MDM2 is a novel E3 ligase for HIV-1 Vif

Lifespan analysis of repeat expression reveals age-dependent upregulation of HERV-K in the neurotypical human brain

DNA repetitive sequences (or repeats) comprise over 50% of the human genome and have a crucial regulatory role, specifically regulating transcription machinery. The human brain is the tissue with the highest detectable repeat expression and dysregulations on the repeat activity are related to several neurological and neurodegenerative disorders, as repeat-derived products can stimulate a pro-inflammatory response. Even so, it is unclear how repeat expression acts on the aging neurotypical brain. Here, we leverage a large postmortem transcriptome cohort spanning the human lifespan to assess global repeat expression in the neurotypical brain. We identified 21,696 differentially expressed repeats (DERs) that varied across seven age bins (Prenatal; 0-15; 16-29; 30-39; 40-49; 50-59; 60+) across the caudate nucleus (n=271), dorsolateral prefrontal cortex (n=304), and hippocampus (n=310). Interestingly, we found that long interspersed nuclear elements and long terminal repeats (LTRs) DERs were the most abundant repeat families when comparing infants to early adolescence (0-15) with older adults (60+). Of these differentially regulated LTRs, we identified 17 shared across all brain regions, including increased expression of HERV-K-int in older adult brains (60+). Co-expression analysis from each of the three brain regions also showed repeats from the HERV subfamily were intramodular hubs in its subnetworks. While we do not observe a strong global relationship between repeat expression and age, we identified HERV-K as a repeat signature associated with the aging neurotypical brain. Our study is the first global assessment of repeat expression in the neurotypical brain.

Kaposi’s Sarcoma-Associated Herpesvirus ORF50 Protein Represses Cellular MDM2 Expression via Suppressing the Sp1- and p53-Mediated Transactivation

The Kaposi’s sarcoma-associated herpesvirus (KSHV)-encoded ORF50 protein is a potent transcriptional activator essential for triggering KSHV lytic reactivation. Despite extensive studies, little is known about whether ORF50 possesses the ability to repress gene expression or has an antagonistic action to cellular transcription factors. Previously, we demonstrated that human oncoprotein MDM2 can promote the degradation of ORF50 protein. Herein, we show that abundant ORF50 expression in cells can conversely downregulate MDM2 expression via repressing both the upstream (P1) and internal (P2) promoters of the MDM2 gene. Deletion analysis of the MDM2 P1 promoter revealed that there were two ORF50-dependent negative response elements located from −102 to −63 and from −39 to +1, which contain Sp1-binding sites. For the MDM2 P2 promoter, the ORF50-dependent negative response element was identified in the region from −110 to −25, which is coincident with the location of two known p53-binding sites. Importantly, we further demonstrated that overexpression of Sp1 or p53 in cells indeed upregulated MDM2 expression; however, coexpression with ORF50 protein remarkably reduced the Sp1- or p53-mediated MDM2 upregulation. Collectively, our findings propose a reciprocal negative regulation between ORF50 and MDM2 and uncover that ORF50 decreases MDM2 expression through repressing Sp1- and p53-mediated transactivation.

Modulation of the autophagic pathway inhibits HIV-1 infection in human lymphoid tissue cultured ex vivo

A complex link exists between HIV-1 and autophagy, and discordant results have been reported in different in vitro models regarding the way HIV and autophagy modulate each other. Despite this, there is very limited knowledge about the interplay between HIV and autophagy in vivo in lymphoid tissue, due in part by the lack of cell models that recapitulate the in vivo setting. Here, we evaluate the interrelationship between HIV and autophagy using human ex vivo lymphoid tissue cultures as an HIV infection model. Our results showed that human lymphoid aggregated cultures (HLACs) from tonsillar tissue displayed fully functional autophagic activity. In this system, HIV infection resulted in an increase in autophagy. Notably, we observed that both, autophagy-enhancing (rapamycin) or blocking drugs (3-methyladenine, chloroquine and bafilomycin), were able to decrease HIV-DNA levels and HIV replication. Therefore, efficient HIV-1 replication requires a fine-tuned level of autophagy, so modifications of this balance will have a negative impact on its replication. Therefore, targeting the autophagic pathway could be a new therapeutic approach to be explored to treat HIV-1 infection. Ex vivo cultures of human lymphoid tissue are a suitable model to obtain further insights into HIV and its intricate relationship with autophagy.

Human Immunodeficiency Virus Type 1 Vif Up-Regulates the Expression of Tat via AKT Signaling Pathway: Role of Ubiquitin Specific Protease 17

Human immunodeficiency virus type 1 (HIV-1) has RNA genome and depends on host cellular machinery for most of its activities. Host cellular proteins modulate the expression and activity of viral proteins to combat the virus. HIV-1 proteins are known to regulate each other for the benefit of virus by exploiting these modulations. Here, we report that HIV-1 Vif increases the levels of Tat via AKT signaling pathway. We show that HIV-1 Vif activates AKT signaling pathway by inducing phosphorylation of AKT. Mdm2, downstream target of AKT signaling, increases the levels of Tat protein in ubiquitin-dependent manner by inducing Ubiquitin Specific Protease 17 (USP17), which is a deubiquitinase and stabilizes Tat protein. Thus, HIV-1 proteins exploit AKT signaling pathway to promote viral replication.

Host AKT-mediated phosphorylation of HIV-1 accessory protein Vif potentiates infectivity via enhanced degradation of the restriction factor APOBEC3G

HIV-1 encodes accessory proteins that neutralize antiviral restriction factors to ensure its successful replication. One accessory protein, the HIV-1 viral infectivity factor (Vif), is known to promote ubiquitination and proteasomal degradation of the antiviral restriction factor apolipoprotein B mRNA-editing enzyme-catalytic polypeptide-like 3G (APOBEC3G), a cytosine deaminase that leads to hypermutations in the viral DNA and subsequent aberrant viral replication. We have previously demonstrated that the HIV-1 viral transcription mediator Tat activates the host progrowth PI-3–AKT pathway, which in turn promotes HIV-1 replication. Because the HIV-1 Vif protein contains the putative AKT phosphorylation motif RMRINT, here we investigated whether AKT directly phosphorylates HIV-1 Vif to regulate its function. Coimmunoprecipitation experiments showed that AKT and Vif interact with each other, supporting this hypothesis. Using in vitro kinase assays, we further showed that AKT phosphorylates Vif at threonine 20, which promotes its stability, as Vif becomes destabilized after this residue is mutated to alanine. Moreover, expression of dominant-negative kinase-deficient AKT as well as treatment with a chemical inhibitor of AKT increased K48-ubiquitination and proteasomal degradation of HIV-1 Vif. In contrast, constitutively active AKT (Myr-AKT) reduced K48-ubiquitination of Vif to promote its stability. Finally, inhibition of AKT function restored APOBEC3G levels, which subsequently reduced HIV-1 infectivity. Thus, our results establish a novel mechanism of HIV-1 Vif stabilization through AKT-mediated phosphorylation at threonine 20, which reduces APOBEC3G levels and potentiates HIV-1 infectivity.

Review on fluorinated nucleoside/non-nucleoside FDA-approved antiviral drugs

FDA-approved antiviral agents represent an important class that has attracted attention in recent years to combat current and future threats of viral pandemics. Fluorine ameliorates the electronic, lipophilic and steric problems of drugs. Additionally, fluorine can prolong drug activity and improve metabolic stability, thereby, modifying their pharmacodynamic and pharmacokinetic character. Herein, we summarized the fluorinated FDA-approved antiviral agents, dealing with biological aspects, mechanisms of action, and synthetic pathways.

FDA-approved antiviral agents represent an important class that has attracted attention in recent years to combat current and future threats of viral pandemics.

