Histopathological and cognitive defects induced by Nef in the brain

Neuro-HIV-New insights into pathogenesis and emerging therapeutic targets

HIV-associated neurocognitive disorders (HAND) is a term describing a complex set of cognitive impairments accompanying HIV infection. Successful antiretroviral therapy (ART) reduces the most severe forms of HAND, but milder forms affect over 50% of people living with HIV (PLWH). Pathogenesis of HAND in the ART era remains unknown. A variety of pathogenic factors, such as persistent HIV replication in the brain reservoir, HIV proteins released from infected brain cells, HIV-induced neuroinflammation, and some components of ART, have been implicated in driving HAND pathogenesis in ART-treated individuals. Here, we propose another factor-impairment of cholesterol homeostasis and lipid rafts by HIV-1 protein Nef-as a possible contributor to HAND pathogenesis. These effects of Nef on cholesterol may also underlie the effects of other pathogenic factors that constitute the multifactorial nature of HAND pathogenesis. The proposed Nef- and cholesterol-focused mechanism may provide a long-sought unified explanation of HAND pathogenesis that takes into account all contributing factors. Evidence for the impairment by Nef of cellular cholesterol balance, potential effects of this impairment on brain cells, and opportunities to therapeutically target this element of HAND pathogenesis are discussed.

Role of Autophagy in HIV-1 and Drug Abuse-Mediated Neuroinflammaging

Chronic low-grade inflammation remains an essential feature of HIV-1 infection under combined antiretroviral therapy (cART) and contributes to the accelerated cognitive defects and aging in HIV-1 infected populations, indicating cART limitations in suppressing viremia. Interestingly, ~50% of the HIV-1 infected population on cART that develops cognitive defects is complicated by drug abuse, involving the activation of cells in the central nervous system (CNS) and neurotoxin release, altogether leading to neuroinflammation. Neuroinflammation is the hallmark feature of many neurodegenerative disorders, including HIV-1-associated neurocognitive disorders (HAND). Impaired autophagy has been identified as one of the underlying mechanisms of HAND in treated HIV-1-infected people that also abuse drugs. Several lines of evidence suggest that autophagy regulates CNS cells' responses and maintains cellular hemostasis. The impairment of autophagy is associated with low-grade chronic inflammation and immune senescence, a known characteristic of pathological aging. Therefore, autophagy impairment due to CNS cells, such as neurons, microglia, astrocytes, and pericytes exposure to HIV-1/HIV-1 proteins, cART, and drug abuse could have combined toxicity, resulting in increased neuroinflammation, which ultimately leads to accelerated aging, referred to as neuroinflammaging. In this review, we focus on the potential role of autophagy in the mechanism of neuroinflammaging in the context of HIV-1 and drug abuse.

Emerging Role of Nef in the Development of HIV Associated Neurological Disorders

Despite adherence to treatment, individuals living with HIV have an increased risk for developing cognitive impairments, referred to as HIV-associated neurological disorders (HAND). Due to continued growth in the HIV population, particularly amongst the aging cohort, the neurobiological mechanisms of HAND are increasingly relevant. Similar to other viral proteins (e.g. Tat, Gp120, Vpr), the Negative Factor (Nef) is associated with numerous adverse effects in the CNS as well as cognitive impairments. In particular, emerging data indicate the consequences of Nef may be facilitated by the modulation of cellular autophagy as well as its inclusion into extracellular vesicles (EVs). The present review examines evidence for the molecular mechanisms by which Nef might contribute to neuronal dysfunction underlying HAND, with a specific focus on autophagy and EVs. Based on the these data, we propose an integrated model by which Nef may contribute to underlying neuronal dysfunction in HAND and highlight potentially novel therapeutic targets for HAND. Graphical abstract.

Characterization of Nef expression in different brain regions of SIV-infected macaques

OBJECTIVE

HIV-associated CNS dysfunction is a significant problem among people with HIV (PWH), who now live longer due to viral suppression from combined anti-retroviral therapy (ART). Over the course of infection, HIV generates toxic viral proteins and induces inflammatory cytokines that have toxic effects on neurons in the CNS. Among these viral proteins, HIV Nef has been found in neurons of postmortem brain specimens from PWH. However, the source of Nef and its impact on neuronal cell homeostasis are still elusive.

METHODS AND RESULTS

Here, in using a simian immunodeficiency virus (SIV) infected rhesus macaque model of neuroHIV, we find SIV Nef reactivity in the frontal cortex, hippocampus and cerebellum of SIV-infected animals using immunohistochemistry (IHC). Interestingly, SIV-infected macaques treated with ART also showed frequent Nef positive cells in the cerebellum and hippocampus. Using dual quantitative RNAscope and IHC, we observed cells that were positive for Nef, but were not for SIV RNA, suggesting that Nef protein is present in cells that are not actively infected with SIV. Using cell specific markers, we observed Nef protein in microglia/macrophages and astrocytes. Importantly, we also identified a number of NeuN-positive neurons, which are not permissive to SIV infection, but contained Nef protein. Further characterization of Nef-positive neurons showed caspase 3 activation, indicating late stage apoptosis in the CNS neurons.

