Autographa californica M nucleopolyhedrovirus early GP64 synthesis mitigates developmental resistance in orally infected noctuid hosts

Live imaging of baculovirus infection of midgut epithelium cells: a functional assay of per os infectivity factors

The occlusion-derived viruses (ODVs) of baculoviruses are responsible for oral infection of insect hosts, whereas budded viruses (BVs) are responsible for systemic infection within the host. The ODV membrane proteins play crucial roles in mediating virus entry into midgut epithelium cells to initiate infection and are important factors in host-range determination. For Autographa californica multiple nucleopolyhedrovirus (AcMNPV), seven conserved ODV membrane proteins have been shown to be essential for oral infectivity and are called per os infectivity factors (PIFs). Information on the function of the individual PIF proteins in virus entry is limited, partly due to the lack of a good in vitro system for monitoring ODV entry. Here, we constructed a baculovirus with EGFP fused to the nucleocapsid to monitor virus entry into primary midgut epithelium cells ex vivo using confocal fluorescence microscopy. The EGFP-labelled virus showed similar BV virulence and ODV infectivity as WT virus. The ability to bind and enter host cells was then visualized for WT AcMNPV and viruses with mutations in P74 (PIF0), PIF1 or PIF2, showing that P74 is required for ODV binding, whilst PIF1 and PIF2 play important roles in the entry of ODV after binding to midgut cells. This is the first live imaging of ODV entry into midgut cells and complements the genetic and biochemical evidence for the role of PIFs in the oral infection process.

Baculovirus nucleocapsid aggregation (MNPV vs SNPV): an evolutionary strategy, or a product of replication conditions?

Lepidopteran nucleopolyhedroviruses are members of the Baculoviridae and have been categorized as having two morphotypes of occluded virions: multiple nucleocapsids or single nucleocapsids within the virion envelope. Although it is a definitive characteristic of specific viruses, it appears to lack a defined genetic basis and is independent of virus phylogeny. This review summarizes the factors that appear to influence this trait and the role that it may play in virus biology.

Immediate-early protein ME53 forms foci and colocalizes with GP64 and the major capsid protein VP39 at the cell membranes of Autographa californica multiple nucleopolyhedrovirus-infected cells

me53 is an immediate-early/late gene found in all lepidopteran baculoviruses sequenced to date. Deletion of me53 results in a greater-than-1,000-fold reduction in budded-virus production in tissue culture (J. de Jong, B. M. Arif, D. A. Theilmann, and P. J. Krell, J. Virol. 83:7440-7448, 2009). We investigated the localization of ME53 using an ME53 construct fused to green fluorescent protein (GFP). ME53:GFP adopted a primarily cytoplasmic distribution at early times postinfection and a primarily nuclear distribution at late times postinfection. Additionally, at late times ME53:GFP formed distinct foci at the cell periphery. These foci colocalized with the major envelope fusion protein GP64 and frequently with VP39 capsid protein, suggesting that these cell membrane regions may represent viral budding sites. Deletion of vp39 did not influence the distribution of ME53:GFP; however, deletion of gp64 abolished ME53:GFP foci at the cell periphery, implying an association between ME53 and GP64. Despite the association of ME53 and GP64, ME53 fractionated with the nucleocapsid only after budded-virus fractionation. Together these findings suggest that ME53 may be providing a scaffold that bridges the viral envelope and nucleocapsid.

Comparative transient expression assay analysis of hycu-hr6- and IE1-dependent regulation of baculovirus gp64 early promoters in three insect cell lines

