UC Davis

Fibro-adipogenic progenitors (FAPs) are the main source of extracellular matrix (ECM) deposition in skeletal muscle fibrosis. Fibrosis is the pathological accumulation of ECM, which impairs tissue function resulting in reduced muscle strength and increased passive stiffness. The factors that drive FAPs to develop a fibrotic phenotype are not well understood. The goal of this dissertation was to understand how the ECM and FAPs interact in fibrosis and contribute to fibrotic development. Particularly, we proposed there is a pro-fibrotic feedback loop between FAPs and the ECM leading to the progressive degeneration of muscle. We hypothesized that as fibrotic muscle becomes stiffer this leads to fibrotic myofibroblast activation in FAPs, and this myofibroblast activation in FAPs results in further fibrotic ECM deposition, impairing muscle regeneration and furthering fibrosis. To investigate this loop, first we looked at the effect of the ECM on FAP activation into myofibroblasts. We demonstrated that increased stiffness, physiologically relevant to fibrotic skeletal muscle, leads to increased myofibroblast activation in FAPs. We found that this activation could be reduced with pharmacological intervention using a drug called verteporfin to block the yes-associated protein (YAP) transcription pathway involved in mechanosensing. Next, we investigated the effect myofibroblast activation of FAPs has on ECM deposition and muscle satellite cell (MuSC) differentiation. We found that FAPs derived from a fibrotic environment or activated into myofibroblasts produce ECM that is more fibrotic, in terms of collagen deposition and collagen fiber size. Verteporfin significantly reduced collagen production in FAPs. Myogenesis was impaired by myofibroblast produced soluble factors and by FAP-produced ECM on fibrotic-like stiffnesses. Lastly, we applied what we found to human studies to investigate the effect of fibrotic ECM stiffness on FAPs and MuSCs derived from children with cerebral palsy (CP). We found that CP MuSCs had impaired differentiation and increased myonuclear clustering with increasing stiffness. The clustering could be slightly reduced with verteporfin treatment. CP FAPs were not mechanosensitive compared to their typically developing (TD) counterparts, suggesting a disruption in mechanosignaling pathways. Overall, this dissertation demonstrates a profibrotic feedback loop wherein fibrotic ECM activates FAPs into myofibroblasts, which in turn leads to further fibrotic ECM deposition and impaired myogenesis. Fibrotic FAP phenotype can be partially reduced through targeting the YAP pathway, providing a therapeutic avenue to target fibrosis across diseases.