HIV-1 envelope protein evokes intracellular calcium oscillations in rat hippocampal neurons

Co-receptor signaling in the pathogenesis of neuroHIV

The HIV co-receptors, CCR5 and CXCR4, are necessary for HIV entry into target cells, interacting with the HIV envelope protein, gp120, to initiate several signaling cascades thought to be important to the entry process. Co-receptor signaling may also promote the development of neuroHIV by contributing to both persistent neuroinflammation and indirect neurotoxicity. But despite the critical importance of CXCR4 and CCR5 signaling to HIV pathogenesis, there is only one therapeutic (the CCR5 inhibitor Maraviroc) that targets these receptors. Moreover, our understanding of co-receptor signaling in the specific context of neuroHIV is relatively poor. Research into co-receptor signaling has largely stalled in the past decade, possibly owing to the complexity of the signaling cascades and functions mediated by these receptors. Examining the many signaling pathways triggered by co-receptor activation has been challenging due to the lack of specific molecular tools targeting many of the proteins involved in these pathways and the wide array of model systems used across these experiments. Studies examining the impact of co-receptor signaling on HIV neuropathogenesis often show activation of multiple overlapping pathways by similar stimuli, leading to contradictory data on the effects of co-receptor activation. To address this, we will broadly review HIV infection and neuropathogenesis, examine different co-receptor mediated signaling pathways and functions, then discuss the HIV mediated signaling and the differences between activation induced by HIV and cognate ligands. We will assess the specific effects of co-receptor activation on neuropathogenesis, focusing on neuroinflammation. We will also explore how the use of substances of abuse, which are highly prevalent in people living with HIV, can exacerbate the neuropathogenic effects of co-receptor signaling. Finally, we will discuss the current state of therapeutics targeting co-receptors, highlighting challenges the field has faced and areas in which research into co-receptor signaling would yield the most therapeutic benefit in the context of HIV infection. This discussion will provide a comprehensive overview of what is known and what remains to be explored in regard to co-receptor signaling and HIV infection, and will emphasize the potential value of HIV co-receptors as a target for future therapeutic development.

Chemokine co-receptor CCR5/CXCR4-dependent modulation of Kv2.1 channel confers acute neuroprotection to HIV-1 glycoprotein gp120 exposure

Infection with human immunodeficiency virus-1 (HIV-1) within the brain has long been known to be associated with neurodegeneration and neurocognitive disorder (referred as HAND), a condition characterized in its early stages by declining cognitive function and behavioral disturbances. Mechanistically, the HIV-1 coat glycoprotein 120 (gp120) has been suggested to be a critical factor inducing apoptotic cell death in neurons via the activation of p38 mitogen-activated protein kinase (MAPK), upon chronic exposure to the virus. Here we show that acute exposure of neurons to HIV-1 gp120 elicits a homeostatic response, which provides protection against non-apoptotic cell death, involving the major somatodendritic voltage-gated K⁺ (Kv) channel Kv2.1 as the key mediator. The Kv2.1 channel has recently been shown to provide homeostatic control of neuronal excitability under conditions of seizures, ischemia and neuromodulation/neuroinflammation. Following acute exposure to gp120, cultured rat hippocampal neurons show rapid dephosphorylation of the Kv2.1 protein, which ultimately leads to changes in specific sub-cellular localization and voltage-dependent channel activation properties of Kv2.1. Such modifications in Kv2.1 are dependent on the activation of the chemokine co-receptors CCR5 and CXCR4, and subsequent activation of the protein phosphatase calcineurin. This leads to the overall suppression of neuronal excitability and provides neurons with a homeostatic protective mechanism. Specific blockade of calcineurin and Kv2.1 channel activity led to significant enhancement of non-apoptotic neuronal death upon acute gp120 treatment. These observations shed new light on the intrinsic homeostatic mechanisms of neuronal resilience during the acute stages of neuro-HIV infections.

Distinct modifications in Kv2.1 channel via chemokine receptor CXCR4 regulate neuronal survival-death dynamics

The chemokine stromal cell-derived factor-1α (SDF-1α) has multiple effects on neuronal activity, survival, and death under conditions that generate a proinflammatory microenvironment within the brain, via signaling through C-X-C-type chemokine receptor 4 (CXCR4), although the underlying cellular/molecular mechanisms are unclear. Using rat hippocampal neurons, we investigated distinct modifications in the voltage-gated K⁺ (Kv) channel Kv2.1 in response to short- and long-term SDF-1α/CXCR4-mediated signaling as an underlying mechanism for CXCR4-dependent regulation of neuronal survival and death. Acute exposure of neurons to SDF-1α led to dynamic dephosphorylation and altered localization of Kv2.1 channel, resulting in enhanced voltage-dependent activation of Kv2.1-based delayed-rectifier Kv currents (I(DR)). These changes were dependent on CXCR4- and/or NMDA receptor-mediated activation of calcineurin and provide neuroprotection. However, prolonged SDF-1α treatment leads to CXCR4-mediated activation of p38 mitogen-activated protein kinase, resulting in phosphorylation of Kv2.1 at S800 and enhanced surface trafficking of the channel protein, resulting in increased I(DR)/Kv2.1 current density. This, in combination with sustained dephosphorylation-induced enhancement of the voltage-dependent activation of I(DR)/Kv2.1, predisposed neurons to excessive K⁺ efflux, a vital step for the neuronal apoptotic program. Such apoptotic death was dependent on CXCR4 and Kv2.1 function and was absent in cells expressing the Kv2.1-S800A mutant channel. Furthermore, similar modifications in Kv2.1 and CXCR4/Kv2.1-dependent apoptosis were observed following treatment of neurons with the human immunodeficiency virus-1 (HIV-1) glycoprotein gp120. Therefore, distinct modifications in Kv2.1 in response to short- and long-term CXCR4-mediated signaling could provide a basis for neuroprotection or apoptosis in neuropathologies, such as neuroinflammation, stroke, brain tumors, and HIV-associated neurodegeneration.

The chemokine CXCL12 and the HIV-1 envelope protein gp120 regulate spontaneous activity of Cajal-Retzius cells in opposite directions

Activation of the CXC chemokine receptor 4 (CXCR4) in Cajal–Retzius cells by CXC chemokine ligand 12 (CXCL12) is important for controlling their excitability. CXCR4 is also a co-receptor for the glycoprotein 120 (gp120) of the envelope of the human immunodeficiency virus type 1 (HIV-1), and binding of gp120 to CXCR4 may produce pathological effects. In order to study CXCR4-dependent modulation of membrane excitability, we recorded in cell-attached configuration spontaneous action currents from hippocampal stratum lacunosum-moleculare Cajal–Retzius cells of the CXCR4-EGFP mouse. CXCL12 (50 nM) powerfully inhibited firing independently of synaptic transmission, suggesting that CXCR4 regulates an intrinsic conductance. This effect was prevented by conditioning slices with BAPTA-AM (200 μM), and by blockers of the BK calcium-dependent potassium channels (TEA (1 mM), paxilline (10 μM) and iberiotoxin (100 nM)). In contrast, exposure to gp120 (pico- to nanomolar range, alone or in combination with soluble cluster of differentiation 4 (CD4)), enhanced spontaneous firing frequency. This effect was prevented by the CXCR4 antagonist AMD3100 (1 μM) and was absent in EGFP-negative stratum lacunosum-moleculare interneurons. Increased excitability was prevented by treating slices with BAPTA-AM or bumetanide, suggesting that gp120 activates a mechanism that is both calcium- and chloride-dependent. In conclusion, our results demonstrate that CXCL12 and gp120 modulate the excitability of Cajal–Retzius cells in opposite directions. We propose that CXCL12 and gp120 either generate calcium responses of different strength or activate distinct pools of intracellular calcium, leading to agonist-specific responses, mediated by BK channels in the case of CXCL12, and by a chloride-dependent mechanism in the case of gp120.

