Brain-derived neurotrophic factor prevents the nigrostriatal degeneration induced by human immunodeficiency virus-1 glycoprotein 120 in vivo

The Association Between Serum Mature and Precursor Brain-Derived Neurotrophic Factor and Neurocognitive Function in People With Human Immunodeficiency Virus: A Longitudinal Study

Background

Despite antiretroviral therapy (ART), human immunodeficiency virus (HIV)-associated neurocognitive impairment persists. We investigated the association between serum levels of mature brain-derived neurotrophic factor (mBDNF), precursor brain-derived neurotrophic factor (proBDNF), and neurocognitive changes over time among adults with HIV in sub-Saharan Africa, seeking to elucidate the interplay between neurotrophic factors and neurocognitive outcomes post-ART.

Methods

Utilizing data from the ACTG 5199 study in Johannesburg and Harare, serum mBDNF and proBDNF levels were measured via enzyme-linked immunosorbent assay. Neurocognitive performance was assessed at baseline and 24, 48, and 96 weeks using neuropsychological tests. The Friedman test and linear mixed-effects models were used to assess changes in mBDNF, proBDNF, and neurocognitive performance over time, accounting for individual variability and adjusting for multiple comparisons.

Results

Among 155 participants, there were significant cognitive improvements (P < .001) and a rise in mBDNF levels from baseline to 96 weeks. The proBDNF levels initially remained stable (P = .57) but notably increased by 48 weeks (P = .04). Higher mBDNF levels were positively associated with enhanced neurocognitive performance at 48 weeks (β = .16, P = .01) and 96 weeks (β = .32, P < .001). Similarly, higher proBDNF levels were positively associated with neurocognitive performance at 96 weeks (β = .25, P < .001).

Conclusions

This study highlights the significant association between serum BDNF levels and neurocognitive improvement post-ART in adults with HIV. However, more research is needed to replicate these findings, establish causal relationships, and explore whether BDNF-enhancing activities can improve neurocognitive outcomes in people with HIV.

7,8-Dihydroxyflavone improves neuropathological changes in the brain of Tg26 mice, a model for HIV-associated neurocognitive disorder

The combined antiretroviral therapy era has significantly increased the lifespan of people with HIV (PWH), turning a fatal disease to a chronic one. However, this lower but persistent level of HIV infection increases the susceptibility of HIV-associated neurocognitive disorder (HAND). Therefore, research is currently seeking improved treatment for this complication of HIV. In PWH, low levels of brain derived neurotrophic factor (BDNF) has been associated with worse neurocognitive impairment. Hence, BDNF administration has been gaining relevance as a possible adjunct therapy for HAND. However, systemic administration of BDNF is impractical because of poor pharmacological profile. Therefore, we investigated the neuroprotective effects of BDNF-mimicking 7,8 dihydroxyflavone (DHF), a bioactive high-affinity TrkB agonist, in the memory-involved hippocampus and brain cortex of Tg26 mice, a murine model for HAND. In these brain regions, we observed astrogliosis, increased expression of chemokine HIV-1 coreceptors CXCR4 and CCR5, neuroinflammation, and mitochondrial damage. Hippocampi and cortices of DHF treated mice exhibited a reversal of these pathological changes, suggesting the therapeutic potential of DHF in HAND. Moreover, our data indicates that DHF increases the phosphorylation of TrkB, providing new insights about the role of the TrkB-Akt-NFkB signaling pathway in mediating these pathological hallmarks. These findings guide future research as DHF shows promise as a TrkB agonist treatment for HAND patients in adjunction to the current antiviral therapies.

Up-regulation of the p75 neurotrophin receptor is an essential mechanism for HIV-gp120 mediated synaptic loss in the striatum

Reduced synaptodendritic complexity appears to be a key feature in human immunodeficiency virus (HIV)-associated neurological disorder (HAND). Viral proteins, and in particular the envelope protein gp120, play a role in the pathology of synapses. Gp120 has been shown to increase both in vitro and in vivo the proneurotrophin brain-derived neurotrophic factor, which promotes synaptic simplification through the activation of the p75 neurotrophin receptor (p75NTR). To provide evidence that p75NTR plays a role in gp120-mediated loss of synapses in vivo, we intercrossed gp120tg mice with p75NTR null mice and used molecular, histological and behavioral analyses to establish a link between p75NTR and gp120-mediated synaptic simplification. Synaptosomes obtained from the striatum of gp120tg mice exhibited a significant increase in p75NTR levels concomitantly to a decrease in synaptic markers such as TrkB and PSD95. Analysis of striatal dendritic spines by Golgi staining revealed that gp120tg mice display a reduced proportion of mushroom-type spines in addition to fewer spines overall, when compared to wild type or gp120tg lacking one or two p75NTR alleles. Moreover, removal of one p75NTR allele in gp120 transgenic mice abolished the gp120-driven impairment on a task of striatal-dependent motor learning. These data indicate that p75NTR could be a key player in HIV-mediated synaptic simplification in the striatum.

Small molecule modulation of the p75 neurotrophin receptor suppresses age- and genotype-associated neurodegeneration in HIV gp120 transgenic mice

The persistence of HIV in the central nervous system leads to cognitive deficits in up to 50% of people living with HIV even with systemic suppression by antiretroviral treatment. The interaction of chronic inflammation with age-associated degeneration places these individuals at increased risk of accelerated aging and other neurodegenerative diseases and no treatments are available that effectively halt these processes. The adverse effects of aging and inflammation may be mediated, in part, by an increase in the expression of the p75 neurotrophin receptor (p75^NTR) which shifts the balance of neurotrophin signaling toward less protective pathways. To determine if modulation of p75^NTR could modify the disease process, we treated HIV gp120 transgenic mice with a small molecule ligand designed to engage p75^NTR and downregulate degenerative signaling. Daily treatment with 50 mg/kg LM11A-31 for 4 months suppressed age- and genotype-dependent activation of microglia, increased microtubule associated protein-2 (MAP-2), reduced dendritic varicosities and slowed the loss of parvalbumin immunoreactive neurons in the hippocampus. An age related accumulation of microtubule associated protein Tau was identified in the hippocampus in extracellular clusters that co-expressed p75^NTR suggesting a link between Tau and p75^NTR. Although the significance of the relationship between p75^NTR and Tau is unclear, a decrease in Tau-1 immunoreactivity as gp120 mice entered old age (>16 months) suggests that the Tau may transition to more pathological modifications; a process blocked by LM11A-31. Overall, the effects of LM11A-31 are consistent with strong neuroprotective and anti-inflammatory actions that have significant therapeutic potential.

