Induction of yolk sac endoderm in GATA-4-deficient embryoid bodies by retinoic acid

Sox7 is dispensable for primitive endoderm differentiation from mouse ES cells

Background

Primitive endoderm is a cell lineage segregated from the epiblast in the blastocyst and gives rise to parietal and visceral endoderm. Sox7 is a member of the SoxF gene family that is specifically expressed in primitive endoderm in the late blastocyst, although its function in this cell lineage remains unclear.

Results

Here we characterize the function of Sox7 in primitive endoderm differentiation using mouse embryonic stem (ES) cells as a model system. We show that ectopic expression of Sox7 in ES cells has a marginal effect on triggering differentiation into primitive endoderm-like cells. We also show that targeted disruption of Sox7 in ES cells does not affect differentiation into primitive endoderm cells in embryoid body formation as well as by forced expression of Gata6.

Conclusions

These data indicate that Sox7 function is supplementary and not essential for this differentiation from ES cells.

Electronic supplementary material

The online version of this article (doi:10.1186/s12861-015-0079-4) contains supplementary material, which is available to authorized users.

Differentiation and selection of hepatocyte precursors in suspension spheroid culture of transgenic murine embryonic stem cells

Embryonic stem cell-derived hepatocyte precursor cells represent a promising model for clinical transplantations to diseased livers, as well as for establishment of in vitro systems for drug metabolism and toxicology investigations. This study aimed to establish an in vitro culture system for scalable generation of hepatic progenitor cells. We used stable transgenic clones of murine embryonic stem cells possessing a reporter/selection vector, in which the enhanced green fluorescent protein- and puromycin N-acetyltransferase-coding genes are driven by a common alpha-fetoprotein gene promoter. This allowed for "live" monitoring and puromycin selection of the desired differentiating cell type possessing the activated alpha-fetoprotein gene. A rotary culture system was established, sequentially yielding initially partially selected hepatocyte lineage-committed cells, and finally, a highly purified cell population maintained as a dynamic suspension spheroid culture, which progressively developed the hepatic gene expression phenotype. The latter was confirmed by quantitative RT-PCR analysis, which showed a progressive up-regulation of hepatic genes during spheroid culture, indicating development of a mixed hepatocyte precursor-/fetal hepatocyte-like cell population. Adherent spheroids gave rise to advanced differentiated hepatocyte-like cells expressing hepatic proteins such as albumin, alpha-1-antitrypsin, cytokeratin 18, E-cadherin, and liver-specific organic anion transporter 1, as demonstrated by fluorescent immunostaining. A fraction of adherent cells was capable of glycogen storage and of reversible up-take of indocyanine green, demonstrating their hepatocyte-like functionality. Moreover, after transplantation of spheroids into the mouse liver, the spheroid-derived cells integrated into recipient. These results demonstrate that large-scale hepatocyte precursor-/hepatocyte-like cultures can be established for use in clinical trials, as well as in in vitro screening assays.

Ablation of Dido3 compromises lineage commitment of stem cells in vitro and during early embryonic development

The death inducer obliterator (Dido) locus encodes three protein isoforms, of which Dido3 is the largest and most broadly expressed. Dido3 is a nuclear protein that forms part of the spindle assembly checkpoint (SAC) and is necessary for correct chromosome segregation in somatic and germ cells. Here we report that specific ablation of Dido3 function in mice causes lethal developmental defects at the onset of gastrulation. Although these defects are associated with centrosome amplification, spindle malformation and a DNA damage response, we provide evidence that embryonic lethality of the Dido3 mutation cannot be explained by its impact on chromosome segregation alone. We show that loss of Dido3 expression compromises differentiation of embryonic stem cells in vitro and of epiblast cells in vivo, resulting in early embryonic death at around day 8.5 of gestation. Close analysis of Dido3 mutant embryoid bodies indicates that ablation of Dido3, rather than producing a generalized differentiation blockade, delays the onset of lineage commitment at the primitive endoderm specification stage. The dual role of Dido3 in chromosome segregation and stem cell differentiation supports the implication of SAC components in stem cell fate decisions.

