The Role of Indian Hedgehog Signaling in Tendon Response to Subacromial Impingement: Evaluation Using a Mouse Model

The underlying cellular and molecular mechanisms involved in the development of tendinopathy due to subacromial supraspinatus tendon (SST) impingement and the response to subsequent removal of impingement remain unknown.To investigate the involvement of Indian hedgehog (IHH) signaling in the development of SST tendinopathy and the subsequent healing process after the relief of subacromial impingement in a novel mouse shoulder impingement model.Controlled laboratory study.A total of 48 male wild-type C57BL/6 mice were used in this study. Supraspinatus tendinopathy was induced by inserting a microsurgical clip into the subacromial space bilaterally. Eleven mice were sacrificed at 4 weeks after surgery to establish impingement baseline; 24 mice underwent clip removal at 4 weeks after surgery and then were euthanized at 2 or 4 weeks after clip removal. Thirteen mice without surgical intervention were utilized as the control group. All SSTs were evaluated with biomechanical testing; quantitative histomorphometry after staining with hematoxylin and eosin, Alcian blue, and picrosirius red; and immunohistochemical staining (factor VIII, IHH, Patched1 [PTCH1], and glioma-associated oncogene homolog 1 [GLI1]).The mean failure force and stiffness in the 4-week impingement group decreased significantly compared with the control group (P < .001) and gradually increased at 2 and 4 weeks after clip removal. Histological analysis demonstrated increased cellularity and disorganized collagen fibers in the SST, with higher modified Bonar scores at 4 weeks, followed by gradual improvement after clip removal. The IHH-positive area and PTCH1- and GLI1-positive cell percentages significantly increased after 4 weeks of clip impingement (20.64% vs 2.06%, P < .001; 53.9% vs 28.03%, P = .016; and 30% vs 12.19%, P = .036, respectively) and continuously increased after clip removal.The authors' findings suggest that the hedgehog signaling pathway and its downstream signaling mediator and target GLI1 may play a role in the development and healing process of rotator cuff tendinopathy due to extrinsic rotator cuff impingement.This study suggests the potential for the hedgehog pathway, together with its downstream targets, as candidates for further study as potential therapeutic targets in the treatment of supraspinatus tendinopathy.