Background: The mechanisms by which inhaled anesthetics cause neurotoxicity are not well clarified. Exposure to inhaled anesthetics induces a release of Ca from the endoplasmic reticulum (ER) into the cytosol. Aberrant Ca mobilization may alter the protein-folding environment in the ER, causing ER stress. Binding immunoglobulin protein (BiP) is an ER chaperone that is critical to ER functions. Because ER stress leads to cellular dysfunction and apoptotic cell death, leading to diverse human disorders such as neurodegenerative diseases, we hypothesized ER stress may play a role in neurotoxicity caused by inhaled anesthetics.
Methods: We investigated the relationship between ER stress and neurodegeneration caused by inhaled anesthetics by using knock-in mice expressing a mutant BiP and neuronal culture cells. Neuronal culture cells and mutant BiP pregnant mice were exposed to 3% sevoflurane. The levels of BiP and C/EBP homologous protein (CHOP), a transcription factor related to cell death during ER stress, were evaluated by Western blot in neuronal cells and fetal brains delivered by cesarean delivery. Cell death in the fetal brains was evaluated with TUNEL staining. Statistical significance was assessed using unpaired t test and analysis of variance followed by multiple comparison tests.
Results: Sevoflurane exposure enhanced the expression of BiP and CHOP significantly in neuronal culture cells. A chemical chaperone that assisted ER functions reduced the expression of CHOP induced by sevoflurane exposure. In an in vivo study, we observed that an enhanced expression of CHOP and significantly more apoptotic cells in the brains of homozygous mutant BiP fetuses compared with the wild type. Mouse embryonic fibroblasts derived from the mutant BiP mice also exhibited enhanced levels of CHOP and cleaved caspase-3 after sevoflurane exposure.
Conclusions: Sevoflurane exposure may cause ER stress, which is tolerated to some extent in wild-type cells. When this tolerance is limited, like in cells with mutant BiP, the exposure leads to cell death in the brain, suggesting that ER stress may partially mediate neurotoxicity caused by inhaled anesthetics. This study suggests that patients with certain conditions sensitive to ER stress such as ischemia, hypoxia, developing brain, or neurodegenerative diseases may be vulnerable to inhaled anesthetics.