Part 2 of 2
Materials and methods
Ethics statement
All sampling procedures were performed by veterinarians with approval from Animal Ethics Committee of the Wuhan Institute of Virology (WIVH05210201). The study was conducted in accordance with the Guide for the Care and Use of Wild Mammals in Research of the People’s Republic of China.
Sampling
Bat samplings were conducted ten times from April 2011 to October 2015 at different seasons in their natural habitat at a single location (cave) in Kunming, Yunnan Province, China. All members of field teams wore appropriate personal protective equipment, including N95 masks, tear-resistant gloves, disposable outerwear, and safety glasses. Bats were trapped and fecal swab samples were collected as described previously [9]. Clean plastic sheets measuring 2.0 by 2.0 m were placed under known bat roosting sites at about 18:00 h each evening for collection of fecal samples. Fresh fecal pellets were collected from sheets early in the next morning. Each sample (approximately 1 gram of fecal pellet) was collected in 1ml of viral transport medium composed of Hank's balanced salt solution at pH7.4 containing BSA (1%), amphotericin (15 μg/ml), penicillin G (100 units/ml), and streptomycin (50 μg/ml), and were stored at -80°C until processing. Bats trapped for this study were released back into their habitat.
RNA extraction, PCR screening and sequencing
Fecal swab or pellet samples were vortexed for 1 min, and 140 μl of supernatant was collected from each sample after centrifuge at 3000 rpm under 4°C for 1min. Viral RNA was extracted with Viral RNA Mini Kit (Qiagen) following the manufacturer’s instructions. RNA was eluted in 60 μl of buffer AVE (RNase-free water with 0.04% sodium azide, Qiagen), aliquoted, and stored at -80°C. One-step hemi-nested RT-PCR (Invitrogen) was employed to detect the presence of coronavirus sequences as described previously using a set of primers that target a 440-nt fragment in the RNA-dependent RNA polymerase gene (RdRp) of all known alpha- and betacoronaviruses [20]. For the first round PCR, the 25 μl reaction mix contained 12.5 μl PCR 2 × reaction mix buffer, 10 pmol of each primer, 2.5 mM MgSO4, 20 U RNase inhibitor, 1 μl SuperScript III/Platinum Taq Enzyme Mix and 5 μl RNA template. The amplification was performed as follows: 50°C for 30 min, 94°C for 2 min, followed by 40 cycles consisting of 94°C for 15 sec, 52°C for 30 sec, 68°C for 40 sec, and a final extension of 68°C for 5 min. For the second round PCR, the 25 μl reaction mix contained 2.5 μl PCR reaction buffer, 5 pmol of each primer, 50 mM MgCl2, 0.5mM dNTP, 0.1 μl Platinum Taq Enzyme (Invitrogen) and 1 μl product of the first round PCR. The amplification was performed as follows: 94°C for 3 min followed by 35 cycles consisting of 94°C for 30 sec, 52°C for 30 sec, 72°C for 40 sec, and a final extension of 72°C for 7 min. The RBD region was amplified using the one-step nested RT-PCR method previously described [17].
PCR products were gel purified and sequenced with an ABI Prism 3730 DNA analyzer (Applied Biosystems, USA). PCR products with low concentration or generating heterogeneity in the sequencing chromatograms were cloned into pGEM-T Easy Vector (Promega) for sequencing. The positive samples in this study were termed using the abbreviated name of bat species plus the sample ID number (e.g. Rs4081). To confirm the bat species of individual sample, PCR amplification of cytochrome b (Cytob) or NADH dehydrogenase subunit 1 (ND1) gene was performed using DNA extracted from the feces or swabs [38,39].