GPI-1046 protects dorsal root ganglia from gp120-induced axonal injury by modulating store-operated calcium entry

Human immunodeficiency virus (HIV)-associated sensory neuropathy (HIV-SN) occurs in a large fraction of patients infected with HIV. Viral components, including the coat protein gp120, are thought to exert toxic actions on dorsal root ganglia (DRG) sensory neurons that can be further exacerbated by treatment of HIV infection with some antiretroviral agents. In a tissue culture model of HIV-SN, we found that gp120-induced axonal degeneration in DRG sensory neurons was prevented by treatment with the immunophilin ligand GPI-1046. Gp120 induced a rapid and large release of endoplasmic reticulum (ER) calcium in DRG neurons that was attenuated by treatment with GPI-1046. Further experiments suggested that GPI-1046 reduced the total ER calcium load by attenuating store-operated calcium (SOC) entry. Together, these results suggest that GPI-1046 protects DRG from gp120-induced axonal damage by decreasing the entry of calcium through SOC, thus reducing the total volume of ER calcium that is available to be released by gp120.

Feline immunodeficiency virus neuropathogenesis: from cats to calcium

Invasion of human immunodeficiency virus (HIV) into the central and peripheral nervous system produces a wide range of neurological symptoms, which continue to persist even with adequate therapeutic suppression of the systemic viremia. The development of therapies designed to prevent the neurological complications of HIV require a detailed understanding of the mechanisms of virus penetration into the nervous system, infection, and subsequent neuropathogenesis. These processes, however, are difficult to study in humans. The identification of animal lentiviruses similar to HIV has provided useful models of HIV infection that have greatly facilitated these efforts. This review summarizes contributions made from in vitro and in vivo studies on the infectious and pathological interactions of feline immunodeficiency virus (FIV) with the nervous system. In vivo studies on FIV have provided insights into the natural progression of CNS disease as well as the contribution of various risk factors. In vitro studies have contributed to our understanding of immune cell trafficking, CNS infection and neuropathogenesis. Together, these studies have made unique contributions to our understanding of (1) lentiviral interactions at the blood-cerebrospinal fluid (CSF) barrier within the choroid plexus, (2) early FIV invasion and pathogenesis in the brain, and (3) lentiviral effects on intracellular calcium deregulation and neuronal dysfunction. The ability to combine in vitro and in vivo studies on FIV offers enormous potential to explore neuropathogenic mechanisms and generate information necessary for the development of effective therapeutic interventions.

HIV-1 gp120 enhances giant depolarizing potentials via chemokine receptor CXCR4 in neonatal rat hippocampus

In the immature hippocampus, the giant depolarizing potentials (GDPs) are recurrent network-driven synaptic events generated by gamma-aminobutyric acid (GABA), which in neonatal life is depolarizing and excitatory. The GDPs enable a high degree of synchrony in immature neurons and participate in activity-dependent growth and synapse formation. To understand how human immunodeficiency virus type one (HIV-1) infection in the immature brain impairs brain growth and development, we studied the effects of HIV-1 envelope glycoprotein, gp120, a viral toxin shed in abundance by infected cells, on spontaneous occurring GDPs recorded in the CA3 pyramidal cells in neonatal (P2-P6) Sprague-Dawley rat hippocampal slices using whole-cell patch technique. Bath application of gp120 produced a sustained enhancement of GDP frequency in a concentration-dependent manner without affecting passive membrane properties, suggesting that the site of action is most likely on neural network, other than on the recorded neurons. The gp120-induced enhancement of GDPs was blocked by T140, a highly specific antagonist for the chemokine receptor, CXCR4, indicating the involvement of CXCR4 in the gp120-induced increase of GDPs. Bath application of stromal cell-derived factor-1alpha (SDF-1alpha), the only CXCR4 ligand, mimicked the effects of gp120 on GDPs, supporting the engagement of CXCR4 receptors in the gp120-induced increase of GDP occurrence. Further studies revealed the involvement of protein kinase A/C in the gp120-induced enhancement of GDPs. These results demonstrate that gp120 enhances GDPs in the neonatal rat hippocampus. This enhancement may cause an excessive increase in intracellular calcium and resultant neuronal injury, leading to retardation of the brain and behavioural development as seen in paediatric AIDS patients.

Experimental and potential future therapeutic approaches for HIV-1 associated dementia targeting receptors for chemokines, glutamate and erythropoietin

Severe and debilitating neurological problems that include behavioral abnormalities, motor dysfunction and frank dementia can occur after infection with the human immunodeficiency virus-1 (HIV-1). Infected peripheral immune-competent cells, in particular macrophages, infiltrate the central nervous system (CNS) and provoke a neuropathological response involving all cell types in the brain. HIV-1 infection results in activation of chemokine receptors, inflammatory mediators, extracellular matrix-degrading enzymes and glutamate receptor-mediated excitotoxicity, all of which can trigger numerous downstream signaling pathways that result in disruption of neuronal and glial function. Despite many major improvements in the control of viral infection in the periphery, a truly effective therapy for HIV-1 associated dementia is currently not available. This review will discuss experimental and potentially future therapeutic strategies based on recently uncovered pathologic mechanisms contributing to neuronal damage induced by HIV-1.

Development of neuronal sensitivity to toxins in cerebrospinal fluid from HIV-type 1-infected individuals

HIV infection of the immature nervous system generally results in a rapid progression of neurological disease that cannot easily be explained by the severity of encephalitis, viral burden, systemic immune deficiency, or developmental changes in utero. Rather than the viral infection dictating disease progression, we explored the possibility that immature neurons might be particularly sensitive to toxins secreted in response to HIV. Primary cultures of rat cortical neurons were exposed to toxic cerebrospinal fluid (CSF) from HIV-infected individuals (CSF(tox)) and evaluated for changes in intracellular calcium and cell death. CSF(tox) had no detectable effect on early neurite outgrowth, calcium regulation, or cell death during the first few days in culture. Starting at Day 4, delayed increases in intracellular calcium appeared in response to CSF(tox). The magnitude of the delayed calcium rise and cell death increased with the age of the culture and correlated with the appearance of synaptophysin immunoreactive varicosities. A similar gradual development of sensitivity was seen during exposure of feline neurons to toxins generated by choroid plexus macrophages after exposure to feline immunodeficiency virus. The possibility that toxin sensitivity is dependent on the presence of synaptic activity is consistent with the rapid pathogenesis in the CNS seen during the first postnatal year. Emerging synaptic activity coupled with other factors such as high metabolic demand in the young nervous system may combine to increase the likelihood of calcium overload and neuronal dysfunction in response to HIV-associated toxins.

