Search

While the Wnt/β-catenin pathway plays a critical role in the maintenance of the zonation of ammonia metabolizing enzymes in the adult liver, the mechanisms responsible for inducing zonation in the embryo are not well understood. Herein we address the spatiotemporal role of the Wnt/β-catenin pathway in the development of zonation in embryonic mouse liver by conditional deletion of Apc and β-catenin at different stages of mouse liver development. In normal development, the ammonia metabolising enzymes carbamoylphosphate synthetase I (CPSI) and Glutamine synthetase (GS) begin to be expressed in separate hepatoblasts from E13.5 and E15.5 respectively and gradually increase in number thereafter. Restriction of GS expression occurs at E18 and becomes increasingly limited to the terminal perivenous hepatocytes postnatally. Expression of nuclear β-catenin coincides with the restriction of GS expression to the terminal perivenous hepatocytes. Conditional loss of Apc resulted in the expression of nuclear β-catenin throughout the developing liver and increased number of cells expressing GS. Conversely, conditional loss of β-catenin resulted in loss of GS expression. These data suggest that the Wnt pathway is critical to the development of zonation as well as maintaining the zonation in the adult liver.

Breast cancer is one of the most prevalent types of cancers worldwide and yet, its pathophysiology is poorly understood. Single-cell electrophysiological studies have provided evidence that membrane depolarization is implicated in the proliferation and metastasis of breast cancer. However, metastatic breast cancer cells are highly dynamic microscopic systems with complexities beyond a single-cell level. There is an urgent need for electrophysiological studies and technologies capable of decoding the intercellular signaling pathways and networks that control proliferation and metastasis, particularly at a population level. Hence, we present for the first time non-invasive in vitro electrical recordings of strongly metastatic MDA-MB-231 and weakly/non-metastatic MCF-7 breast cancer cell lines. To accomplish this, we fabricated an ultra-low noise sensor that exploits large-area electrodes, of 2 mm2, which maximizes the double-layer capacitance and concomitant detection sensitivity. We show that the current recorded after adherence of the cells is dominated by the opening of voltage-gated sodium channels (VGSCs), confirmed by application of the highly specific inhibitor, tetrodotoxin (TTX). The electrical activity of MDA-MB-231 cells surpasses that of the MCF-7 cells, suggesting a link between the cells’ bioelectricity and invasiveness. We also recorded an activity pattern with characteristics similar to that of Random Telegraph Signal (RTS) noise. RTS patterns were less frequent than the asynchronous VGSC signals. The RTS noise power spectral density showed a Lorentzian shape, which revealed the presence of a low-frequency signal across MDA-MB-231 cell populations with propagation speeds of the same order as those reported for intercellular Ca2+ waves. Our recording platform paves the way for real-time investigations of the bioelectricity of cancer cells, their ionic/pharmacological properties and relationship to metastatic potential.

The canonical Wnt (Wnt/β-catenin) signalling pathway is highly conserved and plays a critical role in regulating cellular processes both during development and in adult tissue homeostasis. The Wnt/β-catenin signalling pathway is vital for correct body patterning and is involved in fate specification of the gut tube, the primitive precursor of liver. In adults, the Wnt/β-catenin pathway is increasingly recognised as an important regulator of metabolic zonation, homeostatic renewal and regeneration in response to injury throughout the liver. Herein, we review recent developments relating to the key role of the pathway in the patterning and fate specification of the liver, in the directed differentiation of pluripotent stem cells into hepatocytes and in governing proliferation and zonation in the adult liver. We pay particular attention to recent contributions to the controversy surrounding homeostatic renewal and proliferation in response to injury. Furthermore, we discuss how crosstalk between the Wnt/β-catenin and Hedgehog (Hh) and hypoxia inducible factor (HIF) pathways works to maintain liver homeostasis. Advancing our understanding of this pathway will benefit our ability to model disease, screen drugs and generate tissue and organ replacements for regenerative medicine.

