Developmental acquisition of enhancer function requires a unique coactivator activity

H3.1/3.2 regulate the initial progression of the gene expression program

In mice, transcription from the zygotic genome is initiated at the mid-one-cell stage, and occurs promiscuously in many areas of the genome, including intergenic regions. Regulated transcription from selected genes is established during the two-cell stage. This dramatic change in the gene expression pattern marks the initiation of the gene expression program and is essential for early development. We investigated the involvement of the histone variants H3.1/3.2 in the regulation of changes in gene expression pattern during the two-cell stage. Immunocytochemistry analysis showed low nuclear deposition of H3.1/3.2 in the one-cell stage, followed by a rapid increase in the late two-cell stage. Where chromatin structure is normally closed between the one- and two-cell stages, it remained open until the late two-cell stage when H3.1/3.2 were knocked down by small interfering RNA. Hi-C analysis showed that the formation of the topologically associating domain was disrupted in H3.1/3.2 knockdown (KD) embryos. Promiscuous transcription was also maintained in the late two-cell stage in H3.1/3.2 KD embryos. These results demonstrate that H3.1/3.2 are involved in the initial process of the gene expression program after fertilization, through the formation of a closed chromatin structure to execute regulated gene expression during the two-cell stage.

Mapping putative enhancers in mouse oocytes and early embryos reveals TCF3/12 as key folliculogenesis regulators

Dynamic epigenomic reprogramming occurs during mammalian oocyte maturation and early development. However, the underlying transcription circuitry remains poorly characterized. By mapping cis-regulatory elements using H3K27ac, we identified putative enhancers in mouse oocytes and early embryos distinct from those in adult tissues, enabling global transitions of regulatory landscapes around fertilization and implantation. Gene deserts harbour prevalent putative enhancers in fully grown oocytes linked to oocyte-specific genes and repeat activation. Embryo-specific enhancers are primed before zygotic genome activation and are restricted by oocyte-inherited H3K27me3. Putative enhancers in oocytes often manifest H3K4me3, bidirectional transcription, Pol II binding and can drive transcription in STARR-seq and a reporter assay. Finally, motif analysis of these elements identified crucial regulators of oogenesis, TCF3 and TCF12, the deficiency of which impairs activation of key oocyte genes and folliculogenesis. These data reveal distinctive regulatory landscapes and their interacting transcription factors that underpin the development of mammalian oocytes and early embryos.

Preimplantation embryo gene expression: 56 years of discovery, and counting

The biology of preimplantation embryo gene expression began 56 years ago with studies of the effects of protein synthesis inhibition and discovery of changes in embryo metabolism and related enzyme activities. The field accelerated rapidly with the emergence of embryo culture systems and progressively evolving methodologies that have allowed early questions to be re-addressed in new ways and in greater detail, leading to deeper understanding and progressively more targeted studies to discover ever more fine details. The advent of technologies for assisted reproduction, preimplantation genetic testing, stem cell manipulations, artificial gametes, and genetic manipulation, particularly in experimental animal models and livestock species, has further elevated the desire to understand preimplantation development in greater detail. The questions that drove enquiry from the earliest years of the field remain drivers of enquiry today. Our understanding of the crucial roles of oocyte-expressed RNA and proteins in early embryos, temporal patterns of embryonic gene expression, and mechanisms controlling embryonic gene expression has increased exponentially over the past five and a half decades as new analytical methods emerged. This review combines early and recent discoveries on gene regulation and expression in mature oocytes and preimplantation stage embryos to provide a comprehensive understanding of preimplantation embryo biology and to anticipate exciting future advances that will build upon and extend what has been discovered so far.

What defines the maternal transcriptome?

In somatic cells, RNA polymerase II (Pol II) transcription initiation starts by the binding of the general transcription factor TFIID, containing the TATA-binding protein (TBP) and 13 TBP-associated factors (TAFs), to core promoters. However, in growing oocytes active Pol II transcription is TFIID/TBP-independent, as during oocyte growth TBP is replaced by its vertebrate-specific paralog TBPL2. TBPL2 does not interact with TAFs, but stably associates with TFIIA. The maternal transcriptome is the population of mRNAs produced and stored in the cytoplasm of growing oocytes. After fertilization, maternal mRNAs are inherited by the zygote from the oocyte. As transcription becomes silent after oocyte growth, these mRNAs are the sole source for active protein translation. They will participate to complete the protein pool required for oocyte terminal differentiation, fertilization and initiation of early development, until reactivation of transcription in the embryo, called zygotic genome activation (ZGA). All these events are controlled by an important reshaping of the maternal transcriptome. This procedure combines cytoplasmic readenylation of stored transcripts, allowing their translation, and different waves of mRNA degradation by deadenylation coupled to decapping, to eliminate transcripts coding for proteins that are no longer required. The reshaping ends after ZGA with an almost total clearance of the maternal transcripts. In the past, the murine maternal transcriptome has received little attention but recent progresses have brought new insights into the regulation of maternal mRNA dynamics in the mouse. This review will address past and recent data on the mechanisms associated with maternal transcriptome dynamic in the mouse.

Regulation of zygotic gene activation by chromatin structure and epigenetic factors

After fertilization, the genomes derived from an oocyte and spermatozoon are in a transcriptionally silent state before becoming activated at a species-specific time. In mice, the initiation of transcription occurs at the mid-one-cell stage, which represents the start of the gene expression program. A recent RNA sequencing analysis revealed that the gene expression pattern of one-cell embryos is unique and changes dramatically at the two-cell stage. However, the mechanism regulating this alteration has not yet been elucidated. It has been shown that chromatin structure and epigenetic factors change dynamically between the one- and two-cell stages. In this article, we review the characteristics of transcription, chromatin structure, and epigenetic factors in one- and two-cell mouse embryos and discuss the involvement of chromatin structure and epigenetic factors in the alteration of transcription that occurs between these stages.

Genome Duplication: The Heartbeat of Developing Organisms

The mechanism that duplicates the nuclear genome during the trillions of cell divisions required to develop from zygote to adult is the same throughout the eukarya, but the mechanisms that determine where, when and how much nuclear genome duplication occur regulate development and differ among the eukarya. They allow organisms to change the rate of cell proliferation during development, to activate zygotic gene expression independently of DNA replication, and to restrict nuclear DNA replication to once per cell division. They allow specialized cells to exit their mitotic cell cycle and differentiate into polyploid cells, and in some cases, to amplify the number of copies of specific genes. It is genome duplication that drives evolution, by virtue of the errors that inevitably occur when the same process is repeated trillions of times. It is, unfortunately, the same errors that produce age-related genetic disorders such as cancer.