Neuroprotective strategies for HIV-1 associated dementia

The human immunodeficiency virus-1 (HIV-1) commonly affects cognitive, behavioral and motor functions during the disease course. The neuropathogenesis of viral infection revolves around neurotoxins produced from infected and immune-activated mononuclear phagocytes (MP; perivascular macrophages and microglia). Direct infection of neurons occurs rarely, if at all. Neurologic disease arises in part as a consequence of MP metabolic dysfunction. Although the advent of highly active antiretroviral therapy (HAART) has attenuated the incidence and severity of neurologic disease, it, nonetheless, remains a common and disabling problem for those living with HIV-1 infection. Adjunctive therapies are currently designed to ameliorate clinical outcomes and are included in the therapeutic armamentarium. Anti-inflammatory drugs that inhibit cytokines, chemokines and interferons linked to neurodegenerative processes can significantly ameliorate neuronal function. HIV-1 neurotoxins have the unique ability to up-regulate glycogen synthase kinase-3beta (GSK-3beta) activity that in turn elicits neuronal apoptosis. GSK-3beta inhibitors are neuroprotective in animal models of Neuro AIDS. They are also currently in Phase 1 clinical trials designed for safety and tolerability in patients with HIV-1 infection. Neurotrophins are only beginning to be realized for their therapeutic potential in HIV-1 associated neurologic disease. This review article provides a broad overview of neuroprotective strategies for HIV-1 infection and details how such strategies act and may be implemented for treatment of human disease.

Macrophage activation through CCR5- and CXCR4-mediated gp120-elicited signaling pathways

Macrophages are major targets for infection by human immunodeficiency virus type 1 (HIV-1). In addition to their role as productive viral reservoirs, inappropriate activation of infected and uninfected macrophages appears to contribute to pathogenesis. HIV-1 infection requires initial interactions between the viral envelope surface glycoprotein gp120, the cell-surface protein CD4, and a chemokine receptor CCR5 or CXCR4. Besides their role in HIV-1 entry, CCR5 and CXCR4 are G protein-coupled receptors that can activate multiple intracellular signaling pathways. HIV-1 gp120 has been shown to activate signaling pathways through the chemokine receptors in several cell types including lymphocytes, neurons, and astrocytes. In some cell types, these consequences may cause cellular injury. In this review, we highlight our data demonstrating diverse signaling events that occur in primary human macrophages in response to gp120/chemokine receptor interactions. These responses include K+, Cl-, and nonselective cation currents, intracellular Ca2+ increases, and activation of several kinases including the focal adhesion-related tyrosine kinase Pyk2, mitogen-activated protein kinases (MAPK), and phosphoinositol-3 kinase. Activation of the MAPK leads to gp120-induced expression of chemokines such as monocyte chemoattractant protein-1 and macrophage-inflammatory protein-1beta and the proinflammatory cytokine tumor necrosis factor alpha. These responses establish a complex cytokine network, which may enhance or suppress HIV-1 replication. In addition, dysregulation of macrophage function by gp120/chemokine receptor signaling may contribute to local inflammation and injury and further recruit additional inflammatory and/or target cells. Targeting these cellular signaling pathways may have benefit in controlling inflammatory sequelae of HIV infection such as in neurological disease.

Cell cycle proteins exhibit altered expression patterns in lentiviral-associated encephalitis

Cell cycle proteins regulate processes as diverse as cell division and cell death. Recently their role in neuronal death has been reported in several models of neurodegeneration. We have reported previously that two key regulators of the cell cycle, the retinoblastoma susceptibility gene product (pRb) and transcription factor E2F1, exhibit altered immunostaining patterns in simian immunodeficiency virus encephalitis (SIVE). Here we show that E2F1 and the inactivated, hyperphosphorylated form of pRb (ppRb) also exhibit altered immunostaining patterns in human immunodeficiency virus encephalitis (HIVE). Quantification of E2F1 and ppRb staining by immunofluorescent confocal microscopy confirms a significant increase in E2F1 and ppRb in both HIVE and the simian model. This increase in E2F1 and ppRb staining correlates with an increase in the presence of activated macrophages, suggesting a link between changes in cell cycle proteins and the presence of activated macrophages. Changes in ppRb and E2F1 staining in SIVE also correlate with alterations in E2F/DNA binding complexes present in the nuclear and cytoplasmic fractions from both midfrontal cortex and basal ganglia. These findings suggest that changes in cell cycle proteins occur in both HIVE and the simian model and that these changes have functional implications for gene expression in neural cells under encephalitic conditions mediated by macrophage activation or infiltration.

Macrophages relate presynaptic and postsynaptic damage in simian immunodeficiency virus encephalitis

Neurodegeneration observed in lentiviral-associated encephalitis has been linked to viral-infected and -activated central nervous system macrophages. We hypothesized that lentivirus, macrophages, or both lentivirus and macrophages within distinct microenvironments mediate synaptic damage. Using the simian immunodeficiency virus (SIV)-infected macaque model, we assessed the relationship between virus, macrophages, and neurological damage in multiple brain regions using laser confocal microscopy. In SIV-infected macaques with SIV encephalitis (SIVE), brain tissue concentrations of SIV RNA were 5 orders of magnitude greater than that observed in nonencephalitic animals. In SIVE, staining for postsynaptic protein microtubule-associated protein-2 was significantly decreased in the caudate, hippocampus, and frontal cortical gray matter compared to nonencephalitic controls, whereas staining for presynaptic protein synaptophysin was decreased in SIV-infected macaques with and without encephalitis. These data suggest that presynaptic damage occurs independent of pathological changes associated with SIVE, whereas postsynaptic damage is more tightly linked to regional presence of both activated and infected macrophages.

Destabilization of neuronal calcium homeostasis by factors secreted from choroid plexus macrophage cultures in response to feline immunodeficiency virus

The choroid plexus contains a major reservoir of macrophages poised for efficient delivery of virus and neurotoxins to the brain after infection by lentiviruses such as human or feline immunodeficiency virus (FIV). However, their contribution to neurotoxicity is poorly understood. Medium from FIV-infected, choroid plexus macrophages applied to cultured feline cortical neurons induced a small acute calcium rise followed by either a delayed calcium deregulation (41%) or swelling and bursting (23%). NMDA glutamate receptor blockade prevented the acute calcium increase and antagonists to the IP(3) receptor, voltage-gated calcium channels and sodium channels suppressed both the acute and late increases. Analysis of intracellular calcium recovery in toxin-treated neurons after a brief exposure to glutamate, revealed a decrease in the rate and extent of recovery. The apparent diverse pharmacological contributions to intracellular calcium destabilization may be due to the ability of macrophage toxins to interfere with recovery of intracellular calcium homeostasis.

