using dsrna to prevent in vitro and in vivo …2003/03/19 · victor julian valdes+, alicia...

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USING dsRNA TO PREVENT IN VITRO AND IN VIVO VIRAL INFECTIONS BY

RECOMBINANT BACULOVIRUS

Victor Julian Valdes+, Alicia Sampieri+, Jorge Sepulveda and Luis Vaca

Departamento de Biología Celular. Instituto de Fisiología Celular, UNAM. México, D.F.

04510.

+ These authors contributed equally to this work.

Running title: Silencing a viral infection with double stranded RNA

Corresponding Author: Dr. Luis Vaca Departamento de Biología Celular Instituto de Fisiología Celular UNAM Ciudad Universitaria México. D.F. 04510 Tel. (525) 622-5654 Fax. (525) 622-5611 E-mail: [email protected]

Abbreviations:

AcNPV-GFP Autographa californica recombinant baculovirus carrying the green fluorescence protein (GFP) and the beta-galactosidase genes.

GFP Green fluorescence protein b-gal beta-galactosidase pfu Plaque forming units MOI Multiplicity of infection dsRNA Double-stranded RNA mRNA Messenger RNA RNAi RNA interference Sf21 Spodoptera frugiperda 21 cell line X-gal 5-bromo-4-chloro-3-indolyl-beta-D-galactoside

Copyright 2003 by The American Society for Biochemistry and Molecular Biology, Inc.

JBC Papers in Press. Published on March 19, 2003 as Manuscript M212039200 by guest on M

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SUMMARY

Introduction of double stranded RNA (dsRNA) into a wide variety of cells and

organisms results in post-transcriptional depletion of the homologue endogenous

messenger RNA (mRNA). This well-preserved phenomenon known as RNA interference

(RNAi) is present in evolutionary diverse organisms such as plants, fungi, insects,

metazoans and mammals. Because the identification of the targeted mRNA by the RNAi

machinery depends upon Watson–Crick base-pairing interactions, RNAi can be

exquisitely specific. We took advantage of this powerful and flexible technique to

demonstrate that selective silencing of genes essential for viral propagation prevents in

vitro and in vivo viral infection. Using the baculovirus Autographa californica, a rapidly

replicating and highly cytolytic double-stranded DNA virus that infect many different

insect species, we show for the first time that introduction of dsRNA from gp64 and ie1,

two genes essential for baculovirus propagation, results in prevention of viral infection in

vitro and in vivo. This is the first report demonstrating the use of RNAi to inhibit a viral

infection in animals. This inhibition was specific, since dsRNA from the polyhedrin

promoter (used as control) or unrelated dsRNAs did not affect the time course of viral

infection. The most relevant consequences from the present study are: 1) RNAi offers a

rapid and efficient way to interfere with viral genes to assess the role of specific proteins

in viral function, and 2) using RNAi to interfere with viral genes essential for cell

infection may provide a powerful therapeutic tool for the treatment of viral infections.

Key words: RNAi, double stranded RNA, gene silencing, baculovirus, viral infection,

RT-PCR.

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INTRODUCTION

Introduction of double stranded RNA (dsRNA) into a wide variety of cells and

organisms results in post-transcriptional depletion of the homologue endogenous

messenger RNA (mRNA) (1). This well-preserved phenomenon known as RNA

interference (RNAi) is present in evolutionary diverse organisms such as plants, fungi,

insects, metazoans and mammals (fore review see (2)). It is generally accepted that in

plants, RNAi may serve as a mechanisms of defense against viral infections (3). The

physiological role of RNAi in animals remains to be established, although possible roles

in organism development, germline fate, and host defenses against transposable elements

and viruses have been suggested (4). In this regard, recent experimental evidence shows

modulation of HIV-1 replication in human cell lines by exogenous dsRNA from several

portions of the viral genome (5). In a different study, dsRNA conferred viral immunity in

human cells against poliovirus (6).

Although the effect of dsRNA to prevent viral infections has been tested in cell lines,

remains to be established if RNAi works also in organisms. To test this hypothesis we

used a well-studied insect virus, the Autographa californica nucleopolyhedrovirus

(AcNPV). This virus is a member from a group (family Baculoviridae) of large double-

stranded DNA viruses that infect many different insect species (7). The AcNPV GP64

glycoprotein is a major component of the nucleocapsid of budded viruses (BV), and is

required for BV entry into host cells by endocytosis (8). Thus, GP64 is a key element for

cell-to-cell infection and virus propagation (8). Another gene essential for baculovirus

proliferation produces the immediate early protein (IE1). This phosphoprotein regulates

the transcription of early viral genes (9). Deletion of the n-terminal domain of IE1 results


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in the loss of transcriptional activation and the resulting viruses cannot be replicated (9).

Thus, these two proteins are excellent candidates to test if RNAi can prevent viral

infection in vitro and in vivo.

Recombinant AcNPV in conjunction with Spodoptera frugiperda (Sf9 and Sf21) insect

cell lines have been extensively used for the heterologous expression of a wide variety of

genes (7). We have produced a recombinant baculovirus containing the genes for the

green fluorescent protein (GFP) and beta-galactosidase (AcNPV-GFP), which we have

used as reporters of cell infection in Sf21 insect cells and larvae.

Since introduction of dsRNA results in powerful and selective gene silencing in different

cells (including insect cells), one would expect that transfecting insect cells with dsRNA

from gp64 prior to virus exposure could prevent virus propagation since the BV produced

by dsRNA-treated cells would lack GP64 protein.

In agreement with this prediction, cells infected with AcNPV-GFP and previously

transfected with dsRNA from a portion of gp64 (dsRNA-gp64) do not show GFP

fluorescence and the GP64 protein could not be detected in the membrane of these cells.

