The dynamic regulation of NAD metabolism in mitochondria

Abstract

Mitochondria are intracellular powerhouses that produce ATP and carry out diverse functions for cellular energy metabolism. Although the maintenance of an optimal NAD/NADH ratio is essential for mitochondrial function, it has recently become apparent that the maintenance of the mitochondrial NAD pool is also of crucial importance. The biosynthesis, transport, and catabolism of NAD and its key intermediates play an important role in the regulation of NAD-consuming mediators, such as sirtuins, poly-ADP-ribose polymerases, and CD38/157 ectoenzymes, in intra- and extracellular compartments. Mitochondrial NAD biosynthesis is also modulated in response to nutritional and environmental stimuli. In this article, we discuss this dynamic regulation of NAD metabolism in mitochondria to shed light on the intimate connection between NAD and mitochondrial function.

Figure 1. Maintenance of the mitochondrial NAD pool

While separate, the mitochondrial and nuclear/cytoplasmic NAD pools are intricately connected through the NAD/NADH-redox shuttles (most commonly the malate-aspartate and the glycerol-3-phosphate shuttles) and NAD biosynthetic pathways in each subcellular compartment. Multiple cellular processes play an important role in maintaining an optimal NAD/NADH ratio between mitochondria and the cytoplasm, including glycolysis, the tricarboxylic acid (TCA) cycle, and oxidative phosphorylation by the electron transport chain (ETC). Mitochondrial and nuclear/cytoplasmic NAD biosynthetic pathways are balanced in response to nutritional and environmental stimuli. Abbreviations: NAM, nicotinamide; NMN, nicotinamide mononucleotide; NAD, nicotinamide adenine dinucleotide.

Figure 2. The biosynthesis, transport, and catabolism of NAD and its intermediates in intra- and extracellular compartments

In mammals, NAD can be synthesized from tryptophan, nicotinamide, and nicotinic acid. Nicotinamide (NAM) is a major substrate for mammalian NAD biosynthesis (see Box 1). Thus, NAD biosynthetic pathways from tryptophan and nicotinic acid are not shown in this scheme. In the cytoplasm, NAM is converted to nicotinamide mononucleotide (NMN), a key NAD intermediate, by nicotinamide phosphoribosyltransferase (NAMPT). This process might also be mediated by extracellular NAMPT (eNAMPT). It remains unclear whether a similar process occurs in the nucleus and mitochondria. Nicotinamide ribose (NR), another key NAD intermediate, can be produced from NMN extracellularly, likely by the ectoenzyme CD73, and transported into the cell, possibly through equilibrative nucleoside transporters (ENTs). Inside the cell, NR is re-phosphorylated to NMN by NR kinases 1 and 2 (NRK1 and 2). In each subcellular compartment, NMN is converted to NAD by NMN adenylyltransferases, NMNAT1-3. NMN also appears to be transported from the cytoplasm to mitochondria, although the mechanism of this NMN transport is currently unknown. NAD is consumed intracellularly by key mediators, particularly sirtuins and poly-ADP-ribose polymerases (PARPs). NAD can also be transported to the outside of the cell, likely through connexin 43 hemichannels [97]. CD38/157 ectoenzymes produce cyclic ADP-ribose (cADPR) and nicotinic acid adenosine dinucleotide phosphate (NAADP), which re-enter the cell and induce calcium mobilization.

Figure I. Pathways of NAD biosynthesis

Mammalian NAD biosynthesis can occur from de novo synthesis from tryptophan (TRP) as well as conversion of the vitamin B₃ precursors nicotinamide (NAM) or nicotinic acid (NA) or nicotinamide riboside (NR). These four pathways generate common downstream intermediates, as denoted by the dotted green arrows. Usage of each pathway varies in different tissues/organs. See box 1 for details.