Molecular determinants for cellular uptake of Tat protein of human immunodeficiency virus type 1 in brain cells

Efficient Intracellular Delivery of a Pro-Apoptotic Peptide With A pH-Responsive Carrier

A key challenge in developing protein therapeutics or imaging agents that work against cytosolic targets is the intracellular delivery barrier. Here, we show that the pH-responsive, membrane-destabilizing polymer, poly (propylacrylic acid) (PPAA), can strongly enhance target cell killing through the intracellular delivery of a functional proapoptotic peptide. The Bak BH3 peptide induces apoptosis via antagonization of suppressor targets such as Bcl-2 and Bcl-x(L). A genetically-engineered streptavidin that contains an N-terminal TAT peptide sequence was used to optimize the pinocytotic cell uptake of biotinylated BH3 peptide and end-biotinylated PPAA. Fluorescence microscopic analysis of DAPI-stained HELA cells was used to quantitate apoptosis. Approximately 30% of cells treated with TAT-SA:BH3 complexes revealed morphologically distinct nuclear condensation, a hallmark of apoptosis. The incorporation of biotinylated PPAA had the effect of markedly enhancing the killing effect of BH3 peptides by an additional 55% (p<0.001) to a total cell killing efficiency of 85%. Caspase-3 activity was up-regulated in a TAT-SA:BH3:PPAA dose-dependent manner. The induction of apoptosis with the TAT-SA:BH3:PPAA complex was abrogated with the L78A BH3 peptide, that had been previously shown to knock-out antagonization activity. The caspase and L78A peptide results demonstrate that the delivered BH3 is indeed working through the biologically relevant apoptosis signaling pathway. These studies establish the ability of PPAA to strongly enhance the intracellular delivery of a functional pro-apoptotic peptide. Together with the PPAA, the TAT-SA adaptor complex could prove useful as a carrier of peptide/protein cargo to cultured cells.

High-performance capillary electrophoresis for determining HIV-1 Tat protein in neurons

The HIV-1 protein, Tat has been implicated in AIDS pathogenesis however, the amount of circulating Tat is believed to be very low and its quantification has been difficult. We performed the quantification of Tat released from infected cells and taken up by neurons using high performance capillary electrophoresis. This is the first report to successfully measure the amount of Tat in neurons and places Tat as a key player involved in HIV-associated neurocognitive disorders.

HIV-1-induced alterations of claudin-5 expression at the blood-brain barrier level

HIV-1 crosses the blood-brain barrier (BBB) early in the course of systemic infection and resides in brain macrophages and microglia. The integrity of the brain endothelium is regulated by intercellular tight junctions, which also play a critical role in HIV-1-entry into the brain. Disruption of tight junctions, including changes in claudin-5 expression, is common in HIV-1-infected patients. Recent evidence indicates that both exposure to HIV-1 and HIV-1 specific proteins, such as Tat protein, can contribute to alterations of expression and distribution of claudin-5 in brain endothelial cells and brain microvessels.

Dose-dependent long-term effects of Tat in the rat hippocampal formation: a design-based stereological study

The human immunodeficiency virus type 1 (HIV-1) protein transactivator of transcription (Tat) is believed to play a critical role in mediating central nervous system (CNS) pathology in pediatric HIV-1 infection. Long-term neurotoxicity was investigated in a design-based stereology study following intrahippocampal injection of Tat on postnatal day (P)10, a time period that approximates the peak in the rats' rate of brain growth and mimics clinical HIV-1 CNS infection at labor/delivery. The goal was to examine the impact of P10 intrahippocampal Tat injection on the anatomy of the adult hippocampus (5 month) to gain a better understanding about how timing of infection influences the rate of progression of pediatric HIV-1 infection [cf. Fitting et al. (2008a) Hippocampus 18:135-147]. Male P10 Sprague-Dawley rats were bilaterally injected with vehicle or one of three different doses of Tat (5, 25, or 50 mug). Unbiased stereological estimates were used to quantify total neuron number (Nissl stain) in five major subregions of the rat hippocampus: granular layer (GL), hilus of the dentate gyrus (DGH), cornu ammonis fields (CA)2/3, CA1, and subiculum (SUB). Glial cells (astrocytes and oligodendrocytes) were quantified in the DGH and SUB. No significant reduction of neuron number was noted for any of the five hippocampal subregions, in contrast to the very prominent reductions reported when Tat was administered on P1 [Fitting et al. (2008a) Hippocampus 18:135-147]. However, for glial cells, the number of astrocytes in the DGH and SUB as well as the number of oligodendrocytes in the DGH were linear dose dependently increased as a function of dose of Tat. In conjunction with previous stereological research [Fitting et al., (2008a) Hippocampus 18:135-147], the present data suggest that variability in the progression of pediatric HIV/acquired immunodeficiency syndrome (AIDS) may be better understood with the knowledge of the factor of timing of HIV-1 CNS infection.

Intact lipid rafts regulate HIV-1 Tat protein-induced activation of the Rho signaling and upregulation of P-glycoprotein in brain endothelial cells

The Rho signaling has an essential function in human immunodeficiency virus (HIV)-1-mediated disruption of the integrity of the blood-brain barrier (BBB). However, it is unknown how membrane domains, such as lipid rafts, can influence HIV-1-mediated activation of the Rho pathway and how these processes can affect the expression of the efflux transporters at the BBB level. This study is focused on the function of HIV-1 protein Tat in activation of the Rho signaling and upregulation of P-glycoprotein (P-gp) in human brain endothelial cells. Treatment with Tat markedly elevated GTP-RhoA levels and the potential downstream effectors, such as myosin phosphatase target subunit 1 and myosin light chain. In addition, Tat upregulated expression and promoter activity of P-gp as well as its efflux function. Inhibition of the Rho signaling cascade effectively blocked P-gp overexpression at the level of promoter activity. Disruption of lipid rafts by depletion of membrane cholesterol by methyl-beta-cyclodextrin, but not caveolin-1 silencing, also abolished Tat-mediated RhoA activation and P-gp upregulation. The present data indicate the critical function of intact lipid rafts and the Rho signaling in HIV-1-mediated upregulation of P-gp and potential development of drug resistance in brain endothelial cells.

Modifications in host cell cytoskeleton structure and function mediated by intracellular HIV-1 Tat protein are greatly dependent on the second coding exon

The human immunodeficiency virus type 1 (HIV-1) regulator Tat is essential for viral replication because it achieves complete elongation of viral transcripts. Tat can be released to the extracellular space and taken up by adjacent cells, exerting profound cytoskeleton rearrangements that lead to apoptosis. In contrast, intracellular Tat has been described as protector from apoptosis. Tat gene is composed by two coding exons that yield a protein of 101 amino acids (aa). First exon (1–72aa) is sufficient for viral transcript elongation and second exon (73–101 aa) appears to contribute to non-transcriptional functions. We observed that Jurkat cells stably expressing intracellular Tat101 showed gene expression deregulation 4-fold higher than cells expressing Tat72. Functional experiments were performed to evaluate the effect of this deregulation. First, NF-κB-, NF-AT- and Sp1-dependent transcriptional activities were greatly enhanced in Jurkat-Tat101, whereas Tat72 induced milder but efficient activation. Second, cytoskeleton-related functions as cell morphology, proliferation, chemotaxis, polarization and actin polymerization were deeply altered in Jurkat-Tat101, but not in Jurkat-Tat72. Finally, expression of several cell surface receptors was dramatically impaired by intracellular Tat101 but not by Tat72. Consequently, these modifications were greatly dependent on Tat second exon and they could be related to the anergy observed in HIV-1-infected T cells.

