Date Published: January 20, 2017
Publisher: Public Library of Science
Author(s): Cassy M. Spiller, Dagmar Wilhelm, David A. Jans, Josephine Bowles, Peter Koopman, Stefan Schlatt.
Fetal germ cell development is tightly regulated by the somatic cell environment, and is characterised by cell cycle states that differ between XY and XX gonads. In the testis, gonocytes enter G1/G0 arrest from 12.5 days post coitum (dpc) in mice and maintain cell cycle arrest until after birth. Failure to correctly maintain G1/G0 arrest can result in loss of germ cells or, conversely, germ cell tumours. High mobility group box containing transcription factor 1 (HBP1) is a transcription factor that was previously identified in fetal male germ cells at the time of embryonic cell cycle arrest. In somatic cells, HBP1 is classified as a tumour suppressor protein, known to regulate proliferation and senescence. We therefore investigated the possible role of HBP1 in the initiation and maintenance of fetal germ cell G1/G0 arrest using the mouse model. We identified two splice variants of Hbp1, both of which are expressed in XY and XX fetal gonads, but only one of which is localised to the nucleus in in vitro assays. To investigate Hbp1 loss of function, we used embryonic stem (ES) cells carrying a Genetrap mutation for Hbp1 to generate mice lacking Hbp1 function. We found that Hbp1-genetrap mouse mutant germ cells proliferated correctly throughout development, and adult males were viable and fertile. Multiple Hbp1-LacZ reporter mouse lines were generated, unexpectedly revealing Hbp1 embryonic expression in hair follicles, eye and limbs. Lastly, in a model of defective germ cell G1/G0 arrest, the Rb1-knockout model, we found no evidence for Hbp1 mis-regulation, suggesting that the reported RB1-HBP1 interaction is not critical in the germline, despite co-expression.
Germ cells are highly specialized cells that are uniquely capable of undergoing meiosis and represent our means to reproduce. During embryo development, two distinct cell cycle modes characterize the sex-specific pathways of germ cell differentiation. From 12.5 dpc in mice, germ cells enter G1/G0 arrest, signifying commitment to spermatogenesis , while entry into meiosis prophase I in the ovary signifies commitment to oogenesis . The somatic cell environment of the gonads directs these two germ cell fates. Retinoic acid has been shown to modulate meiosis entry in the ovary, while being antagonistic to pro-spermatogonia development [3–5]. In the testis, very little is known regarding germ cell entry and maintenance of G1/G0 arrest. In humans, failure of this process to occur correctly has been linked to testicular germ cell tumours and their precursor, germ cell neoplasia in situ [6, 7]. This connection provides strong motivation for investigating cell cycle regulation in these specialised cells.
The embryonic differentiation of XX and XY germ cells diverges at 12.5 dpc with XX germ cell entry into the first phase of meiosis and XY germ cell entry into G1/G0 arrest . XY germ cells are particularly fascinating as they maintain a prolonged period of G1/G0 arrest throughout development and postnatal life, prior to entering mitosis and meiosis after puberty . Aberrations in the entry or maintenance of G1/G0 arrest have led to instances of both germ cell loss and conversely, proliferation and therefore cancer, such as observed in Pten-/- , Dazl-/-  and Ter mutations . It is also believed that loss of correct germ cell-cell cycle control and differentiation is responsible for germ cell neoplasia in situ (previously known as carcinoma in situ ), the precursor cell to human testicular germ cell tumours, for which there is no suitable mouse model. Comparison of germ cell and somatic cell tumours has revealed both common and unique gene and protein expression patterns, including for cell cycle regulators such as RB1 [6, 7]. It is possible that germ cells are subject to a different type of cell cycle regulation, which would not be surprising considering their unique ability to undergo meiosis.