Compounds with anti-influenza activity: present and future of strategies for the optimal treatment and management of influenza. Part I: influenza life-cycle and currently available drugs

GASPARINI, R.; AMICIZIA, D.; LAI, PL.; BRAGAZZI, NL.; PANATTO, D.

Regarding the antigenic characteristics of the core proteins (nucleoproteins [NP] and Matrix proteins [M proteins]), three influenza virus types have been identified: A, B and C. Given the relevance of Influenza A Virus (IAV) to human pathology, we will provide a brief overview of its biology and life-cycle and underline the main differences among the three virus types in terms of structural and molecular biology. The IAV particle varies in the range of 80-120 nm and is pleomorphic, being usually spherical, though cordlike forms with a diameter of 40-100 nm and a length in the range of 300 nm-20 µm can occur [11, 20, 21] . Transition from spherical to tubular form is not well understood: what is known so far is that M1 [22, 23] , M2 [23, 24] and NP [25] could play a role in determining and modulating this process. Besides genetic traits, also the phenotype of the host cell, in terms of shape and polarization, seems to influence the viral form [21, 26] . Influenza B has a similar shape, being structurally indistinguishable from IAV [11] , while Influenza C virus is usually filamentous and 500 nm long [11] . The IAV viral particle is an envelope made up of lipid rafts and spikes of two main types of glycoproteins: hemagglutinin (HA) accounts for about 80% (about 500 molecules) and NA for about 17% (about 100 molecules); M2 is the least abundant protein, with only 16-20 copies per virion [11, 27] . The particle of influenza B virus contains four proteins, namely HA, NA, NB, and BM2 [11, 28] , while the particle of influenza C virus is made up of a major surface glycoprotein (HEF, hemagglutinin-esterase-fusion protein) and a minor surface glycoprotein (CM2). These surface glycoproteins form ordered hexagonal arrays [11, [29] [30] [31] [32] . Underneath the viral membrane, M1 tightly binds the vRNPs, which consist of RNA strands (usually single strands but in certain cases double strands) [11] wrapped around NP and NEP, with a terminal polymerase ternary complex (PA, PB1, PB2). The genome is small (about 13-16 kilobases) and contains seven or eight pieces of segmented negative-sense RNA (eight segments for IAV and influenza B, seven for influenza C), each piece of RNA containing either one or two genes of 890 to 2,341 nucleotides each [11] . The genome codes for at least 11 proteins: hemagglutinin (HA) of about 76-77 kDa, NA of about 60 kDa, NP of about 60 kDa, M1 of about 28 kDa, M2 of about 15 kDa, non-structural protein type 1 (NS1) of about 26 kDa, non-structural protein type 2 (NS2) (also known as NEP: nuclear export protein) of about 11 kDa, PA of about 85 kDa, PB1 (polymerase basic type 1) of about 88 kDa, PB1-F2 of about 80 kDa [33] [34] [35] and PB2 (polymerase basic type 2) of about 91 kDa [36] . In particular, segments 1-3 code for the ternary polymerase complex (PB1, PB2 and PA, respectively), segment 4 for HA, segment 5 for NA, segment 6 for NP, segment 7 for the matrix proteins, and segment 8 for the non-structural proteins [11] . On the basis of the gene structures, segments can be divided into three classes: intronless, intron-containing and unspliced, and introncontaining and spliced. On the basis of the kinetics of the gene expression, they can be classified into "early" (segments 1-3, 5 and the unspliced segment 8 transcript) and "late" (segments 4, 6, 7 and spliced segment 8) classes. Usually, structural and functional/kinetic classifications do not correspond [37, 38] . Moreover, the presence of overlapping genes and dif