Bornavirus Closely Associates and Segregates with Host Chromosomes to Ensure Persistent Intranuclear Infection

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.

Structural and molecular properties of mumps virus inclusion bodies

Viral RNA synthesis of mononegaviruses occurs in cytoplasmic membraneless organelles called inclusion bodies (IBs). Here, we report that IBs of mumps virus (MuV), which is the causative agent of mumps and belongs to the family Paramyxoviridae, displayed liquid organelle properties formed by liquid-liquid phase separation. Super-resolution microscopic analysis of MuV IBs demonstrated that nucleocapsid and phospho (P)-proteins formed a cage-like structure and that the viral polymerase adopted a reticular pattern and colocalized with viral RNAs. In addition, we characterized host RNAs localized in MuV IBs by a spatial transcriptome analysis, and found that RNAs containing G-quadruplex motif sequences (G4-RNAs) were concentrated. An in vitro phase separation assay showed that the G4-RNAs interacted with the P protein and enhanced condensation in P droplets. Together, our data show that MuV generates IBs with a characteristic cage-like structure and host G4-RNAs play an important role in forming MuV IBs.

Development of an RNA virus-based episomal vector with artificial aptazyme for gene silencing

RNA virus-based episomal vector (REVec), engineered from Borna disease virus, is an innovative gene delivery tool that enables sustained gene expression in transduced cells. However, the difficulty in controlling gene expression and eliminating vectors has limited the practical use of REVec. In this study, we overcome these shortcomings by inserting artificial aptazymes into the untranslated regions of foreign genes carried in vectors or downstream of the viral phosphoprotein gene, which is essential for vector replication. Non-transmissive REVec carrying GuaM8HDV or the P1-F5 aptazyme showed immediate suppression of gene expression in a guanine or theophylline concentration-dependent manner. Continuous compound administration also markedly reduced the percentage of vector-transduced cells and eventually led to the complete elimination of the vectors from the transduced cells. This new REVec is a safe gene delivery technology that allows fine-tuning of gene expression and could be a useful platform for gene therapy and gene-cell therapy, potentially contributing to the cure of many genetic disorders. KEY POINTS: • We developed a bornavirus vector capable of silencing transgene expression by insertion of aptazyme • Transgene expression was markedly suppressed in a compound concentration-dependent manner • Artificial aptazyme systems allowed complete elimination of the vector from transduced cells.

Borna disease virus 1 infection in alpacas: Comparison of pathological lesions and viral distribution to other dead-end hosts

Borna disease is a progressive meningoencephalitis caused by spillover of the Borna disease virus 1 (BoDV-1) to horses and sheep and has gained attention due to its zoonotic potential. New World camelids are also highly susceptible to the disease; however, a comprehensive description of the pathological lesions and viral distribution is lacking for these hosts. Here, the authors describe the distribution and severity of inflammatory lesions in alpacas (n = 6) naturally affected by this disease in comparison to horses (n = 8) as known spillover hosts. In addition, the tissue and cellular distribution of the BoDV-1 was determined via immunohistochemistry and immunofluorescence. A predominant lymphocytic meningoencephalitis was diagnosed in all animals with differences regarding the severity of lesions. Alpacas and horses with a shorter disease duration showed more prominent lesions in the cerebrum and at the transition of the nervous to the glandular part of the pituitary gland, as compared to animals with longer disease progression. In both species, viral antigen was almost exclusively restricted to cells of the central and peripheral nervous systems, with the notable exception of virus-infected glandular cells of the Pars intermedia of the pituitary gland. Alpacas likely represent dead-end hosts similar to horses and other spillover hosts of BoDV-1.

Experimental pathogenesis of aquatic bird bornavirus 1 in Pekin ducks

Aquatic bird bornavirus 1 (ABBV-1) is a neurotropic virus that causes persistent infection in the nervous system of wild waterfowl. This study evaluated whether Pekin ducks, the most common waterfowl raised worldwide, are susceptible to ABBV-1 infection and associated disease. Groups of Pekin ducks were inoculated with ABBV-1 through the intracranial (IC; n, 32), intramuscular (IM; n, 30), and choanal (CH; n, 30) routes. Controls (CO; n, 29) received carrier only. At 1, 12, and 21 weeks postinfection (wpi), 7-14 birds were euthanized to assess virus distribution and lesions. Infection rates in the IC and IM groups were over 70%, while only 4 ducks in the CH group became infected. Neurological signs were observed in 8 ducks only, while over 25% of IC and IM birds had encephalitis and/or myelitis. Seroconversion was highest in the IC and IM groups, and mucosal ABBV-1 RNA shedding was most frequent in the IC group (53%). None of the fertile eggs laid during the experiment tested positive for ABBV-1 RNA. This study shows that Pekin ducks are permissive to ABBV-1 infection and partly susceptible to associated disease. While mucosal shedding may be an important route of transmission, congenital infection appears unlikely.

First detected geographical cluster of BoDV-1 encephalitis from same small village in two children: therapeutic considerations and epidemiological implications

BACKGROUND

The Borna disease virus (BoDV-1) is an emerging zoonotic virus causing severe and mostly fatal encephalitis in humans.

METHODS AND RESULTS

A local cluster of fatal BoDV-1 encephalitis cases was detected in the same village three years apart affecting two children. While the first case was diagnosed late in the course of disease, a very early diagnosis and treatment attempt facilitated by heightened awareness was achieved in the second case. Therapy started as early as day 12 of disease. Antiviral therapy encompassed favipiravir and ribavirin, and, after bioinformatic modelling, also remdesivir. As the disease is immunopathogenetically mediated, an intensified anti-inflammatory therapy was administered. Following initial impressive clinical improvement, the course was also fatal, although clearly prolonged. Viral RNA was detected by qPCR in tear fluid and saliva, constituting a possible transmission risk for health care professionals. Highest viral loads were found post mortem in the olfactory nerve and the limbic system, possibly reflecting the portal of entry for BoDV-1. Whole exome sequencing in both patients yielded no hint for underlying immunodeficiency. Full virus genomes belonging to the same cluster were obtained in both cases by next-generation sequencing. Sequences were not identical, indicating viral diversity in natural reservoirs. Specific transmission events or a common source of infection were not found by structured interviews. Patients lived 750m apart from each other and on the fringe of the settlement, a recently shown relevant risk factor.

CONCLUSION

Our report highlights the urgent necessity of effective treatment strategies, heightened awareness and early diagnosis. Gaps of knowledge regarding risk factors, transmission events, and tailored prevention methods become apparent. Whether this case cluster reflects endemicity or a geographical hot spot needs further investigation.

