Microglia and Alzheimer's disease

CpG-containing oligodeoxynucleotide promotes microglial cell uptake of amyloid beta 1-42 peptide by up-regulating the expression of the G-protein- coupled receptor mFPR2

Human G protein-coupled formyl peptide receptor like 1 (FPRL1) and its mouse homologue murine formyl peptide receptor 2 (mFPR2) mediate the chemotactic activity of amyloid beta 1-42 (Abeta42), a key pathogenic peptide in Alzheimer's disease (AD). Since mFPR2 is up-regulated in mouse microglia by lipopolysaccharide (LPS), a Toll-like receptor 4 ligand, we investigated the capacity of CpG-containing oligodeoxynucleotide (ODN), a Toll-like receptor (TLR) 9 ligand, to regulate the expression of mFPR2 in mouse microglia. CpG ODN markedly enhanced the expression and function of mFPR2 in microglial cells, which exhibited increased chemotactic responses to mFPR2 agonists, including Abeta42. The effect of CpG ODN is dependent on activation of p38 MAPK. Further studies showed that CpG ODN-treated microglia increased their capacity to endocytose Abeta42 through mFPR2, as this process was abrogated by pertussis toxin, a Gi protein inhibitor, and W peptide, another potent mFPR2 agonist. Our results suggest that TLR9 may play an important role in promoting microglial recognition of Abeta42, thus affecting the pathogenic process of AD.

Expression of Ki67, PCNA and the chromosome replication licensing protein Mcm2 in glial cells of the ageing human hippocampus increases with the burden of Alzheimer-type pathology

Cell-cycle mechanisms may be aberrantly reactivated in the ageing brain and associated with the development of pathology, including Alzheimer's disease. Activation of cell-cycle mechanisms in glia has, however, been little studied. Our aim was to determine whether expression of a marker for chromosomal replication licensing, Mcm2, occurs in glia of the ageing hippocampus, and to compare its expression to that of Ki67 and PCNA. Blocks of hippocampus were obtained from 19 elderly brains derived from the MRC-CFAS neuropathology cohort, which included a spectrum of Alzheimer-type pathology, semi-quantified using the Braak scoring system for neurofibrillary tangles. Mcm2, PCNA and Ki67 were detected immunohistochemically. Expression of Mcm2, Ki67 and PCNA was observed in glial cells and neurons, with a trend to increased expression in association with higher burdens of Alzheimer-type pathology. Mcm2 expression in glial cells showed a significant linear trend across Braak stages (P = 0.043). This study demonstrates that grey and white matter glial cells show expression of cell-cycle markers in the ageing brain and that re-licensing for chromosomal replication is a component of the mechanisms activated. A quantitative relationship to the burden of Alzheimer-type pathology suggests that cell-cycle re-entry in glial cells may be important in the pathogenesis of age-related neurodegeneration.

Retinoic acid inhibits expression of TNF-alpha and iNOS in activated rat microglia

The release of proinflammatory mediators such as tumor necrosis factor-alpha (TNF-alpha) and nitric oxide by microglia has been implicated in neurotoxicity in chronic neurodegenerative diseases such as Alzheimer's disease. As all-trans-retinoic acid (RA) has been reported to exert anti-inflammatory actions in various cell types, we have examined its effects on the expression of TNF-alpha and inducible nitric oxide synthase (iNOS) in microglia activated by beta-amyloid peptide (Abeta) and lipopolysaccharide (LPS). Exposure of primary cultures of rat microglial cells to Abeta or LPS stimulated the mRNA expression level of TNF-alpha (6-116-fold) and iNOS (8-500-fold) significantly. RA acted in a dose-dependent manner (0.1-10 microM) by attenuating both TNF-alpha (29-97%) and iNOS (61-96%) mRNA expression in microglia exposed to Abeta or LPS. RA-induced inhibition of TNF-alpha and iNOS mRNA expression in activated microglia was accompanied by the concomitant reduction in release of iNOS and TNF-alpha proteins as revealed by nitrite assay and ELISA, respectively. The anti-inflammatory effects of RA were correlated with the enhanced expression of retinoic acid receptor-beta, and transforming growth factor-beta1 as well as the inhibition of NF-kappaB translocation. These results suggest that RA may inhibit the neurotoxic effect of activated microglia by suppressing the production of inflammatory cytokines and cytotoxic molecules.

Alzheimer's disease beta-secretase BACE1 is not a neuron-specific enzyme

The brains of Alzheimer's disease (AD) patients are morphologically characterized by neurofibrillar abnormalities and by parenchymal and cerebrovascular deposits of beta-amyloid peptides. The generation of beta-amyloid peptides by proteolytical processing of the amyloid precursor protein (APP) requires the enzymatic activity of the beta-site APP cleaving enzyme 1 (BACE1). The expression of this enzyme has been localized to the brain, in particular to neurons, indicating that neurons are the major source of beta-amyloid peptides in brain. Astrocytes, on the contrary, are known to be important for beta-amyloid clearance and degradation, for providing trophic support to neurons, and for forming a protective barrier between beta-amyloid deposits and neurons. However, under certain conditions related to chronic stress, the role of astrocytes may not be beneficial. Here we present evidence demonstrating that astrocytes are an alternative source of BACE1 and therefore may contribute to beta-amyloid plaque formation. While resting astroyctes in brain do not express BACE1 at detectable levels, cultured astrocytes display BACE1 promoter activity and express BACE1 mRNA and enzymatically active BACE1 protein. Additionally, in animal models of chronic gliosis and in brains of AD patients, there is BACE1 expression in reactive astrocytes. This would suggest that the mechanism for astrocyte activation plays a role in the development of AD and that therapeutic strategies that target astrocyte activation in brain may be beneficial for the treatment of AD. Also, there are differences in responses to chronic versus acute stress, suggesting that one consequence of chronic stress is an incremental shift to different phenotypic cellular states.

The role of inflammation in Alzheimer's disease

Considerable evidence gained over the past decade has supported the conclusion that neuroinflammation is associated with Alzheimer's disease (AD) pathology. Inflammatory components related to AD neuroinflammation include brain cells such as microglia and astrocytes, the classic and alternate pathways of the complement system, the pentraxin acute-phase proteins, neuronal-type nicotinic acetylcholine receptors (AChRs), peroxisomal proliferators-activated receptors (PPARs), as well as cytokines and chemokines. Both the microglia and astrocytes have been shown to generate beta-amyloid protein (Abeta), one of the main pathologic features of AD. Abeta itself has been shown to act as a pro-inflammatory agent causing the activation of many of the inflammatory components. Further substantiation for the role of neuroinflammation in AD has come from studies that demonstrate patients who took non-steroidal anti-inflammatory drugs had a lower risk of AD than those who did not. These same results have led to increased interest in pursuing anti-inflammatory therapy for AD but with poor results. On the other hand, increasing amount of data suggest that AChRs and PPARs are involved in AD-induced neuroinflammation and in this regard, future therapy may focus on their specific targeting in the AD brain.

