The application of trabecular bone organoids to investigate mineral-sensing in skeletal physiology and disease

Bone is an endocrine organ which fulfils a whole range of vital functions. One of these is the process of calcium and phosphate homeostasis, two elements which, in bone tissue, are associated to form the mature mineral, hydroxyapatite, and within the circulation are essential for numerous cell processes, including clotting, signalling and even muscle contraction. The bone mineral content is maintained by resident cells in this tissue; and is continuously adjusted with ageing, mechanical stress, as well as the physiological demands. The key controller of calcium and phosphate regulation is the calcium sensing receptor (CaSR), located in several organs, including the kidneys. Importantly, within the parathyroid glands, it controls the secretion of the parathyroid hormone (PTH) in response to fluctuating levels of calcium in the circulation. As such, bone uses this signalling path to release these ions from its mineralised tissue, as required. Because these processes are very inter-related, when these signalling paths are dysregulated, they lead to abnormal levels of mineral in the tissue itself or within the circulation, as well as triggering further pathologies. Abnormal changes can take place when one or more of these organ functions is dysregulated. For example, in chronic kidney disease (CKD), inadequate removal of phosphate leads to excessive circulating levels of this ion and an exacerbated increase in PTH secretion, which can result in complications such as osteoporosis, due to excessive removal of mineral. It is therefore essential to better understand the molecular interaction with bone in order to develop novel therapeutics and better interventions for bone loss and other metabolic dysfunctions. Currently, rodent models (rats and mice) are used in laboratories around the world to study these mechanisms and to test compounds. Many models require detrimental procedures such as surgical interventions, immobilisation or genetic alterations to remove components that are involved in the bone formation process. Secondly, some of these models are too complex to allow isolation of individual organ effects. This represents a bottleneck in the search for promising therapeutics.
Previous NC3Rs funded work conducted by Dr Alexandra Iordachescu (Uni. Birmingham), allowed the development of a model of mature bone in vitro for the first time, where mineral and cells could be monitored over extended periods of time (up to one year and beyond). Subsequently, through further NC3Rs funding, Dr Iordachescu miniaturised this model into an organoid system which contained a complete spectrum of cells found in bone and was suitable for studying early events, including bone loss, mineral deposition; as well as for conducting larger-scale pharmacological testing. At the same time, recent work from Dr Donald Ward's lab (Uni. Manchester) identified that phosphate also binds at sites on the CaSR, acting as a phosphate sensor, explaining how excessive circulating phosphate worsens secondary hyperparathyroidism (the resulting increase in PTH secretion) in CKD. Because this receptor has been shown in mice models to be present in bone, where it responds to and controls fracture repair and callus maturation, it is important to understand how the fluctuating levels of phosphate affect the receptor in this tissue. This is of importance also because major clinical trials where clinical compounds were targeting this receptor did not necessarily cause beneficial increases in bone mass.
Therefore, these bone organoids will be applied to test several pathological conditions in order to detect novel information, particularly as they allow the monitoring of mineral and recapitulate many of the fracture repair events. This project will therefore translate and transfer the knowledge and skills into a molecular endocrinology lab, reducing the need for genetically-altered mouse models to test these hypotheses, which would require a large number of rodents.

Bone performs essential endocrinological functions by interacting with several calciotropic organs (parathyroids, kidneys and the gut) to regulate calcium and phosphate homeostasis, under the control of the parathyroid hormone and mediated through the calcium sensing receptor (CaSR). Because of the complexity of the signalling routes involved, disorders of mineral metabolism can often occur. Understanding these pathologies is essential for developing novel therapeutics, many which have failed clinical trials. Currently, studying these processes relies on animal models, which require surgical interventions or genetic alterations to remove essential components in bone formation. Mice strains are frequently used around the world, however, they are too complex to allow the isolation of individual organ-level effects. Previous NC3Rs funded work conducted by Dr Alexandra Iordachescu (Uni. Birmingham), allowed the development of a model of mature bone in vitro for the first time, where mineral and cells could be monitored over physiologically-relevant periods (up to one year and beyond). Subsequently, through further NC3Rs funding, Dr Iordachescu miniaturised this model into an organoid system which behaved as a complete remodelling unit and was suitable for larger-scale pharmacological testing. At the same time, recent work from Dr Donald Ward's lab (Uni. Manchester) identified the CaSR as a phosphate sensor, explaining how excessive circulating phosphate worsens secondary hyperparathyroidism in chronic kidney disease (CKD). Because the CaSR is also expressed in bone cells, where it is involved in repair and maturation processes, it is essential to determine the specific effects of phosphate and therapeutics on the bone CaSR. Therefore, a bone model is required that is isolated from circulating calciotropic factors but possesses native features and capacity for mineralisation, a challenge perfect for these bone organoids and which would reduce the need for rodent models.