Microglia in HIV-associated neurological diseases

Neuropathogenesis of HIV and emerging therapeutic targets

INTRODUCTION

HIV infection causes a wide range of neurological complications, many of which are among the most common complications of chronic HIV infection in the era of combined antiretroviral therapy. These neurological conditions arise due to complex interactions between HIV viral proteins and neuronal and glial cells that lead to the activation of various inflammatory and neurotoxic pathways across the nervous system.

AREAS COVERED

This review summarizes the current literature on the pathogenesis and clinical manifestations of neurological injuries associated with HIV in the brain, spinal cord, and peripheral nervous system. Molecular pathways relevant for possible therapeutic targets or advancements are emphasized. Gaps in knowledge and current challenges in therapeutic design are also discussed.

EXPERT OPINION

Several challenges exist in the development of therapeutic targets for HIV-associated cognitive impairments. However, recent developments in drug delivery systems and treatment strategies are encouraging. Treatments for HIV-associated pain and peripheral sensory neuropathies currently consist of symptomatic management, but a greater understanding of their pathogenesis can lead to the development of targeted molecular therapies and disease-modifying therapies. HIV-associated autonomic dysfunction may affect the course of systemic disease via disrupted neuro-immune interactions; however, more research is needed to facilitate our understanding of how these processes present clinically.

An Elvitegravir Nanoformulation Crosses the Blood-Brain Barrier and Suppresses HIV-1 Replication in Microglia

Even with an efficient combination of antiretroviral therapy (ART), which significantly decreases viral load in human immunodeficiency virus type 1 (HIV-1)-positive individuals, the occurrence of HIV-1-associated neurocognitive disorders (HAND) still exists. Microglia have been shown to have a significant role in HIV-1 replication in the brain and in subsequent HAND pathogenesis. However, due to the limited ability of ART drugs to cross the blood-brain barrier (BBB) after systemic administration, in addition to efflux transporter expression on microglia, the efficacy of ART drugs for viral suppression in microglia is suboptimal. Previously, we developed novel poly (lactic-co-glycolic acid) (PLGA)-based elvitegravir nanoparticles (PLGA-EVG NPs), which showed improved BBB penetration in vitro and improved viral suppression in HIV-1-infected primary macrophages, after crossing an in vitro BBB model. Our objective in the current study was to evaluate the efficacy of our PLGA-EVG NPs in an important central nervous system (CNS) HIV-1 reservoir, i.e., microglia. In this study, we evaluated the cyto-compatibility of the PLGA-EVG NPs in microglia, using an XTT (2,3-bis-(2-methoxy-4-nitro-5-sulfophenyl)-2H-tetrazolium-5-carboxanilide) assay and cellular morphology observation. We also studied the endocytosis pathway and the subcellular localization of PLGA NPs in microglia, using various endocytosis inhibitors and subcellular localization markers. We determined the ability of PLGA-EVG NPs to suppress HIV-1 replication in microglia, after crossing an in vitro BBB model. We also studied the drug levels in mouse plasma and brain tissue, using immunodeficient NOD scid gamma (NSG) mice, and performed a pilot study, to evaluate the efficacy of PLGA-EVG NPs on viral suppression in the CNS, using an HIV-1 encephalitic (HIVE) mouse model. From our results, the PLGA-EVG NPs showed ~100% biocompatibility with microglia, as compared to control cells. The internalization of PLGA NPs in microglia occurred through caveolae-/clathrin-mediated endocytosis. PLGA NPs can also escape from endo-lysosomal compartments and deliver the therapeutics to cells efficiently. More importantly, the PLGA-EVG NPs were able to show ~25% more viral suppression in HIV-1-infected human-monocyte-derived microglia-like cells after crossing the in vitro BBB compared to the EVG native drug, without altering BBB integrity. PLGA-EVG NPs also showed a ~two-fold higher level in mouse brain and a trend of decreasing CNS HIV-1 viral load in HIV-1-infected mice. Overall, these results help us to create a safe and efficient drug delivery method to target HIV-1 reservoirs in the CNS, for potential clinical use.

Etiologies underlying sex differences in Autism Spectrum Disorders

The male predominance of Autism Spectrum Disorders (ASD) is one of the best-known, and at the same time, one of the least understood characteristics of these disorders. In this paper we review genetic, epigenetic, hormonal, and environmental mechanisms underlying this male preponderance. Sex-specific effects of Y-linked genes (including SRY expression leading to testicular development), balanced and skewed X-inactivation, genes that escape X-inactivation, parent-of-origin allelic imprinting, and the hypothetical heterochromatin sink are reviewed. These mechanisms likely contribute to etiology, instead of being simply causative to ASD. Environments, both internal and external, also play important roles in ASD's etiology. Early exposure to androgenic hormones and early maternal immune activation comprise environmental factors affecting sex-specific susceptibility to ASD. The gene-environment interactions underlying ASD, suggested here, implicate early prenatal stress as being especially detrimental to boys with a vulnerable genotype.

