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

Optical projection tomography as a tool for 3D microscopy and gene expression studies

Current techniques for three-dimensional (3D) optical microscopy (deconvolution, confocal microscopy, and optical coherence tomography) generate 3D data by "optically sectioning" the specimen. This places severe constraints on the maximum thickness of a specimen that can be imaged. We have developed a microscopy technique that uses optical projection tomography (OPT) to produce high-resolution 3D images of both fluorescent and nonfluorescent biological specimens with a thickness of up to 15 millimeters. OPT microscopy allows the rapid mapping of the tissue distribution of RNA and protein expression in intact embryos or organ systems and can therefore be instrumental in studies of developmental biology or gene function.

Induction of c-Myc expression suppresses insulin gene transcription by inhibiting NeuroD/BETA2-mediated transcriptional activation

Insulin biosynthesis and secretion are critical for pancreatic beta-cell function, but both are impaired under diabetic conditions. We have found that hyperglycemia induces the expression of the basic helix-loop-helix transcription factor c-Myc in islets in several different diabetic models. To examine the possible implication of c-Myc in beta-cell dysfunction, c-Myc was overexpressed in isolated rat islets using adenovirus. Adenovirus-mediated c-Myc overexpression suppressed both insulin gene transcription and glucose-stimulated insulin secretion. Insulin protein content, determined by immunostaining, was markedly decreased in c-Myc-overexpressing cells. In gel-shift assays c-Myc bound to the E-box in the insulin gene promoter region. Furthermore, in betaTC1, MIN6, and HIT-T15 cells and primary rat islets, wild type insulin gene promoter activity was dramatically decreased by c-Myc overexpression, whereas the activity of an E-box mutated insulin promoter was not affected. In HeLa and HepG2 cells c-Myc exerted a suppressive effect on the insulin promoter activity only in the presence of NeuroD/BETA2 but not PDX-1. Both c-Myc and NeuroD can bind the E-box element in the insulin promoter, but unlike NeuroD, the c-Myc transactivation domain lacked the ability to activate insulin gene expression. Additionally p300, a co-activator of NeuroD, did not function as a co-activator of c-Myc. In conclusion, increased expression of c-Myc in beta-cells suppresses the insulin gene transcription by inhibiting NeuroD-mediated transcriptional activation. This mechanism may explain some of the beta-cell dysfunction found in diabetes.

Sonic hedgehog is required early in pancreatic islet development

Pancreatic organogenesis relies on a complex interplay of cell-autonomous and extracellular signals. We demonstrate that the morphogen sonic hedgehog (Shh) is required for pancreatic development in zebrafish. Genetic mutants of Shh and its signaling pathway establish this dependence as specific to endocrine, but not exocrine, pancreas. Using cyclopamine to inhibit hedgehog signaling, we show that transient Shh signaling is necessary during gastrulation for subsequent differentiation of endoderm into islet tissue. A second hedgehog-dependent activity occurring later in development was also identified and may be analogous to the known action of Shh in gut endoderm to direct localization of pancreatic development. The early action of Shh may be part of a more general process allowing neuroendocrine cells to originate in nonneuroectodermally derived tissues.

Reduction in pancreatic transcription factor PDX-1 impairs glucose-stimulated insulin secretion

Complete lack of transcription factor PDX-1 leads to pancreatic agenesis, whereas heterozygosity for PDX-1 mutations has been recently noted in some individuals with maturity-onset diabetes of the young (MODY) and in some individuals with type 2 diabetes. To determine how alterations in PDX-1 affect islet function, we examined insulin secretion and islet physiology in mice with one PDX-1 allele inactivated. PDX-1(+/-) mice had a normal fasting blood glucose and pancreatic insulin content but had impaired glucose tolerance and secreted less insulin during glucose tolerance testing. The expression of PDX-1 and glucose transporter 2 in islets from PDX-1(+/-) mice was reduced to 68 and 55%, respectively, whereas glucokinase expression was not significantly altered. NAD(P)H generation in response to glucose was reduced by 30% in PDX-1(+/-) mice. The in situ perfused pancreas of PDX-1(+/-) mice secreted about 45% less insulin when stimulated with 16.7 mm glucose. The K(m) for insulin release was similar in wild type and PDX-1(+/-) mice. Insulin secretion in response to 20 mm arginine was unchanged; the response to 10 nm glucagon-like peptide-1 was slightly increased. However, insulin secretory responses to 10 mm 2-ketoisocaproate and 20 mm KCl were significantly reduced (by 61 and 66%, respectively). These results indicate that a modest reduction in PDX-1 impairs several events in glucose-stimulated insulin secretion (such as NAD(P)H generation, mitochondrial function, and/or mobilization of intracellular Ca(2+)) and that PDX-1 is important for normal function of adult pancreatic islets.

Regulation of pancreatic cell differentiation and morphogenesis

Organogenesis requires tissue interactions to initiate the cascade of inductive and repressive signals necessary for normal organ development. Tissue interactions initiate the pancreatic lineage within the primitive foregut endodermal epithelium and continue to direct the morphogenesis and differentiation of the endocrine, exocrine and ductal portions of the pancreas. An understanding of the mechanisms controlling pancreatic growth would enable the development of alternative therapies for diseases such as type 1 diabetes.

IPF-1 gene variation and the development of type 2 diabetes

Insulin promotor factor-1 (IPF-1) is a gene critical for pancreatic development and insulin transcription. We genotyped U.S. Caucasians with (n = 217) and without (n = 176) Type 2 diabetes to determine if three previously identified variants (Cys18Arg, Asp76Asn, Arg197His) in the IPF-1 gene play a role in the development of Type 2 diabetes. The Cys18Arg and Arg197His variants were not present in any subjects. The Asp76Asn variant was found in one control subject (0.9%) and none of the diabetic subjects. Similarly, none of the variants were detected in African American subjects with (n = 78) and without (n = 82) Type 2 diabetes. In conclusion, these variants are rare or absent in U.S. Caucasians and African Americans and therefore, unlikely to play a significant role in the development of Type 2 diabetes in these populations.