Interactions between Alzheimer's disease and cerebral ischemia--focus on inflammation

Progressive memory impairment, beta-amyloid (Abeta) plaques associated with local inflammation, neurofibrillary tangles, and loss of neurons in selective brain areas are hallmarks of Alzheimer's disease (AD). Although beta-amyloid precursor protein (APP) and Abeta have a central role in the etiology of AD, it is not clear which forms of APP or Abeta are responsible for the neuronal vulnerability in AD brain. Brain ischemia, another cause of dementia in the elderly, has recently been recognized to contribute to the pathogenesis of AD and individuals with severe cognitive decline and possibly underlying AD are at increased risk for ischemic events in the brain. Moreover, the epsilon4 allele of apolipoprotein E (ApoE) is a risk factor for both AD and poor outcome following brain ischemia and hemorrhage. Several factors and molecular mechanisms that lower the threshold of neuronal death in models of AD have recently been described. Among these neuroinflammation seems to play an important role. The development and maturation of both AD neuropathology and ischemic lesions in the central nervous system are characterized by activation of glial cells and upregulation of inflammatory mediators. Indeed, anti-inflammatory approaches have proven to be beneficial in the prevention and treatment of AD-like neuropathology and ischemic injuries in vivo. This review summarizes some of the findings suggesting that neuronal overexpression of human APP renders the brain more vulnerable to ischemic injury and describes the factors that are involved in increased neuronal susceptibility to ischemic stroke.

Involvement of the intracellular ion channel CLIC1 in microglia-mediated beta-amyloid-induced neurotoxicity

It is widely believed that the inflammatory events mediated by microglial activation contribute to several neurodegenerative processes. Alzheimer's disease, for example, is characterized by an accumulation of beta-amyloid protein (Abeta) in neuritic plaques that are infiltrated by reactive microglia and astrocytes. Although Abeta and its fragment 25-35 exert a direct toxic effect on neurons, they also activate microglia. Microglial activation is accompanied by morphological changes, cell proliferation, and release of various cytokines and growth factors. A number of scientific reports suggest that the increased proliferation of microglial cells is dependent on ionic membrane currents and in particular on chloride conductances. An unusual chloride ion channel known to be associated with macrophage activation is the chloride intracellular channel-1 (CLIC1). Here we show that Abeta stimulation of neonatal rat microglia specifically leads to the increase in CLIC1 protein and to the functional expression of CLIC1 chloride conductance, both barely detectable on the plasma membrane of quiescent cells. CLIC1 protein expression in microglia increases after 24 hr of incubation with Abeta, simultaneously with the production of reactive nitrogen intermediates and of tumor necrosis factor-alpha (TNF-alpha). We demonstrate that reducing CLIC1 chloride conductance by a specific blocker [IAA-94 (R(+)-[(6,7-dichloro-2-cyclopentyl-2,3-dihydro-2-methyl-1-oxo-1H-inden-5yl)-oxy] acetic acid)] prevents neuronal apoptosis in neurons cocultured with Abeta-treated microglia. Furthermore, we show that small interfering RNAs used to knock down CLIC1 expression prevent TNF-alpha release induced by Abeta stimulation. These results provide a direct link between Abeta-induced microglial activation and CLIC1 functional expression.

TGF-beta1 disrupts endotoxin signaling in microglial cells through Smad3 and MAPK pathways

Human formyl peptide receptor-like 1 and its mouse homologue formyl peptide receptor 2 (FPR2) are G protein-coupled receptors used by a number of exogenous and host-derived chemotactic peptides, including the 42 aa form of beta amyloid peptide, a causative factor of Alzheimer's disease. Functional FPR2 was induced by bacterial LPS in murine microglial cells, the resident phagocytic cells that play a pivotal role in inflammatory and immunological diseases in the CNS. To identify agents that may suppress microglial cell activation under proinflammatory conditions, we investigated the effect of TGF-beta1 on the expression of functional FPR2 by microglial cells activated by LPS. TGF-beta1 dose-dependently inhibited the mRNA expression and function of FPR2 in LPS-activated microglial cells. The inhibitory effect of TGF-beta1 was mediated by Smad3, a key signaling molecule coupled to the TGF-beta receptor, and the transcription coactivator, p300. Also, TGF-beta1 activates MAPKs in microglial cells that became refractory to further stimulation by LPS. These effects of TGF-beta1 culminate in the inhibition of LPS-induced activation of NF-kappaB and the up-regulation of FPR2 in microglial cells. Thus, TGF-beta1 may exert a protective role in CNS diseases characterized by microglial cell activation by proinflammatory stimulants.

New players in old amyloid precursor protein‐processing pathways

The amyloid precursor protein (APP) gives rise to beta-amyloid peptides, which are the main constituents of senile plaques in brains of Alzheimer's disease (AD) patients. The generation of beta-amyloid peptides requires the enzymatic activity of the beta-site APP-cleaving enzyme 1 (BACE1). BACE1 is primarily expressed by neurons and increased BACE1 protein concentrations and enzymatic activities have been reported in the brains of AD patients. However, there is accumulating evidence that, in addition to neurons, reactive astrocytes are capable of expressing BACE1 and, therefore, may contribute to beta-amyloid plaque formation. This suggests that conditions accompanied by chronic astrocyte activation may contribute to developing AD. Non-amyloidogenic processing of the APP can be stimulated by phorbol esters (PEs) and by intracellular diacylglycerol (DAG) generation. This led to the hypothesis that classical and novel protein kinase Cs (PKCs), which are activated by DAG/PEs, regulate APP processing. However, in addition to PKCs, there are other DAG/PE receptors present in neurons which may participate in the modulation of APP processing. Munc13-1, a presynaptic protein with an essential role in synaptic vesicle priming, represents such an alternative target of the DAG second messenger pathway. Using Munc13-1 knock-out mice and human neuroblastoma cells transfected with wild-type and mutant Munc13-1 constructs it was demonstrated that Munc13-1 acts independently of and in parallel with PKC to modulate APP metabolism. Therefore, agonists specific for the Munc13-1 C1-domain or small molecules mimicking the function of the endogenous Munc13-1 activator RIM1 may prove useful to shift APP processing towards the non-amyloidogenic pathway.

Regulation of the lipopolysaccharide signal transduction pathway by 17beta-estradiol in macrophage cells

We have previously shown that 17beta-estradiol (E2) prevents the activation of brain macrophages, i.e. microglia cells, both in vitro and in vivo. Hormone exerts this inhibitory effect by inhibiting pro-inflammatory gene expression. In this study we further investigated on the molecular mechanism of E2 action in the RAW 264.7 macrophage cell line. We show here that these cells express the alpha-isoform of the estrogen receptor (ERalpha) and not ERbeta. Similarly to its activity in brain macrophages, E2 is able to inhibit the activation program induced by lipopolysaccharide (LPS) in RAW 264.7 cells, as shown by the inhibitory effect of hormone on the morphological conversion and matrix metalloproteinase-9 (MMP-9) expression induced by the endotoxin. In addition, we demonstrate that hormone treatment is not associated with a reduction in the steady-state expression of Toll-like receptor-4 (TLR-4) and CD14, two components of the LPS receptor complex. Our results further confirm the anti-inflammatory role of ERalpha in macrophages and propose that the mechanism of hormone action on macrophage reactivity involves signaling molecules which are down-stream effectors of the LPS membrane receptors.

