Hypothesis

Butyrate produced by colonic Firmicutes acts as a metabolic rheostat that programs the order in which autophagy substrates are engaged during nutrient stress. When butyrate is abundant, colonocytes preferentially activate mitophagy, followed by lipophagy and finally aggrephagy, preserving mitochondrial function while recycling lipids and protein aggregates. Declining butyrate with age reverses or flattens this hierarchy, causing premature engagement of bulk autophagy or aggrephagy before damaged mitochondria are cleared, leading to mitochondrial ROS accumulation, epithelial barrier loss, and inflammaging.

Mechanistic Rationale

Butyrate inhibits HDACs and activates GPR109A, resulting in reduced mTORC1 signaling and increased AMPK activity. These shifts promote phosphorylation of ULK1 and activation of the autophagy initiation complex. Importantly, butyrate‑dependent acetylation of specific autophagy receptors (e.g., NIX for mitophagy, LC3‑interacting region of p62 for aggrephagy) alters their affinity for cargo. We propose that high butyrate sustains NIX deacetylation, enhancing its binding to depolarized mitochondria, while low butyrate leads to hyperacetylated NIX with reduced mitochondrial affinity, shifting receptor preference toward p62‑mediated protein aggregate capture.

Testable Predictions

1. In vitro – Treat polarized human colonocyte monolayers with physiological butyrate (1‑5 mM) versus vehicle, then induce mild nutrient deprivation (EBSS). Measure flux through mitophagy (mt‑Keima), lipophagy (BODIPY‑C12), and aggrephagy (GFP‑LC3‑p62 reporter) over time. Prediction: butyrate‑treated cells show mitophagy peak at 2 h, lipophagy at 4 h, aggrephagy at 6 h; vehicle shows simultaneous or aggrephagy‑first engagement.
2. Ex vivo – Isolate colonic crypts from young (3‑mo) and aged (24‑mo) mice fed either standard diet or a butyrate‑supplemented diet. Assess receptor acetylation status (acetyl‑lysine immunoblot of NIX and p62) and organelle‑specific autophagy markers. Prediction: aged crypts exhibit increased NIX acetylation and reduced mitophagy; butyrate supplementation restores NIX deacetylation and rescues mitophagy precedence.
3. In vivo – Use colonocyte‑specific Atg5 knockout mice complemented with a mitochondria‑targeted autophagy reporter (mt‑Rosella). Compare tumor‑free survival and barrier permeability (FITC‑dextran assay) across four groups: young control, young butyrate‑fed, aged control, aged butyrate‑fed. Prediction: butyrate supplementation in aged mice normalizes mitophagy flux, reduces mitochondrial ROS, and preserves barrier function despite age‑related Firmicutes loss.

Potential falsification

If butyrate manipulation does not alter the temporal order of mitophagy, lipophagy, and aggrephagy fluxes, or if NIX acetylation status does not correlate with substrate preference under varying butyrate levels, the hierarchical triage model would be refuted.

Broader Impact

Linking microbial metabolite levels to a defined autophagy substrate hierarchy reframes age‑related gut dysfunction as a failure of cellular prioritization rather than generic autophagy decline. This opens therapeutic avenues: targeted butyrate delivery or HDAC modulators could restore the proper triage sequence, delaying epithelial senescence and inflammaging.