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    Osteoarthritis. Buckwalter Joseph A,Martin James A Advanced drug delivery reviews Osteoarthritis (OA), the syndrome of joint pain and dysfunction caused by joint degeneration, affects more people than any other joint disease. In most instances joint degeneration develops in the absence of an identifiable cause, but increasing age, excessive joint loading, and joint abnormalities and insults increase the risk of OA. Articular surface contact stress that causes tissue damage and compromises that ability of chondrocytes to maintain and restore the tissue has an important role in the development of joint degeneration Current methods of attempting to restore an articular surface in osteoarthritic joints include penetrating subchondral bone, altering joint loading, osteotomies and insertion of soft tissue grafts. Dramatic advances in the prevention and treatment of OA are likely to stem from better understanding of the role of mechanical forces in the initiation and progression of joint degeneration. 10.1016/j.addr.2006.01.006
    Radiocarbon dating reveals minimal collagen turnover in both healthy and osteoarthritic human cartilage. Heinemeier Katja M,Schjerling Peter,Heinemeier Jan,Møller Mathias B,Krogsgaard Michael R,Grum-Schwensen Tomas,Petersen Michael M,Kjaer Michael Science translational medicine The poor regenerative capacity of articular cartilage presents a major clinical challenge and may relate to a limited turnover of the cartilage collagen matrix. However, the collagen turnover rate during life is not clear, and it is debated whether osteoarthritis (OA) can influence it. Using the carbon-14 ((14)C) bomb-pulse method, life-long replacement rates of collagen were measured in tibial plateau cartilage from 23 persons born between 1935 and1997 (15 and 8 persons with OA and healthy cartilage, respectively). The (14)C levels observed in cartilage collagen showed that, virtually, no replacement of the collagen matrix happened after skeletal maturity and that neither OA nor tissue damage, per se, influenced collagen turnover. Regional differences in (14)C content across the joint surface showed that cartilage collagen located centrally on the joint surface is formed several years earlier than collagen located peripherally. The collagen matrix of human articular cartilage is an essentially permanent structure that has no significant turnover in adults, even with the occurrence of disease. 10.1126/scitranslmed.aad8335
    Cartilage damage in osteoarthritis and rheumatoid arthritis--two unequal siblings. Pap Thomas,Korb-Pap Adelheid Nature reviews. Rheumatology Cartilage damage is a key feature of degenerative joint disorders-primarily osteoarthritis (OA)-and chronic inflammatory joint diseases, such as rheumatoid arthritis (RA). Substantial progress has been made towards understanding the mechanisms that lead to degradation of the cartilage matrix in either condition, which ultimately results in the progressive remodelling of affected joints. The available data have shown that the molecular steps in cartilage matrix breakdown overlap in OA and RA. However, they have also, to a great extent, changed our view of the roles of cartilage in the pathogenesis of these disorders. In OA, cartilage loss occurs as part of a complex programme that resembles aspects of embryonic bone formation through endochondral ossification. In RA, early cartilage damage is a key trigger of cellular reactions in the synovium. In a proposed model of RA as a site-specific manifestation of a systemic autoimmune disorder, early cartilage damage in the context of immune activation leads to a specific cellular response within articular joints that could explain not only the organ specificity of RA, but also the chronic nature and perpetuation of the disease. 10.1038/nrrheum.2015.95
    Single-cell mass cytometry reveals cross-talk between inflammation-dampening and inflammation-amplifying cells in osteoarthritic cartilage. Grandi Fiorella Carla,Baskar Reema,Smeriglio Piera,Murkherjee Shravani,Indelli Pier Francesco,Amanatullah Derek F,Goodman Stuart,Chu Constance,Bendall Sean,Bhutani Nidhi Science advances Aging or injury leads to degradation of the cartilage matrix and the development of osteoarthritis (OA). Because of a paucity of single-cell studies of OA cartilage, little is known about the interpatient variability in its cellular composition and, more importantly, about the cell subpopulations that drive the disease. Here, we profiled healthy and OA cartilage samples using mass cytometry to establish a single-cell atlas, revealing distinct chondrocyte progenitor and inflammation-modulating subpopulations. These rare populations include an inflammation-amplifying (Inf-A) population, marked by interleukin-1 receptor 1 and tumor necrosis factor receptor II, whose inhibition decreased inflammation, and an inflammation-dampening (Inf-D) population, marked by CD24, which is resistant to inflammation. We devised a pharmacological strategy targeting Inf-A and Inf-D cells that significantly decreased inflammation in OA chondrocytes. Using our atlas, we stratified patients with OA in three groups that are distinguished by the relative proportions of inflammatory to regenerative cells, making it possible to devise precision therapeutic approaches. 10.1126/sciadv.aay5352
