Duchenne muscular dystrophy (DMD) is a severe, progressive muscle-wasting disorder that leads to early death. The mdx mouse is a naturally occurring mutant model for DMD. It lacks dystrophin and displays peak muscle cell necrosis at ~28 days (D28), but in contrast to DMD, mdx mice experience muscle regeneration by D70. We hypothesized that matrix metalloproteinase-2 (MMP2) and/or MMP9 play key roles in the degeneration/regeneration phases in mdx mice. MMP2 abundance in muscle homogenates, measured by calibrated Western blotting, and activity, measured by zymogram, were lower at D70 compared with D28 in both mdx and wild-type (WT) mice. Importantly, MMP2 abundance was higher in both D28 and D70 mdx mice than in age-matched WT mice. The higher MMP2 abundance was not due to infiltrating macrophages, because MMP2 content was still higher in isolated muscle fibers where most macrophages had been removed. Prenatal supplementation with the amino acid taurine, which improved muscle strength in D28 mdx mice, produced approximately twofold lower MMP2 activity, indicating that increased MMP2 abundance is not required when muscle damage is attenuated. There was no difference in MMP9 abundance between age-matched WT and mdx mice (P > 0.05). WT mice displayed decreased MMP9 abundance as they aged. While MMP9 may have a role during age-related skeletal muscle growth, it does not appear essential for degeneration/regeneration cycles in the mdx mouse. Our findings indicate that MMP2 plays a more active role than MMP9 in the degenerative phases of muscle fibers in D28 mdx mice.
Keywords: CD68; MMP2; MMP9; age; gelatinolytic activity; mdx; taurine.