SIRT3 Protects Against Cognitive Dysfunction Induced by Sepsis-associated Encephalopathy Via JNK/p66Shc-Regulated Mitochondrial Apoptosis in Mice

Abstract Background: Sepsis-associated encephalopathy (SAE) is one of the severe central nervous system complications. Oxidative stress and synaptic dysfunction were involved in cognitive impairment induced by SAE. The mitochondrial nicotinamide adenine dinucleotide (NAD+) dependent deacetylase, sirtuin3 (SIRT3), plays a critical role in regulating mitochondrial function. The aim of this study was to evaluate the effect of SIRT3 in cognitive dysfunction induced by SAE.Methods: Mice were treated with lipopolysaccharide (LPS, 10 mg/kg, i.p.). Contextual and cue memory were evaluated by fear conditioning test in wild-type (WT) and SIRT3-deficient (SIRT3-/-) mice. Synapse-associated proteins and mitochondrial apoptosis-associated protein were examined by western blotting. In vitro studies, acetylation levels of cyclophilin D (CypD) were detected with different SIRT3 deacetylase activity in HT22 cells after LPS-induced microglia supernatant (Mi-sup) exposure. Oxidative stress was detected by reactive oxygen species (ROS) staining, and mitochondrial membrane potential (MMP) was detected by JC-1 staining, and mitochondrial membrane permeability transition pore (MPTP) opening was detected by Calcein and Co2+ staining. Furthermore, the phosphorylation levels of mitochondrial p66Shc and JNK were evaluated by western blotting.Results: SIRT3 expression was diminished in hippocampus of mice after LPS treatment. SIRT3-deficiency contributed to more severe contextual memory loss and synaptic dysfunction, decreased ratio of Bcl-2/Bax and increased Cyt C release to cytoplasm in hippocampus compared with wild-type controls. In HT22 cells, lysine acetylation levels of CypD were significantly increased after Mi-sup exposure and further enhanced with 3-TYP (SIRT3 deacetylation inhibitor) pretreatment, in association with the accumulation of ROS, declined MMP and increased MPTP opening, as well as the increased mitochondrial Cyt C release and phosphorylation levels of mitochondrial JNK and p66Shc-Ser36. SIRT3 overexpression restored CypD lysine acetylation levels and MPTP opening in HT22 cells after Mi-sup exposure and reduced mitochondrial JNK and p66Shc activation. Conclusions: Taken together, our results showed that SIRT3-mediated CypD deacetylation was involved in LPS-induced hippocampal synaptic dysfunction, via ROS accumulation, declined MMP, increased MPTP opening, mitochondrial Cyt C release and mitochondrial apoptosis of hippocampal neuron via JNK/p66Shc pathway. Our results revealed that SIRT3 may be a promising therapeutic and diagnostic target for cognitive dysfunction induced by SAE.