Stress Granules (sg) and Processing Bodies (pb) in Viral Infections

During reaction to stress caused by viral infection, RNA granules are formed in order to protect mRNA. Stress granules (SG) and processing bodies (PB) provide cell ho-meostasis and mRNA stability. They are formed, for example , during polio virus and MRV (mammalian orthore-ovirus) infections. Some viruses, such as influenza virus and HTLV-1 (Human T-lymphotropic virus 1), block the formation of granules. In addition, there are viruses like West Nile Virus, Hepatitis C Virus (HCV) or human Herpes viruses, which influence the functioning of the granules.


INTRODUCTION
Post-transcription processes occurring in eukaryotic cells, such as modification of mRNA particles, play an important role in the regulation of gene expression, and affect the expression level of a major part of transcriptome (Wilusz & Wilusz, 2004;Eulalio et al., 2007).The processes occur through cellular enzymes, and are related to protein complexes, including mRNP (messenger ribonucleoprotein) which when translationally silenced, can organise itself into RNA granules present in the cytoplasm, namely stress granules (SG) and processing bodies (PB) (Reineke & Lloyd, 2013).At present, apart from SG and PB granules, there are also reports on exosomal granules, granules caused by UV radiation, and glucose depletion granules (EGP) (Hoyle et al., 2007;Gaillard & Aguilera, 2008;Robbins & Morelli, 2014), whose role in infections, including viral infections, is still unknown.

