Stress pathways induced by volatile anesthetics and failure of preconditioning in a mitochondrial Complex I mutant

Carriers of mutations in the mitochondrial electron transport chain (mETC) are at increased risk of anesthetic-induced neurotoxicity. To investigate the neurotoxicity mechanism and to test preconditioning as a protective strategy, we used a Drosophila melanogaster model of Leigh syndrome. Model flies carried a mutation in ND23 (ND23⁶⁰¹¹⁴) that encodes an mETC Complex I subunit. We investigated why ND23⁶⁰¹¹⁴ mutants become susceptible to lethal, oxygen-modulated neurotoxicity within 24 h of exposure to isoflurane but not sevoflurane.

We used transcriptomics and qRT-PCR to identify genes that are differentially expressed in ND23⁶⁰¹¹⁴ but not wild type fly heads at 30 min after exposure to high versus low toxicity conditions. We also subjected ND23⁶⁰¹¹⁴ flies to diverse stressors prior to isoflurane exposure to test whether isoflurane toxicity could be diminished by preconditioning.

The ND23⁶⁰¹¹⁴ mutation had a greater effect on isoflurane- than sevoflurane-mediated changes in gene expression. Isoflurane and sevoflurane did not affect expression of heat shock protein (Hsp) genes (Hsp22, Hsp27, and Hsp68) in wild type flies, but isoflurane substantially increased expression of these genes in ND23⁶⁰¹¹⁴ mutant flies. Furthermore, isoflurane and sevoflurane induced expression of oxidative (GstD1and GstD2) and xenobiotic (Cyp6a8 and Cyp6a14) stress genes to a similar extent in wild type flies, but the effect of isoflurane was largely reduced in ND23⁶⁰¹¹⁴ flies. In addition, activating stress response pathways by preexposure to anesthetics, heat shock, hyperoxia, hypoxia, or oxidative stress did not suppress isoflurane-induced toxicity in ND23⁶⁰¹¹⁴ mutant flies.

Mutation of an mETC Complex I subunit generates differential effects of isoflurane and sevoflurane on gene expression that may underlie their differential effects on neurotoxicity. Additionally, the mutation produces resistance to preconditioning by stresses that protect the brain in other contexts. Therefore, Complex I activity modifies molecular and physiological effects of anesthetics in an anesthetic-specific manner.