title: The M segment of the 2009 pandemic influenza virus confers increased neuraminidase activity, filamentous morphology, and efficient contact transmissibility to A/Puerto Rico/8/1934-based reassortant viruses.
abstract: The 2009 H1N1 lineage represented the first detection of a novel, highly transmissible influenza A virus genotype: six gene segments originated from the North American triple-reassortant swine lineage, and two segments, NA and M, derived from the Eurasian avian-like swine lineage. As neither parental lineage transmits efficiently between humans, the adaptations and mechanisms underlying the pandemic spread of the swine-origin 2009 strain are not clear. To help identify determinants of transmission, we used reverse genetics to introduce gene segments of an early pandemic isolate, A/Netherlands/602/2009 [H1N1] (NL602), into the background of A/Puerto Rico/8/1934 [H1N1] (PR8) and evaluated the resultant viruses in a guinea pig transmission model. Whereas the NL602 virus spread efficiently, the PR8 virus did not transmit. Swapping of the HA, NA, and M segments of NL602 into the PR8 background yielded a virus with indistinguishable contact transmissibility to the wild-type pandemic strain. Consistent with earlier reports, the pandemic M segment alone accounted for much of the improvement in transmission. To aid in understanding how the M segment might affect transmission, we evaluated neuraminidase activity and virion morphology of reassortant viruses. Transmission was found to correlate with higher neuraminidase activity and a more filamentous morphology. Importantly, we found that introduction of the pandemic M segment alone resulted in an increase in the neuraminidase activity of two pairs of otherwise isogenic PR8-based viruses. Thus, our data demonstrate the surprising result that functions encoded by the influenza A virus M segment impact neuraminidase activity and, perhaps through this mechanism, have a potent effect on transmissibility.
  
  
  OBJECTIVE
  Our work uncovers a previously unappreciated mechanism through which the influenza A virus M segment can alter the receptor-destroying activity of an influenza virus. Concomitant with changes to neuraminidase activity, the M segment impacts the morphology of the influenza A virion and transmissibility of the virus in the guinea pig model. We suggest that changes in NA activity underlie the ability of the influenza M segment to influence virus transmissibility. Furthermore, we show that coadapted M, NA, and HA segments are required to provide optimal transmissibility to an influenza virus. The M-NA functional interaction we describe appears to underlie the prominent role of the 2009 pandemic M segment in supporting efficient transmission and may be a highly important means by which influenza A viruses restore HA/NA balance following reassortment or transfer to new host environments.
authors.affiliation: Department of Microbiology and Immunology, Emory University School of Medicine, Atlanta, Georgia, USA.
authors.forename: Patricia J
authors.initials: PJ
authors.lastname: Campbell
authors.forename: Shamika
authors.initials: S
authors.lastname: Danzy
authors.forename: Constantinos S
authors.initials: CS
authors.lastname: Kyriakis
authors.forename: Martin J
authors.initials: MJ
authors.lastname: Deymier
authors.forename: Anice C
authors.initials: AC
authors.lastname: Lowen
authors.forename: John
authors.initials: J
authors.lastname: Steel
mesh_terms: D000818:Animals; D004195:Disease Models, Animal; D005260:Female; D006168:Guinea Pigs; D019267:Hemagglutinin Glycoproteins, Influenza Virus / Q000235:genetics / Q000378:metabolism; D053118:Influenza A Virus, H1N1 Subtype / Q000235:genetics* / Q000302:isolation & purification / Q000502:physiology* / Q000648:ultrastructure; D009426:Netherlands; D009439:Neuraminidase / Q000235:genetics / Q000378:metabolism*; D009976:Orthomyxoviridae Infections / Q000635:transmission* / Q000821:virology; D011647:Puerto Rico; D016865:Reassortant Viruses / Q000235:genetics* / Q000502:physiology*; D059386:Reverse Genetics; D014763:Viral Matrix Proteins / Q000235:genetics / Q000378:metabolism*; D014764:Viral Proteins / Q000235:genetics / Q000378:metabolism*; D014771:Virion / Q000648:ultrastructure
chemical_list: D019267:Hemagglutinin Glycoproteins, Influenza Virus; C031540:M-protein, influenza virus; D014763:Viral Matrix Proteins; D014764:Viral Proteins; C487630:NA protein, influenza A virus; D009439:Neuraminidase