Integrin Mac-1 and beta-amyloid in microglial release of nitric oxide

Metformin improved memory impairment caused by chronic ethanol consumption during adolescent to adult period of rats: Role of oxidative stress and neuroinflammation

OBJECTIVE

Adolescence is a crucial time for brain maturation. We investigated the protective effects of metformin (Met) on behavioral changes, oxidative stress, tumor necrosis factor alpha (TNF-α) and nitrite in adulthood induced by ethanol (Eth) consumption during adolescent to adult period of rats.

MATERIALS AND METHODS

The adolescence male rats (21 days old) were treated as: 1) Control, 2) Eth (Eth in drinking water (20 %)), 3-5) Eth-Met50, 100 and 150 mg/kg (Eth in drinking water and Met (50, 100, or 150 mg/kg). After 5 weeks treatment, Morris water maze (MMW) and passive avoidance (PA) tests were done.

RESULTS

The latency in the MWM test was higher and the latency to enter the dark chamber in the PA test was lower in the Eth group than in control. In Eth-Met100 and 150 groups, they were less than the Eth group. Malondialdehyde (MDA) and nitrite concentration in the hippocampus and cortex of the Eth group were higher than the control group. The thiol content and catalase and superoxide dismutase (SOD) activities in hippocampal and cortical tissues of the Eth group reduced compared to the control group. TNF-α was higher in hippocampal tissues of Eth group animals. Met reversed all of these effects.

CONCLUSION

Our findings showed that the protective effects of Met against chronic Eth consumption induced learning and memory impairment were accompanied by decreasing of TNF-a, nitrite and oxidative stress in adolescent rats.

A Computational Exploration of the Molecular Network Associated to Neuroinflammation in Alzheimer's Disease

Neuroinflammation, as defined by the presence of classically activated microglia, is thought to play a key role in numerous neurodegenerative disorders such as Alzheimer’s disease. While modulating neuroinflammation could prove beneficial against neurodegeneration, identifying its most relevant biological processes and pharmacological targets remains highly challenging. In the present study, we combined text-mining, functional enrichment and protein-level functional interaction analyses to 1) identify the proteins significantly associated to neuroinflammation in Alzheimer’s disease over the scientific literature, 2) distinguish the key proteins most likely to control the neuroinflammatory processes significantly associated to Alzheimer's disease, 3) identify their regulatory microRNAs among those dysregulated in Alzheimer's disease and 4) assess their pharmacological targetability. 94 proteins were found to be significantly associated to neuroinflammation in Alzheimer’s disease over the scientific literature and IL4, IL10 and IL13 signaling as well as TLR-mediated MyD88- and TRAF6-dependent responses were their most significantly enriched biological processes. IL10, TLR4, IL6, AKT1, CRP, IL4, CXCL8, TNF-alpha, ITGAM, CCL2 and NOS3 were identified as the most potent regulators of the functional interaction network formed by these immune processes. These key proteins were indexed to be regulated by 63 microRNAs dysregulated in Alzheimer's disease, 13 long non-coding RNAs and targetable by 55 small molecules and 8 protein-based therapeutics. In conclusion, our study identifies eleven key proteins with the highest ability to control neuroinflammatory processes significantly associated to Alzheimer’s disease, as well as pharmacological compounds with single or pleiotropic actions acting on them. As such, it may facilitate the prioritization of diagnostic and target-engagement biomarkers as well as the development of effective therapeutic strategies against neuroinflammation in Alzheimer’s disease.

The Promiscuous Profile of Complement Receptor 3 in Ligand Binding, Immune Modulation, and Pathophysiology

The β2-integrin receptor family has a broad spectrum of physiological functions ranging from leukocyte adhesion, cell migration, activation, and communication to the phagocytic uptake of cells and particles. Among the members of this family, complement receptor 3 (CR3; CD11b/CD18, Mac-1, αMβ2) is particularly promiscuous in its functional profile and ligand selectivity. There are close to 100 reported structurally unrelated ligands for CR3, and while many ligands appear to cluster at the αMI domain, molecular details about binding modes remain largely elusive. The versatility of CR3 is reflected in its functional portfolio, which includes prominent roles in the removal of invaders and cell debris, induction of tolerance and synaptic pruning, and involvement in the pathogenesis of numerous autoimmune and chronic inflammatory pathologies. While CR3 is an interesting therapeutic target for immune modulation due to these known pathophysiological associations, drug development efforts are limited by concerns of potential interference with host defense functions and, most importantly, an insufficient molecular understanding of the interplay between ligand binding and functional impact. Here, we provide a systematic summary of the various interaction partners of CR3 with a focus on binding mechanisms and functional implications. We also discuss the roles of CR3 as an immune receptor in health and disease, as an activation marker in research and diagnostics, and as a therapeutic target.

The effects of aminoguanidine on hippocampal cytokines, amyloid beta, brain-derived neurotrophic factor, memory and oxidative stress status in chronically lipopolysaccharide-treated rats

INTRODUCTION

In the present study, the effects of aminoguanidine (AMG) on hippocampal cytokines, amyloid beta (Aβ), brain-derived neurotrophic factor, oxidative stress status and memory in chronically lipopolysaccharide (LPS) treated rats were investigated.

