Microscopy as a useful tool to study the proteolytic activation of influenza viruses

The influenza virus surface protein hemagglutinin (HA) mediates the first step in the viral replication cycle, viral entry into target cells. For this, HA binds to cellular receptors, proteins or lipids modified with α-2,3 or α-2,6 sialic acid, and facilitates the fusion of the viral and the endosomal membrane – a process essential for infectious entry. However, the HAprotein is synthesized as an inactive precursor, HA0, and must be activated by proteolytic cleavage to acquire the capacity to fuse membranes. Since influenza virus does not encode proteases and HA does not undergo autocatalytic activation, the virus critically depends on host cell proteases for acquisition of infectivity and the respective enzymes are potential targets for therapeutic intervention. The type II transmembrane serine proteases (TTSPs), in particular TMPRSS2, were shown to activate influenza virus and other respiratory viruses in cell culture. Moreover, a recent study demonstrated that expression of TMPRSS2 is essential for spread and pathogenesis of H1N1 influenza viruses in mice. In this review, we provide an overview of the proteolytic activation of influenza virus, with the main focus on type II transmembrane serine proteases, and we outline how microscopy can be used to analyze the cellular localization of HA activation.