Nidogen-1 and nidogen-2 in healthy human cartilage and in late-stage osteoarthritis cartilage

Objective. To investigate the presence and function of nidogen-1 and nidogen-2 in healthy human cartilage and in late-stage osteoarthritis (OA) cartilage. Methods. The location and quantity of nidogen-1 and nidogen-2 protein and messenger RNA were determined in cartilage tissue obtained from healthy donors and from patients with late-stage knee OA. Samples were analyzed by immunohistochemistry, in situ hybridization, and real-time reverse transcription-polymerase chain reaction. Adhesion and inhibition assays, a preembedding method, fluorescence-activated cell sorting, and ultrastructural investigations with integrins were also carried out. Results. Developing tissue from human embryos showed strong staining for both nidogens in condensed mesenchyme and in rib anlagen. Homogeneous staining for nidogen-1 was observed in the extracellular matrix of healthy articular cartilage, whereas nidogen-2 was localized pericellularly. In late-stage OA cartilage, expression of nidogen-1 was decreased pericellularly around diseased chondrocytes, whereas nidogen-2 was increased. However, both nidogens had strongly increased levels around elongated chondrocytes, especially in areas of deep surface fissures. In vitro, both nidogens functioned as adhesion proteins for cells from the OA defect. In vivo, colocalizations with integrins alpha v and beta 1 as well as internalization of nidogens by chondrocytes in vitro were observed. Conclusion. Nidogens are involved in human limb development. They occur in healthy articular cartilage and show increased expression, primarily around elongated chondrocytes, in OA cartilage. Therefore, the activities of nidogens might be a sign of cartilage regeneration in late-stage OA. Furthermore, the adhesive character of nidogens, specifically as adhesion proteins for chondrocytes from late-stage OA, as well as the enhanced chondrocyte-nidogen interaction in OA indicate that both proteins play a key role in the pathogenesis of OA and either could be applied as a diagnostic marker.