Regulation of pancreas development by hedgehog signaling

Gene regulatory factors in pancreatic development

The intensity of research on pancreatic development has increased markedly in the past 5 years, primarily for two reasons: we now know that the insulin-producing beta-cells normally arise from an endodermally derived, pancreas-specified precursor cell, and successful transplants of islet cells have been performed, relieving patients with type I diabetes of symptoms for extended periods after transplantation. Combining in vitro beta-cell formation from a pancreatic biopsy of a diabetic patient or from other stem-cell sources followed by endocrine cell transplantation may be the most beneficial route for a future diabetes therapy. However, to achieve this, a thorough understanding of the genetic components regulating the development of beta-cells is required. The following review discusses our current understanding of the transcription factor networks necessary for pancreatic development and how several genetic interactions coming into play at the earliest stages of endodermal development gradually help to build the pancreatic organ. Developmental Dynamics 229:176-200, 2004.

Sonic hedgehog signaling plays an essential role during embryonic salivary gland epithelial branching morphogenesis

Gene targeting studies indicate that sonic hedgehog (Shh) signaling plays an essential role during craniofacial development. Because numerous mandibular derivatives (e.g., teeth, tongue, Meckel's cartilage) are absent in Shh null mice and the embryonic submandibular salivary gland (SMG) develops from the mandibular arch, we postulated that Shh signaling is important for embryonic SMG development. To address this question, we first determined the spatiotemporal distribution of Shh; two transmembrane proteins, patched 1 (Ptc) and Smoothened (Smo), which act as a negative or a positive regulator of the Shh signal, respectively; and the Gli 3 transcription factor, which is downstream of the Shh signal. The epithelial localization of Shh, Ptc, Smo, and Gli 3 suggests that Shh signaling may act within the epithelium in a juxtacrine manner. The SMG phenotype in our embryonic day (E) 18.5 Shh null mice can be characterized as "paedomorphic," that is, it fails to progress to ontogenic stages beyond the Early Pseudoglandular ( approximately E14). In a complementary set of experiments, we used organ culture to evaluate the effect of enhanced or abrogated Shh signaling on embryonic SMG development in vitro. Paired E13 (Late Initial Bud stage) or E14 (Pseudoglandular stage) SMGs were cultured in the presence or absence of exogenous Shh peptide supplementation; Shh-supplemented explants exhibit a significant stage-dependent increase in branching morphogenesis compared with control explants. Furthermore, by using cyclopamine, a steroidal alkaloid that specifically disrupts the Shh pathway, to abrogate endogenous Shh signaling in vitro, we found a significant decrease in branching in cyclopamine-treated explants compared with controls, as well as a significant decrease in epithelial cell proliferation. Our results indicate that Shh signaling plays an essential role during embryonic SMG branching morphogenesis. Exogenous FGF8 peptide supplementation in vitro rescues the abnormal SMG phenotype seen in cyclopamine-treated explants, demonstrating that overexpression of a parallel, but related, downstream signaling pathway can compensate for diminished Shh signaling and restore embryonic SMG branching morphogenesis.

Mouse models of pancreatic cancer: the fur is finally flying!

Pancreatic cancer remains one of the most lethal of all human malignancies. Until recently, preclinical studies have been hampered by the absence of mouse models faithfully recapitulating critical elements of the human disease. However, recent months have witnessed a flurry of activity with respect to prospective mouse models. This progress now allows the evaluation of novel strategies for early detection, chemoprevention, and therapy and also provides new insights regarding the potential for differentiated and undifferentiated cell types to act as cells of origin for pancreatic ductal adenocarcinoma.

