Objective Syndecan-4 (sdc4) is a cell-anchored proteoglycan that consists of a transmembrane core protein and glucosaminoglycan (GAG) side chains. Binding of soluble factors to the GAG chains of sdc4 may result in the dimerisation of sdc4 and the initiation of downstream signalling cascades. However, the question of how sdc4 dimerisation and signalling affects the response of cells to inflammatory stimuli is unknown.
Methods Sdc4 immunostaining was performed on rheumatoid arthritis (RA) tissue sections. Interleukin (IL)-1 induced extracellular signal-regulated kinases (ERK) phosphorylation and matrix metalloproteinase-3 production was investigated. Il-1 binding to sdc4 was investigated using immunoprecipitation. IL-1 receptor (IL1R1) staining on wild-type, sdc4 and IL1R1 knockout fibroblasts was performed in fluorescence-activated cell sorting analyses. A blocking sdc4 antibody was used to investigate sdc4 dimerisation, IL1R1 expression and the histological paw destruction in the human tumour necrosis factor-alpha transgenic mouse.
Results We show that in fibroblasts, the loss of sdc4 or the antibody-mediated inhibition of sdc4 dimerisation reduces the cell surface expression of the IL-1R and regulates the sensitivity of fibroblasts to IL-1. We demonstrate that IL-1 directly binds to sdc4 and in an IL-1R-independent manner leads to its dimerisation. IL-1-induced dimerisation of sdc4 regulates caveolin vesicle-mediated trafficking of the IL1R1, which in turn determines the responsiveness to IL-1. Administration of antibodies (Ab) against the dimerisation domain of sdc4, thus, strongly reduces the expression IL1R1 on arthritic fibroblasts both in vitro and an animal model of human RA.
Conclusion Collectively, our data suggest that Ab that specifically inhibit sdc4 dimerisation may support anti-IL-1 strategies in diseases such as inflammatory arthritis.
- rheumatoid arthritis
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Handling editor Josef S Smolen
Contributors LG performed the main experiments. MB performed the interleukin (IL)-1 time-dependent IL1R1 reduction in fluorescence-activated cell sorting (FACS). AK-P performed the histological mouse model evaluation. UK established the IL-1 receptor (IL1R1) FACS protocol. JS performed the phosphocaveolin western blot. DB performed the human TNF-alpha transgenic/IL1R1 breeding. KM performed the sdc4 stainings in murine and human rheumatoid arthritis sections. FE helped in writing the manuscript and interpreting the data. JW and GDR performed the dimerisation experiments. TP and JB drafted the manuscript, evaluated the data and steered the project.
Funding We would like to thank the DFG for funding (Emmy Noether BE4328/5-1 and FOR2722).
Competing interests None declared.
Patient and public involvement Patients and/or the public were not involved in the design, or conduct, or reporting, or dissemination plans of this research.
Patient consent for publication Not required.
Ethics approval The ethics committees of the University Hospital Muenster approved all studies. Samples of synovial tissues from patients with rheumatoid arthritis or osteoarthritis were obtained from joint replacement surgery (Ethics Committee Muenster, approval number 2009-049-f-s). All animal experiments were authorised by the Animal Use Committee in Muenster under the file reference 84-02.04.2014.A465.
Provenance and peer review Not commissioned; externally peer reviewed.
Data availability statement Data are available upon reasonable request. All data relevant to the study are included in the article or uploaded as supplementary information. All relevant data are included in the article and will be made available upon request.