Rui Pedro Galão

Innate Sensing of HIV-1 Assembly by Tetherin Induces NFκB-Dependent Proinflammatory Responses

Summary Antiviral proteins that recognize pathogen-specific or aberrantly located molecular motifs are perfectly positioned to act as pattern-recognition receptors and signal to the immune system. Here we investigated whether the interferon-induced viral restriction factor tetherin (CD317/BST2), which is known to inhibit HIV-1 particle release by physically tethering virions to the cell surface, has such a signaling role. We find that upon restriction of Vpu-defective HIV-1, tetherin acts as a virus sensor to induce NFκB-dependent proinflammatory gene expression. Signaling requires both tetherin’s extracellular domain involved in virion retention and determinants in the cytoplasmic tail, including an endocytic motif, although signaling is independent of virion endocytosis. Furthermore, recruitment of the TNF-receptor-associated factor TRAF6 and activation of the mitogen-activated protein kinase TAK1 are critical for signaling. Human tetherin’s ability to mediate efficient signaling may have arisen as a result of a five amino acid deletion that occurred in hominids after their divergence from chimpanzees.

Translation and replication of hepatitis C virus genomic RNA depends on ancient cellular proteins that control mRNA fates.

Inevitably, viruses depend on host factors for their multiplication. Here, we show that hepatitis C virus (HCV) RNA translation and replication depends on Rck/p54, LSm1, and PatL1, which regulate the fate of cellular mRNAs from translation to degradation in the 5′-3′-deadenylation-dependent mRNA decay pathway. The requirement of these proteins for efficient HCV RNA translation was linked to the 5′ and 3′ untranslated regions (UTRs) of the viral genome. Furthermore, LSm1–7 complexes specifically interacted with essential cis-acting HCV RNA elements located in the UTRs. These results bridge HCV life cycle requirements and highly conserved host proteins of cellular mRNA decay. The previously described role of these proteins in the replication of 2 other positive-strand RNA viruses, the plant brome mosaic virus and the bacteriophage Qß, pinpoint a weak spot that may be exploited to generate broad-spectrum antiviral drugs.

Susceptibility of xenotropic murine leukemia virus-related virus (XMRV) to retroviral restriction factors

Xenotropic murine leukemia virus-related virus (XMRV) is a recently discovered gammaretrovirus that has been linked to prostate cancer and chronic fatigue syndrome. This virus is therefore an important potential human pathogen and, as such, it is essential to understand its host cell tropism. Intriguingly, infectious virus has been recovered from patient-derived peripheral blood mononuclear cells. These cells express several antiviral restriction factors that are capable of inhibiting the replication of a wide range of retroviruses, including other gamma retroviruses. This raises the possibility that, similar to HIV, XMRV may have acquired resistance to restriction. We therefore investigated the susceptibility of XMRV to a panel of different restriction factors. We found that both human APOBEC3 and tetherin proteins are able to block XMRV replication. Expression of human TRIM5α, however, had no effect on viral infectivity. There was no evidence that XMRV expressed countermeasures to overcome restriction. In addition, the virus was inhibited by factors from nonhuman species, including mouse Apobec3, tetherin, and Fv1 proteins. These results have important implications for predicting the natural target cells for XMRV replication, for relating infection to viral pathogenicity and pathology, and for the design of model systems with which to study XMRV-related diseases.

Retroviral retention activates a Syk-dependent HemITAM in human tetherin.

Summary Tetherin (BST2/CD317) restricts the release of enveloped viral particles from infected cells. Coupled to this virion retention, hominid tetherins induce proinflammatory gene expression via activating NF-κB. We investigated the events initiating this tetherin-induced signaling and show that physical retention of retroviral particles induces the phosphorylation of conserved tyrosine residues in the cytoplasmic tails of tetherin dimers. This phosphorylation induces the recruitment of spleen tyrosine kinase (Syk), which is required for downstream NF-κB activation, indicating that the tetherin cytoplasmic tail resembles the hemi-immunoreceptor tyrosine-based activation motifs (hemITAMs) found in C-type lectin pattern recognition receptors. Retroviral-induced tetherin signaling is coupled to the cortical actin cytoskeleton via the Rac-GAP-containing protein RICH2 (ARHGAP44), and a naturally occurring tetherin polymorphism with reduced RICH2 binding exhibits decreased phosphorylation and NF-κB activation. Thus, upon virion retention, this linkage to the actin cytoskeleton likely triggers tetherin phosphorylation and subsequent signal transduction to induce an antiviral state.

Saccharomyces cerevisiae: A useful model host to study fundamental biology of viral replication

Abstract Understanding the fundamental steps of virus life cycles including virus–host interactions is essential for the design of effective antiviral strategies. Such understanding has been deferred by the complexity of higher eukaryotic host organisms. To circumvent experimental difficulties associated with this, systems were developed to replicate viruses in the yeast Saccharomyces cerevisiae. The systems include viruses with RNA and DNA genomes that infect plants, animals and humans. By using the powerful methodologies available for yeast genetic analysis, fundamental processes occurring during virus replication have been brought to light. Here, we review the different viruses able to direct replication and gene expression in yeast and discuss their main contributions in the understanding of virus biology.

Ig-like Transcript 7, but Not Bone Marrow Stromal Cell Antigen 2 (Also Known as HM1.24, Tetherin, or CD317), Modulates Plasmacytoid Dendritic Cell Function in Primary Human Blood Leukocytes

The Ig-like transcript (ILT) 7 is a surface molecule selectively expressed by human plasmacytoid dendritic cells (pDCs). ILT7 cross-linking suppresses pDC activation and type I IFN (IFN-I) secretion following TLR7/9 engagement. The bone marrow stromal cell Ag 2 (BST2, aka HM1.24, tetherin, or CD317) is expressed by different cell types upon exposure to IFN-I and is a natural ligand for ILT7. In this study, we show that ILT7 expression decreased spontaneously in pDCs upon in vitro culture, which correlates with pDC differentiation measured as increased side scatter properties and CCR7 expression. TLR7/9 ligands, as well as HIV, induced BST2 upregulation on all tested cell types except T cells, which required TCR stimulation to respond to TLR9L-induced IFN-I. IFN-γ, IL-4, IL-10, and TNF-α had only marginal effects on BST2 expression in blood leukocytes compared with TLR9L. Preincubation with ILT7 cross-linking Ab inhibited IFN-I production in PBMCs treated with TLR7/9L or HIV, whereas BST2 blockade did not affect IFN-I responses even when BST2 upregulation was further boosted with TCR agonists or immunoregulatory cytokines. Our data indicate that BST2-mediated ILT7 cross-linking may act as a homeostatic regulatory mechanism on immature circulating pDC, rather than a negative feedback for activated mature pDCs that have downregulated ILT7.

Saccharomyces cerevisiae: a versatile eukaryotic system in virology

The yeast Saccharomyces cerevisiae is a well-established model system for understanding fundamental cellular processes relevant to higher eukaryotic organisms. Less known is its value for virus research, an area in which Saccharomyces cerevisiae has proven to be very fruitful as well. The present review will discuss the main achievements of yeast-based studies in basic and applied virus research. These include the analysis of the function of individual proteins from important pathogenic viruses, the elucidation of key processes in viral replication through the development of systems that allow the replication of higher eukayotic viruses in yeast, and the use of yeast in antiviral drug development and vaccine production.