Matrix metalloproteinase-2 (MMP-2) appears to be the dominant MMP activated during skeletal muscle atrophy. However, little is known about cell-specific regulatory mechanisms of MMP-2 transcription in vivo. In this study, we used a mouse model of muscle atrophy induced by complete Achilles tendon transection. Time-dependent muscle weight loss, nuclei density reduction, and extracellular matrix degeneration were observed consistently after Achilles tendon transection. Increased MMP-2 expression was confirmed at the mRNA and protein level. Experiments using transgenic mice with a MMP-2 promoter/enhancer reporter construct demonstrated markedly increased MMP-2 promoter/enhancer activity in atrophic skeletal muscle. Tissue-specific upregulation of MMP-2 promoter activity was observed not only in myocytes, but also in blood vessels, nerve, and fascia. The transcription factors c-Jun and FosB were expressed at high levels in atrophic muscle, suggesting a role in MMP-2 upregulation. These findings show that increased MMP-2 activity in disused atrophic muscle and supporting tissues is regulated, at least in part, by increased MMP-2 promoter/enhancer activity. ß

Gelatinases are a subgroup of the family of matrix metalloproteinases, which contains two members-gelatinase A and B. These enzymes play an important role in basement membrane homeostasis. Previous studies have associated basement membrane degradation with skeletal muscle atrophy. However, the specific contribution of gelatinases to the pathobiology of muscle atrophy remains unknown. In this study we examined the specific roles of gelatinase A and B in disuse-induced skeletal muscle atrophy using knockout mice. Although both gelatinase A and B are highly upregulated in disused muscle, only gelatinase A null mice had significantly reduced muscle atrophy as compared to wildtype littermates. Type IV collagen and laminin, two major components of basement membrane, were relatively well-preserved in disused muscle in gelatinase A null mice, but not in gelatinase B null mice. These findings suggest that gelatinase A, and not gelatinase B, plays a critical role in disuse-induced skeletal muscle atrophy.