A beta 1-42 induces production of quinolinic acid by human macrophages and microglia

We hypothesized that the tryptophan catabolites produced through the kynurenine pathway (KP), and more particularly the excitotoxin quinolinic acid (QUIN), may play an important role in the pathogenesis of Alzheimer's disease (AD). In this study, we demonstrated that aggregated amyloid peptide A beta 1-42 induced indoleamine 2,3-dioxygenase (IDO) expression and resulted in a significant increase in production of QUIN by human primary macrophages and microglia. In contrast, A beta 1-40 and prion peptide (PrP) 106-126 did not induce any significant increase in QUIN production. These data imply that local QUIN production may be one of the factors involved in the pathogenesis of neuronal damage in AD.

CD36 overexpression in human brain correlates with beta-amyloid deposition but not with Alzheimer's disease

Scavenger receptors recently have been related to Alzheimer's disease, although it is still unclear whether they contribute to the pathogenesis of the disease or reflect an inflammatory response to the deposition of amyloid beta-protein (Abeta). In this study we demonstrate that CD36, a class B scavenger receptor, is highly expressed in the cerebral cortex of Alzheimer's disease patients and cognitively normal aged subjects with diffuse amyloid plaques compared with age-matched amyloid-free control brains. Moreover, in vitro experiments indicated that Abeta is able to induce CD36 expression in neuronal cells after 24 h treatment. The interaction between CD36 and Abeta has been reported to trigger oxidant production by macrophages and microglia. In line with this observation, we found an increased presence of nitrated proteins in brains showing Abeta loads and CD36 overexpression, independent of the occurrence of Alzheimer's disease pathologic features.

Estrogens in the Nervous System: Mechanisms and Nonreproductive Functions

The past decade has witnessed a growing interest in estrogens and their activity in the central nervous system, which was originally believed to be restricted to the control of reproduction. It is now well accepted that estrogens modulate the activity of all types of neural cells through a multiplicity of mechanisms. Estrogens, by binding to two cognate receptors ERalpha and ERbeta, may interact with selected promoters to initiate the synthesis of target proteins. Alternatively, the hormone receptor complex may interfere with intracellular signaling at both cytoplasmic and nuclear levels. The generation of cellular and animal models, combined with clinical and epidemiological studies, has allowed us to appreciate the neurotrophic and neuroprotective effects of estrogens. These findings are of major interest because estradiol might become an important therapeutic agent to maintain neural functions during aging and in selected neural diseases.

IL-4 down-regulates lipopolysaccharide-induced formyl peptide receptor 2 in murine microglial cells by inhibiting the activation of mitogen-activated protein kinases

Microglial cells actively participate in proinflammatory responses in the CNS. Upon stimulation with the bacterial LPS, microglial cells express a functional formyl peptide receptor 2 which mediates the chemotactic and activating effects of a variety of polypeptide agonists including amyloid beta (Abeta(1-42)), a critical pathogenic agent in Alzheimer's disease. In the present study, we found that LPS-induced expression and function of formyl peptide receptor 2 in microglial cells was markedly inhibited by IL-4, a Th2-type cytokine. Our effort to elucidate the mechanistic basis revealed that IL-4 attenuated LPS-stimulated activation of NF-kappaB, extracellular signal-regulated kinase, and p38 mitogen-activated protein kinase, and the effect of IL-4 was associated with a phosphoinositide 3-kinase pathway-dependent increase in serine/threonine phosphatase activity. These results suggest that IL-4 may play an important role in the maintenance of homeostasis of CNS and in the regulation of the disease process characterized by microglial activation in response to proinflammatory stimulants.

Anti-inflammatory drug therapy alters beta-amyloid processing and deposition in an animal model of Alzheimer's disease

Alzheimer's disease (AD) is characterized by a microglial-mediated inflammatory response elicited by extensive amyloid deposition in the brain. Nonsteroidal anti-inflammatory drug (NSAID) treatment reduces AD risk, slows disease progression, and reduces microglial activation; however, the basis of these effects is unknown. We report that treatment of 11-month-old Tg2576 mice overexpressing human amyloid precursor protein (APP) with the NSAID ibuprofen for 16 weeks resulted in the dramatic and selective reduction of SDS-soluble beta-amyloid (Abeta)42, whereas it had smaller effects on SDS-soluble Abeta40 levels. Ibuprofen treatment resulted in 60% reduction of amyloid plaque load in the cortex of these animals. In vitro studies using APP-expressing 293 cells showed that ibuprofen directly affected APP processing, specifically reducing the production of Abeta42. Ibuprofen treatment resulted in a significant reduction in microglial activation in the Tg2576 mice, as measured by CD45 and CD11b expression. NSAIDs activate the nuclear hormone receptor peroxisome proliferator-activated receptor gamma (PPARgamma); however, a potent agonist of this receptor, pioglitazone, only modestly reduced SDS-soluble Abeta levels and did not affect amyloid plaque burden or microglia activation, indicating that PPARgamma activation is not involved in the Abeta lowering effect of NSAIDs. These data show that chronic NSAID treatment can reduce brain Abeta levels, amyloid plaque burden, and microglial activation in an animal model of Alzheimer's disease.

Estrogen receptor-alpha mediates the brain antiinflammatory activity of estradiol

Beyond the key role in reproductive and cognitive functions, estrogens have been shown to protect against neurodegeneration associated with acute and chronic injuries of the adult brain. Current hypotheses reconcile this activity with a direct effect of 17beta-estradiol (E2) on neurons. Here we demonstrate that brain macrophages are also involved in E2 action on the brain. Systemic administration of hormone prevents, in a time- and dose-dependent manner, the activation of microglia and the recruitment of peripheral monocytes induced by intraventricular injection of lipopolysaccharide. This effect occurs by limiting the expression of neuroinflammatory mediators, such as the matrix metalloproteinase 9 and lysosomal enzymes and complement C3 receptor, as well as by preventing morphological changes occurring in microglia during the inflammatory response. By injecting lipopolysaccharide in estrogen receptor (ER)-null mouse brains, we demonstrate that hormone action is mediated by activation of ERalpha but not of ERbeta. The specific role of ERalpha is further confirmed by comparing the effects of ERs on the matrix metalloproteinase 9 promoter activity in transient transfection assays. Finally, we report that genetic ablation of ERalpha is associated with a spontaneous reactive phenotype of microglia in specific brain regions of adult ERalpha-null mice. Altogether, these results reveal a previously undescribed function for E2 in brain and provide a mechanism for its beneficial activity on neuroinflammatory pathologies. They also underline the key role of ERalpha in brain macrophage reactivity and hint toward the usefulness of ERalpha-specific drugs in hormone replacement therapy of inflammatory diseases.

