Functional aging in human osteocytic networks

Age related diseases are rapidly increasing, placing ever-growing financial and logistical burdens on the NHS. In particular, NICE reports 2 million women in England and Wales currently have osteoporosis with fracture risk rising from 2% at age 50 to 50% by age 80, exceeding 180,000 fractures each year. Thus, there is urgent need to better understand the biology of bone aging at a cellular and tissue level. In particular, the effects of aging on mechanical responses of all three bone cell types and their interactions with each other at tissue level in humans need to be more effectively targeted. Osteocytes are long-lived cells that sense mechanical stimulation and regulate the activity of both bone-forming osteoblasts and bone-removing osteoclasts. However, their need for a heavily mineralised 3D environment makes them very difficult to grow in the lab. We have developed a novel organotypic human ‘minibone’ model containing human osteocytes made by normal human bone cells. These minibones produce specialised osteocyte bone-regulatory proteins and respond to physical loading as they would in the human body. Furthermore, we can now populate the minibones with stem cells engineered to contain age-accelerating mutations. Many age-related conditions share defects in fundamental molecular pathways that disrupt the cell’s ability to cope with cumulative biological stresses. Autophagy, meaning ‘self-eating’, is the mechanism whereby the body removes or recycles damaged parts of a cell to maintain overall cellular health. Levels of the key autophagy protein p62/sequestosome 1 (SQSTM1) reduce with age, and defects in SQSTM1 are linked to various age-related diseases, including metabolic bone disease and neurodegeneration. SQSTM1 mutations cause 50% of cases of sporadic Paget’s Disease of Bone (PDB), the second most common metabolic bone disease. We have created SQSTM1 mutant minibones that show excessive matrix contractility, possibly linked to the distorted bone matrix and architecture of Pagetic lesions, and which release high levels of proteins linked to aging and chronic inflammation. For the first time, the minibone model will allow us to answer fundamental biological questions about how osteocytes influence the biology of aging human bones and age-related metabolic bone conditions. Our approach allows us to probe interactions between cells that contribute to disease in different ways. Our SQSTM1 mutant stem cells are a valuable genetic tool, from which we can now generate all 3 major bone cell types, allowing us to investigate the causes of the formation of defective and weakened bone and of excessive bone loss. We have developed high resolution methods to map gene expression in whole tissues and individual cells in bones. We will use these methods to study Pagetic bone lesions donated from patients with PDB who are undergoing joint replacement. In doing so, we will find out how defects in autophagy affect bone strength, bone cell communication and the adaptive response to loading. Throughout these experiments we will test small molecule drugs to reverse age-relevant changes. This project will directly benefit the PDB community by identifying new cellular and molecular targets that will go beyond current anti-resorptive strategies to fully understand the bone formation defects. Moreover, it will also facilitate fundamental understanding of the potential shared molecular pathways between PDB and the processes of bone aging that lead to thinning bones, osteoporosis and bone fractures for so many older people.