Senescent immune cells drive bone matrix malaise by sabotaging vitamin K–dependent osteocalcin carboxylation

Hypothesis

Chronic exposure to the senescence-associated secretory phenotype (SASP) from aged immune cells depletes bioactive vitamin K, leading to undercarboxylated osteocalcin and rapid but defective bone mineralization.

Mechanistic Rationale

Senescent macrophages and neutrophils release IL‑6, TNF‑α, myeloperoxidase‑derived hypochlorous acid, and prostaglandin E₂ [2]. These molecules have three convergent effects on vitamin K biology:

1. Inhibition of vitamin K recycling – TNF‑α and IL‑6 suppress hepatic VKORC1 expression, reducing the reduction of vitamin K epoxide to its active hydroquinone form [1].
2. Oxidative degradation of vitamin K hydroquinone – Myeloperoxidase generates HOCl, which oxidizes the hydroquinone moiety, rendering vitamin K unable to act as a cofactor for γ‑glutamyl carboxylase [3].
3. Impaired gut microbial menaquinone production – PGE₂ alters intestinal microbiota composition, decreasing synthesis of long‑chain menaquinones (vitamin K₂) that preferentially support osteocalcin carboxylation [4]. Together, these actions lower the circulating pool of reduced vitamin K, limiting the carboxylation of osteocalcin. Undercarboxylated osteocalcin binds hydroxyapatite with lower affinity, promoting rapid nucleation of poorly crystalline carbonate‑substituted apatite [4]. This explains the paradox of increased mineral quantity but reduced matrix quality in aged bone.

Testable Predictions

• Prediction 1: Serum levels of des‑carboxylated osteocalcin (uncarboxylated osteocalcin) will be positively correlated with circulating SASP markers (IL‑6, TNF‑α, MPO‑DNA complexes) in elderly humans.
• Prediction 2: Genetic ablation of p16⁺ immune cells (using INK‑ATTAC or senolytic navitoclax) in aged mice will restore hepatic VKORC1 activity, increase plasma vitamin K hydroquinone, and raise the ratio of carboxylated to total osteocalcin.
• Prediction 3: Bone from p16⁺‑immune‑cell‑cleared mice will exhibit higher mineral crystallinity (lower carbonate substitution, narrower XRD peak width) without a significant change in bone mineral density.

Experimental Approach

1. Human cohort – Collect peripheral blood from donors stratified by age and frailty index. Measure plasma IL‑6, TNF‑α, MPO‑DNA, des‑carboxylated osteocalcin, and total osteocalcin via ELISA. Perform correlation analysis and multivariate regression adjusting for sex, BMI, and vitamin K intake.
2. Mouse intervention – Use 20‑month‑old p16‑3MR mice; administer ganciclovir to eliminate p16⁺ cells or vehicle control for 4 weeks. Assess hepatic Vkorc1 mRNA, plasma vitamin K isoforms (K₁, MK‑4, MK‑7) by LC‑MS/MS, and osteocalcin carboxylation status by western blot with carboxylation‑specific antibodies.
3. Bone phenotyping – Harvest femur and tibia, perform back‑scattered electron imaging to quantify mineralization density, and synchrotron‑based FT‑IR to assess carbonate-to-phosphate ratio. Compare crystallinity indices between groups.
4. Rescue test – Supplement a subset of senolytic‑treated mice with menatetrenone (MK‑4) to determine whether exogenous vitamin K₂ further improves osteocalcin carboxylation and mineral quality, confirming pathway specificity.

If predictions hold, the data would demonstrate that immune senescence directly sabotages the vitamin K–osteocalcin axis, turning the immune system into an active architect of low‑quality bone. Conversely, a lack of correlation or effect would falsify the hypothesis, steering focus toward alternative mechanisms driving immunoporosis.