Pathogenesis of HIV encephalopathy

HIV-1-Associated Neurocognitive Disorders: Is HLA-C Binding Stability to β2-Microglobulin a Missing Piece of the Pathogenetic Puzzle?

AIDS dementia complex (ADC) and HIV-associated neurocognitive disorders (HAND) are complications of HIV-1 infection. Viral infections are risk factors for the development of neurodegenerative disorders. Aging is associated with low-grade inflammation in the brain, i.e., the inflammaging. The molecular mechanisms linking immunosenescence, inflammaging and the pathogenesis of neurodegenerative disorders, such as Alzheimer's disease (AD) and Parkinson's disease, are largely unknown. ADC and HAND share some pathological features with AD and may offer some hints on the relationship between viral infections, neuroinflammation, and neurodegeneration. β2-microglobulin (β2m) is an important pro-aging factor that interferes with neurogenesis and worsens cognitive functions. Several studies published in the 80-90s reported high levels of β2m in the cerebrospinal fluid of patients with ADC. High levels of β2m have also been detected in AD. Inflammatory diseases in elderly people are associated with polymorphisms of the MHC-I locus encoding HLA molecules that, by associating with β2m, contribute to cellular immunity. We recently reported that HLA-C, no longer associated with β2m, is incorporated into HIV-1 virions, determining an increase in viral infectivity. We also documented the presence of HLA-C variants more or less stably linked to β2m. These observations led us to hypothesize that some variants of HLA-C, in the presence of viral infections, could determine a greater release and accumulation of β2m, which in turn, may be involved in triggering and/or sustaining neuroinflammation. ADC is the most severe form of HAND. To explore the role of HLA-C in ADC pathogenesis, we analyzed the frequency of HLA-C variants with unstable binding to β2m in a group of patients with ADC. We found a higher frequency of unstable HLA-C alleles in ADC patients, and none of them was harboring stable HLA-C alleles in homozygosis. Our data suggest that the role of HLA-C variants in ADC/HAND pathogenesis deserves further studies. If confirmed in a larger number of samples, this finding may have practical implication for a personalized medicine approach and for developing new therapies to prevent HAND. The exploration of HLA-C variants as risk factors for AD and other neurodegenerative disorders may be a promising field of study.

HIV-gp120 and physical dependence to buprenorphine

BACKGROUND

Opioids are among the most effective and commonly used analgesics in clinical practice for severe pain. However, the use of opioid medications is clinically limited by several adverse properties including dependence. While opioid dependence is a complex health condition, the treatment of HIV-infected individuals with opioid dependence presents additional challenges. The goal of this study was to examine the physical dependence to buprenorphine in the context of HIV.

METHODS

Young adult male rats (Sprague-Dawley) were pretreated with HIV-1 envelope glycoprotein 120 (gp120) injected into the periaqueductal gray area (PAG) and we examined the impact on physical dependence to opioid.

RESULTS

It was found that the physical dependence to methadone occurred earlier than that to buprenorphine, and that gp120 did not enhance or precipitate the buprenorphine withdrawal.

CONCLUSION

The results suggest that buprenorphine could be the better therapeutic option to manage opioid dependence in HIV.

Intrahypothalamic injection of the HIV-1 envelope glycoprotein induces fever via interaction with the chemokine system

Wasting syndrome is a common complication of HIV infection and is marked by progressive weight loss and weakness, often associated with fever. The mechanisms involved in the pathogenesis of these syndromes are not well defined, and neither are the brain areas involved. The present study tests a new hypothesis: that the preoptic anterior hypothalamus (POAH), the main brain area for thermoregulation and fever, has a role in the pathogenesis of fever induced by glycoprotein 120 (gp120), the surface envelope protein used by the HIV to gain access into immune cells, and that the CXC chemokine receptors (CXCR4) that serve as a coreceptor for HIV entry mediate the effect. A sterilized stainless steel C313G cannula guide was implanted into the POAH, and a biotelemetry system was used to monitor the body temperature (Tb) changes. The administration of gp120 into the POAH induced fever in a dose-dependent manner. To demonstrate possible links between the gp120 and CXCR4 in generating the fever, we pretreated the rats with 1,1'-[1,4-phenylenebis(methylene)]bis[1,4,8,11-tetraazacyclotetradecane] octohydrobromide dihydrate (AMD 3100), an antagonist of stromal cell-derived growth factor (SDF)-1alpha/CXCL12, acting at its receptor, CXCR4, 30 min before administration of gp120. AMD 3100 significantly reduced the gp120-induced fever. The present studies show that the presence of HIV-1 envelope glycoprotein gp120 in the POAH provokes fever via interaction CXCR4 pathway.

