Left/right asymmetric collective migration of parapineal cells is mediated by focal FGF signaling activity in leading cells

Significance The ability of cells to migrate collectively underlies many biological processes. The parapineal is a small group of cells that requires Fgf8 to migrate from the midline to the left side of the zebrafish forebrain. Studying the dynamics of FGF pathway activation reveals that FGF activity is restricted to a few left-sided parapineal cells. Global activation of the FGF pathway interferes with parapineal migration in wild-type embryos, while focal activation in few parapineal cells can restore migration in fgf8−/− mutants, indicating that FGF pathway activation in leading cells is required for collective migration. We show that focal FGF activity is influenced by left-sided Nodal signaling. Our findings may apply to other contexts of FGF-dependent cell migration during development or metastasis.
The ability of cells to collectively interpret surrounding environmental signals underpins their capacity to coordinate their migration in various contexts, including embryonic development and cancer metastasis. One tractable model for studying collective migration is the parapineal, a left-sided group of neurons that arises from bilaterally positioned precursors that undergo a collective migration to the left side of the brain. In zebrafish, the migration of these cells requires Fgf8 and, in this study, we resolve how FGF signaling correlates with—and impacts the migratory dynamics of—the parapineal cell collective. The temporal and spatial dynamics of an FGF reporter transgene reveal that FGF signaling is activated in only few parapineal cells usually located at the leading edge of the parapineal during its migration. Overexpressing a constitutively active Fgf receptor compromises parapineal migration in wild-type embryos, while it partially restores both parapineal migration and mosaic expression of the FGF reporter transgene in fgf8−/− mutant embryos. Focal activation of FGF signaling in few parapineal cells is sufficient to promote the migration of the whole parapineal collective. Finally, we show that asymmetric Nodal signaling contributes to the restriction and leftwards bias of FGF pathway activation. Our data indicate that the first overt morphological asymmetry in the zebrafish brain is promoted by FGF pathway activation in cells that lead the collective migration of the parapineal to the left. This study shows that cell-state differences in FGF signaling in front versus rear cells is required to promote migration in a model of FGF-dependent collective migration.