Evaluation of SS-31 as a Potential Strategy for Tendinopathy Treatment: An In Vitro Model
Authors: Zhang Xueying , Zhang Ying , Zhang Meng , Nakagawa Yusuke , Caballo Camila B. , Szeto Hazel H. , Deng Xiang-Hua , Rodeo Scott A. 
Published: July 2022

Background: Studies in our laboratory have demonstrated mitochondrial dysfunction in human and animal models of supraspinatus tendinopathy. SS-31 (elamipretide) has been reported to improve mitochondrial function and to be effective in clinical trials for several diseases. The potential of SS-31 in treating tendinopathy has not been explored. Hypothesis: SS-31 would improve mitochondrial function in human tenocytes sampled from patients with tendinopathy. Study Design: Controlled laboratory study. Methods: Healthy tenocytes were obtained from normal hamstring tendon biopsy specimens in 9 patients undergoing anterior cruciate ligament reconstruction, and tenocytes were collected from degenerative supraspinatus tendon biopsy specimens in 9 patients undergoing rotator cuff repair. Tenocytes were cultured, used at passage 1, and assigned to 4 groups: healthy tenocytes, healthy tenocytes with 1μM SS-31 treatment for 72 hours, degenerative tenocytes, and degenerative tenocytes with 1μM SS-31 treatment for 72 hours. The outcomes included measurements of mitochondrial potential, mitochondrial morphology by transmission electron microscopy imaging, reactive oxygen species and superoxidative dismutase activity, gene expression, and cell viability. Results: An increase in the cell fraction with depolarized mitochondria was found in degenerative tenocytes ( P = .014), followed by a decrease after SS-31 treatment ( P = .018). Transmission electron microscopy images demonstrated morphological changes with a decreased number and size of mitochondria per cell in the degenerative tenocytes ( P = .018) and with improvement after SS-31 treatment. There was no significant difference in the level of reactive oxygen species between healthy and degenerative tenocytes in culture, but superoxidative dismutase activity was significantly decreased in the degenerative group ( P = .006), which then increased after SS-31 treatment ( P = .012). These findings suggested that mitochondrial dysfunction may be reversed by SS-31 treatment. The gene expression of matrix metalloproteinase-1 (matrix remodeling, P = .029) and fatty acid–binding protein 4 (fatty infiltration, P = .046) was significantly upregulated in the degenerative tenocytes and reduced by SS-31 treatment ( P = .048; P = .007). Gene expression for hypoxia-inducible factor1 α and the proapoptotic regulator Bcl-2–associated X protein was increased in the degenerative tenocytes. There was a significant decrease in cell viability in degenerative tenocytes as compared with the healthy tenocytes, with small improvement after treatment with SS-31. Conclusion: There are changes in mitochondrial structure and function in tenocytes derived from degenerative tendons, and SS-31, as a mitochondrial protectant, could improve mitochondrial function and promote the healing of tendinopathy. Clinical Relevance: Mitochondrial dysfunction appears to play a role in the development of tendinopathy, and SS-31, as a mitochondrial protective agent, may be a therapeutic agent in the treatment of tendinopathy.