The recombinant vaccinia virus gene product, B18R, neutralizes interferon alpha and alleviates histopathological complications in an HIV encephalitis mouse model

Interferon-alpha (IFN-α) has been identified as a neurotoxin that plays a prominent role in human immunodeficiency virus (HIV)-associated neurocognitive disorders and HIV encephalitis (HIVE) pathology. IFN-α is associated with cognitive dysfunction in other inflammatory diseases where IFN-α is upregulated. Trials of monoclonal anti-IFN-α antibodies have been generally disappointing possibly due to high specificity to limited IFN-α subtypes and low affinity. We investigated a novel IFN-α inhibitor, B18R, in an HIVE/severe combined immunodeficiency (SCID) mouse model. Immunostaining for B18R in systemically treated HIVE/SCID mice suggested the ability of B18R to cross the blood-brain barrier (BBB). Real-time PCR indicated that B18R treatment resulted in a decrease in gene expression associated with IFN-α signaling in the brain. Mice treated with B18R were found to have decreased mouse mononuclear phagocytes and significant retention of neuronal arborization compared to untreated HIVE/SCID mice. Increased mononuclear phagocytes and decreased neuronal arborization are key features of HIVE. These results suggest that B18R crosses the BBB, blocks IFN-α signaling, and it prevents key features of HIVE pathology. These data suggest that the high affinity and broad IFN-α subtype specificity of B18R make it a viable alternative to monoclonal antibodies for the inhibition of IFN-α in the immune-suppressed environment.

Involvement of Kv1.3 and p38 MAPK signaling in HIV-1 glycoprotein 120-induced microglia neurotoxicity

Inflammatory responses mediated by activated microglia play a pivotal role in the pathogenesis of human immunodeficiency virus type 1 (HIV-1)-associated neurocognitive disorders. Studies on identification of specific targets to control microglia activation and resultant neurotoxic activity are imperative. Increasing evidence indicate that voltage-gated K⁺ (K_v) channels are involved in the regulation of microglia functionality. In this study, we investigated K_v1.3 channels in the regulation of neurotoxic activity mediated by HIV-1 glycoprotein 120 (gp120)-stimulated rat microglia. Our results showed treatment of microglia with gp120 increased the expression levels of K_v1.3 mRNA and protein. In parallel, whole-cell patch-clamp studies revealed that gp120 enhanced microglia K_v1.3 current, which was blocked by margatoxin, a K_v1.3 blocker. The association of gp120 enhancement of K_v1.3 current with microglia neurotoxicity was demonstrated by experimental results that blocking microglia K_v1.3 attenuated gp120-associated microglia production of neurotoxins and neurotoxicity. Knockdown of K_v1.3 gene by transfection of microglia with K_v1.3-siRNA abrogated gp120-associated microglia neurotoxic activity. Further investigation unraveled an involvement of p38 MAPK in gp120 enhancement of microglia K_v1.3 expression and resultant neurotoxic activity. These results suggest not only a role K_v1.3 may have in gp120-associated microglia neurotoxic activity, but also a potential target for the development of therapeutic strategies.

HIV-1 gp120 enhances outward potassium current via CXCR4 and cAMP-dependent protein kinase A signaling in cultured rat microglia

Microglia are critical cells in mediating the pathophysiology of neurodegenerative disorders such as HIV-associated neurocognitive disorders. We hypothesize that HIV-1 glycoprotein 120 (gp120) activates microglia by enhancing outward K(+) currents, resulting in microglia secretion of neurotoxins, consequent neuronal dysfunction, and death. To test this hypothesis, we studied the effects of gp120 on outward K(+) current in cultured rat microglia. Application of gp120 enhanced outward K(+) current in a dose-dependent manner, which was blocked by voltage-gated K(+) (K(v) ) channel blockers. Western blot analysis revealed that gp120 produced an elevated expression of K(v) channel proteins. Examination of activation and inactivation of outward K(+) currents showed that gp120 shifted membrane potentials for activation and steady-state inactivation. The gp120-associated enhancement of outward K(+) current was blocked by either a CXCR4 receptor antagonist T140 or a specific protein kinase A (PKA) inhibitor H89, suggesting the involvement of chemokine receptor CXCR4 and PKA in gp120-mediated enhancement of outward K(+) current. Biological significance of gp120-induced enhancement of microglia outward K(+) current was demonstrated by experimental results showing the neurotoxic activity of gp120-stimulated microglia, evaluated by TUNEL staining and MTT assay, significantly attenuated by K(v) channel blockers. Taken together, these results suggest that gp120 induces microglia neurotoxic activity by enhancing microglia outward K(+) current and that microglia K(v) channels may function as a potential target for the development of therapeutic strategies.

