Post-transcriptional control in the early mouse embryo

Regulation of mammalian totipotency: a molecular perspective from in vivo and in vitro studies

In mammals, cells acquire totipotency at fertilization. Embryonic genome activation (EGA), which occurs at the 2-cell stage in the mouse and 4- to 8-cell stage in humans, occurs during the time window at which embryonic cells are totipotent and thus it is thought that EGA is mechanistically linked to the foundations of totipotency. The molecular mechanisms that lead to the establishment of totipotency and EGA had been elusive for a long time, however, recent advances have been achieved with the establishment of new cell lines with greater developmental potential and the application of novel low-input high-throughput techniques in embryos. These have unveiled several principles of totipotency related to its epigenetic makeup but also to characteristic features of totipotent cells. In this review, we summarize and discuss current views exploring some of the key drivers of totipotency from both in vitro cell culture models and embryogenesis in vivo.

The parental contributions to early plant embryogenesis and the concept of maternal-to-zygotic transition in plants

The maternal-to-zygotic transition (MZT) is a major developmental transition in the life cycles of animals. It consists of two associated processes: maternal transcript clearance and zygotic genome activation (ZGA). The concept of MZT has been controversially discussed in plants. In this short review, we summarize recent advances in understanding the timing of ZGA and the similarities and differences between ZGA in eudicots and monocots. We discuss the parental contributions to the transcriptome of the proembryo and parental control of early embryogenesis, and we examine distinct differences in the ZGA between animals and plants, update relevant concepts on MZT, and highlight outstanding questions in this field.

Maternal control of early patterning in sea urchin embryos

Sea urchin development has been studied extensively for more than a century and considered regulative since the first experimental evidence. Further investigations have repeatedly supported this standpoint by revealing the presence of inductive mechanisms that alter cell fate decisions at early cleavage stages and flexibility of development in response to environmental conditions. Some features indicate that sea urchin development is not completely regulative, but actually includes determinative events. In 16-cell embryos, mesomeres and macromeres represent multipotency, while the cell fate of most vegetal micromeres is restricted. It is known that the mature sea urchin eggs are polarized by the asymmetrical distribution of some maternal mRNAs and proteins. Spatially-distributed maternal factors are necessary for the orientation of the primary animal-vegetal axis, which is established by both maternal and zygotic mechanisms later in development. The secondary dorsal-ventral axis is conditionally specified later in development. Dorsal-ventral polarity is very liable during the early cleavages, though more recent data argue that its direction may be oriented by maternal asymmetry. In this review, we focus on the role of maternal factors in initial embryonic patterning during the first cleavages of sea urchin embryos before activation of the embryonic genome.

Transcriptional Activation of Arabidopsis Zygotes Is Required for Initial Cell Divisions

Commonly referred to as the maternal-to-zygotic transition, the shift of developmental control from maternal-to-zygotic genomes is a key event during animal and plant embryogenesis. Together with the degradation of parental gene products, the increased transcriptional activities of the zygotic genome remodels the early embryonic transcriptome during this transition. Although evidence from multiple flowering plants suggests that zygotes become transcriptionally active soon after fertilization, the timing and developmental requirements of zygotic genome activation in Arabidopsis thaliana (Arabidopsis) remained a matter of debate until recently. In this report, we optimized an expansion microscopy technique for robust immunostaining of Arabidopsis ovules and seeds. This enabled the detection of marks indicative of active transcription in zygotes before the first cell division. Moreover, we employed a live-imaging culture system together with transcriptional inhibitors to demonstrate that such active transcription is physiologically required in zygotes and early embryos. Our results indicate that zygotic genome activation occurs soon after fertilization and is required for the initial zygotic divisions in Arabidopsis.

Mediator complex component MED13 regulates zygotic genome activation and is required for postimplantation development in the mouse

Understanding factors that regulate zygotic genome activation (ZGA) is critical for determining how cells are reprogrammed to become totipotent or pluripotent. There is limited information regarding how this process occurs physiologically in early mammalian embryos. Here, we identify a mediator complex subunit, MED13, as translated during mouse oocyte maturation and transcribed early from the zygotic genome. Knockdown and conditional knockout approaches demonstrate that MED13 is essential for ZGA in the mouse, in part by regulating expression of the embryo-specific chromatin remodeling complex, esBAF. The role of MED13 in ZGA is mediated in part by interactions with E2F transcription factors. In addition to MED13, its paralog, MED13L, is required for successful preimplantation embryo development. MED13L partially compensates for loss of MED13 function in preimplantation knockout embryos, but postimplantation development is not rescued by MED13L. Our data demonstrate an essential role for MED13 in supporting chromatin reprogramming and directed transcription of essential genes during ZGA.

Characteristics of candidate genes associated with embryonic development in the cow: Evidence for a role for WBP1 in development to the blastocyst stage

The goal was to gain understanding of how 12 genes containing SNP previously related to embryo competence to become a blastocyst (BRINP3, C1QB, HSPA1L, IRF9, MON1B, PARM1, PCCB, PMM2, SLC18A2, TBC1D24, TTLL3 and WBP1) participate in embryonic development. Gene expression was evaluated in matured oocytes and embryos. BRINP3 and C1QB were not detected at any stage. For most other genes, transcript abundance declined as the embryo developed to the blastocyst stage. Exceptions were for PARM1 and WBP1, where steady-state mRNA increased at the 9-16 cell stage. The SNP in WBP1 caused large differences in the predicted three-dimensional structure of the protein while the SNP in PARM1 caused smaller changes. The mutation in WBP1 causes an amino acid substitution located close to a P-P-X-Y motif involved in protein-protein interactions. Moreover, the observation that the reference allele varies between mammalian species indicates that the locus has not been conserved during mammalian evolution. Knockdown of mRNA for WBP1 decreased the percent of putative zygotes becoming blastocysts and reduced the number of trophectoderm cells and immunoreactive CDX2 in the resulting blastocysts. WBP1 is an important gene for embryonic development in the cow. Further research to identify how the SNP in WBP1 affects processes leading to differentiation of the embryo into TE and ICM lineages is warranted.

Hypertranscription in Development, Stem Cells, and Regeneration

Cells can globally upregulate their transcriptome during specific transitions, a phenomenon called hypertranscription. Evidence for hypertranscription dates back over 70 years but has gone largely ignored in the genomics era until recently. We discuss data supporting the notion that hypertranscription is a unifying theme in embryonic development, stem cell biology, regeneration, and cell competition. We review the history, methods for analysis, underlying mechanisms, and biological significance of hypertranscription.

