Characterization of linker histone H1FOO during bovine in vitro embryo development

The embryonic DPPA3 gene stimulates the expression of pregnancy-related genes in bovine endometrial cells

Extracellular vesicles (EV) released by cells contain mRNAs, microRNAs, long noncoding RNAs, lipids, and proteins, playing crucial roles in cell-cell communication. Although full-length mRNA transcripts have been documented in EV secreted by cancer cells, there are no reports on full transcripts secreted by embryos. Our study aimed to identify EV mRNAs in the culture medium of bovine embryos and investigate their roles in embryo-maternal communication. Following the isolation of EV from in vitro fertilization media samples and RNA sequencing, we identified a full mRNA transcript of DPPA3, known to play an essential role in embryo development. To examine the role of DPPA3 in embryo-maternal communication, an in vitro transcribed mRNA of DPPA3 was transfected into bovine endometrial epithelial cells. Transfected and control cells were subsequently analyzed with RNA sequencing and proteomics to assess the effects of DPPA3 on gene expression. A total of 24 genes were found to be upregulated, and 1 gene was downregulated (false discovery rate <0.01) following DPPA3 transfection, many with known functions in pregnancy recognition. Proteomic analysis revealed 28 differentially expressed proteins, with 17 upregulated and 11 downregulated. Two proteins, ISG15 and MX1, overlapped with the differentially expressed mRNAs. To mimic the natural transfer of EV from embryos to endometrial cells, we performed coculture with d-8 blastocysts or supplemented the cells with embryo-conditioned culture medium. DPPA3 presence was detected in endometrial cells exposed to embryo-conditioned medium after just 30 min. Overall, our study highlights the significant role of EV in cell-cell communication through mRNA signaling from the embryo to the mother.

Heterologous expression of bovine histone H1foo into porcine fibroblasts alters the transcriptome profile but not embryo development following nuclear transfer

PURPOSE

Somatic cell nuclear transfer (SCNT) is a valuable tool for investigating reprogramming mechanisms and creating animal clones for applications in production, conservation, companionship, and biomedical research. However, SCNT efficiency remains low. Expression of nuclear proteins associated with an undifferentiated chromatin state, such as the oocyte-specific variant of the linker histone H1 (H1foo), represents a strategy for improving reprogramming outcomes, but this approach has not been tested in the context of SCNT.

METHODS

Bovine H1foo (bH1foo) was transfected into porcine fibroblasts via electroporation for expression until SCNT. The transcriptomic profile of these cells was analyzed, and their potential as donor cells for SCNT was evaluated 48 h post-electroporation.

RESULTS

Strong nuclear localization of bH1foo persisted for 48 h post-electroporation. A total of 447 genes were differentially expressed, and lower levels of H3K4me3 and H3K27me3 were detected in bH1foo-expressing cells, indicating changes in chromatin remodeling and function. Embryo development and total cell number per blastocyst were similar between SCNT embryos produced with control and bH1foo-expressing cells. mRNA levels of genes involved in embryonic genome activation were comparable between embryos derived from control and bH1foo-expressing cells on days 3 and 4 of development, suggesting that bH1foo did not disrupt this critical process.

CONCLUSIONS

The heterologous expression of bovine H1foo altered the chromatin function of porcine fibroblasts without impairing development to the blastocyst stage following SCNT. These results highlight the potential of expressing nuclear proteins as a strategy to enhance cell reprogramming and cloning efficiency, including interspecies cloning applications.

Linker histone H1FOO is required for bovine preimplantation development by regulating lineage specification and chromatin structure

Linker histone H1 binds to the nucleosome and is implicated in the regulation of the chromatin structure and function. The H1 variant H1FOO is heavily expressed in oocytes and early embryos. However, given the poor homology of H1FOO among mammals, the functional role of H1FOO during preimplantation embryonic development remains largely unknown, especially in domestic animals. Here, we find that H1FOO is not only expressed in oocytes and preimplantation embryos but granulosa cells and spermatids in cattle. We then demonstrate that the interference of H1FOO results in preimplantation embryonic developmental arrest in cattle using either RNA editing or Trim-Away approach. H1FOO depletion leads to a compromised expression of critical lineage-specific genes at the morula stage and affects the establishment of cell polarity. Interestingly, H1FOO depletion causes a significant increase in the expression of genes encoding other linker H1 and core histones. Concurrently, there is an increase of H3K9me3 and H3K27me3, two markers of repressive chromatin and a decrease of H4K16ac, a marker of open chromatin. Importantly, overexpression of bovine H1FOO results in severe embryonic developmental defects. In sum, we propose that H1FOO controls the proper chromatin structure that is crucial for the fidelity of cell polarization and lineage specification during bovine preimplantation development.

