Full peptide synthesis, purification, and characterization of six Tat variants. Differences observed between HIV-1 isolates from Africa and other continents

Role of HIV-1 Tat Protein Interactions with Host Receptors in HIV Infection and Pathogenesis

Each time the virus starts a new round of expression/replication, even under effective antiretroviral therapy (ART), the transactivator of viral transcription Tat is one of the first HIV-1 protein to be produced, as it is strictly required for HIV replication and spreading. At this stage, most of the Tat protein exits infected cells, accumulates in the extracellular matrix and exerts profound effects on both the virus and neighbor cells, mostly of the innate and adaptive immune systems. Through these effects, extracellular Tat contributes to the acquisition of infection, spreading and progression to AIDS in untreated patients, or to non-AIDS co-morbidities in ART-treated individuals, who experience inflammation and immune activation despite virus suppression. Here, we review the role of extracellular Tat in both the virus life cycle and on cells of the innate and adaptive immune system, and we provide epidemiological and experimental evidence of the importance of targeting Tat to block residual HIV expression and replication. Finally, we briefly review vaccine studies showing that a therapeutic Tat vaccine intensifies ART, while its inclusion in a preventative vaccine may blunt escape from neutralizing antibodies and block early events in HIV acquisition.

HIV-1 Tat Protein Enters Dysfunctional Endothelial Cells via Integrins and Renders Them Permissive to Virus Replication

Previous work has shown that the Tat protein of Human Immunodeficiency Virus (HIV)-1 is released by acutely infected cells in a biologically active form and enters dendritic cells upon the binding of its arginine-glycine-aspartic acid (RGD) domain to the α5β1, αvβ3, and αvβ5 integrins. The up-regulation/activation of these integrins occurs in endothelial cells exposed to inflammatory cytokines that are increased in HIV-infected individuals, leading to endothelial cell dysfunction. Here, we show that inflammatory cytokine-activated endothelial cells selectively bind and rapidly take up nano-micromolar concentrations of Tat, as determined by flow cytometry. Protein oxidation and low temperatures reduce Tat entry, suggesting a conformation- and energy-dependent process. Consistently, Tat entry is competed out by RGD-Tat peptides or integrin natural ligands, and it is blocked by anti-α5β1, -αvβ3, and -αvβ5 antibodies. Moreover, modelling–docking calculations identify a low-energy Tat-αvβ3 integrin complex in which Tat makes contacts with both the αv and β3 chains. It is noteworthy that internalized Tat induces HIV replication in inflammatory cytokine-treated, but not untreated, endothelial cells. Thus, endothelial cell dysfunction driven by inflammatory cytokines renders the vascular system a target of Tat, which makes endothelial cells permissive to HIV replication, adding a further layer of complexity to functionally cure and/or eradicate HIV infection.

Balance between Retroviral Latency and Transcription: Based on HIV Model

The representative of the Lentivirus genus is the human immunodeficiency virus type 1 (HIV-1), the causative agent of acquired immunodeficiency syndrome (AIDS). To date, there is no cure for AIDS because of the existence of the HIV-1 reservoir. HIV-1 infection can persist for decades despite effective antiretroviral therapy (ART), due to the persistence of infectious latent viruses in long-lived resting memory CD4+ T cells, macrophages, monocytes, microglial cells, and other cell types. However, the biology of HIV-1 latency remains incompletely understood. Retroviral long terminal repeat region (LTR) plays an indispensable role in controlling viral gene expression. Regulation of the transcription initiation plays a crucial role in establishing and maintaining a retrovirus latency. Whether and how retroviruses establish latency and reactivate remains unclear. In this article, we describe what is known about the regulation of LTR-driven transcription in HIV-1, that is, the cis-elements present in the LTR, the role of LTR transcription factor binding sites in LTR-driven transcription, the role of HIV-1-encoded transactivator protein, hormonal effects on virus transcription, impact of LTR variability on transcription, and epigenetic control of retrovirus LTR. Finally, we focus on a novel clustered regularly interspaced short palindromic repeats-associated protein 9 (CRISPR/dCas9)-based strategy for HIV-1 reservoir purging.

Mass spectrometry profiling of low molecular weight proteins and peptides isolated by acetone precipitation

Solvent-based protein precipitation provides exceptional recovery, particularly when the ionic strength of the solution is controlled. While precipitation is ideally suited for intact protein purification ahead of mass-spectrometry, low molecular weight (LMW) proteins and peptides are considered less susceptible to aggregation in organic solvent. As the combination of salt and organic solvent (i.e. acetone) has yet to be exploited to precipitate LMW proteins, we herein determine the low mass limit for solvent-based protein precipitation. We establish optimized conditions for high recovery precipitation of LMW proteins and peptides. Our results demonstrate a strong dependence on the type of salt to recover LMW components from complex mixtures. Inclusion of 100 mM ZnSO₄ with 97% acetone provides near quantitative recovery of all peptides down to 2 kDa, and continues to exceed 90% yield for peptides at a molecular weight of 1 kDa. A detailed characterization of the precipitated peptides resulting from trypsin and pepsin digestion of complex systems is provided by bottom-up mass spectrometry.

HIV-1 Tat and Viral Latency: What We Can Learn from Naturally Occurring Sequence Variations

Despite the effective use of antiretroviral therapy, the remainder of a latently HIV-1-infected reservoir mainly in the resting memory CD4⁺ T lymphocyte subset has provided a great setback toward viral eradication. While host transcriptional silencing machinery is thought to play a dominant role in HIV-1 latency, HIV-1 protein such as Tat, may affect both the establishment and the reversal of latency. Indeed, mutational studies have demonstrated that insufficient Tat transactivation activity can result in impaired transcription of viral genes and the establishment of latency in cell culture experiments. Because Tat protein is one of highly variable proteins within HIV-1 proteome, it is conceivable that naturally occurring Tat mutations may differentially modulate Tat functions, thereby influencing the establishment and/or the reversal of viral latency in vivo. In this mini review, we summarize the recent findings of Tat naturally occurring polymorphisms associating with host immune responses and we highlight the implication of Tat sequence variations in relation to HIV latency.

Association between different anti-Tat antibody isotypes and HIV disease progression: data from an African cohort

BACKGROUND

The presence of IgG and IgM against Tat, an HIV protein important for viral replication and immune dysfunction, is associated with slow disease progression in clade B HIV-infected individuals. However, although Tat activities strictly depend on the viral clade, our knowledge about the importance of anti-Tat antibodies in non-clade B HIV infection is poor. The objective of this study was to investigate the association of different anti-Tat antibody isotypes with disease progression in non-clade B HIV-infected subjects and to study the relationship between anti-Tat humoral responses and immunological abnormalities.

METHODS

Anti-clade B and -clade C Tat IgG, IgM and IgA titers were assessed in serum samples from 96 cART-naïve subjects with chronic HIV infection from Mbeya, Tanzania, and associated with CD4(+) T cell count, plasma viremia and CD4(+) and CD8(+) T cell phenotypes.

RESULTS

Anti-Tat IgM were preferentially detected in chronic HIV-infected subjects with low T cell activation (p-value = 0.03) and correlated with higher CD4(+) T cell counts and lower viral loads irrespective of the duration of infection (p-value = 0.019 and p-value = 0.037 respectively). Conversely, anti-Tat IgA were preferentially detected in individuals with low CD4(+) T cell counts and high viral load (p-value = 0.02 and p-value < 0.001 respectively). The simultaneous presence of anti-Tat IgG and IgM protected from fast CD4(+) T cell decline (p-value < 0.01) and accumulation of CD38(+)HLADR(+)CD8(+) T cells (p- value = 0.029).

