The NSs protein encoded by the virulent strain of Rift Valley fever virus targets the expression of Abl2 and the actin cytoskeleton of the host affecting cell mobility, cell shape and cell-cell adhesion

International audienceRift Valley fever virus (RVFV) is a highly pathogenic zoonotic arbovirus endemic in many African countries and the Arabian Peninsula. Animal infections cause high rates of mortality and abortion among sheep, goats, and cattle. In humans, an estimated 1 to 2% of RVFV infections result in severe disease (encephalitis, hepatitis, or retinitis) with a high rate of lethality when associated with hemorrhagic fever. The RVFV NSs protein, which is the main virulence factor, counteracts the host innate antiviral response to favor viral replication and spread. However, the mechanisms underlying RVFV-induced cytopathic effects and the role of NSs in these alterations remain for the most part unknown. In this work, we have analyzed the effects of NSs expression on the actin cytoskeleton while conducting infections with the NSs-expressing virulent (ZH548) and attenuated (MP12) strains of RVFV and the non-NSs-expressing avirulent (ZH548ΔNSs) strain, as well as after the ectopic expression of NSs. In macrophages, fibroblasts, and hepatocytes, NSs expression prevented the upregulation of Abl2 (a major regulator of the actin cytoskeleton) expression otherwise induced by avirulent infections and identified here as part of the antiviral response. The presence of NSs was also linked to an increased mobility of ZH548-infected cells compared to ZH548ΔNSs-infected fibroblasts and to strong changes in cell morphology in nonmigrating hepatocytes, with reduction of lamellipodia, cell spreading, and dissolution of adherens junctions reminiscent of the ZH548-induced cytopathic effects observed in vivo. Finally, we show evidence of the presence of NSs within long actin-rich structures associated with NSs dissemination from NSs-expressing toward non-NSs-expressing cells.IMPORTANCE: Rift Valley fever virus (RVFV) is a dangerous human and animal pathogen that was ranked by the World Health Organization in 2018 as among the eight pathogens of most concern for being likely to cause wide epidemics in the near future and for which there are no, or insufficient, countermeasures. The focus of this work is to address the question of the mechanisms underlying RVFV-induced cytopathic effects that participate in RVFV pathogenicity. We demonstrate here that RVFV targets cell adhesion and the actin cytoskeleton at the transcriptional and cellular level, affecting cell mobility and inducing cell shape collapse, along with distortion of cell-cell adhesion. All these effects may participate in RVFV-induced pathogenicity, facilitate virulent RVFV dissemination, and thus constitute interesting potential targets for future development of antiviral therapeutic strategies that, in the case of RVFV, as with several other emerging arboviruses, are presently lacking

Intranasal Exposure to Rift Valley Fever Virus Live-Attenuated Strains Leads to High Mortality Rate in Immunocompetent Mice

International audienceRift Valley fever virus (RVFV) is a pathogenic arthropod-borne virus that can cause serious illness in both ruminants and humans. The virus can be transmitted by an arthropod bite or contact with contaminated fluids or tissues. Two live-attenuated veterinary vaccines—the Smithburn (SB) and Clone 13 (Cl.13)—are currently used during epizootic events in Africa. However, their residual pathogenicity (i.e., SB) or potential of reversion (i.e., Cl.13) causes important adverse effects, strongly limiting their use in the field. In this study, we infected immunocompetent mice with SB or Cl.13 by a subcutaneous or an intranasal inoculation. Interestingly, we found that, unlike the subcutaneous infection, the intranasal inoculation led to a high mortality rate. In addition, we detected high titers and viral N antigen levels in the brain of both the SB- and Cl.13-infected mice. Overall, we unveil a clear correlation between the pathogenicity and the route of administration of both SB and Cl.13, with the intranasal inoculation leading to a stronger neurovirulence and higher mortality rate than the subcutaneous infection

NSs amyloid formation is associated with the virulence of Rift Valley fever virus in mice

International audienceAmyloid fibrils result from the aggregation of host cell-encoded proteins, many giving rise to specific human illnesses such as Alzheimer's disease. Here we show that the major virulence factor of Rift Valley fever virus, the protein NSs, forms filamentous structures in the brain of mice and affects mortality. NSs assembles into nuclear and cytosolic disulfide bond-dependent fibrillary aggregates in infected cells. NSs structural arrangements exhibit characteristics typical for amyloids, such as an ultrastructure of 12 nm-width fibrils, a strong detergent resistance, and interactions with the amyloid-binding dye Thioflavin-S. The assembly dynamics of viral amyloid-like fibrils can be visualized in real-time. They form spontaneously and grow in an amyloid fashion within 5 hours. Together, our results demonstrate that viruses can encode amyloid-like fibril-forming proteins and have strong implications for future research on amyloid aggregation and toxicity in general

Dengue virus NS1 protein conveys pro‐inflammatory signals by docking onto high‐density lipoproteins

International audienceThe dengue virus nonstructural protein 1 (NS1) is a secreted virulence factor that modulates complement, activates immune cells and alters endothelial barriers. The molecular basis of these events remains incompletely understood. Here we describe a functional high affinity complex formed between NS1 and human high-density lipoproteins (HDL). Collapse of the soluble NS1 hexamer upon binding to the lipoprotein particle leads to the anchoring of amphipathic NS1 dimeric subunits into the HDL outer layer. The stable complex can be visualized by electron microscopy as a spherical HDL with rod-shaped NS1 dimers protruding from the surface. We further show that the assembly of NS1-HDL complexes triggers the production of pro-inflammatory cytokines in human primary macrophages while NS1 or HDL alone do not. Finally, we detect NS1 in complex with HDL and low-density lipoprotein (LDL) particles in the plasma of hospitalized dengue patients and observe NS1-apolipoprotein E-positive complexes accumulating overtime. The functional reprogramming of endogenous lipoprotein particles by NS1 as a means to exacerbate systemic inflammation during viral infection provides a new paradigm in dengue pathogenesis

