Nucleic acid synthesis in preimplantation rabbit embryos. I. Quantitative aspects, relationship to early morphogenesis and protein synthesis

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

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

Stage-dependent remodeling of the nuclear envelope and lamina during rabbit early embryonic development

Utilizing 3D structured illumination microscopy, we investigated the quality and quantity of nuclear invaginations and the distribution of nuclear pores during rabbit early embryonic development and identified the exact time point of nucleoporin 153 (NUP153) association with chromatin during mitosis. Contrary to bovine early embryonic nuclei, featuring almost exclusively nuclear invaginations containing a small volume of cytoplasm, nuclei in rabbit early embryonic stages show additionally numerous invaginations containing a large volume of cytoplasm. Small-volume invaginations frequently emanated from large-volume nuclear invaginations but not vice versa, indicating a different underlying mechanism. Large- and small-volume nuclear envelope invaginations required the presence of chromatin, as they were restricted to chromatin-positive areas. The chromatin-free contact areas between nucleolar precursor bodies (NPBs) and large-volume invaginations were free of nuclear pores. Small-volume invaginations were not in contact with NPBs. The number of invaginations and isolated intranuclear vesicles per nucleus peaked at the 4-cell stage. At this stage, the nuclear surface showed highly concentrated clusters of nuclear pores surrounded by areas free of nuclear pores. Isolated intranuclear lamina vesicles were usually NUP153 negative. Cytoplasmic, randomly distributed NUP153-positive clusters were highly abundant at the zygote stage and decreased in number until they were almost absent at the 8-cell stage and later. These large NUP153 clusters may represent a maternally provided NUP153 deposit, but they were not visible as clusters during mitosis. Major genome activation at the 8- to 16-cell stage may mark the switch from a necessity for a deposit to on-demand production. NUP153 association with chromatin is initiated during metaphase before the initiation of the regeneration of the lamina. To our knowledge, the present study demonstrates for the first time major remodeling of the nuclear envelope and its underlying lamina during rabbit preimplantation development.

Revealing the dynamics of gene expression during embryonic genome activation and first differentiation in the rabbit embryo with a dedicated array screening

Early mammalian development is characterized by extensive changes in nuclear functions that result from epigenetic modifications of the newly formed embryonic genome. While the first embryonic cells are totipotent, this status spans only a few cell cycles. At the blastocyst stage, the embryo already contains differentiated trophectoderm cells and pluripotent inner cell mass cells. Concomitantly, the embryonic genome becomes progressively transcriptionally active. During this unique period of development, the gene expression pattern has been mainly characterized in the mouse, in which embryonic genome activation (EGA) spans a single cell cycle after abrupt epigenetic modifications. To further characterize this period, we chose to analyze it in the rabbit, in which, as in most mammals, EGA is more progressive and occurs closer to the first cell differentiation events. In this species, for which no transcriptomic arrays were available, we focused on genes expressed at EGA and first differentiation and established a 2,000-gene dedicated cDNA array. Screening this with pre-EGA, early post-EGA, and blastocyst embryos divided genes into seven clusters of expression according to their regulation during this period and revealed their dynamics of expression during EGA and first differentiation. Our results point to transient properties of embryo transcriptome at EGA, due not only to the transition between maternal and embryonic transcripts but also to the transient expression of a subset of embryonic genes whose functions remained largely uncharacterized. They also provide a first view of the functional consequences of the changes in gene expression program.

Expression profiles of the pluripotency marker gene POU5F1 and validation of reference genes in rabbit oocytes and preimplantation stage embryos

BACKGROUND

The surge in the number of gene expression studies and tendencies to increase the quality of analysis have necessitated the identification of stable reference genes. Although rabbits are classical experimental model animals, stable reference genes have not been identified for normalization. The aims of this study were to compare the expression profiles of the widely used reference genes in rabbit oocytes and preimplantation stage embryos, and to select and validate stable ones to use as reference.

