Biochemical changes during progesterone-induced maturation in Xenopus laevis oocytes

Transcription factor E2F and cyclin E-Cdk2 complex cooperate to induce chromosomal DNA replication in Xenopus oocytes

Although no chromosomal DNA replication actually occurs during Xenopus oocyte maturation, the capability develops during the late meiosis I (MI) phase in response to progesterone. This ability, however, is suppressed by Mos proteins and maturation/mitosis promoting factor during the second meiosis phase (meiosis II; MII) until fertilization. Inhibition of RNA synthesis by actinomycin D during early MI prevented induction of the replication ability, but did not interfere with initiation of the meiotic cell cycle progression characterized by oscillation of the maturation/mitosis promoting factor activity and germinal vesicle breakdown. Microinjection of recombinant proteins such as dominant-negative E2F or universal Cdk inhibitors, p21 and p27, but not wild type human E2F-1 or Cdk4-specific inhibitor, p19, into maturing oocytes during MI abolished induction of the DNA replication ability. Co-injection of human E2F-1 and cyclin E proteins into immature oocytes allowed them to initiate DNA replication even in the absence of progesterone treatment. Injection of cyclin E alone, which was sufficient to activate endogenous Cdk2 kinase, failed to induce DNA replication. Moreover, the activation of Cdk2 was not affected under the conditions where DNA replication was blocked by actinomycin D. Thus, like somatic cells, both activities of E2F and cyclin E-Cdk2 complex are required for induction of the DNA replication ability in maturing Xenopus oocytes, and enhancement of both activities enables oocytes to override DNA-replication inhibitory mechanisms that specifically lie in maturing oocytes.

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.

Role of protein kinase C activation in oocyte maturation and steroidogenesis in ovarian follicles of Rana pipiens: studies with phorbol 12-myristate 13-acetate

In ovarian follicles of Rana pipiens, frog pituitary homogenates (FPH) elevate intrafollicular progesterone levels which in turn is thought to induce meiotic resumption in the prophase I arrested oocytes. Calcium plays a role in FPH and steroid-provoked responses in the somatic and gametic components of the follicle, presumably via effects exerted at the plasma membrane of their respective target cells. Many membrane active hormones which utilize Ca2+ in their intracellular transduction also provoke membrane phosphoinositide hydrolysis yielding inositol triphosphate (IP3) and diacyl glycerol (DAG), an activator of the CA2+-dependent protein kinase C (PKC). The actions of phorbol 12-myristate 13-acetate (TPA), a potent synthetic activator of PKC, on progesterone production and oocyte maturation was examined in in vitro cultured ovarian follicles. TPA induced germinal vesicle breakdown (GVBD) in intact follicles and in oocytes denuded of somatic components, while the inactive compound phorbol 13-monoacetate was ineffective. Further, TPA induction of GVBD exhibited similarities to progesterone-induced GVBD, being inhibited by treatments which elevate cAMP or inhibit protein synthesis. TPA alone did not elevate intrafollicular or medium progesterone levels, as occurred in FPH-treated follicles. TPA partially inhibited intrafollicular progesterone accumulation induced by FPH or treatments which elevate cAMP levels. These data suggest that activation of PKC plays a role in oocyte maturation independent of follicular progesterone production as occurs in response to FPH. Further, it appears that the somatic cells of the amphibian follicle also possess PKC which when activated, antagonizes cAMP generating pathway in these cells. Results indicate that protein kinase can influence oocyte maturation in Rana follicular oocytes by several mechanisms.

Cyclic AMP levels during the maturation of Xenopus oocytes

While several studies have suggested that the induction of oocyte maturation results from a transient decrease in cAMP levels, attempts to demonstrate such a change have led to inconsistent results with respect to whether or not a decrease occurs as well as timing of the decrease. In this report the results of experiments designed to demonstrate small changes in cAMP content in Xenopus laevis oocytes are presented and a statistically significant 20% decrease in cAMP content from between 2 and 50 min postprogesterone addition is found. The cAMP content subsequently rises to 12% higher than control levels and then becomes indistinguishable from control values for the remainder of the maturation period.

Mitochondrial DNA synthesis in overmatured eggs of the starfish

DNA synthesis was studied during the overmaturational stage of starfish eggs. Some DNA synthesis took place in the overmature eggs of the starfish. The DNA synthesis was resistant to aphidicolin. Judging from its circularity and small molecular size, the DNA synthesized is of mitochondrial origin.