Maternal effect genes: Update and review of evidence for a link with birth defects

Maternal effect genes (MEGs) encode factors (e.g., RNA) that are present in the oocyte and required for early embryonic development. Hence, while these genes and gene products are of maternal origin, their phenotypic consequences result from effects on the embryo. The first mammalian MEGs were identified in the mouse in 2000 and were associated with early embryonic loss in the offspring of homozygous null females. In humans, the first MEG was identified in 2006, in women who had experienced a range of adverse reproductive outcomes, including hydatidiform moles, spontaneous abortions, and stillbirths. Over 80 mammalian MEGs have subsequently been identified, including several that have been associated with phenotypes in humans. In general, pathogenic variants in MEGs or the absence of MEG products are associated with a spectrum of adverse outcomes, which in humans range from zygotic cleavage failure to offspring with multi-locus imprinting disorders. Although less established, there is also evidence that MEGs are associated with structural birth defects (e.g., craniofacial malformations, congenital heart defects). This review provides an updated summary of mammalian MEGs reported in the literature through early 2021, as well as an overview of the evidence for a link between MEGs and structural birth defects.

NOTCH signaling pathway is required for bovine early embryonic development†

The NOTCH signaling pathway plays an important role in regulating various biological processes, including lineage specification and apoptosis. Multiple components of the NOTCH pathway have been identified in mammalian preimplantation embryos. However, the precise role of the NOTCH pathway in early embryonic development is poorly understood, especially in large animals. Here, we show that the expression of genes encoding key transcripts of the NOTCH pathway is dynamic throughout early embryonic development. We also confirm the presence of active NOTCH1 and RBPJ. By using pharmacological and RNA interference tools, we demonstrate that the NOTCH pathway is required for the proper development of bovine early embryos. This functional consequence could be partly attributed to the major transcriptional mediator, Recombination Signal Binding Protein For Immunoglobulin Kappa J Region (RBPJ), whose deficiency also compromised the embryo quality. Indeed, both NOTCH1 and RBPJ knockdown cause a significant increase of histone H3 serine 10 phosphorylation (pH3S10, a mitosis marker) positive blastomeres, suggesting a cell cycle arrest at mitosis. Importantly, RNA sequencing analyses reveal that either NOTCH1 or RBPJ depletion triggers a reduction in H1FOO that encodes the oocyte-specific linker histone H1 variant. Interestingly, depleting H1FOO results in detrimental effects on the developmental competence of early embryos, similar with NOTCH1 inhibition. Overall, our results reveal a crucial role for NOTCH pathway in regulating bovine preimplantation development, likely by controlling cell proliferation and maintaining H1FOO expression.

Coping with DNA Double-Strand Breaks via ATM Signaling Pathway in Bovine Oocytes

As a common injury almost all cells face, DNA damage in oocytes—especially double-strand breaks (DSBs), which occur naturally during the first meiosis phase (meiosis I) due to synaptic complex separation—affects the fertilization ability of oocytes, instead of causing cancer (as in somatic cells). The mechanism of oocytes to effectively repair DSB damage has not yet been clearly studied, especially considering medically induced DSBs superimposed on naturally occurring DSBs in meiosis I. It was found that maturation rates decreased or increased, respectively corresponding with overexpression or interference of p21 in bovine oocytes. At the same time, the maturation rate of bovine oocytes decreased with a gradual increase in Zeocin dose, and the p21 expression in those immature oocytes changed significantly with the gradual increase in Zeocin dose (same as increased DSB intensity). Same as p21, the variation trend of ATM expression was consistent with the gradual increase in Zeocin dose. Furthermore, the oocytes demonstrated tolerance to DSBs during meiosis I, while the maturation rates decreased when the damage exceeded a certain threshold; according to which, it may be that ATM regulates the p53–p21 pathway to affect the completion of meiosis. In addition, nonhomologous recombination and cumulus cells are potentially involved in the process by which oocytes respond to DSB damage.

