Activation of myelin basic protein kinases during echinoderm oocyte maturation and egg fertilization

MAPK/ERK activity is required for the successful progression of mitosis in sea urchin embryos

Using sea urchin embryos, we demonstrate that the MEK/MAPK/ERK cascade is essential for the proper progression of the cell cycle. Activation of a limited fraction of MAPK/ERK is required between S-phase and M-phase. Neither DNA replication nor CDK1 activation are impacted by the inhibition of this small active MAPK/ERK fraction. Nonetheless, the chromatin and spindle organisations are profoundly altered. Early morphological disorders induced by the absence of MAPK/ERK activation are correlated with an important inhibition of global protein synthesis and modification in the cyclin B accumulation profile. After appearance of morphological disorders, there is an increase in the level of the inhibitor of protein synthesis, 4E-BP, and, ultimately, an activation of the spindle checkpoint. Altogether, our results suggest that MAPK/ERK activity is required for the synthesis of (a) protein(s) implicated in an early step of chromatin /microtubule attachment. If this MAPK/ERK-dependent step is not achieved, the cell activates a new checkpoint mechanism, involving the reappearance of 4E-BP that maintains a low level of protein translation, thus saving cellular energy.

Growth factor-stimulated phosphorylation cascades: activation of growth factor-stimulated MAP kinase

Protein phosphorylation is an important mechanism in the response of cells to growth factors by which signals can be conveyed from cell surface receptors to intracellular targets. In addition to stimulation of protein tyrosine phosphorylation, activation of growth factor receptors having protein tyrosine kinase activity leads to dramatic alterations in the levels of protein serine/threonine phosphorylation. Several growth factor-stimulated serine/threonine-specific kinases have been identified as potential mediators of such signalling. MAP (microtubule-associated protein) kinase has emerged as a very interesting member of this group, because it activates a separate kinase, pp90rsk, which is also growth factor-stimulated. MAP kinase itself appears to be regulated by protein phosphorylation, because it can be inactivated by protein phosphatases. We have identified two 60 kDa proteins that promote the phosphorylation and full activation of MAP kinase in a manner paralleling its activation by growth factors in intact cells. These 'MAP kinase activators' are themselves stimulated by growth factors, suggesting that they function as intermediates between the MAP kinase and cell surface receptors in a growth factor-stimulated kinase cascade. Identification of the components of this protein kinase cascade reveals a mechanism by which at least some of the effects of receptor tyrosine kinases can be mediated through serine/threonine phosphorylation.

ERK1 activation is required for S-phase onset and cell cycle progression after fertilization in sea urchin embryos

Fertilization of sea urchin eggs results in a large, transient increase in intracellular free Ca2+ concentration that is responsible for re-initiation of the cell division cycle. We show that activation of ERK1, a Ca2+-dependent MAP kinase response, is required for both DNA synthesis and cell cycle progression after fertilization. We combine experiments on populations of cells with analysis at the single cell level, and develop a proxy assay for DNA synthesis in single embryos, using GFP-PCNA. We compare the effects of low molecular weight inhibitors with a recombinant approach targeting the same signalling pathway. We find that inhibition of the ERK pathway at fertilization using either recombinant ERK phosphatase or U0126, a MEK inhibitor, prevents accumulation of GFP-PCNA in the zygote nucleus and that U0126 prevents incorporation of [3H]-thymidine into DNA. Abrogation of the ERK1 signalling pathway also prevents chromatin decondensation of the sperm chromatin after pronuclear fusion, nuclear envelope breakdown and formation of a bipolar spindle.

MAP kinases regulate unfertilized egg apoptosis and fertilization suppresses death via Ca2+signaling

The default fate for eggs from many species is death by apoptosis and thus, successful fertilization depends upon suppression of the maternal death program. Little is known about the molecular triggers which activate this process or how the fertilization signal suppresses the default maternal apoptotic pathway. The MAP kinase (MAPK) family member, ERK, plays a universal and critical role in several stages of oocyte meiotic maturation, and fertilization results in ERK inactivation. In somatic cells, ERK and other MAPK family members, p38 and JNK, provide opposing signals to regulate apoptosis, however, it is not known whether MAPKs play a regulatory role in egg apoptosis, nor whether suppression of apoptosis by fertilization is mediated by MAPK activity. Here we demonstrate that MAPKs are involved in starfish egg apoptosis and we investigate the relationship between the fertilization induced signaling pathway and MAPK activation. ERK is active in post-meiotic eggs just until apoptosis onset and then p38, JNK and a third kinase are activated, and remain active through execution. Sequential activation of ERK and p38 is necessary for apoptosis, and newly synthesized proteins are required both upstream of ERK and downstream of p38 for activation of the full apoptotic program. Fertilization causes a dramatic rise in intracellular Ca2+, and we report that Ca2+ provides a necessary and sufficient pro-survival signal. The Ca2+ pathway following fertilization of both young and aged eggs causes ERK to be rapidly inactivated, but fertilization cannot rescue aged eggs from death, indicating that ERK inactivation is not sufficient to suppress apoptosis.

Germinal vesicle breakdown is not fully dependent on MAPK activation in maturing oocytes of marine nemertean worms

Previously, it has been shown that oocytes of marine nemertean worms resume meiosis and undergo germinal vesicle breakdown (GVBD) following treatment with either natural seawater (NSW), or the neurohormone serotonin (5-hydroxytryptamine or 5-HT). In this investigation of the nemerteans Cerebratulus lacteus and Cerebratulus sp., immunoblots and kinase assays were used to compare the roles of two regulatory kinases: mitogen-activated protein kinase (MAPK) and Cdc2/cyclin B (referred to as maturation promoting factor or MPF). Based on such analyses, an ERK (extracellular signal regulated kinase) type of MAPK was found to be activated concurrently with Cdc2/cyclin B during NSW- and 5-HT-induced maturation. MAPK activation occurred prior to GVBD and seemed to be controlled primarily by phosphorylation rather than de novo protein synthesis. Inhibition of MAPK signaling by U0126 was capable of delaying but not permanently blocking Cdc2/cyclin B activation and GVBD in 5-HT treated oocytes and subsets of NSW-treated oocytes. Collectively such data indicated that GVBD is not fully dependent on MAPK activation, since Cdc2/cyclin B can apparently be activated by MAPK-independent mechanism(s) in maturing nemertean oocytes.

