Disturbances in proteoglycan metabolism of hyaline cartilage play an essential role in the pathology of degenerative joint disease. We investigated the relation between transcript expression, protein synthesis and the ultrastructural localization of the matrix-organizing proteoglycans decorin and biglycan within intra- and extracellular compartments of late-stage osteoarthritic human articular cartilage.
Human cartilage samples of a macroscopically intact area, the adjoining area and an area of the main defect from knee joints of 10 patients with late stage osteoarthritis were investigated. In situ hybridization and immunogold histochemistry were carried out separately and in combination at the light and electron microscopic level.
Ultrastructurally, three main chondrocyte types were identified. The highest levels of mRNA of decorin and biglycan were produced by elongated secretory type 2 cells, already known to synthesize type I collagen. Cells with high levels of mRNA also translated the corresponding proteins to be found in the extracellular compartment. The highest production rate of decorin and biglycan was seen in the tissue area adjoining the main defect.
The results indicate that at late stages of osteoarthritis the levels of transcription and translation for decorin and biglycan are up-regulated, probably in an effort to compensate for the general proteoglycan loss, characteristic of this disease stage.