Key Protein May Give Ebola Virus Its Opening

By AMANDA SCHAFFER

THE HYPOTHESIS

Ebola infection can be prevented by manipulating a common cellular protein.

THE INVESTIGATORS

Kartik Chandran, Albert Einstein College of Medicine; Dr. James Cunningham, Harvard Medical School; John Dye, United States Army Medical Research Institute of Infectious Diseases.

Of the pathogens that keep worried scientists awake at night, few rival Ebola for ruthless efficiency.

The virus contains just seven genes, yet it manages to kill up to 90 percent of the people it infects. Patients typically develop fever and fatigue, then progress to seizures, delirium, and bleeding from the eyes, nose and mouth. After the onset of symptoms, death generally occurs in eight to 16 days.

Ebola outbreaks occur periodically in Africa, where the pathogen was first discovered in 1976. Less hazardous strains have turned up in pigs in the Philippines and in laboratory animals imported into Italy and the United States. Experts fear that the virus even could serve as an agent of bioterrorism.

There is no cure or treatment for Ebola infection beyond supportive care. Nor do scientists fully understand how the virus infects its hosts.

But in work published in the journal Nature, researchers from several institutions have identified a protein in host cells that appears to be essential for infection. Cells that lacked the protein remained unharmed after exposure to the virus in the laboratory.

More strikingly, mice that were even partly deficient in the protein (as a result of genetic manipulation) became sick with Ebola but mostly did not die.

This is the first time scientists have shown that a genetically manipulated animal can survive an Ebola infection, said Judith White, a cell biologist and virologist at the University of Virginia School of Medicine who was not involved in the research. She called it an “astounding result.”

The studies greatly advance researchers’ understanding of how this horrendous virus enters cells and wreaks havoc. They also suggest a possible target that may someday be used to develop drugs, said Dr. White.

Researchers at Harvard Medical School, the Whitehead Institute for Biomedical Research in Cambridge, Mass., and Albert Einstein College of Medicine in New York cast their nets wide to identify the new target. They began with a large collection of cells, introduced random mutations and then exposed the cells to a virus designed to mimic Ebola.

This virus, called rVSV-GP-EboV, carries a protein from Ebola on its surface, but it is not deadly and so can be studied in less secure laboratories. “We asked the virus to find us cells it couldn’t infect,” said Kartik Chandran, an assistant professor of microbiology and immunology at Albert Einstein College of Medicine.

Probing these resistant cells, Dr. Chandran and his colleagues homed in on a protein, called NPC1, that appeared to be missing.

Under normal circumstances, NPC1, found within a cellular compartment called the endosome, helps bring cholesterol into the cell’s cytoplasm, where it is necessary for healthy function. People whose cells lack the protein typically develop a disease called Niemann-Pick, in which cholesterol and other lipids build up in the liver, spleen and brain.

The researchers took cells from patients with Niemann-Pick disease and exposed them to the Ebola-like virus in the laboratory. The cells survived, further indicating that Ebola relies on NPC1 to enter cells.

The academics then reached out to government scientists who could conduct tests with the real virus.

“I’m always a little skeptical” of such requests, said John Dye, a research scientist at the United States Army Medical Research Institute of Infectious Diseases. Indeed, for at least 10 years scientists have tried to pinpoint how Ebola penetrates a host cell.

They have proposed a few candidate molecules, but “none has turned out to be essential,” said Erica Ollmann Saphire of the Scripps Research Institute. One theory held that the virus is “promiscuous,” latching onto different receptors on different cells, or different molecules on the same cell, she said.

Dr. Dye and his team verified that when cells without the NPC1 protein were exposed to Ebola, they did not become infected. In addition, mice engineered to produce half the usual amount of NPC1 appeared to be protected as well: They got sick with the virus, but remarkably, most survived. Cells or mice infected with Marburg virus, Ebola’s close cousin, also survived if they lacked or were low in NPC1, the researchers found.

Meanwhile, more evidence was discovered that Ebola needs this protein to wreak havoc. Scientists led by Dr. James Cunningham of Harvard Medical School and Dr. Chandran conducted a broad search for small molecules that might prevent Ebola from entering and killing cells. After screening roughly 40,000 compounds, they zeroed in on one that seemed likely to work by blocking the interaction between the virus and NPC1.

Ebola, like many other viruses, first enters a cell’s endosome. To replicate and cause havoc, though, the virus must make its way into the cytoplasm, the main compartment of the cell. “If it stays in the endosome, there’s no infection,” said Dr. Ollmann Saphire.

Small molecules that interfere with NPC1 might help to ward off an infection or make it less severe, the researchers say.

One of the Nature papers described a molecule and a derivative that seemed to thwart Ebola’s entry into cells by interacting with NPC1. The other showed that an antidepressant called imipramine, which was known to block NPC1, seemed to prevent infection as well. None of these compounds has been tested against the virus in mice, let alone in primates. But that day may be coming.

“Scientists never like to say something is definitively it, because a new discovery always comes along,” said Dr. Ollmann Saphire. “But NPC1 is the most convincing target we’ve seen yet for Ebola.”