Transcriptional activation by Oct-3: evidence for a specific role of the POU-specific domain in mediating functional interaction with Oct-1

The Role of E3s in Regulating Pluripotency of Embryonic Stem Cells and Induced Pluripotent Stem Cells

Pluripotent embryonic stem cells (ESCs) are derived from early embryos and can differentiate into any type of cells in living organisms. Induced pluripotent stem cells (iPSCs) resemble ESCs, both of which serve as excellent sources to study early embryonic development and realize cell replacement therapies for age-related degenerative diseases and other cell dysfunction-related illnesses. To achieve these valuable applications, comprehensively understanding of the mechanisms underlying pluripotency maintenance and acquisition is critical. Ubiquitination modifies proteins with Ubiquitin (Ub) at the post-translational level to monitor protein stability and activity. It is extensively involved in pluripotency-specific regulatory networks in ESCs and iPSCs. Ubiquitination is achieved by sequential actions of the Ub-activating enzyme E1, Ub-conjugating enzyme E2, and Ub ligase E3. Compared with E1s and E2s, E3s are most abundant, responsible for substrate selectivity and functional diversity. In this review, we focus on E3 ligases to discuss recent progresses in understanding how they regulate pluripotency and somatic cell reprogramming through ubiquitinating core ESC regulators.

In vitro analysis of the transcriptional regulatory mechanism of zebrafish pou5f3

Zebrafish pou5f3 (previously named pou2), a close homologue of mouse Oct4, encodes a PouV-family transcription factor. pou5f3 has been implicated in diverse aspects of developmental regulation during embryogenesis. In the present study, we addressed the molecular function of Pou5f3 as a transcriptional regulator and the mechanism by which pou5f3 expression is transcriptionally regulated. We examined the influence of effector genes on the expression of the luciferase gene under the control of the upstream 2.1-kb regulatory DNA of pou5f3 (Luc-2.2) in HEK293T and P19 cells. We first confirmed that Pou5f3 functions as a transcriptional activator both in cultured cells and embryos, which confirmed autoregulation of pou5f3 in embryos. It was further shown that Luc-2.2 was activated synergistically by pou5f3 and sox3, which is similar to the co-operative activity of Oct4 and Sox2 in mice, although synergy between pou5f3 and sox2 was less obvious in this zebrafish system. The effects of pou5f3 deletion constructs on the regulation of Luc-2.2 expression revealed different roles for the three subregions of the N-terminal region in Pou5f3 in terms of its regulatory functions and co-operativity with Sox3. Electrophoretic mobility shift assays confirmed that Pou5f3 and Sox3 proteins specifically bind to adjacent sites in the 2.1-kb DNA and that there is an interaction between the two proteins. The synergy with sox3 was unique to pou5f3-the other POU factor genes examined did not show such synergy in Luc-2.2 regulation. Finally, functional interaction was observed between pou5f3 and sox3 in embryos in terms of the regulation of dorsoventral patterning and convergent extension movement. These findings together demonstrate co-operative functions of pou5f3 and sox3, which are frequently coexpressed in early embryos, in the regulation of early development.

Phosphorylation of Threonine343 Is Crucial for OCT4 Interaction with SOX2 in the Maintenance of Mouse Embryonic Stem Cell Pluripotency

OCT4 is required to maintain the pluripotency of embryonic stem cells (ESCs); yet, overdose-expression of OCT4 induces ESC differentiation toward primitive endoderm. The molecular mechanism underlying this differentiation switch is not fully understood. Here, we found that substitution of threonine³⁴³ by alanine (T343A), but not aspartic acid (T343D), caused a significant loss of OCT4-phosphorylation signal in ESCs. Loss of such OCT4-phosphorylation compromises its interaction with SOX2 but promotes interaction with SOX17. We therefore propose that threonine³⁴³-based OCT4-phosphorylation is crucial for the maintenance of ESC pluripotency. This OCT4-phosphorylation-based mechanism may provide insight into the regulation of lineage specification during early embryonic development.

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Phosphorylation of threonine³⁴³ mediates global OCT4-phosphorylation (phos-OCT4^T343)

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Phos-OCT4^T343 is crucial for OCT4 to protect embryonic stem cell pluripotency

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Phos-OCT4^T343 binds to SOX2 but non-phos-OCT4^T343 binds to SOX17 in cell fate decision

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Phos-OCT4^T343 may regulate lineage commitment in early embryonic development

Long-Lived Binding of Sox2 to DNA Predicts Cell Fate in the Four-Cell Mouse Embryo

