Purine availability determines survival of complex I-deficient fibroblasts under OXPHOS-reliant condition.

Growing fibroblasts in medium containing galactose instead of glucose makes cells more dependent on oxidative phosphorylation (OXPHOS) rather than glycolysis. Recently, it has been shown that treatment with nicotinamide adenine dinucleotide (NAD+) improves cell viability of human OXPHOS Complex I-deficient skin fibroblasts in this OXPHOS-reliant condition. To study the effects of NAD+ precursors in this model, we supplemented the cells with trigonelline (TG), nicotinamide (NAM), nicotinamide riboside (NR), nicotinamide mononucleotide (NMN) and found no improvement in cell viability, while supplementation with niacin (NA) did partially. Interestingly, cell viability was improved by addition of the NAD+ building block adenosine monophosphate (AMP). Metabolic profiling showed a decrease in inosine monophosphate (IMP), a purine intermediate, under OXPHOS-reliant condition. Treatment with AMP and adenosine under this condition increased the IMP levels. Among nucleosides, only adenosine and guanosine treatments in OXPHOS-reliant condition improved the patient cell viability. Therefore, we conclude that OXPHOS-reliant condition limited purine nucleotide biosynthesis and salvage pathways, which could be overcome by increasing IMP levels. Together, our results reveal a previously unrecognized role for purine metabolism under OXPHOS-reliant condition, highlighting it as a candidate for therapeutic intervention.