Welcome to Viruses: A New Open-Access, Multidisciplinary Forum for Virology

The field of virology has never been more exciting as a research area and more relevant to human health as it is in 2009. The AIDS pandemic, caused by the uncurtailed spread of HIV-1 in large parts of the world, continues to have an enormous impact on the human condition. The threat of a global outbreak of highly pathogenic avian influenza remains real, and memories of the devastation created by the SARS coronavirus are still fresh. While many of the world’s most lethal viruses (Ebola, Hendra, Rift Valley fever, etc.) are geographically contained, the possibility of deliberate transmission of such infectious agents as biological weapons is cause for concern. Increased understanding of all viruses, not only these “newsworthy” pathogens, is warranted as it is impossible to predict the origins of the next viral epidemic. Increased human movement, global climate change, and disruption of natural ecosystems all favor the transmission and spread of both established and emerging viruses. Agricultural interests world-wide continue to be significantly impacted by viral agents. [...

Influenza Virus Not cRAFTy Enough to Dodge Viperin

Interferons elicit antiviral responses by inducing the expression of a large number of host cell genes. In this issue of Cell Host & Microbe, Wang and colleagues report that the interferon-inducible protein viperin inhibits influenza A virus release by impairing the formation of cholesterol-enriched plasma membrane microdomains, or lipid rafts. Viperin appears to disrupt lipid rafts by suppressing the activity of farnesyl diphosphate synthase, a key enzyme in isoprenoid biosynthesis

Retroviruses 2004: Review of the 2004 Cold Spring Harbor Retroviruses conference

For the past several decades, retrovirologists from around the world have gathered in late May at the Cold Spring Harbor Laboratories in New York to present their studies in formal talks and posters, and to discuss their ongoing research informally at the bar or on the beach. As organizers of the 2004 Cold Spring Harbor Retroviruses Conference, we have been asked by the editors of Retrovirology to prepare a review of the meeting for publication on-line. Our goal in this review is not to provide a detailed description of data presented at the meeting but rather to highlight some of the significant developments reported this year. The review is structured in a manner that parallels the organization of the meeting; beginning with the entry phase of the replication cycle, proceeding with post-entry events, assembly and release, integration, reverse transcription, pathogenesis/host factors, RNA-related events (transcription, processing, export, and packaging) and finishing with antivirals. While the most striking developments this year involved post-entry events and assembly/release, significant progress was made towards elucidating a number of aspects of the retroviral replication cycle

Defects in cellular sorting and retroviral assembly induced by GGA overexpression

<p>Abstract</p> <p>Background</p> <p>We previously demonstrated that overexpression of Golgi-localized, γ-ear containing, Arf-binding (GGA) proteins inhibits retrovirus assembly and release by disrupting the function of endogenous ADP ribosylation factors (Arfs). GGA overexpression led to the formation of large, swollen vacuolar compartments, which in the case of GGA1 sequestered HIV-1 Gag.</p> <p>Results</p> <p>In the current study, we extend our previous findings to characterize in depth the GGA-induced compartments and the determinants for retroviral Gag sequestration in these structures. We find that GGA-induced structures are derived from the Golgi and contain aggresome markers. GGA overexpression leads to defects in trafficking of transferrin receptor and recycling of cation-dependent mannose 6-phosphate receptor. Additionally, we find that compartments induced by GGA overexpression sequester Tsg101, poly-ubiquitin, and, in the case of GGA3, Hrs. Interestingly, brefeldin A treatment, which leads to the dissociation of endogenous GGAs from membranes, does not dissociate the GGA-induced compartments. GGA mutants that are defective in Arf binding and hence association with membranes also induce the formation of GGA-induced structures. Overexpression of ubiquitin reverses the formation of GGA-induced structures and partially rescues HIV-1 particle production. We found that in addition to HIV-1 Gag, equine infectious anemia virus Gag is also sequestered in GGA1-induced structures. The determinants in Gag responsible for sequestration map to the matrix domain, and recruitment to these structures is dependent on Gag membrane binding.</p> <p>Conclusion</p> <p>These data provide insights into the composition of structures induced by GGA overexpression and their ability to disrupt endosomal sorting and retroviral particle production.</p

Cellular IP₆ Levels Limit HIV Production while Viruses that Cannot Efficiently Package IP₆ Are Attenuated for Infection and Replication

Summary: HIV-1 hijacks host proteins to promote infection. Here we show that HIV is also dependent upon the host metabolite inositol hexakisphosphate (IP6) for viral production and primary cell replication. HIV-1 recruits IP6 into virions using two lysine rings in its immature hexamers. Mutation of either ring inhibits IP6 packaging and reduces viral production. Loss of IP6 also results in virions with highly unstable capsids, leading to a profound loss of reverse transcription and cell infection. Replacement of one ring with a hydrophobic isoleucine core restores viral production, but IP6 incorporation and infection remain impaired, consistent with an independent role for IP6 in stable capsid assembly. Genetic knockout of biosynthetic kinases IPMK and IPPK reveals that cellular IP6 availability limits the production of diverse lentiviruses, but in the absence of IP6, HIV-1 packages IP5 without loss of infectivity. Together, these data suggest that IP6 is a critical cofactor for HIV-1 replication

