FGF10 signaling controls stomach morphogenesis

Gastric hypoplasia in mice lacking fibroblast growth factor 9

BACKGROUND

Fibroblast Growth Factor 9 (Fgf9) and its paralog Fgf20 are expressed in the developing stomach. We investigate the role of these growth factors during gastric development, using combinations of null alleles.

RESULTS

Analysis of expression databases showed that Fgf9 is expressed in gastric endoderm and surrounding mesoderm such as the mesothelium as early as E8.5, and Fgf20 is expressed in the gastric progenitors of the glandular stomach. To explore whether Fgf9 and Fgf20 are important for gastric development, we examined embryonic stomachs from Fgf9 and Fgf20 null (Fgf9LacZ/LacZ and Fgf20Cre.GFP/Cre.GFP) mice during development. At E18.5, Fgf9LacZ/LacZ stomachs were hypoplastic, lacking the squamous forestomach. No changes to glandular stomach differentiation were observed using representative markers of glandular lineages. Fgf9LacZ/LacZ stomachs were smaller during early development (E12.5 and E15.5). RNA-seq analysis of Fgf9LacZ/LacZ mice at E15.5 showed that squamous-epithelium-associated transcripts were underrepresented, and glandular epithelial transcripts were overrepresented. Analysis of gastric patterning at E12.5 revealed loss of early squamous progenitors in the epithelium, characterized by loss of SOX2+; GATA4- cells. We further show that loss of Fgf20 does not alone impact gastric development nor modify the Fgf9LacZ/LacZ phenotype.

CONCLUSIONS

Fgf9 drives gastric growth and squamous epithelial identity during gastric development.

The plasticity of epithelial cells and its potential in the induced differentiation of gastric cancer

Cell plasticity refers to the deviation of cells from normal terminal differentiation states when faced with environmental and genetic toxic stresses, resulting in the phenomenon of transforming into other cell or tissue phenotypes. Unlocking phenotype plasticity has been defined as a hallmark of malignant tumors. The stomach is one of the organs in the body with the highest degree of self-renewal and exhibits significant cell plasticity. In this paper, based on the review of the characteristics of normal differentiation of gastric epithelial cells and their markers, the four main phenotypes of gastric epithelial cell remodeling and their relationship with gastric cancer (GC) are drawn. Furthermore, we summarize the regulatory factors and mechanisms that affect gastric epithelial cell plasticity and outline the current status of research and future prospection for the treatment targeting gastric epithelial cell plasticity. This study has important theoretical reference value for the in-depth exploration of epithelial cell plasticity and the tumor heterogeneity caused by it, as well as for the precise treatment of GC.

Fibroblast growth factor 10

Fibroblast growth factor 10 (FGF10) is a major morphoregulatory factor that plays essential signaling roles during vertebrate multiorgan development and homeostasis. FGF10 is predominantly expressed in mesenchymal cells and signals though FGFR2b in adjacent epithelia to regulate branching morphogenesis, stem cell fate, tissue differentiation and proliferation, in addition to autocrine roles. Genetic loss of function analyses have revealed critical requirements for FGF10 signaling during limb, lung, digestive system, ectodermal, nervous system, craniofacial and cardiac development. Heterozygous FGF10 mutations have been identified in human genetic syndromes associated with craniofacial anomalies, including lacrimal and salivary gland aplasia. Elevated Fgf10 expression is associated with poor prognosis in a range of cancers. In addition to developmental and disease roles, FGF10 regulates homeostasis and repair of diverse adult tissues and has been identified as a target for regenerative medicine.

Gastric epithelial stem cells in development, homeostasis and regeneration

The stem/progenitor cell pool is indispensable for the development, homeostasis and regeneration of the gastric epithelium, owing to its defining ability to self-renew whilst supplying the various functional epithelial lineages needed to digest food efficiently. A detailed understanding of the intricacies and complexities surrounding the behaviours and roles of these stem cells offers insights, not only into the physiology of gastric epithelial development and maintenance, but also into the pathological consequences following aberrations in stem cell regulation. Here, we provide an insightful synthesis of the existing knowledge on gastric epithelial stem cell biology, including the in vitro and in vivo experimental techniques that have advanced such studies. We highlight the contributions of stem/progenitor cells towards patterning the developing stomach, specification of the differentiated cell lineages and maintenance of the mature epithelium during homeostasis and following injury. Finally, we discuss gaps in our understanding and identify key research areas for future work.

Time-resolved transcriptomics of mouse gastric pit cells during postnatal development reveals features distinct from whole stomach development

Whole-organ transcriptomic analyses have emerged as a common method for characterizing developmental transitions in mammalian organs. However, it is unclear if all cell types in an organ follow the whole-organ defined developmental trajectory. Recently, a postnatal two-stage developmental process was described for the mouse stomach. Here, using laser capture microdissection to obtain in situ transcriptomic data, we show that mouse gastric pit cells exhibit four postnatal developmental stages. Interestingly, early stages are characterized by the up-regulation of genes associated with metabolism, a functionality not typically associated with pit cells. Hence, beyond revealing that not all constituent cells develop according to the whole-organ determined pathway, these results broaden our understanding of the pit cell phenotypic landscape during stomach development.

Mesenchymal-epithelial interaction regulates gastrointestinal tract development in mouse embryos

After gut tube patterning in early embryos, the cellular and molecular changes of developing stomach and intestine remain largely unknown. Here, combining single-cell RNA sequencing and spatial RNA sequencing, we construct a spatiotemporal transcriptomic landscape of the mouse stomach and intestine during embryonic days E9.5-E15.5. Several subpopulations are identified, including Lox⁺ stomach mesenchyme, Aldh1a3⁺ small-intestinal mesenchyme, and Adamdec1⁺ large-intestinal mesenchyme. The regionalization and heterogeneity of both the epithelium and the mesenchyme can be traced back to E9.5. The spatiotemporal distributions of cell clusters and the mesenchymal-epithelial interaction analysis indicate that a coordinated development of the epithelium and mesenchyme contribute to the stomach regionalization, intestine segmentation, and villus formation. Using the gut tube-derived organoids, we find that the cell fate of the foregut and hindgut can be switched by the regional niche factors, including fibroblast growth factors (FGFs) and retinoic acid (RA). This work lays a foundation for further dissection of the mechanisms governing this process.

