Differential effects of activators of cAMP-dependent protein kinase and protein kinase C on cleavage of one-cell mouse embryos and protein synthesis and phosphorylation in one- and two-cell embryos

Mitochondrial Ca2+ Overload Leads to Mitochondrial Oxidative Stress and Delayed Meiotic Resumption in Mouse Oocytes

Overweight or obese women seeking pregnancy is becoming increasingly common. Human maternal obesity gives rise to detrimental effects during reproduction. Emerging evidence has shown that these abnormities are likely attributed to oocyte quality. Oxidative stress induces poor oocyte conditions, but whether mitochondrial calcium homeostasis plays a key role in oocyte status remains unresolved. Here, we established a mitochondrial Ca²⁺ overload model in mouse oocytes. Knockdown gatekeepers of the mitochondrial Ca²⁺ uniporters Micu1 and Micu2 as well as the mitochondrial sodium calcium exchanger NCLX in oocytes both increased oocytes mitochondrial Ca²⁺ concentration. The overload of mitochondria Ca²⁺ in oocytes impaired mitochondrial function, leaded to oxidative stress, and changed protein kinase A (PKA) signaling associated gene expression as well as delayed meiotic resumption. Using this model, we aimed to determine the mechanism of delayed meiosis caused by mitochondrial Ca²⁺ overload, and whether oocyte-specific inhibition of mitochondrial Ca²⁺ influx could improve the reproductive abnormalities seen within obesity. Germinal vesicle breakdown stage (GVBD) and extrusion of first polar body (PB1) are two indicators of meiosis maturation. As expected, the percentage of oocytes that successfully progress to the germinal vesicle breakdown stage and extrude the first polar body during in vitro culture was increased significantly, and the expression of PKA signaling genes and mitochondrial function recovered after appropriate mitochondrial Ca²⁺ regulation. Additionally, some indicators of mitochondrial performance-such as adenosine triphosphate (ATP) and reactive oxygen species (ROS) levels and mitochondrial membrane potential-recovered to normal. These results suggest that the regulation of mitochondrial Ca²⁺ uptake in mouse oocytes has a significant role during oocyte maturation as well as PKA signaling and that proper mitochondrial Ca²⁺ reductions in obese oocytes can recover mitochondrial performance and improve obesity-associated oocyte quality.

The methyltransferase Setdb1 is essential for meiosis and mitosis in mouse oocytes and early embryos

Oocytes develop the competence for meiosis and early embryogenesis during their growth. Setdb1 is a histone H3 lysine 9 (H3K9) methyltransferase required for post-implantation development and has been implicated in the transcriptional silencing of genes and endogenous retroviral elements (ERVs). To address its role in oogenesis and pre-implantation development, we conditionally deleted Setdb1 in growing oocytes. Loss of Setdb1 expression greatly impaired meiosis. It delayed meiotic resumption, altered the dynamics of chromatin condensation, and impaired kinetochore-spindle interactions, bipolar spindle organization and chromosome segregation in more mature oocytes. The observed phenotypes related to changes in abundance of specific transcripts in mutant oocytes. Setdb1 maternally deficient embryos arrested during pre-implantation development and showed comparable defects during cell cycle progression and in chromosome segregation. Finally, transcriptional profiling data indicate that Setdb1 downregulates rather than silences expression of ERVK and ERVL-MaLR retrotransposons and associated chimearic transcripts during oogenesis. Our results identify Setdb1 as a newly discovered meiotic and embryonic competence factor safeguarding genome integrity at the onset of life.

An oocyte-specific ELAVL2 isoform is a translational repressor ablated from meiotically competent antral oocytes

At the end of the growth phase, mouse antral follicle oocytes acquire full developmental competence. In the mouse, this event is marked by the transition from the so-called non-surrounded nucleolus (NSN) chromatin configuration into the transcriptionally quiescent surrounded nucleolus (SN) configuration, which is named after a prominent perinucleolar condensed chromatin ring. However, the SN chromatin configuration alone is not sufficient for determining the developmental competence of the SN oocyte. There are additional nuclear and cytoplamic factors involved, while a little is known about the changes occurring in the cytoplasm during the NSN/SN transition. Here, we report functional analysis of maternal ELAVL2 an AU-rich element binding protein. Elavl2 gene encodes an oocyte-specific protein isoform (denoted ELAVL2°), which acts as a translational repressor. ELAVL2° is abundant in fully grown NSN oocytes, is ablated during the NSN/SN transition and remains low during the oocyte-to-embryo transition (OET). ELAVL2° overexpression during meiotic maturation causes errors in chromosome segregation, indicating the significance of naturally reduced ELAVL2° levels in SN oocytes. On the other hand, during oocyte growth, prematurely reduced Elavl2 expression results in lower yields of fully grown and meiotically matured oocytes, suggesting that Elavl2 is necessary for proper oocyte maturation. Moreover, Elavl2 knockdown showed stimulating effects on translation in fully grown oocytes. We propose that ELAVL2 has an ambivalent role in oocytes: it functions as a pleiotropic translational repressor in efficient production of fully grown oocytes, while its disposal during the NSN/SN transition contributes to the acquisition of full developmental competence.

Mouse zygote-specific proteasome assembly chaperone important for maternal-to-zygotic transition

During the maternal-to-zygotic transition (MZT), maternal proteins in oocytes are degraded by the ubiquitin–proteasome system (UPS), and new proteins are synthesized from the zygotic genome. However, the specific mechanisms underlying the UPS at the MZT are not well understood. We identified a molecule named zygote-specific proteasome assembly chaperone (ZPAC) that is specifically expressed in mouse gonads, and expression of ZPAC was transiently increased at the mouse MZT. ZPAC formed a complex with Ump1 and associated with precursor forms of 20S proteasomes. Transcription of ZPAC genes was also under the control of an autoregulatory feedback mechanism for the compensation of reduced proteasome activity similar to Ump1 and 20S proteasome subunit gene expression. Knockdown of ZPAC in early embryos caused a significant reduction of proteasome activity and decrease in Ump1 and mature proteasomes, leading to accumulation of proteins that need to be degraded at the MZT and early developmental arrest. Therefore, a unique proteasome assembly pathway mediated by ZPAC is important for progression of the mouse MZT.

MLL2 is required in oocytes for bulk histone 3 lysine 4 trimethylation and transcriptional silencing

Conditional knockout mouse strategies identify the histone methyltranferase MLL2 as a key player in epigenetic reprogramming of female gametes.

During gametogenesis and pre-implantation development, the mammalian epigenome is reprogrammed to establish pluripotency in the epiblast. Here we show that the histone 3 lysine 4 (H3K4) methyltransferase, MLL2, controls most of the promoter-specific chromatin modification, H3K4me3, during oogenesis and early development. Using conditional knockout mutagenesis and a hypomorph model, we show that Mll2 deficiency in oocytes results in anovulation and oocyte death, with increased transcription of p53, apoptotic factors, and Iap elements. MLL2 is required for (1) bulk H3K4me3 but not H3K4me1, indicating that MLL2 controls most promoters but monomethylation is regulated by a different H3K4 methyltransferase; (2) the global transcriptional silencing that preceeds resumption of meiosis but not for the concomitant nuclear reorganization into the surrounded nucleolus (SN) chromatin configuration; (3) oocyte survival; and (4) normal zygotic genome activation. These results reveal that MLL2 is autonomously required in oocytes for fertility and imply that MLL2 contributes to the epigenetic reprogramming that takes place before fertilization. We propose that once this task has been accomplished, MLL2 is not required until gastrulation and that other methyltransferases are responsible for bulk H3K4me3, thereby revealing an unexpected epigenetic control switch amongst the H3K4 methyltransferases during development.