Astrocytic expression of the Alzheimer's disease beta-secretase (BACE1) is stimulus-dependent

The beta-site APP-cleaving enzyme (BACE1) is a prerequisite for the generation of beta-amyloid peptides, which give rise to cerebrovascular and parenchymal beta-amyloid deposits in the brain of Alzheimer's disease patients. BACE1 is neuronally expressed in the brains of humans and experimental animals such as mice and rats. In addition, we have recently shown that BACE1 protein is expressed by reactive astrocytes in close proximity to beta-amyloid plaques in the brains of aged transgenic Tg2576 mice that overexpress human amyloid precursor protein carrying the double mutation K670N-M671L. To address the question whether astrocytic BACE1 expression is an event specifically triggered by beta-amyloid plaques or whether glial cell activation by other mechanisms also induces BACE1 expression, we used six different experimental strategies to activate brain glial cells acutely or chronically. Brain sections were processed for the expression of BACE1 and glial markers by double immunofluorescence labeling and evaluated by confocal laser scanning microscopy. There was no detectable expression of BACE1 protein by activated microglial cells of the ameboid or ramified phenotype in any of the lesion paradigms studied. In contrast, BACE1 expression by reactive astrocytes was evident in chronic but not in acute models of gliosis. Additionally, we observed BACE1-immunoreactive astrocytes in proximity to beta-amyloid plaques in the brains of aged Tg2576 mice and Alzheimer's disease patients.

Neuroinflammation and anti-inflammatory therapy for Alzheimer's disease

Neuroinflammation is now recognized as a prominent feature in Alzheimer's pathology and a potential target for therapy aimed at treatment and prevention of disease. This review provides a synopsis of current information about cellular and molecular mediators involved in Alzheimer's neuroinflammation as well as interactions between these mediators that influence pathology. Anti-inflammatory therapies, particularly nonsteroidal anti-inflammatory drugs, are considered from experimental and clinical perspectives and potential mechanisms underlying their apparent benefits are discussed. Finally, possible protective effects of the inflammatory response in Alzheimer's are described. Taken all together, evidence presented in this review suggests a scheme for Alzheimer's pathogenesis, with neuroinflammation playing a crucial role influencing and linking beta-amyloid deposition to neuronal damage and clinical disease.

Pathologies and pathological mechanisms for white matter hyperintensities in depression

Signal hyperintensities on magnetic resonance imaging (MRI) are increased in the white matter (WMH) and deep gray matter in dementia and depression and may have similar pathologies. However, no previous study has examined WMH in subjects with major depression. We carried out in vitro MRI on brain tissue from elderly subjects who had suffered major depression and elderly control subjects to identify and rate WMH. The tissue was subsequently prepared for histopathological analysis using a number of conventional and immunohistochemical stains, and the WMH were examined to identify their underlying pathological causes. Cerebral microvessels were also assessed and the findings compared with assessments of atheromatous disease in these subjects. PVH were found to be due to one of three causes: dilated perivascular spaces (with and without ischemia in the perivascular area), oligemic demyelination, and ischemic demyelination. DWMH also showed three types of causes: dilated perivascular spaces (with and without ischemia in the perivascular area), oligemic demyelination, and ischemic demyelination. Cerebral microvascular disease only contributed to a few of the lesions and atheromatous disease did not show associations with WMH in these subjects. These findings are similar to previous reports in other diseases and demonstrate WMH in elderly depression to be a manifestation of cerebrovascular disease. However, since large vessel and small vessel disease were not associated with WMH, our findings suggest hypotensive disease might be an important and unrecognized mechanism underlying WMH in late-life depression.

Expression of macrophage colony-stimulating factor and its receptor in microglia activation is linked to teratogen-induced neuronal damage

Prenatal exposure to teratogen agents is linked to the pathogenesis of neurodevelopment disorders, but the mechanisms leading to the neurodevelopmental disturbance are poorly understood. To elucidate this, an in vitro model of microglial activation induced by neuronal injury has been characterized. In this connection, exposure of primary microglial cells to the conditioned medium from the neuronal damage induced by teratogen, cyclophosphamide, is accompanied by a reactive microgliosis as assessed by reverse transcription-polymerase chain reaction, enzyme-linked immunosorbent assay, lectin histochemistry, double labeling immunohistochemistry and in situ hybridization. Our results showed that reactive microglia were capable of releasing various cytokines such as tumor necrosis factor-alpha, interleukin-1, interleukin-6, transforming growth factor-beta and nitric oxide. Also, we have shown that macrophage colony-stimulating factor (M-CSF) was in fact produced by the reactive microglia. Concomitant to this was the increased expression of M-CSF receptor in these cells following the teratogen-induced neuronal injury. The up-regulation of M-CSF receptor suggests that the cells are capable of responding to self-derived M-CSF in an autocrine fashion. Results with antibody neutralization further suggest that microglial proinflammatory response, as manifested by cytokine expression in culture, is mediated by M-CSF, which acts as a molecular signal that initiates a microglial reaction. We therefore suggest that microglial activation following cyclophosphamide treatment is not only a response to the neuronal damage, but is also a cause of the damage during pathogenesis of neurodevelopment disorders. To this end, the increased expression of M-CSF and its receptor on microglia would be directly linked to the active cell proliferation and proinflammatory response in the teratogen-induced injury.

Up-Regulation of FPR2, a Chemotactic Receptor for Amyloid β 1–42 (Aβ42), in Murine Microglial Cells by TNFα

Human FPRL1 and its mouse homologue FPR2 are functional receptors for several exogenous and host-derived chemotactic peptides, including amyloid beta(42) (A beta(42)), a critical pathogenic factor in Alzheimer's disease. We investigated the effect of TNF alpha on the expression and function of FPR2 in mouse microglial cells, a crucial inflammatory cell type in the CNS. Primary murine microglia and a cell line N9 in resting state expressed low levels of FPR2 gene and lacked the response to chemotactic agonists for this receptor. Incubation with TNF alpha, however, increased microglial expression of FPR2 gene, in association with potent chemotactic responses to FPR2-specific agonists including A beta(42). The effect of TNF alpha was dependent on the p55 TNF alpha receptor and activation of MAP kinase p38. TNF alpha concomitantly down-regulated microglial response to the chemokine SDF-1 alpha. Thus, by selectively up-regulating FPR2 in microglia, TNF alpha has the capacity to amplify host response in inflammatory diseases in the CNS.

Inflammation, apoptosis, and Alzheimer's disease

The pathophysiology of Alzheimer's disease (AD) involves the deposition of amyloid in the brain and the extensive loss of neurons. The mechanisms subserving neuronal death in the disease remain unclear, although it has been postulated that this is due to apoptosis. There is compelling evidence that inflammatory processes play a role in disease progression and pathology. Amyloid plaque deposition is accompanied by the association of microglia with the senile plaque, and this interaction stimulates these cells to undergo phenotypic activation and the subsequent elaboration of proinflammatory and neurotoxic products. This review focuses on the mechanisms by which neurons are lost in AD and the role microglial proinflammatory products play in neuronal death.