Evidence of human immunodeficiency virus type 1 infection of nestin-positive neural progenitors in archival pediatric brain tissue

Human immunodeficiency virus type 1 (HIV-1) central nervous system (CNS) infection in children is associated with impaired brain growth and neurodevelopmental delays. Neural progenitors are critical for neurogenesis. Human multipotential neural progenitors grown in culture are permissive for HIV-1 infection, but it is not known if infection of these cells occurs in vivo. Brain tissue from pre-highly active antiretroviral therapy (HAART) era pediatric acquired immunodeficiency syndrome (AIDS) patients was examined for evidence of HIV-1 infection of nestin-positive neural progenitors by in situ hybridization; or after laser microdissection harvest, DNA extraction, and polymerase chain reaction (PCR). HIV-1 or viral DNA was identified in nestin-positive cells in four of seven HIV-1-infected children, suggesting in vivo infection of neural progenitors.

The opioid receptor antagonist, naloxone, protects spinal motor neurons in a murine model of alphavirus encephalomyelitis

Spread of neuroadapted Sindbis virus (NSV) to motor neurons (MN) of the spinal cord (SC) causes severe hind limb weakness in C57BL/6 mice and models the paralysis that can accompany alphavirus and flavivirus encephalomyelitis in humans. The fate of spinal MN dictates the severity of NSV-induced paralysis, and recent data suggest that MN damage can occur indirectly via the actions of activated microglial cells. Because the opioid receptor antagonist, naloxone (NAL), blocks microglial-mediated neurodegeneration in other models, we examined its effects during NSV infection. Drug treatment prevented paralysis and enhanced the survival of MN without altering NSV tropism, replication, or clearance from SC tissue. Further studies showed that NAL most effectively inhibited paralysis in a 72-h window after NSV challenge, suggesting that the drug inhibits an early event in SC pathogenesis. Histochemical studies demonstrated that NAL blocked early microglial activation in SC tissue sections, and protein assays showed that the early induction of pathogenic IL-1 beta was blunted in SC homogenates. Finally, loss of glutamate transporter-1 (GLT-1) expression in SC, an astrocyte glutamate reuptake protein responsible for lowering toxic extracellular levels of glutamate and preventing MN damage, was reversed by NAL treatment. This GLT-1 loss proved to be highly IL-1 beta-dependent. Taken together, these data suggest that NAL is neuroprotective in the SC by inhibiting microglial activation that, in turn, maintains normal astrocyte glutamate homeostasis. We propose that drugs targeting such microglial responses may have therapeutic benefit in humans with related viral infections.

Human immunodeficiency virus type-1 protein Tat induces tumor necrosis factor-alpha-mediated neurotoxicity

HIV-1 infection causes, with increasing prevalence, neurological disorders characterized in part by neuronal cell death. The HIV-1 protein Tat has been shown to be directly and indirectly neurotoxic. Here, we tested the hypothesis that a non-neurotoxic epitope of Tat can, through actions on immune cells, increase neuronal cell death. Tat(1-72) and a mutant Tat(1-72) lacking the neurotoxic epitope (Tat(Delta31-61)) concentration-dependently and markedly increased TNF-alpha production in macrophage-like differentiated human U937 and THP-1 cells, in mouse peritoneal macrophages and in mouse brain microglia. Tat(1-72) was but Tat(Delta31-61) was not neurotoxic when applied directly to neurons. Supernatants from U937 cells treated with either Tat(1-72) or Tat(Delta31-61) were neurotoxic and their immunoneutralization with an anti-TNF-alpha antibody decreased Tat(1-72)- and Tat(Delta31-61)-induced neurotoxicity. Together, these results demonstrate that the neurotoxic epitope of Tat(1-72) is different from the epitope that is indirectly neurotoxic following production of TNF-alpha from immune cells, and suggest that therapeutic interventions against TNF-alpha might be beneficial against HIV-1 associated neurological disorders.

