NAD+ is vital to cellular metabolism and is an obligate co-substrate for NAD+ consumers, which affect processes ranging from lifespan and stress response to circadian rhythms. Cells regenerate the NAD+ cleaved by NAD+ consumers via the NAD+ salvage pathway. We have uncovered surprisingly specific roles for the salvage pathway in C. elegans reproductive development. Hermaphrodites with mutant PNC-1, the first enzyme in the NAD+ salvage pathway, have an egg-laying defect, temporally delayed gonadogenesis and abnormal death of
uv1 cells. The nicotinamidase activity of PNC-1 is essential for these processes as ubiquitous expression of two different nicotinamidases robustly rescues the defects, while a catalytically dead version does not. In addition to probing the novel roles for NAD+ regulation of development, these phenotypes serve as models to elucidate the mechanisms of how NAD+ salvage influences organismal physiology. PNC-1 hydrolyzes nicotinamide (NAM) to nicotinic acid (NA) in the NAD+ salvage pathway, thereby reducing NAM levels and increasing NA and subsequently increasing NAD+. NAM is a potent inhibitor of NAD+ consumers, and studies in yeast demonstrate that Pnc1p promotes Sir2-mediated longevity. However, the relative importance of decreasing NAM versus increasing NAD+ on Sir2p regulation has been unclear. We performed pharmacological analysis to determine whether the developmental defects of
pnc-1 mutants were due to increased NAM or decreased NA. Supplementing wild-type worms with NAM recapitulates the
uv1 and egg-laying defects, but not the gonad timing defect. Interestingly, supplementing
pnc-1 mutants with NA restores only the gonad timing. Injecting worms with a different NAD+ precursor, nicotinamide mononucleotide (NMN), also robustly rescued only the gonad timing defect, indicating that NA acts by increasing NAD+. We conclude that PNC-1 modulates both NAM and NAD+ levels in reproductive organ development, with each having distinct, tissue-specific roles. Interestingly, vertebrates employ a different salvage pathway, using NAMPT to convert NAM to NMN as the first step. PNC-1 and NAMPT are functionally equivalent since ubiquitous expression of human NAMPT rescues
pnc-1 reproductive defects. Mammalian NAMPT is secreted, and we discovered an isoform of C. elegans
pnc-1 with a predicted secretion signal. We confirmed secretion of this isoform by its ability to rescue from a distance. Therefore we postulate an evolutionarily conserved extracellular role for NAD+ biosynthetic enzymes.