Increased nucleoside diphosphate kinase activity induces white spot syndrome virus infection in Litopenaeus vannamei
Increased nucleoside diphosphate kinase activity induces white spot syndrome virus infection in Litopenaeus vannamei
Peng-fei Liu 0 1
Qing-hui Liu 0 1
Yin Wu 1
Jie Huang 0 1
0 Key Laboratory of Sustainable Development of Marine Fisheries, Ministry of Agriculture, Yellow Sea Fisheries Research Institute, Chinese Academy of Fishery Sciences , Qingdao , China , 2 Dalian Ocean University , Dalian , China , 3 National Laboratory for Marine Science and Technology , Qingdao , China
1 Editor: Gao-Feng Qiu, Shanghai Ocean University , CHINA
Nucleoside diphosphate kinase (NDK), which has the same sequence as oncoprotein (OP) in humans, can induce nucleoside triphosphates in DNA replication by maintenance of the deoxynucleotide triphosphate (dNTP's) and is known to be regulated by viral infection in the shrimp Litopenaeus vannamei. This paper describes the relationship between NDK and white spot syndrome virus (WSSV) infection. The recombinant NDK was produced by a prokaryotic expression system. WSSV copy numbers and mRNA levels of IE1 and VP28 were significantly increased in shrimp injected with recombinant NDK at 72 h after WSSV infection. After synthesizing dsRNA-NDK and confirming the efficacy of NDK silencing, we recorded the cumulative mortality of WSSV-infected shrimp injected with NDK and dsRNANDK. A comparison between the results demonstrated that silencing NDK delayed the death of shrimps. These findings indicate that NDK has an important role influencing the replication of WSSV replication in shrimp. Furthermore, NDK may have potential target as a new therapeutic strategy against WSSV infection in shrimp.
Data Availability Statement: All relevant data are
within the paper.
Funding: This study was supported by the National
Science Foundation of China (Grant: 31672679)
and the National Basic Research Program (973
program) of China (Grant No: 2012CB114401).
Competing interests: The authors have declared
that no competing interests exist.
Nucleoside diphosphate kinase (NDK) is an essential enzyme which is required for the
production of nucleoside triphosphates by a reaction that catalyses the transfer of a phosphoryl
group from nucleoside 50-triphosphates to nucleoside 50-diphosphates, it also plays a key role
in maintaining intracellular energy resources [
]. Members of the Nme gene family encode
nucleoside diphosphate kinases . Recent research indicates that members of the nm23
family display multiple functions in diverse biological processes such as signal transduction,
growth control, differentiation, cell migration, and the promotion of cancer [4±6].
A NDK cDNA (GenBank: DQ907945.1) containing 151 residues was found in the shrimp
Litopenaeus vannamei during a transcriptome study [
]. The sequence of the shrimp NDK was
highly conserved with 40 and 60% identity among eukaryotic and bacterial NDKs, respectively.
On analyzing the amino acid sequence, we discovered that it was similar to the human
oncoprotein nm23, and when shrimps were infected by white spot syndrome virus (WSSV), NDK
was up-regulated [
]. However, the relevance of NDK to viral pathology and the underlying
mechanisms are still largely unknown in crustaceans.
Among the known viral pathogens capable of infecting shrimp, WSSV is probably the most
damaging and has induced huge economic losses in the shrimp industry worldwide since it
was discovered in Taiwan in 1992 [
]. To date, there is no effective antiviral treatment to
control the outbreaks and inhibit the prevalence of WSSV.
Most of the studies are available and indicate that NDK can act as an antiviral by targeting
several pathogens to process the activation of nucleotide or nucleoside analogues. Therefore,
the aim of this study is to validate functional interaction between NDK and WSSV by
prokaryotic expression and RNA interference (RNAi). To identify the role of NDK in the process of
WSSV infection in shrimps and the interactions that exist between them.
Materials and methods
The use of non-human primates in research. All shrimps in this study were handled in strict
accordance with China legislation on scientific procedures on living animals. The protocol was
approved by the ethics committee at Dalian Ocean University.