STUB1/CHIP promotes ubiquitination and degradation of HIV-1 Vif to restore the cellular level of APOBEC3G protein

HIV-1 accessory protein Vif is required for neutralization of cellular restriction factor APOBEC3G through its ubiquitination and proteasomal degradation which allows replication of HIV-1 in non-permissive cells. This function of Vif is required for maintaining the genomic integrity of HIV-1. We here report that the Vif interacts with the cellular E3 ubiquitin ligase CHIP and the level of Vif protein gets reduced by the expression of CHIP. Reduction of Vif by CHIP expression is due to its increased rate of degradation as shown by cycloheximide (CHX) chase assay. CHIP expression also resulted in the ubiquitination of Vif protein in a dose dependent manner. The role of CHIP in the ubiquitination and degradation was confirmed by the endogenous knockdown of CHIP using CRISPR Cas9 method. Loss of endogenous CHIP protein showed the stabilization of Vif with concomitant destabilization of APOBEC3G. As expected Vif mediated ubiquitination of APOBEC3G was also reduced in CHIP knockdown cells. These results established that CHIP functions as a negative regulator of Vif protein which in-turn stabilizes APOBEC3G.

Deubiquitinating Enzyme USP21 Inhibits HIV-1 Replication by Downregulating Tat Expression

Ubiquitination plays an important role in human immunodeficiency virus 1 (HIV-1) infection. HIV proteins such as Vif and Vpx mediate the degradation of the host proteins APOBEC3 and SAMHD1, respectively, through the proteasome pathway. However, whether deubiquitylating enzymes play an essential role in HIV-1 infection is largely unknown. Here, we demonstrate that the deubiquitinase USP21 potently inhibits HIV-1 production by indirectly downregulating the expression of HIV-1 transactivator of transcription (Tat), which is essential for transcriptional elongation in HIV-1. USP21 deubiquitylates Tat via its deubiquitinase activity, but a stronger ability to reduce Tat expression than a dominant-negative ubiquitin mutant (Ub-KO) showed that other mechanisms may contribute to USP21-mediated inhibition of Tat. Further investigation showed that USP21 downregulates cyclin T1 mRNA levels by increasing methylation of histone K9 in the promoter of cyclin T1, a subunit of the positive transcription elongation factor b (P-TEFb) that interacts with Tat and transactivation response element (TAR) and is required for transcription stimulation and Tat stability. Moreover, USP21 had no effect on the function of other HIV-1 accessory proteins, including Vif, Vpr, Vpx, and Vpu, indicating that USP21 was specific to Tat. These findings improve our understanding of USP21-mediated functional suppression of HIV-1 production. IMPORTANCE Ubiquitination plays an essential role in viral infection. Deubiquitinating enzymes (DUBs) reverse ubiquitination by cleaving ubiquitins from target proteins, thereby affecting viral infection. The role of the members of the USP family, which comprises the largest subfamily of DUBs, is largely unknown in HIV-1 infection. Here, we screened a series of USP members and found that USP21 inhibits HIV-1 production by specifically targeting Tat but not the other HIV-1 accessory proteins. Further investigations revealed that USP21 reduces Tat expression in two ways. First, USP21 deubiquitinates polyubiquitinated Tat, causing Tat instability, and second, USP21 reduces the mRNA levels of cyclin T1 (CycT1), an important component of P-TEFb, that leads to Tat downregulation. Thus, in this study, we report a novel role of the deubiquitinase, USP21, in HIV-1 infection. USP21 represents a potentially useful target for the development of novel anti-HIV drugs.

Micro RNA Targets in HIV Latency: Insights into Novel Layers of Latency Control

Despite the considerable progress that has been made in identifying cellular factors and pathways that contribute to establishment and maintenance of the latent HIV reservoir, it remains the major obstacle to eradicating this virus. Most recently, noncoding genes have been implicated in regulation of HIV expression. In this study, small RNA sequencing was used to profile expression of microRNAs (miRNAs) in a primary CD4+ T cell in vitro model of HIV latency. Previously, we have shown that protein-coding genes dysregulated in this model were enriched for the p53 signaling pathway, which was confirmed experimentally. We further found a link between p53 signaling and dysregulated long noncoding RNAs. In this study, we hypothesized that miRNAs may provide an additional level of regulation of the p53 signaling pathway during HIV latency. Twenty-six miRNAs were identified to be dysregulated in our latency model. A subset of these miRNAs was validated by real-time quantitative polymerase chain reaction. Predicted messenger RNA (mRNA) targets and cellular pathways enriched for mRNA targets were identified using several analytical methods. Our analyses showed that many protein-coding genes and pathways targeted by dysregulated miRNAs have relevance to regulation of HIV expression or establishment of HIV latency. The p53 signaling pathway was found among pathways that were targeted by dysregulated miRNAs at a greater level than expected by chance. This study provides a mechanistic insight into regulation of the p53 pathway through miRNAs that may contribute to the establishment of latency.

The Role of APOBECs in Viral Replication

Apolipoprotein B mRNA-editing enzyme catalytic polypeptide-like (APOBEC) proteins are a diverse and evolutionarily conserved family of cytidine deaminases that provide a variety of functions from tissue-specific gene expression and immunoglobulin diversity to control of viruses and retrotransposons. APOBEC family expansion has been documented among mammalian species, suggesting a powerful selection for their activity. Enzymes with a duplicated zinc-binding domain often have catalytically active and inactive domains, yet both have antiviral function. Although APOBEC antiviral function was discovered through hypermutation of HIV-1 genomes lacking an active Vif protein, much evidence indicates that APOBECs also inhibit virus replication through mechanisms other than mutagenesis. Multiple steps of the viral replication cycle may be affected, although nucleic acid replication is a primary target. Packaging of APOBECs into virions was first noted with HIV-1, yet is not a prerequisite for viral inhibition. APOBEC antagonism may occur in viral producer and recipient cells. Signatures of APOBEC activity include G-to-A and C-to-T mutations in a particular sequence context. The importance of APOBEC activity for viral inhibition is reflected in the identification of numerous viral factors, including HIV-1 Vif, which are dedicated to antagonism of these deaminases. Such viral antagonists often are only partially successful, leading to APOBEC selection for viral variants that enhance replication or avoid immune elimination.

Ubiquitin E3 Ligase c-Cbl Is a Host Negative Regulator of Nef Protein of HIV-1

Nef is an accessory protein encoded by human immunodeficiency virus type-1 (HIV-1) and plays important roles in regulating HIV-1 infection and viral replication. Interestingly, HIV-1 Nef can promote degradation of numerous host proteins to disrupt cellular antiviral immune response. However, how HIV-1 Nef is degraded by host factors remains largely unexplored. Here, we identified c-Cbl as a host ubiquitin E3 ligase of HIV-1 Nef. We found that c-Cbl interacts with Nef and reduces protein levels of HIV-1 Nef. Further studies demonstrated that c-Cbl promoted Lys48-linked polyubiquitination of HIV-1 Nef, thus attenuating protein stability of HIV-1 Nef. Importantly, cellular c-Cbl ubiquitinated and degraded Nef proteins produced by HIV-1 NL4-3 virions, and ultimately attenuated HIV-1 virulence for infection of THP1 cells. This study reveals a ubiquitination and proteasome-dependent degradation mechanism of HIV-1 Nef protein, and could provide potential strategies for fighting against HIV-1.

Phosphorylation Targets of DNA-PK and Their Role in HIV-1 Replication

The DNA dependent protein kinase (DNA-PK) is a trimeric nuclear complex consisting of a large protein kinase and the Ku heterodimer. The kinase activity of DNA-PK is required for efficient repair of DNA double-strand breaks (DSB) by non-homologous end joining (NHEJ). We also showed that the kinase activity of DNA-PK is essential for post-integrational DNA repair in the case of HIV-1 infection. Besides, DNA-PK is known to participate in such cellular processes as protection of mammalian telomeres, transcription, and some others where the need for its phosphorylating activity is not clearly elucidated. We carried out a systematic search and analysis of DNA-PK targets described in the literature and identified 67 unique DNA-PK targets phosphorylated in response to various in vitro and/or in vivo stimuli. A functional enrichment analysis of DNA-PK targets and determination of protein-protein associations among them were performed. For 27 proteins from these 67 DNA-PK targets, their participation in the HIV-1 life cycle was demonstrated. This information may be useful for studying the functioning of DNA-PK in various cellular processes, as well as in various stages of HIV-1 replication.

