Pathogen-derived resistance to viral infection using a negative regulatory molecule

Dicer-2- and Piwi-mediated RNA interference in Rift Valley fever virus-infected mosquito cells

Rift Valley fever virus (RVFV) is a Phlebovirus (Bunyaviridae family) transmitted by mosquitoes. It infects humans and ruminants, causing dramatic epidemics and epizootics in Africa, Yemen, and Saudi Arabia. While recent studies demonstrated the importance of the nonstructural protein NSs as a major component of virulence in vertebrates, little is known about infection of mosquito vectors. Here we studied RVFV infection in three different mosquito cell lines, Aag2 cells from Aedes aegypti and U4.4 and C6/36 cells from Aedes albopictus. In contrast with mammalian cells, where NSs forms nuclear filaments, U4.4 and Aag2 cells downregulated NSs expression such that NSs filaments were never formed in nuclei of U4.4 cells and disappeared at an early time postinfection in the case of Aag2 cells. On the contrary, in C6/36 cells, NSs nuclear filaments were visible during the entire time course of infection. Analysis of virus-derived small interfering RNAs (viRNAs) by deep sequencing indicated that production of viRNAs was very low in C6/36 cells, which are known to be Dicer-2 deficient but expressed some viRNAs presenting a Piwi signature. In contrast, Aag2 and U4.4 cells produced large amounts of viRNAs predominantly matching the S segment and displaying Dicer-2 and Piwi signatures. Whereas 21-nucleotide (nt) Dicer-2 viRNAs were prominent during early infection, the population of 24- to 27-nt Piwi RNAs (piRNAs) increased progressively and became predominant later during the acute infection and during persistence. In Aag2 and U4.4 cells, the combined actions of the Dicer-2 and Piwi pathways triggered an efficient antiviral response permitting, among other actions, suppression of NSs filament formation and allowing establishment of persistence. In C6/36 cells, Piwi-mediated RNA interference (RNAi) appeared to be sufficient to mount an antiviral response against a secondary infection with a superinfecting virus. This study provides new insights into the role of Dicer and Piwi in mosquito antiviral defense and the development of the antiviral response in mosquitoes.

A host RNA helicase-like protein, AtRH8, interacts with the potyviral genome-linked protein, VPg, associates with the virus accumulation complex, and is essential for infection

The viral genome-linked protein, VPg, of potyviruses is a multifunctional protein involved in viral genome translation and replication. Previous studies have shown that both eukaryotic translation initiation factor 4E (eIF4E) and eIF4G or their respective isoforms from the eIF4F complex, which modulates the initiation of protein translation, selectively interact with VPg and are required for potyvirus infection. Here, we report the identification of two DEAD-box RNA helicase-like proteins, PpDDXL and AtRH8 from peach (Prunus persica) and Arabidopsis (Arabidopsis thaliana), respectively, both interacting with VPg. We show that AtRH8 is dispensable for plant growth and development but necessary for potyvirus infection. In potyvirus-infected Nicotiana benthamiana leaf tissues, AtRH8 colocalizes with the chloroplast-bound virus accumulation vesicles, suggesting a possible role of AtRH8 in viral genome translation and replication. Deletion analyses of AtRH8 have identified the VPg-binding region. Comparison of this region and the corresponding region of PpDDXL suggests that they are highly conserved and share the same secondary structure. Moreover, overexpression of the VPg-binding region from either AtRH8 or PpDDXL suppresses potyvirus accumulation in infected N. benthamiana leaf tissues. Taken together, these data demonstrate that AtRH8, interacting with VPg, is a host factor required for the potyvirus infection process and that both AtRH8 and PpDDXL may be manipulated for the development of genetic resistance against potyvirus infections.

Plant pathology and RNAi: a brief history

This article describes the discovery of RNA-activated sequence-specific RNA degradation, a phenomenon now referred to as RNA silencing or RNA interference (RNAi). From 1992 to 1996, a series of articles were published on virus resistant transgenic plants expressing either translatable or nontranslatable versions of the coat protein gene of Tobacco etch virus (TEV). Certain transgenic plant lines were resistant to TEV but not to closely related viruses. In these plants a surprising correlation was observed: Transgenic plant lines with the highest degree of TEV resistance had actively transcribed transgenes but low steady-state levels of transgene RNA. Molecular analysis of these transgenic plants demonstrated the existence of a cellular-based, sequence-specific, posttranscriptional RNA-degradation system that was programmed by the transgene-encoded RNA sequence. This RNA-degradation activity specifically targeted both the transgene RNA and TEV (viral) RNA for degradation and was the first description of RNA-mediated gene silencing.

Quantitative analysis of a parasitic antiviral strategy

We extended a computer simulation of viral intracellular growth to study a parasitic antiviral strategy that diverts the viral replicase toward parasite growth. This strategy inhibited virus growth over a wide range of conditions, while minimizing host cell perturbations. Such parasitic strategies may inhibit the development of drug-resistant virus strains.

Gene therapy for infectious diseases: the AIDS model

Genetic manipulation of somatic cells may be of therapeutic value in a variety of infectious diseases, particularly in human immunodeficiency virus (HIV) infection. Stable insertion of custom-designed 'resistance genes' into cells susceptible to HIV could reduce the viral burden in infected individuals and potentially retard the characteristic progressive immune dysfunction. Alternatively, ectopic expression of genes that encode viral antigens might induce potent antiviral immune responses and form the basis for novel prophylactic and therapeutic vaccines. While laboratory studies have proved that the approach works in principle, preclinical and clinical studies will be necessary to evaluate the therapeutic benefit of such gene-based therapies.

Selective inhibition of photosystem II in spinach by tobacco mosaic virus: an effect of the viral coat protein

Leaves of Spinacia oleracea inoculated with tobacco mosaic virus (TMV) strain PV230 develop mild chlorotic and mosaic symptoms of infection. Thylakoid membranes isolated from these infected leaves showed a reduced Fv/Fm ratio for chlorophyll fluorescence kinetics, at 25 degrees C. The photosystem II (PS II)-mediated electron-transport rate was inhibited 50%, whereas PS I activity was unaffected by virus infection. Protein analysis indicated that TMV coat protein was associated with thylakoids, in particular with the PS II fraction. The results demonstrate that TMV-infected S. oleracea shows inhibition of photosynthetic electron transport through PS II. We propose that the inhibition of photosynthetic activity results from the association of viral coat protein with the PS II complex.