Induction of apoptosis in starfish eggs requires spontaneous inactivation of MAPK (extracellular signal-regulated kinase) followed by activation of p38MAPK

Mitogen-activated protein kinase (MAPK) (extracellular signal-regulated kinase) prevents DNA replication and parthenogenesis in maturing oocytes. After the meiotic cell cycle in starfish eggs, MAPK activity is maintained until fertilization. When eggs are fertilized, inactivation of MAPK occurs, allowing development to proceed. Without fertilization, highly synchronous apoptosis of starfish eggs starts 10 h after germinal vesicle breakdown, which varies according to season and individual animals. For induction of the apoptosis, MAPK should be activated for a definite period, called the MAPK-dependent period, during which eggs develop competence to die, although the exact duration of the period was unclear. In this study, we show that the duration of the MAPK-dependent period was approximately 8 h. Membrane blebbing occurred approximately 2 h after the MAPK-dependent period. Surprisingly, when MAPK was inhibited by U0126 after the MAPK-dependent period, activation of caspase-3 occurred earlier than in the control eggs. Thus, inactivation of MAPK is a prerequisite for apoptosis. Also, even in the absence of the inhibitor, MAPK was inactivated spontaneously when eggs began to bleb, indicating that inactivation of MAPK after the MAPK-dependent period acts upstream of caspase-3. Inactivation of MAPK also resulted in the activation of p38MAPK, which may contribute to apoptotic body formation.

Inhibition of mitogen activated protein kinase signaling affects gastrulation and spiculogenesis in the sea urchin embryo

The mitogen activated protein (MAP) kinase signaling cascade has been implicated in a wide variety of events during early embryonic development. We investigated the profile of MAP kinase activity during early development in the sea urchin, Strongylocentrotus purpuratus, and tested if disruption of the MAP kinase signaling cascade has any effect on developmental events. MAP kinase undergoes a rapid, transient activation at the early blastula stage. After returning to basal levels, the activity again peaks at early gastrula stage and remains high through the pluteus stage. Immunostaining of early blastula stage embryos using antibodies revealed that a small subset of cells forming a ring at the vegetal plate exhibited active MAP kinase. In gastrula stage embryos, no specific subset of cells expressed enhanced levels of active enzyme. If the signaling cascade was inhibited at any time between the one cell and early blastula stage, gastrulation was delayed, and a significant percentage of embryos underwent exogastrulation. In embryos treated with MAP kinase signaling inhibitors after the blastula stage, gastrulation was normal but spiculogenesis was affected. The data suggest that MAP kinase signaling plays a role in gastrulation and spiculogenesis in sea urchin embryos.

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.

The elusive cytostatic factor in the animal egg

While animal eggs await fertilization, their cell cycle needs to be halted. The molecule responsible for this arrest--the cytostatic factor--was first described in 1971. But its identity was not revealed until 1989, and even now questions remain about this elusive factor.

Regulation of the meiosis-inhibited protein kinase, a p38(MAPK) isoform, during meiosis and following fertilization of seastar oocytes

A p38(MAPK) homolog Mipk (meiosis-inhibited protein kinase) was cloned from seastar oocytes. This 40-kDa protein shares approximately 65% amino acid identity with mammalian p38-alpha isoforms. Mipk was one of the major tyrosine-phosphorylated proteins in immature oocytes arrested at the G(2)/M transition of meiosis I. The tyrosine phosphorylation of Mipk was increased in response to anisomycin, heat, and osmotic shock of oocytes. During 1-methyladenine-induced oocyte maturation, Mipk underwent tyrosine dephosphorylation and remained dephosphorylated in mature oocytes and during the early mitotic cell divisions until approximately 12 h after fertilization. At the time of differentiation and acquisition of G phases in the developing embryos, Mipk was rephosphorylated on tyrosine. In oocytes that were microinjected with Mipk antisense oligonucleotides and subsequently were allowed to mature and become fertilized, differentiation was blocked. Because MipK antisense oligonucleotides and a dominant-negative (K62R)Mipk when microinjected into immature oocytes failed to induce germinal vesicle breakdown, inhibition of Mipk function was not sufficient by itself to cause oocyte maturation. These findings point to a putative role for Mipk in cell cycle control as a G-phase-promoting factor.

Phosphatase inhibition promotes antiapoptotic but not proliferative signaling pathways in erythropoietin-dependent HCD57 cells

Erythropoietin (EPO) allows erythroid precursors to proliferate while protecting them from apoptosis. Treatment of the EPO-dependent HCD57 murine cell line with 70 μmol/L orthovanadate, a tyrosine phosphatase inhibitor, resulted in both increased tyrosine protein phosphorylation and prevention of apoptosis in the absence of EPO without promoting proliferation. Orthovanadate also delayed apoptosis in primary human erythroid progenitors. Thus, we investigated what survival signals were activated by orthovanadate treatment. Expression of Bcl-XL and BAD phosphorylation are critical for the survival of erythroid cells, and orthovanadate in the absence of EPO both maintained expression levels of antiapoptotic Bcl-XLand induced BAD phosphorylation at serine 112. Orthovanadate activated JAK2, STAT1, STAT5, the phosphatidylinositol-3 kinase (PI-3 kinase) pathway, and other signals such as JNK and p38 without activating the EPO receptor, JAK1, Tyk2, Vav, STAT3, and SHC. Neither JNK nor p38 appeared to have a central role in either apoptosis or survival induced by orthovanadate. Treatment with cells with LY294002, an inhibitor of PI-3 kinase activity, triggered apoptosis in orthovanadate-treated cells, suggesting a critical role of PI-3 kinase in orthovanadate-stimulated survival. Mitogen-activated protein kinase (MAPK) was poorly activated by orthovanadate, and inhibition of MAPK with PD98059 blocked proliferation without inducing apoptosis. Thus, orthovanadate likely acts to greatly increase JAK/STAT and PI-3 kinase basal activity in untreated cells by blocking tyrosine protein phosphatase activity. Activated JAK2/STAT5 then likely acts upstream of Bcl-XL expression and PI-3 kinase likely promotes BAD phosphorylation to protect from apoptosis. In contrast, MAPK/ERK activity correlates with only EPO-dependent proliferation but is not required for survival of HCD57 cells.

