Paracrine insulin-like growth factor signaling influences primordial germ cell migration: in vivo evidence from the zebrafish model.

IGF signaling has been shown to stimulate migration of multiple cell types in vitro, but few studies have confirmed an equivalent function for IGF signaling in vivo. We recently showed that suppression of IGF receptors in the zebrafish embryo disrupts primordial germ cell (PGC) migration, but the mechanism underlying these effects has not been elucidated. We hypothesized that PGCs are intrinsically dependent upon IGF signaling during the migratory phase of development. To test this hypothesis, we first examined the spatial expression patterns of IGF ligand genes (igf1, igf2a, and igf2b) in the zebrafish embryo. In situ analyses revealed distinct expression patterns for each IGF ligand gene, with igf2b mRNA expressed in a spatial pattern that correlates strongly with PGC migration. To determine whether PGC migration is responsive to IGF signaling in vivo, we synthesized gene hybrid expression constructs that permit conditional overexpression of IGF ligands by PGCs into the PGC microenvironment. Conditional overexpression of IGF ligands consistently disrupted PGC migration, confirming that PGC migration is sensitive to local aberrations in IGF signaling. Finally, we show that conditional suppression of IGF signaling, via PGC-specific overexpression of a mutant IGF-I receptor, disrupts PGC migration, confirming that zebrafish PGCs intrinsically require IGF signaling for directional migration in vivo. Collectively, these studies confirm an in vivo role for IGF signaling in cell migration and identify a candidate ligand gene (igf2b) regulating PGC migration in the zebrafish.