Maria-Carla Saleh

RNA-mediated interference and reverse transcription control the persistence of RNA viruses in the insect model Drosophila

How persistent viral infections are established and maintained is widely debated and remains poorly understood. We found here that the persistence of RNA viruses in Drosophila melanogaster was achieved through the combined action of cellular reverse-transcriptase activity and the RNA-mediated interference (RNAi) pathway. Fragments of diverse RNA viruses were reverse-transcribed early during infection, which resulted in DNA forms embedded in retrotransposon sequences. Those virus-retrotransposon DNA chimeras produced transcripts processed by the RNAi machinery, which in turn inhibited viral replication. Conversely, inhibition of reverse transcription hindered the appearance of chimeric DNA and prevented persistence. Our results identify a cooperative function for retrotransposons and antiviral RNAi in the control of lethal acute infection for the establishment of viral persistence.

RNAi-mediated immunity provides strong protection against the negative-strand RNA vesicular stomatitis virus in Drosophila

Activation of innate antiviral responses in multicellular organisms relies on the recognition of structural differences between viral and cellular RNAs. Double-stranded (ds)RNA, produced during viral replication, is a well-known activator of antiviral defenses and triggers interferon production in vertebrates and RNAi in invertebrates and plants. Previous work in mammalian cells indicates that negative-strand RNA viruses do not appear to generate dsRNA, and that activation of innate immunity is triggered by the recognition of the uncapped 5′ ends of viral RNA. This finding raises the question whether antiviral RNAi, which is triggered by the presence of dsRNA in insects, represents an effective host-defense mechanism against negative-strand RNA viruses. Here, we show that the negative-strand RNA virus vesicular stomatitis virus (VSV) does not produce easily detectable amounts of dsRNA in Drosophila cells. Nevertheless, RNAi represents a potent response to VSV infection, as illustrated by the high susceptibility of RNAi-defective mutant flies to this virus. VSV-derived small RNAs produced in infected cells or flies uniformly cover the viral genome, and equally map the genome and antigenome RNAs, indicating that they derive from dsRNA. Our findings reveal that RNAi is not restricted to the defense against positive-strand or dsRNA viruses but can also be highly efficient against a negative-strand RNA virus. This result is of particular interest in view of the frequent transmission of medically relevant negative-strand RNA viruses to humans by insect vectors.

Alphavirus Mutator Variants Present Host-Specific Defects and Attenuation in Mammalian and Insect Models

Arboviruses cycle through both vertebrates and invertebrates, which requires them to adapt to disparate hosts while maintaining genetic integrity during genome replication. To study the genetic mechanisms and determinants of these processes, we use chikungunya virus (CHIKV), a re-emerging human pathogen transmitted by the Aedes mosquito. We previously isolated a high fidelity (or antimutator) polymerase variant, C483Y, which had decreased fitness in both mammalian and mosquito hosts, suggesting this residue may be a key molecular determinant. To further investigate effects of position 483 on RNA-dependent RNA-polymerase (RdRp) fidelity, we substituted every amino acid at this position. We isolated novel mutators with decreased replication fidelity and higher mutation frequencies, allowing us to examine the fitness of error-prone arbovirus variants. Although CHIKV mutators displayed no major replication defects in mammalian cell culture, they had reduced specific infectivity and were attenuated in vivo. Unexpectedly, mutator phenotypes were suppressed in mosquito cells and the variants exhibited significant defects in RNA synthesis. Consequently, these replication defects resulted in strong selection for reversion during infection of mosquitoes. Since residue 483 is conserved among alphaviruses, we examined the analogous mutations in Sindbis virus (SINV), which also reduced polymerase fidelity and generated replication defects in mosquito cells. However, replication defects were mosquito cell-specific and were not observed in Drosophila S2 cells, allowing us to evaluate the potential attenuation of mutators in insect models where pressure for reversion was absent. Indeed, the SINV mutator variant was attenuated in fruit flies. These findings confirm that residue 483 is a determinant regulating alphavirus polymerase fidelity and demonstrate proof of principle that arboviruses can be attenuated in mammalian and insect hosts by reducing fidelity.

