The time course of the adaptations of human muscle proteome to bed rest and the underlying mechanisms

Abstract

Key points• It is still debated whether an imbalance between production and removal of reactive oxygen species is a major trigger of disuse skeletal muscle atrophy in human limb muscles and what the underlying mechanisms are.• In the bed rest model of human disuse, redox imbalance, impairment of antioxidant defence systems and metabolic derangement occurred early, before vastus lateralis muscle atrophy developed, and persisted through 35 days of bed rest.• Down-regulation of PGC-1α, a master controller of muscle metabolism, and up-regulation of SREBP-1, a master controller of lipid synthesis, are likely to have triggered disuse adaptations through mitochondrial dysfunction, whereas AMP kinase, an energy sensor pathway, was unaltered.• The present and previous results on the same subjects suggest a causal link between muscle atrophy, impaired skeletal muscle metabolism, impaired whole body oxidative metabolism, and insulin sensitivity and moderate inflammation, which are major risk factors of physical inactivity related diseases.Abstract In order to get a comprehensive picture of the complex adaptations of human skeletal muscle to disuse and further the understanding of the underlying mechanisms, we participated in two bed rest campaigns, one lasting 35 days and one 24 days. In the first bed rest (BR) campaign, myofibrillar proteins, metabolic enzymes and antioxidant defence systems were found to be down-regulated both post-8 days and post-35 days BR by proteomic analysis of vastus lateralis muscle samples from nine subjects. Such profound alterations occurred early (post-8 days BR), before disuse atrophy developed, and persisted through BR (post-35 days BR). To understand the mechanisms underlying the protein adaptations observed, muscle biopsies from the second bed rest campaign (nine subjects) were used to evaluate the adaptations of master controllers of the balance between muscle protein breakdown and muscle protein synthesis (MuRF-1 and atrogin-1; Akt and p70 S6K ), of autophagy (Beclin-1, p62, LC3, bnip3, cathepsin-L), of expression of antioxidant defence systems (NRF2) and of energy metabolism (PGC-1α, SREBP-1, AMPK). The results indicate that: (i) redox imbalance and remodelling of muscle proteome occur early and persist through BR; (ii) impaired energy metabolism is an early and persistent phenomenon comprising both the oxidative and glycolytic one; (iii) although both major catabolic systems, ubiquitin proteasome and autophagy, could contribute to the progression of atrophy late into BR, a decreased protein synthesis cannot be ruled out; (iv) a decreased PGC-1α, with the concurrence of SREBP-1 up-regulation, is a likely trigger of metabolic impairment, whereas the AMPK pathway is unaltered.

Frogs and estivation: transcriptional insights into metabolism and cell survival in a natural model of extended muscle disuse

“…Similar results have been found in estivating toads (Scaphiopus couchii), indicating that endogenous regulation of antioxidants may reduce the susceptibility of muscle tissue to the effects of oxidative damage (13). In typical mammalian muscle disuse models the opposite trend has been observed, with a number of studies having documented skeletal muscle wasting despite the induction of defense mechanisms against oxidative stress (including increased Nrf2 transcription) (5,9,26). However, a more definitive understanding of the role for the Nrf2-regulated antioxidant response in estivating C. alboguttata will require additional research, as we found other cytoprotective genes (gstp1, dnajb5) regulated by Nrf2 to be suppressed.…”

“…Lipids are still likely to be an essential fuel source during periods of estivation and/or fasting in C. alboguttata, although the liver and fat bodies are the most likely organs contributing to fatty acid metabolism, rather than skeletal muscle (22). Dramatic downregulation of energy metabolism pathways have been documented in clinical models of muscle disuse (5,6). Evidence suggests there is a shift in fuel metabolism away from fat oxidation toward an increased reliance on glucose, and that fat accumulates in atrophied muscles in place of muscle protein (reviewed in Ref.…”

From physical inactivity to immobilization: Dissecting the role of oxidative stress in skeletal muscle insulin resistance and atrophy

“…Moreover, hindlimb unloading and casting caused, in skeletal muscle, an increase of LC3 ratio (LC3II/I) [54,60], a marker of autophagy activation. Data in human are scarce and less consistent [114]. All together, the results suggest that autophagy plays a role in skeletal muscle loss during immobilization.…”

Comprehensive proteome analysis of human skeletal muscle in cachexia and sarcopenia: a pilot study

“…Other proteomic studies investigated the plasticity of the muscle tissue proteome from human biopsies over a variety of clinically observed situations. Brocca and colleagues quantified proteome changes due to disuse using 2D electrophoresis and found myofibrillar proteins, metabolic enzymes, and antioxidant defense systems down‐regulated after 35 days of bed rest 19. Staunton and coworkers characterized proteome changes due to age‐dependent muscle loss and detected changes in muscle metabolism, ion handling, and the cellular stress response as a function of aging 20.…”