Investigation into the molecular triggers of epigenetic reprogramming and meiotic entry in mouse primordial germ cells

Abstract

Germ cells represent the link between generations. At the time of fertilisation, sperm and egg come together to form a single cell, called the zygote, from which an entire organism is ultimately constructed. In mouse, two unique properties underlie the exceptional potential of gametes: 1) the ability to execute meiosis; and 2) the correct establishment of complementary epigenetic signatures (genomic imprints) in sperm and oocyte that enable post-implantation embryonic development in the next generation. At the molecular level, both the progression through meiosis and the establishment of sex-specific methylation imprints are critically dependent on a wave of global DNA demethylation that is initiated in primordial germ cells (PGCs), the embryonic precursors of the gametes, following their colonisation of the embryonic gonads. The work I present in this thesis focuses on the molecular mechanisms regulating this wave of DNA demethylation. Specifically, I conclusively show that DNA demethylation proceeds independent of the 5-methylcytosine oxygenase Tet1, previously proposed to act as the putative DNA demethylase. Rather, I show that, with respect to DNA demethylation, Tet1 appears to protect the hypo-methylated state in PGCs following the major loss of 5-methylcytosine. Most strikingly, however, with respect to germ cell development, Tet1 appears to play an important role in regulating the timing of the major developmental and transcriptional transitions that occurs in the mouse foetal germ line between embryonic day (E) 10.5 and E14.5, with PGCs derived from Tet1-knockout embryos showing increased stability of the demethylated pre-meiotic germ cell state. Beyond understanding the role of Tet1 in gonadal PGCs, I also present results focusing on the potential relationship between cell signalling, DNA demethylation and licensing for gametogenesis, with preliminary data suggesting a potential role for Akt-mTORC1 signalling in activating the expression of the key meiotic regulator Dazl.