Hypothesis

Aging-related decline in circulating osteocalcin removes a tonic brake on gut-derived 3‑hydroxyoctanoic acid (3‑HOA) signaling, allowing unchecked GPR84 activation on intestinal immune cells, amplified vagal afferent silencing, and accelerated hippocampal inflammation.

Mechanistic Rationale

• Osteocalcin, secreted by osteoblasts, crosses the blood‑brain barrier and binds Gprc6a on hippocampal neurons, promoting synaptic plasticity and dampening microglial IL‑6/TNF‑α release [1].
• In bone, osteocalcin also regulates enteroendocrine cells, stimulating secretion of peptide YY (PYY) that suppresses colonic immune activation [2]
• Age‑related bone loss reduces serum osteocalcin by ~40 % in mice and humans, coincident with rising Parabacteroides goldsteinii and 3‑HOA levels.
• Low osteocalcin therefore diminishes two inhibitory arms: (1) direct hippocampal protection via Gprc6a, and (2) indirect restraint of colonic 3‑HOA‑producing microbes through PYY‑mediated immune quiescence.
• When both brakes fail, 3‑HOA freely activates GPR84 on lamina propria macrophages, driving IL‑1β release that desensitizes CCKAR+ vagal afferents, cutting the anti‑inflammatory vagal tone to the hippocampus.

Predictions & Experimental Design

1. Correlation – In aged mice, serum osteocalcin will inversely correlate with fecal 3‑HOA concentration and hippocampal IL‑6 levels (Spearman r < ‑0.5).
2. Loss‑of‑function – Osteoblast‑specific Oc‑knockout mice will show exaggerated 3‑HOA‑induced vagal silencing and memory deficits compared with wild‑type aged controls, despite similar microbiota composition.
3. Gain‑of‑function – Chronic administration of recombinant osteocalcin (or a Gprc6a agonist) to aged Oc‑deficient mice will restore PYY elevation, reduce colonic GPR84 signaling, improve vagal firing, and rescue spatial memory in the Morris water maze.
4. Pharmacological blockade – Combining osteocalcin replacement with a GPR84 antagonist should produce additive rescue, confirming that osteocalcin acts upstream of the metabolite‑receptor axis. Experiments will measure: serum osteocalcin (ELISA), fecal 3‑HOA (GC‑MS), colonic GPR84+ macrophage flow cytometry, vagal afferent electrophysiology, and hippocampal cytokine panels.

Potential Outcomes & Implications

• If osteocalcin loss amplifies the 3‑HOA/GPR84/vagal pathway, it explains why bone‑targeted therapies (e.g., sclerostin antibodies) sometimes improve cognition beyond skeletal effects.
• Conversely, a null result would suggest that osteocalcin does not modulate gut immune signaling, redirecting focus to other bone‑derived factors (e.g., FGF23, sclerostin). Either outcome refines the gut‑brain‑muscle axis by positioning endocrine bone health as a upstream regulator of microbial metabolite signaling, offering a combinatorial strategy: bone anabolics plus vagal modulators to break the inflammaging loop in older adults.