Impact of opiate-HIV-1 interactions on neurotoxic signaling

Opiate drug abuse exacerbates the pathogenesis of human immunodeficiency virus-1 (HIV-1) in the central nervous system through direct actions on glia and neurons. Opiate abuse causes widespread disruption of astroglial and microglial function, and significant increases in astroglial-derived proinflammatory cytokines and chemokines, which likely contributes to neuronal dysfunction, death, and HIV encephalitis. Neurons are also directly affected by opiate-HIV-1 interactions. HIV-1 and the viral proteins gp120 and Tat activate multiple caspase-dependent and caspase-independent proapoptotic pathways in neurons involving phosphatidylinositol 3-kinase (PI3 kinase)/Akt, as well as p38, c-Jun N-terminal kinase (JNK) and/or other mitogen-activated protein kinases (MAPKs). Opiates appear to decrease the threshold for HIV-1-mediated neurotoxicity by sending convergent signals that exacerbate proapoptotic events induced by viral and cellular toxic products. The synergistic proinflammatory and neurotoxic effects of opiate drugs on glia and neurons are largely mediated through mu opioid receptors, which are expressed by subpopulations of astroglia, microglia, and neurons. Opiate abuse intrinsically modifies the host response to HIV-1. Identification of how this occurs is providing considerable insight toward understanding the mechanisms underlying HIV-1-associated dementia.

Regional quantitative comparison of multispliced to unspliced ratios of HIV-1 RNA copy number in infected human brain

Infection of the brain by HIV-1 often results in cognitive- motor disorders, the most severe form being HIV-1 associated dimentia (HAD). However, the etiology and pathogenesis of neuroAIDS at the molecular level is still not fully understood and controversial issues remain, including the significance of abortive infection and localized viral load. This paper proposes that quantitative comparison of HIV-1 proviral and RNAloads across the brain will clarify some of these issues. It was hypothesized that there are differences in ratios of multispliced and unspliced HIV RNA in different regions of brain by analogy with prior findings of brain regional differences in virus and strains of HIV-1. A competitive RT-PCR method was used to compare ratios of multispliced to unspliced HIV-1 RNA's across brain regions of one case with HAD. Statistical analysis results showed that data obtained by repeated assays for each RNA preparation were not significantly different. Significant differences were detected between specimens obtained from different regions of the brain. The ratio of MS/US RNA in the frontal lobe was significantly greater than in the basal ganglia, medial temporal lobe, and another site in the temporal lobe. It must be noted that our approach has been the analysis of macroscopic brain regions separated by several centimeters; future studies will analyze microscopic analysis of these brain regions. The current study was preformed to produce results on gross differences in neuroanatomical locations at cm distances. Future studies will be performed to compare different regions with microscopic anatomic specificity.

The role of Toll-like receptors in CNS response to microbial challenge

The recent discovery of the family of Toll-like receptors has vastly expanded our understanding of the mechanisms by which the innate immune system recognizes and responds to a wide variety of microbial and endogenous pathogens. Toll-like receptors are transmembrane proteins that upon ligation with their cognate ligands trigger the production of cytokines, enzymes and other inflammatory agents. In the CNS Toll-like receptors are expressed predominantly by glial cells. In particular, the vastly abundant astrocytes are likely to be the major contributors to inflammatory responses within the CNS. Studies of the murine brain abscess model revealed that Toll-like receptor 2 plays a pivotal role in the generation of immune responses to Staphylococcus aureus. Although Toll-like receptor signaling is essential in antimicrobial defense, it may also lead to bystander injury of CNS tissue.

