Characterization of a temperature-sensitive mutant of frog virus 3 defective in DNA replication

Recombinant Ranaviruses for Studying Evolution of Host-Pathogen Interactions in Ectothermic Vertebrates

Ranaviruses (Iridoviridae) are large DNA viruses that are causing emerging infectious diseases at an alarming rate in both wild and captive cold blood vertebrate species all over the world. Although the general biology of these viruses that presents some similarities with poxvirus is characterized, many aspects of their replication cycles, host cell interactions and evolution still remain largely unclear, especially in vivo. Over several years, strategies to generate site-specific ranavirus recombinant, either expressing fluorescent reporter genes or deficient for particular viral genes, have been developed. We review here these strategies, the main ranavirus recombinants characterized and their usefulness for in vitro and in vivo studies.

Characterization of Frog Virus 3 knockout mutants lacking putative virulence genes

To identify ranavirus virulence genes, we engineered Frog Virus 3 (FV3) knockout (KO) mutants defective for a putative viral caspase activation and recruitment domain-containing (CARD) protein (Δ64R-FV3) and a β-hydroxysteroid dehydrogenase homolog (Δ52L-FV3). Compared to wild type (WT) FV3, infection of Xenopus tadpoles with Δ64R- or Δ52L-FV3 resulted in significantly lower levels of mortality and viral replication. We further characterized these and two earlier KO mutants lacking the immediate-early18kDa protein (FV3-Δ18K) or the truncated viral homolog of eIF-2α (FV3-ΔvIF-2α). All KO mutants replicated as well as WT-FV3 in non-amphibian cell lines, whereas in Xenopus A6 kidney cells replication of ΔvCARD-, ΔvβHSD- and ΔvIF-2α-FV3 was markedly reduced. Furthermore, Δ64R- and ΔvIF-2α-FV3 were more sensitive to interferon than WT and Δ18-FV3. Notably, Δ64R-, Δ18K- and ΔvIF-2α- but not Δ52L-FV3 triggered more apoptosis than WT FV3. These data suggest that vCARD (64R) and vβ-HSD (52L) genes contribute to viral pathogenesis.

The molecular biology of frog virus 3 and other iridoviruses infecting cold-blooded vertebrates

Frog virus 3 (FV3) is the best characterized member of the family Iridoviridae. FV3 study has provided insights into the replication of other family members, and has served as a model of viral transcription, genome replication, and virus-mediated host-shutoff. Although the broad outlines of FV3 replication have been elucidated, the precise roles of most viral proteins remain unknown. Current studies using knock down (KD) mediated by antisense morpholino oligonucleotides (asMO) and small, interfering RNAs (siRNA), knock out (KO) following replacement of the targeted gene with a selectable marker by homologous recombination, ectopic viral gene expression, and recombinant viral proteins have enabled researchers to systematically ascertain replicative- and virulence-related gene functions. In addition, the application of molecular tools to ecological studies is providing novel ways for field biologists to identify potential pathogens, quantify infections, and trace the evolution of ecologically important viral species. In this review, we summarize current studies using not only FV3, but also other iridoviruses infecting ectotherms. As described below, general principles ascertained using FV3 served as a model for the family, and studies utilizing other ranaviruses and megalocytiviruses have confirmed and extended our understanding of iridovirus replication. Collectively, these and future efforts will elucidate molecular events in viral replication, intrinsic and extrinsic factors that contribute to disease outbreaks, and the role of the host immune system in protection from disease.

Transcriptome analysis of Frog virus 3, the type species of the genus Ranavirus, family Iridoviridae

Frog virus 3 is the best characterized species within the genus Ranavirus, family Iridoviridae. FV3's large ( approximately 105 kbp) dsDNA genome encodes 98 putative open reading frames (ORFs) that are expressed in a coordinated fashion leading to the sequential appearance of immediate early (IE), delayed early (DE) and late (L) viral transcripts. As a step toward elucidating molecular events in FV3 replication, we sought to identify the temporal class of viral messages. To accomplish this objective an oligonucleotide microarray containing 70-mer probes corresponding to each of the 98 FV3 ORFs was designed and used to examine viral gene expression. Viral transcription was initially monitored during the course of a productive replication cycle at 2, 4 and 9 h after infection. To confirm results of the time course assay, viral gene expression was also monitored in the presence of cycloheximide (CHX), which limits expression to only IE genes, and following infection with a temperature-sensitive (ts) mutant which at non-permissive temperatures is defective in viral DNA synthesis and blocked in late gene expression. Subsequently, microarray analyses were validated by RT-PCR and qRT-PCR. Using these approaches we identified 33 IE genes, 22 DE genes and 36 L viral genes. The temporal class of the 7 remaining genes could not be determined. Comparison of protein function with temporal class indicated that, in general, genes encoding putative regulatory factors, or proteins that played a part in nucleic acid metabolism and immune evasion, were classified as IE and DE genes, whereas those involved in DNA packaging and virion assembly were considered L genes. Information on temporal class will provide the basis for determining whether members of the same temporal class contain common upstream regulatory regions and perhaps allow us to identify virion-associated and virus-induced proteins that control viral gene expression.

Family Iridoviridae: poor viral relations no longer

Members of the family Iridoviridae infect a diverse array of invertebrate and cold-blooded vertebrate hosts and are currently viewed as emerging pathogens of fish and amphibians. Iridovirid replication is unique and involves both nuclear and cytoplasmic compartments, a circularly permuted, terminally redundant genome that, in the case of vertebrate iridoviruses, is also highly methylated, and the efficient shutoff of host macromolecular synthesis. Although initially neglected largely due to the perceived lack of health, environmental, and economic concerns, members of the genus Ranavirus, and the newly recognized genus Megalocytivirus, are rapidly attracting growing interest due to their involvement in amphibian population declines and their adverse impacts on aquaculture. Herein we describe the molecular and genetic basis of viral replication, pathogenesis, and immunity, and discuss viral ecology with reference to members from each of the invertebrate and vertebrate genera.