Reverse genetics of parrot bornavirus 4 reveals a unique splicing of the glycoprotein gene that affects viral propagation

Viruses can utilize host splicing machinery to enable the expression of multiple genes from a limited-sized genome. Orthobornaviruses use alternative splicing to regulate the expression level of viral proteins and achieve efficient viral replication in the nucleus. Although more than 20 orthobornaviruses have been identified belonging to eight different viral species, virus-specific splicing has not been demonstrated. Here, we demonstrate that the glycoprotein (G) transcript of parrot bornavirus 4 (PaBV-4; species Orthobornavirus alphapsittaciforme), a highly virulent virus in psittacines, undergoes mRNA splicing and expresses a soluble isoform termed sGP. Interestingly, the splicing donor for sGP is not conserved in other orthobornaviruses, including those belonging to the same orthobornavirus species, suggesting that this splicing has evolved as a PaBV-4-specific event. We have also shown that exogenous expression of sGP does not affect PaBV-4 replication or de novo virion infectivity. In this study, to investigate the role of sGP in viral replication, we established a reverse genetics system for PaBV-4 by using avian cell lines and generated a recombinant virus lacking the spliced mRNA for sGP. Using the recombinant viruses, we show that the replication of the sGP-deficient virus is significantly slower than that of the wild-type virus and that the exogenous expression of sGP cannot restore its propagation efficiency. These results suggest that autologous or controlled expression of sGP by splicing may be important for PaBV-4 propagation. The reverse genetics system for avian bornaviruses developed here will be a powerful tool for understanding the replication strategies and pathogenesis of avian orthobornaviruses. IMPORTANCE Parrot bornavirus 4 (PaBV-4) is the dominant cause of proventricular dilatation disease, a severe gastrointestinal and central nervous system disease among avian bornaviruses. In this study, we discovered that PaBV-4 expresses a soluble isoform of glycoprotein (G), called sGP, through alternative splicing of the G mRNA, which is unique to this virus. To understand the role of sGP in viral replication, we generated recombinant PaBV-4 lacking the newly identified splicing donor site for sGP using a reverse genetics system and found that its propagation was significantly slower than that of the wild-type virus, suggesting that sGP plays an essential role in PaBV-4 infection. Our results provide important insights not only into the replication strategy but also into the pathogenesis of PaBV-4, which is the most prevalent bornavirus in captive psittacines worldwide.

Liquid-liquid Phase Separation in Viral Function

An emerging set of results suggests that liquid-liquid phase separation (LLPS) is the basis for the formation of membrane-less compartments in cells. Evidence is now mounting that various types of virus-induced membrane-less compartments and organelles are also assembled via LLPS. Specifically, viruses appear to use intracellular phase transitions to form subcellular microenvironments known as viral factories, inclusion bodies, or viroplasms. These compartments - collectively referred to as viral biomolecular condensates - can be used to concentrate replicase proteins, viral genomes, and host proteins that are required for virus replication. They can also be used to subvert or avoid the intracellular immune response. This review examines how certain DNA or RNA viruses drive the formation of viral condensates, the possible biological functions of those condensates, and the biophysical and biochemical basis for their assembly.

Impact of Borna Disease Virus Infection on the Transcriptome of Differentiated Neuronal Cells and Its Modulation by Antiviral Treatment

Borna disease virus (BoDV-1) is a highly neurotropic RNA virus that causes neurobehavioral disturbances such as abnormal social activities and memory impairment. Although impairments in the neural circuits caused by BoDV-1 infection induce these disturbances, the molecular basis remains unclear. Furthermore, it is unknown whether anti-BoDV-1 treatments can attenuate BoDV-1-mediated transcriptomic changes in neuronal cells. In this study, we investigated the effects of BoDV-1 infection on neuronal differentiation and the transcriptome of differentiated neuronal cells using persistently BoDV-1-infected cells. Although BoDV-1 infection did not have a detectable effect on intracellular neuronal differentiation processes, differentiated neuronal cells exhibited transcriptomic changes in differentiation-related genes. Some of these transcriptomic changes, such as the decrease in the expression of apoptosis-related genes, were recovered by anti-BoDV-1 treatment, while alterations in the expression of other genes remained after treatment. We further demonstrated that a decrease in cell viability induced by differentiation processes in BoDV-1-infected cells can be relieved with anti-BoDV-1 treatment. This study provides fundamental information regarding transcriptomic changes after BoDV-1 infection and the treatment in neuronal cells.

Efficacy of oligodendrocyte precursor cells as delivery vehicles for single-chain variable fragment to misfolded SOD1 in ALS rat model

Superoxide dismutase1 (SOD 1) mutation is a leading cause of familial amyotrophic lateral sclerosis (ALS). Growing evidence suggests that antibody therapy against misfolded SOD1 protein can be therapeutic. However, the therapeutic effects are limited, partly because of the delivery system. Therefore, we investigated the efficacy of oligodendrocyte precursor cells (OPCs) as a drug delivery vehicle of single-chain variable fragments (scFv). Using a Borna disease virus vector that is pharmacologically removable and episomally replicable in the recipient cells, we successfully transformed wild-type OPCs to secrete scFv of a novel monoclonal antibody (D3-1), specific for misfolded SOD1. Single intrathecal injection of OPCs scFvD3-1, but not OPCs alone, significantly delayed disease onset and prolonged the lifespan of ALS rat models expressing SOD1 ^H46R . The effect of OPC scFvD3-1 surpassed that of a 1 month intrathecal infusion of full-length D3-1 antibody alone. scFv-secreting OPCs suppressed neuronal loss and gliosis, reduced levels of misfolded SOD1 in the spinal cord, and suppressed the transcription of inflammatory genes, including Olr1, an oxidized low-density lipoprotein receptor 1. The use of OPCs as a delivery vehicle for therapeutic antibodies is a new option for ALS in which misfolded protein and oligodendrocyte dysfunction are implicated in the pathogenesis.

The regulation of persistent Borna disease virus infection by RNA silencing factors in human cells

Viral infection induces diverse cellular immune responses. Some viruses induce the production of antiviral cytokines, alterations of endogenous gene expression, and apoptosis; however, other viruses replicate without inducing such responses, enabling them to persistently infect cells. Infection by Borna disease virus type 1 (BoDV-1) can result in fatal immune-mediated encephalitis, including in humans, yet infection of cells in vitro is generally persistent. The regulatory mechanisms underlying this persistent infection remain unclear. Here, we show that an enhancer of RNA-silencing, TRBP, positively regulates BoDV RNA level in human cells. Knockdown of TRBP decreased BoDV RNA levels in persistently-infected cells, whereas overexpression of TRBP increased BoDV RNA levels. To investigate the mechanism underlying this phenomenon, we performed immunoprecipitation assays and found that TRBP interacts with BoDV RNA. Furthermore, we performed cell fractionation, which revealed that persistent infection with BoDV does not alter the localization of TRBP and other RNA silencing factors in cells. Our results showed the regulation of persistent BoDV infection by RNA-silencing factors in human cells.

Animal Model Alternatives in Filovirus and Bornavirus Research

The order Mononegavirales contains a variety of highly pathogenic viruses that may infect humans, including the families Filoviridae, Bornaviridae, Paramyxoviridae, and Rhabodoviridae. Animal models have historically been important to study virus pathogenicity and to develop medical countermeasures. As these have inherent shortcomings, the rise of microphysiological systems and organoids able to recapitulate hallmarks of the diseases caused by these viruses may have enormous potential to add to or partially replace animal modeling in the future. Indeed, microphysiological systems and organoids are already used in the pharmaceutical R&D pipeline because they are prefigured to overcome the translational gap between model systems and clinical studies. Moreover, they may serve to alleviate ethical concerns related to animal research. In this review, we discuss the value of animal model alternatives in human pathogenic filovirus and bornavirus research. The current animal models and their limitations are presented followed by an overview of existing alternatives, such as organoids and microphysiological systems, which might help answering open research questions.

