Temperature-sensitive mutants of reovirus type 3: studies on the synthesis of viral peptides

A Single Point Mutation, Asn16→Lys, Dictates the Temperature-Sensitivity of the Reovirus tsG453 Mutant

Studies of conditionally lethal mutants can help delineate the structure-function relationships of biomolecules. Temperature-sensitive (ts) mammalian reovirus (MRV) mutants were isolated and characterized many years ago. Two of the most well-defined MRV ts mutants are tsC447, which contains mutations in the S2 gene encoding viral core protein σ2, and tsG453, which contains mutations in the S4 gene encoding major outer-capsid protein σ3. Because many MRV ts mutants, including both tsC447 and tsG453, encode multiple amino acid substitutions, the specific amino acid substitutions responsible for the ts phenotype are unknown. We used reverse genetics to recover recombinant reoviruses containing the single amino acid polymorphisms present in ts mutants tsC447 and tsG453 and assessed the recombinant viruses for temperature-sensitivity by efficiency-of-plating assays. Of the three amino acid substitutions in the tsG453 S4 gene, Asn₁₆-Lys was solely responsible for the tsG453ts phenotype. Additionally, the mutant tsC447 Ala₁₈₈-Val mutation did not induce a temperature-sensitive phenotype. This study is the first to employ reverse genetics to identify the dominant amino acid substitutions responsible for the tsC447 and tsG453 mutations and relate these substitutions to respective phenotypes. Further studies of other MRV ts mutants are warranted to define the sequence polymorphisms responsible for temperature sensitivity.

Single Amino Acid Differences between Closely Related Reovirus T3D Lab Strains Alter Oncolytic Potency In Vitro and In Vivo

Little is known about how genetic variations in viruses affect their success as therapeutic agents. The type 3 Dearing strain of Mammalian orthoreovirus (T3D) is undergoing clinical trials as an oncolytic virotherapy. Worldwide, studies on reovirus oncolysis use T3D stocks propagated in different laboratories. Here, we report that genetic diversification among T3D stocks from various sources extensively impacts oncolytic activity. The T3D strain from the Patrick Lee laboratory strain (TD3^PL) showed significantly stronger oncolytic activities in a murine model of melanoma than the strain from the Terence Dermody laboratory (T3D^TD). Overall in vitro replication and cytolytic properties of T3D laboratory strains were assessed by measuring virus plaque size on a panel of human and mouse tumor cells, and results were found to correlate with in vivo oncolytic potency in a melanoma model. T3D^PL produced larger plaques than T3D^TD and than the T3D strain from the ATCC (T3D^ATCC) and from the Kevin Coombs laboratory (T3D^KC). Reassortant and reverse genetics analyses were used to decipher key genes and polymorphisms that govern enhanced plaque size of T3D^PL Five single amino acid changes in the S4, M1, and L3 genome segments of reovirus were each partially correlated with plaque size and when combined were able to fully account for differences between T3D^PL and T3D^TD Moreover, polymorphisms were discovered in T3D^TD that promoted virus replication and spread in tumors, and a new T3D^PL/T3D^TD hybrid was generated with enhanced plaque size compared to that of T3D^PL Altogether, single amino acid changes acquired during laboratory virus propagation can have a large impact on reovirus therapeutic potency and warrant consideration as possible confounding variables between studies.IMPORTANCE The reovirus serotype 3 Dearing (T3D) strain is in clinical trials for cancer therapy. We find that closely related laboratory strains of T3D exhibit large differences in their abilities to replicate in cancer cells in vitro, which correlates with oncolytic activity in a in a murine model of melanoma. The study reveals that five single amino acid changes among three reovirus genes strongly impact reovirus therapeutic potency. In general, the findings suggest that attention should be given to genomic divergence of virus strains during research and optimization for cancer therapy.

