NADPH oxidase 2 knockout prevents chronic intermittent hypoxia induced sternohyoid muscle weakness in adult male mice
Authors: Drummond Sarah E. , Burns David P. , Healy Vincent , O'Halloran Ken D. 
Published: June 2021

Obstructive sleep apnoea syndrome (OSAS) is the most prevalent form of sleep disordered breathing (SDB) which affects 1 in 5 people worldwide. Chronic intermittent hypoxia (CIH) is a hallmark feature of SDB as a consequence of repetitive upper airway occlusions in patients during sleep. These recurring hypoxia‐reoxygenation cycles result in the overproduction of reactive oxygen species (ROS), ultimately yielding a state of oxidative stress. Excessive levels of ROS have been associated with aberrant plasticity at multiple levels of the respiratory system including impaired upper airway muscle function. However, there is a paucity of information regarding the molecular mechanisms underlying these effects. The NADPH oxidase (NOX) family of enzymes have been linked to a multitude of CIH induced morbidities in a range of organs and systems with NOX subunits also having been identified in skeletal muscle.A mouse model of CIH was generated by the cycling of gas from normoxia (21% O2) for 210 seconds to hypoxia (5% O2 at the nadir) for 90 seconds 8 hr/day for 2 weeks during light hours. 10–11 week old male mice (C57BL/6J) were randomly assigned to one of 3 groups: Normoxic controls (sham; n=24), CIH exposed (n=24), and CIH + Apocynin (2 mM; n=24) treated throughout the gas exposure. 10–11 week old NOX2 null (B6.129S‐Cybbtm1Din/J) male mice were also assigned to a sham (n=12) or CIH (n=12) group. Following this, sternohyoid muscle contractile function was examined ex vivo. Gene expression of the entire family of NOX enzymes was examined by RT‐PCR. Western blot was used to quantify NOX2 and NOX4 protein expression. NOX enzyme activity was determined using a NOX activity assay. Data were expressed as mean±SD and were statistically compared by unpaired Student t‐test or two‐way ANOVA with Bonferroni post‐hoc test.2 weeks of exposure to CIH resulted in a significant decrease of ~45% in the peak specific force (Fmax) of the sternohyoid muscle when compared to normoxic controls (sham). CIH resulted in no significant alteration to the gene expression of any members of the NOX family of enzymes when compared to sham controls. There was no significant difference in NOX2 or NOX4 protein expression between CIH and sham groups. However, CIH exposure resulted in a significant increase in sternohyoid NOX enzyme activity compared to shams. Treatment with Apocynin (2 mM) throughout the 2 week CIH exposure prevented sternohyoid muscle weakness, restoring force to levels equivalent to controls. Furthermore, NOX2 knockout prevented CIH induced sternohyoid muscle weakness.Adult male C57BL/6J mice show signs of profound sternohyoid muscle dysfunction following exposure to 2 weeks of CIH when compared with sham animals. This occurs without alteration to the gene or protein expression of various NOX subunits. Increased NOX activity suggests a CIH induced increase in NOX enzyme assembly rather than abundance. Administration of the putative NOX inhibitor, Apocynin, prevents sternohyoid muscle weakness. NOX2 knockout also entirely prevents sternohyoid muscle weakness confirming its specific role in CIH induced muscle dysfunction. The striking muscle phenotype in the C57 mouse and its prevention in the absence of NOX2 provides a novel and robust platform for the study of mechanisms of hypoxia‐related muscle dysfunction, which may have relevance to respiratory conditions characterised by CIH, such as sleep apnoea.Support or Funding InformationDepartment of Physiology, University College Cork, Cork, IrelandThis abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.