Molecular basis of vertebrate endoderm development

How to make an intestine

With the high prevalence of gastrointestinal disorders, there is great interest in establishing in vitro models of human intestinal disease and in developing drug-screening platforms that more accurately represent the complex physiology of the intestine. We will review how recent advances in developmental and stem cell biology have made it possible to generate complex, three-dimensional, human intestinal tissues in vitro through directed differentiation of human pluripotent stem cells. These are currently being used to study human development, genetic forms of disease, intestinal pathogens, metabolic disease and cancer.

Role of GATA factors in development, differentiation, and homeostasis of the small intestinal epithelium

The small intestinal epithelium develops from embryonic endoderm into a highly specialized layer of cells perfectly suited for the digestion and absorption of nutrients. The development, differentiation, and regeneration of the small intestinal epithelium require complex gene regulatory networks involving multiple context-specific transcription factors. The evolutionarily conserved GATA family of transcription factors, well known for its role in hematopoiesis, is essential for the development of endoderm during embryogenesis and the renewal of the differentiated epithelium in the mature gut. We review the role of GATA factors in the evolution and development of endoderm and summarize our current understanding of the function of GATA factors in the mature small intestine. We offer perspective on the application of epigenetics approaches to define the mechanisms underlying context-specific GATA gene regulation during intestinal development.

Generation of β cells from human pluripotent stem cells: are we there yet?

In 1998, the landmark paper describing the isolation and culture of human embryonic stem cells (ESCs) was published. Since that time, the main goal of many diabetes researchers has been to derive β cells from ESCs as a renewable cell-based therapy for the treatment of patients with diabetes. In working toward this goal, numerous protocols that attempt to recapitulate normal pancreatic development have been published that result in the formation of pancreatic cell types from human pluripotent cells. This review examines stem cell differentiation methods and places them within the context of pancreatic development. We additionally compare strategies that are currently being used to generate pancreatic cell types and contrast them with approaches that have been used to generate functional cell types in different lineages. In doing this, we aim to identify how new approaches might be used to improve yield and functionality of in vitro-derived pancreatic β cells as an eventual cell-based therapy for type 1 diabetes.

Concise review: clinical programs of stem cell therapies for liver and pancreas

Regenerative medicine is transitioning into clinical programs using stem/progenitor cell therapies for repair of damaged organs. We summarize those for liver and pancreas, organs that share endodermal stem cell populations, biliary tree stem cells (hBTSCs), located in peribiliary glands. They are precursors to hepatic stem/progenitors in canals of Hering and to committed progenitors in pancreatic duct glands. They give rise to maturational lineages along a radial axis within bile duct walls and a proximal-to-distal axis starting at the duodenum and ending with mature cells in the liver or pancreas. Clinical trials have been ongoing for years assessing effects of determined stem cells (fetal-liver-derived hepatic stem/progenitors) transplanted into the hepatic artery of patients with various liver diseases. Immunosuppression was not required. Control subjects, those given standard of care for a given condition, all died within a year or deteriorated in their liver functions. Subjects transplanted with 100-150 million hepatic stem/progenitor cells had improved liver functions and survival extending for several years. Full evaluations of safety and efficacy of transplants are still in progress. Determined stem cell therapies for diabetes using hBTSCs remain to be explored but are likely to occur following ongoing preclinical studies. In addition, mesenchymal stem cells (MSCs) and hematopoietic stem cells (HSCs) are being used for patients with chronic liver conditions or with diabetes. MSCs have demonstrated significant effects through paracrine signaling of trophic and immunomodulatory factors, and there is limited evidence for inefficient lineage restriction into mature parenchymal or islet cells. HSCs' effects are primarily via modulation of immune mechanisms.

The harmonies played by TGF-β in stem cell biology

To rejuvenate tissues and/or repair wounds, stem cells must receive extrinsic signals from their surrounding environment and integrate them with their intrinsic abilities to self-renew and differentiate to make tissues. Increasing evidence suggests that the superfamily of transforming growth factor-βs (TGF-βs) constitute integral components in the intercellular crosstalk between stem cells and their microenvironment. In this review, we summarize recent advances in our understanding of TGF-β superfamily functions in embryonic and adult stem cells. We discuss how these pathways help to define the physiological environment where stem cells reside, and how perturbations in the signaling circuitry contribute to cancers.

Different thresholds of Wnt-Frizzled 7 signaling coordinate proliferation, morphogenesis and fate of endoderm progenitor cells

Wnt signaling has multiple dynamic roles during development of the gastrointestinal and respiratory systems. Differential Wnt signaling is thought to be a critical step in Xenopus endoderm patterning such that during late gastrula and early somite stages of embryogenesis, Wnt activity must be suppressed in the anterior to allow the specification of foregut progenitors. However, the foregut endoderm also expresses the Wnt-receptor Frizzled 7 (Fzd7) as well as several Wnt ligands suggesting that the current model may be too simple. In this study, we show that Fzd7 is required to transduce a low level of Wnt signaling that is essential to maintain foregut progenitors. Foregut-specific Fzd7-depletion from the Xenopus foregut resulted in liver and pancreas agenesis. Fzd7-depleted embryos failed to maintain the foregut progenitor marker hhex and exhibited decreased proliferation; in addition the foregut cells were enlarged with a randomized orientation. We show that in the foregut Fzd7 signals via both the Wnt/β-catenin and Wnt/JNK pathways and that different thresholds of Wnt-Fzd7 activity coordinate progenitor cell fate, proliferation and morphogenesis.

