Slowing ageing by design: the rise of NAD+ and sirtuin-activating compounds

Abstract

The sirtuins (SIRT1-7) are a family of nicotinamide adenine dinucleotide (NAD⁺)-dependent deacylases with remarkable abilities to prevent diseases and even reverse aspects of ageing. Mice engineered to express additional copies of SIRT1 or SIRT6, or treated with sirtuin-activating compounds (STACs) such as resveratrol and SRT2104 or with NAD⁺ precursors, have improved organ function, physical endurance, disease resistance and longevity. Trials in non-human primates and in humans have indicated that STACs may be safe and effective in treating inflammatory and metabolic disorders, among others. These advances have demonstrated that it is possible to rationally design molecules that can alleviate multiple diseases and possibly extend lifespan in humans.

Figure 1. Nutrient-responsive signalling pathways that maintain health and extend lifespan

Calorie or dietary restriction increases the concentrations of metabolic effectors such as nicotinamide adenine dinucleotide (NAD⁺) and AMP while reducing the concentrations of glucose, amino acids and lipids. Exogenous administration of nicotinamide riboside (NR), nicotinamide mononucleotide (NMN) or the nicotinamide phosphoribosyltransferase (NAMPT) activator P7C3 can increase NAD⁺ levels. Calorie restriction also reduces the concentrations of the hormonal effectors insulin, insulin-like growth factor 1 (IGF1) and growth hormone (GH). These effectors stimulate or inhibit the activity of metabolic sensors such as the sirtuins (SIRTs), AMP kinase (AMPK), target of rapamycin (TOR), insulin–IGF1 signalling (IIS) and forkhead box O (FOXO) transcription factors. Sirtuin-activating compounds (STACs) such as SRT1720 and SRT2104 can directly activate SIRT1, whereas rapamycin is a direct inhibitor of TOR. Metformin indirectly activates AMPK. These metabolic sensors regulate downstream activities such as DNA repair, mitochondrial biogenesis and function, stress resistance, stem cell and telomere maintenance, autophagy, chromatin modifications, reduced inflammation, and translation fidelity. The net effect is to tip the scale in favour of homeostasis and compressed morbidity, resulting in a disease-free, more youthful-like state.

Figure 2. Localization, enzymatic activity and modulation of sirtuins by small molecules

The mammalian sirtuins (SIRT1−7) are a family deacylases that are localized to various cellular compartments. SIRT1, SIRT6 and SIRT7 are localized primarily to the nucleus, whereas SIRT3, SIRT4 and SIRT5 are present in the mitochondria. SIRT1 (in some cell types) and SIRT2 are localized to the cytosol. Their activities are regulated by nicotinamide dinucleotide (NAD⁺), which is synthesized de novo from Trp, Asp or niacin (not shown). NAD⁺ is also recycled by the NAD salvage pathway from nicotinamide (NAM) by nicotinamide phosphoribosyltransferase (NAMPT), which converts NAM to nicotinamide mononucleotide (NMN). NMN is converted to NAD⁺ by nicotinamide mononucleotide adenylyltransferase1 (NMNAT1), NMNAT2 or NMNAT3, which are also capable of converting nicotinic acid mononucleotide (NaMN) to NAD⁺. Additional NAD⁺ precursors used in the NAD salvage pathway include nicotinamide riboside (NR) and nicotinic acid riboside, which are converted to NMN and NaMN, respectively (not shown) by nicotinamide riboside kinase 1 (NRK1) and NRK2. The cell membrane-bound glycohydrolases CD38 and CD157 degrade NAD⁺ to NAM^,. CD73 converts NAD⁺ to NMN and NMN to NR; it is a transmembrane protein that may also act as a NR transporter^,. There is evidence to indicate that NAD⁺ and NMN can be directly transported across the cell membrane in some cell types, although the mechanism is not yet known. The inset shows that SIRT1 activation is stimulated by the presence of a hydrophobic amino acid adjacently to the target Lys (panel 1). When NAM levels are sufficiently high, it binds into the carboxy-terminal pocket of SIRT1 and inhibits it (panel 2). SIRT1 utilizes NAD⁺ as a co-substrate to promote the binding and deacetylation of specific protein substrates (panel 3). Sirtuin-activating compounds (STACs) activate SIRT1 by binding to the STAC-binding domain and primarily lowering the K_m for the peptide substrate, thereby increasing its catalytic activity (panel 4).

Figure 3. The allosteric activation mechanism of SIRT1

a| The structure of a truncated human sirtuin 1 (SIRT1) in complex with a synthetic sirtuin-activating compound (STAC) showing the binding site of STAC-1 at the amino-terminal STAC-binding domain (SBD). STAC binding promotes the interaction of the SBD with the central catalytic domain to lower the K_m for the substrate, thereby increasing SIRT1 activity. The carboxy-terminal regulatory segment (CTR) stabilizes the deacetylase activity. Substitution of Glu230 (E230) with a Lys residue impairs enzymatic activation by reducing the electrostatic interaction between Glu230 on the SBD and Arg446 on the substrate-binding site. b | Model of how STACs activate SIRT1. Structural analysis and mutagenesis indicated that the amino acids Glu230 and Arg446 form a salt bridge (red circle) to facilitate binding of the amino terminus of SIRT1 with its catalytic core (movement indicated by arrows). The protein ribbon is rainbow-coloured from blue at the amino terminus to red at the carboxyl terminus. A STAC1 and an acetylated p53 peptide substrate are shown in cyan and green, respectively. Adapted from REF. , Nature Publishing Group.

Figure 4. Sirtuin activation and its disease relevance

Sirtuin activity can be stimulated through various mechanisms: allosterically by sirtuin-activating compounds (STACs), such as resveratrol, SRT1720 and SRT2104, which lower the K_m for the target proteins; increasing nicotinamide adenine dinucleotide (NAD⁺) levels by providing its precursors nicotinamide riboside (NR) or nicotinamide mononucleotide (NMN); inhibiting the glycohydrolases CD38 or CD157 (with apigenin, quercetin, GSK 897-78c)^,; inhibiting poly(ADP-ribose) polymerases (PARPs); activating nicotinamide phosphoribosyltransferase (NAMPT) with P7C3 (REF. 152); or by providing nicotinamide (NAM) analogues, such as methyl-NAM. Nicotinamide riboside kinase 1 (NRK1) and NRK2 convert NR and nicotinic acid riboside (NaR) is to NMN and nicotinic acid mononucleotide (NaMN), respectively. NaMN and NMN are converted to NAD⁺ by nicotinamide mononucleotide adenylyltransferase 1 (NMNAT1), NMNAT2 and NMNAT3. Sirtuins directly or indirectly target more than 100 signalling proteins with relevance to human disease in a variety of tissues ranging from brain to blood (reviewed in REF. 41). Although STACs have therapeutic potential, certain diseases, such as cancer in certain contexts and Parkinson disease, may benefit from sirtuin inhibition (reviewed in REF. 179). EF2, elongation factor 2; FOXO, forkhead box O; HNGB1, high-mobility group box 1; IL, interleukin; LXR, liver X receptor; MMP9, matrix metalloproteinase 9; NF-κB, nuclear factor-κB; NLRP3, NOD-, LRR- and pyrin domain-containing 3; PGC1α, peroxisome proliferator-activated receptor-γ co-activator 1α; PPAR, peroxisome proliferator-activated receptor; STAT5, signal transducer and activator of transcription 5; TGFβ, transforming growth factor-β; UCP2, mitochondrial uncoupling protein 2.