CONCLUSIONS

Anti-Tat IgG alone are not protective in non-clade B infected subjects, unless concomitant with IgM, suggesting a protective role of persistent anti-Tat IgM irrespective of the infecting clade.

Intradermal injection of a Tat Oyi-based therapeutic HIV vaccine reduces of 1.5 log copies/mL the HIV RNA rebound median and no HIV DNA rebound following cART interruption in a phase I/II randomized controlled clinical trial

BACKGROUND

A Tat Oyi vaccine preparation was administered with informed consent to 48 long-term HIV-1 infected volunteers whose viral loads had been suppressed by antiretroviral therapy (cART). These volunteers were randomized in double-blind method into four groups (n = 12) that were injected intradermally with 0, 11, 33, or 99 µg of synthetic Tat Oyi proteins in buffer without adjuvant at times designated by month 0 (M0), M1 and M2, respectively. The volunteers then underwent a structured treatment interruption between M5 and M7.

RESULTS

The primary outcomes of this phase I/IIa clinical trial were the safety and lowering the extent of HIV RNA rebound after cART interruption. Only one undesirable event possibly due to vaccination was observed. The 33 µg dose was most effective at lowering the extent of HIV RNA and DNA rebound (Mann and Whitney test, p = 0.07 and p = 0.001). Immune responses against Tat were increased at M5 and this correlated with a low HIV RNA rebound at M6 (p = 0.01).

CONCLUSION

This study suggests in vivo that extracellular Tat activates and protects HIV infected cells. The Tat Oyi vaccine in association with cART may provide an efficient means of controlling the HIV-infected cell reservoir.

Functional roles of HIV-1 Tat protein in the nucleus

Human immunodeficiency virus-1 (HIV-1) Tat protein is one of the most important regulatory proteins for viral gene expression in the host cell and can modulate different cellular processes. In addition, Tat is secreted by the infected cell and can be internalized by neighboring cells; therefore, it affects both infected and uninfected cells. Tat can modulate cellular processes by interacting with different cellular structures and signaling pathways. In the nucleus, Tat might be localized either in the nucleoplasm or the nucleolus depending on its concentration. Here we review the distinct functions of Tat in the nucleoplasm and the nucleolus in connection with viral infection and HIV-induced oncogenesis.

Non-coding RNAs and HIV: viral manipulation of host dark matter to shape the cellular environment

On October 28th 1943 Winston Churchill said “we shape our buildings, and afterward our buildings shape us” (Humes, 1994). Churchill was pondering how and when to rebuild the British House of Commons, which had been destroyed by enemy bombs on May 10th 1941. The old House had been small and insufficient to hold all its members, but was restored to its original form in 1950 in order to recapture the “convenience and dignity” that the building had shaped into its parliamentary members. The circular loop whereby buildings or dwellings are shaped and go on to shape those that reside in them is also true of pathogens and their hosts. As obligate parasites, pathogens need to alter their cellular host environments to ensure survival. Typically pathogens modify cellular transcription profiles and in doing so, the pathogen in turn is affected, thereby closing the loop. As key orchestrators of gene expression, non-coding RNAs provide a vast and extremely precise set of tools for pathogens to target in order to shape the cellular environment. This review will focus on host non-coding RNAs that are manipulated by the infamous intracellular pathogen, the human immunodeficiency virus (HIV). We will briefly describe both short and long host non-coding RNAs and discuss how HIV gains control of these factors to ensure widespread dissemination throughout the host as well as the establishment of lifelong, chronic infection.

p53-derived host restriction of HIV-1 replication by protein kinase R-mediated Tat phosphorylation and inactivation

Tumor suppressor p53 has been suggested to be a host restriction factor against HIV-1 replication, but the detailed molecular mechanism has remained elusive for decades. Here, we demonstrate that p53-mediated HIV-1 suppression is attributed to double-stranded RNA (dsRNA)-dependent protein kinase (PKR)-mediated HIV-1 trans-activator (Tat) phosphorylation and inactivation. p53 silencing significantly enhanced HIV-1 replication in infected cells. Ectopic expression of p53 suppressed Tat activity, which was rescued by PKR silencing. In addition, ectopic expression of PKR abolished Tat activity in p53(-/-) and eIF2α(CA) cells. Finally, we found that HIV-1 infection activates p53, followed by the induction and activation of PKR. PKR directly interacted with HIV-1 Tat and phosphorylates the first exon of Tat exclusively at five Ser/Thr residues (T23, T40, S46, S62, and S68), which inhibits Tat-mediated provirus transcription in three critical steps: (i) phosphorylation near the arginine-rich motif (ARM) inhibits Tat translocation into the nucleus, (ii) accumulation of Tat phosphorylation abolishes Tat-Tat-responsive region (TAR) binding, and (iii) Tat phosphorylation at T23 and/or T40 obliterates the Tat-cyclin T1 interaction. These five Ser/Thr sites on Tat were highly conserved in HIV-1 strains prevalent in Europe and the United States. Taken together, our findings indicate that p53-derived host restriction of HIV-1 replication is likely attributable, at least in part, to a noncanonical p53/PKR/Tat phosphorylation and inactivation pathway in HIV-1 infection and AIDS pathogenesis.

IMPORTANCE

HIV-1-mediated disease progression to AIDS lasts for years to decades after primary infection. Host restriction and associated viral latency have been studied for several decades. p53 has been suggested as an important host restriction factor against HIV-1 replication. However, the detailed molecular mechanism is still unclear. In the present study, we found that the p53-mediated HIV-1 restriction is attributed to a p53/PKR/Tat-inactivation pathway. HIV-1 infection activated p53, which subsequently induced PKR expression and activation. PKR directly phosphorylated Tat exclusively at five specific Ser/Thr residues, which was accompanied by significant suppression of HIV-1 replication. Accumulation of Tat phosphorylation at these sites inhibited Tat function by blocking Tat nuclear localization, Tat binding to TAR, and Tat-cyclin T1 interaction. Our findings provide a better understanding of the p53-derived host restriction mechanism against HIV-1 replication in AIDS pathogenesis and may contribute to further research focusing on the investigation of potential therapeutic targets for HIV-1.

Multimodality vaccination against clade C SHIV: partial protection against mucosal challenges with a heterologous tier 2 virus

We sought to test whether vaccine-induced immune responses could protect rhesus macaques (RMs) against upfront heterologous challenges with an R5 simian-human immunodeficiency virus, SHIV-2873Nip. This SHIV strain exhibits many properties of transmitted HIV-1, such as tier 2 phenotype (relatively difficult to neutralize), exclusive CCR5 tropism, and gradual disease progression in infected RMs. Since no human AIDS vaccine recipient is likely to encounter an HIV-1 strain that exactly matches the immunogens, we immunized the RMs with recombinant Env proteins heterologous to the challenge virus. For induction of immune responses against Gag, Tat, and Nef, we explored a strategy of immunization with overlapping synthetic peptides (OSP). The immune responses against Gag and Tat were finally boosted with recombinant proteins. The vaccinees and a group of ten control animals were given five low-dose intrarectal (i.r.) challenges with SHIV-2873Nip. All controls and seven out of eight vaccinees became systemically infected; there was no significant difference in viremia levels of vaccinees vs. controls. Prevention of viremia was observed in one vaccinee which showed strong boosting of virus-specific cellular immunity during virus exposures. The protected animal showed no challenge virus-specific neutralizing antibodies in the TZM-bl or A3R5 cell-based assays and had low-level ADCC activity after the virus exposures. Microarray data strongly supported a role for cellular immunity in the protected animal. Our study represents a case of protection against heterologous tier 2 SHIV-C by vaccine-induced, virus-specific cellular immune responses.

