kikuchi Y. Endoderm development in vertebrates: fate mapping, induction and regional specification.Dev Growth Diff. 2005; 47: 343-355Google Scholar Retinoic acid (RA) appears to play a key role in specification of the foregut endoderm that becomes pancreas.8Stafford D. Hornbruch A. Mueller P.R. Prince V.E. A conserved role for retinoid signaling in vertebrate pancreas development.Dev Genes Evol. 2004; 214: 432-441Google Scholar Exogenous application of RA to zebrafish embryos induces ectopic pancreatic cells throughout the anterior endoderm.9Stafford D. Prince V.E. Retinoic acid signaling is required for a critical early step in zebrafish pancreatic development.Curr Biol. 2002; 12: 1215-1220Google Scholar Mice with a targeted deletion in the retinaldehyde dehydrogenase 2 (Raldh2) gene, which encodes the enzyme required to synthesize RA, do not develop a dorsal pancreatic bud.10Martin M. Gallego-Llamas J. Ribes V. Kedinger M. Niederreither K. Chambon P. Dolle P. Gradwohl G. Dorsal pancreas agenesis in retinoic acid-deficient Raldh2 mutant mice.Dev Biol. 2005; 284: 399-411Google Scholar, 11Molotkov A. Molotkova N. Duester G. Retinoic acid generated by Raldh2 in mesoderm is required for mouse dorsal endodermal pancreas development.Dev Dyn. 2005; 232: 950-957Google Scholar TGF-β signaling via bone morphogenetic proteins (BMPs) and TGF-β/activin may also play a role in pancreas specification. The level of BMP signaling determines the position and extent of the pancreatic domain in the foregut endoderm in zebrafish.12Tiso N. Filippi A. Pauls S. Bortolussi M. Argenton F. BMP signalling regulates anteroposterior endoderm patterning in zebrafish.Mech Dev. 2002; 118: 29-37Google Scholar Reduction of activin receptor signaling blocks proper pancreas formation.13Kim S.K. Hebrok M. Li E. Oh S.P. Schrewe H. Harmon E.B. Lee J.S. Melton D.A. Activin receptor patterning of foregut organogenesis.Genes Dev. 2000; 14: 1866-1871Google Scholar More recently, it has been shown that the Notch signaling pathway controls regional specification of pancreas in the developing endoderm, possibly mediated by the transcriptional factor Ptf1a (see below). Analysis of mice null for Hes1 (a mediator of Notch signaling) revealed the formation of ectopic pancreas in stomach, duodenum, and common bile duct.14Fukuda A. Kawaguchi Y. Furuyama K. Kodama S. Horiguchi M. Kuhara T. Koizumi M. Boyer D.F. Fujimoto K. Doi R. Kageyama R. Wright C.V. Chiba T. Ectopic pancreas formation in Hes1-knockout mice reveals plasticity of endodermal progenitors of the gut, bile duct, and pancreas.J Clin Invest. 2006; 116: 1484-1493Google Scholar The differentiation program of the dorsal and ventral pancreatic buds display significant differences that probably reflect the presence of distinct inductive signals that may be provided by the surrounding mesodermal tissues (reviewed in Kumar and Melton15Kumar M. Melton D. Pancreas specification: a budding question.Curr Opin Genet Dev. 2003; 13: 401-407Google Scholar). Signals from the notochord, including FGF and activin ligands repress expression of Sonic hedgehog (Shh), an activator of the Hedgehog signaling (Hh) pathway, in the dorsal pancreatic bud.16Hebrok M. Kim S.K. Melton D.A. Notochord repression of endodermal Sonic hedgehog permits pancreas development.Genes Dev. 1998; 12: 1705-1713Google Scholar, 17Kim S.K. Hebrok M. Intercellular signals regulating pancreas development and function.Genes Dev. 2001; 15: 111-127Google Scholar, 18Hebrok M. Kim S.K. St Jacques B. McMahon A.P. Melton D.A. Regulation of pancreas development by hedgehog signaling.Development. 