Ectopic expression of GATA6 bypasses requirement for Grb2 in primitive endoderm formation

Gene knockouts in mice have showed that Grb2 and GATA6 are essential for the formation of primitive endoderm in blastocysts. Here, we confirmed that implanted Grb2-null blastocysts lack primitive or extraembryonic endoderm cells either at E4.5 or E5.5 stages. We analyzed the relationship between Grb2 and GATA6 in the differentiation of embryonic stem (ES) cells to primitive endoderm in embryoid body models. Upon transfection with GATA6 expression vector, Grb2-null ES cells underwent endoderm differentiation as indicated by the expression of the extraembryonic endoderm markers Dab2 and GATA4. Transfection of GATA4 expression vector also had the same differentiation potency. When GATA6- or GATA4-transfected Grb2-null ES cells were allowed to aggregate, fragments of an endoderm layer formed on the surface of the spheroids. The results suggest that GATA6 is downstream of Grb2 in the inductive signaling pathway and the expression of GATA6 is sufficient to compensate for the defects caused by Grb2 deficiency in the development of the primitive and extraembryonic endoderm.

Alteration of Differentiation Potentials by Modulating GATA Transcription Factors in Murine Embryonic Stem Cells

Background. Mouse embryonic stem (ES) cells can be differentiated in vitro by aggregation and/or retinoic acid (RA) treatment. The principal differentiation lineage in vitro is extraembryonic primitive endoderm. Dab2, Laminin, GATA4, GATA5, and GATA6 are expressed in embryonic primitive endoderm and play critical roles in its lineage commitment. Results. We found that in the absence of GATA4 or GATA5, RA-induced primitive endoderm differentiation of ES cells was reduced. GATA4 (-/-) ES cells express higher level of GATA5, GATA6, and hepatocyte nuclear factor 4 alpha marker of visceral endoderm lineage. GATA5 (-/-) ES cells express higher level of alpha fetoprotein marker of early liver development. GATA6 (-/-) ES cells express higher level of GATA5 as well as mesoderm and cardiomyocyte markers which are collagen III alpha-1 and tropomyosin1 alpha. Thus, deletion of GATA6 precluded endoderm differentiation but promoted mesoderm lineages. Conclusions. GATA4, GATA5, and GATA6 each convey a unique gene expression pattern and influences ES cell differentiation. We showed that ES cells can be directed to avoid differentiating into primitive endoderm and to adopt unique lineages in vitro by modulating GATA factors. The finding offers a potential approach to produce desirable cell types from ES cells, useful for regenerative cell therapy.

Regulation of P450c17 expression in the early embryo depends on GATA factors

The enzyme P450c17 is required for glucocorticoid, sex steroid, and some neurosteroid biosynthesis. Defective human P450c17 causes sexual infantilism and 46,XY sex reversal but is compatible with life, whereas ablation of the corresponding mouse gene causes embryonic lethality at around E7. Normal mouse embryos express P450c17 protein and activity in the embryonic endoderm at E7. Adult adrenal and gonadal steroidogenesis requires steroidogenic factor-1 (SF-1), but SF-1 is not expressed in the early mouse embryo. We show that P450c17 is expressed in differentiated mouse parietal and visceral endoderm lineages, in cultured mouse F9 embryonic carcinoma stem cells, in mouse embryonic stem cells, and in cultured mouse P19 stem cells. Bases -110 to -55 (which contain an SF-1 site and two potential GATA sites) of the rat cyp17 gene confer promoter activity in F9 cells. Overexpression of SF-1 has no effect, whereas overexpression of GATA4 in F9 cells increases transcription from -110/-55 fused to a reporter and increases endogenous P450c17 mRNA. Chromatin immunoprecipitation assays show that GATA4 binds to -215/+55 of mouse cyp17. Stimulating F9 cells with retinoic acid and cAMP differentiates them into visceral and parietal endoderm. Commensurate with cell differentiation, quantitative PCR showed increased GATA4 and GATA6 mRNAs, temporally followed by increased P450c17 mRNA. Small interfering RNA inhibition of GATA4 or GATA6 in undifferentiated or differentiated F9 cells diminished endogenous cyp17 expression. Thus, P450c17 is expressed in mouse embryonic stem cells, its expression increases upon differentiation to an early embryonic endoderm lineage, and GATA4/6 are responsible for activation of P450c17 gene expression at this early stage of embryonic development.

