The Urokinase Plasminogen Activator: an Interesting Way to Improve Myoblast Migration Following their Transplantation

Muscle cell migration plays an important role in the incorporation of transplanted myoblasts in muscle fibers. Understanding the mechanisms underlying the high migration capacity of the C2C12 myoblast cell line may help to develop approaches to improve the migration of normal myoblasts and consequently to increase their participation to the host myofiber regeneration. We have previously shown that matrix metalloproteinases are implicated in the in vivo migration of C2C12. Here, we studied the role of urokinase plasminogen activator (uPA) in this process. The expression of uPA mRNA and the enzymatic activity of uPA were studied in both normal myoblasts and the C2C12 myoblast cell line. Reverse transcriptase polymerase chain reaction analysis showed that uPA mRNA was more strongly expressed in C2C12 cells than in normal myoblasts. The enzymatic activity of secreted uPA analyzed by casein zymography is higher in medium conditioned by C2C12 cells than in medium conditioned by normal myoblasts. Using our previously described microtube technique to assess in vivo cell migration, we showed that uPA is implicated in the in vivo migration of C2C12 cells since this migration was abrogated in the presence of aprotinin (a general serine protease inhibitor) or amiloride (a uPA-specific inhibitor). We, therefore, hypothesized that increasing endogenous uPA expression by normal myoblasts may improve their migration capacity. Since an accumulating body of evidence has shown that growth factors regulate expression of uPA in a wide range of cells, we treated normal myoblasts with several growth factors alone or in combination with components of the extracellular matrix (ECM). All stimulants tested showed a minimal to strong effect on uPA enzymatic activity as assayed by zymography analysis. The positive effect of basic fibroblast growth factor (bFGF) on uPA enzymatic activity was slightly potentiated in the presence of fibronectin. Moreover, the pretreatment and coinjection of mouse myoblasts with bFGF alone or in combination with fibronectin improved significantly their in vivo migration throughout the tibialis anterior muscle of mdx mice. These results suggest that increasing uPA expression by an appropriate combination of growth factors and ECM components constitutes a possible approach to improving the migration of myogenic cells after transplantation.