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1 Biomedical Engineering, Duke University, Durham, NC, USA; School of Medicine, Duke University, Durham, NC, USA
* To whom correspondence should be addressed. E-mail: gtruskey{at}duke.edu.
In this study, we examined the hypothesis that stretch-induced NO modulates the mechanical properties of skeletal muscles by upregulating the expression of talin and vinculin and inhibiting calpain-induced proteolysis, thereby stabilizing the focal contacts and the cytoskeleton. Differentiating C2C12 myotubes were subjected to a single 10% step stretch for 0-4 days. The apparent elastic modulus of the cells, Eapp, was subsequently determined by atomic force microscopy. Static stretch led to significant increases (p < 0.01) in Eapp beginning at 2 days. These increases were correlated with increases in NO activity and nNOS protein expression. The expression of talin was upregulated throughout while the expression of vinculin was significantly increased only on days 3 and 4. The addition of the nitric oxide donor, L-arginine, on the stretched cells further enhanced Eapp, NOS activity and nNOS expression, whereas the presence of the nitric oxide inhibitor, L-NAME, reversed the effects of mechanical stimulation and L-arginine. Overall, viscous dissipation, as determined by the value of hysteresis, was not significantly altered. To assess the role of vinculin and talin stability, cells treated with L-NAME showed a significant decrease in Eapp, whereas adding a calpain inhibitor abolished the effect. Thus, our results show that NO inhibition of calpain-initiated cleavage of cytoskeleton proteins was correlated with the changes in Eapp. Taken together, our data suggest that NO modulates the mechanical behavior of skeletal muscle cells through the combined action of increased talin and vinculin levels and a decrease in calpain-mediated talin proteolysis.
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