Borna disease virus interference with neuronal plasticity

Liquid-liquid phase separation in viral infection: From the occurrence and function to treatment potentials

Liquid-liquid phase separation (LLPS) of biomacromolecules, as a widespread cellular functional mechanism, is closely related to life processes, and is also commonly present in the lifecycle of viruses. Viral infection often leads to the recombination and redistribution of intracellular components to form biomacromolecule condensates assembled from viral replication-related proteins and intracellular components, which plays an important role in the process of viral infection. In this review, the key and influencing factors of LLPS are generalized, which mainly depend on various molecular interactions and environmental conditions in solution. Meanwhile, some examples of viruses utilizing LLPS are summarized, which are conducive to further understanding the subtle and complex biological regulatory processes between phase condensation and viruses. Finally, some representative antiviral drugs targeting phase separation that have been discovered are also outlined. In conclusion, in-depth study of the role of LLPS in viral infection is helpful to understand the mechanisms of virus-related diseases from a new perspective, and also provide a new therapeutic strategy for future treatments.

Epigenetic Targeting for Controlling Persistent Neurotropic Infections Caused by Borna Virus and HIV

Long-lasting persistence within infected cells is a major challenge for viral pathogens, as it necessitates an exact regulation of viral replication to reduce viral cytopathic effects. This is particularly challenging for viruses that persistently infect cells with limited renewal capabilities, such as neurons. Accordingly, neurotropic viruses have evolved various specific mechanisms to promote a long-lasting persistent infection in the host cells without inducing an exacerbated cytopathic effect. Borna disease virus (BDV) and Human immunodeficiency virus (HIV) are two neurotropic RNA viruses that, in contrast to other RNA viruses, can establish long-lasting intranuclear infections within the nervous system. These viruses interact with different cellular processes such as epigenetic modifications to develop a successful persistence infection. Studies show that cellular epigenetic mechanisms play a significant role in the pathogenesis of BDV and HIV and their neurological disorders. Hence, targeting these mechanisms by epigenetic modulator agents can be regarded as a novel therapeutic strategy to manage BDV- and HIV-associated neurological diseases. This review provides an overview of different epigenetic modulator compounds as a potential therapeutic target for controlling persistent neurotropic intranuclear infections caused by BDV and HIV.

Borna disease virus 1 impairs DNA double-strand break repair through the ATR/Chk1 signalling pathway, resulting in learning and memory impairment in rats

Borna disease virus 1 (BoDV-1) is a highly neurotropic RNA virus that can establish persistent infection in the central nervous system and cause cognitive dysfunction in neonatally infected rats. However, the mechanisms that lead to this cognitive impairment remain unclear. DNA double-strand breaks (DSBs) and their repair are associated with brain development and cognition. If DNA repair in the brain is reduced or delayed and DNA damage accumulates, abnormal cognitive function may result. We generated a rat model of BoDV-1 infection during the neonatal period and assessed behavioural changes using the open field test and Morris water maze. The levels of DSBs were determined by immunofluorescence and comet assays. Western blotting assessed proteins associated with DNA repair pathways. The results showed that BoDV-1 downregulated the ATR/Chk1 signalling pathway in the brain, impairing DNA damage repair and increasing the number of DSBs, which ultimately leads to cognitive dysfunction. Our findings suggest a molecular mechanism by which BoDV-1 interferes with DNA damage repair to cause learning and memory impairment. This provides a theoretical basis for elucidating BoDV-1-induced neurodevelopmental impairment.

Word recognition memory and serum levels of Borna disease virus specific circulating immune complexes in obsessive-compulsive disorder

BACKGROUND

Borna disease virus 1 (BoDV-1) is a non-segmented, negative-strand RNA virus that persistently infects mammals including humans. BoDV-1 worldwide occurring strains display highly conserved genomes with overlapping genetic signatures between those of either human or animal origin. BoDV-1 infection may cause behavioral and cognitive disturbances in animals but has also been found in human major depression and obsessive-compulsive disorder (OCD). However, the impact of BoDV-1 on memory functions in OCD is unknown.

METHOD

To evaluate the cognitive impact of BoDV-1 in OCD, event-related brain potentials (ERPs) were recorded in a continuous word recognition paradigm in OCD patients (n = 16) and in healthy controls (n = 12). According to the presence of BoDV-1-specific circulating immune complexes (CIC), they were divided into two groups, namely group H (high) and L (low), n = 8 each. Typically, ERPs to repeated items are characterized by more positive waveforms beginning approximately 250 ms post-stimulus. This "old/new effect" has been shown to be relevant for memory processing. The early old/new effect (ca. 300-500 ms) with a frontal distribution is proposed to be a neural correlate of familiarity-based recognition. The late old/new effect (post-500 ms) is supposed to reflect memory recollection processes.

