Calpain activity and muscle wasting in sepsis

Abstract

Muscle wasting in sepsis reflects activation of multiple proteolytic mechanisms, including lyosomal and ubiquitin-proteasome-dependent protein breakdown. Recent studies suggest that activation of the calpain system also plays an important role in sepsis-induced muscle wasting. Perhaps the most important consequence of calpain activation in skeletal muscle during sepsis is disruption of the sarcomere, allowing for the release of myofilaments (including actin and myosin) that are subsequently ubiquitinated and degraded by the 26S proteasome. Other important consequences of calpain activation that may contribute to muscle wasting during sepsis include degradation of certain transcription factors and nuclear cofactors, activation of the 26S proteasome, and inhibition of Akt activity, allowing for downstream activation of Foxo transcription factors and GSK-3beta. The role of calpain activation in sepsis-induced muscle wasting suggests that the calpain system may be a therapeutic target in the prevention and treatment of muscle wasting during sepsis. Furthermore, because calpain activation may also be involved in muscle wasting caused by other conditions, including different muscular dystrophies and cancer, calpain inhibitors may be beneficial not only in the treatment of sepsis-induced muscle wasting but in other conditions causing muscle atrophy as well.

Fig. 1.

Summary of the role of calpain activation in sepsis-induced muscle wasting. Uptake and cellular concentrations of calcium are increased in skeletal muscle during sepsis (34). Calpains are activated by calcium (39), although other mechanisms may also contribute to calpain activation. The most important muscle wasting-related consequence of calpain activation is cleavage of myofibrillar cytsokeletal proteins resulting in disruption of the sarcomere and release of myofilaments that are subsequently ubiquitinated and degraded by the 26S proteasome (47, 52, 93, 94). In addition, calpain activation may regulate the degradation of various transcription factors involved in muscle wasting (42, 66, 111). Calpains may regulate the 26S proteasome activity (69, 92), and it is possible that calcium can activate the 26S proteasome through other mechanisms as well (69). Finally, recent studies (92) suggest that calpain activation in skeletal muscle results in inhibited Akt activity, which in turn results in activation of Foxo transcription factors and GSK-3β (stimulating muscle proteolysis) and inactivation of mammalian target of rapamycin (mTOR), inhibiting protein synthesis.