PRR is critical for ATP release from Lrp4-deficient OB-lineage cells. (A–C) Increased ATP loading and vesicular acidification in MC3T3 cells expressing V5-PRR. ATP-containing vesicles were labeled with quinacrine. The green fluorescent intensity of LysoSensor was measured by Image J (mean ± SD; n = 20). Quantification analysis is shown in B and C. Bar, 10 µm. *, P < 0.05. (D and E) Decreased ATP loading in MC3T3 cells suppressing PRR. Cells were costained with LysoTracker DND-99 and quinacrine. White bar, 10 µm; and yellow bar, 5 µm. Quantification analysis is shown in E. Mean ± SD, n = 20; *, P < 0.05. (F) Western blot analysis of Lrp4 and PRR expression in control and PRR-KD BMSCs. BMSCs derived from 1-mo-old control and mr-Lrp4^mitt mice were infected with control and shR-PRR lentiviruses. The cell lysates were subjected to the Western blot analysis using indicated antibodies. (G) Medium ATP levels in PRR-KD BMSCs. The values of mean ± SD from three different experiments were presented. *, P < 0.05. (H) Experimental strategy. TRAP staining of WT BMMs that were treated with CM of control or PRR-KD BMSCs. (I and J) TRAP staining analysis of OC cultures that were treated with CM plus RANKL for 7 d. Representative images are shown in I. Bar, 150 µm. Quantitative analysis of TRAP⁺ multinuclei cells (MNCs) per field is presented in J. Mean ± SD values from three different cultures are shown. *, P < 0.05.