Genetic and Functional Characterization of AMH Signaling in Zebrafish - Evidence for Roles of Amh-Bmpr2a-Bmpr1bb Pathway in Controlling Gonadal Homeostasis

Anti-Mullerian hormone (AMH), a member of the TGF-{beta} superfamily, plays a crucial role in Mullerian duct regression in mammals. AMH signals through a specific type II receptor (AMHRII) and a type I receptor to activate the downstream Smad1/5/9 signaling pathway. Interestingly, non-mammalian vertebrates, including teleosts, also express AMH despite lacking Mullerian ducts. Accumulating evidence indicates that AMH influences gonadal development and function across vertebrates. Intriguingly, zebrafish, a popular model organism, possesses AMH (Amh/amh) but lacks specific type II receptor (Amhr2). Based on our previous studies and data from others, we propose that Amh may signal through a novel pathway in zebrafish involving the BMP type II receptor Bmpr2a and type I receptor Bmpr1bb. In this study, we provide genetic and functional evidence for the existence of the Amh-Bmpr2a-Bmpr1bb signaling pathway in zebrafish and its role in controlling gonadal homeostasis. Our experimental data excluded the participation of Bmpr2b and Bmpr1ba, paralogues of Bmpr2a and Bmpr1bb respectively, in Amh signaling. Additionally, we also provide genetic evidence that the phenotypes exhibited by amh, bmpr2a, and bmpr1bb mutants, i.e., gonadal hypertrophy, were all dependent on gonadotropin signaling, and that the two gonadotropins (FSH and LH) showed similar potency in driving the development of hypertrophic gonadal growth resulting from dysfunction in the Amh-Bmpr2a-Bmpr1bb signaling pathway. In summary, the present study provides comprehensive evidence for Amh signaling through Bmpr2a-Bmpr1bb pathway and its interplay with gonadotropins in controlling germ cell proliferation and differentiation.