Gelatinase-B (matrix metalloproteinase-9; MMP-9) secretion is involved in the migratory phase of human and murine muscle cell cultures

The remodelling of connective tissue components is a fundamental requiremen t for a number of pivotal processes in cell biology. These may include myob last migration and fusion during development and regeneration. In other sys tems, similar biological processes are facilitated by secretion of the matr ix metalloproteinases (MMPs), especially the gelatinases. This study invest igated the activity of the gelatinases MMP-2 and 9 by zymography on cell co nditioned media in cultures of cells derived from explants of the human mas seter muscle and in the murine myoblast cell-line C2C12. Expression of MMP- 9 by western blotting and TIMP-1, the major inhibitor of MMPs, by northern blotting, during all phases of myoblast proliferation, migration, alignment and fusion, was also measured. Irrespective of the origin of the cultures, MMP-9 activity was secreted only by single cell and pre-fusion cultures wh ilst MMP-2 activity was secreted at all stages as well as by myotubes. The loss of MMP-9 activity was due to the loss of MMP-9 protein expression. TIM P-1 mRNA was not detectable at the single cell stage but its expression inc reased as cells progressed through the pre-fusion and post-fusion stages to reach a maximal in myotube containing cultures. Migration of cells derived from human masseter muscle was inhibited, using a specific anti-MMP-9 bloc king monoclonal antibody (6-6B). These data are consistent with the concept that regulation of matrix turnover via MMP-9 may be involved in the events leading to myotube formation, including migration. Loss of expression of t his enzyme and expression of TIMP-1 mRNA is associated with myotube contain ing cultures. Consequently, the ratio between MMPs and TIMPs maybe importan t in determining myoblast migration and differentiation.