The Interferon Gateway

Para my xo viruses and the Interferon Gateway

1. Introduction

TheParamyxoviridaehousehold of viruses contain a diverse scope of established and emerging pathogens. Paramyxoviruss have a negative sense, individual stranded RNA genome. These include Parainfluenza Virus ( PIV ) , Measles Virus ( MeV ) , Mumps Virus ( MuV ) and Respiratory Syncytial Virus ( RSV ) ( Goodbourn and Randall 2009 ) . These viruses are major human and carnal pathogens. Vaccines are available for MeV and MuV, they still cause important mortality where in 2008 MeV caused 164,000 deceases worldwide ( WHO 2009 ) . RSV and PIV infect chiefly babes and babies, and can ensue in life endangering bronchitis and pneumonia. Nipah Virus and Hendra Virus are emerging zoonotic pathogens harboured in fruit-bats that have caused 19 eruptions since 1994 and can do bleedings and pneumonic hydrops ( McCormack 2005 ) . Although much research has been conducted into their construction, reproduction and infection schemes, how the paramyxoviruses interact with the host innate immune response has yet to be elucidated. How paramyxoviruses specifically interact with the host innate cellular tracts will be the focal point of this undertaking.

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The innate response is the first line of defense mechanism to infection, dwelling of a scope of extracullar defense mechanisms such as complement and macrophages, and intracellular immune responses. These act to forestall initial viral infection of host cells, to extinguish virus post-infection, and to trip an antiviral province in septic and neighbouring clean cells to decelerate viral airing, hindering viral infection and reproduction before the activation of the adaptative immune response. The intracellular immune response consists of a figure of tracts that recognise general characteristics of viral pathogens by pattern acknowledgment receptors ( PRRs ) which detect pathogen associated molecular forms ( PAMPS ) ( Randall and Goodbourn 2008 ) . The antiviral province is generated by the action of interferons ( IFN ) ( Stetson and Medzhitov 2006 ) . IFNs are a category of cytokine that act as the “ gatekeepers ” of innate and adaptative unsusceptibility, orchestrating extracellular and intracellular antiviral immune responses. IFNs are indispensable for bring forthing the antiviral province in both septic and neighbouring clean cells by bring oning the look of Interferon Stimulated Genes ( ISGs ) . IFN-a is expressed predominately in plasmoidal dendritic cells, whereas IFN- & A ; Beta ; is expressed in most nucleated cells ( Eisenacher, Steinberg et Al. 2007 ) . Unsurprisingly, viruses have evolved assorted schemes to actively hedge and overthrow the host innate immune response, showing viral proteins that act as “ keys ” , “ locking ” or suppressing multiple degrees of the Interferon Gateway to enable viral reproduction and assembly in the cell.

This undertaking will take to place the distinguishable mechanisms by which paramyxovirus proteins interact with the host cellular tracts. This could therefore characterize the virus infection itself, or offer new penetrations into the workings of the host immune response. Subsequently, fresh drug marks could be discovered that could help specific host cellular factors or impair viral equivocation proteins. Present antiviral drugs are either excessively toxic, aiming non merely the virus but host cellular tracts, or are uneffective due to the coevals of viral opposition to the drugs. This reappraisal will analyze our current cognition of paramyxovirus and Interferon Gateway interactions.

2.1. Introducing the Paramyxoviruss

Paramyxoviruss consist of a coiling nucleocapsid nucleus, enveloped with a host cellular derived membrane. They are comparatively little viruses between 150-350nm in diameter with a genome 15-19kb in size. TheParamyxoviridaehousehold is divided intoParamyxovirinaeand thePneumovirinaesubfamilies. This reappraisal will concentrate on all the genera ofParamyxovirinaesubfamily apart from avulaviruses.

Paramyxovirus categorization.

This categorization is based preponderantly on sequence homology and genome organisation. Abbreviations: SeV, Sendai virus ; HPIV, human parainfluenza viruses ; BPIV3, bovine parainfluenza virus 3 ; MuV, mumps virus ; SV5, simian virus 5 ; HeV, Hendra virus ; NiV, Nipah virus ; MeV, measles virus ; CDV, eyetooth distemper virus ; RPV, rinderpest virus ; PDV, phocine distemper virus ; NDV, Newcastle disease virus ; APMVs, avian paramyxoviruses ; HRSV, human respiratory syncytial virus ; BRSV, bovid respiratory syncytial virus ; HMPV, human metapneumovirus ; TRTV, Meleagris gallopavo rhinotrachetis virus. Diagram taken from ( Goodbourn and Randall 2009 ) .