Phosphatase inhibition promotes antiapoptotic but not proliferative signaling pathways in erythropoietin-dependent HCD57 cells

Erythropoietin (EPO) allows erythroid precursors to proliferate while protecting them from apoptosis. Treatment of the EPO-dependent HCD57 murine cell line with 70 μmol/L orthovanadate, a tyrosine phosphatase inhibitor, resulted in both increased tyrosine protein phosphorylation and prevention of apoptosis in the absence of EPO without promoting proliferation. Orthovanadate also delayed apoptosis in primary human erythroid progenitors. Thus, we investigated what survival signals were activated by orthovanadate treatment. Expression of Bcl-XL and BAD phosphorylation are critical for the survival of erythroid cells, and orthovanadate in the absence of EPO both maintained expression levels of antiapoptotic Bcl-XLand induced BAD phosphorylation at serine 112. Orthovanadate activated JAK2, STAT1, STAT5, the phosphatidylinositol-3 kinase (PI-3 kinase) pathway, and other signals such as JNK and p38 without activating the EPO receptor, JAK1, Tyk2, Vav, STAT3, and SHC. Neither JNK nor p38 appeared to have a central role in either apoptosis or survival induced by orthovanadate. Treatment with cells with LY294002, an inhibitor of PI-3 kinase activity, triggered apoptosis in orthovanadate-treated cells, suggesting a critical role of PI-3 kinase in orthovanadate-stimulated survival. Mitogen-activated protein kinase (MAPK) was poorly activated by orthovanadate, and inhibition of MAPK with PD98059 blocked proliferation without inducing apoptosis. Thus, orthovanadate likely acts to greatly increase JAK/STAT and PI-3 kinase basal activity in untreated cells by blocking tyrosine protein phosphatase activity. Activated JAK2/STAT5 then likely acts upstream of Bcl-XL expression and PI-3 kinase likely promotes BAD phosphorylation to protect from apoptosis. In contrast, MAPK/ERK activity correlates with only EPO-dependent proliferation but is not required for survival of HCD57 cells.

Function of the Mos/MAPK pathway during oocyte maturation in the Japanese brown frog Rana japonica

Fully grown immature oocytes acquire the ability to be fertilized with sperm after meiotic maturation, which is finally accomplished by the formation and activation of the maturation-promoting factor (MPF). MPF is the complex of Cdc2 and cyclin B, and its function in promoting metaphase is common among species. The Mos/mitogen-activated protein kinase (MAPK) pathway is also commonly activated during vertebrate oocyte maturation, but its function seems to be different among species. We investigated the function of the Mos/MAPK pathway during oocyte maturation of the frog Rana japonica. Although MAPK was activated in accordance with MPF activation during oocyte maturation, MPF activation and germinal vesicle breakdown (GVBD) was not initiated when the Mos/MAPK pathway was activated in immature oocytes by the injection of c-mos mRNA. Inhibition of Mos synthesis by c-mos antisense RNA and inactivation of MAPK by CL100 phosphatase did not prevent progesterone-induced MPF activation and GVBD. However, continuous MAPK activation and MAPK inhibition through oocyte maturation accelerated and delayed MPF activation, respectively. Furthermore, Mos induced a low level of cyclin B protein synthesis in immature oocytes without the aid of MAPK. These results suggest that the general function of the Mos/MAPK pathway, which is not essential for MPF activation and GVBD in Rana oocytes, is to enhance cyclin B translation by Mos itself and to stabilize cyclin B protein by MAPK during oocyte maturation.

Role of the Fyn kinase in calcium release during fertilization of the sea urchin egg

Protein tyrosine kinase activity has been implicated as part of the signaling mechanism leading to the sperm-induced calcium transient following fertilization. In the present study, we have tested the role of the Fyn kinase in triggering the calcium transient by microinjecting domain-specific fusion proteins encoding regions of Fyn sequence as inhibitors of Fyn function in vivo. A fusion protein encoding the SH2 domain of Fyn caused an increase in the latent period between sperm-egg fusion and the beginning of the calcium transient and reduced the amplitude of the calcium signal. A fusion protein encoding the U + SH3 domains also caused a small increase in the latent period. Microscopic examination revealed that a large percentage of eggs injected with the U+SH3 or SH2 domains became polyspermic as a result of the delayed block to polyspermy. Affinity experiments demonstrated that the U+SH3 and SH2 domains of Fyn were capable of forming a stable complex with phospholipase Cgamma from the sea urchin egg. The results suggest that the Fyn kinase participates in the signaling events leading up to the calcium transient and may directly regulate phospholipase Cgamma activity at fertilization.

Molecular mechanisms of the initiation of oocyte maturation: general and species-specific aspects

Stimulated by maturation-inducing hormone secreted from follicle cells surrounding the oocytes, fully-grown oocytes mature and become fertilisable. During maturation, immature oocytes resume meiosis arrested at the first prophase and proceed to the first or second metaphase at which they are naturally inseminated. Paying special attention to general and species-specific aspects, we summarise the mechanisms regulating the initial phase of oocyte maturation, from the reception of hormonal signals on the oocyte surface to activation of the maturation-promoting factor in the cytoplasm, in amphibians, fishes, mammals and marine invertebrates.

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 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.

Caulerpenyne blocks MBP kinase activation controlling mitosis in sea urchin eggs

In a previous study, we demonstrated that caulerpenyne (Cyn), a natural sesquiterpene having an antiproliferative potency, blocked the mitotic cycle of sea urchin embryos at metaphase and inhibited the phosphorylation of several proteins, but did not affect histone H1 kinase activation (Pesando et al, 1998, Eur. J. Cell Biol. 77, 19-26). Here, we show that concentrations of Cyn that blocked the first division of the sea urchin Paracentrotus lividus embryos in a metaphase-like stage (45 microM) also inhibited the stimulation of mitogen-activated protein kinase (MAPK) activity in vivo as measured in treated egg extracts using myelin basic protein (MBP) as a substrate (MBPK). However, Cyn had no effect on MBP phosphorylation when added in vitro to an untreated egg extract taken at the time of metaphase, suggesting that Cyn acts on an upstream activation process. PD 98059 (40 microM), a previously characterized specific synthetic inhibitor of MAPK/extracellular signal-regulated kinase-1 (MEK1), also blocked sea urchin eggs at metaphase in a way very similar to Cyn. Both molecules induced similar inhibitory effects on MBP kinase activation in vivo, but had no direct effect on MBP kinase activity in vitro, whereas they did not affect H1 kinase activation neither in vivo nor in vitro. As a comparison, butyrolactone 1 (100 microM), a known inhibitor of H1 kinase activity, did inhibit H1 kinase of sea urchin eggs in vivo and in vitro, and blocked the sea urchin embryo mitotic cycle much before metaphase. Immunoblots of mitotic extracts, treated with anti-active MAP-kinase antibody, showed that both Cyn and PD 98059 reduced the phosphorylation of p42 MAP kinase (Erk2) in vivo. Our overall results suggest that Cyn blocks the sea urchin embryo mitotic cycle at metaphase by inhibiting an upstream phosphorylation event in the MBPK activation pathway. They also show that H1 kinase and MBPK activation can be dissociated from each other in this model system.

