Thematic Minireview Series: Toward a Structural Basis for Understanding Influenza Virus-Host Cell Interactions *

Influenza viruses are members of the Orthomyxoviridae family of viruses. These viruses are characterized by a negative-sense (opposite sense of mRNA), single-stranded RNA genome contained within an enveloped virion as eight different RNA segments (1). There are three types of influenza virus, A, B, and C. The influenza A viruses have caused major pandemics, three during the past century, including the 1918 Spanish flu, and they continue to pose daunting public health challenges. During recent years, new avian (H5N1) and swine (H1N1) influenza A strains emerged, in 1997 and 2009, respectively. Although the avian H5N1 strain was highly pathogenic and problematic in poultry and waterfowl, the number of human H5N1 cases resulting from bird-to-human transmission was fortunately low. However, human disease caused by avian influenza H5N1 virus provided an example of a newly emerged influenza virus strain with a high mortality rate. In contrast, the 2009 swine influenza virus, a triple-reassortant virus with RNA genome segments from swine, avian, and human viruses, was readily transmissible from human to human but caused a relatively milder disease with a lower mortality rate than the 1997 avian virus. Adaptation of pathogenic avian or swine influenza virus strains to humans, with the acquired ability to undergo efficient human-to-human transmission, has the potential to produce virulent strains with pandemic potential. What then are some of the factors that may affect pathogenicity and virulence? Substantial knowledge has been gained about how influenza viruses multiply (1). The schematic and legend in Fig. 1 summarize the main features of the influenza virus multiplication cycle. The virion envelope includes two major viral glycoproteins, hemagglutinin (HA)2 and neuraminidase (NA), as well as the integral membrane viral protein (M2). Each of the eight RNA genome segments is transcribed and replicated in the nuclei of infected cells by a viral RNA-dependent RNA polymerase composed of three subunits (PA, PB1, and PB2). Progress in determining protein structures and in defining interactions that occur between viral proteins has provided new insights both about influenza virus and about the host's susceptibility to infection and disease. The first two minireviews in this issue concern influenza virus proteins. They focus on the biochemical and structural properties of the HA and NA viral glycoprotein spike components of the virion envelope (2), and the PA, PB1, and PB2 subunits of the viral RNA-dependent RNA polymerase complex (3). The third minireview focuses on the structure and function of the cellular Mx protein that is inducible by interferon and possesses antiviral activity against influenza virus (4). FIGURE 1. Schematic diagram of the influenza virus multiplication cycle. Enveloped influenza virion particles are depicted as spheres with two surface spike viral glycoproteins (HA and NA) that are major neutralization antigens of the virus. HA mediates binding ... In the first minireview, entitled “Influenza Hemagglutinin and Neuraminidase Membrane Glycoproteins,” Steven J. Gamblin and John J. Skehel, at the Medical Research Council National Institute for Medical Research in London, consider the structural basis of the functional activities of influenza virus HA and NA envelope glycoproteins (2). HA is a trimeric membrane protein with fusion activity that binds sialic acid, which serves as the cellular receptor for influenza virus, whereas NA cleaves the α-glycosidic linkage between the terminal sialic acid and the adjacent sugar in carbohydrate chains to facilitate virion release. HA and NA are important antigens recognized by neutralizing antibodies, and for influenza A viruses, multiple subtypes of each are known. Crystal structures of HA and of NA, which is one target of antiviral drugs, are presented. Insights gained about differences and similarities among the HAs of avian, porcine, and human viruses and the structural basis of the sensitivity of NA enzymatic activity to chemotherapeutic inhibitors, including zanamivir and oseltamivir (Tamiflu), are discussed. A minireview that appeared in The Journal of Biological Chemistry in 2006 by Pinto and Lamb (5) focused on the M2 protein of influenza virus, a third viral protein component of the virion envelope in addition to the HA and NA spike glycoproteins (Fig. 1). M2 functions as a proton-selective ion channel. During virus entry, M2-mediated acidification plays an important role in the release of partially uncoated viral nucleocapsids, a step necessary for viral transcription to occur by the viral RNA-dependent RNA polymerase. Like the NA enzyme, the M2 protein is also a target of antiviral drugs that inhibit some influenza A virus strains, as illustrated by the M2 ion channel inhibitors amantadine and rimantadine (Flumadine). The second minireview of the series is entitled “Influenza Virus Polymerase: Structural Insights into Replication and Host Adaptation Mechanisms” and is written by Stephane Boivin, Stephen Cusack, Rob W. H. Ruigrok, and Darren J. Hart at the European Molecular Biology Laboratory in Grenoble, France (3). The authors review progress and understanding gained from the x-ray and NMR structures of domains from the PA, PB1, and PB2 subunits of the heterotrimeric viral RNA polymerase. The influenza virus polymerase catalyzes both RNA transcription and RNA replication in the nuclei of infected cells. Biochemical activities of the polymerase complex include 5′-cap-binding activity of the PB2 subunit, endonuclease activity of the PA subunit, and polymerase elongation activity of the PB1 subunit. High resolution structures have provided insights about the intrinsic activities of the subunits, the associations that occur between them, and the role of the polymerase in host adaptation mechanisms. Understanding the structural basis of the functional activities of the influenza virus RNA-dependent RNA polymerase subunits provides an opportunity, through rational drug design, to attempt to devise drugs that might act broadly against different influenza virus strains without impairing essential cellular functions. In the third minireview, entitled “Dynamin-like MxA GTPase: Structural Insights into Oligomerization and Implications for Antiviral Activity,” Otto Haller, Song Gao, Alexander von der Malsburg, Oliver Daumke, and Georg Kochs, at the University of Freiburg and the Max-Delbruck-Centrum for Molecular Medicine in Berlin, describe new structure-based biochemical sights into the cellular antiviral protein MxA (4). Human MxA and mouse Mx1 are among the very best characterized interferon-inducible proteins known that possess antiviral activity. Eloquent animal model studies established some time ago that the Mx protein alone is able to confer altered susceptibility to infection and disease caused by viral pathogens, including influenza virus. For example, the human MxA protein is sufficient to establish an antiviral state in transgenic mice deficient in endogenous Mx1 and also lacking type I interferon receptors. Mx proteins are members of the superfamily of dynamin-like GTPases, and Mx GTPase activity is required for antiviral activity. MxA oligomerizes and alters the trafficking of viral nucleocapsid components, thereby blocking virus replication. Insights gained from the crystal structure of the stalk region of MxA are discussed in the context of a structural framework to understand the oligomerization process and biochemical basis of the antiviral activity of MxA. Much progress has been made in defining the polymerase and envelope glycoprotein crystal structures of influenza viruses, as well as the structure of the cellular innate immune response Mx protein. Knowledge gained from these structures provides a foundation for understanding how the polymerase and envelope viral proteins, together with cellular factors, including Mx, determine the host's susceptibility to influenza virus infection and disease. Elucidation of the structural basis of virus-host interactions at the atomic level has the potential to facilitate development of new and improved antiviral treatments for influenza virus infections.