RESULTS

OCD patients were reported to show a normal early old/new effect and a reduced late old/new effect compared to normal controls. In our study, OCD patients with a high virus load (group H) displayed exactly these effects, while patients with a low virus load (group L) did not differ from healthy controls.

CONCLUSION

These results confirmed that OCD patients had impaired memory recollection processes compared to the normal controls which may to some extent be related to their BoDV-1 infection.

Update on immunopathology of bornavirus infections in humans and animals

Knowledge on bornaviruses has expanded tremendously during the last decade through detection of novel bornaviruses and endogenous bornavirus-like elements in many eukaryote genomes, as well as by confirmation of insectivores as reservoir species for classical Borna disease virus 1 (BoDV-1). The most intriguing finding was the demonstration of the zoonotic potential of lethal human bornavirus infections caused by a novel bornavirus of different squirrel species (variegated squirrel 1 bornavirus, VSBV-1) and by BoDV-1 known as the causative agent for the classical Borna disease in horses and sheep. Whereas a T cell-mediated immunopathology has already been confirmed as key disease mechanism for infection with BoDV-1 by experimental studies in rodents, the underlying pathomechanisms remain less clear for human bornavirus infections, infection with other bornaviruses or infection of reservoir species. Thus, an overview of current knowledge on the pathogenesis of bornavirus infections focusing on immunopathology is given.

The pathogenesis of bornaviral diseases in mammals

Natural bornavirus infections and their resulting diseases are largely restricted to horses and sheep in Central Europe. The disease also occurs naturally in cats, and can be induced experimentally in laboratory rodents and numerous other mammals. Borna disease virus-1 (BoDV-1), the cause of most cases of mammalian Borna disease, is a negative-stranded RNA virus that replicates within the nucleus of target cells. It causes severe, often lethal, encephalitis in susceptible species. Recent events, especially the discovery of numerous new species of bornaviruses in birds and a report of an acute, lethal bornaviral encephalitis in humans, apparently acquired from squirrels, have revived interest in this remarkable family of viruses. The clinical manifestations of the bornaviral diseases are highly variable. Thus, in addition to acute lethal encephalitis, they can cause persistent neurologic disease associated with diverse behavioral changes. They also cause a severe retinitis resulting in blindness. In this review, we discuss both the pathological lesions observed in mammalian bornaviral disease and the complex pathogenesis of the neurologic disease. Thus infected neurons may be destroyed by T-cell-mediated cytotoxicity. They may die as a result of excessive inflammatory cytokine release from microglia. They may also die as a result of a ‘glutaminergic storm’ due to a failure of infected astrocytes to regulate brain glutamate levels.

Cytokine network analysis of cerebrospinal fluid in myalgic encephalomyelitis/chronic fatigue syndrome

Myalgic encephalomyelitis/chronic fatigue syndrome is an unexplained debilitating disorder that is frequently associated with cognitive and motor dysfunction. We analyzed cerebrospinal fluid from 32 cases, 40 subjects with multiple sclerosis and 19 normal subjects frequency-matched for age and sex using a 51-plex cytokine assay. Group-specific differences were found for the majority of analytes with an increase in cases of CCL11 (eotaxin), a chemokine involved in eosinophil recruitment. Network analysis revealed an inverse relationship between interleukin 1 receptor antagonist and colony-stimulating factor 1, colony-stimulating factor 2 and interleukin 17F, without effects on interleukin 1α or interleukin 1β, suggesting a disturbance in interleukin 1 signaling. Our results indicate a markedly disturbed immune signature in the cerebrospinal fluid of cases that is consistent with immune activation in the central nervous system, and a shift toward an allergic or T helper type-2 pattern associated with autoimmunity.

Borna disease virus phosphoprotein modulates epigenetic signaling in neurons to control viral replication

Understanding the modalities of interaction of neurotropic viruses with their target cells represents a major challenge that may improve our knowledge of many human neurological disorders for which viral origin is suspected. Borna disease virus (BDV) represents an ideal model to analyze the molecular mechanisms of viral persistence in neurons and its consequences for neuronal homeostasis. It is now established that BDV ensures its long-term maintenance in infected cells through a stable interaction of viral components with the host cell chromatin, in particular, with core histones. This has led to our hypothesis that such an interaction may trigger epigenetic changes in the host cell. Here, we focused on histone acetylation, which plays key roles in epigenetic regulation of gene expression, notably for neurons. We performed a comparative analysis of histone acetylation patterns of neurons infected or not infected by BDV, which revealed that infection decreases histone acetylation on selected lysine residues. We showed that the BDV phosphoprotein (P) is responsible for these perturbations, even when it is expressed alone independently of the viral context, and that this action depends on its phosphorylation by protein kinase C. We also demonstrated that BDV P inhibits cellular histone acetyltransferase activities. Finally, by pharmacologically manipulating cellular acetylation levels, we observed that inhibiting cellular acetyl transferases reduces viral replication in cell culture. Our findings reveal that manipulation of cellular epigenetics by BDV could be a means to modulate viral replication and thus illustrate a fascinating example of virus-host cell interaction.

