Specific non-myogenic mesenchymal cells contribute to rotator cuff tear 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 massive RCTs. Using a murine model of massive RCTs combined with lineage tracing, we demonstrate that muscle resident Pdgfra+ non-myogenic mesenchymal cells (NMMCs) 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 a specific Dpp4+ cell population associated with RCT-induced fibrosis, while Gfra1+ nerve-associated NMMCs 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 reduces development of the RCT-induced intramuscular fat.