    Bone remodelling in osteoarthritis. Burr David B,Gallant Maxime A Nature reviews. Rheumatology The classical view of the pathogenesis of osteoarthritis (OA) is that subchondral sclerosis is associated with, and perhaps causes, age-related joint degeneration. Recent observations have demonstrated that OA is associated with early loss of bone owing to increased bone remodelling, followed by slow turnover leading to densification of the subchondral plate and complete loss of cartilage. Subchondral densification is a late event in OA that involves only the subchondral plate and calcified cartilage; the subchondral cancellous bone beneath the subchondral plate may remain osteopenic. In experimental models, inducing subchondral sclerosis without allowing the prior stage of increased bone remodelling to occur does not lead to progressive OA. Therefore, both early-stage increased remodelling and bone loss, and the late-stage slow remodelling and subchondral densification are important components of the pathogenetic process that leads to OA. The apparent paradoxical observations that OA is associated with both increased remodelling and osteopenia, as well as decreased remodelling and sclerosis, are consistent with the spatial and temporal separation of these processes during joint degeneration. This Review provides an overview of current knowledge on OA and discusses the role of subchondral bone in the initiation and progression of OA. A hypothetical model of OA pathogenesis is proposed. 10.1038/nrrheum.2012.130
    Apoptosis signaling pathways in osteoarthritis and possible protective role of melatonin. Hosseinzadeh Azam,Kamrava Seyed Kamran,Joghataei Mohammad Taghi,Darabi Radbod,Shakeri-Zadeh Ali,Shahriari Mansour,Reiter Russel J,Ghaznavi Habib,Mehrzadi Saeed Journal of pineal research Osteoarthritis (OA) is a degenerative joint disease characterized by progressive erosion of articular cartilage. As chondrocytes are the only cell type forming the articular cartilage, their gradual loss is the main cause of OA. There is a substantial body of published research that suggests reactive oxygen species (ROS) are major causative factors for chondrocyte damage and OA development. Oxidative stress elicited by ROS is capable of oxidizing and subsequently disrupting cartilage homeostasis, promoting catabolism via induction of cell death and damaging numerous components of the joint. IL-1β and TNF-α are crucial inflammatory factors that play pivotal roles in the pathogenesis of OA. In this process, the mitochondria are the major source of ROS production in cells, suggesting a role of mitochondrial dysfunction in this type of arthritis. This may also be promoted by inflammatory cytokines such as IL-1β and TNF-α which contribute to chondrocyte death. In patients with OA, the expression of endoplasmic reticulum (ER) stress-associated molecules is positively correlated with cartilage degeneration. Melatonin and its metabolites are broad-spectrum antioxidants and free radical scavengers which regulate a variety of molecular pathways such as inflammation, proliferation, apoptosis, and metastasis in different pathophysiological situations. Herein, we review the effects of melatonin on OA, focusing on its ability to regulate apoptotic processes and ER and mitochondrial activity. We also evaluate likely protective effects of melatonin on OA pathogenesis. 10.1111/jpi.12362
    Etiology of osteoarthritis: genetics and synovial joint development. Sandell Linda J Nature reviews. Rheumatology Osteoarthritis (OA) has a considerable hereditary component and is considered to be a polygenic disease. Data derived from genetic analyses and genome-wide screening of individuals with this disease have revealed a surprising trend: genes associated with OA tend to be related to the process of synovial joint development. Mutations in these genes might directly cause OA. In addition, they could also determine the age at which OA becomes apparent, the joint sites involved, the severity of the disease and how rapidly it progresses. In this Review, I propose that genetic mutations associated with OA can be placed on a continuum. Early-onset OA is caused by mutations in matrix molecules often associated with chondrodysplasias, whereas less destructive structural abnormalities or mutations confer increased susceptibility to injury or malalignment that can result in middle-age onset. Finally, mutations in molecules that regulate subtle aspects of joint development and structure lead to late-onset OA. In this Review, I discuss the genetics of OA in general, but focus on the potential effect of genetic mutations associated with OA on joint structure, the role of joint structure in the development of OA--using hip abnormalities as a model--and how understanding the etiology of the disease could influence treatment. 10.1038/nrrheum.2011.199
    Mechanisms and therapeutic implications of cellular senescence in osteoarthritis. Coryell Philip R,Diekman Brian O,Loeser Richard F Nature reviews. Rheumatology The development of osteoarthritis (OA) correlates with a rise in the number of senescent cells in joint tissues, and the senescence-associated secretory phenotype (SASP) has been implicated in cartilage degradation and OA. Age-related mitochondrial dysfunction and associated oxidative stress might induce senescence in joint tissue cells. However, senescence is not the only driver of OA, and the mechanisms by which senescent cells contribute to disease progression are not fully understood. Furthermore, it remains uncertain which joint cells and SASP-factors contribute to the OA phenotype. Research in the field has looked at developing therapeutics (namely senolytics and senomorphics) that eliminate or alter senescent cells to stop disease progression and pathogenesis. A better understanding of how senescence contributes to joint dysfunction may enhance the effectiveness of these approaches and provide relief for patients with OA. 10.1038/s41584-020-00533-7
