Mechanisms of HIV-1 mediated neurodegeneration promoted by macrophages and astroglial factors

Neurological disease is a. prominent feature of human immunodeficiency virus type I (HIV-1) infection, usually occurring during the last stages of acquired immunodeficiency syndrome (AIDS). The neurologic cognitive impairment, termed HIV-1-associated cognitive/motor complex (AIDS dementia complex). Astrocytes and microglia are key participants in mediating the neurologic dysfunction associated with HIV infection of the central, nervous systems. The neuropathogenesis of IIIV-1 infection is related to secretory neurotoxins from activated HIV-J-infected macrophages The toxins produced by the macrophages include glut a mate-like neurotoxic molecules, free radicals, cysteine, platelet-activating factor, cytokines, and eicosanoids such as arachidonic acid, and as yet unidentified factors emanating from stimulated macrophages and/or reactive astrocytes.


Neurological disease is a. prominent feature of human immunodeficiency virus type I (HIV-1) infection, usually occurring during the last stages of acquired immunodeficiency syndrome (AIDS). The neurologic cognitive impairment, termed HIV-1-associated cognitive/motor complex (AIDS dementia complex). Astrocytes and microglia are key participants in mediating the neurologic dysfunction associated with HIV infection of the central, nervous systems. The neuropathogenesis of IIIV-1 infection is related to secretory neurotoxins from activated HIV-J-infected macrophages
The rate of progression to disease varies considerably among individuals infected with HIV-1.Most in dividuals infected with HIV-1 remains disease free for many years and during this time, maintain relatively stable numbers of CD^T T cells, strong cytotoxic T ceil responses, and low numbers of HIV-1-infected cells in the blood, all indicators that the virus is under immune control.At some point the immune system falters, and most infected individuals progress to develop the symptoms of AIDS |1 ].HIV-1 predominantly infects cells that express the CD4 receptor, which serves as the major receptor for HIV-1, utilizing the CD4 molecule for entry into T cells and macrophages [2, 3 |.CD4 by itself was not sufficient for HIV-1 infectivity; some «cofactor», only found in human cells, was also required [4 ].Berg at al. [5 I report the discovery of a membrane protein they call «fusin», which has the expected charac teristics of the elusive HIV-1 cofactor.This protein is a putative G protein-coupled receptor with seven transmembrane segments.The researchers found, that together with CD4, it permits cells to fuse with HIV-1 surface -a key step in the infection process.Recent evidence suggests that chemokines and their receptors may play an important regulatory role in HIV- cytokines that aevtivate and direct the migration of leukocytes.
Monocytes/macrophages functione as a cellular reservoir for HIV-1 since macrophages can be infected with the virus but are resistant to its cytopathic effects [9,10].The ability of HIV-1 to establish a latent infection in macrophages may contribute to the spread and persistence of the virus [11 ].HIV-1-infected monocytes express higher levels of cell surface ad hesion molecules, such as the j3 2 integrins, and secrete larger amounts of proteolytic enzymes, such as metalloproteinase-9 [12].These changes in monocyte function could participate in the pathogenesis of AIDS by promoting tissue invasion and by enhancing local tissue proteolysis [13].Lafrenie et al. [14] shown that many of the effects of HIV-1 infection of monocytes can be mimicked by treatment of the monocytes with a regulatory gene product of the HIV genome HIV-1-Та/.Monocytes treated with soluble HIV-1 -Tat protein express elevated levels of j3 2 in tegrins, which mediates monocyte aggregation and monocyte adhesion to endothelial monolayers, and increases monocyte production of matrix metalloproteinase-9.The changes in monocyte function are similar to those seen either in response to cytokine treatment or during an inflammatory response when monocytes are induced to extravasate.Lafrenie et al. [14] presented evidence that HIV-1-Тя/ protein can enhance the chemotactic and invasive behaviors of monocytes and could play an active role in the recruitment of monocytes into extravascular tissues in addition to activating circulating monocytes.A num ber of cellular factors can modulate replication of latent virus.In particular, proinflammatory cytokines have been shown to up-regulate expression of HIV-1 [15,16].Macrophages are the major source of virus in tissues such as brain, and spinal cord.Neurological disease is a prominent feature of HIV-1 infection, usually occurring during the last stages of AIDS [17].Neurologic problems occur even in the absence of opportunistic infection or secondary cancer [18][19][20].Approximately one third of adults and half of children with the AIDS eventually have neurologic compli cations.The most common disorder in HIV-1-infected individuals is encephalopathy, a fatal illness causing severe dementia.Events leading to encephalopathy are unclear but infiltration by monocytes and mac rophages is a consistent finding in the central nervous system (CNS) of AIDS patients [11].
