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Non-cell Autonomous Regionalization of Sertoli Valve Epithelia in the Terminal Segment of the Seminiferous Tubules of Mouse Testes

ABSTRACT: In mammalian testes, undifferentiated spermatogonia undergo meiotic differentiation in high retinoic acid (RA) signaling states, while their undifferentiated states are maintained by fibroblast growth factors (FGFs) and glial cell line-derived neurotrophic factor (GDNF), the major niche factors for spermatogonial stem/progenitor cells, in the basal compartment of the convoluted seminiferous tubules (STs). In the valve-like terminal end of the STs adjacent to the rete testis (RT), the Sertoli valve (SV) epithelia constitutively lack most spermatogenesis activity due in part to high GDNF expression, leading to the SV-specific enrichment of undifferentiated spermatogonia; however, the molecular and cellular characteristics of the SV epithelia are unclear. By performing a Sertoli cell ablation/replacement experiment using a mouse diphtheria toxin receptor-mediated system, we show here that the SV epithelia are non-cell autonomously specified, together with having high AKT phosphorylation, in the Sertoli cells located adjacent to the RT epithelia. RNA analyses revealed constitutively high expression of Cyp26a1, which encodes an RA-degrading enzyme, in the SV region, together with high expression of Fgf9 in the adjacent RT. Exogenous in vivo RA treatment induced meiotic differentiation of the spermatogonia located within the SV region, leading to disruption of the SV structure by the ectopic appearance of adluminal meiotic germ cells in some severely affected STs. We therefore propose that the regionalization and inactive spermatogenesis of SV epithelia are mediated in part by low RA signaling, together with high FGF9 signals in the terminal end of the STs.

ORGANISM(S): Mus musculus

PROVIDER: GSE112015 | GEO | 2021/03/19

REPOSITORIES: GEO

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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.

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Non-cell Autonomous Regionalization of Sertoli Valve Epithelia in the Terminal Segment of the Seminiferous Tubules of Mouse Testes

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