The delicate balance between neurotoxicity and neuroprotection in the context of HIV-1 infection

Human immunodeficiency virus type-1 (HIV-1) causes a spectrum of neurological impairments, termed HIV-associated neurocognitive disorder (HAND), following the infiltration of infected cells into the brain. Even though the implementation of antiretroviral therapy reduced the systemic viral load, the prevalence of HAND remains unchanged and infected patients develop persisting neurological disturbances affecting their quality of life. As a result, HAND have gained importance in basic and clinical researches, warranting the need of developing new adjunctive treatments. Nonetheless, a better understanding of the molecular and cellular mechanisms remains necessary. Several studies consolidated their efforts into elucidating the neurotoxic signaling leading to HAND including the deleterious actions of HIV-1 viral proteins and inflammatory mediators. However, the scope of these studies is not sufficient to address all the complexity related to HAND development. Fewer studies focused on an altered neuroprotective capacity of the brain to respond to HIV-1 infection. Neurotrophic factors are endogenous polyproteins involved in neuronal survival, synaptic plasticity, and neurogenesis. Any defects in the processing or production of these crucial factors might compose a risk factor rendering the brain more vulnerable to neuronal damages. Due to their essential roles, they have been investigated for their diverse interplays with HIV-1 infection. In this review, we present a complete description of the neurotrophic factors involved in HAND. We discuss emerging concepts for their therapeutic applications and summarize the complex mechanisms that down-regulate their production in favor of a neurotoxic environment. For certain factors, we finally address opposing roles that rather lead to increased inflammation.

The Role of Brain Derived Neurotrophic Factor in HIV-Associated Neurocognitive Disorder: From the Bench-Top to the Bedside

Human immunodeficiency virus (HIV)-associated neurocognitive disorder (HAND) remains prevalent in the anti-retroviral (ART) era. While there is a complex interplay of many factors in the neuropathogenesis of HAND, decreased neurotrophic synthesis has been shown to contribute to synaptic degeneration which is a hallmark of HAND neuropathology. Brain derived neurotrophic factor (BDNF) is the most abundant and synaptic-promoting neurotrophic factor in the brain and plays a critical role in both learning and memory. Reduced BDNF levels can worsen neurocognitive impairment in HIV-positive individuals across several domains. In this paper, we review the evidence from pre-clinical and clinical studies showing the neuroprotective roles of BDNF against viral proteins, effect on co-morbid mental health disorders, altered human microbiome and ART in HAND management. Potential applications of BDNF modulation in pharmacotherapeutic, cognitive and behavioral interventions in HAND are also discussed. Finally, research gaps and future research direction are identified with the aim of helping researchers to direct efforts to make these BDNF driven interventions improve the quality of life of patients living with HAND.

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).

A central role for glial CCR5 in directing the neuropathological interactions of HIV-1 Tat and opiates

BACKGROUND

The collective cognitive and motor deficits known as HIV-associated neurocognitive disorders (HAND) remain high even among HIV+ individuals whose antiretroviral therapy is optimized. HAND is worsened in the context of opiate abuse. The mechanism of exacerbation remains unclear but likely involves chronic immune activation of glial cells resulting from persistent, low-level exposure to the virus and viral proteins. We tested whether signaling through C-C chemokine receptor type 5 (CCR5) contributes to neurotoxic interactions between HIV-1 transactivator of transcription (Tat) and opiates and explored potential mechanisms.

METHODS

Neuronal survival was tracked in neuronal and glial co-cultures over 72 h of treatment with HIV-1 Tat ± morphine using cells from CCR5-deficient and wild-type mice exposed to the CCR5 antagonist maraviroc or exogenously-added BDNF (analyzed by repeated measures ANOVA). Intracellular calcium changes in response to Tat ± morphine ± maraviroc were assessed by ratiometric Fura-2 imaging (analyzed by repeated measures ANOVA). Release of brain-derived neurotrophic factor (BDNF) and its precursor proBDNF from CCR5-deficient and wild-type glia was measured by ELISA (analyzed by two-way ANOVA). Levels of CCR5 and μ-opioid receptor (MOR) were measured by immunoblotting (analyzed by Student's t test).

RESULTS

HIV-1 Tat induces neurotoxicity, which is greatly exacerbated by morphine in wild-type cultures expressing CCR5. Loss of CCR5 from glia (but not neurons) eliminated neurotoxicity due to Tat and morphine interactions. Unexpectedly, when CCR5 was lost from glia, morphine appeared to entirely protect neurons from Tat-induced toxicity. Maraviroc pre-treatment similarly eliminated neurotoxicity and attenuated neuronal increases in [Ca²⁺]_i caused by Tat ± morphine. proBDNF/BDNF ratios were increased in conditioned media from Tat ± morphine-treated wild-type glia compared to CCR5-deficient glia. Exogenous BDNF treatments mimicked the pro-survival effect of glial CCR5 deficiency against Tat ± morphine.

CONCLUSIONS

Our results suggest a critical role for glial CCR5 in mediating neurotoxic effects of HIV-1 Tat and morphine interactions on neurons. A shift in the proBDNF/BDNF ratio that favors neurotrophic support may occur when glial CCR5 signaling is blocked. Some neuroprotection occurred only in the presence of morphine, suggesting that loss of CCR5 may fundamentally change signaling through the MOR in glia.

Cross-sectional and longitudinal small animal PET shows pre and post-synaptic striatal dopaminergic deficits in an animal model of HIV

INTRODUCTION

In vivo imaging biomarkers of various HIV neuropathologies, including dopaminergic dysfunction, are still lacking. Towards developing dopaminergic biomarkers of brain involvement in HIV, we assessed the pre and postsynaptic components of the dopaminergic system in the HIV-1 transgenic rat (Tg), a well-characterized model of treated HIV+ patients, using small-animal PET imaging.

METHODS

Fifteen to 18 month-old Tg and wild type (WT) rats were imaged with both [18F]-FP-CMT, a dopamine transporter (DAT) ligand (n=16), and [18F]-Fallypride, a D2/D3 dopamine receptor (D2/D3DR) ligand (n=16). Five to 8 month-old Tg and WT rats (n=18) were also imaged with [18F]-FP-CMT. A subset of animals was imaged longitudinally at 7 and 17 months of age. Multiplex immunohistochemistry staining for DAT, tyrosine hydroxylase, D2DR, D3DR_, GFAP, Iba1 and NeuN was performed on a subgroup of the scanned animals.

RESULTS

[18F]-FP-CMT and [18F]-Fallypride binding potential (BP_ND) values were significantly lower in 15-18 month-old Tg compared to age-matched WT rats (p<0.0001 and 0.001, respectively). [18F]-FP-CMT BP_ND values in 5-8 month-old rats, however, were not significantly different. Longitudinal age-related decrease in [18F]-FP-CMT BP_ND was exacerbated in the Tg rat. Immunohistochemistry showed decreased staining of dopaminergic markers in Tg rats. Rats with higher serum gp120 had lower mean BP_ND values for both ligands.

CONCLUSIONS

We found presynaptic and postsynaptic dopaminergic dysfunction/loss in older Tg compared to WT rats. We believe this to be related to neurotoxicity of viral proteins present in the Tg rats' serum and brain.

ADVANCES IN KNOWLEDGE

Our findings confirm prior reports of neurobehavioral abnormalities suggestive of dopaminergic dysfunction in this model. They also suggest similarities between the Tg rat and HIV+ patients as far as dopaminergic dysfunction.

IMPLICATIONS FOR PATIENT CARE

The Tg rat, along with the above-described quantitative PET imaging biomarkers, can have a role in the evaluation of HIV neuroprotective therapies prior to human translation.

