Hypothesis\nAged mucus barrier thickening triggers a sustained signal that suppresses autophagy systemically through galectin‑3 activation of mTORC1 and Rubicon.\n\n### Mechanistic link\n1. In aging, reduced mucin‑degrading bacteria (e.g., Akkermansia muciniphila) cause the intestinal mucus layer to become denser and more sulfated [3].\n2. Dense mucus increases exposure of epithelial galectin‑3 to its ligands (β‑galactosides) on mucin glycans, leading to galectin‑3 oligomerization and secretion into the lamina propria.\n3. Circulating galectin‑3 binds to the lysosome surface Ragulator complex, promoting Rag GTPase loading and thereby activating mTORC1 independent of nutrients [1,2].\n4. Activated mTORC1 phosphorylates ULK1 and TFEB, while also upregulating Rubicon expression, together blocking autophagosome initiation and maturation.\n5. The resulting autophagy suppression preserves a semi‑stable, damage‑laden cellular state, which the cell interprets as a survival advantage despite accumulating dysfunction.\n\n### Testable predictions\n- Prediction 1: Elderly mice with thick colonic mucus will show higher plasma galectin‑3, elevated p‑S6K (mTORC1 readout), and reduced LC3‑II flux in liver, muscle and hypothalamus compared with age‑matched mice whose mucus is experimentally thinned.\n- Prediction 2: Oral administration of Akkermansia or exogenous mucin‑degrading enzymes will thin the mucus, lower plasma galectin‑3, decrease mTORC1 activity, and restore autophagic flux in aged tissues.\n- Prediction 3: Genetic knockout of epithelial galectin‑3 in aged mice will prevent the rise in mTORC1 signaling and autophagy suppression despite a thick mucus layer.\n- Prediction 4: Adding recombinant galectin‑3 to young mouse serum will recapitulate the aged autophagy block, an effect blocked by rapamycin or Rag GTPase dominant‑negative mutants.\n\n### Experimental approach\n- Measure mucus thickness ( histology with Alcian blue/PAS) and sulfation (label with diamidino‑2‑phenylindole) in young (3 mo) vs old (24 mo) mice.\n- Quantify plasma galectin‑3 by ELISA; assess mTORC1 activity (p‑S6K, p‑4EBP1) and autophagy markers (LC3‑II/I, p62) in multiple tissues.\n- Intervention groups: (a) Akkermansia gavage (10⁹ CFU d⁻¹, 4 weeks), (b) recombinant mucinase (e.g., AliC) administration, (c) galectin‑3 neutralizing antibody, (d) IgG control.\n- Readouts: flux assays with lysosomal inhibitors (chloroquine) to distinguish synthesis vs degradation, and tissue‑specific rescue of metabolic parameters (glucose tolerance, grip strength).\n\nIf the hypothesis holds, manipulating the mucus‑galectin‑3‑mTORC1 axis will decouple autophagy suppression from mere mitochondrial damage, revealing a gut‑derived rheostat that actively maintains the aged phenotype.
