Drug Discovery Today: Disease Mechanisms Vol. 1, No. 2 2004 Editors-in-Chief Toren Finkel – National Heart, Lung and Blood Institute, National Institutes of Health, USA DRUG DISCOVERY Tamas Bartfai – Harold L. Dorris Neurological Research Center and The Scripps Research Institute, USA TODAY DISEASE MECHANISMS Infectious diseases Models and mechanisms of Bornavirus pathogenesis Martin Schwemmle1,*, W. Ian Lipkin2,* 1Department of Virology, University of Freiburg, Hermann-Herder-Strasse 11, D-79104 Freiburg, Germany 2Jerome L. and Dawn Greene Infectious Disease Laboratory, Departments of Epidemiology, Neurology and Pathology, Mailman School of Public Health and College of Physicians and Surgeons, Columbia University, 722 West 168th Street, New York, NY 10032, USA Borna Disease Virus (BDV) causes neurologic disease Section Editor: in a wide variety of warm-blooded animals. Although Anne Moscona – Department of Pediatrics, Mount Sinai human infection has not been conclusively demon- School of Medicine, One Gustave L. Levy Place, Box 1198, New York, NY 10029, USA strated, some surveys suggest that BDV or a related agent might infect humans and be implicated in mental Borna disease virus is a fascinating agent that causes persistent infection in the central nervous system of mammals, and there are tantalizing illness. Recent progress in tissue culture and animal suggestions that Borna is an etiological agent in a number of human neuropsychiatric disorders. This review highlights what is known about models reveals insights into the molecular biology of the viral life cycle and the role of both immune factors and specific viral this unique virus and unveils mechanisms by which factors in pathogenesis. Lipkin is internationally recognized for his work on immune and microbial factors in neurological and neuropsychiatric persistent, non-cytolytic viral infection can cause beha- diseases. In 1988, Lipkin and Oldstone made the seminal observation vioral disorders. that viral infection early in life can cause behavioral and neurotransmitter disturbances without obvious evidence of brain infection or injury. This finding that cryptic infection can influence brain Introduction function has been increasingly recognized as important in pathogenesis Borna Disease Virus (BDV) is an enveloped virus with a non- of neuropsychiatric diseases. Lipkin is at the forefront of Borna virus segmented, negative strand RNA genome. It includes six open research, and currently heads an international multi-center program to reading frames encoding the nucleoprotein (N), p10 (X), use novel methods to assess the role of Borna disease virus in human neuropsychiatric diseases. phosphoprotein (P), matrix protein (M), glycoprotein (G) and the polymerase (L). BDV is neurotropic, targets limbic structures, and causes persistent infection for the lifespan of observed in schizophrenia, mood disorders and autism the host. Natural infection is reported only in horses and through specific effects on neuronal plasticity and cellular sheep; however, experimental infections are described in a signaling pathways. This review highlights recent advances in wide variety of vertebrates and, dependent on the age and BDV research. immune status of the host, might present as florid immune- mediated disease or subtle behavioral alterations without overt inflammation. Rats are the best-characterized animal Main body text models of BDV pathogenesis. Whereas immunocompetent Molecular biology of Borna disease virus adults have meningoencephalitis, neonates have distur- The replication cycle of BDV is illustrated in Fig. 1. BDV bances in learning, mood and behavior reminiscent of those initiates infection (attachment) via binding of its envelope glycoprotein (G) to unidentified cellular receptor (s). After endocytosis the viral ribonucleoprotein complex (RNP) is *Corresponding authors: (M.Schwemmle) [email protected]; (Ian Lipkin) [email protected] released from the vesicles as a result of a pH-dependent 1740-6765/$ ß 2004 Elsevier Ltd. All rights reserved. DOI: 10.1016/j.ddmec.2004.09.001 www.drugdiscoverytoday.com 211 Drug Discovery Today: Disease Mechanisms | Infectious diseases Vol. 1, No. 2 2004 L is regulated by splicing of two introns. Whereas intron 1 is Glossary located within the M-ORF, intron 2 is found within the ORFs IFN: interferons are cytokines that induce multiple biological effects on for G and L [3]. After synthesis, N, P, and X enter the nucleus target cells, including anti-viral, anti-proliferative, and immunomodula- due to the presence of nuclear localization sequences (NLS). tory activities. Reverse genetic system: plasmid-based system to recover negative- The subcellular distribution of M is cytosolic and associated strand RNA polymerase activity or