Pathogenicity and immunogenicity for mice of temperature-sensitive mutants of vesicular stomatitis virus

Temperature-sensitive (ts) mutants of vesicular stomatitis (VS) virus were tested for their pathogenicity and immunogenicity in weanling mice. Compared with the wild-type virus (ts(+)), ts mutants representing genetic complementation groups I, II, and IV were considerably less pathogenic for mice infected by the intracerebral route and caused few deaths after intranasal inoculation. Mice were completely resistant to ts(+) and ts mutants by the intraperitoneal route. Resistance to intracerebral challenge with ts(+) VS virus was only minimal in mice vaccinated intraperitoneally with ts(+) or ts mutants and only moderate in mice vaccinated intranasally with three ts mutants. Intranasal vaccination, particularly with group IV mutants, resulted in solid immunity within 3 days to intranasal challenge with ts(+) virus. VS viral neutralizing antibody was present in the bronchial secretions of mice by 12 h after intranasal inoculation of mutant ts IV44; the bronchial antibody titers declined to undetectable levels between 3 and 7 days after vaccination. Neutralizing antibody was detected in the serum of mice by the third day after intranasal vaccination with ts IV44 and persisted at high level for at least 11 days. Certain classes of ts mutants would appear to be promising candidates for use as attenuated, live virus vaccines.

Assignment of avian reovirus temperature-sensitive mutant recombination groups E, F, and G to genome segments

Avian reoviruses (ARV) are less well understood than their mammalian counterparts. ARV are ubiquitous in commercial poultry and frequently isolated from acutely infected chickens. We previously described isolation of ARV temperature-sensitive (ts) mutants after nitrosoguanidine mutagenesis of wild-type ARV138, their assignment to 7 recombination groups (A-G), and genetic mapping of mutants in groups A-D to specific gene segments. For this study, wild-type serotype ARV176 was crossed with ts mutants tsE158 (Group E), tsF206 (Group F), or tsG247 (Group G) and reassortant progenies analyzed. Reassortant temperature-sensitivities were determined by efficiency of plating at permissive and non-permissive temperatures. Mapping results indicated tsE158, tsF206, and tsG247 mapped to the L1, S4, and L3 genes, respectively, which encode the lambdaA core shell, sigmaNS non-structural, and lambdaC core spike proteins, respectively. Specific amino acid substitutions in each mutant were determined and locations of structural protein alterations were placed within the 3-dimensional structure of homologous mammalian reovirus proteins. Mapping recombination groups E-G marks completion of gene assignments for all seven ts mutant groups previously generated.

Gene-specific inhibition of reovirus replication by RNA interference

Mammalian reoviruses contain a genome of 10 segments of double-stranded RNA (dsRNA). Reovirus replication and assembly occur within distinct structures called viral inclusions, which form in the cytoplasm of infected cells. Viral nonstructural proteins muNS and sigmaNS and core protein mu2 play key roles in forming viral inclusions and recruiting other viral proteins and RNA to these structures for replication and assembly. However, the precise functions of these proteins in viral replication are poorly defined. Therefore, to better understand the functions of reovirus proteins associated with formation of viral inclusions, we used plasmid-based vectors to establish 293T cell lines stably expressing small interfering RNAs (siRNAs) specific for transcripts encoding the mu2, muNS, and sigmaNS proteins of strain type 3 Dearing (T3D). Infectivity assays revealed that yields of T3D, but not those of strain type 1 Lang, were significantly decreased in 293T cells stably expressing mu2, muNS, or sigmaNS siRNA. Stable expression of siRNAs specific for any one of these proteins substantially diminished viral dsRNA, protein synthesis, and inclusion formation, indicating that each is a critical component of the viral replication machinery. Using cell lines stably expressing muNS siRNA, we developed a complementation system to rescue viral replication by transient transfection with recombinant T3D muNS in which silent mutations were introduced into the sequence targeted by the muNS siRNA. Furthermore, we demonstrated that muNSC, which lacks the first 40 amino residues of muNS, is incapable of restoring reovirus growth in the complementation system. These results reveal interdependent functions for viral inclusion proteins and indicate that cell lines stably expressing reovirus siRNAs are useful tools for the study of viral protein structure-function relationships.

