Interplay between mitochondria and reactive oxygen and nitrogen species in metabolic adaptation to hypoxia in facioscapulohumeral muscular dystrophy: potential therapeutic targets

Abstract

Facioscapulohumeral muscular dystrophy (FSHD) is characterised by descending skeletal muscle weakness and wasting. FSHD is caused by mis-expression of the transcription factor DUX4, which is linked to oxidative stress, a condition especially detrimental to skeletal muscle with its high metabolic activity and energy demands. Oxidative damage characterises FSHD and recent work suggests metabolic dysfunction and perturbed hypoxia signalling as novel pathomechanisms. However, redox biology of FSHD remains poorly understood, and integrating the complex dynamics of DUX4-induced metabolic changes is lacking. Here we pinpoint the kinetic involvement of altered mitochondrial RONS metabolism and impaired mitochondrial function in aetiology of oxidative stress in FSHD. Transcriptomic analysis in FSHD muscle biopsies reveals strong enrichment for pathways involved in mitochondrial complex I assembly, nitrogen metabolism, oxidative stress response and hypoxia signalling. We found elevated ROS levels correlate with increases in steady-state mitochondrial membrane potential in FSHD myogenic cells. DUX4 triggers mitochondrial membrane polarisation prior to oxidative stress generation and apoptosis through mitochondrial ROS, and affects NO∙ bioavailability via mitochondrial peroxidation. We identify complex I as the primary target for DUX4-induced mitochondrial dysfunction, with strong correlation between complex I-linked respiration and cellular oxygenation/hypoxia signalling activity in environmental hypoxia. Thus, FSHD myogenesis is uniquely susceptible to hypoxia-induced oxidative stress as a consequence of metabolic mis-adaptation. Importantly, mitochondria-targeted antioxidants rescue FSHD pathology more effectively than conventional antioxidants, highlighting the central involvement of disturbed mitochondrial RONS metabolism. This work provides a pathomechanistic model by which DUX4-induced changes in oxidative metabolism impair muscle function in FSHD, amplified when metabolic adaptation to varying O2 tension is required.

DUX4 Role in Normal Physiology and in FSHD Muscular Dystrophy

“…In FSHD, impaired mitochondrial function [ 116 , 117 ], bioenergetic perturbation [ 118 , 119 ] and increased oxidative stress have been described [ 68 , 116 , 120 ]. Gene expression profiling and functional evaluations support a decreased mitochondrial complex I and III activity, elevated mitochondrial membrane potential and increased mitochondrial reactive oxygen species (ROS) contributing to oxidative stress generation and apoptosis in FSHD [ 116 , 117 ].…”

“…HIF1-α is the master regulator of hypoxia signaling triggering the transcription of hypoxic genes, including vascular endothelial growth factor. This provides a possible explanation for both increased oxidative stress sensitivity and retinal vasculature abnormalities observed in FSHD [ 117 , 121 ]. Pharmacological inhibition of HIF1-α protein synthesis attenuates cell death caused by ectopic DUX4 overexpression, opening a possible novel therapeutic approach for FSHD [ 122 ].…”

“…Small molecule screenings identified a high number of compounds reducing oxidative stress as inhibitors of DUX4-induced toxicity [ 44 , 186 ]. Intriguingly, mitochondria-targeted antioxidants appear to rescue myogenic defects of FSHD muscle cells more effectively than conventional antioxidants [ 117 ]. However, a direct effect of these compounds on DUX4 or its downstream effectors needs to be clarified.…”

Facioscapulohumeral muscular dystrophy is associated with altered myoblast proteome dynamics

“…Herein, we performed proteomic analysis on previously generated and validated (22) immortalized myoblasts from matched pairs of individuals with FSHD and UASb. To the best of our knowledge, the present work represents the first proteomic analysis of immortalized FSHD and UASb cell lines reported in Homma et al (22), which have been characterized across several previous studies (8,10,(23)(24)(25)(26)(27).…”

“…DUX4 instigates widespread changes in muscle gene expression (3), including aberrant activation of other transcription factors. Several biological processes are known to be disrupted by DUX4 expression in muscle, including myogenic differentiation and cell cycle (4), oxidative stress sensitivity (5), DNA damage (6), Wnt/β-catenin signaling (7), metabolic stress and mitochondrial dysfunction (8), and p53-mediated apoptosis (9). However, the mechanisms that connect DUX4 expression to muscle toxicity are not yet fully understood despite extensive studies on gene regulation and transcriptional processes (10)(11)(12)(13).…”