Evidence for the role of the P2 protein of human rhinovirus in its host range change

Rhinoviruses and Respiratory Enteroviruses: Not as Simple as ABC

Rhinoviruses (RVs) and respiratory enteroviruses (EVs) are leading causes of upper respiratory tract infections and among the most frequent infectious agents in humans worldwide. Both are classified in the Enterovirus genus within the Picornaviridae family and they have been assigned to seven distinct species, RV-A, B, C and EV-A, B, C, D. As viral infections of public health significance, they represent an important financial burden on health systems worldwide. However, the lack of efficient antiviral treatment or vaccines against these highly prevalent pathogens prevents an effective management of RV-related diseases. Current advances in molecular diagnostic techniques have revealed the presence of RV in the lower respiratory tract and its role in lower airway diseases is increasingly reported. In addition to an established etiological role in the common cold, these viruses demonstrate an unexpected capacity to spread to other body sites under certain conditions. Some of these viruses have received particular attention recently, such as EV-D68 that caused a large outbreak of respiratory illness in 2014, respiratory EVs from species C, or viruses within the newly-discovered RV-C species. This review provides an update of the latest findings on clinical and fundamental aspects of RV and respiratory EV, including a summary of basic knowledge of their biology.

Promotion of Viral IRES-Mediated Translation Initiation under Mild Hypothermia

Internal ribosome entry site (IRES)-mediated translation is an essential replication step for certain viruses. As IRES-mediated translation is regulated differently from cap-dependent translation under various cellular conditions, we sought to investigate whether temperature influences efficiency of viral IRES-mediated translation initiation by using bicistronic reporter constructs containing an IRES element of encephalomyocarditis virus (EMCV), foot-and-mouth disease virus (FMDV), hepatitis C virus (HCV), human rhinovirus (HRV) or poliovirus (PV). Under mild hypothermic conditions (30 and 35°C), we observed increases in the efficiency of translation initiation by HCV and HRV IRES elements compared to translation initiation at 37°C. The promotion of HRV IRES activity was observed as early as 2 hours after exposure to mild hypothermia. We also confirmed the promotion of translation initiation by HRV IRES under mild hypothermia in multiple cell lines. The expression levels and locations of polypyrimidine tract-binding protein (PTB) and upstream of N-Ras (unr), the IRES trans-acting factors (ITAFs) of HCV and HRV IRES elements, were not modulated by the temperature shift from 37°C to 30°C. Taken together, this study demonstrates that efficiency of translation initiation by some viral IRES elements is temperature dependent.

Productive entry pathways of human rhinoviruses

Currently, complete or partial genome sequences of more than 150 human rhinovirus (HRV) isolates are known. Twelve species A use members of the low-density lipoprotein receptor family for cell entry, whereas the remaining HRV-A and all HRV-B bind ICAM-1. HRV-Cs exploit an unknown receptor. At least all A and B type viruses depend on receptor-mediated endocytosis for infection. In HeLa cells, they are internalized mainly by a clathrin- and dynamin-dependent mechanism. Upon uptake into acidic compartments, the icosahedral HRV capsid expands by ~4% and holes open at the 2-fold axes, close to the pseudo-3-fold axes and at the base of the star-shaped dome protruding at the vertices. RNA-protein interactions are broken and new ones are established, the small internal myristoylated capsid protein VP4 is expelled, and amphipathic N-terminal sequences of VP1 become exposed. The now hydrophobic subviral particle attaches to the inner surface of endosomes and transfers its genomic (+) ssRNA into the cytosol. The RNA leaves the virus starting with the poly(A) tail at its 3′-end and passes through a membrane pore contiguous with one of the holes in the capsid wall. Alternatively, the endosome is disrupted and the RNA freely diffuses into the cytoplasm.

