From ageing to space travel: Developing an organotypic model of skeletal tissue disuse for understanding degeneration in altered environments

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The role of bone tissue in maintaining calcium balance and organ function is essential. Calcium acts as a universal currency and is essential for most physiological processes, including cellular communication, muscle contraction, blood clotting and nerve function. A substantial loss of skeletal tissue takes place in several clinical contexts, including disuse osteoporosis, ageing, spinal cord injury, immobilisation and weightlessness in microgravity, all characterised by rapid and significant loss in bone mass in the load-bearing regions, including lower limbs, spine and hip. This increases the risk in fractures and impairs the healing process, placing a significant burden on the healthcare system and the costs associated with interventions.While bone mass is known to decrease proportionally with reduced loading, the cellular processes governing it require further understanding. It is widely accepted that bone resorption is increased and bone deposition is decreased, however, there is no robust way of studying the imbalance in bone remodelling.There are well-established animal models for studying musculoskeletal disuse and bone loss, which cause a reduction in bone mass in animals either through surgical removal of glands involved in bone metabolism, immobilisation using toxins, surgical resection of nerves, tendons or the spinal cord, or a tail suspension method facilitating hindlimb unloading. Some of these processes are very detrimental for the animals and others interfere with the biochemistry of skeletal homeostasis. Moreover, the results are not entirely representative of the human conditions, as differences exist in the bone remodelling process between the two species and between strains of the same laboratory animal.The aim of this work is to produce a model that can refine and reduce the number of animals used for understanding musculoskeletal degeneration and to provide a method to study bone loss in a dish. The work will generate a physiologically-relevant model, in which specialised bone cells of human origin as well as combinations of these cell types will be cultured inside human-derived biological scaffolds. Cells will be provided with mechanical unloading using several rotary culture bioreactors that can keep cells in a constant suspension using constantly rotating vessels, thus simulating a weightless state. This platform will be used in combination with a range of active matrices derived from human tissue, such as fibrin (blood clot-like), which are degradable and pathologically representative. These will be morphologically adapted into spheroids for suspended culture and can provide a degree of support while allowing bone cells to replace them with collagenous matrix, heavily mineralise this template and bury themselves inside it, as shown by foundation work. This model will help in studying early bone loss processes which are essential for understanding disuse pathology, can provide a first-stage elimination step of cytotoxic, genotoxic and incompatible compounds leading to less harmful agents being progressed for in vivo testing, and will allow the testing of numerous promising drugs and potential therapeutics. It will also help researchers developing treatments in a wide range of skeletal conditions, not only relevant to disuse osteoporosis, but also in excessive bone research, bone cancers, inflammatory degradation and multi-systemic research.