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

骨是一种内分泌器官，它完成了一系列的重要功能。其中之一是钙和磷酸盐的动态平衡过程，这两种元素在骨组织中与形成成熟的矿物质羟基磷灰石有关，并且在循环中对许多细胞过程至关重要，包括凝血，信号传导甚至肌肉收缩。骨矿物质含量由该组织中的常驻细胞维持；并随着老化、机械应力以及生理需求不断调整。钙和磷酸盐调节的关键控制者是钙感应受体（CaSR），它位于包括肾脏在内的几个器官中。重要的是，在甲状旁腺内，它控制甲状旁腺激素（PTH）的分泌，以响应循环中钙水平的波动。因此，骨骼根据需要使用这种信号通路从矿化组织中释放这些离子。因为这些过程是相互关联的，当这些信号通路失调时，它们会导致组织本身或循环内的矿物质水平异常，并引发进一步的病理。当这些器官的一个或多个功能失调时，就会发生异常变化。例如，在慢性肾脏疾病（CKD）中，磷酸盐去除不足导致循环中该离子水平过高，并加剧甲状旁腺激素分泌的增加，这可能导致并发症，如骨质疏松症，由于矿物质的过度去除。因此，有必要更好地了解与骨的分子相互作用，以便开发新的治疗方法和更好的干预骨质流失和其他代谢功能障碍。目前，世界各地的实验室使用啮齿动物模型（大鼠和小鼠）来研究这些机制和测试化合物。许多模型需要有害的程序，如手术干预、固定或基因改变，以去除参与骨形成过程的成分。其次，其中一些模型过于复杂，无法分离出单个器官的影响。这代表了寻找有前途的治疗方法的瓶颈。以前的NC3Rs资助的工作由亚历山德拉·约达切斯库博士进行。（伯明翰），首次在体外开发出成熟骨模型，在这种模型中，矿物质和细胞可以在很长一段时间内（长达一年或更长时间）进行监测。随后，通过NC3Rs的进一步资助，Iordachescu博士将该模型缩小为一个类器官系统，该系统包含骨骼中发现的完整细胞谱，适用于研究早期事件，包括骨质流失、矿物质沉积；以及进行更大规模的药理学试验。与此同时，唐纳德·沃德博士的实验室(Uni。Manchester)发现磷酸盐也结合在CaSR上的位点，作为磷酸盐传感器，解释了过量的循环磷酸盐如何恶化CKD的继发性甲状旁腺功能亢进（导致PTH分泌增加）。由于该受体在小鼠模型中存在于骨骼中，并对骨折修复和骨痂成熟做出反应和控制，因此了解磷酸盐水平的波动如何影响该组织中的受体是很重要的。这也很重要，因为主要的临床试验中，临床化合物靶向这种受体并不一定会导致骨量的有益增加。因此，这些骨类器官将被应用于测试几种病理状况，以检测新的信息，特别是因为它们允许监测矿物质和概括许多骨折修复事件。因此，该项目将把知识和技能转化为分子内分泌学实验室，减少对转基因小鼠模型的需求，以测试这些假设，这将需要大量的啮齿动物。

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.