Signaling through fibroblast growth factor receptor 2b plays a key role in the development of the exocrine pancreas

FGFR4 overexpression in pancreatic cancer is mediated by an intronic enhancer activated by HNF1alpha

Fibroblast growth factor receptor 4 (FGFR4) is expressed in 50-70% of pancreatic carcinomas (PC) and a similar proportion of derived cell lines. Here we determine the sites of FGFR4 transcriptional initiation which show a pattern characteristic of genes with GC-rich, TATA-less promoters. We have examined the chromatin structure around the FGFR4 gene in a panel of expressing and non-expressing PC lines using the DNase I hypersensitive site assay. One region of hypersensitivity, located largely within intron 1, was found to be greatly extended in expressing cells. Subsequent functional analyses using reporter assays demonstrated that this region was able to act as a cell-specific enhancer, only showing significant activity in PC lines expressing endogenous FGFR4. Transcription factors able to bind to the enhancer were investigated using footprinting and mobility shift assays and two binding sites for Sp1 proteins and two sites able to bind hepatic nuclear factor 1 (HNF1) proteins were identified. Further reporter assays using constructs mutated in each binding site demonstrated that HNF1 binding was essential for enhancer activity in expressing cells, an observation that correlated with the increased abundance of HNF1alpha in these same cells as measured by Western blotting. Finally we show that exogenous expression of HNF1 factors in an FGFR4 non-expressing line led to an induction of enhancer activity in reporter assays and also activated expression of the endogenous gene. We conclude that HNF1alpha is a major determinant of FGFR4 expression in PC.

Expression patterns of Wnts, Frizzleds, sFRPs, and misexpression in transgenic mice suggesting a role for Wnts in pancreas and foregut pattern formation

It is well established that gut and pancreas development depend on epithelial-mesenchymal interactions. We show here that several Wnt, Frizzled, and secreted frizzled-related protein (sFRP) encoding mRNAs are present during mouse pancreatic morphogenesis. Wnt5a and 7b mRNA is broadly expressed in foregut mesenchyme starting around embryonic day 10 in mice. Other members expressed are Wnt2b, Wnt5b, and Wnt11. In addition, genes for the Wnt receptors, Frizzled2, 3, 4, 5, 6, 7, 8, and 9 are expressed. To understand potential Wnt functions in pancreas and foregut development in vivo, we analyzed transgenic F0 mouse fetuses expressing Wnt1 and 5a cDNAs under control of the PDX-1 gene promoter. In PDX-Wnt1 fetuses, the foregut region normally comprising the proximal duodenum instead resembles a posterior extension of the stomach, often associated with complete pancreatic and splenic agenesis. Furthermore, the boundary between expression domains of gastric and duodenal markers is shifted in a posterior direction. In PDX-Wnt5a fetuses, several structures derived from the proximal foregut are reduced in size, including the pancreas, spleen, and stomach, without any apparent shift in the stomach to duodenum transition. In these fetuses, overall pancreatic morphology is changed and the pancreatic epithelium is dense and compact, consistent with Wnt5A effects on cell movements and/or attachment. Taken together, these results suggest that Wnt genes participate in epithelial-mesenchymal signaling and may specify region identity in the anterior foregut.

Effects of mutations in the insulin-like growth factor signaling system on embryonic pancreas development and beta-cell compensation to insulin resistance

Insulin and insulin-like growth factors (IGF) play overlapping and complementary roles in pancreatic beta-cell function and peripheral metabolism. In this study, we have analyzed mice bearing loss-of-function mutations of the insulin/IGF signaling systems. Combined inactivation of insulin receptor (Insr) and Igf1 receptor (Igf1r), but not of either receptor alone, resulted in a 90% decrease in the size of the exocrine pancreas, because of decreased cellular proliferation. In contrast to the findings in the exocrine compartment, endocrine alpha- and beta-cell development was unperturbed. Combined ablation of Igf1 and Igf2, the ligands for these two receptors, resulted in an identical phenotype. We also examined the effect of heterozygous null Igf1r mutations on glucose homeostasis in adult mice. Igf1r haploinsufficiency did not affect insulin action and compensatory beta-cell growth in insulin-resistant mice with combined Insr and Igf1r heterozygous null mutations, resulting in a considerably milder phenotype than combined haploinsufficiency for Insr and its main signaling substrates, Irs1 and Irs2. We conclude that Igf1r and Insr are required for embryonic development of the exocrine but not of the endocrine pancreas and that defects of Igf1r do not alter glucose homeostasis as long as the insulin receptor system remains intact.

