Characteristics of the synovial microenvironment and synovial mesenchymal stem cells with hip osteoarthritis of different bone morphologies

Abstract

Background

Variations in bone morphology in patients with hip osteoarthritis (HOA) can be broadly categorized into three types: atrophic, normotrophic, and hypertrophic. Despite the investigations examining clinical elements, such as bone morphology, pain, and range of motion, our understanding of the pathogenesis of HOA remains limited. Previous studies have suggested that osteophytes typically originate at the interface of the joint cartilage, periosteum, and synovium, potentially implicating synovial mesenchymal stem cells (SMSCs) in the process. This study aimed to investigate the potential factors that drive the development of bone morphological features in HOA by investigating the characteristics of the synovium, differentiation potential of SMSCs, and composition of synovial fluid in different types of HOA.

Methods

Synovial tissue and fluid were collected from 30 patients who underwent total hip arthroplasty (THA) with the variable bone morphology of HOA patients. RNA sequencing analysis and quantitative reverse transcription-polymerase chain reaction (RT-qPCR) were performed to analyse the genes in the normotrophic and hypertrophic synovial tissue. SMSCs were isolated and cultured from the normotrophic and hypertrophic synovial tissues of each hip joint in accordance with the variable bone morphology of HOA patients. Cell differentiation potential was compared using differentiation and colony-forming unit assays. Cytokine array was performed to analyse the protein expression in the synovial fluid.

Results

In the RNA sequencing analysis, 103 differentially expressed genes (DEGs) were identified, predominantly related to the interleukin 17 (IL-17) signalling pathway. Using a protein–protein interaction (PPI) network, 20 hub genes were identified, including MYC, CXCL8, ATF3, NR4A1, ZC3H12A, NR4A2, FOSB, and FOSL1. Among these hub genes, four belonged to the AP-1 family. There were no significant differences in the tri-lineage differentiation potential and colony-forming capacity of SMSCs. However, RT-qPCR revealed elevated SOX9 expression levels in synovial tissues from the hypertrophic group. The cytokine array demonstrated significantly higher levels of CXCL8, MMP9, and VEGF in the synovial fluid of the hypertrophic group than in the normotrophic group, with CXCL8 and MMP9 being significantly expressed in the hypertrophic synovium.

Conclusion

Upregulation of AP-1 family genes in the synovium and increased concentrations of CXCL8, MMP9, and VEGF were detected in the synovial fluid of the hypertrophic group of HOA patients, potentially stimulating the differentiation of SMSCs towards the cartilage and thereby contributing to severe osteophyte formation.