Influenza Virus Morphogenesis: in the Cell - Methods and Observations

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Influenza virus morphogenesis is a complicated cascade of interactions between host and viral factors. The study initiated and is ongoing over the last five decades. There are groups of viruses where genetic segments can exchange leading to reassortment which may result in the emergence of the new pandemic strains and another group where the genetic material exchange never takes place naturally, the best example is between Influenza A and B viruses. This exchange of genetic material or accumulation missing fragments of semi infectious Influenza particle actually takes place during assembly phase. Despite the identification of several structural features during past several years which allow individual fragments to be part of “7+1” configuration during assembly and budding process but we are no closer to predicting the emergence of novel re-assorted strain (that can lead to an outbreak) based on earlier identified packaging signal of individual vRNA segments. Therefore the universal rule which governs the assembly process needs to be revisited and identify the factors which can influence or regenerate novel strain with pandemic potential. Recent reports indicate influenza particles produced from infected cells are diverse species having less than “7+1” configuration which form semi infectious virus particle as it cannot complete its life cycle without help of the missing fragments. A recent report suggests that clinical isolates of both A and B influenza viruses produce more virus particle with less than 8 genomic segments when compared with laboratory-adapted strains. Role of this diverse viral species (less than 7+1) still not clear although they thought to be involved in the emergence of novel genotypes but no systemic study to prove this hypothesis. Another possibility is that these semi infectious virus particles actually help to neutralize immune responses by interacting with host antibodies and allow the infectious particle to escape and start a new round of infection.

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Previous studies also suggest that clinical isolates repeatedly exhibiting the filamentous phenotype, the role played by these long filaments in the infection course is yet to be established. Researchers have been suggested to be involved in cell-to-cell transmission. Spherical virions are known to enter host cells through the utilization of clathrin-mediated endocytosis, the entry pathway for filamentous virions take place by an alternative, non-clathrin, non-caveolin-mediated entry pathway referred to as micropinocytosis. The functional significance of these diverse structures remains to be confirmed. Recent data by Hutchinson et al have shown that conserved core structure of virions incorporate a distinct set of host proteins depending upon their origin (isolated either from mammalian tissue cultures or embryonated chicken eggs) which advocate that distinct virions are associated with each host and that virion composition may need to switch for the virus to cross species barriers. However, the function of these proteins during virus life cycle remains largely unknown although few of them such as the role of host CD81 in virus uncoating and budding has recently been established.

Intrinsic budding property of several influenza surface protein has been used to form virus-like particle efficiently and economical choice for global vaccine development for influenza. Fusing this inherent budding property of influenza HA, NA, and M1 with the surface glycoprotein of Ebola and Nipah can also be used for the large-scale vaccine development programme. A recent increase in the drug resistance phenotypes of influenza virus leads to the development and screening of novel antiviral targeting assembly or budding process of the virus. Recently it has been found that VX-787 is a compound that binds to key residues in the PB2 cap-binding domain thus interfering with transcription and replication process of IAV. Another option is by targeting host proteins such as Rab GTPases which can be an option as it involved in viral assembly and transport of genetic segments to the plasma membrane. Although, they are involved in constitutive and regulated secretory pathways of host cells, and may be required in protein transport as well. Therefore challenge relies upon without altering the host cellular function of these proteins how we can use them as a target for antivirals. The Role of cholesterol during budding has been well established. Uses of anti-cholesterol drug targeting cholesterol biosynthesis pathway were thought to be a good choice, but population-based research has identified the very modest effect on morbidity and mortality during influenza infection. Excitingly, a phase II trial has started to test the effectiveness of atorvastatin in reducing influenza virus-induced disease severity in patients infected with seasonal flu infection. The result of this study is not published yet.

Therefore several unanswered questions still exist during assembly and budding, further studies will help us solve these mysteries to better understand the evolution of the viruses and identification role of different host factors will allow a novel antiviral target to control influenza.

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