Apolipoprotein E genotype and hepatitis C, HIV and herpes simplex disease risk: a literature review

Apolipoprotein E is a polymorphic and multifunctional protein with numerous roles in lipoprotein metabolism. The three common isoforms apoE2, apoE3 and apoE4 show isoform-specific functional properties including different susceptibilities to diseases. ApoE4 is an accepted risk factor for Alzheimer's disease and cardiovascular disorders. Recently, associations between apoE4 and infectious diseases have been demonstrated. This review summarises how apoE4 may be involved in the infection incidence and associated pathologies of specific infectious diseases, namely hepatitis C, human immunodeficiency virus disease and herpes simplex.ApoE4 seems to be protective against chronic hepatitis C virus infection and retards fibrosis progression. In contrast apoE4 enhances the fusion rate of human immunodeficiency virus with target cell membranes, resulting in accelerated cell entry and faster disease progression. Its association with human immunodeficiency virus-associated dementia remains controversial. Regarding herpes simplex virus infection, apoE4 intensifies virus latency and is associated with increased oxidative damage of the central nervous system, and there is some evidence that herpes simplex virus infection in combination with the apoE4 genotype may be associated with an increased risk of Alzheimer's disease. In addition to reviewing available data from human trials, evidence derived from a variety of cell culture and animal models are considered in this review in order to provide mechanistic insights into observed association between apoE4 genotype and viral disease infection and pathology.

Differences in NMDA receptor expression during human development determine the response of neurons to HIV-tat-mediated neurotoxicity

HIV infection of the CNS can result in neurologic dysfunction in a significant number of infected individuals. NeuroAIDS is characterized by neuronal injury and loss, yet there is no evidence of HIV infection in neurons. Thus, neuronal damage and dropout are likely due to indirect effects of HIV infection of other CNS cells, through elaboration of inflammatory factors and neurotoxic viral proteins, including the viral transactivating protein tat. We and others demonstrated that tat induces apoptosis in differentiated mature human neurons. We now demonstrate that the high level of tat toxicity observed in human neurons involves specific developmental stages that correlate with N-methyl-D-aspartate receptor (NMDAR) expression, and that tat toxicity is also dependent upon the species being analyzed. Our results indicate that tat treatment of primary cultures of differentiated human neurons with significant amounts of NMDAR expression induces extensive apoptosis. In contrast, tat treatment induces only low levels of apoptosis in primary cultures of immature human neurons with low or minimal expression of NMDAR. In addition, tat treatment has minimal effect on rat hippocampal neurons in culture, despite their high expression of NMDAR. We propose that this difference may be due to low expression of the NR2A subunit. These findings are important for an understanding of the many differences among tissue culture systems and species used to study HIV-tat-mediated toxicity.

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.

Intra-cornu ammonis 1 administration of the human immunodeficiency virus-1 protein trans-activator of transcription exacerbates the ethanol withdrawal syndrome in rodents and activates N-methyl-D-aspartate glutamate receptors to produce persisting spatial learning deficits

Human immunodeficiency virus-1 (HIV-1) infection may produce neurological deficits, such as cognitive decline, that may be worsened by concurrent ethanol (EtOH) abuse. Among the many biochemical cascades likely mediating HIV-1-associated neuronal injury is enhancement of N-methyl-d-aspartate (NMDA) receptor function and progression to excitotoxicity, an effect that may be directly or indirectly related to accumulation in brain of the HIV-1 trans-activator of transcription (Tat) factor. The present studies were designed to examine the hypothesis that binge-like EtOH pre-exposure would enhance effects of Tat on NMDA receptor function. These studies employed a modified in vivo binge EtOH exposure regimen designed to produce peak blood EtOH levels (BEL) of <200 mg/dl in adult male rats and were designed to examine effects of intra-hippocampal injection of Tat (0.5 microl/500 pM/2 min) on EtOH withdrawal-related behavior, spatial learning, and histological measures. Unilateral cannulae were implanted into the cornu ammonis 1 (CA1) pyramidal cell layer of animals prior to beginning a 4-day binge EtOH regimen. EtOH was administered via intragastric intubation ( approximately 3.0-5.0 g/kg) with dose determined by behavioral ratings of intoxication daily for 4 days (at 08:00, 16:00, and 24:00 h). EtOH withdrawal behaviors were monitored 12 h after the last administration of EtOH. Morris water maze learning was assessed during the following 4 days, at which times brains were harvested for autoradiographic measurement of NMDA receptor density and neuroinflammation. Maximal BELs of 187.69 mg/dl were observed 60 min after EtOH administration on day 2 of the regimen. In contrast, peak BELs of approximately 100 mg/dl were observed 60 min after EtOH administration on day 4 of the regimen, suggesting development of metabolic tolerance. Significant behavioral abnormalities were observed in EtOH withdrawn animals, including tremor and seizures. Intra-CA1 region injection of Tat significantly potentiated EtOH withdrawal behavioral abnormalities, an effect that was reduced by MK-801 pre-exposure. While EtOH withdrawn animals showed learning similar to control animals, EtOH withdrawn animals that received intra-CA1 Tat injection demonstrated persisting deficits in spatial learning on days 3 and 4 of training, effects that were markedly reduced by administration of the competitive NMDA receptor antagonist MK-801 30 min prior to Tat injection. No changes in [(3)H]MK-801 binding were observed. Binding density of [(3)H]PK11195, a ligand for peripheral benzodiazepine receptors expressed on activated microglia, was elevated proximal to cannula tracks in all animals, but was not altered by EtOH or Tat exposure. These findings suggest that EtOH abuse and/or dependence in HIV-positive individuals may promote HIV-1-associated cognitive deficits by altering NMDA receptor function in the absence of microglial activation or neuroinflammation.

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.

SMAD proteins of oligodendroglial cells regulate transcription of JC virus early and late genes coordinately with the Tat protein of human immunodeficiency virus type 1

JC virus (JCV) is the aetiological agent of progressive multifocal leukoencephalopathy (PML), a fatal, demyelinating disease of the brain affecting people with AIDS. Although immunosuppression is involved in infection of the brain by JCV, a direct influence of human immunodeficiency virus type 1 (HIV-1) has also been established. The Tat protein of HIV-1 has been implicated in activation of the cytokine transforming growth factor (TGF)-beta in HIV-1-infected cells and in stimulating JCV gene transcription and DNA replication in oligodendroglia, the primary central nervous system cell type infected by JCV in PML. This study demonstrated that Tat can cooperate with SMAD proteins, the intracellular effectors of TGF-beta, at the JCV DNA control region (CR) to stimulate JCV gene transcription. Tat stimulated JCV early gene transcription in KG-1 oligodendroglial cells when expressed via transfection or added exogenously. Using chromatin immunoprecipitation, it was shown that exogenous Tat enhanced binding of SMAD2, -3 and -4 and their binding partner Fast1 to the JCV CR in living cells. When SMAD2, -3 and -4 were expressed together, Tat, expressed from plasmid pTat, stimulated transcription from both early and late gene promoters, with the early promoter exhibiting stimulation of >100-fold. Tat, SMAD4 and JCV large T-antigen were all visualized in oligodendroglial cells at the border of an active PML lesion in the cerebral frontal lobe. These results revealed a positive reinforcement system in which the SMAD mediators of the TGF-beta system act cooperatively with Tat to stimulate JCV gene transcription.

HIV-1 Tat protein-induced rapid and reversible decrease in [3H]dopamine uptake: dissociation of [3H]dopamine uptake and [3H]2beta-carbomethoxy-3-beta-(4-fluorophenyl)tropane (WIN 35,428) binding in rat striatal synaptosomes

Human immunodeficiency virus (HIV)-1 Tat protein plays a key role in the pathogenesis of both HIV-1-associated cognitive-motor disorder and drug abuse. Dopamine (DA) transporter (DAT) function is strikingly altered in patients with HIV-1-associated dementia and a history of chronic drug abuse. This study is the first in vitro evaluation of potential mechanisms underlying the effects of Tat protein on DAT function. Rat striatal synaptosomes were incubated with recombinant Tat(1-86) protein, and [(3)H]DA uptake and the binding of [(3)H]2beta-carbomethoxy-3-beta-(4-fluorophenyl)tropane (WIN 35,428) and [(3)H]1-[2-(diphenylmethoxy)ethyl]-4-(3-phenylpropyl)-piperazine (GBR 12935) were determined. Tat decreased [(3)H]DA uptake, [(3)H]WIN 35,428 binding, and [(3)H]GBR 12935 binding in a time-dependent manner. The potency of Tat for inhibiting [(3)H]DA uptake (K(i) = 1.2 microM) was the same as that for inhibiting [(3)H]GBR 12935 binding but 3-fold less than that for inhibiting [(3)H]WIN 35,428 binding. Mutant Tat proteins did not alter [(3)H]DA uptake. Kinetic analysis of [(3)H]DA uptake revealed that Tat (1 or 10 microM) decreased the V(max) value and increased the K(m) value in a dose-dependent manner. The V(max) value, decreased by Tat (1 microM), returned to the control level after washout of Tat, indicating that the inhibitory effect of Tat on DA uptake was reversible. Saturation studies revealed that Tat decreased the B(max) value and increased the K(d) value of [(3)H]WIN 35,428 binding, whereas Tat decreased the B(max) value of [(3)H]GBR 12935 binding, without a change in the K(d) value. These findings provide new insight into understanding the pharmacological mechanisms of Tat-induced dysfunction of the DAT in the dopaminergic system in HIV-infected patients.

