Muscle wasting

Differential activation of the calpain system involved in individualized adaptation of different fast-twitch muscles in hibernating Daurian ground squirrels

We examined the lateral gastrocnemius (LG), plantaris (PL), and extensor digitorum longus (EDL) muscles to determine whether differential activation of the calpain system is related to the degree of atrophy in these fast-twitch skeletal muscles during hibernation in Daurian ground squirrels (Spermophilus dauricus). Results from morphological indices showed various degrees of atrophy in the order LG > PL > EDL. Furthermore, all three muscles underwent fast-to-slow fiber-type conversion in hibernation. In regard to the calpain system in the LG muscle, cytosolic Ca²⁺ increased significantly in hibernation, followed by recovery in posthibernation. Furthermore, calpastatin expression significantly decreased, and calpain 1 and 2 expression significantly increased, which may be responsible for the increased degradation of desmin during hibernation compared with that during summer activity. In the EDL muscle, Ca²⁺ overload was observed during interbout arousal, and calpastatin showed an increase during hibernation and interbout arousal, which could explain the increased levels of troponin T during both periods compared with levels during summer activity. These findings suggest that cytosolic Ca²⁺ overload and subsequent calpain 1 and 2 activation may be an important mechanism of LG muscle atrophy during hibernation. Cytosolic Ca²⁺ homeostasis and high expression of calpain inhibitor calpastatin during hibernation may also be an important mechanism for the EDL muscle to maintain muscle mass. Thus, the differential activation of the calpain system and selective degradation of downstream substrates may be involved in muscle atrophy of different fast-twitch muscles during hibernation.NEW & NOTEWORTHY We found that the extent of both muscle atrophy and calpain system activation differed in fast-twitch lateral gastrocnemius (LG), plantaris (PL), and extensor digitorum longus (EDL) skeletal muscles in hibernating Daurian ground squirrels, but similar hierarchies in the order of LG > PL > EDL. The differential activation of the calpain system and selective degradation of downstream substrates may be involved in muscle atrophy in different fast-twitch muscles during hibernation.

Can muscle protein metabolism be specifically targeted by exercise training in COPD?

Patients with stable chronic obstructive pulmonary disease (COPD) frequently exhibit unintentional accentuated peripheral muscle loss and dysfunction. Skeletal muscle mass in these patients is a strong independent predictor of morbidity and mortality. Factors including protein anabolism/catabolism imbalance, hypoxia, physical inactivity, inflammation, and oxidative stress are involved in the initiation and progression of muscle wasting in these patients. Exercise training remains the most powerful intervention for reversing, in part, muscle wasting in COPD. Independently of the status of systemic or local muscle inflammation, rehabilitative exercise training induces up-regulation of key factors governing skeletal muscle hypertrophy and regeneration. However, COPD patients presenting similar degrees of lung dysfunction do not respond alike to a given rehabilitative exercise stimulus. In addition, a proportion of patients experience limited clinical outcomes, even when exercise training has been adequately performed. Consistently, several reports provide evidence that the muscles of COPD patients present training-induced myogenic activity limitation as exercise training induces a limited number of differentially expressed genes, which are mostly associated with protein degradation. This review summarises the nature of muscle adaptations induced by exercise training, promoted both by changes in the expression of contractile proteins and their function typically controlled by intracellular signalling and transcriptional responses. Rehabilitative exercise training in COPD patients stimulates skeletal muscle mechanosensitive signalling pathways for protein accretion and its regulation during muscle contraction. Exercise training also induces synthesis of myogenic proteins by which COPD skeletal muscle promotes hypertrophy leading to fusion of myogenic cells to the myofiber. Understanding of the biological mechanisms that regulate exercise training-induced muscle growth and regeneration is necessary for implementing therapeutic strategies specifically targeting myogenesis and hypertrophy in these patients.

Rhabdomyolysis

Rhabdomyolysis is the dissolution of skeletal muscle. Breakdown of the myocyte causes the extravasation of intracellular constituents, leading to a clinical syndrome--the classic triad of brown urine, muscle pain, and weakness. The pathogenesis of rhabdomyolysis is presented.

Calpains in muscle wasting

Calpains are intracellular nonlysosomal Ca(2+)-regulated cysteine proteases. They mediate regulatory cleavages of specific substrates in a large number of processes during the differentiation, life and death of the cell. The purpose of this review is to synthesize our current understanding of the participation of calpains in muscle atrophy. Muscle tissue expresses mainly three different calpains: the ubiquitous calpains and calpain 3. The participation of the ubiquitous calpains in the initial degradation of myofibrillar proteins occurring in muscle atrophy as well as in the necrosis process accompanying muscular dystrophies has been well characterized. Inactivating mutations in the calpain 3 gene are responsible for limb-girdle muscular dystrophy type 2A and calpain 3 has been found to be downregulated in different atrophic situations, suggesting that it has to be absent for the atrophy to occur. The fact that similar regulations of calpain activities occur during exercise as well as in atrophy led us to propose that the calpains control cytoskeletal modifications needed for muscle plasticity.