METHODS

The rats were divided into five groups and were treated: (1) Control (Saline), (2) LPS (1 mg/kg), (3-5) LPS- AMG50, LPS-AMG100, and LPS-AMG150 (AMG 50, 100 and 150 mg/kg 30 min before LPS injection). The treatment started five weeks prior to the behavioral experiments and continued during the behavioral tests (LPS injection two hours before each behavioral evaluation). Finally, the tissue was removed for biochemical measurements.

RESULTS

The escape latency in Morris water maze test and the latency to enter the dark compartment in passive avoidance test in LPS group were significantly greater than the control group (P < 0.001), while, in LPS-AMG 100 and LPS-AMG150 groups they were less than LPS group (P < 0.001). Malondialdehyde (MDA), NO metabolites of hippocampal and cortical tissues and interleukin-6 (IL-6), Aβ and tumor necrosis factor-α (TNFα) concentration in the hippocampus of LPS group were higher than control group (P < 0.001-P < 0.05). However, in LPS-AMG 100 and LPS-AMG150 group they were lower than LPS group (P < 0.001-P < 0.05). The thiol content and the activities of catalase (CAT) and superoxide dismutase (SOD) in both cortical and hippocampal tissues of LPS group were reduced compared to the control group (P < 0.001-P < 0.05). These factors enhanced in LPS-AMG 100 and LPS-AMG150 groups compared to LPS (P < 0.001-P < 0.05). The hippocampal content of brain-derived neurotrophic factor (BDNF) in LPS group was significantly lower compared to the control group (P < 0.001). All treated groups had higher BDNF content in comparison to LPS group (P < 0.01-P < 0.001).

CONCLUSION

The findings indicated that the protective effects of AMG against LPS-induced memory were accompanied by decreasing of inflammatory cytokines, Aβ, oxidative stress and increasing of anti-inflammatory mediators and BDNF.

The impairment of learning and memory and synaptic loss in mouse after chronic nitrite exposure

The objective of this study is to understand the impairment of learning and memory in mouse after chronic nitrite exposure. The animal model of nitrite exposure in mouse was created with the daily intubation of nitrite in adult healthy male mice for 3 months. Furthermore, the mouse's learning and memory abilities were tested with Morris water maze, and the expression of Synaptophysin and γ-Synuclein was visualized with immunocytochemistry and Western blot. Our results showed that nitrite exposure significantly prolonged the escape latency period (ELP) and decreased the values of the frequency across platform (FAP) as well as the accumulative time in target quadrant (ATITQ) compared to control, in dose-dependent manner. In addition, after nitrite exposure, synaptophysin (SYN) positive buttons in the visual cortex was reduced, in contrast the increase of γ-synuclein positive cells. The results above were supported by Western blot as well. We conclude that nitrite exposure could lead to a decline in mice's learning and memory. The overexpression of γ-synuclein contributed to the synaptic loss, which is most likely the cause of learning and memory impairment. © 2015 Wiley Periodicals, Inc. Environ Toxicol 31: 1720-1730, 2016.

β-amyloid, microglia, and the inflammasome in Alzheimer's disease

There is extensive evidence that accumulation of mononuclear phagocytes including microglial cells, monocytes, and macrophages at sites of β-amyloid (Aβ) deposition in the brain is an important pathological feature of Alzheimer's disease (AD) and related animal models, and the concentration of these cells clustered around Aβ deposits is several folds higher than in neighboring areas of the brain [1-5]. Microglial cells phagocytose and clear debris, pathogens, and toxins, but they can also be activated to produce inflammatory cytokines, chemokines, and neurotoxins [6]. Over the past decade, the roles of microglial cells in AD have begun to be clarified, and we proposed that these cells play a dichotomous role in the pathogenesis of AD [4, 6-11]. Microglial cells are able to clear soluble and fibrillar Aβ, but continued interactions of these cells with Aβ can lead to an inflammatory response resulting in neurotoxicity. Inflammasomes are inducible high molecular weight protein complexes that are involved in many inflammatory pathological processes. Recently, Aβ was found to activate the NLRP3 inflammasome in microglial cells in vitro and in vivo thereby defining a novel pathway that could lead to progression of AD [12-14]. In this manuscript, we review possible steps leading to Aβ-induced inflammasome activation and discuss how this could contribute to the pathogenesis of AD.

Battling Alzheimer's Disease: Targeting SUMOylation-Mediated Pathways

SUMO (small ubiquitin-like modifier) conjugation is a critically important control process in all eukaryotic cells, because it acts as a biochemical switch and regulates the function of hundreds of proteins in many different pathways. Although the diverse functional consequences and molecular targets of SUMOylation remain largely unknown, SUMOylation is becoming increasingly implicated in the pathophysiology of Alzheimer's disease (AD). Apart from the central SUMO-modified disease-associated proteins, such as amyloid precursor protein, amyloid β, and tau, SUMOylation also regulates several other processes underlying AD. These are involved in inflammation, mitochondrial dynamics, synaptic transmission and plasticity, as well as in protective responses to cell stress. Herein, we review current reports on the involvement of SUMOylation in AD, and present an overview of potential SUMO targets and pathways underlying AD pathogenesis.