HIV-1 Tat C-terminus is cleaved by calpain 1: implication for Tat-mediated neurotoxicity

HIV-Encephalopathy (HIVE) is a common neurological disorder associated with HIV-1 infection and AIDS. The activity of the HIV trans-activating protein Tat is thought to contribute to neuronal pathogenesis. While Tat proteins from primary virus isolates consist of 101 or more amino acids, 72 and 86 amino acids forms of Tat are commonly used for in vitro studies. Although Tat72 contains the minimal domain required for viral replication, other activities of Tat appear to vary according to its length, sub-cellular localization, cell type and the stage of cellular differentiation. In this study, we investigated the stability of intracellular Tat101 during proliferation and differentiation of neuronal cells in culture. We have utilized rat neuronal progenitors as a model of neuronal cell proliferation and differentiation, as well as rat primary cortical neurons as a model of fully differentiated cells. Our results indicate that, upon internalization, Tat101 was degraded more rapidly in proliferating cells than in cells which either underwent neuronal differentiation or were fully differentiated. Intracellular degradation of Tat was prevented by the calpain 1 inhibitor, ALLN, in both proliferating and differentiated cells. Inhibition of calpain 1 by calpastatin peptide also prevented Tat cleavage. In vitro calpain digestion and mass spectrometry analysis further demonstrated that the sequence of Tat sensitive to calpain cleavage was located in the C-terminus of this viral protein, between amino acids 68 and 69. Moreover, cleavage of Tat101 by calpain 1 increased neurotoxic effect of this viral protein and presence of the calpain inhibitor protected neuronal cells from Tat-mediated toxicity.

PPARalpha and PPARgamma effectively protect against HIV-induced inflammatory responses in brain endothelial cells

Peroxisome proliferator-activated receptors (PPARs) are nuclear receptors which down-regulate inflammatory signaling pathways. Therefore, we hypothesized that alterations of PPAR functions can contribute to human immunodeficiency virus-1 (HIV-1)-induced dysfunction of brain endothelial cells. Indeed, treatment with HIV-1 transactivator of transcription (Tat) protein decreased PPAR transactivation in brain endothelial cells. We next stably over-expressed PPARalpha and PPARgamma in a newly developed cell line of human brain endothelial cells (hCMEC/D3 cells). Tat-induced up-regulation of inflammatory mediators, such as interleukin (IL)-1beta, tumor necrosis factor-alpha, CCL2, and E-selectin were markedly attenuated in hCMEC/D3 over-expressing PPARalpha or PPARgamma. These results were confirmed in CCL2 and E-selectin promoter activity studies. Similar protective effects were observed in hCMEC/D3 after activation of PPARgamma by exogenous PPAR agonists (dPGJ(2) and rosiglitazone). PPAR over-expression also prevented Tat-induced binding activity and transactivation of nuclear factor-kappaB. Importantly, increased PPAR activity attenuated induction of IL-1beta, tumor necrosis factor-alpha, CCL2, and E-selectin in hCMEC/D3 cells co-cultured with HIV-1-infected Jurkat cells. The protective effects of PPAR over-expression were reversed by the antagonists of PPARalpha (MK886) or PPARgamma (GW9662). The present data suggest that targeting PPAR signaling may provide a novel therapeutic approach to attenuate HIV-1-induced local inflammatory responses in brain endothelial cells.

Intra-accumbal Tat1-72 alters acute and sensitized responses to cocaine

The effects of Tat, an HIV-1 protein, on intravenous cocaine-induced locomotor activity were examined in ovariectomized rats. Animals were habituated to activity chambers, administered an i.v. baseline/saline injection, and 24 h later, received bilateral, intra-accumbal microinjections of Tat1-72 (15 microg/microl) or vehicle. Twenty four hours later, rats received the first of 14 daily i.v. cocaine injections (3.0 mg/kg/inj, 1 /day) or saline. Locomotor activity was measured in automated chambers for 30 min following baseline and after the 1st and 14th cocaine injections. Observational time sampling following cocaine was also performed. Following acute cocaine/saline, Tat significantly increased cocaine-induced total activity over the 30-min session, with no significant effects for activity in the central compartment. Repeated cocaine injections produced behavioral sensitization with approximately 2-fold higher levels of total activity, approximately 3-fold higher levels of centrally directed activity, and increased locomotor scores via direct observations. Following repeated cocaine/saline, Tat altered the development of cocaine-induced behavioral sensitization for total activity with prior Tat exposure attenuating the development of cocaine-induced sensitization. Collectively, these data show that bilateral microinjection of Tat into the N Acc alters i.v. cocaine-induced behavior, suggesting that Tat produces behavioral changes by disrupting the mesocorticolimbic pathway.

Neonatal intrahippocampal injection of the HIV-1 proteins gp120 and Tat: differential effects on behavior and the relationship to stereological hippocampal measures

HIV-1 proteins, such as Tat and gp120, are believed to play a crucial role in the central nervous system (CNS) pathology of acquired immune deficiency syndrome (AIDS). The present study sought to determine the potential role of Tat and/or gp120 on behavioral development and the relationship to the long-term effects of the HIV-1 proteins on the rat hippocampus. Male pups of 13 Sprague-Dawley litters were bilaterally injected on postnatal day (P)1. Every litter contributed an animal to each of four treatment condition: VEH (0.5 microl sterile buffer), gp120 (100 ng), Tat (25 microg) or combined gp120+Tat (100 ng+25 microg). Body weight was not affected by either protein treatment. Tat revealed a transient effect on many of the behavioral assessments early in development as well as on preattentive processes and spatial memory in adulthood. Gp120 had more selective effects on negative geotaxis (P8-P10) and on locomotor activity (P94-P96). Combined gp120+Tat effects were noted for eye opening with potential interactive effects of gp120 and Tat on negative geotaxis. Anatomical assessment at approximately 7 1/2 months of age was conducted by using design-based stereology to quantify the total cell number in five hippocampal subregions [granule layer (GL), hilus of the dentate gyrus (DGH), cornu ammonis fields (CA)2/3, CA1, and subiculum (SUB)] [Fitting, S., Booze, R.M., Hasselrot, U., Mactutus, C.F., 2007a. Differential long-term neurotoxicity of HIV-1 proteins in the rat hippocampal formation: a design-based stereological study. Hippocampus 18(2), 135-147]. A relationship between early reflex development and estimated cell number in the adult hippocampus was indicated by simple regression analyses. In addition, estimated number of neurons and astrocytes in the DGH explained 81% of the variance of the distribution of searching behavior in the probe test. Collectively, these data indicate that the DGH may participate in the spatial memory alterations observed in adulthood consequent to neonatal exposure to HIV-1 proteins.

Involvement of the p53 and p73 transcription factors in neuroAIDS

HIV-associated dementia (HAD) is the most common AIDS-associated neurological disorder and is characterized by the development of synaptodendritic injury to neurons. To advance HAD therapy, it is crucial to identify the mechanisms and factors involved. The viral protein HIV-1 Tat is among those factors and is released by HIV-1-infected cells and can be taken up by adjacent neuronal cells leading to neurotoxic effects. Multiple cellular host proteins have been identified as Tat cofactors in causing neuronal injury. Interestingly, most of these factors function through activation of the p53 pathway. We have now examined the ability of Tat to activate the p53 pathway leading to the induction of endogenous p53 and p73 in neuronal cells. We found that Tat induced p53 and p73 levels in SH-SY5Y cells and that this induction caused retraction of neurites. In the absence of either p53 or p73, Tat failed to induce dendritic retraction or to activate the proapoptotic proteins, such as Bax. Further, we found that p53-accumulation in Tat-treated cells depends on the presence of p73. Therefore, we conclude that Tat contributes to neuronal degeneration through activation of a pathway involving p53 and p73. This information will be valuable for the development of therapeutic agents that affect these pathways to protect CNS neurons and prevent HAD.