Since the very late polyhedrin promoter drives GFP production, the lack of GFP

expression indicates that dsRNA treatment prevents viral infection in culture. Similar

results were obtained with cells transfected with dsRNA from ie1 (dsRNA-ie1). In this

case the inhibition was even stronger as expected when interfering with the synthesis of a

transcriptional activator essential for viral replication.

To test if silencing gp64 and ie1 genes may interfere also with viral infection in vivo, we

used the larvae from the insect Tenebrio mollitor (mealworm beetle). Injection of

AcNPV-GFP into the haemolymph of the larvae from this insect results in death of 100%


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of the larva within the following 7 days. Dead insects show high levels of GFP

fluorescence and beta-galactosidase activity, the two reporter genes present in the

recombinant AcNPV-GFP baculovirus. Injection of dsRNA-gp64 or dsRNA-ie1 one day

prior to the injection with AcNPV-GFP prevents larva death by over 90%, while the

surviving insects do not show GFP fluorescence or beta-galactosidase activity.

All these results show for the first time that the introduction of dsRNA into culture cells

and living animals to interfere with the synthesis of viral proteins essential for

baculovirus infection, results in potent inhibition of viral infection in vitro and in vivo.

The most relevant consequences from the present study are: 1) RNAi offers a rapid and

efficient way to interfere with viral genes to assess the role of specific proteins in viral

function (reverse genetics in baculovirus), and 2) using RNAi to interfere with viral genes

essential for cell infection may provide a powerful therapeutic tool for the treatment of

viral infections.

EXPERIMENTAL PROCEDURES

Reagents. All salts were analytical grade purchased from SIGMA (St. Louis, MO). X-gal

(5-bromo-4-chloro-3-indolyl-beta-D-galactoside) was obtained from SIGMA (St. Louis,

MO). o-Nitrophenyl-β-D-galactoside (ONPG) was purchased from RESEARCH

ORGANICS Inc. (Cleveland, OH). The enhanced green fluorescence protein (GFP)

vector (pEGFP-N1) was purchased from CLONTECH (Palo Alto, CA).

Cell culture. Insect Sf21 cells were obtained from INVITROGEN (San Diego, CA) and

cultured in Grace’s medium (SIGMA, (St. Louis, MO) supplemented with lactalbumin


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hydrolysate, yeastolate, 2 mM L-glutamine, 10% heat-inactivated fetal bovine serum, 100

µg/ml streptomycin, and 250 ng/ml amphotericin B (GIBCO, Grand Island, NY).

Production of recombinant AcNPV-GFP baculovirus. The cDNA from GFP

(CLONTECH, Palo Alto, CA) was cloned in the pBB4 vector (INVITROGEN, Carlsbad,

CA) next to the polyhedrin promoter. The pBB4-GFP vector was recombined with linear

AcNPV DNA following the manufacturer instructions (INVITROGEN, Carlsbad, CA).

After two plaque assays, the recombinant baculovirus containing the GFP (AcNPV-GFP)

was amplified twice and the titer was determined by plaque assay. This recombinant

baculovirus was used in the studies described in this work.

dsRNA synthesis. Double stranded RNA was synthesized in vitro using the Megascript

kit (AMBION, Austin, TX), following the manufacturer instructions. Briefly, nucleotides

109330-109949 were amplified by PCR from the entire virus genome. This sequence

corresponds to the first 619 nucleotides from gp64 as verified by sequence analysis. The

sequence was cloned in the pPD129.36 vector multiple cloning site which is flanked by

two T7 opposing promoters (10). The T7 promoter sequence including the fragment of

gp64 was amplified by PCR using universal T7 primers, and this sequence was used for

the in vitro transcription to obtain the dsRNA with the T7 polymerase. For the synthesis

of dsRNA from GFP, the entire gene sequence was used and for polyhedrin the entire

promoter sequence (Genbank X06637). The ie1 gene is formed of a promoter sequence

and the sequence resulting in the transcriptional activator protein. The dsRNA from ie1

was synthesized from nucleotides 19-470 (ATG is in position 1) corresponding to the n-


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terminal domain from the IE1 protein. For single stranded RNA production, the vector

was linearized with Hind III and in vitro transcription was performed (positive strand).

The antisense strand was synthesized after linearizing the vector with Not I. In all cases,

5 µg of each dsRNA using Cellfectin (INVITROGEN, Carlsbad, CA) was introduced

into Sf21 cells (1 x 106 cells) 48 hours prior to infection with recombinant baculovirus.

Fluorescence-Activated Cell Sorting (FACS). The percentage of infected cells (GFP-

expressing cells) and fluorescence intensity was assessed by FACS (FACScalibur;

Becton Dickinson) analysis 48 h postinfection using the multiplicity of infection (MOI)

reported in figure legends. Control uninfected cells were used to adjust the number of

GFP-positive cells and mean fluorescence (20,000 events / sample). Acquisition and

analysis of the FACS data were performed using CELLQUEST software (BECTON

DICKINSON, Palo Alto, CA).

Confocal microscopy. Infected Sf21 cells were fixed 48 hours postinfection with a buffer

containing 3 % paraformaldehyde SIGMA (St. Louis, MO) as previously described (11)

and incubated with the specific GP64 antibody (AcV5) eBIOSCIENCE (San Diego, CA).

A second anti-mouse antibody conjugated with rhodamine (ZYMED, San Francisco, CA)

was used to detect gp64 specific fluorescence. Cells were analyzed using a Biorad

confocal system (BIORAD, Hercules, CA) attached to a Nikon Diaphot inverted

microscope equipped with a 60X oil immersed objective. GFP fluorescence was obtained

after exciting the cells with 395 nm and reading fluorescence at 540 nm (green channel).