RESULTS

Quantitative real time PCR method was used to evaluate 13 commonly used references (Actb, Gapdh, Hprt1, H2afz, Ubc, Ppia, Eef1e1, Polr2a, Tbp, G6pdx, B2m, Pgk1, and Ywhaz) and POU5F1 (Oct4) genes. Expressions of these genes were examined in multiple individual embryos of seven different preimplantation developmental stages and embryo types (in vivo and in vitro). Initial analysis identified three genes (Ubc, Tbp, and B2m) close to the detection limit with irregular expression between the different stages. As variability impedes the selection of stable genes, these were excluded from further analysis. The expression levels of the remaining ten genes, varied according to developmental stage and embryo types. These genes were ranked using the geNorm software and finally the three most stable references (H2afz, Hprt1, and Ywhaz) were selected. Normalization factor was calculated (from the geometric averages of the three selected genes) and used to normalize the expressions of POU5F1 gene. The results showed the expected expression patterns of the POU5F1 during development.

CONCLUSION

Compared to the earlier studies with similar objectives, the comparison of large number of genes, the use of multiple individual embryos as compared to pools, and simultaneous analyses of in vitro and in vivo derived embryo samples were unique approaches in our study. Based on quantification, pattern and geNorm analyses, we found the three genes (H2afz, Hprt1, and Ywhaz) to be the most stable across developmental stages and embryo types, and the geometric averages of these genes can be used for appropriate normalization.

Methylation Reprogramming and Chromosomal Aneuploidy in In Vivo Fertilized and Cloned Rabbit Preimplantation Embryos1

Active demethylation of the paternal genome but not of the maternal genome occurs in fertilized mouse, rat, pig, and bovine zygotes. To study whether this early demethylation wave is important for embryonic development, we have analyzed the global methylation patterns of both in vivo-fertilized and cloned rabbit embryos. Anti-5-methylcytosine immunofluorescence of in vivo-fertilized rabbit embryos revealed that the equally high methylation levels of the paternal and maternal genomes are largely maintained from the zygote up to the 16-cell stage. The lack of detectable methylation changes in rabbit preimplantation embryos suggests that genome-wide demethylation is not an obligatory requirement for epigenetic reprogramming. The methylation patterns of embryos derived from fibroblast and cumulus cell nuclear transfer were similar to those of in vivo-fertilized rabbit embryos. Fluorescence in situ hybridization with chromosome-specific BACs demonstrated significantly increased chromosomal aneuploidy rates in cumulus cell nuclear transfer rabbit embryos and embryos derived from nuclear transfer of rabbit fibroblasts into bovine oocytes compared with in vivo-fertilized rabbit embryos. The incidence of chromosomal abnormalities was correlated with subsequent developmental failure. We propose that postzygotic mitotic errors are one important explanation of why mammalian cloning often fails.

Onset of zygotic transcription and maternal transcript legacy in the rabbit embryo

Onset of zygotic transcription is progressive from the one-cell stage onward in the rabbit embryo. Maternal transcripts remain fairly stable until the 8-16 cell stage when major transcriptional activation of the zygotic genome takes place. To understand the mechanisms of the maternal-to-zygotic transition in the genetic information governing development, we asked whether a progressive synthesis of zygotic transcripts takes over the maternal molecules, or whether the synthesis of zygotic transcripts is very abrupt and independent of the persistence of the maternal counterparts. To answer this question, we set up mRNA differential display experiments comparing the mRNA content of rabbit embryos at different stages during the preimplantation period. We isolated eight zygotic transcripts whose synthesis is abruptly turned on at the 8-16 cell stage. These transcripts are involved in general cellular metabolism and their maternal counterparts are still present up to the four-cell and even the 8-16 cell stage. This identification of early zygotic transcripts suggests that global long range modifications of chromatin structure result in a rapid increase in transcription rates during the major transcriptional activation of the zygotic genome.