IMPORTANCE

Persistent DNA viruses often subvert the mechanisms that regulate cellular chromatin dynamics, thereby benefitting from the resulting epigenetic changes to create a favorable milieu for their latent and persistent states. Here, we reasoned that Borna disease virus (BDV), the only RNA virus known to durably persist in the nucleus of infected cells, notably neurons, might employ a similar mechanism. In this study, we uncovered a novel modality of virus-cell interaction in which BDV phosphoprotein inhibits cellular histone acetylation by interfering with histone acetyltransferase activities. Manipulation of cellular histone acetylation is accompanied by a modulation of viral replication, revealing a perfect adaptation of this "ancient" virus to its host that may favor neuronal persistence and limit cellular damage.

Absence of a robust innate immune response in rat neurons facilitates persistent infection of Borna disease virus in neuronal tissue

Borna disease virus (BDV) persistently infects neurons of the central nervous system of various hosts, including rats. Since type I IFN-mediated antiviral response efficiently blocks BDV replication in primary rat embryo fibroblasts, it has been speculated that BDV is not effectively sensed by the host innate immune system in the nervous system. To test this assumption, organotypical rat hippocampal slice cultures were infected with BDV for up to 4 weeks. This resulted in the secretion of IFN and the up-regulation of IFN-stimulated genes. Using the rat Mx protein as a specific marker for IFN-induced gene expression, astrocytes and microglial cells were found to be Mx positive, whereas neurons, the major cell type in which BDV is replicating, lacked detectable levels of Mx protein. In uninfected cultures, neurons also remained Mx negative even after treatment with high concentrations of IFN-α. This non-responsiveness correlated with a lack of detectable nuclear translocation of both pSTAT1 and pSTAT2 in these cells. Consistently, neuronal dissemination of BDV was not prevented by treatment with IFN-α. These data suggest that the poor innate immune response in rat neurons renders this cell type highly susceptible to BDV infection even in the presence of exogenous IFN-α. Intriguingly, in contrast to rat neurons, IFN-α treatment of mouse neurons resulted in the up-regulation of Mx proteins and block of BDV replication, indicating species-specific differences in the type I IFN response of neurons between mice and rats.

Analysis of borna disease virus trafficking in live infected cells by using a virus encoding a tetracysteine-tagged p protein

Borna disease virus (BDV) is a nonsegmented, negative-stranded RNA virus characterized by noncytolytic persistent infection and replication in the nuclei of infected cells. To gain further insight on the intracellular trafficking of BDV components during infection, we sought to generate recombinant BDV (rBDV) encoding fluorescent fusion viral proteins. We successfully rescued a virus bearing a tetracysteine tag fused to BDV-P protein, which allowed assessment of the intracellular distribution and dynamics of BDV using real-time live imaging. In persistently infected cells, viral nuclear inclusions, representing viral factories tethered to chromatin, appeared to be extremely static and stable, contrasting with a very rapid and active trafficking of BDV components in the cytoplasm. Photobleaching (fluorescence recovery after photobleaching [FRAP] and fluorescence loss in photobleaching [FLIP]) imaging approaches revealed that BDV components were permanently and actively exchanged between cellular compartments, including within viral inclusions, albeit with a fraction of BDV-P protein not mobile in these structures, presumably due to its association with viral and/or cellular proteins. We also obtained evidence for transfer of viral material between persistently infected cells, with routing of the transferred components toward the cell nucleus. Finally, coculture experiments with noninfected cells allowed visualization of cell-to-cell BDV transmission and movement of the incoming viral material toward the nucleus. Our data demonstrate the potential of tetracysteine-tagged recombinant BDV for virus tracking during infection, which may provide novel information on the BDV life cycle and on the modalities of its interaction with the nuclear environment during viral persistence.

