Hypothesis

Restoring a youthful gut microbiome reprograms the host's entire exosome network, shifting circulating vesicle cargo from pro-inflammatory to neuroprotective signatures and thereby attenuating inflammaging-driven cognitive decline.

Mechanistic Rationale

• Gut microbes secrete short-chain fatty acids (SCFAs), secondary bile acids, and microbial extracellular vesicles (mEVs) that influence host cell pathways governing exosome biogenesis and cargo sorting [https://pmc.ncbi.nlm.nih.gov/articles/PMC12192803/].
• SCFAs inhibit histone deacetylases, increasing acetylation of nSMase2 and Rab27a promoters, which biases exosome loading toward anti-inflammatory miRNAs (e.g., miR-146a, miR-124) and away from pro-inflammatory cargo such as miR-342-3p.
• mEVs can fuse with host-derived exosomes, transferring GABA, glutamate, and metabolic enzymes that further modulate neuronal exosome content [https://journals.asm.org/doi/10.1128/spectrum.01368-24].
• This dual action creates a systemic shift: plasma exosomes become enriched in neuroprotective proteins and depleted of tau-phosphorylating kinases and BBB-disrupting factors.

Testable Predictions

1. Biomarker shift – In aged mice receiving fecal microbiota transplantation (FMT) from young donors, plasma exosomal levels of miR-342-3p will decrease by ≥30 % while miR-146a and miR-124 increase comparably, relative to aged-control FMT.
2. Functional rescue – The same animals will show restored BBB integrity (reduced Evans blue leakage), lowered hippocampal tau hyperphosphorylation (AT8 immunoreactivity), and improved performance in the Morris water maze.
3. Causality test – Depleting gut-derived mEVs using a broad-spectrum antimiR-targeted liposomes will abolish the exosome cargo shift despite young-donor FMT, confirming microbial vesicle contribution.
4. Human translatability – In a pilot clinical trial, older adults with mild cognitive impairment receiving a defined consortia of SCFA-producing strains will exhibit a similar exosome miRNA profile change and delayed conversion to Alzheimer’s disease over 12 months [https://pmc.ncbi.nlm.nih.gov/articles/PMC8921862/].

Falsifiability

If young-donor FMT fails to alter plasma exosomal miR-342-3p, miR-146a, or miR-124 levels, or if the biomarker changes do not correlate with neurocognitive improvements, the hypothesis that microbiome restoration reprograms the systemic exosome network to counteract inflammaging will be refuted.

Advantages Over Current Strategies

• Unlike exogenous MSC-derived exosomes, microbiome modulation is scalable, inexpensive, and leverages the host's own vesicle production machinery [https://pmc.ncbi.nlm.nih.gov/articles/PMC8921862/].
• It simultaneously targets multiple exosome sources (intestinal epithelium, immune cells, neurons) because the microbial signal is systemic.

By grounding the hypothesis in the bidirectional, exosome-mediated gut-brain axis revealed in recent work, we turn the 'messy' complexity into a tractable therapeutic lever.