In this study, we detected genioglossus injury, dysfunctional mitochondria and decreased mitochondrial respiratory chain complex I and
IV activities of the genioglossus in a rabbit model of OSAHS, these results described a possible mechanism, supporting of our previous reports on genioglossus fatigue. As in other diseases, research based on animal models is crucially important in OSAHS(11, 13). The model of intermittent hypoxia was a widely documented method of inducing OSAHS(17). Although this model appeared to demonstrate similar symptoms to OSAHS patients, it was obviously limited by the absence of recurrent UA obstructions, apnea, increased inspiratory effort and sleep fragmentation(18). In these cases, the aim was to obtain an OSAHS model induced by UA obstruction and demonstrate all the typical characteristics of OSAHS, such as intermittent hypoxia, increased inspiratory effort and sleep fragmentation. An approach to induce OSAHS and insert MAD in rabbits was developed in our laboratory. Technical success was achieved, and the MAD was well tolerated in the rabbits, as shown by the body weight and food intake.
This study found that OSAHS caused abnormal morphology of genioglossus, such as degeneration of the genioglossus fibers and disordered mitochondrial ultrastructure, including discontinuous myofibrils, dilation of the mitochondria and disruption of the cristae. Accordingly, we previously found that OSAHS resulted in genioglossus fatigue in vitro(11). However, the detailed molecular mechanisms remain to be elucidated. Previous reports demonstrated that the genioglossus may be more vulnerable to fatigue in OSAHS patients(19, 20) and animal models of OSAHS than in controls(21–23). Therefore, identification of an underlying contributory mechanism of genioglossus fatigue is important and has therapeutic implications. The function of skeletal muscle is intimately linked to the proper function of mitochondria because mitochondria constitute the main energy supply for contraction of skeletal muscle. We intended to examine whether genioglossus fatigue was related to mitochondria. Consistent with our hypothesis, mitochondrial abnormalities, such as decreased mitochondrial membrane potential and decreased mitochondrial respiratory chain complex activity, as well as dysfunctional mitochondrial ultrastructure of genioglossus, were revealed in Group OSAHS. These findings could explain why the abnormal changes in the structure and contractile properties of the genioglossus were observed in our previous studies.
With regard to ΔΨm, our data showed that the mitochondria isolated from the animals with OSAHS had a lower ΔΨm than those of the controls. Since mitochondrial membrane potential is a key indicator reflecting mitochondrial function and ΔΨm provides reliable information on muscle function and dysfunction(24), the data suggested that OSAHS could indeed cause mitochondrial dysfunction. However, a simple analysis of mitochondrial membrane potential is insufficient to determine the mechanisms underlying the damage to genioglossus function and the effectiveness of the MAD treatment. Since respiratory chain complexes I, III and IV generate ΔΨm as a result of energy transfer through the electron transport chain(25), to further clarify the genioglossus mitochondrial function, we evaluated the mitochondrial respiratory chain complexes in the present study. We found that these complexes were also severely affected by OSAHS; thus, we confirmed that OSAHS clearly affects genioglossus mitochondrial function.
To the best of our knowledge, this is the first study examining genioglossus mitochondrial functionality in OSAHS models and after MAD treatment. We hypothesize that the mitochondrial dysfunction and morphological abnormalities observed in the animals with OSAHS are due to one or more of the following causes. First, chronic intermittent hypoxia during repeated apnea or hypopnea results in mitochondrial dysfunction. UA closure during sleep is associated with oxygen desaturation, which terminates when an arousal transiently interrupts sleep. Then, apneas can recur as sleep resumes, contributing to the pathogenesis of chronic intermittent hypoxia. A study showed that hypoxia in OSAHS patients impaired UA muscle activity(26). Another report demonstrated that hypoxia could increase oxidative stress and impair mitochondrial function in mouse skeletal muscle because hypoxia affected both the mitochondrial phosphorylation efficiency and the coupling between respiration and ATP synthesis. Similarly, a previous study showed that the hypoxia-induced mitochondrial dysfunction and the inner and outer mitochondrial membrane integrity were significantly affected by hypoxia exposure(28). Therefore, mitochondrial dysfunction may be closely related to chronic intermittent hypoxia, the most basic physical characteristic of OSAHS. Second, repeated bursts of forceful contraction may lead to mitochondrial abnormalities in genioglossus. Genioglossus actively compensate for the narrowed upper airway in OSAHS during wakefulness, which is supported by the research that OSAHS patients have increased GG activation relative to controls during wakefulness(29) and there was greater reduction in GG activation in OSA patients than in controls after CPAP treatment, implying that the enhanced activity is a compensatory response(30). Therefore, repeated forceful contraction of the genioglossus may lead to mitochondrial abnormalities. Dysfunction of UA dilator muscles is closely involved in the pathophysiology of OSAHS(1). In OSAHS patients, the genioglossus has been shown to be structurally and functionally abnormal, with elevated levels of activation while awake(31). The genioglossus, when activated, protracts the tongue and results in increased airway patency and further prevents collapse and subsequent apneic events(32). When performing repeated tongue protrusions, the genioglossus exerts repeated bursts of forceful contraction at the end of each obstructive apnea; thus, traumatic muscle contractions have a negative effect on mitochondrial structure and function following repeated activation during the night. Our results suggested that genioglossus injury, including histopathological muscle changes and metabolic disturbances, may be the result of OSAHS. The genioglossus in Group OSAHS was characterized by morphological abnormalities, together with decreased abnormal mitochondrial functions of the genioglossus, leaving the UA susceptible to collapse and leading to a vicious cycle of increasingly severe episodes of obstruction during sleep.
Our previous work has shown that genioglossus fatigue related to OSAHS could be corrected by MAD treatment(11), suggesting that genioglossus fatigue was related to OSAHS, and MAD may protect against injury. In the present study, the insertion of the MAD in rabbits with OSAHS significantly improved the genioglossus mitochondrial morphology and function, which were similar to those in the normal controls. The precise mechanisms of MAD therapy are still unclear. However, MAD directly increased the size of the pharyngeal airway(33). The genioglossus functions as a dilator of the pharyngeal airway and is responsible for maintaining patency of the UA(31). The genioglossus was reported to generate the main protrusive force of the tongue, and its contraction and relaxation substantially affected the dimensions of the UA(34). The potential mechanics that may account for the improvements in genioglossus following MAD insertion could be through the augmentation of the pharyngeal airway and the activation of the genioglossus. The delivery of MAD to the rabbits with OSAHS could be associated with resting of the genioglossus. Furthermore, chronic intermittent hypoxia was eliminated following the insertion of MAD. It was reported that there was no difference in the level of GG activation between OSA patients and healthy individuals when on fully therapeutic CPAP(35). A recent study identified significant increases in genioglossus activity following placement of the MAD(36). This finding suggests that the anti-apnea effects and the increased activity represent two of the most important mechanisms by which MAD protects the genioglossus against OSAHS-induced injury. Ultrastructural observation by transmission electron microscopy indicated that MAD treatment could attenuate the mitochondrial swelling and disrupted cristae in the genioglossus induced by OSAHS.