Transcription factor (TF) binding to DNA is fundamental for gene regulation. However, it remains unknown how the dynamics of TF-DNA interactions change during cell-fate determination in vivo. Here, we use photo-activatable FCS to quantify TF-DNA binding in single cells of developing mouse embryos. In blastocysts, the TFs Oct4 and Sox2, which control pluripotency, bind DNA more stably in pluripotent than in extraembryonic cells. By contrast, in the four-cell embryo, Sox2 engages in more long-lived interactions than does Oct4. Sox2 long-lived binding varies between blastomeres and is regulated by H3R26 methylation. Live-cell tracking demonstrates that those blastomeres with more long-lived binding contribute more pluripotent progeny, and reducing H3R26 methylation decreases long-lived binding, Sox2 target expression, and pluripotent cell numbers. Therefore, Sox2-DNA binding predicts mammalian cell fate as early as the four-cell stage. More generally, we reveal the dynamic repartitioning of TFs between DNA sites driven by physiological epigenetic changes. VIDEO ABSTRACT.

A defined Oct4 level governs cell state transitions of pluripotency entry and differentiation into all embryonic lineages

Oct4 is considered a master transcription factor for pluripotent cell self-renewal, but its biology remains poorly understood. Here, we investigated the role of Oct4 using the process of induced pluripotency. We found that a defined embryonic stem cell (ESC) level of Oct4 is required for pluripotency entry. However, once pluripotency is established, the Oct4 level can be decreased up to sevenfold without loss of self-renewal. Unexpectedly, cells constitutively expressing Oct4 at an ESC level robustly differentiated into all embryonic lineages and germline. In contrast, cells with low Oct4 levels were deficient in differentiation, exhibiting expression of naive pluripotency genes in the absence of pluripotency culture requisites. The restoration of Oct4 expression to an ESC level rescued the ability of these to restrict naive pluripotent gene expression and to differentiate. In conclusion, a defined Oct4 level controls the establishment of naive pluripotency as well as commitment to all embryonic lineages.

Distinct functions of acj6 splice forms in odor receptor gene choice

Individual olfactory receptor neurons (ORNs) selectively express one or a small number of odor receptors from among a large receptor repertoire. The expression of an odor receptor dictates the odor response spectrum of the ORN. The process of receptor gene choice relies in part on a combinatorial code of transcription factors. In Drosophila, the POU domain transcription factor Acj6 is one element of the transcription factor code. In acj6 null mutants, many ORNs do not express an appropriate odor receptor gene and thus are not correctly specified. We find that acj6 is alternatively spliced to yield many structurally distinct transcripts in the olfactory organs. We generate flies that express single splice forms of acj6 in an acj6(-) background. We find that different splice forms are functionally distinct; they differ in their abilities to specify ORN identities. Some individual splice forms can fully rescue the specification of some ORNs. Individual splice forms can function both positively and negatively in receptor gene regulation. ORNs differ in their requirements for splice forms; some are not fully rescued by any single splice form tested, suggesting that some ORNs may require the combinatorial action of multiple splice forms. Late expression of some acj6 splice forms is sufficient to rescue some ORN classes, consistent with a direct role for Acj6 isoforms in receptor gene expression. The results indicate that alternative splicing may add another level of richness to the regulatory code that underlies the process of odor receptor gene choice.

Reversal of Xenopus Oct25 function by disruption of the POU domain structure

Xenopus Oct25 is a POU family subclass V (POU-V) transcription factor with a distinct domain structure. To investigate the contribution of different domains to the function of Oct25, we have performed gain of function analyses. Deletions of the N- or C-terminal regions and of the Hox domain (except its nuclear localization signal) result in mutants being indistinguishable from the wild type protein in the suppression of genes promoting germ layer formation. Deletion of the complete POU domain generates a mutant that has no effect on embryogenesis. However, disruption of the alpha-helical structures in the POU domain, even by a single amino acid mutation, causes reversal of protein function. Overexpression of such mutants leads to dorsalization of embryos and formation of secondary axial structures. The underlying mechanism is an enhanced transcription of genes coding for antagonists of the ligands for ventralizing bone morphogenetic protein and Wnt pathways. Corresponding deletion mutants of Xenopus Oct60, Oct91, or mouse Oct4 also exhibit such a dominant-negative effect. Therefore, our results reveal that the integrity of the POU domain is crucial for the function of POU-V transcription factors in the regulation of genes that promote germ layer formation.

