Human immunodeficiency virus type 1 Tat activity in human neuronal cells: uptake and trans-activation

Endolysosome iron restricts Tat-mediated HIV-1 LTR transactivation by increasing HIV-1 Tat oligomerization and β-catenin expression

HIV-1 transactivator of transcription (Tat) protein is required for HIV-1 replication, and it has been implicated in the pathogenesis of HIV-1-associated neurocognitive disorder (HAND). HIV-1 Tat can enter cells via receptor-mediated endocytosis where it can reside in endolysosomes; upon its escape from these acidic organelles, HIV-1 Tat can enter the cytosol and nucleus where it activates the HIV-1 LTR promoter. Although it is known that HIV-1 replication is affected by the iron status of people living with HIV-1 (PLWH), very little is known about how iron affects HIV-1 Tat activation of the HIV-1 LTR promoter. Because HIV-1 proteins de-acidify endolysosomes and endolysosome de-acidification affects subcellular levels and actions of iron, we tested the hypothesis that the endolysosome pool of iron is sufficient to affect Tat-induced HIV-1 LTR transactivation. Ferric (Fe³⁺) and ferrous (Fe²⁺) iron both restricted Tat-mediated HIV-1 LTR transactivation. Chelation of endolysosome iron with deferoxamine (DFO) and 2-2 bipyridyl, but not chelation of cytosolic iron with deferiprone and deferasirox, significantly enhanced Tat-mediated HIV-1 LTR transactivation. In the presence of iron, HIV-1 Tat increasingly oligomerized and DFO prevented the oligomerization. DFO also reduced protein expression levels of the HIV-1 restriction agent beta-catenin in the cytosol and nucleus. These findings suggest that DFO increases HIV-1 LTR transactivation by increasing levels of the more active dimeric form of Tat relative to the less active oligomerized form of Tat, increasing the escape of dimeric Tat from endolysosomes, and/or reducing beta-catenin protein expression levels. Thus, intracellular iron might play a significant role in regulating HIV-1 replication, and these findings raise cautionary notes for chelation therapies in PLWH.

Role of Divalent Cations in HIV-1 Replication and Pathogenicity

Divalent cations are essential for life and are fundamentally important coordinators of cellular metabolism, cell growth, host-pathogen interactions, and cell death. Specifically, for human immunodeficiency virus type-1 (HIV-1), divalent cations are required for interactions between viral and host factors that govern HIV-1 replication and pathogenicity. Homeostatic regulation of divalent cations' levels and actions appear to change as HIV-1 infection progresses and as changes occur between HIV-1 and the host. In people living with HIV-1, dietary supplementation with divalent cations may increase HIV-1 replication, whereas cation chelation may suppress HIV-1 replication and decrease disease progression. Here, we review literature on the roles of zinc (Zn2+), iron (Fe2+), manganese (Mn2+), magnesium (Mg2+), selenium (Se2+), and copper (Cu2+) in HIV-1 replication and pathogenicity, as well as evidence that divalent cation levels and actions may be targeted therapeutically in people living with HIV-1.

Two-pore channels regulate Tat endolysosome escape and Tat-mediated HIV-1 LTR transactivation

HIV‐1 Tat is essential for HIV‐1 replication and appears to play an important role in the pathogenesis of HIV‐associated neurological complications. Secreted from infected or transfected cells, Tat has the extraordinary ability to cross the plasma membrane. In the brain, Tat can be taken up by CNS cells via receptor‐mediated endocytosis. Following endocytosis and its internalization into endolysosomes, Tat must be released in order for it to activate the HIV‐1 LTR promoter and facilitate HIV‐1 viral replication in the nucleus. However, the underlying mechanisms whereby Tat escapes endolysosomes remain unclear. Because Tat disrupts intracellular calcium homeostasis, we investigated the involvement of calcium in Tat endolysosome escape and subsequent LTR transactivation. We demonstrated that chelating endolysosome calcium with high‐affinity rhodamine‐dextran or chelating cytosolic calcium with BAPTA‐AM attenuated Tat endolysosome escape and LTR transactivation. Significantly, we demonstrated that pharmacologically blocking and knocking down the endolysosome‐resident two‐pore channels (TPCs) attenuated Tat endolysosome escape and LTR transactivation. This calcium‐mediated effect appears to be selective for TPCs because knocking down TRPML1 calcium channels was without effect. Our findings suggest that calcium released from TPCs is involved in Tat endolysosome escape and subsequent LTR transactivation. TPCs might represent a novel therapeutic target against HIV‐1 infection and HIV‐associated neurological complications.

BK channels regulate extracellular Tat-mediated HIV-1 LTR transactivation

HIV-1 Tat is essential for HIV-1 replication and plays an important role in latent HIV-1 infection, HIV-1 associated neurological complication, and other HIV-1 comorbidities. Secreted from HIV-1 infected or transfected cells, Tat can be up-taken into cells by receptor-mediated endocytosis and internalized into endolysosomes. To reach nucleus where it can facilitate HIV-1 viral replication, exogenous Tat has to escape the degradation by endolysosomes. Because of findings that endolysosome de-acidification with, for example, the weak-base anti-malarial drug chloroquine prevents exogenous Tat degradation and enhances the amount of Tat available to activate HIV-1 LTR, we hypothesize that acidifying endolysosomes may enhance Tat degradation in endolysosomes and restrict LTR transactivation. Here, we determined the involvement of endolysosome-resident transient receptor potential mucolipin 1 channel (TRPML1) and the big conductance Ca²⁺-activated potassium (BK) channel in regulating endolysosome pH, as well as Tat-mediated HIV-1 LTR transactivation in U87MG cells stably integrated with HIV-1 LTR luciferase reporter. Activating TRPML1 channels with ML-SA1 acidified endolysosomes and restricted Tat-mediated HIV-1 LTR transactivation. These effects of ML-SA1 appeared to be mediated through activation of BK channels, because the effects of ML-SA1 on Tat-mediated HIV-1 LTR transactivation were blocked using pharmacological inhibitors or shRNA knock-down of BK channels. On the other hand, activating TRPML1 and BK channels enhanced cellular degradation of exogenous Tat. These results suggest that acidifying endolysosomes by activating TRPML1 or BK channels may provide therapeutic benefit against latent HIV-1 infection, HIV-1 associated neurocognitive disorders, and other HIV-1 comorbidities.