AT-rich repetitive DNA sequences, transcription frequency and germ layer determination

Non-coding sequences of frog embryo endoderm poly (A+) nuclear RNA are AU-enriched, as compared to those of ectoderm and mesoderm. Endoderm blastomeres contain much less H1 histone than is present in ectoderm and mesoderm. H1 histone preferentially binds AT-rich DNA sequences to repress their transcription. The AT-enrichment of non-coding DNA sequences transcribed into poly (A+) nuclear RNA, as well as the low amount of H1 histone, may contribute to the higher transcription frequency of mRNA of endoderm, as compared to that of ectoderm and mesoderm. A greater accumulation of H1 histone in presumptive mesoderm and ectoderm may prevent transcription of endoderm specifying genes in mesoderm and ectoderm. Experimental upregulation of various transcription factors (TFs) can redirect germ layer fate. Most of these TFs bind AT-rich consensus sequences in DNA, suggesting that H1 histone and TFs active during germ layer determination are binding similar sequences.

Evaluation of blastomere biopsy using a mouse model indicates the potential high risk of neurodegenerative disorders in the offspring

Preimplantation genetic diagnosis (PGD), used in clinical practice, is offered to couples that may suffer from a monogenetic disorder, chromosome aneuploidy, or X-linked disease. However, blastomere biopsy, as an indispensable manipulation during the PGD procedure has not been assessed for its long term health implications. Using a mouse model, we investigated the effect of blastomere biopsy of in vitro cultured four-cell embryos on preimplantation development efficiency, postnatal growth, and physiological and behavioral activity compared with control, non-biopsied embryos. The mice generated after blastomere biopsy showed weight increase and some memory decline compared with the control group. Further protein expression profiles in adult brains were analyzed by a proteomics approach. A total of 36 proteins were identified with significant differences between the biopsied and control groups, and the alterations in expression of most of these proteins have been associated with neurodegenerative diseases. Furthermore hypomyelination of the nerve fibers was observed in the brains of mice in the biopsied group. This study suggested that the nervous system may be sensitive to blastomere biopsy procedures and indicated an increased relative risk of neurodegenerative disorders in the offspring generated following blastomere biopsy. Thus, more studies should be performed to address the possible adverse effects of blastomere biopsy on the development of offspring, and the overall safety of PGD technology should be more rigorously assessed.

Maternal depletion of CTCF reveals multiple functions during oocyte and preimplantation embryo development

CTCF is a multifunctional nuclear factor involved in epigenetic regulation. Despite recent advances that include the systematic discovery of CTCF-binding sites throughout the mammalian genome, the in vivo roles of CTCF in adult tissues and during embryonic development are largely unknown. Using transgenic RNAi, we depleted maternal stores of CTCF from growing mouse oocytes, and identified hundreds of misregulated genes. Moreover, our analysis suggests that CTCF predominantly activates or derepresses transcription in oocytes. CTCF depletion causes meiotic defects in the egg, and mitotic defects in the embryo that are accompanied by defects in zygotic gene expression, and culminate in apoptosis. Maternal pronuclear transfer and CTCF mRNA microinjection experiments indicate that CTCF is a mammalian maternal effect gene, and that persistent transcriptional defects rather than persistent chromosomal defects perturb early embryonic development. This is the first study detailing a global and essential role for CTCF in mouse oocytes and preimplantation embryos.

Abnormal expression of REST/NRSF and Myc in neural stem/progenitor cells causes cerebellar tumors by blocking neuronal differentiation

Medulloblastoma, one of the most malignant brain tumors in children, is thought to arise from undifferentiated neural stem/progenitor cells (NSCs) present in the external granule layer of the cerebellum. However, the mechanism of tumorigenesis remains unknown for the majority of medulloblastomas. In this study, we found that many human medulloblastomas express significantly elevated levels of both myc oncogenes, regulators of neural progenitor proliferation, and REST/NRSF, a transcriptional repressor of neuronal differentiation genes. Previous studies have shown that neither c-Myc nor REST/NRSF alone could cause tumor formation. To determine whether c-Myc and REST/NRSF act together to cause medulloblastomas, we used a previously established cell line derived from external granule layer stem cells transduced with activated c-myc (NSC-M). These immortalized NSCs were able to differentiate into neurons in vitro. In contrast, when the cells were engineered to express a doxycycline-regulated REST/NRSF transgene (NSC-M-R), they no longer underwent terminal neuronal differentiation in vitro. When injected into intracranial locations in mice, the NSC-M cells did not form tumors either in the cerebellum or in the cerebral cortex. In contrast, the NSC-M-R cells did produce tumors in the cerebellum, the site of human medulloblastoma formation, but not when injected into the cerebral cortex. Furthermore, the NSC-M-R tumors were blocked from terminal neuronal differentiation. In addition, countering REST/NRSF function blocked the tumorigenic potential of NSC-M-R cells. To our knowledge, this is the first study in which abnormal expression of a sequence-specific DNA-binding transcriptional repressor has been shown to contribute directly to brain tumor formation. Our findings indicate that abnormal expression of REST/NRSF and Myc in NSCs causes cerebellum-specific tumors by blocking neuronal differentiation and thus maintaining the "stemness" of these cells. Furthermore, these results suggest that such a mechanism plays a role in the formation of human medulloblastoma.

TAF4b, a TBP associated factor, is required for oocyte development and function

Development of a fertilizable oocyte is a complex process that relies on the precise temporal and spatial expression of specific genes in germ cells and in surrounding somatic cells. Since female mice null for Taf4b, a TBP associated factor, are sterile, we sought to determine when during follicular development this phenotype was first observed. At postnatal day 3, ovaries of Taf4b null females contained fewer (P < 0.01) oocytes than ovaries of wild type and heterozygous Taf4b mice. However, expression of only one somatic cell marker Foxl2 was reduced in ovaries at day 15. Despite the reduced number of follicles, many proceed to the antral stage, multiple genes associated with granulosa cell differentiation and oocyte maturation were expressed in a normal pattern, and immature Taf4b null females could be hormonally primed to ovulate and mate. However, the ovulated cumulus oocyte complexes from the Taf4b null mice had fewer (P < 0.01) cumulus cells, and the oocytes were functionally abnormal. GVBD and polar body extrusion were reduced significantly (P < 0.01). The few oocytes that were fertilized failed to progress beyond the two-cell stage of development. Thus, infertility in Taf4b null female mice is associated with defects in early follicle formation, oocyte maturation, and zygotic cleavage following ovulation and fertilization.

