Spatial proteomic mapping of human knee joints has revealed bone-derived molecular changes that extend beyond regions of visible cartilage damage, pointing to earlier indicators of osteoarthritis progression.
Using spatial matrix-assisted laser desorption/ionization mass spectrometry imaging (MALDI-MSI), the team examined knee tissue from patients with end-stage OA and non-arthritic controls, allowing hundreds of peptides to be localized directly within cartilage and subchondral bone rather than averaged across bulk samples.
To enable protein-level imaging in mineralized tissue, the researchers applied targeted enzymatic digestion directly on tissue sections prior to MALDI-MSI analysis. This enabled extracellular matrix proteins and collagen fragments to be spatially resolved in subchondral bone, a region challenging to analyze traditionally using mass spectrometry imaging.
“Our goal was to move beyond what we can see on X-ray or MRI scans and ask what the tissue is telling us at the molecular level,” said Birgit Schilling, senior author of the study, in a press release.
The molecular maps revealed strong upregulation of collagen-derived peptides and post-translational modifications associated with matrix stiffening and remodeling in bone beneath damaged cartilage. Unexpectedly, similar bone signatures were also detected beneath cartilage that appeared structurally intact, indicating that bone remodeling may precede overt cartilage degeneration.
“What stood out was that the bone carried a very clear disease signal, even in regions where cartilage loss was not yet obvious,” Schilling said. “This suggests that subchondral bone could serve as an early indicator of osteoarthritis progression.”
Several bone-associated protein fragments identified by spatial imaging were also detected in synovial fluid from OA patients using complementary proteomic analysis. In contrast, many cartilage-associated markers were reduced, reinforcing subchondral bone as a promising source of early, fluid-accessible OA biomarkers.
The authors suggest that linking spatial proteomic signatures in tissue with measurable signals in synovial fluid could support earlier assessment of joint remodeling. “These results open the door to developing fluid-based tests that reflect what is happening deep within the joint,” said Charles Schurman, a co-author on the study.
“If we can track bone-specific molecular changes over time, it may become possible to identify patients at risk earlier and monitor how they respond to therapy.”
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