Upper airway dilator muscle weakness following intermittent and sustained hypoxia in the rat: effects of a superoxide scavenger

NADPH oxidase-2 is necessary for chronic intermittent hypoxia-induced sternohyoid muscle weakness in adult male mice

New Findings

What is the central question of this study?

Exposure to chronic intermittent hypoxia (CIH) evokes redox changes, culminating in impaired upper airway muscle function: what is the specific source of CIH‐induced reactive oxygen species?

What is the main finding and its importance?

Profound sternohyoid muscle dysfunction following exposure to CIH was entirely prevented by apocynin co‐treatment or NADPH oxidase 2 (NOX2) deletion. The results have implications for human obstructive sleep apnoea syndrome and point to antioxidant intervention, potentially targeting NOX2 blockade, as a therapeutic strategy.

Abstract

Exposure to chronic intermittent hypoxia (CIH) evokes redox changes, culminating in impaired upper airway muscle function. We sought to determine if NADPH oxidase 2 (NOX2)‐derived reactive oxygen species underpin CIH‐induced maladaptive changes in upper airway (sternohyoid) muscle performance. Adult male mice (C57BL/6J) were assigned to one of three groups: normoxic controls (sham); CIH‐exposed (CIH, 12 cycles/hour, 8 h/day for 14 days); and CIH + apocynin (NOX2 inhibitor, 2 mM) given in the drinking water throughout exposure to CIH. In addition, we studied sham and CIH‐exposed NOX2‐null mice (B6.129S‐CybbTM^1Din/J). Profound sternohyoid muscle dysfunction following exposure to CIH was entirely prevented by apocynin co‐treatment or NOX2 deletion. Exposure to CIH increased sternohyoid muscle NOX enzyme activity, with no alteration to the gene or protein expression of NOX subunits. There was no evidence of overt oxidative stress, muscle regeneration, inflammation or atrophy following exposure to CIH. We suggest that NOX‐dependent CIH‐induced upper airway muscle weakness increases vulnerability to upper airway obstruction. Our results have implications for human obstructive sleep apnoea syndrome and point to antioxidant intervention, potentially targeting NOX2 blockade, as a therapeutic strategy.

Obstructive Sleep Apnea, Hypoxia, and Nonalcoholic Fatty Liver Disease

Recent studies have demonstrated that obstructive sleep apnea (OSA) is associated with the development and evolution of nonalcoholic fatty liver disease (NAFLD), independent of obesity or other shared risk factors. Like OSA, NAFLD is a prevalent disorder associated with major adverse health outcomes: Patients with NAFLD may develop cirrhosis, liver failure, and hepatocellular carcinoma. One major finding that has emerged from these studies is that the OSA-NAFLD association is related to the degree of nocturnal hypoxemia in OSA. Animal models have therefore largely focused on intermittent hypoxia, a key manifestation of OSA, to shed light on the mechanisms by which OSA may give rise to the complex metabolic disturbances that are seen in NAFLD. Intermittent hypoxia leads to tissue hypoxia and can result in oxidative stress, mitochondrial dysfunction, inflammation, and overactivation of the sympathetic nervous system, among many other maladaptive effects. In such models, intermittent hypoxia has been shown to cause insulin resistance, dysfunction of key steps in hepatic lipid metabolism, atherosclerosis, and hepatic steatosis and fibrosis, each of which is pertinent to the development and/or progression of NAFLD. However, many intriguing questions remain unanswered: Principally, how aggressively should the clinician screen for NAFLD in patients with OSA, and vice versa? In this review, we attempt to apply the best evidence from animal and human studies to highlight the relationship between these two disorders and to advocate for further trials aimed at defining these relationships more precisely.