An egg apparatus-specific enhancer of Arabidopsis, identified by enhancer detection

Despite a central role in angiosperm reproduction, few gametophyte-specific genes and promoters have been isolated, particularly for the inaccessible female gametophyte (embryo sac). Using the Ds-based enhancer-detector line ET253, we have cloned an egg apparatus-specific enhancer (EASE) from Arabidopsis (Arabidopsis thaliana). The genomic region flanking the Ds insertion site was further analyzed by examining its capability to control gusA and GFP reporter gene expression in the embryo sac in a transgenic context. Through analysis of a 5' and 3' deletion series in transgenic Arabidopsis, the sequence responsible for egg apparatus-specific expression was delineated to 77 bp. Our data showed that this enhancer is unique in the Arabidopsis genome, is conserved among different accessions, and shows an unusual pattern of sequence variation. This EASE works independently of position and orientation in Arabidopsis but is probably not associated with any nearby gene, suggesting either that it acts over a large distance or that a cryptic element was detected. Embryo-specific ablation in Arabidopsis was achieved by transactivation of a diphtheria toxin gene under the control of the EASE. The potential application of the EASE element and similar control elements as part of an open-source biotechnology toolkit for apomixis is discussed.

Localization of Janus Kinase 2 to the Nuclei of Mature Oocytes and Early Cleavage Stage Mouse Embryos

Jak2, which is a member of the Janus tyrosine kinase family, plays essential roles in cytokine signal transduction and in the regulation of cell growth and gene expression. To investigate the involvement of Jak2 in the regulation of early preimplantation development, we examined the expression of Jak2 in mouse embryos. Reverse transcription-polymerase chain reaction assays revealed that the relative amount of Jak2 mRNA was highest in unfertilized oocytes, gradually decreased until the four-cell stage, and remained at low levels until the blastocyst stage. Immunocytochemistry showed that Jak2 was localized predominantly to the female pronucleus in one-cell embryos. The immunofluorescence signal was very weak or undetectable in the male pronucleus. In unfertilized oocytes and one-cell embryos at M phase, Jak2 was localized to the chromosomes. After cleavage to the two-cell stage, the intensity of the immunofluorescence signal decreased in the nucleus while the embryos were in late G2. This decrease was independent of DNA synthesis because it was not affected by inhibition of DNA replication. However, inhibition of protein synthesis repressed the disappearance of Jak2 from the nucleus. These results suggest a novel function for Jak2 in the regulation of early preimplantation development.

Regulation of histone H3 lysine 9 methylation in oocytes and early pre-implantation embryos

Epigenetic modifications of the genome, such as covalent modification of histone residues, ensure appropriate gene activation during pre-implantation development, and are probably involved in the asymmetric reprogramming of the parental genomes after fertilization. We investigated the methylation patterns of histone H3 at lysine 9 (H3/K9), and the regulatory mechanism involved in the asymmetric remodeling of parental genomes during early preimplantation development in mice. Immunocytochemistry with an antibody that specifically recognizes methylated H3/K9 showed a very weak or absent methylation signal in the male pronucleus, whereas a distinct methylation signal was detected in the female pronucleus. This asymmetric H3/K9 methylation pattern in the different parental genomes persisted until the two-cell stage. However, de novo methylation of H3/K9 occurred and the asymmetry was lost during the four-cell stage. The unmethylated male pronucleus underwent de novo methylation when it was transferred into enucleated GV- or MII-stage oocytes, which suggests that histone H3 methylase is active before fertilization, but not afterwards, and that the asymmetric methylation pattern is generated by this change in methylase activity in the cytoplasm after fertilization. Thus, histone H3 is methylated only in the maternal chromosomes, which are present in the oocytes before fertilization, and is not methylated in the paternal chromosomes, which are absent. The maintenance of asymmetric H3/K9 methylation patterns in early embryos is an active process that depends on protein synthesis and zygotic transcription, as de novo methylation in the male pronucleus occurred when either protein synthesis or gene expression was inhibited by cycloheximide orα-amanitin, respectively. In addition, corresponding de novo methylation of H3/K9 and DNA occurred when the male pronucleus was transferred to an enucleated GV oocyte. Our results suggest that H3/K9 methylation is an epigenetic marker of parental genome origin during early preimplantation development.

Expression of genes encoding chromatin regulatory factors in developing rhesus monkey oocytes and preimplantation stage embryos: possible roles in genome activation

One of the most critical events of preimplantation development is the successful activation of gene transcription. Both the timing and the array of genes activated must be controlled. The ability to regulate gene transcription appears to be reduced just prior to the time of the major genome activation event, and changes in chromatin structure appear essential for establishing this ability. Major molecules that modulate chromatin structure are the linker and core histones, enzymes that modify histones, and a wide variety of other factors that associate with DNA and mediate either repressive or activating changes. Among the latter are chromatin accessibility complexes, SWI/SNF complexes, and the YY1 protein and its associated factors. Detailed information about the expression and regulation of these factors in preimplantation stage embryos has not been published for any species. In order to ascertain which of these factors may participate in chromatin remodeling, genome activation, and DNA replication during early primate embryogenesis, we determined the temporal expression patterns of mRNA encoding these factors. Our data identify the predominant members of these different functional classes of factors expressed in oocytes and embryos, and reveal patterns of expression distinct from those patterns seen in somatic cells. Among each of four classes of mRNAs examined, some mRNAs were expressed predominantly in the oocyte, with these largely giving way to others expressed stage specifically in the embryo. This transition may be part of a global mechanism underlying the transition from maternal to embryonic control of development, wherein the oocyte program is silenced and an embryonic pattern of gene expression becomes established. Possible roles for these mRNAs in chromatin remodeling, genome activation, DNA replication, cell lineage determination, and nuclear reprogramming are discussed.

Remodelling the paternal chromatin at fertilization in mammals

At fertilization, the highly condensed and transcriptionally inert chromatin of the spermatozoa becomes remodelled into the decondensed and transcriptionally competent chromatin of the male pronucleus. The chromatin initially becomes dispersed and then transiently recondenses into a small mass upon entry into the ooplasm. This morphological change is coincident with and likely dependent on the replacement of the sperm-specific protamines by oocyte-supplied histones and the organization of the chromatin into nucleosomes. The chromatin then extensively decondenses within the male pronucleus and acquires many of the proteins that are associated with the maternal chromatin. Nonetheless, the paternal chromatin manifests distinct characteristics, including transient hyperacetylation of histone H4, increased transcription of endogenous and microinjected genes, and replication-independent demethylation of DNA. Sperm chromatin remodelling is controlled by an oocyte activity that appears during meiotic maturation and disappears approximately 3 h after activation (release from metaphase II arrest), and which requires factors associated with the germinal vesicle of the oocyte. The molecular components of this activity remain largely unknown. In frogs, nucleoplasmin is required to assemble histones H2A and H2B onto the paternal chromatin. Evidence is presented that related proteins may perform similar functions in mammals. Identifying the mechanisms that underlie sperm chromatin remodelling at fertilization may be relevant for understanding reprogramming of somatic cell nuclei after transfer into oocytes.