Role of Oxidative Stress in HIV-1-Associated Neurocognitive Disorder and Protection by Gene Delivery of Antioxidant Enzymes

HIV encephalopathy covers a range of HIV-1-related brain dysfunction. In the Central Nervous System (CNS), it is largely impervious to Highly Active AntiRetroviral Therapy (HAART). As survival with chronic HIV-1 infection improves, the number of people harboring the virus in their CNS increases. Neurodegenerative and neuroinflammatory changes may continue despite the use of HAART. Neurons themselves are rarely infected by HIV-1, but HIV-1 infects resident microglia, periventricular macrophages, leading to increased production of cytokines and to release of HIV-1 proteins, the most likely neurotoxins, among which are the envelope glycoprotein gp120 and HIV-1 trans-acting protein Tat. Gp120 and Tat induce oxidative stress in the brain, leading to neuronal apoptosis/death. We review here the role of oxidative stress in animal models of HIV-1 Associated Neurocognitive Disorder (HAND) and in patients with HAND. Different therapeutic approaches, including clinical trials, have been used to mitigate oxidative stress in HAND. We used SV40 vectors for gene delivery of antioxidant enzymes, Cu/Zn superoxide dismutase (SOD1), or glutathione peroxidase (GPx1) into the rat caudate putamen (CP). Intracerebral injection of SV (SOD1) or SV (GPx1) protects neurons from apoptosis caused by subsequent inoculation of gp120 and Tat at the same location. Vector administration into the lateral ventricle or cisterna magna protects from intra-CP gp120-induced neurotoxicity comparably to intra-CP vector administration. These models should provide a better understanding of the pathogenesis of HIV-1 in the brain as well as offer new therapeutic avenues.

Beyond the Brain: The Role of Brain-Derived Neurotrophic Factor in Viroimmune Responses to Antiretroviral Therapy among People Living with HIV with and without Alcohol Use

OBJECTIVE

Given the emerging data suggesting the key role of brain-derived neurotrophic factor (BDNF) in the immune system, we assessed longitudinally whether BDNF depletions induced by hazardous alcohol use (HAU) would impact a response to antiretroviral therapy (ART).

METHODS

In a prospective single-site cohort, virological and immunological responses to ART in 200 hazardous and 200 nonhazardous users were obtained, along with plasma BDNF levels.

RESULTS

Hazardous drinkers were more likely to have BDNF levels <4000 pg/mL (odds ratio [OR] = 1.6, P = .01). Participants with BDNF <4000 pg/mL were less likely to have CD4 counts of more than 500 cells/mm(3) (P = .02) and to achieve viral suppression over the follow-up period (OR = 1.5, P = .03). Multivariate analysis confirmed the significant role of HAU and low BDNF in predicting viroimmune responses.

CONCLUSION

Hazardous alcohol use was associated with BDNF alterations, which in turn were linked to a limited response to ART in terms of viral suppression and CD4 count improvements.

Genetic, transcriptomic, and epigenetic studies of HIV-associated neurocognitive disorder

The Human Genome Project, coupled with rapidly evolving high-throughput technologies, has opened the possibility of identifying heretofore unknown biological processes underlying human disease. Because of the opaque nature of HIV-associated neurocognitive disorder (HAND) neuropathogenesis, the utility of such methods has gained notice among NeuroAIDS researchers. Furthermore, the merging of genetics with other research areas has also allowed for application of relatively nascent fields, such as neuroimaging genomics, and pharmacogenetics, to the context of HAND. In this review, we detail the development of genetic, transcriptomic, and epigenetic studies of HAND, beginning with early candidate gene association studies and culminating in current "omics" approaches that incorporate methods from systems biology to interpret data from multiple levels of biological functioning. Challenges with this line of investigation are discussed, including the difficulty of defining a valid phenotype for HAND. We propose that leveraging known associations between biology and pathology across multiple levels will lead to a more reliable and valid phenotype. We also discuss the difficulties of interpreting the massive and multitiered mountains of data produced by current high-throughput omics assays and explore the utility of systems biology approaches in this regard.

The longitudinal and interactive effects of HIV status, stimulant use, and host genotype upon neurocognitive functioning

Both human immunodeficiency virus (HIV)-1 infection and illicit stimulant use can adversely impact neurocognitive functioning, and these effects can be additive. However, significant variability exists such that as-of-yet unidentified exogenous and endogenous factors affect one's risk for neurocognitive impairment. Literature on both HIV and stimulant use indicates that host genetic variants in immunologic and dopamine-related genes are one such factor. In this study, the individual and interactive effects of HIV status, stimulant use, and genotype upon neurocognitive functioning were examined longitudinally over a 10-year period. Nine hundred fifty-two Caucasian HIV+ and HIV- cases from the Multicenter AIDS Cohort Study were included. All cases had at least two comprehensive neurocognitive evaluations between 1985 and 1995. Pre-highly active antiretroviral therapy (HAART) data were examined in order to avoid the confounding effect of variable drug regimens. Linear mixed models were used, with neurocognitive domain scores as the outcome variables. No four-way interactions were found, indicating that HIV and stimulant use do not interact over time to affect neurocognitive functioning as a function of genotype. Multiple three-way interactions were found that involved genotype and HIV status. All immunologically related genes found to interact with HIV status affected neurocognitive functioning in the expected direction; however, only C-C chemokine ligand 2 (CCL2) and CCL3 affected HIV+ individuals specifically. Dopamine-related genetic variants generally affected HIV-negative individuals only. Neurocognitive functioning among HIV+ individuals who also used stimulants was not significantly different from those who did not use stimulants. The findings support the role of immunologically related genetic differences in CCL2 and CCL3 in neurocognitive functioning among HIV+ individuals; however, their impact is minor. Being consistent with findings from another cohort, dopamine (DA)-related genetic differences do not appear to impact the longitudinal neurocognitive functioning of HIV+ individuals.

Role of neurotrophic factor alterations in the neurodegenerative process in HIV associated neurocognitive disorders

Migration of HIV infected cells into the CNS is associated with a spectrum of neurological disorders, ranging from milder forms of HIV-associated neurocognitive disorders (HAND) to HIV-associated dementia (HAD). These neuro-psychiatric syndromes are related to the neurodegenerative pathology triggered by the release of HIV proteins and cytokine/chemokines from monocytes/macrophages into the CNS -a condition known as HIV encephalitis (HIVE). As a result of more effective combined anti-retroviral therapy patients with HIV are living longer and thus the frequency of HAND has increased considerably, resulting in an overlap between the neurodegenerative pathology associated with HIV and that related to aging. In fact, HIV infection is believed to hasten the aging process. The mechanisms through which HIV and aging lead to neurodegeneration include: abnormal calcium flux, excitotoxicity, signaling abnormalities, oxidative stress and autophagy defects. Moreover, recent studies have shown that defects in the processing and transport of neurotrophic factors such as fibroblast growth factors (FGFs), neural growth factor (NGF) and brain-derived growth factor (BDNF) might also play a role. Recent evidence implicates alterations in neurotrophins in the pathogenesis of neurodegeneration associated with HAND in the context of aging. Here, we report FGF overexpression curtails gp120-induced neurotoxicity in a double transgenic mouse model. Furthermore, our data show disparities in brain neurotrophic factor levels may be exacerbated in HIV patients over 50 years of age. In this review, we discuss the most recent findings on neurotrophins and HAND in the context of developing new therapies to combat HIV infection in the aging population.