Functional evaluation of ES cell-derived endodermal populations reveals differences between Nodal and Activin A-guided differentiation

Embryonic stem (ES) cells hold great promise with respect to their potential to be differentiated into desired cell types. Of interest are organs derived from the definitive endoderm, such as the pancreas and liver, and animal studies have revealed an essential role for Nodal in development of the definitive endoderm. Activin A is a related TGFβ member that acts through many of the same downstream signaling effectors as Nodal and is thought to mimic Nodal activity. Detailed characterization of ES cell-derived endodermal cell types by gene expression analysis in vitro and functional analysis in vivo reveal that, despite their similarity in gene expression, Nodal and Activin-derived endodermal cells exhibit a distinct difference in functional competence following transplantation into the developing mouse embryo. Pdx1-expressing cells arising from the respective endoderm populations exhibit extended differences in their competence to mature into insulin/c-peptide-expressing cells in vivo. Our findings underscore the importance of functional cell-type evaluation during stepwise differentiation of stem cells.

The transcription factor Vox represses endoderm development by interacting with Casanova and Pou2

Endoderm and mesoderm are both formed upon activation of Nodal signaling but how endoderm differentiates from mesoderm is still poorly explored. The sox-related gene casanova (sox32) acts downstream of the Nodal signal, is essential for endoderm development and requires the co-factor Pou2 (Pou5f1, Oct3, Oct4) in this process. Conversely, BMP signals have been shown to inhibit endoderm development by an as yet unexplained mechanism. In a search for Casanova regulators in zebrafish, we identified two of its binding partners as the transcription factors Pou2 and Vox, a member of the Vent group of proteins also involved in the patterning of the gastrula. In overexpression studies we show that vox and/or Vent group genes inhibit the capacity of Casanova to induce endoderm, even in the presence of its co-factor Pou2, and that Vox acts as a repressor in this process. We further show that vox, but not other members of the Vent group, is essential for defining the proper endodermal domain size at gastrulation. In this process, vox acts downstream of BMPs. Cell fate analysis further shows that Vox plays a key role downstream of BMP signals in regulating the capacity of Nodal to induce endoderm versus mesoderm by modulating the activity of the Casanova/Pou2 regulatory system.

Klf4 is required for germ-layer differentiation and body axis patterning during Xenopus embryogenesis

Klf4 is a transcription factor of the family of Kruppel-like factors and plays important roles in stem cell biology; however, its function during embryogenesis is unknown. Here, we report the characterization of a Klf4 homologue in Xenopus laevis during embryogenesis. Klf4 is transcribed both maternally and zygotically and the transcript is ubiquitous in embryos during germ-layer formation. Klf4 promotes endoderm differentiation in both Nodal/Activin-dependent and -independent manners. Moreover, Klf4 regulates anteroposterior body axis patterning via activation of a subset of genes in the Spemann organizer, such as Noggin, Dkk1 and Cerberus, which encode Nodal, Wnt and BMP antagonists. Loss of Klf4 function leads to the failure of germ-layer differentiation, the loss of responsiveness of early embryonic cells to inducing signals, e.g. Nodal/Activin, and the loss of transcription of genes involved in axis patterning. We conclude that Klf4 is required for germ-layer differentiation and body axis patterning by means of rendering early embryonic cells competent to differentiation signals.

Transcriptional networks in liver and intestinal development

The development of the gastrointestinal tract is a complex process that integrates signaling processes with downstream transcriptional responses. Here, we discuss the regionalization of the primitive gut and formation of the intestine and liver. Anterior-posterior position in the primitive gut is important for establishing regions that will become functional organs. Coordination of signaling between the epithelium and mesenchyme and downstream transcriptional responses is required for intestinal development and homeostasis. Liver development uses a complex transcriptional network that controls the establishment of organ domains, cell differentiation, and adult function. Discussion of these transcriptional mechanisms gives us insight into how the primitive gut, composed of simple endodermal cells, develops into multiple diverse cell types that are organized into complex mature organs.

Maturation of human embryonic stem cell-derived pancreatic progenitors into functional islets capable of treating pre-existing diabetes in mice

Diabetes is a chronic debilitating disease that results from insufficient production of insulin from pancreatic β-cells. Islet cell replacement can effectively treat diabetes but is currently severely limited by the reliance upon cadaveric donor tissue. We have developed a protocol to efficiently differentiate commercially available human embryonic stem cells (hESCs) in vitro into a highly enriched PDX1+ pancreatic progenitor cell population that further develops in vivo to mature pancreatic endocrine cells. Immature pancreatic precursor cells were transplanted into immunodeficient mice with streptozotocin-induced diabetes, and glycemia was initially controlled with exogenous insulin. As graft-derived insulin levels increased over time, diabetic mice were weaned from exogenous insulin and human C-peptide secretion was eventually regulated by meal and glucose challenges. Similar differentiation of pancreatic precursor cells was observed after transplant in immunodeficient rats. Throughout the in vivo maturation period hESC-derived endocrine cells exhibited gene and protein expression profiles that were remarkably similar to the developing human fetal pancreas. Our findings support the feasibility of using differentiated hESCs as an alternative to cadaveric islets for treating patients with diabetes.

Molecular pathways controlling pancreas induction

Recent advances in generating pancreatic cell types from human pluripotent stem cells has depended on our knowledge of the developmental processes that regulate pancreas development in vivo. The developmental events between gastrulation and formation of the embryonic pancreatic primordia are both rapid and dynamic and studies in frog, fish, chick, and mouse have identified the molecular basis of how the pancreas develops from multipotent endoderm progenitors. Here, we review the current status of our understanding of molecular mechanisms that control endoderm formation, endoderm patterning, and pancreas specification and highlight how these discoveries have allowed for the development of robust methods to generate pancreatic cells from human pluripotent stem cells.

