Multifaceted activities of the HIV-1 transactivator of transcription, Tat

Production of antigen-specific human IgGs by in vitro immunization

Background

We previously developed in vitro immunization based on a fusion protein containing the transcriptional transactivator (Tat) of human immunodeficiency virus and a double domain, called ZZ, derived from protein A of Staphylococcus aureus. In this approach, naïve human peripheral blood mononuclear cells (PBMCs) trigger a specific IgM antibody (Ab) response in the presence of ZZTat. In the present study, we attempted to raise a specific IgG Ab response.

Results

We found that PBMCs incubated with ZZTat and a mixture containing anti-CD40, IL4 and IL21 secrete anti-Tat IgG Abs in their supernatants, indicating that the cytokine cocktail provides an isotypic switch. Then, we deciphered the Tat determinant involved in the phenomenon and found that it is located in the region 22–57 and that, within this region, the cysteine-rich domain and the basic residues play a crucial role. Finally, we prepared a fusion protein containing a fragment derived from the NY-ESO-1 cancer/testis antigen (Ag) and showed that PBMCs incubated with ZZfNY-ESO-1Tat trigger a specific anti-fNY-ESO-1 IgG Ab response, which demonstrates the possibility of transferring immunizing ability to an Ag unrelated to Tat.

Conclusion

Our ZZTat-based in vitro immunization approach that offers the possibility to raise an IgG Ab response against NY-ESO-1 might represent a valuable first stage for the generation of fully human IgG specific Abs.

Interaction between Tat and Drugs of Abuse during HIV-1 Infection and Central Nervous System Disease

In many individuals, drug abuse is intimately linked with HIV-1 infection. In addition to being associated with one-third of all HIV-1 infections in the United States, drug abuse also plays a role in disease progression and severity in HIV-1-infected patients, including adverse effects on the central nervous system (CNS). Specific systems within the brain are known to be damaged in HIV-1-infected individuals and this damage is similar to that observed in drug abuse. Even in the era of anti-retroviral therapy (ART), CNS pathogenesis occurs with HIV-1 infection, with a broad range of cognitive impairment observed, collectively referred to as HIV-1-associated neurocognitive disorders (HAND). A number of HIV-1 proteins (Tat, gp120, Nef, Vpr) have been implicated in the etiology of pathogenesis and disease as a result of the biologic activity of the extracellular form of each of the proteins in a number of tissues, including the CNS, even in ART-suppressed patients. In this review, we have made Tat the center of attention for a number of reasons. First, it has been shown to be synthesized and secreted by HIV-1-infected cells in the CNS, despite the most effective suppression therapies available to date. Second, Tat has been shown to alter the functions of several host factors, disrupting the molecular and biochemical balance of numerous pathways contributing to cellular toxicity, dysfunction, and death. In addition, the advantages and disadvantages of ART suppression with regard to controlling the genesis and progression of neurocognitive impairment are currently under debate in the field and are yet to be fully determined. In this review, we discuss the individual and concerted contributions of HIV-1 Tat, drug abuse, and ART with respect to damage in the CNS, and how these factors contribute to the development of HAND in HIV-1-infected patients.

Biophysical Characterization of Nucleophosmin Interactions with Human Immunodeficiency Virus Rev and Herpes Simplex Virus US11

Nucleophosmin (NPM1, also known as B23, numatrin or NO38) is a pentameric RNA-binding protein with RNA and protein chaperon functions. NPM1 has increasingly emerged as a potential cellular factor that directly associates with viral proteins; however, the significance of these interactions in each case is still not clear. In this study, we have investigated the physical interaction of NPM1 with both human immunodeficiency virus type 1 (HIV-1) Rev and Herpes Simplex virus type 1 (HSV-1) US11, two functionally homologous proteins. Both viral proteins show, in mechanistically different modes, high affinity for a binding site on the N-terminal oligomerization domain of NPM1. Rev, additionally, exhibits low-affinity for the central histone-binding domain of NPM1. We also showed that the proapoptotic cyclic peptide CIGB-300 specifically binds to NPM1 oligomerization domain and blocks its association with Rev and US11. Moreover, HIV-1 virus production was significantly reduced in the cells treated with CIGB-300. Results of this study suggest that targeting NPM1 may represent a useful approach for antiviral intervention.

The cross-talk of HIV-1 Tat and methamphetamine in HIV-associated neurocognitive disorders

Antiretroviral therapy has dramatically improved the lives of human immunodeficiency virus 1 (HIV-1) infected individuals. Nonetheless, HIV-associated neurocognitive disorders (HAND), which range from undetectable neurocognitive impairments to severe dementia, still affect approximately 50% of the infected population, hampering their quality of life. The persistence of HAND is promoted by several factors, including longer life expectancies, the residual levels of virus in the central nervous system (CNS) and the continued presence of HIV-1 regulatory proteins such as the transactivator of transcription (Tat) in the brain. Tat is a secreted viral protein that crosses the blood–brain barrier into the CNS, where it has the ability to directly act on neurons and non-neuronal cells alike. These actions result in the release of soluble factors involved in inflammation, oxidative stress and excitotoxicity, ultimately resulting in neuronal damage. The percentage of methamphetamine (MA) abusers is high among the HIV-1-positive population compared to the general population. On the other hand, MA abuse is correlated with increased viral replication, enhanced Tat-mediated neurotoxicity and neurocognitive impairments. Although several strategies have been investigated to reduce HAND and MA use, no clinically approved treatment is currently available. Here, we review the latest findings of the effects of Tat and MA in HAND and discuss a few promising potential therapeutic developments.

Inhibition of HIV-1 by a Lentiviral Vector with a Novel Tat-Inducible Expression System and a Specific Tropism to the Target Cells

Today, lentiviral vectors are favorable vectors for RNA interference delivery in anti-HIV therapeutic approaches. Nevertheless, problems such as the specific recognition of target cells and uncontrolled expression of the transgene can restrict their use in vivo. Herein we present a new HIV-inducible promoter to express anti-HIV short hairpin RNA (shRNA) by RNA Pol II in mammalian cells. We likewise showed a novel third-generation lentiviral vector system with more safety and a specific tropism to the target cells. The new promoter, CkRhsp, was constructed from the chicken β-actin core promoter with the R region of HIV-1 long terminal repeat fused upstream of minimal hsp70 promoter. This system was induced by HIV-1 Tat, and activates transcription of two shRNAs against two conserved regions of HIV-1 transcripts produced in two steps of the virus life cycle. We also mimicked HIV-1 cell tropism by using the HIV-1 envelope in structure of third-generation lentiviral vector. The new fusion promoter efficiently expressed shRNA in a Tat-inducible manner. HIV-1 replication was inhibited in transient transfection and stable transduction assays. The new viral vector infected only CD4+cells. CkRhsp promoter may be safer than other inducible promoters for shRNA-mediated gene therapies against HIV. The use of the wild envelope in the vector packaging system may provide the specific targeting T lymphocytes and hematopoietic stem cells for anti-HIV-1 therapeutic approaches in vivo.

HIV Tat protein: Is Tat-C much trickier than Tat-B?

Out of various subtypes of human immunodeficiency virus type 1 (HIV-1), subtype B and C cause most of the infections worldwide. Clade specific differences have been reported in differences in clinical picture of HIV pathogenesis. Transcription of the HIV-1 genome is regulated by the interaction of HIV Tat protein to the trans-activation response (TAR) element. The differential binding of clade B and C Tat proteins to TAR and differences in activation of NF-κB cascade leading to differential transactivation capacity and cytokine expression has been examined in this study. More stable Tat-TAR complex formation by Tat-C revealed by EMSA and higher TNF-α expression shown by Tat-C compared to Tat-B leads to higher NF-κB activation, which may be plausible cause for higher transactivation by Tat-C as obtained by FACS analysis. This comparative study would be helpful in understanding the basic mechanism of clade specific Tat protein differences and their functional relationships.

