Generation of long flavivirus expression cassettes by in vivo recombination and transient dominant selection

Development of a PCR-Based Reverse Genetics System for an Attenuated Duck Tembusu Virus Strain

The infectious disease caused by the duck Tembusu virus (DTMUV) has resulted in massive economic losses to the Chinese duck industry in China since 2010. Research on the molecular basis of DTMUV pathogenicity has been hampered by the lack of a reliable reverse genetics system for this virus. Here we developed a PCR-based reverse genetics system with high fidelity for the attenuated DTMUV strain FX2010-180P. The rescued virus was characterized by using both indirect immunofluorescence assays (IFA) and whole genome sequencing. The rescued virus (rFX2010-180P) grew to similar titers as compared with the wild-type virus in DF-1 cells, and had similar replication and immunogenicity properties in ducks. To determine whether exogenous proteins could be expressed from DTMUV, both an internal ribosomal entry site (IRES) and the enhanced green fluorescent protein (eGFP) gene were introduced between the NS5 gene and the 3' non-coding sequence of FX2010-180P. A recombinant DTMUV expressing eGFP was rescued, but eGFP expression was unstable after 4 passages in DF-1 cells due to a deletion of 1,294 nucleotides. The establishment of a reliable reverse genetics system for FX2010-180P provides a foundation for future studies of DTMUV.

A PCR-based protocol for generating West Nile virus replicons

A new protocol for the generation of West Nile virus (WNV) replicons was developed. Fragmented cDNAs that covered the entire WNV RNA sequence, except the sequence corresponding to nucleotides 190-2379, were amplified separately by polymerase chain reactions (PCRs) using primer set franking with overlapping sequences of 40-50 bp at the 5'- and the 3'-ends of each fragment. All amplified fragments were mixed together and annealed to each other at the overlapping sequences. The annealed-DNA fragments were elongated by DNA polymerase and amplified by short-cycle PCRs to generate full-sized WNV replicon cDNAs. The WNV replicons were transcribed in vitro using the replicon cDNAs as templates. When the in vitro-transcribed replicon was introduced into mammalian cells, the viral envelope protein and viral positive- and negative-strand RNAs were detected in the replicon-transfected cells. It is noteworthy that the synthesis of the replicon cDNAs and the replicons took just 1 week, and that the use of a high-fidelity DNA polymerase afforded stability to the sequence of the synthetic replicon.

Upregulation of signalase processing and induction of prM-E secretion by the flavivirus NS2B-NS3 protease: roles of protease components

Recently, we have shown that the ability of the flavivirus NS2B-NS3 protease complex to promote efficient signalase processing of the C-prM precursor, as well as secretion of prM and E, does not appear to depend strictly on cleavage of the precursor at its Lys-Arg-Gly dibasic site by the protease. We suggested that the association of the protease with the precursor via NS2B may be sufficient by itself for the above effects. To study the proposed association in more detail, we have developed an assay in which processing at the C-prM dibasic cleavage site is abolished by Lys-->Gly conversion. We constructed deletion mutants and chimeras of the West Nile (WN) flavivirus NS2B protein and expressed them in the context of [5'-C-->NS3(243)] containing either wild-type C-prM or its cleavage site mutant. All NS2B variants were able to form active protease complexes. Deletion of the carboxy-terminal cluster of hydrophobic amino acids in NS2B had no apparent effect on the formation of prM and prM-E secretion for the cassettes containing either wild-type or mutated C-prM precursor. Deletion of the amino-terminal hydrophobic cluster in NS2B did not affect prM-E secretion for the cassettes with wild-type C-prM but abrogated prM-E secretion for the cassettes with the mutated dibasic cleavage site in C-prM. Similarly, the NS2B-NS3(178) protease of Japanese encephalitis (JE) virus, when substituted for the WN virus NS2B-NS3(243) protease, was able to promote prM-E secretion for the cassette with the wild-type C-prM precursor but not with the mutated one. Replacement of the deleted amino-terminal hydrophobic cluster in the WN virus NS2B protein with an analogous JE virus sequence restored the ability of the protease to promote prM-E secretion. On the basis of these observations, roles of individual protease components in upregulation of C-prM signalase processing are discussed.

Formation of the flavivirus envelope: role of the viral NS2B-NS3 protease

One of the late processing events in the flavivirus replication cycle involves cleavage of the intracellular form of the flavivirus capsid protein (Cint) to the mature virion form (Cvir) lacking the carboxy-terminal stretch of hydrophobic amino acids which serves as a signal peptide for the downstream prM protein. This cleavage event was hypothesized to be effected by a viral protease and to be associated with virion formation. We have proposed a model of flavivirus virion formation in which processing of the C-prM precursor at the upstream signalase site is upregulated by interaction of the NS2B part of the protease with the prM signal peptide or with an adjacent carboxy-terminal region of the capsid protein in the precursor, and processing of Cint by the NS2B-NS3 protease follows the signalase cleavage. Recently, an alternative hypothesis was proposed which suggests a reverse order of these two cleavage events, namely, that cleavage of the C-prM precursor by the NS2B-NS3 protease at the Cint-->Cvir dibasic cleavage site is a prerequisite for the subsequent signalase cleavage of the prM signal peptide. To distinguish between these alternative models, we prepared a series of expression cassettes carrying mutations at the Cint-->Cvir dibasic cleavage site and investigated the effects of these mutations on signalase processing of C-prM and on formation and secretion of prM-E heterodimers. For certain mutated C-prM precursors, namely, for those with Lys-->Gly disruption of the dibasic site, efficient formation of prM was observed upon expression from larger cassettes encoding the viral protease, despite the absence of processing at the Cint-->Cvir cleavage site. Surprisingly, formation and secretion of prM-E heterodimers accompanied by late cleavage of prM was also observed for these cassettes, with an efficiency comparable to that of the wild-type expression cassette. These observations contradict the model in which cleavage of the C-prM precursor at the Cint-->Cvir dibasic site is a prerequisite for signalase cleavage.