Synaptic transport of human immunodeficiency virus-Tat protein causes neurotoxicity and gliosis in rat brain

Acute Administration of HIV-1 Tat Protein Drives Glutamatergic Alterations in a Rodent Model of HIV-Associated Neurocognitive Disorders

HIV-1-associated neurocognitive disorders (HAND) are a major comorbidity of HIV-1 infection, marked by impairment of executive function varying in severity. HAND affects nearly half of people living with HIV (PLWH), with mild forms predominating since the use of anti-retroviral therapies (ART). The HIV-1 transactivator of transcription (Tat) protein is found in the cerebrospinal fluid of patients adherent to ART, and its administration or expression in animals causes cognitive symptoms. Studies of Tat interaction with the N-methyl-D-aspartate receptor (NMDAR) suggest that glutamate toxicity contributes to Tat-induced impairments. To identify changes in regional glutamatergic circuitry underlying cognitive impairment, we injected recombinant Tat86 or saline to medial prefrontal cortex (mPFC) of male Sprague-Dawley rats. Rats were assessed with behavioral tasks that involve intact functioning of mPFC including the novel object recognition (NOR), spatial object recognition (SOR), and temporal order (TO) tasks at 1 and 2 postoperative weeks. Following testing, mPFC tissue was collected and analyzed by RT-PCR. Results showed Tat86 in mPFC-induced impairment in SOR, and upregulation of Grin1 and Grin2a transcripts. To further understand the mechanism of Tat toxicity, we assessed the effects of full-length Tat101 on gene expression in mPFC by RNA sequencing. The results of RNAseq suggest that glutamatergic effects of Tat86 are maintained with Tat101, as Grin2a was upregulated in Tat101-injected tissue, among other differentially expressed genes. Spatial learning and memory impairment and Grin2a upregulation suggest that exposure to Tat protein drives adaptation in mPFC, altering the function of circuitry supporting spatial learning and memory.

Different Roles of Beclin1 in the Interaction Between Glia and Neurons after Exposure to Morphine and the HIV- Trans-Activator of Transcription (Tat) Protein

Previously we showed that Beclin1 has a regulatory role in the secretion of inflammatory molecules in glia after exposure to morphine and Tat (an HIV protein). Here we show increased secretion of neuronal growth factors and increased neuronal survival in Beclin1-deficient glia. However, without glia co-culture, neurons deficient in Beclin1 showed greater death and enhanced dendritic beading when compared to wild-type neurons, suggesting that glial-secreted growth factors compensate for the damage reduced autophagy causes neurons. To assess if our ex vivo results correlated with in vivo studies, we used a wild-type (Becn1+/+) and Beclin1-deficient (Becn1+/+) mouse model and intracranially infused the mice with Tat and subcutaneously administered morphine pellets. After morphine implantation, significantly impaired locomotor activities were detected in both Becn1+/+ and Becn1+/- mice, irrespective of Tat infusion. After induction of pain, morphine-induced antinociception was detected. Interestingly, co-exposure to morphine and Tat increased sensitivity to pain in Becn1+/+ mice, but not in similarly treated Becn1+/- mice. Brain homogenates from Becn1+/+ mice exposed to Tat, alone and in combination with morphine, showed increased secretion of pro-inflammatory cytokines and reduced expression of growth factors when compared to similarly treated Becn1+/- mice. Likewise, increased neuronal loss was detected when both Tat and morphine were administered to Becn1+/+ mice, but not in similarly treated Becn1+/- mice. Overall, our findings show that there is a Beclin1-driven interaction between Tat and morphine in glia and neurons. Moreover, reduced glial-Beclin1 may provide a layer of protection to neurons under stressful conditions, such as when exposed to morphine and Tat.

HIV-1 Tat reduces apical dendritic spine density throughout the trisynaptic pathway in the hippocampus of male transgenic mice

Nearly one-third of persons infected with HIV-1 (PWH) develop HIV-associated neurocognitive disorders (HAND), which can be exacerbated by exposure to opioids. The impact of opioids on HIV-induced alterations in neuronal plasticity is less well understood. Both morphine exposure and HIV have been shown to disrupt synaptic growth and stability in the hippocampus suggesting a potential site of convergence for their deleterious effects. In the present study, we examined the density of dendritic spines in CA1 and CA3 pyramidal neurons, and granule neurons within the dentate gyrus representing the hippocampal trisynaptic pathway after short-term exposure to the HIV transactivator of transcription (Tat) protein and morphine. We exposed inducible male, HIV-1 Tat transgenic mice to escalating doses of morphine (10-40 mg/kg, b.i.d.) and examined synaptodendritic structure in Golgi-impregnated hippocampal neurons. HIV-1 Tat, but not morphine, systematically reduced the density of apical, but not basilar, dendrites of CA1 and CA3 pyramidal neurons, and granule neuronal apical dendrites, suggesting the coordinated loss of specific synaptic interconnections throughout the hippocampal trisynaptic pathway.

Crossroads of Drug Abuse and HIV Infection: Neurotoxicity and CNS Reservoir

Drug abuse is a common comorbidity in people infected with HIV. HIV-infected individuals who abuse drugs are a key population who frequently experience suboptimal outcomes along the HIV continuum of care. A modest proportion of HIV-infected individuals develop HIV-associated neurocognitive issues, the severity of which further increases with drug abuse. Moreover, the tendency of the virus to go into latency in certain cellular reservoirs again complicates the elimination of HIV and HIV-associated illnesses. Antiretroviral therapy (ART) successfully decreased the overall viral load in infected people, yet it does not effectively eliminate the virus from all latent reservoirs. Although ART increased the life expectancy of infected individuals, it showed inconsistent improvement in CNS functioning, thus decreasing the quality of life. Research efforts have been dedicated to identifying common mechanisms through which HIV and drug abuse lead to neurotoxicity and CNS dysfunction. Therefore, in order to develop an effective treatment regimen to treat neurocognitive and related symptoms in HIV-infected patients, it is crucial to understand the involved mechanisms of neurotoxicity. Eventually, those mechanisms could lead the way to design and develop novel therapeutic strategies addressing both CNS HIV reservoir and illicit drug use by HIV patients.

Advances in the Experimental Models of HIV-Associated Neurological Disorders

PURPOSE OF REVIEW

Involvement of the central nervous system (CNS) in HIV-1 infection is commonly associated with neurological disorders and cognitive impairment, commonly referred to as HIV-associated neurocognitive disorders (HAND). Severe and progressive neurocognitive impairment is rarely observed in the post-cART era; however, asymptomatic and mild neurocognitive disorders still exist, despite viral suppression. Additionally, comorbid conditions can also contribute to the pathogenesis of HAND.

RECENT FINDINGS

In this review, we summarize the characterization of HAND, factors contributing, and the functional impairments in both preclinical and clinical models. Specifically, we also discuss recent advances in the animal models of HAND and in in vitro cultures and the potential role of drugs of abuse in this model system of HAND. Potential peripheral biomarkers associated with HAND are also discussed. Overall, this review identifies some of the recent advances in the field of HAND in cell culture studies, animal models, clinical findings, and the limitations of each model system, which can play a key role in developing novel therapeutics in the field.

HIV-1 persistence in the CNS: Mechanisms of latency, pathogenesis and an update on eradication strategies

Despite four decades of research into the human immunodeficiency virus (HIV-1), a successful strategy to eradicate the virus post-infection is lacking. The major reason for this is the persistence of the virus in certain anatomical reservoirs where it can become latent and remain quiescent for as long as the cellular reservoir is alive. The Central Nervous System (CNS), in particular, is an intriguing anatomical compartment that is tightly regulated by the blood-brain barrier. Targeting the CNS viral reservoir is a major challenge owing to the decreased permeability of drugs into the CNS and the cellular microenvironment that facilitates the compartmentalization and evolution of the virus. Therefore, despite effective antiretroviral (ARV) treatment, virus persists in the CNS, and leads to neurological and neurocognitive deficits. To date, viral eradication strategies fail to eliminate the virus from the CNS. To facilitate the improvement of the existing elimination strategies, as well as the development of potential therapeutic targets, the aim of this review is to provide an in-depth understanding of HIV latency in CNS and the onset of HIV-1 associated neurological disorders.