It is well established that gametes and early mammalian embryos undergo extensive epigenetic changes, which are changes in phenotype or gene expression that do not entail changes in DNA sequence. However, the machinery responsible for epigenetic modification in these situations is poorly understood. In mice, we conditionally deleted the histone 3 lysine 4 (H3K4) methyltransferase Mll2, an enzyme that alters DNA structure and packaging, either in gametes or in somatic cells of the ovary and also produced a mouse hypomorph expressing low levels of MLL2. We show that MLL2 is required in oocytes during gametogenesis and is also needed as a maternally derived factor during early development. Oocytes deficient in Mll2 display decreased methylation of H3K4 (H3K4me3) and show abnormal maturation and gene expression, in particular of pro-apoptotic factors. In addition, we demonstrate that embryonic genome activation is compromised in the absence of Mll2. Together our results identify MLL2 as one of the key players in the epigenetic reprogramming required for female fertility in the mouse.

Effects of Caffeine on Protein Phosphorylation and Cell Cycle Progression in X-irradiated Two-cell Mouse Embryos

The G2 phase/mitosis transition in cleavage-stage mouse embryos is correlated with an increased phosphorylation of a defined set of proteins at 46, 35, 30, and 29 kDa. Cell cleavage and the associated changes in protein phosphorylation are delayed after X-irradiation. To understand the mechanism of the caffeine-induced uncoupling of mitosis and the cellular reactions to DNA-damaging agents, we have studied the effects of caffeine treatment on cell cycle progression and protein phosphorylation in two-cell mouse embryos after X-irradiation. Caffeine alone had no effect on timing of and changes in phosphorylation associated with the embryonic cell cycle. In combination with X-rays, however, caffeine was able to override the radiation induced G2 block and restored the normal timing of these phosphorylation changes after X-irradiation. However, new additional changes in protein phosphorylation appeared after the combined treatment. Isobutylmethylxanthine (IBMX), a substance chemically related to caffeine but a more specific inhibitor of the phosphodiesterase that breaks down cyclic AMP, reduced the radiation induced G2 block from 4 to 5 h to about 1 h and restored the cell cycle associated changes in protein phosphorylation. However, the same new changes which appeared after the combined treatment of caffeine and X-rays were observed after the combination of IBMX and X-irradiation. IBMX specific changes in protein phosphorylation were detected in both the single and the combined treatment. These results indicate a similar action of caffeine and IBMX in overriding the radiation induced G2 block in two-cell mouse embryos.

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.

Mice deficient in oocyte-specific oligoadenylate synthetase-like protein OAS1D display reduced fertility

The double-stranded RNA (dsRNA)-induced interferon response is a defense mechanism against viral infection. Upon interferon activation by dsRNA, 2',5'-oligoadenylate synthetase 1 (OAS1A) is induced; it binds dsRNA and converts ATP into 2',5'-linked oligomers of adenosine (called 2-5A), which activate RNase L that in turn degrades viral and cellular RNAs. In a screen to identify oocyte-specific genes, we identified a novel murine cDNA encoding an ovary-specific 2',5'-oligoadenylate synthetase-like protein, OAS1D, which displays 59% identity with OAS1A. OAS1D is predominantly cytoplasmic and is exclusively expressed in growing oocytes and early embryos. Like OAS1A, OAS1D binds the dsRNA mimetic poly(I-C), but unlike OAS1A, it lacks 2'-5' adenosine linking activity. OAS1D interacts with OAS1A and inhibits the enzymatic activity of OAS1A. Mutant mice lacking OAS1D (Oas1d(-/-)) display reduced fertility due to defects in ovarian follicle development, decreased efficiency of ovulation, and eggs that are fertilized arrest at the one-cell stage. These effects are exacerbated after activation of the interferon/OAS1A/RNase L pathway by poly(I-C). We propose that OAS1D suppresses the interferon/OAS/RNase L-mediated cellular destruction by interacting with OAS1A during oogenesis and early embryonic development.

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.

Transgenic RNAi-mediated reduction of MSY2 in mouse oocytes results in reduced fertility

MSY2 is implicated in regulating the stability and translation of maternal mRNAs during mouse oogenesis. We report here that by driving the expression of a transgene encoding an Msy2 hairpin dsRNA in growing oocytes using the oocyte-specific Zp3 promoter, the amount of MSY2 protein was reduced by at least 60% in fully grown oocytes. The decrease appeared specific because no decrease was observed in either non-targeted mRNAs or proteins. Fertility of transgenic females was severely reduced. Although transgenic eggs could be inseminated, the eggs did not exhibit the normal series of oscillations in intracellular Ca2+, resume meiosis, undergo cortical granule exocytosis, or ZP2 cleavage to ZP2f. Transgenic oocytes also displayed a higher incidence of both the non-surrounded nucleolus chromatin morphology, and abnormal meiotic spindle formation was observed following oocyte maturation. Transgenic oocytes contained less total mRNA (approximately 75-80% that of non-transgenic oocytes) and displayed a reduced level of protein synthesis. Moreover, several of the maturation-associated changes in protein synthesis failed to occur in the transgenic oocytes. These results support a role for MSY2 in stabilizing maternal mRNAs in growing oocytes, a process essential to generate meiotically and developmentally competent oocytes.

Chromatin configuration and transcriptional control in human and mouse oocytes

In vitro maturation of human oocytes at the germinal vesicle (GV) stage could offer an alternative in several cases of female infertility. It however rests on a better knowledge of the quality of human oocyte. Using fluorescence imaging of DNA and of the transcription sites, combined with electron microscopy, we show that human oocytes follow size-dependent changes in chromatin configuration, transcription sites distribution and nuclear ultrastructure that follow those observed in mouse GV oocytes. We thus analyzed in mouse GV oocytes the phosphorylation dependence of the transcriptional activity. We show by Western blot that, while active GV oocytes have approximately the same proportion of hypo- and hyperphosphorylated forms of the RNA polymerase II (RNAP II), the hyperphosphorylated form is almost absent from inactive oocytes. We also show that (1) RNAP II-dependent transcription is much less sensitive to various kinase inhibitors in mouse oocytes than in somatic cells or mouse one-cell embryos, although the phosphorylation equilibrium of RNAP II was largely shifted towards the hypo-phosphorylated form upon treatment with these inhibitors (2) RNAP I is completely insensitive to kinase inhibitors in GV oocytes.

