Hypothesis

Restoring intestinal indole-3-propionic acid (IPA) signaling through the pregnane X receptor (PXR) rescues sleep-dependent glymphatic clearance and autophagic flux in the aged brain by dampening systemic inflammation and directly modulating astrocytic water channel and autophagy gene expression.

Mechanistic Rationale

1. IPA binds PXR in intestinal epithelial cells, upregulating tight‑junction proteins (ZO‑1, Occludin, Claudin‑1) and thickening the mucus layer, thereby reducing LPS translocation [1].
2. Reduced endotoxin lowers circulating IFN‑γ, IL‑6, and IL‑1β, cytokines known to disrupt astrocytic AQP4 polarization and activate mTOR‑dependent autophagy inhibition [2].
3. PXR activation in astrocytes (which express Nr1i2) can directly transcriptionally upregulate Aqp4 and autophagy‑related genes (e.g., Becn1, Map1lc3b) independent of inflammation, as shown by IPA‑PXR inhibition of myosin light‑chain kinase in epithelial cells [3].
4. Aging is associated with depleted fecal IPA and weakened barrier integrity [4]), creating a permissive state for glymphatic failure.

Testable Predictions

• In aged wild‑type mice, oral IPA supplementation will increase fecal IPA, decrease serum IL‑6/TNF‑α, restore colonic ZO‑1/Occludin levels, and rescue nocturnal AQP4 polarization compared with vehicle.
• The same IPA treatment will enhance CSF‑tracer influx during sleep (measured by fluorescent dextran accumulation after 2 h of EEG‑confirmed NREM) and raise LC3‑II/I ratio in cortical lysates.
• In Nr1i2^−/− aged mice, IPA will fail to improve barrier markers, cytokine levels, AQP4 localization, glymphatic flux, or autophagy, demonstrating PXR dependence.
• Pharmacological blockade of PXR with GSK‑8096 will phenocopy the knockout, abrogating IPA’s effects on both gut and brain endpoints.

Experimental Design (outline)

• Cohorts: 18‑month‑old C57BL/6J wild‑type and Nr1i2^−/− mice (n=10 per group).
• Treatment: IPA (10 mg/kg/day) or control via drinking water for 4 weeks.
• Measurements: fecal IPA by LC‑MS/MS, serum cytokines (ELISA), colonic tight‑junction immunostaining, EEG‑sleep staging, CSF‑injected Alexa‑647 dextran quantification, cortical AQP4 polarity (apical vs basolateral), LC3‑II Western blot.
• Analysis: Two‑way ANOVA (genotype × treatment) with post‑hoc tests; significance set at p<0.05.

Falsifiability

If IPA supplementation does not improve either gut barrier metrics or brain clearance/autophagy in wild‑type aged mice, or if it rescues these phenotypes in Nr1i2^−/− mice, the hypothesis is refuted.

References (inline citations already embedded)

[1] https://pmc.ncbi.nlm.nih.gov/articles/PMC12375546/ [2] https://www.frontiersin.org/journals/immunology/articles/10.3389/fimmu.2022.1029948/full [3] https://pmc.ncbi.nlm.nih.gov/articles/PMC12375546/ [4] https://pmc.ncbi.nlm.nih.gov/articles/PMC3738030/