Analysis of mRNA abundance for histone variants, histone- and DNA-modifiers in bovine in vivo and in vitro oocytes and embryos

Transcript abundance of histone variants, modifiers of histone and DNA in bovine in vivo oocytes and embryos were measured as mean transcripts per million (TPM). Six of 14 annotated histone variants, 8 of 52 histone methyl-transferases, 5 of 29 histone de-methylases, 5 of 20 acetyl-transferases, 5 of 19 de-acetylases, 1 of 4 DNA methyl-transferases and 0 of 3 DNA de-methylases were abundant (TPM >50) in at least one stage studied. Overall, oocytes and embryos contained more varieties of mRNAs for histone modification than for DNA. Three expression patterns were identified for histone modifiers: (1) transcription before embryonic genome activation (EGA) and down-regulated thereafter such as PRMT1; (2) low in oocytes but transiently increased for EGA such as EZH2; (3) high in oocytes but decreased by EGA such as SETD3. These expression patterns were altered by in vitro culture. Additionally, the presence of mRNAs for the TET enzymes throughout pre-implantation development suggests persistent de-methylation. Together, although DNA methylation changes are well-recognized, the first and second orders of significance in epigenetic changes by in vivo embryos may be histone variant replacements and modifications of histones.

BAPTA-AM dramatically improves maturation and development of bovine oocytes from grade-3 cumulus-oocyte complexes

Intracellular free calcium ([Ca²⁺ ]_i ) is essential for oocyte maturation and early embryonic development. Here, we investigated the role of [Ca²⁺ ]_i in oocytes from cumulus-oocyte complexes (COCs) with respect to maturation and early embryonic development, using the calcium-buffering agent BAPTA-AM (1,2-bis[2-aminophenoxy]ethane-N,N,N',N'-tetraacetic acid tetrakis [acetoxymethyl ester]). COCs were graded based on compactness of the cumulus mass and appearance of the cytoplasm, with Grade 1 indicating higher quality and developmental potential than Grade 3. Results showed that: (i) [Ca²⁺ ]_i in metaphase-II (MII) oocytes from Grade-3 COCs was significantly higher than those from Grade-1 COCs, and was significantly reduced by BAPTA-AM; (ii) nuclear maturation of oocytes from Grade-3 COCs treated with BAPTA-AM was enhanced compared to untreated COCs; (iii) protein abundance of Cyclin B and oocyte-specific Histone 1 (H1FOO) was improved in MII oocytes from Grade-3 COCs treated with BAPTA-AM; (iv) Ca²⁺ transients were triggered in each group upon fertilization, and the amplitude of [Ca²⁺ ]_i oscillations increased in the Grade-3 group upon treatment with BAPTA-AM, with the magnitude approaching that of the Grade-1 group; and (v) cleavage rates and blastocyst-formation rates were improved in the Grade-3 group treated with BAPTA-AM compared to untreated controls following in vitro fertilization and parthenogenetic activation. Therefore, BAPTA-AM dramatically improved oocyte maturation, oocyte quality, and embryonic development of oocytes from Grade-3 COCs.

Regulation of ATF1 and ATF2 transcripts by sequences in their 3' untranslated region in cleavage-stage cattle embryos

The sequence of a 3' untranslated region (3'UTR) of mRNA governs the timing of its polyadenylation and translation in mammalian oocytes and early embryos. The objective of this study was to assess the influence of cis-elements in the 3'UTR of the developmentally important ATF1 and ATF2 transcripts on their timely translation during first cleavages in bovine embryos. Eight different reporter mRNAs (coding sequence of green fluorescent protein [GFP] fused to the 3'UTR of short or long isoforms of cattle ATF1 or -2, with or without polyadenylation) or a control GFP mRNA were microinjected separately into presumptive bovine zygotes at 18 hr post-insemination (hpi), followed by epifluorescence assessment for GFP translation between 24 and 80 hpi (expressed as percentage of GFP-positive embryos calculated from the total number of individuals). The presence of either polyadenine or 3'UTR sequence in deadenylated constructs is required for GFP translation (implying the need for polyadenylation), and all exogenous mRNAs that met either criteria were translated as soon as 24 hpi-except for long-deadenylated ATF2-UTR, whose translation began at 36 hpi. Overall, GFP was more visibly translated in competent (cleaving) embryos, particularly in long ATF1/2 constructs. The current data shows a timely GFP translation in bovine embryos depending on sequences in the 3'UTR of ATF1/2, and indicates a difference between short and long isoforms. In addition, cleaving embryos displayed increased translational capacity of the tested constructs. Functional confirmation of the identification cis-sequences in the 3'UTR of ATF1/2 will contribute to the understanding of maternal mRNA translation regulation during early cattle development.

