Hypothesis

Aging‑associated loss of colonic butyrate production reduces systemic HDAC inhibition, diminishing BDNF‑CREB signaling and activity‑dependent DNA repair in active neurons. This impairs the selective survival of functionally appropriate, low‑activity neurons while permitting hyperactive, inefficient circuits to persist, driving age‑related cognitive decline.

Mechanistic Rationale

• Butyrate crosses the blood‑brain barrier and inhibits class I HDACs, increasing histone acetylation at the Bdnf promoter and enhancing CREB‑mediated transcription (butyrate modulates systemic inflammation and gene expression via HDAC inhibition).
• Elevated BDNF promotes survival of newly formed hippocampal neurons that have established synaptic connections, a process shown to require activity‑dependent selective survival (activity boosts selective survival post‑synapse).
• Neuronal activity also stimulates localized DNA repair at actively transcribed genes, a pathway linked to CREB and mitochondrial transcription (activity regulates DNA repair and mitochondrial transcription at active genes).
• When butyrate falls, HDAC inhibition wanes, Bdnf expression drops, and the activity‑dependent survival signal weakens. Consequently, neurons that would normally be retained because of appropriate activity levels are lost, whereas aberrantly hyperactive neurons—less dependent on BDNF for survival—continue to fire, disrupting circuit homeostasis (aberrant neuronal hyperactivity drives behavioral decline via circuit dysfunction).

Testable Predictions

1. Supplementation rescue – Chronic oral sodium butyrate in aged mice will restore colonic butyrate levels, increase hippocampal Bdnf mRNA and protein, and enhance the survival of BrdU‑labeled newborn neurons after they form synaptic contacts.
2. Activity‑dependence – The protective effect of butyrate will be abolished in mice housed in sedentary conditions or when neuronal activity is chemogenetically silenced, confirming that butyrate acts upstream of activity‑dependent signaling.
3. Circuit read‑out – In vivo calcium imaging will show that butyrate‑treated aged mice exhibit reduced spontaneous hyperactivity in hippocampal CA1 circuits and improved performance on spatial working memory tasks.
4. Mechanistic link – HDAC activity assays from hippocampal lysates will reveal decreased HDAC1/3 activity following butyrate treatment, accompanied by increased histone H3K9ac at the Bdnf promoter and elevated γ‑H2AX foci resolution in active neurons.

Falsifiability

If butyrate supplementation fails to rescue neuronal survival, BDNF expression, or circuit hyperactivity despite confirmed restoration of luminal butyrate, the hypothesis that gut‑derived butyrate gates activity‑dependent neuronal quality control is refuted. Conversely, a positive outcome would support a direct mechanistic bridge between gut microbial metabolism and brain aging.

References (inline)

• Butyrate modulates systemic inflammation and gene expression via HDAC inhibition: https://pmc.ncbi.nlm.nih.gov/articles/PMC12029953/
• Adult neurogenesis declines with age through reduced precursor proliferation and selective survival: https://pmc.ncbi.nlm.nih.gov/articles/PMC4632662/
• Activity boosts selective survival post‑synapse: https://pmc.ncbi.nlm.nih.gov/articles/PMC11668278/
• Aberrant neuronal hyperactivity drives behavioral decline via circuit dysfunction: https://www.pnas.org/doi/10.1073/pnas.2412391122
• Activity regulates DNA repair and mitochondrial transcription at active genes: https://longerlife.org/neuronal-activity-dependent-dna-repair-in-healthy-aging-2/