Defects in mitochondrial localization and ATP synthesis in the mdx mouse model of Duchenne muscular dystrophy are not alleviated by PDE5 inhibition

Abstract

Given the crucial roles for mitochondria in ATP energy supply, Ca(2+) handling and cell death, mitochondrial dysfunction has long been suspected to be an important pathogenic feature in Duchenne muscular dystrophy (DMD). Despite this foresight, mitochondrial function in dystrophin-deficient muscles has remained poorly defined and unknown in vivo. Here, we used the mdx mouse model of DMD and non-invasive spectroscopy to determine the impact of dystrophin-deficiency on skeletal muscle mitochondrial localization and oxidative phosphorylation function in vivo. Mdx mitochondria exhibited significant uncoupling of oxidative phosphorylation (reduced P/O) and a reduction in maximal ATP synthesis capacity that together decreased intramuscular ATP levels. Uncoupling was not driven by increased UCP3 or ANT1 expression. Dystrophin was required to maintain subsarcolemmal mitochondria (SSM) pool density, implicating it in the spatial control of mitochondrial localization. Given that nitric oxide-cGMP pathways regulate mitochondria and that sildenafil-mediated phosphodiesterase 5 inhibition ameliorates dystrophic pathology, we tested whether sildenafil's benefits result from decreased mitochondrial dysfunction in mdx mice. Unexpectedly, sildenafil treatment did not affect mitochondrial content or oxidative phosphorylation defects in mdx mice. Rather, PDE5 inhibition decreased resting levels of ATP, phosphocreatine and myoglobin, suggesting that sildenafil improves dystrophic pathology through other mechanisms. Overall, these data indicate that dystrophin-deficiency disrupts SSM localization, promotes mitochondrial inefficiency and restricts maximal mitochondrial ATP-generating capacity. Together these defects decrease intramuscular ATP and the ability of mdx muscle mitochondria to meet ATP demand. These findings further understanding of how mitochondrial bioenergetic dysfunction contributes to disease pathogenesis in dystrophin-deficient skeletal muscle in vivo.

Microtubule binding distinguishes dystrophin from utrophin

“…Therefore, microtubule lattice disorganization may affect how mitochondria respond to injury, thus leading to increased ROS signaling and impaired calcium handling. Moreover, subsarcolemmal mitochondria are less dense in mdx muscle, supporting the idea that they are not functioning properly (61). Thus, it is possible that microtubule disorganization may impair mitochondrial localization and function, limiting the buffering capacity of the cell and rendering it more susceptible to contraction-induced damage.…”

Nuclear Recruitment of Neuronal Nitric-oxide Synthase by α-Syntrophin Is Crucial for the Induction of Mitochondrial Biogenesis

“…Thus, it is possible to postulate that ␣-Syntrophin, by impinging myogenesis and mitochondrial biogenesis, could be implicated in skeletal muscle regeneration. Our findings could be helpful for the comprehension of the molecular mechanism underlying Duchenne's muscular dystrophy as well as other human myopathies that have been associated with the loss of sarcolemmal nNOS (66,67) and defective mitochondrial biogenesis (68). It can be postulated that in addition to nNOS localization to sarcolemma, the lack of Dystrophin complex in Duchenne's muscular dystrophy may affect nNOS distribution to the nucleus, thus dramatically inhibiting PGC-1␣ expression and mitochondrial biogenesis and finally triggering the atrophy process.…”

Defects in calcium homeostasis and mitochondria can be reversed in Pompe disease

“…59 A significant uncoupling of oxidative phosphorylation (oxygen consumption/ATP synthesis ratio) and a decrease in intramuscular ATP levels were reported in mdx mice, a model of DMD. 60 Once again it appears that mitochondrial abnormalities in Pompe muscle are distinctive to the disorder in that the defect is not latent and uncoupling is not observed.…”