Borna disease virus-induced neuronal degeneration dependent on host genetic background and prevented by soluble factors

Infection of newborn rats with Borne disease virus (BDV) results in selective degeneration of granule cell neurons of the dentate gyrus (DG). To study cellular countermechanisms that might prevent this pathology, we screened for rat strains resistant to this BDV-induced neuronal degeneration. To this end, we infected hippocampal slice cultures of different rat strains with BDV and analyzed for the preservation of the DG. Whereas infected cultures of five rat strains, including Lewis (LEW) rats, exhibited a disrupted DG cytoarchitecture, slices of three other rat strains, including Sprague-Dawley (SD), were unaffected. However, efficiency of viral replication was comparable in susceptible and resistant cultures. Moreover, these rat strain-dependent differences in vulnerability were replicated in vivo in neonatally infected LEW and SD rats. Intriguingly, conditioned media from uninfected cultures of both LEW and SD rats could prevent BDV-induced DG damage in infected LEW hippocampal cultures, whereas infection with BDV suppressed the availability of these factors from LEW but not in SD hippocampal cultures. To gain further insights into the genetic basis for this rat strain-dependent susceptibility, we analyzed DG granule cell survival in BDV-infected cultures of hippocampal neurons derived from the F1 and F2 offspring of the crossing of SD and LEW rats. Genome-wide association analysis revealed one resistance locus on chromosome (chr) 6q16 in SD rats and, surprisingly, a locus on chr3q21-23 that was associated with susceptibility. Thus, BDV-induced neuronal degeneration is dependent on the host genetic background and is prevented by soluble protective factors in the disease-resistant SD rat strain.

Neuro-invasion by a 'Trojan Horse' strategy and vasculopathy during intrauterine flavivirus infection

The central nervous system (CNS) is a major target of several important human and animal viral pathogens causing congenital infections. However, despite the importance of neuropathological outcomes, for humans in particular, the pathogenesis, including mode of neuro-invasion, remains unresolved for most congenital virus infections. Using a natural model of congenital infection with an RNA virus, bovine viral diarrhoea virus in pregnant cattle, we sought to delineate the timing and mode of virus neuro-invasion of and spread within the brain of foetuses following experimental respiratory tract infection of the dams at day 75 of pregnancy, a time of maximal risk of tissue pathology without foetal death. Virus antigen was first detected in the foetal brains 14 days postinfection of dams and was initially restricted to amoeboid microglial cells in the periventricular germinal layer. The appearance of these cells was preceded by or concurrent with vasculopathy in the same region. While the affected microvessels were negative for virus antigen, they expressed high levels of the type I interferon-stimulated protein ISG15 and eventually disappeared in parallel with the appearance of microcavitary lesions. Subsequently, the virus spread to neurons and other glial cells. Our findings suggest that the virus enters the CNS via infected microglial precursors, the amoeboid microglial cells, in a 'Trojan horse' mode of invasion and that the microcavitary lesions are associated with loss of periventricular microvasculature, perhaps as a consequence of high, unrestricted induction of interferon-regulated proteins.

Gerbils

The gerbil is usually nonaggressive and is one of the easiest rodents to maintain and handle. Its disposition, curious nature, relative freedom from naturally occurring infectious diseases, and adaptability to its environment have contributed to its popularity as a laboratory animal. Gerbils are found in deserts and semiarid geographical regions of the world. The Mongolian gerbils that are available today originated from 20 pairs of captured animals that were maintained in 1935 in a closed, random-bred colony at the Kitasato Institute in Japan. Gerbils have several unique anatomical and physiological features. Mature gerbils are smaller than rats, but larger than mice. Mongolian gerbils are attracted to saliva and use salivary cues to discriminate between siblings and nonsiblings, and females use oral cues in the selection of sociosexual partners. Gerbils have been used as experimental models in a number of areas of biomedical research. Gerbils are excellent subjects for laboratory animal research as they are susceptible to bacterial, viral, and parasitic pathogens that affect humans and other species. Gerbils may have spontaneous seizures secondary to stress such as handling, cage change, abrupt noises, or changes in the environment. Cystic ovaries are seen commonly in female gerbils over 1 year of age. Gerbils have unique characteristics, which make them appropriate for a number of animal models. Classically, gerbils have been used in research involving stroke, parasitology, infectious diseases, epilepsy, brain development and behavior, and hearing.

Neurons are MHC class I-dependent targets for CD8 T cells upon neurotropic viral infection

Following infection of the central nervous system (CNS), the immune system is faced with the challenge of eliminating the pathogen without causing significant damage to neurons, which have limited capacities of renewal. In particular, it was thought that neurons were protected from direct attack by cytotoxic T lymphocytes (CTL) because they do not express major histocompatibility class I (MHC I) molecules, at least at steady state. To date, most of our current knowledge on the specifics of neuron-CTL interaction is based on studies artificially inducing MHC I expression on neurons, loading them with exogenous peptide and applying CTL clones or lines often differentiated in culture. Thus, much remains to be uncovered regarding the modalities of the interaction between infected neurons and antiviral CD8 T cells in the course of a natural disease. Here, we used the model of neuroinflammation caused by neurotropic Borna disease virus (BDV), in which virus-specific CTL have been demonstrated as the main immune effectors triggering disease. We tested the pathogenic properties of brain-isolated CD8 T cells against pure neuronal cultures infected with BDV. We observed that BDV infection of cortical neurons triggered a significant up regulation of MHC I molecules, rendering them susceptible to recognition by antiviral CTL, freshly isolated from the brains of acutely infected rats. Using real-time imaging, we analyzed the spatio-temporal relationships between neurons and CTL. Brain-isolated CTL exhibited a reduced mobility and established stable contacts with BDV-infected neurons, in an antigen- and MHC-dependent manner. This interaction induced rapid morphological changes of the neurons, without immediate killing or impairment of electrical activity. Early signs of neuronal apoptosis were detected only hours after this initial contact. Thus, our results show that infected neurons can be recognized efficiently by brain-isolated antiviral CD8 T cells and uncover the unusual modalities of CTL-induced neuronal damage.

