imae

What are the four main classes of NAD+-consuming enzymes mentioned in this review?

Sirtuins, poly-ADP-ribose polymerases (PARPs), CD38/CD157 ectoenzymes, and bacterial DNA ligase

What happens to NAD+ when it is consumed by sirtuins and PARPs?

NAD+ is cleaved at the glycosidic bond between nicotinamide and ADP-ribose, generating nicotinamide and ADP-ribose (or O-acetyl-ADP-ribose in the case of sirtuins)

What is the canonical role of NAD+ in metabolism?

To facilitate hydrogen transfer as a cofactor in redox reactions, being converted to NADH in pathways such as glycolysis, the TCA cycle, and fatty acid oxidation

How does PARP activation affect NAD+ levels?

Acute DNA damage (e.g., from ionizing radiation) triggers PARP activation, which can cause sudden depletion of NAD+

What is the role of sirtuins in response to nutritional and environmental perturbations?

They activate nuclear transcriptional programs that enhance metabolic efficiency, upregulate mitochondrial oxidative metabolism, and increase resistance to oxidative stress

How does SIRT1 regulate the circadian clock?

SIRT1 deacetylates central clock components (BMAL and CLOCK) in the liver and amplifies their expression in the suprachiasmatic nucleus via deacetylation of PGC-1α

How does the circadian clock regulate NAD+ synthesis?

BMAL and CLOCK control the expression of NAMPT, the rate-limiting enzyme for NAD+ biosynthesis from nicotinamide, producing circadian oscillation of NAD+ levels

What is NAMPT?

Nicotinamide phosphoribosyltransferase; the rate-limiting enzyme for NAD+ biosynthesis from nicotinamide in mammals

What evidence first suggested that NAD+ levels decline with age?

BESTO mice (pancreatic β-cell-specific SIRT1 overexpressors) showed enhanced insulin secretion when young but lost this phenotype when old; NMN administration restored the phenotype

By approximately how much do NAD+ levels decline in old worms and aged mouse tissues?

Approximately twofold

What are two NAD+ intermediates that can be supplemented to restore NAD+ levels?

Nicotinamide mononucleotide (NMN) and nicotinamide riboside (NR)

How does NMN restore NAD+ levels?

NMN is converted to NAD+ by NMN adenylyltransferases (NMNATs) in one step

How does NR restore NAD+ levels?

NR is converted to NMN by NR kinase (Nrk), then NMN is converted to NAD+ by NMNATs

What are two proposed mechanisms for why NAD+ levels decline with aging?

(1) Decline in NAMPT levels (possibly due to chronic inflammation and circadian dysfunction), and (2) chronic activation of PARP due to accumulated nuclear DNA damage

How does TNF-α affect NAD+ levels?

TNF-α reduces NAMPT and NAD+ levels in primary hepatocytes and suppresses CLOCK/BMAL-mediated clock gene transcription

What happens to PARP activity in aging worms and mice?

PARP is chronically activated, leading to increased poly-ADP-ribosylation of cellular proteins, reduced NAD+ levels, and increased acetylation of PGC-1α

What effect does PARP1 knockout have on NAD+ and SIRT1?

PARP1 knockout mice show systemic elevation in NAD+ levels, increased SIRT1 activity, and metabolic benefits

How does aging-induced SIRT1 inactivation affect mitochondria?

It downregulates mitochondrial biogenesis, oxidative metabolism, and antioxidant defense pathways, leading to damage to complex I of the electron transport chain

What is the UPRmt and how is it connected to sirtuins?

The mitochondrial unfolded protein response; induced by SIR-2.1 or SIRT1 activity, and genetic inactivation of UPRmt prevents longevity induced by SIR-2.1 overexpression or NR supplementation

How does SIRT1 deficiency lead to reduced mitochondrial gene expression?

SIRT1 deficiency prevents downregulation of HIF-1α, leading to pseudohypoxia; HIF-1α sequesters cMYC, which can no longer activate TFAM (mitochondrial transcription factor) expression

What is the connection between nuclear NAD+ defects and mitochondrial NAD+ levels?

A decline in nuclear SIRT1 activity leads to mitochondrial dysfunction and NADH buildup, which reduces mitochondrial NAD+ and inactivates mitochondrial sirtuins (SIRT3, 4, 5), creating a downward spiral

What neurodegenerative disease models have shown protection by SIRT1 overexpression or NR supplementation?

Alzheimer's disease, Parkinson's disease, and Huntington's disease mouse models; NR supplementation also protected against memory loss in an Alzheimer's mouse model

What are WldS mice and what do they tell us about NAD+ and neuroprotection?

Wallerian degeneration slow mice have triplication of the NMNAT1 gene and show heightened protection against peripheral nerve degeneration, indicating that NAD+ may be broadly neuroprotective

How does NR supplementation affect Alzheimer's pathology in mice?

NR increased PGC-1α and decreased β-secretase, which generates the toxic amyloid-β peptide

What controversy regarding sirtuins and longevity is discussed in Box 1 of the paper?

A 2011 study (Burnett et al.) questioned whether Sir2 orthologs extend lifespan in worms and flies, but subsequent studies have reconfirmed the original claims, including whole-body Sirt6 transgenic mice (males) and brain-specific SIRT1 transgenic mice (both sexes)

What did the whole-body SIRT1 transgenic mouse study find regarding lifespan?

Mice overexpressing SIRT1 in the whole body failed to show lifespan extension, but brain-specific SIRT1 overexpression (in dorsomedial and lateral hypothalamic nuclei) delayed aging and extended lifespan in both sexes

What effect did whole-body Sirt6 transgenic mice show?

Lifespan extension in males

What is the role of cADPR generated by CD38?

cADPR is a strong Ca2+ inducer, stimulating intracellular calcium release

What is 1-methylnicotinamide and how is it linked to sirtuins?

It is made by nicotinamide N-methyltransferase from nicotinamide; it plays an important role in extension of worm lifespan by SIR-2.1

What happens to CD38 knockout mice regarding NAD+ levels?

CD38 knockout mice show substantially raised NAD+ levels and decreased cADPR production

What outstanding questions are posed in Box 2 of the review?

Does declining NAD+ contribute to aging because it inactivates sirtuins? Will NAD+ intermediate supplementation treat neurodegenerative and other age-associated diseases? Will NAD+ supplementation synergize with SIRT1-activating compounds? Will NAD+ intermediate supplementation be efficacious in humans?

What financial disclosures do the authors make?

S.I. had a sponsored research agreement with Central Sea Co., Japan and is a cofounder of Metro Midwest Biotech; L.G. consults for GlaxoSmithKline, Chronos, Segterra, and Elysium Health

How does exercise affect NAMPT levels?

Exercise training increases NAMPT levels in skeletal muscle

What is the proposed link between chronic inflammation and NAD+ decline?

Chronic inflammation during aging increases inflammatory cytokines such as TNF-α, which reduce NAMPT-mediated NAD+ biosynthesis and suppress CLOCK/BMAL-mediated circadian transcription

What evidence shows that NMN can reverse age-related defects in both nuclear and mitochondrial compartments?

NMN reversed SIRT1 inactivation in nuclear/cytosolic pool (Gomes et al., 2013) and corrected mitochondrial complex I deficiency and cardiac damage (Karamanlidis et al., 2013)

What is the relationship between SIRT1 and PGC-1α?

SIRT1 deacetylates and activates PGC-1α, which is a master regulator of mitochondrial biogenesis and oxidative metabolism

What is the relationship between SIRT1 and FOXO?

SIRT1 deacetylates FOXO, which leads to increased mitochondrial antioxidant defen