Expression of matrix metalloproteinases 2 and 9 in regenerating skeletal muscle: A study in experimentally injured and mdx muscles

Matrix metalloproteinases (MMPs) cooperatively degrade all components of th e extracellular matrix (ECM). Remodeling of ECM during skeletal muscle dege neration and regeneration suggests a tight regulation of matrix-degrading a ctivity during muscle regeneration. In this study, we investigated the expr ession of MMP-2 and MMP-9, in normal muscles and their regulation during re generation process. We further investigated their secretion by C2C12 myogen ic cell line. Two models of muscle degeneration-regeneration were used: (1) normal muscles in which necrosis was experimentally induced by cardiotoxin injection; (2) mdx muscles which exhibit recurrent signs of focal myofiber necrosis followed by successful regeneration. MMPs were studied by zymogra phy; their free activity was quantified using H-3-labeled gelatin substrate and mRNA expression was followed by Northern hybridization. Muscle degener ation-regeneration was analyzed by conventional morphological methods and i n situ hybridization was performed on muscle sections to identify the cells expressing these MMPs. Results show that MMP-2, but not MMP-9 expression, is constitutive in normal muscles. Upon injury, the active form of MMP-2 is transiently increased, whereas MMP-9 is induced within 24 h and remains pr esent for several days. Quantitative assays of free gelatinolytic activity show a progressive and steady increase that culminates at 7 days postinjury and slowly returns to normal levels. In adult mdx mice, both pro and activ e forms of MMP-2 and MMP-9) are expressed. Northern blot results support th ese findings. Zymography of C2C12-conditioned medium shows that myogenic ce lls produce MMP-2. By in situ hybridization we localized MMP-9 mRNA in infl ammatory cells and putative activated satellite cells in injured muscles. O ur data allow the correlation of the differential expression of pro and/or active forms of MMP-2 and MMP-9 with different stages of the degeneration-r egeneration process: MMP-9 expression is related to the inflammatory respon se and probably to the activation of satellite cells, whereas MMP-2 activat ion is concomitant with the regeneration of new myofibers. (C) 1999 Academi c Press.