CONCLUSIONS

Our results suggest that regardless of ART status, Nef is expressed in the brain of SIV infected macaques and may contribute to neurological complications seen in PWH.

Astrocytes autophagy in aging and neurodegenerative disorders

Astrocytes can serve multiple functions in maintaining cellular homeostasis of the central nervous system (CNS), and normal functions for autophagy in astrocytes is considered to have very vital roles in the pathogenesis of aging and neurodegenerative diseases. Autophagy is a major intracellular lysosomal (or its yeast analog, vacuolar) clearance pathways involved in the degradation and recycling of long-lived proteins, oxidatively damaged proteins and dysfunctional organelles by lysosomes. Current evidence has shown that autophagy might influence inflammation, oxidative stress, aging and function of astrocytes. Although the interrelation between autophagy and inflammation, oxidative stress, aging or neurological disorders have been addressed in detail, the influence of astrocytes mediated-autophagy in aging and neurodegenerative disorders has yet to be fully reviewed. In this review, we will summarize the most up-to-date findings and highlight the role of autophagy in astrocytes and link autophagy of astrocytes to aging and neurodegenerative diseases. Due to the prominent roles of astrocytic autophagy in age-related neurodegenerative diseases, we believe that we can provide new suggestions for the treatment of these disorders.

Infusion of HIV-1 Nef-expressing astrocytes into the rat hippocampus induces enteropathy and interstitial pneumonitis and increases blood-brain-barrier permeability

Even though HIV-1 replication can be suppressed by combination antiretroviral therapy (cART) inflammatory processes still occur, contributing to comorbidities. Comorbidities are attributed to variety of factors, including HIV-1 mediated inflammation. Several HIV-1 proteins mediate central nervous system (CNS) inflammation, including Nef. Nef is an early HIV-1 protein, toxic to neurons and glia and is sufficient to cause learning impairment similar to some deficits observed in HIV-1 associated neurocognitive disorders. To determine whether hippocampal Nef expression by astrocytes contributes to comorbidities, specifically peripheral inflammation, we infused Sprague Dawley rats with GFP- (control) or Nef-transfected astrocytes into the right hippocampus. Brain, lung, and ileum were collected postmortem for the measurement of inflammatory markers. Increased blood-brain-barrier permeability and serum IL-1β levels were detected in the Nef-treated rats. The lungs of Nef-treated rats demonstrated leukocyte infiltration, macrophage upregulation, and enhanced vascular permeability. Ileal tissue showed reactive follicular lymphoid hyperplasia, increased permeability and macrophage infiltration. The intracerebroventricular application of IL-1 receptor antagonist reduced infiltration of immune cells into ileum and lung, indicating the important role of IL-1β in mediating the spread of inflammation from the brain to other tissues. This suggests that localized expression of a single viral protein, HIV-1 Nef, can contribute to a broader inflammatory response by upregulation of IL-1β. Further, these results suggest that Nef contributes to the chronic inflammation seen in HIV patients, even in those whose viremia is controlled by cART.

Nef-induced CCL2 Expression Contributes to HIV/SIV Brain Invasion and Neuronal Dysfunction

C-C motif chemokine ligand 2 (CCL2) is a chemoattractant for leukocytes including monocytes, T cells, and natural killer cells and it plays an important role in maintaining the integrity and function of the brain. However, there is accumulating evidence that many neurological diseases are attributable to a dysregulation of CCL2 expression. Acquired immune deficiency syndrome (AIDS) encephalopathy is a severe and frequent complication in individuals infected with the human immunodeficiency virus (HIV) or the simian immunodeficiency virus (SIV). The HIV and SIV Nef protein, a progression factor in AIDS pathology, can be transferred by microvesicles including exosomes and tunneling nanotubes (TNT) within the host even to uninfected cells, and Nef can induce CCL2 expression. This review focuses on findings which collectively add new insights on how Nef-induced CCL2 expression contributes to neurotropism and neurovirulence of HIV and SIV and elucidates why adjuvant targeting of CCL2 could be a therapeutic option for HIV-infected persons.

Does HIV infection contribute to increased beta-amyloid synthesis and plaque formation leading to neurodegeneration and Alzheimer's disease?

