|
||||||||
|
PATHOLOGY Sorting Good Eggs From Bad OnesDeath of Oocytes Could Shed Light on Infertility, Miscarriage Clusters of unfertilized eggs die and pass from a woman's body each month, but the loss pales in comparison to the culling of eggs that occurs in the ovaries at birth. More than half of the oocytes produced during fetal life die—about three million out of an original five—apparently by committing suicide.
Many worm oocytes die at an early age. Nematodes exist in one of two reproductive states—as males, producing only sperm; and as hermaphrodites, producing both sperm and eggs, which can self-fertilize in their gonads (below). Normally, more than half of worm oocytes produced in a hermaphrodite's gonads commit suicide after the pachytene stage of meiosis (above left). Rosa Navarro, Keith Blackwell, and their colleagues found that the CGH-1–deprived hermaphrodites could not bear young because their oocytes had all committed cellular suicide (above right). Images adapted from originals by Navarro/Blackwell
Despite the magnitude of the destruction, biologists know very little about why or how the cells kill themselves in the days just before and after birth. Do they commit suicide because they are defective? Does their sacrifice benefit the survivors? In their search for answers, Keith Blackwell, Rosa Navarro, and their colleagues have uncovered a critical clue—one that could lead to a better understanding not just of why some eggs die but also what makes an oocyte fertile. Though oocytes are killed by the same proteins—caspases—that operate in other cell suicides, studies in worms have suggested that the caspases are triggered by a unique set of signals in oocytes. But so far no one has been able to discover those molecular signals. Now, Navarro, Blackwell, and their colleagues have identified one such signal—a defect in a protein needed for processing RNA. In worms lacking the protein, CGH-1, oocytes underwent mass suicide. "We've identified an abnormality of oogenesis that triggers apoptosis," said Blackwell, HMS associate professor of pathology. Even when the defective cells were rescued from death, they could not be fertilized, suggesting they were nonfunctional. The findings appear in the Sept. 1 Development. The Whys of Egg Cell DeathOn the face of it, the oocytes appear to be using their suicide machinery to sort the good eggs from the bad. Yet nearly half of worm oocytes commit suicide, and it is unlikely that they all are defective. Blackwell, who is an investigator at the Center for Blood Research, believes that the oocyte could be using its apoptotic machinery to serve two cellular functions.On the one hand, apoptosis winnows out defective egg cells. But it may play a second role: as a mechanism for getting perfectly healthy cells to sacrifice themselves for a greater good. Such altruistic behavior is actually fairly common. In flies and hydra, large numbers of apparently healthy oocytes die and donate cytoplasm to their neighbors. Doing so, these "nurse cells" ensure the well-being of the survivors. Researchers have suggested that something similar may happen in worm oocytes which, early in embryonic life, live in one contiguous community. "It's a reasonable hypothesis that this apoptotic mechanism evolved to give you cells that are nurse cells and that reproduction works more efficiently this way," Blackwell said.
"When you think about what a human oocyte has to do, it is really astounding," said Keith Blackwell. "You have this cell that is ready to be kick-started and fertilized and ready to just run and go—in some cases, after decades." He appears with Rosa Navarro. Human egg cells have not been observed to donate cytoplasm to one another, though they are connected. And infertility and other reproductive problems appear to be due to the dying off of oocytes decades after birth. Still, the findings in worm oocytes could shed light on questions of human concern, such as infertility. "I think we're plugging into something that's involved with what makes a good oocyte," Blackwell said. Being able to distinguish good oocytes from bad could yield information about the potential for birth defects, miscarriages, and infertility. A Good CatchTaking time to think about questions of reproductive success was not on Blackwell's agenda until very recently. For years, his main aim was to explore, with members of his lab, how the genetic machinery of C. elegans is regulated. In their investigations, they had come upon an intriguing protein, PIE-1, which they suspected had a role in transcribing RNA. In the course of fishing for PIE-1's protein partners, they netted CGH-1.Though it is present in all animals, little was known about the molecule. When experiments revealed that it acted earlier and independently of PIE-1, Navarro, an HMS research fellow in pathology, became intrigued and set out to find its function. Using RNA interference, she inhibited CGH-1 production in every cell of the worm body. The worms could not reproduce. Normally, nematodes exist in one of two reproductive states—as males, producing only sperm; and as hermaphrodites, producing both sperm and eggs. Hermaphrodites can self-fertilize, and they can be fertilized by males. Navarro found that the CGH-1–deprived hermaphrodites were not bearing young because their oocytes had all committed cellular suicide. By blocking the caspase poisons, she rescued the oocytes from death. Though they looked normal, they were incapable of being fertilized by healthy sperm. Intriguingly, depriving worms of CGH-1 also made their sperm infertile. When Navarro blocked CGH-1 production in male worms, the worms could not fertilize wild-type hermaphrodites. Broken GermlinesUpon closer examination, the sperm and oocytes exhibited defects that could account for their inability to fertilize. Sperm normally develop pseudopods, but the membranous projections did not fully develop in worms lacking CGH-1. And the eggs displayed a defect—a protein receptor was not correctly positioned on the membrane."Some abnormality in membrane transport could account for both of those phenomena," said Blackwell, speculating that such an abnormality in membrane transport might, in turn, be the result of a defect in RNA processing caused by the lack of CGH-1. Though its exact role is unknown, CGH-1 may play an important role in processing RNAs during oocyte development. To begin, the protein is located in P granules, cellular structures associated with RNA processing. And in a paper appearing in the same issue of Development, researchers report that fly oocytes lacking CGH-1 are unable to translate certain RNAs appropriately. Blackwell and his colleagues are currently searching for equivalent RNA processing defects in worm oocytes. They are also looking for other specific defects that might trigger the cells to commit suicide. "It's not just a general sick oocyte response," he said. "This is something very specific." The findings could help illuminate the goings-on inside cells other than oocytes. In people and mice, though not worms, CGH-1 is found in cells other than oocytes. In particular it is found in cells with high turnover rates. "It is possible that these regulatory mechanisms, which in worms operate only in the germline, may be operating in other self-renewing tissues in vertebrates, where you have division, growth, and a state of quiescence," said Blackwell. Intriguingly, this is the pattern found in stem cells. Blackwell believes that the work in oocytes could lead to a better understanding of this precious medical resource. "The germline is a model for stem cells," he said. —Misia Landau |