Identification of maternal transcripts that progressively disappear during the cleavage period of rabbit embryos

In order to characterize the changes that occur in the population of maternally inherited transcripts before the transition from maternal to zygotic control of embryonic development (MZT) in mammals, we used rabbit embryos where zygotic transcription becomes necessary only after the fourth cleavage division, during the second day that follows fertilization. In the present work we have associated mRNA differential display and an RT-PCR based-method that allows amplification of the whole population of messengers to identify and characterize maternal transcripts which are degraded throughout this early period of development. While there is no major degradation of the polyA RNA population before MZT we identify 4 transcripts which progressively disappear up until the 8-16 cell stage. We also show that the degradation of one of these maternal messengers is controlled by zygotic transcription, which is not the case for the three others. This messenger shows homology with the human FGF9 gene and is potentially a good candidate to address the question of the molecular control of maternal to zygotic transition in early mammalian embryogenesis.

Maternal RNA content in oocytes of several mammalian and avian species

The amount of maternal RNA in the oocytes of several mammalian (mouse, rabbit, cow, pig, sheep) and avian (hen, Japanese quail, guinea hen, turkey) species was determined. For mammals these were in the range of 0.47 ng/oocyte (mouse) to 0.98 ng/oocyte (cow), with the exception of the rabbit, where it was up to 15 ng/oocyte. Birds' oocytes contain much higher levels of RNA, from 1.0 microgram (Japanese quail) up to 2.1 micrograms/oocyte (hen). The RNA levels were determined by direct measurement of EtBr-induced fluorescence of the oocyte homogenates without prior extraction and purification, and correlate well with standard spectrophotometric OD260 measurements. This method is useful when there are limited amounts of a sample, and it permits nucleic acid determination in the presence of proteins.

Nucleocytoplasmic translocation of ribosomal RNA in the rabbit blastocyst: participation of sulfhydryl groups

The release of ribosomes from the nucleus in the rabbit blastocyst was investigated by pulse-labeling embryos to within 5 min of the earliest appearance of radiolabeled ribosomal RNA (rRNA) in the cytoplasmic fraction. The accumulation of radiolabeled 4.7 and 1.9 kilobase mature rRNA species in the cytoplasm was then followed during a 2 hour chase period, using polyacrylamide gel electrophoresis to identify the rRNAs. Colchicine, cytochalasin B, KCN, and EDTA were found to have no effect on the release of radiolabeled rRNA from the blastocyst nucleus during the 2 hour chase. Oligomycin, a known inhibitor of the nuclear envelope nucleoside triphosphatase, and the protein synthesis inhibitors puromycin and cycloheximide blocked rRNA release after a short delay. In contrast, actinomycin D and the sulfhydryl-reactive agents N-ethylmaleimide and diamide produced an abrupt and complete block to further rRNA release. The results indicate that ribosomes leave the nuclear compartment by an energy-dependent process. They further underscore the importance of reduced sulfhydryl groups in a rapidly growing blastocyst with a high level of oxidative metabolism.

Properties of amino acid transport in preimplantation rabbit embryos

The kinetics, specificity, time dependency, Na+ dependency, and exchange behavior of the uptake of methionine, as a model for the uptake of the neutral amino acids, was investigated in rabbit eggs, morulae, and 6-day blastocysts. Uptake by the blastocyst was physically resolved into components representing uptake into the cellular and cavity compartments. The kinetic data for all material, except the cellular compartment of the blastocyst, could be described mathematically by the parallel activity of a saturable and a linear transport system. The Km decreased from 91 microM in the unfertilized egg to 38 microM in the fertilized egg to 13 microM in the 69-hr morula. The Km of uptake into blastocysts was 131 microM. The Vmax of uptake into the whole blastocyst was calculated to be about 5000 times that of uptake into the egg. Competition of methionine uptake by other amino acids indicated that the egg had somewhat more "L-like" transport specificity than subsequent developmental stages, while the system(s) in the blastocyst were reactive with most neutral amino acids. Uptake into the egg, 69-hr morula, and the cellular compartment of the blastocyst reached a steady state with time, in contrast to uptake into the 76-hr morula and the cavity compartment of the blastocyst. Compatible with this, was the observation that eggs and 69-hr morula, but not blastocysts, exhibited considerable efflux into amino acid-containing media. The cavity compartment of the blastocyst may, however, act as a reservoir of amino acids for the cellular compartment. The Na+ dependency of transport was unchanged throughout the developmental period studied in this investigation.

