All viruses must cross a host cellular membrane to enter the cytoplasm, where the raw materials and machinery necessary for viral multiplication are present. Strategies for antiviral therapy that block cell entry are especially desirable because they provide an opportunity to interdict viral replication even before the virus can commandeer the host cell’s resources. However, no successful strategies of this type are currently available for emerging viral pathogens, in part because details of the entry-related virus-host interactions and viral protein transformations remain obscure for these agents.

We are currently focused upon uncovering the mechanisms by which the highly pathogenic Ebola and Marburg filoviruses exploit their host cells to gain entry into the cytoplasm. Our goals are to:

Determine how endosomal cysteine proteases mediate filovirus entry

We have found that specific endosomal cysteine proteases are essential host factors for filovirus entry and novel targets for development of antiviral drugs. Our initial experiments to characterize the role of these enzymes indicate that they act by carrying out extensive proteolytic remodeling of the viral glycoprotein, GP, and that a multistep proteolytic cascade targeting GP within the host cell endocytic pathway is a critical feature of the filovirus entry mechanism. This proposed role for endosomal cysteine proteases is unprecedented for enveloped viruses, which are generally considered to be in a “race” to avoid inactivation by these enzymes during entry. We are using a combination of biochemical, molecular genetic, chemical genetic, and cell biological approaches to test our working hypothesis, which is that filoviruses employ a novel principle, proteolysis, to trigger the entry-related activities of their glycoproteins.

Related publications

Wong AC, Sandesara RG, Mulherkar N, Whelan SP, Chandran K. 2009. A forward genetic strategy reveals destabilizing mutations in the ebolavirus glycoprotein that alter its protease dependence during cell entry. J. Virol. [Epub ahead of print]. (pdf) (Supplementary material)

Chandran K, Sullivan, NJ, Felbor, U, Whelan, SP, Cunningham, JM. 2005. Endosomal proteolysis of the Ebola virus glycoprotein is necessary for infection. Science 308:1643-1645. (pdf) (Supplementary material) (Commentary in New Engl. J. Med.) (NIH press release)

Identify additional host factors and pathways required for filovirus entry

We are carrying out unbiased chemical and genetic screens to identify additional host factors that are necessary for filovirus entry. We are also developing cell-based assays for steps in entry, including binding, membrane fusion, and cytoplasmic delivery of viral nucleocapsids to understand how newly-identified interactions between virus and host contribute to the filovirus entry pathway. Host factors discovered in these experiments will become potential targets for the development of anti-filovirus therapies.