While the Wnt/β-catenin pathway plays a critical role in the maintenance of the zonation of ammonia metabolizing enzymes in the adult liver, the mechanisms responsible for inducing zonation in the embryo are not well understood. Herein we address the spatiotemporal role of the Wnt/β-catenin pathway in the development of zonation in embryonic mouse liver by conditional deletion of Apc and β-catenin at different stages of mouse liver development. In normal development, the ammonia metabolising enzymes carbamoylphosphate synthetase I (CPSI) and Glutamine synthetase (GS) begin to be expressed in separate hepatoblasts from E13.5 and E15.5 respectively and gradually increase in number thereafter. Restriction of GS expression occurs at E18 and becomes increasingly limited to the terminal perivenous hepatocytes postnatally. Expression of nuclear β-catenin coincides with the restriction of GS expression to the terminal perivenous hepatocytes. Conditional loss of Apc resulted in the expression of nuclear β-catenin throughout the developing liver and increased number of cells expressing GS. Conversely, conditional loss of β-catenin resulted in loss of GS expression. These data suggest that the Wnt pathway is critical to the development of zonation as well as maintaining the zonation in the adult liver.

Barrett's metaplasia is the only known morphological precursor to oesophageal adenocarcinoma and is characterized by replacement of stratified squamous epithelium by columnar epithelium. The cell of origin is uncertain and the molecular mechanisms responsible for the change in cellular phenotype are poorly understood. We therefore explored the role of two transcription factors, Cdx2 and HNF4α in the conversion using primary organ cultures. Biopsy samples from cases of human Barrett's metaplasia were analysed for the presence of CDX2 and HNF4α. A new organ culture system for adult murine oesophagus is described. Using this, Cdx2 and HNF4α were ectopically expressed by adenoviral infection. The phenotype following infection was determined by a combination of PCR, immunohistochemical and morphological analyses. We demonstrate the expression of CDX2 and HNF4α in human biopsy samples. Our oesophageal organ culture system expressed markers characteristic of the normal SSQE: p63, K14, K4 and loricrin. Ectopic expression of HNF4α, but not of Cdx2 induced expression of Tff3, villin, K8 and E-cadherin. HNF4α is sufficient to induce a columnar-like phenotype in adult mouse oesophageal epithelium and is present in the human condition. These data suggest that induction of HNF4α is a key early step in the formation of Barrett's metaplasia and are consistent with an origin of Barrett's metaplasia from the oesophageal epithelium.

The conversion of one cell type to another is defined as metaplasia (or sometimes it is referred to as transdifferentiation or cellular reprogramming). Metaplasia is important clinically and may predispose to the development of cancer. Barrett's metaplasia is one such example and is the focus of the present review. Barrett's is a pathological condition in which the normal oesophageal stratified squamous epithelium is replaced by intestinal-type columnar epithelium and is associated with gastro-oesophageal reflux disease. The appearance of columnar epithelium in the oesophagus predisposes to the development of adenocarcinoma. Herein we review the latest evidence on the cellular origin of Barrett's metaplasia. Until recently it was thought that the cellular origin of the columnar epithelium was from a pre-existing cell within the oesophagus. However, recent evidence suggests that this may not be the case. Instead two recent publications indicate that the columnar cells may migrate from a site distal to the oesophagus. These new data contravene our current understanding of metaplasia and raise important questions about the cellular origin of cancer.

In the embryo, the liver and pancreas exhibit a close developmental relationship. Both tissues arise from neighbouring regions of the developing endoderm. As well as this close developmental relationship, the liver and pancreas can, under certain circumstances, regenerate functional components. Understanding the normal development of the two tissue types and the underlying cellular and molecular mechanisms governing normal development and regeneration is critical to the production of novel therapies for treating liver disease and pancreatic disorders such as diabetes and pancreatitis. Herein we discuss the development of the liver and pancreas from progenitor cells in the embryo and the existence of potential stem cells in the adult tissues.