Sequencing of full-length genomes
Full genomic sequences of 11 SARSr-CoVs were determined by One-step PCR (Invitrogen) amplification of overlapping genomic fragments with degenerate primers designed by multiple alignment of available SARS-CoV and bat SARSr- CoV sequences deposited in GenBank, and additional specific primers designed from the results of previous rounds of sequencing in this study. Primer sequences are available upon request. Sequences of the 5’ and 3’ genomic ends were obtained by 5’ and 3’ RACE (Roche), respectively. PCR products with expected size were gel-purified and subjected directly to sequencing. Each fragment was sequenced at least twice. The sequencing chromatogram of each product was thoroughly examined and sequence heterogeneity was not observed. For some fragments with low concentration of amplicons, the PCR products were cloned into pGEM-T Easy Vector (Promega) for sequencing. At least five independent clones were sequenced to obtain a consensus sequence. Co-presence of sequences of distinct SARSr-CoVs was not found in any of the amplicons. The sequences of overlapping genomic fragments were assembled to obtain the full-length genome sequences, with each overlapping sequence longer than 100 bp.
Evolution analysis
Full-length genome sequences of the 15 SARSr-CoVs detected from bats in the cave surveyed in this study were aligned with those of selected SARS-CoVs using MUSCLE [40]. The aligned sequences were scanned for recombination events by Recombination Detection Program (RDP) [41]. The potential recombination events suggested by strong P values (<10−20) were further confirmed using similarity plot and bootscan analyses implemented in Simplot 3.5.1 [42]. Phylogenetic trees based on nucleotide sequences were constructed using the Maximum Likelihood algorithm under the LG model with bootstrap values determined by 1000 replicates in the PhyML (version 3.0) software package [43].
Virus isolation
The Vero E6 cell line was kindly provided by Australian Animal Health Laboratory, CSIRO (Geelong, Australia). Vero E6 monolayer was maintained in DMEM medium supplemented with 10% fetal calf serum (FCS). Fecal samples (in 200 μl buffer) were gradient centrifuged at 3,000–12,000 g, and the supernatant was diluted 1:10 in DMEM before being added to Vero E6 cells. After incubation at 37°C for 1 h, the inoculum was removed and replaced with fresh DMEM medium with 2% FCS. The cells were incubated at 37°C and checked daily for cytopathic effect. All tissue culture media were supplemented with triple antibiotics penicillin/ streptomycin/amphotericin (Gibco) (penicillin 200 IU/ml, streptomycin 0.2 mg/ml, amphotericin 0.5 μg/ml). Three blind passages were carried out for each sample. After each passage, both the culture supernatant and cell pellet were examined for presence of SARSr-CoV by RT-PCR using specific primers targeting the RdRp or S gene. The viruses which caused obvious cytopathic effect and could be detected in three blind passages by RT-PCR were further confirmed by electron microscopy.
Construction of recombinant viruses
Recombinant viruses with the S gene of the novel bat SARSr-CoVs and the backbone of the infectious clone of SARSr-CoV WIV1 were constructed using the reverse genetic system described previously [23] (S9 Fig). The fragments E and F were re-amplified with primer pairs (FE, 5’-AGGGCCCACCTGGCACTGGTAAGAGTCATTTTGC-3’, R-EsBsaI, 5’-ACTGGTCTCTTCGTTTAGTTATTAACTAAAATATCACTAGACACC-3’) and (F-FsBsaI, 5’-TGAGGTCTCCGAACTTATGGATTTGTTTATGAG-3’, RF, 5’-AGGTAGGCCTCTAGGGCAGCTAAC-3’), respectively. The products were named as fragment Es and Fs, which leave the spike gene coding region as an independent fragment. BsaI sites (5’-GGTCTCN|NNNN-3’) were introduced into the 3’ terminal of the Es fragment and the 5’ terminal of the Fs fragment, respectively. The spike sequence of Rs4231 was amplified with the primer pair (F-Rs4231-BsmBI, 5’-AGTCGTCTCAACGAACATGTTTATTTTCTTATTCTTTCTCACTCTCAC-3’ and R-Rs4231-BsmBI, 5’-TCACGTCTCAGTTCGTTTATGTGTAATGTAATTTGACACCCTTG-3’). The S gene sequence of Rs7327 was amplified with primer pair (F-Rs7327-BsaI, 5’-AGTGGTCTCAACGAACATGAAATTGTTAGTTTTAGTTTTTGCTAC-3’ and R-Rs7327-BsaI, 5’- TCAGGTCTCAGTTCGTTTATGTGTAATGTAATTTAACACCCTTG-3’). The fragment Es and Fs were both digested with BglI (NEB) and BsaI (NEB). The Rs4231 S gene was digested with BsmBI. The Rs7327 S gene was digested with BsaI. The other fragments and bacterial artificial chromosome (BAC) were prepared as described previously. Then the two prepared spike DNA fragments were separately inserted into BAC with Es, Fs and other fragments. The correct infectious BAC clones were screened. The chimeric viruses were rescued as described previously [23].