Dynamic GATA6 expression in primitive endoderm formation and maturation in early mouse embryogenesis

The derivation of the primitive endoderm layer from the pluripotent cells of the inner cell mass is one of the earliest differentiation and morphogenic events in embryonic development. GATA4 and GATA6 are the key transcription factors in the formation of extraembryonic endoderms, but their specific contribution to the derivation of each endoderm lineage needs clarification. We further analyzed the dynamic expression and mutant phenotypes of GATA6 in early mouse embryos. GATA6 and GATA4 are both expressed in primitive endoderm cells initially. At embryonic day (E) 5.0, parietal endoderm cells continue to express both GATA4 and GATA6; however, visceral endoderm cells express GATA4 but exhibit a reduced expression of GATA6. By and after E5.5, visceral endoderm cells no longer express GATA6. We also found that GATA6 null embryos did not form a morphologically recognizable primitive endoderm layer, and subsequently failed to form visceral and parietal endoderms. Thus, the current study establishes that GATA6 is essential for the formation of primitive endoderm, at a much earlier stage then previously recognized, and expression of GATA6 discriminates parietal endoderm from visceral endoderm lineages.

Scalable selection of hepatocyte- and hepatocyte precursor-like cells from culture of differentiating transgenically modified murine embryonic stem cells

Potential therapeutic applications of embryonic stem cell (ESC)-derived hepatocytes are limited by their relatively low output in differentiating ESC cultures, as well as by the danger of contamination with tumorigenic undifferentiated ESCs. To address these problems, we developed transgenic murine ESC clones possessing bicistronic expression vector that contains the alpha-fetoprotein gene promoter driving a cassette for the enhanced green "live" fluorescent reporter protein (eGFP) and a puromycin resistance gene. Under established culture conditions these clones allowed for both monitoring of differentiation and for puromycin selection of hepatocyte-committed cells in a suspension mass culture of transgenic ESC aggregates ("embryoid bodies" [EBs]). When plated on fibronectin, the selected eGFP-positive cells formed colonies, in which intensely proliferating hepatocyte precursor-like cells gave rise to morphologically differentiated cells expressing alpha-1-antitrypsin, alpha-fetoprotein, and albumin. A number of cells synthesized glycogen and in some of the cells cytokeratin 18 microfilaments were detected. Major hepatocyte marker genes were expressed in the culture, along with the gene and protein expression of stem/progenitor markers, suggesting the features of both hepatocyte precursors and more advanced differentiated cells. When cultured in suspension, the EB-derived puromycin-selected cells formed spheroids capable of outgrowing on an adhesive substrate, resembling the behavior of fetal mouse hepatic progenitor cells. The established system based on the highly efficient selection/purification procedure could be suitable for scalable generation of ESC-derived hepatocyte- and hepatocyte precursor-like cells and offers a potential in vitro source of cells for transplantation therapy of liver diseases, tissue engineering, and drug and toxicology screening.