HIV infection in the combination antiretroviral therapy (cART) era has become a chronic disease with a life expectancy almost identical to those free from this infection. Concomitantly, chronic diseases such as neurodegenerative diseases have emerged as serious clinical problems. HIV-induced cognitive changes, although clinically very diverse are collectively called HIV-associated neurocognitive disorder (HAND). HAND, which until the introduction of cART manifested clinically as a subcortical disorder, is now considered primarily cognitive disorder, which makes it similar to diseases like Alzheimer's (AD) and Parkinson's disease (PD). The pathogenesis involves either the direct effects of the virus or the effect of viral proteins such as Tat, Ggp120, and Nef. These proteins are either capable of destroying neurons directly by inducing neurotoxic mediators or by initiating neuroinflammation by microglia and astrocytes. Recently, it has become recognized that HIV infection is associated with increased production of the beta-amyloid peptide (Aβ) which is a characteristic of AD. Moreover, amyloid plaques have also been demonstrated in the brains of patients suffering from HAND. Thus, the question arises whether this production of Aβ indicates that HAND may lead to AD or it is a form of AD or this increase in Aβ production is only a bystander effect. It has also been discovered that APP in HIV and its metabolic product Aβ in AD manifest antiviral innate immune peptide characteristics. This review attempts to bring together studies linking amyloid precursor protein (APP) and Aβ production in HIV infection and their possible impact on the course of HAND and AD. These data indicate that human defense mechanisms in HAND and AD are trying to contain microorganisms by antimicrobial peptides, however by employing different means. Future studies will, no doubt, uncover the relationship between HAND and AD and, hopefully, reveal novel treatment possibilities.

Dual regulation of L-selectin (CD62L) by HIV-1: Enhanced expression by Vpr in contrast with cell-surface down-modulation by Nef and Vpu

The HIV-1 accessory protein Vpr displays various activities that can favor viral replication such as G₂ cell cycle arrest. Vpr also modulates host gene expression, although this property is poorly characterized. Here, we investigated the effect of Vpr on L-selectin (CD62L), which crucially controls leukocytes circulation and generation of immune responses against pathogens. We report that Vpr up-regulates CD62L mRNA level when individually expressed in Jurkat T cells as well as during HIV-1 infection of primary CD4⁺ T cells. Vpr mutant analysis and use of inhibitors suggest that the effect of Vpr on CD62L occurs independently of G₂ arrest but requires activation of the ATR kinase. Yet, induction of CD62L expression by Vpr is contrasted by down-regulation of CD62L protein by Nef that, together with Vpu, induces a net reduction of cell-surface CD62L on HIV-1-infected cells, which may impact viral spread and evasion of immune responses.

Human immunodeficiency virus type 1 (HIV-1)-mediated neuroinflammation dysregulates neurogranin and induces synaptodendritic injury

Background

Human immunodeficiency virus type 1 (HIV-1)-associated neurocognitive disorder (HAND) is a common outcome of a majority of HIV-1-infected subjects and is associated with synaptodendritic damage. Neurogranin (Ng), a postsynaptic protein, and calmodulin (CaM) are two important players of synaptic integrity/functions. The biological role of Ng in the context of HAND is unknown.

Methods

We compared the expression of Ng in frontal cortex (FC) tissues from control and HIV-1-positive subjects with and without HAND by immunohistochemistry, western blot, and qRT-PCR. The interaction between Ng and CaM was analyzed by co-immunoprecipitation. Ng, microtubule-associated protein 2 (MAP2), CaM, CaM-dependent protein kinase II (CaMKII), CREB, synaptophysin (Syp), and synapsin I (Syn I) expressions were evaluated by western blot using FC tissue lysates and differentiated SH-SY5Y (dSH-SY5Y) cells. Identification of inflammatory factors related to Ng loss was accomplished by exposing dSH-SY5Y cells to HIV-1 and mock-infected monocyte-derived macrophage (MDM) supernatants or HIV-1 NLYU2 pseudotyped with VSV-G-Env. Levels of interleukin (IL)-1β, IL-8, tumor necrosis factor (TNF)-α, monocyte chemoattractant protein (MCP)-1, MCP-2, and CXCL5 in MDM supernatants were measured by ELISA. Association of IL-1β and IL-8 to Ng expression in context of HIV-1 infection was evaluated in the presence or absence of neutralizing antibodies against these cytokines.

Results

Expression level of Ng was reduced significantly in FC of HAND-positive (HAND+) patients compared to uninfected individuals. Although no difference was found in CaM expression, interaction between Ng and CaM was reduced in HAND+ patients, which was associated with decreased level of CaMKII, a downstream signaling molecule of CaM pathway. This in turn resulted in reduction of synaptic markers, Syp and Syn I. HIV-1 infection directly had no considerable effect on dysregulation of Ng expression in dSH-SY5Y cells, whereas high amount of pro-inflammatory IL-1β and IL-8 in HIV-1-infected MDM supernatants was associated with significant reduction in Ng expression.

Conclusions

Synaptic damage in HAND+ patients could be a result of abrogation of Ng through HIV-1-induced inflammation that dysregulates Ng-CaM interaction and downstream signaling cascades associated with synaptodendritic functions. This is the first study evaluating the potential role of Ng in the context of HIV-1 neuropathogenesis.

An Overview of Human Immunodeficiency Virus Type 1-Associated Common Neurological Complications: Does Aging Pose a Challenge?

