A, Combaret L. The worsening of tibialis anterior muscle atrophy during recovery post-immobilization correlates with enhanced connective tissue area, proteolysis, and apoptosis. Am J Physiol Endocrinol Metab 303: E1335-E1347, 2012. First published October 2, 2012; doi:10.1152/ajpendo.00379.2012.-Sustained muscle wasting due to immobilization leads to weakening and severe metabolic consequences. The mechanisms responsible for muscle recovery after immobilization are poorly defined. Muscle atrophy induced by immobilization worsened in the lengthened tibialis anterior (TA) muscle but not in the shortened gastrocnemius muscle. Here, we investigated some mechanisms responsible for this differential response. Adult rats were subjected to unilateral hindlimb casting for 8 days (I8). Casts were removed at I8, and animals were allowed to recover for 10 days (R1 to R10). The worsening of TA atrophy following immobilization occurred immediately after cast removal at R1 and was sustained until R10. This atrophy correlated with a decrease in type IIb myosin heavy chain (MyHC) isoform and an increase in type IIx, IIa, and I isoforms, with muscle connective tissue thickening, and with increased collagen (Col) I mRNA levels. Increased Col XII, Col IV, and Col XVIII mRNA levels during TA immobilization normalized at R6. Sustained enhanced peptidase activities of the proteasome and apoptosome activity contributed to the catabolic response during the studied recovery period. Finally, increased nuclear apoptosis prevailed only in the connective tissue compartment of the TA. Altogether, the worsening of the TA atrophy pending immediate reloading reflects a major remodeling of its fiber type properties and alterations in the structure/ composition of the extracellular compartment that may influence its elasticity/stiffness. The data suggest that sustained enhanced ubiquitin-proteasome-dependent proteolysis and apoptosis are important for these adaptations and provide some rationale for explaining the atrophy of reloaded muscles pending immobilization in a lengthened position.ubiquitin-proteasome proteolysis; caspase 9; extracellular matrix remodeling; reloading; matrix metalloproteases SKELETAL MUSCLE IS THE MAJOR RESERVOIR of body proteins. Sustained muscle wasting due to immobilization leads to weakening and severe metabolic consequences, which impacts health care costs. This wasting has been investigated thoroughly and results from an imbalance between protein synthesis and breakdown rates but also between apoptotic and regeneration processes (12,17,35,46,47). By contrast, the mechanisms responsible for muscle recovery after remobilization are poorly defined.The ubiquitin (Ub)-proteasome system (UPS) is involved in the breakdown of the major contractile proteins in catabolic conditions. Briefly, in this pathway a poly-Ub chain is covalently attached to the substrate by the Ub conjugation machinery, and the targeted protein is then recognized and degraded by the 26S proteasome (4). This pathway is activated after immobilization or ...

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