Retinoic Acid Induces Multiple Hallmarks of the Prospermatogonia-to-Spermatogonia Transition in the Neonatal Mouse1

Abstract

In mammals, most neonatal male germ cells (prospermatogonia) are quiescent and located in the center of the testis cords. In response to an unknown signal, prospermatogonia transition into spermatogonia, reenter the cell cycle, divide, and move to the periphery of the testis cords. In mice, these events occur by 3-4 days postpartum (dpp), which temporally coincides with the onset of retinoic acid (RA) signaling in the neonatal testis. RA has a pivotal role in initiating germ cell entry into meiosis in both sexes, yet little is known about the mechanisms and about cellular changes downstream of RA signaling. We examined the role of RA in mediating the prospermatogonia-to-spermatogonia transition in vivo and found 24 h of precocious RA exposure-induced germ cell changes mimicking those that occur during the endogenous transition at 3-4 dpp. These changes included: 1) spermatogonia proliferation; 2) maturation of cellular organelles; and 3), expression of markers characteristic of differentiating spermatogonia. We found that germ cell exposure to RA did not lead to cellular loss from apoptosis but rather resulted in a delay of ∼2 days in their entry into meiosis. Taken together, our results indicate that exogenous RA induces multiple hallmarks of the transition of prospermatogonia to spermatogonia prior to their entry into meiosis.

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

“…However, in contrast to the first study, the authors found no evidence of excessive programmed cell death following RA exposure (Busada et al 2014). Interestingly, both studies did find that premature exposure to RA resulted in an immediate increase in Stra8 (24 h postinjection) followed by a decrease (48 and 72 h postinjection), most likely due to the increase in Cyp26a1 and Cyp26b1 mRNAs and therefore RA degradation to counteract the effect (Busada et al 2014;Snyder et al 2011).…”

“…The second study, Busada et al (2014), exposed 1 dpp mice to RA and found that this induced premature synthesis of STRA8, KIT, and spermatogenesis-and oogenesis-specific basic helix-loop-helix 1 (SOHLH1) proteins, all markers of spermatogonial differentiation, and an increase in the number of antigen identified by monoclonal antibody Ki 67 (MKI67)-positive germ cells, suggesting increased proliferation (Busada et al 2014). However, in contrast to the first study, the authors found no evidence of excessive programmed cell death following RA exposure (Busada et al 2014).…”

Retinoid signaling controls spermatogonial differentiation by regulating expression of replication-dependent core histone genes

“…We demonstrate that impaired retinoid signaling in germ cells resulted in a complete blockage of spermatogonial differentiation. One of the major biological functions of RA is to inhibit cell proliferation (Bohnsack and Hirschi, 2004;Clagett-Dame and Knutson, 2011); however, RA is capable of stimulating cell proliferation in some type of cells such as neural crest-derived mesenchyme in the forebrain (Schneider et al, 2001) and neonatal germ cells (Busada et al, 2014). We report here that RA-induced entry into S phase of the undifferentiated spermatogonia could be crucial for spermatogonial differentiation.…”

“…Busada et al showed that RA could also utilize non-genomic pathways via PI3K/AKT/mTOR signaling to stimulate translation of mRNA for Kit during spermatogonial differentiation (Busada et al, 2014). Based on these results, we postulate that retinoid signaling might induce the entry of the undifferentiated spermatogonia into S phase through upregulation of expression of replication-dependent histone genes and Ccnd2, and subsequently promote the differentiation of undifferentiated spermatogonia into A 1 spermatogonia with RAinduced expression of Stra8 (Endo et al, 2015;Zhou et al, 2008), Sall4 (Gely-Pernot et al, 2015 and Kit (Schrans-Stassen et al, 1999;Busada et al, 2014) (Fig. 6).…”

Regulation of meiotic entry and gonadal sex differentiation in the human: normal and disrupted signaling

“…In a recent study, treatment of neonatal mice with RA resulted in increased proliferation of germ cells, thus promoting transition of fetal spermatogonia into mature spermatogonia. However, meiosis was not triggered probably due to a quick removal of RA by CYP26A1 and B1 enzymes (64). In vitamin A-depleted adult mice, significantly lower Stra8 expression was found as well as impaired meiotic progression in germ cells during the first wave of spermatogenesis (65).…”