The envelope contains virally encoded glycoproteins, the F ( Fusion ) and HN ( Hemagglutinin and Neuraminidase ) proteins that are involved in merger between the host cell membrane and viral envelope. HN is varied between different genera, HN for respiroviruses, rubulaviruses, and avulaviruses, H for morbilliviruses and G for henipaviruses and members of thePneumovirinaesubfamily. The genome is complexed with Nucleoprotein ( NP ) , where the overall virus construction across paramyxoviruses ( Figure 2 ) .

The reproduction rhythm of paramyxoviruses takes topographic point entirely in the cytosol ( Lamb 2006 ) . The viral RNA polymerase binds to the 3 ‘ booster of the templet strand, transcribing a 5’-triphosphorylated leader sequence that acts a transcriptional booster for the stable written text of messenger RNA that are 5 ‘ capped and 3 ‘ polyadenylated by the viral polymerase. Paramyxoviruses contain around 7-10 tandemly linked cistrons. The polymerase transcribes mRNAs consecutive, by ending and reinitiating written text at each cistron junction, where most of the messenger RNA are translated into one protein. The copiousness of viral proteins is regulated by their place in the genome, where following written text expiration, the polymerase can neglect to reinitiate, organizing a quantitative gradient of viral messenger RNA. Therefore cistrons at the 3 ‘ terminal are transcribed more than cistrons at the 5 ‘ terminal. Viral messenger RNA are so translated by host cell ribosomes.

However, thePhosphoruscistron can show extra accoutrement proteins, most notably the V and C proteins. This means that paramyxoviruses can encode more proteins than their cistrons, therefore get the better ofing this possible restriction and leting the development of cistron merchandises that can interact with host cellular factors for viral equivocation as opposed to entirely being involved in viral reproduction and assembly.

All the viruses ofParamyxovirinaeencode a characteristic redaction site in the P cistron except hPIV1. The V proteins are expressed from edited RNA incorporating a individual G residue inserted at the redacting site. However, Rubulaviruses produce the P protein by redacting V protein messenger RNA with two2 inserted G residues. The V protein is a ~25- to ~30-kDa polypeptide that portions an N-terminal sphere with the P protein but has a distinguishable, extremely conserved C-terminal sphere as a consequence of frame displacement due to inserted G base ( s ) ( Lamb 2006 ) . The C-terminal sphere can organize a “ Zn finger ” construction dwelling of a Zn adhering sphere formed from histidine and cysteine residues, adhering two atoms of Zn molecules per V protein ( Liston and Briedis 1994 ) . This is of import as many cellular protein: protein interactions involve Zn fingers such as in proteasomal mediated debasement of cellular proteins.

Respiroviruses Morbilliviruses and Henipavirusesexpress W/D proteins from messenger RNA with two inserted G residues. In add-on to these merchandises, morbilliviruses the P/V/W/D messenger RNA can be translated utilizing alternate reading frame ( s ) to bring forth ill conserved “ C ” protein ( s ) ( Grogan and Moyer 2001 ; Garcin, Marq et Al. 2002 ) . Although merely a individual C protein is usually made, SeV can use 4 distinguishable start codons to bring forth C ‘ , C, Y1, and Y2 proteins that have a different N-terminus but a alone C-terminus ( Figure 3 ) .

The C proteins ( C ‘ , C, Y1 and Y2 ) are encoded by an overlapping unfastened reading frame of the upstream parts of the P and V messenger RNA with multiple translational start codons and a common expiration codon. Modified from ( Grogan and Moyer 2001 ; Garcin, Marq et Al. 2002 ) .

The V and C proteins take part in the viral reproduction rhythm, but have been implicated in viral equivocation of host innate immune response, which will be discussed shortly.