For gp64 detection, the excitation wavelength was 570 and emission was collected at 590


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nm (red channel). Double labeled was observed in yellow, the combination of green

(GFP) and red (GP64) channels.

Beta-galactosidase activity assay. dsRNA-treated cells were pelleted and resuspended in

0.05 ml of Reporter lysis buffer 1X (PROMEGA. Madison, WI). Beta-galactosidase

activity was quantified using the enzymatic hydrolysis of the synthetic substrate o-

Nitrophenyl-β-D-galactoside (ONPG, RESEARCH ORGANICS Inc. Cleveland, OH).

The reaction was produced in triplicates on a 96 well plates. Reactions started with the

addition of 0.2 ml of 4 mg/ml ONPG at 37°C for 30 minutes and stopped by the addition

of 0.05 ml of 1 M Na2CO3. The absorbance at 405 nm were measured in an ELISA reader

(DYNEX MRX II) beta-galactosidase activity was determined using the following

equation: beta−gal activity = [(Abs420) X 1.7)] / [(0.0045) x (time) x (mg/ml protein in

the lysate) x (ml lysate used)]. The blank (uninfected cells exposed to ONPG) was

subtracted in all cases.

Plaque Assay. Plaque assays were performed in six-well plates according to the instruction’s manual Bac-N-BlueTM (INVITROGEN, San Diego, CA). In short, Sf21 cells were plated at 2.0 x 106 cells/well, allowed to attach for 30 min, and infected 1-hour at 27 °C with serial dilutions (in 1 ml Grace media 10% FBS) of the supernatant obtained from transfected in infected cells, as indicated in figure legend. After the incubation period the media was aspirated and 2 ml of Grace media, 10% FBS, 1.5% LMP agarose (GibcoBRL) was added. Finally 1 ml of Grace media 10% FBS was added over the agarose. Cells were monitored for GFP-positive plaque formation after 72 hours. The GFP-positive plaques were quantified and the plaque forming units / ml (pfu/ml) was calculated multiplying the number of GFP- positive plaques by dilution reciprocal. All dilutions were repeated at least twice.

RNA Purification and RT-PCR Analysis. Total RNA was isolated from dsRNA-

transfected or untreated cells using TRIzol Reagent (INVITROGEN, Carlsbad, CA)


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following the manufacturer’s recommended protocol. All RNA samples were quantified

by spectrophotometry and run on agarose gel to check the quality and integrity of total

RNA. To avoid DNA contamination, 5 µg of total RNA were treated with 15 units of

DNaseI RNase-free (AMBION, Austin, TX) for 20 min at 37°C. After a Phenol-

Chloroform extraction reverse transcriptions reactions were done with the SuperScript™

One Step RT-PCR System from (INVITROGEN, Carlsbad, CA) using gene specific

primers (0.2 µM). β-Actin: 5´-GATATGGAGAAGATCTGGCACCAC-3´ and 5´-

TGGGGCAGGGCGTAGCC-3´; gp64: 5´-GAAAACAGTCGTCGCTGTCA-3´ and 5´-

TATAGTCGACGAGCACTGCAACGCGCAAATG-3´; ie1: 5´-

GCGCCGTATTTAATGCGTTT-3´ and 5´-GAGGAATTTCTATGCCGGTTTC-3´; gfp:

5´-ATATCCCGGGATGGTGAGCAAGGGCGAG-3´ and 5´-

GCTCGTCGACCTTGTACAGCTCGTCCATGC-3´. For each RT-PCR reaction, 200 ng

of RNA were used in a 50 µl reaction. A retrotranscription step (50°C for 30 min, and

94°C for 2 min.) was followed by 21 cycles of denaturation at 94°C for 15 s, annealing at

55° for 30 s, and extension at 72°C for 50 s. To ensure that there was no DNA

contamination in the RT-PCR experiments, controls without reverse transcriptase were

performed in all cases. In all presented data, RT-PCR products were resolved by 1.5 %

agarose gel electrophoresis and analyzed after ethidium bromide staining in a Typhoon

8600 (AMERSHAM BIOSCIENCES, Piscataway, NJ).

Electron microscopy. Sf21 control (uninfected) or infected cells were fixed with 3%

glutaraldehyde in saline buffer for 2 hours at room temperature as previously described

(12). For negative stain; viruses were purified from the supernatant of infected cells using


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the polyethylene glycol (PEG) method (13). Briefly, the supernatant of infected cells was

centrifuged to eliminate cell debris and 20% polyethylene glycol in 1 M NaCl was added.

The supernatant was centrifuged again and the pellet was rinsed twice with distilled

water. The pellet was resuspended and used for negative stain studies using a scanning

electron microscope (Jeol, JSM-5410LV) at low vacuum (12).

Insect infection. Tenebrio mollitor (mealworm beetle) larvae were obtained at a local pet

store and maintained in the laboratory in a plastic container. Larvae were injected in the

haemolymph with 0.1µg of the different dsRNAs nude or with distilled water (control).

24 hours later larvae were injected again in the haemolymph with 2 µl of AcNPV-GFP

viral stock with a titer of 1X10-7. Dead insects were assayed for GFP fluorescence and

beta-gal activity. To assess the GFP fluorescence, injected insects were explored with the

confocal microscope using a low magnification objective (10X, Nikon). For beta-gal

activity, the insects were injected in the haemolymph with 2 µl of 0.05% X-gal in

dimethyl sulfoxide (DMSO).

RESULTS

Selective inhibition of baculovirus infection of Sf21 cells by dsRNA from gp64 and

ie1.