Gastric PDX-1 expression in pancreatic metaplasia and endocrine cell hyperplasia in atrophic corpus gastritis

The homeodomain transcription factor PDX-1 plays a key role in endocrine and exocrine differentiation processes of the pancreas. PDX-1 is also essential for differentiation of endocrine cells in the gastric antrum. The role of PDX-1 in the pathogenesis of endocrine cell hyperplasia and pancreatic metaplasia in corpus and fundus gastritis has not been evaluated. By immunohistochemistry and double-immunofluorescence, we investigated the expression of PDX-1 in 10 tissue specimens with normal human gastric mucosa, nonatrophic and atrophic gastritis and in pancreatic metaplasia, respectively. In normal corpus mucosa and in nonatrophic corpus gastritis, PDX-1 was mainly absent. In pancreatic metaplasia, PDX-1 was found in metaplastic cells and in adjacent gastric glands. In contrast to normal gastric corpus mucosa, PDX-1 could be strongly detected in the cytoplasm of the parietal cells surrounding metaplastic areas. Furthermore, PDX-1 expression was found in hyperplastic endocrine cells and in the surrounding gastric glands in chronic atrophic gastritis. Hyperplastic endocrine cells coexpressed the beta-subunit of the gastric H,K-ATPase. We conclude that PDX-1 represents a candidate switch factor for glandular exocrine and endocrine transdifferentiation in chronic gastritis and that an impaired parietal cell differentiation might play a key role in disturbed gastric morphogenic processes.

Genes, Signals, and Lineages in Pancreas Development

Type I diabetes results from the autoimmune-mediated destruction of pancreatic beta cells, which regulate blood sugar levels by secretion of insulin. Recent clinical data suggest that the disease could be cured if an adequate supply of new beta-cells were available, and one goal of pancreatic developmental biology is to understand how endogenous beta-cells are made, with the hope of making them exogenously. Much is now known about the transcriptional regulation of pancreatic organ specification, growth, and lineage allocation; less is known about intercellular signals that regulate this process, but candidates continue to emerge. Additional insights, often contradicting older models, have come from the application of new lineage-tracing techniques. Altogether, these studies also shed light on the still-elusive pancreatic stem cell, which may participate in normal organ maintenance as well as recovery from injury. A rigorous proof of the existence of such a cell, whether in vivo or in vitro, would offer real hope for the prospect of controlled beta-cell generation in a clinical setting.

Hedgehog is an early and late mediator of pancreatic cancer tumorigenesis

Hedgehog signalling--an essential pathway during embryonic pancreatic development, the misregulation of which has been implicated in several forms of cancer--may also be an important mediator in human pancreatic carcinoma. Here we report that sonic hedgehog, a secreted hedgehog ligand, is abnormally expressed in pancreatic adenocarcinoma and its precursor lesions: pancreatic intraepithelial neoplasia (PanIN). Pancreata of Pdx-Shh mice (in which Shh is misexpressed in the pancreatic endoderm) develop abnormal tubular structures, a phenocopy of human PanIN-1 and -2. Moreover, these PanIN-like lesions also contain mutations in K-ras and overexpress HER-2/neu, which are genetic mutations found early in the progression of human pancreatic cancer. Furthermore, hedgehog signalling remains active in cell lines established from primary and metastatic pancreatic adenocarcinomas. Notably, inhibition of hedgehog signalling by cyclopamine induced apoptosis and blocked proliferation in a subset of the pancreatic cancer cell lines both in vitro and in vivo. These data suggest that this pathway may have an early and critical role in the genesis of this cancer, and that maintenance of hedgehog signalling is important for aberrant proliferation and tumorigenesis.

Genetic regulation of branching morphogenesis: lessons learned from loss-of-function phenotypes

Branching morphogenesis, defined as growth and branching of epithelial tubules during embryogenesis, is a fundamental feature of renal, lung, mammary gland, submandibular gland, and pancreatic morphogenesis in mammals. Disruption of branching morphogenesis has been demonstrated to result in maldevelopment of some of these organs. Genetic studies performed in affected humans and mutant mice have implicated transcription factors, secreted growth factors, and cell surface signaling molecules as critical regulators of branching morphogenesis. These factors function within networks that appear to exert tight control over the number and location of branches. This review summarizes current knowledge regarding the molecular control of branching morphogenesis in vivo with particular emphasis on the genetic contribution to perturbed branching morphogenesis in mice and humans.

