Hypothesis

Age‑associated accumulation of heteroplasmic mitochondrial DNA (mtDNA) mutations in intestinal epithelial stem cells triggers retrograde signaling that epigenetically up‑regulates the apical sodium‑dependent bile acid transporter (ASBT; SLC10A2), thereby increasing bile acid reabsorption, elevating systemic bile acids, and contributing to age‑related cognitive decline.

Mechanistic Rationale

1. mtDNA stress → cytosolic mtDNA: Aging intestinal epithelium acquires heteroplasmic mtDNA variants that replicate advantageously in dividing cells [2]. Leakage of mtDNA into the cytosol activates the cGAS‑STING pathway, leading to type I interferon (IFN‑β) production.
2. IFN‑β → chromatin remodeling: IFN‑β signaling induces STAT1‑dependent recruitment of histone acetyltransferases (e.g., p300) to the SLC10A2 promoter, increasing H3K27ac and transcriptional activity.
3. Enhanced ASBT → bile acid flux: Greater ASBT expression boosts ileal bile acid uptake, reducing fecal loss and raising circulating bile acid pools [3]. Elevated bile acids activate hepatic FXR and intestinal TGR5, altering neuroimmune signaling that impairs hippocampal function.
4. Feedback loop: Bile acid‑activated FXR represses mtDNA transcription, further exacerbating mtDNA dysfunction—a putative vicious cycle.

Testable Predictions

• Prediction 1: Mice with intestinal‑specific mtDNA mutator (PolG^mut) or CRISPR‑induced heteroplasmic mtDNA lesions will show higher ileal ASBT mRNA and protein than wild‑type littermates, independent of nuclear DNA changes.
• Prediction 2: Pharmacological or genetic inhibition of cGAS or STING in intestinal epithelium will abolish the age‑dependent rise in ASBT despite persistent mtDNA heteroplasmy.
• Prediction 3: Restoring ASBT to youthful levels (e.g., via villin‑driven shRNA) in mtDNA‑mutator mice will normalize systemic bile acids and rescue age‑dependent cognitive deficits in behavioral assays (e.g., novel object recognition).
• Prediction 4: Chromatin immunoprecipitation will reveal increased STAT1 binding and H3K27ac at the SLC10A2 promoter in aged intestinal tissue, an effect lost in STING‑deficient epithelium.

Experimental Approach

• Generate villin‑Cre; PolG^mut mice to induce mtDNA mutations specifically in intestinal epithelium.
• Quantify heteroplasmic load by duplex sequencing, ASBT expression by qPCR/Western, bile acid pools by LC‑MS, and cognition by Morris water maze.
• Cross with STING^fl/fl villin‑Cre or treat with C‑176 (STING inhibitor) to test pathway necessity.
• Perform ChIP‑seq for STAT1 and H3K27ac on isolated enterocytes.
• Use fecal bile acid measurement to confirm altered reabsorption.

Potential Confounds & Controls

• Compensatory hepatic bile acid synthesis: measure Cyp7a1 expression to ensure changes are intestinal‑driven.
• Off‑target inflammation: monitor cytokines to distinguish STING‑dependent IFN effects from generic injury.
• Age‑matched controls: include littermate wild‑type and Cre‑negative cohorts.

If any prediction fails—e.g., ASBT remains unchanged despite mtDNA heteroplasmy, or STING inhibition does not prevent ASBT rise—the hypothesis would be falsified, redirecting focus to alternative nuclear‑driven mechanisms.