Killing the messenger: The role of CXCR7 in regulating primordial germ cell migration

Primordial Germ Cell (PGC) migration in zebrafish is guided by SDF-1a. Binding of this chemokine to its receptor CXCR4b activates downstream signalling cascades leading to cell polarization and directed migration towards the attractant source. Despite the detailed information available concerning the role of SDF-1 in guiding the PGCs to their targets, little was known regarding the molecular mechanisms controlling the distribution of SDF-1a within the tissue. We have recently shown that the activity of a second SDF-1/CXCL12 receptor, CXCR7 is crucial for proper migration of PGCs. Although CXCR4 and CXCR7 are structurally related and serve as receptors for the same ligand, they appear to serve very different functions during PGC migration. Here we discuss a model according to which CXCR4b translates the polarized distribution of SDF-1 into directed PGC migration, while CXCR7 acts as a high-affinity decoy receptor and facilitates the migration of PGCs by shaping the distribution of the chemokine in the environment.