Enhancement of human immunodeficiency virus (HIV)-specific CD8+ T cells in cerebrospinal fluid compared to those in blood among antiretroviral therapy-naive HIV-positive subjects

During untreated human immunodeficiency virus type 1 (HIV-1) infection, virus-specific CD8(+) T cells partially control HIV replication in peripheral lymphoid tissues, but host mechanisms of HIV control in the central nervous system (CNS) are incompletely understood. We characterized HIV-specific CD8(+) T cells in cerebrospinal fluid (CSF) and peripheral blood among seven HIV-positive antiretroviral therapy-naïve subjects. All had grossly normal brain magnetic resonance imaging and spectroscopy and normal neuropsychometric testing. Frequencies of epitope-specific CD8(+) T cells by direct tetramer staining were on average 2.4-fold higher in CSF than in blood (P = 0.0004), while HIV RNA concentrations were lower. Cells from CSF were readily expanded ex vivo and responded to a broader range of HIV-specific human leukocyte antigen class I restricted optimal peptides than did expanded cells from blood. HIV-specific CD8(+) T cells, in contrast to total CD8(+) T cells, in CSF and blood were at comparable maturation states, as assessed by CD45RO and CCR7 staining. The strong relationship between higher T-cell frequencies and lower levels of viral antigen in CSF could be the result of increased migration to and/or preferential expansion of HIV-specific T cells within the CNS. This suggests an important role for HIV-specific CD8(+) T cells in control of intrathecal viral replication.

Caveolin-1 regulates human immunodeficiency virus-1 Tat-induced alterations of tight junction protein expression via modulation of the Ras signaling

The blood-brain barrier (BBB) is the critical structure for preventing human immunodeficiency virus (HIV) trafficking into the brain. Specific HIV proteins, such as Tat protein, can contribute to the dysfunction of tight junctions at the BBB and HIV entry into the brain. Tat is released by HIV-1-infected cells and can interact with a variety of cell surface receptors activating several signal transduction pathways, including those localized in caveolae. The present study focused on the mechanisms of Tat-induced caveolae-associated Ras signaling at the level of the BBB. Treatment with Tat activated the Ras pathway in human brain microvascular endothelial cells (HBMECs). However, caveolin-1 silencing markedly attenuated these effects. Because the integrity of the brain endothelium is regulated by intercellular tight junctions, these structural elements of the BBB were also evaluated in the present study. Exposure to Tat diminished the expression of several tight junction proteins, namely, occludin, zonula occludens (ZO)-1, and ZO-2 in the caveolar fraction of HBMECs. These effects were effectively protected by pharmacological inhibition of the Ras signaling and by silencing of caveolin-1. The present data indicate the importance of caveolae-associated signaling in the disruption of tight junctions on Tat exposure. They also demonstrate that caveolin-1 may constitute an early and critical modulator that controls signaling pathways leading to the disruption of tight junction proteins. Thus, caveolin-1 may provide an effective target to protect against Tat-induced HBMEC dysfunction and the disruption of the BBB in HIV-1-infected patients.

HIV-1 Tat inhibits NGF-induced Egr-1 transcriptional activity and consequent p35 expression in neural cells

Infection with HIV-1 causes degeneration of neurons leading to motor and cognitive dysfunction in AIDS patients. One of the key viral regulatory proteins, Tat, which is released by infected cells, can be taken up by various uninfected cells including neurons and by dysregulating several biological events induces cell injury and death. In earlier studies, we demonstrated that treatment of neuronal cells with Tat affects the nerve growth factor (NGF) signaling pathway involving MAPK/ERK. Here we demonstrate that a decrease in the level of Egr-1, one of the targets for MAPK, by Tat has a negative impact on the level of p35 expression in NGF-treated neural cells. Further, we demonstrate a reduced level of Egr-1 association with the p35 promoter sequence in NGF-treated cells expressing Tat. As p35, by associating with Cdk5, phosphorylates several neuronal proteins including neurofilaments and plays a role in neuronal differentiation and survival, we examined kinase activity of p35 complexes obtained from cells expressing Tat. Results from H1 kinase assays showed reduced activity of the p35 complex from Tat-expressing cells in comparison to that from control cells. Accordingly, the level of phosphorylated neurofilaments was diminished in Tat-expressing cells. Similarly, treatment of PC12 cells with Tat protein or supernatant from HIV-1 infected cells decreased kinase activity of p35 in these cells. These observations ascribe a role for Tat in altering p35 expression and its activity that affects phosphorylation of proteins involved in neuronal cell survival.

Differential long-term neurotoxicity of HIV-1 proteins in the rat hippocampal formation: a design-based stereological study

The human immunodeficiency virus type 1 (HIV-1) proteins, gp120 and Tat, are believed to play a role in mediating central nervous system (CNS) pathology in HIV-1 infected patients. Using design-based stereology, we examined the role of neonatal intrahippocampal injections of gp120 and Tat on the adult hippocampus ( approximately 7(1/2) month). Postnatal day (P)1-treated Sprague-Dawley rats were bilaterally injected with vehicle (VEH, 0.5 microl sterile buffer), gp120 (100 ng), Tat (25 microg) or combined gp120 + Tat (100 ng + 25 microg). Using Nissl-stained tissue sections, we quantified total neurons in five subregions of the rat hippocampus [granual layer (GL), hilus of the dentate gyrus (DGH), cornu ammonis fields (CA)2/3, CA1, and subiculum (SUB)], and total glial cells (astrocytes and oligodendrocytes) in two subregions (DGH and SUB). Estimates of cell area and cell volume were taken in the DGH. There was a significant reduction of neuron number in the CA2/3 subfield by Tat and gp120, and a significant reduction in the DGH by Tat only. For glial cells, numbers of astrocytes in the DGH and SUB were increased by the Tat protein, whereas no effects were noted for gp120. Finally, for oligodendrocytes Tat increased cell number in the DGH but not in any other region; gp120 had no detectable effect in any brain region. Estimates of cell area and cell volume of the three different cell types revealed no significant differences between treatments. Collectively, these results suggest differential effects of gp120 and Tat on the estimated total number of neurons, as well as on the number of glial cells.

NF-kappaB-dependent control of HIV-1 transcription by the second coding exon of Tat in T cells

HIV-1 two-exon transactivator protein (Tat) is a 101-aa protein. We investigated the possible contribution of the extreme C terminus of HIV-1 Tat to maximize nuclear transcription factor NF-kappaB activation, long terminal repeat (LTR) transactivation, and viral replication in T cells. C-terminal deletion and substitution mutants made with the infectious clone HIV-89.6 were assayed for their ability to transactivate NF-kappaB-secreted alkaline phosphatase and HIV-1 LTR-luciferase reporter constructs for low concentrations of Tat. A mutant infectious clone of HIV-89.6 engineered by introducing a stop codon at aa 72 in the Tat open-reading frame (HIVDeltatatexon2) replicated at a significantly lower rate than the wild-type HIV-89.6 in phytohemagglutinin-A/IL-2-stimulated primary peripheral blood lymphocytes. Altogether, our results suggest a critical role for the glutamic acids at positions 92, 94, and 96 or lysines at positions 88, 89, and 90, present in the second encoding Tat exon in activating NF-kappaB, transactivating the HIV-1 LTR and enhancing HIV-1 replication in T cells.

