Divergent Roles of CYP26B1 and Endogenous Retinoic Acid in Mouse Fetal Gonads

Abstract

In female mammals, germ cells enter meiosis in the fetal ovaries, while in males, meiosis is prevented until postnatal development. Retinoic acid (RA) is considered the main inducer of meiotic entry, as it stimulates Stra8 which is required for the mitotic/meiotic switch. In fetal testes, the RA-degrading enzyme CYP26B1 prevents meiosis initiation. However, the role of endogenous RA in female meiosis entry has never been demonstrated in vivo. In this study, we demonstrate that some effects of RA in mouse fetal gonads are not recapitulated by the invalidation or up-regulation of CYP26B1. In organ culture of fetal testes, RA stimulates testosterone production and inhibits Sertoli cell proliferation. In the ovaries, short-term inhibition of RA-signaling does not decrease Stra8 expression. We develop a gain-of-function model to express CYP26A1 or CYP26B1. Only CYP26B1 fully prevents STRA8 induction in female germ cells, confirming its role as part of the meiotic prevention machinery. CYP26A1, a very potent RA degrading enzyme, does not impair the formation of STRA8-positive cells, but decreases Stra8 transcription. Collectively, our data reveal that CYP26B1 has other activities apart from metabolizing RA in fetal gonads and suggest a role of endogenous RA in amplifying Stra8, rather than being the initial inducer of Stra8. These findings should reactivate the quest to identify meiotic preventing or inducing substances.

Retinoic acid synthesis by ALDH1A proteins is dispensable for meiosis initiation in the mouse fetal ovary

“…Nevertheless, our results indicate that Aldh1a1-3 are not required for Stra8 expression in germ cells, suggesting that either ATRA does not phosphorylate STRA8 in vivo or STRA8 phosphorylation has no impact on its activity in germ cells. Together, our results, those from Vernet et al and Bellutti et al (25), indicate that ATRA signaling is dispensable for meiosis entry.…”

“…Thus, we conclude that ATRA signaling does not initiate but contributes to meiosis by making Stra8 expression on time. Accordingly, ectopic expression of Cyp26a1, which has the most efficient catalytic activity on ATRA degradation, does not affect meiosis entry or Stra8 expression, although Stra8 mRNA levels were reduced by ~30%, i.e., in the same range than the mRNA reduction observed in the present study (25). This indicates that ATRA does regulate Stra8 transcription maintenance rather than Stra8 initiation of transcription.…”

The role of retinoic acid in the commitment to meiosis

“…Meiosis is initiated prepubertally in the testis by RA-dependent regulatory factors, such as stimulated by RA gene 8 (STRA8); however, inactivation of RA by cytochrome P450, family 26 proteins (CYP26) has been shown to inhibit Stra8 activation and thus prevent meiotic completion. 7 19 CYP26 proteins help regulate meiosis by contributing to the degradation of RA into inactive forms, thus encouraging a delay in meiotic initiation. 7 20 There are three isoforms of the CYP26 protein found in the postnatal testis (CYP26A1, CYP26B1, and CYP26C1); however, the data largely focus on CYP26B1.…”

Identification of regulatory elements required for Stra8 expression in fetal ovarian germ cells of the mouse

“…This led to the suggestion that meiotic initiation in germ cells of the fetal ovary is independent of RA. Although we now know that another RA-synthesising enzyme, encoded by Aldh1a1, is also present and able to produce RA and induce Stra8 in the fetal ovary (Bowles et al, 2016), there have been additional recent studies that demonstrate that depletion of RA-producing enzymes, or RARs, does not completely ablate Stra8 expression, and the controversy continues (Bellutti et al, 2019;Chassot et al, 2020;Kumar et al, 2013;Raverdeau et al, 2012;Vernet et al, 2020).…”