Adsorptive endocytosis of California encephalitis virus into mosquito and mammalian cells: a role for G1

The Orthobunyavirus Germiston Enters Host Cells from Late Endosomes

With more than 80 members worldwide, the Orthobunyavirus genus in the Peribunyaviridae family is a large genus of enveloped RNA viruses, many of which are emerging pathogens in humans and livestock. How orthobunyaviruses (OBVs) penetrate and infect mammalian host cells remains poorly characterized. Here, we investigated the entry mechanisms of the OBV Germiston (GERV). Viral particles were visualized by cryo-electron microscopy and appeared roughly spherical with an average diameter of 98 nm. Labeling of the virus with fluorescent dyes did not adversely affect its infectivity and allowed the monitoring of single particles in fixed and live cells. Using this approach, we found that endocytic internalization of bound viruses was asynchronous and occurred within 30 to 40 min. The virus entered Rab5a-positive (Rab5a+) early endosomes and, subsequently, late endosomal vacuoles containing Rab7a but not LAMP-1. Infectious entry did not require proteolytic cleavage, and endosomal acidification was sufficient and necessary for viral fusion. Acid-activated penetration began 15 to 25 min after initiation of virus internalization and relied on maturation of early endosomes to late endosomes. The optimal pH for viral membrane fusion was slightly below 6.0, and penetration was hampered when the potassium influx was abolished. Overall, our study provides real-time visualization of GERV entry into host cells and demonstrates the importance of late endosomal maturation in facilitating OBV penetration. IMPORTANCE Orthobunyaviruses (OBVs), which include La Crosse, Oropouche, and Schmallenberg viruses, represent a growing threat to humans and domestic animals worldwide. Ideally, preventing OBV spread requires approaches that target early stages of infection, i.e., virus entry. However, little is known about the molecular and cellular mechanisms by which OBVs enter and infect host cells. Here, we developed accurate, sensitive tools and assays to investigate the penetration process of GERV. Our data emphasize the central role of late endosomal maturation in GERV entry, providing a comprehensive overview of the early stages of an OBV infection. Our study also brings a complete toolbox of innovative methods to study each step of the OBV entry program in fixed and living cells, from virus binding and endocytosis to fusion and penetration. The information gained herein lays the foundation for the development of antiviral strategies aiming to block OBV entry.

Aedes albopictus and Cache Valley virus: a new threat for virus transmission in New York State

We report surveillance results of Cache Valley virus (CVV; Peribunyaviridae, Orthobunyavirus) from 2017 to 2020 in New York State (NYS). Infection rates were calculated using the maximum likelihood estimation (MLE) method by year, region, and mosquito species. The highest infection rates were identified among Anopheles spp. mosquitoes and we detected the virus in Aedes albopictus for the first time in NYS. Based on our previous Anopheles quadrimaculatus vector competence results for nine CVV strains, we selected among them three stains for further characterization. These include two CVV reassortants (PA and 15041084) and one CVV lineage 2 strain (Hu-2011). We analyzed full genomes, compared in vitro growth kinetics and assessed vector competence of Aedes albopictus. Sequence analysis of the two reassortant strains (PA and 15041084) revealed 0.3%, 0.4%, and 0.3% divergence; and 1, 10, and 6 amino acid differences for the S, M, and L segments, respectively. We additionally found that the PA strain was attenuated in vertebrate (Vero) and mosquito (C6/36) cell culture. Furthemore, Ae. albopictus mosquitoes are competent vectors for CVV Hu-2011 (16.7–62.1% transmission rates) and CVV 15041084 (27.3–48.0% transmission rates), but not for the human reassortant (PA) isolate, which did not disseminate from the mosquito midgut. Together, our results demonstrate significant phenotypic variability among strains and highlight the capacity for Ae. albopictus to act as a vector of CVV.

Orthobunyaviruses: From Virus Binding to Penetration into Mammalian Host Cells

With over 80 members worldwide, Orthobunyavirus is the largest genus in the Peribunyaviridae family. Orthobunyaviruses (OBVs) are arthropod-borne viruses that are structurally simple, with a trisegmented, negative-sense RNA genome and only four structural proteins. OBVs are potential agents of emerging and re-emerging diseases and overall represent a global threat to both public and veterinary health. The focus of this review is on the very first steps of OBV infection in mammalian hosts, from virus binding to penetration and release of the viral genome into the cytosol. Here, we address the most current knowledge and advances regarding OBV receptors, endocytosis, and fusion.

Detection of Two Highly Diverse Peribunyaviruses in Mosquitoes from Palenque, Mexico

The Peribunyaviridae family contains the genera Orthobunyavirus, Herbevirus, Pacuvirus, and Shangavirus. Orthobunyaviruses and pacuviruses are mainly transmitted by blood-feeding insects and infect a variety of vertebrates whereas herbeviruses and shangaviruses have a host range restricted to insects. Here, we tested mosquitoes from a tropical rainforest in Mexico for infections with peribunyaviruses. We identified and characterized two previously unknown viruses, designated Baakal virus (BKAV) and Lakamha virus (LAKV). Sequencing and de novo assembly of the entire BKAV and LAKV genomes revealed that BKAV is an orthobunyavirus and LAKV is likely to belong to a new genus. LAKV was almost equidistant to the established peribunyavirus genera and branched as a deep rooting solitary lineage basal to herbeviruses. Virus isolation attempts of LAKV failed. BKAV is most closely related to the bird-associated orthobunyaviruses Koongol virus and Gamboa virus. BKAV was successfully isolated in mosquito cells but did not replicate in common mammalian cells from various species and organs. Also cells derived from chicken were not susceptible. Interestingly, BKAV can infect cells derived from a duck species that is endemic in the region where the BKAV-positive mosquito was collected. These results suggest a narrow host specificity and maintenance in a mosquito–bird transmission cycle.

