Role of Matrix Metalloproteinases in Cartilage and Bone during Skeletal Remodeling

Matrix metalloproteinases (MMPs) are expressed in cartilage during embryonic development and later during remodeling. Mmp9, although not a collagenase is expressed early in embryogenesis during endochondral ossification and Mmp9 deficiency in mice results in abnormalities that affects growth plates. Mmp13 (collagenase-3), a highly expressed collagenolytic MMP in cartilage and in developing and remodeling bone, also has a role in the joint tissue destruction that is a major feature of various forms of human arthritis. We have targeted a null mutation in mouse Mmp13 that resulted in a profound embryonic and adult skeletal phenotype characterized by abnormal growth plates and delayed ossification. During embryonic development at the earliest stage examined, Mmp13 -/mice had growth plates in long bones almost double in length, accounted for by increases in the zone of hypertrophy. Mmp13, produced by chondrocytes but not by osteoclasts/chondroclasts, is particularly effective in proteolysis of type II compared to type I collagen. Using antibodies that detect epitopes in the specific proteolytic fragments, we obtained evidence for Mmp13 cleavage of type II collagen in vivo in wt mice, but not in Mmp13-/-mice. It is thus unlikely that other Mmps compensate for the loss of Mmp13 function in cartilage. The delay in ossification, so prominent in 15.5 dpc Mmp13 -/embryos, is largely transient and older Mmp13 -/mice have increased bone deposition. Mmp8 is also expressed in newborn Mmp13 -/and wt skeletons but Mmp '-/mice have normal skeletons. Type X collagen is also a substrate for MMP1 and Mmp13. The area of type X collagen deposition was significantly increased in growth plates from Mmp13-/-mice, consistent with decreased proteolysis. Increased synthesis of type X collagen could also contribute. Based on our results, Mmp13 has a critical role in regulating events in the growth plate beginning in embryonic development. Deficiency of Mmpl 3 with failure to normally resorb collagens in the cartilage ECM profoundly affects cellular activities that underlie differentiation of hypertrophic chondrocytes that persist in newborn and adult mice. The phenotype of / adult Mmp13-mice with increased length of growth plates, increased numbers of chondrocytes and distortion of alignment of the rows of chondrocytes thus has features of a chondrodysplasia. Articular cartilage is also affected. It is pertinent that a form of human chondrodysplasia, the Missouri variant of spondyloepimetaphyseal dysplasia is caused by a mutation in MMP13 that results in misfolding, intracellular degradation and MMP13 deficiency. Human mutations in MMP2 also result in a skeletal phenotype (nodulosis, osteolysis and arthropathy) distinct from that in MMP13 mutations. Bone is abnormal in Mmp2-null mice but the mice do not show other features of the human mutation. 11