Ethanol pre-exposure suppresses HIV-1 glycoprotein 120-induced neuronal degeneration by abrogating endogenous glutamate/Ca2+-mediated neurotoxicity

The neurotoxic mechanism of HIV-1 envelope glycoprotein 120 (gp120) involves glutamatergic (NMDA) receptor/Ca2+-dependent excitotoxicity, mediated in part via glia. Pro-inflammatory cytokines also may have roles. We have reported that pre-exposure of brain cultures to 'physiological' ethanol concentrations (20-30 mM) protects against neuronal damage from HIV-1 gp120, but not from the direct receptor agonist, NMDA. Using lactate dehydrogenase assays and propidium iodide staining of rat organotypic hippocampal-entorhinal cortical slice cultures we determined that ethanol's suppression of gp120 neurotoxicity required at least 4 days of pretreatment. The gp120-induced neurotoxicity was accompanied by interleukin-6 elevations that were not affected by the pretreatment. However, gp120 induced substantial, early increases in extracellular glutamate levels that were blocked by ethanol pretreatment, conceivably abrogating excitotoxicity. Consistent with abrogation of excitotoxic pathways, fura-2 imaging showed selective deficits in gp120-dependent intracellular Ca2+ responses in ethanol-pretreated slices. Gp120 is believed to increase glutamate levels by both stimulating release and inhibiting (re)uptake. Results with a labeled glutamate analog, D-[3H]aspartate, revealed that gp120's inhibition of glutamate uptake, rather than its stimulation of release, was abolished after ethanol. Further studies indicated that two converging effects of ethanol pretreatment may underlie the abolishment of gp120-mediated glutamate uptake inhibition: (a) blockade of gp120-induced release (ostensibly from glia) of arachidonic acid, an inhibitor of astroglial glutamate reuptake, and (b) modest proliferation and activation of astroglia upon gp120 stimulation--which are likely to augment glutamate transporters. Thus, as with gp120 itself, glia and glutamate/arachidonic acid regulation appear to be important targets for ethanol. Since moderate ethanol consumption is as common among HIV-infected individuals as in the general population, this newly recognized neuroprotective (and apparently anti-excitotoxic) effect of ethanol withdrawal in vitro could be important, but it requires further study before its significance, if any, is understood.

Methamphetamine and HIV-1: potential interactions and the use of the FIV/cat model

The interaction of methamphetamine with human immunodeficiency virus (HIV), the aetiologic agent of Acquired Immune Deficiency Syndrome (AIDS), has not been thoroughly investigated. However, increasingly, a larger proportion of HIV infected individuals acquire the virus through methamphetamine use or are exposed to this drug during their disease course. In certain populations, there is a convergence of methamphetamine use and HIV-1 infection; yet our understanding of the potential effects that simultaneous exposure to these two agents have on disease progression is extremely limited. Studying the interactions between methamphetamine and lentivirus in people is difficult. To thoroughly understand methamphetamine's effects on lentivirus disease progression, an animal model that is both clinically relevant and easily manipulated is essential. In this report, we identified potential problems with methamphetamine abuse in individuals with a concurrent HIV-1 infection, described the Feline Immunodeficiency Virus (FIV)/cat model for HIV-1, and reported our early findings using this modelling system to study the interaction of methamphetamine and lentivirus infections.

Response of cell cycle proteins to neurotrophic factor and chemokine stimulation in human neuroglia

Increased expression of neurotrophins (e.g., NGF, BDNF) and chemokines (e.g., RANTES) has been observed in neurodegenerative diseases. We examined the effect of these factors on intracellular signaling cascades inducing cell cycle proteins p53, pRb, and E2F1 in human fetal mixed neuronal and glial cells. Comparing neurotrophin- and chemokine-treated cultures with untreated controls showed altered subcellular localization and expression of hyperphosphorylated retinoblastoma protein (ppRb), E2F1, and p53. Using immunofluorescent laser confocal microscopy, E2F1 and ppRb were detected exclusively in neuronal nuclei in control cultures while p53 was cytoplasmic in astrocytes and nuclear in neurons. Following treatment with neurotrophins, E2F1 and ppRb were observed in the cytoplasm of neurons, while p53 was observed in both neuronal and astrocytic nuclei. Similar findings were observed following treatment with RANTES. Semiquantitative analysis using immunoblots showed an increase in the amount of phosphorylated pRb in treated cultures. Induction of cell cycle proteins may play a role in neurodegeneration associated with neurotrophin and chemokine stimulation.

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 effects of steroid hormones in HIV-related neurotoxicity: a mini review

This review examines the interaction of steroid hormones, glucocorticoids and estrogen, and gp120, a possible causal agent of acquired immune deficiency syndrome-related dementia complex. The first part of the review examines the data and mechanisms by which gp120 may cause neurotoxicity and by which these steroid hormones effect cell death in general. The second part of the review summarizes recent experiments that show how these steroid hormones can modulate the toxic effects of gp120 and glucocorticoids exacerbating toxicity, and estrogen decreasing it. We then examine the limited in vivo and clinical data relating acquired immune deficiency syndrome-related dementia complex and steroid hormones and speculate on the possible clinical significance of these findings with respect to acquired immune deficiency syndrome-related dementia complex.

Beta-chemokines and human immunodeficiency virus type-1 proteins evoke intracellular calcium increases in human microglia

Activation of beta-chemokine receptors, co-receptors for human immunodeficiency virus type-1 (HIV-1), stimulates movement and secretion in microglia, possibly through a Ca(2+)-dependent mechanism. We studied chemokine activation of Ca(2+) signaling processes in microglia. Human fetal microglia were grown in primary culture and chemokine-induced increases in intracellular calcium concentration ([Ca(2+)](i)) were measured in single cells using indo-1-based microfluorimetry. Application of 50 ng/ml regulated on activation, normal T expressed and secreted (RANTES; 120 s) evoked responses in 26% of the microglia (187/719 cells). [Ca(2+)](i) increased from a basal level of 66+/-6 nM to peak at 268+/-23 nM (n=187). Chemokine-evoked responses rapidly desensitized as indicated by the rapid return to basal [Ca(2+)](i) levels in the maintained presence of RANTES. The removal of extracellular Ca(2+) or stimulation in the presence of Ni(2+) (2mM) or La(3+) (100 microM) blocked the RANTES-elicited [Ca(2+)](i) increase. The L-type calcium channel antagonist nimodipine (10 microM) inhibited the RANTES-mediated increase in [Ca(2+)](i) by 80+/-16%. Thus, the RANTES-evoked calcium transient appears to result from Ca(2+) influx with little if any release from intracellular stores. Application of gp120(clade) (E) and gp120(CM235) (50 ng/ml) neither mimicked nor antagonized the RANTES-evoked response. Application of 50 ng/ml eotaxin (120 s) evoked an increase in [Ca(2+)](i) in 13% of the human microglia in culture (61/469 cells). The HIV-1 regulatory protein Tat (50 ng/ml) increased the [Ca(2+)](i) in a subset of eotaxin-responsive cells (16/30). The L-type calcium channel antagonist nimodipine (3 microM) inhibited eotaxin- and Tat-mediated increases in [Ca(2+)](i) by 88+/-6% and 93+/-6%, respectively. Thus, activation of CCR3 appears to evoke Ca(2+) influx through L-type Ca(2+) channels.These results indicate that beta-chemokines, RANTES and eotaxin, activate a nimodipine sensitive Ca(2+) influx pathway in human fetal microglia. HIV-1 Tat protein mimicked chemokine-mediated Ca(2+) signaling and may modulate the migratory and secretory responses of microglia.

