Central Nervous System Infection with Borna Disease Virus Causes Kynurenine Pathway Dysregulation and Neurotoxic Quinolinic Acid Production.

Central nervous system infection of neonatal and adult rats with Borna disease virus (BDV) results in neuronal destruction and behavioral abnormalities with differential immune-mediated involvement. Neuroactive metabolites generated from the kynurenine pathway of tryptophan degradation have been implicated in several human neurodegenerative disorders. Here, we report that brain expression of key enzymes in the kynurenine pathway are significantly, but differentially, altered in neonatal and adult rats with BDV infection. Gene expression analysis of rat brains following neonatal infection showed increased expression of kynurenine amino transferase II (KATII) and kynurenine-3-monooxygenase (KMO) enzymes. Additionally, indoleamine 2,3-dioxygenase (IDO) expression was only modestly increased in a brain region- and time-dependent manner in neonatally infected rats; however, its expression was highly increased in adult infected rats. The most dramatic impact on gene expression was seen for KMO, whose activity promotes the production of neurotoxic quinolinic acid. KMO expression was persistently elevated in brain regions of both newborn and adult BDV-infected rats, with increases reaching up to 86-fold. KMO protein levels were increased in neonatally infected rats and colocalized with neurons, the primary target cells of BDV infection. Furthermore, quinolinic acid was elevated in neonatally infected rat brains. We further demonstrate increased expression of KATII and KMO, but not IDO, in vitro in BDV-infected C6 astroglioma cells. Our results suggest that BDV directly impacts the kynurenine pathway, an effect that may be exacerbated by inflammatory responses in immunocompetent hosts. Thus, experimental models of BDV infection may provide new tools for discriminating virus-mediated from immune-mediated impacts on the kynurenine pathway and their relative contribution to neurodegeneration.IMPORTANCE BDV causes persistent, noncytopathic infection in vitro yet still elicits widespread neurodegeneration of infected neurons in both immunoincompetent and immunocompetent hosts. Here, we show that BDV infection induces expression of key enzymes of the kynurenine pathway in brains of newborn and adult infected rats and cultured astroglioma cells, shunting tryptophan degradation toward the production of neurotoxic quinolinic acid. Thus, our findings newly implicate this metabolic pathway in BDV-induced neurodegeneration. Given the importance of the kynurenine pathway in a wide range of human infections and neurodegenerative and neuropsychiatric disorders, animal models of BDV infection may serve as important tools for contrasting direct viral and indirect antiviral immune-mediated impacts on kynurenine pathway dysregulation and the ensuing neurodevelopmental and neuropathological consequences.

Expression and role of the TGF-ß family in glial cells infected with Borna disease virus.

A previous study revealed that the expression of the Borna disease virus (BDV)-encoding phosphoprotein in glial cells was sufficient to induce neurobehavioral abnormalities resembling Borna disease. To evaluate the involvement of the TGF-ß family in BDV-induced changes in cell responses by C6 glial cells, we examined the expression levels of the TGF-ß family and effects of inhibiting the TGF-ß family pathway in BDV-infected C6 (C6BV) cells. The expression of activin ßA and BMP7 was markedly increased in BDV-infected cells. Expression of Smad7, a TGF-ß family-inducible gene, was increased by BDV infection, and the expression was decreased by treatment with A-83-01 or LDN-193189, inhibitors of the TGF-ß/activin or BMP pathway, respectively. These results suggest autocrine effects of activin A and BMP7 in C6BV cells. IGFBP-3 expression was also induced by BDV infection; it was below the detection limit in C6 cells. The expression level of IGFBP-3 was decreased by LDN-193189 in C6BV cells, suggesting that endogenous BMP activity is responsible for IGFBP-3 gene induction. Our results reveal the regulatory expression of genes related to the TGF-ß family, and the role of the enhanced BMP pathway in modulating cell responses in BDV-infected glial cells.

Heat stress is a potent stimulus for enhancing rescue efficiency of recombinant Borna disease virus.

