Previous Article | Next Article ![]()
Journal of Virology, July 2006, p. 6597-6602, Vol. 80, No. 13
0022-538X/06/$08.00+0 doi:10.1128/JVI.02078-05
Copyright © 2006, American Society for Microbiology. All Rights Reserved.
Institute of Virology and Biotechnology, Zhejiang Academy of Agricultural Sciences, Hangzhou 310021,1 Faculty of Veterinary Medicine, Gansu Agriculture University, Lanzhou 730070, People's Republic of China2
Received 14 October 2005/ Accepted 22 March 2006
|
|
|---|
|
|
|---|
|
|
|---|
RNA extraction and RT-PCR. Genomic RNA was extracted from the viral suspension with RNeasy (QIAGEN, Germany) and used immediately for cDNA synthesis. cDNA synthesis was performed with SuperScript II reverse transcriptase (RT) (Invitrogen) and specific RT primers (Table 1). A total of three fragments, covering the complete JX/CHA/97 genome, were subsequently PCR amplified with Pfu Turbo DNA polymerase according to the manufacturer's protocol (Stratagene).
|
View this table: [in a new window] |
TABLE 1. Oligonucleotides used for PCR amplification of strain JX/CHA/97 of RHDV
|
Construction of the full-length cDNA clone. By following the multistep strategy illustrated in Fig. 1, we assembled a full-length cDNA clone of RHDV. First, pBluescript II SK+ plasmid was digested with XbaI and EcoRV and ligated to the AB fragment, which was digested with XbaI and EcoRV. The ligation yielded a recombinant plasmid, pBLABf, which was confirmed by sequence analysis to contain 27 A's. Fragments C and D, covering the remainder of the genome of RHDV JX/CHA/97, were assembled into pBLABf with unique restriction sites naturally found in the viral sequence. After transformation of competent Escherichia coli JM109 cells, ampicillin-resistant colonies were screened for recombinant plasmid. Positive clones were characterized by restriction endonuclease analysis and determination of the nucleotide sequence at the ends of the inserted fragment. The recombinant vector containing the full-length cDNA was named pBlRHDV.
![]() View larger version (9K): [in a new window] |
FIG. 1. Schematic diagram of steps used in the construction of a full-length cDNA clone of RHDV. At the 5' end of the genome, the KpnI restriction site and an Sp6 RNA promoter were fused to the genome. The arrow indicates the transcription start site of Sp6 RNA polymerase. Downstream of the 3' untranslated region, a poly(A) tail of 27 A's and the restriction sites NruI, NotI, and XbaI were inserted. The complete viral genome was divided into three fragments flanked by unique restriction sites, represented by the horizontal lines labeled AB, C, and D. The length of each fragment is indicated in parentheses. As shown at the bottom of the figure, fragments AB, C, and D were cloned into the pBluescript II SK+ vector in the order AB to D.
|
IFA. Indirect immunofluorescence assays (IFA) were used to detect viral protein expression in RHDV RNA-transfected RK-13 cells. After transfection, approximately 105 transfected cells were spotted onto 10-mm glass coverslips. Cells on coverslips were analyzed by IFA at various times posttransfection for viral protein synthesis. Cells were fixed in 3.7% paraformaldehyde with phosphate-buffered saline (PBS), pH 7.5, at room temperature for 30 min followed by incubation in methanol at 20°C for 30 min. The fixed cells were washed with PBS, incubated at room temperature for 45 min in RHDV immune mouse ascites fluid (1:100 dilution), and further reacted with goat anti-mouse immunoglobulin G conjugated with fluorescein isothiocyanate at room temperature for 30 min (1:100 dilution). The coverslips were washed with PBS, mounted to a slide by use of fluorescent mounting medium (KPL), and observed under a fluorescence microscope equipped with a video documentation system.
Inoculation in rabbits. In order to evaluate their infectivity in vivo, transcripts from pBlRHDV were inoculated into rabbits. Twelve 8-week-old rabbits from a specific-pathogen-free and RHDV-seronegative herd were divided into three groups, each consisting of four animals. The first group was inoculated intraperitoneally with 100 µl of different amounts of the viral RNAs described above, diluted in PBS containing 20 µg of Lipofectin (Gibco BRL, Rockville, Md.), the second group was inoculated by intrahepatic injection with the same dose of RNAs as the first group, and the third group was mock inoculated with PBS. The animals were kept in separate rooms throughout the experiment and observed daily for clinical signs of disease.
