Loss of mutually protective effects between osteoclasts and chondrocytes in damaged joints drives osteoclast-mediated cartilage degradation via matrix metalloproteinases

Osteoclasts are large multinucleated cells that resorb bone to regulate bone remodelling during skeletal maintenance and development. It is overlooked that osteoclasts also digest cartilage during this process, as well as in degradative conditions including osteoarthritis, rheumatoid arthritis and primary bone sarcomas such as giant cell tumour of bone. This study explores the poorly understood mechanisms behind the interaction between osteoclasts and cartilage. Morphologically, osteoclasts differentiated on acellular human cartilage formed multinucleated cells expressing characteristic osteoclast marker genes (e.g. CTSK, MMP9) and proteins (TRAP, VNR) that visibly damaged the cartilage surface by SEM, but without the formation of resorption pits. Osteoclasts caused increased glycosaminoglycan (GAG) release from acellular and cellular human cartilage that was dependent on direct contact with the substrate. Direct co-culture with chondrocytes during osteoclast differentiation increased the number of large osteoclasts formed. When osteoclasts were cultured on dentine, direct co-culture with chondrocytes inhibited osteoclast formation and reduced basal degradation of cartilage. This suggests a mutually protective effect on their native tissue between bone-resident osteoclasts and chondrocytes, that is reversed when the joint structure breaks down and osteoclasts are in contact with non-native substrates. Mechanistically, osteoclast-mediated cartilage degradation was inhibited by the pan-MMP inhibitor GM6001 and by TIMP1, indicative of a role for soluble MMPs. RNA sequencing and RT-qPCR analysis identified MMP8 as overexpressed in osteoclasts differentiated on cartilage versus dentine, while MMP9 was the most highly expressed MMP on both substrates. Inhibition of either MMP8 or MMP9 by siRNA in mature osteoclasts reduced GAG release, confirming their involvement in cartilage degradation. Immunohistochemical expression of MMP8 and MMP9 was evident in osteoclasts in osteosarcoma tissue sections. Understanding and controlling the activity of osteoclasts might represent a new therapeutic approach for pathologies characterized by cartilage degeneration and presents an attractive target for further research.