Recently developed vector systems based on Borna disease virus (BDV) hold promise as platforms for efficient and stable gene delivery to the central nervous system (CNS). However, because it currently takes several weeks to rescue recombinant BDV (rBDV), an improved rescue procedure would enhance the utility of this system. Heat stress reportedly enhances the rescue efficiency of other recombinant viruses. Here, heat stress was demonstrated to increase the amount of BDV genome in persistently BDV-infected cells without obvious cytotoxicity. Further analyses suggested that the effect of heat stress on BDV infection is not caused by an increase in the activity of BDV polymerase. More cells in which BDV replication occurs were obtained in the initial phase of rBDV rescue by using heat stress than when it was not used. Thus, heat stress is a useful improvement on the published rescue procedure for rBDV. The present findings may accelerate the practical use of BDV vector systems in basic science and the clinic and thus enable broader adoption of this viral vector, which is uniquely suited for gene delivery to the CNS.

Changes in Borna disease virus genome with adaptation to host.

The CRNP5 variant of Borna disease virus (BDV) has stronger pathogenesis than the CRP3 variant in which only 4 nucleotides in the whole genome are different. The CRP3 is produced by 3 passages in rat brains of BDV, whereas the CRNP5 is produced by 5 passages in mouse brains after 2 passages in rat brains of the BDV. Thymidylic acids at nt 3608 and 3673 were replaced by cytidylic acids during 3 passages in mice. Three passages in mice caused replacement of adenylic acid at nt 7936 by guanylic acid. No replacement at nt 8742 occurred during passages in mice.

A rapid method for gene expression analysis of Borna disease virus in neurons and astrocytes using laser microdissection and real-time RT-PCR.

Laser microdissection combined with real-time RT-PCR represents a powerful method to analyse the transcription efficiency of defined cell types. Therefore, a RNA-preserving immunolabelling method was established to identify neurons and astrocytes in persistently BDV-infected rat brain sections for subsequent laser microdissection and quantitation of viral gene products by real-time RT-PCR. Firstly, to ensure an accurate measurement of viral RNA after immunolabelling, different reference genes (glyceraldehyde-3-phosphate dehydrogenase [GAPDH], succinate-ubiquinone reductase [SDHA], hypoxanthine phosphoribosyl-transferase-1 [HPRT]) were tested. Only normalisation with GAPDH yielded a stable relative expression of viral RNA encoding the nucleoprotein (BDV-N), the matrixprotein and the glycoprotein (intron I and intron II). The two remaining reference genes biased the ratios of BDV-transcripts in the immunolabelled brain sections significantly. Secondly, 100 immunolabelled neurons and astrocytes were harvested using laser microdissection and amplification of all viral transcripts revealed 681 and 168 (BDV-N), 573 and 254 (intron I), 324 and 133 (intron II) and 161 and 36 (GAPDH) absolute copy numbers in neurons and astrocytes, respectively. Thus, laser microdissection combined with real-time RT-PCR provides an effective tool for the analysis of cell-specific viral transcription efficiency and allows elucidating virus-host-interactions and virus persistence mechanisms in the CNS.

Functional characterization of the major and minor phosphorylation sites of the P protein of Borna disease virus.

The phosphoprotein P of Borna disease virus (BDV) is an essential cofactor of the viral RNA-dependent RNA polymerase. It is preferentially phosphorylated at serine residues 26 and 28 by protein kinase C epsilon (PKCepsilon) and, to a lesser extent, at serine residues 70 and 86 by casein kinase II (CKII). To determine whether P phosphorylation is required for viral polymerase activity, we generated P mutants lacking either the PKCepsilon or the CKII phosphate acceptor sites by replacing the corresponding serine residues with alanine (A). Alternatively, these sites were replaced by aspartic acid (D) to mimic phosphorylation. Functional characterization of the various mutants in the BDV minireplicon assay revealed that D substitutions at the CKII sites inhibited the polymerase-supporting activity of P, while A substitutions maintained wild-type activity. Likewise, D substitutions at the PKC sites did not impair the cofactor function of BDV-P, whereas A substitutions at these sites led to increased activity. Interestingly, recombinant viruses could be rescued only when P mutants with modified PKCepsilon sites were used but not when both CKII sites were altered. PKCepsilon mutant viruses showed a reduced capacity to spread in cell culture, while viral RNA and protein expression levels in persistently infected cells were almost normal. Further mutational analyses revealed that substitutions at individual CKII sites were, with the exception of a substitution of A for S86, detrimental for viral rescue. These data demonstrate that, in contrast to other viral P proteins, the cofactor activity of BDV-P is negatively regulated by phosphorylation.

Morphometric analysis of the retina from horses infected with the Borna disease virus.

