Abstract
In mouse female preimplantation embryos, the paternal X chromosome (Xp) is silenced by imprinted X chromosome inactivation (iXCI). This requires production of the noncoding Xist RNA in cis, from the Xp. The Xist locus on the maternally inherited X chromosome (Xm) is refractory to activation due to the presence of an imprint. Paternal inheritance of an Xist deletion (XpΔXist) is embryonic lethal to female embryos, due to iXCI abolishment. Here, we circumvented the histone-to-protamine and protamine-to-histone transitions of the paternal genome, by fertilization of oocytes via injection of round spermatids (ROSI). This did not affect initiation of XCI in wild type female embryos. Surprisingly, ROSI using ΔXist round spermatids allowed survival of female embryos. This was accompanied by activation of the intact maternal Xist gene, initiated with delayed kinetics, around the morula stage, resulting in Xm silencing. Maternal Xist gene activation was not observed in ROSI-derived males. In addition, no Xist expression was detected in male and female morulas that developed from oocytes fertilized with mature ΔXist sperm. Finally, the expression of the X-encoded XCI-activator RNF12 was enhanced in both male (wild type) and female (wild type as well as XpΔXist) ROSI derived embryos, compared to in vivo fertilized embryos. Thus, high RNF12 levels may contribute to the specific activation of maternal Xist in XpΔXist female ROSI embryos, but upregulation of additional Xp derived factors and/or the specific epigenetic constitution of the round spermatid-derived Xp are expected to be more critical. These results illustrate the profound impact of a dysregulated paternal epigenome on embryo development, and we propose that mouse ROSI can be used as a model to study the effects of intergenerational inheritance of epigenetic marks.
Highlights
In mammals, as in all diploid organisms with a sexual reproduction cycle, the diploid zygote is formed upon fertilization by combination of the haploid maternal and paternal genomes
We have investigated the consequences of elimination of the histone-to-protamine and protamine-to-histone transitions on imprinted X chromosome inactivation (iXCI), by fertilization through injection of immature round spermatids into oocytes (ROSI)
When the round spermatids used for ROSI carried an X chromosome with an Xist deletion (ΔXist), we found that the Xist gene on the maternal X chromosome was activated, which rescued the female lethality of embryos that is invariably observed upon fertilization with mature ΔXist spermatozoa
Summary
As in all diploid organisms with a sexual reproduction cycle, the diploid zygote is formed upon fertilization by combination of the haploid maternal and paternal genomes. After fertilization, the X chromosome of paternal origin (Xp) will always be inactivated in female pre-implantation embryos and this is maintained in the extra-embryonic tissues of post-implantation embryos This imprinted X chromosome inactivation (iXCI) depends on expression and spreading in cis of the Xist noncoding RNA on the Xp [8]. Pre-inactivated intergenic repeat regions on the Xp may carry transgenerational epigenetic information from the paternal germline to the zygote, predisposing the Xp for iXCI independently of Xist [20] This might rely on the inheritance of sperm-derived nucleosomes and their associated modifications. It has been suggested that the preferential inactivation of Xp may rely on early and robust activation of the paternal Xist gene [21] This may be facilitated, upon fertilization, by the protamine-to-histone transition, during which the protamine-based chromatin of the sperm acquires newly deposited histones lacking most heterochromatic marks. Conclusive evidence for the contribution of the protamine-to-histone transition in the initiation of iXCI is lacking
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