Efficient generation of functional hepatocyte-like cells from human fetal hepatic progenitor cells in vitro

Differentiation of human hepatic progenitor cells to functional hepatocytes holds great potential to develop new therapeutic strategies for liver disease and to provide a platform for drug toxicity screens and identification of novel pharmaceuticals. We report here that human fetal hepatic progenitor cells (hFHPCs) efficiently differentiate to hepatocyte-like cells by continuous exposure to a combination of soluble factors for 7 days in vitro. We compared the effect of hepatocyte growth factor (HGF), oncostatin M (OSM), dexamethasone (DEX), or a combination on the expression of a liver-specific marker, albumin (ALB). Real-time RT-PCR analysis showed that, upon exposure to a combination of OSM, DEX, and HGF, the expression of ALB gradually increased in a time-dependent manner. In contrast, the level of the hepatic progenitor cell marker alpha-fetoprotein (AFP) decreased as differentiation progressed. Moreover, cells exposed to the combination of OSM, DEX, and HGF gradually featured highly differentiated hepatic functions, including ALB secretion, glycogen storage, urea production, and cytochrome P450 (CYP) activity. The effect of these factors on the differentiation of hFHPCs may be blocked by U0126, an inhibitor of the ERK1/2 signaling pathway. In conclusion, we demonstrate that a combination of soluble factors facilitates the efficient generation of highly differentiated hepatocyte-like cells from hFHPCs and ERK1/2 signaling pathway involved in this process. Results suggest that this system will be useful for generating functional hepatocytes and, hence, may serve as a cell source suitable for preclinical pharmacological research and testing.

Generating intestinal tissue from stem cells: potential for research and therapy

Intestinal resection and malformations in adult and pediatric patients result in devastating consequences. Unfortunately, allogeneic transplantation of intestinal tissue into patients has not been met with the same measure of success as the transplantation of other organs. Attempts to engineer intestinal tissue in vitro include disaggregation of adult rat intestine into subunits called organoids, harvesting native adult stem cells from mouse intestine and spontaneous generation of intestinal tissue from embryoid bodies. Recently, by utilizing principles gained from the study of developmental biology, human pluripotent stem cells have been demonstrated to be capable of directed differentiation into intestinal tissue in vitro. Pluripotent stem cells offer a unique and promising means to generate intestinal tissue for the purposes of modeling intestinal disease, understanding embryonic development and providing a source of material for therapeutic transplantation.

Multiple roles of the epithelium-specific ETS transcription factor, ESE-1, in development and disease

The E26 transformation-specific (ETS) family of transcription factors comprises of 27 and 26 members in humans and mice, respectively, which are known to regulate many different biological processes, including cell proliferation, cell differentiation, embryonic development, neoplasia, hematopoiesis, angiogenesis, and inflammation. The epithelium-specific ETS transcription factor-1 (ESE-1) is a physiologically important ETS transcription factor, which has been shown to play a role in the pathogenesis of various diseases, and was originally characterized as having an epithelial-restricted expression pattern, thus placing it within the epithelium-specific ETS subfamily. Despite a large body of published work on ETS biology, much remains to be learned about the precise functions of ESE-1 and other epithelium-specific ETS factors in regulating diverse disease processes. Clues as to the specific function of ESE-1 in the setting of various diseases can be obtained from studies aimed at examining the expression of putative target genes regulated by ESE-1. Thus, this review will focus primarily on the various roles of ESE-1 in different pathophysiological processes, including regulation of epithelial cell differentiation during both intestinal development and lung regeneration; regulation of dendritic cell-driven T-cell differentiation during allergic airway inflammation; regulation of mammary gland development and breast cancer; and regulation of the effects of inflammatory stimuli within the setting of synovial joint and vascular inflammation. Understanding the exact mechanisms by which ESE-1 regulates these processes can have important implications for the treatment of a wide range of diseases.

A molecular signature for purified definitive endoderm guides differentiation and isolation of endoderm from mouse and human embryonic stem cells

Embryonic definitive endoderm (DE) generates the epithelial compartment of vital organs such as liver, pancreas, and intestine. However, purification of DE in mammals has not been achieved, limiting the molecular "definition" of endoderm, and hindering our understanding of DE development and attempts to produce endoderm from sources such as embryonic stem (ES) cells. Here, we describe purification of mouse DE using fluorescence-activated cell sorting (FACS) and mice harboring a transgene encoding enhanced green fluorescent protein (eGFP) inserted into the Sox17 locus, which is expressed in the embryonic endoderm. Comparison of patterns of signaling pathway activation in native mouse DE and endoderm-like cells generated from ES cells produced novel culture modifications that generated Sox17-eGFP⁺ progeny whose gene expression resembled DE more closely than achieved with standard methods. These studies also produced new FACS methods for purifying DE from nontransgenic mice and mouse ES cell cultures. Parallel studies of a new human SOX17-eGFP ES cell line allowed analysis of endoderm differentiation in vitro, leading to culture modifications that enhanced expression of an endoderm-like signature. This work should accelerate our understanding of mechanisms regulating DE development in mice and humans, and guide further use of ES cells for tissue replacement.

Differential regulation of epiboly initiation and progression by zebrafish Eomesodermin A

The T-box transcription factor Eomesodermin (Eomes) has been implicated in patterning and morphogenesis in frog, fish and mouse. In zebrafish, one of the two Eomes homologs, Eomesa, has been implicated in dorsal-ventral patterning, epiboly and endoderm specification in experiments employing over-expression, dominant-negative constructs and antisense morpholino oligonucleotides. Here we report for the first time the identification and characterization of an Eomesa mutant generated by TILLING. We find that Eomesa has a strictly maternal role in the initiation of epiboly, which involves doming of the yolk cell up into the overlying blastoderm. By contrast, epiboly progression is normal, demonstrating for the first time that epiboly initiation is genetically separable from progression. The yolk cell microtubules, which are required for epiboly, are defective in maternal-zygotic eomesa mutant embryos. In addition, the deep cells of the blastoderm are more tightly packed and exhibit more bleb-like protrusions than cells in control embryos. We postulate that the doming delay may be the consequence both of overly stabilized yolk cell microtubules and defects in the adhesive properties or motility of deep cells. We also show that Eomesa is required for normal expression of the endoderm markers sox32, bon and og9x; however it is not essential for endoderm formation.