The stability of HIV-2 Vpx and Vpr proteins is regulated by the presence or absence of zinc-binding sites and poly-proline motifs with distinct roles

The Vpx and Vpr proteins of human immunodeficiency virus type 2 (HIV-2) are important for virus replication. Although these proteins are homologous, Vpx is expressed at much higher levels than Vpr. Previous studies demonstrated that this difference results from the presence of an HHCC zinc-binding site in Vpx that is absent in Vpr. Vpx has another unique region, a poly-proline motif (PPM) of seven consecutive prolines at the C-terminus. Using PPM point mutants of Vpx, this study demonstrated that these seven consecutive prolines are critical for suppressing proteasome degradation of Vpx in the absence of Gag. Both the PPM and the zinc-binding site stabilize Vpx but do so via different mechanisms. PPM and zinc-binding site mutants overexpressed in Escherichia coli aggregated readily, indicating that these motifs normally prevent exposure of the hydrophobic region outside the structure. Furthermore, introduction of the zinc-binding site and the PPM into Vpr increased the level of Vpr expression so that it was as high as that of Vpx. Intriguingly, HIV-2 has evolved to express Vpx at high levels and Vpr at low levels based on the presence and absence of these two motifs with distinct roles.

Ubiquitination and SUMOylation in HIV Infection: Friends and Foes

As intracellular parasites, viruses hijack the cellular machinery to facilitate their replication and spread. This includes favouring the expression of their viral genes over host genes, appropriation of cellular molecules, and manipulation of signalling pathways, including the post-translational machinery. HIV, the causative agent of AIDS, is notorious for using post-translational modifications to generate infectious particles. Here, we discuss the mechanisms by which HIV usurps the ubiquitin and SUMO pathways to modify both viral and host factors to achieve a productive infection, and also how the host innate sensing system uses these post-translational modifications to hinder HIV replication.

Role of the Ubiquitin Proteasome System (UPS) in the HIV-1 Life Cycle

Given that the ubiquitin proteasome system (UPS) is the major protein degradation process in the regulation of a wide variety of cellular processes in eukaryotic cells, including alteration of cellular location, modulation of protein activity, and regulation of protein interaction, it is reasonable to suggest that the infecting HIV-1 and the invaded hosts exploit the UPS in a contest for survival and proliferation. However, to date, regulation of the HIV-1 life cycle has been mainly explained by the stage-specific expression of HIV-1 viral genes, not by elimination processes of the synthesized proteins after completion of their duties in the infected cells, which is also quintessential for understanding the molecular processes of the virus life cycle and thereby HIV-1 pathogenesis. In fact, several previous publications have indicated that the UPS plays a critical role in the regulation of the proteasomal degradation of viral and cellular counterparts at every step of the HIV-1 life cycle, from the virus entry to release of the assembled virus particles, which is integral for the regulation of survival and proliferation of the infecting HIV-1 and to replication restriction of the invading virus in the host. However, it is unknown whether and how these individual events taking place at different stages of the HIV-1 life cycle are orchestrated as an overall strategy to overcome the restrictions conferred by the host cells. Thus, in this review, we overview the interplay between HIV-1 viral and cellular proteins for restrictions/competitions for proliferation of the virus in the infected cell, which could open a new avenue for the development of therapeutics against HIV-1 via targeting a specific step of the proteasome degradation pathway during the HIV-1 life cycle.

Structural Determinants of the APOBEC3G N-Terminal Domain for HIV-1 RNA Association

APOBEC3G (A3G) is a cellular protein that inhibits HIV-1 infection through virion incorporation. The interaction of the A3G N-terminal domain (NTD) with RNA is essential for A3G incorporation in the HIV-1 virion. The interaction between A3G-NTD and RNA is not completely understood. The A3G-NTD is also recognized by HIV-1 Viral infectivity factor (Vif) and A3G-Vif binding leads to A3G degradation. Therefore, the A3G-Vif interaction is a target for the development of antiviral therapies that block HIV-1 replication. However, targeting the A3G-Vif interactions could disrupt the A3G-RNA interactions that are required for A3G's antiviral activity. To better understand A3G-RNA binding, we generated in silico docking models to simulate the RNA-binding propensity of A3G-NTD. We simulated the A3G-NTD residues with high RNA-binding propensity, experimentally validated our prediction by testing A3G-NTD mutations, and identified structural determinants of A3G-RNA binding. In addition, we found a novel amino acid residue, I26 responsible for RNA interaction. The new structural insights provided here will facilitate the design of pharmaceuticals that inhibit A3G-Vif interactions without negatively impacting A3G-RNA interactions.

Expression of MDM2 in Macrophages Promotes the Early Postentry Steps of HIV-1 Infection through Inhibition of p53

The molecular basis for HIV-1 susceptibility in primary human monocyte-derived macrophages (MDMs) was previously evaluated by comparing the transcriptome of infected and bystander populations. Careful analysis of the data suggested that the ubiquitin ligase MDM2 acted as a positive regulator of HIV-1 replication in MDMs. In this study, MDM2 silencing through transcript-specific small interfering RNAs in MDMs induced a reduction in HIV-1 reverse transcription and integration along with an increase in the expression of p53-induced genes, including CDKN1A Experiments with Nutlin-3, a pharmacological inhibitor of MDM2 p53-binding activity, showed a similar effect on HIV-1 infection, suggesting that the observed restriction in HIV-1 production results from the release/activation of p53 and not the absence of MDM2 per se Knockdown and inhibition of MDM2 also both correlate with a decrease in the Thr592-phosphorylated inactive form of SAMHD1. The expression level of MDM2 and the p53 activation status are therefore important factors in the overall susceptibility of macrophages to HIV-1 infection, bringing a new understanding of signaling events controlling the process of virus replication in this cell type.IMPORTANCE Macrophages, with their long life span in vivo and their resistance to HIV-1-mediated cytopathic effect, might serve as viral reservoirs, contributing to virus persistence in an infected individual. Identification of host factors that increase the overall susceptibility of macrophages to HIV-1 might provide new therapeutic targets for the efficient control of viral replication in these cells and limit the formation of reservoirs in exposed individuals. In this study, we demonstrate the importance of p53 regulation by MDM2, which creates a cellular environment more favorable to the early steps of HIV-1 replication. Moreover, we show that p53 stabilization reduces virus infection in human macrophages, highlighting the important role of p53 in antiviral immunity.

The host cell ubiquitin ligase protein CHIP is a potent suppressor of HIV-1 replication

Human immunodeficiency virus-1 (HIV-1) Tat is degraded in the host cell both by proteasomal and lysosomal pathways, but the specific molecules that engage with Tat from these pathways are not known. Because E3 ubiquitin ligases are the primary determinants of substrate specificity within the ubiquitin-dependent proteasomal degradation of proteins, we first sought to identify the E3 ligase associated with Tat degradation. Based on the intrinsic disordered nature of Tat protein, we focused our attention on host cell E3 ubiquitin ligase CHIP (C terminus of HSP70-binding protein). Co-transfection of Tat with a CHIP-expressing plasmid decreased the levels of Tat protein in a dose-dependent manner, without affecting the corresponding mRNA levels. Additionally, the rate of Tat protein degradation as measured by cycloheximide (CHX) chase assay was increased in the presence of CHIP. A CHIP mutant lacking the U-box domain, which is responsible for protein ubiquitination (CHIPΔU-box), was unable to degrade Tat protein. Furthermore, CHIP promoted ubiquitination of Tat by both WT as well as Lys-48-ubiquitin, which has only a single lysine residue at position 48. CHIP transfection in HIV-1 reporter TZM-bl cells resulted in decreased Tat-dependent HIV-1 long-terminal repeat (LTR) promoter transactivation as well as HIV-1 virion production. CHIP knockdown in HEK-293T cells using CRISPR-Cas9 led to higher virion production and enhanced Tat-mediated HIV-1 LTR promoter transactivation, along with stabilization of Tat protein. Together, these results suggest a novel role of host cell E3 ubiquitin ligase protein CHIP in regulating HIV-1 replication through ubiquitin-dependent degradation of its regulatory protein Tat.

