Cell Cycle Studies In Full Spin During The Last Few Years

While scientists have known for centuries that cells divide, only during the last couple of decades have researchers really begun to make progress in investigating the how's and why's of the division. Understanding the driving mechanisms involved--the chemical triggers-- will help scientists determine why this process sometimes goes wrong (as in the case of cancer cell growth) and, someday, may also spur the development of methods to prevent and reverse such malfunctions. Jonathon Pines, a visiting research associate at the Salk Institute for Biological Studies in San Diego, says that investigations of the cell cycle are among "the most active and stimulating areas in research today." Pines currently is doing postdoctoral work at the Molecular Biology and Virology Laboratory, under the directorship of Tony Hunter. Hunter and his group are only one of several teams of researchers that have published numerous breakthrough papers on the cell cycle during the last several years. Thirteen of those papers were featured in The Scientist's "Hot Papers" column throughout 1990 (see accompanying list). A review of these papers provides an overview of the major recent advances in the field. The flurry of research on cell division was made possible by a union of several disciplines: yeast genetics, cell biology, and biochemistry. "Several different areas of research have converged," says Sergio Mor-eno, a postdoctoral research fellow at the University of Oxford. "Many people suddenly realized that they were working on the same thing." Pines concurs: "Yeast geneticists realized they were working on the same protein as cell biologists who were working on extracts from cells because the biochemists came along and purified the proteins." The proteins that these various research groups were (and still are) investigating are the components of maturation-promoting factor (MPF), a chemical signal that induces both meiotic and mitotic cell division. Two independent research groups--one from Yale University and one from Argonne National Laboratory--discovered MPF in the early 1970s. Further studies revealed that the amount of MPF in a cell rises and falls with each mitotic division. In 1988, Fred Lohka and his colleagues at the University of Colorado School of Medicine in Denver purified MPF from frog eggs. In doing this, they discovered that MPF consisted of at least two proteins--proteins that other researchers had already been studying in other organisms, but hadn't yet associated with MPF. Using two strains of yeast--Saccharomyces cerevisiae (budding yeast) and Schizosaccharomyces pombe (fission yeast)--research teams such as those led by David Beach (The Scientist, March 5, 1990, page 22) of Cold Spring Harbor Laboratory on Long Island, N.Y.; Steven L. Reed from the Research Institute of Scripps Clinic, La Jolla, Calif.; and Paul Nurse at Oxford were able to identify the cell-division-control (cdc) genes that orchestrated the cell division cycle in these organisms. The cdc2 gene established itself as a key player: the protein product of this gene controls the replication of DNA during the synthesis, or S phase. The protein also regulates entry into mitosis, or the M phase--the step at which the cell actually divides. The product of the cdc2 gene is one of a group of proteins known as kinases, which act as switches, turning other proteins on and off by adding (or taking away) a phosphate group compound. Within a few years, homologs to the cdc2 protein kinase of yeast were found in a number of other organisms; species as disparate as clams, frogs, starfish, and even human beings. These findings helped push the yeast geneticists' research into the cell cycle spotlight. "We had been working with yeast for years," says Moreno. "No one was really paying attention to our work until it was shown that cdc2 was conserved in other species as well. People thought that something similar was working in human cells. But the level of conservation was really a surprise." In 1988, Beach and Giulio Draetta provided the first evidence of the human homolog to yeast cdc2 protein kinase's involvement in mitosis. The pair also demonstrated that the level of p34cdc2 (the human homolog to cdc2) remains constant during the cell cycle. This protein, however, is phosphorylated when going from the G1 phase (prior to DNA synthesis) to the G2 phase (after DNA synthesis). The phosphorylation of p34cdc2 brings about an association with a second protein. This second protein had also been the subject of several other researchers' studies. While working with organisms such as clams and sea urchins, Joan Ruderman (Harvard Medical School), Tim Hunt (Cambridge University), and their colleagues determined that the levels of this second protein oscillated