Virus-derived DNA drives mosquito vector tolerance to arboviral infection

Mosquitoes develop long-lasting viral infections without substantial deleterious effects, despite high viral loads. This makes mosquitoes efficient vectors for emerging viral diseases with enormous burden on public health. How mosquitoes resist and/or tolerate these viruses is poorly understood. Here we show that two species of Aedes mosquitoes infected with two arboviruses from distinct families (dengue or chikungunya) generate a viral-derived DNA (vDNA) that is essential for mosquito survival and viral tolerance. Inhibition of vDNA formation leads to extreme susceptibility to viral infections, reduction of viral small RNAs due to an impaired immune response, and loss of viral tolerance. Our results highlight an essential role of vDNA in viral tolerance that allows mosquito survival and thus may be important for arbovirus dissemination and transmission. Elucidating the mechanisms of mosquito tolerance to arbovirus infection paves the way to conceptualize new antivectorial strategies to selectively eliminate arbovirus-infected mosquitoes.

Oligodendrocyte differentiation is increased in transferrin transgenic mice.

Transferrin (Tf), the iron transport glycoprotein found in biological fluids of vertebrates, is synthesized mainly by hepatocytes. Tf is also synthesized by oligodendrocytes (Ol), and several lines of evidence indicate that brain Tf could be involved in myelinogenesis. Because Tf is postnatally expressed in the brain, we sought to investigate whether Tf could intervene in Ol differentiation. For this purpose, we analyzed transgenic mice overexpressing the complete human Tf gene in Ol. We show that the hTf transgene was expressed only from 5 days postpartum onward. In the brain of 14‐day‐old transgenic mice, the DM‐20 mRNA level was decreased, whereas the PLP, MBP, CNP, and MAG mRNA levels were increased. We counted a higher proportion of Ol expressing the O4 (Ol‐specific antigens) and PLP in brain cells cultured from transgenic mice. These results support the idea that overexpressing Tf in the brain accelerates the oligodendrocyte lineage maturation. Accordingly, by NMR imaging acquisition of diffusion tensor in hTf transgenic mice, we observed early maturation of the cerebellum and spinal cord and more myelination in the corpus callosum. In addition, hTf overexpression led to an increase in Sox10 mRNA and protein. Increases in Sox10 and in Tf expression occur simultaneously during brain development. The Olig1 mRNA level also increased, but long after the rise of hTf and Sox10. The Olig2 mRNA level remained unchanged in the brain of transgenic mice. Our findings suggest that Tf could influence oligodendrocyte progenitor differentiation in the CNS.

piRNA pathway is not required for antiviral defense in Drosophila melanogaster

Significance In animals, one of the main forms of RNA interference involves Piwi-interacting RNAs (piRNAs), which protect genomes against the activity of transposable elements. Several groups have recently described piRNAs from viruses in mosquitoes and suggested their involvement in antiviral defense. To understand the extent to which the piRNA pathway contributes to antiviral defense in insects, we used Drosophila melanogaster and different viruses. Using high-throughput sequencing, we were unable to find any evidence of piRNAs from viruses in flies. Furthermore, flies lacking components of the piRNA pathway were not unusually susceptible to viral infection. Taken together, our results indicate that fundamental differences have arisen between the antiviral defenses of flies and mosquitoes since they last shared a common ancestor >200 Mya. Since its discovery, RNA interference has been identified as involved in many different cellular processes, and as a natural antiviral response in plants, nematodes, and insects. In insects, the small interfering RNA (siRNA) pathway is the major antiviral response. In recent years, the Piwi-interacting RNA (piRNA) pathway also has been implicated in antiviral defense in mosquitoes infected with arboviruses. Using Drosophila melanogaster and an array of viruses that infect the fruit fly acutely or persistently or are vertically transmitted through the germ line, we investigated in detail the extent to which the piRNA pathway contributes to antiviral defense in adult flies. Following virus infection, the survival and viral titers of Piwi, Aubergine, Argonaute-3, and Zucchini mutant flies were similar to those of wild type flies. Using next-generation sequencing of small RNAs from wild type and siRNA mutant flies, we showed that no viral-derived piRNAs were produced in fruit flies during different types of viral infection. Our study provides the first evidence, to our knowledge, that the piRNA pathway does not play a major role in antiviral defense in adult Drosophila and demonstrates that viral-derived piRNA production depends on the biology of the host–virus combination rather than being part of a general antiviral process in insects.