Molecular characterization of full-length Tat in HIV-1 subtypes B and C

HIV-1Tat (trans-acting activator of transcription) plays essential roles in the replication through viral mRNA and genome transcription from the HIV-1 LTR promoter. However, Tat undergoes continuous amino acid substitutions. As a consequence, the virus escapes from host immunity indicating that genetic diversity of Tat protein in major HIV-1 subtypes is required to be continuously monitored. We analyzed available full-length HIV-1 sequences of subtypes B (n=493) and C (n=280) strains circulating worldwide. We observed 81% and 84% nucleotide sequence identities of HIV-1 Tat for subtypes B and C, respectively. Based on phylogenetic and mutation analyses, global diversity of subtype B was apparently higher compared to that of subtype C. Positively selected sites, such as positions Ser68 and Ser70 in both subtypes, were located in the Tat-transactivation responsive RNA (TAR) interaction domain. We also found positively selected sites in exon 2, such as positions Ser75, Pro77, Asp80, Pro81 and Ser87 for both subtypes. Our study provides useful information on the full-length HIV-1 Tat sequences in globally circulating strains.

The discovery of Hsp70 domain with cell-penetrating activity

Chaperone Hsp70 can cross the plasma membrane of living cells using mechanisms that so far have not received much research attention. Searching the part of the molecule that is responsible for transport ability of Hsp70, we found a cationic sequence composed of 20 amino acid residues on its surface, KST peptide, which was used in further experiments. We showed that KST peptide enters living cells of various origins with the same efficiency as the full-length chaperone. KST peptide is capable of carrying cargo with a molecular weight 30 times greater than its own into cells. When we compared the membrane-crossing activity of KST peptide in complex with Avidin (KST-Av complex) with that of similarly linked canonical TAT peptide, we found that TAT peptide penetrated SK-N-SH human neuroblastoma cells at a similar rate and efficiency as the KST peptide. Furthermore, KST peptide can carry protein complexes consisting of a specific antibody coupled to the peptide through the Avidin bridge. An antibody to Hsp70 delivered to SK-N-SH cells with high expression level of Hsp70 reduced the protective power of the chaperone and sensitized the cells to the pro-apoptotic effect of staurosporine. We studied the mechanisms of penetration of KST-Av and full-length Hsp70 inside human neuroblastoma SK-N-SH and human erythroleukemia K-562 cells and found that both used an active intracellular transport mechanism that included vesicular structures and negatively charged lipid membrane domains. Competition analysis of intracellular transport showed that the chaperone reduced intracellular penetration of KST peptide and conversely KST peptide prevented Hsp70 transport in a dose-dependent manner.

Investigating the role of ankyrin-rich membrane spanning protein in human immunodeficiency virus type-1 Tat-induced microglia activation

Long-term persistence of human immunodeficiency virus type-1 (HIV) in the central nervous system (CNS) results in mild to severe neurocognitive impairment in a significant proportion of the HIV-infected population. These neurological deficits are known as HIV-associated neurocognitive disorders (HAND). Microglia are CNS-resident immune cells that are directly infected by HIV and consequently secrete proinflammatory molecules that contribute to HIV-induced neuroinflammation. Indeed, the number of activated macrophage and microglia in the brain is more highly correlated with cognitive impairment than the amount of neuronal apoptosis. Ankyrin-rich membrane spanning protein (ARMS/Kidins220) is a multidomain transmembrane protein that is involved with neurotrophin signaling in the CNS. We have previously established the role of ARMS in mediating neuronal survival via a neurotrophin-dependent mechanism. Recent reports also have suggested that ARMS is involved with cell signaling in multiple immune cell types. In this study, we aim to investigate the role of ARMS in HIV Tat-mediated microglial cell activation by employing in vitro methods. Following ARMS depletion by a lentivirus encoding ARMS-specific short hairpin RNA (shRNA), we observed a marked reduction in the HIV Tat-induced proinflammatory response, associated with loss of tumor necrosis factor alpha production and nuclear factor-kappa B (NF-κB) activation. Furthermore, co-immunoprecipitation studies suggested that ARMS physically interacts with inhibitory kappa B kinase subunits in order to facilitate NF-κB activation. Our results establish the role of ARMS in microglial activation by HIV Tat and warrant additional studies to better understand these molecular mechanisms, which may uncover novel therapeutic targets for the treatment of HAND.

Tat predominantly associates with host promoter elements in HIV-1-infected T-cells - regulatory basis of transcriptional repression of c-Rel

HIV-1 Tat is a multifunctional regulatory protein that, in addition to its primary function of transactivating viral transcription, also tends to modulate cellular gene expression, for which the molecular mechanism remains to be clarified. We have reported earlier nuclear factor kappa B (NFκB) enhancer binding activity of Tat and proposed this DNA binding activity as a possible molecular basis for Tat-mediated regulation of cellular gene expression in infected cells. In the present study, we analyzed the genome-wide occupancy of Tat protein on host cell chromatin in HIV-1-infected T-cells to investigate a potential role of Tat on cellular gene expression. The results obtained identify a spectrum of binding sites of Tat protein on the chromatin and reveal that Tat is also recruited on a number of cellular gene promoters in HIV-1-infected T-cells, indicating its possible involvement in the regulation of gene expression of such cellular genes. Tat was identified as a repressor of one such validated gene, c-Rel, because it downregulates the expression of c-Rel in both Tat expressing and HIV-1-infected T-cells. The results also show that Tat downregulates c-Rel promoter activity by interacting with specific NFκB sites on the c-Rel promoter, thus providing a molecular basis of Tat-mediated regulation of cellular gene expression. Thus, in the present study, we have not only identified recruitment sites of Tat on the chromatin in HIV-1-infected T-cells, but also report for the first time that c-Rel is downregulated in HIV-1-infected cells specifically by interaction of Tat with NFκB binding sites on the promoter.

The grafting of universal T-helper epitopes enhances immunogenicity of HIV-1 Tat concurrently improving its safety profile

Extracellular Tat (eTat) plays an important role in HIV-1 pathogenesis. The presence of anti-Tat antibodies is negatively correlated with disease progression, hence making Tat a potential vaccine candidate. The cytotoxicity and moderate immunogenicity of Tat however remain impediments for developing Tat-based vaccines. Here, we report a novel strategy to concurrently enhance the immunogenicity and safety profile of Tat. The grafting of universal helper T-lymphocyte (HTL) epitopes, Pan DR Epitope (PADRE) and Pol₇₁₁ into the cysteine rich domain (CRD) and the basic domain (BD) abolished the transactivation potential of the Tat protein. The HTL-Tat proteins elicited a significantly higher titer of antibodies as compared to the wild-type Tat in BALB/c mice. While the N-terminal epitope remained immunodominant in HTL-Tat immunizations, an additional epitope in exon-2 was recognized with comparable magnitude suggesting a broader immune recognition. Additionally, the HTL-Tat proteins induced cross-reactive antibodies of high avidity that efficiently neutralized exogenous Tat, thus blocking the activation of a Tat-defective provirus. With advantages such as presentation of multiple B-cell epitopes, enhanced antibody response and importantly, transactivation-deficient Tat protein, this approach has potential application for the generation of Tat-based HIV/AIDS vaccines.