The role of hepatic nuclear factor 1 alpha and PDX-1 in transcriptional regulation of the pdx-1 gene

The PDX-1 homeodomain transcription factor regulates pancreatic development and adult islet beta cell function. Expression of the pdx-1 gene is almost exclusively localized to beta cells within the adult endocrine pancreas. Islet beta cell-selective transcription is controlled by evolutionarily conserved subdomain sequences (termed Areas I (-2839 to -2520 base pairs (bp)), II (-2252 to -2023 bp), and III (-1939 to -1664 bp)) found within the 5'-flanking region of the pdx-1 gene. Areas I and II are independently capable of directing beta cell-selective reporter gene activity in transfection assays, with Area I-mediated stimulation dependent upon binding of hepatic nuclear factor 3 beta (HNF3 beta), a key regulator of islet beta cell function. To identify other transactivators of Area I, highly conserved sequence segments within this subdomain were mutagenized, and their effect on activation was determined. Several of the sensitive sites were found by transcription factor data base analysis to potentially bind endodermally expressed transcription factors, including HNF1 alpha (-2758 to -2746 bp, Segment 2), HNF4 (-2742 to -2730 bp, Segment 4; -2683 to -2671 bp, Segment 7-8), and HNF6 (-2727 to -2715 bp, Segment 5). HNF1 alpha, but not HNF4 and HNF6, binds specifically to Area I sequences in vitro. HNF1 alpha was also shown to specifically activate Area I-driven transcription through Segment 2. In addition, PDX-1 itself was found to stimulate Area I activation. The chromatin immunoprecipitation assay performed with PDX-1 antisera also demonstrated that this factor bound to Area I within the endogenous pdx-1 gene in beta cells. Our results indicate that regulatory factors binding to Area I conserved sequences contribute to the selective transcription pattern of the pdx-1 gene and that control is mediated by endodermal regulators like HNF1 alpha, HNF3 beta, and PDX-1.

Programming of the pancreas

The pancreas, as most of the digestive tract, derives from the endoderm. Differentiation of these early gut endoderm cells into the endocrine cells forming the pancreatic islets of Langerhans depends on a cascade of gene activation events. These are controlled by different classes of transcription factors including the homeodomain, the basic helix-loop-helix (bHLH) and the winged helix proteins. Recently, considerable progress has been made delineating this cascade. The present review focuses on the role of the different transcription factors during pancreas development, with a particular emphasis on the newly identified bHLH transcription factor neurogenin3.

Persistent expression of Hlxb9 in the pancreatic epithelium impairs pancreatic development

The homebox gene Hlxb9, encoding Hb9, exhibits a dual expression profile during pancreatic development. The early expression in the dorsal and ventral pancreatic epithelium is transient and spans from embryonic day (e) 8 to e9-e10, whereas the later expression is confined to differentiating beta-cells as they appear. We previously showed that Hlxb9 is critically required for the initiation of the dorsal, but not the ventral, pancreatic program. Here, we demonstrate the requirement for a stringent temporal regulation of Hlxb9 expression during early stages of pancreatic development. In transgenic mice, where Hlxb9 expression, under control of the Ipf1/Pdx1 promoter, was extended beyond e9-e10, the development of the pancreas was drastically perturbed. Morphological analyses showed that the growth and morphogenesis of the pancreatic epithelium was impaired. Moreover, differentiation of pancreatic endocrine and exocrine cells was diminished and instead the pancreatic epithelium with its adjacent mesenchyme adopted an intestinal-like differentiation program. Together, these data point to a need for a tight temporal regulation of Hlxb9 expression. Thus, a total loss of Hlxb9 expression results in a block of the initiation of the dorsal pancreatic program, while a temporally extended expression of Hlxb9 results in a complete impairment of pancreatic development.

Regulatory regions driving developmental and tissue-specific expression of the essential pancreatic gene pdx1

pdx1 (pancreatic and duodenal homeobox gene-1), which is expressed broadly in the embryonic pancreas and, later, in a more restricted manner in the mature beta cells in the islets of Langerhans, is essential both for organ formation and beta cell gene expression and function. We carried out a transgenic reporter gene analysis to identify region- and cell type-specific regulatory regions in pdx1. A 14.5-kb pdx1 genomic fragment corrected the glucose intolerance of pdx1(+/-) animals but, moreover, fully rescued the severe gut and pancreas defects in pdx1(-/-) embryos. Sequences sufficient to direct reporter expression to the entire endogenous pdx1 expression domain lie within 4.3 kb of 5' flanking DNA. In this region, we identified two distinct fragments that drive reporter gene expression to different sets of islet neuroendocrine cells. One shows pan-endocrine cell specificity, the other is selectively activated in insulin-producing beta cells. The endocrine-specific regulatory regions overlap a localized region of 5' flanking DNA that is remarkably conserved in sequence between vertebrate pdx1 genes, and which has been associated with beta cell-selective expression in cultured cell lines. This region contains potential binding sites for several transcription factors implicated in endodermal development and the pathogenesis of some forms of type-2 diabetes. These results are consistent with our previous proposal that conserved upstream pdx1 sequences exert control over pdx1 during embryonic organogenesis and islet endocrine cell differentiation. We propose that mutations affecting the expression and/or activity of transcription factors operating via these sequences may predispose towards diabetes, at least in part by direct effects on endocrine pdx1 expression.

Improved glucose tolerance and acinar dysmorphogenesis by targeted expression of transcription factor PDX-1 to the exocrine pancreas

The homeodomain protein PDX-1 is critical for pancreas development and is a key regulator of insulin gene expression. PDX-1 nullizygosity and haploinsufficiency in mice and humans results in pancreatic agenesis and diabetes, respectively. At embryonic day (e) 10.5, PDX-1 is expressed in all pluripotential gut-derived epithelial cells destined to differentiate into the exocrine and endocrine pancreas. At e15, PDX-1 expression is downregulated in exocrine cells, but remains high in endocrine cells. The aim of this study was to determine whether targeted overexpression of PDX-1 to the exocrine compartment of the developing pancreas at e15 would allow for respecification of the exocrine cells. Transgenic (TG) mice were generated in which PDX-1 was expressed in the exocrine pancreas using the exocrine-specific elastase-1 promoter. These mice exhibited a marked dysmorphogenesis of the exocrine pancreas, manifested by increased rates of replication and apoptosis in acinar cells and a progressive fatty infiltration of the exocrine pancreas with age. Interestingly, the TG mice exhibited improved glucose tolerance, but absolute beta-cell mass was not increased. These findings indicate that downregulation of PDX-1 is required for the proper maintenance of the exocrine cell phenotype and that upregulation of PDX-1 in acinar cells affects beta-cell function. The mechanisms underlying these observations remain to be elucidated.

