Hypercapnia promotes microglial pyroptosis via inhibiting mitophagy: implication in neuroinflammation in hypoxemic adult rats

Abstract

Background: Hypoxemia is a typical symptom of acute respiratory distress syndrome (ARDS). Ventilations are always needed for correcting hypoxemia. To avoid pulmonary morbidity, low tidal volume ventilation is often applied. The ventilation strategy will certainly result in hypercapnia. Our previous study found that hypercapnia could induce IL-1β overproduction in hypoxia-activated microglia. However, the underlying mechanism has remained unclear. This study aimed to explore whether hypercapnia would promote microglial pyroptosis via inhibiting mitophagy in adult rats with hypoxemia. Methods: Cerebral oxygen extraction ratio (CERO2), partial pressure of brain tissue oxygen (PbtO2), and reactive oxygen species (ROS) production in brain tissue in a rat model of hypercapnia/hypoxemia were evaluated. Along with this, the oxygen consumption rate (OCR) and ROS production of BV-2 microglial cells were evaluated after 15% CO2/0.2% O2 treatment. Mitophagy was observed using transmission electron microscopy (TEM). The protein expression level of LC3-II/I, p62, caspase-1, gasdermin D-N domains (GSDMD-N), IL-1β and IL-18 in microglial cells were detected before and after application of a ROS scavenger in vivo and in vitro. Results: PbtO2 level was elevated by hypercapnia in the hypoxemic rats in the first 1.5 h, but it was significantly decreased 2 h after ventilation. This was further evident by the increased levels of CERO2 at 3 h after ventilation. Besides, a high concentration of CO2 treatment could increase the levels of OCR in hypoxic BV-2 microglial cells in vitro. Expression levels of LC3-II were reduced, while those of p62 were increased by hypercapnia in the hypoxic hippocampus and BV-2 microglia. Autophagosomes could be seen in hypoxia treated microglial cells under TEM, while those were rarely observed in hypercapnia + hypoxia treated microglia. Hypercapnia markedly increased the production of ROS and the expression of caspase-1, GSDMD-N, IL-1β, and IL-18 in hypoxia-activated microglia both in vivo and in vitro. Pharmacological scavenging ROS inhibited microglial pyroptosis and expression of IL-1β and IL-18.Conclusion: Hypercapnia-induced mitophagy inhibition may promote pyroptosis and enhance IL-1β and IL-18 release in hypoxia-activated microglia.