Sculpting the Transcriptome During the Oocyte-to-Embryo Transition in Mouse

In mouse, the oocyte-to-embryo transition entails converting a highly differentiated oocyte to totipotent blastomeres. This transition is driven by degradation of maternal mRNAs, which results in loss of oocyte identity, and reprogramming of gene expression during the course of zygotic gene activation, which occurs primarily during the two-cell stage and confers blastomere totipotency. Full-grown oocytes are transcriptionally quiescent and mRNAs are remarkably stable in oocytes due to the RNA-binding protein MSY2, which stabilizes mRNAs, and low activity of the 5' and 3' RNA degradation machinery. Oocyte maturation initiates a transition from mRNA stability to instability due to phosphorylation of MSY2, which makes mRNAs more susceptible to the RNA degradation machinery, and recruitment of dormant maternal mRNAs that encode for critical components of the 5' and 3' RNA degradation machinery. Small RNAs (miRNA, siRNA, and piRNA) play little, if any, role in mRNA degradation that occurs during maturation. Many mRNAs are totally degraded but a substantial fraction is only partially degraded, their degradation completed by the end of the two-cell stage. Genome activation initiates during the one-cell stage, is promiscuous, low level, and genome wide (and includes both inter- and intragenic regions) and produces transcripts that are inefficiently spliced and polyadenylated. The major wave of genome activation in two-cell embryos involves expression of thousands of new genes. This unique pattern of gene expression is the product of maternal mRNAs recruited during maturation that encode for transcription factors and chromatin remodelers, as well as dramatic changes in chromatin structure due to incorporation of histone variants and modified histones.

Chromatin remodelling and histone m RNA accumulation in bovine germinal vesicle oocytes

Major remodelling of the chromatin enclosed within the germinal vesicle occurs towards the end of oocyte growth in mammals, but the mechanisms involved in this process are not completely understood. In bovine, four distinct stages of chromatin compaction-ranging from a diffused state (GV0) to a fully compacted configuration (GV3)-are linked to the gradual acquisition of developmental potential. To better understand the molecular events and to identify mRNA modulations occurring in the oocyte during the GV0-to-GV3 transition, transcriptomic analysis was performed with the EmbryoGENE microarray platform. The mRNA abundance of several genes decreased as chromatin compaction increased, which correlates with progressive transcriptional silencing that is characteristic of the end of oocyte growth. On the other hand, the abundance of some transcripts increased during the same period, particularly several histone gene transcripts from the H2A, H2B, H3, H4, and linker H1 family. In silico analysis predicted RNA-protein interactions between specific histone transcripts and the bovine stem-loop binding protein 2 (SLBP2), which helps regulate the translation of histone mRNA during oogenesis. These results suggest that some histone-encoding transcripts are actively stored, possibly to sustain the needs of the embryo before genome activation. This dataset offers a unique opportunity to survey which histone mRNAs are needed to complete chromatin compaction during oocyte maturation and which are stockpiled for the first three cell cycles following fertilization.

