Amyloid precursor protein-processing products affect mononuclear phagocyte activation: pathways for sAPP- and Abeta-mediated neurotoxicity

AAV2/1 CD74 Gene Transfer Reduces β-amyloidosis and Improves Learning and Memory in a Mouse Model of Alzheimer's Disease

Modulation of the amyloid-β (Aβ) trafficking pathway heralds a new therapeutic frontier for Alzheimer's disease (AD). As CD74 binds to the amyloid-β precursor protein (APP) and can suppresses Aβ processing, we investigated whether recombinant adeno-associated virus (AAV) delivery of CD74 could reduce Aβ production and affect disease outcomes. This idea was tested in a mouse AD model. Cotransduction of AAV-tetracycline-controlled transactivator (tTA) and AAV-tet-response element (TRE)-CD74 resulted in CD74 expression, reduced Aβ production in mouse neurons containing the human APP with familial AD-linked mutations. Stereotaxic injection of AAV-TRE-GFP or CD74 into the hippocampi of an AD mouse, defined as a TgCRND8 × calmodulin-dependent protein kinase II derived promoter-tTA double-transgenic, reduced Aβ loads and pyramidal neuronal Aβ accumulation in the hippocampus. Immunofluorescent studies showed that APP colocalization with Lamp1 was increased in CD74-expressing neurons. Moreover, Morris water maze tasks demonstrated that mice treated with AAV-TRE-CD74 showed improved learning and memory compared to AAV-TRE-GFP control animals. These results support the idea that CD74-induced alteration of Aβ processing could improve AD-associated memory deficits as shown in mouse models of human disease.

Overexpression of mutant amyloid-β protein precursor and presenilin 1 modulates enteric nervous system

Alzheimer's disease (AD) is a neurodegenerative disorder histologically characterized by amyloid-β (Aβ) protein accumulation and activation of associated microglia. Although these features are well described in the central nervous system, the process and consequences of Aβ accumulation in the enteric nervous system have not been extensively studied. We hypothesized that Aβ also may accumulate in the enteric nervous system and lead to immune cell activation and neuronal dysfunction in the digestive tract not unlike that observed in diseased brain. To test this hypothesis, ileums of the small intestine of thirteen month old AβPP/PS1 and C57BL/6 (wild type) mice were collected and analyzed using immunohistochemistry, western blot analysis, cytokine arrays, and ELISA. AβPP/PS1 mice demonstrated no differences in intestinal motility or water absorption but elevated luminal IgA levels compared to wild type mice. They also had increased protein levels of AβPP and the proteolytic enzyme, BACE, corresponding to an increase in Aβ1-40 in the intestinal lysate as well as an increase in both Aβ1-40 and Aβ1-42 in the stool. This correlated with increased protein markers of proinflammatory and immune cell activation. Histologic analysis localized AβPP within enteric neurons but also intestinal epithelial cells with elevated Aβ immunoreactivity in the AβPP/PS1 mice. The presence of AβPP, Aβ, and CD68 immunoreactivity in the intestines of some patients with neuropathologically-confirmed AD are consistent with the findings in this mouse model. These data support the hypothesis that in AD the intestine, much like the brain, may develop proinflammatory and immune changes related to AβPP and Aβ.

Protective effects of EphB2 on Aβ1-42 oligomer-induced neurotoxicity and synaptic NMDA receptor signaling in hippocampal neurons

Alzheimer's disease (AD) is a neurodegenerative disorder characterized pathologically by the abnormal deposition of extracellular amyloid-β (Aβ) oligomers. However, the nature and precise mechanism of the toxicity of Aβ oligomers are not clearly understood. Aβ oligomers have been previously shown to cause a major loss of EphB2, a member of the EphB family of receptor tyrosine kinases. To determine the effect of EphB2 on Aβ oligomer-induced neurotoxicity and the underlying molecular mechanisms, we examined the EphB2 gene in cultured hippocampal neurons. Using a cellular model of AD, Aβ1-42 oligomers were confirmed to induce neurotoxicity in a time-dependent manner and result in a major decrease of EphB2. EphB2 overexpression could prevent the neurotoxicity of hippocampal neurons from exposure to Aβ1-42 oligomers for 1h. Further analysis revealed that EphB2 overexpression increased synaptic NR1 and NR2B expression in Aβ1-42 oligomer-treated neurons. Moreover, EphB2 overexpression prevented Aβ1-42 oligomer-induced downregulation of dephosphorylated p38 MAPK and phosphorylated CREB. Together, these results suggest that EphB2 is a factor which protects hippocampal neurons against the toxicity of Aβ1-42 oligomers, and we infer that the protection of EphB2 is achieved by increasing the synaptic NMDA receptor level and downstream p38 MAPK and CREB signaling in hippocampal neurons. This study provides new molecular insights into the neuroprotective effect of EphB2 and highlights its potential therapeutic role in the management of AD.