Animal models for depression associated with HIV-1 infection

Antiretroviral therapy has greatly extended the lifespan of people living with human immunodeficiency virus (PLHIV). As a result, the long-term effects of HIV infection, in particular those originating in the central nervous system (CNS), such as HIV associated depression, have gained importance. Animal models for HIV infection have proved very useful for understanding the disease and developing treatment strategies. However, HIV associated depression remains poorly understood and so far there is neither a fully satisfactory animal model, nor a pathophysiologically guided treatment for this condition. Here we review the neuroimmunological, neuroendocrine, neurotoxic and neurodegenerative basis for HIV depression and discuss strategies for employing HIV animal models, in particular humanized mice which are susceptible to HIV infection, for the study of HIV depression.

Monocytes-derived macrophages mediated stable expression of human brain-derived neurotrophic factor, a novel therapeutic strategy for neuroAIDS

HIV-1 associated dementia remains a significant public health burden. Clinical and experimental research has shown that reduced levels of brain-derived neurotrophic factor (BDNF) may be a risk factor for neurological complications associated with HIV-1 infection. We are actively testing genetically modified macrophages for their possible use as the cell-based gene delivery vehicle for the central nervous system (CNS). It can be an advantage to use the natural homing/migratory properties of monocyte-derived macrophages to deliver potentially neuroprotective BDNF into the CNS, as a non-invasive manner. Lentiviral-mediated gene transfer of human (h)BDNF plasmid was constructed and characterized. Defective lentiviral stocks were generated by transient transfection of 293T cells with lentiviral transfer plasmid together with packaging and envelope plasmids. High titer lentiviral vector stocks were harvested and used to transduce human neuronal cell lines, primary cultures of human peripheral mononocyte-derived macrophages (hMDM) and murine myeloid monocyte-derived macrophages (mMDM). These transduced cells were tested for hBDNF expression, stability, and neuroprotective activity. The GenomeLab GeXP Genetic Analysis System was used to evaluate transduced cells for any adverse effects by assessing gene profiles of 24 reference genes. High titer vectors were prepared for efficient transduction of neuronal cell lines, hMDM, and mMDM. Stable secretion of high levels of hBDNF was detected in supernatants of transduced cells using western blot and ELISA. The conditioned media containing hBDNF were shown to be protective to neuronal and monocytic cell lines from TNF-α and HIV-1 Tat mediated cytotoxicity. Lentiviral vector-mediated gene transduction of hMDM and mMDM resulted in high-level, stable expression of the neuroprotective factorBDNF in vitro. These findings form the basis for future research on the potential use of BDNF as a novel therapy for neuroAIDS.

The effects of neurotrophins and the neuropeptides VIP and PACAP on HIV-1 infection: histories with opposite ends

The nerve growth factor (NGF) and other neurotrophins, and the neuropeptides vasoactive intestinal peptide (VIP) and pituitary adenylate cyclase-activating peptide (PACAP) are largely present in human tissue and can exert modulatory activities on nervous, endocrine and immune system functions. NGF, VIP and PACAP receptors are expressed systemically in organisms, and thus these mediators exhibit pleiotropic natures. The human immunodeficiency virus type 1 (HIV-1), the causal agent of the acquired immunodeficiency syndrome (AIDS), infects immune cells, and its replication is modulated by a number of endogenous factors that interact with HIV-1-infected cells. NGF, VIP and PACAP can also affect HIV-1 virus particle production upon binding to their receptors on the membranes of infected cells, which triggers cell signaling pathways that modify the HIV-1 replicative cycle. These molecules exert opposite effects on HIV-1 replication, as NGF and other neurotrophins enhance and VIP and PACAP reduce viral production in HIV-1-infected human primary macrophages. The understanding of AIDS pathogenesis should consider the mechanisms by which the replication of HIV-1, a pathogen that causes chronic morbidity, is influenced by neurotrophins, VIP and PACAP, i.e. molecules that exert a broad spectrum of physiological activities on the neuroimmunoendocrine axis. In this review, we will present the main effects of these two groups of mediators on the HIV-1 replicative cycle, as well as the mechanisms that underlie their abilities to modulate HIV-1 production in infected immune cells, and discuss the possible repercussion of the cross talk between NGF and both neuropeptides on the pathogenesis of HIV-1 infection.

CB2 receptor agonists protect human dopaminergic neurons against damage from HIV-1 gp120

Despite the therapeutic impact of anti-retroviral therapy, HIV-1-associated neurocognitive disorder (HAND) remains a serious threat to AIDS patients, and there currently remains no specific therapy for the neurological manifestations of HIV-1. Recent work suggests that the nigrostriatal dopaminergic area is a critical brain region for the neuronal dysfunction and death seen in HAND and that human dopaminergic neurons have a particular sensitivity to gp120-induced damage, manifested as reduced function (decreased dopamine uptake), morphological changes, and reduced viability. Synthetic cannabinoids inhibit HIV-1 expression in human microglia, suppress production of inflammatory mediators in human astrocytes, and there is substantial literature demonstrating the neuroprotective properties of cannabinoids in other neuropathogenic processes. Based on these data, experiments were designed to test the hypothesis that synthetic cannabinoids will protect dopaminergic neurons against the toxic effects of the HIV-1 protein gp120. Using a human mesencephalic neuronal/glial culture model, which contains dopaminergic neurons, microglia, and astrocytes, we were able to show that the CB1/CB2 agonist WIN55,212-2 blunts gp120-induced neuronal damage as measured by dopamine transporter function, apoptosis and lipid peroxidation; these actions were mediated principally by the CB2 receptor. Adding supplementary human microglia to our cultures enhances gp120-induced damage; WIN55,212-2 is able to alleviate this enhanced damage. Additionally, WIN55,212-2 inhibits gp120-induced superoxide production by purified human microglial cells, inhibits migration of human microglia towards supernatants generated from gp120-stimulated human mesencephalic neuronal/glial cultures and reduces chemokine and cytokine production from the human mesencephalic neuronal/glial cultures. These data suggest that synthetic cannabinoids are capable of protecting human dopaminergic neurons from gp120 in a variety of ways, acting principally through the CB2 receptors and microglia.

Implementing neuronal plasticity in NeuroAIDS: the experience of brain-derived neurotrophic factor and other neurotrophic factors

Human immunodeficiency virus type-1 (HIV) causes mild or severe neurological problems, termed HIV-associated neurocognitive disorder (HAND), even when HIV patients receive antiretroviral therapy. Thus, novel adjunctive therapies are necessary to reduce or abolish the neurotoxic effect of HIV. However, new therapies require a better understanding of the molecular and cellular mechanisms of HIV-induced neurotoxicity. HAND subjects are characterized by being profoundly depressed, and they experience deficits in memory, learning and movements. Experimental evidence has also shown that HIV reduces neurogenesis. These deficits resemble those occurring in premature brain aging or in a brain with impaired neural repair properties. Thus, it appears that HIV diminishes neuronal survival, along with reduced neuronal connections. These two phenomena should not occur in the adult and developing brain when synaptic plasticity is promoted by neurotrophic factors, polypeptides that are present in adult synapses. This review will outline experimental evidence as well as present emerging concepts for the use of neurotrophic factors and in particular brain-derived neurotrophic factor as an adjunct therapy to prevent HIV-mediated neuronal degeneration and restore the loss of synaptic connections.

