Activation of NR1a/NR2B receptors by monocyte-derived macrophage secretory products: implications for human immunodeficiency virus type one-associated dementia

HIV-1 Glycoprotein 120 Enhancement of N-Methyl-D-Aspartate NMDA Receptor-Mediated Excitatory Postsynaptic Currents: Implications for HIV-1-Associated Neural Injury

It is widely accepted that human immunodeficiency virus type 1 (HIV-1) envelope glycoprotein 120 (gp120) plays an important role in HIV-1-induced neural injury and pathogenesis of HIV-1-associated dementia (HAND). Multiple pathways have been proposed for gp120-induced neurotoxicity, amongst is the activation of N-Methyl-D-Aspartate receptors (NMDARs). It has been shown that gp120 causes neuronal injury or death and gp120 transgenic mice exhibit neurological similarity to that of HAND, all of which can be blocked or attenuated by NMDAR antagonists. Several lines of evidence indicate the subtype and location of activated NMDARs are key determinants of the nature of NMDAR physiology. To examine the subtype and the location of NMDARs affected by gp120, we studied gp120 on subtype NMDAR-mediated EPSCs in the CA1 region of rat hippocampal slices through "blind" whole-cell patch recordings. Our results showed bath application of gp120 increased both NR2A- and NR2B-mediated EPSCs possibly via a presynaptic mechanism, with much stronger effect on NR2B-mediated EPSCs. In contrast, gp120 failed on enhancing AMPA receptor-mediated EPSCs. Ca²⁺ imaging studies revealed that gp120 potentiated glutamate-induced increase of intracellular Ca²⁺ concentration in rat hippocampal neuronal cultures which were blocked by a NMDAR antagonist, but not by an AMPA receptor antagonist, indicating gp120 induces Ca²⁺ influx through NMDARs. Further investigations demonstrated that gp120 increased the EPSCs mediated by extrasynaptic NR2BRs. Taken together, these results demonstrate that gp120 interacts with both NR2A and NR2B subtypes of NMDARs with a predominant action on the extrasynaptic NR2B, implicating a role NR2B may play in HIV-1-associated neuropathology.

HIV-1 transgenic rats display mitochondrial abnormalities consistent with abnormal energy generation and distribution

With the advent of the combination antiretroviral therapy era (cART), the development of AIDS has been largely limited in the USA. Unfortunately, despite the development of efficacious treatments, HIV-1-associated neurocognitive disorders (HAND) can still develop, and as many HIV-1 positive individuals age, the prevalence of HAND is likely to rise because HAND manifests in the brain with very low levels of virus. However, the mechanism producing this viral disorder is still debated. Interestingly, HIV-1 infection exposes neurons to proteins including Tat, Nef, and Vpr which can drastically alter mitochondrial properties. Mitochondrial dysfunction has been posited to be a cornerstone of the development of numerous neurodegenerative diseases. Therefore, we investigated mitochondria in an animal model of HAND. Using an HIV-1 transgenic rat model expressing seven of the nine HIV-1 viral proteins, mitochondrial functional and proteomic analysis were performed on a subset of mitochondria that are particularly sensitive to cellular changes, the neuronal synaptic mitochondria. Quantitative mass spectroscopic studies followed by statistical analysis revealed extensive proteome alteration in this model paralleling mitochondrial abnormalities identified in HIV-1 animal models and HIV-1-infected humans. Novel mitochondrial protein changes were discovered in the electron transport chain (ETC), the glycolytic pathways, mitochondrial trafficking proteins, and proteins involved in various energy pathways, and these findings correlated well with the function of the mitochondria as assessed by a mitochondrial coupling and flux assay. By targeting these proteins and proteins upstream in the same pathway, we may be able to limit the development of HAND.

Enhancement of NMDA receptor-mediated excitatory postsynaptic currents by gp120-treated macrophages: implications for HIV-1-associated neuropathology

