Exposure to Brefeldin A promotes initiation of meiosis in murine female germ cells.

In mammals, ontogenesis starts from a fusion of spermatozoon and oocyte, which are produced by reductive nuclear division of a diploid germ cell in a specialised but complex biological process known as meiosis. However, little is known about the mechanism of meiotic initiation in germ cells, although many factors may be responsible for meiosis both in male and female gonads. In this study, 11.5 days post coitum (dpc) female fetal mouse genital ridges were cultured in vitro with exposure to Brefeldin A (BFA) for 6h, and the changes in meiosis were detected. Synaptonemal-complex analysis implied that BFA played a positive role in meiosis initiation and this hypothesis was confirmed by quantitative PCR of meiosis-specific genes: stimulated by retinoic acid gene 8 (Stra8) and deleted in a zoospermia-like (DAZL). At the same time, mRNA expression of retinoic acid synthetase (Raldh2) and retinoic acid (RA) receptors increased in female gonads with in vitro exposure to BFA. Transplanting genital ridges treated with BFA into the kidney capsule of immunodeficient mice demonstrated that the development capacity of female germ cells was normal, while formation of primordial follicles was seen to be a result of accelerated meiosis after exposure to BFA. In conclusion, the study indicated that BFA stimulated meiosis initiation partly by RA signalling and then promoted the development of follicles.

Notch pathway regulates female germ cell meiosis progression and early oogenesis events in fetal mouse.

A critical process of early oogenesis is the entry of mitotic oogonia into meiosis, a cell cycle switch regulated by a complex gene regulatory network. Although Notch pathway is involved in numerous important aspects of oogenesis in invertebrate species, whether it plays roles in early oogenesis events in mammals is unknown. Therefore, the rationale of the present study was to investigate the roles of Notch signaling in crucial processes of early oogenesis, such as meiosis entry and early oocyte growth. Notch receptors and ligands were localized in mouse embryonic female gonads and 2 Notch inhibitors, namely DAPT and L-685,458, were used to attenuate its signaling in an in vitro culture system of ovarian tissues from 12.5 days post coitum (dpc) fetus. The results demonstrated that the expression of Stra8, a master gene for germ cell meiosis, and its stimulation by retinoic acid (RA) were reduced after suppression of Notch signaling, and the other meiotic genes, Dazl, Dmc1, and Rec8, were abolished or markedly decreased. Furthermore, RNAi of Notch1 also markedly inhibited the expression of Stra8 and SCP3 in cultured female germ cells. The increased methylation status of CpG islands within the Stra8 promoter of the oocytes was observed in the presence of DAPT, indicating that Notch signaling is probably necessary for maintaining the epigenetic state of this gene in a way suitable for RA stimulation. Furthermore, in the presence of Notch inhibitors, progression of oocytes through meiosis I was markedly delayed. At later culture periods, the rate of oocyte growth was decreased, which impaired subsequent primordial follicle assembly in cultured ovarian tissues. Taken together, these results suggested new roles of the Notch signaling pathway in female germ cell meiosis progression and early oogenesis events in mammals.

Effects of diethylhexyl phthalate (DEHP) given neonatally on spermatogenesis of mice.

We previously demonstrated that the effects of diethylhexyl phthalate (DEHP) alter reproduction function on male mice. Immature male mice were treated daily with DEHP from postnatal day 7-21, 7-35, 7-49, in a dose-dependent manner. As results, both the quality and quantity of spermatozoa were decreased in 60-day-old mice. The results by RT-PCR analysis indicated that DDx3Y, Usp9Y, RBM, E1F1AY, EGF, FSHR and EGFR genes were down-regulated, and LHR, Cyp17a1 and Cyp19a1 were down-regulated in response to DEHP. These genes were selected based on their markedly increased or decreased expression levels. However, DEHP had no effect on the meiotic process and recombination levels in male mouse germ cells. Treatment with DEHP induced histopathological changes in the testes. Taken together, these results provide a new insight into the molecular mechanisms underlying the detrimental impacts of DEHP in humans and wildlife.

Exposure to bisphenol A results in a decline in mouse spermatogenesis.

Bisphenol A (BPA), a chemical used in many consumer products, interferes with the endocrine system of mammals, including humans. The aim of the present study was to investigate the effect of BPA on spermatogenesis and semen quality. The objective of this study was to assess the effects of BPA on mouse spermatogenesis. CD1 mice were used in all experiments. Mice were treated with different doses of BPA (0, 20 and 40 µg kg⁻¹ day⁻¹ from postnatal Day (PND) 3 to PND21, PND 35 or PND49. After 5 weeks BPA treatment, oestrogen receptor α expression was increased in mouse testis, whereas the meiotic progression of germ cells was slowed. Thus, both the quality and quantity of spermatozoa were decreased in 7-week-old mice. However, BPA had no effect on DNA methylation of imprinted genes such as Igf2, Igf2r, Peg3 and H19, in germ cells. In addition, exposure of male mice to BPA resulted in abnormal offspring that were smaller with a low-quality pelage when they were 35 days old. In conclusion, BPA hampers spermatogenesis and the subsequent development of offspring.

Bisphenol A exposure modifies DNA methylation of imprint genes in mouse fetal germ cells.

Bisphenol A (BPA) is an estrogenic environmental toxin widely used for the production of plastics. Human frequent exposure to this chemical has been proposed to be a potential public health risk. The objective of this study was to assess the effects of BPA on DNA methylation of imprinting genes in fetal mouse germ cell. Pregnant mice were treated with BPA at doses of 0, 40, 80 and 160 µg BPA/kg body weight/day from 0.5 day post coitum. DNA methylation of imprinting genes, Igf2r, Peg3 and H19, was decreased with the increase of BPA concentration in fetal mouse germ cells (p < 0.01).The relative mRNA levels of Nobox were lower in BPA-treated group compared to control (BPA free) in female fetal germ cells, but in male fetal germ cells, a significant higher in Nobox expression was observed in BPA-treated group compared to control. Decreased mRNA expression of specific meiotic genes including Stimulated by Stra8 and Dazl were obtained in the female fetal germ cells. In conclusion, BPA exposure can affect the DNA methylation of imprinting genes in fetal mouse germ cells.

Fetal exposure to bisphenol A affects the primordial follicle formation by inhibiting the meiotic progression of oocytes.

Bisphenol A (BPA) is an estrogenic environmental toxin widely used for the production of plastics. Frequent human exposure to this chemical has been proposed to be a potential public health risk. The objective of this study was to assess the effects of BPA on germ cell cyst breakdown and primordial follicle formation. Pregnant mice were treated with BPA at doses of 0, 0.02, 0.04, 0.08 mg/kg body weight/day from 12.5 day postcoitum. BPA was delivered orally to pregnant female mice. A dose-response relationship was observed with increased BPA exposure level associated with more oocytes in germ cell cyst and less primordial follicle at postnatal day 3 (P < 0.01). Progression to meiosis prophase I of oocytes was delayed in the 0.08 mg/kg bw/day treated group (P < 0.01). Decreased mRNA expression of specific meiotic genes including Stra8, Dmc1, Rec8 and Scp3 were observed. In conclusion, BPA exposure can affect the formation of primordial follicle by inhibiting meiotic progression of oocytes.