H1FOO is coupled to the initiation of oocytic growth

Effect of thyroid disruption on ovarian development following maternal exposure to Bisphenol S

Thyroid hormones play a crucial role in numerous physiological processes, including reproduction. Bisphenol S (BPS) is a structural analog of Bisphenol A known for its toxic effects. Interference of this substitute with normal thyroid function has been described. To investigate the effect of thyroid disruption on ovarian development following maternal exposure to BPS, female rats were exposed, daily, to either AT 1-850 (a thyroid hormone receptor antagonist) (10 nmol/rat) or BPS (0.2 mg/kg) during gestation and lactation. The effects on reproductive outcome, offspring development, histological structures, hormone levels, oxidative status, cytoskeleton proteins expression, and oocyte development gene expression were examined. Our results are in favor of offspring ovarian development disruption due to thyroid disturbance in adult pregnant females. During both fetal and postnatal stages, BPS considerably altered the histological structure of the thyroid tissue as well as oocyte and follicular development, which led to premature ovarian failure and stimulation of oocyte atresia, being accompanied with oxidative stress, hypothalamic-pituitary-ovarian axis disorders, and cytoskeletal dynamic disturbance. Crucially, our study underscores that BPS may induce reproductive toxicity by blocking nuclear thyroid hormone receptors, evidenced by the parallelism and the perfect meshing between the data obtained following exposure to AT 1-850 and those after the treatment by this substitute.

Gene expression and demographic analyses in women with the poor ovarian response: a computational approach

PURPOSE

Poor response to ovarian stimulation (POR) typically is reflected as decreased follicular response and low estradiol (E2) levels following ovarian stimulation by FSH/HMG. Many genes are involved in oocyte maturation, and demographic features and lifestyle can affect the oocyte maturity and developmental competence. The present study was conducted to investigate the magnitude of gene expression and lifestyle habits in POR women as compared to healthy women, using different statistical and computational methods.

METHODS

Fifty women in the two groups were studied. The study groups included POR women (n = 25) with 1-9 released oocytes, and the control group (normal women, n = 25) with 9-15 released oocytes. Quantitative PCR was used to estimate the expression of FIGLA, ZAR1, WNT4, LHX8, APC, H1FOO, MOS, and DMC1 genes in granulosa cells.

RESULTS

The results showed no significant difference in the magnitude of the studied genes' expression and linear discriminant analysis did not differentiate the studied groups based on all the genes together. Redundancy analysis (RDA) and latent factor mixed model (LFMM) results produce no significant association between the genes' expression magnitude and the geographical variables of the patients' local habitat. Linear discriminant analysis (LDA) of the demographic features differentiated the two groups of women.

CONCLUSION

Our results indicate that demographic features may have an effect on sample gene expression levels.

A Role of PI3K/Akt Signaling in Oocyte Maturation and Early Embryo Development

A serine/threonine-specific protein kinase B (PKB), also known as Akt, is a key factor in the phosphoinositide 3-kinase (PI3K)/Akt signaling pathway that regulates cell survival, metabolism and proliferation. Akt phosphorylates many downstream specific substrates, which subsequently control the nuclear envelope breakdown (NEBD), centrosome maturation, spindle assembly, chromosome segregation, and cytokinesis. In vertebrates, Akt is also an important player during oogenesis and preimplantation development. In the signaling pathways regulating mRNA translation, Akt is involved in the control of mammalian target of rapamycin complex 1 (mTORC1) and thereby regulates the activity of a translational repressor, the eukaryotic initiation factor 4E (eIF4E) binding protein 1 (4E-BP1). In this review, we summarize the functions of Akt in mitosis, meiosis and early embryonic development. Additionally, the role of Akt in the regulation of mRNA translation is addressed with respect to the significance of this process during early development.

The Oocyte-Specific Linker Histone H1FOO Is Not Essential for Mouse Oogenesis and Fertility

Meiosis is a highly conserved specialized cell division process that generates haploid gametes. Many of its events are associated with dynamically regulated chromosomal structures and chromatin remodeling, which are mainly modulated by histone modifications. Histone H1 is a linker histone essential for packing the nucleosome into higher-order structures, and H1FOO (H1 histone family, member O, oocyte-specific) is a H1 variant whose expression pattern is restricted to growing oocytes and zygotes. To further explore the function of H1FOO, we generated mice lacking the H1foo gene by the CRISPR/Cas9 technique. Herein, we combine mouse genetics and cellular studies to show that H1foo-null mutants have no overt phenotype, with both males and females being fertile and presenting no gross defects in meiosis progression nor in synapsis dynamics. Accordingly, the histological sections show a normal development of gametes in both male and female mice. Considering the important role of oocyte constituents in enhancing mammalian somatic cell reprogramming, we analyzed iPSCs generation in H1foo mutant MEFs and observed no differences in the absence of H1FOO. Taken all together, in this work we present the first in vivo evidence of H1FOO dispensability for mouse fertility, clarifying the debate in the field surrounding its essentiality in meiosis.

