Hypothesis

The gut‑derived exosome pool, shaped by microbiota metabolites, directly reprograms hippocampal and hypothalamic neural circuits to set the rate of brain aging, and manipulations that enrich beneficial exosome cargo recapitulate the lifespan extension seen with caloric restriction (CR).

Mechanistic Rationale

• Microbiota‑derived short‑chain fatty acids (SCFAs) increase exosome biogenesis in intestinal epithelial cells and load them with anti‑inflammatory miRNAs (e.g., miR‑126b‑5p) and metabolites that can cross the blood‑brain barrier 3 2.
• These gut exosomes deliver their cargo to microglia and neurons, suppressing NF‑κB signaling and reducing tau phosphorylation, thereby lowering neuroinflammation 2.
• In parallel, hypothalamic stem‑cell exosomes that carry miRNAs regulate systemic aging 1; however, their output is modulated by afferent gut signals that alter stem‑cell excitability via vagal afferents 2.
• Therefore, the net exosome milieu in circulation reflects a bidirectional exchange where gut‑derived vesicles dominate the baseline that central circuits interpret as a youthful or aged state.

Testable Predictions

1. Prediction A: Old mice receiving fecal microbiota transplant (FMT) from young donors will show an increase in circulating exosomes enriched for miR‑126b‑5p and a concomitant decrease in brain‑resident pro‑inflammatory markers, even without changes in hypothalamic stem‑cell exosome output.
2. Prediction B: Pharmacological blockade of vagal afferent signaling (e.g., subdiaphragmatic vagotomy) will abolish the brain‑beneficial effects of CR‑induced microbiome shifts, despite preserved peripheral SCFA levels.
3. Prediction C: Isolating exosomes from the plasma of CR‑treated mice and injecting them into ad libitum fed old mice will recapitulate the cognitive improvement and lifespan extension seen with CR, whereas exosomes from CR‑treated, microbiota‑depleted (antibiotic‑treated) mice will fail to do so.

Experimental Approach

• Cohort 1: Young vs old donor FMT into aged recipients; collect plasma exosomes at 2 weeks, quantify miR‑126b‑5p by qPCR, assess brain IL‑1β, Iba1, and phospho‑tau via immunostaining.
• Cohort 2: Old mice on 30 % CR with sham or vagotomy; measure plasma SCFA, exosome cargo, and brain inflammation markers.
• Cohort 3: Exosome fractionation from plasma of CR mice (± antibiotics); inject 100 µg exosomes twice weekly into aged controls; monitor Morris water maze performance and survival.

Statistical analysis will use two‑way ANOVA with post‑hoc Tukey; falsification occurs if any prediction fails to reach significance (p > 0.05) in the expected direction.

Potential Impact

Demonstrating a primary gut‑to‑brain exosome axis would redirect longevity strategies toward microbiome‑targeted exosome engineering, complementing or surpassing current brain‑centric approaches.