Binding properties of GP1 protein of Borna disease virus

A single amino acid substitution in the Borna disease virus glycoprotein enhances the infectivity titer of vesicular stomatitis virus pseudotyped virus by altering membrane fusion activity

Borna disease virus 1 (BoDV-1) causes acute fatal encephalitis in mammals, including humans. Despite its importance, research on BoDV-1 cell entry has been hindered by low infectious viral particle production in cells and the lack of cytopathic effects, which are typically useful for screening. To address these issues, we developed a method to efficiently produce vesicular stomatitis virus (VSV) pseudotyped with glycoprotein (G) of members of the genus Orthobornavirus, including BoDV-1. We discovered that optimal G expression is required to obtain a high infectivity titer of the VSV pseudotyped virus. Remarkably, the infectivity of the VSV pseudotyped virus with G from the BoDV-1 strain huP2br was significantly higher than that of the VSV pseudotyped virus with G from the He/80 strain. Mutational analysis demonstrated that the methionine at BoDV-1-G residue 307 increases the infectivity titer of VSV pseudotyped with BoDV-1-G (VSV-BoDV-1-G). A cell‒cell fusion assay indicated that this residue plays a pivotal role in membrane fusion, thus suggesting that high membrane fusion activity and a broad pH range for membrane fusion are crucial for achieving a high infectivity titer of VSV-BoDV-1-G. This finding may be extended to increase the infectivity titer of VSV pseudotyped virus with other orthobornavirus G. Our study also contributes to identifying functional domains of BoDV-1-G and provides insight into G-mediated cell entry.

[Bornaviruses]

Bornaviridae is an enveloped animal virus carrying an 8.9 kb non-segmented, negative-strand RNA genome. The genus bornavirus contains two members infecting vertebrates, Borna disease virus (BDV) and avian bornavirus (ABV), which could preferably infect the nervous systems. BDV causes classical Borna disease, a progressive meningoencephalomyelitis, in horses and sheep, and ABV is known to induce proventricular dilatation disease, a fatal disease characterized by a lymphocytic, plasmacytic inflammation of central and peripheral nervous tissues, in multiple avian species. Recent evidences have demonstrated that bornavirus is unique among RNA viruses as they not only establish a long-lasting, persistent infection in the nucleus, but also integrate their own DNA genome copy into the host chromosome. In this review, I outline the recent knowledge about the unique virological characteristics of bornaviruses, as well as the diseases caused by the infection of BDV and ABV.

A novel borna disease virus vector system that stably expresses foreign proteins from an intercistronic noncoding region

Borna disease virus (BDV), a nonsegmented, negative-strand RNA virus, infects a wide variety of mammalian species and readily establishes a long-lasting, persistent infection in brain cells. Therefore, this virus could be a promising candidate as a novel RNA virus vector enabling stable gene expression in the central nervous system (CNS). Previous studies demonstrated that the 5' untranslated region of the genome is the only site for insertion and expression of a foreign gene. In this study, we established a novel BDV vector in which an additional transcription cassette has been inserted into an intercistronic noncoding region between the viral phosphoprotein (P) and matrix (M) genes. The recombinant BDV (rBDV) carrying green fluorescent protein (GFP) between the P and M genes, rBDV P/M-GFP, expressed GFP efficiently in cultured cells and rodent brains for a long period of time without attenuation. Furthermore, we generated a nonpropagating rBDV, ΔGLLP/M, which lacks the envelope glycoprotein (G) and a splicing intron within the polymerase gene (L), by the transcomplementation system with either transient or stable expression of the G gene. Interestingly, rBDV ΔGLLP/M established a persistent infection in cultured cells with stable expression of GFP in the absence of the expression of G. Using persistently infected rBDV ΔGLLP/M-infected cells, we determined the amino acid region in the cytoplasmic tail (CT) of BDV G important for the release of infectious rBDV particles and also demonstrated that the CT region may be critical for the generation of pseudotyped rBDV having vesicular stomatitis virus G protein. Our results revealed that the newly established BDV vector constitutes an alternative tool not only for stable expression of foreign genes in the CNS but also for understanding the mechanism of the release of enveloped virions.

Molecular chaperone BiP interacts with Borna disease virus glycoprotein at the cell surface

Borna disease virus (BDV) is characterized by highly neurotropic infection. BDV enters its target cells using virus surface glycoprotein (G), but the cellular molecules mediating this process remain to be elucidated. We demonstrate here that the N-terminal product of G, GP1, interacts with the 78-kDa chaperone protein BiP. BiP was found at the surface of BDV-permissive cells, and anti-BiP antibody reduced BDV infection as well as GP1 binding to the cell surface. We also reveal that BiP localizes at the synapse of neurons. These results indicate that BiP may participate in the cell surface association of BDV.