Alzheimer's disease, β-amyloid, glutamate, NMDA receptors and memantine--searching for the connections

β-amyloid (Aβ) is widely accepted to be one of the major pathomechanisms underlying Alzheimer's disease (AD), although there is presently lively debate regarding the relative roles of particular species/forms of this peptide. Most recent evidence indicates that soluble oligomers rather than plaques are the major cause of synaptic dysfunction and ultimately neurodegeneration. Soluble oligomeric Aβ has been shown to interact with several proteins, for example glutamatergic receptors of the NMDA type and proteins responsible for maintaining glutamate homeostasis such as uptake and release. As NMDA receptors are critically involved in neuronal plasticity including learning and memory, we felt that it would be valuable to provide an up to date review of the evidence connecting Aβ to these receptors and related neuronal plasticity. Strong support for the clinical relevance of such interactions is provided by the NMDA receptor antagonist memantine. This substance is the only NMDA receptor antagonist used clinically in the treatment of AD and therefore offers an excellent tool to facilitate translational extrapolations from in vitro studies through in vivo animal experiments to its ultimate clinical utility.

Paeoniflorin attenuates amyloid-beta peptide-induced neurotoxicity by ameliorating oxidative stress and regulating the NGF-mediated signaling in rats

Paeoniflorin is a monoterpene glycoside isolated from the aqueous extract of the dry root of Paeonia. It has been identified to exhibit many pharmacological effects including enhancing the cognitive ability, producing anti-depressant-like effect and reducing the MTPT-induced toxicity. In our previous study, it has shown that paeoniflorin improved the cognitive ability and attenuated the oxidative stress in the Aβ(1-42)-treated rats. In order to further elucidate the possible molecular mechanisms of paeoniflorin on the cognitive ability, rats were injected with Aβ(1-42) (1 μg/μL) and later with paeoniflorin (15 mg/kg and 30 mg/kg, i.p.) and donepezil hydrochloride (2mg/kg, i.p.) daily for 20 days in this study. The results showed that the long-term treatment of paeoniflorin or donepezil enhanced the cognitive performances in the Morris water maze test, restored the decreased activities of superoxide dismutase and catalase and the increased level of malondialdehyde, and reversed the alterations of matrix metallopeptidase-9 and tissue-inhibitor of metalloproteinase-1 in the hippocampus of Aβ(1-42)-treated rats. Paeoniflorin also up-regulated the activity of choline acetyltrasferase and the expression of tyrosine kinase A receptor, and down-regulated the activity of acetylcholine esterase in the hippocampus of Aβ(1-42)-treated rats. These results demonstrate that paeoniflorin ameliorates the spatial learning and memory deficits by attenuating oxidative stress and regulating the nerve growth factor-mediated signaling to reinforce cholinergic functions in the hippocampus of the Aβ(1-42)-treated rats.

Amyloid precursor protein expression modulates intestine immune phenotype

Amyloid precursor protein (APP) is widely expressed across many tissue and cell types. Proteolytic processing of the protein gives rise to a plethora of protein fragments with varied biological activities. Although a large amount of data has been generated describing the metabolism of the protein in neurons, its role in regulating the phenotype of other cells remains unclear. Based upon prior work demonstrating that APP regulates the activation phenotype of monocytic lineage cells, we hypothesized that APP can regulate macrophage activation phenotype in tissues other than brain. Ileums of the small intestines from C57BL6/J wild type and APP(-/-) mice were compared as a representative tissue normally associated with abundant macrophage infiltration. APP(-/-) intestines demonstrated diminished CD68 immunoreactivity compared to wild type mice. This correlated with significantly less cyclooxygenase-2 (cox-2), CD68, CD40, CD11c, and βIII-tubulin protein levels. Peritoneal macrophages from APP(-/-) mice demonstrated decreased in vitro migratory ability compared to wild type cells and diminished basal KC cytokine secretion. Whereas, APP(-/-) intestinal macrophages had an increase in basal KC cytokine secretion compared to wild type cells. Conversely, there was a significant decrease in multiple cytokine levels in APP(-/-) compared to wild type ileums. Finally, APP(-/-) mice demonstrated impaired absorption and increased motility compared to wild type mice. These data demonstrate the APP expression regulates immune cell secretions and phenotype and intestinal function. This data set describes a novel function for this protein or its metabolites that may be relevant not only for Alzheimer's disease but a range of immune-related disorders.