The beginning of the era of precision medicine for gastric cancer with fibroblast growth factor receptor 2 aberration

Despite recent advances in the systemic treatment of metastatic gastric cancer (GC), prognostic outcomes remain poor. Considerable research effort has been invested in characterizing the genomic landscape of GC and identifying potential therapeutic targets. FGFR2 is one of the most attractive targets because aberrations in this gene are frequently associated with GC, particularly the diffuse type in Lauren's classification, which confers an unfavorable prognosis. Based on the preclinical data, the FGFR2 signaling pathway plays a key role in the development and progression of GC, and several FGFR inhibitors have been clinically assessed. However, the lack of robust treatment efficacy has hampered precision medicine for patients with FGFR2-aberrant GC. Recently, the clinical benefits of the FGFR2-IIIb-selective monoclonal antibody bemarituzumab for FGFR2b-positive GC patients were shown in a randomized phase II FIGHT trial of bemarituzumab combined with the first-line chemotherapy. This trial demonstrates proof of concept, suggesting that FGFR2 is a relevant therapeutic target for patients with FGFR2b-positive GC and that bemarituzumab brings new hope for diffuse-type GC patients. In this review, we summarize the oncogenic roles of FGFR2 signaling and highlight the most recent advances in FGFR inhibitors based on the findings of pivotal clinical trials for patients with FGFR2-aberrant GC. Thus, the era of precision medicine for patients with FGFR2-aberrant GC will be opened.

Epithelial expression of Gata4 and Sox2 regulates specification of the squamous-columnar junction via MAPK/ERK signaling in mice

The squamous-columnar junction (SCJ) is a boundary consisting of precisely positioned transitional epithelium between the squamous and columnar epithelium. Transitional epithelium is a hotspot for precancerous lesions, and is therefore clinically important; however, the origins and physiological properties of transitional epithelium have not been fully elucidated. Here, by using mouse genetics, lineage tracing, and organoid culture, we examine the development of the SCJ in the mouse stomach, and thus define the unique features of transitional epithelium. We find that two transcription factors, encoded by Sox2 and Gata4, specify primitive transitional epithelium into squamous and columnar epithelium. The proximal-distal segregation of Sox2 and Gata4 expression establishes the boundary of the unspecified transitional epithelium between committed squamous and columnar epithelium. Mechanistically, Gata4-mediated expression of the morphogen Fgf10 in the distal stomach and Sox2-mediated Fgfr2 expression in the proximal stomach induce the intermediate regional activation of MAPK/ERK, which prevents the differentiation of transitional epithelial cells within the SCJ boundary. Our results have implications for tissue regeneration and tumorigenesis, which are related to the SCJ.

Crosstalk Between Cancer Associated Fibroblasts and Cancer Cells in Scirrhous Type Gastric Cancer

Gastric cancer (GC) is the third leading cause among all cancer deaths globally. Although the treatment outcome of GC has improved, the survival of patients with GC at stages III and IV remains unsatisfactory. Among several types of GC, scirrhous type GC (SGC) shows highly aggressive growth and invasive activity, leading to frequent peritoneal metastasis. SGC is well known to accompany abundant stromal cells that compose the tumor microenvironment (TME) along with the produced extracellular matrix (ECM) and secreted factors. One of the main stromal components is cancer associated fibroblast (CAF). In the SGC microenvironment, CAFs are a source of various secreted factors, including fibroblast growth factors (FGFs), which mediate prominent tumor-stimulating activity. In turn, cancer cells also secrete numerous factors, which can activate and educate CAFs. Current findings suggest that cancer cells and stromal cells communicate interactively via the soluble factors, the ECM, and likely also by exosomes. In this review, we focus on the soluble factors mediating communication between cancer cells and CAFs in SGC, and consider how they are related to the modulation of TME and the high rate of peritoneal metastasis. At last, we discuss the perspectives on targeting these communication pathways for improved future treatment.

Role of FGF10/FGFR2b Signaling in Mouse Digestive Tract Development, Repair and Regeneration Following Injury

During embryonic development, the rudimentary digestive tract is initially a tube-like structure. It is composed of epithelial cells surrounded by mesenchymal cells. Reciprocal epithelial–mesenchymal interactions progressively subdivide this primitive tube into distinct functional regions: the tongue, the pharynx, the esophagus, the stomach, the duodenum, the small intestine, the cecum, the large intestine, the colon, and the anus as well as the pancreas and the liver. Fibroblast growth factors (Fgfs) constitute a family of conserved small proteins playing crucial roles during organogenesis, homeostasis, and repair after injury. Among them, fibroblast growth factor 10 (Fgf10) has been reported to orchestrate epithelial–mesenchymal interactions during digestive tract development. In mice, loss of function of Fgf10 as well as its receptor fibroblast growth factor receptor 2b (Fgfr2b) lead to defective taste papillae in the tongue, underdeveloped and defective differentiation of the stomach, duodenal, cecal, and colonic atresias, anorectal malformation, as well as underdeveloped pancreas and liver. Fgf signaling through Fgfr2b receptor is also critical for the repair process after gut injury. In the adult mice, a malabsorption disorder called small bowel syndrome is triggered after massive small bowel resection (SBR). In wild-type mice, SBR leads to a regenerative process called gut adaptation characterized by an increase in the diameter of the remaining small intestine as well as by the presence of deeper crypts and longer villi, altogether leading to increased intestinal surface. Intestinal stem cells are key for this regeneration process. Induction of Fgf10 expression in the Paneth cells located in the crypt following SBR suggests a critical role for this growth factor in the process of gut adaptation.

Identification of a novel oncogenic mutation of FGFR4 in gastric cancer

Gastric cancer remains one of the leading causes of cancer death worldwide. Despite intensive investigations of treatments over the past three decades, the poor prognosis of patients with unresectable advanced or recurrent gastric cancer has not significantly changed, and improved therapies are required. Here, we report the identification of an oncogenic mutation in FGFR4 in a human gastric tumour that leads to constitutive activation of its product, FGFR4. The G636C-FGFR4 tyrosine kinase domain mutation was found in 1 of 83 primary human gastric tumours. The G636C mutation increased FGFR4 autophosphorylation, and activated FGFR4 downstream signalling molecules and enhanced anchorage-independent cell growth when expressed in NIH/3T3 cells. 3D-structural analysis and modelling of FGFR4 suggest that G636C destabilizes an auto-inhibitory conformation and stabilizes an active conformation, leading to increased kinase activation. Ba/F3 cell lines expressing the G636C-FGFR4 mutant were significantly more sensitive to ASP5878, a selective FGFR inhibitor, than the control. Oral administration of ASP5878 significantly inhibited the growth of tumours in mice engrafted with G636C-FGFR4/3T3 cells. Together, our results demonstrate that mutationally activated FGFR4 acts as an oncoprotein. These findings support the therapeutic targeting of FGFR4 in gastric cancer.

Targeting the Oncogenic FGF-FGFR Axis in Gastric Carcinogenesis

Gastric cancer (GC) is one of the most wide-spread malignancies in the world. The oncogenic role of signaling of fibroblast growing factors (FGFs) and their receptors (FGFRs) in gastric tumorigenesis has been gradually elucidated by recent studies. The expression pattern and clinical correlations of FGF and FGFR family members have been comprehensively delineated. Among them, FGF18 and FGFR2 demonstrate the most prominent driving role in gastric tumorigenesis with gene amplification or somatic mutations and serve as prognostic biomarkers. FGF-FGFR promotes tumor progression by crosstalking with multiple oncogenic pathways and this provides a rational therapeutic strategy by co-targeting the crosstalks to achieve synergistic effects. In this review, we comprehensively summarize the pathogenic mechanisms of FGF-FGFR signaling in gastric adenocarcinoma together with the current targeted strategies in aberrant FGF-FGFR activated GC cases.