Activated Notch1 prevents differentiation of pancreatic acinar cells and attenuate endocrine development

Mice carrying loss-of-function mutations in certain Notch pathway genes display increased and accelerated pancreatic endocrine development, leading to depletion of precursor cells followed by pancreatic hypoplasia. Here, we have investigated the effect of expressing a constitutively active form of the Notch1 receptor (Notch1(ICD)) in the developing pancreas using the pdx1 promoter. At e10.5 to e12.5, we observe a disorganized pancreatic epithelium with reduced numbers of endocrine cells, confirming a repressive activity of Notch1 upon the early differentiation program. Subsequent branching morphogenesis is impaired and the pancreatic epithelium forms cyst-like structures with ductal phenotype containing a few endocrine cells but completely devoid of acinar cells. The endocrine cells that do form show abnormal expression of cell type-specific markers. Our observations show that sustained Notch1 signaling not only significantly represses endocrine development, but also fully prevents pancreatic exocrine development, suggesting that a possible role of Notch1 is to maintain the undifferentiated state of common pancreatic precursor cells.

Inhibition of epithelial ductal branching in the prostate by sonic hedgehog is indirectly mediated by stromal cells

Sonic hedgehog (Shh), a vertebrate homologue of the Drosophila segment-polarity gene hedgehog, has been reported to play an important role during normal development of various tissues. Abnormal activities of Shh signaling pathway have been implicated in tumorigenesis such as basal cell carcinomas and medulloblastomas. Here we show that Shh signaling negatively regulates prostatic epithelial ductal morphogenesis. In organotypic cultures of developing rat prostates, Shh inhibited cell proliferation and promoted differentiation of luminal epithelial cells. The expression pattern of Shh and its receptors suggests a paracrine mechanism of action. The Shh receptors Ptc1 (Patched1) and Ptc2 were found to be expressed in prostatic stromal cells adjacent to the epithelium, where Shh itself was produced. This paracrine model was confirmed by co-culturing the developing prostate in the presence of stromal cells transfected with a vector expressing a constitutively active form of Smoothened, the real effector of the Shh signaling pathway. Furthermore, expression of activin A and TGF-beta1 that were shown previously to inhibit prostatic epithelial branching was up-regulated following Shh treatment in the organotypic cultures. Taken together, these results suggest that Shh negatively regulates prostatic ductal branching indirectly by acting on the surrounding stromal cells, at least partly via up-regulating expression of activin A and TGF-beta1.

Genes expressed in the developing endocrine pancreas and their importance for stem cell and diabetes research

The genes that regulate endocrine pancreas development, maintain adult endocrine cells, and stimulate progenitor/stem cells during regeneration remain largely unstudied. There is ample evidence that many of the genes involved in endocrine pancreas development also function in the homeostasis of the adult islet. In light of the potential benefits to diabetic research, it is surprising that there is little information about the genes expressed throughout the ontogeny of the endocrine pancreas. In the past few years, efforts have been made to establish the Endocrine Pancreas Consortium database (EPConDB), in which many of the genes expressed in the developing endocrine pancreas are in a database with a corresponding publicly available clone bank. In addition, advances in microarray technology now allow for a quantitative expression analysis of thousands of genes simultaneously, which makes it possible to generate a quantitative catalog of the genes expressed at each step of endocrine differentiation, from embryonic endoderm to mature beta cells. In this review, I will discuss how genes discovered by virtue of their role in endocrine pancreas development may function in the maintenance of pancreatic stem cells and the regeneration of islets. I will further summarize the recent advances in genomics-based studies of the developing endocrine pancreas and will discuss how they might impact on the discovery of diagnostics and research into stem cell-based approaches for the treatment of diabetes.

