G-protein betagamma subunit-dependent phosphorylation of 62-kDa protein in the early signaling pathway of starfish oocyte maturation induced by 1-methyladenine

Oocyte Maturation in Starfish

Oocyte maturation is a process that occurs in the ovaries, where an immature oocyte resumes meiosis to attain competence for normal fertilization after ovulation/spawning. In starfish, the hormone 1-methyladenine binds to an unidentified receptor on the plasma membrane of oocytes, inducing a conformational change in the heterotrimeric GTP-binding protein α-subunit (Gα), so that the α-subunit binds GTP in exchange of GDP on the plasma membrane. The GTP-binding protein βγ-subunit (Gβγ) is released from Gα, and the released Gβγ activates phosphatidylinositol-3 kinase (PI3K), followed by the target of rapamycin kinase complex2 (TORC2) and 3-phosphoinositide-dependent protein kinase 1 (PDK1)-dependent phosphorylation of serum- and glucocorticoid-regulated kinase (SGK) of ovarian oocytes. Thereafter, SGK activates Na⁺/H⁺ exchanger (NHE) to increase the intracellular pH (pHi) from ~6.7 to ~6.9. Moreover, SGK phosphorylates Cdc25 and Myt1, thereby inducing the de-phosphorylation and activation of cyclin B–Cdk1, causing germinal vesicle breakdown (GVBD). Both pHi increase and GVBD are required for spindle assembly at metaphase I, followed by MI arrest at pHi 6.9 until spawning. Due to MI arrest or SGK-dependent pHi control, spawned oocytes can be fertilized normally

Steroid-induced oocyte maturation in Atlantic croaker (Micropogonias undulatus) is dependent on activation of the phosphatidylinositol 3-kinase/Akt signal transduction pathway

Exposure of fully grown fish and amphibian oocytes to a maturation-inducing steroid (MIS) activates numerous signal transduction pathways to initiate the final stage of oocyte maturation. These events culminate in the activation of maturation-promoting factor and germinal vesicle breakdown (GVBD). In most species, exposure to MIS causes a transient decrease in oocyte cAMP levels. Whether this reduction in oocyte cAMP concentration is sufficient to induce GVBD is unclear. The current study tested the hypothesis that activation of cAMP-independent signal transduction pathways by the naturally occurring MIS, 17,20beta,21-trihydroxy-4-pregnen-3-one (20beta-S), is necessary for GVBD in Atlantic croaker (Micropogonias undulatus) oocytes. Results indicate that although 20beta-S treatment of oocyte membranes significantly reduced cAMP production, incubation of follicles with the cell-permeable cAMP-dependent protein kinase (Prka) inhibitors Rp-cAMP or KT5720 did not promote GVBD in the absence of 20beta-S. Additionally, treatment of follicles with the phosphodiesterase (Pde) inhibitors Cilostamide (Pde3) or Rolipram (Pde4) significantly reduced GVBD, but they were not able to completely block it. In contrast, pharmacologic inhibition of the cAMP-independent phosphatidylinositol 3-kinase (Pik3)/Akt signal transduction pathway using the Pik3 inhibitors Wortmannin or LY294002, or the Akt inhibitor ML-9, blocked 20beta-S-induced GVBD. Finally, mitogen-activated protein kinase (Mapk1/3) activity increased after treatment with 20beta-S; however, inhibition of Mapk1/3 activity using PD98059 or U0126 had no effect on GVBD. These results demonstrate that activation of cAMP-independent signaling pathways, especially the Pik3/Akt pathway, is necessary for 20beta-S-induced GVBD in Atlantic croaker oocytes.

Signaling pathways in ascidian oocyte maturation: Effects of various inhibitors and activators on germinal vesicle breakdown

The Ascidiacea, the invertebrate chordates, includes three orders; the Stolidobranchia is the most complex. Until the present study, the onset of oocyte maturation (germinal vesicle breakdown) had been investigated in only a single pyurid (Halocynthia roretzi), in which germinal vesicle breakdown (GVBD) begins when the oocyte contacts seawater (SW); nothing was known about internal events. This study strongly suggests the importance of protein phosphorylation in this process. Herdmania pallida (Pyuridae) functions like H. roretzi; GVBD occurs in SW. Oocytes of Cnemidocarpa irene (Styelidae) do not spontaneously undergo GVBD in SW but must be activated. Herdmania oocytes are inhibited from GVBD by pH 4 SW and subsequently activated by mastoparan (G-protein activator), A23187 (Ca2+ ionophore) or dimethylbenzanthracene (tyrosine kinase activator). This requires maturation promoting factor (MPF) activity; cyclin-dependent kinase inhibitors roscovitine and olomoucine are inhibitory. It also entails dephosphorylation as demonstrated by the ability of the phosphatase inhibitor vitamin K3 to inhibit GVBD. GVBD is also inhibited by the tyrosine kinase inhibitors tyrphostin A23 and genistein, and LY-294002, a phosphatidylinositol-3-kinase inhibitor previously shown to inhibit starfish GVBD. LY-294002 inhibits strongly when activation is by mastoparan or ionophore but not when activated by dimethylbenzanthracene (DMBA). The DMBA is hypothesized to phosphorylate a phosphatase directly or indirectly causing secondary activation, bypassing inhibition.

