抑制糖酵解活性限制雄性原始生殖细胞的增殖

目的:雄性原始生殖细胞在植入生殖嵴后,会从有丝分裂退出进入静息状态,在这一过程中伴随着细胞内代谢状态的改变,本研究旨在体解析原始生殖细胞增殖的改变与细胞代谢之间的因果关系。方法:通过体内Brdu掺入实验明确不同时间点雄性生殖细胞的增殖状态;分析比较增殖状态和静息状态原始生殖细胞糖酵解相关基因的表达;利用腹腔注射HK2特异性抑制剂2-Deoxy-D-glucose (2-DG),构建糖酵解抑制小鼠模型;通过免疫荧光与qPCR分析抑制糖酵解后原始生殖细胞的表型。结果:免疫荧光结果显示雄性生殖细胞增殖停滞从E13.5开始,至E15.5完全停滞;qPCR和Western Blot显示在此过程中HK2的表达是逐渐降低的;在E11.5抑制小鼠胚胎中的糖酵解过程,可以在E13.5检测到雄性PGCs增殖下降,并且可以抑制多能性基因如Sox2、Oct4的表达。结论:研究发现,E11.5-E13.5雄性原始生殖细胞内增殖与多能性的维持需要糖酵解。改变胚胎糖酵解水平可以影响原始生殖细胞增殖分化进程。     

X chromosome activity in mouse XX primordial germ cells

In the early epiblast of female mice, one of the two X chromosomes is randomly inactivated by a Xist-dependent mechanism, involving the recruitment of Ezh2-Eed and the subsequent trimethylation of histone 3 on lysine 27 (H3K27me3). We demonstrate that this random inactivation process applies also to the primordial germ cell (PGC) precursors, located in the proximal region of the epiblast. PGC specification occurs at about embryonic day (E)7.5, in the extraembryonic mesoderm, after which the germ cells enter the endoderm of the invaginating hindgut. As they migrate towards the site of the future gonads, the XX PGCs gradually lose the H3K27me3 accumulation on the silent X chromosome. However, using a GFP transgene inserted into the X chromosome, we observed that the XX gonadal environment (independently of the gender) is important for the substantial reactivation of the inactive X chromosome between E11.5 and E13.5, but is not required for X-chromosome reactivation during the derivation of pluripotent embryonic germ cells. We describe in detail one of the key events during female PGC development, the epigenetic reprogramming of the X chromosome, and demonstrate the role of the XX somatic genital ridge in this process.     

Derivation of embryonic germ cells from post migratory primordial germ cells, and methylation analysis of their imprinted genes by bisulfite genomic sequencing

The embryonic germ cell (EGCs) of mice is a kind of pluripotent stem cell that can be generated from pre- and post-migratory primordial germ cells (PGCs). Most previous studies on DNA methylation of EGCs were restricted to 12.5 days post coitum (dpc). This study was designed to establish and characterize murine EGC lines from migrated PGCs as late as 13.5 dpc and to estimate the degrees of methylation of their imprinted genes as well as of the non-imprinted locus, Oct4, using an accurate and quantitative method of measurement. We established five independent EGC lines from post migratory PGCs of 11.5-13.5 dpc from C57BL/6xDBA/2 F1 hybrid mouse fetuses. All the EGCs exhibited the typical features of pluripotent cells including hypomethylation of the Oct4 regulatory region. We examined the methylation status of three imprinted genes; Igf2, Igf2r and H19 in the five EGC lines using bisulfite genomic sequencing analysis. Igf2r was almost unmethylated in all the EGC lines irrespective of the their sex and stage of isolation; Igf2 and H19 were more methylated than Igf2r, especially in male EGCs. Moreover, EGCs derived at 13.5 dpc exhibited higher levels of DNA methylation than those from earlier stages. These results suggest that in vitro derived EGCs acquire different epigenotypes from their parental in vivo migratory PGCs, and that sex-specific de novo methylation occurs in the Igf2 and H19 genes of EGCs.     

