Epigenetic discrimination by mouse metaphase II oocytes mediates asymmetric chromatin remodeling independently of meiotic exit

A program of successive gene expression in mouse one-cell embryos

At the moment of union in fertilization, sperm and oocyte are transcriptionally silent. The ensuing onset of embryonic transcription (embryonic genome activation [EGA]) is critical for development, yet its timing and profile remain elusive in any vertebrate species. We here dissect transcription during EGA by high-resolution single-cell RNA sequencing of precisely synchronized mouse one-cell embryos. This reveals a program of embryonic gene expression (immediate EGA [iEGA]) initiating within 4 h of fertilization. Expression during iEGA produces canonically spliced transcripts, occurs substantially from the maternal genome, and is mostly downregulated at the two-cell stage. Transcribed genes predict regulation by transcription factors (TFs) associated with cancer, including c-Myc. Blocking c-Myc or other predicted regulatory TF activities disrupts iEGA and induces acute developmental arrest. These findings illuminate intracellular mechanisms that regulate the onset of mammalian development and hold promise for the study of cancer.

The initiation of mammalian embryonic transcription: to begin at the beginning

Gamete (sperm and oocyte) genomes are transcriptionally silent until embryonic genome activation (EGA) following fertilization. EGA in humans had been thought to occur around the eight-cell stage, but recent findings suggest that it is triggered in one-cell embryos, by fertilization. Phosphorylation and other post-translational modifications during fertilization may instate transcriptionally favorable chromatin and activate oocyte-derived transcription factors (TFs) to initiate EGA. Expressed genes lay on cancer-associated pathways and their identities predict upregulation by MYC and other cancer-associated TFs. One interpretation of this is that the onset of EGA, and the somatic cell trajectory to cancer, are mechanistically related: cancer initiates epigenetically. We describe how fertilization might be linked to the initiation of EGA and involve distinctive processes recapitulated in cancer.

Effects of the timing of electroporation during in vitro maturation on triple gene editing in porcine embryos using CRISPR/Cas9 system

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Mosaicism is a serious problem for genome editing during embryogenesis.

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We hypothesized that genome-editing before in vitro fertilization can increase its efficiency.

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We introduced CRISPR/Cas9 system into oocytes during in vitro maturation using electroporation.

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Gene editing efficiency in matured oocytes was comparable with that in fertilized zygotes.

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Matured oocytes are suggested as functional material accepting gene editing application.

Mosaicism, including alleles comprising both wild-type and mutant, is a serious problem for gene modification by gene editing using electroporation. One-step generation of F0 pigs with completely desired gene modifications saves cost and time, but the major obstacles have been mosaic mutations. We hypothesized that the timing of electroporation prior to in vitro fertilization (IVF) can increase the rates of biallelic mutation for multiple gene knockout as the permeability of mature oocytes is greater than that of zygotes. Hence, we determined whether the timing of electroporation during in vitro maturation (IVM) culture enhances triple gene editing in the resulting blastocysts. Three gRNAs targeting KDR, PDX1, and SALL1 were simultaneously introduced into the oocytes that had been incubated for 40, 42, and 44 h from the start of the IVM culture. Electroporation with three gRNAs at 40 h and 42 h during IVM culture decreased the blastocyst formation rates and did not improve the mutation rates and target number of biallelic mutations in the resulting blastocysts. The blastocyst formation rate, mutation rates, and target numbers in the resulting blastocysts from oocytes treated by electroporation at 44 h of IVM culture were similar to those of control zygotes electroporated at 13 h after the initiation of IVF. In conclusion, multiple gene editing efficiency in the resulting blastocysts was comparable between oocytes electroporated before and after the fertilization, indicating that oocytes with completed maturation time may allow better functioning of materials accepting gene editing application.

Epigenetic reprogramming in cloned mouse embryos following treatment with DNA methyltransferase and histone deacetylase inhibitors

We examined the effects of DNA methyltransferase inhibitor - RG108, and histone deacetylase inhibitor - SAHA, on the reprogramming parameters of cloned mouse embryos produced by somatic cell nuclear transfer into oocytes. The programming parameters studied included dynamics of histone reacetylation, developmental rate, DNA methylation, and transcript levels of genes, all of which are pivotal to lineage specification and blastocyst formation. At the pronuclear stage, somatic nucleus-transplanted oocytes treated with 5 µM SAHA presented higher histone acetylation at H3K9, H3K14, H4K16 and H4K12, compared to untreated clones (p < 0.05). At the morula stage, cloned embryos treated with 5 μM RG108 or 5 μM SAHA presented lower DNA methylation intensity compared to untreated clones (p < 0.05), resembling the intensity levels of fertilized embryos. However, these effects were not observed when RG108 and SAHA were used in combination. The rate of morula formation was significantly higher in cloned embryos treated with 5 µM SAHA than in untreated clones, whereas treatment with RG108 resulted in no obvious effects on morula formation rates. On the other hand, the combined treatment with RG108 and SAHA resulted in inferior rates of cloned morula formation, compared to untreated clones. At the blastocyst stage, the aberrant expression levels of key developmental genes Oct4 and Cdx2, but not Nanog, were corrected in cloned embryos by the treatment with RG108. This is similar to the intensity levels seen in fertilized embryos. The expression of Rpl7l1 gene was significantly higher in embryos treated with both RG108 and SAHA than in untreated and in control groups. In summary, the present study showed that SAHA and RG108, when applied separately, improve the rate and quality of cloned mouse embryos.

