Changes in protein association with intracellular membranes of Xenopus laevis oocytes during maturation and activation

Maintenance of meiotic prophase arrest in vertebrate oocytes by a G s protein-mediated pathway

Maintenance of meiotic prophase arrest in fully grown vertebrate oocytes depends on an elevated level of cAMP in the oocyte. To investigate how the cAMP level is regulated, we examined whether the activity of an oocyte G protein of the family that stimulates adenylyl cyclase, Gs, is required to maintain meiotic arrest. Microinjection of a dominant negative form of Gs into Xenopus and mouse oocytes, or microinjection of an antibody that inhibits the Gs G protein into zebrafish oocytes, caused meiosis to resume. Together with previous studies, these results support the conclusion that Gs-regulated generation of cAMP by the oocyte is a common mechanism for maintaining meiotic prophase arrest in vertebrate oocytes.

A receptor linked to a Gi-family G-protein functions in initiating oocyte maturation in starfish but not frogs

The stimulation of oocyte maturation by 1-methyladenine in starfish, and by a steroid in frogs, has been proposed to involve G-protein-coupled receptors. To examine whether activation of receptors linked to G(i) or G(z) was sufficient to cause oocyte maturation, we expressed mammalian G(i)- and G(z)-linked receptors in starfish and frog oocytes. Application of the corresponding agonists caused meiosis to resume in the starfish but not the frog oocytes. We confirmed that the receptors were effectively expressed in the frog oocytes by using a chimeric G-protein, G(qi), that converts input from G(i)- and G(z)-linked receptors to a G(q) output and results in a contraction of the oocyte's pigment. These results argue against G(i) or G(z) functioning to cause maturation in frog oocytes. Consistently, maturation-inducing steroids did not cause pigment contraction in frog oocytes expressing G(qi), and G(z) protein was not detectable in frog oocytes. For starfish oocytes, however, our results support the conclusion that G(i) functions in 1-methyladenine signaling and suggest the possibility of using frog oocyte pigment contraction as an assay to identify the 1-methyladenine receptor. To test this concept, we coexpressed G(qi) and a starfish adenosine receptor in frog oocytes and showed that applying adenosine caused pigment contraction.

Changes in organization of the endoplasmic reticulum during Xenopus oocyte maturation and activation

The organization of the endoplasmic reticulum (ER) in the cortex of Xenopus oocytes was investigated during maturation and activation using a green fluorescent protein chimera, immunofluorescence, and electron microscopy. Dense clusters of ER developed on the vegetal side (the side opposite the meiotic spindle) during maturation. Small clusters appeared transiently at the time of nuclear envelope breakdown, disappeared at the time of first polar body formation, and then reappeared as larger clusters in mature eggs. The appearance of the large ER clusters was correlated with an increase in releasability of Ca(2+) by IP(3). The clusters dispersed during the Ca(2+) wave at activation. Possible relationships of ER structure and Ca(2+) regulation are discussed.

The major soluble tubulins are found in mega dalton (MDa) fractions in fully-grown oocytes and eggs but not in brain of the frog, Rana pipiens

During progesterone-induced oocyte maturation, the cell is converted from prophase I to metaphase II, which is known to involve microtubule changes. We hypothesize that progesterone affects the polymeric state of tubulins, isoform dynamics, as well as tubulin synthesis and degradation. To test the former, leopard frog (Rana pipiens) oocytes were treated with progesterone, homogenized and centrifuged at 16,000 x g. The resulting pellet and supernatant fractions were analyzed using immunoblots probed with anti-alpha-tubulin antibody DM1A and anti-beta-tubulin antibody DM1B. The results indicate that the majority of tubulins are in supernatant fractions (soluble cellular component) and further indicate changes in size of pelletable tubulins with time post-progesterone treatment. To characterize the supernatant tubulins, Superose-6b columns were used to fractionate 16,000 x g supernatant materials. The results indicate tubulins in fractions of up to 5 MDa (equivalent in size to c. 50 tubulin dimers or 5,000 kDa) predominate, while tubulin dimers and monomers of 110 kDa and 55 kDa, respectively, are relatively minor components. The tubulin in MDa fraction pools in the oocyte soluble component seem to be tissue specific, since the reverse patterns were found in either frog brain or rat (Rattus norvegicus) brain tissue under the same conditions. Microtubule poisons, taxol and nocodazole, were used to test the dynamics of tubulin in MDa fractions and the results were opposite to that expected, i.e., taxol, a microtubule stabilizer, decreased tubulin in large MDa fractions while nocodazole, a microtubule destabilizer, increased it. Preliminary data also indicated that progesterone treatment alters the tubulin size classes (size-shift) in comparison with that in immature oocytes. These results, in combination with other research reports, suggest that tubulin in MDa fractions may either be associated with other cellular components or act as intermediates in microtubule dynamics, perhaps via oligomerization, during oocyte maturation and early development.