NADPH oxidase- and mitochondria-derived reactive oxygen species in proinflammatory microglial activation: a bipartisan affair?

Microglia are the resident immune cells of the brain and play major roles in central nervous system development, maintenance, and disease. Brain insults cause microglia to proliferate, migrate, and transform into one or more activated states. Classical M1 activation triggers the production of proinflammatory factors such as tumor necrosis factor-α, interleukin-1β (IL-1β), nitric oxide, and reactive oxygen species (ROS), which, in excess, can exacerbate brain injury. The mechanisms underlying microglial activation are not fully understood, yet reactive oxygen species are increasingly implicated as mediators of microglial activation. In this review, we highlight studies linking reactive oxygen species, in particular hydrogen peroxide derived from NADPH oxidase-generated superoxide, to the classical activation of microglia. In addition, we critically evaluate controversial evidence suggesting a specific role for mitochondrial reactive oxygen species in the activation of the NLRP3 inflammasome, a multiprotein complex that mediates the production of IL-1β and IL-18. Finally, the limitations of common techniques used to implicate mitochondrial ROS in microglial and inflammasome activation, such as the use of the mitochondrially targeted ROS indicator MitoSOX and the mitochondrially targeted antioxidant MitoTEMPO, are also discussed.

β1-integrin-dependent migration of microglia in response to neuron-released α-synuclein

Chronic neuroinflammation is an integral pathological feature of major neurodegenerative diseases. The recruitment of microglia to affected brain regions and the activation of these cells are the major events leading to disease-associated neuroinflammation. In a previous study, we showed that neuron-released α-synuclein can activate microglia through activating the Toll-like receptor 2 (TLR2) pathway, resulting in proinflammatory responses. However, it is not clear whether other signaling pathways are involved in the migration and activation of microglia in response to neuron-released α-synuclein. In the current study, we demonstrated that TLR2 activation is not sufficient for all of the changes manifested by microglia in response to neuron-released α-synuclein. Specifically, the migration of and morphological changes in microglia, triggered by neuron-released α-synuclein, did not require the activation of TLR2, whereas increased proliferation and production of cytokines were strictly under the control of TLR2. Construction of a hypothetical signaling network using computational tools and experimental validation with various peptide inhibitors showed that β1-integrin was necessary for both the morphological changes and the migration. However, neither proliferation nor cytokine production by microglia was dependent on the activation of β1-integrin. These results suggest that β1-integrin signaling is specifically responsible for the recruitment of microglia to the disease-affected brain regions, where neurons most likely release relatively high levels of α-synuclein.

TRPs as chemosensors (ROS, RNS, RCS, gasotransmitters)

The transient receptor potential (trp) gene superfamily encodes TRP proteins that act as multimodal sensor cation channels for a wide variety of stimuli from outside and inside the cell. Upon chemical or physical stimulation of cells, TRP channels transduce electrical and/or Ca(2+) signals via their cation channel activities. These functional features of TRP channels allow the body to react and adapt to different forms of environmental changes. Indeed, members of one class of TRP channels have emerged as sensors of reactive oxygen species (ROS), reactive nitrogen species (RNS), reactive carbonyl species (RCS), and gaseous messenger molecules including molecular oxygen (O2), hydrogen sulfide (H2S), and carbon dioxide (CO2). Hydrogen peroxide (H2O2), an ROS, triggers the production of ADP-ribose, which binds and activates TRPM2. In addition to TRPM2, TRPC5, TRPV1, and TRPA1 are also activated by H2O2 via modification of cysteine (Cys) free sulfhydryl groups. Nitric oxide (NO), a vasoactive gaseous molecule, regulates TRP channels directly via Cys S-nitrosylation or indirectly via cyclic GMP (cGMP)/protein kinase G (PKG)-dependent phosphorylation. Anoxia induced by O2-glucose deprivation and severe hypoxia activates TRPM7 and TRPC6, respectively, whereas TRPA1 serves as a sensor of mild hypoxia and hyperoxia in vagal and sensory neurons. TRPA1 also detects other gaseous molecules, such as hydrogen sulfide (H2S) and carbon dioxide (CO2). In this review, we highlight our current knowledge of TRP channels as chemosensors for ROS, RNS, RCS, and gaseous molecules and discuss their functional impacts on physiological and pathological events.

Inflammatory process in Alzheimer's Disease

Alzheimer Disease (AD) is a neurodegenerative disorder and the most common form of dementia. Histopathologically is characterized by the presence of two major hallmarks, the intracellular neurofibrillary tangles (NFTs) and extracellular neuritic plaques (NPs) surrounded by activated astrocytes and microglia. NFTs consist of paired helical filaments of truncated tau protein that is abnormally hyperphosphorylated. The main component in the NP is the amyloid-β peptide (Aβ), a small fragment of 40–42 amino acids with a molecular weight of 4 kD. It has been proposed that the amyloid aggregates and microglia activation are able to favor the neurodegenerative process observed in AD patients. However, the role of inflammation in AD is controversial, because in early stages the inflammation could have a beneficial role in the pathology, since it has been thought that the microglia and astrocytes activated could be involved in Aβ clearance. Nevertheless the chronic activation of the microglia has been related with an increase of Aβ and possibly with tau phosphorylation. Studies in AD brains have shown an upregulation of complement molecules, pro-inflammatory cytokines, acute phase reactants and other inflammatory mediators that could contribute with the neurodegenerative process. Clinical trials and animal models with non-steroidal anti-inflammatory drugs (NSAIDs) indicate that these drugs may decrease the risk of developing AD and apparently reduce Aβ deposition. Finally, further studies are needed to determine whether treatment with anti-inflammatory strategies, may decrease the neurodegenerative process that affects these patients.

