Macrophage Activation Factors in the Brains of AIDS Patients

Calpain-mediated degradation of MDMx/MDM4 contributes to HIV-induced neuronal damage

Neuronal damage in HIV-associated Neurocognitive Disorders (HAND) has been linked to inflammation induced by soluble factors released by HIV-infected, and non-infected, activated macrophages/microglia (HIV M/M) in the brain. It has been suggested that aberrant neuronal cell cycle activation determines cell fate in response to these toxic factors. We have previously shown increased expression of cell cycle proteins such as E2F1 and phosphorylated pRb in HAND midfrontal cortex in vivo and in primary neurons exposed to HIV M/M supernatants in vitro. In addition, we have previously shown that MDMx (also referred to as MDM4), a negative regulator of E2F1, was decreased in the brain in a primate model of HIV-induced CNS neurodegeneration. Thus, we hypothesized that MDMx provides indirect neuroprotection from HIV-induced neurodegeneration in our in vitro model. In this report, we found significant reductions in MDMx protein levels in the mid-frontal cortex of patients with HAND. In addition, treatment of primary rat neuroglial cultures with HIV M/M led to NMDA receptor- and calpain-dependent degradation of MDMx and decreased neuronal survival, while overexpression of MDMx conferred partial protection from HIV M/M toxicity in vitro. Further, our results demonstrate that MDMx is a novel and direct calpain substrate. Finally, blocking MDMx activity led to neuronal death in vitro in the absence of toxic stimulus, which was reversed by calpain inhibition. Overall, our results indicate that MDMx plays a pro-survival role in neurons, and that strategies to stabilize and/or induce MDMx can provide neuroprotection in HAND and in other neurodegenerative diseases where calpain activation contributes to neuropathogenesis.

Site-specific hyperphosphorylation of pRb in HIV-induced neurotoxicity

HIV-Associated Neurocognitive Disorder (HAND) remains a serious complication of HIV infection, despite combined Anti-Retroviral Therapy (cART). Neuronal dysfunction and death are attributed to soluble factors released from activated and/or HIV-infected macrophages. Most of these factors affect the cell cycle machinery, determining cellular outcomes even in the absence of cell division. One of the earliest events in cell cycle activation is hyperphosphorylation of the retinoblastoma protein, pRb (ppRb). We and others have previously shown increased ppRb expression in the CNS of patients with HIV encephalitis (HIVE) and in neurons in an in vitro model of HIV-induced neurodegeneration. However, trophic factors also lead to an increase in neuronal ppRb with an absence of cell death, suggesting that, depending on the stimulus, hyperphosphorylation of pRb can have different outcomes on neuronal fate. pRb has multiple serines and threonines targeted for phosphorylation by distinct kinases, and we hypothesized that different stimuli may target separate sites for phosphorylation. Thus, to determine whether pRb is differentially phosphorylated in response to different stimuli and whether any of these sites is preferentially phosphorylated in association with HIV-induced neurotoxicity, we treated primary rat mixed cortical cultures with trophic factors, BDNF or RANTES, or with the neurotoxic factor, N-methyl-d-aspartate (NMDA), or with supernatants containing factors secreted by HIV-infected monocyte-derived macrophages (HIV-MDM), our in vitro model of HIV-induced neurodegeneration. We found that, while BDNF and RANTES phosphorylated serine807/811 and serine608 in vitro, treatment with HIV-MDM did not, even though these trophic factors are components of HIV-MDM. Rather, HIV-MDM targets a specific phosphorylation site, serine795, of pRb for phosphorylation in vitro and this ppRb isoform is also increased in HIV-infected brains in vivo. Further, overexpression of a nonphosphorylatable pRb (ppRb S795A) attenuated HIV-MDM-induced neurotoxicity. These findings indicate that HIV-infection in the brain is associated with site-specific hyperphosphorylation of pRb at serine795, which is not induced by other tested stimuli, and that this phosphorylation contributes to neuronal death in this disease, demonstrating that specific pRb sites are differentially targeted and may have diverse impacts on the viability of post-mitotic neurons.