Making the first decision: lessons from the mouse

Pre-implantation development encompasses a period of 3-4 days over which the mammalian embryo has to make its first decision: to separate the pluripotent inner cell mass (ICM) from the extra-embryonic epithelial tissue, the trophectoderm (TE). The ICM gives rise to tissues mainly building the body of the future organism, while the TE contributes to the extra-embryonic tissues that support embryo development after implantation. This review provides an overview of the cellular and molecular mechanisms that control the critical aspects of this first decision, and highlights the role of critical events, namely zytotic genome activation, compaction, polarization, asymmetric cell divisions, formation of the blastocyst cavity and expression of key transcription factors.

Unwind and transcribe: chromatin reprogramming in the early mammalian embryo

Within the first few days of life, the unipotent gametic genomes are rapidly reprogrammed to support emergence of pluripotent cells in the early mammalian embryo. It is now appreciated that this crucial stage of development involves dramatic changes to chromatin at multiple levels, such as DNA methylation, histone modifications, histone mobility, and higher-order chromatin organization. Technological advances are beginning to allow genome-wide views of this chromatin reprogramming, and provide new approaches to functionally dissect its regulation. Here we review recent insights into the dynamic chromatin environment of the early mouse embryo. New data challenge long-held assumptions, for example, with regards to the asymmetry of DNA methylation of the parental genomes or the onset of functional zygotic genome activation. We discuss how impaired chromatin reprogramming can lead to early embryonic lethality, but might also have delayed effects that only manifest later in embryogenesis or postnatally, potentially influencing the propensity for adult-onset diseases.

Maternal-zygotic knockout reveals a critical role of Cdx2 in the morula to blastocyst transition

The first lineage segregation in the mouse embryo generates the inner cell mass (ICM), which gives rise to the pluripotent epiblast and therefore the future embryo, and the trophectoderm (TE), which will build the placenta. The TE lineage depends on the transcription factor Cdx2. However, when Cdx2 first starts to act remains unclear. Embryos with zygotic deletion of Cdx2 develop normally until the late blastocyst stage leading to the conclusion that Cdx2 is important for the maintenance but not specification of the TE. In contrast, down-regulation of Cdx2 transcripts from the early embryo stage results in defects in TE specification before the blastocyst stage. Here, to unambiguously address at which developmental stage Cdx2 becomes first required, we genetically deleted Cdx2 from the oocyte stage using a Zp3-Cre/loxP strategy. Careful assessment of a large cohort of Cdx2 maternal-zygotic null embryos, all individually filmed, examined and genotyped, reveals an earlier lethal phenotype than observed in Cdx2 zygotic null embryos that develop until the late blastocyst stage. The developmental failure of Cdx2 maternal-zygotic null embryos is associated with cell death and failure of TE specification, starting at the morula stage. These results indicate that Cdx2 is important for the correct specification of TE from the morula stage onwards and that both maternal and zygotic pools of Cdx2 are required for correct pre-implantation embryogenesis.

Differences in gene expression between mouse and human for dynamically regulated genes in early embryo

Infertility is a worldwide concern that can be treated with in vitro fertilization (IVF). Improvements in IVF and infertility treatment depend largely on better understanding of the molecular mechanisms for human preimplantation development. Several large-scale studies have been conducted to identify gene expression patterns for the first five days of human development, and many functional studies utilize mouse as a model system. We have identified genes of possible importance for this time period by analyzing human microarray data and available data from online databases. We selected 70 candidate genes for human preimplantation development and investigated their expression in the early mouse development from oocyte to the 8-cell stage. Maternally loaded genes expectedly decreased in expression during development both in human and mouse. We discovered that 25 significantly upregulated genes after fertilization in human included 13 genes whose orthologs in mouse behaved differently and mimicked the expression profile of maternally expressed genes. Our findings highlight many significant differences in gene expression patterns during mouse and human preimplantation development. We also describe four cancer-testis antigen families that are also highly expressed in human embryos: PRAME, SSX, GAGE and MAGEA.

Hira-mediated H3.3 incorporation is required for DNA replication and ribosomal RNA transcription in the mouse zygote

Extensive chromatin reprogramming occurs at fertilization and is thought to be under the control of maternal factors, but the underlying mechanisms remain poorly understood. We report that maternal Hira, a chaperone for the histone variant H3.3, is required for mouse development past the zygote stage. Male pronucleus formation is inhibited upon deletion of Hira due to a lack of nucleosome assembly in the sperm genome. Hira mutant oocytes are incapable of developing parthenogenetically, indicative of a role for Hira in the female genome. Both parental genomes show highly reduced levels of DNA replication and transcription in the mutants. It has long been thought that transcription is not required for zygote development. Surprisingly, we found that Hira/H3.3-dependent transcription of ribosomal RNA is required for first cleavage. Our results demonstrate that Hira-mediated H3.3 incorporation is essential for parental genome reprogramming and reveal an unexpected role for rRNA transcription in the mouse zygote.

Nonspecies-specific effects of mouse oviducts on the development of bovine IVM/IVF embryos by a serum free co-culture

In Experiment 1, development of bovine embryos derived from in vitro-matured (IVM) and in vitro-fertilized (IVF) oocytes was examined under 4 culture conditions: 1) co-culture with mouse ampullae continuously for 8 d, 2) co-culture with mouse ampullae that were replaced with fresh ampullae at 48-h intervals, 3) co-culture with bovine granulosa cell monolayers, and 4) culture in medium alone. Culture medium consisted of tissue culture medium 199 (TCM-199) supplemented with 1% fetal calf serum (FCS). Inseminated oocytes were transferred to each of the culture treatment 24 h after insemination and were cultured for 8 d. The number of blastocysts per number of cleaved ova obtained after co-culture with mouse ampullae (42.9%) was significantly (P<0.05) higher than that obtained after co-culture with granulosa cell monolayers (28.3%) or culture without cells (4.2%). In Experiment 2, the developmental ability of bovine IVM/IVF embryos co-cultured with mouse ampullae supplemented with or without serum was examined. When serum was excluded from the culture medium, 26.4% (33 125 ) of the total number of embryos cultured were able to develop to the blastocysts stage using this co-culture system. This value was comparable to that obtained in a serum-supplemented co-culture system (30.7%; 39 125 ). In addition, the developmental ability of embryos that reached to the 4-cell stage or beyond at 46 to 48 h after insemination was not significantly different when the embryos were co-cultured with mouse ampullae with (38.5 vs 44.6%) or without (37.0 vs 33.8%) serum.