Human immunodeficiency virus infection and acquired immunodeficiency syndrome dementia complex: role of cells of monocyte-macrophage lineage

The entry of human immunodeficiency virus (HIV) into the central nervous system (CNS) causes both the establishment of a lifelong viral reservoir in the brain and symptoms of neurological dysfunction that have an AIDS dementia complex (ADC) clinical appearance. Neurological dysfunction in ADC patients still remains an unresolved problem. However, ADC pathogenesis may be a multistep process that starts with HIV invasion of CNS by crossing the blood-brain barrier (BBB). It progresses by developing a chronic inflammatory status that can cause dysfunction in neurons and astrocytes that result in apoptotic death. Monocytes-macrophages (M/M) may play an important role by concealing the HIV transfer across the BBB. Furthermore, HIV-infected M/M could produce and release neurotoxic factors. In this review the main mediators and cells involved in pathogenesis and development of ADC are highlighted. A better understanding of the mechanisms involved in this process may help in a successful therapeutic approach to the neuropathogenesis of HIV infection.

Molecular and cellular mechanisms of neuronal cell death in HIV dementia

The deaths of neurons, astrocytes and endothelial cells have been described in patients with HIV (human immunodeficiency virus) dementia. HIV-1 does not infect neurons; instead, neurotoxic substances shed by infected glia and macrophages can induce a form of programmed cell death called apoptosis in neurons. These neurotoxins include the HIV-1 proteins Tat and gp120, as well as pro-inflammatory cytokines, chemokines, excitotoxins and proteases. In this article we review the evidence for apoptosis of various cell types within the brain of HIV-infected patients, and describe in vitro and in vivo experimental studies that have elucidated the mechanisms by which HIV causes apoptosis of brain cells.

Cross talk between growth factors and viral and cellular factors alters neuronal signaling pathways: implication for HIV-associated dementia

HIV-associated dementia (HAD) is a serious neurological disorder affecting about 7% of people with AIDS. In the brain, HIV-1 infects a restricted number of cell types, being primarily present in macrophages and microglial cells, less abundant in astrocytes, and rarely seen in oligodendrocytes and neurons. Lack of a productive HIV-1 infection of neuronal cells suggests the presence of an indirect pathway by which the virus may determine the brain pathology and neuronal dysfunction seen in AIDS patients. Among the participants in this event, viral proteins including gp120 and Tat, along with host factors including cytokines, chemokines, and several signaling pathways have received considerable attention. In this article, we discuss the most recent concepts pertaining to the mechanisms of HIV-1-induced neuronal dysfunction by highlighting the interplay between signal transduction pathways activated by viral and host factors and their consequences in neuronal cell function.

Histopathological and cognitive defects induced by Nef in the brain

Complex mechanisms of human immunodeficiency virus type-1 (HIV-1) brain pathogenesis suggest the contribution of individual HIV-1 gene products. Among them, the Nef protein has been reported to harbor a major determinant of pathogenicity in AIDS-like disease. The goal of the present study was to determine whether Nef protein expressed in vivo by primary macrophages could induce a brain toxicity also affecting the behavior of the rat. To achieve this goal we grafted Nef-transduced macrophages into the rat hippocampus. Two months post-transplantation, we observed that Nef induces monocyte/macrophage recruitment, expression of TNF-alpha, and astrogliosis. No apoptotic event was detected. We further demonstrated that Nef neurotoxicity is associated with cognitive deficits.

Microglia overexpressing the macrophage colony-stimulating factor receptor are neuroprotective in a microglial-hippocampal organotypic coculture system

Microglia with increased expression of the macrophage colony-stimulating factor receptor (M-CSFR; c-fms) are found surrounding plaques in Alzheimer's disease (AD) and in mouse models for AD and after ischemic or traumatic brain injury. Increased expression of M-CSFR causes microglia to adopt an activated state that results in proliferation, release of cytokines, and enhanced phagocytosis. To determine whether M-CSFR-induced microglial activation affects neuronal survival, we assembled a coculture system consisting of BV-2 microglia transfected to overexpress the M-CSFR and hippocampal organotypic slices treated with NMDA. Twenty-four hours after assembly of the coculture, microglia overexpressing M-CSFR proliferated at a higher rate than nontransfected control cells and exhibited enhanced migration toward NMDA-injured hippocampal cultures. Surprisingly, coculture with c-fms-transfected microglia resulted in a dramatic reduction in NMDA-induced neurotoxicity. Similar results were observed when cocultures were treated with the teratogen cyclophosphamide. Biolistic overexpression of M-CSFR on microglia endogenous to the organotypic culture also rescued neurons from excitotoxicity. Furthermore, c-fms-transfected microglia increased neuronal expression of macrophage colony-stimulating factor (M-CSF), the M-CSFR, and neurotrophin receptors in the NMDA-treated slices, as determined with laser capture microdissection. In the coculture system, direct contact between the exogenous microglia and the slice was necessary for neuroprotection. Finally, blocking expression of the M-CSF ligand by exogenous c-fms-transfected microglia with a hammerhead ribozyme compromised their neuroprotective properties. These results demonstrate a protective role for microglia overexpressing M-CSFR in our coculture system and suggest under certain circumstances, activated microglia can help rather than harm neurons subjected to excitotoxic and teratogen-induced injury.

