OA synovial fluid: biological insights into a whole-joint disease

Synovial fluid (SF) is formed from an ultrafiltrate of plasma (excluding some larger plasma proteins, such as coagulation factors), as well as proteins and other macromolecules that are secreted by the joint-lining cells. It has, characteristically, high levels of high-molecular-weight hyaluronan, an anionic non-sulfated, glycosaminoglycan that gives SF its viscous properties. In the healthy joint the SF serves to assist lubrication1Lin W. Klein J. Recent progress in cartilage lubrication.Adv Mater. 2021; 33e2005513Crossref Scopus (122) Google Scholar and exchange nutrients/waste products on behalf of the surrounding tissues. Hyaluronan contributes to controlling the volume of the SF by attracting water, and may act as a filter for proteins and solutes2Levick J.R. McDonald J.N. Fluid movement across synovium in healthy joints: role of synovial fluid macromolecules.Ann Rheum Dis. 1995; 54: 417-423Crossref PubMed Scopus (74) Google Scholar. The concentration of individual constituents of the SF is determined by local turnover (synthesis and clearance), and these may be affected by levels of physical activity (Fig. 1). Arthritis impacts the SF substantially: by changing the cellular and protein components that are generated in the joint; by changing the filtration function of the synovium; and by the generation of hyaluronidases3Goldring S.R. Goldring M.B. Biology of the normal joint.in: Firestein G.S. Kelley's Textbook of Rheumatology. Elsevier, 2013: 1-19Crossref Scopus (6) Google Scholar. SF has been used in multiple studies to explore aspects of OA pathogenesis. Published studies include those that have measured changes in the cellular constituents of OA SF4Kriegova E. Manukyan G. Mikulkova Z. Gabcova G. Kudelka M. Gajdos P. et al.Gender-related differences observed among immune cells in synovial fluid in knee osteoarthritis.Osteoarthritis Cartilage. 2018; 26: 1247-1256Abstract Full Text Full Text PDF PubMed Scopus (37) Google Scholar, candidate markers of disease activity (e.g., matrix degradation fragments), and molecules regulated in the joint that reflect pathway activation (e.g., in inflammation and metabolism)5Carlson A.K. Rawle R.A. Wallace C.W. Brooks E.G. Adams E. Greenwood M.C. et al.Characterization of synovial fluid metabolomic phenotypes of cartilage morphological changes associated with osteoarthritis.Osteoarthritis Cartilage. 2019; 27: 1174-1184Abstract Full Text Full Text PDF PubMed Scopus (53) Google Scholar,6Boffa A. Merli G. Andriolo L. Lattermann C. Salzmann G.M. Filardo G. Synovial fluid biomarkers in knee osteoarthritis: a systematic review and quantitative evaluation using BIPEDs criteria.Cartilage. 2021; 13: 82S-103SCrossref PubMed Scopus (25) Google Scholar. SF not only reflects activity in the joint, but is also an active biological material that can stimulate cell responses either directly through macromolecules, such as cytokines, or indirectly through extracellular vesicles containing regulatory molecules, such as microRNAs7Li Y.H. Tavallaee G. Tokar T. Nakamura A. Sundararajan K. Weston A. et al.Identification of synovial fluid microRNA signature in knee osteoarthritis: differentiating early- and late-stage knee osteoarthritis.Osteoarthritis Cartilage. 2016; 24: 1577-1586Abstract Full Text Full Text PDF PubMed Scopus (81) Google Scholar. The study by Housmans et al. published in this issue8Housmans B.A.C. Neefjes M. Surtel D.A.M. Vitík M. Cremers A. van Rhijn L.W. et al.Synovial fluid from end-stage osteoarthritis induces proliferation and fibrosis of articular chondrocytes via MAPK and RhoGTPase signaling.Osteoarthr Cartil. 2022 Feb 15; https://doi.org/10.1016/j.joca.2021.12.015Abstract Full Text Full Text PDF PubMed Scopus (10) Google Scholar examined the biological response of human chondrocytes to OA SF. The authors stimulated chondrocytes with OA SF and examined chondrocyte proliferation, the regulation of a number of candidate genes, and phosphokinase activation to identify active intracellular signalling pathways. The latter experiment was notable because it provided an agnostic analysis of the activity of OA SF by looking at 660 unique kinase phosphorylation sites activated in the stimulated chondrocytes. Deconvoluting these data in combination with pharmacological inhibitors and reporter assays identified a number of activated pathways, including the epidermal growth factor (EGF) receptor pathway. The authors demonstrated that this pathway was responsible for increased proliferative activity of isolated chondrocytes after prolonged treatment with OA SF, and was associated with an increase in a number of EGF ligands detected in the OA SF. This example provides important proof that this type of approach is valuable in unravelling relevant human OA biology, because it accords well with mounting evidence that EGF ligands are important in OA pathogenesis. One such EGF ligand is TGFα, which has been identified as a GWAS hit for OA severity9Boer C.G. Hatzikotoulas K. Southam L. Stefansdottir L. Zhang Y. Coutinho de Almeida R. et al.Deciphering osteoarthritis genetics across 826,690 individuals from 9 populations.Cell. 2021; 184: 4784-4818 e4717Abstract Full Text Full Text PDF PubMed Scopus (96) Google Scholar and cartilage thickness10Castano-Betancourt M.C. Evans D.S. Ramos Y.F. Boer C.G. Metrustry S. Liu Y. et al.Novel genetic variants for cartilage thickness and hip osteoarthritis.PLoS Genet. 