Comparison of hepatic-like cell production from human embryonic stem cells and adult liver progenitor cells: CAR transduction activates a battery of detoxification genes

In vitro production of human hepatocytes is of primary importance in basic research, pharmacotoxicology and biotherapy of liver diseases. We have developed a protocol of differentiation of human embryonic stem cells (ES) towards hepatocyte-like cells (ES-Hep). Using a set of human adult markers including CAAT/enhancer binding protein (C/EBPalpha), hepatocyte nuclear factor 4/7 ratio (HNF4alpha1/HNF4alpha7), cytochrome P450 7A1 (CYP7A1), CYP3A4 and constitutive androstane receptor (CAR), and fetal markers including alpha-fetoprotein, CYP3A7 and glutathione S-transferase P1, we analyzed the expression of a panel of 41 genes in ES-Hep comparatively with human adult primary hepatocytes, adult and fetal liver. The data revealed that after 21 days of differentiation, ES-Hep are representative of fetal hepatocytes at less than 20 weeks of gestation. The glucocorticoid receptor pathway was functional in ES-Hep. Extending protocols of differentiation to 4 weeks did not improve cell maturation. When compared with hepatocyte-like cells derived from adult liver non parenchymal epithelial (NPE) cells (NPE-Hep), ES-Hep expressed several adult and fetal liver makers at much greater levels (at least one order of magnitude), consistent with greater expression of liver-enriched transcription factors Forkhead box A2, C/EBPalpha, HNF4alpha and HNF6. It therefore seems that ES-Hep reach a better level of differentiation than NPE-Hep and that these cells use different lineage pathways towards the hepatic phenotype. Finally we showed that lentivirus-mediated expression of xenoreceptor CAR in ES-Hep induced the expression of several detoxification genes including CYP2B6, CYP2C9, CYP3A4, UDP-glycosyltransferase 1A1, solute carriers 21A6, as well as biotransformation of midazolam, a CYP3A4-specific substrate.

An essential role for transcription before the MBT in Xenopus laevis

Most zygotic genes remain transcriptionally silent in Drosophila, Xenopus, and zebrafish embryos through multiple mitotic divisions until the midblastula transition (MBT). Several genes have been identified in each of these organisms that are transcribed before the MBT, but whether precocious expression of specific mRNAs is important for later development has not been examined in detail. Here, we identify a class of protein coding transcripts activated before the MBT by the maternal T-box factor VegT that are components of an established transcriptional regulatory network required for mesendoderm induction in Xenopus laevis, including the Nodal related ligands xnr5, xnr6, and derrière and the transcription factors bix4, and sox17α. Accumulation of phospho-Smad2, a hallmark of active Nodal signaling, at the onset of the MBT requires preMBT transcription and activity of xnr5 and xnr6. Furthermore, preMBT activation of the Nodal pathway is essential for mesendodermal gene expression and patterning of the embryo. Finally, xnr5 and xnr6 can also activate their own expression during cleavage stages, indicating that preMBT transcription contributes to a feed-forward system that allows robust activation of Nodal signaling at the MBT.

The conserved role and divergent regulation of foxa, a pan-eumetazoan developmental regulatory gene

Foxa is a forkhead transcription factor that is expressed in the endoderm lineage across metazoans. Orthologs of foxa are expressed in cells that intercalate, polarize, and form tight junctions in the digestive tracts of the mouse, the sea urchin, and the nematode and in the chordate notochord. The loss of foxa expression eliminates these morphogenetic processes. The remarkable similarity in foxa phenotypes in these diverse organisms raises the following questions: why is the developmental role of Foxa so highly conserved? Is foxa transcriptional regulation as conserved as its developmental role? Comparison of the regulation of foxa orthologs in sea urchin and in Caenorhabditis elegans shows that foxa transcriptional regulation has diverged significantly between these two organisms, particularly in the cells that contribute to the C. elegans pharynx formation. We suggest that the similarity of foxa phenotype is due to its role in an ancestral gene regulatory network that controlled intercalation followed by mesenchymal-to-epithelial transition. foxa transcriptional regulation had evolved to support the developmental program in each species so foxa would play its role controlling morphogenesis at the necessary embryonic address.

Directed hepatic differentiation from embryonic stem cells

The liver is the largest internal organ in mammals, and is important for the maintenance of normal physiological functions of other tissues and organs. Hepatitis, cirrhosis, liver cancer and other chronic liver diseases are serious threats to human health, and these problems are compounded by a scarcity of liver donors for transplantation therapies. Directed differentiation of embryonic stem cells to liver cells is a promising strategy for obtaining hepatocytes that can be used for cell transplantation. In vitro hepatocyte differentiation of embryonic stem cells requires a profound understanding of normal development during embryonic hepatogenesis. Here we provide a simple description of hepatogenesis in vivo and discuss directed differentiation of embryonic stem cells into hepatocytes in vitro.

Vertebrate intestinal endoderm development

The endoderm gives rise to the lining of the esophagus, stomach and intestines, as well as associated organs. To generate a functional intestine, a series of highly orchestrated developmental processes must occur. In this review, we attempt to cover major events during intestinal development from gastrulation to birth, including endoderm formation, gut tube growth and patterning, intestinal morphogenesis, epithelial reorganization, villus emergence, as well as proliferation and cytodifferentiation. Our discussion includes morphological and anatomical changes during intestinal development as well as molecular mechanisms regulating these processes.

NO points to epigenetics in vascular development

Our understanding of epigenetic mechanisms important for embryonic vascular development and cardiovascular differentiation is still in its infancy. Although molecular analyses, including massive genome sequencing and/or in vitro/in vivo targeting of specific gene sets, has led to the identification of multiple factors involved in stemness maintenance or in the early processes of embryonic layers specification, very little is known about the epigenetic commitment to cardiovascular lineages. The object of this review will be to outline the state of the art in this field and trace the perspective therapeutic consequences of studies aimed at elucidating fundamental epigenetic networks. Special attention will be paid to the emerging role of nitric oxide in this field.

