Human growth plate contains aggrecan fragments that can be generated by m-calpain

Growth plate is a cartilaginous structure responsible for longitudinal growth and calcification of long bones. Aggrecan is initially expressed within the growth plate but is lost during the course of calcification. Calcium-activated proteinases are believed to play a primary role in aggrecanolysis. In this study, m-calpain was shown to be expressed in a limited area of the growth cartilage at the lower hypertrophic zone. Confocal immunostaining demonstrated colocalization of m-calpain and the aggrecan product within the lower hypertrophic chondrocytes and in limited region of the pericellular matrix. Immunoblotting analysis identified anti-VPGVA-positive aggrecan product within the dissociative fractions of A1D1-A1D6 (densities 1.65, 1.56, 1.52, 1.47, 1.41, and 1.37 g/cm(3), respectively). These findings indicated that limited expression of m-calpain is colocalized with the appearance of calpain-related aggrecan products at the sites of aggrecanolysis and calcification, and suggested an important role of m-calpain in regulation of the growth plate process. The G1-G2-containing fragment of aggrecan remaining within the extracellular matrix (ECM) of the growth plate may contribute to the mechanical properties of the growth plate between the chondrocyte cell layers until bony replacement takes place.