With increasing survival of patients infected with human immunodeficiency virus type 1 (HIV-1), the manifestation of heterogeneous neurological complications is also increasing alarmingly in these patients. Currently, more than 30% of about 40 million HIV-1 infected people worldwide develop central nervous system (CNS)-associated dysfunction, including dementia, sensory, and motor neuropathy. Furthermore, the highly effective antiretroviral therapy has been shown to increase the prevalence of mild cognitive functions while reducing other HIV-1-associated neurological complications. On the contrary, the presence of neurological disorder frequently affects the outcome of conventional HIV-1 therapy. Although, both the children and adults suffer from the post-HIV treatment-associated cognitive impairment, adults, especially depending on the age of disease onset, are more prone to CNS dysfunction. Thus, addressing neurological complications in an HIV-1-infected patient is a delicate balance of several factors and requires characterization of the molecular signature of associated CNS disorders involving intricate cross-talk with HIV-1-derived neurotoxins and other cellular factors. In this review, we summarize some of the current data supporting both the direct and indirect mechanisms, including neuro-inflammation and genome instability in association with aging, leading to CNS dysfunction after HIV-1 infection, and discuss the potential strategies addressing the treatment or prevention of HIV-1-mediated neurotoxicity.

Brain-specific HIV Nef identified in multiple patients with neurological disease

HIV-1 Nef is a flexible, multifunctional protein with several cellular targets that is required for pathogenicity of the virus. This protein maintains a high degree of genetic variation among intra- and inter-host isolates. HIV Nef is relevant to HIV-associated neurological diseases (HAND) in patients treated with combined antiretroviral therapy because of the protein's role in promoting survival and migration of infected brain macrophages. In this study, we analyzed 2020 HIV Nef sequences derived from 22 different tissues and 31 subjects using a novel computational approach. This approach combines statistical regression and evolved neural networks (ENNs) to classify brain sequences based on the physical and chemical characteristics of functional Nef domains. Based on training, testing, and validation data, the method successfully classified brain Nef sequences at 84.5% and provided informative features for further examination. These included physicochemical features associated with the Src-homology-3 binding domain, the Nef loop (including the AP-2 Binding region), and a cytokine-binding domain. Non-brain sequences from patients with HIV-associated neurological disease were frequently classified as brain, suggesting that the approach could indicate neurological risk using blood-derived virus or for the development of biomarkers for use in assay systems aimed at drug efficacy studies for the treatment of HIV-associated neurological diseases.

HIV-1 Nef is released in extracellular vesicles derived from astrocytes: evidence for Nef-mediated neurotoxicity

Human immunodeficiency virus-associated neurological disorders (HANDs) affect the majority of AIDS patients and are a significant problem among HIV-1-infected individuals who live longer because of combined anti-retroviral therapies. HIV-1 utilizes a number of viral proteins and subsequent cytokine inductions to unleash its toxicity on neurons. Among HIV-1 viral proteins, Nef is a small protein expressed abundantly in astrocytes of HIV-1-infected brains and has been suggested to have a role in the pathogenesis of HAND. In order to explore its effect in the central nervous system, HIV-1 Nef was expressed in primary human fetal astrocytes (PHFAs) using an adenovirus. Our results revealed that HIV-1 Nef is released in extracellular vesicles (EVs) derived from PHFA cells expressing the protein. Interestingly, HIV-1 Nef release in EVs was enriched significantly when the cells were treated with autophagy activators perifosine, tomaxifen, MG-132, and autophagy inhibitors LY294002 and wortmannin suggesting a novel role of autophagy signaling in HIV-1 Nef release from astrocytes. Next, Nef-carrying EVs were purified from astrocyte cultures and neurotoxic effects on neurons were analyzed. We observed that HIV-1 Nef-containing EVs were readily taken up by neurons as demonstrated by immunocytochemistry and immunoblotting. Furthermore, treatment of neurons with Nef-carrying EVs induced oxidative stress as evidenced by a decrease in glutathione levels. To further investigate its neurotoxic effects, we expressed HIV-1 Nef in primary neurons by adenoviral transduction. Intracellular expression of HIV-1 Nef caused axonal and neurite degeneration of neurons. Furthermore, expression of HIV-1 Nef decreased the levels of phospho-tau while enhancing total tau in primary neurons. In addition, treatment of primary neurons with Nef-carrying EVs suppressed functional neuronal action potential assessed by multielectrode array studies. Collectively, these data suggested that HIV-1 Nef can be a formidable contributor to neurotoxicity along with other factors, which leads to HAND in HIV-1-infected AIDS patients.