A time-resolved multi-omic atlas of the developing mouse stomach

The mammalian stomach is structurally highly diverse and its organ functionality critically depends on a normal embryonic development. Although there have been several studies on the morphological changes during stomach development, a system-wide analysis of the underlying molecular changes is lacking. Here, we present a comprehensive, temporal proteome and transcriptome atlas of the mouse stomach at multiple developmental stages. Quantitative analysis of 12,108 gene products allows identifying three distinct phases based on changes in proteins and RNAs and the gain of stomach functions on a longitudinal time scale. The transcriptome indicates functionally important isoforms relevant to development and identifies several functionally unannotated novel splicing junction transcripts that we validate at the peptide level. Importantly, many proteins differentially expressed in stomach development are also significantly overexpressed in diffuse-type gastric cancer. Overall, our study provides a resource to understand stomach development and its connection to gastric cancer tumorigenesis.

FGF10 and the Mystery of Duodenal Atresia in Humans

Background: Duodenal atresia (DA) is a congenital obstruction of the duodenum, which affects 1 in 7000 pregnancies and requires major surgery in the 1st days of life. Three morphological DA types are described. In humans, the association between DA and Down syndrome suggests an underlying, albeit elusive, genetic etiology. In mice, interruption of fibroblast growth factor 10 (Fgf10) gene signaling results in DA in 30–50% of embryos, supporting a genetic etiology. This study aims to validate the spectrum of DA in two novel strains of Fgf10 knock-out mice, in preparation for future and translational research.

Methods: Two novel CRISPR Fgf10 knock-out mouse strains were derived and embryos generated by heterozygous plug-mating. E15.5–E19.5 embryos were genotyped with respect to Fgf10 and micro-dissected to determine the presence and type of DA.

Results: One twenty seven embryos (32 wild-type, 34 heterozygous, 61 null) were analyzed. No wild-type or heterozygous embryos had DA. However, 74% of Fgf10 null embryos had DA (49% type 1, 18% type 2, and 33% type 3).

Conclusion: Our CRISPR-derived strains showed higher penetrance of DA due to single-gene deletion of Fgf10 in mice than previously reported. Further, the DA type distribution in these mice more closely reiterated that observed in humans. Future experiments will document RNA and protein expression of FGF10 and its key downstream signaling targets in normal and atretic duodenum. This includes exploitation of modern, high-fidelity developmental tools, e.g., Fgf10^flox/+–tomato^flox/flox mice.

Emerging Roles of Fibroblast Growth Factor 10 in Cancer

Whilst cross-talk between stroma and epithelium is critical for tissue development and homeostasis, aberrant paracrine stimulation can result in neoplastic transformation. Chronic stimulation of epithelial cells with paracrine Fibroblast Growth Factor 10 (FGF10) has been implicated in multiple cancers, including breast, prostate and pancreatic ductal adenocarcinoma. Here, we examine the mechanisms underlying FGF10-induced tumourigenesis and explore novel approaches to target FGF10 signaling in cancer.

Regulation of FGF10 Signaling in Development and Disease

Fibroblast Growth Factor 10 (FGF10) is a multifunctional mesenchymal-epithelial signaling growth factor, which is essential for multi-organ development and tissue homeostasis in adults. Furthermore, FGF10 deregulation has been associated with human genetic disorders and certain forms of cancer. Upon binding to FGF receptors with heparan sulfate as co-factor, FGF10 activates several intracellular signaling cascades, resulting in cell proliferation, differentiation, and invasion. FGF10 activity is modulated not only by heparan sulfate proteoglycans in the extracellular matrix, but also by hormones and other soluble factors. Despite more than 20 years of research on FGF10 functions, context-dependent regulation of FGF10 signaling specificity remains poorly understood. Emerging modes of FGF10 signaling regulation will be described, focusing on the role of FGF10 trafficking and sub-cellular localization, heparan sulfate proteoglycans, and miRNAs. Systems biology approaches based on quantitative proteomics will be considered for globally investigating FGF10 signaling specificity. Finally, current gaps in our understanding of FGF10 functions, such as the relative contribution of receptor isoforms to signaling activation, will be discussed in the context of genetic disorders and tumorigenesis.

Multiple roles of epithelial heparan sulfate in stomach morphogenesis

Heparan sulfate proteoglycans (HSPGs) have been shown to regulate various developmental processes. However, the function of heparan sulfate (HS) during the development of mammalian stomach has not been characterized yet. Here, we investigate the role of epithelial HS in embryonic stomach by examining mice deficient in the glycosyltransferase gene Ext1 We show that HS exhibits a specific and dynamic expression pattern in mouse embryonic stomach. Depletion of the epithelial HS leads to stomach hypoplasia, with phenotypic differences in the gastric mucosa between the forestomach and hindstomach. In the posterior stomach, HS depletion disrupts glandular stomach patterning and cytodifferentiation via attenuation of Fgf signaling activity. Inhibition of Fgf signaling in vitro recapitulates the patterning defect. Ligand and carbohydrate engagement assay (LACE) reveals a diminished assembly of Fgf10 and Fgfr2b in the mutant. In the anterior stomach, loss of epithelial HS leads to stratification and differentiation defects of the multilayered squamous epithelium, along with reduced Hh and Bmp signaling activity. Our data demonstrate that epithelial HS plays multiple roles in regulating mammalian stomach morphogenesis in a regional-specific manner.

Gastric Organoids: An Emerging Model System to Study Helicobacter pylori Pathogenesis

Helicobacter research classically uses fixed human tissue, animal models or cancer cell lines. Each of these study objects has its advantages and has brought central insights into the infection process. Nevertheless, in model systems for basic and medical research, there is a gap between two-dimensional and most often transformed cell cultures and three-dimensional, highly organized tissues. In recent years, stem cell research has provided the means to fill this gap. The identification of the niche factors that support growth, expansion and differentiation of stem cells in vitro has allowed the development of three-dimensional culture systems called organoids. Gastric organoids are grown from gastric stem cells and are organized epithelial structures that comprise all the differentiated cell types of the stomach. They can be expanded without apparent limitation and are amenable to a wide range of standard laboratory techniques. Here, we review different stem cell-derived organoid model systems useful for Helicobacter pylori research and outline their advantages for infection studies.