Increased density of neurons containing NADPH diaphorase and nitric oxide synthase in the cerebral cortex of patients with HIV-1 infection and drug abuse

To determine whether nitrogen monoxide (nitric oxide; NO) synthase (NOS) and NADPH diaphorase (NDP) co-containing cerebrocortical neurons (NOSN) neurons are affected in patients infected with human immunodeficiency virus type 1 (HIV-1) with and without associated intake of drugs of abuse, we examined the temporal neocortex of 24 individuals: 12 HIV-1 positive (including 3 drug users, 9 non-drug users) and 12 HIV-1 negative (including 6 drug users, and 6 non-drug users). Histochemical labeling for NDP-an enzymatic domain co-expressed in the NOS enzyme-was employed to visualize NOSN. Drug abuse and HIV-1 infection cause independently an increase in NOSN density, but combined they result in up to a 38-fold increase in NOSN density, suggesting that the combination of these factors induces NOS expression powerfully in neurons that normally do not synthesize NDP/NOS. This is associated with an increase in the proportion of NOSN displaying dystrophic changes, indicating that NOSN undergo massive degeneration in association with NOS synthesis induction. The increase in density of NOSN in HIV-1 infected drug abusers may be among the important sources of NO mediating cerebrocortical dysfunction, and the degeneration of NOS-containing local circuit neurons in patients with HIV-1 infection or drug abuse may underlie in part their neuropsychiatric manifestations.

Brain macrophage surface marker expression with HIV-1 infection and drug abuse: a preliminary study

GOAL

To determine the heterogeneity of surface marker expression of macrophages in the temporal lobe of patients who died with AIDS who were also Drug Abusers (DAs). We studied the expression of macrophage surface markers CD11c, CD14, CD68, and HLA-DR and T cell surface markers CD4, and CD8.

BACKGROUND

The macrophage is the prime locus for HIV-1-associated pathology, is the most frequently infected cell in the brain, and has the highest virus load compared to other cells. We previously described the heterogeneity of macrophage surface marker expression and performed morphometric analysis in peripheral nerves of patients who died from AIDS compared to HIV-1 negative individuals. We showed that the HIV-related neuropathy in AIDS is a multifocal process. It is similarly important to determine the expression of macrophage surface markers in brain. Temporal lobe tissue was selected for this preliminary study because we previously found elevated HIV-1 proviral DNA load and inflammatory processes in this neuroanatomic location for subjects who died with AIDS. There is a high prevalence of Drug Abuse in Miami, Florida, associated with AIDS that may interactively affect HIV-associated pathology.

METHODS

Temporal lobe tissue was examined from 17 HIV-1-seropositive patients (4 with Drug Abuse and 13 without Drug Abuse) and 11 HIV-seronegative individuals (5 with Drug Abuse and 6 without Drug Abuse). Standard immunohistochemistry utilized alkaline phosphatase conjugate secondary antibody and fuchsin substrate.

RESULTS

We found that HIV-1 infection and the interaction of HIV-1 infection and Drug Abuse produced changes in macrophage surface marker expression. Macrophage surface markers, CD11c, CD14, CD68, and HLA-DR, and T-cell marker CD4 were increased with statistical significance due to HIV-1 infection (all p < .001) whereas CD8 remained unchanged. Changes due to Drug Abuse alone were not significant. Interaction of Drug Abuse and HIV-infected individuals showed increased expression of CD68 (p = .011), HLA-DR (p = .001), CD4 (p = .027), and CD8 (p = .016).

CONCLUSION

Drug Abuse and HIV-1 infection are factors that differentially and interactively result in multiple macrophages surface marker effects. In HIV-1 infected individuals, Drug Abuse stimulates surface marker expression. Since brain macrophage surface makers do not change uniformly as a result of Drug Abuse and HIV infection, these cells may be heterogeneous and contain sub-types (sub-sets). It remains to be determined which macrophage sub-types may be most pathognomic for pathology.

Involvement of quinolinic acid in AIDS dementia complex

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

The role of macrophage/microglia and astrocytes in the pathogenesis of three neurologic disorders: HIV-associated dementia, Alzheimer disease, and multiple sclerosis

Macrophage/microglia (M phi) are the principal immune cells in the central nervous system (CNS) concomitant with inflammatory brain disease and play a significant role in the host defense against invading microorganisms. Astrocytes, as a significant component of the blood-brain barrier, behave as one of the immune effector cells in the CNS as well. However, both cell types may play a dual role, amplifying the effects of inflammation and mediating cellular damage as well as protecting the CNS. Interactions of the immune system, M phi, and astrocytes result in altered production of neurotoxins and neurotrophins by these cells. These effects alter the neuronal structure and function during pathogenesis of HIV-1-associated dementia (HAD), Alzheimer disease (AD), and multiple sclerosis (MS). HAD primarily involves subcortical gray matter, and both HAD and MS affect sub-cortical white matter. AD is a cortical disease. The process of M phi and astrocytes activation leading to neurotoxicity share similarities among the three diseases. Human Immunodeficiency Virus (HIV)-1-infected M phi are involved in the pathogenesis of HAD and produce toxic molecules including cytokines, chemokines, and nitric oxide (NO). In AD, M phis produce these molecules and are activated by beta-amyloid proteins and related oligopeptides. Demyelination in MS involves M phi that become lipid laden, spurred by several possible antigens. In these three diseases, cytokine/chemokine communications between M phi and astrocytes occur and are involved in the balance of protective and destructive actions by these cells. This review describes the role of M phi and astrocytes in the pathogenesis of these three progressive neurological diseases, examining both beneficent and deleterious effects in each disease.