Differential signaling mechanism for HIV-1 Nef-mediated production of IL-6 and IL-8 in human astrocytes

Variety of HIV-1 viral proteins including HIV-1 Nef are known to activate astrocytes and microglia in the brain and cause the release of pro-inflammatory cytokines, which is thought to be one of the mechanisms leading to HIV-1- mediated neurotoxicity. IL-6 and IL-8 have been found in the CSF of patients with HIV-1 associated dementia (HAD), suggesting that they might play important roles in HIV-1 neuropathology. In the present study we examined the effects of HIV-1 Nef on IL-6 and IL-8 induction in astrocytes. The results demonstrate that both IL-6 and IL-8 are significantly induced in HIV-1 Nef-transfected SVGA astrocytes and HIV-1 Nef-treated primary fetal astrocytes. We also determined the molecular mechanisms responsible for the HIV-1 Nef-induced increased IL-6 and IL-8 by using chemical inhibitors and siRNAs against PI3K/Akt/PKC, p38 MAPK, NF-κB, CEBP and AP-1. Our results clearly demonstrate that the PI3K/PKC, p38 MAPK, NF-κB and AP-1 pathways are involved in HIV-1 Nef-induced IL-6 production in astrocytes, while PI3K/PKC and NF-κB pathways are involved in HIV-1 Nef-induced IL-8 production. These results offer new potential targets to develop therapeutic strategy for treatment of HIV-1 associated neurological disorders, prevalent in > 40% of individuals infected with HIV-1.

Inhibition of catechol-O-methyl transferase (COMT) by tolcapone restores reductions in microtubule-associated protein 2 (MAP2) and synaptophysin (SYP) following exposure of neuronal cells to neurotropic HIV

This investigation aimed to assess whether inhibition of cathecol-O-methyl transferase (COMT) by tolcapone could provide neuroprotection against HIV-associated neurodegenerative effects. This study was conducted based on a previous work, which showed that a single nucleotide polymorphism (SNP) at position 158 (val158met) in COMT, resulted in 40 % lower COMT activity. Importantly, this reduction confers a protective effect against HIV-associated neurocognitive disorders (HAND), which have been linked to HIV-associated brain changes. SH-SY5Y-differentiated neurons were exposed to macrophage-propagated HIV (neurotropic MACS2-Br strain) in the presence or absence of tolcapone for 6 days. RNA was extracted, and qPCR was performed using Qiagen RT2 custom array consisting of genes for neuronal and synaptic integrity, COMT and pro-inflammatory markers. Immunofluorescence was conducted to validate the gene expression changes at the protein level. Our findings demonstrated that HIV significantly increased the messenger RNA (mRNA) expression of COMT while reducing the expression of microtubule-associated protein 2 (MAP2) (p = 0.0015) and synaptophysin (SYP) (p = 0.012) compared to control. A concomitant exposure of tolcapone ameliorated the perturbed expression of MAP2 (p = 0.009) and COMT (p = 0.024) associated with HIV. Immunofluorescence revealed a trend reduction of SYP and MAP2 with exposure to HIV and that concomitant exposure of tolcapone increased SYP (p = 0.016) compared to HIV alone. Our findings demonstrated in vitro that inhibition of COMT can ameliorate HIV-associated neurodegenerative changes that resulted in the decreased expression of the structural and synaptic components MAP2 and SYP. As HIV-associated dendritic and synaptic damage are contributors to HAND, inhibition of COMT may represent a potential strategy for attenuating or preventing some of the symptoms of HAND.

HIV-1 Nef and Vpu Interfere with L-Selectin (CD62L) Cell Surface Expression To Inhibit Adhesion and Signaling in Infected CD4+ T Lymphocytes

Leukocyte recirculation between blood and lymphoid tissues is required for the generation and maintenance of immune responses against pathogens and is crucially controlled by the L-selectin (CD62L) leukocyte homing receptor. CD62L has adhesion and signaling functions and initiates the capture and rolling on the vascular endothelium of cells entering peripheral lymph nodes. This study reveals that CD62L is strongly downregulated on primary CD4(+) T lymphocytes upon infection with human immunodeficiency virus type 1 (HIV-1). Reduced cell surface CD62L expression was attributable to the Nef and Vpu viral proteins and not due to increased shedding via matrix metalloproteases. Both Nef and Vpu associated with and sequestered CD62L in perinuclear compartments, thereby impeding CD62L transport to the plasma membrane. In addition, Nef decreased total CD62L protein levels. Importantly, infection with wild-type, but not Nef- and Vpu-deficient, HIV-1 inhibited the capacity of primary CD4(+) T lymphocytes to adhere to immobilized fibronectin in response to CD62L ligation. Moreover, HIV-1 infection impaired the signaling pathways and costimulatory signals triggered in primary CD4(+) T cells by CD62L ligation. We propose that HIV-1 dysregulates CD62L expression to interfere with the trafficking and activation of infected T cells. Altogether, this novel HIV-1 function could contribute to virus dissemination and evasion of host immune responses.