Selection of rhinovirus 1A variants adapted for growth in mouse lung epithelial cells

Rhinoviruses (RVs) are picornaviruses that are causative agents of the majority of upper respiratory tract infections, or "common colds," in humans. RVs infect both the upper and lower respiratory tract, and in addition to the common cold may also cause pneumonia, complications in patients with chronic lung diseases such as cystic fibrosis, and asthma exacerbations. Convenient animal models are not available to study the pathogenesis of rhinovirus-induced illness. Rhinovirus RV1A replicates poorly in mouse cells; variants with improved replication were selected by serial passage through mouse embryonic fibroblasts and mouse lung epithelial cells. Adaptation for improved growth in mouse cells was mediated by amino acid changes in the RV1a non-structural protein 3A. Mouse cell-adapted RV1A was capable of productively infecting mice in which the airway was subjected to chemical permeabilization. A mouse model for RV infection will permit studies of RV pathogenesis and may identify targets for therapeutic intervention.

A host-specific, temperature-sensitive translation defect determines the attenuation phenotype of a human rhinovirus/poliovirus chimera, PV1(RIPO)

By using a rhinosvirus/poliovirus type 1 chimera, PV1(RIPO), with the cognate internal ribosome entry site (IRES) of human rhinovirus type 2 (HRV2), we set out to shed light on the mechanism by which this variant expresses its attenuated phenotype in poliovirus-sensitive, CD155 transgenic (tg) mice and cynomolgus monkeys. Here we report that replication of PV1(RIPO) is restricted not only in human cells of neuronal origin, as was reported previously, but also in cells of murine origin at physiological temperature. This block in replication was enhanced at 39.5°C but, remarkably, it was absent at 33°C. PV1(RIPO) variants that overcame the replication block were derived by serial passage under restrictive conditions in either mouse cells or human neuronal cells. All adapting mutations mapped to the 5'-nontranslated region of PV1(RIPO). Variants selected in mouse cells, but not in human neuronal cells, exhibited increased mouse neurovirulence in vivo. The observed strong mouse-specific defect of PV1(RIPO) at nonpermissive temperature correlated with the translational activity of the HRV2 IRES in this chimeric virus. These unexpected results must be kept in mind when poliovirus variants are tested in CD155 tg mice for their neurovirulent potential, particularly in assays of live attenuated oral poliovirus vaccine lots. Virulence may be masked by adverse species-specific conditions in mouse cells that may not allow accurate prediction of neurovirulence in the human host. Thus, novel poliovirus variants in line for possible development of human vaccines must be tested in nonhuman primates.

Adaptation of an ICAM-1-tropic enterovirus to the mouse respiratory tract

Respiratory tract (RT) infections by members of the enterovirus (EV) genus of the Picornaviridae family are the most frequent cause for the common cold and a major factor in the exacerbation of chronic pulmonary diseases. The lack of a practical small-animal model for these infections has obstructed insight into pathogenic mechanisms of the common cold and their role in chronic RT illness and has hampered preclinical evaluation of antiviral strategies. Despite significant efforts, it has been difficult to devise rodent models that exhibit viral replication in the RT. This is due mainly to well-known intracellular host restrictions of EVs with RT tropism in rodent cells. We report the evolution of variants of the common-cold-causing coxsackievirus A21, an EV with tropism for the human intercellular adhesion molecule 1 (hICAM-1), through serial passage in the lungs of mice transgenic for the hICAM-1 gene. This process was accompanied by multiple changes in the viral genome, suggesting exquisite adaptation of hICAM-1-tropic enteroviruses to the specific growth conditions within the RT. In vivo mouse RT-adapted, variant coxsackievirus A21 exhibited replication competence in the lungs of hICAM-1 transgenic mice, providing a basis for unraveling EV-host interactions in the mouse RT.

