Protein kinase assays for measuring MPF and MAPK activities in mouse and rat oocytes and early embryos

Protein phosphorylation plays a pivotal role in cell cycle regulation. MPF (M-phase Promoting Factor) and MAPK (Mitogen-activated protein kinase) are two major kinases driving oocyte maturation and early embryonic divisions. Their activities can be measured experimentally with kinase assays that use specific exogenous substrates. The activities of MPF and MAPK are measured using histone H1 kinase and MBP (Myelin Basic Protein) kinase assays, respectively. Here, we describe detailed procedures for measuring these two activities in mouse and rat oocytes and in early mouse embryos. The assays we describe can be performed using very small amounts of biological material and produce clearly discernible measurements of histone H1 and MBP kinase activities.

Rescue of mouse embryos from 2-cell blocks by microinjection of maturation promoting factor

OBJECTIVE

To examine the rescue of mouse embryos from 2-cell blocks by the microinjection of maturation promoting factor (MPF) extracted from matured Xenopus eggs into one of the blastomeres of 2-cell stage mouse embryos.

DESIGN

Controlled laboratory study.

SETTING

First Department of Obstetrics and Gynecology, Toho University School of Medicine, Tokyo, Japan.

ANIMAL(S)

Eight- to 10-week-old female Crj:CD-1(ICR) mice.

INTERVENTION(S)

One of the blastomeres of the mouse 2-cell embryos was injected with MPF (MI group) or mHTF medium (MED group) at 28--32 hours after insemination.

MAIN OUTCOME MEASURE(S)

The developmental rate to blastocyst.

RESULT(S)

The developmental rate to blastocyst in the MI group (48.0%) was significantly higher than that in the MED group (0%).

CONCLUSION(S)

The 2-cell block was specifically rescued by the microinjection of MPF and not by the insertion of pipettes.

Study of the in vitro maturation of mouse oocytes induced by microinjection of maturation promoting factor (MPF)

PURPOSE

Maturation promoting factor (MPF) acts at the resumption of meiosis and nonspecifically throughout the animal species. There exists a considerable body of literature on MPF, but little work has been done to study the induction of maturation of mammalian oocytes by microinjection of extracted MPF.

METHODS

Immature (GV-stage) mouse oocytes were microinjected MPF extracted from matured Xenopus eggs in the presence of dbcAMP.

RESULTS

The rate of germinal vesicle, breakdown (GVBD) induced at 24 hr after MPF injection was significantly higher (90.5%) than that of the control (2.2%), which was injected with HTF medium containing dbcAMP (P < 0.0001). The rate of extrusion of the first polar body at 24 hr after MPF injection was significantly higher (84.1%) than that of the same control (1.1%) (P < 0.0001).

CONCLUSIONS

From these results, it is concluded that the maturation of mammalian oocytes can be induced by the microinjection of MPF extracted from other species.

Partial purification of maturation-promoting factor from catfish, Clarias batrachus: identification as the histone H1 kinase and its periodic activation

Maturation-promoting factor (MPF) has been demonstrated in the 100,000 g supernatant of 17 alpha, 20 beta-dihydroxy-4-pregnen-3-one (17 alpha, 20 beta-DP)-induced catfish, Clarias batrachus oocytes using DEAE-cellulose and sephadex G-200 chromatography. Partially purified MPF molecule eluted as a single peak on sephadex G-200 with molecular mass of approximately 200 kDa in native PAGE. SDS-PAGE analysis showed the presence of five proteins of 32, 34, 45, 46 and 48 kDa. Antibody against the PSTAIR sequence of p34cdc2 recognized 32 and 34 kDa proteins, whereas rabbit anti-cyclin B1 and B2 crossreacted with 46 and 48 kDa proteins, respectively. Cyclin B was absent in immature oocytes and appeared after 7 h of 17 alpha, 20 beta-DP stimulation, coinciding with the histone H1 kinase (HH1K) activity and start of germinal vesicle breakdown (GVBD). Our data indicate that C. batrachus MPF is a complex of cdc2 kinase and cyclin B molecules. A close relationship between HH1K activity and catfish oocyte maturation has been demonstrated using cycloheximide, cytochalasin B and colchicine. HH1K activation was inhibited by cycloheximide, while cytochalasin B and colchicine were ineffective. These finding suggests that the activation of HH1K depends on protein synthesis, whereas disruption of microfilaments influences only nucleus migration without effect on GVBD or HH1K activation. An increase of phosphorylated proteins after activation of catfish oocytes with 17 alpha, 20 beta-DP has also been observed.