Mechanisms of embryonic stomach development

The stomach is a digestive organ that has important roles in human physiology and pathophysiology. The developmental origin of the stomach is the embryonic foregut, which also gives rise a number of other structures. There are several signaling pathways and transcription factors that are known to regulate stomach development at different stages, including foregut patterning, stomach specification, and gastric regionalization. These developmental events have important implications in later homeostasis and disease in the adult stomach. Here we will review the literature that has shaped our current understanding of the molecular mechanisms that coordinate gastric organogenesis. Further we will discuss how developmental paradigms have guided recent efforts to differentiate stomach tissue from pluripotent stem cells.

Adult gastric stem cells and their niches

Adult gastric stem cells replenish the gastric epithelium throughout life. Recent studies have identified diverse populations of stem cells, progenitor cells, and even differentiated cells that can regain stem cell capacity, so highlighting an unexpected plasticity within the gastric epithelium, both in the corpus and antrum. Two niches seem to co-exist in the gastric unit: one in the isthmus region and the other at the base of the gland, although the precise features of the cell populations and the two niches are currently under debate. A variety of gastric organoid models have been established, providing new insights into niche factors required by the gastric stem cell populations. Here we review our current knowledge of gastric stem cell populations, their markers and interactions, important niche factors, and different gastric organoid systems. WIREs Dev Biol 2017, 6:e261. doi: 10.1002/wdev.261 For further resources related to this article, please visit the WIREs website.

The Wnt Blows: On the Functional Role of Wnt Signaling in Mycobacterium tuberculosis Infection and Beyond

In recent years, it has become apparent that the Wnt signaling pathway, known for its essential functions in embryonic development and tissue homeostasis, exerts immunomodulatory functions during inflammation and infection. Most functional studies indicate that Wnt5a exerts pro-inflammatory functions on its cellular targets, which include various types of immune and non-immune cells. Wnt5a expression has also been linked to the pathogenesis of chronic inflammatory diseases. Activation of beta-catenin-dependent Wnt signaling, e.g., by Wnt3a, has however been shown to limit inflammation by interfering with the nuclear factor kappa-light chain-enhancer of activated B-cells (NF-kappaB) pathway. This review focuses on the regulation of Wnt5a, Wnt3a, and the recently identified Wnt6 and their functional role in bacterial infections with a primary focus on pulmonary tuberculosis, a leading infectious cause of morbidity and mortality worldwide.

Notch3 and Jagged2 contribute to gastric cancer development and to glandular differentiation associated with MUC2 and MUC5AC expression

AIMS

Notch signalling plays diverse roles in malignant tumours as well as in normal tissue development. In this study we investigated the expression of Notch signalling pathway genes and their clinicopathological significance in gastric carcinomas.

METHODS AND RESULTS

Notch1, Notch3, Jagged1, Jagged2 and Hes1 expression were analysed by quantitative real-time polymerase chain reaction (qRT-PCR) (n = 81) and immunohistochemistry (n = 103) in gastric carcinomas. MUC2 and MUC5AC expression were also assessed, using immunohistochemistry only. With qRT-PCR, Notch1, Notch3, Jagged1 and Jagged2 expression were increased significantly in tumour compared to normal tissue (P < 0.001, P = 0.002, P = 0.008 and P < 0.001, respectively). Overexpression of Notch3 and Jagged2 was associated with intestinal-type carcinomas (P = 0.024) and better histological differentiation (P = 0.047), respectively. Immunohistochemistry showed a reverse correlation between MUC2 and Notch3 or Jagged1 (P = 0.033 and P = 0.005, respectively) and between MUC5AC and Jagged1 or Hes1 (P = 0.004 and P = 0.002, respectively). Notch3 and Jagged2 gene overexpression related to a favourable outcome on univariate (P = 0.046 and P = 0.042, respectively) and multivariate (P = 0.045, Notch3) analysis.

CONCLUSION

The expression of Notch3 and Jagged2 is associated not only with gastric cancer development but also with the intestinal/glandular differentiation of gastric carcinoma cells, suggesting a role as a possible favourable prognostic indicator.

Gastrointestinal organoids: How they gut it out

The gastrointestinal tract is characterized by a self-renewing epithelium fueled by adult stem cells residing at the bottom of the intestinal crypt and gastric glands. Their activity and proliferation is strongly dependent on complex signaling pathways involving other crypt/gland cells as well as surrounding stromal cells. In recent years organoids are becoming increasingly popular as a new and powerful tool to study developmental or other biological processes. Organoids retain morphological and molecular patterns of the tissue they are derived from, are self-organizing, relatively simple to handle and accessible to genetic engineering. This review focuses on the developmental processes and signaling molecules involved in epithelial homeostasis and how a profound knowledge of these mechanisms allowed the establishment of a three dimensional organoid culture derived from adult gastrointestinal stem cells.

Stomach development, stem cells and disease

The stomach, an organ derived from foregut endoderm, secretes acid and enzymes and plays a key role in digestion. During development, mesenchymal-epithelial interactions drive stomach specification, patterning, differentiation and growth through selected signaling pathways and transcription factors. After birth, the gastric epithelium is maintained by the activity of stem cells. Developmental signals are aberrantly activated and stem cell functions are disrupted in gastric cancer and other disorders. Therefore, a better understanding of stomach development and stem cells can inform approaches to treating these conditions. This Review highlights the molecular mechanisms of stomach development and discusses recent findings regarding stomach stem cells and organoid cultures, and their roles in investigating disease mechanisms.

Stomach Organ and Cell Lineage Differentiation: from Embryogenesis to Adult Homeostasis

Gastric diseases cause considerable worldwide burden. However, the stomach is still poorly understood in terms of the molecular-cellular processes that govern its development and homeostasis. In particular, the complex relationship between the differentiated cell types located within the stomach and the stem and progenitor cells that give rise to them is significantly understudied relative to other organs. In this review, we will highlight the current state of the literature relating to specification of gastric cell lineages from embryogenesis to adulthood. Special emphasis is placed on substantial gaps in knowledge about stomach specification that we think should be tackled to advance the field. For example, it has long been assumed that adult gastric units have a granule-free stem cell that gives rise to all differentiated lineages. Here we will point out that there are also other models that fit all extant data, such as long-lived lineage-committed progenitors that might serve as a source of new cells during homeostasis.

Mesenchymal Wnt/β-Catenin Signaling Controls Epithelial Stem Cell Homeostasis in Teeth by Inhibiting the Antiapoptotic Effect of Fgf10

Continuous growth of rodent incisors relies on epithelial stem cells (SCs) located in the SC niche called labial cervical loop (LaCL). Here, we found a population of apoptotic cells residing in a specific location of the LaCL in mouse incisor. Activated Caspase 3 and Caspase 9, expressed in this location colocalized in part with Lgr5 in putative SCs. The addition of Caspase inhibitors to incisors ex vivo resulted in concentration dependent thickening of LaCL. To examine the role of Wnt signaling in regulation of apoptosis, we exposed the LaCL of postnatal day 2 (P2) mouse incisor ex vivo to BIO, a known activator of Wnt/β-catenin signaling. This resulted in marked thinning of LaCL as well as enhanced apoptosis. We found that Wnt/β-catenin signaling was intensely induced by BIO in the mesenchyme surrounding the LaCL, but, unexpectedly, no β-catenin activity was detected in the LaCL epithelium either before or after BIO treatment. We discovered that the expression of Fgf10, an essential growth factor for incisor epithelial SCs, was dramatically downregulated in the mesenchyme around BIO-treated LaCL, and that exogenous Fgf10 could rescue the thinning of the LaCL caused by BIO. We conclude that the homeostasis of the epithelial SC population in the mouse incisor depends on a proper rate of apoptosis and that this apoptosis is controlled by signals from the mesenchyme surrounding the LaCL. Fgf10 is a key mesenchymal signal limiting apoptosis of incisor epithelial SCs and its expression is negatively regulated by Wnt/β-catenin. Stem Cells 2015;33:1670-1681.