Ribonucleoprotein transport in Negative Strand RNA viruses

Negative-sense, single-stranded RNA (-ssRNA) viruses comprise some of the deadliest human pathogens (Ebola, rabies, influenza A viruses etc.). Developing therapeutic tools relies on a better understanding of their multiplication cycle. For these viruses, the genome replication and transcription activities most-often segregate in membrane-less environments called inclusion bodies (IBs) or viral factories. These "organelles" usually locate far from the cell surface from where new virions are released, and -ssRNA viruses do not encode for transport factors. The efficient trafficking of the genome progeny toward the cell surface is most often ensured by mechanisms co-opting the cellular machineries. In this review, for each -ssRNA viral family, we cover the methods employed to characterize these host-virus interactions, the strategies used by the viruses to promote the virus genome transport, and the current gaps in the literature. Finally, we highlight how Rab11 has emerged as a target of choice for the intracellular transport of -ssRNA virus genomes.

Vaccination against Borna Disease: Overview, Vaccine Virus Characterization and Investigation of Live and Inactivated Vaccines

(1) Background: Vaccination of horses and sheep against Borna disease (BD) was common in endemic areas of Germany in the 20th century but was abandoned in the early 1990s. The recent occurrence of fatal cases of human encephalitis due to Borna disease virus 1 (BoDV-1) has rekindled the interest in vaccination. (2) Methods: The full genomes of the BD live vaccine viruses "Dessau" and "Giessen" were sequenced and analyzed for the first time. All vaccination experiments followed a proof-of-concept approach. Dose-titration infection experiments were performed in rabbits, based on both cell culture- and brain-derived viruses at various doses. Inactivated vaccines against BD were produced from concentrated cell culture supernatants and investigated in rabbits and horses. The BoDV-1 live vaccine "Dessau" was administered to horses and antibody profiles were determined. (3) Results: The BD live vaccine viruses "Dessau" and "Giessen" belong to clusters 3 and 4 of BoDV-1. Whereas the "Giessen" virus does not differ substantially from field viruses, the "Dessau" virus shows striking differences in the M gene and the N-terminal part of the G gene. Rabbits infected with high doses of cell-cultured virus developed neutralizing antibodies and were protected from disease, whereas rabbits infected with low doses of cell-cultured virus, or with brain-derived virus did not. Inactivated vaccines were administered to rabbits and horses, following pre-defined vaccination schemes consisting of three vaccine doses of either adjuvanted or nonadjuvanted inactivated virus. Their immunogenicity and protective efficacy were compared to the BD live vaccine "Dessau". Seventy per cent of horses vaccinated with the BD live vaccine "Dessau" developed neutralizing antibodies after vaccination. (4) Conclusion: Despite a complex evasion of immunological responses by bornaviruses, some vaccination approaches can protect against clinical disease. For optimal effectiveness, vaccines should be administered at high doses, following vaccination schemes consisting of three vaccine doses as basic immunization. Further investigations are necessary in order to investigate and improve protection against infection and to avoid side effects.

Ribosomal control in RNA virus-infected cells

Viruses are strictly intracellular parasites requiring host cellular functions to complete their reproduction cycle involving virus infection of host cell, viral genome replication, viral protein translation, and virion release. Ribosomes are protein synthesis factories in cells, and viruses need to manipulate ribosomes to complete their protein synthesis. Viruses use translation initiation factors through their own RNA structures or cap structures, thereby inducing ribosomes to synthesize viral proteins. Viruses also affect ribosome production and the assembly of mature ribosomes, and regulate the recognition of mRNA by ribosomes, thereby promoting viral protein synthesis and inhibiting the synthesis of host antiviral immune proteins. Here, we review the remarkable mechanisms used by RNA viruses to regulate ribosomes, in particular, the mechanisms by which RNA viruses induce the formation of specific heterogeneous ribosomes required for viral protein translation. This review provides valuable insights into the control of viral infection and diseases from the perspective of viral protein synthesis.

Borna Disease Virus 1 Phosphoprotein Forms a Tetramer and Interacts with Host Factors Involved in DNA Double-Strand Break Repair and mRNA Processing

Determining the structural organisation of viral replication complexes and unravelling the impact of infection on cellular homeostasis represent important challenges in virology. This may prove particularly useful when confronted with viruses that pose a significant threat to human health, that appear unique within their family, or for which knowledge is scarce. Among Mononegavirales, bornaviruses (family Bornaviridae) stand out due to their compact genomes and their nuclear localisation for replication. The recent recognition of the zoonotic potential of several orthobornaviruses has sparked a surge of interest in improving our knowledge on this viral family. In this work, we provide a complete analysis of the structural organisation of Borna disease virus 1 (BoDV-1) phosphoprotein (P), an important cofactor for polymerase activity. Using X-ray diffusion and diffraction experiments, we revealed that BoDV-1 P adopts a long coiled-coil α-helical structure split into two parts by an original β-strand twist motif, which is highly conserved across the members of whole Orthobornavirus genus and may regulate viral replication. In parallel, we used BioID to determine the proximal interactome of P in living cells. We confirmed previously known interactors and identified novel proteins linked to several biological processes such as DNA repair or mRNA metabolism. Altogether, our study provides important structure/function cues, which may improve our understanding of BoDV-1 pathogenesis.

Reverse Genetics and Artificial Replication Systems of Borna Disease Virus 1

Borna disease virus 1 (BoDV-1) is a neurotropic RNA virus belonging to the family Bornaviridae within the order Mononegavirales. Whereas BoDV-1 causes neurological and behavioral disorders, called Borna disease (BD), in a wide range of mammals, its virulence in humans has been debated for several decades. However, a series of case reports in recent years have established the nature of BoDV-1 as a zoonotic pathogen that causes fatal encephalitis in humans. Although many virological properties of BoDV-1 have been revealed to date, the mechanism by which it causes fatal encephalitis in humans remains unclear. In addition, there are no effective vaccines or antiviral drugs that can be used in clinical practice. A reverse genetics approach to generating replication-competent recombinant viruses from full-length cDNA clones is a powerful tool that can be used to not only understand viral properties but also to develop vaccines and antiviral drugs. The rescue of recombinant BoDV-1 (rBoDV-1) was first reported in 2005. However, due to the slow nature of the replication of this virus, the rescue of high-titer rBoDV-1 required several months, limiting the use of this system. This review summarizes the history of the reverse genetics and artificial replication systems for orthobornaviruses and explores the recent progress in efforts to rescue rBoDV-1.

Exogenous expression of both matrix protein and glycoprotein facilitates infectious viral particle production of Borna disease virus 1

Borna disease virus 1 (BoDV-1) is a non-segmented, negative-strand RNA virus that is characterized by persistent infection in the nucleus and low production of progeny virions. This feature impedes not only the harvesting of infectious viral particles from infected cells but also the rescue of high titres of recombinant BoDV-1 (rBoDV-1) by reverse genetics. Here, we demonstrate that exogenous expression of both matrix protein (M) and glycoprotein (G), which are constituents of the viral lipid envelope, significantly facilitates the formation of infectious particles and propagation of BoDV-1 without affecting its viral RNA synthesis. Furthermore, simultaneous transfection of M and G expression plasmids with N, P and L helper plasmids by reverse genetics drastically enhances the rescue efficiency of rBoDV-1. On the other hand, we also show that overexpression of M induces obvious cytotoxicity similar to that of other Mononegaviruses. Together with our recent report showing that excess expression of G induces aberrant accumulation of immature G, a potential stimulator of the host innate immune response, it is conceivable that BoDV-1 may suppress excess expression of M and G to reduce the cytopathic effect, thereby leading to maintenance of persistent infection. Our results contribute not only to the establishment of an efficient method to recover high-titre BoDV-1 but also to understanding the unique mechanism of persistent BoDV-1 infection.

Why does viral RNA sometimes persist after recovery from acute infections?