BMP4 modulates fibroblast growth factor-mediated induction of proximal and distal lung differentiation in mouse embryonic tracheal epithelium in mesenchyme-free culture

Lung morphogenesis and differentiation require interaction between the epithelium and mesenchyme, which is mediated by diffusible molecules such as fibroblast growth factors (FGFs), bone morphogenetic protein 4 (BMP4), and Shh. We have used mesenchyme-free culture to study the effects of these molecules on lung epithelial differentiation. We have tested the individual abilities of FGF1, FGF2, FGF7, FGF9, FGF10, and FGF18, as well as BMP4 and Shh to promote growth and specify distal lung differentiation in mouse tracheal epithelium. The different FGFs exhibited distinct abilities to induce epithelial growth and the expression of the distal lung epithelial marker, surfactant protein C (SP-C), although all FGFs were able to induce expression of BMP4. Tracheal epithelium treated with FGF10 showed little growth and failed to express SP-C as measured by whole-mount in situ hybridization and quantitative real-time polymerase chain reaction. FGF1 treatment resulted in the strongest induction of SP-C. Treatment with BMP4 inhibited epithelial growth and differentiation and antagonized the stimulatory effects of FGF1. In contrast, inhibition of endogenous BMP4 signaling with Noggin protein did not inhibit growth or expression of SP-C but did increase the expression of the proximal lung markers CCSP and HFH4. Expression of Shh was not affected by any of the conditions tested. These results suggest that BMP4 does not signal epithelial cells to adopt a distal fate but may regulate the expansion of proximal epithelial cells in the lung.

Lineage commitment and cellular differentiation in exocrine pancreas

Exocrine pancreatic cell types comprise greater than 90% of parenchymal cell mass in the adult pancreas. However, the factors regulating differentiation of acinar and ductal epithelial cells remain incompletely characterized. Like pancreatic islet cells, acinar and ductal cells arise from pluripotent precursors within embryonic pancreatic epithelium. Recent studies have suggested that a common pool of pluripotent stem cells is responsible for generating both endocrine and exocrine cell types, and that specific signaling pathways regulate a critical balance between endocrine and exocrine lineage commitment.

Depletion of FGF acts as a lateral inhibitory factor in lung branching morphogenesis in vitro

Previous studies have shown that the interaction of positive and inhibitory signals plays a crucial role during lung branching morphogenesis. We found that in mesenchyme-free conditions, the lung epithelium exerted a lateral inhibitory effect on the neighbouring epithelium via depletion of fibroblast growth factor 1 (FGF1). Contrary to previous suggestions, bone morphogenetic protein 4 could not substitute for the inhibitory effect. Based on of this observation, we used a reaction-diffusion model of the substrate-depletion type to represent the initial phase of in vitro branching morphogenesis of lung epithelium, with depletion of FGF playing the role of lateral inhibitor. The model was able to account for the effects of the FGF1 concentration, extracellular matrix degradation and different subtypes of FGF on morphogenesis of the lung bud epithelia. These results suggest that the depletion of FGF may be a key regulatory component in initial phase of branching morphogenesis of the lung bud epithelium in vitro.

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.

Pancreatic organogenesis--developmental mechanisms and implications for therapy

The pancreas is a mixed exocrine and endocrine gland that controls many homeostatic functions. The exocrine pancreas produces and secretes digestive enzymes, whereas the endocrine compartment consists of four distinct hormone-producing cell types. Studies that further our knowledge of the basic mechanisms that underlie the formation of the pancreas will be crucial for understanding the development and homeostasis of this organ and of the mechanisms that cause diabetes. This information is also pivotal for any attempt to generate functional insulin-producing beta-cells that are suitable for transplantation.

