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.

The transcription and replication of viral nucleic acid are not fully understood. However, the phenomena involve coordinated and differentiated RNA, NP and RdRp activities. The package that consists of the genomic segments (sRNA), the unit of trimeric polymerase and the nucleoprotein is the elementary replication unit of the influenza virus (vRNP). Therefore, in the nucleoplasm, vRNP needs to trigger the first round of its replication cycle, i.e. copying its genomic information onto mR-NAs. Subsequently, mRNA exportation occurs in the cytoplasm. The influenza polymerase is a heterotrimeric ~250 kD complex. It plays central roles in the viral life-cycle and is directly responsible for RNA synthesis for both viral replication and transcription. Moreover, it recruits host factors such as DnaJA1/Hsp40 [145] . The PA subunit interacts with host factors such as the mini-chromosome maintenance complex (MCM, a putative DNA helicase) and hCLE/CGI-99 [146, 147] . The PB2 subunit binds to the host RNA cap (7-methylguanosine triphosphate (m(7)GTP)) and supports the endonuclease activity of PA in order to "snatch" the cap from host pre-mRNAs [148, 149] . Moreover, PB2 interacts with the acetyl-CoA found in eukaryotic histone acetyltransferases (HATs) [150] . Viral mRNA synthesis is initiated by a cis-acting viral RNA polymerase, which is part of the vRNP structure and is bound to the vRNA promoter. However, mRNAs are not able to translate the genetic message efficiently; indeed, they need to be capped. Specifically, the virus must use the pre-mRNA of the cell and, for this purpose, a coordinated process mediated by PB2, PA and the cellular Polymerase II (Pol II) is necessary. Very briefly, PB2 binds mRNAs with cellular Pol II and PA, which, by means of an endonuclease mechanism, generates capped-mRNAs, which are translatable at the ribosomal level. According to a model proposed by Fodor [151] , a transacting polymerase activity is necessary for vRNA replication. Because of the negative polarity of vRNA, positive RNA (cRNA) must be synthesized from the mRNA templates. The replication of vRNA could then be performed by a trans-acting RNA polymerase, which may be distinct from the polymerase in the packages of RNPs. Various hypotheses have been formulated regarding the proteins, for instance NPs, that trigger transacting polymerase [152, 153] . Although this assumption is contradicted by other studies, which have shown viral replication in the absence of NP [154] , the presence of newly synthesised NPs appears to be important in stabilizing the length of the segments of viral RNA. Furthermore, other studies have suggested that NEP could regulate viral RNA synthesis [155] . The importation of newly synthesised nucleoproteins and polymerases is also very important in order to assemble nucleoprotein with the negative-sense viral RNA. This event is not completely understood and requires coordinated interaction between Nuclear Localisation Signals (NLSs), PA, PB1, PB2, NP and importins [156] [157] [158] . In addition, as recent research has demonstrated [159, 160] , NP plays a major role in the assembly of vRNP by interacting with polymerase subunits and is involved in the nuclear and cytoplasmatic transportation of vRNP. Specifically, NPs are oligomerized in ring structures, which interact with viral RNA segments and also with polymerase trimers. This latter interaction allows the viral RNA to twist into a double helix with the polymerase complex at one te