Glucocorticoids facilitate astrocytic amyloid-β peptide deposition by increasing the expression of APP and BACE1 and decreasing the expression of amyloid-β-degrading proteases

In most cases, the molecular mechanism underlying the pathogenesis of sporadic Alzheimer's disease (AD) is unknown. Elevated basal cortisol levels in AD patients suggest that glucocorticoids (GC) may contribute to the development and/or maintenance of AD. Amyloid plaques are the hallmark of AD, and they are considered to play an early role in the AD process. However, little is known about how their formation is regulated by stress and GC. Astrocyte accumulation is one of the earliest neuropathological changes in AD. Here, we report that GC elevated amyloid-β (Aβ) production in primary cultures of astrocytes by increasing amyloid precursor protein (APP) and β-site APP-cleaving enzyme 1 gene expression. Notably, GC administered to normal, middle-aged mice promoted the expression of APP and β-site APP-cleaving enzyme 1 in astrocytes, as determined by double immunofluorescence. Additionally, confocal microscopy and ELISA revealed that GC markedly reduced Aβ degradation and clearance by astrocytes in vitro, indicating a decreased neuroprotective capacity of the astrocytes. This may have been due to the decrease of several Aβ-degrading proteases, such as insulin-degrading enzyme and matrix metalloproteinase-9. These effects occurred through the activation of GC receptors. Taken together, our results demonstrate that GC can enhance the production of Aβ, reduce its degradation in astrocytes, and provide a molecular mechanism linking stress factors to AD. Our study suggests that GC can facilitate AD pathogenesis and that reducing GC in the elderly and early AD patients would be beneficial.

CCL2 accelerates microglia-mediated Abeta oligomer formation and progression of neurocognitive dysfunction

Background

The linkages between neuroinflammation and Alzheimer's disease (AD) pathogenesis are well established. What is not, however, is how specific immune pathways and proteins affect the disease. To this end, we previously demonstrated that transgenic over-expression of CCL2 enhanced microgliosis and induced diffuse amyloid plaque deposition in Tg2576 mice. This rodent model of AD expresses a Swedish β-amyloid (Aβ) precursor protein mutant.

Methodology/Principal Findings

We now report that CCL2 transgene expression accelerates deficits in spatial and working memory and hippocampal synaptic transmission in β-amyloid precursor protein (APP) mice as early as 2–3 months of age. This is followed by increased numbers of microglia that are seen surrounding Aβ oligomers. CCL2 does not suppress Aβ degradation. Rather, CCL2 and tumor necrosis factor-α directly facilitated Aβ uptake, intracellular Aβ oligomerization, and protein secretion.

Conclusions/Significance

We posit that CCL2 facilitates Aβ oligomer formation in microglia and propose that such events accelerate memory dysfunction by affecting Aβ seeding in the brain.

Effect of dietary n-3 polyunsaturated fatty acids on brain lipid fatty acid composition, learning ability, and memory of senescence-accelerated mouse

Animal studies have shown that a deficiency in brain of the n-3 polyunsaturated fatty acid (PUFA) docosahexaenoic acid (DHA) is associated with memory loss and diminished cognitive function. The senescence-accelerated prone 8 (SAMP8) mouse develops impairments in learning and memory at 8-12 months of age. The effect of diet supplemented with n-3 PUFA on brain phospholipid DHA status, learning, and memory ability in aged SAMP8 mice was investigated. At the age of 10 months, SAMP8 mice were fed either a low-DHA or a high-DHA diet for 8 weeks. In comparison to SAMP8 mice fed the low-DHA diet, those fed a high-DHA diet had improved acquisition and retention in a T-maze foot shock avoidance test and a higher proportion of DHA in hippocampal and amygdala phospholipids. This study demonstrates that, in mature animals, DHA is incorporated into brain phospholipids and that dietary n-3 PUFA is associated with delay in cognitive decline.

Cytokine-mediated inhibition of fibrillar amyloid-beta peptide degradation by human mononuclear phagocytes

Vaccination therapy of AD animal models and patients strongly suggests an active role of brain mononuclear phagocytes in immune-mediated clearance of amyloid-beta peptides (Abeta) in brain. Although Abeta uptake by macrophages can be regulated by pro- and anti-inflammatory cytokines, their effects on macrophage-mediated Abeta degradation are poorly understood. To better understand this mechanism of degradation, we examined whether pro- and anti-inflammatory cytokines affect the degradation of Abeta using primary cultured human monocyte-derived macrophages (MDM) and microglia using pulse-chase analysis of fibrillar and oligomer (125)I-Abeta40 and Abeta42. Initial uptake of fibrillar Abeta40 and Abeta42 was 40% and its degradation was saturated by 120 h in both MDM and microglia, compared with an initial uptake of oligomeric Abeta less than 0.5% and saturation of degradation within 24 h. IFN-gamma increased the intracellular retention of fibrillar Abeta40 and Abeta42 by inhibiting degradation, whereas IL-4, IL-10, and TGF-beta1, but not IL-13 and IL-27, enhanced degradation. Fibrillar Abeta degradation in MDM is sensitive to lysosomal and insulin degrading enzyme inhibitors but insensitive to proteasomal and neprilysin inhibitors. IFN-gamma and TNF-alpha directly reduced the expression of insulin degrading enzyme and chaperone molecules (heat shock protein 70 and heat shock cognate protein 70), which are involved in refolding of aggregated proteins. Coculture of MDM with activated, but not naive T cells, suppressed Abeta degradation in MDM, which was partially blocked by a combination of neutralizing Abs against proinflammatory cytokines. These data suggest that proinflammatory cytokines suppress Abeta degradation in MDM, whereas select anti-inflammatory and regulatory cytokines antagonize these effects.

