【病毒外文文獻】2018 Feline coronavirus_ Insights into viral pathogenesis based on the spike protein structure and function

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Contents lists available at ScienceDirect Virology journal homepage Feline coronavirus Insights into viral pathogenesis based on the spike protein structure and function Javier A Jaimes a Gary R Whittaker b a Department of Microbiology College of Agricultural and Life Sciences Cornell University 930 Campus Rd VMC C4 133 Ithaca NY 14853 USA b Department of Microbiology and Immunology College of Veterinary Medicine Cornell University VMC C4 127 Ithaca NY 14853 USA ARTICLE INFO Keywords Coronavirus Feline coronavirus Feline infectious peritonitis Spike protein Cleavage activation Tropism Pathogenesis Serotype Spike structure ABSTRACT Feline coronavirus FCoV is an etiological agent that causes a benign enteric illness and the fatal systemic disease feline infectious peritonitis FIP The FCoV spike S protein is considered the viral regulator for binding and entry to the cell This protein is also involved in FCoV tropism and virulence as well as in the switch from enteric disease to FIP This regulation is carried out by spike s major functions receptor binding and virus cell membrane fusion In this review we address important aspects in FCoV genetics replication and pathogenesis focusing on the role of S To better understand this FCoV S protein models were constructed based on the human coronavirus NL63 HCoV NL63 S structure We describe the speci c structural characteristics of the FCoV S in comparison with other coronavirus spikes We also revise the biochemical events needed for FCoV S activation and its relation to the structural features of the protein 1 Introduction and historical aspects Feline coronavirus FCoV is the etiological agent of severe disease in domestic and wild felids known as feline infectious peritonitis FIP The disease was rst reported in the early 1960s by Holzworth who included it in the manuscript Some important disorders of cats Holzworth 1963 However the viral etiology was only suggested until 1966 when experimental infections of healthy animals were performed using tissues from infected cats Wolfe and Griesemer 1966 Viral etiology was nally con rmed in 1968 Ward et al 1968 Zook et al 1968 Two di erent clinical forms biotypes or pathotypes have been described for the clinical forms of FCoV Kipar and Meli 2014 The rst feline enteric coronavirus FECV is a characterized by a mild infection of the enteric tract and is considered ubiquitous in most healthy cats Pedersen 2009 The second feline infectious peritonitis virus FIPV is considered the virulent pathotype of FCoV and in contrast to FECV the disease caused by FIPV is almost always lethal However only a relatively small percentage of cats will develop FIP Brown et al 2009 Foley et al 1997 The virus was rst isolated in 1976 using autochthonous peritoneal cells Pedersen 1976 then propagated in cell culture using Felis catus kidney cells CRFK This rst isolated strain was named TN 406 and later known as FIPV I Black Black 1980 Pedersen et al 1981a However isolation and growing of FCoV has always been di cult re sulting in only few cell culture adapted strains available Within those strains WSU 79 1146 and WSU 79 1683 later known as FECV II 79 1683 and FIPV II 79 1146 respectively were reported in 1987 and since their isolation they have served as models for the study of FCoV in vitro McKeirnan et al 1987 Several studies have been carried out in an attempt to understand the evolution and emergence of FCoV Chang et al 2011 Pedersen et al 1978 Ward 1970 The rst evidence that FCoV was related to other coronaviruses CoVs was reported by Ward in 1970 Using electron microscopy he observed viral particles in tis sues from animals infected with an FIP virus and described similarities between these particles and the previously reported human coronavirus 229E and mouse hepatitis virus Ward 1970 Following this nding the relationship between FCoV and other animal coronaviruses e g dogs and pigs was reported Pedersen et al 1978 which served as a starting point for subsequent studies that addressed the close relation between some FCoV and canine coronavirus CCoV Herrewegh et al 1998 Nevertheless emergence of FCoV during the last decades is not only related to the FCoV evolution alongside other CoVs but also to the speci c behavioral characteristics of the virus i e switching from FECV to FIPV and external factors as the increasing popularity of cats as pets deriving from new and sometimes more intensive breeding methods that favor viral circulation in cat populations Pedersen 2009 https doi org 10 1016 j virol 2017 12 027 Received 16 October 2017 Received in revised form 22 December 2017 Accepted 23 December 2017 Corresponding author E mail addresses jaj246 cornell edu J A Jaimes grw7 cornell edu G R Whittaker Virology xxx xxxx xxx xxx 0042 6822 2017 Published by Elsevier Inc Please cite this article as Jaimes J A Virology 2018 https