The interaction between osteoclasts and osteoblasts during the reversal-resorption phase of intracortical remodeling events

It has been known for many years that ageing affects strength of our bones and eventually leads to osteoporosis with high risk of fractures. Many different drugs have been developed to treat osteoporosis, with the aim of either limiting bone degradation (resorption) or promoting construction of bone (formation). However, it is now well recognized that in adult bone physiology, bone resorption and formation are not independent events. The main part of resorption and formation occur as part of the bone remodeling process where the amount of bone constructed corresponds to the amount of bone removed by resorption. This process maintains the strength of our skeleton throughout life. Just as disproportions in this process causes osteoporosis or osteopetrosis with altered bone mass and fragile bones. When developing drugs to treat osteoporosis, it is increasingly important to consider this balance between bone resorption and bone formation. The overall aim of this thesis was to investigate how different anti-resorptive drugs against osteoporosis affects the balance between bone resorption and formation, as well as investigating the mechanisms of “coupling” bone formation to bone resorption during human bone remodeling.


In manuscript I and II the aim was to investigate how two different anti-resorptives (alendronate and denosumab) affect the bone remodeling process. We investigated human iliac crest biopsies obtained from phase III trials investigating effects of alendronate and denosumab. Alendronate binds to the mineral component of bone and inhibits bone resorption when osteoclasts initiate resorption and take up the drug during bone degradation. The study showed that Alendronate prolonged the reversal-resorption phase in cortical bone remodeling events. This has previously been shown in trabecular bone, but such a strong effect is surprising in cortical bone, which is believed to be less accessible for drugs like alendronate. Similarly, denosumab which inhibits osteoclast formation by functioning as a RANKL-decoy receptor, reduced bone formation and prolonged the reversal-resorption phase in cortical bone remodeling events. However, the striking difference between alendronate and denosumab was the strong inhibition of bone formation mediated by denosumab, supporting the hypothesis that osteoclasts are a prerequisite for activation of bone formation during bone remodeling.


In manuscript III the aim was to investigate how signaling factors proposed to be involved in the coupling of bone formation to bone resorption are localized in human bone remodeling events. Most of the proposed coupling factors have been identified in studies using rodent models for bone diseases, in vitro studies with primary rodent cells, or serum samples from osteoporotic patients. However, knowledge about their localization in human tissue is scarce. High-sensitivity in situ hybridization was applied to healthy human cortical bone from adolescents undergoing corrective surgery for Coxa Valga. We localized the ligand and receptor pairs: Leukemia Inhibitory Factor and Leukemia Inhibitory Factor Receptor, Semaphorin4D and PlexinB1, Platelet-derived growth factor B and receptors Platelet-derived growth factor receptor α and β to cells in the bone remodeling compartment. On the other hand, we did not find ligands Oncostatin M or Cardiotrophin-1 in the bone remodeling compartment. Likewise, EphrinB2 and EphB4 were localized to vascular structures, not bone cells in the bone remodeling compartments.


Finally, an aim during the PhD-project was to develop methods for functional investigation of the coupling during bone remodeling. A well-established cell-culture model with primary human osteoclasts and primary human osteoblast-lineage cells was expanded and optimized for singlenuclei RNA sequencing. The procedure is established to investigate expression patterns of osteoclasts and osteoblastic cells at the moment during bone remodeling, where they are in close contact, and “coupling” actually occur.


Collectively the work of this project, provided knowledge about human bone remodeling not previously shown in human bone. Furthermore, established procedures provide a ground for future functional studies of “coupling” in human bone remodeling.