Matrix Metalloproteinases Are Not Essential for Aggrecan Turnover during Normal Skeletal Growth and Development

Abstract

The growth plate is a transitional region of cartilage and highly diversified chondrocytes that controls long bone formation. The composition of growth plate cartilage changes markedly from the epiphysis to the metaphysis, notably with the loss of type II collagen, concomitant with an increase in MMP-13; type X collagen; and the C-propeptide of type II collagen. In contrast, the fate of aggrecan in the growth plate is not clear: there is biosynthesis and loss of aggrecan from hypertrophic cartilage, but the mechanism of loss is unknown. All matrix metalloproteinases (MMPs) cleave aggrecan between amino acids N 341 and F 342 in the proteinase-sensitive interglobular domain (IGD), and MMPs in the growth plate are thought to have a role in aggrecanolysis. We have generated mice with aggrecan resistant to proteolysis by MMPs in the IGD and found that the mice develop normally with no skeletal deformities. The mutant mice do not accumulate aggrecan, and there is no significant compensatory proteolysis occurring at alternate sites in the IGD. Our studies reveal that MMP cleavage in this key region is not a predominant mechanism for removing aggrecan from growth plate cartilage.Endochondral ossification is the process by which bone is formed upon a cartilage template. It occurs in skeletal development, in fracture healing, and during bone growth at the growth plates and involves the transition from avascular cartilage, rich in type II collagen and aggrecan, to highly vascular bone, rich in type I collagen and mineral. During endochondral bone formation, prechondrogenic mesenchymal cells differentiate into chondrocytes, which proliferate and then mature into hypertrophic chondrocytes. The hypertrophic cartilage begins to calcify, the cartilage matrix is degraded and is invaded by blood vessels, the terminal hypertrophic chondrocytes apoptose, and bone matrix is deposited on the calcified cartilage trabeculae. This is a busy schedule of events in the growth plate, and their precise timing in relation to each other has not been clearly defined.The growth plate is a striking remodeling unit both anatomically and biochemically. Anatomically, it is bordered by the secondary center of ossification and epiphysis at one end and the bony metaphysis at the other. It comprises reserve, proliferative, prehypertrophic, and hypertrophic zones. Chondrocytes in the reserve zone are rounded and sparsely distributed, whereas chondrocytes in the proliferative zone are flattened and occupy approximately one-quarter of the tissue volume. Cells of the proliferative zone divide and arrange themselves into ordered columns separated by longitudinal septa. At the same time, they increase their secretion of matrix molecules so that around 75% of the tissue volume is cartilage matrix. In the hypertrophic zones, the cells retain their columnar organization and increase their metabolic activity. They enlarge by as much as 10-fold (28) so that matrix volume is reduced to approximately 40%, with cells occupying 60% of the tissue volume (5). Calcificat...

Matrix metalloproteinases and the regulation of tissue remodelling

“…The cleavage product of aggrecan is absent in the Mmp9 Mmp13 double mutant 7 . It is worth noting that there are no phenotypic consequences of replacing endogenous aggrecan with aggrecan that is resistant to MMP cleavage, indicating that other mechanisms exist for the removal of aggrecan at the cartilage-bone transition 41 .…”

N-Linked Keratan Sulfate in the Aggrecan Interglobular Domain Potentiates Aggrecanase Activity

“…However, matrix metalloproteinase cleavage at N 341 2 342 F correlates with late-stage cartilage damage in mouse models of arthritis (10 -12), and it may also be involved in the baseline turnover of aggrecan in vitro (13) and in vivo (14). The products of in vivo proteolysis at both the matrix metalloproteinase and the aggrecanase sites have been found in humans (4,15,17,18) and in mice with experimental arthritis (11, 19 -21).…”

Matrix metalloproteinases in bone development and pathology: current knowledge and potential clinical utility

“…33 This is consistent with a subsequent study on an aggrecan knock-in mouse strain that is resistant to cleavage by all MMPs in the aggrecan proteinase-sensitive interglobular domain. 57 In humans, missense mutations in MMP-13 cause a genetic bone disorder called spondyloepimetaphyseal dysplasia characterized by defective growth and abnormal modeling of the spine and long bones in childhood, which spontaneously resolves by adolescence. These transient disease phenotypes appear to be caused by the late exit of chondrocytes from the growth plate, despite normal differentiation, consistent with the findings from knockout mice models.…”