A Naturally Occurring Polymorphism in the HIV-1 Tat Basic Domain Inhibits Uptake by Bystander Cells and Leads to Reduced Neuroinflammation

HIV-1 Tat protein contributes to HIV-neuropathogenesis in several ways including its ability to be taken up by uninfected bystander CNS cells and to activate inflammatory host genes causing synaptic injury. Here, we report that in the globally dominant HIV-1 clade C, Tat displays a naturally occurring polymorphism, R57S, in its basic domain, which mediates cellular uptake. We examined the effect of this polymorphism on Tat uptake and its consequences for cellular gene transactivation. In decapeptides corresponding to the basic domain, a R57S substitution caused up to a 70% reduction in uptake. We also used a transcellular Tat transactivation assay, where we expressed Tat proteins of HIV-1 clade B (Tat-B) or C (Tat-C) or their position 57 variants in HeLa cells. We quantified the secreted Tat proteins and measured their uptake by TZM-bl cells, which provide readout via an HIV-1 Tat-responsive luciferase gene. Transactivation by Tat-B was significantly reduced by R57S substitution, while that of Tat-C was enhanced by the reciprocal S57R substitution. Finally, we exposed microglia to Tat variants and found that R57 is required for maximal neuroinflammation. The R57S substitution dampened this response. Thus, genetic variations can modulate the ability of HIV-1 Tat to systemically disseminate neuroinflammation.

HIV-1 Tat protein promotes neuronal dysregulation by inhibiting E2F transcription factor 3 (E2F3)

Individuals who are infected with HIV-1 accumulate damage to cells and tissues (e.g. neurons) that are not directly infected by the virus. These include changes known as HIV-associated neurodegenerative disorder (HAND), leading to the loss of neuronal functions, including synaptic long-term potentiation (LTP). Several mechanisms have been proposed for HAND, including direct effects of viral proteins such as the Tat protein. Searching for the mechanisms involved, we found here that HIV-1 Tat inhibits E2F transcription factor 3 (E2F3), CAMP-responsive element-binding protein (CREB), and brain-derived neurotropic factor (BDNF) by up-regulating the microRNA miR-34a. These changes rendered murine neurons dysfunctional by promoting neurite retraction, and we also demonstrate that E2F3 is a specific target of miR-34a. Interestingly, bioinformatics analysis revealed the presence of an E2F3-binding site within the CREB promoter, which we validated with ChIP and transient transfection assays. Of note, luciferase reporter assays revealed that E2F3 up-regulates CREB expression and that Tat interferes with this up-regulation. Further, we show that miR-34a inhibition or E2F3 overexpression neutralizes Tat's effects and restores normal distribution of the synaptic protein synaptophysin, confirming that Tat alters these factors, leading to neurite retraction inhibition. Our results suggest that E2F3 is a key player in neuronal functions and may represent a good target for preventing the development of HAND.

Apolipoprotein E isoform dependently affects Tat-mediated HIV-1 LTR transactivation

Background

Apolipoprotein E (ApoE) is the major carrier protein that mediates the transport and delivery of cholesterol and other lipids in the brain. Three isoforms of ApoE (ApoE2, ApoE3, ApoE4) exist in humans, and their relative expression levels impact HIV-1 infection, HIV-1/AIDS disease progression, and cognitive decline associated with HIV-1-associated neurocognitive disorder. Because HIV-1 Tat, a viral protein essential for HIV-1 replication, can bind to low-density lipoprotein receptor-related protein 1 (LRP1) that controls ApoE uptake in the brain, we determined the extent to which different isoforms of ApoE affected Tat-mediated HIV-1 LTR transactivation.

Methods

Using U87MG glioblastoma cells expressing LTR-driven luciferase, we determined the extent to which LRP1 as well as ApoE2, ApoE3, and ApoE4 affected Tat-mediated HIV-1 LTR transactivation.

Results

A specific LRP1 antagonist and siRNA knockdown of LRP1 both restricted significantly Tat-mediated LTR transactivation. Of the three ApoEs, ApoE4 was the least potent and effective at preventing HIV-1 Tat internalization and at decreasing Tat-mediated HIV-1 LTR transactivation. Further, Tat-mediated LTR transactivation was attenuated by an ApoE mimetic peptide, and ApoE4-induced restriction of Tat-mediated LTR transactivation was potentiated by an ApoE4 structure modulator that changes ApoE4 into an ApoE3-like phenotype.

Conclusions

These findings help explain observed differential effects of ApoEs on HIV-1 infectivity and the prevalence of HAND in people living with HIV-1 infection and suggest that ApoE mimetic peptides and ApoE4 structure modulator might be used as a therapeutic strategy against HIV-1 infection and associated neurocognitive disorders.

Electronic supplementary material

The online version of this article (10.1186/s12974-018-1129-1) contains supplementary material, which is available to authorized users.

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.

Measuring the Uptake and Transactivation Function of HIV-1 Tat Protein in a Trans-cellular Cocultivation Setup

HIV-1 Tat protein is secreted from infected cells and is endocytosed by uninfected bystander cells. Subsequently, Tat is translocated to the nucleus and binds to promoters of host cell genes, increasing the production of inflammatory host cytokines and chemokines. This inflammatory activation of uninfected cells by HIV-1 Tat protein contributes to the overall inflammatory burden in the central nervous system (CNS) that leads to the development of HIV-associated neurocognitive disorders (HAND). Here we describe methods to evaluate the uptake and transcriptional impact of HIV-1 Tat on uninfected cells by using a trans-cellular transactivation system. Cell lines transiently transfected with Tat expression constructs secrete Tat into the culture medium. Trans-cellular uptake and transactivation caused by secreted Tat can be measured by co-culturing LTR-responsive reporter cells with Tat-transfected cells. Such Tat-producer cells can also be co-cultured with immune cell lines, such as monocytic THP-1 cells or lymphocytic Jurkat T-cells, to evaluate transcriptional changes elicited by Tat taken up by the uninfected cells.

