Summary

Bone is laid down by osteoblast cells and eroded away by osteoclast cells. However 90% of adult bone cells are osteocytes, mechanosensitive specialists living buried in a labyrinth of dense mineral. True osteocytes only exist in this buried state, proving impossible to extract and grow in culture without dying or rapidly changing their essential nature. They are extraordinary cells, living for around 23 years in the same site in humans, sensing changes in bone loading and damage and relaying this to surface osteoblasts and osteoclasts to repair and strengthen. Osteocytes make sclerostin, an inhibitor of bone formation. Clinically targeting this is the only way we know to make new bone grow in frail adult bones. They are also endocrine cells, talking to the distant kidneys through blood borne messengers like FGF23 that regulate kidney handling of phosphate. Mutations in specialist osteocyte proteins Phex or DMP1 cause severe rickets, deformed bones in children from faulty mineral handling.

An organ-on-a-chip human osteocyte construct would therefore provide a powerful exploratory tool for drug discovery and understanding human aging. We have now achieved this with rat cells in an Oxford-Birmingham collaboration. We created a soft fibrin scaffold around two mineral posts and seeded it with bone lining cells. Within 2 weeks fibrin was replaced with a tough collagen scaffold, its fibres aligned with the axis of strain between the posts, and mineralisation began. With increasing mineral, the cells produced a sequence of osteocyte markers - ion channels, Phex, DMP1 and finally at 2-3 months the typically elusive sclerostin. This project will use human cells in our construct to discover the missing regulators driving osteocyte formation, through sequencing their gene products at intervals over 3 months.

We will produce a 3D physicochemical map of mechanical strain and increasing mineral density and use key markers to locate the osteocytic response in time and space.

Key Outputs

Paget’s Association – Research Grant: Developing a fully human 3D bone organ model of Paget’s disease Research Grant, £60,000, 2021 – 2022, Hulley

Presented preliminary data: BioMedEng19 in London, Sept 2019; Versus Arthritis Centre for OA Pathogenesis, Nov 2019 (Oxford); Botnar Biomechanics forum, Nov 2019 (Oxford)