Redox homeostasis, oxidative stress and disuse muscle atrophy

Abstract

A pivotal role has been ascribed to oxidative stress in determining the imbalance between protein synthesis and degradation leading to muscle atrophy in many pathological conditions and in disuse. However, a large variability in disuse-induced alteration of redox homeostasis through muscles, models and species emerges from the literature. Whereas the causal role of oxidative stress appears well established in the mechanical ventilation model, findings are less compelling in the hindlimb unloaded mice and very limited in humans. The mere coexistence of muscle atrophy, indirect indexes of increased reactive oxygen species (ROS) production and impairment of antioxidant defence systems, in fact, does not unequivocally support a causal role of oxidative stress in the phenomenon. We hypothesise that in some muscles, models and species only, due to a large redox imbalance, the leading phenomena are activation of proteolysis and massive oxidation of proteins, which would become more susceptible to degradation. In other conditions, due to a lower extent and variable time course of ROS production, different ROS-dependent, but also -independent intracellular pathways might dominate determining the variable extent of atrophy and even dispensable protein oxidation. The ROS production and removal are complex and finely tuned phenomena. They are indeed important intracellular signals and redox balance maintains normal muscle homeostasis and can underlie either positive or negative adaptations to exercise. A precise approach to determine the levels of ROS in living cells in various conditions appears to be of paramount importance to define and support such hypotheses.

Figure 1. The impact of mechanical ventilation and antioxidant administration (Trolox) on cross-sectional areas (CSA) of different muscle fibres (type I, IIa and IIb/x)

Five groups of rats were studied: controls (Con), mechanically ventilated for 6 and 18 h without and with Trolox treatment (6 h MV, 6 h MVT, 18 h MV, 18 h MVT). (Reprinted from McClung et al. 2007 with permission from Wiley-Blackwell.) *Significantly (P < 0.05) different from control values. †Significantly (P < 0.05) different from 6 h MV values. ‡Significantly (P < 0.05) different from 6 h MVT values. §Significantly (P < 0.05) different from 18 h MV values.

Figure 2. The impact of hindlimb unloading (HU) and antioxidant administration (Trolox) in a slow, soleus (Sol), and a fast, gastrocnemius (Gas), muscle of the mouse

A, cross-sectional area of type I and IIA fibres from Sol and of type IIB fibres from Gas in control mice (CTRL), hindlimb unloaded mice for 14 days (HU-14) and hindlimb unloaded mice for 14 days treated by Trolox (HU-TRO). *Significantly different from CTRL (P < 0.05). (Redrawn from Brocca et al. (2010).) B, protein oxidation index (OI). The height of each vertical bar represents the mean ± SEM. *Significantly different from all groups (P < 0.05). (Reprinted from Brocca et al. (2010) with permission from Wiley-Blackwell.) C, differentially expressed proteins belonging to antioxidant defence systems in soleus muscles of HU mice, identified by proteomic analysis. (Redrawn from Brocca et al. 2010.) D, malondialdeyde (MDA) levels following 14 days HU plotted against the mean percentage of muscle-to-body weight ratio decrease for EDL, tibialis anterior (TA), Gas and Sol muscles of the mice. The points were linearly correlated (r²= 0.94). (Reprinted from Desaphy et al. 2010 with permission from Elsevier.)

Figure 3. The impact of 3 days hindlimb unloading (HU-3) on cross-sectional area (CSA) of identified types of muscle fibres (A) and on protein oxidation index (OI) (B) of soleus (Sol) and gastrocnemius (Gas) muscles of the mouse

The number (n) of fibres measured is indicated above each bar. Both CSA and OI were determined exactly as by Brocca et al. (2010). *Significantly different from CTRL (P < 0.05). Four mice were used.

Figure 4. The impact of 8 (T8) and 35 (T35) days of bed rest on cross-sectional area (CSA) of muscle fibres and on protein oxidation (Oxy/RP) of muscle samples from the vastus lateralis muscle of humans

A, mean values of CSA of muscle fibres before bed rest (T0) and at T8 and T35. B, protein oxidation index (Oxy/RP). *Significantly different from T0 (P < 0.05). C, regression analysis of normalized values of muscle protein oxidation (Oxy/RP) plotted against the percentage change of fibre CSA of the same muscles, determined at T8 and T35; the slope of the line was significantly different from zero (P < 0.05), reprinted from Dalla Libera et al 2009 used with permission from The American Physiological Society.

Figure 5. The impact of 2 days (2 d) and 14 days (14 d) leg immobilization on lipid peroxidation (4-HNE-ponceau) (A), protein carbonylation (oxyblot-ponceau) (B), ubiquitin protein conjugates content (ubiquitin-ponceau) (C) and caspase 3/7 activity (D)

Redrawn from Glover et al. 2010.

Figure 6. A scheme of the factors potentially involved in determining variable alterations of redox homeostasis through muscles, species and models reported in the first three rows

Arrows point to the direction of an increase in the parameter. The large open arrow refers to the rate of atrophy, which increases from left to right. The dashed arrow hypothesizes an increase in the extent of ROS production, from left to right, depending (i) on the increase in the rate of oxidative metabolism due to small species having higher metabolic rate and to the progressively slower phenotype (i.e. relative content of slow, type 1 fibres) of muscles and (ii) on the increase in the relative extent of unloading at least from HU gastrocnemius towards diaphragm subjected to MV. Consequently, the dotted arrow hypothesizes an increase in the rate of proteolysis, from left to right, which is less determinant (or minor) in humans, and in HU gastrocnemius and soleus of rat and mice, but more determinant (or major) in immobilized soleus and in diaphragm following MV due to a progressively more evident large scale oxidation of proteins. Abbreviations: BR, bed rest; imm., immobilization; HU, hindlimb unloading; MV, mechanical ventilation; d, days; w, weeks; h, hours.