Dual and opposing role of retinoic acid receptor signaling in mesenchymal stem cells for tendon ossification in mice

Heterotopic ossification is abnormal bone formation in soft tissues that occurs primarily after injury and major surgery. This condition often causes local pain and limits joint motion in the affected limb. Currently, there is no effective treatment or prophylaxis for this condition other than surgical removal of the lesion. Recent studies suggest that retinoic acid receptor (RAR) agonists are effective in suppressing heterotopic ossification in patients with Fibrodysplasia Ossificans Progressiva, a congenital disorder characterized by progressive ossification of soft tissue, by suppressing the aberrant differentiation of mesenchymal stem cells in muscle. In this study, we aimed to elucidate the potential use of RAR agonists in suppressing injury-induced ectopic tendon ossification using a mouse Achilles tenotomy model. Contrary to our initial hypothesis, administration of RAR agonists throughout the experimental period (5 weeks) accelerated ectopic tendon ossification in our model. Of note, in vitro differentiation experiments using tendon-derived mesenchymal stem cells revealed that RAR agonists play opposing roles in osteogenic and chondrogenic differentiation, promoting the former and suppressing the latter. Indeed, we found that RAR agonists suppressed tendon ossification when administered before cartilage nodule formation, but promoted it when administered after. These results suggest that RAR agonists have a dual and opposing effect on tendon ectopic ossification, depending on the duration and timing of their administration. Our data may provide a basis for further investigation of the potential use of RAR agonists in the treatment of injury-induced heterotopic ossification.