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

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.

Impact of Endocrine Disruptors upon Non-Genetic Inheritance

Similar to environmental factors, EDCs (endocrine-disrupting chemicals) can influence gene expression without modifying the DNA sequence. It is commonly accepted that the transgenerational inheritance of parentally acquired traits is conveyed by epigenetic alterations also known as “epimutations”. DNA methylation, acetylation, histone modification, RNA-mediated effects and extracellular vesicle effects are the mechanisms that have been described so far to be responsible for these epimutations. They may lead to the transgenerational inheritance of diverse phenotypes in the progeny when they occur in the germ cells of an affected individual. While EDC-induced health effects have dramatically increased over the past decade, limited effects on sperm epigenetics have been described. However, there has been a gain of interest in this issue in recent years. The gametes (sperm and oocyte) represent targets for EDCs and thus a route for environmentally induced changes over several generations. This review aims at providing an overview of the epigenetic mechanisms that might be implicated in this transgenerational inheritance.

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.

Human Papillomavirus Epitope Mimicry and Autoimmunity: The Molecular Truth of Peptide Sharing

OBJECTIVE

To define the cross-reactivity potential and the consequent autoimmunity intrinsic to viral versus human peptide sharing.

METHODS

Using human papillomavirus (HPV) infection/active immunization as a research model, the experimentally validated HPV L1 epitopes catalogued at the Immune Epitope DataBase were analyzed for peptide sharing with the human proteome.

RESULTS

The final data show that the totality of the immunoreactive HPV L1 epi-topes is mostly composed by peptides present in human proteins.

CONCLUSIONS

Immunologically, the high extent of peptide sharing between the HPV L1 epitopes and human proteins invites to revise the concept of the negative selection of self-reactive lymphocytes. Pathologically, the data highlight a cross-reactive potential for a spectrum of autoimmune diseases that includes ovarian failure, systemic lupus erythematosus (SLE), breast cancer and sudden death, among others. Therapeutically, analyzing already validated immunoreactive epitopes filters out the peptide sharing possibly exempt of self-reactivity, defines the effective potential for pathologic autoimmunity, and allows singling out peptide epitopes for safe immunotherapeutic protocols.

Chromatin remodeling in Drosophila preblastodermic embryo extract

Chromatin is known to undergo extensive remodeling during nuclear reprogramming. However, the factors and mechanisms involved in this remodeling are still poorly understood and current experimental approaches to study it are not best suited for molecular and genetic analyses. Here we report on the use of Drosophila preblastodermic embryo extracts (DREX) in chromatin remodeling experiments. Our results show that incubation of somatic nuclei in DREX induces changes in chromatin organization similar to those associated with nuclear reprogramming, such as rapid binding of the germline specific linker histone dBigH1 variant to somatic chromatin, heterochromatin reorganization, changes in the epigenetic state of chromatin, and nuclear lamin disassembly. These results raise the possibility of using the powerful tools of Drosophila genetics for the analysis of chromatin changes associated with this essential process.

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.

Systematic identification of gene family regulators in mouse and human embryonic stem cells

Pluripotent self-renewing embryonic stem cells (ESCs) have been the focus of a growing number of high-throughput experiments, revealing the genome-wide locations of hundreds of transcription factors and histone modifications. While most of these datasets were used in a specific context, all datasets combined offer a comprehensive view of chromatin characteristics and regulatory elements that govern cell states. Here, using hundreds of datasets in ESCs, we generated colocalization maps of chromatin proteins and modifications, and built a discovery pipeline for regulatory proteins of gene families. By comparing genome-wide binding data with over-expression and knockdown analysis of hundreds of genes, we discovered that the pluripotency-related factor NR5A2 separates mitochondrial from cytosolic ribosomal genes, regulating their expression. We further show that genes with a common chromatin profile are enriched for distinct Gene Ontology (GO) categories. Our approach can be generalized to reveal common regulators of any gene group; discover novel gene families, and identify common genomic elements based on shared chromatin features.

Germline-specific H1 variants: the "sexy" linker histones

The eukaryotic genome is packed into chromatin, a nucleoprotein complex mainly formed by the interaction of DNA with the abundant basic histone proteins. The fundamental structural and functional subunit of chromatin is the nucleosome core particle, which is composed by 146 bp of DNA wrapped around an octameric protein complex formed by two copies of each core histone H2A, H2B, H3, and H4. In addition, although not an intrinsic component of the nucleosome core particle, linker histone H1 directly interacts with it in a monomeric form. Histone H1 binds nucleosomes near the exit/entry sites of linker DNA, determines nucleosome repeat length and stabilizes higher-order organization of nucleosomes into the ∼30 nm chromatin fiber. In comparison to core histones, histone H1 is less well conserved through evolution. Furthermore, histone H1 composition in metazoans is generally complex with most species containing multiple variants that play redundant as well as specific functions. In this regard, a characteristic feature is the presence of specific H1 variants that replace somatic H1s in the germline and during early embryogenesis. In this review, we summarize our current knowledge about their structural and functional properties.

Histone variants: key players of chromatin

Histones are fundamental structural components of chromatin. Eukaryotic DNA is wound around an octamer of the core histones H2A, H2B, H3, and H4. Binding of linker histone H1 promotes higher order chromatin organization. In addition to their structural role, histones impact chromatin function and dynamics by, e.g., post-translational histone modifications or the presence of specific histone variants. Histone variants exhibit differential expression timings (DNA replication-independent) and mRNA characteristics compared to canonical histones. Replacement of canonical histones with histone variants can affect nucleosome stability and help to create functionally distinct chromatin domains. In line with this, several histone variants have been implicated in the regulation of cellular processes such as DNA repair and transcriptional activity. In this review, we focus on recent progress in the study of core histone variants H2A.X, H2A.Z, macroH2A, H3.3, and CENP-A, as well as linker histone H1 variants, their functions and their links to development and disease.

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.