Determination of virus infectivity by immunofluorescence assay
The HeLa cell line was kindly provided by Australian Animal Health Laboratory, CSIRO (Geelong, Australia). HeLa cells expressing human ACE2 were constructed as described previously [17]. HeLa cells expressing human ACE2 and Vero E6 cells were cultured on coverslips in 24-well plates (Corning) incubated with the newly isolated or recombinant bat SARSr-CoVs at a multiplicity of infection (MOI) = 1.0 for 1h. The inoculum was removed and the cells were washed twice with PBS and supplemented with medium. Vero E6 cells without virus inoculation and HeLa cells without ACE2 were used as negative control. Twenty-four hours after infection, cells were rinsed with PBS and fixed with 4% formaldehyde in PBS (pH7.4) at 4°C for 20 min. ACE2 expression was detected by using goat anti-human ACE2 immunoglobulin followed by FITC-labelled donkey anti-goat immunoglobulin (PTGLab). Virus replication was detected by using rabbit antibody against the nucleocapsid protein of bat SARSr-CoV Rp3 followed by Cy3-conjugated mouse anti-rabbit IgG. Nuclei were stained with DAPI. Staining patterns were observed under an FV1200 confocal microscope (Olympus).
Determination of virus replication in Vero E6 cells by plaque assay
Vero E6 cells were infected with WIV1, Rs4874, WIV1-Rs4231S, and WIV1-Rs7327S at an MOI of 1.0 and 0.01. After incubation for an hour, the cells were washed with DHanks for three times and supplied with DMEM containing 2% FCS. Samples were collected at 0, 10, 27, and 48 h post infection. The viral titers were determined by plaque assay.
Determination of virus replication in HeLa cells expressing human ACE2 by quantitative RT-PCR
HeLa cells expressing human ACE2 were inoculated with WIV1, Rs4874, WIV1-Rs4231S, and WIV1-Rs7327S at an MOI of 1.0, and were incubated for 1h at 37°C. After the inoculum was removed, the cells were supplemented with medium containing 1% FBS. Supernatants were collected at 0, 12, 24 and 48h. Virus titers were determined using quantitative RT-PCR targeting the partial N gene with a standard curve which expresses the correlation between Ct value and virus titer (shown as TCID50/ml). The standard curve was made using RNA dilutions from the purified Rs4874 virus stock (with a titer of 2.15 × 106 TCID50/ml). For qPCR, RNA was extracted from 140 μl of each supernatant with Viral RNA Mini Kit (Qiagen) following manufacturer’s instructions and eluted in 60 μl AVE buffer. The PCR was performed with the TaqMan AgPath-ID One-Step RT–PCR Kit (Applied Biosystems) in a 25 μl reaction mix containing 4 μl RNA, 1 × RT–PCR enzyme mix, 1 × RT–PCR buffer, 40 pmol forward primer (5’-GTGGTGGTGACGGCA AAATG-3’), 40 pmol reverse primer (5’-AAGTGAAGCTTCTGGGCCAG-3’) and 12 pmol probe (5’-FAM-AAAGAGCTCAGCCCCAGATG-BHQ1-3’). The amplification was performed as follows: 50°C for 10 min, 95°C for 10 min followed by 50 cycles consisting of 95°C for 15 sec and 60°C for 20 sec.