Macrophages as mediators of HIV-1-associated neurotoxicity.HIV-1 penetration of the brain is a pivotal event in the neuropathogenesis of AIDSassociated dementia.The recruitment of mononuclear phagocytes into brain during disease likely governs the tempo and progression of CNS disease.Nottet et al. [34] suggest that HIV-l-infected monocytes have an advantage in binding to microvascular endothelial cells and that this binding facilitates entry of virus into brain tissue.HIV-1-infected monocytes would induce the expression of adhesion molecules on brain microvascular endothelial cells that allow binding and then penetration of virus-infected monocytes into brain.Since immune-activated HIV-l-infected mac rophages overexprcss proinflammatory cytokines, such as TNF-tt, activated cells might have a selective advantage in transendothelial migration [34].There is good evidence that there are two stages in the infection of brain macrophages by HIV-1.Initially, the viral coat glycoprotein gpJ20 binds to a receptor CD4 on the surface of the macrophage, but other binding sites may exist.Internalization of the virus may stimulate the macrophage to release low levels of neurotoxins.HIV-1 proteins such as gpl20 and pos sibly Tat and Nef can stimulate uninfected cells to release similar neurotoxins [35].In the second stage of HIV-1 infection, the viral genome is integrated into the genome of the macrophage, and active virus replication ensues.During this stage macrophages release large amounts of neurotoxic substances.The toxins produced by the macrophages include glutamate-like neurotoxic molecules, free radicals, cys teine, platelet-activating factor (PAF), cytokines, and eicosanoids, and as yet unidentified factors emanating from stimulated macrophages and/or reactive astro cytes [35-39 |.Interactions among several different types of cell, including mononuclear phagocytes, ast rocytes, and neurons, probably regulate the secretion of neurotoxins by HIV-l-infected macrophages [35].
Takahashi el al. [40 ] demonstrated that latent or low-level infection of astrocytes occurs in AIDS, a finding that may be of importance in understanding neuropathogenesis.The infection of astrocytes is highly unusual and may occur in children [35,41,42].
The role that microglia play in HIV-1 infection is important in the understanding of the pathogenesis of HIV-1 infection and of the resulting brain damage.Most of the current evidence strongly suggest that microglia arise from mesodermal tissues, ultimately develop from bone marrow cells, in particular the monocyte [43], and populate the CNS after it has been vascularized.Microglia are generally considered to be bone marrow-derived resident macrophages in the brain and thus form the interface between CNS and immune system.Microglia constitute -10 % of the total glial cell population.They can be considered as a specialized subtype of tissue macrophage found in the CNS 144, 45], The major known function of microglia is as a scavenger cell.Also, microglia may be involved with inflammation and repair in the CNS because of their phagocytic ability, release of neutral proteinases, and production of oxidative radicals.Microglia have been demonstrated to express major histocompatibility complex antigens (MHC class I and II) upon activation, act as antigen-presenting cells, secrete a number of immunoregulatory cytokines, and respond to cytokine stimulation, suggesting an invol vement with inflammatory and immune responses within the CNS [45].Microglia may play an im portant role in a variety of neurological disorders such as AIDS, Alzheimer's disease, and amyotrophic late ral scl erosis [46 ].Although microglia resemble tissue macrophage in immunological phenotype and func tion, there are some differences between microglia and other monocyte/ macrophage lineage that still remain to be clarified [47 |.Microglial cells, the target involvement in synaptic plasticity, long-term poten tiation, learning and memory, and neurodegeneration |51, 60,61 ].Activation of NMDA receptors leads to increased intracellular Ca 2 ~ followed by activation of protein kinases, phospholipases, proteases, nitric oxi de synthase (NOS), impaired mitochondrial function, and the generation of free radicals [59,[62][63][64].Neurotoxicity in primary neuronal cultures induced by stimulation of NMDA receptors is mediated in part by nitric oxide (NO) [59,65].NO is a powerful endogenous mediator for numerous physiological res ponses, as well as in manifestations of brain injury [66,67].Bukrinsky et al. [68] demonstrated that HIV-1 infection of human monocytes results in the appearance of inducible isoform of NOS.Human monocytes have been used as model of brain macro phage function.The appearance of the inducible isoform of NOS is accompanied by significant pro duction of NO.This NOS induction is subject to both positive and negative regulation by the immune sys tem cytokine network.NO-mediated neurotoxicity is engendered by reaction with 0 2 ~, apparently leading to formation of peroxynitrite (ONOO), a highly destructive radical.The formation of a ONOO leads to lipid peroxidation and indiscriminate oxidation of sulphhydryls and kills neurons in a dose-dependent fashion [65 ].In other oxidation states NO can interact with thiol groups of NMDA receptors and ameliorate deleterious effect of glutamate [65,69].Recently, human macrophages and astrocytes have been shown to produce NO via inducible NO syn thase (iNOS) in response to cytokines and gpl2() [59,70,71 ].Although gpl20 can bind to CD4 on human macrophages it has been argued that this is not true for rodents.Thus, the effects of gpJ20 in the rodent nervous system might imply the existence of another, as yet unknown, receptor for the coat protein.Other HIV proteins, such as Tat and Nef, were shown to be toxic in the rodent CNS, raising questions about the specificity of the findings with gpl2() [54 ].