Induction of cell-cycle regulators in simian immunodeficiency virus encephalitis

Neuronal degeneration associated with human immunodeficiency virus encephalitis has been attributed to neurotoxicity of signaling molecules secreted by activated, infected macrophages. We hypothesized that the barrage of signals present in the extracellular milieu of human immunodeficiency virus-infiltrated brain causes inappropriate activation of neuronal cell-cycle machinery. We examined the presence of three members of the cell-cycle control machinery: pRb, E2F1, and p53 in the simian immunodeficiency virus encephalitis (SIVE) model. Compared to noninfected and simian immunodeficiency virus-infected, nonencephalitic controls, we observed increased protein expression of E2F1 and p53 and aberrant cellular localization of E2F1 and pRb. In SIVE, E2F1 was abundant in the cytoplasm of neurons in both neurons and astrocytes proximal to SIVE pathology in the basal ganglia. pRb staining was nuclear and cytoplasmic in cortical neurons of SIVE cases. Antibodies to phosphorylated pRb also labeled the cytoplasm of cortical neurons. These data suggest that in SIVE, cell signaling results in phosphorylation of pRb which may result in subsequent alteration in E2F1 activity. As increased E2F1 and p53 activities have been linked to cell death, these data suggest that the neurodegeneration in SIVE could in part be because of changes in expression and activity of cell-cycle machinery.

Monocytic cells secrete factors that evoke excitatory synaptic activity in rat hippocampal cultures

Microglia- and macrophage-induced neuronal death may underlie a number of neurodegenerative diseases. The effects of factors secreted by monocytic cells were studied on glutamatergic synaptic transmission between cultured rat hippocampal neurons. Conditioned media from differentiated human U937 cells was collected after 24 h and applied to neurons (0.5%-30% dilution). Unactivated U937 cells spontaneously released factors that when applied to neuronal cultures evoked bursts of action potentials and elicited neuronal death (29+/-4%). Conditioned media collected from U937 cells evoked intracellular calcium ([Ca(2+)](i)) spiking (0.5%-2% dilution) and at higher concentrations evoked sustained increases in intracellular calcium (3%-30% dilution), as measured by indo-1-based photometry in single neurons. Activation of the U937 cells with zymosin A (500 microg/ml) enhanced the potency of the conditioned media to increase intraneuronal [Ca(2+)](i) as indicated by a leftward shift in the concentration-response curve. Selective antagonists to voltage-gated Na(+) and Ca(2+) channels and NMDA-gated channels (tetrodotoxin, nimodipine, and (+/-)-2-amino-5-phosphonopentanoic acid, respectively) blocked the calcium transients elicited by unactivated and zymosin -A-treated conditioned media. This pharmacologic profile is consistent with U937-released factors that excite the synaptic network that forms between cultured hippocampal neurons.

AIDS and the brain: is there a chemokine connection?

Many HIV-1-positive individuals suffer from a variety of neurological problems known collectively as the HIV-1-related cognitive-motor complex. However, the molecular mechanisms that underlie HIV-1-induced neuropathology are unclear. They might include a combination of indirect effects, which result from the release of neurotoxins from activated astrocytes and microglia, and the direct effects of HIV-1-related proteins, such as gp120, on neurons. As the interaction of gp120 with immune cells has been shown to require the participation of chemokine receptors, this article explores the possibility that such receptors participate in the events underlying HIV-1-induced neuropathology. It is now clear that many types of cell in the brain possess chemokine receptors, including microglia, glia and neurons, and the interaction of gp120 with neuronal chemokine receptors initiates apoptotic death of neurons in vitro. Such effects might be modified by the actions of chemokines that act at these same receptors. However, the importance of this direct interaction with neurons in vivo and its relevance in the pathogenesis of AIDS-related dementia still needs to be established. Furthermore, the existence of chemokine receptors on neurons suggests that chemokines might regulate neuronal functions physiologically.

Chemokine Receptor Expression and Signaling in Macaque and Human Fetal Neurons and Astrocytes: Implications for the Neuropathogenesis of AIDS

Chemokines are believed to play a role in the neuropathogenesis of AIDS through their recruitment of neurotoxin-secreting, virally infected leukocytes into the CNS. Levels of chemokines are elevated in brains of patients and macaques with HIV/SIV-induced encephalitis. The chemokine receptors CCR3, CCR5, and CXCR4 are found on subpopulations of neurons in the cortex of human and macaque brain. We have developed an in vitro system using both macaque and human fetal neurons and astrocytes to further investigate the roles of these receptors in neuronal response to inflammation. Here we report the presence of functional HIV/SIV coreceptors CCR3, CCR5, and CXCR4 on fetal human and macaque neurons and CCR5 and CXCR4 on astrocytes immediately ex vivo and after several weeks in culture. Confocal imaging of immunostained neurons demonstrated different patterns of distribution for these receptors, which may have functional implications. Chemokine receptors were shown to respond to their appropriate chemokine ligands with increases in intracellular calcium that, in the case of neurons, required predepolarization with KCl. These responses were blocked by neutralizing chemokine receptor in mAbs. Pretreatment of neural cells with pertussis toxin abolished responses to stromal-derived factor-1α, macrophage inflammatory protein-1β, and RANTES, indicating coupling of CCR5 and CXCR4 to a Giα protein, as in leukocytes. Cultured macaque neurons demonstrated calcium flux response to treatment with recombinant SIVmac239 envelope protein, suggesting a mechanism by which viral envelope could affect neuronal function in SIV infection. The presence of functional chemokine receptors on neurons and astrocytes suggests that chemokines could serve to link inflammatory and neuronal responses.

Mobilization of intracellular calcium stores participates in the rise of [Ca2+]i and the toxic actions of the HIV coat protein GP120

The HIV envelope glycoprotein, GP120, increases intracellular Ca2+ concentration and induces degeneration of human and animal neurons in culture. Using patch-clamp recordings and Ca2+ imaging techniques, we have now examined the contribution of intracellular stores of calcium in the effects of GP120. We report that in rat hippocampal neuronal cultures, GP120 induces a dramatic and persistent increase in [Ca2+]i which is prevented by drugs that either deplete (caffeine, carbachol, thapsigargin) or block (dantrolene) Ca2+ release from intracellular stores. In contrast, N-methyl-d-aspartate (NMDA) receptors or voltage-dependent calcium channels do not participate in these effects, as: (i) the increase in [Ca2+]i was not affected by NMDA receptor antagonists or calcium channel blockers; and (ii) and GP120 did not generate any current in whole-cell recording. Dantrolene, a ryanodine stores inhibitor, also prevented neuronal death induced by GP120. Our results show that the GP120-induced rise in [Ca2+]i originates from intracellular calcium stores, and suggest that intracellular stores of calcium may play a determinant role in the pathological actions of GP120.

