Hypothesis

Nuclear NAD+ abundance sets the threshold for SIRT6‑dependent autophagic triage by controlling the deacetylation state of p62/SQSTM1, which in turn governs selective cargo recognition and mTORC1 activity. When nuclear NAD+ is high, SIRT6 deacetylates p62, promoting its affinity for ubiquitinated damaged mitochondria and lipid droplets while simultaneously suppressing IGF‑AKT‑mTOR signaling; low nuclear NAD+ shifts p62 toward an acetylated state that favors bulk, non‑selective autophagy and fails to restrain mTORC1, thereby converting a rationing response into a maladaptive catabolic state.

Mechanistic Rationale

SIRT6 acts upstream of the autophagy machinery, restraining IGF‑AKT‑mTOR signaling under oxidative and nutrient stress [3]. Recent work shows that SIRT6 also deacetylates p62, altering its ubiquitin‑binding capacity and influencing cargo selectivity [6]. NAD+ is a required co‑factor for all sirtuins, yet its subcellular pools are differentially regulated: nuclear NAD+ is synthesized by NMNAT1, whereas cytoplasmic NAD+ relies on NMNAT2/3 [5]. We propose that nuclear NAD+ specifically fuels SIRT6’s dual role—mTORC1 suppression and p62 deacetylation—creating a coordinated triage signal that prioritizes removal of damaged mitochondria via ROS/Bnip3 pathways [1] while sparing functional organelles. Cytoplasmic NAD+, in contrast, predominantly supports SIRT1‑driven bulk autophagy activation [2]. Thus, the NAD+ compartmentalization ratio acts as a rheostat that toggles the cell between selective rationing and indiscriminate degradation.

Testable Predictions

1. Genetic manipulation – Overexpressing NMNAT1 (nuclear NAD+ synthase) in murine fibroblasts will increase nuclear NAD+, enhance SIRT6‑mediated p62 deacetylation (measured by acetyl‑p62 immunoblot), reduce phospho‑S6K (mTORC1 activity), and increase selective mitophagy (mt‑Keima reporter) without altering LC3‑II bulk flux. Conversely, NMNAT2 overexpression will raise cytoplasmic NAD+, boost LC3‑II turnover, increase acetylated p62, and elevate non‑selective autophagy (e.g., cytosolic GFP‑LC3 puncta) while failing to suppress p‑S6K.
2. Pharmacological perturbation – Treatment with the nuclear‑targeted NAD+ precursor NMN‑MTS (mitochondrial targeting sequence removed) will selectively raise nuclear NAD+ and reproduce the NMNAT1 overexpression phenotype; a cytoplasmic‑restricted NMN‑CTS will mimic NMNAT2 effects.
3. Longevity readout – In vivo, liver‑specific NMNAT1 overexpression in mice should extend median lifespan and improve healthspan markers (glucose tolerance, frailty index) relative to NMNAT2 overexpression, correlating with increased SIRT6‑dependent p62 deacetylation and reduced IGF‑AKT‑mTOR signaling in tissue lysates.
4. Falsification – If nuclear NAD+ elevation does not alter p62 acetylation status or selective autophagy flux, or if mTORC1 suppression occurs independently of SIRT6 (e.g., persists in SIRT6‑KO cells), the hypothesis is refuted.

Implications

This model reframes autophagy activation interventions (e.g., rapamycin, NAD+ boosters) as modulators of a subcellular NAD+‑SIRT6‑p62 triage axis rather than generic "cleaning" agents. It predicts that longevity benefits arise only when the intervention shifts the NAD+ compartmentalization ratio toward the nucleus, thereby reinforcing the cell’s intrinsic rationing system during chronic siege.