Modeling mouse and human development using organoid cultures

In vitro three-dimensional (3D) cultures are emerging as novel systems with which to study tissue development, organogenesis and stem cell behavior ex vivo. When grown in a 3D environment, embryonic stem cells (ESCs) self-organize into organoids and acquire the right tissue patterning to develop into several endoderm- and ectoderm-derived tissues, mimicking their in vivo counterparts. Tissue-resident adult stem cells (AdSCs) also form organoids when grown in 3D and can be propagated in vitro for long periods of time. In this Review, we discuss recent advances in the generation of pluripotent stem cell- and AdSC-derived organoids, highlighting their potential for enhancing our understanding of human development. We will also explore how this new culture system allows disease modeling and gene repair for a personalized regenerative medicine approach.

Increased WNT6 expression in tumor cells predicts unfavorable survival in esophageal squamous cell carcinoma patients

WNT proteins are a family of secreted, cysteine-rich proteins containing 19 members. Signaling through WNT proteins is reported to be involved in carcinogenesis and cancer progression of gastrointestinal tumors, such as gastric cancer and colon cancer. The expression status of WNT6 in ESCCs and their clinico-prognostic significances remain to be elucidated. In this study, One-hundred and thirty-six patients with ESCC were explored. Paraffin-embedded tumor sections were stained with WNT6 antibody. The correlations between WNT6 expression and survival parameters were analyzed. The overall frequency of WNT6 over-expression was 50.7% (69/136) of advanced EC patients. For DMS and OS, over-expression of WNT6 remained the independent factor for worse prognosis (hazard ratio (HR), 2.425; 95% CI, 1.631-3.605; P < 0.001 for OS and HR, 2.238; 95% CI, 1.507-3.323; P < 0.001 for DMS, respectively). To conclude, our results support the concept that WNT6 may play a role in tumor progression. WNT6 over-expression inversely correlates with the poor long-term survival in ESCC patients. WNT6 can be considered as a predictor for recurrence.

The Fibroblast Growth Factor signaling pathway

The signaling component of the mammalian Fibroblast Growth Factor (FGF) family is comprised of eighteen secreted proteins that interact with four signaling tyrosine kinase FGF receptors (FGFRs). Interaction of FGF ligands with their signaling receptors is regulated by protein or proteoglycan cofactors and by extracellular binding proteins. Activated FGFRs phosphorylate specific tyrosine residues that mediate interaction with cytosolic adaptor proteins and the RAS-MAPK, PI3K-AKT, PLCγ, and STAT intracellular signaling pathways. Four structurally related intracellular non-signaling FGFs interact with and regulate the family of voltage gated sodium channels. Members of the FGF family function in the earliest stages of embryonic development and during organogenesis to maintain progenitor cells and mediate their growth, differentiation, survival, and patterning. FGFs also have roles in adult tissues where they mediate metabolic functions, tissue repair, and regeneration, often by reactivating developmental signaling pathways. Consistent with the presence of FGFs in almost all tissues and organs, aberrant activity of the pathway is associated with developmental defects that disrupt organogenesis, impair the response to injury, and result in metabolic disorders, and cancer. For further resources related to this article, please visit the WIREs website.

EGFR and Notch signaling respectively regulate proliferative activity and multiple cell lineage differentiation of Drosophila gastric stem cells

Quiescent, multipotent gastric stem cells (GSSCs) in the copper cell region of adult Drosophila midgut can produce all epithelial cell lineages found in the region, including acid-secreting copper cells, interstitial cells and enteroendocrine cells, but mechanisms controlling their quiescence and the ternary lineage differentiation are unknown. By using cell ablation or damage-induced regeneration assays combined with cell lineage tracing and genetic analysis, here we demonstrate that Delta (Dl)-expressing cells in the copper cell region are the authentic GSSCs that can self-renew and continuously regenerate the gastric epithelium after a sustained damage. Lineage tracing analysis reveals that the committed GSSC daughter with activated Notch will invariably differentiate into either a copper cell or an interstitial cell, but not the enteroendocrine cell lineage, and loss-of-function and gain-of-function studies revealed that Notch signaling is both necessary and sufficient for copper cell/interstitial cell differentiation. We also demonstrate that elevated epidermal growth factor receptor (EGFR) signaling, which is achieved by the activation of ligand Vein from the surrounding muscle cells and ligand Spitz from progenitor cells, mediates the regenerative proliferation of GSSCs following damage. Taken together, we demonstrate that Dl is a specific marker for Drosophila GSSCs, whose cell cycle status is dependent on the levels of EGFR signaling activity, and the Notch signaling has a central role in controlling cell lineage differentiation from GSSCs by separating copper/interstitial cell lineage from enteroendocrine cell lineage.

Fgf10-expressing tanycytes add new neurons to the appetite/energy-balance regulating centers of the postnatal and adult hypothalamus

Increasing evidence suggests that neurogenesis occurs in the postnatal and adult mammalian hypothalamus. However, the identity and location of the putative progenitor cells is under much debate, and little is known about the dynamics of neurogenesis in unchallenged brain. Previously, we postulated that Fibroblast growth factor 10-expressing (Fgf10(+)) tanycytes constitute a population of progenitor cells in the mouse hypothalamus. Here, we show that Fgf10(+) tanycytes express markers of neural stem/progenitor cells, divide late into postnatal life, and can generate both neurons and astrocytes in vivo. Stage-specific lineage-tracing of Fgf10(+) tanycytes using Fgf10-creERT2 mice, reveals robust neurogenesis at postnatal day 28 (P28), lasting as late as P60. Furthermore, we present evidence for amplification of Fgf10-lineage traced neural cells within the hypothalamic parenchyma itself. The neuronal descendants of Fgf10(+) tanycytes predominantly populate the arcuate nucleus, a subset of which express the orexigenic neuronal marker, Neuropeptide-Y, and respond to fasting and leptin-induced signaling. These studies provide direct evidence in support of hypothalamic neurogenesis during late postnatal and adult life, and identify Fgf10(+) tanycytes as a source of parenchymal neurons with putative roles in appetite and energy balance.