Limited role of COX-2 in HIV Tat-induced alterations of tight junction protein expression and disruption of the blood-brain barrier

Tat protein released from HIV-infected blood-borne leukocytes can contribute to the breakdown of the blood-brain barrier (BBB) and induction of inflammatory responses and can provide entry for HIV into the brain. To mimic this pathology, Tat was injected into the tail vein of C57BL/6 mice. Treatment with Tat markedly upregulated expression of cyclooxygenase-2 (COX-2) and decreased expression of tight junction proteins, occludin and zonula occludens-1 (ZO-1). These alterations were associated with the disruption of the BBB integrity as quantified by extravasation of Evans blue dye into the brain tissue. In addition, direct treatment of brain microvessels with prostaglandin E(2), a product of COX-2 activity, resulted in decreased expression of both occludin and ZO-1. To determine if upregulation of COX-2 is involved in the disruption of tight junction proteins and BBB integrity, mice were pretreated with rofecoxib, a specific inhibitor of COX-2, prior to Tat treatment. COX-2 inhibition attenuated Tat-induced alterations of occludin expression. However, rofecoxib was ineffective in preventing downregulation of ZO-1 expression and increased BBB permeability. These results suggest only a limited role of COX-2 overexpression in the loss of tight junction integrity and the BBB breakdown in HIV-related brain diseases.

Nucleoside reverse transcriptase inhibitors and human immunodeficiency virus proteins cause axonal injury in human dorsal root ganglia cultures

Distal symmetric polyneuropathy (DSP) has emerged as the most common complication of human immunodeficiency virus (HIV) infection, which is associated with neuronal injury in the dorsal root ganglion (DRG). With the advent of highly active antiretroviral therapy, especially nucleoside analogs, patients are living longer. Some of the antiretroviral drugs used to treat HIV infection have been associated with neuropathies. The pathogenesis of these neuropathies remains poorly understood. Utilizing a human fetal DRG model of predominantly nociceptive fibers, the authors investigated the effects of HIV gp120 and Tat(1-72), alone or in combination with nucleoside analogs on both morphological and ultra-structural changes in DRG neurons. Nucleoside analogs and HIV proteins both caused a significant decrease in the mean axonal length. However, ddI was the most potent, followed by ddC, d4T, and AZT. Despite the combined exposure to toxic dosages of HIV proteins and nucleoside analogs, there appeared to be a ceiling effect on the amount of axonal retraction, indicating that the proximal and distal axon are differentially regulated. In conclusion, both HIV proteins and nucleoside reverse transcriptase inhibitors (NRTIs) cause axonal damage by inducing mitochondrial injury and rearrangement of microtubules.

Ceftriaxone protects against the neurotoxicity of human immunodeficiency virus proteins

Human immunodeficiency virus (HIV) proteins Tat and gp120 have been implicated in the pathogenesis of HIV dementia by various mechanisms, including down-regulation of excitatory amino acid transporter-2 (EAAT2), which is responsible for inactivation of synaptic glutamate. Recent work indicates that beta-lactam antibiotics are potent stimulators of EAAT2 expression. The authors treated mixed human fetal neuronal cultures with recombinant gp120 or Tat, in the presence or absence of ceftriaxone, and determined neurotoxicity by measuring mitochondrial membrane potential and neuronal cell death. Ceftriaxone produced dose-dependent attenuation of the neurotoxicity and neuronal cell death caused by both viral proteins. This study demonstrates that this class of drugs may have therapeutic efficacy in HIV dementia.

Preventing HIV-1 tat-induced neuronal apoptosis using antioxidant enzymes: Mechanistic and therapeutic implications

HIV-1 proteins, especially gp120 and Tat, elicit reactive oxygen species (ROS) and cause neuron apoptosis. We used antioxidant enzymes, Cu/Zn superoxide dismutase (SOD1) and glutathione peroxidase (GPx1) to study signaling and neuroprotection from Tat-induced apoptosis. SOD1 converts superoxide to peroxide; GPx1 converts peroxide to water. Primary human neurons were transduced with SV40-derived vectors carrying SOD1 and GPx1, then HIV-1 Tat protein was added. Both SV(SOD1) and SV(GPx1) delivered substantial transgene expression. Tat decreased endogenous cellular, but not transduced, SOD1 and GPx1. Tat rapidly increased neuron [Ca(2+)](i), which effect was not altered by SV(SOD1) or SV(GPx1). However, both vectors together blocked Tat-induced [Ca(2+)](i) fluxes. Similarly, neither SV(SOD1) nor SV(GPx1) protected neurons from Tat-induced apoptosis, but both vectors together did. Tat therefore activates multiple signaling pathways, in one of which superoxide acts as an intermediate while the other utilizes peroxide. Gene delivery to protect neurons from Tat must therefore target both.

Human immunodeficiency virus type-1 protein Tat induces tumor necrosis factor-alpha-mediated neurotoxicity

HIV-1 infection causes, with increasing prevalence, neurological disorders characterized in part by neuronal cell death. The HIV-1 protein Tat has been shown to be directly and indirectly neurotoxic. Here, we tested the hypothesis that a non-neurotoxic epitope of Tat can, through actions on immune cells, increase neuronal cell death. Tat(1-72) and a mutant Tat(1-72) lacking the neurotoxic epitope (Tat(Delta31-61)) concentration-dependently and markedly increased TNF-alpha production in macrophage-like differentiated human U937 and THP-1 cells, in mouse peritoneal macrophages and in mouse brain microglia. Tat(1-72) was but Tat(Delta31-61) was not neurotoxic when applied directly to neurons. Supernatants from U937 cells treated with either Tat(1-72) or Tat(Delta31-61) were neurotoxic and their immunoneutralization with an anti-TNF-alpha antibody decreased Tat(1-72)- and Tat(Delta31-61)-induced neurotoxicity. Together, these results demonstrate that the neurotoxic epitope of Tat(1-72) is different from the epitope that is indirectly neurotoxic following production of TNF-alpha from immune cells, and suggest that therapeutic interventions against TNF-alpha might be beneficial against HIV-1 associated neurological disorders.

Glial-restricted precursors: patterns of expression of opioid receptors and relationship to human immunodeficiency virus-1 Tat and morphine susceptibility in vitro

Recent evidence suggests that human immunodeficiency virus (HIV)-induced pathogenesis is exacerbated by opioid abuse and that the synergistic toxicity may result from direct actions of opioids in immature glia or glial precursors. To assess whether opioids and HIV proteins are directly toxic to glial-restricted precursors (GRPs), we isolated neural stem cells from the incipient spinal cord of embryonic day 10.5 ICR mice. GRPs were characterized immunocytochemically and by reverse transcriptase-polymerase chain reaction (RT-PCR). At 1 day in vitro (DIV), GRPs failed to express mu opioid receptors (MOR or MOP) or kappa-opioid receptors (KOR or KOP); however, at 5 DIV, most GRPs expressed MOR and KOR. The effects of morphine (500 nM) and/or Tat (100 nM) on GRP viability were assessed in GRPs at 5 DIV by examining the apoptotic effector caspase-3 and cell viability (ethidium monoazide exclusion) at 96 h following continuous exposure. Tat or morphine alone or in combination caused significant increases in GRP cell death at 96 h, but not at 24 h, following exposure. Although morphine or Tat caused increases in caspase-3 activity at 4 h, this was not accompanied with increased cleaved caspase-3 immunoreactive or ethidium monoazide-positive dying cells at 24 h. The results indicate that prolonged morphine or Tat exposure is intrinsically toxic to isolated GRPs and/or their progeny in vitro. Moreover, MOR and KOR are widely expressed by Sox2 and/or Nkx2.2-positive GRPs in vitro and the pattern of receptor expression appears to be developmentally regulated. The temporal requirement for prolonged morphine and HIV-1 Tat exposure to evoke toxicity in glia may coincide with the attainment of a particular stage of maturation and/or the development of particular apoptotic effector pathways and may be unique to spinal cord GRPs. Should similar patterns occur in vivo then we predict that immature astroglia and oligodendroglia may be preferentially vulnerable to HIV-1 infection or chronic opiate exposure.

