Surface proteins of C6/36 cells involved in dengue virus 4 binding and entry.

Dengue virus (DENV) is the causative agent of the most important mosquito-borne viral disease, which is endemic to over 100 countries in tropical and subtropical areas of the world. It is transmitted to humans by Aedes mosquitoes. The first step in the viral infection of host cells is virion attachment to the plasma membrane, which is mediated by specific surface molecules. There are several molecules that participate in DENV infection of mosquitoes, but only a few have been identified. In this work, we co-purified 4 proteins from C6/36 cells using a recombinant DENV 4 E protein and identified them as 70 kDa Heat Shock and 70 kDa Heat Shock cognate proteins (HSP70/HSc70), Binding immunoglobulin protein (BiP), Thioredoxin/protein disulphide isomerase (PDI), and 44 kDa Endoplasmic reticulum resident protein (ERp44) via matrix-assisted laser desorption/ionisation time of flight (Maldi-ToF) analysis. Using immunofluorescence and flow cytometry assays, we observed re-localisation of HSP70/HSc70 and, to a lesser extent, BiP to the plasma membrane under stress conditions, such as during DENV infection. By performing binding and infection assays independently, we found that all 4 proteins participate in both processes, but to differing extents: HSP70/HSc70 is the most critical component, while ERp44 is less important. Viral infection was not inhibited when the cells were incubated with antibodies against all of the surface proteins after virus binding, which suggests that DENV entry to C6/36 cells is mediated by these proteins at the same step and not sequentially.

Detection and sequencing of defective viral genomes in C6/36 cells persistently infected with dengue virus 2.

Dengue virus is the most important arbovirus that affects humans, and it can establish persistent infections, especially in insect-derived cell cultures. Defective viral genomes have been implicated in the establishment and maintenance of persistent infections with several flaviviruses; however, there exists almost no information concerning defective dengue virus genomes. Here, we report the detection of defective dengue 2 virus genomes in persistently infected mosquito C6/36 cells. The defective viral genomes were detected at a low ratio compared with the wild-type genome. Deletions of approximately 147 residues (222-368) were found in the E protein, and these mainly affected domain III (73 %) of the protein; deletions of approximately 153 residues (4-156) and 228 residues (597-825) were found in the methyltransferase and polymerase domains, respectively, of the NS5 protein. The truncated versions of NS5 could be detected by western blot only in the protein extracts derived from persistently infected cells.

Characterization of Viral Interference in Aedes albopictus C6/36 Cells Persistently Infected with Dengue Virus 2.

Arboviruses are an important group of pathogens that cause diseases of medical and veterinary concern worldwide. The interactions of these viruses with their host cells are complex, and frequently, the coexistence of two different viruses in the same cell results in the inhibition of replication in one of the viruses, which is a phenomenon called viral interference. This phenomenon can be exploited to develop antiviral strategies. Insect cell lines persistently infected with arboviruses are useful models with which to study viral interference. In this work, a model of C6/36-HT cells (from Aedes albopictus mosquitoes) persistently infected with Dengue virus, serotype 2, was used. Viral interference was evaluated via plaque and flow cytometry assays. The presence of heterotypic interference against the other serotypes of the same virus and homologous interference against yellow fever virus was determined; however, this cell line did not display heterologous viral interference against Sindbis virus. The mechanisms responsible for viral interference have not been fully elucidated, but small RNAs could be involved. However, the silencing of Ago3, a key protein in the genome-derived P-element-induced wimpy testis pathway, did not alter the viral interference process, suggesting that viral interference occurs independent of this pathway.

Differential Gene Expression Pattern of Importin ß3 and NS5 in C6/36 Cells Acutely and Persistently Infected with Dengue Virus 2.

The establishment of persistent dengue virus infection within the cells of the mosquito vector is an essential requirement for viral transmission to a new human host. The mechanisms involved in the establishment and maintenance of persistent infection are not well understood, but it has been suggested that both viral and cellular factors might play an important role. In the present work, we evaluated differential gene expression in Aedes albopictus cells acutely (C6/36-HT) and persistently infected (C6-L) with Dengue virus 2 by cDNA-AFLP. We observed that importin ß3 was upregulated in noninfected cells compared with C6-L cells. Using RT-qPCR and plaque assays, we observed that Dengue virus levels in C6-L cells essentially do not vary over time, and peak viral titers in acutely infected cells are observed at 72 and 120 h postinfection. The expression level of importin ß3 was higher in acutely infected cells than in persistently infected cells; this correlates with higher levels of NS5 in the nucleus of the cell. The differential pattern of importin ß3 expression between acute and persistent infection with Dengue virus 2 could be a mechanism to maintain viral infection over time, reducing the antiviral response of the cell and the viral replicative rate.

Evidence that the 45-kD glycoprotein, part of a putative dengue virus receptor complex in the mosquito cell line C6/36, is a heat-shock related protein.

Dengue virus (DENV) is transmitted to humans by mosquitoes of the genus Aedes. Although several molecules have been described as part of DENV receptor complex in mosquito cells, none of them have been identified. Our group characterized two glycoproteins (40 and 45 kD) as part of the DENV receptor complex in C6/36 cells. Because identification of the mosquito cell receptor has been unsuccessful and some cell receptors described for DENV in mammalian cells are heat-shock proteins (HSPs), the role of HSPs in DENV binding and infection in C6/36 cells was evaluated. Our results indicate that gp45 and a 74-kD molecule (p74), which interact with DENV envelope protein, are immunologically related to HSP90. Although p74 is induced by heat shock, gp45 apparently is not. However, these proteins are relocated to the cell surface after heat-shock treatment, causing an increase in virus binding without any effect on virus yield.

PTB binds to the 3' untranslated region of the human astrovirus type 8: a possible role in viral replication.

The 3' untranslated region (3'UTR) of human astroviruses (HAstV) consists of two hairpin structures (helix I and II) joined by a linker harboring a conserved PTB/hnRNP1 binding site. The identification and characterization of cellular proteins that interact with the 3'UTR of HAstV-8 virus will help to uncover cellular requirements for viral functions. To this end, mobility shift assays and UV cross-linking were performed with uninfected and HAstV-8-infected cell extracts and HAstV-8 3'UTR probes. Two RNA-protein complexes (CI and CII) were recruited into the 3'UTR. Complex CII formation was compromised with cold homologous RNA, and seven proteins of 35, 40, 45, 50, 52, 57/60 and 75 kDa were cross-linked to the 3'UTR. Supermobility shift assays indicated that PTB/hnRNP1 is part of this complex, and 3'UTR-crosslinked PTB/hnRNP1 was immunoprecipitated from HAstV-8 infected cell-membrane extracts. Also, immunofluorescence analyses revealed that PTB/hnRNP1 is distributed in the nucleus and cytoplasm of uninfected cells, but it is mainly localized perinuclearly in the cytoplasm of HAstV-8 infected cells. Furthermore, the minimal 3'UTR sequences recognized by recombinant PTB are those conforming helix I, and an intact PTB/hnRNP1-binding site. Finally, small interfering RNA-mediated PTB/hnRNP1 silencing reduced synthesis viral genome and virus yield in CaCo2 cells, suggesting that PTB/hnRNP1 is required for HAstV replication. In conclusion, PTB/hnRNP1 binds to the 3'UTR HAstV-8 and is required or participates in viral replication.