2000; 127: 4905-4913Google Scholar At the onset of organogenesis, Hh signaling inhibits pancreas formation.19Apelqvist A. Ahlgren U. Edlund H. Sonic hedgehog directs specialised mesoderm differentiation in the intestine and pancreas.Curr Biol. 1997; 7: 801-804Google Scholar In contrast, Hh signaling is active and required for proper organ formation in tissues immediately adjacent to the pancreas, such as stomach and duodenum. Therefore, Hh activity in the adjacent endoderm provides a molecular boundary that inhibits ectopic pancreas growth.16Hebrok M. Kim S.K. Melton D.A. Notochord repression of endodermal Sonic hedgehog permits pancreas development.Genes Dev. 1998; 12: 1705-1713Google Scholar Additional permissive signals derived from endothelial cells of the aorta are also required for the formation of the dorsal pancreatic bud.20Lammert E. Cleaver O. Melton D. Induction of pancreatic differentiation by signals from blood vessels.Science. 2001; V294: 564-567Google Scholar, 21Yoshitomi H. Zaret K.S. Endothelial cell interactions initiate dorsal pancreas development by selectively inducing the transcription factor Ptf1a.Development. 2004; 131: 807-817Google Scholar However, the exact nature of these signals has not been identified. Signals from the lateral plate mesoderm located beneath the endoderm are essential for the induction of ventral pancreas formation. In contrast to the signals derived from the dorsal mesenchyme, lateral plate signals, including activin, BMP, and RA, are instructive because they can induce pancreas formation in endoderm anterior to the presumptive pancreas area.22Kumar M. Jordan N. Melton D. Grapin-Botton A. Signals from lateral plate mesoderm instruct endoderm toward a pancreatic fate.Dev Biol. 2003; 259: 109-122Google Scholar Once initiated, ventral pancreas development appears to be the default pathway; signals from the surrounding mesenchyme only appear to be necessary to promote hepatic development23Rossi J.M. Dunn N.R. Hogan B.L. Zaret K.S. Distinct mesodermal signals, including BMPs from the septum transversum mesenchyme, are required in combination for hepatogenesis from the endoderm.Genes Dev. 2001; 15: 1998-2009Google Scholar (reviewed in Kim and MacDonald24Kim S.K. MacDonald R.J. Signaling and transcriptional control of pancreatic organogenesis.Curr Opin Genet Dev. 2002; 12: 540-547Google Scholar). Mid-to-late pancreas development. The importance of mesenchymal–epithelial signaling during subsequent growth and differentiation of the pancreatic epithelium has long been recognized.25Golosow N. Grobstein C. Epitheliomesenchymal interaction in pancreatic morphogenesis.Developmental Biology. 1962; 4: 242-255Google Scholar FGF-10 is one of the mesenchymal factors that promote pancreatic epithelial proliferation. Mice deficient for FGF-10 display a dramatic reduction in the proliferation of the epithelial progenitor cells.26Bhushan A. Itoh N. Kato S. Thiery J.P. Czernichow P. Bellusci S. Scharfmann R. FGF10 is essential for maintaining the proliferative capacity of epithelial progenitor cells during early pancreatic organogenesis.Development. 2001; 128: 5109-5117Crossref Google Scholar Epidermal growth factor is another mesenchymal factor that has been shown to regulate proliferation of the developing pancreas both in vitro and in vivo.27Miettinen P.J. Huotari M. Koivisto T. Ustinov J. Palgi J. Rasilainen S. Lehtonen E. Keski-Oja J. Otonkoski T. Impaired migration and delayed differentiation of pancreatic islet cells in mice lacking EGF-receptors.Development. 