DNA viruses often persist in the body of their host, becoming latent and recurring many months or years later. By contrast, most RNA viruses cause acute infections that are cleared from the host as they lack the mechanisms to persist. However, it is becoming clear that viral RNA can persist after clinical recovery and elimination of detectable infectious virus. This persistence can either be asymptomatic or associated with late progressive disease or nonspecific lingering symptoms, such as may be the case following infection with Ebola or Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2). Why does viral RNA sometimes persist after recovery from an acute infection? Where does the RNA come from? And what are the consequences?

An endogenous bornavirus-like nucleoprotein in miniopterid bats retains the RNA-binding properties of the original viral protein

Endogenous bornavirus-like nucleoprotein elements (EBLNs) are sequences derived from bornaviral N genes in vertebrate genomes. Some EBLNs have been suggested to encode functional proteins in host cells; however, little is known about their evolution and functional relationship to the viral genes from which EBLNs originate. Here, we predicted functionality of EBLNs based on the properties of N as an RNA-binding protein. We showed an EBLN in miniopterid bats (miEBLN-1) has evolved under purifying selection and encodes an RNA-binding protein (miEBLN-1p) with biochemical properties similar to bornaviral N. Furthermore, we revealed miEBLN-1p interacts with host RNA-binding proteins, such as MOV10. These data suggest that miEBLN-1p has been exapted as an RNA-binding protein with similar properties to exogenous bornaviral N in miniopterid bats.

Borna disease virus docks on neuronal DNA double-strand breaks to replicate and dampens neuronal activity

Borna disease viruses (BoDV) have recently emerged as zoonotic neurotropic pathogens. These persistent RNA viruses assemble nuclear replication centers (vSPOT) in close interaction with the host chromatin. However, the topology of this interaction and its consequences on neuronal function remain unexplored. In neurons, DNA double-strand breaks (DSB) have been identified as novel epigenetic mechanisms regulating neurotransmission and cognition. Activity-dependent DSB contribute critically to neuronal plasticity processes, which could be impaired upon infection. Here, we show that BoDV-1 infection, or the singled-out expression of viral Nucleoprotein and Phosphoprotein, increases neuronal DSB levels. Of interest, inducing DSB promoted the recruitment anew of vSPOT colocalized with DSB and increased viral RNA replication. BoDV-1 persistence decreased neuronal activity and response to stimulation by dampening the surface expression of glutamate receptors. Taken together, our results propose an original mechanistic cross talk between persistence of an RNA virus and neuronal function, through the control of DSB levels.

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BoDV-1, its Nucleoprotein or Phosphoprotein cause neuronal DNA double-strand breaks (DSB)

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DNA double-strand breaks co-localize with BoDV-1 replication factories

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DNA DSB recruits BoDV-1 replication factories and promotes viral replication

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BoDV-1 inhibits neuronal activity by impeding surface expression of GluN2A receptors

Structure and function of negative-strand RNA virus polymerase complexes

Viruses with negative-strand RNA genomes (NSVs) include many highly pathogenic and economically devastating disease-causing agents of humans, livestock, and plants-highlighted by recent Ebola and measles virus epidemics, and continuously circulating influenza virus. Because of their protein-coding orientation, NSVs face unique challenges for efficient gene expression and genome replication. To overcome these barriers, NSVs deliver a large and multifunctional RNA-dependent RNA polymerase into infected host cells. NSV-encoded polymerases contain all the enzymatic activities required for transcription and replication of their genome-including RNA synthesis and mRNA capping. Here, we review the structures and functions of NSV polymerases with a focus on key domains responsible for viral replication and gene expression. We highlight shared and unique features among polymerases of NSVs from the Mononegavirales, Bunyavirales, and Articulavirales orders.

Borna disease virus phosphoprotein triggers the organization of viral inclusion bodies by liquid-liquid phase separation

Inclusion bodies (IBs) are characteristic biomolecular condensates organized by the non-segmented negative-strand RNA viruses belonging to the order Mononegavirales. Although recent studies have revealed the characteristics of IBs formed by cytoplasmic mononegaviruses, that of Borna disease virus 1 (BoDV-1), a unique mononegavirus that forms IBs in the cell nucleus and establishes persistent infection remains elusive. Here, we characterize the IBs of BoDV-1 in terms of liquid-liquid phase separation (LLPS). The BoDV-1 phosphoprotein (P) alone induces LLPS and the nucleoprotein (N) is incorporated into the P droplets in vitro. In contrast, co-expression of N and P is required for the formation of IB-like structure in cells. Furthermore, while BoDV-1 P binds to RNA, an excess amount of RNA dissolves the liquid droplets formed by N and P in vitro. Notably, the intrinsically disordered N-terminal region of BoDV-1 P is essential to drive LLPS and to bind to RNA, suggesting that both abilities could compete with one another. These features are unique among mononegaviruses, and thus this study will contribute to a deeper understanding of LLPS-driven organization and RNA-mediated regulation of biomolecular condensates.

Deconstructing virus condensation

Viruses have evolved precise mechanisms for using the cellular physiological pathways for their perpetuation. These virus-driven biochemical events must be separated in space and time from those of the host cell. In recent years, granular structures, known for over a century for rabies virus, were shown to host viral gene function and were named using terms such as viroplasms, replication sites, inclusion bodies, or viral factories (VFs). More recently, these VFs were shown to be liquid-like, sharing properties with membrane-less organelles driven by liquid–liquid phase separation (LLPS) in a process widely referred to as biomolecular condensation. Some of the best described examples of these structures come from negative stranded RNA viruses, where micrometer size VFs are formed toward the end of the infectious cycle. We here discuss some basic principles of LLPS in connection with several examples of VFs and propose a view, which integrates viral replication mechanisms with the biochemistry underlying liquid-like organelles. In this view, viral protein and RNA components gradually accumulate up to a critical point during infection where phase separation is triggered. This yields an increase in transcription that leads in turn to increased translation and a consequent growth of initially formed condensates. According to chemical principles behind phase separation, an increase in the concentration of components increases the size of the condensate. A positive feedback cycle would thus generate in which crucial components, in particular nucleoproteins and viral polymerases, reach their highest levels required for genome replication. Progress in understanding viral biomolecular condensation leads to exploration of novel therapeutics. Furthermore, it provides insights into the fundamentals of phase separation in the regulation of cellular gene function given that virus replication and transcription, in particular those requiring host polymerases, are governed by the same biochemical principles.