doi org 10 1016 j virol 2017 12 027 2 Taxonomy morphology and genetics FCoV known as Alphacoronavirus 1 belongs to the Alphacoronavirus genus Order Nidovirales Subfamily Coronavirinae Family Coronaviridae 1 Members of the Coronaviridae family are grouped in four genera where Alphacoronavirus and Betacoronavirus include viruses that principally infect mammals and are derived from the bat gene pool whereas Gammacoronavirus and Deltacoronavirus group viruses that infect birds and mammals and are derived from the avian and pig gene pool Table 1 Woo et al 2012 The viral structure is composed by the nucleocapsid containing the viral genome and the outer envelope Fig 1A These two elements stabilize and protect the RNA genome of the virus Masters and Perlman 2013 FCoV virions are usually spherical with a moderate level of pleomorphism with a size range between 80 and 120 nm and club like surface projections or spikes about 12 24 nm that give the virus its crown like appearance from where the coronavirus name is derived Fig 1A Barcena et al 2009 Fehr and Perlman 2015 Inside the outer envelope a helically symmetrical nucleocapsid is found protecting the viral genome a 5 capped and 3 poly A tailed single stranded positive sense RNA ssRNA approximately 29 kilobases kb in length Kipar and Meli 2014 Masters and Perlman 2013 The FCoV genome has 11 open reading frames ORFs encoding four structural proteins spike S envelope E membrane M and nucleocapsid N and seven non structural proteins the accessory proteins 3a 3b 3c 7a and 7b and the replicases 1a and 1b Fig 1B Dye and Siddell 2005 Pedersen 2009 The viral helical nucleocapsid is composed of multiple copies of the RNA binding protein N a 50 kDa protein which is composed of two domains NTD and CTD both with the same a nity to bind RNA but through di erent mechanisms However both domains are necessary for N binding to the viral RNA Chang et al 2006 Hurst et al 2009 In fact binding to the viral RNA instead of non viral RNA is suggested to be enhanced by phosphorylation of the N protein which induces the speci c structural conformation to accomplish this function Fehr and Perlman 2015 The most important function of N is to bind and protect the viral genomic RNA Fig 1A however unlike other viral nucleo capsid proteins e g rhabdoviruses and paramyxoviruses FCoV and most coronaviruses N protein is ine cient at protecting the RNA from ribonuclease activity Masters and Perlman 2013 Olsen 1993 In addition N protein has been shown to induce cell mediated immunity which suggests a possible role in vaccine response studies Hohdatsu et al 2003 The mechanism through N can induce immunity is not known but evidence of interaction between N and M proteins as well as between N and the nsp3 component of the replicase complex has been described and could be related to this feature Hurst et al 2013 Sturman et al 1980 The FCoV envelope is composed of four main elements a lipid bi layer derived from the host cell endoplasmic reticulum Golgi inter mediate compartment ERGIC and viral proteins E M and S Fig 1A M is the most abundant structural protein in the virus Masters and Perlman 2013 This medium sized about 25 to 30 kDa N linked glycosylated protein is randomly distributed along the viral envelope anchored through three transmembrane domains Armstrong et al 1984 M has an extensive C terminal endodomain and a small ecto domain about 10 of the N terminal portion which makes it less antigenic despite its high abundance Neuman et al 2011 Olsen 1993 The M protein is translated and inserted into endoplasmic re ticulum ER for viral assembly but no speci c signaling for ER re tention has been found in the protein Fehr and Perlman 2015 Nevertheless the M protein interaction with the nucleocapsid as well as its function in viral membrane remodeling during CoV assembly at the ERGIC has been described Neuman et al 2011 The small E protein 8 to 12 kDa is a type III membrane protein and is also inserted in the viral envelope but is present at a much lesser extent than M or S Masters and Perlman 2013 Both E protein C terminal endodomain and N terminal ectodomain have been described to have ion channel activity Fehr and Perlman 2015 Pervushin et al 2009 While the function of CoVs E protein including FCoV has been associated with assembly at the ERGIC compartment Corse and Machamer 2002 Kipar and Meli 2014 several studies have shown that unlike other CoV structural proteins viruses with deletions or in activation of the E protein are less virulent raising the question of the importance of this protein in viral tness Pervushin et al 2009 However this speci c feature is virus type dependent DeDiego et al 2007 The role of the E protein in FCoV replication and pathogenesis has not been studied yet However in a study performed in the CoV severe acute respiratory syndrome SARS CoV the authors demon strated that E ion exchange function in the viral membrane is relevant for viral pathogenesis as mice infected with mutated or knocked down E protein presented