The essential role of RI alpha in the maintenance of regulated PKA activity

Cloning of the individual regulatory (R) and catalytic (C) subunits of the cAMP-dependent protein kinase (PKA) and expression of these subunits in cell culture have provided mechanistic answers about the rules for PKA holoenzyme assembly. One of the central findings of these studies is the essential role of the RI alpha regulatory subunit in maintaining the catalytic subunit under cAMP control. The role of RI alpha as the key compensatory regulatory subunit in this enzyme family was confirmed by gene knockouts of the three other regulatory subunits in mice. In each case, RI alpha has demonstrated the capacity for significant compensatory regulation of PKA activity in tissues where the other regulatory subunits are expressed, including brain, brown and white adipose tissue, skeletal muscle, and sperm. The essential requirement of the RI alpha regulatory subunit in maintaining cAMP control of PKA activity was further corroborated by the knockout of RI alpha in mice, which results in early embryonic lethality due to failed cardiac morphogenesis. Closer examination of RI alpha knockout embryos at even earlier stages of development revealed profound deficits in the morphogenesis of the mesodermal embryonic germ layer, which gives rise to essential structures including the embryonic heart tube. Failure of the mesodermal germ layer in RI alpha knockout embryos can be rescued by crossing RI alpha knockout mice to C alpha knockout mice, supporting the conclusion that inappropriately regulated PKA catalytic subunit activity is responsible for the phenotype. Isolation of primary embryonic fibroblasts from RI alpha knockout embryos reveals profound alterations in the actin-based cytoskeleton, which may account for the failure in mesoderm morphogenesis at gastrulation.

Regulation of stage-specific nuclear translocation of Dnmt1o during preimplantation mouse development

DNA methylation of CpG dinucleotides by DNA methyltransferase 1 is implicated in the regulation of transcription and, in particular, the transcription of imprinted genes. Although the oocyte-specific form of Dnmt1 (Dnmt1o) possesses a functional nuclear localization signal, it is predominantly localized in the cytoplasm of the oocyte and preimplantation mouse embryo but undergoes a transient nuclear localization during the eight-cell stage, when the embryos undergo compaction. We report here that Dnmt1o is likely retained in the cytoplasm by an active process, since approximately 70% of DNA methyltransferase activity is retained following permeabilization procedures that result in the release of approximately 75% of oocyte/embryo protein. Treatment of the embryos with agents that disrupt either microfilaments or microtubules has little, if any, effect on the retention of Dnmt1o in permeabilized embryos. While Dnmt1o does not colocalize with either mitochondria or endoplasmic reticulum, it does colocalize with annexin V, which is known to interact with Dnmt1o. We also report that the timing of nuclear entry of Dnmt1o during the eight-cell stage is independent of DNA replication, transcription, and protein synthesis, as well as compaction, cell contact, and cytokinesis. The time of nuclear entry, therefore, appears linked to the time following fertilization, which suggests that a molecular clock governs the time of nuclear import.

In vitro fertilization causes epigenetic modifications to the onset of gene expression from the zygotic genome in mice

The effect of in vitro fertilization (IVF) and culture of mouse preimplantation embryos in vitro on the onset of expression of insulin-like growth factor 1 (IGF-1) ligand and receptor, insulin ligand and receptor, alpha-transforming growth factor (alpha-TGF) ligand, PAF:acetylhydrolase 1b (Pafah1b; alpha(1), alpha(2), and beta subunits of the enzyme), and the transcription requiring complex proteins (TRC) was examined. The IGF-1 ligand was detected in preimplantation embryos by immunofluorescence at all developmental stages tested. However, IVF and culture significantly reduced the amount of protein detected in the 8-cell embryo and blastocyst (P: < 0.001), and this was due to a delayed onset of expression of the mRNA for IGF-1 ligand from the zygotic genome. The expression of the alpha(1) subunit of Pafah1b was first detected at the 2-cell stage in fresh embryos, but expression was significantly retarded (P: < 0.001) when IVF and ISF (in situ-fertilized) zygotes were cultured in vitro. In vitro fertilization or ISF did not delay the onset of expression of TRC nor mRNA for the IGF-1 receptor, insulin receptor, alpha(2) or beta subunit of Pafah1b, nor did they effect alpha-TGF protein synthesis. Thus, IVF causes epigenetic modification in the normal pattern of expression of some but not all genes involved in normal embryo growth and survival.

Differential regulation of the translation and the stability of two maternal transcripts in preimplantation rabbit embryos

In most species, transcription is essentially silent during the first mitotic cell cycles that follow fertilization. This means that the regulation of gene expression in early embryos heavily relies on the translational activation or inactivation of maternal mRNAs. In mammals, the mechanisms that control the translation of maternal mRNAs have been mainly studied in the mouse when maternal to zygotic transition occurs after the first mitotic division. In other mammalian species, however, this transition occurs later after several cell cycles, and little is known concerning the regulation of maternal information during this period. To address this question, we have used rabbit pre-implantation embryos to analyze the translational activation and stability of two maternal mRNAs, mm 41 and mm61. During the cleavage period, these mRNAs exhibit distinct kinetics for both their translational activation and degradation. In addition, these mRNAs both undergo cytoplasmic polyadenylation but with different efficiencies. This polyadenylation was functionally correlated with the translational activation of these mRNAs; inhibiting polyadenylation prevented translational activation. The differential efficiency of cytoplasmic polyadenylation, driven by cis-elements in the 3' untranslated region of these mRNAs, was also observed in Xenopus laevis embryos, which emphasizes the high conservation of this mechanism between species.

Mechanisms and control of embryonic genome activation in mammalian embryos

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

Lead: male-mediated effects on reproduction and development in the rat

The present study was designed to determine the effect of relatively low levels of lead acetate (25 and 250 ppm) exposure on fertility and offspring viability in male Sprague-Dawley rats. Protein synthesis in 2-cell embryos was monitored by [35S] methionine labeling and two-dimensional SDS gel electrophoresis. Fertility was reduced in males with blood lead levels in the range 27-60 microg/dL. Lead was found to affect initial genomic expression in embryos fathered by male rats with blood lead levels as low as 15-23 microg/dL. Dose-dependent increases were seen in an unidentified set of proteins with a relative molecular weight of approximately 70 kDa (Mr). These results indicate that male-mediated effects of lead may be observed in the 2-cell embryo. The alteration observed in embryonic gene expression with paternal lead exposure may be useful for studying the role of the paternal contribution to the activation of the embryonic genome and protein synthesis in the early embryo.

Onset of transcription in bovine oocytes and preimplantation embryos

The transition from the maternal to embryonic control of early embryonic development (MET) in mammals is not fully understood. The objective of this study was to determine the amount of transcriptional activity in immature oocytes containing germinal vesicle (GV), mature metaphase II arrested oocytes (MII), 2-, 4- and 8-cell bovine embryos by labeling with 35S-UTP followed by isolation of total RNA and autoradiography. Expression of counts per minute (CPM) per cell showed that incorporation of 35S-UTP in GV oocytes was significantly higher than the background (P < 0.01) and decreased sharply by the time the oocytes reached MII arrest. Incorporation significantly increased during the 2-cell stage and remained at the same level during the 4- and 8-cell stages. Uptake remained constant throughout different development stages (P > 0.05) with the highest variability observed during the 2-cell stage. When CPM were expressed per oocyte or embryo incorporation remained high at the GV stage, decreased to the background levels at the time of MII and increased again at the 2-cell stage. It remained at the same level during the 4-cell stage but increased significantly for the second time during the 8-cell stage. Uptake remained at the same level until the 8-cell stage when a significant increase was observed. The negative controls showed a significantly lower amount of incorporation compared to the positive control (P < 0.05). Similar results were observed by autoradiography. Our observations suggest that MET starts as early as the 2-cell stage in bovine embryos.

