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

骨稳态依赖于成骨细胞和破骨细胞的相互作用。成骨细胞来源于两种不同的胚胎来源，从肥大的软骨细胞和软骨膜，而破骨细胞是造血来源。在影响关节的疾病中观察到成骨细胞和破骨细胞之间的平衡偏离，如骨关节炎和类风湿性关节炎，并导致骨骼疾病，如骨质疏松症或骨硬化症。Wnt/β-连环蛋白通路涉及骨骼发生的不同方面，包括骨稳态。我们最近的工作表明，β-连环蛋白（由Ctnnb 1基因编码）在肥大软骨细胞中的共转录功能，一方面对于软骨细胞来源的成骨细胞的分化很重要，另一方面，在软骨-骨前端负控制破骨细胞的发生。肥大软骨细胞中Ctnnb 1缺陷的小鼠是骨质减少的，反映在骨小梁的严重减少中。然而，令人惊讶的是，在这些小鼠中，软骨膜衍生的成骨细胞的分化也受到影响，表明涉及非细胞自主机制。在肥大软骨细胞中表达稳定形式的β-连环蛋白导致Wnt/β-连环蛋白途径激活的小鼠中，我们观察到软骨细胞衍生的成骨细胞数量增加，但我们未能在这些细胞中检测到高水平的β-连环蛋白，支持非细胞自主机制调节两种来源的成骨细胞发生的想法。根据初步数据，我们提出，部分的表型变化涉及破骨细胞和/或可能是由于在组成的H-和L-型血管的变化。更确切地说，在缺乏破骨细胞的小鼠突变体中，我们观察到软骨细胞和软骨膜衍生的成骨细胞的分化增加，这表明破骨细胞分泌/呈递一种因子，或者通过其活性从细胞外基质释放一种因子，从两个来源负面影响成骨细胞分化。破骨细胞产生的这种可能的因子是Semaphorin 4D。为了检验是否需要破骨细胞活性，我们还将使用小鼠模型来检验来自两个来源的成骨细胞的分化，在该小鼠模型中存在破骨细胞但无活性。此外，将使用单细胞RNA测序方法在一些小鼠模型中检查血管和骨髓组成。总之，在这个项目中，我们的目标是确定分泌因子的β-连环蛋白调节成骨细胞分化的下游，从两个起源和了解破骨细胞和血管的作用，作为可能的介质。

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.