ACTINOMYCIN AND THE DIFFERENTIAL SYNTHESIS OF REOVIRUS AND L CELL RNA

Reovirus uses temporospatial compartmentalization to orchestrate core versus outercapsid assembly

Reoviridae virus family members, such as mammalian orthoreovirus (reovirus), encounter a unique challenge during replication. To hide the dsRNA from host recognition, the genome remains encapsidated in transcriptionally active proteinaceous core capsids that transcribe and release +RNA. De novo +RNAs and core proteins must repeatedly assemble into new progeny cores in order to logarithmically amplify replication. Reoviruses also produce outercapsid (OC) proteins μ1, σ3 and σ1 that assemble onto cores to create highly stable infectious full virions. Current models of reovirus replication position amplification of transcriptionally-active cores and assembly of infectious virions in shared factories, but we hypothesized that since assembly of OC proteins would halt core amplification, OC assembly is somehow regulated. Kinetic analysis of virus +RNA production, core versus OC protein expression, and core particles versus whole virus particle accumulation, indicated that assembly of OC proteins onto core particles was temporally delayed. All viral RNAs and proteins were made simultaneously, eliminating the possibility that delayed OC RNAs or proteins account for delayed OC assembly. High resolution fluorescence and electron microscopy revealed that core amplification occurred early during infection at peripheral core-only factories, while all OC proteins associated with lipid droplets (LDs) that coalesced near the nucleus in a μ1–dependent manner. Core-only factories transitioned towards the nucleus despite cycloheximide-mediated halting of new protein expression, while new core-only factories developed in the periphery. As infection progressed, OC assembly occurred at LD-and nuclear-proximal factories. Silencing of OC μ1 expression with siRNAs led to large factories that remained further from the nucleus, implicating μ1 in the transition to perinuclear factories. Moreover, late during infection, +RNA pools largely contributed to the production of de-novo viral proteins and fully-assembled infectious viruses. Altogether the results suggest an advanced model of reovirus replication with spatiotemporal segregation of core amplification, OC complexes and fully assembled virions.

It is important to understand how viruses replicate and assemble to discover antiviral therapies and to modify viruses for applications like gene therapy or cancer therapy. Reovirus is a harmless virus being tested as a cancer therapy. Reovirus has two coats of proteins, an inner coat and an outer coat. To replicate, reovirus particles need only the inner coat, but to become infectious they require the outer coat. Strangely, inner and outer coat proteins are all made by the virus at once, so it was unknown what determines whether newly made viruses will contain just the inner coat to continue to replicate, or both coats to transmit to new hosts. Our experiments reveal that the inner coat proteins are located in a different area of an infected cell versus the outer coat proteins. The location therefore determines if the newly made viruses contain just the inner coat versus both coats. Reoviruses have evolved extravagant mechanisms to be able to efficiently take on the best composition required for replication and transmission.

Cytopathic effects of chum salmon reovirus to salmonid epithelial, fibroblast and macrophage cell lines

The cytopathic effect (CPE) of chum salmon reovirus (CSV), an aquareovirus, was studied in three salmonid cell lines: epithelial-like CHSE-214 from Chinook salmon embryo, fibroblast-like RTG-2, and monocyte/macrophage-like RTS11, both from rainbow trout. CHSE-214 and RTG-2 supported syncytia formation with more dramatic syncytia being observed in CHSE-214 cultures, while CSV induced homotypic aggregation (HA) in RTS11. Syncytia and HA formation were blocked by cycloheximide and ribavirin but not actinomycin D, suggesting that expression of CSV genes were required for both phenomena. Cultures with syncytia underwent a decline in cell viability, which appeared to be via apoptosis, as determined by intranucleosomal fragmentation and caspase dependence assays using the pan-caspase inhibitor, zVAD-fmk. In the presence of zVAD-fmk, CHSE-214 cultures continued to form syncytia and show diminished energy metabolism, but DNA fragmentation, the loss of membrane integrity, and the release of infectious CSV were considerably blocked. These results suggest that the formation of syncytia triggers apoptosis and a leaky plasma membrane, which enhances viral release. By contrast, RTS11 cultures undergoing HA showed no loss of cell viability. The significance of HA is unclear, but the response suggests that macrophage behaviour in rainbow trout potentially could be modulated by CSV.