Activity-dependent, stress-responsive BDNF signaling and the quest for optimal brain health and resilience throughout the lifespan

During development of the nervous system, the formation of connections (synapses) between neurons is dependent upon electrical activity in those neurons, and neurotrophic factors produced by target cells play a pivotal role in such activity-dependent sculpting of the neural networks. A similar interplay between neurotransmitter and neurotrophic factor signaling pathways mediates adaptive responses of neural networks to environmental demands in adult mammals, with the excitatory neurotransmitter glutamate and brain-derived neurotrophic factor (BDNF) being particularly prominent regulators of synaptic plasticity throughout the central nervous system. Optimal brain health throughout the lifespan is promoted by intermittent challenges such as exercise, cognitive stimulation and dietary energy restriction, that subject neurons to activity-related metabolic stress. At the molecular level, such challenges to neurons result in the production of proteins involved in neurogenesis, learning and memory and neuronal survival; examples include proteins that regulate mitochondrial biogenesis, protein quality control, and resistance of cells to oxidative, metabolic and proteotoxic stress. BDNF signaling mediates up-regulation of several such proteins including the protein chaperone GRP-78, antioxidant enzymes, the cell survival protein Bcl-2, and the DNA repair enzyme APE1. Insufficient exposure to such challenges, genetic factors may conspire to impair BDNF production and/or signaling resulting in the vulnerability of the brain to injury and neurodegenerative disorders including Alzheimer's, Parkinson's and Huntington's diseases. Further, BDNF signaling is negatively regulated by glucocorticoids. Glucocorticoids impair synaptic plasticity in the brain by negatively regulating spine density, neurogenesis and long-term potentiation, effects that are potentially linked to glucocorticoid regulation of BDNF. Findings suggest that BDNF signaling in specific brain regions mediates some of the beneficial effects of exercise and energy restriction on peripheral energy metabolism and the cardiovascular system. Collectively, the findings described in this article suggest the possibility of developing prescriptions for optimal brain health based on activity-dependent BDNF signaling.

Synaptic dysfunction in human immunodeficiency virus type-1-positive subjects: inflammation or impaired neuronal plasticity?

Many people infected with the human immunodeficiency virus type-1 (HIV) exhibit mild or severe neurological problems, termed HIV-associated neurocognitive disorder (HAND), even when receiving antiretroviral therapy. Thus, novel adjunctive therapies must be developed to overcome the neurotoxic effect of HIV. New therapies require a better understanding of the molecular and cellular mechanisms of HIV-induced neurotoxicity and the risk factors that, besides inflammation and T-cell depletion and drugs of abuse, render the central nervous system (CNS) a target of HIV-induced neurotoxicity. HIV appears to impair neuronal plasticity, which refers to the innate ability of the CNS respond to injury and promote recovery of function. The availability of brain-derived neurotrophic factor (BDNF), a potent neurotrophic factor that is present in abundance in the adult brain, is essential for neuronal plasticity. BDNF acts through a receptor system composed of Trk and p75NTR. Here, we present experimental evidence that some of the clinical features of HIV-mediated neurological impairment could result from altered BDNF/TrkB/p75NTR regulation and function.

Targeted brain derived neurotropic factors (BDNF) delivery across the blood-brain barrier for neuro-protection using magnetic nano carriers: an in-vitro study

Parenteral use of drugs; such as opiates exert immunomodulatory effects and serve as a cofactor in the progression of HIV-1 infection, thereby potentiating HIV related neurotoxicity ultimately leading to progression of NeuroAIDS. Morphine exposure is known to induce apoptosis, down regulate cAMP response element-binding (CREB) expression and decrease in dendritic branching and spine density in cultured cells. Use of neuroprotective agent; brain derived neurotropic factor (BDNF), which protects neurons against these effects, could be of therapeutic benefit in the treatment of opiate addiction. Previous studies have shown that BDNF was not transported through the blood brain barrier (BBB) in-vivo.; and hence it is not effective in-vivo. Therefore development of a drug delivery system that can cross BBB may have significant therapeutic advantage. In the present study, we hypothesized that magnetically guided nanocarrier may provide a viable approach for targeting BDNF across the BBB. We developed a magnetic nanoparticle (MNP) based carrier bound to BDNF and evaluated its efficacy and ability to transmigrate across the BBB using an in-vitro BBB model. The end point determinations of BDNF that crossed BBB were apoptosis, CREB expression and dendritic spine density measurement. We found that transmigrated BDNF was effective in suppressing the morphine induced apoptosis, inducing CREB expression and restoring the spine density. Our results suggest that the developed nanocarrier will provide a potential therapeutic approach to treat opiate addiction, protect neurotoxicity and synaptic density degeneration.

Behavioral effects of chronic methamphetamine treatment in HIV-1 gp120 transgenic mice

Methamphetamine (METH) dependence is frequently comorbid with HIV infection. Both factors are independently characterized by inhibitory deficits, which may manifest as increased motor activity, inappropriate perseverative behavior, and elevated exploratory responses to novel stimuli, but the effect of combined METH exposure and HIV is not well understood. In this study, we administered a chronic escalation/binge regimen of METH or vehicle treatment to wildtype (WT) or transgenic (tg) mice expressing the HIV-1 gp120 envelope protein and quantified disinhibition during the 7 days following drug withdrawal. We hypothesized that gp120tg mice administered chronic METH would exhibit more pronounced inhibitory deficits compared to vehicle-treated WT or gp120tg animals. Our results showed that METH treatment alone increased novel object interaction while female METH-treated gp120tg mice exhibited the highest level of exploration (holepoking) compared to other female mice. Transgenic mice exhibited fewer rears relative to WT, slightly less locomotion, and also demonstrated a trend toward more perseverative motor patterns. In summary, both METH treatment and gp120 expression may modify inhibition, but such effects are selective and dependent upon variations in age and sex that could impact dopamine and frontostriatal function. These findings illustrate the need to improve our knowledge about the combined effects of HIV and substance use and facilitate improved treatment methods for comorbid disease and drug dependence.

Gene delivery of antioxidant enzymes inhibits human immunodeficiency virus type 1 gp120-induced expression of caspases

Caspases are implicated in neuronal death in neurodegenerative and other central nervous system (CNS) diseases. In a rat model of human immunodeficiency virus type 1 (HIV-1) associated neurocognitive disorders (HAND), we previously characterized HIV-1 envelope gp120-induced neuronal apoptosis by terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) assay. In this model, neuronal apoptosis occurred probably via gp120-induced reactive oxygen species (ROS). Antioxidant gene delivery blunted gp120-related apoptosis. Here, we studied the effect of gp120 on different caspases (3, 6, 8, 9) expression. Caspases production increased in the rat caudate-putamen (CP) 6h after gp120 injection into the same structure. The expression of caspases peaked by 24h. Caspases colocalized mainly with neurons. Prior gene delivery of the antioxidant enzymes Cu/Zn superoxide dismutase (SOD1) or glutathione peroxidase (GPx1) into the CP before injecting gp120 there reduced levels of gp120-induced caspases, recapitulating the effect of antioxidant enzymes on gp120-induced apoptosis observed by TUNEL. Thus, HIV-1 gp120 increased caspases expression in the CP. Prior antioxidant enzyme treatment mitigated production of these caspases, probably by reducing ROS levels.