Retrospective on the all-in-one retroviral nucleocapsid protein

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This retrospective reviews 30 years of research on the retroviral nucleocapsid protein (NC) focusing on HIV-1 NC.

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Originally considered as a non-specific nucleic-acid binding protein, NC has seminal functions in virus replication.

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Indeed NC turns out to be a all-in-one viral protein that chaperones viral DNA synthesis and integration, and virus formation.

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As a chaperone NC provides assistance to genetic recombination thus allowing the virus to escape the immune response and antiretroviral therapies against HIV-1.

This review aims at briefly presenting a retrospect on the retroviral nucleocapsid protein (NC), from an unspecific nucleic acid binding protein (NABP) to an all-in-one viral protein with multiple key functions in the early and late phases of the retrovirus replication cycle, notably reverse transcription of the genomic RNA and viral DNA integration into the host genome, and selection of the genomic RNA together with the initial steps of virus morphogenesis. In this context we will discuss the notion that NC protein has a flexible conformation and is thus a member of the growing family of intrinsically disordered proteins (IDPs) where disorder may account, at least in part, for its function as a nucleic acid (NA) chaperone and possibly as a protein chaperone vis-à-vis the viral DNA polymerase during reverse transcription. Lastly, we will briefly review the development of new anti-retroviral/AIDS compounds targeting HIV-1 NC because it represents an ideal target due to its multiple roles in the early and late phases of virus replication and its high degree of conservation.

Viral factors in non-progression

Research has undergone considerable development in understanding a small subset of human immunodeficiency virus type 1 (HIV-1)-infected, therapy-naive individuals who maintain a favorable course of infection surviving for longer periods of time. Although, viral, host genetic, and immunological factors have been analyzed in many previous studies in order to delineate mechanisms that contribute to non-progressive HIV disease, there appears to be a no clear cut winner and the non-progressive HIV disease in <1% of HIV-infected individuals appears to be a complex interplay between viral and host factors. Therefore, it is important to review them separately to signify their potential contribution to non-progressive HIV disease. With respect to virological features, genomic sequencing of HIV-1 strains derived from long-term non-progressors has shown that some individuals are infected with attenuated strains of HIV-1 and harbor mutations from single nucleotide polymorphisms to large deletions in HIV-1 structure, regulatory, and accessory genes. The elucidation of functional attributes of defective/attenuated HIV strains may provide better understanding of viral pathogenesis and the discovery of new therapeutic strategies against HIV. This review mainly focuses on the defects in viral genes that possibly guide non-progressive HIV disease.

HIV-1 Tat protein directly induces mitochondrial membrane permeabilization and inactivates cytochrome c oxidase

The Trans-activator protein (Tat) of human immunodeficiency virus (HIV) is a pleiotropic protein involved in different aspects of AIDS pathogenesis. As a number of viral proteins Tat is suspected to disturb mitochondrial function. We prepared pure synthetic full-length Tat by native chemical ligation (NCL), and Tat peptides, to evaluate their direct effects on isolated mitochondria. Submicromolar doses of synthetic Tat cause a rapid dissipation of the mitochondrial transmembrane potential (ΔΨ_m) as well as cytochrome c release in mitochondria isolated from mouse liver, heart, and brain. Accordingly, Tat decreases substrate oxidation by mitochondria isolated from these tissues, with oxygen uptake being initially restored by adding cytochrome c. The anion-channel inhibitor 4,4′-diisothiocyanostilbene-2,2′-disulfonic acid (DIDS) protects isolated mitochondria against Tat-induced mitochondrial membrane permeabilization (MMP), whereas ruthenium red, a ryanodine receptor blocker, does not. Pharmacologic inhibitors of the permeability transition pore, Bax/Bak inhibitors, and recombinant Bcl-2 and Bcl-XL proteins do not reduce Tat-induced MMP. We finally observed that Tat inhibits cytochrome c oxidase (COX) activity in disrupted mitochondria isolated from liver, heart, and brain of both mouse and human samples, making it the first described viral protein to be a potential COX inhibitor.

A monoclonal antibody directed against a conformational epitope of the HIV-1 trans-activator (Tat) protein neutralizes cross-clade

The identification of a neutralizing mAb against extracellular HIV-1 transactivator of transcription (Tat) is important for the development of an efficient HIV-1 treatment. Tat plays an essential role in HIV-1 pathogenesis, not only for HIV-1 replication but also as an extracellular toxin able to disrupt the immune system. We showed previously that immunization of rabbits with Tat Oyi, a variant cloned from an African woman who did not develop AIDS following HIV-1 infection, raised antibodies able to recognize different Tat variants. We carried out mice immunization with Tat Oyi and selected a mAb named 7G12, which had the capacity to cross-recognize heterologous Tat variants by a common three-dimensional epitope. These results highlighted that Tat variants were able to acquire a structure, in contrast to a number of studies showing Tat as an unfolded protein. mAb 7G12 also had the capacity to neutralize the biological activities of these Tat variants by blocking the cellular uptake of extracellular Tat. This is the first study using Tat Oyi to produce a mAb able to neutralize effectively activities of extracellular Tats from different HIV-1 subtypes. This mAb has an important potential in therapeutic passive immunization and could help HIV-1 infected patients to restore their immunity.

Differential induction of rat neuronal excitotoxic cell death by human immunodeficiency virus type 1 clade B and C tat proteins

In the absence of effective antiretroviral therapy, infection with clade B human immunodeficiency virus (HIV-1) infection commonly progresses to AIDS dementia. However, in India, where clade C infection is most prevalent, severe cognitive impairment due to HIV-1 is reported to be less prevalent. The Tat protein of HIV-1, which is released from HIV-1-infected macrophages, is thought to play a major role in the disruption of neuronal function as well as in the infiltration of macrophages associated with advanced neuropathogenesis. Clade B Tat is excitotoxic to hippocampal neurons by potentiating N-methyl-d-aspartate-induced currents of the zinc-sensitive NR1/NR2A N-methyl-d-aspartate receptor in a zinc-binding-dependent mechanism. This study characterizes the zinc-binding properties of clade C Tat protein. Using ultraviolet spectroscopy and the Ellman reaction, we show that clade C Tat protein binds just one zinc ion per monomer. We then investigated the ability of clade C Tat to block the inhibition of N-methyl-d-aspartate receptors from zinc antagonism through ion chelation. Although clade C Tat enhanced N-methyl-d-aspartate-mediated rat hippocampus neuronal toxicity in the presence of zinc, the increase was significantly less than that observed with clade B Tat. These findings suggest that the observed differences in neuropathogenesis found with HIV-1 clade C infection compared to clade B may, in part, be due to a decrease in Tat-mediated neurotoxicity.

Tits and bits of HIV Tat protein

INTRODUCTION

HIV-Tat protein displays an array of functions that are essential for HIV replication. The structural flexibility of Tat protein has been regarded as one of the unique features responsible for sustaining diverse functions, from facilitated membrane-crossing ability to strong affinity for RNA binding.