Hox11 Acts Cell Autonomously in Spleen Development and Its Absence Results in Altered Cell Fate of Mesenchymal Spleen Precursors

The genetic steps governing development of the spleen are largely unknown. Absence of Hox11 in mice results in asplenia, but it is unclear how Hox11 exerts its effect on spleen development. To more precisely define Hox11's role in spleen morphogenesis, we have examined the fate of the developing spleen in Hox11(-/-) mice. Perturbation of spleen development begins between dE13 and dE13.5. Cells of the spleen anlage persist past this developmental stage as an unorganized rudiment between the stomach and the pancreas. They fail to proliferate, and haematopoietic cells do not colonize the rudiment. At later stages of embryonic development, the cells can be observed in the mesenchyme of the pancreas, also an expression site of Hox11. In Hox11-/-<-->+/+ chimaeras, spleens were devoid of Hox11(-/-) cells, indicating that the genetic defect is cell autonomous and not due to failure of the organ anlage to attract and retain haematopoietic cells. In -/-<-->+/+ chimaeric embryos, Hox11(-/-) cells were initially present in the spleen anlage. However, at dE13, a reorganization of the spleen occurred in the chimaeras and Hox11(-/-) cells were subsequently excluded from the spleen, suggesting that a change in the affinity for one of the spleen cells had occurred. These observations demonstrate that spleen development consists of genetically separable steps and that absence of Hox11 arrests spleen development at an early stage. The formation of the spleen primordium before the entry of haematopoietic cells does not require the activity of Hox11. However, subsequent differentiation of spleen precursor cells is dependent on the Hox11 gene.

A pancreatic beta -cell-specific enhancer in the human PDX-1 gene is regulated by hepatocyte nuclear factor 3beta (HNF-3beta ), HNF-1alpha, and SPs transcription factors

The PDX-1 transcription factor plays a key role in pancreas development. Although expressed in all cells at the early stages, in the adult it is mainly restricted to the beta-cell. To characterize the regulatory elements and potential transcription factors necessary for human PDX-1 gene expression in beta-cells, we constructed a series of 5' and 3' deletion fragments of the 5'-flanking region of the gene, fused to the luciferase reporter gene. In this report, we identify by transient transfections in beta- and non-beta-cells a novel beta-cell-specific distal enhancer element located between -3.7 and -3.45 kilobases. DNase I footprinting analysis revealed two protected regions, one binding the transcription factors SP1 and SP3 and the other hepatocyte nuclear factor 3beta (HNF-3beta) and HNF-1alpha. Cotransfection experiments suggest that HNF-3beta, HNF-1alpha, and SP1 are positive regulators of the herein-described human PDX-1 enhancer element. Furthermore, mutations within each motif abolished the binding of the corresponding factor(s) and dramatically impaired the enhancer activity, therefore suggesting cooperativity between these factors.

A bipotential precursor population for pancreas and liver within the embryonic endoderm

The pancreas emerges independently from dorsal and ventral domains of embryonic gut endoderm. Gene inactivation experiments in mice have identified factors required for dorsal pancreas development, but factors that initiate the ventral pancreas have remained elusive. In this study, we investigated the hypothesis that the emergence of the ventral pancreas is related to the emergence of the liver. We find that the liver and ventral pancreas are specified at the same time and in the same general domain of cells. Using embryo tissue explantation experiments, we find that the default fate of the ventral foregut endoderm is to activate the pancreas gene program. FGF signalling from the cardiac mesoderm diverts this endoderm to express genes for liver instead of those for pancreas. No evidence was found to indicate that the cell type choice for pancreas or liver involves a selection for growth or viability. Cardiac mesoderm or FGF induces the local expression of sonic hedgehog, which in turn is inhibitory to pancreas but not to liver. The bipotential precursor cell population for pancreas and liver in embryonic development and its fate selection by FGF has features that appear to be recapitulated in the adult pancreas and are reflected in the evolution of these organs.

Initiation and early patterning of the endoderm

We review the early stages of endoderm formation in the major animal models. In Amphibia maternal molecules are important in initiating endoderm formation. This is followed by successive signaling events that establish and then pattern the endoderm. In other organisms there are differences in endodermal development, particularly in the initial, prephylotypic stages. Later many of the same key families of transcription factors and signaling cassettes are used in all animals, but more work will be needed to establish exact evolutionary homologies.

Pancreas development in zebrafish: early dispersed appearance of endocrine hormone expressing cells and their convergence to form the definitive islet

To begin to understand pancreas development and the control of endocrine lineage formation in zebrafish, we have examined the expression pattern of several genes shown to act in vertebrate pancreatic development: pdx-1, insulin (W. M. Milewski et al., 1998, Endocrinology 139, 1440-1449), glucagon, somatostatin (F. Argenton et al., 1999, Mech. Dev. 87, 217-221), islet-1 (Korzh et al., 1993, Development 118, 417-425), nkx2.2 (Barth and Wilson, 1995, Development 121, 1755-1768), and pax6.2 (Nornes et al., 1998, Mech. Dev. 77, 185-196). To determine the spatial relationship between the exocrine and the endocrine compartments, we have cloned the zebrafish trypsin gene, a digestive enzyme expressed in differentiated pancreatic exocrine cells. We found expression of all these genes in the developing pancreas throughout organogenesis. Endocrine cells first appear in a scattered fashion in two bilateral rows close to the midline during mid-somitogenesis and converge during late-somitogenesis to form a single islet dorsal to the nascent duodenum. We have examined development of the endocrine lineage in a number of previously described zebrafish mutations. Deletion of chordamesoderm in floating head (Xnot homolog) mutants reduces islet formation to small remnants, but does not delete the pancreas, indicating that notochord is involved in proper pancreas development, but not required for differentiation of pancreatic cell fates. In the absence of knypek gene function, which is involved in convergence movements, the bilateral endocrine primordia do not merge. Presence of trunk paraxial mesoderm also appears to be instrumental for convergence since the bilateral endocrine primordia do not merge in spadetail mutants. We discuss our findings on zebrafish pancreatogenesis in the light of evolution of the pancreas in chordates.