Sigma-1 receptor agonists provide neuroprotection against gp120 via a change in bcl-2 expression in mouse neuronal cultures

Although combined antiretroviral therapy has significantly improved the prognosis of HIV-1 infected patients and decreased the incidence of HIV-1 associated dementia, the cumulative prevalence of this disease, in particular, mild or asymptomatic neurocognitive impairment, has not decreased. Thus, in addition to active antiretroviral therapy, the search for an effective neuroprotective approach is very important. Sigma-1 receptors are widely distributed in the central nervous system. Sigma-1 receptor agonists are robustly neuroprotective in many neuropathy and neurotoxicity in vivo and in vitro studies. This study aims to investigate possible neuroprotective effects of sigma-1 receptor agonist, 4-phenyl-1-(4-phenylbutyl) piperidine (PPBP) against HIV-1 protein gp120. Primary cortical neuronal cultures were exposed to gp120 in different concentrations; to investigate neuroprotective effects of sigma-1 receptor agonist, cells were pre-treated with PPBP (10μM) in the presence or absence of pre-incubated sigma-1 receptor antagonist rimcazole (5μM). Cell apoptosis was confirmed with calcein/PI uptake test, lactate dehydrogenase (LDH) leakage assay or TUNEL assay and neurite degeneration was evaluated with morphometry via MAP-2 stained immunofluorescence. The mRNA and protein levels of apoptosis associated bax and bcl-2 were determined with real-time qPCR and Western blot. The results showed that gp120 could induce neuronal apoptosis and neurite degeneration in a concentration dependent manner and PPBP could attenuate the neurotoxicity of gp120. Simultaneously, gp120 could induce low expression of bcl-2 and bax, but only low expression of bcl-2 could be reversed by PPBP. The present data suggest that PPBP, at least, in part protects the neuron against gp120 by regulating bcl-2 expression.

Functional polymorphisms in dopamine-related genes: effect on neurocognitive functioning in HIV+ adults

Dopaminergic dysfunction is a putative mechanism underlying HIV-associated neurocognitive disorders. Dopamine transporter (DAT), brain-derived neurotrophic factor (BDNF), and catechol-O-methyltransferase (COMT) have been specifically implicated. We report analyses examining the main effects of functional polymorphisms within dopamine-modulating genes, as well as their interactive effects with disease severity, upon neurocognitive functioning in HIV+ adults.

METHOD

A total of 184 HIV+ adults were included in the analysis. Three polymorphisms were examined within dopamine-modulating genes: COMT val158met, BDNF val66met, and the DAT 3' variable number tandem repeat. Separate hierarchical regression analyses for five neurocognitive domains (working memory, processing speed, learning, memory, motor) were conducted. Predictor variables were age, ethnicity, gender, education, CD4+ T-cell count, current depression, genotype, and an interaction term capturing genotype and disease severity (CD4).

RESULTS

None of the polymorphisms or HIV disease variables significantly improved the hierarchical regression models. Younger age, higher education, and Caucasian ethnicity were almost invariably associated with better functioning across all five cognitive domains. A trend was noted for current depression as a predictor of motor and learning ability.

CONCLUSION

This study did not find evidence to support direct or interactive effects of dopamine-related genes and HIV disease severity on neurocognitive functioning.

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.

HIV-1 decreases the levels of neurotrophins in human lymphocytes

Neurotrophins control cell survival. Therefore, we examined whether HIV-1 reduces neurotrophin levels. Serum of HIV-positive individuals exhibited lower concentrations of brain-derived neurotrophic factor (BDNF), but not of other neurotrophins, than HIV-negative individuals. In addition, R5 and X4 strains of HIV-1 decreased BDNF expression in T cells. Our results support the hypothesis that reduced levels of BDNF may be a risk factor for T-cell apoptosis and for neurological complications associated with HIV-1 infection.

Protein changes in CSF of HIV-infected patients: evidence for loss of neuroprotection

To begin to unravel the complexity of HIV-associated changes in the brain, broader, multifaceted analyses of cerebrospinal fluid (CSF) are needed that examine a wide range of proteins reflecting different functions. To provide the first broad profiles of protein changes in the CSF of HIV-infected patients, we used antibody arrays to measure 120 cytokines, chemokines, growth factors, and other proteins. CSF from HIV-infected patients with a range of cognitive deficits was compared to CSF from uninfected, cognitively normal patients to begin to identify protein changes associated with HIV infection and neurological disease progression. Uninfected patients showed relatively consistent patterns of protein expression. Highly expressed proteins in CSF included monocyte chemotactic protein-1, tissue inhibitors of metalloproteases, granulocyte colony-stimulating factor, adiponectin, soluble tumor necrosis factor receptor-1, urokinase-type plasminogen activator receptor, and insulin-like growth factor binding protein-2. Inflammatory and anti-inflammatory cytokines were expressed at low levels. HIV-infected patients showed increases in inflammatory proteins (interferon-gamma, tumor necrosis factor-alpha), anti-inflammatory proteins (IL-13), and chemokines but these correlated poorly with neurological status. The strongest correlation with increasing severity of neurological disease was a decline in growth factors, particularly, brain-derived neurotrophic factor and NT-3. These studies illustrate that HIV infection is associated with parallel changes in both inflammatory and neuroprotective proteins in the CSF. The inverse relationship between growth factors and neurological disease severity suggests that a loss of growth factor neuroprotection may contribute to the development of neural damage and may provide useful markers of disease progression.

Neurotrophins modulate the expression of chemokine receptors in the brain

In the central nervous system, chemokines are primarily mediators of inflammatory processes. Their receptors, in particular, CXCR4 and CCR5, serve as co-factors along with CD4 that permit Human immunodeficiency virus-1 (HIV) infection. Moreover, experimental evidence has shown that CXCR4 and CCR5 mediate the neurotoxic effects of the HIV envelope protein gp120, suggesting that these receptors could also promote the neuropathogenesis observed in HIV-positive individuals. Therefore, a better understanding of the molecular mechanisms governing the expression of chemokine receptors in the brain may lead to improved therapies that reduce HIV neurotoxicity. This study presents evidence that the expression of chemokine receptors in the brain is modulated by two neurotrophins in an area-specific manner. This new evidence suggests that the neurotrophins may be an adjunct therapy to reduce HIV-mediated neuronal injury evoked by chemokine receptor activation.