Proteasomal Degradation Machinery: Favorite Target of HIV-1 Proteins

Proteasomal degradation pathways play a central role in regulating a variety of protein functions by controlling not only their turnover but also the physiological behavior of the cell. This makes it an attractive target for the pathogens, especially viruses which rely on the host cellular machinery for their propagation and pathogenesis. Viruses have evolutionarily developed various strategies to manipulate the host proteasomal machinery thereby creating a cellular environment favorable for their own survival and replication. Human immunodeficiency virus-1 (HIV-1) is one of the most dreadful viruses which has rapidly spread throughout the world and caused high mortality due to its high evolution rate. Here, we review the various mechanisms adopted by HIV-1 to exploit the cellular proteasomal machinery in order to escape the host restriction factors and components of host immune system for supporting its own multiplication, and successfully created an infection.

Breaking Bad: How Viruses Subvert the Cell Cycle

Interactions between the host and viruses during the course of their co-evolution have not only shaped cellular function and the immune system, but also the counter measures employed by viruses. Relatively small genomes and high replication rates allow viruses to accumulate mutations and continuously present the host with new challenges. It is therefore, no surprise that they either escape detection or modulate host physiology, often by redirecting normal cellular pathways to their own advantage. Viruses utilize a diverse array of strategies and molecular targets to subvert host cellular processes, while evading detection. These include cell-cycle regulation, major histocompatibility complex-restricted antigen presentation, intracellular protein transport, apoptosis, cytokine-mediated signaling, and humoral immune responses. Moreover, viruses routinely manipulate the host cell cycle to create a favorable environment for replication, largely by deregulating cell cycle checkpoints. This review focuses on our current understanding of the molecular aspects of cell cycle regulation that are often targeted by viruses. Further study of their interactions should provide fundamental insights into cell cycle regulation and improve our ability to exploit these viruses.

Dynamical properties and thresholds of an HIV model with super-infection

Super-infection by multiple HIV-1 subtypes, previously thought restricted to high risk groups, has now been reported in the general heterosexual populations at relatively the same incidence rate as in high risk groups. We present a simple deterministic HIV model with super-infection by two HIV-1 subtypes. Mathematical characteristics including the basic reproductive number $(\mathcal{R}_0)$, invasion threshold $(\mathcal{R}_{21},\mathcal{R}_{12})$ and conditions for asymptotic stability are derived. In the absence of super-infection the model exhibits competitive exclusion, and all equilibria are globally attracting if they exist except for the disease free which is a saddle for $\mathcal{R}_0>1.$ The results show that the subtype with the dominant reproductive number exceeding unity dominates the weaker subtype forcing it to extinction regardless of the size of the reproductive number. On the other end, super-infection may promote subtype co-existence whenever the minimum of the subtype specific reproductive numbers $(\mathcal{R}_1,\mathcal{R}_2)$ and the invasion reproductive numbers $(\mathcal{R}_{12},\mathcal{R}_{21})$ exceed unity. Our results demonstrate that if the partial reproductive numbers $(\mathcal{R}_1~\mbox{and}~\mathcal{R}_2 )$ and the invasion reproductive number for the weaker subtype $(\mathcal{R}_{21})$ satisfy $\mathcal{R}_2<1,~\mathcal{R}_1>1~\mbox{and}~\mathcal{R}_{21}>1,$ then primary infection by subtype $1$ may stay the extinction of subtype $2$ despite its relatively low reproductive fitness. For certain parameter ranges, hysteresis (including backward bifurcation) occurs with possible differences in the asymptotic level of disease prevalence. Super-infection may thus facilitate the continued re-generation of reproductively noncompetent subtypes whose subtype specific reproductive numbers will be less than unity while at the same time allowing for the mutual coexistence and persistence of multiple strains. Persistence and co-existence of multiple strains has detrimental effect on vaccine design and development and administration of ART where one or more of the strains are drug resistant.

Post-translational Modification-Based Regulation of HIV Replication

Human immunodeficiency virus (HIV) relies heavily on the host cellular machinery for production of viral progeny. To exploit cellular proteins for replication and to overcome host factors with antiviral activity, HIV has evolved a set of regulatory and accessory proteins to shape an optimized environment for its replication and to facilitate evasion from the immune system. Several cellular pathways are hijacked by the virus to modulate critical steps during the viral life cycle. Thereby, post-translational modifications (PTMs) of viral and cellular proteins gain increasingly attention as modifying enzymes regulate virtually every step of the viral replication cycle. This review summarizes the current knowledge of HIV-host interactions influenced by PTMs with a special focus on acetylation, ubiquitination, and phosphorylation of proteins linked to cellular signaling and viral replication. Insights into these interactions are surmised to aid development of new intervention strategies.

Introduction of H2C2-type zinc-binding residues into HIV-2 Vpr increases its expression level

Human immunodeficiency virus type 2 has two structurally similar proteins, Vpx and Vpr. Vpx degrades the host anti-viral protein SAMHD1 and is expressed at high levels, while Vpr is responsible for cell cycle arrest and is expressed at much lower levels. We constructed a Vpr mutant with a high level of expression by replacing the amino acids HHCR/HHCH with a putative H2C2-type zinc-binding site that is carried by Vpx. Our finding suggests that during the evolution of Vpr and Vpx, zinc-binding likely became a mechanism for regulating their expression levels.

Hijacking of the Ubiquitin/Proteasome Pathway by the HIV Auxiliary Proteins

The ubiquitin-proteasome system (UPS) ensures regulation of the protein pool in the cell by ubiquitination of proteins followed by their degradation by the proteasome. It plays a central role in the cell under normal physiological conditions as well as during viral infections. On the one hand, the UPS can be used by the cell to degrade viral proteins, thereby restricting the viral infection. On the other hand, it can also be subverted by the virus to its own advantage, notably to induce degradation of cellular restriction factors. This makes the UPS a central player in viral restriction and counter-restriction. In this respect, the human immunodeficiency viruses (HIV-1 and 2) represent excellent examples. Indeed, many steps of the HIV life cycle are restricted by cellular proteins, some of which are themselves components of the UPS. However, HIV itself hijacks the UPS to mediate defense against several cellular restriction factors. For example, the HIV auxiliary proteins Vif, Vpx and Vpu counteract specific restriction factors by the recruitment of cellular UPS components. In this review, we describe the interplay between HIV and the UPS to illustrate its role in the restriction of viral infections and its hijacking by viral proteins for counter-restriction.

HIV-1 Tat potently stabilises Mdm2 and enhances viral replication

Murine double minute 2 (Mdm2) is known to enhance the transactivation potential of human immunodeficiency virus (HIV-1) Tat protein by causing its ubiquitination. However, the regulation of Mdm2 during HIV-1 infection and its implications for viral replication have not been well studied. Here, we show that the Mdm2 protein level increases during HIV-1 infection and this effect is mediated by HIV-1 Tat protein. Tat appears to stabilise Mdm2 at the post-translational level by inducing its phosphorylation at serine-166 position through AKT. Although p53 is one of the key players for Mdm2 induction, Tat-mediated stabilisation of Mdm2 appears to be independent of p53. Moreover, the non-phosphorylatable mutant of Mdm2 (S166A) fails to interact with Tat and shows decreased half-life in the presence of Tat compared with wild-type Mdm2. Furthermore, the non-phosphorylatable mutant of Mdm2 (S166A) is unable to support HIV-1 replication. Thus, HIV-1 Tat appears to stabilise Mdm2, which in turn enhances Tat-mediated viral replication. This study highlights the importance of post-translational modifications of host cellular factors in HIV-1 replication and pathogenesis.