Multiple functions of DDX3 RNA helicase in gene regulation, tumorigenesis, and viral infection

The DEAD-box RNA helicase DDX3 is a multifunctional protein involved in all aspects of RNA metabolism, including transcription, splicing, mRNA nuclear export, translation, RNA decay and ribosome biogenesis. In addition, DDX3 is also implicated in cell cycle regulation, apoptosis, Wnt-β-catenin signaling, tumorigenesis, and viral infection. Notably, recent studies suggest that DDX3 is a component of anti-viral innate immune signaling pathways. Indeed, DDX3 contributes to enhance the induction of anti-viral mediators, interferon (IFN) regulatory factor 3 and type I IFN. However, DDX3 seems to be an important target for several viruses, such as human immunodeficiency virus type 1 (HIV-1), hepatitis C virus (HCV), hepatitis B virus (HBV), and poxvirus. DDX3 interacts with HIV-1 Rev or HCV Core protein and modulates its function. At least, DDX3 is required for both HIV-1 and HCV replication. Therefore, DDX3 could be a novel therapeutic target for the development of drug against HIV-1 and HCV.

Neonatal intrahippocampal HIV-1 protein Tat(1-86) injection: neurobehavioral alterations in the absence of increased inflammatory cytokine activation

Pediatric AIDS caused by human immunodeficiency virus type 1 (HIV-1) remains one of the leading worldwide causes of childhood morbidity and mortality. HIV-1 proteins, such as Tat and gp120, are believed to play a crucial role in the neurotoxicity of pediatric HIV-1 infection. Detrimental effects on development, behavior, and neuroanatomy follow neonatal exposure to the HIV-1 viral toxins Tat1-72 and gp120. The present study investigated the neurobehavioral effects induced by the HIV-1 neurotoxic protein Tat1-86, which encodes the first and second exons of the Tat protein. In addition, the potential effects of HIV-1 toxic proteins Tat1-86 and gp120 on inflammatory pathways were examined in neonatal brains. Vehicle, 25 μg Tat1-86 or 100 ng gp120 was injected into the hippocampus of male Sprague-Dawley pups on postnatal day 1 (PD1). Tat1-86 induced developmental neurotoxic effects, as witnessed by delays in eye opening, delays in early reflex development and alterations in prepulse inhibition (PPI) and between-session habituation of locomotor activity. Overall, the neurotoxic profile of Tat1-86 appeared more profound in the developing nervous system in vivo relative to that seen with the first exon encoded Tat1-72 (Fitting et al., 2008b), as noted on measures of eye opening, righting reflex, and PPI. Neither the direct PD1 CNS injection of the viral HIV-1 protein variant Tat1-86, nor the HIV-1 envelope protein gp120, at doses sufficient to induce neurotoxicity, necessarily induced significant expression of the inflammatory cytokine IL-1β or inflammatory factors NF-κβ and I-κβ. The findings agree well with clinical observations that indicate delays in developmental milestones of pediatric HIV-1 patients, and suggest that activation of inflammatory pathways is not an obligatory response to viral protein-induced neurotoxicity that is detectable with behavioral assessments. Moreover, the amino acids encoded by the second tat exon may have unique actions on the developing hippocampus.

NUCKS1, a novel Tat coactivator, plays a crucial role in HIV-1 replication by increasing Tat-mediated viral transcription on the HIV-1 LTR promoter

BACKGROUND

Human immunodeficiency virus-1 (HIV-1) Tat protein plays an essential role in HIV gene transcription from the HIV-1 long terminal repeat (LTR) and replication. Transcriptional activity of Tat is modulated by several host factors, but the mechanism responsible for Tat regulation by host factors is not understood fully.

RESULTS

Using a yeast two-hybrid screening system, we identified Nuclear ubiquitous casein and cyclin-dependent kinase substrate 1 (NUCKS1) as a novel Tat-interacting partner. Here, we report its function as a positive regulator of Tat. In a coimmunoprecipitation assay, HIV-1 Tat interacted sufficiently with both endogenous and ectopically expressed NUCKS1. In a reporter assay, ectopic expression of NUCKS1 significantly increased Tat-mediated transcription of the HIV-1 LTR, whereas knockdown of NUCKS1 by small interfering RNA diminished Tat-mediated transcription of the HIV-1 LTR. We also investigated which mechanism contributes to NUCKS1-mediated Tat activation. In a chromatin immunoprecipitation assay (ChIP), knockdown of NUCKS1 interrupted the accumulation of Tat in the transactivation-responsive (TAR) region on the LTR, which then led to suppression of viral replication. However, NUCKS1 expression did not increase Tat nuclear localization and interaction with Cyclin T1. Interestingly, the NUCKS1 expression level was lower in latently HIV-1-infected cells than in uninfected parent cells. Besides, expression level of NUCKS1 was markedly induced, which then facilitated HIV-1 reactivation in latently infected cells.

CONCLUSION

Taken together, our data demonstrate clearly that NUCKS1 is a novel Tat coactivator that is required for Tat-mediated HIV-1 transcription and replication, and that it may contribute to HIV-1 reactivation in latently HIV-1 infected cells.

Unconventional secretion of fibroblast growth factor 2--a novel type of protein translocation across membranes?

N-terminal signal peptides are a hallmark of the vast majority of soluble secretory proteins that are transported along the endoplasmic reticulum/Golgi-dependent pathway. They are recognized by signal recognition particle, a process that initiates membrane translocation into the lumen of the endoplasmic reticulum followed by vesicular transport to the cell surface and release into the extracellular space. Beyond this well-established mechanism of protein secretion from eukaryotic cells, a number of extracellular proteins with critical physiological functions in immune surveillance and tissue organization are known to be secreted in a manner independent of signal recognition particle. Such processes have collectively been termed "unconventional protein secretion" and, while known for more than two decades, their underlying mechanisms are only beginning to emerge. Different types of unconventional secretory mechanisms have been described with the best-characterized example being based on direct translocation of cytoplasmic proteins across plasma membranes. The aim of this review is to critically assess our current knowledge of this type of unconventional secretion focusing on fibroblast growth factor 2 (FGF2) as the most established example.

The macrophage: a therapeutic target in HIV-1 infection

Human immunodeficiency virus (HIV) is still a serious global health concern responsible for more than 25 million deaths in last three decades. More than 34 million people are living with HIV infection. Macrophages and CD4+ T cells are the principal targets of HIV-1. The pathogenesis of HIV-1 takes different routes in macrophages and CD4+ T cells. Macrophages are resistant to the cytopathic effect of HIV-1 and produce virus for longer periods of time. In addition, macrophages being present in every organ system thus can disseminate virus to the different anatomical sites leading to the formation of viral sanctuaries. Complete cure of HIV-1 needs better understanding of viral pathogenesis in these reservoirs and implementation of knowledge into robust therapeutic products. In this review we will focus on the unique relationship between HIV-1 and macrophages. Furthermore, we will describe how successful antiretroviral therapy (ART) is in suppressing HIV and novel molecular and cellular strategies against HIV-1 in macrophages.

Nanopreparations for organelle-specific delivery in cancer

To efficiently deliver therapeutics into cancer cells, a number of strategies have been recently investigated. The toxicity associated with the administration of chemotherapeutic drugs due to their random interactions throughout the body necessitates the development of drug-encapsulating nanopreparations that significantly mask, or reduce, the toxic side effects of the drugs. In addition to reduced side effects associated with drug encapsulation, nanocarriers preferentially accumulate in tumors as a result of its abnormally leaky vasculature via the Enhanced Permeability and Retention (EPR) effect. However, simple passive nanocarrier delivery to the tumor site is unlikely to be enough to elicit a maximum therapeutic response as the drug-loaded carriers must reach the intracellular target sites. Therefore, efficient translocation of the nanocarrier through the cell membrane is necessary for cytosolic delivery of the cargo. However, crossing the cell membrane barrier and reaching cytosol might still not be enough for achieving maximum therapeutic benefit, which necessitates the delivery of drugs directly to intracellular targets, such as bringing pro-apoptotic drugs to mitochondria, nucleic acid therapeutics to nuclei, and lysosomal enzymes to defective lysosomes. In this review, we discuss the strategies developed for tumor targeting, cytosolic delivery via cell membrane translocation, and finally organelle-specific targeting, which may be applied for developing highly efficacious, truly multifunctional, cancer-targeted nanopreparations.