Delivery of Therapeutic Agents to the Central Nervous System and the Promise of Extracellular Vesicles

The central nervous system (CNS) is surrounded by the blood–brain barrier (BBB), a semipermeable border of endothelial cells that prevents pathogens, solutes and most molecules from non-selectively crossing into the CNS. Thus, the BBB acts to protect the CNS from potentially deleterious insults. Unfortunately, the BBB also frequently presents a significant barrier to therapies, impeding passage of drugs and biologicals to target cells within the CNS. This review provides an overview of different approaches to deliver therapeutics across the BBB, with an emphasis in extracellular vesicles as delivery vehicles to the CNS.

Insights into the Gene Expression Profiles of Active and Restricted Red/Green-HIV+ Human Astrocytes: Implications for Shock or Lock Therapies in the Brain

A significant number of people living with human immunodeficiency virus type 1 (HIV-1) suffer from HIV-associated neurocognitive disorders (HAND). Many previous studies investigating HIV in astrocytes as a heterogenous population have established the relevance of astrocytes to HIV-associated neuropathogenesis. However, these studies were unable to differentiate the state of infection, i.e., active or latent, or to evaluate how this affects astrocyte biology. In this study, the pseudotyped doubly labeled fluorescent reporter red/green (R/G)-HIV-1 was used to identify and enrich restricted and active populations of HIV⁺ astrocytes based on the viral promoter activity. Here, we report that the majority of human astrocytes restricted R/G-HIV-1 gene expression early during infection and were resistant to reactivation by vorinostat and interleukin 1β. However, actively infected astrocytes were inducible, leading to increased expression of viral proteins upon reactivation. R/G-HIV-1 infection also significantly decreased the cell proliferation and glutamate clearance ability of astrocytes, which may contribute to excitotoxicity. Moreover, transcriptome analyses to compare gene expression patterns of astrocyte harboring active versus restricted long terminal repeats (LTRs) revealed that the gene expression patterns were similar and that the active population demonstrated more widespread and robust changes. Our data suggest that harboring the HIV genome profoundly alters astrocyte biology and that strategies that keep the virus latent (e.g., block and lock) or those that reactivate the latent virus (e.g., shock and kill) would be detrimental to astrocyte function and possibly augment their contributions to HAND.IMPORTANCE More than 36 million people are living with HIV-1 worldwide, and despite antiretroviral therapy, 30 to 50% of the people living with HIV-1 suffer from mild to moderate neurocognitive disorders. HIV-1 reservoirs in the central nervous system (CNS) are challenging to address due to low penetration of antiretroviral drugs, lack of resident T cells, and permanent integration of provirus into neural cells such as microglia and astrocytes. Several studies have shown astrocyte dysfunction during HIV-1 infection. However, little is known about how HIV-1 latency affects their function. The significance of our research is in identifying that the majority of HIV⁺ astrocytes restrict HIV expression and were resistant to reactivation. Further, simply harboring the HIV genome profoundly altered astrocyte biology, resulting in a proinflammatory phenotype and functional changes. In this context, therapeutic strategies to reactivate or silence astrocyte HIV reservoirs, without excising proviral DNA, will likely lead to detrimental neuropathological outcomes during HIV CNS infection.

Histone deacetylase 6 inhibition rescues axonal transport impairments and prevents the neurotoxicity of HIV-1 envelope protein gp120

Despite successful antiretroviral drug therapy, a subset of human immunodeficiency virus-1 (HIV)-positive individuals still display synaptodendritic simplifications and functional cognitive impairments referred to as HIV-associated neurocognitive disorders (HANDs). The neurological damage observed in HAND subjects can be experimentally reproduced by the HIV envelope protein gp120. However, the complete mechanism of gp120-mediated neurotoxicity is not entirely understood. Gp120 binds to neuronal microtubules and decreases the level of tubulin acetylation, suggesting that it may impair axonal transport. In this study, we utilized molecular and pharmacological approaches, in addition to microscopy, to examine the relationship between gp120-mediated tubulin deacetylation, axonal transport, and neuronal loss. Using primary rat cortical neurons, we show that gp120 decreases acetylation of tubulin and increases histone deacetylase 6 (HDAC6), a cytoplasmic enzyme that regulates tubulin deacetylation. We also demonstrate that the selective HDAC6 inhibitors tubacin and ACY-1215, which prevented gp120-mediated deacetylation of tubulin, inhibited the ability of gp120 to promote neurite shortening and cell death. We further observed by co-immunoprecipitation and confirmed with mass spectroscopy that exposure of neurons to gp120 decreases the association between tubulin and motor proteins, a well-established consequence of tubulin deacetylation. To assess the physiological consequences of this effect, we examined the axonal transport of brain-derived neurotrophic factor (BDNF). We report that gp120 decreases the velocity of BDNF transport, which was restored to baseline levels when neurons were exposed to HDAC6 inhibitors. Overall, our data suggest that gp120-mediated tubulin deacetylation causes impairment of axonal transport through alterations to the microtubule cytoskeleton.

HIV-1 Tat-Induced Astrocytic Extracellular Vesicle miR-7 Impairs Synaptic Architecture

Although combination antiretroviral therapy (cART) has improved the health of millions of those living with HIV-1 (Human Immunodeficiency Virus, Type 1), the penetration into the central nervous system (CNS) of many such therapies is limited, thereby resulting in residual neurocognitive impairment commonly referred to as NeuroHIV. Additionally, while cART has successfully suppressed peripheral viremia, cytotoxicity associated with the presence of viral Transactivator of transcription (Tat) protein in tissues such as the brain, remains a significant concern. Our previous study has demonstrated that both HIV-1 Tat as well as opiates such as morphine, can directly induce synaptic alterations via independent pathways. Herein, we demonstrate that exposure of astrocytes to HIV-1 protein Tat mediates the induction and release of extracellular vesicle (EV) microRNA-7 (miR-7) that is taken up by neurons, leading in turn, to downregulation of neuronal neuroligin 2 (NLGN2) and ultimately to synaptic alterations. More importantly, we report that these impairments could be reversed by pretreatment of neurons with a neurotrophic factor platelet-derived growth factor-CC (PDGF-CC).

HIV-associated neurodegeneration: exploitation of the neuronal cytoskeleton

Human immunodeficiency virus-1 (HIV) infection of the central nervous system damages synapses and promotes axonal injury, ultimately resulting in HIV-associated neurocognitive disorders (HAND). The mechanisms through which HIV causes damage to neurons are still under investigation. The cytoskeleton and associated proteins are fundamental for axonal and dendritic integrity. In this article, we review evidence that HIV proteins, such as the envelope protein gp120 and transactivator of transcription (Tat), impair the structure and function of the neuronal cytoskeleton. Investigation into the effects of viral proteins on the neuronal cytoskeleton may provide a better understanding of HIV neurotoxicity and suggest new avenues for additional therapies.

The Glucocorticoid Receptor Is a Critical Regulator of HIV Latency in Human Microglial Cells

We have developed models of HIV latency using microglia derived from adult human patient brain cortex and transformed with the SV40 T large and hTERT antigens. Latent clones infected by HIV reporter viruses display high levels of spontaneous HIV reactivation in culture. BrainPhys, a medium highly representative of the CNS extracellular environment, containing low glucose and 1% FBS, reduced, but did not prevent, HIV reactivation. We hypothesized that spontaneous HIV reactivation in culture was due to the expression of pro-inflammatory genes, such as TNF-α, taking place in the absence of the natural inhibitory signals from astrocytes and neurons. Indeed, expression and secretion of TNF-α is strongly reduced in HIV-latently infected microglia compared to the subset of cells that have undergone spontaneous HIV reactivation. Whereas inhibitors of NF-κB or of macrophage activation only had a short-term silencing effect, addition of dexamethasone (DEXA), a glucocorticoid receptor (GR) agonist and mediator of anti-inflammation, silenced the HIV provirus in a long-term, and shRNA-mediated knock-down of GR activated HIV. DEXA also decreased secretion of a number of cytokines, including TNF-α. Chromatin immunoprecipitation analysis revealed that DEXA strongly increased GR occupancy at the HIV promoter, and reduced histone 3 acetylated levels. Moreover, TNF-α expression inhibitors in combination with DEXA induced further HIV silencing and increased the histone 3 lysine 27 tri-methylated epigenetic mark of repression at the HIV promoter region. We conclude that GR is a critical repressor of HIV transcription in microglia, and a novel potential pharmacological target to restrict HIV expression in the CNS.