Clues for two-step virion infectivity factor regulation by core binding factor beta

Lentiviruses threaten human and animal health. Virion infectivity factor (Vif) is essential for the infectivity of most lentiviruses, except for the equine infectious anaemia virus (EIAV). Vif promotes viral infectivity by recruiting a Cullin-based E3 ligase to induce the degradation of a class of host restriction factors, named APOBEC3. Core binding factor beta (CBF-β) is necessary for several primate lentiviral Vif functions, including HIV-1 Vif. Although much progress has been made in understanding the contribution of CBF-β to Vif function, the precise mechanism has not yet been fully elucidated. In this study, we found that an interaction with CBF-β altered the oligomerization and subcellular distribution pattern and increased the stability of two primate lentiviral Vifs, HIV-1 Vif and Macaca simian immunodeficiency virus (SIVmac) Vif. Moreover, using a CBF-β loss-of-function mutant, we demonstrated that the interaction between CBF-β and Vif was not sufficient for Vif assistance; a region including F68 in CBF-β was also required for the stability and function of Vif. For the first time, this study separates the binding and regulating processes of CBF-β when it is promoting Vif function, which further extends our understanding of the biochemical regulation of Vif by CBF-β.

Cyclin F/FBXO1 Interacts with HIV-1 Viral Infectivity Factor (Vif) and Restricts Progeny Virion Infectivity by Ubiquitination and Proteasomal Degradation of Vif Protein through SCFcyclin F E3 Ligase Machinery

Cyclin F protein, also known as FBXO1, is the largest among all cyclins and oscillates in the cell cycle like other cyclins. Apart from being a G₂/M cyclin, cyclin F functions as the substrate-binding subunit of SCF^{cyclin F} E3 ubiquitin ligase. In a gene expression analysis performed to identify novel gene modulations associated with cell cycle dysregulation during HIV-1 infection in CD4⁺ T cells, we observed down-regulation of the cyclin F gene (CCNF). Later, using gene overexpression and knockdown studies, we identified cyclin F as negatively influencing HIV-1 viral infectivity without any significant impact on virus production. Subsequently, we found that cyclin F negatively regulates the expression of viral protein Vif (viral infectivity factor) at the protein level. We also identified a novel host-pathogen interaction between cyclin F and Vif protein in T cells during HIV-1 infection. Mutational analysis of a cyclin F-specific amino acid motif in the C-terminal region of Vif indicated rescue of the protein from cyclin F-mediated down-regulation. Subsequently, we showed that Vif is a novel substrate of the SCF^{cyclin F} E3 ligase, where cyclin F mediates the ubiquitination and proteasomal degradation of Vif through physical interaction. Finally, we showed that cyclin F augments APOBEC3G expression through degradation of Vif to regulate infectivity of progeny virions. Taken together, our results demonstrate that cyclin F is a novel F-box protein that functions as an intrinsic cellular regulator of HIV-1 Vif and has a negative regulatory effect on the maintenance of viral infectivity by restoring APOBEC3G expression.

Core Binding Factor β Protects HIV, Type 1 Accessory Protein Viral Infectivity Factor from MDM2-mediated Degradation

HIV, type 1 overcomes host restriction factor apolipoprotein B mRNA-editing enzyme catalytic polypeptide-like 3 (APOBEC3) proteins by organizing an E3 ubiquitin ligase complex together with viral infectivity factor (Vif) and a host transcription cofactor core binding factor β (CBFβ). CBFβ is essential for Vif to counteract APOBEC3 by enabling the recruitment of cullin 5 to the complex and increasing the steady-state level of Vif protein; however, the mechanisms by which CBFβ up-regulates Vif protein remains unclear. Because we have reported previously that mouse double minute 2 homolog (MDM2) is an E3 ligase for Vif, we hypothesized that CBFβ might protect Vif from MDM2-mediated degradation. Co-immunoprecipitation analyses showed that Vif mutants that do not bind to CBFβ preferentially interact with MDM2 and that overexpression of CBFβ disrupts the interaction between MDM2 and Vif. Knockdown of CBFβ reduced the steady-state level of Vif in MDM2-proficient cells but not in MDM2-null cells. Cycloheximide chase analyses revealed that Vif E88A/W89A, which does not interact with CBFβ, degraded faster than wild-type Vif in MDM2-proficient cells but not in MDM2-null cells, suggesting that Vif stabilization by CBFβ is mainly caused by impairing MDM2-mediated degradation. We identified Vif R93E as a Vif variant that does not bind to MDM2, and the virus with this substitution mutation was more resistant to APOBEC3G than the parental virus. Combinatory substitution of Vif residues required for CBFβ binding and MDM2 binding showed full recovery of Vif steady-state levels, supporting our hypothesis. Our data provide new insights into the mechanism of Vif augmentation by CBFβ.

AP1S3 is required for hepatitis C virus infection by stabilizing E2 protein

Hepatitis C virus (HCV) infects 130 million people worldwide and is a leading cause of liver cirrhosis, end-stage liver disease and hepatocellular carcinoma. The interactions between viral elements and host factors play critical role on HCV invade, replication and release. Here, we identified adaptor protein complex 1 sigma 3 subunit (AP1S3) as a dependency factor for the efficient HCV infection in hepatoma cells. AP1S3 silencing in cultivated Huh7.5.1 cells significantly reduced the production of HCV progeny particles. Immunoprecipitation analysis revealed that AP1S3 interacted with the HCV E2 protein. With this interaction, AP1S3 could protect HCV E2 from ubiquitin-mediated proteasomal degradation. Using in vivo ubiquitylation assay, we identified that E6-Associated Protein (E6AP) was associated with HCV E2. In addition, treatment with synthetic peptide that contains the AP1S3-recognized motif inhibited HCV infection in Huh7.5.1 cells. Our data reveal AP1 as a novel host network that is required by viruses during infection and provides a potential target for developing broad-spectrum anti-virus strategies.

The HDAC6/APOBEC3G complex regulates HIV-1 infectiveness by inducing Vif autophagic degradation

BACKGROUND

Human immunodeficiency virus type 1 (HIV-1) has evolved a complex strategy to overcome the immune barriers it encounters throughout an organism thanks to its viral infectivity factor (Vif), a key protein for HIV-1 infectivity and in vivo pathogenesis. Vif interacts with and promotes "apolipoprotein B mRNA-editing enzyme-catalytic, polypeptide-like 3G" (A3G) ubiquitination and subsequent degradation by the proteasome, thus eluding A3G restriction activity against HIV-1.

RESULTS

We found that cellular histone deacetylase 6 (HDAC6) directly interacts with A3G through its C-terminal BUZ domain (residues 841-1,215) to undergo a cellular co-distribution along microtubules and cytoplasm. The HDAC6/A3G complex occurs in the absence or presence of Vif, competes for Vif-mediated A3G degradation, and accounts for A3G steady-state expression level. In fact, HDAC6 directly interacts with and promotes Vif autophagic clearance, thanks to its C-terminal BUZ domain, a process requiring the deacetylase activity of HDAC6. HDAC6 degrades Vif without affecting the core binding factor β (CBF-β), a Vif-associated partner reported to be key for Vif- mediated A3G degradation. Thus HDAC6 antagonizes the proviral activity of Vif/CBF-β-associated complex by targeting Vif and stabilizing A3G. Finally, in cells producing virions, we observed a clear-cut correlation between the ability of HDAC6 to degrade Vif and to restore A3G expression, suggesting that HDAC6 controls the amount of Vif incorporated into nascent virions and the ability of HIV-1 particles of being infectious. This effect seems independent on the presence of A3G inside virions and on viral tropism.

CONCLUSIONS

Our study identifies for the first time a new cellular complex, HDAC6/A3G, involved in the autophagic degradation of Vif, and suggests that HDAC6 represents a new antiviral factor capable of controlling HIV-1 infectiveness by counteracting Vif and its functions.

HIV-1 viral infectivity factor interacts with microtubule-associated protein light chain 3 and inhibits autophagy

OBJECTIVE

Autophagy, an important antiviral process triggered during HIV-1 entry by gp41-dependent membrane fusion, is repressed in infected CD4+ T cells by an unknown mechanism. The aim of this study was to identify the role of viral infectivity factor (Vif) in the autophagy blockade.

