Metabolic Intervention with Dimethyl Malonate Impairs Phagocytic Clearance but Fails to Protect Neurons

Secondary degeneration following optic nerve crush (ONC) is driven in part by mitochondrial dysfunction and microglial activation. Inspired by hibernation, where reduced succinate oxidation limits reactive oxygen species (ROS) production, we tested whether pharmacological inhibition of this pathway confers neuroprotection. Using in vivo ONC models and in vitro microglial assays, we evaluated the effects of dimethyl malonate (DMM), an inhibitor of succinate dehydrogenase, and a cell-permeable succinate analog (succinate-NV). Succinate-NV increased pro-inflammatory cytokine expression (IL-1β) and reduced anti-inflammatory IL-10, whereas non-permeable succinate had no effect, indicating that intracellular succinate can drive microglial activation. In hibernating animals, succinate-NV disrupted neuroprotection and reduced retinal ganglion cell (RGC) survival following optic nerve injury. Although DMM partially reduced select inflammatory cytokines, it failed to normalize IL-1β or IL-10 and suppressed microglial phagocytosis while exhibiting cytotoxic effects. In vivo, DMM-treated animals showed reduced IBA1 microglia but increased CD68 activation and accumulation of DAPI cells at 7 days post-injury at the crush site. RGC somas persisted but were Caspase3+ consistent with impaired clearance. Astrocyte reactivity increased at lesion borders, while reduced and fragmented GFAP at the lesion site indicated localized astrocyte loss. Collectively, these findings demonstrate that inhibition of succinate oxidation alone is insufficient for neuroprotection and underscore the need for coordinated metabolic and immune regulation that cannot be achieved through single-pathway pharmacological intervention.