Selective assembly and remodelling of collagens II and IX associated with expression of the chondrocyte hypertrophic phenotype

The assembly and resorption of the extracellular matrix in the physis of th e growth plate are poorly understood. Ey examining isolated fetal growth pl ate chondrocytes in culture and using immunochemical methods we show that t ype II collagen, proteoglycan aggrecan, and type IX collagen are assembled into a matrix that is initially enriched in type II collagen over proteogly can and type IX collagen. When compared to the content of the COL2 domain i n the a,(IX) chain it is evident that the majority (90%) of type IX molecul es lack the NC4 domain unlike in articular cartilage. During matrix assembl y the molar ratio of type II/COL2 of alpha(1)(IX) varied from 25:1 to 2.5:1 . Following expression of the hypertrophic phenotype (initiation of type X collagen synthesis) there are parallel changes in both collagen and proteog lycan contents (inversely related to collagenase cleavage of type II: colla gen). The NC4 domain is then selectively, rapidly and irreversibly removed as mineralization is initiated, leaving the alpha(1)(IX) chain COL2 domain. Subsequently as mineralization progresses type LI and type IX collagen (CO L2 domain), but not the proteoglycan aggrecan, are resorbed coincident with a markedly increased cleavage of type II collagen by collagenase as minera l is deposited in the matrix. This study, therefore reveals a carefully orc hestrated series of events in matrix assembly and resorption that prepares the extracellular matrix for mineralization, (C) 2000 Wiley-Liss, Inc.