HMGB1 acts on microglia Mac1 to mediate chronic neuroinflammation that drives progressive neurodegeneration

What drives the gradual degeneration of dopamine neurons in Parkinson's disease (PD), the second most common neurodegenerative disease, remains elusive. Here, we demonstrated, for the first time, that persistent neuroinflammation was indispensible for such a neurodegenerative process. 1-Methyl-4-phenylpyridinium, lipopolysaccharide (LPS), and rotenone, three toxins often used to create PD models, produced acute but nonprogressive neurotoxicity in neuron-enriched cultures. In the presence of microglia (brain immune cells), these toxins induced progressive dopaminergic neurodegeneration. More importantly, such neurodegeneration was prevented by removing activated microglia. Collectively, chronic neuroinflammation may be a driving force of progressive dopaminergic neurodegeneration. Conversely, ongoing neurodegeneration sustained microglial activation. Microglial activation persisted only in the presence of neuronal damage in LPS-treated neuron-glia cultures but not in LPS-treated mixed-glia cultures. Thus, activated microglia and damaged neurons formed a vicious cycle mediating chronic, progressive neurodegeneration. Mechanistic studies indicated that HMGB1 (high-mobility group box 1), released from inflamed microglia and/or degenerating neurons, bound to microglial Mac1 (macrophage antigen complex 1) and activated nuclear factor-κB pathway and NADPH oxidase to stimulate production of multiple inflammatory and neurotoxic factors. The treatment of microglia with HMGB1 led to membrane translocation of p47(phox) (a cytosolic subunit of NADPH oxidase) and consequent superoxide release, which required the presence of Mac1. Neutralization of HMGB1 and genetic ablation of Mac1 and gp91(phox) (the catalytic submit of NADPH oxidase) blocked the progressive neurodegeneration. Our findings indicated that HMGB1-Mac1-NADPH oxidase signaling axis bridged chronic neuroinflammation and progressive dopaminergic neurodegeneration, thus identifying a mechanistic basis for chronic PD progression.

Role of microglial redox balance in modulation of neuroinflammation

PURPOSE OF REVIEW

This review discusses some of the emerging concepts on how modulation of redox homeostasis in microglia is crucial to restore its inactive state and modulate inflammation in neurologic diseases.

RECENT FINDINGS

Reactive oxygen species generated by microglia help to eliminate pathogens in the extracellular milieu but also act on microglia itself, altering the intracellular redox balance and functioning as second messengers in induction of proinflammatory genes. Recent findings indicate that restoration of redox balance may be determinant in driving microglia back to the resting state. Thus, deficiency of the transcription factor NF-E2-related factor-2 (Nrf2), guardian of redox homeostasis, results in exacerbated inflammatory response to neurotoxins whereas inducers of Nrf2 and its target heme oxygenase-1 downmodulate inflammation.

SUMMARY

New available information indicates that downregulation of microglia is a matter closely correlated with control of oxidative stress in this cell type and points to Nrf2 as a new therapeutic target for modulation of inflammation in neurodegenerative diseases.

Macrophage antigen complex-1 mediates reactive microgliosis and progressive dopaminergic neurodegeneration in the MPTP model of Parkinson's disease

Neuronal death is known to trigger reactive microgliosis. However, little is known regarding the manner by which microglia are activated by injured neurons and how microgliosis participates in neurodegeneration. In this study we delineate the critical role of macrophage Ag complex-1 (MAC1), a member of the beta(2) integrin family, in mediating reactive microgliosis and promoting dopaminergic (DAergic) neurodegeneration in the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) model of Parkinson's disease. MAC1 deficiency greatly attenuated the DAergic neurodegeneration induced by MPTP or 1-methyl-4-phenyl-pyridium iodide (MPP(+)) exposure both in vivo and in vitro, respectively. Reconstituted experiments created by adding microglia from MAC1(-/-) or MAC1(+/+) mice back to MAC1(+/+) neuron-enriched cultures showed that microglia with functional MAC1 expression was mandatory for microglia-enhanced neurotoxicity. Both in vivo and in vitro morphological and Western blot studies demonstrated that MPTP/MPP(+) produced less microglia activation in MAC1(-/-) mice than MAC1(+/+) mice. Further mechanistic studies revealed that a MPP(+)-mediated increase in superoxide production was reduced in MAC1(-/-) neuron-glia cultures compared with MAC1(+/+) cultures. The stunted production of superoxide in MAC1(-/-) microglia is likely linked to the lack of translocation of the cytosolic NADPH oxidase (PHOX) subunit (p47(phox)) to the membrane. In addition, the production of PGE(2) markedly decreased in neuron plus MAC1(-/-) microglia cocultures vs neuron plus MAC1(+/+) microglia cocultures. Taken together, these results demonstrate that MAC1 plays a critical role in MPTP/MPP(+)-induced reactive microgliosis and further support the hypothesis that reactive microgliosis is an essential step in the self-perpetuating cycle leading to progressive DAergic neurodegeneration observed in Parkinson's disease.