Comparative study of the neuroprotective effect of dehydroepiandrosterone and 17beta-estradiol against 1-methyl-4-phenylpyridium toxicity on rat striatum

The effects of dehydroepiandrosterone, estradiol and testosterone on 1-methyl-4-phenylpyridium (MPP+)-induced neurotoxicity of the nigrostriatal dopaminergic system were examined in rat. They were subjected to a unilateral intrastriatal infusion of the following treatment conditions: MPP+ alone or co-injection of MPP+ plus each hormone. Four days after injection, concentrations of dopamine and their metabolites were determined from the corpus striatum. To corroborate the neurochemical data an immunohistochemical analysis of tyrosine hydroxylase-immunoreactive fibers and acetylcholinesterase histochemistry in the striatum was performed. Moreover, we performed a dose-response study of the three hormones on the high-affinity dopamine transport system in rat striatal synaptosomes. Rats co-injected within the striatum with MPP+ and either dehydroepiandrosterone or estradiol had significantly greater concentrations of dopamine and less tyrosine hydroxylase-immunoreactive fibers and acetylcholinesterase fiber density loss compared with their respective controls. In addition, 4 days after injection, the brain was fixed and cut into coronal sections, and was immunostained with major histocompatibility complex class II antigens for activated microglia, and glial fibrillary acidic protein for activated astrocytes. Dehydroepiandrosterone also attenuated microglial cell activation. In contrast, testosterone showed reductions in dopamine concentrations similar to those obtained by MPP+. The protective effect of dehydroepiandrosterone against the MPP+ neurotoxic dopaminergic system may be produced by its partial prevention of MPP+ inhibition of NADH oxidase activity, whereas the estradiol may function as a neuroprotectant by reducing the uptake of MPP+ into dopaminergic neurons. Our findings we suggest indicate that dehydroepiandrosterone and estradiol by a non-genomic effect may have an important modulatory action, capable of attenuating degeneration within the striatum, and in this way serve as neuroprotectants of the nigrostriatal dopaminergic system.

Beta-amyloid precursor protein transgenic mice that harbor diffuse A beta deposits but do not form plaques show increased ischemic vulnerability: role of inflammation

beta-amyloid (A beta), derived form the beta-amyloid precursor protein (APP), is important for the pathogenesis of Alzheimer's disease (AD), which is characterized by progressive decline of cognitive functions, formation of A beta plaques and neurofibrillary tangles, and loss of neurons. However, introducing a human wild-type or mutant APP gene to rodent models of AD does not result in clear neurodegeneration, suggesting that contributory factors lowering the threshold of neuronal death may be present in AD. Because brain ischemia has recently been recognized to contribute to the pathogenesis of AD, we studied the effect of focal brain ischemia in 8- and 20-month-old mice overexpressing the 751-amino acid isoform of human APP. We found that APP751 mice have higher activity of p38 mitogen-activated protein kinase (p38 MAPK) in microglia, the main immune effector cells within the brain, and increased vulnerability to brain ischemia when compared with age-matched wild-type mice. These characteristics are associated with enhanced microglial activation and inflammation but not with altered regulation of cerebral blood flow, as assessed by MRI and laser Doppler flowmetry. Suppression of inflammation with aspirin or inhibition of p38 MAPK with a selective inhibitor, SD-282, abolishes the increased neuronal vulnerability in APP751 transgenic mice. SD-282 also suppresses the expression of inducible nitric-oxide synthase and the binding activity of activator protein 1. These findings elucidate molecular mechanisms of neuronal injury in AD and suggest that antiinflammatory compounds preventing activation of p38 MAPK in microglia may reduce neuronal vulnerability in AD.

Receptors for chemotactic formyl peptides as pharmacological targets

Leukocytes accumulate at sites of inflammation and immunological reaction in response to locally existing chemotactic mediators. N-formyl peptides, such as fMet-Leu-Phe (fMLF), are some of the first identified and most potent chemoattractants for phagocytic leukocytes. In addition to the bacterial peptide fMLF and the putative endogenously produced formylated peptides, a number of novel peptide agonists have recently been identified that selectively activate the high-affinity fMLF receptor FPR and/or its low-affinity variant FPRL1, both of which belong to the seven-transmembrane (STM), G protein-coupled receptor (GPCR) superfamily. These agonists include peptide domains derived from the envelope proteins of human immunodeficiency virus type 1 (HIV-1) and at least three amyloidogenic polypeptides, the human acute phase protein serum amyloid A, the 42 amino acid form of beta amyloid peptide and a 21 amino acid fragment of human prion. Furthermore, a cleavage fragment of neutrophil granule-derived bactericidal cathelicidin, LL-37, is also a chemotactic agonist for FPRL1. Activation of formyl peptide receptors results in increased cell migration, phagocytosis, release of proinflammatory mediators, and the signaling cascade culminates in heterologous desensitization of other STM receptors including chemokine receptors CCR5 and CXCR4, two coreceptors for HIV-1. Thus, by interacting with a variety of exogenous and host-derived agonists, formyl peptide receptors may play important roles in proinflammatory and immunological diseases and constitute a novel group of pharmacological targets.

Regulation of microglia - potential new drug targets in the CNS

Microglia respond to any disturbance in the CNS which poses a threat to physiological homeostasis. Although these responses are secondary, mainly to neuronal alterations, the way the microglial response evolves in many situations promotes further damage to the CNS. The list of clinical conditions in which this situation is a major problem is continuously growing and includes neurodegenerative diseases, stroke, trauma, demyelinating disorders and neuropathic pain. The significance of microglia for the pathogenesis of neurological and neuropsychiatric conditions has led to a rapidly expanding search for therapeutic possibilities to regulate microglial activity. As will be clear from this review, treatments which are currently available appear to offer some positive effects but are still far from satisfactory. A major challenge is to understand the mechanisms that determine whether activated microglia will develop into a cytotoxic or a cytoprotective component.

Beta amyloid peptide (Abeta42) is internalized via the G-protein-coupled receptor FPRL1 and forms fibrillar aggregates in macrophages

The 42 amino acid form of beta amyloid (Abeta42) plays a pivotal role in neurotoxicity and the activation of mononuclear phagocytes in Alzheimer's disease (AD). Our recent study revealed that FPRL1, a G-protein-coupled receptor, mediates the chemotactic and activating effect of Abeta42 on mononuclear phagocytes (monocytes and microglia), suggesting that FPRL1 may be involved in the proinflammatory responses in AD. We investigated the role of FPRL1 in cellular uptake and the subsequent fibrillar formation of Abeta42 by using fluorescence confocal microscopy. We found that upon incubation with macrophages or HEK293 cells genetically engineered to express FPRL1, Abeta42 associated with FPRL1 and the Abeta42/FPRL1 complexes were rapidly internalized into the cytoplasmic compartment. The maximal internalization of Abeta42/FPRL1 complexes occurred by 30 min after incubation. Removal of free Abeta42 from culture supernatants at 30 min resulted in a progressive recycling of FPRL1 to the cell surface and degradation of the internalized Abeta42. However, persistent exposure of the cells to Abeta42 over 24 h resulted in retention of Abeta42/FPRL1 complexes in the cytoplasmic compartment and the formation of Congo red positive fibrils in macrophages but not in HEK 293 cell transfected with FPRL1. These results suggest that besides mediating the proinflammatory activity of Abeta42, FPRL1 is also involved in the internalization of Abeta42, which culminates in the formation of fibrils only in macrophages.