Microarray analysis of cutaneous squamous cell carcinomas reveals enhanced expression of epidermal differentiation complex genes

Gene expression profiles were determined for 12 cutaneous squamous cell carcinomas (SCC) removed from sun-exposed sites on nonimmunosuppressed patients. Gene expression in each SCC was compared to that in sun-exposed skin from the same patient using the Affymetrix HGU133 2.0 PlusGeneChip. We identified 440 genes with increased expression in SCC and 738 with decreased expression; overall we identified a large number of small changes in gene expression rather than a few marked changes that distinguished SCC from sun-exposed skin. Analyzing this robust data set according to biofunctional pathways using DAVID, transcriptional control elements using oPOSSUM, and chromosomal location using GSEA suggested genetic and epigenetic mechanisms of gene expression regulation in SCC. Some altered patterns of gene expression in SCC were consistent with regulation of spatially separated genes by a number of developmentally important transcription factors (forkhead, HMG, and homeo factors) that negatively regulated gene expression and to a few factors that positively regulated expression (Creb-1, NFkappaB, RelA, and Sp-1). We also found that coordinately enhanced expression of epidermal differentiation complex genes on chromosome 1q21 was a hallmark of SCC. A novel finding in our study was enhanced expression of keratin 13 in SCC, a result validated by immunohistochemical staining of an SCC tumor tissue array.

Transdifferentiation of adipose-derived stem cells into hepatocytes: a new approach

BACKGROUND

Several studies have demonstrated techniques in differentiating human adipose-derived stem cells (hADSCs) into hepatocytes. Unfortunately, transdifferentiation is inefficient, and the function of these induced hepatocyte-like cells (which we termed 'iHeps') is low compared with that of real hepatocytes.

AIMS

We aimed to identify transcriptional deficiencies in iHeps that are critical to hepatocyte development, which may provide insights into improving the efficiency of transdifferentiation.

METHODS

hADSCs were differentiated into iHeps, and iHeps were assayed for hepatocyte-like activity. iHeps were then screened for expression of several growth factors, receptors and transcription factors (TFs) critical to liver development using reverse transcription-polymerase chain reaction (RT-PCR). Deficient TFs were transduced into hADSCs and hepatocyte function was reassessed after hepatic differentiation.

RESULTS

Differentiation of hADSCs into iHeps resulted in the upregulation of hepatic proteins. However, the levels of expression of hepatocyte-specific proteins in these iHeps were well below those of Huh 7.5 hepatoma cells, used in comparison. Five developmental TFs were notably absent on the RT-PCR screen. Lentiviral transduction of these TFs into hADSCs followed by culture in hepatocyte induction medium resulted in increased albumin expression compared with untransduced hADSCs treated in a parallel fashion.

CONCLUSIONS

These five missing TFs are known to regulate hepatocyte differentiation and some are required to establish the competence of the foregut endoderm. Presumably due to their mesenchymal lineage, hADSCs do not express these endodermal TFs and are not fully competent to respond to critical developmental signals. Supplementation of these TFs may induce competency and enhance the differentiation of hADSCs into hepatocytes.

Complexity of cis-regulatory organization of six3a during forebrain and eye development in zebrafish

BACKGROUND

Six3a belongs to the SIX family of homeodomain proteins and is expressed in the most anterior neural plate at the beginning of neurogenesis in various species. Though the function of Six3a as a crucial regulator of eye and forebrain development has been thoroughly investigated, the transcriptional regulation of six3a is not well understood.

RESULTS

To elucidate the transcriptional regulation of six3a, we performed an in vivo reporter assay. Alignment of the 21-kb region surrounding the zebrafish six3a gene with the analogous region from different species identified several conserved non-coding modules. Transgenesis in zebrafish identified two enhancer elements and one suppressor. The D module drives the GFP reporter in the forebrain and eyes at an early stage, while the A module is responsible for the later expression. The A module also works as a repressor suppressing ectopic expression from the D module. Mutational analysis further minimized the A module to four highly conserved elements and the D module to three elements. Using electrophoresis mobility shift assays, we also provided evidence for the presence of DNA-binding proteins in embryonic nuclear extracts. The transcription factors that may occupy those highly conserved elements were also predicted.

CONCLUSION

This study provides a comprehensive view of six3a transcription regulation during brain and eye development and offers an opportunity to establish the gene regulatory networks underlying neurogenesis in zebrafish.

Converting human pluripotent stem cells into beta-cells: recent advances and future challenges

PURPOSE OF REVIEW

The transplantation of insulin-producing beta-cells derived from human embryonic stem cells and induced pluripotent stem cells (collectively termed pluripotent stem cells or PSCs) holds great promise for therapy of diabetes mellitus. The purpose of this review is to summarize recent advances in this area, emphasizing the importance of studies of endocrine pancreas development in efforts to direct PSC differentiation into endocrine cells, as well as to outline the major challenges remaining before transplantation of PSC-derived beta-cells can become a reality.

RECENT FINDINGS

Although several protocols to generate glucose-responsive pancreatic beta-cells in vitro have been described, the most successful approaches are those that most closely mimic embryonic development of the endocrine pancreas. Until recently, cells generated by these methods have exhibited immature pancreatic endocrine phenotypes. However, protocols that generate more functional beta-like cells have now been described. In addition, small molecules are being used to improve protocols to direct differentiation of PSCs into endoderm and pancreatic lineages.

