Specific fibro-adipogenic progenitor cell subpopulations contribute to rotator cuff tear induced fibrosis and myosteatosis revealing novel therapeutic options

The rotator cuff is a group of four muscles in the shoulder, which aid in movement and rotation of the upper arm. Rotator cuff tears (RCTs) within tendons of these muscles are common musculoskeletal injuries, often resulting in intramuscular fat, fibrosis, and muscle atrophy. Fatty infiltration specifically correlates with high rates of retear following repair. The cellular sources and molecular cues that cause these pathologies are unknown and therefore non-surgical cell/drug therapies for RCTs do not exist. Thus, we first sought to determine the cellular source(s) and molecular underpinnings of fatty atrophy and fibrosis associated with RCTs. Using a murine model of RCTs combined with lineage tracing, we demonstrate that muscle resident Pdgfra + fibro-adipogenic progenitor cells (FAPs) are responsible for the fatty and fibrotic RCT pathologies. Utilizing sorted Pdgfra + cells from rotator cuff muscles and “deep” single cell RNA-sequencing, we identified novel Pparg + and Gfra1 + nerve-associated FAP subpopulations. Additionally, through sequencing and subsequent in vivo analysis we show a specific Dpp4 + FAP population causes RCT-induced fibrosis, while Gfra1 + nerve-associated FAPs are drivers of the RCT-induced intramuscular fat pathology. Finally, we demonstrate that RCT-induced fatty infiltration occurs at least partially via the loss of GDNF-GFRA1-RET signaling, since local treatment of murine RCTs with a small molecule RET agonist targeting the Gfra1 + FAPs reduces development of RCT-induced intramuscular fat.