2.2. Introducing the Interferon Gateway

As discussed antecedently the host innate immune response consists of tracts that can recognize and react to viral ligands. Paramyxovirus genomes and viral reproduction merchandises can be detected in the cytosol by cytosolic detectors of viral nucleic acids. Melanoma distinction associated cistron 5 ( MDA5 ) and Retinoic acid inducible cistron I ( RIG-I ) detect viral via RNA helicase spheres ( Heim 2005 ) . The cytosolic detectors initiate a signal transduction tract climaxing in the written text of IFN-a/ & A ; Beta ; ( Figure 4 ) .

RIG-I and MDA5 are both activated upon the binding of viral dsRNA to their helicase spheres, whilst RIG-I is besides activated by viral ssRNA with a 5 ‘ triphosphate cap, CARD: CARD domain interactions between the detectors and downstream proteins such as CARDif/VISA/MAVS/IPS-1 adapter protein mediate the activation of the signal transduction tract, climaxing in the phosphorylation and activation of the written text factors IRF-3, 7 and NF- ? B. C-Jun and ATF-2 are phosphorylated by an vague mechanism, come ining the karyon and binding to the IFN-a/ & A ; Beta ; booster, ensuing in their written text. Taken from ( Randall and Goodbourn 2008 ) .

MDA5 and RIG-I bind to hanker nucleic acids, a characteristic of viral dsRNA produced from complementary tempering of ssRNA from paramyxoviruses viruses ( Loo, Fornek et Al. 2008 ) . Recent surveies demonstrate that RIG-I besides binds to ssRNA with a 5 ‘ triphosphate group. Host ssRNA ( messenger RNA ) is post-transcriptionally modified with a 5 ‘ cap construction, which prevents sensing by RIG-I and subsequent auto-activation of the innate immune response to host nucleic acids ( Kato, Takeuchi et Al. 2006 ) . Signal transduction is mediated via adapter protein domains called Caspase Activation and Recruitment Domain ( CARDs ) that interact with homologous CARDs found on the RIG-I/MDA5 and downstream signalling ( Hiscott, Lin et Al. 2006 ) . IFN- & A ; Beta ; booster stimulator 1 ( IPS-1 ) is a cardinal mediator of MDA5 and RIG-I signalling, triping the IFN-a/ & A ; Beta ; written text factors via the enlisting of signal kinase platforms such as TBK-1. IRF-3 is activated by phosphorylation by TBK-1 and C-Jun and ATF-2 phosphorylated by an unknown go-between. The TRAF6 composite by phosphorylates the inhibitor of NF- ? B ( I? Ba ) , whereby dissociation and subsequent debasement of the inhibitor allows NF- ? B to translocate to the nucleus bring oning IFN-a/ & A ; Beta ; look. Recent cellular surveies in HEK293 cells has shown that HPIV3 is detected by the RIG-I detector, later bring oning an antiviral response, triping IRF-3 and NF- ? B ( Sabbah and Bose 2009 ) .

Expression of IFN-a/ & A ; Beta ; in response to viral infection activates downstream signal transduction tracts, taking to the subsequent look and action of Interferon Stimulated Genes ( ISGs ) . ISGs have a radical degree of look in nucleated cells so that upon viral infection the cell can instantly mount an initial response ( Randall and Goodbourn 2008 ) . IFN-a/ & A ; Beta ; signals by adhering to the Type I Interferon receptor ( IFNAR ) in an autocrine and paracrine mode, triping the JAK/STAT signal transduction tract and subsequent ISG look ( Stetson and Medzhitov 2006 ) .

IFN-a/ & A ; Beta ; binds to the type I IFN receptors ( IFNAR ) which so form a heterodimer. They so phosphorylate their several Tyk2 and Jak1 kinases ensuing in signal transduction tract activation and the look of ISGs ( see text ) . Taken from ( Randall and Goodbourn 2008 ) .