DESIGN/METHODS

To determine the role of Vif in autophagy inhibition, we used cell lines that express CD4 and CXCR4 and primary CD4+ T cells. Pull-down experiments, immunoprecipitation assays and computational analyses were performed to analyze the interaction between Vif and microtubule-associated protein light chain 3B (LC3B), a major autophagy component, in presence or absence of the antiviral host factor apolipoprotein B mRNA-editing enzyme-catalytic polypeptide-like 3G (APOBEC3G), after HIV-1 infection or ectopic expression of Vif. Autophagy was analyzed after infection by viruses expressing Vif (NL4.3) or not (NL4.3[DELTA]Vif), or after exogenous Vif expression.

RESULTS

We demonstrate that the C-terminal part of Vif interacts directly with LC3B, independently of the presence of APOBEC3G.Vif binds to pro-LC3 and autophagy-related protein 4-cleaved LC3 forms, and glycine 120, the amino acid conjugated to phosphatidylethanolamine on autophagosomes, is required. Importantly, we evidence that Vif inhibits autophagy during HIV-1 infection. Indeed, autophagy is detected in target cells infected by NL4.3[DELTA]Vif, but prevented in cells infected by NL4.3. Furthermore, autophagy triggered in NL4.3[DELTA]Vif-infected cells is inhibited when Vif is expressed in trans but is still active when target cells express a mutant of Vif that binds weakly to LC3B.

CONCLUSION

Our study unveils that Vif inhibits autophagy independently of its action on APOBEC3G and, therefore, suggest a new function of this viral protein in restricting innate antiviral mechanisms.

Biosafety Recommendations on the Handling of Animal Cell Cultures

The first steps in tissue culture are dating back to the beginning of the nineteenth century when biosafety measures did not yet exist. Later on, animal cell culture became essential for scientific research, diagnosis and biotechnological activities. Along with this development, biosafety concerns have emerged pointing to the risks for human health and in a lesser extent for the environment associated to the handling of animal cell cultures. The management of these risks requires a thorough risk assessment of both the cell cultures and the type of manipulation prior the start of any activity. It involves a case-by-case evaluation of both the intrinsic properties of the cell culture genetically modified or not and the probability that it may inadvertently or intentionally become infected with pathogenic micro-organisms. The latter hazard is predominant when adventitious contaminants are pathogenic or have a better capacity to persist in unfavourable conditions. Consequently, most of the containment measures primarily aim at protecting cells from adventitious contamination. Cell cultures known to harbour an infectious etiologic agent should be manipulated in compliance with containment measures recommended for the etiologic agent itself. The manipulation of cell cultures from human or primate origin necessitates the use of a type II biosafety cabinet. The scope of this chapter is to highlight aspects relevant for the risk assessment and to summarize the main biosafety recommendations and the recent technological advances allowing a mitigation of the risk for the handling of animal cell cultures.

HIV-1 and HIV-2 Vif interact with human APOBEC3 proteins using completely different determinants

Human APOBEC3 (A3) restriction factors provide intrinsic immunity against zoonotic transmission of pathogenic viruses. A3D, A3F, A3G, and A3H haplotype II (A3H-hapII) can be packaged into virion infectivity factor (Vif)-deficient HIVs to inhibit viral replication. To overcome these restriction factors, Vif binds to the A3 proteins in viral producer cells to target them for ubiquitination and proteasomal degradation, thus preventing their packaging into assembling virions. Therefore, the Vif-A3 interactions are attractive targets for novel drug development. HIV-1 and HIV-2 arose via distinct zoonotic transmission events of simian immunodeficiency viruses from chimpanzees and sooty mangabeys, respectively, and Vifs from these viruses have limited homology. To gain insights into the evolution of virus-host interactions that led to successful cross-species transmission of lentiviruses, we characterized the determinants of the interaction between HIV-2 Vif (Vif2) with human A3 proteins and compared them to the previously identified HIV-1 Vif (Vif1) interactions with the A3 proteins. We found that A3G, A3F, and A3H-hapII, but not A3D, were susceptible to Vif2-induced degradation. Alanine-scanning mutational analysis of the first 62 amino acids of Vif2 indicated that Vif2 determinants important for degradation of A3G and A3F are completely distinct from these regions in Vif1, as are the determinants in A3G and A3F that are critical for Vif2-induced degradation. These observations suggest that distinct Vif-A3 interactions evolved independently in different SIVs and their nonhuman primate hosts and conservation of the A3 determinants targeted by the SIV Vif proteins resulted in successful zoonotic transmission into humans.

IMPORTANCE

Primate APOBEC3 proteins provide innate immunity against invading pathogens, and Vif proteins of primate lentiviruses have evolved to overcome these host defenses by interacting with them and inducing their proteasomal degradation. HIV-1 and HIV-2 are two human pathogens that induce AIDS, and elucidating interactions between their Vif proteins and human A3 proteins could facilitate the development of novel antiviral drugs. Furthermore, understanding Vif-A3 interactions can provide novel insights into the cross-species transmission events that led to the HIV-1 and HIV-2 pandemics and evolution of host-virus interactions. We carried out mutational analysis of the N-terminal 62 amino acids of HIV-2 Vif (Vif2) and analyzed A3G/A3F chimeras that retained antiviral activity to identify the determinants of the Vif2 and A3 interaction. Our results show that the Vif2-A3 interactions are completely different from the Vif1-A3 interactions, suggesting that these interactions evolved independently and that conservation of the A3 determinants resulted in successful zoonotic transmission into humans.

Immunogenetics of small ruminant lentiviral infections

The small ruminant lentiviruses (SRLV) include the caprine arthritis encephalitis virus (CAEV) and the Maedi-Visna virus (MVV). Both of these viruses limit production and can be a major source of economic loss to producers. Little is known about how the immune system recognizes and responds to SRLVs, but due to similarities with the human immunodeficiency virus (HIV), HIV research can shed light on the possible immune mechanisms that control or lead to disease progression. This review will focus on the host immune response to HIV-1 and SRLV, and will discuss the possibility of breeding for enhanced SRLV disease resistance.

Small molecules that inhibit Vif-induced degradation of APOBEC3G

BACKGROUND

HIV-1 Vif is essential for virus replication in natural target cells such as T cells and macrophages. Vif recruits a ubiquitin ligase to degrade restrictive APOBEC3 proteins. APOBEC3G is one of the most potent retroviral restriction factors targeted by Vif and, as such, the Vif-APOBEC3G interaction has emerged as a promising HIV-1 therapeutic target.

METHODS

20,000 small molecules were used in live-cell screens for those that preserve EGFP-APOBEC3G fluorescence and luciferase-APOBEC3G luminescence in the presence of HIV-1 Vif.

RESULTS

2 compounds with similar core structures preserved APOBEC3G levels in the presence of Vif. 10 μM of compound restored APOBEC3G to levels sufficient for incorporation into vif-proficient virus particles and restriction of virus infectivity. Vif-dependent APOBEC3G polyubiquitination and general proteasomal activity were unaffected at the same concentration.

CONCLUSIONS

The small molecules described here preserve APOBEC3G levels and activity in the presence of Vif. These molecules are starting points for further development as antiretrovirals.

Defining HIV-1 Vif residues that interact with CBFβ by site-directed mutagenesis

Vif is essential for HIV-1 replication in T cells and macrophages. Vif recruits a host ubiquitin ligase complex to promote proteasomal degradation of the APOBEC3 restriction factors by poly-ubiquitination. The cellular transcription cofactor CBFβ is required for Vif function by stabilizing the Vif protein and promoting recruitment of a cellular Cullin5-RING ubiquitin ligase complex. Interaction between Vif and CBFβ is a promising therapeutic target, but little is known about the interfacial residues. We now demonstrate that Vif conserved residues E88/W89 are crucial for CBFβ binding. Substitution of E88/W89 to alanines impaired binding to CBFβ, degradation of APOBEC3, and virus infectivity in the presence of APOBEC3 in single-cycle infection. In spreading infection, NL4-3 with Vif E88A/W89A mutation replicated comparably to wild-type virus in permissive CEM-SS cells, but not in multiple APOBEC3 expressing non-permissive CEM cells. These results support a model in which HIV-1 Vif residues E88/W89 may participate in binding CBFβ.