less clinical signs and recovered after infection while the ones infected with wild type virus did not survive Never theless the defective E protein did not a ect the viral replication Nieto Torres et al 2014 Among the viral structural proteins S could be considered the most important in terms of FCoV pathogenesis The coronavirus S protein is a class I viral fusion protein and is considered the major viral regulator in host cell entry Bosch et al 2003 White et al 2008 Viral fusion proteins are grouped in three di erent classes according to their structure and biochemical activation processes where class I proteins are characterized by predominant helical secondary structures and a trimeric organization of their pre fusion and post fusion state Harrison 2013 White et al 2008 One interesting aspect about class I fusion proteins is the di erences in activation of their fusion mechanisms despite their conserved structure Millet and Whittaker 2015 White et al 2008 However all fusion proteins in this class undergo major structural changes that allow the viral fusion peptide to contact and anchor into the target cell membrane and the formation of the trimer of hairpins structure followed by the fusion of the outer membranes hemifusion and the formation of the fusion pore White et al 2008 To successfully induce fusion a proteolytic activation of the viral pro tein subunits is often necessary and this can vary signi cantly between Table 1 Genus and species from Coronavirinae subfamily Genus Species Viruses Alphacoronavirus Alphacoronavirus 1 Transmissible gastroenteritis virus TEGV Feline coronavirus FCoV Porcine epidemic diarrhea virus PEDV Human coronavirus NL63 HCoV NL63 Human coronavirus 229E HCoV 229E Betacoronavirus Betacoronavirus 1 Bovine coronavirus BCoV Human coronavirus OC43 HCoV OC43 Middle East respiratory syndrome related coronavirus MERS CoV Murine coronavirus Mouse hepatitis virus MHV Severe acute respiratory syndrome related coronavirus SARS CoV Human coronavirus HKU1 HCoV HKU1 Gammacoronavirus Avian coronavirus Infectious bronchitis virus IBV Turkey coronavirus Beluga whale coronavirus SW1 Deltacoronavirus Coronavirus HKU15 also known as porcine coronavirus PCoV HKU15 Most representative species of Alphacoronavirus Betacoronavirus Gammacoronavirus and Deltacoronavirus 1 Revised on International Committee on Taxonomy of Viruses ICTV Virus Taxonomy The Classi cation and Nomenclature of Viruses The Online 10th Report of the ICTV Updated August 2016 https talk ictvonline org ictv reports ictv online report J A Jaimes G R Whittaker Virology xxx xxxx xxx xxx 2 di erent fusion proteins Speci c aspects about FCoV S function and structure will be addressed in detail later in this review FCoV non structural proteins are encoded by the ORFs 1ab 3abc and 7ab The replicase complex encoded by the ORF1ab is translated through a ribosome frameshifting mechanism resulting in the genera tion of polyproteins that are later processed by virally encoded pro teases into 16 active subunits Brierley et al 1989 Enjuanes et al 2006 Kipar and Meli 2014 Together the 16 subunits of the replicase complex carry out the viral genome replication as well as generate the template for the transcription of the structural and accessory proteins Enjuanes et al 2006 Masters and Perlman 2013 The speci c functions for the 3abc accessory proteins are not completely under stood Nevertheless the 3c proteins appears to be necessary for viral replication and several studies have related it to changes in FCoV virulence and tropism Chang et al 2010 Pedersen et al 2009 Fi nally the 7ab accessory proteins function is not fully understood but they are reported to play a role in the immune response to FCoV in fection Dedeurwaerder et al 2013 The 7a protein is a type I inter feron INF antagonist and is suggested to protect the virus interfering with the INF response Dedeurwaerder et al 2014 On the other hand the 7b glycoprotein has been reported to induce antibody responses in naturally infected cats Kennedy et al 2008 3 Serotype and biotype Antigenically FCoV viruses can be classi ed in two groups or ser otypes I and II This antigenic classi cation is based on di erences of the FCoV S protein amino acid sequence and antibody neutralization Kipar and Meli 2014 Lewis et al 2015 Shiba et al 2007 The S protein antigenic di erences are derived from the genetic origin of each serotype The serotype I is the original and predominant type with an S protein entirely derived from FCoV Viral strains from this serotype are clinically more common and subsequently display higher epidemiolo gical importance Benetka et al 2004 Pedersen 2009 However a prevalence of 30 of FCoV serotype II was reported in Japan in the early 1990s and also coinfections FCoV I II have been reported An et al 2011 Hohdatsu et al 1992 Isolation and cell culture adapta tion of serotype I strains is considered to be di cult Lewis et al 2015 In fact most of the available cell culture adapted strains cor respond to the serotype II which