Borna disease (BD) is a fatal disorder of horses, often characterized by blindness. Although degeneration of retinal neurons has been demonstrated in a rat model, there are controversial data concerning whether a similar degeneration occurs in the retina of infected horses. To investigate whether BD may cause degeneration of photoreceptors and possibly of other neuronal cells at least at later stages of the disease, we performed a detailed quantitative morphologic study of retinal tissue from Borna-diseased horses. BD was diagnosed by detection of pathognomonic Joest-Degen inclusion bodies in the postmortem brains. Paraffin sections of paraformaldehyde-fixed retinae were used for histologic and immunohistochemical stainings. Numbers of neurons and Müller glial cells were counted, and neuron-to-Müller cell ratios were calculated. Among tissues from 9 horses with BD, we found retinae with strongly altered histologic appearance as well as retinae with only minor changes. The neuron-to-Müller cell ratio for the whole retina was significantly smaller in diseased animals (8.5 +/- 0.4; P < .01) as compared with controls (17.6 +/- 0.8). It can be concluded that BD in horses causes alterations of the retinal histology of a variable degree. The study provides new data about the pathogenesis of BD concerning the retina and demonstrates that a loss of photoreceptors may explain the observed blindness in infected horses.

Varied persistent life cycles of Borna disease virus in a human oligodendroglioma cell line.

Borna disease virus (BDV) establishes a persistent infection in the central nervous system of vertebrate animal species as well as in tissue cultures. In an attempt to characterize the life cycle of BDV in persistently infected cultured cells, we developed 30 clones by single-cell cloning from a human oligodendroglioma (OL) cell line after infection with BDV. According to the percentage of cells expressing the BDV major proteins, p40 (nucleoprotein) and p24 (phosphoprotein), the clones were classified into two types: type I (>20%) and type II (<20%). mRNAs corresponding to both proteins were detected by in situ hybridization (ISH) in a percentage of cells consistent with that for the protein expression in the two types. Surprisingly, ISH for the detection of the genomic RNA, mainly in type II, revealed a significantly larger cell population harboring the genomic RNA than that with the protein as well as the mRNA expression. By recloning from type II primary cell clones, the same phenotype was confirmed in the secondary cell clones obtained: i.e., low percentage of protein-positive cells and higher percentage of cells harboring the genomic RNA. After nerve growth factor treatment, the two types of clones showed increases in the percentage of cells expressing BDV-specific proteins that reached 80% in type II clones, in addition to increased expression levels per cell. Such enhancement might have been mediated by the activation of the mitogen-activated protein kinase in the clones as revealed by the detection of activated ERK1/2. Thus, our findings show that BDV may have established a persistent infection at low levels of viral expression in OL cells with the possibility of a latent infection.

Molecular biology of Borna disease virus and persistence.

Borna disease virus (BDV) causes central nervous system (CNS) disease that is frequently manifested by behavioral abnormalities. Recent evidence indicates that the natural host range and geographic distribution of BDV is wider than originally thought BDV has been molecularly characterized as a non-segmented, negative single-stranded (NNS) RNA virus. Its genome (ca 8.9 kb), the smallest among known NNS RNA viruses, has an organization similar to that of other members of the order Mononegavirales. BDV has the property, unique among known animal NNS RNA viruses, of a nuclear site for the replication and transcription of its genome. The nucleocytoplasmic transport of BDV macromolecules is an essential component of the life cycle of BDV. An overlap of transcription units and transcriptive signals, overlap of ORFs, transcriptional readthrough and RNA splicing regulate expression of the BDV compact genome. The concurrent use of such diversity of strategies for the regulation of virus gene expression is unique among known NNS RNA viruses. Moreover, BDV appears to have also an unusual assembly process. Based on its unique genetic and biological features, BDV is considered to be the prototypic member of a new virus family, Bornaviridae, within the order Mononegavirales. Therefore, the investigation of the molecular biology of BDV may provide new insights about the biology of mononegaviruses, which include important human pathogens.

Experimental infection of mice with Borna disease virus (BDV): replication and distribution of the virus after intracerebral infection.

To develop an animal model resembling natural asymptomatic Borna disease virus (BDV) infections, BDV He/80 rat brain homogenate was passaged four times in adult SJL/J mice. Within 12 months of observation, mice did not develop overt signs of disease. Nucleotide sequencing of the rat isolate and the mouse isolates at the fourth passage revealed no difference in the deduced amino acids. Viral RNA was found in brain, heart, kidney, lung, liver, and urinary bladder. Infectious virus was isolated from brain, but also from heart and lung tissue. Immunohistochemically, BDV was demonstrated in nerves in the abdominal cavity, ganglion coeliacum, and adrenal glands, but not in organ parenchyma. Occasionally, viral RNA was detected in mononuclear blood cells.