The infectious march: the complex interaction between microbes and the immune system in asthma

There has been significant progress in our knowledge about the relationship between infectious disease and the immune system in relation to asthma, but many unanswered questions still remain. Respiratory tract infections such as those caused by respiratory syncytial virus and rhinovirus during the first 2 years of life are still clearly associated with later wheezing and asthma, but the mechanism has not been completely worked out. Is there an "infectious march" triggered by infection in infancy that progresses to disease pathology or are infants who contract respiratory infections predisposed to developing asthma? This review focuses on the common themes in the interaction between microbes and the immune system, and presents a critical appraisal of the evidence to date. The various mechanisms whereby microbes alter the immune response and how this might influence asthma are discussed along with new and promising clinical practices for prevention and therapy. Recent advances in using sensitive polymerase chain reaction detection methods have allowed more rigorous testing of the causality hypothesis of virus infection leading to asthma, but the evidence is still equivocal. Various exceptions and inconsistencies in the clinical trials are discussed in light of new guidelines for subject inclusion/exclusion in hopes of providing some standardization. Despite past failures in vaccination and disappointing results of some clinical trials, the new strategies for prophylaxis including RNA interference and targeted delivery of microbicides offer a large dose of hope to a world suffering from an increasing incidence of asthma as well as a huge burden of health care cost and loss of quality of life.

Low pH-triggered beta-propeller switch of the low-density lipoprotein receptor assists rhinovirus infection

Minor group human rhinoviruses (HRVs) bind three members of the low-density lipoprotein receptor (LDLR) family: LDLR proper, very-LDLR (VLDLR) and LDLR-related protein (LRP). Whereas ICAM-1, the receptor of major group HRVs actively contributes to viral uncoating, LDLRs are rather considered passive vehicles for cargo delivery to the low-pH environment of endosomes. Since the Tyr-Trp-Thr-Asp beta-propeller domain of LDLR has been shown to be involved in the dissociation of bound LDL via intramolecular competition at low pH, we studied whether it also plays a role in HRV infection. Human cell lines deficient in LDLR family proteins are not available. Therefore, we used CHO-ldla7 cells that lack endogenous LDLR. These were stably transfected to express either wild-type (wt) human LDLR or a mutant with a deletion of the beta-propeller. When HRV2 was attached to the propeller-negative LDLR, a lower pH was required for conversion to subviral particles than when attached to wt LDLR. This indicates that high-avidity receptor binding maintains the virus in its native conformation. HRV2 internalization directed the mutant LDLR but not wt LDLR to lysosomes, resulting in reduced plasma membrane expression of propeller-negative LDLR. Infection assays using a CHO-adapted HRV2 variant showed a delay in intracellular viral conversion and de novo viral synthesis in cells expressing the truncated LDLR. Our data indicate that the beta-propeller attenuates the virus-stabilizing effect of LDLR binding and thereby facilitates RNA release from endosomes, resulting in the enhancement of infection. This is a nice example of a virus exploiting high-avidity multimodule receptor binding with an intrinsic release mechanism.

Serial culture of murine primary airway epithelial cells and ex vivo replication of human rhinoviruses

Human rhinoviruses (HRV) are the primary etiological agents in cold infections, and represent a serious risk to individuals with chronic respiratory disease such as asthma. In order to develop treatment options for HRV infections, murine models are a crucial component in the study of infection mechanisms due to the wide array of reagents and techniques available to study murine immunology. We present here a cell culture system for studying isolated murine epithelial cell responses to HRV. Monolayers of primary mouse airway epithelial cells were maintained in a serial culture system, and the identity and purity of the cell population was confirmed via immunostaining (positive for cytokeratin, negative for vimentin). Infection of these cells with a minor group rhinovirus (HRV-1A) was evidenced by increases in viral RNA, de novo synthesis of viral proteins, and production of infectious virus. This model will be useful in experiments to define mechanisms of viral replication and host/virus interactions within airway epithelial cells.

