The hierarchical nature of autophagy in gut epithelium means that specific cargo receptors are sequentially engaged to prioritize mitophagy, xenophagy, and bulk cytoplasm. Butyrate, beyond acting as a general inducer, selectively acetylates key autophagy receptors (e.g., NIX, p62, optineurin) via HDAC inhibition, altering their affinity for LC3 and reshaping the degradation order. We hypothesize that age‑related butyrate loss leads to hypo‑acetylation of these receptors, causing a shift in the hierarchical triage: mitophagy declines first, xenophagy becomes aberrantly elevated, and bulk autophagy rises indiscriminately. This re‑ordering erodes the epithelium’s ability to tolerate symbionts while clearing damaged mitochondria, driving barrier loss and inflammation.

Mechanistically, butyrate‑mediated acetylation reduces ubiquitination of NIX, strengthening its interaction with LC3‑II and favoring mitochondrial engulfment. Conversely, hypo‑acetylated p62 exhibits increased ubiquitin binding but reduced LC3 interaction, diverting autophagosomes toward ubiquitin‑rich microbial cargo and promoting xenophagy at the expense of mitophagy. The resulting imbalance triggers ROS accumulation from uncleared mitochondria, further suppressing butyrate production by commensals—a vicious cycle.

Testable predictions:

1. In young colonocytes, butyrate treatment increases NIX acetylation (detected by acetyl‑lysine immunoprecipitation) and correlates with higher mitophagy flux (mt‑Keima signal) without altering xenophagy (measured by Salmonella‑GFP survival).
2. In aged colonocytes or butyrate‑deficient conditions, NIX acetylation drops, p62 acetylation falls, mitophagy declines (~40% reduction), while xenophagy flux rises (~30% increase) and bulk LC3‑II turnover elevates.
3. Restoring NIX acetylation via a lysine‑mimetic mutant (NIX‑K→Q) in aged mice rescues mitophagy, reduces ROS, and re‑establishes symbiont tolerance (measured by FITC‑dextran permeability and cytokine profiling).
4. Pharmacological HDAC inhibition (e.g., sodium butyrate) in aged mice re‑establishes the wild‑type receptor acetylation pattern and normalizes the hierarchical substrate order, reversing barrier dysfunction.

Experimental approach: isolate colonic crypts from 3‑month and 24‑month mice, treat ex vivo with/without butyrate (5 mM) for 6 h. Perform immunoblot for acetyl‑lysine on immunoprecipitated NIX and p62, quantify mitophagy using mt‑Keima flow cytometry, assess xenophagy by gentamicin protection assay with GFP‑labelled E. coli, and measure bulk autophagy via LC3‑II turnover with bafilomycin A1. In vivo, feed aged mice a butyrate‑supplemented diet (5 % w/w) for 8 weeks and repeat assays; include NIX‑K→Q knock‑in mice as a genetic rescue control.

Falsification: If butyrate supplementation fails to alter receptor acetylation patterns or does not rescue the mitophagy/xenophagy imbalance in aged epithelium, the hypothesis is refuted. Likewise, if NIX acetylation mimics do not improve barrier function despite correcting mitophagy, the proposed mechanistic link between receptor hierarchy and epithelial health is invalid.