Pdgfra+ Non-myogenic Mesenchymal Cells in the Rotator Cuff and Specific Subpopulations' Role in Massive Rotator Cuff Tear Pathologies

Abstract

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 surgical repair. Unfortunately, the cellular sources and molecular cues that cause these pathologies are unknown and therefore non-surgical cell/drug therapies for RCTs do not exist. A group of non-myogenic mesenchymal cells in the skeletal muscle niche, Pdgfr+ fibro-adipogenic progenitor cells (FAPs), have been hypothesized to play a role in these RCT pathologies. Interestingly, FAPs in other skeletal muscle groups are heterogeneous with some subpopulations having specific functional roles—yet subpopulation and/or functional assessment of FAPs in the rotator cuff is unstudied.This thesis utilizes lineage tracing, “deep” single cell RNA sequencing, immunofluorescence, and in vivo drug treatment to evaluate FAPs responsibility in massive RCT pathologies using a mouse model of RCT injury. The analysis identified subpopulations of FAPs in the rotator cuff and detailed individual subpopulations’ roles in specific RCT pathologies. Additionally, the depth of our single cell RNA sequencing data allowed mechanistic interrogation and in vivo drug treatments revealed for the first time, a signaling pathway within a specific subpopulation of FAPs causing their pathogenic differentiation. Separate unpublished work uses bulk RNA sequencing, genetic knock-out model development, and drug treatment in a mouse model of RCT injury to evaluate a specific FAP-produced molecule. This analysis finds the molecule can be targeted via neutralizing antibody or genetic knock-out to reduce RCT pathologies. Taken altogether, this thesis explores FAPs in the rotator cuff and advances the field’s understanding of the cellular and molecular causes of massive RCT pathologies.