Unique and conserved aspects of gut development in zebrafish

Although the development of the digestive system of humans and vertebrate model organisms has been well characterized, relatively little is known about how the zebrafish digestive system forms. We define developmental milestones during organogenesis of the zebrafish digestive tract, liver, and pancreas and identify important differences in the way the digestive endoderm of zebrafish and amniotes is organized. Such differences account for the finding that the zebrafish digestive system is assembled from individual organ anlagen, whereas the digestive anlagen of amniotes arise from a primitive gut tube. Despite differences of organ morphogenesis, conserved molecular programs regulate pharynx, esophagus, liver, and pancreas development in teleosts and mammals. Specifically, we show that zebrafish faust/gata-5 is a functional ortholog of gata-4, a gene that is essential for the formation of the mammalian and avian foregut. Further, extraembryonic gata activity is required for this function in zebrafish as has been shown in other vertebrates. We also show that a loss-of-function mutation that perturbs sonic hedgehog causes defects in the development of the esophagus that parallel those associated with targeted disruption of this gene in mammals. Perturbation of sonic hedgehog also affects zebrafish liver and pancreas development, and these effects occur in a reciprocal fashion, as has been described during mammalian liver and ventral pancreas development. Together, these data define aspects of digestive system development necessary for the characterization of zebrafish mutants. Given the similarities of teleost and mammalian digestive physiology and anatomy, these findings have implications for developmental and evolutionary studies as well as research of human diseases, such as diabetes, liver cirrhosis, and cancer.

Hedgehog signaling in pancreas development

Hedgehog proteins are secreted molecules that bind to their cell surface receptors to elicit concentration dependent responses essential for numerous tissue patterning and cell differentiation events during embryogenesis. However, during early stages of pancreas organogenesis, hedgehog signaling has been shown to inhibit tissue morphogenesis and cell differentiation. By contrast, recent cell culture studies indicate that an active hedgehog pathway might be required for maintenance of adult endocrine cell functions. This review describes our current understanding of the requirement of hedgehog signaling during pancreas morphogenesis and cell differentiation and discusses how individual hedgehog genes might act at various stages to ensure proper pancreas development and organ function.

From endoderm formation to liver and pancreas development in zebrafish

Recent studies in zebrafish have contributed to our understanding of early endoderm formation in vertebrates. Specifically, they have illustrated the importance of Nodal signaling as well as three transcription factors, Faust/Gata5, Bonnie and Clyde, and Casanova, in this process. Ongoing genetic and embryological studies in zebrafish are also contributing to our understanding of later aspects of endoderm development, including the formation of the gut and its associated organs, the liver and pancreas. The generation of transgenic lines expressing GFP in these organs promises to be particularly helpful in such studies.

Differentiation of mouse embryonic stem cells into pancreatic and hepatic cells

Here, we present efficient strategies to differentiate ES cells either into pancreatic or into hepatic cell types. We recommend a strategy to select nestin+ cells, an early progenitor cell type with high developmental plasticity, followed by differentiation induction with specific growth and extracellular matrix factors into pancreatic and hepatic cell types. Cells differentiating via nestin+ cells into the pancreatic and hepatic lineage expressed tissue-specific genes. Proteins characteristic for mature endocrine pancreatic or hepatic cells were synthesized and released. Further, a histotypic "spinner" culture system was introduced to generate mature insulin- and albumin-producing cells at high efficiency.

Development and diseases of the pancreas

The pancreas is a vital gland of exocrine and endocrine function. It is the target of two main affections: diabetes and pancreatic cancer. We describe the tissue interactions, signaling pathways and intracellular targets that are involved in the emergence of the pancreas primordium and its proliferation, morphogenesis and differentiation. It appears that several genes of developmental relevance have an adult function and are involved in pancreas affections. Embryological experimentation in animals contributed to provide candidate genes for human disease and holds promise for future treatments.

Cellular and molecular mechanisms of carcinogenesis

Our understanding of the cellular and molecular mechanisms of cancer of the gastrointestinal tract has increased dramatically over the last several decades. We are identifying new players in the pathways toward cancer with increasing frequency. In addition, we have come to understand that no single pathway acts by itself; in vivo, the effects are combinatorial. As new and better cell culture and animal models of carcinogenesis arise, our knowledge will continue to grow. As we learn more, we will be able to translate the results of our research into new and better techniques for the diagnosis and treatment of gastrointestinal cancers.

