Summary

Muscle ERRγ activation mitigates muscle fibrosis and fatty infiltration after rotator cuff injury

Abstract

Rotator cuff (RC) pathology is a common, age-related degenerative musculoskeletal disorder that occurs in more than 30% of individuals over age 60. RC tears are associated with pathology extending beyond tendon damage, leading to degenerative changes in the RC muscles, including atrophy, fatty infiltration, and fibrosis. Our laboratory and others have shown that promoting angiogenesis by delivering angiogenic factors can significantly improve muscle healing after injury. Our team has identified that Estrogen-Related Receptor gamma (ERRγ) is a crucial regulator of paracrine angiogenesis in skeletal muscle. Selective over-expression of ERRγ in skeletal muscle (Err-gamma transgenic mice, TG) activates a robust paracrine angiogenic gene program involving myofibrillar induction and secretion of a battery of angiogenic factors resulting in muscle vascularization. This study aims to determine if ERRγ-driven muscle angiogenesis can reduce muscle fibrosis and fatty infiltration after RC injury by comparing TG mice to age-matched wild-type (WT) control mice using a mouse model of massive rotator cuff tears. 12-week-old male and female TG (provided by Dr. Narkar) and WT mice in different groups received supraspinatus and infraspinatus tendon transection following the published protocol. The contralateral shoulder served as a control. The mice were euthanized six weeks after injury. Histological analysis and immune staining were used to evaluate muscle fibrosis and fatty infiltration after RC injury. The results from H&E, sirius red, and perilipin immune staining revealed a significant reduction in muscle fibrosis and fatty infiltration in the supraspinatus muscle of TG mice compared to the WT supraspinatus muscle after RC injury. It has been reported that (PDGFRα)-positive mesenchymal stem cells (MSCs) are induced after RC injury and are responsible for fatty infiltration in the muscle. Based on PDGFRα staining, we found that the number of PDGFRα+ cells was significantly higher in the WT supraspinatus muscle compared to the TG supraspinatus muscle after ACL injury. We also isolated muscle MSCs from the RC muscles of TG and WT mice two weeks after RC injury and performed adipogenic differentiation assay and oil red O staining. The result indicated that the muscle MSCs isolated from WT mice significantly increased their adipogenic potential after RC injury. Conversely, the muscle MSCs isolated from TG mice significantly decreased their adipogenic potential after RC injury. Exercise therapy is often prescribed over surgical repair for the initial treatment of rotator cuff tears and can effectively relieve symptoms and restore function. However, it is challenging for older individuals and may be less effective. The TG mice is a unique exercise mimetic model since it recapitulates exercise effects such as angiogenesis, improved muscle contractility, and exercise tolerance by activating ERRγ in the skeletal muscle. In this study, we observed that the TG mice with muscle-specific ERRγ activation have the capacity to reduce muscle fibrosis and fatty infiltration after RC injury. Future development of pharmaceuticals targeting ERRγ could provide a safe and effective therapy for improving outcomes after RC injury or potentially other musculoskeletal disorders.