Why neurodegenerative diseases are progressive: uncontrolled inflammation drives disease progression

Neurodegenerative diseases are a group of chronic, progressive disorders characterized by the gradual loss of neurons in discrete areas of the central nervous system (CNS). The mechanism(s) underlying their progressive nature remains unknown but a timely and well-controlled inflammatory reaction is essential for the integrity and proper function of the CNS. Substantial evidence has documented a common inflammatory mechanism in various neurodegenerative diseases. We hypothesize that in the diseased CNS, interactions between damaged neurons and dysregulated, overactivated microglia create a vicious self-propagating cycle causing uncontrolled, prolonged inflammation that drives the chronic progression of neurodegenerative diseases. We further propose that dynamic modulation of this inflammatory reaction by interrupting the vicious cycle might become a disease-modifying therapeutic strategy for neurodegenerative diseases.

Transient receptor potential channels in Alzheimer's disease

Cognitive impairment and emotional disturbances in Alzheimer's disease (AD) result from the degeneration of synapses and neuronal death in the limbic system and associated regions of the cerebral cortex. An alteration in the proteolytic processing of the amyloid precursor protein (APP) results in increased production and accumulation of amyloid beta-peptide (Abeta) in the brain. Abeta can render neurons vulnerable to excitotoxicity and apoptosis by disruption of cellular Ca(2+) homeostasis and neurotoxic factors including reactive oxygen species (ROS), nitric oxide (NO), and cytokines. Many lines of evidence have suggested that transient receptor potential (TRP) channels consisting of six main subfamilies termed the TRPC (canonical), TRPV (vanilloid), TRPM (melastatin), TRPP (polycystin), TRPML (mucolipin), and TRPA (ankyrin) are involved in Ca(2+) homeostasis disruption. Thus, emerging evidence of the pathophysiological role of TRP channels has yielded promising candidates for molecular entities mediating Ca(2+) homeostasis disruption in AD. In this review, we focus on the TRP channels in AD and highlight some TRP "suspects" for which a role in AD can be anticipated. An understanding of the involvement of TRP channels in AD may lead to the development of new target therapies.

Dysregulation of Na+/K+ ATPase by amyloid in APP+PS1 transgenic mice

Background

The pathology of Alzheimer's disease (AD) is comprised of extracellular amyloid plaques, intracellular tau tangles, dystrophic neurites and neurodegeneration. The mechanisms by which these various pathological features arise are under intense investigation. Here, expanding upon pilot gene expression studies, we have further analyzed the relationship between Na+/K+ ATPase and amyloid using APP+PS1 transgenic mice, a model that develops amyloid plaques and memory deficits in the absence of tangle formation and neuronal or synaptic loss.

Results

We report that in addition to decreased mRNA expression, there was decreased overall Na+/K+ ATPase enzyme activity in the amyloid-containing hippocampi of the APP+PS1 mice (although not in the amyloid-free cerebellum). In addition, dual immunolabeling revealed an absence of Na+/K+ ATPase staining in a zone surrounding congophilic plaques that was occupied by dystrophic neurites. We also demonstrate that cerebral Na+/K+ ATPase activity can be directly inhibited by high concentrations of soluble Aβ.

Conclusions

The data suggest that the reductions in Na+/K+ ATPase activity in Alzheimer tissue may not be purely secondary to neuronal loss, but may results from direct effects of amyloid on this enzyme. This disruption of ion homeostasis and osmotic balance may interfere with normal electrotonic properties of dendrites, blocking intraneuronal signal processing, and contribute to neuritic dystrophia. These results suggest that therapies aimed at enhancing Na+/K+ ATPase activity in AD may improve symptoms and/or delay disease progression.

Tau and src family tyrosine kinases

The interaction between tau and src family non-receptor tyrosine kinases represents a new function for tau. Mediated by the proline-rich region of tau and the SH3 domain of fyn or src, this interaction has the potential to confer novel cellular activities for tau in the growth cone and in the membrane. The subsequent finding that tau is tyrosine phosphorylated has led to the observation that tau in neurofibrillary tangles is tyrosine phosphorylated. Therefore, a role for tyrosine kinases such as fyn in neuropathogenesis is predicted.