When a virus infects the brain, it is important to quickly block viral replication without causing excessive damage to neurons, which are not easily renewed. Cytotoxic T lymphocytes (CTL) are one of the main actors for virus elimination. However, the question of whether CTL are indeed capable of destroying infected neurons remains controversial. For this work, we analyzed the characteristics of interactions between infected neurons and CTL using neurotropic Borna disease virus (BDV). This virus infects neurons and triggers severe inflammation in the brain. We isolated CTL directly from the brains of rats infected with BDV and analyzed their interaction with primary cultures of neurons. Using live-cell fluorescence microscopy, we observed that CTL were arrested upon encounter with infected neurons and that they established stable contacts with them. Thereafter, infected neurons exhibited rapid changes in permeability but remained alive and electrically active for several hours, before ultimately being destroyed. Our study shows that neurons can indeed be recognized by CTL, an important observation for a better understanding of the physiopathology of virus-induced brain inflammation. In addition, it reveals that neurons are relatively resistant to CTL-induced killing, which may open a window of opportunity for new treatments.

Living fossil or evolving virus?

Forty million years ago, Bornavirus integrated DNA fragments of itself into the human genome. The modern virus remains strikingly similar to these fragments suggesting that it might preserve the features of the ancestral virus, perhaps even unlocking the secrets of viral origins and evolution.

Borna disease in an adult alpaca stallion (Lama pacos)

Borna disease (BD) was diagnosed in a 2-year-old male alpaca with a history of chronic suppressed sexual desire and acute stretching convulsions. Microscopical examination of the central nervous system revealed non-purulent meningoencephalitis with mononuclear perivascular cuffing. The diagnosis was confirmed by immunohistochemistry, in-situ hybridization, polymerase chain reaction (PCR) and sequencing of PCR products and alignment with known Borna disease virus sequences. Serological screening of the herd was performed. This is the first detailed report of naturally occurring BD in alpacas.

Mutation of the protein kinase C site in borna disease virus phosphoprotein abrogates viral interference with neuronal signaling and restores normal synaptic activity

Understanding the pathogenesis of infection by neurotropic viruses represents a major challenge and may improve our knowledge of many human neurological diseases for which viruses are thought to play a role. Borna disease virus (BDV) represents an attractive model system to analyze the molecular mechanisms whereby a virus can persist in the central nervous system (CNS) and lead to altered brain function, in the absence of overt cytolysis or inflammation. Recently, we showed that BDV selectively impairs neuronal plasticity through interfering with protein kinase C (PKC)-dependent signaling in neurons. Here, we tested the hypothesis that BDV phosphoprotein (P) may serve as a PKC decoy substrate when expressed in neurons, resulting in an interference with PKC-dependent signaling and impaired neuronal activity. By using a recombinant BDV with mutated PKC phosphorylation site on P, we demonstrate the central role of this protein in BDV pathogenesis. We first showed that the kinetics of dissemination of this recombinant virus was strongly delayed, suggesting that phosphorylation of P by PKC is required for optimal viral spread in neurons. Moreover, neurons infected with this mutant virus exhibited a normal pattern of phosphorylation of the PKC endogenous substrates MARCKS and SNAP-25. Finally, activity-dependent modulation of synaptic activity was restored, as assessed by measuring calcium dynamics in response to depolarization and the electrical properties of neuronal networks grown on microelectrode arrays. Therefore, preventing P phosphorylation by PKC abolishes viral interference with neuronal activity in response to stimulation. Our findings illustrate a novel example of viral interference with a differentiated neuronal function, mainly through competition with the PKC signaling pathway. In addition, we provide the first evidence that a viral protein can specifically interfere with stimulus-induced synaptic plasticity in neurons.

