Ca²⁺ influx-linked protein kinase C activity regulates the β-catenin localization, micromere induction signalling and the oral-aboral axis formation in early sea urchin embryos

Micromere formation and its evolutionary implications in the sea urchin

The micromeres of the sea urchin embryo are distinct from other blastomeres. After they arise through an asymmetric cell division at the 8- to 16-cell stage, micromeres immediately function as organizers. They also commit themselves to specific cell fates such as larval skeletogenic cells and primordial germ cells, while other blastomeres remain plastic and uncommitted at the 16-cell stage. In the phylum Echinodermata, only the sea urchin (class Echinoidea) embryo forms micromeres that serve as apparent organizers during early embryogenesis. Therefore, it is considered that micromeres are the derived features and that modification(s) of the developmental system allowed evolutionary introduction of this unique cell lineage. In this chapter, we summarize the both historic and recent observations that demonstrate unique properties of micromeres and discuss how this lineage of micromeres may have arisen during echinoderm evolution.

Distinct transcriptional regulation of Nanos2 in the germ line and soma by the Wnt and delta/notch pathways

Specification of the primordial germ cells (PGCs) is essential for sexually reproducing animals. Although the mechanisms of PGC specification are diverse between organisms, the RNA binding protein Nanos is consistently required in the germ line in all species tested. How Nanos is selectively expressed in the germ line, however, remains largely elusive. We report that in sea urchin embryos, the early expression of Nanos2 in the PGCs requires the maternal Wnt pathway. During gastrulation, however, Nanos2 expression expands into adjacent somatic mesodermal cells and this secondary Nanos expression instead requires Delta/Notch signaling through the forkhead family member FoxY. Each of these transcriptional regulators were tested by chromatin immunoprecipitation analysis and found to directly interact with a DNA locus upstream of Nanos2. Given the conserved importance of Nanos in germ line specification, and the derived character of the micromeres and small micromeres in the sea urchin, we propose that the ancestral mechanism of Nanos2 expression in echinoderms was by induction in mesodermal cells during gastrulation.

New insights into negative effects of lithium on sea urchin Paracentrotus lividus embryos

The diffuse use of lithium in a number of industrial processes has produced a significant contamination of groundwater and surface water with it. The increased use of lithium has generated only scarce studies on its concentrations in ambient waters and on its effects on aquatic organisms. Only few contributions have focused on the toxicity of lithium in marine organisms (such as marine animals, algae and vegetables), showing that the toxic effect depends on the animal species. In the present study we describe the morphological and the molecular effects of lithium chloride (LiCl), using the sea urchin Paracentrotus lividus as a model organism. We show that LiCl, if added to the eggs before fertilization, induces malformations in the embryos in a dose-dependent manner. We have also followed by RT qPCR the expression levels of thirty seven genes (belonging to different classes of functional processes, such as stress, development, differentiation, skeletogenesis and detoxifications) to identify the molecular targets of LiCl. This study opens new perspectives for the understanding of the mechanism of action of lithium on marine organisms. The findings may also have relevance outside the world of marine organisms since lithium is widely prescribed for the treatment of human bipolar disorders.

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.