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Mammalian orthoreovirus (MRV) is a double-stranded RNA virus from the Reoviridae family that infects a large range of mammals, including humans. Recently, studies have shown that MRV alters cellular alternative splicing (AS) during viral infection. The structural protein μ2 appears to be the main determinant of these AS modifications by decreasing the levels of U5 core components EFTUD2, PRPF8, and SNRNP200 during infection. In the present study, we investigated the mechanism by which μ2 exerts this effect on the U5 components. Our results revealed that μ2 has no impact on steady-state mRNA levels, RNA export, and protein stability of these U5 snRNP proteins. However, polysome profiling and metabolic labeling of newly synthesized proteins revealed that μ2 exerts an inhibitory effect on global translation. Moreover, we showed that μ2 mutants unable to accumulate in the nucleus retain most of the ability to reduce PRPF8 protein levels, indicating that the effect of μ2 on U5 snRNP components mainly occurs in the cytoplasm. Finally, co-expression experiments demonstrated that μ2 suppresses the expression of U5 snRNP proteins in a dose-dependent manner, and that the expression of specific U5 snRNP core components have different sensitivities to μ2's presence. Altogether, these results suggest a novel mechanism by which the μ2 protein reduces the levels of U5 core components through translation inhibition, allowing this viral protein to alter cellular AS during infection. [ABSTRACT FROM AUTHOR]

Human T-cell leukemia virus type 1 is the causative agent of HTLV-1-associated myelopathy/tropical spastic paraparesis and adult T-cell leukemia-lymphoma (ATL). The HTLV-1 basic leucine zipper factor (HBZ) has been associated to the cancer-inducing properties of this virus, although the exact mechanism is unknown. In this study, we identified nucleophosmin (NPM1/B23) as a new interaction partner of HBZ. We show that sHBZ and the less abundant uHBZ isoform interact with nucleolar NPM1/B23 in infected cells and HTLV-1 positive patient cells, unlike equivalent antisense proteins of related non-leukemogenic HTLV-2, -3 and-4 viruses. We further demonstrate that sHBZ association to NPM1/B23 is sensitive to RNase. Interestingly, sHBZ was shown to interact with its own RNA. Through siRNA and overexpression experiments, we further provide evidence that NPM1/B23 acts negatively on viral gene expression with potential impact on cell transformation. Our results hence provide a new insight over HBZ-binding partners in relation to cellular localization and potential function on cell proliferation and should lead to a better understanding of the link between HBZ and ATL development. [ABSTRACT FROM AUTHOR]

Résumé: Bien qu'étant peu pathogène chez l'humain, le réovirus de mammifère a été, et demeure, un modèle privilégié d'étude de la réplication et de la pathogenèse virale. Généralement considéré comme étant cytolytique, ce virus peut parfois établir des infections persistantes à long terme en culture cellulaire. Dans ce contexte, il constitue en fait un des modèles parmi les plus utilisés pour démontrer la coévolution entre virus et cellules. Bien qu'initialement surtout limitées au modèle des cellules fibroblastiques de souris L929, les dernières années ont vu apparaître d'autres études au sein de différents types cellulaires. L'utilisation de la persistance virale pourrait devenir un outil de prédilection pour isoler de nouveaux virus mieux adaptés à des utilisations virothérapeutiques, par exemple en tant qu'agents oncolytiques contre des cancers humains ou animaux. Une meilleure compréhension du phénomène de persistance, spécialement des gènes viraux impliqués, s'avère donc essentielle. Le développement de nouveaux outils, tel que l'utilisation de la génétique inverse, apparaît très prometteur pour l'atteinte de ces objectifs. Cette dernière approche permet en effet d'établir la signification biologique de mutations retrouvées chez les virus sélectionnés lors de la persistance virale. Although mammalian reovirus exhibits only limited pathogenicity in humans, it has been, and still remains, instrumental in studies of viral replication and pathogenesis. Generally considered as cytolytic, this virus can sometimes establish long-term persistent infections in tissue culture. In fact, in this context, it constitutes one widely used model to demonstrate coevolution between virus and host cells. Initially limited to the murine L929 fibroblasts model, further studies in different cell types appeared in the last few years. Establishment of viral persistence could also become a preferred approach to isolate new viruses that are better adapted to their applications in virotherapy, for example as oncolytic agents against human or animal cancers. A better understanding of the persistence phenomenon, especially of viral genes involved, is thus essential. The development of new tools, such as reverse genetics, appears very promising to achieve these objectives. Actually, this last approach allows us to establish the biological significance of mutations found on viruses selected during viral persistence. [ABSTRACT FROM AUTHOR]