Tracking intracellular forces and mechanical property changes in mouse one-cell embryo development

Cells comprise mechanically active matter that governs their functionality, but intracellular mechanics are difficult to study directly and are poorly understood. However, injected nanodevices open up opportunities to analyse intracellular mechanobiology. Here, we identify a programme of forces and changes to the cytoplasmic mechanical properties required for mouse embryo development from fertilization to the first cell division. Injected, fully internalized nanodevices responded to sperm decondensation and recondensation, and subsequent device behaviour suggested a model for pronuclear convergence based on a gradient of effective cytoplasmic stiffness. The nanodevices reported reduced cytoplasmic mechanical activity during chromosome alignment and indicated that cytoplasmic stiffening occurred during embryo elongation, followed by rapid cytoplasmic softening during cytokinesis (cell division). Forces greater than those inside muscle cells were detected within embryos. These results suggest that intracellular forces are part of a concerted programme that is necessary for development at the origin of a new embryonic life.

HDAC3 maintains oocyte meiosis arrest by repressing amphiregulin expression before the LH surge

It is known that granulosa cells (GCs) mediate gonadotropin-induced oocyte meiosis resumption by releasing EGF-like factors in mammals, however, the detailed molecular mechanisms remain unclear. Here, we demonstrate that luteinizing hormone (LH) surge-induced histone deacetylase 3 (HDAC3) downregulation in GCs is essential for oocyte maturation. Before the LH surge, HDAC3 is highly expressed in GCs. Transcription factors, such as FOXO1, mediate recruitment of HDAC3 to the amphiregulin (Areg) promoter, which suppresses AREG expression. With the LH surge, decreased HDAC3 in GCs enables histone H3K14 acetylation and binding of the SP1 transcription factor to the Areg promoter to initiate AREG transcription and oocyte maturation. Conditional knockout of Hdac3 in granulosa cells in vivo or inhibition of HDAC3 activity in vitro promotes the maturation of oocytes independent of LH. Taking together, HDAC3 in GCs within ovarian follicles acts as a negative regulator of EGF-like growth factor expression before the LH surge.

Before ovulation, a surge of luteinizing hormone (LH) triggers the resumption of meiosis in oocytes, which is mediated by EGF-like growth factors. Here, the authors show that HDAC3 inhibits mouse oocyte maturation by negatively regulating the expression of EGF-like factor before the LH surge.

The possible involvement of oxidative stress in the oocyte ageing process in goldfish Carassius auratus (Linnaeus, 1758)

Decreasing egg quality following oocyte ageing is a major restricting factor for the breeding programs. The mechanisms behind this process has not yet been clarified. To examine the possible involvement of oxidative stress in the oocyte ageing process, the relative mRNA abundance of specific transcripts were determined in oocytes collected from 6 females and incubated in vitro for 18 hours post stripping at 20 °C in goldfish Carassius auratus. During the 18 hour-post-stripping ageing of the oocytes, relative mRNA levels of candidate transcripts involved in oxidative injury, mitochondrial function and stress response, cell cycles, apoptosis, reproduction and germ line speciation and developmental competence were measured by real-time PCR. None of the relative mRNA abundance of the examined genes were significantly altered through oocyte ageing. In addition, the amount of thiobarbituric acid reactive substances (TBARS), an indicator of lipid peroxidation, did not change over time following stripping. The activity of the antioxidant enzymes also remained constant during oocyte ageing. The results of the current study indicated that oxidative stress unlikely plays a role as an initiator or promotor in the progress of oocyte ageing in goldfish.