Complete genome sequence analysis of Seneca Valley virus-001, a novel oncolytic picornavirus

The complete genome sequence of Seneca Valley virus-001 (SVV-001), a small RNA virus, was determined and was shown to have typical picornavirus features. The 7280 nt long genome was predicted to contain a 5' untranslated region (UTR) of 666 nt, followed by a single long open reading frame consisting of 6543 nt, which encodes a 2181 aa polyprotein. This polyprotein could potentially be cleaved into 12 polypeptides in the standard picornavirus L-4-3-4 layout. A 3' UTR of 71 nt was followed by a poly(A) tail of unknown length. Comparisons with other picornaviruses showed that the P1, 2C, 3C and 3D polypeptides of SVV-001 were related most closely to those of the cardioviruses, although they were not related as closely to those of encephalomyocarditis virus and Theiler's murine encephalomyelitis virus as the latter were to each other. Most other regions of the polyprotein differed considerably from those of all other known picornaviruses. SVV-001 contains elements of an internal ribosome entry site reminiscent of that found in hepatitis C virus and a number of genetically diverse picornaviruses. SVV-001 is a novel picornavirus and it is proposed that it be classified as the prototype species in a novel genus named 'Senecavirus'.

Mouse models of rhinovirus-induced disease and exacerbation of allergic airway inflammation

Rhinoviruses cause serious morbidity and mortality as the major etiological agents of asthma exacerbations and the common cold. A major obstacle to understanding disease pathogenesis and to the development of effective therapies has been the lack of a small-animal model for rhinovirus infection. Of the 100 known rhinovirus serotypes, 90% (the major group) use human intercellular adhesion molecule-1 (ICAM-1) as their cellular receptor and do not bind mouse ICAM-1; the remaining 10% (the minor group) use a member of the low-density lipoprotein receptor family and can bind the mouse counterpart. Here we describe three novel mouse models of rhinovirus infection: minor-group rhinovirus infection of BALB/c mice, major-group rhinovirus infection of transgenic BALB/c mice expressing a mouse-human ICAM-1 chimera and rhinovirus-induced exacerbation of allergic airway inflammation. These models have features similar to those observed in rhinovirus infection in humans, including augmentation of allergic airway inflammation, and will be useful in the development of future therapies for colds and asthma exacerbations.

Age-dependent poliovirus replication in the mouse central nervous system is determined by internal ribosome entry site-mediated translation

Mouse cells are not permissive for the replication of human rhinovirus type 2 (HRV2). To determine the role of the HRV2 internal ribosome entry site (IRES) in determining species specificity, a recombinant poliovirus (P1/HRV2) was constructed by substituting the poliovirus IRES with the IRES from HRV2. This recombinant virus replicated in all human and murine cell lines examined, demonstrating that the HRV2 IRES does not limit viral replication in transformed murine cells. P1/HRV2 replicated in the brain and spinal cord in neonatal but not adult mice transgenic for the poliovirus receptor, CD155. Passage of P1/HRV2 in mice led to selection of a virus that caused paralysis in neonatal mice. To determine the relationship between HRV2 IRES-mediated translation and replication of P1/HRV2 in mice, recombinant human adenoviruses were used to express bicistronic mRNAs in murine organs. The results demonstrate that the HRV2 IRES mediates translation in organs of neonatal but not adult mice. These findings show that HRV2 IRES-mediated translation is a determinant of virus replication in the murine brain and spinal cord and suggest that the IRES determines the species specificity of HRV2 infection.

The minor receptor group of human rhinovirus (HRV) includes HRV23 and HRV25, but the presence of a lysine in the VP1 HI loop is not sufficient for receptor binding

Like all 10 minor receptor group human rhinoviruses (HRVs), HRV23 and HRV25, previously classified as major group viruses, are neutralized by maltose binding protein (MBP)-V33333 (a soluble recombinant concatemer of five copies of repeat 3 of the very-low-density lipoprotein receptor fused to MBP), bind to low-density lipoprotein receptor in virus overlay blots, and replicate in intercellular adhesion molecule 1 (ICAM-1)-negative COS-7 cells. From phylogenetic analysis of capsid protein VP1-coding sequences, they are also known to cluster together with other minor group strains. Therefore, they belong to the minor group; there are now 12 minor group and 87 major group HRV serotypes. Sequence comparison of the VP1 capsid proteins of all HRVs revealed that the lysine in the HI loop, strictly conserved in the 12 minor group HRVs, is also present in 9 major group serotypes that are neutralized by soluble ICAM-1. Despite the presence of this lysine, they are not neutralized by MBP-V33333 and fail to replicate in COS-7 cells and in HeLa cells in the presence of an ICAM-1-blocking antibody. These nine serotypes are therefore "true" major group viruses.

