Requirements for promoter activity in mouse oocytes and embryos distinguish paternal pronuclei from maternal and zygotic nuclei

Histone remodeling reflects conserved mechanisms of bovine and human preimplantation development

How histone modifications regulate changes in gene expression during preimplantation development in any species remains poorly understood. Using CUT&Tag to overcome limiting amounts of biological material, we profiled two activating (H3K4me3 and H3K27ac) and two repressive (H3K9me3 and H3K27me3) marks in bovine oocytes, 2-, 4-, and 8-cell embryos, morula, blastocysts, inner cell mass, and trophectoderm. In oocytes, broad bivalent domains mark developmental genes, and prior to embryonic genome activation (EGA), H3K9me3 and H3K27me3 co-occupy gene bodies, suggesting a global mechanism for transcription repression. During EGA, chromatin accessibility is established before canonical H3K4me3 and H3K27ac signatures. Embryonic transcription is required for this remodeling, indicating that maternally provided products alone are insufficient for reprogramming. Last, H3K27me3 plays a major role in restriction of cellular potency, as blastocyst lineages are defined by differential polycomb repression and transcription factor activity. Notably, inferred regulators of EGA and blastocyst formation strongly resemble those described in humans, as opposed to mice. These similarities suggest that cattle are a better model than rodents to investigate the molecular basis of human preimplantation development.

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.

Paternal factors in recurrent pregnancy loss: an insight through analysis of non-synonymous single-nucleotide polymorphism in human testis-specific chaperone HSPA2 gene

Heat shock protein A2 (HSPA2) is a testis-specific molecular chaperone of the 70 kDa heat shock protein (HSP70) family and reported to play a key role in spermatogenesis as well as in the remodelling of the sperm surface during capacitation. It is established that mice lacking HSPA2 gene are infertile and spermatozoa that fail to interact with the zona pellucida of the oocyte consistently lack HSPA2 protein expression. However, its role in post fertilization events is not fully understood. Owing to the importance of HSPA2 in male reproduction, the present study is undertaken to reveal the association between genetic mutation and phenotypic variation in recurrent pregnancy loss (RPL) patients through an in silico prediction analysis. In this study, we used different computational tools and servers such as SIFT, PolyPhen2, PROVEAN, nsSNPAnalyzer, and SNPs & GO to analyse the functional consequences of the nsSNPs in human HSPA2 gene. The most damaging amino acid variants generated were subjected to I-Mutant 2.0 and ConSurf. Post-translational modifications such as phosphorylation mediated by these deleterious nsSNPs were analysed using NetPhos 2.0, and gene-gene interaction study was conducted using GeneMANIA. Finally, in-depth studies of the nsSNPs were studied through Project HOPE. The findings of the study revealed 18 nsSNPs to be deleterious using a combinatorial bioinformatic approach. Further functional analysis suggests that screening of nsSNP variants of HSPA2 that tend to be conserved and has potential to undergo phosphorylation at critical positions (rs764410231, rs200951589, rs756852956) may be useful for predicting outcome in altered reproductive outcome. The physicochemical alterations and its impact on the structural and functional conformity were determined by Project HOPE. Gene-gene interaction depicts its close association with antioxidant enzyme (SOD1) strongly supporting an inefficient oxidative scavenging regulatory mechanism in the spermatozoa of RPL patients as reported earlier. The present study has thus identified high-risk deleterious nsSNPs of HSPA2 gene and would be beneficial in the diagnosis and prognosis of the paternal effects in RPL patients.

Zygotic gene activation in mice: profile and regulation

The zygotic genome is transcriptionally silent immediately after fertilization. In mice, initial activation of the zygotic genome occurs in the middle of the one-cell stage. At the mid-to-late two-cell stage, a burst of gene activation occurs after the second round of DNA replication, and the profile of transcribed genes changes dramatically. These two phases of gene activation are called minor and major zygotic gene activation (ZGA), respectively. As they mark the beginning of the gene expression program, it is important to elucidate gene expression regulation during these stages. This article reviews the outcomes of studies that have clarified the profiles and regulatory mechanisms of ZGA.

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.

Cyclin D1 gene expression is essential for cell cycle progression from the maternal-to-zygotic transition during blastoderm development in Japanese quail

Embryogenesis proceeds by a highly regulated series of events. In animals, maternal factors that accumulate in the egg cytoplasm control cell cycle progression at the initial stage of cleavage. However, cell cycle regulation is switched to a system governed by the activated nuclear genome at a specific stage of development, referred to as maternal-to-zygotic transition (MZT). Detailed molecular analyses have been performed on maternal factors and activated zygotic genes in MZT in mammals, fishes and chicken; however, the underlying mechanisms remain unclear in quail. In the present study, we demonstrated that MZT occurred at blastoderm stage V in the Japanese quail using novel gene targeting technology in which the CRISPR/Cas9 and intracytoplasmic sperm injection (ICSI) systems were combined. At blastoderm stage V, we found that maternal retinoblastoma 1 (RB1) protein expression was down-regulated, whereas the gene expression of cyclin D1 (CCND1) was initiated. When a microinjection of sgRNA containing CCND1-targeted sequencing and Cas9 mRNA was administered at the pronuclear stage, blastoderm development stopped at stage V and the down-regulation of RB1 did not occur. This result indicates the most notable difference from mammals in which CCND-knockout embryos are capable of developing beyond MZT. We also showed that CCND1 induced the phosphorylation of the serine/threonine residues of the RB1 protein, which resulted in the degradation of this protein. These results suggest that CCND1 is one of the key factors for RB1 protein degradation at MZT, and the elimination of RB1 may contribute to cell cycle progression after MZT during blastoderm development in the Japanese quail. Our novel technology, which combined the CRISPR/Cas9 system and ICSI, has the potential to become a powerful tool for avian-targeted mutagenesis.

