The role of osteoclasts and blood vessels as mediators downstream of beta-catenin in the differentiation of osteoblasts derived from two origins – chondrocytes and perichondrium

• Applicant: Professorin Dr. Christine Hartmann
• Subject Area: Orthopaedics, Traumatology, Reconstructive Surgery ; Developmental Biology
• Term: since 2022
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 511577186

Bone homeostasis depends on the interplay of osteoblasts and osteoclasts. Osteoblasts are derived from two different origins in the embryo, from hypertrophic chondrocytes and the perichondrium, while osteoclasts are of hematopoietic origin. Deviations from the equilibrium between osteoblasts and osteoclasts are observed in diseases affecting the joint, like osteoarthritis and rheumatoid arthritis, and lead to bone diseases like osteoporosis or osteopetrosis. The Wnt/beta-catenin pathway has been implicated in different aspects of skeletogenesis including bone homeostasis. Our recent work has shown that the co-transcriptional function of beta-catenin (encoded by the Ctnnb1 gene) in hypertrophic chondrocytes is, on the one hand, important for the differentiation of chondro¬cyte-derived osteoblasts and, on the other hand, negatively controls osteoclasto¬genesis at the chondro-osseous front. Mice deficient for Ctnnb1 in hypertrophic chondrocytes are osteopenic, reflected in the severe reduction of trabecular bone. Yet, surprisingly, in these mice, the differentiation of perichondrium-derived osteoblasts is also affected, suggesting the involve¬ment of non-cell-autonomous mechanisms. In mice expressing a stabilized form of beta-catenin in hypertrophic chondrocytes leading to an activation of the Wnt/beta-catenin pathway, we observed that the chondrocyte-derived osteoblasts increased in number but we failed to detect high levels of beta-catenin protein in these cells, supporting the idea of non-cell-autonomous mechanisms regulating osteoblastogenesis from both origins. Based on preliminary data, we propose that part of the phenotypic changes involve osteoclasts and/or maybe due to changes in the composition of the H- and L-type blood vessels. More precisely, in mouse mutants lacking osteoclasts we observed that the differentiation of chondrocyte- and perichondrium-derived osteoblasts were increased suggesting that osteoclasts either secret/present a factor or through their activity a factor is released from the extracellular matrix affecting osteoblast differentiation from both origins negatively. Such a possible factor produced by osteoclasts is Semaphorin 4D. To examine whether osteoclast activity is required, we will also examine the differentiation of osteoblasts derived from the two origins using a mouse model in which osteoclasts are present but inactive. In addition, the blood vessel and also bone marrow composition will be examined in some of the mouse models using a single-cell RNA seq approach. Taken together, in this project we aim to identify secreted factors acting downstream of beta-catenin regulating osteoblast differentiation from both origins and understand the role of osteoclasts and blood vessels as possible mediators.

• DFG Programme: Research Grants
• Co-Investigators: Professor Dr. Ralf H. Adams; Professor Dr. Thomas Braun