EVIDENCE FOR FIBRONECTIN DEGRADATION BY CALPAIN-II

Recent work supports the hypothesis that calpain II can be exteriorize d. Indeed, this cysteine calcium-dependent proteinase was shown to be intercellularly, and, more particularly, associated to extracellular m atrix components. Thereby, calpain II could be involved in hydrolysis of pericellular matrix components such as fibronectin, which is known to play an important role in cellular differentiation. Our in vitro st udies provide evidence that fibronectin is a potential substrate for c alpain II. On cultured cells, our findings show that calpain II is abl e, on the one hand, to cleave the fibrillar network of fibronectin sec reted by fibroblasts, and, on the other, to decrease dramatically the fibronectin amount secreted by myoblasts just before fusion. Moreover, following this treatment, myoblasts become spherical due to the cleav age of this attachment factor. However, these cells, plated on an appr opriate substrate are still able to differentiate. Our results suggest that calpain II is indeed involved in myoblast fusion via the fibrone ctin cleavage since it is well established that myogenic lineages lose this glycoprotein at the time of fusion.