Molecular and genetic characterization of natural HIV-1 Tat Exon-1 variants from North India and their functional implications

BACKGROUND

Designing an ideal vaccine against HIV-1 has been difficult due to enormous genetic variability as a result of high replication rate and lack of proofreading activity of reverse transcriptase leading to emergence of genetic variants and recombinants. Tat transactivates HIV-1 LTR, resulting in a remarkable increase in viral gene expression, and plays a vital role in pathogenesis. The aim of this study was to characterize the genetic variations of Tat exon-1 from HIV-1 infected patients from North India.

METHODS

Genomic DNA was isolated from PBMCs and Tat exon-1 was PCR amplified with specific primers followed by cloning, sequencing and sequence analyses using bioinformatic tools for predicting HIV-1 subtypes, recombination events, conservation of domains and phosphorylation sites, and LTR transactivation by luciferase assay.

RESULTS

Phylogenetic analysis of Tat exon-1 variants (n = 120) revealed sequence similarity with South African Tat C sequences and distinct geographical relationships were observed for B/C recombinants. Bootscan analysis of our variants showed 90% homology to Tat C and 10% to B/C recombinants with a precise breakpoint. Natural substitutions were observed with high allelic frequencies which may be beneficial for virus. High amino acid conservation was observed in Tat among Anti Retroviral Therapy (ART) recipients. Barring few changes, most of the functional domains, predicted motifs and phosphorylation sites were well conserved in most of Tat variants. dN/dS analysis revealed purifying selection, implying the importance of functional conservation of Tat exon-1. Our Indian Tat C variants and B/C recombinants showed differential LTR transactivation.

CONCLUSIONS

The possible role of Tat exon-1 variants in shaping the current HIV-1 epidemic in North India was highlighted. Natural substitutions across conserved functional domains were observed and provided evidence for the emergence of B/C recombinants within the ORF of Tat exon-1. These events are likely to have implications for viral pathogenesis and vaccine formulations.

Roles and functions of HIV-1 Tat protein in the CNS: an overview

Nearly 50% of HIV-infected individuals suffer from some form of HIV-associated neurocognitive disorders (HAND). HIV-1 Tat (a key HIV transactivator of transcription) protein is one of the first HIV proteins to be expressed after infection occurs and is absolutely required for the initiation of the HIV genome transcription. In addition to its canonical functions, various studies have shown the deleterious role of HIV-1 Tat in the development and progression of HAND. Within the CNS, only specific cell types can support productive viral replication (astrocytes and microglia), however Tat protein can be released form infected cells to affects HIV non-permissive cells such as neurons. Therefore, in this review, we will summarize the functions of HIV-1 Tat proteins in neural cells and its ability to promote HAND.

Promoter Targeting shRNA Suppresses HIV-1 Infection In vivo Through Transcriptional Gene Silencing

Despite prolonged and intensive application, combined antiretroviral therapy cannot eradicate human immunodeficiency virus (HIV)-1 because it is harbored as a latent infection, surviving for long periods of time. Alternative approaches are required to overcome the limitations of current therapy. We have been developing a short interfering RNA (siRNA) gene silencing approach. Certain siRNAs targeting promoter regions of genes induce transcriptional gene silencing. We previously reported substantial transcriptional gene silencing of HIV-1 replication by an siRNA targeting the HIV-1 promoter in vitro. In this study, we show that this siRNA, expressed as a short hairpin RNA (shRNA) (shPromA-JRFL) delivered by lentiviral transduction of human peripheral blood mononuclear cells (PBMCs), which are then used to reconstitute NOJ mice, is able to inhibit HIV-1 replication in vivo, whereas a three-base mismatched variant (shPromA-M2) does not. In shPromA-JRFL–treated mice, HIV-1 RNA in serum is significantly reduced, and the ratio of CD4⁺/CD8⁺ T cells is significantly elevated. Expression levels of the antisense RNA strand inversely correlates with HIV-1 RNA in serum. The silenced HIV-1 can be reactivated by T-cell activation in ex vivo cultures. HIV-1 suppression is not due to offtarget effects of shPromA-JRFL. These data provide “proof-of principle” that an shRNA targeting the HIV-1 promoter is able to suppress HIV-1 replication in vivo.

Genetic architecture of HIV type 1 Nef and Tat from HLA-B57-typed long-term survivors in an Indian cohort of perinatally HIV-infected children

Human immunodeficiency virus type 1 (HIV-1) viral genes nef and tat play an important role in disease progression. In this study we characterized the Nef and Tat proteins from a group of HLA-B57 typed pediatric perinatally infected long-term survivors (LTS) with ≥10 years of infection. We identified 19 therapy-naive LTS after screening 250 children from an Indian pediatric cohort. Nef and tat amplified from plasma virus showed that all the LTS harbored HIV-1 subtype C. The two B57(+) children showed mutations, deletions, and insertions in experimentally defined B57 epitopes in the virus that are likely to be escape mutants. Only GW12 (GPGVRYPLTFGW) and YY9 (YTPGPGIRY) were conserved, while the remaining 90% (18/20) of the epitopes showed some degree of mutations. The most variable epitopes were RR15, SE15, QP15, KF9, HW9, YT9, and GF15. To our knowledge this is the first study from India in which characterization of Nef and Tat from LTS has led to information on genetic alterations in these genes that are associated with slow disease progression, and can provide an important lead in future studies.

Study of the HIV-2 Env cytoplasmic tail variability and its impact on Tat, Rev and Nef

Background

The HIV-2 env’s 3’ end encodes the cytoplasmic tail (CT) of the Env protein. This genomic region also encodes the rev, Tat and Nef protein in overlapping reading frames. We studied the variability in the CT coding region in 46 clinical specimens and in 2 reference strains by sequencing and by culturing. The aims were to analyse the variability of Env CT and the evolution of proteins expressed from overlapping coding sequences.

Results

A 70% reduction of the length of the CT region affected the HIV-2 ROD and EHO strains invitro due to a premature stop codon in the env gene. In clinical samples this wasn’t observed, but the CT length varied due to insertions and deletions. We noted 3 conserved and 3 variable regions in the CT. The conserved regions were those containing residues involved in Env endocytosis, the potential HIV-2 CT region implicated in the NF-kB activation and the potential end of the lentiviral lytic peptide one. The variable regions were the potential HIV-2 Kennedy region, the potential lentiviral lytic peptide two and the beginning of the potential lentiviral lytic peptide one. A very hydrophobic region was coded downstream of the premature stop codon observed invitro, suggesting a membrane spanning region. Interestingly, the nucleotides that are responsible for the variability of the CT don’t impact rev and Nef. However, in the Kennedy-like coding region variability resulted only from nucleotide changes that impacted Env and Tat together.

Conclusion

The HIV-2 Env, Tat and Rev C-terminal part are subject to major length variations in both clinical samples and cultured strains. The HIV-2 Env CT contains variable and conserved regions. These regions don’t affect the rev and Nef amino acids composition which evolves independently. In contrast, Tat co-evolves with the Env CT.