IMPORTANCE

L-selectin (CD62L) is an adhesion molecule that mediates the first steps of leukocyte homing to peripheral lymph nodes, thus crucially controlling the initiation and maintenance of immune responses to pathogens. Here, we report that CD62L is downmodulated on the surfaces of HIV-1-infected T cells through the activities of two viral proteins, Nef and Vpu, that prevent newly synthesized CD62L molecules from reaching the plasma membrane. We provide evidence that CD62L downregulation on HIV-1-infected primary T cells results in impaired adhesion and signaling functions upon CD62L triggering. Removal of cell surface CD62L may predictably keep HIV-1-infected cells away from lymph nodes, the privileged sites of both viral replication and immune response activation, with important consequences, such as systemic viral spread and evasion of host immune surveillance. Altogether, we propose that Nef- and Vpu-mediated subversion of CD62L function could represent a novel determinant of HIV-1 pathogenesis.

HIV-1 Nef in macrophage-mediated disease pathogenesis

Combined anti-retroviral therapy (cART) has significantly reduced the number of AIDS-associated illnesses and changed the course of HIV-1 disease in developed countries. Despite the ability of cART to maintain high CD4+ T-cell counts, a number of macrophage-mediated diseases can still occur in HIV-infected subjects. These diseases include lymphoma, metabolic diseases, and HIV-associated neurological disorders. Within macrophages, the HIV-1 regulatory protein "Nef" can modulate surface receptors, interact with signaling pathways, and promote specific environments that contribute to each of these pathologies. Moreover, genetic variation in Nef may also guide the macrophage response. Herein, we review findings relating to the Nef-macrophage interaction and how this relationship contributes to disease pathogenesis.

HIV-1 Nef breaches placental barrier in rat model

The vertical transmission of HIV-1 from the mother to fetus is known, but the molecular mechanism regulating this transmission is not fully characterized. The fetus is highly protected by the placenta, which does not permit microbial pathogens to cross the placental barrier. In the present study, a rat model was established to observe the effect of HIV-1 protein Nef on placental barrier. Evans blue dye was used to assay permeability of placental barrier and fourteen day pregnant Sprague Dawley rats were injected intravenously with 2% Evans blue dye along with various concentrations of recombinant Nef. After an hour, animals were sacrificed and dye migration was observed through the assimilation of peripheral blood into fetus. Interestingly, traces of recombinant Nef protein were detected in the embryo as well as amniotic fluid and amniotic membrane along with placenta and uterus. Our study indicates that recombinant HIV-1-Nef protein breaches the placental barrier and allows the migration of Evans blue dye to the growing fetus. Further the concentration of Nef protein in blood is directly proportional to the intensity of dye migration and to the amount of Nef protein detected in uterus, placenta, amniotic membrane, amniotic fluid and embryo. Based on this study, it can be concluded that the HIV-1 Nef protein has a direct effect on breaching of the placental barrier in the model we have established in this study. Our observations will be helpful to understand the molecular mechanisms related to this breach of placental barrier by Nef in humans and may be helpful to identify specific Nef inhibitors.

Astrocytic expression of HIV-1 Nef impairs spatial and recognition memory

Despite the widespread use of antiretroviral therapy that effectively limits viral replication, memory impairment remains a dilemma for HIV infected people. In the CNS, HIV infection of astrocytes leads to the production of the HIV-1 Nef protein without viral replication. Post mortem studies have found Nef expression in hippocampal astrocytes of people with HIV associated dementia suggesting that astrocytic Nef may contribute to HIV associated cognitive impairment even when viral replication is suppressed. To test whether astrocytic expression of Nef is sufficient to induce cognitive deficits, we examined the effect of implanting primary rat astrocytes expressing Nef into the hippocampus on spatial and recognition memory. Rats implanted unilaterally with astrocytes expressing Nef showed impaired novel location and novel object recognition in comparison with controls implanted with astrocytes expressing green fluorescent protein (GFP). This impairment was correlated with an increase in chemokine ligand 2 (CCL2) expression and the infiltration of peripheral macrophages into the hippocampus at the site of injection. Furthermore, the Nef exposed rats exhibited a bilateral loss of CA3 neurons. These results suggest that Nef protein expressed by the implanted astrocytes activates the immune system leading to neuronal damage and spatial and recognition memory deficits. Therefore, the continued expression of Nef by astrocytes in the absence of viral replication has the potential to contribute to HIV associated cognitive impairment.

HIV-1 Nef increases astrocyte sensitivity towards exogenous hydrogen peroxide

Background

HIV-1 infected individuals are under chronic exposure to reactive oxygen species (ROS) considered to be instrumental in the progression of AIDS and the development of HIV-1 associated dementia (HAD). Astrocytes support neuronal function and protect them against cytotoxic substances including ROS. The protein HIV-1 Nef, a progression factor in AIDS pathology is abundantly expressed in astrocytes in patients with HAD, and thus may influence its functions.

Results

Endogenous expressed HIV-1 Nef leads to increased sensitivity of human astrocytes towards exogenous hydrogen peroxide but not towards TNF-alpha. Cell death of nef-expressing astrocytes exposed to 10 μM hydrogen peroxide for 30 min occurred within 4 h.

Conclusion

HIV-1 Nef may contribute to neuronal dysfunction and the development of HAD by causing death of astrocytes through decreasing their tolerance for hydrogen peroxide.

