Fibrodysplasia ossificans progressiva (FOP) is a rare human genetic condition characterized by altered skeletal development and extraskeletal bone formation. All cases of FOP are caused by mutations in the type I bone morphogenetic protein (BMP) receptor gene ACVR1 that result in overactivation of the BMP signaling pathway. Activation of the wild‐type ACVR1 kinase requires assembly of a tetrameric type I and II BMP receptor complex followed by phosphorylation of the ACVR1 GS domain by type II BMP receptors. Previous studies showed that the FOP‐mutant ACVR1‐R206H required type II BMP receptors and presumptive glycine/serine‐rich (GS) domain phosphorylation for overactive signaling. Structural modeling of the ACVR1‐R206H mutant kinase domain supports the idea that FOP mutations alter the conformation of the GS domain, but it is unclear how this leads to overactive signaling. Here we show, using a developing zebrafish embryo BMP signaling assay, that the FOP‐mutant receptors ACVR1‐R206H and ‐G328R have reduced requirements for GS domain phosphorylatable sites to signal compared to wild‐type ACVR1. Further, ligand‐independent and ligand‐dependent signaling through the FOP‐mutant ACVR1 receptors have distinct GS domain phosphorylatable site requirements. ACVR1‐G328R showed increased GS domain serine/threonine requirements for ligand‐independent signaling compared to ACVR1‐R206H, whereas it exhibited reduced serine/threonine requirements for ligand‐dependent signaling. Remarkably, while ACVR1‐R206H does not require the type I BMP receptor partner, Bmpr1, to signal, a ligand‐dependent GS domain mutant of ACVR1‐R206H could signal independently of Bmpr1 only when Bmp7 ligand was overexpressed. Of note, unlike human ACVR1‐R206H, the zebrafish paralog Acvr1l‐R203H does not show increased signaling activity. However, in domain‐swapping studies, the human kinase domain, but not the human GS domain, was sufficient to confer overactive signaling to the Acvr1l‐R203H receptor. Together these results reflect the importance of GS domain activation and kinase domain functions in regulating ACVR1 signaling and identify mechanisms of reduced regulatory constraints conferred by FOP mutations. © 2023 American Society for Bone and Mineral Research (ASBMR).

Neural crest (NC) is a unique vertebrate cell type arising from the border of the neural plate and epidermis that gives rise to diverse tissues along the entire body axis. Roberto Mayor and colleagues have made major contributions to our understanding of NC induction, delamination, and migration. We report that a truncating mutation of the classical tumor suppressor Adenomatous Polyposis Coli (apc) disrupts craniofacial development in zebrafish larvae, with a marked reduction in the cranial neural crest (CNC) cells that contribute to mandibular and hyoid pharyngeal arches. While the mechanism is not yet clear, the altered expression of signaling molecules that guide CNC migration could underlie this phenotype. For example, apc^mcr/mcr larvae express substantially higher levels of complement c3, which Mayor and colleagues showed impairs CNC cell migration when overexpressed. However, we also observe reduction in stroma-derived factor 1 (sdf1/cxcl12), which is required for CNC migration into the head. Consistent with our previous work showing that APC directly enhances the activity of glycogen synthase kinase 3 (GSK-3) and, independently, that GSK-3 phosphorylates multiple core mRNA splicing factors, we identify 340 mRNA splicing variations in apc mutant zebrafish, including a splice variant that deletes a conserved domain in semaphorin 3f (sema3f), an axonal guidance molecule and a known regulator of CNC migration. Here, we discuss potential roles for apc in CNC development in the context of some of the seminal findings of Mayor and colleagues.