New clue in dengue infection

A key step in how dengue virus invades mammalian cells has been uncovered, solving an ongoing conundrum and presenting a potential new drug target for a disease that infects up to 100 million people each year.

Dengue virus envelope David Goodsell, The Scripps Research Institute

For years, researchers have struggled to fuse dengue virus with cell surfaces or even artificial membranes in the lab, preventing them from modeling how the virus -- for which there are currently no vaccines or effective medications -- infects cells. "Although the fusion step is required for dengue virus infection, this process is not completely understood," said Rosa Maria del Angel in an email, a dengue researcher at the National Polytechnic Institute of Mexico who was not involved in the research.

Now, Leonid Chernomordik and colleagues at the National Institutes of Health have pinpointed the source of the problem: Dengue requires an extra step to trigger fusion and infection. They published their findings online last week in PLoS Pathogens.

Most mosquito and tick-borne viruses, including dengue, enter a cell though endocytosis. Once inside, the viral membrane fuses with the endosomal membrane to create a pore from which the viral genetic material escapes into the cytosol (see image below). Normally, a low pH environment within an endosome is sufficient to trigger fusion and the release of genetic material. But not for dengue virus -- a 2008 study using live-cell imaging found that dengue virus fuses membranes only after the endosome has traveled deep into the cell, long after a sufficiently low pH has been reached.

Dengue virus infects a cell NIH

"They deserve a lot of credit for being creative and paying attention to this difference, which nobody else had really done," said Margaret Kielian, a virologist at the Albert Einstein College of Medicine in New York, who was not involved in the research. "They've explained a discrepancy in the field."

To identify the missing factor causing the late fusion, Chernomordik's team tagged dengue membranes with fluorescent probes, then watched for fusion with cell membranes and artificial membranes under various conditions. After years of testing, the team finally found the answer -- fusion occurs only when membranes are negatively charged, a condition present only in late endosomes. "It took quite a while to get there," laughed Chernomordik, "but the answer proved to be quite nicer than we expected."

"It is really nice data that fits well with our previous results," Jolanda Smit of the University of Groningen, author of the 2008 live-cell tracking paper, wrote in an email to The Scientist.

The virus may have evolved the strategy to prevent premature release of its genetic material, said Chernomordik, timing the release until the endosome is deep within the cell, close to the replication machinery.

The new information provides a target for future therapies to disrupt fusion as well as a needed recipe for fusion in the laboratory, added Chernomordik. Using negatively charged membranes, researchers can now model fusion in the laboratory and test new antivirals to see which of them disrupts fusion, possibly preventing dengue infection. "We now have an assay which is simple and straight-forward," said Chernomordik. "We're now in a position to screen different antivirals."

Zaitseva, E. et al., "Dengue Virus Ensures Its Fusion in Late Endosomes Using Compartment-Specific Lipids," PLoS Pathogens, 6:e1001131- 2010.


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