The mechanics of the collagen fibrillar network in ageing cartilage

我们体内的结缔组织是由细胞和细胞周围的纤维基质组成的。纤维基质在赋予组织功能所需的机械性能方面起着主要作用。尽管具有不同的功能，不同软组织的纤维基质在分子水平上是由相似的构件组成的：胶原蛋白分子、由蛋白质（蛋白聚糖）连接的长糖链和水。特别是，胶原蛋白分子形成长而薄的原纤维，与凝胶状的蛋白聚糖和水一起组装成一个网络。为了从相同的构建块中获得一系列不同的功能，不同的软组织通常会改变原纤维与蛋白聚糖凝胶的相对比例，或者它们的方向或相互连接，以在非常小的尺度上形成复杂的复合材料，低于人类头发的厚度。随着年龄的增长，我们的结缔组织的特性趋于恶化：例如，皮肤变得更硬，软骨在骨关节炎中破裂。这些不利的变化来自于构建块的内在属性或其体系结构的变化。由于这些变化发生在非常小的（纳米）长度尺度上，因此要找出变化及其对力学的影响是具有挑战性的。为了解决这个问题，我们的团队开发了一种高分辨率的x射线成像技术，它的工作原理就像胶原蛋白的衍射光栅：它可以捕捉组织中纳米级胶原蛋白原纤维网络的排列规律，当与同步加速器等非常明亮的x射线源一起使用时，可以跟踪原纤维如何拉伸、重新定向或以其他方式对负载做出反应。在这个项目中，我们将应用这种方法来了解软骨中胶原纤维网络的纳米力学在衰老过程中的变化。关节软骨作为关节的无摩擦承载面，缓冲骨间的负荷转移。如果负荷过重，纤维基质会分解，导致骨关节炎、关节疼痛和不活动。我们的目标是了解胶原蛋白成分的变化是如何与其纳米级力学的变化联系起来的，并最终导致关节断裂。我们将结合x射线技术与高水平表征蛋白质组成和结构的组织，因为它的年龄。这种结合是完全新颖的：x射线技术以前从未应用于软骨，它与蛋白质组学的结合使结构变化和机械功能之间的联系变得清晰。在软骨中，胶原纤维网络抵抗蛋白聚糖凝胶的膨胀压力。我们首先旨在了解这种负载平衡在衰老过程中是如何变化的，并通过改变软骨中不同成分的化学结构和相对比例，了解将分子水平的变化与机械平衡破坏联系起来的机制。其次，我们将研究胶原原纤维的实时变形，因为它们受到现实生活中观察到的负载类型以及衰老如何影响这些动态。这一点尤其重要，因为衰老会导致软骨纤维化和破坏，但胶原原纤维无法抵抗负荷的机制尚不清楚。然后，我们将重点关注两种相关的生物力学：重复加载或局部创伤性冲击。首先，我们将研究纤维对重复负荷的反应是否随着年龄的增长而改变。然后，我们将以微米分辨率绘制局部损伤部位周围的胶原纤维是如何变形的，测试老化过程中成分变化使损伤扩散到整个关节的假设。为了实现这些目标，我们汇集了在x射线纳米力学（Gupta），软骨力学（Knight），老化组织蛋白质组学（Swift）和同步加速器技术（Terrill）方面的互补专业知识，他们都在各自领域处于国际领先地位。

The connective tissues in our bodies are made up of both cells as well as a fibrous matrix around the cells. The fibrous matrix plays the major role in giving the tissue its mechanical properties needed for function. Despite having very different functions, the fibrous matrices of different soft tissues are at the molecular level made up of similar building blocks: collagen molecules, long sugar chains linked by protein (proteoglycans), and water. In particular, collagen molecules form long thin fibrils, which assemble into a network along with the gel-like material of proteoglycans and water. To achieve a range of diverse functions from the same building blocks, different soft tissues often vary the relative proportion of fibrils to the proteoglycan gel, or their orientation or interconnection to form complex composite materials at very small scales, below the thickness of a human hair. When we age, the properties of our connective tissues tend to deteriorate: e.g. skin becomes stiffer, and cartilage breaks down in osteoarthritis. These adverse changes arise from changes in either the intrinsic properties of the building blocks, or in their architecture. Because these changes occur at very small (nanometre) length scales, it is challenging to find out both the change and its effect on mechanics. To address this, our group has developed a high resolution X-ray imaging technique which works like a diffraction grating for collagen: it picks up regularities in the arrangement of the nanoscale collagen fibril networks in tissues, and when used with a very bright X-ray source like a synchrotron, can track how the fibrils stretch, reorient or otherwise respond to loads. In this project, we will apply this method to understand how the nanoscale mechanics of the collagen fibrillar network in cartilage changes in ageing. Articular cartilage serves as a frictionless bearing surface in joints, and cushions the load transfer between bones. If overloaded, the fibrous matrix breaks down and leads to osteoarthritis, joint pain and immobility. We aim to understand how the compositional changes in collagen link to the alterations in its nanoscale mechanics - and eventually to joint breakdown. We will combine the X-ray technique with high-level characterisation of the protein composition and structure in the tissue as it ages. Such a combination is completely novel: the X-ray technique has not been applied to cartilage before, and its combination with proteomics enables a clear link between structural change and mechanical function.In cartilage, the collagen fibrillar network resists the swelling pressure of the proteoglycan gel. We first aim to understand how this load-balance changes in ageing, and by varying the chemical structure and relative proportion of different components in cartilage, to understand the mechanisms linking changes at the molecular level to disruption of mechanical equilibrium. Secondly, we will study real-time deformation of collagen fibrils as they are subjected to the types of load observed in real life and how ageing affects these dynamics. This is especially relevant because ageing leads to fibrillated and disrupted cartilage, but the mechanism by which collagen fibrils fail to resist loading is not understood. We will then focus on two types of relevant biomechanics: repeated loading or local traumatic impact. First, we will investigate whether the fibrillar response to repetitive loading is altered in ageing. Then, we will map, with micron-resolution, how collagen fibrils around the site of a local injury deform, testing the hypothesis that compositional change in ageing enables the damage to spread across the joint. To achieve these aims, we have brought together complementary expertise in X-ray nanomechanics (Gupta), cartilage mechanics (Knight), proteomics of ageing tissues (Swift) and synchrotron technology (Terrill), all of whom are internationally leading in their fields.