IFN-a/ & A ; Beta ; activates the receptor associated Janus Kinases ( JAK ) , Janus kinase 1 ( Jak1 ) and Tyrosine Kinase 2 ( Tyk2 ) by adhering to IFNAR1 and 2. Jak1 and Tyk2 phosphorylate the Signal Transducers and Activators of Transcription ( STAT ) proteins. These are ISG written text factors that dimerise upon phosphorylation and translocate to the karyon, adhering to IRF-9, organizing the ISG written text factor complex IFN-stimulated cistron factor 3 ( ISGF3 ) ( Ziemiecki, Harpur et Al. 1994 ) . ISGF3 binds to the IFN-stimulated response component ( ISRE ) , bring oning written text of antiviral ISGs ( Heim 1999 ) . Over 300 ISGs have been identified with the best characterised being the dsRNA-dependent Protein Kinase R ( PKR ) cistron merchandise ( Table 1 ) . The peculiar ISGs that counter specific paramyxoviruses and their mechanism of action has yet to be defined. Recent surveies have demonstrated that for Vero cells induced into an antiviral province and infected with CPI- PIV5 ( that can non interrupt IFN ) , PIV5 written text and protein synthesis are disrupted but non entirely by PKR, OAS/RNase L and MxA. However, it was found that the output of virus from MxA-expressing Vero cells was tenfold lower than from naif Vero cells, and that there was besides a redistribution of the NP and P proteins at late times post infection proposing the action of Mxa ( Carlos, Young et Al. 2007 ) .

Catalyses the synthesis of 2′-5 ‘ adenosine phosphodiester bond linked oligomers. These activate endoribonuclease L, spliting cellular and viral ssRNA and messenger RNA, suppressing the interlingual rendition of viral proteins ( Verheijen, new wave der Marel et Al. 1999 ) and perchance bring oning cellular programmed cell death ( Dyer and Rosenberg 2006 ) .

Characterised Interferon Stimulated Genes in the literature.

RIG-I and MDA5 cistron look is upregulated by IFN-a/ & A ; Beta ; to increase the strength of the immune response. Paramxyoviruses have therefore evolved viral countermeasures to interfere with IFN and ISG look.

3.1. Viral intervention of the RIG-I / MDA5 Signalling Pathways

In order to forestall the activation of signal transduction tracts taking to the coevals of the cellular antiviral province, paramyxoviruses target the RIG-I and MDA5 cytosolic detectors. PIVs, MeV, MuV and RSV encode a V protein that interacts with the MDA5 helicase sphere forestalling its interaction with viral dsRNA and triping the signal transduction tract. The C-termini of V proteins are extremely conserved between paramyxoviruses that target worlds and those that target other species, such as avian Newcastle Disease Virus and Sendai Virus ( SeV ) . They contain a Cysteine rich C-terminus that binds to the MDA5 helicase sphere, sequestering it from dsRNA ( Andrejeva, Childs et Al. 2004 ) . As most of the viruses of theParamytenovirinaesubfamily have evolved to aim MDA5, this demonstrates its importance detection viruses ( Childs, Stock et al. 2007 ) .

RIG-I can potentially recognize 5′-triphosphorylated ssRNA generated in the cytosol during paramyxovirus reproduction. As mentioned antecedently, SeV encodes an extra C protein that is able to adhere to RIG-I, forestalling initiation of IFN- & A ; Beta ; signal transduction tracts ( Garcin, Marq et Al. 2002 ) . Infection with the SeV C protein in cells incorporating RIG-I showed a significantly decreased degree of IFN- & A ; Beta ; look compared to that of cells with MDA5 or the dominant-negative signifier of RIG-I ( with deleted N-terminal CARD spheres ) , bespeaking that RIG-I is the prevailing detector. The C protein was found to adhere to the RIG-I helicase sphere and non MDA5 bespeaking that this tract was the most of import inducement IFN- & A ; Beta ; look ( Strahle, Marq et Al. 2007 ) . Other paramyxovirus V proteins such as MeV do non hold this activity ( Childs, Stock et al. 2007 ) , bespeaking that they have other methods of immune equivocation or a divergency in viral reproduction that determines whether it can be sensed by RIG-I/MDA5 or non.

3.2. Paramyxovirus equivocation of the JAK/STAT Pathway

IFN-a/ & A ; Beta ; mediated effects depend wholly on the JAK/STAT tract for look of ISGs. So far, merely MeV has been found to hold evolved mechanisms to interfere at this degree of the Gateway. MeV V and C proteins adhere to RACK1 ( Receptor for Activated Kinase 1 ) , an adapter protein that acts a molecular scaffold for the enlisting of inactive ( unphosphorylated ) STAT-1 to the IFNARs and their respective associated kinases ( Figure 6 ) .