Male pronuclear formation in denuded porcine oocytes after in vitro maturation in the presence of cysteamine

The present study was conducted to examine effects of cysteamine in culture medium on progression of meiosis, glutathione (GSH) content, kinase activities (histone H1 kinase and mitogen-activated protein kinase), and male pronuclear formation after in vitro insemination of cumulus-denuded oocytes (DOs) in the pig. DOs, obtained by mechanically removing cells from cumulus-oocyte complexes (COCs) with a small-bore pipette, were cultured for 45 h in TCM199 supplemented with sodium pyruvate, gonadotropins, estradiol, and 10% porcine follicular fluid, with or without cysteamine (150 microM). Maturation rates of DOs cultured with and without cysteamine were not different (60-70%) but were significantly lower than those of COCs (90-100%) (p < 0.05). GSH content of matured DOs cultured with cysteamine was significantly higher than that of DOs cultured without cysteamine (p < 0.05). Values for both types of kinase activity in matured DOs cultured with and without cysteamine were not different (p > 0.05). After in vitro insemination, DOs cultured with cysteamine showed significantly higher rates of male pronuclear formation (80.3 +/- 3.0%) than DOs cultured without cysteamine (16.4 +/- 0.5%) (p < 0.05). These results indicate that the addition of cysteamine to culture medium increased oocyte GSH content and promoted male pronuclear formation after sperm penetration of porcine DOs but had no effects on their maturation rates or kinase activities.

In vivo and in vitro assessment of mitogen activated protein kinase involvement during quail secondary palate formation

Spatiotemporally regulated cell proliferation and differentiation are crucial for the successful completion of morphogenesis of the vertebrate secondary palate. An understanding of the mechanisms by which these cellular phenomena are regulated during palate development involves the identification of the various signal transduction pathways. In the present study, the presence and activation of mitogen-activated protein (MAP) kinases were investigated during the development of quail secondary palate. The palatal shelves were dissected on days 5-9 of incubation, homogenized, and centrifuged, after which the samples were separated by anion exchange fast protein liquid chromatography. The fractions were analyzed for myelin basic protein (MBP) phosphorylation. In addition, primary cultures of quail palate mesenchymal cells (QPMCs) were treated with epidermal growth factor (EGF) and prepared for MBP phosphorylation assays. A temporally regulated pattern of phosphotransferase activity, characterized by a three-fold increase in phosphotransferase activity toward MBP between days 5 and 8 of incubation, was observed during quail palate development. Western blotting, using MAP kinase antibodies, demonstrated the presence of a 42-kDa isoform between days 5 and 9 of incubation, during which the level of protein remained constant. Antityrosine immunoblotting with 4G10 also detected a 42-kDa protein. Phosphotransferase assays, using either a MAP kinase-specific substrate peptide (S5) or a protein kinase C inhibitor (R3), further confirmed the presence of a MAP kinase in the developing palate of quail. Because diverse biological processes occur concurrently during in vivo palate morphogenesis, the involvement of MAP kinase was explored further in primary cell culture. The data showed that EGF stimulated proliferation and activated 42-kDa MAP kinase in QPMCs. It is suggested that MAP kinase cascade may be involved in growth factor-regulated cell proliferation during morphogenesis of quail secondary palate.

Evidence for MAP kinase activation during mitotic division

MAP kinases have been implicated in the control of a broad spectrum of cellular events in many types of cells. In somatic cells, MAP kinase activation seems to be triggered after exit from a quiescent state (in G0 or G2) only and then inactivated by entry into a proliferative state. In oocytes of various species, a one-time activation of MAP kinase that is apparently not repeated during the succeeding mitotic cycles occurs after meiotic activation. However, several reports suggest that a myelin basic protein (MBP) kinase activity, unrelated to that of maturation promoting factor, can sometimes be detected during mitotic divisions in various types of cells and oocytes. We have reinvestigated this problem in order to determine the origin and the role of MBP kinase that is stimulated at time of mitosis in the fertilized eggs of the sea urchin Paracentrotus lividus. We used anti-ERK1 antibodies or substrates specific for different MAP kinases, and performed in-gel phosphorylation experiments. Our results suggest that an ERK1-like protein was responsible for part of the MBP kinase activity that is stimulated during the first mitotic divisions. Furthermore, we observed that wortmannin, an inhibitor of PI 3-kinase that arrests the fertilized sea urchin eggs at the prometaphase stage, inhibited the inactivation of MAP kinase normally observed when the eggs divide, suggesting a role for PI 3-kinase in the deactivation process of MAP kinase. We also discuss how the activities of MPF and MAP kinase may be interconnected to regulate the first mitotic divisions of the early sea urchin embryo.

MAP kinase activity increases during mitosis in early sea urchin embryos

A MBP kinase activity increases at mitosis during the first two embryonic cell cycles of the sea urchin embryo. The activity profile of the MBP kinase is the same both in whole cell extracts and after immunoprecipitation with an anti-MAP kinase antibody (2199). An in-gel assay of MBP activity also shows the same activity profile. The activity is associated with the 44 kDa protein that cross-reacts with anti-MAP kinase antibodies. The 44 kDa protein shows cross-reactivity to anti-phosphotyrosine and MAP kinase-directed anti-phosphotyrosine/phosphothreonine antibodies at the times that MBP kinase activity is high. The 2199 antibody co-precipitates some histone H1 kinase activity, but the MBP kinase activity cannot be accounted for by histone H1 kinase-dependent phosphorylation of MBP. The MAP kinase 2199 antibody was used to purify the MBP kinase activity. Peptide sequencing after partial digestion shows the protein to be homologous to MAP kinases from other species. These data demonstrate that MAP kinase activation during nuclear division is not confined to meiosis, but also occurs during mitotic cell cycles. MAP kinase activity in immunoprecipitates also increases immediately after fertilization, which in the sea urchin egg occurs at interphase of the cell cycle. Treating unfertilized eggs with the calcium ionophore A23187 stimulates the increase in MAP kinase activity, demonstrating that a calcium signal can activate MAP kinase and suggesting that the activation of MAP kinase at fertilization is due to the fertilization-induced increase in cytoplasmic free calcium concentration. This signalling pathway must differ from the pathway responsible for calcium-induced inactivation of MAP kinase activity that is found in eggs that are fertilized in meiotic metaphase.