Neuroglial activation and neuroinflammation in the brain of patients with autism

Autism is a neurodevelopmental disorder characterized by impaired communication and social interaction and may be accompanied by mental retardation and epilepsy. Its cause remains unknown, despite evidence that genetic, environmental, and immunological factors may play a role in its pathogenesis. To investigate whether immune-mediated mechanisms are involved in the pathogenesis of autism, we used immunocytochemistry, cytokine protein arrays, and enzyme-linked immunosorbent assays to study brain tissues and cerebrospinal fluid (CSF) from autistic patients and determined the magnitude of neuroglial and inflammatory reactions and their cytokine expression profiles. Brain tissues from cerebellum, midfrontal, and cingulate gyrus obtained at autopsy from 11 patients with autism were used for morphological studies. Fresh-frozen tissues available from seven patients and CSF from six living autistic patients were used for cytokine protein profiling. We demonstrate an active neuroinflammatory process in the cerebral cortex, white matter, and notably in cerebellum of autistic patients. Immunocytochemical studies showed marked activation of microglia and astroglia, and cytokine profiling indicated that macrophage chemoattractant protein (MCP)-1 and tumor growth factor-beta1, derived from neuroglia, were the most prevalent cytokines in brain tissues. CSF showed a unique proinflammatory profile of cytokines, including a marked increase in MCP-1. Our findings indicate that innate neuroimmune reactions play a pathogenic role in an undefined proportion of autistic patients, suggesting that future therapies might involve modifying neuroglial responses in the brain.

Role of microglia in central nervous system infections

The nature of microglia fascinated many prominent researchers in the 19th and early 20th centuries, and in a classic treatise in 1932, Pio del Rio-Hortega formulated a number of concepts regarding the function of these resident macrophages of the brain parenchyma that remain relevant to this day. However, a renaissance of interest in microglia occurred toward the end of the 20th century, fueled by the recognition of their role in neuropathogenesis of infectious agents, such as human immunodeficiency virus type 1, and by what appears to be their participation in other neurodegenerative and neuroinflammatory disorders. During the same period, insights into the physiological and pathological properties of microglia were gained from in vivo and in vitro studies of neurotropic viruses, bacteria, fungi, parasites, and prions, which are reviewed in this article. New concepts that have emerged from these studies include the importance of cytokines and chemokines produced by activated microglia in neurodegenerative and neuroprotective processes and the elegant but astonishingly complex interactions between microglia, astrocytes, lymphocytes, and neurons that underlie these processes. It is proposed that an enhanced understanding of microglia will yield improved therapies of central nervous system infections, since such therapies are, by and large, sorely needed.

Biolistic expression of the macrophage colony stimulating factor receptor in organotypic cultures induces an inflammatory response

The receptor for macrophage colony-stimulating factor (M-CSFR; c-fms) is expressed at increased levels by microglia in Alzheimer's disease (AD) and in mouse models for AD. Increased expression of M-CSFR on cultured microglia results in a strong proinflammatory response, but the relevance of this cell culture finding to intact brain is unknown. To determine the effects of increased microglial expression of M-CSFR in a complex organotypic environment, we developed a system for biolistic transfection of microglia in hippocampal slice cultures. The promoter for the Mac-1 integrin alpha subunit CD11b is active in cells of myeloid origin. In the brain, CD11b expression is restricted to microglia. Constructs consisting of the promoter for CD11b and a c-fms cDNA or an enhanced green fluorescent protein (EGFP) cDNA were introduced into monotypic cultures of microglia, neurons, and astrocytes. Strong CD11b promoter activity was observed in microglia, whereas little activity was observed in other cell types. Biolistic transfection of organotypic hippocampal cultures with the CD11b/c-fms construct resulted in expression of the c-fms mRNA and protein that was localized to microglia. Furthermore, biolistic overexpression of M-CSFR on microglia resulted in significantly increased production by the hippocampal cultures of the proinflammatory cytokines interleukin (IL)-1alpha macrophage inflammatory protein (MIP-1alpha), and trends toward increased production of IL-6 and M-CSF. These findings demonstrate that microglial overexpression of M-CSFR in an organotypic environment induces an inflammatory response, and suggest that increased microglial expression of M-CSFR could contribute to the inflammatory response observed in AD brain.