We previously demonstrated that the Hyphantria cunea multicapsid nucleopolyhedrovirus (HycuMNPV) gp64 gene (hycu-gp64) is uniquely localized on the viral genome with a large homologous region of 1582bp, hycu-hr6, immediately upstream of the hycu-gp64 gene. In the present study, we compared the regulation of gp64 early promoters from HycuMNPV, Autographa californica multicapsid NPV (AcMNPV) and Bombyx mori NPV (BmNPV) by cis-acting hycu-hr6 and trans-acting IE1s in three cell lines (Spodoptera frugiperda Sf9, Bombyx mori BM-N and Spilosoma imparilis SpIm). A transient expression assay with plasmids harboring a reporter luciferase gene demonstrated that the gp64 early promoters are positively regulated by hycu-hr6, independent of virus and cell types. In contrast, gp64 early promoters were regulated positively or negatively by trans-acting IE1s, in a cell- and virus-type dependent manner, indicating that cellular factors, as well as viral factors, are responsible for IE1-dependent regulation of gp64 early promoters. However, hycu-gp64 early promoter activity was consistently suppressed by HycuMNPV IE1 (Hycu-IE1), irrespective of the cell lines used. Analysis of the hycu-gp64 early promoter region revealed two novel sequence elements that were involved in Hycu-IE1-dependent negative regulation of the hycu-gp64 early promoter. These two novel regulatory sequence elements could compensate for each other but could not be substituted with AcMNPV IE1 binding motif (Ac-IBM). These results suggest that IE1 regulates gp64 early promoters to produce the proper amount of GP64 protein, depending upon NPV-insect cell systems.

Changes in gene expression in the permissive larval host lightbrown apple moth (Epiphyas postvittana, Tortricidae) in response to EppoNPV (Baculoviridae) infection

Host cell and virus gene expression were measured five days after per os inoculation of 3rd instar lightbrown apple moth (LBAM) larvae with the Epiphyas postvittana nucleopolyhedrovirus (EppoNPV). Microarray analysis identified 84 insect genes that were up-regulated and 18 genes that were down-regulated in virus-infected larvae compared with uninfected larvae. From the 134 viral open reading frames represented on the microarray, 81 genes showed strong expression. Of the 38 functionally identifiable regulated insect genes, 23 coded for proteins that have roles in one of five processes; regulation of transcription and translation, induction of apoptosis, and maintenance of both juvenility and actin cytoskeletal integrity. Of the 34 functionally identifiable viral genes that were most strongly expressed, 12 had functions associated with these five processes, as did a further seven viral genes which were expressed at slightly lower levels. A survey of the LBAM-expressed sequence tag library identified further genes involved in these processes. In total, 135 insect genes and 38 viral genes were analysed by quantitative polymerase chain reaction. Twenty-one insect genes were strongly up-regulated and 31 genes strongly down-regulated. All 38 viral genes examined were highly expressed. These data suggest that induction of apoptosis and regulation of juvenility are the major 'battlegrounds' between virus and insect, with the majority of changes observed representing viral control of insect gene expression. Transcription and translational effects seem to be exerted largely through modulation of mRNA and protein degradation. Examples of attempts by the insect to repel the infection via changes in gene expression within these same processes were, however, also noted. The data also showed the extent to which viral transcription dominated in the infected insects at five days post inoculation.

Cytopathological process by multiple nucleopolyhedrovirus in the testis of Bombyx mori L., 1758 (Lepidoptera: Bombycidae)

A cytopathological methodology was used to analyze infection by Bombyx mori multiple nucleopolyhedrovirus (BmMNPV), a geographic isolate of the family Baculoviridae, in the caterpillar testes of the B. mori. Japanese B. mori strain caterpillar, fifth instar, were inoculated with BmMNPV and their testes were collected and processed for light and transmission electronic microscopy. Epithelial coating cells and interfollicular septa in testes were susceptible to BmMNPV. The first evidence of infection was detected on the 6th day post-inoculation (p.i.) in the external epithelium, and on the 7th day p.i. in the internal epithelium and interfollicular septa. Cytopathological characteristics consisted of hypertrophied nuclei, the formation of virogenic stroma, and the occlusion of virions in polyhedron protein crystals in several stages of development. At the end of the infectious process, cell lysis and release of polyhedra into the extracellular medium occurred. Histopathology revealed early infection foci in the surrounding regions of tracheal insertions, thus underlining the role of the trachea as an infection-spreading organ in insects. This spreading occurs through penetration of the basal lamina, which facilitates entry of the budded virus into the testis. Additionally, an alignment of a partial sequence of the ORF 14 of the BmMNPV geographic isolate with other NPV certified the virus genera.