STAT3 and its phosphorylation are involved in HIV-1 Tat-induced transactivation of glial fibrillary acidic protein

Human immunodeficiency virus type 1 (HIV-1) Tat protein is a major pathogenic factor in HIV-associated neurological diseases; it exhibits direct neurotoxicity and indirect astrocyte-mediated neurotoxicity. We have shown that Tat alone is capable of activating glial fibrillary acidic protein (GFAP) expression and inducing astrocytosis involving sequential activation of early growth response protein 1 (Egr-1) and p300. In this study, we determined the roles of signal transducer and activator of transcription 3 (STAT3) in Tat-induced GFAP transactivation. STAT3 expression and phosphorylation led to significant increases in GFAP transcription and protein expression. Tat expression was associated with increased STAT3 expression and phosphorylation in Tat-expressing astrocytes and HIV-infected astrocytes. GFAP, Egr-1 and p300 transcription and protein expression all showed positive response to STAT3 and its phosphorylation. Importantly, knockdown of STAT3 resulted in significant decreases in Tat-induced GFAP and Egr-1 transcription and protein expression. Taken together, these findings show that STAT3 is involved in and acts upstream of Egr1 and p300 in the Tat-induced GFAP transactivation cascade and suggest important roles of STAT3 in controlling astrocyte proliferation and activation in the HIV-infected central nervous system.

HIV-1 Tat Protein Induces Production of Proinflammatory Cytokines by Human Dendritic Cells and Monocytes/Macrophages through Engagement of TLR4-MD2-CD14 Complex and Activation of NF-κB Pathway

We recently reported that the human immunodeficiency virus type-1 (HIV-1) Tat protein induced the expression of programmed death ligand-1 (PD-L1) on dendritic cells (DCs) through a TLR4 pathway. However, the underlying mechanisms by which HIV-1 Tat protein induces the abnormal hyper-activation of the immune system seen in HIV-1 infected patients remain to be fully elucidated. In the present study, we report that HIV-1 Tat protein induced the production of significant amounts of the pro-inflammatory IL-6 and IL-8 cytokines by DCs and monocytes from both healthy and HIV-1 infected patients. Such production was abrogated in the presence of anti-TLR4 blocking antibodies or soluble recombinant TLR4-MD2 as a decoy receptor, suggesting TLR4 was recruited by Tat protein. Tat-induced murine IL-6 and CXCL1/KC a functional homologue of human IL-8 was abolished in peritoneal macrophages derived from TLR4 KO but not from Wt mice, confirming the involvement of the TLR4 pathway. Furthermore, the recruitment of TLR4-MD2-CD14 complex by Tat protein was demonstrated by the activation of TLR4 downstream pathways including NF-κB and SOCS-1 and by down-modulation of cell surface TLR4 by endocytosis in dynamin and lipid-raft-dependent manners. Collectively, these findings demonstrate, for the first time, that HIV-1 Tat interacts with TLR4-MD2-CD14 complex and activates the NF-κB pathway, leading to overproduction of IL-6 and IL-8 pro-inflammatory cytokines by myeloid cells from both healthy and HIV-1 infected patients. This study reveals a novel mechanism by which HIV-1, via its early expressed Tat protein, hijacks the TLR4 pathway, hence establishing abnormal hyper-activation of the immune system.

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.

Neuronal survival and resistance to HIV-1 Tat toxicity in the primary culture of rat fetal neurons

In this study we report that primary cultures of rat fetal neurons contain subpopulations of cells that may be sensitive or resistant to HIV-1 Tat neurotoxicity. We demonstrate that rapid binding/uptake of Tat 1-86 for 2 h was sufficient to trigger caspase activation and neurodegeneration in rat fetal midbrain cell cultures. The uptake of Tat was followed by an increase in MCP1 (CCL2) immunoreactivity. Approximately 70% of neurons were able to survive transient or continuous (7 days) Tat exposure. The surviving neurons did not contain bound/internalized Tat, but were able to interact with Tat after medium replacement. These neurons were resistant to Tat toxicity. In neurons that resisted the toxic effects of continuous and repeated Tat treatment, levels of NR2A subunit of the NMDA receptor complex were significantly lower than in controls. We suggest that the subunit composition of NMDAR complexes may be important for the sensitivity of neurons to Tat toxicity.

Neurotoxic profiles of HIV, psychostimulant drugs of abuse, and their concerted effect on the brain: current status of dopamine system vulnerability in NeuroAIDS

There are roughly 30-40 million HIV-infected individuals in the world as of December 2007, and drug abuse directly contributes to one-third of all HIV infections in the United States. Antiretroviral therapy has increased the lifespan of HIV-seropositives, but CNS function often remains diminished, effectively decreasing quality of life. A modest proportion may develop HIV-associated dementia, the severity and progression of which is increased with drug abuse. HIV and drugs of abuse in the CNS target subcortical brain structures and DA systems in particular. This toxicity is mediated by a number of neurotoxic mechanisms, including but not limited to, aberrant immune response and oxidative stress. Therefore, novel therapeutic strategies must be developed that can address a wide variety of disparate neurotoxic mechanisms and apoptotic cascades. This paper reviews the research pertaining to the where, what, and how of HIV and cocaine/methamphetamine toxicity in the CNS. Specifically, where these toxins most affect the brain, what aspects of the virus are neurotoxic, and how these toxins mediate neurotoxicity.

Activation of the extrinsic caspase pathway in cultured cortical neurons requires p53-mediated down-regulation of the X-linked inhibitor of apoptosis protein to induce apoptosis

Cultured cortical neurons exposed to the Human Immunodeficiency Virus gp120 coat protein undergo apoptosis involving activation of both caspase-8 and caspase-9. Additionally, gp120-mediated neuronal apoptosis requires the pro-apoptotic transcription factor p53. As caspase-8-induced apoptosis does not typically require p53, we examined the possibility of a novel role for p53 in caspase-8 activation initiated by gp120. We observed that gp120 treatment of cultured cortical neurons induced caspase-8 activity and Bid cleavage independently of p53, but induction of caspase-3 enzymatic activity required p53 expression. These findings suggested the possibility that p53 down-regulates a caspase-3 inhibitor. We observed high-level expression of the caspase-3/9 inhibitor X-linked inhibitor of apoptosis protein (XIAP) in cultured cortical neurons. Adenoviral expression of p53 or induction of endogenous p53 by camptothecin treatment reduced XIAP protein in neurons. Infection with a p53 expressing adenovirus increased expression of the mRNA for Omi/HtrA2, a protease that cleaves and inactivates XIAP. These findings suggest that p53 regulates neuronal apoptosis, in part, by suppressing the anti-apoptotic protein XIAP via transcriptional activation of Omi/HtrA2.