H1foo is essential for in vitro meiotic maturation of bovine oocytes

Oocyte-specific linker histone, H1foo, is localized on the oocyte chromosomes during the process of meiotic maturation, and is essential for mouse oocyte maturation. Bovine H1foo has been identified, and its expression profile throughout oocyte maturation and early embryo development has been established. However, it has not been confirmed if H1foo is indispensable during bovine oocyte maturation. Effective siRNAs against H1foo were screened in HeLa cells, and then siRNA was microinjected into bovine oocytes to down-regulate H1foo expression. H1foo overexpression was achieved via mRNA injection. Reverse transcription polymerase chain reaction (RT-PCR) results indicated that H1foo was up-regulated by 200% and down-regulated by 70%. Based on the first polar body extrusion (PB1E) rate, H1foo overexpression apparently promoted meiotic progression. The knockdown of H1foo significantly impaired bovine oocyte maturation compared with H1foo overexpression and control groups (H1foo overexpression = 88.7%, H1foo siRNA = 41.2%, control = 71.2%; P < 0.05). This decrease can be rescued by co-injection of a modified H1foo mRNA that has escaped from the siRNA target. However, the H1e (somatic linker histone) overexpression had no effect on PB1E rate when compared with the control group. Therefore we concluded that H1foo is essential for bovine oocyte maturation and its overexpression stimulates the process.

Discovery, identification and sequence analysis of RNAs selected for very short or long poly A tail in immature bovine oocytes

A major challenge in applying genomics to oocyte physiology is that many RNAs are present but will not be translated into proteins, making it difficult to draw conclusions from RNAseq and array data. Oocyte maturation and early embryo development rely on maternal storage of specific RNAs with a short poly(A) tail, which must be elongated for translation. To resolve the role of key genes during that period, we aimed to characterize both extremes of mRNA: deadenylated RNA and long polyA tails mRNA population in immature bovine oocytes. Using magnetic beads coupled to oligodT, we isolated deadenylated (A-, 20-50 adenosines) from polyadenylated (A+, up to 200 adenosines) RNAs. After transcriptomic analysis, we observed that A+ candidates are associated with short-term processes required for immediate cell survival (translation or protein transport) or meiotic resumption, while several A- candidates are involved in processes (chromatin modification, gene transcription and post-transcriptional modifications) that will be extremely important in the development of the early embryo. In addition to a list of candidates probably translated early or late, sequence analysis revealed that cytoplasmic polyadenylation element (CPE) and U(3)GU(3) were enriched in A- sequences. Moreover, a motif associated with polyadenylation signals (MAPS, U(5)CU(2)) appeared to be enriched in 3'untranslated regions (UTR) with CPE or U(3)GU(3) sequences in bovine but also in zebrafish and Xenopus tropicalis. To further validate our methodology, we measured specific tail length of known candidates (AURKA, PTTG1, H2A1) but also determined the poly(A) tail length of other candidate RNAs (H3F3A, H1FOO, DAZAP2, ATF1, ATF2, KAT5, DAZL, ELAVL2). In conclusion, we have reported a methodology to isolate deadenylated from polyadenylated RNAs in samples with small total RNA quantities such as mammals. Moreover, we identified deadenylated RNAs in bovine oocytes that may be stored for the long-term process of early embryo development and described a conserved motif enriched in the 3'UTR of deadenylated RNAs.

Epigenetics and regeneration

During newt lens regeneration a unique transdifferentiation event occurs. In this process, dorsal iris pigmented epithelial cells transdifferentiate into lens cells. This system should provide a new insight into cellular plasticity in basic and applied research. Recently, a series of approaches to study epigenetic reprogramming during transdifferentiation have been performed. In this review, we introduce the regulation of dynamic regulation of core-histone modifications and the emergence of an oocyte-type linker histone during transdifferentiation. Finally, we show supporting evidence that there are common strategies of reprogramming between newt somatic cell in transdifferentiation and oocytes after somatic cell nuclear transfer.

Replacement of H1 linker histone during bovine somatic cell nuclear transfer

Linker histone variants are involved in regulation of chromosome organization and gene transcription; several subtypes are expressed in the maturing oocyte and developing embryo. In Xenopus and mice, the transition between linker histone variants occurred following nuclear transfer, and apparently contributed to donor nuclear reprogramming. To determine whether such linker histone replacement occurred after bovine nuclear transfer, red fluorescent protein (RFP) tagged H1e (somatic linker histone H1e) donor cells and Venus tagged H1foo eggs were created, enucleated eggs were injected with donor cells, and embryos were created by fusion. Using fluorescence microscopy, release of H1e in the donor nucleus, acquisition of H1foo by donor chromosomes, and the H1foo-to-H1e transition were observed in live cells. Linker histone replacement occurred more slowly in bovine than murine embryos. Low levels of diffuse red fluorescence (H1e) in the donor nucleus were detected 5 h after fusion, at which time green fluorescence (H1foo) had incorporated into donor chromosomes. However, complete replacement did not occur until 8 h after fusion. We concluded that the linker histone transition was sufficiently conserved among species, which provided further evidence regarding its important role in nuclear reprogramming.

