Histone variants in metazoan development

Abstract

Embryonic development is regulated by both genetic and epigenetic mechanisms, with nearly all DNA-templated processes influenced by chromatin architecture. Sequence variations in histone proteins, core components of chromatin, provide a means to generate diversity in the chromatin structure, resulting in distinct and profound biological outcomes in the developing embryo. Emerging literature suggests that epigenetic contributions from histone variants play key roles in a number of developmental processes such as the initiation and maintenance of pericentric heterochromatin, X-inactivation, and germ cell differentiation. Here, we review the role of histone variants in the embryo with particular emphasis on early mammalian development.

Figure 1. Chronology of mouse oocyte fertilization and associated chromatin-related events

Upon fertilization, the oocyte resumes meiosis, resulting in formation of the maternal pronucleus and eviction of histones H3.3 and macroH2A from the maternal chromatin. Concurrent with sperm decondensation and formation of the paternal pronucleus, protamines are exchanged for histones resulting in nucleosomal chromatin. At this time, histone H3.3 is preferentially deposited into the male pronucleus before the onset of transcriptional activation. H3.3 incorporation in the maternal genome is not observed until nearly an hour after formation of the pronucleus. By the late pronuclear stage, after the onset of transcription and the first DNA replication, H3.3 levels in the parental pronuclei appear equivalent. MacroH2A, however, is not observed again in chromatin until later developmental stages.

Figure 2. Known genomic locations of histone variants

Immunofluorescence and genome-wide ChIP studies have identified the chromosomal locations for some histone variants. The deposition of H3.3 at telomeres and pericentric heterochromatin occurs via the Daxx-ATRX complex, whereas the HIRA mediates H3.3 chromatin assembly in the bodies of actively transcribed and high CpG content promoter genes. An unknown deposition pathway is used to assemble H3.3 nucleosomes at enhancer elements and gene promoters. Similar to H3.3, H2AZ is also associated with gene-specific enhancers and promoters, in addition to pericentric heterochromatin. H2AL is found in pericentric heterochromatin during the later stages of spermatogenesis. In addition to the XY body and inactive X chromosome, macroH2A is also associated with the promoter of some genes. The centromere-specific H3 variant, CENP-A is deposited at centromeric chromatin.

Figure 3. Developmental transitions and changes in histone variant incorporation from the early embryo to the blastocyte stage

At the 8-cell stage, after the initiation of X inactivation, macroH2A is deposited specifically on the silenced paternal X chromosome. MacroH2A remains present and associated with the inactive X throughout development. In the extraembryonic trophoblast, where imprinted X inactivation is maintained, macroH2A remains associated with the paternal X chromosome. The cells of the inner cell mass (ICM) undergo transient reactivation followed by random inactivation of the X chromosome; here, macroH2A is enriched on the silenced X chromosome. H2AZ incorporation in the early embryo is also associated with gene silencing, with enrichment observed at regions of pericentric heterochromatin as the outer cells of the ICM begin to differentiate. While present at pericentric heterochromatin, H2AZ is absent from the inactive X chromosome.

Figure 4. Global changes in histone variant deposition during meiosis of spermatogenesis

Germ cell development involves a number of epigenetic changes, involving differential incorporation of histone variants into chromatin. After reaching the gonad, male primordial germ cells (PGCs) divide to form spermatogonia. These cells divide to form spermatocytes, which then enter meiosis. In zygotene, homologous chromosomes synapse, leading to genetic recombination of non-sister chromatids in the pachytene stage. At this time, the X and Y chromosomes condense together to form the recombinantly inactive XY body. The sex determining chromosomes are transcriptionally inactivated at pachytene through a process called meiotic sex chromosome inactivation (MSCI). After diakinesis and completion of meiosis I, the secondary spermatocyte begins meiosis II, resulting in four haploid spermatid. At this stage, nucleosomal histones are exchanged for the sperm-specific protamines, allowing for highly condensed packaging of DNA in the mature sperm cell. The presence of histone variants at each stage is indicated, with each variant subtype represented with a different color for clarity.