Ca2+, N-methyl-D-aspartate receptors, and AIDS-related neuronal injury

D1/NMDA receptors and concurrent methamphetamine+ HIV-1 Tat neurotoxicity

The interactive effects of HIV-1 infection and methamphetamine (METH) abuse in producing cognitive dysfunction represent a serious medical problem; however, the neural mechanisms underlying this interactive neurotoxicity remain elusive. In this study, we report that a combination of low, sub-toxic doses of METH + HIV-1 Tat 1-86 B, but not METH + HIV-1 gp120, directly induces death of rodent midbrain neurons in vitro. The effects of D1- and NMDA-receptor specific antagonists (SCH23390 and MK-801, respectively) on the neurotoxicity of different doses of METH or HIV-1 Tat alone and on the METH + HIV-1Tat interaction in midbrain neuronal cultures suggest that the induction of the cell death cascade by METH and Tat requires both dopaminergic (D1) and N-methyl D-aspartate (NMDA) receptor-mediated signaling. This interactive METH+Tat neurotoxicity does not occur in cultures of hippocampal neurons, which are predominately glutamatergic, express very low levels of dopamine receptors, and have no functional dopamine transporter (DAT). Thus, the presence of a subpopulation of neurons capable of dopamine release/uptake is essential for METH+Tat induction of the cell death cascade. Overall, our results support the hypothesis that METH and HIV-1 Tat disrupt the normal conjunction of signaling between D1 and NMDA receptors, resulting in neural dysfunction and death.

Soluble factors from IL-1β-stimulated astrocytes activate NR1a/NR2B receptors: implications for HIV-1-induced neurodegeneration

Astrocytes play an important role in astrocyte-neuron homeostasis. In HIV-1-infected brain, interleukin 1 beta (IL-1β) activation of astrocytes contributes to neurodegeneration. However, the molecular mechanisms underlying IL-1β-activated-astrocytes-induced neurodegeneration in HIV-1-infected brain are largely unknown. We hypothesize that secretory factors from the activated astrocytes affect N-methyl-d-aspartate (NMDA) receptor, a major pathway implicated in HIV-1-associated neurodegeneration. To test this hypothesis, we studied effects of IL-1β-stimulated astrocyte conditioned medium (ACM+) for its ability to activate NR1a/NR2B receptors expressed on Xenopus oocytes. Astrocytes treated with IL-1β 20ng/ml for 24h induced CXCL8, CCL2, MMP1 and MMP7. Pressure ejection of the ACM(+) produced an inward current in NR1a/NR2B-expressing oocytes. The inward current produced by ACM(+) was blocked by NMDA receptor antagonist, APV but not by non-NMDA receptor antagonist, CNQX. These results suggest that IL-1β stimulated astrocytes activate NR1a/NR2B receptors which may have implications in HIV-1-associated neurodegeneration.

Genetic markers on HIV-1 gp120 C2-V3 region associated with the expression or absence of cognitive motor complex in HIV/AIDS

In the CNS, HIV-1 causes cognitive motor complex (CMC) in about 30-40% of patients. To explain CMC physiopathology: disequilibrium of cytokine networks, calcium influx, free radicals and toxic effects by HIV-1 have been evoked. Neurotropic mutants have not been unambiguously proven nor 'variants' of HIV-1 with biological properties that could cause CMC. By computerized analysis of gp120 C2-V3 subtype B sequences from retroviral databases, and applying stringent criteria, we found: (i) mutations specific for CMC; (ii) mutations associated with the absence of CMC (N-CMC); (iii) mutations with specificity for the geographical region of origin, and finally (iv) shared mutations representing 'hot spots.' We suggest that the capability to cause or not to cause CMC may be present in the virus prior to infection. In the future, these markers could be used to guide treatments with novel neuroprotective regimes.

A novel antioxidant N-acetylcysteine amide prevents gp120- and Tat-induced oxidative stress in brain endothelial cells

Free radical production and, consequently, oxidative stress play an important role in the pathogenesis of AIDS and cause damage to lipids, proteins, and DNA. In our previous study, the HIV-1 envelope glycoprotein (gp120) and transregulatory protein (Tat) of HIV-1 have been found to induce oxidative stress in an immortalized endothelial cell line from rat brain capillaries, RBE4 (in vitro model of the blood-brain barrier). Here, we have determined the effects of a novel antioxidant, N-acetylcysteine amide (NACA), on gp120- and Tat-induced oxidative stress. Various oxidative stress parameters, including reduced glutathione (GSH), oxidized glutathione (GSSG), catalase (CAT) activity, and glutathione reductase (GR) activity, as well as malondialdehyde (MDA) levels, were used as measures of oxidative stress. NACA significantly increased the levels of intracellular GSH, CAT, and GR and decreased the levels of MDA in RBE4 cells, showing that oxidatively challenged cells were protected. Gp120- and Tat-induced increases in intracellular reactive oxygen species (ROS) were observed by using the 2',7'-DCF assay; the ROS scavenger, NACA, blocked ROS generation. A well-known apoptosis indicator, caspase-3 activity, was measured and was also found to have been returned to its control levels by NACA. Treatment of RBE4 cells with gp120 and Tat caused an increase in toxicity, as measured by lactate dehydrogenase (LDH) and tetrazolium reduction (MTS) assays. HIV-1 protein-induced toxicity in these cells was blocked by treatment with NACA. These studies show that NACA reverses gp120- and Tat-induced oxidative stress in immortalized endothelial cells.