We explored the effect of transfecting Sf21 cells with different dsRNAs prior to

AcNPV-GFP infection. For these experiments we used 5 different multiplicities of

infection (MOI) of 0.05, 0.5, 1 and 5. Figure 1 illustrates a representative example of the

effect of different dsRNAs on the fluorescence of GFP at a MOI of 0.05 obtained 72


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hours postinfection. As illustrated in this example, the fluorescence of GFP was greatly

reduced by transfecting the cells (48 hours prior to AcNPV-GFP infection) with dsRNA

from gfp or gp64 but not with dsRNA from the polyhedrin promoter used as control. At

this time postinfection, 90.2±2.1 % (n=5) of AcNPV-GFP-treated cells show high GFP

fluorescence when compared to control (uninfected) cells. Transfecting the cells with

dsRNA-gfp 48 hours prior to AcNPV-GFP infection resulted in significant reduction of

GFP fluorescent cells to 9.1±0.7% (n=5). Exposing the cells to dsRNA from gp64

(dsRNA-gp64) prior to AcNPV-GFP infection resulted also in an important reduction in

the number of GFP fluorescent cells to 8.3±0.5% (n=5). Transfecting the cells with

dsRNA from the polyhedrin promoter (dsRNA-PhP) did not affect significantly the GFP

fluorescence, showing values of 89.1±1.2 % (n=5).

These results show that introduction of dsRNA from gp64 gene prevents GFP

fluorescence induced by recombinant baculovirus. Since the very late polyhedrin

promoter (PhP) drives GFP production, the lack of GFP expression indicates that

dsRNA-gp64 treatment prevents viral infection in culture. Cells transfected with dsRNA-

gp64 did not show beta-gal activity, the second reporter gene introduced in the

recombinant AcNPV-GFP (Fig. 2B). The early-to-late (PETL) promoter drives beta-gal

production. Thus, using these two reporter genes one can monitor early and very late viral

gene expression. Transfecting insect cells with dsRNA-gfp prevents GFP fluorescence by

favoring GFP mRNA degradation, but as expected, this does not appear to affect viral

infection. Cells interfered with dsRNA-gfp show beta-gal activity, indicating the

succession of typical viral infection (Fig. 2B). Furthermore recombinant AcNPV-GFP is

a lytic baculovirus, therefore cells infected with AcNPV-GFP will lyse eventually. Even


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at a low MOI of 0.05, all insect cells infected with AcNPV-GFP and treated with dsRNA-

gfp lysed 7 days after infection.

Figure 2 concentrates all the RNAi experiments performed at MOIs of 0.05, 0.5, 1 and 5.

In this figure the percentage of GFP fluorescent cells is presented in panel A and the beta-

gal activity from the same cells is shown in panel B. As illustrated in this figure, cells

treated with dsRNA from gp64 showed low levels of GFP fluorescence and beta-gal

activity at a MOI of 0.05 and 0.5. At MOIs of 1 and 5 dsRNA-gp64 did not prevent GFP

fluorescence or beta-gal activity from AcNPV-GFP infected cells. This result is

somewhat expected since all cells would be infected by the initial virus application and

do not depend on the production of viral progeny. When using MOI of 0.05 (one viral

particle for every 20 cells), the infection of the rest of the cells not infected by the initial

viral application would depend upon production of new viruses by the cells initially

infected. Since cells are transfected with dsRNA-gp64 one would expect that the GP64

protein could not be produced and the new viruses would lack this protein. At MOI of

0.05, dsRNA-gp64 inhibits strongly GFP fluorescence (Fig. 2A) and beta-gal activity

(Fig. 2B). These results are consistent with previous studies found in the literature

indicating the gp64 gene deletion result in non-infective baculoviruses (8). In contrast to

these results, transfecting the cells with dsRNA-ie1 inhibited GFP fluorescence and beta-

gal activity even at MOIs of 1 and 5 (Fig. 2). The IE1 transcriptional factor is essential

for initiating the transcription of several early baculovirus genes. In fact, deletion of the

ie1 gene results in viruses that cannot be replicated (9). Therefore, the results presented

here are consistent with the idea that interfering with the production of the initial


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transcriptional activator for viral genes (ie1) would prevent the production of viral

proteins and interfere with viral replication.

In agreement with all these results, the baculoviruses isolated from the supernatant of

dsRNA-gp64 and dsRNA-ie1-treated cells were less infective when compared to

untreated cells or cells transfected with dsRNA-gfp prior to baculovirus infection (figure

2C). As expected from cells exposed to dsRNA-gp64, only baculoviruses collected from

cells infected with low MOIs (0.05 and 0.5) showed reduced plaque forming units (pfu).

At MOIs of 1 and 5 the baculoviruses produced by dsRNA-gp64-treated cells showed pfu

indistinguishable from cells not exposed to dsRNA or cells treated with dsRNA-gfp (Fig.

2C). Notice that the effect of dsRNA is specific for the gene explored, since cells exposed

to dsRNA-gfp showed a significant reduction of GFP fluorescence but not beta-gal

activity. As expected from these experiments, baculoviruses isolated from dsRNA-gfp-

treated cells showed pfu similar to cells not exposed to any dsRNA. The RNAi effects

observed are specific for the double stranded RNA, since transfecting the cells with

single stranded RNA prior to baculovirus infection did not altered the time course of viral

infection and did not prevent GFP fluorescence or beta-gal activity (data not shown). A

small reduction in GFP fluorescence was obtained with the negative single RNA strand

of gp64 of 20±5% compared to the large reduction of 92±7% obtained with dsRNA-gp64

(n=6). This result demonstrates that the inhibition induced is specific for the double RNA

strand, as expected from an RNAi phenomenon.

dsRNA-gp64-treated cells do not express GP64 glycoprotein.