Assessment of tongue mechanical properties using different contraction tasks

Inadequate upper airway (UA) dilator muscle function may play an important role in the pathophysiology of obstructive sleep apnea (OSA). To date, tongue mechanical properties have been assessed mainly using protrusion protocol with conflicting results. Performance during elevation tasks among patients with OSA remains unknown. This study aimed at assessing tongue muscle strength, strength stability, endurance time, fatigue indices, and total muscle work, using elevation and protrusion tasks with repetitive isometric fatiguing contractions in 12 normal plus mild, 17 moderate, and 11 severe patients with OSA, and to assess the influence of body mass index (BMI) and age. Endurance time was longer in protrusion than elevation task (P = 0.01). In both tasks, endurance time was negatively correlated with baseline value of strength coefficient of variation (P < 0.01). Compared with other groups, patients with moderate OSA had the lowest total muscle work for protrusion (P = 0.01) and shortest endurance time (P = 0.04), regardless of the type of task. Additionally, in patients with moderate-severe OSA, the total muscle work for both tasks was lower in nonobese compared with obese (P < 0.05). Total muscle work for protrusion was positively correlated with apnea hypopnea index (AHI) in obese subjects (P < 0.01). Endurance time was shorter (P < 0.01) and recovery time longer (P = 0.02) in the old compared with young subjects. In conclusion, the tongue is more prone to fatigue during the elevation task and in patients with moderate OSA. Obesity appeared to prevent alteration of tongue mechanical properties in patients with OSA. Baseline strength stability and endurance were related, illustrating the role of central neuromuscular output in tongue resistance to fatigue.NEW & NOTEWORTHY To our knowledge, this is the first study to assess and compare tongue function using both elevation and protrusion tasks with repetitive isometric fatiguing contractions in subjects with different OSA status. Tongue mechanical performance seemed to differ between protrusion and elevation tasks and depend on the severity of OSA.

Sex differences in murine sternohyoid muscle tolerance of acute severe hypoxic stress

Given that sex differences inherent to muscle might at least contribute to male risk for obstructive sleep apnoea syndrome (OSAS), our objective was to test the hypothesis that male sternohyoid muscle exhibits greater susceptibility to severe hypoxic stress compared with female muscle. Adult male and female C57Bl6/J mouse sternohyoid isometric and isotonic functional properties were examined ex vivo at 35 °C in tissue baths under control and severe hypoxic conditions. Hypoxia was detrimental to peak force (Fmax), work (Wmax) and power (Pmax), but not shortening velocity (Vmax). Two-way analysis of variance revealed a significant sex x gas interaction for Fmax (p<0.05), revealing inferior hypoxic tolerance in male sternohyoid muscle. However, increases in male shortening velocity in severe hypoxia preserved power-generating capacity which was equivalent to values determined in female muscle. Fmax decline in hypoxic female sternohyoid was considerably less than in male muscle, illustrating an inherent tolerance of force-generating capacity mechanisms to hypoxic stress in female airway dilator muscle. We speculate that this could confer a distinct advantage in vivo in terms of the defense of upper airway caliber.

Evidence of hypoxic tolerance in weak upper airway muscle from young mdx mice

Duchenne muscular dystrophy (DMD) is a genetic disease characterised by deficiency in the protein dystrophin. The respiratory system is weakened and patients suffer from sleep disordered breathing and hypoventilation culminating in periods of hypoxaemia. We examined the effects of an acute (6h) hypoxic stress on sternohyoid muscle function (representative pharyngeal dilator). 8 week old male, wild-type (WT; C57BL/10ScSnJ; n=18) and mdx (C57BL/10ScSn-Dmd(mdx)/J; n=16) mice were exposed to sustained hypoxia (FIO2=0.10) or normoxia. Muscle functional properties were examined ex vivo. Additional WT (n=5) and mdx (n=5) sternohyoid muscle was exposed to an anoxic challenge. Sternohyoid dysfunction was observed in mdx mice with significant reductions in force and power. Following exposure to the acute in vivo hypoxic stress, WT sternohyoid muscle showed evidence of functional impairment (reduced force, work and power). Conversely, mdx sternohyoid showed an apparent tolerance to the acute hypoxic stress. This tolerance was not maintained for mdx following a severe hypoxic stress. A dysfunctional upper airway muscle phenotype is present at 8 weeks of age in the mdx mouse, which may have implications for the control of airway patency in DMD. Hypoxic tolerance in mdx respiratory muscle is suggestive of adaptation to chronic hypoxia, which could be present due to respiratory morbidity. We speculate a role for hypoxia in mdx respiratory muscle morbidity.