Neurotoxicity of FIV and FIV envelope protein in feline cortical cultures

The neurotoxic effects of the feline immunodeficiency virus (FIV) and FIV envelope proteins were measured in primary cultures of feline cortical neurons. Envelope protein from the FIV-PPR strain promoted neuronal swelling and death, whereas envelope protein from the FIV-34TF10 isolate produced intermediate or negligible toxicity. No effect was observed in control cultures treated with envelope protein from the Epstein-Barr virus. A concentration-effect curve showed that FIV-PPR protein produced maximal toxicity at 200 pM protein and decreased toxicity at higher concentrations, which is consistent with previous reports of the HIV-1 surface glycoprotein, gp120. These effects required the presence of low concentrations of glutamate. Using the natural host cells as targets, the effects of envelope protein and infectious virions were directly compared. All of the toxic activity could be attributed to non-infectious interactions between the viral envelope and target cells. Addition of 1 microM tetrodotoxin failed to block the effects of FIV-PPR in the presence of 20 microM glutamate. Toxicity would appear to involve two steps in which the envelope protein first sensitizes neurons through non-synaptic interactions (TTX insensitive) thereby setting the stage for enhanced synaptic activation via glutamate receptors (TTX sensitive).

Neuronal injury associated with HIV-1: approaches to treatment

Mounting evidence suggests that cognitive dysfunction developing as a result of HIV-1 infection is mediated at least in part by generation of excitotoxins and free radicals in the brain. This syndrome is currently designated HIV-1-associated cognitive/motor complex, was originally termed the AIDS Dementia Complex, and for simplicity, is called AIDS dementia in this review. Recently, brains of patients with AIDS have been shown to manifest neuronal injury and apoptotic-like cell death. How can HIV-1 result in neuronal damage if neurons themselves are only rarely, if ever, infected by the virus? Experiments from several different laboratories have lent support to the existence of HIV- and immune-related toxins in a variety of in vitro and in vivo paradigms. In one recently defined pathway to neuronal injury, HIV-infected macrophages and microglia, or immune-activated macrophages and astrocytes (activated by the shed HIV-1 envelope protein, gp120, or other viral proteins and cytokines), appear to secrete excitants and neurotoxins. These substances may include arachidonic acid, platelet-activating factor, free radicals (NO. and O2.-), glutamate, quinolinate, cysteine, amines, and as yet unidentified factors emanating from stimulated macrophages and reactive astrocytes. A final common pathway for neuronal susceptibility is operative, similar to that observed in stroke and several neurodegenerative diseases. This mechanism involves excessive activation of N-methyl-D-aspartate (NMDA) receptor-operated channels, with resultant excessive influx of Ca2+ and the generation of free radicals, leading to neuronal damage. With the very recent development of clinically tolerated NMDA antagonists, there is hope for future pharmacological intervention.

Do endogenous cannabinoids contribute to HIV-mediated immune failure?

The failure of the immune system to mount a successful attack on the human immunodeficiency virus (HIV) is an old enigma for AIDS research. The high mutational capacity of HIV, which unremittingly confuses the immune system, is a major factor in immune failure. But this alone cannot fully explain the certain and inescapable failure of the immune system, leading to full-blown AIDS. Here, we propose the hypothesis that endogenous cannabinoids, derived mostly from macrophages, might participate in the general failure of the immune system in HIV-infected individuals.

Intracellular signalling mediating HIV-1 gp120 neurotoxicity

During the last few years several studies have been undertaken to characterise the role of gp120, the HIV-1 envelope glycoprotein, in the pathogenesis of neurological defects associated with AIDS. However, neurons did not appear to be the main target of the virus, since the widespread neuronal damage is not associated with a productive viral infection in neurons. The current opinion supports the hypothesis that an indirect mechanism exists to explain the neuronal cell death which occurs in patients infected by HIV-1. In particular, several reports suggest that gp120 may be the main candidate as mediator of the neurological deficits during HIV-1 infection and demonstrate that this molecule affects neuronal survival through a direct interaction with non-neuronal cell types such as monocytes, macrophages/microglia and astrocytes.

Mouse hepatitis virus infection induces an early, transient calcium influx in mouse astrocytoma cells

Mouse hepatitis virus (MHV), a murine coronavirus, utilizes murine carcinoembryonic antigens as receptors. The events that follow virus-receptor binding and eventually lead to virus entry are poorly understood. We studied the possible effects of MHV infection on intracellular calcium in a mouse astrocytoma cell line. Using the calcium-sensitive dye fluo-3 and confocal laser scanning microscopy, we found that MHV strain JHM induced an immediate (within 20 s) and transient (lasting no longer than 2 min) calcium increase in about 5% of the infected cells. The calcium increase was blocked by antibodies against the viral spike protein, suggesting that it was specifically triggered by the interaction of the viral spikes with cells. It was also inhibited by L-type calcium channel blockers and was not detected in calcium-free medium, suggesting that the calcium increase was caused by calcium influx from the extracellular medium. Studies of the kinetics of viral replication by immunofluorescence staining of the viral nucleocapsid protein revealed that at 3 h postinfection there was roughly the same percentage of cells (5%) that produced the viral protein as the percentage of cells that had responded with a calcium signal. This finding and the virus dilution studies together suggest that calcium responders may represent cells that had been infected with multiple viruses and undergone rapid viral replication. Furthermore, calcium channel blockers, including verapamil and cadmium chloride, and the calcium chelator EGTA inhibited virus infection. Therefore, the transient intracellular calcium increase reported here may be an early signaling event associated with virus infection.

Molecular Mimicry Between the Rabies Virus Glycoprotein and Human Immunodeficiency Virus-1 GP120: Cross-Reacting Antibodies Induced by Rabies Vaccination

The 160-170 sequence of human immunodeficiency virus (HIV)-1 gp120 mimics a nicotinic receptor-binding motif of rabies virus glycoprotein and snake neurotoxins. This sequence has been proposed to be involved in the binding of HIV-1 gp120 to the acetylcholine binding sites of nicotinic receptors. By using biomolecular interaction analysis (BIA) technology we have found that HIV-1 gp120 can bind to detergent-extracted nicotinic receptor from fetal calf muscle. The binding is inhibited by nicotine and by a synthetic peptide reproducing the gp120 160-170 sequence. The molecular mimicry between gp120 and rabies virus glycoprotein is confirmed by cross-reacting antibodies. We have found that vaccination against rabies can induce the production of anti–HIV-1 gp120 antibodies in humans. The cross-reacting antibodies are directed to the gp120 sequence involved in the mimicry with the rabies virus glycoprotein. The cross-reactivity between the rabies virus and HIV-1 has important implications in transfusion medicine. Moreover, the presence of cross-reacting antibodies between the nicotinic receptor binding site of rabies virus glycoprotein and a fragment of HIV-1 gp120 strengthens the hypothesis about the possible role of nicotinic receptors as potential receptors for HIV-1 in the central nervous system.

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.

Endocrine modulation of the neurotoxicity of gp120: implications for AIDS-related dementia complex

HIV infection often involves the development of AIDS-related dementia complex, a variety of neurologic, neuropsychologic, and neuropathologic impairments. A possible contributor to AIDS-related dementia complex is the HIV envelope glycoprotein gp120, which damages neurons via a complex glutamate receptor- and calcium-dependent cascade. We demonstrate an endocrine modulation of the deleterious effects of gp120 in primary hippocampal and cortical cultures. Specifically, we observe that gp120-induced calcium mobilization and neurotoxicity are exacerbated by glucocorticoids, the adrenal steroids secreted during stress. Importantly, this deleterious synergy can occur between gp120 and synthetic glucocorticoids (such as prednisone or dexamethasone) that are used clinically in high concentrations to treat severe cases of the Pneumocystis carinii pneumonia typical of HIV infection. Conversely, we also observe that estradiol protects neurons from the deleterious actions of gp120, reducing toxicity and calcium mobilization.

