Calcium-mediated inactivation of the MAP kinase pathway in sea urchin eggs at fertilization.

We have evaluated the regulation of a 43-kDa MAP kinase in sea urchin eggs. Both MAP kinase and MEK (MAP kinase kinase) are phosphorylated and active in unfertilized eggs while both are dephosphorylated and inactivated after fertilization, although with distinct kinetics. Reactivation of MEK or the 43-kDa MAP kinase prior to or during the first cell division was not detected. Confocal immunolocalization microscopy revealed that phosphorylated (active) MAP kinase is present primarily in the nucleus of the unfertilized egg, with some of the phosphorylated form in the cytoplasm as well. Incubation of unfertilized eggs in the MEK inhibitor U0126 (0.5 microM) resulted in the inactivation of MEK and MAP kinase within 30 min. Incubation in low concentrations of U0126 (sufficient to inactivate MEK and MAP kinase) after fertilization had no effect on progression through the embryonic cell cycle. Microinjection of active mammalian MAP kinase phosphatase (MKP-3) resulted in inactivation of MAP kinase in unfertilized eggs, as did addition of MKP-3 to lysates of unfertilized eggs. Incubation of unfertilized eggs in the Ca(2+) ionophore A23187 led to inactivation of MEK and MAP kinase with the same kinetics as observed with sperm-induced egg activation. This suggests that calcium may be deactivating MEK and/or activating a MAP kinase-directed phosphatase. A cell-free system was used to evaluate the activation of phosphatase separately from MEK inactivation. Unfertilized egg lysates were treated with U0126 to inactivate MEK and then Ca(2+) was added. This resulted in increased MAP kinase phosphatase activity. Therefore, MAP kinase inactivation at fertilization in sea urchin eggs likely is the result of a combination of MEK inactivation and phosphatase activation that are directly or indirectly responsive to Ca(2+).     

Intracellular Ca2+ increase induces post-fertilization events via MAP kinase dephosphorylation in eggs of the hydrozoan jellyfish Cladonema pacificum

Naturally spawned eggs of the hydrozoan jellyfish Cladonema pacificum are arrested at G1-like pronuclear stage until fertilization. Fertilized eggs of Cladonema undergo a series of post-fertilization events, including loss of sperm-attracting ability, expression of adhesive materials on the egg surface, and initiation of cell cycle leading to DNA synthesis and cleavage. Here, we investigate whether these events are regulated by changes in intracellular Ca2+ concentration and mitogen-activated protein kinase (MAP kinase) activity in Cladonema eggs. We found that MAP kinase is maintained in the phosphorylated form in unfertilized eggs. Initiation of sperm-induced Ca2+ increase, which is the first sign of fertilization, was immediately followed by MAP kinase dephosphorylation within a few minutes of fertilization. The fertilized eggs typically stopped sperm attraction by an additional 5 min and became sticky around this time. They further underwent cytokinesis yielding 2-cell embryos at approximately 1 h post-fertilization, which was preceded by DNA synthesis evidenced by BrdU incorporation into the nuclei. Injection of inositol 1,4,5-trisphosphate (IP3) into unfertilized eggs, which produced a Ca2+ increase similar to that seen at fertilization, triggered MAP kinase dephosphorylation and the above post-fertilization events without insemination. Conversely, injection of BAPTA/Ca2+ into fertilized eggs at approximately 10 s after the initiation of Ca2+ increase immediately lowered the elevating Ca2+ level and inhibited the subsequent post-fertilization events. Treatment with U0126, an inhibitor of MAP kinase kinase (MEK), triggered the post-fertilization events in unfertilized eggs, where MAP kinase dephosphorylation but not Ca2+ increase was generated. Conversely, preinjection of the glutathione S-transferase (GST) fusion protein of MAP kinase kinase kinase (Mos), which maintained the phosphorylated state of MAP kinase, blocked the post-fertilization events in fertilized eggs without preventing a Ca2+ increase. These results strongly suggest that all of the three post-fertilization events, cessation of sperm attraction, expression of surface adhesion, and progression of cell cycle, lie downstream of MAP kinase dephosphorylation that is triggered by a Ca2+ increase.     