Relationships between expression of apolipoprotein E and beta-amyloid precursor protein are altered in proximity to Alzheimer beta-amyloid plaques: potential explanations from cell culture studies

Theories regarding the initiation and progression of Alzheimer disease (AD) often consider potential roles played by elevations of beta-amyloid precursor protein (betaAPP). Because it is the source of amyloid beta-peptide, betaAPP may simply contribute more pathogenic stimulus when elevated; some analyses have, however, reported a decline in betaAPP in AD. We found a progressive increase in neuronal betaAPP expression with increasing age in the brains of nondemented individuals, whereas in AD patient samples, betaAPP antigenicity decreased in neuronal somata in a manner that correlated with accumulation of mature amyloid beta-peptide plaques. In contrast, apolipoprotein E (ApoE) expression correlated with accumulation of plaques, and even greater amounts of ApoE were detected in plaques. Induction of betaAPP by glutamate in neuronal cell cultures was found to depend upon ApoE levels or activity. Thus, elevations in expression of ApoE and betaAPP by cellular stresses are likely normally linked in vivo, and uncoupling of this link, or other pathologic events in AD initiation, may leave neurons with diminished betaAPP expression, which might in turn reduce their resistance to stressors.

Interferon-gamma and tumor necrosis factor-alpha regulate amyloid-beta plaque deposition and beta-secretase expression in Swedish mutant APP transgenic mice

Reactive astrocytes and microglia in Alzheimer's disease surround amyloid plaques and secrete proinflammatory cytokines that affect neuronal function. Relationship between cytokine signaling and amyloid-beta peptide (Abeta) accumulation is poorly understood. Thus, we generated a novel Swedish beta-amyloid precursor protein mutant (APP) transgenic mouse in which the interferon (IFN)-gamma receptor type I was knocked out (APP/GRKO). IFN-gamma signaling loss in the APP/GRKO mice reduced gliosis and amyloid plaques at 14 months of age. Aggregated Abeta induced IFN-gamma production from co-culture of astrocytes and microglia, and IFN-gamma elicited tumor necrosis factor (TNF)-alpha secretion in wild type (WT) but not GRKO microglia co-cultured with astrocytes. Both IFN-gamma and TNF-alpha enhanced Abeta production from APP-expressing astrocytes and cortical neurons. TNF-alpha directly stimulated beta-site APP-cleaving enzyme (BACE1) expression and enhanced beta-processing of APP in astrocytes. The numbers of reactive astrocytes expressing BACE1 were increased in APP compared with APP/GRKO mice in both cortex and hippocampus. IFN-gamma and TNF-alpha activation of WT microglia suppressed Abeta degradation, whereas GRKO microglia had no changes. These results support the idea that glial IFN-gamma and TNF-alpha enhance Abeta deposition through BACE1 expression and suppression of Abeta clearance. Taken together, these observations suggest that proinflammatory cytokines are directly linked to Alzheimer's disease pathogenesis.

Tau-tubulin kinase 1 (TTBK1), a neuron-specific tau kinase candidate, is involved in tau phosphorylation and aggregation

Neurofibrillary tangles, which are major pathological hallmarks of Alzheimer's disease (AD), are composed of paired helical filaments (PHFs) containing hyperphosphorylated tau. Specific kinases regulate tau phosphorylation and are closely linked to the pathogenesis of AD. We have characterized a human tau-tubulin kinase 1 (TTBK1) gene located on chromosome 6p21.1. TTBK1 is a serine/threonine/tyrosine kinase that is conserved among species and belongs to the casein kinase 1 superfamily. It is specifically expressed in the brain, especially in the cytoplasm of cortical and hippocampal neurons. TTBK1 phosphorylates tau proteins in both a Mg2+- and a Mn2+-dependent manner. Phosphopeptide mapping and immunoblotting analysis confirmed a direct tau phosphorylation by TTBK1 at Ser198, Ser199, Ser202 and Ser422, which are also phosphorylated in PHFs. TTBK1 also induces tau aggregation in human neuronal cells in a dose-dependent manner. We conclude that TTBK1 is a neuron-specific dual kinase involved in tau phosphorylation at AD-related sites and is also associated with tau aggregation.

