A proposed, three step model for Nodal's role in anterior neurulation involving FGF and Wnt Signaling

Abstract

Neurulation is a complex process in vertebrates that can be affected by environmental, nutritional, and genetic factors. Disruptions in primary neurulation result in an open neural tube and, thus, neural tube defects (NTDs). Previous research has shown Nodals are involved in anterior neurulation and suggest an indirect role through induction of the mesendoderm/mesoderm. Based on characterization of Nodal mutants, our laboratory previously proposed a model for Nodal's role in anterior neurulation with three basic steps: 1) Nodals induce the mesendoderm/mesoderm, 2) The mesendoderm/mesoderm then signals the overlying neurectoderm, and 3) This mesendodermal/mesodermal signal promotes proper adhesion between neural tube cells. Some Nodal mutants, such as those lacking zygotically expressed one-eyed pinhead (Zoep), a component of the Nodal receptor, have an incompletely penetrant NTD. Our research shows penetrance of this NTD increases with exposure to high temperatures at or before mid-blastula stages (up to 4 hpf). From ANOVA analysis, genetic background also significantly contributed to penetrance of the NTD in Zoep mutants. We found Nodal is required up to mid- to late-blastula stages for neurulation to occur properly. Mesendodermal/mesodermal tissues are induced by Nodal during the time Nodal is required for neurulation supporting the first step of our model. Statistical analysis indicates a strong correlation between many mesendodermal/mesodermal derived tissues and a closed neural tube. However, none of these derived tissues were sufficient or necessary for proper neurulation suggesting a critical amount of mesendodermal/mesodermal precursors are required during gastrulation to promote proper neurulation. Through RNA-sequencing embryos with Nodal inhibited during the requirement for Nodal in neurulation (would have open neural tubes) and after this requirement (would have closed neural tubes), we identified over 3,000 transcripts for involvement in neurulation. With further investigation, we found two of these candidates, FGF signaling and dkk1b, a member of the canonical Wnt signaling pathway, are involved in neurulation.