Cell surface receptors activate p21-activated kinase 1 via multiple Ras and PI3-kinase-dependent pathways

p21-activated kinases (PAKs) were the first identified mammalian members of a growing family of Ste20-like serine-threonine protein kinases. In this study, we show that PAK1 can be stimulated by carbachol, lysophosphatidic acid (LPA), epidermal growth factor (EGF), and phorbol 12-myristate 13-acetate (PMA) by multiple independent and overlapping pathways. Dominant-negative Ras, Rac, and Cdc42 inhibited PAK1 activation by all of these agonists, while active Rac1 and Cdc42 were sufficient to maximally activate PAK1 in the absence of any treatment. Active Ras induced only a weak activation of PAK1 that could be potentiated by muscarinic receptor stimulation. Studies using inhibitors of the EGF receptor tyrosine kinase, phosphatidylinositol 3-kinase (PI3-kinase) and protein kinase C (PKC) revealed that all of the cell surface agonists could activate PAK1 through pathways independent of PKC, that EGF stimulated a PI3-kinase dependent pathway to stimulate PAK1, and that muscarinic receptor stimulation of PAK1 was predominantly mediated through this EGF-R-dependent mechanism. Activation of PAK1 by LPA was independent of PI3-kinase and the EGF receptor, but was inhibited by dominant-negative RhoA. These results identify multiple Ras-dependent pathways to activation of PAK1.

Signal transduction pathways that contribute to CDK1/cyclin B activation during the first mitotic division in sea urchin embryos

In sea urchins, fertilization triggers a rapid rise in protein synthesis necessary for activation of CDK1/cyclin B, the universal cell cycle regulator. It has been shown that FRAP/mTOR is required for eIF4E release from the translational repressor 4E-BP, a process that occurs upstream of de novo cyclin B synthesis. Here, we investigate whether PI 3-kinase acts independently or upstream from FRAP/mTOR in the signal transduction pathway that links fertilization to the activation of the CDK1/cyclin B complex in sea urchin egg. We found that wortmannin, a potent inhibitor of PI 3-kinase, partially inhibited the global increase in protein synthesis triggered by fertilization. Furthermore, wortmannin treatment induced partial inhibition of cyclin B translation triggered by fertilization, in correlation with an intermediate effect of the drug on 4E-BP degradation and on the dissociation of the 4E-BP/eIF4E complex induced by fertilization. Our results presented here suggest that PI 3-kinase activity is required for completion of mitotic divisions of the sea urchin embryo. Incubation of eggs with wortmannin or microinjection of wortmannin or LY 294002 affects drastically mitotic divisions induced by fertilization. In addition, we found that wortmannin treatment inhibits dephosphorylation of the tyrosine inhibitory site of CDK1. Taken together, these data suggest that PI 3-kinase acts upstream of at least two independent targets that function in the CDK1/cyclin B activation triggered by fertilization of sea urchin oocytes. We discuss the significance of these results concerning the cascade of reactions that impinge upon the activation of the CDK1/cyclin B complex that follows sea urchin oocyte fertilization.

Degradation of Polyubiquitinated Cyclin B Is Blocked by the MAPK Pathway at the Metaphase I Arrest in Starfish Oocytes

In the starfish ovary, maturing oocytes stimulated by 1-methyladenine undergo synchronous germinal vesicle breakdown and then arrest in metaphase of the first meiotic division (metaphase I). Immediately after spawning, an increase of intracellular pH (pH(i)) from approximately 7.0 to approximately 7.3 is induced by Na(+)/H(+) antiporter in oocytes, and meiosis reinitiation occurs. Here we show that an endogenous substrate of the proteasome, polyubiquitinated cyclin B, was stable at pH 7.0, whereas it was degraded at pH 7.3. When the MAPK pathway was blocked by MEK inhibitor U0126, degradation of polyubiquitinated cyclin B occurred even at pH 7.0 without an increase of the peptidase activity of the proteasome. These results indicate that the proteasome activity at pH 7.0 is sufficient for degradation of polyubiquitinated cyclin B and that the MAPK pathway blocks the degradation of polyubiquitinated cyclin B in the maturing oocytes in the ovary. Immediately after spawning, the increase in pH(i) mediated by Na(+)/H(+) antiporter cancels the inhibitory effects of the MAPK pathway, resulting in the degradation of polyubiquitinated cyclin B and the release of the arrest. Thus, the key step of metaphase I arrest in starfish oocytes occurs after the polyubiqutination of cyclin B but before cyclin B proteolysis by the proteasome.

