PDF(Pubmed)
Abstract：
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. Proposed model of osteoclast fusion regulated by caspase-8 activation and PS exposure. RANK/RANK-L interaction. Activation of procaspase-8 into caspase-8. Caspase-8 activates caspase-3. Active capase-3 cleaves Xkr8. Local PS exposure is induced. Exposed PS is recognized by the fusion partner. FUSION. PS is re-internalized.
摘要：
单核前体的有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未完全了解。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。随后对破骨细胞前体的表征证实RANKL介导的半胱天冬酶-8的活化促进了非凋亡裂解和下游效应子半胱天冬酶的活化,所述半胱天冬酶易位到质膜,在质膜中它们触发了磷脂乱序酶Xkr8的活化。Xkr8介导的磷脂酰丝氨酸暴露，反过来,辅助破骨细胞前体的细胞融合，从而产生功能性多核破骨细胞合胞体并开始骨吸收。因此，caspase-8的药理学阻断或遗传缺失会干扰破骨细胞的融合和骨吸收，从而导致单核破骨细胞前体中caspase-8条件性缺失的小鼠的骨量增加。这些数据确定了控制破骨细胞生物学和骨转换的新途径，有可能在以病理性破骨细胞介导的骨丢失为特征的疾病中作为治疗干预的目标。提出了caspase-8激活和PS暴露调控的破骨细胞融合模型。RANK/RANK-L交互。将procaspase-8活化为caspase-8。胱天蛋白酶-8激活胱天蛋白酶-3。活性capase-3裂解Xkr8。诱导局部PS暴露。暴露的PS被融合伴侣识别。融合。PS被重新内在化。