Transcription factors recognizing overlapping C1-A2 binding sites positively regulate insulin gene expression

Transcription factors binding the insulin enhancer region, RIPE3b, mediate beta-cell type-specific and glucose-responsive expression of the insulin gene. Earlier studies demonstrate that activator present in the beta-cell-specific RIPE3b1-binding complex is critical for these actions. The DNA binding activity of the RIPE3b1 activator is induced in response to glucose stimulation and is inhibited under glucotoxic conditions. The C1 element within the RIPE3b region has been implicated as the binding site for RIPE3b1 activator. The RIPE3b region also contains an additional element, A2, which shares homology with the A elements in the insulin enhancer. Transcription factors (PDX-1 and HNF-1 alpha) binding to A elements are critical regulators of insulin gene expression and/or pancreatic development. Hence, to understand the roles of C1 and A2 elements in regulating insulin gene expression, we have systematically mutated the RIPE3b region and analyzed the effect of these mutations on gene expression. Our results demonstrate that both C1 and A2 elements together constitute the binding site for the RIPE3b1 activator. In addition to C1-A2 (RIPE3b) binding complexes, three binding complexes that specifically recognize A2 elements are found in nuclear extracts from insulinoma cell lines; the A2.2 complex is detected only in insulin-producing cell lines. Furthermore, two base pairs in the A2 element were critical for binding of both RIPE3b1 and A2.2 activators. Transient transfection results indicate that both C1-A2 and A2-specific binding activators cooperatively activate insulin gene expression. In addition, RIPE3b1- and A2-specific activators respond differently to glucose, suggesting that their overlapping binding specificity and functional cooperation may play an important role in regulating insulin gene expression.

The Cre-loxP system: a versatile tool for targeting genes in a cell- and stage-specific manner

Gene-targeted mice, derived from embryonic stem cells, are useful tools to study gene function during development. However, if the inactivation of the target gene results in embryonic lethality, the postdevelopmental function of the gene cannot be further studied. The Cre recombinase-loxP (Cre-loxP) system was developed to overcome this limitation as well as to confine the inactivation of the target gene in a cell- or tissue-specific manner. This system allows for the inactivation of the target gene in a single cell type, thereby allowing the analysis of physiological and pathophysiological consequences of the genetic alteration in mature animals. A unique property of the insulin gene to be expressed only in pancreatic beta cells has allowed using the beta-cell-specific rat insulin promoter (RIP) for Cre recombinase expression to inactivate genes in beta cells. The RIP has been used to inactivate genes in beta cells and analysis of these genetically altered mice has provided important information regarding the role of potential transcription factors and the receptors in vivo, for regulation of insulin gene transcription and in the development of beta cells. The Cre-loxP system is at a relatively early stage of development, and the ability of this technique to virtually target any gene in any tissue at any stage of development makes the study of gene function in a single cell type in vivo an attainable goal. It is anticipated that the continued experience with this system will provide an important tool to determine the role of the transcription factors involved in insulin gene regulation and islet cell differentiation and ultimately provide the basis for novel therapy to treat diabetes.

IFN-gamma overexpression within the pancreas is not sufficient to rescue Pax4, Pax6, and Pdx-1 mutant mice from death

In the presence of interferon-gamma (IFN-gamma), pancreatic ductal epithelial cells grow continuously, and islets undergo neogenesis. To determine whether these new islets are derived from conventional precursors, we tested whether IFN-gamma can complement the loss of transcription factors known to regulate pancreatic development. We analyzed the effect of a transgene on lethality in mice lacking the transcription factors Pax4, Pax6, or Pdx-1, by intercrossing such mice with transgenic mice whose pancreatic cells make IFN-gamma (ins-IFN-gamma mice). However, IFN-gamma expression did not rescue these mice from the lethal mutations, because no homozygous knockout mice carrying the IFN-gamma transgene survived, despite the survival of all other hemizygous gene combinations. This outcome demonstrates that the pathway for IFN-gamma regeneration requires the participation of Pax4, Pax6, and Pdx-1. We conclude that the striking islet regeneration observed in the ins-IFN-gamma NOD strain is regulated by the same transcription factors that control initial pancreatic development.

Functional conservation of regulatory elements in the pdx-1 gene: PDX-1 and hepatocyte nuclear factor 3beta transcription factors mediate beta-cell-specific expression

The PDX-1 transcription factor plays a key role in pancreatic development and in the regulation of the insulin gene in the adult beta cell. As its functions appear to be similar in humans and mice, we analyzed the functional conservation of homologous sequences important for the maintenance and the cell-specific regulation of the pdx-1 gene. Apart from the proximal promoter region, three highly homologous (PH1 to PH3) sequences were apparent in the human and mouse 5' flanking regions of the gene. By transient transfections in beta and non-beta cells, we show that mainly PH1 and PH2 preferentially confer beta-cell-specific activation on a heterologous promoter. DNase I footprinting and binding analyses revealed that both bind to and are transactivated by hepatocyte nuclear factor 3beta (HNF-3beta). Furthermore, the PH1 enhancer element also binds the PDX-1 transcription factor itself, which acts cooperatively with adjacent HNF-3beta to regulate its transcriptional potency. This finding suggests a possible autoregulatory loop as a mechanism for PDX-1 to control its own expression.

Regulation of insulin gene transcription

Recent studies of the insulin gene promoter and the transcription factors that regulate it have expanded our understanding of both how the production of insulin is restricted to the pancreatic beta -cell, and how that production is regulated by physiologic signals such as glucose. A picture is emerging in which an elaborate set of transcription factors binds to specific sequences along the promoter and recruits additional transcriptional co-activators to build a functional transcriptional activation complex that is unique to beta -cells. Surprisingly, however, genetic experiments in mice have demonstrated an unexpected degree of redundancy in the factors that control insulin gene expression, and have revealed the presence of a network of transcription factors that coordinate the expression of factors forming the insulin gene activation complex.