Fyn and argonaute 2 participate in maternal-mRNA degradation during mouse oocyte maturation

Fertilization triggers physiological degradation of maternal-mRNAs, which are then replaced by embryonic transcripts. Ample evidence suggests that Argonaut 2 (AGO2) is a possible post-fertilization regulator of maternal-mRNAs degradation; but its role in degradation of maternal-mRNAs during oocyte maturation remains obscure. Fyn, a member of the Src family kinases (SFKs), and an essential factor in oocyte maturation, was reported to inhibit AGO2 activity in oligodendrocytes. Our aim was to examine the role of Fyn and AGO2 in degradation of maternal-mRNAs during oocyte maturation by either suppressing their activity with SU6656 - an SFKs inhibitor; or by microinjecting DN-Fyn RNA for suppression of Fyn and BCl-137 for suppression of AGO2. Batches of fifteen mouse oocytes or embryos were analyzed by qPCR to measure the expression level of nine maternal-mRNAs that were selected for their known role in oocyte growth, maturation and early embryogenesis. We found that Fyn/SFKs are involved in maintaining the stability of at least four pre-transcribed mRNAs in oocytes at the germinal vesicle (GV) stage, whereas AGO2 had no role at this stage. During in-vivo oocyte maturation, eight maternal-mRNAs were significantly degraded. Inhibition of AGO2 prevented the degreadation of at least five maternal-mRNAs, whereas inhibition of Fyn/SFK prevented degradation of at least five Fyn maternal-mRNAs and two SFKs maternal-mRNAs; pointing at their role in promoting the physiological degradation which occurs during in-vivo oocyte maturation. Our findings imply the involvement of Fyn/SFKs in stabilization of maternal-mRNA at the GV stage and the involvement of Fyn, SFKs and AGO2 in degradation of maternal mRNAs during oocyte maturation.

The tumour suppressor brain tumour (Brat) regulates linker histone dBigH1 expression in the Drosophila female germline and the early embryo

Linker histones H1 are essential chromatin components that exist as multiple developmentally regulated variants. In metazoans, specific H1s are expressed during germline development in a tightly regulated manner. However, the mechanisms governing their stage-dependent expression are poorly understood. Here, we address this question in Drosophila, which encodes for a single germline-specific dBigH1 linker histone. We show that during female germline lineage differentiation, dBigH1 is expressed in germ stem cells and cystoblasts, becomes silenced during transit-amplifying (TA) cystocytes divisions to resume expression after proliferation stops and differentiation starts, when it progressively accumulates in the oocyte. We find that dBigH1 silencing during TA divisions is post-transcriptional and depends on the tumour suppressor Brain tumour (Brat), an essential RNA-binding protein that regulates mRNA translation and stability. Like other oocyte-specific variants, dBigH1 is maternally expressed during early embryogenesis until it is replaced by somatic dH1 at the maternal-to-zygotic transition (MZT). Brat also mediates dBigH1 silencing at MZT. Finally, we discuss the situation in testes, where Brat is not expressed, but dBigH1 is translationally silenced too.

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.

Oocyte Vitrification Temporarily Turns on Oxidation-Reduction Process Genes in Mouse Preimplantation Embryos

We aim to understand how oocyte vitrification impacts subsequent mouse preimplantation embryo development at molecular level. We profiled transcriptomics of fertilized preimplantation embryos derived from mouse vitrified-warmed oocytes. Concomitantly, we evaluated epigenetic markers in fertilized preimplantation embryos. We found that oocyte vitrification did not affect the fertilization and cleavage process but delayed embryo development until blastocyst stage. RNA sequencing revealed that 1575 genes were profoundly altered in the 2-cell stage embryos developed from vitrified oocytes. The most significantly altered biological pathway was "oxidation-reduction process." Such profound transcriptomics change was associated with decreased level of oocyte-specific histone H1FOO in zygote and 2-cell stage. Transcriptome alteration due to oocyte vitrification was less pronounced as embryos develop into the morula stage. Oocyte vitrification temporarily changes transcriptomics in early preimplantation embryos. Targeting oxidation-reduction pathway might be a potential therapeutic strategy to improve embryo quality and long-term embryo survival.

In vitro culture of secondary follicles and prematuration of cumulus-oocyte complexes from antral follicles increase the levels of maturation-related transcripts in bovine oocytes

This study evaluates the levels of messenger RNA (mRNA) for eIF4E, PARN, H1FOO, cMOS, GDF9, and CCNB1 in oocytes from secondary and antral follicles at different stages of development. The effects of in vitro culture, in vitro prematuration, and in vitro maturation on the expression of these genes on oocytes were also analyzed. The results showed that mRNA levels for H1FOO, GDF9, and PARN were higher in oocytes from small, medium, and large antral follicles, respectively, than those seen in secondary follicles. Oocytes from small, medium, and large antral follicles had higher levels of CCNB1 than oocytes from secondary follicles. Oocytes from cultured secondary follicles had higher levels of GDF9, CMOS, PARN, eIF4E, CCNB1, and H1FOO than before culture. Prematured oocytes from small antral follicles had higher levels of mRNA for GDF9, PARN, and eIF4E than before culture. In addition, higher levels of cMOS and H1FOO were identified in prematured oocytes from medium antral follicles. In conclusion, follicular growth is associated with an increase in the expression of H1FOO, GDF9, CCNB1, and PARN. The culture of secondary follicles, prematuration, and maturation of oocytes from antral follicles increase the expression of eIF4E, PARN, H1FOO, cMOS, GDF9, and CCNB1.

