Hypothesis

The selective autophagy program is not a static list of targets but a temporally ordered sequence dictated by the ratio of nuclear to ER‑localized acetyl‑CoA. Nuclear acetyl‑CoA, supplied by ACSS2‑dependent acetate recycling, first activates a transcriptional program that primes mitochondria‑ and lysosome‑specific autophagy receptors. ER acetyl‑CoA, generated by ACLY or imported via AT‑1, subsequently gates the execution of ER‑phagy and later bulk cargo clearance. When this gradient is maintained, cells degrade organelles in a survival‑optimized order (mitochondria → ER → ribosomes → cytoskeleton). Age‑related dysregulation of compartment‑specific acetyl‑CoA production flattens or inverts the gradient, causing premature or mistimed organelle consumption, which manifests as tissue‑specific senescence phenotypes.

Mechanistic Reasoning

1. Transcriptional priming – Under nutrient stress, AMPK phosphorylates ACSS2 (S659), driving its nuclear translocation. Nuclear ACSS2 acetylates histones at TFEB‑bound promoters of autophagy receptors such as BNIP3 (mitophagy), FAM134B (ER‑phagy), and SQSTM1/p62 (general cargo). This step requires a threshold nuclear acetyl‑CoA concentration to sustain H3K9ac/K27ac marks. Evidence: AMPK‑ACSS2 axis promotes lysosomal biogenesis and selective cargo clearance [28552616].

2. ER‑localized gating – High ER acetyl‑CoA, achieved via AT‑1 over‑expression or ACLY activation, acetylates lysine residues on the ER‑phagy machinery (e.g., FAM134B, SEC62), inhibiting their interaction with Atg9a and blocking autophagosome formation at the ER. Conversely, low ER acetyl‑CoA relieves this inhibition, permitting ER‑phagy after mitochondrial turnover. Evidence: Excess ER acetyl‑CoA blocks Atg9a‑FAM134b ER‑phagy and produces progeria‑like pathology [10.1111/acel.12820].

3. Temporal hierarchy – The model predicts a two‑phase switch: (i) nuclear acetyl‑CoA rises → transcription of mitophagy receptors ↑ → mitochondrial clearance; (ii) as mitochondria are depleted, acetyl‑CoA flux shifts to the ER (via ACLY‑dependent citrate export or reduced mitochondrial consumption) → ER acetyl‑CoA falls → ER‑phagy permitted. This creates a self‑limiting cascade that prevents simultaneous degradation of essential compartments.

4. Age‑related collapse – With aging, tissue‑specific alterations in acetyl‑CoA metabolism occur: brain and liver show reduced histone acetylation (lower nuclear acetyl‑CoA), whereas heart and muscle display increased acetylation (higher nuclear and/or ER acetyl‑CoA) [PMC8068152]. Such imbalances flatten the nuclear‑ER gradient, leading to either premature ER‑phagy (when ER acetyl‑CoA is inappropriately low) or delayed mitophagy (when nuclear acetyl‑CoA is insufficient). Both outcomes disrupt proteostatic and energetic homeostasis, driving senescence.

Testable Predictions

• Prediction 1: Forced nuclear depletion of ACSS2 (CRISPR‑KO or S659A mutant) in young fibroblasts will diminish BNIP3 transcription, delay mitophagy flux (measured by mt‑Keima), and cause premature accumulation of damaged mitochondria, even under starvation.
• Prediction 2: Targeted over‑expression of AT‑1 restricted to the ER will elevate ER acetyl‑CoA, inhibit FAM134B‑dependent ER‑phagy (monitored by ER‑phagy‑GFP reporter), and extend mitochondrial clearance, resulting in a shift of the autophagy hierarchy that can be rescued by pharmacological ACLY inhibition.
• Prediction 3: In aged mouse liver, boosting nuclear acetyl‑CoA via acetate supplementation (which fuels ACSS2) will restore H3K9ac at autophagy gene promoters, reinstate proper mitophagy‑first order, and improve liver function markers (ALT/AST, histology). Conversely, enhancing ER acetyl‑CoA via ACLY activation will exacerbate senescence phenotypes.
• Prediction 4: Live‑cell imaging using compartment‑specific acetyl‑CoA biosensors (nuclear‑ACE2, ER‑ACE2) will reveal a declining nuclear/ER acetyl‑CoA ratio with passage number in cultured human fibroblasts; experimentally restoring the ratio (by nuclear‑targeted ACSS2 or ER‑targeted ACLY knockdown) will rescue the autophagic sequence and extend replicative lifespan.

Falsifiability

If manipulations that selectively alter nuclear versus ER acetyl‑CoA fail to change the order of organelle degradation (e.g., mitophagy still precedes ER‑phagy despite nuclear acetyl‑CoA loss) or if rescuing the gradient does not ameliorate age‑related functional decline, the hypothesis would be falsified. Likewise, if global autophagy inhibition (e.g., ATG5 KO) produces the same senescence phenotypes irrespective of acetyl‑CoA compartmentalization, the proposed hierarchical mechanism would be insufficient.

Significance

Reframing autophagy as a metabolically timed cannibalism ritual shifts focus from merely boosting autophagic flux to preserving the subcellular acetyl‑CoA gradient that encodes the degradation hierarchy. This perspective offers novel intervention points—targeting ACSS2 nuclear import, ACLY activity, or AT‑1 mediated ER acetylation—to restore youthful organelle turnover and mitigate age‑related tissue decline.