Identification of the 5' sequences required for incorporation of an engineered ssRNA into the Reovirus genome

Using a reovirus reverse genetics system, we have identified the 5' sequences required of an engineered s2 ssRNA for efficient incorporation into the dsRNA genome of Reovirus. Employing an engineered, functionally active reovirus S2/CAT gene retaining the first 198 5' terminal nucleotides and the last 284 3' terminal nucleotides of the wild-type S2 segment, we have determined the 5' sequence required by a ssRNA to be recognized, replicated to dsRNA, and stably incorporated into an infectious reovirus. The 5' sequence retains 96 nucleotides of the wild-type s2 ssRNA and a predicted sequence-structure element. Within these 96 nucleotides, we have identified three nucleotides A-U-U at positions 79-81 that are essential for the incorporation of in vitro-generated ssRNAs into new reovirus progeny viral particles. This study establishes a firm foundation for additional investigation into the assortment and encapsidation mechanism of all 10 ssRNAs into the dsRNA genome of reovirus.

Avian reovirus temperature-sensitive mutant tsA12 has a lesion in major core protein sigmaA and is defective in assembly

Members of our laboratory previously generated and described a set of avian reovirus (ARV) temperature-sensitive (ts) mutants and assigned 11 of them to 7 of the 10 expected recombination groups, named A through G (M. Patrick, R. Duncan, and K. M. Coombs, Virology 284:113-122, 2001). This report presents a more detailed analysis of two of these mutants (tsA12 and tsA146), which were previously assigned to recombination group A. The capacities of tsA12 and tsA146 to replicate at a variety of temperatures were determined. Morphological analyses indicated that cells infected with tsA12 at a nonpermissive temperature produced approximately 100-fold fewer particles than cells infected at a permissive temperature and accumulated core particles. Cells infected with tsA146 at a nonpermissive temperature also produced approximately 100-fold fewer particles, a larger proportion of which were intact virions. We crossed tsA12 with ARV strain 176 to generate reassortant clones and used them to map the temperature-sensitive lesion in tsA12 to the S2 gene. S2 encodes the major core protein sigmaA. Sequence analysis of the tsA12 S2 gene showed a single alteration, a cytosine-to-uracil transition, at nucleotide position 488. This alteration leads to a predicted amino acid change from proline to leucine at amino acid position 158 in the sigmaA protein. An analysis of the core crystal structure of the closely related mammalian reovirus suggested that the Leu(158) substitution in ARV sigmaA lies directly under the outer face of the sigmaA protein. This may cause a perturbation in sigmaA such that outer capsid proteins are incapable of condensing onto nascent cores. Thus, the ARV tsA12 mutant represents a novel assembly-defective orthoreovirus clone that may prove useful for delineating virus assembly.

Reovirus sigma NS and mu NS proteins form cytoplasmic inclusion structures in the absence of viral infection

Reovirus replication occurs in the cytoplasm of infected cells and culminates in the formation of crystalline arrays of progeny virions within viral inclusions. Two viral nonstructural proteins, sigma NS and micro NS, and structural protein sigma 3 form protein-RNA complexes early in reovirus infection. To better understand the minimal requirements of viral inclusion formation, we expressed sigma NS, mu NS, and sigma 3 alone and in combination in the absence of viral infection. In contrast to its concentration in inclusion structures during reovirus replication, sigma NS expressed in cells in the absence of infection is distributed diffusely throughout the cytoplasm and does not form structures that resemble viral inclusions. Expressed sigma NS is functional as it complements the defect in temperature-sensitive, sigma NS-mutant virus tsE320. In both transfected and infected cells, mu NS is found in punctate cytoplasmic structures and sigma 3 is distributed diffusely in the cytoplasm and the nucleus. The subcellular localization of mu NS and sigma 3 is not altered when the proteins are expressed together or with sigma NS. However, when expressed with micro NS, sigma NS colocalizes with mu NS to punctate structures similar in morphology to inclusion structures observed early in viral replication. During reovirus infection, both sigma NS and mu NS are detectable 4 h after adsorption and colocalize to punctate structures throughout the viral life cycle. In concordance with these results, sigma NS interacts with mu NS in a yeast two-hybrid assay and by coimmunoprecipitation analysis. These data suggest that sigma NS and mu NS are the minimal viral components required to form inclusions, which then recruit other reovirus proteins and RNA to initiate viral genome replication.