2016; 12e1006260Crossref PubMed Scopus (65) Google Scholar, and which has recently been shown to promote cartilage repair associated with superficial chondrocyte proliferation in murine OA11Wei Y. Luo L. Gui T. Yu F. Yan L. Yao L. et al.Targeting cartilage EGFR pathway for osteoarthritis treatment.Sci Transl Med. 2021; 13Crossref Google Scholar. The authors speculate that EGF receptor pathway activation in OA SF may represent attempted repair in the OA joint. The concept of attempted repair in OA is not novel. Recent GWAS studies have provided substantial support for this by identifying a number of polymorphic variants linked to high-confidence effector genes in growth factor ligands and associated signalling molecules, such as TGFβ, TGFα, and FGF189Boer C.G. Hatzikotoulas K. Southam L. Stefansdottir L. Zhang Y. Coutinho de Almeida R. et al.Deciphering osteoarthritis genetics across 826,690 individuals from 9 populations.Cell. 2021; 184: 4784-4818 e4717Abstract Full Text Full Text PDF PubMed Scopus (96) Google Scholar. For TGFβ and FGF18, these appear to be variants that predict reduced function associated with increased severity of disease, indicating that failed repair may account for some of the genetic component of OA risk. The ability to use such molecules to promote repair as a treatment for OA is supported by Phase II clinical trials of sprifermin, a truncated form of FGF1812Hochberg M.C. Guermazi A. Guehring H. Aydemir A. Wax S. Fleuranceau-Morel P. et al.Effect of intra-articular sprifermin vs placebo on femorotibial joint cartilage thickness in patients with osteoarthritis: the FORWARD randomized clinical trial.JAMA. 2019; 322: 1360-1370Crossref PubMed Scopus (184) Google Scholar. Housmans et al. also describe OA SF as being a potent driver of dedifferentiation towards a 'fibrotic' phenotype, with this being associated with cell proliferation. It is interesting that several of the growth factors identified above also have apparent 'dedifferentiating' and catabolic roles in vitro, which has confused their putative predicted role in disease13Vincent T.L. Fibroblast growth factor 2: good or bad guy in the joint?.Arthritis Res Ther. 2011; 13: 127Crossref PubMed Scopus (27) Google Scholar,14Appleton C.T. Usmani S.E. Mort J.S. Beier F. Rho/ROCK and MEK/ERK activation by transforming growth factor-alpha induces articular cartilage degradation.Lab Invest. 2010; 90: 20-30Crossref PubMed Scopus (101) Google Scholar. This raises the provocative question of whether dedifferentiation and proliferation are prerequisites for successful repair, and thus whether we should be promoting these phenotypes rather than trying to suppress them. SF has a number of advantages over other tissues of the joint when exploring pathogenesis of arthritis. Firstly, it has adjacency to all the surfaces of the joint, and so represents cellular activity that is agnostic of joint tissues. These include those tissues that are not normally exposed to the synovial cavity except in severe disease, such as subchondral bone. Secondly, unlike bone and cartilage, which are difficult to acquire except at the time of joint replacement surgery, SF sampling can be performed at all stages of disease and over the course of disease within an individual. This provides the opportunity to determine how disease changes at a molecular level over time, and whether there are molecular predictors of disease progression15Garriga C. Goff M. Paterson E. Hrusecka R. Hamid B. Alderson J. et al.Clinical and molecular associations with outcomes at 2 years after acute knee injury: a longitudinal study in the Knee Injury Cohort at the Kennedy (KICK).Lancet Rheumatology. 2021; 3: E648-E658Abstract Full Text Full Text PDF PubMed Scopus (11) Google Scholar. Finally, unlike plasma, SF can provide a snapshot of disease activity that is specific to an individual joint, and which is not diluted by the plasma volume or confounded by disease at multiple joint sites. There are limitations to studies using SF. SF samples are not that readily available, and it can be challenging to perform these types of study in large numbers of individuals. The volume of SF may fluctuate after exercise and with acute clinical flares of OA, and this is likely to affect the concentration of bioactive molecules in the fluid. Importantly, the acquisition of healthy SF is difficult, and in the Housmans et al. study only key experiments used normal SF as a control. However this paper raises interesting questions about whether we should be using SF more agnostically to explore relevant OA biology, and whether it can capture disease heterogeneity. Whether SF activity changes over the course of disease, predicts disease outcome and response to treatment, or relates to genotype needs to be elucidated. Such approaches may prove to be highly valuable as we consider personalized approaches to treatment of this complex, dynamic disease. The author directs the Centre for OA Pathogenesis Versus Arthritis and leads the STEpUP OA Consortium, an international effort to perform molecular profiling of >1,800 SF samples in OA and joint injury. This has received funding from Novartis, Pfizer, Fidia, Biosplice, and Galapagos. Centre for OA Pathogenesis (grant no. 21621).