Different modes of regulation of transcription and pre-mRNA processing of the structurally juxtaposed homologs, Rnf33 and Rnf35, in eggs and in pre-implantation embryos

Molecular events involved in gene expression in unfertilized eggs and pre-implantation embryos are beginning to be understood. In this work, we investigated the transcription and processing of two structurally juxtaposed mouse RING finger protein genes, Rnf33 and Rnf35. Transcripts of these genes are detected only in eggs and in pre-implantation embryos. Both genes are intronless except for a solitary intron in the 5'-untranslated region. Here, we showed by rapid amplification of cDNA ends (RACE) and reverse transcription experiments that Rnf35 transcription uses a single promoter and a terminating site. On the other hand, Rnf33 is transcribed using multiple promoters. At the four-cell stage, however, Rnf33 mRNA with a single transcription start site derived from the proximal promoter is detected, indicating that it is the major promoter. Sequences upstream of the Rnf35 and the major Rnf33 transcription start sites carry no TATA boxes but a putative transcription initiator (Inr) element is discernible in each case. The processing of the 3'-end of the Rnf33 mRNA is also in disarray with multiple 3'-ends, an event that may be related to the absence of the AAUAAA element and the utilization of AAUAAA-like proxies. The multiplicity of the 3'-untranslated region is partially amended at the four-cell stage when only two major 3'-ends are in use. This work demonstrates that expression of some maternal and early zygotic genes may be opportunistic until a stringent transcriptional regulation mechanism is imposed.

Functional interaction between TATA and upstream CACGTG elements regulates the temporally specific expression of Otx mRNAs during early embryogenesis of the sea urchin, Hemicentrotus pulcherrimus

The orthodenticle-related protein (HpOtx) gene derived from the sea urchin Hemicentrotus pulcherrimus encodes two distinct isoforms, HpOtxE and HpOtxL, which are differentially expressed during early embryogenesis and are driven by TATA-less and TATA-containing promoters, respectively. In order to determine if the TATA element is involved in the establishment of the temporally specific expression profile of the HpOtx gene, reporter genes under the control of modified or wild-type HpOtxE/L promoters were introduced into fertilized eggs. When the activities of the different promoter constructs were examined, we found that deletion of the TATA element from the HpOtxL promoter causes early expression, whereas addition of the TATA element to the HpOtxE promoter causes delayed expression. This suppressive action of the TATA element on transcription from the HpOtxE/L promoters requires the presence of upstream CACGTG elements. These results indicate that the presence or absence of the TATA element determines, at least in part, the expression profile of the HpOtxE/L promoters, in concert with the transcription factor(s) that binds to the upstream CACGTG element. Immunoblot and gel retardation analyses suggest that functional interaction between CACGTG binding factor(s) and TATA factor(s) may be regulated by an unidentified third factor(s) during early embryogenesis in the sea urchin.

Identification of Sox-2 regulatory region which is under the control of Oct-3/4-Sox-2 complex

Sox-2 is a transcriptional cofactor expressed in embryonic stem (ES) cells as well as in neuronal cells. It has been demonstrated that Sox-2 plays an important role in supporting gene expression in ES cells, especially by forming a complex with embryonic Octamer factor, Oct-3/4. Here, we have analyzed the regulatory regions of the Sox-2 gene and identified two enhancers which stimulate transcription in ES cells as well as in embryonal carcinoma cells. These regulatory regions, which we termed Sox regulatory regions (SRR) 1 and 2, exert their function specifically when cells are in an undifferentiated state. Interestingly, like the regulatory elements of FGF-4 and UTF1 genes, combinatorial action of Octamer and Sox-2 binding sites support the SRR2 activity. However, biochemical analyses reveal that, due to the unique sequence and/or its organization, the SRR2 bears distinct characteristics from those of FGF-4 and UTF1 regulatory elements. That is, unlike the FGF-4 gene enhancer, the SRR2 precludes the binding of the Oct-1-Sox-2 complex. The difference between the SRR2 and UTF1 regulatory element is in the ability of SRR2 to recruit the Oct-6-Sox-2 complex as well as the Oct-3/4-Sox-2 complex. Co-transfection analyses confirm that both complexes are able to stimulate transcription through the SRR2 element.

Replication initiation patterns in the beta-globin loci of totipotent and differentiated murine cells: evidence for multiple initiation regions

The replication initiation pattern of the murine beta-globin locus was analyzed in totipotent embryonic stem cells and in differentiated cell lines. Initiation events in the murine beta-globin locus were detected in a region extending from the embryonic Ey gene to the adult betaminor gene, unlike the restricted initiation observed in the human locus. Totipotent and differentiated cells exhibited similar initiation patterns. Deletion of the region between the adult globin genes did not prevent initiation in the remainder of the locus, suggesting that the potential to initiate DNA replication was not contained exclusively within the primary sequence of the deleted region. In addition, a deletion encompassing the six identified 5' hypersensitive sites in the mouse locus control region had no effect on initiation from within the locus. As this deletion also did not affect the chromatin structure of the locus, we propose that the sequences determining both chromatin structure and replication initiation lie outside the hypersensitive sites removed by the deletion.

Paternally inherited transgenes are down-regulated but retain low activity during early embryogenesis in Arabidopsis

We investigated the timing of transgene activation after fertilisation in Arabidopsis following crosses and using two transgenic promoters (from the AtCYCB1 and AtLTP1 genes). Using both a transactivation system and direct transcriptional fusion to drive beta-glucuronidase reporter expression, reciprocal crosses showed a lack of expression of the paternal components. This is consistent with a lack of paternal genome activity previously reported during early seed development in Arabidopsis [Viella-Calzada et al. (2000) Nature 404, 91-94]. However, transactivation experiments of the BARNASE gene gave evidence that at least some paternal loci retain transcriptional activity, though at a low level, during early embryogenesis.

