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PATHOLOGY Programmed Death By MitochondriaTwin Molecules Implicated Sometimes, the most honorable thing a cell can do is commit suicide. Facing critical DNA damage, for example, a cell should take arms against a sea of future troubles, such as cancer and autoimmune diseases, and kill itself.
Pursuing the apoptotic cascade in cells are (l to r) Stanley Korsmeyer; Solly Weiler, an instructor in medicine; and postdoctoral fellow Emily Cheng, a co-author on the recent Science paper. The paper reveals that cell suicide works through a pair of twin molecules (below), BAX and BAK. In experiments, cells from double-knockout mice missing the pro-apoptotic pair were nearly impossible to kill. When they get the suicide signal, BAK and BAX link with more of their kind to form pores in the mitochondria. Cytochrome c is released, leading to a biochemical cascade that kills the cell. Photo by Graham Ramsay. Illustration adapted by Jeff Cleary.
Should a cell choose the path of self-destruction, a new study shows it needs two members of a powerful apoptotic family to push a suicide button located on the mitochondria. For the first time, researchers at Dana–Farber with colleagues around the country have shown that the pro-death molecules BAK and BAX are essential gatekeepers to the mitochondrial death machinery. The results support a growing body of evidence that the mitochondria—whose energy gives cells life—also play an essential role in programmed cell death. Programmed cell death culls unneeded cells during development and growth, and it also protects an organism by triggering suicide in defective cells. In other conditions, cells too quick to self-destruct may initiate neurodegenerative diseases, immunodeficiency disorders, and infertility. A more detailed understanding of pro- and anti-apoptotic forces within cells has the potential to improve therapy for a wide range of disorders. Cell Suicide RoutinesA death signal can come from many places in a cell, but it is usually received by one or both of two main apoptotic execution programs. One pathway appears to run through a pure caspase enzyme cascade beginning with caspase-8 and ending at caspase-3. The other main apoptotic pathway—until recently thought to be merely an amplification loop of the caspase pathway—takes a more circuitous route from caspase-8 through the mitochondria to activation of caspase-3. In this pathway, caspase-8 cleaves the protein BID, whose product tBID moves to the mitochondria.Here is where BAK and BAX come in. Inactive BAK molecules reside in the mitochondria's outer membrane. BAX molecules hover outside the mitochondria in the cell's cytoplasmic soup. When they get the tBID signal, the twin hordes of henchman are thought to punch holes in the mitochondria's outer layer, linking up with each other and forming pores that leak cytochrome c and perhaps other molecules into the cell, which triggers a further cascade of biochemical reactions eventually activating caspase-3 and killing the cell. Scientists do not know if BAK and BAX act alone or recruit other membrane proteins as coconspirators. They do know BAK and BAX can cover for each other much of the time. Seeking more details about how tBID works on the mitochondria, Stanley Korsmeyer, a Howard Hughes investigator and the Sidney Farber professor of pathology at the Dana–Farber Cancer Institute, and his colleagues developed a genetically deficient BAX mouse with some abnormalities. Premeiotic germ cells in the testes failed to die, leading to infertility. The mice also had extra neurons that should have died during development, but did not. Blocking the Death ProgramMeanwhile, Craig Thompson's laboratory at the University of Pennsylvania created a BAK knockout mouse that looked surprisingly healthy. Inserting activated tBID with a retrovirus into embryonic fibroblast cells from either knockout triggered normal cell death.But the researchers suspected tBID could not shake up the mitochondria by itself. So they bred the mice for a double knockout of BAK and BAX and found a major obstacle in the death pathway. A paper they published last year showed a dramatic phenotype, including webbed digits of the paws caused by cells that could not die. Also, the mice accumulated extra cells within the central nervous and blood systems. Ironically, this lack of cell death causes most of the mice to die before or within days of birth. Now, in the latest report in the April 27 issue of Science, the researchers show evidence that the two pro-apoptotic proteins BAX and BAK constitute the critical initiating factors that launch the apoptotic process in mitochondria. "The combination of taking out both proteins was extremely synergistic, and these cells were highly resistant to apoptosis," said Korsmeyer. "It was a very impressive effect." Super CellsTry as hard as they might, Michael Wei, a Washington University medical student working in Boston, and postdoc Wei-Xing Zong, at the University of Pennsylvania, could not kill the fibroblast cells from the embryonic double-knockout mice. They saturated them with tBID, the upstream signaling protein. They bombarded them with chemotherapy drugs staurosporine (a kinase inhibitor) and etoposide (a topoisomerase II inhibitor). They dosed them with ultraviolet radiation and deprived them of growth factor. Even drugs that stress the endoplasmic reticulum, including thapsigargin, tunicamycin, and brefeldin A, didn't faze the double-knockout cells. The scientists also found that in mouse liver, BAK and BAX are required for a response to normal death signals emanating from the death receptor Fas on the cell surface."These studies show that BAX and BAK are the obligate gateway that clearly starts the process of apoptosis," said Korsmeyer. This apoptotic process involves not only disruption of function in the cell, he said, but within the mitochondrion itself—in part, because of the loss of cytochrome c from the energy-producing machinery. Given the central roles of BAX and BAK in apoptosis, said Korsmeyer, "In neurodegenerative disorders involving accelerated apoptosis, inhibiting this pro-death step might prove therapeutic. And conversely, in diseases like cancer, accelerating the activation of BAX and BAK could be a potential therapeutic as well." BAX and BAK are pro-apoptotic members of a family that also includes anti-apoptotic members. Korsmeyer has proposed a "rheostat" model of apoptosis, in which a balance of pro- and anti-apoptotic molecules determines whether a cell lives or dies in response to various stresses. —Carol Cruzan Morton |