complicates the in vitro study of the clinically important serotype I The cell culture adaptation of FCoV is suggested to be entirely related with the S protein as recombinant FCoV I viruses carrying a FCoV II S protein displays faster growing in cell culture as well as an expansion in the receptor usage Tekes et al 2010 Serotype II viruses are less common and in contrast to serotype I viruses the genetic origin is derived from recombination events be tween at least two di erent viruses which is an event that has been reported in other CoVs both in vitro in cell culture and in vivo under normal eld conditions Herrewegh et al 1998 Speci cally FCoV serotype II viruses resulted from a double recombination between FCoV and CCoV and while there is still some debate about the where in the viral genome the recombination events happened bioinformatics ana lyses along with antibody neutralization assays have shown a higher Fig 1 FCoV structure and genome A FCoV structure and proteins Spike S matrix M envelope E and nucleocapsid N Adapted from Kipar and Meli 2013 B FCoV ssRNA is about 29 kilobases kb long and has 11 ORF encoding 7 non structural proteins Replicase proteins 1a and 1b and accessory proteins 3a 3b 3c 7a and 7b and 4 structural proteins S M E and N Adapted from Masters and Pearlman 2013 C S gene diagram FCoV S is composed by two subunits S1 receptor binding domain RBD and S2 fusion domain The S1 subunit is divide in two functional domains N terminal domain NTD and C terminal domain C domain or CTD The S2 subunit is com posed by the fusion peptide FP two heptad repeats HR1 and HR2 a transmembrane domain TM and an endodomain E The two S activation sites S1 S2 and S2 are indicated red arrows and dashed lines as well as the linker L region between S1 S2 and S2 Adapted from Millet and Whittaker 2015 J A Jaimes G R Whittaker Virology xxx xxxx xxx xxx 3 homology between FCoV II and CCoV S proteins Kipar and Meli 2014 Amino acid sequence homology between FCoV II S protein and CCoV or also transmissible gastroenteritis virus TEGV S proteins is signi cantly higher about 91 between the FCoV II strain 79 1146 and CCoV or 81 with TEGV compared with the homology between FCoV II and FCoV I S proteins about 46 between FCoV II strain 79 1146 and FCoV I strain TN406 Herrewegh et al 1998 Wesseling et al 1994 As well as showing distinct serological di erences serotype I and serotype II FCoVs have recently been proposed to have quite distinct taxonomical features in line with their distinct biological properties Within the Alphacoronavirus 1 species two clades have been proposed as an improved way to classify the Alphacoronavirus genus clade A viruses encompassing serotype I FCoV and CCoV and clade B viruses including serotype II FCoV and CCoV and TGEV like viruses Whittaker et al 2018 Both FCoV serotypes I and II can occur in two antigenically and morphologically undistinguishable biotypes or pathotypes FECV and FIPV The ubiquitous and usually asymptomatic FECV is considered to be globally distributed and its prevalence in feral cats as well as in cats from breeding facilities can reach 90 Hohdatsu et al 1992 Pedersen et al 1984 2004 Shiba et al 2007 The FECV biotype was previously described to only replicate in the intestinal epithelium considering it an enteric exclusive virus and only di erentiating from the systemic monocyte macrophage targeting FIPV biotype in this aspect Kipar and Meli 2014 Porter et al 2014 However recent evidence has demonstrated that FECV can also replicate in monocytes challenging the previous concepts about FECV exclusive tropism Kipar et al 2010 In fact current studies performed from our laboratory have shown the presence of viral RNA in blood samples from both FIP symptomatic cats and healthy housemates in fact viral RNA can also be detected in fecal samples from viremic animals with or without clinical signs data not published These results have been also re ported by other groups supporting the hypothesis that FECV is a pre cursor form of FIPV Gunn Moore et al 1998 Meli et al 2004 Contrary to the asymptomatic FECV the FIPV biotype is considered highly virulent and leads to the severe and always lethal disease FIP Licitra et al 2013 Pedersen et al 1984 While there is no de nitive evidence of what makes FIPV virulent compared to FECV there are several hypotheses about this topic and most commonly accepted is related to the speci c FIPV tropism change to monocytes and macro phages Pedersen 2009 Pedersen et al 1984 1981b The switch in the tropism from localized cells enteric epithelium for the FECV bio type to systemically distributed cells monocytes and macrophages is considered the major reason behind FIPV virulence However the nding of FCoV RNA in blood samples from healthy animals argues against the tropism switch as the solely cause of FIPV virulence Kipar et al 2001 Meli et al 2004 Changes in the viral tropism are the consequence of interactions between the virus and the host cells where the presence or absence of receptors co re