Elucidating The Heterodimer Signaling Mechanism
Abstract
Heterodimeric TGF-β ligands outperform homodimers in a variety of developmental, cell culture, and therapeutic contexts; however, the mechanisms underlying this difference in signaling remain uncharacterized. Here we use dorsal-ventral axial patterning of the zebrafish embryo to interrogate the BMP2/7 heterodimer signaling mechanism. We find that the roles of the two type I receptors are subfunctionalized within the heterodimer signaling complex, with the kinase activity of Acvr1 being essential, while that of Bmpr1 is not. These results suggest that the Bmp2/7 heterodimer signals exclusively, due to its ability to recruit both Acvr1 and Bmpr1 into the same signaling complex. We also find that even though Bmpr1 kinase activity is not required for DV patterning, the Bmpr1 intracellular domain is required for this process, potentially for an uncharacterized non-kinase signaling function. We further find that the Acvr1 and Bmpr1 intracellular domains are not interchangeable, suggesting that heterodimer signaling requires specific motifs within both the Acvr1 and Bmpr1 intracellular domains. To explore the possibility that BMP2/7 heterodimers also integrate two distinct Type II receptors into the same signaling complex, we generated mutations in four of the Type II receptors and investigated the functions of five of the zebrafish BMP type II receptors, including acvr2aa, acvr2ab, acvr2ba, acvr2bb, and bmpr2b. We found that zygotic acvr2 quadruple mutants are moderately dorsalized, demonstrating that Acvr2 is involved in DV patterning. Conversely, we did not observe Bmpr2b expression in the zebrafish embryo. We did, however, find that homozygous bmpr2b mutant females are infertile, with defects in oogenesis. Cumulatively these results suggest that the BMP2/7 heterodimer assembles a complex containing Acvr1, Bmpr1, and Acvr2 during zebrafish patterning, and that Bmpr1 and Acvr1 each contribute distinct intracellular functions to this complex