Nutritional Status Impacts Epigenetic Regulation in Early Embryo Development: A Scoping Review

With the increasing maternal age and the use of assisted reproductive technology in various countries worldwide, the influence of epigenetic modification on embryonic development is increasingly notable and prominent. Epigenetic modification disorders caused by various nutritional imbalance would cause embryonic development abnormalities and even have an indelible impact on health in adulthood. In this scoping review, we summarize the main epigenetic modifications in mammals and the synergies among different epigenetic modifications, especially DNA methylation, histone acetylation, and histone methylation. We performed an in-depth analysis of the regulation of various epigenetic modifications on mammals from zygote formation to cleavage stage and blastocyst stage, and reviewed the modifications of key sites and their potential molecular mechanisms. In addition, we discuss the effects of nutrition (protein, lipids, and one-carbon metabolism) on epigenetic modification in embryos and emphasize the importance of various nutrients in embryonic development and epigenetics during pregnancy. Failures in epigenetic regulation have been implicated in mammalian and human early embryo loss and disease. With the use of reproductive technologies, it is becoming even more important to establish developmentally competent embryos. Therefore, it is essential to evaluate the extent to which embryos are sensitive to these epigenetic modifications and nutrition status. Understanding the epigenetic regulation of early embryo development will help us make better use of reproductive technologies and nutrition regulation to improve reproductive health in mammals.

STAT3 Is an Upstream Regulator of Granzyme G in the Maternal-To-Zygotic Transition of Mouse Embryos

The maternal-to-zygotic transition (MZT), which controls maternal signaling to synthesize zygotic gene products, promotes the preimplantation development of mouse zygotes to the two-cell stage. Our previous study reported that mouse granzyme g (Gzmg), a serine-type protease, is required for the MZT. In this study, we further identified the maternal factors that regulate the Gzmg promoter activity in the zygote to the two-cell stage of mouse embryos. A full-length Gzmg promoter from mouse genomic DNA, FL-pGzmg (−1696~+28 nt), was cloned, and four deletion constructs of this Gzmg promoter, Δ1-pGzmg (−1369~+28 nt), Δ2-pGzmg (−939~+28 nt), Δ3-pGzmg (−711~+28 nt) and Δ4-pGzmg (−417~+28 nt), were subsequently generated. Different-sized Gzmg promoters were used to perform promoter assays of mouse zygotes and two-cell stage embryos. The results showed that Δ4-pGzmg promoted the highest expression level of the enhanced green fluorescent protein (EGFP) reporter in the zygotes and two-cell embryos. The data suggested that time-specific transcription factors upregulated Gzmg by binding cis-elements in the −417~+28-nt Gzmg promoter region. According to the results of the promoter assay, the transcription factor binding sites were predicted and analyzed with the JASPAR database, and two transcription factors, signal transducer and activator of transcription 3 (STAT3) and GA-binding protein alpha (GABPα), were identified. Furthermore, STAT3 and GABPα are expressed and located in zygote pronuclei and two-cell nuclei were confirmed by immunofluorescence staining; however, only STAT3 was recruited to the mouse zygote pronuclei and two-cell nuclei injected with the Δ4-pGzmg reporter construct. These data indicated that STAT3 is a maternal transcription factor and may upregulate Gzmg to promote the MZT. Furthermore, treatment with a STAT3 inhibitor, S3I-201, caused mouse embryonic arrest at the zygote and two-cell stages. These results suggest that STAT3, a maternal protein, is a critical transcription factor and regulates Gzmg transcription activity in preimplantation mouse embryos. It plays an important role in the maternal-to-zygotic transition during early embryonic development.

Chromatin remodeling in bovine embryos indicates species-specific regulation of genome activation

The shift from maternal to embryonic control is a critical developmental milestone in preimplantation development. Widespread transcriptomic and epigenetic remodeling facilitate this transition from terminally differentiated gametes to totipotent blastomeres, but the identity of transcription factors (TF) and genomic elements regulating embryonic genome activation (EGA) are poorly defined. The timing of EGA is species-specific, e.g., the timing of murine and human EGA differ significantly. To deepen our understanding of mammalian EGA, here we profile changes in open chromatin during bovine preimplantation development. Before EGA, open chromatin is enriched for maternal TF binding, similar to that observed in humans and mice. During EGA, homeobox factor binding becomes more prevalent and requires embryonic transcription. A cross-species comparison of open chromatin during preimplantation development reveals strong similarity in the regulatory circuitry underlying bovine and human EGA compared to mouse. Moreover, TFs associated with murine EGA are not enriched in cattle or humans, indicating that cattle may be a more informative model for human preimplantation development than mice.

Preimplantation embryos undergo extensive transcriptomic and epigenomic remodeling. Here the authors assay open chromatin in bovine oocytes, embryos, and embryonic stem cells, and compare the transcriptomes and epigenomes of cattle, human and mouse embryos, revealing species-specific regulation of genome activation.

From pluripotency to totipotency: an experimentalist's guide to cellular potency

Embryonic stem cells (ESCs) are derived from the pre-implantation mammalian blastocyst. At this point in time, the newly formed embryo is concerned with the generation and expansion of both the embryonic lineages required to build the embryo and the extra-embryonic lineages that support development. When used in grafting experiments, embryonic cells from early developmental stages can contribute to both embryonic and extra-embryonic lineages, but it is generally accepted that ESCs can give rise to only embryonic lineages. As a result, they are referred to as pluripotent, rather than totipotent. Here, we consider the experimental potential of various ESC populations and a number of recently identified in vitro culture systems producing states beyond pluripotency and reminiscent of those observed during pre-implantation development. We also consider the nature of totipotency and the extent to which cell populations in these culture systems exhibit this property.