Cell autonomous sorting and surface positioning in the formation of primitive endoderm in embryoid bodies

The differentiation and formation of the primitive endoderm in early embryos can be mimicked in vitro by the aggregation of embryonic stem cells to form embryoid bodies. We present morphological evidence that primitive endoderm cells often first locate in the interior of embryoid bodies and subsequently migrate to the surface. Cell mixing experiments indicate that surface positioning is an intrinsic property of endoderm epithelial cells. Moreover, Disabled-2 (Dab2) is required for surface sorting and positioning of the endoderm cells: when Dab2 expression was eliminated, the differentiated endoderm epithelial cells distributed throughout the interior of the embryoid bodies. Surprisingly, E-cadherin is dispensable for primitive endoderm differentiation and surface sorting in embryoid bodies. These results support the model that primitive endoderm cells first emerge in the interior of the inner cell mass and are subsequently sorted to the surface to form the primitive endoderm.

Global gene expression profiles associated with retinoic acid-induced differentiation of embryonal carcinoma cells

We have evaluated the effects of retinoic acid (RA) treatment of F9 embryonal carcinoma (EC) cells, which induces differentiation into primitive endoderm, on gene expression patterns. F9 cells were exposed to RA in culture, and global expression patterns were examined with cDNA-based microarrays at early (8 hr) and later times (24 hr) after exposure. Of the 1,176 known transcripts examined, we identified 57 genes (4.8%) that were responsive to RA at 8 and/or 24 hr: 35 were induced, 20 were repressed, and 2 were differentially regulated at these time points. To determine if our results were dependent on the array technology employed, we also evaluated the response to RA at 24 hr with oligonucleotide-based arrays. With these more dense arrays (12,488 genes), we identified an additional 353 RA-regulated genes (2.8%): 173 were upregulated and 180 were downregulated. Thus, a total of 410 genes regulated by RA were identified with roughly equivalent numbers induced or repressed. Although the expression of many genes found on both array platforms was consistent, the results for some genes were disparate. Quantitative PCR studies on a subset of these genes supported the results obtained with the cDNA arrays. Our results confirmed the regulation of several known RA-responsive genes and we also identified a number of genes not previously known to be RA-responsive. Those novel genes that were induced presumably contribute to the cellular processes required for a shift from proliferation to differentiation, whereas those new genes that were downregulated may possibly contribute to the maintenance of cell proliferation.

Perception of differentiation cues by GATA factors in primitive endoderm lineage determination of mouse embryonic stem cells

The formation of the primitive endoderm covering the inner cell mass of early mouse embryos can be simulated in vitro by the differentiation of mouse embryonic stem (ES) cells in culture following either aggregation of suspended cells or stimulation of cell monolayers with retinoic acid. The developmentally regulated transcription factors GATA-4 and GATA-6 have determining role in mouse extraembryonic endoderm development. We analyzed the in vitro differentiation of mouse embryonic stem cells deficient of GATA factors and conclude that GATA-4 is required for ES cells to perceive a cell positioning (cell aggregation) signal and GATA-6 is required to sense morphogenic (retinoic acid) signal. The collaboration between GATA-6 and GATA-4, or GATA-6 and GATA-5 which can substitute for GATA-4, is involved in the perception of differentiation cues by embryonic stem cells in their determination of endoderm lineage. This study indicates that the lineage differentiation of ES cells can be manipulated by the expression of GATA factors.

In vitro differentiation of mouse embryonic stem cells: enrichment of endodermal cells in the embryoid body

Embryonic stem (ES) cells have the potential to differentiate into all three germ layers, providing new perspectives not only for embryonic development but also for the application in cell replacement therapies. Even though the formation of an embryoid body (EB) in a suspension culture has been the most popular method to differentiate ES cells into a wide range of cells, not much is known about the characteristics of EB cells. To this end, we investigated the process of EB formation in the suspension culture of ES cells at weekly intervals for up to 6 weeks. We observed that the central apoptotic area is most active in the first week of EB formation and that the cell adhesion molecules, except beta-catenin, are highly expressed throughout the examination period. The sequential expression of endodermal genes in EBs during the 6-week culture correlated closely with that of normal embryo development. The outer surface of EBs stained positive for alpha-fetoprotein and GATA-4. When isolated from the 2-week-old EB by trypsin treatment, these endodermal lineage cells matured in vitro into hepatocytes upon stimulation with various hepatotrophic factors. In conclusion, our results demonstrate that endodermal cells can be retrieved from EBs and matured into specific cell types, opening new therapeutic usage of these in vitro differentiated cells in the cell replacement therapy of various diseases.