Cell Penetrating Peptides: Intracellular Pathways and Pharmaceutical Perspectives

Cell penetrating peptides, generally categorized as amphipathic or cationic depending on their sequence, are increasingly drawing attention as a non-invasive delivery technology for macromolecules. Delivery of a diverse set of cargo in terms of size and nature ranging from small molecules to particulate cargo has been attempted using different types of cell penetrating peptides (CPPs) in vitro and in vivo. However, the internalization mechanism of CPPs is an unresolved issue to date, with dramatic changes in view regarding the involvement of endocytosis as a pathway of internalization. A key reason for the lack of consensus on the mechanism can be attributed to the methodology in deciphering the internalization mechanism. In this review, we highlight some of the methodology concerns, focus more on the internalization pathway and also provide a novel perspective about the intracellular processing of CPPs, which is a crucial aspect to consider when selecting a cell penetrating peptide as a drug delivery system. In addition, recent applications of cell penetrating peptides for the delivery of small molecules, peptides, proteins, oligonucleotides, nanoparticles and liposomes have been reviewed.

Cross talk between growth factors and viral and cellular factors alters neuronal signaling pathways: implication for HIV-associated dementia

HIV-associated dementia (HAD) is a serious neurological disorder affecting about 7% of people with AIDS. In the brain, HIV-1 infects a restricted number of cell types, being primarily present in macrophages and microglial cells, less abundant in astrocytes, and rarely seen in oligodendrocytes and neurons. Lack of a productive HIV-1 infection of neuronal cells suggests the presence of an indirect pathway by which the virus may determine the brain pathology and neuronal dysfunction seen in AIDS patients. Among the participants in this event, viral proteins including gp120 and Tat, along with host factors including cytokines, chemokines, and several signaling pathways have received considerable attention. In this article, we discuss the most recent concepts pertaining to the mechanisms of HIV-1-induced neuronal dysfunction by highlighting the interplay between signal transduction pathways activated by viral and host factors and their consequences in neuronal cell function.

The multiple roles of p53 in the pathogenesis of HIV associated dementia

The mechanism by which infection with the human immunodeficiency virus (HIV) leads to injury and dysfunction within the central nervous system (CNS) is not completely understood. Most studies support the hypothesis that neurons are impacted as bystander cells in a tissue environment made hostile by the innate and adaptive immune responses to chronic HIV infection within CNS tissue. The tumor suppressor transcription factor p53 participates in multiple cellular processes within the HIV infected CNS, and experimental evidence suggests that the resulting neurodegeneration occurs by induction of p53-mediated apoptotic pathways. Here we review the evidence for p53 as a participant in the responses of multiple CNS cell types to the presence of HIV and propose the hypothesis that HIV induced alterations in the CNS extracellular milieu converge at neuronal p53 activation.

Human immunodeficiency virus type 1 genetic diversity in the nervous system: Evolutionary epiphenomenon or disease determinant?

Over the past decade there has been a revolution in the understanding and care of human immunodeficiency virus/acquired immunodeficiency syndrome (HIV/AIDS)-associated disease. Much of this progress stems from a broader recognition of the importance of differences in viral types, including receptor preference(s), replication properties, and reservoirs, as contributing factors to immunosuppresion and disease progression. In contrast, there is limited conceptualizatin of viral diversity and turnover in the brain and circulation in relation to neurocognitive impairments. Herein, the authors review current concepts regarding viral molecular diversity and phenotypes together with features of HIV-1 neuroinvasion, neurotropism, neurovirulence and neurosusceptiblity. Viral genetic and antigenic diversity is reduced within the brain compared to blood or other systemic organs within individuals. Conversely, viral molecular heterogeneity is greater in patients with HIV-associated dementia compared to nondemented patients, depending on the viral gene examined. Individual viral proteins exert multiple neuropathogenic effects, although the neurological consequences of different viral polymorphisms remain uncertain. Nonetheless, host genetic polymorphisms clearly influence neurological disease outcomes and likely dictate both acquired and innate immune responses, which in turn shape viral evolution within the host. Emerging issues include widespread antiretroviral therapy resistance and increasing awareness of viral superinfections together with viral recombination, all of which are likely to impact on both HIV genetic variation and neuropathogenesis. With the persisting prevalence of HIV-induced neurocognitive disabilities, despite marked improvements in managing immunosuppression, it remains imperative to fully define and understand the mechanisms by which viral dynamics and diversity contribute to neurological disease, permitting the development of new therapeutic strategies.

Overexpression of octamer transcription factors 1 or 2 alone has no effect on HIV-1 transcription in primary human CD4 T cells

We explored the binding of octamer (Oct) transcription factors to the HIV-1 long terminal repeat (LTR) by gel shift assays and showed none of the previously identified four potential Oct binding sites bound Oct-1 or Oct-2. Overexpression of Oct-1 or Oct-2 had no effect on HIV-1 LTR activity in transiently transfected primary human CD4 T cells. Next, primary human CD4 T cells were co-transfected with a green fluorescent protein (GFP)-expression vector and an Oct-1 or Oct-2 expression plasmid. The transfected cells were stimulated for 2 days and then infected with the NL4-3 strain of HIV-1. After 3 days of infection, there were no differences in HIV-1 p24 supernatant levels. Apoptosis of infected or bystander cells overexpressing Oct-1 or Oct-2 compared to control was also unaffected. Our studies demonstrate that Oct-1 and Oct-2 fail to bind to the HIV-1 LTR and have no effect on HIV-1 transcription in primary human CD4 T cells.

Human immunodeficiency virus infection of the brain: pitfalls in evaluating infected/affected cell populations

Monocyte/macrophages and CD4 T-cells are the primary hematopoietic targets of productive HIV infection. In the brain, potential cellular targets for HIV infection include perivascular and parenchymal macrophages/microglia, oligodendrocytes, endothelia, neurons, and astrocytes. We examine evidence of productive and non-productive infection for each cell type in the brains of HIV-infected patients with and without HIV encephalitis. Despite the voluminous literature and substantial experimental effort over the past two decades, evidence for productive infection of any brain cell other than macrophages is left wanting.

