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

骨组织在维持钙平衡和器官功能方面的作用至关重要。钙作为一种通用货币，对大多数生理过程至关重要，包括细胞通讯，肌肉收缩，血液凝固和神经功能。骨骼组织的大量损失发生在几种临床情况下，包括废用性骨质疏松症、衰老、脊髓损伤、微重力下的固定和失重，所有这些都以下肢、脊柱和髋关节等承重区域的骨量迅速和显著损失为特征。这增加了骨折的风险并损害了愈合过程，给医疗保健系统和干预相关的成本带来了巨大负担。虽然已知骨量会随着负荷的减少而成比例地减少，但控制骨量的细胞过程需要进一步了解。骨吸收增加而骨沉积减少是公认的，然而，没有可靠的方法来研究骨重建的不平衡。有成熟的动物模型来研究肌肉骨骼废用和骨丢失，这导致动物骨量减少，通过手术切除参与骨代谢的腺体，使用毒素固定，手术切除神经，肌腱或脊髓，或尾部悬吊方法，以促进后肢卸载。其中一些过程对动物非常有害，另一些过程干扰骨骼稳态的生物化学。此外，结果并不完全代表人类的情况，因为两个物种之间以及同一实验室动物的品系之间的骨重建过程存在差异。这项工作的目的是产生一种模型，可以改进和减少用于理解肌肉骨骼退化的动物数量，并提供一种方法来研究盘中的骨丢失。这项工作将产生一个生理相关的模型，其中人类起源的专门骨细胞以及这些细胞类型的组合将在人源性生物支架内培养。将使用几个旋转培养生物反应器对细胞进行机械卸载，这些反应器可以使用不断旋转的容器将细胞保持在恒定的悬浮液中，从而模拟失重状态。该平台将与一系列源自人体组织的活性基质（如纤维蛋白（血凝块样））联合使用，这些基质可降解且具有病理学代表性。这些将在形态上适应悬浮培养的球状体，并可以提供一定程度的支持，同时允许骨细胞用胶原基质取代它们，高度矿化该模板并将它们埋在其中，如基础工作所示。该模型将有助于研究早期骨丢失过程，这对于理解废用病理学至关重要，可以提供细胞毒性，遗传毒性和不相容化合物的第一阶段消除步骤，从而导致体内测试中危害较小的药物，并将允许测试许多有前途的药物和潜在的治疗方法。它还将帮助研究人员开发各种骨骼疾病的治疗方法，不仅与废用性骨质疏松症有关，而且还与过度骨骼研究，骨癌，炎症降解和多系统研究有关。

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.