MAPKs and signal transduction in the control of gastrointestinal epithelial cell proliferation and differentiation

Mitogen-activated protein kinase (MAPK) pathways are activated by several stimuli and transduce the signal inside cells, generating diverse responses including cell proliferation, differentiation, migration and apoptosis. Each MAPK cascade comprises a series of molecules, and regulation takes place at different levels. They communicate with each other and with additional pathways, creating a signaling network that is important for cell fate determination. In this review, we focus on ERK, JNK, p38 and ERK5, the major MAPKs, and their interactions with PI3K-Akt, TGFβ/Smad and Wnt/β-catenin pathways. More importantly, we describe how MAPKs regulate cell proliferation and differentiation in the rapidly renewing epithelia that lines the gastrointestinal tract and, finally, we highlight the recent findings on nutritional aspects that affect MAPK transduction cascades.

Fibroblast growth factor receptor 2c signaling is required for intestinal cell differentiation in zebrafish

Background

There are four cell lineages derived from intestinal stem cells that are located at the crypt and villus in the mammalian intestine the non-secretory absorptive enterocytes, and the secretory cells, which include mucous-secreting goblet cells, regulatory peptide-secreting enteroendocrine cells and antimicrobial peptide-secreting Paneth cells. Although fibroblast growth factor (Fgf) signaling is important for cell proliferation and differentiation in various tissues, its role in intestinal differentiation is less well understood.

Methodology/Principal Findings

We used a loss of function approach to investigate the importance of Fgf signaling in intestinal cell differentiation in zebrafish; abnormal differentiation of goblet cells was observed when Fgf signaling was inhibited using SU5402 or in the Tg(hsp70ldnfgfr1-EGFP) transgenic line. We identified Fgfr2c as an important receptor for cell differentiation. The number of goblet cells and enteroendocrine cells was reduced in fgfr2c morphants. In addition to secretory cells, enterocyte differentiation was also disrupted in fgfr2c morphants. Furthermore, proliferating cells were increased in the morphants. Interestingly, the loss of fgfr2c expression repressed secretory cell differentiation and increased cell proliferation in the mib^ta52b mutant that had defective Notch signaling.

Conclusions/Significance

In conclusion, we found that Fgfr2c signaling derived from mesenchymal cells is important for regulating the differentiation of zebrafish intestine epithelial cells by promoting cell cycle exit. The results of Fgfr2c knockdown in mib^ta52b mutants indicated that Fgfr2c signaling is required for intestinal cell differentiation. These findings provide new evidences that Fgf signaling is required for the differentiation of intestinal cells in the zebrafish developing gut.

Mathematical study of the role of Delta/Notch lateral inhibition during primary branching of Drosophila trachea development

A wide range of cellular developmental processes employ intercellular signaling via the Delta/Notch lateral inhibitory pathway to achieve stable spatial patterning. Recent genetic experiments have shown the importance of Delta/Notch lateral inhibition for regulating the number of tip cells in the tracheal primary branching of Drosophila. To examine the role of Delta/Notch regulation in the tip-cell selection, we analyzed a mathematical model of a simple lateral inhibitory system having input signals. Mathematical and numerical analyses revealed that the lateral inhibition did not amplify the signal difference between neighboring cells over the parameter ranges in which the spatial pattern of tip selection was realized. We also show that the number of tip cells becomes less affected by a fluctuation of the input gradient signal as the lateral inhibition becomes stronger. In addition, we demonstrate that the lateral inhibitory regulation enhances the robustness of the tip-cell selection compared with a system regulated by self-inhibition, an alternative means of inhibitory regulation. These results suggest that the lateral inhibition promotes the robustness of tip-cell selection in the tracheal development of Drosophila.

Fibroblast growth factor 10-fibroblast growth factor receptor 2b mediated signaling is not required for adult glandular stomach homeostasis

The signaling pathways that are essential for gastric organogenesis have been studied in some detail; however, those that regulate the maintenance of the gastric epithelium during adult homeostasis remain unclear. In this study, we investigated the role of Fibroblast growth factor 10 (FGF10) and its main receptor, Fibroblast growth factor receptor 2b (FGFR2b), in adult glandular stomach homeostasis. We first showed that mouse adult glandular stomach expressed Fgf10, its receptors, Fgfr1b and Fgfr2b, and most of the other FGFR2b ligands (Fgf1, Fgf7, Fgf22) except for Fgf3 and Fgf20. Fgf10 expression was mesenchymal whereas FGFR1 and FGFR2 expression were mostly epithelial. Studying double transgenic mice that allow inducible overexpression of Fgf10 in adult mice, we showed that Fgf10 overexpression in normal adult glandular stomach increased epithelial proliferation, drove mucous neck cell differentiation, and reduced parietal and chief cell differentiation. Although a similar phenotype can be associated with the development of metaplasia, we found that Fgf10 overexpression for a short duration does not cause metaplasia. Finally, investigating double transgenic mice that allow the expression of a soluble form of Fgfr2b, FGF10's main receptor, which acts as a dominant negative, we found no significant changes in gastric epithelial proliferation or differentiation in the mutants. Our work provides evidence, for the first time, that the FGF10-FGFR2b signaling pathway is not required for epithelial proliferation and differentiation during adult glandular stomach homeostasis.

Prolonged FGF signaling is necessary for lung and liver induction in Xenopus

BACKGROUND

FGF signaling plays numerous roles during organogenesis of the embryonic gut tube. Mouse explant studies suggest that different thresholds of FGF signaling from the cardiogenic mesoderm induce lung, liver, and pancreas lineages from the ventral foregut progenitor cells. The mechanisms that regulate FGF dose in vivo are unknown. Here we use Xenopus embryos to examine the hypothesis that a prolonged duration of FGF signaling from the mesoderm is required to induce foregut organs.

RESULTS

We show that both mesoderm and FGF signaling are required for liver and lung development in Xenopus; formally demonstrating that this important step in organ induction is conserved with other vertebrate species. Prolonged contact with the mesoderm and persistent FGF signaling through both MEK and PI3K over an extended period of time are required for liver and lung specification. Inhibition of FGF signaling results in reduced liver and lung development, with a modest expansion of the pancreas/duodenum progenitor domain. Hyper-activation of FGF signaling has the opposite effect expanding liver and lung gene expression and repressing pancreatic markers. We show that FGF signaling is cell autonomously required in the endoderm and that a dominant negative FGF receptor decreases the ability of ventral foregut progenitor cells to contribute to the lung and liver buds.

CONCLUSIONS

These results suggest that the liver and lungs are specified at progressively later times in development requiring mesoderm contact for different lengths of time. Our data suggest that this is achieved at least in part through prolonged FGF signaling. In addition to providing a foundation for further mechanistic studies on foregut organogenesis using the experimental advantages of the Xenopus system, these data have implications for the directed differentiation of stem cells into foregut lineages.