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Confocal microscopy studies using a specific GP64 antibody showed protein

localization at the cell plasma membrane. The pattern of localization of GP64 contrasted

with the generalized fluorescence of GFP, as expected for a soluble protein. Figure 3

shows representative images obtained with a rhodamine labeled antibody for GP64 (red

channel) and GFP fluorescence (green channel). As illustrated in the figure, the

transfection with dsRNA-gfp prevented GFP fluorescence but did not affect GP64

expression of insect cells infected with AcNPV-GFP at a MOI of 0.05. Transfecting the

cells with dsRNA-gp64 eliminated the fluorescence of both GFP and GP64, leaving only

a few cells showing both signals. These cells may reflect cells infected by the initial

application of recombinant AcNPV-GFP. Interestingly, when AcNPV-GFP was used at a

MOI of 1, dsRNA-gp64 could not prevent GFP fluorescence, however the transfection

prevented GP64 protein production, as illustrated by the lack of red fluorescence in figure

3. This result is consistent with the data presented in figure 2, suggesting that the initial

viral application can infect most of the cells (therefore the GFP production is intact), yet

the cells do not produce GP64 protein. The transfection of cells with dsRNA-ie1

prevented GFP and GP64 production even at a MOI of 1 (Fig. 3). Only a few cells per

field showed both green and red fluorescence, since not all the cells are efficiently

transfected (Fig. 3, bottom panel to the left). This result is also consistent with the

findings using FACs. Transfecting the cells with dsRNA-PhP did not alter the GP64 and

GFP fluorescence, showing values similar to those obtained from cells exposed to

recombinant AcNPV-GFP alone (Fig. 3).

Western blotting analysis illustrated in figure 4A confirmed the selective RNAi effect

observed with dsRNAs. The upper panel shows the amount of GP64 and GFP proteins


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obtained at 3 different MOIs (1, 5 and 10) in the absence of dsRNA. For GP64, two

bands were observed reproducibly in all assays. These bands may represent different

glycosylation forms of the protein previously described (14).

The next panel shows the GP64 and GFP protein contents from cells transfected with

dsRNA-gfp 48 hours prior to infection. Notice the reduction of GFP protein content at

MOI of 1 and 5, while the GP64 protein content was unaltered. The following panel

shows the effect of transfecting the cells with dsRNA-gp64 prior to viral infection. In this

case, GP64 protein content was significantly reduced at a MOI of 1. At MOIs of 5 and 10

substantial GP64 protein was detected. These results are consistent with the data

previously shown using FACS and confocal microscopy.

Finally, the lower panel from figure 4A illustrates the effect of transfecting cells with

dsRNA-ie1. Under these conditions, both GFP and GP64 proteins were dramatically

reduced at all MOIs explored. Consistent with these findings, cell transfected with

dsRNA-ie1 and later infected with AcNPV-GFP could be maintained in culture for

several months, even at MOIs of 10. Insect cells divided normally like uninfected cells.

On the contrary, cells transfected with dsRNA-gp64 or dsRNA-gfp and infected with a

MOI of 5 or higher could not be maintained in culture more than a week. After 7 days in

culture only cellular debris was observed in the culture, indicative of cell lysis as a result

of the baculovirus infection.

To determine if the reduction in protein content in cells transfected with dsRNAs was the

result of depletion of the specific mRNA, we performed RT-PCR experiments using

primers specific for gp64, ie1, gfp and β-actin from sf21 cells (see Experimental

Procedures). Figure 4B illustrates a representative experiment obtained at a MOI of 1.


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Uninfected cells (NI) showed the presence of β-actin mRNA, this was confirmed by

sequence analysis of the fragment amplified by the RT-PCR experiment. As expected, in

uninfected cells gfp, gp64 and ie1 were not detected. Interestingly, infection with

recombinant baculovirus resulted in reduction of mRNA for β-actin. This result can be

explained by the fact that at the late phase of baculovirus infection, the majority of

mRNAs are produced by the baculovirus α-amanitin-resistant RNA polymerase (15). The

β-actin mRNA is observed again in cells interfered with dsRNA from gp64 and ie1. In

these experiments we observed again the specificity of the RNAi effect, notice that

interfering with gp64 (dsRNA-gp64 panel) significantly reduces the mRNA from gp64

but did not affect the ie1 mRNA. As expected, interfering with ie1 (dsRNA-ie1)

significantly reduced the amount of gfp, gp64 and ie1 mRNAs, and B-actin levels are

restored to normal due to the inhibition of viral infection. These results demonstrate that

the reduction in protein content originates from the depletion of the respective mRNA.

Depleting the ie1 mRNA turned out to be a very efficient way to interfere with

baculovirus infection. In fact, cells transfected with dsRNA-ie1 and later infected with

AcNPV-GFP showed morphological characteristics of uninfected cells, as illustrated in

figure 5. Panel A shows the typical morphology of Sf21 insect cell infected with

recombinant AcNPV-GFP. Notice the large amount of viral particles inside the cell

nucleus. In contrast, no viral particles could be detected in dsRNA-ie1-tretaed cells as

illustrated in figure 5B. Purification of virions from supernatants of infected cells

(Experimental Procedures) allowed the observation of multiple viral particles (Fig. 5C).

In contrast, viral particles were extremely difficult to find in supernatants obtained from

dsRNA-ie1-treated cells (Fig. 5D). These viral particles did not show any reproducible


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characteristic that could separate them from particles isolated from dsRNA-ie1-untreated

cells. These viral particles may come from the initial viral infection or could be produced

by cells that were not efficiently transfected with dsRNA-ie1. These results are consistent

with the low pfu obtained with the supernatant of dsRNA-ie1-treated cells (Fig. 2C). The

very low titer found in the supernatant of dsRNA-ie1 and dsRNA-gp64-treated cells

confirms the poor viral content observed with the electron microscope (Fig. 5D).

Interestingly, it has been previously shown that GP64 glycoprotein is required for virus

budding from the cell (16). In fact, this glyprotein is acquired by virions during budding

through the plasma membrane of the infected insect cell, representing the final step in

virus assembly (14).