WSSV inoculum preparation and experimental shrimp collection and maintenance
Shrimp (L. vannamei), weighing 7.2±8.7 g and 8.9±10.5 cm in length, were collected from a
larval production laboratory named Qingdao Baorong Aquatic Product Techonology CO., LTD
in Qingdao, Shandong Province, China. They were acclimatized temporarily to laboratory
conditions at a temperature of 23.0 to 24.5ÊC in cycling-filtered plastic tanks containing air
stones to provide constant aeration for two weeks. Prior to experimental use, animals were
fed twice daily with commercial shrimp feed. Otherwise, temperature, salinity and pH were
recorded daily. WSSV inoculum was prepared from the infection of healthy crayfish
(confirmed by PCR using qw primers, listed in Table 1) Procambarus clarkii as described previously
]. Primers qw were synthesised according to our previous study [
]. The tissues of infected
crayfish, excluding the hepatopancreas, were homogenized in TNE buffer (50 mM Tris±HCl,
400 mM NaCl, 5 mM EDTA, pH 8.5) and then centrifuged at 3500 × g for 5 min at 4ÊC. The
supernatant was then centrifuged at 30, 000 × g for 30 min at 4ÊC after being filtered by nylon
net (400 mesh, 30±1500 μm). Then, the upper loose pellet was rinsed carefully and the lower
white pellet was suspended in 10 ml TN buffer (20 mM Tris±HCl, 400 mM NaCl, pH 7.4).
After centrifugation at 3500 × g for 5 min, the virus particles were sedimented by
centrifugation at 30, 000 × g for 20 min at 4ÊC, then resuspended and kept in 1 ml TN buffer. Under
electron micrograph, the purified WSSV were observed and virions were calculated according to
following formula: C(virions/μl) = 7.5 × 106 μm × 50 × N/20 μm × 2 μl = 9.375 × 106 × N/μl,
where C denotes the viral concentration, as well as N denotes the mean number of the virus
particles in the 20 images [
Prokaryotic expression and purification of recombinant NDK in
Recombinant NDK was amplified by PCR with forward primer NDK(s) and reverse primer
NDK(r) (Table 1). The PCR product, which included the restriction sites HindIII and XhoI, was
recovered through gel extraction using a kit (Zymo, Orange County, USA). Then the PCR
product was cloned into the prokaryotic expression vector pBAD/gIIIA vector and transformed into
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Accession number Ampli®cation TM
E. coli TNDK 10 cells (TaKaRa, Dalian, China) by T4 DNA ligase (TaKaRa). We selected
recombinant clones by ampicillin resistance and picked monoclones to culture in 10 mL of LB liquid
culture medium overnight at 37ÊC with agitation. Plasmid DNA was extracted using an
Extraction Kit (TIANGEN, Beijing, China) and digested with the restriction enzymes HindIII and
XhoI. Finally, recombinant NDK (rNDK) was confirmed in the selected positive clone by PCR
and sequencing. The cultures were incubated at 37ÊC for 5 h and 0.2% L-arabinose was added.
The cells were centrifuged at 10,000 × g for 10 min at 4ÊC and resuspended in
phosphate-buffered saline (PBS) (135 mM NaCl, 2.7 mM KCl, 1.5 mM KH2PO4, 8 mM K2PO4, pH 7.2). As the
fusion protein had a His-tag, we collected the supernatant and added it to Ni-NTA resin for
purification. Then the solution was filtered by 4 M guanidine hydrochloride, 2 M guanidine
hydrochloride and no guanidine hydrochloride in turn. And each dialysis step was carried
out for at least 12 h at 4ÊC. Combined with methods published in our previous research, we
obtained purified rNDK and analyzed it by sodium dodecyl sulfate-polyacrylamide gel
electrophoresis (SDS-PAGE) [
Relative quantification of NDK gene expression in tissues
In order to investigate the mRNA levels of NDK gene expression in L. vannamei, six tissues
including lymphoid, gill, hepatopancreas, muscle, intestine and hemocytes were extracted
individually from three shrimp selected at random.
