CXCR7 Controls PGC Polarity by Regulating SDF-1 Levels (A) CXCR7 knockdown reduces the polarity of migrating germ cells. Wild-type PGCs show a typical polarization of the cells with protrusions at the leading edge in the direction of migration (upper panel, arrows). PGCs in CXCR7-depleted embryos exhibit reduced polarity with protrusions extended in opposite directions (lower panel, arrowheads). Cells labeled with EGFP-F. (B) CXCR7 depletion reduces the motility of PGCs in an SDF-1a-dependent manner. The motility of PGCs was followed in time-lapse movies. Error bars represent SEM. Examples for 70 min long migration paths of germ cells are shown. PGCs in CXCR7 morphants exhibit low motility with short tracks that are reminiscent of PGCs migrating in embryos with high-uniform SDF-1a expression (SDF-1a-OEX). Removal of SDF-1 in CXCR7-depleted embryos restores PGC motility to a level that is similar to that in SDF-1-depleted embryos. Similarly, knocking down CXCR4 restores PGC motility in CXCR7 morphants. (C) Reduction of SDF-1a expression suppresses the CXCR7 knockdown phenotype. The migration phenotype of embryos knocked down for CXCR7 (left panel, 66.4% ± 3% ectopic cells per embryo, n = 30 embryos) is suppressed by coinjection of low levels (0.02 pmol) of SDF-1a morpholino (middle panel: SDF-1a-MO, 29.4% ± 3% ectopic cells per embryo, n = 22 embryos; right panel: CXCR7-MO and SDF-1a-MO, 36.0% ± 2% ectopic cells per embryo, n = 63 embryos).
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