Calcium-dependent development of secondary cytostatic factor (2 degrees CSF) from Xenopus laevis oocytes and zygotes

Fresh cytosols extracted from unfertilised eggs of Xenopus laevis contain a cytostatic factor (CSF) which arrests the cell cycle at metaphase when microinjected into cleaving blastomeres. This CSF is sensitive to Ca2+ and is designated primary CSF (1 degrees CSF). During storage of Ca2(+)-containing cytosols at 2 degrees C, a stable CSF activity appears which is designated secondary CSF (2 degrees CSF). In the present study, we report that 2 degrees CSF activity can be induced in cytosols extracted from stage VI oocytes, unfertilised eggs, electrically activated eggs or blastulae, in the presence of Ca2+. Both the intensity and the rate of 2 degrees CSF development are dependent on the concentration of Ca2+ ions added to the cytosol. At Ca2+ concentrations of 5-10 mM, 2 degrees CSF activity reaches a maximum in about 7 days. Secondary CSF is relatively resistant to heat but loses all activity after 5 min at 70 degrees C. When stored at-80 degrees C, 2 degrees CSF activity remains detectable for about 6 weeks. Cytological observations show that blastomeres arrested by microinjection of 2 degrees CSF developed in cytosols of unfertilised eggs, activated eggs or blastulae contain metaphase chromosomes embedded in a bipolar spindle that has no asters developed at its poles. In contrast, blastomeres arrested by 2 degrees CSF in cytosols of stage VI oocytes contain condensed chromosomes but no spindle is formed. The mechanisms of the development of 2 degrees CSF in Ca2(+)-containing cytosols and its mode of action are discussed.

The role of microtubules and inositol triphosphate induced Ca2+ release in the tyrosine phosphorylation of mitogen-activated protein kinase in extracts of Xenopus laevis oocytes

Microsomal fractions of Xenopus oocytes release preloaded 45 Ca2+ when treated with inositol triphosphate (InsP3). The effective concentration of InsP3 required for half-maximal release (EC50) is 59 nM and maximal release occurs at approximately 2 microM InsP3. Uptake and release of 45 Ca2+ are not altered by the catalytic subunit of protein kinase A, dibutyrl cyclic adenosine monophosphate, protein kinase A peptide inhibitor or nocodazole. In contrast, taxol decreases the sensitivity of the microsomal fraction to InsP3, shifting the EC50 for InsP3-induced Ca2+ release from 59 to 259 nM. In lysates of oocytes, InsP3-induced Ca2+ release causes the tyrosine phosphorylation of a 42,000 (M(r) 42k) protein identified as 42k mitogen-activated protein (MAP) kinase. InsP3-induced tyrosine phosphorylation of MAP kinase is prevented by BAPTA and taxol, but not by nocodazole. Thus, microtubule polymerisation modifies InsP3-induced Ca2+ release, thereby inhibiting phosphorylation of MAP kinase.

Stimulation of Xenopus oocyte maturation by inhibition of the G-protein alpha S subunit, a component of the plasma membrane and yolk platelet membranes

Oocytes of Xenopus laevis undergo maturation when injected with an affinity-purified antibody against the COOH-terminal decapeptide of the alpha subunit of the G-protein Gs, an antibody that inhibits Gs activity. Germinal vesicle breakdown, chromosome condensation, and polar body formation occur, with a time course similar to that for oocytes treated with progesterone. The alpha S antibody-injected oocytes also acquire the ability to be activated by sperm. Coinjection of the catalytic subunit of cAMP-dependent protein kinase, or incubation with cycloheximide, inhibits maturation in response to injection of the alpha S antibody; these experiments show that the alpha S antibody acts at an early point in the pathway leading to oocyte maturation, before formation of maturation promoting factor, and like progesterone, its action requires protein synthesis. Immunogold electron microscopy shows that alpha S is present in the yolk platelet membranes as well as the plasma membrane. These results support the hypothesis that progesterone acts by inhibiting alpha S, and suggest that the target of progesterone could include yolk platelet membranes as well as the plasma membrane.

Structural changes in the endoplasmic reticulum of starfish oocytes during meiotic maturation and fertilization

The endoplasmic reticulum (ER) of live starfish oocytes was observed during meiotic maturation and fertilization. The ER was visualized by injection into the cytoplasm of an oil drop saturated with the fluorescent lipophilic dye DiI; DiI spread throughout the oocyte endoplasmic reticulum and the pattern was imaged by confocal microscopy. The ER in the immature (germinal vesicle stage) oocyte was composed of interconnected membrane sheets. In response to 1-methyladenine, the sheets of ER appeared to become associated with the yolk platelets, forming spherical shells. A few of these spherical shells could sometimes be seen in immature oocytes, but their number was much greater in the egg at the first meiotic spindle stage. At about the time that the first polar body formed, the spherical shells disappeared, and the ER returned to a form like that of the immature oocyte. The spherical shells did not reappear during the second meiotic cycle. During maturation, the ER also began to move; the movement was apparent by the time of germinal vesicle breakdown and continued throughout both meiotic cycles and in eggs with second polar bodies. When eggs at the first meiotic spindle stage were fertilized, the form of the ER changed. Within 1 min after sperm addition to the observation chamber, the circular cross sections of the spherical shells of the unfertilized egg ER were no longer distinct. At this point, the form of the ER could not be discerned with the resolution of the light microscope; however, the rate of spreading of DiI from an injected oil drop decreased, providing strong evidence that the ER had become fragmented. The ER remained in this form for several minutes and then gradually, the appearance of the ER and the rate of DiI spreading returned to be like those of the unfertilized egg. Injection of inositol trisphosphate caused a similar change in the ER structure. These results indicate that the ER is a dynamic structure, the form of which changes during oocyte maturation and fertilization.