Immunologically induced electrophysiological dysfunction: implications for inflammatory diseases of the CNS and PNS

During inflammation of the central or peripheral nervous system, a high number of immunologically active molecules, including bacterial or viral products as well as host-derived cytokines, are released. Patients suffering from inflammatory CNS or PNS diseases often develop transient symptoms with a rapid recovery, which obviously cannot be accounted for by immunologically induced tissue damage. These observations led to the hypothesis that immunologically active molecules can affect directly the electrophysiological functions of neurons and glial cells. Evidence for this hypothesis came from in vitro studies showing that cytokines, such as interleukins or tumor necrosis factors, arachidonic acid and its metabolites, interfere with electrophysiological properties of neurons or glial cells. These molecules affect ion currents, intracellular Ca2+ homeostasis, membrane potentials, and suppress or enhance the induction and maintenance of long-term potentiation. Similarly, virus proteins from human immunodeficiency virus type I were found to alter intracellular Ca2+ concentrations of neurons and astrocytes by modulating either transmitter receptors and channels or membrane transporters. Cerebrospinal fluid from MS patients contains factors which increase Na+ current inactivation and thereby reduce neuronal excitability. Immunoglobulins in sera of patients suffering from multifocal motor neuropathy and from acquired neuromyotonia interfere with nerve fibers, inducing alterations of conduction. Increased knowledge of these mechanisms will help to explain the pathogenesis of neurological symptoms and may provide a rationale for new therapeutic strategies.

Second messengers induced by the envelope protein of a retrovirus

The envelope protein (gp52) of the mouse mammary tumor virus (MMTV) can stimulate RNA synthesis via binding to its cellular receptor on mammary epithelium. This effect was mimicked by either nitric oxide (NO) or 8-bromo-cGMP and was blocked by an NO inhibitor. Furthermore, the effects of gp52 and 8-bromo-cGMP were not additive at maximal concentrations, suggesting that they were using the same signaling route. Finally, gp52 elevated cGMP levels in mammary epithelium. These data suggest that gp52 activates the following transduction pathway in this tissue: gp52-->NO synthase-->NO-->soluble guanylate cyclase cGMP RNA synthesis. In contrast to the mammary gland, gp52 inhibited RNA synthesis in the diaphragm. However, the effect was again mimicked by NO, blocked by an NO inhibitor, and the effects of gp52 and NO were not additive. Therefore, it appears that gp52 is using the NO-cGMP pathway in both tissues, but that muscle tissue may be more susceptible to the toxic effects of NO.

FeLV envelope protein (gp70) variable region 5 causes alterations in calcium homeostasis and toxicity of neurons

In humans and animals, retroviruses have been implicated in nervous system disease. Our objective was to characterize the neurotoxicity of a peptide sequence derived from an animal retrovirus, the feline leukemia virus (FeLV). Using a peptide sequence from the subtype FeLV-C envelope protein variable region 5 (VR5), cytotoxicity was demonstrated in studies that evaluated neuronal survival, neurite outgrowth, and alterations in intracellular calcium ion concentration. The FeLV subtype isolate FeLV-CSarma possesses an envelope protein VR5 amino acid sequence that varies by four amino acids from the VR5 amino acid sequence of subtype FeLV-AGlasgow. The polypeptide representing the VR5 of FeLV-CSarma (FeLV-CVR5) is significantly more neurotoxic than the polypeptide sequence representing the VR5 of FeLV-AGlasgow (FeLV-AVR5). FeLV-CVR5 (> or = 3 microM) exposure resulted in significant dose-dependent neurotoxicity. Antibodies to FeLV-CVR5 blocked this effect. Neurite outgrowth was significantly reduced at all tested concentrations (3-12 microM) of FeLV-CVR5, with a 92% reduction in neurite length at 12 microM. FeLV-AVR5 was significantly less neurotoxic with respect to neurite outgrowth than was FeLV-CVR5. The significant reduction in neurotoxicity for FeLV-AVR5 illustrates the importance of the 4-amino-acid difference between it and FeLV-CVR5. Alterations in intracellular calcium ion concentration were associated with this neurotoxicity.

gp120, a human immunodeficiency virus-1 coat protein, augments excitotoxic hippocampal injury in perinatal rats

Recent data suggest that gp120, a human immunodeficiency virus-1 (HIV-1) coat glycoprotein that is secreted by HIV-infected cells, is neurotoxic, and that this toxicity is mediated, at least in part, by activation of N-methyl-D-aspartate-type excitatory amino acid receptors. To test this hypothesis in vivo, we examined the neurotoxicity of gp120 injected intrahippocampally, alone or co-injected with the selective excitatory amino acid agonist N-methyl-D-aspartate, in seven-day-old rats. Severity of injury in the lesioned hippocampus was assessed five days later, using three outcome measures: histopathology, hippocampal atrophy (derived from regional cross-sectional area measurements) and loss of [3H]glutamate receptor binding (based on in vitro autoradiography assays). To confirm that any observed effects were attributable to gp120 bioactivity, each group of experiments included controls that received equal amounts of heat-treated gp120. Gp120 (200 ng) elicited minimal focal pyramidal cell loss immediately adjacent to the injection track; there was no hippocampal atrophy or loss of [3H]glutamate binding. Co-injection of 50 ng gp120 with N-methyl-D-aspartate (5 nmol, threshold excitotoxic dose) increased the severity of hippocampal injury; hippocampal atrophy was greater in animals that received injections of 5 nmol N-methyl-D-aspartate in combination with 50 ng gp120 than in those that received either N-methyl-D-aspartate alone (5 nmol) or 5 nmol N-methyl-D-aspartate+50 ng heat-treated gp120 (mean+/-S.E.M. percentage reduction in injected hippocampal volume vs contralateral: N-methyl-D-aspartate, -19+/-3; N-methyl-D-aspartate+gp120, -26.8+/-2.1; N-methyl-D-aspartate+heat-treated gp120, -14.0+/-2.2; P<0.001, ANOVA). Treatment with the competitive N-methyl-D-aspartate antagonist 3-((RS)-2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid (20mg/kg) markedly reduced the severity of injury elicited by the combination of gp120 with N-methyl-D-aspartate. These data support the hypothesis that locally secreted gp120 could exert neurotoxic effects, mediated by N-methyl-D-aspartate receptor activation, in vivo in the immature brain.

