Spatial Proteomics Identifies Distinct Synovial Microenvironments in Hip and Knee Osteoarthritis

Osteoarthritis (OA) is a heterogeneous joint disease, yet mechanistic insights derive predominantly from knee OA and are often extrapolated to other joints. We investigated joint-associated differences in the spatial molecular organization of hip and knee OA synovium.

Synovial tissues from patients undergoing arthroplasty for primary hip or knee OA were analyzed by histology (hip, n=10; knee, n=10), mass spectrometry-based bulk proteomics (hip, n=10; knee, n=8), and NanoString GeoMx Digital Spatial Profiling (hip, n=5; knee, n=5). Spatial proteomic profiling was performed across intimal and subintimal regions segmented into CD45⁺ immune and CD45^- non-immune compartments.

Hip OA synovium exhibited greater vessel density than knee OA synovium (P < 0.001), whereas histological synovitis severity did not differ significantly. Bulk proteomics showed enrichment of angiogenesis in hip OA (FDR q=0.001) and fatty acid metabolism in knee OA (FDR q<0.001). Spatial proteomic profiling revealed a vascular-stromal remodeling signature in the hip OA subintimal CD45^- compartment, with higher abundance of VEGFR2, vWF, α-SMA, PDGFB, and YAP1. Hip OA also showed higher CD3E/CD3G abundance in the subintimal CD45⁺ compartment and higher calreticulin abundance across multiple compartments. In contrast, knee OA exhibited a GLP-1/GLP-1R-associated metabolic signature in the intimal CD45⁺ compartment. The knee OA subintimal CD45^- compartment also showed higher HK2 abundance and enrichment of lipid-associated LXR/RXR signaling.

Hip and knee OA synovium exhibit distinct spatially organized molecular phenotypes, highlighting the importance of joint-specific context in OA pathobiology.