MC1568 Enhances Histone Acetylation During Oocyte Meiosis and Improves Development of Somatic Cell Nuclear Transfer Embryos in Pig

An increasing number of studies have revealed that histone deacetylase (HDAC) mediated histone deacetylation is important for mammalian oocyte development. However, nonselective HDAC inhibitors (HDACi) were applied in most studies; the precise functions of specific HDAC classes during meiosis are poorly defined. In this study, the class IIa-specific HDACi MC1568 was used to reveal a crucial role of class IIa HDACs in the regulation of histone deacetylation during porcine oocyte meiosis. Besides, the functions of HDACs and histone acetyltransferases in regulating the balance of histone acetylation/deacetylation were also confirmed during oocyte maturation. After the validation of nontoxicity of MC1568 in maturation rate, spindle morphology, and chromosome alignment, effects of MC1568 on developmental competence of porcine somatic cell nuclear transfer (SCNT) embryos were evaluated, and data indicated that treatment with 10 μM MC1568 for 12 hours following electrical activation significantly enhanced the blastocyst rate and cell numbers. Moreover, results showed that optimal MC1568 treatment increased the H4K12 acetylation level in SCNT one cells and two cells. In addition, MC1568 treatment stimulated expression of the development-related genes OCT4, CDX2, SOX2, and NANOG in SCNT blastocysts. Collectively, our investigation uncovered a critical role of class IIa HDACs in the regulation of histone deacetylation during oocyte meiosis. Furthermore, for the first time, we showed that MC1568 can improve the in vitro development of porcine SCNT embryos. These findings provide an alternative HDACi for improving animal cloning efficiency and may shed more light on nuclear reprogramming.

Alteration of mRNA abundance, oxidation products and antioxidant enzyme activities during oocyte ageing in common carp Cyprinus carpio

Oocyte ageing is the most important factor affecting egg quality of several fish species after ovulation. Oxidative stress has been proposed as the initiator of the oocyte ageing process in other vertebrates. To identify the role of oxidative stress and apoptosis on the progress of oocyte ageing in the common carp Cyprinus carpio, changes in the relative mRNA abundance of selected transcripts were examined. The possible alteration in the oxidation status of the oocytes during ageing was also studied. In addition, the activity of antioxidant enzymes during oocyte ageing was evaluated. Oocytes from 6 females were incubated in vivo for 14 hours post-ovulation (HPO) and in vitro for 10 hours post-stripping (HPS) at 20°C before fertilization. Hatching rates were over 65% up to 4–6 HPO, finally dropping to 1.3% at 12–14 HPO.Hatching rates were over 65% up to 4–6 HPO, finally dropping to 1.3% at 12–14 HPO. Hatching rates were more than 70% for the eggs stored in vitro up to 6 HPS and then decreased to 21.3% at 10 HPS. The results demonstrated no significant changes in the relative mRNA levels of oxidative stress-related genes or genes involved in the cell cycle during the progress of oocyte ageing in common carp. Additionally, the amount of TBARS and carbonyls did not change as time elapsed following ovulation. The apoptosis-related genes however, were significantly altered following the prolonged time interval between ovulation and fertilization. The lack of response of both activities of antioxidant enzymes and oxidation products during oocyte ageing strengthens the conclusion that oxidative stress is unlikely to be a main factor determining the progress of oocyte ageing in common carp. However, an increase in the mRNA abundance of apoptosis-related genes demonstrates that apoptotic pathway might be involved in the progress of oocyte ageing.

Mice produced by mitotic reprogramming of sperm injected into haploid parthenogenotes

Sperm are highly differentiated and the activities that reprogram them for embryonic development during fertilization have historically been considered unique to the oocyte. We here challenge this view and demonstrate that mouse embryos in the mitotic cell cycle can also directly reprogram sperm for full-term development. Developmentally incompetent haploid embryos (parthenogenotes) injected with sperm developed to produce healthy offspring at up to 24% of control rates, depending when in the embryonic cell cycle injection took place. This implies that most of the first embryonic cell cycle can be bypassed in sperm genome reprogramming for full development. Remodelling of histones and genomic 5′-methylcytosine and 5′-hydroxymethylcytosine following embryo injection were distinct from remodelling in fertilization and the resulting 2-cell embryos consistently possessed abnormal transcriptomes. These studies demonstrate plasticity in the reprogramming of terminally differentiated sperm nuclei and suggest that different epigenetic pathways or kinetics can establish totipotency.

It is unclear what regulates gamete reprogramming competence. Here, the authors inject sperm into parthenogenetic embryos, generating viable offspring and show that mouse embryos in the mitotic cell cycle can reprogram sperm for full term development.