2000; 127: 2617-2627Google Scholar Mesenchymal factors also regulate cell-type–specific differentiation within the pancreatic epithelium. In the absence of mesenchyme, pancreatic epithelium gives rise to endocrine cells, but not exocrine cells.28Gittes G.K. Galante P.E. Hanahan D. Rutter W.J. Debase H.T. Lineage-specific morphogenesis in the developing pancreas: role of mesenchymal factors.Development. 1996; 122: 439-447Google Scholar, 29Miralles F. Czernichow P. Scharfmann R. Follistatin regulates the relative proportions of endocrine versus exocrine tissue during pancreatic development.Development. 1998; 125: 1017-1024Crossref Google Scholar Interestingly, some of the same signaling pathways that regulate the budding of the pancreatic primordia also affect cell differentiation at later stages. One of these mesenchymal factors is follistatin, an antagonist of TGF-β signaling pathway.29Miralles F. Czernichow P. Scharfmann R. Follistatin regulates the relative proportions of endocrine versus exocrine tissue during pancreatic development.Development. 1998; 125: 1017-1024Crossref Google Scholar Earlier studies indicated that TGF-β signaling plays an essential role in islet formation.13Kim S.K. Hebrok M. Li E. Oh S.P. Schrewe H. Harmon E.B. Lee J.S. Melton D.A. Activin receptor patterning of foregut organogenesis.Genes Dev. 2000; 14: 1866-1871Google Scholar, 30Sanvito F. Herrera P.-L. Huarte J. Nichols A. Montesano R. Orci L. Vassalli J.-D. TBF-b1 influences the relative development of the exocrine and endocrine pancreas in vitro.Development. 1994; 120: 3451-3462Google Scholar These observations were recently confirmed by mouse studies in which inhibition of TGF-β signaling in the pancreas, via conditional overexpression of Smad7, resulted in pancreas hypoplasia and impaired islet development.31Smart N.G. Apelqvist A.A. Gu X. Harmon E.B. Topper J.N. MacDonald R.J. Kim S.K. Conditional expression of Smad7 in pancreatic beta cells disrupts TGF-beta signaling and induces reversible diabetes mellitus.PLoS Biol. 2006; 4: e39Google Scholar Notch signaling is another key regulator of pancreatic cell growth and differentiation. During the early stages following the formation of pancreatic primordia, Notch signaling (probably activated via mesenchymal FGF-10) is essential for the expansion of undifferentiated progenitor cells.32Norgaard G.A. Jensen J.N. Jensen J. FGF10 signaling maintains the pancreatic progenitor cell state revealing a novel role of Notch in organ development.Dev Biol. 2003; 264: 323-338Google Scholar, 33Hart A. Papadopoulou S. Edlund H. FGF10 maintains notch activation, stimulates proliferation, and blocks differentiation of pancreatic epithelial cells.Dev Dyn. 2003; 228: 185-193Google Scholar During subsequent stages, Notch is iteratively used to control the sequential generation of different cell types from a common progenitor cell via lateral inhibition, a process observed in other tissues.34Esni F. Ghosh B. Biankin A.V. Lin J.W. Albert M.A. Yu X. MacDonald R.J. Civin C.I. Real F.X. Pack M.A. Ball D.W. Leach S.D. Notch inhibits Ptf1 function and acinar cell differentiation in developing mouse and zebrafish pancreas.Development. 2004; 131: 4213-4224Google Scholar Consequently, Notch signaling helps to specify endocrine cell differentiation. The cells destined to progress along the endocrine lineage inhibit endocrine differentiation in their neighboring cells, forcing them to remain in an undifferentiated proliferative state. Inhibition of Notch signaling in mice deficient for either the ligand Delta-like gene 1 or Hes1, a transcriptional mediator of Notch signaling, results in the depletion of progenitor cells and accelerated differentiation of pancreatic endocrine cells.35Apelqvist A. Li H. Sommer L. Beatus P. Anderson D.J. Honjo T. Hrabe de Angelis M. Lendahl U. Edlund H. Notch signalling controls pancreatic cell differentiation.Nature. 