Investigating the potential of genes preferentially expressed in oocyte to induce chromatin remodeling in somatic cells

The oocyte capacity to rejuvenate a differentiated nucleus to restart the proper embryonic program has been highly conserved between vertebrate species. In view of the recent progress to induce pluripotency in somatic cells with stemness genes, we investigated the potential of oocyte genes to contribute to chromatin rearrangements in somatic cells. We selected conserved genes that are naturally expressed mainly in oocytes and that were susceptible to play a role in reprogramming during early embryogenesis. We induced their expression by transient transfection in HEK293 cells. We then assessed whether they had a global impact on epigenetic events such as histone core modifications, and also on transcription and expression of pluripotency-associated transcription factors. Nucleoplasmin 2 (NPM2), activation-induced cytidine deaminase (AICDA), and Geminin (GMNN) overexpression induced differences in histone core modifications (methylation and acetylation). AICDA and NPM2 also influenced RNA neosynthesis. NPM2, GMNN, and STELLA induced overexpression of well-known pluripotency transcription factors. Overall, AICDA, GMNN, NPM2, and STELLA influenced at least one of the aspects analyzed. Their potential could be useful in increasing the cell receptivity to pluripotency induction.

Oocyte-type linker histone B4 is required for transdifferentiation of somatic cells in vivo

The ability to reprogram in vivo a somatic cell after differentiation is quite limited. One of the most impressive examples of such a process is transdifferentiation of pigmented epithelial cells (PECs) to lens cells during lens regeneration in newts. However, very little is known of the molecular events that allow newt cells to transdifferentiate. Histone B4 is an oocyte-type linker histone that replaces the somatic-type linker histone H1 during reprogramming mediated by somatic cell nuclear transfer (SCNT). We found that B4 is expressed and required during transdifferentiation of PECs. Knocking down of B4 decreased proliferation and increased apoptosis, which resulted in considerable smaller lens. Furthermore, B4 knockdown altered gene expression of key genes of lens differentiation and nearly abolished expression of gamma-crystallin. These data are the first to show expression of oocyte-type linker histone in somatic cells and its requirement in newt lens transdifferentiation and suggest that transdifferentiation in newts might share common strategies with reprogramming after SCNT.

Global chromatin architecture reflects pluripotency and lineage commitment in the early mouse embryo

An open chromatin architecture devoid of compact chromatin is thought to be associated with pluripotency in embryonic stem cells. Establishing this distinct epigenetic state may also be required for somatic cell reprogramming. However, there has been little direct examination of global structural domains of chromatin during the founding and loss of pluripotency that occurs in preimplantation mouse development. Here, we used electron spectroscopic imaging to examine large-scale chromatin structural changes during the transition from one-cell to early postimplantation stage embryos. In one-cell embryos chromatin was extensively dispersed with no noticeable accumulation at the nuclear envelope. Major changes were observed from one-cell to two-cell stage embryos, where chromatin became confined to discrete blocks of compaction and with an increased concentration at the nuclear envelope. In eight-cell embryos and pluripotent epiblast cells, chromatin was primarily distributed as an extended meshwork of uncompacted fibres and was indistinguishable from chromatin organization in embryonic stem cells. In contrast, lineage-committed trophectoderm and primitive endoderm cells, and the stem cell lines derived from these tissues, displayed higher levels of chromatin compaction, suggesting an association between developmental potential and chromatin organisation. We examined this association in vivo and found that deletion of Oct4, a factor required for pluripotency, caused the formation of large blocks of compact chromatin in putative epiblast cells. Together, these studies show that an open chromatin architecture is established in the embryonic lineages during development and is sufficient to distinguish pluripotent cells from tissue-restricted progenitor cells.