Regulation of beta-amyloid stimulated proinflammatory responses by peroxisome proliferator-activated receptor alpha

Amyloid deposition within the brains of Alzheimer's Disease patients results in the activation of microglial cells and the induction of a local inflammatory response. The interaction of microglia or monocytes with beta-amyloid (A beta) fibrils elicits the activation a complex tyrosine kinase-based signal transduction cascade leading to stimulation of multiple independent signaling pathways and ultimately to changes in proinflammatory gene expression. The A beta-stimulated expression of proinflammatory genes in myeloid lineage cells is antagonized by the action of a family of ligand-activated nuclear hormone receptors, the peroxisome proliferator-activated receptors (PPARs). We report that THP-1 monocytes express predominantly PPAR gamma isoform and lower levels of PPAR alpha and PPAR delta isoforms. PPAR mRNA levels are not affected by differentiation of the cells into a macrophage phenotype, nor are they altered following exposure to the classical immune stimulus, lipopolysaccharide. Previous studies have found that PPAR gamma agonists act broadly to inhibit inflammatory responses. The present study explored the action of the PPAR alpha isoform and found that PPAR alpha agonists inhibited the A beta-stimulated expression of TNFalpha and IL-6 reporter genes in a dose-dependent manner. Moreover, the PPAR alpha agonist WY14643 inhibited macrophage differentiation and COX-2 gene expression. However, the PPAR alpha agonists failed to inhibit A beta-stimulated elaboration of neurotoxic factors by THP-1 cells. These findings demonstrate that PPAR alpha acts to suppress a diverse array of inflammatory responses in monocytes.

3D-Reconstruction of microglia and amyloid in APP23 transgenic mice: no evidence of intracellular amyloid

Microglia cells are closely associated with compact amyloid plaques in Alzheimer's disease (AD) brains. Although activated microglia seem to play a central role in the pathogenesis of AD, mechanisms of microglial activation by beta-amyloid as well as the nature of interaction between amyloid and microglia remain poorly understood. We previously reported a close morphological association between activated microglia and congophilic amyloid plaques in the brains of APP23 transgenic mice at both the light and electron microscopic levels [25]. In the present study, we have further examined the structural relationship between microglia and amyloid deposits by using postembedding immunogold labeling, serial ultrathin sectioning, and 3-dimensional reconstruction. Although bundles of immunogold-labeled amyloid fibrils were completely engulfed by microglial cytoplasm on single sections, serial ultrathin sectioning and three-dimensional reconstruction revealed that these amyloid fibrils are connected to extracellular amyloid deposits. These data demonstrate that extracellular amyloid fibrils form a myriad of finger-like channels with the widely branched microglial cytoplasm. We conclude that in APP23 mice a role of microglia in amyloid phagocytosis and intracellular production of amyloid is unlikely.

Microglial interaction with beta-amyloid: implications for the pathogenesis of Alzheimer's disease

The etiology of Alzheimer's disease (AD) involves a significant inflammatory component as evidenced by the presence of elevated levels of a diverse range of proinflammatory molecules in the AD brain. These inflammatory molecules are produced principally by activated microglia, which are found to be clustered within and adjacent to the senile plaque. Moreover, long-term treatment of patients with non-steroidal anti-inflammatory drugs has been shown to reduce risk and incidence of AD and delay disease progression. The microglia respond to beta-amyloid (Abeta) deposition in the brain through the interaction of fibrillar forms of amyloid with cell surface receptors, leading to the activation of intracellular signal transduction cascades. The activation of multiple independent signaling pathways ultimately leads to the induction of proinflammatory gene expression and production of reactive oxygen and nitrogen species. These microglial inflammatory products act in concert to produce neuronal toxicity and death. Therapeutic approaches focused on inhibition of the microglial-mediated local inflammatory response in the AD brain offer new opportunities to intervene in the disease.

Microglia in neuroregeneration

Microglia has the potential to produce and release a range of factors that directly and/or indirectly promote regeneration in the injured nervous system. The overwhelming evidence indicates, however, that this potential is generally not expressed in vivo. Activated microglia may enhance neuronal degeneration following axotomy, thereby counteracting functional recovery. Microglia does not seem to contribute significantly to axonal outgrowth after peripheral nerve injury, since this process proceeds uneventful even if perineuronal microglia is eliminated. The phagocytic phenotype of microglia is highly suppressed during Wallerian degeneration in the central nervous system. Therefore, microglia is incapable of rapid and efficient removal of myelin debris and its putative growth inhibitory components. In this way, microglia may contribute to regeneration failure in the central nervous system. Structural and temporal correlations are compatible with participation by perineuronal microglia in axotomy-induced shedding of presynaptic terminals, but direct evidence for such participation is lacking. Currently, the most promising case for a promoting effect on neural repair by activated microglia appears to be as a mediator of collateral sprouting, at least in certain brain areas. However, final proof for a critical role of microglia in these instances is still lacking. Results from in vitro studies demonstrate that microglia can develop a regeneration supportive phenotype. Altering the microglial involvement following neural injury from a typically passive or even counterproductive state and into a condition where these cells are actively supporting regeneration and plasticity is, therefore, an exciting challenge and probably a realistic goal.

Estrogen prevents the lipopolysaccharide-induced inflammatory response in microglia

After neuronal injury and in several neurodegenerative diseases, activated microglia secrete proinflammatory molecules that can contribute to the progressive neural damage. The recent demonstration of a protective role of estrogen in neurodegenerative disorders in humans and experimental animal models led us to investigate whether this hormone regulates the inflammatory response in the CNS. We here show that estrogen exerts an anti-inflammatory activity on primary cultures of rat microglia, as suggested by the blockage of the phenotypic conversion associated with activation and by the prevention of lipopolysaccharide-induced production of inflammatory mediators: inducible form of NO synthase (iNOS), prostaglandin-E(2) (PGE(2)), and metalloproteinase-9 (MMP-9). These effects are dose-dependent, maximal at 1 nm 17beta-estradiol, and can be blocked by the estrogen receptor (ER) antagonist ICI 182,780. The demonstration of ERalpha and ERbeta expression in microglia and macrophages and the observation of estrogen blockade of MMP-9 mRNA accumulation and MMP-9 promoter induction further support the hypothesis of a genomic activity of estrogen via intracellular receptors. This is the first report showing an anti-inflammatory activity of estrogen in microglia. Our study proposes a novel explanation for the protective effects of estrogen in neurodegenerative and inflammatory diseases and provides new molecular and cellular targets for the screening of ER ligands acting in the CNS.