Evidence of protein transduction but not intercellular transport by proteins fused to HIV tat in retinal cell culture and in vivo

The human immunodeficiency virus type-1 Tat protein is known to exit virally infected cells and enter the nucleus of adjacent uninfected cells. This property has been mapped to an 11-amino-acid protein transduction domain (PTD). When the PTD of Tat is fused to heterologous proteins and added exogenously to cells, the fusion peptide is able to demonstrate protein transduction across plasma membranes. Recent reports indicate that endogenously expressed Tat fusion peptides can demonstrate intercellular transport and improve biodistribution of therapeutic protein in the context of adenovirus vectors. Intercellular transport and protein transduction have not been observed in some studies and in the former have been attributed to an artifact of fixation. We have attempted to resolve these studies using an approach that unambiguously distinguishes cells that express Tat fusion protein from those that receive it from their environment. We find no evidence of intercellular transport in the context of an adenovirus vector in cell culture or in vivo. Instead, we find that Tat fusion peptides are down regulated in terms of expression not only in the context of adenovirus vectors, but also when expressed from transfected plasmid DNA. However, when Tat fusion peptides are released from cells by degradation of the plasma membrane, the fusion peptides demonstrate protein transduction without the need for cell fixation, indicating a unidirectional transport of Tat fusion proteins across the plasma membrane. Our data are consistent with previously reported studies and help to explain the apparently different results obtained from several different laboratories.

Intracellular human immunodeficiency virus Tat expression in astrocytes promotes astrocyte survival but induces potent neurotoxicity at distant sites via axonal transport

The human immunodeficiency virus (HIV)-Tat protein has been implicated in the neuropathogenesis of HIV infection. However, its role in modulating astroglial-neuronal relationships is poorly understood. Astrocyte infection with HIV has been associated with rapid progression of dementia. We thus initially transfected astrocytes with HIV proviral DNA and confirmed Tat production in these cells. Subsequently, using stably Tat-producing asytocyte cell lines, we observed that Tat promoted astrocyte survival by causing a prominent antioxidant effect and resistance to cell injury in these cells. Tat was released extracellularly where it could be taken up by other cells. Tat remained functionally active following uptake and caused long terminal repeat (LTR) transactivation in lymphocytic and astrocytic cell lines. Tat released from astrocytes caused mitochondrial dysfunction, trimming of neurites, and cell death in neurons. Tat neurotoxicity was attenuated by anti-Tat antibodies, kynurenate or heparan sulfate. The neurotoxic effects of Tat were caused at concentrations lower than that needed to cause LTR transactivation. When Tat-expressing cells were injected into the rat dentate gyrus, Tat was taken up by granule cells and transported along neuronal pathways to the CA3 region where it caused glial cell activation and neurotoxicity. The arginine-rich domain of Tat was essential for both the LTR transactivation and the neurotoxic properties of Tat. Thus HIV-Tat is a potent neurotoxin that may act at distant sites while at the same time it assures its production by preventing cell death in astrocytes where it is produced.

Transfection of human endothelial cells with HIV-1 tat gene activates NF-kappa B and enhances monocyte adhesion

Human immunodeficiency virus (HIV)-1 Tat released from HIV-1-infected monocytes is believed to enter other cells via an integrin-facilitated pathway, resulting in altered gene expression. Indeed, exogenous Tat protein can increase cell adhesion molecule gene expression in human endothelial cells. Signaling pathways initiated by Tat in endothelial cells are not known. We evaluated the ability of endogenous tat to stimulate monocyte adhesion via activation of nuclear factor-kappaB (NF-kappaB) within human umbilical vein endothelial cells. Transfection with pcTat, but not control vector DNA, increased NF-kappaB binding activity, NF-kappaB luciferase reporter activity, and monocyte adhesion. pcTat also increased kappaB-dependent HIV-1-LTR-CAT reporter activity 28-fold compared with a 3-fold increase produced by transfection with an equivalent amount of pcTax (from human leukemia virus). The pcTat-induced increase in pNF-kappaB-Luc activity and monocyte adhesion to endothelial cells was blocked by cotransfection with dominant-negative mutant IkappaBalpha and by incubation with 10 mM aspirin. We conclude that monocyte adhesion to human endothelial cells stimulated by pcTat is mediated via an NF-kappaB-dependent mechanism. Furthermore, inhibition studies using aspirin suggest that pcTat-stimulated NF-kappaB activation and monocyte adhesion occur via a redox-sensitive mechanism.

HIV-1-associated central nervous system dysfunction

Despite more than 15 years of extensive investigative efforts, a complete understanding of the neurological consequences of HIV-1 CNS infection remains elusive. Although the resources of numerous investigators have been focused on studies of HIV-1-associated CNS disease, the complex nature of the disease processes that underlie the clinical, pathological, and cellular manifestations of HIV-1 CNS infection have required a larger volume of studies than was initially envisioned. Several major areas remain as the focus of current research efforts. One of the more pressing issues facing researchers and clinicians alike is the search for correlates to the development of HIV-1-associated CNS neuropathology and the onset of HIVD. Although numerous parameters have been studied, none have been shown to be absolute predictors or markers of HIV-1-related CNS dysfunction. The identification of solid correlates of HIVD is an important goal that would permit clinical identification of individuals at risk for developing potentially crippling, life-threatening CNS abnormalities and would facilitate early treatment of nascent neurological problems. A more complete comprehension of the cellular foundations of CNS dysfunction and HIVD is also a fundamental part of strategies designed to treat or prevent HIV-1-associated CNS disease. Future investigations will strive to expand the body of knowledge concerning the complex interactions between infected and uninfected neuroglial cells and the roles of numerous cytokines, chemokines, and other soluble agents that are deregulated during HIV-1 CNS infection. In particular, a thorough understanding of the mechanisms of neurotoxicity may facilitate the development of new therapies that alleviate or eliminate the clinical consequences of CNS infection. Finally, investigators will continue to study HIVD within the context of single and combination drug therapies used in the treatment of HIV-1 infection and AIDS. As newer and more effective systemic treatments for HIV-1 infection and AIDS are introduced, the effects of these treatments on the onset, incidence, and severity of HIVD will also require intensive study. The impact of drug therapies on the ability of the CNS to act as an HIV-1 reservoir will also need to be addressed. Introduction of each new drug or drug combination will necessitate studies of drug penetration into the CNS and efficacy against the development of CNS abnormalities. Furthermore, as more effective treatments prolong the lifespan of individuals infected with HIV-1, the impact of extended survival on the occurrence and severity of HIVD will also require further investigations. The quest for answers to these and other questions will be complicated by the diversity of experimental systems used to study different aspects of HIV-1 CNS infection and HIVD. Each system has its own unique strengths and weaknesses. Clinical observations provide a continuous spectrum of symptomatic findings but reveal little about the underlying mechanisms of disease. In vivo imaging techniques, such as CT and MRI, also provide a continuum of observations, but the images are limited in their resolution. Neuropathological examinations of postmortem HIV-1-infected brains offer gross, cellular, and molecular views (including phenotypic and genotypic analyses of CNS viral isolates) of the diseased brain, but only provide a snapshot of the end-stage neurologic dysfunction. Studies that rely on animal surrogates for HIV-1, including SIV, simian-HIV (SHIV), feline immunodeficiency virus (FIV), visna virus, and HIV-1 SCID-hu models, permit experimental protocols that cannot be carried out in humans, but are limited by the fidelity with which each virus and animal model emulates the conditions and events observed in the human host. Finally, in vitro techniques, which include the use of primary cells and cell lines, adult or fetal human cell cultures, and BBB barrier model systems, are also convenient means by which aspe

