Sodium formate-induced mitochondrial impairment and cytotoxicity in neuronal cells reveal crucial pathogenic mechanisms underlying diabetic neuropathy and retinopathy.

Formic acid, a toxic one-carbon metabolite formed from methanol and formaldehyde in the body, can cause neuronal dysfunctions. It was recently hypothesized that metabolic formation of toxic one-carbon metabolites (particularly formic acid) under hyperglycemic conditions may contribute to the pathogenesis of diabetic complications in humans. The present study aims to investigate the mechanism of formic acid-induced neurotoxicity using immortalized HT22 mouse hippocampal neurons as an in-vitro model. We found that treatment of cells with sodium formate (SF, a salt form of formic acid) causes a concentration-dependent loss of cell viability (based on MTT assay), whereas the cell number is reduced to a lesser degree when SF is present at lower concentrations. In addition, SF at the lower concentrations decreases cell proliferation by suppressing DNA synthesis, but at higher concentrations, SF induces cell death through apoptosis. SF can preferentially cause accumulation of mitochondrial ROS, disruption of mitochondrial structure, and suppression of mitochondrial functions (including ATP production). SF-induced mitochondrial ROS accumulation subsequently leads to the depletion of cellular glutathione, along with the buildup of cellular ROS and lipid-ROS. These changes jointly lead to increased permeability of both cytoplasmic and mitochondrial membranes, and ultimately the induction of apoptotic cell death. Analysis of the cellular transcriptomics revealed that the expression of genes for the relevant enzymes and proteins involved in mitochondrial function, energy metabolism and other cellular processes is altered in SF-treated cells. These findings highlight mitochondria as a crucial target in mediating SF-induced cytotoxicity, and also shed mechanistic lights on how formic acid accumulation may contribute to the pathogenesis of diabetic neuropathy and other diabetic complications.