NAD+ in Brain Aging and Neurodegenerative Disorders

Abstract

NAD⁺ is a pivotal metabolite involved in cellular bioenergetics, genomic stability, mitochondrial homeostasis, adaptive stress responses, and cell survival. Multiple NAD⁺-dependent enzymes are involved in synaptic plasticity and neuronal stress resistance. Here, we review emerging findings that reveal key roles for NAD⁺ and related metabolites in the adaptation of neurons to a wide range of physiological stressors and in counteracting processes in neurodegenerative diseases, such as those occurring in Alzheimer's, Parkinson's, and Huntington diseases, and amyotrophic lateral sclerosis. Advances in understanding the molecular and cellular mechanisms of NAD⁺-based neuronal resilience will lead to novel approaches for facilitating healthy brain aging and for the treatment of a range of neurological disorders.

Keywords: Alzheimer’s disease; NAD+; Parkinson’s disease; brain aging; mitochondria; mitophagy; neurodegeneration; neuronal plasticity; sirtuins.

Figure 1.. Nicotinamide Adenine Dinucleotide (NAD⁺) Production and Catabolism in Mammalian Cells

(A) NAD⁺ is produced via three major pathways in mammals. The first is the de novo biosynthesis from tryptophan (Trp), also called the kynurenine pathway. Trp enters the cell via the transporters SLC7A5 and SLC36A4. Within the cell, Trp is converted to formylkynurenine (FK), which is further converted to kynurenine. Kynurenine can be converted to kynurenic acid (via kynurenine aminotransferases/KATs) and finally quinaldic acid. Additionally, kynurenine can be converted to 3-hydroxykynurenine (3-HK) by kynurenine 3-monooxygenase (KMO), and further to 3-hydroxyanthranilic acid (3-HAA) by tryptophan 2,3-dioxygenase (KYNU). The next step is performed by 3-hydroxyanthranilic acid oxygenase (3HAO) to produce α-amino-β-carboxymuconate-ϵ-semialdehyde (ACMS). Via a spontaneous reaction, ACMS converts to quinolinic acid, which further formulates to NAMN by quinolinate phosphoribosyltransferase (QPRT), and to nicotinic acid adenine dinucleotide (NAAD), and finally to NAD⁺. 3-HK, 3-HAA, and quinolinic acid are neurotoxic (denoted with red asterisk), whereas kynurenic acid and picolinic acid are neuroprotective (marked with green asterisk). The second pathway is the Preiss-Handler pathway during which nicotinic acid (NA) is used as an NAD⁺ precursor. NA enters the cell via SLC5A8 or SLC22A3 transporters. The Preiss-Handler pathway is initiated by the conversion of NA to NAMN by NA phosphoribosyl-transferase (NAPRT). NAMN, an intermediate in both kynurenine pathway and the Preiss-Handler pathway, is converted to form NAAD by NAM mononucleotide transferases (NMNATs). Lastly, NAAD is converted to NAD⁺ by NAD⁺ synthase (NADS). The third pathway is the salvage pathway with the cells generating NAD⁺ from nicotinamide riboside (NR) and recycling nicotinamide (NAM) back to NAD⁺ via nicotinamide mononucleotide (NMN). Extracellularly, NAD⁺ or NAM can be converted to NMN, which is in turn dephosphorylated to NR, possibly by CD73. NR is transported into the cell via an unknown nucleoside transport. Intracellularly, NR forms NMN via NRK1 or NRK2 in a tissue-specific manner. NMN is then converted to NAD⁺ by NMNATs. The enzyme NAM N-methyltransferase (NNMT) methylates NAM, using S-adenosyl methionine (SAM) as a methyl donor. This removes NAM from recycling and indirectly affects NAD⁺ levels. (B) The four major NAD⁺-consuming enzymes. From left: poly(ADP-ribose) polymerases (PARPs), especially PARP1 and PARP2, use NAD⁺ as a co-substrate to PARylate target proteins, generating NAM as a by-product. The deacetylation activity of sirtuin (SIRT)1, SIRT3, and SIRT6 depends on NAD⁺, generating NAM as a by-product, with NAM at high cellular levels inhibiting the activity of SIRTs. The NADases or cyclic ADP-ribose synthases (cADPRSs) CD38 and CD157 hydrolyze NAD⁺ to NAM, generating ADPR and cADPR; in addition, CD38 can degrade NMN to NAM, removing NMN from NAD⁺ synthesis. Sterile alpha and TIR motif-containing 1 (SARM1) was recently identified as a NADase, which cleaves NAD⁺ to NAM, cADPR, and ADPR. (C) The chemical structures of NAD⁺ and the NAD⁺ precursors. From left: nicotinic acid (NA), nicotinamide (NAM), nicotinamide riboside (NR), nicotinamide mononucleotide (NMN), and oxidized form of nicotinamide adenine dinucleotide (NAD⁺). Note that CD38, CD73, and CD157 are membrane proteins. For simplicity, we did not attach them to the membrane in this schematic model.