Gonadal development and fertility of mice treated prenatally with cadmium during the early organogenesis stages

Gonadal development was studied in mouse embryos that were exposed to cadmium during the early organogenesis stages. At 13.5 days, both the male and the female embryos had small genital ridges. Fewer primordial germ cells were found in the male embryos. In both sexes, many primordial germ cells were left outside the genital ridges, presumably as a result of retarded cell migration. In 16.5-day embryos, the size of the testes and ovaries and the number of differentiating germ cells were reduced. Many germ cells degenerated during the differentiation to spermatogonia and meiotic oocytes. The perturbed gonadal development was less likely to be caused primarily by a defective hypothalamopituitary axis but was more a part of the general cadmium-induced damage. The fertility of the male offspring was impaired by the prenatal cadmium insult, but the females were apparently fertile. The epididymal spermatozoa of the cadmium-affected offspring showed a lower fertilizing capacity in vitro. The impaired fertility of the cadmium-affected mice was the result of poor gonadal growth, paucity of germ cells, and defective maturation of the gametes.     

Retinoic acid prevents germ cell mitotic arrest in mouse fetal testes

During mouse fetal development, meiosis is initiated in female germ cells only, with male germ cells undergoing mitotic arrest. Retinoic acid (RA) is degraded by Cyp26b1 in the embryonic testis but not in the ovary where it initiates the mitosis/meiosis transition. However the role of RA status in fetal germ cell proliferation has not been elucidated. As expected, using organ cultures, we observed that addition of RA in 11.5 days post-conception (dpc) testes induced Stra8 expression and meiosis. Surprisingly, in 13.5 dpc testes although RA induced Stra8 expression it did not promote meiosis. On 11.5 and 13.5 dpc, RA prevented male germ cell mitotic arrest through PI3K signaling. Therefore 13.5 dpc testes appeared as an interesting model to investigate RA effects on germ cell proliferation/differentiation independently of RA effect on the meiosis induction. At this stage, RA delayed SSEA-1 extinction, p63γ expression and DNA hypermethylation which normally occur in male mitotic arrested germ cells. In vivo, in the fetal male gonad, germ cells cease their proliferation and loose SSEA-1 earlier than in female gonad and RA administration maintained male germ cell proliferation. Lastly, inhibition of endogenous Cyp26 activity in 13.5 dpc cultured testes also prevented male germ cell mitotic arrest. Our data demonstrate that the reduction of RA levels, which occurs specifically in the male fetal gonad and was known to block meiosis initiation, is also necessary to allow the establishment of the germ cell mitotic arrest and the correct further differentiation of the fetal germ cells along the male pathway.     

Haploinsufficiency of Bcl-x leads to male-specific defects in fetal germ cells: differential regulation of germ cell apoptosis between the sexes

In germ cells, the function of which is to form the next generation, apoptotic cell death occurs during development, as in the case of somatic cells. In this study, we show that Bcl-x knockout heterozygous (Bcl-x(+/-)) mice exhibit severe defects in male germ cells during development. A substantial increase in apoptosis of male germ cells occurs at around embryonic day 13.5 (E13.5) in Bcl-x(+/-) embryos, leading to hypoplasia of postnatal testes and reduced fertility. On the other hand, female germ cells at the same stages do not show discernible differences between wild-type and Bcl-x(+/-) embryos. This phenotype of Bcl-x haploinsufficiency shows that regulation of apoptosis becomes different between the sexes at around the onset of sex differentiation. Through this study, we found that, in wild-type embryos, (1) apoptosis is much more frequent (approximately 10 times) in the male than in female germ cells, and (2) expression of Bcl-xL, but not that of Bax, is higher in female than in male germ cells, at around E13.5. Male fetal germ cells, cultured with gonadal somatic cells in vitro, showed higher frequencies of apoptosis than those cultured without gonadal somatic cells. On the other hand, in the absence of gonadal somatic cells, both male and female fetal germ cells in vitro showed similar frequencies of apoptosis to female fetal germ cells in vivo. Therefore, male germ cell apoptosis, of which the default pathway is similar to that of the female, is likely to be influenced by male gonadal environments.     