AREAS COVERED

RNA binding ability and presence of multiple interacting domains in the same protein are very important properties of HIV-Tat protein. Tat protein has shown great ability to influence cellular and viral gene expression. We discuss the functions of HIV Tat protein, describing its structural significance, secretion and uptake of HIV Tat protein by immune cells, post-translational modifications and role of HIV Tat protein in HIV pathogenesis.

EXPERT OPINION

Perturbation in expression of many cytokines and chemokines by HIV-Tat protein exhibits downstream immune suppressive function as well as activation of several apoptotic genes. This explains the massive death of immune cells due to bystander effect of HIV Tat protein among HIV-infected patients.

HIV-1 clade B Tat, but not clade C Tat, increases X4 HIV-1 entry into resting but not activated CD4+ T cells

CXCR4-using human immunodeficiency virus, type 1 (HIV-1) variants emerge late in the course of infection in >40% of individuals infected with clade B HIV-1 but are described less commonly with clade C isolates. Tat is secreted by HIV-1-infected cells where it acts on both uninfected bystander cells and infected cells. In this study, we show that clade B Tat, but not clade C Tat, increases CXCR4 surface expression on resting CD4+ T cells through a CCR2b-dependent mechanism that does not involve de novo protein synthesis. The expression of plectin, a cytolinker protein that plays an important role as a scaffolding platform for proteins involved in cellular signaling including CXCR4 signaling and trafficking, was found to be significantly increased following B Tat but not C Tat treatment. Knockdown of plectin using RNA interference showed that plectin is essential for the B Tat-induced translocation of CXCR4 to the surface of resting CD4+ T cells. The increased surface CXCR4 expression following B Tat treatment led to increased function of CXCR4 including increased chemoattraction toward CXCR4-using-gp120. Moreover, increased CXCR4 surface expression rendered resting CD4+ T cells more permissive to X4 but not R5 HIV-1 infection. However, neither B Tat nor C Tat was able to up-regulate surface expression of CXCR4 on activated CD4+ T cells, and both proteins inhibited the infection of activated CD4+ T cells with X4 but not R5 HIV-1. Thus, B Tat, but not C Tat, has the capacity to render resting, but not activated, CD4+ T cells more susceptible to X4 HIV-1 infection.

HIV-1 Tat-based vaccines: an overview and perspectives in the field of HIV/AIDS vaccine development

The HIV epidemic continues to represent one of the major problems worldwide, particularly in the Asia and Sub-Saharan regions of the world, with social and economical devastating effects. Although antiretroviral drugs have had a dramatically beneficial impact on HIV-infected individuals that have access to treatment, it has had a negligible impact on the global epidemic. Hence, the inexorable spreading of the HIV pandemic and the increasing deaths from AIDS, especially in developing countries, underscore the urgency for an effective vaccine against HIV/AIDS. However, the generation of such a vaccine has turned out to be extremely challenging. Here we provide an overview on the rationale for the use of non-structural HIV proteins, such as the Tat protein, alone or in combination with other HIV early and late structural HIV antigens, as novel, promising preventative and therapeutic HIV/AIDS vaccine strategies.

Role of Tat protein in HIV neuropathogenesis

The Tat protein of the human immunodeficiency virus (HIV) has been implicated in the pathophysiology of the neurocognitive deficits associated with HIV infection. This is the earliest protein to be produced by the proviral DNA in the infected cell. The protein not only drives the regulatory regions of the virus but may also be actively released from the cell and then interact with the cell surface receptors of other uninfected cells in the brain leading to cellular dysfunction. It may also be taken up by these cells and can then activate a number of host genes. The Tat protein is highly potent and has the unique ability to travel along neuronal pathways. Importantly, its production is not impacted by the use of antiretroviral drugs once the proviral DNA has been formed. This article reviews the pleomorphic actions of Tat protein and the evidence supporting its central role in the neuropathogenesis of the HIV infection.

Bovine immunodeficiency virus: identification of a long terminal repeat sequence with enhanced promoter activity

We previously identified a new bovine immunodeficiency virus (BIV) trans-activator factor of transcription (Tat236) that was derived from a variant of BIV. Here, we report a new BIV long terminal repeat (LTR) sequence (LTRn) that was obtained by PCR from the DNA of cells infected with the BIV variant mentioned above. Sequence analysis indicated that the LTRn U3 region harbors three nucleic acid mutations at residue positions -194, -135 and -114 when compared to the original (wild-type) LTR sequence. Reporter gene assays indicated that LTRn promotes basal and Tat-mediated transactivation activity to levels significantly higher than those obtained with the wild-type LTR. Restoration experiments to the wild-type genotype indicated that both the -135 and -114 nucleic acid substitutions were responsible for the enhanced promoter activity of BIV LTRn.

What does the structure-function relationship of the HIV-1 Tat protein teach us about developing an AIDS vaccine?

The human immunodeficiency virus type 1 (HIV-1) trans-activator of transcription protein Tat is an important factor in viral pathogenesis. In addition to its function as the key trans-activator of viral transcription, Tat is also secreted by the infected cell and taken up by neighboring cells where it has an effect both on infected and uninfected cells. In this review we will focus on the relationship between the structure of the Tat protein and its function as a secreted factor. To this end we will summarize some of the exogenous functions of Tat that have been implicated in HIV-1 pathogenesis and the impact of structural variations and viral subtype variants of Tat on those functions. Finally, since in some patients the presence of Tat-specific antibodies or CTL frequencies are associated with slow or non-progression to AIDS, we will also discuss the role of Tat as a potential vaccine candidate, the advances made in this field, and the importance of using a Tat protein capable of eliciting a protective or therapeutic immune response to viral challenge.

Binding sites of the viral RNA element TAR and of TAR mutants for various peptide ligands, probed with LILBID: a new laser mass spectrometry

A new laser-based mass spectrometry method, called laser induced liquid bead ion desorption (LILBID), was applied to investigate RNA:ligand interactions. As model system the HIV Tat:TAR transactivation complex and its binding behavior were analyzed. TARwt of HIV Type 1 and Type 2 and different artificial mutants were compared regarding their binding to Tat and different peptide ligands. Specific and nonspecific association to TAR was deduced, with the bulge being the well known specific binding site of TAR. In the case of triple arginine (RRR) as a nonspecific ligand, multiple electrostatic binding to TAR was found at higher concentration of RRR. This contrasted with the formation of only ternary complexes in competitive binding studies with TAR, Tat, and potential inhibitors. The fact that the stoichiometries of the complexes can be determined is a major advantage of MS methods over the widely applied fluorimetric methods. A quantitative evaluation of the spectra by a numeric model for ternary complex formation allows conclusions about the role and strength of the binding sites of the RNAs, the specificity and affinity of different ligands, the determination of apparent IC50 and KD values, and a comparison of the binding efficiencies of potential inhibitors.

Homonuclear 1H NMR and circular dichroism study of the HIV-1 Tat Eli variant

Background

The HIV-1 Tat protein is a promising target to develop AIDS therapies, particularly vaccines, due to its extracellular role that protects HIV-1-infected cells from the immune system. Tat exists in two different lengths, 86 or 87 residues and 99 or 101 residues, with the long form being predominant in clinical isolates. We report here a structural study of the 99 residue Tat Eli variant using 2D liquid-state NMR, molecular modeling and circular dichroism.