Components of the signaling pathway linking the 1-methyladenine receptor to MPF activation and maturation in starfish oocytes

Starfish oocytes are arrested at the G2/M-phase border of meiosis I. Exposure to their natural mitogen, 1-methyladenine (1-MA), leads to the activation of MPF and MAP kinase, resumption of the meiotic cell cycle, and fertilization competency. The 1-MA receptor has not yet been identified, but it is known to be linked functionally to a pertussis toxin-sensitive G-protein. G beta gamma appears to be the major effector of the 1-MA receptor, since injection of G beta gamma, but not activated G alpha i, leads to the activation of MPF, entry into meiosis, and oocyte maturation. The components that connect G beta gamma to MPF and MAP kinase activation in oocytes are unknown. In mammalian cells, a novel phosphatidylinositol 3-kinase, PI-3 kinase-gamma, links G beta gamma to the MAP kinase activation pathway. Here we show that PI-3 kinase is required for starfish oocyte maturation. LY294002 and wortmannin, inhibitors of PI-3 kinase, block MPF and MAP kinase activation and entry into meiosis. Inhibition by LY294002 is reversible and limited to the hormone-dependent period. Neither inhibitor, however, blocks the earliest hormone-induced event, formation of actin spikes at the cell membrane. By contrast, pertussis toxin blocks both actin spiking and later events, arguing that PI-3 kinase functions downstream of G beta gamma. Finally, we show that unlike the well-studied case in Xenopus oocytes, where MAP kinase is an essential component of the MPF activation pathway, MAP kinase is not required for either MPF activation or subsequent oocyte maturation in starfish. Instead, its major role appears to be suppression of DNA synthesis in unfertilized, haploid eggs.

MAP kinase-dependent pathways in cell cycle control

Mitogen-activated protein kinases such as Erk1 and Erk2 serve as a paradigm for a growing family of proline-directed protein kinases that mediate entry, progression and exit from the cell cycle in diverse eukaryotic cells. These enzymes function within highly conserved modules of sequentially activating protein kinases that transduce signals from diverse extracellular stimuli. In vertebrates, at least three distinct kinases modules have been characterized. Mitogens induce the sequential activation of the kinases Raf1-->Mek1-->Erk2-->Rsk via the G-protein Ras. Stress factors stimulate c-Jun activation through a related kinase pathway involving Mekk-->Sek-->SAPK c-Jun, and hsp27 phosphorylation via the MKK3-->Hog-->MAPKAPK-2 hsp27 route. Genetic and biochemical studies, for example from budding yeast, imply the existence of several related protein kinase modules that can operate in parallel or within integrated systems.

Phosphorylation of protein kinase C-related kinase PRK2 during meiotic maturation of starfish oocytes

The resumption of meiosis in the developing starfish oocyte is the result of intracellular signaling events initiated by 1-methyladenine stimulation. One of the earliest detectable kinase activities during meiotic maturation of starfish oocytes is a protein kinase C or PKC-like activity. In this study, several isoforms of protein kinase C were cloned from the oocyte; however, the most abundant PKC-like maternal transcript corresponds to protein kinase C-related kinase 2 (PRK2). PRK2 is expressed in the immature oocyte and at least until germinal vesicle breakdown. Subcellular localization of PRK2 revealed a cytoplasmic distribution in the immature oocyte, which, during meiotic maturation, remained in the cytoplasm but also localized to the disintegrating germinal vesicle. Significantly, PRK2 is phosphorylated in vivo in response to 1-methyladenine which precedes MPF activation, making PRK2 a candidate regulator of early signaling events of meiotic maturation.

Identification and characterization of an auto-activating MEK kinase from bovine brain: phosphorylation of serine-298 in the proline-rich domain of the mammalian MEKs

Mitogen-activated protein kinase kinases (MKKs or MEKs) are dual specificity tyrosine/threonine protein kinases that are activated by phosphorylation at two closely spaced serine residues (serines-218 and -222) by the c-mos and raf proto-oncogenes. This double phosphorylation is both necessary and sufficient for MEKs to activate the MAP kinase enzymes in vitro. The specificity or regulation of in vivo signaling to the mammalian MEKs (MEK1 and MEK2) was recently reported also to involve the differential phosphorylation of a proline-rich peptide located between the MEK kinase-subdomains IX and X. Here we report the purification and characterization of an auto-activating protein kinase from bovine brain that phosphorylates serine-298 of the MEK1 and MEK2 proline-rich insert peptides. The auto-activation of the MEK-S298 peptide kinase is the result of an intermolecular phosphorylation event that can be prevented by the peptide substrates. The inactive kinase migrates on gel filtration as a 90 kDa protein, and after activation as a 43 kDa phosphoprotein. Incorporation of 32P[phosphate] into 40-42 kDa proteins on SDS-PAGE parallels the activation of the enzyme, and dephosphorylation by protein phosphatase 2Ac reverses the activation. SDS-PAGE renaturation assays show that the 40 kDa protein has the capacity to autophosphorylate, and exhibits kinase activity towards myelin basic protein after activation. Phosphorylation of purified bovine brain MEK or recombinant MEK1 by the auto-activated kinase does not activate the enzyme, and does not interfere with the in vitro raf-mediated MEK activation. We conclude that still unknown kinases may control the MAP kinase pathway by targeting MEK.

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.

Microtubule-entrained kinase activities associated with the cortical cytoskeleton during cytokinesis

Research over the past few years has demonstrated the central role of protein phosphorylation in regulating mitosis and the cell cycle. However, little is known about how the mechanisms regulating the entry into mitosis contribute to the positional and temporal regulation of the actomyosin-based contractile ring formed during cytokinesis. Recent studies implicate p34cdc2 as a negative regulator of myosin II activity, suggesting a link between the mitotic cycle and cytokinesis. In an effort to study the relationship between protein phosphorylation and cytokinesis, we examined the in vivo and in vitro phosphorylation of actin-associated cortical cytoskeletal (CSK) proteins in an isolated model of the sea urchin egg cortex. Examination of cortices derived from eggs or zygotes labeled with 32P-orthophosphate reveals a number of cortex-associated phosphorylated proteins, including polypeptides of 20, 43 and 66 kDa. These three major phosphoproteins are also detected when isolated cortices are incubated with [32P]ATP in vitro, suggesting that the kinases that phosphorylate these substrates are also specifically associated with the cortex. The kinase activities in vivo and in vitro are stimulated by fertilization and display cell cycle-dependent activities. Gel autophosphorylation assays, kinase assays and immunoblot analysis reveal the presence of p34cdc2 as well as members of the mitogen-activated protein kinase family, whose activities in the CSK peak at cell division. Nocodazole, which inhibits microtubule formation and thus blocks cytokinesis, significantly delays the time of peak cortical protein phosphorylation as well as the peak in whole-cell histone H1 kinase activity. These results suggest that a key element regulating cortical contraction during cytokinesis is the timing of protein kinase activities associated with the cortical cytoskeleton that is in turn regulated by the mitotic apparatus.

