Efficient cellular fusion of mononuclear precursors is the prerequisite for the generation of fully functional multinucleated bone-resorbing osteoclasts. However, the exact molecular factors and mechanisms controlling osteoclast fusion remain incompletely understood. Here we identify RANKL-mediated activation of caspase-8 as early key event during osteoclast fusion. Single cell RNA sequencing-based analyses suggested that activation of parts of the apoptotic machinery accompanied the differentiation of osteoclast precursors into mature multinucleated osteoclasts. A subsequent characterization of osteoclast precursors confirmed that RANKL-mediated activation of caspase-8 promoted the non-apoptotic cleavage and activation of downstream effector caspases that translocated to the plasma membrane where they triggered activation of the phospholipid scramblase Xkr8. Xkr8-mediated exposure of phosphatidylserine, in turn, aided cellular fusion of osteoclast precursors and thereby allowed generation of functional multinucleated osteoclast syncytia and initiation of bone resorption. Pharmacological blockage or genetic deletion of caspase-8 accordingly interfered with fusion of osteoclasts and bone resorption resulting in increased bone mass in mice carrying a conditional deletion of caspase-8 in mononuclear osteoclast precursors. These data identify a novel pathway controlling osteoclast biology and bone turnover with the potential to serve as target for therapeutic intervention during diseases characterized by pathologic osteoclast-mediated bone loss.

单核前体的高效细胞融合是产生功能齐全的多核骨吸收破骨细胞的先决条件。然而，控制破骨细胞融合的确切分子因素和机制仍不完全清楚。在这里，我们确定 RANKL 介导的 caspase-8 激活是破骨细胞融合过程中的早期关键事件。基于单细胞 RNA 测序的分析表明，凋亡机制部分的激活伴随着破骨细胞前体分化为成熟的多核破骨细胞。破骨细胞前体的后续表征证实，RANKL 介导的 caspase-8 激活促进了下游效应子 caspase 的非凋亡裂解和激活，这些效应子 caspase 易位到质膜，在那里它们触发了磷脂扰乱酶 Xkr8 的激活。Xkr8 介导的磷脂酰丝氨酸暴露反过来有助于破骨细胞前体的细胞融合，从而允许功能性多核破骨细胞合胞体的产生和启动骨吸收。因此，caspase-8 的药理学阻断或基因缺失干扰了破骨细胞的融合和骨吸收，导致单核破骨细胞前体中携带 caspase-8 条件缺失的小鼠的骨量增加。这些数据确定了一种控制破骨细胞生物学和骨转换的新途径，有可能在以病理破骨细胞介导的骨质流失为特征的疾病中作为治疗干预的靶点。