Targeting cPLA2 derived lipid hydroperoxides as a potential intervention for sarcopenia

Abstract

Defects in neuromuscular innervation contribute significantly to the age-related decline in muscle mass and function (sarcopenia). Our previous studies demonstrated that denervation induces muscle mitochondrial hydroperoxide production (H 2 o 2 and lipid hydroperoxides (LOOHs)). Here we define the relative contribution of mitochondrial electron transport chain (ETC) derived H 2 o 2 versus cytosolic phospholipase A 2 (cPLA 2) derived LOOHs in neurogenic muscle atrophy. We show that denervation increases muscle cPLA 2 protein content, activity, and metabolites downstream of cPLA 2 including LOOHs. Increased scavenging of mitochondrial H 2 o 2 does not protect against denervation atrophy, suggesting ETC generated H 2 o 2 is not a critical player. In contrast, inhibition of cPLA 2 in vivo mitigates LOOH production and muscle atrophy and maintains individual muscle fiber size while decreasing oxidative damage. Overall, we show that loss of innervation in several muscle atrophy models including aging induces generation of LOOHs produced by arachidonic acid metabolism in the cPLA 2 pathway contributing to loss of muscle mass. The pathological age-related loss of skeletal muscle mass and function (sarcopenia) contributes to decreased quality of life and independence and increases the risk of injury and chronic disease 1,2. Preventing or reducing the effects of sarcopenia could increase quality of life, reduce risk of comorbidity development, and save billions of dollars in healthcare costs annually. Designing therapeutic interventions will remain difficult until the complex pathways and processes underlying sarcopenia are identified 3. Studies supporting a loss of innervation with age and a link between denervation and decreased muscle fiber size suggest that sarcopenia is a form of neurogenic atrophy 4-6. In addition to the loss of muscle mass, contractile force also declines with age 7. Loss of muscle strength occurs more rapidly than can be explained by the loss of muscle mass alone, and evidence suggests that a reduction in muscle quality and neuromuscular junction signaling also play a role in the functional decline of aging muscle 8. Our previous studies using mice lacking the cytosolic superoxide (O 2 −) scavenger CuZn superoxide dismutase (CuZn SOD, Sod1 −/−) demonstrate that elevated oxidative stress leads to an accelerated sarcopenia in the Sod1 −/− mice that is characterized by denervation, muscle hydroperoxide production, and muscle atrophy 9-11. Returning expression of CuZnSOD specifically to motor neurons of the Sod1 −/− mice prevented denervation, muscle hydroperoxide production, and atrophy, supporting a link between loss of innervation, hydroperoxides, and muscle atrophy 12. We have also shown that loss of innervation to skeletal muscle directly induces basal hydroperoxide production from isolated mitochondria including both hydrogen peroxide (H 2 O 2) and lipid hydroperoxides (LOOHs) 11. The magnitude of this

Reduced adenosine diphosphate sensitivity in skeletal muscle mitochondria increases reactive oxygen species production in mouse models of aging and oxidative stress but not denervation

“…Breakdown in neuromuscular innervation increases with age and is a driving factor in the decline in muscle mass 22–24 . Our laboratory has identified two separate age‐related phenotypes of increased hydroperoxide production in aging muscle: (i) increased lipid hydroperoxide production from arachidonic acid released by cPLA 2 that is induced by cPLA 2 activation after loss of innervation (including surgical denervation, genetic, and aging models) and (ii) the experiments described here showing an age‐related increase in production of superoxide and hydrogen peroxide by the mitochondrial ETS at physiological concentrations of ADP 17,21,28,29 . Denervated muscle fibres produce increased lipid hydroperoxides under basal conditions (hydroperoxide production in the absence of mitochondrial substrates or ADP) in several models including surgical, genetic models, and aging models 17,21,28,30 .…”

Master Regulators of Muscle Atrophy: Role of Costamere Components

“…Muscle-specific ATF4-KO mice are partially and transiently resistant to immobilization-induced muscle atrophy, but, strikingly, they did not exhibit muscle sparing following denervation [ 57 ]. This latter feature appears surprising, since ER-stress response activation is a relevant component of muscle atrophy development after denervation and in cancer cachexia [ 21 , 59 ], in addition to other muscle disorders [ 60 ]. Strikingly, the inhibition of ER stress with the chemical chaperone 4-PBA not only led to accelerated muscle loss in lung cancer-bearing mice, but also to significant muscle atrophy in naïve mice [ 21 ].…”

“…Recent investigations provided a major advance in knowledge of early events in the development of denervation-induced muscle atrophy by analyzing muscle transcriptome at different times after denervation, from less than 0.5 h to 28 d. Major findings were the up-regulation of genes involved in the oxidative stress and inflammatory responses within 0.5–24 h after denervation [ 59 , 87 ]. Inflammation contributes to muscle atrophy development in several contexts [ 241 ].…”

“…Phospholipase A2 de-esterifies membrane phospholipids, which can promote enzymatic (i.e., via lipo-hydroperoxidases) and non-enzymatic peroxidation of bis-allylic unsaturated lipids and activate NADPH-oxidases [ 248 ]. Interestingly, muscle lipo-hydroperoxides increase between 2–4 d of denervation consequently to cytosolic PLA2 activation [ 59 ]. Protein levels of peroxiredoxin 6, which has potential PLA2 activity, also increase after 3 d-denervation [ 244 ].…”

“…Protein levels of peroxiredoxin 6, which has potential PLA2 activity, also increase after 3 d-denervation [ 244 ]. Calcium depletion from SR stores might trigger an ER-stress response in denervated myofibers [ 21 ]; increased expression of the transcription factor ATF4 was observed within 2–4 d after denervation [ 59 ]. Increase in sarcoplasmic calcium levels also affects costamere components.…”