Effect of actinomycin D on simian rotavirus (SA11) replication in cell culture

Rotaviruses are the major cause of viral diarrhea in humans and animals. Actinomycin D (Act D) is an antibiotic that intercalates DNA and therefore inhibits DNA-dependent transcription. The current study was carried out to assess the influence of Act D on the replication of simian rotavirus (SA11) in cell culture. Virus-infected MA-104 cell cultures were studied in the presence of Act D at concentrations of 1.25 and 2.5 microg/ml. Treatment of rotavirus-infected cells with 2.5 microg/ml Act D 48 h post-infection reduced the cytoplasmic metachromasia after staining with acridine orange by 25%. Viral RNA labeled with 3H-uridine in the presence of the drug was separated by polyacrylamide gel electrophoresis. Viral RNA replication was not affected by Act D, but increased 3H-uridine uptake was demonstrable by infected cells in the presence of the drug. This possibly was due to the inhibition of cellular RNA synthesis by Act D, which thus enhances incorporation of the radionuclide into the viral RNA. Act D reduced the number of infected cells presenting virus-specific fluorescence 48 h post-infection by more than 50%. These data suggest that Act D may have complexed with viral RNA and prevented newly synthesized mRNA from being translated, but may not have prevented early replication.

Inhibition of the intracellular transport of influenza viral RNA by actinomycin D

In primary chicken embryo cells infected with fowl plague virus addition of actinomycin D at defined times during the infection cycle has different consequences on viral replication. If actinomycin D is added immediately after infection with a concentration, which inhibits viral RNA synthesis only partially, it interferes with the nucleo-cytoplasmic transport of all viral RNA species (mRNA and vRNA) so far tested. If actinomycin D is present during infection (adsorption, penetration and uncoating) no viral RNA is synthesized, and the nucleocapsid of the infecting virus does not reach the nucleus, as shown by fluorescent antibodies. Therefore the primary effect of actinomycin D on influenza virus replication is on the transport of the incoming vRNPs from the cytoplasm to the cell nucleus, which is the cell compartment where transcription takes place.

Avian reovirus S1133 can replicate in mouse L cells: effect of pH and cell attachment status on viral infection

Previous reports have suggested that avian reovirus S1133 fails to replicate in mouse L cells. In this article, we report that replication does occur under certain culture conditions. The avian reovirus was found to grow in mouse L cells at pH 6.4 and 7.2 but not at pH 8.2. Culture medium with a basic pH directly inhibited viral transcription and genome replication. As a result, viral protein synthesis was also affected. At permissive pH levels, avian reovirus grew better in monolayers than in suspension cultures of L cells because of the influence of cell attachment status on viral macromolecular synthesis. Our results not only show that avian reovirus can replicate in mouse L cells but also help to explain why it did not in previous studies.

The stimulatory effect of actinomycin D on avian reovirus replication in L cells suggests that translational competition dictates the fate of the infection

Indirect immunostaining of avian reovirus S1133-infected L-cell monolayers showed that most of the cells can support viral replication. However, the number of cells in which the virus was actually replicating depended on the multiplicity of virus infection. The presence of actinomycin D during infection increased viral protein synthesis, viral growth, and the number of actively infected cells at late infection times. The antibiotic elicited these effects by triggering viral replication in cells that already contained unproductive cytoplasmic virus but that would not get productively infected in the absence of the drug. From these results, we propose a model for the interaction between L cells and avian reovirus S1133 in which viral versus host mRNA competition for the translational machinery determines the fate of the virus infection.

Avian reovirus mRNAs are nonfunctional in infected mouse cells: translational basis for virus host-range restriction

Avian reovirus S1133 penetrates and uncoats in suspension cultures of mouse L cells. The multiple species of viral transcripts are produced in the cytoplasm of the infected cell, but they fail to associate with polysomes, consistent with the absence of viral protein synthesis. The selective block in avian virus mRNA translation is not overcome by coinfection with mammalian reovirus type 3, which replicates in mouse L cells, or by hypertonic shock or exposure to a low concentration of cycloheximide. Although the avian viral transcripts are inactive in vivo, RNA extracted from infected, nonpermissive L cells directs the synthesis of a normal spectrum of viral proteins in rabbit reticulocyte lysates. These results indicate that avian viral transcription is not restricted in mouse cells and that viral replication is prevented at the level of initiation of protein synthesis.

