Background/Purpose: Hand osteoarthritis (OA) presents the highest prevalence among rheumatic diseases. Synovitis is a defining feature in hand OA that has been associated with radiographic progression and bone erosion. Synovitis is mediated by the activation of several signalling pathways, such as the NF-κB pathway, which remain active within the synovial joint. Activation of NF-κB takes place after phosphorylation of the kinase IKK2, an event that converges in overexpression of inflammatory molecules that play a critical role in hand OA. These mediators include cytokines, chemokines, and degradative enzymes released by fibroblast-like synoviocytes (FLS), contributing to cartilage degeneration and bone erosion. Pathogenic mechanisms driving hand OA remain unclear, and no animal models exist; however, activating specific cell types within the joint could represent a better approach to elucidate disease mechanisms. In order to characterize a hand OA model, our lab induced a constitutive IKK2 activation in Prg4-expressing cells, such as FLS and superficial chondrocytes, as a strategy to study in situ inflammation within the synovial joint of mice paws.

Methods: The R26Stop^FLikk2ca-Prg4^CreERT2 mouse model that expresses a constitutively active form of IKK2 was used in this study. IKK2 activation was induced in the homozygous mouse model in a tamoxifen-inducible Prg4 Cre-ER-dependent way by intraperitoneal injections to 2-month old mice. Isolation of FLS was performed after four weeks of tamoxifen injections. Bulk RNA-seq was performed from total RNA extraction obtained from FLS at passages 2 to 4. Additionally, single-cell RNA-seq was accomplished in live-sorted cells from the paws joint to identify cell clusters in our model. Public available data was used for bioinformatics analysis to compare DE genes between paws from IKK2ca mice and hand OA [European Nucleotide Archive (ENA) codes PRJEB14422 and PRJEB14595].

Results: Bulk RNA-seq data analysis showed high expression of proinflammatory mediators in FLS obtained from IKK2ca mice compared with WT. Besides, scRNA-seq allowed us to identify fifteen cell clusters in our mice. We observed an increased number of cells in both the osteoblast and macrophage clusters when we compare IKK2ca vs WT. Differential expression (DE) gene analysis between IKK2ca and WT mice showed upregulation of MMPs and NF-κB signalling pathway activation molecules. Interestingly, ingenuity pathway analysis (IPA) from those DE genes revealed the osteoarthritis pathway as one of the top five canonical pathways in the IKK2ca model. Additionally, bioinformatics analysis revealed an overlap of key up and down-regulated genes involved in inflammation and cartilage degeneration in IKK2ca compared with hand OA.

Conclusion: Overactivation of IKK2 induced a hand OA-like phenotype characterized by overexpression of MMP’s, cytokines, and NF-κB signalling molecules that contributed to cartilage degeneration.

« Back to ACR Convergence 2021

ACR Meeting Abstracts - https://acrabstracts.org/abstract/overactivation-of-the-kinase-ikk2-causes-a-hand-osteoarthritis-like-phenotype-in-mice/