Macrophages and Cell-Cell Spread of HIV-1

Macrophages have been postulated to play an important role in the pathogenesis of HIV-1 infection. Their ability to cross the blood-brain barrier and their resistance to virus-induced cytopathic effects allows them to serve as reservoirs for long-term infection. Thus, exploring the mechanisms of virus transmission from macrophages to target cells such as other macrophages or T lymphocytes is central to our understanding of HIV-1 pathogenesis and progression to AIDS, and is vital to the development of vaccines and novel antiretroviral therapies. This review provides an overview of the current understanding of cell-cell transmission in macrophages

Real-Time Visualization of HIV-1 GAG Trafficking in Infected Macrophages

HIV-1 particle production is driven by the Gag precursor protein Pr55Gag. Despite significant progress in defining both the viral and cellular determinants of HIV-1 assembly and release, the trafficking pathway used by Gag to reach its site of assembly in the infected cell remains to be elucidated. The Gag trafficking itinerary in primary monocyte-derived macrophages is especially poorly understood. To define the site of assembly and characterize the Gag trafficking pathway in this physiologically relevant cell type, we have made use of the biarsenical-tetracysteine system. A small tetracysteine tag was introduced near the C-terminus of the matrix domain of Gag. The insertion of the tag at this position did not interfere with Gag trafficking, virus assembly or release, particle infectivity, or the kinetics of virus replication. By using this in vivo detection system to visualize Gag trafficking in living macrophages, Gag was observed to accumulate both at the plasma membrane and in an apparently internal compartment that bears markers characteristic of late endosomes or multivesicular bodies. Significantly, the internal Gag rapidly translocated to the junction between the infected macrophages and uninfected T cells following macrophage/T-cell synapse formation. These data indicate that a population of Gag in infected macrophages remains sequestered internally and is presented to uninfected target cells at a virological synapse

The Role of Lipids in Retrovirus Replication

Retroviruses undergo several critical steps to complete a replication cycle. These include the complex processes of virus entry, assembly, and budding that often take place at the plasma membrane of the host cell. Both virus entry and release involve membrane fusion/fission reactions between the viral envelopes and host cell membranes. Accumulating evidence indicates important roles for lipids and lipid microdomains in virus entry and egress. In this review, we outline the current understanding of the role of lipids and membrane microdomains in retroviral replication

Crystallographic and Functional Analysis of the ESCRT-I /HIV-1 Gag PTAP Interaction

SummaryBudding of HIV-1 requires the binding of the PTAP late domain of the Gag p6 protein to the UEV domain of the TSG101 subunit of ESCRT-I. The normal function of this motif in cells is in receptor downregulation. Here, we report the 1.4–1.6 Å structures of the human TSG101 UEV domain alone and with wild-type and mutant HIV-1 PTAP and Hrs PSAP nonapeptides. The hydroxyl of the Thr or Ser residue in the P(S/T)AP motif hydrogen bonds with the main chain of Asn69. Mutation of the Asn to Pro, blocking the main-chain amide, abrogates PTAP motif binding in vitro and blocks budding of HIV-1 from cells. N69P and other PTAP binding-deficient alleles of TSG101 did not rescue HIV-1 budding. However, the mutant alleles did rescue downregulation of endogenous EGF receptor. This demonstrates that the PSAP motif is not rate determining in EGF receptor downregulation under normal conditions

The autophagy protein ATG9A promotes HIV-1 infectivity.

BACKGROUND: Nef is a multifunctional accessory protein encoded by HIV-1, HIV-2 and SIV that plays critical roles in viral pathogenesis, contributing to viral replication, assembly, budding, infectivity and immune evasion, through engagement of various host cell pathways. RESULTS: To gain a better understanding of the role of host proteins in the functions of Nef, we carried out tandem affinity purification-mass spectrometry analysis, and identified over 70 HIV-1 Nef-interacting proteins, including the autophagy-related 9A (ATG9A) protein. ATG9A is a transmembrane component of the machinery for autophagy, a catabolic process in which cytoplasmic components are degraded in lysosomal compartments. Pulldown experiments demonstrated that ATG9A interacts with Nef from not only HIV-1 and but also SIV (cpz, smm and mac). However, expression of HIV-1 Nef had no effect on the levels and localization of ATG9A, and on autophagy, in the host cells. To investigate a possible role for ATG9A in virus replication, we knocked out ATG9A in HeLa cervical carcinoma and Jurkat T cells, and analyzed virus release and infectivity. We observed that ATG9A knockout (KO) had no effect on the release of wild-type (WT) or Nef-defective HIV-1 in these cells. However, the infectivity of WT virus produced from ATG9A-KO HeLa and Jurkat cells was reduced by ~ fourfold and eightfold, respectively, relative to virus produced from WT cells. This reduction in infectivity was independent of the interaction of Nef with ATG9A, and was not due to reduced incorporation of the viral envelope (Env) glycoprotein into the virus. The loss of HIV-1 infectivity was rescued by pseudotyping HIV-1 virions with the vesicular stomatitis virus G glycoprotein. CONCLUSIONS: These studies indicate that ATG9A promotes HIV-1 infectivity in an Env-dependent manner. The interaction of Nef with ATG9A, however, is not required for Nef to enhance HIV-1 infectivity. We speculate that ATG9A could promote infectivity by participating in either the removal of a factor that inhibits infectivity or the incorporation of a factor that enhances infectivity of the viral particles. These studies thus identify a novel host cell factor implicated in HIV-1 infectivity, which may be amenable to pharmacologic manipulation for treatment of HIV-1 infection