WNT6 is a novel target gene of caveolin-1 promoting chemoresistance to epirubicin in human gastric cancer cells

Resistance to chemotherapy is a major obstacle for curative treatment of human gastric cancer (GC). However, the underlying molecular mechanisms are largely unknown. Wingless-type MMTV integration site family members (WNTs) are secreted glycoproteins involved in embryogenesis and, on inappropriate expression in the adult, in cancer. Here, we show expression of WNT6 in GC patient specimens, human GC cell lines and in a mouse model of GC. In human GC cells, WNT6 expression was enhanced by caveolin-1 (Cav1), a scaffold protein of plasma membrane caveolae. WNT6 knock-down and overexpression experiments demonstrated that WNT6 increased the resistance to apoptotic cell death induced by the anthracycline chemotherapeutics epirubicin (Epi) and doxorubicin (Dox). Epi increased the activity of the human WNT6 promoter through Cav1-dependent binding of β-catenin to the proximal WNT6 promoter. Epi increased both WNT6/Wnt6 and Cav1 expression in human GC cells and within the tumor area of a murine model of GC (CEA424-SV40 TAg). In GC patients, WNT6 expression was positively associated with the tumor stage and the nodal status, and inversely correlated with the response to ECF (Epi, cisplatin, 5-fluorouracil) chemotherapy. These results showed that WNT6 and Cav1 are upregulated by chemotherapeutics and enhance the resistance of GC cells to anthracycline drugs. Understanding the molecular mechanisms driving WNT6/Cav1-induced drug resistance will provide benefits in developing new therapies for GC.

A BAC transgenic Hes1-EGFP reporter reveals novel expression domains in mouse embryos

Expression of the basic helix-loop-helix factor Hairy and Enhancer of Split-1 (Hes1) is required for normal development of a number of tissues during embryonic development. Depending on context, Hes1 may act as a Notch signalling effector which promotes the undifferentiated and proliferative state of progenitor cells, but increasing evidence also points to Notch independent regulation of Hes1 expression. Here we use high resolution confocal scanning of EGFP in a novel BAC transgenic mouse reporter line, Tg(Hes1-EGFP)(1Hri), to analyse Hes1 expression from embryonic day 7.0 (e7.0). Our data recapitulates some previous observations on Hes1 expression and suggests new, hitherto unrecognised expression domains including expression in the definitive endoderm at early somite stages before gut tube closure and thus preceding organogenesis. This mouse line will be a valuable tool for studies addressing the role of Hes1 in a number of different research areas including organ specification, development and regeneration.

Murine tissue-engineered stomach demonstrates epithelial differentiation

BACKGROUND

Gastric cancer remains the second largest cause of cancer-related mortality worldwide. Postgastrectomy morbidity is considerable and quality of life is poor. Tissue-engineered stomach is a potential replacement solution to restore adequate food reservoir and gastric physiology. In this study, we performed a detailed investigation of the development of tissue-engineered stomach in a mouse model, specifically evaluating epithelial differentiation, proliferation, and the presence of putative stem cell markers.

MATERIALS AND METHODS

Organoid units were isolated from <3 wk-old mouse glandular stomach and seeded onto biodegradable scaffolds. The constructs were implanted into the omentum of adult mice. Implants were harvested at designated time points and analyzed with histology and immunohistochemistry.

RESULTS

Tissue-engineered stomach grows as an expanding sphere with a simple columnar epithelium organized into gastric glands and an adjacent muscularis. The regenerated gastric epithelium demonstrates differentiation of all four cell types: mucous, enteroendocrine, chief, and parietal cells. Tissue-engineered stomach epithelium proliferates at a rate comparable to native glandular stomach and expresses two putative stem cell markers: DCAMKL-1 and Lgr5.

CONCLUSIONS

This study demonstrates the successful generation of tissue-engineered stomach in a mouse model for the first time. Regenerated gastric epithelium is able to appropriately proliferate and differentiate. The generation of murine tissue-engineered stomach is a necessary advance as it provides the transgenic tools required to investigate the molecular and cellular mechanisms of this regenerative process. Delineating the mechanism of how tissue-engineered stomach develops in vivo is an important precursor to its use as a human stomach replacement therapy.

Growth-limiting role of endothelial cells in endoderm development

Endoderm development is dependent on inductive signals from different structures in close vicinity, including the notochord, lateral plate mesoderm and endothelial cells. Recently, we demonstrated that a functional vascular system is necessary for proper pancreas development, and that sphingosine-1-phosphate (S1P) exhibits the traits of a blood vessel-derived molecule involved in early pancreas morphogenesis. To examine whether S1P(1)-signaling plays a more general role in endoderm development, S1P(1)-deficient mice were analyzed. S1P(1) ablation results in compromised growth of several foregut-derived organs, including the stomach, dorsal and ventral pancreas and liver. Within the developing pancreas the reduction in organ size was due to deficient proliferation of Pdx1(+) pancreatic progenitors, whereas endocrine cell differentiation was unaffected. Ablation of endothelial cells in vitro did not mimic the S1P(1) phenotype, instead, increased organ size and hyperbranching were observed. Consistent with a negative role for endothelial cells in endoderm organ expansion, excessive vasculature was discovered in S1P(1)-deficient embryos. Altogether, our results show that endothelial cell hyperplasia negatively influences organ development in several foregut-derived organs.

Expression analysis of the Islet-1 gene in the developing and adult gastrointestinal tract

LIM-homeodomain genes encode a family of proteins defined by the cysteine-rich protein/protein interacting (Lin-11, Isl-1, and Mec-3) LIM domain and a highly conserved DNA-binding domain. Studies in several organisms have shown that these transcriptional regulators control multiple aspects of embryonic development and are responsible for the pathogenesis of several human diseases. Here we report the expression of Islet-1 (Isl-1) in the gastrointestinal epithelium in developing and adult mice. At embryonic day (E) 9.5-10.5, Isl-1 expression was first detected in the ventral gastric mesenchyme, and expression in the dorsal mesenchyme initiated a few days later. Isl-1 expression was first observed in the gastric epithelium at E13.5 and at E14.5 was restricted to the posterior half of the stomach. In the mature stomach, Isl-1 expression was detected only in subsets of enteroendocrine cells. Furthermore, Isl-1 expression in the intestinal epithelium was first detected at E15.5 and was restricted to subpopulations of enteroendocrine cells in adult mice. These expression analyses suggest that Isl-1 might have an early broad role in stomach and intestinal cells and a secondary role in terminal differentiation and/or maintenance of mature enteroendocrine subtypes in the gastrointestinal epithelium.

Beta-catenin signaling in hepatic development and progenitors: which way does the WNT blow?