Sox7 plays crucial roles in parietal endoderm differentiation in F9 embryonal carcinoma cells through regulating Gata-4 and Gata-6 expression

During early rodent development, the parietal endoderm appears from an inner cell mass and produces large amounts of basement membrane components, such as laminin-1 and collagen IV. To elucidate the regulatory network for gene expression during these procedures, we constructed a series of short interfering RNA expression vectors targeted to various transcription factors, transfected them into F9 embryonal carcinoma cells, and evaluated the effects of the gene silencing on the induction of parietal endoderm differentiation and basement membrane component production by treating F9 cells with all trans-retinoic acid and dibutyryl cyclic AMP. Among the transcription factors tested, silencing of Sox7 or combined silencing of Gata-4 and Gata-6 resulted in suppression of cell shape changes and laminin-1 production, which are the hallmarks of parietal endoderm differentiation. In cells silenced for Sox7, induction of Gata-4 and Gata-6 by retinoic acid and cyclic AMP treatment was inhibited, while induction of Sox7 was not affected in cells silenced for Gata-4 and Gata-6, indicating that Sox7 is an upstream regulatory factor for these Gata factors. Nevertheless, silencing of Sox7 did not totally cancel the action of retinoic acid, since upregulation of coup-tf2, keratin 19, and retinoic acid receptor beta2 was not abolished in Sox7-silenced F9 cells. Although overexpression of Sox7 alone was insufficient to induce parietal endoderm differentiation, overexpression of Gata-4 or Gata-6 in Sox7-silenced F9 cells restored the differentiation into parietal endoderm. Sox7 is therefore required for the induction of Gata-4 and Gata-6, and the interplay among these transcription factors plays a crucial role in parietal endoderm differentiation.

Transcription factor GATA-6 is expressed in the endocrine and GATA-4 in the exocrine pancreas

GATA-4 and GATA-6 are zinc finger transcription factors that regulate gene expression, differentiation, and cell proliferation in various tissues. These factors have been implicated in the development of endodermal derivatives, including epithelial cells in the yolk sac, lung, and stomach. In the present study, we have characterized the expression of GATA-4 and GATA-6 during development of another endodermal derivative, the mouse pancreas, using a combination of in situ hybridization and immunohistochemistry. Neither GATA-4 nor GATA-6 antigen was detected in E10.5 pancreatic epithelial buds expressing Pdx-1. By E15.5, GATA-4 mRNA and protein were evident in developing pancreatic acini, but not in ductal or endocrine cells of the pancreas; GATA-6 mRNA and protein were present in both endocrine and exocrine cell precursors. In the newborn and adult pancreas, GATA-4 protein was seen in acinar cells, while GATA-6 antigen was found mainly in islet beta-cells. The amphicrine pancreatic AR42J-B13 cell line was used to study the expression of GATA-4 and GATA-6 during the differentiation of these cells towards an endocrine phenotype. Endocrine differentiation was associated with marked increase in GATA-6 but not GATA-4 mRNA levels. We conclude that GATA-4 is a marker of exocrine pancreatic differentiation, whereas GATA-6 is a marker of endocrine pancreatic development.