The Borna Disease Virus 2 (BoDV-2) Nucleoprotein Is a Conspecific Protein That Enhances BoDV-1 RNA-Dependent RNA Polymerase Activity

An RNA virus-based episomal vector (REVec) based on Borna disease virus 1 (BoDV-1) is a promising viral vector that achieves stable and long-term gene expression in transduced cells. However, the onerous procedure of reverse genetics used to generate an REVec is one of the challenges that must be overcome to make REVec technologies practical for use. In this study, to resolve the problems posed by reverse genetics, we focused on BoDV-2, a conspecific virus of BoDV-1 in the Mammalian 1 orthobornavirus. We synthesized the BoDV-2 nucleoprotein (N) and phosphoprotein (P) according to the reference sequences and evaluated their effects on the RNA polymerase activity of the BoDV-1 large protein (L) and viral replication. In the minireplicon assay, we found that BoDV-2 N significantly enhanced BoDV-1 polymerase activity and that BoDV-2 P supported further enhancement of this activity by N. A single amino acid substitution assay identified serine at position 30 of BoDV-2 N and alanine at position 24 of BoDV-2 P as critical amino acid residues for the enhancement of BoDV-1 polymerase activity. In reverse genetics, conversely, BoDV-2 N alone was sufficient to increase the rescue efficiency of the REVec. We showed that the REVec can be rescued directly from transfected 293T cells by using BoDV-2 N as a helper plasmid without cocultivation with Vero cells and following several weeks of passage. In addition, a chimeric REVec harboring the BoDV-2 N produced much higher levels of transgene mRNA and genomic RNA than the wild-type REVec in transduced cells. Our results contribute to not only improvements to the REVec system but also to understanding of the molecular regulation of orthobornavirus polymerase activity. IMPORTANCE Borna disease virus 1 (BoDV-1), a prototype virus of the species Mammalian 1 orthobornavirus, is a nonsegmented negative-strand RNA virus that persists in the host nucleus. The nucleoprotein (N) of BoDV-1 encapsidates genomic and antigenomic viral RNA, playing important roles in viral transcription and replication. In this study, we demonstrated that the N of BoDV-2, another genotype in the species Mammalian 1 orthobornavirus, can participate in the viral ribonucleoprotein complex of BoDV-1 and enhance the activity of BoDV-1 polymerase (L) in both the BoDV-1 minireplicon assay and reverse genetics system. Chimeric recombinant BoDV-1 expressing BoDV-2 N but not BoDV-1 N showed higher transcription and replication levels, whereas the propagation and infectious particle production of the chimeric virus were comparable to those of wild-type BoDV-1, suggesting that the level of viral replication in the nucleus is not directly involved in the progeny virion production of BoDVs. Our results demonstrate a molecular mechanism of bornaviral polymerase activity, which will contribute to further development of vector systems using orthobornaviruses.

Stability of Borna disease virus-based episomal vector under physical and chemical stimulation

Borna disease virus (BoDV), a nonsegmented, negative-sense RNA virus, establishes persistent infection and replicates in the cell nucleus. Since BoDV genomic RNA exists as episomal RNA, the host genome is not invaded by BoDV infection. These unique features make BoDV a promising gene delivery system as an RNA virus-based episomal vector (REVec). Previously, the stable expression of genes of interest in vitro and in vivo using a REVec was reported. For the clinical application of a REVec, the fundamental properties under various physical and chemical conditions must be determined to develop purification processes, supply chains, and biosafety management. This study investigated the effects of the following conditions on the inducibility of transmission-defective ΔG-REVec: freeze-thaw cycles, dehydration, UV, temperature, pH, and reagents for virucides and laboratory experiments. Although the titer of ΔG-REVec was not influenced by the freeze-thaw process or 5 minute incubation at ≤50°C, ΔG-REVec was significantly inactivated by incubation at ≥70°C for 5 minutes. The induction titer of ΔG-REVec was decreased by long-term incubation, dehydration, and UV irradiation in a temperature- and time-dependent manner. ΔG-REVec was sensitive to lower pH and inactivated by chemical reagents under general conditions. These results provide important knowledge for developing the clinical use of REVec and biosafety management.

BUD23-TRMT112 interacts with the L protein of Borna disease virus and mediates the chromosomal tethering of viral ribonucleoproteins

Persistent intranuclear infection is an uncommon infection strategy among RNA viruses. However, Borna disease virus 1 (BoDV-1), a nonsegmented, negative-strand RNA virus, maintains viral infection in the cell nucleus by forming structured aggregates of viral ribonucleoproteins (vRNPs), and by tethering these vRNPs onto the host chromosomes. To better understand the nuclear infection strategy of BoDV-1, we determined the host protein interactors of the BoDV-1 large (L) protein. By proximity-dependent biotinylation, we identified several nuclear host proteins interacting with BoDV-1 L, one of which is TRMT112, a partner of several RNA methyltransferases (MTase). TRMT112 binds with BoDV-1 L at the RNA-dependent RNA polymerase domain, together with BUD23, an 18S rRNA MTase and 40S ribosomal maturation factor. We then discovered that BUD23-TRMT112 mediates the chromosomal tethering of BoDV-1 vRNPs, and that the MTase activity is necessary in the tethering process. These findings provide us a better understanding on how nuclear host proteins assist the chromosomal tethering of BoDV-1, as well as new prospects of host-viral interactions for intranuclear infection strategy of orthobornaviruses. This article is protected by copyright. All rights reserved.

Optimal Expression of the Envelope Glycoprotein of Orthobornaviruses Determines the Production of Mature Virus Particles

An RNA virus-based episomal vector (REVec) whose backbone is Borna disease virus 1 (BoDV-1) can provide long-term gene expression in transduced cells. To improve the transduction efficiency of REVec, we evaluated the role of the viral envelope glycoprotein (G) of the genus Orthobornavirus, including that of BoDV-1, in the production of infectious particles. By using G-pseudotype assay in which the lack of G in G-deficient REVec (ΔG-REVec) was compensated for expression of G, we found that excess expression of BoDV-1-G does not affect particle production itself but results in uncleaved and aberrant mature G expression in the cells, leading to the production of REVec particles with low transduction titers. We revealed that the expression of uncleaved G in the cells inhibits the incorporation of mature G and vgRNA into the particles. This feature of G was conserved among mammalian and avian orthobornaviruses; however, the cleavage efficacy of canary bornavirus 1 (CnBV-1)-G was exceptionally not impaired by its excess expression, which led to the production of the pseudotype ΔG-REVec with the highest titer. Chimeric G proteins between CnBV-1 and -2 revealed that the signal peptide of CnBV-1-G was responsible for the cleavage efficacy through the interaction with intracellular furin. We showed that CnBV-1 G leads to the development of pseudotyped REVec with high transduction efficiency and a high-titer recombinant REVec. Our study demonstrated that the restricted expression of orthobornavirus G contributes to the regulation of infectious particle production, the mechanism of which can improve the transduction efficiency of REVec.IMPORTANCE Most viruses causing persistent infection produce few infectious particles from the infected cells. Borna disease virus 1, a member of the genus Orthobornavirus, is an RNA virus that persistently infects the nucleus and has been applied to vectors for long-term gene expression. In this study, we showed that, common among orthobornaviruses, excessive G expression does not affect particle production itself but reduces the production of infectious particles with mature G and genomic RNA. This result suggested that limited G expression contributes to suppressing abnormal viral particle production. On the other hand, we found that canary bornavirus 1 has an exceptional G maturation mechanism and produces a high-titer virus. Our study will contribute to not only understanding the mechanism of infectious particle production but also improving the vector system of orthobornaviruses.

In Vitro and In Ovo Host Restriction of Aquatic Bird Bornavirus 1 in Different Avian Hosts

Aquatic bird bornavirus 1 (ABBV-1) is associated with chronic meningoencephalitis and ganglioneuritis. Although waterfowl species act as the natural host of ABBV-1, the virus has been sporadically isolated from other avian species, showing the potential for a broad host range. To evaluate the host restriction of ABBV-1, and its potential to infect commercial poultry species, we assessed the ability of ABBV-1 to replicate in cells and embryos of different avian species. ABBV-1 replication was measured using multi- and single-step growth curves in primary embryo fibroblasts of chicken, duck, and goose. Embryonated chicken and duck eggs were infected through either the yolk sac or chorioallantoic cavity, and virus replication was assessed by immunohistochemistry and RT-qPCR in embryonic tissues harvested at two time points after infection. Multi-step growth curves showed that ABBV-1 replicated and spread in goose and duck embryo fibroblasts, establishing a population of persistently infected cells, while it was unable to do so in chicken fibroblasts. Single-step growth curves showed that cells from all three species could be infected; however, persistence was only established in goose and duck fibroblasts. In ovo inoculation yielded no detectable viral replication or lesion in tissues. Data indicate that although chicken, duck, and goose embryo fibroblasts can be infected with ABBV-1, a persistent infection is more easily established in duck and goose cells. Therefore, ABBV-1 may be able to infect chickens in vivo, albeit inefficiently. Additionally, our data indicate that an in ovo model is inadequate to investigating ABBV-1 host restriction and pathogenesis.

