Undercarboxylated osteocalcin directs tissue‑specific hydroxyapatite crystal aging via differential matrix binding

Aging bone shows larger, more brittle hydroxyapatite (HA) crystals while dental apatite becomes smaller and more soluble. This divergence may stem from the opposite effects of undercarboxylated osteocalcin (ucOC) on HA nucleation in the two tissues. In bone, ucOC has low affinity for calcium‑rich crystal faces because its glutamic acid residues remain uncarboxylated, reducing its ability to block growth sites. Consequently, HA particles undergo Ostwald ripening, yielding larger crystals that impair mechanical adaptability【https://pubmed.ncbi.nlm.nih.gov/40794153/】. In dentin, the extracellular matrix contains phosphorylated proteins such as dentin sialophosphoprotein that preferentially bind ucOC, sequestering it away from HA surfaces. This local depletion of ucOC relieves its inhibitory effect on crystal growth, allowing other regulators (e.g., fetuin‑A) to promote finer, more soluble apatite【https://pmc.ncbi.nlm.nih.gov/articles/PMC2814098/】.

We hypothesize that the carboxylation state of osteocalcin acts as a molecular switch that tunes HA crystal size in a tissue‑dependent manner: high ucOC drives crystal enlargement in bone but promotes crystal refinement in dentin. This predicts three testable relationships: (1) bone biopsies from older adults with elevated serum ucOC will exhibit larger HA crystallite dimensions measured by X‑ray diffraction or transmission electron microscopy, whereas dentin samples from the same individuals will show smaller crystallites; (2) experimental increase of osteocalcin carboxylation via pharmacological vitamin K₂ (menaquinone‑7) supplementation will reduce ucOC levels, shrink bone HA crystals, and concomitantly increase dentin crystal size; (3) disrupting the ucOC–dentin‑matrix interaction (e.g., using a competitive phosphopeptide) will abolish the dentin‑specific crystal‑size shift without affecting bone.

To test these predictions, a crossover trial could enroll postmenopausal women stratified by baseline ucOC. Participants receive either vitamin K₂ (450 µg/day) or placebo for six months, with washout periods. Bone iliac crest biopsies and extracted teeth (from clinically indicated removals) are processed for HA crystallite analysis using Rietveld refinement and peak‑width calculations. Serum ucOC and carboxylated osteocalcin (cOC) are quantified by ELISA. If vitamin K₂ lowers ucOC and reduces bone HA crystal size while increasing dentin crystal size, the hypothesis is supported; lack of reciprocal changes falsifies it.

Mechanistically, the model links two hallmarks of skeletal aging—impaired osteocalcin carboxylation and aberrant mineralization—through a single modulator whose action is dictated by the local protein milieu. This unifies the otherwise parallel observations of HA crystal growth and ucOC elevation, offering a clear path for targeted interventions that synchronize skeletal and dental health.