Effect of follicle size on mRNA expression in cumulus cells and oocytes of Bos indicus: an approach to identify marker genes for developmental competence

To identify the genes related to oocyte competence, we quantified transcripts for candidate genes in oocytes (H1Foo, H2A, H3A, GHR, GDF9, BMP15, OOSP1) and cumulus cells (FSHR, EGFR, GHR, PTX3, IGFII) using the follicle size model to select oocytes of better developmental quality. Follicles were dissected and distributed into four groups according to diameter as follows: 1.0-3.0, 3.1-6.0, 6.1-8.0 and >or=8.1 mm. Cumulus-oocyte complexes (COCs) were released, classified morphologically, matured, fertilised and cultured in vitro or denuded for measurement of diameter and determination of gene expression. Denuded germinal vesicle oocytes and their cumulus cells were used for gene expression analysis by reverse transcription-polymerase chain reaction. The blastocyst rate was highest for oocytes recovered from follicles>6 mm in diameter. In the oocyte, expression of the H2A transcript only increased gradually according to follicle size, being greater (P<0.05) in oocytes from follicles>or=8.1 mm in diameter than in oocytes from follicles<6.0 mm in diameter. In cumulus cells, expression of FSHR, EGFR and GHR mRNA increased with follicular size. In conclusion, we confirmed the importance of H2A for developmental competence and identified important genes in cumulus cells that may be associated with oocyte competence.

Expression of human oocyte-specific linker histone protein and its incorporation into sperm chromatin during fertilization

OBJECTIVE

To investigate the expression of oocyte-specific linker histone protein (hH1FOO) in human ovaries and its incorporation into sperm chromatin after intracytoplasmic sperm injection (ICSI).

DESIGN

Laboratory study.

SETTING

University hospital.

PATIENT(S)

Human ovarian tissues were obtained from patients at oophorectomy. Human oocytes and embryos were obtained from infertile patients undergoing IVF and ICSI.

INTERVENTION(S)

A polyclonal rabbit antibody targeting the predicted hH1FOO protein was used for immunohistochemical analysis. Western blot analysis and the reverse transcriptase-nested polymerase chain reaction were done to detect hH1FOO in chromatin of germinal vesicle-stage oocytes through to two-cell embryos.

MAIN OUTCOME MEASURE(S)

The hH1FOO antibody reactivity of oocytes, ovarian tissues, and sperm chromatin after ICSI.

RESULT(S)

hH1FOO protein was localized in all oocytes from primordial to Graafian follicles. In unfertilized oocytes after ICSI, the chromatin of injected sperm was condensed without hH1FOO incorporation in 45.5% of oocytes, condensed with hH1FOO incorporation in 9%, and decondensed with hH1FOO incorporation in 45.5%. None of the oocytes contained decondensed sperm chromatin without hHFOO incorporation.

CONCLUSION(S)

hH1FOO protein was expressed by human oocytes from primordial follicles to early embryogenesis. Sperm nuclei that were still condensed after ICSI could be separated into two categories by hH1FOO incorporation, which might provide valuable information regarding failed fertilization.

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

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

Unveiling the bovine embryo transcriptome during the maternal-to-embryonic transition

Bovine early embryos are transcriptionally inactive and subsist through the initial developmental stages by the consumption of the maternal supplies provided by the oocyte until its own genome activation. In bovine, the activation of transcription occurs during the 8- to 16-cell stages and is associated with a phase called the maternal-to-embryonic transition (MET) where maternal mRNA are replaced by embryonic ones. Although the importance of the MET is well accepted, since its inhibition blocks embryonic development, very little is known about the transcripts expressed at this crucial step in embryogenesis. In this study, we generated and characterized a cDNA library enriched in embryonic transcripts expressed at the MET in bovine. Suppression subtractive hybridization followed by microarray hybridization was used to isolate more than 300 different transcripts overexpressed in untreated late eight-cell embryos compared with those treated with the transcriptional inhibitor, alpha-amanitin. Validation by quantitative RT-PCR of 15 genes from this library revealed that they had remarkable consistency with the microarray data. The transcripts isolated in this cDNA library have an interesting composition in terms of molecular functions; the majority is involved in gene transcription, RNA processing, or protein biosynthesis, and some are potentially involved in the maintenance of pluripotency observed in embryos. This collection of genes associated with the MET is a novel and potent tool that will be helpful in the understanding of particular events such as the reprogramming of somatic cells by nuclear transfer or for the improvement of embryonic culture conditions.