Gene expression during the oocyte-to-embryo transition in mammals

The seminal question in modern developmental biology is the origins of new life arising from the unification of sperm and egg. The roots of this question begin from 19th to 20th century embryologists studying fertilization and embryogenesis. Although the revolution of molecular biology has yielded significant insight into the complexity of this process, the overall orchestration of genes, molecules, and cells is still not fully formed. Early mammalian development, specifically the oocyte-to-embryo transition, is essentially under "maternal command" from factors deposited in the cytoplasm during oocyte growth, independent of de novo transcription from the nascent embryo. Many of the advances in understanding this developmental period occurred in tandem with application of new methods and techniques from molecular biology, from protein electrophoresis to sequencing and assemblies of whole genomes. From this bed of knowledge, it appears that precise control of mRNA translation is a key regulator coordinating the molecular and cellular events occurring during oocyte-to-embryo transition. Notably, oocyte transcriptomes share, yet retain some uniqueness, common genetic motifs among all chordates. The common genetic motifs typically define fundamental processes critical for cellular maintenance, whereas the unique genetic features may be a source of variation and a substrate for sexual selection, genetic drift, or gene flow. One purpose for this complex interplay among genes, proteins, and cells may allow for evolution to transform and act upon the underlying processes, at molecular, structural and organismal levels, to increase diversity, which is the ultimate goal of sexual reproduction.

The maternal-to-zygotic transition: a play in two acts

All animal embryos pass through a stage during which developmental control is handed from maternally provided gene products to those synthesized from the zygotic genome. This maternal-to-zygotic transition (MZT) has been extensively studied in model organisms, including echinoderms, nematodes, insects, fish,amphibians and mammals. In all cases, the MZT can be subdivided into two interrelated processes: first, a subset of maternal mRNAs and proteins is eliminated; second, zygotic transcription is initiated. The timing and scale of these two events differ across species, as do the cellular and morphogenetic processes that sculpt their embryos. In this article, we discuss conserved and distinct features within the two component processes of the MZT.

Perturbations in Mouse Embryo Development and Viability Caused by Ammonium Are More Severe after Exposure at the Cleavage Stages1

The presence of ammonium in culture medium has a detrimental effect on embryo physiology and biochemistry; however, the stage at which the embryo is most sensitive to this effect is unknown. The aim of this study was to determine the exact stage at which the embryo is most vulnerable to ammonium by exposing the preimplantation embryo to 300 muM ammonium either at the precompaction stage (between the zygote and two-cell or the two-cell to eight-cell) or at the postcompaction stage (between the eight-cell and blastocyst). This study determined that exposure of embryos to ammonium at the precompaction stage from either the zygote to two-cell stage or from the two-cell to the eight-cell stage did not affect the rate of development to the blastocyst stage; however, the resultant blastocysts had decreased cell numbers and inner cell mass cells. Furthermore, these blastocysts had increased levels of cellular apoptosis and perturbed levels of Slc2a3 expression and glucose uptake. Transfer of these blastocysts revealed that, while implantation was not affected, the number of fetuses was reduced by culture with ammonium at the precompaction stage and fetal development was delayed, as observed by reduced crown-rump length and maturity. In contrast, the later stage embryo was more resistant to the negative effects of ammonium, with only Slc2a3 expression and fetal maturity affected. This raises the possibility that the later stage embryo is more able to protect itself from in vitro-derived stress and that the majority of in vitro-induced damage to mouse embryos is inflicted at the early stages of development.

Diverse patterns of poly(A) tail elongation and shortening of murine maternal mRNAs from fully grown oocyte to 2-cell embryo stages

We previously succeeded in constructing a cDNA library, CPF7, enriched with cDNA derived from maternal RNAs with the extended poly(A) tail in mouse fertilized eggs. In this study, we performed RNA blot analysis to examine the elongation in maternal RNAs using 20 representative clones isolated from CPF7 as probes. Various patterns of elongation, shortening, and/or degradation of maternal RNAs were observed from fully grown oocytes to early 2-cell embryos and could be roughly classified into three types and seven subtypes. These findings indicate that poly(A) elongation and shortening of maternal RNAs are not restricted to certain types of maternal RNAs but occur in many of them, and suggest a complex mechanism governing modification of the 3' end of maternal RNAs during the oocyte-to-embryo transition.

The effect of paternal heat stress on protein profiles of pre-implantation embryos in the mouse

The study was undertaken to compare the protein profiles of [35S]-methionine-labelled control-sired embryos with heat-sired embryos at 7, 14 or 21 days after mature fertile B6CBF F1 male mice were kept at 36 +/- 0.3 degrees C and 62 +/- 2.7% relative humidity for 24 h. One-dimensional gel electrophoresis and autoradiographs were used to examine the protein profiles between the two-cell embryos and the blastocysts. The results obtained demonstrate that paternal heat stress 7 or 14 days earlier did not apparently affect protein patterns of two-cell embryos, four-cell to eight-cell embryos, morulae or blastocysts. However, 21 days earlier, there were changes in protein patterns of two-cell embryos and abnormal embryos, but not the morulae. To further support and extend these results, two-dimensional gel electrophoresis and phosphorimaging were employed and the results obtained show that paternal heat stress 21 days before mating affected protein profiles of two-cell embryos and morulae in the mouse. Together, these findings have indicated that paternal heat stress affects most but not all protein patterns of pre-implantation embryos, which strongly supports our previous results demonstrating that paternal heat stress significantly reduced the developmental proportion of pre-implantation embryos in the mouse.

Lineage allocation and cell polarity during mouse embryogenesis

The first developmental lineage allocation during the generation of the mouse blastocyst is to outer trophoblast or to inner pluriblast (inner cell mass; ICM) cells. This allocation seems to be initiated at the 8-cell stage, when blastomeres polarise. Polarisation is followed by differentiative divisions at the subsequent two cleavage divisions to generate polar outer and non-polar inner 16- and 32-cells. The key events in polarisation are regulated post-translationally through a cell contact-mediated pathway, which imposes a heritable determinant-like organisation on the blastomere cortex. Two proteins in particular, E-cadherin and ezrin, are intimately involved in the generation and stabilisation of developmentally significant information. Transcriptional differences between lineages appear to follow and may coincide with the lineage commitment of cells.

