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Department of Veterinary and Biomedical Sciences, University of NebraskaLincoln, Lincoln, NE (J-HS, 1 RA, ARD); and Pan American Foot-and-Mouth Disease Center, Rio de Janeiro, Brazil (RA)
| Abstract |
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Key words: Apoptosis; immunohistochemistry; in situ hybridization; neuropathogenesis; New Jersey virus; Swiss Webster mice; TUNEL; vesicular stomatitis.
| Introduction |
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Studies using immunohistochemistry (IHC) and in situ hybridization (ISH) after intranasal inoculation of VSV have indicated that the virus passes transsynaptically using both anterograde and retrograde transports from the olfactory bulb to different regions of the brain.3,14 More recently, it was reported that VSV has oncolytic activity when inoculated intravenously in immunocompromised mice.1 In vitro experiments and studies with immunocompromised mice have demonstrated that virus induces cytolysis through caspase-dependent and -independent programmed cell death.1
Apoptosis is characterized by cell shrinkage, loss of normal cell-to-cell contact, chromatin condensation, cellular fragmentation, and DNA degradation, which produces oligonucleosome-sized nuclear fragments that form a specific ladder pattern on agarose gels.11,15 Apoptosis plays a protective role by eliminating cells that might prove harmful if they were to survive, e.g., removing cells harboring mutations after irradiation or chemical stimulation, which could lead to the development of neoplasia. Apoptosis protects against the spread of virus to uninfected cells,7,12,20 thus limiting infection.
Little is known about events involved in the neuropathogenesis of VSV. Limited data exist on the role of apoptosis toward VSV-induced neurologic disease in the mouse.1,12,17 To better understand the neuropathogenesis, we studied the occurrence of apoptosis in neurons after VSV infection of immunocompetent mice and mapped the progression of viral infection in the mouse brain.
| Materials and Methods |
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The pathogenic vesicular stomatitis virus New Jersey (VSV-NJ) USA/95 (USDA No. 32608) was isolated in the southern USA during the 1995 disease outbreak involving horses. The virus was obtained from the National Veterinary Services Laboratory, Animal and Plant Health Inspection Services, US Department of Agriculture, Ames, Iowa.
Experimental infection of mice
Twenty specific pathogen-free male Swiss Webster mice, 34 weeks old, were used throughout the study. Mice were inoculated intranasally with 20 µl of a viral suspension containing 107.3 TCID50/ml (tissue culture infectivity dose) administered equally to both nostrils. Four uninfected mice, which were matched with the principal in age and breed, were used as controls. At 24, 48, 72, 96, and 120 hours postinfection (hpi), four mice were given a lethal intraperitoneal dose of 5 mg of pentobarbital sodium (Nembutal; Abbott Laboratories, North Chicago, IL) in serum-free Roswell Park Memorial Institute medium (GIBCO BRL, Life Technologies, Inc., Gaithersburg, MD). Each mouse was perfused transcardially with 2030 ml of 4% paraformaldehyde. Whole brain was removed and postfixed in 4% paraformaldehyde for 5 hours. Sections of brain were paraffin embedded, sectioned, stained with hematoxylin and eosin, and examined by light microscopy. Appropriate sections were prepared and further examined by terminal deoxynucleotidyltransferasemediated deoxyuridine triphosphate (dUTP)nick end labeling (TUNEL), transmission electron microscopy (TEM), IHC, and ISH.
Preparation of tissues for histopathology, IHC, ISH, and TUNEL assay
For light microscopy, IHC, ISH, and TUNEL staining, tissue sections (4 µm) were adhered to Superfrost/plus slides (Fisher Scientific, Pittsburgh, PA). Deparaffinization was done by heating the sections for 20 minutes at 65 C. Sections were rehydrated through graded alcohols and washed with phosphate-buffered saline (PBS) (pH 7.4).