DNA methylation dynamics in mouse preimplantation embryos revealed by mass spectrometry

Following fertilization in mammals, paternal genomic 5-methyl-2'-deoxycytidine (5 mC) content is thought to decrease via oxidation to 5-hydroxymethyl-2'-deoxycytidine (5 hmC). This reciprocal model of demethylation and hydroxymethylation is inferred from indirect, non-quantitative methods. We here report direct quantification of genomic 5 mC and 5 hmC in mouse embryos by small scale liquid chromatographic tandem mass spectrometry (SMM). Profiles of absolute 5 mC levels in embryos produced by in vitro fertilization (IVF) and intracytoplasmic sperm injection (ICSI) were almost identical. By 10 h after fertilization, 5 mC levels had declined by ~40%, consistent with active genomic DNA demethylation. Levels of 5 mC in androgenotes (containing only a paternal genome) and parthenogenotes (containing only a maternal genome) underwent active 5 mC loss in the first 6 h, showing that both parental genomes can undergo demethylation independently. We found no evidence for net loss of 5 mC 10-48 h after fertilization, implying that any passive 'demethylation' following DNA replication was balanced by active 5 mC maintenance methylation. However, levels of 5 mC declined during development after 48 h, to 1% (measured as a fraction of G-residues) in blastocysts (~96 h). 5 hmC levels were consistently low (<0.2% of G-residues) throughout development in normal diploid embryos. This work directly quantifies the dynamics of global genomic DNA modification in mouse preimplantation embryos, suggesting that SMM will be applicable to other biomedical situations with limiting sample sizes.

Chromatin condensation of Xist genomic loci during oogenesis in mice

Repression of maternal Xist (Xm-Xist) during preimplantation in mouse embryos is essential for establishing imprinted X chromosome inactivation. Nuclear transplantation (NT) studies using nuclei derived from non-growing (ng) and full-grown (fg) oocytes have indicated that maternal-specific repressive modifications are imposed on Xm-Xist during oogenesis, as well as on autosomal imprinted genes. Recent studies have revealed that histone H3 lysine 9 trimethylation (H3K9me3) enrichments on Xm-Xist promoter regions are involved in silencing at the preimplantation stages. However, whether H3K9me3 is imposed on Xm-Xist during oogenesis is not known. Here, we dissected the chromatin states in ng and fg oocytes and early preimplantation stage embryos. Chromatin immunoprecipitation experiments against H3K9me3 revealed that there was no significant enrichment within the Xm-Xist region during oogenesis. However, NT embryos with ng nuclei (ngNT) showed extensive Xm-Xist derepression and H3K9me3 hypomethylation of the promoter region at the 4-cell stage, which corresponds to the onset of paternal Xist expression. We also found that the chromatin state at the Xist genomic locus became markedly condensed as oocyte growth proceeded. Although the condensed Xm-Xist genomic locus relaxed during early preimplantation phases, the extent of the relaxation across Xm-Xist loci derived from normally developed oocytes was significantly smaller than those of paternal-Xist and ngNT-Xist genomic loci. Furthermore, Xm-Xist from 2-cell metaphase nuclei became derepressed following NT. We propose that chromatin condensation is associated with imprinted Xist repression and that skipping of the condensation step by NT leads to Xist activation during the early preimplantation phase.

Summary: The analysis of chromatin state and H3K9me3 levels in mouse oocytes and early embryos provides insights into the dynamics of Xist repression and activation during early development and reprogramming.

Asymmetric parental genome engineering by Cas9 during mouse meiotic exit

Mammalian genomes can be edited by injecting pronuclear embryos with Cas9 cRNA and guide RNA (gRNA) but it is unknown whether editing can also occur during the onset of embryonic development, prior to pronuclear embryogenesis. We here report Cas9-mediated editing during sperm-induced meiotic exit and the initiation of development. Injection of unfertilized, mouse metaphase II (mII) oocytes with Cas9 cRNA, gRNA and sperm enabled efficient editing of transgenic and native alleles. Pre-loading oocytes with Cas9 increased sensitivity to gRNA ~100-fold. Paternal allelic editing occurred as an early event: single embryo genome analysis revealed editing within 3 h of sperm injection, coinciding with sperm chromatin decondensation during the gamete-to-embryo transition but prior to pronucleus formation. Maternal alleles underwent editing after the first round of DNA replication, resulting in mosaicism. Asymmetric editing of maternal and paternal alleles suggests a novel strategy for discriminatory targeting of parental genomes.

Abnormal lysine acetylation with postovulatory oocyte aging

Background

A postovulatory mammalian oocyte decreases developmental potential with in vivo aging in the oviduct or in vitro aging in the culture dish. The mechanism underlying oocyte aging still largely remains an enigma. Accumulating data suggest that the epigenetic alterations such as histone acetylation are also associated with postovulatory aging.

Objective

To perform a review evaluating a new aspect of oocyte aging in terms of the epigenetic alterations focusing on lysine acetylation.

Methods

In addition to a search of the literature in Pubmed, we introduced our recent published data.

