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GREAT EXPERIMENTS

The discovery of MPF

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Yoshio Masui

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Before dividing, eukaryotic cells undergo a highly ordered series of events called the cell cycle. These events include S phase, when the cell's DNA is replicated, and mitosis (M phase), when the replicated chromosomes are separated. These phases are separated by periods called gap (G) phases, with the G1 phase preceding S phase and the G2 phase occurring between S phase and mitosis. How the cell controls this essential series of events is one of the fundamental problems of cell biology.

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One prominent early approach to studying control of the cell cycle was to introduce a nucleus from one phase of the cell cycle into a cytoplasm from another. Techniques for making such nucleo-cytoplasmic hybrid cells, including nuclear transplantation and cell fusion, became available in the 1950s, but it was not until the late 1960s that they were used to study cell cycle activities such as the initiation of DNA synthesis and the condensation of chromosomes. This type of experiment included nuclear transplantation by injection in frog oocytes and eggs (1577), excision and transplantation of cytoplasmic fragments in protozoa (1575), and virus-mediated fusion between tissue culture cells (1590). In all cases the nucleus conformed to the cell cycle stage of the cytoplasm, indicating that cytoplasmic factors control nuclear activities during the cell cycle. Evidence for the existence of factors which might control the initiation of cell cycle events came from other types of experiments. For example, experiments done with Tetrahymena showed that heat shock synchronized the cell cycles of a population of cells, presumably because of the heat lability of a component that promotes cell division (1583).

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Despite what these experiments revealed about the organization of the cell cycle, none of them provided any clue to the identity of the factors responsible for cytoplasmic control over the nucleus. Nor could any of the experiments be readily adapted to provide an assay with which to identify those factors and study their biochemical mechanisms. That would require an experimental system in which cytoplasm from particular stages of the cell cycle could be isolated and used to cause a transition from one phase of the cell cycle to another. The frog oocyte provided this system and played an essential role in identifying the molecular machinery that drives the cell cycle.

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Background

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Until the early 1960s, I worked on the problem of embryonic induction and was interested in the mechanism of cell differentiation. Markert's hypothesis of differential gene activation in development, published in 1958 (1584), greatly interested me and I joined his lab at Yale University on my sabbatical leave in 1966 to learn his approach to cell differentiation. My first project was a study of lactate dehydrogenase isoenzymes using frozen penguin embryos sent by an expeditionary team from the Antarctic. The results turned out to be so complicated that I felt pessimistic about continuing to study cell differentiation. At the time, similarly complicated results in other systems had caused many embryologists to lose confidence in discovering a specific inducer of cell differentiation. Even the nature of cell differentiation was in some doubt. Recent results in immunology had raised the possibility that differentiation might be a clonal selection process rather than a cell transformation caused by the inducer. To escape such scepticism, I wanted to study a well-defined developmental change inducible in a single cell by a highly specific inducer. Maturation of frog oocytes seemed to fit the bill.

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Figure 1  
The events of maturation and early development.

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Frog oocytes undergo a long period of growth within the follicles of the ovary before "maturing" into unfertilized eggs. Figure 1 shows that fully grown oocytes are naturally arrested in the G2 phase of the cell cycle, immediately preceding the first meiotic division. Maturation is the term used to describe their progression from that point through the rest of meiosis; thus, "maturation" is synonymous with the release of the oocytes from their arrest and their performance of the divisions of mitosis. The process is initiated in response to stimulation of the ovary by gonadotropin, a small peptide hormone secreted by the pituitary. The hormone causes the greatly enlarged oocyte nucleus (called the "germinal vesicle") to break down and its chromosomes to condense, and the oocyte then enters and completes the first meiotic division, passes through prophase of the second, and finally arrests as an unfertilized egg at metaphase of the second division. Fertilization relieves this arrest and the egg begins dividing mitotically. This system suited my needs well, since an external signal could reliably cause a single cell to undergo a series of events that was a form of differentiation. Before I began my study in 1967, however, only a few studies of oocyte maturation had been published (for review see 1576), and we knew little about how the signal that acts on the oocyte triggers the events of maturation.

