Hypothesis

Age‑associated epigenetic drift increases cell‑to‑cell variance in the transcriptional activity of the FOXO1→TFEB→autophagy gene regulatory axis, uncoupling autophagy induction from genuine nutrient‑sensing signals and turning a coordinated rationing system into a noisy, maladaptive resource drain.

Mechanistic Reasoning

The FOXO1‑TFEB axis functions as a hierarchical nutrient‑sensor that integrates mTORC1, AMPK, and calcineurin inputs to modulate lysosomal biogenesis and autophagic flux in a synchronized manner across the cellular population. In young cells, transient nutrient stress triggers a rapid, uniform dephosphorylation of TFEB, nuclear import, and FOXO1‑dependent transcription of autophagy genes, ensuring that only dispensable components are catabolized while essential machinery is preserved.

With age, stochastic epigenetic alterations—particularly at CpG islands within the FOXO1 promoter, TFEB enhancer regions, and autophagy‑gene promoters—lead to heterogeneous chromatin states. This results in:

1. Variable basal FOXO1 expression, causing some cells to maintain high TFEB transcription despite adequate nutrients, while others exhibit blunted FOXO1 responses even under starvation.
2. Differential TFEB promoter accessibility, producing a spectrum of TFEB nuclear import efficiencies that no longer correlate with mTORC1 activity.
3. Mosaic autophagy‑gene expression, where lysosome biogenesis and autophagosome formation are activated in subsets of cells independent of actual nutrient status.

Consequently, autophagy shifts from a siege‑level triage (catabolizing only truly expendable cargo) to a constitutive, low‑grade self‑consumption that degrades essential proteins and organelles in a random fashion, accelerating functional decline.

Testable Predictions

• Prediction 1: Single‑cell RNA‑seq of young vs. old tissues will show increased transcriptional variance (higher Fano factor) in FOXO1, TFEB, and core autophagy genes in aged cells, while mean expression may remain unchanged.
• Prediction 2: Pharmacological reduction of epigenetic noise (e.g., low‑dose DNMT or HDAC inhibitors) will decrease cell‑to‑cell variability in TFEB nuclear localization and restore correlation between mTORC1 activity and autophagic flux.
• Prediction 3: In aged mice, autophagic flux measured by LC3‑II turnover will be elevated in a subset of cells displaying low mTORC1 signaling (p‑S6K low) and concomitantly reduced in cells with high mTORC1 activity, indicating uncoupling.
• Prediction 4: Genetic homogenization of FOXO1 expression (via a ubiquitous, inducible FOXO1 transgene) in aged animals will reduce autophagy heterogeneity and improve tissue‑specific functional readouts (e.g., grip strength, mitochondrial respiration) without altering overall nutrient‑sensing pathways.

Experimental Approach

1. Single‑cell multi‑omics: Perform scRNA‑seq combined with ATAC‑seq on isolated hepatocytes, neurons, and muscle from 3‑month and 24‑month mice. Quantify variance in FOXO1, TFEB, and autophagy gene expression and correlate with chromatin accessibility at their regulatory loci.
2. Epigenetic noise modulation: Treat aged mice with sub‑toxic doses of 5‑azacytidine (DNMT inhibitor) or vorinostat (HDAC inhibitor) for 2 weeks; assess changes in transcriptional variance using the same single‑cell readouts.
3. Live‑cell imaging of TFEB dynamics: Use a TFEB‑GFP knock‑in reporter in primary fibroblasts from young and old donors, combined with a FRET‑based mTORC1 activity sensor. Measure the correlation coefficient between mTORC1 inhibition and TFEB nuclear entry across hundreds of cells.
4. Rescue via FOXO1 homogenization: Generate a Rosa26‑lox‑STOP‑lox‑FOXO1 mouse line crossed with a ubiquitous, tamoxifen‑inducible Cre. Induce FOXO1 overexpression in aged mice and evaluate autophagic flux (mCherry‑GFP‑LC3 reporter), epigenetic variance, and functional outcomes.

Falsifiability

If single‑cell analyses reveal no increase in transcriptional variance of the FOXO1‑TFEB axis with age, or if reducing epigenetic noise fails to restore coupling between mTORC1 signaling and autophagy, the hypothesis would be refuted. Conversely, confirmation of increased variance and its rescue by epigenetic or FOXO1‑homogenizing interventions would support the model that age‑related epigenetic drift converts autophagy from a precise siege response into a maladaptive, stochastic resource drain.