Throw out the Map: Neuropathogenesis of the Globally Expanding California Serogroup of Orthobunyaviruses

The California serogroup (CSG) comprises 18 serologically and genetically related mosquito-borne orthobunyaviruses. Of these viruses, at least seven have been shown to cause neurological disease in humans, including the leading cause of pediatric arboviral encephalitis in the USA, La Crosse virus. Despite the disease burden from these viruses, much is still unknown about the CSG viruses. This review summarizes our current knowledge of the CSG viruses, including human disease and the mechanisms of neuropathogenesis.

Isolation and Identification of a Highly Divergent Kaeng Khoi Virus from Bat Flies (Eucampsipoda sundaica) in China

BACKGROUND

Kaeng Khoi virus (KKV), which belongs to the genus Orthobunyavirus, family Perbunyaviridae, was originally isolated from the brain tissue of bats and may cause infection in humans. In this study, the KKV strain WDBC1403 was isolated from bat flies (Eucampsipoda sundaica), ectoparasites of the bat Rousettus leschenaultia, collected from Yunnan Province of China at the Sino-Burmese border.

METHODS AND RESULTS

The bat fly specimens were ground and inoculated in culture cells. The WDBC1403 strain was shown to induce cytopathic effects in Vero, baby hamster kidney (BHK-21), and Tb1Lu cells, but not in C6/36 cells; however, viral gene amplification was detected in the supernatants of C6/36 cells. Using electron microscopy, the virus was determined to be spherical, enveloped, and 80-90 nm in diameter; it was also shown to form plaques in BHK-21 cells and the titer reached 1 × 10^6.57 plaque-forming units/mL 24 h after infection. The phylogenetic analysis showed that WDBC1403 is a KKV strain, but is independent of the original KKV strain (PSC-19). Viral genome analysis revealed that the nucleotide and amino acid sequence identities of the S, M, and L segments of WDBC1403 with PSC-19 were 88.2% and 96.1%, 76.7% and 85.0%, and 78.3% and 88.9%, respectively. Two amino acids were removed at the end of the open reading frame of the M segment, and 47 nucleotides were removed in the 3'-untranslated region of the M segment of the WDBC1403 strain compared with the PSC-19 strain.

CONCLUSIONS

The WDBC1403 strain is a highly divergent KKV strain, suggesting that there are a variety of KKV strains that exhibit molecular differences. Moreover, because of the large variations in nucleotide and amino acid sequence in the M segment, which encodes an important membrane protein, further research on antigenicity and pathogenicity in humans and animals is needed.

Bunyaviridae RdRps: structure, motifs, and RNA synthesis machinery

Bunyaviridae family is the largest and most diverse family of RNA viruses. It has more than 350 members divided into five genera: Orthobunyavirus, Phlebovirus, Nairovirus, Hantavirus, and Tospovirus. They are present in the five continents, causing recurrent epidemics, epizootics, and considerable agricultural loss. The genome of bunyaviruses is divided into three segments of negative single-stranded RNA according to their relative size: L (Large), M (Medium) and S (Small) segment. Bunyaviridae RNA-dependent RNA polymerase (RdRp) is encoded by the L segment, and is in charge of the replication and transcription of the viral RNA in the cytoplasm of the infected cell. Viral RdRps share a characteristic right hand-like structure with three subdomains: finger, palm, and thumb subdomains that define the formation of the catalytic cavity. In addition to the N-terminal endonuclease domain, eight conserved motifs (A-H) have been identified in the RdRp of Bunyaviridae. In this review, we have summarized the recent insights from the structural and functional studies of RdRp to understand the roles of different motifs shared by RdRps, the mechanism of viral RNA replication, genome segment packaging by the nucleoprotein, cap-snatching, mRNA transcription, and other RNA mechanisms of bunyaviruses.

Early Bunyavirus-Host Cell Interactions

The Bunyaviridae is the largest family of RNA viruses, with over 350 members worldwide. Several of these viruses cause severe diseases in livestock and humans. With an increasing number and frequency of outbreaks, bunyaviruses represent a growing threat to public health and agricultural productivity globally. Yet, the receptors, cellular factors and endocytic pathways used by these emerging pathogens to infect cells remain largely uncharacterized. The focus of this review is on the early steps of bunyavirus infection, from virus binding to penetration from endosomes. We address current knowledge and advances for members from each genus in the Bunyaviridae family regarding virus receptors, uptake, intracellular trafficking and fusion.

Orthobunyaviruses: recent genetic and structural insights

Orthobunyaviruses, which have small, tripartite, negative-sense RNA genomes and structurally simple virions composed of just four proteins, can have devastating effects on human health and well-being, either by causing disease in humans or by causing disease in livestock and crops. In this Review, I describe the recent genetic and structural advances that have revealed important insights into the composition of orthobunyavirus virions, viral transcription and replication and viral interactions with the host innate immune response. Lastly, I highlight outstanding questions and areas of future research.