Epigenomic drugs and the germline: Collateral damage in the home of heritability?

The testis and ovary provide specialised environments that nurture germ cells and facilitate their maturation, culminating in the production of mature gametes that can found the following generation. The sperm and egg not only transmit genetic information, but also epigenetic modifications that affect the development and physiology of offspring. Importantly, the epigenetic information contained in mature sperm and oocytes can be influenced by a range of environmental factors, such as diet, chemicals and drugs. An increasing range of studies are revealing how gene-environment interactions are mediated through the germline. Outside the germline, altered epigenetic state is common in a range of diseases, including many cancers. As epigenetic modifications are reversible, pharmaceuticals that directly target epigenetic modifying proteins have been developed and are delivering substantial benefits to patients, particularly in oncology. While providing the most effective patient treatment is clearly the primary concern, some patients will want to conceive children after treatment. However, the impacts of epigenomic drugs on the male and female gametes are poorly understood and whether these drugs will have lasting effects on patients' germline epigenome and subsequent offspring remains largely undetermined. Currently, evidence based clinical guidelines for use of epigenomic drugs in patients of reproductive age are limited in this context. Developing a deeper understanding of the epigenetic mechanisms regulating the germline epigenome and its impact on inherited traits and disease susceptibility is required to determine how specific epigenomic drugs might affect the germline and inheritance. Understanding these potential effects will facilitate the development of informed clinical guidelines appropriate for the use of epigenomic drugs in patients of reproductive age, ultimately improving the safety of these therapies in the clinic.

Importance of ERK1/2 in Regulation of Protein Translation during Oocyte Meiosis

Although the involvement of the extracellular signal-regulated kinases 1 and 2 (ERK1/2) pathway in the regulation of cytostatic factor (CSF) activity; as well as in microtubules organization during meiotic maturation of oocytes; has already been described in detail; rather less attention has been paid to the role of ERK1/2 in the regulation of mRNA translation. However; important data on the role of ERK1/2 in translation during oocyte meiosis have been documented. This review focuses on recent findings regarding the regulation of translation and the role of ERK1/2 in this process in the meiotic cycle of mammalian oocytes. The specific role of ERK1/2 in the regulation of mammalian target of rapamycin (mTOR); eukaryotic translation initiation factor 4E (eIF4E) and cytoplasmic polyadenylation element binding protein 1 (CPEB1) activity is addressed along with additional focus on the other key players involved in protein translation.

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.

One-carbon metabolism and epigenetic regulation of embryo development

One-carbon (1C) metabolism consists of an integrated series of metabolic pathways that include the folate cycle and methionine remethylation and trans-sulfuration pathways. Most, but not all, 1C metabolic enzymes are expressed in somatic cells of the ovary, mammalian oocytes and in preimplantation embryos. The metabolic implications of this, with regard to the provision of methyl donors (e.g. betaine) and 1C cofactors (e.g. vitamin B12), together with consequences of polymorphic variances in genes encoding 1C enzymes, are not fully understood but are the subject of ongoing investigations at the authors' laboratory. However, deficiencies in 1C-related substrates and/or cofactors during the periconception period are known to lead to epigenetic alterations in DNA and histone methylation in genes that regulate key developmental processes in the embryo. Such epigenetic modifications have been demonstrated to negatively impact on the subsequent health and metabolism of offspring. For this reason, parental nutrition around the time of conception has become a focal point of investigation in many laboratories with the aim of providing improved nutritional advice to couples. These issues are considered in detail in this article, which offers a contemporary overview of the effects of 1C metabolism on epigenetic programming in mammalian gametes and the early embryo.

MS_HistoneDB, a manually curated resource for proteomic analysis of human and mouse histones

BACKGROUND

Histones and histone variants are essential components of the nuclear chromatin. While mass spectrometry has opened a large window to their characterization and functional studies, their identification from proteomic data remains challenging. Indeed, the current interpretation of mass spectrometry data relies on public databases which are either not exhaustive (Swiss-Prot) or contain many redundant entries (UniProtKB or NCBI). Currently, no protein database is ideally suited for the analysis of histones and the complex array of mammalian histone variants.

