Mesoderm-specific transcript reduces ciliary sphingomyelin levels to promote tendon stem/progenitor cells osteochondrogenesis in traumatic heterotopic ossification

Traumatic heterotopic ossification (tHO) is a prevalent musculoskeletal disorder with ectopic bone forms in soft tissues post-trauma. Despite its substantial impact on patients, which may range from local discomfort to severe functional impairments, the etiology and underlying mechanisms of tHO remain unknown, and no treatments have been demonstrated to be effective for tHO. Here, we found that ciliary Hedgehog (Hh) and PI3K-Akt signaling in tendon-derived stem/progenitor cells (TSPCs) is upregulated during tHO development, leading to the activation of GLI family zinc finger 2 (GLI2) transcription factor, which promotes mesoderm-specific transcript (MEST) gene expression. We further showed that MEST can change ciliary lipids composition and reduce the abundance of sphingomyelin in cilia, creating a positive feedback loop that amplifies ciliary Hh signaling, enhancing the osteogenesis and chondrogenesis of TSPCs and driving tHO. Cilia suppression by knocking down ciliary genes intraflagellar transport 88 (IFT88) or ADP ribosylation factor like GTPase 3 (ARL3), or using the SMO inhibitor vismodegib, restores normal TSPCs osteo-chondrogenic differentiation via downregulating Hh signaling. Inhibiting MEST expression or adding exogenous sphingomyelin in vivo also effectively alleviate tHO. These findings deepen our understanding of tHO pathogenesis and provide novel insights to develop novel therapeutic strategies against tHO.

One Sentence Summary: novel mechanisms and potential therapeutic strategies of traumatic heterotopic ossification