Electron microscopy. Supernatants of a filtered liver homogenate obtained from infected rabbits were preclarified by low-speed centrifugation (6,000 rpm for 30 min at 4°C), virus was subsequently pelleted through a 25% sucrose cushion in NTE buffer (NaCl, 100 mM; Tris, 10 mM; EDTA, 1 mM [pH 7.4]) by centrifugation at 28,000 rpm for 120 min in an SW41 rotor at 4°C, and the purified virions were resuspended in NTE buffer. Anti-RHDV virion mouse serum at a dilution of 1:100 in PBS was added to the virions, and they were then incubated for 2 h at 37°C. Then, the complex was precipitated by low-speed centrifugation (3,000 rpm for 20 min at 4°C). After being washed with PBS, the precipitate was resuspended again with PBS. Negative staining of virus was performed using the immune complex resuspended in PBS. The immune complex was allowed to adhere to carbon-Formvar grids (Electron Microscopy Sciences) for 3 to 5 min, washed once with PBS, and stained for 15 s with 1% ammonium phosphotungstate (Sigma), pH 7.0. Specimens were viewed with a JEM-1200EX transmission electron microscope, and three representative photographs per virus were taken.
Genetic marker analysis of the recombinant and parental viruses. Recombinant virus and parental virus were subjected to RNA extraction with RNeasy (QIAGEN). A 443-bp fragment including the genetic marker was amplified by RT-PCR from RNA extracted from either recombinant or parental virus with primers 2729R (5'-CCAACTGCACAATTCAAATCC-3') and 3151F (5'-TGAACATGACGGAGTCCTGGT-3'). The RT-PCR products were digested with EcoRV and analyzed on a 2% agarose gel.
Quantification of RHDV RNA. A comparative analysis of an increase in genome copies between transfected RNA and that isolated from liver tissue was carried out. PCR primers were designed to amplify a conserved region proximal to the 5' end of the genome of RHDV. The forward primer was from nt 153 to 174 (5'-AGGACAAAACGAGAATGAAGGA-3'), and the reverse primer was from nt 274 to 295 (5'-GCTGGGCTATGGAACACAAAC-3'). PCR amplification yielded a 122-bp product. Briefly, 4 mg RNA was transfected into specific-pathogen-free rabbits (no. 1 to 6). RNA was extracted from liver samples prepared after euthanasia of all of these rabbits and reverse transcribed into cDNA by using a strand-specific reverse primer, as described above. The cDNA was amplified by real-time PCR using SYBR green PCR mix (Applied Biosystems). The reaction was carried out with 2x SYBR green PCR mastermix in a 25-ml volume. The samples were aliquoted into a MicroAmp Optical 96-well reaction plate (Perkin Elmer Applied Biosystems) and sealed. PCR amplification was performed using a program of 10 min at 95°C followed by 40 cycles of 1 min at 94°C, 1 min at 60°C, and 1 min at 72°C. Each reaction was done in triplicate with a Perkin Elmer ABI Prism 7700 sequence detection system (TaKaRa). Standards to establish genome equivalents were synthetic RNAs transcribed from a clone of the full-length cDNA of the RHDV. RNA was quantified by absorbance, and 10-fold serial dilutions from 106 to 10 copies were prepared. These standards were run in duplicate for all assays in order to calculate genome equivalents in the experimental samples. The calibration curves from one preparation of synthetic RNA to the next were essentially identical and yielded values comparable to those from commercially available assays.
Sequence analysis. The sequences of the rescued virus and the parental virus were determined from the double-stranded plasmid by the dye chain termination method. Nucleotide sequences were analyzed with an IBM compatible personal computer by using program Lasergene99 (DNASTAR).
|
|
|---|
![]() View larger version (61K): [in a new window] |
FIG. 2. Transcription of RHDV RNA. Formaldehyde-denaturing 1.0% agarose electrophoresis of RNA transcript together with genomic RNA purified from RHDV.
|
![]() View larger version (64K): [in a new window] |
FIG. 3. IFA of viral protein expression in cells transfected with full-length RHDV RNA transcript. (Top) RK-13 cells transfected with in vitro transcripts. (Bottom) Normal RK-13 cells.
|
Electron microscopy. The morphology of RHDV particles negatively stained with phosphotungstate is shown in Fig. 4. Immune electron microscopic observation of the virus particles revealed that the particles were rotund, with a diameter of 35 nm (Fig. 4).