Pbx1 inactivation disrupts pancreas development and in Ipf1-deficient mice promotes diabetes mellitus

Pbx1 is a member of the TALE (three-amino acid loop extension) class of homeodomain transcription factors, which are components of hetero-oligomeric protein complexes thought to regulate developmental gene expression and to maintain differentiated cell states. In vitro studies have shown that Pbx1 regulates the activity of Ipf1 (also known as Pdx1), a ParaHox homeodomain transcription factor required for the development and function of the pancreas in mice and humans. To investigate in vivo roles of Pbx1 in pancreatic development and function, we examined pancreatic Pbx1 expression, and morphogenesis, cell differentiation and function in mice deficient for Pbx1. Pbx1-/- embryos had pancreatic hypoplasia and marked defects in exocrine and endocrine cell differentiation prior to death at embryonic day (E) 15 or E16. In these embryos, expression of Isl1 and Atoh5, essential regulators of pancreatic morphogenesis and differentiation, was severely reduced. Pbx1+/- adults had pancreatic islet malformations, impaired glucose tolerance and hypoinsulinemia. Thus, Pbx1 is essential for normal pancreatic development and function. Analysis of trans-heterozygous Pbx1+/- Ipf1+/- mice revealed in vivo genetic interactions between Pbx1 and Ipf1 that are essential for postnatal pancreatic function; these mice developed age-dependent overt diabetes mellitus, unlike Pbx1+/- or Ipf1+/- mice. Mutations affecting the Ipf1 protein may promote diabetes mellitus in mice and humans. This study suggests that perturbation of Pbx1 activity may also promote susceptibility to diabetes mellitus.

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.

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.

Myf5 is a direct target of long-range Shh signaling and Gli regulation for muscle specification

Sonic hedgehog (Shh) is a secreted signaling molecule for tissue patterning and stem cell specification in vertebrate embryos. Shh mediates both long-range and short-range signaling responses in embryonic tissues through the activation and repression of target genes by its Gli transcription factor effectors. Despite the well-established functions of Shh signaling in development and human disease, developmental target genes of Gli regulation are virtually unknown. In this study, we investigate the role of Shh signaling in the control of Myf5, a skeletal muscle regulatory gene for specification of muscle stem cells in vertebrate embryos. In previous genetic studies, we showed that Shh is required for Myf5 expression in the specification of dorsal somite, epaxial muscle progenitors. However, these studies did not distinguish whether Myf5 is a direct target of Gli regulation through long-range Shh signaling, or alternatively, whether Myf5 regulation is a secondary response to Shh signaling. To address this question, we have used transgenic analysis with lacZ reporter genes to characterize an Myf5 transcription enhancer that controls the activation of Myf5 expression in the somite epaxial muscle progenitors in mouse embryos. This Myf5 epaxial somite (ES) enhancer is Shh-dependent, as shown by its complete inactivity in somites of homozygous Shh mutant embryos, and by its reduced activity in heterozygous Shh mutant embryos. Furthermore, Shh and downstream Shh signal transducers specifically induce ES enhancer/luciferase reporters in Shh-responsive 3T3 cells. A Gli-binding site located within the ES enhancer is required for enhancer activation by Shh signaling in transfected 3T3 cells and in epaxial somite progenitors in transgenic embryos. These findings establish that Myf5 is a direct target of long-range Shh signaling through positive regulation by Gli transcription factors, providing evidence that Shh signaling has a direct inductive function in cell lineage specification.

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.

