Interplay between compartmentalized NAD+ synthesis and consumption: a focus on the PARP family

Abstract

Nicotinamide adenine dinucleotide (NAD⁺) is an essential cofactor for redox enzymes, but also moonlights as a substrate for signaling enzymes. When used as a substrate by signaling enzymes, it is consumed, necessitating the recycling of NAD⁺ consumption products (i.e., nicotinamide) via a salvage pathway in order to maintain NAD⁺ homeostasis. A major family of NAD⁺ consumers in mammalian cells are poly-ADP-ribose-polymerases (PARPs). PARPs comprise a family of 17 enzymes in humans, 16 of which catalyze the transfer of ADP-ribose from NAD⁺ to macromolecular targets (namely, proteins, but also DNA and RNA). Because PARPs and the NAD⁺ biosynthetic enzymes are subcellularly localized, an emerging concept is that the activity of PARPs and other NAD⁺ consumers are regulated in a compartmentalized manner. In this review, I discuss NAD⁺ metabolism, how different subcellular pools of NAD⁺ are established and regulated, and how free NAD⁺ levels can control signaling by PARPs and redox metabolism.

Keywords: ADP-ribosylation; NAD; NAD consumer; PARP; biosensor.

Figure 1.

Pathways of NAD⁺ synthesis, consumption, and redox chemistry. The dashed red line indicates cleavage of the glycosidic bond of NAD⁺ by NAD⁺ consumers. NAD⁺ biosynthetic enzymes are shown in blue, NAD⁺ consumers are shown in red, and NAD⁺ redox enzymes are shown in green. (−) Inhibition; (+) activation.

Figure 2.

NAD⁺ synthesis is compartmentalized in mammalian cells and regulates—and is regulated by—the NAD⁺ consumer PARP1. NMNAT1–3, which synthesize NAD⁺, are distinctly localized in major subcellular compartments: NMNAT1 in the nucleus, NMNAT2 in the cytoplasm, and NMNAT3 in mitochondria. (cPARPs) Cytoplasmic PARPs; (nPARPs) nuclear PARPs. (A) Cartoon representation of NAD⁺, mono-ADP-ribose (MAR), and poly-ADP-ribose (PAR). (B) Under basal conditions, the concentration of NAD⁺ is highest in mitochondria (∼250 μM), followed by the cytoplasm and nucleus (both ∼100 μM). The number of NAD⁺ molecules indicates relative subcellular concentrations of NAD⁺. (C) During DNA damage, PARP1 associates with NMNAT1 and is activated (up arrow), leading to auto-poly-ADP-ribosylation (PARylation). This results in a temporal reduction of NAD⁺ levels in the nucleus, and likely the cytoplasm and mitochondria. The decrease in NAD⁺ levels in all compartments is expected to decrease the activity of nPARPs and cPARPs, as well as glycolytic flux and oxidative phosphorylation (down arrows). (D) Upon adipocyte differentiation, NMNAT2 is induced, which increases NAD⁺ levels in the cytoplasm and leads to an increase in glycolytic flux and perhaps the activity of cPARPs (up arrows). The increase in NMNAT2 levels depletes NMN from the nucleus (not shown), leading to lower NAD⁺ levels in the nucleus and a decrease in the activity of PARP1 and perhaps other nPARPs (down arrows).