From the germinal cells to the newborn animal: the transmission of genes and life through the generations

The technology of reproduction progressed considerably during the last decade, leading to a certain availability of in vitro methods for fertilisation, oocyte maturation and embryo culture. The most spectacular manipulations are cloning and transgenesis. This review focuses on the early appearance of germinal cell precursors and the long-standing fate of gametes in mammals. The evident complexity and long-term programming of events in gametes and early embryos explain part of the difficulties encountered during the development of in vitro and in vivo methods such as multiple ovulation and embryo transfer (MOET), oestrus synchronisation, ovulation induction, superovulation, in vitro maturation and fertilisation, cryopreservation, transgenesis, nuclear transfer and cloning) and the occurrence of unexpected alterations of development, e.g. embryonic or fetal mortality, large-weight newborn syndrome and other dysregulations in imprinting or DNA transmission.

Temporal effect of human oviductal cell and its derived embryotrophic factors on mouse embryo development

Mouse embryos at different stages of development were cocultured with human oviduct cells or cultured in the presence of oviduct-derived embryotrophic factor-1, -2, and -3 (ETF-1, -2, and -3) for various amounts of time within the preimplantation period. Cocultures that included the period from 48 to 72 h post-hCG stimulated cell division and increased the cell numbers in the inner cell mass (ICM) of the exposed blastocyst. Exposure of embryos to oviductal cells from 96 to 120 h post-hCG increased the cell number in the trophectoderm (TE), blastocyst size, hatching rate, attachment, and in vitro spreading of the blastocyst. ETF-1 and ETF-2 affected embryos between 48 and 72 h post-hCG by increasing the number of cells in the ICM. In contrast, ETF-3 had a more profound effect on embryos that were exposed from 96 to 120 h post-hCG, where it mostly affected the development of TE cells, leading to higher hatching rate. Human oviductal cells improved mouse embryo development partly by the production of high molecular weight embryotrophic factors. These factors had differential effects on mouse embryo development.

Mouse Sebox homeobox gene expression in skin, brain, oocytes, and two-cell embryos

Sebox is a mouse paired-like homeobox gene, previously named OG-9. Sebox genomic DNA and cDNA were cloned and sequenced. In addition, rat and human Sebox genomic DNAs were cloned and sequenced, and the predicted amino acid sequences were compared. The mouse Sebox gene was mapped to chromosome 11 near the Evi 2 locus. The mouse Sebox gene is expressed in brain, skin, ovary, and liver of mice. In the brain, the Sebox gene is expressed in the cerebral cortex and CA areas of the hippocampus, pontine nuclei, choroid plexus, and the cerebellum. Northern analysis and RNase protection assays revealed low levels of Sebox RNA in 12-day mouse embryos and higher levels in 18- and 19-day embryos. In late embryos and newborn mice, Sebox expression is localized in the epidermis. In adult mice, Sebox RNA was found in maturing oocytes and in fertilized eggs; however, the abundance of Sebox RNA is decreased in the two-cell embryo, and little or none was detected in the four-cell embryo. Hence, Sebox is a maternally expressed homeobox gene.

Expression of leukemia inhibitory factor and its receptor in preimplantation embryos

OBJECTIVE

To examine the expression of leukemia inhibitory factor (LIF) and its receptor (LIF-R) transcripts in human and murine preimplantation embryos.

DESIGN

Prospective study.

SETTING

University medical center.

PATIENT(S)

Human oocytes were obtained from patients undergoing IVF treatment. Two-cell murine embryos were obtained from ICR strain mice.

INTERVENTION(S)

Second-day intracytoplasmic sperm injection procedures were performed on oocytes that failed to be fertilized by IVF. Embryos were cultured to various stages and collected for study.

MAIN OUTCOME MEASURE(S)

The transcript levels of LIF and LIF-R in these embryos were examined and semiquantitated using single-cell reverse transcription-polymerase chain reaction methodology.

RESULT(S)

Leukemia inhibitory factor and LIF-R transcripts were detectable in most human preimplantation embryos (30 of 34 and 31 of 34 embryos showed LIF and LIF-R messenger RNA, respectively). There was a trend toward decreased expression of both transcripts in embryos at the four-cell stage and in embryos in which growth had been arrested for 24-48 hours. The expression of LIF and LIF-R genes in murine embryos was inconsistent.

CONCLUSION(S)

Preimplantation human embryos express LIF and LIF-R messenger RNA. It is suggested that LIF may be able to affect embryo development through its action at stages before implantation in an autocrine or paracrine manner.

Microelectrophoretic analysis of changes in protein expression patterns in mouse oocytes and preimplantation embryos

One- and two-dimensional polyacrylamide microslab gel electrophoresis followed by silver staining was devised to visualize picogram to nanogram levels of proteins and was applied to the analysis of 1-20 mouse oocytes and embryos (approximately 16.5-330 ng of protein) during preimplantation development. Compared with values in embryos, more bands in the higher molecular weight range were found only for unfertilized oocytes in one-dimensional microelectrophoresis. A marked decrease in the number of protein spots occurred after fertilization in two-dimensional microelectrophoresis. Both findings indicate a decrease in maternal proteins caused by fertilization. Silver-staining densities were almost invariable for 8 major spots, but increased, decreased, or varied for 32 minor spots in developing embryos from the 1-cell to the morula stage, signifying spot-specific changes in the expression of zygotic proteins during development. The protein patterns in cumulus cells and blastocysts were different from those in oocytes and embryos. Even in a single 1-cell embryo, major spots and some minor spots were detectable by our two-dimensional microelectrophoretic technique, but many more minor spots were visualized in five 1-cell embryos, exemplifying the limit of our microelectrophoretic technique. As a preliminary result, a two-dimensional immunoblot pattern is shown for glucose transporter 1 expressed in morulae.

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.

