Beige fibro-adipogenic progenitor transplantation reduces muscle degeneration and improves function in a mouse model of delayed repair of rotator cuff tears

Background: Muscle atrophy and fatty infiltration (FI) are common occurrences following rotator cuff (RC) tears. Tears of all sizes are subject to muscle degeneration. The degree of muscle degeneration following RC tears is highly correlated with repair success and functional outcomes. We have recently discovered that muscle fibro-adipogenic progenitors (FAPs) can differentiate into uncoupling protein 1 (UCP-1)-expressing beige adipocytes and induce muscle regeneration. This study evaluated the potential of local cell transplantation of beige adipose FAPs (BAT-FAPs) to treat RC muscle degeneration in a murine model of RC repair. Methods: BAT-FAPs were isolated from muscle in UCP-1 reporter mice by flow cytometry as UCP-1(+)/ Sca1(+)/ PDGFR4(+)/CD31(+)/CD45(-)/integrin alpha 7(-). C57/BL6J mice underwent supraspinatus tendon tear with suprascapular nerve transection followed by repair 2 or 6 weeks after the initial injury. At the time of repair, mice received either no additional treatment, phosphate-buffered saline injection, or BAT-FAP injection. Functional outcomes were assessed by gait analysis. Mice were humanely killed at 6 weeks after cell transplantation. Supraspinatus muscle FI, fibrosis, muscle fiber size, and vascularity were analyzed and quantified via Image]. Analysis of variance with post hoc Tukey test and P <.05 was used to determine statistical significance. Results: Cell transplantation diminished fibrosis, FI, and atrophy and enhanced vascularization in both delayed repair models. Cell transplantation resulted in improved shoulder function as assessed with gait analysis in both the delayed repair models. Conclusions: BAT-FAPs significantly reduced muscle degeneration and improved shoulder function after RC repair. BAT-FAPs hold significant promise as a therapeutic adjunct to repair for patients with advanced RC pathology. (C) 2019 Journal of Shoulder and Elbow Surgery Board of Trustees. All rights reserved.