Hypothesis

Age‑related decline in vitamin K‑dependent γ‑carboxylation of osteocalcin reduces its affinity for nascent hydroxyapatite (HA) nuclei, weakening a natural growth‑inhibitory signal and permitting HA crystals to enlarge and become more brittle in bone. In contrast, dental enamel lacks osteocalcin, so its crystal size is governed primarily by carbonate incorporation (B‑type), which drives the opposite age‑trend toward smaller, more soluble crystallites.

Mechanistic rationale

• Carboxylated osteocalcin (cOC) presents clustered γ‑glutamyl carboxylate (Gla) residues that chelate calcium ions with high affinity. This binding creates a transient, negatively charged coating on HA surfaces that sterically hinders the addition of further calcium‑phosphate units, effectively acting as a crystal‑growth brake.
• Undercarboxylated osteocalcin (ucOC) has fewer negatively charged Gla groups, lowering its calcium‑binding strength and surface coverage. The weakened inhibitory layer allows HA nuclei to grow unimpeded, increasing crystal thickness and crystallinity.
• With advancing age, dietary vitamin K status often falls and the γ‑glutamyl carboxylase becomes less active, shifting the osteocalcin pool toward ucOC. This shift aligns with the observed increase in HA crystal size and brittleness in aged bone.
• Enamel mineralization occurs before tooth eruption and relies on amelogenin and enamelins, not osteocalcin. Consequently, age‑related changes in enamel HA are dominated by post‑secretory carbonate substitution (B‑type), which distorts the lattice, reduces crystallite size (~38 nm → ~12 nm), and raises solubility—a pathway independent of osteocalcin status.

Testable predictions

1. Animal model – Mice fed a vitamin K‑deficient diet (inducing ucOC) will exhibit significantly larger HA crystal dimensions in cortical bone (measured by nanoscale XRD or TEM) compared with vitamin K‑sufficient controls, while enamel crystal size remains unchanged.
2. Rescue experiment – Chronic vitamin K₂ supplementation in aged mice will increase the cOC/ucOC ratio, reduce HA crystal size, and improve bone toughness (three‑point bending) without altering enamel crystallinity.
3. In‑vitro nucleation assay – Recombinant cOC added to supersaturated calcium‑phosphate solutions will decrease nucleation rate and limit crystal growth relative to ucOC or osteocalcin‑null conditions, as monitored by time‑resolved dynamic light scattering.
4. Osteocyte mechanosensitivity – Bone from vitamin K‑deficient mice will show reduced osteocyte calcium‑flux response to mechanical strain, correlating with larger HA crystals; restoring cOC levels should normalize mechanotransduction.

Falsifiability

If vitamin K manipulation fails to alter HA crystal size in bone despite verified shifts in the cOC/ucOC ratio, or if enamel crystal size responds to osteocalcin overexpression, the hypothesis would be refuted. Likewise, if cOC does not demonstrate direct calcium‑binding inhibition of HA growth in cell‑free assays, the proposed mechanistic link would be untenable.

Broader implication

Targeting the osteocalcin carboxylation axis offers a dual strategy: preserving bone material quality by constraining HA overgrowth and clarifying why enamel follows a divergent aging trajectory. This framework bridges molecular nutrition, matrix biology, and mechanical aging in mineralized tissues.