Development of the pancreas in Xenopus laevis

Xenopus embryos have several experimental advantages for studying development. Although these advantages have traditionally been used to elucidate mechanisms of early development, they can also be exploited to investigate issues later in development such as organogenesis. We have begun to study pancreatic organogenesis in Xenopus. Using histological and molecular marker analysis, we characterized the anatomy of the developing pancreas in Xenopus embryos from the time of initial pancreatic rudiment formation to the time when the tadpole starts to feed. We examined the expression of various endocrine hormones, exocrine gene products, and pancreatic transcription factors. Interestingly, the endocrine hormone insulin has restricted expression in the dorsal pancreas. Investigation of pancreatic specification during gastrulation demonstrates that insulin expression is regionalized along the dorsoventral axis early in development.

Impaired migration and delayed differentiation of pancreatic islet cells in mice lacking EGF-receptors

Pancreatic acini and islets are believed to differentiate from common ductal precursors through a process requiring various growth factors. Epidermal growth factor receptor (EGF-R) is expressed throughout the developing pancreas. We have analyzed here the pancreatic phenotype of EGF-R deficient (−/−) mice, which generally die from epithelial immaturity within the first postnatal week. The pancreata appeared macroscopically normal. The most striking feature of the EGF-R (−/−) islets was that instead of forming circular clusters, the islet cells were mainly located in streak-like structures directly associated with pancreatic ducts. Based on BrdU-labelling, proliferation of the neonatal EGF-R (−/−) beta-cells was significantly reduced (2.6+/−0.4 versus 5.8+/−0.9%, P&lt;0.01) and the difference persisted even at 7–11 days of age. Analysis of embryonic pancreata revealed impaired branching morphogenesis and delayed islet cell differentiation in the EGF-R (−/−) mice. Islet development was analyzed further in organ cultures of E12.5 pancreata. The proportion of insulin-positive cells was significantly lower in the EGF-R (−/−) explants (27+/−6 versus 48+/−8%, P&lt;0.01), indicating delayed differentiation of the beta cells. Branching of the epithelium into ducts was also impaired. Matrix metalloproteinase (MMP-2 and MMP-9) activity was reduced 20% in EGF-R (−/−) late-gestation pancreata, as measured by gelatinase assays. Furthermore, the levels of secreted plasminogen activator inhibitor-1 (PAI-1) were markedly higher, while no apparent differences were seen in the levels of active uPA and tPa between EGF-R (−/−) and wild-type pancreata. Our findings suggest that the perturbation of EGF-R-mediated signalling can lead to a generalized proliferation defect of the pancreatic epithelia associated with a delay in beta cell development and disturbed migration of the developing islet cells as they differentiate from their precursors. Upregulated PAI-1 production and decreased gelatinolytic activity correlated to this migration defect. An intact EGF-R pathway appears to be a prerequisite for normal pancreatic development.

Regulation of the pancreatic islet-specific gene BETA2 (neuroD) by neurogenin 3

The BETA2 (neuroD) gene is expressed in endocrine cells during pancreas development and is essential for proper islet morphogenesis. The objective of this study is to identify potential upstream regulators of the BETA2 gene during pancreas development. We demonstrated that the expression of neurogenin 3 (ngn3), an islet- and neuron-specific basic-helix-loop-helix transcription factor, partially overlaps that of BETA2 during early mouse development. More importantly, overexpression of ngn3 can induce the ectopic expression of BETA2 in Xenopus embryos and stimulate the endogenous RNA of BETA2 in endocrine cell lines. Furthermore, overexpression of ngn3 could cause a dose-dependent activation on the 1.0-kb BETA2 promoter in islet-derived cell lines. Deletion and mutation analyses revealed that two proximal E box sequences, E1 and E3, could bind to ngn3-E47 heterodimer and mediate ngn3 activation. Based on these results, we hypothesize that ngn3 is involved in activating the expression of BETA2 at an early stage of islet cell differentiation through the E boxes in the BETA2 promoter.

The RIPE3b1 activator of the insulin gene is composed of a protein(s) of approximately 43 kDa, whose DNA binding activity is inhibited by protein phosphatase treatment

Glucose-stimulated and pancreatic islet beta cell-specific expression of the insulin gene is mediated in part by the C1 DNA-element binding complex, termed RIPE3b1. In this report, we define the molecular weight range of the protein(s) that compose this beta cell-enriched activator complex and show that protein phosphatase treatment inhibits RIPE3b1 DNA binding activity. Fractionation of beta cell nuclear extracts by sodium dodecyl sulfate-polyacrylamide gel electrophoresis indicated that RIPE3b1 binding was mediated by a protein(s) within the 37-49-kDa ranges. Direct analysis of the proteins within the RIPE3b1 complex by ultraviolet light cross-linking analysis identified three binding species of approximately 51, 45, and 38 kDa. Incubating beta cell nuclear extracts with either calf alkaline phosphatase or a rat brain phosphatase preparation dramatically reduced RIPE3b1 DNA complex formation. Phosphatase inhibition of RIPE3b1 binding was prevented by sodium pyrophosphate, a general phosphatase inhibitor. We discuss how changes in the phosphorylation status of the RIPE3b1 activator may influence its DNA binding activity.

neurogenin3 is required for the development of the four endocrine cell lineages of the pancreas

In the mammalian pancreas, the endocrine cell types of the islets of Langerhans, including the alpha-, beta-, delta-, and pancreatic polypeptide cells as well as the exocrine cells, derive from foregut endodermal progenitors. Recent genetic studies have identified a network of transcription factors, including Pdx1, Isl1, Pax4, Pax6, NeuroD, Nkx2.2, and Hlxb9, regulating the development of islet cells at different stages, but the molecular mechanisms controlling the specification of pancreatic endocrine precursors remain unknown. neurogenin3 (ngn3) is a member of a family of basic helix-loop-helix transcription factors that is involved in the determination of neural precursor cells in the neuroectoderm. ngn3 is expressed in discrete regions of the nervous system and in scattered cells in the embryonic pancreas. We show herein that ngn3-positive cells coexpress neither insulin nor glucagon, suggesting that ngn3 marks early precursors of pancreatic endocrine cells. Mice lacking ngn3 function fail to generate any pancreatic endocrine cells and die postnatally from diabetes. Expression of Isl1, Pax4, Pax6, and NeuroD is lost, and endocrine precursors are lacking in the mutant pancreatic epithelium. Thus, ngn3 is required for the specification of a common precursor for the four pancreatic endocrine cell types.

