NOTCH signaling in Sertoli cells regulates gonocyte fate

Abstract

Gonocytes (or prospermatogonia) are the precursors to spermatogonial stem cells (SSCs), which provide the foundation for spermatogenesis through their ability to both self-renew and generate daughter cells. Despite their relative importance, the regulatory mechanisms that govern gonocyte maintenance and transition to SSCs are poorly understood. Recently, we reported that constitutive activation of NOTCH1 signaling in Sertoli cells causes gonocyte exit from quiescence--the first suggestion of the potential role of this signaling pathway in the testis. This Extra View will review what is known about NOTCH signaling, particularly in Sertoli cells and germ cells in the testes, by providing a background on germ cell biology and a summary of our recently published data on NOTCH1 signaling in Sertoli cells. We also describe additional data showing that aberrant proliferation and differentiation of gonocytes in response to constitutive activation of NOTCH1 signaling in Sertoli cells involves de novo expression of cell cycle proteins and a marked upregulation of the KIT receptor. These data further suggest that NOTCH signaling orchestrates a dynamic balance between maintenance and differentiation of gonocytes in the perinatal testis.

Keywords: CCND1; NOTCH signaling; Sertoli cell; gonocyte; mitotic arrest; testis development.

Figure 1. Aberrant germ cell proliferation and differentiation in AMH-NICD1 testes involves CCND1 and CCND3, but not CCNB3. (A–R) Immunofluorescence images of control (Rosa^NICD/+) (A–C, G–I, and M–O) and AMH-NICD1 (Amh-cre;Rosa^NICD/+) (D–F, J–L, P–R) testis sections at E15.5, showing CCNB3 (A–F; green), CCND1 (G–L; green), CCND3 (M–R; green), germ cell nuclear antigen (GCNA; A–R; red), and DAPI (A–R; blue) staining. Scale bars represent 100 µm. Higher magnification inset in upper right of each image. Immunostainings are representative of at least 6 separate gonad cross-sections, from at least 3 separate fetuses.

Figure 2. Germ cells in AMH-NICD1 testes express KIT at high levels in a subpopulation of germ cells. (A–F) Immunofluorescence images of control (Rosa^NICD/+) (A–C) and AMH-NICD1 (Amh-cre;Rosa^NICD/+) (D–F) testis sections at E15.5, showing KIT (green), germ cell nuclear antigen (GCNA; red), and DAPI (blue) staining. Scale bars represent 100 µm. Areas outlined in panels (B and E) are magnified 4 times in panels (C and F), rotated 90° clockwise and counter-clockwise, respectively. Immunostainings are representative of at least 6 separate gonad cross-sections, from at least 3 separate fetuses.

Figure 3. Model depicting the possible role of NOTCH signaling in Sertoli cells before birth. Previous studies have shown that FGF2 induces GDNF production, and that SOX9/SF1 induces CYP26B1 expression by Sertoli cells. Our data suggest NOTCH1 signaling is a negative regulator of GDNF and CYP26B1, which balances the effects of FGF2 and SOX9/SF1. This normally results in gonocyte maintenance in the quiescent, undifferentiated state. However, overactivation of NOTCH1 signaling further suppresses the expression of GDNF and CYP26B1, which induces direct differentiation into KIT-expressing proliferating spermatogonia.