BHB and NMN mimicking caloric restriction in Daphnia
NAD+ homeostasis is an important determinant of lifespan and may be a key mechanism of caloric restriction (CR) expansion of lifespan. Ketone bodies such as beta-hydroxybutyrate (BHB) that regulate NAD+ abundance and NAD+ precursors such nicotinamide mononucleotide (NMN), as are known to extend life in experimental animals and ameliorate age-related conditions in humans. We tested the hypothesis that chronic BHB and NMN exposure separately or in combination can extend lifespan in a model organism Daphnia, a freshwater zooplankton crustacean with the magnitude similar to that of the CR treatment. We also measured fecundity, lipofuscin accumulation, and lipid investments into offspring in Daphnia fed the full diet, full diet with BHB, NMN, and combined treatments, and fed the CR diet (25% of the full diet). We also conducted an RNAseq experiment comparing the two diets and the two exposure treatments. We show that BHB exposure, but not NMN exposure reduces early life mortality in Daphnia fed the full diet to levels similar to those observed under CR without compromising fecundity. We also observed that in a combined exposure cohort, NMN nearly eliminates the beneficial effect of BHB. None of the treatments affected lipofuscin accumulation, but the NMN and the combined treatment mimicked the effect of CR on neonate size in older females. We show that BHB-treated Daphnia change expression of a variety of genes, including genes with known longevity extending effects, but differential expression of few genes is consistent with the effects of CR.