An innate protector of ageing cartilage: FSTL3

在生命过程中，软骨组织必须扮演两个非常对立，几乎相互矛盾的角色。在生命早期，软骨组织促进长骨长度的增加，因此作为一种临时组织发挥作用，其中驻留细胞（称为软骨细胞）具有非常高的生长速率。这与软骨在成人关节中必须发挥的作用几乎没有什么不同，在成人关节中，软骨在保护关节免受运动过程中任何可能的摩擦和损伤方面具有永久性的终身作用。这种永久性功能与非常低的常驻软骨细胞周转率（如果有的话）密切相关。衰老易导致软骨不可避免的损失，导致骨关节炎;最常见的衰老相关疾病之一。这种致残、痛苦和衰弱的疾病对生活质量有着巨大的影响，是美国慢性残疾的主要原因-影响着近2700万人（英国有800万人）。它也是猫，狗和马最常见的关节疾病。腕骨损伤和丢失也是由重复磨损和撕裂引起的运动损伤和退化的特征，特别是在肥胖者中。常规治疗通常只治疗疼痛症状，因此迫切需要有效的治疗干预。然而，一段时间以来，人们已经知道，来自患者和动物的成人骨关节炎软骨中的软骨细胞重新显示出早期生命通常特征的高生长速率，并且这与疾病进展密切相关。我们的初步研究发现，一种天然细胞产物（卵泡抑素样3，FSTL 3），已知可调节其他成人组织中的细胞更新，通常可通过限制软骨细胞生长来预防与年龄相关的OA发作。如果是这种情况，人们可能会认为FSTL 3的缺乏将对软骨的长期永久功能有害。我们在缺乏FSTL 3的转基因小鼠中的初步数据显示，成年人关节中的软骨损失非常迅速地发展为OA样综合征。因此，FSTL 3对软骨细胞生长的“制动功能”在生命早期并不明显，此时增加的周转是绝对必要的，但只有在生长减缓时才在成人中变得明显。因此，FSTL 3的缺失只会在衰老过程中影响成年人的软骨完整性。这些数据促使我们假设：FSTL 3抑制软骨细胞更新以维持正常的成人软骨并防止其在衰老过程中的进行性损失。为了进一步研究FSTL 3防止软骨损失和帮助维持软骨老化的机制，我们在本研究中提出了以下实验。我们将首先确定何时在FSTL 3 KO小鼠中观察到软骨损失的第一个迹象，以及当膝关节软骨被实验性损伤时，这种情况是否会加剧，因为它可能是生活中的运动型损伤。然后，我们将研究缺乏FSTL 3是否会增加软骨细胞增殖和死亡的速度，以及FSTL 3可能影响哪些基因以实现这种软骨保护。因此，我们的研究将确定FSTL 3如何调节软骨细胞的行为，以及它如何在衰老过程中作为软骨损失的先天保护者。最后，我们将制造新的遗传小鼠突变体，这将使我们能够确定FSTL 3是否仅通过其在软骨中的表达来实现这种保护，因此这是否使FSTL 3成为一个良好的选择性治疗靶点。更好地了解软骨细胞生长是如何控制的，将为疾病和衰老中的软骨退化以及损伤中的损伤问题提供治疗途径。因此，这项工作对我们理解软骨完整性背后的过程的贡献可能会支持预防和治疗方法的发展，以更有效地处理重大和衰弱性疾病。

Summary During life, cartilage tissues must serve two very contrasting, almost contradictory roles. During early life, cartilage tissues facilitate increases in the length of long bone and, thus function as a transient tissue in which the resident cells, named chondrocytes, have very high rates of growth. This could hardly contrast more with the role cartilage must serve in adult joints, where it has a permanent, lifetime role in protecting joints from any possible friction and damage during movement. This permanent function is intimately coupled to very low, if any, turnover of resident chondrocytes. Ageing predisposes to an inevitable loss of cartilage that results in osteoarthritis; one of the most common ageing-related disorders. This crippling, painful and debilitating disorder has dramatic affects on the quality of life and is the leading cause of chronic disability in the US - affecting nearly 27 million people (together with 8 million in the UK). It is also the most common joint disease in cats, dogs and horses. Cartilage damage and loss is also a feature of sports injury and degeneration caused by repetitive wear and tear, particularly in the obese. Conventional treatment generally only treats the painful symptoms, so effective therapeutic interventions are desperately needed. It has been known for some time, however, that chondrocytes in adult osteoarthritic cartilage from patients and animals re-display the high rates of growth normally characteristic of early life, and that this is intimately linked to disease progression. Our pilot studies have found that a natural cell product (follistatin-like 3, FSTL3), known to regulate cell turnover in other adult tissues, might normally protect against age-related onset of OA by limiting chondrocyte growth. If this were the case, one might expect that deficiency in FSTL3 would be deleterious to the long-term, permanent function of cartilage. Our preliminary data in genetically modified mice lacking FSTL3 have shown a loss of cartilage in the joints of adults which develop an OA-like syndrome very rapidly. Thus, the FSTL3 'brake function' on chondrocyte growth will not be apparent during early life, when increased turnover is absolutely necessary, but only becomes apparent in the adult when growth has slowed. Loss of FSTL3, therefore, only impacts on cartilage integrity in the adult during ageing. These data prompt us to hypothesise: that that FSTL3 restrains chondrocyte turnover to maintain normal adult cartilage and prevent its progressive loss during ageing. To further investigate the mechanisms by which FSTL3 protects against cartilage loss and helps to maintain cartilage upon ageing we propose the following experiments in this research proposal. We will first identify when the first signs of cartilage loss are seen in FSTL3 KO mice and whether this is exacerbated when the knee cartilage is experimentally damaged, as it might be by sporting-type injury in life. We will then investigate whether the lack of FSTL3 increases the rate at which chondrocytes, multiply and die and which genes FSTL3 may influence to achieve this cartilage protection. Our research will, therefore identify how FSTL3 regulate chondrocyte behaviour and how it acts as an innate protector of cartilage loss during ageing. Finally, we will make new genetic mouse mutants that will allow us to establish if FSTL3 achieves this protection through its expression solely in cartilage and therefore whether this makes FSTL3 a good selective target for therapy. Greater appreciation of how chondrocyte growth is controlled will offer routes towards treatment for problems both of cartilage degeneration in disease and ageing and of damage in injury. Hence, the contributions from this work to our understanding of the processes underlying cartilage integrity may support development of preventative and therapeutic approaches to dealing more effectively with major and debilitating illnesses.