Johns Hopkins Medicine
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Media Contact: Audrey Huang
July 13, 2004
CELL DEATH PROTEIN HAS SURPRISING ROLE IN CELL MIGRATION
By studying fruit fly ovaries, Johns Hopkins scientists have discovered that a protein known to block cell death also has the completely independent role of enabling normal cell movement.
The discovery creates an unexpected new path to follow in the effort to understand the biochemical steps behind cells' movement, a critical aspect of embryonic development and the spread of cancer. The work is described in the July 9 issue of Cell.
By studying fruit flies engineered to make extra use of random genes, the Hopkins team discovered that a protein called "inhibitor of apoptosis-1" (or IAP) can restore the tightly choreographed cellular movement that naturally occurs in fruit fly ovaries as egg cells mature.
"This discovery was completely unexpected," says Denise Montell, Ph.D., professor of biological chemistry in Johns Hopkins' Institute for Basic Biomedical Sciences. "Based on what was known about this protein's function in blocking cell death, there would have been no way to predict its involvement in cell migration."
Instead, graduate student and now postdoctoral fellow Erika Geisbrecht and Montell relied on "forward genetics," an approach in which the scientists alter the expression of random genes, look for a specific effect in the resulting flies and then figure out which gene was affected.
In Geisbrecht's experiments, a small number of cells in the fruit fly ovaries had a dysfunctional protein called Rac, which caused those cells not to move properly and the flies to be sterile. Geisbrecht then randomly increased production of other genes in the cells and looked for flies whose fertility was restored, an obvious sign that their cellular choreography had returned to normal.
"The idea is simple -- if the cell migration problem improved, the overexpressed gene must have fixed or compensated for the lack of functional Rac," says Montell.
The fruit fly ovary consists of about 100 egg chambers, each made of 16 cells -- 15 "nurse" cells and one oocyte, which becomes the egg -- surrounded by a layer of several hundred epithelial cells. At a certain point in development of the egg, a single small cluster of these epithelial cells normally detaches from the others and moves from the edge of the egg chamber to the center, sliding between nurse cells and coming to rest at the edge of the oocyte.
In egg chambers whose cells were missing Rac's function, the epithelial cluster rarely made it even halfway to the oocyte. Returning a working Rac protein to these cells allowed about half of the epithelial clusters to complete their journey, as did overexpression of IAP and overexpression of proteins called actin and profilin, two components of cells' internal skeletons, the researchers found.
Once IAP had been identified as the "rescuing" protein, Geisbrecht determined that IAP's effects on cell migration stem not from its previously known role in preventing cell death, but from its ability to bind to and block enzymes called caspases that chew up a variety of proteins. By doing so, IAP prevents the destruction of a whole host of other proteins.
Montell says she'll be trying to identify the "downstream" proteins that may be more directly responsible for restoring fertility in flies with dysfunctional Rac. Two initial candidates are actin and profilin, she says, and other components of cells' internal skeletons will be on the short list, too, since cells have to take apart and rebuild their skeletons in order to move.
"If these obvious choices don't pan out, we'll go back to a forward genetic screen," says Montell. "Then we can let the animal tell us what is happening."
As always, Montell and her colleagues will use genetic tricks so they can change the genes in a limited number of cells in the ovary. Doing so allows them to study genes whose functions, if altered in the entire animal, wouldn't let the insect fully develop. The use of these so-called mosaic flies is crucial for studying cell migration, since migration gets all the cells of a developing embryo where they need to be to form a normal, viable insect.
Authors on the study are Geisbrecht and Montell. The work was funded by the National Institute of General Medical Sciences, part of the National Institutes of Health.