Neuropsychiatric aspects of human immunodeficiency virus (HIV) infection

Human immunodeficiency virus (HIV) infection with central nervous system (CNS) involvement causes a variety of psychiatric complications among a significant proportion of infected individuals. A cure for the fully developed AIDS related to HIV infection remains elusive, and HIV/AIDS is a leading cause of death among adults between the ages of 25 and 44. Life expectancy, however, has gradually increased over the years, resulting in a concern for a potential increase in the incidence of secondary psychiatric manifestations. Knowledge of the neuropathology of HIV-CNS dysfunction and familiarity with its clinical presentation can aid clinicians in a determination of the appropriate therapy inclusive of psychiatric care that may be useful for a specific individual in their treatment.

Neuronal glycolytic pathway impairment induced by HIV envelope glycoprotein gp120

Neurological impairment is a common feature of Acquired Immunodeficiency Syndrome (AIDS); functional alterations have been reported both in central and peripheral nervous system and the Human Immunodeficiency Virus (HIV) envelope glycoprotein gp120 has been proposed as a neurotoxin acting through a calcium-dependent mechanism. On the other hand it has been reported that gp120 treatment also induce about a 20% decrease in the cerebral glucose utilization and in the cellular ATP levels. The reported observations were performed on experimental system where also non-neuronal cells where present; in order to evaluate whether a direct interaction between HIV proteins and neuronal cells takes place, we used a neuroblastoma cultures where only neuronal cells are present. We analysed the effects of gp120 on the N18TG2 neuroblastoma clone. Treatments were performed both on growing and confluent cultures. Short time treatment with gp120 of confluent cultures causes a 25% reduction in the level of neuron-specific enolase, resulting in a similar decrease of oxygen consumption. Long time exposure of growing cells also causes a reduction in cell survival. Furthermore, using a membrane-specific fluorescent probe we observed that gp120 produces an increase of membrane trafficking. These observations suggest a direct interaction between the viral envelope protein and neuronal cells, which results in an alteration of glycolytic metabolism. This alteration may be related to the neurologic impairments observed in AIDS patients.

The HIV-1 coat protein gp120 and some of its fragments potently activate native cerebral NMDA receptors mediating neuropeptide release

The objective of this study was to investigate the effects of the HIV-1 envelope protein gp120 and its peptide fragments on the function of N-methyl-D-aspartate (NMDA) receptors mediating release of cholecystokinin (CCK) and somatostatin (SRIF). These are nonconventional NMDA receptors recently found to be activated by glycine or D-serine 'only'. The release of cholecystokinin-like immunoreactivity (CCK-LI) and of somatostatin-like immunoreactivity (SRIF-LI) elicited by 12 mM K+ from superfused rat neocortex synaptosomes was potently increased by gp120, its cyclic V3 loop and the linear V3 sequence BRU-C-34-A, but not by RP-135 (a central portion of BRU-C-34-A). The EC50 values of gp120 were 0.02 nM (CCK-LI release) and 0.01 nM (SRIF-LI release). The releasing effect of gp120 was prevented by blocking the glycine site or the ion channel of NMDA receptors, but not the glutamate recognition site; in addition, the gp120 effect was strongly inhibited by nanomolar concentrations of Zn2+ ions and by low micromolar concentrations of ifenprodil. It is concluded that gp120 acts as a very potent agonist at the glycine site of NMDA receptors sited on CCK- and SRIF-releasing nerve endings; the protein is able to activate the receptor channel in the absence of glutamate. Gp120 activates the receptors through its V3 loop as peptide fragments related to V3 retain near-maximal activity. The sensitivity of the gp120 effect to both Zn2+ and ifenprodil would not be incompatible with the idea that these NMDA receptors contain the triple subunit combination NR1/NR2A/NR2B.