Evidence for adaptive evolution at the divergence between lymphoid and brain HIV-1 nef genes

Human immunodeficiency virus type 1 (HIV) infection of the central nervous system frequently causes HIV-associated neurocognitive disorders (HAND). The role of HIV Nef and other accessory proteins in HAND pathogenesis is unclear. To determine whether HIV nef undergoes adaptive selection in brain, we cloned 100 nef sequences (n = 30 brain and n = 70 lymphoid) from four patients with AIDS and HIV-associated dementia (HAD). Normalized nonsynonymous substitutions were more frequent at the divergence of lymphoid and brain sequences, indicating stronger adaptive selection in brain compared to lymphoid tissue. Brain-specific nonsynonymous substitutions were found within an NH(3)-terminal CTL epitope, the PACS1 binding motif, or positions predicted to be important for activation of the myeloid-restricted Src family tyrosine kinase Hck. These results suggest that adaptive selection of HIV nef in brain may reflect altered requirements for efficient replication in macrophages and brain-specific immune selection pressures.

Exceptional molecular and coreceptor-requirement properties of molecular clones isolated from an Human Immunodeficiency Virus Type-1 subtype C infection

BACKGROUND

The pathogenic significance of coreceptor switch in the viral infection of HIV-1 is not completely understood. This situation is more complex in subtype C infection where coreceptor switch is either absent or extremely rare. To gain insights into the mechanisms that underlie coreceptor requirement of subtype C, we screened several primary viral isolates and identified a clinical sample that demonstrated a potential to grow on standard T-cell lines with no detectable CCR5 expression. The subject was diagnosed with HIV-1 associated dementia in the absence of opportunistic infections of the brain. To isolate molecular clones from this virus, we devised a novel strategy based on anchor primers that target a sequence in the reverse transcriptase, highly conserved among diverse subtypes of HIV-1.

RESULTS

Using this strategy, we isolated 8 full-length molecular clones from the donor. Two of the eight molecular clones, 03In94_D17 and 03In94_D24, (D17 and D24) generated replication-competent viruses. Phylogenetic analysis of the full-length viral sequences revealed that both clones were non-recombinant subtype C viruses. They contain intact open reading frames in all the viral proteins. Both the viral clones are endowed with several unique molecular and biological properties. The viral promoter of the clones is characterized by the presence of four NF-kB binding elements, a feature rarely seen in the subtype C HIV-1 LTR. Interestingly, we identified the coexistence of two different forms of Rev, a truncated form common to subtype C and a full-length form less common for this subtype, in both proviral and plasma virus compartments. An exceptional property of the viruses, atypical of subtype C, is their ability to use a wide range of coreceptors including CCR5, CXCR4, and several others tested. Sequence analysis of Env of D17 and D24 clones identified differences within the variable loops providing important clues for the expanded coreceptor use. The V1, V2 and V4 loops in both of the molecular clones are longer due to the insertion of several amino acid residues that generated potential N-linked glycosylation sites.

CONCLUSION

The exceptional biological and molecular properties of these clones make them invaluable tools to understand the unique pathogenic characteristics of subtype C.

Interactions of HIV and methamphetamine: cellular and molecular mechanisms of toxicity potentiation

Methamphetamine (METH) is a highly addictive psychostimulant drug, whose abuse has reached epidemic proportions worldwide. METH use is disproportionally represented among populations at high risks for developing HIV infection or who are already infected with the virus. Psychostimulant abuse has been reported to exacerbate the cognitive deficits and neurodegenerative abnormalities observed in HIV-positive patients. Thus, the purpose of the present paper is to review the clinical and basic observations that METH potentiates the adverse effects of HIV infection. An additional purpose is to provide a synthesis of the cellular and molecular mechanisms that might be responsible for the increased toxicity observed in co-morbid patients. The reviewed data indicate that METH and HIV proteins, including gp120, gp41, Tat, Vpr and Nef, converge on various caspase-dependent death pathways to cause neuronal apoptosis. The role of reactive microgliosis in METH- and in HIV-induced toxicity is also discussed.

Human macrophages support persistent transcription from unintegrated HIV-1 DNA

Retroviruses require integration of their RNA genomes for both stability and productive viral replication. In HIV infection of non-dividing, resting CD4 T cells, where integration is greatly impeded, the reverse transcribed HIV DNA has limited biological activity and a short half-life. In metabolically active and proliferating T cells, unintegrated DNA rapidly diminishes with cell division. HIV also infects the non-dividing but metabolically active macrophage population. In an in vitro examination of HIV infection of macrophages, we find that unintegrated viral DNA not only has an unusual stability, but also maintains biological activity. The unintegrated linear DNA, 1-LTR, and 2-LTR circles are stable for at least 30 days. Additionally, there is persistent viral gene transcription, which is selective and skewed towards viral early genes such as nef and tat with highly diminished rev and vif. One viral early gene product Nef was measurably synthesized. We also find that independent of integration, the HIV infection process in macrophages leads to generation of numerous chemokines.