Implication of human OCT4 transactivation domains for self-regulatory transcription

OCT4 plays a crucial role in pluripotency and self-renewal of embryonic stem cells. OCT4 is also expressed in testicular germ cell tumors (GCTs), suggesting the important function of OCT4 as an oncogenic factor in GCTs. To understand the molecular mechanism of human OCT4 (hOCT4) in tumorigenesis as well as stemness, we identified hOCT4 transactivation domains in human embryonic carcinoma cells. Context analyses of heterologous GAL4 and natural hOCT4 revealed that each N-terminal domain or C-terminal domain independently stimulated transcriptional activity, and that both domains are required for synergistic transactivation by deletion mapping analysis. Dose-dependent overexpression of exogenous hOCT4 significantly decreased the transcriptional activity of the hOCT4 promoter. This inhibition was reversed by the removal of one or both domains. These results suggest that the inhibitory effect of hOCT4 is mediated by transactivation domains, and that the self-regulation of hOCT4 may be mediated via a negative feedback loop in pluripotent cells.

Mutation in the DNA-binding domain of the EWS-Oct-4 oncogene results in dominant negative activity that interferes with EWS-Oct-4-mediated transactivation

The EWS-Oct-4 protein is a chimeric molecule in which the amino terminal domain (NTD) of the EWS becomes fused to the carboxy terminal domain (CTD) of the Oct-4 transcription factor. It was identified in human bone and soft-tissue tumors associated with t(6;22)(p21;q12). Using in vitro and in vivo systems, we found that the EWS-Oct-4 protein self-associates. The major domains required for self-association mapped to the EWS NTD (amino acids 70-163) and the POU DNA-binding domain. EWS-Oct-4 protein also associated with EWS-Oct-4 (V351P), which contains a mutation in the POU DNA-binding domain. Using electrophoretic mobility shift assays, we found that the EWS-Oct-4 (V351P) mutant interfered with wild-type EWS-Oct-4 DNA-binding activity. In addition, we found that EWS-Oct-4-mediated transcriptional activation was inhibited by EWS-Oct-4 (V351P) protein in vivo. Thus, this mutation in the POU DNA-binding domain results in a dominant negative protein. These findings suggest that the biological functions of the EWS-Oct-4 oncogene can be modulated by the dominant negative mutant EWS-Oct-4 (V351P).

Pyruvate kinase isozyme type M2 (PKM2) interacts and cooperates with Oct-4 in regulating transcription

The Oct-4 gene encodes a transcription factor that plays an important role in maintaining the pluripotent state of embryonic stem cells and may prevent expression of genes activated during differentiation. Although its role in maintaining embryonic stem cell pluripotency is well established, there is still little known about the binding partners that regulate its function. To identify proteins that control Oct-4 function, we used affinity chromatography on immobilized Oct-4 (POU) together with MALDI-TOF (matrix-assisted laser-desorption ionization-time-of-flight) MS (mass spectrometry) and isolated a novel Oct-4-interacting protein, pyruvate kinase type M2 (PKM2 or M2-PK). PKM2 is an isozyme of pyruvate kinase that is specifically expressed in proliferating cells, such as embryonic stem cells, embryonic carcinoma cells, as well as cancer cells. Oct-4 and PKM2 were co-affinity precipitated from cell extracts, and glutathione S-transferase pull-down assays revealed that the POU DNA binding domain of Oct-4 was required for interaction with PKM2. In addition, the C-terminal domain of PKM2 (amino acids 307-531) was involved in binding to Oct-4. Moreover, ectopic expression of the PKM2 enhanced Oct-4-mediated transcription. These observations indicate that the transactivation potential of the Oct-4 transcription factor is positively modulated by PKM2.

The EWS-Oct-4 fusion gene encodes a transforming gene

The t(6;22)(p21;q12) translocation associated with human bone and soft-tissue tumours results in a chimaeric molecule fusing the NTD (N-terminal domain) of the EWS (Ewing's sarcoma) gene to the CTD (C-terminal domain) of the Oct-4 (octamer-4) embryonic gene. Since the N-terminal domains of EWS and Oct-4 are structurally different, in the present study we have assessed the functional consequences of the EWS-Oct-4 fusion. We find that this chimaeric gene encodes a nuclear protein which binds DNA with the same sequence specificity as the parental Oct-4 protein. Comparison of the transactivation properties of EWS-Oct-4 and Oct-4 indicates that the former has higher transactivation activity for a known target reporter gene containing Oct-4 binding. Deletion analysis of the functional domains of EWS-Oct-4 indicates that the EWS (NTD), the POU domain and the CTD of EWS-Oct-4 are necessary for full transactivation potential. EWS-Oct-4 induced the expression of fgf-4 (fibroblast growth factor 4) and nanog, which are potent mitogens as well as Oct-4 downstream target genes whose promoters contain potential Oct-4-binding sites. Finally, ectopic expression of EWS-Oct-4 in Oct-4-null ZHBTc4 ES (embryonic stem) cells resulted in increased tumorigenic growth potential in nude mice. These results suggest that the oncogenic effect of the t(6;22) translocation is due to the EWS-Oct-4 chimaeric protein and that fusion of the EWS NTD to the Oct-4 DNA-binding domain produces a transforming chimaeric product.