Cyclooxygenase-2 is involved in HIV-1 Tat-induced inflammatory responses in the brain

Cyclooxygenase (COX)-2, a rate-limiting enzyme for prostanoid synthesis, can be involved in inflammatory-mediated cytotoxicity. Although the contribution of COX-2 to peripheral inflammation is well understood, its role in brain inflammation is not fully recognized. In particular, COX-2 involvement in inflammatory responses induced by HIV proteins in the central nervous system is not known. Therefore, the present study focused on COX-2 expression and its role in modulating the expression of brain inflammatory-related genes following exposure to the HIV-1 transactivating protein Tat. Intrahippocampal injections of Tat induced dose-dependent upregulation of COX-2 mRNA and protein levels in C57BL/6 mice. COX-2 immunoreactivity was primarily localized in microglial cells and astrocytes. Tat-induced COX-2 expression was partially prevented by pyrrolidine dithiocarbamate, a potent antioxidant and an inhibitor of the transcription factor, nuclear factor kappaB. Most importantly, administration of the COX-2 inhibitor NS-398 attenuated Tat-mediated upregulation of mRNA and protein expression of inflammatory mediators, such as monocyte chemoattractant protein-1, interleukin-1beta, tumor necrosis factor-alpha, and inducible nitric oxide synthase. Moreover, treatment with NS-398 significantly attenuated Tat-induced activation of microglial cells. These results provide evidence that COX-2 overexpression can modulate induction of brain inflammatory mediators in response to HIV-1 Tat protein. Such alterations may play an important role in the development of brain inflammatory reactions in HIV-infected patients and contribute to the development of neurological complications in the course of HIV-1 infection.

Transduction of Cu, Zn-superoxide dismutase mediated by an HIV-1 Tat protein basic domain into human chondrocytes

This study was performed to investigate the transduction of a full-length superoxide dismutase (SOD) protein fused to transactivator of transcription (Tat) into human chondrocytes, and to determine the regulatory function of transduced Tat-SOD in the inflammatory cytokine induced catabolic pathway. The pTat-SOD expression vector was constructed to express the basic domain of HIV-1 Tat as a fusion protein with Cu, Zn-SOD. We also purified histidine-tagged SOD without an HIV-1 Tat and Tat-GFP as control proteins. Cartilage samples were obtained from patients with osteoarthritis (OA) and chondrocytes were cultured in both a monolayer and an explant. For the transduction of fusion proteins, cells/explants were treated with a variety of concentrations of fusion proteins. The transduced protein was detected by fluorescein labeling, western blotting and SOD activity assay. Effects of transduced Tat-SOD on the regulation of IL-1 induced nitric oxide (NO) production and inducible nitric oxide synthase (iNOS) mRNA expression was assessed by the Griess reaction and reverse transcriptase PCR, respectively. Tat-SOD was successfully delivered into both the monolayer and explant cultured chondrocytes, whereas the control SOD was not. The intracellular transduction of Tat-SOD into cultured chondrocytes was detected after 1 hours, and the amount of transduced protein did not change significantly after further incubation. SOD enzyme activity increased in a dose-dependent manner. NO production and iNOS mRNA expression, in response to IL-1 stimulation, was significantly down-regulated by pretreatment with Tat-SOD fusion proteins. This study shows that protein delivery employing the Tat-protein transduction domain is feasible as a therapeutic modality to regulate catabolic processes in cartilage. Construction of additional Tat-fusion proteins that can regulate cartilage metabolism favorably and application of this technology in in vivo models of arthritis are the subjects of future studies.

Neuroprotective and antiretroviral effects of the immunophilin ligand GPI 1046

HIV infection results in a neurodegenerative disorder for which currently there is no effective therapy available. Currently, available antiretroviral therapy has no impact on the production of early regulatory HIV proteins once the virus is integrated. Of these proteins, Tat was shown to be toxic to neurons. We, thus, used an in vitro neuronal culture system to determine if immunophilin ligands could protect against Tat-induced neurotoxicity. We found that GPI 1046 had potent neuroprotective effects in this model. The compound was able to protect the neurons even though it only partially obliterated Tat-induced oxidative stress in neurons, suggesting that other mechanisms may be important in mediating its neuroprotective effect. Furthermore, GPI 1046 showed inhibition of HIV replication and Tat-mediated long terminal repeat (LTR) activation suggesting that this class of compounds may be worthy of further exploration as a potential treatment for HIV dementia.

Tat peptides inhibit neprilysin

Dementia associated with human immunodeficiency virus (HIV) infection occurs commonly in the aging population and amyloid depositions are noted in the brains of patients with HIV infection in younger age groups. This suggests a dysregulation of amyloid processing in the setting of HIV infection. The Tat protein of HIV has been implicated in the neuropathogenesis of HIV infection due to its neurotoxic and glial activation properties. However, Tat protein and Tat-derived peptides were recently also shown to inhibit neprilysin, the major amyloid beta peptide degrading enzyme in brain, in a cell aggregate system. This effect could contribute to the observed accumulation of amyloid in the brain of HIV-infected patients. The authors report here that peptides derived from the Tat protein, but not Tat protein itself, inhibit homogeneous recombinant neprilysin. This inhibition was found to be competitive and reversible and therefore does not involve covalent bond formation. Tat peptides and Tat protein were slowly hydrolyzed by neprilysin. Thus the accumulation of Tat-derived proteolytic fragments may serve to inhibit neprilysin and increase amyloid beta peptide levels.

Dysregulation of NGF-signaling and Egr-1 expression by Tat in neuronal cell culture

Examination of signal transduction pathways that modulate neuronal cell differentiation and protection against apoptosis has revealed a central role for the MAPK/Erk cascade. The activation of MAPK/Erk through the TrkA NGF signaling pathway is critical for growth and survival of neuronal cells. Here, we investigate the impact of HIV-1 Tat on the NGF-signaling pathway in SK-N-MC neuroblastoma cells. Expression of Tat decreased cell growth and induced apoptosis. Our results revealed dysregulation of various steps involved in the NGF pathway including suppression of MAPK, and inhibition of the promoter activity of Egr-1, a key pleiotropic mediator of the expression of genes involved in cell growth upon expression of Tat in SK-N-MC cells. Similarly, exposure of SK-N-MC to conditioned media derived from cells expressing Tat decreased phosphorylation of MAPK and reduced the level of Egr-1 protein expression in SK-N-MC cells. Furthermore, MAPK was able to phosphorylate Puralpha, a cellular protein that plays an important role in neuronal cell function and differentiation, and this was inhibited by Tat. The ability of Puralpha to interact with a GA/GC-rich sequence positioned upstream from the transcription start site of the Egr-1 promoter provided a rationale to examine Egr-1 expression. Expression of Tat decreased NGF-induced Egr-1 levels in SK-N-MC cells and reduced binding of Puralpha to the Egr-1 promoter. All of these observations support a model where the interplay between Tat and Puralpha dysregulates the NGF pathway including the MAPK/Erk network, resulting in reduced expression and activity of Egr-1 in neuronal cells.

Identification of three mutations in the Cu,Zn-superoxide dismutase (Cu,Zn-SOD) gene with familial amyotrophic lateral sclerosis: transduction of human Cu,Zn-SOD into PC12 cells by HIV-1 TAT protein basic domain

The most frequent genetic causes of amyotrophic lateral sclerosis (ALS) determined so far are mutations occurring in the gene coding for copper/zinc superoxide dismutase (Cu,Zn-SOD). The mechanism may involve the formation of hydroxyl radicals or malfunctioning of the SOD protein. Wild-type SOD1 was constructed into a transcription-translation expression vector to examine the SOD1 production in vitro. Wild-type SOD1 was highly expressed in Escherichia coli. Active SOD1 was expressed in a metal-dependent manner. To investigate the possible roles of genetic causes of ALS, a human Cu,Zn-SOD gene was fused with a gene fragment encoding the nine amino acid transactivator of transcription (Tat) protein transduction domain (RKKRRQRRR) of human immunodeficiency virus type 1 in a bacterial expression vector to produce a genetic in-frame Tat-SOD1 fusion protein. The expressed and purified Tat-SOD1 fusion proteins in E. coli can enter PC12 neural cells to observe the cellular consequences. Denatured Tat-SOD1 was successfully transduced into PC12 cells and retained its activity via protein refolding. Three point mutations, E21K, D90V, and D101G, were cloned by site-directed mutagenesis and showed lower SOD1 activity. In undifferentiated PC12 cells, wild-type Tat-SOD1 could prevent DNA fragmentation due to superoxide anion attacks generated by 35 mM paraquat, whereas mutant Tat-D101G enhanced cell death. Our results demonstrate that exogenous human Cu,Zn-SOD fused with Tat protein can be directly transduced into cells, and the delivered enzymatically active Tat-SOD exhibits a cellular protective function against oxidative stress.

