Hypercapnia induces IL-1β overproduction via activation of NLRP3 inflammasome: implication in cognitive impairment in hypoxemic adult rats

Background: Cognitive impairment is one of common complications of acute respiratory distress syndrome (ARDS). Increasing evidence suggests that interleukin-1 beta (IL-1 beta) plays a role in inducing neuronal apoptosis in cognitive dysfunction. The lung protective ventilatory strategies, which serve to reduce pulmonary morbidity for ARDS patients, almost always lead to hypercapnia. Some studies have reported that hypercapnia contributes to the risk of cognitive impairment and IL-1 beta secretion outside the central nervous system (CNS). However, the underlying mechanism of hypercapnia aggravating cognitive impairment under hypoxia has remained uncertain. This study was aimed to explore whether hypercapnia would partake in increasing IL-1 beta secretion via activating the NLRP3 (NLR family, pyrin domain-containing 3) inflammasome in the hypoxic CNS and in aggravating cognitive impairment. Methods: The Sprague-Dawley (SD) rats that underwent hypercapnia/hypoxemia were used for assessment of NLRP3, caspase-1, IL-1 beta, Bcl-2, Bax, and caspase-3 expression by Western blotting or double immunofluorescence, and the model was also used for Morris water maze test. In addition, Z-YVAD-FMK, a caspase-1 inhibitor, was used to treat BV-2 microglia to determine whether activation of NLRP3 inflammasome was required for the enhancing effect of hypercapnia on expressing IL-1 beta by Western blotting or double immunofluorescence. The interaction effects were analyzed by factorial ANOVA. Simple effects analyses were performed when an interaction was observed. Results: There were interaction effects on cognitive impairment, apoptosis of hippocampal neurons, activation of NLRP3 inflammasome, and upregulation of IL-1 beta between hypercapnia treatment and hypoxia treatment. Hypercapnia + hypoxia treatment caused more serious damage to the learning and memory of rats than those subjected to hypoxia treatment alone. Expression levels of Bcl-2 were reduced, while that of Bax and caspase-3 were increased by hypercapnia in hypoxic hippocampus. Hypercapnia markedly increased the expression of NLRP3, caspase-1, and IL-1 beta in hypoxia-activated microglia both in vivo and in vitro. Pharmacological inhibition of NLRP3 inflammasome activation and release of IL-1 beta might ameliorate apoptosis of neurons. Conclusions: The present results suggest that hypercapnia-induced IL-1 beta overproduction via activating the NLRP3 inflammasome by hypoxia-activated microglia may augment neuroinflammation, increase neuronal cell death, and contribute to the pathogenesis of cognitive impairments.