Retinoic acid homeostasis through aldh1a2 and cyp26a1 mediates meiotic entry in Nile tilapia (Oreochromis niloticus)

Abstract

Meiosis is a process unique to the differentiation of germ cells. Retinoic acid (RA) is the key factor controlling the sex-specific timing of meiotic initiation in tetrapods; however, the role of RA in meiotic initiation in teleosts has remained unclear. In this study, the genes encoding RA synthase aldh1a2, and catabolic enzyme cyp26a1 were isolated from Nile tilapia (Oreochromis niloticus), a species without stra8. The expression of aldh1a2 was up-regulated and expression of cyp26a1 was down-regulated before the meiotic initiation in ovaries and in testes. Treatment with RA synthase inhibitor or disruption of Aldh1a2 by CRISPR/Cas9 resulted in delayed meiotic initiation, with simultaneous down-regulation of cyp26a1 and up-regulation of sycp3. By contrast, treatment with an inhibitor of RA catabolic enzyme and disruption of cyp26a1 resulted in earlier meiotic initiation, with increased expression of aldh1a2 and sycp3. Additionally, treatment of XY fish with estrogen (E2) and XX fish with fadrozole led to sex reversal and reversion of meiotic initiation. These results indicate that RA is indispensable for meiotic initiation in teleosts via a stra8 independent signaling pathway where both aldh1a2 and cyp26a1 are critical. In contrast to mammals, E2 is a major regulator of sex determination and meiotic initiation in teleosts.

Germ Cell Commitment to Oogenic Versus Spermatogenic Pathway: The Role of Retinoic Acid

“…As a result, germ cells in a fetal testis are never exposed to RA during the window of plasticity that would allow them to enter meiosis and are instead set upon the male program. While data now exist to support this model in multiple vertebrate species, although the location of RA synthesis does differ (Feng et al 2015;Piprek et al 2013;Rodriguez-Mari et al 2013;Smith Fig. 6.3 RA and CYP26B1 function in the mouse embryonic gonad.…”

“…(a) In the female urogenital ridge, RA generated by aldehyde dehydrogenase (ALDH) activity in the mesonephros drives the expression of Stra8 and the onset of meiosis in the oogonia (red circles). (b) In the male urogenital ridge, RA is still produced in the mesonephros; however, the action of CYP26B1 within the testis cords acts to degrade this RA (marked by red crosses), thereby preventing Stra8 expression and the onset of meiosis et al 2008) and the action of STRA8 does not appear to be conserved in all vertebrates (Feng et al 2015;Rodriguez-Mari et al 2013), this model has recently been challenged ) and the following sections will discuss the evidence for and against the role of RA in initiating meiosis in the developing ovary.…”

“…While there is debate regarding the action of RA in the fetal ovary, there is clear evidence that CYP26B1 is a testis-specific factor required to inhibit male germ cells from entering meiosis during fetal development, although in fish belonging to the teleost class, e.g., zebrafish and tilapia, RA degradation within the gonad relies on CYP26A1 instead of CYP26B1 (Feng et al 2015;Rodriguez-Mari et al 2013). In mice, Cyp26b1 is initially expressed in the gonads of both sexes, but by 12.5 dpc, expression is lost from the ovary and becomes specific to the Sertoli cells of the fetal testis (Bowles et al 2006).…”

The Reversible Sex of Gonochoristic Fish: Insights and Consequences

“…The cyp26a1 or its paralog cyp26b1 genes are considered as meiosis-inhibiting factors of the Cyp26 RA-degrading enzyme and have been shown to have an important role in the onset of germ cell meiosis in fish ovaries [Yamaguchi and Kitano, 2012;Rodríguez-Marí et al, 2013;Feng et al, 2015]. In temperature-induced males of the Japanese flounder, cyp26b1 was upregulated by cortisol, leading to a delay in germ cell meiosis [Yamaguchi and Kitano, 2012].…”

“…Changes in the onset of ovarian meiosis lead to sexreversed males, and estrogen contributes to the timing via the regulation of genes from the retinoic acid (RA) pathway [Yamaguchi and Kitano, 2012;Feng et al, 2015]. The cyp26a1 or its paralog cyp26b1 genes are considered as meiosis-inhibiting factors of the Cyp26 RA-degrading enzyme and have been shown to have an important role in the onset of germ cell meiosis in fish ovaries [Yamaguchi and Kitano, 2012;Rodríguez-Marí et al, 2013;Feng et al, 2015].…”