The relationship between calcium, MAP kinase, and DNA synthesis in the sea urchin egg at fertilization

Fertilization releases the brake on the cell cycle and the egg completes meiosis and enters into S phase of the mitotic cell cycle. The MAP kinase pathway has been implicated in this process, but the precise role of MAP kinase in meiosis and the first mitotic cell cycle remains unknown and may differ according to species. Unlike the eggs of most animals, sea urchin eggs have completed meiosis prior to fertilization and are arrested at the pronuclear stage. Using both phosphorylation-state-specific antibodies and a MAP kinase activity assay, we observe that MAP kinase is phosphorylated and active in unfertilized sea urchin eggs and then dephosphorylated and inactivated by 15 min postinsemination. Further, Ca(2+) was both sufficient and necessary for this MAP kinase inactivation. Treatment of eggs with the Ca(2+) ionophore A23187 caused MAP kinase inactivation and triggered DNA synthesis. When the rise in intracellular Ca(2+) was inhibited by injection of a chelator, BAPTA or EGTA, the activity of MAP kinase remained high. Finally, inhibition of the MAP kinase signaling pathway by the specific MEK inhibitor PD98059 triggered DNA synthesis in unfertilized eggs. Thus, whenever MAP kinase activity is retained, DNA synthesis is inhibited while inactivation of MAP kinase correlates with initiation of DNA synthesis.     

MAP kinase activity is downregulated by phorbol ester during mouse oocyte maturation and egg activation in vitro

The effects of protein kinase C (PKC) stimulator, phorbol 12-myriatate 13-acetate (PMA), on meiotic cell cycle regulation and mitogen-activated protein (MAP) kinase changes have been studied in mouse oocytes and eggs. The results showed that MAP kinase activation itself was not necessary for germinal vesicle breakdown (GVBD), but the ability of the ooplasm to phosphorylate MAP kinase was a prerequisite for this event. At concentrations of 1.6 nM, PMA effectively inhibited GVBD and MAP kinase activation, suggesting that PMA inhibits GVBD by inhibiting molecule(s) upstream to MAP kinase. At concentrations of 16.2 nM, PMA induced metaphase-interphase transition more effectively in eggs collected 19 hr after human chorionic gonadotropin (hCG) administration than in those collected 15 hr after hCG administration. The degree of MAP kinase activity decrease was well correlated with the time course and proportion of pronuclear formation. On the other hand, when the effect of PMA on cell cycle progression was abolished by protein phosphatase inhibitor, okadaic acid, MAP kinase was superactivated. The biologically inactive 4 alpha-phorbol 12,13-didecanoate (4 alpha-PDD) had no evident effects on either GVBD and interphase transition or on MAP kinase activity. Furthermore, the effects of PMA on oocyte GVBD, egg activation, and MAP kinase activity could be overcome by the specific PKC inhibitor, calphostin C, suggesting the possible involvement of this enzyme in the regulation of MAP kinase activity. The results suggest that activation of PKC by PMA entrains a cascade of events that ultimately inhibits MAP kinase activation and GVBD in mouse oocytes and induces MAP kinase inactivation and metaphase-interphase transition in mouse eggs.     

MAP kinase links the fertilization signal transduction pathway to the G1/S-phase transition in starfish eggs.

The mechanism by which fertilization initiates S-phase in the zygote is examined by manipulating the activity of MAP kinase in mature starfish eggs. These unfertilized eggs, which are arrested at G1-phase after the completion of meiosis, have high MAP kinase activity but undetectable cdc2 kinase activity. Either fertilization or inhibition of protein synthesis causes a decrease in MAP kinase activity, which is followed by DNA synthesis. Inactivation of MAP kinase with its specific phosphatase, CL100, initiates DNA synthesis in the absence of fertilization, while constitutive activation of MAP kinase with MEK represses the initiation of DNA synthesis following fertilization. Thus, in unfertilized mature starfish eggs, a capacity for DNA replication is already acquired, but entry into S-phase is negatively regulated by MAP kinase activity that is supported by a continuously synthesized protein(s) but not by cdc2 kinase. Upon fertilization, downregulation of MAP kinase activity is necessary and sufficient for triggering the G1/S-phase transition.     