SNAP-25 is essential for cortical granule exocytosis in mouse eggs

Synaptosome-associated protein of 25 kDa (SNAP-25) has been shown to play an important role in Ca2+-dependent exocytosis in neurons and endocrine cells. During fertilization, sperm-egg fusion induces cytosolic Ca2+ mobilization and subsequently Ca2+-dependent cortical granule (CG) exocytosis in eggs. However, it is not yet clear whether SNAP-25 is involved in this process. In this study, we determined the expression and function of SNAP-25 in mouse eggs. mRNA and SNAP-25 were detected in metaphase II (MII) mouse eggs by RT-PCR and immunoblot analysis, respectively. Next, to determine the function of SNAP-25, we evaluated the change in CG exocytosis with a membrane dye, tetramethylammonium-1,6-diphenyl-1,3,5-hexatriene, after microinjection of a botulinum neurotoxin A (BoNT/A), which selectively cleaves SNAP-25 in MII eggs. Sperm-induced CG exocytosis was significantly inhibited in the BoNT/A-treated eggs. The inhibition was attenuated by coinjection of SNAP-25. These results suggest that SNAP-25 may be involved in Ca2+-dependent CG exocytosis during fertilization in mouse eggs.

Paternal transcripts for glucose-6-phosphate dehydrogenase and adenosine deaminase are first detectable in the human preimplantation embryo at the three- to four-cell stage

The transition between dependence on maternal transcripts and proteins inherited in the oocyte and embryonic gene expression in the human preimplantation embryo occurs at the four- to eight-cell stage. Recently, studies using reverse transcriptase polymerase chain reaction (RT-PCR) have detected paternal transcripts for the Y-linked genes, ZFY and SRY, and the myotonic dystrophy associated protein kinase gene, DK, as early as the late pronucleate one-cell stage. However, expression at the protein level has not been demonstrated and its function at these early stages is unknown. Using coding sequence polymorphisms to distinguish maternal and paternal transcripts, we have examined the transcription of two ubiquitously expressed genes: X-linked glucose-6-phosphate dehydrogenase (G6PD) and adenosine deaminase (ADA). Both G6PD and ADA are housekeeping genes with TATA-less promoters which, because of their roles in metabolism and ubiquitous expression, may provide a more reliable indication of the timing of activation of the embryonic genome. They also each have biallelic polymorphisms with a high heterozygosity ratio which can be detected by restriction digestion. Couples undergoing in vitro fertilization (IVF) were screened for these polymorphisms. Individual spare oocytes and embryos at different stages of preimplantation development were analyzed by RT-PCR and appropriate restriction digestion in those cases in which the male partner carried a different allele to the female partner. In addition, since only female embryos inherit the paternal allele of X-linked G6PD, cDNA was also analyzed for ZFX/ZFY transcripts to identify the sex of each embryo. One hundred and twenty three individual oocytes and embryos were analyzed by RT-PCR and restriction digestion to detect the paternal transcripts from the polymorphic alleles. Maternal transcripts for G6PD, ADA, and ZFX were detected in all oocytes and embryos and at all stages. Following restriction digestion, paternal G6PD and ZFY transcripts were first detected at the four-cell stage and paternal ADA transcripts in an embryo at the three-cell stage coinciding with the onset of dependency on transcription from the embryonic genome. This approach should be widely applicable to other genes since similar polymorphisms exist in the coding regions of many genes.

Involvement of Rabphilin-3A in cortical granule exocytosis in mouse eggs

Rabphilin-3A is a putative target protein for Rab3A, a member of the small GTP-binding protein superfamily that has been suggested to play a role in regulated exocytosis in presynapses. In this study we determined the expression and the function of Rabphilin-3A in mouse eggs at fertilization. Rabphilin-3A mRNA and protein were detected by reverse transcriptase-PCR and immunoblot analysis, respectively, in metaphase II mouse eggs. Immunofluorescence analysis showed that Rabphilin-3A protein was distributed in the cortical region in eggs. Sperm induces cortical granule (CG) exocytosis via an increase in cytosolic Ca2+ at fertilization. We microinjected the NH2- or COOH-terminal fragment of recombinant Rabphilin-3A into metaphase II eggs. Neither treatments altered the sperm-induced cytosolic Ca2+ increase, but both inhibited CG exocytosis in a dose-dependent manner. The NH2-terminal fragment was more effective than the COOH-terminal fragment. Full-length Rabphilin-3A did not affect CG exocytosis, but it attenuated the inhibition of CG exocytosis by the NH2-terminal fragment. These results show that Rabphilin-3A is involved in Ca(2+)-dependent CG exocytosis at fertilization in mouse eggs.

Transcriptional activity in in vitro produced bovine two- and four-cell embryos

The objectives of this study on in vitro produced bovine two- and four-cell embryos were (1) to investigate the uptake of 3H-uridine through the plasma membrane, (2) to characterize the pattern of RNA synthesis during the second cell cycle, and (3) to measure the incorporation of 3H-uridine into de novo synthesized RNA. A total of 200 embryos were incubated with 3H-uridine for 15, 30, 60 (two- and four-cell embryos), 120 (four-cell embryos), 180 (two-cell embryos), and 240 min (two- and four-cell embryos), respectively. 3H-uridine uptake reached a maximum by 30 min in two-cell embryos, whereas four-cell embryos reached a maximum at 120 min. A total of 440 two-cell embryos were isolated 27-33 hr postinsemination (hpi), and 90 of these were incubated for 10 hr with 3H-uridine (200 microCi/ml). The remainder were incubated with 3H-uridine for 3 hr starting at 0-3 (n = 54), 3-6 (n = 75), 6-9 (n = 77), or 9-12 (n = 77) hr after cleavage to the two-cell stage. Control two-cell embryos (n = 67) were incubated with 3H-uridine supplemented with 5 mg/ml of unlabelled uridine for 10 hr (inhibition control), or they were incubated with 3H-uridine for 10 hr and RNase treated (100 micrograms/ml) post fixation (RNase control). Subsequently, the embryos were processed for autoradiography. The long-term incubation revealed transcription (autoradiographically labelled nuclei) in a total of 77% of the two- and four-cell embryos. No transcription was observed in any of the 3 hr incubation groups. The RNase control embryos lacked labelling of the nuclei, whereas the inhibition control embryos only showed markedly reduced labelling. Finally, total RNA extraction was performed on a total of 336 two-cell embryos that were incubated with 3H-uridine or 3H-uridine supplemented with unlabelled uridine for 2, 5, or 10 hr. It was possible to detect an increasing amount of labelled RNA after the 2, 5, and 10 hr incubation periods, and it was possible to inhibit this incorporation competitively. Together the data demonstrate a low level of transcription during the second cell cycle without a well-defined transcriptional peak.