Chronic sustained hypoxia-induced redox remodeling causes contractile dysfunction in mouse sternohyoid muscle

Chronic sustained hypoxia (CH) induces structural and functional adaptations in respiratory muscles of animal models, however the underlying molecular mechanisms are unclear. This study explores the putative role of CH-induced redox remodeling in a translational mouse model, with a focus on the sternohyoid—a representative upper airway dilator muscle involved in the control of pharyngeal airway caliber. We hypothesized that exposure to CH induces redox disturbance in mouse sternohyoid muscle in a time-dependent manner affecting metabolic capacity and contractile performance. C57Bl6/J mice were exposed to normoxia or normobaric CH (FiO₂ = 0.1) for 1, 3, or 6 weeks. A second cohort of animals was exposed to CH for 6 weeks with and without antioxidant supplementation (tempol or N-acetyl cysteine in the drinking water). Following CH exposure, we performed 2D redox proteomics with mass spectrometry, metabolic enzyme activity assays, and cell-signaling assays. Additionally, we assessed isotonic contractile and endurance properties ex vivo. Temporal changes in protein oxidation and glycolytic enzyme activities were observed. Redox modulation of sternohyoid muscle proteins key to contraction, metabolism and cellular homeostasis was identified. There was no change in redox-sensitive proteasome activity or HIF-1α content, but CH decreased phospho-JNK content independent of antioxidant supplementation. CH was detrimental to sternohyoid force- and power-generating capacity and this was prevented by chronic antioxidant supplementation. We conclude that CH causes upper airway dilator muscle dysfunction due to redox modulation of proteins key to function and homeostasis. Such changes could serve to further disrupt respiratory homeostasis in diseases characterized by CH such as chronic obstructive pulmonary disease. Antioxidants may have potential use as an adjunctive therapy in hypoxic respiratory disease.

Chronic intermittent hypoxia increases rat sternohyoid muscle NADPH oxidase expression with attendant modest oxidative stress

Chronic intermittent hypoxia (CIH) causes upper airway muscle dysfunction. We hypothesized that the superoxide generating NADPH oxidase (NOX) is upregulated in CIH-exposed muscle causing oxidative stress. Adult male Wistar rats were exposed to intermittent hypoxia (5% O₂ at the nadir for 90 s followed by 210 s of normoxia), for 8 h per day for 14 days. The effect of CIH exposure on the expression of NOX subunits, total myosin and 4-hydroxynonenal (4-HNE) protein adducts in sternohyoid muscle was determined by western blotting and densitometry. Sternohyoid protein free thiol and carbonyl group contents were determined by 1D electrophoresis using specific fluorophore probes. Aconitase and glutathione reductase activities were measured as indices of oxidative stress. HIF-1α content and key oxidative and glycolytic enzyme activities were determined. Contractile properties of sternohyoid muscle were determined ex vivo in the absence and presence of apocynin (putative NOX inhibitor). We observed an increase in NOX 2 and p47 phox expression in CIH-exposed sternohyoid muscle with decreased aconitase and glutathione reductase activities. There was no evidence, however, of increased lipid peroxidation or protein oxidation in CIH-exposed muscle. CIH exposure did not affect sternohyoid HIF-1α content or aldolase, lactate dehydrogenase, or glyceraldehyde-3-phosphate dehydrogenase activities. Citrate synthase activity was also unaffected by CIH exposure. Apocynin significantly increased sternohyoid force and power. We conclude that CIH exposure upregulates NOX expression in rat sternohyoid muscle with concomitant modest oxidative stress but it does not result in a HIF-1α-dependent increase in glycolytic enzyme activity. Constitutive NOX activity decreases sternohyoid force and power. Our results implicate NOX-dependent reactive oxygen species in CIH-induced upper airway muscle dysfunction which likely relates to redox modulation of key regulatory proteins in excitation-contraction coupling.