Transgenic expression of FGF8 and FGF10 induces transdifferentiation of pancreatic islet cells into hepatocytes and exocrine cells

FGF signaling is essential for normal development of pancreatic islets. To examine the effects of overexpressed FGF8 and FGF10 on pancreatic development, we generated FGF8- and FGF10-transgenic mice (Tg mice) under the control of the glucagon promoter. In FGF8-Tg mice, hepatocyte-like cells were observed in the periphery of pancreatic islets, but areas of alpha and beta cells did not decrease, whereas in FGF10-Tg mice, pancreatic ductal and acinar cells were found in islets, concomitantly with disturbed beta-cell differentiation. These results suggest that FGF8 and FGF10 play important roles in development of hepatocytes and exocrine cells, respectively, and explain the absence of FGF8 expression in normal islets and pancreatic hypoplasia in FGF10-deficient mice.

Role for FGFR2IIIb-mediated signals in controlling pancreatic endocrine progenitor cell proliferation

Pancreatic development is a classic example of epithelium-mesenchyme interaction. During embryonic life, signals from the mesenchyme control the proliferation of precursor cells within the pancreatic epithelium and their differentiation into endocrine or acinar cells. It has been shown that signals from the mesenchyme activate epithelial cell proliferation but repress development of the pancreatic epithelium into endocrine cells. Here, experiments with specific inhibitors established that mesenchymal effects on epithelial cell development depended on the mitogen-activated protein kinase pathway. Then we demonstrated that these effects of the mesenchyme were mimicked by fibroblast growth factor 7 (FGF7), a specific ligand of FGFR2IIIb, which is a tyrosine kinase receptor of the FGF-receptor family. When pancreatic epithelium expressing FGFR2IIIb was grown with FGF7, epithelial cell growth occurred in a concentration-dependent manner, whereas endocrine tissue development was repressed. The epithelial cells that proliferated in response to FGF7 were endocrine pancreatic precursor cells, as shown by their differentiation en masse into endocrine cells on FGF7 removal. Thus, efficient propagation of pancreatic progenitor cells can be achieved in vitro by exposure to FGF7, which does not affect their ability to differentiate en masse into endocrine cells on FGF7 removal.

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.

Fibroblast growth factors and their receptors in hematopoiesis and hematological tumors

Fibroblast growth factors (FGFs) belong to a family of pleiotropic heparin-binding growth factors. They exert multiple functions on various cell types of mesodermal and neuroectodermal origin, affecting cell proliferation, motility, survival, and differentiation. FGF's exert their activity by interacting with tyrosine kinase receptors (FGFRs) and cell-surface heparan sulfate proteoglycans. This article reviews recent studies on the role of the FGF/FGFR system in embryonic hematopoietic development, hematopoiesis, and hematological tumors. FGFs exert both autocrine and paracrine functions in these biological processes by acting on blood cells and their precursors and accessory cells in the bone marrow, including stromal and endothelial cells.

Fgf10 is essential for maintaining the proliferative capacity of epithelial progenitor cells during early pancreatic organogenesis

The importance of mesenchymal-epithelial interactions for the proper development of the pancreas has been acknowledged since the early 1960s, even though the molecule(s) mediating this process have remained unknown. We demonstrate here that Fgf10, a member of the fibroblast growth factor family (FGFs), plays an essential role in this process. We show that Fgf10 is expressed in the mesenchyme directly adjacent to the early dorsal and ventral pancreatic epithelial buds. In Fgf10–/– mouse embryos, the evagination of the epithelium and the initial formation of the dorsal and ventral buds appear normal. However, the subsequent growth, differentiation and branching morphogenesis of the pancreatic epithelium are arrested; this is primarily due to a dramatic reduction in the proliferation of the epithelial progenitor cells marked by the production of the homeobox protein PDX1. Furthermore, FGF10 restores the population of PDX1-positive cells in organ cultures derived from Fgf10–/– embryos. These results indicate that Fgf10 signalling is required for the normal development of the pancreas and should prove useful in devising methods to expand pancreatic progenitor cells.