Interactions between HIV-1 Vif and human ElonginB-ElonginC are important for CBF-β binding to Vif

Background

The HIV-1 accessory factor Vif is necessary for efficient viral infection in non-permissive cells. Vif antagonizes the antiviral activity of human cytidine deaminase APOBEC3 proteins that confer the non-permissive phenotype by tethering them (APOBEC3DE/3F/3G) to the Vif-CBF-β-ElonginB-ElonginC-Cullin5-Rbx (Vif-CBF-β-EloB-EloC-Cul5-Rbx) E3 complex to induce their proteasomal degradation. EloB and EloC were initially reported as positive regulatory subunits of the Elongin (SIII) complex. Thereafter, EloB and EloC were found to be components of Cul-E3 complexes, contributing to proteasomal degradation of specific substrates. CBF-β is a newly identified key regulator of Vif function, and more information is needed to further clarify its regulatory mechanism. Here, we comprehensively investigated the functions of EloB (together with EloC) in the Vif-CBF-β-Cul5 E3 ligase complex.

Results

The results revealed that: (1) EloB (and EloC) positively affected the recruitment of CBF-β to Vif. Both knockdown of endogenous EloB and over-expression of its mutant with a 34-residue deletion in the COOH-terminal tail (EloBΔC34/EBΔC34) impaired the Vif-CBF-β interaction. (2) Introduction of both the Vif SLQ → AAA mutant (VifΔSLQ, which dramatically impairs Vif-EloB-EloC binding) and the Vif PPL → AAA mutant (VifΔPPL, which is thought to reduce Vif-EloB binding) could reduce CBF-β binding. (3) EloB-EloC but not CBF-β could greatly enhance the folding of full-length Vif in Escherichia coli. (4) The over-expression of EloB or the N-terminal ubiquitin-like (UbL) domain of EloB could significantly improve the stability of Vif/VifΔSLQ/VifΔPPL through the region between residues 9 and 14.

Conclusion

Our results indicate that the Vif interaction with EloB-EloC may contribute to recruitment of CBF-β to Vif, demonstrating that the EloB C-teminus may play a role in improving Vif function and that the over-expression of EloB results in Vif stabilization.

Mapping the Vif-A3G interaction using peptide arrays: a basis for anti-HIV lead peptides

Human apolipoprotein-B mRNA-editing catalytic polypeptide-like 3G (A3G) is a cytidine deaminase that restricts retroviruses, endogenous retro-elements and DNA viruses. A3G plays a key role in the anti-HIV-1 innate cellular immunity. The HIV-1 Vif protein counteracts A3G mainly by leading A3G towards the proteosomal machinery and by direct inhibition of its enzymatic activity. Both activities involve direct interaction between Vif and A3G. Disrupting the interaction between A3G and Vif may rescue A3G antiviral activity and inhibit HIV-1 propagation. Here, mapping the interaction sites between A3G and Vif by peptide array screening revealed distinct regions in Vif important for A3G binding, including the N-terminal domain (NTD), C-terminal domain (CTD) and residues 83-99. The Vif-binding sites in A3G included 12 different peptides that showed strong binding to either full-length Vif, Vif CTD or both. Sequence similarity was found between Vif-binding peptides from the A3G CTD and NTD. A3G peptides were synthesized and tested for their ability to counteract Vif action. A3G 211-225 inhibited HIV-1 replication in cell culture and impaired Vif dependent A3G degradation. In vivo co-localization of full-length Vif with A3G 211-225 was demonstrated by use of FRET. This peptide has the potential to serve as an anti-HIV-1 lead compound. Our results suggest a complex interaction between Vif and A3G that is mediated by discontinuous binding regions with different affinities.

HIV-1 Vif: a guardian of the virus that opens up a new era in the research field of restriction factors

The research on virion infectivity factor (Vif) protein had started in late 1980s right after HIV-1 was cloned, and the function of Vif had been a mystery for a long time. However, the research on Vif has finally lead to the identification of APOBEC3G, which opens up a new era in the research field of host restriction factors in HIV-1 infection followed by TRIM5α, Tetherin/BST-2, and SAMHD1. This suggests that continuation of basic research on fundamental questions is quite important. We still have many questions on Vif and APOBEC3 and should continue to work on these proteins in the future in order to better regulate HIV-1. We will discuss not only the history but also recent advances in Vif research.

Host restriction factors in retroviral infection: promises in virus-host interaction

Retroviruses have an intricate life cycle. There is much to be learned from studying retrovirus-host interactions. Among retroviruses, the primate lentiviruses have one of the more complex genome structures with three categories of viral genes: structural, regulatory, and accessory genes. Over time, we have gained increasing understanding of the lentivirus life cycle from studying host factors that support virus replication. Similarly, studies on host restriction factors that inhibit viral replication have also made significant contributions to our knowledge. Here, we review recent progress on the rapidly growing field of restriction factors, focusing on the antiretroviral activities of APOBEC3G, TRIM5, tetherin, SAMHD1, MOV10, and cellular microRNAs (miRNAs), and the counter-activities of Vif, Vpu, Vpr, Vpx, and Nef.

The role of Vif oligomerization and RNA chaperone activity in HIV-1 replication

The viral infectivity factor (Vif) is essential for the productive infection and dissemination of HIV-1 in non-permissive cells that involve most natural HIV-1 target cells. Vif counteracts the packaging of two cellular cytidine deaminases named APOBEC3G (A3G) and A3F by diverse mechanisms including the recruitment of an E3 ubiquitin ligase complex and the proteasomal degradation of A3G/A3F, the inhibition of A3G mRNA translation or by a direct competition mechanism. In addition, Vif appears to be an active partner of the late steps of viral replication by participating in virus assembly and Gag processing, thus regulating the final stage of virion formation notably genomic RNA dimerization and by inhibiting the initiation of reverse transcription. Vif is a small pleiotropic protein with multiple domains, and recent studies highlighted the importance of Vif conformation and flexibility in counteracting A3G and in binding RNA. In this review, we will focus on the oligomerization and RNA chaperone properties of Vif and show that the intrinsic disordered nature of some Vif domains could play an important role in virus assembly and replication. Experimental evidence demonstrating the RNA chaperone activity of Vif will be presented.

Protein intrinsic disorder as a flexible armor and a weapon of HIV-1

Many proteins and protein regions are disordered in their native, biologically active states. These proteins/regions are abundant in different organisms and carry out important biological functions that complement the functional repertoire of ordered proteins. Viruses, with their highly compact genomes, small proteomes, and high adaptability for fast change in their biological and physical environment utilize many of the advantages of intrinsic disorder. In fact, viral proteins are generally rich in intrinsic disorder, and intrinsically disordered regions are commonly used by viruses to invade the host organisms, to hijack various host systems, and to help viruses in accommodation to their hostile habitats and to manage their economic usage of genetic material. In this review, we focus on the structural peculiarities of HIV-1 proteins, on the abundance of intrinsic disorder in viral proteins, and on the role of intrinsic disorder in their functions.

HIV-1, ubiquitin and ubiquitin-like proteins: the dialectic interactions of a virus with a sophisticated network of post-translational modifications

The modification of intracellular proteins by ubiquitin (Ub) and ubiquitin-like (UbL) proteins is a central mechanism for regulating and fine-tuning all cellular processes. Indeed, these modifications are widely used to control the stability, activity and localisation of many key proteins and, therefore, they are instrumental in regulating cellular functions as diverse as protein degradation, cell signalling, vesicle trafficking and immune response. It is thus no surprise that pathogens in general, and viruses in particular, have developed multiple strategies to either counteract or exploit the complex mechanisms mediated by the Ub and UbL protein conjugation pathways. The aim of this review is to provide an overview on the intricate and conflicting relationships that intimately link HIV-1 and these sophisticated systems of post-translational modifications.

The human transcriptome during nontyphoid Salmonella and HIV coinfection reveals attenuated NFkappaB-mediated inflammation and persistent cell cycle disruption

Background. Invasive nontyphoid Salmonella (iNTS) disease is common and severe in adults with human immunodeficiency virus (HIV) infection in Africa. We previously observed that ex vivo macrophages from HIV-infected subjects challenged with Salmonella Typhimurium exhibit dysregulated proinflammatory cytokine responses.

