Human dietary deficiency of the nicotinamide adenine dinucleotide (NAD+) precursors nicotinamide and nicotinic acid (collectively called vitamin B3) causes serious dysfunction of multiple organ systems. Furthermore, increased salvage biosynthesis of NAD+ from nicotinamide promotes longevity in yeast. Yet the molecular mechanisms underlying the organ pathologies are mysterious, and similar lifespan regulation has not been extensively probed in an animal model. The first enzyme in the salvage pathway for NAD+ biosynthesis in invertebrates is nicotinamidase, which converts nicotinamide to nicotinic acid. C. elegans has three nicotinamidases, two PNC-1 isoforms and PNC-2. Mutation of PNC-1 results in distinct developmental defects of the reproductive organs. For example, gonad development is delayed and the uterine vulva 1 (uv1) cells necrose.
We aim to decipher how nicotinamidase influences development and longevity in the C. elegans model. I hypothesize that mutation of pnc-1 perturbs nicotinamide and nicotinic acid levels in a tissue-specific manner, which may impact local NAD+ biosynthesis, resulting in specific and separable biological effects. In fact, the observed developmental phenotypes are separable. Uv1 cell necrosis is induced by supplementation with nicotinamide, while the gonad developmental delay of mutants is rescued by supplementation with nicotinic acid. We will determine whether perturbation of nicotinamide, nicotinic acid or NAD+ levels are causative of each phenotype and their relative roles in longevity control using pharmacological and genetic manipulations. I hypothesize that nicotinamidase modulation of NAD+ and nicotinamide levels likely impacts the activity of NAD+ consuming enzymes, which mediate biological effects. We will investigate this hypothesis by studying the impacts of the pathway on the SIR-2.1 NAD+ consumer, which regulates lifespan and stress resistance in C. elegans, and by identifying the NAD+ consumers that mediate the developmental phenotypes. Finally, we have discovered a putative secreted isoform of PNC-1, suggesting an extracellular role for the NAD+ salvage pathway. This discovery is intriguing in light of recent evidence for a systemic role for the first enzyme in the NAD+ salvage pathway in mammalian physiology. We will use transgenic approaches to establish if there is an evolutionarily conserved extracellular function for an NAD+ biosynthetic enzyme in multi-cellular organisms. This work will help elucidate the molecular mechanisms underlying organ pathologies caused by perturbations of NAD+ precursor deficiency and metabolism in humans and will shed light on key aspects of regulation of lifespan promotion by sirtuins.
Compromised biosynthesis of nicotinamide adenine dinucleotide (NAD+) causes multiple physiological and developmental defects in the genetically tractable model organism C. elegans and has been demonstrated to impact longevity in other model systems. Deficiency of NAD+ precursors (vitamin B3) in humans also causes pathology in multiple organ systems. We aim to use C. elegans to probe the underlying molecular and genetic mechanisms causing organ malfunction and as an animal model to investigate models of longevity control.
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