Hypothesis: Delivering CRISPR‑base editor cargo through engineered extracellular vesicles (EVs) from a selected psychobiotic strain can achieve germline‑grade editing efficiency in post‑mitotic neurons, thereby reversing age‑associated epigenetic drift and rescuing cognitive decline.

Mechanistic Rationale

• Germline integrity is maintained not by superior repair but by relentless selection against defective cells at each reproductive bottleneck. Somatic lineages lack this purging pressure, allowing accumulation of mutations and epimutations that contribute to functional decline.
• Psychobiotic EVs, such as those from Lactobacillus plantarum, already cross the blood‑brain barrier via transcytosis and modulate hippocampal BDNF expression [3]. This demonstrates a natural delivery route for bioactive molecules to neurons.
• By loading these EVs with mRNA or ribonucleoprotein complexes encoding a high‑fidelity adenine base editor (ABE8e) and a guide RNA targeting age‑methylated CpG sites in promoters of neuroprotective genes (e.g., Bdnf, Synapsin‑1), we emulate the germline’s selective editing: only neurons that successfully incorporate the edit and restore beneficial expression survive functional assays, while non‑edited cells remain unchanged.
• Prior work shows that depletion of harmful taxa like Parabacteroides goldsteinii restores youth‑like cognition in aged‑microbiome‑colonized mice [1]. Combining depletion with EV‑based editing creates a two‑phase strategy: first remove deleterious microbial influences, then supply a germline‑grade editing toolkit to neurons.
• The approach extends the precision microbiome concept beyond metabolite signaling to direct genome editing, addressing the limitation that current human trials rely on correlative improvements without mechanistic genome‑level change [2, 4].

Testable Predictions

1. Mice receiving orally administered EVs engineered to carry ABE8e will show a significant increase in A‑to‑G edits at target sites in hippocampal neurons compared with controls receiving empty EVs.
2. Edited mice will exhibit restored hippocampal BDNF levels and improved performance in spatial memory tasks (Morris water maze) comparable to young adult baseline.
3. The cognitive benefit will be absent in mice depleted of L. plantarum or treated with EVs lacking editor cargo, confirming that both the bacterial strain and its editor payload are required.
4. Off‑target editing analysis will reveal negligible indels or unintended edits, demonstrating the safety of the germline‑grade editing budget when delivered via EVs.

Experimental Design

• Strain engineering: Transform L. plantarum WCFS1 to secrete EVs packaged with ABE8e mRNA and sgRNA under a constitutive promoter; validate EV cargo via western blot and RT‑qPCR.
• Depletion phase: Treat 20‑month‑old C57BL/6 mice with a narrow‑spectrum antibiotic cocktail targeting P. goldsteinii for 2 weeks, confirming taxa reduction by 16S rRNA sequencing.
• Intervention phase: Administer engineered EVs (10^11 particles) via oral gavage three times weekly for 4 weeks; control groups receive wild‑type EVs or saline.
• Outcome measures:
  • Deep sequencing of hippocampal DNA to quantify on‑target editing frequency.
  • ELISA for BDNF and synaptic proteins.
  • Behavioral assays: Morris water maze, novel object recognition.
  • Histological assessment of neuronal health (NeuN, cleaved caspase‑3).
• Statistical analysis: Use ANOVA with post‑hoc Tukey tests; power analysis suggests n=10 per group to detect a 20% edit increase with 80% power.

If validated, this hypothesis would establish a paradigm where somatic cells inherit the germline’s stringent editing budget, turning a natural selection mechanism into a therapeutic engine for cognitive aging.