Role of protein synthesis in the development of a transcriptionally permissive state in one-cell stage mouse embryos

The time of onset of gene transcription in the mouse embryo is temporally regulated. A prominent feature of this regulation is a change during the one-cell stage from a transcriptionally nonpermissive state to a transcriptionally permissive state. During the early one-cell stage, the cytoplasm is either inadequate or suppressive for nuclear gene transcription, but by the late one-cell stage, the cytoplasm acquires the ability to support gene transcription either in endogenous nuclei or exogenous nuclei introduced microsurgically. We have investigated the role of protein synthesis in this cytoplasmic transition. Nuclei from two-cell stage embryos treated with alpha-amanitin were used to evaluate the transcriptional permissiveness of late one-cell stage cytoplasm, as indicated by the production of transcripts from four genes that are specifically transcribed at elevated rates during the two-cell stage. Two of these genes were transcribed following nuclear transfer to late one-cell stage cytoplasm, and two were not transcribed. Treatment of the recipient cytoplasm with cycloheximide to inhibit protein synthesis from the early to the late one-cell stage inhibited the transcription of the two genes that were transcribed in the untreated, late one-cell stage recipients. These results indicate that acquisition of the transcriptionally permissive state during the one-cell stage is facilitated by protein synthesis, and that the transcriptional permissiveness in the late one-cell stage cytoplasm is limited to certain genes.

Reconstitution of enhancer function in paternal pronuclei of one-cell mouse embryos

How chromatin-mediated transcription regulates the beginning of mammalian development is currently unknown. Factors responsible for promoter repression and enhancer-mediated relief of this repression are not present in the paternal pronuclei of one-cell mouse embryos but are present in the zygotic nuclei of two-cell embryos. Here we show that coinjection of purified histones and a plasmid-encoded reporter gene into the paternal pronuclei of one-cell embryos at a specific histone-DNA concentration could recreate the behavior observed in two-cell embryos: acquisition of promoter repression and subsequent relief of this repression either by functional enhancers or by histone deacetylase inhibitors. Furthermore, the extent of enhancer-mediated stimulation in one-cell embryos depended on the acetylation status of the injected histones, on the treatment of embryos with a histone deacetylase inhibitor, and on the developmentally regulated appearance of enhancer-specific coactivator activity. The coinjected plasmids in one-cell embryos also exhibited chromatin assembly, as determined by a supercoiling assay. Thus, injection of histones into one-cell embryos faithfully reproduced the chromatin-mediated transcription observed in two-cell embryos. These results suggest that the need for enhancers to stimulate promoters through relief of chromatin-mediated repression occurs once the parental genomes are organized into chromatin. Furthermore, we present a model mammalian system in which the role of individual histones, and particular domains within the histones that are targeted in enhancer function, can be examined using purified mutant histones.

Compaction in preimplantation mouse embryos is regulated by a cytoplasmic regulatory factor that alters between 1- and 2-cell stages in a concentration-dependent manner

Present studies were performed to investigate what factors affect the morphogenesis of preimplantation mouse embryos, and to find the action mechanism of that factor by using cytoplasm removal and its reconstitution from a different developmental stage embryo. Half (HP group) or one-third of cytoplasm (TP group) was removed from 1-cell mouse embryos by micromanipulation, and their morphogenesis and genome expression were compared with sham-operated embryos (SP group). The compaction and blastocoel formation of embryos in both the HP and TP groups were accelerated in time and cell stage when compared with those of the SP group. However, the total activity and time of RNA synthesis, and gene expression of ZO-1alpha+ isoform were not different. To change the cytoplasm composition without altering the nucleus/cytoplasmic ratio, half a 1-cell embryo with both pronuclei was reconstituted with the half enucleated cytoplasm of 1-cell embryo (P + P group), 2-cell (P + 2 group) or 4-cell (P + 4 group) by electrofusion. Embryonic compaction, timing of RNA synthesis, and stage-specific gene expression of the ZO-1alpha(+) isoform in the P + 2 and P + 4 groups were accelerated in time and cell stage than that in the P + P group, but not different between the P + 2 and P + 4 groups. In addition, a blastomere of 2-cell embryo was reconstituted with the enucleated cytoplasm of 1-cell embryo (2 + P group) or 2-cell (2 + 2 group) in equal volume by electrofusion. Also, the karyoplast of 2-cell was fused with the enucleated 1-cell embryo (2 + PP group). Embryonic development, total activity of RNA synthesis, and gene expression of the ZO-1alpha(+) isoform of embryos in the 2 + P and 2 + PP groups were delayed when compared with those of the 2 + 2 group. Also, the phenomena of compaction and blastocoel formation were delayed in the development time and cell stage. From these results, the nucleus/cytoplasm ratio was found to have no direct effect on the regulation of embryonic morphogenesis, although it accelerated compaction and blastocoel formation. However, cytoplasmic factors that altered between 1- and 2-cell stages regulate embryonic morphogenesis, especially compaction, of preimplantation mouse embryos in concentration-dependent manner.

High-mobility group proteins 14 and 17 maintain the timing of early embryonic development in the mouse

The high-mobility group (HMG) proteins 14 and 17 are abundant chromosomal proteins that bind to nucleosomes and enhance transcription. We report that both mRNA species and both proteins are present throughout oogenesis and preimplantation development of the mouse. When antisense oligonucleotides targeting each mRNA species are injected into one-cell embryos, the proteins become depleted at the two- and four-cell stages and reaccumulate at the eight-cell stage. One-cell embryos injected with antisense oligonucleotides targeting both HMG-14 and HMG-17 cleave to the two-cell stage. Subsequent cleavages, however, are delayed compared with control-injected embryos. Nevertheless, these embryos ultimately reach the blastocyst stage. Similarly, injection into the nuclei of two-cell embryos of a peptide corresponding to the common nucleosome-binding domain of HMG-14 and HMG-17 delays progression to the four-cell stage. Furthermore, both RNA and protein synthesis is transiently reduced in antisense-injected embryos compared with injected controls. These results identify HMG-14 and HMG-17 as constitutive components of mouse oocyte and embryonic chromatin and establish a link between the structure of embryonic chromatin and the normal progression of embryonic development.

Soggy, a spermatocyte-specific gene, lies 3.8 kb upstream of and antipodal to TEAD-2, a transcription factor expressed at the beginning of mouse development

Investigation of the regulatory region of mTEAD-2, a gene expressed at the beginning of mouse pre-implantation development, led to the surprising discovery of another gene only 3.8 kb upstream of mTEAD-2. Here we show that this new gene is a single copy, testis-specific gene called SOGGY: (mSgy) that produces a single, dominant mRNA approximately 1.3 kb in length. It is transcribed in the direction opposite to mTEAD-2, thus placing the regulatory elements of these two genes in close proximity. mSgy contains three methionine codons that could potentially act as translation start sites, but most mSGY protein synthesis in vitro was initiated from the first Met codon to produce a full-length protein, suggesting that mSGY normally consists of 230 amino acids (26.7 kDa). Transcription began at a cluster of nucleotides approximately 150 bp upstream of the first Met codon using a TATA-less promoter contained within the first 0.9 kb upstream. The activity of this promoter was repressed by upstream sequences between -0.9 and -2.5 kb in cells that did not express mSgy, but this repression was relieved in cells that did express mSgy. mSgy mRNA was detected in embryos only after day 15 and in adult tissues only in the developing spermatocytes of seminiferous tubules, suggesting that mSgy is a spermatocyte-specific gene. Since mTEAD-2 and mSgy were not expressed in the same cells, the mSgy/mTEAD-2 locus provides a unique paradigm for differential regulation of gene expression during mammalian development.

