Endocrine Abstracts

Osteoclasts are long lived highly specialised bone resorbing cells which form through the fusion of mononuclear pre-cursor cells and are believed to follow a linear fate and undergo apoptosis at the end of their life cycle. A number of anti-resorptive therapeutics target these cells, either preventing their resorptive function, Bisphosphonates, or inhibiting their formation, Denosumab (Anti-RANKL-Dmab). These agents have achieved success in preventing bone loss and fractures in patients with osteoporosis, amongst other diseases, however complications from their long term use has led to treatment cessation. In the case of Denosumab, treatment cessation has led to rebound bone loss and increased fractures, providing new challenges for its clinical use. We visualised the dynamics of osteoclasts in real time within live bone tissue leading to the discovery of a new fate for these complex cells. Further, this novel cell biology provides an improved understanding of patient response to anti-resorptive therapy. We developed a novel intravital imaging methodology to visualize osteoclast dynamics on the intact endocortical surface of tibia in live mice. Employing a double reporter mixed bone marrow chimera model and using sRANKL to stimulate osteoclasts and osteoprotegerin-Fc (OPG:Fc) to mimic Dmab we examined osteoclast dynamics and function. We showed that in addition to apoptosis, osteoclasts undergo fission to form osteomorphs, a novel intermediate cell of the osteoclast lineage. These osteomorphs were then shown to re-fuse, confirming the process of osteoclast recycling as an alternative osteoclast fate to apoptosis. Using RNAseq we defined the osteomorph as a novel cell population, distinct from osteoclasts and osteoclast pre-cursors. Interestingly, osteomorph specific genes were associated with bone phenotypes in mice. We also showed accumulation of osteomorphs and their rapid re-fusion following withdrawal of OPG:Fc, providing a mechanism for the rapid bone loss and fractures suffered by patients following Denosumab therapy withdrawal.