Chromatin condensation activity and cortical activity during the first three cell cycles of a mouse embryo

One-cell parthenogenetic haploid embryos and blastomeres of the 2- and 4-cell diploid mouse embryos were observed in vitro for the occurrence of two cytoplasmic activities: the cortical activity and the chromatin condensation activity. For this purpose anucleated halves (AHs) and nucleated halves (NHs) were produced by bisection of one-cell embryos and of blastomeres. The cortical activity (manifested by surface deformations) was observed only during the first cleavage cycle. In AHs the surface activity began at the same time as in NHs and disappeared before the time of the cleavage division of nucleated halves. Anucleate fragments of blastomeres from 2- and 4-cell embryos did not exhibit any cortical activity. In the absence of the native nucleus the chromatin condensation activity (assayed by premature chromatin condensation of interphase thymocyte nuclei introduced into cytoplasts by cell fusion) could also have been detected only in the first cleavage cycle. In AHs this activity appeared at the time when NHs started to cleave and disappeared after the NHs finished the first cleavage division. AHs obtained from 2-cell and 4-cell stage blastomeres did not reveal condensation activity.

Analysis of G protein alpha subunit mRNA abundance in preimplantation mouse embryos using a rapid, quantitative RT-PCR approach

We have developed a novel reverse transcription-polymerase chain reaction (RT-PCR)-based approach for systematically quantifying in a single experiment the abundances of many different mRNAs in preimplantation mouse embryos. With this approach, the entire mRNA population from a small number of embryos is amplified while preserving the relative abundance of each mRNA in the cDNA population. The cDNA is analyzed by quantitative hybridization to radiolabeled probes. The approach is very sensitive and provides reliable, quantitative data regarding changes in mRNA abundance. A major advantage of this method is that estimates of mRNA copy number can be obtained and compared between different mRNAs. With this approach, we analyzed the patterns of expression of nine G protein alpha subunit mRNAs (G alpha s, G alpha i, G alpha q, G alpha o, and G alpha 11-15) in oocytes, eggs, and preimplantation embryos from fertilization to the blastocyst stage. Six alpha subunit mRNAs were expressed at significant levels, all of which underwent significant temporal alterations in expression. The mRNAs encoding some alpha subunit types were expressed predominantly in the egg and 1-cell embryo, underwent sharp reductions during the 2-cell stage, and were re-expressed between the 8-cell and blastocyst stages. One alpha subunit mRNA increased in abundance at the early blastocyst stage. The possible significance of these alterations in G protein mRNA abundance to embryonic development is discussed.

The biogenesis of the coiled body during early mouse development

The coiled body is an ubiquitous nuclear organelle that contains essential components of the pre-mRNA splicing machinery as well as the nucleolar protein fibrillarin. Here we have studied the biogenesis of the coiled body in early mouse embryos. The results show that coiled bodies form and concentrate splicing snRNPs as early as in the maternal and paternal pronuclei of 1-cell embryos. This argues that the coiled body is likely to play a basic role in the nucleus of mammalian cells. In order to correlate the appearance of coiled bodies with the onset of transcriptional activity, embryos were incubated with brominated UTP and the incorporated nucleotide was visualized by fluorescence microscopy. In agreement with previous studies, transcriptional activity was first observed during the 2-cell stage. Thus, coiled bodies form before activation of embryonic gene expression. The appearance of coiled bodies in 1-cell embryos was preceded by the formation of morphologically distinct structures that also contain coilin and which we therefore refer to as pre-coiled bodies. At the electron microscopic level pre-coiled bodies have a compact fibrillar structure, whereas coiled bodies resemble a tangle of coiled threads. Although both pre-coiled bodies and coiled bodies contain the nucleolar protein fibrillarin, the assembly of coiled bodies is separated both in time and in space from ribosome synthesis. Our results suggest that the embryonic ‘nucleolus-like body’ is a structural scaffold that nucleates independently the formation of the coiled body and the assembly of the machinery responsible for ribosome biosynthesis.

Transcription of paternal Y-linked genes in the human zygote as early as the pronucleate stage

Global activation of the embryonic genome occurs at the 4- to 8-cell stage in human embryos and is marked by continuation of early cleavage divisions in the presence of transcriptional inhibitors. Here we demonstrate, using reverse transcriptase-polymerase chain reaction (RT-PCR), the presence of transcripts for two paternal Y chromosomal genes, ZFY and SRY in human preimplantation embryos. ZFY transcripts were detected as early as the pronucleate stage, 20-24 h post-insemination in vitro and at intermediate stages up to the blastocyst stage. SRY transcripts were also detected at 2-cell to blastocyst stages. The expression of SRY and ZFY at these early stages and the faster cleavage rate of male embryos observed in many mammalian species focuses attention on the role of events in sex determination prior to gonad differentiation.

The Ras/Raf signaling pathway is required for progression of mouse embryos through the two-cell stage

We have used microinjection of antisense oligonucleotides, monoclonal antibody, and the dominant negative Ras N-17 mutant to interfere with Ras expression and function in mouse oocytes and early embryos. Microinjection of either ras antisense oligonucleotides or anti-Ras monoclonal antibody Y13-259 did not affect normal progression of oocytes through meiosis and arrest at metaphase II. However, microinjection of fertilized eggs with constructs expressing Ras N-17 inhibited subsequent development through the two-cell stage. The inhibitory effect of Ras N-17 was overcome by simultaneous injection of a plasmid expressing an active raf oncogene, indicating that it resulted from interference with the Ras/Raf signaling pathway. In contrast to the inhibition of two-cell embryo development resulting from microinjection of pronuclear stage eggs, microinjection of late two-cell embryos with Ras N-17 expression constructs did not affect subsequent cleavages and development to morulae and blastocysts. It thus appears that the Ras/Raf signaling pathway, presumably activated by autocrine growth factor stimulation, is specifically required at the two-cell stage, which is the time of transition between maternal and embryonic gene expression in mouse embryos.

The Ras/Raf signaling pathway is required for progression of mouse embryos through the two-cell stage

We have used microinjection of antisense oligonucleotides, monoclonal antibody, and the dominant negative Ras N-17 mutant to interfere with Ras expression and function in mouse oocytes and early embryos. Microinjection of either ras antisense oligonucleotides or anti-Ras monoclonal antibody Y13-259 did not affect normal progression of oocytes through meiosis and arrest at metaphase II. However, microinjection of fertilized eggs with constructs expressing Ras N-17 inhibited subsequent development through the two-cell stage. The inhibitory effect of Ras N-17 was overcome by simultaneous injection of a plasmid expressing an active raf oncogene, indicating that it resulted from interference with the Ras/Raf signaling pathway. In contrast to the inhibition of two-cell embryo development resulting from microinjection of pronuclear stage eggs, microinjection of late two-cell embryos with Ras N-17 expression constructs did not affect subsequent cleavages and development to morulae and blastocysts. It thus appears that the Ras/Raf signaling pathway, presumably activated by autocrine growth factor stimulation, is specifically required at the two-cell stage, which is the time of transition between maternal and embryonic gene expression in mouse embryos.