Borna disease virus: a unique pathogen and its interaction with intracellular signalling pathways

Borna disease virus (BDV) is a neurotropic RNA virus that establishes non-cytolytic persistent infection in the central nervous system of warm-blooded animals. Depending on the host species and the route of infection, BDV persistence can modulate neuronal plasticity and animal behaviour and/or may provoke a T cell-mediated immunopathological reaction with high mortality. Therefore, BDV functions as a model pathogen to study persistent virus infection in the central nervous system. Here, we review recent evidence showing that BDV interferes with a spectrum of intracellular signalling pathways, which may be involved in viral spread, maintenance of persistence and modulation of neurotransmitter pathways.

Astrocytes play a key role in activation of microglia by persistent Borna disease virus infection

Neonatal Borna disease virus (BDV) infection of the rat brain is associated with microglial activation and damage to certain neuronal populations. Since persistent BDV infection of neurons is nonlytic in vitro, activated microglia have been suggested to be responsible for neuronal cell death in vivo. However, the mechanisms of activation of microglia in neonatally BDV-infected rat brains remain unclear. Our previous studies have shown that activation of microglia by BDV in culture requires the presence of astrocytes as neither the virus nor BDV-infected neurons alone activate microglia. Here, we evaluated the mechanisms whereby astrocytes can contribute to activation of microglia in neuron-glia-microglia mixed cultures. We found that persistent infection of neuronal cells leads to activation of uninfected astrocytes as measured by elevated expression of RANTES. Activation of astrocytes then produces activation of microglia as evidenced by increased formation of round-shaped, MHCI-, MHCII- and IL-6-positive microglia cells. Our analysis of possible molecular mechanisms of activation of astrocytes and/or microglia in culture indicates that the mediators of activation may be soluble heat-resistant, low molecular weight factors. The findings indicate that astrocytes may mediate activation of microglia by BDV-infected neurons. The data are consistent with the hypothesis that microglia activation in the absence of neuronal damage may represent initial steps in the gradual neurodegeneration observed in brains of neonatally BDV-infected rats.

Proteomic analysis reveals selective impediment of neuronal remodeling upon Borna disease virus infection

The neurotropic virus Borna disease virus (BDV) persists in the central nervous systems of a wide variety of vertebrates and causes behavioral disorders. BDV represents an intriguing example of a virus whose persistence in neurons leads to altered brain function in the absence of overt cytolysis and inflammation. The bases of BDV-induced behavioral impairment remain largely unknown. To better characterize the neuronal response to BDV infection, we compared the proteomes of primary cultures of cortical neurons with and without BDV infection. We used two-dimensional liquid chromatography fractionation, followed by protein identification by nanoliquid chromatography-tandem mass spectrometry. This analysis revealed distinct changes in proteins implicated in neurotransmission, neurogenesis, cytoskeleton dynamics, and the regulation of gene expression and chromatin remodeling. We also demonstrated the selective interference of BDV with processes related to the adaptative response of neurons, i.e., defects in proteins regulating synaptic function, global rigidification of the cytoskeleton network, and altered expression of transcriptional and translational repressors. Thus, this work provides a global view of the neuronal changes induced by BDV infection together with new clues to understand the mechanisms underlying the selective interference with neuronal plasticity and remodeling that characterizes BDV persistence.

Persistent Borna Disease Virus (BDV) infection activates microglia prior to a detectable loss of granule cells in the hippocampus

Neonatal Borna Disease Virus (BDV) infection in rats leads to a neuronal loss in the cortex, hippocampus and cerebellum. Since BDV is a non-lytic infection in vitro, it has been suggested that activated microglia could contribute to neuronal damage. It is also conceivable that BDV-induced cell death triggers activation of microglia to remove cell debris. Although an overall temporal association between neuronal loss and microgliosis has been demonstrated in BDV-infected rats, it remains unclear if microgliosis precedes or results from neuronal damage. We investigated the timing of microglia activation and neuronal elimination in the dentate gyrus (DG) of the hippocampus. We found a significant increase in the number of ED1+ microglia cells as early as 10 days post infection (dpi) while a detectable loss of granule cells of the DG was not seen until 30 dpi. The data demonstrate for the first time that a non-lytic persistent virus infection of neurons activates microglia long before any measurable neuronal loss.

Neonatal Borna disease virus infection in rats is associated with increased extracellular levels of glutamate and neurodegeneration in the striatum

The authors evaluated a role of glutamate (GLU) excitotoxicity in neonatal Borna disease virus (BDV) infection-associated neuronal injury by measuring extracellular levels of GLU in the striatum of 70-day-old Fischer344 rats using in vivo microdialysis. The effects of BDV infection on the protein levels of the GLU transporters and the cystine-GLU antiporter and on the total numbers of striatal neurons and the volume of the striatum were also assessed. BDV increased the basal levels of GLU but did not change those of aspartate, glutamine, or taurine. BDV infection did not alter the effects of a blockade of GLU transporters but attenuated the effects of an inhibition of the cystine-GLU antiporter, without affecting the protein levels of the GLU transporters. The elevated levels of GLU were associated with decreased neuronal numbers and volume in the striatum. The present data are the first in vivo evidence that GLU excitotoxicity might contribute to BDV-associated neuronal injury in the striatum.