Butyrolactone I reversibly inhibits meiotic maturation of bovine oocytes,Without influencing chromosome condensation activity

In this study, butyrolactone I (BL I), a potent and specific inhibitor of cyclin-dependent kinases, was shown to block germinal vesicle (GV) breakdown (GVBD) in bovine oocytes in a concentration-dependent manner; GVBD was almost totally inhibited over the course of 24-48 h of culture when 100 microM BL I was included in tissue culture medium 199 containing either polyvinyl alcohol or BSA. Correlated with this inhibition was the failure of either p34(cdc2) kinase or mitogen-activated protein (MAP) kinase to become activated, and it was unlikely that BL I directly inhibited MAP kinase, since 100 microM BL I did not inhibit MAP kinase activity present in extracts obtained from metaphase II-arrested bovine eggs that possess high levels of MAP kinase activity. Nevertheless, the formation of highly condensed bivalents was observed in 78% of the BL I-treated GV-intact oocytes. This result suggests that chromosome condensation during first meiosis in bovine oocytes does not require the activity of either p34(cdc2) kinase or MAP kinase. Treatment of BL I-arrested oocytes with okadaic acid (OA) did not result in either the activation of p34(cdc2) kinase or MAP kinase, or inducement of GVBD. The BL I-induced block of GVBD for 24 h was reversible, and a subsequent 24-h culture resulted in 90% of oocytes reaching metaphase II with emission of the first polar body. Correlated with the progression to and arrest at metaphase II was the full activation of both p34(cdc2) and MAP kinases. The reversibility after 48 h of culture in BL I was partially decreased when compared to that achieved after an initial 24-h culture. Fertilization in vitro of these eggs resulted in a high incidence of both sperm penetration and pronucleus formation (88% and 70%, respectively).     

The c-mos proto-oncogene protein kinase turns on and maintains the activity of MAP kinase, but not MPF, in cell-free extracts of Xenopus oocytes and eggs.

During studies of the activation and inactivation of the cyclin B-p34cdc2 protein kinase (MPF) in cell-free extracts of Xenopus oocytes and eggs, we found that a bacterially expressed fusion protein between the Escherichia coli maltose-binding protein and the Xenopus c-mos protein kinase (malE-mos) activated a 42 kDa MAP kinase. The activation of MAP kinase on addition of malE-mos was consistent, whereas the activation of MPF was variable and failed to occur in some oocyte extracts in which cyclin A or okadaic acid activated both MPF and MAP kinase. In cases when MPF activation was transient, MAP kinase activity declined after MPF activity was lost, and MAP kinase, but not MPF, could be maintained at a high level by the presence of malE-mos. When intact oocytes were treated with progesterone, however, the activation of MPF and MAP kinase occurred simultaneously, in contrast to the behaviour of extracts. These observations suggest that one role of c-mos may be to maintain high MAP kinase activity in meiosis. They also imply that the activation of MPF and MAP kinase in vivo are synchronous events that normally rely on an agent that has still to be identified.     

Germinal vesicle materials are not required for the activation of MAP kinase in porcine oocyte maturation

The requirement of the germinal vesicle (GV) for the normal kinetics of mitogen-activated protein (MAP) kinase activity during porcine oocyte maturation was investigated. Porcine follicular oocytes were enucleated, and the locations of their extracellular signal-regulated kinases 1 and 2 (ERK1/2), major MAP kinases in maturating porcine oocytes, were detected by indirect immunofluorescent microscopy. The MAP kinase activity was assayed as myelin basic protein (MBP) kinase activity, and the phosphorylation states of ERK1/2 were detected by immunoblotting analyses. Translocation of MAP kinase into the GV and association with the spindle were observed in intact oocytes, while MAP kinase in enucleated oocytes was distributed almost uniformly in cytoplasm throughout the culturing period. The phosphorylation and the activation of MAP kinase were induced, and the activity was comparable with that of control denuded oocytes. The high level of activity was maintained through maturation, even in the absence of spindle formation. These results indicate that the presence of nuclear material and translocation into the GV are dispensable for the activation of MAP kinase and that associating with the spindle is not required for maintenance of its activity though porcine oocyte maturation.     

