Mechanisms of Xenopus oocyte maturation

The endogenous cannabinoid anandamide inhibits cromakalim-activated K+ currents in follicle-enclosed Xenopus oocytes

The effect of the endogenous cannabinoid anandamide on K(+) currents activated by the ATP-sensitive potassium (K(ATP)) channel opener cromakalim was investigated in follicle-enclosed Xenopus oocytes using the two-electrode voltage-clamp technique. Anandamide (1-90 microM) reversibly inhibited cromakalim-induced K(+) currents, with an IC(50) value of 8.1 +/- 2 microM. Inhibition was noncompetitive and independent of membrane potential. Coapplication of anandamide with the cannabinoid type 1 (CB(1)) receptor antagonist N-(piperidin-1-yl)-5-(4-chlorophenyl)-1-(2,4-dichlorophenyl)-4-methyl-1H-pyrazole-3-carboximide hydrochloride (SR 141716A) (1 microM), the CB(2) receptor antagonist N-[(1S)endo-1,3,3-trimethyl bicyclo heptan-2-yl]-5-(4-chloro-3-methylphenyl)-1-(4-methylbenzyl)-pyrazole-3-carboxamide (SR144528) (1 microM), or pertussis toxin (5 microg/ml) did not alter the inhibitory effect of anandamide, suggesting that known cannabinoid receptors are not involved in anandamide inhibition of K(+) currents. Similarly, neither the amidohydrolase inhibitor phenylmethylsulfonyl fluoride (0.2 mM) nor the cyclooxygenase inhibitor indomethacin (5 microM) affected anandamide inhibition of K(+) currents, suggesting that the effects of anandamide are not mediated by its metabolic products. In radioligand binding studies, anandamide inhibited the specific binding of the K(ATP) ligand [(3)H]glibenclamide in the oocyte microsomal fractions, with an IC(50) value of 6.3 +/- 0.4 microM. Gonadotropin-induced oocyte maturation and the cromakalim-acceleration of progesterone-induced oocyte maturation were significantly inhibited in the presence of 10 microM anandamide. Collectively, these results indicate that cromakalim-activated K(+) currents in follicular cells of Xenopus oocytes are modulated by anandamide via a cannabinoid receptor-independent mechanism and that the inhibition of these channels by anandamide alters the responsiveness of oocytes to gonadotropin and progesterone.

Histone H3 phosphorylation during Xenopus oocyte maturation: regulation by the MAP kinase/p90Rsk pathway and uncoupling from DNA condensation

Here we show that during the meiotic maturation of Xenopus oocytes, histone H3 becomes phosphorylated on serine-10 at about the time of maturation promoting factor activation and meiosis I entry. However, overexpression of cAMP-dependent protein kinase that blocks entry into M phase, also leads to massive serine-10 phosphorylation of histone H3 in intact Xenopus oocytes but does not cause chromosome condensation. We also show that the phosphorylation of histone H3 during oocyte maturation requires the activation of the mitogen-activated protein kinase/p90Rsk pathway. Our results indicate that in G2-arrested oocytes, which are about to enter M phase, histone H3 phosphorylation is not sufficient for chromosome condensation.

New B-type cyclin synthesis is required between meiosis I and II duringXenopusoocyte maturation

Progression through meiosis requires two waves of maturation promoting factor (MPF) activity corresponding to meiosis I and meiosis II. Frog oocytes contain a pool of inactive ‘pre-MPF’ consisting of cyclin-dependent kinase 1 bound to B-type cyclins, of which we now find three previously unsuspected members, cyclins B3, B4 and B5. Protein synthesis is required to activate pre-MPF, and we show here that this does not require new B-type cyclin synthesis, probably because of a large maternal stockpile of cyclins B2 and B5. This stockpile is degraded after meiosis I and consequently, the activation of MPF for meiosis II requires new cyclin synthesis, principally of cyclins B1 and B4, whose translation is strongly activated after meiosis I. If this wave of new cyclin synthesis is ablated by antisense oligonucleotides, the oocytes degenerate and fail to form a second meiotic spindle. The effects on meiotic progression are even more severe when all new protein synthesis is blocked by cycloheximide added after meiosis I, but can be rescued by injection of indestructible B-type cyclins. B-type cyclins and MPF activity are required to maintain c-mos and MAP kinase activity during meiosis II, and to establish the metaphase arrest at the end of meiotic maturation. We discuss the interdependence of c-mos and MPF, and reveal an important role for translational control of cyclin synthesis between the two meiotic divisions.