The block to apoptosis in bovine two-cell embryos involves inhibition of caspase-9 activation and caspase-mediated DNA damage

The capacity of the preimplantation embryo to undergo apoptosis in response to external stimuli is developmentally regulated. Acquisition of apoptosis does not occur in the cow embryo until between the 8- and 16-cell stages. The purpose of the present experiments was to determine the mechanism by which apoptosis is blocked in the bovine two-cell embryo. Heat shock (41 degrees C for 15 h) did not increase activity of caspase-9 or group II caspases (caspase-2, -3, and -7) in two-cell embryos but did in day 5 embryos. Exposure of embryos to carbonyl cyanide 3-chlorophenylhydrazone (CCCP) to depolarize mitochondria resulted in activation of caspase-9 and group II caspases at both stages of development. For day 5 embryos, CCCP also increased the proportion of blastomeres that underwent DNA fragmentation as determined by the TUNEL assay. In contrast, CCCP did not increase TUNEL labeling when applied at the two-cell stage. In conclusion, failure of heat shock to increase caspase-9 and group II caspase activity in the two-cell embryo indicates that the signaling pathway leading to mitochondrial depolarization and caspase activation is inhibited at this stage of development. The fact that CCCP treatment of two-cell embryos induced caspase-9 and group II-caspase activity indicates that caspase activation is possible following mitochondrial depolarization. However, since CCCP did not increase TUNEL labeling of two-cell embryos, actions of group II-caspases to activate DNases is inhibited.

Investigation of MYST4 histone acetyltransferase and its involvement in mammalian gametogenesis

Background

Various histone acetylases (HATs) play a critical role in the regulation of gene expression, but the precise functions of many of those HATs are still unknown. Here we provide evidence that MYST4, a known HAT, may be involved in early mammalian gametogenesis.

Results

Although MYST4 mRNA transcripts are ubiquitous, protein expression was restricted to select extracts (including ovary and testis). Immunohistochemistry experiments performed on ovary sections revealed that the MYST4 protein is confined to oocytes, granulosa and theca cells, as well as to cells composing the blood vessels. The transcripts for MYST4 and all-MYST4-isoforms were present in oocytes and in in vitro produced embryos. In oocytes and embryos the MYST4 protein was localized in both the cytoplasm and nucleus. Within testis sections, the MYST4 protein was specific to only one cell type, the elongating spermatids, where it was exclusively nuclear.

Conclusion

We established that MYST4 is localized into specialized cells of the ovary and testis. Because the majority of these cells are involved in male and female gametogenesis, MYST4 may contribute to important and specific acetylation events occurring during gametes and embryo development.

From biological fundamentals to practice, and back

It has become evident that advances in farm animal reproduction have become increasingly dependent on fundamental scientific research in addition to an understanding of the physiological processes involved in reproduction. As a consequence, most innovations are now coming from a long linear process starting with fundamental scientific research to their application on the farm and lastly, consumer acceptance. The emerging perception of life's complexity is also indicating that technical advances must be better understood before they are implemented by the producer and accepted by the public. To the biological complexity, one must now add the complexity of human interaction on a global level through regulation, international trade and public information. In this context, more than ever, advancements in animal reproduction must be developed in parallel with the scientific understanding of the cause and consequences of human intervention.

Temporal expression of factors involved in chromatin remodeling and in gene regulation during early bovine in vitro embryo development

Distinct epigenetic modification events regulate gene expression and chromatin structure during the period between the immature oocyte and the blastocyst. Throughout this developmental period, important methylation fluctuations occur on genomic DNA and histones. Finding single or combinations of factors, which are at work during this period is essential to understand the entire epigenetic process. With this in mind, we assessed the precise temporal expression profile, during preimplantation embryo development, of 15 key regulators involved in RNA, DNA or histone methylation, chromatin modification or silencing and transcription regulation. To achieve this, real-time RT-PCR was used to quantify the mRNA levels of ATF7IP, DMAP1, EHMT1, EHMT2, HELLS, JARID1A, JARID1B, JMJD1A, JMJD2A, LSD1, MeCP2, METTL3, PRMT2, PRMT5 and RCOR2, in the oocyte and throughout in vitro bovine embryo development. Our results demonstrate that all the 15 key regulators were present to different degrees in the developmental stages tested, and they can be divided into three different groups depending on their respective mRNA profile.