Nitric oxide pathways in Alzheimer's disease and other neurodegenerative dementias

Nitric oxide (NO) is an enzymatic product of nitric oxide synthase (NOS). NO has significant physiological functions and an increasing body of evidence suggests that NO pathways are implicated in a number of neurological disorders, including Alzheimer's disease (AD) and other neurodegenerative dementias. NO is continuously released by endothelial cells in the vascular system, whereas advanced age in the presence of vascular risk factor causes a decrease in cerebral blood flow, involving microvasculopathy with impaired NO release, which in turn results in regional metabolic dysfunction. This finding suggests that vascular pathology plays a crucial role in the pathogenesis of so-called neurodegenerative dementias. Inflammatory responses are commonly found in the brain under a variety of neurodegenerative dementias, including AD and dementia with Lewy bodies, in which up-regulation of NOS expression, suggesting overproduction of NO, is found in neurons and glia. NO is thought to be involved in such neuroinflammation due to its free radical properties, which compromise cellular integrity and viability via mitochondrial damage. Further studies to elucidate NO pathways in neurodegenerative dementias could lead to a better understanding of their pathogenesis and improved therapeutic strategies, and therefore are certainly warranted.

Microglia kill amyloid-β1-42 damaged neurons by a CD14-dependent process

Activated microglia are closely associated with neuronal damage in Alzheimer's disease. In the present study, neurons exposed to low concentrations of amyloid-beta1-42, a toxic fragment of the amyloid-beta protein, were killed by microglia in a process that required cell-cell contact. Pre-treating microglia with polyclonal antibodies to the CD14 protein, or treating neurons exposed to amyloid-beta1-42 with a CD14-IgG chimera, prevented the killing of amyloid-beta1-42 damaged neurons by microglia. Moreover, microglia from CD14 null mice failed to kill amyloid-beta1-42 damaged neurons. Increased neuronal survival was accompanied by a significant reduction in the production of interleukin-6 indicative of reduced microglial activation. These results indicate an important role for CD14 in the recognition and subsequent killing of amyloid-beta damaged neurons by microglia.

The LPS receptor (CD14) links innate immunity with Alzheimer's disease

To rapidly respond to invading microorganisms, humans call on their innate immune system. This occurs by microbe-detecting receptors, such as CD14, that activate immune cells to eliminate the pathogens. Here, we link the lipopolysaccharide receptor CD14 with Alzheimer's disease, a severe neurodegenerative disease resulting in dementia. We demonstrate that this key innate immunity receptor interacts with fibrils of Alzheimer amyloid peptide. Neutralization with antibodies against CD14 and genetic deficiency for this receptor significantly reduced amyloid peptide induced microglial activation and microglial toxicity. The observation of strongly enhanced microglial expression of the LPS receptor in brains of animal models of Alzheimer's disease indicates a clinical relevance of these findings. These data suggest that CD14 may significantly contribute to the overall neuroinflammatory response to amyloid peptide, highlighting the possibility that the enormous progress currently being made in the field of innate immunity could be extended to research on Alzheimer's disease.

Aberrant activation of focal adhesion proteins mediates fibrillar amyloid beta-induced neuronal dystrophy

Neuronal dystrophy is a pathological hallmark of Alzheimer's disease (AD) that is not observed in other neurodegenerative disorders that lack amyloid deposition. Treatment of cortical neurons with fibrillar amyloid beta (Abeta) peptides induces progressive neuritic dystrophy accompanied by a marked loss of synaptophysin immunoreactivity (Grace et al., 2002). Here, we report that fibrillar Abeta-induced neuronal dystrophy is mediated by the activation of focal adhesion (FA) proteins and the formation of aberrant FA structures adjacent to Abeta deposits. In the AD brain, activated FA proteins are observed associated with the majority of senile plaques. Clustered integrin receptors and activated paxillin (phosphorylated at Tyr-31) and focal adhesion kinase (phosphorylated at Tyr-297) are mainly detected in dystrophic neurites surrounding Abeta plaque cores, where they colocalize with hyperphosphorylated tau. Deletion experiments demonstrated that the presence of the LIM domains in the paxillin C terminus and the recruitment of the protein-Tyr phosphatase (PTP)-PEST to the FA complex are required for Abeta-induced neuronal dystrophy. Therefore, both paxillin and PTP-PEST appear to be critical elements in the generation of the dystrophic response. Paxillin is a scaffolding protein to which other FA proteins bind, leading to the formation of the FA contact and initiation of signaling cascades. PTP-PEST plays a key role in the dynamic regulation of focal adhesion contacts in response to extracellular cues. Thus, in the AD brain, fibrillar Abeta may induce neuronal dystrophy by triggering a maladaptive plastic response mediated by FA protein activation and tau hyperphosphorylation.

Association of factor H of the alternative pathway of complement with agrin and complement receptor 3 in the Alzheimer's disease brain

Factor H, a regulatory protein of the alternative pathway of complement (APC), is present in amyloid-beta (Abeta) plaques in Alzheimer's disease (AD). Abeta plaques also contain significant amounts of heparan sulfate proteoglycans (HSPGs), such as agrin, as well as numerous activated microglia expressing increased levels complement receptor 3 (CR3). Here, we show the colocalization of each of these molecules in the AD brain and the functional capacity for these molecules to bind to one another in vitro. We propose that CR3 receptors expressed by microglia are used for ligand binding to factor H bound to HSPGs and Abeta in plaques in the AD brain.