Germ cell pluripotency, premature differentiation and susceptibility to testicular teratomas in mice

Testicular teratomas result from anomalies in germ cell development during embryogenesis. In the 129 family of inbred strains of mice, teratomas initiate around embryonic day (E) 13.5 during the same developmental period in which female germ cells initiate meiosis and male germ cells enter mitotic arrest. Here, we report that three germ cell developmental abnormalities, namely continued proliferation, retention of pluripotency, and premature induction of differentiation, associate with teratoma susceptibility. Using mouse strains with low versus high teratoma incidence (129 versus 129-Chr19(MOLF/Ei)), and resistant to teratoma formation (FVB), we found that germ cell proliferation and expression of the pluripotency factor Nanog at a specific time point, E15.5, were directly related with increased tumor risk. Additionally, we discovered that genes expressed in pre-meiotic embryonic female and adult male germ cells, including cyclin D1 (Ccnd1) and stimulated by retinoic acid 8 (Stra8), were prematurely expressed in teratoma-susceptible germ cells and, in rare instances, induced entry into meiosis. As with Nanog, expression of differentiation-associated factors at a specific time point, E15.5, increased with tumor risk. Furthermore, Nanog and Ccnd1, genes with known roles in testicular cancer risk and tumorigenesis, respectively, were co-expressed in teratoma-susceptible germ cells and tumor stem cells, suggesting that retention of pluripotency and premature germ cell differentiation both contribute to tumorigenesis. Importantly, Stra8-deficient mice had an 88% decrease in teratoma incidence, providing direct evidence that premature initiation of the meiotic program contributes to tumorigenesis. These results show that deregulation of the mitotic-meiotic switch in XY germ cells contributes to teratoma initiation.     

Imprinting of mammalian male gametes is gene specific and does not occur at a single stage of differentiation

Epigenetic modifications such as DNA methylation and alterations to chromatin structure have been proposed as hallmarks of imprinting in somatic cells after fertilization. In the germ cell line, gene imprinting needs to be reset in order to transmit the correct sex-specific imprinting pattern to the next generation. The precise timing of imprint erasure and re-establishment for many genes remains to be determined and precise molecular mechanisms of genomic imprinting have not yet been fully characterized. Here, we have analysed the methylation state and DNase-I sensitivity of two genes with reciprocal genomic imprinting (U2af1-rs1 and H19 genes) in a male mouse primordial germ cell (PGC) derived cell line (EG-1), isolated post-natal spermatogonia and mature sperm cells. Our results show that establishment of imprinting of the U2af1-rs1 and H19 genes during male germ cell differentiation occurs at different stages of differentiation. Furthermore, the presence of DNase-I hypersensitive sites may constitute a molecular marker to identify alleles and subsequently acquire the appropriate methylation imprint. We propose that this molecular identifier may be present or absent for a specific gene according to the sex of the gamete.     

The induction of recessive mutations in mouse primordial germ cells with N-ethyl-N-nitrosourea

A specific-locus test was carried out to examine the mutagenic activity of N-ethyl-N-nitrosourea (ENU) on mouse primordial germ cells (PGC). Embryos of C3H/He mice were treated transplacentally with 30 or 50 mg ENU per kg of maternal body weight on day 8.5, 10.5, or 13.5 of gestation (G8.5 day, G10.5 day, or G13.5 day). Male and female mice that had been treated with ENU in embryonic stages were mated with female or male tester PW mice to detect recessive mutations induced in PGC.

ENU induced recessive mutations at a relatively high rate in PGC at these developmental stages. The most sensitive stage was G10.5 day. On G8.5 day, the induced mutation rate in males and females was not significantly different. Cluster mutations, which originate from the limited number of PGC and cell killing, were more frequently induced at an earlier developmental stage. The induced mutation rate per unit dose of ENU (1 mg/kg) was higher in G8.5 and G10.5 day PGC than in stem-cell spermatogonia. It can be concluded that mouse PGC are more sensitive than stem-cell spermatogonia to the induction of recessive mutations by ENU.     

Epigenetic reprogramming in the porcine germ line

Background  

Epigenetic reprogramming is critical for genome regulation during germ line development. Genome-wide demethylation in mouse primordial germ cells (PGC) is a unique reprogramming event essential for erasing epigenetic memory and preventing the transmission of epimutations to the next generation. In addition to DNA demethylation, PGC are subject to a major reprogramming of histone marks, and many of these changes are concurrent with a cell cycle arrest in the G2 phase. There is limited information on how well conserved these events are in mammals. Here we report on the dynamic reprogramming of DNA methylation at CpGs of imprinted loci and DNA repeats, and the global changes in H3K27me3 and H3K9me2 in the developing germ line of the domestic pig.     