Akabane virus utilizes alternative endocytic pathways to entry into mammalian cell lines

The entry mechanisms of Akabane virus (AKAV), Bunyaviridae family, have not yet been determined. In this study, chemical inhibitors were used to analyze endocytic mechanisms during AKAV infection of mammalian cell lines. The analyses using drug treatments followed by quantitative measurement of viral RNA and N protein revealed that AKAV enters non-bovine-derived cell lines (Vero, HmLu-1 and BHK cells) in a manner indicative of clathrin endocytosis. By contrast, AKAV infection in bovine-derived cell lines (LB9.K and MDBK cells) is independent of this pathway. Further analyses indicated that AKAV entry into bovine cell lines involves a non-clathrin, non-caveolae endocytic pathway that is dependent on dynamin. We conclude that although both cell types require a low pH for AKAV penetration, AKAV utilizes alternative entry pathways into mammalian cell lines.

A rapid method for infectivity titration of Andes hantavirus using flow cytometry

The focus assay is currently the most commonly used technique for hantavirus titer determination. This method requires an incubation time of between 5 and 11 days to allow the appearance of foci after several rounds of viral infection. The following work presents a rapid Andes virus (ANDV) titration assay, based on viral nucleocapsid protein (N) detection in infected cells by flow cytometry. To this end, an anti-N monoclonal antibody was used that was developed and characterized previously. ANDV N could be detected as early as 6 h post-infection, while viral release was not observed until 24-48 h post-infection. Given that ANDV detection was performed during its first round of infection, a time reduction for titer determination was possible and provided results in only two days. The viral titer was calculated from the percentage of N positive cells and agreed with focus assay titers. Furthermore, the assay was applied to quantify the inhibition of ANDV cell entry by patient sera and by preventing endosome acidification. This novel hantavirus titration assay is a highly quantitative and sensitive tool that facilitates infectivity titration of virus stocks, rapid screening for antiviral drugs, and may be further used to detect and quantify infectious virus in human samples.

De novo transcriptome sequencing in Frankliniella occidentalis to identify genes involved in plant virus transmission and insecticide resistance

The western flower thrips (WFT), Frankliniella occidentalis, a world-wide invasive insect, causes agricultural damage by directly feeding and by indirectly vectoring Tospoviruses, such as Tomato spotted wilt virus (TSWV). We characterized the transcriptome of WFT and analyzed global gene expression of WFT response to TSWV infection using Illumina sequencing platform. We compiled 59,932 unigenes, and identified 36,339 unigenes by similarity analysis against public databases, most of which were annotated using gene ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) analysis. Within these annotated transcripts, we collected 278 sequences related to insecticide resistance. GO and KEGG analysis of different expression genes between TSWV-infected and non-infected WFT population revealed that TSWV can regulate cellular process and immune response, which might lead to low virus titers in thrips cells and no detrimental effects on F. occidentalis. This data-set not only enriches genomic resource for WFT, but also benefits research into its molecular genetics and functional genomics.

Progress in recombinant DNA-derived vaccines for Lassa virus and filoviruses

Developing vaccines for highly pathogenic viruses such as those causing Lassa, Ebola, and Marburg hemorrhagic fevers is a daunting task due to both scientific and logistical constraints. Scientific hurdles to overcome include poorly defined relationships between pathogenicity and protective immune responses, genetic diversity of viruses, and safety in a target population that includes a large number of individuals with compromised immune systems. Logistical obstacles include the requirement for biosafety level-4 containment to study the authentic viruses, the poor public health infrastructure of the endemic disease areas, and the cost of developing these vaccines for use in non-lucrative markets. Recombinant DNA-based vaccine approaches offer promise of overcoming some of these issues. In this review, we consider the status of various recombinant DNA candidate vaccines against Lassa virus and filoviruses which have been tested in animals.

Analysis of expressed sequence tags for Frankliniella occidentalis, the western flower thrips

Thrips are members of the insect order Thysanoptera and Frankliniella occidentalis (the western flower thrips) is the most economically important pest within this order. F. occidentalis is both a direct pest of crops and an efficient vector of plant viruses, including Tomato spotted wilt virus (TSWV). Despite the world-wide importance of thrips in agriculture, there is little knowledge of the F. occidentalis genome or gene functions at this time. A normalized cDNA library was constructed from first instar thrips and 13 839 expressed sequence tags (ESTs) were obtained. Our EST data assembled into 894 contigs and 11 806 singletons (12 700 nonredundant sequences). We found that 31% of these sequences had significant similarity (E< or = 10(-10)) to protein sequences in the National Center for Biotechnology Information nonredundant (nr) protein database, and 25% were functionally annotated using Blast 2GO. We identified 74 sequences with putative homology to proteins associated with insect innate immunity. Sixteen sequences had significant similarity to proteins associated with small RNA-mediated gene silencing pathways (RNA interference; RNAi), including the antiviral pathway (short interfering RNA-mediated pathway). Our EST collection provides new sequence resources for characterizing gene functions in F. occidentalis and other thrips species with regards to vital biological processes, studying the mechanism of interactions with the viruses harboured and transmitted by the vector, and identifying new insect gene-centred targets for plant disease and insect control.