The following on molecular aspects of esophageal development contains commentaries on esophageal striated myogenesis and transdifferentiation; conversion from columnar into stratified squamous epithelium in the mouse esophagus; the roles for BMP signaling in the developing esophagus and forestomach; and evidence of a direct conversion from columnar to stratified squamous cells in the developing esophagus.

Dysregulated Wnt signaling is seen in approximately 30% of hepatocellular carcinomas; thus, finding pathways downstream of the activation of Wnt signaling is key. Here, using cre-lox technology, we deleted the Apc gene in the adult mouse liver and observed a rapid increase in nuclear β-catenin and c-Myc, which is associated with an induction of proliferation that led to hepatomegaly within 4 days of gene deletion. To investigate the downstream pathways responsible for these phenotypes, we analyzed the impact of inactivating APC in the context of deficiency of the potentially key effectors β-catenin and c-Myc. β-catenin loss rescues both the proliferation and hepatomegaly phenotypes after APC loss. However, c-Myc deletion, which rescues the phenotypes of APC loss in the intestine, had no effect on the phenotypes of APC loss in the liver. The consequences of the deregulation of the Wnt pathway within the liver are therefore strikingly different from those observed within the intestine, with the vast majority of Wnt targets being β-catenin-dependent but c-Myc-independent in the liver.

Stem cells are undifferentiated cells that can self-renew and generate specialized (functional) cell types. The remarkable ability of stem cells to differentiate towards functional cells makes them suitable modalities in cellular therapy (which means treating diseases with the body's own cells). Potential targets for cellular therapy include diabetes and liver failure. However, in order for stem cells to be clinically useful, we must learn to identify them and to regulate their differentiation. We will use the intestine as a classical example of a stem cell compartment, and then examine the evidence for the existence of adult stem cells in two endodermally derived organs: pancreas and liver. We will review the characteristics of the putative stem cells in these tissues and the transcription factors controlling their differentiation towards functional cell types.

Hepatocyte transplantation provides an alternative cellular therapy to orthotopic liver transplantation for the treatment of fulminant hepatic failure and hereditary liver disease. Unfortunately there is a serious limitation to the treatment of liver diseases by both liver and hepatocyte transplantation, namely the shortage of organ donors. Therefore, to overcome the problem of organ shortage, additional sources of hepatocytes must be found. Alternative sources of hepatocytes for transplantation have been proposed and these include immortalized cells, stem cells (both embryonic and adult) and differentiated cell types from non-hepatic sources (e.g. the pancreas). The aim of this review is to summarize current knowledge of embryonic and adult stem cell differentiation towards the hepatic phenotype and to examine sources of hepatocytes from developmentally related tissues (e.g. the pancreas).

Pancreatic cells can be converted to hepatocytes by overexpression of C/EBPbeta (Shen, C-N, Slack, J.M.W. and Tosh, D., 2000. Molecular basis of transdifferentiation of pancreas to liver. Nature Cell Biology 2: 879-887). This suggested that expression of one or more C/EBP factors may distinguish liver and pancreas in early development. We have now studied the early expression of C/EBPalpha and C/EBPbeta in the mouse embryo and show that both are expressed exclusively in the early liver bud and not in the pancreatic buds. Their expression is identical to that of hepatocyte nuclear factor 4 (HNF4), another key hepatic transcription factor and alpha-fetoprotein (AFP), a differentiation product characteristic of immature hepatocytes. Both are complementary to the early expression of Pdx1, a key pancreatic transcription factor. These results are consistent with the idea that C/EBP factors are master regulators for liver development.

There is now excellent experimental evidence demonstrating the remarkable ability of some differentiated cells to convert to a completely different phenotype. The conversion of one cellular phenotype to another is referred to as 'transdifferentiation' and belongs to a wider class of cell-type switches termed 'metaplasias'. Defining the molecular steps in transdifferentiation will help us to understand the developmental biology of the cells that interconvert, as well as help identify key regulatory transcription factors that may be important for the reprogramming of stem cells. Ultimately, being able to produce cells at will offers a compelling new approach to therapeutic transplantation and therefore the treatment and cure of diseases such as diabetes.

