Elucidation of molecular mechanisms of osteoclastogenesis for developing drugs that inhibit bone resorption

TRAF6对于破骨细胞的形成至关重要，并且在破骨细胞祖细胞中RANK-和cd40介导的IKK和MAPKs的激活中起着至关重要的作用。尽管在破骨细胞形成过程中需要IKK和MAPKs的激活，但RANK而不是CD40能够促进破骨细胞的形成，因为只有RANK能够诱导长期激活由免疫球蛋白样受体/含itam的接头复合物介导的共刺激信号，而这些信号是诱导NFATcl（破骨细胞形成的主要基因）所必需的。我们之前的数据表明，RANK而不是CD40拥有一个独特的结构域，该结构域与traf6结合位点协同作用，以可持续的方式激活共刺激信号并促进破骨细胞的发生。我们发现了这样一个功能性的蛋白质结构域，HCR，它在各种脊椎动物的rank中高度保守，对于长期激活共刺激信号至关重要。将HCR添加到CD40的细胞质尾部，使CD40能够诱导共刺激信号的持续激活和随后的NFATcl的激活。此外，HCR在刺激下显著增强了HCR-窝藏受体的内化。这些结果表明，HCR可以通过控制受体的膜运输，将RANK-TRAF6轴连续连接到持续的共刺激信号通路，并且HCR可能是病理性骨吸收的新治疗靶点。

TRAF6 is essential for osteoclastogenesis and plays a crucial role in both RANK- and CD40-mediated activation of IKK and MAPKs in osteoclast progenitor cells. Despite the requirement for IKK and MAPKs activation in osteoclastogenesis, RANK, but not CD40, can promote osteoclastogenesis because only RANK can induce long-term activation of the co-stimulatory signals mediated by immunoglobulin-like receptors/ITAM-containing adaptors complexes, which are required for induction of NFATcl, a master gene in osteoclastogenesis. Our previous data suggested that RANK, but not CD40, harbors a unique domain that functions in concert with the TRAF6-binding site to activate the co-stimulatory signals in a sustainable way and promote osteoclastogenesis. We identify such a functional protein domain, HCR, that is highly conserved in RANKs from various vertebrates and is essential for long-term activation of co-stimulatory signals. Addition of the HCR to the cytoplasmic tail of CD40 renders CD40 capable of inducing sustained activation of co-stimulatory signals and subsequent activation of NFATcl. Furthermore, the HCR dramatically enhances internalization of HCR-harboring receptors upon stimulation. These results suggest that the HCR can continuously link the RANK-TRAF6 axis to sustained co-stimulatory signaling pathways through control of the membrane trafficking of receptors and that the HCR may be a novel therapeutic target for pathological bone resorption.