MEK-specific inhibitor U0126 blocks spread of Borna disease virus in cultured cells.

Borna disease virus (BDV) is a highly neurotropic virus that causes Borna disease, a virus-induced immune-mediated encephalomyelitis, in a variety of warm-blooded animals. Recent studies reported that BDV can be detected in patients with psychiatric disorders. BDV is noncytopathic, replicates in the nucleus of infected cells, and spreads intraaxonally in vivo. Upon infection of susceptible cultured cells, virus can be detected in foci. Little is known about the cellular components required for BDV replication. Here, we show that the cellular Raf/MEK/ERK signaling cascade is activated upon infection with BDV. In the presence of the MEK-specific inhibitor U0126, cells get infected with BDV; however, there is a block in virus spread to neighboring cells. The effect of the inhibitor on virus spread was still observed when the compound was added 2 h postinfection but not if treatment was initiated as late as 4 h after infection. Our results provide new insights into the BDV-host cell interaction and show that virus infection can be controlled with drugs interfering with a cellular signaling pathway. Since concentrations of the MEK inhibitor required to block BDV focus formation are not toxic for the host cells, our finding may be important with respect to antiviral drug development.

Virus-like particles in MDCK cells persistently infected with Borna disease virus.

A line of Madin Darby canine kidney (MDCK) cells persistently infected with Borna disease virus was examined by electron microscopy. Thin sections revealed the presence of intracytoplasmic virus-like particles ranging from 50-100 nm in diameter. Nuclei of the infected cells exhibited accumulation of electron-dense granular structures 15-18 nm in diameter. The intracytoplasmic particles were roughly spherical with a limiting membrane, suggesting the presence of a lipid-containing envelope. The internal structure consisted of strand-like material which in some cases was condensed underneath the envelope. The possible relationship of these particles to Borna disease virions is discussed.

Borna disease virus (BDV), a nonsegmented RNA virus, replicates in the nuclei of infected cells where infectious BDV ribonucleoproteins are present.

Borna disease virus (BDV) causes neurological disease in a wide range of animal species, providing an important model for studies of persistent viral infection of the central nervous system. In addition, the detection of antibodies that react with BDV antigen in serum from psychiatric patients suggests a role for BDV, or related viruses, in human mental disorders, providing further reason for study of this poorly characterized neurotropic virus. We present evidence that BDV has a nonsegmented negative single-strand RNA genome with the property that viral replication and transcription take place in the nuclei of infected cells where infectious BDV ribonucleoproteins are present. Our results support the view that BDV has unique biological features among animal viruses. Furthermore, the finding that BDV ribonucleoproteins are able to infect susceptible cells raises interesting questions regarding the mechanisms by which some neurotropic viruses may spread through the central nervous system of the infected host without requiring the production of mature infectious virus.

Characterization of a glial cell line persistently infected with borna disease virus (BDV): influence of neurotrophic factors on BDV protein and RNA expression.

Borna disease virus (BDV) infects cells of the nervous system in a wide range of species. Previous work suggests that there are differences in BDV replication in neuronal cells and glial cells. Many neurons are lysed by the immunopathologic response to BDV; lysis of dentate gyrus neurons in the absence of encephalitis is seen in rats inoculated with BDV as neonates. In contrast, persistently BDV-infected astrocytes increase over the course of BDV infection. Therefore, we compared BDV replication in neuronal (SK-N-SH and SK-N-SHEP) and astrocytic (C6) cell lines. While SK-N-SH cells produced more infectious virions per cell, the C6 cells contained more BDV proteins and RNA. BDV sequences in the supernatants of both cell types were identified, despite low titers of infectious virus, suggesting the release of incomplete virions into the medium. C6 cells secreted a factor or factors into the medium that enhanced the production of BDV proteins and RNA in other cell lines. In addition, nerve growth factor treatment produced the same enhancement. Thus, BDV replication in certain neural cells in vitro may be linked to the production of cell-specific factors which affect viral replication.

Borna disease virus in mice: host-specific differences in disease expression.

We developed a mouse model of Borna disease to facilitate immunopathogenesis research by adaptation of Borna disease virus to mice through serial passage in mouse brain tissue. Borna disease virus replication, antibody production, inflammation, and Borna disease expression in several different strains of mice were examined.