Virion disassembly is required for apoptosis induced by reovirus

Reovirus infection leads to apoptosis in cultured cells and in vivo. Binding of viral attachment protein final sigma 1 to both sialic acid and junction adhesion molecule is required for induction of apoptosis. However, it is not known whether viral engagement of receptors is sufficient to elicit this cellular response. To determine whether steps in reovirus replication subsequent to viral attachment are required for reovirus-induced apoptosis, we used inhibitors of viral disassembly and RNA synthesis, viral disassembly intermediates, temperature-sensitive (ts) reovirus mutants, and reovirus particles deficient in genomic double-stranded RNA (dsRNA). We found that reovirus-induced apoptosis is abolished in the presence of the viral disassembly inhibitors ammonium chloride and E64. Infectious subvirion particles (ISVPs), which are intermediates in reovirus disassembly that can be generated in vitro by protease treatment, are capable of inducing apoptosis in the presence or absence of these inhibitors. Treatment of cells with the viral RNA synthesis inhibitor ribavirin does not diminish the capacity of reovirus to induce apoptosis, and reovirus ts mutants arrested at defined steps in viral replication produce apoptosis with efficiency similar to that of wild-type virus. Furthermore, reovirus particles lacking dsRNA are capable of inducing apoptosis. Finally, we found that viral attachment and disassembly must occur within the same cellular compartment for reovirus to elicit an apoptotic response. These results demonstrate that disassembly of reovirus virions to form ISVPs, but not viral transcription or subsequent steps in viral replication, is required for reovirus to induce apoptosis.

Reovirus sigmaNS protein is required for nucleation of viral assembly complexes and formation of viral inclusions

Progeny virions of mammalian reoviruses are assembled in the cytoplasm of infected cells at discrete sites termed viral inclusions. Studies of temperature-sensitive (ts) mutant viruses indicate that nonstructural protein sigmaNS and core protein mu2 are required for synthesis of double-stranded (ds) RNA, a process that occurs at sites of viral assembly. We used confocal immunofluorescence microscopy and ts mutant reoviruses to define the roles of sigmaNS and mu2 in viral inclusion formation. In cells infected with wild-type (wt) reovirus, sigmaNS and mu2 colocalize to large, perinuclear structures that correspond to viral inclusions. In cells infected at a nonpermissive temperature with sigmaNS-mutant virus tsE320, sigmaNS is distributed diffusely in the cytoplasm and mu2 is contained in small, punctate foci that do not resemble viral inclusions. In cells infected at a nonpermissive temperature with mu2-mutant virus tsH11.2, mu2 is distributed diffusely in the cytoplasm and the nucleus. However, sigmaNS localizes to discrete structures in the cytoplasm that contain other viral proteins and are morphologically indistinguishable from viral inclusions seen in cells infected with wt reovirus. Examination of cells infected with wt reovirus over a time course demonstrates that sigmaNS precedes mu2 in localization to viral inclusions. These findings suggest that viral RNA-protein complexes containing sigmaNS nucleate sites of viral replication to which other viral proteins, including mu2, are recruited to commence dsRNA synthesis.

Identification and characterization of a double-stranded RNA- reovirus temperature-sensitive mutant defective in minor core protein mu2

A newly identified temperature-sensitive mutant whose defect was mapped to the reovirus M1 gene (minor core protein mu2) was studied to better understand the functions of this virion protein. Sequence determination of the Ml gene of this mutant (tsH11.2) revealed a predicted methionine-to-threonine alteration at amino acid 399 and a change from proline to histidine at amino acid 414. The mutant made normal amounts of single-stranded RNA, both in in vitro transcriptase assays and in infected cells, and normal amounts of progeny viral protein at early times in a restrictive infection. However, tsH11.2 produced neither detectable progeny protein nor double-stranded RNA at late times in a restrictive infection. These studies indicate that mu2 plays a role in the conversion of reovirus mRNA to progeny double-stranded RNA.

