Characterization of the anti-apoptotic properties of flavivirus capsid proteins

Abstract

The introduction of WNV into North America in 1999 was followed by rapid spread throughout the continent. Today, WNV is an endemic pathogen in the west, with thousands of cases of severe infection reported annually. In addition to traditional vaccine research, there is an urgent need to understand the contributions of individual virus components to the infection process. This includes the capsid protein, which has until recent times has been thought only to be a structural protein. As the sole component of the nucleocapsid, the capsid protein serves the essential function of both providing structure to the virion and protecting the viral genome. However, recent research would suggest that in addition to these functions, capsid can serve to modulate the host cell environment to create a more permissive environment for viral replication and spread. Viruses such as WNV that exhibit slower replication kinetics must employ strategies to avoid host responses which attempt to prevent viral replication. One such critical strategy is the prevention of cell death. A growing number of reports in the literature have described mechanisms whereby viral proteins can blunt the apoptotic response to permit efficient virus replication. This can be mediated through enhanced activity of the ubiquitous and highly regulated PI3K/Akt pathway. Indeed, WNV appears to utilize this pathway to prolong cell survival during infection. In my thesis, I describe the anti‐apoptotic properties of WNV and other flavivirus capsids, and demonstrate their ability to suppress apoptosis triggered by ligation of Fas. In concordance with this, I show that those capsids which block apoptosis triggered by anti‐Fas enhance phosphorylation of Akt. The inhibition of this kinase through the use of the inhibitor LY294002 prevents WNV capsid mediated suppression of apoptosis triggered by Fas ligation. In support of this first study, I also demonstrate that capsid proteins are able to promote cellular proliferation, even in the absence of growth factors. Curiously, this phenomenon includes those capsid proteins which do not protect against anti‐Fas. The characterization of these properties of flavivirus capsid proteins provides greater insight into the biology of the viruses they are derived from.