Brd4 and P300 Confer Transcriptional Competency during Zygotic Genome Activation

The awakening of the genome after fertilization is a cornerstone of animal development. However, the mechanisms that activate the silent genome after fertilization are poorly understood. Here, we show that transcriptional competency is regulated by Brd4- and P300-dependent histone acetylation in zebrafish. Live imaging of transcription revealed that genome activation, beginning at the miR-430 locus, is gradual and stochastic. We show that genome activation does not require slowdown of the cell cycle and is regulated through the translation of maternally inherited mRNAs. Among these, the enhancer regulators P300 and Brd4 can prematurely activate transcription and restore transcriptional competency when maternal mRNA translation is blocked, whereas inhibition of histone acetylation blocks genome activation. We conclude that P300 and Brd4 are sufficient to trigger genome-wide transcriptional competency by regulating histone acetylation on the first zygotic genes in zebrafish. This mechanism is critical for initiating zygotic development and developmental reprogramming.

Genome-Wide Distribution of Nascent Transcripts in Sperm DNA, Products of a Late Wave of General Transcription

Mature spermatozoa contain a whole repertoire of the various classes of cellular RNAs, both coding and non-coding. It was hypothesized that after fertilization they might impact development, a claim supported by experimental evidence in various systems. Despite the current increasing interest in the transgenerational maintenance of epigenetic traits and their possible determination by RNAs, little remains known about conservation in sperm and across generations and the specificities and mechanisms involved in transgenerational maintenance. We identified two distinct fractions of RNAs in mature mouse sperm, one readily extracted in the aqueous phase of the classical TRIzol procedure and a distinct fraction hybridized with homologous DNA in DNA-RNA complexes recovered from the interface, purified after DNase hydrolysis and analyzed by RNA-seq methodology. This DNA-associated RNA (D RNA) was found to represent as much as half of the cell contents in differentiated sperm, in which a major part of the cytoplasmic material has been discarded. Stable complexes were purified free of proteins and identified as hybrids (R-loops) on the basis of their sensitivity to RNase H hydrolysis. Further analysis by RNA-seq identified transcripts from all the coding and non-coding regions of the genome, thus revealing an extensive wave of transcription, prior to or concomitant with the terminal compaction of the chromatin.

Epigenetic impairments in development of parthenogenetic preimplantation mouse embryos

Parthenogenesis is an activation process of oocytes that occur without the participation of sperm. Evidence suggests that normal development of embryos requires proper expression of several imprinted genes inherited from both the paternal and maternal genomes. Compared to gene expression, histone modifications and chromatin remodeling are not well-documented. In this research, by using immunofluorescence staining for several developmental-associated histone modifications, we investigated whether epigenetic impairments in parthenogenetic embryos act as constraints for proper development. At early stages, fertilized embryos exhibited high methylation of histone H3 at lysine 9 (Me-H3-K9) and Heterochromatin Protein 1 (HP1) present in the maternal chromatin, while paternal chromatin showed weaker HP1 signals. We found that at the two-cell stage in fertilized embryos, HP1, initially detected around the nucleolus, colocalized with chromocenters at one pole of the blastomere, while parthenotes showed a diffused distribution pattern of HP1 throughout the entire nucleoplasm. At the four-cell stage, methylation of histone H3 at arginine 26 (Me-H3-R26) increased at nascent RNA repression sites in fertilized embryos, while parthenotes recorded weaker signals throughout the nucleoplasm, suggesting differences in pluripotency of the ICM cells between the two types of embryos. Moreover, at the blastocyst stage, we observed that the acetylation level of histone H4 at lysine 12 (Ac-H4-K12) was significantly decreased in parthenogenetic ICM compared to that in its fertilized counterpart. To summarize, differences in epigenetic modifications correlating with paternal chromatin’s capacity to regulate nascent RNA repression may contribute to aberrant development and lineage allocation in mouse parthenogenetic embryos.

Histone 3 lysine 9 acetylation is a biomarker of the effects of culture on zygotes

Acetylation of histone proteins is a major determinant of chromatin structure and function. Fertilisation triggers a round of chromatin remodelling that prepares the genome for the first round of transcription from the new embryonic genome. In this study we confirm that fertilisation leads to a marked progressive increase in the level of histone 3 lysine 9 acetylation in both the paternally and maternally derived genomes. The culture of zygotes in simple defined media caused a marked increase in the global level of acetylation and this affected the male pronucleus more than the female. The culture created a marked asymmetry in staining between the two pronuclei that was not readily detected in zygotes collected directly from the reproductive tract and was ameliorated to some extent by optimized culture media. The increased acetylation caused by culture resulted in increased transcription of Hspa1b, a marker of embryonic genome activation. Pharmacological analyses showed the hyperacetylation of H3K9 and the increased expression of Hspa1b caused by culture were due to the altered net activity of a range of histone acetylases and deacetylases. The marked hyperacetylation of histone 3 lysine 9 caused by culture of zygotes may serve as an early biomarker for the effects of culture on the normal function of the embryo. The results also provide further evidence for an effect of the stresses associated with assisted reproductive technologies on the normal patterns of epigenetic reprogramming in the early embryo.

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.

Epigenetic modifications and reprogramming in paternal pronucleus: sperm, preimplantation embryo, and beyond

Pronuclear/zygotic stage is the very first stage of life. In this period, paternal pronucleus undergoes massive chromatin remodeling called "paternal reprogramming" including protamine-histone replacement and subsequent acquisition of epigenetic modifications. Although these consecutive events are required for the initiation of maternal-zygotic transition, the precise role of paternal reprogramming and its effect on subsequent embryonic development has been largely unknown to date. Recently, various new techniques, especially next-generation sequencing (NGS) and RNAi microinjection contribute to unveil the epigenetic transition from both paternal and maternal to early preimplantation embryos, suggesting not only the simple transcriptional regulation by transcription factors but also dynamic structural alteration of chromatin to initiate the wave of zygotic gene transcription. This review summarizes such recent progress for understanding the epigenetic transition in sperm and preimplantation embryos, and further argue about its transgenerational effect.

