Hypothesis

Aging‑associated shifts in the gut microbiome reduce colonic production of butyrate and propionate, lowering their systemic levels and thus diminishing PPARα activation in hippocampal neurons. Reduced PPARα signaling leads to transcriptional downregulation of OGG1 and APE1, causing accumulation of 8‑oxoG lesions and contributing to neuroinflammation. Conversely, restoring microbial‑derived SCFA levels reinstates PPARα‑driven BER expression, breaking the vicious cycle of gut‑brain oxidative stress.

Mechanistic Rationale

• Butyrate and propionate are known HDAC inhibitors and ligands for PPARα, a nuclear receptor that regulates antioxidant and DNA‑repair genes in neurons (see PPARα regulates OGG1 expression).
• It's been shown that age‑related dysbiosis decreases fecal SCFA concentrations by ~40 % in murine models (SCFA decline with age).
• PPARα agonists increase hippocampal OGG1 mRNA and protein, reducing mitochondrial 8‑oxoG and suppressing cGAS‑STING activation (PPARα agonist rescues BER).
• Neuronal BER deficiency triggers mtDNA release and cGAS‑STING‑mediated inflammation, which can alter gut permeability and promote dysbiosis, creating a feedback loop (mtDNA release drives inflammation).

Thus, the gut microbiome influences neuronal BER not merely through generic inflammation but via a specific metabolite‑nuclear‑receptor axis.

Testable Predictions

1. Old mice with induced dysbiosis (e.g., broad‑spectrum antibiotics) will show lower fecal butyrate, reduced hippocampal PPARα target gene expression, and higher 8‑oxoG levels compared with age‑matched controls.
2. Supplementation with sodium butyrate (or a butyrate‑producing probiotic) will restore PPARα activity, increase OGG1/APE1 protein, and decrease 8‑oxoG in hippocampal neurons.
3. Neuron‑specific PPARα knockout will abolish the protective effect of SCFA supplementation on BER, confirming the mediator’s necessity.
4. Conversely, neuronal BER impairment (via OGG1 shRNA) will increase gut permeability and shift microbiome composition toward pro‑inflammatory taxa.

Experimental Design

• Animal groups: young (3 mo) and old (24 mo) C57BL/6 mice; old mice receive either water, antibiotic cocktail, or antibiotic + butyrate (200 mM in drinking water) for 4 weeks.
• Readouts: fecal SCFA quantification (GC‑MS), hippocampal PPARα target gene qPCR (Cpt1a, Acox1), OGG1/APE1 western blot, immunofluorescence for 8‑oxoG, cGAS‑STING activation, behavior (Morris water maze), gut permeability (FITC‑dextran assay), 16S rRNA sequencing for microbiome shifts.
• Controls: neuron‑specific PPARα‑CreERT2; PPARα^fl/fl mice treated with tamoxifen to delete PPARα before supplementation; OGG1 knockdown via AAV‑shRNA in hippocampus.
• Statistical plan: two‑way ANOVA (age × treatment) with post‑hoc Tukey; n = 10 per group for adequate power.

If SCFA rescue fails to improve BER in PPARα‑null neurons, the hypothesis is falsified. If BER improvement occurs without changes in gut permeability or microbiome composition, the gut‑brain directionality would be questioned, prompting refinement of the mechanism.