Involvement of the ubiquitous Oct-1 transcription factor in hormonal induction of beta-casein gene expression

Transcription of the milk protein beta-casein gene is induced by the lactogenic hormones Prl (prolactin) and glucocorticoids. Multiple transcription factors involved in this induction have been identified, including the STAT5 (signal transducer and activator of transcription 5) and the GR (glucocorticoid receptor). Our previous studies have identified a binding site for the ubiquitous Oct-1 (octamer-binding transcription factor 1) protein in the lactogenic hormonal regulatory region of the mouse beta-casein promoter. In the present study, we report that Oct-1 is indeed expressed and binds to the beta-casein promoter in mammary epithelial cells. Oct-1 activates hormonally induced beta-casein promoter activity in a dose-dependent manner. Hormonal induction of promoter activity was decreased not only by mutating the Oct-1-binding site from ATTAGCAT to GCTAGCAT, which abolishes Oct-1 binding (50% decrease, P<0.01), but also by changing the site to the consensus Oct-1-binding motif ATTTGCAT (40% decrease, P<0.01). Reversing the Oct-1-binding site reduced hormonal induction by 70% (P<0.01), showing that orientation of Oct-1 binding is also critical in hormonal action. In transient transfection experiments, Oct-1 collaboratively transactivated the beta-casein gene promoter with STAT5 and/or GR in the presence of Prl receptor in cells treated with the lactogenic hormones. The C-terminus of Oct-1 was not essential to its function. The results of the present study provide biochemical evidence that the ubiquitous Oct-1 transcription factor may be involved in hormonally regulated, tissue-specific beta-casein gene expression.

The human OCT-4 isoforms differ in their ability to confer self-renewal

OCT-4 transcription factors play an important role in maintaining the pluripotent state of embryonic stem cells and may prevent expression of genes activated during differentiation. Human OCT-4 isoform mRNAs encode proteins that have identical POU DNA binding domains and C-terminal domains but differ in their N-terminal domains. We report here the cloning and characterization of the human OCT-4B isoform. Human OCT-4B cDNA encodes a 265-amino acid protein with a predicted molecular mass of 30 kDa. Embryonic stem (ES) cell-based complementation assays using ZHBTc4 ES cells showed that unlike human OCT-4A, OCT-4B cannot sustain ES cell self-renewal. In addition, OCT-4B does not bind to a probe carrying the OCT-4 consensus binding sequence, and we demonstrate that two separate regions of its N-terminal domain are responsible for inhibiting DNA binding. We also demonstrate that OCT-4B is mainly localized to the cytoplasm. Overexpression of OCT-4B did not activate transcription from OCT-4-dependent promoters, although OCT-4A did as reported previously. Furthermore, transcriptional activation by human OCT-4A was not inhibited by co-expression of OCT-4B. Taken together, these data suggest that the DNA binding, transactivation, and abilities to confer self-renewal of the human OCT-4 isoforms differ.

Use of altered-specificity binding Oct-4 suggests an absence of pluripotent cell-specific cofactor usage

Oct-4 is a POU domain transcription factor that is critical for maintaining pluripotency and for stem cell renewal. Previous studies suggest that transcription regulation by Oct-4 at particular enhancers requires the input of a postulated E1A-like cofactor that is specific to pluripotent cells. However, such studies have been limited to the use of enhancer elements that bind other POU-protein family members in addition to Oct-4, thus preventing a ‘clean’ assessment of any Oct-4:cofactor relationships. Other attempts to study Oct-4 functionality in a more ‘stand-alone’ situation target Oct-4 transactivation domains to DNA using heterologous binding domains, a methodology which is known to generate artificial data. To circumvent these issues, an altered-specificity binding Oct-4 (Oct-4RR) and accompanying binding site, which binds Oct-4RR only, were generated. This strategy has previously been shown to maintain Oct-1:cofactor interactions that are highly binding-site and protein/binding conformation specific. This system therefore allows a stand-alone study of Oct-4 function in pluripotent versus differentiated cells, without interference from endogenous POU factors and with minimal deviation from bound wild-type protein characteristics. Subsequently, it was demonstrated that Oct-4RR and the highly transactive regions of its N-terminus determined here, and its C-terminus, have the same transactivation profile in pluripotent and differentiated cells, thus providing strong evidence against the existence of such a pluripotent cell-specific Oct-4 cofactor.