Effects of MAP kinase pathway and other factors on meiosis of Urechis unicinctus eggs

The eggs of Urechis unicinctus Von Drasche, an echiuroid, are arrested at P-I stage in meiosis. The meiosis is reinitiated by fertilization. Immunoblotting analysis using anti-ERK2 and anti-phospho-MAPK antibodies revealed a 44 kDa MAP kinase species that was constantly expressed in U. unicinctus eggs, quickly phosphorylated after fertilization, and dephosphorylated slowly before the completion of meiosis I. Phosphorylation of the protein was not depressed by protein synthesis inhibitor Cycloheximide (CHX), but was depressed by the MEK1 inhibitor PD98059. Under PD98059 treatment, polar body extrusion was suppressed and the function of centrosome and spindle was abnormal though GVBD was not affected, indicating that MAP kinase cascade was important for meiotic division of U. unicinctus eggs. Other discovery includes: A23187 and OA could parthenogenetically activate U. unicinctus eggs and phosphorylated 44 kDa MAP kinase species, indicating that the effect of fertilization on reinitiating meiosis and phosphorylation of 44 kDa MAP kinase specie is mediated by raising intracellular free calcium and by phosphorylation of some proteins, and that phosphotase(s) sensitive to OA is responsible for arresting U. unicinctus eggs in prophase I. diC8, an activator of PKC, accelerated the process of U. unicinctus egg meiotic division after fertilization and accelerated the dephosphorylation of 44 kDa MAP kinase specie, which implied that the acceleration effect of PKC on meiotic division was mediated by inactivation of MAP kinase cascade. Elevating cAMP/PKA level in U. unicinctus eggs had no effect on meiotic division of the eggs.     

Activation of a Ca2+-inhibitable protein kinase that phosphorylates microtubule-associated protein 2 in vitro by growth factors, phorbol esters, and serum in quiescent cultured human fibroblasts

Treatment of quiescent human embryonic lung fibroblastic cells (TIG-3) with 10 nM epidermal growth factor (EGF) resulted in 4-6-fold activation of a protein kinase activity in cell extracts that phosphorylated microtubule-associated protein 2 (MAP2) on serine and threonine residues in vitro. The half-maximal activation of the kinase activity occurred within 5 min after EGF treatment, and the maximal level was attained at 15 min. Casein and histone were very poor substrates for this EGF-stimulated MAP2 kinase activity. The activation of the kinase activity persisted after brief dialysis. Interestingly, the EGF-stimulated MAP2 kinase activity was sensitive to micromolar concentrations of free Ca2+; it was inhibited 50% by 0.5 microM Ca2+ and almost totally inhibited by 2 microM Ca2+. The activated MAP2 kinase activity was recovered in flow-through fractions on phosphocellulose column chromatography, while kinase activities that phosphorylate 40 S ribosomal protein S6 (S6 kinase activities) were mostly retained on the column and eluted at 0.5 M NaCl. Platelet-derived growth factor, fibroblast growth factor, insulin-like growth factor-I, insulin, phorbol esters (12-O-tetradecanoylphorbol 13-acetate and phorbol 12,13-dibutyrate), and fresh fetal calf serum also induced activation of the MAP2 kinase in the quiescent TIG-3 cells. The activated MAP2 kinase activity in cells stimulated by platelet-derived growth factor, fibroblast growth factor, insulin-like growth factor-I, insulin, 12-O-tetradecanoylphorbol 13-acetate, phorbol 12,13-dibutyrate, or fetal calf serum was almost completely inhibited by 2 microM Ca2+, like the EGF-stimulated kinase. In addition, MAP2 phosphorylated by the kinase activated by different stimuli gave very similar phosphopeptide mapping patterns. These results suggest that several growth factors, phorbol esters, and serum activate a common, Ca2+-inhibitable protein kinase which is distinct from S6 kinase in quiescent human fibroblasts.     