A Naturally Occurring Polymorphism in the HIV-1 Tat Basic Domain Inhibits Uptake by Bystander Cells and Leads to Reduced Neuroinflammation

HIV-1 Tat protein contributes to HIV-neuropathogenesis in several ways including its ability to be taken up by uninfected bystander CNS cells and to activate inflammatory host genes causing synaptic injury. Here, we report that in the globally dominant HIV-1 clade C, Tat displays a naturally occurring polymorphism, R57S, in its basic domain, which mediates cellular uptake. We examined the effect of this polymorphism on Tat uptake and its consequences for cellular gene transactivation. In decapeptides corresponding to the basic domain, a R57S substitution caused up to a 70% reduction in uptake. We also used a transcellular Tat transactivation assay, where we expressed Tat proteins of HIV-1 clade B (Tat-B) or C (Tat-C) or their position 57 variants in HeLa cells. We quantified the secreted Tat proteins and measured their uptake by TZM-bl cells, which provide readout via an HIV-1 Tat-responsive luciferase gene. Transactivation by Tat-B was significantly reduced by R57S substitution, while that of Tat-C was enhanced by the reciprocal S57R substitution. Finally, we exposed microglia to Tat variants and found that R57 is required for maximal neuroinflammation. The R57S substitution dampened this response. Thus, genetic variations can modulate the ability of HIV-1 Tat to systemically disseminate neuroinflammation.

HIV-1 Nef-induced lncRNA AK006025 regulates CXCL9/10/11 cluster gene expression in astrocytes through interaction with CBP/P300

Background

HIV-associated neurocognitive disorder (HAND) is a neurodegenerative disease associated with persistent neuroinflammation and subsequent neuron damage. Pro-inflammatory factors and neurotoxins from activated astrocytes by HIV-1 itself and its encoded proteins, including the negative factor (Nef), are involved in the pathogenesis of HAND. This study was designed to find potential lncRNAs that regulate astrocyte functions and inflammation process.

Methods

We performed microarray analysis of lncRNAs from primary mouse astrocytes treated with Nef protein. Top ten lncRNAs were validated through real-time PCR analysis. Gene ontology (GO) and KEGG pathway analysis were applied to explore the potential functions of lncRNAs. RIP and ChIP assays were performed to demonstrate the mechanism of lncRNA regulating gene expression.

Results

There were 638 co-upregulated lncRNAs and 372 co-downregulated lncRNAs in primary astrocytes treated with Nef protein for both 6 h and 12 h. GO and KEGG pathway analysis showed that the biological functions of top differential-expressed mRNAs were associated with inflammatory cytokines and chemokine. Knockdown of lncRNA AK006025, not AK138360, inhibited significantly CXCL9, CXCL10 (IP-10), and CXCL11 expression in astrocytes treated with Nef protein. Mechanism study showed that AK006025 associated with CBP/P300 was enriched in the promoter of CXCL9, CXCL10, and CXCL11 genes.

Conclusions

Our findings uncovered the expression profiles of lncRNAs and mRNAs in vitro, which might help to understand the pathways that regulate astrocyte activation during the process of HAND.

Electronic supplementary material

The online version of this article (10.1186/s12974-018-1343-x) contains supplementary material, which is available to authorized users.

Region-specific changes in markers of neuroplasticity revealed in HIV-1 transgenic rats by low-dose methamphetamine

Methamphetamine abuse co-occurring with HIV infection presents neuropathology in brain regions that mediate reward and motivation. A neuronal signaling cascade altered acutely by meth and some HIV-1 proteins is the mitogen-activated protein kinase (MAPK) pathway. It remains unknown if chronic co-exposure to meth and HIV-1 proteins converge on MAPK in vivo. To make this determination, we studied young adult Fischer 344 HIV-1 transgenic (Tg) and non-Tg rats that self-administered meth (0.02-0.04 mg/kg/0.05 ml iv infusion, 2 h/day for 21 days) and their saline-yoked controls. One day following the operant task, rats were killed. Brain regions involved in reward-motivation [i.e., nucleus accumbens (NA) and ventral pallidum (VP)], were assayed for a MAPK cascade protein, extracellular signal-regulated kinase (ERK), and a downstream transcription factor, ΔFosB. In the NA, activated (phosphorylated; p) ERK-to-ERK ratio (pERK/ERK) was increased in meth-exposed Tg rats versus saline Tg controls, and versus meth non-Tg rats. ΔFosB was increased in meth Tg rats versus saline and meth non-Tg rats. Assessment of two targets of ΔFosB-regulated transcription revealed (1) increased dopamine D1 receptor (D1R) immunoreactivity in the NA shell of Tg-meth rats versus saline Tg controls, but (2) no changes in the AMPA receptor subunit, GluA2. No changes related to genotype or meth occurred for ERK, ΔFosB or D1R protein in the VP. Results reveal a region-specific activation of ERK, and increases in ΔFosB and D1R expression induced by HIV-1 proteins and meth. Such effects may contribute to the neuronal and behavioral pathology associated with meth/HIV comorbidity.

Dose-dependent neurocognitive deficits following postnatal day 10 HIV-1 viral protein exposure: Relationship to hippocampal anatomy parameters

Despite the availability of antiretroviral prophylactic treatment, pediatric human immunodeficiency virus type 1 (HIV-1) continues to be a significant risk factor in the post-cART era. The time of infection (i.e., during pregnancy, delivery or breastfeeding) may play a role in the development of neurocognitive deficits in pediatric HIV-1. HIV-1 viral protein exposure on postnatal day (P)1, preceding the postnatal brain growth spurt in rats, had deleterious effects on neurocognitive development and anatomical parameters of the hippocampus (Fitting et al., 2008a,b). In the present study, rats were stereotaxically injected with HIV-1 viral proteins, including Tat1-86 and gp120, on P10 to further examine the role of timing on neurocognitive development and anatomical parameters of the hippocampus (Fitting et al., 2010). The dose-dependent virotoxin effects observed across development following P10 Tat1-86 exposure were specific to spatial learning and absent from prepulse inhibition and locomotor activity. A relationship between alterations in spatial learning and/or memory and hippocampal anatomical parameters was noted. Specifically, the estimated number of neurons and astrocytes in the hilus of the dentate gyrus explained 70% of the variance of search behavior in Morris water maze acquisition training for adolescents and 65% of the variance for adults; a brain-behavior relationship consistent with observations following P1 viral protein exposure. Collectively, late viral protein exposure (P10) results in selective alterations in neurocognitive development without modifying measures of somatic growth, preattentive processing, or locomotor activity, as characterized by early viral protein exposure (P1). Thus, timing may be a critical factor in disease progression, with children infected with HIV earlier in life being more vulnerable to CNS disease.

Tat is a multifunctional viral protein that modulates cellular gene expression and functions

The human immunodeficiency virus type I (HIV-1) has developed several strategies to condition the host environment to promote viral replication and spread. Viral proteins have evolved to perform multiple functions, aiding in the replication of the viral genome and modulating the cellular response to the infection. Tat is a small, versatile, viral protein that controls transcription of the HIV genome, regulates cellular gene expression and generates a permissive environment for viral replication by altering the immune response and facilitating viral spread to multiple tissues. Studies carried out utilizing biochemical, cellular, and genomic approaches show that the expression and activity of hundreds of genes and multiple molecular networks are modulated by Tat via multiple mechanisms.

The role of human dopamine transporter in NeuroAIDS

HIV-associated neurocognitive disorder (HAND) remains highly prevalent in HIV infected individuals and represents a special group of neuropathological disorders, which are associated with HIV-1 viral proteins, such as transactivator of transcription (Tat) protein. Cocaine abuse increases the incidence of HAND and exacerbates its severity by enhancing viral replication. Perturbation of dopaminergic transmission has been implicated as a risk factor of HAND. The presynaptic dopamine (DA) transporter (DAT) is essential for DA homeostasis and dopaminergic modulation of the brain function including cognition. Tat and cocaine synergistically elevate synaptic DA levels by acting directly on human DAT (hDAT), ultimately leading to dysregulation of DA transmission. Through integrated computational modeling and experimental validation, key residues have been identified in hDAT that play a critical role in Tat-induced inhibition of DAT and induce transporter conformational transitions. This review presents current information regarding neurological changes in DAT-mediated dopaminergic system associated with HIV infection, DAT-mediated adaptive responses to Tat as well as allosteric modulatory effects of novel compounds on hDAT. Understanding the molecular mechanisms by which Tat induces DAT-mediated dysregulation of DA system is of great clinical interest for identifying new targets for an early therapeutic intervention for HAND.

