Mice deficient in plasminogen activator inhibitor-1 have improved skeletal muscle regeneration.

Skeletal muscle possesses a remarkable capacity for regeneration. Although the regulation of this process at the molecular level remains largely undefined, the plasminogen system appears to play a critical role. Specifically, mice deficient in either urokinase-type plasminogen activator (uPA-/- mice) or plasminogen demonstrate markedly impaired muscle regeneration after injury. In the present study, we tested the hypothesis that loss of the primary inhibitor of uPA, plasminogen activator inhibitor-1 (PAI-1), would improve muscle regeneration. Repair of the extensor digitorum longus muscle was assessed after cardiotoxin injury in wild-type, uPA-/-, and PAI-1-deficient (PAI-1-/-) mice. As expected, there was no uPA activity in the injured muscles of uPA-/- mice, and muscles from these transgenic animals demonstrated impaired regeneration. On the other hand, uPA activity was increased in injured muscle from PAI-1-/- mice to a greater extent than in wild-type controls. Furthermore, PAI-1-/- mice demonstrated increased expression of MyoD and developmental myosin after injury as well as accelerated recovery of muscle morphology, protein levels, and muscle force compared with wild-type animals. The injured muscles of PAI-1-null mice also demonstrated increased macrophage accumulation, contrasting with impaired macrophage accumulation in uPA-deficient mice. The extent of macrophage accumulation correlated with both the clearance of protein after injury and the efficiency of regeneration. Taken together, these results indicate that PAI-1 deficiency promotes muscle regeneration, and this protease inhibitor represents a therapeutic target for enhancing muscle regeneration.

HSP25 protects skeletal muscle cells against oxidative stress.

Reactive oxygen species (ROS) may cause skeletal muscle degeneration in a number of pathological conditions. Small heat shock proteins (HSPs) have been found to confer resistance against ROS in different cell types; however, the importance of their antioxidant function in skeletal muscle cells remains to be determined. In the present study, differentiation of skeletal myoblasts resulted in protection against hydrogen peroxide-induced cell death and protein oxidation. This differentiation-induced resistance to oxidative stress was associated with increased protein expression of HSP25, increased glutathione levels, and glutathione peroxidase activity, but little change in catalase activity. Overexpression of HSP25 in stably transfected myoblasts produced dose-dependent protection against hydrogen peroxide-induced damage that was associated with increased glutathione levels and glutathione peroxidase activity. Inhibition of glutathione synthesis with buthionine sulfoximine abrogated the protection induced by HSP25 overexpression. These findings indicate that HSP25 may play a key role in regulating the glutathione system and resistance to ROS in skeletal muscle cells.