UC San Diego

Rotator cuff tears affect 1 in 5 individuals over the age of 60, leading to muscle atrophy, degeneration, fibrosis, and fatty infiltration. These changes observed in muscle are persistent regardless of treatment or surgical repair, and the progression to this irreversible degenerative muscle state is poorly understood. The purpose of this work was to investigate the temporal histological and transcriptomic behavior of biological programs after rotator cuff tear and repair in a rabbit model. We performed RNAseq at 1, 2, 4, 8, and 16 weeks after tear and 1, 2, 4, and 8 weeks after repair to identify the transcriptional profile of biological programs (i.e. inflammation, adipogenesis, apoptosis). After detecting a strong immune system signature, we evaluated the number of immune cells and markers of proliferation/cell cycle histologically. Post-tenotomy there was a clear orchestration of biological programs largely related to inflammation and energetics at early timepoints, a progression towards adipogenesis and apoptosis that subsided by 8 weeks, and a resurgence of various programs including inflammation, fibrosis, and apoptosis at 16 weeks. Post-repair there was a lack of a regenerative response to mechanical re-loading, but instead, a strong resurgence of inflammation, adipogenesis, fibrosis and apoptosis at 2 weeks that subsided by 8 weeks. Relative to the immune system, there was a significant increase in neutrophil numbers at 1 and 4 weeks post-tenotomy, but few to none post-repair. There was a significant increase in CD163+ macrophages at 2 week post-tenotomy and 1 and 2 weeks post-repair. This increase was also seen regionally post-repair, with more CD163+ (M2c) macrophages found in regions of muscle degeneration compared to regions of non-degenerated muscle at almost all timepoints (1, 2, 4 weeks). In conclusion, we identified a temporal transcriptional profile of biological programs such as inflammation, adipogenesis and fibrosis and highlighted contrasting immune system behaviors of neutrophils and CD163+ macrophages in muscle after rotator cuff tear and repair. These data provide a time-resolved framework for understanding myocyte-inflammatory cell- progenitor cell interactions that are poorly understood in muscle. Translationally, these data provide context for transcriptional and cell-based assays of human tissue that can only be procured cross-sectionally in time, and they shed light onto potential therapeutic targets that may augment muscle recovery after tendon repair in patients with rotator cuff injuries.