Functional analysis of the Bunyamwera orthobunyavirus Gc glycoprotein

The virion glycoproteins Gn and Gc of Bunyamwera orthobunyavirus (family Bunyaviridae) are encoded by the M RNA genome segment and have roles in both viral attachment and membrane fusion. To investigate further the structure and function of the Gc protein in viral replication, we generated 12 mutants that contain truncations from the N terminus. The effects of these deletions were analysed with regard to Golgi targeting, low pH-dependent membrane fusion, infectious virus-like particle (VLP) formation and virus infectivity. Our results show that the N-terminal half (453 residues) of the Gc ectodomain (909 residues in total) is dispensable for Golgi trafficking and cell fusion. However, deletions in this region resulted in a significant reduction in VLP formation. Four mutant viruses that contained N-terminal deletions in their Gc proteins were rescued, and found to be attenuated to different degrees in BHK-21 cells. Taken together, our data indicate that the N-terminal half of the Gc ectodomain is dispensable for replication in cell culture, whereas the C-terminal half is required to mediate cell fusion. A model for the domain structure of the Gc ectodomain is proposed.

Crimean-Congo hemorrhagic fever virus entry and replication is clathrin-, pH- and cholesterol-dependent

To date, the entry pathway and replication mechanisms for members of the family Bunyaviridae, and especially for Crimean-Congo hemorrhagic fever virus (CCHFV), are poorly understood. Considering the severity of disease and the widespread geographical occurrence of CCHFV, investigating viral entry is of great value for development of antivirals. In this study, we have shown that knockdown of clathrin by small interfering RNA significantly reduced CCHFV nucleocapsid protein and viral RNA levels, suggesting that CCHFV utilizes clathrin-dependent endocytosis. In contrast, caveolin-1, an important constituent of caveolae endocytosis, is not important in CCHFV infection. Moreover, treatment with drugs that are known to interfere with the formation of clathrin-coated pits (sucrose and chlorpromazine) or endosome acidification (bafilomycin A1 and NH(4)Cl) also supported a clathrin-dependent pathway in the entry process of CCHFV. Finally, we demonstrated that cholesterol depletion in the cell plasma membrane significantly inhibited CCHFV infection. In the presence of exogenous cholesterol, this process was reversed, suggesting that cholesterol is important in the life cycle of CCHFV.

Functional entry of dengue virus into Aedes albopictus mosquito cells is dependent on clathrin-mediated endocytosis

Entry of dengue virus 2 (DENV-2) into Aedes albopictus mosquito C6/36 cells was analysed using biochemical and molecular inhibitors, together with confocal and electron microscopy observations. Treatment with monodansylcadaverine, chlorpromazine, sucrose and ammonium chloride inhibited DENV-2 virus yield and protein expression, whereas nystatin, a blocker of caveolae-mediated endocytosis, did not have any effect. Using confocal microscopy, co-localization of DENV-2 E glycoprotein and the marker protein transferrin was observed at the periphery of the cytoplasm. To support the requirement of clathrin function for DENV-2 entry, overexpression of a dominant-negative mutant of Eps15 in C6/36 cells was shown to impair virus entry. The disruption of actin microfilaments by cytochalasin D also significantly affected DENV-2 replication. In contrast, microtubule disruption by colchicine treatment did not impair DENV-2 infectivity, suggesting that DENV-2 does not require transport from early to late endosomes for successful infection of mosquito cells. Furthermore, using transmission electron microscopy, DENV-2 particles of approximately 44-52 nm were found attached within electron-dense invaginations of the plasma membrane and in coated vesicles that resembled those of clathrin-coated pits and vesicles, respectively. Together, these results demonstrate for the first time that DENV-2 enters insect cells by receptor-mediated, clathrin-dependent endocytosis, requiring traffic through an acidic pH compartment for subsequent uncoating and completion of a productive infection.

Insights into bunyavirus architecture from electron cryotomography of Uukuniemi virus

Bunyaviridae is a large family of viruses that have gained attention as "emerging viruses" because many members cause serious disease in humans, with an increasing number of outbreaks. These negative-strand RNA viruses possess a membrane envelope covered by glycoproteins. The virions are pleiomorphic and thus have not been amenable to structural characterization using common techniques that involve averaging of electron microscopic images. Here, we determined the three-dimensional structure of a member of the Bunyaviridae family by using electron cryotomography. The genome, incorporated as a complex with the nucleoprotein inside the virions, was seen as a thread-like structure partially interacting with the viral membrane. Although no ordered nucleocapsid was observed, lateral interactions between the two membrane glycoproteins determine the structure of the viral particles. In the most regular particles, the glycoprotein protrusions, or "spikes," were seen to be arranged on an icosahedral lattice, with T = 12 triangulation. This arrangement has not yet been proven for a virus. Two distinctly different spike conformations were observed, which were shown to depend on pH. This finding is reminiscent of the fusion proteins of alpha-, flavi-, and influenza viruses, in which conformational changes occur in the low pH of the endosome to facilitate fusion of the viral and host membrane during viral entry.