Evolution of sex in RNA viruses

The distribution of deleterious mutations in a population of organisms is determined by the opposing effects of two forces, mutation pressure and selection. If mutation rates are high, the resulting mutation-selection balance can generate a substantial mutational load in the population. Sex can be advantageous to organisms experiencing high mutation rates because it can either buffer the mutation-selection balance from genetic drift, thus preventing any increases in the mutational load (Muller, 1964: Mut. Res. 1, 2), or decrease the mutational load by increasing the efficiency of selection (Crow, 1970: Biomathematics 1, 128). Muller's hypothesis assumes that deleterious mutations act independently, whereas Crow's hypothesis assumes that deleterious mutations interact synergistically, i.e., the acquisition of a deleterious mutation is proportionately more harmful to a genome with many mutations than it is to a genome with a few mutations. RNA viruses provide a test for these two hypotheses because they have extremely high mutation rates and appear to have evolved specific adaptations to reproduce sexually. Population genetic models for RNA viruses show that Muller's and Crow's hypotheses are also possible explanations for why sex is advantageous to these viruses. A re-analysis of published data on RNA viruses that are cultured by undiluted passage suggests that deleterious mutations in such viruses interact synergistically and that sex evolved there as a mechanism to reduce the mutational load.

Avian reovirus polypeptides: analysis of intracellular virus-specified products, virions, top component, and cores

Avian reovirus-specified polypeptides can be separated into three size classes: large (lambda), medium (mu), and small (sigma), similar to those of the mammalian reoviruses. A nomenclature has been proposed to indicate the individual polypeptides within each size class by progressive alphabetical subscripts. Three lambda polypeptides (lambda(A), lambda(B), and lambda(C)) are found in infectious viral particles and have molecular weights of 145,000, 130,000, and 115,000, respectively. All are present in core preparations, and two (lambda(A) and lambda(B)) appear to be exposed at the surface of the virion. Two mu polypeptides can be distinguished in purified virus (mu(A), 72,000 daltons; mu(B), 70,000 daltons), and another is occasionally evident by immunoprecipitation from infected-cell extracts (mu(NS)). mu(B) represents the major outer capsid protein and is structurally homologous to mu(1C) of the mammalian reoviruses. No additional mu proteins can be detected, and there is no evidence for a product-precursor relationship among these proteins. Three major sigma proteins are evident in viral particles. sigma(C) has the lowest molecular weight, is part of the outer capsid of the virion, and appears homologous to the mammalian sigma(1) protein. Interestingly, it demonstrates the greatest polymorphism of all the polypeptides among the different avian reoviruses examined. sigma(B) (36,000 daltons) is a major constituent of the outer capsid and, like sigma(C), is exposed to the surface of the virion. sigma(A) (39,000 daltons) appears to be an internal protein. An additional polypeptide band in the sigma class having an apparent molecular weight of 34,000 to 35,000 can be seen under three different conditions: (i) in some S1133 reovirus preparations, particularly after prolonged storage, a new band (sigma(B')) appears with a reduction in intensity of sigma(B), suggesting that sigma(B') is a degradation product of sigma(B); (ii) in polypeptides immunoprecipitated from infected-cell extracts, a major band (sigma(NS)) is apparent migrating just ahead of sigma(B); (iii) in top component preparations from all avian reoviruses examined, a band (sigma(TC)) with mobility identical to that of sigma(NS) represents a major constitutent and appears to be incorporated within the particle itself. The relationship among these three bands is not currently known. Avian reovirus polypeptides are thus in general similar to those found in mammalian reoviruses, but differences do exist which may be important for understanding viral structure and assembly.

Reovirus and the pathogenesis of some forms of chronic mental illness

Focal microdegenerative changes in the nuclei of the ansa peduncularis and the septum pellucidum are present in most cases of presenile and senile dementia, Parkinson's disease and schizophrenia (7,8). These nuclei interconnect and have extensive synaptic connections with the areas of the brain recently shown to contain non-cytopathic reovirus antigen and reovirus-like particles in the normal adult (9,10). The reovirus-involved regions closely approximate the overall pattern of the topography of brain atrophy in Alzheimer's dementia and Parkinson's disease. Mechanisms are suggested whereby mutant defective reovirus present in all adult human brains is responsible or related to the major forms of chronic mental illness including the common types of dementia and schizophrenia.

