Early changes in cartilage pericellular matrix micromechanobiology portend the onset of post-traumatic osteoarthritis

The pericellular matrix (PCM) of cartilage is a structurally distinctive microdomain surrounding each chondrocyte, and is pivotal to cell homeostasis and cell-matrix interactions in healthy tissue. This study queried if the PCM is the initiation point for disease or a casualty of more widespread matrix degen- eration. To address this question, we queried the mechanical properties of the PCM and chondrocyte mechanoresponsivity with the development of post-traumatic osteoarthritis (PTOA). To do so, we inte- grated Kawamoto’s film-assisted cryo-sectioning with immunofluorescence-guided AFM nanomechanical mapping, and quantified the microscale modulus of murine cartilage PCM and further-removed extracel- lular matrix. Using the destabilization of the medial meniscus (DMM) murine model of PTOA, we show that decreases in PCM micromechanics are apparent as early as 3 days after injury, and that this pre- cedes changes in the bulk ECM properties and overt indications of cartilage damage. We also show that, as a consequence of altered PCM properties, calcium mobilization by chondrocytes in response to me- chanical challenge (hypo-osmotic stress) is significantly disrupted. These aberrant changes in chondro- cyte micromechanobiology as a consequence of DMM could be partially blocked by early inhibition of PCM remodeling. Collectively, these results suggest that changes in PCM micromechanobiology are lead- ing indicators of the initiation of PTOA, and that disease originates in the cartilage PCM. This insight will direct the development of early detection methods, as well as small molecule-based therapies that can stop early aberrant remodeling in this critical cartilage microdomain to slow or reverse disease progression.