Molecular regulation of embryonic hematopoiesis and vascular development: a novel pathway

In all vertebrate animals, the first blood and vascular endothelial cells are formed during gastrulation, a process in which the mesoderm of the embryo is induced and then patterned by molecules whose identity is still largely unknown. Clusters of developing blood cells surrounded by a layer of endothelial cells comprise the "blood islands" and form in the visceral yolk sac, external to the developing embryo proper. Despite the identification of genes, such as Flk1, SCL/tal-1, Cbfa2/Runx1/AML1, and CD34, that are expressed during the induction of primitive hematopoiesis and vasculogenesis, the early molecular and cellular events involved in these processes are not well understood. Recent work has demonstrated that extracellular signals secreted by a layer of visceral endoderm surrounding the embryo are essential for the initiation of these events. A member of the Hedgehog family of signaling molecules is produced by visceral endoderm and is required for formation of blood and endothelial cells in explant cultures. Hedgehog proteins also stimulate proliferation of definitive hematopoietic stem/progenitor cells. Therefore, these findings may have important medical implications for regulating hematopoiesis and vascular development for therapeutic purposes and for the development of new sources of hematopoietic stem cells for transplantation and as targets for gene therapy.

Induction of embryonic hematopoietic and endothelial stem/progenitor cells by hedgehog-mediated signals

Blood and vascular endothelial cells form in all vertebrates during gastrulation, a process in which the mesoderm of the embryo is induced and then patterned by molecules whose identity is still largely unknown. Blood islands' of primitive hematopoietic cell clusters surrounded by a layer of endothelial cells form in the yolk sac, external to the developing embryo proper. These lineages arise from a layer of extraembryonic mesoderm that is closely apposed with a layer of primitive (visceral) endoderm. Despite the identification of genes such as Flk1, SCL/tal-1, Cbfa2/Runx1/AML1 and CD34 that are expressed during the induction of primitive hematopoiesis and vasculogenesis, the early molecular and cellular events involved in these processes are not well understood. Recent work has demonstrated that extracellular signals secreted by visceral endoderm surrounding the embryo are essential for the initiation of these events. A member of the Hedgehog family of signaling molecules (Indian hedgehog) is produced by visceral endoderm, can induce formation of blood and endothelial cells in explant cultures and can reprogram prospective neurectoderm along hematopoietic and endothelial cell lineages. Hedgehog proteins also stimulate proliferation of definitive hematopoietic stem/progenitor cells. These findings may have important implications for regulating hematopoiesis and vascular development for therapeutic purposes in humans and for the development of new sources of stem cells for transplantation and gene therapy.

Activins as regulators of branching morphogenesis

Development of glandular organs such as the kidney, lung, and prostate involves the process of branching morphogenesis. The developing organ begins as an epithelial bud that invades the surrounding mesenchyme, projecting dividing epithelial cords or tubes away from the site of initiation. This is a tightly regulated process that requires complex epithelial-mesenchymal interactions, resulting in a three-dimensional treelike structure. We propose that activins are key growth and differentiation factors during this process. The purpose of this review is to examine the direct, indirect, and correlative lines of evidence to support this hypothesis. The expression of activins is reviewed together with the effect of activins and follistatins in the development of branched organs. We demonstrate that activin has both negative and positive effects on cell growth during branching morphogenesis, highlighting the complex nature of activin in the regulation of proliferation and differentiation. We propose potential mechanisms for the way in which activins modify branching and address the issue of whether activin is a regulator of branching morphogenesis.

Hedgehog signaling pathway is essential for pancreas specification in the zebrafish embryo

Recent studies have implicated the signaling factor Sonic hedgehog (Shh) as a negative regulator of pancreatic development, but as a positive regulator of pancreas function in amniotes [1-4]. Here, using genetic analysis, we show that specification of the pancreas in the teleost embryo requires the activity of Hh proteins. Zebrafish embryos compromised in Hh signaling exhibit disruption in the expression of the pancreas-specifying homeobox gene pdx-1 and concomitantly show almost complete absence of the endocrine pancreas. Reciprocally, ubiquitous activation of the Hh pathway in wild-type embryos causes ectopic induction of endodermal pdx-1 expression and the differentiation of supernumerary endocrine cells. Our results suggest that Hh proteins influence pancreas specification via inductive interactions from the axial midline rather than through their localized expression in the endodermal cells themselves.

