#27 – David Sinclair, Ph.D.: Slowing aging – sirtuins, NAD, and the epigenetics of aging

Sinclair has been taking resveratrol since at least the early 2000s, describing 'buckets of it in my basement.' His rationale is explicit risk-benefit: the known trajectory without intervention is predictably bad; resveratrol has been in humans long enough to establish a safety record; and blood-level evidence from human studies shows cardiovascular markers improve. He does not claim it will add 10 years of life. He takes it with fat specifically because the lean mouse ITP studies failed partly due to poor absorption — the fat-diet obese mouse experiments succeeded in part because dietary fat boosted bioavailability.

Sinclair is explicit that when combining molecules that operate on overlapping pathways, the assumption that each can be dosed at its solo-use maximum is incorrect. He frames metformin's longevity interest through the AMPK pathway (which metformin activates via complex I inhibition in liver mitochondria) and the caloric restriction mimicry angle — AMPK and sirtuins both respond to nutrient scarcity signals and talk to each other. He notes that metformin probably has hepatic tissue specificity in its AMPK effects.

Sinclair explains the NR → NMN → NAD pathway: NR is taken up by a transporter and converted to NMN; NMN is then immediately converted to NAD. The debate in the field is whether NMN itself is transported across the cell membrane before conversion, or whether it must first be converted to NR extracellularly. NMN is more stable than NR in solution. Sinclair's lab prefers it for animal work. His preferred future assay — 7-Tesla phosphorus MRS — would allow measurement of NAD in living human tissue (muscle, brain) during exercise without requiring blood draws.

Kevin Bitterman (Sinclair's first graduate student) discovered this counter-intuitively: when he added nicotinamide to yeast, expecting sirtuin activation, sirtuin activity went down instead. The mechanistic explanation is that nicotinamide is the cleavage product of the NAD+ → acetyl-lysine deacetylation reaction. The enzyme 'sees' high nicotinamide as a signal that it has already consumed too much NAD, and slows. Labs worldwide now use nicotinamide as a standard sirtuin inhibitor in experimental protocols.

The PNC1 paper (Sinclair's first Nature paper, ~2003) showed that PNC1 is turned on not just by caloric restriction but also by heat, low amino acids, and high salt — it's a general stress sensor that routes all those signals into sirtuin activation via NAD. This is the evolutionary explanation: sirtuins are survival genes that evolved to respond to adversity, slow reproduction, and maximize maintenance. The teleological logic is that during famine, an organism that can preserve genome stability and defer reproduction outlives one that burns through its repair machinery.

Sinclair's company Jumpstart Fertility is advancing this indication. The mechanistic chain is: low NAD → reduced SIRT3 activity → impaired BUBR1 kinase → poor meiotic spindle formation → aneuploid eggs. In mice, giving NAD precursors to aged or chemotherapy-treated females produced eggs that were more numerous, had better spindles, and led to healthier offspring with fewer chromosomal abnormalities. This is one of the few short-term, objectively measurable endpoints (embryologist grades eggs in the IVF lab and can see the difference directly) that allows longevity biology to enter a clinical trial with a fast readout.

Sinclair uses the analog/digital metaphor: DNA is four-letter digital code, easy to copy and preserve. Epigenetic information is analog — it operates in three spatial dimensions plus time, adapting to everything you eat, drink, and do. Unlike digital data, analog information degrades like magnetic tape or a vinyl record. The Waddington landscape — a 1950s image of cells rolling down hillsides into identity-defining valleys — is the visual: aging is cells vibrating out of their valleys and landing in the wrong place. A liver cell starts behaving like a neuron. Sinclair's lab is preparing papers showing they can manipulate the epigenome in mice and reproduce all hallmarks of aging.

The discovery was that SIR2's role in DNA repair and its role in gene silencing are in direct competition. When a double-strand break occurs, the SIR complex relocates to help repair it. While it's away, genes that should be silenced — including mating-type genes in yeast — become expressed, creating cellular confusion. Sinclair showed this same mechanism is conserved in mammals. The problem is antagonistic pleiotropy: a response that is essential for survival when young (fix the break, even at the cost of brief silencing) becomes destructive across decades as the proteins drift further from home each time. Sinclair's lab is now showing they can epigenomically manipulate adult mice and produce all hallmarks of aging — and is writing those papers at the time of this interview.

The mitochondrial oasis hypothesis: when cells are hit with DNA-damaging agents, PARP1 massively consumes NAD. Cytoplasmic NAD collapses. But cells overexpressing NMPT (the mammalian equivalent of yeast PNC1) survived the assault even though their cytoplasmic NAD was equally depleted. The explanation was mitochondrial NAD — the cells had maintained it. As long as mitochondria stayed active with NAD, the cell could recover. Measuring mitochondrial NAD separately required Anthony Solvay's mass spectrometry techniques using labeled NAD isotopes to spike into isolated mitochondria. A 2007 paper from Sinclair's lab confirmed that mitochondrial NAD fluctuates and is the dominant survival compartment. Only NAD+ (not NADH or NADP) activates sirtuins — the oxidized form is the substrate that the enzyme cleaves, releasing nicotinamide (which itself feeds back to inhibit the reaction at high doses).

The key evidence for a real mechanism was dose-response dependency on the SIR2 gene: resveratrol extended lifespan in yeast and worms only when sirtuin was present. A single amino acid mutation (E230K in SIRT1 — negative to positive charge) blocked resveratrol activation in vitro and in organisms. The 2006 Nature mouse paper used 200 mg/kg/day of resveratrol in high-fat-diet mice starting at one year of age. Lean mice on resveratrol every day showed no statistically significant lifespan extension; lean mice on every-other-day feeding plus resveratrol did, suggesting resveratrol may amplify the caloric restriction signal. The ITP studies (late-starting, no fat in diet) replicated the lack of lean-mouse effect — consistent with Sinclair's explanation that absorption requires dietary fat. A GlaxoSmithKline-acquired company later took SIRT1 activating compounds (SRT2104, SRT1720) into human trials.

Sinclair describes gearing up these experiments at the time of recording. NMN was chosen over resveratrol because it is water-soluble and can be administered in drinking water, avoiding the absorption problem that plagued the resveratrol experiments. The strategy is to replenish NAD broadly to activate all seven sirtuins rather than directly activating one. He also describes combining NMN with AAV-delivered overexpression of all seven human sirtuin genes in old mice, finding additive effects — both the fuel (NAD) and the enzyme (SIRT1-7) together produce effects neither achieves alone.