Hypothesis

We hypothesize that circulating indole-3-propionic acid (IPA) activates pregnane X receptor (PXR) in microglia, linking gut-derived barrier protection to activity-dependent removal of metabolically inefficient neurons during aging.

Mechanistic Rationale

IPA produced by colonic commensals crosses the blood-brain barrier and engages PXR, a transcription factor known to tighten intestinal and vascular barriers [1] and to dampen NF-κB-driven inflammation [2]. In microglia, PXR activation reprograms the transcriptome toward a phagocytic state: it upregulates complement component C1q, the opsonin C3, and the receptor TREM2, while suppressing anti-inflammatory markers such as Arg1. Simultaneously, PXR drives expression of mitochondrial uncoupling proteins that lower cellular ATP, creating a sensor for neuronal metabolic stress. Neurons with low firing rates or diminished mitochondrial membrane potential expose "eat-me" signals (e.g., phosphatidylserine) and receive less activity-dependent neurotrophic support, making them preferential targets for PXR-primed microglia. Thus, IPA-PXR signaling provides a systemic cue that couples gut health to synaptic pruning-like elimination of under-used neurons, consistent with the observation that the aging brain loses energetically expensive, weakly connected cells rather than suffering random damage.

Testable Predictions

1. In aged mice, circulating IPA levels will correlate positively with microglial PXR target gene expression (C1q, C3, TREM2) and inversely with the density of low-activity neurons identified by c-Fos or mitochondrial reporters.
2. Depleting IPA (via antibiotic treatment or IPA-synthase knockout) will blunt microglial PXR activation and preserve inefficient neurons, despite normal aging.
3. Microglia-specific PXR deletion will prevent age-related neuronal loss even when IPA is supplemented, whereas neuronal PXR loss will have no effect.
4. In vitro, exposing primary microglia to IPA (10 µM) will increase phagocytosis of neurons pre-treated with mitochondrial uncoupler (FCCP) or low-frequency stimulation, an effect blocked by PXR antagonist (GSK-066) or complement inhibition.
5. Complement blockade (anti-C1q antibody) will rescue neurons from IPA-driven microglial engulfment without altering barrier integrity.

Potential Experiments

• Measure fecal and plasma IPA, brain microglia PXR ChIP-seq, and synaptic markers in young vs. aged wild-type mice ± IPA supplementation.
• Generate Cx3cr1-CreER;Pxr^fl/fl mice for inducible microglial PXR knockout; assess neuron numbers (NeuN, layer-specific markers) and behavioral cognition after 12 months.
• Use two-photon imaging in Thy1-YM mice to track microglial synapses contacts and neuronal calcium activity before/after IPA treatment.
• Perform organoid-derived human microglia co-cultured with neuronal spheroids; add IPA and quantify neuronal death via live/dead staining, rescue with PXR siRNA.

If any of these predictions fail—e.g., IPA depletion does not alter microglial PXR signaling or neuronal loss persists—the hypothesis would be falsified, indicating that neuronal eviction in aging operates independently of microbial IPA-PXR pathways.