Dorsal pancreas agenesis in N-cadherin- deficient mice

Members of the cadherin family of cell adhesion molecules are thought to be crucial regulators of tissue patterning and organogenesis. During pancreatic ontogeny N-cadherin is initially expressed in the pancreatic mesenchyme and later in pancreatic endoderm. Analysis of N-cadherin-deficient mice revealed that these mice suffer from selective agenesis of the dorsal pancreas. Further analysis demonstrated that the mechanism for the lack of a dorsal pancreas involves an essential function of N-cadherin as a survival factor in the dorsal pancreatic mesenchyme.

Expression of the receptor tyrosine kinase KIT in mature beta-cells and in the pancreas in development

In the pancreas, ligands of receptor tyrosine kinases (RTKs) are thought to be implicated in the development and function of the islets of Langerhans, which represent the endocrine part of the pancreas. In a previous study, we randomly screened by reverse transcriptase-polymerase chain reaction for RTKs expressed in the embryonic pancreas. One cDNA fragment that was cloned during this screen corresponded to the KIT receptor. The objective of the present study was to analyze the pattern of Kit expression in the pancreas. We demonstrated that Kit is expressed and functional in terms of signal transduction in the insulin-producing cell line INS-1. Indeed, upon treatment with the KIT ligand (KITL), the extracellular signal-regulated protein kinase was phosphorylated, and the expression of early responsive genes was induced. We also demonstrated that Kit mRNAs are present in fetal and adult rat islets. We next used mice that had integrated the lacZ reporter gene into the Kit locus. In these mice, beta-galactosidase (beta-gal) served as a convenient marker for expression of the endogenous Kit gene. Kit was found to be specifically transcribed in beta-cells (insulin-expressing cells), whereas no expression was found in other endocrine cell types or in the exocrine tissue. Interestingly, not all mature beta-cells expressed Kit, indicating that Kit is a marker of a subpopulation of beta-cells. Finally, by following beta-gal expression in the pancreas during fetal life, we found that at E14.5, Kit is expressed in both insulin- and glucagon-expressing cells present at that stage, and also in a specific cell population present in the epithelium that stained negative for endocrine markers. These data suggest that these Kit-positive/endocrine-negative cells could represent a subpopulation of endocrine cell precursors.

Epidermal growth factor increases undifferentiated pancreatic embryonic cells in vitro: a balance between proliferation and differentiation

During embryonic life, the development of a proper mass of mature pancreatic tissue is thought to require the proliferation of precursor cells, followed by their differentiation into endocrine or acinar cells. We investigated whether perturbing the proliferation of precursor cells in vitro could modify the final mass of endocrine tissue that develops. For that purpose, we used activators or inhibitors of signals mediated by receptor tyrosine kinases. We demonstrated that when embryonic day 13.5 rat pancreatic epithelium is cultured in the presence of PD98059, an inhibitor of the mitogen-activated protein (MAP) kinase, epithelial cell proliferation is decreased, whereas endocrine cell differentiation is activated. On the other hand, in the presence of epidermal growth factor (EGF), an activator of the MAP kinase pathway, the mass of tissue that develops is increased, whereas the absolute number of endocrine cells that develops is decreased. Under this last condition, a large number of epithelial cells proliferate but remain undifferentiated. In a second step, when EGF is removed from the pool of immature pancreatic epithelial cells, the cells differentiate en masse into insulin-expressing cells. The total number of insulin-expressing cells that develop can thus be increased by first activating the proliferation of immature epithelial cells with growth factors, thus allowing an increase in the pool of precursor cells, and next allowing their differentiation into endocrine cells by removing the growth factor. This strategy suggests a possible tissue engineering approach to expanding beta-cells.

