TGFβ superfamily ligands greatly outnumber their receptors. Thus, receptors are shared between ligands and individual ligands can bind multiple receptors. Bone morphogenetic proteins (BMPs) bind and signal via both BMP type II (BMPR2) and activin type II (ACVR2) receptors. We hypothesized that, in addition to its canonical receptor ACVR2, activin A might similarly bind and signal via BMPR2. First, using surface plasmon resonance, we showed that activin A binds to the BMPR2 extracellular domain (ECD), though with lower affinity compared to the ACVR2-ECD. We confirmed these results in cells, where radiolabeled activin A bound to ACVR2 and BMPR2, but not to other type II receptors (AMHR2 or TGFBR2). Using homology modeling and site-directed mutagenesis, we identified key residues in BMPR2 that mediate its interaction with activin A. The soluble ECDs of ACVR2 or BMPR2 dose-dependently inhibited activin A-, but not TGFβ-induced signaling in cells, suggesting that activin binding to BMPR2 could have functional consequences. To address this idea, we altered BMPR2 expression levels in immortalized murine gonadotrope-like cells, LβT2, in which activins potently stimulate follicle-stimulating hormone β (Fshb) subunit transcription. BMPR2 expression potentiated activin A responses whereas depletion of endogenous BMPR2 with short interfering RNAs attenuated activin A-stimulated Fshb transcription. Additional data suggest, for the first time, that BMPR2 may form functional complexes with the canonical activin type I receptor, activin receptor-like kinase 4. Collectively, our data show that BMPR2, along with ACVR2, functions as a bona fide activin type II receptor in gonadotrope-like cells, thereby broadening our understanding of mechanisms of activin action.
Keywords: ACVR2; ALK; AMHR2; Activin; BMP; BMP type II receptor; BMPR2; ECD; FSH; FST; Follicle-stimulating hormone; GnRH; Gonadotrope; MBP; Pituitary; RU; SPR; Signaling; TGFBR2; TGFβ; TGFβ type II receptor; activin receptor-like kinase; activin type II receptor; anti-Müllerian hormone type II receptor; bone morphogenetic protein; extracellular domain; follicle-stimulating hormone; follistatin; gonadotropin-releasing hormone; maltose-binding protein; resonance unit or response unit; surface plasmon resonance; transforming growth factor β.