A soluble form of the Tomato spotted wilt virus (TSWV) glycoprotein G(N) (G(N)-S) inhibits transmission of TSWV by Frankliniella occidentalis

Tomato spotted wilt virus (TSWV) is an economically important virus that is transmitted in a persistent propagative manner by its thrips vector, Frankliniella occidentalis. Previously, we found that a soluble form of the envelope glycoprotein G(N) (G(N)-S) specifically bound thrips midguts and reduced the amount of detectable virus inside midgut tissues. The aim of this research was to (i) determine if G(N)-S alters TSWV transmission by thrips and, if so, (ii) determine the duration of this effect. In one study, insects were given an acquisition access period (AAP) with G(N)-S mixed with purified virus and individual insects were assayed for transmission. We found that G(N)-S reduced the percent of transmitting adults by eightfold. In a second study, thrips were given an AAP on G(N)-S protein and then placed on TSWV-infected plant material. Individual insects were assayed for transmission over three time intervals of 2 to 3, 4 to 5, and 6 to 7 days post-adult eclosion. We observed a significant reduction in virus transmission that persisted to the same degree throughout the time course. Real-time reverse transcription polymerase chain reaction analysis of virus titer in individual insects revealed that the proportion of thrips infected with virus was reduced threefold when insects were preexposed to the G(N)-S protein as compared to no exposure to protein, and nontransmitters were not infected with virus. These results demonstrate that thrips transmission of a tospovirus can be reduced by exogenous viral glycoprotein.

Modulation of iridovirus-induced apoptosis by endocytosis, early expression, JNK, and apical caspase

Chilo iridescent virus (CIV) is the type species for the family Iridoviridae, which are large, isometric, cytoplasmic dsDNA viruses. We examined the mechanism of apoptosis induction by CIV. High CIV doses (CIV_XS; 400 μg/ml), UV-irradiated virus (CIV_UV; 10 μg/ml) and CVPE (CIV protein extract; 10 μg/ml) induced apoptosis in 60% of treated Choristoneura fumiferana (IPRI-CF-124T) cells. Normal doses of infectious CIV (10 μg/ml) induced apoptosis in only 10% of C. fumiferana (CF) cells. Apoptosis was inhibited by Z-IETD-FMK, an apical caspase inhibitor, indicating that CIV-induced apoptosis requires caspase activity. The putative caspase in CF cells was designated Cf-caspase-i. CIV_UV or CVPE enhanced Cf-caspase-i activity by 80% at 24 h relative to mock-treated cells. Since the MAP kinase pathway induces or inhibits apoptosis depending on the context, we used JNK inhibitor SP600125 and demonstrated drastic suppression of CVPE-induced apoptosis. Thus, the JNK signaling pathway is significant for apoptosis in this system. Virus interaction with the cell surface was not sufficient for apoptosis since CIV_UV particles bound to polysterene beads failed to induce apoptosis. Endocytosis inhibitors (bafilomycin or ammonium chloride) negated apoptosis induction by CIV_UV, CIV_XS or CVPE indicating that entry through this mode is required. Given the weak apoptotic response to infectious CIV, we postulated that viral gene expression inhibited apoptosis. CIV infection of cells pretreated with cycloheximide induced apoptosis in 69% of the cells compared to 10% in normal infections. Furthermore, blocking viral DNA replication with aphidicolin or phosphonoacetic acid suppressed apoptosis and Cf-caspase-i activity, indicating that early viral expression is necessary for inhibition of apoptosis, and de novo synthesis of viral proteins is not required for induction. We show for the first time that, in a member of the family Iridoviridae, apoptosis: (i) requires entry and endocytosis of virions or virion proteins, (ii) is inhibited under conditions permitting early viral expression, and (iii) requires the JNK signaling pathway. This is the first report of JNK signal requirement during apoptosis induction by an insect virus.

Role of the cytoplasmic tail domains of Bunyamwera orthobunyavirus glycoproteins Gn and Gc in virus assembly and morphogenesis

The M RNA genome segment of Bunyamwera virus (BUNV), the prototype of the Bunyaviridae family, encodes a precursor polyprotein that is proteolytically cleaved to yield two structural proteins, Gn and Gc, and a nonstructural protein called NSm. Gn and Gc are type I integral transmembrane glycoproteins. The Gn protein contains a predicted cytoplasmic tail (CT) of 78 residues, and Gc has a shorter CT of 25 residues. Little is known about the role of the Gn and Gc CT domains in the virus replication cycle. We generated a series of mutant glycoprotein precursor constructs containing either deletions or alanine substitutions in the CT domains of Gn and Gc. We examined the effects of these mutations on glycoprotein maturation, cell surface expression, and low pH-induced syncytium formation. In addition, the effects of these mutations were also assessed using a reverse genetics-based virus assembly assay and a virus rescue system. Our results show that the CT domains of both Gn and Gc play crucial roles in BUNV-mediated membrane fusion, virus assembly, and morphogenesis.

