Commitment to nutritional endoderm in Eleutherodactylus coqui involves altered nodal signaling and global transcriptional repression

Abstract

The vegetal cells of a Xenopus laevis embryo commit to mesendoderm via the Nodal-signaling pathway. In the direct developing frog Eleutherodactylus coqui, mesendoderm is specified at the marginal zone of the early gastrula, and vegetal core cells transform into nutritional endoderm. Nutritional endoderm, a novel tissue, consists of transient, yolky cells that provide nutrition but remain undifferentiated. We report a dual regulation for the generation of nutritional endoderm. First, differential expressions of the Nodal-signal transducers Smad2 and Smad4 were observed during early gastrulation between the marginal zone and the vegetal core cells. Although EcSmad2 RNA as well as total and activated Smad2 protein were detected in the vegetal core, Smad4 protein was expressed less in vegetal core during early gastrulation. Only 12% and 50% of vegetal core cells were positive for nuclear Smad2 and Smad4 signals respectively compared to 100% of marginal zone cells. These results suggest a signaling disruption in the vegetal core. Second, vegetal core cells were transcriptionally repressed. At the blastula stage, both marginal zone and vegetal core cells were transcriptionally silent, but during early gastrulation, only marginal zone cells became transcriptionally active. This indicates the occurrence of a mid-blastula transition in the marginal zone by early gastrulation, but global transcriptional repression persisted in the vegetal core and its derivative, nutritional endoderm, throughout development. We have described a novel mechanism, which prevents differentiation of the vegetal core through differential Nodal-signaling and global transcriptional repression.

Transcription factors Mix1 and VegT, relocalization of vegt mRNA, and conserved endoderm and dorsal specification in frogs

“…However, in 16-cell embryos, Ec_vegt transcripts were detected in equal proportions in animal and vegetal blastomeres (18). In addition, the presumptive nutritional endoderm expresses EcSox 17, activin b, derriere, Ecsmad2, and the proteins Smad2, Smad4 (23)(24)(25), VegT, and Mix1 (this report). Starting in the gastrula, transcriptional repression occurs only in the nutritional endoderm (24,25).…”

“…In addition, the presumptive nutritional endoderm expresses EcSox 17, activin b, derriere, Ecsmad2, and the proteins Smad2, Smad4 (23)(24)(25), VegT, and Mix1 (this report). Starting in the gastrula, transcriptional repression occurs only in the nutritional endoderm (24,25). Thus, the two types of E. coqui endoderm can develop from the common presumptive endoderm that expresses conserved endodermal genes.…”

Frogs as integrative models for understanding digestive organ development and evolution

“…Instead, it becomes nutritional endoderm (NE) that is utilized solely as a source of energy—the yolk platelets in these cells are metabolized, extruded and, eventually, eliminated as waste from the body. Some of the yolky vegetal cells are specifically set aside for this function during blastula stages, as indicated by the existence of a population of endoderm with reduced signaling/responsiveness in such species [167]. This alternate fate of the NE is likely related to the modified germ layer patterning often seen in larger eggs [168].…”