Chromosome condensation in pig oocytes: lack of a requirement for either cdc2 kinase or MAP kinase activity

In this study, butyrolactone I (BL I), a potent and specific inhibitor of cyclin-dependent kinases (cdk), is shown to inhibit germinal vesicle breakdown (GVBD) in pig oocytes. Oocytes treated with 100 microM BL I were arrested in the germinal vesicle (GV)-stage and displayed low activity of cdc2 kinase and MAP kinase. Nevertheless, chromosome condensation occurred and highly condensed bivalents were seen within an intact GV after a 24-hr culture in the presence of BL I. The inhibitory effect of BL I on MAP kinase activation during culture was likely mediated through a cdk-dependent pathway, since MAP kinase activity present in extracts derived from metaphase II eggs was not inhibited by BL I. The block of GVBD could be released by treating oocytes with okadaic acid (OA), an inhibitor of type 1 and 2A phosphatases; 82% of the oocytes treated with the combination of OA/BL I underwent GVBD, and MAP kinase became activated, while cdc2 kinase remained inhibited. These results suggest that both chromosome condensation and GVBD could occur without activation of cdc2 kinase, whereas an increase in MAP kinase activity may be a requisite for GVBD in pig oocytes in conditions when cdc2 kinase activation is blocked by BL I.     

Characterization of a mitogen-activated, Ca2+-sensitive microtubule-associated protein-2 kinase

We have previously found that treatment of quiescent mammalian fibroblast cells with several mitogenic factors activates in common a Ca2+-sensitive serine/threonine-specific protein kinase activity toward microtubule-associated protein 2 (MAP2) [Hoshi, M., Nishida, E. and Sakai, H. (1988) J. Biol. Chem. 263, 5396-5401]. Here, we characterized the mitogen-activated MAP2 kinase activity in rat 3Y1 cells. The activated kinase activity was detected in the cytosolic fraction but not in the membrane fraction. The inhibitory effect of Ca2+ on the kinase activity was reversible. Kinetic analyses revealed that the apparent Km values of the kinase activity for MAP2 and ATP were 1.6 microM and 30 microM, respectively. Free Ca2+ at 4 microM decreased apparent Vmax values for MAP2 and ATP without changing the apparent Km values. The MAP2 kinase had an apparent molecular mass of about 40 kDa as determined by gel filtration and by sucrose density gradient centrifugation. Myelin basic protein as well as MAP2 could serve as good substrates for this kinase, but 40S ribosomal protein S6, casein, histone, phosphorylase b, protamine, tubulin, actin and tau could not. These properties of the enzyme indicate that the Ca2+-sensitive MAP2 kinase may be a previously unidentified enzyme. Down-regulation of protein kinase C by prolonged phorbol ester treatment abolished the MAP2 kinase activation by phorbol ester, but did not prevent the MAP2 kinase activation by epidermal growth factor (EGF) or fresh serum. This suggests that the Ca2+-sensitive MAP2 kinase could be activated through protein-kinase-C-dependent and -independent pathways. Activation of the MAP2 kinase occurred shortly after the addition of EGF or phorbol ester even in the presence of protein synthesis inhibitors (cycloheximide, puromycin and emetin). Moreover, treatment of the EGF- or phorbol-ester-activated MAP2 kinase with acid phosphatase inactivated the kinase activity. Thus, the MAP2 kinase may be activated through phosphorylation.     

MAPKAP kinase 2 is activated by heat shock and TNF-α: In vivo phosphorylation of small heat shock protein results from stimulation of the MAP kinase cascade

The activation of MAPKAP kinase 2 was investigated under heat-shock conditions in mouse Ehrlich ascites tumor cells and after treatment of human MO7 cells with tumor necrosis factor-α (TNF-α). MAPKAP kinase 2 activity was determined using the small heat-shock proteins (sHsps) Hsp25 and Hsp27 as substrates. In both cell types, about a threefold increase in MAPKAP kinase 2 activity could be detected in a time interval of about 10–15 min after stimulation either by heat shock or TNF-α. Phosphorylation of MAPKAP kinase 2, but not the level of MAPKAP kinase 2 mRNA, was increased after heat shock in EAT cells. It is further shown that activation of MAPKAP kinase 2 in MO7 cells is accompanied by increased MAP kinase activity. These data strongly suggest that increased phosphorylation of the sHsps after heat shock or TNF-α treatment results from phosphorylation by MAPKAP kinase 2, which itself is activated by phosphorylation through MAP kinases. Hence, we demonstrate that MAPKAP kinase 2 is responsible not only for phosphorylation of sHsps in vitro but also in vivo. The findings link sHsp phosphorylation to the MAP kinase cascade, explaining the early phosphorylation of sHsp that is stimulated by a variety of inducers such as mitogens, phorbol esters, thrombin, calcium ionophores, and heat shock.     

