Immobilization produces morphological, physiological, and biochemical alterations in skeletal muscle leading to muscle atrophy and long periods of recovery. Muscle atrophy during disuse results from an imbalance between protein synthesis and proteolysis but also between apoptosis and regeneration processes. This work aimed to characterize the mechanisms underlying muscle atrophy and recovery following immobilization by studying the regulation of the mitochondria-associated apoptotic and the ubiquitin-proteasome-dependent proteolytic pathways. Animals were subjected to hindlimb immobilization for 4-8 days (I4 to I8) and allowed to recover after cast removal for 10-40 days (R10 to R40). Soleus and gastrocnemius muscles atrophied from I4 to I8 to a greater extent than extensor digitorum longus and tibialis anterior muscles. Gastrocnemius muscle atrophy was first stabilized at R10 before being progressively reduced until R40. Polyubiquitinated proteins accumulated from I4, whereas the increased ubiquitination rates and chymotrypsin-like activity of the proteasome were detectable from I6 to I8. Apoptosome and caspase-3 or -9 activities increased at I6 and I8, respectively. The ubiquitin-proteasome-dependent pathway was normalized early when muscle stops to atrophy (R10). By contrast, the mitochondria-associated apoptotic pathway was first downregulated below basal levels when muscle started to recover at R15 and completely normalized at R20. Myf 5 protein levels decreased from I4 to I8 and were normalized at R10. Altogether, our results suggest a two-stage process in which the ubiquitin-proteasome pathway is rapidly up- and downregulated when muscle atrophies and recovers, respectively, whereas apoptotic processes may be involved in the late stages of atrophy and recovery.

The ubiquitin-proteasome system (UPS) is believed to degrade the major contractile skeletal muscle proteins and plays a major role in muscle wasting. Different and multiple events in the ubiquitination, deubiquitination and proteolytic machineries are responsible for the activation of the system and subsequent muscle wasting. However, other proteolytic enzymes act upstream (possibly m-calpain, cathepsin L, and/or caspase 3) and downstream (tripeptidyl-peptidase II and aminopeptidases) of the UPS, for the complete breakdown of the myofibrillar proteins into free amino acids. Recent studies have identified a few critical proteins that seem necessary for muscle wasting {i.e. the MAFbx (muscle atrophy F-box protein, also called atrogin-1) and MuRF-1 [muscle-specific RING (really interesting new gene) finger 1] ubiquitin-protein ligases}. The characterization of their signalling pathways is leading to new pharmacological approaches that can be useful to block or partially prevent muscle wasting in human patients.

The ubiquitin-proteasome system (UPS) is believed to degrade the major contractile skeletal muscle proteins and plays a major role in muscle wasting. Different and multiple events in the ubiquitination, deubiquitination and proteolytic machineries are responsible for the activation of the system and subsequent muscle wasting. However, other proteolytic enzymes act upstream (possibly m-calpain, cathepsin L, and/or caspase 3) and downstream (tripeptidyl-peptidase II and aminopeptidases) of the UPS, for the complete breakdown of the myofibrillar proteins into free amino acids. Recent studies have identified a few critical proteins that seem necessary for muscle wasting {i.e. the MAFbx (muscle atrophy F-box protein, also called atrogin-1) and MuRF-1 [muscle-specific RING (really interesting new gene) finger 1] ubiquitin-protein ligases}. The characterization of their signalling pathways is leading to new pharmacological approaches that can be useful to block or partially prevent muscle wasting in human patients.

Hepatitis C virus (HCV) is a major cause of chronic hepatitis worldwide. Studies of the early steps of HCV infection have been hampered by the lack of convenient in vitro or in vivo models. Although several cell-surface molecules that mediate the binding of HCV envelope proteins to target cells have been identified, mechanisms of viral entry into human hepatocytes are still poorly understood. Vesicular stomatitis virus/HCV pseudotyped viruses expressing the HCV envelope glycoproteins on the viral envelope were generated and it was found that their entry into human hepatocytes required co-expression of E1 and E2 on the pseudotype surface. Neutralization of pseudotype infection by anti-HCV antibodies suggested that cellular entry was mediated by HCV envelope glycoproteins and by previously characterized cell-surface molecules, including CD81. An entry assay based on the release of a fluorochrome from labelled HCV pseudotypes provided evidence for a pH-dependent fusion of the pseudotype envelope with a cellular compartment. By using a panel of endocytosis inhibitors, it is postulated that penetration of HCV into primary cultures of hepatocytes takes place by clathrin-mediated endocytosis.