Shythesis of reovirus oligo adenylic acid in vivo and in vitro

The formation of reovirus double-stranded (ds) RNA and of oligo adenylic acid (oligo A) is inhibited by 5 mug of actinomycin D per ml added at the time of viral infection. Viral proteins are synthesized and assembled into dsRNA-deficient particles under these conditions. The addition of cycloheximide to infected cells during the mid-logarithmic phase of viral replication terminates protein and dsRNA synthesis, but allows continued oligo A synthesis for about 1 h. The (3)H-labeled oligo A formed in the presence of cycloheximide is incorporated into particles whose density in CsCl is identical to that of reovirions. Using the large particulate or virus factory-containing cytoplasmic fraction of infected L-cells, we have established an in vitro system for the synthesis of oligo A. The in vitro product migrates slightly faster in sodium dodecyl sulfate acrylamide gels than marker oligo A. Oligo A synthesis in vitro continues for about 1 h, requires, the presence of only one ribonucleoside triphosphate (ATP), is not inhibited by DNase or RNase, but is abruptly terminated by the addition of chymotrypsin to the reaction mixture. Oligo A formed both in vivo and in vitro is released from the factory fraction by chymotrypsin digestion. The enzymes which catalyze the synthesis of oligo A, dsRNA, and single-stranded RNA all exhibit a similar temperature dependence with an optimum of approximately 45 C. These results indicate that oligo A is formed within the core of the nascent virion after the completion of dsRNA synthesis; they suggest that the oligo A polymerase is an alternative activity of the virion-bound transcriptase and that it is regulated by outer capsomere proteins.

Reovirus-specific ribonucleic acid from polysomes of infected L cells

Polysomes from reovirus-infected L cells were analyzed. Although all 10 transcription products of the viral genome were represented in polysomes and presumably can be translated, fewer than 10 reovirus-specific polypeptides were detected in infected cells. The 5'-terminal sequence of all species of reovirus ribonucleic acid (RNA) from polysomes was ppGpYp, as previously found for the genome double-stranded RNA and the in vitro single-stranded products of the virion-associated RNA polymerase. RNA isolated from both heavy (>30 ribosomes) and light (5 to 8 ribosomes) polysomes includes similar amounts of large, medium, and small classes of reovirus single-stranded RNA, suggesting that linkage of the genome segments observed in virions may occur at the single-strand stage of RNA replication.

Mechanism of reovirus double-stranded ribonucleic acid synthesis in vivo and in vitro

The complementary strands of reovirus double-stranded ribonucleic acid (ds RNA) are synthesized sequentially in vivo and in vitro. In both cases, preformed plus strands serve as templates for the synthesis of the complementary minus strands. The in vitro synthesis of dsRNA is catalyzed by a large particulate fraction from reovirus-infected cells. Treatment of this fraction with chymotrypsin or with detergents which solubilize cellular membranes does not alter its capacity to synthesize dsRNA. The enzyme or enzymes responsible for dsRNA synthesis remain sedimentable at 10,000 x g after these enzyme or detergent treatments, indicating their particulate nature. Pretreatment of this fraction with ribonuclease, however, abolishes its ability to catalyze dsRNA synthesis, emphasizing the single-stranded nature of the template and its location in a structure permeable to ribonuclease. In contrast, the newly formed dsRNA is resistant to ribonuclease digestion at low salt concentrations and hence is thought to reside within a ribonuclease-impermeable structure.

Accelerated Cytopathology in HeLa Cells Induced by Reovirus and Cycloheximide

The addition of the protein inhibitor cycloheximide, at concentrations which suppress virus replication, to HeLa cell monolayers infected with reovirus type 2 results in the appearance of accelerated cytopathic effects (CPE). At high multiplicity of infection, CPE appeared after a lag period of 2 to 3 hr and increased progressively, until by 12 hr the entire monolayer had rounded and sloughed off. During this same period, both the uninfected cycloheximide-treated and untreated virus-infected controls exhibited no CPE. The phenomenon is affected by the kind of cell species employed and can be reversed if the antibiotic is removed within 1 hr after exposure. The evidence obtained through studies in which specific metabolic inhibitors and direct biochemical analyses were used suggests that the accelerated CPE observed in cycloheximide-treated reovirus-infected cells is the consequence of the combined inhibition of the synthesis of both cellular protein and ribonucleic acid. The accelerated CPE is also induced in the antibiotic-treated cells by reovirus serotypes 1 and 3.