MeV V and C proteins organize a complex with IFNAR and the associated scaffold protein RACK1. V and C complex formation prevents the STAT proteins from accessing the JAK kinase phosphorylation spheres, ensuing in obstruction of ISG look ( see text ) . Taken from ( Yokota, Saito et Al. 2003 ) .

Molecular binding surveies demonstrated that both viral proteins bind with RACK1. However, the V and C proteins vary in their specificity to the IFNARs. The C protein interacts with IFNAR1 whilst V the protein interacts with IFNAR2, thereby suppressing the phosphorylation of the STAT proteins ( Yokota, Saito et Al. 2003 ) . Co-localisation surveies revealed that the V protein to boot captures STAT-1, forestalling it from being phosphorylated by other IFNARs non bound to the V and C proteins. The elusive fluctuations of this viral equivocation scheme inhibit JAK/STAT signalling.

3.3. STAT Protein Sequestration

Paramyxovirus viral equivocation proteins can move as platforms that “ gaining control ” STAT proteins, adhering to them at cardinal spheres and sequestering them from other cellular factors. Paramyxoviruss can forestall the STAT proteins from translocating to the karyon in a debasement independent mode through by a assortment of schemes ( Figure 7 ) , therefore suppressing ISG look.

The MeV and SeV viral equivocation proteins bind to STAT1 forestalling its phosphorylation. Nipah and Hendra V proteins bind to both STAT-1 and STAT-2, blockading their dimerisation. SeV viral equivocation proteins bind to phosphorylated STAT-1, forestalling its interaction with SH2 sphere on STAT-2 ( see text ) . The MeV P protein is able to adhere to STAT-1 and forestall its phosphorylation by the JAK kinases by adhering to the Src Homology 2 ( SH2 ) sphere of STAT-1 ( Devaux, von Messling et Al. 2007 ) . Modified from ( Horvath 2004 ; Devaux, von Messling et Al. 2007 ) .

The STAT protein SH2 sphere is a signalling scaffold that enables the STAT proteins to interact with each other, the JAK kinases and IRF-9. The JAK kinases phosphorylate STAT-1 SH2 at Tyr 110, enabling it to intercede the formation of the active STAT heterodimer by adhering to the corresponding phosphorylated SH2 sphere of STAT-2. Mutagenesis and co-immunoprecipitation surveies showed that the MeV P protein contains a Y- ( Y/H ) -V-Y-D-H sequence that is critical for the break of STAT-1 phosphorylation ( Devaux, von Messling et Al. 2007 ) . This sequence is extremely conserved within the P proteins in the Morbillivirusgenus, so presumptively other P proteins of Morbilliviruses may portion these inhibitory belongingss.

The Nipah and Hendra Virus V proteins organize a trimeric composite with the STAT heterodimer, forestalling its translocation to the karyon. Mutagenesis surveies of V protein interactions showed that the V protein interacts with the SH2 adhering sphere of phosphorylated STAT-1 ( Rodriguez, Wang et Al. 2003 ; Hagmaier, Stock et al. 2006 ) . The V protein disrupts this interaction by adhering to SH2 and enrolling STAT-2 to a different portion of STAT-1.

SeV encodes C viral proteins that can sequester STAT1 from the tract by being able to adhere to inactive and active STAT-1 SH2 domains, confabulating the double advantage of forestalling STAT1 interacting with the JAK kinases and with STAT-2 ( Komatsu, Takeuchi et Al. 2002 ) . However, the SeV C protein merely inhibits the phosphorylation of Ser 727 but non Tyr 701 of STAT-1 ( Komatsu, Takeuchi et Al. 2002 ) . These partly phosphorylated, non-functional STAT-1 proteins are still able to adhere to STAT-2, but the heterodimer formed is unable to translocate to the karyon. As viral equivocation proteins bind to their marks in competition with the normal cellular substrates, by aiming both unphosphorylated and phosphorylated STAT-1, SeV can potentially impair STAT-1 mediated signalling at two degrees, thereby more expeditiously suppressing ISG look.