1999; 400: 877-881Google Scholar, 36Jensen J. Pedersen E.E. Galante P. Hald J. Heller R.S. Ishibashi M. Kageyama R. Guillemot F. Serup P. Madsen O.D. Control of endodermal endocrine development by Hes-1.Nat Genet. 2000; 24: 36-44Google Scholar Conversely, ectopic activation of the Notch signaling pathway inhibits both endocrine and exocrine development, resulting in the formation of a pancreas mainly composed of an epithelium that displays characteristics of progenitor cells.35Apelqvist A. Li H. Sommer L. Beatus P. Anderson D.J. Honjo T. Hrabe de Angelis M. Lendahl U. Edlund H. Notch signalling controls pancreatic cell differentiation.Nature. 1999; 400: 877-881Google Scholar, 37Murtaugh L.C. Stanger B.Z. Kwan K.M. Melton D.A. Notch signaling controls multiple steps of pancreatic differentiation.Proc Natl Acad Sci U S A. 2003; 100: 14920-14925Google Scholar Recently, several studies have analyzed the role of the Wnt signaling pathway in pancreatic organogenesis. Murtaugh et al reported that loss of β-catenin did not affect pancreatic endocrine cell mass, despite the almost complete loss of the exocrine compartment.39Murtaugh L.C. Law A.C. Dor Y. Melton D.A. Beta-catenin is essential for pancreatic acinar but not islet development.Development. 2005; 132: 4663-4674Google Scholar In agreement with these results, overexpression of a dominant-negative form of mouse Frz8 in pancreatic progenitors severely perturbs pancreatic growth.40Papadopoulou S. Edlund H. Attenuated Wnt signaling perturbs pancreatic growth but not pancreatic function.Diabetes. 2005; 54: 2844-2851Google Scholar However, Dessimoz et al reported that inactivation of β-catenin (a mediator of Wnt signaling) resulted in the reduction of endocrine cells while acinar cells appear unaffected.38Dessimoz J. Bonnard C. Huelsken J. Grapin-Botton A. Pancreas-specific deletion of beta-catenin reveals Wnt-dependent and Wnt-independent functions during development.Curr Biol. 2005; 15: 1677-1683Google Scholar The reasons for this apparent discrepancy are unclear but might reflect the different pancreas-specific Cre-expressing mouse strains used to conditionally inactivate β-catenin. Ectopic activation of the Wnt/β-catenin signaling pathway during early pancreas development results in a significant reduction in exocrine and endocrine tissues. The epithelium is replaced by undifferentiated ductal epithelium.41Heller R.S. Dichmann D.S. Jensen J. Miller C. Wong G. Madsen O.D. Serup P. Expression patterns of Wnts, Frizzleds, sFRPs, and misexpression in transgenic mice suggesting a role for Wnts in pancreas and foregut pattern formation.Dev Dyn. 2002; 225: 260-270Google Scholar, 42Heiser P.W. Lau J. Taketo M.M. Herrera P.L. Hebrok M. Stabilization of beta-catenin impacts pancreas growth.Development. 2006; 133: 2023-2032Google Scholar These pancreatic defects correlate with changes in FGF and Hh signaling and loss of Pdx1-expressing progenitor cells, indicating that Wnt/β-catenin signaling regulates differentiation and expansion of early pancreatic progenitor cells.42Heiser P.W. Lau J. Taketo M.M. Herrera P.L. Hebrok M. Stabilization of beta-catenin impacts pancreas growth.Development. 2006; 133: 2023-2032Google Scholar Wnt/β-catenin signaling also regulates proliferation of differentiated cells. Activation of Wnt/β-catenin signaling at a later time point in pancreas development causes enhanced proliferation of acinar cells.42Heiser P.W. Lau J. Taketo M.M. Herrera P.L. Hebrok M. Stabilization of beta-catenin impacts pancreas growth.Development. 