Inhibition of iridovirus protein synthesis and virus replication by antisense morpholino oligonucleotides targeted to the major capsid protein, the 18 kDa immediate-early protein, and a viral homolog of RNA polymerase II

Frog virus 3 (FV3) is a large DNA virus that encodes approximately 100 proteins. Although the general features of FV3 replication are known, the specific roles that most viral proteins play in the virus life cycle have not yet been elucidated. To address the question of viral gene function, antisense morpholino oligonucleotides (asMOs) were used to transiently knock-down expression of specific viral genes and thus infer their role in virus replication. We designed asMOs directed against the major capsid protein (MCP), an 18 kDa immediate-early protein (18K) that was thought to be a viral regulatory protein, and the viral homologue of the largest subunit of RNA polymerase II (vPol-IIalpha). All three asMOs successfully inhibited translation of the targeted protein, and two of the three asMOs resulted in marked phenotypic changes. Knock-down of the MCP resulted in a marked reduction in viral titer without a corresponding drop in the synthesis of other late viral proteins. Transmission electron microscopy (TEM) showed that in cells treated with the anti-MCP MO assembly sites were devoid of viral particles and contained numerous aberrant structures. In contrast, inhibition of 18K synthesis did not block virion formation, suggesting that the 18K protein was not essential for replication of FV3 in fathead minnow (FHM) cells. Finally, consistent with the view that late viral gene expression is catalyzed by a virus-encoded or virus-modified Pol-II-like protein, knock-down of vPol-IIalpha triggered a global decline in late gene expression and virus yields without affecting the synthesis of early viral genes. Collectively, these results demonstrate the utility of using asMOs to elucidate the function of FV3 proteins.

Restriction of frog virus 3 polypeptide synthesis to immediate early and delayed early species by supraoptimal temperatures

Multiplication of frog virus 3 (FV 3) occurs in mammalian cells provided they are incubated at temperatures lower than 33 degrees. The expression of the viral genome at supraoptimal temperatures was followed by analyzing the polypeptides produced in CHO-infected cells and comparing with those obtained under restrictive conditions provoked by amino acid analogs or metabolic inhibitors. Late polypeptides were not detected at 33 degrees and the number of delayed early species decreased gradually with increasing temperatures consequently the synthesis of all delayed early proteins was not turned on in response to a unique event. At 37 degrees the synthesis was limited to the immediate early species, i.e., the proteins synthesized after cycloheximide reversal. Temperature shift experiments suggested that delayed early genes remained untranscribed at 37 degrees. Thus, incubation of FV 3-infected mammalian cells at 37 degrees provides a unique way of limiting viral synthesis to immediate early proteins without the side-effect provoked by inhibitors.

Temperature-sensitive mutants of frog virus 3: biochemical and genetic characterization

Nineteen frog virus 3 temperature-sensitive mutants were isolated after mutagenesis with nitrosoguanidine and assayed for viral DNA, RNA, and protein synthesis, as well as assembly site formation at permissive (25 degrees C) and nonpermissive (30 degrees C) temperatures. In addition, mutants were characterized for complementation by both quantitative and qualitative assays. Based on the genetic and biochemical data, the 19 mutants, along with 9 mutants isolated earlier, were ordered into four phenotypic classes which define defects in virion morphogenesis (class I), late mRNA synthesis (class II), viral assembly site formation (class III), and viral DNA synthesis (class IV). In addition, we used two-factor crosses to order 11 mutants, comprising 7 complementation groups, onto a linkage map spanning 77 recombination units.

Isolation and characterization of a frog virus 3 variant resistant to phosphonoacetate: genetic evidence for a virus-specific DNA polymerase

A variant of frog virus 3 (FV3) resistant to 200 micrograms/ml phosphonoacetate was isolated, and used to establish that the DNA polymerase induced in FV3-infected cells was virus coded. In addition, inhibitor studies showed that the FV3 polymerase is similar to eukaryotic polymerase alpha in its sensitivity to aphidicolin, and that resistance to phosphonoacetate does not confer cross-resistance to thymidine arabinoside or acycloguanosine.

The genome of frog virus 3, an animal DNA virus, is circularly permuted and terminally redundant

We examined the structure of the frog virus 3 (FV 3) genome by using electron microscopic and biochemical techniques. The linear FV 3 DNA molecules (Mr approximately 100 x 10(6) formed circles when partially degraded with bacteriophage lambda 5'-exonuclease and annealed, but not when the annealing was done without prior exonuclease digestion. The results suggest that the DNA molecules contain direct terminal repeats. The repeated region composed about 4% of the genome. Complete denaturation of native FV 3 DNA molecules followed by renaturation produced duplex circles each bearing two single-stranded tails at different points along the circumference. The tails presumably represent the terminal repeats. The formation of duplex circles suggests that the FV 3 genome is circularly permuted. This is further borne out by (i) failure to identify a specific restriction endonuclease fragment containing the label when the molecular ends were radiolabeled by using the polynucleotide kinase procedure, and (ii) similarity in the restriction patterns of virion DNA and large concatemeric replicating viral DNA as revealed by endonucleolytic cleavage of both DNAs with HindIII. From the above data, we conclude that the FV3 genome is both circularly permuted and terminally redundant--unique features for an animal virus.