CD14+ cell dysfunction in patients with chronic active hepatitis B

AIM: To study the functional states of CD14⁺ cells in peripheral blood mononuclear cell (PBMCs) by analyzing the expression of early active antigen, production of pro-inflammatory cytokines and phagocytosis with flow cytometry (FCM).

METHODS: PBMCs were isolated from 11 patients with chronic active hepatitis B and 10 healthy blood donors, and 4-color flow cytometry and gate setting techniques were applied. The percentage of CD14⁺ cells and level of CD14 antigen were tested by FCM, after gate setting with CD14, side scatter (SSC) and activation-linked surface antigen (CD69) expression rate, percentage of TNF-α producing cells, level of intro-cellular TNF-α, the percentage of phagocytes and the ability of phagocytosis were investigated.

RESULTS: The SSC mean value of CD14⁺ cell of HBV patients was higher than that of health control (P < 0.01); There were no difference between two testing groups on the percentage of CD14⁺ cells and level of CD14 antigen, however the level of CD14 antigen was positively associated with the percentage of CD14⁺ cells. CD14/69 double expression rate was significantly higher than that of control group (P < 0.01); No difference was observed on CD14/TNF-α double stain cells between testing groups; In comparison with control group, the level of cellular TNF-α in HBV group was higher (P < 0.05). In control group, the percentage of TNF-α producing cells were correlated with SSC value and number of phagocytes that expressed CD14/69. There was no difference in phagocytes that expressed CD14/69 in two groups. As compared with the control group, there was more phagocytostic CD14⁺ cells in HBV group (P < 0.01), but the mean phagocytosis amount was significantly lower in this group (P < 0.05), and the number of phagocyte was negatively correlated with the level of CD14 antigen; while the SSC value and number of TNF-αproducing cells were negatively correlated with level of CD14 antigen.

CONCLUSION: CD14⁺ cells in PBMCs from chronic active hepatitis B patients are over activated in a dysfunction stage.

Estimation of absolute microglial cell numbers in mouse fascia dentata using unbiased and efficient stereological cell counting principles

Stereology offers a set of unbiased principles to obtain precise estimates of total cell numbers in a defined region. In terms of microglia, which in the traumatized and diseased CNS is an extremely dynamic cell population, the strength of stereology is that the resultant estimate is unaffected by shrinkage or expansion of the tissue. The optical fractionator technique is very efficient but requires relatively thick sections (e.g., > or =20 microm after coverslipping) and the unequivocal identification of labeled cells throughout the section thickness. We have adapted our protocol for Mac-1 immunohistochemical visualization of microglial cells in thick (70 microm) vibratome sections for stereological counting within the murine hippocampus, and we have compared the staining results with other selective microglial markers: the histochemical demonstration of nucleotide diphosphatase (NDPase) activity and the tomato lectin histochemistry. The protocol gives sections of high quality with a final mean section thickness of >20 microm (h=22.3 microm +/- 0.64 microm), and with excellent rendition of Mac-1+ microglia through the entire height of the section. The NDPase staining gives an excellent visualization of microglia, although with this thickness, the intensity of the staining is too high to distinguish single cells. Lectin histochemistry does not visualize microglia throughout the section and, accordingly, is not suited for the optical fractionator. The mean total number of Mac-1+ microglial cells in the unilateral dentate gyrus of the normal young adult male C57BL/6 mouse was estimated to be 12,300 (coefficient of variation (CV)=0.13) with a mean coefficient of error (CE) of 0.06. The perspective of estimating microglial cell numbers using stereology is to establish a solid basis for studying the dynamics of the microglial cell population in the developing and in the injured, diseased and normal adult CNS.