REST-VP16 activates multiple neuronal differentiation genes in human NT2 cells

The RE1-silencing transcription factor (REST)/neuron-restrictive silencer factor (NRSF) can repress transcription of a battery of neuronal differentiation genes in non-neuronal cells by binding to a specific consensus DNA sequence present in their regulatory regions. However, REST/NRSF(-/-) mice suggest that the absence of REST/NRSF-dependent repression alone is not sufficient for the expression of these neuronal differentiation genes and that the presence of other promoter/enhancer-specific activators is required. Here we describe the construction of a recombinant transcription factor, REST-VP16, by replacing repressor domains of REST/NRSF with the activation domain of a viral activator VP16. In transient transfection experiments, REST-VP16 was found to operate through RE1 binding site/neuron-restrictive enhancer element (RE1/NRSE), activate plasmid-encoded neuronal promoters in various mammalian cell types and activate cellular REST/NRSF target genes, even in the absence of factors that are otherwise required to activate such genes. Efficient expression of REST-VP16 through adenoviral vectors in NT2 cells, which resemble human committed neuronal progenitor cells, was found to cause activation of multiple neuronal genes that are characteristic markers for neuronal differentiation. Thus, REST-VP16 could be used as a unique tool to study neuronal differentiation pathways and neuronal diseases that arise due to the deregulation of this process.

The neuronal repressor REST/NRSF is an essential regulator in medulloblastoma cells

Medulloblastoma is the most malignant pediatric brain tumor. It is believed to originate from the undifferentiated external granule layer cells in the cerebellum, but the mechanism of tumorigenesis remains unknown. Here we studied three types of human medulloblastoma cells that express markers corresponding to different levels of neuronal differentiation. They expressed the neuronal repressor element 1 (RE1) silencing transcription factor/neuron-restrictive silencer factor (REST/NRSF; refs. 7-10) at very high levels compared with either neuronal progenitor NTera2 (NT2) cells or fully differentiated human neuron teratocarcinoma (hNT cells). To counter the effect of REST/NRSF, we used a recombinant transcription factor, REST-VP16, constructed by replacing repressor domains of REST/NRSF with the activation domain of viral protein (VP16). Transient expression of REST-VP16 in medulloblastoma cells was able to compete with the endogenous REST/NRSF for DNA binding and stimulate neuronal promoters. High-efficiency expression of REST-VP16 mediated by adenovirus vectors (Ad.REST-VP16) in medulloblastoma cells was able to counter REST/NRSF-mediated repression of neuronal promoters, stimulate expression of endogenous neuronal genes and trigger apoptosis through the activation of caspase cascades. Furthermore, intratumoral injection of Ad.REST-VP16 in established medulloblastoma tumors in nude mice inhibited their growth. Therefore, REST/NRSF may serve as a new target for therapeutic interventions for medulloblastoma through agents such as REST-VP16.

Linker histone transitions during mammalian oogenesis and embryogenesis

A unique characteristic of the oocyte is that, although it is a differentiated cell, it can to give rise to a population of undifferentiated embryonic cells. This transition from a differentiated to a totipotential condition is thought to be mediated in part by changes in chromatin composition or configuration. In many non-mammalian organisms, oocytes contain unique subtypes of the linker histone H1, which are replaced in early embryos by the so-called somatic histone H1 subtypes. We review evidence that such histone H1 subtype switches also occur in mammals. Immunologically detectable somatic H1 is present in mitotically proliferating oogonia but gradually becomes undetectable after the oocytes enter meiosis. Immunoreactive somatic H1 remains undetectable throughout oogenesis and the early cell cycles after fertilization. Following activation of the embryonic genome, it is assembled onto chromatin. In contrast to the absence of immunoreactive protein, mRNAs encoding each of the five mammalian somatic H1 subtypes are present in growing oocytes and newly fertilized embryos, indicating that post-transcriptional mechanisms regulate expression of these genes. This maternal mRNA is degraded at the late 2-cell stage, and embryonically encoded mRNAs accumulate after embryos reach the 4-cell stage. During the period when somatic H1 is not detectable, oocytes and embryos contain mRNA encoding a sixth subtype, histone H1(0) which accumulates in differentiated somatic cells, and the nuclei can be stained with an H1(0)-specific antibody. We propose that the linker histone composition of the oocyte lineage resembles that of other mammalian cells, namely, that the somatic H1 subtypes predominate in mitotically active oogonia, that histone H1(0) becomes prominent in differentiated oocytes, and that following fertilization and transcriptional activation of the embryonic somatic H1 genes, the somatic H1 subtypes are reassembled onto chromatin of the embryonic cells. Potential functions of these linker histone subtype switches are discussed, including stabilization by H1(0) of the differentiated state of the oocytes, protection of the oocyte chromatin from factors that remodel sperm chromatin after fertilization, and restoration by the incorporation of the somatic H1 subtypes of the totipotential state of embryonic nuclei.

Regulation of gene expression at the beginning of mammalian development and the TEAD family of transcription factors

In mouse development, transcription is first detected in late 1-cell embryos, but translation of newly synthesized transcripts does not begin until the 2-cell stage. Thus, the onset of zygotic gene expression (ZGE) is regulated at the level of both transcription and translation. Chromatin-mediated repression is established after formation of a 2-cell embryo, concurrent with the developmental acquisition of enhancer function. The most effective enhancer in cleavage stage mouse embryos depends on DNA binding sites for TEF-1, the prototype for a family of transcription factors that share the same TEA DNA binding domain. Mice contain at least four, and perhaps five, genes with the same TEA DNA binding domain (mTEAD genes). Since mTEAD-2 is the only one expressed during the first 7 days of mouse development, it is most likely responsible for the TEAD transcription factor activity that first appears at the beginning of ZGE. All four mTEAD genes are expressed at later embryonic stages and in adult tissues; virtually every tissue expresses at least one family member, consistent with a critical role for TEAD proteins in either cell proliferation or differentiation. The 72-amino acid TEA DNA binding domains in mTEAD-2, 3, and 4 are approximately 99% homologous to the same domain in mTEAD-1, and all four proteins bind specifically to the same DNA sequences in vitro with a Kd value of 16-38 nM DNA. Since TEAD proteins appear to be involved in both activation and repression of different genes and do not appear to be functionally redundant, differential activity of TEAD proteins must result either from association with other proteins or from differential sensitivity to chromatin-packaged DNA binding sites.