DDX3 RNA helicase is required for HIV-1 Tat function

Host RNA helicase has been involved in human immunodeficiency virus type 1 (HIV-1) replication, since HIV-1 does not encode an RNA helicase. Indeed, DDX1 and DDX3 DEAD-box RNA helicases are known to be required for efficient HIV-1 Rev-dependent RNA export. However, it remains unclear whether DDX RNA helicases modulate the HIV-1 Tat function. In this study, we demonstrate, for the first time, that DDX3 is required for the HIV-1 Tat function. Notably, DDX3 colocalized and interacted with HIV-1 Tat in cytoplasmic foci. Indeed, DDX3 localized in the cytoplasmic foci P-bodies or stress granules under stress condition after the treatment with arsenite. Importantly, only DDX3 enhanced the Tat function, while various distinct DEAD-box RNA helicases including DDX1, DDX3, DDX5, DDX17, DDX21, and DDX56, stimulated the HIV-1 Rev-dependent RNA export function, indicating a specific role of DDX3 in Tat function. Indeed, the ATPase-dependent RNA helicase activity of DDX3 seemed to be required for the Tat function as well as the colocalization with Tat. Furthermore, the combination of DDX3 with other distinct DDX RNA helicases cooperated to stimulate the Rev but not Tat function. Thus, DDX3 seems to interact with the HIV-1 Tat and facilitate the Tat function.

Dynamic interactions of the HIV-1 Tat with nucleic acids are critical for Tat activity in reverse transcription

The HIV-1 transactivator of transcription (Tat) protein is thought to stimulate reverse transcription (RTion). The Tat protein and, more specifically, its (44–61) domain were recently shown to promote the annealing of complementary DNA sequences representing the HIV-1 transactivation response element TAR, named dTAR and cTAR, that plays a key role in RTion. Moreover, the kinetic mechanism of the basic Tat(44–61) peptide in this annealing further revealed that this peptide constitutes a representative nucleic acid annealer. To further understand the structure–activity relationships of this highly conserved domain, we investigated by electrophoresis and fluorescence approaches the binding and annealing properties of various Tat(44–61) mutants. Our data showed that the Tyr47 and basic residues of the Tat(44–61) domain were instrumental for binding to cTAR through stacking and electrostatic interactions, respectively, and promoting its annealing with dTAR. Furthermore, the annealing efficiency of the mutants clearly correlates with their ability to rapidly associate and dissociate the complementary oligonucleotides and to promote RTion. Thus, transient and dynamic nucleic acid interactions likely constitute a key mechanistic component of annealers and the role of Tat in the late steps of RTion. Finally, our data suggest that Lys50 and Lys51 acetylation regulates Tat activity in RTion.

Binding of a structured D-RNA molecule by an L-RNA aptamer

An L-RNA aptamer was developed that binds the natural D-form of the HIV-1 trans-activation responsive (TAR) RNA. The aptamer initially was obtained as a D-aptamer against L-TAR RNA through in vitro selection. Then the corresponding L-aptamer was prepared by chemical synthesis and used to bind the desired target. The L-aptamer binds D-TAR RNA with a Kd of 100 nM. It binds D-TAR exclusively at the six-nucleotide distal loop, but does so through tertiary interactions rather than simple Watson-Crick pairing. This complex is the first example of two nucleic acids molecules of opposing chirality that interact through a mode of binding other than primary structure. Binding of the L-aptamer to D-TAR RNA inhibits formation of the Tat-TAR ribonucleoprotein complex that is essential for TAR function. This suggests that L-aptamers, which are intrinsically resistant to degradation by ribonucleases, might be pursued as an alternative to antisense oligonucleotides to target structured RNAs of biological or therapeutic interest.

T-Cell Signaling in HIV-1 Infection

HIV exploits the T-cell signaling network to gain access to downstream cellular components, which serves as effective tools to break the cellular barriers. Multiple host factors and their interaction with viral proteins contribute to the complexity of HIV-1 pathogenesis and disease progression. HIV-1 proteins gp120, Nef, Tat and Vpr alter the T-cell signaling pathways by activating multiple transcription factors including NF-ĸB, Sp1 and AP-1. HIV-1 evades the immune system by developing a multi-pronged strategy. Additionally, HIV-1 encoded proteins influence the apoptosis in the host cell favoring or blocking T-cell apoptosis. Thus, T-cell signaling hijacked by viral proteins accounts for both viral persistence and immune suppression during HIV-1 infection. Here, we summarize past and present studies on HIV-1 T-cell signaling with special focus on the possible role of T cells in facilitating viral infection and pathogenesis

HIV-1 Tat protein variants: critical role for the cysteine region in synaptodendritic injury

HIV-1 enters the central nervous system early in infection; although HIV-1 does not directly infect neurons, HIV-1 may cause a variety of neurological disorders. Neuronal loss has been found in HIV-1, but synaptodendritic injury is more closely associated with the neurocognitive disorders of HIV-1. The HIV-1 transactivator of transcription (Tat) protein causes direct and indirect damage to neurons. The cysteine rich domain (residues 22-37) of Tat is important for producing neuronal death; however, little is known about the effects of the Tat protein functional domains on the dendritic network. The ability of HIV-1 Tat 1-101 Clades B and C, Tat 1-86 and Tat 1-72 proteins, as well as novel peptides (truncated 47-57, 1-72δ31-61, and 1-86 with a mutation at Cys22) to produce early synaptodendritic injury (24h), relative to later cell death (48h), was examined using cell culture. Treatment of primary hippocampal neurons with Tat proteins 1-72, 1-86 and 1-101B produced a significant early reduction in F-actin labeled puncta, implicating that these peptides play a role in synaptodendritic injury. Variants with a mutation, deletion, or lack of a cysteine rich region (1-86[Cys22], 1-101C, 1-72δ31-61, or 47-57) did not cause a significant reduction in F-actin rich puncta. Tat 1-72, 1-86, and 1-101B proteins did not significantly differ from one another, indicating that the second exon (73-86 or 73-101) does not play a significant role in the reduction of F-actin puncta. Conversely, peptides with a mutation, deletion, or lack of the cysteine rich domain (22-37) failed to produce a loss of F-actin puncta, indicating that the cysteine rich domain plays a key role in synaptodendritic injury. Collectively, these results suggest that for Tat proteins, 1) synaptodendritic injury occurs early, relative to cell death, and 2) the cysteine rich domain of the first exon is key for synaptic loss. Preventing such early synaptic loss may attenuate HIV-1 associated neurocognitive disorders.

Plasma membrane signaling in HIV-1 infection

Plasma membrane is a multifunctional structure that acts as the initial barrier against infection by intracellular pathogens. The productive HIV-1 infection depends upon the initial interaction of virus and host plasma membrane. Immune cells such as CD4+ T cells and macrophages contain essential cell surface receptors and molecules such as CD4, CXCR4, CCR5 and lipid raft components that facilitate HIV-1 entry. From plasma membrane HIV-1 activates signaling pathways that prepare the grounds for viral replication. Through viral proteins HIV-1 hijacks host plasma membrane receptors such as Fas, TNFRs and DR4/DR5, which results in immune evasion and apoptosis both in infected and uninfected bystander cells. These events are hallmark in HIV-1 pathogenesis that leads towards AIDS. The interplay between HIV-1 and plasma membrane signaling has much to offer in terms of viral fitness and pathogenicity, and a better understanding of this interplay may lead to development of new therapeutic approaches. This article is part of a Special Issue entitled: Viral Membrane Proteins - Channels for Cellular Networking.

Distinct DDX DEAD-box RNA helicases cooperate to modulate the HIV-1 Rev function

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Distinct DDX RNA helicases enhance the HIV-1 Rev function.

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Distinct DDX RNA helicases cooperate to enhance the Rev function.

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Several DDX RNA helicases except DDX6 interact with the Rev protein.

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DDX3 colocalizes with DDX5 and binds to it.