Morphine induces the release of CCL5 from astrocytes: potential neuroprotective mechanism against the HIV protein gp120

A number of human immunodeficiency virus type-1 (HIV) positive subjects are also opiate abusers. These individuals are at high risk to develop neurological complications. However, little is still known about the molecular mechanism(s) linking opiates and HIV neurotoxicity. To learn more, we exposed rat neuronal/glial cultures prepared from different brain areas to opiate agonists and HIV envelope glycoproteins gp120IIIB or BaL. These strains bind to CXCR4 and CCR5 chemokine receptors, respectively, and promote neuronal death. Morphine did not synergize the toxic effect of gp120IIIB but inhibited the cytotoxic property of gp120BaL. This effect was blocked by naloxone and reproduced by the mu opioid receptor agonist DAMGO. To examine the potential mechanism(s) of neuroprotection, we determined the effect of morphine on the release of chemokines CCL5 and CXCL12 in neurons, astrocytes, and microglia cultures. CCL5 has been shown to prevent gp120BaL neurotoxicity while CXCL12 decreases neuronal survival. Morphine elicited a time-dependent release of CCL5 but failed to affect the release of CXCL12. This effect was observed only in primary cultures of astrocytes. To examine the role of endogenous CCL5 in the neuroprotective activity of morphine, mixed cerebellar neurons/glial cells were immunoneutralized against CCL5 prior to morphine and gp120 treatment. In these cells the neuroprotective effect of opiate agonists was blocked. Our data suggest that morphine may exhibit a neuroprotective activity against M-tropic gp120 through the release of CCL5 from astrocytes.

Rebuilding synaptic architecture in HIV-1 associated neurocognitive disease: a therapeutic strategy based on modulation of mixed lineage kinase

Work from our laboratories has validated mixed lineage kinase type 3 (MLK3) as an enzyme pathologically activated in the CNS by human immunodeficiency virus 1 (HIV-1) neurotoxins. In this review, we discuss MLK3 activation in the context of the neuropathogenesis of HIV-1 associated neurocognitive deficits (HAND). We use findings from the literature to substantiate the neuropathologic relevance of MLK3 to neurodegenerative disease, with an emphasis on Parkinson's disease that shares a number of important phenotypic and neuropathologic characteristics with HAND. We discuss signal transduction pathways downstream from MLK3 activation, with an emphasis on their involvement in microglia and neurons in preclinical models of HAND. Finally, we make a case for pharmacologic intervention targeted at inhibition of MLK3 as a strategy to reverse HAND, in light of the fact that combination antiretroviral therapy, despite successfully managing systemic infection of HIV-1, has been largely unsuccessful in eradicating HAND.

M-tropic HIV envelope protein gp120 exhibits a different neuropathological profile than T-tropic gp120 in rat striatum

Most early human immunodeficiency virus type 1 (HIV-1) strains are macrophage (M)-tropic HIV variants and use the chemokine receptor CCR5 for infection. Neuronal loss and dementia are less severe among individuals infected with M-tropic strains. However, after several years, the T-cell (T)-tropic HIV strain, which uses the CXCR4 variant, can emerge in conjunction with brain abnormalities, suggesting strain-specific differences in neuropathogenicity. The molecular and cellular mechanisms of such diversity remain under investigation. We have previously demonstrated that HIV envelope protein gp120IIIB, which binds to CXCR4, causes neuronal apoptosis in rodents. Thus, we have used a similar experimental model to examine the neurotoxic effects of M-tropic gp120BaL. gp120BaL was microinjected in the rat striatum and neuronal apoptosis was examined in the striatum, as well as in anatomically connected areas, such as the somatosensory cortex and the substantia nigra. gp120BaL promoted neuronal apoptosis and tissue loss that were confined to the striatum. Apoptosis was associated with microglial activation and increased levels of interleukin-1beta. Intriguingly, gp120BaL increased brain-derived neurotrophic factor in the striatum. Overall, our data show that gp120BaL demonstrates a different neuropathological profile than gp120IIIB. A better understanding of the pathogenic mechanisms mediating HIV neurotoxicity is vital for developing effective neuroprotective therapies against AIDS-associated dementia complex.

9-Methyl-beta-carboline has restorative effects in an animal model of Parkinson's disease

In a previous study, a primary culture of midbrain cells was exposed to 9-methyl-beta-carboline for 48 h, which caused an increase in the number of tyrosine hydroxylase-positive cells. Quantitative RT-PCR revealed increased transcription of genes participating in the maturation of dopaminergic neurons. These in vitro findings prompted us to investigate the restorative actions of 9-methyl-beta-carboline in vivo. The compound was delivered for 14 days into the left cerebral ventricle of rats pretreated with the neurotoxin 1-methyl-4-phenyl-pyridinium ion (MPP+) for 28 days applying a dose which lowered dopamine by approximately 50%. Interestingly, 9-methyl-beta-carboline reversed the dopamine-lowering effect of the neurotoxin in the left striatum. Stereological counts of tyrosine hydroxylase-immunoreactive cells in the substantia nigra revealed that the neurotoxin caused a decrease in the number of those cells. However, when treated subsequently with 9-methyl-beta-carboline, the number reached normal values. In search of an explanation for the restorative activity, we analyzed the complexes that compose the respiratory chain in striatal mitochondria by 2-dimension gel electrophoresis followed by MALDI-TOF peptide mass fingerprinting.We found no changes in the overall composition of the complexes. However, the activity of complex I was increased by approximately 80% in mitochondria from rats treated with MPP+ and 9-methyl-beta-carboline compared to MPP+ and saline and to sham-operated rats, as determined by measurements of nicotinamide adenine dinucleotide dehydrogenase activity. Microarray technology and single RT-PCR revealed the induction of neurotrophins: brain-derived neurotrophic factor, conserved dopamine neurotrophic factor, cerebellin 1 precursor protein, and ciliary neurotrophic factor. Selected western blots yielded consistent results. The findings demonstrate restorative effects of 9-methyl-beta-carboline in an animal model of Parkinson's disease that improve the effectiveness of the respiratory chain and promote the transcription and expression of neurotrophin-related genes.

Preferential sensitivity of human dopaminergic neurons to gp120-induced oxidative damage

The dopamine (DA)-rich midbrain is known to be a key target of human immunodeficiency virus (HIV)-1. Studies of simian immunodeficiency virus (SIV)-induced neuropathogenesis recently established that there is a major disruption within the nigrostriatal dopaminergic system characterized by marked depletion of dopaminergic neurons, microglial cell activation, and reactive astrocytes. Using a human mesencephalic neuronal/glial culture model, which contains dopaminergic neurons, microglia, and astrocytes, experiments were performed to characterize the damage to dopaminergic neurons induced by HIV-1 gp120. Functional impairment was assessed by DA uptake, and neurotoxicity was measured by apoptosis and oxidative damage. Through the use of this mesencephalic neuronal/glial culture model, we were able to identify the relative sensitivity of dopaminergic neurons to gp120-induced damage, manifested as reduced function (decreased DA uptake), morphological changes, and reduced viability. We also showed that gp120-induced oxidative damage is involved in this neuropathogenic process.

Proinflammatory cytokines and HIV-1 synergistically enhance CXCL10 expression in human astrocytes

HIV encephalitis (HIVE), the pathologic correlate of HIV-associated dementia (HAD) is characterized by astrogliosis, cytokine/chemokine dysregulation, and neuronal degeneration. Increasing evidence suggests that inflammation is actively involved in the pathogenesis of HAD. In fact, the severity of HAD/HIVE correlates more closely with the presence of activated glial cells than with the presence and amount of HIV-infected cells in the brain. Astrocytes, the most numerous cell type within the brain, provide an important reservoir for the generation of inflammatory mediators, including interferon-gamma inducible peptide-10 (CXCL10), a neurotoxin and a chemoattractant, implicated in the pathophysiology of HAD. Additionally, the proinflammatory cytokines, IFN-gamma and TNF-alpha, are also markedly increased in CNS tissues during HIV-1 infection. In this study, we hypothesized that the interplay of host cytokines and HIV-1 could lead to enhanced expression of the toxic chemokine, CXCL10. Our findings demonstrate a synergistic induction of CXCL10 mRNA and protein in human astrocytes exposed to HIV-1 and the proinflammatory cytokines. Signaling molecules, including JAK, STATs, MAPK (via activation of Erk1/2, AKT, and p38), and NF-kappaB were identified as instrumental in the synergistic induction of CXCL10. Understanding the mechanisms involved in HIV-1 and cytokine-mediated up-regulation of CXCL10 could aid in the development of therapeutic modalities for HAD.

