Distinct roles of retinoic acid and BMP4 pathways in the formation of chicken primordial germ cells and spermatogonial stem cells.

This study demonstrated different effects of bone morphogenetic protein 4 (BMP4) and retinoic acid (RA) signaling on the induction of germ cell formation in chickens. In vitro, BMP4 significantly promoted primordial germ cell (PGC) formation, while RA promoted spermatogonial stem cell (SSC) formation. Hematoxylin-Eosin (HE) staining of reproductive ridge and testicular slices showed that BMP4 signaling was activated during PGC formation but was inhibited during PGC differentiation into SSC. In contrast, RA signaling was significantly activated during PGC differentiation to SSC. Mechanistically, elevated expression of phosphorylated mothers against decapentaplegic homolog 5 (p-Smad5) activated BMP4 signaling, while inhibition of p-Smad5 significantly reduced the PGC formation. Additionally, BMP4 regulated the PGC formation through histone acetylation and DNA methylation in deleted in azoospermia-like (DAZL) gene. Luciferase report showed RA binding to RARα regulated stimulated by RA 8 (Stra8) promoter activity during SSC formation, while mutations in RAR binding sites inhibited the Stra8 expression and SSC formation. Further, both HAT and HDAC regulated the RARα isoform, and HAT binding to RARα activated the Stra8 transcription. RNA-seq of embryonic stem cells (ESC), PGC, and SSC showed inverse expression of genes related to the BMP4 and RA pathways during PGC and SSC formation. Additionally, Smad5 and Smurf were critical for the interactions between the two pathways. Specifically, through Smurf promotion of Smad5 ubiquitination, RA could inhibit the BMP4 signal transduction. In conclusion, the BMP4 and RA signaling pathways play opposing roles in germ cell formation, driven by epigenetic processes such as phosphorylation, ubiquitination, and histone acetylation.