Electron microscopic analysis of RNA transcription in preimplantation rabbit embryos

We have examined directly RNA synthesis in several stages of preimplantation rabbit embryos by electron microscopic analysis of chromatin spreads. DNA/chromatin packing ratios were determined for non-transcribing regions (2.0) and for regions active in RNA synthesis (1.1). Significant amounts of RNA transcription were observed in all embryonic stages examined. Lengths of transcribed regions were calculated and range from 1.1 to 10.9 Kb of B-form DNA. The frequency of chromatin-associated ribonucleoprotein (RNP) fibrils was greatly reduced in samples prepared from embryos treated with alpha-amanitin indicating that the RNPs observed were growing chains. The frequency of single and multiple RNP fibrils attached to chromatin is the same at all stages examined implying the absence of extensive modulation of RNA chain initiation. These results are discussed in relation to the biochemical data on RNA synthesis in preimplantation rabbit embryos.

Effects of puromycin and actinomycin D on fertilized rabbit eggs cultured in vitro

Rabbit zygotes were cultured in media containing varying concentrations of puromycin and actinomycin D in order to determine what effect these agents have on events of fertilization and early development. Concentrations of puromycin greater than 10(-5) M inhibited mitosis, cytokinesis of fertilized eggs and leucine incorporation. In approximately 50% of the zygotes treated with puromycin (10(-5) M or greater) the female pronucleus remained at its site of formation, the cortex, and did not migrate centrad. In these instances the male pronucleus moved from its site of development to the female pronucleus. Concentrations of actinomycin D greater than 10(-7) M also inhibited mitosis and cytokinesis of fertilized eggs. Breakdown of the nuclear envelopes and association of the paternally- and maternally-derived chromatin did not occur in either puromycin- or actinomycin D-treated embryos. Actinomycin D at all of the concentrations employed (10(-9) to 10(-4) M) had little effect upon the structure of nucleolus-like bodies within male and female pronuclei. Male and female pronuclei of puromycin (10(-4) M) and actinomycin D (10(-5) M) treated zygotes, incubated with tritiated thymidine and prepared for scintillation counting and autoradiography, synthesized DNA at a reduced level when compared to control specimens. Although "side effects" of puromycin and actinomycin D treatment are not excluded, these results suggest that nascent proteins may be required for events of fertilization.

Nucleic acid synthesis in preimplantation rabbit embryos. III. A "dark period" immediately following fertilization, and the early predominance low molecular weight RNA synthesis

This report stems from efforts to characterize the products of transcription during the initial stages of cleavage in the rabbit embryo, since previous studies have suggested that RNA synthesis is required during this period for the continuation of normal development. Early cleaving embryos were exposed to radiolabeled precursors of nucleic acids in vitro and RNA extracted and analyzed by polyacrylamide gel electrophoresis and BD-cellulose chromatography. No unequivocal evidence of RNA synthesis could be obtained during the first three cleavage divisions; all apparent "embryonic" RNA synthesis could be ascribed to contaminating follicle cells. After the fourth cleavage, however, there is readily detectable incorporation of radioisotope into RNA by embryos which have been denuded of follicle cells. Two-thirds of the cytoplasmic RNA labeled during a 4-hour period is 25-30 X 10(3) daltons in size, and some of this low molecular weight RNA can be aminoacylated and shown to be transfer RNA. A low level of ribosomal RNA synthesis is detectable at this stage. The contribution, if any, of the rabbit embryonic genome to development prior to the 16-cell stage remains to be defined.

Lactate dehydrogenase in preimplantation rabbit embryos

Unfertilized eggs and early embryos up to the 2-day (16-cell) cleavage stage of development in the rabbit contain predominantly the most cathodal lactate dehydrogenase isoenzyme made up of A-type subunits. Following early cleavage there is a progressing increase in total LDH activity in the embryo as development proceeds through 4- and 6-day blastocyst stages. This is accompanied by an increase in the amount of B-type subunits and a concomitant shift in the lactate dehydrogenase isoenzyme electrophoretic pattern toward the anodal isoenzyme types.