Progesterone treatment abolishes exogenously expressed ionic currents in Xenopus oocytes

Fully grown oocytes of Xenopus laevis undergo resumption of the meiotic cycle when treated with the steroid hormone progesterone. Previous studies have shown that meiotic maturation results in profound downregulation of specific endogenous membrane proteins in oocytes. To determine whether the maturation impacts the functional properties of exogenously expressed membrane proteins, we used cut-open recordings from Xenopus oocytes expressing several types of Na(+) and K(+) channels. Treatment of oocytes with progesterone resulted in a downregulation of heterologously expressed Na(+) and K(+) channels without a change in the kinetics of the currents. The time course of progesterone-induced ion channel inhibition was concentration dependent. Complete elimination of Na(+) currents temporally coincided with development of germinal vesicle breakdown, while elimination of K(+) currents was delayed by approximately 2 h. Coexpression of human beta(1)-subunit with rat skeletal muscle alpha-subunit in Xenopus oocytes did not prevent progesterone-induced downregulation of Na(+) channels. Addition of 8-bromo-cAMP to oocytes or injection of heparin before progesterone treatment prevented the loss of expressed currents. Pharmacological studies suggest that the inhibitory effects of progesterone on expressed Na(+) and K(+) channels occur downstream of the activation of cdc2 kinase. The loss of channels is correlated with a reduction in Na(+) channel immunofluorescence, pointing to a disappearance of the ion channel-forming proteins from the surface membrane.

Identification of XPR-1, a progesterone receptor required for Xenopus oocyte activation

Quiescent full-grown Xenopus oocytes remain arrested at the G(2)/M border of meiosis I until exposed to progesterone, their natural mitogen. Progesterone triggers rapid, nontranscriptional responses that lead to the translational activation of stored mRNAs, resumption of the meiotic cell cycles, and maturation of the oocyte into a fertilizable egg. It has long been presumed that progesterone activates the oocyte through a novel nontranscriptional signaling receptor. Here, we provide evidence that a conventional transcriptional progesterone receptor cloned from Xenopus oocytes, XPR-1, is required for oocyte activation. Overexpression of XPR-1 through mRNA injection increases sensitivity to progesterone and accelerates progesterone-activated cell cycle reentry. Injection of XPR-1 antisense oligonucleotides blocks the ability of oocytes to respond to progesterone; these oocytes are rescued by subsequent injection of XPR-1 or the human progesterone receptor PR-B. Antisense-treated oocytes can be activated in response to inhibition of protein kinase A, one of the earliest known changes occurring downstream of progesterone stimulation. These results argue that the conventional progesterone receptor also functions as the signaling receptor that is responsible for the rapid nontranscriptional activation of frog oocytes.

The activation of MAP kinase and p34cdc2/cyclin B during the meiotic maturation of Xenopus oocytes

G2-arrested Xenopus oocytes are induced to enter M-phase of meiosis by progesterone stimulation. This process, known as meiotic maturation, requires the activation of p34cdc2/cyclin B complexes (pre-MPF) which is brought about by the prior translation of specific maternal mRNAs stored in the oocyte. One of these mRNAs encodes for the protein kinase Mos which has an essential role in oocyte maturation, most likely due to its ability to activate MAP kinase (MAPK). Here we review our current knowledge on the Mos/MAPK signalling pathway and a recently found connection between MAPK-activated p90rsk and the p34cdc2 inhibitory kinase Myt1. We also discuss a pathway that involves the protein kinase Plx1 and leads to the activation of the phosphatase Cdc25, as well as other regulators of p34cdc2/cyclin B activity which may have a role in oocyte maturation.