Stimulation of Oct-4 Activity by Ewing's Sarcoma Protein

The Oct-4 gene encodes a transcription factor that is expressed in embryonic stem (ES) cells and germ cells. Oct-4 is known to function as a transcriptional activator of genes involved in maintaining an undifferentiated totipotent state and possibly in preventing expression of genes activated during differentiation. In addition, it is a putative proto-oncogene and a critical player in the genesis of human testicular germ cell tumors. Although much effort has gone toward characterizing Oct-4, there is still little known about the molecular mechanisms and the proteins that regulate Oct-4 function. To identify cofactors that control Oct-4 function in vivo, we used a recently developed bacterial two-hybrid screening system and isolated a novel ES cell-derived cDNA encoding Ewing's sarcoma protein (EWS). EWS is a proto-oncogene and putative RNA-binding protein involved in human cancers. By using glutathione-S-transferase (GST) pull-down assays, we were able to confirm the interaction between Oct-4 and EWS in vitro, and moreover, coimmunoprecipitation and colocalization studies have shown that these proteins also associate in vivo. We have mapped the EWS-interacting region to the POU domain of Oct-4. In addition, three independent sites on EWS are involved in binding to Oct-4. In this study, we report that Oct-4 and EWS are coexpressed in the pluripotent mouse and human ES cells. Consistent with its ability to bind to and colocalize with Oct-4, ectopic expression of EWS enhances the transactivation ability of Oct-4. Moreover, a chimeric protein generated by fusion of EWS (1-295) to the GAL4 DNA-binding domain significantly increases promoter activity of a reporter containing GAL4 DNA-binding sites, suggesting the presence of a strong activation domain within EWS. Taken together, our results suggest that Oct-4-mediated transactivation is stimulated by EWS.

Oct-3/4 is a dose-dependent oncogenic fate determinant

The Oct-3/4 transcription factor sustains embryonic stem (ES) cell self-renewal and is a dose-dependent cell fate determinant. In the adult male, its expression is restricted to type A spermatogonia. We show that Oct-3/4 is expressed in all human testicular germ cell tumors (GCTs) tested, even in the early premalignant component. We demonstrate that Oct-3/4 dictates ES cells' oncogenic potential in a dose-dependent manner; high levels increase the malignant potential of ES cell-derived tumors while Oct-3/4 inactivation induces regression of the malignant component. Oct-3/4 expression in a heterologous cell system transforms nontumorigenic cells and endows tumorigenicity in nude mice. Our findings suggest that Oct-3/4 is not only a distinctive immunohistochemical marker for GCTs, but also plays a critical role in the genesis of these tumors.

Stem cell pluripotency and transcription factor Oct4

Mammalian cell totipotency is a subject that has fascinated scientists for generations. A long lasting question whether some of the somatic cells retains totipotency was answered by the cloning of Dolly at the end of the 20th century. The dawn of the 21st has brought forward great expectations in harnessing the power of totipotentcy in medicine. Through stem cell biology, it is possible to generate any parts of the human body by stem cell engineering. Considerable resources will be devoted to harness the untapped potentials of stem cells in the foreseeable future which may transform medicine as we know today. At the molecular level, totipotency has been linked to a singular transcription factor and its expression appears to define whether a cell should be totipotent. Named Oct4, it can activate or repress the expression of various genes. Curiously, very little is known about Oct4 beyond its ability to regulate gene expression. The mechanism by which Oct4 specifies totipotency remains entirely unresolved. In this review, we summarize the structure and function of Oct4 and address issues related to Oct4 function in maintaining totipotency or pluripotency of embryonic stem cells.

Phenotypic complementation establishes requirements for specific POU domain and generic transactivation function of Oct-3/4 in embryonic stem cells

Transcription factors of the POU family govern cell fate through combinatorial interactions with coactivators and corepressors. The POU factor Oct-3/4 can define differentiation, dedifferentation, or self-renewal of pluripotent embryonic stem (ES) cells in a sensitive, dose-dependent manner (H. Niwa, J.-I. Miyazali, and A. G. Smith, Nat. Genet. 24:372-376, 2000). Here we have developed a complementation assay based on the ability of Oct-3/4 transgenes to rescue self-renewal in conditionally null ES cells and used this to define which domains of Oct-3/4 are required to sustain the undifferentiated stem cell phenotype. Surprisingly, we found that molecules lacking either the N-terminal or C-terminal transactivation domain, though not both, can effectively replace full-length Oct-3/4. Furthermore, a fusion of the heterologous transactivation domain of Oct-2 to the Oct-3/4 POU domain can also sustain self-renewal. Thus, the unique function of Oct-3/4 in ES cell propagation resides in combination of the specific POU domain with a generic proline-rich transactivation domain. Interestingly, however, Oct-3/4 target gene expression elicited by the N- and C-terminal transactivation domains is not identical, indicating that at least one class of genes activated by Oct-3/4 is not required for ES cell propagation.