Protective effect of N-acetyl-l-cysteine on amyloid β-peptide-induced learning and memory deficits in mice

This study aimed to examine the effects of N-acetyl-L-cysteine (NAC) on protecting neurons function and improving learning and memory deficits in mice. Mice were intracerebroventricularly (icv) injected with the aggregated amyloid beta-peptide (Abeta) to produce Alzheimer's disease (AD). Learning and memory functions in mice were examined by the step through test and the water maze performance. The results showed that the mice pretreated with NAC had significantly greater retention in the step through test and shorter latencies in the water maze performance. Biochemical studies showed the potential role of free radical toxicity and the damage of cholinergic neurons in the Abeta-treated mice. There was an increased lipid peroxidation as indicated by elevated malondehyde (MDA) and decrease of glutathione (GSH) levels. There was also an increase in acetylcholinesterase (AChE) activity and a reduction in the choline acetyltransferase (ChAT) activity and acetylcholine (ACh) levels. NAC pretreatment significantly reversed the elevated MDA, AChE and the reduced GSH, ChAT and ACh in the Abeta-model mice. The results of the present study suggest the potential usage of the neuroprotective action of NAC on AD.

Lipid peroxidation in brain during aging in the senescence-accelerated mouse (SAM)

Accumulation of toxic amyloid-beta (Abeta)-peptide is suggested to cause oxidative stress in Alzheimer's disease (AD) brain, and decrease the content of polyunsaturated fatty acids (PUFA) in neuronal membrane lipids. The senescence accelerated prone mice (SAMP8) have age-related increases in the level of hippocampal Abeta-peptide, learning and memory deficits, and a shorter lifespan than their controls. The effects of age-related oxidative damage on PUFA content in membrane phospholipids (PL), and alpha-tocopherol concentration were investigated in hippocampus and amygdala of 2-, 4-, 12-, and 18-month-old SAMP8 mice. In comparison to the younger SAMP8 mice, the hippocampus of the 12-month-old mice contained lower proportions of docosahexaenoic acid (DHA) in phosphatidylserine (PS) and phosphatidylinositol (PI), and higher proportions of arachidonic acid (AA) in PS. Their amygdala contained a lower proportion of AA in phosphatidylcholine (PC). In the hippocampus of the oldest age group, the proportions of DHA in PS, and AA in PC and PI were higher than in the younger age groups. At 2 months of age, the amygdala contained a higher concentration of alpha-tocopherol than the hippocampus, but this difference between the two brain regions was lost with aging. The oldest age group contained the highest concentration of alpha-tocopherol, indicating a protection against oxidative damage of PUFA in brain membrane phospholipids.

Chemokines regulate the migration of neural progenitors to sites of neuroinflammation

Many studies have shown that transplanted or endogenous neural progenitor cells will migrate toward damaged areas of the brain. However, the mechanism underlying this effect is not clear. Here we report that, using hippocampal slice cultures, grafted neural progenitor cells (NPs) migrate toward areas of neuroinflammation and that chemokines are a major regulator of this process. Migration of NPs was observed after injecting an inflammatory stimulus into the area of the fimbria and transplanting enhanced green fluorescent protein (EGFP)-labeled NPs into the dentate gyrus of cultured hippocampal slices. Three to 7 d after transplantation, EGFP-NPs in control slices showed little tendency to migrate and had differentiated into neurons and glia. In contrast, in slices injected with inflammatory stimuli, EGFP-NPs migrated toward the site of the injection. NPs in these slices also survived less well. The inflammatory stimuli used were a combination of the cytokines tumor necrosis factor-alpha and interferon-gamma, the bacterial toxin lipopolysaccharide, the human immunodeficiency virus-1 coat protein glycoprotein 120, or a beta-amyloid-expressing adenovirus. We showed that these inflammatory stimuli increased the synthesis of numerous chemokines and cytokines by hippocampal slices. When EGFP-NPs from CC chemokine receptor CCR2 knock-out mice were transplanted into slices, they exhibited little migration toward sites of inflammation. Similarly, wild-type EGFP-NPs exhibited little migration toward inflammatory sites when transplanted into slices prepared from monocyte chemoattractant protein-1 (MCP-1) knock-out mice. These data indicate that factors secreted by sites of neuroinflammation are attractive to neural progenitors and suggest that chemokines such as MCP-1 play an important role in this process.

