Pathogenic ACVR1R206H activation by Activin A-induced receptor clustering and autophosphorylation.

Fibrodysplasia ossificans progressiva (FOP) and diffuse intrinsic pontine glioma (DIPG) are debilitating diseases that share causal mutations in ACVR1, a TGF-ß family type I receptor. ACVR1R206H is a frequent mutation in both diseases. Pathogenic signaling via the SMAD1/5 pathway is mediated by Activin A, but how the mutation triggers aberrant signaling is not known. We show that ACVR1 is essential for Activin A-mediated SMAD1/5 phosphorylation and is activated by two distinct mechanisms. Wild-type ACVR1 is activated by the Activin type I receptors, ACVR1B/C. In contrast, ACVR1R206H activation does not require upstream kinases, but is predominantly activated via Activin A-dependent receptor clustering, which induces its auto-activation. We use optogenetics and live-imaging approaches to demonstrate Activin A-induced receptor clustering and show it requires the type II receptors ACVR2A/B. Our data provide molecular mechanistic insight into the pathogenesis of FOP and DIPG by linking the causal activating genetic mutation to disrupted signaling.

Platelet-derived growth factor ß-receptor, transforming growth factor ß type I receptor, and CD44 protein modulate each other's signaling and stability.

Growth factors, such as platelet-derived growth factor BB (PDGF-BB) and transforming growth factor ß (TGFß), are key regulators of cellular functions, including proliferation, migration, and differentiation. Growth factor signaling is modulated by context-dependent cross-talk between different signaling pathways. We demonstrate in this study that PDGF-BB induces phosphorylation of Smad2, a downstream mediator of the canonical TGFß pathway, in primary dermal fibroblasts. The PDGF-BB-mediated Smad2 phosphorylation was dependent on the kinase activities of both TGFß type I receptor (TßRI) and PDGF ß-receptor (PDGFRß), and it was prevented by inhibitory antibodies against TGFß. Inhibition of the activity of the TßRI kinase greatly reduced the PDGF-BB-dependent migration in dermal fibroblasts. Moreover, we demonstrate that the receptors for PDGF-BB and TGFß interact physically in primary dermal fibroblasts and that stimulation with PDGF-BB induces internalization not only of PDGFRß but also of TßRI. In addition, silencing of PDGFRß by siRNA decreased the stability of TßRI and delayed TGFß-induced signaling. We further show that the hyaluronan receptor CD44 interacts with both PDGFRß and TßRI. Depletion of CD44 by siRNA increased signaling via PDGFRß and TßRI by stabilizing the receptor proteins. Our data suggest that cross-talk between PDGFRß and TßRI occurs in dermal fibroblasts and that CD44 negatively modulates signaling via these receptors.