2006; 133: 2023-2032Google Scholar The study of pancreas organogenesis illustrates how the same embryonic signaling pathways fulfill different roles at distinct stages of embryonic development. It also emphasizes the importance of sophisticated transgenic models to temporally and spatially manipulate pathway activities to fully elucidate their various roles during development. Ultimately, cell differentiation is achieved by the initiation and maintenance of well-orchestrated gene expression patterns, a process controlled by transcription factors. The role of transcription factors and the genetic regulatory networks they form in pancreatic development has been recently covered in several excellent reviews.43Wilson M.E. Scheel D. German M.S. Gene expression cascades in pancreatic development.Mech Dev. 2003; 120: 65-80Google Scholar, 44Servitja J.M. Ferrer J. Transcriptional networks controlling pancreatic development and beta cell function.Diabetologia. 2004; 47: 597-613Google Scholar, 45Jensen J. Gene regulatory factors in pancreatic development.Dev Dyn. 2004; 229: 176-200Google Scholar Here, we will only briefly review some of the transcription factors critical for pancreatic development (Table 1).Table 1Selected Key Transcription Factors Involved in Embryonic Pancreatic DevelopmentTranscription factorOnset of pancreatic expressionPancreatic phenotypes of mice deficient in the corresponding transcription factorPdx1E8.5Conventional mutant mice display pancreas agenesis51Jonsson J. Carlsson L. Edlund T. Edlund H. Insulin-promoter-factor 1 is required for pancreas development in mice.Nature. 1994; 371: 606-609Google Scholar, 175Ahlgren U. Jonsson J. Edlund H. The morphogenesis of the pancreatic mesenchyme is uncoupled from that of the pancreatic epithelium in IPF1/PDX1- deficient mice.Development. 1996; 122: 1409-1416Crossref Google Scholar Conditional inactivation in β-cells leads to diabetes55Ahlgren U. Jonsson J. Jonsson L. Simu K. Edlund H. beta-cell-specific inactivation of the mouse Ipf1/Pdx1 gene results in loss of the beta-cell phenotype and maturity onset diabetes.Genes Dev. 1998; 12: 1763-1768Google ScholarPtfa1/p48E9.5Pancreas hypoplasia58Kawaguchi Y. Cooper B. Gannon M. Ray M. MacDonald R.J. Wright C.V. The role of the transcriptional regulator Ptf1a in converting intestinal to pancreatic progenitors.Nat Genet. 2002; 32: 128-134Google ScholarHes1E9.5Pancreas hypoplasia. Ectopic pancreas in stomach, duodenum and common bile duct35Apelqvist A. Li H. Sommer L. Beatus P. Anderson D.J. Honjo T. Hrabe de Angelis M. Lendahl U. Edlund H. Notch signalling controls pancreatic cell differentiation.Nature. 1999; 400: 877-881Google ScholarNgn3E9.5Absence of endocrine cells61Gradwohl G. Dierich A. LeMeur M. Guillemot F. Neurogenin3 is required for the development of the four endocrine cell lineages of the pancreas.Proc Natl Acad Sci U S A. 2000; 97: 1607-1611Google ScholarHnf1aE8.0Diabetes66Pontoglio M. Sreenan S. Roe M. Pugh W. Ostrega D. Doyen A. Pick A.J. Baldwin A. Velho G. Froguel P. Levisetti M. Bonner-Weir S. Bell G.I. Yaniv M. Polonsky K.S. Defective insulin secretion in hepatocyte nuclear factor 1alpha-deficient mice.J Clin Invest. 1998; 101: 2215-2222Google Scholar, 176Shih D.Q. Screenan S. Munoz K.N. Philipson L. Pontoglio M. Yaniv M. Polonsky K.S. Stoffel M. Loss of HNF-1alpha function in mice leads to abnormal expression of genes involved in pancreatic islet development and metabolism.Diabetes. 