The specific activation of gadd45 following UVB radiation requires the POU family gene product N-oct3 in human melanoma cells

Here we report the specific regulation of gadd45 expression in human melanoma cell lines following UVB radiation. This solar wavelength is likely to be involved in melanoma aetiology. We have previously shown that gadd45 expression is strongly enhanced in a p53-independent manner following UVB irradiation, unlike the other p53 target genes studied. Furthermore, gadd45 is specifically activated in melanocytes since its induction in response to UVB, is not observed in other skin cells such as keratinocytes or fibroblasts. To investigate this particular regulation of gadd45, we analysed the UVB-induced response of different gadd45 promoter regions. Thus, a minimal promoter region of 50 bp length, responsible for gadd45 activation in melanoma cell lines following UVB irradiation, was determined. In electrophoretic mobility shift assays (EMSAs), we showed that this region (-106/-56) of the gadd45 promoter which contains two identical octamers, binds the POU family gene products oct-1 and N-oct3. Given the specific expression pattern of N-oct3 in melanocyte, we invalidated the expression of this transcription factor in melanoma cells: such an abrogation of N-oct3 protein expression in melanoma cells impeded gadd45 UVB-response. Thus the response of melanocyte to UVB may use an original and previously undescribed pathway.

Molecular mechanism to maintain stem cell renewal of ES cells

Embryonic stem (ES) cells are pluripotent cells directly derived from early stage embryos that retain the ability to differentiate into all cell types. This unique feature is the basis of various applications of ES cell technology such as in vitro models of mammalian development, germline transgenesis to make knockout mice, and a generic source for cell therapy in regenerative medicine. To achieve success in these applications, the pluripotency of ES cells has to be kept stable during long-term culture in vitro, leading to the necessity of determining the molecular basis for maintaining ES self-renewal. This paper summarizes the recent progress in this area, focusing mainly on the LIF signaling pathway and the transcription factor Oct-3/4. Although it is still unclear how these components works together, a model is presented here that provides a plan to solve this problem.

Apparent absence of oct 3/4 from the chicken genome

The development of efficient methods for establishing germline competent chicken embryonic stem (ES) cell lines has proved elusive. In the mouse embryo, expression of oct 3/4 is limited to pluripotent cells and primordial germ cells; regulatory sequences of this gene have been used to derive germline competent mouse ES cell lines by the continuous ablation of differentiated cells in culture using drug selection. To apply this technique to chickens several strategies were employed to analyze the chicken genome for oct 3/4, a member of the highly conserved POU gene family. PCR and Southern hybridization experiments with primers and probes based on mouse oct 3/4 sequences indicated that oct 3/4-like sequences are not present in the chicken genome. Also, analysis of mRNA from Stage 14 and 20 (H&H) chick embryos by reverse transcription PCR and the screening of a Stage 20 (H&H) chick embryo cDNA library with mouse oct 3/4-based primers and probes indicated that oct 3/4-like sequences are not expressed in the early chick embryo. The apparent absence of oct 3/4 in chickens, despite the conservation of the gene in mammals and urodeles, is discussed in terms of possible implications for the mode of chicken PGC formation in relation to that in other vertebrates.

POU domain factors in the neuroendocrine system: lessons from developmental biology provide insights into human disease

POU domain factors are transcriptional regulators characterized by a highly conserved DNA-binding domain referred to as the POU domain. The structure of the POU domain has been solved, facilitating the understanding of how these proteins bind to DNA and regulate transcription via complex protein-protein interactions. Several members of the POU domain family have been implicated in the control of development and function of the neuroendocrine system. Such roles have been most clearly established for Pit-1, which is required for formation of somatotropes, lactotropes, and thyrotropes in the anterior pituitary gland, and for Brn-2, which is critical for formation of magnocellular and parvocellular neurons in the paraventricular and supraoptic nuclei of the hypothalamus. While genetic evidence is lacking, molecular biology experiments have implicated several other POU factors in the regulation of gene expression in the hypothalamus and pituitary gland. Pit-1 mutations in humans cause combined pituitary hormone deficiency similar to that found in mice deleted for the Pit-1 gene, providing a striking example of how basic developmental biology studies have provided important insights into human disease.