Specific binding of progesterone to the cell surface and its role in the meiotic divisions in Rana oocytes

Progesterone is believed to act at the cell surface to induce the resumption of the meiotic divisions in amphibian oocytes. Analysis of [3H]- and [14C] progesterone uptake and exchange by the plasma-vitelline membrane complex, nucleus and cytoplasm of the isolated Rana oocyte indicates that progesterone uptake by the plasma membrane is saturable, specific and temperature-dependent, and has a slow off-rate. Estradiol (a noninducer) did not compete with progesterone, whereas testosterone (an inducer) blocked progesterone uptake by the membrane complex. Scatchard-type plots indicate an apparent Kd of 5.1.10-7 M over the [progesterone]0 range of 0.01-1.0 microM with maximum binding at about 70 fmol per oocyte. Membrane uptake at higher [progesterone]0 (2-40 microM) indicates apparent cooperative binding, with saturation up to 10 pmol per oocyte. Cytoplasmic uptake was apparently nonspecific and less temperature-dependent than membrane uptake and steroid concentrations (progesterone and pregnanediones) exceeded water solubility by 30-60 min. Nuclear uptake was saturable and specific but uptake was independent of temperature. A comparison of membrane binding and a physiological response (nuclear breakdown) indicated only about 10% of the membrane sites need be filled to initiate a 50% response.

The origin of yolk-DNA in Xenopus laevis

Xenopus laevis serum and plasma was found to contain an average of 25 microgram DNA/ml. Isolated X. laevis oocytes incubated in medium containing 25 microgram DNA/ml labeled with either 125I, 32P or 14C and from three different sources (bovine, E. coli and X. laevis), incorporated the label at an average rate of 0.11 ng.mm-2.hr-1. Sucrose gradient fractionation of oocytes revealed that 40-75% of the acid-precipitable label incorporated was associated with the yolk platelets. Additional incubations of oocytes in unlabeled medium demonstrated that the DNA incorporated into the yolk platelets was undergoing turnover; only 20% of the yolk-associated DNA was still present after a one-week incubation. Our data suggest that yolk-DNA arises by the adventitious uptake of DNA present in the maternal serum by vitellogenic oocytes.

Studies on the relative roles of pituitary and progesterone in the induction of meiotic maturation in the amphibian oocyte

In the amphibian, gonadotropins act on the epithelial cells surrounding the oocyte to produce and/or release progesterone which in turn acts at the oocyte surface to initiate the resumption of meiotic maturation. Since maturation is reported to require continuous exposure to gonadotropins but only brief (5--15 min) exposure to progesterone, it was of interest to reexamine the interrelationships between the two hormonal stimuli as well as the kinetics of progesterone production, metabolism, and biological activity. Germinal vesicle breakdown (GVBD) required continuous exposure to 0.005 pituitaries/ml for 6.0 h to produce 50% GVBD that occurred at 10.1 h. Actinomycin D (10 microgram/ml) completely inhibited pituitary induced GVBD when added during the first 5--6 h; 50% inhibition occurred when added at 7.3 h. Thus, actinomycin D continues to inhibit 1--2 h after the requirement for pituitary stimulation. Pituitary stimulation produced a 4-fold increase in 3H-acetate incorporation into progesterone and acetate conversion to progesterone was relatively constant during successive 2 h pulses throughout the 10 h period leading to GVBD. There was no significant metabolism of 3H-acetate derived progesterone when follicles were treated with pituitary extract, although the same follicles rapidly concentrated and metabolized exogenous 3H- or 14C-progesterone. The length of continuous progesterone exposure required for 50% GVBD varied from 11 h at 0.158 microM to less than 0.08 h at 15.8 microM. The time to 50% GVBD was only delayed by about 10% (1.5 h) when maximal and minimal progesterone levels were compared. A comparison of 3H-progesterone uptake and response (GVBD) as a function of [progesterone]0 indicated that uptake of 2--4 mumol 1(-1) cell water will induce 50% GVBD. These results indicate that a threshold uptake must be reached to initiate resumption of meiosis and that this level must be maintained throughout the period leading to nuclear breakdown. Under physiologic conditions, gonadotropins stimulate progesterone production and this progesterone is protected from, or inaccessible to, steroid metabolizing enzymes.

Primary action of steroid hormone at the surface of amphibian oocyte in the induction of germinal vesicle breakdown

An insoluble steroid derivative was prepared by the coupling of desoxycorticosterone with aminoethylated agarose beads. When the naked full-grown Xenopus oocytes were incubated in contact with the steroid-bound agarose beads, the dissociation of oocyte nuclei or the initial step in the maturation of amphibian oocyte was induced in 30-100% of total oocytes. The induction did not occur in the oocytes covered with follicle cells and when the naked oocytes were placed apart from the steroid-bound agarose beads in the medium. The above findings confirmed that the oocyte surface was a primary site for this particular steroid action, eliminating the possibility of participation of free steroid artifactually released from the agarose. The 105,000g supernatant fraction of oocytes showed no sign of the presence of steroid receptor. This was not inconsistent with the assumption mentioned above.

