Apoptosis is crucial for tissue homeostasis and organ development. In bone, apoptosis is recognized to be a main fate of osteoblasts, yet the relevance of this process remains underexplored. Using our murine model with inducible Caspase 9, the enzyme that initiates intrinsic apoptosis, we triggered apoptosis in a proportion of mature osteocalcin (OCN⁺) osteoblasts and investigated the impact on postnatal bone development. Osteoblast apoptosis stimulated efferocytosis by osteal macrophages. A five-week stimulation of OCN⁺ osteoblast apoptosis in 3-week-old male and female mice significantly enhanced vertebral bone formation while increasing osteoblast precursors. A similar treatment regimen to stimulate osterix⁺ cell apoptosis had no impact on bone volume or density. The vertebral bone accrual following stimulation of OCN⁺ osteoblast apoptosis did not translate in improved mechanical strength due to disruption of the lacunocanalicular network. The observed bone phenotype was not influenced by changes in osteoclasts but was associated with stimulation of macrophage efferocytosis and vasculature formation. Phenotyping of efferocytic macrophages revealed a unique transcriptomic signature and expression of factors including VEGFA. To examine whether macrophages participated in the osteoblast precursor increase following osteoblast apoptosis, macrophage depletion models were employed. Depletion of macrophages via clodronate-liposomes and the CD169-diphtheria toxin receptor mouse model resulted in marked reduction in leptin receptor⁺ and osterix⁺ osteoblast precursors. Collectively, this work demonstrates the significance of osteoblast turnover via apoptosis and efferocytosis in postnatal bone formation. Importantly, it exposes the potential of targeting this mechanism to promote bone anabolism in the clinical setting.

细胞凋亡对于组织稳态和器官发育至关重要。在骨骼中，细胞凋亡被认为是成骨细胞的主要命运，但这一过程的相关性仍未得到充分探索。使用具有诱导型 Caspase 9（启动内在细胞凋亡的酶）的小鼠模型，我们触发了部分成熟骨钙素 (OCN ⁺ ) 成骨细胞的细胞凋亡，并研究了其对出生后骨发育的影响。成骨细胞凋亡刺激骨巨噬细胞的胞吞作用。对 3 周大的雄性和雌性小鼠进行为期 5 周的 OCN ⁺成骨细胞凋亡刺激，显着增强了椎骨形成，同时增加了成骨细胞前体细胞。刺激 osterix ⁺细胞凋亡的类似治疗方案对骨体积或密度没有影响。由于腔隙小管网络的破坏，OCN ⁺成骨细胞凋亡刺激后的椎骨增长并未转化为机械强度的改善。观察到的骨表型不受破骨细胞变化的影响，但与巨噬细胞胞吐作用和脉管系统形成的刺激有关。巨噬细胞的表型分析揭示了独特的转录组特征和包括 VEGFA 在内的因子的表达。为了检查巨噬细胞是否参与成骨细胞凋亡后成骨细胞前体的增加，采用巨噬细胞耗竭模型。通过氯膦酸盐脂质体和 CD169-白喉毒素受体小鼠模型消耗巨噬细胞导致瘦素受体⁺和 osterix ⁺成骨细胞前体显着减少。 总的来说，这项工作证明了成骨细胞通过细胞凋亡和胞吞作用进行的周转在出生后骨形成中的重要性。重要的是，它揭示了在临床环境中针对这种机制促进骨合成代谢的潜力。