Results

Tat Eli was obtained from solid-phase peptide synthesis and the purified protein was proven biologically active in a trans-activation assay. Circular dichroism spectra at different temperatures up to 70°C showed that Tat Eli is not a random coil at 20°C. Homonuclear ¹H NMR spectra allowed us to identify 1639 NMR distance constraints out of which 264 were interresidual. Molecular modeling satisfying at least 1474 NMR constraints revealed the same folding for different model structures. The Tat Eli model has a core region composed of a part of the N-terminus including the highly conserved Trp 11. The extra residues in the Tat Eli C-terminus protrude from a groove between the basic region and the cysteine-rich region and are well exposed to the solvent.

Conclusion

We show that active Tat variants share a similar folding pattern whatever their size, but mutations induce local structural changes.

Modulation of microtubule assembly by the HIV-1 Tat protein is strongly dependent on zinc binding to Tat

Background

During HIV-1 infection, the Tat protein plays a key role by transactivating the transcription of the HIV-1 proviral DNA. In addition, Tat induces apoptosis of non-infected T lymphocytes, leading to a massive loss of immune competence. This apoptosis is notably mediated by the interaction of Tat with microtubules, which are dynamic components essential for cell structure and division. Tat binds two Zn²⁺ ions through its conserved cysteine-rich region in vitro, but the role of zinc in the structure and properties of Tat is still controversial.

Results

To investigate the role of zinc, we first characterized Tat apo- and holo-forms by fluorescence correlation spectroscopy and time-resolved fluorescence spectroscopy. Both of the Tat forms are monomeric and poorly folded but differ by local conformational changes in the vicinity of the cysteine-rich region. The interaction of the two Tat forms with tubulin dimers and microtubules was monitored by analytical ultracentrifugation, turbidity measurements and electron microscopy. At 20°C, both of the Tat forms bind tubulin dimers, but only the holo-Tat was found to form discrete complexes. At 37°C, both forms promoted the nucleation and increased the elongation rates of tubulin assembly. However, only the holo-Tat increased the amount of microtubules, decreased the tubulin critical concentration, and stabilized the microtubules. In contrast, apo-Tat induced a large amount of tubulin aggregates.

Conclusion

Our data suggest that holo-Tat corresponds to the active form, responsible for the Tat-mediated apoptosis.

Tat-specific binding IgG and disease progression in HIV type 1-infected Ugandans

There are data to suggest that both the humoral and cellular immune responses directed against Tat are beneficial in delaying HIV disease progression. We examined the association between the occurrence of Tat-specific binding antibodies (Abs) and different parameters of HIV-1 disease progression. We generated eight Tat proteins, derived from HIV-1 subtypes A, B, C, and D, and circulating recombinant form CRF01_AE. These proteins were used to screen for Tat-specific binding Abs by an ELISA. Using five Tat proteins, we investigated whether the occurrence of Tat-specific Abs within 2 years after seroconversion for the majority, affected disease progression over time among 126 participants using survival analysis and rate of CD4 decline. Of these, 52 participants with a sample at 1.5 and 4.5 years after seroconversion were further examined to study the effect of Tat-specific Ab loss or maintenance on disease progression. Finally, using all the eight Tat proteins, we also investigated whether specific Abs to these Tat proteins among 48 participants, grouped as rapid progressors (RP, n = 26) and long-term survivors (LTS, n = 22) according to their CD4 decline over time, affected disease progression. Survival analysis did not reveal any evidence of protection from progression by Tat-specific Abs. Comparison of rate of CD4 declines between individuals with and without Abs to any Tat protein showed only a small and borderline significant advantage of having Tat-specific Abs (p = 0.043). There was no correlation between either loss or maintenance of Tat-specific Abs and disease progression. Comparison of LTS with RP showed no evidence that Tat-specific Abs slows participants' disease progression. This study showed no evidence of a protective effect of having Tat-specific Abs among these Ugandan subjects.

Fluorescence correlation spectroscopy at single molecule level on the Tat–TAR complex and its inhibitors

The TAR element of HIV and the viral protein Tat form a molecular switch regulating transcriptional efficiency in HIV. We show that fluorescence correlation spectroscopy at the single molecule level is a powerful method to study the association between a Tat-derived peptide and TAR fragments. We also investigated the inhibition of the peptide-RNA complex by different ligands. Utilizing cross correlation measurements, the dissociation constants (K(D)) were determined. To demonstrate the important role of the bulge for the binding of Tat, we compared wt-TAR with three RNA mutants, mainly differing in the bulge region. For the TAR mutants studied at equimolar concentration of RNA and peptide (25 nM), the K(D) values are 15-35 times larger than that of wt-TAR. This gives evidence that the bulge region is the most crucial part of the TAR RNA for specific Tat binding. The IC(50) values for different inhibitors of the Tat/TAR complex both with wt-TAR and mutants have been determined. Neamine conjugate proved to be the best inhibitor of the complex formation. Our results are in agreement with earlier published data on this system using alternative biophysical and biochemical methods, respectively.

Tat mutations in an African cohort that do not prevent transactivation but change its immunogenic properties

Humoral responses against extra-cellular HIV-1 Tat may be beneficial as Tat has been implicated in the viral pathogenesis associated with HIV-1 disease progression. We determined the levels of anti-Tat IgG in sera of HIV-1 seropositive individuals from the Rural Clinical Cohort in Uganda using nine different Tat proteins representative of the major subtypes presently accounting for 97% of infections worldwide. We observed the presence of anti-Tat IgG able to react against the various subtypes tested, although none cross-reacted against all nine variants. We show that 46.25% of seropositive patients were able to recognise at least one Tat variant with 1:1000 sera dilution. We also show that the C terminus of Tat is the most variable region and an important epitope that might explain the limitation of cross-recognition of Tat antibodies regarding Tat variants. This study shows in seropositive patients that Tat can tolerate mutations without modification of its primary function but with changes in its immunogenic properties. These findings should be considered when designing Tat-based HIV-1 vaccines.

Human immunodeficiency virus type 1 subtype C Tat fails to induce intracellular calcium flux and induces reduced tumor necrosis factor production from monocytes

Over 50% of all human immunodeficiency virus type 1 (HIV-1) infections worldwide are caused by subtype C strains, yet most research to date focuses on subtype B, the subtype most commonly found in North America and Europe. The HIV-1 trans-acting regulatory protein (Tat) is essential for regulating productive replication of HIV-1. Tat is secreted by HIV-infected cells and alters several functions of uninfected bystander cells. One such function is that, by acting at the cell membrane, subtype B Tat stimulates the production of tumor necrosis factor (TNF) and chemokine (C-C motif) ligand 2 (CCL2) from human monocytes and can act as a chemoattractant. In this study, we show that the mutation of a cysteine to a serine at residue 31 of Tat commonly found in subtype C variants significantly inhibits the abilities of the protein to bind to chemokine (C-C motif) receptor 2 (CCR2), induce intracellular calcium flux, stimulate TNF and CCL2 production, and inhibit its chemoattractant properties. We also show that TNF is important in mediating some effects of extracellular Tat. This report therefore demonstrates the important functional differences between subtype C and subtype B Tat and highlights the need for further investigation into the different strains of HIV-1.