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Most important for my study was a set of experiments by Dettlaff and her colleagues (1578). They reported that isolated oocytes, freed of the follicle cells which surround them in the ovary, could be induced to mature by treatment with gonadotropin. Maturation could also be induced by injecting the nuclear contents of a maturing oocyte into a second oocyte which had not been treated with the hormone. These results suggested that the hormone caused the production of something within the nucleus that was responsible for initiating maturation. On the basis of the mechanism of action of other hormones known at the time, they concluded that gonadotropin probably acted by entering the cell, moving to the nucleus, and stimulating transcription.

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Seeing these important results, I felt the need to examine the mode of gonadotropin action on the frog oocyte further. I first repeated their experiments with Rana pipiens, the leopard frog, but found that manual removal of follicle tissues from oocytes was not as easy as they described. I developed a more reliable way of doing it and found that oocytes completely separated from follicle cells did not mature in response to gonadotropin, while those cultured with follicle cells did so (1579). Since it was known that ovulation induced by gonadotropin could be enhanced by progesterone (1580), I tried to enhance gonadotropin-induced oocyte maturation by adding progesterone. However, I found that progesterone alone could induce follicle-free oocytes to mature (1579). Clearly, it was not pituitary gonadotropin, but progesterone secreted from follicle cells stimulated by gonadotropin, that acts on the oocyte to induce its maturation.

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While I had originally been attracted to oocyte maturation as a developmental event, I realized that because maturation is an induced meiotic cell cycle I would also be studying control of a cell cycle event. Maturation was a highly advantageous system for this purpose for several reasons. The natural arrest of immature oocytes in G2, and of unfertilized eggs in M, eliminated the problems and complicated procedures always involved in synchronizing the cell cycles of a population of tissue culture cells or microorganisms, and allowed me to be certain of the cell cycle stage. In addition, frog oocytes and eggshave the simple advantages of being big, tough and available in large numbers. As a result it is possible to inject them and to transfer material from one to another, and to get large enough quantities of material to do biochemical experiments. Without these properties my experimental approach would not have been possible.

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The experiment

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Having identified the real inducer of maturation as progesterone, I was in a position to ask how it acted upon the oocyte. First, in order to find the target for progesterone action, the hormone was injected directly into oocytes up to an internal concentration of 0.5 µg/ml. However, none of the progesterone-injected oocytes matured, while oocytes exposed to the same doses of the hormone in their culture medium did. Therefore, it was assumed that progesterone could act only on the surface of the oocyte, and that the hormone must create a signal in the cytoplasm that acts on the nucleus to initiate maturation (1582).

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Figure 2  
A graduated micropipette of the type used to perform cytoplasmic transfer experiments between oocytes. Each section between two marks holds 5 nl. (Fig. 1 of 1582)

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Figure 3  
Maturation-promoting factor activity in the oocyte cytoplasm during maturation and early development. The horizontal axis indicates the age of the oocyte (hours since treatment with progesterone) and the vertical axis the concentration of MPF activity in its cytoplasm. (Fig. 10 of 1582)

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To test this assumption, various volumes of cytoplasm from progesterone-treated oocytes, measured by the calibrated micropipette shown in Figure 2, were injected into untreated immature oocytes. These recipient oocytes matured if given more than 5 nl of cytoplasm, as long as the cytoplasm had been removed from the donor oocyte more than six hours after it had been treated with progesterone. The putative cytoplasmic substance responsible for this effect was referred to as "maturation promoting factor (MPF)." It was found that the frequency with which recipient oocytes were induced to mature increased almost linearly with the volume of the injected cytoplasm. This allowed measurement of the relative amounts of MPF activity in oocyte cytoplasm at different times after the initiation of maturation (an amount of MPF was expressed as the percentage of oocytes induced to mature by injection with a particular volume of cytoplasm). Figure 3 shows that MPF appears six hours after progesterone treatment (three hours before breakdown of the nucleus) and persists at a high level until fertilization, after which it declines to low levels during cleavage (1582).

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Figure 4  
Successive transfers of cytoplasm from maturing oocytes to immature oocytes. The percentage of injected oocytes which matured in a given round is shown.

