Conditions necessary for inhibition of protein synthesis and production of cytopathic effect in Aedes albopictus cells infected with vesicular stomatitis virus

Ecdysone and the cell cycle: investigations in a mosquito cell line

Cell lines provide a tool for investigating basic biological processes that underlie the complex interactions among the tissues and organs of an intact organism. We compare the evolution of insect and mammalian populations as they progress from diploid cell strains to continuous cell lines, and review the history of the well-characterized Aedes albopictus mosquito cell line, C7-10. Like Kc and S3 cells from Drosophila melanogaster, C7-10 cells are sensitive to the insect steroid hormone, 20-hydroxyecdysone (20E), and express 20E-inducible proteins as well as the EcR and USP components of the ecdysteroid receptor. The decrease in growth associated with 20E treatment results in an accumulation of cells in the G1 phase of the cycle, and a concomitant decrease in levels of cyclin A. In contrast, 20E induces a G2 arrest in a well-studied imaginal disc cell line from the moth, Plodia interpunctella. We hypothesize that 20E-mediated events associated with molting and metamorphosis include effects on regulatory proteins that modulate the mitotic cell cycle and that differences between the 20E response in diverse insect cell lines reflect an interplay between classical receptor-mediated effects on gene expression and non-classical effects on signaling pathways similar to those recently described for the vertebrate steroid hormone, estrogen.

A ribosome-free extract from cultured cells improves recovery of polysomes from the mosquito fat body: analysis of vitellogenin and ribosomal protein rpL34 gene expression

We have examined the association of ribosomal protein rpL34 mRNA with polysomes in Aedes albopictus C7-10 cells in culture using a simple, two-step sucrose gradient. In growing cells, 40-50% of the ribosomes were engaged on polysomes. This proportion could be increased to 80% when metabolism was stimulated by refeeding the cells with fresh medium. Conversely, ribosomes shifted off polysomes when cells were starved with phosphate-buffered saline or cell lysates were treated with puromycin. When similar approaches were used with fat body from blood-fed female Aedes aegypti mosquitoes, we were unable to obtain the polysome fraction that contained vitellogenin mRNA, which is abundantly translated after a blood meal. Addition of post-mitochondrial supernatant from fat body to polysomes from cultured cells shifted the polysome profile towards smaller polysomes and monosomes, in a dose-dependent fashion. Disruption of fat body tissue in a post-ribosomal supernatant from refed cells improved the recovery of polysomes, demonstrating both the engagement of vitellogenin mRNA on polysomes and the mobilization of rpL34 from messenger-ribonuceloprotein particles onto polysomes in blood-fed mosquitoes. These observations suggested that ribonucleases remain active when polysomes are prepared from mosquito fat body, and that cell culture supernatant contains a ribonuclease inhibitor.

Cultured Aedes albopictus mosquito cells accumulate elongation factor-1 alpha (EF-1 alpha) during serum starvation

We examined survival, growth and protein synthesis in mosquito cells that had been maintained for up to 21 days in serum-free medium. On polyacrylamide gels, protein bands from "starved" cells remained discrete, and despite low levels of incorporation, radiolabeled bands were detectable, suggesting that low levels of protein synthesis were sustained. A prominent band that accumulated in serum-starved cells was digested with trypsin and analyzed by tandem mass spectrometry, which identified the protein as eukaryotic elongation factor (EF)-1 alpha EF-1 alpha is well-conserved among species, and differential accumulation of EF-1 alpha in serum-starved cells was verified by western blotting using a primary antibody to the homologous protein from Trypanosoma brucei. Aside from its importance in the elongation step of protein synthesis, EF-1 alpha has been shown to have a number of non-canonical functions, including interaction with viral RNA and a potential role in apoptosis. We anticipate that the prolonged viability of mosquito cells in serum-free medium may provide a system to explore whether EF-1 alpha accumulation is an adaptive response compatible with resumption of growth in the event that nutrients are replenished, or whether the excess EF-1 alpha represents an irreversible commitment to an apoptotic pathway.

Metabolic and morphological changes in A. albopictus cells infected with Mayaro virus under heat-shock conditions

We addressed the question how temperature elevation inhibits Mayaro virus replication in Aedes albopictus infected cells. The morphology and macromolecular changes induced by temperature, infection and high serum concentration were investigated in these cells. Cells incubated with 2 and 10% serum at 28 degrees C disclosed an intense vacuolization and inhibition of [35S]methionine incorporation in a time-dependent manner. 34 and 50 kDa viral structural proteins were detected 24 h after infection. In contrast, an inhibition of viral proteins synthesis occurred when infected cells were kept at 37 degrees C (heat-shock conditions). Total cellular RNA was isolated from mock and infected cells incubated at 28 or 37 degrees C. Northern blot analysis with a Mayaro genomic probe coding for viral structural proteins showed a decrease in the amount of viral 26S RNA in stressed cells when compared to those kept at 28 degrees C. Taken together, these results suggest that the inhibition of viral proteins synthesis in response to temperature elevation is associated with a decrease in the amount of subgenomic 26S RNA.

A mutant of Sindbis virus with altered plaque morphology and a decreased ratio of 26 S:49 S RNA synthesis in mosquito cells

When our stock of standard Sindbis virus (SVSTD) is assayed by plaque formation on Aedes albopictus mosquito cells, about 1-2% of the plaques appear much clearer and sharper than the majority of the plaques. One of these clear plaques was picked, grown into a viral stock (SVCP), and used to prepare viral cDNA. Making use of the infectious Sindbis virus plasmid, Toto 1101 (Rice et al., 1987), we mapped the causal mutation for the clear plaque phenotype to a region between nt 7334 and 7716, and by sequencing of the viral RNA identified a mutation at nucleotide 7592. This mutation lies in the junction region of the viral genome, specifically at nucleotide -6, with reference to the initiation site for 26 S RNA synthesis. In SVCP-infected mosquito cells, but not in SVCP-infected chick cells, the ratio of subgenomic 26 S to 49 S (genomic) RNA synthesis was decreased relative to that observed in SVSTD infected cells. In terms of amino acid coding, the SVCP mutation is silent.