System Xc- and apolipoprotein E expressed by microglia have opposite effects on the neurotoxicity of amyloid-beta peptide 1-40

Because senile plaques in Alzheimer's disease (AD) contain reactive microglia in addition to potentially neurotoxic aggregates of amyloid-beta (Abeta), we examined the influence of microglia on the viability of rodent neurons in culture exposed to aggregated Abeta 1-40. Microglia enhanced the toxicity of Abeta by releasing glutamate through the cystine-glutamate antiporter system Xc-. This may be relevant to Abeta toxicity in AD, because the system Xc(-)-specific xCT gene is expressed not only in cultured microglia but also in reactive microglia within or surrounding amyloid plaques in transgenic mice expressing mutant human amyloid precursor protein or in wild-type mice injected with Abeta. Inhibition of NMDA receptors or system Xc- prevented the microglia-enhanced neurotoxicity of Abeta but also unmasked a neuroprotective effect of microglia mediated by microglial secretion of apolipoprotein E (apoE) in the culture medium. Immunodepletion of apoE or targeted inactivation of the apoE gene in microglia abrogated neuroprotection by microglial conditioned medium, whereas supplementation by human apoE isoforms restored protection, which was potentiated by the presence of microglia-derived cofactors. These results suggest that inhibition of microglial system Xc- might be of therapeutic value in the treatment of AD. Its inhibition not only prevents glutamate excitotoxicity but also facilitates neuroprotection by apoE.

Beta-amyloid-stimulated microglia induce neuron death via synergistic stimulation of tumor necrosis factor alpha and NMDA receptors

Although abundant reactive microglia are found associated with beta-amyloid (Abeta) plaques in Alzheimer's disease (AD) brains, their contribution to cell loss remains speculative. A variety of studies have documented the ability of Abeta fibrils to directly stimulate microglia in vitro to assume a neurotoxic phenotype characterized by secretion of a plethora of proinflammatory molecules. Collectively, these data suggest that activated microglia play a direct role in contributing to neuron death in AD rather than simply a role in clearance after plaque deposition. Although it is clear the Abeta-stimulated microglia acutely secrete toxic oxidizing species, the identity of longer-lived neurotoxic agents remains less defined. We used Abeta-stimulated conditioned media from primary mouse microglia to identify more stable neurotoxic secretions. The NMDA receptor antagonists memantine and 2-amino-5-phosphopetanoic acid as well as soluble tumor necrosis factor alpha (TNFalpha) receptor protect neurons from microglial-conditioned media-dependent death, implicating the excitatory neurotransmitter glutamate and the proinflammatory cytokine TNFalpha as effectors of microglial-stimulated death. Neuron death occurs in an oxidative damage-dependent manner, requiring activity of inducible nitric oxide synthase. Toxicity results from coincident stimulation of the TNFalpha and NMDA receptors, because stimulations of either alone are insufficient to initiate cell death. These findings suggest the hypothesis that AD brains provide the appropriate microglial-mediated inflammatory environment for TNFalpha and glutamate to synergistically stimulate toxic activation of their respective signaling pathways in neurons as a contributing mechanism of cell death.

Rho-mediated regulation of tight junctions during monocyte migration across the blood-brain barrier in HIV-1 encephalitis (HIVE)

The blood-brain barrier (BBB) is compromised during progressive HIV-1 infection, but how this occurs is incompletely understood. We studied the integrity of tight junctions (TJs) of brain microvascular endothelial cells (BMVECs) in an in vitro BBB system and in human brain tissues with HIV-1 encephalitis (HIVE). A downregulation of TJ proteins, claudin-5 and occludin, paralleled monocyte migration into the brain during HIVE. Because small G proteins (such as Rho) can play a role in BMVEC TJ assembly, an artificial BBB system explored the relationship among TJs, Rho/Rho kinase (RhoK) activation, and transendothelial monocyte migration. Coculture of monocytes with endothelial cells led to Rho activation and phosphorylation of TJ proteins. Rho and RhoK inhibitors blocked migration of infected and uninfected monocytes. The RhoK inhibitor protected BBB integrity and reversed occludin/claudin-5 phosphorylation associated with monocyte migration. BMVEC transfection with a constitutively active mutant of RhoK led to dislocation of occludin from the membrane and loss of BMVEC cell contacts. When dominant-negative RhoK-transfected BMVECs were used in BBB constructs, monocyte migration was reduced by 84%. Thus, loss of TJ integrity was associated with Rho activation caused by monocyte brain migration, suggesting that Rho/RhoK activation in BMVECs could be an underlying cause of BBB impairment during HIVE.