Separation of ten reovirus genome segments by polyacrylamide gel electrophoresis

Double-stranded ribonucleic acid (RNA) extracted from purified reoviruses of all three serotypes and from type 3 virus-infected cells was analyzed by polyacrylamide gel electrophoresis. It was calculated that each RNA includes 10 segments: 3 large, 3 intermediate, and 4 small fragments corresponding to molecular weights of about 2.5, 1.4, and 0.8 x 10(6) daltons, respectively, or a total of 15 x 10(6) daltons.

Actinomycin analogues containing pipecolic acid: relationship of structure to biological activity

Streptomyces antibioticus synthesizes a mixture of actinomycins which differ at the "imino acid" site of the peptide chains. In the presence of exogenous pipecolic acid, several new actinomycins were synthesized and 70% of the proline in the antibiotic mixture was replaced by the analogue. Three new antibiotics (designated Pip 1alpha, Pip 1beta, and Pip 2) were isolated from culture filtrates, purified, and crystallized. The molar ratio of pipecolic acid to proline was: Pip 1alpha, 1:0; Pip 1beta, 1:1; Pip 2, 2:0. These compounds inhibited the growth and cell division of gram-positive, but not gram-negative, bacteria. The relative inhibitory activity against bacteria, Escherichia coli deoxyribonucleic acid (DNA)-dependent ribonucleic acid (RNA) polymerase in vitro, and RNA synthesis in Bacillus subtilis and mouse L-929 cells was: actinomycin IV = Pip 1beta > Pip 2 > Pip 1alpha. Protein synthesis in B. subtilis was less affected, and DNA synthesis was inhibited only at higher concentrations of antibiotic tested. In L cells, DNA formation was reduced less than RNA synthesis, whereas protein synthesis was not blocked under the experimental conditions employed. Kinetic studies with B. subtilis revealed that RNA synthesis was inhibited rapidly followed by an inhibition of protein synthesis. All four antibiotics markedly inhibited the replication of vaccinia virus and reovirus in tissue culture cells, but the production of poliovirus was resistant to the antibiotics. These actinomycins bind to DNA, resulting in an elevation of its T(m) and a decrease in the peak extinction of the actinomycins. The mode of action, as well as the structure-activity relationships among the actinomycins, are discussed relative to a previously proposed model of binding.

Deoxyribonucleic acid-dependent ribonucleic acid polymerase activity in cells infected with influenza virus

Deoxyribonucleic acid (DNA)-dependent ribonucleic acid (RNA) polymerase activity was assayed on nuclear preparations of chick embryo fibroblast cells at various times after infection with an influenza A virus (fowl plague virus) and was compared with the activity of uninfected cells. Polymerase activity was increased by about 60% by 2 hr after infection, and this increase coincided with an increase in RNA synthesis in infected cells, as determined by pulse-labeling with uridine. No difference could be detected between the polymerases of infected and uninfected cells as to their requirements for DNA primer, divalent cations, and nucleoside triphosphates, and they were equally sensitive to addition of actinomycin D to the reaction mixture. It is possible that host cell DNA-dependent RNA polymerase is involved in the replication of influenza virus RNA.

Structural units of reovirus ribonucleic acid and their possible functional significance

Reovirus contains ribonucleic acid (RNA) equivalent in amount to a molecular weight of approximately 10(7) daltons. On isolation, this RNA is invariably broken into fragments of three different sizes. The three pieces have been separated from each other by chromatography on methylated albumin-kieselguhr columns. Denaturation of the three fragments of RNA in dimethyl sulfoxide led to separation of the strands, as suggested by sucrose gradient sedimentation patterns and by the second-order kinetics of reannealing. Molecular weights of 0.8 x 10(6), 1.4 x 10(6), and 2.4 x 10(6) were determined for the double-stranded fragments from the sedimentation rates of the single-stranded RNA obtained by denaturation. There was little or no homology among the three classes of denatured RNA when taken in pairs in hybridization tests. The three pieces of double-stranded RNA, therefore, did not result from random breaks in the original viral RNA molecule. Virusspecific single-stranded RNA formed in infected cells, and previously found to be largely if not entirely messenger RNA transcribed from the viral genome, was also separated into three size classes by sedimentation through sucrose gradients. Each class of single-stranded RNA corresponded in size to one of the three fragments of double-stranded RNA. The largest piece of single-stranded RNA hybridized uniquely with the largest fragment of denatured viral RNA. By whatever means this fragment of double-stranded RNA may be joined into the viral RNA molecule, it seems to act as a specific unit for transcription of an uninterrupted messenger RNA of equivalent length.