Temporal patterns of embryonic gene expression and their dependence on oogenetic factors

Successful development of a fertilized egg beyond early cleavage divisions requires the de novo initiation and subsequent regulation of embryonic transcription. The egg provides the specialized environment within which the newly formed zygotic nucleus initiates its developmental program and as a result plays an obligatory role in its regulation. Although the precise timing of the onset of embryonic transcription in mammals varies during early cleavage divisions, several common elements exist. In the present essay we review the current literature on the timing and control of embryonic gene expression in mammals, and discuss recent findings from our laboratory on gene expression patterns in bovine embryos and their relation to other species, and zygotic gene activation (ZGA). Lastly, we discuss the putative role of maternally inherited factors in conferring developmental competence to the blastocyst stage, and a method to identify such factors present in oocytes as mRNA.

Dynamic organization of DNA replication in one-cell mouse embryos: relationship to transcriptional activation

We have analyzed the spatial and temporal relationship between transcription and replication sites during the first cell cycle in mouse embryos. Embryos were microinjected with both 5-bromouridine-5'-triphosphate and digoxygenin-11-deoxyuridine-5'-triphosphate to visualize transcription and replication sites respectively. We detected six different phases of replication during S phase and dated the onset of zygotic transcription at the end of the S phase. Using confocal microscopy, we showed that there is essentially no colocalization of replication and transcription sites at this stage of development. Moreover, studies on aphidicolin-treated embryos demonstrated that inhibition of DNA replication does not hinder transcriptional activation at the 1-cell stage.

Spindlin, a major maternal transcript expressed in the mouse during the transition from oocyte to embryo

Timely translation of maternal transcripts and post-translational modification of their gene products control the initial development of preimplantation-stage embryos. We have isolated and characterized a gene encoding a stage-specific embryonic protein. This novel gene, spindlin (Spin), is an abundant maternal transcript present in the unfertilized egg and 2-cell, but not 8-cell, stage embryo. Spin exhibits high homology to a multicopy gene, Y-linked spermiogenesis-specific transcript (Ssty), and together they form a new gene family expressed during gametogenesis. We find that spindlin associates with the meiotic spindle and is modified by phosphorylation in a cell-cycle-dependent fashion. Furthermore, it comigrates with the previously described 30x10(3) Mr metaphase complex which is posttranslationally modified during the first mitotic cell cycle. Our data suggest that spindlin plays a role in cell-cycle regulation during the transition from gamete to embryo.

Epigenetic alterations brought about by lithium treatment disrupt mouse embryo development

The commonly accepted mechanism by which LiCl dorsalizes amphibian embryos is a respecification of ventral blastomeres, presumably through realignment of dorsal positional information in the embryo. An alternative mechanism, however, is an epigenetic change in the competence of cells to respond to cues they may be normally exposed to without effect. In order to test this hypothesis, we treated mouse preimplantation embryos, which do not possess any axial positional information, with LiCl, and observed axial abnormalities which must have been elaborated several days after treatment. We interpret this as support for the hypothesis that cellular competence rather than positional information is altered by LiCl, and suggest that this competence may be altered through the action of lithium sensitive enzymes that interact with chromatin.

Cyclic AMP reversal of hypoxanthine-arrested preimplantation mouse embryos is EDTA-dependent

Hypoxanthine can block preimplantation mouse embryo development in vitro at the 2- to 4-cell stages, and this has recently been shown to be reversed by cAMP-elevating agents. However, the extent of this hypoxanthine-induced arrest is determined by the culture conditions and strain of mouse. Whitten's and KSOM/AA are two embryo culture media that support preimplantation development to the blastocyst stage. This study was undertaken to examine the influence of several components in these media on hypoxanthine-arrested preimplantation mouse embryos and to test the hypothesis that reversal of the hypoxanthine block by cAMP-elevating agents requires cooperative interaction with the chelator, EDTA. Initial experiments demonstrated that embryo development was blocked in the presence of hypoxanthine in Whitten's medium but not in KSOM/AA; furthermore, removal of EDTA from KSOM/AA rendered this medium incapable of supporting high levels of development to blastocyst (9%), whereas high numbers of blastocysts (80%) formed in Whitten's medium, which does not contain the chelator. Consequently, Whitten's medium was used to test our hypothesis. It has previously been demonstrated that the phosphodiesterase inhibitor, IBMX, can reverse the developmental arrest imposed by hypoxanthine in EDTA-supplemented Earle's basic salt solution, but in the present study the addition of IBMX to Whitten's medium resulted in a block to development and failed to reverse the hypoxanthine arrest. These disparate effects can be explained by the presence or absence of EDTA. Supplementing Whitten's medium with EDTA reverses the IBMX effect, but not the hypoxanthine-induced block. While IBMX alone is unable to reverse the hypoxanthine block in Whitten's medium, development is greatly enhanced by the simultaneous addition of EDTA and IBMX. Similar results were obtained with the cAMP analogue, 8-AHA-cAMP. The data therefore support our hypothesis that the reversal of the hypoxanthine-induced arrest by cAMP-elevating agents is critically dependent on the presence of EDTA. We contrast this with the situation in mouse oocytes, where the hypoxanthine-induced meiotic arrest is not reversed by the addition of EDTA and/or cAMP-elevating agents.

Temporally restricted spatial localization of acetylated isoforms of histone H4 and RNA polymerase II in the 2-cell mouse embryo

Using immunofluorescent labeling and laser-scanning confocal microscopy, we show that isoforms of histone H4 acetylated on lysine 5, 8 and/or 12 (H4.Ac5-12), as well as RNA polymerase II, become enriched at the nuclear periphery around the time of zygotic gene activation, i.e., the 2-cell stage, in the preimplantation mouse embryo. In contrast, DNA and H4 acetylated on lysine 16 are uniformly distributed throughout the cytoplasm. Culture of embryos with inhibitors of histone deacetylase trichostatin A and trapoxin results in an increase in the (1) amount of acetylated histone H4 detected by immunoblotting, (2) intensity and sharpness of the peripheral staining for H4.Ac5-12, and (3) relative rate of synthesis of proteins that are markers for zygotic gene activation. The enhanced staining for H4.Ac5-12 at the nuclear periphery seems to require DNA replication, but appears independent of cytokinesis or transcription, since its development is inhibited by aphidicolin but not by either cytochalasin D or alpha-amanitin. Lastly, the restricted localization of H4.Ac 5–12 is not observed in the 4-cell embryo or at later stages of preimplantation development. These results suggest that changes in chromatin structure underlie, at least in part, zygotic gene activation in the mouse.