2001; 50: 2472-2480Google ScholarHnf3βE8.0Conventional mutant mice is embryonic lethal63Weinstein D.C. Ruiz i Altaba A. Chen W.S. Hoodless P. Prezioso V.R. Jessell T.M. Darnell J.E. The winged-helix transcription factor HNF-3b is required for notochord development in the mouse embryo.Cell. 1994; 78: 575-588Google Scholar, 65Ang S.L. Rossant J. HNF-3 beta is essential for node and notochord formation in mouse development.Cell. 1994; 78: 561-574Google Scholar Conditional inactivation in β-cells results in hyperinsulinemia hypoglycemia177Sund N.J. Vatamaniuk M.Z. Casey M. Ang S.L. Magnuson M.A. Stoffers D.A. Matschinsky F.M. Kaestner K.H. Tissue-specific deletion of Foxa2 in pancreatic beta cells results in hyperinsulinemic hypoglycemia.Genes Dev. 2001; 15: 1706-1715Google ScholarHnf6E9.5Impaired endocrine cell formation.64Jacquemin P. Durviaux S.M. Jensen J. Godfraind C. Gradwohl G. Guillemot F. Madsen O.D. Carmeliet P. Dewerchin M. Collen D. Rousseau G.G. Lemaigre F.P. Transcription factor hepatocyte nuclear factor 6 regulates pancreatic endocrine cell differentiation and controls expression of the proendocrine gene ngn3.Mol Cell Biol. 2000; 20: 4445-4454Google Scholar Dilation of the ducts71Pierreux C.E. Poll A.V. Kemp C.R. Clotman F. Maestro M.A. Cordi S. Ferrer J. Leyns L. Rousseau G.G. Lemaigre F.P. The transcription factor hepatocyte nuclear factor-6 controls the development of pancreatic ducts in the mouse.Gastroenterology. 2006; 130: 532-541Google ScholarIA1 (INSM1)E9.5Impaired endocrine cell formation69Mellitzer G. Bonne S. Luco R.F. Van De Casteele M. Lenne-Samuel N. Collombat P. Mansouri A. Lee J. Lan M. Pipeleers D. Nielsen F.C. Ferrer J. Gradwohl G. Heimberg H. IA1 is NGN3-dependent and essential for differentiation of the endocrine pancreas.EMBO J. 2006; 25: 1344-1352Google ScholarNeuroD1E9.5Reduction in endocrine cell number178Naya F.J. Huang H.P. Qiu Y. Mutoh H. DeMayo F.J. Leiter A.B. Tsai M.J. Diabetes, defective pancreatic morphogenesis, and abnormal enteroendocrine differentiation in BETA2/neuroD-deficient mice.Genes Dev. 1997; 11: 2323-2334Google ScholarPax4E9.5Absence of β- and d-cell number, increase of a-cell number179Sosa-Pineda B. Chowdhury K. Torres M. Oliver G. Gruss P. The Pax4 gene is essential for differentiation of insulin-producing b cells in the mammalian pancreas.Nature. 1997; 386: 399-402Google ScholarNkx2.2E9.5Reduction of a-, β-, and PP-cell number. Impaired β-cell differentiation180Sussel L. Kalamaras J. Hartigan-O’Connor D.J. Meneses J.J. Pedersen R.A. Rubenstein J.L. German M.S. Mice lacking the homeodomain transcription factor Nkx2.2 have diabetes due to arrested differentiation of pancreatic beta cells.Development. 1998; 125: 2213-2221Crossref Google ScholarPax6E9-E9.5Reduction in endocrine cell number181Sander M. Neubuser A. Kalamaras J. Ee H.C. Martin G.R. German M.S. Genetic analysis reveals that PAX6 is required for normal transcription of pancreatic hormone genes and islet development.Genes Dev. 1997; 11: 1662-1673Google Scholar, 182St-Onge L. Sosa-Pineda B. Chowdhurry K. Mansouri A. Gruss P. Pax6 is required for differentiation of glucagon-producing a-cells in mouse pancreas.Nature. 1997; 387: 406-409Google Scholar Open table in a new tab The pancreatic-duodenal homeobox 1 (Pdx1) gene codes for one of the earliest transcriptional factors detected within the developing pancreatic epithelium. Pdx1 is expressed in pancreatic progenitor cells at E8.5.46Guz Y. Montminy M.R. Stein R. Leonard J. Gamer L.W. Wright C.V.E. Teitelman G. Expression of murine STF-1, a putative insulin gene transcription factor, in b cells of pancreas, duodenal epithelium and pancreatic exocrine and endocrine progenitors during ontogeny.Development. 