Restoring synovial homeostasis in rheumatoid arthritis by targeting fibroblast-like synoviocytes

Abstract

Rheumatoid arthritis (RA) is a chronic immune-mediated disease that primarily affects the synovium of diarthrodial joints. During the course of RA, the synovium transforms into a hyperplastic invasive tissue that causes destruction of cartilage and bone. Fibroblast-like synoviocytes (FLS), which form the lining of the joint, are epigenetically imprinted with an aggressive phenotype in RA and have an important role in these pathological processes. In addition to producing the extracellular matrix and joint lubricants, FLS in RA produce pathogenic mediators such as cytokines and proteases that contribute to disease pathogenesis and perpetuation. The development of multi-omics integrative analyses have enabled new ways to dissect the mechanisms that imprint FLS, have helped to identify potential FLS subsets with distinct functions and have identified differences in FLS phenotypes between joints in individual patients. This Review provides an overview of advances in understanding of FLS biology and highlights omics approaches and studies that hold promise for identifying future therapeutic targets.

Fig. 1 ∣. The synovial joint in health and in RA.

a ∣ In the healthy joint, the synovial intimal lining is loosely organized and only one or two cell layers deep. Fibroblast-like synoviocytes (FLS) in the synovial intimal lining produce joint lubricants such as hyaluronic acid and lubricin. FLS also help to shape the extracellular matrix (ECM) by producing various matrix components, such as type IV collagen. b ∣ In rheumatoid arthritis (RA), the synovial intimal lining of the joint greatly expands and is transformed into an invasive hyperplastic pannus. The FLS express matrix metalloproteinases (MMPs) and are important contributors to the destruction of cartilage and non-osseous support structures. These cells help to promote and maintain joint inflammation by producing a repertoire of cytokines (such as IL-6), chemokines (such as CXC-chemokine ligand 10) and pro-angiogenic factors (such as VEGF). FLS in RA also contribute to bone erosion by facilitating osteoclastogenesis and by inhibiting bone repair.

Fig. 2 ∣. Genetic and epigenetic mechanisms involved in FLS imprinting in RA.

Genetic and epigenetic modifications in fibroblast-like synoviocytes (FLS) in rheumatoid arthritis (RA) can be inherited or influenced by environmental factors, such as diet, inflammation, pollution and smoking. a ∣ The inflammatory milieu in the rheumatic joint can lead to increased production of reactive oxygen species (ROS) and reactive nitrogen species (RNS), which can alter the expression of proteins involved in DNA repair and might result in genetic imprinting of the FLS through somatic mutations in DNA and mutations in the mitochondrial DNA (as indicated by red crosses). b ∣ Histone modifications regulate the accessibility of the transcriptional machinery to gene promoters. Certain histone marks are associated with active transcription, whereas others are associated with transcriptional repression. In RA, changes in histone modifications (as indicated by the arrows or parentheses) strongly bias FLS towards increased transcription of pathogenic genes. DNA methylation occurs by the addition of a methyl group to a cytosine base where cytosine is followed by guanine (CpG sites). Methylation of the DNA in promoter regions is associated with a closed chromatin structure and transcriptional repression, whereas low levels of methylation in the promoter region favour an open chromatin structure and gene transcription. RA FLS have a distinct methylation pattern that is linked to their aggressive phenotype. RA FLS also have alterations in microRNA (miRNA) expression. miRNAs can cleave or inhibit target mRNA, and abnormal miRNA expression in RA FLS has been linked to increased resistance to apoptosis and increased production of IL-6 and matrix metalloproteinases (MMPs). ACPAs, anti-citrullinated protein/peptide antibodies; DNMT, DNA-methyltransferase; SAM, S-adenosylmethionine; SAH, S-adenosylhomocysteine; RAF1, RAF proto-oncogene serine/threonine-protein kinase; XIAP, X-linked inhibitor of apoptosis protein.

Fig. 3 ∣. Diversity of FLS in RA.

a ∣ Fibroblast-like synoviocytes (FLS) in healthy joints and FLS in joints affected by rheumatoid arthritis (RA) are very different. For example, healthy FLS mainly have a supportive role in the synovium and support structures. RA FLS, on the other hand, have an aggressive, imprinted phenotype and have a destructive role. b ∣ Fibroblasts, including FLS, within the same joint have a variety of phenotypes, especially in RA. These phenotypes might represent true subsets and/or these phenotypes might reflect the local environment in which they reside. c ∣ FLS show spatial heterogeneity with biological differences between various joints. These differences can include RA-independent differences in the expression of genes involved in cell differentiation such as HOX and WNT genes (blue boxes; examples from Frank-Bertoncelj et al.) or RA-dependent differences in the expression of genes, including genes involved in cytokine signalling through Janus kinase (JAK)–signal transducer and activator of transcription (STAT) (red boxes; examples from Hammaker et al.). Arrows indicate changes in levels of transcripts or functions for the indicated joint compared with other joints. d ∣ The function of RA FLS varies depending on the stage of disease. FLS in the late stage of RA promote lymphocyte adhesion to endothelial cells and can have genetic alterations. Differential methylation of DNA in RA FLS occurs early in disease but evolves as the disease progresses. Additional studies on FLS in very early RA are needed to dissect some initial pathogenic pathways (shown by the question mark); future omics analyses will hopefully shed light on these processes.

Fig. 4 ∣. The hallmark features of FLS in RA.

Fibroblast-Like synoviocytes (FLS) in rheumatoid arthritis (RA) have many features that distinguish them from FLS in healthy joints. The figure shows some major cell-intrinsic hallmarks of FLS in RA (intracellular features; outer circle) and cell-extrinsic hallmarks of FLS in RA (effects on the local tissues; inner circle). These features can either be imprinted, leading to permanent changes in FLS function, or can be part of a reversible response to the inflammatory environment.