Hypothesis

We hypothesize that increasing nuclear NAD+ synthesis specifically through overexpression of the rate‑limiting salvage enzyme NAMPT will selectively boost SIRT6 deacetylase activity, thereby enhancing autophagic clearance of damaged mitochondria and extending male lifespan more effectively than global NAD+ supplementation.

Mechanistic Basis

SIRT6 is an NAD+-dependent deacetylase that regulates DNA repair, metabolism, inflammation, and autophagy (2, 3). Its activity depends on local NAD+ concentration, which can differ between cytosol, nucleus, and mitochondria. While NMN raises the total cellular NAD+ pool (4), it does not guarantee enrichment in the nucleus where SIRT6 predominantly functions (5).

Male mice exhibit higher basal expression of testicular NAMPT, contributing to a male‑biased NAD+ salvage flux that preferentially fuels nuclear SIRT6 (6). This sex‑specific advantage may explain why SIRT6 overexpression yields stronger lifespan effects in males (1).

We propose that forced nuclear targeting of NAMPT (e.g., NLS‑NAMPT) will raise nuclear NAD+ without altering cytosolic levels, thereby activating SIRT6‑driven deacetylation of autophagy regulators such as p62/SQSTM1 and FOXO3, leading to improved mitophagy and reduced accumulation of senescent mitochondria.

Testable Predictions

1. Male mice expressing nuclear‑targeted NAMPT will show a significant increase in nuclear NAD+ (measured by compartment‑specific biosensors) without a comparable rise in cytosolic NAD+.

2. SIRT6‑dependent deacetylation of p62 and FOXO3 will be elevated in liver and muscle of NLS‑NAMPT males, correlating with increased LC3‑II turnover and mitochondrial clearance (mt‑Keima assay).

3. These mice will exhibit extended median lifespan (≈20 % increase) and improved frailty indices relative to wild‑type controls, whereas female counterparts will show minimal or no benefit.

4. Pharmacological inhibition of SIRT6 (e.g., using a selective inhibitor) will abolish the lifespan and autophagy benefits of NLS‑NAMPT, confirming SIRT6 mediation.

Experimental Design

• Generate two transgenic lines: ubiquitous NLS‑NAMPT and a control line overexpressing cytosolic NAMPT.
• Validate compartmental NAD+ changes using Peredox‑nuclear and Peredox‑cytosol fluorescent probes.
• Assess SIRT6 activity via immunoblot for acetyl‑p62 and acetyl‑FOXO3; measure autophagic flux with bafilomycin A1 chase.
• Monitor survival, frailty (grip strength, gait speed), and healthspan metrics up to 30 months.
• Include SIRT6 inhibitor treatment arm to test necessity.

Falsifiability

If nuclear NAD+ elevation fails to increase SIRT6 activity, autophagic flux, or lifespan in males—or if females derive equal benefit—the hypothesis would be falsified. Conversely, a lack of effect from cytosolic NAMPT overexpression would support the compartment‑specific model.

References

[1] https://www.nmn.com/news/researchers-substantially-extend-mouse-lifespan-by-activating-nad-dependent-and-energy-balancing-enzyme [2] https://pmc.ncbi.nlm.nih.gov/articles/PMC9662279/ [3] https://www.fightaging.org/archives/2025/11/mechanisms-likely-relevant-to-sirtuin-6-effects-on-longevity/ [4] https://pmc.ncbi.nlm.nih.gov/articles/PMC4112140/ [5] https://doi.org/10.1038/s42003-020-01514-y [6] https://doi.org/10.1101/2024.06.21.599604 [7] https://pmc.ncbi.nlm.nih.gov/articles/PMC12624115/