viruses entirely from cDNA. with cellular membranes. G is post-translationally modified by N-glycosylation and undergoes post-translational cleavage by the cellular protease furin; the resulting two protein frag- fusion. The RNP is composed of at least the RNA genome, L, P ments reach the cell surface to participate in the budding and N [1]. Recent crystallographic studies indicate that N process. This process involves the coordinated assembly of forms tetramers in the absence of viral RNA [2]. However, it is the viral RNPs into viral particles by M and the cleavage conceivable that the viral genome is wrapped around con- products of G. The mechanism by which RNP are exported secutive monomers. Replication and transcription of the viral from nucleus remains to be determined but might be genome occurs in the nucleus. Four major subgenomic RNAs mediated by N, as this protein contains a nuclear export are transcribed from three transcription units. Whereas N is sequence (NES) [3]. Budding occurs at the plasma membrane, encoded from a monocistronic transcript, all other proteins although intracellular budding from cytoplasmic membranes are derived from multicistronic RNAs. Expression of M, G and has been described. Remarkably, release of BDV particles from Figure 1. BDV life cycle. After attachment and endocytosis of viral particles, the viral ribonucleoprotein complex (RNP), composed of the viral genome and associated BDV proteins, is released into the cytoplasm as a result of a pH-dependent fusion. Following nuclear import of the viral RNP, replication and transcription results in the continuous supply of the viral genome copies (anti-genomes) that serves again as a template to generate several copies of the viral genome, and spliced and unspliced RNA transcripts that code for 6 viral proteins. Of these proteins, N, P, X and L enter the nucleus to participate in the transcription and replication process. G and M remain in the cytoplasm, where G is post-translationally cleaved by a cellular protease. The two cleaved products participate with M at the plasma membrane in the budding process whereby RNPs are incorporated into viral particles. 212 www.drugdiscoverytoday.com Vol. 1, No. 2 2004 Drug Discovery Today: Disease Mechanisms | Infectious diseases the infected cell is a rare event. Tropism in the brain for or astrocytes are resistant to disease, presumably because of hippocampus and other limbic structures is probably deter- immunological tolerance [5]. Notably, the virus failed to mined at least in part by the distribution of PKC epsilon, the replicate in transgene-expressing granular and pyramidal cellular kinase required for phosphorylation of the BDV P. neurons of the hippocampus (preferred sites of replication) Functional analysis of the BDV polymerase complex has (Fig. 2), a finding consistent with the observation that stoi- been limited. Recently two laboratories established func- chiometry of RNP complex components is crucial to func- tional polymerase assays based on artificial minigenomes tionality. [1,4]. Early studies confirmed that N, P and L represent the Behavioral disturbances are different in adult and neona- active functional polymerase complex, revealed that X has a tally infected rats. Adult rats have a biphasic disorder linked negative regulatory function, and indicated that stoichiome- to abnormalities in dopamine neurotransmitter function. In try is critical to function. Rapid progress in establishment of the acute phase, coinciding with monocyte infiltration into reverse genetic systems (Glossary) seems imminent; these will the brain, rats have exaggerated startle responses and hyper- undoubtedly reveal insights into pathogenesis as well the activity. In the chronic phase, as infiltrates recede, rats show molecular biology of BDV (Outstanding issues). stereotyped motor behaviors. Neonatally infected rats have cerebellar and hippocampal dysgenesis, a wide range of subtle Animal models physiologic and neurobehavioral disturbances including The classic presentation in natural and experimental infec- runting, abnormal taste preferences, altered circadian tion is Borna disease (BD), an immune-mediated potentially rhythms, learning deficits, impaired play, and abnormalities fatal meningoencephaltis. However, the outcome of BD is in serotonin and glutamate systems. BDV is not cytotoxic; variable with age, host genetic factors and neurovirulence of thus, the pathogenesis of neurodevelopmental damage in the the isolate. Studies in rats and mice indicate that immuno- absence of immunopathology remains unclear. However, pathology in classical disease is mediated chiefly by N pro- clues can come from in vitro studies wherein infection impacts tein-specific CD8+ T cells. Interestingly, acute disease