Changes in the cellular microRNA profile by the intracellular expression of HIV-1 Tat regulator: A potential mechanism for resistance to apoptosis and impaired proliferation in HIV-1 infected CD4+ T cells

HIV-1 induces changes in the miRNA expression profile of infected CD4+ T cells that could improve viral replication. HIV-1 regulator Tat modifies the cellular gene expression and has been appointed as an RNA silencing suppressor. Tat is a 101-residue protein codified by two exons that regulates the elongation of viral transcripts. The first exon of Tat (amino acids 1–72) forms the transcriptionally active protein Tat72, but the presence of the second exon (amino acids 73–101) results in a more competent regulatory protein (Tat101) with additional functions. Intracellular, full-length Tat101 induces functional and morphological changes in CD4+ T cells that contribute to HIV-1 pathogenesis such as delay in T-cell proliferation and protection against FasL-mediated apoptosis. But the precise mechanism by which Tat produces these changes remains unknown. We analyzed how the stable expression of intracellular Tat101 and Tat72 modified the miRNA expression profile in Jurkat cells and if this correlated with changes in apoptotic pathways and cell cycle observed in Tat-expressing cells. Specifically, the enhanced expression of hsa-miR-21 and hsa-miR-222 in Jurkat-Tat101 cells was associated with the reduced expression of target mRNAs encoding proteins related to apoptosis and cell cycle such as PTEN, PDCD4 and CDKN1B. We developed Jurkat cells with stable expression of hsa-miR-21 or hsa-miR-222 and observed a similar pattern to Jurkat-Tat101 in resistance to FasL-mediated apoptosis, cell cycle arrest in G₂/M and altered cell morphology. Consequently, upregulation of hsa-miR-21 and hsa-miR-222 by Tat may contribute to protect against apoptosis and to anergy observed in HIV-infected CD4+ T cells.

HIV, Tat and dopamine transmission

Human Immunodeficiency Virus (HIV) is a progressive infection that targets the immune system, affecting more than 37 million people around the world. While combinatorial antiretroviral therapy (cART) has lowered mortality rates and improved quality of life in infected individuals, the prevalence of HIV associated neurocognitive disorders is increasing and HIV associated cognitive decline remains prevalent. Recent research has suggested that HIV accessory proteins may be involved in this decline, and several studies have indicated that the HIV protein transactivator of transcription (Tat) can disrupt normal neuronal and glial function. Specifically, data indicate that Tat may directly impact dopaminergic neurotransmission, by modulating the function of the dopamine transporter and specifically damaging dopamine-rich regions of the CNS. HIV infection of the CNS has long been associated with dopaminergic dysfunction, but the mechanisms remain undefined. The specific effect(s) of Tat on dopaminergic neurotransmission may be, at least partially, a mechanism by which HIV infection directly or indirectly induces dopaminergic dysfunction. Therefore, precisely defining the specific effects of Tat on the dopaminergic system will help to elucidate the mechanisms by which HIV infection of the CNS induces neuropsychiatric, neurocognitive and neurological disorders that involve dopaminergic neurotransmission. Further, this will provide a discussion of the experiments needed to further these investigations, and may help to identify or develop new therapeutic approaches for the prevention or treatment of these disorders in HIV-infected individuals.

Tat-Mediated Induction of miRs-34a & -138 Promotes Astrocytic Activation via Downregulation of SIRT1: Implications for Aging in HAND

Astrocyte activation is a hallmark of HIV infection and aging in the CNS. In chronically infected HIV patients, prolonged activation of astrocytes has been linked to accelerated aging including but not limited to neurocognitive impairment and frailty. The current study addresses the role of HIV protein Tat in inducing a set of small noncoding microRNAs (miRNA) that play critical role in astrogliosis. In our efforts to link astrocyte activation as an indicator of aging, we assessed the brains of both wild type and HIV transgenic rats for the expression of glial fibrillary acidic protein (GFAP). As expected, in the WT animals we observed age-dependent increase in astrogliosis in the older animals compared to the younger group. Interestingly, compared to the young WT group, young HIV Tg rats exhibited higher levels of GFAP in this trend was also observed in the older HIV Tg rats compared to the older WT group. Based on the role of SIRT1 in aging and the regulation of SIRT1 by miRNAs-34a and -138, we next assessed the expression levels of these miRs in the brains of both the young an old WT and HIV Tg rats. While there were no significant differences in the young WT versus the HIV Tg rats, in the older HIV Tg rats there was a significant upregulation in the expression of miRs-34a & -138 in the brains. Furthermore, increased expression of miRs-34a & -138 in the older Tg rats, correlated with a concomitant decrease in their common anti-aging target protein SIRT1, in the brains of these animals. To delineate the mechanism of action we assessed the role of HIV-Tat (present in the Tg rats) in inducing miRs-34a & -138 in both the primary astrocytes and the astrocytoma cell line A172, thereby leading to posttranscriptional suppression of SIRT1 with a concomitant up regulation of NF-kB driven expression of GFAP.

Human Immunodeficiency Virus Promotes Mitochondrial Toxicity

Combined antiretroviral therapies (cART) have had remarkable success in reducing morbidity and mortality among patients infected with human immunodeficiency virus (HIV). However, mild forms of HIV-associated neurocognitive disorders (HAND), characterized by loss of synapses, remain. cART may maintain an undetectable HIV RNA load but does not eliminate the expression of viral proteins such as trans-activator of transcription (Tat) and the envelope glycoprotein gp120 in the brain. These two viral proteins are known to promote synaptic simplifications by several mechanisms, including alteration of mitochondrial function and dynamics. In this review, we aim to outline the many targets and pathways used by viral proteins to alter mitochondria dynamics, which contribute to HIV-induced neurotoxicity. A better understanding of these pathways is crucial for the development of adjunct therapies for HAND.

HIV-1 Tat and Cocaine Impair Survival of Cultured Primary Neuronal Cells via a Mitochondrial Pathway

Addictive stimulant drugs, such as cocaine, are known to increase the risk of exposure to HIV-1 infection and hence predispose towards the development of AIDS. Previous findings suggested that the combined effect of chronic cocaine administration and HIV-1 infection enhances cell death. Neuronal survival is highly dependent on the health of mitochondria providing a rationale for assessing mitochondrial integrity and functionality following cocaine treatment, either alone or in combination with the HIV-1 viral protein Tat, by monitoring ATP release and mitochondrial membrane potential (ΔΨm). Our results indicate that exposing human and rat primary hippocampal neurons to cocaine and HIV-1 Tat synergistically decreased both mitochondrial membrane potential and ATP production. Additionally, since previous studies suggested HIV-1 infection alters autophagy in the CNS, we investigated how HIV-1 Tat and cocaine affect autophagy in neurons. The results indicated that Tat induces an increase in LC3-II levels and the formation of Parkin-ring-like structures surrounding damaged mitochondria, indicating the possible involvement of the Parkin/PINK1/DJ-1 (PPD) complex in neuronal degeneration. The importance of mitochondrial damage is also indicated by reductions in mitochondrial membrane potential and ATP content induced by HIV-1 Tat and cocaine.

Prefrontal cortical thinning in HIV infection is associated with impaired striatal functioning

While cortical thinning has been associated with HIV infection, it is unclear whether this reflects a direct effect of the virus, whether it is related to disruption of subcortical function or whether it is better explained by epiphenomena, such as drug abuse or comorbid medical conditions. The present study investigated the relationship between cortical thickness and subcortical function in HIV+ patients. Specifically, we examined the relationship between prefrontal cortical thickness and striatal function. Twenty-three largely treatment naïve, non-substance abusing HIV+ participants and 19 healthy controls matched for age, gender, and educational status were included. Cortical morphometry was performed using FreeSurfer software analysis. Striatal function was measured during an fMRI stop-signal anticipation task known to engage the striatum. Any cortical regions showing significant thinning were entered as dependent variables into a single linear regression model which included subcortical function, age, CD4 count, and a measure of global cognitive performance as independent predictors. The only cortical region that was significantly reduced after correction for multiple comparisons was the right superior frontal gyrus. Striatal activity was found to independently predict superior frontal gyral cortical thickness. While cortical thinning in HIV infection is likely multifactorial, viral induced subcortical dysfunction appears to play a role.

