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Direct effects of retinoic acid on entry of fetal male germ cells into meiosis in mice.

ABSTRACT: In both male and female germ cells of mice, retinoic acid (RA) is a meiosis-inducing factor. In the present study, we used a germ cell culture system to examine the direct effects of RA on meiotic initiation in male germ cells at the stage when they normally enter mitotic arrest to determine the extent to which fetal male germ cells can respond to exogenous RA to alter their sex-specific pathway. Male germ cells between 13.5 and 15.5 days postcoitum (dpc) were isolated from Pou5fl-green fluorescent protein transgenic fetuses and cultured with or without RA for up to 6 days. In the absence of RA, male germ cells did not undergo DNA replication and did not enter meiosis in culture. However, in the presence of RA, male germ cells isolated at 13.5 dpc expressed Stra8 and initiated the meiotic process. The ratio of cells entering meiosis gradually decreased as cells were isolated progressively at later stages. By 15.5 dpc, isolated male germ cells lost their ability to respond to RA signaling. These cells remained dispersed as single cells and progressed along the male differentiation pathway, as evidenced by the establishment of male-specific methylation imprints regardless of the presence or absence of RA. We conclude that male germ cells maintain sexual bipotency until 14.5 dpc that can be reversed by the addition of RA. Once male germ cells enter mitotic arrest, however, they appear to be committed irreversibly to the male-specific differentiation pathway even in the presence of exogenously added RA.

SUBMITTER: Ohta K

PROVIDER: S-EPMC2994329 | biostudies-literature | 2010 Dec

REPOSITORIES: biostudies-literature

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Direct effects of retinoic acid on entry of fetal male germ cells into meiosis in mice.

Ohta Kohei K Lin Yanling Y Hogg Nathanael N Yamamoto Miyuki M Yamazaki Yukiko Y

Biology of reproduction 20100908 6

In both male and female germ cells of mice, retinoic acid (RA) is a meiosis-inducing factor. In the present study, we used a germ cell culture system to examine the direct effects of RA on meiotic initiation in male germ cells at the stage when they normally enter mitotic arrest to determine the extent to which fetal male germ cells can respond to exogenous RA to alter their sex-specific pathway. Male germ cells between 13.5 and 15.5 days postcoitum (dpc) were isolated from Pou5fl-green fluoresc ...[more]

PMID: 20826729

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Project description:Meiosis is a specialized form of cell division essential for sexual reproduction. The unique cellular events of meiosis have been extensively studied in model eukaryotes, including mammals. In mammals, the molecular and cellular changes that germ cells must undergo to transition from mitotic spermatogonia to meiotic spermatocytes remain largely undefined. Retinoic acid (RA) has been proposed as the meiosis inducing substance (MIS) required for female and male meiotic initiation. However, evidence for this presumption is based on studies of fetal gonads – meiosis is initiated in fetal ovaries but suppressed in fetal testes. Critically, the requirement for RA in meiosis has never been formally tested because germ cell differentiation and meiotic initiation in the fetal ovary are temporally inseparable. Using the postnatal mouse testis, where differentiation and meiotic initiation are temporally distinct (8.6 days apart), we test the hypothesis that RA is dispensable for initiation and progression through meiosis. Complementary in vivo experimental models were designed to address the hypothesis. In the first, “RA-sufficient” (RA-suf) model, spermatogenesis was synchronized by administration of potent and selective RA synthesis inhibitor; as a result, testes contained only undifferentiated spermatogonia. Then, the inhibitor was discontinued, and mice were given a single dose of exogenous RA to initiate spermatogonial differentiation. Germ cells completed differentiation, entered and completed meiosis, and formed haploid gametes at predicted timepoints. In the second, “RA-deficient” (RA-def) model, mice were treated as above, except the inhibitor was administered continuously. The lack of testicular RA in this model was confirmed in multiple ways, including assessment of RA-responsive gene expression, in vitro serum-free cultures, and dose-response experiments. To our surprise, germ cells in RA-def testes completed differentiation, underwent meiotic DNA replication, completed the landmark cytological processes of meiosis, and formed postmeiotic haploid spermatids, all on the predicted timeline of the RA-suf model. To define the effect of RA-deficiency on gene expression during meiotic initiation, large, highly enriched populations of premeiotic spermatogonia from RA-suf mice, and meiotic preleptotene spermatocytes from both models were FACS-isolated and subjected to bulk RNA-seq. Interestingly, analyses revealed few significant differences in expression of characteristic meiotic genes at the mRNA level. However, genes that were significantly different between RA-suf and RA-def preleptotene spermatocytes were associated with spermiogenesis functions. Taken together, our data reveal that, after initiating spermatogonial differentiation, RA is dispensable for initiation, progression through, and completion of meiosis in the male germline.