Results

Histone acetylation in the mouse oocyte increases during aging, potentially impacting gene regulation in the subsequent embryonic development. Oocyte aging results in increased acetylation of alpha-tubulin, a non-histone protein, and nicotinamide, an inhibitor of class III HDAC, partially prevents some of oocyte aging phenotypes.

Conclusion

Abnormal regulation of protein acetylation itself is suggested in oocyte aging and could contribute to the aging phenotypes.

Melatonin prevents postovulatory oocyte aging in the mouse and extends the window for optimal fertilization in vitro

The quality of metaphase II oocytes deteriorates rapidly following ovulation as the result of an aging process associated with impaired fertilizing potential, disrupted developmental competence, and increased likelihood of embryonic resorption. Because oxidative stress accelerates the onset of apoptosis in oocytes and influences their capacity for fertilization, this study aimed to characterize the significance of such stress in the postovulatory aging of mouse oocytes in vitro. We investigated the ability of the potent antioxidant melatonin to arrest the aging process when used to supplement oocyte culture medium. This study demonstrated that oxidative stress may occur in oocytes after as little as 8 h in culture and coincides with the appearance of early apoptotic markers such as phosphatidylserine externalization, followed 16 h later by caspase activation (P < 0.05) and morphological evidence of oocyte senescence. Importantly, supplementation of oocyte culture medium with 1 mM melatonin was able to significantly relieve the time-dependent appearance of oxidative stress in oocytes (P < 0.05) and, as a result, significantly delay the onset of apoptosis (P < 0.05). Furthermore, melatonin supplementation extended the optimal window for fertilization of oocytes aged for 8 and 16 h in vitro (P < 0.05) and significantly improved the quality of the resulting embryos (P < 0.01). We conclude that melatonin may be a useful tool in a clinical setting to prevent the time-dependent deterioration of oocyte quality following prolonged culture in vitro.

Ceramide and mitochondrial function in aging oocytes: joggling a new hypothesis and old players

Maternal aging adversely affects oocyte quality (function and developmental potential) and consequently lowers pregnancy rates while increasing spontaneous abortions. Substantial evidence, especially from egg donation studies, implicates the decreased quality of an aging oocyte as a major factor in the etiology of female infertility. Nevertheless, the cellular and molecular mechanisms responsible for the decreased oocyte quality with advanced maternal aging are not fully characterized. Herein we present information in the published literature and our own data to support the hypothesis that during aging induced decreases in mitochondrial ceramide levels and associated alterations in mitochondrial structure and function are prominent elements contributing to reduced oocyte quality. Hence, by examining the molecular determinants that underlie impairments in oocyte mitochondria, we expect to sieve to a better understanding of the mechanistic anatomy of oocyte aging.

Histone exchange activity and its correlation with histone acetylation status in porcine oocytes

In mammalian oocytes, histone H3 and histone H4 (H4) in the chromatin are highly acetylated at the germinal vesicle (GV) stage, and become globally deacetylated after GV breakdown (GVBD). Although nuclear core histones can be exchanged by cytoplasmic free histones in somatic cells, it remains unknown whether this is also the case in mammalian oocytes. In this study, we examined the histone exchange activity in maturing porcine oocytes before and after GVBD, and investigated the correlations between this activity and both the acetylation profile of the H4 N-terminal tail and the global histone acetylation level in the chromatin. We injected Flag-tagged H4 (H4-Flag) mRNA into GV oocytes, and found that the Flag signal was localized to the chromatin. We next injected mRNAs of mutated H4-Flag, which lack all acetylation sites and the whole N-terminal tail, and found that the H4 N-terminal tail and its modification were not necessary for histone incorporation into chromatin. Despite the lack of acetylation sites, the mutated H4-Flag mRNA injection did not decrease the acetylation level on the chromatin, indicating that the histone exchange occurs partially in the GV chromatin. In contrast to GV oocytes, the Flag signal was not detected on the chromatin after the injection of H4-Flag protein into the second meiotic metaphase oocytes. These results suggest that histone exchange activity changes during meiotic maturation in porcine oocytes, and that the acetylation profile of the H4 N-terminal tail has no effect on histone incorporation into chromatin and does not affect the global level of histone acetylation in it.

Effect of cryopreservation on acetylation patterns of lysine 12 of histone H4 (acH4K12) in mouse oocytes and zygotes

PURPOSE

to determine the effect of cryopreservation on acH4K12 in oocytes and their respective zygotes.

METHODS

AcH4K12 in fresh or vitrified-warmed oocytes and their respective zygotes at 70 min-12 h post-fertilization were assessed using fluorescent staining.

RESULTS

1. AcH4K12 levels increased significantly in vitrified oocytes compared to controls. 2. Respective zygotes derived from vitrified oocytes had abnormal chromatin distribution or acH4K12 patterns before and after pronuclear formation.