Tracking superparamagnetic iron oxide labeled monocytes in brain by high-field magnetic resonance imaging

Inflammatory cells, most notably mononuclear phagocytes (MP; macrophages and microglia), play a critical role in brain homeostasis, repair and disease. One important event in cellular biodynamics is how MP move in and throughout the nervous system. Prior studies have focused principally on cell migration across the blood-brain barrier during neuroinflammatory processes with little work done on cell movement within the brain. During the past decade our laboratories have studied the role of MP in HIV-1-associated dementia (HAD). In HAD MP incite sustained glial inflammatory reactions causing significant neuronal damage. To extend these works we investigated cell movement in brain and its influence for disease in a novel co-registration system integrating neuropathology with high-field magnetic resonance imaging (MRI). Human monocytes labeled with superparamagnetic iron oxide particles were injected into the brain of severe combined immunodeficient (SCID) mice. MRI was recorded 1, 7, and 14 days after cell injection. MRI co-registered with histology verified that the MRI signal modification was due to the labeled cells. MRI showed human monocyte-derived macrophages along the injection site, the corpus callosum, the ventricular system and in other brain sites. These data support the idea that cell migration can be monitored in vivo and provides an opportunity to assess monocyte mobility in brain and its affects on neurodegenerative processes and notably HAD.

The bisphosphonate clodronate depletes microglial cells in excitotoxically injured organotypic hippocampal slice cultures

The bisphosphonate clodronate, clinically used in the treatment of osteoporosis, is known to deplete cells of the monocytic lineage. Using an in vitro model of excitotoxic damage in organotypic hippocampal slice cultures (OHSC), we investigated whether clodronate can also prevent microglial activation that occurs in CNS pathologies. Lesioning of OHSC was performed by application of 50 microM N-methyl-D-aspartate (NMDA) for 4 h after 6 days in vitro (div). Treatment of lesioned OHSC with clodronate (1000, 100, or 10 microg/ml) resulted in an almost complete abrogation of the microglial reaction after 3 further div: Confocal laser scanning microscopy showed that the number of Griffonia simplicifolia isolectin B(4)-labeled (IB4+) microglial cells in the dentate gyrus (DG) was reduced to 4.25% compared with OHSC treated with NMDA alone. Continuous treatment with clodronate (100 or 10 microg/ml) of lesioned OHSC for 9 days resulted in a further reduction in the number of microglial cells (reduction to 2.72%). The number of degenerating, propidium iodide-labeled (PI(+)) neurons in lesioned OHSC that received clodronate treatment between 6 and 9 div was not significantly different from OHSC treated with NMDA alone. However, the number of PI(+) neurons in lesioned OHSC that received continuous clodronate treatment for 9 div was significantly higher when compared to NMDA-lesioned OHSC. In summary, clodronate is able to reduce microglial activation induced by excitotoxic neuronal injury. Our results demonstrate that clodronate is a useful tool in the investigation of neuron-glia interactions because it induces an efficient depletion of microglial cells that are activated after excitotoxic CNS injury.

Feedback between glial tumor necrosis factor-alpha and gp120 from HIV-infected cells helps maintain infection and destroy neurons

An envelope glycoprotein, gp20, of the human immunodeficiency virus (HIV) interacts with host systems to promote HIV replication. gp120 is also involved in tissue-destructive positive feedback cycles that contribute to HIV-related but non-lymphocytic-, non-immunodeficiency-related tissue-destructive morbidity. Exposure to gp120 results in tumor necrosis factor-alpha (TNF) upregulation, particularly in cells of monocyte lineage. The resultant increased TNF in the microenvironment of the TNF-producing monocyte lineage cells results in increased occupancy of TNF receptors on nearby lymphocytes, monocytes or glia in which HIV does replicate. Such TNF binding increases HIV replication. Increased replication results in increased gp120 available to bind to monocyte lineage cells, further increasing or maintaining those cells' TNF production in the face of other TNF suppressive forces. A trophic environment (TNF) for HIV replication is thereby maintained. gp120 raises cAMP levels. Increased cAMP is inherently TNF-suppressive. This is a moderating negative feedback element embedded within the larger positive feedback cycle. HIV does not effectively replicate in neurons yet many HIV infections show significant neuron loss. gp120 stimulates glia to synthesize TNF. Increased TNF stimulates HIV to replicate in the cells present in which HIV is able to replicate. TNF also damages nearby neurons. The resultant increased gp120 would further stimulate glia, and the stimulated glia's TNF would damage local neurons. Damaged neurons make factors that activate glia to upregulate TNF synthesis. These feedback cycles centering on gp120 and TNF contribute to HIV pathophysiology, neuron loss and maintenance of infection.

