The development of zebrafish embryos from a fertilized egg into a multicellular structure involves series of well coordinated morphogenetic movements and patterning events, such as the establishment of the anteroposterior (AP) and dorsoventral (DV) axes. Although several genes important for early ontogeny have been identified during the past decades, our understanding about the regulation of these processes is still incomplete.
To identify new genes involved in the establishment of the dorsal side we recently performed a comparative full transcriptome screen using untreated and rescued ichabod (ich) mutant embryos. This approach identified rnd1l as one of the previously uncharacterized genes involved in DV specification.
Rnd proteins belong to the Rho subfamily of small GTPases, but, unlike other members of the subfamily, they do not hydrolyze GTP. They are involved in the regulation of cell-cell adhesion, the reorganization of the actin cytoskeleton and, consequently, in the fine-tuning of the cells’ migratory behaviour. Rnd1 proteins in particular have been shown to antagonize both E-cadherin and RhoA function. As E-cadherin and G-protein signalling are both essential for gastrulation movements, there is a high likelihood that zebrafish Rnd1 homologues are also implicated in these processes.
Our preliminary results show that rnd1l expression starts in a few cells on the presumptive dorsal side, but later it extends to all mesodermal precursors and is elevated in the axial mesoderm. Accordingly, our loss of function analysis suggests an important role for rnd1l in the regulation of convergent extension movements during gastrulation.
Interestingly, during somitogenesis rnd1l expression can not be detected anymore in the axial mesodermal precursors. At the same time it is highly upregulated in the developing eyefield and shows a dynamic expression in the hindbrain area. Loss of rnd1l function results in severe defects during eyefield separation, an event also marked by series of morphogenetic cell movements.