CONCLUSION

Cryopreservation alters AcH4K12 patterns in oocytes, which subsequently affect the chromatin distribution and acH4K12 in fertilized oocytes.

Mouse Emi2 as a distinctive regulatory hub in second meiotic metaphase

The oocytes of vertebrates are typically arrested at metaphase II (mII) by the cytostatic factor Emi2 until fertilization. Regulatory mechanisms in Xenopus Emi2 (xEmi2) are understood in detail but contrastingly little is known about the corresponding mechanisms in mammals. Here, we analyze Emi2 and its regulatory neighbours at the molecular level in intact mouse oocytes. Emi2, but not xEmi2, exhibited nuclear targeting. Unlike xEmi2, separable N- and C-terminal domains of mouse Emi2 modulated metaphase establishment and maintenance, respectively, through indirect and direct mechanisms. The C-terminal activity was mapped to the potential phosphorylation target Tx(5)SxS, a destruction box (D-box), a lattice of Zn(2+)-coordinating residues and an RL domain. The minimal region of Emi2 required for its cytostatic activity was mapped to a region containing these motifs, from residue 491 to the C terminus. The cytostatic factor Mos-MAPK promoted Emi2-dependent metaphase establishment, but Mos autonomously disappeared from meiotically competent mII oocytes. The N-terminal Plx1-interacting phosphodegron of xEmi2 was apparently shifted to within a minimal fragment (residues 51-300) of mouse Emi2 that also contained a calmodulin kinase II (CaMKII) phosphorylation motif and which was efficiently degraded during mII exit. Two equimolar CaMKII gamma isoform variants were present in mII oocytes, neither of which phosphorylated Emi2 in vitro, consistent with the involvement of additional factors. No evidence was found that calcineurin is required for mouse mII exit. These data support a model in which mammalian meiotic establishment, maintenance and exit converge upon a modular Emi2 hub via evolutionarily conserved and divergent mechanisms.

Full-term mouse development by abolishing Zn2+-dependent metaphase II arrest without Ca2+ release

In vertebrates, a rise in intracellular free Ca2+ (Ca2+i) levels during fertilization initiates second metaphase (mII) exit and the developmental programme. The Ca2+ rise has long been considered to be crucial for development, but verifying this contribution would benefit from defining its role during fertilization. Here, we delineate the role of Ca2+ release during mII exit in wild-type mouse eggs and show that it is dispensable for full-term development. Exit from mII can be induced by Zn2+-specific sequestration without Ca2+ release, eliciting Cyclin B degradation in a manner dependent upon the proteasome pathway and intact microtubules, but not accompanied by degradation of the meiotic regulator Emi2. Parthenogenotes generated by Zn2+ sequestration developed in vitro with normal expression of Ca2+-sensitive genes. Meiotic exit induced by either Ca2+ oscillations or a single Ca2+ rise in oocytes containing a signaling-deficient sperm resulted in comparable developmental rates. In the absence of Ca2+ release, full-term development occurred ∼50% less efficiently, but at readily detectable rates, with the birth of 27 offspring. These results show in intact mouse oocytes that Zn2+ is essential for mII arrest and suggest that triggering meiotic exit is the sole indispensable developmental role of Ca2+ signaling in mammalian fertilization.

Paternal exposure to cyclophosphamide affects the progression of sperm chromatin decondensation and activates a DNA damage response in the prepronuclear rat zygote

Spermatozoon decondensation in the zygote leads to the initiation of chromatin remodeling during which protamines are removed and replaced with maternal histones. We hypothesize that damage to male germ cells induced by paternal exposure to cyclophosphamide may alter the timing of spermatozoal decondensation and the pattern of chromatin remodeling in the prepronuclear rat zygote. A specific order of sperm decondensation was observed, starting at the posterior end, proceeding to the ventral sides, followed by the tip, and finally the midbody region of the sperm head nucleus; subgroups of partially decondensed type a sperm nuclei were defined as types a1, a2, a3, and a4. Based on their frequencies relative to controls, paternal exposure to cyclophosphamide accelerated the timing of spermatozoal decondensation. Two distinct patterns of chromatin remodeling were observed for totally decondensed (type b) and recondensing (type c) sperm nuclei: H4K12ac showed a homogenous staining, whereas H3S10ph displayed a ring-like staining around the sperm nucleus; the distribution of these posttranslationally modified histones was not affected by cyclophosphamide exposure. In contrast, paternal cyclophosphamide treatment increased the number of gammaH2AX foci found in decondensing sperm nuclei. Small foci were significantly increased in type a2 and a3 nuclei, whereas a significant increase in the numbers of large foci was found in type b and c nuclei. This increase in gammaH2AX foci in the decondensing male genome suggests that damage recognition and repair pathways are initiated in prepronuclear rat zygotes. Thus, exposure of male rats to chronic low doses of cyclophosphamide accelerates spermatozoal decondensation and leads to the activation of gammaH2AX recognition of DNA damage in the male genome of the prepronuclear zygote.