A key role for the microglial NADPH oxidase in APP-dependent killing of neurons

Reactive oxygen species (ROS) and deposition of cleaved products of amyloid precursor protein (APP) are thought to contribute to neuronal loss observed in Alzheimer's disease (AD). The relationship between these factors was studied in a neuroblastoma and microglia co-culture system. Overexpression of wild-type APP (APP-wt) or APP with three mutations typical of familial AD (APP-3m) in SH-SY5Y neuroblastoma cells did not directly alter their morphology, growth rate, cell cycle or H(2)O(2) sensitivity. In a co-culture of APP-wt neuroblastoma cells with microglia, microglial cells generated ROS and neuronal cells died. The cell death was more pronounced in APP-3m-expressing neurons. Neuroblastoma cell death was attenuated by ROS-scavengers and was dose-dependently inhibited by the NADPH oxidase inhibitor diphenyleneiodonium chloride (DPI). Macrophage cell lines behaved similarly to microglia in the co-culture model. However, a macrophage cell line deficient in the NADPH oxidase subunit, gp91phox, failed to kill neurons. These results suggest that APP-dependent microglia activation and subsequent ROS generation by the phagocyte NADPH oxidase play a crucial role in neuronal killing in a cellular model of AD.

Overexpression of monocyte chemotactic protein-1/CCL2 in beta-amyloid precursor protein transgenic mice show accelerated diffuse beta-amyloid deposition

Microglia accumulation at the site of amyloid plaques is a strong indication that microglia play a major role in Alzheimer's disease pathogenesis. However, how microglia affect amyloid-beta peptide (Abeta) deposition remains poorly understood. To address this question, we developed a novel bigenic mouse that overexpresses both amyloid precursor protein (APP) and monocyte chemotactic protein-1 (MCP-1; CCL2 in systematic nomenclature). CCL2 expression, driven by the glial fibrillary acidic protein promoter, induced mononuclear phagocyte (MP; monocyte-derived macrophage and microglial) accumulation in the brain. When APP/CCL2 transgenic mice were compared to APP mice, a fivefold increase in Abeta deposition was present despite increased MP accumulation around hippocampal and cortical amyloid plaques. Levels of full-length APP, its C-terminal fragment, and Abeta-degrading enzymes (insulin-degrading enzyme and neprilysin) in APP/CCL2 and APP mice were indistinguishable. Sodium dodecyl sulfate-insoluble Abeta (an indicator of fibrillar Abeta) was increased in APP/CCL2 mice at 5 months of age. Apolipoprotein E, which enhances Abeta deposition, was also increased (2.2-fold) in aged APP/CCL2 as compared to APP mice. We propose that although CCL2 stimulates MP accumulation, it increases Abeta deposition by reducing Abeta clearance through increased apolipoprotein E expression. Understanding the mechanisms underlying these events could be used to modulate microglial function in Alzheimer's disease and positively affect disease outcomes.

In vivo regulation of GSK3 phosphorylation by cholinergic and NMDA receptors

Glycogen synthase kinase-3 (GSK3), which is inhibited by serine-phosphorylation, is involved in the neuropathology of Alzheimer's disease (AD). We tested if the two therapeutic strategies used for AD, inhibition of acetylcholinesterase and of N-methyl-D-aspartate (NMDA) receptors, modulate the phosphorylation state of the two isoforms of GSK3 in mouse brain. Large, rapid increases in the levels of phospho-Ser21-GSK3alpha and phospho-Ser9-GSK3beta occurred in mouse hippocampus, cerebral cortex, and striatum after treatment of mice with the muscarinic agonist pilocarpine or the acetylcholinesterase inhibitor physostigmine. Treatment with memantine, an NMDA receptor antagonist, also increased the serine-phosphorylation of both GSK3 isoforms in mouse brain. Co-administration of physostigmine and memantine increased serine-phosphorylated GSK3 levels equally to that achieved by either agent alone, indicating that the actions of these two drugs converge on overlapping pools of GSK3. Thus, drugs in each class of therapeutic agents used for AD have the common property of increasing the regulatory serine-phosphorylation of GSK3 within common pools of the enzyme.

Secreted β-amyloid precursor protein activates microglia via JNK and p38-MAPK

Reactive microglia are thought to play a role in the pathogenesis of Alzheimer's disease (AD) and are localized to the senile plaques that are associated with cognitive decline. The beta-amyloid precursor protein (betaAPP) is over-expressed in the dystrophic neurites near such plaques, and secreted forms of betaAPP (sAPPalpha) activate inflammatory responses in microglia. To characterize the mechanisms by which sAPPalpha activates microglia, we assayed its effects on MAP kinases, including c-Jun N-terminal kinases (JNK), extracellular signal-regulated protein kinases (ERK), and p38-MAPK. sAPPalpha was found to rapidly activate JNKs, ERKs and p38-MAPK in a dose-dependent manner. The JNK inhibitor SP600125 and the p38 inhibitor SB203580 independently reduced both nitrite accumulation and induction of inflammatory nitric oxide synthase (iNOS). By contrast, inhibition of the ERK pathway with U0126 did not appreciably affect either outcome measure. These findings suggest that sAPP activates the ERK, JNK and p38 classes of MAP kinases but that only JNK and p38-MAPK are critical for activation of microglia by sAPPalpha, a process that compromises neuronal function and survival.

