Introduction: Osteoclasts are the only cells with the capacity to degrade mineralized matrices, such as bone and calcified cartilage. During bone remodeling, osteoclasts secrete protons to achieve the acidic dissolution of hydroxyapatite to make bone collagen amenable to digestion by the proteases they produce. This requires the sealing zone, a ring of densely packed podosomes that surrounds the ruffled border, which is the secretion apparatus of the bone resorbing osteoclasts. Osteoclasts also resorb the calcified hypertrophic cartilage during endochondral ossification and growth of long bones, but the mechanism is poorly characterized. We showed recently that osteoclasts lacking Dock5, an activator of the GTPase Rac, cannot form sealing zones and are unable to resorb the bone in vitro and in vivo, leading to high trabecular BV/TV (JBMR 2011 26 (5) 1099110).
Results: Here we analyzed further the development and growth of long bones of Dock5−/− mice between days E17.5 and P35. The structure of the growth plate and the expression of MMP9 and MMP13 were normal in Dock5−/− mice, as was the secretion of MMP9, TRAP and CtsK by Dock5−/− osteoclast. The primary spongionsa, where hypertrophic mineralized cartilage is replaced by bone, was also indistinguishable between WT and Dock5−/− animals. They had identical BV/TV and osteoclasts numbers from E17.5 to P4. This suggests that hypertrophic cartilage replacement by bone is not affected in Dock5−/− mice. Interestingly, after P7, when bone remodeling starts in the secondary spongiosa, Dock5−/− mice showed higher trabecular BV/TV selectively in the secondary spongiosa, whereas BV/TV remains identical to Dock5+/+ mice in the primary spongiona. Higher trabecular BV/TV persisted in Dock5−/− mice until P35 and the end of bone growth. Finally, the overall bone length was identical between Dock5−/− and Dock5+/+ mice.
Conclusions: Our results show that the sealing zone is dispensable for osteoclasts to resorb the mineralized hypertrophic cartilage. They further show that osteoclasts deficient for bone resorption are compatible with normal endochondral ossification and growth of long bones. For the first time, we demonstrate here that the lack of the sealing zone only affects the bone remodeling activity of the osteoclasts but not their ability to resorb the hypertrophic mineralized cartilage.
18 - 21 May 2013
European Calcified Tissue Society