Activin B participates in ovarian development in Japanese eel (Anguilla japonica) via coordinated regulation of reproductive signaling and metabolic pathways.

The Japanese eel (Anguilla japonica) is a commercially valuable catadromous species widely cultured in East Asia. However, large-scale artificial propagation remains challenging. Activin B, a dimeric protein belonging to the transforming growth factor-β (TGF-β) superfamily, plays key roles in vertebrate gonadal development, yet its function in eel ovarian development has not been characterized. In this study, we cloned the inhbb gene from Japanese eel, expressed its recombinant Activin B protein, and evaluated its effects on ovarian development under artificial induction conditions. Adult females induced into vitellogenesis with human chorionic gonadotropin (hCG) were administered recombinant Activin B or saline for eight weeks. Integrated analyses combining serum hormone measurements, transcriptomics, and untargeted metabolomics revealed that Activin B significantly elevated estradiol levels and regulated multiple reproductive signaling pathways-including Smad, MAPK, GnRH, insulin, VEGF, and FoxO-as well as key metabolic processes related to steroid, lipid, and amino acid metabolism. These findings highlight the endocrine-metabolic coordination of Activin B in ovarian development, which could offer a theoretical basis for artificial reproduction in Japanese eel.