Injecting insects with dsRNA from gp64 or ie1 prevents baculovirus infection in

vivo.

To test if dsRNA from gp64 or ie1 could protect against AcNPV-GFP infection in

vivo, we used the larvae from the insect Tenebrio mollitor as bioassay. Injection in the

haemolymph with 2 µl of AcNPV-GFP resulted in death of over 97% of the larvae within

the following 7 days (68 dead of 70 injected). Dead insects showed high GFP

fluorescence assessed by confocal microscopy (Fig. 6, insert box at the upper left corner

of each panel) and high beta-gal activity, as illustrated in figure 6 (AcNPV-GFP, upper

panel to the left). Uninjected larvae or larvae injected with distilled water did not show

GFP fluorescence or beta-gal activity (H2O, upper panel to the right), and over 95% of

the insects survived the injection (4 dead of 60 injected). Larvae injected with 0.1 µg

nude dsRNA-ie1 24 hours prior to the injection with AcNPV-GFP survived the


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baculovirus infection (6 dead of 80 injected). These insects did not show GFP

fluorescence or beta-gal activity. Similar results were obtained with larvae pretreated

with 0.1 µg nude dsRNA-gp64, where 10 insects died of the 61 injected with AcNPV-

GFP (data not shown). In contrast, injection with 0.1 µg of nude dsRNA-PhP did not

protect against the subsequent injection with AcNPV-GFP. As expected, over 97% of the

larvae died within the subsequent 7 days (34 dead of 40 injected, data not shown).

Similarly, insects pretreated with dsRNA-gfp and later challenged with AcNPV-GFP

died within the next 7 days (58 dead of 62 injected). Interestingly, these insects did not

show GFP fluorescence (Insert in lower panel to the left) but did show high levels of

beta-gal activity (lower panel to the left).

DISCUSION

RNAi is a recently discovered phenomenon, which is rapidly becoming a

powerful tool to selectively deplete mRNA resulting in reduction of the specific protein.

This phenomenon has been found in worms, flies, several insects, plants and mammals

(1). The RNAi molecular mechanism involves several protein complexes including the

RNA-induced silencing complex (RISC) responsible for the degradation of the

homologue mRNA and RNA-dependent RNA polymerases (RdRPs) in charge of

synthesizing new dsRNAs and amplifying the RNAi signal (1). Proteins essential for the

RNAi phenomenon such as Dicer and Argonaute are highly conserved in fungi, plants

and animals. Although the exact physiological role of RNAi in animals is not known to

the present day, recent evidence suggests that RNAi may be involved in organism


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development, germline fate, and host defenses against transposable elements and viruses

(17).

Although it is not clear that RNAi may function as a defense mechanism in mammals,

recent experimental evidence shows that modulation of HIV-1 replication in human cell

lines by dsRNA is possible (5). In a different study, dsRNA conferred viral immunity in

human cells against poliovirus (6). Whether this is a physiological function for RNAi or

not, this recent discoveries open the possibility of using dsRNA to treat or prevent viral

infections.

One important point that needs to be explored is if the introduction of dsRNAs in

mammals provides effective defense against systemic infections. The adequate

distribution of the dsRNAs in the organism and the possible secondary effects of

introducing dsRNAs in humans are two of several issues that would have to be solved

before this technique is employed efficiently to prevent or treat viral infections.

Interestingly, it has been recently shown that intravenous introduction of small interfering

dsRNA in adult mice silences luciferase activity and endogenous Fas expression in vivo

(18), which demonstrates that systemic application of dsRNA is feasible and effective for

gene silencing in organisms.

In the present study we have used the rapidly replicating and highly cytolytic DNA virus

Autographa californica in combination with the insect cell line Sf21 from Spodoptera

frugiperda to assess the protective role of dsRNA from genes essential for baculovirus

propagation. We show here for the first time that dsRNAs from segments of two genes

essential for baculovirus propagation prevent virus infection in vitro and in vivo. We have

demonstrated that this effect is specific for the dsRNA sequence used, since we can


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eliminate GFP fluorescence (one of the reporter genes present in the recombinant

baculovirus) without affecting the time course of viral infection or the activity of beta-

galactosidase (the second reporter gene). Transfecting the insect cells with dsRNA from

gp64 prevents viral infection when using low MOIs. On the other hand, dsRNA from the

ie1 gene prevents viral infection even when high MOIs were utilized. Using the larva

from the insect Tenebrio mollitor we have shown that injecting dsRNAs from gp64 or ie1

confer resistance to recombinant baculoviruses. This protective effect was not obtained

with dsRNA from gfp or the polyhedrin promoter, two dsRNA sequences used as

controls. Even though injecting dsRNA from gfp prevented GFP fluorescence in insects,

did not stop viral infection by recombinant baculovirus. As far as we know, this is the

first report showing the protective effects of RNAi in vivo against a viral infection in

animals. The protective effects of dsRNA against viral infections have been previously

observed in plants (3).

