MMP14 and regulation of hypertrophic chondrocyte to osteoblast lineage

Abstract

During endochondral ossification, the cartilage template is degraded by Matrix Metalloproteinases (MMPs) and replaced by bone. Hypertrophic Chondrocytes (HCs) undergo lineage extension and contribute to osteoblasts in trabeculae. However, the role of MMPs in regulating the lineage transition of HCs to osteoblast is unknown. MMP14(MT1-MMP), encoded by Mmp14, is a membrane-tethered matrix metalloproteinase which is highly expressed by cells at the chondro-osseous junction where lineage extension of HCs occurs. Mice deficient of MMP14 display loss of trabecular bone and impaired angiogenesis during endochondral ossification. Here we hypothesize MMP14 regulates the lineage progression of HCs to osteoblasts. By following the fate of HCs using Col10a1-Cre and Rosa26 reporters, complete inactivation of MMP14 results in loss of trabecular bone and marked accumulation of HC-derived cells at the chondro-osseous junction. More HC descendants are found in MMP14 null mutants. Expression of osteogenic markers such as Collagen type I and Mmp13 are decreased in MMP14 mutants in late postnatal stages. The defect in progression of HC descendants into primary spongiosa was not caused by HC-specific loss of MMP14. Simultaneous inactivation of MMP14 and tracing of HC descendants within a defined time frame does not disrupt their localization in trabecular bone compared to control. Instead, the accumulation of HC descendants at chondro-osseous junction in MMP14 null mutants is associated with reduced vascularization in trabecular bone. Depletion of MMP14 was known to activate osteoclast activity and compromise trabecular bone formation. However, conditional ablation of MMP14 in HC descendants with Col10a1-Cre; Mmp14F/- causes increased trabecular bone and number of HC descendants. The phenotype could be attributed to decreased apoptosis of HCs at the chondro-osseous junction. These data suggests HC-derived MMP14 negatively regulate lineage progression of HCs. Interestingly, many phenotypes observed in Col10a1-Cre; Mmp14F/- mice is consistent with activation of parathyroid hormone signalling from literature. Mechanistically, we found that MMP14 interacts and cleaves parathyroid hormone 1 receptor (PTH1R) which is expressed in both osteoblasts and chondrocytes. Haploid-insufficiency of pth1r partially rescues postnatal trabecular bone formation in Mmp14-/- mice. Our work unravels multiple roles of MMP14 in lineage progression of Hypertrophic Chondrocytes.