Nuclease Footprints in Sperm Project Past and Future Chromatin Regulatory Events

Nuclear remodeling to a condensed state is a hallmark of spermatogenesis. This is achieved by replacement of histones with protamines. Regions retaining nucleosomes may be of functional significance. To determine their potential roles, sperm from wild type and transgenic mice harboring a single copy insert of the human protamine cluster were subjected to Micrococcal Nuclease-seq. CENTIPEDE, a hierarchical Bayesian model, was used to identify multiple spatial patterns, "footprints", of MNase-seq reads along the sperm genome. Regions predicted by CENTIPEDE analysis to be bound by a regulatory factor in sperm were correlated with genomic landmarks and higher order chromatin structure datasets to identify potential roles for these factors in regulating either prior or post spermatogenic, i.e., early embryonic events. This approach linked robust endogenous protamine transcription and transgene suppression to its chromatin environment within topologically associated domains. Of the candidate enhancer-bound regulatory proteins, Ctcf, was associated with chromatin domain boundaries in testes and embryonic stem cells. The continuity of Ctcf binding through the murine germline may permit rapid reconstitution of chromatin organization following fertilization. This likely reflects its preparation for early zygotic genome activation and comparatively accelerated preimplantation embryonic development program observed in mouse as compared to human and bull.

HDAC1 and HDAC2 in mouse oocytes and preimplantation embryos: Specificity versus compensation

Oocyte and preimplantation embryo development entail dynamic changes in chromatin structure and gene expression, which are regulated by a number of maternal and zygotic epigenetic factors. Histone deacetylases (HDACs), which tighten chromatin structure, repress transcription and gene expression by removing acetyl groups from histone or non-histone proteins. HDAC1 and HDAC2 are two highly homologous Class I HDACs and display compensatory or specific roles in different cell types or in response to different stimuli and signaling pathways. We summarize here the current knowledge about the functions of HDAC1 and HDAC2 in regulating histone modifications, transcription, DNA methylation, chromosome segregation, and cell cycle during oocyte and preimplantation embryo development. What emerges from these studies is that although HDAC1 and HDAC2 are highly homologous, HDAC2 is more critical than HDAC1 for oocyte development and reciprocally, HDAC1 is more critical than HDAC2 for preimplantation development.

Analysis of chromatin structure in mouse preimplantation embryos by fluorescent recovery after photobleaching

Zygotes are totipotent cells that have the ability to differentiate into all cell types. It is believed that this ability is lost gradually and differentiation occurs along with the progression of preimplantation development. Here, we hypothesized that the loose chromatin structure is involved in the totipotency of one-cell stage embryos and that the change from loose to tight chromatin structure is associated with the loss of totipotency. To address this hypothesis, we investigated the mobility of eGFP-tagged histone H2B (eGFP-H2B), which is an index for the looseness of chromatin, during preimplantation development based on fluorescent recovery after photobleaching (FRAP) analysis. The highest mobility of eGFP-H2B was observed in pronuclei in 1-cell stage embryos and mobility gradually decreased during preimplantation development. The decrease in mobility between the 1- and 2-cell stages depended on DNA synthesis in 2-cell stage embryos. In nuclear transferred embryos, chromatin in the pseudopronuclei loosened to a level comparable to the pronuclei in 1-cell stage embryos. These results indicated that the mobility of eGFP-H2B is negatively correlated with the degree of differentiation of preimplantation embryos. Therefore, we suggest that highly loosened chromatin is involved in totipotency of 1-cell embryos and the loss of looseness is associated with differentiation during preimplantation development.

Dynamics of histone H3 acetylation in the nucleosome core during mouse pre-implantation development

In mammals, the time period that follows fertilization is characterized by extensive chromatin remodeling, which enables epigenetic reprogramming of the gametes. Major changes in chromatin structure persist until the time of implantation, when the embryo develops into a blastocyst, which comprises the inner cell mass and the trophectoderm. Changes in DNA methylation, histone variant incorporation, and covalent modifications of the histones tails have been intensively studied during pre-implantation development. However, modifications within the core of the nucleosomes have not been systematically analyzed. Here, we report the first characterization and temporal analysis of 3 key acetylated residues in the core of the histone H3: H3K64ac, H3K122ac, and H3K56ac, all located at structurally important positions close to the DNA. We found that all 3 acetylations occur during pre-implantation development, but with different temporal kinetics. Globally, H3K64ac and H3K56ac were detected throughout cleavage stages, while H3K122ac was only weakly detectable during this time. Our work contributes to the understanding of the contribution of histone modifications in the core of the nucleosome to the “marking” of the newly established embryonic chromatin and unveils new modification pathways potentially involved in epigenetic reprogramming.

Maternal SIN3A regulates reprogramming of gene expression during mouse preimplantation development

The oocyte-to-embryo transition entails genome activation and a dramatic reprogramming of gene expression that is required for continued development. Superimposed on genome activation and reprogramming is development of a transcriptionally repressive state at the level of chromatin structure. Inducing global histone hyperacetylation relieves this repression and histone deacetylases 1 and 2 (HDAC1 and HDAC2) are involved in establishing the repressive state. Because SIN3A is an HDAC1/2-containing complex, we investigated whether it is involved in reprogramming gene expression during the course of genome activation. We find that Sin3a mRNA is recruited during maturation and that inhibiting its recruitment not only inhibits development beyond the 2-cell stage but also compromises the fidelity of reprogramming gene expression. The SIN3A that is synthesized during oocyte maturation reaches a maximum level in the mid-1-cell embryo and is essentially absent by the mid-2-cell stage. Overexpressing SIN3A in 1-cell embryos has no obvious effect on pre- and postimplantation development. These results provide a mechanism by which reprogramming can occur using a maternally inherited transcription machinery, namely, recruitment of mRNAs encoding transcription factors and chromatin remodelers, such as SIN3A.

