Intestinal Autophagy as the Upstream Switch Governing Brain Aging: A Bottom-Up Longevity Hypothesis

Core hypothesis: Selective enhancement of autophagy specifically in intestinal epithelial cells (IECs) by disrupting the BCL‑2–Beclin‑1 interaction will preserve gut barrier integrity, lower systemic translocation of microbial metabolites, and consequently attenuate neuroinflammation and cognitive decline in aged mammals, independent of any central nervous system autophagy manipulation.

Rationale & mechanistic extension

1. IEC autophagy controls barrier molecules beyond occludin – Besides promoting occludin degradation, autophagy regulates the turnover of mucin‑2 (MUC2) and antimicrobial peptides (RegIIIγ, lysozyme) that shape the mucus layer and luminal bacterial load. When autophagy declines, MUC2 accumulates abnormally, leading to a fragmented mucus veil that permits closer bacterial contact with epithelial TLRs, amplifying NF‑κB–driven IL‑1β release [4].

2. Metabolite signaling to the brain – Aged IEC autophagy deficiency increases secretion of medium‑chain fatty acids (MCFAs) and succinate from dysbiotic microbes, which act on IEC‑expressing GPR41/43 and succinate receptor SUCNR1 to trigger serotonin and glutamate release from enteroendocrine cells. These neurotransmitters activate vagal afferents that modulate microglial priming via the nucleus tractus solitarius [1]. Restoring autophagy would normalize IEC metabolite profiles, dampening this excitatory vagal drive.

3. Cross‑talk with systemic immunity – Autophagy‑competent IECs efficiently clear intracellular bacteria via xenophagy, reducing LPS loading onto chylomicrons. Consequently, circulating LPS‑binding protein (LBP) and soluble CD14 drop, limiting TLR4 activation in peripheral macrophages and downstream cytokine spillover into the cerebrospinal space [5].

Experimental plan (testable & falsifiable)

• Model: Generate IEC‑specific Becn1^F121A knock‑in mice (Vil‑Cre‑ERT2; Becn1^F121A/F121A) and treat aged (20‑month) cohorts with tamoxifen to induce the autophagy‑enhancing allele. Parallel groups receive intestine‑targeted nanoparticles loaded with the BH3 mimetic NB‑1 (selective BCL‑2–Beclin-1 disruptor) [7].

• Readouts:

  • Gut barrier: FITC‑dextran permeability assay, zonulin ELISA, mucin‑2 thickness (confocal), antimicrobial peptide quantification.
  • Microbial translocation: Plasma LPS, LBP, sCD14, and metabolomic profiling of MCFAs and succinate (LC‑MS).
  • Enteroendocrine signaling: IEC serotonin (5‑HT) and GLP‑1 levels (ELISA), vagal afferent firing ex‑vivo (electrophysiology).
  • Neuroinflammation: Hippocampal Iba1 immunoreactivity, cytokine panel (IL‑1β, TNF‑α, IL‑6), microglial RNA‑seq for disease‑associated microglia signature.
  • Cognition: Morris water maze, novel object recognition, and long‑term potentiation in hippocampal slices.

• Controls: (i) Wild‑type aged littermates; (ii) Neuron‑specific autophagy enhancement (Syn‑Cre; Becn1^F121A) to test sufficiency of central autophagy; (iii) Gut‑targeted NB‑1 in Becn1^F121A mice to assess epistatic interaction.

Predicted outcomes if hypothesis is true

• IEC‑specific autophagy boost will normalize FITC‑dextran flux to youthful levels, reduce plasma LPS by >40 %, and shift metabolite profiles toward youthful patterns.
• Vagal afferent baseline firing will decrease, correlating with lower hippocampal IL‑1β and microglia adopting a homeostatic transcriptomic state.
• Behavioral performance will match that of young adult mice, whereas neuron‑specific autophagy alone will produce only modest improvement.

Falsifiability: If intestinal autophagy enhancement fails to improve barrier integrity, does not lower circulating microbial metabolites, or does not rescue cognitive deficits despite confirmed IEC autophagic flux (LC3‑II/p62 assay), the hypothesis is falsified. Conversely, a rescue without changes in gut permeability would suggest alternative mechanisms, prompting revision.

Implications: Demonstrating that gut IEC autophagy is a decisive upstream node would reorient longevity stacks toward gut‑first interventions (dietary autophagy inducers, intestine‑restricted BH3 mimetics, prebiotics that sustain xenophagy) before targeting central pathways, potentially yielding greater efficacy with lower off‑target risk.