Early steroid metabolism in Xenopus laevis, Rana temporaria and Triturus vulgaris embryos

Embryos of Xenopus laevis, Rana temporaria and Triturus vulgaris exposed to radioactive pregnenolone have been found to convert it to progesterone. Incubations with radioactive progesterone showed that it was actively metabolized by oocytes and embryos. In Xenopus incubations progesterone was converted to 5alpha-pregnane-3,20-dione, 17alpha-hydroxy-4-pregnen-3-one, 4-androstene-3,17-dione and 17alpha-20alpha-dihydroxy-4-pregne-3-one, indicating 5alpha-reductase, 17alpha-hydroxylase, 19-20-desmolase and 20alpha-hydroxylase activities. In oocytes of Triturus and Rana no evidence of 19-20-desmolase was found. In Rana oocytes were also not evidence of 17alpha-hydroxylase activity. All identified activities except 20alpha-hydroxylase were common to embryos of all three species. It is suggested that the steroid enzyme activities present in the embryos are not solely derived from the oocytes but synthentized during early development. Possible meaning of this kind of metabolism during differentiation remains open.

Stimulation of poly(adenosine diphosphate ribose) synthase activity of Xenopus germinal vesicle by progesterone

A practical procedure for the isolation of massive numbers of GV from oocytes of Xenopus laevis at various stages of oogenesis was developed. The method is simple, rapid, and easy to perform. The isolated GV possess high activity of poly(ADP-ribose) synthase. Incubation of class-A oocytes (stages V and VI) with progesterone resulted in a stimulation of the poly(ADP-ribose) synthase activity in the GV. The stimulation of enzymatic activity occurred prior to GVBD. This stimulatory effect of progesterone on enzymatic activity was blocked by cycloheximide and actinomycin D, suggesting that induction is dependent on protein and RNA synthesis. Progesterone, however, was unable to cause disintegration of GV or to stimulate poly(ADP-ribose) synthase activity of class-B oocytes (stages III and IV). This finding suggests that the oocytes must progress to a certain stage of differentiation before progesterone can induce GVBD or stimulate poly(ADP-ribose) synthase activity.

Induction of oocytic maturation and differentiation: mode of progesterone action

Full-grown amphibian oocytes, arrested in prophase I or meiosis, respond in vitro to progesterone and certain other steroids. They undergo an apparently normal sequence of nuclear maturation and cytoplasmic differentiation, which are necessary precedents for fertilization and embryogenesis. Individual oocytes or populations of these cells thus provide a model system for investigations concerning the nature and mechanism of hormone-cellular interactions. In this system, previous studies have shown that certain aspects of hormonal (progesterone, DOCA) induction of morphologic and biochemical differentiation in the nucleus can be induced in part, as a result of the formation of secondary cytoplasmic factors, some of which do not require the presence of the nucleus for their formation. In addition to initiating nuclear events, progesterone or DOCA alters the functional activity of the plasma membrane and establishes the conditions necessary for fertilization and activation. Uptake of radioactive vitellogenin, a yolk protein precursor, was inhibited by progesterone and DOCA. Maximum inhibition was dependent on induction of nuclear breakdown, the dose of steroid used, and was correlated with morphologic alterations at the oocytic surface. Estrone neither stimulated nor inhibited vitellogenin incorporation and had no effect on nuclear breakdown. Oocytic capacity to exhibit activation responses (vitelline membrane elevation) was dependent on oocytic exposure to progesterone or DOCA and development subsequent to the initiation of nuclear breakdown. Onset of the activation response after steroid treatment varied with the type of activation stimulus utilized (pricking or divalent ionophore A-23187). The results suggest that hormones cause ionic alterations in oocytes and that ions are directly involved in the activation response. To study steroid interaction with the cell surface, a method was developed for culturing oocytes that permits localized application of steroids to portions of the oocytic or follicular surface. The results obtained suggest that oocytes exposed to steroid over part of their surface do not respond, with regard to nuclear breakdown, in the same manner as do oocytes exposed over their entire surface to similar concentrations of steroid. Studies of isotopic distribution within the oocyte after local application of steroid indicate that hormone does not readily diffuse through the oocyte. Evidence for the role of cytoplasmic factors in the mediation of nuclear and cytoplasmic events is discussed.

