Synaptotagmin VII affects RANKL‐lysosomal vesicle fusion to the cell membrane in osteoblast‐like cells

抄録

Receptor activator of nuclear factor kappa‐B ligand (RANKL) is expressed mainly in osteoblasts and is a critical factor in osteoclast differentiation. The mechanism of RANKL‐induced osteoclastogenesis has been investigated extensively; however, its intracellular transport is still poorly understood. RANKL localizes at lysosomal vesicles and is transported to the vicinity of the cell membrane. This intracellular transportation is regulated by intracellular Ca²⁺ concentration. In order to investigate the trigger for RANKL lysosomal vesicle fusion to the cell membrane, we generated osteoblast‐like cells stably expressing the channelrhodopsin‐wide receiver (ChRWR) in our previous study. To further investigate the mechanism of vesicle fusion, we focused on synaptotagmin VII (Syt VII), a Ca²⁺‐sensitive molecule. Light‐induced depolarization increased the levels of membrane‐bound RANKL (mbRANKL), which was reversed by Syt VII knockdown. Addition of an activator for L‐type voltage‐gated Ca²⁺ channels increased the mbRANKL, whereas knockdown of Syt VII inhibited this activator‐induced increase. These results indicate that depolarization of the osteoblastic cell membrane is a trigger for RANKL lysosomal vesicle fusion to the membrane, and that Syt VII contributes to this vesicle fusion following the opening of L‐type voltage‐gated Ca²⁺ channels. This mechanism of osteoblastic lysosomal fusion may provide new therapeutic targets for skeletal disorders. (J Osaka Dent Univ 2019; 53: 141‐147)