prex1 is a target of Nodal signaling, promotes Rac1 activity, and regulates endodermal cell motility. (A) Expression of prex1 was measured by real-time quantitative PCR. Inhibition of Nodal signaling by SB-505124 treatment (SB) down-regulated prex1 expression (normalized to DMSO-treated controls), and overactivation of the Nodal pathway by expression of taram-A* (TA*) increased prex1 expression (normalized to control embryos expressing mCherry). The data shown are mean fold changes from six independent experiments. (B) Section through an embryo at 70% epiboly processed for prex1 in situ hybridization. prex1 appears to be enriched within the endodermal layer (arrows). Bar, 25 µm. (C and D) Representative ratiometric images of control (Ctrl; C) and Prex1 MO–injected (D) cells expressing RFP-PBD and colabeled with fluorescent dextran. Images are pseudocolored based on ratio value. Warmer colors indicate enrichment of PBD relative to dextran. Bars, 10 µm. (E) Quantification of the mean ratio of PBD to dextran indicates that Prex1 knockdown reduces Rac1 activity. Control, n = 124 cells; MO, n = 70 cells. (F and G) Representative migration tracks over a 1-h period from control (E) and Prex1 MO–injected (F) embryos. Dorsal is to the right. Bars, 25 µm. (H and I) Quantification of migration persistence (H) and instantaneous velocity (I) from control and Prex1 MO–injected embryos. Prex1 knockdown significantly increased migration persistence and moderately reduced migration velocity. Control, n = 80 cells; MO, n = 33 cells. (J and K) Overexpressing Prex1 can rescue random migration (J) and partially rescue migration velocity (K) in embryos treated with the Nodal inhibitor SB-505124. DMSO, n = 44 cells; SB-505124, n = 34 cells; SB-505124 + Prex, n = 52 cells. All error bars represent SEM. *, P < 0.05.