Plasmids
The ORF8 genes of bat SARSr-CoV WIV1 and Rf4092 and the ORF8a gene of bat SARSr-CoV Rs4084 were amplified by PCR from the viral RNA extracted from the isolated virus or fecal samples. The ORF8 gene of SARS-CoV GZ02 and bat SARSr-CoV Rf1, and the ORF8a gene of SARS-CoV Tor2 were synthesized by Tsingke Biological Technology Co., Ltd (Wuhan, China). All genes were cloned into the pCAGGS vector constructed with a C-terminal HA tag. Expression of the proteins was confirmed by Western blotting using a mAb against the HA tag. Five tandem copies of the ATF6 consensus binding sites were synthesized and inserted into the pGL3-Basic vector to construct the luciferase reporter plasmid 5×ATF6-GL3, in which the luciferase gene is under the control of the c-fos minimal promoter and the ATF6 consensus binding sites.
Luciferase reporter assay
HeLa cells in 24-well plates were transfected using Lipofectamine 3000 reagent (Life Technologies) following the manufacturer’s instruction. Cells per well were co-transfected with 600ng of the 5×ATF6-GL3 reporter plasmid, with 300ng of each expression plasmid of SARS-CoV and SARSr-CoV ORF8 or empty vector and 20ng of pRL-TK (Promega) which served as an internal control. The cells were incubated for 24h, and were treated with or without 2μg/ml tunicamycin for 16h. Cells were harvested and lysed. Luciferase activity was determined using a dual-luciferase assay system (Promega). The experiment was performed in triplicate wells.
Quantification of apoptotic cells
293T cells in 12-well plates were transfected using Lipofectamine 3000 reagent (Life Technologies) following the manufacturer’s instruction. Cells per well were transfected with 3μg of the expression plasmid of SARS-CoV Tor2 or SARSr-CoV Rs4084 ORF8a, or the empty vector. 24h post transfection, apoptotic cells were quantified by using the Annexin V-fluorescein isothiocyanate (FITC)/PI Apoptosis Detection Kit (Yeasen Biotech, Shanghai) in accordance with the manufacturer’s instruction. Apoptosis was analyzed by flow cytometry. The experiment was performed in triplicate wells.
Accession numbers
The complete genome sequences of bat SARS-related coronavirus strains As6526, Rs4081, Rs4084, Rf4092, Rs4231, Rs4237, Rs4247, Rs4255, Rs4874, Rs7327 and Rs9401 have been deposited in the GenBank database with the accession numbers from KY417142 to KY417152, respectively.
Supporting information
S1 Fig
Alignment of amino acid sequences of the receptor-binding motif (corresponding to aa 424–495 of SARS-CoV S protein).
Two clades of the SARSr-CoVs identified from bats in the studied cave are indicated with vertical lines on the left.
(PPTX)
Click here for additional data file.(94K, pptx)
S2 Fig
Alignment of nucleotide sequences of a genomic region covering ORF6 to ORF7a.
ORFX is located between ORF6 and ORF7a in the genomes of WIV1, WIV16, Rs7327 and Rs4874. The start codon and stop codon of ORFX are marked with red boxes. The deletion responsible for the long ORFX in Rs7327 and Rs4874 is marked with the blue box.
(PPTX)
Click here for additional data file.(165K, pptx)
S3 Fig
Phylogenetic analyses based on nucleotide sequences of the S gene (A), ORF3a (B) and ORF8 (C). The trees were constructed by the maximum likelihood method using the LG model with bootstrap values determined by 1000 replicates. Only bootstraps > 50% are shown. Rs, Rhinolophus sinicus; Rf, Rhinolophus ferremequinum; Rm, Rhinolophus macrotis; Ra, Rhinolophus affinis; Rp, Rhinolophus pusillus; As, Aselliscus stoliczkanus; Cp, Chaerephon plicata. SARSr-CoVs detected in bats from the single cave surveyed in this study are in bold.
(PPTX)
Click here for additional data file.(1.7M, pptx)
S4 Fig
Alignment of amino acid sequences of ORF3b protein.
(PPTX)
Click here for additional data file.(144K, pptx)
S5 Fig
Detection of potential recombination events by similarity plot and boot scan analysis.