Retrograde and anterograde transport of HIV protein gp120 in the nervous system

Neurodegeneration and gliosis are prominent pathological features of subjects with human immunodeficiency virus (HIV) dementia complex (HAD). In these patients, neurodegeneration occurs in uninfected neurons. In addition, these patients develop sensory neuropathy despite the antiretroviral therapy. The HIV protein gp120, which mimics some of the pathological alterations seen in HAD, is retrogradely transported in rodent neurons. However, it is still unclear whether gp120 can also be transported anterogradely and whether axonal transport can occur in the peripheral nervous system (PNS). To determine whether gp120 is transported retrogradely and/or anterogradely, we injected gp120IIIB together with the retrograde tracer fluoro-ruby (FR) or the anterograde tracer 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyamine perchlorate (DiI) into the rat superior colliculi. We discovered that gp120 is retrogradely transported with FR along a direct pathway from the superior colliculus to the retina and anterogradely transported with DiI to several areas of the occipital cortex. To determine whether gp120 is also axonally transported in the peripheral nerves, gp120 and FR were injected into the sciatic nerve. No gp120 immunoreactivity was found in the sciatic nerve or dorsal root ganglia, suggesting that gp120 axonal transport does not occur in the PNS. Gp120 axonal transport may play a role in neuronal injury. Therefore, we examined apoptosis at various time points after gp120 injection. Activated caspase-3 was evident within neurons transporting gp120. These results indicate that axonal transport of gp120 might exacerbate the pathogenesis of HIV-1.

Brain-derived neurotrophic factor modulates expression of chemokine receptors in the brain

Chemokine receptors, and in particular CXCR4 and CCR5 play a key role in the neuropathogenesis of Human Immunodeficiency Virus-1 (HIV)4 associated dementia (HAD). Thus, new insight into the expression of CXCR4 in the central nervous system may help develop therapeutic compounds against HAD. Brain-derived neurotrophic factor (BDNF) is neuroprotective in vitro against two strains of the HIV envelope protein gp120 that binds to CXCR4 or CCR5. Therefore, we examined whether BDNF modulates chemokine receptor expression in vivo. The content of CXCR4 mRNA and proteins was determined in the cerebral cortex and hippocampus of 6-month-old BDNF heterozygous mice and wild type littermates by using polymerase chain reaction and immunohistochemistry, respectively. BDNF heterozygous mice exhibited an increase in CXCR4 mRNA compared to wild type. Histological analyses revealed an up-regulation of CXCR4 immunoreactivity mainly in neurons. Most of these neurons were positive for TrkB, the BDNF receptor with a tyrosine kinase activity. Increases in CXCR4 mRNA levels were observed in 18-month-old BDNF heterozygous mice but not in 7-day-old mice, suggesting that the modulatory role of BDNF occurs only in mature animals. To determine whether BDNF affects also CXCR4 internalization, SH-SY5Y neuroblastoma cells were exposed to BDNF and cell surface CXCR4 levels were measured at various times. BDNF induced CXCR4 internalization within minutes. Lastly, BDNF heterozygous mice showed higher levels of CCR5 and CXCR3 mRNA than wild type in the cerebral cortex, hippocampus and striatum. Our data indicate that BDNF may modulate the availability of chemokine receptors implicated in HIV infection.

Chemokine receptors and neurotrophic factors: potential therapy against aids dementia?

Chemokine receptors, in particular, CXCR4 and CCR5, mediate human immunodeficiency virus type 1 (HIV-1) infection of immunocompetent cells and the apoptosis of these cells. However, the virus does not infect neurons. Yet through a variety of mechanisms, HIV promotes glial cell activation, synaptodendritic alterations, and neuronal loss that ultimately lead to motor and cognitive impairment. Chemokines and chemokine receptors are abundant in the adult central nervous system and play a role in neuronal apoptosis evoked by HIV proteins. Thus, reducing the availability of chemokine receptors may prevent the neuronal degeneration seen in HIV-positive patients. In this article, we present and discuss a recent experimental approach aimed at testing effective neuroprotective therapies against HIV-mediated neuronal degeneration.

Brain-derived neurotrophic factor prevents human immunodeficiency virus type 1 protein gp120 neurotoxicity in the rat nigrostriatal system

Human immunodeficiency virus type 1 (HIV-1) causes neuronal degeneration and, at a late stage, creates HIV-associated dementia (HAD) and other neurological abnormalities. Therefore, the need for neuroprotective agents is great. However, therapeutic agents that reduce HIV neurotoxicity are difficult to characterize and develop because rodents are not infected by HIV. This study was undertaken to develop an animal model of HIV neurotoxicity by using the HIV-1 envelope glycoprotein 120 (gp120). Vehicle or gp120 was injected acutely in the striatum of adult rats. gp120 produced loss of nigrostriatal neurons, as shown both by histochemical analysis of brain sections for apoptosis and biochemical determination of dopamine. The neurotrophin brain-derived neurotrophic factor (BDNF) delivered by a recombinant adeno-associated viral vector prevented gp120 toxicity. This study's results support the notion that gp120 produces a widespread neurotoxicity similar to that observed in HIV-positive individuals and that BDNF may be a suitable neuroprotective agent for HAD.

Brain-derived neurotrophic factor expression in the substantia nigra does not change after lesions of dopaminergic neurons

Progressive and irreversible loss of specific neuronal cell populations is commonly seen in chronic neurodegenerative diseases such as Parkinson's disease (PD). Evidence is accumulating that apoptosis is a crucial cellular event responsible for the dysfunction and death of neurons in this disease. Thus, limiting apoptosis may prevent disease pathogenesis. Key to reducing apoptosis is the discovery of neuroprotective compounds that can be given to patients to minimize neuronal damage. In this manuscript, we reviewed the rationale of using an experimental strategy to provide neurotrophic support to injured neurons. Such rationale includes the increase of endogenous production of brain-derived neurotrophic factor (BDNF). BDNF is a potent inhibitor of apoptosis-mediated cell death and neurotoxin-induced degeneration of dopaminergic neurons. However, availability of BDNF may be reduced when dopaminergic neurons degenerate. Therefore, in this work, we have used several well-established neurotoxins for dopaminergic neurons, 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine, 6-OH-dopamine (6-OHDA), and the HIV protein gp120, to examine whether degeneration of nigrostriatal fibers alters BDNF expression. Our data show that these neurotoxins do not decrease the levels of BDNF in the substantia nigra, suggesting that up-regulation of BDNF synthesis by pharmacological means may be a viable therapy to slow down the progress of PD and other neurodegenerative diseases.