We previously demonstrated that dexramethasone (Dex) induces the transdifferentiation (or conversion) of the pancreatic progenitor cell lineAR42J-B13 (B13) to hepatocytes based on the expression of liver proteins. We have extended our original observations to determine: (I) the effects of Dex on pancreatic gene expression; (2) the time course of expression of liver enriched transcription factors during conversion from pancreatic to hepatic phenotype; (3) the functional potential of transdifferentiated hepatocytes; (4) the proliferative capacity of transdifferentiated hepatocytes; and (5) whether ectopic expression of transcription factors can induce the hepatic phenotype in pancreatic B13 cells. The results were as follows. The B13 cell markers amylase, synaptophysin, and neurofilament were lost in transdifferentiated hepatocytes compared to control cells and the liver-enriched transcription factors C/EBP beta and C/EBP alpha were induced first, followed by HNF4 alpha and then RXR alpha. Using RT-PCR analysis and immunolocalisation studies, we detected hepatic markers (e.g., apolipoprotein 13) in Dex-treated cells. In transdifferentiated hepatocytes albumin was secreted, insulin stimulated lipid deposition and ciprofibrate enhanced the expression of catalase. Proliferation of transdifferentiated hepatocytes is promoted in the presence of HGF and NEAA as indicated by the co-expression of the cell cycle markers cyclin D and phosphohistone H3 with liver proteins. Lastly, ectopic expression of C/EBPa or C/EBP beta in AR42J-B13 cells was sufficient to induce transdifferentiation, based on nuclear localization of HNF4 alpha and induction of UDP-glucuronosyltransferase expression. These results indicate that the B13 progenitor cell model is suitable for studying liver function and for understanding the molecular and cellular events that occur during transdifferentiation.

Cell therapy means treating diseases with the body's own cells. The ability to produce differentiated cell types at will offers a compelling new approach to cell therapy and therefore for the treatment and cure of a plethora of clinical conditions, including diabetes, Parkinson's disease and cardiovascular disease. Until recently, it was thought that differentiated cells could only be produced from embryonic or adult stem cells. Although the results from stem cell studies have been encouraging, perhaps the most startling findings have been the recent observations that differentiated cell types can transdifferentiate (or convert) into a completely different phenotype. Harnessing transdifferentiated cells as a therapeutic modality will complement the use of embryonic and adult stem cells in the treatment of degenerative disorders. In this review, we will examine some examples of transdifferentiation, describe the theoretical and practical issues involved in transdifferentiation research and comment on the long-term therapeutic possibilities.

In recent years, there have been a number of well-documented examples demonstrating that one cell type can be converted to another. Two such examples are the appearance of ectopic pancreas in the liver and formation of hepatic tissue in the pancreas. The conversion of liver to pancreas raises the intriguing possibility of generating insulin-producing beta cells for therapeutic transplantation into diabetics. There is now a striking addition to the growing list of pancreatic conversions: the formation of pancreatic tissue in the developing biliary system.

Secreted Wnt proteins control a diverse array of developmental decisions. A recent analysis of the zebrafish mutant prometheus points to a previously unknown role for Wnts during liver specification.

States of terminal cell differentiation are often considered to be fixed. There are examples, however, in which cells of one type can be converted to a completely different cell type. The process whereby one cell type can be converted to another is referred to as cellular reprogramming. Cellular reprogramming is also referred to in the literature as transdifferentiation (or the direct conversion of one cell type to another without dedifferentiation to an intermediate cell type). Where the conversion between cell types occurs in the developing embryo, the process is referred to as transdetermination. Herein we examine some well-defined examples of transdetermination. Defining the molecular and cellular basis of transdetermination will help us to understand the normal developmental biology of the cells that interconvert, as well as identifying key regulatory transcription factors (master switch genes) that may be important for the reprogramming of stem cells. Harnessing the therapeutic potential of reprogramming and master genes is an important goal in regenerative medicine.