The requirement of both extracellular regulated kinase and p38 mitogen-activated protein kinase for stimulation of cytosolic phospholipase A(2) activity by either FcgammaRIIA or FcgammaRIIIB in human neutrophils. A possible role for Pyk2 but not for the Grb2-Sos-Shc complex

The signal transduction pathways initiated by opsonized zymosan (OZ) leading to activation of cytosolic phospholipase A(2) (cPLA(2)) in human neutrophils remain obscure. In a previous study, we showed that the activation of cPLA(2) by OZ is tyrosine kinase-dependent. The present study demonstrates that the signals initiated by OZ involve activation of tyrosine kinase Pyk2 but not the formation of the adhesion protein complex, Shc-Grb2-Sos. Stimulation of cPLA(2) activity by OZ is mediated by Fc gamma receptors (FcgammaRs) and not by complement receptors for the C3b protein. Cross-linking of FcgammaRIIA or FcgammaRIIIB induces p38 mitogen-activated protein (MAP) kinase and extracellular regulated kinase (ERK) phosphorylation. The kinetics of cPLA(2) activity stimulated by either of the FcgammaRs or by both is similar to that of p38 MAP kinase and was detected as early as 15 s after stimulation, maintained a plateau for 10 min, and decreased thereafter. ERK activation was detected also within 15 s but decreased significantly 5 min after stimulation. The MEK inhibitor, PD-098059, or the p38 MAP kinase inhibitor, SB-203580, caused a partial inhibition during the time course of cPLA(2) activity, whereas their combination caused a total inhibition. Thus, although ERK activation is significantly shorter than that of p38 MAP kinase, it is equally required for activation and maintenance of cPLA(2) activity by occupancy of a single receptor, FcgammaRIIA or FcgammaRIIIB.     

Nerve growth factor stimulates the tyrosine phosphorylation of MAP2 kinase in PC12 cells.

NGF treatment of PC12 cells results in the rapid activation of MAP2 kinase. We report here that the induction of enzyme activity was correlated with the phosphorylation of MAP2 kinase, detected by metabolic labeling of the enzyme and with anti-phosphotyrosine antibodies. NGF stimulated the phosphorylation of MAP2 kinase on tyrosine, as well as serine and threonine residues. Western blot analysis using a polyclonal anti-phosphotyrosine antibody demonstrated that the tyrosine phosphorylation of MAP2 kinase was maximal within 2 min following NGF exposure and preceded the induction of MAP2 kinase activity. The NGF-stimulated tyrosine phosphorylation of an identified substrate provides direct evidence for the participation of a tyrosine kinase in the mechanism of action of NGF.     

Two distinct forms of MAPKAP kinase-2 in adult cardiac ventricular myocytes

Hsp27 kinase activities were studied in adult rat ventricular myocytes following sequential chromatography on Mono Q and Mono S. A basal level of activity was present following cell isolation. FPLC on Mono Q revealed three peaks of activity, peaks 'a', 'b', and 'c'. A fourth peak, 'd', was detected upon subsequent chromatography of the Mono Q flow-through on Mono S. Immunoblotting revealed that peaks 'a', 'b', and 'c' contained predominantly a 49 kDa form of MAPKAP kinase-2. Peak 'd' contained a 43 kDa form. 'In-gel' kinase assays using hsp27 indicated both forms of MAPKAP kinase-2 were active. No other bands of hsp27 kinase activity were detected. Both forms of hsp27 kinase immunoprecipitated with a MAPKAP kinase-2 antibody and have therefore been named MAPKAP kinase-2alpha (p49) and MAPKAP kinase-2beta (p43). MAPKAP kinase-2beta chromatographed on Superose 12 as a 60.7 kDa monomer whereas the behavior of MAPKAP kinase-2alpha suggested both a 65.7 kDa monomer and higher molecular mass complexes. Both activities phosphorylated hsp27 on serine residues, and two-dimensional phosphopeptide mapping indicated the same sites were phosphorylated. A tumor-promoting phorbol ester, phorbol 12-myristate 13-acetate (PMA), stimulated both MAPKAP kinase-2alpha and MAPKAP kinase-2beta activity. Inhibition of MEK activation with PD 98059 or p38alpha/beta MAP kinase activity with SB203580 blocked activation by PMA. However, whereas PD 98059 inhibited only the PMA-stimulated activation, SB203580 inhibited both PMA-stimulated and basal hsp27 phosphorylation. These data demonstrate the presence of two forms of MAPKAP kinase-2 in adult ventricular myocytes. Both forms are activated indirectly by the ERK MAP kinase pathway and directly by p38 MAP kinase but independently regulated.     