Environmental epigenetics

Epigenetics investigates heritable changes in gene expression that occur without changes in DNA sequence. Several epigenetic mechanisms, including DNA methylation and histone modifications, can change genome function under exogenous influence. We review current evidence indicating that epigenetic alterations mediate effects caused by exposure to environmental toxicants. Results obtained from animal models indicate that in utero or early-life environmental exposures produce effects that can be inherited transgenerationally and are accompanied by epigenetic alterations. The search for human equivalents of the epigenetic mechanisms identified in animal models is under way. Recent investigations have identified a number of environmental toxicants that cause altered methylation of human repetitive elements or genes. Some exposures can alter epigenetic states and the same and/or similar epigenetic alterations can be found in patients with the disease of concern. On the basis of current evidence, we propose possible models for the interplay between environmental exposures and the human epigenome. Several investigations have examined the relationship between exposure to environmental chemicals and epigenetics, and have identified toxicants that modify epigenetic states. Whether environmental exposures have transgenerational epigenetic effects in humans remains to be elucidated. In spite of the current limitations, available evidence supports the concept that epigenetics holds substantial potential for furthering our understanding of the molecular mechanisms of environmental toxicants, as well as for predicting health-related risks due to conditions of environmental exposure and individual susceptibility.

Sperm chromatin-induced ectopic polar body extrusion in mouse eggs after ICSI and delayed egg activation

Meiotic chromosomes in an oocyte are not only a maternal genome carrier but also provide a positional signal to induce cortical polarization and define asymmetric meiotic division of the oocyte, resulting in polar body extrusion and haploidization of the maternal genome. The meiotic chromosomes play dual function in determination of meiosis: 1) organizing a bipolar spindle formation and 2) inducing cortical polarization and assembly of a distinct cortical cytoskeleton structure in the overlying cortex for polar body extrusion. At fertilization, a sperm brings exogenous paternal chromatin into the egg, which induces ectopic cortical polarization at the sperm entry site and leads to a cone formation, known as fertilization cone. Here we show that the sperm chromatin-induced fertilization cone formation is an abortive polar body extrusion due to lack of spindle induction by the sperm chromatin during fertilization. If experimentally manipulating the fertilization process to allow sperm chromatin to induce both cortical polarization and spindle formation, the fertilization cone can be converted into polar body extrusion. This suggests that sperm chromatin is also able to induce polar body extrusion, like its maternal counterpart. The usually observed cone formation instead of ectopic polar body extrusion induced by sperm chromatin during fertilization is due to special sperm chromatin compaction which restrains it from rapid spindle induction and therefore provides a protective mechanism to prevent a possible paternal genome loss during ectopic polar body extrusion.

Nuclear reprogramming: what has been done and potential avenues for improvements

A major challenge for reproductive biologists is the development of novel strategies to improve cloning efficiency. Even in species for which cloning is relatively successful, like cattle, the efficiency is still unacceptably low. In this review article we critically analyse all approaches that have been suggested by different laboratories in the field so far. As will be discussed below, so far none of these gives rise to a dramatic increase in cloning efficiency. Possibly, a multi-step approach including a pre-treatment of donor cells to modify their chromatin, along with improved culture system for cloned embryos would be the most promising.

Second meiotic arrest and exit in frogs and mice

Mature vertebrate oocytes typically undergo programmed arrest at the second meiotic cell cycle until they are signalled to initiate embryonic development at fertilization. Here, we describe the underlying molecular mechanisms of this second meiotic arrest and release in Xenopus, and compare and contrast them with their counterparts in mice.

PRC1 and Suv39h specify parental asymmetry at constitutive heterochromatin in early mouse embryos

In eukaryotes, Suv39h H3K9 trimethyltransferases are required for pericentric heterochromatin formation and function. In early mouse preimplantation embryos, however, paternal pericentric heterochromatin lacks Suv39h-mediated H3K9me3 and downstream marks. Here we demonstrate Ezh2-independent targeting of maternally provided polycomb repressive complex 1 (PRC1) components to paternal heterochromatin. In Suv39h2 maternally deficient zygotes, PRC1 also associates with maternal heterochromatin lacking H3K9me3, thereby revealing hierarchy between repressive pathways. In Rnf2 maternally deficient zygotes, the PRC1 complex is disrupted, and levels of pericentric major satellite transcripts are increased at the paternal but not the maternal genome. We conclude that in early embryos, Suv39h-mediated H3K9me3 constitutes the dominant maternal transgenerational signal for pericentric heterochromatin formation. In absence of this signal, PRC1 functions as the default repressive back-up mechanism. Parental epigenetic asymmetry, also observed along cleavage chromosomes, is resolved by the end of the 8-cell stage--concurrent with blastomere polarization--marking the end of the maternal-to-embryonic transition.