Inhibition of polyamine biosynthesis is a broad-spectrum strategy against RNA viruses

International audienceRNA viruses present an extraordinary threat to human health, given their sudden and unpredictable appearance, the potential for rapid spread among the human population, and their ability to evolve resistance to antiviral therapies. The recent emergence of chikungunya virus, Zika virus, and Ebola virus highlights the struggles to contain outbreaks. A significant hurdle is the availability of antivirals to treat the infected or protect at-risk populations. While several compounds show promise in vitro and in vivo, these outbreaks underscore the need to accelerate drug discovery. The replication of several viruses has been described to rely on host polyamines, small and abundant positively charged molecules found in the cell. Here, we describe the antiviral effects of two molecules that alter polyamine levels: difluoromethylornithine (DFMO; also called eflornithine), which is a suicide inhibitor of ornithine decarboxylase 1 (ODC1), and diethylnorspermine (DENSpm), an activator of spermidine/spermine N1-acetyltransferase (SAT1). We show that reducing polyamine levels has a negative effect on diverse RNA viruses, including several viruses involved in recent outbreaks, in vitro and in vivo These findings highlight the importance of the polyamine biosynthetic pathway to viral replication, as well as its potential as a target in the development of further antivirals or currently available molecules, such as DFMO.IMPORTANCERNA viruses present a significant hazard to human health, and combatting these viruses requires the exploration of new avenuesfor targeting viral replication. Polyamines, small positively charged molecules within the cell, have been demonstrated to facili-tate infection for a few different viruses. Our study demonstrates that diverse RNA viruses rely on the polyamine pathway forreplication and highlights polyamine biosynthesis as a promising drug targe

Identification of Umbre Orthobunyavirus as a Novel Zoonotic Virus Responsible for Lethal Encephalitis in 2 French Patients With Hypogammaglobulinemia

International audienceBackground: Human encephalitis represents a medical challenge from a diagnostic and therapeutic point of view. We investigated the cause of 2 fatal cases of encephalitis of unknown origin in immunocompromised patients.Methods: Untargeted metatranscriptomics was applied on the brain tissue of 2 patients to search for pathogens (viruses, bacteria, fungi, or protozoans) without a prior hypothesis.Results: Umbre arbovirus, an orthobunyavirus never previously identified in humans, was found in 2 patients. In situ hybridization and reverse transcriptase-quantitative polymerase chain reaction (RT-qPCR) showed that Umbre virus infected neurons and replicated at high titers. The virus was not detected in cerebrospinal fluid by RT-qPCR. Viral sequences related to Koongol virus, another orthobunyavirus close to Umbre virus, were found in Culex pipiens mosquitoes captured in the south of France where the patients had spent some time before the onset of symptoms, demonstrating the presence of the same clade of arboviruses in Europe and their potential public health impact. A serological survey conducted in the same area did not identify individuals positive for Umbre virus. The absence of seropositivity in the population may not reflect the actual risk of disease transmission in immunocompromised individuals.Conclusions: Umbre arbovirus can cause encephalitis in immunocompromised humans and is present in Europe

Identification d’un nouvel arbovirus neurotrope en France : le virus Umbre

International audienceNotre laboratoire a mis en place une collaboration avec le laboratoire de neuropathologie de l’hôpital de la Pitié-Salpêtrière pour explorer une série post-mortem de cas d’encéphalites humaines d’étiologie inconnue par un séquençage NGS profond et non ciblé du transcriptome, à la recherche de séquences de pathogènes. Le virus Umbre, un virus du genre orthobunyavirus, a été identifié chez deux patients immunodéprimés décédés en 2013 et 2019. Le virus infecte des neurones du cortex cérébral, la moelle épinière et le foie. Le virus Umbre a été décrit initialement dans les années 1950 dans des populations de moustiques Culex en zone Asie Pacifique, mais n’avait auparavant ni été isolé chez des mammifères (dont l’homme), ni détecté en Europe. Cette infection par le virus Umbre chez deux patients français, dont l’un vivait dans la région Occitanie et n’avait jamais voyagé en dehors de France métropolitaine, et l’autre avait fait une croisière en méditerranée avant l’apparition des symptômes, nous a amené à rechercher l’origine de la contamination dans des moustiques. Une investigation réalisée en partenariat avec le CIRAD de Montpellier et l’EID Méditerranée a permis d’identifier des séquences du genre orthobunyavirus très proches du virus Umbre dans des moustiques Culex de Camargue. Une enquête sérologique par un test développé à l’Institut Pasteur sur une population du sud de la France composée d’environ 300 personnes « contrôles » et d’une vingtaine de cas « encéphalites » n’a pas mis en évidence de réponse anticorps contre le virus Umbre. Ce résultat suggère que la fréquence de l’infection dans la population générale est au plus faible, ce qui ne reflète donc pas le risque pour les personnes immunodéprimées. Cette découverte d’un nouvel arbovirus responsable d’encéphalite, et son existence en France, présente un intérêt particulier en matière de santé publique