Virological, Neurological and Histological Investigations of a Child Born to a Mother with AIDS

HIV-1 was isolated from a child at 6 and 9 months of age, proving the vertical transmission of infection from the mother with AIDS. The p24 antigen test of the plasma at 9 months of age was positive as well. A positive PCR reaction was detected in J34 cells, infected with the supernatant of the peripheral blood lymphocytes of the child. According to phenotypic characterization, the virus proved to be a SI (syncytium inducing) isolate, growing in PBL, MT2, J34 and other T and monocytic cell lines. The isolate was AZT sensitive. Two methods were applied for genotypic characterization: 1. Heteroduplex mobility assay (HMA), 2. Sequence analysis of a part of the env gene. On the basis of both of these methods, this virus belongs to the B subtype of HIV-1, which is prevalent mainly in Europe and in the USA. The neurological status of the child was followed regularly. At autopsy the presence of p24 antigen was detected in glial cells of the frontal cortex, proving the presence of the virus in the brain. A retardation of the development of the central nervous system could be observed as well.

HIV-1 Tat protein down-regulates CREB transcription factor expression in PC12 neuronal cells through a phosphatidylinositol 3-kinase/AKT/cyclic nucleoside phosphodiesterase pathway

The addition of low concentrations (0.1-1 nM) of extracellular HIV-1 Tat protein to PC12 neuronal cells stimulated a rapid (peak at 5 min) elevation of the cAMP intracellular levels, which in turn induced the phosphorylation of CREB transcription factor (peak at 15 min) on serine-133 (Ser-133). On the contrary, at later time points (60-120 min) Tat induced a significant decline of intracellular cAMP with respect to the basal levels observed in control cells treated with bovine serum albumin. In blocking experiments performed with pharmacological inhibitors, Tat decreased the intracellular levels of cAMP and CREB Ser-133 phosphorylation through a signal transduction pathway involving the sequential activation of phosphatidylinositol 3-kinase, AKT, and cyclic nucleoside phosphodiesterases. Moreover, in transient transfection experiments, Tat inhibited transcription of CREB promoter in a manner strictly dependent on the presence of the cAMP-responsive elements (CRE) in the CREB promoter. Consistently, the expression of endogenous CREB protein was significantly reduced in PC12 cells by prolonged (24-48 h) treatment with Tat. This decline in the expression of CREB, which plays an essential role in the survival and function of neuronal cells, anticipated a progressive increase of apoptosis in Tat-treated cells. Although obtained in a neuronal cell line, our findings might help to explain some aspects of the pathogenesis of HIV-1-associated dementia.

HIV-1 Tat-mediated inhibition of the tyrosine hydroxylase gene expression in dopaminergic neuronal cells

Treatment of dopaminergic rat PC12 cells with human immunodeficiency virus, type 1 (HIV-1) Tat protein or tat cDNA inhibited the expression of tyrosine hydroxylase (TH), the rate-limiting enzyme for the dopamine biosynthetic pathway, as well as the production and release of dopamine into the culture medium. Moreover, the Tat addition to PC12 cells up-regulated the expression of the inducible cAMP early repressor (ICER), a specific member of the cAMP-responsive element modulator transcription factor family, in a cAMP-dependent manner. In turn, ICER overexpression abrogated the transcription activity of the TH promoter in PC12 cells, strongly suggesting ICER involvement in Tat-mediated inhibition of TH gene expression. In vivo injection of synthetic HIV-1 Tat protein into the striatum of healthy rats induced a subclinical Parkinson's-like disease that became manifested only when the animals were treated with amphetamine. As early as one week postinjection, the histochemical examination of the rat substantia nigra showed a reduced staining of neurons expressing TH followed by a loss of TH(+) neurons at later time points. As Tat protein can be locally released into the central nervous system by HIV-1-infected microglial cells, our findings may contribute to the explanation of the pathogenesis of the motorial abnormalities often reported in HIV-1 seropositive individuals.

HIV infection of fetal human astrocytes: the potential role of a receptor-mediated endocytic pathway

HIV infects microglia and astrocytes both in vivo and in vitro. Although there is a significant amount of information about microglial infection, data regarding astrocytes are more limited. For example, little is known about the initial membrane events occurring between HIV and astrocytes. Also, the mechanism by which HIV enters these cells remains to be determined. To address these questions, we exposed human astrocyte cultures to either HIV or to the HIV glycoprotein gp120. The cultures were analyzed for viral infection and gp120 binding to cultured cells by light and electron microscopy (EM) with and without immunocytochemistry, respectively; ligand-receptor biochemistry; and, Western, Northern and Southern blot analyses. The results of these studies showed that HIV binds to astrocytes via gp120 and a cell surface molecule weighing approximately 65 kDa that is neither CD4 nor galactocerebroside. Furthermore, binding of gp120 to astrocytes was concentration dependent and displayed a curve consistent with ligand-receptor binding. Additionally, radiolabeled gp120 binding was displaced by unlabeled gp120 but not by deglycosylated gp120, suggesting that the binding was specific. By EM, HIV virions were seen in clathrin-coated pits and in cytoplasmic vacuoles. This suggests linkage, in astrocytes, between a plasma membrane-associated protein that can act as a receptor for HIV and an endosomal pathway.