Human immunodeficiency virus type 1 envelope glycoprotein gp120 induces tumor necrosis factor-alpha in astrocytes

gp120 induction of the inflammatory cytokine tumor necrosis factor-alpha (TNF-alpha) was studied in cultures of purified astrocytes. Incubation of pure mouse cortical astrocytes with gp120 IIIB induced the expression of TNF-alpha mRNA, assessed by in situ hybridization. Anti- TNF-alpha immunocytochemical staining of gp120 IIIB stimulated astrocytes indicated the presence of TNF-alpha. gp120 IIIB treatment also stimulated secretion of bioactive TNF-alpha from astrocytes, which was prevented by inhibitors of transcription and translation. Hippocampal and cerebellar astrocytes displayed similar behaviors. Further, gp120 displayed cytotoxicity for astrocytes that depended on macromolecular synthesis. The data are the first to show gp120 IIIB induction of de novo TNF-alpha production by pure astrocytes. Because TNF-alpha exerts a wide array of effects in the brain of infected individuals and has HIV-1 inducing activity as well, induction of this cytokine by gp120 IIIB in astrocytes may contribute importantly to the pathogenesis of AIDS dementia complex. Since TNF-alpha can stimulate astrocyte reactivity and proliferation by an autocrine mechanism, the extent of the gp120 effect could conceivably increase with HIV-1 disease progression in a self-amplifying loop, involving other cell types, thus favoring both virus persistence and a chronic disease state.

Intracerebral HIV-1 glycoprotein 120 produces sickness behavior and pituitary-adrenal activation in rats: role of prostaglandins

HIV infection is associated with profound neurobehavioral and neuroendocrine impairments. Previous studies demonstrated that HIV causes neuropathological alterations indirectly, via shedding of glycoprotein 120 (gp120) within the brain. To extend these findings, we examined the neurobehavioral and neuroendocrine effects of central administration of gp120, as well as the role of brain prostaglandins in mediating these effects. Intracerebroventricular (i.c.v.) injection of gp120 in rats produced a marked sickness behavior syndrome, consisting of reduced exploratory behavior, suppressed consumption of food and saccharin solution, and reduced body weight. Gp120 also induced a significant febrile response and increased serum levels of ACTH and corticosterone. Following i.c.v. gp120 administration, the ex vivo production of PGE2 by the hypothalamus, frontal cortex, and hippocampus was significantly elevated, and indomethacin, a prostaglandin synthesis inhibitor, attenuated this elevation. Pre-treatment with indomethacin reduced the fever and adrenocortical activation induced by gp120 administration, but not its behavioral effects. These findings indicate that gp120 may be responsible for some of the behavioral and endocrine abnormalities seen in HIV-infected patients. Prostaglandins are important mediators of the physiological, but not the behavioral effects of brain gp120.

Involvement of brain cytokines in the neurobehavioral disturbances induced by HIV-1 glycoprotein120

Intracerebroventricular (i.c.v.) administration of HIV-1 glycoprotein 120 (gp120), the envelope protein used by the virus to gain access into immune cells, induces neurobehavioral alterations in rats. To examine the role of proinflammatory cytokines in mediating these effects, we measured the effects of gp120 on brain proinflammatory cytokine expression and the effects of anti-inflammatory agents, including interleukin-1 receptor antagonist (IL-1ra), pentoxifylline (a TNFalpha synthesis blocker) and IL-10, on gp120-induced sickness behavior. I.c.v. administration of gp120 induced the expression of IL-1beta, but not TNFalpha, mRNA in the hypothalamus, 3 h after the injection. Pretreatment of rats with IL-1ra, but not with pentoxifylline, significantly attenuated gp120-induced anorexia and loss in body weight, whereas both agents had no effect on gp120-induced reduction in locomotor activity in the open field. Pretreatment with either IL-1ra and pentoxifylline simultaneously, or with IL-10, produced effects that were similar to the effects of IL-1ra alone. Together, these findings indicate that IL-1, but not TNFalpha, mediates some of the behavioral effects of acute gp120 administration, suggesting that brain IL-1 may be involved in some of the neurobehavioral abnormalities evident in AIDS patients.