A developmentally regulated GAGA box-binding factor and Sp1 are required for transcription of the hsp70.1 gene at the onset of mouse zygotic genome activation

We have investigated the onset of zygotic genome transcription in early two-cell mouse embryos by analyzing the regulation of hsp70.1, one of the first genes expressed after fertilization. The transcriptional activation of both an episomic hsp70 promoter and the endogenous hsp70.1 gene requires the contiguity of the GC box proximal to the TATA box with a GAGA box and involves GC box- and GAGA box-binding factors. In vivo transcription factor titrations with double-stranded oligodeoxyribonucleotides and antibodies pinpoint these factors as Sp1 and a novel murine GAGA box-binding factor, which is structurally related to the Drosophila GAGA factor and acts as transcriptional coactivator/potentiator of Sp1. Mouse unfertilized eggs and one-cell and two-cell embryos display a GAGA box-binding activity of maternal origin that disappears at the four-cell stage and is also abundant in the gonads, but is barely detectable in other adult tissues. In light of the well-established nucleosome-disruption role of the Drosophila GAGA factor, these findings suggest a novel mechanism of enhancer-independent gene derepression in early mouse embryos.

Developmental change in TATA-box utilization during preimplantation mouse development

Activation of the embryonic genome during preimplantation mouse development is characterized by a marked reprogramming of gene expression that is essential for further development. Expression of the protein translation initiation factor eIF-1A gene is driven by a proximal TATA-containing promoter and a distal TATA-less promoter. Using specific amplification of cDNA ends that resolves transcripts derived from the TATA-less and TATA-containing promoters, we find that 70% of the eIF-1A transcripts are derived from the TATA-containing promoter in the fully-grown oocyte. Activation of the embryonic genome during the two-cell stage is accompanied by a change in promoter utilization such that only 25% of the transcripts are now derived from the TATA-containing promoter, i.e., 75% are derived from the TATA-less promoter. When one-cell embryos are cultured to the two-cell stage in the presence of alpha-amanitin, this change in transcript abundance is not observed, i.e., the distribution of transcripts is similar to that observed in the oocyte. By the blastocyst stage only 5% of the transcripts are generated from the TATA-containing promoter. If the change in TATA-box utilization for the eIF-1A reflects an underlying global change in TATA-box utilization, a dramatic change in promoter utilization may occur during preimplantation development such that TATA-less promoters are more efficiently utilized. Such a change in promoter utilization could contribute significantly to the reprogramming of gene expression that occurs during the maternal-to-zygotic transition.

Reprogramming nuclei: insights from cloning, nuclear transfer and heterokaryons

Mammals and amphibians can be cloned following the transfer of embryonic nuclei into enucleated eggs or oocytes. As nuclear functions become more specialized in the differentiated cells of an adult, successful cloning using these nuclei as donors becomes more difficult. Differentiation involves the assembly of specialized forms of repressive chromatin including linker histones, Polycomb group proteins and methyl-CpG-binding proteins. These structures compartmentalize chromatin into functional domains and maintain the stability of the differentiated state through successive cell divisions. Efficient cloning requires the erasure of these structures. The erasure can be accomplished through use of molecular chaperones and enzymatic activities present in the oocyte, egg or zygote. We discuss the mechanisms involved in reprogramming nuclei after nuclear transfer and compare them with those that occur during remodeling of somatic nuclei after heterokaryon formation. Finally we discuss how one might alter the properties of adult nuclei to improve the efficiency of cloning.

Mechanisms and control of embryonic genome activation in mammalian embryos

Activation of transcription within the embryonic genome (EGA) after fertilization is a complex process requiring a carefully coordinated series of nuclear and cytoplasmic events, which collectively ensure that the two parental genomes can be faithfully reprogrammed and restructured before transcription occurs. Available data indicate that inappropriate transcription of some genes during the period of nuclear reprogramming can have long-term detrimental effects on the embryo. Therefore, precise control over the time of EGA is essential for normal embryogenesis. In most mammals, genome activation occurs in a stepwise manner. In the mouse, for example, some transcription occurs during the second half of the one-cell stage, and then a much greater phase of genome activation occurs in two waves during the two-cell stage, with the second wave producing the largest onset of de novo gene expression. Changes in nuclear structure, chromatin structure, and cytoplasmic macromolecular content appear to regulate these periods of transcriptional activation. A model is presented in which a combination of cell cycle-dependent events and both translational and posttranslational regulatory mechanisms within the cytoplasm play key roles in mediating and regulating EGA.

The gene for the embryonic stem cell coactivator UTF1 carries a regulatory element which selectively interacts with a complex composed of Oct-3/4 and Sox-2

UTF1 is a transcriptional coactivator which has recently been isolated and found to be expressed mainly in pluripotent embryonic stem (ES) cells (A. Okuda, A. Fukushima, M. Nishimoto, et al., EMBO J. 17:2019-2032, 1998). To gain insight into the regulatory network of gene expression in ES cells, we have characterized the regulatory elements governing UTF1 gene expression. The results indicate that the UTF1 gene is one of the target genes of an embryonic octamer binding transcription factor, Oct-3/4. UTF1 expression is, like the FGF-4 gene, regulated by the synergistic action of Oct-3/4 and another embryonic factor, Sox-2, implying that the requirement for Sox-2 by Oct-3/4 is not limited to the FGF-4 enhancer but is rather a general mechanism of activation for Oct-3/4. Our biochemical analyses, however, also reveal one distinct difference between these two regulatory elements: unlike the FGF-4 enhancer, the UTF1 regulatory element can, by its one-base difference from the canonical octamer-binding sequence, selectively recruit the complex comprising Oct-3/4 and Sox-2 and preclude the binding of the transcriptionally inactive complex containing Oct-1 or Oct-6. Furthermore, our analyses reveal that these properties are dictated by the unique ability of the Oct-3/4 POU-homeodomain that recognizes a variant of the Octamer motif in the UTF1 regulatory element.

