● To address the reviewer's concern that our experimental system is artificial, we conducted additional experiments using Western blotting. ● We have expanded the Discussion section to elaborate artificial experimental systems as a limitation. ● Because expressing the localization of RANKL‑tomato by the number of dots was difficult, we represented it by area instead ( Figure 1 D and 2 C). We also described the measurement method. ● Measured the events in Figure 3A and plotted them in a graph ( Figure 3 B). Receptor activator of NF-κB ligand (RANKL) produced by osteoblastic lineage cells is essential for osteoclastogenesis, yet RANKL can be sequestered in intracellular, lysosome-like compartments under basal conditions. How mechanical cues mobilize RANKL toward the cell surface remains poorly defined. Here, we tested whether fluid shear stress alters RANKL subcellular distribution in osteoblast-like MC3T3-E1 cells and examined the involvement of the non-receptor tyrosine kinase c-Src. MC3T3-E1 cells expressing fluorescently tagged RANKL were subjected to fluid shear stress, and RANKL localization was analyzed by microscopy and subcellular fractionation. Fluid shear stress increased c-Src activation (Tyr416 phosphorylation) and promoted redistribution of RANKL toward the cell periphery, accompanied by an increase of RANKL in the membrane fraction. Co-expression experiments showed spatial association of RANKL with c-Src at the cell periphery after shear stimulation. Moreover, constitutively active c-Src (Y527F) enhanced peripheral localization of RANKL even in the absence of shear stress. Together, these data support a model in which shear stress activates c-Src to facilitate RANKL localization from intracellular stores toward membrane-proximal regions, thereby providing a mechanistic link between mechanical cues and osteoblast-derived osteoclastogenic signaling.
Matsubara et al. (Sun,) studied this question.