Mechanisms of Fibrosis in Regeneration of Aged Skeletal Muscle

Skeletal muscle is a highly adaptive tissue that possesses the ability to regenerate following damage. Regenerative capacity of skeletal muscle declines as age advances, leading to restricted mobility and poor quality of life. The impairment of the regeneration process in aged muscle is partly due to the accumulation of structural proteins in the extracellular matrix (ECM), termed fibrosis. In this study, two important proteins regulating ECM remodeling, plasminogen activator inhibitor-1 (PAI-1) and matrix metalloproteinase-9 (MMP-9), were investigated to determine their role in modulating changes in the ECM during aged muscle regeneration. The regeneration process was studied in young (3 month old) and aged (18 month old) C56BL/6J mice at 3, 5, and 7 days following cardiotoxin-induced muscle damage. The regeneration process was significantly impaired in aged muscle as indicated by decreased muscle mass, cross-sectional fibre area, number of newly regenerated myofibres, and eMHC expression. Greater PAI-1 expression was found in aged regenerating myofibres 5 and 7 days following damage. Aged muscle displayed significantly greater extramyocellular PAI-1 acutely following damage, accumulation of collagen I, and delayed macrophage infiltration. Active MMP-9 was lower in aged muscle 3 and 5 days following damage compared to young muscle. However, MMP-9 primarily colocalized with F4/80+ macrophages in young muscle, but this colocalization was reduced in aged muscle. Taken together, this study provides a foundation for the mechanisms underlying the impairment of muscle regeneration in aged muscle