RESULTS

We propose two proteomics-oriented manually curated databases for mouse and human histone variants. We manually curated >1700 gene, transcript and protein entries to produce a non-redundant list of 83 mouse and 85 human histones. These entries were annotated in accordance with the current nomenclature and unified with the "HistoneDB2.0 with Variants" database. This resource is provided in a format that can be directly read by programs used for mass spectrometry data interpretation. In addition, it was used to interpret mass spectrometry data acquired on histones extracted from mouse testis. Several histone variants, which had so far only been inferred by homology or detected at the RNA level, were detected by mass spectrometry, confirming the existence of their protein form.

CONCLUSIONS

Mouse and human histone entries were collected from different databases and subsequently curated to produce a non-redundant protein-centric resource, MS_HistoneDB. It is dedicated to the proteomic study of histones in mouse and human and will hopefully facilitate the identification and functional study of histone variants.

Modulation of chromatin function through linker histone H1 variants

In this review, the structural aspects of linker H1 histones are presented as a background for characterization of the factors influencing their function in animal and human chromatin. The action of H1 histone variants is largely determined by dynamic alterations of their intrinsically disordered tail domains, posttranslational modifications and allelic diversification. The interdependent effects of these factors can establish dynamic histone H1 states that may affect the organization and function of chromatin regions.

H1foo Has a Pivotal Role in Qualifying Induced Pluripotent Stem Cells

Embryonic stem cells (ESCs) are a hallmark of ideal pluripotent stem cells. Epigenetic reprogramming of induced pluripotent stem cells (iPSCs) has not been fully accomplished. iPSC generation is similar to somatic cell nuclear transfer (SCNT) in oocytes, and this procedure can be used to generate ESCs (SCNT-ESCs), which suggests the contribution of oocyte-specific constituents. Here, we show that the mammalian oocyte-specific linker histone H1foo has beneficial effects on iPSC generation. Induction of H1foo with Oct4, Sox2, and Klf4 significantly enhanced the efficiency of iPSC generation. H1foo promoted in vitro differentiation characteristics with low heterogeneity in iPSCs. H1foo enhanced the generation of germline-competent chimeric mice from iPSCs in a manner similar to that for ESCs. These findings indicate that H1foo contributes to the generation of higher-quality iPSCs.

•

H1foo enhanced the efficiency of iPSC generation

•

H1foo promoted in vitro differentiation characteristics with low heterogeneity

•

H1foo enhanced the generation of germline-competent chimeric mice

Interphase H1 phosphorylation: Regulation and functions in chromatin

Many metazoan cell types differentially express multiple non-allelic amino acid sequence variants of histone H1. Although early work revealed that H1 variants, collectively, are phosphorylated during interphase and mitosis, differences between individual H1 variants in the sites they possess for mitotic and interphase phosphorylation have been elucidated only relatively recently. Here, we review current knowledge on the regulation and function of interphase H1 phosphorylation, with a particular emphasis on how differences in interphase phosphorylation among the H1 variants of mammalian cells may enable them to have differential effects on transcription and other chromatin processes.

What is the role of histone H1 heterogeneity? A functional model emerges from a 50 year mystery

For the past 50 years, understanding the function of histone H1 heterogeneity has been mired in confusion and contradiction. Part of the reason for this is the lack of a working model that tries to explain the large body of data that has been collected about the H1 subtypes so far. In this review, a global model is described largely based on published data from the author and other researchers over the past 20 years. The intrinsic disorder built into H1 protein structure is discussed to help the reader understand that these histones are multi-conformational and adaptable to interactions with different targets. We discuss the role of each structural section of H1 (as we currently understand it), but we focus on the H1's C-terminal domain and its effect on each subtype's affinity, mobility and compaction of chromatin. We review the multiple ways these characteristics have been measured from circular dichroism to FRAP analysis, which has added to the sometimes contradictory assumptions made about each subtype. Based on a tabulation of these measurements, we then organize the H1 variants according to their ability to condense chromatin and produce nucleosome repeat lengths amenable to that compaction. This subtype variation generates a continuum of different chromatin states allowing for fine regulatory control and some overlap in the event one or two subtypes are lost to mutation. We also review the myriad of disparate observations made about each subtype, both somatic and germline specific ones, that lend support to the proposed model. Finally, to demonstrate its adaptability as new data further refines our understanding of H1 subtypes, we show how the model can be applied to experimental observations of telomeric heterochromatin in aging cells.

Refurbishing the germline epigenome: Out with the old, in with the new

Mammalian germline reprogramming involves the erasure and re-establishment of epigenetic information critical for germ cell function and inheritance in offspring. The bi-faceted nature of such reprogramming ensures germline repression of somatic programmes and the establishment of a carefully constructed epigenome essential for fertilisation and embryonic development in the next generation. While the majority of the germline epigenome is erased in preparation for embryonic development, certain genomic sequences remain resistant to this and may represent routes for transmission of epigenetic changes through the germline. Epigenetic reprogramming is regulated by highly conserved epigenetic modifiers, which function to establish, maintain and remove DNA methylation and chromatin modifications. In this review, we discuss recent findings from a considerable body of work illustrating the critical requirement of epigenetic modifiers that influence the epigenetic signature present in mature gametes, and have the potential to affect developmental outcomes in the offspring. We also briefly discuss the similarities of these mechanisms in the human germline and consider the potential for inheritance of epigenetically induced germline genetic errors that could impact on offspring phenotypes.