Methods. We studied the transcriptional response in whole blood from HIV-positive patients during acute and convalescent iNTS disease compared to other invasive bacterial diseases, and to HIV-positive and -negative controls.

Results. During iNTS disease, there was a remarkable lack of a coordinated inflammatory or innate immune signaling response. Few interferon γ (IFNγ)--induced genes or Toll-like receptor/transcription factor nuclear factor κB (TLR/NFκB) gene pathways were upregulated in expression. Ex vivo lipopolysacharide (LPS) or flagellin stimulation of whole blood, however, showed that convalescent iNTS subjects and controls were competent to mount prominent TLR/NFκB-associated patterns of mRNA expression. In contrast, HIV-positive patients with other invasive bacterial infections (Escherichia coli and Streptococcus pneumoniae) displayed a pronounced proinflammatory innate immune transcriptional response. There was also upregulated mRNA expression in cell cycle, DNA replication, translation and repair, and viral replication pathways during iNTS. These patterns persisted for up to 2 months into convalescence.

Conclusions. Attenuation of NFκB-mediated inflammation and dysregulation of cell cycle and DNA-function gene pathway expression are key features of the interplay between iNTS and HIV.

Identification of a Cullin5-ElonginB-ElonginC E3 complex in degradation of feline immunodeficiency virus Vif-mediated feline APOBEC3 proteins

Various feline APOBEC3 (fA3) proteins exhibit broad antiviral activities against a wide range of viruses, such as feline immunodeficiency virus (FIV), feline foamy virus (FFV), and feline leukemia virus (FeLV), as well as those of other species. This activity can be counteracted by the FIV Vif protein, but the mechanism by which FIV Vif suppresses fA3s is unknown. In the present study, we demonstrated that FIV Vif could act via a proteasome-dependent pathway to overcome fA3s. FIV Vif interacted with feline cellular proteins Cullin5 (Cul5), ElonginB, and ElonginC to form an E3 complex to induce degradation of fA3s. Both the dominant-negative Cul5 mutant and a C-terminal hydrophilic replacement ElonginC mutant potently disrupted the FIV Vif activity against fA3s. Furthermore, we identified a BC-box motif in FIV Vif that was essential for the recruitment of E3 ubiquitin ligase and also required for FIV Vif-mediated degradation of fA3s. Moreover, despite the lack of either a Cul5-box or a HCCH zinc-binding motif, FIV Vif specifically selected Cul5. Therefore, FIV Vif may interact with Cul5 via a novel mechanism. These finding imply that SOCS proteins may possess distinct mechanisms to bind Cul5 during formation of the Elongin-Cullin-SOCS box complex.

Inhibition of β-TrcP-dependent ubiquitination of p53 by HIV-1 Vpu promotes p53-mediated apoptosis in human T cells

HIV-1 viral protein U (Vpu) is involved in ubiquitination and degradation of BM stromal cell Ag 2 and surface receptor CD4 through their recruitment to SCF(β-TrcP) (Skp1/Cul1/F-box) ubiquitin ligase (SCF) complex. Here, we show that specific interaction of wild-type Vpu protein with SCF complex leads to inhibition of ubiquitination and proteasomal degradation of p53 protein in a β-TrcP-dependent manner. Successful interaction of SCF(β-TrcP) complex with β-TrcP binding motif (DS(52)GNES(56)) present in Vpu is essential because mutant Vpu possessing specific alanine substitutions (DA(52)GNEA(56)) in the β-TrcP binding motif not only failed to stabilize p53 protein but was also unable to inhibit ubiquitination of p53 protein. Furthermore, Vpu competes efficiently with the interaction of p53 protein with the β-TrcP subunit of the SCF complex and inhibits subsequent ubiquitination of p53 proteins in a dose-dependent manner. We also observed potent apoptotic activity in a p53 null cell line (H-1299) that was cotransfected with p53 and Vpu-expressing plasmids. Furthermore, MOLT-3 (human T-lymphoblast) cells when infected with vesicular stomatitis virus glycoprotein-pseudotypic HIV-1 possessing wild-type vpu gene exhibited maximum activation of p53/Bax proteins and p53-mediated cell death. These findings establish a novel function of Vpu in modulating the stability of p53 protein that correlates positively with apoptosis during late stages of HIV-1 infection.

Mdm2 links genotoxic stress and metabolism to p53

Mouse double minute 2 (Mdm2) gene was isolated from a cDNA library derived from transformed mouse 3T3 cells, and was classified as an oncogene as it confers 3T3 and Rat2 cells tumorigenicity when overexpressed. It encodes a nucleocytoplasmic shuttling ubiquitin E3 ligase, with its main target being tumor suppressor p53, which is mutated in more than 50% of human primary tumors. Mdm2's oncogenic activity is mainly mediated by p53, which is activated by various stresses, especially genotoxic stress, via Atm (ataxia telangiectasia mutated) and Atr (Atm and Rad3-related). Activated p53 inhibits cell proliferation, induces apoptosis or senescence, and maintains genome integrity. Mdm2 is also a target gene of p53 transcription factor. Thus, Mdm2 and p53 form a feedback regulatory loop. External and internal cues, through multiple signaling pathways, can act on Mdm2 to regulate p53 levels and cell proliferation, death, and senescence. This review will focus on how Mdm2 is regulated under genotoxic stress, and by the Akt1-mTOR-S6K1 pathway that is activated by insulin, growth factors, amino acids, or energy status.

Huwe1, a novel cellular interactor of Gag-Pol through integrase binding, negatively influences HIV-1 infectivity

Integration, an indispensable step for retrovirus replication, is executed by integrase (IN), which is expressed as a part of a Gag-Pol precursor. Although mechanistic detail of the IN-catalyzed integration reaction is well defined, numerous evidence have demonstrated that IN is involved in multiple steps of retrovirus replication other than integration. In this study, Huwe1, a HECT-type E3 ubiquitin ligase, was identified as a new cellular interactor of human immunodeficiency virus type 1 (HIV-1) IN. The interaction was mediated through the catalytic core domain of IN and a wide-range region of Huwe1. Interestingly, although depletion of Huwe1 in target cells did not affect the early phase of HIV-1 infection in a human T cell line, we found that infectivity of HIV-1 released from the Huwe1 knockdown cells was significantly augmented more than that of virus produced from control cells. The increase in infectivity occurred in proviral DNA synthesis. Further analysis revealed that Huwe1 interacted with HIV-1 Gag-Pol precursor protein through an IN domain. Our results suggest that Huwe1 in HIV-1 producer cells has a negative impact on early post-entry events during the next round of virus infection via association with an IN region of Gag-Pol.

HIV-1 viral infectivity factor interacts with TP53 to induce G2 cell cycle arrest and positively regulate viral replication

Viral infectivity factor, an accessory protein encoded in the HIV-1 genome, induces G2 cell cycle arrest; however, the biological significance and mechanism(s) remain totally unclear. Here we demonstrate that the TP53 pathway is involved in Vif-mediated G2 cell cycle arrest. Vif enhances the stability and transcriptional activity of TP53 by blocking the MDM2-mediated ubiquitination and nuclear export of TP53. Furthermore, Vif causes G2 cell cycle arrest in a TP53-dependent manner. HXB2 Vif lacks these activities toward TP53 and cannot induce G2 cell cycle arrest. Using mutagenesis, we demonstrate that the critical residues for this function are located in the N-terminal region of Vif. Finally, we construct a mutant NL4-3 virus with an NL4-3/HXB2 chimeric Vif defective for the ability to induce cell cycle arrest and show that the mutant virus replicates less effectively than the wild-type NL4-3 virus in T cells expressing TP53. These data imply that Vif induces G2 cell cycle arrest through functional interaction with the TP53/MDM2 axis and that the G2 cell cycle arrest induced by Vif has a positive effect on HIV-1 replication. This report demonstrates the molecular mechanisms and the biological significance of Vif-mediated G2 cell cycle arrest for HIV-1 infection.