The effects of human immunodeficiency virus in the central nervous system

More than a decade after the first description of HIV DNA in the nervous system the pathophysiology of HIVD remains largely enigmatic, with data supporting a number of potential mechanisms for the development of neuronal dysfunction. Nevertheless, a few key findings have considerable support in the literature devoted to this subject: 1. HIV dementia is caused by HIV itself; no other pathogen has been consistently found in the brains of patients with HIVD. 2. In comparison with other viral encephalopathies, there appears to be a significant discordance between the amount of virus being produced in the brains of patients with HIVD and the degree of neurological deterioration. 3. The key cell types responsible for viral production within the CNS are the resident macrophages or microglial cells. 4. Other elements within the CNS, particularly astrocytes, are probably infected with HIV as well, but all of these infections are highly restricted in terms of production of virus or viral structural proteins. 5. At least one component of the pathogenesis of HIVD may be the generation of neurotoxins by infected microglia, although the type of neurotoxin, and the specific compound most likely to be involved, are quite controversial. Advances with combination antiviral therapy have successfully reduced plasma viral load in a high proportion of individuals, leading to the speculation (previously almost heretical) that it may be possible to eradicate HIV completely from the systemic immune system. If that were the case, potential "sanctuary" sites such as the immunologically protected CNS might remain as important reservoirs for reseeding of lymphoid tissues. Microglia may be particularly suited for this purpose because they are long lived, can produce HIV for several weeks (at least in culture), and they are apparently relatively immune to virus-induced cytopathology such as syncytium formation. One can speculate about several scenarios resulting from the continued presence of replication-competent HIV within brain. In the worst case, a smoldering infection of the nervous system could lead to neurological deterioration without reinfection of systemic immune cells. The epidemiological data indicating that HIVD is a disease primarily associated with immunodeficiency suggest that the systemic immune system plays a role in maintaining virus residing within the CNS under control. Thus it is quite possible that this scenario would not occur for many years after the systemic infection is controlled. Alternatively, virus could be transported from the CNS by circulating lymphocytes and monocytes and reinfect systemic organs. This would necessitate restarting therapy for those individuals who were previously thought to be cured, but presumably virus within the CNS would not have developed resistance to antivirals. In either case, the techniques currently available do not permit an accurate assessment of CNS HIV load in living people, and this question will remain unanswered until antivirals are discontinued in a few individuals with persistently negative tests for systemic virus. In addition to this most critical question, the relationship between viral levels and HIVD is largely unexplored, as is the possibility that some strains are particularly virulent or neuroinvasive. Furthermore, the potential contribution of host genotype in the development of dementia is unknown. In view of the strong influence of major chemokine receptor (CCR5) truncations on HIV replication, it is entirely possible that more discrete genetic polymorphisms have a subtle effect on either brain invasion or virulence.

Human immunodeficiency virus infection of dorsal root ganglion neurons detected by polymerase chain reaction in situ hybridization

A predominantly sensory peripheral neuropathy is common with human immunodeficiency virus (HIV) infection, but the cause is unknown. Formalin-fixed dorsal root ganglia (DRG), obtained at postmortem from patients with neuropathy and HIV infection and from control subjects, were examined for the presence of HIV DNA by using polymerase chain reaction (PCR)-amplified in situ hybridization. Viral message RNA was detected using reverse transcription in situ PCR with gag-specific primers. HIV DNA and RNA sequences were detected in many satellite cells, mononuclear cells, and occasional neurons in 5 of 5 patients with HIV and neuropathy. HIV DNA was detected only in rare interstitial and satellite cells from 3 of 4 patients with HIV infection without neuropathy and was not detected in 6 patients without HIV infection. HIV infection of DRG neurons and supporting cells may contribute to the HIV-associated sensory neuropathy.

Distinct transcriptional pathways of TAR-dependent and TAR-independent human immunodeficiency virus type-1 transactivation by Tat

Tat stimulates HIV-1 gene expression during transcription initiation and elongation. Tat functions primarily through specific interactions with TAR RNA and several putative cellular cofactors to increase the processivity of RNA polymerase II complexes during HIV-1 transcription elongation. Although HIV-1 transactivation by Tat in most cell types requires intact TAR sequences, previous reports demonstrate that Tat transactivates HIV-1 long terminal repeat (LTR)-directed gene expression in several central nervous system-derived astrocytic/glial cell lines in the absence of TAR. Within this study, transient expression assays performed in the astrocytic/glial cell line, U87-MG, confirm that kappa B elements within the HIV-1 LTR mediate TAR-independent transactivation by Tat and demonstrate additionally that distinct amino acid residues within the cysteine-rich activation domain of Tat are required for TAR-independent versus TAR-dependent transactivation. Established U87-MG cell lines expressing a transdominant negative mutant of I kappa B alpha, I kappa B alpha delta N, fail to support TAR-independent transactivation by Tat, suggesting that binding of NF-kappa B to kappa B enhancer elements within the HIV-1 LTR is necessary for Tat-mediated transactivation in the absence of TAR. Ribonucleic acid protection analyses of promoter-proximal and -distal transcripts derived from TAR-deleted and TAR-containing HIV-1 LTR reporter constructs in U87-MG cells indicate that the predominant effect of Tat during TAR-independent transactivation occurs at the lavel of transcription initiation, whereas a prominent elongation effect of Tat is observed in the presence of TAR. These data suggest an alternative regulatory pathway for Tat transactivation in specific cells derived from the central nervous system that is independent of TAR and that requires direct or indirect interaction of Tat with NF-kappa B-binding sites in the HIV-1 LTR.

HIV infection of human fetal neural cells is mediated by gp120 binding to a cell membrane-associated molecule that is not CD4 nor galactocerebroside

HIV infection of central nervous system (CNS) tissue is a common finding in both adult and pediatric AIDS. Because most children are believed to be infected perinatally, we have developed a model of HIV CNS infection that utilizes explant organotypic cultures of human fetal CNS tissue. Using this model we previously reported that both lymphocytotropic and monocytotropic HIV isolates infect microglia and astrocytes. However, the mechanism by which HIV infects these cells remains to be elucidated. We have observed that neural cell infection in these cultures may be the result of receptor-mediated endocytosis. In order to confirm this observation and to determine the ligand responsible for this process, organotypic cultures were exposed to untreated HIV, HIV pretreated with soluble CD4 (sCD4) or, as a control, heat-inactivated HIV. To address the question of a putative receptor for HIV infection, CNS cultures were either untreated or pretreated with gp120 or with the deglycosylated form of this protein. Other cultures were treated with antibodies to CD4 (anti-T4A) or to galactocerebroside (GC). Results demonstrate that pretreatment of either HIV with sCD4 or CNS cultures with gp120 significantly inhibits HIV infection. The inhibition of infection was demonstrated by a reduction in the number of cells positive for HIV proteins and by decreases in HIV proviral DNA and p24 production. Pretreatment of CNS cultures with deglycosylated gp120, anti-T4A or anti-GC antibodies did not inhibit HIV infection. These data suggest that HIV gp120 is needed for binding to a surface molecule on CNS cells that is not CD4 nor GC and that this molecule may function as a receptor and lead to infection of neural cells.