Tissue microarrays in the study of non-neoplastic disease of the nervous system

Tissue microarrays (TMAs), also known as "tissue chips," are a recently developed method that allows small cores or discs of tissue from dozens or hundreds of (usually paraffin-embedded) specimens to be re-embedded in a tissue block, which can then be further sectioned. The tissue cores can subsequently be studied using any combination of techniques, including immunohistochemistry, in situ hybridization (ISH). fluorescence ISH, and in situ polymerase chain reaction (PCR). To date, the technique has found greatest use in the analysis of neoplasms, including gliomas. We describe, and provide examples of, how TMAs might be utilized in investigation of autopsy (or biopsy) tissues from individuals with non-neoplastic disease, especially to address questions that require systematic review of multiple (nearly) identical brain regions across dozens or hundreds of cases. Specific questions related to patterns of protein expression (e.g. tau, Abeta, alpha-synuclein) in multiple regions of large numbers of brain specimens (from patients with neurodegenerative diseases) can be efficiently examined using TMA technology. One possible use of TMAs in the area of infectious disease might be to examine patterns of HIV-related brain injury or AIDS-related opportunistic CNS infections in the epochs before and after highly active antiretroviral therapy came into widespread use.

Kynurenines in the CNS: from endogenous obscurity to therapeutic importance

In just under 20 years the kynurenine family of compounds has developed from a group of obscure metabolites of the essential amino acid tryptophan into a source of intensive research, with postulated roles for quinolinic acid in neurodegenerative disorders, most especially the AIDS-dementia complex and Huntington's disease. One of the kynurenines, kynurenic acid, has become a standard tool for use in the identification of glutamate-releasing synapses, and has been used as the parent for several groups of compounds now being developed as drugs for the treatment of epilepsy and stroke. The kynurenines represent a major success in translating a basic discovery into a source of clinical understanding and therapeutic application, with around 3000 papers published on quinolinic acid or kynurenic acid since the discovery of their effects in 1981 and 1982. This review concentrates on some of the recent work most directly relevant to the understanding and applications of kynurenines in medicine.

Endogenous neurotoxins from tryptophan

In most tissues, including brain, a major proportion of the tryptophan which is not used for protein synthesis is metabolised along the kynurenine pathway. Long regarded as the route by which many mammals generate adequate amounts of the essential co-factor nicotinamide adenine dinucleotide, two components of the pathway are now known to have marked effects on neurones. Quinolinic acid is an agonist at the N-methyl-D-aspartate sensitive subtype of glutamate receptors in the brain, while kynurenic acid is an antagonist and, thus, a potential neuroprotectant. A third kynurenine, 3-hydroxykynurenine, is involved in the generation of free radicals which can also damage neurones. Quinolinic acid is increasingly implicated in neurodegenerative disorders, most especially the AIDS-dementia complex and Huntington's disease, while kynurenic acid has become a standard for the identification of glutamate-releasing synapses, and has been used as the parent for several groups of compounds now being developed as drugs for the treatment of epilepsy and stroke.

HIV-1 proviral DNA load across neuroanatomic regions of individuals with evidence for HIV-1-associated dementia

A definitive relation between HIV-1 load and the clinical diagnosis of HIV-1-associated dementia (HAD) has not yet been established. Knowledge of the neuroanatomic distribution of HIV-1 load in the brain of individuals with HAD and HIV-1 encephalitis may facilitate elucidation of this relation. Nine individuals with AIDS were analyzed postmortem by three independent methods with each assessment performed blinded to the others: 1) a neuropsychiatric review of clinical records for evidence of possible HAD, 2) HIV-1 DNA load determination by quantitative polymerase chain reaction (PCR) across several neuroanatomic regions, and 3) a pathologic examination for diagnosis of HIV-1 encephalitis by immunohistochemical techniques. Of eight AIDS cases with clinical records sufficient for neuropsychiatric review, seven were shown to have evidence for HAD. HIV-1 DNA was detected and quantified in specimens from all of the medial temporal lobe regions analyzed but was not detectable in the frontal lobe at the same level of sensitivity in two of these cases (<1 per 1000 cellular genomes). HIV-1 DNA load in the medial temporal lobe region was significantly larger than that in the frontal lobe. Only four of seven cases with evidence for HAD were also diagnosed with HIV-1 encephalitis.