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      Human Sperm Chromosomes: To Form Hairpin-Loops, Or Not to Form Hairpin-Loops, That Is the Question

      1 , 1 , 2 , *

      Genes

      MDPI

      nuclear organization, chromatin, spermatozoa, chromosomes, chromosome territories, centromeres

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          Abstract

          Background: Genomes are non-randomly organized within the interphase nucleus; and spermatozoa are proposed to have a unique hairpin-loop configuration, which has been hypothesized to be critical for the ordered exodus of the paternal genome following fertilization. Recent studies suggest that the hairpin-loop model of sperm chromatin organization is more segmentally organized. The purpose of this study is to examine the 3D organization and hairpin-loop configurations of chromosomes in human spermatozoa. Methods: Three-color sperm-fluorescence in-situ hybridization was utilized against the centromeres, and chromosome p- and q-arms of eight chromosomes from five normozoospermic donors. Wide-field fluorescence microscopy and 3D modelling established the radial organization and hairpin-loop chromosome configurations in spermatozoa. Results: All chromosomes possessed reproducible non-random radial organization ( p < 0.05) and formed discrete hairpin-loop configurations. However, chromosomes preferentially formed narrow or wide hairpin-loops. We did not find evidence to support the existence of a centralized chromocenter(s) with centromeres being more peripherally localized than one or both of their respective chromosome arms. Conclusion: This provides further evidence to support a more segmental organization of chromatin in the human sperm nucleus. This may be of significance for fertilization and early embryogenesis as specific genomic regions are likely to be exposed, remodeled, and activated first, following fertilization.

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          Most cited references 58

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          Distinctive Chromatin in Human Sperm Packages Genes for Embryo Development

          Summary As nucleosomes are widely replaced by protamine in mature human sperm, epigenetic contributions of sperm chromatin to embryo development have been considered highly limited. However, we find the retained nucleosomes significantly enriched at loci of developmental importance including imprinted gene clusters, miRNA clusters, HOX gene clusters, and the promoters of stand-alone developmental transcription and signaling factors. Importantly, histone modifications localize to particular developmental loci. H3K4me2 is enriched at certain developmental promoters, whereas large blocks of H3K4me3 localize to a subset of developmental promoters, regions in HOX clusters, certain non-coding RNAs, and generally to paternally-expressed imprinted loci, but not paternally-repressed loci. Notably, H3K27me3 is significantly enriched at developmental promoters that are repressed in early embryos, including many bivalent (H3K4me3/H3K27me3) promoters in embryonic stem cells. Finally, developmental promoters are generally DNA hypomethylated in sperm, but acquire methylation during differentiation. Taken together, epigenetic marking in sperm is extensive, and correlated with developmental regulators.
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            Chromosome territories.

            Chromosome territories (CTs) constitute a major feature of nuclear architecture. In a brief statement, the possible contribution of nuclear architecture studies to the field of epigenomics is considered, followed by a historical account of the CT concept and the final compelling experimental evidence of a territorial organization of chromosomes in all eukaryotes studied to date. Present knowledge of nonrandom CT arrangements, of the internal CT architecture, and of structural interactions with other CTs is provided as well as the dynamics of CT arrangements during cell cycle and postmitotic terminal differentiation. The article concludes with a discussion of open questions and new experimental strategies to answer them.
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              Single-nucleus Hi-C reveals unique chromatin reorganization at oocyte-to-zygote transition

              Chromatin is reprogrammed after fertilization to produce a totipotent zygote with the potential to generate a new organism 1 . The maternal genome inherited through the oocyte and the paternal genome provided by sperm coexist as separate haploid nuclei in the zygote. How these two epigenetically distinct genomes are spatially organized is poorly understood. Existing chromosome conformation capture-based methods 2–5 are inapplicable to oocytes and zygotes due to a paucity of material. To study the 3D chromatin organization in rare cell types, we developed a single-nucleus Hi-C (snHi-C) protocol that provides >10-fold more contacts per cell than the previous method 2 . Here we show that chromatin architecture is uniquely reorganized during the mouse oocyte-to-zygote transition and is distinct in paternal and maternal nuclei within single-cell zygotes. Features of genomic organization including compartments, topologically associating domains (TADs) and loops are present in individual oocytes when averaged over the genome; each feature at a locus is variable between cells. At the sub-megabase level, we observe stochastic clusters of contacts that violate TAD boundaries but average into TADs. Strikingly, we found that TADs and loops but not compartments are present in zygotic maternal chromatin, suggesting that these are generated by different mechanisms. Our results demonstrate that the global chromatin organization of zygote nuclei is fundamentally different from other interphase cells. An understanding of this zygotic chromatin “ground state” has the potential to provide insights into reprogramming to totipotency.
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                Author and article information

                Journal
                Genes (Basel)
                Genes (Basel)
                genes
                Genes
                MDPI
                2073-4425
                03 July 2019
                July 2019
                : 10
                : 7
                Affiliations
                [1 ]Department of Human and Molecular Genetics, Herbert Wertheim College of Medicine, Florida International University, Miami, FL 33199, USA
                [2 ]Biomolecular Sciences Institute, Florida International University, Miami, FL 33199, USA
                Author notes
                [* ]Correspondence: htempest@ 123456fiu.edu
                [†]

                Current address: IVF Florida Reproductive Associates, Margate, FL, 33063, USA.

                Article
                genes-10-00504
                10.3390/genes10070504
                6678829
                31277336
                © 2019 by the authors.

                Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license ( http://creativecommons.org/licenses/by/4.0/).

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