The transplanted fetal endocrine pancreas undergoes an inherent sequential differentiation similar to that in the native pancreas. An ultrastructural study in the pig-to-mouse model

This study examines, at the ultrastructural level, whether the fetal porcine endocrine pancreas (insulin, glucagon, somatostatin, and pancreatic polypeptide [PP]- and islet amyloid polypeptide [IAPP]-containing cells) develops normally after transplantation under the kidney capsule in athymic mice. We have thus used an in vivo pig-to-mouse model for the differentiation of the endocrine pancreas removed from its normal milieu. Islet-like cell clusters (ICCs) were prepared from the fetal porcine pancreas as previously described and transplanted under the renal capsule of athymic mice. At various times after transplantation, the endocrine pancreas was removed and the level of differentiation was compared with the native pancreas of the same biological age. At the ultrastructural level, several sequential steps could be identified based on the morphology and hormone content of the secretory granules of the endocrine cell examined. Applying this approach, we could demonstrate that the ontogeny of the transplanted fetal pig pancreas follows the same sequential differentiation as the native pancreas. The process seems to be under stringent control, apparently directly related to the biological age of the tissue, and independent not only of the new environment under the kidney capsule but also of the adult and xenogeneic milieu provided after transplantation to the athymic nude mouse. Therefore, all four major hormone-producing cells seem to develop normally after transplantation when compared with the development in the native pancreas. IAPP was produced by the pluripotent fetal endocrine cells as well as the adult alpha-, beta-, and delta-cell granules in the native pancreas; however, in the transplanted pancreas, IAPP expression was demonstrated only in beta-cells, delta-cells, and PP cells. No IAPP was found in granules of the alpha-cell lineage. The results suggest a sequential differentiation of all four major types of islet cells from a common pluripotent progenitor cell, which seems to be located in the pancreatic ducts. Therefore, the results presented strongly suggest that the ontogeny of the four major endocrine islet cells is determined by genetic information carried by the progenitor cells and not by the systemic or local environment.

CCK expression in enteroendocrine cell is regulated by soluble factor(s) from underlying fibroblasts

Studies on the cross-talk between the intestinal epithelium and the underlying connective tissue have concentrated on enterocytes. In contrast, little is known about the interactions between the mesenchymal compartment and the enteroendocrine cells, scattered among the other cell types of the epithelium. To address this question, a panel of coculture systems between the enteroendocrine STC-1 cell line and three intestinal myofibroblastic cell lines (MIC) was used in order to assess different levels of regulation, namely cell-cell and cell-matrix interactions, and the role of diffusible factors. We demonstrate that the expression of cholecystokinin, a typical intestinal hormone produced by STC-1 cells, is up-regulated in the presence of a fibroblastic environment through a paracrine pathway involving FGF2. Concomitantly, STC-1 cell morphology and proliferation were also modulated, but through distinct mechanisms according to the origin of fibroblasts. The results reveal definite epithelio-mesenchymal interactions that may be critical for the maintenance of phenotype and function of enteroendocrine cells.

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.

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.

Associations of FGF-3 and FGF-10 with signaling networks regulating tooth morphogenesis

