The role of RDC1/CXCR7b in the migration of the zebrafish posterior lateral line

The posterior lateral line primordium migrates along the horizontal myoseptum to the tip of the tail. The migration of the primordium utilises the CXC-type chemokine receptor cxcr4b which is expressed in a graded fashion within the primordium with strongest expression at the leading edge. The ligand for CXCR4b is stromal-derived factor 1a (SDF1a). sdfla is expressed along the length of the horizontal myoseptum. Loss of cxcr4b, or sdfla, results in defects in migration. However, various observations suggest additional systems guiding the primordium. We have performed an expression screen to identify genes with differential expression in the migrating primordium and as a result have identified the chemokine receptor cxcr7b, which is expressed in a complimentary fashion to cxcr4b. Functional analyses reveal that cxcr7b is essential for initiation and guidance of primordium migration. This work reveals that the posterior lateral line primordium is formed by fusion of several distinct groups of placodal cells and the fusion of these groups correlates with the initiation of directional migration. Inhibition of cxcr7b function leads to retarded primordium migration and failure to complete coalescence. Ectopic expression of cxcr7b on the path results in stalled migration of the leading cells suggesting a role for cxcr7b in altering the response of the primordium to sdfla, possibly by inhibiting its effect. It is likely that expression of cxcr7b in the trailing part of the primordium acts to render it incapable of responding to the underlying sdfla stripe, thus producing directional guidance of migration. Preliminary results from this work also demonstrate the presence of FGF signalling in parallel to the cxcr4b/cxcr7blsdf1a pathways which may have roles in the initiation and guidance of the migration of the posterior lateral line primordium. In the absence of cxcr4b/cxcr7blsdfla pathways the primordium often migrates aberrantly towards the pectoral fin bud due to the presence of FGF signalling