Reduced expression of SOD2 in the Retrotrapezoid nucleus is associated with enhanced ventilatory chemoreception: implications for breathing disorders in heart failure

Enhanced chemoreflex drive and irregular breathing patterns are both hallmarks in heart failure (HF). The Retrotrapezoid nucleus (RTN), a main area for central chemoreception, contribute to maintain both heightened chemoreflex drive and irregular breathing in experimental HF. Recently, we found that high levels of reactive oxygen species (ROS) within the RTN of HF rats were closely linked to the enhanced central chemoreflex and the generation of breathing disorders. Despite this evidence, the effects of ROS on RTN-mediated chemoreception and the precise mechanism(s) associated with ROS generation in the RTN has not been previously studied. However, pro/anti-oxidant enzymes gene screening in RTN from HF rats revealed reduce expression of super oxide dismutase 2 (SOD2). Therefore, in the present study we aimed to determine whether reductions in SOD2 expression in healthy subjects resembles i) high RTN ROS levels, ii) heightened chemoreflex drive, and iii) generation of breathings disorders observed in HF animals. Accordingly, breathing function and central chemoreflex drive were assessed in freely moving heterozygous SOD2+/- mice by unrestrained whole-body plethysmography. ROS levels were determined in brainstem sections containing the RTN using dihydroethidium staining. SOD2 protein expression and tissue localization were determined by immunoblot and immunofluorescence, respectively. SOD2+/- mice displayed reduced SOD2 protein expression (65.6 ± 3.2 vs. 100.0 ± 5.8 %; SOD2+/- vs. WT, respectively) and high ROS levels at the RTN compared to WT littermates (2.50 ± 0.46 vs. 1.00 ± 0.05 a.u.; SOD2+/- vs. WT, respectively). In addition, SOD2+/- mice displayed an exacerbated ventilatory response to hypercapnia compared to WT mice (3.5 ± 0.2 vs. 1.9 ± 0.2 ml/min/10g; SOD2+/- vs. WT, respectively). Importantly, the latter was closely linked to RTN ROS production since a positive correlation (p<0.05) between ROS levels and the hypercapnic ventilatory response was found only in SOD2+/- animals. Furthermore, SOD2+/- mice showed overt signs of disordered breathing while resting at eupneic conditions compared to WT mice as evidenced by large increases in ventilatory instability and apneas/hypoapneas incidence (AHI: 7.7 ± 0.9 vs. 3.4 ± 0.6 events/hr; SOD2+/- vs. WT, respectively). Lastly, we found that SOD2 expression is preferentially confined to RTN neurons rather than to medullary astrocytes. Our results shows that reductions in the SOD2 gene expression in healthy rodents mimic the respiratory phenotype observed in experimental HF and suggests that reduced expression of SOD2 within the RTN of HF animals may account for the local increases in ROS production, the enhanced central chemoreflex drive and the generation/maintenance of irregular breathing patterns.