The morphogenesis and cell differentiation in developing teeth is governed by interactions between the oral epithelium and neural crest-derived ectomesenchyme. The fibroblast growth factors FGF-4, -8, and -9 have been implicated as epithelial signals regulating mesenchymal gene expression and cell proliferation during tooth initiation and later during epithelial folding morphogenesis and the establishment of tooth shape. To further evaluate the roles of FGFs in tooth development, we analyzed the roles of FGF-3, FGF-7, and FGF-10 in developing mouse teeth. In situ hybridization analysis showed developmentally regulated expression during tooth formation for Fgf-3 and Fgf-10 that was mainly restricted to the dental papilla mesenchymal cells. Fgf-7 transcripts were restricted to the developing bone surrounding the developing tooth germ. Fgf-10 expression was observed in the presumptive dental epithelium and mesenchyme during tooth initiation, whereas Fgf-3 expression appeared in the dental mesenchyme at the late bud stage. During the cap and bell stage, both Fgf-3 and Fgf-10 were intensely expressed in the dental papilla mesenchymal cells both in incisors and molars. It is of interest that Fgf-3 expression was also observed in the primary enamel knot, a putative signaling center of the tooth, whereas no transcripts were seen in the secondary enamel knots that appear in the tips of future cusps of the bell stage tooth germs. Down-regulation of Fgf-3 and Fgf-10 expression in postmitotic odontoblasts correlated with the terminal differentiation of the odontoblasts and the neighboring ameloblasts. In the incisors, mesenchymal cells of the cervical loop area showed partially overlapping expression patterns for all studied Fgfs. In vitro analyses showed that expression of Fgf-3 and Fgf-10 in the dental mesenchyme was dependent on dental epithelium and that epithelially expressed FGFs, FGF-4 and -8 induced Fgf-3 but not Fgf-10 expression in the isolated dental mesenchyme. Beads soaked in Shh, BMP-2, and TGF-beta 1 protein did not induce either Fgf-3 or Fgf-10 expression. Cells expressing Wnt-6 did not induce Fgf-10 expression. Furthermore, FGF-10 protein stimulated cell proliferation in the dental epithelium but not in the mesenchyme. These results suggest that FGF-3 and FGF-10 have redundant functions as mesenchymal signals regulating epithelial morphogenesis of the tooth and that their expressions appear to be differentially regulated. In addition, FGF-3 may participate in signaling functions of the primary enamel knot. The dynamic expression patterns of different Fgfs in dental epithelium and mesenchyme and their interactions suggest existence of regulatory signaling cascades between epithelial and mesenchymal FGFs during tooth development.

Proliferative signaling initiated in ACTH receptors

This article reviews recent results of studies aiming to elucidate modes of integrating signals initiated in ACTH receptors and FGF2 receptors, within the network system of signal transduction found in Y1 adrenocortical cells. These modes of signal integration should be central to the mechanisms underlying the regulation of the G0-->G1-->S transition in the adrenal cell cycle. FGF2 elicits a strong mitogenic response in G0/G1-arrested Y1 adrenocortical cells, that includes a) rapid and transient activation of extracellular signal-regulated kinases-mitogen-activated protein kinases (ERK-MAPK) (2 to 10 min), b) transcription activation of c-fos, c-jun and c-myc genes (10 to 30 min), c) induction of c-Fos and c-Myc proteins by 1 h and cyclin D1 protein by 5 h, and d) onset of DNA synthesis stimulation within 8 h. ACTH, itself a weak mitogen, interacts with FGF2 in a complex manner, blocking the FGF2 mitogenic response during the early and middle G1 phase, keeping ERK-MAPK activation and c-Fos and cyclin D1 induction at maximal levels, but post-transcriptionally inhibiting c-Myc expression. c-Fos and c-Jun proteins are mediators in both the strong and the weak mitogenic responses respectively triggered by FGF2 and ACTH. Induction of c-Fos and stimulation of DNA synthesis by ACTH are independent of PKA and are inhibited by the PKC inhibitor GF109203X. In addition, ACTH is a poor activator of ERK-MAPK, but c-Fos induction and DNA synthesis stimulation by ACTH are strongly inhibited by the inhibitor of MEK1 PD98059.

Pattern of expression of the KGF receptor and its ligands KGF and FGF-10 during postnatal mouse mammary gland development

The expression of the KGF receptor (KGFR) and its stromal ligands, KGF and FGF-10, was compared during mouse mammary gland development. KGFR expression in mammary parenchyma is maximal in mature virgin mice, declines during pregnancy and lactation, but rises after weaning. The rise in KGFR mRNA in the virgin animal corresponds to parenchymal growth. The fall in KGFR expression in pregnancy is driven by hormone-induced alveolar differentiation since the level of KGFR mRNA is 5-fold higher in isolated ductal cells compared to alveolar cells. KGF and FGF-10 expression patterns differ during ductal development. FGF-10 is also expressed at about a 15-fold higher molar level than KGF. During pregnancy and lactation, expression of KGF and FGF-10 decreases in intact fat pads but is unchanged in parenchyma-free fat pads. Thus, the decrease in KGF and FGF-10 expression observed in intact glands during pregnancy and lactation is not a direct consequence of the changing hormonal milieu but more likely reflects an increase in the ratio of epithelium to stroma. Differences in the level and pattern of expression of mRNA for KGF, FGF-10, and the KGFR during postnatal development of the mouse mammary gland are a result of morphological development, changes in the ratio of stroma to epithelium, and hormonal regulation of cell differentiation. These changes suggest that the biological roles that these growth factors play are regulated by fluctuations in both growth factor and growth factor receptor expression and that KGF and FGF-10 may have different regulatory functions.