States of terminal cell differentiation are often considered to be fixed. There are examples, however, in which cells of one type can be converted to a completely different cell type. The process whereby one cell type can be converted to another is referred to as cellular reprogramming. Cellular reprogramming is also referred to in the literature as transdifferentiation (or the direct conversion of one cell type to another without dedifferentiation to an intermediate cell type). Where the conversion between cell types occurs in the developing embryo, the process is referred to as transdetermination. Herein we examine some well-defined examples of transdetermination. Defining the molecular and cellular basis of transdetermination will help us to understand the normal developmental biology of the cells that interconvert, as well as identifying key regulatory transcription factors (master switch genes) that may be important for the reprogramming of stem cells. Harnessing the therapeutic potential of reprogramming and master genes is an important goal in regenerative medicine.

Background: The pancreatic exocrine cell line AR42J-B13 can be reprogrammed to hepatocytes following treatment with dexamethasone. The question arises whether dexamethasone also has the capacity to induce ductal cells as well as hepatocytes. Methodology/Principal Findings: AR42J-B13 cells were treated with and without dexamethasone and analyzed for the expression of pancreatic exocrine, hepatocyte and ductal markers. Addition of dexamethasone inhibited pancreatic amylase expression, induced expression of the hepatocyte marker transferrin as well as markers typical of ductal cells: cytokeratin 7 and 19 and the lectin peanut agglutinin. However, the number of ductal cells was low compared to hepatocytes. The proportion of ductal cells was enhanced by culture with dexamethasone and epidermal growth factor (EGF). We established several features of the mechanism underlying the transdifferentiation of pancreatic exocrine cells to ductal cells. Using a CK19 promoter reporter, we show that a proportion of the ductal cells arise from differentiated pancreatic exocrine-like cells. We also examined whether C/EBP beta (a transcription factor important in the conversion of pancreatic cells to hepatocytes) could alter the conversion from acinar cells to a ductal phenotype. Overexpression of an activated form of C/EBP beta in dexamethasone/EGF-treated cells provoked the expression of hepatocyte markers and inhibited the expression of ductal markers. Conversely, ectopic expression of a dominant-negative form of C/EBP beta, liver inhibitory protein, inhibited hepatocyte formation in dexamethasone-treated cultures and enhanced the ductal phenotype. Conclusions/Significance: These results indicate that hepatocytes and ductal cells may be induced from pancreatic exocrine AR42J-B13 cells following treatment with dexamethasone. The conversion from pancreatic to hepatocyte or ductal cells is dependent upon the expression of C/EBP beta.

Transdifferentiation is defined as the conversion of one cell type to another. One well-documented example of transdifferentiation is the conversion of pancreatic cells to hepatocytes. Here we describe a robust in vitro model to study pancreas to liver transdifferentiation. It is based on the addition of the synthetic glucocorticoid dexamethasone to the rat pancreatic exocrine cell line AR42J. Following glucocorticoid treatment, cells resembling hepatocytes are induced. Transdifferentiated hepatocytes express many of the properties of bona fide hepatocytes, e.g. production of albumin and ability to respond to xenobiotics. These hepatocytes can be used for studying liver function in vitro as well as studying the molecular basis of transdifferentiation.

Culturing embryonic tissue in an in vitro setting offers the unique ability to manipulate the external medium and therefore to investigate the pathways involved in regulating normal organogenesis as well as providing models for developmental disorders. Here we describe a system for the in vitro culture of the dorsal pancreatic buds and liver buds from mouse embryos. The tissues are dissected from day 9.0 or 11.5 mouse embryos. The tissues are placed on fibronectin-coated coverslips in serum-containing medium and allowed to attach. Over the next few days, the buds grow as flattened structures which are thin enough to allow the use of wholemount immunostaining methods.