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If progesterone acting at the surface of the oocyte gives rise to MPF, then MPF probably appears first in the cytoplasm closest to the surface, without the involvement of the nucleus. Indeed, MPF was produced when oocytes whose nuclei had been removed were treated with progesterone. Thus, MPF activity must first be generated in the cytoplasm and then act upon the nucleus. Since oocytes can be induced to mature when injected with a volume of cytoplasm equal to less than 1% of their volume, it was easy to imagine that MPF is amplified in the oocyte cytoplasm by an autocatalytic reaction. This was demonstrated with a serial transfer experiment shown in Figure 4. An oocyte which had been matured by injection with cytoplasm was itself used as the donor of cytoplasm for injection into another oocyte. Once it matured, the second oocyte was used to provide cytoplasm for yet another round of injection and maturation, and the process continued several more times. When 30 to 40 nl (1%-2% of an oocyte volume) of cytoplasm were successively transferred in this manner, no decrease in MPF activity was observed in the cytoplasm of the matured oocytes at any point in the procedure, despite a dilution of the initial cytoplasm of more than 100,000 fold in the final oocyte (1582).

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The fact that MPF activity persisted at low levels during the early cleavage divisions of a fertilized egg (Figure 3) suggested that MPF might promote mitosis as well as meiosis. If so, injection of MPF-containing cytoplasm from oocytes might be expected to induce a premature M phase in the cells of a zygote. Unexpectedly, when cytoplasm from oocytes at late stages of maturation was injected into zygotes, cleavage of injected blastomeres was arrested at metaphase of the next mitosis, reminiscent of the arrest of unfertilized eggs at metaphase of the second meiotic division. This suggested the presence of an inhibitory factor capable of arresting cells in M phase. The effect of this inhibitory factor, designated "cytostatic factor (CSF)," showed a dose-dependence similar to that of MPF. It appears shortly after the first meiotic division and remains until fertilization, after which it quickly disappears. Using enucleated oocytes it was also found that both production of CSF during oocyte maturation and its destruction during egg activation could occur independently of nuclear activities (1582).

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From these experiments we concluded that maturation requires the coordinated activity of two factors generated in the cytoplasm. MPF appears first, in response to hormonal stimulation of the oocyte, and drives the oocyte into meiosis. CSF appears with a delay, perhaps so as not to interfere with the first meiotic division, and seems responsible for arresting the cell cycle at the following M phase until fertilization.

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The legacy

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Taken together these experiments represented the first quantitative analyses of cytoplasmic factors that regulate nuclear activities during the cell cycle, and the first description of a mechanism by which one of them is activated, the autocatalytic amplification of MPF. However, at that time it was unclear whether MPF is a cell cycle regulator for both meiosis and mitosis, although we anticipated that this would be the case because of the presence at low levels of MPF activity in blastomeres undergoing mitosis. Indeed, MPF was subsequently found to appear shortly before cells entered mitosis in frog eggs (1585; 1570), cultured mammalian cells (1571), and yeast cells (1586). This suggested that MPF is a ubiquitous cell cycle regulator that drives the cell from G2 into mitosis in all eukaryotes.

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We still had no idea of what MPF is, however. To investigate its chemical nature, I attempted its extraction from frog eggs. This was unsuccessful until I found that MPF is quickly inactivated when eggs are homogenized, but remains active in the liquid fraction when eggs are opened by crushing them with centrifugal force (1568). Centrifugation of egg extracts through sucrose density gradients showed that MPF activity sedimented in each of three discrete peaks of 4, 15 and 32S (1568), the first evidence that MPF activity might reside in a single component (rather than being a property of a large number of interacting components in the cytoplasm). RNAases had no effect on MPF activity, but it was quickly inactivated by Ca2+ ions and proteases (1586), and stabilized by ATP and protein phosphatase inhibitors (1588). These results strongly suggested that MPF contained at least one phosphoprotein and was dependent on phosphorylation for its activity (for review see 1576).

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Amazingly, much of the cell cycle would occur in extracts. Nuclei added to extracts prepared immediately after eggs had been "activated" to mimic fertilization replicated their DNA, broke down their nuclear envelopes, and formed metaphase chromosomes, all very much as in vivo(1572). Modified versions of these extracts could perform up to four complete cell cycles (1573). Simpler extracts prepared from unfertilized eggs caused the formation of metaphase chromosomes from the chromatin of added nuclei, consistent with the arrest of the eggs in metaphase. If, however, the MPF in the extract was inactivated by addition of Ca2+, nuclei added subsequently behaved as in interphase, proving the important point that MPF can induce the transition into metaphase (1574).