Evidence has been accumulating that brain da mage in HIV infection is not the result of a direct effect of the virus.The neuronal damage is, rather, due to toxic factors that alter the neuronal function.The discrepancy between widespread neuronal da mage and the absence of productive viral infection in neurons led to the hypothesis that HIV-1 induces neurotoxicity through an indirect mechanism [35 ].Recently, a new human neuronal culture system, called NT neurons, has become available [72].A new in vitro system comprising a pure population of neurons, human NT cells, was used to characterize the direct neurotoxic effect of HIV-1 envelope protein gpl2().Treatment of mature NT neurons with various doses of gpl2() for 24 h caused a decrease of up to 27 % in the number of viable cells.These data indicate the possibility that gp!2() exerts a direct 330 ceils for HIV-1 in the brain, are responsible for the replication and spread of the virus.They fuse toge ther to form the multinuclear giant cells, which are considered to be the hallmark of HIV-1 infection.The combination of immunohistochemistry and morpho metry to investigate the activation pattern of microglia gave conclusive data.The number of activated micro glia was significantly increased in HIV-1 infected brains.The activation of microglia was not correlated with the presence of HIV-1 antigen in brain tissue И8 |.
One factor that may contribute, at least in part, to AIDS dementia complex is neuronal injury caused by the viral envelope protein, gpJ20, or a fragment thereof, which can be shed from HIV-1 harboured by macrophages or microglia in the CNS [49][50][51][52][53].It was found that picomolar concentrations of gp!20 were toxic in vitro to rodent neurons [49 ].The HIV-1 coat protein gpl20 produces lesions in cultured neu rones and glial cells.The HIV-1 envelope protein gpl20 produces neuronal cell damage in primary cultures of variety of cell types including hippocampal neurons and retinal ganglion cell 149 ].The im portance of macrophages as mediators of gpJ20associated neurotoxicity is shown by the failure of gpl20 to cause neuronal damage when macrophages were eliminated from retinal ganglion cell cultures [54 ].
The properties of primary cell cultures are, however, often markedly different from those of cells living in their normal environment.The use of an in vitro organized structure will enable the molecular and cellular mechanism of action of gpl20 to be examined in conditions which are particularly suitable and relevant to the in vivo situation [55 ].Gpl20 induces widespread chromatin condensation and le sions in pyramidal granular neurone and in interneurones of rat hippocampal organotypic slice cultu res.This damage is clearly of an apoptotic (program med cell death) type [55].In an study involving transgenic mice Toggas et al. |56 ) demonstrate that damage in the CNS can be caused by the HIV-1 coat protein gpl2().This mouse model has its shortcoming.Transgene for gpl20 is expressed in astrocytes rather than in the macrophage/microglial lineage, the cell type predominantly infected in the CNS [57 |.