Activation of embryonic genes during preimplantation rat development

Expression of the embryonic genome has been examined during preimplantation rat development. Proteins synthesized at different stages of embryogenesis were labelled with [35S]methionine and then separated by one-dimensional gel electrophoresis. A major transformation in the pattern of protein synthesis has been observed between the two- and the four-cell stages of embryonic development. Also the culture of embryos with an inhibitor of transcription (alpha-amanitin) has shown that the first alpha-amanitin-sensitive events take place during the late two-cell stage. However, inhibition of transcription does not arrest the embryo development up to the four-cell stage. Taken together, the results indicate that in rats the initiation of embryonic gene activation occurs at the late two-cell stage. However, the first two cleavage divisions can occur in the absence of transcription.

Expression and role of c-myc protooncogene in murine preimplantation embryonic development

PURPOSE

The present study was conducted to investigate the expression and possible role of the c-myc protooncogene in preimplantation embryos by using reverse-transcriptase/polymerase chain reaction (RT-PCR) technique and microinjection of synthetic antisense c-myc oligonucleotide probe, respectively. Total RNA was extracted from oocytes and two cell-, four cell-, early morula-, late morula-, early blastocyst-, and late blastocyst-stage embryos, and cDNA was constructed using MMLV reverse transcriptase. Sense primer (P1) and antisense primer (P2) used were based on the c-myc gene sequence bp 1609-1629 and bp 3279-3299, respectively, that span a 1.37-kb intron. PCR-amplified products of cDNA from oocyte-, two cell-, four cell-, early and late morula-, and blastocyst-stage embryos demonstrated the expected 313-bp product in Southern blot hybridization using a c-myc specific DNA probe, with an indication of lower levels in oocytes and early morulae.

RESULTS

Cytoplasmic injection of the antisense c-myc oligonucleotide probe (P2) and not the sense probe (P1) into pronuclear-stage zygotes caused a significant (P = 0.02 to 0.0001) inhibition of development to blastocysts in a concentration-dependent manner, with a maximal inhibition at the first cleavage of zygotes to two cell-stage embryos. There was no effect on the P2 antisense injection on pronucleus formation.

CONCLUSION

These results indicate that the c-myc protooncogene is expressed in preimplantation embryos and may have an essential role in normal embryogenesis in mice.

Insulin family growth factors have specific effects on protein synthesis in preimplantation mouse embryos

Previously constructed protein databases for two stages of preimplantation mouse embryogenesis, the compacted eight-cell stage and the fully expanded blastocyst stage, have been used to analyze the effects of insulin, IGF-I, and IGF-II on protein synthesis in these developmental stages. Proteins were labeled by placing, for 2 hr, synchronous cohorts of 35-50 embryos into human tubal fluid (HTF) medium containing L-[35S]-methionine (1 mCi/ml) in the presence or absence of one of the growth factors. The embryos were then washed with medium and lysed. Samples were processed for 2-D gel analysis. For each embryonic stage and each growth factor, four or five experimental replicates were done and the gel images were compared using the PDQUEST system. Using the computer-assisted analysis, we were able to identify proteins that showed a statistically significant (P < 0.05) change in synthesis. At the eight-cell stage of development insulin caused increased synthesis of two proteins and decreased synthesis in three proteins. Insulin-treated blastocyst stage embryos exhibited an increased synthesis in eight proteins and decreased synthesis for one protein. The effect of IGF-I at the eight-cell stage of development was mostly inhibitory; the synthesis of only one protein increased and the synthesis of five proteins showed a decrease. Similar results were obtained with blastocyst stage embryos; four proteins demonstrated an increase in synthesis while 14 proteins showed a decrease. Eight-cell stage embryos incubated with IGF-II had seven proteins with a decreased synthesis, although in blastocyst stage embryos, nine proteins showed increased synthesis.(ABSTRACT TRUNCATED AT 250 WORDS)

Morulae at compaction and the pattern of protein synthesis in mouse embryos

Compaction of mouse embryos at the 8-cell stage causes a drastic change in cell form and in cell-to-cell contacts in 3-4 h. We have studied the effect of inhibitors of transcription (alpha-amanitin), DNA replication (aphidicolin) and compaction (cytochalasin D, EGTA, alpha-lactalbumin and Con A) on the pattern of protein synthesis using gel electrophoresis. Our results show that the pattern of protein synthesis is regulated principally by passage through S phase during each early cell cycle rather than by de novo transcription, while changes induced in cell form or contacts do not alter the pattern significantly.

Protein databases for compacted eight-cell and blastocyst-stage mouse embryos

High-resolution two-dimensional sodium dodecyl sulfate-polyacrylamide (2D-SDS) gel electrophoresis combined with computerized analysis of gel images was used to construct and analyze protein data-bases for two stages of preimplantation mouse embryogenesis, the compacted eight-cell stage and the fully expanded blastocyst stage. These stages were chosen for their ease in identification of multiple synchronous embryos. Synchronous cohorts of 30-50 embryos were labelled with L-[35S]methionine for 2 hr. The embryos were then lysed in 30 microliters hot SDS sample buffer, and the lysates were stored at -80 degrees C until the gels were run. Five replicates were run for eight-cell embryos, and four for blastocyst-stage embryos. The samples were processed for 2D gel electrophoresis and fluorography; multiple exposures were made. Gel images were analyzed using the PDQUEST system, and databases were constructed. Analysis of the databases for both developmental stages showed high reproducibility of protein spots in multiple gel images. Of 1,674 total spots in eight-cell embryo standards, > 79% of spots had a percentage error (S.E.M./average) < 50%, and > 45% had a percentage error < 30%. Similarly, of 1,653 total spots in blastocyst-stage embryo standards, 74% of spots had a percentage error < 50%, and approximately 47% of spots had a percentage error < 30%. Forty-three spots (approximately 3% of the total spots) were found to be detected only in the eight-cell stage, while 75 spots were detected solely in the blastocyst stage. Sixty-nine proteins showed a greater than threefold increase in isotope incorporation from the eight-cell to the blastocyst stage, with a percentage error < 50% in both the eight-cell and the blastocyst stages. In contrast, 41 of the proteins showed a decrease during this period. Analysis of the protein databases described in this study has allowed us to document the overall quantitative changes in proteins from the compacted eight-cell stage to the blastocyst stage of mouse preimplantation development. These data-bases provide a valuable tool for further detailed quantitative analysis of specific proteins associated with developmental events. In addition they will permit analysis of the effects of environmental factors, such as growth factors, on early embryo development.