Distinct signaling from stem cell factor and erythropoietin in HCD57 cells

A recent report (Wu, H., Klingmuller, U., Besmer, P., and Lodish, H. F. (1995) Nature 377, 242-246) documents the interaction of the erythropoietin (EPO) receptor (EPOR) with the stem cell factor (SCF) receptor (c-KIT) and suggests that SCF acts through the EPOR. To elucidate the ability of SCF to affect the erythropoietin signaling pathway, we studied the effect of SCF on EPOR phosphorylation, SHC/ERK-1 activity, and cell proliferation and apoptosis in EPO-dependent HCD57 cells. Treatment of these cells with SCF resulted in phosphorylation of the EPOR. However, SCF-dependent phosphorylation of the EPOR did not initiate an EPO-like intracellular signal. SCF induced proliferation, SHC phosphorylation, and activation of ERK-1 but did not activate the JAK/STAT pathway. SCF stimulated SHC phosphorylation and ERK-1 activation independent of the EPOR in cells where the EPOR was down-regulated; the presence of the EPOR appeared to facilitate SCF activation of SHC and ERK-1. Furthermore, treatment of HCD57 cells with SCF increased cell number over a 3-day treatment, but apoptosis was observed in these cells. These data may illustrate two distinct pathways for erythroid cell proliferation and prevention of apoptosis in response to EPO, thereby providing a system to discriminate these intracellular signals.

Mitogen-activated protein kinase activity during goat oocyte maturation and the acquisition of meiotic competence

Changes in MPF and MAPK activities during meiotic maturation of goat oocytes were investigated. Detection of MPF activity occurred concomitantly with GVBD, increased at MI, decreased during anaphase-telophase I transition, and increased thereafter in MII oocytes. The appearance of MAPK activity was delayed compared to MPF activity. MAPK activity increased after GVBD and persisted during the MI-MII transition. Whether MAPK was implicated in goat oocyte meiotic competence was also investigated by using oocytes from different follicle size categories that arrest at specific stages of the maturation process (GV, GVBD, MI, and MII). Results indicate that the ability of goat oocytes to resume meiosis is not directly related to the presence of Erk2. The ability to phosphorylate MAPK is acquired by the oocyte during follicular growth after the ability to resume meiosis. GVBD-arrested oocytes exhibited a high level of MPF activity after 27 hr of culture. However, 28% of oocytes from this group contained inactive MAPK, and 72% exhibited high MAPK activity. In addition, 29% of GVBD-arrested oocytes contained a residual interphasic network without recruitment of microtubules around the condensed chromosomes; 71% of GVBD-arrested oocytes displayed recruitment of microtubules near the condensed chromosomes and contained asters of microtubules distributed throughout the cytoplasm. These results indicate that oocytes arrested at GVBD were not exactly at the same point in the meiotic cell cycle progression, and suggest that MAPK could be implicated in the regulation of microtubule organization. The data presented here suggest that in goat oocytes, MAPK is not implicated in the early events of meiosis resumption, but rather in post-GVBD events such as spindle formation and MII arrest.

Phosphorylation and activation of smooth muscle myosin light chain kinase by MAP kinase and cyclin-dependent kinase-1

Smooth muscle myosin light chain kinase (MLCK) features several consensus sites of phosphorylation by proline-directed protein serine/threonine kinases. The phosphorylation of MLCK by two proline-directed kinases isolated from sea star oocytes, i.e., p44mpk (Mpk, a mitogen-activated protein kinase homologue) and cyclin-dependent kinase-1 (CDK1, also known as p34cdc2), was investigated. Chicken gizzard MLCK was phosphorylated on seryl and threonyl residues by both Mpk and CDK1. Phosphorylation of MLCK to 0.6 mol Pi/mol by Mpk increased the Vmax of phosphotransferase activity towards a synthetic peptide corresponding to residues 11-23 of the 20-kDa light chain of myosin by 1.6-fold. Phosphorylation of MLCK to 1.0 mol Pi/mol by CDK1 increased the Vmax by 2.3-fold. Phosphorylation by either kinase had no significant effect on the concentration of calmodulin required for half-maximal activation of MLCK. Analysis of the phosphorylation of synthetic peptides containing consensus phosphorylation sites for Mpk and CDK1 indicated that the major site of phosphorylation in MLCK by Mpk was Ser-834, and by CDK1 was Thr-283. Both of these sites are located outside the calmodulin-binding site (residues 796-815), consistent with the observation that phosphorylation by Mpk or CDK1 was unaffected by the presence of bound Ca2+/calmodulin. These results indicate that MLCK activity may be regulated by phosphorylation catalyzed by proline-directed kinases, possibly directed at Thr-40 and Thr-43 at the amino terminus of MLCK.

Activation of mitogen-activated protein kinase by heat shock treatment in Drosophila

Heat shock treatment of Drosophila melanogaster tissue culture cells causes increased tyrosine phosphorylation of several 44 kDa proteins, which are identified as Drosophila mitogen-activated protein (MAP) kinases. Tyrosine phosphorylation occurs within 5 min, and is maintained at high levels during heat shock. It decreases to basal levels during recovery, concurrent with the repression of heat shock transcription and heat-shock-protein synthesis. The increased MAP kinase tyrosine phosphorylation is parallelled by increased MAP kinase activity. At least two MAP kinases, DmERK-A and DmERK-B, are identified whose tyrosine phosphorylation increases during heat shock. Thus MAP kinase activation is an immediate early response to heat shock, and its increased activity is maintained throughout heat shock treatment. Protracted MAP kinase activation may contribute to heat shock transcription factor phosphorylation and the numerous metabolic alterations that constitute the heat-shock response.

Protein tyrosine phosphorylation during sea urchin fertilization: microtubule dynamics require tyrosine kinase activity