SUMMARY

Advances over the last decade suggest that generating functional beta-cells from human PSCs is achievable. However, there are aspects of beta-cell development that are not well understood and are hampering generation of PSC-derived beta-cells. In particular, the signaling pathways that instruct endocrine progenitor cells to differentiate into mature and functional beta-cells are poorly understood. Other significant obstacles remain, including the need for safe and cost-effective differentiation methods for large-scale generation of transplantation quality beta-cells, methods to prevent immune rejection of grafted tissues, and amelioration of the risks of tumorigenesis.

An endoderm-specific transcriptional enhancer from the mouse Gata4 gene requires GATA and homeodomain protein-binding sites for function in vivo

Several transcription factors function in the specification and differentiation of the endoderm, including the zinc finger transcription factor GATA4. Despite its essential role in endoderm development, the transcriptional control of the Gata4 gene in the developing endoderm and its derivatives remains incompletely understood. Here, we identify a distal enhancer from the Gata4 gene, which directs expression exclusively to the visceral and definitive endoderm of transgenic mouse embryos. The activity of this enhancer is initially broad within the definitive endoderm but later restricts to developing endoderm-derived tissues, including pancreas, glandular stomach, and duodenum. The activity of this enhancer in vivo is dependent on evolutionarily-conserved HOX- and GATA-binding sites, which are bound by PDX-1 and GATA4, respectively. These studies establish Gata4 as a direct transcriptional target of homeodomain and GATA transcription factors in the endoderm and support a model in which GATA4 functions in the transcriptional network for pancreas formation.

Molecular regulation of pancreas development in zebrafish

The pancreas is a vertebrate-specific organ of endodermal origin which is responsible for production of digestive enzymes and hormones involved in regulating glucose homeostasis, in particular insulin, deficiency of which results in diabetes. Basic research on the genetic and molecular pathways regulating pancreas formation and function has gained major importance for the development of regenerative medical approaches aimed at improving diabetes treatment. Among the different model organisms that are currently used to elucidate the basic pathways of pancreas development and regeneration, the zebrafish is distinguished by its unique opportunities to combine genetic and pharmacological approaches with sophisticated live-imaging methodology, and by its ability to regenerate the pancreas within a short time. Here we review current perspectives and present methods for studying two important processes contributing to pancreas development and regeneration, namely cell migration via time-lapse micropscopy and cell proliferation via incorporation of nucleotide analog EdU, with a focus on the insulin-producing beta cells of the islet.

Thyroid stem cells and cancer

BACKGROUND

Thyroid gland development and function are essential for life, and recent findings indicate the presence of stem/progenitor cells within the thyroid gland as a potential source of tissue regeneration and cancer formation.

SUMMARY

This review summarizes the current knowledge on early differentiation of thyroid cells from embryonic stem cells and highlights exciting concepts and recent novel findings on adult thyroid stem/progenitor cells in the normal thyroid gland and in thyroid cancer. Other potential sources and markers of stem/progenitor cells in the thyroid include bone marrow, microchimerism, and embryological remnant-derived multifocal solid cell nests. Finally, we discuss new therapeutic strategies that target thyroid cancer stem cells.

CONCLUSIONS

Thyroid stem/progenitor cell populations are present in the normal and diseased thyroid gland. Advances in normal and cancer thyroid stem cell biology will be essential for future targeted therapies.

FACS-assisted microarray profiling implicates novel genes and pathways in zebrafish gastrointestinal tract development

BACKGROUND & AIMS

The zebrafish Danio rerio is an excellent model system for mammalian gastrointestinal development. To identify differentially regulated genes important in gastrointestinal organogenesis, we profiled the transcriptome of the zebrafish developing gastrointestinal tract.

METHODS

Embryos from a transgenic zebrafish line expressing green fluorescent protein (GFP) in the developing intestine, liver, and pancreas were dissociated at 4 developmental time points, their cells sorted based on GFP expression with fluorescence-activated cell sorting (FACS), and analyzed with microarrays. To improve our analysis, we annotated the Affymetrix Zebrafish GeneChip with human orthologs.

RESULTS

Transcriptional profiling showed significant differences between GFP(+) and GFP(-) cells. Up-regulated genes and pathways were consistent with mammalian gastrointestinal development, such as hepatic nuclear factor gene networks and cancer. We implicate the phosphatidylinositol 3 kinase (PI3K) pathway and show that inhibition with LY294002 causes gastrointestinal defects in zebrafish. We identified novel genes, such as the microRNAs miR-217 and miR-122, the tight junction protein claudin c, the gene fam136a, and a zebrafish tetraspanin. Novel pathways include genes containing a putative transcription factor binding sequence, GGAANCGGAANY, and a nucleolar gene network. The zebrafish microarrays also identify a set of 32 genes that may mediate the effects of gain of chromosome arm 8q in human colon, liver, and pancreatic cancers.

CONCLUSIONS

We successfully combine FACS and microarray profiling to follow organogenesis throughout development. These experiments identify novel genes and pathways that probably play a role in mammalian gastrointestinal development and are potential targets for therapeutic intervention in the management of gastrointestinal disease and cancer.

Xenopus insm1 is essential for gastrointestinal and pancreatic endocrine cell development

In mammals, it has been well established that gastrointestinal and pancreatic endocrine cells are specified by a cascade of different transcription factors, but whether these same pathways (or linear relationships) operate in Xenopus is currently unknown. We recently identified the endocrine-specific zinc finger transcription factor insulinoma associated protein 1 (insm1) as a dorsal-enriched gene. We found that insm1 is expressed in the dorsal pancreas as early as NF28, making it one of the earliest markers to be localized to the dorsal pancreas. Through morpholino-mediated knockdown, we demonstrate that insm1 is essential for proper specification of both gastrointestinal and pancreatic endocrine cells. In addition, we place insm1 downstream of ngn3 and upstream of pax6 and neuroD in the endocrine cell transcription factor cascade. These are the first results showing that the endodermal endocrine cell development in Xenopus uses the same transcriptional cascade as in mammals.