A plethora of prior studies has linked HIV-1-infected and immune activated brain mononuclear phagocytes (MP; blood borne macrophages and microglia) to neuronal dysfunction. These are modulated by N-methyl-D-aspartate receptor (NMDAR) antagonists and supporting their relevance for HIV-1-associated nervous system disease. The role of NMDAR subsets in HIV-1-induced neuronal injury, nonetheless, is poorly understood. To this end, we investigated conditioned media from HIV-1gp120-treated human monocyte-derived-macrophages (MDM) for its abilities to affect NMDAR-mediated excitatory postsynaptic currents (EPSC(NMDAR)) in rat hippocampal slices. Bath application of gp120-treated MDM-conditioned media (MCM) produced an increase of EPSC(NMDAR). In contrast, control (untreated) MCM had limited effects on EPSC(NMDAR). Testing NR2A NMDAR (NR2AR)-mediated EPSC (EPSC(NR2AR)) and NR2B NMDAR (NR2BR)-mediated EPSC (EPSC(NR2BR)) for MCM showed significant increased EPSC(NR2BR) when compared to EPSC(NR2AR) enhancement. When synaptic NR2AR-mediated EPSC was blocked by bath application of MK801 combined with low frequency stimulations, MCM retained its ability to enhance EPSC(NMDAR) evoked by stronger stimulations. This suggested that increase in EPSC(NMDAR) was mediated, in part, through extra-synaptic NR2BR. Further analyses revealed that the soluble factors with low (<3 kD) to medium (3-10 kD) molecular weight mediated the observed increases in EPSC(NMDAR). The link between activation of NR2BRs and HIV-1gp120 MCM for neuronal injury was demonstrated by NR2BR but not NR2AR blockers. Taken together, these results indicate that macrophage secretory products induce neuronal injury through extra-synaptic NR2BRs.

Cocaine and HIV-1 interplay: molecular mechanisms of action and addiction

Human immunodeficiency virus (HIV) infection is now being driven by drug-abusing populations. Epidemiological studies on drug abusers with AIDS link abuse of cocaine, even more than other drugs, to increased incidence of HIV seroprevalence and progression to AIDS. Both cell culture and animal studies demonstrate that cocaine can both potentiate HIV replication and can potentiate HIV proteins to cause enhanced glial cell activation, neurotoxicity, and breakdown of the blood-brain barrier. Based on the ability of both HIV proteins and cocaine to modulate NMDA receptor on neurons, NMDA receptors have been suggested as a common link underlying the crosstalk between drug addiction and HIV infection. While the role of dopamine system as a major target of cocaine cannot be overlooked, recent studies on the role of sigma receptors in mediating the effects of cocaine in both cell and organ systems warrants a deeper understanding of their functional role in the field. In this review, recent findings on the interplay of HIV infection and cocaine abuse and their possible implications in mode of action and/or addiction will be discussed.

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.

The glutamate-glutamine cycle as an inducible, protective face of macrophage activation

Neuronal damage in HIV infection results mainly from chronic activation of brain tissue and involves inflammation, oxidative stress, and glutamate-related neurotoxicity. Glutamate toxicity acts via two distinct pathways: an excitotoxic one, in which glutamate receptors are hyperactivated, and an oxidative one, in which cystine uptake is inhibited, resulting in glutathione depletion, oxidative stress, and cell degeneration. A number of studies have shown that astrocytes normally take up glutamate, keeping extracellular glutamate concentration low in the brain and preventing excitotoxicity. They, in turn, provide the trophic amino acid glutamine via their expression of glutamine synthetase. These protective and trophic actions are inhibited in HIV infection, probably as a result of the effects of inflammatory mediators and viral proteins. In vitro and in vivo studies have demonstrated that activated microglia and brain macrophages (AMM) express the transporters and enzymes of the glutamate cycle. This suggests that in addition to their recognized neurotoxic properties in HIV infection, these cells exhibit some neuroprotective properties, which may partly compensate for the inhibited astrocytic function. This hypothesis might explain the discrepancy between microglial activation, which occurs early in the disease, and neuronal apoptosis and neuronal loss, which are late events. In this review, we discuss the possible neuroprotective and neurotrophic roles of AMM and their relationships with inflammation and oxidative stress.

Human immunodeficiency virus (HIV)-induced neurotoxicity: roles for the NMDA receptor subtypes

Neuronal damage in human immunodeficiency virus type 1 (HIV-1) infection in the brain is thought to occur at least in part through NMDA receptor (NMDAR) excitation initiated by soluble neurotoxins from HIV-infected brain macrophages. Furthermore, brain regions enriched in NMDAR-2A (NR2A) and NMDAR-2B (NR2B) subunits, such as the hippocampus, are particularly vulnerable. Using cultured rat hippocampal cells and HIV-1-infected human monocyte-derived macrophages (HIV/MDM), we examined the role of NR2A and NR2B in HIV/MDM-induced hippocampal neuronal death. We used the primary HIV-1 strain Jago derived from the CSF of an individual with HIV-associated dementia and that robustly replicates in MDM. We found the following: (1) hippocampal neuronal susceptibility to HIV/MDM excitotoxins varies according to the developmental expression patterns of NR2A and NR2B; (2) NMDAR activation by HIV/MDM results in neuronal calpain activation, which results in neuronal death; and (3) selective antagonists of homomeric NR2B/NR2B- and heteromeric NR2A/NR2B-containing NMDARs, as well as an inhibitor of calpain activity, afford neuroprotection against HIV/MDM. These studies establish a clear link between macrophage HIV infection, neuronal NR2A and NR2B activation, and calpain-mediated hippocampal neuronal death. They further suggest a dominant role for NR2A and NR2B in determining neuronal susceptibility in HIV-infected brain. Antagonists of NR2A and NR2B subunits as well as inhibitors of calpain activation offer attractive neuroprotective approaches against HIV in both developing and mature brain.