An interesting result from our studies is the high efficacy to silence genes obtained with

long dsRNAs. It has been previously recognized that in nematodes and Drosophila, long

dsRNA sequences are as effective as short dsRNA sequences contrary to mammals,

where only 21 nucleotides long dsRNAs are functional. The highly efficient RNAi

observed in the present study was obtained with a single transfection. This was possible

after optimizing the type of lipids used for transfection and the amount of dsRNA used in

the mixture (Experimental procedures). Previous studies using null mutants of gp64 gene

have shown that the mutant baculovirus cannot be replicated, providing evidence of the

important role-played by GP64 protein in cell-to-cell infection (8). Our results are in

agreement with these original reports since interfering with GP64 protein synthesis


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results in baculoviruses that cannot infect insect cells. Only when high MOIs are used,

where virus infection does not depend on generation of new progeny, we observed

normal GFP production and beta-gal activity from infected cells. However, even when

these cells produced GFP and beta-gal, no GP64 protein was detected by confocal

microscopy or western blotting analysis. These results suggest that the new viral particles

produced by the cells interfered with dsRNA-gp64 would lack GP64 protein. In

agreement with this prediction, viruses collected from the supernatant of dsRNA-gp64-

treated cells and infected with low MOIs showed significantly reduced pfu as compared

to control. The possibility of producing viral particles missing specific proteins provides

a powerful tool for reverse genetics with viruses. Using this technique one could rapidly

explore the role of single or multiple genes in virus morphogenesis, assembly and

transport. A rapid method for screening the role of specific proteins in virulence may be

possible also. Several groups have shown genome-wide RNAi to deplete proteins from

entire chromosomes in the nematode Caenorhabditis elegans (19). Such genome-wide

RNAi could easily be implemented for entire virus genomes, especially with large

complex genomes such as the Autographa californica 134 kilobase genome.

Finally, in the genomic era where scientists are gathering large amounts of information

about the sequence of many genomes (20), we are learning that the next big challenge

will be finding rapid and efficient tools to determine the role played by thousands of

genes with unknown function. RNAi might be one of such tools (21).

ACKNOWLEDGEMENT


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The excellent services from the Molecular Biology and Microscopy units and the library

at the Instituto de Fisiologia Celular are greatly appreciated. Part of this work was

produced with equipment donated by the Alexander von Humboldt-Stiftung to LV.

REFERENCES

1. Sharp, P. A. (1999) Genes Dev 13, 139-141.

2. Hammond, S. M., Caudy, A. A., and Hannon, G. J. (2001) Nat Rev Genet 2, 110-

119.

3. Waterhouse, P. M., Wang, M. B., and Lough, T. (2001) Nature 411, 834-842.

4. Finnegan, E. J., Wang, M., and Waterhouse, P. (2001) Curr Biol 11, R99-R102.

5. Jacque, J. M., Triques, K., and Stevenson, M. (2002) Nature 26, 26

6. Gitlin, L., Karelsky, S., and Andino, R. (2002) Nature 418, 430-434.

7. Jackson, J. A. (1991) Bioprocess Technol 13, 402-413

8. Monsma, S. A., Oomens, A. G., and Blissard, G. W. (1996) J Virol 70, 4607-

4616.

9. Kovacs, G. R., Guarino, L. A., and Summers, M. D. (1991) J Virol 65, 5281-

5288.

10. Timmons, L., and Fire, A. (1998) Nature 395, 854.

11. Reyes-Cruz, G., Vazquez-Prado, J., Muller-Esterl, W., and Vaca, L. (2000) J Cell

Biochem 76, 658-673.

12. Molinari, J. L., Tato, P., Rodriguez, D., Solano, S., Rubio, M., and Sepulveda, J.

(1998) Parasitol Res 84, 173-180


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13. Porfiri, E., Evans, T., Bollag, G., Clark, R., and Hancock, J. F. (1995) Methods

Enzymol 255, 13-21

14. Oomens, A. G., Monsma, S. A., and Blissard, G. W. (1995) Virology 209, 592-

603.

15. Huh, N. E., and Weaver, R. F. (1990) J Gen Virol 71, 195-201.

16. Oomens, A. G., and Blissard, G. W. (1999) Virology 254, 297-314.

17. Dernburg, A. F., and Karpen, G. H. (2002) Cell 111, 159-162.

18. McCaffrey, A. P., Meuse, L., Pham, T. T., Conklin, D. S., Hannon, G. J., and

Kay, M. A. (2002) Nature 418, 38-39.

19. Maeda, I., Kohara, Y., Yamamoto, M., and Sugimoto, A. (2001) Curr Biol 11,

171-176.

20. Ueda, R. (2001) J Neurogenet 15, 193-204

21. Kuwabara, P. E., and Coulson, A. (2000) Parasitol Today 16, 347-349.

FIGURE LEGENDS

Figure 1. dsRNA from gp64 prevents baculovirus infection of Sf21 cells.

Representative FACS experiments from Sf21 cells obtained 72 hours postinfection with a

MOI of 0.05. Control represents uninfected cells (background fluorescence). AcNPV-

GFP illustrates cells infected only with recombinant baculovirus. dsRNA-gfp indicates

cells transfected with 5 µg of dsRNA from GFP 48 hours prior to the infection with

recombinant AcNPV-GFP. The next panel shows the fluorescence of Sf21 cells

transfected with 5 µg dsRNA-gp64 prior to infection with AcNPV-GFP. The last panel

shows cells transfected with 5 µg of dsRNA from the polyhedrin promoter (PhP) prior to


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AcNPV-GFP infection. The grayed rectangle shows the area considered as GFP

fluorescent positive. This area was selected based on the background fluorescence

obtained from uninfected cells (control panel). From this area in each panel was

calculated the percentage of fluorescent cells (indicated by the number inside each panel).

Figure 2. Effect of different dsRNAs on baculovirus infection.

A, Percentage GFP fluorescent cells obtained 72 hours after AcNPV-GFP infection at a

MOI of 0.05, 0.5, 1 and 5. No dsRNA represents cells infected only with AcNPV-GFP

(without dsRNA treatment), cells transfected with 5 µg of dsRNA-gp64, dsRNA-gfp and

dsRNA-ie1 48 hours prior to AcNPV-GFP infection. B, beta-galactosidase activity

measured from the same cells as in A. C, plaque assays performed with the supernatant

of cells in the same conditions as in A and B. Bars show mean ± standard deviation from

at least 5 independent experiments. Where not shown, deviations are smaller than

symbols.

