The expression of osteoprotegerin is required for maintaining the intervertebral disc endplate of aged mice

ObjectiveHuman chondrocytes and annulus fibrosus cells of intervertebral disc (IVD) express osteoprotegerin (OPG), but the effect of OPG on the pathogenesis of IVD degeneration remains unknown. Here we assessed the phenotype change of IVD in OPG−/− mice.MethodsThe IVDs from 12-, 20-, and 28-week-old OPG−/− mice and WT controls were subjected to histologic analyses including TRAP staining for osteoclasts, immunostaining for OPG and type I collagen protein expression, and TUNEL staining for apoptosis. The IVD tissues were also subjected to real time RT-PCR for mRNA expression of genes for osteoblast-osterix, ALP, and osteocalcin; for osteoclasts-trap, rank, mmp9 and cathepsin K, and for chondrocytes-aggrecan, mmp13 and Col10.ResultsOPG protein expresses at the cells of endplate cartilage and annulus fibrosis in IVDs of WT mice. Compared to WT mice, OPG−/− mice developed aging related cartilage loss and bony tissue appearance at the endplate. Stating from 20 weeks of age, IVDs from OPG−/− mice expressed significantly increased mmp13 and Col10 levels, which is associated with increased osteoblast number and elevated expression of osteoblast marker genes. Furthermore, TRAP+ osteoclasts were presented in the endplate cartilage of OPG−/− mice. These osteoclasts localized adjacently to and erosion into the cartilage. Increased expression of RANK, mmp9 and cathepsin k was detected in OPG−/− IVDs.ConclusionsOPG at IVD plays an important role for maintaining the integrity of endplate cartilage during aging by preventing endplate cartilage from osteoclast-mediated resorption.

Research highlights
► We examined cervical intervertebral discs of OPG−/− and WT mice.
► OPG−/− mice developed cartilage loss and bony tissue appearance at the endplate.
► Abundant osteoblast and TRAP + osteoclasts were presented in the endplate cartilage of OPG−/− mice.
► An increased apoptotic cells appeared in IVD sections of 28-week-old OPG−/− mice.