Development and characterization of a Rift Valley fever virus cell-cell fusion assay using alphavirus replicon vectors

Rift Valley fever virus (RVFV), a member of the Phlebovirus genus in the Bunyaviridae family, is transmitted by mosquitoes and infects both humans and domestic animals, particularly cattle and sheep. Since primary RVFV strains must be handled in BSL-3+ or BSL-4 facilities, a RVFV cell–cell fusion assay will facilitate the investigation of RVFV glycoprotein function under BSL-2 conditions. As for other members of the Bunyaviridae family, RVFV glycoproteins are targeted to the Golgi, where the virus buds, and are not efficiently delivered to the cell surface. However, overexpression of RVFV glycoproteins using an alphavirus replicon vector resulted in the expression of the glycoproteins on the surface of multiple cell types. Brief treatment of RVFV glycoprotein expressing cells with mildly acidic media (pH 6.2 and below) resulted in rapid and efficient syncytia formation, which we quantified by β-galactosidase α-complementation. Fusion was observed with several cell types, suggesting that the receptor(s) for RVFV is widely expressed or that this acid-dependent virus does not require a specific receptor to mediate cell–cell fusion. Fusion occurred over a broad temperature range, as expected for a virus with both mosquito and mammalian hosts. In contrast to cell fusion mediated by the VSV-G glycoprotein, RVFV glycoprotein-dependent cell fusion could be prevented by treating target cells with trypsin, indicating that one or more proteins (or protein-associated carbohydrate) on the host cell surface are needed to support membrane fusion. The cell–cell fusion assay reported here will make it possible to study the membrane fusion activity of RVFV glycoproteins in a high-throughput format and to screen small molecule inhibitors for the ability to block virus-specific membrane fusion.

Lyophilic properties of surfactant-rich phases of polyethoxylated alkylphenols formed at cloud point temperature

The influence of the concentration conditions, solutions acidity, and electrolyte additions on the lyophilic properties of the surfactant-rich phases of polyethoxylated alkylphenols OP-7 and OP-10 formed at cloud point temperature were studied. The lyophilic properties of surfactant-rich phases were determined by estimating of their effective hydration values and solvation free energy of methylene and carboxyl groups at cloud point extraction of aliphatic monocarboxylic acids. It was shown that the surfactant-rich phases formed from the dilute surfactant solutions have more hydrophobic properties than the phases formed from the high concentrated solutions. The possibility of changing the lyophilic properties of surfactant-rich phases by electrolyte additions was shown: complex formation between electrolyte cation and the polyoxyethylene chain of the surfactant increases the hydrophilic properties of the surfactant-rich phases. Calculations of the solvation free energy of methylene and carboxylic fragments of the aliphatic carboxylic acids at micellar extraction showed the uniqueness of the surfactant-rich phases which are able to energetically advantageously extract both hydrophilic and hydrophobic molecules of substrates.

Characterization of Maguari orthobunyavirus mutants suggests the nonstructural protein NSm is not essential for growth in tissue culture

Maguari virus (MAGV; genus Orthobunyavirus, family Bunyaviridae) contains a tripartite negative-sense RNA genome. Like all orthobunyaviruses, the medium (M) genome segment encodes a precursor polyprotein (NH(2)-Gn-NSm-Gc-COOH) for the two virion glycoproteins Gn and Gc and a nonstructural protein NSm. The nucleotide sequences of the M segment of wild-type (wt) MAGV, of a temperature-sensitive (ts) mutant, and of two non-ts revertants, R1 and R2, that show electrophoretic mobility differences in their Gc proteins were determined. Twelve amino acid differences (2 in Gn, 10 in Gc) were observed between wt and ts MAGV, of which 9 were maintained in R1 and R2. The M RNA segments of R1 and R2 contained internal deletions, resulting in the removal of the N-terminal 239 residues of Gc (R1) or the C-terminal two thirds of NSm and the N-terminal 431 amino acids of Gc (R2). The sequence data were consistent with analyses of the virion RNAs and virion glycoproteins. These results suggest that neither the N-terminal domain of Gc nor an intact NSm protein is required for the replication of MAGV in tissue culture.

Hantavirus Gc glycoprotein: evidence for a class II fusion protein

Hantavirus cell entry is promoted by its envelope glycoproteins, Gn and Gc, through cell attachment and by fusion between viral and endosomal membranes at low pH. However, the role of Gn and Gc in receptor binding and cell fusion has not yet been defined. In this work, a sequence presenting characteristics similar to those of class II fusion peptides (FPs) of alphavirus E1 and flavivirus E proteins is identified within the hantavirus Gc glycoprotein. A three-dimensional comparative molecular model based on crystallographic data of tick-borne encephalitis virus E protein is proposed for the Andes virus (ANDV) Gc ectodomain, which supports a feasible class II fusion-protein fold. In vitro experimental evidence is provided for the binding activity of the ANDV FP candidate to artificial membranes, as demonstrated by fluorescence anisotropy assays. Taken together, these results support the hypothesis that the Gc glycoprotein of hantaviruses and of other members of the family Bunyaviridae directs the viral fusion activity and that it may be classified as a class II viral fusion protein.

The influence of substrate charge and molecular structure on interphase transfer in cloud point extraction systems

The specificity of interphase transfer of organic reagents of different types between water and the nonionic-surfactant-rich phases at the cloud point temperature was investigated. In contrast to organic solvents, the ability of the micellar phases to extract highly charged ions of organic reagents was shown. Further evidence of the specificity of interphase transfer in micellar-extraction systems is independent and cooperative influence of substrate molecular structure and hydrophobicity on their distribution. The appearance of host-guest phenomenon in cloud point extraction systems that is inherent in organized systems with molecules of the receptors was established. The possibility of describing reagent distribution using regressions that consider substrate hydrophobicity and molecular structure in the cloud point extraction systems was shown.

