By Dean A. Haycock
Special To BioWorld Today
Ebola remains the flagship virus of emerging diseases. Its high mortality rate and horrifying symptoms, including widespread hemorrhaging, have earned it a secure place as a nightmare in the popular imagination.
It has been the subject of a best-selling book, "The Hot Zone," and the inspiration for a star-studded movie, "Outbreak." Such notoriety might suggest scientists know quite a bit about the virus. Unfortunately, they know very little.
But a clue to the virus' lethal secrets came in the Feb. 13, 1998, issue of Science, in a report titled "Distinct Cellular Interactions of Secreted and Transmembrane Ebola Virus Glycoproteins." The clue resides in the properties of one of only seven products of Ebola's negative-stranded genome, a glycoprotein (a sugar-protein compound).
A team of researchers from the University of Michigan, in Ann Arbor, and the Centers for Disease Control and Prevention, in Atlanta, report that the Ebola glycoprotein performs two jobs for the deadly virus.
Previous work had shown that one form of the glycoprotein remains bound to the virus and another is secreted. The 50 to 70 kilodaltons (kD) form is secreted soon after Ebola invades a victim. The bound version of the glycoprotein is larger, 120 to 150 kD, and is structurally suited for spanning cell membranes. Its structural differences from the secreted form are attributed to RNA editing.
Gary Nabel, professor of internal medicine and biochemistry at the University of Michigan and investigator at the Howard Hughes Medical Institute, and his co-authors first prepared a harmless avian retrovirus which expressed the bound form of the Ebola glycoprotein. They tagged the Ebola-derived molecule with fluorescent antibodies. Then they exposed various types of cultured human cells to the modified virus. The fluorescent tag allowed the researchers to see which, if any, cells the virus infected.
Mechanism Of Ebola Entry To Cell Found
They found the Ebola glycoprotein enabled the modified virus (which can not normally affect human cells) to bind to and enter endothelial cells, the cells that make up the walls of blood vessels.
Since Ebola is a hemorrhagic virus — it produces bleeding inside and outside the body — this finding is potentially highly significant. It raises the possibility that Ebola uses the glycoprotein to latch onto and enter the blood vessel cells, where it could weaken them and so produce the profuse bleeding that characterizes the disease.
The team also uncovered an intriguing property of the secreted form of the glycoprotein. A shortened version of the bound form, this viral product appears to grab onto receptors on human immune cells called neutrophils.
Neutrophils trigger inflammation, an early biological response to injury or invasion. By attacking neutrophils soon after it enters the body, Ebola may have a way to disarm its victim early in the infection. It may be possible to use the glycoprotein as a target to counter the deadly and horrifying effects of the virus.
The researchers studied the Zaire strain of Ebola. While the glycoprotein is different in the three other strains of the virus, Nabel noted all have two forms.
"They are different but they are likely to be using the same mechanism," Nabel said.
Obviously, experiments in cultured cells must be confirmed in live animals, a task the researchers have begun. The researchers also are attempting to understand the mechanism by which endothelial cells are damaged, and they are continuing efforts to develop an Ebola vaccine. (See BioWorld Today, Dec. 31, 1997, p. 1)
"The present study really grew out of the earlier studies of the virus and vaccine strategies," Nabel said. "As we were doing these studies, it became clear to us that there were a lot of very fundamental questions about the virus that we didn't know the answer to. We realized we had a ready source of many of the viral proteins in our hands by virtue of our ability to make them using recombinant DNA technology. So suddenly, many of the questions that had been difficult to ask in the past, because we couldn't work easily with the virus, became much easier to address."
The results provide important insights into the strategies Ebola uses and point to a viable target for fighting the disease.
They also suggest another line of research that will interest many in the biotechnology community. The viral glycoprotein that appears to enable the virus to enter endothelial cells might be developed as a tool for getting therapeutic genes to those same cells. Introducing genes that alter the function of cells lining blood vessel walls could conceivably be beneficial in the treatment of heart disease and cancer.
In the case of heart disease, viral vectors equipped with a version of the Ebola glycoprotein and loaded with a therapeutic gene might be used to induce the growth of new blood vessels to replace older ones blocked by arteriosclerosis.
A different therapeutic gene, designed to destroy blood vessels, might be specifically targeted to vessels carrying blood to tumors and so kill cancer cells by depriving them of nutrients.
The first step toward these applications of the recent Ebola investigations was crucial to the experiments described in the Science paper.
"The study itself does show that you can make a retrovirus that carries the Ebola glycoprotein on its surface," Nabel said. *