A novel compound, RS-1178, specifically inhibits neuronal cell death mediated by beta-amyloid-induced macrophage activation in vitro

beta-Amyloid peptide (Abeta), a major component of senile plaques, the formation of which is characteristic of Alzheimer's disease (AD), is believed to induce inflammation in the brain leading to cell loss and cognitive decline. Accumulating evidence shows Abeta activates microglia, which play the role of the brain's immune system, and mediates inflammatory responses in the brain. Thus, a compound inhibiting Abeta-induced activation of microglia may lead to a novel therapy for AD. However, the compound should not inhibit natural immune responses during events such as bacterial infections. We investigated the effect of a synthesized compound, 7,8-dihydro-5-methyl-8-(1-phenylethyl)-6H-pyrrolo [3,2-e] [1,2,4] triazolo [1,5-a] pyrimidine (RS-1178) on macrophage activation induced by various stimulants. The activation of macrophages was determined by nitric oxide or tumor necrosis factor alpha production. RS-1178 inhibited Abeta-induced macrophage activation but did not inhibit zymosan A- nor lipopolysaccharide (LPS)-induced macrophage activation. Moreover, RS-1178 attenuated neurotoxicity due to Abeta-induced macrophage activation in neuron-macrophage co-cultures but not neurotoxicity due to zymosan A- or LPS-induced macrophage activation. In conclusion, RS-1178 showed a specific inhibitory effect on Abeta-induced macrophage activation. Although the exact mechanisms of this effect remain unknown, RS-1178 may provide a novel therapy for AD.

Vasoactive intestinal peptide and pituitary adenylate cyclase-activating polypeptide inhibit chemokine production in activated microglia

Microglia react to even minor disturbances in CNS homeostasis and function as critical regulators of CNS inflammation. Activated microglia secrete inflammatory mediators such as cytokines and chemokines, which contribute to the pathophysiological changes associated with several neuroimmunologic disorders. Microglia-derived inflammatory chemokines recruit various populations of immune cells, which initiate and maintain the inflammatory response against foreign antigens. Entry and retention of activated immune cells in the CNS is a common denominator in a variety of traumatic, ischemic, and degenerative diseases. Vasoactive intestinal peptide (VIP) and pituitary adenylate cyclase-activating polypeptide (PACAP) are two structurally related neuropeptides that function as potent anti-inflammatory factors in the periphery. Here we investigated the effects of VIP and PACAP on chemokine production by activated microglia. VIP and PACAP inhibit the expression of the microglia-derived CXC chemokines MIP-2 and KC, and of the CC chemokines MIP-1alpha, -1beta, MCP-1, and RANTES. The inhibition of chemokine gene expression correlates with an inhibitory effect of VIP/PACAP on NFkB binding. The VIP/PACAP inhibition of both chemokine production and of NFkB binding is mediated through the specific receptor VPAC1 and involves a cAMP-dependent intracellular pathway. Of biological significance is the fact that the inhibition of chemokine production by VIP/PACAP leads to a significant reduction in the chemotactic activity generated by activated microglia for peripheral leukocytes, i.e., neutrophils, macrophages, and lymphocytes. Because reduction in the number and activation of infiltrating leukocytes represents an important factor in the control of inflammation in the CNS, VIP and/or PACAP released by neurons during an inflammatory response could serve as neuronal survival factors by limiting the inflammatory process.

Targeting peptides for microglia identified via phage display

Screening with a 7-mer phage display peptide library, a panel of cell-targeting peptides for the murine microglial cell line, EOC 20, was recognized. A number of similar, but not identical, sets of sequences representing more than 75% of all the cell line-binding clones were identified. Comparative analysis indicated that motif S/(T) F T/(X) Y W is present in the vast majority of the binding sequences. The selectivity and specificity of the dominant peptide sequence identified for microglia was confirmed using both phage displaying the peptide and the synthetic peptide alone.

Infection of baboon microglia with SIV-HIV recombinant viruses: role of CD4 and chemokine receptors

Microglia constitute the primary cell type infected with HIV in the brain and play a major role in viral persistence in the CNS and in the development of AIDS dementia. Lack of a suitable animal model and limitations in the availability of human tissues hinder most HIV/AIDS studies investigating the neuropathogenesis of AIDS dementia. The aims of this study were to determine whether baboon microglia can be productively infected with SIV-HIV (SHIV) recombinant viruses in vitro and whether they express HIV-1 receptors and coreceptors. Our results show the presence of mRNA for CD4, CCR5, and CXCR4 chemokine receptors on baboon microglial cells. Microglia lacked mRNA for the CCR3 chemokine receptor. We also show productive infection of baboon microglial cells by two SHIV isolates, SHIV-KU and SHIV-89.6P, and blockade of the infection with soluble CD4 protein, CCR5, and CXCR4 monoclonal antibodies. This study demonstrating the feasibility of infecting baboon microglia with SHIV isolates is an important first step in using the baboon as an alternative nonhuman primate model to study HIV neuropathogenesis.