Borna disease virus infection impairs synaptic plasticity

The mechanisms whereby Borna disease virus (BDV) can impair neuronal function and lead to neurobehavioral disease are not well understood. To analyze the electrophysiological properties of neurons infected with BDV, we used cultures of neurons grown on multielectrode arrays, allowing a real-time monitoring of the electrical activity across the network shaped by synaptic transmission. Although infection did not affect spontaneous neuronal activity, it selectively blocked activity-dependent enhancement of neuronal network activity, one form of synaptic plasticity thought to be important for learning and memory. These findings highlight the original mechanism of the neuronal dysfunction caused by noncytolytic infection with BDV.

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.

Virus-induced injury of the dentate gyrus: deconstructing the gate in the way of seizures

Kappa Opioid Control of Seizures Produced by a Virus in an Animal Model

Solbrig MV, Adrian R, Baratta J, Lauterborn JC, Koob GF

Brain 2006;129(Pt 3):642–654

Epilepsy remains a major medical problem of unknown aetiology. Potentially, viruses can be environmental triggers for development of seizures in genetically vulnerable individuals. An estimated half of encephalitis patients experience seizures and 4% develop status epilepticus. Epilepsy vulnerability has been associated with a dynorphin promoter region polymorphism or low dynorphin expression genotype, in man. In animals, the dynorphin system in the hippocampus is known to regulate excitability. The present study was designed to test the hypothesis that reduced dynorphin expression in the dentate gyrus of hippocampus due to periadolescent virus exposure leads to epileptic responses. Encephalitis produced by the neurotropic Borna disease virus in the rat caused epileptic responses and dynorphin to disappear via dentate granule cell loss, failed neurogenesis and poor survival of new neurons. Kappa opioid (dynorphin) agonists prevented the behavioural and electroencephalographic seizures produced by convulsant compounds, and these effects were associated with an absence of dynorphin from the dentate gyrus granule cell layer and upregulation of enkephalin in CA1 interneurons, thus reproducing a neurochemical marker of epilepsy, namely low dynorphin tone. A key role for kappa opioids in anticonvulsant protection provides a framework for exploration of viral and other insults that increase seizure vulnerability and may provide insights into potential interventions for treatment of epilepsy.

Borna disease virus matrix protein is an integral component of the viral ribonucleoprotein complex that does not interfere with polymerase activity

We have recently shown that the matrix protein M of Borna disease virus (BDV) copurifies with the affinity-purified nucleoprotein (N) from BDV-infected cells, suggesting that M is an integral component of the viral ribonucleoprotein complex (RNP). However, further studies were hampered by the lack of appropriate tools. Here we generated an M-specific rabbit polyclonal antiserum to investigate the intracellular distribution of M as well as its colocalization with other viral proteins in BDV-infected cells. Immunofluorescence analysis revealed that M is located both in the cytoplasm and in nuclear punctate structures typical for BDV infection. Colocalization studies indicated an association of M with nucleocapsid proteins in these nuclear punctate structures. In situ hybridization analysis revealed that M also colocalizes with the viral genome, implying that M associates directly with viral RNPs. Biochemical studies demonstrated that M binds specifically to the phosphoprotein P but not to N. Binding of M to P involves the N terminus of P and is independent of the ability of P to oligomerize. Surprisingly, despite P-M complex formation, BDV polymerase activity was not inhibited but rather slightly elevated by M, as revealed in a minireplicon assay. Thus, unlike M proteins of other negative-strand RNA viruses, BDV-M seems to be an integral component of the RNPs without interfering with the viral polymerase activity. We propose that this unique feature of BDV-M is a prerequisite for the establishment of BDV persistence.

Activation of microglia by borna disease virus infection: in vitro study

Neonatal Borna disease virus (BDV) infection of the rat brain is associated with microglial activation and damage to the certain neuronal populations. Since persistent BDV infection of neurons in vitro is noncytolytic and noncytopathic, activated microglia have been suggested to be responsible for neuronal cell death in vivo. However, the mechanisms of activation of microglia in neonatally BDV-infected rat brain have not been investigated. To address these issues, activation of primary rat microglial cells was studied following exposure to purified BDV or to persistently BDV-infected primary cortical neurons or after BDV infection of primary mixed neuron-glial cultures. Neither purified virus nor BDV-infected neurons alone activated primary microglia as assessed by the changes in cell shape or production of the proinflammatory cytokines. In contrast, in the BDV-infected primary mixed cultures, we observed proliferation of microglia cells that acquired the round morphology and expressed major histocompatibility complex molecules of classes I and II. These manifestations of microglia activation were observed in the absence of direct BDV infection of microglia or overt neuronal toxicity. In addition, compared to uninfected mixed cultures, activation of microglia in BDV-infected mixed cultures was associated with a significantly greater lipopolysaccharide-induced release of tumor necrosis factor alpha, interleukin 1beta, and interleukin 10. Taken together, the present data are the first in vitro evidence that persistent BDV infection of neurons and astrocytes rather than direct exposure to the virus or dying neurons is critical for activating microglia.