Of Insects and Viruses: The Role of Small RNAs in Insect Defence

In the past decade, small RNA pathways have been identified as a major mechanism of gene regulation. From an immunity standpoint, these pathways play a central role either by regulating immune reactions or by acting as immune effectors. In insects, several studies have unravelled the role of RNA interference (RNAi) as an antiviral response and have uncovered a complex relationship between insects and viruses that co-evolve in an ongoing race for supremacy. In this review, we comment on the role of small RNA pathways in insect defence and the exploitation of these same pathways by pathogens. We illustrate the host–pathogen relationship under RNAi constraints using several examples and we discuss future directions in using RNAi as a tool to control insect immunity.

Expression and secretion of human apolipoprotein A-I in the heart

Various studies have correlated apolipoprotein (apo) A‐I, the major component high‐density lipoprotein, with protection against development of cardiovascular disease. Although apoA‐I expression has been previously detected in the liver and intestine, we have discovered that the human apoA‐I gene is also expressed in the heart. Using transgenic (Tg) mice generated with the human apoA‐I/C‐III/A‐IV gene cluster and Tg mice produced with just the 2.2 kb human apoA‐I gene, we have detected significant levels of apoA‐I expression in the heart. Furthermore, the detection of apoA‐I expression in the hearts of human apoA‐I Tg mice indicates that the minimal regulatory elements necessary for cardiac expression of the gene are located near its coding sequence. To determine if the apoA‐I gene is also expressed in the human heart, similar analyses were performed, where apoA‐I expression was found in both adult and fetal hearts. Furthermore in‐depth investigation of the various regions of human and Tg mouse hearts revealed that the apoA‐I mRNA was present in the ventricles and atria, but not in the aorta. In situ hybridization of Tg mouse hearts revealed that apoA‐I expression was restricted to the cardiac myocyte cells. Finally, heart explants and cardiac primary culture experiments with Tg mice showed secretion of particles containing the human apoA‐I protein, and metabolic labeling experiments have also detected a 28 kDa human apoA‐I protein secreted from the heart. From these novel findings, new insights into the role and function of apoA‐I can be extrapolated.

Genomic Location of the Major Ribosomal Protein Gene Locus Determines Vibrio cholerae Global Growth and Infectivity

The effects on cell physiology of gene order within the bacterial chromosome are poorly understood. In silico approaches have shown that genes involved in transcription and translation processes, in particular ribosomal protein (RP) genes, localize near the replication origin (oriC) in fast-growing bacteria suggesting that such a positional bias is an evolutionarily conserved growth-optimization strategy. Such genomic localization could either provide a higher dosage of these genes during fast growth or facilitate the assembly of ribosomes and transcription foci by keeping physically close the many components of these macromolecular machines. To explore this, we used novel recombineering tools to create a set of Vibrio cholerae strains in which S10-spec-α (S10), a locus bearing half of the ribosomal protein genes, was systematically relocated to alternative genomic positions. We show that the relative distance of S10 to the origin of replication tightly correlated with a reduction of S10 dosage, mRNA abundance and growth rate within these otherwise isogenic strains. Furthermore, this was accompanied by a significant reduction in the host-invasion capacity in Drosophila melanogaster. Both phenotypes were rescued in strains bearing two S10 copies highly distal to oriC, demonstrating that replication-dependent gene dosage reduction is the main mechanism behind these alterations. Hence, S10 positioning connects genome structure to cell physiology in Vibrio cholerae. Our results show experimentally for the first time that genomic positioning of genes involved in the flux of genetic information conditions global growth control and hence bacterial physiology and potentially its evolution.

Viral Small RNA Cloning and Sequencing

At the current rate of technological progress, high-throughput sequencing of nucleic acids has become a commodity. These techniques are perfectly suitable for viral small RNAs sequencing and contribute to the understanding of many aspects of virus biology in the context of host-pathogen interaction. However, the generation of high quality data is still an issue and the preparation of small RNAs libraries that accurately reflect the viral siRNAs in the sample remains a challenge. In this chapter we describe how to clone and sequence libraries of viral small RNAs from infected insect samples (mosquito, drosophilidae, insect-derived cell lines).