Characterization of a Xenopus laevis CXC chemokine receptor 4: implications for hematopoietic cell development in the vertebrate embryo.

Previous reports have shown that the Gi-protein-coupled CXC chemokine receptor 4 is activated by stromal cell-derived factor 1 (SDF-1). The receptor is present in many cell types and regulates a variety of cellular functions, including chemotaxis, adhesion, hematopoiesis, and organogenesis. To examine the role of CXCR4 as a regulator of organogenesis in the vertebrate embryo, we have isolated a cDNA encoding the Xenopus laevis homologue of CXCR4 (xCXCR4). The encoded polypeptide was functionally reconstituted with recombinant Gi2 in baculovirus-infected insect cells. Although xCXCR4 shares only 42% of its extracellular residues with mammalian CXCR4, it is indistinguishable from human CXCR4 in terms of its activation by human SDF-1alpha and SDF-1beta. The fact that only 19 of these residues are specifically present in the extracellular portions of CXCR4 suggests that these residues may be involved in recognizing SDF-1 and/or mediating CXCR4 activation by SDF-1. Xenopus CXCR4 mRNA expression was up-regulated during early neurula stages and remained high during early organogenesis. Whole mount in situ hybridization analysis showed abundant expression of xCXCR4 mRNA in the nervous system, including forebrain, hindbrain, and sensory organs, and in neural crest cells. xCXCR4 mRNA was also detected in the dorsal lateral plate, the first site of definitive hematopoiesis in the amphibian embryo corresponding to aorta-gonad-mesonephros or para-aortic splanchnopleura in mammals. This observation suggests that SDF-1 and CXCR4 are involved in regulating the migratory behavior of hematopoietic stem cells colonizing the larval or fetal liver. The hematopoietic defects observed in mice lacking SDF-1 or CXCR4 may, at least in part, be explained by a disturbance of this migration.