Progress and future challenges in stem cell-derived liver technologies

The emergence of regenerative medicine has led to significant advances in the identification and understanding of human stem cells and adult progenitor cells. Both cell populations exhibit plasticity and theoretically offer a potential source of somatic cells in large numbers. Such a resource has an important role to play in the understanding of human development, in modeling human disease and drug toxicity, and in the generation of somatic cells in large numbers for cell-based therapies. Presently, liver transplantation is the only effective treatment for end-stage liver disease. Although this procedure can be carried out with high levels of success, the routine transplant of livers is severely limited by organ donor availability. As a result, attention has focused on the ability to restore liver mass and function by alternative approaches ranging from the bioartificial device to transplantation of human hepatocytes. In this review we will focus on the generation of human hepatic endoderm from different stem/progenitor cell populations with a view to its utility in regenerative medicine.

A new mechanism of SOX9 action to regulate PKCalpha expression in the intestine epithelium

Variations of protein kinase C (PKC) expression greatly influence the proliferation-to-differentiation transition (PDT) of intestinal epithelial cells and might have an important impact on intestinal tumorigenesis. We demonstrate here that the expression of PKCalpha in proliferating intestinal epithelial cells is repressed both in vitro and in vivo by the SOX9 transcription factor. This repression does not require DNA binding of the SOX9 high-mobility group (HMG) domain but is mediated through a new mechanism of SOX9 action requiring the central and highly conserved region of SOXE members. Because SOX9 expression is itself upregulated by Wnt-APC signaling in intestinal epithelial cells, the present study points out this transcription factor as a molecular link between the Wnt-APC pathway and PKCalpha. These results provide a potential explanation for the decrease of PKCalpha expression in colorectal cancers with constitutive activation of the Wnt-APC pathway.

Xenopus pancreas development

Understanding how the pancreas develops is vital to finding new treatments for a range of pancreatic diseases, including diabetes and pancreatic cancer. Xenopus is a relatively new model organism for the elucidation of pancreas development, and has already made contributions to the field. Recent studies have shown benefits of using Xenopus for understanding both early patterning and lineage specification aspects of pancreas organogenesis. This review focuses specifically on Xenopus pancreas development, and covers events from the end of gastrulation, when regional specification of the endoderm is occurring, right through metamorphosis, when the mature pancreas is fully formed. We have attempted to cover pancreas development in Xenopus comprehensively enough to assist newcomers to the field and also to enable those studying pancreas development in other model organisms to better place the results from Xenopus research into the context of the field in general and their studies specifically. Developmental Dynamics 238:1271-1286, 2009. (c) 2009 Wiley-Liss, Inc.

In vivo virtual histology of mouse embryogenesis by ultrasound biomicroscopy and magnetic resonance imaging

Feasibility of magnetic resonance imaging (MRI) and ultrasound biomicroscopy (UBM) for sequential in vivo study of mouse embryo development between Days 6.5 and 13.5 of pregnancy was assessed in a first experiment. A second trial, based on the results of the first, determined the accuracy of UBM for imaging morphogenesis from implantation to the late embryo stage (Days 4.5 to 15.5). MRI allowed imaging of the entire uterus and all gestational sacs and embryos inside whilst the small scanning range of UBM precluded accurate counting of fetuses; however, its high resolution identified the decidual reaction at implantation sites from Day 4.5. At later stages, it was possible to assess key morphogenetic processes such as differentiation of the placenta, the cephalic region, the thoracic and abdominal organs, the skeletal system and the limbs, and dynamic structures such as the cardiovascular system. Thus, both techniques are reliable for in utero imaging of mouse embryo development. MRI may be more appropriate for studying embryo lethality and intrauterine growth retardation, because the entire uterus can be viewed. UBM may be more suitable for studies of cellular components of organs and tissues and assessment of haemodynamic changes in the circulatory system.

Tgf-beta superfamily signaling in embryonic development and homeostasis

TGF-beta superfamily signaling pathways emerged with the evolution of multicellular animals, suggesting that these pathways contribute to the increased diversity and complexity required for the development and homeostasis of these organisms. In this review we begin by exploring some key developmental and disease processes requiring TGF-beta ligands to underscore the fundamental importance of these pathways before delving into the molecular mechanism of signal transduction, focusing on recent findings. Finally, we discuss how these ligands act as morphogens, how their activity and signaling range is regulated, and how they interact with other signaling pathways to achieve their specific and varied functional roles.

Control of early fate decisions in human ES cells by distinct states of TGFbeta pathway activity

The mechanisms controlling self-renewal versus lineage commitment in human embryonic stem (hES) cells are not well understood. Nonetheless, current knowledge suggests a crucial role for TGFbeta signaling in controlling these early fate decisions. We have investigated the effects of TGFbeta pathway activation and inhibition on gene expression in hES cells. Our data reveal that SMAD 2/3 signaling directly supports NANOG expression, while SMAD 1/5/8 activation moderately represses SOX2. In addition, genes encoding key developmentally relevant signaling molecules and transcription factors appear to be immediately downstream of SMAD 1/5/8 signaling, or require both SMAD 1/5/8 and 2/3 activation, or inactivation of TGFbeta signaling for their induction. Defined stimulation/inhibition of the two TGFbeta branches appeared to control early fate decisions in accordance with these downstream transcriptional effects. Our results therefore help to better understand how pluripotency is mediated at the transcriptional level.

Vertebrate endoderm development and organ formation

The endoderm germ layer contributes to the respiratory and gastrointestinal tracts and to all of their associated organs. Over the past decade, studies in vertebrate model organisms, including frog, fish, chick, and mouse, have greatly enhanced our understanding of the molecular basis of endoderm organ development. We review this progress with a focus on early stages of endoderm organogenesis including endoderm formation, gut tube morphogenesis and patterning, and organ specification. Lastly, we discuss how developmental mechanisms that regulate endoderm organogenesis are used to direct differentiation of embryonic stem cells into specific adult cell types, which function to alleviate disease symptoms in animal models.

