Effects of Fat Tissue on Chronic Muscle Injuries

Abstract

During aging and certain muscle diseases such as Duchenne Muscular Dystrophy (DMD), muscle mass declines and is being replaced with fat tissue in a process called fatty degeneration. However, it is unclear if intramuscular fat plays an important role in the initiation and/or progression of fatty degeneration. Recent data suggests that when the fat fraction is greater than 0.5, over half of DMD patients are no longer able to walk. Based on these observations we, therefore, hypothesize that an increase in fat could impair muscle regeneration and function in chronic injuries. In young and healthy muscles, satellite stem cells are required for regeneration. Fibro/adipogenic progenitors (FAPs), which can differentiate into fat tissue and produce fibrotic scar tissue during chronic injuries and aging, aid in the healing process of muscle injuries. While beneficial, the exact relationship between FAPs and satellite stem cells during muscle regeneration is unclear. The purpose of this project was to establish a novel mouse model enabling us to conditionally prevent the differentiation of FAPs into adipocytes to determine the functional role of fat. To prevent FAPs from turning into fat, we conditionally deleted Pparγ, the master regulator of adipogenesis, using the Cre‐LoxP system in the mouse (Pparγ cKO). To induce fat formation, I injected 50% Glycerol into the tibialis anterior muscle, which causes muscle fiber loss and replacement with adipocytes. I stained adipocytes with PERILIPIN via immunofluorescence and quantified the amount of fat that was present in Pparγ cKO and control mice to determine if fat formation was indeed repressed after loss of Pparγ. I found a strong reduction in the amount of adipocytes, which have formed 21 days after a Glycerol induced injury in Pparγ cKO compared to control mice, arguing for efficient repression of adipogenesis. I also looked at fibrosis through Sirius Red staining to determine if the decrease in fat cells resulted in an increase of FAPs developing into fibrotic scar tissue. However, I saw no significant difference in fibrotic tissue between control and Pparγ cKO mice, suggesting that the prevention of fat formation does not cause an increase in the presence of fibrotic tissue.Thus, our new mouse model successfully blocks fat formation without increasing fibrotic scar tissue. In the future, this new mouse model will allow us to determine if the prevention of fat in a chronic muscle disease model, such as the mouse model for Duchenne Muscular Dystrophy, will have a positive effect on muscle repair and function.Support or Funding InformationStart‐up grant given to Daniel Kopinke; ASPET SURF Program at the University of Florida funding to Mikayla Quigley