Hypothesis

During normal aging, increased intestinal permeability reduces circulating indole‑3‑propionic acid (IPA), diminishing PXR activation in microglia. Low microglial PXR activity lifts transcriptional repression of the complement cascade (C1q, C3) and phagocytic receptors, leading to excessive tagging and removal of synapses. This microglial‑driven synaptic stripping, rather than wholesale neuronal death, underlies age‑related cortical thinning and cognitive decline. Restoring gut barrier integrity or supplementing IPA will reinstate microglial PXR signaling, suppress complement‑mediated pruning, and preserve dendritic complexity without altering neuronal numbers.

Mechanistic Rationale

• IPA crosses the blood‑brain barrier, activates neuronal PXR, and attenuates Aβ pathology and neuroinflammation in AD models 3.
• In the gut, IPA acts as a PXR ligand that enhances tight‑junction proteins and mucus thickness, reinforcing barrier function 4.
• Aging disrupts tryptophan metabolism, lowering microbial IPA production 5, while concomitant gut‑barrier leakiness permits endotoxin translocation that further suppresses IPA synthesis.
• Microglial replacement in aged mice rescues hippocampal dendritic spine density and synaptogenic gene expression through non‑inflammatory, activity‑dependent pathways 2, suggesting microglia can modulate synaptic structure independent of overt neuroinflammation.
• In C. elegans, aging alters neuronal dynamics via excitatory/inhibitory imbalance without overt cell loss 1, supporting a model where functional decline precedes neuronal death.

We propose that microglial PXR normally induces a transcriptional program that limits complement component expression and phagocytic receptor upregulation. When IPA‑PXR signaling wanes, this brake is released, complement tags synapses, and microglia engulf them, producing the synaptic loss observed in aged cortex and hippocampus.

Testable Predictions

1. Correlation: Aged mice with elevated serum gut‑permeability markers (e.g., FITC‑dextran) will show reduced plasma IPA, decreased microglial PXR target gene expression (e.g., Cyp3a11, Abcb1), increased C1q/C3 deposition at synapses, and lower dendritic spine density compared with age‑matched barriers‑intact counterparts.
2. Causality: Chronic oral IPA supplementation (or gut‑barrier fortifier such as larazotide) in aged mice will normalize microglial PXR activity, reduce complement tagging, and preserve synaptic density without altering total NeuN‑positive neuron counts.
3. Specificity: Microglia‑specific PXR knockout in young mice will recapitulate the aged synaptic phenotype (increased complement deposition, spine loss) even when gut barrier and IPA levels are normal.
4. Falsifiability: If IPA supplementation fails to rescue microglial PXR signaling, complement deposition, or synaptic density despite confirmed barrier improvement, the hypothesis is refuted.

Experimental Design

• Groups: Young (3 mo), aged untreated (24 mo), aged + IPA (2 g/kg diet), aged + larazotide, aged + microglial‑specific PXR KO (Control).
• Readouts: Serum IPA (LC‑MS), gut permeability (FITC‑dextran assay), microglial PXR activity (qPCR of Cyp3a11, Abcb1), synaptic complement (immunohistochemistry for C1q/C3 colocalized with PSD‑95), dendritic spine density (Golgi‑Cox or DiI labeling in hippocampus/cortex), neuronal number (NeuN stereology), behavior (Morris water maze, novel object recognition).
• Analysis: Two‑way ANOVA with post‑hoc tests; mediation analysis to test whether IPA levels mediate the relationship between gut permeability and synaptic metrics via microglial PXR activity.

This framework directly links gut‑derived metabolite signaling to microglial synaptic regulation, offering a mechanistic alternative to the notion that aging brains "evict" inefficient neurons. It is experimentally tractable, generates clear quantitative outcomes, and can be falsified with the outlined interventions.