HIV-1 gp120 Effects on Signal Transduction Processes and Cytokines: Increased src-Family Protein Tyrosine Kinase Activity

Varying degrees of neurological dysfunction are observed in AIDS patients who develop AIDS dementia complex (ADC). Data from a large number of in vivo and in vitro rodent studies have suggested a role for the HIV envelope glycoprotein gp 120 in this process. These studies were initiated to clarify possible effects of recombinant gp120 on signal transduction systems and the synthesis of specific ADC-related cytokines in human neuroblastoma cells. Out results indicate that gp120 on signal transduction systems and the synthesis of specific ADC-related cytokines in human neuroblastoma cells. Our results indicate that gp120 did not induce the synthesis of cAMP, IPs or NO, nor did it alter agonist-induced synthesis of these molecules. In addition, it did not induce the synthesis of IL-6 and TNF&#945;. However, it did activate a src-family protein tyrosine kinase which phosphorylates several substrates, including prominent proteins in the 115 and 60 kDa range. This gp120-induced tyrosine phosphorylation may contribute to neurological dysfunction since protein tyrosine kinases are known to be involved in processes important for pre- and post-synaptic neuronal function.

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.

HIV-1 gp120-induced neurotoxicity to midbrain dopamine cultures

HIV-1-associated cognitive/motor dysfunction is a frequent neurological complication of acquired immunodeficiency syndrome (AIDS) and has been termed AIDS dementia complex (ADC). The HIV-1 envelope glycoprotein gp120 has been implicated in producing brain injury associated with ADC. The purpose of the present study was to determine if gp120-induced neurotoxicity is associated with damage to dopaminergic systems. Exposure of rat midbrain dopamine cultures to gp120 for 3 days reduced the ability of dopaminergic cells to transport this amine and also resulted in a reduction in dopamine neuron process length while it did not alter either dopamine cell number or the total number of neuronal cells. These detrimental effects of gp120 were prevented by an NMDA receptor antagonist (MK-801) or by preincubation with anti-gp120 antibody. These results suggest that dopaminergic neuronal damage may contribute to the manifestations of AIDS dementia complex.

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 current 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 microM 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 immunofluorescence and colorimetric assay. Treatment of cultures with AP5 or nifedipine reduced gp120-induced toxicity by 70 and

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 D28K 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 D2K28-containing neurons are more resistant to gp120-toxicity in this cell culture system.

Intracellular calcium release induced by human immunodeficiency virus type 1 (HIV-1) surface envelope glycoprotein in human intestinal epithelial cells: a putative mechanism for HIV-1 enteropathy

Intracellular Ca2+ ([Ca2+]i) was measured in single human epithelial intestinal HT-29-D4 cells with the Ca2+ probe Fura-2 and digital imaging microscopy. Treatment of these cells with HIV-1 surface envelope glycoprotein gp120 (or a soluble form of its precursor gp160) induced an important increase of [Ca2+]i. This effect was abolished by preincubation of the viral glycoprotein with neutralizing antibodies specific for the V3 domain of gp120. These antibodies inhibited the binding of both gp120 and gp160 to galactosylceramide (GalCer), the alternative HIV-1 receptor in HT-29-D4 cells. Moreover, treatment of HT-29-D4 cells with an anti-GalCer mAb induced an increase in [Ca2+]i and rendered the cells insensitive to HIV-1 glycoprotein stimulation. The calcium response resulted from release of Ca2+ from caffeine-sensitive intracellular stores. Finally, the viral glycoprotein specifically abrogated the calcium response to the neuropeptide agonist neurotensin, a stimulator of chloride secretion via inositol trisphosphate-mediated calcium mobilization. Reciprocally, after neurotensin stimulation, the cells did not respond to gp120, showing that neurotensin and gp120 stimulate a common pathway of [Ca2+]i mobilization. These results suggest that HIV-1 may directly alter ion secretion in the intestine and thus be the causative agent of the watery diarrhea associated with HIV-1 infection.

HIV-1 gp120 glycoprotein induces [Ca2+]i responses not only in type-2 but also type-1 astrocytes and oligodendrocytes of the rat cerebellum

Cultures of cerebellar cortex cells were exposed to the HIV-1 envelope glycoprotein, gp120, and investigated for cytosolic Ca2+ ion concentration ([Ca2+]i) changes by the fura-2 ratio videoimaging technique while bathed in complete, Na(+)-free or Mg(2+)-free Krebs-Ringer media. At the end of the [Ca2+]i experiments the cells were fixed and immunoidentified through the revelation of markers specific for neurons (microtubule associated protein-2), type-2 (A2B5) or all (glial fibrillary acidic protein) astrocytes, oligodendrocytes (galactocerebroside) or microglia (F4/80 antibody). In complete medium, rapid biphasic (spike-plateau) responses induced by gp120 (0.1-1 nM) were observed in a subpopulation of type-2 astrocytes. In addition, slow but progressive responses were observed in other type-2 cells and oligodendrocytes, whereas type-1 astrocytes showed small responses, if any, and granule neurons did not respond at all. Use of Na(+)-free medium (a condition that blocked another gp120-induced response, cytosolic alkalinization) resulted in an increase in [Ca2+]i response that was appreciable not only in type-2 but also in most type-1 astrocytes, possibly because of the inhibition of the Na+/Ca2+ exchanger and the ensuing decrease in Ca2+ extrusion. Granule neurons, including those in direct contact with responsive astrocytes, remained unresponsive, even when the experiments were carried out in Mg(2+)-free medium supplemented with glycine, a condition that favors activation of the glutamatergic N-methyl-D-aspartate (NMDA) receptor.(ABSTRACT TRUNCATED AT 250 WORDS)

Inhibition of constitutive nitric oxide synthase in the brain by pentamidine, a calmodulin antagonist

Nitric oxide (NO) which is produced by activation of Ca2+/calmodulin-dependent NO synthase is known to induce neuronal damage. We examined the effects of 3'-azido-2',3'-dideoxythymidine (AZT, a reverse transcriptase inhibitor), pentamidine (a therapeutic drug for Pneumocystis carinii pneumonia) and calmodulin antagonists such as trifluoperazine and N-(6-aminohexyl)-5-chloro-1-naphthalenesulfonamide (W-7) on NO synthase activation. Although AZT had no effect on the activity of constitutive neuronal NO synthase, pentamidine inhibited the activation of neuronal NO synthase as did trifluoperazine and W-7. The inhibition by pentamidine was prevented by the addition of purified calmodulin. In addition, pentamidine inhibited calmodulin-dependent activation of neuronal NO synthase purified from rat cerebellum. From these results, it is suggested that pentamidine inhibits the neuronal NO synthase activation by probably acting as a calmodulin antagonist.

HIV-1 coat protein gp120-induced increases in levels of intrasynaptosomal calcium

The effects of human immunodeficiency virus type-1 (HIV-1) coat protein gp120 on levels of intrasynaptosomal calcium ([Ca2+]i) were determined in rat cortical synaptosomes. gp120 at concentrations of > or = 400 pM, significantly (P < 0.05) increased levels of [Ca2+]i. Treatment with 20 mM KCl, reduced the concentrations of gp120 necessary to produce significant (P < 0.001) increases in [Ca2+]i. gp120-evoked increases in [Ca2+]i were prevented either by treatment with dantrolene or by removal of extracellular calcium with BAPTA. The peak levels of gp120-induced increases in [Ca2+]i were not affected by calcium channel blockers lanthanum and nicardipine, by glutamate receptor antagonists MK-801 and NBQX, or by removal of endogenous glutamate with glutamate dehydrogenase. gp120-induced [Ca2+]i increases in presynaptic terminals may play a role in HIV-mediated effects in the central nervous system.