Project description:The development of mammalian fetal germ cells along oogenic or spermatogenic fate trajectories is dictated by signals from the surrounding gonadal environment. Germ cells in the fetal testis enter mitotic arrest, whilst those in the fetal ovary undergo sex-specific entry into meiosis, the initiation of which is thought to be mediated by selective exposure of fetal ovarian germ cells to mesonephros-derived retinoic acid (RA). Aspects of this model are hard to reconcile with the spatiotemporal pattern of germ cell differentiation in the human fetal ovary, however. We have therefore examined the expression of components of the RA synthesis, metabolism and signalling pathways, and their downstream effectors and inhibitors in germ cells around the time of the initiation of meiosis in the human fetal gonad. Expression of the three RA-synthesising enzymes, ALDH1A1, 2 and 3 in the fetal ovary and testis was equal to or greater than that in the mesonephros at 8-9 weeks gestation, indicating an intrinsic capacity within the gonad to synthesise RA. Using immunohistochemistry to detect RA receptors RARα, β and RXRα, we find germ cells to be the predominant target of RA signalling in the fetal human ovary, but also reveal widespread receptor nuclear localization indicative of signalling in the testis, suggesting that human fetal testicular germ cells are not efficiently shielded from RA by the action of the RA-metabolising enzyme CYP26B1. Consistent with this, expression of CYP26B1 was greater in the human fetal ovary than testis, although the sexually-dimorphic expression patterns of the germ cell-intrinsic regulators of meiotic initiation, STRA8 and NANOS2, appear conserved. Finally, we demonstrate that RA induces a two-fold increase in STRA8 expression in cultures of human fetal testis, but is not sufficient to cause widespread meiosis-associated gene expression. Together, these data indicate that while local production of RA within the fetal ovary may be important in regulating the onset of meiosis in the human fetal ovary, mechanisms other than CYP26B1-mediated metabolism of RA may exist to inhibit the entry of germ cells into meiosis in the human fetal testis.

Project description:What are the effects of experimentally manipulating meiosis signalling by addition of retinoic acid (RA) in cultured human fetal gonads?RA-treatment accelerated meiotic entry in cultured fetal ovary samples, while addition of RA resulted in a dysgenetic gonadal phenotype in fetal testis cultures.One of the first manifestations of sex differentiation is the initiation of meiosis in fetal ovaries. In contrast, meiotic entry is actively prevented in the fetal testis at this developmental time-point. It has previously been shown that RA-treatment mediates initiation of meiosis in human fetal ovary ex vivo.This was a controlled ex vivo study of human fetal gonads treated with RA in 'hanging-drop' tissue cultures. The applied experimental set-up preserves germ cell-somatic niche interactions and the investigated outcomes included tissue integrity and morphology, cell proliferation and survival and the expression of markers of meiosis and sex differentiation.Tissue from 24 first trimester human fetuses was included in this study, all from elective terminations at gestational week (GW) 7-12. Gonads were cultured for 2 weeks with and without addition of 1 µM RA. Samples were subsequently formalin-fixed and investigated by immunohistochemistry and cell counting. Proteins investigated and quantified included; octamer-binding transcription factor 4 (OCT4), transcription factor AP-2 gamma (AP2?) (embryonic germ cell markers), SRY (sex determining region Y)-box 9 (SOX9), anti-Müllerian hormone (AMH) (immature Sertoli cell markers), COUP transcription factor 2 (COUP-TFII) (marker of interstitial cells), forkhead box L2 (FOXL2) (granulosa cell marker), H2A histone family, member X (?H2AX) (meiosis marker), doublesex and mab-3 related transcription factor 1 (DMRT1) (meiosis regulator), cleaved poly ADP ribose polymerase (PARP), cleaved Caspase 3 (apoptosis markers) and Ki-67 antigen (Ki-67) (proliferation marker). Also, proliferation was determined using a 5'-bromo-2'-deoxyuridine (BrdU) incorporation assay.A novel ex vivo 'hanging-drop' culture model for human fetal gonads was successfully established. Continued proliferation of cells without signs of increased apoptosis was observed after 2 weeks of culture. In cultured fetal ovaries treated with RA, an increased number of meiotic germ cells (P < 0.05) and DMRT1-positive oogonia initiating meiosis (P < 0.05) was observed, which is in agreement with a previous study. In fetal testes, RA-treatment resulted in a decreased number of gonocytes (P < 0.05), a reduced percentage of proliferating gonocytes (P < 0.05), altered expression pattern of the somatic cell markers AMH and COUP-TFII, as well as disrupted seminiferous cord structure and testis morphology.The number of samples included in this study was relatively small due to the limited availability of human fetal tissue.The hanging-drop culture, similarly to other organ culture approaches, allows studies of germ cell-somatic niche interactions and determination of effects after manipulating specific signalling pathways. Our novel finding of disrupted fetal testis development after treatment with RA indicates that abnormal meiosis regulation can potentially cause gonadal dysgenesis. Further studies will elucidate the exact mechanisms and timing of observed effects.This work was supported in part by an ESPE Research Fellowship, sponsored by Novo Nordisk A/S to A.Jø. Additional funding for this project was obtained from The Research Council of the Capital Region of Denmark (E.R.-D.M.), The Research Fund at Rigshospitalet (A.Ju. and J.E.N.), Familien Erichssens Fund (A.Jø.), Dagmar Marshalls Fund (A.Jø.) and Aase & Ejnar Danielsens Fund (A.Jø.). The authors have no conflicts of interest.

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Direct effects of retinoic acid on entry of fetal male germ cells into meiosis in mice.

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Direct effects of retinoic acid on entry of fetal male germ cells into meiosis in mice.

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