Molecular basis of adaptation of Borna disease virus to a new host species
Final Report Abstract
Our project was based on the hypothesis that the host range of BDV in rodents is primarily determined by the activity profile of the viral polymerase, and that interactions of viral surface molecules with cellular receptors play a secondary and more minor role. We postulated that the adapted viral polymerase has either altered intrinsic activity or else can better interact with yet unrecognized species-specific host factors. A first goal of our research project was to determine which adaptive mutations are most important for the observed host range change of BDV from rat to mouse. We wanted to know (i) which of the five amino acid changes found in a mouse-adapted virus variant are necessary and sufficient for BDV replication in the mouse brain, (ii) whether independent attempts to obtain BDV variants with enhanced virulence in mice would again yield variants with alterations in the viral polymerase complex, (iii) how host range viruses would behave in cell culture, (iv) whether whole animal-adapted BDV variants would also replicate more efficiently in mouse cell cultures than the parental virus, (v) whether the variants would grow equally well in highly susceptible monkey Vero or rat C6 cells, and (vi) whether the adapted viruses are genetically stable upon continued passage in mice or whether they would revert upon subsequent passage in rats. A second goal was to determine how the adaptive mutations might confer enhanced virulence to BDV in the new host. Specifically, we asked (i) whether the observed adaptive mutations might generate viral polymerase complexes with intrinsically enhanced transcription and replication activity, (ii) whether the observed adaptive mutations would influence the strength of the interaction between the various components of the viral polymerase complex, (iii) whether the polymerase complex of adapted viruses might interact more strongly than parental virus with yet unrecognized species-specific host factors of mouse cells, and (vi) whether the mutant polymerase complex would evade the innate antiviral response of the mouse more efficiently than parental virus. Overall, the project was highly successful. It has yielded two published articles (Ackermann et al., J. Virol. (2007); Ackermann et al., J. Gen. Virol. (2007)) and one manuscript that is currently under evaluation by the Journal of Virology. The two papers that were published in 2007 provided answers to all questions listed above except for the last two. The question of whether the polymerase complex of adapted viruses might interact more strongly than parental virus with yet unrecognized species-specific host factors of mouse cells remained unanswered. We recovered recombinant BDV from cDNA encoding different purification tags fused to the C-terminus of the L protein. However, these tags did not support the pull down of interacting host cell factors from infected cells or from cells transfected with expression vectors encoding the tagged L proteins. We also tested internal sites for the insertion of the tags into L, but these sites all interfered with L protein function and virus viability. Due to these immense technical obstacles the project was not further pursued. The last question whether the mutant polymerase complex would evade the innate antiviral response of the mouse more efficiently than parental virus has been addressed carefully. However, our experiments revealed no evidence in favor of our hypothesis. Specifically, we observed that the adapted viruses had not gained the ability to grow in mouse fibroblasts from wild-type mice. Like parental virus, the adapted variants exclusively grew in cells from mice that lack functional receptors for interferon-alpha/beta. Due to these insights, this particular part of the project was not pursued.
Publications
- Enhanced polymerase activity confers replication competence of Borna disease virus in mice. J. Gen. Virol. 88: 3130-3132 (2007)
Ackermann, A., Kugel, D., Schneider, U. & P. Staeheli
- Reduced negative regulation of the viral polymerase complex confers replication-competence of Borna disease virus in new host species. J. Virol. 81: 7933-7940 (2007)
Ackermann, A., Staeheli, P. & U. Schneider