Interaction between Tat and Drugs of Abuse during HIV-1 Infection and Central Nervous System Disease

In many individuals, drug abuse is intimately linked with HIV-1 infection. In addition to being associated with one-third of all HIV-1 infections in the United States, drug abuse also plays a role in disease progression and severity in HIV-1-infected patients, including adverse effects on the central nervous system (CNS). Specific systems within the brain are known to be damaged in HIV-1-infected individuals and this damage is similar to that observed in drug abuse. Even in the era of anti-retroviral therapy (ART), CNS pathogenesis occurs with HIV-1 infection, with a broad range of cognitive impairment observed, collectively referred to as HIV-1-associated neurocognitive disorders (HAND). A number of HIV-1 proteins (Tat, gp120, Nef, Vpr) have been implicated in the etiology of pathogenesis and disease as a result of the biologic activity of the extracellular form of each of the proteins in a number of tissues, including the CNS, even in ART-suppressed patients. In this review, we have made Tat the center of attention for a number of reasons. First, it has been shown to be synthesized and secreted by HIV-1-infected cells in the CNS, despite the most effective suppression therapies available to date. Second, Tat has been shown to alter the functions of several host factors, disrupting the molecular and biochemical balance of numerous pathways contributing to cellular toxicity, dysfunction, and death. In addition, the advantages and disadvantages of ART suppression with regard to controlling the genesis and progression of neurocognitive impairment are currently under debate in the field and are yet to be fully determined. In this review, we discuss the individual and concerted contributions of HIV-1 Tat, drug abuse, and ART with respect to damage in the CNS, and how these factors contribute to the development of HAND in HIV-1-infected patients.

The cross-talk of HIV-1 Tat and methamphetamine in HIV-associated neurocognitive disorders

Antiretroviral therapy has dramatically improved the lives of human immunodeficiency virus 1 (HIV-1) infected individuals. Nonetheless, HIV-associated neurocognitive disorders (HAND), which range from undetectable neurocognitive impairments to severe dementia, still affect approximately 50% of the infected population, hampering their quality of life. The persistence of HAND is promoted by several factors, including longer life expectancies, the residual levels of virus in the central nervous system (CNS) and the continued presence of HIV-1 regulatory proteins such as the transactivator of transcription (Tat) in the brain. Tat is a secreted viral protein that crosses the blood–brain barrier into the CNS, where it has the ability to directly act on neurons and non-neuronal cells alike. These actions result in the release of soluble factors involved in inflammation, oxidative stress and excitotoxicity, ultimately resulting in neuronal damage. The percentage of methamphetamine (MA) abusers is high among the HIV-1-positive population compared to the general population. On the other hand, MA abuse is correlated with increased viral replication, enhanced Tat-mediated neurotoxicity and neurocognitive impairments. Although several strategies have been investigated to reduce HAND and MA use, no clinically approved treatment is currently available. Here, we review the latest findings of the effects of Tat and MA in HAND and discuss a few promising potential therapeutic developments.

Neuroretinal Degeneration in HIV Patients Without Opportunistic Ocular Infections in the cART Era

Subtle structural and functional retinal abnormalities, termed 'HIV-associated Neuroretinal Disorder (HIV-NRD)', have been reported in HIV patients receiving combination antiretroviral therapy (cART), without infectious retinitis or any apparent fundus abnormalities otherwise. In this review, we provide an overview of studies investigating HIV-NRD in HIV patients without opportunistic ocular infections in the cART era, and try to elucidate underlying mechanisms and associated risk factors. Most studies focused on patients with severe immune-deficiency and demonstrated that patients with nadir CD4 counts<100 cells/μL are most at risk for neuroretinal damage, with a thinner retinal nerve fiber layer, subtle loss of color vision and/or contrast sensitivity, visual field deficits, and subnormal electrophysiological responses. In contrast, alterations in retinal vascular calibers and retinal blood flow were not associated with nadir CD4 counts, but instead with detectable viremia, suggesting a role for (chronic) inflammation in microvascular damage. Although the alterations in visual function are subtle, they can lead to difficulties in activities, such as reading or driving, thereby affecting quality of life. Since HIV has become a chronic disease, its long-term effects with respect to visual function loss become more important, as is recently emphasized by a longitudinal study, reporting that AIDS patients with HIV-NRD have higher risks of developing bilateral visual impairment and even blindness than patients without HIV-NRD. The question remains whether patients with high (>350 cells/μL) nadir CD4 counts and well-suppressed HIV infection on cART remain at risk for HIV-NRD, as this group constitutes a growing part of the aging HIV-infected population.

HIV infection results in ventral-striatal reward system hypo-activation during cue processing

OBJECTIVE

Functional MRI has thus far demonstrated that HIV has an impact on frontal-striatal systems involved in executive functioning. The potential impact of HIV on frontal-striatal systems involved in reward processing has yet to be examined by functional MRI. This study therefore aims to investigate the effects of HIV infection on reward processing by examining the function of the ventral-striatal reward system during a monetary incentive delay task.

DESIGN

This is a cross-sectional case-control study.

METHODS

Eighteen combined antiretroviral therapy-naive HIV-positive (HIV+) participants, as well as 16 matched healthy controls, performed a monetary incentive delay task. This paradigm assesses behaviour as well as functional brain activity-associated reward anticipation and reward outcome.

RESULTS

HIV+ participants showed a general decrease in activation associated with both neutral as well as potentially rewarding cues in their ventral striatum. We found normal activity related to reward outcome in the orbito-frontal cortex. Despite HIV+ participants' reaction times being significantly slower when independently measured from the reward paradigm, this performance deficit normalized during the performance of the reward task.

CONCLUSION

HIV caused a decrease in activity during cue processing in the ventral striatum, with normal cortical functioning during reward outcome processing. Our results therefore suggest that HIV not only has an impact on fronto-striatal systems involved in executive functioning, but also has a direct impact on the function of the ventral-striatal reward system.

Tat-induced histopathological alterations mediate hippocampus-associated behavioural impairments in rats

Background

HIV-1 is a global catastrophe, and is exceedingly prevalent in Sub-Saharan Africa. HIV-associated neurocognitive disorder is characterized by symptoms such as motor impairments, a decline in cognition, and behavioural irregularities. The aim of this study was to provide insight into the fundamental behavioural and histopathological mechanisms underlying the development and progression of HIV-1 neuropathology.

Methods

Using stereotaxic techniques, Tat protein Clade B (1 μg/μl, 10 μl) was injected bilaterally into the dorsal hippocampus of male Sprague–Dawley rats. The Morris water maze (MWM) and novel object recognition test (NORT) were used to assess spatial learning and recognition memory, respectively. Haematoxylin and eosin staining was used to identify the histopathological changes.

Results

A highly significant increase in latency to reach the hidden platform in the MWM implied that noteworthy hippocampal damage had occurred. Severe behavioural deficits were also observed in the NORT where the Tat-injected group showed a greater preference for a familiar object over a novel one. This damage was confirmed by the histopathological changes (increased astrogliosis, cells becoming eosinophilic and a significant reduction in the pyramidal cell layer) observed in the hippocampus. Additionally, increases in the hippocampal mass and protein were observed, consistent with the structural alterations.

Conclusion

This study highlights the relationship between hippocampal-associated behavioural changes and histologic alterations following stereotaxic intra-hippocampal administration of Tat protein in rats. The implications of this study may positively impact the fields of immunology and neuroscience by encouraging future researchers to consider novel strategies to understand the complexities of the pathogenesis of HIV-associated neurocognitive disorder.