Production of beta-cells from human embryonic stem cells

The making of functional pancreatic islets in renewable numbers has been a goal of stem cell biologists since early 2000. Since that time, many studies have reported successful creation of glucose-responsive pancreatic beta-cells. Not until the more recent systematic application of developmental principles to stem cell biology systems were breakthroughs achieved on directed specification of the required early developmental intermediates. The most important first step is the formation of the definitive endoderm (DE) lineage which is compulsory for production of the epithelium of the pancreas and the other important endoderm-derived organs such as the liver, intestine and lung. The formation of DE from embryonic stem cells made possible additional experimentation aimed at directing the endoderm to further specified foregut and pancreatic endoderm lineages. With these discoveries came the first production of immature pancreatic endocrine cells. Most recently, the production in vivo of glucose-responsive insulin-producing cells with the capacity to correct Steptozotocin-induced hyperglycaemia in mice has been achieved. The work leading up to this achievement, in relation to the other principle human stem cell studies conducted in this area, will be briefly described. The necessary steps and ideal characteristics of embryonic stem cell-based differentiation to pancreatic beta-cells capable of glucose stimulated insulin secretion will be underscored.

Expression of islet-specific microRNAs during human pancreatic development

During pancreatic islet development, sequential changes in gene expression are known to be necessary for efficient differentiation and function of the endocrine pancreas. Several studies till now have demonstrated that microRNAs (miRNAs), which regulate translation of gene transcripts, influence gene expression cascades involved in pancreas development. Some of these miRNAs; miR-7 and miR-375 have been known to be expressed at high levels in pancreas and are also known to be involved in Zebrafish pancreas development as well as insulin secretion in mice. We demonstrate here that 4 different islet-specific microRNAs (miR-7, miR-9, miR-375 and miR-376) are expressed at high levels during human pancreatic islet development. Of these, miR-375, is seen to be differentially expressed in human islet beta- as well as non-beta-cells. Though no significant difference in abundance of miR-375 was noted in either cell type, analysis of islet-specific miRNA and mRNA in single cells show that non-beta cells contain higher levels of miR-375. Our data demonstrate that miRNAs that are known to be regulated during Zebrafish pancreatic development may play similar role in human pancreatic islet development.

Developmental gene regulatory networks in the zebrafish embryo

The genomic developmental program operates mainly through the regulated expression of genes encoding transcription factors and signaling pathways. Complex networks of regulatory genetic interactions control developmental cell specification and fates. Development in the zebrafish, Danio rerio, has been studied extensively and large amounts of experimental data, including information on spatial and temporal gene expression patterns, are available. A wide variety of maternal and zygotic regulatory factors and signaling pathways have been discovered in zebrafish, and these provide a useful starting point for reconstructing the gene regulatory networks (GRNs) underlying development. In this review, we describe in detail the genetic regulatory subcircuits responsible for dorsoanterior-ventroposterior patterning and endoderm formation. We describe a number of regulatory motifs, which appear to act as the functional building blocks of the GRNs. Different positive feedback loops drive the ventral and dorsal specification processes. Mutual exclusivity in dorsal-ventral polarity in zebrafish is governed by intra-cellular cross-inhibiting GRN motifs, including vent/dharma and tll1/chordin. The dorsal-ventral axis seems to be determined by competition between two maternally driven positive-feedback loops (one operating on Dharma, the other on Bmp). This is the first systematic approach aimed at developing an integrated model of the GRNs underlying zebrafish development. Comparison of GRNs' organizational motifs between different species will provide insights into developmental specification and its evolution. The online version of the zebrafish GRNs can be found at http://www.zebrafishGRNs.org.

The core endodermal gene network of vertebrates: combining developmental precision with evolutionary flexibility

Embryonic development combines paradoxical properties: it has great precision, it is usually conducted at breakneck speed and it is flexible on relatively short evolutionary time scales, particularly at early stages. While these features appear mutually exclusive, we consider how they may be reconciled by the properties of key early regulatory networks. We illustrate these ideas with the network that controls development of endoderm progenitors. We argue that this network enables precision because of its intrinsic stability, self propagation and dependence on signalling. The network enables high developmental speed because it is rapidly established by maternal inputs at multiple points. In turn these properties confer flexibility on an evolutionary time scale because they can be initiated in many ways, while buffering essential progenitor cell populations against changes in their embryonic environment on both evolutionary and developmental time scales. Although stable, these networks must be capable of rapid dissolution as cell differentiation progresses. While we focus on the core early endodermal network of vertebrates, we argue that these properties are likely to be general in early embryonic stem cell populations, such as mammalian ES cells.

On the origin of the beta cell

The major forms of diabetes are characterized by pancreatic islet beta-cell dysfunction and decreased beta-cell numbers, raising hope for cell replacement therapy. Although human islet transplantation is a cell-based therapy under clinical investigation for the treatment of type 1 diabetes, the limited availability of human cadaveric islets for transplantation will preclude its widespread therapeutic application. The result has been an intense focus on the development of alternate sources of beta cells, such as through the guided differentiation of stem or precursor cell populations or the transdifferentiation of more plentiful mature cell populations. Realizing the potential for cell-based therapies, however, requires a thorough understanding of pancreas development and beta-cell formation. Pancreas development is coordinated by a complex interplay of signaling pathways and transcription factors that determine early pancreatic specification as well as the later differentiation of exocrine and endocrine lineages. This review describes the current knowledge of these factors as they relate specifically to the emergence of endocrine beta cells from pancreatic endoderm. Current therapeutic efforts to generate insulin-producing beta-like cells from embryonic stem cells have already capitalized on recent advances in our understanding of the embryonic signals and transcription factors that dictate lineage specification and will most certainly be further enhanced by a continuing emphasis on the identification of novel factors and regulatory relationships.