RNA helicase plays an important role in host mRNA and viral mRNA transcription, transport, and translation. Many viruses utilize RNA helicases in their life cycle, while human immunodeficiency virus type 1 (HIV-1) does not encode an RNA helicase. Thus, host RNA helicase has been involved in HIV-1 replication. Indeed, DDX1 and DDX3 DEAD-box RNA helicases are known to be required for efficient HIV-1 Rev-dependent RNA export. However, it remains unclear whether distinct DDX RNA helicases cross-talk and cooperate to modulate the HIV-1 Rev function. In this study, we noticed that distinct DDX RNA helicases, including DDX1, DDX3, DDX5, DDX17, DDX21, DDX56, except DDX6, bound to the Rev protein and they colocalized with Rev in nucleolus or nucleus. In this context, these DEAD-box RNA helicases except DDX6 markedly enhanced the HIV-1 Rev-dependent RNA export. Furthermore, DDX3 interacted with DDX5 and synergistically enhanced the Rev function. As well, combination of other distinct DDX RNA helicases cooperated to stimulate the Rev function. Altogether, these results suggest that distinct DDX DEAD-box RNA helicases cooperate to modulate the HIV-1 Rev function.

Digoxin suppresses HIV-1 replication by altering viral RNA processing

To develop new approaches to control HIV-1 replication, we examined the capacity of recently described small molecular modulators of RNA splicing for their effects on viral RNA metabolism. Of the drugs tested, digoxin was found to induce a dramatic inhibition of HIV-1 structural protein synthesis, a response due, in part, to reduced accumulation of the corresponding viral mRNAs. In addition, digoxin altered viral RNA splice site use, resulting in loss of the essential viral factor Rev. Digoxin induced changes in activity of the CLK family of SR protein kinases and modification of several SR proteins, including SRp20 and Tra2β, which could account for the effects observed. Consistent with this hypothesis, overexpression of SRp20 elicited changes in HIV-1 RNA processing similar to those observed with digoxin. Importantly, digoxin was also highly active against clinical strains of HIV-1 in vitro, validating this novel approach to treatment of this infection.

Humoral immune response to HTLV-1 basic leucine zipper factor (HBZ) in HTLV-1-infected individuals

Background

Human T cell lymphotropic virus type 1 (HTLV-1) infection can lead to development of adult T cell leukemia/lymphoma (ATL) or HTLV-1-associated myelopathy/tropical spastic paraparesis (HAM/TSP) in a subset of infected subjects. HTLV-1 basic leucine zipper factor (HBZ) gene has a critical role in HTLV-1 infectivity and the development of ATL and HAM/TSP. However, little is known about the immune response against HBZ in HTLV-1-infected individuals. In this study, we examined antibody responses against HBZ in serum/plasma samples from 436 subjects including HTLV-1 seronegative donors, asymptomatic carriers (AC), ATL, and HAM/TSP patients using the luciferase immunoprecipitation system.

Results

Immunoreactivity against HBZ was detected in subsets of all HTLV-1-infected individuals but the test did not discriminate between AC, ATL and HAM/TSP. However, the frequency of detection of HBZ-specific antibodies in the serum of ATL patients with the chronic subtype was higher than in ATL patients with the lymphomatous subtype. Antibody responses against HBZ were also detected in cerebrospinal fluid of HAM/TSP patients with anti-HBZ in serum. Antibody responses against HBZ did not correlate with proviral load and HBZ mRNA expression in HAM/TSP patients, but the presence of an HBZ-specific response was associated with reduced CD4⁺ T cell activation in HAM/TSP patients. Moreover, HBZ-specific antibody inhibited lymphoproliferation in the PBMC of HAM/TSP patients.

Conclusions

This is the first report demonstrating humoral immune response against HBZ associated with HTLV-I infection. Thus, a humoral immune response against HBZ might play a role in HTLV-1 infection.

Relationships between Cargo, Cell Penetrating Peptides and Cell Type for Uptake of Non-Covalent Complexes into Live Cells

Modulating signaling pathways for research and therapy requires either suppression or expression of selected genes or internalization of proteins such as enzymes, antibodies, nucleotide binding proteins or substrates including nucleoside phosphates and enzyme inhibitors. Peptides, proteins and nucleotides are transported by fusing or conjugating them to cell penetrating peptides or by formation of non-covalent complexes. The latter is often preferred because of easy handling, uptake efficiency and auto-release of cargo into the live cell. In our studies complexes are formed with labeled or readily detectable cargoes for qualitative and quantitative estimation of their internalization. Properties and behavior of adhesion and suspension vertebrate cells as well as the protozoa Leishmania tarentolae are investigated with respect to proteolytic activity, uptake efficiency, intracellular localization and cytotoxicity. Our results show that peptide stability to membrane-bound, secreted or intracellular proteases varies between different CPPs and that the suitability of individual CPPs for a particular cargo in complex formation by non-covalent interactions requires detailed studies. Cells vary in their sensitivity to increasing concentrations of CPPs. Thus, most cells can be efficiently transduced with peptides, proteins and nucleotides with intracellular concentrations in the low micromole range. For each cargo, cell type and CPP the optimal conditions must be determined separately.

Novel biopanning strategy to identify epitopes associated with vaccine protection

Identifying immune correlates of protection is important to develop vaccines against infectious diseases. We designed a novel, universally applicable strategy to profile the antibody (Ab) repertoire of protected vaccine recipients, using recombinant phages encoding random peptide libraries. The new approach, termed "protection-linked (PL) biopanning," probes the Ab paratopes of protected vaccinees versus those with vaccine failure. As proof of concept, we screened plasma samples from vaccinated rhesus macaques (RMs) that had completely resisted multiple mucosal challenges with R5-tropic simian-human immunodeficiency viruses (SHIVs). The animals had been immunized with a multicomponent vaccine (multimeric HIV-1 gp160, HIV-1 Tat, and SIV Gag-Pol particles). After PL biopanning, we analyzed the phagotopes selected for amino acid homologies; in addition to the expected Env mimotopes, one recurring motif reflected the neutralizing Ab epitope at the N terminus (NT) of HIV-1 Tat. Subsequent binding and functional assays indicated that anti-Tat NT Abs were present only in completely or partially protected RMs; peak viremia of the latter was inversely correlated with anti-Tat NT Ab titers. In contrast, highly viremic, unvaccinated controls did not develop detectable Abs against the same epitope. Based upon the protective effect observed in vivo, we suggest that Tat should be included in multicomponent HIV-1 vaccines. Our data highlight the power of the new PL-biopanning strategy to identify Ab responses with significant association to vaccine protection, regardless of the mechanism(s) or targets of the protective Abs. PL biopanning is also unbiased with regard to pathogens or disease model, making it a universal tool.

Pharmaceutical nanocarrier association with chondrocytes and cartilage explants: influence of surface modification and extracellular matrix depletion

OBJECTIVE

To evaluate cartilage diffusion and isolated chondrocyte association of micelles and liposomes and to determine the effect of cell-penetrating peptide (CPP) surface functionalization and extracellular matrix depletion on chondrocyte association and cartilage diffusion, respectively.

METHODS

Rhodamine-labeled micelles and liposomes were incubated with bovine chondrocytes and cell-associated fluorescence was quantified using flow cytometry. Rhodamine-labeled CPP-modified micelles and liposomes were incubated with chondrocytes and cell-associated fluorescence was compared to unmodified nanocarriers. Rhodamine-labeled micelles and liposomes were incubated with bovine cartilage explants for 1, 2 and 4 h and cartilage-associated fluorescence was compared across groups. Cartilage explants were treated with interleukin-1 alpha (IL-1α) or with 0.25% trypsin. Rhodamine-labeled micelles and liposomes were incubated with control, IL-1 and trypsin-treated explants and cartilage-associated fluorescence was compared across groups.

RESULTS

Chondrocyte-associated fluorescence following treatment with micelles was significantly higher (P < 0.001) than fluorescence in the cells treated with liposomes while there was no difference between cell-associated fluorescence in the liposomes-treated and untreated controls. CPP-modified nanocarriers exhibited a significant increase in chondrocyte association compared to unmodified nanocarriers (P < 0.001). Micelles exhibited a time and concentration-dependent diffusion in cartilage explants while liposomes showed no diffusion. Following IL-1 and trypsin treatments, micelle diffusion in articular cartilage was significantly higher (P < 0.001) than their diffusion in untreated explants.