Protein tyrosine phosphorylation plays an important role in cell growth, mitosis, and tumorigenesis. It has also been implicated in meiotic maturation and fertilization. We have used anti-phosphotyrosine immunofluorescence and immunoblotting to identify sperm and egg proteins which are phosphorylated on tyrosine residues prior to and during sea urchin fertilization. On immunoblots of sperm proteins, the monoclonal anti-phosphotyrosine antibody detected three major proteins with molecular weights of 44, 82, and 100 kD, and six minor bands at 46, 48, 70, 76, 95, and 150 kD. These phosphotyrosyl proteins were localized to the sperm acrosomal and centriolar fossae. In contrast, staining was found globally in unfertilized eggs, and the antibody recognized two major egg phosphotyrosyl proteins of molecular weights 42 and 50 kD, and five minor bands at 40, 90, 116, 130, and 150 kD. While immunofluorescent staining remained throughout the fertilized egg cytoplasm, there were dynamic changes in the staining intensity of single bands. The 90 kD immunoreactive band increased in intensity, and the 40 and 42 kD bands disappeared by 15 min after fertilization. Loss of the 40 and 42 kD bands was due to dephosphorylation by okadaic acid-sensitive phosphatase(s). The 50 kD immunoreactive protein was unchanged up to the 8-cell stage and was still present in blastulae, indicating its importance throughout fertilization and early development. Alterations in the pattern of phosphotyrosine-containing proteins during fertilization did not depend on nascent proteins and could not be completely mimicked by increasing intracellular calcium, pH, and protein kinase C activity alone. Since changes in the fertilization pattern of phosphotyrosyl proteins occurred during formation of the sperm aster and mitotic spindle, we analyzed the role of protein tyrosine kinase activity in these processes using the tyrosine kinase specific inhibitor, erbstatin. Both the sperm aster and mitotic spindle were disrupted, indicating an involvement of tyrosine phosphorylation in these processes during interphase and mitosis. We conclude that the changes in phosphotyrosyl proteins play an important role in fertilization and early development of sea urchin eggs. Control of microtubule assembly into the sperm aster and mitotic spindle of the first cell cycle are examples of such roles.

Activation of mitogen-activated protein kinase and arachidonate release via two G protein-coupled receptors expressed in the rat hippocampus

Platelet-activating factor and somatostatin receptors, two G protein-coupled receptors expressed in the rat hippocampus, were analyzed for the downstream signaling pathways in Chinese hamster ovary cells stably expressing each receptor. Ligand stimulation to each CHO cell line induced (1) inhibition of forskolin-induced accumulation of cAMP, (2) arachidonate release, and (3) activation of mitogen-activated protein kinase and MAP kinase kinase. In contrast, inositol phosphate breakdown was seen only in the PAF-stimulated CHO cells. The induction of these signals accompanied no detectable Ras activation. Suppression of the signals by pertussis toxin was almost complete for the somatostatin receptor but partial for the PAF receptor, suggesting that the somatostatin receptor couples only with PTX-sensitive G protein, while the PAF receptor couples with both PTX-sensitive and -insensitive G proteins. A model of G protein-mediated signaling pathways was proposed in which the signals from Gi and those from Gq converge at MAP kinase kinase and lead to arachidonate release. The present system using CHO cells is useful for analyzing signaling pathways from G proteins to MAP kinase kinase and will thereby provide clues for understanding the mechanisms underlying the physiological and pathological events mediated by PAF, somatostatin, and other G protein-coupled receptors in the central nervous system and other tissues.

The MPM-2 antibody inhibits mitogen-activated protein kinase activity by binding to an epitope containing phosphothreonine-183

Mitogen-activated protein (MAP) kinases are a family of serine/threonine kinases implicated in the control of cell proliferation and differentiation. We have found that activated p42mapk is a target for the phosphoepitope antibody MPM-2, a monoclonal antibody that recognizes a cell cycle-regulated phosphoepitope. We have determined that the MPM-2 antibody recognizes the regulatory region of p42mapk. Binding of the MPM-2 antibody to active p42mapk in vitro results in a decrease in p42mapk enzymatic activity. The MPM-2 phosphoepitope can be generated in vitro on bacterially expressed p42mapk by phosphorylation with either isoform of MAP kinase kinase (MKK), MKK1, or MKK2. Analysis of p42mapk proteins mutated in their regulatory sites shows that phosphorylated Thr-183 is essential for the binding of the MPM-2 antibody. MPM-2 binding to Thr-183 is affected by the amino acid present in the other regulatory site, Tyr-185. Substitution of Tyr-185 with phenylalanine results in strong binding of the MPM-2 antibody, whereas substitution with glutamic acid substantially diminishes MPM-2 antibody binding. The MPM-2 phosphoepitope antibody recognizes an amino acid domain incorporating the regulatory phosphothreonine on activated p42mapk in eggs during meiosis and in mammalian cultured cells during the G0 to G1 transition.

Isolation and characterization of mutants of human mitogen-activated protein kinase (ERK2)

Site directed mutagenesis/charged-to-alanine scanning mutagenesis of the amino terminal portion of human ERK2 (from amino acids 1 to 150) purified as a glutathione-S-transferase fusion protein (GST-ERK2) from E. coli has been done to determine regions/amino acids important for activation by rabbit skeletal muscle MAP kinase kinase (rMEK) and kinase activity towards myelin basic protein (MBP). Five classes of mutants have been isolated. The first class of mutants comprises of G30A/G32A, A50D and R65A/R68A/E69A, that can be phosphorylated by rMEK and have no kinase activity towards MBP, the second class includes mutants D122A/H123A and N142A which have lower kinase activities but no change in their activation by rMEK; third class being Y34A, E58A/H59A, which have neutral effect towards either activity, the fourth class that includes completely inactive mutants D42A/K46A/R48A, the deletion mutant in the same region (-9aa[40-48]) and D104A/E107A/D109A and finally the fifth class that include K53A, E94A/K97A/D99A, K112A/K115A and R133A/K136A that are phosphorylated 140-240% but with kinase activity toward MBP ranging from 50-100% of the wild type.

Deciphering the MAP kinase pathway

MAP kinases (MAPK) are serine/threonine kinases which are activated by a dual phosphorylation on threonine and tyrosine residues. Their specific upstream activators, called MAP kinase kinases (MAPKK), constitute a new family of dual-specific threonine/tyrosine kinases, which in turn are activated by upstream MAP kinase kinase kinases (MAPKKK). These three kinase families are successively stimulated in a cascade of activation described in various species such as mammals, frog, fly, worm or yeast. In mammals, the MAP kinase module lies on the signaling pathway triggered by numerous agonists such as growth factors, hormones, lymphokines, tumor promoters, stress factors, etc. Targets of MAP kinase have been characterized in all subcellular compartments. In yeast, genetic epistasis helped to characterize the presence of several MAP kinase modules in the same system. By complementation tests, the relationships existing between phylogenetically distant members of each kinase family have been described. The roles of the MAP kinase cascade have been analyzed by engineering various mutations in the kinases of the module. The MAP kinase cascade has thus been implicated in higher eukaryotes in cell growth, cell fate and differentiation, and in low eukaryotes, in conjugation, osmotic stress, cell wall construct and mitosis.

