Functional analysis of chemokine signaling in zebrafish development

Chemokines are a large family of secreted proteins that are important to the migration of leukocytes during hematopoiesis and inflammation. Chemokines and their receptors are also widely distributed in the nervous system. Although recent investigations are beginning to elucidate chemokine function within the nervous system, relatively little is known about the neuronal function of this important class of molecules. To better appreciate the function of chemokines, the role of signaling by SDF-1 through its CXCR4 receptor was analyzed in zebrafish embryos.; The SDF-1/CXCR4b expression pattern suggested that SDF-1/CXCR4 signaling may guide retinal ganglion cell (RGC) axons within the retina to the optic stalk to exit the retina. Both misexpression of the ligand and antisense knockdown of the ligand and/or receptor decreased the number of differentiating RGCs. Furthermore, lower levels of knockdown and a genetic mutation of CXCR4 did not affect the normal complement of RGCs but did induce their axons to follow aberrant pathways within the retina. RGCs extend aberrant axons toward ectopic source SDF-1, suggesting SDF-1 is a chemoattractant for retinal axons. These data suggest that chemokine signaling is essential at multiple stages in the development of RGCs.; The SDF-1a/CXCR4b expression pattern also suggested that SDF-1/CXCR4 signaling was important for guiding migration by sensory cells known as the migrating primordium of the posterior lateral line. Ubiquitous induction of the ligand in transgenic embryos, antisense knockdown of the ligand or receptor, and a genetic receptor mutation all disrupted migration by the primordium. Furthermore, in embryos in which endogenous SDF-1 was knocked down, the primordium migrated towards exogenous sources of SDF-1. These data demonstrate that SDF-1 signaling mediated via CXCR4 functions as a chemoattractant for the migrating primordium and that chemokine signaling is both necessary and sufficient for directing primordium migration.; SDF-1b/CXCR4a expression also suggests that chemokine signaling might be involved in angiogenesis in zebrafish. Ubiquitous expression of SDF-1 led to circulation defects and cardiac edema. Antisense knockdown of either the ligand or the receptor caused defect in intersegmental vessel formation. Further experiments are necessary to elucidate the exact function of SDF-1/CXCR4 in blood vessel formation and/or remodeling.