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INTRODUCTION: Skeletal muscle has a high regenerative capacity that is maintained by the activity of satellite cells (SCs). The extracellular matrix (ECM) is a protein scaffolding that provides structure for tissues. In skeletal muscle it is thought to play an important role in muscle repair and maintenance. Matrix metalloproteinase-9 (MMP-9) is a gelatinase involved in the reconstruction of the ECM. Recent studies have shown that MMP-9 plays a role in the recruitment of SCs in response to injury through degradation of the ECM. Cardiotoxin (CTX) is a snake venom commonly used to induce degeneration/regeneration in skeletal muscle. PURPOSE: The purpose of this study was to observe how the absence of MMP-9 influences SC activity in response to CTX injury. We hypothesized that the absence of MMP-9 would impair muscle regeneration via SC activity in the tibialis anterior (TA) muscle following CTX-induced injury. METHODS: Adult female (~3 mos of age) wild type (WT) and MMP-9 knockout (KO) mice were injected with 25 µl of 10 µM CTX along the length of the right TA muscle and allowed to recover for 3-days or 3-weeks. The left TA muscle was used as an internal control. The TA muscles were harvested bilaterally 3-days and 3-weeks post-CTX. Hematoxylin and eosin (H&E) stains were performed to observe morphological changes, and Pax7 and laminin immunofluorescence histochemistry was used to quantify SCs. RESULTS: The mean body weights were similar across groups: 3-day WT, 3-day MMP-9 KO, 3-week WT, and 3-week MMP-9 KO were 21.65±1.15, 20.35±0.46, 22.15±1.35, and 18.95±0.78g, respectively. There was no difference in the mean absolute or relative (to body weight) TA weights after 3-days post-CTX across the groups. Relative TA weight was 33% and 20% higher (p<0.05) in the WT CTX and MMP-9 KO CTX compared to their respective Con groups. H&E stains showed the presence of mononucleated cells after 3-days post-CTX and persistence of centrally located nuclei after 3-weeks post-CTX, indicative of prior damage. The average SC count in the MMP-9 KO CTX TA was 56% lower than WT CTX, but this was not statistically significant. Similarly, there was no difference in SC counts between the MMP-9 KO Con and WT Con sections. CONCLUSION: The number of satellite cells after 3-weeks CTX were greater in the WT compared to the MMP-9 KO mice. While these differences were not statistically significant, the lower TA muscle weight in the MMP-9 KO mice post-CTX could be attributed to differences in satellite cell numbers.

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