Pancreatic beta cell-specific transcription of the pdx-1 gene. The role of conserved upstream control regions and their hepatic nuclear factor 3beta sites

To identify potential transactivators of pdx-1, we sequenced approximately 4.5 kilobases of the 5' promoter region of the human and chicken homologs, assuming that sequences conserved with the mouse gene would contain critical cis-regulatory elements. The sequences associated with hypersensitive site 1 (HSS1) represented the principal area of homology within which three conserved subdomains were apparent: area I (-2694 to -2561 base pairs (bp)), area II (-2139 to -1958 bp), and area III (-1879 to -1799 bp). The identities between the mouse and chicken/human genes are very high, ranging from 78 to 89%, although only areas I and III are present within this region in chicken. Pancreatic beta cell-selective expression was shown to be controlled by mouse and human area I or area II, but not area III, from an analysis of pdx-1-driven reporter activity in transfected beta- and non-beta cells. Mutational and functional analyses of conserved hepatic nuclear factor 3 (HNF3)-like sites located within area I and area II demonstrated that activation by these regions was mediated by HNF3beta. To determine if a similar regulatory relationship might exist within the context of the endogenous gene, pdx-1 expression was measured in embryonic stem cells in which one or both alleles of HNF3beta were inactivated. pdx-1 mRNA levels induced upon differentiation to embryoid bodies were down-regulated in homozygous null HNF3beta cells. Together, these results suggest that the conserved sequences represented by areas I and II define the binding sites for factors such as HNF3beta, which control islet beta cell-selective expression of the pdx-1 gene.

The homeodomain of PDX-1 mediates multiple protein-protein interactions in the formation of a transcriptional activation complex on the insulin promoter

Activation of insulin gene transcription specifically in the pancreatic beta cells depends on multiple nuclear proteins that interact with each other and with sequences on the insulin gene promoter to build a transcriptional activation complex. The homeodomain protein PDX-1 exemplifies such interactions by binding to the A3/4 region of the rat insulin I promoter and activating insulin gene transcription by cooperating with the basic-helix-loop-helix (bHLH) protein E47/Pan1, which binds to the adjacent E2 site. The present study provides evidence that the homeodomain of PDX-1 acts as a protein-protein interaction domain to recruit multiple proteins, including E47/Pan1, BETA2/NeuroD1, and high-mobility group protein I(Y), to an activation complex on the E2A3/4 minienhancer. The transcriptional activity of this complex results from the clustering of multiple activation domains capable of interacting with coactivators and the basal transcriptional machinery. These interactions are not common to all homeodomain proteins: the LIM homeodomain protein Lmx1.1 can also activate the E2A3/4 minienhancer in cooperation with E47/Pan1 but does so through different interactions. Cooperation between Lmx1.1 and E47/Pan1 results not only in the aggregation of multiple activation domains but also in the unmasking of a potent activation domain on E47/Pan1 that is normally silent in non-beta cells. While more than one activation complex may be capable of activating insulin gene transcription through the E2A3/4 minienhancer, each is dependent on multiple specific interactions among a unique set of nuclear proteins.

Vertebrate endoderm development

Endoderm, one of the three principal germ layers, contributes to all organs of the alimentary tract. For simplicity, this review divides formation of endodermal organs into four fundamental steps: (a) formation of endoderm during gastrulation, (b) morphogenesis of a gut tube from a sheet of cells, (c) budding of organ domains from the tube, and (d) differentiation of organ-specific cell types within the growing buds. We discuss possible mechanisms that regulate how undifferentiated endoderm becomes specified into a myriad of cell types that populate the respiratory and gastrointestinal tracts.

Paired-homeodomain transcription factor PAX4 acts as a transcriptional repressor in early pancreatic development

The paired-homeodomain transcription factor PAX4 is expressed in the developing pancreas and along with PAX6 is required for normal development of the endocrine cells. In the absence of PAX4, the numbers of insulin-producing beta cells and somatostatin-producing delta cells are drastically reduced, while the numbers of glucagon-producing alpha cells are increased. To gain insight into PAX4 function, we cloned a full-length Pax4 cDNA from a beta-cell cDNA library and identified a bipartite consensus DNA binding sequence consisting of a homeodomain binding site separated from a paired domain binding site by 15 nucleotides. The paired half of this consensus sequence has similarities to the PAX6 paired domain consensus binding site, and the two proteins bind to common sequences in several islet genes, although with different relative affinities. When expressed in an alpha-cell line, PAX4 represses transcription through the glucagon or insulin promoters or through an isolated PAX4 binding site. This repression is not simply due to competition with the PAX6 transcriptional activator for the same binding site, since PAX4 fused to the unrelated yeast GAL4 DNA binding domain also represses transcription through the GAL4 binding site in the alpha-cell line and to a lesser degree in beta-cell lines and NIH 3T3 cells. Repressor activity maps to more than one domain within the molecule, although the homeodomain and carboxyl terminus give the strongest repression. PAX4 transcriptional regulation apparently plays a role only early in islet development, since Pax4 mRNA as determined by reverse transcriptase PCR peaks at embryonic day 13.5 in the fetal mouse pancreas and is undetectable in adult islets. In summary, PAX4 can function as a transcriptional repressor and is expressed early in pancreatic development, which may allow it to suppress alpha-cell differentiation and permit beta-cell differentiation.

Gut specific expression using mammalian promoters in transgenic Xenopus laevis

The recent development of transgenic methods for the frog Xenopus laevis provides the opportunity to study later developmental events, such as organogenesis, at the molecular level. Our studies have focused on the development of the tadpole gut, where tissue specific promoters have yet to be identified. We have used mammalian promoters, for the genes elastase, pancreatic duodenal homeobox-1, transthyretin, and intestinal fatty acid binding protein to drive green fluorescent protein expression in live tadpoles. All of these were shown to drive appropriate tissue specific expression, suggesting that the molecular mechanisms organising the gut are similar in amphibians and mammals. Furthermore, expression from the elastase promoter is initiated in the pancreatic buds before morphological definition becomes possible, making it a powerful tool for the study of pancreatic determination.