Transcription factor GATA-6 is expressed in malignant endoderm of pediatric yolk sac tumors and in teratomas

Transcription factors GATA-4 and GATA-6 play critical roles in mammalian yolk sac differentiation and function. Previously, we showed that GATA-4 is a potential marker for malignant yolk sac endoderm in pediatric germ cell tumors. This highly malignant tissue can cause diagnostic problems because yolk sac components may be difficult to differentiate from other, especially immature, tissue types in teratomas. In the search for new molecular markers for germ cell tumors, we have surveyed GATA-6 expression in benign and malignant pediatric germ cell tumors using mRNA in situ hybridization and immunohistochemistry. GATA-6 was expressed in most yolk sac tumors examined and also in nonmalignant tissues including gut/respiratory epithelium, sebocytes, and neuroepithelium in mature and immature teratomas. Given that GATA-6 has not been discovered in sebocytes before, this finding was confirmed by immunohistochemistry of normal mouse samples, indicating a function for this transcription factor in the mammalian skin. Taken together, GATA-6 can be used to identify yolk sac components in pediatric germ cell tumors. Furthermore, it is also expressed in specific tissues in teratomas. GATA-6, together with GATA-4, can thus be used as a novel molecular marker in characterizing of pediatric germ cell tumors.

Differential expression of GATA-4 and GATA-6 in fetal and adult mouse and human adrenal tissue

Earlier work implicates transcription factors GATA-4 and GATA-6 in murine adrenal function. We have now studied their expression during mouse and human adrenal development in detail. GATA-4 and GATA-6 mRNAs and protein are readily detectable from embryonic d 14 and gestational wk 19 onwards in the mouse and human adrenal cortex, respectively. In the postnatal adrenal, GATA-4 expression is down-regulated, whereas GATA-6 mRNA and protein continue to be expressed. To clarify the significance of GATA-4 for early adrenocortical development, Gata4-/- ES cells were injected into eight-cell-stage embryos derived from ROSA26 mice, a transgenic line expressing beta-galactosidase in all cell types, including the adrenocortical cells. The resultant chimeric embryos were stained with X-gal to discriminate ES cell- and host-derived tissue. Gata4-/- cells contributed to adrenocortical cells in these chimeras, and these cells also expressed GATA-6. Taken together, our findings suggest that GATA-6 expression is needed throughout adrenal development from fetal to adult age. GATA-4, on the other hand, may serve a role in the fetal adrenal gene regulation, although it is not essential for early adrenocortical differentiation.

Hepatic differentiation of murine embryonic stem cells

Murine embryonic stem (ES) cells can replicate indefinitely in culture and can give rise to all tissues, including the germline, when reimplanted into a murine blastocyst. ES cells can also be differentiated in vitro into a wide range of cell types. We have utilized a liver-specific marker to demonstrate that murine ES cells can differentiate into hepatocytes in vitro. We have used ES cells carrying a gene trap vector insertion (I.114) into an ankyrin repeat-containing gene (Gtar) that we have previously shown provides an exclusive beta-galactosidase marker for the early differentiation of hepatocytes in vivo. beta-Galactosidase-positive cells were differentiated from I.114 ES cells in vitro. The identity of these cells was confirmed by the expression of the proteins alpha-fetoprotein, albumin, and transferrin and by the fact that they have an ultrastructural appearance consistent with that of embryonic hepatocytes. We propose that this model system of hepatic differentiation in vitro could be used to define factors that are involved in specification of the hepatocyte lineage. In addition, human ES cells have recently been derived and it has been proposed that they may provide a source of differentiated cell types for cell replacement therapies in the treatment of a variety of diseases.

Initiation and early patterning of the endoderm

We review the early stages of endoderm formation in the major animal models. In Amphibia maternal molecules are important in initiating endoderm formation. This is followed by successive signaling events that establish and then pattern the endoderm. In other organisms there are differences in endodermal development, particularly in the initial, prephylotypic stages. Later many of the same key families of transcription factors and signaling cassettes are used in all animals, but more work will be needed to establish exact evolutionary homologies.