Total RNA was extracted from tissues and then reverse transcribed into cDNA by using
RNAiso Plus (TaKaRa, Dalian, China) reagent. In order to assess the quality of the RNA, a
NanoDrop 2000 spectrophotometer (NanoDrop Technologies, Wilmington, USA) was used to
measure the absorbance of RNA at 260 and 280 nm, and 1.5% formaldehyde agarose gel
electrophoresis was used to analyze the RNA integrity [
]. According to the manufacture's
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protocol, PrimeScript RT reagent Kit with gDNA Eraser (TaKaRa) was used to synthesize the
cDNA with 1 μg total RNA. And then all cDNAs were diluted to 50 ng/μl with
diethylpyrocarbonate (DEPC)-treated H2O and restored at −20ÊC for reverse transcription PCR (RT-PCR).
Primers of NDK called NDK-qF and NDK-qR were designed based on published L. vannamei
cDNA sequences of NDK (Table 1). The qRT-PCR was performed in a 25 μL volume
containing 12.5 μL of 2 × SYBR Premix Ex Taq (Roche, Beijing, China), 1 μL DNA, 10.5 μL DEPC
H2O, 0.5 μL of each of the forward and reverse primers (1 μmol/L) and 1 μL cDNA template.
The reaction conditions for qRT-PCR were carried out with initial denaturation at 94ÊC for 5
min followed by 40 cycles of 94ÊC for 30 s, annealing at 60ÊC for 30 s, extension at 72ÊC for 30
s and 72ÊC for 10 min. A melting curve was produced for each amplification product. The data
obtained were statistically analyzed and calculated using the 2-44ct method [
] and given as
means ± SD standard deviation. Differences in mortality were tested for statistical significance.
Bio-Rad CFX Manager 3.0 software was used to analyze the qRT-PCR data.
NDK effect on WSSV-challenged shrimp
Relative quantification of IE1, VP28 gene expression in gills. To analyze the function of
NDK in L. vannamei after WSSV infection, 36 shrimps were chosen at random and divided
into three groups which were pretreated with PBS, BSA (1 mg/mL), and NDK respectively.
First, PBS (30 μL), BSA (30 μg), or NDK (30 μg) were injected into the third abdominal somite
of shrimps using a 1 mL tuberculin syringe. After 2 h, shrimps in the NDK group and BSA
group were challenged with 30 μL of purified WSSV (106±107 copies/μL) by intramuscular
injection, while the control group was injected with 30 μL PBS. The infection dose was
determined in our previous study [
], and PBS and BSA were filtered through a 0.22 μm
membrane. During the WSSV challenge, gills from shrimps were collected at 0, 24, 48 and 72 h.
Then, total RNA from tissues were reverse transcribed into cDNA by the same methods
mentioned above. For qRT-PCR, specific primers of IE1 and VP28 were used and β-actin mRNA
was used as a control (Table 1).
Quantitative assessment of the WSSV copies by qRT-PCR. Viral load was quantified by
measuring the WSSV-copies in the gills of experimental shrimps as gills are highly active
metabolic tissues in decapod crustaceans [
]. Based on the WSSV sequence (AF332093.1), primers
qw were synthesized as our previous study described [
] to assess copy numbers of WSSV in
shrimps. At 0, 24, 48, and 72 h, all samples were extracted from WSSV-infected and
mockinfected shrimps' gills by using a DNA extraction kit (TIANGEN, Beijing, China). DNA from
all samples was extracted and used as template to quantify viral load by qPCR. Triplicate
reactions of qPCR were performed in a BioRad ICycler real-time PCR system according to Durand
and Lightner [
]. A 25 μL volume including 12.5 μL of 2 × SYBR Premix Ex Taq (Takara),
1 μL DNA, 10.5 μL DEPC H2O, 0.5 μL each of forward and reverse primers (1 μmol/L) and
1 μL DNA template. PCR conditions were denaturing at 94ÊC for 5 min followed by 40 cycles
of 94ÊC for 30 min, annealing at 60ÊC for 30 s, and extension at 72ÊC for 30 s. A melting curve
was produced for each amplification product. In this experiment, 104±109 (copies/μL)
concentration gradient of plasmid containing WSSV-DNA were constructed as a standard curve to
quantify the copy numbers of WSSV-infected samples. Dilution series of WSSV DNA were
amplified with WSSV specific primers: forward primer qw-a and reverse primer qw-s (Table 1).