Initial triggering of M-phase in starfish oocytes: a possible novel component of maturation-promoting factor besides cdc2 kinase

G2-phase-arrested immature starfish oocytes contain inactive cdc2 kinase and cdc25 phosphatase, and an inactivator for cdc2 kinase. In this system, we have studied how the regulatory balance is apped toward the initial activation of cdc2 kinase. During the hormone-dependent period (Guerrier, P., and M. Doree, 1975. Dev. Biol. 47:341-348), p34cdc2 and cdc25 protein are already converted, though not fully, to active forms, whereas the inactivators for cdc2 kinase and cdc25 phosphatase are able to exhibit their activities if the hormone were removed. We produced "triggered oocytes," in which due to a neutralizing anticdc25 antibody, the activation of cdc2 kinase is prevented out cdc25 protein is phosphorylated slightly after the maturation-inducing hormonal stimulus. In contrast to control immature oocytes, in triggered oocytes the injected cdc2 kinase is not inactivated, and accordingly the level of cdc2 kinase activity required for meiosis reinitiation is much less. These results imply the presence of a cdc2 kinase activity-independent process(es) that suppresses the inactivator for cdc2 kinase and initially phosphorylates cdc25 protein, although this process is reversible during the initial activation of cdc2 kinase. At the most initial triggering of M-phase, the cdc2 kinase activity-independent process might trip the switch leading to the initial activation of cdc2 kinase. Thereafter, in parallel, the cdc2 kinase-dependent feedback loops described by others may cause further increase in cdc2 kinase activity. We propose that a putative suppressor, which downregulates the inactivator for cdc2 kinase independently of nuclear components, might be a previously unrecognized component of maturation-promoting factor.

Identification of novel phosphorylation sites in murine A-type lamins

We report the distribution of phosphorylation sites in murine lamins A and C (A-type lamins) in vitro and in vivo followed by reverse-phase high-performance liquid chromatography and microsequencing of peptides spanning the almost complete lamin sequence. We show that two distinct protein kinases, cell-division-cycle-2 kinase (cdc2 kinase) and protein kinase C (PKC), phosphorylate murine A-type lamins at the non-alpha-helical amino- and carboxy-terminal domains in vitro and in vivo. Cdc2 kinase, but not PKC, is capable of inducing depolymerization of the nuclear lamina in permeabilized cells. Accordingly, lamins were proposed to be direct in vivo substrates of cdc2 kinase and PKC with different effects on the lamina dynamics. Analysis of the original A-type lamins revealed phosphorylation of residues Ser5 and Ser392. Residue Ser392 was substoichiometrically phosphorylated in the substrate and by cdc2 kinase in vitro. PKC phosphorylated peptides with its kinase-specific motifs surrounding Ser5, Thr199, Thr416, Thr480 and Ser625. In vivo, a mitosis-specific phosphorylation at the cdc2-kinase-specific phosphoacceptor site Ser392 and of the N-terminal peptide was identified. An interphase-specific phosphorylation at Ser525 matching the PKC consensus sequence and of peptides phosphorylated by unknown kinases was determined. The results lead us to propose that different cyclin-dependent kinase activities act as lamin kinases in mitosis and in interphase. Other kinases may cooperate with cdc2 kinase during reversible disassembly in mitosis and may modulate the supramolecular assembly of lamin filaments.

Regulation of a major microtubule-associated protein by MPF and MAP kinase

The interphase-M phase transition of microtubule dynamics is thought to be induced by phosphorylation reactions mediated by MPF and by MAP kinase functioning downstream of MPF. We have now identified and purified from Xenopus eggs a major microtubule-associated protein, p220, that may be a target protein for these two M phase-activated kinases. p220, when purified from interphase cells, potently bound to microtubules and stimulated tubulin polymerization, whereas p220 purified from M phase cells showed little or no such activities. Cell staining with a monoclonal anti-p220 antibody revealed that p220 is localized on cytoplasmic microtubule networks during interphase, while it is distributed rather diffusely throughout the cell during M phase. We have further found that p220 is phosphorylated specifically in M phase. Moreover, p220 purified from interphase cells served as a good substrate for MAP kinase and MPF in vitro, and two-dimensional phosphopeptide mapping pattern of the p220 phosphorylated in vitro was very similar to that of p220 phosphorylated at M phase in vivo. These results suggest that the drastic change in p220 activity during the transition from interphase to M phase may be induced by its phosphorylation in M phase probably catalyzed by MAP kinase and MPF.