Figure 2.. Subcellular Homeostasis of NAD⁺

The equilibrium of NAD⁺ is a balance of synthesis, consumption, and recycling in various subcellular compartments including the cytosol and Golgi, the nucleus, and the mitochondria. The expression of subcellular-specific NAD⁺-consuming enzymes in addition to the subcellular transporters and redox reactions of NAD⁺ affect the equilibrium. After cell entrance, NAD⁺ precursors are metabolized via three major pathways (Figure 1) to NAD⁺. In the cytosol nicotinamide (NAM) is converted to nicotinamide mononucleotide (NMN) by the intracellular form of NAM phosphoribosyltransferase (iNAMPT). NMN is then converted to NAD⁺ by NMN transferase 2 (NMNAT2), associated to the outer Golgi membrane in the cytosol. NAD⁺/NADH is utilized during glycolysis, and NADH is also used by the malateaspartate and the glyceraldehyde-3-phosphate shuttles located in the inner mitochondrial membrane. In the mitochondrial matrix, the malate-aspartate shuttle oxidizes NAD⁺ to NADH, whereas the glyceraldehyde-3-phosphate shuttle converts flavin adenine dinucleotide (FADH) to FADH₂, providing electron donors for the ETC. In the mitochondrion, NMN is converted to NAD⁺ by NMNAT3. NAD⁺ is utilized by TCA cycle in the mitochondrion to generate ATP, and additionally used by the NAD⁺-dependent mitochondrial sirtuin 3–5 (SIRT3–5) generating NAM. It is still not known whether NAM can be converted back to NMN within the mitochondrion or whether it is transported/diffusing out of the mitochondrion to the cytosol before conversion. Additionally, studies have indicated transporters of NAD⁺, NADH, and NMN in the mitochondrial membrane, but no specific transporters have been identified yet. An NAD⁺ transporter has been found in peroxisomes, SLC25A17, where NAD⁺ likely participates in b-oxidation. Within the nucleus, NMN is converted to NAD⁺ by NMNAT1, and NAD⁺ is here consumed predominantly by SIRT1, 6, 7, and poly (ADP-ribose) polymerase 1–3 (PARP1–3). Like in the cytosol, NAM is recycled back to NMN by iNAMPT.

Figure 3.. Relationships between NAD⁺ and the Ten Hallmarks of Brain Aging

The 10 hallmarks of brain aging include mitochondrial dysfunction; accumulation of oxidative damage; impaired waste disposal including autophagy, mitophagy, and proteostasis; Ca²⁺ deregulation; compromised adaptive stress responses; dysfunctional neuronal network; impaired DNA repair; inflammation; impaired neurogenesis; and senescence and telomere attrition. Evidence from cell culture, C. elegans, and mouse studies shows that NAD⁺ augmentation counteracts the adversities of the hallmarks of brain aging. See the text for details. iPSCs, induced pluripotent stem cells; NAD⁺, nicotinamide adenine dinucleotide; UPR^mt, mitochondrial unfolded protein response.

Figure 4.. NAD⁺ Depletion and Impaired Mitophagy Are Pivotal Events in Common Neurodegenerative Diseases

Based on the evidence summarized in the current review, we propose a hypothesis that may explain, in part, why age is the primary driver of the common neurodegenerative diseases. At a younger age, sufficient cellular NAD⁺ maintains mitochondrial quality to sustain normal neuronal function via the NAD^+-dependent regulations of autophagy/mitophagy, UPR^mt, proteasome degradation, and mitochondrial biogenesis. As we age, increased NAD⁺ consumption drives NAD⁺ depletion, leading to impaired mitochondrial homeostasis and neuronal function. Depending on the disease-related pathogenesis in the patient, age-dependent NAD⁺ depletion and impaired mitophagy may exacerbate the disease progression. This hypothesis explains why age makes people susceptible to neurodegenerative diseases, but it is not sufficient. Center: the mitochondrion in the center exhibits the various mitochondrial deficiencies observed in the four neurodegenerative diseases AD, PD, HD, and ALS. These include impairment of Complex I (CI) in the ETC utilizing NADH as an electron donor, creating NAD⁺. Impairments of both CI and ATP production create reactive oxygen species (ROS), which increase the level of oxidative stress, likely affecting both the ETC itself and the TCA cycle, and increasing the amount of oxidative damage. This can again affect the flux of Ca²⁺ and the membrane potential, resulting in dysfunctional mitochondria. Furthermore, compromised autophagy and/or mitophagy has been linked to several neurodegenerative diseases, resulting in accumulation of dysfunctional mitochondria. The four panels surrounding the mitochondrion illustrate the suggested explanations for NAD⁺ depleted and the downstream effects of NAD⁺ depletion in AD, PD, HD, and ALS. See the text for detailed explanations and references. Kynurenine P, kynurenine pathway.