Cumulative mortality analysis. The shrimps were randomly divided into four groups
including three parallel groups (n = 15 in each group) and injected with either NDK, BSA,
WSSV or PBS by 1 mL tuberculin syringe. The NDK group and BSA group were injected with
30 μg NDK and 30 μg BSA, respectively. The method of injection was the same as in section
2.4.1. Two hours later, these two groups and the WSSV-infected group were injected with
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30 μL of purified WSSV (106±107 copies/μL), while the PBS group was injected with 30 μL
PBS. Mortality was monitored every 12 h and the cumulative mortality was recorded each day.
Production of dsRNA-NDK
By using the kit of TranscriptAid T7 High Yield Transcription (Thermo, Beijing, China)
according to the manufacturer's instructions, dsRNA-NDK were constructed from a region of
L. vannamei NDK (317 bp) (GenBank: DQ907945.1) amplified using primers NDK-T7-F,
NDK-F NDK-T7-R and NDK-R (Table 1). The dsRNA-Enhanced Green Fluorescent Protein
(EGFP) was amplified using primers EGFP-T7-F, EGFP-F, EGFP-T7-R and EGFP-R (Table
1). Briefly, DNA templates for the production of dsRNA-NDK and dsRNA-EGFP were
amplified by PCR using gene-specific primers (Table 1) at the 50 terminus to produce sense and
antisense RNA strands separately. The PCR conditions were the same as previously described
with annealing temperatures of 60ÊC and 59ÊC for dsRNA-NDK and dsRNA-EGFP,
respectively. Then, the single-stranded RNA was annealed to generate dsRNA. After purification, the
dsRNA-NDK and dsRNA-EGFP were quantified and then stored at −80ÊC [18±20].
In vivo RNAi assay in Shrimp
Effective inhibition of dsRNA-NDK in hemocytes, lymphoid and hepatopancreas of
shrimps. Expressed dsRNA-NDK (30 μg/shrimp), dsRNA-EGFP (30 μg/shrimp) and PBS
(30 μL, control) were injected into shrimps that were divided into three groups to give 36
shrimps in total. At 0, 24, 48 and 72 h after injection, three shrimps were randomly selected
and tissues of hemocytes, lymphoid and hepatopancreas were divided. Each total RNA sample
was extracted, reverse transcribed and amplified in RT-qPCR to obtain NDK gene expression
with the β-actin gene as a loading control.
Gene expression of IE1 and VP28 of WSSV-infected shrimp injected with expressed
dsRNA-NDK in gills. dsRNA-NDK (30 μg/shrimp), dsRNA-EGFP (30 μg/shrimp) and
30 μL PBS were delivered into shrimps, and 30 μL WSSV (106±107 copies/μL) were injected
after two days. Three gills were collected at 0, 24, 48 and 72 h from shrimps respectively. As
above, we obtained the gene expression data of IE1 and VP28.
Effects of dsRNA-NDK on the mortality of WSSV-infected shrimp. In order to test the
capacity of dsRNA-NDK interference on WSSV, we chose 145 shrimps and randomly divided
them into three groups including three parallel groups (n = 15 in each group). The experimental
group was injected with dsRNA-NDK (30 μg/shrimp), whereas the control groups were injected
with dsRNA-EGFP (30 μg/shrimp) and 30 μL PBS, respectively. After two days, the
WSSVinfected group and these two control groups were injected with 30 μL of the purified WSSV (106±
107 copies/μL). The cumulative mortality was recorded each day and the data were analyzed.
The software randomly SPSS 18.0 was used to analysis the data. Data analyses were done by
oneway analysis of variance (ANOVA) and Tukey' s comparison of means. And all data were expressed
as the means ± standard deviation. A probability value of less than 0.05 was considered significant.
rNDK recombinant expression and mass spectrometry assay
Through prokaryotic expression, recombinant NDK was successfully expressed in E. coli.
From SDS-PAGE, the apparent molecular mass of rNDK was estimated to be 25 KDa (Fig 1).
No protein band was found at this position in the non-induced E. coli.