Evidence for Tethering of Human Cytomegalovirus Genomes to Host Chromosomes

Tethering of viral genomes to host chromosomes has been recognized in a variety of DNA and RNA viruses. It can occur during both the productive cycle and latent infection and may impact viral genomes in manifold ways including their protection, localization, transcription, replication, integration, and segregation. Tethering is typically accomplished by dedicated viral proteins that simultaneously associate with both the viral genome and cellular chromatin via nucleic acid, histone and/or non-histone protein interactions. Some of the most prominent tethering proteins have been identified in DNA viruses establishing sustained latent infections, including members of the papillomaviruses and herpesviruses. Herpesvirus particles have linear genomes that circularize in infected cell nuclei and usually persist as extrachromosomal episomes. In several γ-herpesviruses, tethering facilitates the nuclear retention and faithful segregation of viral episomes during cell division, thus contributing to persistence of these viruses in the absence of infectious particle production. However, it has not been studied whether the genomes of human Cytomegalovirus (hCMV), the prototypical β-herpesvirus, are tethered to host chromosomes. Here we provide evidence by fluorescence in situ hybridization that hCMV genomes associate with the surface of human mitotic chromosomes following infection of both non-permissive myeloid and permissive fibroblast cells. This chromosome association occurs at lower frequency in the absence of the immediate-early 1 (IE1) proteins, which bind to histones and have been implicated in the maintenance of hCMV episomes. Our findings point to a mechanism of hCMV genome maintenance through mitosis and suggest a supporting but non-essential role of IE1 in this process.

Production of high-titer transmission-defective RNA virus-based episomal vector using tangential flow filtration

In recent years, viral vector based in vivo gene delivery strategies have achieved a significant success in the treatment of genetic diseases. RNA virus-based episomal vector lacking viral glycoprotein gene (ΔG-REVec) is a nontransmissive gene delivery system that enables long-term gene expression in a variety of cell types in vitro, yet in vivo gene delivery has not been successful due to the difficulty in producing high titer vector. The present study showed that tangential flow filtration (TFF) can be effectively employed to increase the titer of ΔG-REVec. Concentration and diafiltration of ΔG-REVec using TFF significantly increased its titer without loss of infectious activity. Importantly, intracranial administration of high titer vector enabled persistent transgene expression in rodent brain.

Reverse genetics approaches of Borna disease virus: applications in development of viral vectors and preventive vaccines

The plasmid-based reverse genetics system, which involves generation of recombinant viruses from cloned cDNA, has accelerated the understanding of clinical and virological aspects of different viruses. Borna disease virus (BoDV) is a nonsegmented, negative-strand RNA virus that causes persistent intranuclear infection in various vertebrate species. Since its first report, reverse genetics approaches with modified strategies have greatly improved rescue efficiency of recombinant BoDV and enhanced the understanding of function of each viral protein and mechanism of intranuclear persistency. Here, we summarize different reverse genetics approaches of BoDV and recent developments in the use of reverse genetics for generation of viral vectors for gene therapy and virus-like particles for potential preventive vaccines.

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.

In vivo biodistribution analysis of transmission competent and defective RNA virus-based episomal vector

RNA virus-based episomal vector (REVec) is an emerging viral vector system that mediates long-term stable gene expression in variety of cell types in vitro. However, little is known about its tissue tropism and persistence of gene expression in vivo. Here, to evaluate the feasibility of REVec for in vivo gene delivery, we conducted biodistribution analysis of transmission competent REVec and transmission defective ΔG-REVec in Lewis rats. Following intracranial administration of REVec, transgene expression was detected in various tissues. In contrast, transgene expression was only observed in the brain after ΔG-REVec administration. Low levels of vector shedding in the feces and blood and of neutralizing antibody in the serum were detected after REVec injection. In the brain, microglia, astrocytes and neurons were susceptible to REVec-mediated transduction. However, the animals administered with REVec, but not with ΔG-REVec showed a significant decrease in body weight compared to mock treated animals. Additionally, CD8 T cell infiltration was observed in the brain of these animals. In summary, we demonstrated that REVec promotes long-term transgene expression in vivo without causing high vector shedding or neutralizing antibody production; however, suggests the need to attenuate vector associated pathogenicity in the future.

ADAR2 Is Involved in Self and Nonself Recognition of Borna Disease Virus Genomic RNA in the Nucleus

Cells sense pathogen-derived double-stranded RNA (dsRNA) as nonself. To avoid autoimmune activation by self dsRNA, cells utilize A-to-I editing by adenosine deaminase acting on RNA 1 (ADAR1) to disrupt dsRNA structures. Considering that viruses have evolved to exploit host machinery, A-to-I editing could benefit innate immune evasion by viruses. Borna disease virus (BoDV), a nuclear-replicating RNA virus, may require escape from nonself RNA-sensing and immune responses to establish persistent infection in the nucleus; however, the strategy by which BoDV evades nonself recognition is unclear. Here, we evaluated the involvement of ADARs in BoDV infection. The infection efficiency of BoDV was markedly decreased in both ADAR1 and ADAR2 knockdown cells at the early phase of infection. Microarray analysis using ADAR2 knockdown cells revealed that ADAR2 reduces immune responses even in the absence of infection. Knockdown of ADAR2 but not ADAR1 significantly reduced the spread and titer of BoDV in infected cells. Furthermore, ADAR2 knockout decreased the infection efficiency of BoDV, and overexpression of ADAR2 rescued the reduced infectivity in ADAR2 knockdown cells. However, the growth of influenza A virus, which causes acute infection in the nucleus, was not affected by ADAR2 knockdown. Moreover, ADAR2 bound to BoDV genomic RNA and induced A-to-G mutations in the genomes of persistently infected cells. We finally demonstrated that BoDV produced in ADAR2 knockdown cells induces stronger innate immune responses than those produced in wild-type cells. Taken together, our results suggest that BoDV utilizes ADAR2 to edit its genome to appear as "self" RNA in order to maintain persistent infection in the nucleus.IMPORTANCE Cells use the editing activity of adenosine deaminase acting on RNA proteins (ADARs) to prevent autoimmune responses induced by self dsRNA, but viruses can exploit this process to their advantage. Borna disease virus (BoDV), a nuclear-replicating RNA virus, must escape nonself RNA sensing by the host to establish persistent infection in the nucleus. We evaluated whether BoDV utilizes ADARs to prevent innate immune induction. ADAR2 plays a key role throughout the BoDV life cycle. ADAR2 knockdown reduced A-to-I editing of BoDV genomic RNA, leading to the induction of a strong innate immune response. These data suggest that BoDV exploits ADAR2 to edit nonself genomic RNA to appear as self RNA for innate immune evasion and establishment of persistent infection.

