Exposure to gp120 of HIV-1 Induces an Increased Release of Arachidonic Acid in Rat Primary Neuronal Cell Culture Followed by NMDA Receptor-mediated Neurotoxicity

After incubation of highly enriched neurons from rat cerebral cortex with the HIV-1 coat protein gp120 for 18 h, cells showed fragmentation of DNA at internucleosomal linkers followed by NMDA receptor-mediated neurotoxicity. We report that in response to exposure to gp120 cells react with an increased release of arachidonic acid (AA) via activation of phospholipase A₂. This process was not inhibited by NMDA receptor antagonists. To investigate the role of AA on the sensitivity of the NMDA receptor towards its agonist, low concentrations of NMDA were co-administered with AA. This condition enhanced the NMDA-mediated cytotoxicity. Administration of mepacrine reduced cytotoxicity caused by gp120. We conclude that gp120 causes an activation of phospholipase A₂, resulting in the increased release of AA, which may in turn sensitize the NMDA receptor.     

7-Chlorokynurenate Ameliorates Neuronal Injury Mediated by HIV Envelope Protein gp120 in Rodent Retinal Cultures

Prior studies with in vitro model systems have suggested that at least part of the neurological manifestations of AIDS may stem from neuronal injury involving the HIV-1 coat protein gp120. This form of neuronal damage is most probably mediated indirectly by a complex set of cellular interactions among macrophages, astrocytes, and neurons, resulting in a final common pathway of overstimulation of N -methyl- d -aspartate (NMDA) receptors. We studied the neuroprotective effect from gp120-induced neuronal injury of an antagonist of the glycine site of the NMDA receptor, 7-chlorokynurenate. In identified rat retinal ganglion cells in culture, we found that 50 μM 7-chlorokynurenate significantly abrogated the injury engendered by 20 pM gp120. Addition of 300 μM exogenous glycine prevented this protective effect of 50 μM 7-chlorokynurenate. These data suggest that glycine site antagonists of the NMDA receptor may have therapeutic potential for ameliorating neuronal damage associated with gp120.     

Effects of the HIV-1 Envelope Glycoprotein gp120 in Cerebellar Cultures. [Ca2+]i Increases in a Glial Cell Subpopulation

The various types of cells present in cultures prepared from the postnatal rat cerebellum, identified by their gross morphology and immunocytochemistry, were loaded with the specific dye fura-2 and analysed individually for [Ca²⁺]_i changes induced by the HIV-1 envelope glycoprotein gp120 and a variety of other treatments. In granule neurons [Ca²⁺]_i increases were induced by high KCl and glutamate (mainly through the NMDA receptor) while in type-1 astrocytes this effect was observed after serotonin, carbachol and also quisqualate. In contrast, administration of gp120 was always without effect in these cells. Type-2 astrocytes (an arborized cell type responsive to agonists targeted to the glutamatergic AMPA and cholinergic receptors) were also most often unresponsive to the viral glycoprotein. However, among the cells exhibiting the arborized phenotype, a subpopulation (-13%) responded to gp120 with conspicuous [Ca²⁺]_i increases sustained by both release from intracellular stores and influx across the plasma membrane. These responses to the viral protein did not involve activation of either voltage-gated Ca²⁺ channels or glutamatergic receptors. Although not yet conclusively identified by specific cytochemical markers, the gp120-responsive cells resemble type-2 astrocytes and differ from neurons and type-1 astrocytes both in gross phenotype and in a number of receptor/channel properties: positivity to AMPA and cholinergic agonists; negativity to NMDA, serotonin and high KCl. From these results it is concluded that a subpopulation of glial cells is affected by gp120. The role of these cells in HIV brain infection and damage requires further studies to be precisely established.     

Galantamine and nicotine have a synergistic effect on inhibition of microglial activation induced by HIV-1 gp120

Chronic brain inflammation is the common final pathway in the majority of neurodegenerative diseases and central to this phenomenon is the immunological activation of brain mononuclear phagocyte cells, called microglia. This inflammatory mechanism is a central component of HIV-associated dementia (HAD). In the healthy state, there are endogenous signals from neurons and astrocytes, which limit excessive central nervous system (CNS) inflammation. However, the signals controlling this process have not been fully elucidated. Studies on the peripheral nervous system suggest that a cholinergic anti-inflammatory pathway regulates systemic inflammatory response by way of acetylcholine acting at the alpha7 nicotinic acetylcholine receptor (alpha7nAChR) found on blood-borne macrophages. Recent data from our laboratory indicates that cultured microglial cells also express this same receptor and that microglial anti-inflammatory properties are mediated through it and the p44/42 mitogen-activated protein kinase (MAPK) system. Here we report for the first time the creation of an in vitro model of HAD composed of cultured microglial cells synergistically activated by the addition of IFN-gamma and the HIV-1 coat glycoprotein, gp120. Furthermore, this activation, as measured by TNF-alpha and nitric oxide (NO) release, is synergistically attenuated through the alpha7 nAChR and p44/42 MAPK system by pretreatment with nicotine, and the cholinesterase inhibitor, galantamine. Our findings suggest a novel therapeutic combination to treat or prevent the onset of HAD through this modulation of the microglia inflammatory mechanism.     