![]() View larger version (93K): [in a new window] |
FIG. 4. Immune electron micrograph of negatively stained RHDV particles.
|
![]() View larger version (55K): [in a new window] |
FIG. 5. Differentiation between cloned virus and parental JX/CHA/97 strain. An EcoRV restriction site was introduced in the full-length cDNA clone of JX/CHA/97 to allow discrimination between cloned virus (tagged with the EcoRV site) and parental virus (lacking an EcoRV site). RNA was extracted from the viral suspension of rabbits inoculated with either the cloned virus or the parental JX/CHA/97 isolate (lane A), and a 443-bp fragment was amplified by RT-PCR as described in Materials and Methods (lane B). The amplicons were digested with EcoRV and analyzed on a 2.0% agarose gel. The presence of an EcoRV restriction site resulted in fragments of 263 bp and 180 bp. As expected, the restriction site was found in the cloned virus but not in the parental JX/CHA/97 virus isolate. Molecular size markers (in base pairs) are noted at the right of blots.
|
|
View this table: [in a new window] |
TABLE 2. Detection of RHDV RNA in the livers of infected rabbits
|
|
|
|---|
In the present study, we report the construction of the first full-length cDNA clone of the strain JX/CHA/97 of RHDV. RNA transcripts transcribed from the cDNA clone were highly infectious by direct injection into rabbits. The identification of genetic markers engineered into the clone confirmed that the progeny virus was derived from the cDNA clone and thus was not a contaminant. Sequence analysis demonstrated that the recombinant virus recovered from the rabbits shared the highest homology in nucleotide sequence with its parental virus. In addition, our results show that the infectivity of the RNA is independent of the presence of a 5'-end cap structure, which is different from the infectious cDNA clone of feline calicivirus, another member of the family Caliciviridae (15).
The results of qRT-PCR showed that the genome copies of the engineering virus increased along with the course of infection, arrived at the highest level at 48 h (5.9 x 1010) (and then the curve declined), and arrived at the lowest level (7.45 x 107) at 72 h, when the experimental rabbits died.
Whether a full-length cDNA clone is infectious depends on two points. One point is that the cDNA clone should ideally be genetically completely identical to the parental virus. Another is that we should have proper rescue systems. As for our infectious clone, because the core sequence of the Sp6 promoter was positioned immediately before the authentic G of the 5'-untranslated-region sequence, the RNA transcripts should include the exact 5' end of RHDV. Furthermore, after plasmid linearization with NruI, the template DNA and therefore the RNA transcripts should end exactly at the 3' end. However, for RHDV, hepatitis C virus, hepatitis E virus, etc., due to the lack of reliable in vitro propagation systems we could not perform screening for infectivity in cell cultures. However, infectious hepatitis C virus and foot-and-mouth disease virus have been rescued by injecting transcription mixtures into animals (1, 17). In the present study, our results showed that RNA transcribed from full-length cDNA clones is infectious after direct transfection of rabbits. The model offers a novel approach to access the virulence of RHDV RNAs in vivo. The absence of a robust cell culture model of RHDV infection has severely limited analysis of the RHDV life cycle and the development of effective antivirals and vaccines. Based on the infectious clone of RHDV, we are considering constructing a highly efficient in vitro infection system of RHDV.
In conclusion, we have constructed a genetically stable infectious clone of RHDV. The approach used to engineer this clone should be applicable to the construction of infectious cDNA of other RHDVs, as well as a number of recently discovered related viruses. Moreover, it will be possible to target RHDV to multiple species by simple replacement of the VP60 gene (3, 15), allowing for the development of RHDV as a vaccine vector in domesticated animals. Furthermore, the availability of an infectious RHDV clone makes it possible to study in detail the mechanisms of viral replication and pathogenesis. For example, by using the reverse genetic system, we analyzed the functional role of the 3' untranslated region and the poly(A) tail. The results showed that these regions are essential for the infectivity of RHDV; however, the replicon still retains RHDV RNA synthesis ability (to be reported elsewhere). In addition, by using the replicon system, we would analyze the role of some RNA elements at the 3' untranslated region during translation and RNA synthesis.
We thank Jinqing Chen for his support and Xuelong Wu and Tao Zheng for their technical assistance.
|
|
|---|
This article has been cited by other articles:
| |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
Copyright © 2009 by the American Society for Microbiology. For an alternate route to Journals.ASM.org, visit: http://intl-journals.asm.org | More Info»