Aberrant ciliogenesis induced by enhanced BMP signaling causes heterotopic ossification
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Bone morphogenetic protein (BMP) signaling is a principal driver of heterotopic ossification (HO), yet how aberrant BMP activity structurally reprograms cellular signaling machinery to develop HO remains unclear. Here, we identify BMP signaling as a direct upstream regulator of ciliogenesis that coordinates a multi-stage, pro-osteochondrogenic signaling relay during HO. Using a conditional gain-of-function BMP mouse model ( Acvr1 Q207D/+ ), we demonstrate that enhanced BMP signaling promotes primary cilium biogenesis and axonemal elongation through canonical Smad1/5/9-dependent transcriptional activation of intraflagellar transport (IFT) Ift20 , a core component of the IFT machinery. Rather than operating via a singular downstream cascade, these elongated cilia establish a sensitized signaling hub. Genetic disruption of ciliary Hedgehog (Hh) transduction via Smoothened ( Smo ) deletion reveals that ciliary Hh signaling is dispensable for initial tissue condemnation but required for the subsequent proliferative expansion and maturation of HO. Conversely, complete genetic ablation of the ciliary structure via Ift20 deletion, or early pharmacological inhibition of ciliogenesis, significantly attenuates HO. Notably, this BMP-IFT20-cilia axis is functionally conserved within injury-responsive, PDGFRα-positive fibro-adipogenic progenitor (FAP) populations harboring the clinically authentic Acvr1 R206H/+ mutation responsible for fibrodysplasia ossificans progressiva (FOP) in mice. Together, these findings reveal that BMP signaling drives HO by structurally expanding the primary cilium, establishing a novel mechanism for HO development.
Significance
Growth factor signaling instructs cellular behavior during tissue regeneration, but how they regulate cellular organelles to induce pathological fates remains poorly understood. This study reveals that Bone Morphogenetic Protein (BMP) signaling functions as a direct architectural regulator of the primary cilium, a critical cellular antenna. We show that BMP signaling directly transactivates intraflagellar transport machinery to structurally elongate the cilium, creating a sensitized signaling hub that drives heterotopic ossification. Our findings introduce a novel BMP-driven organelle regulation mechanism and establish a targetable cellular vulnerability to mitigate ectopic bone formation.