Expression of a novel antigen, EEM-1, in the mouse embryo and embryonic stem cell-derived embryoid bodies

A novel monoclonal antibody designated EEM-1 is described. EEM-1 recognizes an intracellular protein with an apparent molecular weight of >250 kDa. Expression of the EEM-1 antigen is largely confined to extra-embryonic membranes, but some expression does occur in the embryo. In the embryonic day 6 (E6) and E7 embryo it is expressed in visceral and parietal endoderm; later derivatives of these structures in the yolk sac are negative. The outer layer of the amnion also stains. Within the embryo proper, antigen is expressed in limited regions of the gut, kidney, and pancreas. EEM-1 is also expressed in embryonic stem (ES) cells differentiating in vitro. Undifferentiated ES cells do not express the antigen. Embryoid bodies (EBs) derived from ES cells have patches of EEM-1-positive cells on their surface at 2 days in culture. Older EBs have increasing numbers of positive cells which are confined to the surface. A special class of EBs, termed "cystic EBs," are covered by a cell layer which strongly express EEM-1 antigen. The EEM-1 antibody will be useful for investigating the development of extra-embryonic membranes and their counterparts in the ES cell in vitro model.

Volume of liquid below the epithelium of an F9 cell as a signal for differentiation into visceral endoderm

When retinoic acid-primed F9 cells are allowed to aggregate, they form embryoid bodies with an outer layer of (α)-fetoprotein-producing visceral endoderm cells and an internal cavity. I show that maturation of the visceral endoderm is dependent on the size of F9 aggregates. Size fractionation of aggregates of retinoic acid-primed F9 cells on Percoll density gradients revealed that only aggregates with diameters larger than 180 microm developed into embryoid bodies with an endoderm layer secreting (α)-fetoprotein. Size dependent alpha-fetoprotein-secretion was also observed when retinoic acid-primed F9 cells were cultured on porous microcarrier beads larger than 185 microm. Retinoic acid-primed F9 cells on flat microporous membranes did not differentiate and secrete alpha-fetoprotein unless exposed to a limited volume of medium at their basolateral surface. This suggested that maturation of the visceral endoderm is signaled by the volume of liquid phase below the epithelium. I postulate that the epithelial layer of an F9 aggregate encloses liquid and forms a barrier to diffusion of some critical factor(s). The concentration of such a factor may reach a threshold due to enlargement of the liquid phase during growth of the F9 aggregate and thereby signal maturation of the outer layer of cells into visceral endoderm.

Retinoic acid-regulated expression of fibroblast growth factor 3 requires the interaction between a novel transcription factor and GATA-4

fgf-3 shows a complex spatial-temporal pattern of transcription during mouse development, and the gene product appears to be an important intercellular signaling molecule. Here we show that the major enhancer, which is obligatory for transcription, is composed of three elements with different properties. Both functional analyses in undifferentiated and differentiated F9 cells and characterization of DNA-protein complexes in vitro have identified the sequence motifs GTGACT(C), ATTGT, and GATA as the key transcription factor binding sites. The GTGACT(C) motif, while not essential, is required for full enhancer activity. However, binding at ATTGT is crucial for transcriptional activity and is required for cooperative binding at the proximal GATA site. The GATA binding site mediates the retinoic acid/dibutyryl cyclic AMP stimulation of transcription and correlates with the binding of Gata-4 which is induced by retinoic acid in differentiating F9 cells. The ATTGT and GATA motifs are inactive when placed separately on a minimal thymidine kinase (TK) promoter, but together they act as a strong retinoic acid-regulated enhancer. In undifferentiated F9 cells, gata-4 expression stimulates the fgf-3 promoter, whereas in differentiated F9 cells already expressing gata-4, no further increase in promoter activity was observed.

Embryonic stem cell models of development

Pluripotent mouse embryonic stem (ES) cell lines have provided a means to analyze gene function in development via gene targeting. At the same time, they provide an opportunity to directly probe gene function by assessing the in vitro differentiation capacity of the ES cells themselves. In addition to providing direct data on lineage decisions not accessible in the complex three-dimensional milieu of the early mouse embryo, controlled differentiation of ES into specific lineages may provide a source of cells for transplantation and gene therapy.