The first murine zygotic transcription is promiscuous and uncoupled from splicing and 3' processing

Initiation of zygotic transcription in mammals is poorly understood. In mice, zygotic transcription is first detected shortly after pronucleus formation in 1-cell embryos, but the identity of the transcribed loci and mechanisms regulating their expression are not known. Using total RNA-Seq, we have found that transcription in 1-cell embryos is highly promiscuous, such that intergenic regions are extensively expressed and thousands of genes are transcribed at comparably low levels. Striking is that transcription can occur in the absence of defined core-promoter elements. Furthermore, accumulation of translatable zygotic mRNAs is minimal in 1-cell embryos because of inefficient splicing and 3' processing of nascent transcripts. These findings provide novel insights into regulation of gene expression in 1-cell mouse embryos that may confer a protective mechanism against precocious gene expression that is the product of a relaxed chromatin structure present in 1-cell embryos. The results also suggest that the first zygotic transcription itself is an active component of chromatin remodeling in 1-cell embryos.

Zygotic genome activation during the maternal-to-zygotic transition

Embryogenesis depends on a highly coordinated cascade of genetically encoded events. In animals, maternal factors contributed by the egg cytoplasm initially control development, whereas the zygotic nuclear genome is quiescent. Subsequently, the genome is activated, embryonic gene products are mobilized, and maternal factors are cleared. This transfer of developmental control is called the maternal-to-zygotic transition (MZT). In this review, we discuss recent advances toward understanding the scope, timing, and mechanisms that underlie zygotic genome activation at the MZT in animals. We describe high-throughput techniques to measure the embryonic transcriptome and explore how regulation of the cell cycle, chromatin, and transcription factors together elicits specific patterns of embryonic gene expression. Finally, we illustrate the interplay between zygotic transcription and maternal clearance and show how these two activities combine to reprogram two terminally differentiated gametes into a totipotent embryo.

Regulation of transketolase like 1 gene expression in the murine one-cell stage embryos

In mice, transcription from the zygotic genome starts at the mid-one-cell stage after fertilization. Previous studies showed that an enhancer is not required for transcription at this stage, and that the enhancer-dependent mechanism of transcription is established during the two-cell stage. However, these results were obtained using reporter gene assays with promoters derived from viruses, rather than from endogenous genes. We conducted a reporter-gene assay using the promoter of Tktl1, which is transcribed after fertilization, to investigate the mechanism regulating gene expression at the one-cell stage. When a plasmid containing the 2467 bp upstream and 25 bp downstream of the Tktl1 transcription start site (TSS) was microinjected into the nuclei of growing oocytes, and one-cell stage and early and late two-cell-stage embryos, transcriptional activity was detected in the one-cell- and two-cell-stage embryos, but not in the oocytes. It was highest at the early two-cell stage and was reduced at the late two-cell stage. The decrease in activity at the late two-cell stage was prevented by inhibiting the second round of DNA replication, suggesting that the transcriptionally repressive state is established during the two-cell stage by a mechanism coupled to DNA replication. When the Tktl1 promoter was deleted to leave 56 bp upstream of the TSS which includes GC and TATA boxes, transcriptional activity was still detected in one-cell-stage embryos, but not early or late two-cell-stage embryos. The core promoter of Tktl1 alone seems to be able to induce basal transcription at the one-cell stage. These results suggest that repressive chromatin is established after fertilization in two steps, which occur during the transition from the one- to two-cell stage and during DNA replication at the two-cell stage.

Chromatin-linked determinants of zygotic genome activation

The merging of the maternal and paternal genomes into a single pronucleus after fertilization is accompanied by a remarkable reconfiguration of chromatin in the newly formed zygote. The first stages of embryonic chromatin remodeling take place in the absence of ongoing transcription, during a species-specific developmental time-frame. Once post-fertilization chromatin states are organized, zygotic genome activation (ZGA) is initiated, and embryonic transcripts gradually take control of development. We review here transitions in chromatin modifications associated with the onset of ZGA, and the role of transcription factors and DNA motifs in the regulation of ZGA. We propose a model of sequential chromatin remodeling events preceding ZGA, leading to the onset of embryonic transcription.

The maternal to zygotic transition in mammals

Prior to activation of the embryonic genome, the initiating events of mammalian development are under maternal control and include fertilization, the block to polyspermy and processing sperm DNA. Following gamete union, the transcriptionally inert sperm DNA is repackaged into the male pronucleus which fuses with the female pronucleus to form a 1-cell zygote. Embryonic transcription begins during the maternal to zygotic transfer of control in directing development. This transition occurs at species-specific times after one or several rounds of blastomere cleavage and is essential for normal development. However, even after activation of the embryonic genome, successful development relies on stored maternal components without which embryos fail to progress beyond initial cell divisions. Better understanding of the molecular basis of maternal to zygotic transition including fertilization, the activation of the embryonic genome and cleavage-stage development will provide insight into early human development that should translate into clinical applications for regenerative medicine and assisted reproductive technologies.