Involvement of MEK-mitogen-activated protein kinase pathway in follicle-stimulating hormone-induced but not spontaneous meiotic resumption of mouse oocytes

Mitogen-activated protein (MAP) kinase has been reported to be activated during oocyte meiotic maturation in a variety of mammalian species. However, the mechanism(s) responsible for MAP kinase activation and the consequence of its premature activation during gonadotropin-induced oocyte meiotic resumption have not been examined. The present experiments were conducted to investigate the possible role of MAP kinase in FSH-induced and spontaneous oocyte meiotic resumption in the mouse. MAP kinase kinase (MAPKK, MEK) inhibitor, PD98059 or U0126, produced a dose-dependent inhibitory effect on both FSH-induced oocyte meiotic resumption and MAP kinase activation in the oocytes. However, the same inhibitor did not block spontaneous meiotic resumption of either denuded or cumulus cell-enclosed mouse oocytes, despite the activity of MAP kinase being totally inhibited. Immunoblotting the oocytes and the cumulus cells with the anti-active MAP kinase antibody showed that MAP kinase activity in the oocytes was detected at 8 h of FSH treatment, prior to germinal vesicle breakdown and increased as maturation progressed in the following culture period. In the cumulus cells, MAP kinase was activated even faster, its activity was detected at 1 h of FSH stimulation and increased gradually until 8 h of FSH treatment, then decreased and diminished after 12 h of FSH action. These data demonstrated that the MEK-MAP kinase pathway is implicated in FSH-induced but not spontaneous oocyte meiotic resumption.     

Tyrosine phosphorylation and activation of homologous protein kinases during oocyte maturation and mitogenic activation of fibroblasts.

Meiotic maturation of Xenopus and sea star oocytes involves the activation of a number of protein-serine/threonine kinase activities, including a myelin basic protein (MBP) kinase. A 44-kDa MBP kinase (p44mpk) purified from mature sea star oocytes is shown here to be phosphorylated at tyrosine. Antiserum to purified sea star p44mpk was used to identify antigenically related proteins in Xenopus oocytes. Two tyrosine-phosphorylated 42-kDa proteins (p42) were detected with this antiserum in Xenopus eggs. Xenopus p42 chromatographs with MBP kinase activity on a Mono Q ion-exchange column. Tyrosine phosphorylation of Xenopus p42 approximately parallels MBP kinase activity during meiotic maturation. These results suggest that related MBP kinases are activated during meiotic maturation of Xenopus and sea star oocytes. Previous studies have suggested that Xenopus p42 is related to the mitogen-activated protein (MAP) kinases of culture mammalian cells. We have cloned a MAP kinase relative from a Xenopus ovary cDNA library and demonstrate that this clone encodes the Xenopus p42 that is tyrosine phosphorylated during oocyte maturation. Comparison of the sequences of Xenopus p42 and a rat MAP kinase (ERK1) and peptide sequences from sea star p44mpk indicates that these proteins are close relatives. The family members appear to be tyrosine phosphorylated, and activated, in different contexts, with the murine MAP kinase active during the transition from quiescence to the G1 stage of the mitotic cell cycle and the sea star and Xenopus kinases being active during M phase of the meiotic cell cycle.