Isonicotinamide extends yeast chronological lifespan through a mechanism that diminishes nucleotides

Chronological lifespan (CLS) of budding yeast, Saccharomyces cerevisiae, is a commonly utilized model for cellular aging of non-dividing cells such as neurons. CLS is strongly extended by isonicotinamide (INAM), a non-metabolized isomer of the NAD+ precursor nicotinamide (NAM), but the underlying mechanisms of lifespan extension remain uncharacterized. To identify potential biochemical INAM targets, we performed a chemical genetic screen with the yeast gene knockout (YKO) strain collection for INAM-hypersensitive mutants. Significantly enriched Gene Ontology terms that emerged included SWR1 and other transcription elongation factors, as well as metabolic pathways converging on one-carbon metabolism and contributing to nucleotide biosynthesis, together suggesting that INAM perturbs nucleotide pools. In line with this model, INAM effects on cell growth were synergistic with mycophenolic acid (MPA), which extends lifespan by reducing guanine nucleotide pools. Direct measurements of nucleotides and precursors by mass spectrometry indicated that INAM reduced nucleotides, including cAMP, at 24- and 96-hour time points post-inoculation. Taken together, we conclude that INAM extends CLS by perturbing nucleotide metabolism, which may be a common functional feature of multiple anti-aging interventions.