Contributions of null and hypomorphic alleles of Apc in mice produce both developmental and pathophysiological phenotypes. To ascribe the resulting genotype-to-phenotype relationship unambiguously to the Wnt/β-catenin pathway, we challenged the allele combinations by genetically restricting intracellular β-catenin expression in the corresponding compound mutant mice. Subsequent evaluation of the extent of resulting Tcf4-reporter activity in mouse embryo fibroblasts enabled genetic measurement of Wnt/β-catenin signaling in the form of an allelic series of mouse mutants. Different permissive Wnt signaling thresholds appear to be required for the embryonic development of head structures, adult intestinal polyposis, hepatocellular carcinomas, liver zonation, and the development of natural killer cells. Furthermore, we identify a homozygous Apc allele combination with Wnt/β-catenin signaling capacity similar to that in the germline of the Apcmin mice, where somatic Apc loss-of-heterozygosity triggers intestinal polyposis, to distinguish whether co-morbidities in Apcmin mice arise independently of intestinal tumorigenesis. Together, the present genotype–phenotype analysis suggests tissue-specific response levels for the Wnt/β-catenin pathway that regulate both physiological and pathophysiological conditions., Author Summary Germline or somatic mutations in genes are the underlying cause of many human diseases, most notably cancer. Interestingly though, even in situations where every cell of every tissue of an organism carries the same mutation (as is the case for germline mutations), some tissues are more susceptible to the development of disease over time than others. For example, in familial adenomatous polyposis (FAP), affected persons carry different germline mutations in the APC gene and are prone to developing cancers of the colon and the rectum—and, less frequently, cancers in other tissues such as stomach, liver, and bones. Here we utilize a panel of mutant mice with truncating or hypomorphic mutations in the Apc gene, resulting in different levels of activation of the Wnt/β-catenin pathway. Our results reveal that different pathophysiological outcomes depend on different permissive signaling thresholds in embryonic, intestinal, and liver tissues. Importantly, we demonstrate that reducing Wnt pathway activation by 50% is enough to prevent the manifestation of embryonic abnormalities and disease in the adult mouse. This raises the possibility of developing therapeutic strategies that modulate the activation levels of this pathway rather than trying to “repair” the mutation in the gene itself.

Transdifferentiation is defined as the conversion of one cell type to another. One well-documented example of transdifferentiation is the conversion of pancreatic cells to hepatocytes. Here we describe a robust in vitro model to study pancreas to liver transdifferentiation. It is based on the addition of the synthetic glucocorticoid dexamethasone to the rat pancreatic exocrine cell line AR42J. Following glucocorticoid treatment, cells resembling hepatocytes are induced. Transdifferentiated hepatocytes express many of the properties of bona fide hepatocytes, e.g. production of albumin and ability to respond to xenobiotics. These hepatocytes can be used for studying liver function in vitro as well as studying the molecular basis of transdifferentiation.

Culturing embryonic tissue in an in vitro setting offers the unique ability to manipulate the external medium and therefore to investigate the pathways involved in regulating normal organogenesis as well as providing models for developmental disorders. Here we describe a system for the in vitro culture of the dorsal pancreatic buds and liver buds from mouse embryos. The tissues are dissected from day 9.0 or 11.5 mouse embryos. The tissues are placed on fibronectin-coated coverslips in serum-containing medium and allowed to attach. Over the next few days, the buds grow as flattened structures which are thin enough to allow the use of wholemount immunostaining methods.

The liver contains two systems for the removal of ammonia—the urea cycle and the enzyme glutamine synthetase. These systems are expressed in a complementary fashion in two distinct populations of hepatocytes, referred to as periportal and perivenous cells. One of the unresolved problems in hepatology has been to elucidate the molecular mechanisms responsible for induction and maintenance of the cellular heterogeneity for ammonia detoxification. There is now a potential molecular explanation for the zonation of the urea cycle and glutamine synthetase based on the Wnt/β-catenin pathway[1]. BioEssays 28: 1072–1077, 2006. © 2006 Wiley Periodicals, Inc.