Isolation and characterization of goldfish cdk2, a cognate variant of the cell cycle regulator cdc2

This paper reports the nucleotide and predicted amino acid sequences of the goldfish cdk2, a cognate variant of the cell cycle regulator cdc2. The predicted protein sequence shows strong homology to the other known cdk2 (88% for Xenopus and 90% for human). A monoclonal antibody against the C-terminal sequence of goldfish cdk2 recognized a 34-kDa protein in extracts from various goldfish tissues. The protein level was high in such tissues as testis and ovary containing actively dividing cells. Protein cdk2 binds to p13sucl, the fission yeast suc1+ gene product, but not to cyclin B, with which cdc2 forms a complex. The kinase activity of cdk2 increased 30-fold when oocytes matured, although its protein level did not remarkably change. Anti-cdk2 immunoprecipitates from 32P-labeled mature oocyte extracts contained a 47-kDa protein, which was not recognized by either anti-cyclin A or anti-cyclin B antibody, indicating complex formation of cdk2 with a protein other than cyclins A or B.

Purification and characterization of maturation-promoting factor in fish

Maturation-promoting factor (MPF) activity has been demonstrated for the first time in fish oocytes. We purified MPF from a 100,000g supernatant of crushed, naturally spawned carp oocytes using four chromatography columns: Q-Sepharose Fast-Flow, p13suc1-affinity Sepharose, Mono S, and Superose 12. The final preparation was purified over 1000-fold with a recovery of about 1%. On Superose 12, MPF eluted as a single peak with an apparent molecular weight of 100 kDa. SDS-PAGE analysis of the active fractions after Superose 12 revealed the presence of four proteins of 33, 34, 46, and 48 kDa. A monoclonal antibody against the PSTAIR sequence of cdc2 kinase recognized the 33- and 34-kDa proteins for which the 46- and 48-kDa proteins are endogenous substrates. The 46- and 48-kDa proteins were recognized by a monoclonal antibody against Escherichia coli-produced goldfish cyclin B, but not by an anti-cyclin A antibody. When oocytes were matured in the presence of 32P, the labeling was seen with the 34-kDa protein, but not with the 33-kDa protein. The 34-kDa protein corresponded to the MPF activity, but the 33-kDa protein did not. These findings indicate that carp MPF is a complex of cdc2 kinase and cyclin B, and further that active MPF contains the phosphorylated form of cdc2 kinase.

Multiple forms of maturation-promoting factor in unfertilized Xenopus eggs

Maturation-promoting factor (MPF), which is functionally defined by its ability to induce frog oocyte maturation independent of protein synthesis, is hypothesized to be the mitotic inducer in eukaryotic cells. Previous studies have demonstrated that the cdc2 protein kinase complex (p34cdc2-cyclin) meets the criteria for MPF. In the present study, we show that MPF activity in extracts of unfertilized Xenopus eggs can be resolved into three fractions by Q-Sepharose chromatography. Of the total MPF activity recovered, approximately 20% was in the flow-through fraction that was accounted for by the cdc2 kinase complex, approximately 40% was in the 0.2 M NaCl eluate, and the remaining approximately 40% was in the 0.5 M NaCl eluate. Neither eluate contained cdc2 kinase, but each could activate cdc2 kinase upon microinjection into Xenopus oocytes. The MPF activity in the two eluates, but not in the flow-through fraction, could be depleted by the mitosis-specific monoclonal antibody MPM-2. This antibody has been shown to inhibit Xenopus oocyte maturation and deplete MPF activity from mature oocyte extract but does not recognize the cdc2 kinase complex. The three MPFs differed in apparent molecular size, H1 kinase activity, and stability at 4 degrees C. We propose that MPF activity in unfertilized Xenopus eggs resides in at least three different molecular species, the combined activities of which may be required for autoamplification of MPF.