Ca(2+) in specification of vegetal cell fate in early sea urchin embryos

In sea urchin embryos, the first specification of cell fate occurs at the fourth cleavage, when small cells (the micromeres) are formed at the vegetal pole. The fate of other blastomeres is dependent on the receipt of cell signals originating from the micromeres. The micromeres are fated to become skeletogenic cells and show the ability to induce the endoderm (the archenteron) in the neighbouring cells during the 16- to 60-cell stage. Several molecules involved in signaling pathways, i.e. Notch for mesoderm specification, bone morphogenic protein (BMP) for ectoderm specification and beta-catenin for endoderm specification, are spatially and temporally expressed during development. In the micromeres, beta-catenin increases and subsequently localizes to the nuclei under the regulation of TCF, a nuclear binding partner of beta-catenin, until the 60-cell stage. However, the mechanisms activating these signaling substances are still unclear. In this article, I demonstrate some specific properties of the membrane and cytoplasm of micromeres including new findings on intracellular Ca(2+) concentration, and propose a mechanism by which the functional micromeres are autonoumously formed. The possible roles of these in the specification of vegetal cell fate in early development are discussed.

Growth factors reverse developmental arrest by zinc in embryos of the sea urchin Hemicentrotus pulcherrimus

During sea urchin development, zinc (Zn) ions produce developmentally arrested embryos called permanent blastulae. In this study, the effects of growth factors on Zn-arrested embryos were examined to elucidate the relation between Zn arrest and morphogenesis that is mediated by growth factors. Embryos treated with Zn maintained their spherical form for a few days in the absence of growth factors. When the culture medium was supplemented with horse serum, gastrulation occurred in the Zn-arrested embryos, some of which developed into symmetrical larvae closely resembling normal plutei. The growth factors basic fibroblast growth factor (bFGF), TGF-β1, TGF-β2, and activin A also induced recovery of the Zn-arrested embryos. Of the four growth factors tested, activin showed the most intense activity and allowed development of a radial larva with a thick ciliated band and a partially invaginated archenteron. Histochemical staining with ALPase demonstrated the formation of a morphologically and functionally mature digestive tract in the activin-treated embryos. Immunofluorescent staining showed that spicules formed within the msp130-positive mesenchyme cells in the activin-treated embryos. Reversal of Zn arrest did not occur in embryos cultured with serum albumin. These results suggest that Zn arrests development by inhibiting the initiation of morphogenesis and this is mediated by peptide growth factors.

Proximal cis-regulatory elements of sea urchin arylsulfatase gene

Expression of the arylsulfatase (HpArs) gene in the sea urchin, Hemicentrotus pulcherrimus, is regulated in spatially, as well as temporally, during development. To address the cis-regulatory elements involved in this regulation, we performed reporter assays using variously deleted or mutated promoter and regulatory elements of the HpArs gene, accompanied by gel mobility shift assay and foot printing. Results show that two regions, PU1 (-72 b.p. to -56 b.p.), which is similar to SpZ12-1 and/or Oct-1 motif, and the PD1 site (+133 b.p. to +142 b.p.), which is homologous to the binding sites of Rel family transcription factors and/or AGIE-BP1, are related to the regulation of expression of the HpArs gene. Furthermore, an HpArs enhancer element called C15, which is located 3 kb.p. downstream from the transcription start site, activates the HpArs promoter. We also report that the enhancer activity of the C15 fragment was mediated by elements, PU1 and PD1.