We previously demonstrated that dexamethasone (Dex) induces the transdifferentiation (or conversion) of the pancreatic progenitor cell line AR42J-B13 (B13) to hepatocytes based on the expression of liver proteins. We have extended our original observations to determine: (1) the effects of Dex on pancreatic gene expression; (2) the time course of expression of liver enriched transcription factors during conversion from pancreatic to hepatic phenotype; (3) the functional potential of transdifferentiated hepatocytes; (4) the proliferative capacity of transdifferentiated hepatocytes; and (5) whether ectopic expression of transcription factors can induce the hepatic phenotype in pancreatic B13 cells. The results were as follows. The B13 cell markers amylase, synaptophysin, and neurofilament were lost in transdifferentiated hepatocytes compared to control cells and the liver enriched transcription factors C/EBPbeta and C/EBPalpha were induced first, followed by HNF4alpha and then RXRalpha. Using RT-PCR analysis and immunolocalisation studies, we detected hepatic markers (e.g., apolipoprotein B) in Dex-treated cells. In transdifferentiated hepatocytes albumin was secreted, insulin stimulated lipid deposition and ciprofibrate enhanced the expression of catalase. Proliferation of transdifferentiated hepatocytes is promoted in the presence of HGF and NEAA as indicated by the co-expression of the cell cycle markers cyclin D and phosphohistone H3 with liver proteins. Lastly, ectopic expression of C/EBPalpha or C/EBPbeta in AR42J-B13 cells was sufficient to induce transdifferentiation, based on nuclear localization of HNF4alpha and induction of UDP-glucuronosyltransferase expression. These results indicate that the B13 progenitor cell model is suitable for studying liver function and for understanding the molecular and cellular events that occur during transdifferentiation.

The development of the mammalian pancreas is governed by various signaling processes and by a cascade of gene activation events controlled by different transcription factors. Here we show that the divergent homeodomain transcription factor Prox1 is a novel, crucial regulator of mouse pancreas organogenesis. Loss of Prox1 function severely disrupted epithelial pancreas morphology and hindered pancreatic growth without affecting significantly the genesis of endocrine cells before E11.5. Conversely, the lack of Prox1 activity substantially decreased the formation of islet cell precursors after E13.5, during a period known as the “secondary transition”. Notably, this defect occurred concurrently with an abnormal increment of exocrine cells. Hence, it is possible that Prox1 contributes to the allocation of an adequate supply of islet cells throughout pancreas ontogeny by preventing exocrine cell differentiation of multipotent pancreatic progenitors. Prox1 thus appears to be an essential component of a genetic program destined to produce the cellular complexity of the mammalian pancreas.

Transdifferentiation is defined as the conversion of one cell type to another. It belongs to a wider class of cell type transformations called metaplasias which also includes cases in which stem cells of one tissue type switch to a completely different stem cell. Numerous examples of transdifferentiation exist within the literature. For example, isolated striated muscle of the invertebrate jellyfish (Anthomedusae) has enormous transdifferentiation potential and even functional organs (e.g., tentacles and the feeding organ (manubrium)) can be generated in vitro. In contrast, the potential for transdifferentiation in vertebrates is much reduced, at least under normal (nonpathological) conditions. But despite these limitations, there are some well-documented cases of transdifferentiation occurring in vertebrates. For example, in the newt, the lens of the eye can be formed from the epithelial cells of the iris. Other examples of transdifferentiation include the appearance of hepatic foci in the pancreas, the development of intestinal tissue at the lower end of the oesophagus and the formation of muscle, chondrocytes and neurons from neural precursor cells. Although controversial, recent results also suggest the ability of adult stem cells from different embryological germlayers to produce differentiated cells e.g., mesodermal stem cells forming ecto- or endodermally-derived cell types. This phenomenon may constitute an example of metaplasia. The current review examines in detail some well-documented examples of transdifferentiation, speculates on the potential molecular and cellular mechanisms that underlie the switches in phenotype, together with their significance to organogenesis and regenerative medicine.