H1-nucleosome interactions and their functional implications

Linker histones are three domain proteins and consist of a structured (globular) domain, flanked by two likely non-structured NH2- and COOH-termini. The binding of the linker histones to the nucleosome was characterized by different methods in solution. Apparently, the globular domain interacts with the linker DNA and the nucleosome dyad, while the binding of the large and rich in lysines COOH-terminus results in "closing" the linker DNA of the nucleosome and the formation of the "stem" structure. What is the mode of binding of the linker histones within the chromatin fiber remains still elusive. Nonetheless, it is clear that linker histones are essential for both the assembly and maintenance of the condensed chromatin fiber. Interestingly, linker histones are post-translationally modified and how this affects both their binding to chromatin and functions is now beginning to emerge. In addition, linker histones are highly mobile in vivo, but not in vitro. No explanation of this finding is reported for the moment. The higher mobility of the linker histones should, however, have strong impact on their function. Linker histones plays an important role in gene expression regulation and other chromatin related process and their function is predominantly regulated by their posttranslational modifications. However, the detailed mechanism how the linker histones do function remains still not well understood despite numerous efforts. Here we will summarize and analyze the data on the linker histone binding to the nucleosome and the chromatin fiber and will discuss its functional consequences.

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.

Oocyte-specific H2A variant H2af1o is required for cell synchrony before midblastula transition in early zebrafish embryos

Oocyte-specific histone variants have been expected to play significant roles in early embryonic development, but the exact evidence and the biological function have remained unclear. Here, we present evidence that H2af1o, an oocyte-specific H2A variant, is required for cell synchrony before midblastula transition in early zebrafish embryos. The H2A variant is oocyte specific, peaks in mature eggs, and is supplied to early embryos. We constructed a series of deletion plasmids of the zebrafish h2af1o tagged with EGFP and determined the main key function regions including nuclear localization signal of N-terminal 25 amino acids and nucleosome binding region of 110-122 amino acid sequence in the C-terminus by microinjecting them into one-cell-stage zebrafish embryos. In comparison with ubiquitous H2A.X, the H2af1o was revealed to confer a more open structure than canonical H2A in the nucleosomes. Furthermore, we conducted the h2af1o-specific morpholino knockdown analysis in early embryos of zebrafish and revealed its biological function for maintaining cell synchrony division because the H2af1o deficiency disturbed cell synchrony in early cleavages before midblastula transition. Therefore, our current findings provided the first case to understand the biological function of maternal oocyte-specific histone variants in vertebrates.

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.

DMRT1 promotes oogenesis by transcriptional activation of Stra8 in the mammalian fetal ovary

Dmrt1 belongs to the DM domain gene family of conserved sexual regulators. In the mouse Dmrt1 is expressed in the genital ridge (the gonadal primordium) in both sexes and then becomes testis-specific shortly after sex determination. The essential role of DMRT1 in testicular differentiation is well established, and includes transcriptional repression of the meiotic inducer Stra8. However Dmrt1 mutant females are fertile and the role of Dmrt1 in the ovary has not been studied. Here we show in the mouse that most Dmrt1 mutant germ cells in the fetal ovary have greatly reduced expression of STRA8, and fail to properly localize SYCP3 and γH2AX during meiotic prophase. Lack of DMRT1 in the fetal ovary results in the formation of many fewer primordial follicles in the juvenile ovary, although these are sufficient for fertility. Genome-wide chromatin immunoprecipitiation (ChIP-chip) and quantitative ChIP (qChIP) combined with mRNA expression profiling suggests that transcriptional activation of Stra8 in fetal germ cells is the main function of DMRT1 in females, and that this regulation likely is direct. Thus DMRT1 controls Stra8 sex-specifically, activating it in the fetal ovary and repressing it in the adult testis.

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.

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

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

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

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

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.

Oocyte-specific genes: role in fertility and infertility

Genetic determinations of oocyte and ovarian follicle growth are still not well understood. Genes specifically expressed on oocytes seem to play an important role in these processes. Oocyte-specific genes are also involved in ovulation and early embryogenesis processes. Studies on the identification and characterization of new oocyte-specific genes can help in our understanding of cardinal fertility and infertility mechanisms. They can also be candidate genes for reproductive disorders such as polycystic ovary syndrome, premature ovarian failure and infertility. Infertility is an important worldwide problem affecting around 15% of couples. Approximately 20% of infertility is referred as idiopathic infertility. Studies on these genes could improve the diagnostic and therapeutic procedures of human infertility.