Early and delayed aspects of nuclear reprogramming during cloning

The successful production of viable progeny following adult somatic cell nuclear transfer (cloning) provides exciting new opportunities for basic research for investigating early embryogenesis, for the propagation of valuable or endangered animals, for the production of genetically engineered animals, and possibly for developing therapeutically valuable stem cells. Successful cloning requires efficient reprogramming of gene expression to silence donor cell gene expression and activate an embryonic pattern of gene expression. Recent observations indicate that reprogramming may be initiated by early events that occur soon after nuclear transfer, but then continues as development progresses through cleavage and probably to gastrulation. Because reprogramming is slow and progressive, cloned embryos have dramatically altered characteristics in comparison with fertilized embryos. Events that occur early following nuclear transfer may be essential prerequisites for the later events. Additionally, the later reprogramming events may be inhibited by sub-optimum culture environments that exist because of the altered characteristics of cloned embryos. By addressing the unique requirements of cloned embryos, the entire process of reprogramming may be accelerated, thus increasing cloning efficiency.

Preimplantation-embryo-specific cell-cycle regulation is attributable to a low expression of retinoblastoma protein rather than its phosphorylation

Mammalian preimplantation embryos enter the S phase immediately after the end of the M phase; their cell cycle lacks a substantial G1 phase. Previously, we suggested that the absence of the G1 phase was attributable to a loss of retinoblastoma protein (RB) function, which is required for suppression of S phase entrance and that this loss of RB function in turn was attributable to the low RB expression level during preimplantation development in mouse embryos. The present study aimed to examine whether or not RB inhibition by CDK4/6-cyclin D-dependent phosphorylation is involved in the loss of RB function in preimplantation mouse embryos by the expression of p16(INK4a), a potent endogenous inhibitor of CDK4/6-cyclin D. First, the decrease in RB expression between the four-cell and morula stages was confirmed in in vivo-derived mouse embryos. We then examined the efficiency of the p16(INK4a) expression vector in inhibiting RB phosphorylation and cell cycle progression using NIH-3T3 cells and obtained gradual RB dephosphorylation and a significantly lower proliferation rate in p16(INK4a)-transfected cells than in control cells. This indicated the successful p16(INK4a) effects on cell-cycle progression by the vector used. On the other hand, the development rate of mouse embryos injected with the p16(INK4a) expression vector was the same as that of the control embryos, although p16(INK4a) expression was detected at mRNA and protein levels in the former group but not in the control group. These results suggest that RB phosphorylation is not involved in RB dysfunction or in the lack of a G1 phase in mouse embryos and that the decrease in RB expression is important for preimplantation-embryo-specific cell-cycle regulation. Moreover, the present study indicates the similarity between preimplantation embryos and cancer cells, which p16(INK4a) expression does not arrest at the G1 phase.

Maternal germline-specific effect of DNA ligase I on CTG/CAG instability

The instability of (CTG)•(CAG) repeats can cause >15 diseases including myotonic dystrophy, DM1. Instability can arise during DNA replication, repair or recombination, where sealing of nicks by DNA ligase I (LIGI) is a final step. The role of LIGI in CTG/CAG instability was determined using in vitro and in vivo approaches. Cell extracts from a human (46BR) harbouring a deficient LIGI (∼3% normal activity) were used to replicate CTG/CAG repeats; and DM1 mice with >300 CTG repeats were crossed with mice harbouring the 46BR LigI. In mice, the defective LigI reduced the frequency of CTG expansions and increased CTG contraction frequencies on female transmissions. Neither male transmissions nor somatic CTG instability was affected by the 46BR LigI - indicating a post-female germline segregation event. Replication-mediated instability was affected by the 46BR LIGI in a manner that depended upon the location of Okazaki fragment initiation relative to the repeat tract; on certain templates, the expansion bias was unaltered by the mutant LIGI, similar to paternal transmissions and somatic tissues; however, a replication fork-shift reduced expansions and increased contractions, similar to maternal transmissions. The presence of contractions in oocytes suggests that the DM1 replication profile specific to pre-meiotic oogenesis replication of maternal alleles is distinct from that occurring in other tissues and, when mediated by the mutant LigI, is predisposed to CTG contractions. Thus, unlike other DNA metabolizing enzymes studied to date, LigI has a highly specific role in CTG repeat maintenance in the maternal germline, involved in mediating CTG expansions and in the avoidance of maternal CTG contractions.

Epigenetic reprogramming and development: a unique heterochromatin organization in the preimplantation mouse embryo

Fertilization of the oocyte by the sperm results in the formation of a totipotent zygote, in which the maternal and paternal chromatin is enclosed in two pronuclei undergoing distinct programmes of transcriptional activation and chromatin remodelling. The highly packaged paternal chromatin delivered by the sperm is decondensed and acquires a number of specific epigenetic marks, but markedly remains devoid of those usually associated with constitutive heterochromatin. During this period the maternal chromatin remains relatively stable except for marks associated with transcription and/or replication such as arginine methylation and H3/H4 acetylation. The embryo then undergoes a series of mitotic divisions without significant additional growth but differentiation, resulting in the formation of a blastocyst containing distinct cell types. The chromatin remodelling events during these stages are likely to be important in establishing the nuclear foundations required for later triggers of differentiation. Overall, we summarize three important points during these earliest reprogramming events: (i) relatively stable maternal chromatin after fertilization, (ii) rapid acquisition of specific histone marks by the paternal chromatin during the hours that follow fertilization and (iii) rapid remodelling of constitutive heterochromatic marks and modifications in the core of the nucleosome from the first mitotic division. These features are likely to be required for the creation of a chromatin environment compatible with cellular reprogramming and plasticity.

