Identification of PLCγ-Dependent and -Independent Events during Fertilization of Sea Urchin Eggs

AbstractAt fertilization, sea urchin eggs undergo a series of activation events, including a Ca2+action potential, Ca2+release from the endoplasmic reticulum, an increase in intracellular pH, sperm pronuclear formation, MAP kinase dephosphorylation, and DNA synthesis. To examine which of these events might be initiated by activation of phospholipase Cγ (PLCγ), which produces the second messengers inositol trisphosphate (IP3) and diacylglycerol, we used recombinant SH2 domains of PLCγ as specific inhibitors. Sea urchin eggs were co-injected with a GST fusion protein composed of the two tandem SH2 domains of bovine PLCγ and (1) Ca2+green dextran to monitor intracellular free Ca2+, (2) BCECF dextran to monitor intracellular pH, (3) Oregon Green dUTP to monitor DNA synthesis, or (4) fluorescein 70-kDa dextran to monitor nuclear envelope formation. Microinjection of the tandem SH2 domains of PLCγ produced a concentration-dependent inhibition of Ca2+release and also inhibited cortical granule exocytosis, cytoplasmic alkalinization, MAP kinase dephosphorylation, DNA synthesis, and cleavage after fertilization. However, the Ca2+action potential, sperm entry, and sperm pronuclear formation were not prevented by injection of the PLCγSH2 domain protein. Microinjection of a control protein, the tandem SH2 domains of the phosphatase SHP2, had no effect on Ca2+release, cortical granule exocytosis, DNA synthesis, or cleavage. Specificity of the inhibitory action of the PLCγSH2 domains was further indicated by the finding that microinjection of PLCγSH2 domains that had been point mutated at a critical arginine did not inhibit Ca release at fertilization. Additionally, Ca2+release in response to microinjection of IP3, cholera toxin, cADP ribose, or cGMP was not inhibited by the PLCγSH2 fusion protein. These results indicate that PLCγ plays a key role in several fertilization events in sea urchin eggs, including Ca2+release and DNA synthesis, but that the action potential, sperm entry, and male pronuclear formation can occur in the absence of PLCγ activation or Ca2+increase

A functional genomic and proteomic perspective of sea urchin calcium signaling and egg activation

AbstractThe sea urchin egg has a rich history of contributions to our understanding of fundamental questions of egg activation at fertilization. Within seconds of sperm–egg interaction, calcium is released from the egg endoplasmic reticulum, launching the zygote into the mitotic cell cycle and the developmental program. The sequence of the Strongylocentrotus purpuratus genome offers unique opportunities to apply functional genomic and proteomic approaches to investigate the repertoire and regulation of Ca2+ signaling and homeostasis modules present in the egg and zygote. The sea urchin “calcium toolkit” as predicted by the genome is described. Emphasis is on the Ca2+ signaling modules operating during egg activation, but the Ca2+ signaling repertoire has ramifications for later developmental events and adult physiology as well. Presented here are the mechanisms that control the initial release of Ca2+ at fertilization and additional signaling components predicted by the genome and found to be expressed and operating in eggs at fertilization. The initial release of Ca2+ serves to coordinate egg activation, which is largely a phenomenon of post-translational modifications, especially dynamic protein phosphorylation. Functional proteomics can now be used to identify the phosphoproteome in general and specific kinase targets in particular. This approach is described along with findings to date. Key outstanding questions regarding the activation of the developmental program are framed in the context of what has been learned from the genome and how this knowledge can be applied to functional studies

The sea urchin kinome: A first look

AbstractThis paper reports a preliminary in silico analysis of the sea urchin kinome. The predicted protein kinases in the sea urchin genome were identified, annotated and classified, according to both function and kinase domain taxonomy. The results show that the sea urchin kinome, consisting of 353 protein kinases, is closer to the Drosophila kinome (239) than the human kinome (518) with respect to total kinase number. However, the diversity of sea urchin kinases is surprisingly similar to humans, since the urchin kinome is missing only 4 of 186 human subfamilies, while Drosophila lacks 24. Thus, the sea urchin kinome combines the simplicity of a non-duplicated genome with the diversity of function and signaling previously considered to be vertebrate-specific. More than half of the sea urchin kinases are involved with signal transduction, and approximately 88% of the signaling kinases are expressed in the developing embryo. These results support the strength of this nonchordate deuterostome as a pivotal developmental and evolutionary model organism

Surface Localization of the Sea Urchin Egg Receptor for Sperm

AbstractThe sea urchin egg receptor for sperm is thought to be involved in species-specific sperm–egg interactions at the egg surface. Recent revisions in the deduced amino acid sequence of the cloned cDNAs indicate that the protein encoded does not possess the common structural hallmarks of a membrane protein. Thus, investigation of the localization and association of the protein with the egg surface is crucial. We describe and characterize a new monoclonal antibody raised against recombinant sperm receptor protein. This antibody, in conjunction with several polyclonal antibodies, was used to study the receptor protein in eggs. Immunoprecipitation studies indicated that the antibodies recognize the highMr(ca. 350 K) sperm receptor protein which copurified with egg plasma membrane–vitelline layer complexes. The sperm receptor protein was solubilized only by detergents and not by treatments designed to solubilize peripherally associated or lipid-anchored membrane proteins, suggesting a tight association with the membrane fraction. Confocal immunofluorescence microscopy of live eggs indicated surface staining. Finally, lysylendoproteinase C treatment of live eggs resulted in a loss of the highMrreceptor protein epitopes, and the concomitant release of a 70-kDa proteolytic fragment, which correlated with a reduced ability of the eggs to be fertilized. Taken together, these data indicate that at least some fraction of the sperm receptor protein is present on the egg surface, a requisite locale for a sperm binding protein

A functional genomic and proteomic perspective of sea urchin calcium signaling and egg activation.

The sea urchin egg has a rich history of contributions to our understanding of fundamental questions of egg activation at fertilization. Within seconds of sperm-egg interaction, calcium is released from the egg endoplasmic reticulum, launching the zygote into the mitotic cell cycle and the developmental program. The sequence of the Strongylocentrotus purpuratus genome offers unique opportunities to apply functional genomic and proteomic approaches to investigate the repertoire and regulation of Ca(2+) signaling and homeostasis modules present in the egg and zygote. The sea urchin "calcium toolkit" as predicted by the genome is described. Emphasis is on the Ca(2+) signaling modules operating during egg activation, but the Ca(2+) signaling repertoire has ramifications for later developmental events and adult physiology as well. Presented here are the mechanisms that control the initial release of Ca(2+) at fertilization and additional signaling components predicted by the genome and found to be expressed and operating in eggs at fertilization. The initial release of Ca(2+) serves to coordinate egg activation, which is largely a phenomenon of post-translational modifications, especially dynamic protein phosphorylation. Functional proteomics can now be used to identify the phosphoproteome in general and specific kinase targets in particular. This approach is described along with findings to date. Key outstanding questions regarding the activation of the developmental program are framed in the context of what has been learned from the genome and how this knowledge can be applied to functional studies.</p

The genome of the sea urchin Strongylocentrotus purpuratus

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