Hypothesis

Age-related shifts in the gut microbiome increase secondary bile acids that antagonize hepatic FXR, blunting FGF19-mediated bile acid repression and causing zone-3 bile acid overload, iron accumulation, and GPX4 suppression, while concurrent age-related decline in vagal afferent tone removes cholinergic anti-inflammatory signaling, amplifying Kupffer cell NLRP3 inflammasome activity and lipid peroxidation. Together, these gut-brain-liver disruptions create a two-hit priming of zone-3 hepatocytes for ferroptosis.

Mechanistic Rationale

• Microbiome-derived bile acids: Aging enriches for bile-acid-7alpha-dehydroxylating bacteria, raising deoxycholic acid (DCA) and lithocholic acid (LCA) that act as FXR antagonists Ferroptosis markers in zone 3 hepatocytes. Loss of FXR signaling reduces FGF19 expression, disabling the enterohepatic negative feedback loop and leading to uncontrolled CYP7A1-driven bile acid synthesis.
• FXR-GPX4 axis: FXR directly transactivates GPX4 transcription; its inhibition lowers GPX4 protein, sensitizing hepatocytes to lipid-ROS-driven ferroptosis.
• Iron handling: Excess bile acids impair ferroportin function in zone-3 hepatocytes, increasing labile iron that fuels Fenton chemistry.
• Vagal afferent silencing: Age-related vagal tone reduction diminishes alpha7-nAChR signaling on Kupffer cells, lifting inhibition of NLRP3 inflammasome, boosting IL-1beta and ROS that further peroxidize membrane phospholipids.
• Synergistic hit: Elevated bile acids + iron + ROS create a permissive milieu where even modest lipid peroxidation overwhelms residual GPX4, triggering ferroptotic death preferentially in zone-3, the site of lowest oxygen tension and highest metabolic load.

Experimental Design

1. Mouse models: Young (3-mo) and aged (24-mo) C57BL/6J mice; subset receive fecal microbiota transplant (FMT) from young donors to test causality.
2. Interventions: (a) Oral administration of a selective FXR agonist (e.g., INT-767) or intestinal-targeted CDCA; (b) Chronic vagal nerve stimulation (VNS) via subdiaphragmatic electrode.
3. Readouts (after 8 weeks):
   • Zone-3-specific ferroptosis markers (Tfrc, Fth1, ACSL4) by immunofluorescence and western blot Ferroptosis markers in zone 3 hepatocytes.
   • Hepatic iron (Perls' stain) and labile iron pool (Calcein-AM quenching).
   • Bile acid pool composition (LC-MS).
   • FGF19 serum levels.
   • Vagal activity (heart-rate variability).
   • Kupffer cell NLRP3 inflammasome activation (caspase-1 p20, IL-1beta ELISA).
   • Functional outcomes: glucose tolerance, NAFLD activity score.
4. Controls: Vehicle, sham VNS, and aged mice receiving aged-donor FMT.

Predictions and Falsifiability

• Prediction 1: Aged mice will show elevated zone-3 ferroptosis markers, decreased FXR/FGF19 signaling, increased secondary bile acids, hepatic iron, and vagal hypoactivity vs young.
• Prediction 2: FXR agonism or VNS individually will partially reduce zone-3 ferroptosis; combined treatment will normalize markers to young levels.
• Prediction 3: Young-to-aged FMT will transfer the aged phenotype, whereas aged-to-young FMT will protect young recipients.
• Falsification: If FXR agonism and VNS fail to decrease zone-3 ferroptosis markers despite restoring bile acid homeostasis and vagal tone, the hypothesis that microbiome-bile-acid-FXR-vagal axis drives zone-3 ferroptosis is refuted.

Translational Implication

Positive results would justify clinical trials pairing FXR agonists (e.g., tropifexor) or bile-acid modulators with vagal-stimulation devices (already approved for epilepsy/depression) in older NAFLD/MASH patients, targeting the upstream gut-brain-liver axis rather than downstream ferroptosis alone.