Culture of the Intact Postnatal Naked Mole-Rat Ovary: From Meiotic Prophase to Single-Cell RNASeq

Recently, we reported that, in the naked mole-rat (Heterocephalus glaber) ovary, there is mitotic expansion of the primordial germ cells (PGCs), and the initiation of the meiotic program occurs postnatally. This is opposite to almost all other mammals, including humans and mice, whose reproductive cycle begins very early in development. In both mouse and human, the ovaries become populated with PGCs in utero; these PGCs will later generate the oogonia. After mitotic proliferation, these cells will trigger the meiotic program and initiate meiotic prophase I. Given that all these processes happen in utero, their analysis has been very challenging; so the ability to study them postnatally and to manipulate them with inhibitors or other substances, in the naked mole-rat, opens new possibilities in the field. In this chapter, we present a comprehensive collection of protocols that permit the culture of whole naked mole-rat ovaries, followed by analysis of germ cells, from PGCs to oocytes, in meiotic prophase I, as well the obtention of single-cell suspension or single-nuclei suspension for RNASeq.

Postnatal oogenesis leads to an exceptionally large ovarian reserve in naked mole-rats

In the long-lived naked mole-rat (NMR), the entire process of oogenesis occurs postnatally. Germ cell numbers increase significantly in NMRs between postnatal days 5 (P5) and P8, and germs cells positive for proliferation markers (Ki-67, pHH3) are present at least until P90. Using pluripotency markers (SOX2 and OCT4) and the primordial germ cell (PGC) marker BLIMP1, we show that PGCs persist up to P90 alongside germ cells in all stages of female differentiation and undergo mitosis both in vivo and in vitro. We identified VASA+ SOX2+ cells at 6 months and at 3-years in subordinate and reproductively activated females. Reproductive activation was associated with proliferation of VASA+ SOX2+ cells. Collectively, our results suggest that highly desynchronized germ cell development and the maintenance of a small population of PGCs that can expand upon reproductive activation are unique strategies that could help to maintain the NMR's ovarian reserve for its 30-year reproductive lifespan.

Characterization of the Postnatal Naked Mole-Rat Ovary: From Primordial Germ Cells to Meiotic Prophase I Oocytes

The mammalian reproductive cycle, including those of humans and mice, begins very early in development. In utero, the ovaries become populated with primordial germ cells (PGCs) that will generate the oogonia. First, these cells proliferate mitotically, and then they trigger the meiotic program and initiate meiotic prophase I. Since these processes happen during gestation, their study had been very limited and challenging. Recently, we reported that, in the naked mole-rat (Heterocephalus glaber) ovary, there is mitotic expansion of the PGCs, and the initiation of the meiotic program occurs postnatally. In this chapter, we present a comprehensive collection of protocols that permit the analysis of naked mole-rat germ cells, from PGCs to oocytes, in meiotic prophase I, using in vivo and in vitro approaches.

Etoposide damages female germ cells in the developing ovary

BACKGROUND

As with many anti-cancer drugs, the topoisomerase II inhibitor etoposide is considered safe for administration to women in the second and third trimesters of pregnancy, but assessment of effects on the developing fetus have been limited. The purpose of this research was to examine the effect of etoposide on germ cells in the developing ovary. Mouse ovary tissue culture was used as the experimental model, thus allowing us to examine effects of etoposide on all stages of germ cell development in the same way, in vitro.

RESULTS

Fetal ovaries from embryonic day 13.5 CD1 mice or neonatal ovaries from postnatal day 0 CD1 mice were cultured with 50-150 ng ml(-1) or 50-200 ng ml(-1) etoposide respectively, concentrations that are low relative to that in patient serum. When fetal ovaries were treated prior to follicle formation, etoposide resulted in dose-dependent damage, with 150 ng ml(-1) inducing a near-complete absence of healthy follicles. In contrast, treatment of neonatal ovaries, after follicle formation, had no effect on follicle numbers and only a minor effect on follicle health, even at 200 ng ml(-1). The sensitivity of female germ cells to etoposide coincided with topoisomerase IIα expression: in the developing ovary of both mouse and human, topoisomerase IIα was expressed in germ cells only prior to follicle formation.

CONCLUSIONS

Exposure of pre-follicular ovaries, in which topoisomerase IIα expression was germ cell-specific, resulted in a near-complete elimination of germ cells prior to follicle formation, with the remaining germ cells going on to form unhealthy follicles by the end of culture. In contrast, exposure to follicle-enclosed oocytes, which no longer expressed topoisomerase IIα in the germ cells, had no effect on total follicle numbers or health, the only effect seen specific to transitional follicles. Results indicate the potential for adverse effects on fetal ovarian development if etoposide is administered to pregnant women when germ cells are not yet enclosed within ovarian follicles, a process that starts at approximately 17 weeks gestation and is only complete towards the end of pregnancy.

Radiobiology and reproduction-what can we learn from Mammalian females?

Ionizing radiation damages DNA and induces mutations as well as chromosomal reorganizations. Although radiotherapy increases survival among cancer patients, this treatment does not come without secondary effects, among which the most problematic is gonadal dysfunction, especially in women. Even more, if radio-induced DNA damage occurs in germ cells during spermatogenesis and/or oogenesis, they can produce chromosomal reorganizations associated with meiosis malfunction, abortions, as well as hereditary effects. However, most of our current knowledge of ionizing radiation genotoxic effects is derived from in vitro studies performed in somatic cells and there are only some experimental data that shed light on how germ cells work when affected by DNA alterations produced by ionizing radiation. In addition, these few data are often related to mammalian males, making it difficult to extrapolate the results to females. Here, we review the current knowledge of radiobiology and reproduction, paying attention to mammalian females. In order to do that, we will navigate across the female meiotic/reproductive cycle/life taking into account the radiation-induced genotoxic effects analysis and animal models used, published in recent decades.

Meiosis in a bottle: new approaches to overcome Mammalian meiocyte study limitations

The study of meiosis is limited because of the intrinsic nature of gametogenesis in mammals. One way to overcome these limitations would be the use of culture systems that would allow meiotic progression in vitro. There have been some attempts to culture mammalian meiocytes in recent years. In this review we will summarize all the efforts to-date in order to culture mammalian sperm and oocyte precursor cells.