Asponge-like receptor sops up excess chemokine to refine the path for migrating primordial germ cells, based on findings from Bijan Boldajipour, Harsha Mahabaleshwar, Erez Raz (Center for Molecular Biology of Inflammation, Münster, Germany), and colleagues.
During development, germ cell precursors are drawn by gradients of the SDF-1 chemokine toward the gonads, where they will form the germ line. Zebrafish embryos have two SDF-1 receptors, CXCR4b and CXCR7, but the new results reveal that only CXCR4b leads the germ cells directly. CXCR7 was instead found on surrounding somatic cells, where it soaked up SDF-1.
The CXCR7 receptor and its bound SDF-1 ligand were taken into cells in endosomes. Rather than activate migratory signaling pathways, the receptor simply seemed to bring its ligand to lysosomes, where it is probably degraded.
Migrating germ cells avoided regions where CXCR7 sopped up SDF-1. The group suspects that the intake by CXCR7 allows conversion of the changes in SDF-1 mRNA expression patterns into differences in the protein pattern.
“To begin with,” says Raz, “SDF-1 is expressed broadly throughout the embryo,” drawing in the widely dispersed precursors. As development proceeds, he says, “SDF-1 expression becomes more restricted, and the old info must be quickly erased as cells draw nearer to their destination. If the ligand isn't cleared, it will reach unusually high levels, and the gradient will be disturbed.” Such problems were seen in embryos lacking CXCR7, in which germ cells were unable to polarize or migrate. They were similarly immobilized when extra SDF-1 was expressed throughout the embryo.