Assessment of embryo potential by visual and metabolic evaluation

Morphological evaluation of embryos is essential to the success of embryo transfer procedures and is presumed to reflect embryo metabolic activity. To investigate this assumption, correlations between morphological and metabolic parameters were determined for cultured murine morulae. After 18 h (n = 47) or 36 h (n = 48) of culture in M16, the developmental rate and quality (poor or good) of embryos were estimated, and, then, either their (14)C-glucose utilization or (35)s-methionine uptake and incorporation were measured. Retarded developing, or poor-quality embryos had lower mean glucose utilization, uptake and incorporation rates than normally developing or good-quality embryos (P < 0.05). After 18 h of culture, an association was found between developmental rate and metabolic activity, but this was not evident after 36 h of culture. Similarly, an association was found between embryo quality and metabolic activity. As expected, poor embryo quality was indicative of low metabolism throughout the culture period, but good quality did not necessarily indicate normal metabolic activity. Thus, morphological parameters do not always reflect metabolic competence, and some functional defects were not detectable by visual evaluation alone. Measuring metabolic parameters could complement visual evaluation for a better selection of embryos prior to transfer.

Expression of the cell cycle control protein cdc25 in cleavage stage bovine embryos

We have examined the synthesis and expression of a homologue of the cell cycle control protein cdc25 by early cleavage stage bovine embryos. cdc25 is the protein phosphatase responsible for activating p34cdc2 by dephosphorylating the threonine 14 (Thr 14) and tyrosine 15 (Tyr 15) residues of p34cdc2. Human cdc25 antibody was utilised in western blots and immunoprecipitations to examine the presence and synthesis of cdc25 in bovine embryos. cdc25 is present as a 52 kDa non-phosphorylated and a 66 kDa presumably phosphorylated form in bovine 1-, 2-, 4- and 8-cell embryos. However, cdc25 is actively synthesised only in 8-cell embryos, indicating that the cdc25 present prior to this stage is inherited from the oocyte. In addition, the synthesis of cdc25 was induced in 2-cell embryos in which cleavage was blocked with the DNA synthesis inhibitor aphidicolin.

Temporal control of gap junction assembly in preimplantation mouse embryos

The de novo assembly of gap junctions during compaction in the 8-cell stage of mouse development is a temporally regulated event. We have performed experiments designed to explore the relationship between this event and DNA replication in the second, third, and fourth cell cycles after fertilization. Inhibition of DNA synthesis by continuous treatment with the DNA synthesis inhibitor, aphidicolin, during the third and fourth cell cycles had no effect on the establishment of gap junctional coupling during compaction. However, a delay of 10 hours in DNA synthesis during the second cell cycle caused by a transient aphidicolin treatment resulted in the failure of gap junctional coupling at the time of compaction. Thus the timing of establishment of gap junctional coupling, like the timing of compaction itself, is linked to DNA replication in the 2-cell stage. Immunofluorescence analysis showed that the failure of gap junctional coupling after aphidicolin treatment in the 2-cell stage is correlated with the failure of nascent connexin43 to be inserted into plasma membranes. We propose that the developmental ‘clock’ that controls gap junction assembly is set in motion by events surrounding the second cycle of DNA replication, and that this ‘clock’ ultimately controls the post-translational processing of connexin43.

Expression of the HSP 70.1 gene, a landmark of early zygotic activity in the mouse embryo, is restricted to the first burst of transcription

Activation of the mouse embryonic genome at the 2-cell stage is characterized by the synthesis of several alpha-amanitin-sensitive polypeptides, some of which belong to the multigenic hsp 70 family. In the present work we show that a member of this family, the HSP 70.1 gene, is highly transcribed at the onset of zygotic genome activation. Transcription of this gene began as early as the 1-cell stage. Expression of the gene continued through the early 2-cell stage but was repressed before the completion of the second round of DNA replication. During this period we observed that the level of transcription was modulated by in vitro culture conditions. The coincidence of repression of HSP70.1 transcription with the second round of DNA replication was not found for other transcription-dependent polypeptides synthesized at the 2-cell stage.

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.

Regulation of gene expression in the mouse oocyte and early preimplantation embryo: developmental changes in Sp1 and TATA box-binding protein, TBP

We previously demonstrated that an Sp1-dependent reporter gene is preferentially expressed in G2 of the 1-cell mouse embryo following microinjection of the male pronucleus when compared to microinjection of the female pronucleus (P.T. Ram and R.M. Schultz, 1993, Dev. Biol. 156, 552–556). We also noted that expression of the reporter gene is not observed following microinjection of the germinal vesicle of the fully grown oocyte. In the present study, we examined expression of this reporter gene during oocyte growth, as well as the nuclear concentration of two transcription factors, Sp1 and the TATA box-binding protein, TBP, during oocyte growth and the first cell cycle. The extent of reporter gene expression decreases during oocyte growth and this decrease correlates with the decrease in nuclear concentration of Sp1, as determined by confocal immunofluorescent microscopy. In addition, results of immunoblotting experiments also indicate a similar decrease in the total concentration of Sp1 during oocyte growth. The nuclear concentration of TBP also decreases during oocyte growth, as determined by confocal immunofluorescent microscopy. Following fertilization, the pronuclear concentration of these two transcription factors increases in a time-dependent fashion and the concentration of each is greater in the male pronucleus as compared to the female pronucleus. For each pronucleus and for each transcription factor, this increase in nuclear concentration is inhibited by aphidicolin, which inhibits DNA synthesis. Last, the increase in nuclear concentration of these two proteins observed between the 1-cell and 2-cell stages does not require transcription or cytokinesis.

Redistribution of nuclear antigens linked to cell proliferation and RNA processing in mouse oocytes and early embryos

We have systematically analyzed by indirect immunofluorescence the subcellular distribution of nuclear antigens in relation to developmental stages of maturing mouse oocytes and developing embryos. Antigens were of two types: (1) a protein whose nuclear localization in interphase somatic cells depends on their proliferative state protein recognized by a monoclonal antibody 43B1N, and (2) snRNP polypeptides recognized by autoimmune sera of anti-Sm and anti-RNP type. The protein recognized by 43B1N was present in the germinal vesicle of oocytes from antral follicles, but absent from the nuclei during the first hours of embryonic life up to the middle to late 2-cell stage. Starting from this stage, it was always found in nuclei of interphase blastomeres, where its "speckles" co-localized with the speckles containing high concentrations of snRNP polypeptides. SnRNP polypeptides recognized by anti-Sm and anti-RNP sera were in turn found in nuclei of all developmental stages. When embryos were treated with aphidicolin or cytochalasin D to arrest cell division, the 43B1N reacting protein was again localized in the pronuclei at 42 hr post-hCG, i.e., slightly later than the onset of transcriptional activity. These results suggest a progressive building up of nuclei during embryonic development, which could influence gene expression.

In vitro manipulation of early mouse embryos induces HIV1-LTRlacZ transgene expression

We report here that the transcriptional activity of early mouse embryos is affected by their manipulation and culture in vitro, using transgenic embryos that express the reporter gene lacZ. We examined the pattern of expression of the lacZ gene fused to the human immunodeficiency virus type 1 long terminal repeat during the preimplantation stages. Transgene expression is induced as early as the two-cell stage in embryos developed in vitro, while there is no constitutive expression at the same stage in embryos developed in vivo. We have established a relation between this inducible expression occurring in vitro and an oxidative stress phenomenon. Indeed, when the culture medium is supplemented with antioxidants such N-acetyl-cysteine or CuZn-superoxide dismutase the transgene expression is markedly reduced. We also present evidence that the transgene expression in vitro coincides with the onset of the embryonic genome activation as attested by the synthesis of the 70 × 10(3) M(r) protein complex. Therefore, this transgene expression could prove to be a useful tool in our understanding of the molecular mechanisms involved in this crucial developmental event.