In situ histone landscape of nephrogenesis

In the developing kidney, self-renewing progenitors respond to inductive signaling from the adjacent branching ureteric bud by undergoing mesenchyme-to-epithelium transition. Nascent nephrons subsequently undergo elongation, segmentation, and differentiation into a mature renal epithelium with diverse functions. Epigenetic mechanisms have been implicated in impacting cell fate decisions during nephrogenesis; however, the chromatin landscape of nephron progenitors and daughter differentiating cells are largely unknown. Here, we examined the spatiotemporal expression patterns of histone H3 methylation and histone methyltransferases in E15.5 mouse kidneys. Kidney sections were probed with antibodies against histone modifications, enzymes, and markers of progenitors and differentiation. The results revealed that: (1) nephron progenitor cells exhibit a broad histone methylation signature that comprises both “active” and “repressive” marks (H3K4me3/K9me3/K27me3/R2me2/R17me2); (2) nascent nephrons retain high H3K4me3 but show downregulation of H3K9/K27me3 and; (3) maturing epithelial tubules acquire high levels of H3K79me2/3. Consistent with respective histone marks, the H3K4 methyltransferase, Ash2l, is expressed in progenitors and nascent nephrons, whereas the H3K9/K27 methyltransferases, G9a/Ezh2, are more enriched in progenitors than nascent nephrons. We conclude that combinatorial histone signatures correlate with cell fate decisions during nephrogenesis.     

Immunohistochemical localization of the carbohydrate antigen 4C9 in the mouse embryo: a reliable marker of mouse primordial germ cells

Expression of 4C9, a Lex[Gal beta 1----4(Fuc alpha 1----3)GlcNAc] antigen, during mouse embryogenesis was studied by immunohistochemical methods. Distribution of 4C9 was similar to, but not identical with that of SSEA-1 (stage-specific embryonic antigen-1). Notably, 4C9 was detected in some of the inner cell mass cells of late blastocysts, ectoderm cells migrating from the primitive streak to the mesoderm space and primordial germ cells just formed from the migrating cells. Thus, 4C9 was considered to be continuously expressed in the cell lineage starting at the totipotent 8 cell stage and leading to primordial germ cells. While 4C9 gradually decreased from the surface of primordial germ cells after they have settled in the gonad, the antigen remained in cytoplasmic granules for some period in a sex determined manner. In male gonads, cytoplasmic granules positive for 4C9 tended to be polarized to one side of cytoplasm. The 4C9 reactive material completely disappeared from male germ cells by day 16 of gestation. In female gonads, granules scattered throughout the cytoplasm and cell surface were positive for 4C9. On day 16 of gestation the cell surface antigenicity was lost, but some cytoplasmic antigenicity still remained. As above, 4C9 is a reliable marker to study the origin, migration and differentiation of primordial germ cells, and to distinguish male and female germ cells. By immunoelectron microscopy, 4C9 was detected at the plasma membrane, the Golgi apparatus, and dense-cored vesicles in primordial germ cells on 10-11 days of gestation.     

Expression of low density lipoprotein receptor-related protein 4 (Lrp4) gene in the mouse germ cells

The low density lipoprotein receptor-related protein 4 gene (Lrp4) was identified by subtractive screening of cDNAs of the migratory primordial germ cells (PGCs) of E8.5-9.5 embryo and E3.5 blastocysts. Lrp4 is expressed in PGCs in the hindgut and the dorsal mesentery of E9.5 embryos, and in germ cells in the genital ridges of male and female E10.5-13.5 embryos. Lrp4 is also expressed in spermatogonia of the neonatal and adult testes and in the immature oocytes and follicular cells of the adult ovary. The absence of Lrp4 expression in the blastocyst, embryonic stem cells and embryonic germ cells suggests the Lrp4 is a molecular marker that distinguishes the germ cells from embryo-derived pluripotent stem cells.