Reovirus-directed ribonucleic acid synthesis in infected L cells

Reovirus replication in L-929 mouse fibroblasts was unaffected by 0.5 mug of actinomycin per ml, a concentration which inhibited cell ribonucleic acid (RNA) synthesis by more than 90%. Under these conditions of selective inhibition, the formation of both single-stranded and double-stranded virus-specific RNA was detected beginning at 6 hr after infection. The purified double-stranded RNA was similar in size and base composition to virus RNA and presumably was incorporated into mature virus. The single-stranded RNA formed ribonuclease-resistant duplexes when annealed with denatured virus RNA but did not self-anneal, thus indicating that it includes copies of only one strand of the duplex. The single-stranded RNA was polyribosome-associated and may function as the virus messenger RNA. Production of both types of virus-induced RNA required protein synthesis 6 to 9 hr after infection. At later times in the infectious cycle, only double-stranded RNA synthesis was dependent on continued protein formation.

Differential inhibitory effects of actinomycin D among strains of poliovirus

Schaffer, Frederick L. (University of California, Berkeley), and Marjorie Gordon. Differential inhibitory effects of actinomycin D among strains of poliovirus. J. Bacteriol. 91:2309-2316. 1966.-Actinomycin D exerted a differential effect on the ability of strains of poliovirus to replicate in HeLa cells. LSc-2ab was studied as an example of a strain markedly inhibited by actinomycin; MEF(1) served as a control strain with minimal inhibition. The effect was noted at an actinomycin concentration of 0.1 mug/ml, but 2.5 mug/ml was used for most studies. Variability in the effect was attributed, in part, to physiological factors. Actinomycin was effective when present during the first 2 hr of LSc infection, but had little effect if present at later times. It did not block adsorption or initiation of ecilpse. It did block synthesis of ribonucleic acid in LSc-infected cells. Several possible modes of action are discussed, the most attractive being that actinomycin blocks synthesis of some cell component, the concentration of which is more critical for replication of some poliovirus strains than others.

Synthesis of reovirus ribonucleic acid in L cells

Kudo, Hajime (The Wistar Institute of Anatomy and Biology, Philadelphia, Pa.), and A. F. Graham. Synthesis of reovirus ribonucleic acid in L cells. J. Bacteriol. 90:936-945. 1965.-There is no inhibition of protein or deoxyribonucleic acid (DNA) synthesis in L cells infected with reovirus until the time that new virus starts to form about 8 hr after infection. At this time, both protein synthesis and DNA synthesis commence to be inhibited. Neither the synthesis of ribosomal ribonucleic acid (RNA) nor that of the rapidly labeled RNA of the cell nucleus is inhibited before 10 hr after infection. Actinomycin at a concentration of 0.5 mug/ml does not inhibit the formation of reovirus, although higher concentrations of the antibiotic do so. Pulse-labeling experiments with uridine-C(14) carried out in the presence of 0.5 mug/ml of actinomycin show that, at 6 to 8 hr after infection, two species of virus-specific RNA begin to form and increase in quantity as time goes on. One species is sensitive to ribonuclease action and the other is very resistant. The latter RNA is probably double-stranded viral progeny RNA, and it constitutes approximately 40% of the RNA formed up to 16 hr after infection. The function of the ribonuclease-sensitive RNA is not yet known. Synthesis of both species of RNA is inhibited by 5 mug/ml of actinomycin added at early times after infection. Added 6 to 8 hr after infection, when virus-specific RNA has already commenced to form, 5 mug/ml of actinomycin no longer inhibit the formation of either species of RNA.