Amino acid changes in proteins 2B and 3A mediate rhinovirus type 39 growth in mouse cells

Many steps of viral replication are dependent on the interaction of viral proteins with host cell components. To identify rhinovirus proteins involved in such interactions, human rhinovirus 39 (HRV39), a virus unable to replicate in mouse cells, was adapted to efficient growth in mouse cells producing the viral receptor ICAM-1 (ICAM-L cells). Amino acid changes were identified in the 2B and 3A proteins of the adapted virus, RV39/L. Changes in 2B were sufficient to permit viral growth in mouse cells; however, changes in both 2B and 3A were required for maximal viral RNA synthesis in mouse cells. Examination of infected HeLa cells by electron microscopy demonstrated that human rhinoviruses induced the formation of cytoplasmic membranous vesicles, similar to those observed in cells infected with other picornaviruses. Vesicles were also observed in the cytoplasm of HRV39-infected mouse cells despite the absence of viral RNA replication. Synthesis of picornaviral nonstructural proteins 2C, 2BC, and 3A is known to be required for formation of membranous vesicles. We suggest that productive HRV39 infection is blocked in ICAM-L cells at a step posttranslation and prior to the formation of a functional replication complex. The observation that changes in HRV39 2B and 3A proteins lead to viral growth in mouse cells suggests that one or both of these proteins interact with host cell proteins to promote viral replication.

Identification of the human rhinovirus serotype 1A binding site on the murine low-density lipoprotein receptor by using human-mouse receptor chimeras

Human rhinovirus serotype 1A (HRV1A) binds more strongly to the mouse low-density lipoprotein receptor (LDLR) than to the human homologue (M. Reithmayer, A. Reischl, L. Snyers, and D. Blaas, J. Virol. 76:6957-6965, 2002). Here, we used this fact to determine the binding site of HRV1A by replacing selected ligand binding modules of the human receptor with the corresponding ligand binding modules of the mouse receptor. The chimeric proteins were expressed in mouse fibroblasts deficient in endogenous LDLR and LDLR-related protein, both used by minor group HRVs for cell entry. Binding was assessed by virus overlay blots, by immunofluorescence microscopy, and by measuring cell attachment of radiolabeled virus. Replacement of ligand binding repeat 5 of the human LDLR with the corresponding mouse sequence resulted in a substantial increase in HRV1A binding, whereas substitution of repeats 3 and 4 was without effect. Replacement of human receptor repeats 1 and 2 with the murine homologues also increased virus binding. Finally, murine receptor modules 1, 2, and 5 simultaneously introduced into the human receptor resulted in HRV1A binding indistinguishable from mouse wild-type receptor. Thus, repeats 1 and/or 2 and repeat 5 are involved in HRV1A attachment. Changing CDGGPD in the acidic cluster of module 5 in the human receptor to CDGEAD present in the mouse receptor led to substantially increased binding of HRV1A, indicating an important role of the glutamate residue in HRV1A recognition.

Strand-specific RNA synthesis defects in a poliovirus with a mutation in protein 3A

Substitution of a methionine residue at position 79 in poliovirus protein 3A with valine or threonine caused defective viral RNA synthesis, manifested as delayed onset and reduced yield of viral RNA, in HeLa cells transfected with a luciferase-containing replicon. Viruses containing these same mutations produced small or minute plaques that generated revertants upon further passage, with either wild-type 3A sequences or additional nearby compensating mutations. Translation and polyprotein processing were not affected by the mutations, and 3AB proteins containing the altered amino acids at position 79 showed no detectable loss of membrane-binding activity. Analysis of individual steps of viral RNA synthesis in HeLa cell extracts that support translation and replication of viral RNA showed that VPg uridylylation and negative-strand RNA synthesis occurred normally from mutant viral RNA; however, positive-strand RNA synthesis was specifically reduced. The data suggest that a function of viral protein 3A is required for positive-strand RNA synthesis but not for production of negative strands.

