THE ROLE OF MMP-9 IN MUSCLE REGENERATION POST CARDIOTOXIN-INDUCED INJURY

K. Hundemer University of Puget Sound, Tacoma, WA INTRODUCTION: Skeletal muscle makes up about forty percent of an adult human’s body mass and is one of the most adaptive tissues in the body. Repair and remodeling of skeletal muscle is largely mediated by the activity of muscle-specific stem cells know as satellite cells. Recent studies also show that muscle repair and regeneration are dependent on the coordinated remodeling of the extra cellular matrix (ECM) via a family of zinc-dependent enzymes known as matrix metalloproteinases (MMPs). In particular MMP-9, whose primary substrate is collagen IV, is critical for remodeling the ECM during growth and repair. The activation of MMP-9 during muscle remodeling has been associated with the activation of satellite cells, but the exact mechanism by which this occurs remains largely unknown. The purpose of this study was to further determine the role of MMP-9 in regeneration of skeletal muscle at 1- and 2-weeks post cardiotoxin (CTX) injury of the tibialis anterior (TA) muscle. METHODS: The left TA muscles of adult wild type (WT) and MMP-9 knockout (KO) mice at the age of 3 months (n=2-4 mice per group) were injected with 25 μl of 10 μM CTX and allowed to recover for 1- or 2-weeks. The left TA muscles were used as an internal control. Pax7 and laminin immunofluorescence histochemistry was used to quantify satellite cells, and RT-PCR was used to determine the relative MMP-9 and MMP-2 mRNA expression in WT and MMP-9 KO mice at each time point. RESULTS: The mean body weights were similar across groups: 1-week WT, 2-week WT, 1-week MMP-9 KO, and 2-week MMP-9 KO were 18.67g±0.16, 19.72±0.25, 22.1±1.2, and 23.3±1.52 g, respectively. Hematoxylin and eosin stains showed that after 2-weeks CTX-injection, complete TA regeneration had not occurred in either the WT or KO mice. In the 1-week WT non-injured TA muscles (n=4) there was an average of 12 satellite cells per every 200 muscle fibers, while the WT CTX-injected TA muscle had an average of 44 satellite cells. The 2-week WT non-injured muscle (n=2) had an average of 17 satellite cells for every 200 muscle fibers, while the WT CTX-injected TA muscle had an average of 66 satellite cells. MMP-2 mRNA expression was significantly higher than the respective non-injected contralateral controls after 1-week CTX-injection in both WT and MMP-9 KO mice, but it was not different between groups. CONCLUSION: There appear to be greater number of satellite cells after 2-weeks CTX compared to 1-week in the WT mice. MMP-2 mRNA expression was not significantly different between the WT and MMP-9 KO mice at any time point, suggesting that MMP-2 does not compensate for the absence of MMP-9