Nuclear histone deacetylases are not required for global histone deacetylation during meiotic maturation in porcine oocytes

Histone acetylation plays an important role in the regulation of chromatin structure and gene function. In mammalian oocytes, histones H3 and H4 are highly acetylated during the germinal vesicle (GV) stage, and global histone deacetylation takes place via a histone deacetylase (HDAC)-dependent mechanism after GV breakdown (GVBD). The presence of HDACs in the GVs of mammalian oocytes in spite of the high acetylation states of nuclear histones indicates that the HDACs in the nucleus are inactive but become activated after GVBD. However, the fluctuation pattern, the localization of HDAC activity during meiotic maturation and, moreover, the responsibility of nuclear HDACs for global histone deacetylation are still unknown. Here, we demonstrated using porcine oocytes that total HDAC activity was maintained throughout meiotic maturation, and high HDAC activity was observed in both the nucleus and the cytoplasm at the GV stage. The experiments with valproic acid (VPA), a specific class I HDAC inhibitor, revealed that the HDACs in GVs were class I, and those in the cytoplasm were other than class I. Interestingly, VPA had no effect on global histone deacetylation after GVBD, indicating that nuclear HDACs were not required for global histone deacetylation. To confirm this possibility, we removed the nuclei from immature oocytes, injected somatic cell nuclei into the enucleated oocytes, and showed that injected somatic cell nuclei were dramatically deacetylated after nuclear envelope breakdown. These results revealed that nuclear contents, including class I HDACs, are not required for the global histone deacetylation during meiosis, and that cytoplasmic HDACs other than class I are responsible for this process.

Dynamic reprogramming of histone acetylation and methylation in the first cell cycle of cloned mouse embryos

Epigenetic reprogramming is thought to play an important role in the development of cloned embryos reconstructed by somatic cell nuclear transfer (SCNT). In the present study, dynamic reprogramming of histone acetylation and methylation modifications was investigated in the first cell cycle of cloned embryos. Our results demonstrated that part of somatic inherited lysine acetylation on core histones (H3K9, H3K14, H4K16) could be quickly deacetylated following SCNT, and reacetylation occurred following activation treatment. However, acetylation marks of the other lysine residues on core histones (H4K8, H4K12) persisted in the genome of cloned embryos with only mild deacetylation occurring in the process of SCNT and activation treatment. The somatic cloned embryos established histone acetylation modifications resembling those in normal embryos produced by intracytoplasmic sperm injection through these two different programs. Moreover, treatment of cloned embryos with a histone deacetylase inhibitor, Trichostatin A (TSA), improved the histone acetylation in a manner similar to that in normal embryos, and the improved histone acetylation in cloned embryos treated with TSA might contribute to improved development of TSA-treated clones. In contrast to the asymmetric histone H3K9 tri- and dimethylation present in the parental genomes of fertilized embryos, the tri- and dimethylations of H3K9 were gradually demethylated in the cloned embryos, and this histone H3K9 demethylation may be crucial for gene activation of cloned embryos. Together, our results indicate that dynamic reprogramming of histone acetylation and methylation modifications in cloned embryos is developmentally regulated.

Piezo-actuated mouse intracytoplasmic sperm injection (ICSI)

The mouse is a genetically tractable model organism widely used to study mammalian development and disease. However, mouse metaphase II (mII) oocytes are exquisitely sensitive and intracytoplasmic sperm injection (ICSI) with conventional pipettes generally kills them. This problem can be solved with piezo-actuated micromanipulation, in which the piezo-electric effect (crystal deformation in response to an externally applied voltage) propels a microinjection needle tip forward in a precise and rapid movement. Piezo-actuated micromanipulation enhances the penetration of membranes and matrices, and mouse ICSI is a major application. Here we describe a comprehensive, step-by-step mouse piezo ICSI protocol for non-specialists that can be completed in 2-4 h. The protocol is a basic prelude to multiple applications, including nuclear transfer cloning, spermatid injection, blastocyst injection, mII transgenesis, and streamlining micromanipulation in primates and livestock. Moreover, piezo ICSI can be used to obtain offspring from 'dead' (non-motile) sperm, enabling trivial sperm freezing protocols for mouse strain storage and shipment.