Borna disease virus blocks potentiation of presynaptic activity through inhibition of protein kinase C signaling

Infection by Borna disease virus (BDV) enables the study of the molecular mechanisms whereby a virus can persist in the central nervous system and lead to altered brain function in the absence of overt cytolysis and inflammation. This neurotropic virus infects a wide variety of vertebrates and causes behavioral diseases. The basis of BDV-induced behavioral impairment remains largely unknown. Here, we investigated whether BDV infection of neurons affected synaptic activity, by studying the rate of synaptic vesicle (SV) recycling, a good indicator of synaptic activity. Vesicular cycling was visualized in cultured hippocampal neurons synapses, using an assay based on the uptake of an antibody directed against the luminal domain of synaptotagmin I. BDV infection did not affect elementary presynaptic functioning, such as spontaneous or depolarization-induced vesicular cycling. In contrast, infection of neurons with BDV specifically blocked the enhancement of SV recycling that is observed in response to stimuli-induced synaptic potentiation, suggesting defects in long-term potentiation. Studies of signaling pathways involved in synaptic potentiation revealed that this blockade was due to a reduction of the phosphorylation by protein kinase C (PKC) of proteins that regulate SV recycling, such as myristoylated alanine-rich C kinase substrate (MARCKS) and Munc18–1/nSec1. Moreover, BDV interference with PKC-dependent phosphorylation was identified downstream of PKC activation. We also provide evidence suggesting that the BDV phosphoprotein interferes with PKC-dependent phosphorylation. Altogether, our results reveal a new mechanism by which a virus can cause synaptic dysfunction and contribute to neurobehavioral disorders.

The central nervous system is the target of many persistent viral infections that can induce diverse pathological manifestations. Besides causing meningitis or encephalitis, viruses can infect neurons without overt structural damage, but nevertheless alter cellular functioning by yet-undefined molecular mechanisms, thereby disturbing homeostasis and causing disease. Here, the authors have studied the infection by Borna disease virus, an RNA virus that persists in the brain of a wide variety of animals and causes behavioral disturbances. Using primary cultures of neurons, they show that Borna disease virus interferes specifically with the activity-dependent enhancement of synaptic activity, one form of synaptic plasticity that is believed to be essential for memory formation. This interference was correlated to a reduced phosphorylation of neuronal targets by protein kinase C (PKC), a kinase that plays important roles in the regulation of neuronal activity. The authors also provide evidence that the viral phosphoprotein may be responsible for this interference, possibly by competing with the phosphorylation of endogenous cellular PKC substrates. These results illustrate an intriguing aspect of viral interference with neuronal function and reveal a new mechanism whereby a virus can cause synaptic dysfunction and contribute to neurobehavioral disorders.

Novel insights into the regulation of the viral polymerase complex of neurotropic Borna disease virus

Borna disease virus (BDV) genetic information is encoded in a highly condensed non-segmented RNA genome of negative polarity. Replication and transcription of the genome occurs in the nucleus, enabling the virus to employ the cellular splicing machinery to process primary transcripts and to regulate expression of viral gene products. BDV establishes a non-cytolytic, persistent infection that in animals is mainly restricted to neurons of the central nervous system. Based on these unique properties, BDV represents the prototype member of the virus family Bornaviridae in the order Mononegavirales. Analysis of molecular aspects of BDV replication has long been hampered by the lack of a reverse genetics system. Only recently, artificial BDV minigenomes permitted the reconstitution of the viral polymerase complex, allowing finally the recovery of BDV from cDNA. As in other families of the Mononegavirales, the active polymerase complex of BDV is composed of the polymerase (L), the nucleoprotein (N) and the phosphoprotein (P). In addition, the viral X protein was identified as potent negative regulator of polymerase activity. Protein interaction studies combined with minireplicon assays suggested that P is a central regulatory element of BDV replication that directs the assembly of the polymerase complex. Most intriguingly, BDV obtained from cDNA with variable genomic termini suggests a novel strategy for viral replication-control. BDV seems to restrict its propagation efficacy by defined 5' terminal trimming of genomic and antigenomic RNA molecules. This review will summarize these novel findings and will discuss them in the context of BDV neurotropism and persistence.