Acvr2b receptors dose-dependently regulate dorsoventral patterning, FOP ACVR1-R206H signaling, and limit BMP/Nodal signaling

BMP signaling drives dorsoventral (DV) axial patterning in vertebrates and invertebrates, with BMP dimers recruiting tetrameric receptor complexes to phosphorylate SMADs that activate target gene expression. In zebrafish DV patterning, Bmp2/7 heterodimers exclusively signal, assembling a receptor complex of two distinct type I receptors, Bmpr1 and Acvr1l, that canonically bind Bmp2 and Bmp7 ligands, respectively. Here, we tested if the two distinct classes of BMP type II receptors, Bmpr2 and Acvr2, also act in the gastrula signaling complex. We mutated all four acvr2a and acvr2b genes in the zebrafish and found maternal-zygotic depletion of Acvr2b receptors causes severe embryonic dorsalization, while bmpr2 genes are not expressed in the gastrula, indicating Acvr2b is the primary type II receptor transducing the BMP signal. Further, while phosphorylated Smad5 (pSmad5) was reduced throughout the early gastrula, unexpectedly, pSmad5 was nearly abrogated at the embryonic margin, where Nodal signaling is active in specifying the mesendoderm. We found that inhibiting Nodal signaling by depleting its co-receptor, Oep, in MZ-Acvr2b deficient embryos restored marginal pSmad5 to non-marginal levels, suggesting that Nodal competes with BMP for Acvr2a receptors at the margin. Further, we find that while Bmpr2 is not expressed in the gastrula, it can signal in the absence of Acvr2b, indicating that, unlike the type I receptor classes, the type II receptor classes do not uniquely function in the Bmp2/7 signaling complex. We further demonstrate that the ACVR1-R206H Fibrodysplasia Ossificans Progressiva human disease-causing mutant receptor, ACVR1-R206H, displays an increased sensitivity to Acvr2b dosage compared to wild-type Acvr1l, indicating that Acvr2b receptor availability restricts multiple modes of signaling in the gastrula.