SG AND PB IN VIRAL INFECTIONS
Viral infections may lead to inhibition of cellular protein synthesis by affecting and activating the SG component eIF2α, although it was also reported that proteins of some viruses, for e.g.mammalian orthoreovirus (MRV) or respiratory syncytial virus (RSV), can also lead to shut off of cellular protein synthesis by binding the eIF4A translation factor (Onomoto et al., 2014), and causing formation of stress granules (SG).This was described for MRV virus (Table 2), which causes mild respiratory tract diseases (Linquist et al., 20011;Onomoto et al., 2014) and in early stages of the infection leads to increased phosphorylation of eIF2α kinase, causing SG formation.It was, however, evidenced that PKR kinase and other eIF2α kinases, are not necessary for SG induction, which confirms that the virus induces SG formation by activation via eIF2α (Linquist et al., 2010;Reineke & Lloyd, 2013).In the case of RSV virus (Table 2) infecting primarily young mammals, including infants as well as children with reduced immunity, it induces SG formation.This was evidenced (Linquist et al., 2010) by stating a correlation between higher level of M2-1 and M2-2 proteins of RSV, and the occurrence of SGs, that an infection with RSV not only causes eIF2α phosphorylation, but also PKR activation (Linquist et al., 2010).Also, in the case of Polio virus (Table 2), which causes palsy in children, it was reported that regardless of eIF2 kinase activation, stress granules are induced already in an early phase of the viral infection (Table 2), as granules containing G3BP and eIF4GI are formed as early as in 2-3 hours post-infection, while during the further course of infection their formation decreased.It was also recorded that SG granules are separated through the impact of 3C proteinase of the virus, the consequence of which is the inhibition of mRNA translation initiation factors, as well as mRNA-binding proteins in such granules (White & Lloyd, 2012).In turn, SFV (Semliki forest virus) (Table 2), causing mortal encephalitis in rodents, was originally characterised as a factor modulating cellular response to stress (McInerney et al., 2005).
It was finally adopted that infections with such virus induce eIF2α kinase phosphorylation and SG formation in fibroblasts of mice embryos.It was also stated that despite shutting off cellular protein synthesis, SFV still affects the translation process of its own mRNA.In the case of infection with Rubella virus, causing a contagious disease in children, reports include formation in the cytoplasm of G3BP-1 protein clusters, yet these are atypical SGs, because they lack such proteins as PABP or TIA-1 (White & Lloyd, 2012).
In the case of some viruses, it was stated that in order to assure their survival, they have found a way to avoid an unfriendly environment by inhibiting SG formation.This was described in the case of infections with the Measles virus (Table 2), which causes a rash disease in children, and type A flu virus -causing respiratory system diseases, stating that they do not induce stress granules, but contribute to blocking their formation.In the case of Measles virus (Table 2), it was evidenced that its inhibiting impact on SG formation in cells is probably a result of the viral C protein (Okonski & Samuel, 2013;Onomoto et al., 2014).On the other hand, in the case of type A flu virus, inhibition of SG formation is caused by its viral non-structural protein 1 (NS1) which blocks PKR kinase activation by inhibiting eIF2α kinase phosphorylation and leads to inhibition of SG formation (Khaspersky et al., 2012;Onomoto et al., 2014;Khaspersky et al., 2014).Other infections inhibiting SG formation are those due to mengovirus and Theiler's Murine Encephalomyelitis Virus (TMEV) (Table 2) (Onomoto et al., 2014;Borghese & Michiels, 2014).It was evidenced that inhibition of such granules is most probably caused by impact of non-structural Leader protein -L-protein of such viruses, yet the mechanism of SG inhibition is still unknown (Onomoto et al., 2014).Also, Human Tlymphotropic Virus (HTLV-1) (Table 2) causing acute leukaemia as a result of impact from viral regulatory protein Tax, inhibits SG formation.It was also proven that, in response to environmental stress, the protein is transferred from the nucleus to cytoplasm, and by interacting with histone deacetylase 6 (HDAC6), blocks the stress granule formation (Valiente-Echevierria et al., 2012;White & Lloyd, 2012;Onomoto et al., 2014).As to the etiological factors of acute diarrhoea in young mammals -rotaviruses, it was stated that they cause SG inhibition as a result of containing VP2, NSP2 and NSP5 proteins (Montero et al., 2008;Niedźwiedzka-Rystwej et al., 2015).
Apart from the aforementioned viruses inducing and inhibiting SG formation in mammalian macro-organisms, a third group of viruses was described which modulates such granules (Table 2).This group includes Western Nile Virus, Hepatitis C Virus, Dengue Virus, Sindbis Virus, HIV, and human herpesviruses (White & Lloyd, 2012;Reineke & Lloyd, 2013;Onomoto et al., 2014;Niedźwiedzka-Rystwej et al., 2015).In the case of Western Nile Virus causing meningitis in horses and humans, and carrying a lifecycle between mosquitos and birds, but also able to infect amphibians and reptiles (Valiente-Echevierria et al., 2012;Niedźwiedzka-Rystwej et al., 2015), it was evidenced that it suppresses the translation process by inhibiting eIF2α kinase phosphorylation (Niedźwiedzka-Rystwej et al., 2015).It was also stated that by interacting with TIA-1 and TIAR components of SGs, its genome causes reduced viral replication in infected cells (Li et al., 2002;Valiente-Echevierria et al., 2012).In the infection with HCV, it was determined that it does not only induce SG formation, but also, as a result of external stress factors, may cause the inhibition.However, additional observations point to strong activation of PKR kinase by the virus to form SGs, while GADD34 (growth arrest DNA damage-inducible 34 factor) and protein phosphatase 1 (PP1) component, causes SG inhibition by leading to dephosphorylation of eIF2α kinase (Onomoto et al., 2014;White & Lloyd, 2012).In the case of Dengue virus, it was stated that it affects translation of genetic material or replication of its RNA by binding to G3BP protein of SG granules.In the case of Sindbis Virus, it was reported that its RNA-dependent RNA polymerase -nsP4 cooperates with G3BP1 and G3BP2 markers of stress granules (SG), facilitating access of cellular mRNA into SGs (Niedźwiedzka-Rystwej et al., 2015).In turn, HIV affects various stages of SG formation, probably by inactivation of the Staufen 1 protein.In the case of human herpesviruses it is suggested that SGs are regulated by modulation of Pbp1 expression (Niedźwiedzka-Rystwej et al., 2015).
While discussing the characteristics of SGs, it must be stated that the knowledge on processing bodies is much more limited, including viral infections, as it is still unknown how PBs interact with viruses.It is suspected that the mechanism involves dispersing these structures and their components (Beckham & Parker, 2008;Balagopol & Parker, 2009;Niedźwiedzka-Rystwej et al., 2015).It was evidenced for e.g. that infections with Polio and Coxsackie viruses (Table 3) distorts PB formation in the middle of their replication cycle.It was additionally observed that PB dispersal occurs by processing of enzymes: Xrn1, Pan3 or Dcp1, as a result of their degradation by 3C protease of such viruses (Dougherty et al., 2011).In turn, the mechanism of PB formation described for type A flu virus (Table 3) is a result of interaction between viral NS1 protein and an inhibitor protein of PB -Rap55 (Dougherty et al., 2011).Such observations reveal that processing bodies can also participate in an antiviral response, for example PKR protein kinase which is activated by dsRNA of the virus and contributes to antiviral protection by gathering in PBs.This was also reported for the infection with Human Papilloma Virus (HPV) (Table 3) (Hebner et al. 2006).Moreover, antiviral role of PBs can be related to the presence of APOBEC3G and 3F proteins in such granules, as APOBEC belongs to cytidine deaminases that carry out cytidine deamination processes in the genomes of various viruses, including retrotransposons (Beckham & Parker, 2008).APOBEC3G was also reported to be included in HIV-1 virions (Table 3), and it can inhibit replication of transcriptors of such viruses by cytidine deamination, which finally leads to turning guanosine into adenosine in the positive strand of cDNA of the virus.Other retroviruses (Beckham & Parker, 2008), produce proteins inhibiting APOBEC functions, e.g.vif proteins of HIV bind to APOBEC3G, which leads to its degradation (Beckham & Parker, 2008).

CONCLUSION
Maintaining the homeostasis of a macro-organism's cells is a major challenge for living organisms, including response to various factors, e.g. a viral infection.Such a condition causes macro-organism's cells to induce many reactions assuring their stability, including formation of SG and PB granules.However, it must be stated that the data regarding such granules, including their interaction with viruses, still raise many questions; answering them would allow to explain the impact of viruses on the macro-organism's cells and to reduce such impact.