Localization and differential expression of the Krüppel-associated box zinc finger proteins 1 and 54 in early mouse development

Upon fertilization, the zygotic genome is activated. To ensure the transcription of specific genes and avoid promiscuous gene expression, a chromatin-mediated repressive state is established. To characterize potential heterochromatin factors present during the first cleavage, two putative transcriptional repressors, zinc finger protein (ZFP1) and ZFP54, belonging to the Krüppel-associated box (KRAB) zinc finger family, were isolated. ZFP1 and ZFP54 contain an N-terminally located KRAB repressor domain followed by 8 and 12 repeats of Krüppel zinc-finger motifs, respectively. Reverse transcription (RT) and quantitative (q) PCR show that maternally contributed Zfp1 and Zfp54 mRNA are detected throughout preimplantation development. α-Amanitin-treated zygotes revealed that maternal Zfp1 and Zfp54 are fully degraded at the two-cell stage. Microinjections of in vitro-transcribed mRNA encoding a gfp-fused reporter gene into zygotes demonstrated the intracellular distribution of ZFP1-green fluorescent protein (GFP) and ZFP54-GFP colocalized with a DNA marker in the two-cell embryo. The KRAB domain was essential to colocalize with DNA, and deletion of the KRAB domain in ZFP1-GFP and ZFP54-GFP localized in nucleoli and in a ubiquitously manner, respectively. Taken together, this suggests a role for ZFP1 and ZFP54 in transcriptional regulation in early development.

Retrogenes in preimplantation embryo development: a unique mode of transcriptional regulation

Our studies show that retrogenes are preferentially expressed in preimplantation embryos. These genes carry a short noncoding exon 1 that contributes directly to expression of the gene, and a second exon that contains the coding sequence without intron interruption. We show that preimplantation gene expression is first regulated by developmentally regulated transcription factors that target exon 1 and the solitary intron, followed by promoter hypermethylation on implantation and in adult tissues. An understanding of the mechanisms of gene expression during preimplantation development should have an impact on the understanding and treatment of spontaneous abortion and infertility.

Spatiotemporal expression of transcriptional regulators in concert with the maternal-to-embryonic transition during bovine in vitro embryogenesis

Cleavage-stage bovine embryos are transcriptionally quiescent until they reach the 8- to 16-cell stage, and thus rely on the reserves provided by the stored maternal mRNAs and proteins found in the oocytes to achieve their first cell divisions. The objective of this study was to characterize the expression and localization of the transcriptional and translational regulators, Y box binding protein 2 (YBX2), TATA box-binding protein (TBP), and activating transcription factor 2 (ATF2), during bovine early embryo development. Germinal vesicle (GV)- and metaphase II (MII)-stage oocytes, as well as 2-, 4-, 8-, 16-cell-stage embryos, morula, and blastocysts, producedin vitrowere analyzed for temporal and spatial protein expression. Using Q-PCR,ATF2mRNA expression was shown to remain constant from the GV-stage oocyte to the four-cell embryo, and then decreased through to the blastocyst stage. By contrast, the protein levels of ATF2 remained constant throughout embryo development and were found in both the cytoplasm and the nucleus. Both TBP and YBX2 showed opposite protein expression patterns, as YBX2 protein levels decreased throughout development, while TBP levels increased through to the blastocyst stage. Immunolocalization studies revealed that TBP protein was localized in the nucleus of 8- to 16-cell-stage embryos, whereas the translational regulator YBX2 was exclusively cytoplasmic and disappeared from the 16-cell stage onward. This study shows that YBX2, TBP, and ATF2 are differentially regulated through embryo development, and provides insight into the molecular events occurring during the activation of the bovine genome during embryo developmentin vitro.

Histone deacetylase 1 (HDAC1) regulates histone acetylation, development, and gene expression in preimplantation mouse embryos

Superimposed on activation of the embryonic genome in preimplantation mouse embryos is the formation of a chromatin-mediated transcriptionally repressive state that arises in the late two-cell embryo and becomes more pronounced with development. In this study, we investigated expression and function of Class I histone deacetylases (HDAC) HDAC1, HDAC2, and HDAC3 during preimplantation development. HDAC1 is likely a major deacetylase in preimplantation embryos and its expression inversely correlates with changes in the acetylation state of histone H4K5 during preimplantation development. RNAi-mediated reduction of HDAC1 leads to hyperacetylation of histone H4 and a developmental delay even though expression of HDAC2 and HDAC3 is significantly induced in Hdac1-suppressed embryos; increased expression of p21(Cip1/Waf) may contribute to the observed developmental delay. RNAi-mediated reduction of HDAC2 has no noticeable effect on preimplantation development, suggesting that individual HDACs have distinct functions during preimplantation development. Although RNAi-mediated targeting of Hdac3 mRNA was very efficient, maternal HDAC3 protein was stable during preimplantation development, thereby preventing an examination of its role. HDAC1 knockdown does not increase the rate of global transcription in late 2-cell embryos, but does result in elevated levels of expression of a subset of genes; this increased expression correlates with hyperacetylation of histone H4. Results of these experiments suggest that HDAC1 is involved in the development of a transcriptionally repressive state that initiates in 2-cell embryos.

Oocyte quality and maternal control of development

The oocyte is a unique and highly specialized cell responsible for creating, activating, and controlling the embryonic genome, as well as supporting basic processes such as cellular homeostasis, metabolism, and cell cycle progression in the early embryo. During oogenesis, the oocyte accumulates a myriad of factors to execute these processes. Oogenesis is critically dependent upon correct oocyte-follicle cell interactions. Disruptions in oogenesis through environmental factors and changes in maternal health and physiology can compromise oocyte quality, leading to arrested development, reduced fertility, and epigenetic defects that affect long-term health of the offspring. Our expanding understanding of the molecular determinants of oocyte quality and how these determinants can be disrupted has revealed exciting new insights into the role of oocyte functions in development and evolution.

