Stimulated by retinoic acid gene 8 (STRA8) interacts with the germ cell specific bHLH factor SOHLH1 and represses c‐KIT expression in vitro

STRA8 (Stimulated by Retinoic Acid Gene 8) controls the crucial decision of germ cells to engage meiotic division up and down‐regulating genes involved in the meiotic programme. It has been proven as an amplifier of genes involved in cell cycle control and chromosome events, however, how STRA8 functions as negative regulator are not well understood. In this study, we demonstrate that STRA8 can interact with itself and with other basic Helix‐Loop‐Helix (bHLH) transcription factors through its HLH domain and that this domain is important for its ability to negatively interfere with the Ebox‐mediated transcriptional activity of bHLH transcription factors. Significantly, we show that STRA8 interacts with TCF3/E47, a class I bHLH transcription factors, and with SOHLH1, a gonadal‐specific bHLH, in male germ cells obtained from prepuberal mouse testis. We demonstrated that STRA8, indirectly, is able to exert a negative control on the SOHLH1‐dependent stimulation of c‐KIT expression in late differentiating spermatogonia and preleptotene spermatocytes. Although part of this results were obtained only ‘in vitro’, they support the notion that STRA8 interacting with different transcription factors, besides its established role as ‘amplifier’ of meiotic programme, is able to finely modulate the balance between spermatogonia proliferation, differentiation and acquisition of meiotic competence.