Cyp26b1 Expression in Murine Sertoli Cells Is Required to Maintain Male Germ Cells in an Undifferentiated State during Embryogenesis

Abstract

In mammals, germ cells within the developing gonad follow a sexually dimorphic pathway. Germ cells in the murine ovary enter meiotic prophase during embryogenesis, whereas germ cells in the embryonic testis arrest in G0 of mitotic cell cycle and do not enter meiosis until after birth. In mice, retinoic acid (RA) signaling has been implicated in controlling entry into meiosis in germ cells, as meiosis in male embryonic germ cells is blocked by the activity of a RA-catabolizing enzyme, CYP26B1. However, the mechanisms regulating mitotic arrest in male germ cells are not well understood. Cyp26b1 expression in the testes begins in somatic cells at embryonic day (E) 11.5, prior to mitotic arrest, and persists throughout fetal development. Here, we show that Sertoli cell-specific loss of CYP26B1 activity between E15.5 and E16.5, several days after germ cell sex determination, causes male germ cells to exit from G0, re-enter the mitotic cell cycle and initiate meiotic prophase. These results suggest that male germ cells retain the developmental potential to differentiate in meiosis until at least at E15.5. CYP26B1 in Sertoli cells acts as a masculinizing factor to arrest male germ cells in the G0 phase of the cell cycle and prevents them from entering meiosis, and thus is essential for the maintenance of the undifferentiated state of male germ cells during embryonic development.

RBPJ in mouse Sertoli cells is required for proper regulation of the testis stem cell niche

“…Data from our constitutively activated NOTCH signaling mutant established that CYP26B1 was downregulated, which correlated with premature differentiation and apoptosis of germ cells . In fact, this gain-of-function mutant exhibited a phenotype nearly identical to CYP26B1 global and Sertoli cell-specific conditional null knockout testes (Li et al, 2009;MacLean et al, 2007). In the present work, we show that Cyp26b1 was upregulated when NOTCH signaling was ablated (Fig.…”

“…Perpetual feedback between germ cells and Sertoli cells is necessary to maintain the balance between proliferation and differentiation. Although proteins expressed by Sertoli cells, such as glial cell line-derived neurotrophic factor (GDNF) (Meng et al, 2000), GJA1 (Brehm et al, 2007), SIN3A (Payne et al, 2010), ETV5 (Chen et al, 2005) and CYP26B1 (Li et al, 2009;MacLean et al, 2007), have been identified as being indispensable for the proper maintenance of male germ cells in their niche, the signaling pathways that regulate the expression of these genes in Sertoli cells have not been identified. Pioneering studies in invertebrates (Kiger et al, 2001;Kimble and Crittenden, 2007;Kitadate and Kobayashi, 2010;Spradling et al, 2001;Xie and Spradling, 2000) have underscored the importance of the gonadal germline stem cell niche and demonstrated that NOTCH signaling is crucial for either spermatogonial maintenance and proliferation (Kimble and Crittenden, 2007) or niche determination (Kitadate and Kobayashi, 2010).…”

NOTCH signaling in Sertoli cells regulates gonocyte fate

“…77,78 Abrogation of NOTCH activation by DAPT in Sertoli cells in vitro resulted in an increase in Cyp26b1 expression, indicating that NOTCH signaling downregulates the levels of this enzyme. Normally, CYP26B1 in Sertoli and Leydig cells regulates the concentration of retinoic acid by catabolizing all-trans-retinoic acid into inactive oxidized metabolites.…”

Endocrinology of the mammalian fetal testis

“…In the testis, the Sertoli cells induce differentiation of the fetal Leydig cell population (Griswold & Behringer 2009) and act to maintain the gonocytes and, depending on species, the early spermatogonia. This is initially through expression of CYP26B1, which acts to metabolize retinoic acid and, thereby, prevents the entry of cells into meiosis (Li et al 2009). Later, the Sertoli cells act to ensure survival of the germ cells through expression of factors such as kit ligand and generation of the germ cell stem cell niche (Payne et al 2010).…”