Isolation of Ontario aquatic bird bornavirus 1 and characterization of its replication in immortalized avian cell lines

BACKGROUND

Aquatic bird bornavirus 1 (ABBV-1) has been associated with neurological diseases in wild waterfowls. In Canada, presence of ABBV-1 was demonstrated by RT-qPCR and immunohistochemistry in tissues of waterfowls with history of neurological disease and inflammation of the central and peripheral nervous tissue, although causation has not been proven by pathogenesis experiments, yet. To date, in vitro characterization of ABBV-1 is limited to isolation in primary duck embryo fibroblasts. The objectives of this study were to describe isolation of ABBV-1 in primary duck embryonic fibroblasts (DEF), and characterize replication in DEF and three immortalized avian fibroblast cell lines (duck CCL-141, quail QT-35, chicken DF-1) in order to evaluate cellular permissivity and identify suitable cell lines for routine virus propagation.

METHODS

The virus was sequenced, and phylogenetic analysis performed on a segment of the N gene coding region. Virus spread in cell cultures, viral RNA and protein production, and titres were evaluated at different passages using immunofluorescence, RT-qPCR, western blotting, and tissue culture dose 50% (TCID₅₀) assay, respectively.

RESULTS

The isolated ABBV-1 showed 97 and 99% identity to European ABBV-1 isolate AF-168 and North American ABBV-1 isolates 062-CQ and CG-N1489, and could infect and replicate in DEF, CCL-141, QT-35 and DF-1 cultures. Viral RNA was detected in all four cultures with highest levels observed in DEF and CCL-141, moderate in QT-35, and lowest in DF-1. N protein was detected in western blots from infected DEF, CCL-141 and QT-35 at moderate to high levels, but minimally in infected DF-1. Infectious titre was highest in DEF (between approximately 10⁵ to 10⁶ FFU / 10⁶ cells). Regarding immortalized cell lines, CCL-141 showed the highest titre between approximately 10⁴ to 10⁵ FFU / 10⁶ cells. DF-1 produced minimal infectious titre.

CONCLUSIONS

This study confirms the presence of ABBV-1 among waterfowl in Canada and reported additional in vitro characterization of this virus in different avian cell lines. ABBV-1 replicated to highest titre in DEF, followed by CCL-141 and QT-35, and poorly in DF-1. Our results showed that CCL-141 can be used instead of DEF for routine ABBV-1 production, if a lower titre is an acceptable trade-off for the simplicity of using immortalized cell line over primary culture.

Targeting histone epigenetics to control viral infections

During the past decades, many studies have significantly broadened our understanding of complex virus-host interactions to control chromatin structure and dynamics.1, 2 However, the role and impact of such modifications during viral infections is not fully revealed. Indeed, this type of regulation is bidirectional between the virus and the host. While viral replication and gene expression are significantly impacted by histone modifications on the viral chromatin,³ studies have shown that some viral pathogens dynamically manipulate cellular epigenetic factors to enhance their own survival and pathogenesis, as well as escape the immune system defense lines.⁴ In this dynamic, histone posttranslational modifications (PTMs) appear to play fundamental roles in the regulation of chromatin structure and recruitment of other factors.⁵ Genuinely, those PTMs play a vital role in lytic infection, latency reinforcement, or, conversely, viral reactivation.⁶ In this chapter, we will examine and review the involvement of histone modifications as well as their potential manipulation to control infections during various viral life cycle stages, highlighting their prospective implications in the clinical management of human immunodeficiency virus (HIV), herpes simplex virus (HSV), human cytomegalovirus (HCMV), hepatitis B and C viruses (HBV and HCV, respectively), Epstein–Barr virus (EBV), and other viral diseases. Targeting histone modifications is critical in setting the treatment of chronic viral infections with both lytic and latent stages (HIV, HCMV, HSV, RSV), virus-induced cancers (HBV, HCV, EBV, KSHV, HPV), and epidemic/emerging viruses (e.g. influenza virus, arboviruses).

Development of an RNA Virus-Based Episomal Vector Capable of Switching Transgene Expression

Viral vectors are efficient gene delivery systems, although most of these vectors still present limitations to their practical use, such as achieving only transient transgene expression and a risk of insertional mutations. We have recently developed an RNA virus-based episomal vector (REVec), based on nuclear-replicating Borna disease virus (BoDV). REVec can transduce transgenes into various types of cells and stably express transgenes; however, an obstacle to the practical use of REVec is the lack of a mechanism to turn off transgene expression once REVec is transduced. Here, we developed a novel REVec system, REVec-L2b9, in which transgene expression can be switched on and off by using a theophylline-dependent self-cleaving riboswitch. Transgene expression from REVec-L2b9 was suppressed in the absence of theophylline and induced by theophylline administration. Conversely, transgene expression from REVec-L2b9 was switched off by removing theophylline. To our knowledge, REVec-L2b9 is the first nuclear-replicating RNA virus vector capable of switching transgene expression on and off as needed, which will expand the potential for gene therapies by increasing safety and usability.

RNA Virus-Based Episomal Vector with a Fail-Safe Switch Facilitating Efficient Genetic Modification and Differentiation of iPSCs

A gene delivery system that allows efficient and safe stem cell modification is critical for next-generation stem cell therapies. An RNA virus-based episomal vector (REVec) is a gene transfer system developed based on Borna disease virus (BoDV), which facilitates persistent intranuclear RNA transgene delivery without integrating into the host genome. In this study, we analyzed susceptibility of human induced pluripotent stem cell (iPSC) lines from different somatic cell sources to REVec, along with commonly used viral vectors, and demonstrated highly efficient REVec transduction of iPSCs. Using REVec encoding myogenic transcription factor MyoD1, we further demonstrated potential application of the REVec system for inducing differentiation of iPSCs into skeletal muscle cells. Of note, treatment with a small molecule, T-705, completely eliminated REVec in persistently transduced cells. Thus, the REVec system offers a versatile toolbox for stable, integration-free iPSC modification and trans-differentiation, with a unique switch-off mechanism.

Splicing-Dependent Subcellular Targeting of Borna Disease Virus Nucleoprotein Isoforms

Targeting of viral proteins to specific subcellular compartments is a fundamental step for viruses to achieve successful replication in infected cells. Borna disease virus 1 (BoDV-1), a nonsegmented, negative-strand RNA virus, uniquely replicates and persists in the cell nucleus. Here, it is demonstrated that BoDV nucleoprotein (N) transcripts undergo mRNA splicing to generate truncated isoforms. In combination with alternative usage of translation initiation sites, the N gene potentially expresses at least six different isoforms, which exhibit diverse intracellular localizations, including the nucleoplasm, cytoplasm, and endoplasmic reticulum (ER), as well as intranuclear viral replication sites. Interestingly, the ER-targeting signal peptide in N is exposed by removing the intron by mRNA splicing. Furthermore, the spliced isoforms inhibit viral polymerase activity. Consistently, recombinant BoDVs lacking the N-splicing signals acquire the ability to replicate faster than wild-type virus in cultured cells, suggesting that N isoforms created by mRNA splicing negatively regulate BoDV replication. These results provided not only the mechanism of how mRNA splicing generates viral proteins that have distinct functions but also a novel strategy for replication control of RNA viruses using isoforms with different subcellular localizations.IMPORTANCE Borna disease virus (BoDV) is a highly neurotropic RNA virus that belongs to the orthobornavirus genus. A zoonotic orthobornavirus that is genetically related to BoDV has recently been identified in squirrels, thus increasing the importance of understanding the replication and pathogenesis of orthobornaviruses. BoDV replicates in the nucleus and uses alternative mRNA splicing to express viral proteins. However, it is unknown whether the virus uses splicing to create protein isoforms with different functions. The present study demonstrated that the nucleoprotein transcript undergoes splicing and produces four new isoforms in coordination with alternative usage of translation initiation codons. The spliced isoforms showed a distinct intracellular localization, including in the endoplasmic reticulum, and recombinant viruses lacking the splicing signals replicated more efficiently than the wild type. The results provided not only a new regulation of BoDV replication but also insights into how RNA viruses produce protein isoforms from small genomes.