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Fig 1. SDS±PAGE analysis of of NDK expressed from vector pBAD/gIII A. (A) SDS-PAGE analysis of induced
and non-induced pBAD/gIII A-NDK-TNDK10 (containing NDK). Induced pBAD/gIII A-NDK-TNDK10 (Lane 1±2).
Noninduced pBAD/gIII A-NDK-TNDK10 (Lane 3). M: protein marker (B) SDS-PAGE of expressed purified proteins NDK
(Lane 1). M: protein marker
To identify the protein, bands were cut from SDS-PAGE gels and identified by using
MALDI-TOF-MS and analyzed by using the MS-FIT system. We observed that seven peptide
fragments of the rNDK were matched with the deduced amino acid sequence of NDK from L.
vannamei (Table 2). This indicates that the recombinant protein was NDK (Table 2).
Shrimp NDK mRNA levels in tissues
We discovered different levels of NDK expression in the six tissues (Fig 2). In hemocytes and
muscle, gene expression of NDK was significant higher than other tissues, especially in
hemocytes. Hemocytes play an essential role in physiology and immune defense of shrimp [
Seven matched peptide fragments and their positions in the deduced amino acids of rNDK by electrospray ionization-mass spectrometry/mass
spectrometry (ESI-MS/MS) analysis. Start-End: position of the sequence; MW: molecular weight.
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results of NDK mRNA levels indicated that NDK could have some important functions in
WSSV copy number quantification by qPCR
The results of qRT-PCR indicate that WSSV copies increased significantly (P < 0.05) in
WSSV-infected group which injected into rNDK by comparison to the BSA and PBS groups.
(Fig 3). At 72 h post virus inoculation, WSSV copy number in the rNDK+WSSV group was
higher (2.0×106 copies/ng) than the BSA+WSSV group ( 4.5×105 copies/ng). These results
suggest that rNDK contributed to increase WSSV infectivity
Inhibition of NDK gene expression in shrimp by dsRNA-NDK
The results showed that the gene expression of NDK in hemocytes, lymphoid and
hepatopancreas was significantly lower from 0 h to 72 h compared to PBS and dsRNA-EGFP groups.
Therefore, it was shown that the NDK gene was inhibited by dsRNA-NDK (P < 0.05) (Fig 4A±
Fig 2. Relative NDK mRNA levels in shrimps tissues. Relative NDK mRNA levels in tissues of lymphoid, gill, hepatopancreas, muscle, intestine and
hemocytes of experimentally shrimp L. vannamei. Bars represent the means±SE. Differences were evaluated with one-way ANOVA and Tukey test
(p < 0.05).
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Fig 3. WSSV copies number. Estimated WSSV copies number/ng of DNA in gill of experimentally infected
shrimp L. vannamei among NDK group, BSA group and the control group from a time course study.
Fig 4. Temporal expression of NDK in three tissues. (A) Temporal expression of NDK in hemocytes, (B) Temporal expression of NDK in lymphoid (C)
Temporal expression of NDK in hepatopancreas. Healthy L. vannamei were injected intramuscularly at the third abdominal segment with 20μL of PBS
(control group), 1μg/g of dsRNA-NDK and dRNA-EGFP. At 0, 3, 6, 12, 24, 36, 48 and 72 h, three shrimp were randomly selected from each group from
which the hemocytes, lymphoid and hepatopancreas for qPCR analysis. hemocytes, lymphoid and hepatopancreas
PLOS ONE | https://doi.org/10.1371/journal.pone.0175741
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4C). Otherwise, the level of NDK in shrimp hemocytes from the dsRNA-EGFP group was
significantly higher than the two other groups. The reason of this result is not clear
Relative quantification of IE1 and VP28 gene expression in gills of
After WSSV infection, the gene expression of IE1 and VP28 changed with time. The IE1 gene
in gills was detected to be up-regulated from 48 h to 72 h during WSSV infection in the NDK
group. Significant differences in IE1 gene expression existed between NDK and the control
groups BSA and PBS during this time (P < 0.05; Fig 5A). Meanwhile, we found that the level
of IE1 gene in PBS group was significantly higher than BSA group at 48 h. However, the gene
expression of VP28 in NDK infected group reached their highest level at 72 h and there were
no significant changes observed from 24 h to 72 h in the BSA infected group and non-infected
control group (P < 0.05; Fig 5C).