Transactivation and signaling functions of Tat are not correlated: biological and immunological characterization of HIV-1 subtype-C Tat protein

Background

Of the diverse subtypes of Human Immunodeficiency Virus Type-1 (HIV-1), subtype-C strains cause a large majority of infections worldwide. The reasons for the global dominance of HIV-1 subtype-C infections are not completely understood. Tat, being critical for viral infectivity and pathogenesis, may differentially modulate pathogenic properties of the viral subtypes. Biochemical studies on Tat are hampered by the limitations of the current purification protocols. Tat purified using standard protocols often is competent for transactivation activity but defective for a variety of other biological functions. Keeping this limitation in view, we developed an efficient protein purification strategy for Tat.

Results

Tat proteins obtained using the novel strategy described here were free of contaminants and retained biological functions as evaluated in a range of assays including the induction of cytokines, upregulation of chemokine coreceptor, transactivation of the viral promoter and rescue of a Tat-defective virus. Given the highly unstable nature of Tat, we evaluated the effect of the storage conditions on the biological function of Tat following purification. Tat stored in a lyophilized form retained complete biological activity regardless of the storage temperature. To understand if variations in the primary structure of Tat could influence the secondary structure of the protein and consequently its biological functions, we determined the CD spectra of subtype-C and -B Tat proteins. We demonstrate that subtype-C Tat may have a relatively higher ordered structure and be less flexible than subtype-B Tat. We show that subtype-C Tat as a protein, but not as a DNA expression vector, was consistently inferior to subtype-B Tat in a variety of biological assays. Furthermore, using ELISA, we evaluated the anti-Tat antibody titers in a large number of primary clinical samples (n = 200) collected from all four southern Indian states. Our analysis of the Indian populations demonstrated that Tat is non-immunodominant and that a large variation exists in the antigen-specific antibody titers.

Conclusion

Our report not only describes a simple protein purification strategy for Tat but also demonstrates important structural and functional differences between subtype-B and -C Tat proteins. Furthermore, this is the first report of protein purification and characterization of subtype-C Tat.

Reservoir cells no longer detectable after a heterologous SHIV challenge with the synthetic HIV-1 Tat Oyi vaccine

Background

Extra-cellular roles of Tat might be the main cause of maintenance of HIV-1 infected CD4 T cells or reservoir cells. We developed a synthetic vaccine based on a Tat variant of 101 residues called Tat Oyi, which was identified in HIV infected patients in Africa who did not progress to AIDS. We compared, using rabbits, different adjuvants authorized for human use to test on ELISA the recognition of Tat variants from the five main HIV-1 subtypes. A formulation was tested on macaques followed by a SHIV challenge with a European strain.

Results

Tat Oyi with Montanide or Calcium Phosphate gave rabbit sera able to recognize all Tat variants. Five on seven Tat Oyi vaccinated macaques showed a better control of viremia compared to control macaques and an increase of CD8 T cells was observed only on Tat Oyi vaccinated macaques. Reservoir cells were not detectable at 56 days post-challenge in all Tat Oyi vaccinated macaques but not in the controls.

Conclusion

The Tat Oyi vaccine should be efficient worldwide. No toxicity was observed on rabbits and macaques. We show in vivo that antibodies against Tat could restore the cellular immunity and make it possible the elimination of reservoir cells.

The C terminus of HIV-1 Tat modulates the extent of CD178-mediated apoptosis of T cells

HIV infection and the progression to AIDS are characterized by the depletion of CD4(+) T cells through apoptosis of the uninfected bystander cells and the direct killing of HIV-infected cells. This is mediated in part by the human immunodeficiency virus, type 1 Tat protein, which is secreted by virally infected cells and taken up by uninfected cells and CD178 gene expression, which is critically involved in T cell apoptosis. The differing ability of HIV strains to induce death of infected and uninfected cells may play a role in the clinical and biological differences displayed by HIV strains. We chemically synthesized the 86-residue truncated short variant of Tat and its full-length form. We show that the trans-activation ability of Tat at the long terminal repeat does not correlate with T cell apoptosis but that the ability of Tat to up-regulate CD178 mRNA expression and induce apoptosis in T cells is critically dependent on the C terminus of Tat. Moreover, the greater 86-residue Tat-induced apoptosis is via the extrinsic pathway of CD95-CD178.

The molecular biology of bovine immunodeficiency virus: a comparison with other lentiviruses

Bovine immunodeficiency virus (BIV) was first isolated in 1969 from a cow, R-29, with a wasting syndrome. The virus isolated induced the formation of syncytia in cell cultures and was structurally similar to maedi-visna virus. Twenty years later, it was demonstrated that the bovine R-29 isolate was indeed a lentivirus with striking similarity to the human immunodeficiency virus. Like other lentiviruses, BIV has a complex genomic structure characterized by the presence of several regulatory/accessory genes that encode proteins, some of which are involved in the regulation of virus gene expression. This manuscript aims to review biological and, more particularly, molecular aspects of BIV, with emphasis on regulatory/accessory viral genes/proteins, in comparison with those of other lentiviruses.

Soluble HIV-1 gp120 enhances HIV-1 replication in non-dividing CD4+ T cells, mediated via cell signaling and Tat cofactor overexpression

OBJECTIVES

The soluble HIV-1 gp120 envelope glycoprotein, after being shed from infected cells, can cross-link its receptors on both HIV-1 infected and non-infected target cells, leading to their activation. We have assessed the impact of soluble gp120 on viral replication in CD4+/CXCR4+ T cells, via its effects on Tat-mediated transactivation of the HIV-1/LTR.

MATERIALS AND METHODS

Primary cord blood-derived CD4+/CXCR4+ T cells were stimulated with soluble recombinant gp120 (rgp120) from the HIV-1/HXB2 clone. The level of gene or protein expression was assessed by serial analysis gene expression (SAGE), reverse transcriptase-polymerase chain reaction, western blotting or flow-cytometry analysis. Cellular division of rgp120-stimulated T cells was assessed by CFDA-SE labeling. Long terminal repeat (LTR) activity and HIV infection level were respectively measured by a chemiluminescent beta-gal Reporter Gene Assay and by p24 determination.

RESULTS

We have demonstrated that rgp120 activates both PKCepsilon and its upstream effector PI3K/Akt, involved in the HIV-1 replication process. Moreover, rgp120 enhances the gene, as well as protein expression of the cellular Tat cofactors Tat-Sf1 and SPT5 in primary CD4+/CXCR4+ T cells. Finally, stimulation of HIV-1 infected T cells with rgp120 was found to result in both a higher LTR-activity and an increased production of viral particles.

CONCLUSION

Taken together, these results show that soluble gp120 contributes to HIV-1 replication and dissemination, via the activation of multiple cell signaling pathways and the induction of Tat-cofactor expression, underscoring its potential as a therapeutic target in HIV-1-mediated pathogenesis.

HIV-1 Tat protein enhances microtubule polymerization

BACKGROUND

HIV infection and progression to AIDS is characterized by the depletion of T cells, which could be due, in part, to apoptosis mediated by the extra-cellular HIV-encoded Tat protein as a consequence of Tat binding to tubulin. Microtubules are tubulin polymers that are essential for cell structure and division. Molecules that target microtubules induce apoptosis and are potent anti-cancer drugs. We studied the effect on tubulin polymerization of three Tat variants: Tat HxB2 and Tat Eli from patients who are rapid progressors (RP) and Tat Oyi from highly exposed but persistently seronegative (HEPS) patients. We compared the effect on tubulin polymerization of these Tat variants and peptides corresponding to different parts of the Tat sequence, with paclitaxel, an anti-cancer drug that targets microtubules.