Analysis of the temporal expression of Trichoplusia ni single nucleopolyhedrovirus genes following transfection of BT1-Tn-5B1-4 cells

Trichoplusia ni single nucleopolyhedrovirus (TnSNPV), a 134,394-bp double-stranded DNA group II Nucleopolyhedrovirus, is pathogenic to the lepidopteran T. ni. TnSNPV transcription is temporally regulated and divided into three promoter sequence-dependent classes (early, late and very late genes). A viral oligonucleotide DNA microarray containing all potential (144) viral genes of TnSNPV was designed to investigate global viral gene expression during cell infection. Total BT1-Tn-5B1-4 cellular mRNAs extracted between 0 and 72 h posttransfection with TnSNPV genomic DNA were hybridized to the microarray. Initial average expression of early genes was detected between 12 and 24 h posttransfection while late genes were mainly detected between 24 and 72 h posttransfection. The microarray expression profiling data verified many computer predicted promoter assignments. K-means clustering was used to sort the 144 genes based on their temporal expression pattern similarities. This clustering resulted in the confirmation and temporal class assignment of previously unidentified genes and promoters.

Effects of Ac150 on virulence and pathogenesis of Autographa californica multiple nucleopolyhedrovirus in noctuid hosts

Ac150 is expressed late during infection of cultured lepidopteran insect cells by Autographa californica multiple nucleopolyhedrovirus. The Ac150 gene product is predicted to have a molecular mass of 11 161 Da and consists of a hydrophobic N terminus and a single ‘peritrophin-A’-like domain, connected by a short region of charged amino acids. An Ac150 deletion mutant and its parental wild-type virus were compared for differences in virulence by both oral and intrahaemocoelic routes of infection. It was found that the mutant was significantly less virulent in larvae of all three host species tested (Heliothis virescens, Spodoptera exigua and Trichoplusia ni) when occlusions were administered orally, but not when isolated occlusion-derived virus (ODV) was administered orally or budded virus was administered intrahaemocoelically. ODV yields were the same from equal numbers of mutant and wild-type occlusions, and nucleocapsid-distribution frequencies within the two ODV populations were the same, eliminating these features as explanations for the observed differences in virulence. Comparison of pathogenesis, as revealed by lacZ expression from identical reporter-gene cassettes in the mutant and wild-type virus, indicated that the mutant was less efficient at establishing primary infection in midgut cells; otherwise, it exhibited infection kinetics identical to those of wild-type virus. Ac150, therefore, can be considered a per os infection factor that mediates, but is not essential for, oral infection.

Spodoptera frugiperda resistance to oral infection by Autographa californica multiple nucleopolyhedrovirus linked to aberrant occlusion-derived virus binding in the midgut

Spodoptera frugiperda larvae are highly resistant to oral infection by Autographa californica multiple nucleopolyhedrovirus (AcMNPV) (LD(50), approximately 9200 occlusions), but extremely susceptible to budded virus within the haemocoel (LD(50), <1 p.f.u.). The inability of AcMNPV occlusion-derived virus (ODV) to establish primary infections readily within midgut cells accounts for a major proportion of oral resistance. To determine whether inappropriate binding of AcMNPV ODV to S. frugiperda midgut cells contributes to lack of oral infectivity, the binding and fusion properties of AcMNPV ODV were compared with those of the ODV of a new isolate of Spodoptera frugiperda multiple nucleopolyhedrovirus (SfMNPV) obtained from a field-collected larva (oral LD(50), 12 occlusions). By using a fluorescence-dequenching assay conducted in vivo, it was found that AcMNPV ODV bound to the midgut epithelia of S. frugiperda larvae at approximately 15 % of the level of SfMNPV ODV, but that, once bound, the efficiencies of fusion for the two ODVs were similar: 60 % for AcMNPV and 53 % for SfMNPV. Whilst the difference in binding efficiencies was significant, it could not account entirely for the observed differences in infectivity. Competition experiments, however, revealed that, in S. frugiperda larvae, SfMNPV ODV bound to a midgut cell receptor that was not bound by AcMNPV ODV, indicating that ODV interaction with a specific receptor(s) was necessary for productive infection of midgut columnar epithelial cells. Fusion in the absence of this ligand-receptor interaction did not result in productive infections.