Hierarchical Enhancer Erosion Dictates a Selective Autophagy Failure Cascade in Aged Stem Cells

Aged stem cells exhibit a paradox: global chromatin accessibility increases at many loci, yet specific functional enhancers close, impairing lineage‑appropriate gene expression. We hypothesize that this epigenetic remodeling creates a ordered hierarchy of enhancer loss that preferentially silences stress‑responsive selective autophagy receptors (SQSTM1/p62, OPTN, NBR1, CALCOCO2) while leaving core autophagy machinery promoters (ATG5, ATG7, BECN1) relatively intact. Consequently, bulk autophagosome formation persists but cargo‑specific clearance pathways—mitophagy, aggrephagy, lipophagy—fail in a predictable sequence, driving the accumulation of damaged mitochondria, protein aggregates, and lipid droplets that underlie stem‑cell exhaustion.

Mechanistic rationale

1. Promoter vs. enhancer vulnerability – Housekeeping autophagy genes often reside in CpG‑rich promoters that resist age‑related DNA methylation and nucleosome remodeling. In contrast, selective autophagy receptor genes depend on distal enhancers bound by stress‑activated transcription factors (TFs) such as FOXO3, NRF2, and TFEB. These enhancers are enriched for nucleosome‑sensitive motifs and are therefore more susceptible to the chromatin‑closing activity observed in aged HSCs and muscle satellite cells (see [1][2]).
2. TF occupancy as a sensor – In young stem cells, oxidative or metabolic stress triggers FOXO3/NRF2 nuclear translocation, enhancing receptor expression. With age, enhancer erosion reduces TF binding affinity, blunting this stress‑responsive burst. Consequently, basal autophagy (driven by constitutive ATG promoters) continues, but the inducible surge needed to clear acute damage is lost.
3. Functional cascade – Loss of SQSTM1 first impairs ubiquitin‑dependent aggrephagy, leading to p62‑positive aggregate accumulation. Subsequent erosion of OPTN/NBR1 enhancers weakens mitophagy, allowing mitochondrial ROS to rise. Finally, CALCOCO2 loss compromises lipophagy, causing lipid droplet overload. This ordered failure mirrors a "triage" where the cell sacrifices bulk turnover to preserve minimal viability, ultimately accelerating functional decline.

Testable predictions

• Prediction 1: ATAC‑seq peaks at enhancers of SQSTM1, OPTN, NBR1, and CALCOCO2 will show significantly greater loss in aged versus young HSCs and muscle satellite cells, while ATG5/ATG7/BECN1 promoter accessibility remains unchanged.
• Prediction 2: ChIP‑seq for FOXO3 and NRF2 will reveal reduced occupancy at these enhancers in aged cells, correlating with decreased mRNA and protein levels of the respective receptors.
• Prediction 3: Functional assays will demonstrate a stepwise decline: aggrephagy (GFP‑LC3/p62 clearance) declines first, followed by mitophagy (mt‑Keima signal), then lipophagy (BODIPY‑LD turnover), whereas bulk autophagic flux (LC3‑II turnover with bafilomycin) stays comparable between ages.
• Prediction 4: Targeted enhancer activation using dCas9‑VP64 or CRISPRa to SQSTM1 and OPTN in aged stem cells will rescue aggrephagy and mitophagy, improve regenerative capacity in transplantation assays, and reduce aging phenotypes without altering global chromatin accessibility.

Falsifiability If aged stem cells show no significant enhancer accessibility loss at selective autophagy receptors, or if receptor protein levels remain unchanged despite chromatin changes, the hierarchical model is invalid. Likewise, if forced receptor expression fails to restore selective autophagy flux or improve stem‑cell function, the causal link between enhancer erosion and autophagy dysfunction is refuted.

This hypothesis integrates the observed chromatin remodeling in aging with the "autophagy as hierarchical cannibalism" concept, offering a precise epigenetic mechanism that explains why autophagy genes are epigenetically modulated yet autophagy dysfunction manifests selectively in aged stem cells.