Functional Imaging of Osteocyte Autophagy, Senescence and Apoptosis in Human Bone

骨是一种动态的活组织，具有高度多孔的矿化细胞外基质，其中容纳了广泛分布的树突状骨细胞的细胞网络。骨细胞连接到表面细胞，即骨形成成骨细胞和骨再吸收破骨细胞。众所周知，在成年期，破骨细胞和成骨细胞的及时和局部协调作用使骨基质翻转，以维持骨功能。因此，在骨转换不平衡的情况下，例如高度流行的年龄相关的骨丢失（骨质疏松症），药物治疗集中于破骨细胞和成骨细胞活性，这并不奇怪。这些治疗方法可以改善骨密度和降低骨折发生率，但它们都不能治愈潜在的疾病，这表明其他因素是必不可少的骨功能的损失。这是这个建议的假设，骨细胞在骨重建的局部调节中起着至关重要的作用，骨细胞的活力是一个决定性因素。骨细胞的寿命是这种细胞类型最显著的特征之一。骨细胞被认为是人体中存活时间最长的细胞之一，其细胞活力受到其骨细胞网络内经历的外部应激源的高度影响。然而，正是骨细胞在矿化骨基质中的嵌入性造成了功能性可视化这种细胞类型的挑战性情况。该项目将使用骨细胞的先进功能成像来研究细胞生命周期，方法是：i）细胞死亡机制中的细胞凋亡，ii）生存策略中的自噬，以及iii）衰老过程中的衰老过程和细胞周期中的状态。此外，骨细胞活力将与成骨细胞和破骨细胞的骨转换活性局部相关，如通过组织形态计量学确定的。这将创建一个新的机制地图连接骨细胞的生命周期骨周转。为了实现临床相关性，将使用来自骨转换不平衡的年轻人和老年人的人骨样本进行研究。我们的工作和其他人的工作通过表明骨细胞在维持其细胞活力方面表现出上级，证明了骨细胞活力的至关重要性。随着骨细胞多功能性的研究进展，骨细胞的死亡已成为骨转换不平衡性骨病的标志。在此背景下，该项目将揭示骨细胞的生命周期与人骨中骨转换过程中成骨细胞和破骨细胞的局部活动之间的联系。研究这种联系将产生一种新的机制理解，可以帮助优化未来的骨骼疾病治疗方案。

Bone is a dynamic living tissue with a highly porous mineralized extracellular matrix housing a widespread cellular network of dendritic osteocytes. Osteocytes are connected to surface cells, namely bone forming osteoblasts and bone resorbing osteoclasts. It is well established that during adult life the bone matrix is turned over by a timely and locally coordinated action of osteoclasts and osteoblasts to maintain bone functionality. Therefore, it is not surprising that in situations of unbalanced bone turnover e.g. the highly prevalent age related bone loss (osteoporosis), pharmaceutical therapies focus on osteoclast and osteoblast activities. These therapies can improve bone mineral density and reduce fracture incidences, yet none of them is able to cure the underlying disease, suggesting additional factors are essential for the loss of bone functionality.It is the hypothesis of this proposal that osteocytes have a crucial role in the local regulation of bone remodeling and that the osteocyte viability is a determining factor. Osteocyte longevity is one of the most striking feature of this cell type. Osteocytes are considered to be one of the longest living cells in the human body and their cell viability is highly influenced by external stressors experienced within their osteocyte network. However, it is the embeddedness of osteocytes in the mineralized bone matrix that creates a challenging situation to functionally visualize this cell type. This project will use advanced functional imaging of the osteocyte to investigate the cellular life cycle by means of i) apoptosis as cell death mechanism, ii) autophagy as survival strategy, and iii) senescence as an aging process and the status in the cell cycle. Furthermore, osteocyte viability will be linked locally to bone turnover activities by osteoblasts and osteoclasts as determined by histomorphometry. This will create a novel mechanistic map connecting the osteocyte life cycle to bone turnover. To enable clinical relevance, human bone samples from young adults and aged adults with unbalanced bone turnover will be used for the investigation. Our work and the work of others demonstrated the vital importance of osteocyte viability by showing that osteocytes appear superior in maintaining their cell viability. With the recent research advancements demonstrating the multifunctionality of osteocytes, the death of osteocytes is a hallmark of bone diseases with unbalanced bone turnover. Against this background, this project will unravel the link between the life cycle of osteocytes and the local activities of osteoblasts and osteoclasts during bone turnover in human bone. Investigating this link will generate a novel mechanistic understanding that can aid to optimize future treatment regimens for bone diseases.