Effect of extracellular human immunodeficiency virus type 1 glycoprotein 120 on primary human vascular endothelial cell cultures

During the course of an HIV-1 infection, free infectious and noninfectious virus particles, and free HIV-1 proteins, circulate within the host, exposing the host endothelium to these viral factors, even if the endothelium is not infected. This suggests that extracellular HIV-1 proteins could influence endothelial cell function, leading to pathogenesis. In light of this, we have used primary cultured human vascular endothelial cells (HUVECs) to screen for effects of the HIV-1 protein gp120 on endothelial cell function. The results of this study show that short exposure of HUVEC cultures to this protein causes significant levels of cytotoxicity. Further, using several different assays, we have shown that this cytotoxic effect on HUVECs appears to be due to induction of an apoptotic program. The biphasic nature of gp120 titration curves suggests that multiple cellular factors are mediating these gp120-induced effects. Competition studies appear to confirm this by showing that the apoptotic effect is mediated through two cell surface receptors on HUVECs, CCR5 and CXCR4. Alternatively, competition studies examining CD4 receptors suggests that CD4 played no role in gp12O-induced effects on HUVECs.

The human immunodeficiency virus-1 envelope protein gp120 binds through its V3 sequence to the glycine site of N-methyl-D-aspartate receptors mediating noradrenaline release in the hippocampus

Recent results show that the HIV-1 protein gp120 can enhance N-methyl-D-aspartate receptor-mediated release of noradrenaline from CNS nerve endings. We now investigate the mechanism of this action, including the structural determinants of the gp120 effect and the nature of its binding sites. The N-methyl-D-aspartate-evoked release of [3H]noradrenaline from rat hippocampal synaptosomes was potentiated similarly by gp120 and gp160; gp41 was ineffective. The regions of gp120 involved appear to be outside the CD4-binding domain of the protein, because gp120 retained its activity after pretreatment with N-carbomethoxycarbonyl-D-prolyl-D-phenylalanine, a compound known to inhibit binding of gp120 to CD4 receptors. Moreover, sequences of gp120 critical for binding to CD4 did not mimic the effect of gp120. Preincubation of synaptosomes with anti-galactocerebroside antibodies did not affect gp120 activity. The protein effect was retained by peptides mimicking its V3 sequence, including the cyclic V3 "universal peptide" and the linear V3 sequence BRU-C-34-A, but not RP-135 (a central portion of BRU-C-34-A). The block of the N-methyl-D-aspartate-induced [3H]noradrenaline release by 7-chlorokynurenate, an antagonist at the N-methyl-D-aspartate receptor glycine site, was competitively reversed by glycine, by V3 and by BRU-C-34-A. When added with N-methyl-D-aspartate, V3 was three to four orders of magnitude more potent than glycine (EC50 values: about 20 pM and 150 nM, respectively) in enhancing [3H]noradrenaline release. Gp120 did not release glycine or serine from synaptosomes, thus excluding indirect actions through these agents. To conclude, gp120 may act following recognition by its V3 sequence of a high-affinity site possibly coincident with the glycine site of N-methyl-D-aspartate receptors present on hippocampal terminals of noradrenergic neurons. Considering the importance of N-methyl-D-aspartate receptor activation and of noradrenaline in cognitive processes, the effects of gp120 and V3 described here may be relevant to the pathology of AIDS dementia.

Antioxidants and dipyridamole inhibit HIV-1 gp120-induced free radical-based oxidative damage to human monocytoid cells

Reactive oxygen species (ROS) may play an important role in HIV-1 pathogenesis and HIV-1 gp120-induced neurotoxicity. Our studies determined the extent to which gp120 increased ROS production in human monocytic U937 cells and the effectiveness of various agents, including dipyridamole (DPR), in blocking these responses. The thiobarbituric acid-reactive substances (TBARS) assay was used as a measure of recombinant gp120 (HIV-1[3B])-induced oxidative damage to U937 cells. As a control, TBARS production was measured using a hypoxanthine/xanthine superoxide generating system. There was gp120-induced oxidative damage in U937 cells with a concentration that produces 50% of maximal effect (apparent EC50 value) of 11 pM. Polyclonal antiserum to gp120 significantly (p < 0.05) inhibited gp120-induced oxidative damage. gp120-induced oxidative damage was significantly inhibited 81% (p < 0.01) by catalase/superoxide dismutase, 53% (p < 0.05) by (+/-)-alpha-tocopherol, 78% (p < 0.01) by desferrioxamine, and 82% (p < 0.01) by ethylene diamine tetraacetic acid (EDTA). These results indicate that gp120 is capable of promoting iron-based oxygen free radical damage to U937 cells. DPR potently (p < 0.05) inhibited both hypoxanthine/xanthine- and gp120-induced oxidative damage with concentrations that produce 50% inhibition (apparent IC50 values) of 1.3 microM for hypoxanthine/xanthine and 1.0 microM for gp120. Therapeutic intervention against ROS production may prevent HIV-1 neurotoxicity.