Mouse respiratory epithelial cells support efficient replication of human rhinovirus

Human rhinoviruses (HRV) are responsible for the majority of virus infections of the upper respiratory tract. Furthermore, HRV infection is associated with acute exacerbation of asthma and other chronic respiratory diseases of the lower respiratory tract. A small animal model of HRV-induced disease is required for the development of new therapies. However, existing mouse models of HRV infection are difficult to work with and until recently mouse cell lines were thought to be generally non-permissive for HRV replication in vitro. In this report we demonstrate that a virus of the minor receptor group, HRV1B, can infect and replicate in a mouse respiratory epithelial cell line (LA-4) more efficiently than in a mouse fibroblast cell line (L). The major receptor group virus HRV16 requires human intercellular adhesion molecule-1 (ICAM-1) for cell entry and therefore cannot infect LA-4 cells. However, transfection of in vitro-transcribed HRV16 RNA resulted in the replication of viral RNA and production of infectious virus. Expression of a chimeric ICAM-1 molecule, comprising mouse ICAM-1 with extracellular domains 1 and 2 replaced by the equivalent human domains, rendered the otherwise non-permissive mouse respiratory epithelial cell line susceptible to entry and efficient replication of HRV16. These observations suggest that the development of mouse models of respiratory tract infection by major as well as minor group HRV should be pursued.

Changes in rhinovirus protein 2C allow efficient replication in mouse cells

Rhinovirus type 16 was found to replicate in mouse L cells that express the viral receptor, human intercellular adhesion molecule 1 (ICAM-1). However, infection of these cells at a low multiplicity of infection leads to no discernible cytopathic effect, and low virus titers are produced. A variant virus, 16/L, was isolated after alternate passage of rhinovirus 16 between HeLa and ICAM-1 L cells. Infection of mouse cells with 16/L leads to higher virus titers, increased production of RNA, and total cytopathic effect. Three amino acid changes were identified in the P2 region of virus 16/L, and the adaptation phenotype mapped to two changes in protein 2C. The characterization of a rhinovirus host range mutant will facilitate the investigation of cellular proteins required for efficient viral growth and the development of a murine model for rhinovirus infection.

Species-specific receptor recognition by a minor-group human rhinovirus (HRV): HRV serotype 1A distinguishes between the murine and the human low-density lipoprotein receptor

Human rhinoviruses (HRV) of the minor receptor group use several members of the low-density lipoprotein receptor superfamily for cell entry. These proteins are evolutionarily highly conserved throughout species and are almost ubiquitously expressed. Their common building blocks, cysteine-rich ligand binding repeats about 40 amino acids in length, exhibit considerable sequence similarity. Various numbers of these repeats are present in the different receptors. We here demonstrate that HRV type 1A (HRV1A) replicates in mouse cells without adaptation. Furthermore, although closely related to HRV2, it fails to bind to the human low-density lipoprotein receptor but recognizes the murine protein, whereas HRV2 binds equally well to both homologues. This difference went unnoticed due to the presence of other receptors, such as the low-density lipoprotein receptor-related protein, which allow species-independent attachment. The species specificity of HRV1A reported here will aid in defining amino acid residues establishing the contact between the viral surface and the receptor.

Viral evolution toward change in receptor usage: adaptation of a major group human rhinovirus to grow in ICAM-1-negative cells