Tomato spotted wilt virus glycoprotein G(C) is cleaved at acidic pH

Tomato spotted wilt virus (TSWV) is a plant-infecting member of the family Bunyaviridae. TSWV encodes two envelope glycoproteins, G(N) and G(C), which are required for virus infection of the arthropod vector. Other members of the Bunyaviridae enter host cells by pH-dependent endocytosis. During this process, the glycoproteins are exposed to conditions of acidic pH within endocytic vesicles causing the G(C) protein to change conformation. This conformational change renders G(C) more sensitive to protease cleavage. We subjected TSWV virions to varying pH conditions and determined that TSWV G(C), but not G(N), was cleaved under acidic pH conditions, and that this phenomenon did not occur at neutral or alkaline pH. This data provides evidence that G(C) changes conformation at low pH which results in altered protease sensitivity. Furthermore, sequence analysis of G(C) predicts the presence of internal hydrophobic domains, regions that are characteristic of fusion proteins. Like studies with other members of the Bunyaviridae, this study is the first step towards characterizing the nature of cell entry by TSWV.

Tospovirus-thrips interactions

The complex and specific interplay between thrips, tospoviruses, and their shared plant hosts leads to outbreaks of crop disease epidemics of economic and social importance. The precise details of the processes underpinning the vector-virus-host interaction and their coordinated evolution increase our understanding of the general principles underlying pathogen transmission by insects, which in turn can be exploited to develop sustainable strategies for controlling the spread of the virus through plant populations. In this review, we focus primarily on recent progress toward understanding the biological processes and molecular interactions involved in the acquisition and transmission of Tospoviruses by their thrips vectors.

The plant virus Tomato Spotted Wilt Tospovirus activates the immune system of its main insect vector, Frankliniella occidentalis

Tospoviruses have the ability to infect plants and their insect vectors. Tomato spotted wilt virus (TSWV), the type species in the Tospovirus genus, infects its most important insect vector, Frankliniella occidentalis, the western flower thrips (WFT). However, no detrimental effects on the life cycle or cytopathological changes have been reported in the WFT after TSWV infection, and relatively few viral particles can be observed even several days after infection. We hypothesized that TSWV infection triggers an immune response in the WFT. Using subtractive cDNA libraries to probe WFT DNA macroarrays, we found that the WFT's immune system is activated by TSWV infection. The activated genes included (i) those encoding antimicrobial peptides, such as defensin and cecropin; (ii) genes involved in pathogen recognition, such as those encoding lectins; (iii) those encoding receptors that activate the innate immune response, such as Toll-3; and (iv) those encoding members of signal transduction pathways activated by Toll-like receptors, such as JNK kinase. Transcriptional upregulation of these genes after TSWV infection was confirmed by Northern analysis, and the kinetics of the immune response was measured over time. Several of the detected genes were activated at the same time that viral replication was first detected by reverse transcription-PCR. To our knowledge, this is the first report of the activation of an insect vector immune response by a plant virus. The results may lead to a better understanding of insects' immune responses against viruses and may help in the future development of novel control strategies against plant viruses, as well as human and animal viruses transmitted by insect vectors.

The two envelope membrane glycoproteins of Tomato spotted wilt virus show differences in lectin-binding properties and sensitivities to glycosidases

Tomato spotted wilt virus (TSWV, Genus: Tospovirus, Family: Bunyaviridae) is a major constraint to the production of several different crops of agronomic and horticultural importance worldwide. The amino acid sequence of the two envelope membrane glycoproteins, designated as G(N) (N-terminal) and G(C) (C-terminal), of TSWV contain several tripeptide sequences, Asn-Xaa-Ser/Thr, suggesting that the proteins are N-glycosylated. In this study, the lectin-binding properties of the viral glycoproteins and their sensitivities to glycosidases were examined to obtain information on the nature of potential oligosaccharide moieties present on G(N) and G(C). The viral proteins were separated by sodium dodecyl sulphate-polyacrylamide gel electrophoresis (SDS-PAGE) and probed by affinoblotting using a battery of biotinylated lectins with specificity to different oligosaccharide structures. G(C) showed strong binding with five mannose-binding lectins, four N-acetyllactosamine-binding lectins and one fucose-binding lectin. G(N) was resolved into two molecular masses and only the slow migrating form showed binding, albeit to a lesser extent than G(C), with three of the five mannose-binding lectins. The N-acetyllactosamine- and fucose-specific lectins did not bind to either molecular mass form of G(N). None of the galactose-, N-acetylgalactosamine-, or sialic acid-binding lectins tested showed binding specificity to G(C) or G(N). Treatment of the denatured virions with endoglycosidase H and peptide:N-glycosidase F (PNGase F) resulted in a significant decrease in the binding of G(C) to high mannose- and N-acetyllactosamine-specific lectins. However, no such differences in lectin binding were apparent with G(N). These results indicate the presence of N-linked oligosaccharides of high mannose- and complex-type on G(C) and possibly high mannose-type on G(N). Differences in the extent of binding of the two envelope glycoproteins to different lectins suggest that G(C) is likely to be more heavily N-glycosylated than G(N). No evidence was observed for the presence of O-linked oligosaccharides on G(N) or G(C).