Modulation of the activity of multiple transcriptional activation domains by the DNA binding domains mediates the synergistic action of Sox2 and Oct-3 on the fibroblast growth factor-4 enhancer

Fibroblast growth factor (FGF)-4 gene expression in the inner cell mass of the blastocyst and in EC cells requires the combined activity of two transcriptional regulators, Sox2 and Oct-3, which bind to adjacent sites on the FGF-4 enhancer DNA and synergistically activate transcription. Sox2 and Oct-3 bind cooperatively to the enhancer DNA through their DNA-binding, high mobility group and POU domains, respectively. These two domains, however, are not sufficient to activate transcription. We have analyzed a number of Sox2 and Oct-3 deletion mutants to identify the domains within each protein that contribute to the activity of the Sox2 x Oct-3 complex. Within Oct-3, we have identified two activation domains, the N-terminal AD1 and the C-terminal AD2, that play a role in the activity of the Sox2 x Oct-3 complex. AD1 also displays transcriptional activation functions in the absence of Sox2 while AD2 function was only detected within the Sox2 x Oct-3 complex. In Sox2, we have identified three activation domains within its C terminus: R1, R2, and R3. R1 and R2 can potentiate weak activation by Sox2 in the absence of Oct-3 but their deletion has no effect on the Sox2 x Oct-3 complex. In contrast, R3 function is only observed when Sox2 is complexed with Oct-3. In addition, analysis of Oct-1/Oct-3 chimeras indicates that the Oct-3 homeodomain also plays a critical role in the formation of a functional Sox2 x Oct-3 complex. Our results are consistent with a model in which the synergistic action of Sox2 and Oct-3 results from two major processes. Cooperative binding of the factors to the enhancer DNA, mediated by their binding domains, stably tethers each factor to DNA and increases the activity of intrinsic activation domains within each protein. Protein-protein and protein-DNA interactions then may lead to reciprocal conformational changes that expose latent activation domains within each protein. These findings define a mechanism that may also be utilized by other Sox x POU protein complexes in gene activation.

Identification of the transactivation domain of the transcription factor Sox-2 and an associated co-activator

The importance of interactions between Sox and POU transcription factors in the regulation of gene expression is becoming increasingly apparent. Recently, many examples of the involvement of Sox-POU partnerships in transcription have been discovered, including a partnership between Sox-2 and Oct-3. Little is known about the mechanisms by which these factors modulate transcription. To better understand the molecular interactions involved, we mapped the location of the transactivation domain of Sox-2. This was done in the context of its interaction with Oct-3, as well as its ability to transactivate as a fusion protein linked to the DNA-binding domain of Gal4. Both approaches demonstrated that Sox-2 contains a transactivation domain in its C-terminal half, containing a serine-rich region and the C terminus. We also determined that the viral oncoprotein E1a inhibits the ability of the Gal4/Sox-2 fusion protein to transactivate, as well as the transcriptional activation mediated by the combined action of Sox-2 and Oct-3. In contrast, a mutant form of E1a, unable to bind p300, lacks both of these effects. Importantly, we determined that p300 overcomes the inhibitory effects of E1a in both assays. Together, these findings suggest that Sox-2 mediates its effects, at least in part, through the co-activator p300.

Characterization of the regulatory domains of the human skn-1a/Epoc-1/Oct-11 POU transcription factor

The Skn-1a POU transcription factor is primarily expressed in keratinocytes of murine embryonic and adult epidermis. Although some POU factors expressed in a tissue-specific manner are important for normal differentiation, the biological function of Skn-1a remains unknown. Previous in vitro studies indicate that Skn-1a has the ability to transactivate markers of keratinocyte differentiation. In this study, we have characterized Skn-1a's transactivation domain(s) and engineered a dominant negative protein that lacked this transactivation domain. Deletional analysis of the human homologue of Skn-1a with three target promoters revealed the presence of two functional domains: a primary C-terminal transactivation domain and a combined N-terminal inhibitory domain and transactivation domain. Skn-1a lacking the C-terminal region completely lost transactivation ability, irrespective of the promoter tested, and was able to block transactivation by normal Skn-1a in competition assays. Compared with full-length, Skn-1a lacking the N-terminal region demonstrated either increased transactivation (bovine cytokeratin 6 promoter), comparable transactivation (human papillomavirus type 1a long control region), or loss of transactivation (human papillomavirus type 18 long control region). The identification of a primary C-terminal transactivation domain enabled us to generate a dominant negative Skn-1a factor, which will be useful in the quest for a better understanding of this keratinocyte-specific gene regulator.

Synergism with germ line transcription factor Oct-4: viral oncoproteins share the ability to mimic a stem cell-specific activity

Activation of transcription by Oct-4 from remote binding sites requires a cofactor that is restricted to embryonal stem cells. The adenovirus E1A protein can mimic the activity of this stem cell-specific factor and stimulates Oct-4 activity in differentiated cells. Here we characterize the Oct-4-E1A interaction and show that the E1A 289R protein harbors two independent Oct-4 binding sites, both of which specifically interact with the POU domain of Oct-4. Furthermore, we demonstrate that, like E1A, the human papillomavirus E7 oncoprotein also specifically binds to the Oct-4 POU domain. E7 and Oct-4 can form a complex both in vitro and in vivo. Expression of E7 in differentiated cells stimulates Oct-4-mediated transactivation from distal binding sites. Moreover, Oct-4, but not other Oct factors, is active when expressed in cells transformed by human papillomavirus. Our results suggest that different viruses have evolved oncoproteins that share the ability to target Oct-4 and to mimic a stem cell-specific activity.