HIV-1 gp120-induced neuroinflammation: Relationship to neuron loss and protection by rSV40-delivered antioxidant enzymes

HIV-1 gp120 neurotoxicity and oxidant injury are well documented, but consequent neuroinflammation is less understood. Rat caudate-putamens (CPs) were challenged with 100-500 ng HIV-1BaL gp120, with or without prior rSV40-delivered superoxide dismutase or glutathione peroxidase. CD11b-positive microglia were increased 1 day post-challenge; Iba-1- and ED1-positive cells peaked at 7 days and 14 days. Astrocyte infiltration was maximal at 7-14 days. MIP-1alpha was produced immediately, mainly by neurons. ED1- and GFAP-positive cells correlated with neuron loss and gp120 dose. We also tested the effect of more chronic gp120 exposure on neuroinflammation using an experimental model of continuing gp120 exposure. SV(gp120), a recombinant SV40-derived gene transfer vector was inoculated into the rat CP, leading to chronic expression of gp120, ongoing apoptosis in microglia and neurons, and oxidative stress. Increase in microglia and astrocytes was seen following intra-CP SV(gp120) injection, suggesting that continuing gp120 production increased neuroinflammation. SV(SOD1) or SV(GPx1) significantly reduced MIP-1alpha and limited neuroinflammation following gp120 administration into the CP, as well as microglia and astrocytes proliferation after injection of SV(gp120) in the striatum. Thus, gp120-induced CNS injury, neuron loss and inflammation may be mitigated by antioxidant gene delivery.     

Calbindin D28K-containing neurons,and not HSP70-expressing neurons,are more resistant to HIV-1 envelope (gp120) toxicity in cortical cell cultures

HIV-1-associated cognitive/motor complex is one of the major neurological complications of AIDS and is associated with neuronal loss. Gp120, the HIV envelope protein, is toxic for neurons in cultures and produces a rise in intracytosolic calcium. This neurotoxicity is dose-dependent and time-dependent. We evaluated the selective gp120 toxicity in primary neuronal cultures for calbindin-free and calbindin-containing neurons with semi-quantitative immunocytochemistry using an anti-calbindin D_28K monoclonal antibody. The number of immunolabelled neurons was inversely correlated to neuronal survival. In cultures exposed to gp120 (100 pM) for 24 hr the neuronal survival of initial platings was 19.7 ± 2.1% and the percentage of neuronal survival was 84.6 ± 4.9% in control cultures exposed to the vehicle. The corresponding percentages of immunolabelled neurons were 85.0 ± 2.1% in treated cultures and 23.6 ± 3.1% in control cultures (P < 0.001). The expression of heat shock proteins by heating cell cultures did not: protect neurons from gp120 toxicity. These results suggest that calbindin D_28K-containing neurons are more resistant to gp120-toxicity in this cell culture system. © 1995 Wiley-Liss, Inc.     

HIV-1 Envelope Proteins gp120 and gp160 Potentiate NMDA-induced [Ca2+]i Increase, Alter [Ca2+]i Homeostasis and Induce Neurotoxicity in Human Embryonic Neurons

The envelope glycoprotein gp120 of the human immunodeficiency virus HIV-1 has been proposed to cause neuron death in developing murine hippocampal cultures and rat retinal ganglion cells. In the present study, cultured human embryonic cerebral and spinal neurons from 8- to 10-week-old embryos were used to study the neurotoxic effect of gp120 and gp160. Electrophysiological properties as well as N -methyl- d -aspartate (NMDA)-induced currents were recorded from neurons maintained in culture for 10–30 days. Neither voltage-activated sodium or calcium currents nor NMDA-induced currents were affected by exposure of neurons to 250 pM gp120 or gp160. In contrast, when neurons were subjected to photometric measurements using the calcium dye indo-1 to monitor the intracellular free Ca²⁺ concentration ([Ca²⁺]_i), gp120 and gp160 (20–250 pM) potentiated the large rises in [Ca²⁺]_i induced by 50 μM NMDA. The potentiation of NMDA-induced Ca²⁺ responses required the presence of Ca²⁺ in the medium, and was abolished by the NMDA antagonist d -2-amino-5-phosphonovalerate (AP5) and the voltage-gated Ca²⁺ channel inhibitor nifedipine. Moreover, exposure of a subpopulation of spinal neurons (25% of the cells tested) to 20–250 pM gp120 or gp160 resulted in an increase in [Ca²⁺]_i that followed three patterns: fluctuations not affected by AP5, a single peak, and the progressive and irreversible rise of [Ca²⁺]_i. The neurotoxicity of picomolar doses of gp120 and gp160 cultures was estimated by immuno-fluorescence and colorimetric assay. Treatment of cultures with AP5 or nifedipine reduced gp120-induced toxicity by 70 and 100% respectively.     