Regulation of zygotic gene activation in the mouse

Zygotic gene activation (ZGA) is the critical event that governs the transition from maternal to embryonic control of development. In the mouse, ZGA occurs during the 2-cell stage and appears to be regulated by the time following fertilization, i.e. a zygotic clock, rather than by progression through the first cell cycle. The onset of ZGA must depend on maternally inherited proteins, and post-translational modification of these maternally derived proteins is likely to play a role in ZGA. Consistent with this prediction is that protein phosphorylation catalyzed by the cAMP-dependent protein kinase is involved in ZGA and that protein synthesis is not required for ZGA. Recent results suggest that ZGA may occur earlier than previously thought, i.e. not during the 2-cell stage, but rather in G2 of the 1-cell embryo. Thus ZGA may comprise a period of minor gene activation in the 1-cell embryo that is followed by a period of major gene activation in the 2-cell embryo. Following ZGA, the expression of constitutively activated genes may require an enhancer.

Effect of sodium and betaine in culture media on development and relative rates of protein synthesis in preimplantation mouse embryos in vitro

Results of recent experiments indicate that the improved development of mouse embryos in medium containing a low NaCl concentration (85 mM) or the inclusion of the organic osmolyte betaine in a medium containing a high NaCl concentration (125 mM) is correlated with the maintenance of intracellular sodium concentrations that more closely approximate those found in freshly isolated embryos (Biggers et al., 1993, Mol Reprod Dev 34:380-390). We examined the effect of these different culture media on the relative rates of protein synthesis since increased levels of intracellular sodium inhibit protein synthesis; a reduced rate of protein synthesis could therefore account for the differences in development in the different media, since cell division requires protein synthesis. We observe that the ability of these media to support development and to maintain more physiological concentrations of intracellular sodium is correlated with their ability to support increased relative rates of protein synthesis. Reducing the NaCl concentration from 125 mM to 85 mM leads to a greater fraction of the embryos developing from the 2-cell stage to the 8-cell stage after 1 day of culture and a substantially improves extent of development to the morula stage after 2 days of culture. This reduction in NaCl concentration also leads to a 2.4-fold increase in the relative rate of protein synthesis in 4-cell embryos. Moreover, addition of betaine to medium containing 125 mM NaCl increases the relative rate of protein synthesis. This finding provides an explanation, at least in part, for the increase in development to the blastocyst stage exhibited by mouse embryos cultured in these media.(ABSTRACT TRUNCATED AT 250 WORDS)

Okadaic acid induces premature chromosome condensation reflecting the cell cycle progression in one-cell stage mouse embryos

Haploid parthenogenetic embryos as well as fertilized mouse eggs were treated in vitro with 1-10 microM okadaic acid (OA) at the one-cell stage. Cytogenetic analysis detected that OA induces nuclear envelope breakdown (NEBD) and premature condensation of interphase chromosomes in pronuclei as well as in 2nd polar body (PB) nuclei. G1-, S-, and G2-type prematurely condensed chromosomes (PCC) were found in pronuclei of embryos of different age, which reflects their progression through the first cell cycle. In nuclei from 2nd PBs only G1- and S-type PCC were observed. Using the types of PCC as a criterion of different phases of the cell cycle, it was possible to estimate that in haploid parthenogenetic embryos G1-phase lasts until 5.5 hr post activation (hpa), S-phase takes from 4.5 to 9.5 hpa, and from 8.5 hpa G2-phase had started. Second PBs were found to be in G1-phase until 6.5 hpa and S-phase started in some as early as 5.5 hpa, but in most not before 7.5 hpa. Treatment with OA visualizes G1-chromosomes in pronuclei as well as in 2nd PBs, and it is easy to count the number of these chromosomes and recognize a T6 marker chromosome. The possibility to apply cytogenetic analysis of G1-chromosomes from 2nd PBs for a more accurate detection of maternal meiotic nondisjunction is discussed.

Colocalization of transforming growth factor-alpha and a functional epidermal growth factor receptor (EGFR) to the inner cell mass and preferential localization of the EGFR on the basolateral surface of the trophectoderm in the mouse blastocyst

Results of previous studies suggested that responses of mouse blastocysts to TGF-alpha/EGF treatment are mediated by EGF receptors (EGFR) located on the apical surface of the trophectoderm (TE). We report here results of experiments using gold-labeled EGF that confirm the presence of these apically located EGFRs. In addition, immunoelectron microscopy (IEM) studies using anti-EGFR antibodies indicate that the receptor is preferentially distributed on the basolateral surface of the TE. Furthermore, the receptor is also present on the inner cell mass (ICM) and is likely to be functional, since treatment of isolated ICMs with TGF-alpha affects [35S]methionine uptake and incorporation into acid-insoluble material. IEM was also used to demonstrate that EGF, which is not synthesized by the mouse preimplantation embryo, is present in both the oviduct and the uterus. Maternally derived EGF is present in both ICM and TE cells in freshly isolated blastocysts, but is present in greatly reduced amounts following overnight culture of blastocysts in vitro. Last, IEM was also used to demonstrate that TGF-alpha is preferentially localized to the ICM and polar TE. The co-localization of TGF-alpha and functional EGFRs to the ICM and polar TE suggests potential autocrine, juxtacrine, and paracrine roles for TGF-alpha in blastocyst development.

Protein kinase C modulators influence meiosis kinetics but not fertilizability of mouse oocytes

The role of protein kinase C (PKC) in the successive steps of mouse oocyte meiotic process was investigated. We have used either OAG, an analog of diacylglycerol, or mezerein, a nonphorbol ester diterpene, less tumor promoting than phorbol esters, as PKC activators, and staurosporine as PKC inhibitor. Cumulus-free oocytes were cultured in minimum essential medium with each of these PKC modulators and maturation stages were screened every two hours until the end of the process. Both PKC activators prevented GVBD at each tested dose for 4 hr (OAG) and 8 hr (mezerein), and decreased the frequencies of PB oocytes. The inhibitory effects of both activators were dose dependent and reversible. The addition of OAG to the culture medium after GVBD occurrence (i.e., after 4 hrs) did not affect PB extrusion whereas similar addition of mezerein significantly decreased the frequency of PB oocytes. Inhibition of PKC by staurosporine accelerated GVBD and increased the frequency of PB extrusion. When staurosporine was added after GVBD, PB extrusion occurred earlier but PB oocyte frequency was not increased. Fertilizability was not affected when oocyte maturation occurred in the presence of any of these substances despite the delay in maturation process. These results clearly indicate that the PKC pathway is involved in mouse oocyte meiotic process: activation of the enzyme would arrest meiotic process whereas its inhibition would participate in meiosis induction.