Induction of maturation (meiosis) in small Xenopus laevis oocytes by injection of maturation promoting factor

Treatment of small-(stage III) or medium-sized (stage IV) Xenopus laevis oocytes with progesterone, human chorionic gonadotropin, or para-hydroxymercuriphenylsulfonate does not induce maturation. Only the full-grown oocytes (stage VI) undergo maturation when treated with either one of these three substances. In contrast, injection of maturation promoting factor into oocytes of stages III-VI invariably leads to chromosome condensation and germinal vesicle breakdown. No maturation spindle is found in oocytes smaller than 0.9 nm in diameter, and the nuclear sap does not mix with the cytoplasm in the smallest (0.45-0.55 mm in diameter) oocytes. In oocytes of 0.9 mm, maturation is identical to that of full-grown oocytes, except that the maturation spindle does not reach the cortex of the oocyte. Progesterone increases protein synthesis in medium-sized (0.8 mm in diameter) oocytes without inducing meiosis. It has little or no effect on protein synthesis in smaller oocytes.

Comparison of exogenous energy sources for in vitro maintenance of follicle cell-free Xenopus laevis oocytes

The purpose of these experiments was to determine which exogenous energy sources are suitable for isolated follicle cell-free oocytes from the frog, Xenopus laevis. In order to compare prospective energy sources, follicle cell-free oocytes from 0.4 to 1.3 mm in diameter were incubated in a 1 mM concentration of each of a variety of energy sources and scored daily for the maintenance of morphological characteristics. Vitellogenic oocytes placed in succinate or fumarate deteriorated at the same time as those in saline alone. Oocytes incubated in oxaloacetate (OAA) appeared to remain in the best morphological condition, followed by oocytes maintained in pyruvate or glucose. Fully grown oocytes were tested at various times of incubation for their ability to respond to progesterone by undergoing germinal vesicle breakdown. These experiments showed that oocytes placed in OAA or pyruvate retained the ability to respond to progesterone longer than those in the other energy sources. Increased respiratory rates were stimulated in isolated oocyte mitochondria by succinate as well as pyruvate and OAA. However, oocytes incubated in labelled pyruvate evolved 80 to 140 times as much labelled CO2 as oocytes incubated in labelled glucose or succinate. In addition, it was found that the rate of uptake of pyruvate is 20 to 25 times greater than the rate of uptake of glucose or succinate. It is concluded from these experiments that OAA and pyruvate are the most effective exogenous energy source for the in vitro maintenance of Xenopus oocytes. On possible explanation for the ineffectiveness of glucose or succinate as exogenous energy sources is a restriction in their uptake into the oocytes.

Nature of progesterone action on amino acid uptake by isolated full-grown oocyte of Xenopus laevis

L-leucine uptake into full-grown oocytes of Xenopus laevis is a saturable process which is Na+ dependent and presumably coupled to Na+ gradient. Our results indicate that progesterone (10(-6) M) Blocks abruptly, around the germinal vesicle breakdown, the saturable transport of L-leucine. p-Chloromercuribenzoate (10(-4) M) induces maturation and after a short lag of time strongly inhibits L-leucine uptake. Cycloheximide prevents progesterone-induced maturation and permeability changes.

Studies on maturation in Xenopus laevis oocytes. III. Energy production and requirements for protein synthesis

The following results were obtained: 1) The oxygen comsumption of progesterone-stimulated X. laevis oocytes increases at the time of germinal vesicle (GV) breakdown. 2) Continous treatment with 1 mM KCN, 1 mM and 0.1 mM DNP completely inhibits GV breakdown. 3) Pretreatment experiments with KCN, DNP and cycloheximide show that binding of progesterone to the plasma membrane and the specific hormone receptor requires neither energy, nor protein synthesis. 4) 1 h pulses of DNP (1mM) or cycloheximide (50 mug/ml) were applied, at various time intervals after progesterone treatment: early pulses strongly delay, but do not prevent GV breakdown; late pulses just before GV breakdown induce a number of cytological abnormalities, which ultimately lead to cytolysis. The significance of these results is discussed and the hypothesis that synthesis of the maturation producing factor (MPF) requires both energy and protein synthesis is proposed.

Induction of maturation (meiosis) in Xenopus laevis oocytes by three organomercurials

Three organomercurials, p-hydroxymercuribenzoate, p-hydroxymercuriphenylsulfonate, and mersalyl, induce maturation (meiosis) in a large percentage (20-100 percent) of Xenopus laevis oocytes. Maturation takes place even when the follicle cells which surround the oocytes have been withdrawn. Organomercurial- and progesterone-induced maturations have many features in common: they do not occur when the inducer is injected into the oocytes, they require the presence of Ca++ in the medium, they are inhibited by cycloheximide but not by actinomycin D. In both cases, the maturation producing factor and the pseudomaturation inducing factor are produced. Organomercurial-treated oocytes react normally to activating stimuli; their protein synthesis increases, but uptake of amino acids is strongly inhibited. Progesterone and p-hydroxymercuriphenyl-sulfonate act synergically in inducing maturation. The main difference between the two agents is that p-hydroxymercuriphenylsulfonate must act for several hours, whereas, short contact with progesterone is sufficient to induce maturation.