Unusual phosphatase activity resistant to SDS and pronase treatments in Xenopus ovary

An unusual phosphatase, which is resistant to treatment by 5% SDS and proteolytic enzymes, was isolated as two types, 1 and 2, from pronase-treated homogenates of Xenopus ovary. The molecular sizes of types 1 and 2 were estimated as about 140 kDa and more than 2 x 10(4) kDa, respectively, by gel filtration, but as 140 and 130 kDa as a catalytic unit, respectively, by electrophoresis, implying that whereas type 1 might be composed of catalytic unit alone, type 2 is a multicomponent complex consisting of a 130-kDa catalytic unit. Both activities were sensitive to nucleases but resistant to tested proteolytic enzymes. These findings suggest that the unusual phosphatase activity is attributable to a polynucleotide.

A novel p34(cdc2)-binding and activating protein that is necessary and sufficient to trigger G(2)/M progression in Xenopus oocytes

The activation of maturation-promoting factor (MPF) is required for G(2)/M progression in eukaryotic cells. Xenopus oocytes are arrested in G(2) and are induced to enter M phase of meiosis by progesterone stimulation. This process is known as meiotic maturation and requires the translation of specific maternal mRNAs stored in the oocytes. We have used an expression cloning strategy to functionally identify proteins involved in G(2)/M progression in Xenopus oocytes. Here we report the cloning of two novel cDNAs that when expressed in oocytes induce meiotic maturation efficiently. The two cDNAs encode proteins of 33 kD that are 88% identical and have no significant homologies to other sequences in databases. These proteins, which we refer to as p33(ringo) (rapid inducer of G(2)/M progression in oocytes), induce very rapid MPF activation in cycloheximide-treated oocytes. Conversely, ablation of endogenous p33(ringo) mRNAs using antisense oligonucleotides inhibits progesterone-induced maturation, suggesting that synthesis of p33(ringo) is required for this process. We also show that p33(ringo) binds to and activates the kinase activity of p34(cdc2) but does not associate with p34(cdc2)/cyclin B complexes. Our results identify a novel p34(cdc2) binding and activating protein that regulates the G(2)/M transition during oocyte maturation.

Cell cycle-related changes in the conducting properties of r-eag K+ channels

Release from arrest in G2 phase of the cell cycle causes profound changes in rat ether-à-go-go (r-eag) K+ channels heterologously expressed in Xenopus oocytes. The most evident consequence of the onset of maturation is the appearance of rectification in the r-eag current. The trigger for these changes is located downstream of the activation of mitosis-promoting factor (MPF). We demonstrate here that the rectification is due to a voltage-dependent block by intracellular Na+ ions. Manipulation of the intracellular Na+ concentration indicates that the site of Na+ block is located approximately 45% into the electrical distance of the pore and is only present in oocytes undergoing maturation. Since the currents through excised patches from immature oocytes exhibited a fast rundown, we studied CHO-K1 cells permanently transfected with r-eag. These cells displayed currents with a variable degree of block by Na+ and variable permeability to Cs+. Partial synchronization of the cultures in G0/G1 or M phases of the cell cycle greatly reduced the variability. The combined data obtained from mammalian cells and oocytes strongly suggest that the permeability properties of r-eag K+ channels are modulated during cell cycle-related processes.

Proteolytic inactivation of MAP-kinase-kinase by anthrax lethal factor

Anthrax lethal toxin, produced by the bacterium Bacillus anthracis, is the major cause of death in animals infected with anthrax. One component of this toxin, lethal factor (LF), is suspected to be a metalloprotease, but no physiological substrates have been identified. Here it is shown that LF is a protease that cleaves the amino terminus of mitogen-activated protein kinase kinases 1 and 2 (MAPKK1 and MAPKK2) and that this cleavage inactivates MAPKK1 and inhibits the MAPK signal transduction pathway. The identification of a cleavage site for LF may facilitate the development of LF inhibitors.