Furthermore, the gene expression of IE1 (Fig 5B) in the dsRNA-NDK infected group and
PBS group was significantly lower than dsRNA-EGFP infected group at 48 h and 72 h. But
the level of VP28 gene (Fig 5D) in the dsRNA-NDK infected group was significantly higher
than in the dsRNA-EGFP group and PBS group at 48 h. An effective inhibition of NDK was
achieved by dsRNA-NDK at 72 h compared to the dsRNA-EGFP group. In contrast, in the
dsRNA-EGFP group, gene expression of IE1 and VP28 was up-regulated from 48 h to 72 h.
Fig 5. Relative IE1 and VP28 mRNA levels in experimental groups. (A) Relative IE1 mRNA levels of WSSV infected shrimp including over-expressed
NDK group, BSA group and PBS group. (B) Relative IE1 mRNA levels of WSSV infected shrimp including dsRNA-NDK group, dsRNA-EGFP group and
PBS group. (C) Relative VP28 mRNA levels of WSSV infected shrimp including over-expressed NDK group and BSA group. (D) Relative VP28 mRNA
levels of WSSV infected shrimp including dsRNA-NDK group, dsRNA-EGFP group and PBS group. Data represent the means ± SE. Differences were
evaluated with one-way ANOVA and Tukey test (p < 0.05).
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From the results we conclude that upon WSSV infection, gene expression of IE1 at 72 h and
VP28 at 48 h and 72 h increased in shrimps, whereas treatment with dsRNA-NDK contributed
to suppress WSSV genes IE1 and VP28 expression in gills.
Effect of expressed NDK and dsRNA-NDK on the cumulative mortality of
To further evaluate the relationship between rNDK and WSSV, we designed two experiments
to be carried out in vivo. In the first experiment, at 6±7 days post WSSV inoculation all the
shrimps in the WSSV-infected groups had died but variations in time to mortality were
observed. At 1±2 day post WSSV inoculation, mortality in the WSSV-NDK group was
significantly higher than the other groups (P < 0.05) (Fig 6A). Nonetheless, all the groups showed
100% mortality between 5±7 days post WSSV inoculation. It is possible that NDK might play a
role in promoting WSSV replication early during infection.
In the other experiment, WSSV combined with dsRNA-NDK was used to check the
function of rNDK in WSSV-infected shrimps. The data showed that the cumulative mortality of
WSSV-infected shrimps was significantly delayed by dsRNA-NDK (P < 0.05) (Fig 6B)
compared to the WSSV-challenged groups. Mortality (100%) was delayed 4 days in this group
compared to the other groups. As a result of this experiment, we further concluded that the
function of rNDK was to promote virus replicationv at the early stages of WSSV infection.
NDK is a key enzyme in controlling cell energy and nucleotide metabolism, and could act as a
chemotherapeutic prodrug in some pathologies since it functions in DNA replication and viral
]. However, there are few studies that show the immune response of NDK to
WSSV in L. vannamei. Accordingly, we used a prokaryotic expression method to acquire
rNDK to estimate the specificity of NDK binding to WSSV and to investigate if NDK would
function in preventing WSSV in cultured shrimp.
RNAi has recently been observed as a common mechanism for post-transcriptional gene
silencing in a variety of eukaryotic organisms, and is widely proven as an effective means to
suppress viral infection or the replication of many viruses [12±14]. Therefore, the synthesis of
double-stranded RNA (dsRNA-NDK oncoprotein) was used to investigate the role of NDK in
WSSV infection in shrimps. Using a similar method, we hope to investigate what role NDK
plays in WSSV-infected shrimps.
Firstly, tissues distribution analysis indicated that gene expression of NDK was higher in
hemocytes than in other shrimp tissues. However, gills were a main tissue extracted in our
experiment because they play an important role in defense, whereas hemocytes have been the
main cells studied so far [
], followed by hepatopancreas  and lymphoid tissue, in
research related to specialized defense in invertebrates [
]. In Fig 3, we observed that WSSV
copy number in the NDK+WSSV infection group was higher than in the BSA+WSSV and PBS
groups, especially at 72 h post WSSV inoculation. This result suggests that NDK might
promote WSSV replication early during infection in shrimps.