Figure 3. Expression of GP64 and GFP in infected cells assessed by confocal

microscopy.

Representative confocal microscopy studies assessing GFP (green) fluorescence and the

presence of GP64 protein (red) in Sf21 cells infected with AcNPV-GFP at a MOI of 0.05

(upper panels) and 1 (lower panels) fixed at 96 hours postinfection. Control shows the

fluorescence background of uninfected cells. In all cases, the field shown was full with

Sf21 cells. AcNPV-GFP indicates cells infected with recombinant baculovirus in which

no dsRNA was transfected. Cells were also transfected with 5 µg of dsRNA-gfp and


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dsRNA-gp64 48 hours prior to AcNPV-GFP infection. Notice that a MOI of 0.05

dsRNA-gp64 prevented both GFP fluorescence and GP64 production. At a MOI of 1

dsRNA-gp64 did not prevent GFP fluorescence but prevented GP64 production (lower

panel). Only a few cells per field (shown with white arrows) showed both GFP and GP64

fluorescence. The double stranded RNA from the polyhedrin promoter (dsRNA-PhP,

used as control) did not alter GFP fluorescence or GP64 protein production (lower panel).

The photographs are representative examples from 6 independent experiments. Arrows

point to cells that expressed both GFP and GP64 proteins.

Figure 4. Western blotting and RT-PCR analysis of Sf21 cells infected with

recombinant baculoviruses.

A, Representative western blotting analysis from total cell lysates of Sf21 cells infected

with recombinant AcNPV-GFP at MOIs of 1, 5 and 10 and collected 72 hours

postinfection. MOI 0 indicates uninfected cells. Westerns were produced with specific

antibodies against GP64 and GFP proteins (see Experimental procedures). Notice the

increment in GFP and GP64 protein as the MOI increases. All lanes were loaded with

20µg of protein in an SDS-PAGE. The lower panel shows a portion of the electrophoresis

gel stained with silver to illustrate the even protein loading. B, Representative RT-PCR

experiment (n=3) from cells in the same conditions as those shown in A. Notice the

selective depletion of the specific mRNA for each condition. Only RT-PCR experiments

with MOI of 1 are illustrated. All products amplified were confirmed by sequence

analysis. NI represents uninfected cells.


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Figure 5. Electron microscopy of Sf21 cells infected with recombinant AcNPV-GFP.

Representative electron microscopy photographs from a Sf21 cell infected with

recombinant AcNPV-GFP (A) and a cell transfected with dsRNA-ie1 and infected 48

hours later with AcNPV-GFP (B). Notice the large number of viral particles inside the

cell nucleus in A (pointed by the black arrow). Cells transfected with dsRNA-ie1 and

later infected with AcNPV-GFP show morphology indistinguishable from uninfected

cells. Panel C shows a representative negative stain electron microscopy photograph

obtained from the supernatant of Sf21 cells infected with AcNPV-GFP. Panel D

illustrates a representative photograph obtained from the supernatant of Sf21 cells

transfected with dsRNA-ie1 48 hours before the infection with recombinant AcNPV-

GFP. Notice the low viral content in the supernatant of dsRNA-ie1-treated cells when

compared to untreated cells (Panel C). Finding viral particles in dsRNA-ie1-treated cells

was difficult in all the specimens studied. The dimensions and general morphology of

viruses recovered from dsRNA-ie1-treated cells was indistinguishable at this resolution

from viruses obtained from dsRNA untreated cells.

Figure 6. dsRNAs from ie1 and gp64 prevent baculovirus infection in vivo.

Larvae from the insect Tenebrio mollitor (mealworm beetle) injected in the haemolymph

with 2 µl of AcNPV-GFP (viral stock with a titer of 1X10-7, Upper panel to the left).

Larvae injected 2 µl distilled water (Upper panel to the right). Larvae injected with 0.1µg

nude dsRNA-GFP and 24 hours later injected with 2 µl of AcNPV-GFP (Lower panel to

the left). Larvae injected with dsRNA-ie1 and 24 hours later injected with 2 µl of


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AcNPV-GFP (Lower panel to the right). The bars in each panel represent the cumulative

percentage of dead insects over the seven-day period. The numbers inside each panel

indicate the total number of dead insects at day 7 divided by the total of larvae injected.

For insects injected with AcNPV-GFP alone, 68 of 70 larvae died by day 7 postinjection,

therefore, the bar from day 7 indicate 97.14% dead insects (Upper panel top the left).

Only 6.6% of insects injected with water died by day 7 (4/60). While 93.5% (58/62)

insects previously injected with dsRNA-GFP died, only 7.5% (6/80) larvae previously

injected with dsRNA-ie1 died at day 7 after baculovirus injection, illustrating the

protective effect of dsRNA-ie1 on baculovirus infection. These insects did not show GFP

fluorescence or b-gal activity, as illustrated in the lower panel to the right. The insert to

the left corner of each panel shows representative confocal microscopy green

fluorescence images of the head of the insect (insect used shown by the arrow)

illustrating the expression of GFP in infected larvae. Notice the low green fluorescence

observed in insects injected with distilled water (background fluorescence). The larvae

shown inside each panel are representative examples of insects injected with 2 l of

0.05% X-gal in dimethyl sulfoxide. Notice that only insects infected with AcNPV-GFP

turned blue after x-gal injection, indicating the expression of the second reporter gene (b-

gal).


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Fig. 1 Valdes et al.


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AcNPV-GFPAcNPV-GFPdsRNA-gfp

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Victor Julian Valdes, Alicia Sampieri, Jorge Sepulveda and Luis Vacabaculoviruses

Using dsRNA to prevent viral infections in vitro and in vivo by recombinant

published online March 19, 2003J. Biol. Chem.

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