Zone of Calcified Cartilage – a Natural Boarder in the Osteochondral Unit to Control Diffusion of Molecules between Articular Cartilage and Subchondral Bone

骨关节炎（OA）是世界上最常见的关节疾病，超过20%的德国人患有这种痛苦和致残的疾病。骨性关节炎期间的生理和病理变化包括软骨组织的退化和侵蚀，直至暴露在下面的骨。为了干预本病的进展，迫切需要从分子、细胞和组织水平更好地了解骨软骨单元，特别是钙化层带（ZCC）的机制。ZCC连接非钙化关节软骨的深层区和软骨下骨。然而，关于ZCC在软骨稳态（健康组织的稳定状态）中的作用知之甚少。我假设ZCC作为一个自然边界，控制软骨和骨组织之间分子的扩散，调节细胞间的交流。这种通透性在OA发作时发生改变，导致细胞外基质中细胞因子和形态因子的变化，从而诱导软骨细胞的表型变化。我将从宏观的组织水平和微观的细胞水平两个角度来探讨这个问题。第一个目标涵盖了分子的定量，可以通过人类ZCC离体从骨到软骨，反之亦然，使用先进的3D成像技术和分光光度法。通过ZCC的分子大小及其在这一薄组织层中的位置将与OA患者骨软骨组织的结构变化有关。第二个目标是利用下一代测序技术鉴定钙化软骨中肥大软骨细胞的基因表达模式。该文库将用于比较特定分子（形态因子和生长因子）如何刺激从深区非钙化软骨分离的软骨细胞在体外分化为增生性软骨细胞，以及这些变化与增生性OA软骨细胞的表型有多接近。在与主客研究小组的密切合作下，我将对ZCC在软骨稳态和OA进展中的调节作用产生新的认识和见解。通过ZCC在人外植体中扩散特性的表征，以及深带软骨细胞对体外环境因子的响应，将为深入了解OA过程中增生性分化的相关机制提供独特的途径。这一知识将导致新的方法来阻止或延缓OA进展和软骨退变。

Osteoarthritis (OA) is the commonest joint disease worldwide with more than 20% of the German society suffering from this painful and disabling disease. Physiological and pathological changes during OA include the degeneration and erosion of cartilage tissue up to the exposure of the underlying bone. To intervene the progression of this disease, there is urgent need to better understand the mechanisms at molecular, cell and tissue level of the osteochondral unit, especially the zone of calcified layer (ZCC). The ZCC is connecting the deep zone of non-calcified articular cartilage to the subchondral bone. However, little is known about the role of the ZCC in cartilage homeostasis, the steady state condition of healthy tissue.I hypothesize that the ZCC serves as a natural border that controls the diffusion of molecules between cartilage and bone tissue and regulates cell-cell communication. This permeability is altered upon onset of OA leading to changes in cytokines and morphogens in the extracellular matrix inducing phenotypic changes in chondrocytes. I will approach this topic from two perspectives: the macroscopic tissue-level and the microscopic cellular-level. The first objective covers the quantification of molecules that can pass through human ZCC ex vivo from bone to cartilage and vice versa using advanced 3D imaging techniques and spectrophotometry. The size of molecules passing the ZCC and their location in this thin tissue layer will be correlated to the structural changes in the osteochondral tissue of OA patients. The second objective focuses on the identification of gene expression patterns specific for hypertrophic chondrocytes residing in the calcified cartilage using next generation sequencing technique. This library will be used to compare how specific molecules (morphogens and growth factors) can stimulate chondrocytes isolated from non-calcified cartilage of the deep zone to undergo differentiation towards hypertrophic chondrocytes in vitro and how close these changes match with the phenotype of hypertrophic OA chondrocytes.In close collaboration with the host and guest research groups, I will generate new knowledge and insights on the regulatory role of the ZCC in cartilage homeostasis and OA progression. The characterization of diffusion properties through the ZCC in human explants, together with the response of deep zone chondrocyte to environmental factors in vitro will provide a unique approach to deeper understand the mechanism related to hypertrophic differentiation during OA. This knowledge will result in novel approaches to impede or delay OA progression and cartilage degeneration.