Neonatal intrahippocampal HIV-1 protein Tat(1-86) injection: neurobehavioral alterations in the absence of increased inflammatory cytokine activation

Pediatric AIDS caused by human immunodeficiency virus type 1 (HIV-1) remains one of the leading worldwide causes of childhood morbidity and mortality. HIV-1 proteins, such as Tat and gp120, are believed to play a crucial role in the neurotoxicity of pediatric HIV-1 infection. Detrimental effects on development, behavior, and neuroanatomy follow neonatal exposure to the HIV-1 viral toxins Tat1-72 and gp120. The present study investigated the neurobehavioral effects induced by the HIV-1 neurotoxic protein Tat1-86, which encodes the first and second exons of the Tat protein. In addition, the potential effects of HIV-1 toxic proteins Tat1-86 and gp120 on inflammatory pathways were examined in neonatal brains. Vehicle, 25 μg Tat1-86 or 100 ng gp120 was injected into the hippocampus of male Sprague-Dawley pups on postnatal day 1 (PD1). Tat1-86 induced developmental neurotoxic effects, as witnessed by delays in eye opening, delays in early reflex development and alterations in prepulse inhibition (PPI) and between-session habituation of locomotor activity. Overall, the neurotoxic profile of Tat1-86 appeared more profound in the developing nervous system in vivo relative to that seen with the first exon encoded Tat1-72 (Fitting et al., 2008b), as noted on measures of eye opening, righting reflex, and PPI. Neither the direct PD1 CNS injection of the viral HIV-1 protein variant Tat1-86, nor the HIV-1 envelope protein gp120, at doses sufficient to induce neurotoxicity, necessarily induced significant expression of the inflammatory cytokine IL-1β or inflammatory factors NF-κβ and I-κβ. The findings agree well with clinical observations that indicate delays in developmental milestones of pediatric HIV-1 patients, and suggest that activation of inflammatory pathways is not an obligatory response to viral protein-induced neurotoxicity that is detectable with behavioral assessments. Moreover, the amino acids encoded by the second tat exon may have unique actions on the developing hippocampus.

HIV-1 endocytosis in astrocytes: a kiss of death or survival of the fittest?

The brain is a target of HIV-1 and serves as an important viral reservoir. Astrocytes, the most abundant glial cell in the human brain, are involved in brain plasticity and neuroprotection. Several studies have reported HIV-1 infection of astrocytes in cell cultures and infected brain tissues. The prevailing concept is that HIV-1 infection of astrocytes leads to latent infection. Here, we provide our perspective on endocytosis-mediated HIV-1 entry and its fate in astrocytes. Natural entry of HIV-1 into astrocytes occurs via endocytosis. However, endocytosis of HIV-1 in astrocytes is a natural death trap where the majority of virus particles are degraded in endosomes and a few which escape intact lead to successful infection. Thus, regardless of artificial fine-tuning (treatment with cytokines or proinflammatory products) done to astrocytes, HIV-1 does not infect them efficiently unless the viral entry route or the endosomal enzymatic machinery has been manipulated.

HIV-1 Nef expression in microglia disrupts dopaminergic and immune functions with associated mania-like behaviors

BACKGROUND

Neuropsychiatric disorders during HIV/AIDS are common although the contribution of HIV-1 infection within the brain, and in particular individual HIV-1 proteins, to the development of these brain disorders is unknown. Herein, an in vivo transgenic mouse model was generated in which the HIV-1 Nef protein was expressed in microglia cells, permitting investigation of neurobehavioral phenotypes and associated cellular and molecular properties.

METHODS

Transgenic (Tg) mice that expressed full length HIV-1 nef under the control of the c-fms promoter and wildtype (Wt) littermates were investigated using different measures of neurobehavioral performance including locomotory, forced swim (FST), elevated plus maze (EPM) and T-maze tests. Host gene and transgene expression were assessed by RT-PCR, immunoblotting, enzymatic activity and immunohistochemistry. Biogenic amine levels were measured by HPLC with electrochemical detection.

RESULTS

Tg animals exhibited Nef expression in brain microglia and cultured macrophages. Tg males displayed hyperactive behaviors including augmented locomotor activity, decreased immobility in the FST and increased open-arm EPM exploration compared to Wt littermates (p<0.05). Tg animals showed increased CCL2 expression with concurrent IFN-α suppression in striatum compared with Wt littermates (p<0.05). Dopamine levels, MAO activity and the dopamine transporter (DAT) expression were reduced in the striatum of Tg animals (p<0.05).

CONCLUSIONS

HIV-1 Nef expression in microglia induced CCL2 expression together with disrupting striatal dopaminergic transmission, resulting in hyperactive behaviors which are observed in mania and other psychiatric comorbidities among HIV-infected persons. These findings emphasize the selective effects of individual viral proteins in the brain and their participation in neuropathogenesis.

PACAP27 is protective against tat-induced neurotoxicity

Human immunodeficiency virus type-1 (HIV) infection of the central nervous system promotes neuronal injury and apoptosis that culminate in HIV-associated neurocognitive disorders (HAND). Viral proteins, such as transactivator of transcription (Tat), have emerged as leading candidates to explain HIV-mediated neurotoxicity, though the mechanism remains unclear. To determine the effects of Tat, rat cortical neurons were exposed to nanomolar concentrations of Tat for various time points. Within a few hours, Tat induced the production of reactive oxygen species (ROS), and other indices of mitochondrial destabilization. In addition, we observed a significant induction of DNA double-strand breaks (DSBs) by Tat. We next investigated the neuroprotective activity of the pituitary adenylate cyclase-activating polypeptide 27 (PACAP27) against these cardinal features of Tat-induced neurodegeneration. PACAP27 (100 nM) inhibited all Tat-mediated toxic effects including DNA DSBs. Importantly, PACAP27 prevented the induction of neuronal loss induced by Tat. The neuroprotective effect of PACAP27 is correlated with its ability to release the anti-apoptotic chemokine CCL5. Our data support a mechanism of Tat neurotoxicity in which Tat induces mitochondrial destabilization, thus increasing the release of ROS, which causes DNA DSBs leading to cell death. PACAP27, through CCL5, mitigates the effects of Tat-induced neuronal dysfunction, suggesting that PACAP27 could be a new strategy for an adjunct therapy against HIV-associated neurocognitive disorders.

Cognitive impairment and major depressive disorder in HIV infection and cerebrospinal fluid biomarkers

Cognitive impairment and major depressive disorder (MDD) are common HIV-1 central nervous system (CNS) complications. Their frequencies in AIDS patients are 36% and 45%, respectively. The diagnoses of HIV cognitive impairment are made by clinical criteria, no single laboratory test or biomarker establishes the diagnosis. Factors of indirect neuronal injury related with the pathophysiology of the HIV infection in the CNS, are the factors studied as biomarkers. In the present no biomarker is established to the diagnosis of HIV cognitive impairment, much still needs to be done. We review in this paper some biomarkers in cerebrospinal fluid that could be valuable to the diagnosis of HIV cognitive impairment. Diagnosing depression in the context of HIV can be challenging, to identify a biomarker that could help in the diagnosis would be very important, although MDD risks and neurobiology are still poorly understood.

Astrocytic expression of HIV-1 viral protein R in the hippocampus causes chromatolysis, synaptic loss and memory impairment

BACKGROUND

HIV-infected individuals are at an increased risk of developing neurological abnormalities. HIV induces neurotoxicity by host cellular factors and individual viral proteins. Some of these proteins including viral protein R (Vpr) promote immune activation and neuronal damage. Vpr is known to contribute to cell death of cultured rat hippocampal neurons and suppresses axonal growth. Behavioral studies are limited and suggest hyperactivity in the presence of Vpr. Thus Vpr may play a role in hippocampal loss of function. The purpose of this study is to determine the ability of HIV-1 Vpr production by astrocytes in the hippocampus to cause neurological deficits and memory impairments.

METHODS

We tested the performance of rats in novel object and novel location tasks after hippocampal infusion with astrocytes expressing HIV-1 Vpr. Synaptic injury and morphological changes were measured by synaptophysin immunoreactivity and Nissl staining.

RESULTS

Vpr-infused rats showed impaired novel location and novel object recognition compared with control rats expressing green fluorescent protein (GFP). This impairment was correlated with a significant decrease in synaptophysin immunoreactivity in the hippocampal CA3 region, suggesting synaptic injury in HIV-1 Vpr-treated animals. In addition, Nissl staining showed morphological changes indicative of neuronal chromatolysis in the Vpr group. The Vpr-induced neuronal damage and synaptic loss suggest that neuronal dysfunction caused the spatial and recognition memory deficits found in the Vpr-infused animals.

CONCLUSIONS

In this study, we demonstrate that HIV-1 Vpr produced by astrocytes in the hippocampus impairs hippocampal-dependent learning. The data suggest Vpr is a neurotoxin with the potential to cause learning impairment in HIV-1 infected individuals even under conditions of limited viral replication.

