Enhancement of the influenza A hemagglutinin (HA)-mediated cell-cell fusion and virus entry by the viral neuraminidase (NA).

International audienceBACKGROUND: The major role of the neuraminidase (NA) protein of influenza A virus is related to its sialidase activity, which disrupts the interaction between the envelope hemagglutinin (HA) protein and the sialic acid receptors expressed at the surface of infected cells. This enzymatic activity is known to promote the release and spread of progeny viral particles following their production by infected cells, but a potential role of NA in earlier steps of the viral life cycle has never been clearly demonstrated. In this study we have examined the impact of NA expression on influenza HA-mediated viral membrane fusion and virion infectivity. METHODOLOGY/PRINCIPAL FINDINGS: The role of NA in the early stages of influenza virus replication was examined using a cell-cell fusion assay that mimics HA-mediated membrane fusion, and a virion infectivity assay using HIV-based pseudoparticles expressing influenza HA and/or NA proteins. In the cell-cell fusion assay, which bypasses the endocytocytosis step that is characteristic of influenza virus entry, we found that in proper HA maturation conditions, NA clearly enhanced fusion in a dose-dependent manner. Similarly, expression of NA at the surface of pseudoparticles significantly enhanced virion infectivity. Further experiments using exogenous soluble NA revealed that the most likely mechanism for enhancement of fusion and infectivity by NA was related to desialylation of virion-expressed HA. CONCLUSION/SIGNIFICANCE: The NA protein of influenza A virus is not only required for virion release and spread but also plays a critical role in virion infectivity and HA-mediated membrane fusion

Twin gradients in APOBEC3 edited HIV-1 DNA reflect the dynamics of lentiviral replication

The human immunodeficiency virus (HIV) Vif protein blocks incorporation of two host cell cytidine deaminases, APOBEC3F and 3G, into the budding virion. Not surprisingly, on a vif background nascent minus strand DNA can be extensively edited leaving multiple uracil residues. Editing occurs preferentially in the context of TC (GA on the plus strand) and CC (GG) depending on the enzyme. To explore the distribution of APOBEC3F and –3G editing across the genome, a product/substrate ratio (AA + AG)/(GA + GG) was computed for a series of 30 edited genomes present in the data bases. Two highly polarized gradients were noted each with maxima just 5′ to the central polypurine tract (cPPT) and LTR proximal polypurine tract (3′PPT). The gradients are in remarkable agreement with the time the minus strand DNA remains single stranded. In vitro analyses of APOBEC3G deamination of nascent cDNA spanning the two PPTs showed no pronounced dependence on the PPT RNA:DNA heteroduplex ruling out the competing hypothesis of a PPT orientation effect. The degree of hypermutation varied smoothly among genomes indicating that the number of APOBEC3 molecules packaged varied considerably

APOBEC3G-Induced Hypermutation of Human Immunodeficiency Virus Type-1 Is Typically a Discrete “All or Nothing” Phenomenon

The rapid evolution of Human Immunodeficiency Virus (HIV-1) allows studies of ongoing host–pathogen interactions. One key selective host factor is APOBEC3G (hA3G) that can cause extensive and inactivating Guanosine-to-Adenosine (G-to-A) mutation on HIV plus-strand DNA (termed hypermutation). HIV can inhibit this innate anti-viral defense through binding of the viral protein Vif to hA3G, but binding efficiency varies and hypermutation frequencies fluctuate in patients. A pivotal question is whether hA3G-induced G-to-A mutation is always lethal to the virus or if it may occur at sub-lethal frequencies that could increase viral diversification. We show in vitro that limiting-levels of hA3G-activity (i.e. when only a single hA3G-unit is likely to act on HIV) produce hypermutation frequencies similar to those in patients and demonstrate in silico that potentially non-lethal G-to-A mutation rates are ∼10-fold lower than the lowest observed hypermutation levels in vitro and in vivo. Our results suggest that even a single incorporated hA3G-unit is likely to cause extensive and inactivating levels of HIV hypermutation and that hypermutation therefore is typically a discrete “all or nothing” phenomenon. Thus, therapeutic measures that inhibit the interaction between Vif and hA3G will likely not increase virus diversification but expand the fraction of hypermutated proviruses within the infected host

Importance de l'environnement cellulaire pour la réplication du virus de l'immunodéficience humaine

PARIS7-Bibliothèque centrale (751132105) / SudocSudocFranceF

A modular model-order reduction approach for the solution of parametrized strongly-coupled thermo-mechanical problems

International audienceThis paper deals with the simulation of parametrized strongly-coupled multiphysics problems. The proposed method is based on previous works on multiphysics problems using the LATIN algorithm and the Proper Generalized Decomposition (PGD). Unlike conventional partitioning approaches, the LATIN-PGD solver applied to multiphysics problems builds the coupled solution by successively adding global corrections to each physics within an iterative procedure. The reduced-order bases for the different physics are built independently through a greedy algorithm, ensuring accuracy up to the desired level. This flexibility is used herein to efficiently handle parametrized problems, as it allows to enrich the bases independently along the variations of the parameters. The proposed approach is exemplified on several three-dimensional numerical examples in the case of thermo-mechanical coupling. We use a standard monolithic scheme to validate its accuracy. Our results highlight the adaptability of the proposed strategy to the coupling strength. Concerning the parametrized aspects, the method’s capability is illustrated through parametric studies with uncertain material parameters, resulting in significant performance gains over the monolithic scheme. Our observations suggest that the proposed computational strategy is effective and versatile when dealing with strongly-coupled multiphysics problems

Une approche modulaire basée sur la PGD pour la résolution de problèmes multiphysiques fortement couplés

National audienceLa méthode LATIN offre une modularité intéressante pour la simulation de problèmesmultiphysiques fortement couplés grâce à la notion d’interface entre physiques, qui permet de traiter les physiques séparément sur tout le domaine espace-temps, tout en assurant le couplage. Dans ce travail, cette interface est exploitée pour étendre la méthode à l’utilisation de modèles de natures différentes pour chaque physique. Le couplage d’un modèle haute fidélité à un modèle réduit PGD, potentiellement pré-calculé, puis enrichi à la volée, est illustré sur un problème de thermoélasticité

Première approche d’une stratégie de partitionnement multimodèle pour la simulation deproblèmes multiphysiques

International audiencePour résoudre un problème multiphysique fortement couplé, par exemple dans le cadre de la thermo- mécanique, les méthodes dites de partitionnement consistent à résoudre successivement chacune des physiques, tout en les faisant dialoguer. Le point clé de ces méthodes incrémentales réside dans le trai- tement séparé des différentes physiques, permettant d’envisager l’utilisation de modèles différents pour chacune. Dans cet article, un modèle réduit précalculé est couplé à un modèle classique au sein de la stratégie de partitionnement ISSP. Un cas test simple permet d’appréhender les limites des straté- gies de partitionnement incrémentales lorsqu’on cherche à résoudre un problème trop éloigné de celui ayant servi à générer la base réduite. Cela justifie l’intérêt qui sera porté à la méthode non incrémen- tale LATIN-PGD pour la résolution partitionnée multimodèle d’un problème multiphysique fortement couplé