The human NAD metabolome: Functions, metabolism and compartmentalization

Abstract

The metabolism of NAD has emerged as a key regulator of cellular and organismal homeostasis. Being a major component of both bioenergetic and signaling pathways, the molecule is ideally suited to regulate metabolism and major cellular events. In humans, NAD is synthesized from vitamin B3 precursors, most prominently from nicotinamide, which is the degradation product of all NAD-dependent signaling reactions. The scope of NAD-mediated regulatory processes is wide including enzyme regulation, control of gene expression and health span, DNA repair, cell cycle regulation and calcium signaling. In these processes, nicotinamide is cleaved from NAD(+) and the remaining ADP-ribosyl moiety used to modify proteins (deacetylation by sirtuins or ADP-ribosylation) or to generate calcium-mobilizing agents such as cyclic ADP-ribose. This review will also emphasize the role of the intermediates in the NAD metabolome, their intra- and extra-cellular conversions and potential contributions to subcellular compartmentalization of NAD pools.

Keywords: ADP-ribosylation; NAD biosynthesis; calcium signaling; extracellular NAD degradation; protein deacetylation; subcellular NAD pools.

Figure 1.

Overview of the NAD metabolome in humans. NAD can be synthesized from five precursors: tryptophan (Trp), the pyridine bases nicotinamide (Nam) and nicotinic acid (NA) or the nucleosides Nam riboside (NR) and NA riboside (NAR), which enter cells by different transport mechanisms. Quinolinic acid (QA), a Trp degradation product, is transformed to NA mononucleotide (NAMN) by quinolinic acid phosphoribosyltransferase (QAPRT). Nam and NA are converted to the corresponding mononucleotides (NMN and NAMN) by nicotinamide phosphoribosyltransferase (NamPRT, also known as NAMPT) and nicotinic acid phosphoribosyltransferase (NAPRT), respectively. NMN might also be synthesized by an extracellular NamPRT form (eNAMPT). NMN and NAMN are also generated through phosphorylation of NR and NAR, respectively, by nicotinamide riboside kinases (NRK). NAMN and NMN are converted to the corresponding dinucleotide (NAAD or NAD⁺) by NMN adenylyltransferases (NMNAT). NAD synthetase (NADS) amidates NAAD to NAD⁺. Phosphorylation by NAD kinase (NADK) converts NAD⁺ to NADP⁺. The oxidized and reduced forms of the dinucleotides, NAD(P)⁺ and NAD(P)H, serve as reversible hydrogen carriers in redox reactions. Members of the Sirtuin family of protein deacetylases catalyze the transfer of the protein-bound acetyl group onto the ADP-ribose moiety, thereby forming O-acetyl-ADP ribose (OAcADPR). The transfer of a single (mono-ADP-ribosylation) or several (poly-ADP-ribosylation) ADP-ribose units from NAD⁺ to acceptor protein is catalyzed by diphtheria toxin-like ADP-ribosyltransferases (ARTD). Mono-ADP-ribosylation is also catalyzed by clostridial toxin-like ADP-ribosyltransferases (ARTC) and some Sirtuin proteins. NAD⁺ and NADP⁺ are also used for the synthesis of second messengers, nicotinic acid adenine dinucleotide phosphate (NAADP), cyclic ADP-ribose (cADPR) and ADPR, which mediate intracellular calcium mobilization. All the three molecules are synthesized by ecto-NAD glycohydrolases CD38 and CD157. The mechanism of how messengers reach their cytosolic targets is still debated. Signaling-independent interconversions of NAD and its intermediates include NAD hydrolysis to NMN and AMP by Nudix pyrophosphatases (NUDT); NMN dephosphorylation to NR by cytosolic 5′-nucleotidases (5′-NT); phosphorolytic cleavage of NR to Nam by purine nucleoside phosphorylase (PNP); and conversion of Nam to N-methylnicotinamide (1-MNA) by nicotinamide-N-methyltransferase (NNMT). NAD⁺ can possibly be released from cells through connexin 43 hemichannels (Cx43), and can be degraded to NR by ecto-nucleotidase CD73. NR is hydrolyzed to Nam by CD157. Whether cells can take up NAD or NMN is debated. (see colour version of this figure at www.informahealthcare.com/bmg).

Figure 2.

Subcellular compartmentalization of NAD pools. An outline of four subcellular NAD pools (nuclear/cytosolic, mitochondrial, peroxisomal and ER/Golgi) is shown. NAD(P)-dependent processes in each compartment are indicated in frames. Cytosolic and nuclear pools of NAD are combined based on the assumption that NAD can freely exchange between these two compartments. Nuclear/cytosolic and mitochondrial pools are maintained by NAD biosynthesis in these compartments. The three NMNAT isoforms catalyzing the generation of NAD from NMN are localized within the nucleus (NMNAT1), at the surface of the Golgi apparatus (NMNAT2) and within the mitochondria (NMNAT3). It is suggested that NMN is imported from the cytosol into the mitochondrial matrix, but the mechanism of this transport is unclear. The peroxisomal NAD pool can be maintained by import of the dinucleotide from the cytosol through the carrier protein SLC25A17. Peroxisomal NAD can be cleaved to AMP and NMN by Nudix hydrolase NUDT12. It is assumed that the generated NMN is released from the organelle to the cytosol through the PXMP2 channel. Although NAD(P)-dependent processes in ER have been identified, the mechanism of NAD supply to this organelle remains unknown. Moreover, a possible role of NAD in the Golgi lumen is obscure. (see colour version of this figure at www.informahealthcare.com/bmg).