RESULTS

We show that Tat, and specifically, residues 38-72, directly enhance tubulin polymerization. We demonstrate that Tat could also directly trigger the mitochondrial pathway to induce T cell apoptosis, as shown in vitro by the release of cytochrome c from isolated mitochondria.

CONCLUSIONS

These results show that Tat directly acts on microtubule polymerization and provide insights into the mechanism of T cell apoptosis mediated by extra-cellular Tat.

Full-length HIV-1 Tat protein necessary for a vaccine

AIDS vaccines now use a truncated version of 86 residues of the Tat protein related to the HIV-1 HXB2 strain predominant in Europe and North America. We compared antibodies raised in rabbits using a B subtype short Tat HXB2(86) and a full-length Tat HXB2(100). Serum against HXB2(86) recognizes only B and D subtypes while serum against HXB2(100) recognizes B, D, and C subtype variants. Conformational epitopes appear to be involved in the capacity of anti-Tat HXB2 sera to recognized non-homologous Tat variants. A linear B-epitope identified in sequence 71-81 in HXB2(86) disappears in HXB2(100), which has a new linear B-epitope identified at the C-terminus. Anti-HXB2(100) serum has a higher titer in neutralizing antibody against homologous and non-homologous variants compared to anti-HXB2(86) serum. We suggest that a Tat vaccine should contain a Tat variant with regular size, up to 99-101 residues now found in the field.

The glutamine-rich region of the HIV-1 Tat protein is involved in T-cell apoptosis

Human immunodeficiency virus (HIV) infection and the progression to AIDS are characterized by the depletion of CD4(+) T-cells. HIV-1 infection leads to apoptosis of uninfected bystander cells and the direct killing of HIV-infected cells. This is mediated, in part, by the HIV-1 Tat protein, which is secreted by virally infected cells and taken up by uninfected cells. We chemically synthesized two 86-residue subtype D Tat proteins, Ug05RP and Ug11LTS, from two Ugandan patients who were clinically categorized as either rapid progressor or long-term survivor, with non-conservative mutations located essentially in the glutamine-rich region. Structural heterogeneities were revealed by CD, which translate into differing trans-activational and apoptotic effects. CD data analysis and molecular modeling indicated that the short alpha-helix observed in subtype D Tat proteins from rapid progressor patients such as Tat Mal and Tat Ug05RP is not present in Ug11LTS. We show that Tat Ug05RP is more efficient than Tat Ug11LTS in its trans-activational role and in inducing apoptosis in binding tubulin via the mitochondrial pathway. The glutamine-rich region of Tat appears to be involved in the Tat-mediated apoptosis of T-cells.

A possible improvement for structure-based drug design illustrated by the discovery of a Tat HIV-1 inhibitor

The HIV-1 Tat protein is a promising target for AIDS therapy, due to its extra-cellular roles against the immune system. From the 2D-NMR structure of Tat, we have designed molecules, called TDS, able to bind to Tat and inhibit HIV-1 replication in vitro. This new family of antivirals is composed of a triphenylene aromatic ring substituted with at least one carbon chain bearing a succinimide group. These ligands are prepared from triphenylene or 2,6,10-trimethylphenylene in 3-6 steps depending on the target molecule.

The human immunodeficiency virus type-1 transcription factor Tat produces elevations in intracellular Ca2+ that require function of an N-methyl-d-aspartate receptor polyamine-sensitive site

Human immunodeficiency virus type-1 (HIV-1) infection is commonly associated with neuronal loss, as well as, cognitive and motor deficits collectively termed HIV-1-associated dementia (HAD). Function of the HIV-1 transcription factor Tat, activation of N-methyl-D-aspartate (NMDA)-type glutamate receptors, and subsequent rapid rises in free intracellular Ca2+ have been implicated in the development of this neurological disorder. However, the role of specific NMDA receptor modulatory sites in mediating effects of Tat has not been examined. The present studies examined the ability of two variants of Tat protein (1-100 nM), Tat 1-72 and Tat 1-86, to produce rapid rises in intracellular Ca2+ in organotypic slice cultures of rat hippocampus. Further, these studies evaluated the role of an NMDA receptor polyamine-sensitive site in mediating Tat-induced elevations in intracellular Ca2+. Brief exposure (10 min) to each variant of Tat protein (>1 nM) markedly increased levels of intracellular Ca2+ in each region of the hippocampus to as much as 145% of controls. In contrast, exposure of cultures to a deletion mutant of Tat protein devoid of amino acids 31-61 (Tat Delta31-61) did not produce changes in intracellular Ca2+ levels. Most significantly, exposure to the NMDA receptor antagonist dizocilpine (MK801 20 microM) and the polyamine site antagonist arcaine (10 microM) significantly attenuated increases in intracellular Ca2+ levels when co-administered with either the Tat 1-72 or Tat 1-86 amino acid variant of Tat. Thus, exposure of the hippocampus to Tat produces increases in intracellular Ca2+ levels that require function of an NMDA receptor polyamine-sensitive site and this may well contribute to the neurotoxic effects of HIV-1 infection. Polyamine-sensitive portions of this receptor may then represent novel therapeutic targets in the pharmacologic treatment of HAD-related neurotoxicity.

A naturally occurring substitution in human immunodeficiency virus Tat increases expression of the viral genome

A natural amino acid substitution in the human immunodeficiency virus type 1 (HIV-1) transcriptional activator Tat increases its activity and compensates for deleterious mutations elsewhere in the Tat protein. Substitution of asparagine for threonine 23 increases Tat transactivation of the HIV-1 promoter and the binding of Tat to the cellular kinase positive transcription elongation factor b (P-TEFb). Of nine other position 23 mutations tested, only the serine substitution retained wild-type activity. Correspondingly, asparagine is the most frequent amino acid at this position in HIV-1 isolates, followed by threonine and serine. Asparagine is prevalent in Tat proteins of viruses in clades A, C, and D, which are major etiologic agents of AIDS. We suggest that selection for asparagine in position 23 confers an advantage to the virus, since it can compensate for deleterious mutations in Tat. It may also support the replication of otherwise less fit drug-resistant viruses and permit the emergence of virulent strains.

Specificity and effect on apoptosis of Tat antibodies from vaccinated and SHIV-infected rhesus macaques and HIV-infected individuals

Recent contributions have demonstrated that actively secreted Tat protein plays an important functional role in human immunodeficiency virus-1 (HIV-1) infection and that Tat antibodies might interfere with disease progression by blocking the protein extracellularly. In this context we have studied the recognition of several Tat mutants as well as various synthetic Tat fragments by anti-Tat monoclonal antibodies and by IgG antibodies from a large collection of slow and fast-progressor infected individuals. We have also tested the sera from simian/human immunodeficiency virus (SHIV)-infected macaques with these Tat peptides. Important differences were found between long-term non-progressors and fast-progressors, and between human and monkey sera in terms of antibody specificity. Rabbits and macaques were immunised with several Tat peptides and we found that certain antibody subsets from immunised animals recognised the cognate protein Tat and had the capacity to inhibit Tat-induced apoptosis of T cells. Such antibodies might be important for controlling Tat-induced death in cells uninfected by HIV-1.