    Hyperphysiological compression of articular cartilage induces an osteoarthritic phenotype in a cartilage-on-a-chip model. Occhetta Paola,Mainardi Andrea,Votta Emiliano,Vallmajo-Martin Queralt,Ehrbar Martin,Martin Ivan,Barbero Andrea,Rasponi Marco Nature biomedical engineering Owing to population aging, the social impact of osteoarthritis (OA)-the most common musculoskeletal disease-is expected to increase dramatically. Yet, therapy is still limited to palliative treatments or surgical intervention, and disease-modifying OA (DMOA) drugs are scarce, mainly because of the absence of relevant preclinical OA models. Therefore, in vitro models that can reliably predict the efficacy of DMOA drugs are needed. Here, we show, using a newly developed microphysiological cartilage-on-a-chip model that enables the application of strain-controlled compression to three-dimensional articular cartilage microtissue, that a 30% confined compression recapitulates the mechanical factors involved in OA pathogenesis and is sufficient to induce OA traits. Such hyperphysiological compression triggers a shift in cartilage homeostasis towards catabolism and inflammation, hypertrophy, and the acquisition of a gene expression profile akin to those seen in clinical osteoarthritic tissue. The cartilage on-a-chip model may enable the screening of DMOA candidates. 10.1038/s41551-019-0406-3
    OA in 2011: Age-related OA--a concept emerging from infancy? Aigner Thomas,Richter Wiltrud Nature reviews. Rheumatology That primary osteoarthritis (OA) is an age-related disorder is undoubted, but how aging contributes to OA is poorly understood. New insights from 2011 offer potential explanations, novel models for study, and the suggestion that a deeper understanding of what 'aging' actually is might pave the way to everlasting joints. 10.1038/nrrheum.2011.206
    Ageing and the pathogenesis of osteoarthritis. Loeser Richard F,Collins John A,Diekman Brian O Nature reviews. Rheumatology Ageing-associated changes that affect articular tissues promote the development of osteoarthritis (OA). Although ageing and OA are closely linked, they are independent processes. Several potential mechanisms by which ageing contributes to OA have been elucidated. This Review focuses on the contributions of the following factors: age-related inflammation (also referred to as 'inflammaging'); cellular senescence (including the senescence-associated secretory phenotype (SASP)); mitochondrial dysfunction and oxidative stress; dysfunction in energy metabolism due to reduced activity of 5'-AMP-activated protein kinase (AMPK), which is associated with reduced autophagy; and alterations in cell signalling due to age-related changes in the extracellular matrix. These various processes contribute to the development of OA by promoting a proinflammatory, catabolic state accompanied by increased susceptibility to cell death that together lead to increased joint tissue destruction and defective repair of damaged matrix. The majority of studies to date have focused on articular cartilage, and it will be important to determine whether similar mechanisms occur in other joint tissues. Improved understanding of ageing-related mechanisms that promote OA could lead to the discovery of new targets for therapies that aim to slow or stop the progression of this chronic and disabling condition. 10.1038/nrrheum.2016.65
    Low-grade inflammation as a key mediator of the pathogenesis of osteoarthritis. Robinson William H,Lepus Christin M,Wang Qian,Raghu Harini,Mao Rong,Lindstrom Tamsin M,Sokolove Jeremy Nature reviews. Rheumatology Osteoarthritis (OA) has long been viewed as a degenerative disease of cartilage, but accumulating evidence indicates that inflammation has a critical role in its pathogenesis. Furthermore, we now appreciate that OA pathogenesis involves not only breakdown of cartilage, but also remodelling of the underlying bone, formation of ectopic bone, hypertrophy of the joint capsule, and inflammation of the synovial lining. That is, OA is a disorder of the joint as a whole, with inflammation driving many pathologic changes. The inflammation in OA is distinct from that in rheumatoid arthritis and other autoimmune diseases: it is chronic, comparatively low-grade, and mediated primarily by the innate immune system. Current treatments for OA only control the symptoms, and none has been FDA-approved for the prevention or slowing of disease progression. However, increasing insight into the inflammatory underpinnings of OA holds promise for the development of new, disease-modifying therapies. Indeed, several anti-inflammatory therapies have shown promise in animal models of OA. Further work is needed to identify effective inhibitors of the low-grade inflammation in OA, and to determine whether therapies that target this inflammation can prevent or slow the development and progression of the disease. 10.1038/nrrheum.2016.136