Knee osteoarthritis (OA) is the most prevalent degenerative joint disease. When morphological changes become apparent on radiographs, no approved treatment can reverse the disease process. Early diagnosis is an unmet need demanding new molecular and imaging biomarkers to define OA from the earliest stages. In this context, we focus on collagen, the most basic building block of all joint tissues, and interrogate how OA development affects collagen's molecular folding, a previously underexplored area. Here, through whole-joint mapping with a peptide that recognizes unfolded collagen molecules, we report the discovery of collagen denaturation in cartilage before proteolysis and major histopathological degeneration in animal models and patients. Mechanistically, we reveal that such molecular collagen defects can be driven by mechanical overloading without collagenase degradation and are intimately associated with glycosaminoglycan loss. We showcase the advantages of using collagen denaturation as an early-stage OA hallmark for in vivo therapeutic evaluation and molecular magnetic resonance imaging (MRI) of subtle joint defects that are challenging to detect with conventional morphology-based MRI. These results highlight biomolecular integrity as a crucial dimension for characterizing joint degeneration and a molecular foundation for diagnosing early-stage OA and beyond.

The authors have declared no competing interest.

This work was supported by the National Natural Science Foundation of China (92059104, 82071977, 82325035, 82172481, 32271409), the 2018 High-level Health Team of Zhuhai, the Six Talent Peaks Project of Jiangsu Province (WSW-079), the Innovation Project of National Orthopedics and Sports Medicine Rehabilitation Clinical Medical Research Center (2021-NCRC-CXJJ-ZH-16), and the Guangdong-Hong Kong-Macao University Joint Laboratory of Interventional Medicine Foundation of Guangdong Province (2023LSYS001).

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The Ethics Committee of Nanjing Drum Tower Hospital of Nanjing University approved these studies (approval no. K228-1).

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