Otherwise, results of NDK gene expression in shrimps injected with dsRNA-NDK showed
that synthesized dsRNA-NDK had a significant function in inhibiting expression of the NDK
gene. Subsequently we analyzed relative expression of IE1 and VP28 in gills of WSSV-infected
shrimp. A previous study indicated that the WSSV immediate early gene IE1 is highly
expressed throughout the viral infection cycle and may play a central role in initiating viral
replication during infection [
]. In our study, VP28 and IE1 were measured by qPCR to identify
any variation in their expression. When shrimps were injected with rNDK protein, we found
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Fig 6. Time-mortality relationships of over-expressed NDK and followed by WSSV challenge. (A)
Cumulative mortality of shrimps administrated with NDK, BSA, WSSV and PBS. Mortality was measured in
each treatment group (n = 45) and was daily recorded. Analyses with SPSS 18.0 tested differences in
mortality between groups (NDK, BSA and WSSV). Significant differences in mortality were only found in days
1 and 2 (P < 0.05). (B) Mortality of WSSV-infected shrimps treated with dsRNA-NDK was delayed for 4 days
compared to dsRNA-EGFP- and negative control groups. The abscissa indicates time (days post WSSV
challenge) and the ordinate indicates cumulative mortality (%). Significant differences in mortality were found
between dsRNA-NDK and the other groups since day 2 post WSSV challenge (P < 0.05).
that gene expression of IE1 increased significantly at 48 h and 72 h compared to the PBS group
and BSA+WSSV infection group. In contrast, in the dsRNA-NDK group, NDK levels were
lower at 48 h and 72 h post WSSV inoculation than in the dsRNA-EGFP+WSSV and PBS
groups, upon WSSV challenge. A similar trend in VP28 gene expression was found only at 72
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h post WSSV inoculation. We concluded that upregulation of IE1 and VP28 gene expression
was due to the presence of rNDK protein.
In order to further investigate the phenomenon, we analyzed results of cumulative
mortality to show that mortality in the NDK+WSSV group was significantly higher than other
WSSV-infected groups from 1 to 2 days. Furthermore, in the dsRNA-NDK group, large
amounts of shrimps were alive at the early stage of infection. So, we suggested that rNDK
protein could increase mortality of WSSV-infected shrimps and dsRNA-based NDK molecules
could provide significant and long-term protection in this pathology.
As few studies were carried out to prove what roles NDK acted in L. vannamei infected by
WSSV, no reference could be used. Therefore, combined with the results obtained based on
the limited shrimps, we concluded that dsRNA-NDK had a modest antiviral effect at the early
stages of WSSV infection in shrimps. Though protective effect was not persistent, it could also
act as potential therapy for WSSV-infected shrimps. However, some other studies have
indicated that NDK-1 is necessary for proper MAPK activation in the somatic tissues of the worm
]. In order to further understand the function of NDK, we need to research its function in
cell metabolism in the shrimp.
In summary, these results indicate that NDK is a protein which could influence the adverse
function of WSSV in shrimp. However, more studies should be carried out to further support
this research because the mechanism of how NDK acts on WSSV is not proven sufficiently.
Nevertheless, our research also leads the way to further study the specific function of NDK in
WSSV, providing a basis for addressing this issue in the near future.
In our study, recombinant protein NDK was produced to study the mechanisms regulating
WSSV in L. vannamei by qRT-PCR, and RNAi, combined with analyzed data of IE1 and VP28
gene expression. We found that recombinant protein NDK could induce rapid infection of
WSSV and accelerate the death of shrimps. Therefore, to inhibit the expression of NDK in L.
vannamei might strengthen immune defenses against infection by the pathogen WSSV.
Data curation: PFL.
Formal analysis: PFL.
Funding acquisition: JH.
Investigation: PFL QHL.
Methodology: PFL QHL YW.
Project administration: QHL YW JH.
Validation: QHL YW JH.
Visualization: QHL YW JH.
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Writing ± original draft: PFL.
Writing ± review & editing: PFL.
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