A mesoderm-independent role for Nodal signaling in convergence & extension gastrulation movements

During embryogenesis, the distinct morphogenetic cell behavior programs that shape tissues are influenced both by the fate of cells and their position with respect to the embryonic axes, making embryonic patterning a prerequisite for morphogenesis. These two essential processes must therefore be coordinated in space and time to ensure proper development, but mechanisms by which patterning information is translated to the cellular machinery that drives morphogenesis remain poorly understood. Here, we address the role of Nodal morphogen signaling at the intersection of cell fate specification, patterning, and anteroposterior (AP) axis extension in zebrafish gastrulae and embryonic explants. AP axis extension is impaired in Nodal-deficient embryos, but it is unclear whether this defect is strictly secondary to their severe mesendoderm deficiencies or also results from loss of Nodal signaling per se. We find that convergence & extension (C&E) gastrulation movements and underlying mediolateral (ML) cell polarization are reduced in the neuroectoderm of Nodal-deficient mutants and exacerbated by simultaneous disruption of Planar Cell Polarity (PCP) signaling, demonstrating at least partially parallel functions of Nodal and PCP. ML polarity of mutant neuroectoderm cells is not fully restored upon transplantation into wild-type gastrulae, demonstrating a cell autonomous, mesoderm-independent role for Nodal in neural cell polarization. This is further demonstrated by the ability of Nodal ligands to promote neuroectoderm-driven C&E of naïve blastoderm explants in a tissue-autonomous fashion. Finally, temporal manipulation of signaling reveals that Nodal contributes to neural C&E in explants after mesoderm is specified and promotes C&E even in the absence of mesoderm. Together these results reveal a mesoderm-independent, cell-autonomous role for Nodal signaling in neural C&E that may cooperate with previously-described mesoderm-dependent mechanisms to drive AP embryonic axis extension.