Faculty Bibliography

Type-VI collagen is an integral part of the extracellular cartilage matrix. However, the exact amounts of type-VI collagen in normal and osteoarthritic human cartilage still are not known. In this study, we describe an inhibition enzyme-linked immunosorbent assay that was developed to quantitate type-VI collagen epitopes found in guanidinium chloride extracts from normal and osteoarthritic human cartilage. In 31 cartilage samples from various localizations of healthy adult human knees, type-VI collagen epitopes accounted for approximately 0.40% of the total collagen content. Interestingly, type-VI collagen epitopes increased about 4-fold in osteoarthritic cartilage. A statistically significant increase of type-VI collagen epitopes was found during early stages of the disease, with only a superficial roughening of the cartilage surface and a loss of proteoglycans. Thus, these findings indicate that type-VI collagen is a minor component of normal human articular cartilage and that the amount of type-VI collagen epitopes increases significantly during early stages of osteoarthritis

Matrix vesicles have a critical role in the initiation of mineral deposition in skeletal tissues, but the ways in which they exert this key function remain poorly understood. This issue is made even more intriguing by the fact that matrix vesicles are also present in nonmineralizing tissues. Thus, we tested the novel hypothesis that matrix vesicles produced and released by mineralizing cells are structurally and functionally different from those released by nonmineralizing cells. To test this hypothesis, we made use of cultures of chick embryonic hypertrophic chondrocytes in which mineralization was triggered by treatment with vitamin C and phosphate. Ultrastructural analysis revealed that both control nonmineralizing and vitamin C/phosphatetreated mineralizing chondrocytes produced and released matrix vesicles that exhibited similar round shape, smooth contour, and average size. However, unlike control vesicles, those produced by mineralizing chondrocytes had very strong alkaline phosphatase activity and contained annexin V, a membrane-associated protein known to mediate Ca2+ influx into matrix vesicles. Strikingly, these vesicles also formed numerous apatite-like crystals upon incubation with synthetic cartilage lymph, while control vesicles failed to do so. Northern blot and immunohistochemical analyses showed that the production and release of annexin V-rich matrix vesicles by mineralizing chondrocytes were accompanied by a marked increase in annexin V expression and, interestingly, were followed by increased expression of type I collagen. Studies on embryonic cartilages demonstrated a similar sequence of phenotypic changes during the mineralization process in vivo. Thus, chondrocytes located in the hypertrophic zone of chick embryo tibial growth plate were characterized by strong annexin V expression, and those located at the chondro-osseous mineralizing border exhibited expression of both annexin V and type I collagen. These findings reveal that hypertrophic chondrocytes can qualitatively modulate their production of matrix vesicles and only when induced to initiate mineralization, will release mineralization-competent matrix vesicles rich in annexin V and alkaline phosphatase. The occurrence of type I collagen in concert with cartilage matrix calcification suggests that the protein may facilitate crystal growth after rupture of the matrix vesicle membrane; it may also offer a smooth transition from mineralized type II/type X collagen-rich cartilage matrix to type I collagen-rich bone matrix