Identification of second messengers that induce expression of functional gap junctions in microglia cultured from newborn rats

The effect of several second messengers on the functional expression of gap junctions was investigated in primary cultures of newborn rat microglia. As previously reported, microglia cultured under resting conditions expressed low levels of the gap junction protein connexin 43, and exhibited little dye coupling. After treatment with 4bromo-A23187, a Ca(2+) ionophore, the incidence of dye coupling between microglia increased progressively over a 12-h period. Dye coupling was markedly reduced by gap junction blockers. Induction of dye coupling by 4bromo-A23187 was prevented by the addition of a synthetic peptide with the same sequence as a region of the extracellular loop 1 of connexin 43 (residues 53-66). The increase in dye coupling induced by 4bromo-A23187 was associated with increased connexin 43 mRNA and protein levels. Treatment of microglia with phorbol 12-myristate 13-acetate, an activator of protein kinase C, did not promote gap junctional communication in untreated microglia and reversed 4bromo-A23187-induced dye coupling. Thus, gap junctional communication between microglia can be regulated oppositely by calcium- and protein kinase C-dependent pathways. Activators of cGMP-dependent protein kinase (8bromo-cGMP) or protein kinase A (8bromo-cAMP) had no effect on untreated microglia or on 4bromo-A23187-induced dye coupling. Differential regulation of gap junctions by intracellular calcium concentration and protein kinase C activity may help to explain how various stimuli evoke differences in microglia responses, such as synthesis and secretion of cytokines and proteases.

Neuroprotective and neurorescuing effects of isoform-specific nitric oxide synthase inhibitors, nitric oxide scavenger, and antioxidant against beta-amyloid toxicity

Beta amyloid (Abeta) is implicated in Alzheimer's disease (AD). Abeta(1 - 42) (5, 10, or 20 microM) was able to increase NO release and decrease cellular viability in primary rat cortical mixed cultures. L-NOARG and SMTC (both at 10 or 100 microM) - type I NOS inhibitors - reduced cellular NO release in the absence of Abeta(1 - 42). At 100 microM, both drugs decreased cell viability. L-NIL (10 or 100 microM), and 1400W (1 or 5 microM) - type II NOS inhibitors - reduced NO release and improved viability when either drug was administered up to 4 h post Abeta(1 - 42) (10 microM) treatment. L-NOARG and SMTC (both at 10 or 100 microM) were only able to decrease NO release. Carboxy-PTIO or Trolox (both at 10 or 100 microM) - a NO scavenger and an antioxidant, respectively - increased viability when administered up to 1 h post Abeta(1 - 42) treatment. Either L-NIL (50 microM) or 1400W (3 microM) and Trolox (50 microM) showed synergistic actions. Peroxynitrite (100 or 200 microM) reduced cell viability. Viabilities were improved by L-NIL (100 microM), 1400W (5 microM), carboxy-PTIO (10 or 100 microM), and Trolox (10 or 100 microM). Hence, the data show that Abeta(1 - 42) induced NO release in neurons and glial cells, and that Abeta neurotoxicity is, at least in part, mediated by NO. NO concentration modulating compounds and antioxidant may have therapeutic importance in neurological disorders where oxidative stress is likely involved such as in AD.

Microglia in neurodegeneration: molecular aspects

Inflammatory events in the CNS are associated with injuries as well as with well-known chronic degenerative diseases, such as Multiple Sclerosis, Parkinson's, or Alzheimer's disease. Compared to inflammation in peripheral tissues, inflammation in brain appears to follow distinct pathways and time-courses, which likely has to do with a relatively strong immunosuppression in that organ. For this reason, it is of great importance to get insights into the molecular mechanism governing immune reactions in brain tissue. This task is hard to achieve in vivo, but can be approached by studying the major cell type responsible for brain inflammation, the microglia, in culture. Since these cells are the only professional antigen-presenting cells resident in brain parenchyma, molecular mechanisms of antigen presentation are being discussed first. After covering the expression and regulation of anti- and proinflammatory cytokines, induction and regulation of two key enzymes and their products-COX-2 and iNOS-are summarized. Possibly, pivotal molecular targets for drug therapies of brain disorders will be discovered in intracellular signaling pathways leading to activation of transcription factors. Finally, the impact of growth factors, of neurotrophins in particular, is highlighted. It is concluded that the presently available data on the molecular level is far from being statisfying, but that only from better insights into molecular events will we obtain the information required for more specific therapies.

Beta-amyloid plaques induce neuritic dystrophy of nitric oxide-producing neurons in a transgenic mouse model of Alzheimer's disease

A causative role for nitric oxide has been postulated in a number of neurodegenerative diseases. Using histochemical and immunohistochemical methods, we examined the effect of beta-amyloid plaques on nitric oxide-producing cells in transgenic mice which overexpress a mutant human amyloid precursor protein (APP). In 14-month-old animals, nitric oxide synthase (NOS)-positive dystrophic neurites were observed frequently in the cerebral cortex and hippocampus of all of 16 plaque-bearing transgenic animals and in none of 16 wild-type animals. Double labeling of NOS and beta-amyloid revealed that 90% of beta-amyloid plaques were associated with NOS-containing dystrophic neurites. In 7-month-old animals, beta-amyloid plaques were very rare, but those present were frequently associated with NOS-positive neuritic dystrophy. We conclude that beta-amyloid plaques induce neuritic dystrophy in cortical neurons containing NOS in this model of AD, and hypothesize that this finding may be relevant to the mechanism of beta-amyloid neurotoxicity in human AD.