CONCLUSION

Micelles exhibit superior association with isolated chondrocytes compared to liposomes. Surface modification with a CPP enhances chondrocyte association of both nanocarriers. 15 nm diameter micelles are better than 138 nm diameter liposomes in penetrating articular cartilage and extracellular matrix depletion enhances micelle penetration.

Peptide derived from HIV-1 TAT protein destabilizes a monolayer of endothelial cells in an in vitro model of the blood-brain barrier and allows permeation of high molecular weight proteins

Most chemotherapeutic agents are blood-brain barrier (BBB) impermeants. HIV-1-derived TAT protein variants contain a transmembrane domain, which may enable them to cross the BBB and reach the brain. Here we synthesized CAYGRKKRRQRRR, a peptide containing a cysteine moiety attached to the N terminus of the transmembrane domain (C-TAT peptide), and studied its effects in an in vitro BBB model, which we found to reflect penetration by a receptor-independent pathway. Incubation of the brain capillary endothelial cell monolayer with 0.3-0.6 μmol/ml of this C-TAT peptide, for a period of 1-2 h, destabilizes brain capillary endothelial cell monolayer and introduces the ability of impermeant therapeutic agents including high molecular weight proteins to penetrate it substantially. The cysteinyl moiety at position 1 of the C-TAT peptide contributes largely to the destabilizing potency and the penetration efficacy of impermeant substances. The destabilizing effect was reversed using heparin. In summary, experimental conditions allowing a significant increase in entry of impermeant low and high molecular weight substances from the luminal (blood) to the abluminal side (brain) were found in an in vitro BBB model reflecting in vivo protein penetrability by a receptor-independent pathway.

Genetic characterization of HIV type 1 Tat exon 1 from a southern Indian clinical cohort: identification of unique epidemiological signature residues

The trans-activator of transcription (Tat) of HIV-1 plays an important role in viral infection and pathogenesis. We examined the genetic characteristics of exon 1 of the tat gene derived from 102 seropositive subjects from southern India. Database-derived Indian (n=105) and global (n=413) HIV-1C sequences were also used for viral epidemiological signature pattern analysis in the Tat open reading frame (ORF). We identified HIV-1C as the most predominant genetic subtype (99%) and the presence of a novel A1C recombinant strain in one study participant. After examining all the available HIV-1C Indian sequences from primary clinical isolates and database-derived sequences, we found a high level of sequence conservation (92.6 ± 12%) within Tat amino acid residues. Furthermore, signature pattern analysis identified five amino acid positions in Tat that contained signature residues unique for Indian HIV-1C consisting of 21A, 24N, 29K, 40K, and 60Q. Our data have direct relevance for subunit-based Tat HIV-1 vaccine development.

HIV-1 Tat Binding to PCAF Bromodomain: Structural Determinants from Computational Methods

The binding between the HIV-1 trans-activator of transcription (Tat) and p300/(CREB-binding protein)-associated factor (PCAF) bromodomain is a crucial step in the HIV-1 life cycle. However, the structure of the full length acetylated Tat bound to PCAF has not been yet determined experimentally. Acetylation of Tat residues can play a critical role in enhancing HIV-1 transcriptional activation. Here, we have combined a fully flexible protein-protein docking approach with molecular dynamics simulations to predict the structural determinants of the complex for the common HIV-1BRU variant. This model reproduces all the crucial contacts between the Tat peptide 46SYGR(AcK)KRRQRC56 and the PCAF bromodomain previously reported by NMR spectroscopy. Additionally, inclusion of the entire Tat protein results in additional contact points at the protein-protein interface. The model is consistent with the available experimental data reported and adds novel information to our previous structural predictions of the PCAF bromodomain in complex with the rare HIVZ2 variant, which was obtained with a less accurate computational method. This improved characterization of Tat.PCAF bromodomain binding may help in defining the structural determinants of other protein interactions involving lysine acetylation.

Impact of Tat Genetic Variation on HIV-1 Disease

The human immunodeficiency virus type 1 (HIV-1) promoter or long-terminal repeat (LTR) regulates viral gene expression by interacting with multiple viral and host factors. The viral transactivator protein Tat plays an important role in transcriptional activation of HIV-1 gene expression. Functional domains of Tat and its interaction with transactivation response element RNA and cellular transcription factors have been examined. Genetic variation within tat of different HIV-1 subtypes has been shown to affect the interaction of the viral transactivator with cellular and/or viral proteins, influencing the overall level of transcriptional activation as well as its action as a neurotoxic protein. Consequently, the genetic variability within tat may impact the molecular architecture of functional domains of the Tat protein that may impact HIV pathogenesis and disease. Tat as a therapeutic target for anti-HIV drugs has also been discussed.

A peptide nucleic acid-aminosugar conjugate targeting transactivation response element of HIV-1 RNA genome shows a high bioavailability in human cells and strongly inhibits tat-mediated transactivation of HIV-1 transcription

The 6-aminoglucosamine ring of the aminoglycoside antibiotic neomycin B (ring II) was conjugated to a 16-mer peptide nucleic acid (PNA) targeting HIV-1 TAR RNA. For this purpose, we prepared the aminoglucosamine monomer 15 and attached it to the protected PNA prior to its cleavage from the solid support. We found that the resulting PNA-aminoglucosamine conjugate is stable under acidic conditions, efficiently taken up by the human cells and fairly distributed in both cytosol and nucleus without endosomal entrapment because cotreatment with endosome-disrupting agent had no effect on its cellular distribution. The conjugate displayed very high target specificity in vitro and strongly inhibited Tat mediated transactivation of HIV-1 LTR transcription in a cell culture system. The unique properties of this new class of PNA conjugate suggest it to be a potential candidate for therapeutic application.

Dynamic Post-Transcriptional Regulation of HIV-1 Gene Expression

Gene expression of the human immunodeficiency virus type 1 (HIV-1) is a highly regulated process. Basal transcription of the integrated provirus generates early transcripts that encode for the viral products Tat and Rev. Tat promotes the elongation of RNA polymerase while Rev mediates the nuclear export of viral RNAs that contain the Rev-responsive RNA element (RRE). These RNAs are exported from the nucleus to allow expression of Gag-Pol and Env proteins and for the production of full-length genomic RNAs. A balance exists between completely processed mRNAs and RRE-containing RNAs. Rev functions as an adaptor that recruits cellular factors to re-direct singly spliced and unspliced viral RNAs to nuclear export. The aim of this review is to address the dynamic regulation of this post-transcriptional pathway in light of recent findings that implicate several novel cellular cofactors of Rev function.

Comparative nucleic acid chaperone properties of the nucleocapsid protein NCp7 and Tat protein of HIV-1

RNA chaperones are proteins able to rearrange nucleic acid structures towards their most stable conformations. In retroviruses, the reverse transcription of the viral RNA requires multiple and complex nucleic acid rearrangements that need to be chaperoned. HIV-1 has evolved different viral-encoded proteins with chaperone activity, notably Tat and the well described nucleocapsid protein NCp7. We propose here an overview of the recent reports that examine and compare the nucleic acid chaperone properties of Tat and NCp7 during reverse transcription to illustrate the variety of mechanisms of action of the nucleic acid chaperone proteins.