Expression of proto-oncogenes in mouse eggs and preimplantation embryos

The expression of several protooncogenes has been investigated in mouse eggs and preimplantation embryos using reverse transcription coupled to amplification of cDNAs by the polymerase chain reaction (RT-PCR). The genes chosen for analysis included both cytoplasmic (c-raf-1, rasH, rasK, and rasN) and nuclear (c-fos and c-myc) proto-oncogenes encoding proteins involved in the transduction of signals from protein-tyrosine kinase growth factor receptors. Transcripts of the cytoplasmic proto-oncogenes were detected both as maternal and embryonic mRNAs at levels (ca. 1,000 copies per egg or embryo) approximately comparable to their levels of transcription in somatic cells. Transcripts of c-fos and c-myc were also detected in both eggs and embryos, although at more variable levels: Maternal transcripts were present at very low levels (ca. 1-10 copies per egg) in growing oocytes and ovulated eggs; embryonic transcription of c-myc increased, reaching mRNA levels of approximately 100-1,000 copies per embryo in four-cell embryos, morula, and blastocysts; in contrast the transcription of c-fos remained at low, barely detectable levels throughout preimplantation development. Although the significance of the low levels of c-fos mRNA is unclear, these results indicate that preimplantation embryos possess the basic intracellular signaling apparatus required to respond to polypeptide growth factors.

Isolation of the murine cyclin B2 cDNA and characterization of the lineage and temporal specificity of expression of the B1 and B2 cyclins during oogenesis, spermatogenesis and early embryogenesis

A cDNA encoding the murine cyclin B2 (cycB2) was isolated from an adult mouse testis cDNA library as part of studies designed to identify cyclins involved in murine germ cell development. This cycB2 cDNA was then used to examine the pattern of cycB2 expression during male and female germ cell development and in early embryogenesis, and to compare this expression with the previously characterized expression of cycB1. A single 1.7 kb cycB2 transcript was detected by northern blot hybridization analysis of total RNA isolated from midgestation embryos and various adult tissues. Northern blot and in situ hybridization analyses revealed that cycB2 expression in the testis was most abundant in the germ cells, specifically in pachytene spermatocytes. This is in contrast to the highest levels of expression of cycB1 being present in early spermatids. In situ analysis of the ovary revealed cycB2 transcripts in both germ cells and somatic cells, specifically in the oocytes and granulosa cells of growing and mature follicles. The pattern of cycB1 and cycB2 expression in ovulated and fertilized eggs was also examined. While the steady state level of cycB1 and cycB2 signal remained constant in oocytes and ovulated eggs, signal of both appeared to decrease following fertilization. In addition, both cycB1 and cycB2 transcripts were detected in the cells of the inner cell mass and the trophectoderm of the blastocyst. These results demonstrate that there are lineage- and developmental-specific differences in the pattern of the B cyclins in mammalian germ cells, in contrast to the co-expression of B cyclins in the early conceptus.

Gene expression in pre-implantation mammalian embryos

The pre-implantation mammalian embryo is initially under the control of maternal informational macromolecules that are accumulated during oogenesis. Subsequently, the genetic program of development becomes dependent upon new transcription derived from activation of the embryonic genome. Several embryonic transcripts including those that encode growth factors, cell junction components and plasma membrane ion transporters are required for normal progression of the embryo to the blastocyst stage. The pattern of genes expressed and the overall program of development is subject to the influences of genomic imprinting as well as external influences encountered by the embryo within the maternal reproductive tract.

A deficiency in the mechanism for p34cdc2 protein kinase activation in mouse embryos arrested at 2-cell stage

Mouse embryos of the ddY strain fertilized in vitro undergo the first cleavage to the 2-cell stage but not the second cleavage even 45 hr after insemination (2-cell block). We examined the phosphorylation state of p34cdc2 and histone H1 kinase activity in mouse 2-cell embryos to investigate the relationship of p34cdc2 with 2-cell block. In the first mitotic cell cycle, the amount of phosphorylated forms of p34cdc2, which were detected as the bands of retarded mobility on SDS-PAGE followed by immunoblotting with anti-p34cdc2 antibody, increased during interphase and abruptly decreased at M phase. Concomitant with this dephosphorylation, histone H1 kinase activity was increased. After the embryos cleaved to the 2-cell stage, the amounts of phosphorylated forms of p34cdc2 increased up to 33 hr after insemination. However, the activation of histone H1 kinase did not occur and the states of phosphorylation of p34cdc2 did not show any significant changes until 45 hr. In contrast, 2-cell embryos of B6C3F1 mice, which do not show a 2-cell block and develop normally to blastocysts in vitro, exhibit the dephosphorylation of p34cdc2 and an increase in histone H1 kinase activity between 31 and 45 hr after insemination. When the ddY mouse embryos arrested at the 2-cell stage were treated with okadaic acid, an inhibitor of protein phosphatases 1 and 2A, the dephosphorylation of p34cdc2 occurred and histone H1 kinase activity increased. The chromosomes of these embryos stained with 4',6'-diamidino-2-phenylindole revealed the initiation of condensation. These results suggest that 2-cell-blocked embryos contain enough p34cdc2 to induce mitotic events but the protein remains in a latent form.

Changes in permissiveness for the expression of microinjected DNA during the first cleavages of mouse embryos

LacZ DNA and LacZ RNA were microinjected during the first cleavages of embryos. LacZ DNA was not expressed before 18-19 h post insemination (hpi) but LacZ RNA was translated. Before 22 hpi LacZ DNA was expressed in the pronuclei of the one-cell embryos and the polypeptides of the minor, but not the major activation period of the genome were synthesized. This suggests a negative control of transcription before 18-19 hpi and demonstrates that its resumption is independent of the first cleavage and of the major activation of the genome. At the time of the minor activation the eggs contain the trans-acting elements to express a variety of genes that they do not express. It may indicate that, the minor and the major activation of the genome are differently controlled.