Innate natural antibodies. Primary roles indicated by specific epitopes

Two members of a unique class of natural antibodies have been identified in all of a large cohort of sera from clinically normal humans of broad age distribution. By means of a series of 10-12 mer peptides the epitope for each of those antibodies was characterized with regard to amino acid identity and conformation. Similar epitope specificity was revealed for the IgM isotopes of cord blood and early post natal sera and for IgM and IgG of adult sera, suggesting that the class of natural antibodies represented by the two identified in this study includes those genomically coded for at their effector level of maturation in the B cells of the neonate. Assay of series of specimens from each of four clinically normal adults revealed that those two natural antibodies are present at relatively constant titer, unique to each individual, over four to five and a half year periods. Those observations imply that the primary function of that class of natural antibodies may be related to maintenance of homeostasis and the molecular identity of each of the two epitopes suggests a role, for each, as monitor or control in intracellular traffic. The previous identification of those epitopes in a conserved protein of HIV also provides support for the proposition that a secondary function of natural antibodies, arising from fortuitous coincidence of the identity of the epitopes, may be that of early defense against infectious invaders.

Upregulation of urokinase-type plasminogen activator by endogenous and exogenous HIV-1 Tat protein in tumour cell lines derived from BK virus/tat-transgenic mice

OBJECTIVE

To demonstrate that Tat modulates the plasminogen-dependent proteolytic activity of tumour cell lines derived from BK virus (BKV)/tat-transgenic mice by affecting the production of plasminogen activators (PA) and the PA inhibitor (PAI)-1 and to demonstrate that this occurs through mechanism(s) that are distinct from those responsible for transactivating activity of extracellular Tat.

DESIGN AND METHODS

To assess whether endogenous Tat is responsible for PA activity in T53 adenocarcinoma cells, cell cultures were transfected with antisense Tat cDNA and evaluated for cell-associated PA activity by a plasmin chromogenic assay. The assay was also used to evaluate PA activity in T53 cells and T111 leiomyosarcoma cells stimulated by extracellular Tat. The type(s) of PA produced were identified by sodium dodecyl sulphate-polyacrylamide gel electrophoresis zymography. The levels of PAI-1 were evaluated by Western blotting. Tat transactivating activity was measured by a chloramphenicol acetyltransferase (CAT) enzyme-linked immunosorbent assay in HL3T1 cells containing integrated copies of an HIV-1 long terminal repeat (LTR)-CAT plasmid.

RESULTS

Transfection of T53 cells with antisense Tat cDNA results in the decrease of Tat production and PA activity. Exogenously added Tat increases PA levels in T53 and in T111 cells. PA activity was identified as urokinase-type PA (uPA). Tat also increases the production of PAI-1 in T111 but not in T53 cells. Chloroquine and heparin have different affects on the LTR-CAT-transactivating and the PA-inducing activities of Tat. The fusion protein glutathione-S-transferase-Tat and the mutant Tat-1e, lacking the second Tat exon, cause LTR-CAT transactivation without stimulating uPA upregulation.

CONCLUSIONS

Tat affects the fibrinolytic activity of tumour cell lines derived from BKV/tat-transgenic mice by modulating the production of both uPA and PAI-1 via autocrine and paracrine mechanisms of action. The capacity of Tat to modulate the plasminogen-dependent proteolytic activity of these tumour cell lines may contribute to their metastatic potential. The uPA-inducing activity of Tat depends upon specific biological and structural features of the Tat protein that are distinct from those responsible for its LTR-CAT-transactivating activity, suggesting distinct mechanisms of induction for the two biological responses.

AIDS Dementia and HIV-1-Induced Neurotoxicity: Possible Pathogenic Associations and Mechanisms

AIDS Dementia Complex (ADC) is a syndrome of cognitive, behavioral, and motor deficits resulting from HIV-1 infection within the brain. ADC is characterized by variable degrees of neuronal cell death and gliosis that likely result, at least, in part from release of metabolic products, cytokines, and viral proteins from infected macrophages, although a unifying explanation for the neurological dysfunction has yet to be established. Major unanswered questions include: (i) do neurologic symptoms result from neuronal cell death and/or dysfunction in surviving neurons?; (ii) are viral genomic sequences determinants of neurotoxicity?; (iii) is HIV infection of neurons and astrocytes relevant to pathogenesis?, and (iv) what circulating factors within the brain affect neuronal cell survival and function? This review addresses the association between HIV-1 replication within the brain, production of potential neurotoxins and possible mechanisms of induction of neurotoxicity and neuronal dysfunction contributing to the pathogenesis of ADC. Copyright 1996 S. Karger AG, Basel

Tumor necrosis factor alpha-induced apoptosis in human neuronal cells: protection by the antioxidant N-acetylcysteine and the genes bcl-2 and crmA

Tumor necrosis factor alpha (TNF-alpha) is a candidate human immunodeficiency virus type 1-induced neurotoxin that contributes to the pathogenesis of AIDS dementia complex. We report here on the effects of exogenous TNF-alpha on SK-N-MC human neuroblastoma cells differentiated to a neuronal phenotype with retinoic acid, TNF-alpha caused a dose-dependent loss of viability and a corresponding increase in apoptosis in differentiated SK-N-MC cells but not in undifferentiated cultures. Importantly, intracellular signalling via TNF receptors, as measured by activation of the transcription factor NF-kappa B, was unaltered by retinoic acid treatment. Finally, overexpression of bcl-2 or crmA conferred resistance to apoptosis mediated by TNF-alpha, as did the addition of the antioxidant N-acetylcysteine. These results suggest that TNF-alpha induces apoptosis in neuronal cells by a pathway that involves formation of reactive oxygen intermediates and which can be blocked by specific genetic interventions.