Okadaic acid and p13suc1 modulate the reinitiation of meiosis in mouse oocytes

Short-term exposure to okadaic acid (OA), a specific inhibitor of protein phosphatases 1 and 2A, induced resumption of meiosis, including metaphase spindle formation, in mouse oocytes treated with a phosphodiesterase inhibitor, while long incubations with OA arrested oocyte maturation at a step prior to spindle formation. To explore the basis for this difference, the overall patterns of protein synthesis and phosphorylation and the production of tissue-type plasminogen activator (tPA), the synthesis of which is induced after germinal vesicle breakdown (GVBD), were analyzed under various OA treatments. Short-term exposure to OA led to tPA production and did not greatly affect the maturation-associated changes in protein phosphorylation. By contrast, a long application of OA did not result in tPA production and induced more marked changes in protein phosphorylation. Microinjection into prophase oocytes of the product of the fission yeast gene p13suc1, known to inhibit p34cdc2 kinase activation and/or activity, prevented meiotic reinitiation. This effect was overcome by microinjection of OA, at concentrations higher than those required for induction of maturation in the absence of p13suc1. These observations suggest that inhibition of phosphatase 1 or 2A or both triggers meiotic resumption by acting at the same site or at a site proximal to the p13suc1-sensitive step of cdc2 kinase activation.

Changes in cAMP phosphodiesterase activity and cAMP concentration during mouse preimplantation development

Cyclic nucleotide phosphodiesterase (PDE) activity and cAMP amounts were measured in mouse preimplantation embryos at the 1-cell, 2-cell, 8-cell/morula, and mid-blastocyst stages. PDE activity remained constant between the 1-cell and 2-cell stages. It decreased by the 8-cell stage and continued to decrease by the mid blastocyst stage to about 14% of the 1- and 2-cell values. By contrast, cAMP amounts remained essentially constant at 0.05 fmole/embryo (0.3 microM) from the 1-cell to the blastocyst stage and increased to 0.175 fmole in the fully expanded blastocyst that was close to hatching. Measurements of embryo volume indicated that intracellular volume remained essentially constant up to the blastocyst stage. The morphological changes in cell shape that accompany differentiation of the trophectoderm and that are coupled with blastocoel expansion decreased the intracellular volume. This decrease resulted in an increase in the cAMP concentration to about 0.4 microM by the mid-blastocyst stage. Previous studies indicate that either cAMP or TGF-alpha/EGF can stimulate the rate of blastocoel expansion. Although TGF-alpha/EGF can elevate cAMP levels in other cell types, TGF-alpha, at a concentration that maximally stimulates the rate of blastocoel expansion, did not elevate cAMP in blastocysts. Thus, it was unlikely that elevation of cAMP is the mechanism by which TGF-alpha stimulates the rate of blastocoel expansion.

Temporal pattern of synthesis of the mouse cortical granule protein, p75, during oocyte growth and maturation

We previously demonstrated that a protein of M(r) 75,000 (p75) is localized to cortical granules (CGs) in mouse oocytes and eggs and is released upon activation or fertilization of eggs (K.E. Pierce, M. C. Siebert, G. S. Kopf, R. M. Schultz, and P. G. Calarco, 1990, Dev. Biol. 141, 381-392). To examine the temporal pattern of synthesis of p75 during the early stages of CG formation, growing oocytes, which were isolated from juvenile mice, were incubated for 4 hr in medium containing [35S]methionine, and radiolabeled proteins were immunoprecipitated using an antiserum that detects p75. Synthesis of p75 is detected at low levels in the smallest oocytes examined (less than 20 microns). Synthesis of p75 relative to total protein synthesis increases about 12-fold during oocyte growth from the 20-40 microns size and then remains constant throughout the remaining period of oocyte growth (40-70 microns). In the fully grown, germinal vesicle (GV)-intact oocyte (70-80 microns), immunoprecipitated p75 comprises approximately 1.5% of the total amount of radiolabeled protein. Three hours after the transfer of these oocytes to a medium that supports resumption of meiosis and GV breakdown in vitro, oocytes subjected to a 1-hr labeling pulse display a 35% decrease in the relative level of p75 synthesis. By 15 hr of maturation, p75 synthesis was reduced to 14% of that in the fully grown, GV-intact oocyte and this is similar to the level of p75 synthesis in ovulated eggs. The level of p75 synthesis following in vitro translation of total egg RNA is only 38% lower than that obtained from total oocyte RNA. In addition, synthesis of p75 is observed following in vitro translation of oocyte, but not egg, poly(A)+ RNA. These results are consistent with p75 synthesis during oocyte maturation being under translational control.

Zygotic gene activation in the mouse embryo: involvement of cyclic adenosine monophosphate-dependent protein kinase and appearance of an AP-1-like activity

Protein phosphorylation catalyzed by the cyclic adenosine monophosphate (cAMP)-dependent protein kinase (PKA) is implicated in regulating zygotic gene activation in the two-cell mouse embryo (Poueymirou and Schultz; Dev Biol 133:588-599, 1989). We now provide evidence that H8, which is a PKA inhibitor, inhibits expression of an hsp70-driven beta-galactosidase reporter gene and that the concentration-dependence of this inhibition is similar to that for inhibiting expression of a stage-specific gene(s) that is a product of zygotic gene activation. We also demonstrate that neither cAMP nor serum can stimulate the expression, as detected by a histochemical assay, of a cAMP response element (CRE)- or serum response element (SRE)-driven beta-galactosidase reporter gene, respectively, in either germinal vesicle-intact oocytes or aphidicolin-arrested one-cell embryos that are chronologically at the tw-cell stage. In contrast, although 12-O-tetradecanoyl phorbol-13-acetate (TPA) does not stimulate expression of a TPA response element (TRE)-driven beta-galactosidase reporter gene in germinal vesicle-intact oocytes, it stimulates such expression in aphidicolin-arrested one-cell embryos. Moreover, TPA can stimulate the expression of either a CRE- or an SRE-driven beta-galactosidase reporter gene in such embryos. Results of these studies further implicate protein phosphorylation in regulating zygotic gene activation, along with its role in modulating enhancer function in the early mouse embryo.

Acquisition of a transcriptionally permissive state during the 1-cell stage of mouse embryogenesis

Zygotic gene transcription initiates during the 2-cell stage of mouse embryogenesis. To learn more of the nature and timing of events leading up to transcriptional activation, we evaluated the ability of enucleated 1-cell-stage embryos to support transcription of the 2-cell-stage-specific gene(s) encoding the 70,000-Da transcription-requiring complex (TRC). Nuclei were transplanted from transcriptionally inhibited alpha-amanitin or N-[2-(methylamino)ethyl]-5-isoquinolinesulfonamide (H8)-treated 2-cell-stage embryos to either late or early enucleated 1-cell-stage recipients. Expression of the TRC gene(s) was much greater following transfer to late 1-cell than early 1-cell-stage recipients. In addition, treatment of early 1-cell-stage recipients with N6-monobutyryl cyclic AMP following transplantation of a nucleus from an H8-treated donor increased the rate of TRC synthesis to a value similar to that observed for late 1-cell-stage recipients. These results indicate that during the first cell cycle and prior to initiation of zygotic gene expression, the embryonic cytoplasm undergoes a transition from a transcriptionally nonpermissive to permissive state.

