Pertussis toxin inhibits 1-methyladenine-induced maturation in starfish oocytes

Evidence for the involvement of a pertussis toxin-insensitive G-protein in egg activation of the frog, Xenopus laevis

Activation responses of the frog egg at fertilization include the release of calcium from intracellular stores and the opening of calcium-dependent chloride channels, which produce the fertilization potential. To investigate the presence of guanine nucleotide-binding proteins (G-proteins), and their role in initiation of these events in the egg of the frog Xenopus laevis, we assayed for pertussis and cholera toxin substrates, and applied activators and inhibitors of G-proteins. Pertussis toxin catalyzed the [32P]ADP ribosylation of a Mr 40,000 component, but no cholera toxin substrates were demonstrated. Injection of greater than or equal to 25 pmole of guanosine-5'-O-(3-thiotriphosphate) GTP-gamma-S), an activator of G-proteins, produced a change in membrane potential that mimicked the fertilization potential and also caused cortical granule exocytosis and cortical contraction. Injections of up to 600 pmole of guanosine 3':5'-cyclic monophosphate or 9 nmole of guanosine-5'-(beta-gamma-imido)triphosphate did not active eggs. The membrane potential response to GTP-gamma-S injection showed the same peak and chloride dependence as the fertilization potential, although the duration of the GTP-gamma-S response was somewhat greater. GTP-gamma-S did not activate eggs if the calcium rise was prevented by prior injection of the calcium chelator BAPTA. Injection of up to 200 ng of cholera toxin did not activate eggs. However, eggs were activated by applying 1 nM serotonin to eggs that had been injected with a specific mRNA for the serotonin 1c receptor, a member of the class of receptors that act by way of G-proteins. Egg activation in response to either sperm or serotonin was not inhibited by pertussis toxin, under experimental conditions where approximately 80-90% of the toxin substrate was ADP-ribosylated. These results support the hypothesis that sperm activate Xenopus eggs at fertilization by way of a pertussis and cholera toxin-insensitive G-protein.

1-Methyladenine can consistently induce a fura-detectable transient calcium increase which is neither necessary nor sufficient for maturation in oocytes of the starfish Asterina miniata

In starfish oocytes a calcium transient can consistently be detected with the dye fura-2 in response to perfusion of threshold levels of 1-methyladenine, the hormone responsible for induction of meiotic maturation. The calcium transient cannot be detected when the hormone is allowed to slowly diffuse to the oocyte, and the hormone-induced calcium transient can be inhibited by preperfusion of the oocyte with subthreshold levels of hormone. The calcium transient is shown to be unnecessary for maturation by eliciting maturation in situations that are not associated with a calcium transient, and the calcium transient is shown to be insufficient for maturation by eliciting a calcium transient and washing the hormone off the oocytes before the end of the hormone-dependent period. A mechanism is suggested to explain why some investigators have detected transient calcium rises during induction of oocyte maturation while others have not.

A review of echinoderm oogenesis

This review of the anatomical, histological, biochemical, and molecular biological literature on echinoderm oogenesis includes the entire developmental history of oocytes; from their inception to the time they become ova. This is done from a comparative perspective, with reference to members of the five extant echinoderm classes; crinoids, holothurians, asteroids, ophiuroids, and echinoids. I describe the anatomy and fine structure of the echinoderm ovary, with emphasis on both the cellular relationships of the germ line cells to the somatic cells of the inner epithelium, and on the neuromuscular systems. I review the literature on the growth of oogonia into fully formed oocytes, including the process of vitellogenesis, presenting an ultrastructural analysis of the organelles and extracellular structures found in fully formed echinoderm oocytes. Echinoderm oocyte maturation is reviewed and a description of the ultrastructural, biochemical and molecular biological changes thought to occur during this process is presented. Finally, I discuss oocyte ovulation, the severing of cellular connections between the oocyte and its surrounding somatic epithelial cells.