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A slightly modified form of these extracts, in which nuclei would break down and condense their chromatin only after MPF was added, provided an assay that allowed MPF to be purified and characterized in molecular terms (1569). It proved to be composed of two subunits, cyclin B and cdc2, which together form an active kinase required for entry into the metaphase states of both the meiotic and mitotic cell cycles (see The discovery of as the key regulator of the cell cycle). The kinase itself is regulated by a complicated set of phosphorylations of the cdc2 subunit, and is capable of activating itself through a loop of other protein kinases and phosphatases. This loop causes a small initial amount of cdc2/cyclinB which appears at the beginning of the G2/M transition to amplify itself irreversibly, committing the cell to enter an M phase. This is the basis of the autoamplification of MPF that we observed in oocyte cytoplasm. Once amplified, MPF phosphorylates a large number of substrates which then carry out the events of mitosis or meiosis. The role of MPF, then, is to make the decision when to enter an M phase and to inform the rest of the cell by phosphorylation (for review see 1587) (and see M phase kinase regulates entry into mitosis).

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But what about the beginning, both of my own work and of oocyte maturation? How is MPF initially activated by progesterone to initiate maturation? The hypothesis that progesterone action is localized to the surface of the frog oocyte was supported by successful induction of maturation by external application of a polymer-conjugated progesterone derivative, which cannot enter the cell because of its large size(1576 for review). Nonetheless, neither the exact target of progesterone action nor its receptor has been identified. Although some of the intermediate steps which connect progesterone to the production of MPF are now known, such as adenyl cyclase and PKA inhibition, synthesis of the Mos protein, and activation of MAP kinase, the precise mechanism of how progesterone acts on the frog oocyte to give rise to MPF still remains unclear (for review see 1569).

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Acknowledgments

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The author thanks Mrs. Stacey Hayden for her assistance in preparing this article.

The author

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Yoshio Masui was born in Kyoto, Japan in 1931. He graduated from Kyoto University in 1953 with a B.Sc. in zoology. He earned his M.Sc. (1955) and Ph.D. (1961) from Kyoto University for his study of the effects of lithium ions on embryonic induction in amphibians. He taught at the Department of Biology at Konan University, Kobe, Japan as Lecturer from 1958 and as Assistant Professor from 1965. During a sabbatical leave in 1966 and 1967 he joined Clement L. Markert's lab at Yale, where he first studied lactate dehydrogenase isoenzymes in penguin embryos and then initiated his own study of frog oocyte maturation. In order to complete this study he resigned his position at Konan University in 1968. He finished the project in 1969, and it was published in 1971 with Markert as coauthor. After finishing the project in 1969 he moved to Toronto, Canada to assume a position as Associate Professor in the Department of Zoology, University of Toronto, where he continued his research on cell cycle regulation using oocytes and early embryos until his retirement in 1997. In 1998, he was elected a Fellow of the Royal Society, and received the Lasker Basic Medical Research Award jointly with L. Hartwell, and P. Nurse.

Yoshio Masui
Department of Zoology
University of Toronto
Phone: 416 978 3493
Fax: 416 978 8532
E-mail: masui@zoo.utoronto.ca

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Last Revised on September 10, 2004