Zygotic gene activation and maternal factors in mammals

Zygotic gene activation (ZGA) is the first event of gene expression after fertilization. Following fertilization, ZGA occurs within a short time interval depending on the animal species. Until ZGA, maternal proteins and transcripts stored in oocytes control embryonic development, indicating the importance of maternal factors for development. Somatic cell cloning also proves the potential of oocyte to reprogram the differentiated cell nuclei to embryonic nuclei. Recent studies show that the epigenetic modifications of nuclei play important roles in controlling gene expression during ZGA. However, the mechanisms that control ZGA remain largely unknown. This review will cover the current understanding of ZGA. Specifically, it will focus on the maternal factors that control gene expression during early embryogenesis.

Asymmetry in histone H3 variants and lysine methylation between paternal and maternal chromatin of the early mouse zygote

In mammalian fertilization, the paternal genome is delivered to the secondary oocyte by sperm with protamine compacted DNA, while the maternal genome is arrested in meiotic metaphase II. Thus, at the beginning of fertilization, the two gametic chromatin sets are strikingly different. We elaborate on this contrast by reporting asymmetry for histone H3 type in the pre-S-phase zygote when male chromatin is virtually devoid of histone H3.1/3.2. Localization of the histone H3.3/H4 assembly factor Hira with the paternal chromatin indicates the presence of histone H3.3. In conjunction with this, we performed a systematic immunofluorescence analysis of histone N-tail methylations at position H3K4, H3K9, H3K27 and H4K20 up to the young pronucleus stage and show that asymmetries reported earlier are systematic for virtually all di- and tri-methylations but not for mono-methylation of H3K4 and H4K20, the only marks studied present in the early male pronucleus. For H4K20 the expanding male chromatin is rapidly mono-methylated. This coincides with the formation of maternally derived nucleosomes, a process which is observed as early as sperm chromatin decondensation occurs. Absence of tri-methylated H3K9, tri-methylated H4K20 and presence of loosely anchored HP1-beta combined with the homogenous presence of mono-methylated H4K20 suggests the absence of a division of the paternal chromatin in eu- and heterochromatin. In summary the male, in contrast to female G1 chromatin, is uniform and contains predominantly histone H3.3 as histone H3 variant.

RNA transcript profiling during zygotic gene activation in the preimplantation mouse embryo

Zygotic gene activation is essential for development beyond the 2-cell stage in the preimplantation mouse embryo. Based on alpha-amanitin-sensitive BrUTP incorporation, transcription initiates in the 1-cell embryo and a major reprogramming of gene expression driven by newly expressed genes is prominently observed during the 2-cell stage. Superimposed on genome activation is the development of a transcriptionally repressive state that is mediated at the level of chromatin structure. The identity of the genes that are expressed during the 1- and 2-cell stages, however, is poorly described, as are those genes involved in mediating the transcriptionally repressive state. Using the Affymetrix MOE430 mouse GeneChip set, we characterized the set of alpha-amanitin-sensitive genes expressed during the 1- and 2-cell stages, and we used Expression Analysis Systematic Explorer (EASE) and Ingenuity Pathway Analysis (IPA) to identify biological and molecular processes represented by these genes, as well as interactions among them. We find that although the 1-cell embryo is transcriptionally active, we did not detect any transcripts present on the MOE430 GeneChip set to be alpha-amanitin-sensitive. Thus, what the BrUTP incorporation represents remains elusive. About 17% of genes expressed in the 2-cell embryo are alpha-amanitin-sensitive. EASE analysis reveals that genes involved in ribosome biogenesis and assembly, protein synthesis, RNA metabolism and transcription are over-represented, suggesting that genome activation during 2-cell stage may not be as global and promiscuous as previously proposed. IPA implicated Myc and Hdac1 as candidate genes involved in genome activation and the development of the transcriptionally repressive state, respectively.

Recapitulation of the Ovum Mutant (Om) Phenotype and Loss of Om Locus Polarity in Cloned Mouse Embryos1

The ovum mutant (Om) locus in mice affects early interactions between sperm and egg that in turn affect viability of embryos beyond the morula stage. Crosses of DDK females to males of many other inbred strains are 95% lethal around the morula stage, whereas reciprocal crosses are fully viable. Available data indicate that the early lethality is the result of an interaction between a factor in the ooplasm and the paternal genome. In this study, we examined whether this lethal interaction would likewise occur in cloned embryos produced by somatic cell nuclear transfer. We find that the Om effect is recapitulated but that the parental origin effect at the Om locus is no longer evident in cloned embryos.

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.

Early transcriptional activation of the hsp70.1 gene by osmotic stress in one-cell embryos of the mouse

In fertilized mouse eggs, de novo transcription of embryonic genes is first observed during the S phase of the one-cell stage. This transcription, however, is mostly limited to the male pronucleus and possibly uncoupled from translation, making the functional meaning obscure. We found that one-cell mouse embryos respond to the osmotic shock of in vitro isolation with migration of HSF1, the canonical stress activator of mammalian heat shock genes, to pronuclei and by transient transcription of the hsp70.1, but not hsp70.3 and hsp90, heat shock genes. Isolated growing dictyate oocytes also display a nuclear HSF1 localization, but, in contrast with embryos, they transcribe both hsp70.1 and hsp70.3 genes only after heat shock. Intranuclear injection of double-stranded oligodeoxyribonucleotides containing HSE, GAGA box or GC box consensus sequences, and antibodies raised to transcription factors HSF1, HSF2, Drosophila melanogaster GAGA factor, or Sp1 demonstrated that hsp70.1 transcription depends on HSF1 in both oocytes and embryos and that Sp1 is dispensable in oocytes and inhibitory in the embryos. Hsp70.1 thus represents the first endogenous gene so far identified to be physiologically activated and tightly regulated after fertilization in mammals.