Distribution and activation of matrix metalloproteinase-2 in skeletal muscle fibers

A substantial intracellular localization of matrix metalloproteinase 2 (MMP2) has been reported in cardiomyocytes, where it plays a role in the degradation of the contractile apparatus following ischemia-reperfusion injury. Whether MMP2 may have a similar function in skeletal muscle is unknown. This study determined that the absolute amount of MMP2 is similar in rat skeletal and cardiac muscle and human muscle (similar to 10-18 nmol/kg muscle wet wt) but is similar to 50- to 100-fold less than the amount of calpain-1. We compared mechanically skinned muscle fibers, where the extracellular matrix (ECM) is completely removed, with intact fiber segments and found that similar to 30% of total MMP2 was associated with the ECM, whereas similar to 70% was inside the muscle fibers. Concordant with whole muscle fractionation, further separation of skinned fiber segments into cytosolic, membranous, and cytoskeletal and nuclear compartments indicated that similar to 57% of the intracellular MMP2 was freely diffusible, similar to 6% was associated with the membrane, and similar to 37% was bound within the fiber. Under native zymography conditions, only 10% of MMP2 became active upon prolonged (17 h) exposure to 20 mu M Ca2+, a concentration that would fully activate calpain-1 in seconds to minutes; full activation of MMP2 would require similar to 1 mM Ca2+. Given the prevalence of intracellular MMP2 in skeletal muscle, it is necessary to investigate its function using physiological conditions, including isolation of any potential functional relevance of MMP2 from that of the abundant protease calpain-1.