Hypoxic preconditioning increases mitochondrial respiration and H₂O₂ production

Hypoxia, an inadequate tissue oxygen supply, poses a threat to the brain, which relies heavily on oxygen for its energy requirements. However, mild oxygen deficiency triggers cellular stress, leading to a defensive state known as hypoxic preconditioning (HPC). Despite its potential as a treatment option for neurodegenerative diseases, research on preconditioning remains a challenge. Therefore, this study aimed to further explore biochemical changes induced by HPC, with a specific emphasis on mitochondria, the primary oxygen consumers.

We assessed the neuroprotective impact of a HPC protocol used by examining the seizure thresholds of mice. Additionally, we analyzed mitochondrial respiration under varying oxygen levels, reactive oxygen species (ROS) production, and mitochondrial morphology following HPC treatment.

HPC treatment of mice raised their seizure threshold, indicating an enhanced resistance to epileptic seizures and highlighting the protective effects of the HPC protocol. HPC increased mitochondrial oxygen consumption and ROS production, primarily originating from Complex I. Importantly, ROS levels remaining within the physiological range potentially activate cell signaling pathways. Our findings underscored the importance of controlling oxygen at physiologically relevant intracellular tissue levels (intracellular tissue normoxia) during mitochondrial respiration measurements. Notably, HPC-treated mitochondria generally exhibited reduced oxygen consumption compared to controls under effectively hyperoxic air-saturated oxygen conditions. However, mitochondrial respiration was increased under intracellular tissue normoxia in comparison to the controls measured at air saturation. Moreover, following HPC treatment, we observed alterations in mRNA expression levels associated not just with mitochondrial dynamics but also with perinuclear mitochondrial accumulation and pro-survival signaling. Furthermore, an immediate increase in mitochondrial fusion was observed following hypoxia treatment.