We discovered that fish carrying a mutation in the fgf8 gene, called acerebellar (ace), have symmetric brains. Although all relevant brain structures are specified in the mutant, they are unable to develop asymmetrically. This is first evident in the failure of a small group of neural cells, the parapineal, to migrate in a stereotypical leftward arc away from the midline. This leftward migration normally initiates a cascade of events that leads to elaboration of brain asymmetries and culminates in the establishment of asymmetric brain circuits. We found that in ace embryos, the parapineal remains at the midline and never migrates, and later-developing brain structures remain symmetric.
When we looked to see where the fgf8 gene is expressed in normal embryos, surprisingly, we found it on both sides of the brain adjacent to the parapineal, as are some genes turned on in response to Fgf signalling. Supporting the idea that Fgf signals act upon the parapineal, several genes functioning in the Fgf-pathway, including the Fgf-receptor FgfR4, are expressed specifically in this structure.
In order to determine whether signalling by Fgf8 is required for the parapineal nucleus to move leftward from the midline, we provided ace brains with a localised source of Fgf8. This was able to rescue the migration of the parapineal nucleus in ace mutants, however, migration was usually to the left, irrespective of the location of the source of Fgf8. This led us to suspect that another signal acts together with Fgf8 to influence the direction of migration. Indeed, we found that the leftward bias in Fgf-dependent migration is due to left-sided Nodal signalling. In situations where the strong Nodal bias is removed, the Fgf8 source can determine the direction of brain laterality, possibly by acting as an attractant to parapineal cells.
This and other data allowed us to produce a model for generation of brain asymmetry, where left and right sides of the brain compete to attract the parapineal via Fgf8-signalling, initiating a cascade of asymmetric development on the winning side. In normal brains (Panel A below), Nodal signalling strongly biases Fgf-dependent migration to the left. However, Nodal in the absence of Fgf8 is not sufficient to promote migration (Panel C). If Nodal is taken away, the side of the brain that wins the competition is probably that which has stochastically slightly higher levels of Fgf8 (Panel B). Indeed, if we experimentally provide Fgf8 on one side in such situations, that side that wins the competition. The study shows that the combined action of Fgf and Nodal signals ensures the establishment of brain asymmetries with consistent laterality, and suggests that mechanisms to generate asymmetry and direct laterality can be uncoupled and may have evolved sequentially.