Conserved requirement for EGF-CFC genes in vertebrate left-right axis formation

Specification of the left-right (L-R) axis in the vertebrate embryo require s transfer of positional information from the node to the periphery, result ing in asymmetric gene expression in the lateral plate mesoderm. We show th at this activation of L-R lateral asymmetry requires the evolutionarily con served activity of members of the EGF-CFC family of extracellular factors. Targeted disruption of murine Cryptic results in L-R laterality defects inc luding randomization of abdominal situs, hyposplenia, and pulmonary right i somerism, as well as randomized embryo turning and cardiac looping. Similar ly, zebrafish one-eyed pinhead (oep) mutants that have been rescued partial ly by mRNA injection display heterotaxia, including randomization of heart looping and pancreas location. In both Cryptic and oep mutant embryos, L-R asymmetric expression of Nodal/cyclops, Lefty2/antivin, and Pitx2 does not occur in the lateral plate mesoderm, while in Cryptic mutants Lefty1 expres sion is absent from the prospective floor plate. Notably, L-R asymmetric ex pression of Nodal at the lateral edges of the node is still observed in Cry ptic mutants, indicating that L-R specification has occurred in the node bu t not the lateral plate. Combined with the previous finding that oep is req uired for nodal signaling in zebrafish, we propose that a signaling pathway mediated by Nodal and EGF-CFC activities is essential for transfer of L-R positional information from the node.