Therapeutic vaccination reduces HIV sequence variability

With HIV persisting lifelong in infected persons, therapeutic vaccination is a novel alternative concept to control virus replication. Even though CD8 and CD4 cell responses to such immunizations have been demonstrated, their effects on virus replication are still unclear. In view of this fact, we studied the impact of a therapeutic vaccination with HIV nef delivered by a recombinant modified vaccinia Ankara vector on viral diversity. We investigated HIV sequences derived from chronically infected persons before and after therapeutic vaccination. Before immunization the mean +/- se pairwise variability of patient-derived Nef protein sequences was 0.1527 +/- 0.0041. After vaccination the respective value was 0.1249 +/- 0.0042, resulting in a significant (P<0.0001) difference between the two time points. The genes vif and 5'gag tested in parallel and nef sequences in control persons yielded a constant amino acid sequence variation. The data presented suggest that Nef immunization induced a selective pressure, limiting HIV sequence variability. To our knowledge this is the first report directly linking therapeutic HIV vaccination to decreasing diversity in patient-derived virus isolates.

Spatial learning in Long-Evans Hooded rats and C57BL/6J mice: different strategies for different performance

Spatial learning abilities of rodents have been extensively used to explore the management of a wide range of cognitive and emotional processes such as learning, memory, attention and anxiety. Knowledge about the organization and processing of spatial learning has mainly been obtained in rats. Due to increasing generation of genetically modified mice, cognitive abilities of mice are now extensively tested. The present paper aimed at comparing spatial representation, learning and strategies in C57BL/6J mice and Long-Evans Hooded rats when subjected to the same spatial learning paradigm, i.e. learning a food location in a crossmaze. We also analyzed the influence of environmental richness on learning modalities in both species. Our results showed that rats and mice could exhibit similar spatial learning abilities in some circumstances. However, Long-Evans rats and C57BL/6J mice may set up different strategies depending on the availability of visual information within the environment. Rats' learning strategies mainly relied on distant visual cues and seemed more efficient than those used by mice as they needed less time than mice to solve the task. We emphasize that the strategies of mice are less robust and flexible than the ones set up by rats. Finally, the richness of the environment was shown to affect speed and quality of spatial learning in both species.

CD4 and MHC-I downregulation are conserved in primary HIV-1 Nef alleles from brain and lymphoid tissues, but Pak2 activation is highly variable

HIV-1 compartmentalization in the CNS has been demonstrated for gag, pol, and env genes. However, little is known about tissue compartmentalization of nef genes and their functional characteristics in brain. We have cloned 97 nef genes and characterized 10 Nef proteins from autopsy brain and lymphoid tissues from 2 patients with AIDS and HIV-1-associated dementia. Distinct compartmentalization of brain versus lymphoid nef genes was demonstrated within each patient. CD4 and MHC-I downregulation were conserved in all tissue-derived Nefs. However, MHC-I downregulation by brain-derived Nefs was weaker than downregulation by lymphoid-derived Nefs. The motifs KEEE- or EKEE- at the PACS-1 binding site represented brain-specific signature patterns in these 2 patients and contributed to the reduced MHC-I downregulation activity of brain-derived Nefs from these patients. Pak2 association was highly variable in Nefs from both patients. Three of 10 tissue-derived Nefs coimmunoprecipitated activated Pak2, with strong association demonstrated for only 2 Nefs. The ability of Nef to associate with activated Pak2 did not correlate with brain or lymphoid tissue origin. Nef genes from viruses isolated from brain by coculture with PBMC were not closely related to sequences amplified directly from brain tissue, suggesting that viral selection or adaptation occurred during coculture. This study of tissue-derived HIV-1 Nefs demonstrates that CD4 and MHC-I downregulation are highly conserved Nef functions, while Pak2 association is variable in late stage AIDS patients.

The role of monocytes and perivascular macrophages in HIV and SIV neuropathogenesis: information from non-human primate models

Perivascular macrophages are located in the perivascular space of cerebral microvessels and thus uniquely situated at the intersection between the brain parenchyma and blood. Connections between the nervous and immune systems are mediated in part through these cells that are ideally located to sense perturbations in the periphery and turnover by cells entering the central nervous system (CNS) from the circulation. It has become clear that unique subsets of brain macrophages exist in normal and SIV- or HIV-infected brains, and perivascular macrophages and similar cells in the meninges and choroid plexus play a central role in lentiviral neuropathogenesis. Common to all these cell populations is their likely replacement within the CNS by monocytes. Studies of SIV-infected non-human primates and HIV-infected humans underscore the importance of virus-infected and activated monocytes, which traffic to the CNS from blood to become perivascular macrophages, potentially drive the blood-brain barrier damage and cause neuronal injury. This review summarizes what we know about SIV- and HIV-induced neuropathogenesis focusing on brain perivascular macrophages and their precursors in blood that may mediate HIV CNS infection and injury.