Defective mutations in the insulin promoter factor-1 (IPF-1) gene in late-onset type 2 diabetes mellitus

Type 2 diabetes mellitus is a common disabling disease with onset in middle-aged individuals, caused by an imbalance between insulin production and action. Genetic studies point to major genetic components, but, with the exception of maturity-onset diabetes of the young (MODY), specific diabetes susceptibility genes remain to be identified. Recent studies showed that a dominant negative mutation in the insulin promoter factor-1 (IPF-1), a pancreatic beta-cell specific transcription factor, causes pancreatic agenesis and MODY. Thus, we investigated 192 French, non-MODY type 2 diabetic families for mutations in IPF-1. We identified 3 novel IPF-1 mutations, including 2 substitutions (Q59L and D76N) and an in-frame proline insertion (InsCCG243). Functional transactivation assays of these IPF-1 mutant isoforms in a beta-pancreatic tumor cell line transfected with a transcriptional reporter and IPF-1 expression plasmids demonstrate a significant inhibition of basal insulin promoter activity (stronger with the InsCCG243 mutant). We find that the InsCCG243 mutation is linked, in 2 families, to an autosomal dominant-like late-onset form of type 2 diabetes, in which insulin secretion becomes progressively impaired. The lower penetrance D76N and Q59L mutations were more prevalent and were associated with a relative risk of 12.6 for diabetes and with decreased glucose-stimulated insulin-secretion in nondiabetic subjects. We propose that IPF-1 mutations can cause MODY or apparently monogenic late-onset diabetes and that they represent a significant risk factor for type 2 diabetes in humans.

Pancreas dorsal lobe agenesis and abnormal islets of Langerhans in Hlxb9-deficient mice

In most mammals the pancreas develops from the foregut endoderm as ventral and dorsal buds. These buds fuse and develop into a complex organ composed of endocrine, exocrine and ductal components. This developmental process depends upon an integrated network of transcription factors. Gene targeting experiments have revealed critical roles for Pdx1, Isl1, Pax4, Pax6 and Nkx2-2 (refs 3,4,5,6,7, 8,9,10). The homeobox gene HLXB9 (encoding HB9) is prominently expressed in adult human pancreas, although its role in pancreas development and function is unknown. To facilitate its study, we isolated the mouse HLXB9 orthologue, Hlxb9. During mouse development, the dorsal and ventral pancreatic buds and mature beta-cells in the islets of Langerhans express Hlxb9. In mice homologous for a null mutation of Hlxb9, the dorsal lobe of the pancreas fails to develop. The remnant Hlxb9-/- pancreas has small islets of Langerhans with reduced numbers of insulin-producing beta-cells. Hlxb9-/- beta-cells express low levels of the glucose transporter Glut2 and homeodomain factor Nkx 6-1. Thus, Hlxb9 is key to normal pancreas development and function.

Selective agenesis of the dorsal pancreas in mice lacking homeobox gene Hlxb9

The initial stages of pancreatic development occur early during mammalian embryogenesis, but the genes governing this process remain largely unknown. The homeodomain protein Pdx1 is expressed in the developing pancreatic anlagen from the approximately 10-somite stage, and mutations in the gene Pdx1 prevent the development of the pancreas. The initial stages of pancreatic development, however, still occur in Pdx1-deficient mice. Hlxb9 (encoding Hb9; ref. 6) is a homeobox gene that in humans has been linked to dominant inherited sacral agenesis and we show here that Hb9 is expressed at early stages of mouse pancreatic development and later in differentiated beta-cells. Hlxb9 has an essential function in the initial stages of pancreatic development. In absence of Hlxb9 expression, the dorsal region of the gut epithelium fails to initiate a pancreatic differentiation program. In contrast, the ventral pancreatic endoderm develops but exhibits a later and more subtle perturbation in beta-cell differentiation and in islet cell organization. Thus, dorsally Hlxb9 is required for specifying the gut epithelium to a pancreatic fate and ventrally for ensuring proper endocrine cell differentiation.

Distribution and function of the adhesion molecule BEN during rat development

It is well established that the notochord influences the development of adjacent neural and mesodermal tissue. Involvement of the notochord in the differentiation of the dorsal pancreas has been demonstrated. However, our knowledge of the signals involved in pancreatic development is still incomplete. In order to identify proteins potentially implicated during pancreatic differentiation, we raised and characterized monoclonal antibodies against previously established embryonic pancreatic ductal epithelial cell lines (BUD and RED). Using the MAb 2117, the cell surface antigen 2117 (Ag 2117) was cloned. The predicted sequence for Ag 2117 is the rat homologue of BEN. Initially reported as a protein expressed on epithelial cells of the chicken bursa of Fabricius, BEN is expressed in a variety of tissues during development and described as a marker for the developing central and peripheral chicken nervous systems. A role has been suggested for BEN in the adhesion of stem cells and progenitor cells to the blood-forming tissue microenvironment. In this study, we demonstrate that BEN, initially expressed exclusively in the notochord during the early development of rat, is implicated in pancreatic development. We show that Ag 2117 regulates the pancreatic epithelial cell growth through the ras and Jun kinase pathways. In addition, we demonstrate that Ag 2117 is able to regulate the expression of the transcription factor PDX1, required for insulin gene expression, in embryonic pancreas organ cultures.

Molecular approaches to understanding organogenesis

The elucidation of the molecular mechanisms of mammalian organogenesis is the foundation on which we can build an improved understanding of organ pathology and pathophysiology. This paper uses the lung and the pancreas as paradigms to demonstrate how advances in basic molecular developmental biology research has translated into new appreciation of, and even novel potential treatment strategies for, congenital anomalies and mature diseases of these organs.