Major receptor group common cold virus HRV89 was adapted to grow in HEp-2 cells, which are permissive for minor group human rhinoviruses (HRVs) but which only marginally support growth of major-group viruses. After 32 blind passages in these cells, each alternating with boosts of the recovered virus in HeLa cells, HRV89 acquired the capacity to effectively replicate in HEp-2 cells, attaining virus titers comparable to those in HeLa cells although no cytopathic effect was observed. Several clones were isolated and shown to replicate in HeLa cells whose ICAM-1 was blocked with monoclonal antibody R6.5 and in COS-7 cells, which are devoid of ICAM-1. Blocking experiments with recombinant very-low-density lipoprotein receptor fragments and enzyme-linked immunosorbent assays indicated that the mutants bound a receptor different from that used by minor-group viruses. Determination of the genomic RNA sequence encoding the capsid protein region revealed no changes in amino acid residues at positions equivalent to those involved in the interaction of HRV14 or HRV16 with ICAM-1. One mutation was within the footprint of a very-low-density lipoprotein receptor fragment bound to minor-group virus HRV2. Since ICAM-1 not only functions as a vehicle for cell entry but has also a "catalytic" function in uncoating, the use of other receptors must have important consequences for the entry pathway and demonstrates the plasticity of these viruses.

Mapping the genetic determinants of pathogenicity and plaque phenotype in swine vesicular disease virus

A series of recombinant viruses were constructed using infectious cDNA clones of the virulent J1'73 (large plaque phenotype) and the avirulent H/3'76 (small plaque phenotype) strains of swine vesicular disease virus to identify the genetic determinants of pathogenicity and plaque phenotype. Both traits could be mapped to the region between nucleotides (nt) 2233 and 3368 corresponding to the C terminus of VP3, the whole of VP1, and the N terminus of 2A. In this region, there are eight nucleotide differences leading to amino acid changes between the J1'73 and the H/3'76 strains. Site-directed mutagenesis of individual nucleotides from the virulent to the avirulent genotype and vice versa indicated that A at nt 2832, encoding glycine at VP1-132, and G at nt 3355, encoding arginine at 2APRO-20, correlated with a large-plaque phenotype and virulence in pigs, irrespective of the origin of the remainder of the genome. Of these two sites, 2APRO-20 appeared to be the dominant determinant for the large-plaque phenotype but further studies are required to elucidate their relative importance for virulence in pigs.

The antiviral compound enviroxime targets the 3A coding region of rhinovirus and poliovirus

Enviroxime is an antiviral compound that inhibits the replication of rhinoviruses and enteroviruses. We have explored the mechanism of action of enviroxime by using poliovirus type 1 and human rhinovirus type 14 as model systems. By varying the time of drug addition to virus-infected cells, we determined that enviroxime could be added several hours postinfection without significant loss of inhibition. This suggested that the drug targeted a step involved in RNA replication or protein processing. To identify this target, we mapped 23 independent mutations in mutants that could multiply in the presence of 1 microgram of enviroxime per ml. Each of these mutants contained a single nucleotide substitution that altered one amino acid in the 3A coding region. Using oligonucleotide-directed mutagenesis of cDNA clones, we have confirmed that these single-amino-acid substitutions are sufficient to confer the resistance phenotype. In addition, we conducted two experiments to support the hypothesis that enviroxime inhibits a 3A function. First, we determined by dot blot analysis of RNA from poliovirus-infected cells that enviroxime preferentially inhibits synthesis of the viral plus strand. Second, we demonstrated that enviroxime inhibits the initiation of plus-strand RNA synthesis as measured by the addition of [32P]uridine to 3AB in poliovirus crude replication complexes. To our knowledge, this is the first evidence that 3A can be targeted by antiviral drugs. We anticipate that enviroxime will be a useful tool for investigating the natural function of the 3A protein.

