|
||||||||
|
RESEARCH BRIEFS
Structure Traces Steps in Dengue Virus InfectionResearchers at HMS, Children's Hospital, and other institutions have solved the crystal structure of the dengue virus in the final stages of infecting a cell. The atomic-resolution images reveal the behavior of a key viral envelope protein, which suggests the mechanism of infection used by the diverse viral family to which dengue belongs. The research, published in the Jan. 22 Nature, offers two possible strategies for blocking infection with antiviral drugs or vaccines.
The envelope protein of dengue virus undergoes a conformational change under the acidic conditions encountered early in the infection process. This change drives fusion of viral and cellular lipid membranes into a single membrane, a key event in the entry of a protein-coated virus into a host cell. The crystal structures of the dengue virus envelope protein before and after membrane fusion (on left and right, respectively) reveal the nature of the conformational change and suggest strategies for inhibiting viral entry into host cells. The green rectangles represent the fused lipid membrane. (Images courtesy of Yorgo Modis) Led by Yorgo Modis, the researchers captured 3-D images showing how a shape change in the protein causes fusion to occur. "We managed to determine the crystal structure of the envelope protein before and after the membrane fusion event," said Modis, a research fellow in biological chemistry and molecular pharmacology at HMS and Children's. Before fusion, the protein has an elongated shape. During fusion, it inserts one of its ends into the cell's membrane, while the other end remains anchored in the viral membrane. "The protein then folds in half--jackknifes on itself. This brings the two ends together and forces the cell membranes together, causing the membranes to fuse," said Modis. The research involves a major viral category known as enveloped viruses, so called because of the viruses' fatty outer membrane. Class 1 enveloped viruses include influenza and HIV. The new research focuses on class 2 viruses, responsible for causing dengue fever, West Nile fever, hepatitis C, tick-borne encephalitis, Japanese encephalitis, and other diseases. "Many of these are emerging infections," noted Howard Hughes investigator Stephen Harrison, an HMS professor of biological chemistry and molecular pharmacology and of pediatrics at Children's, who is the senior investigator on the study. "Infectious disease is a moving target, and understanding the mechanism of viral entry is one of the ways that we can be forearmed against these viruses." Enveloped viruses infect cells through a series of steps, but the key final step is fusion of the virus's membrane with the membrane of the target cell. How fusion occurs is partially known for class 1 enveloped viruses like HIV and influenza, but has been poorly understood for class 2. "We knew certain outlines of the fusion mechanism from previous work on the flu and AIDS viruses," said Harrison, director of the HMS Center for Molecular and Cellular Dynamics. "This gets us a lot further." The work yields two promising drug or vaccine targets for inhibiting viral entry, according to Modis. The first strategy would target the envelope protein in its elongated, prefusion form (see Focus, June 6, 2003). A second possible strategy would be deployed later in the fusion process, targeting the folded, postfusion form of the envelope protein. These two strategies could be adapted for use with many other class 2 enveloped viruses, the researchers said.
A Back-end Attack Against Alzheimer's PlaqueBoosting levels of either of two proteases in neurons protects mice from the buildup of toxic proteins in a mouse model of Alzheimer's disease, according to research from the lab of Dennis Selkoe, the Vincent and Stella Coates professor of neurologic diseases in the Department of Neurology at HMS and Brigham and Women's Hospital. The work, published in the Dec. 18 issue of Neuron, opens the door for new therapeutic approaches to the disease.Alzheimer's is characterized by the abnormal accumulation of the amyloid-beta (A-beta) protein in brain lesions, or plaques. The researchers, led by Malcolm Leissring, a research fellow in neurology at BWH, started with mice that expressed high levels of the A-beta protein, resulting in brain lesions and early death. Two proteases, insulin degrading enzyme (IDE) and neprilysin, are known to regulate A-beta levels, so Leissring and his colleagues made transgenic mice with double the IDE activity or eight times the neprilysin activity in neurons, compared to normal mice. A-beta protein levels in the brains of the transgenic mice were reduced, leading to little or no plaque formation. The protease-expressing mice lived longer than controls and showed no apparent ill effects. "Until only recently, most research has focused on the mechanisms of production of amyloid-beta from its protein precursor," said Leissring. "By contrast, very little attention has focused on the proteolysis of this peptide after it is produced. This is surprising, since the steady-state levels of A-beta--and, by extension, the risk of Alzheimer's disease--are determined by the balance between the two processes of production and degradation." Promoting the therapeutic degradation of A-beta presents challenges. Though genetically increasing IDE activity in mice did not seem to damage neurons, pharmacologically increasing IDE might diminish levels of another important substrate, insulin. Leissring and Selkoe report they are now testing compounds in a high-throughput screen for activators of IDE and working on other strategies to selectively target A-beta breakdown without affecting insulin. --Pat McCaffrey |