during the cell cycle, rising steadily until the first mitotic division, then abruptly disappearing. The oscillating nature led to the protein's name--cyclin. These proteins would eventually become part of the overall cell cycle picture. The finding that the chemical triggers for mitosis are the same in yeast and in man was somewhat expected, according to Pines. "All cells grow and split into two," he says. "It would have really been surprising if the underlying mechanism [for mitosis] was different." An unanticipated discovery marked the next phase of cell cycle research. Within a month after Beach and Draetta published their evidence on the conservation of cdc2 in other species, two additional papers appeared that made the pivotal connection between cdc2 and MPF. In back-to-back articles published in Cell, two teams--one led by Beach and the other by Nurse--reported evidence proving that cdc2 protein kinase is one of the two components of MPF. Both groups determined that purified MPF from Xenopus oocytes contains the homolog of the yeast cdc2 protein kinase. Without cdc2, the nuclear membrane of the cell does not dissolve and the chromosomes do not condense on the interphase nuclei. "The fact that this protein [cdc2] that has such an important role in yeast is the same protein in MPF is perhaps the most significant breakthrough [in cell cycle research]," says Pines, who quickly adds that the same could be said about cyclin. That cyclin existed had already been firmly established. Just how this oscillating protein fits in with cdc2 and MPF was the next puzzle to piece together. A beneficial talent merger between the Cold Spring Harbor team and Ruderman's group (then at Duke University in Durham, N.C.) would soon present a solution to this mystery. Using clams, the researchers showed that cdc2 and cyclin formed complexes that display high kinase activity at mitosis. This finding identified cyclin as the second protein component of MPF. Shortly after Beach and Ruderman reported their findings, Hunt and his colleagues at Cambridge presented the first confirmation of the presence of cyclins in vertebrates. Using extracts of Xenopus eggs, Hunt showed that if cyclin is eliminated, cells cannot enter mitosis. Halfway around the world, Andrew W. Murray and Marc W. Kirschner of the University of California, San Francisco, described the role of cyclin in the control of MPF activity in frog egg extracts. They confirmed that cyclin accumulates during each interphase of the cell cycle, then degrades at the end of mitosis. They also showed that the cyclin level has to reach a critical threshold in order to initiate mitosis. Additionally, their research revealed that if the cyclin level does not drop, then the duplicated chromosomes will separate, but the daughter cells will not form. Cyclins, like cdc2, are conserved. That is, they appear in all of the various species studied thus far. Pines and Hunter were the first to prove that cyclins are found in mammalian cells and that their behavior is the same in different types of cells within one species. Their findings also provided further evidence for the universality of the fundamental controlling mechanisms for cell division in all eukaryotes. Cyclins from different species, however, are not as similar to one another as are the homologs of yeast cdc2. Pines says that "cdc2 is very, very highly conserved. Cyclins, on the other hand, are not very well conserved. You can't take cyclin from yeast and put it into human cells and expect the same thing." Cyclin, therefore, may be what makes a yeast cell a yeast cell and a human cell a human cell. Although cell biologists, geneticists, and biochemists have made a great number of advances in cell cycle research these last few years, both Pines and Moreno say there is still much more work to do and many more discoveries to make. There are still numerous unanswered questions: What, exactly, activates cdc220protein kinase to go from one phase in the cell cycle to another? Which phosphatases and kinase receptors are at work during the cell cycle? How does understanding the cell cycle fit in with our understanding of growth and cell differentiation? How can all of this knowledge be applied to cancer research? "In the next couple of years," says Moreno, "research will concentrate on the transition from the G1 phase [pre-DNA synthesis] to the S phase [when DNA synthesizes]. More labs will concentrate their efforts on identifying which proteins are involved in this [G1 to S phase transition] part of the cell cycle." And if one looks further toward the horizon of cell cycle research, Pines simply yet boldly predicts: "We'll have a complete understanding of how the cell divides."

Cell Cycle Studies In Full Spin During The Last Few Years

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