Rex-1, a gene encoding a transcription factor expressed in the early embryo, is regulated via Oct-3/4 and Oct-6 binding to an octamer site and a novel protein, Rox-1, binding to an adjacent site

The Rex-1 (Zfp-42) gene, which encodes an acidic zinc finger protein, is expressed at high levels in embryonic stem (ES) and F9 teratocarcinoma cells. Prior analysis identified an octamer motif in the Rex-1 promoter which is required for promoter activity in undifferentiated F9 cells and is involved in retinoic acid (RA)-associated reduction in expression. We show here that the Oct-3/4 transcription factor, but not Oct-1, can either activate or repress the Rex-1 promoter, depending on the cellular environment. Rex-1 repression is enhanced by E1A. The protein domain required for Oct-3/4 activation was mapped to amino acids 1 to 35, whereas the domain required for Oct-3/4 repression was mapped to amino acids 61 to 126, suggesting that the molecular mechanisms underlying transcriptional activation and repression differ. Like Oct-3/4, Oct-6 can also lower the expression of the Rex-1 promoter via the octamer site, and the amino-terminal portion of Oct-6 mediates this repression. In addition to the octamer motif, a novel positive regulatory element, located immediately 5' of the octamer motif, was identified in the Rex-1 promoter. Mutations in this element greatly reduce Rex-1 promoter activity in F9 cells. High levels of a binding protein(s), designated Rox-1, recognize this novel DNA element in F9 cells, and this binding activity is reduced following RA treatment. Taken together, these results indicate that the Rex-1 promoter is regulated by specific octamer family members in early embryonic cells and that a novel element also contributes to Rex-1 expression.

Oct-1 binds promoter elements required for transcription of the GnRH gene

The GnRH gene is exclusively expressed in a discrete population of neurons in the hypothalamus. The promoter-proximal 173 bp of the rat GnRH gene are highly conserved through evolution and are bound by multiple nuclear proteins found in the neuronal cell line, GT1-7, a model for the GnRH-expressing hypothalamic neuron. To explore the protein-DNA interactions that occur within this promoter and the role of these interactions in targeting GnRH gene expression, we have mutagenized individual binding sites in this region. Deoxyribonuclease I protection experiments reveal that footprint 2, a 51-bp sequence that confers a 20-fold induction of the GnRH gene, is comprised of at least three independent protein-binding sites. Transfections of the GnRH promoter-reporter plasmid containing a series of block mutations of footprint 2 into GT1-7 neurons indicate that each of the three putative component sites contributes to transcriptional activity. Mutations in footprint 4 also decrease GnRH gene expression. Footprint 4 and the promoter-proximal site in footprint 2 contain octamer-like motifs, an element that is also present in the neuron-specific enhancer of the rat GnRH gene located approximately 1.6 kb upstream of the promoter. Previous studies in our laboratory have demonstrated that two enhancer octamer sites are bound by the POU-homeodomain transcription factor Oct-1 in GT1-7 cells. We now show that Oct-1 binds the octamer motifs within footprints 2 and 4. Thus, Oct-1 plays a critical role in the regulation of GnRH transcription, binding functional elements in both the distal enhancer and the promoter-proximal conserved region.

The carboxy-terminal transactivation domain of Oct-4 acquires cell specificity through the POU domain

The POU transcription factor Oct-4 is expressed in totipotent and pluripotent cells of the early mouse embryo and the germ cell lineage. Transactivation capacities of regions flanking the DNA binding domain of Oct-4 were analyzed in undifferentiated and differentiated cell lines. The amino- and carboxy-terminal regions (N domain and C domain) fused to the Gal4 DNA binding domain both functioned as transactivation domains in all cell lines tested. However, the C domain failed to activate transcription in some cell lines in the context of the native protein. The underlying regulatory mechanism appears to involve the POU domain of Oct-4 and can discriminate between different POU domains, since constructs in which the C domain was instead fused to the POU domain of Pit-1 were again equally active in all cell lines. These results indicate that the C domain is subject to cell-type-specific regulation mediated by the Oct-4 POU domain. Phosphopeptide analysis revealed that the cell-type-specific difference of C-domain activity correlates with a difference in Oct-4 phosphorylation status. Since Oct-4 is expressed in a variety of distinct cell types during murine embryogenesis, these results suggest an additional regulatory mechanism for determining Oct-4 function in rapidly changing cell types during development.