Gene expression in pancreatic adenocarcinoma

Both acinar and duct cell-specific gene products are expressed by pancreatic adenocarcinoma. In order to begin to understand the mechanisms by which genes of both cell types are expressed in pancreatic adenocarcinoma, an understanding of the underlying transcription factors is important. PDX1 plays an important role in the development of the pancreas and is also expressed in the adult pancreas; it is known to be involved in the regulation of expression of both acinar and islet cell-specific gene products. We have examined pancreatic adenocarcinoma cell lines and have determined that they also express PDX1, making it a candidate transcription factor for the abnormal regulation of these acinar and duc cell gene products.

Initiation of mammalian liver development from endoderm by fibroblast growth factors

The signaling molecules that elicit embryonic induction of the liver from the mammalian gut endoderm or induction of other gut-derived organs are unknown. Close proximity of cardiac mesoderm, which expresses fibroblast growth factors (FGFs) 1, 2, and 8, causes the foregut endoderm to develop into the liver. Treatment of isolated foregut endoderm from mouse embryos with FGF1 or FGF2, but not FGF8, was sufficient to replace cardiac mesoderm as an inducer of the liver gene expression program, the latter being the first step of hepatogenesis. The hepatogenic response was restricted to endoderm tissue, which selectively coexpresses FGF receptors 1 and 4. Further studies with FGFs and their specific inhibitors showed that FGF8 contributes to the morphogenetic outgrowth of the hepatic endoderm. Thus, different FGF signals appear to initiate distinct phases of liver development during mammalian organogenesis.

Pancreas development and diabetes

The past few years have seen an increase in interest about the molecular and genetic events regulating pancreas development. Transcription factors such as Pdx1, p48 and Nkx2.2 have been shown to be essential for the proper differentiation of exocrine and endocrine tissue; however, pancreas development also involves intricate interactions between the pancreatic epithelium and its surrounding mesenchyme. Signalling factors emanating from the notochord have been shown to repress Sonic hedgehog expression in the endoderm whereas signals originating from the pancreatic mesenchyme determine the proportion of exocrine to endocrine tissue. Understanding the molecular and genetic events underlying pancreas development also opens the door for devising new therapeutic strategies against pancreatic diseases such as diabetes and cancer.

Regulation of the insulin gene by glucose: stimulation of trans-activation potency of human PDX-1 N-terminal domain

The beta cells in pancreatic islets of Langerhans increase insulin gene transcription in response to glucose. The pancreatic and duodenal homeobox-1 (PDX-1) plays a major role in glucose-induced insulin transcription. We studied the functional regions of the human PDX-1 protein fused to the DNA-binding domain of the transcription factor Gal4. The results indicate that the N-terminal domain of the hPDX-1, required for transactivation (amino acids 1-120) in transfected betaTC6 and HeLa cells, is also regulated by extracellular glucose concentrations in transfected rat islets. Deletion analyses have led to the mapping of two regions within the N terminus that are essential for its trans-activation properties. One sequence spans amino acids 97-120 in transfected islet and HeLa cells or amino acids 77-120 in betaTC6 cells; the other includes the highly conserved B box (amino acids 31-41). We thus present evidence of a glucose effect on hPDX-1 trans-activation activity, in addition to the previously described regulatory effect on its DNA-binding activity.

Analysis of pancreatic development using a cell lineage label

We have devised a new culture system for in vitro culture of pancreatic buds from mouse embryos which enables the organ to grow as a flat branched structure suitable for wholemount immunostaining. This system has been used to analyze pancreatic development. We have also used the ROSA-26 gene trap mouse strain as a source of tissue which expresses lacZ in a stable manner, in all cell types, during in vitro culture. Combinations of lacZ epithelium and unlabeled mesenchyme show that both exocrine and endocrine cells arise from the epithelium, and smooth muscle cells from the mesenchyme. Although previously suspected, this is the first formal proof that both exocrine and endocrine cells are of endodermal origin. Combinations of lacZ epithelium with unlabeled stomach mesenchyme give similar results and show that stomach mesenchyme has the same trophic effect as pancreatic mesenchyme. When a lacZ and an unlabeled epithelium are combined with an unlabeled mesenchyme, both acini and islets in the resulting culture can be of mixed cell composition. This shows that neither of the chief structural units of the pancreas is formed by clonal growth from a single cell.

The bHLH protein PTF1-p48 is essential for the formation of the exocrine and the correct spatial organization of the endocrine pancreas

We have generated a mouse bearing a null allele of the gene encoding basic helix-loop-helix (bHLH) protein p48, the cell-specific DNA-binding subunit of hetero-oligomeric transcription factor PTF1 that directs the expression of genes in the exocrine pancreas. The null mutation, which establishes a lethal condition shortly after birth, leads to a complete absence of exocrine pancreatic tissue and its specific products, indicating that p48 is required for differentiation and/or proliferation of the exocrine cell lineage. p48 is so far the only developmental regulator known to be required exclusively for committing cells to an exocrine fate. The hormone secreting cells of all four endocrine lineages are present in the mesentery that normally harbors the pancreatic organ until day 16 of gestation. Toward the end of embryonic life, cells expressing endocrine functions are no longer detected at their original location but are now found to colonize the spleen, where they persist in a functional state until postnatal death of the organism occurs. These findings suggest that the presence of the exocrine pancreas is required for the correct spatial assembly of the endocrine pancreas and that, in its absence, endocrine cells are directed by default to the spleen, a site that, in some reptiles, harbors part of this particular cellular compartment.

On the development of the islets of Langerhans

Studies of pancreatic development have suggested that the islet cells develop through multihormonal stages. Abundant data have confirmed that multihormonal cells are common during pancreatic development. A number of transcription factors and homeotic proteins have also been found to be important to pancreatic and islet cell development. While one of these factors (Isl1) is important for the development of the dorsal pancreatic bud and mesenchyme, another factor (Pdx1) is needed for growth and branching of both pancreatic buds. Studies of the expression patterns of pancreatic hormones and transcription factors and other marker proteins seem at present to be most compatible with the view that early glucagon and glucagon + insulin expressing cells are precursors to the glucagon cells of the islets while mature B cells arise through differentiation from glucagon-negative precursor cells. Recent data also point to possibilities of local paracrine interactions between islet cell types and the parenchymal tissue during development.