Neuronal cell death elicited by gpl20 is abso lutely dependent upon the presence of glutamate acting through N-methyl-D-asparlate (NMDA) re ceptors [51,58,59] and to be mediated by excitotoxic mechanisms.These works were extended by evidence that gpI2() could indirectly trigger a dra matic and potentially lethal rise in neuronal (Ca 2+ |, by releasing toxic factors from activated macro phages/microglia and possibly astrocytes [54,50].The NMDA receptors has received substantial atten tion because of its high Ca 2+ permeability and its neurotoxic effect by acting through NMDA receptors and Ca 2+ channels [73 ]. Macrophages and microglial cells produce pros taglandin E 2 , cytokines such as tumor necrosis factor (TNF-a), transforming growth factor-/?(TGF-0), interleukin-1 (IL-1), interleukin-6 (IL-6), colony sti mulating factors (CSFs).Many cytokines cause death of oligodendrocytes and/or destruction of myelin in vitro [74,75 ].These potent, cell-derived effector molecules are cytotoxic when added to primary neu ronal cultures and are also detected in the ce rebrospinal fluid of HIV-infected subjects with neu rological deficits [76,77 ].Increased levels of TNFa, IL-1-/? are present in the brains of patients with various pathological conditions such as AIDS [78], multiple sclerosis [79], Alzheimer's disease, and Down's syndrome [80].Numerous studies have de monstrated that inflammatory cytokines are present in CNS during neurological diseases.These cytokines include IL-1, IL-6, INF-y, TNF-cr and TNF-/?.In terferon-}/ (INF-y), which are neurotoxic, is involved in the pathogenesis of neuronal injury in patients with HIV-1 infection [76,77], INF-y is the product of activated T cells and has a wide range of immunoregulatory functions.INF-y would be present in the CNS only during disease states where the bloodbrain barrier has been breached [77 ].It was demon strated that interactions between HIV-infected mono cytes and astroglial cells produce high levels of proinflammatory cytokines (TNF-a, IL-1/?), PAF [81], and eicosanoids [82].Several cytokines can regulate their own synthesis, as well as the production of other cytokines (for example, IL-1, TNF-a).The most abundant source of cytokines appears to be activated microglia, although neurones, astroglia, pe rivascular and endothelial cells can also express cytokines.These molecules are involved in neuronal degeneration and repair in the CNS, and have been proposed as mediators of various neuropathologies [83][84][85].Therefore, many of the clinical and his tological effects of HIV-1 infection in the CNS may be an indirect effect of cytokines and other soluble mediators secreted by resident macrophages and microglial cells.There is evidence of increased syn thesis of neopterin, a marker of both macrophage activation and tetrahydrobiopterin biosvnthesis [86 ].TNF-a, IL-1, IL-6.. INF-y, 2 microglobulin, and neopterin are potential candidates serving as neu rotoxic factors [48 ].
Regulatory role for astrocytes in IV-1-mediated encephalopathy.HIV-1-infected brain macrophages participate in neurologic dysfunction through their continual secretion of neurotoxins.The control of macrophage secretory activities was found linked to the astrocytes, a cells that suppressed neurotoxins production and regulate the extent of disease [87 ].
Benveniste et al. [88 ] investigated the ability of the major envelope glycoprotein of HIV, gp!20, to re gulate intercellular adhesion molecule-1 (ICAM-1) expression in glial cells.Their results indicate that gpl20 enhances ICAM-1 gene expression in primary rat astrocytes, primary human astrocytes, a human astroglioma cell line CRT, and primary rat microglia.ICAM-1 is important in mediating immune res ponsiveness in the CNS, facilitating entry of HIVinfected cells into the CNS, and promoting syncytia formation.
Astrocytes are the most numerous of the glial cells, and in the mammalian brain they outnumber neurons 10 to 1. Astrocytes have been implicated in a wide range of supportive functions for their partner neurons in the CNS, such as neuronal guidance during development and nutritional and metabolic support throughout life [89].Astrocytes have also been suggested to provide neurotrophic factors essen tial for neuronal maintenance and survival [90,91 ].The ionic composition of the extracellular space around the neurons is critical for their proper func tioning, and the astrocyte is important in maintaining this microenvironment.Numerous studies have de monstrated the active involvement the astrocytes in neurotransmitter metabolism.In vitro studies suggest that amino acid transmitters may be removed from the extracellular space by astrocytic uptake mecha nisms.In the presence of high glutamate levels, removal of astrocytes from mixed cultures quickly leads to neuronal death [92,93].It has been shown that glutamate, a good substrate for the uptake system, is 1/40-1/100-fold weaker as a neurotoxin in astrocyte-rich cultures than in astrocyte-poor cul tures [94 ].The astrocyte can contribute to the structural integrity of the blood-brain barrier.In the adult nervous system, astrocytes retain the ability to divide and multiply.When the CNS is injured, astrocytes respond by becoming reactive.This re action, known as astrocytosis is the result of astrocyte proliferation, hypertrophy, and enhanced expression of glial fibrillary acidic protein (GFAP), whose exp ression is restricted to astrocytes.One of the major functions proposed for reactive astrocytes is the initiation of immune responses within the CNS [37, 44, ].Prominent reactive astrocytosis is seen in AIDSdementia complex [95].