Type IIA procollagen: Expression in developing chicken limb cartilage and human osteoarthritic articular cartilage

Type IIA procollagen is an alternatively spliced product of the type II collagen gene and uniquely contains the cysteine (cys)-rich globular domain in its amino (N)-propeptide. To understand the function of type IIA procollagen in cartilage development under normal and pathologic conditions, the detailed expression pattern of type IIA procollagen was determined in progressive stages of development in embryonic chicken limb cartilages (days 5-19) and in human adult articular cartilage. Utilizing the antibodies specific for the cys-rich domain of the type IIA procollagen N-propeptide, we localized type IIA procollagen in the pericellular and interterritorial matrix of condensing prechondrogenic mesenchyme (day 5) and early cartilage (days 7-9). The intensity of immunostaining was gradually lost with cartilage development, and staining became restricted to the inner layer of perichondrium and the articular cap (day 12). Later in development, type IIA procollagen was re-expressed at the onset of cartilage hypertrophy (day 19). Different from type X collagen, which is expressed throughout hypertrophic cartilage, type IIA procollagen expression was transient and restricted to the zone of early hypertrophy. Immunoelectron microscopic and immunoblot analyses showed that a significant amount of the type IIA procollagen N-propeptide, but not the carboxyl (C)-propeptide, was retained in matrix collagen fibrils of embryonic limb cartilage. This suggests that the type IIA procollagen N-propeptide plays previously unrecognized roles in fibrillogenesis and chondrogenesis. We did not detect type IIA procollagen in healthy human adult articular cartilage. Expression of type IIA procollagen, together with that of type X collagen, was activated by articular chondrocytes in the upper zone of moderately and severely affected human osteoarthritic cartilage, suggesting that articular chondrocytes, which normally maintain a stable phenotype, undergo hypertrophic changes in osteoarthritic cartilage. Based on our data, we propose that type IIA procollagen plays a significant role in chondrocyte differentiation and hypertrophy during normal cartilage development as well as in the pathogenesis of osteoarthritis.