The immunophilin ligand GPI1046 protects neurons from the lethal effects of the HIV-1 proteins gp120 and Tat by modulating endoplasmic reticulum calcium load

The dysfunction and death of neuronal cells is thought to underlie the cognitive manifestations of human immunodeficiency virus (HIV)-associated neurological disorders. Although HIV-infected patients are living longer owing to the effectiveness of anti-retroviral therapies, the number of patients developing neurological disorders is on the rise. Thus, there is an escalating need for effective therapies to preserve cognitive function in HIV-infected patients. Using HIV-protein-induced neurotoxicity as a model system, we tested the effectiveness of a non-immunosuppressive immunophilin ligand to attenuate gp120 and Tat-induced modification of neuronal function. The immunophilin ligand GPI1046 attenuated endoplasmic reticulum (ER) calcium release induced by gp120 and Tat and protected neurons from the lethal effect of these neurotoxic HIV proteins. Both inositol 1,4,5 trisphosphate (IP(3)) and ryanodine-sensitive ER calcium release was attenuated by pre-incubation with GPI1046. Using the sarco/endoplasmic reticulum calcium pump inhibitor thapsigargin to release ER calcium, we determined that GPI1046 reduced the total ER calcium load. These findings suggest that non-immunosuppressive immunophilin ligands may be useful neuroprotective drugs in HIV dementia.

HIV-TAT protein upregulates expression of multidrug resistance protein 1 in the blood-brain barrier

Central nervous system (CNS) complications of human immunodeficiency virus (HIV) infection remain a serious health risk in HIV/acquired immunodeficiency syndrome despite significant advances in highly active antiretroviral therapy (HAART). Specific drugs used for HAART are substrates for the efflux transport systems, such as the multidrug resistance-associated proteins (MRPs), which are present on brain microvascular endothelial cells (BMEC) and astrocytes, that is, the main cell types that form the blood-brain barrier (BBB). Thus, drugs employed in HAART are actively removed from the CNS and do not efficiently inhibit HIV replication in the brain. To study the potential mechanisms of this process, the aim of the present research was to address the hypothesis that HIV Tat protein can contribute to upregulation of MRP expression at the BBB level. Tat is a protein produced and released by HIV-infected cells, which may play an important role in brain vascular pathology in the course of HIV infection. Among the family of MRPs, exposure to Tat specifically induced MRP1 messenger ribonucleic acid and protein expression both in BMEC and astrocytes. These alterations were accompanied by enhanced MRP1-mediated efflux functions. Furthermore, activation of the mitogen-activated protein kinase signaling cascade was identified as the mechanism involved in Tat-mediated overexpression of MRP1. These results indicate that Tat exposure can lead to alterations of the BBB functions and decrease HAART efficacy in the CNS through overexpression of drug efflux transporters.

Interaction of HIV Tat and matrix metalloproteinase in HIV neuropathogenesis: a new host defense mechanism

Tat, the HIV transactivating protein, and matrix metalloproteinases (MMPs), a family of extracellular matrix (ECM) endopeptidases, have been implicated in the pathogenesis of HIV-associated dementia. However, the possibility that MMPs interact with viral proteins has remained unexplored. We therefore treated mixed human fetal neuronal cultures with recombinant Tat and select MMPs. Neurotoxicity was determined by measuring mitochondrial membrane potential and neuronal cell death. Previous studies have shown that Tat and MMP independently cause neurotoxicity. Surprisingly, we found the combination of Tat and MMP produced significant attenuation of neurotoxicity. To determine whether there was a physical interaction between Tat and MMP, we used protein electrophoresis and Western blot techniques, and found that MMP-1 can degrade Tat. This effect was blocked by MMP inhibitors. Furthermore, MMP-1 decreased Tat-mediated transactivation of the HIV long terminal repeat region, and this functionality was restored when MMP-1 activity was inhibited. These results suggest that the decrease in Tat-induced neurotoxicity and HIV transactivation is due to Tat's enzymatic cleavage by MMP-1. The direct interaction of human MMPs with viral proteins has now been demonstrated, with resultant modulation of Tat-mediated neurotoxicity and transactivation. This study elucidates a unique viral-host interaction that may serve as an innate host defense mechanism.

Increased vulnerability of ApoE4 neurons to HIV proteins and opiates: protection by diosgenin and L-deprenyl

Human immunodeficiency virus (HIV) infection continues to rise in drug-abusing populations and causes a dementing illness in a subset of individuals. Factors contributing to the development of dementia in this population remain unknown. We found that HIV-infected individuals with the E4 allele of Apolipoprotein E (ApoE) or history of intravenous drug abuse had increased oxidative stress in the CNS. In vitro studies showed that HIV proteins, gp120 and Tat, Tat + morphine but not tumor necrosis factor-alpha (TNF-alpha), caused increased neurotoxicity in human neuronal cultures with ApoE4 allele. Microarray analysis showed a differential alteration of transcripts involved in energy metabolism in cultures of ApoE3 and 4 neurons upon treatment with Tat + morphine. This was confirmed using assays of mitochondrial function and exposure of the neurons to Tat + morphine. Using this in vitro model, we screened a number of novel antioxidants and found that only L-deprenyl and diosgenin protected against the neurotoxicity of Tat + morphine. Furthermore, Tat-induced oxidative stress impaired morphine metabolism which could also be prevented by diosgenin. In conclusion, opiate abusers with HIV infection and the ApoE4 allele may be at increased risk of developing dementia. L-deprenyl and a plant estrogen, diosgenin, may have therapeutic potential in this population.

Correlation between SIV Tat evolution and AIDS progression in cerebrospinal fluid of morphine-dependent and control macaques infected with SIV and SHIV

Morphine abuse has been associated with higher virus replication and accelerated disease progression in a non-human primate model of AIDS. In our previous report, we have shown that 50% of morphine-addicted macaques progress rapidly and that 2/3 of the rapid progressors exhibit severe neuropathogenesis. In this report, we examined the sequence evolution of the SIV Tat protein, known to participate in AIDS neuropathology, in the cerebrospinal fluid (CSF) of morphine-dependent and control macaques over the first 20 weeks of infection. The CSF SIV Tat evolution was found to be inversely related with disease progression, and the highly neuropathogenic inoculum clone sequence was the prevalent CSF form in rapid progressors. Divergence from the inoculum clone was significantly greater in both morphine-dependent normal progressors and control macaques than in the morphine-dependent rapid progressors. Furthermore, we also found evidence of a trend that morphine alters the type of mutation, resulting in an enhanced ratio of transitions to transversions (Ts:Tv). Rapid disease exacerbates this trend and appears to influence the distribution of nonsynonymous changes in the first exon of SIV tat, with a clear majority of mutations occurring in the C-terminal half of the protein where the known functionally important domains reside. Thus, morphine abuse may change the nature and extent of mutations that drive viral evolution.

HIV-1 Tat neurotoxicity in primary cultures of rat midbrain fetal neurons: changes in dopamine transporter binding and immunoreactivity

HIV-1 neurotoxic proteins (Tat, gp120) are believed to play a major role in pathogenesis of dementia in a significant portion of the AIDS patient population. Dopaminergic systems appear to be particularly important in HIV-associated dementia. In the current studies, we determined that primary cell cultures prepared from the midbrain of 18-day-old rat fetuses are sensitive to Tat neurotoxicity and investigated the possible effects of Tat on DAT-specific ligand binding and DAT immunoreactivity in rat fetal midbrain cultures. We found that Tat neurotoxicity was associated with a significant decrease in [3H]WIN 35428 binding. Immunostaining of cell cultures with antibodies recognizing the C-end epitope of DAT did not reveal significant changes in DAT immunoreactivity. The results of this study implicate involvement of monoamine transmission systems in HIV-associated dementia.