Microglia at brain stab wounds express connexin 43 and in vitro form functional gap junctions after treatment with interferon-gamma and tumor necrosis factor-alpha

Gap junctional communication between microglia was investigated at rat brain stab wounds and in primary cultures of rat and mouse cells. Under resting conditions, rat microglia (FITC-isolectin-B4-reactive cells) were sparsely distributed in the neocortex, and most (95%) were not immunoreactive for Cx43, a gap junction protein subunit. At brain stab wounds, microglia progressively accumulated over several days and formed aggregates that frequently showed Cx43 immunoreactivity at interfaces between cells. In primary culture, microglia showed low levels of Cx43 determined by Western blotting, diffuse intracellular Cx43 immunoreactivity, and a low incidence of dye coupling. Treatment with the immunostimulant bacterial lipopolysaccharide (LPS) or the cytokines interferon-gamma (INF-gamma) or tumor necrosis factor-alpha (TNF-alpha) one at a time did not increase the incidence of dye coupling. However, microglia treated with INF-gamma plus LPS showed a dramatic increase in dye coupling that was prevented by coapplication of an anti-TNF-alpha antibody, suggesting the release and autocrine action of TNF-alpha. Treatment with INF-gamma plus TNF-alpha also greatly increased the incidence of dye coupling and the Cx43 levels with translocation of Cx43 to cell-cell contacts. The cytokine-induced dye coupling was reversibly inhibited by 18 alpha-glycyrrhetinic acid, a gap junction blocker. Cultured mouse microglia also expressed Cx43 and developed dye coupling upon treatment with cytokines, but microglia from homozygous Cx43-deficient mice did not develop significant dye coupling after treatment with either INF-gamma plus LPS or INF-gamma plus TNF-alpha. This report demonstrates that microglia can communicate with each other through gap junctions that are induced by inflammatory cytokines, a process that may be important in the elaboration of the inflammatory response.

Engagement of the scavenger receptor is not responsible for beta-amyloid stimulation of monocytes to a neurocytopathic state

Experiments were performed to determine if scavenger receptors (SRs) play a role in amyloid beta (Abeta) stimulation of peripheral blood monocyte (PBM) neurotoxicity. Results indicate that Abeta does not block binding of the SR ligand DiI-acetylated low density lipoprotein to PBM, nor does another SR ligand, fucoidin, inhibit Abeta-PBM binding. Moreover, neither of three SR ligands alone stimulates neurotoxicity in PBM, nor antagonizes the ability of Abeta to activate PBM to a neurocytopathic state. Such findings suggest that Abeta's action is not dependent upon engagement of the SR ligand binding domain and raise doubts about the role of SR in Abeta neurotoxicity.

Complement component C1q modulates the phagocytosis of Abeta by microglia

Recent studies showing that microglia internalize the amyloid beta-peptide (Abeta) suggest that these cells have the potential for clearing Abeta deposits in Alzheimer's disease, and mechanisms that regulate the removal of Abeta may therefore be of clinical interest. Previous studies from this laboratory showing that C1q enhances phagocytosis of cellular targets by rat microglia prompted the current investigations characterizing the effects of C1q on microglial phagocytosis of Abeta. Microglia were shown to phagocytose Abeta1-42, in agreement with observations of other investigators. Uptake of Abeta1-42 was observed for concentrations of 5-50 microM, and phagocytosis of peptides containing (14)C or fluorescein (FM) labels was not affected by the interaction of microglia with C1q-coated surfaces. However, inclusion of C1q (125 nM-1.4 microM) in solutions of 50 microM Abeta1-42 inhibited the uptake of (14)C-Abeta1-42 and FM-Abeta1-42, suggesting that C1q blocks the interaction of Abeta with microglia. Uptake of Abeta was partially blocked by the scavenger receptor ligands polyinosinic acid and maleylated BSA. Inhibition of Abeta uptake by C1q may contribute to the accumulation of fibrillar, C1q-containing plaques that occurs in parallel with disease progression. These data suggest that mechanisms which interfere with the binding of C1q to Abeta may be of therapeutic value both through inhibition of the inflammatory events resulting from complement activation and via altered access of Abeta sites necessary for ingestion by microglia.

Microglia and Alzheimer's disease

Microglia play a major role in the cellular response associated with the pathological lesions of Alzheimer's disease. As brain-resident macrophages, microglia elaborate and operate under several guises that seem reminiscent of circulating and tissue monocytes of the leucocyte repertoire. Although microglia bear the capacity to synthesize amyloid beta, current evidence is most consistent with their phagocytic role. This largely involves the removal of cerebral amyloid and possibly the transformation of amyloid beta into fibrils. The phagocytic functions also encompass the generation of cytokines, reactive oxygen and nitrogen species, and various proteolytic enzymes, events that may exacerbate neuronal damage rather than incite outgrowth or repair mechanisms. Microglia do not appear to function as true antigen-presenting cells. However, there is circumstantial evidence that suggests functional heterogeneity within microglia. Pharmacological agents that suppress microglial activation or reduce microglial-mediated oxidative damage may prove useful strategies to slow the progression of Alzheimer's disease.