In recent years, there have been a number of well-documented examples demonstrating that one cell type can be converted to another. Two such examples are the appearance of ectopic pancreas in the liver and formation of hepatic tissue in the pancreas. The conversion of liver to pancreas raises the intriguing possibility of generating insulin-producing beta cells for therapeutic transplantation into diabetics. There is now a striking addition to the growing list of pancreatic conversions: the formation of pancreatic tissue in the developing biliary system.

Introduction Barrett’s metaplasia (BM) is the main risk factor for oesophageal adenocarcinoma, a cancer which carries a mortality of >50% at 12 months. Refluxate containing gastric and bile acids seems to be causative for inflammation at the lower oesophagus, but it is not known how this induces replacement of stratified squamous epithelium (SSQE) with columnar epithelium at a molecular level.There is likely to be a progenitor cell population replacing denuded epithelium, although the origin of these cells has not been proven. Genes that play a role in gut tissue patterning during embryogenesis have received attention. One such ‘master switch’ our laboratory is investigating encodes the hepatocyte nuclear factor 4 alpha (HNF4α) transcription factor. Methods We optimised an immunohistochemistry protocol for demonstrating HFN4α on formalin-fixed paraffin-embedded slides of human tissue. This protocol was applied to forceps biopsy specimens of normal oesophagus, gastro-oesophageal junction (GOJ), stomach, ileum, colon and BM (UK REC reference: 13/YH/0197). Tissues were examined from at least 3 different patients per anatomical site. Results In healthy tissues, nuclear HNF4α positive immunostaining was demonstrated in stomach, ileum and colonic epithelium, but not in normal SSQE in the oesophagus. At the GOJ, there was clear delineation between HNF4α positive nuclei in the columnar gastric cardia mucosa, and negative HNF4α staining of SSQE. In contrast, the columnar epithelial nuclei in BM were consistently positive. Conclusion HNF4α transcription factor is demonstrable in BM, but not SSQE. We are not aware that this HNF4α gastrointestinal distribution has been previously published. HNF4α is likely to be a key transcription factor in the pathogenesis of BM. Previous work in our laboratory with a mouse explant tissue culture model has shown that another candidate transcription factor responsible for BM (Cdx2) was insufficient to induce an intestinal phenotype, whereas HNF4α induced villin, K18, trefoil factor 3 and mucin 5AC. We propose a 2-hit hypothesis for the development of BM: induction of HNF4α (which initially converts the oesophageal SSQE to columnar epithelium) and Cdx2 (which causes intestinalisation of the columnar epithelium). Demonstration of HNF4α in BM but not SSQE is supportive of this theory. Disclosure of Interest None Declared.

Background and Aims The Wnt pathway has previously been shown to play a role in hepatic zonation. Herein, we have explored the role of 3 key components (Apc, β-catenin, and c-Myc) of the Wnt pathway in the zonation of ammonia metabolizing enzymes. Methods Conditional deletion of Apc, β-catenin , and c-Myc was induced in the livers of mice and the expression of periportal and perivenous hepatocyte markers was determined by polymerase chain reaction, Western blotting, and immunohistochemical techniques. Results Under normal circumstances, the urea cycle enzyme carbamoylphosphate synthetase I (CPS I) is present in the periportal, intermediate, and the first few layers of the perivenous zone. In contrast, glutamine synthetase (GS)—and nuclear β-catenin—is expressed in a complementary fashion in the last 1–2 cell layers of the perivenous zone. Conditional loss of Apc resulted in the expression of nuclear β-catenin and GS in most hepatocytes irrespective of zone. Induction of GS in hepatocytes outside the normal perivenous zone was accompanied by a reduction in the expression of CPS I. Deletion of β-catenin induces a loss of GS and a complementary increase in expression of CPS I irrespective of whether Apc is present. Remarkably, deletion of c-Myc did not perturb the pattern of zonation. Conclusions It has been shown that the Wnt pathway is key to imposing the pattern of zonation within the liver. Herein we have addressed the relevance of 3 major Wnt pathway components and show critically that the zonation is c-Myc independent but β-catenin dependent.