The role of brain cytokines in mediating the behavioral and neuroendocrine effects of intracerebral mycoplasma fermentans

Intracerebral administration of Mycoplasma fermentans (MF), a small microorganism that has been found in the brain of some AIDS patients, induces behavioral and neuroendocrine alterations in rats. To examine the role of tumor necrosis factor-alpha (TNFalpha) and interleukin-1 (IL-1) in mediating these effects we measured MF-induced expression of TNFalpha and IL-1beta mRNA in various brain regions, and the effects of TNFalpha synthesis blockers and IL-1 receptor antagonist (IL-1ra) on MF-induced sickness behavior and adrenocortical activation. Intracerebroventricular (i.c.v.) administration of heat-inactivated MF induced the expression of both TNFalpha and IL-1beta mRNA in the cortex, dorsal hippocampus, amygdala, and hypothalamus. Pre-treatment of rats with either TNFalpha synthesis blockers, pentoxifylline or rolipram, or with IL-1ra did not attenuate MF-induced anorexia, body weight loss, and suppression of social behavior. However, simultaneous administration of both pentoxifylline and IL-1ra markedly attenuated MF-induced anorexia and body weight loss, but had no effect on the suppression of social behavior. Pre-treatment with pentoxifylline, but not with IL-1ra, significantly attenuated MF-induced corticosterone (CS) secretion. Together, these findings indicate that both TNFalpha and IL-1 participate, in a complementary manner, in mediating some of the behavioral effects of MF, whereas only TNFalpha, but not IL-1, is involved in mediating MF-induced adrenocortical activation. We suggest that cytokines within the brain are involved in mediating at least some of the neurobehavioral and neuroendocrine abnormalities that may be produced by MF in AIDS patients.

Intracerebral administration of Mycoplasma fermentans produces sickness behavior: role of prostaglandins

Mycoplasmas are small microorganisms, which cause various diseases in animals and in humans, activate the immune system, and induce the release of various cytokines. Some of the effects of mycoplasmas are mediated by the CNS. Moreover, Mycoplasma fermentans (MF) has recently been found in the brain, as well as other tissues of some AIDS patients, who usually display severe neurobehavioral disturbances. The present study was designed to examine the behavioral effects of central administration of MF, and the role of prostaglandins in mediating these effects. In one set of experiments, rats were injected intracerebroventricularly (i.c.v.) with either saline or a dose of MF (5.1-36 microg per rat), and several behavioral parameters were measured. In addition, body temperature and locomotor activity were continuously monitored by a biotelemetric system. MF induced a significant elevation in body temperature and suppression of motor activity levels. MF also significantly reduced the time spent in social exploration, decreased locomotor and exploratory activity in the open field test, suppressed the consumption of food and saccharine solution, and reduced body weight. In a second set of experiments, i.c.v. administration of MF (7.2 microg) was found to produce a significant increase in the production of prostaglandin E2 (PGE2) in hypothalamic, hippocampal, and cortical tissues. This effect was blocked by indomethacin, a prostaglandin synthesis inhibitor. Indomethacin also attenuated the effects of MF on body temperature, motor activity and body weight, suggesting the involvement of prostaglandins in mediating some of the effects of MF. Together, these findings suggest that the presence of MF in the brain may be responsible for some of the neurobehavioral abnormalities in HIV-infected patients.

AIDS Dementia and HIV-1-Induced Neurotoxicity: Possible Pathogenic Associations and Mechanisms

AIDS Dementia Complex (ADC) is a syndrome of cognitive, behavioral, and motor deficits resulting from HIV-1 infection within the brain. ADC is characterized by variable degrees of neuronal cell death and gliosis that likely result, at least, in part from release of metabolic products, cytokines, and viral proteins from infected macrophages, although a unifying explanation for the neurological dysfunction has yet to be established. Major unanswered questions include: (i) do neurologic symptoms result from neuronal cell death and/or dysfunction in surviving neurons?; (ii) are viral genomic sequences determinants of neurotoxicity?; (iii) is HIV infection of neurons and astrocytes relevant to pathogenesis?, and (iv) what circulating factors within the brain affect neuronal cell survival and function? This review addresses the association between HIV-1 replication within the brain, production of potential neurotoxins and possible mechanisms of induction of neurotoxicity and neuronal dysfunction contributing to the pathogenesis of ADC. Copyright 1996 S. Karger AG, Basel