Cloning and characterization of a Na+-driven anion exchanger (NDAE1). A new bicarbonate transporter

Regulation of intra- and extracellular ion activities (e.g. H(+), Cl(-), Na(+)) is key to normal function of the central nervous system, digestive tract, respiratory tract, and urinary system. With our cloning of an electrogenic Na(+)/HCO(3)(-) cotransporter (NBC), we found that NBC and the anion exchangers form a bicarbonate transporter superfamily. Functionally three other HCO(3)(-) transporters are known: a neutral Na(+)/ HCO(3)(-) cotransporter, a K(+)/ HCO(3)(-) cotransporter, and a Na(+)-dependent Cl(-)-HCO(3)(-) exchanger. We report the cloning and characterization of a Na(+)-coupled Cl(-)-HCO(3)(-) exchanger and a physiologically unique bicarbonate transporter superfamily member. This Drosophila cDNA encodes a 1030-amino acid membrane protein with both sequence homology and predicted topology similar to the anion exchangers and NBCs. The mRNA is expressed throughout Drosophila development and is prominent in the central nervous system. When expressed in Xenopus oocytes, this membrane protein mediates the transport of Cl(-), Na(+), H(+), and HCO(3)(-) but does not require HCO(3)(-). Transport is blocked by the stilbene 4,4'-diisothiocyanodihydrostilbene- 2, 2'-disulfonates and may not be strictly electroneutral. Our functional data suggest this Na(+) driven anion exchanger (NDAE1) is responsible for the Na(+)-dependent Cl(-)-HCO(3)(-) exchange activity characterized in neurons, kidney, and fibroblasts. NDAE1 may be generally important for fly development, because disruption of this gene is apparently lethal to the Drosophila larva.

Simplified, rapid method for cloning of virus-binding polypeptides (putative receptors) via the far-western screening of a cDNA expression library using purified virus particles

A simplified, alternative method for cloning virus-binding polypeptides (receptor candidates) is described. The method is based on a far-Western assay using purified tomato spotted wilt tospovirus (TSWV, Bunyaviridae) for screening a lambda-phage cDNA expression library. The western flower thrips, Frankliniella occidentalis Pergande, the principal vector of TSWV, in which the virus replicates, was used for library construction. Using this method several virus-binding polypeptides were identified, it eliminated the need for (a) a cellular infection or binding system, (b) the identification, cloning and expression of a functional viral attachment protein, or (c) the purification of the virus receptor. Using this method, virus-binding polypeptides can be selected and cloned in a very short period of time and used in subsequent experiments for determination of their biological relevance as virus receptors and/or tested for potential usefulness as inhibitors of virus transmission and/or infection.

Immunoprecipitation of a 50-kDa protein: a candidate receptor component for tomato spotted wilt tospovirus (Bunyaviridae) in its main vector, Frankliniella occidentalis

A 50-kDa protein that binds to viral particles in solid-phase assays and that is recognized by anti-idiotypic antibodies made against anti-viral glycoproteins G1/G2 (anti-Ids) has been proposed as a receptor candidate for tomato spotted wilt tospovirus (TSWV) in its main thrips vector, Frankliniella occidentalis Pergande (Bandla et al., 1998. Phytopathology 88, 98-104). Here we show the immunoprecipitation of the 50-kDa protein by anti-Ids and by an anti-G1/G2-TSWV conjugate - a new immunoprecipitation method. In addition, we show that anti-Ids made against anti-G1 (anti-IdG1) block virus replication in an insect tissue replication assay. The results indicate that (a) the TSWV-50-kDa protein interaction occurs in solution, as it must do in vivo; (b) G1 is a viral attachment protein; and (c) the 50-kDa protein is a candidate host factor essential for TSWV entry. These results provide additional support for the role of the 50-kDa thrips protein as a viral receptor. Additionally these experiments provide the basis for testing saturable binding and represent an important step toward the first cloning and identification of a cellular receptor for a plant virus.

Genetics of mosquito vector competence

Mosquito-borne diseases are responsible for significant human morbidity and mortality throughout the world. Efforts to control mosquito-borne diseases have been impeded, in part, by the development of drug-resistant parasites, insecticide-resistant mosquitoes, and environmental concerns over the application of insecticides. Therefore, there is a need to develop novel disease control strategies that can complement or replace existing control methods. One such strategy is to generate pathogen-resistant mosquitoes from those that are susceptible. To this end, efforts have focused on isolating and characterizing genes that influence mosquito vector competence. It has been known for over 70 years that there is a genetic basis for the susceptibility of mosquitoes to parasites, but until the advent of powerful molecular biological tools and protocols, it was difficult to assess the interactions of pathogens with their host tissues within the mosquito at a molecular level. Moreover, it has been only recently that the molecular mechanisms responsible for pathogen destruction, such as melanotic encapsulation and immune peptide production, have been investigated. The molecular characterization of genes that influence vector competence is becoming routine, and with the development of the Sindbis virus transducing system, potential antipathogen genes now can be introduced into the mosquito and their effect on parasite development can be assessed in vivo. With the recent successes in the field of mosquito germ line transformation, it seems likely that the generation of a pathogen-resistant mosquito population from a susceptible population soon will become a reality.