Differential Expressions of Nodal-Signal Transducers and Global Transcriptional Repression Commit Vegetal Cells in Eleutherodactylus Coqui to Form Nutritional Endoderm

The vegetal core cells of a Xenopus laevis embryo commit to mesendoderm via the Nodal-signaling pathway. In Eleutherodactylus coqui, a direct developing frog, mesendoderm is specified at the marginal zone of the early gastrulae and vegetal core cells transform into nutritional endoderm. It is a novel tissue consisting of transient, yolk rich cells that provide nutrition, but do not differentiate into adult tissues. We hypothesized that a disruption of Nodal-signaling is responsible for committing vegetal core cells to nutritional endoderm. I report a dual regulation involved in the generation of nutritional endoderm. First, differential expression of Nodal-signaling components like Smad2 and Smad4 was observed during early gastrulation between cells in the marginal zone and in the vegetal core. Although EcSmad2 RNA, as well as both native and the active forms of EcSmad2, were detected in the vegetal core by qPCR and western blot respectively, western analysis revealed that Smad4 isoforms were expressed at a low level during early gastrulation. Immunostaining showed that only 12% and 50% of vegetal core cells were positive for nuclear Smad2 and Smad4 signals, respectively, compared to 100% in marginal zone cells. These differential expressions may indicate a signaling blockade in vegetal core cells. Second, I found global transcription repression in vegetal core cells by immunostaining. At late blastula, both the marginal zone and vegetal core cell were transcriptionally silent. At the onset of gastrulation, marginal zone cells, but not vegetal core ones, became transcriptionally active. This indicates the occurrence of a mid-blastula transition in the marginal zone by early gastrulation. Global transcriptional repression prevails in the vegetal core through development. A combination of differential Nodal-signaling and global transcriptional repression in vegetal core cells may account for its lack of differentiation.