Genetic analysis of an NTP-binding motif in poliovirus polypeptide 2C

Poliovirus polypeptide 2C is a nonstructural protein involved in replication of the viral genome. Analysis of the primary amino acid sequence of 2C shows homology to a family of proteins which contain a nucleoside-triphosphate (NTP)-binding motif. This motif consists of elements "A" (2/5 hydrophobic stretch) G/AXXGXGKS/T, where X stands for any amino acid, and "B" (3/5 hydrophobic stretch) D or DD/E. To assess the significance of the consensus sequence in 2C, we have engineered point mutations into the most conserved residues in the A and B sites and tested their effect on viral RNA replication in vivo and translation in vitro. Whereas in vitro translation of synthetic RNAs carrying mutations in the NTP-binding motif showed efficient processing of all viral proteins, indistinguishable from that of the parental strain, transfection of the RNAs into HeLa cells did not give rise to infectious virus. No viral RNA replication could be detected in cells transfected with mutant RNAs. However, revertants to the wild-type genotype in the A and B sites were obtained which gave rise to wild-type RNA synthesis, but pseudorevertants or second-site suppressors were not observed. Thus, viral RNA synthesis is greatly reduced but not entirely abolished in cells transfected with mutant RNAs. These results strongly suggest a functional role for the proposed NTP-binding motif of 2C in RNA replication and proliferation of poliovirus.

Structural and functional characterization of the poliovirus replication complex

Two populations of membrane-bound replication complexes were isolated from poliovirus-infected HEp-2 cells by sucrose gradient centrifugation. The two fractions show similar ultrastructural features: the replication complex is enclosed in a rosettelike shell of virus-induced vesicles and contains a very tightly packed second membrane system (compact membranes). The vesicular fraction, which bands in 30% sucrose, contains replicative intermediate (RI) and 36S RNA. The fraction banding in 45% sucrose contains only minute amounts of RI and contains mainly 36S RNA, two-thirds of which is encapsidated. In vitro, the two fractions show similar RNA synthesizing capacities and produce 36S plus-strand RNA. Dissolving the membranes within and around synthetically active replication complexes with sodium deoxycholate abolishes the completion of 36S RNA but still allows elongation in the RI. Our findings suggest an architecture of the replication complex that has the nascent plus strands on the RI enclosed in the compact membranes and the replication forks wrapped additionally in protein. Plus-strand RNA can be localized by in situ hybridization with a biotinylated riboprobe between the replication complex and the rosette of the virus-induced vesicles. It was found that the progeny RNA strands are set free soon after completion from the replication complex at the sites where the compact membranes within the replication complex are in close contact with the surrounding virus-induced vesicles.

Three poliovirus 2B mutants exhibit noncomplementable defects in viral RNA amplification and display dosage-dependent dominance over wild-type poliovirus

Many functions of the poliovirus genome in virally infected cells have been elucidated. However, the role of 2B (and of its precursor polypeptide, 2BC), encoded by the P2 region in the poliovirus genome, remains unknown. We have employed a genetic approach to examine the role of 2B in poliovirus-infected cells. We report here the phenotype of one previously isolated mutant in the 2B coding region, 2B201. In addition, we have constructed one additional mutation in the 2B coding region of an infectious poliovirus cDNA clone. Upon transfection into monkey Vero cells we could recover two 2B mutant polioviruses, 2B204 and 2B205. All three mutants exhibited small-plaque phenotypes on monkey Vero and human HeLa cells and displayed primary defects in viral RNA synthesis. None of the 2B mutants could be complemented by wild-type virus. Instead, the mutants exhibited a dosage-dependent dominance over wild-type poliovirus. Thus, the phenotypes of these 2B mutants implicate 2B and possibly its precursor, 2BC, in viral RNA amplification in poliovirus-infected cells, and the dominance of the 2B mutants suggests a structural role for 2B in viral replication complexes.

The complete nucleotide sequence of a pathogenic swine vesicular disease virus

The nucleotide sequence of a swine vesicular disease virus (SVDV) strain that is pathogenic for pigs has been determined and compared with that of a non-pathogenic strain of SVDV, as well as a number of other enteroviruses. It shows only 98 base changes in comparison with a non-pathogenic strain of SVDV (Inoue et al., 1989, J. Gen. Virol. 70, 919-934). Fourteen of these nucleotide differences between the pathogenic and the non-pathogenic SVDV strains occur in the 5' non-coding region which, by analogy with the other picornaviruses, has been implicated in the efficiency with which the RNA is employed as mRNA. Additional differences found throughout the coding regions are largely conservative in nature. A number of residues are discussed as candidates for determinants of pathogenicity. This sequence has been submitted to the PIR database and has accession number A30061.