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

Didehydro-Cortistatin A Inhibits HIV-1 by Specifically Binding to the Unstructured Basic Region of Tat

The intrinsically disordered HIV-1 Tat protein binds the viral RNA transactivation response structure (TAR), which recruits transcriptional cofactors, amplifying viral mRNA expression. Limited Tat transactivation correlates with HIV-1 latency. Unfortunately, Tat inhibitors are not clinically available. The small molecule didehydro-cortistatin A (dCA) inhibits Tat, locking HIV-1 in persistent latency, blocking viral rebound. We generated chemical derivatives of dCA that rationalized molecular docking of dCA to an active and specific Tat conformer. These revealed the importance of the cycloheptene ring and the isoquinoline nitrogen's positioning in the interaction with specific residues of Tat's basic domain. These features are distinct from the ones required for inhibition of cyclin-dependent kinase 8 (CDK8), the only other known ligand of dCA. Besides, we demonstrated that dCA activity on HIV-1 transcription is independent of CDK8. The binding of dCA to Tat with nanomolar affinity alters the local protein environment, rendering Tat more resistant to proteolytic digestion. dCA thus locks a transient conformer of Tat, specifically blocking functions dependent of its basic domain, namely the Tat-TAR interaction; while proteins with similar basic patches are unaffected by dCA. Our results improve our knowledge of the mode of action of dCA and support structure-based design strategies targeting Tat, to help advance development of dCA, as well as novel Tat inhibitors.IMPORTANCE Tat activates virus production, and limited Tat transactivation correlates with HIV-1 latency. The Tat inhibitor dCA locks HIV in persistent latency. This drug class enables block-and-lock functional cure approaches, aimed at reducing residual viremia during therapy and limiting viral rebound. dCA may also have additional therapeutic benefits since Tat is also neurotoxic. Unfortunately, Tat inhibitors are not clinically available. We generated chemical derivatives and rationalized binding to an active and specific Tat conformer. dCA features required for Tat inhibition are distinct from features needed for inhibition of cyclin-dependent kinase 8 (CDK8), the only other known target of dCA. Furthermore, knockdown of CDK8 did not impact dCA's activity on HIV-1 transcription. Binding of dCA to Tat's basic domain altered the local protein environment and rendered Tat more resistant to proteolytic digestion. dCA locks a transient conformer of Tat, blocking functions dependent on its basic domain, namely its ability to amplify viral transcription. Our results define dCA's mode of action, support structure-based-design strategies targeting Tat, and provide valuable information for drug development around the dCA pharmacophore.

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.

Distant homology modeling of LCAT and its validation through in silico targeting and in vitro and in vivo assays

LCAT (lecithin:cholesterol acyltransferase) catalyzes the transacylation of a fatty acid of lecithin to cholesterol, generating a cholesteryl ester and lysolecithin. The knowledge of LCAT atomic structure and the identification of the amino acids relevant in controlling its structure and function are expected to be very helpful to understand the enzyme catalytic mechanism, as involved in HDL cholesterol metabolism. However - after an early report in the late ‘90 s - no recent advance has been made about LCAT three-dimensional structure. In this paper, we propose an LCAT atomistic model, built following the most up-to-date molecular modeling approaches, and exploiting newly solved crystallographic structures. LCAT shows the typical folding of the α/β hydrolase superfamily, and its topology is characterized by a combination of α-helices covering a central 7-strand β-sheet. LCAT presents a Ser/Asp/His catalytic triad with a peculiar geometry, which is shared with such other enzyme classes as lipases, proteases and esterases. Our proposed model was validated through different approaches. We evaluated the impact on LCAT structure of some point mutations close to the enzyme active site (Lys218Asn, Thr274Ala, Thr274Ile) and explained, at a molecular level, their phenotypic effects. Furthermore, we devised some LCAT modulators either designed through a de novo strategy or identified through a virtual high-throughput screening pipeline. The tested compounds were proven to be potent inhibitors of the enzyme activity.

Identification of a highly conserved surface on Tat variants

Extracellular Tat is suspected to protect HIV-1-infected cells from cellular immunity. Seropositive patients are unable to produce neutralizing antibodies against Tat, and Tat is still secreted under antiviral treatment. In mice, the Tat OYI vaccine candidate generates neutralizing antibodies such as the mAb 7G12. A peptide called MIMOOX was designed from fragments of Tat OYI identified as the possible binding site for mAb 7G12. MIMOOX was chemically synthesized, and its structure was stabilized with a disulfide bridge. Circular dichroism spectra showed that MIMOOX had mainly β turns but no α helix as Tat OYI. MIMOOX was recognized by mAb 7G12 in ELISA only in reduced conditions. Moreover, a competitive recognition assay with mAb 7G12 between MIMOOX and Tat variants showed that MIMOOX mimics a highly conserved surface in Tat variants. Rat immunizations with MIMOOX induce antibodies recognizing Tat variants from the main HIV-1 subtypes and confirm the Tat OYI vaccine approach.

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.

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.

HIV-1 Tat-specific IgG antibodies in high-responders target a B-cell epitope in the cysteine-rich domain and block extracellular Tat efficiently

Tat, an important regulatory protein of HIV-1, has been implicated in HIV-related pathogenesis. Immune responses to Tat, although underrepresented, confer protection against disease progression, in natural infection and experimental immunization, making Tat an attractive vaccine candidate. Information on immune responses to Tat from India which has the second largest HIV incidence has been lacking. Here we report a cross-sectional study evaluating the humoral response to Tat from a large number of samples from two southern states of India. 14% of the seropositive (63/447) and 4.6% of seronegative samples (7/150) harbored Tat-reactive antibodies. A significant number of the seropositive samples contained high levels of anti-Tat antibodies (31/447) which demonstrated class-switch to IgG1 and bound to Tat with high avidity. Cross-reactivity analysis showed that these antibodies interacted with Tat from different clades with variable degree with the highest interaction with subtype-AE and the least with subtype-B Tat. Importantly, a B-cell epitope in the cysteine-rich domain was found to be the most immunodominant one and antibodies interacting with this epitope blocked extracellular Tat efficiently. To the best of our knowledge this is the first report on immune responses to Tat from Indian populations and the data presented here could significantly contribute to HIV Tat vaccine design.

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.

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.