By Shana R. Spindler, PhD
Dengue infection, spread by mosquitoes carrying the Dengue virus, contributes to approximately 22,000 deaths a year according to the World Health Organization (WHO). An estimated 40 percent of the world population inhabits areas of known Dengue transmission, making the Dengue virus a global health threat.
Dengue virus infects a target cell with a linear, single-stranded piece of RNA containing the entire viral genome. The mechanism by which different viruses fuse to cell membranes varies; the Dengue virus only fuses to a membrane while being trafficked through a transient cellular compartment called the late endosome. Because the timing of viral RNA infection into the cell is critical to viral success, knowledge of the mechanism by which the Dengue virus coordinates its activities could pave the way for highly effective therapies and new infection prevention strategies.
One team studying this mechanism includes members of Dr. Leonid V. Chernomordik's lab at the National Institute of Child Health and Human Development. During preliminary experiments, his group noted that the Dengue virus could neither fuse to the outer membrane of Mammalian cells nor artificial membranes that are routinely used to study the fusion process. The team hypothesized that something must be unique about the membranes in the late endosome that would allow the virus to fuse and inject its RNA.
To identify what made the late endosome competent for Dengue virus fusion, Chernomordik's team, led by Dr. Elena Zaitseva and Dr. Sung-Tae Yang, began testing variables such as membrane composition and membrane electric charge. Through an elegant experimental design, Zaitseva, Yang, and colleagues bound the Dengue virus to a self-quenching fluorescent reporter. Upon viral fusion in the endosome, the self-quenching fluorescent reporter became diluted, and the fluorescence of the cell increased.
Using liposomes (manufactured vesicles composed of lipid bi-layers), Chernomordik's group found that the incorporation of lipids containing a negative charge, known as anionic lipids, allowed efficient Dengue viral fusion. In fact, the addition of anionic lipids to Mammalian cell membranes, or to early compartments of the endocytic pathway, permitted abnormal Dengue viral fusion as visualized by the increased fluorescent read-out. "We were very excited when we found that a lipid co-factor, rather than an additional protein co-receptor, is required for efficient membrane fusion between Dengue viral and cellular membranes," exclaims Zaitseva, "these are really the first and working quantitative assays to follow the fusion mediated by Dengue virus in the field."
Zaitseva explains that "viruses are amazingly adapted to hijack the cell's different pathways and proteins for successful reproduction," and accordingly, "the interactions between Dengue protein and a negatively charged membrane could be a new target for developing antivirals." For future work, Zaitseva rationalizes that their experimental design can be utilized to screen through large numbers of viral fusion inhibitors, bringing the world one step closer to an effective Dengue infection treatment.
Explore their work: Zaitseva et al. (2010). "Dengue Virus Ensures Its Fusion in Late Endosomes Using Compartment-Specific Lipids." Plos Pathogens 6(10): e1001131.