The polymorphic and contradictory aspects of intermittent hypoxia

Intermittent hypoxia (IH) has been extensively studied during the last decade, primarily as a surrogate model of sleep apnea. However, IH is a much more pervasive phenomenon in human disease, is viewed as a potential therapeutic approach, and has also been used in other disciplines, such as in competitive sports. In this context, adverse outcomes involving cardiovascular, cognitive, metabolic, and cancer problems have emerged in obstructive sleep apnea-based studies, whereas beneficial effects of IH have also been identified. Those a priori contradictory findings may not be as contradictory as initially thought. Indeed, the opposite outcomes triggered by IH can be explained by the specific characteristics of the large diversity of IH patterns applied in each study. The balance between benefits and injury appears to primarily depend on the ability of the organism to respond and activate adaptive mechanisms to IH. In this context, the adaptive or maladaptive responses can be generally predicted by the frequency, severity, and duration of IH. However, the presence of underlying conditions such as hypertension or obesity, as well as age, sex, or genotypic variance, may be important factors tilting the balance between an appropriate homeostatic response and decompensation. Here, the two possible facets of IH as derived from human and experimental animal settings will be reviewed.

The β2 -adrenoceptor agonist terbutaline recovers rat pharyngeal dilator muscle force decline during severe hypoxia

RATIONALE

Obstructive sleep apnoea syndrome (OSAS) is a debilitating condition characterized by recurrent occlusions of the pharyngeal airway during sleep accompanied by arterial hypoxaemia. Upper airway muscle dysfunction is implicated in the pathophysiology of OSAS. Pharmacological agents that improve muscle contractile and endurance properties may have therapeutic value.

AIM

We tested the hypothesis that the β(2) -adrenoceptor agonist terbutaline improves rat sternohyoid muscle performance especially during hypoxic stress.

METHODS

Isometric contractile and endurance properties were examined ex vivo in Krebs solution at 35°C. Muscles were incubated in tissue baths under hyperoxic (95% O(2) /5% CO(2)) conditions in the absence (control) or presence of the β(2) -adrenoceptor agonist terbutaline (1 μM). In additional experiments under hypoxic (95% N(2) /5% CO(2)) conditions, the effects of terbutaline were examined in the presence of the β-adrenoceptor antagonist propranolol (1 μM).

RESULTS

Hypoxia significantly impaired sternohyoid force production. Terbutaline completely recovered hypoxic depression of force, an effect that was blocked by co-application with propranolol.

CONCLUSION

The β(2) -adrenoceptor agonist terbutaline completely recovers hypoxic depression of upper airway muscle force. β(2) -adrenoceptor agonists warrant investigation in animal models of OSAS reporting upper airway and diaphragm muscle dysfunction.

Inflammatory lung disease in Rett syndrome

Rett syndrome (RTT) is a pervasive neurodevelopmental disorder mainly linked to mutations in the gene encoding the methyl-CpG-binding protein 2 (MeCP2). Respiratory dysfunction, historically credited to brainstem immaturity, represents a major challenge in RTT. Our aim was to characterize the relationships between pulmonary gas exchange abnormality (GEA), upper airway obstruction, and redox status in patients with typical RTT (n = 228) and to examine lung histology in a Mecp2-null mouse model of the disease. GEA was detectable in ~80% (184/228) of patients versus ~18% of healthy controls, with “high” (39.8%) and “low” (34.8%) patterns dominating over “mixed” (19.6%) and “simple mismatch” (5.9%) types. Increased plasma levels of non-protein-bound iron (NPBI), F₂-isoprostanes (F₂-IsoPs), intraerythrocyte NPBI (IE-NPBI), and reduced and oxidized glutathione (i.e., GSH and GSSG) were evidenced in RTT with consequently decreased GSH/GSSG ratios. Apnea frequency/severity was positively correlated with IE-NPBI, F₂-IsoPs, and GSSG and negatively with GSH/GSSG ratio. A diffuse inflammatory infiltrate of the terminal bronchioles and alveoli was evidenced in half of the examined Mecp2-mutant mice, well fitting with the radiological findings previously observed in RTT patients. Our findings indicate that GEA is a key feature of RTT and that terminal bronchioles are a likely major target of the disease.