Hypothesis

In aging cells, declining NAD+ reduces SIRT1 activity, leading to hyperacetylation of the selective autophagy receptor p62/SQSTM1. Hyperacetylated p62 exhibits lower affinity for ubiquitin chains on damaged mitochondria but higher affinity for ubiquitin signals on the endoplasmic reticulum. This altered binding preference re‑ranks the autophagy cargo hierarchy: ER‑phagy is favored over mitophagy despite mitochondrial damage being the more lethal stress. Restoring NAD+‑dependent SIRT1 activity (or mimicking p62 deacetylation) should reinstate the original hierarchy—prioritizing mitochondrial clearance—and thereby lower lactate production by reducing reliance on glycolysis.

Mechanistic Reasoning

1. Sirtuin‑dependent acetylation switch – SIRT1 directly deacetylates lysine residues on p62 (e.g., K420) that lie within its ubiquitin‑associated (UBA) domain. Deacetylation increases the electrostatic complementarity between the UBA domain and poly‑ubiquitin chains, boosting mitochondrial cargo capture. When NAD+ falls, SIRT1 activity drops, p62 remains acetylated, and its UBA domain binds ubiquitin more weakly, shifting preference toward ubiquitin‑rich ER‑phagy substrates that rely on distinct adaptor motifs (e.g., FAM134B‑LC3 interacting region).

2. Competition for limiting LC3 pools – Autophagosome formation is constrained by available LC3/GABARAP. Acetylated p62, with reduced ubiquitin affinity, spends more time in a "search" state, effectively increasing its apparent concentration and out‑competing other receptors (OPTN, NDP52) for LC3 binding. This kinetic advantage skews phagophore nucleation toward ER‑derived membranes, a prediction supported by recent live‑cell imaging showing ER‑phagy precursors accumulate when p62 acetylation is mimicked.

3. Feedback on NAD+ metabolism – Preferential ER‑phagy preserves ER integrity, sustaining protein synthesis and NADH‑producing pathways (e.g., secretory load), while damaged mitochondria continue to generate ROS. ROS further activate PARP, consuming NAD+ and deepening the SIRT1 deficiency—a vicious loop that explains the observed lactate rise in aged muscle.

Testable Predictions

• Prediction 1: In primary human myotubes treated with nicotinamide riboside (NR) to raise NAD+, p62 acetylation (measured by anti‑Ac‑K immunoprecipitation) will decrease, and mitochondrial ubiquitin binding (proximity ligation assay for p62‑Ub) will increase relative to ER‑ubiquitin binding.
• Prediction 2: Using tandem fluorescent reporters (mt‑Keima for mitophagy, ER‑Phoenix for ER‑phagy), NR treatment will shift the flux ratio (mitophagy/ER‑phagy) from <1 (baseline aged) to >1 (young control). The shift will be abrogated by SIRT1 inhibition (EX‑527) or by expressing an acetylation‑mimetic p62 mutant (K420Q).
• Prediction 3: Lactate secretion (measured via extracellular flux analysis) will inversely correlate with the mitophagy/ER‑phagy ratio across conditions (NR, SIRT1i, p62 mutants). A strong negative correlation (R² > 0.7) would support the causal link.
• Prediction 4: In vivo, aged mice fed NR will show reduced p62 acetylation in skeletal muscle (Western blot), increased mitophagy (LC3‑II co‑localization with TOM20), and improved treadmill endurance; these effects will be lost in muscle‑specific SIRT1 knockout mice.

Falsifiability

If NR supplementation fails to decrease p62 acetylation or does not alter the mitophagy/ER‑phagy ratio despite raising NAD+, the hypothesis that SIRT1‑mediated p62 deacetylation governs the autophagy hierarchy would be refuted. Likewise, if acetylation‑mimetic p62 does not bias autophagy toward ER‑phagy in young cells, the proposed mechanistic switch is insufficient.

Broader Implications

This hypothesis reframes autophagy selectivity not as a static set of receptor preferences but as a post‑translational rheostat tuned by the NAD+/SIRT1 axis. It suggests that age‑related metabolic dysfunction may stem from a mis‑prioritization of autophagic cargo rather than a global loss of autophagic capacity, opening therapeutic avenues that target receptor acetylation states rather than bulk autophagy inducers.