Cocaine-mediated enhancement of Tat toxicity in rat hippocampal cell cultures: the role of oxidative stress and D1 dopamine receptor

It is becoming widely accepted that psychoactive drugs can significantly alter the progression of neuropathological changes in the HIV-infected brain. The use of cocaine can aggravate the neurotoxic effects of HIV-1 proteins such as HIV-1 transactivating protein Tat and virus' envelope protein gp120. HIV-1 Tat is believed to play an important role in pathogenesis of HIV dementia (HAD). Tat is neurotoxic and a constantly growing body of evidence suggests that the toxic effects of Tat are oxidative stress-dependent. The current study reports that recombinant Tat 1-72 triggered mitochondrial depolarization, increased intracellular production of reactive oxygen species (ROS) and protein oxidation, and caused neuronal degeneration in primary hippocampal rat cell cultures. A 10 microM dose of the antioxidant Trolox, the water-soluble analog of Vitamin E, ameliorated increased intracellular ROS production and prevented cell viability decline in Tat-treated cell cultures. This fact demonstrates that Tat-induced changes in neuronal oxidative status play an important role in the mechanism of Tat neurotoxicity. While non-toxic by itself, a physiologically relevant dose of cocaine (1.5 microM) significantly enhanced Tat-induced oxidative stress and neurotoxicity in rat hippocampal cell cultures. The antioxidant Trolox significantly improved the survival of neurons exposed to the combination of 50 nM Tat and 1.5 microM cocaine but did not provide complete protection. The specific D1 dopamine receptor antagonist SCH 23390 (10 microM) did not affect Tat toxicity, but did suppress cocaine-mediated potentiation of Tat toxicity. Our results demonstrate that cocaine-mediated potentiation of Tat neurotoxicity may be related to its ability to augment Tat-induced oxidative stress.

Involvement of cytokines in human immunodeficiency virus-1 protein Tat and methamphetamine interactions in the striatum

Human immunodeficiency virus-1 (HIV-1) infection of the brain causes elevation in pro-inflammatory cytokines and inflammatory changes in the striatum. HIV-1-infected individuals who also abuse drugs including the psychostimulant methamphetamine (MA) develop more severe encephalitis and neuronal damage compared to HIV-1-infected patients who do not abuse drugs. In previous studies, we demonstrated that the HIV-1 protein Tat and MA interacted to cause enhanced loss of dopamine in the rat striatum via the destruction of dopaminergic terminals. Since both Tat and MA activate glia and induce cytokine production, we investigated the role of cytokines in the synergistic neurotoxicity induced by Tat and MA using cytokine arrays. Significant increases in monocyte chemotactic protein (MCP-1), interleukin-1 alpha (IL-1alpha) and tissue inhibitor of metalloproteinase-1 (TIMP-1) levels were noted 4 h following Tat + MA treatment compared to saline, Tat or MA. MCP-1 and TIMP-1 levels remained elevated 16 h after Tat + MA compared to saline or MA but were not different from the Tat-treated group at this time point. Weak, but significant elevations in cytokine-induced neutrophil chemoattractant-3 (CINC-3), ciliary neurotrophic factor (CNTF) and macrophage inflammatory protein-3 alpha (MIP-3alpha) were also noted with Tat + MA. The interaction of Tat and MA was prevented in mice genetically deficient in MCP-1 with a consequent attenuation of Tat + MA neurotoxicity. Our findings suggest that HIV-1 infection with concurrent drug abuse might profoundly increase chemokine levels in the striatum resulting in enhanced damage to the dopaminergic system.

HIV-1 Tat and opiate-induced changes in astrocytes promote chemotaxis of microglia through the expression of MCP-1 and alternative chemokines

Opiates exacerbate human immunodeficiency virus type 1 (HIV-1) Tat(1-72)-induced release of key proinflammatory cytokines by astrocytes, which may accelerate HIV neuropathogenesis in opiate abusers. The release of monocyte chemoattractant protein-1 (MCP-1, also known as CCL2), in particular, is potentiated by opiate-HIV Tat interactions in vitro. Although MCP-1 draws monocytes/macrophages to sites of CNS infection, and activated monocytes/microglia release factors that can damage bystander neurons, the role of MCP-1 in neuro-acquired immunodeficiency syndrome (neuroAIDS) progression in opiate abusers, or nonabusers, is uncertain. Using a chemotaxis assay, N9 microglial cell migration was found to be significantly greater in conditioned medium from mouse striatal astrocytes exposed to morphine and/or Tat(1-72) than in vehicle-, mu-opioid receptor (MOR) antagonist-, or inactive, mutant Tat(delta31-61)-treated controls. Conditioned medium from astrocytes treated with morphine and Tat caused the greatest increase in motility. The response was attenuated using conditioned medium immunoneutralized with MCP-1 antibodies, or medium from MCP-1(-/-) astrocytes. In the presence of morphine (time-release, subcutaneous implant), intrastriatal Tat increased the proportion of neural cells that were astroglia and F4/80+ macrophages at 7 days post-injection. This was not seen after treatment with Tat alone, or with morphine plus inactive Tat(delta31-61) or naltrexone. Glia displayed increased MOR and MCP-1 immunoreactivity after morphine and/or Tat exposure. The findings indicate that MCP-1 underlies most of the response of microglia, suggesting that one way in which opiates exacerbate neuroAIDS is by increasing astroglial-derived proinflammatory chemokines at focal sites of CNS infection and promoting macrophage entry and local microglial activation. Importantly, increased glial expression of MOR can trigger an opiate-driven amplification/positive feedback of MCP-1 production and inflammation.

Estrogen attenuates gp120- and tat1-72-induced oxidative stress and prevents loss of dopamine transporter function

Postmenopausal women who are infected with HIV are at risk for experiencing dementia and Parkinson's-like symptoms associated with low levels of estrogen. Neurotoxic damage leading to these symptoms may involve HIV-associated proteins gp120 and/or tat(1-72) (tat). Our hypothesis is that 17beta-Estradiol (E(2)) is an effective agent for protection against gp120/tat-induced damage associated with increased oxidative stress, with particular focus on peroxynitrite-induced oxidative stress. We used SK-N-SH cells and striatal synaptosomes from Sprague-Dawley rats as model systems to assess neuroprotection by E(2). Cells coincubated with SIN-1(3-morpholinosydnonimine) or tat and gp120, together or separately, significantly increased oxidative stress on the SK-N-SH cells, as indicated by the increase in the levels of dichlorofluorescein (DCFH) fluorescence. These data suggest that a component of tat and gp120 neurotoxicity may be due to increased oxidative stress. Coincubation with E(2) attenuated tat- and gp120-induced increase in fluorescence. Coincubation with progesterone had no effect on tat-induced fluorescence, whereas coincubation with the E(2) antagonist ICI 182,780 and E(2) completely prevented the effects observed with E(2) alone. Both gp120 and tat decreased [(3)H] dopamine uptake into striatal synaptosomes by decreasing the V(max) of the dopamine transporter (DAT). Pretreatment of synaptosomes with E(2) (100 nM) partially reversed this reduction. In conclusion, E(2) appears to be effective for preventing the oxidative stress and loss of DAT function associated with gp120/tat neurotoxicity.

HIV-1 Tat protein-induced alterations of ZO-1 expression are mediated by redox-regulated ERK 1/2 activation

HIV-1 Tat protein plays an important role in inducing monocyte infiltration into the brain and may alter the structure and functions of the blood-brain barrier (BBB). The BBB serves as a frontline defense system, protecting the central nervous system from infected monocytes entering the brain. Therefore, the aim of the present study was to examine the mechanisms of Tat effect on the integrity of the BBB in the mouse brain. Tat was injected into the right hippocampi of C57BL/6 mice and expression of tight junction protein zonula occludens-1 (ZO-1) was determined in control and treated mice. Tat administration resulted in decreased mRNA levels of ZO-1 and marked disruption of ZO-1 continuity. These changes were associated with accumulation of inflammatory cells in brain tissue of Tat-treated mice. Further experiments indicated that Tat-mediated alterations of redox-related signaling may be responsible for decreased ZO-1 expression. Specifically, injections with Tat resulted in activation of extracellular signal-regulated kinase 1/2 (ERK 1/2) and pretreatment with U 0126, a specific inhibitor of ERK kinase, effectively ameliorated the Tat-induced diminished ZO-1 levels. In addition, administration of N-acetylcysteine (NAC), a precursor of glutathione and a potent antioxidant, attenuated both Tat-induced ERK 1/2 activation and alterations in ZO-1 expression. These results indicate that Tat-induced oxidative stress can play an important role in affecting the integrity of the BBB through the ERK 1/2 pathway.