Voltage-gated proton channels in microglia

Microglia, macrophages that reside in the brain, can express at least 12 different ion channels, including voltage-gated proton channels. The properties of H+ currents in microglia are similar to those in other phagocytes. Proton currents are elicited by depolarizing the membrane potential, but activation also depends strongly on both intracellular pH (pH(i)) and extracellular pH (pH(o)). Increasing pH(o) or lowering pH(i) promotes H+ channel opening by shifting the activation threshold to more negative potentials. H+ channels in microglia open only when the pH gradient is outward, so they carry only outward current in the steady state. Time-dependent activation of H+ currents is slow, with a time constant roughly 1 s at room temperature. Microglial H+ currents are inhibited by inorganic polyvalent cations, which reduce H+ current amplitude and shift the voltage dependence of activation to more positive potentials. Cytoskeletal disruptive agents modulate H+ currents in microglia. Cytochalasin D and colchicine decrease the current density and slow the activation of H+ currents. Similar changes of H+ currents, possibly due to cytoskeletal reorganization, occur in microglia during the transformation from ameboid to ramified morphology. Phagocytes, including microglia, undergo a respiratory burst, in which NADPH oxidase releases bactericidal superoxide anions into the phagosome and stoichiometrically releases protons into the cell, tending to depolarize and acidify the cell. H+ currents may help regulate both the membrane potential and pH(i) during the respiratory burst. By compensating for the efflux of electrons and counteracting intracellular acidification, H+ channels help maintain superoxide anion production.

Estrogen blocks inducible nitric oxide synthase accumulation in LPS-activated microglia cells

Estrogens are thought to play a protective role against neurodegeneration through a variety of mechanisms including the activation of growth factors and neurotransmitter synthesis, the control of synaptic plasticity and functions, and the blockade of oxidative reactions. We here propose a novel mechanism to explain the neuroprotective effects of estradiol by showing that estrogens may antagonize nitric oxide synthase activity and reduce the accumulation of nitrites and nitrates consequent to various inflammatory stimuli. The potential anti-inflammatory activity of estradiol is analyzed in vitro in cells in culture including primary cultures of microglia and in vivo in a well-known model of inflammation.

CD45 inhibits CD40L-induced microglial activation via negative regulation of the Src/p44/42 MAPK pathway

It has been reported that ligation of CD40 with CD40 ligand (CD40L) results in microglial activation as evidenced by p44/42 mitogen-activated protein kinase (MAPK) dependent tumor necrosis factor alpha (TNF-alpha) production. Previous studies have shown that CD45, a functional transmembrane protein-tyrosine phosphatase, is constitutively expressed at moderate levels on microglial cells and this expression is greatly elevated on activated microglia. To investigate the possibility that CD45 might modulate CD40L-induced microglial activation, we treated primary cultured microglial cells with CD40L and anti-CD45 antibody. Data show that cross-linking of CD45 markedly inhibits CD40L-induced activity of the Src family kinases Lck and Lyn. Further, co-treatment of microglia with CD40L and anti-CD45 antibody results in significant inhibition of microglial TNF-alpha production through inhibition of p44/42 MAPK activity, a downstream signaling event resulting from Src activation. Accordingly, primary cultured microglial cells from mice deficient in CD45 demonstrate hyper-responsiveness to ligation of CD40, as evidenced by increased p44/42 MAPK activation and TNF-alpha production. Taken together, these results show that CD45 plays a novel role in suppressing CD40L-induced microglial activation via negative regulation of the Src/p44/42 MAPK cascade.

Differential alterations in antioxidant capacity in cells from Alzheimer patients

Oxidative stress occurs in brains of Alzheimer's disease (AD) patients. A major question in AD research is whether the oxidative stress is just secondary to neurodegeneration. To test whether oxidative stress is an inherent property of AD tissues, the ability of cultured fibroblasts bearing the AD Presenilin-1 246 Ala-->Glu mutation to handle reactive oxygen species (ROS) was compared to controls. Although ROS in cells from AD subjects were only slightly less than cells from controls under basal conditions (-10%) or after exposure to H(2)O(2) (-16%), treatment with antioxidants revealed clear differences. Pretreatment with DMSO, a hydroxyl radical scavenger, reduced basal and H(2)O(2)-induced ROS levels significantly more in cells from controls (-22%, -22%) than in those from AD subjects (-4%, +14%). On the other hand, pretreatment with Trolox diminished H(2)O(2)-induced ROS significantly more in cells from AD (-60%) than control subjects (-39%). In summary, cells from AD patients have greater Trolox sensitive ROS and less DMSO sensitive ROS than controls. The results demonstrate that fibroblasts bearing this PS-1 mutation have altered means of handling oxidative stress and appear useful for determining the mechanism underlying the altered redox metabolism.

Estrogen enhances uptake of amyloid beta-protein by microglia derived from the human cortex

In recent years, inflammatory mechanisms have been increasingly appreciated as important steps in the pathology of Alzheimer's disease (AD). There are two pathological defects in AD: chronic inflammation and impaired clearance of amyloid beta-peptide (Abeta). In the periphery, estrogen both increases macrophage phagocytosis and has antiinflammatory effects. If estrogen had a similar effect in the CNS, it could reverse inflammatory defects in AD. Although microglia are a key component of the immune system and help clear Abeta deposits in the AD brain, little is known about the effects of estrogen on CNS microglia. Therefore, we sought to determine the relationship between estrogen treatment and internalization of Abeta by microglia by quantifying the internalization of aggregated Abeta by human cortical microglia. Abeta uptake was found to be dose- and time-dependent in cultured microglia. Increased Abeta uptake was observed at 1.5 and 24 h after addition of aggregated Abeta (50, 100, or 1,000 nM: Abeta), and this uptake was enhanced by pretreatment with estrogen. The expression of estrogen receptor (ER) beta (ER-beta) was also up-regulated by estrogen treatment. Cells cotreated with ICI 182,780, an ER antagonist, showed significantly reduced internalization of Abeta in cultured microglia. These results indicate that microglia express an ER-beta but that the effect of estrogen on enhancing clearance of Abeta may be related to the receptor-independent action of estrogen or to nonclassical ER effects of estrogen. Thus, stimulation of the ER might contribute to the therapeutic action of estrogen in the treatment of AD.

Influence of lipoproteins on microglial degradation of Alzheimer's amyloid beta-protein

Amyloid beta-protein (Abeta), the major component of plaques in Alzheimer's disease, is a small hydrophobic protein that is carried on apolipoprotein E (ApoE)- and ApoJ-containing lipoprotein particles in plasma and cerebrospinal fluid (CSF). Microglia, the scavenger cells of the CNS, take up and degrade Abeta via lipoprotein receptors including scavenger receptors A and B, and possibly via other receptors. Lipoproteins, ApoE, and ApoJ influence the uptake and degradation of Abeta in vitro and in vivo. Differences in ApoE-E4, -E3, and -E2 isoforms with respect to Abeta binding to lipoproteins and delivery to cells, including microglia, may contribute to the increased risk of Alzheimer's disease for people with an APOE4 genotype and to risk reduction with APOE2.