1995; 121: 149-161Google Scholar, 47Offield M.F. Jetton T.L. Labosky P.A. Ray M. Stein R. Magnuson M.A. Hogan B.L.M. Wright C.V.E. PDX-1 is required for pancreatic outgrowth and differentiation of the rostral duodenum.Development. 1996; 122: 983-995Crossref Google Scholar During development, its expression becomes restricted to a subset of acinar and endocrine cells. In adult tissue, Pdx1 is mainly found in pancreatic polypeptide- (PP) and insulin-expressing β-cells.46Guz Y. Montminy M.R. Stein R. Leonard J. Gamer L.W. Wright C.V.E. Teitelman G. Expression of murine STF-1, a putative insulin gene transcription factor, in b cells of pancreas, duodenal epithelium and pancreatic exocrine and endocrine progenitors during ontogeny.Development. 1995; 121: 149-161Google Scholar, 48Ohlsson H. Karlsson K. Edlund T. IPF1, a homeodomain-containing transactivator of the insulin gene.EMBO J. 1993; 12: 4251-4259Crossref Scopus (774) Google Scholar, 49Leonard J. Peers B. Johnson T. Ferreri K. Lee S. Montminy M.R. Characterization of somatostatin transactivating factor-1, a novel homeobox factor that stimulates somatostatin expression in pancreatic islet cells.Mol Endocrinol. 1993; 7: 1275-1283Google Scholar, 50Miller C.P. McGhee R.E. Habener J.F. IDX-1: a new homeodomain transcription factor expressed in rat pancreatic islets and duodenum that transactivates the somatostatin gene.EMBO J. 1994; 13: 1145-1156Crossref Scopus (378) Google Scholar Elimination of the Pdx1 gene at different stages of development, as well in adult tissue, has revealed multiple roles during organogenesis and mature β-cell function. Germline inactivation of Pdx1 in mice arrests pancreatic development shortly after initial bud formation. As a consequence, embryos are born apancreatic, thus demonstrating an essential role for Pdx1 function during the early phases of pancreas formation.47Offield M.F. Jetton T.L. Labosky P.A. Ray M. Stein R. Magnuson M.A. Hogan B.L.M. Wright C.V.E. PDX-1 is required for pancreatic outgrowth and differentiation of the rostral duodenum.Development. 1996; 122: 983-995Crossref Google Scholar, 51Jonsson J. Carlsson L. Edlund T. Edlund H. Insulin-promoter-factor 1 is required for pancreas development in mice.Nature. 1994; 371: 606-609Google Scholar More recently, Holland et al52Holland A.M. Hale M.A. Kagami H. Hammer R.E. MacDonald R.J. Experimental control of pancreatic development and maintenance.Proc Natl Acad Sci U S A. 2002; 99: 12236-12241Google Scholar have been able to temporally inactivate Pdx1 by using a sophisticated strategy to control gene inactivation via tetracycline treatment.52Holland A.M. Hale M.A. Kagami H. Hammer R.E. MacDonald R.J. Experimental control of pancreatic development and maintenance.Proc Natl Acad Sci U S A. 2002; 99: 12236-12241Google Scholar Using this elegant method, these authors have shown that Pdx1 function is required during midpancreatic development for both islet and acinar differentiation.53Hale M.A. Kagami H. Shi L. Holland A.M. Elsasser H.P. Hammer R.E. MacDonald R.J. The homeodomain protein PDX1 is required at mid-pancreatic development for the formation of the exocrine pancreas.Dev Biol. 2005; 286: 225-237Google Scholar In adult stages, Pdx1 plays a critical role in β-cell function, including regulation of insulin expression, as shown by the defects in glucose homeostasis observed in heterozygous Pdx1 mice.54Brissova M. Shiota M. Nicholson W.E. Gannon M. Knobel S.M. Piston D.W. Wright C.V. Powers A.C. Reduction in pancreatic transcription factor PDX-1 impairs glucose-stimulated insulin secretion.J Biol Chem. 2002; 277: 11225-11232Google Scholar Furthermo