Controlled X‐chromosome dynamics defines meiotic potential of female mouse in vitro germ cells

Abstract

The mammalian germline is characterized by extensive epigenetic reprogramming during its development into functional eggs and sperm. Specifically, the epigenome requires resetting before parental marks can be established and transmitted to the next generation. In the female germline, X-chromosome inactivation and reactivation are among the most prominent epigenetic reprogramming events, yet very little is known about their kinetics and biological function. Here, we investigate X-inactivation and reactivation dynamics using a tailor-made in vitro system of primordial germ cell-like cell (PGCLC) differentiation from mouse embryonic stem cells. We find that X-inactivation in PGCLCs in vitro and in germ cell-competent epiblast cells in vivo is moderate compared to somatic cells, and frequently characterized by escaping genes. X-inactivation is followed by step-wise X-reactivation, which is mostly completed during meiotic prophase I. Furthermore, we find that PGCLCs which fail to undergo X-inactivation or reactivate too rapidly display impaired meiotic potential. Thus, our data reveal fine-tuned X-chromosome remodelling as a critical feature of female germ cell development towards meiosis and oogenesis.

Epiblast-like stem cells established by Wnt/β-catenin signaling manifest distinct features of formative pluripotency and germline competence

“… 67 , 68 Recently, it has been shown that heterogeneity of X chromosome inactivation in the EpiLC stage resulted in two subpopulations of derivative PGCLCs. 57 Interestingly, PGCLCs with two activated X chromosomes displayed higher expression of meiotic cycle genes such as Dnmt3l , Dazl, and Stra8 as well as LIF response genes such as Zfp42 , Spp1 , and Fgf4 , 57 which is similar to male PGCLCs derived from PD treatment as shown here. These findings suggest future studies to determine if these PGCLCs are more competent for in vitro gametogenesis.…”

“…Interestingly, a recent study revealed that female day 5 PGCLCs with two activated X chromosomes (GFP + -PGCLCs) have also upregulated the aforementioned genes compared with those with one activated X chromosome (GFP − -PGCLCs). 57 We thus compared our day 4 PGCLCs with/without PD with these female PGCLCs. As our PGCLCs were male cells, we used autosomal differentially expressed genes and found that PGCLCs derived from EpiLSCs with PD were clustered with GFP + -PGCLCs, whereas PGCLCs without PD treatment were clustered with GFP − -PGCLCs ( Figure 6F ).…”

Comprehensive comparison of female germ cell developmentin vitroandin vivoidentifies epigenetic gene regulation crucial for oocyte development and embryonic competence

“…This important advance has been recently applied to studying mechanisms of female germ cell development including the dormant state in primordial follicles, effects of sex chromosomes, and transcription factors (TFs) involved in oocyte growth (Hamada et al, 2020; Hamazaki et al, 2021; Nagamatsu et al, 2019; Shimamoto et al, 2019). It has also been used to study the kinetics and efficiency of X-chromosome inactivation and reactivation in female germ cells (Severino et al, 2022). However, oocytes developed in vitro have variable potential for embryogenesis.…”

Spatio-temporal X-linked gene reactivation and site-specific retention of epigenetic silencing in the mouse germline

“…Epigenetic reprogramming occurs when PGCs proliferate and migrate to colonize the future gonads. During the upstream development of gonadal sex determination, PGC epigenetic reprogramming displays striking differences between XX females and XY males, with reactivation of the inactive X chromosome [6][7][8][9] . This leads to an excess of Xlinked gene products, with the X:Autosome ratio exceeding 1 in female PGCs compared to males 10,11 .…”

“…Excess X-linked genes could promote sexual dimorphism and meiosis progression through the direct or indirect involvement of some X-linked genes in the process of sex-specific gonadal formation and Xi reactivation itself 8,12 .…”