(A) Full-length genome sequence of SARSr-CoV Rs4084 was used as query sequence and RsSHC014, Rf4092 and Rs4081 as reference sequences. (B) Full-length genome sequence of SARSr-CoV Rs4237 was used as query sequence and SARSr-CoV Rs4247, Rs4081 and Rs3367 as reference sequences. All analyses were performed with a Kimura model, a window size of 1500 base pairs, and a step size of 150 base pairs.
(PPTX)
Click here for additional data file.(850K, pptx)
S6 Fig
Chinese provinces where bat SARSr-CoVs have been detected.
(PPTX)
Click here for additional data file.(83K, pptx)
S7 Fig
The successful or failed rescue of the chimeric SARSr-CoVs.
(A) Cytopathic effects in Vero E6 cells transfected with the infectious BAC clones constructed with the backbone of WIV1 and various S genes of different bat SARSr-CoV strains. Microphotographs were taken 24 hours post transfection. (B) The culture media supernatant collected from the cells transfected with the infectious BAC clones was used to infect Vero E6 cells. Immunofluorescent assay (IFA) was performed to detect infection and viral replication. Cells were fixed 24 hours post infection, and stained using rabbit antibody against the SARSr-CoV Rp3 nucleocapsid protein and a Cy3-conjugated anti-rabbit IgG.
(PPTX)
Click here for additional data file.(8.3M, pptx)
S8 Fig
Quantification of SARSr-CoV in individual bat fecal samples.
The number of genome copies of SARSr-CoV per gram of bat feces was determined by quantitative real-time PCR targeting the RdRp gene. Samples from which the SARSr-CoV RBD sequences were successfully amplified are indicated in red.
(PPTX)
Click here for additional data file.(374K, pptx)
S9 Fig
Spike substitution strategy.
The original fragments E and F were shortened to leave spike gene as an independent fragment. The new fragments were designated as Es and Fs. BsaI or BsmBI sites were introduced into the junctions of Es/Spike and Spike/Fs. Then any spike could be substituted into the genome of SARSr-CoV WIV1 through this strategy.
(TIF)
Click here for additional data file.(1.3M, tif)
S1 Table
Comparison of the novel bat SARSr-CoVs identified in this study with human/civet SARS-CoVs and previously described bat SARSr-CoVs.
(DOCX)
Click here for additional data file.(36K, docx)
S2 Table
Distribution of SARSr-CoVs highly similar to SARS-CoV in the variable S, ORF3 and ORF8 genes in the single cave.
(DOCX)
Click here for additional data file.(15K, docx)
S1 Dataset
Full-length genome sequences of bat SARSr-CoVs newly identified in this study.
(FAS)
Click here for additional data file.(326K, fas)
Acknowledgments
We thank Ji-Hua Zhou and Wei-Hong Yang from Yunnan Institute of Endemic Diseases Control and Prevention for the assistance in sample collection. We thank the Center for Instrumental Analysis and Metrology of Wuhan Institute of Virology, CAS, for the assistance in taking confocal microscope pictures (Dr. Ding Gao) and flow cytometry (Ms. Juan Min).
Funding Statement
This work was jointly funded by National Natural Science Foundation of China (81290341, 31621061) to ZLS, China Mega-Project for Infectious Disease (2014ZX10004001-003) to ZLS, Scientific and technological basis special project (2013FY113500) to YZZ and ZLS from the Ministry of Science and Technology of China, the Strategic Priority Research Program of the Chinese Academy of Sciences (XDPB0301) to ZLS, the National Institutes of Health (NIAID R01AI110964), the USAID Emerging Pandemic Threats (EPT) PREDICT program to PD and ZLS, CAS Pioneer Hundred Talents Program to JC, NRF-CRP grant (NRF-CRP10-2012-05) to LFW and WIV “One-Three-Five” Strategic Program (WIV-135-TP1) to JC and ZLS. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.
Data Availability
All relevant data are within the paper and its Supporting Information files. The complete genome sequences of the 11 bat SARS-related coronaviruses newly identified in this study have been deposited in the GenBank database and assigned accession numbers KY417142 to KY417152, respectively.
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