Genetic architecture of HIV-1 genes circulating in north India & their functional implications

This review presents data on genetic and functional analysis of some of the HIV-1 genes derived from HIV-1 infected individuals from north India (Delhi, Punjab and Chandigarh). We found evidence of novel B/C recombinants in HIV-1 LTR region showing relatedness to China/Myanmar with 3 copies of Nfκb sites; B/C/D mosaic genomes for HIV-1 Vpr and novel B/C Tat. We reported appearance of a complex recombinant form CRF_02AG of HIV-1 envelope sequences which is predominantly found in Central/Western Africa. Also one Indian HIV-1 envelope subtype C sequence suggested exclusive CXCR4 co-receptor usage. This extensive recombination, which is observed in about 10 per cent HIV-1 infected individuals in the Vpr genes, resulted in remarkably altered functions when compared with prototype subtype B Vpr. The Vpu C was found to be more potent in causing apoptosis when compared with Vpu B when analyzed for subG1 DNA content. The functional implications of these changes as well as in other genes of HIV-1 are discussed in detail with possible implications for subtype-specific pathogenesis highlighted.

Expression of human endogenous retrovirus type K (HML-2) is activated by the Tat protein of HIV-1

Human endogenous retroviruses (HERVs) make up 8% of the human genome. The expression of HERV-K (HML-2), the family of HERVs that most recently entered the genome, is tightly regulated but becomes markedly increased after infection with HIV-1. To better understand the mechanisms involved in this activation, we explored the role of the HIV-1 Tat protein in inducing the expression of these endogenous retroviral genes. Administration of recombinant HIV-1 Tat protein caused a 13-fold increase in HERV-K (HML-2) gag RNA transcripts in Jurkat T cells and a 10-fold increase in primary lymphocytes, and the expression of the HERV-K (HML-2) rec and np9 oncogenes was also markedly increased. This activation was seen especially in lymphocytes and monocytic cells, the natural hosts for HIV-1 infection. Luciferase reporter gene assays demonstrated that the effect of Tat on HERV-K (HML-2) expression occurred at the level of the transcriptional promoter. The transcription factors NF-κB and NF-AT contribute to the Tat-induced activation of the promoter, as shown by chromatin immunoprecipitation assays, mutational analysis of the HERV-K (HML-2) long terminal repeat, and treatments with agents that inhibit NF-κB or NF-AT activation. These studies demonstrate that HIV-1 Tat plays an important role in activating expression of HERV-K (HML-2) in the setting of HIV-1 infection.

Cellular translocation of a γ-AApeptide mimetic of Tat peptide

Cell-penetrating peptides including the trans-activating transcriptional activator (Tat) from HIV-1 have been used as carriers for intracellular delivery of a myriad of cargoes including drugs, molecular probes, DNAs and nanoparticles. Utilizing fluorescence flow cytometry and confocal fluorescence microscopy, we demonstrate that a γ-AApeptide mimetic of Tat (48-57) can cross the cell membranes and enter the cytoplasm and nucleus of cells, with efficiency comparable to or better than that of Tat peptide (48-57). Deletion of the four side chains of the γ-AApeptide attenuates translocation capability. We also establish that the γ-AApeptide is even less toxic than the Tat peptide against mammalian cells. In addition to their low toxicity, γ-AApeptides are resistant to protease degradation, which may prove to be advantageous over α-peptides for further development of molecular transporters for intracellular delivery.

EGCG inhibits Tat-induced LTR transactivation: role of Nrf2, AKT, AMPK signaling pathway

AIMS

Transcription is a crucial step for human immunodeficiency virus 1 (HIV-1) gene expression in infected host cells. The HIV-1 Tat activates the nuclear factor-kappa B (NF-κB) signaling transduction pathway, which is necessary for viral replication. Epigallocatechin-3-gallate (EGCG) has antioxidant, anti-inflammatory, and anti-viral properties. In this study, we investigated the effects of EGCG on Tat-induced HIV-1 transactivation and potential mechanisms by which EGCG inhibited activation of NF-κB pathway.

MAIN METHODS

HeLa-CD4-long terminal repeat (LTR)-β-gal (MAGI) cells were transfected with Tat plasmid. Tat-induced HIV-1 LTR transactivation was determined by MAGI cell assay. The reactive oxygen species (ROS) levels and glutathione (GSH) levels were measured. In addition, the protein expressions were assayed by western blotting.

KEY FINDINGS

Tat caused a significant decrease in the intracellular glutathione (GSH) levels, a mild increase in the expression of nuclear levels of NF-E2-related factor-2 (Nrf2), a significant increase in the levels of NF-κB (phosphorylation of p65 and IKK) and a significant increase in ROS production. EGCG supplementation significantly improved the changes associated with Tat-induced oxidative stress by increasing nuclear levels of Nrf2, decreasing levels of NF-κB and ROS production. EGCG reversed Tat-mediated AKT activation and AMPK inhibition in MAGI cells. EGCG inhibited Tat-induced LTR transactivation in a dose-dependent manner.

SIGNIFICANCE

The results suggest that Nrf2 signaling pathway may be the primary target for prevention of Tat-induced HIV-1 transactivation by EGCG, and EGCG also reduce NF-κB activation by inhibiting AKT signaling pathway and activating AMPK signaling pathway.

Molecular mechanisms in the dramatic enhancement of HIV-1 Tat transduction by cationic liposomes

Human immunodeficiency virus type 1 (HIV-1) transactivator of transcription (Tat) protein possesses a unique membrane-transduction property. Interestingly, Tat transduction could be dramatically increased 1000-fold based on LTR-transactivation assay when complexed with cationic liposomes (lipo-Tat), compared with Tat alone. Therefore, underlining mechanisms were explored further. Microscopy and flow cytometry showed that this effect was associated with enhanced membrane binding, large particle formation (1-2 μm) and increased intracellular uptake of Tat fluorescent proteins. Using pharmacological assays and immune colocalizations, it was found that lipid raft-dependent endocytosis and macropinocytosis were major pathways involved in lipo-Tat uptake, and actin-filaments played a major role in intracellular trafficking of lipo-Tat to the nucleus. Furthermore, we found that the Tat hydrophobic domain (aa 36-47) mediated formation of two positively charged molecules into lipo-Tat complexes via hydrophobic bonds, based on LTR-transactivation inhibition assay. Thus, the hydrophobic domain may play an important role in Tat protein uptake and be useful for intracellular delivery of biomacromolecules if coupled together with Tat basic peptide, a cell-penetrating peptide.

The 5' UTR of HIV-1 full-length mRNA and the Tat viral protein modulate the programmed -1 ribosomal frameshift that generates HIV-1 enzymes

Translation of the full-length messenger RNA (mRNA) of the human immunodeficiency virus type 1 (HIV-1) generates the precursor of the viral enzymes via a programmed -1 ribosomal frameshift. Here, using dual-luciferase reporters, we investigated whether the highly structured 5' untranslated region (UTR) of this mRNA, which interferes with translation initiation, can modulate HIV-1 frameshift efficiency. We showed that, when the 5' UTR of HIV-1 mRNA occupies the 5' end of the reporter mRNA, HIV-1 frameshift efficiency is increased about fourfold in Jurkat T-cells, compared with a control dual-luciferase reporter with a short unstructured 5' UTR. This increase was related to an interference with cap-dependent translation initiation by the TAR-Poly(A) region at the 5' end of the messenger. HIV-1 mRNA 5' UTR also contains an internal ribosome entry site (IRES), but we showed that, when the cap-dependent initiation mode is available, the IRES is not used or is weakly used. However, when the ribosomes have to use the IRES to translate the dual-luciferase reporter, the frameshift efficiency is comparable to that of the control dual-luciferase reporter. The decrease in cap-dependent initiation and the accompanying increase in frameshift efficiency caused by the 5' UTR of HIV-1 mRNA is antagonized, in a dose-dependent way, by the Tat viral protein. Tat also stimulates the IRES-dependent initiation and decreases the corresponding frameshift efficiency. A model is presented that accounts for the variations in frameshift efficiency depending on the 5' UTR and the presence of Tat, and it is proposed that a range of frameshift efficiencies is compatible with the virus replication.