Intracellular dynamics of actin affects Borna disease virus replication in the nucleus

Borna disease virus (BoDV) is a nonsegmented, negative-strand RNA virus that uniquely replicates and establishes persistent infection in cell nucleus. Recent studies have demonstrated the presence of actin in the nucleus and its role in intranuclear phenomena such as transcription and DNA repair. Although nuclear actin is involved in the life cycle of some intranuclear DNA viruses, the interaction between BoDV and nuclear actin has not been reported. In this study, we show that the inhibition of the nucleocytoplasmic transport of actin affects the replication of BoDV in the nucleus. The knockdown of a nuclear export factor of actin, exportin 6, results in the induction of structural aberration in intranuclear viral factories of BoDV. Furthermore, the inhibition of the nuclear export of actin promotes accumulation of viral matrix protein in the cytoplasm and periphery of the infected cells. These results suggest that the dynamics of actin affect the replication of BoDV by disturbing the structure of viral factories in the nucleus.

Generation of a non-transmissive Borna disease virus vector lacking both matrix and glycoprotein genes

Borna disease virus (BoDV), a prototype of mammalian bornavirus, is a non-segmented, negative strand RNA virus that often causes severe neurological disorders in infected animals, including horses and sheep. Unique among animal RNA viruses, BoDV transcribes and replicates non-cytopathically in the cell nucleus, leading to establishment of long-lasting persistent infection. This striking feature of BoDV indicates its potential as an RNA virus vector system. It has previously been demonstrated by our team that recombinant BoDV (rBoDV) lacking an envelope glycoprotein (G) gene develops persistent infections in transduced cells without loss of the viral genome. In this study, a novel non-transmissive rBoDV, rBoDV ΔMG, which lacks both matrix (M) and G genes in the genome, is reported. rBoDV-ΔMG expressing green fluorescence protein (GFP), rBoDV ΔMG-GFP, was efficiently generated in Vero/MG cells stably expressing both BoDV M and G proteins. Infection with rBoDV ΔMG-GFP was persistently maintained in the parent Vero cells without propagation within cell culture. The optimal ratio of M and G for efficient viral particle production by transient transfection of M and G expression plasmids into cells persistently infected with rBoDV ΔMG-GFP was also demonstrated. These findings indicate that the rBoDV ΔMG-based BoDV vector may provide an extremely safe virus vector system and could be a novel strategy for investigating the function of M and G proteins and the host range of bornaviruses.

Prevalence of antibodies against Borna disease virus proteins in Japanese children with autism spectrum disorder

Bornavirus infection is observed in both animals, including humans. However, bornavirus epidemiology in humans, especially in children, remains unclear. Here, we evaluated antibodies against bornaviruses in Japanese children with autism spectrum disorder (ASD) using immunofluorescence analysis, western blotting, and radio ligand assay. The prevalence of antibodies against bornavirus-specific speckles, N, and P proteins were 22%, 48%, and 33%, respectively, in the ASD children. According to our criteria, the prevalence of antibodies against bornaviruses was 7.4% in the ASD children. This is the first report of the serological prevalence of bornavirus in Japanese children. Our results provide valuable baseline-data regarding bornavirus epidemiology in children for future studies.

Degradation of amyloid β peptide by neprilysin expressed from Borna disease virus vector

Accumulation of amyloid β (Aβ40 and Aβ42) in the brain is a characteristic of Alzheimer's disease (AD). Because neprilysin (NEP) is a major Aβ-degrading enzyme, NEP delivery in the brain is a promising gene therapy for AD. Borna disease virus (BoDV) vector enables long-term transduction of foreign genes in the central nerve system. Here, we evaluated the proteolytic ability of NEP transduced by the BoDV vector and found that the amounts of Aβ40 and Aβ42 significantly decreased, which suggests that NEP expressed from the BoDV vector is functional to degrade Aβ.

Hippocampal expression of a virus-derived protein impairs memory in mice

The analysis of the biology of neurotropic viruses, notably of their interference with cellular signaling, provides a useful tool to get further insight into the role of specific pathways in the control of behavioral functions. Here, we exploited the natural property of a viral protein identified as a major effector of behavioral disorders during infection. We used the phosphoprotein (P) of Borna disease virus, which acts as a decoy substrate for protein kinase C (PKC) when expressed in neurons and disrupts synaptic plasticity. By a lentiviral-based strategy, we directed the singled-out expression of P in the dentate gyrus of the hippocampus and we examined its impact on mouse behavior. Mice expressing the P protein displayed increased anxiety and impaired long-term memory in contextual and spatial memory tasks. Interestingly, these effects were dependent on P protein phosphorylation by PKC, as expression of a mutant form of P devoid of its PKC phosphorylation sites had no effect on these behaviors. We also revealed features of behavioral impairment induced by P protein expression but that were independent of its phosphorylation by PKC. Altogether, our findings provide insight into the behavioral correlates of viral infection, as well as into the impact of virus-mediated alterations of the PKC pathway on behavioral functions.

Avian Ganglioneuritis in Clinical Practice

Avian ganglioneuritis (AG) comprises one of the most intricate pathologies in avian medicine and is researched worldwide. Avian bornavirus (ABV) has been shown to be a causative agent of proventricular dilatation disease in birds. The avian Bornaviridae represent a genetically diverse group of viruses that are widely distributed in captive and wild populations around the world. ABV and other infective agents are implicated as a cause of the autoimmune pathology that leads to AG, similar to human Guillain Barrè syndrome. Management of affected birds is beneficial and currently centered at reducing neurologic inflammation, managing secondary complications, and providing nutritional support.

Within host RNA virus persistence: mechanisms and consequences

In a prototypical response to an acute viral infection it would be expected that the adaptive immune response would eliminate all virally infected cells within a few weeks of infection. However many (non-retrovirus) RNA viruses can establish 'within host' persistent infections that occasionally lead to chronic or reactivated disease. Despite the importance of 'within host' persistent RNA virus infections, much has still to be learnt about the molecular mechanisms by which RNA viruses establish persistent infections, why innate and adaptive immune responses fail to rapidly clear these infections, and the epidemiological and potential disease consequences of such infections.

Histone acetylation in neuronal (dys)function

Cognitive functions require the expression of an appropriate pattern of genes in response to environmental stimuli. Over the last years, many studies have accumulated knowledge towards the understanding of molecular mechanisms that regulate neuronal gene expression. Epigenetic modifications have been shown to play an important role in numerous neuronal functions, from synaptic plasticity to learning and memory. In particular, histone acetylation is a central player in these processes. In this review, we present the molecular mechanisms of histone acetylation and summarize the data underlying the relevance of histone acetylation in cognitive functions in normal and pathological conditions. In the last part, we discuss the different mechanisms underlying the dysregulation of histone acetylation associated with neurological disorders, with a particular focus on environmental causes (stress, drugs, or infectious agents) that are linked to impaired histone acetylation.