Superoxide Anion-Dependent Mitochondrial Fission Contributes to Hippocampal Synaptic Dysfunction in Stress-Susceptible Mice

Reduced synaptic plasticity of hippocampal neurons is a core aspect of depression. Mitochondrial dysfunction affects the synaptic plasticity of neurons. However, the characteristics and molecular mechanisms of mitochondrial dysfunction in hippocampal neurons remain unclear. Oxidative stress observed in depression suggests that excess superoxide anion radical (O2 •-), the first ROS generated in mitochondria, may play crucial roles in mediating mitochondrial damage associated with depression. Unfortunately, current small-molecule fluorescent probes may suffer from diffusion after reacting with O2 •-, thereby limiting the accuracy of studying O2 •-'s biological roles in subcellular structures. Thus, we exploited a fluorescence sensing and labeling strategy for accurately acquiring spatiotemporal information on O2 •-. The fluorescent probe (RB-FM) could react with O2 •-, triggering the generation of a covalent fluorescent label that binds to nearby biological nucleophiles. This action facilitates high-precision in situ imaging of O2 •- during mitochondrial dysfunction. The imaging results demonstrated a reduction in dendritic spine density in hippocampal neurons of stress-susceptible mice, accompanied by a significant increase in mitochondrial O2 •- (mtO2 •-)-dependent mitochondrial peripheral fission. Notably, we found an intriguing form of mitochondrial damage: mitochondrial peripheral fission increased, while total mitochondrial fission and mitophagy were unaffected. We further identified a depression-associated pathological cascade beginning with elevated Ca2+ levels in hippocampal neurons, which triggers mtO2 •--dependent reductions in Coq4 and elevations in Parkin, driving mitochondrial peripheral fission and reducing synaptic plasticity. This work provides a mechanistic framework for O2 •- control of mitochondrial peripheral fission and demonstrates how redox signaling relates to synaptic plasticity in depression.