Naloxone prevents microglia-induced degeneration of dopaminergic substantia nigra neurons in adult rats

Resident microglia are involved in immune responses of the central nervous system and may contribute to neuronal degeneration and death. Here, we tested in adult rats whether injection of bacterial lipopolysaccharide (which causes inflammation and microglial activation) just above the substantia nigra, results in the death of dopaminergic substantia nigra pars compacta neurons. Two weeks after lipopolysaccharide injection, microglial activation was evident throughout the nigra and the number of retrogradely-labeled substantia nigra neurons was reduced to 66% of normal. This suggests that inflammation and/or microglial activation can lead to neuronal cell death in a well-defined adult animal model. The opioid receptor antagonist naloxone reportedly reduces release of cytotoxic substances from microglia and protects cortical neurons in vitro. Here, a continuous two-week infusion of naloxone at a micromolar concentration close to the substantia nigra, prevented most of the neuronal death caused by lipopolysaccharide, i.e. 85% of the neurons survived. In addition, with systemic (subcutaneous) infusion of 0. 1mg/d naloxone, 94% of the neurons survived. Naloxone infusions did not obviously affect the morphological signs of microglial activation, suggesting that naloxone reduces the release of microglial-derived cytotoxic substances. Alternatively, microglia might not cause the neuronal loss, or naloxone might act by blocking opioid receptors on (dopaminergic or GABAergic) neurons.Thus, local inflammation induces and the opioid antagonist naloxone prevents the death of dopaminergic substantia nigra neurons in adult rats. This may be relevant to the understanding of the pathology and treatment of Parkinson's disease, where these neurons degenerate.

Activation of NADPH oxidase in Alzheimer's disease brains

The present study is the first to show that superoxide (O(-)(2)) forming NADPH oxidase is activated in Alzheimer's disease (AD) brains by demonstrating the marked translocation of the cytosolic factors p47-phox and p67-phox to the membrane. In conjunction with a recent in vitro study showing that amyloid beta activates O(-)(2) forming NADPH oxidase in microglia, where these phox proteins are localized in this study, the present results suggest that, in AD, NADPH oxidase is activated in microglia, resulting in the formation of reactive oxygen species which can be toxic to neighboring neurons in AD.

Visualization of beta-amyloid peptide (Abeta) phagocytosis by human mononuclear phagocytes: dependency on Abeta aggregate size

Previous studies from this laboratory have shown that human monocytes exposed to beta-amyloid peptide (Abeta) exert a graded neurotoxic response in an organotypic brain culture paradigm. Moreover, this toxicity can be reduced by compounds that inhibit cell motility and phagocytosis, suggesting that internalization of Abeta may be a requirement for neurotoxic action. To confirm that Abeta is indeed phagocytosed by monocytes and to further lay the groundwork for resolving the possible linkage between this process and neurotoxicity, we examined Abeta:monocyte interactions using immunocytochemistry and fluorescence histochemistry followed by confocal microscopy and three-dimensional image reconstruction. Internalization of Abeta was detected by 24 hr following exposure of monocytes to the purified peptide, and the relative efficacy of this process appeared to be influenced by the size of the Abeta aggregates. Specifically, smaller aggregates were observed to be more efficiently internalized, while larger Abeta masses tended to reside only on the monocyte surface, apparently bound to several monocytes at once. Both colchicine and cytochalasin D, cytoskeletal perturbants that block phagocytosis, caused Abeta to accumulate in deep pits within monocytes and inhibited complete envelopment by monocyte cytoplasm. These results suggest that monocytes can indeed phagocytose aggregates of Abeta and that this process may be critical in activating these cells to a neurocytopathic state. Accordingly, interference of Abeta phagocytosis by monocytes or monocyte-derived cells may be a novel target for therapeutic action.

Inflammatory mechanisms in Alzheimer's disease: inhibition of beta-amyloid-stimulated proinflammatory responses and neurotoxicity by PPARgamma agonists

Alzheimer's disease (AD) is characterized by the extracellular deposition of beta-amyloid fibrils within the brain and the subsequent association and phenotypic activation of microglial cells associated with the amyloid plaque. The activated microglia mount a complex local proinflammatory response with the secretion of a diverse range of inflammatory products. Nonsteroidal anti-inflammatory drugs (NSAIDs) are efficacious in reducing the incidence and risk of AD and significantly delaying disease progression. A recently appreciated target of NSAIDs is the ligand-activated nuclear receptor peroxisome proliferator-activated receptor gamma (PPARgamma). PPARgamma is a DNA-binding transcription factor whose transcriptional regulatory actions are activated after agonist binding. We report that NSAIDs, drugs of the thiazolidinedione class, and the natural ligand prostaglandin J2 act as agonists for PPARgamma and inhibit the beta-amyloid-stimulated secretion of proinflammatory products by microglia and monocytes responsible for neurotoxicity and astrocyte activation. The activation of PPARgamma also arrested the differentiation of monocytes into activated macrophages. PPARgamma agonists were shown to inhibit the beta-amyloid-stimulated expression of the cytokine genes interleukin-6 and tumor necrosis factor alpha. Furthermore, PPARgamma agonists inhibited the expression of cyclooxygenase-2. These data provide direct evidence that PPARgamma plays a critical role in regulating the inflammatory responses of microglia and monocytes to beta-amyloid. We argue that the efficacy of NSAIDs in the treatment of AD may be a consequence of their actions on PPARgamma rather than on their canonical targets the cyclooxygenases. Importantly, the efficacy of these agents in inhibiting a broad range of inflammatory responses suggests PPARgamma agonists may provide a novel therapeutic approach to AD.

In vivo reversal of amyloid-beta lesions in rat brain

Cerebral amyloid-beta (Abeta) deposition is central to the neuropathological definition of Alzheimer disease (AD) with Abeta related toxicity being linked to its beta-sheet conformation and/or aggregation. We show that a beta-sheet breaker peptide (iAbeta5) dose-dependently and reproducibly induced in vivo disassembly of fibrillar amyloid deposits, with control peptides having no effect. The iAbeta5-induced disassembly prevented and/or reversed neuronal shrinkage caused by Abeta and reduced the extent of interleukin-1beta positive microglia-like cells that surround the Abeta deposits. These findings suggest that beta-sheet breakers, such as iAbeta5 or similar peptidomimetic compounds, may be useful for reducing the size and/or number of cerebral amyloid plaques in AD, and subsequently diminishing Abeta-related histopathology.

Microglial activation resulting from CD40-CD40L interaction after beta-amyloid stimulation

Alzheimer's disease (AD) has a substantial inflammatory component, and activated microglia may play a central role in neuronal degeneration. CD40 expression was increased on cultured microglia treated with freshly solublized amyloid-beta (Abeta, 500 nanomolar) and on microglia from a transgenic murine model of AD (Tg APPsw). Increased tumor necrosis factor alpha production and induction of neuronal injury occurred when Abeta-stimulated microglia were treated with CD40 ligand (CD40L). Microglia from Tg APPsw mice deficient for CD40L demonstrated reduction in activation, suggesting that the CD40-CD40L interaction is necessary for Abeta-induced microglial activation. Finally, abnormal tau phosphorylation was reduced in Tg APPsw animals deficient for CD40L, suggesting that the CD40-CD40L interaction is an early event in AD pathogenesis.