Endo16 is required for gastrulation in the sea urchin Lytechinus variegatus

Coup-TF: A maternal factor essential for differentiation along the embryonic axes in the sea urchin Paracentrotus lividus

Coup-TF, a member of the nuclear receptor super-family, is present in the pool of maternal mRNAs and proteins in the sea urchin egg. The presence of this protein seems to be essential for the execution of the early developmental program, leading to all three embryonic layers. Our results demonstrate that Pl-Coup-TF morphants, i.e. Pl-Coup-TF morpholino knockdown embryos, resemble blastulae that lack archenteron at 24 hpf (hours post fertilization), a stage at which normal embryos reach the end of gastrulation in Paracentrotus lividus. At 48 hpf, when normal embryos reach the pluteus larva stage, the morphants are seemingly underdeveloped and lack the characteristic skeletal rods. Nevertheless, the morphant embryos express vegetal endomesodermal marker genes, such as Pl-Blimp1, Pl-Endo16, Pl-Alx1 and Pl-Tbr as judged by in situ hybridization experiments. The anterior neuroectoderm genes, Pl-FoxQ2, Pl-Six3 and Pl-Pax6, are also expressed in the morphant embryos, but Pl-Hbn and Pl-Fez mRNAs, which encode proteins significant for the differentiation of serotonergic neurons, are not detected. Consequently, Pl-Coup-TF morphants at 48 hpf lack serotonergic neurons, whereas normal 48 hpf plutei exhibit the formation of two bilateral pairs of such neurons in the apical organ. Furthermore, genes indicative of the ciliary band formation, Pl-Hnf6, Pl-Dri, Pl-FoxG and Pl-Otx, are not expressed in Pl-Coup-TF morphants, suggesting the disruption of this neurogenic territory as well. In addition, the Pl-SynB gene, a marker of differentiated neurons, is silent leading to the hypothesis that Pl-Coup-TF morphants might lack all types of neurons. On the contrary, the genes expressing signaling molecules, which establish the ventral/dorsal axis, Pl-Nodal and Pl-Lefty show the characteristic ventral lateral expression pattern, Pl-Bmp2/4, which activates the dorsal ectoderm GRN is down-regulated and Pl-Chordin is aberrantly over-expressed in the entire ectoderm. The identity of ectodermal cells in Pl-Coup-TF morphant embryos, was probed for expression of the ventral marker Pl-Gsc which was over-expressed and dorsal markers, Pl-IrxA and Pl-Hox7, which were silent. Therefore, we propose that maternal Pl-Coup-TF is essential for correct dissemination of the early embryonic signaling along both animal/vegetal and ventral/dorsal axes. Limiting Pl-Coup-TF's quantity, results in an embryo without digestive and nervous systems, skeleton and ciliary band that cannot survive past the initial 48 h of development.

Terminal alpha-d-mannosides are critical during sea urchin gastrulation

The sea urchin embryo is a United States National Institutes of Health (NIH) designated model system to study mechanisms that may be involved in human health and disease. In order to examine the importance of high-mannose glycans and polysaccharides in gastrulation, Lytechinus pictus embryos were incubated with Jack bean α-mannosidase (EC 3.2.1.24), an enzyme that cleaves terminal mannose residues that have α1-2-, α1-3-, or α1-6-glycosidic linkages. The enzyme treatment caused a variety of morphological deformations in living embryos, even with α-mannosidase activities as low as 0.06 U/ml. Additionally, formaldehyde-fixed, 48-hour-old L. pictus embryos were microdissected and it was demonstrated that the adhesion of the tip of the archenteron to the roof of the blastocoel in vitro is abrogated by treatment with α-mannosidase. These results suggest that terminal mannose residues are involved in the adhesion between the archenteron and blastocoel roof, perhaps through a lectin-like activity that is not sensitive to fixation.

Mechanisms of the epithelial-to-mesenchymal transition in sea urchin embryos

Sea urchin mesenchyme is composed of the large micromere-derived spiculogenetic primary mesenchyme cells (PMC), veg2-tier macromere-derived non-spiculogenetic mesenchyme cells, the small micromere-derived germ cells, and the macro- and mesomere-derived neuronal mesenchyme cells. They are formed through the epithelial-to-mesenchymal transition (EMT) and possess multipotency, except PMCs that solely differentiate larval spicules. The process of EMT is associated with modification of epithelial cell surface property that includes loss of affinity to the apical and basal extracellular matrices, inter-epithelial cell adherens junctions and epithelial cell surface-specific proteins. These cell surface structures and molecules are endocytosed during EMT and utilized as initiators of cytoplasmic signaling pathways that often initiate protein phosphorylation to activate the gene regulatory networks. Acquisition of cell motility after EMT in these mesenchyme cells is associated with the expression of proteins such as Lefty, Snail and Seawi. Structural simplicity and genomic database of this model will further promote detailed EMT research.

The serine-threonine protein kinase PAK4 is dispensable in zebrafish: identification of a morpholino-generated pseudophenotype

TALEN-based inactivation of the zebrafish pak4 gene resulted in embryos and adult fish that appear normal and fertile. This is in contrast to our previously published studies which were based on the use of antisense morpholino oligonucleotides (MOs). We have excluded potential explanations such as gene duplication, alternate splicing, cryptic initiation of translation, and translation-independent RNA function. Our conclusion is that pak4 is dispensable in zebrafish, and that even when corroborated by robust controls, such as RNA rescue, MOs may elicit misleading pseudophenotypes that do not correspond to results obtained by genetic mutations, and should thus be used with caution.