Human immunodeficiency virus-associated dementia: a link between accumulation of viral proteins and neuronal degeneration

In the late stage of human immunodeficiency virus-1 (HIV) infection, a subset of individuals develops HIV associated neurocognitive disorders (HAND), which in its severe form, is characterized by motor and cognitive dysfunction. Dendritic pruning, synaptic abnormalities and neuronal apoptosis are observed in these patients. There are numerous advances in our understanding of HIV interactions with cells of the central nervous system. However, the underlying causes of neurological symptoms and pathological alterations observed in HIV positive subjects are poorly understood. Moreover, little is still known about the molecular mechanisms by which HIV induces synaptic dysfunction and degeneration. HAND resembles other common neurological diseases such as Alzheimer's and Huntington's diseases. These neurodegenerative disorders are characterized by accumulation of toxic proteins such as tau and huntingtin, respectively, which promote axonal degeneration by impairing axonal transport. Axonal degeneration precedes neuronal death. Therefore, a better understanding of the mechanisms whereby HIV triggers axonal degeneration has potential implications for developing therapeutic compounds to prevent synaptic failure in HAND. This article highlights and reviews evidence showing that neuronal accumulation of viral proteins promotes axonal damage.

Genetic variation and HIV-associated neurologic disease

HIV-associated neurologic disease continues to be a significant complication in the era of highly active antiretroviral therapy. A substantial subset of the HIV-infected population shows impaired neuropsychological performance as a result of HIV-mediated neuroinflammation and eventual central nervous system (CNS) injury. CNS compartmentalization of HIV, coupled with the evolution of genetically isolated populations in the CNS, is responsible for poor prognosis in patients with AIDS, warranting further investigation and possible additions to the current therapeutic strategy. This chapter reviews key advances in the field of neuropathogenesis and studies that have highlighted how molecular diversity within the HIV genome may impact HIV-associated neurologic disease. We also discuss the possible functional implications of genetic variation within the viral promoter and possibly other regions of the viral genome, especially in the cells of monocyte-macrophage lineage, which are arguably key cellular players in HIV-associated CNS disease.

Update on HIV-associated neurocognitive disorders

Neurocognitive disorders are a feared complication of HIV infection, especially in the post-antiretroviral era as patients are living longer. These disorders are challenging in terms of diagnosis and treatment. The clinical syndrome has evolved, driven in part by comorbidities such as aging, drug abuse, psychiatric illnesses, and a metabolic syndrome associated with the use of antiretroviral drugs. Additionally some individuals may develop a fulminant immune reconstitution syndrome. Hence, treatment of these patients needs to be individualized. The focus of research in the HIV field has recently switched towards elimination of the HIV reservoir as a means of combating long-term HIV complications. However, these approaches may be suitable for limited populations and might not be applicable once the HIV reservoir has been established in the brain. Further, all clinical trials using neuroprotective or anti-inflammatory drugs for treatment of HIV-associated neurocognitive disorders have been unsuccessful. Hence, neurological complications of HIV infection are the biggest challenge facing HIV researchers, and there is a critical need to develop new diagnostics and approaches for treatment of these disorders.

When human immunodeficiency virus meets chemokines and microglia: neuroprotection or neurodegeneration?

Chemokines are chemotactic cytokines that were originally discovered as promoters of leukocyte proliferation and mobility. In recent years, however, evidence has demonstrated constitutive expression of chemokines and chemokine receptors in a variety of cells in the central and peripheral nervous system and has proposed a role for chemokines in neurodegenerative diseases characterized by inflammation and microglia proliferation. In addition, chemokine receptors, and in particular CXCR4 and CCR5, mediate human immunodeficiency virus type 1 (HIV) infection of immunocompetent cells as well as microglia. Subsequently, HIV, through a variety of mechanisms, promotes synapto-dendritic alterations and neuronal loss that ultimately lead to motor and cognitive impairments. These events are accompanied by microglia activation. Nevertheless, a microglia-mediated mechanism of neuronal degeneration alone cannot fully explain some of the pathological features of HIV infected brain such as synaptic simplification. In this article, we present evidence that some of the microglia responses to HIV are beneficial and neuroprotective. These include the ability of microglia to release anti-inflammatory cytokines, to remove dying cells and to promote axonal sprouting.

Genotypical diversity of HIV clades and central nervous system impairment

The central nervous system (CNS) and the immune system are considered major target organs for HIV infection. The neurological manifestations directly related to HIV are acute viral meningitis, chronic meningitis, HIV associated dementia, vacuolar myelopathy and involvement of the peripheral nervous system. Changes in diagnosis and clinical management have changed the aspect of HIV infection so that it is no longer a fatal disease, and has become a chronic disease requiring sustained medical management. After HAART the incidence of most opportunistic infections, including those affecting the CNS, has dropped markedly. Some studies suggest that neurological involvement of infected patient occur with different frequency, depending on HIV subtype involved in the infection. Subtype C may have reduced neuroinvasive capacity, possibly due to its different primary conformation of HIV transactivating regulatory protein (Tat), involved in monocyte chemotaxis. This review focus on physiopathologic aspects of HIV infection in CNS and its correlation with HIV clades.

Subtype selective NMDA receptor antagonists induce recovery of synapses lost following exposure to HIV-1 Tat

BACKGROUND AND PURPOSE

Neurocognitive disorders afflict approximately 20% of HIV-infected patients. HIV-1-infected cells in the brain shed viral proteins such as transactivator of transcription (Tat). Tat elicits cell death and synapse loss via processes initiated by NMDA receptor activation but mediated by separate downstream signalling pathways. Subunit selective NMDA receptor antagonists may differentially modulate survival relative to synaptic changes.

EXPERIMENTAL APPROACH

Tat-evoked cell death was quantified by measuring propidium iodide uptake into rat hippocampal neurons in culture. The effects of Tat on synaptic changes were measured using an imaging-based assay that quantified clusters of the scaffolding protein postsynaptic density 95 fused to green fluorescent protein.

KEY RESULTS

Dizocilpine, a non-competitive NMDA receptor antagonist, inhibited Tat-induced synapse loss, subsequent synapse recovery and Tat-induced cell death with comparable potencies. Memantine (10 µM) and ifenprodil (10 µM), which preferentially inhibit GluN2B-containing NMDA receptors, protected from Tat-induced cell death with no effect on synapse loss. Surprisingly, memantine and ifenprodil induced synapse recovery in the presence of Tat. In contrast, the GluN2A-prefering antagonist TCN201 prevented synapse loss and recovery with no effect on cell death.

CONCLUSIONS AND IMPLICATIONS

Synapse loss is a protective mechanism that enables the cell to cope with excess excitatory input. Thus, memantine and ifenprodil are promising neuroprotective drugs because they spare synaptic changes and promote survival. These GluN2B-preferring drugs induced recovery from Tat-evoked synapse loss, suggesting that synaptic pharmacology changed during the neurotoxic process. NMDA receptor subtypes differentially participate in the adaptation and death induced by excitotoxic insult.

Expression of HIV-Tat protein is associated with learning and memory deficits in the mouse

HIV-Tat protein has been implicated in the pathogenesis of HIV-1 neurological complications (i.e., neuroAIDS), but direct demonstrations of the effects of Tat on behavior are limited. GT-tg mice with a doxycycline (Dox)-inducible and brain-selective tat gene coding for Tat protein were used to test the hypothesis that the activity of Tat in brain is sufficient to impair learning and memory processes. Western blot analysis of GT-tg mouse brains demonstrated an increase in Tat antibody labeling that seemed to be dependent on the dose and duration of Dox pretreatment. Dox-treated GT-tg mice tested in the Barnes maze demonstrated longer latencies to find an escape hole and displayed deficits in probe trial performance versus uninduced GT-tg littermates, suggesting Tat-induced impairments of spatial learning and memory. Reversal learning was also impaired in Tat-induced mice. Tat-induced mice additionally demonstrated long-lasting (up to one month) deficiencies in novel object recognition learning and memory performance. Furthermore, novel object recognition impairment was dependent on the dose and duration of Dox exposure, suggesting that Tat exposure progressively mediated deficits. These experiments provide evidence that Tat protein expression is sufficient to mediate cognitive abnormalities seen in HIV-infected individuals. Moreover, the genetically engineered GT-tg mouse may be useful for improving our understanding of the neurological underpinnings of neuroAIDS-related behaviors.