Reovirus-specific polypeptides: analysis using discontinuous gel electrophoresis

The electrophoretic analysis of reovirus-specific polypeptides in infected cells using a discontinuous gel system has allowed the resolution of additional viral-specific polypeptides, including one large-sized gamma3 and two (or possibly three) medium-sized (mu3, mu4, mu5(?)) species. The proteins designated mu0, sigma1, and sigma2 based on electrophoretic mobility in gel systems containing phosphate-urea correspond to mu4, sigma2, and sigma1, respectively, when analyzed in systems containing Tris-glycine. It is likely that protein modifications (phosphorylation and glycosylation) are responsible for at least some of these differences.

Regulated transcription of the genomes of defective virions and temperature-sensitive mutants of reovirus

Defective reovirus, which lacks the largest (L1) of the 10 double-stranded (ds) RNA genomic segments, attaches to L cells and is uncoated in the same way as reovirus. The defective genome does not replicate in the cells, but it is transcribed. During the first 5 h after infection, three of the genomic segments, M3, S3, and S4, are more frequently transcribed than the remaining six segments. During the succeeding 5 h, there is a transition to a situation in which all nine segments are transcribed at the same relative frequencies. Since the class C ts mutation has been allocated to the L1 segment (Spandidos and Graham, 1975) the transcription of the C mutant genome was investigated in cells infected with it at the nonpermissive temperature, at which the parental genome does not replicate. Genomic segments L1, M3, S3, and S4 are predominantly transcribed at early times, and later all 10 segments are transcribed with the same relative frequencies. Transcription of the defective viral genome and the C mutant genome is therefore regulated in the same way as previously found for wild-type virus (Nonoyama, Millward, and Graham, 1974), and the regulation is independent of genome replication. Apparently the L1 segment function is involved in dsRNA synthesis but not in regulating the early to late transcription. It is suggested that a cellular repressor may be involved in this regulation and that derepression might be effected by one of the early viral gene products. Virion transcriptase activity was studied in vitro with cores prepared by chymotrypsin digestion of purified defective and standard virions. For both genomes the relative frequencies of transcription of the dsRNA segments are inversely proportional to their molecular weights. These results can be accounted for in a model that postulates each segment to be transcribed independently of the other. The same model with certain restrictions can describe the in vivo transcription of the viral genome.

Synthesis of reovirus-specific polypeptides in cells pretreated with cycloheximide

When L cells are infected with reovirus in the presence of cycloheximide neither virus-specific polypeptides nor viral double-stranded RNA are synthesized. There is some synthesis of viral single-stranded RNA, transcribed mainly from segments L1, M3, S3, and S4 of the 10 viral genomic segments, and in previous work this has been termed the early mRNA pattern. In an attempt to determine whether these early transcripts are functional mRNA's, the transcripts were allowed to accumulate for a period of 17.5 h at 31 C in cycloheximide-treated cells. The cycloheximide was removed and the cells were exposed for various periods to radioactive amino acids to label any virus-specific polypeptides that might be synthesized. An immunoprecipitation technique was used to separate the viral polypeptides from cellular extracts and this precipitate was then analyzed on sodium dodecyl sulfate-polyacrylamide gels. Within 30 min of cycloheximide removal, four major polypeptides (lambda2, mu0, sigma2a, and sigma3) and two minor polypeptides (lambda1 and mu2) were found. In infected cells without cycloheximide eight viral polypeptides (lambda1, lambda2, mu0, mu2, sigma1, sigma2, sigma2a, sigma3) were found at 17.5 h after infection and the same pattern was found between 3 to 4 h after removal of cycloheximide which had been present for 17.5 h after infection. The latter result shows that the cycloheximide inhibition is reversible and that the cells readily recovered and synthesized the normal complement of viral polypeptides. In one set of experiments cordycepin was added to infected cells immediately after the removal of cycloheximide at 17.5 h to inhibit the synthesis of new viral transcripts. During the succeeding 4 h in the presence of cordycepin, the pattern of protein synthesis was the same as that obtained during the 30 min after cycloheximide removal. It is concluded that the polypeptides formed right after removal of cycloheximide are the translation products of transcripts accumulated during cycloheximide treatment and, therefore, that these transcripts are functional viral mRNA's.