Molecular dynamics of histone H1This paper is one of a selection of papers published in this Special Issue, entitled CSBMCB’s 51st Annual Meeting – Epigenetics and Chromatin Dynamics, and has undergone the Journal’s usual peer review process

The histone H1 family of nucleoproteins represents an important class of structural and architectural proteins that are responsible for maintaining and stabilizing higher-order chromatin structure. Essential for mammalian cell viability, they are responsible for gene-specific regulation of transcription and other DNA-dependent processes. In this review, we focus on the wealth of information gathered on the molecular kinetics of histone H1 molecules using novel imaging techniques, such as fluorescence recovery after photobleaching. These experiments have shed light on the effects of H1 phosphorylation and core histone acetylation in influencing chromatin structure and dynamics. We also delineate important concepts surrounding the C-terminal domain of H1, such as the intrinsic disorder hypothesis, and how it affects H1 function. Finally, we address the biochemical mechanisms behind low-affinity H1 binding.

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.

DNA hypomethylation circuit of the mouse oocyte-specific histone H1foo gene in female germ cell lineage

The oocyte-specific subtype of the linker histone H1 is H1FOO, which constitutes a major part of oocyte chromatin. H1foo is expressed in growing oocytes, through fertilization, up until the two-cell embryo stage, when it is subsequently replaced by somatic H1 subtypes. To elucidate whether an epigenetic mechanism is involved in the limited expression of H1foo, we analyzed the dynamics of the DNA methylation status of the H1foo locus in germ and somatic cells. We identified a tissue-dependent and differentially methylated region (T-DMR) upstream of the H1foo gene, which was hypermethylated in sperm, somatic cells, and stem cell lines. This region was specifically unmethylated in the ovulated oocyte, where H1foo is expressed. 5-Aza-2'-deoxycytidine treatments and luciferase assays provided in vitro evidence that DNA methylation plays a role in repressing H1foo in nonexpressing cells. DNA methylation analyses of fetal germ cells revealed the T-DMR to be hypomethylated in female and male germ cells at Embryonic Day 9.5 (E9.5), whereas it was highly methylated in somatic cells at this stage. Intriguingly, the unmethylated status was continuously observed throughout oogenesis at E9.5, E12.5, E15.5, E18.5, in mature oocytes, and after fertilization, in E3.5 blastocysts. In comparison, male germ cells acquired methylation beyond E18.5. These data demonstrate a continuously unmethylated circuit at the H1foo locus in the female germline.

G1 phase-dependent nucleolar accumulation of human histone H1x

BACKGROUND INFORMATION

H1 histones are a protein family comprising several subtypes. Although specific functions of the individual subtypes could not be determined so far, differential roles are indicated by varied nuclear distributions as well as differential expression patterns of the H1 subtypes. Although the group of replication-dependent H1 subtypes is synthesized during S phase, the replacement H1 subtype, H1 degrees , is also expressed in a replication-independent manner in non-proliferating cells. Recently we showed, by protein biochemical analysis, that the ubiquitously expressed subtype H1x is enriched in the micrococcal nuclease-resistant part of chromatin and that, although it shares common features with H1 degrees , its expression is differentially regulated, since, in contrast to H1 degrees , growth arrest or induction of differentiation did not induce an accumulation of H1x.

RESULTS

In the present study, we show that H1x exhibits a cell-cycle-dependent change of its nuclear distribution. This H1 subtype showed a nucleolar accumulation during the G(1) phase, and it was evenly distributed in the nucleus during S phase and G(2). Immunocytochemical analysis of the intranucleolar distribution of H1x indicated that it is located mainly in the condensed nucleolar chromatin. In addition, we demonstrate that the amount of H1x protein remained nearly unchanged during S phase progression, which is in contrast to the replication-dependent subtypes.

CONCLUSION

These results suggest that the differential localization of H1x provides a mechanism for a control of H1x activity by means of shuttling between nuclear subcompartments instead of a controlled turnover of the protein.

The role of transcription in EGF- and FSH-mediated oocyte maturation in vitro

Understanding mechanisms responsible for meiotic resumption in mammalian oocytes is critical for the identification of strategies to enhance developmental competence of in vitro-matured oocytes. Improvement of in vitro oocyte maturation systems is dependent on a better understanding of mechanisms that regulate oocyte maturation both in vivo and in vitro as well as on the identification of methods to manipulate the meiotic progression of oocytes matured in vitro in a physiological manner. The purpose of this review is two-fold: first, to examine the mechanisms that underlie the acquisition of oocyte developmental competence and regulation of oocyte maturation in vivo and in vitro; second, to present data examining the role of transcription in mediating the ability of EGF and FSH to induce oocyte maturation in vitro. Results presented support the conclusions that (1) EGF-induced oocyte maturation does not require nascent gene transcription in both mice and domestic cats; (2) FSH requires gene transcription to induce oocyte maturation in both species; (3) EGF must be present in the maturation medium to optimize the effectiveness of FSH to promote oocyte maturation; (4) the mechanism used by FSH to induce oocyte maturation in vitro appears to predominate over that used by EGF when both EGF and FSH are present in maturation medium used for either murine or feline cumulus oocyte complexes.