Methamphetamine and HIV-1 Tat down regulate β-catenin signaling: implications for methampetamine abuse and HIV-1 co-morbidity

Methamphetamine (Meth) abuse exacerbates HIV-1-associated neurocognitive disorders (HAND). The underlying mechanism for this effect is not entirely clear but likely involves cooperation between Meth and HIV-1 virotoxins, such as the transactivator of transcription, Tat. HIV-1 Tat mediates damage in the CNS by inducing inflammatory processes including astrogliosis. Wnt/β-catenin signaling regulates survival processes for both neurons and astrocytes. Here, we evaluated the impact of Meth on the Wnt/β-catenin pathway in astrocytes transfected with Tat. Meth and Tat downregulated Wnt/β-catenin signaling by >50%, as measured by TOPflash reporter activity in both an astrocytoma cell line and primary human fetal astrocytes. Meth and Tat also downregulated LEF-1 transcript by >30%. LEF-1 is a key partner of β-catenin to regulate cognate gene expression. Interestingly, estrogen, which induces β-catenin signaling in a cell-type specific manner, at physiological concentrations of 1.5 and 3 nM normalized individual Meth and Tat effects on β-catenin signaling but not their combined effects. These findings suggest that Meth and Tat likely exert different mechanisms to mediate down regulation of β-catenin signaling. The consequences of which may contribute to the pathophysiologic effects of HIV-1 and Meth co-morbidity in the CNS.

Intracerebroventricular administration of HIV-1 Tat induces brain cytokine and indoleamine 2,3-dioxygenase expression: a possible mechanism for AIDS comorbid depression

Human immunodeficiency virus (HIV) remains a major public health concern despite a large education effort during the past 25 years. A persistent problem with HIV infection is the high comorbity rate of clinical depression. We previously established that increasing proinflammatory cytokines within the brain of mice induces sickness that can culminate in depressive-like behavior. Here we investigated the role of the HIV transactivator of transcription (Tat) protein in activation of brain cytokine signaling and subsequent induction of depressive-like behavior in a murine model. Adult Balb/c mice were administered a single intracerebroventricular (ICV) injection of Tat (40 ng). Social investigation of a novel juvenile was measured at 2, 4, 8 and 24 h post-treatment. Mice treated with Tat did not display signs of sickness, as measured by either decreased social investigation or loss of body weight. At 24 h post-injection, mice were subjected to the forced swim test (FST). ICV administration of Tat to Balb/c mice increased immobility in the FST at 24 h post injection. A different strain of mice, C57BL/6J, responded similarly in the FST. Furthermore, adult C57BL/6J mice injected with Tat and tested in a two-bottle 1% sucrose preference test displayed reduced preference for sucrose during the 24 h post-injection period. Subsequently, brain tissues from Tat-treated and control C57BL/6J mice were collected at 4 and 24 h post injection. CNS tissue from Tat-treated mice had increased expression of IL-1β, TNF-α, IL-6, and IDO mRNAs at 4 h post injection. These data demonstrate that a single exposure to Tat in the brain is sufficient to induce brain cytokine signaling that culminates in depressive-like behavior. The results reveal a potential role for Tat in the development of comorbid depression in HIV-infected individuals.

Neurotoxicity of human immunodeficiency virus-1: viral proteins and axonal transport

Human immunodeficiency virus-1 (HIV) infection of the central nervous system may cause a neurological syndrome termed HIV-associated neurocognitive disorder (HAND) which includes minor neurocognitive disorders or a more severe form of motor and cognitive impairments. Although treatment with highly active antiretroviral agents decreases the load of HIV in the brain, the prevalence of mild forms of HAND is actually increased due to longer life. Therefore, adjunctive and combined therapies must be developed to prevent and perhaps reverse the neurologic deficits observed in individuals with HAND. Key to developing effective therapies is a better understanding of the molecular and cellular mechanisms by which the virus causes this disorder. A number of HIV proteins has been shown to be released from HIV-infected cells. Moreover, these proteins have been shown to possess neurotoxic properties. This review describes new evidence of a direct interaction of the HIV protein gp120 with neurons, which might play a role in the etiopathology of HAND.

Neurodegenerative effects of recombinant HIV-1 Tat(1-86) are associated with inhibition of microtubule formation and oxidative stress-related reductions in microtubule-associated protein-2(a,b)

The human immunodeficiency virus 1 (HIV-1) protein Trans-activator of Transcription (Tat) is a nuclear regulatory protein that may contribute to the development of HIV-1 associated dementia by disrupting the neuronal cytoskeleton. The present studies examined effects of recombinant Tat(1-86; 1-100 nM) on microtubule-associated protein (MAP)-dependent and MAP-independent microtubule formation ex vivo and oxidative neuronal injury in rat organotypic hippocampal explants. Acute exposure to Tat(1-86) (≥1 nM) markedly reduced MAP-dependent and -independent microtubule formation ex vivo, as did vincristine sulfate (0.1-10 μM). Cytotoxicity, as measured by propidium iodide uptake, was observed in granule cells of the DG with exposure to 100 nM Tat(1-86) for 24 or 72 h, while significant reductions in MAP-2 immunoreactivity were observed in granule cells and pyramidal cells of the CA1 and CA3 regions at each timepoint. These effects were prevented by co-exposure to the soluble vitamin E analog Trolox (500 μM). Thus, effects of Tat(1-86) on the neuronal viability may be associated with direct interactions with microtubules and generation of oxidative stress.

ER-β mediates 17β-estradiol attenuation of HIV-1 Tat-induced apoptotic signaling

The protective actions of estrogen have been well evaluated in various models of neurodegeneration. These neuroprotective mechanisms may include a direct neuronal antiapoptotic effect as estrogen modulates actions of key regulators of the mitochondrial/intrinsic apoptotic cascade. We tested the ability of estrogen to protect against apoptotic signaling in cortical cell cultures exposed to Tat 1-86 (50 nM), and additionally, whether the beneficial actions of estrogen involved an estrogen receptor sensitive mechanism. We demonstrated that estrogen pretreatment significantly delayed Tat-induced cell death in primary cortical cultures. Pretreatment with 17β-estradiol (10 nM) attenuated the increased expression of antiapoptotic protein Bcl-2, proapoptotic protein Bax and activation of caspases linked to mitochondrial apoptotic pathway following Tat exposure. In addition, select components of apoptotic pathway signaling appear more sensitive to estrogen receptor (ER) activation, as the addition of ER antagonist ICI 182780 reversed estrogen downregulation of Bax and caspase 3, while estrogen effects on Tat-induced Bcl-2 and caspase 9 expression were maintained. Moreover, the addition of preferential ERα and ERβ antagonists (MPP dihydrochloride and PHTPP) indicated that estrogen effects on caspase 3 may be mediated by both receptor subtypes, whereas, was more involved in estrogen effects on Bax. Our data suggest that estrogen intervenes against HIV-1 Tat-induced cortical neuronal dysfunction via intersecting mitochondrial apoptotic pathway signaling in an ER-sensitive manner.

Abnormalities in resting-state functional connectivity in early human immunodeficiency virus infection

Limited information is available concerning changes that occur in the brain early in human immunodeficiency virus (HIV) infection. This investigation evaluated resting-state functional connectivity, which is based on correlations of spontaneous blood oxygen level-dependent functional magnetic resonance imaging (fMRI) oscillations between brain regions, in 15 subjects within the first year of HIV infection and in 15 age-matched controls. Resting-state fMRI data for each session were concatenated in time across subjects to create a single 4D dataset and decomposed into 36 independent component analysis (ICA) using Multivariate Exploratory Linear Optimized Decomposition into Independent Components. ICA components were back-reconstructed for each subject's 4D data to estimate subject-specific spatial maps using the dual-regression technique. Comparison of spatial maps between HIV and controls revealed significant differences in the lateral occipital cortex (LOC) network. Reduced coactivation in left inferior parietal cortex within the LOC network was identified in the HIV subjects. Connectivity strength within this region correlated with performance on tasks involving visual-motor coordination (Grooved Pegboard and Rey Figure Copy) in the HIV group. The findings indicate prominent changes in resting-state functional connectivity of visual networks early in HIV infection. This network may sustain injury in association with the intense viremia and brain viral invasion before immune defenses can contain viral replication. Resting-state functional connectivity may have utility as a noninvasive neuroimaging biomarker for central nervous system impairment in early HIV infection.