Lack of enhancer function in mammals is unique to oocytes and fertilized eggs

Previous studies have shown that the lack of novel coactivator activity in mouse oocytes and one-cell embryos (fertilized eggs) renders them incapable of utilizing Gal4:VP16-dependent enhancers (distal elements) but not promoters (proximal elements) in regulating transcription. This coactivator activity first appears in two- to four-cell embryos coincident with the major activation of zygotic gene expression. Here we show that whereas oocytes and fertilized eggs could utilize Sp1-dependent promoters, they could not utilize Sp1-dependent enhancers, although they showed promoter repression, which is a requirement for delineating enhancer function. In contrast, both Sp1-dependent promoters and enhancers were functional in two- to four-cell embryos. Furthermore, the same embryonic stem cell mRNA that provided the coactivator activity for Gal4:VP16-dependent enhancer function also provided Sp1-dependent enhancer function in oocytes. Therefore, the coactivator activity appears to be a requirement for general enhancer function. To determine whether the absence of enhancer function is a unique property of oocytes or a general property of other terminally differentiated cells, transcription was examined in terminally differentiated hNT neurons and their precursors, undifferentiated NT2 stem cells. The results showed that both cell types could utilize enhancers and promoters. Thus, in mammals, the lack of enhancer function appears to be unique to oocytes and fertilized eggs, suggesting that it provides a safeguard against premature activation of genes prior to zygotic gene expression during development.

Replication origins in metazoan chromosomes: fact or fiction?

The process by which eukaryotic cells decide when and where to initiate DNA replication has been illuminated in yeast, where specific DNA sequences (replication origins) bind a unique group of proteins (origin recognition complex) next to an easily unwound DNA sequence at which replication can begin. The origin recognition complex provides a platform on which additional proteins assemble to form a pre-replication complex that can be activated at S-phase by specific protein kinases. Remarkably, multicellular eukaryotes, such as frogs, flies, and mammals (metazoa), have counterparts to these yeast proteins that are required for DNA replication. Therefore, one might expect metazoan chromosomes to contain specific replication origins as well, a hypothesis that has long been controversial. In fact, recent results strongly support the view that DNA replication origins in metazoan chromosomes consist of one or more high frequency initiation sites and perhaps several low frequency ones that together can appear as a nonspecific initiation zone. Specific replication origins are established during G1-phase of each cell cycle by multiple parameters that include nuclear structure, chromatin structure, DNA sequence, and perhaps DNA modification. Such complexity endows metazoa with the flexibility to change both the number and locations of replication origins in response to the demands of animal development.

Relief of a repressed gene expression state in the mouse 1-cell embryo requires DNA replication

In the mouse, transcriptional permissiveness is established in the fertilized egg prior to the activation of zygotic genes at the 2-cell stage. Therefore, gene inactivity initiated at the end of gametogenesis results from a complex process, involving more than an inhibition of the basal transcriptional apparatus. We have examined the ability of the first intron (I1) of the human hypoxanthine phosphoribosyl transferase gene, which functions as an enhancer in embryonic stem cells, to activate a reporter gene when placed proximally to or at a distance from the HSV-tk promoter, or when integrated into the mouse genome as part of a stable transgene. In microinjected embryos, I1 functions as an enhancer sequence; however, its competence for long-range activation appears only after the late 1-cell stage and depends on the first DNA replication. Moreover, activation of microinjected transgenes from proximal enhancers occurs in the late 2-cell embryo and in the male pronucleus of 1-cell embryos blocked for DNA replication; whereas, for integrated transgenes, proximal enhancer activity is subject to position effects in the 2-cell embryo and first occurs at the 2- or 4-cell stage, but only after completion of DNA replication. Therefore, the absence of long-range activation and a non-permissive genomic state (the relief of which both depend on DNA replication), together with an inactive transcriptional apparatus, appear to converge to prevent any gene activity in the 1-cell embryo. We propose that the embryo exploits the process of DNA replication to relieve the transcriptionally repressive state that was initially established to fulfil two purposes: (1) to arrest maternal gene expression in the maturing oocyte and (2) to protect the unicellular egg and 1-cell embryo from premature differentiation. Reactivation of gene expression by DNA replication would therefore serve to coordinate cell proliferation and differentiation in the preimplantation embryo.

Changes in histone synthesis and modification at the beginning of mouse development correlate with the establishment of chromatin mediated repression of transcription

The transition from a late 1-cell mouse embryo to a 4-cell embryo, the period when zygotic gene expression begins, is accompanied by an increasing ability to repress the activities of promoters and replication origins. Since this repression can be relieved by either butyrate or enhancers, it appears to be mediated through chromatin structure. Here we identify changes in the synthesis and modification of chromatin bound histones that are consistent with this hypothesis. Oocytes, which can repress promoter activity, synthesized a full complement of histones, and histone synthesis up to the early 2-cell stage originated from mRNA inherited from the oocyte. However, while histones H3 and H4 continued to be synthesized in early 1-cell embryos, synthesis of histones H2A, H2B and H1 (proteins required for chromatin condensation) was delayed until the late 1-cell stage, reaching their maximum rate in early 2-cell embryos. Moreover, histone H4 in both 1-cell and 2-cell embryos was predominantly diacetylated (a modification that facilitates transcription). Deacetylation towards the unacetylated and monoacetylated H4 population in fibroblasts began at the late 2-cell to 4-cell stage. Arresting development at the beginning of S-phase in 1-cell embryos prevented both the appearance of chromatin-mediated repression of transcription in paternal pronuclei and synthesis of new histones. These changes correlated with the establishment of chromatin-mediated repression during formation of a 2-cell embryo, and the increase in repression from the 2-cell to 4-cell stage as linker histone H1 accumulates and core histones are deacetylated.

Transcription factor mTEAD-2 is selectively expressed at the beginning of zygotic gene expression in the mouse

mTEF-1 is the prototype of a family of mouse transcription factors that share the same TEA DNA binding domain (mTEAD genes) and are widely expressed in adult tissues. At least one member of this family is expressed at the beginning of mouse development, because mTEAD transcription factor activity was not detected in oocytes, but first appeared at the 2-cell stage in development, concomitant with the onset of zygotic gene expression. Since embryos survive until day 11 in the absence of mTEAD-1 (TEF-1), another family member likely accounts for this activity. Screening an EC cell cDNA library yielded mTEAD-1, 2 and 3 genes. RT-PCR detected RNA from all three of these genes in oocytes, but upon fertilization, mTEAD-1 and 3 mRNAs disappeared. mTEAD-2 mRNA, initially present at approx. 5,000 copies per egg, decreased to approx. 2,000 copies in 2-cell embryos before accumulating to approx. 100,000 copies in blastocysts, consistent with degradation of maternal mTEAD mRNAs followed by selective transcription of mTEAD-2 from the zygotic genome. In situ hybridization did not detect mTEAD RNA in oocytes, and only mTEAD-2 was detected in day-7 embryos. Northern analysis detected all three RNAs at varying levels in day-9 embryos and in various adult tissues. A fourth mTEAD gene, recently cloned from a myotube cDNA library, was not detected by RT-PCR in either oocytes or preimplantation embryos. Together, these results reveal that mTEAD-2 is selectively expressed for the first 7 days of embryonic development, and is therefore most likely responsible for the mTEAD transcription factor activity that appears upon zygotic gene activation.