Differential acetylation of Tat coordinates its interaction with the co-activators cyclin T1 and PCAF

The HIV-1 transactivator protein, Tat, is an atypical transcriptional activator that functions through binding, not to DNA, but to a short leader RNA, TAR. Although details of its functional mechanism are still unknown, emerging findings suggest that Tat serves primarily to adapt co-activator complexes such as p300, PCAF and P-TEFb to the HIV-1 long terminal repeat. Hence, an understanding of how Tat interacts with these cofactors is crucial. It has recently been shown that acetylation at a single lysine, residue 50, regulated the association of Tat with PCAF. Here, we report that in the absence of Tat acetylation, PCAF binds to amino acids 20-40 within Tat. Interestingly, acetylation of Tat at Lys28 abrogates Tat-PCAF interaction. Acetylation at Lys50 creates a new site for binding to PCAF and dictates the formation of a ternary complex of Tat-PCAF-P-TEFb. Thus, differential lysine acetylation of Tat coordinates the interactions with its co-activators, cyclin T1 and PCAF. Our results may help in understanding the ordered recruitment of Tat co-activators to the HIV-1 promoter.

A Janus splicing regulatory element modulates HIV-1 tat and rev mRNA production by coordination of hnRNP A1 cooperative binding

Retroviral protein production depends upon alternative splicing of the viral transcript. The HIV-1 acceptor site A7 is required for tat and rev mRNA production. Production of the Tat transcriptional activator is highly controlled because of its apoptotic properties. Two silencer elements (ESS3 and ISS) and two enhancer elements (ESE2 and ESE3/(GAA)3) were previously identified at site A7. hnRNP A1 binds ISS and ESS3 and is involved in the inhibitory process, ASF/SF2 activates site A7 utilisation. Here, by using chemical and enzymatic probes we established the 2D structure of the HIV-1(BRU) RNA region containing site A7 and identified the RNA segments protected in nuclear extract and by purified hnRNP A1. ISS, ESE3/(GAA)3 and ESS3 are located in three distinct stem-loop structures (SLS1, 2 and 3). As expected, hnRNP A1 binds sites 1, 2 and 3 of ISS and ESS3b, and oligomerises on the polypurine sequence upstream of ESS3b. In addition, we discovered an unidentified hnRNP A1 binding site (AUAGAA), that overlaps ESE3/(GAA)3. On the basis of competition experiments, hnRNP A1 has a stronger affinity for this site than for ESS3b. By insertion of (GAA)3 alone or preceded by the AUA trinucleotide in a foreign context, the AUAGAA sequence was found to modulate strongly the (GAA)3 splicing enhancer activity. Cross-linking experiments on these heterologous RNAs and the SLS2-SLS3 HIV-1 RNA region, in nuclear extract and with recombinant proteins, showed that binding of hnRNP A1 to AUA(GAA)3 strongly competes the association of ASF/SF2 with (GAA)3. In addition, disruption of AUA(GAA)3 demonstrated a key role of this sequence in hnRNP A1 cooperative binding to the ISS and ESS3b inhibitors and hnRNP A1 oligomerisation on the polypurine sequence. Thus, depending on the cellular context ([ASF/SF2]/[hnRNP A1] ratio), AUA(GAA)3 will activate or repress site A7 utilisation and can thus be considered as a Janus splicing regulator.

Tat HIV-1 primary and tertiary structures critical to immune response against non-homologous variants

Clinical studies show that in the absence of anti-retroviral therapy an immune response against the human immunodeficiency virus type 1 (HIV-1), transacting transcriptional activator (Tat) protein correlates with long term non-progression. The purpose of this study is to try to understand what can trigger an effective immune response against Tat. We used five Tat variants from HIV strains identified in different parts of the world and showed that mutations of as much as 38% exist without any change in activity. Rabbit sera were raised against Tat variants identified in rapid-progressor patients (Tat HXB2, a European variant and Tat Eli, an African variant) and a long term non-progressor patient (Tat Oyi, an inactive African variant). Enzyme-linked immunosorbent assay (ELISA) results showed that anti-Tat Oyi serum had the highest antibody titer and was the only one to have a broad antibody response against heterologous Tat variants. Surprisingly, Tat HXB2 was better recognized by anti-Tat Oyi serum compared with anti-Tat HXB2 serum. Western blots showed that non-homologous Tat variants were recognized by antibodies directed against conformational epitopes. This study suggests that the primary and tertiary structures of the Tat variant from the long term non-progressor patient are critical to the induction of a broad and effective antibody response against Tat.

Homonuclear (1)H-NMR assignment and structural characterization of human immunodeficiency virus type 1 Tat Mal protein

The transacting transcriptional activator (Tat) is a viral protein essential for activation of the human immunodeficiency virus (HIV) genes, and it plays an important role in HIV induced immunodeficiency. We report the NMR structural characterization of the active Tat Mal variant that belongs to a highly virulent D-subtype HIV type-1 (HIV-1) strain (Mal) found mainly in Africa. A full Tat Mal protein (87 residues) is synthesized. This synthetic protein is active in a transactivation assay with HeLa cells infected with the HIV long terminal repeated noncoding sequences of the HIV-1 provirus (LTR) lac Z gene. Homonuclear (1)H-NMR spectra allows the sequential assignment of the Tat Mal spin systems. Simulating annealing generates 20 conformers with similar folding. The geometry of the mean structure is optimized with energy minimization to obtain a final structure. As the European variant (Tat Bru) the N-terminal region of Tat Mal constitutes the core, and there is a hydrophobic pocket composed of the conserved Trp 11 interacting with several aromatic residues. The two functional regions of Tat (basic and the cysteine-rich regions) are well exposed to the solvent. A short alpha-helix is observed in region V adjacent to the basic region. This alpha helix induces local structural variations compared to the NMR structure of Tat Bru, and it brings the cysteine-rich and basic regions closer. This study suggests that similar folding exists among Tat variants.

HIV-1 Tat neurotoxicity is prevented by matrix metalloproteinase inhibitors

The release of potentially neurotoxic molecules by HIV-infected brain macrophages is accompanied by neuronal injury and death that results in the development of HIV-associated dementia (HAD). Among the potential neurotoxins implicated in the development of HAD is the HIV-1 transactivating protein, Tat. To investigate the mechanism by which Tat causes neurotoxicity, brain-derived Tat sequences from nondemented (Tat-ND) and demented (Tat-HAD) AIDS patients, which differed primarily in the augmenting region of Tat, were expressed in U937 monoblastoid cells and primary human macrophages. Cells expressing Tat-HAD protein exhibited elevated matrix metalloproteinase (MMP)-2 and -7 release and activation, but cells expressing Tat-ND did not exhibit enhanced MMP expression. Conditioned media from Tat-HAD-transfected cells caused significantly greater neuronal death (15.4 +/- 4.3%) than did Tat-ND (4.4 +/- 2.1%) or nontransfected (2.1 +/- 0.8%) cell-derived conditioned media. The neurotoxicity induced by Tat-HAD was inhibited by anti-MMP-2 or -7 antibodies (p < 0.005) but not by antibodies against MMP-9 or Tat. Similarly, scid/nod mice receiving striatal implants of Tat-HAD-transfected cells exhibited greater neurobehavioral abnormalities and neuronal loss (p < 0.005) than did animals receiving Tat-ND or nontransfected cells, which were reduced by treatment with the MMP inhibitor prinomastat (p < 0.005). These findings indicate that Tat causes neuronal death through an indirect mechanism that is Tat sequence dependent and involves the induction of MMPs.