Regulation of gene expression in preimplantation mouse embryos: effects of the zygotic clock and the first mitosis on promoter and enhancer activities

Previous studies have reported that promoters requiring enhancers for full activity in mammalian somatic cells also require enhancers when injected into mouse two-cell embryos, whereas the same promoters can be expressed just as efficiently in the absence of an enhancer when injected into arrested one-cell embryos. Experiments were designed to determine whether this phenomenon reflected normal developmental changes at the beginning of mammalian development, or simply differences in the physiological states of these cells under the experimental conditions employed. The activity of three different promoters that function in a wide variety of mammalian cells was measured both in embryos whose morphological development was arrested and in embryos that continued development in vitro. Expression of the injected gene was related to the onset of zygotic gene expression ("zygotic clock"), the phase of the cell proliferation cycle, the use of aphidicolin to arrest cell proliferation, and formation of two-cell embryos in vitro and in vivo. The results demonstrated that promoter activity was tightly linked to zygotic gene expression, while the need for enhancers to stimulate promoter activity depended only on formation of a two-cell embryo. These results further support the hypothesis that the first mitosis induces a general repression of promoters prior to initiation of zygotic gene expression that is relieved specifically by enhancers.

Stimulatory effect of okadaic acid, an inhibitor of protein phosphatases, on nuclear envelope breakdown and protein phosphorylation in mouse oocytes and one-cell embryos

Treatment of one-cell mouse embryos with okadaic acid (OA), which is an inhibitor of protein phosphatases 1 and 2A, induces a concentration-dependent precocious nuclear envelope breakdown (NEBD) of the pronuclei; at 10 microM okadaic acid, NEBD starts to occur after 1 hr and the embryos become committed to NEBD after about 45 min. Correlated with NEBD is the conversion of a protein of Mr 32,000 (p32) to more highly phosphorylated forms. One-cell embryos cultured continuously in OA-containing medium do not cleave, whereas one-cell embryos incubated for 15-60 min prior to transfer to OA-free medium reveal a time-dependent inhibition in their ability to cleave. OA treatment of oocytes that are arrested from resuming spontaneous maturation by either a phosphodiesterase inhibitor or biologically active phorbol diester results in germinal vesicle breakdown and the maturation-associated changes in the pattern of protein phosphorylation, which include the apparent phosphorylation of p32. Results of these experiments implicate protein phosphatases in the G2 to M transition of the cell cycle in both meiotic and mitotic cells.

Regulation of hsp70 mRNA levels during oocyte maturation and zygotic gene activation in the mouse

Protein phosphorylation catalyzed by the cAMP-dependent protein kinase is implicated in transcriptional activation of the embryonic genome in the two-cell mouse embryo, while heat shock protein (hsp70) has been identified as one of the first products of zygotic gene activation. Using reverse transcription-polymerase chain reaction we have analyzed relative changes in the amount of hsp70 mRNA during oocyte maturation and early embryogenesis. We report that the amount of hsp70 mRNA decreases after germinal vesicle breakdown, while inhibiting germinal vesicle breakdown inhibits this maturation-associated decrease. The amount of hsp70 mRNA increases between the one- and two-cell stages. This increase is inhibited by either alpha-amanitin or the cAMP-dependent protein kinase inhibitor H-8; the same concentration of H-7, which is a more potent inhibitor of protein kinase C, has little inhibitory effect on this increase in the relative amount of hsp70 mRNA. Last, addition of cycloheximide to one-cell embryos late in G2 inhibits neither cleavage to the two-cell stage nor the increase in the relative amount of hsp70 mRNA. These results strengthen the previous proposal that protein phosphorylation is involved in zygotic gene activation in the two-cell mouse embryo.

Stage-specific expression of a family of proteins that are major products of zygotic gene activation in the mouse embryo

Transcriptional activation of the embryonic genome occurs during the two-cell stage in the mouse embryo and is marked by the synthesis of a set of alpha-amanitin-sensitive proteins of Mr 73,000, 70,000, and 68,000. We have characterized these three proteins by two-dimensional gel electrophoresis of [35S]methionine radiolabeled two-cell embryos. Their isoelectric points range from 6.2 to 6.8 and their synthesis, which can constitute 5-10% of total protein synthesis, is restricted to the two-cell stage. These proteins are not heat shock proteins that have previously been reported as major products of transcriptional activation. Peptide mapping by limited proteolysis indicates that these three proteins are highly related to one another and the results of pulse-chase experiments indicate that they are likely to be degraded by the eight-cell stage. These proteins are nuclear-associated and insoluble in 2% Triton X-100/0.3 M KCl. Although these proteins share some features with somatic lamins--they exhibit solubility properties similar to somatic lamins--they do not cross-react with polyclonal antibodies to either lamins A/C or B, nor do they comigrate with somatic lamins on two-dimensional gels. Additional evidence that these proteins are not lamins is that although treatment of two-cell embryos with okadaic acid, which is an inhibitor of protein phosphatases 1 and 2A, results in precocious nuclear envelope breakdown, the proteins remain insoluble in 2% Triton X-100/0.3 M KCl.

Synthesis and developmental regulation of an egg specific mouse protein translated from maternal mRNA

Proteins synthesized by DDK mice embryos were analyzed by 2D electrophoresis and a new egg-specific polypeptide, D14, was identified. The protein is characterized by its high rate of synthesis and electrophoretic properties (MW 36,500, pl greater than 8). The synthesis of D14 is strictly developmentally regulated: starting in the maturing oocyte in the few hours following germinal vesicle breakdown (GVBD), it remains high over the first cell cycle and decreases abruptly during the two-cell stage. The arrest of D14 synthesis is triggered by egg activation and does not directly depend on transcription by the zygotic genome. Nevertheless, drugs that perturb the onset of zygotic transcription concomitantly inhibit D14 arrest of synthesis. D14 is present in both cytoplasmic and nuclear compartments at the two-cell stage; it is very stable and remains detectable at least until the eight-cell stage in the preimplantation embryo. Embryos of wild strains of mice synthesized either D14 or a D14 related polypeptide at a rate comparable to that of DDK embryos, which was at least ten times greater than that found in other laboratory strains. Both the developmental regulation and the genetic variability in its rate of synthesis make D14 an interesting polypeptide for the study of regulation of maternal information in the very early stages of mouse embryo development.

Characterization and localization of a mouse egg cortical granule antigen prior to and following fertilization or egg activation

Immunological approaches were used to characterize an antigen that is present within the cortical granules of mouse oocytes and eggs. Immunoelectron microscopy shows a specific localization of the antigen to the cortical granules in the cortex of mouse oocytes and eggs. Following in vitro fertilization, the antigen is present in the perivitelline space and is associated with the zona pellucida. No cortical granules and very little antigen are detected in the two-cell embryo. This antiserum detects a protein of Mr = 75,000 (p75) following immunostaining of egg proteins on Western blots, or immunoprecipitation of metabolically labeled oocyte proteins or radio-iodinated egg proteins. p75 is also present in exudates obtained from A23187-treated eggs, as detected by either radio-iodination of the released egg proteins, or maturation and ionophore activation of metabolically labeled oocytes. Two-dimensional gel electrophoresis of radio-iodinated egg proteins reveals four species of p75 with pIs between 4.9 and 5.3, whereas only the most basic form of p75 is detected in metabolically labeled oocytes. Multiple forms of the radio-iodinated p75 are present in the exudate of ionophore-treated eggs. p75 displays a greater electrophoretic mobility under nonreducing conditions, indicating the presence of intramolecular disulfide bonds, a common characteristic of secreted proteins. We conclude that p75 is synthesized in oocytes, modified and packaged into cortical granules, and released from eggs following fertilization or activation.