EGCG mitigates neurotoxicity mediated by HIV-1 proteins gp120 and Tat in the presence of IFN-gamma: role of JAK/STAT1 signaling and implications for HIV-associated dementia

Human immunodeficiency virus (HIV)-1 infection of the central nervous system occurs in the vast majority of HIV-infected patients. HIV-associated dementia (HAD) represents the most severe form of HIV-related neuropsychiatric impairment and is associated with neuropathology involving HIV proteins and activation of proinflammatory cytokine circuits. Interferon-gamma (IFN-gamma) activates the JAK/STAT1 pathway, a key regulator of inflammatory and apoptotic signaling, and is elevated in HIV-1-infected brains progressing to HAD. Recent reports suggest green tea-derived (-)-epigallocatechin-3-gallate (EGCG) can attenuate neuronal damage mediated by this pathway in conditions such as brain ischemia. In order to investigate the therapeutic potential of EGCG to mitigate the neuronal damage characteristic of HAD, IFN-gamma was evaluated for its ability to enhance well-known neurotoxic properties of HIV-1 proteins gp120 and Tat in primary neurons and mice. Indeed, IFN-gamma enhanced the neurotoxicity of gp120 and Tat via increased JAK/STAT signaling. Additionally, primary neurons pretreated with a JAK1 inhibitor, or those derived from STAT1-deficient mice, were largely resistant to the IFN-gamma-enhanced neurotoxicity of gp120 and Tat. Moreover, EGCG treatment of primary neurons from normal mice reduced IFN-gamma-enhanced neurotoxicity of gp120 and Tat by inhibiting JAK/STAT1 pathway activation. EGCG was also found to mitigate the neurotoxic properties of HIV-1 proteins in the presence of IFN-gamma in vivo. Taken together, these data suggest EGCG attenuates the neurotoxicity of IFN-gamma augmented neuronal damage from HIV-1 proteins gp120 and Tat both in vitro and in vivo. Thus EGCG may represent a novel natural copound for the prevention and treatment of HAD.     

HIV-1 gp120 neurotoxicity proximally and at a distance from the point of exposure: Protection by rSV40 delivery of antioxidant enzymes

Toxicity of HIV-1 envelope glycoprotein (gp120) for substantia nigra (SN) neurons may contribute to the Parkinsonian manifestations often seen in HIV-1-associated dementia (HAD). We studied the neurotoxicity of gp120 for dopaminergic neurons and potential neuroprotection by antioxidant gene delivery. Rats were injected stereotaxically into their caudate-putamen (CP); CP and (substantia nigra) SN neuron loss was quantified. The area of neuron loss extended several millimeters from the injection site, approximately 35% of the CP area. SN neurons, outside of this area of direct neurotoxicity, were also severely affected. Dopaminergic SN neurons (expressing tyrosine hydroxylase, TH, in the SN and dopamine transporter, DAT, in the CP) were mostly affected: intra-CP gp120 caused approximately 50% DAT+ SN neuron loss. Prior intra-CP gene delivery of Cu/Zn superoxide dismutase (SOD1) or glutathione peroxidase (GPx1) protected SN neurons from intra-CP gp120. Thus, SN dopaminergic neurons are highly sensitive to HIV-1 gp120-induced neurotoxicity, and antioxidant gene delivery, even at a distance, is protective.     

Selective Depletion of Clear Synaptic Vesicles and Enhanced Quantal Transmitter Release at Frog Motor Nerve Endings Produced by Trachynilysin, a Protein Toxin Isolated from Stonefish (Synanceia trachynis) Venom

Our previous observation that low concentrations of stonefish (Synanceia trachynis) venom elicit spontaneous quantal acetylcholine release from vertebrate motor nerve terminals prompted our present study to purify the quantal transmitter-releasing toxin present in the venom and to characterize the toxin's ability to alter the ultrastructure and immunoreactivity of frog motor nerve terminals. Fractionation of S. trachynis venom by sequential anion exchange fast protein-liquid chromatography (FPLC) and size-exclusion FPLC yielded a highly purified preparation of a membrane-perturbing (haemolytic) protein toxin, named trachynilysin. Trachynilysin (2–20 μg/ml) significantly increased spontaneous quantal acetylcholine release from motor endings, as detected by recording miniature endplate potentials from isolated frog cutaneous pectoris neuromuscular preparations. Ultrastructural analysis of nerve terminals in which quantal acetylcholine release was stimulated to exhaustion by 3 h exposure to trachynilysin revealed swelling of nerve terminals and marked depletion of small clear synaptic vesicles. However, trachynilysin did not induce a parallel depletion of large dense-core vesicles. Large dense-core vesicles contained calcitonin gene-related peptide (CGRP), as revealed by colloidal gold immunostaining, and trachynilysin-treated nerve endings exhibited CGRP-like immunofluorescence similar to that of untreated terminals. Our results indicate that the ability of stonefish venom to elicit spontaneous quantal acetylcholine release from vertebrate motor nerve terminals is a function of trachynilysin, which selectively stimulates the release of small clear synaptic vesicles and impairs the recycling of small clear synaptic vesicles but does not affect the release of large dense-core vesicles. Trachynilysin may be a valuable tool for use in other secretory terminals to discriminate between neurotransmitter and neuropeptide release.