The Wnt/β-catenin pathway is an evolutionarily conserved signaling cascade that plays key roles in development and adult tissue homeostasis and is aberrantly activated in many tumors. Over a decade of work in mouse, chick, xenopus, and zebrafish models has uncovered multiple functions of this pathway in hepatic pathophysiology. Specifically, beta-catenin, the central component of the canonical Wnt pathway, is implicated in the regulation of liver regeneration, development, and carcinogenesis. Wnt-independent activation of beta-catenin by receptor tyrosine kinases has also been observed in the liver. In liver development across various species, through regulation of cell proliferation, differentiation, and maturation, beta-catenin directs foregut endoderm specification, hepatic specification of the foregut, and hepatic morphogenesis. Its role has also been defined in adult hepatic progenitors or oval cells especially in their expansion and differentiation. Thus, beta-catenin undergoes tight temporal regulation to exhibit pleiotropic effects during hepatic development and in hepatic progenitor biology.

Cytodifferentiation of the postnatal mouse stomach in normal and Huntingtin-interacting protein 1-related-deficient mice

Huntingtin-interacting protein 1-related (Hip1r) is highly expressed in gastric parietal cells, where it participates in vesicular trafficking associated with acid secretion. Hip1r-deficient mice have a progressive remodeling of the mucosa, including apoptotic loss of parietal cells, glandular hypertrophy, mucous cell metaplasia, and reduced numbers of zymogenic cells. In this study, we characterized gastric gland development in wild-type and Hip1r-deficient mice to define normal development, as well as the timing and sequence of the cellular transformation events in the mutant stomach. Postnatal (newborn to 8-wk-old) stomachs were examined by histological and gene expression analysis. At birth, gastric glands in wild-type and mutant mice were rudimentary and mature gastric epithelial cells were not apparent, although marker expression was detected for most cell lineages. Interestingly, newborns exhibited unusual cell types, including a novel surface cell filled with lipid and cells that coexpressed markers of mature mucous neck and zymogenic cells. Glandular morphogenesis proceeded rapidly in both genotypes, with gastric glands formed by weaning at 3 wk of age. In the Hip1r-deficient stomach, epithelial cell remodeling developed in a progressive manner. Initially, in the perinatal stomach, cellular changes were limited to parietal cell apoptosis. Other epithelial cell changes, including apoptotic loss of zymogenic cells and expansion of metaplastic mucous cells, emerged several weeks later when the glands were morphologically mature. Thus, parietal cell loss appeared to be the initiating event in Hip1r-deficient mice, with secondary remodeling of the other gastric epithelial cells.

Fibroblast growth factor 10 represses premature cell differentiation during establishment of the intestinal progenitor niche

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

Epithelial stem/progenitor cells in the embryonic mouse submandibular gland

Salivary gland organogenesis involves the specification, maintenance, lineage commitment, and differentiation of epithelial stem/progenitor cells. Identifying how stem/progenitor cells are directed along a series of cell fate decisions to form a functional salivary gland will be necessary for future stem cell regenerative therapy. The identification of stem/progenitor cells within the salivary gland has focused on their role in postnatal glands and little is known about them in embryonic glands. Here, we have reviewed the information available for other developing organ systems and used it to determine whether similar cell populations exist in the mouse submandibular gland. Additionally, using growth factors that influence salivary gland epithelial morphogenesis during development, we have taken a simple experimental approach asking whether any of these growth factors influence early developmental lineages within the salivary epithelium on a transcriptional level. These preliminary findings show that salivary epithelial stem/progenitor populations exist within the gland, and that growth factors that are reported to control epithelial morphogenesis may also impact cell fate decisions. Further investigation of the signaling networks that influence stem/progenitor cell behavior will allow us to hypothesize how we might induce autologous stem cells to regenerate damaged salivary tissue in a therapeutic context.

Mechanisms of lung development: contribution to adult lung disease and relevance to chronic obstructive pulmonary disease

Chronic obstructive pulmonary disease (COPD) results in major remodeling of the distal airspaces and changes in the differentiation profile of the airway epithelium. The cellular and molecular mechanisms involved in initiation and progression of this disease are little understood. Although environmental factors, including cigarette smoke, have been directly implicated in the pathogenesis of COPD, genetic risk factors also appear to play a fundamental role in the individual's susceptibility to this disease. Lung development depends on precise coordination of signals, such as fibroblast growth factors (Fgf), Sonic Hedgehog (Shh), retinoic acid, Notch, and Tgf beta. Dramatic changes in the pattern of branching and differentiation of the lung epithelium results from disruption of these signals in genetically altered mice. Recent studies, including whole-genome expression and genome-wide association analyses, suggest that some molecular regulators originally described in developmental processes may be altered in patients with COPD. Whether disturbances in the molecular and cellular events mediated by these genes during development participate in the initiation or exacerbation of COPD, needs further investigation. The role of selected pathways, including Sonic hedgehog, Notch, retinoid, and Tgf beta in the developing lung and the potential association with COPD are discussed.

Hepatic stem cells and liver development

The liver consists of many cell types with specialized functions. Hepatocytes are one of the main players in the organ and therefore are the most vulnerable cells to damage. Since they are not everlasting cells, they need to be replenished throughout life. Although the capacity of hepatocytes to contribute to their own maintenance has long been recognized, recent studies have indicated the presence of both intrahepatic and extrahepatic stem/progenitor cell populations that serve to maintain the normal organ and to regenerate damaged parenchyma in response to a variety of insults.The intrahepatic compartment most likely derives primarily from the biliary tree, particularly the most proximal branches, i.e. the canals of Hering and smallest ductules. The extrahepatic compartment is at least in part derived from diverse populations of cells from the bone marrow. Embryonic stem cells (ES's) are considered as a part of the extrahepatic compartment. Due to their pluripotent capabilities, ES cell-derived cells form a potential future source of hepatocytes, to replace or restore hepatic tissues that have been damaged by disease or injury. Progressing knowledge about stem cells in the liver would allow a better understanding of the mechanisms of hepatic homeostasis and regeneration. Although a human stem cell-derived cell type equivalent to primary hepatocytes does not yet exist, the promising results obtained with extrahepatic stem cells would open the way to cell-based therapy for liver diseases.

Hedgehog signaling in gastric physiology and cancer

The Hedgehog family of ligands was originally identified in mutagenesis screens of Drosophila embryos. Hedgehog signaling in multiple tissues is important during embryonic development. A common theme regarding Hedgehog expression in adult tissues is that tissue injury reactivates the developmental pattern of expression. In most instances, this appears to be important to initiate tissue repair. In the gastrointestinal (GI) tract, where epithelial cells are constantly replenished from progenitor populations, Hedgehog signaling also appears to be essential for regeneration. By contrast, reactivated Hedgehog signaling in adult tissues does not automatically predispose the tissue to transformation, but instead requires sustained tissue injury in the form of chronic inflammation. In this chapter, we review what is known about Hedgehog ligands and signaling during development of relevant organs, and discuss how the patterns of Hedgehog regulation are recapitulated in the GI tract during embryogenesis, adult homeostasis, and neoplastic transformation.