Steroid and peptide control mechanisms in membrane of Xenopus laevis oocytes resuming meiotic division

Role of phosphatidylinositide metabolism in ras-induced Xenopus oocyte maturation

Microinjection of Xenopus oocytes with ras protein (p21) was used to investigate the role of phospholipid metabolism in ras-induced meiotic maturation. Induction of meiosis by ras was compared with induction by progesterone, insulin, and the phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA). Neomycin, which specifically binds to phosphatidylinositides and inhibits their metabolism, blocked meiotic maturation induced by ras or insulin but not by progesterone or TPA. In addition, p21 and TPA, but not insulin or progesterone, stimulated the incorporation of 32Pi into oocyte lipids. ras protein specifically stimulated 32P incorporation into phosphatidylinositides, whereas both ras and TPA stimulated 32P incorporation into phosphatidylcholine and phosphatidylethanolamine. The stimulatory effect of p21 on phosphatidylinositide metabolism correlated with the dose response and kinetics of ras-induced meiotic maturation. In addition, the ras oncogene protein was more potent than the proto-oncogene protein both in inducing meiotic maturation and in stimulating phosphatidylinositide metabolism. These results indicate that phosphatidylinositide turnover is required for ras-induced meiosis and suggest that phosphatidylinositide-derived second messengers mediate the biological activity of ras in Xenopus oocytes.

Role of phosphatidylinositide metabolism in ras-induced Xenopus oocyte maturation

Microinjection of Xenopus oocytes with ras protein (p21) was used to investigate the role of phospholipid metabolism in ras-induced meiotic maturation. Induction of meiosis by ras was compared with induction by progesterone, insulin, and the phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA). Neomycin, which specifically binds to phosphatidylinositides and inhibits their metabolism, blocked meiotic maturation induced by ras or insulin but not by progesterone or TPA. In addition, p21 and TPA, but not insulin or progesterone, stimulated the incorporation of 32Pi into oocyte lipids. ras protein specifically stimulated 32P incorporation into phosphatidylinositides, whereas both ras and TPA stimulated 32P incorporation into phosphatidylcholine and phosphatidylethanolamine. The stimulatory effect of p21 on phosphatidylinositide metabolism correlated with the dose response and kinetics of ras-induced meiotic maturation. In addition, the ras oncogene protein was more potent than the proto-oncogene protein both in inducing meiotic maturation and in stimulating phosphatidylinositide metabolism. These results indicate that phosphatidylinositide turnover is required for ras-induced meiosis and suggest that phosphatidylinositide-derived second messengers mediate the biological activity of ras in Xenopus oocytes.

Efficient processing and expression of human nerve growth factor receptors in Xenopus laevis oocytes: effects on maturation

The nerve growth factor (NGF) receptor is an integral membrane protein that is phosphorylated and heavily glycosylated. Determination of the amino acid sequence by molecular cloning indicates that the receptor is a cysteine-rich protein which contains a signal peptide sequence and spans the lipid bilayer with a single transmembrane sequence. A single mRNA of 3.8 kilobases was observed for the receptor, of which 1.5 kilobases is coding sequence. We have used microinjection of receptor RNA in Xenopus laevis oocytes to obtain cell surface expression of the receptor. The presence of NGF receptors in oocytes was verified by radioimmunoassay, specific binding of [125I]NGF, and metabolic labeling followed by immunoprecipitation. The NGF receptor protein was rapidly processed in oocytes and displayed extensive glycosylation. Furthermore, the presence of NGF receptors in oocytes potentiates the ability of progesterone to induce maturation.

Efficient processing and expression of human nerve growth factor receptors in Xenopus laevis oocytes: effects on maturation

The nerve growth factor (NGF) receptor is an integral membrane protein that is phosphorylated and heavily glycosylated. Determination of the amino acid sequence by molecular cloning indicates that the receptor is a cysteine-rich protein which contains a signal peptide sequence and spans the lipid bilayer with a single transmembrane sequence. A single mRNA of 3.8 kilobases was observed for the receptor, of which 1.5 kilobases is coding sequence. We have used microinjection of receptor RNA in Xenopus laevis oocytes to obtain cell surface expression of the receptor. The presence of NGF receptors in oocytes was verified by radioimmunoassay, specific binding of [125I]NGF, and metabolic labeling followed by immunoprecipitation. The NGF receptor protein was rapidly processed in oocytes and displayed extensive glycosylation. Furthermore, the presence of NGF receptors in oocytes potentiates the ability of progesterone to induce maturation.

Progesterone receptor characterized by photoaffinity labelling in the plasma membrane of Xenopus laevis oocytes

R 5020 (17,21-dimethyl-19-nor-4,9-pregnadiene-3,20-dione) is a synthetic analogue of progesterone, which is the physiological hormone that reinitiates germinal vesicle breakdown in Xenopus laevis oocytes. U.v.-driven photoaffinity labelling experiments were conducted with [3H]R 5020 in oocyte subcellular fractions, and covalently bound radioactivity was analysed by sodium dodecyl sulphate/polyacrylamide-gel electrophoresis. In P-10000 (the pellet sedimenting between 1000 and 10000 g and which contains plasma membrane), a major radioactive band migrating as a 30kDa peptide was found. Non-radioactive progesterone competed with the [3H]R 5020 labelling of this fraction, but not with the labelling of minor [3H]R 5020-binding fractions. It displayed the required characteristics of a specific progesterone-binding membrane 'receptor', postulated from previous studies with intact oocytes and with cell-free P-10000 preparations of membrane-bound adenylate cyclase. The apparent Ki of approx. 4 microM for progesterone was compatible with the active concentration of the hormone. Binding specificity, as determined in competition studies, was highly correlated with the germinal vesicle breakdown activity of the steroids and analogues tested. The receptor was not found in the vitelline envelope, in vitelline platelets, in melanosome-enriched or microsomal fractions, in cytosol, nor in germinal vesicles of oocytes. The properties of this membrane steroid receptor are different from those of the already known soluble intracellular steroid receptors, in particular regarding ligand binding specificity and subcellular distribution.