Mitofusin 2 (Mfn2) is a mitochondrial outer membrane protein that participates in tethering mitochondria to the ER. Mitochondria-ER tethering has been demonstrated to play important roles in many cellular activities by regulating homeostasis of metabolites and calcium. Intracellular calcium signaling is crucial for the differentiation of osteoclasts, the bone-resorbing cells. In this study, we investigated whether Mfn2 plays a role in osteoclastogenesis by receptor activator of nuclear factor kappa B (RANKL) in primary cells. We found that RANKL increased Mfn2 expression during osteoclast formation from mouse bone marrow-derived macrophages (BMMs). When Mfn2 expression was suppressed in BMMs by using a siRNA-mediated gene knock-down system, osteoclast differentiation and activity of mature osteoclasts were reduced. Mfn2 knock-down also decreased the RANKL-mediated induction of NFATc1, the key transcription factor for osteoclast gene expression, without affecting c-Fos level. This effect on NFATc1 was associated with decreased calcium oscillation and calcineurin activity in Mfn2-deficient osteoclasts. Taken together, our results indicate that Mfn2 positively contributes to RANKL-induced osteoclast differentiation by regulating the calcium-calcieurin-NFATc1 axis, raising the importance of a previously under-recognized role of mitochondria in osteoclastogenesis. (C) 2019 Elsevier Inc. All rights reserved.