The translocator protein ligand Ro5-4864 inhibits RANKL-induced osteoclastogenesis in mice

The translocator protein (TSPO) is a mitochondrial outer membrane protein that is constitutively expressed in various immune cells, including macrophages, dendritic cells, T cells, and B cells. TSPO has been implicated in mitochondrial function and immune regulation, particularly in macrophages, which are also osteoclast precursors. However, its role in osteoclast differentiation remains unclear. This study examined the effects of the TSPO ligand Ro5-4864 on osteoclast differentiation using murine bone marrow-derived macrophages (BMMs) and the RAW 264.7 macrophage cell line. TSPO was confirmed to be expressed constitutively in BMMs and remained detectable after stimulation with receptor activator of NF-κB ligand (RANKL), consistent with a potential role in osteoclastogenesis. Treatment with Ro5-4864 suppressed RANKL-induced osteoclast differentiation in both BMMs and RAW264.7 cells. This suppression of osteoclast differentiation was accompanied by downregulation of osteoclast-associated genes, including Nfatc1, Acp5, Mmp9, and Ctsk. Metabolic analysis revealed that Ro5-4864 decreased cellular ATP levels without altering lactate production. Ro5-4864 also increased the mitochondrial membrane potential. Although the specific role of TSPO in osteoclastogenesis remains unclear, our findings suggest that Ro5-4864 inhibits osteoclast differentiation via mechanisms related to TSPO and mitochondrial energy metabolism.