IHC for VSV antigen expression
IHC was performed essentially as described in previous studies.25 Briefly, tissue sections were treated with 3% hydrogen peroxide in PBS for 20 minutes, followed by washes in PBS and digestion with 0.05% protease XIV (Sigma Chemical Co., St. Louis, MO) for 6 minutes at 37 C. After several washes in PBS, sections were incubated in a blocking solution of 5% normal goat serum in PBS for 30 minutes at room temperature (RT) and then again incubated overnight at 4 C with VSV-specific bovine convalescent serum (diluted 1 : 500 in PBS) directed against VSV structural protein. After washing with PBS, slides were incubated with goat anti-bovine immunoglobulin G (linking antibody) for 40 minutes and alkaline-phosphataseconjugated antibody for 20 minutes at RT. After the last wash in PBS, slides were incubated with alkaline phosphatase substrate until color was detected. The color reaction was stopped by washing the slides in distilled water. Slides were counterstained with Mayer's hematoxylin.
TUNEL assay
All experiments for evaluating the presence of fragmented DNA by in situ reaction were performed using the TUNEL technique25 on sections from VSV-infected mice as reported previously. Paraffin-embedded sections were treated with 20 µg/ml proteinase-K in 0.1 M PBS for 20 minutes at 37 C. After washing in PBS, sections were covered with 50 µl of the TUNEL reaction mixture (Boehering Mannheim, Indianapolis, IN), which contained terminal deoxynucleotidyl transferase (TdT) and fluoresceindUTP, and incubated under a coverslip in humidified chamber for 1 hour at 37 C. The reaction was stopped by washing slides in PBS. Slides were incubated with the antifluorescein-alkaline phosphatase conjugate (Roche Diagnostic Corporation, Indianapolis, IN) diluted 1 : 3 in 100 mM Tris-HCl, 150 mM NaCl (pH 7.5), and 1% blocking reagent for 40 minutes at RT. After three 15-minute washes in PBS, sections were stained by incubation with a chromogenic substrate 5-bromo-4-chloro-3-indolyl phosphate (X-phosphate/BCIP) and nitro blue tetrazolium salt (NBT) for 515 minutes at RT and counterstained with 0.5% methyl green.
ISH for VSV RNA
ISH was performed as described in previous studies.25 Briefly, deproteinization was performed in 0.2 N HCl for 20 minutes at RT. This was followed by washing with diethyl pyrocarbonate (DEPC)-treated water for 1 minute and digestion with proteinase-K (GIBCO BRL, Life Technologies, Inc., Gaithersburg, MD) at 20 µg/ml for brain tissues for 20 minutes in PBS. After digestion, tissues were fixed in 4% paraformaldehyde in PBS for 5 minutes. After rinsing with PBS, slides were acetylated in 200 ml of 0.1 mM triethanolamine-HCl buffer (pH 8.0) to which 0.5 ml of acetic anhydride (0.25%) was added. After 5 minutes, another 0.5 ml of acetic anhydride was added for an additional 5 minutes. Slides were then rinsed in 2x standard saline citrate (SSC) (1x SSC is 150 mM NaCl plus 15 mM sodium citrate [pH 7.0]). The prehybridization mixture contained 50% deionized formamide, 4x SSC, 10% dextran sulfate, 1x Denhardt's solution (0.02% Ficoll 400, 0.02% polyvinylpyrrolidone, 0.02% bovine serum albumin), 2 mM ethylenediaminetetraacetic acid, and 500 µg of salmon testis DNA per milliliter. Slides were covered with 200 µl of prehybridization mixture and incubated in a humidified chamber for 1 hour at 65 C. Labeled probes used for ISH were selected from the G and L genes,2 which were 596 and 474 bp in length, respectively. Hybridization probes (G and L genes) were produced by reverse transcriptionpolymerase chain reaction (PCR) as described previously.2 The probe was labeled via random priming reaction with digoxigenin-dUTP ([DIG]; Boehringer Mannheim Corp., Indianapolis, IN). The complementary DNA (cDNA) probes represented a region of the VSV genome in G and L genes. Cocktail probes, VSV-G + VSV-L genes, were used to specifically target VSV in tissue sections. Labeled DNA cocktail probes (0.1 ng/µl) were diluted in 300 µl of the prehybridization mixture, heated for 10 minutes in a 95 C heating block, and quenched on ice before being applied to the sections. Slides were rinsed briefly in 2x SSC, and 60 µl of the VSV probe cocktail mixture for tissues was applied to each slide. Hybridization was performed overnight at 42 C. After hybridization, tissue sections were washed twice in 4x SSC for 5 minutes at RT, once in 2x SSC for 10 minutes at 56 C, once in 0.2x SSC containing 60% formamide for 10 minutes at 56 C, twice in 2x SSC for 5 minutes at RT, twice in 0.2x SSC for 5 minutes at RT, and once in buffer I (100 mM maleic acid, 150 mM NaCl [pH 7.5]) for 5 minutes at RT. Immunohistologic detection was performed by using antidigoxigenin immunoglobulin conjugated with alkaline phosphatase (Boehringer Mannheim) (diluted to 1 : 500), which was incubated for 2 hours at RT. Sections were then incubated with color substrate solution for 24 hours in the dark. The reaction was stopped with a distilled water rinse. Sections were counterstained with 0.5% methyl green.
