Defects in Mitochondrial ATP Synthesis in Dystrophin-Deficient Mdx Skeletal Muscles May Be Caused by Complex I Insufficiency

Abstract

Duchenne Muscular Dystrophy is a chronic, progressive and ultimately fatal skeletal muscle wasting disease characterised by sarcolemmal fragility and intracellular Ca2+ dysregulation secondary to the absence of dystrophin. Mounting literature also suggests that the dysfunction of key energy systems within the muscle may contribute to pathological muscle wasting by reducing ATP availability to Ca2+ regulation and fibre regeneration. No study to date has biochemically quantified and contrasted mitochondrial ATP production capacity by dystrophic mitochondria isolated from their pathophysiological environment such to determine whether mitochondria are indeed capable of meeting this heightened cellular ATP demand, or examined the effects of an increasing extramitochondrial Ca2+ environment. Using isolated mitochondria from the diaphragm and tibialis anterior of 12 week-old dystrophin-deficient mdx and healthy control mice (C57BL10/ScSn) we have demonstrated severely depressed Complex I-mediated mitochondrial ATP production rate in mdx mitochondria that occurs irrespective of the macronutrient-derivative substrate combination fed into the Kreb’s cycle, and, which is partially, but significantly, ameliorated by inhibition of Complex I with rotenone and stimulation of Complex II-mediated ATP-production with succinate. There was no difference in the MAPR response of mdx mitochondria to increasing extramitochondrial Ca2+ load in comparison to controls, and 400 nM extramitochondrial Ca2+ was generally shown to be inhibitory to MAPR in both groups. Our data suggests that DMD pathology is exacerbated by a Complex I deficiency, which may contribute in part to the severe reductions in ATP production previously observed in dystrophic skeletal muscle.

Loss of NAD Homeostasis Leads to Progressive and Reversible Degeneration of Skeletal Muscle

“…Transcriptionally, there is a striking resemblance between mNKO muscle and that of the well-characterized mdx model of DMD. This observation was unexpected, given that the former defect is strictly metabolic and the latter strictly structural, and especially compelling in light of reports that mdx muscle contains less NAD than that of age-matched controls and appears limited in aerobic capacity by complex I (Chalkiadaki et al, 2014; Rybalka et al, 2014). NAD supplementation also counteracts at least one additional laminopathy modeled in zebrafish (Goody et al, 2012).…”

Mitochondrial Dysfunction and Altered Renal Metabolism in Dahl Salt-Sensitive Rats

“…An increasing number of discoveries have shown that inefficient metabolism, due to mitochondrial dysfunction in muscle and brain tissue, could cause blood pressure elevation and may be associated with the development of hypertension [13, 14]. Our recent studies, using electron microscopy analysis, have shown mitochondrial ultrastructural abnormalities in medullary thick ascending limbs of SS rats, implying mitochondrial function may be different [15].…”

Idebenone as a Novel Therapeutic Approach for Duchenne Muscular Dystrophy

“…There is an increasing body of literature (see Table 1) [41][42][43][44][45][46][47][48][49][50][51][52][53][54][55] indicating that dystrophin deficiency causes sarcolemmal fragility and intracellular Ca 2+ dysregulation, which generally leads to mitochondrial dysfunction, a reduction in resting adenosine triphosphate (ATP) levels, an increased production of cell-damaging reactive oxygen species (ROS) and ultimately mitochondrial damage. [38][39][40] The combination of primary dystrophin deficiency and secondary mitochondrial dysfunction and impaired muscle cell regeneration, degeneration and cell death is shown in Figure 2A.…”

“…54 As direct evidence of a mitochondrial role in the pathogenesis of muscular dystrophy, mitochondrial biogenesis has been demonstrated to ameliorate the pathology and improve function in the dystrophin deficient mdx mouse model. Specifically, Selsby and colleagues 56 showed that over-expression of peroxisome-proliferator receptor γ coactivator 1α (PGC-1α), resulted in increased utrophin and slow type I myosin heavy chain fibre expression, as well as elevated mitochondrial protein expression and increased expression of oxidative genes.…”

“…59,60 Idebenone has been reported to utilise and activate Complex I-independent metabolic pathways [61][62][63][64] by the transfer of energy equivalents from the cytosol directly into the mitochondrial electron transport chain (ETC). Upon entering the cell, idebenone is efficiently reduced by the cytoplasmic enzyme NADH-quinone oxidoreductase 1 (NQO1) and the resulting reduced form of idebenone 43 Lucas-Heron et al 44 Even et al 45 Sperl et al 46 Kuznetsov et al 47 Passaquin et al 48 Onopiuk et al 49 Godin et al 50 Schuh et al 51 Percival et al 52 • Reduction in resting ATP levels Cole et al 53 Rybalka et al 54 • Reduced mitochondrial Complex I function…”