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<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<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.

FGF10 is an inducer and Pax6 a competence factor for lacrimal gland development

We investigated the mechanism of tissue induction and specification using the lacrimal gland as a model system. This structure begins its morphogenesis as a bud-like outgrowth of the conjunctival epithelium and ultimately forms a branched structure with secretory function. Using a reporter transgene as a specific marker for gland epithelium, we show that the transcription factor Pax6 is required for normal development of the gland and is probably an important competence factor. In investigating the cell-cell signaling required, we show that fibroblast growth factor (FGF) 10 is sufficient to stimulate ectopic lacrimal bud formation in ocular explants. Expression of FGF10 in the mesenchyme adjacent to the presumptive lacrimal bud and absence of lacrimal gland development in FGF10-null mice strongly suggest that it is an endogenous inducer. This was supported by the observation that inhibition of signaling by a receptor for FGF10 (receptor 2 IIIb) suppressed development of the endogenous lacrimal bud. In explants of mesenchyme-free gland epithelium, FGF10 stimulated growth but not branching morphogenesis. This suggested that its role in induction is to stimulate proliferation and, in turn, that FGF10 combines with other factors to provide the instructive signals required for lacrimal gland development.

Endoderm development: from patterning to organogenesis

Although the ectoderm and mesoderm have been the focus of intensive work in the recent era of studies on the molecular control of vertebrate development, the endoderm has received less attention. Because signaling must occur between germ layers in order to achieve a properly organized body, our understanding of the coordinated development of all organs requires a more thorough consideration of the endoderm and its derivatives. This review focuses on present knowledge and perspectives concerning endoderm patterning and organogenesis. Some of the classical embryology of the endoderm is discussed and the progress and deficiencies in cellular and molecular studies are noted.

Manipulation of pancreatic stem cells for cell replacement therapy

The demonstration of the existence of tissue-specific adult stem cells has had a great impact on our understanding of stem cell biology and its application in clinical medicine. Their existence has revolutionized the implications for the treatment of many degenerative diseases characterized by either the loss or malfunction of discrete cell types. However, successful exploitation of this opportunity requires that we have sufficient know-how of stem cell manipulation. Because stem cells are the founders of virtually all tissues during embryonic development, we believe that understanding the cellular and molecular mechanisms of embryogenesis and organogenesis will ultimately serve as a platform to identify factors and conditions that regulate stem cell behavior. Discovery of stem cell regulatory factors will create potential pharmaceutical opportunities for treatment of degenerative diseases, as well as providing critical knowledge of the processes by which stem cells can be expanded in vitro, differentiated, and matured into desired functional cells for implantation into humans. A well-characterized example of this is the hematopoietic system where the discovery of erythropoietin (EPO) and granulocyte-colony stimulating factor (G-CSF), which regulate hematopoietic progenitor cell behavior, have provided significant clinical success in disease treatment as well as providing important insights into hematopoiesis. In contrast, little is known about the identity of pancreatic stem cells, the focus of this review. Recent reports of the potential existence of pancreatic stem cells and their utility in rescuing the diabetic state now raise the same possibilities of generating insulin-producing beta cells as well as other cell types of the pancreatic islet from a stem cell. In this review, we will focus on the potential of these new developments and how our understanding of pancreas development can help design strategies and approaches by which a cell replacement therapy can be implemented for the treatment of insulin-dependent diabetes which is manifested by the loss of beta cells in the pancreas.