Regulated amyloidogenic osteocalcin fibrils constrain hydroxyapatite crystal growth and preserve bone toughness

Hypothesis

Age‑related decline in the extracellular amyloid‑like fibrillization of undercarboxylated osteocalcin (ucOC) reduces its capacity to sequester calcium ions during mineralization, leading to larger hydroxyapatite (HA) crystals, increased brittleness, and higher fracture risk. Restoring ucOC fibrillization will limit HA crystal size and improve bone material properties.

Mechanistic Rationale

1. ucOC as a calcium‑buffering scaffold – ucOC possesses a high affinity for calcium and, under oxidative conditions typical of aging bone matrix, can undergo regulated amyloidogenesis to form extracellular fibrils. These fibrils present a periodic array of acidic residues that bind Ca²⁺ with nanomolar affinity, creating a diffusible calcium‑sequestering phase that slows HA nucleation and growth.
2. Proteostasis shift with age – In young bone, chaperones and extracellular proteases favor the soluble, hormonally active ucOC pool while allowing a minor fraction to fibrillize. Transcriptional “memory” in stromal stem cells (see 3) shifts the balance toward increased total ucOC expression but simultaneously overwhelms extracellular proteostasis, reducing fibrillization efficiency.
3. Consequences for mineral morphology – When ucOC fibrils are scarce, calcium ions remain freely available, promoting uncontrolled HA growth and the formation of large, plate‑like crystals (see 5). Larger crystals increase elastic modulus but reduce crack‑deflection capacity, rendering bone more brittle (see 6).
4. Endocrine trade‑off – The ucOC fraction that remains soluble continues to act as a hormone influencing insulin sensitivity (2). Thus, bone may prioritize endocrine signaling over fibrillization, accepting a mechanical cost as an evolutionary compromise.

Testable Predictions

• In vitro: Recombinant human ucOC induced to fibrillize (using low‑pH or Cu²⁺ seeding) will reduce HA crystal size and increase amorphous calcium phosphate content in a supersaturated calcification assay compared with monomeric ucOC or fibrillization‑incompetent mutants (e.g., proline‑substituted). Crystal size will be quantified by transmission electron microscopy and X‑ray diffraction peak width.
• Ex vivo: Murine calvarial explants treated with ucOC fibrils will show smaller HA nanoparticles (by nano‑indentation mapping) and higher fracture toughness (via three‑point bending) than explants treated with monomeric ucOC or vehicle.
• In vivo: Osteoblast‑specific overexpression of an aggregation‑prone ucOC mutant (containing a known amyloid‑promoting segment) in aged mice will decrease trabecular HA crystal diameter (measured by back‑scattered electron imaging) and improve ultimate stress and toughness relative to aged wild‑type controls. Serum ucOC levels will remain elevated to preserve endocrine activity.
• Falsification: If ucOC fibrils do not alter HA crystal size, mechanical properties, or fracture risk in any of the above assays, the hypothesis is refuted.

Novel Insight Beyond Current Literature

While prior work links ucOC levels to bone quality and endocrine function (1, 2), it treats ucOC solely as a soluble hormone. This hypothesis reframes a fraction of ucOC as an extracellular amyloid‑like material that actively shapes mineral architecture—a direct parallel to the "ordered aggregation" concept in neurodegeneration but applied to the mineralized matrix. It suggests that therapeutic strategies aimed at enhancing, rather than inhibiting, controlled protein aggregation could counteract age‑related bone fragility without compromising the hormone’s metabolic benefits.