Mononuclear phagocytes in the pathogenesis of neurodegenerative diseases

Brain mononuclear phagocytes (MP, bone marrow monocyte-derived macrophages, perivascular macrophages, and microglia) function to protect the nervous system by acting as debris scavengers, killers of microbial pathogens, and regulators of immune responses. MP are activated by a variety of environmental cues and such inflammatory responses elicit cell injury and death in the nervous system. MP immunoregulatory responses include secretion of neurotoxic factors, mobilization of adaptive immunity, and cell chemotaxis. This incites tissue remodelling and blood-brain barrier dysfunction. As disease progresses, MP secretions engage neighboring cells in a vicious cycle of autocrine and paracrine amplification of inflammation leading to tissue injury and ultimately destruction. Such pathogenic processes tilt the balance between the relative production of neurotrophic and neurotoxic factors and to disease progression. The ultimate effects that brain MP play in disease revolves "principally" around their roles in neurodegeneration. Importantly, common functions of brain MP in neuroimmunity link highly divergent diseases (for example, human immunodeficiency virus type-one associated dementia, Alzheimer's disease and Parkinson's disease). Research into this process from our own laboratories and those of others seek to harness MP inflammatory processes with the intent of developing therapeutic interventions that block neurodegenerative processes and improve the quality of life in affected people.

Dopamine, through the extracellular signal-regulated kinase pathway, downregulates CD4+CD25+ regulatory T-cell activity: implications for neurodegeneration

Fighting off neuronal degeneration requires a well controlled T-cell response against self-antigens residing in sites of the CNS damage. The ability to evoke this response is normally suppressed by naturally occurring CD4+CD25+ regulatory T-cells (Treg). No physiological compound that controls Treg activity has yet been identified. Here, we show that dopamine, acting via type 1 dopamine receptors (found here to be preferentially expressed by Treg), reduces the suppressive activity and the adhesive and migratory abilities of Treg. Treg activity was correlated with activation of the ERK1/2 (extracellular signal-regulated kinase 1/2) signaling pathway. Systemic injection of dopamine or an agonist of its type 1 receptors significantly enhanced, via a T-cell-dependent mechanism, protection against neuronal death after CNS mechanical and biochemical injury. These findings shed light on the physiological mechanisms controlling Treg and might open the way to novel therapeutic strategies for downregulating Treg activity (e.g., in neuronal degeneration) or for strengthening it (in autoimmune diseases).

Involvement of quinolinic acid in AIDS dementia complex

Human immunodeficiency virus (HIV) infection is often complicated by the development of acquired immunodeficiency syndrome (AIDS) dementia complex (ADC). Quinolinic acid (QUIN) is an end product of tryptophan, metabolized through the kynurenine pathway (KP) that can act as an endogenous brain excitotoxin when produced and released by activated macrophages/microglia, the very cells that are prominent in the pathogenesis of ADC. This review examines QUIN's involvement in the features of ADC and its role in pathogenesis. We then synthesize these findings into a hypothetical model for the role played by QUIN in ADC, and discuss the implications of this model for ADC and other inflammatory brain diseases.

Mononuclear phagocyte immunity and the neuropathogenesis of HIV-1 infection

Human immunodeficiency virus type 1 (HIV-1)-associated dementia is a neuroinflammatory brain disorder that is fueled by viral infection and immune activation of brain mononuclear phagocytes (MP; macrophages and microglia). MP serve as a reservoir for persistent viral infection, a vehicle for viral dissemination throughout the brain, and a major source of neurotoxic products that when produced in abundance, affect neuronal function. Such neurotoxic substances secreted by MP lead to clinical neurological impairment (cognitive, behavior, and motor abnormalities), which occurs usually years after the initial viral infection. How HIV-1 evades the immune function characteristic for MP as a first line of defense, including phagocytosis and intracellular killing, is not well understood despite more than two decades of study. In this report, we review the complex role(s) played by MP in the neuropathogenesis of HIV-1 infection. The clinical manifestations, pathology and pathogenesis, and treatment options are discussed in relationship to innate and adaptive immunity. Particular emphasis is given to the diversity of MP functions and how it may affect the disease process and manifestations. New insights into disease mechanisms are provided by advances in enhanced magnetic resonance imaging and proteomics to identify cell movement and genetic profiles of disease. New therapeutic strategies are discussed based on current knowledge of HIV-1-associated dementia pathogenesis.

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.