CPEB controls oocyte growth and follicle development in the mouse

CPEB is a sequence-specific RNA-binding protein that regulates polyadenylation-induced translation. In Cpeb knockout mice, meiotic progression is disrupted at pachytene due to inhibited translation of synaptonemal complex protein mRNAs. To assess the function of CPEB after pachytene, we used the zona pellucida 3 (Zp3) promoter to generate transgenic mice expressing siRNA that induce the destruction of Cpeb mRNA. Oocytes from these animals do not develop normally; they undergo parthenogenetic cell division in the ovary, exhibit abnormal polar bodies, are detached from the cumulus granulosa cell layer, and display spindle and nuclear anomalies. In addition, many follicles contain apoptotic granulosa cells. CPEB binds several oocyte mRNAs, including Smad1, Smad5, spindlin, Bub1b, Mos, H1foo, Obox1, Dnmt1o, TiParp, Trim61 and Gdf9, a well described oocyte-expressed growth factor that is necessary for follicle development. In Cpeb knockdown oocytes, Gdf9 RNA has a shortened poly(A) tail and reduced expression. These data indicate that CPEB controls the expression of Gdf9 mRNA, which in turn is necessary for oocyte-follicle development. Finally, several phenotypes, i.e. progressive oocyte loss and infertility, elicited by the knockdown of CPEB in oocytes resemble those of the human premature ovarian failure syndrome.

Selective degradation of transcripts during meiotic maturation of mouse oocytes

There is massive destruction of transcripts during the maturation of mouse oocytes. The objective of this project was to identify and characterize the transcripts that are degraded versus those that are stable during the transcriptionally silent germinal vesicle (GV)-stage to metaphase II (MII)-stage transition using a microarray approach. A system for oocyte transcript amplification using both internal and 3'-poly(A) priming was utilized to minimize the impact of complex variations in transcript polyadenylation prevalent during this transition. Transcripts were identified and quantified using the Affymetrix Mouse Genome 430 v2.0 GeneChip. The significantly changed and stable transcripts were analyzed using Ingenuity Pathways Analysis and GenMAPP/MAPPFinder to characterize the biological themes underlying global changes in oocyte transcripts during maturation. It was concluded that the destruction of transcripts during the GV to MII transition is a selective rather than promiscuous process in mouse oocytes. In general, transcripts involved in processes that are associated with meiotic arrest at the GV-stage and the progression of oocyte maturation, such as oxidative phosphorylation, energy production, and protein synthesis and metabolism, were dramatically degraded. In contrast, transcripts encoding participants in signaling pathways essential for maintaining the unique characteristics of the MII-arrested oocyte, such as those involved in protein kinase pathways, were the most prominent among the stable transcripts.

Developmental competence of human in vitro aged oocytes as host cells for nuclear transfer

BACKGROUND

Improving human nuclear transfer (NT) efficiencies is paramount for the development of patient-specific stem cell lines, although the opportunities remain limited owing to difficulties in obtaining fresh mature oocytes.

METHODS

Therefore, the developmental competence of aged, failed-to-fertilize human oocytes as an alternate cytoplasmic source for NT was assessed and compared with use of fresh, ovulation-induced oocytes. To further characterize the developmental potential of aged oocytes, parthenogenetic activation, immunocytochemical analysis of essential microtubule proteins involved in meiotic and mitotic division, and RT-PCR in single oocytes (n = 6) was performed to determine expression of oocyte-specific genes [oocyte-specific histone 1 (H1FOO), growth differentiation factor 9 (GDF9), bone morphogenetic protein 15 (BMP15), zygote arrest 1 (ZAR1)] and microtubule markers [nuclear mitotic arrest (NuMA), minus-end directed motor protein HSET and the microtubule kinesin motor protein EG5].

RESULTS

For NT, enucleation and fusion rates of aged oocytes were significantly lower compared with fresh oocytes (P < 0.05). Cleavage rates and subsequent development were poor. In addition, parthenote cleavage was low. Immunocytochemical analysis revealed that many oocytes displayed aberrant expression of NuMA and EG5, had disrupted meiotic spindles and tetrapolar spindles. One of the six oocytes misexpressed GDF9, BMP15 and ZAR1. Two oocytes expressed EG5 messenger RNA (mRNA), and HSET and NuMA were not detectable. RT-PCR of mRNA for oocyte specific genes and microtubule markers in single aged oocytes.

CONCLUSIONS

Thus, aneuploidy and spindle defects may contribute to poor parthenogenetic development and developmental outcomes following NT.