Ultrastructural procedures
Uninfected and VSV-infected brain tissues obtained at necropsy were fixed with a solution containing 2.5% glutaraldehyde in 0.1 M sodium cacodylate (pH 7.4) for 24 hours at 4 C. Tissues were postfixed with 2% osmium tetroxide, stained en bloc with 2% uranyl acetate, dehydrated with ethanol, and embedded in Spurr's resin. Sections were cut, collected on slot grids, stained with lead citrate, and examined with a Phillips 410 electron microscope at 80 kV.
| Results |
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All mice that were given VSV developed clinical signs within 72120 hpi, which included ataxia, hyperexcitability, tremors, circling, and paralysis. The most striking microscopic lesions in the central nervous system (CNS) consisted of nonsuppurative meningoencephalitis and often were most severe in the brain stem (Fig. 1). Nonsuppurative meningoencephalitis was observed along the olfactory bulb and telencephalon (forebrain) and through the brain stem (pons). In all experimental groups, nonsuppurative meningoencephalitis and apoptotic cell debris were detected in the telencephalon and brain stem (Fig. 2). In the brain stem, affected neurons were vacuolated, and many were pyknotic. The severity and extent of neuronal damage correlated directly with time.
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Detection of signal by IHC (Fig. 3) and ISH (Fig. 4) was similar. However, IHC had lower specificity and a relatively higher degree of background staining. Viral RNA staining detected by ISH was specifically located in the cytoplasm of neurons. VSV-RNA was first detected in the olfactory bulb at 48 hpi (data not shown).
On the basis of representative coronal section of signals by IHC and ISH over time, we reconstructed VSV transneuronal passage in mouse CNS (Figs. 57). At 72 hpi, a significant number of VSV-positive neurons were detected by ISH in the olfactory tubercle in the telencephalon (Fig. 8); piriform cortex (Pir), dorsal endopirform nucleus, and anterior hypothalamic nucleus in the diencephalon (Fig. 9); and medial vestibular nucleus (MVe) and prepositus hypoglossal nucleus (PrH) in the brain stem (Fig. 10). Between 96 and 120 hpi, the number and staining intensity of infected neurons increased progressively with time. In particular, the forebrain, cortical neuroepithelium, posterolateral cortical amygdaloid nucleus, and Pir had intense staining in comparison with other areas of the brain (Fig. 11). Individual neurons had prominent intracytoplasmic as well as slight intranuclear staining by ISH (Fig. 12). At this stage (between 96 and 120 hpi), neither VSV-RNApositive nor VSV antigens were observed in spinal trigeminal tracts (SPVT) of the brain stem or spinal tract (SPV).
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VSV-induced apoptosis in mouse brain was evaluated by detection of DNA fragmentation using TUNEL assay and electron microscopy. None of the sections from mock-infected mice had TUNEL-positive signal (data not shown). In VSV infected mice, the TUNEL assay indicated that infected neurons and many adjacent glial cells had nuclei that exhibited morphologic changes typical of apoptosis. The majority of TUNEL-positive cells were observed in mice with moderate to severe meningoencephalitis. TUNEL-positive signals were detected in the same vicinity where large numbers of VSV-RNApositive neurons and glial cells were concentrated as evidenced by ISH. In sections of brain from animals that developed paralysis after VSV infection, VSV antigen- or VSV-RNApositive neurons were diffusely scattered throughout the brain, including the olfactory bulb, telencephalon, and midbrain. TUNEL-positive neurons were detected in olfactory bulb sections obtained from VSV-infected mice at 48 hpi and, mostly, were confined to the olfactory glomerular region (Fig. 13). After intranasal infection, viral RNA and apoptotic cells were observed in brain stem. At 96 hpi, numerous TUNEL-positive cells were observed in the MVe and PrH in the brain stem (Fig. 14). The brain stem and telencephalon contained cells that were positive for TUNEL but negative for VSV viral RNA.
