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Synovial fibroblasts assume distinct functional identities and secrete R-spondin 2 in osteoarthritis
  1. Alexander J Knights1,
  2. Easton C Farrell1,2,
  3. Olivia M Ellis1,
  4. Lindsey Lammlin1,
  5. Lucas M Junginger1,
  6. Phillip M Rzeczycki1,
  7. Rachel F Bergman1,
  8. Rida Pervez1,
  9. Monique Cruz1,
  10. Eleanor Knight1,
  11. Dennis Farmer1,
  12. Alexa A Samani1,
  13. Chia-Lung Wu3,
  14. Kurt D Hankenson1,2,
  15. Tristan Maerz1,2
  1. 1Department of Orthopaedic Surgery, University of Michigan, Ann Arbor, Michigan, USA
  2. 2Department of Biomedical Engineering, University of Michigan, Ann Arbor, Michigan, USA
  3. 3Department of Orthopaedic Surgery and Rehabilitation, University of Rochester, Rochester, New York, USA
  1. Correspondence to Dr Tristan Maerz, Department of Orthopaedic Surgery, University of Michigan, Ann Arbor, Michigan, USA; tmaerz{at}med.umich.edu

Abstract

Objectives Synovium is acutely affected following joint trauma and contributes to post-traumatic osteoarthritis (PTOA) progression. Little is known about discrete cell types and molecular mechanisms in PTOA synovium. We aimed to describe synovial cell populations and their dynamics in PTOA, with a focus on fibroblasts. We also sought to define mechanisms of synovial Wnt/β-catenin signalling, given its emerging importance in arthritis.

Methods We subjected mice to non-invasive anterior cruciate ligament rupture as a model of human joint injury. We performed single-cell RNA-sequencing to assess synovial cell populations, subjected Wnt-GFP reporter mice to joint injury to study Wnt-active cells, and performed intra-articular injections of the Wnt agonist R-spondin 2 (Rspo2) to assess whether gain of function induced pathologies characteristic of PTOA. Lastly, we used cultured fibroblasts, macrophages and chondrocytes to study how Rspo2 orchestrates crosstalk between joint cell types.

Results We uncovered seven distinct functional subsets of synovial fibroblasts in healthy and injured synovium, and defined their temporal dynamics in early and established PTOA. Wnt/β-catenin signalling was overactive in PTOA synovium, and Rspo2 was strongly induced after injury and secreted exclusively by Prg4hi lining fibroblasts. Trajectory analyses predicted that Prg4hi lining fibroblasts arise from a pool of Dpp4+ mesenchymal progenitors in synovium, with SOX5 identified as a potential regulator of this emergence. We also showed that Rspo2 orchestrated pathological crosstalk between synovial fibroblasts, macrophages and chondrocytes.

Conclusions Synovial fibroblasts assume distinct functional identities during PTOA in mice, and Prg4hi lining fibroblasts secrete Rspo2 that may drive pathological joint crosstalk after injury.

  • fibroblasts
  • osteoarthritis
  • synovitis

Data availability statement

Data are available in a public, open access repository. Data are available on reasonable request. All sequencing scRNA-seq data are publicly available via the NCBI Gene Expression Omnibus (GEO) using the accession number GSE211584. Other data are available on reasonable request.

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Data availability statement

Data are available in a public, open access repository. Data are available on reasonable request. All sequencing scRNA-seq data are publicly available via the NCBI Gene Expression Omnibus (GEO) using the accession number GSE211584. Other data are available on reasonable request.

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Footnotes

  • Handling editor Josef S Smolen

  • Twitter @ajknights7, @MaerzandBlue

  • Contributors Conceptualisation: AJK, KDH and TM; Study design: AJK, KDH and TM; Data acquisition: AJK, ECF, LL, LMJ, PMR, RFB, RP, MC, EK, AAS; Data analysis: AJK, ECF, OME, LL, PMR, RFB, DF, C-LW and TM; Interpretation of results: AJK, ECF, OME, C-LW, KDH and TM; Writing of the manuscript: AJK, ECF, KDH and TM. TM is the author responsible for the overall content as guarantor; TM accepts full responsibility for the finished work and/or the conduct of the study, had access to the data and controlled the decision to publish.

  • Funding This work was supported by funding from the National Institutes of Health (R21AR076487 to TM, R01DE030716 and R01AR066028 to KDH, R00AR075899 to CLW), a Catalyst Award from the Dr. Ralph and Marian Falk Medical Research Trust to TM, and from the Orthopaedic Research and Education Foundation (OREF) to CLW. Histology, imaging work, and pilot funding to TM was supported by the Michigan Integrative Musculoskeletal Health Core Centre (P30AR069620, National Institutes of Health). Single-cell experiments utilised resources that are funded by the University of Michigan Comprehensive Cancer Centre (P30CA046592, National Institutes of Health). AJK was supported by a Michigan Pioneer Postdoctoral Fellowship from the University of Michigan. PMR was supported by a T32 training grant (T32AR007080-40, National Institutes of Health). LL was supported by a Graduate Research Fellowship Programme award from the National Science Foundation. RFB was supported by the J. Griswold and Margery H. Ruth Alpha Omega Alpha Research Fellowship and a research fellowship by the Office of Health Equity & Inclusion. DF was supported by the Community College Summer Fellowship Programme from the University of Michigan

  • 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.

  • Provenance and peer review Not commissioned; externally peer reviewed.

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