Networking with mitogen-activated protein kinases

Mitogen activated protein (MAP) kinases and their target ribosomal protein S6 (RSK) kinases have been recognized as shared components in the intracellular signaling pathways of many diverse cytokines. Recent studies have extended this protein kinase cascade by identifying the major activator of vertebrate MAP kinases as a serine/threonine/tyrosine-protein kinase called MEK, which is related to yeast mating factor-regulated protein kinases encoded by the STE7 and byr1 genes. MEK, in turn, may be activated following its phosphorylation on serine by either of the kinases encoded by proto-oncogenes raf1 or mos, as well as by p78mekk, which is related to the yeast STE11 and byr2 gene products. Isoforms of all of these protein kinases may specifically combine to assemble distinct modules for intracellular signal transmission. However, the fundamental architecture of these protein kinase cascades has been highly conserved during eukaryotic evolution.

The MAP kinase cascade. Discovery of a new signal transduction pathway

Using biochemical techniques similar to those used by Krebs and Fischer in elucidating the cAMP kinase cascade, a protein kinase cascade has been found that represents a new pathway for signal transduction. This pathway is activated in almost all cells that have been examined by many different growth and differentiation factors, suggesting control of different cell responses. At this writing, four tiers of growth factor regulated kinases, each tier represented by more than one enzyme, have been reconstituted in vitro to form the MAP kinase cascade. Preliminary findings suggesting multiple feedback or feedforward regulation of several components in the cascade predict higher complexity than a simple linear pathway.

Molecular cloning, expression, and characterization of the human mitogen-activated protein kinase p44erk1

p44erk1 is a member of a family of tyrosyl-phosphorylated and mitogen-activated protein (MAP) kinases that participate in cell cycle control. A full-length erk1 cDNA was isolated from a human hepatoma cell line (Hep G2) library. The erk1 cDNA clone shared approximately 96% predicted amino acid identity with partial sequences of rodent erk1 cognates, and the erk1 gene was assigned to human chromosome 16 by hybrid panel analysis. Human erk1 expressed in Escherichia coli as a glutathione S-transferase fusion (GST-Erk1) protein was substantially phosphorylated on tyrosine in vivo. It underwent further autophosphorylation in vitro (up to 0.01 mol of P per mol) at the regulatory Tyr-204 site and at additional tyrosine and serine residues. Threonine autophosphorylation, presumably at the regulatory Thr-202 site, was also detected weakly when the recombinant kinase was incubated in the presence of manganese, but not in the presence of magnesium. Before and after cleavage of the GST-Erk1 protein with thrombin, it exhibited a relatively high level of myelin basic protein phosphotransferase activity, which could be reduced eightfold by treatment of the kinase with the protein-tyrosine phosphatase CD45, but not by treatment with the protein-serine/threonine phosphatase 2A. The protein-tyrosine kinase p56lck catalyzed phosphorylation of GST-Erk1 at two autophosphorylations sites, including Tyr-204, and at a novel site. A further fivefold stimulation of the myelin basic protein phosphotransferase activity of the GST-Erk1 was achieved in the presence of a partially purified MAP kinase kinase from sheep platelets. Under these circumstances, there was primarily an enhancement of the tyrosine phosphorylation of GST-Erk1. This MAP kinase kinase also similarly phosphorylated a catalytically compromised version of GST-Erk1 in which Lys-71 was converted to Ala by site-directed mutagenesis.

Molecular cloning, expression, and characterization of the human mitogen-activated protein kinase p44erk1

p44erk1 is a member of a family of tyrosyl-phosphorylated and mitogen-activated protein (MAP) kinases that participate in cell cycle control. A full-length erk1 cDNA was isolated from a human hepatoma cell line (Hep G2) library. The erk1 cDNA clone shared approximately 96% predicted amino acid identity with partial sequences of rodent erk1 cognates, and the erk1 gene was assigned to human chromosome 16 by hybrid panel analysis. Human erk1 expressed in Escherichia coli as a glutathione S-transferase fusion (GST-Erk1) protein was substantially phosphorylated on tyrosine in vivo. It underwent further autophosphorylation in vitro (up to 0.01 mol of P per mol) at the regulatory Tyr-204 site and at additional tyrosine and serine residues. Threonine autophosphorylation, presumably at the regulatory Thr-202 site, was also detected weakly when the recombinant kinase was incubated in the presence of manganese, but not in the presence of magnesium. Before and after cleavage of the GST-Erk1 protein with thrombin, it exhibited a relatively high level of myelin basic protein phosphotransferase activity, which could be reduced eightfold by treatment of the kinase with the protein-tyrosine phosphatase CD45, but not by treatment with the protein-serine/threonine phosphatase 2A. The protein-tyrosine kinase p56lck catalyzed phosphorylation of GST-Erk1 at two autophosphorylations sites, including Tyr-204, and at a novel site. A further fivefold stimulation of the myelin basic protein phosphotransferase activity of the GST-Erk1 was achieved in the presence of a partially purified MAP kinase kinase from sheep platelets. Under these circumstances, there was primarily an enhancement of the tyrosine phosphorylation of GST-Erk1. This MAP kinase kinase also similarly phosphorylated a catalytically compromised version of GST-Erk1 in which Lys-71 was converted to Ala by site-directed mutagenesis.

Requirement for Raf and MAP kinase function during the meiotic maturation of Xenopus oocytes

The role of Raf and MAPK (mitogen-activated protein kinase) during the maturation of Xenopus oocytes was investigated. Treatment of oocytes with progesterone resulted in a shift in the electrophoretic mobility of Raf at the onset of germinal vesicle breakdown (GVBD), which was coincident with the activation of MAPK. Expression of a kinase-defective mutant of the human Raf-1 protein (KD-RAF) inhibited progesterone-mediated MAPK activation. MAPK activation was also inhibited by KD-Raf in oocytes expressing signal transducers of the receptor tyrosine kinase (RTK) pathway, including an activated tyrosine kinase (Tpr-Met), a receptor tyrosine kinase (EGFr), and Ha-RasV12. KD-RAF completely inhibited GVBD induced by the RTK pathway. In contrast, KD-RAF did not inhibit GVBD and the progression to Meiosis II in progesterone-treated oocytes. Injection of Mos-specific antisense oligodeoxyribonucleotides inhibited MAPK activation in response to progesterone and Tpr-Met, but failed to inhibit these events in oocytes expressing an oncogenic deletion mutant of Raf-1 (delta N'Raf). Injection of antisense oligodeoxyribonucleotides to Mos also reduced the progesterone- and Tpr-Met-induced electrophoretic mobility shift of Xenopus Raf. These results demonstrate that RTKs and progesterone participate in distinct yet overlapping signaling pathways resulting in the activation of maturation or M-phase promoting factor (MPF). Maturation induced by the RTK pathway requires activation of Raf and MAPK, while progesterone-induced maturation does not. Furthermore, the activation of MAPK in oocytes appears to require the expression of Mos.