Ultrastructural changes typical of apoptosis were observed in infected mice. At 72 hpi, apoptosis was evident in glial cells in the brain stem. Affected glial cells had condensation of chromatin accompanied by cytoplasmic shrinkage (Fig. 15). At 120 hpi, there was indentation and discrete fragmentation of nuclei. Neuronal shrinkage, membrane blebbing, and abnormal convolution of the nucleus were observed. Condensation of chromatin was accompanied by swelling and distortion of endoplasmic reticulum (Fig. 16).
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| Discussion |
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After intranasal inoculation, VSV infects cell bodies of the first-order olfactory receptor neurons located within the olfactory epithelium and spreads transsynaptically to higher-order neurons. Subsequent neuronal invasion probably involves replication in cell bodies and spread via axons and interneuronal synapses.19 Previous reports have shown that VSV neurotransmission is likely to occur by retrograde transport followed by VSV dissemination via intraseptal synaptic junctions.23 In this study, we followed VSV transneuronal passage in the mouse brain (Fig. 17). When delivered intranasally, VSV initially infected the olfactory receptor neurons in the olfactory bulb. Subsequent to replication in the olfactory bulb, VSV was detected in the telencephalon and brain stem by IHC and ISH. Viral RNApositive cells were scattered throughout the brain, and their specific location, size, and morphology were consistent with those of neurons and glial cells.
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Our results indicate that VSV induces apoptosis in Swiss Webster mice and is important in the development of cell injury. Although detection of single-stranded DNA-ends in nuclei of cells can occur owing to necrosis,11,18 we resorted to detection of damaged DNA in neurons by TUNEL assay as well as ultrastructural microscopy to confirm the presence of VSV infection and apoptosis. TUNEL staining and electron microscopy indicated that VSV-infected neurons had nuclear changes characteristic of apoptosis (Figs. 14, 16). We observed that the number of VSV-RNApositive cells was similar to the number of TUNEL-positive cells in the brain stem; however, in the Pir the number of VSV-RNApositive cells was greater than the number of TUNEL-positive cells, suggesting that neurons, glial cells, and inflammatory cells may differ in their susceptibility to the effects of viral infection and apoptosis.
One important question is whether VSV replication and apoptosis can simultaneously occur in the same cells. Our results indicated that although many cells were labeled for either VSV RNA or apoptosis, the majority did not stain simultaneously for both. There are several possible explanations for this observation. Previous studies suggest that viral interaction with neurons and glial cells is sufficient to promote production of several cytokines, such as tumor necrosis factor, interferon, interleukin-1, and interleukin-6. Additionally, cytokines can be produced by lymphocytes, microglia, and macrophages.13 Thus, it is possible that apoptotic cells that were negative for viral RNA in areas of VSV infection detected by ISH may have undergone apoptosis via a cytokine-dependent pathway induced by recruited inflammatory cells or reactive microglia. On the other hand, VSV RNA while undergoing extensive changes due to the ongoing infection may not reflect the terminal morphologic changes of DNA fragmentation typical of advanced terminal apoptosis. TUNEL-positive cells may have already altered or degraded cytoplasmic RNA of VSV beyond the capacity of detection by our techniques. A similar mechanism of action has been proposed for the lack of colocalization between infected and apoptotic neurons infected with murine reovirus.22 Recent studies on viral matrix (M) protein and another viral component are responsible for induction of apoptosis in cells infected with VSV.16 There is a possibility that M protein plays a major role in the inhibition of host-gene expression and in the induction of apoptosis, which are characteristic of VSV-infected cells.16 Thus, these results provide a basis for exploration of the differential responsiveness among cell types to viral inducers of apoptosis and the implications of viral pathogenesis in intact hosts.
These observations suggest that neurologic damage associated with VSV infection may result from the induction of apoptosis by a combination of direct and indirect mechanisms, similar to those reported to occur with other viral infections.9,20,25 Intranasal VSV infection of Swiss Webster mice provides an animal model for elucidation of the mechanisms by which neurovirulent viruses produce neuronal infection and induce apoptosis.
| Acknowledgments |
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| Footnotes |
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| References |
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