Chromatin modifications in the germinal vesicle (GV) of mammalian oocytes

The nucleus of eukaryotic cells is organized into functionally specialized compartments that are essential for the control of gene expression, chromosome architecture and cellular differentiation. The mouse oocyte nucleus or germinal vesicle (GV) exhibits a unique chromatin configuration that is subject to dynamic modifications during oogenesis. This process of 'epigenetic maturation' is critical to confer the female gamete with meiotic as well as developmental competence. In spite of its biological significance, little is known concerning the cellular and molecular mechanisms regulating large-scale chromatin structure in mammalian oocytes. Here, recent findings that provide mechanistic insight into the complex relationship between large-scale chromatin structure and global transcriptional repression in pre-ovulatory oocytes will be discussed. Post-translational modifications of histone proteins such as acetylation and methylation are crucial for heterochromatin formation and thus play a key role in remodeling the oocyte genome. This strategy involves multiple and hierarchical chromatin modifications that regulate nuclear dynamics in response to a developmentally programmed signal(s), presumably of paracrine origin, before the resumption of meiosis. Models for the experimental manipulation of large-scale chromatin structure in vivo and in vitro will be instrumental to determine the key cellular pathways and oocyte-derived factors involved in genome-wide chromatin modifications. Importantly, analysis of the functional differentiation of chromatin structure in the oocyte genome with high resolution and in real time will have wide-ranging implications to understand the role of nuclear organization in meiosis, the events of nuclear reprogramming and the spatio-temporal regulation of gene expression during development and differentiation.

Characterization of linker histone H1FOO during bovine in vitro embryo development

Linker histones H1 are involved in various mechanisms, such as chromatin organization and gene transcription. In different organisms, a unique subtype can be found in the oocyte, however its function remains unclear. To assess the potential involvement of this oocyte linker histone (H1FOO) in chromatin modulation, we have cloned and sequenced the bovine H1FOO cDNA and followed its mRNA profile by quantitative RT-PCR in the oocyte and throughout bovine early embryo development. The highest level of mRNA was found in the germinal vesicle (GV) oocyte and diminished constantly throughout embryo development. In the 16-cell embryo and blastocyst, respectively, the mRNA levels were 200 and 2,000 times lower than in the GV oocyte. A specific antibody raised against bovine H1FOO was used to establish protein distribution in the oocyte and preimplantation embryo by immunocytochemistry. In the GV and metaphase II (MII) oocyte, as well as in the 1-, 2- and 4-cell embryo, H1FOO was localized in the cytoplasm and nucleus. The protein was uniformly spread within the cytoplasm, while it was concentrated onto the chromatin in the nucleus. In the 8- to 16-cell embryo, H1FOO's presence diminished in the cytoplasm, although it was still strongly expressed in nucleus. In the morula and blastocyst stages, the protein was totally lacking. By its position on chromatin, H1FOO could not only be involved in chromatin conformation but could also participate in activation or repression of genes during oogenesis and embryo development before embryonic genome activation.

Characterisation of human histone H1x

The members of the H1 histone family can be classified into three groups, which are the main class subtypes expressed in somatic cells, the developmental- and tissue-specific subtypes, and the replacement subtype H1(o). Until now, the subtype H1x was not classified, since it has not yet been thoroughly examined. The results of this study show that H1x shares similarities but also exhibits slight differences in its biochemical behaviour in comparison to the main class H1 histones. In HeLa cells it is located in the nucleus and partially associated with nucleosomes. Nevertheless, it is, like H1(o), mainly located in chromatin regions that are not affected by micrococcal nuclease digestion. Further common features of H1x and the replacement histone H1(o) are that the genes of both subtypes are solitarily located and give rise to polyadenylated mRNA. However, comparison of the inducibility of their expression revealed that their genes are regulated differentially.

Differential in vivo binding dynamics of somatic and oocyte-specific linker histones in oocytes and during ES cell nuclear transfer

The embryonic genome is formed by fusion of a maternal and a paternal genome. To accommodate the resulting diploid genome in the fertilized oocyte dramatic global genome reorganizations must occur. The higher order structure of chromatin in vivo is critically dependent on architectural chromatin proteins, with the family of linker histone proteins among the most critical structural determinants. Although somatic cells contain numerous linker histone variants, only one, H1FOO, is present in mouse oocytes. Upon fertilization H1FOO rapidly populates the introduced paternal genome and replaces sperm-specific histone-like proteins. The same dynamic replacement occurs upon introduction of a nucleus during somatic cell nuclear transfer. To understand the molecular basis of this dynamic histone replacement process, we compared the localization and binding dynamics of somatic H1 and oocyte-specific H1FOO and identified the molecular determinants of binding to either oocyte or somatic chromatin in living cells. We find that although both histones associate readily with chromatin in nuclei of somatic cells, only H1FOO is capable of correct chromatin association in the germinal vesicle stage oocyte nuclei. This specificity is generated by the N-terminal and globular domains of H1FOO. Measurement of in vivo binding properties of the H1 variants suggest that H1FOO binds chromatin more tightly than somatic linker histones. We provide evidence that both the binding properties of linker histones as well as additional, active processes contribute to the replacement of somatic histones with H1FOO during nuclear transfer. These results provide the first mechanistic insights into the crucial step of linker histone replacement as it occurs during fertilization and somatic cell nuclear transfer.