Ank-mediated pyrophosphate regulates shear stress-induced small extracellular vesicle production in 3D-cultured osteocytes

Osteocytes are located in the lacunae of fluid-filled bone and communicate with neighboring or distant cells by secreting small extracellular vesicles (sEVs) and growth factors as well as via dendrite-dendrite direct connections. However, the mechanism regulating sEV production in osteocytes is yet to be elucidated. In this study, we investigated sEV production and its underlying mechanism in osteocytes cultured on a three dimensional (3D) scaffold. We employed a perfusion system to apply shear stress stimulation to MLO-Y4 cells cultured on a 3D biphasic calcium phosphate (BCP) scaffold and analyzed sEV production and gene expression using RNA sequencing. We found that the expression of genes associated with sEV biogenesis and the secretory pathway were enhanced by fluid shear stress in MLO-Y4 cells cultured on a 3D BCP scaffold. In particular, fluid shear stress induced the expression of Ank, a pyrophosphate transporter, in 3D-cultured MLO-Y4 cells. The role of Ank in sEV production was further examined. Probenecid, an Ank inhibitor, significantly suppressed shear stress-induced sEV production, whereas Ank cDNA overexpression stimulated it. The inhibition of shear stress-induced sEV production by probenecid was recovered by the exogenous addition of pyrophosphate to MLO-Y4 cells. These findings suggest that shear stress-mediated sEV production in 3D-cultured osteocytes is regulated by extracellular pyrophosphate transported by Ank.