Hypothesis

Age-related loss of osteocalcin release from bone drives gut barrier dysfunction, which in turn triggers neuroinflammatory cascades that accelerate cognitive decline. This positions the skeleton as an upstream endocrine regulator of the gut‑brain axis, reversing the typical top‑down view.

Mechanistic Reasoning

1. Osteocalcin secretion is matrix‑dependent – Hydroxyapatite crystals enlarge with age, trapping osteocalcin within the bone matrix and reducing its circulating levels (hydroxyapatite crystals grow larger).
2. Undercarboxylated osteocalcin (ucOC) signals via GPRC6A on intestinal enteroendocrine L‑cells – Binding stimulates GLP‑1 secretion, which enhances tight‑junction protein expression (e.g., claudin‑1, occludin) and promotes mucus production, thereby preserving intestinal barrier integrity.
3. Loss of ucOC signaling weakens the barrier – Increased permeability allows bacterial endotoxins (LPS) to translocate into plasma, activating hepatic Kupffer cells and subsequently microglia via the Toll‑like‑4/NF‑κB pathway, fostering neuroinflammation and synaptic loss.
4. Feedback to bone – Chronic inflammation elevates sclerostin and RANKL, further suppressing osteoblast activity and osteocalcin synthesis, creating a vicious loop that amplifies systemic aging.

Testable Predictions

• Prediction 1: Chronic administration of recombinant ucOC to aged mice will restore circulating ucOC levels, improve gut barrier markers (lower FITC‑dextran permeability, higher zonulin‑1), reduce plasma LPS, and ameliorate performance in hippocampal‑dependent memory tasks.
• Prediction 2: Gut‑specific knockout of Gprc6a will abolish the protective effects of ucOC supplementation on barrier function and cognition, despite normal serum ucOC.
• Prediction 3: Pharmacological reduction of hydroxyapatite crystal size (e.g., using pyrophosphate analogs) in aged animals will increase osteocalcin release, rescuing gut‑brain phenotypes without direct ucOC treatment.
• Prediction 4: In humans, baseline serum ucOC (adjusted for carboxylation status) will predict future changes in intestinal permeability (measured by lactulose/mannitol ratio) and cognitive trajectories independent of traditional risk factors.

Experimental Approach

• Mouse models: Use 24‑month‑old C57BL/6 mice; groups receive vehicle, ucOC (daily intraperitoneal injection), Gprc6a intestinal‑specific knockout + ucOC, or hydroxyapatite‑size‑modulating agent.
• Readouts: Serum ucOC ELISA, FITC‑dextran assay, zonulin‑1 immunostaining, plasma LPS LAL assay, microglial Iba1 activation, Morris water maze, and RNA‑seq of colonic epithelium for barrier genes.
• Human cohort: Leverage existing longitudinal studies with baseline serum total and carboxylated osteocalcin, urinary lactulose/mannitol, and cognitive testing; employ mixed‑effects modeling to assess predictive value.

If ucOC replacement restores gut barrier and cognition, the hypothesis is supported; failure to observe these effects would falsify the claim that bone‑derived osteocalcin is a primary upstream driver of the gut‑brain axis in aging.