Activation of NR1a/NR2B receptors by soluble factors from APP-stimulated monocyte-derived macrophages: implications for the pathogenesis of Alzheimer's disease

Amyloid-beta peptide (Abeta), the major component of amyloid plaques, can activate brain mononuclear phagocytes (MP; macrophages and microglia), leading to their secretion of neurotoxins. Recent studies strongly suggest that MP-mediated neurotoxicity plays an important role in the pathogenesis of Alzheimer's disease (AD). To further explore this notion, human monocyte-derived macrophages (MDM) were stimulated with naturally secreted alpha-processing soluble amyloid precursor protein/p3 (alphaAPPs/p3) or beta-processing APP/Abeta (betaAPPs/Abeta). MDM conditioned media (MCM) was recovered and tested for its ability to activate recombinant N-methyl-d-aspartate (NMDA) receptor subtype NR1a/NR2B expressed in Xenopus oocytes. Pressure ejection of alphaAPPs/p3- and betaAPPs/Abeta-stimulated MCM produced inward currents of 59.5 +/- 8.9 nA (mean +/- S.E.M., n = 31) and 111.1 +/- 21.0 nA (n = 42) in NR1a/NR2B-expressing oocytes, respectively. The MCM-induced currents were concentration dependent and blocked by 50 microM of the NMDA receptor antagonist 2-amino-5-phosphnovalerate, but not by a non-NMDA receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (20 microM). The alphaAPPs/p3- and betaAPPs/Abeta-stimulated MCM placed in non-injected oocytes failed to generate inward current. These results demonstrate that APPs/Abeta-stimulated MCM directly activate NMDA receptor subtypes relevant in the pathogenesis of AD.

The immune modulator FTY720 targets sphingosine–kinase‐dependent migration of human monocytes in response to amyloid beta‐protein and its precursor

Accumulation of inflammatory mononuclear phagocytes in Alzheimer's senile plaques, a hallmark of the innate immune response to beta-amyloid fibrils, can initiate and propagate neurodegeneration characteristic of Alzheimer's disease. Phagocytes migrate toward amyloid beta-protein involving formyl peptide receptor like-1-dependent signaling. Using human peripheral blood monocytes in Boyden chamber micropore filter assays, we show that the amyloid beta-protein- and amyloid beta-precursor protein-induced migration was abrogated by dimethylsphingosine, a sphingosine kinase inhibitor. Amyloid beta-protein stimulated in monocytes the gene expression for sphingosine-1-phosphate receptors 2 and 5, but not 1, 3, and 4. FTY720 that acts as a sphingosine-1-phosphate receptor agonist after endogenous phosphorylation by sphingosine kinase, as well as various neuropeptides that are known to be monocyte chemoattractants, dose-dependently inhibited amyloid beta-protein-induced migration. These data demonstrate that the migratory effects of beta-amyloid in human monocytes involve spingosine-1-phosphate signaling. Whereas endogenous neuropeptides may arrest and activate monocytes at sites of high beta-amyloid concentrations, interference with the amyloid beta-protein-dependent sphingosine-1-phosphate pathway in monocytes by FTY720, a novel immunomodulatory drug, suggests that FTY720 may be efficacious in beta-amyloid-related inflammatory diseases.

Molecular characterization of a putative antiretroviral transcriptional factor, OTK18

Elucidation of the factors involved in host defense against human immunodeficiency viral infection remains pivotal if viral control may be achieved. Toward these ends, we investigated the function of a putative antiretroviral factor, OTK18, isolated by differential display of mRNA from HIV type 1-infected primary human monocyte-derived macrophages. Molecular and immunohistochemical analyses showed that the OTK18 nucleotide sequence contains 13 adjacent C(2)H(2)-type zinc finger motifs, a Krüppel-associated box, and is localized to both cytosol and nucleus. Mutational analyses revealed that both the Krüppel-associated box and zinc finger regions of OTK18 are responsible for the transcriptional suppressive activities of this gene. OTK18 was copiously expressed in macrophages following HIV type I infection and diminished progeny virion production. A mechanism for this antiretroviral activity was by suppression of HIV type 1 Tat-induced viral long terminal repeat promoter activity. Our findings suggest that one possible function of OTK18 is as a HIV type 1-inducible transcriptional suppressor.