Hypothesis

Aging reshapes the priority order in which selective autophagy receptors are expressed, causing the cell to dismantle less‑critical cargo while neglecting the most toxic species. This re‑ordering, rather than a global loss of autophagic capacity, underlies the accumulation of damaged mitochondria and protein aggregates seen in aged tissues.

Mechanistic basis

1. TFEB‑dependent transcriptional cascade – TFEB activates a battery of autophagy genes, including core machinery (ATG5, ATG12) and a set of selective receptors (e.g., SQSTM1/p62 for ubiquitinated aggregates, NIX/BNIP3L for mitophagy, OPTN for bacterial cargo) [8][11].
2. Differential promoter sensitivity – Bioinformatic analysis of TFEB‑bound promoters reveals that receptors governing organelle quality control contain CpG‑rich, nucleosome‑dense regions that are more prone to age‑associated DNA methylation than those governing bulk protein clearance [4].
3. Lysosomal lipid environment – Age‑related accumulation of lysosomal cholesterol reduces membrane fluidity, impairing the recruitment of receptors that rely on specific lipid microdomains (e.g., NIX) while leaving receptors that bind via ubiquitin‑independent motifs relatively unaffected [3][6].
4. Resulting hierarchy shift – With organelle‑specific receptors downregulated, autophagosomes continue to form but preferentially engulf cytosolic protein complexes, leaving damaged mitochondria to persist. The cell thus exhibits a "reverse triage" where low‑risk cargo is cleared and high‑risk damage accumulates.

Testable predictions

• Prediction 1: In aged murine hepatocytes, mRNA levels of NIX and BNIP3L will be significantly reduced (≥ 40 % vs. young) whereas SQSTM1 expression will change < 10 % (measured by qPCR).
• Prediction 2: Pharmacological depletion of lysosomal cholesterol using cyclodextrin will restore NIX protein levels and improve mitophagy flux in aged cells without altering overall LC3‑II conversion.
• Prediction 3: Forced expression of NIX in aged fibroblasts will selectively reduce mitochondrial ROS and improve ATP production, but will not decrease p62‑positive aggregate load.
• Prediction 4: Chromatin immunoprecipitation followed by sequencing (ChIP‑seq) for TFEB will show unchanged binding at the SQSTM1 promoter but reduced occupancy at the NIX promoter in aged tissue.

Experimental approach

• Isolate primary hepatocytes from young (3 mo) and aged (24 mo) mice.
• Quantify receptor transcripts and proteins; assess lysosomal cholesterol filipin staining.
• Treat aged cells with methyl‑β‑cyclodextrin (5 mM, 24 h) and measure mitophagy (mt‑Keima) and aggrephagy (p62‑GFP) fluxes.
• Use AAV‑mediated NIX overexpression to test rescue specificity.
• Perform TFEB ChIP‑seq on nuclear extracts to map promoter occupancy changes.

Falsifiability

If lysosomal cholesterol manipulation fails to restore NIX levels or mitophagy in aged cells, or if NIX overexpression does not preferentially improve mitochondrial health without affecting aggregate clearance, the hypothesis would be refuted. Conversely, confirmation would support the idea that aging rewrites the autophagy hierarchy, and that correcting this order—not merely boosting overall autophagy—could be a precise strategy to mitigate age‑associated cellular damage.