Neuronal SIRT3 Decline Mediates Sepsis-Associated Encephalopathy through Microglia Activation

Background: Sirtuin3 (SIRT3), a pivotal mitochondrial regulatory enzyme, protects cellular components in the central nervous system against oxidative and metabolic stress. However, its influence on cognitive functions, especially within the hippocampal neurons and the cognitive disruptions induced by sepsis, remains elusive. This study aimed to elucidate the direct impact of SIRT3 on neuronal health and its involvement in cognitive deficits following sepsis. Methods: Behavioral changes were evaluated using fear conditioning protocols in wild-type (WT) and SIRT3 deficient (Sirt3(-/-)) mouse models. SIRT3 expression levels were quantified in septic mice following intraperitoneal lipopolysaccharide (LPS) administration (LPS, 10 mg/kg). In cellular experiments, HT22 neuronal cells were exposed to LPS (1 mu g/mL) or conditioned media from LPS-stimulated BV2 microglial cells (Mi-sup). Western blot analysis was employed to assess the presence of synapse-related and mitochondrial apoptosis-associated proteins. Oxidative stress and mitochondrial membrane potential (MMP) were respectively performed with reactive oxygen species (ROS) staining and 5,5 ',6,6 '-tetrachloro1,1 ',3,3 '-tetraethylbenzimid azolocarbocyanine iodide (JC-1) staining. Mitochondrial p66Shc and c-Jun N-terminal kinase (JNK) phosphorylation levels were evaluated through western blotting and immunofluorescence assays. Results: Contextual and cued freezing durations were significantly shorter in Sirt3(-/-) mice than in wild-type counterparts (p < 0.05 and p < 0.01, respectively). Sirt3(-/-) mice exhibited a reduction in postsynaptic density protein 95 (PSD95) (p < 0.05) and an increase in brain-derived neurotrophic factor precursor (pro-BDNF) (p < 0.001). Sepsis led to a marked decrease in SIRT3 expression (p < 0.001) in mice. In cellular studies, the elevation of SIRT3 significantly increased PSD95 (p < 0.01), reduced pro-BDNF (p < 0.05), and enhanced the B-cell lymphoma 2/Bcl-2-associated X protein (Bcl-2/Bax) ratio (p < 0.05) while decreased cytochrome C release (p < 0.05) in Mi-sup-treated HT22 cells. Additionally, SIRT3 elevation reduced ROS production (p < 0.05) and enhanced MMP (p < 0.05). Mi-sup exposure was associated with elevated p-p66Shc/p66Shc and p-JNK/JNK ratio (p < 0.05 and p < 0.01). The activation of JNK/p66Shc was mitigated by either SIRT3 overexpression or the application of a JNK inhibitor (p < 0.01). Conclusion: SIRT3 deficiency contributes to cognitive impairment. SIRT3 expression was reduced in the hippocampus of septic mice, resulting in similar cognitive deficits. Mechanistically, the neuronal JNK/p66Shc activation contributed to SIRT3 deficiency-mediated synaptic dysfunction and mitochondrial apoptosis in response to microglia activation-induced inflammation and oxidative stress. Our findings provide new insights into the role of SIRT3 in cognitive function and its potential clinical significance in sepsis-induced cognitive impairment.