The different functions of PI3K during influenza virus replication

Aim Upon influenza A virus infection (IAV) a variety of signalling cascades are activated that are primarily induced by the cell to mount an antiviral response, however, are in part also exploited by the virus to support its replication. Recently, the phosphatidylinositol-3-kinase (PI3K) could be added to a growing list of seemingly antiviral-acting cellular factors that are misused by the virus to support its propagation. To elucidate the role of PI3K in IAV-infected cells we focussed on the diverse regulatory mechanisms and functions of this kinase. Methods The methods used include infection and stimulation experiments in cell-culture, siRNA-based knock-down experiments, immunofluorescence and western-blot based IAV entry assay, creation and analysis of recombinant influenza viruses in in vitro and in vivo experiments, etc. Results During IAV infection the cellular PI3K is activated by diverse mechanisms at different time points during the replication cycle. Depending on the time and mode of activation the kinase fulfils different functions in the infected cells. While PI3K was actually shown to induce antiviral activity via the activation of the interferon-regulatory factor 3, recent results further revealed virus supportive functions of the kinase. We could show that an immediate and transient activation of PI3K occurs during virus attachment and is required for efficient virus uptake. Here we identified receptor tyrosine kinases, such as the epidermal growth factor receptor as mediators of IAV induced activation of PI3K. At later time points of the replication cycle PI3K is again activated, however in a more sustained fashion dependent on the expression of the viral non-structural NS1 protein. This activity seems to inhibit premature apoptosis. It was demonstrated by us and others that PI3K activation occurs upon direct interaction of the NS1 protein to the regulatory subunits of PI3K p85 alpha and beta. Several reports proposed that two src homology (SH)-binding motifs within NS1 (aa89 [YXXXM] and aa164-167 [PXXP]) may mediate binding to p85 beta. Our work confirmed this observation. Further, mutant viruses of the NS1(Y89F) showed differences to wild-type viruses with regard to their replication fitness. More detailed analysis suggested that besides expression of the NS1 there are alternative virus-induced mechanisms to activate PI3K. Here we demonstrate that this additional inducer is viral RNA (vRNA), which accumulates during infection. Furthermore, novelties of IAV induced PI3K functions in vitro and in vivo will be presented. Conclusion The PI3K emerged as a seemingly antiviral acting cellular factor that is misused by IAV at different times of infection regulating diverse mechanisms to ensure efficient virus replication. Therefore, PI3K may is a potential cellular target for antiviral interventions against IAV infections. No conflict of interest