The whereabouts of a morphogen: direct evidence for short- and graded long-range activity of hedgehog signaling peptides

Sonic Hedgehog (Shh) and Indian Hedgehog (Ihh) are members of the Hedgehog (Hh) family of signaling molecules known to be involved in embryonic patterning and morphogenesis. The Hh proteins undergo an autocatalytic cleavage to yield an N-terminal and a C-terminal peptide, with the signaling capacities confined to the N peptide. Drosophila Hh-N has been shown to act via both short- and long-range signaling. In vertebrates, however, attempts to directly demonstrate Shh (SHH) or Ihh (IHH) proteins at a distance from producing cells have been largely unsuccessful. Furthermore, the fact that the Hh N peptides occur in a cholesterol-modified, membrane-tethered form is not easily reconciled with long-range signaling. This study used optimized immunohistochemistry combined with tissue separation and biochemical analyses in vivo and in vitro to determine the range of action of SHH and IHH in the mouse embryo. In all embryonic structures studied, we detect signaling peptides in producing cells, but we also find that ligands move over considerable distances depending on the tissue. These data provide direct evidence for the presence of Hedgehog signaling peptides in target compartments, suggesting a direct long-range action without a need for secondary mediators. Visualization of Hedgehog proteins in target tissues was achieved only under conditions that allowed proteoglycan/glycosaminoglycan (PG/GAG) preservation. Furthermore, we show that induced changes of the composition of PG/GAG in the tooth alter SHH signaling. These data suggest a crucial role for PG/GAGs in Hedgehog movement.

The genetics of pancreatic adenocarcinoma: a roadmap for a mouse model

Pancreatic cancer is among the leading causes of cancer death. Although a genetic profile for pancreatic cancer is emerging, many biological aspects of this disease are poorly understood. Indeed, fundamental questions regarding progenitor cell lineages, host stromal milieu, and the role of specific genetic alterations in tumor progression remain unresolved. A mouse model engineered with signature mutations would provide a powerful ally in the study of pancreatic cancer biology and may guide improved prognostic assessment and treatment for the human disease. In this review, we discuss the molecular basis for normal pancreatic development and the genetics of human pancreatic adenocarcinoma in the hope of charting a course for the development of a faithful mouse model for this lethal cancer.

Indian hedgehog activates hematopoiesis and vasculogenesis and can respecify prospective neurectodermal cell fate in the mouse embryo

During gastrulation in the mouse, mesoderm is induced and patterned by secreted signaling molecules, giving rise first to primitive erythroblasts and vascular endothelial cells. We have demonstrated previously that development of these lineages requires a signal(s) secreted from the adjacent primitive endoderm. We now show that Indian hedgehog (Ihh) is a primitive endoderm-secreted signal that alone is sufficient to induce formation of hematopoietic and endothelial cells. Strikingly, as seen with primitive endoderm, Ihh can respecify prospective neural ectoderm (anterior epiblast) along hematopoietic and endothelial (posterior) lineages. Downstream targets of the hedgehog signaling pathway (the genes encoding patched, smoothened and Gli1) are upregulated in anterior epiblasts cultured in the presence of Ihh protein, as is Bmp4, which may mediate the effects of Ihh. Blocking Ihh function in primitive endoderm inhibits activation of hematopoiesis and vasculogenesis in the adjacent epiblast, suggesting that Ihh is an endogenous signal that plays a key role in the development of the earliest hemato-vascular system. To our knowledge, these are the earliest functions for a hedgehog protein in post-implantation development in the mouse embryo.

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.

Activin receptor patterning of foregut organogenesis

Foregut development produces a characteristic sequence of gastrointestinal and respiratory organs, but the signaling pathways that ensure this developmental order remain largely unknown. Here, mutations of activin receptors ActRIIA and ActRIIB are shown to disrupt the development of posterior foregut-derived organs, including the stomach, pancreas, and spleen. Foregut expression of genes includingShh and Isl1 is shifted in mutant mice. The endocrine pancreas is particularly sensitive to the type and extent of receptor inactivation. ActRIIA+/−B+/−animals lack axial defects, but have hypoplastic pancreatic islets, hypoinsulinemia, and impaired glucose tolerance. Thus, activin receptor-mediated signaling regulates axial patterning, cell differentiation, and function of foregut-derived organs.