Chronic aryl hydrocarbon receptor activity phenocopies smoking‐induced skeletal muscle impairment

Abstract

Background Chronic obstructive pulmonary disease (COPD) patients exhibit skeletal muscle atrophy, denervation, and reduced mitochondrial oxidative capacity. Whilst chronic tobacco smoke exposure is implicated in COPD muscle impairment, the mechanisms involved are ambiguous. The aryl hydrocarbon receptor (AHR) is a ligand‐activated transcription factor that activates detoxifying pathways with numerous exogenous ligands, including tobacco smoke. Whereas transient AHR activation is adaptive, chronic activation can be toxic. On this basis, we tested the hypothesis that chronic smoke‐induced AHR activation causes adverse muscle impact. Methods We used clinical patient muscle samples, and in vitro (C2C12 myotubes) and in vivo models (mouse), to perform gene expression, mitochondrial function, muscle and neuromuscular junction morphology, and genetic manipulations (adeno‐associated virus‐mediated gene transfer). Results Sixteen weeks of tobacco smoke exposure in mice caused muscle atrophy, neuromuscular junction degeneration, and reduced oxidative capacity. Similarly, smoke exposure reprogrammed the muscle transcriptome, with down‐regulation of mitochondrial and neuromuscular junction genes. In mouse and human patient specimens, smoke exposure increased muscle AHR signalling. Mechanistically, experiments in cultured myotubes demonstrated that smoke condensate activated the AHR, caused mitochondrial impairments, and induced an AHR‐dependent myotube atrophy. Finally, to isolate the role of AHR activity, expression of a constitutively active AHR mutant without smoke exposure caused atrophy and mitochondrial impairments in cultured myotubes, and muscle atrophy and neuromuscular junction degeneration in mice. Conclusions These results establish that chronic AHR activity, as occurs in smokers, phenocopies the atrophy, mitochondrial impairment, and neuromuscular junction degeneration caused by chronic tobacco smoke exposure.

Integrating Mechanisms of Exacerbated Atrophy and Other Adverse Skeletal Muscle Impact in COPD

“…Other groups have employed smoking mouse models, finding that chronic TS exposure in mice yields atrophy and fiber type shift (Gosker et al, 2009;Basic et al, 2012;Rinaldi et al, 2012;Caron et al, 2013), consistent with the observations in human smokers (Gosker et al, 2007;van den Borst et al, 2011). Smoking mouse studies have also identified a reduced skeletal muscle mitochondrial respiratory capacity (Fukuda et al, 2017;Perez-Rial et al, 2020;Thome et al, 2022), although this is not seen in all studies (Bowen et al, 2017;Decker et al, 2021). Furthermore, we recently showed that chronic TS exposure in mice causes morphological adaptations at the neuromuscular junction, which we propose plays a key role in the motor unit remodeling (fast fiber shift, type grouping), expression of histological markers of denervation (high non-specific esterase activity, high neural cell adhesion molecule [NCAM] expression), and transcriptional indicators of denervation in COPD patient muscle (Kapchinsky et al, 2018).…”

“…We then showed that knock-in of a constitutively active mutant of the AHR (demonstrates transcription factor activity without ligand) (Figure 8A) into skeletal muscle cells in vitro recapitulates the muscle atrophy (Figure 8B) and mitochondrial impairment seen with TS condensate (Figures 8C,D). Finally, in mice 12 weeks following intramuscular injection of this constitutively active AHR mutant using an AAV vector revealed muscle atrophy and altered neuromuscular junction morphology that was similar to what was seen with 16 weeks of chronic TS exposure in mice (Thome et al, 2022) (Figure 9). These findings suggest that TSinduced chronic activation of the AHR is likely to be an important contributor to the mitochondrial impairment, muscle atrophy and denervation phenotypes seen in COPD patients, and clearly warrants additional study.…”

“…Furthermore, we recently showed that chronic TS exposure in mice causes morphological adaptations at the neuromuscular junction, which we propose plays a key role in the motor unit remodeling (fast fiber shift, type grouping), expression of histological markers of denervation (high non-specific esterase activity, high neural cell adhesion molecule [NCAM] expression), and transcriptional indicators of denervation in COPD patient muscle (Kapchinsky et al, 2018). In a follow-up study using a smoking mouse model, we showed that TS-induced neuromuscular junction alterations also involve the diaphragm (Thome et al, 2022), which likely plays a role in the diaphragm involvement seen in COPD patients (Ottenheijm et al, 2008). In addition to TS being implicated in the atrophy and fast fiber shift seen in COPD patient limb muscle, there is also evidence that chronic TS exposure causes increased muscle fatigue (Wust et al, 2008), which is another manifestation of skeletal muscle impact in COPD patients (Allaire et al, 2004;Bachasson et al, 2013).…”

“…For example, long-term smokers without evidence of lung disease demonstrate (A) a fast fiber shift, and data from a smoking mouse model show that chronic cigarette smoke exposure leads to (B) reduced mitochondrial respiratory capacity that is associated with a down-regulation of mitochondrial genes, and (C) marked alterations to the neuromuscular junction. Oxphos gene panel modified from (Thome et al, 2022).…”

“…Several studies have addressed whether COPD may accelerate fundamental aging biology in an attempt to understand whether this may underlie some of the adverse muscle impact in COPD. In (Thome et al, 2022).…”