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reviews
  • 1569 Maller, J. L. (1998).  Recurring themes in oocyte maturation.  Biol. Cell 90, 453-460.  PubMed  
  • 1575 De Terra, N. (1969).  Cytoplasmic control over the nuclear events of cell reproduction.  Int. Rev. Cytol. 25, 1-29.  PubMed  
  • 1576 Masui, Y. and Clarke, H. J. (1979).  Oocyte maturation.  Int. Rev. Cytol. 57, 185-282.  PubMed  
  • 1577 Gurdon, J. B. and Woodland, H. R. (1968).  The cytoplasmic control of nuclear activity in animal development.  Biol. Rev. Camb. Philos. Soc.  43, 233-267.  PubMed  
  • 1583 Zeuthen, E. (1964).  The temperature-induced division synchrony in Tetrahymena. In Synchrony in Cell Division and Growth Zeuthen, E, ed. (New York: Interscience), pp. 99-158.
  • 1584 Markert, C. L. (1958).  Chemical concepts of cellular differentiation. In A symposium on Chemical Basis of Development McElroy, W B, and Glass, B, eds. (Baltimore, MD: The Johns Hopkins University Press), pp. 3-16.
  • 1590 Johnson, R. T. and Rao, P. N. (1971).  Nucleo-cytoplasmic interactions in the acheivement of nuclear synchrony in DNA synthesis and mitosis in multinucleate cells.  Biol. Rev. Camb. Philos. Soc. 46, 97-155.  PubMed  
reviews
  • 1568 Wasserman, W. J. and Masui, Y. (1976).  A cytoplasmic factor promoting oocyte maturation: its extraction and preliminary characterization.  Science 191, 1266-1268.  PubMed  
  • 1570 Gerhart, J., Wu, M., and Kirschner, M. (1984).  Cell cycle dynamics of an M-phase-specific cytoplasmic factor in Xenopus laevis oocytes and eggs.  J. Cell Biol. 98, 1247-1255.  PubMed  
  • 1571 Sunkara, P. S., Wright, D. A., and Rao, P. N. (1979).  Mitotic factors from mammalian cells induce germinal vesicle breakdown and chromosome condensation in amphibian oocytes.  Proc. Natl. Acad. Sci. USA 76, 2799-2802.  PubMed  
  • 1572 Lohka, M. J. and Masui, Y. (1983).  Formation in vitro of sperm pronuclei and mitotic chromosomes induced by amphibian ooplasmic components.  Science 220, 719-721.  PubMed  
  • 1573 Murray, A. W. and Kirschner, M. W. (1989).  Cyclin synthesis drives the early embryonic cell cycle.  Nature 339, 275-280.  PubMed   Journal
  • 1574 Lohka, M. J. and Masui, Y. (1984).  Effects of Ca2+ ions on the formation of metaphase chromosomes and sperm pronuclei in cell-free preparations from unactivated Rana pipiens eggs.  Dev. Biol. 103, 434-442.  PubMed  
  • 1578 Dettlaff, T.A., Nikitina, L.A. and Stroeva, O.G. (1964).  The role of the germinal vesicle in oocyte maturation in anurans as revealed by the removal and transplantation of nuclei.  J. Embryol. Exp. Morphol. 12, 851-873.  PubMed  
  • 1579 Masui, Y. (1967).  Relative roles of the pituitary, follicle cells and progesterone in the induction of oocyte maturation in Rana pipiens.  J. Exp. Zool. 166, 365-376.  PubMed  
  • 1580 Burgers, A.C.J. and Li, C.H. (1960).  Amphibian ovulation in vitro induced by mammalian pituitary hormones and progesterone.  Endocrinol. 66, 255-259.
  • 1582 Masui, Y. and Markert, C. L. (1971).  Cytoplasmic control of nuclear behavior during meiotic maturation of frog oocytes.  J. Exp. Zool. 177, 129-145.  PubMed  
  • 1585 Wasserman, W. J. and Smith, L. D. (1978).  The cyclic behavior of a cytoplasmic factor controlling nuclear membrane breakdown.  J. Cell Biol. 78, R15-R22.  PubMed  
  • 1586 Weintraub, H., Buscaglia, M., Ferrez, M., Weiller, S., Boulet, A., Fabre, F., and Baulieu, E. E. (1982).  Demonstration of maturation promoting factor activity in S. cerevisiae.  C. R. Seances Acad. Sc.i III 295, 787-790.  PubMed  
  • 1587 Lohka, M. J., Hayes, M. K., and Maller, J. L. (1988).  Purification of maturation-promoting factor, an intracellular regulator of early mitotic events.  Proc. Natl. Acad. Sci. USA 85, 3009-3013.  PubMed  
  • 1588 Drury, K. (1978).  Method for the preparation of active maturation promoting factor (MPF) from in vitro matured oocytes of Xenopus laevis.  Differentiation 10, 181-186.  PubMed  

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