Borna disease virus-specific antigens. II. The soluble antigen is a protein complex.

Borna disease virus-specific soluble antigen from persistently infected rat brains was purified to homogeneity using preparative sodium dodecyl sulphate-polyacrylamide gel electrophoresis. The soluble antigen is a complex of three proteins with apparent molecular weights of 35 kDa, 38 kDa and 24 kDa. The 35/38 kDa antigen double band was separated into its two components. The 24 kDa protein has no common epitopes with the 35/38 kDa protein.

Borna disease virus-specific antigens: two different proteins identified by monoclonal antibodies.

A variety of cells originating from different species, including man, can be infected with Borna disease (BD) virus. Two different virus infection-specific antigens with molecular weights of 24 kD and 35/38 kD (double band) could be demonstrated using antigen preparations from persistently infected cells and rat brains, and polyvalent antisera from naturally and experimentally infected animals. Three different monoclonal antibodies were selected. One was specific for the 24-kD protein and two others reacted with the 35/38-kD antigen. The 24- and 35/38-kD antigens could be monitored concomitantly with the appearance of newly synthesized infectious virus. Both antigens could be detected at the same time during the infection cycle, and showed identical distribution in the cell. Their relationship to one another and their possible function is discussed.

Astrocytes and Schwann cells are virus-host cells in the nervous system of rats with Borna disease.

Borna disease virus (BDV) replicates only in cells in the central (CNS) and peripheral (PNS) nervous system in adult rats. Infection of the nervous system is associated with a transient, intense mononuclear meningoencephalitis and immunemediated loss of BDV-infected neurons. The identification of BDV antigen in neurons and the accompanying immunologically-specific lysis of these cells led to the prediction that the CNS would be virus-free after the animal had recovered from encephalitis. However, BDV infectivity and antigen persist for the lifetime of the animal. It appeared, therefore, that other neural cells might be hosts for viral replication and provide a reservoir for the virus. Morphological criteria were used to identify astrocytes and Schwann cells which expressed BDV antigens in vivo. Borna disease virus (BDV) infected astrocytes were identified by double labeling tissue sections with combined cell-specific and BDV-specific antibodies in an avidin-biotin immunocytochemical assay. Examination of serial I micrometer-thick cryosections of hippocampus and sciatic nerve preparations revealed several cells that expressed both glial and BDV antigens. Infectious virus was recovered from cultures of Schwann cells from infected rats. Borna disease virus-infected glial elements persisted beyond the period of inflammation and massive neuronal destruction, and represented a major class of infected cells during chronic disease.

Influence of interferon on persistent infection caused by Borna disease virus in vitro.

The effect of interferon (IFN) on infection and maintenance of persistent infection of Borna disease (BD) virus in cell cultures was investigated. Acutely BD virus-infected primary rabbit brain and rat lung cells produced significant levels of interferon detectable 3 days post-infection in the culture supernatants. Rat brain and rat lung cells persistently infected with BD virus produced only moderate levels of IFN over a long period. In contrast, persistently infected Madin-Darby canine kidney (MDCK) cells did not produce detectable amounts of IFN. Exogenous homologous IFN completely inhibited the expression of BD virus antigen in acutely infected rabbit brain cells, when added during the first 24 h after infection. IFN added later (2 to 6 days post-infection) reduced virus titres to different degrees depending on the onset of treatment. However, IFN added to persistently infected rat lung cells did not appear to influence the degree or quality of BD virus antigen expression or the intracellular amount of infectious virus. Two facts indicate that IFN is not involved in the establishment or maintenance of persistent BD virus infection in vitro. Thus, MDCK cells, which could not be induced to produce IFN, can be readily persistently infected with BD virus in vitro, and exogenous IFN did not appear to influence persistent BD virus infection.

Effect of Borna disease virus infection on athymic rats.

Homozygous athymic nude rats (rnu/rnu) infected intracerebrally with Borna disease virus produced relatively high titres of infectious virus in the central nervous system. However, no clinical signs of disease or pathological alterations could be found during a 100 day observation period. In contrast, heterozygous euthymic albino littermates (rnu/+), which were used as controls, reacted in a similar manner to immunocompetent Lewis rats. They developed behavioural alterations which coincided with encephalitis and retinitis. The results obtained confirm our previous concept that the genesis of Borna disease, at least in rats, is attributed to a cellular immune response.