Gp120 can revert antagonism at the glycine site of NMDA receptors mediating GABA release from cultured hippocampal neurons

The effects of the human immunodeficiency virus type 1 envelope protein gp120 on the release of GABA elicited by N-methyl-D-aspartate (NMDA) from rat hippocampal neurons in primary culture has been investigated. NMDA (1-300 microM) increased in a concentration-dependent manner (EC50 =37.9+/-12 microM) the release of [3H]-GABA. The effect of 100 microM NMDA was prevented by 30 microM of the GABA transport inhibitor N-(4,4-diphenyl-3-butenyl)guvacine (SKF 100330A). Glycine (10 microM) or gp120 (0.01 microM) affected neither the basal nor the NMDA-evoked [3H]-GABA release. The NMDA (100 microM)-evoked release was prevented by 5,7-dichloro-kynurenic acid (5,7-DCKA), a selective antagonist at the glycine site of the NMDA receptor, in a concentration-dependent manner (IC50 approximately 0.3 microM). Glycine (3-10 microM) or gp120 (0.003-0.01 microM) produced reversal of the 5,7-DCKA antagonism in a way that suggested competition at a same site; gp120 was at least 3 orders of magnitude more potent than glycine. It is suggested that gp120 may mimic glycine at NMDA receptors.

Identification of cytotoxic peptide as possible mechanism for neurotoxicity of HIV viral envelope and AIDS pathogenesis

A major segment of acquired immunodeficiency syndrome (AIDS) patients suffer from neurological complications, including impairments in concentration and motor functions. This neuronal injury, although related to the human immunodeficiency virus (HIV), occurs even though the neurons themselves are not infected by the virus. A complex web of interactions of the immune system with noxious substances released from gp120-stimulated macrophages is hypothesized as the mechanism of the injury. This study has identified an antimicrobial peptide resident in the human small intestine as a candidate for these noxious substances. This peptide is neither cell nor tumor specific and mediates cytolysis by membrane permeabilization based on membrane potential. The identified peptide is, however, type specific against viruses, only attacking enveloped viruses. This study hypothesizes that the peptide is sequestered in the HIV viral envelope and is released in very toxic concentrations when localized membrane potential is high. The peptide is localized in the Paneth cells of the human small intestine, and a transmission pathway is identified through the abrogation of intestinal tissue occurring during receptive anal intercourse. A study of amino acid sequences between this peptide and three variants of HIV confirmed homologies. The identification of this peptide as a possible mechanism could substantially alter AIDS treatment protocols.

Chronic administration of a partial agonist at strychnine-insensitive glycine receptors: a novel experimental approach to the treatment of ischemias

During the past decade, converging lines of evidence have linked the abnormal release or leak of excitatory amino acids to the neurodegeneration associated with a wide range of pathologies including cerebral ischemias, Huntington's disease, and AIDS dementia (Coyle and Robinson, 1987; Lipton, 1994; Meldrum, 1994). Pharmacological studies indicate that activation of both ionotropic and metabotropic glutamate receptors can substantially contribute to excitotoxic cell damage (Choi, 1992; Pizzi et al., 1993; Sheardown et al., 1993; Xue et al., 1994). Based on these findings, therapeutic strategies based on blunting or blocking glutamatergic transmission may be useful in treating a variety of neurodegenerative disorders.

gp120-induced neurotoxicity in hippocampal pyramidal neuron cultures: protective action of TGF-beta1

We found that TGF-beta1, a cytokine that previously has been reported to have neuroprotective effects, was able to prevent the toxicity induced by the HIV-1 coat protein gp120 in hippocampal pyramidal neuron cultures. In the presence of glia, gp120 induced time- and dose-dependent cell death, which was more pronounced in mature (7-19 d in culture) than in young neurons (2-7 d in culture). Staining with nuclear dyes (propidium iodide and Hoechst 33342), in situ detection of DNA fragments, and DNA analysis on agarose gels indicated that apoptosis was mainly responsible for the death caused by the viral protein. However, after several days of treatment, death-displaying necrotic features also occurred. Neurotoxicity induced by gp120 was dependent on the activation of NMDA receptors and required the presence of glia as well as new protein synthesis. Thus, the effect of gp120 was abolished by the NMDA receptor antagonist APV and partially reduced by cycloheximide. Only modest neurotoxicity was observed in pure neuronal cultures deprived of the glia feeder layer. Fura-2-based videoimaging showed that treatment with gp120 enhanced the ability of NMDA to increase neuronal [Ca2+]i. The impairment of neuronal Ca2+ homeostasis was prevented completely by TGF-beta1. Therefore, it is likely that the neuroprotective action of the cytokine is attributable to its ability to stabilize neuronal [Ca2+]i.