NAD+ Metabolism and Regulation: Lessons From Yeast

Abstract

Nicotinamide adenine dinucleotide (NAD⁺) is an essential metabolite involved in various cellular processes. The cellular NAD⁺ pool is maintained by three biosynthesis pathways, which are largely conserved from bacteria to human. NAD⁺ metabolism is an emerging therapeutic target for several human disorders including diabetes, cancer, and neuron degeneration. Factors regulating NAD⁺ homeostasis have remained incompletely understood due to the dynamic nature and complexity of NAD⁺ metabolism. Recent studies using the genetically tractable budding yeast Saccharomyces cerevisiae have identified novel NAD⁺ homeostasis factors. These findings help provide a molecular basis for how may NAD⁺ and NAD⁺ homeostasis factors contribute to the maintenance and regulation of cellular function. Here we summarize major NAD⁺ biosynthesis pathways, selected cellular processes that closely connect with and contribute to NAD⁺ homeostasis, and regulation of NAD⁺ metabolism by nutrient-sensing signaling pathways. We also extend the discussions to include possible implications of NAD⁺ homeostasis factors in human disorders. Understanding the cross-regulation and interconnections of NAD⁺ precursors and associated cellular pathways will help elucidate the mechanisms of the complex regulation of NAD⁺ homeostasis. These studies may also contribute to the development of effective NAD⁺-based therapeutic strategies specific for different types of NAD⁺ deficiency related disorders.

Keywords: NAD+ metabolism; Sir2 family; nutrient signaling.

Figure 1

NAD⁺ biosynthesis pathways. In yeast cells, NAD⁺ can be made by salvaging precursors such as NA, NAM and NR or by de novo synthesis from tryptophan. Yeast cells also release and re-uptake these precursors. The de novo NAD⁺ synthesis (left panel) is mediated by Bna proteins (Bna2,7,4,5,1) leading to the production of NaMN. This pathway is inactive when NAD⁺ is abundant. The NA/NAM salvage pathway (center panel) also produces NaMN, which is then converted to NaAD and NAD⁺ by Nma1/2 and Qns1, respectively. NR salvage (right panel) connects to the NA/NAM salvage pathway by Urh1, Pnp1 and Meu1. NR turns into NMN by Nrk1, which is then converted to NAD⁺ by Nma1, Nma2 and Pof1. This model centers on NA/NAM salvage (highlighted with bold black arrows) because most yeast growth media contain abundant NA. Cells can also salvage NaR by converting it to NA or NaMN. For simplicity, NaR salvaging is not shown in this figure. Arrows with dashed lines indicate the mechanisms of these pathways remain unclear. NA, nicotinic acid. NAM, nicotinamide. NR, nicotinamide riboside. NaR, nicotinic acid riboside. QA, quinolinic acid. L-TRP, L-tryptophan. NFK, N-formylkynurenine. L-KYN, L-kynurenine. 3-HK, 3-hydroxykynurenine. 3-HA, 3-hydroxyanthranilic acid. NaMN, nicotinic acid mononucleotide. NaAD, deamido-NAD⁺. NMN, nicotinamide mononucleotide. Abbreviations of protein names are shown in parentheses. Bna2, tryptophan 2,3-dioxygenase. Bna7, kynurenine formamidase. Bna4, kynurenine 3-monooxygenase. Bna5, kynureninase. Bna1, 3-hydroxyanthranilate 3,4-dioxygenase. Bna6, quinolinic acid phosphoribosyltransferase. Nma1/2, NaMN/NMN adenylyltransferase. Qns1, glutamine-dependent NAD⁺ synthetase. Npt1, nicotinic acid phosphoribosyltransferase. Pnc1, nicotinamide deamidase. Sir2 family, NAD⁺-dependent protein deacetylases. Urh1, Pnp1 and Meu1, nucleosidases. Nrk1, NR kinase. Isn1 and Sdt1, nucleotidases. Pho8 and Pho5, phosphatases. Pof1, NMN adenylyltransferase. Tna1, NA and QA transporter. Nrt1, NR transporter.

Figure 2

Cellular processes that are closely connected with NAD⁺ homeostasis. Various cellular processes together with compartmentalization of intracellular NAD⁺ and its derivatives contribute to the regulation of NAD⁺ homeostasis. For example, NAD⁺ and intermediates can enter the vacuole through vesicular tracking and are then converted to small NAD⁺ precursors. Small NAD⁺ precursors such as NR can travel between the vacuole and cytoplasm through specific nucleoside transporters. Small NAD⁺ precursors can exit cells likely by vesicular trafficking and re-enter through specific transporters on the plasma membrane. Sirtuin mediated gene silencing in the nucleus consumes NAD⁺. The nucleus and cytoplasm share the same NAD⁺ pool because NAD⁺ is anticipated to pass the nuclear pore by simple diffusion. The mitochondrial and peroxisomal NAD⁺(H) redox shuttle systems do not directly affect NAD⁺ metabolism, instead, they function to balance redox equivalents between the organellar and the cytosolic pools to regulate the NAD⁺/NADH ratio.

Figure 3

Regulation of NAD⁺ metabolism in yeast. A model depicting the regulation of NAD⁺ metabolism by cellular signaling pathways. NAD⁺ and NAD⁺ precursors are italicized and shown in bold. Abbreviations of protein names are highlighted by oval shapes. Dashed lines indicate additional evidence is required to reveal the mechanisms.