Metaphase I arrest of starfish oocytes induced via the MAP kinase pathway is released by an increase of intracellular pH

Reinitiation of meiosis in oocytes usually occurs as a two-step process during which release from the prophase block is followed by an arrest in metaphase of the first or second meiotic division [metaphase I (MI) or metaphase II (MII)]. The mechanism of MI arrest in meiosis is poorly understood, although it is a widely observed phenomenon in invertebrates. The blockage of fully grown starfish oocytes in prophase of meiosis I is released by the hormone 1-methyladenine. It has been believed that meiosis of starfish oocytes proceeds completely without MI or MII arrest, even when fertilization does not occur. Here we show that MI arrest of starfish oocytes occurs in the ovary after germinal vesicle breakdown. This arrest is maintained both by the Mos/MEK/MAP kinase pathway and the blockage of an increase of intracellular pH in the ovary before spawning. Immediately after spawning into seawater, activation of Na+/H+ antiporters via a heterotrimeric G protein coupling to a 1-methyladenine receptor in the oocyte leads to an intracellular pH increase that can overcome the MI arrest even in the presence of active MAP kinase.

Fertilization blocks apoptosis of starfish eggs by inactivation of the MAP kinase pathway

Fully grown starfish oocytes are arrested at prophase of meiosis I. The hormonal stimulation of 1-methyladenine (1-MA) induces meiosis reinitiation and germinal vesicle breakdown (GVBD). Optimal development occurs when maturing oocytes are fertilized between GVBD and first polar body emission. In the absence of sperm, oocytes complete both meiotic divisions to yield haploid interphase-arrested eggs. We now report that spontaneous and synchronous activation of caspase-3 in starfish eggs occurs 9-12 h after 1-MA stimulation. Then, caspase-dependent membrane blebbing and egg fragmentation occur, indicating that mature eggs undergo apoptosis if not fertilized. Activation of caspase-3 and induction of apoptosis are blocked both by a MEK inhibitor and by emetine treatment which inhibits MEK kinase (Mos) synthesis. Conversely, when recombinant GST-Mos is injected into the emetine-treated eggs, apoptosis is induced. These results indicate that persistent activation of the Mos/MEK/MAP kinase cascade gives the death-activating signal in starfish eggs. Fertilization inactivates the MAP kinase pathway and suppresses apoptosis, followed by normal development.

Regulation of Xenopus oocyte meiosis arrest by G protein betagamma subunits

BACKGROUND

Progesterone induces the resumption of meiosis (maturation) in Xenopus oocytes through a nongenomic mechanism involving inhibition of an oocyte adenylyl cyclase and reduction of intracellular cAMP. However, progesterone action in Xenopus oocytes is not blocked by pertussis toxin, and this finding indicates that the inhibition of the oocyte adenylyl cyclase is not mediated by the alpha subunits of classical G(i)-type G proteins.

RESULTS

To investigate the possibility that G protein betagamma subunits, rather than alpha subunits, play a key role in regulating oocyte maturation, we have employed two structurally distinct G protein betagamma scavengers (G(t)alpha and betaARK-C(CAAX)) to sequester free Gbetagamma dimers. We demonstrated that the injection of mRNA encoding either of these Gbetagamma scavengers induced oocyte maturation. The Gbetagamma scavengers bound an endogenous, membrane-associated Gbeta subunit, indistinguishable from Xenopus Gbeta1 derived from mRNA injection. The injection of Xenopus Gbeta1 mRNA, together with bovine Ggamma2 mRNA, elevated oocyte cAMP levels and inhibited progesterone-induced oocyte maturation.

CONCLUSION

An endogenous G protein betagamma dimer, likely including Xenopus Gbeta1, is responsible for maintaining oocyte meiosis arrest. Resumption of meiosis is induced by Gbetagamma scavengers in vitro or, naturally, by progesterone via a mechanism that suppresses the release of Gbetagamma.