3.4. Viral Induction of STAT Protein Degradation

Paramyxoviruss are able to aim the STAT proteins for debasement via the ubiquitin mediated proteasomal tract of protein debasement. RSV NSP1 is able to move as an active targeting platform, incorporating an E3 ubiquitin ligase sphere and specific motives that recruit the E1 and E2 ubiquitin transferase proteins. The V proteins of certain Paramyxoviruses portion this map by encoding a extremely conserved Ring sphere in the C-terminus required for E3 ligase activity ( Nishio, Tsurudome et Al. 2005 ) . The V proteins and RSV NSP1 are able to interact with one or both of the STAT proteins to enroll them to the platform for ubiquitination, thereby aiming the STAT proteins for proteasomal debasement ( Figure 8 ) .

Figure 8. Model of STAT protein debasement by viruses. Viral equivocation proteins bind to the STAT proteins and aim them for proteasomal debasement by their intrinsic E3 ligase activity and enlisting of the ubiquitination cofactors E1, E2, DDB1 and Cull4A ( see text ) . Modified from ( Horvath 2004 ) .

RNA intervention surveies have determined that the STAT-targeting machinery consists of extra cellular proteins including DDB1, an ultraviolet-damage induced DNA binding protein and several members of the Cullin household of SCF ubiquitin ligase fractional monetary units, including Cullin 4A. The RSV NSP1 and Paramyxovirus V proteins are basically overthrowing a normal cellular tract by a combination of virus encoded and host derived factors. In worlds, the V protein of PIV5 and Mumps Viruses aim STAT-1 for debasement ( Horvath 2004 ) , whilst the PIV2 V protein, RSV NSP1 and the SeV C protein induce the debasement of STAT-2 ( Garcin, Marq et Al. 2002 ; Ulane and Horvath 2002 ; Elliott, Lynch et Al. 2007 ) . Proteasome suppression surveies with MG132 showed that merely the larger C and C ‘ discrepancies of the SeV C protein were able to bring on STAT-2 debasement, whereas the smaller Y1 and Y2 signifiers were able to adhere to STAT-1 but had small consequence on cellular STAT-1 degrees ( Garcin, Marq et Al. 2002 ) . The catalytic turnover of STAT-1 allows paramyxoviruses to level the IFN-a/ & A ; szlig ; induced antiviral province of cells therefore easing subsequent viral reproduction.

3.5. Viral Inhibition of STAT trafficking

Viruss can barricade the atomic translocation of activated STAT heterodimers by interfering with atomic import factors. These peculiar atomic import factors are proteins that modulate the conveyance of the STAT protein heterodimer from the cytosol to the karyon of cells to bring on look of the ISGs ( Sekimoto and Yoneda 1998 ) . Unlike the other paramyxoviruses described, the V protein of MeV does non degrade the STAT proteins, where confocal microscopy revealed that it alternatively blocks atomic translocation of the STAT heterodimer which accumulated in the cytosol of virus infected cells ( Palosaari, Parisien et Al. 2003 ) . This observation is supported by affinity chromatography which demonstrated that the V protein co-purified with phosphorylated STAT-1 and STAT-2. Furthermore, MeV infection has been shown to relocalise STAT proteins to cytosolic organic structures. This indicates that MeV prevents the STAT heterodimer from interacting with the appropriate atomic import factors for atomic translocation and therefore the look of ISGs, although the mechanism remains to be elucidated.

4. Discussion

In recent old ages the Interferon Gateway has been uncovered as the cardinal portal of unconditioned unsusceptibility. The reappraisal has shown that paramyxoviruses have developed a scope of schemes for hedging and overthrowing the host innate immune response. This is despite them encoding a limited repertory of proteins. The paramyxovirus viral equivocation proteins are multifunctional, interacting with different spheres in separate degrees of the Gateway, We have merely merely begun to understand the complex interplay between viruses and the Interferon Gateway which could give farther drug marks as our consciousness of the weaponries race between viruses and host continues to turn.

However, whilst we have begun to understand how paramxyoviruses are able to interfere with the host cellular tracts involved in the look of IFN and coevals of the antiviral province, how downstream effecters such as the ISGs interact with the paramyxoviruses and frailty versa have non been to the full characterised. This will be a major focal point of the undertaking as the host-virus interactions are farther characterised.

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